Front. Mater. Sci. 2022, 16(3): 220607
https://doi.org/10.1007/s11706-022-0607-7
REVIEW ARTICLE
Nanoparticles embedded into glass matrices:
glass nanocomposites
Javier FONSECA (✉)
Department of Chemical Engineering, Northeastern University, Boston 02115, USA
© Higher Education Press 2022
ABSTRACT: Research on glass nanocomposites (GNCs) has been very active in the
past decades. GNCs have attracted — and still do — great interest in the fields of
optoelectronics, photonics, sensing, electrochemistry, catalysis, biomedicine, and art. In
this review, the potential applications of GNCs in these fields are briefly described to
show the reader the possibilities of these materials. The most important synthesis
methods of GNCs (melt-quenching, sol-gel, ion implantation, ion-exchange, staining
process, spark plasma sintering, radio frequency sputtering, spray pyrolysis, and
chemical vapor deposition techniques) are extensively explained. The major aim of this
review is to systematize our knowledge about the synthesis of GNCs and to explore the
mechanisms of formation and growth of NPs within glass matrices. The size-controlled
preparation of NPs within glass matrices, which remains a challenge, is essential for
advanced applications. Therefore, a thorough understanding of GNC synthesis
techniques is expected to facilitate the preparation of innovative GNCs.
KEYWORDS:
exchange
glass nanocomposites; melt-quenching; sol-gel; ion implantation; ion-
Contents
1
2
Introduction
Applications
2.1 Optoelectronics
2.1.1 Non-linear optical phenomena
2.1.2 Non-linear optical materials
2.1.2.1 Quantum dots embedded in
glass matrices
2.1.2.2 Metal nanoparticles embedded
in glass matrices
2.2 Photonics
2.3 Sensing
Received March 5, 2022; accepted April 24, 2022
E-mail: fonsecagarcia.j@northeastern.edu
3
2.4 Electrochemistry
2.5 Catalysis
2.6 Biomedicine
2.7 Art
Synthesis methods of GNCs
3.1 Melt-quenching
3.1.1 Introduction
3.1.2 Concomitant synthesis of glass matrix
and nanoparticles
3.1.3 Precipitation of nanoparticles
3.1.3.1 Heat-assisted nanoparticle
precipitation
3.1.3.2 Irradiation-assisted
nanoparticle precipitation
3.1.4 Nanoparticles added to glass precursors
before melting process
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Front. Mater. Sci. 2022, 16(3): 220607
3.2 Sol-gel technique
3.2.1 Introduction
3.2.2 Concomitant synthesis of glass matrix
and nanoparticles
3.2.3 Sol-gel technique combined with
nanoparticle or nanoparticle precursors
impregnation
3.3 Ion implantation
3.3.1 Introduction
3.3.2 Single implantation
3.3.3 Multiple implantation
3.4 Ion-exchange
3.4.1 Introduction
3.4.2 Direct formation of NPs
3.4.3 Thermal annealing in air
3.4.4 Thermal annealing in H2 atmosphere
3.4.5 Laser irradiation techniques
3.5 Less conventional techniques
3.5.1 Staining process
3.5.2 Spark plasma sintering
3.5.3 Radio frequency sputtering
3.5.4 Spray pyrolysis
3.5.5 Chemical vapor deposition
4 Conclusions
Acknowledgements
References
1 Introduction
The concept of nanotechnology was introduced by Nobel
laureate Richard P. Feynman at a meeting of the
American Physical Society in 1959. In his famous lecture
entitled “There’s Plenty of Room at the Bottom”,
Feynman revealed his idea of manipulating matter at
atomic scale. Since then, nanoscience and nanotechnology
have grown rapidly and have been applied in almost every
field related to industry [1‒3]. Nanoscience is defined as
the study of matter at nanometer scales ranging from 1 to
100 nm. Nanotechnology is that technology that utilizes
nanoscience in practical applications [4].
Subnanoclusters and nanoparticles (NPs) have attracted
interest due to their remarkable properties compared to
their bulk analogues. The changes in the properties of the
materials in the nano- and subnano-regimes are related to
their high surface-to-volume ratios and their morphology.
However, subnanoclusters and NPs are often very
unstable and tend to aggregate. The incorporation or
embedding of subnanoclusters or NPs in host materials
solves this problem [5‒8]. Logically, the properties of
embedded NPs depend on the host matrix and the hostguest interactions [9]. Similarly, the geometric
configurations and, in turn, the properties of the
subnanoclusters are strongly affected by the host matrix in
which they are incorporated. It should be mentioned that
currently the incorporation of subnanoclusters with a
narrow size distribution and a constant geometric structure
in porous supports is a great challenge [10].
Glass is an antique invention of mankind. The oldest
manufactured opaque glass, which was found in Egyptian
potteries, dates from the eighth millennium BC [11]. The
concept of glass has gradually evolved over time due to
the progress of science and technology. Nowadays,
glasses may be defined as nonequilibrium, non-crystalline
condensed state of matter that spontaneously relaxes
toward the supercooled liquid state. The structure of
glasses is similar to that of their parent supercooled
liquids. They crystallize in the limit of infinite time [12].
Glasses have been traditionally classified as inorganic,
organic, and metallic, which exhibit ionic-covalent,
covalent, and metallic bonds, respectively. Recently, a
fourth family of glasses based on metal-organic
frameworks (MOFs), which contain coordination bonds,
has been reported [13]. Glasses exhibit high optical
transparency, structural rigidity, compositional flexibility,
property tailoring suitability, and durability. They have
many applications ranging from traditional, such as
windows, mirrors, containers, lighting, lenses, and handcrafted art, to technologically advanced, such as laser
glass, radiation shielding glass, optical communication
fibers, bioglass, armor glass, solar glass, etc. [11].
Glass composites are hybrid materials made of a glass
matrix, which is the continuous phase, and the
reinforcement(s) or dispersed phase(s), which is (are)
present to a lesser extent. Glasses are excellent host
materials. The glass matrices protect the dispersed
phase(s) from the environment. The reinforcement may be
either amorphous or crystalline. The purpose of the
reinforcement(s) is to improve the properties of the glass
matrix. Thus, the physical and chemical properties in the
glass composites are different from those of the
constituent phases [11].
Glass nanocomposites (GNCs) are defined as composite
materials in which the matrix is a glass and at least one of
the reinforcements is nanometric in size. Glasses are an
exceptional matrix for NP growth due to their
Javier FONSECA. NPs embedded into glass matrices: GNCs
advantageous characteristics such as easy and inexpensive
processing, high mechanical strength, high optical
transparency, and excellent chemical durability [14‒15].
In fact, glasses with dispersed NPs were developed during
the Roman Era [16]. The Lycurgus Cup, dating from the
fourth century AD, is one of the most famous specimens.
It is an early example of nanotechnology in human
history. The Lycurgus Cup was made from silicate glass
with Au and Ag NPs (50‒70 nm) incorporated within the
matrix [17].
The properties of the GNCs depend on the glass matrix
material and the reinforcement(s) size, shape, orientation,
degree of loading, and dispersion. In GNCs, there is a
strong reinforcement-interface interaction due to the high
surface free energy of the dispersed phase. The remarkable properties of NPs embedded into glass matrices are
associated with the small size effect, the large grain
boundary effect, and the quantum confine effect. Thus,
NPs may improve the mechanical properties (such as
modulus, strength, and dimensional stability), thermal
properties (such as conductivity, stability, flame retardancy, and heat distortion temperature), electrical
properties (such as conductivity and capacitance), optical
properties (such as absorption, scattering, and photoluminescence (PL)), chemical properties (such as reactivity,
acid-alkali resistance, and durability) and surface
properties (such as appearance) of glass matrices [11].
GNCs have attracted great interest driven by their large
number of potential applications in fields such as
optoelectronics, photonics, sensing, electrochemistry,
catalysis, biomedicine, and art [18‒20]. The size, shape,
number, and size distribution of the NPs determine the
performance of the GNCs in all these fields [14].
Specifically, advanced applications require a well-defined
particle size and size distribution. Size-controlled
preparation of NPs within glass matrices remains a
challenge. Understanding the kinetics growth of NPs is an
essential prerequisite for improving NP syntheses in glass
matrices. In this review, we focus on GNC synthesis
protocols paying particular attention to the kinetics growth
of NPs within glass matrices for each synthesis approach.
Far from being obsolete materials, GNCs have an
important impact on daily life, both at a purely
commercial level and at the frontier research level. The
ancient synergy between NPs and glasses can lead to the
realization of intriguing new materials with interesting
applications. We anticipate a significant increase in
research efforts in the field of GNCs. We expect this
3
review serves as useful reading to motivate researchers at
all levels to move or return to the GNC topic.
In this review, we begin by describing the potential
applications of GNCs in the fields of optoelectronics,
photonics, sensors, electrochemistry, catalysis, biomedicine, and art. The fundamentals behind those fields are
briefly explained to understand the suitability of GNCs in
each application. In the next section of this Review, we
extensively describe the most important synthesis methods
of GNCs: melt-quenching, sol-gel, ion implantation, ionexchange, staining process, spark plasma sintering, radio
frequency sputtering, spray pyrolysis, and chemical vapor
deposition techniques. The aim of this extensive review is
to explore the mechanisms of formation of GNCs. Many
studies are reported for each GNC synthesis technique
paying special attention to the underlying mechanism of
GNC formation and the kinetics growth of NPs within
glass matrices. We conclude this review by highlighting
the importance of GNCs in various fields.
2
Applications
In this section, we describe the potential applications of
GNCs in the fields of optoelectronics, photonics, sensing,
electrochemistry, catalysis, biomedicine, and art. By
doing this, we hope to show the reader the tremendous
possibilities of GNCs.
2.1
Optoelectronics
The study of electronic devices that generate, detect and
control light is known as optoelectronics (or optronics).
Non-linear optical (NLO) glasses have attracted great
attention for decades in the field of optoelectronics due to
their high transparency, ease of manufacture, durability,
and stability [21‒29]. The non-linearity in glasses is
generally quite small, but the non-linear response can be
improved by several orders of magnitude by embedding
NPs. Particularly, all-optical switching devices based on
NLO glasses with embedded NPs can potentially
overcome the speed limitation imposed by electrical
switching devices [30]. It is worth noting that optical
devices can operate in a time range inaccessible to
electronics (< 1 ps) [31]. In non-linear switching devices,
the intensity of an optical signal changes between two
output channels that perform logic operations (Fig. 1).
All-optical switching devices include non-linear
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Front. Mater. Sci. 2022, 16(3): 220607
[34‒35].
Polarization (P, dipole moment per unit volume)
induced by an applied electric field (E) can be expressed
as a Taylor expansion as follows:
(
)
P = ε χ(1) E + χ(2) E 2 + χ(3) E 3 + ...
Fig. 1 Examples of all-optical devices: (a) directional
coupler (self-induced switching); (b) Mach‒Zehnder
interferometer. The response of linear (dashed lines) and nonlinear (solid lines) materials is also illustrated.
directional couplers, non-linear mode sorters, non-linear
distributed couplers, non-linear Mach‒Zehnder interferometer, distributed feedback grating, etc. [32‒33].
Third-order NLO susceptibility (χ(3)), also known as
optical Kerr susceptibility, assesses the applicability of
any material in all-optical switching. Both the non-linear
absorption coefficient (β ∝ Im[χ(3)]) and the non-linear
refractive index (n2 ∝ Re[χ(3)]), which play a fundamental
role in all-optical devices, are related to χ(3). The
incorporation of reinforcements in glass matrices is
currently being studied to improve the non-linearity of the
pristine glass. Both quantum dots (QDs) and metal NPs
embedded in glass matrices have been found to enhance
χ(3). Specifically, metal NPs improve the χ(3) of the
pristine glass at wavelengths very close to that of the
characteristic localized surface plasmon resonance
(LSPR) of the metal NPs [34‒35].
2.1.1
Non-linear optical phenomena
Non-linear susceptibility arises from the response of an
electron to an electromagnetic field in an anharmonic
potential well. The electromagnetic field of a light that
propagates through an optical medium exerts a polarizing
force on all the electrons that make up such medium.
Logically, the polarization is exerted mainly on the
external electrons. The polarization response is linear for
weak electric fields of light waves (weaker than the intraatomic field which is 10 V·nm−1). However, the response
becomes increasingly non-linear for electric fields
comparable to or greater than the intra-atomic field
(1)
where ε is the dielectric constant of the medium, χ(1) is the
linear dielectric susceptibility of the medium and χ(2), χ(3)
and so on are the non-linear coefficients. χ(1) involves all
the phenomena associated with linear optics, such as
reflection, refraction, and interference. Generally, χ(1) is
greater than χ(2) which, in turn, is greater than χ(3) and so
forth [34‒35].
NLO phenomena can be classified as intrinsic (or
structural) or extrinsic (or compositional). Intrinsic NLO
phenomena include all light-induced changes in structure,
such as changes in electronic density, average interatomic
distances, molecular orientation, phase transition, etc. On
the other hand, extrinsic NLO phenomena comprise all
changes in chemical composition induced by light such as
molecular dissociation, polymerization, etc. [34‒35]. As
previously mentioned, n2 and β play a critical role in alloptical devices. In amorphous materials, e.g., glasses, n2
and β are related to the real and imaginary parts of χ(3),
respectively [36]:
12 π [ (3) ]
Re χ
n0
(2)
96 π2 ω [ (3) ]
Im χ
n20 c2
(3)
n2 =
β=
where Re[χ(3)] and Im[χ(3)] are the real and imaginary
parts of χ(3), respectively; ω is the frequency of the
applied electric field, c is the speed of light, n0 is the
linear refractive index and is given by Eq. (4), and n2 and
χ(3) are in e.s.u.
(
[ ])0.5
n0 = 1 + 4 πRe χ(1)
2.1.2
(4)
Non-linear optical materials
A NLO medium must meet several basic requirements.
Proper selection of a NLO material for practical
application requires defining the application, device
characteristics and operating conditions. Anyhow,
materials with inherent χ(3) or high-intensity optical
Javier FONSECA. NPs embedded into glass matrices: GNCs
5
field-induced χ(3) are always needed, regardless of
practical application [34‒35].
The inherent χ(3) of a glass matrix can be improved by
several orders of magnitude by embedding semiconductor
or metal NPs into the glass [37]. These NPs have sizes
smaller than the wavelength of light [38]. Electrons in
these NPs are confined between infinite potential barriers,
leading to the decomposition of the conduction and
valence bands into a set of discrete levels [39]. This
quantum confinement modifies the interaction of electrons
with applied optical fields [40]. Furthermore, when the
size of the NPs is much smaller (< λ/20) than the
wavelength (λ) of the applied optical field, the electric
field that polarizes the electrons of the NPs can be very
different from the macroscopic field outside the NPs in
the surrounding medium [41]. This effect is known as
dielectric or classical confinement. Local field effects
arise from dielectric confinement due to the difference in
dielectric constants between the NPs and the surrounding
glass matrix. In brief, the χ(3) of GNCs is affected by both
quantum and dielectric confinement effects.
2.1.2.2
2.1.2.1 Quantum dots embedded in glass matrices
The GNC non-linear susceptibility (χ(3)
eff ) is related to the
non-linear susceptibility of the metal NPs (χ(3)
m ) according
to the effective medium theory [45]:
The effective dielectric constant ( ε) of a GNC (made of
QDs embedded in a glass matrix), derived from effective
medium theory in the self-consistent field approximation,
can be expressed as [42]:
(1 − p)
εQD − ε
εd − ε
+p
=0
2εQD − εd
2ε + εQD
(5)
where p is the volume fraction of the NPs, and εQD and εd
are the dielectric constants of the embedded QDs and the
glass matrix, respectively.
The optical fields inside (Ein) and outside (Eex) the QDs
are related:
Ein = fQD · Eex
(6)
where fQD is a local field enhancement factor [43]. It can
be expressed as:
[
(
)]−1
εQD
fQD = 1 + A
−1
εd
(7)
where, A is a geometry-dependent factor: A = 1/3 for
spherical particles; 1 > A > 1/3 for oblate spheroidal
shapes; and 0 < A < 1/3 for prolate spheroidal shapes.
Metal nanoparticles embedded in glass matrices
ε of a GNC, made of metal NPs embedded in a glass
matrix, is given by [44]:
ε − εd
εm − εd
=p
ε − 2εd
εm + 2εd
(8)
where εm is the dielectric constant of the metal NP.
Considerations regarding Eq. (8): (a) spherical NPs;
(b) the size of the NPs is much smaller than the
wavelength of light; (c) low-volume fraction (p << 1); and
(d) absence of interactions among NPs.
The optical fields inside (Ein) and outside (Eex) the
metal NPs are related:
Ein = fm · Eex
(9)
where fm is the local field enhancement factor. It describes
the first-order susceptibility near or at LSPR of the metal
NPs. The fm can be expressed as:
fm =
(3)
3εd
εm + 2εd
χeff = p · fm4 · χ(3)
m
(10)
(11)
where, χ(3)
is proportional to the fourth power of the local
eff
field enhancement factor, determined at a frequency close
to LSPR of metal NPs. Therefore, χ(3)
eff increases
considerably under LSPR conditions [46‒51].
The χ(3)
m of the metal NPs includes contributions from (i)
intra-band, (ii) interband, and (iii) hot-electron transitions:
(i) Conduction electrons exhibit non-linear behavior in
metal NPs due to the quantum-size effect [52]. This
contribution vanishes in large particles. In other words,
this process is highly dependent on size.
(ii) Interband transitions between the d-levels and the
conduction band also contribute to the χ(3)
m . This process is
resonant and does not depend on the size of NP [53].
(iii) Hot-electron transition. Strong absorption near the
LSPR, which is dependent on particle size, can generate
hot photo-excited electrons that change the Fermi‒Dirac
distribution and thus the interband transition [54].
As explained above, the NLO response of GNCs with
metal NPs is highly dependent on the LSPRs. The surface
plasmon resonance (SPR) is the resonant oscillation of
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Front. Mater. Sci. 2022, 16(3): 220607
conduction electrons at the interface between a negative
dielectric material (metal) and a positive dielectric
material (glass) stimulated by incident light. Specifically,
LSPR is the collective oscillation of conduction electrons
of metal NPs in resonance with the electromagnetic wave,
which occurs when the metal NPs are smaller than the
mean free path of electrons (Fig. 2) [55]. This localized
field decays rapidly from the metal NPs-glass interface to
the glass background. The LSPR frequency depends on
the nature, size, and shape of the metal NPs [56‒61], the
dielectric properties of the glass matrix [62‒63], the interNP coupling interactions [64‒66], and the wavelength of
the incident light [67]. It is worth emphasizing that the
electron cloud of the glass matrix often perturbates the
electrons of the metal NPs [68‒69]. The resonance
oscillations of the electron cloud of metal NPs allow them
to be used as sensors. In brief, an analyte adsorbed on the
surface of metal NPs generates a change in the electric
field that can be detected as SPR shifts. Furthermore, rareearth (RE) ions experience local field enhancement when
close to plasmonic NPs. This enhancement results in an
increase in the emission intensities of RE ions. The
improvement of luminescence by plasmonic NPs depends
on the LSPR frequency [70‒72], and the orientation and
distance of the luminophore from the metal NP [73‒74].
In addition, energy transfer between plasmonic NPs and
RE ions also enhances the luminescence of those ions
[75].
2.2
Photonics
Glasses doped with RE ions have been extensively
investigated over the past decades due to their
applicability in the field of photonics [76‒78]. Glasses are
considered ideal hosts for RE ions due to their mechanical
and thermal stability and cost-effective preparation. The
PL properties of the RE ions within the glasses are limited
by: (1) concentration quenching due to the solubility of
Fig. 2
LSPR generated by incising light on a metal NP.
the RE ions; and by (2) multiphonon relaxation due to the
phonon energy of such glasses [79]. Therefore, glasses
that allow a large amount of RE ions to be incorporated
and exhibit low phonon energy are very desirable [80‒82].
In addition, the small β and low quantum efficiency of
RE-doped glasses also restrict their applicability [83].
Metal NPs embedded within RE-doped glasses can
overcome these difficulties, favoring the PL of RE ions
and thus leading to high-performance photonic devices
[84‒85]. Specifically, metal NPs, through LSPR, amplify
the local electric field around the RE ions, enhancing the
emission intensity [86‒88]. Therefore, GNCs doped with
RE ions are promising photonic materials [89‒91]. More
research in this field can be expected in the coming years.
2.3
Sensing
Surface-enhanced Raman spectroscopy (SERS) is a
promising analytical technique that enables sensitive
detection with chemical specificity of a single molecule
[92‒93]. SERS overcomes the low Raman scattering cross
sections (10−30‒10−25) of the Raman effect [94]. It is
worth noting that, in Raman experiments, high
concentrations and high laser intensities are generally
required to detect Raman scattering signals. SERS is
based on LSPR at the interface of the surface of metal
NPs and dielectric hosts [95‒97]. The LSPR generally
enhanced the Raman modes of a molecule by a factor of
η2, where η is the enhancement factor. Moreover, the
emitted Raman signals are further improved by the same
LSPR effect. Therefore, the output Raman signal is
enhanced by a factor of η4 [98]. The maximum
enhancement occurs near the plasmon frequency [99].
Non-spherical metal NPs such as metallic nanorods,
nanowires, or nanofiber show improved SERS activity
compared to spherical NPs [93,100]. In other words, the
LSPR is further promoted by lightening rod effects
(LREs) on the surface of non-spherical NPs.
Since the discovery of SERS by Fleischman et al. in
1974 [101], great efforts have been made to develop
inexpensive SERS-active substrates with high sensitivity,
reproducibility, and stability [102‒105]. It is especially
difficult to produce long-term stable SERS substrates with
a high degree of reproducibility. In particular, GNCs are
considered promising SERS-active substrates [106‒109].
Research on SERS-active GNCs is expected to be
expanded in the near future.
Javier FONSECA. NPs embedded into glass matrices: GNCs
2.4 Electrochemistry
An immediate challenge for modern society is to reduce
greenhouse gases by replacing vehicles that run on
internal combustion engine with those that run on safe,
low-cost rechargeable batteries with a high volumetric
energy density and long life cycle [110]. Traditional
rechargeable batteries use an aqueous electrolyte that
rapidly transports H+ cations. The energy-gap “window”
(Eg = 1.23 eV) of an aqueous electrolyte restricts the
discharge voltage of a cell with a long shelf life to Vdis ≤
1.5 V [111]. The organic-liquid Li+ electrolytes of the
lithium-ion battery are inexpensive, easy to prepare, and
have the required high conductivity of Li+. However, they
are flammable. The formation and growth of dendrites
across a liquid electrolyte to the cathode during charge
can cause an internal short-circuit with incendiary
consequences unless the dendrites are blocked [112]. The
organic-liquid electrolyte has an energy-gap window of
around 3 eV that limits the voltage of a cell with a long
life cycle to Vdis ≤ 3.1 V [111]. Furthermore, if the
battery cells have an anode with a chemical potential
(Fermi level) lower than that of lithium metal, a Li+conductive solid-electrolyte interphase (SEI) passivating
layer forms on the anode that prevents electrolyte
reduction. The formation/dissolution of the SEI layer in a
charge/discharge cycle restricts the life cycle of a cell
[113]. The solid-state batteries (SSBs) that use solid
electrolytes (SEs) can be an alternative [114]. Basically, a
SSB is an electrochemical chain characterized by the
continuity of cation transport from one electrode to
another [115]. SEs offer some unique beneficial features:
(1) Blocking dendrite growth and even preventing
dendrite formation, thus eliminating the safety issue.
(2) High current densities and fast charging times.
(3) Stability at elevated temperatures. Thus, SEs
improves battery safety.
(4) No bulk polarization limiting the cell current. Li+
cation is the only mobile species. The associated anion is
immobilized in a rigid macromolecular network.
Therefore, there is no bulk polarization.
SEs can be divided into inorganic solids (crystalline,
glass or glass-ceramic in nature) and organic solid
polymers [116]. However, the lithium-ion conductivity of
the polymer electrolytes is too low for battery operation at
room temperature. In addition, the rate capability of these
electrolytes is limited, preventing fast charging [117].
Polymer-based batteries will certainly not enter the
7
market. On the other hand, the potential of glass SEs has
been demonstrated [113]. Glass SEs are of special interest
because various glass compositions exhibit good lithium
conductivities (10−4 to 10−3 S·cm−1) at room temperature
[118]. The critical question is whether alkali-metal anodes
can be plated through a glass electrolyte efficiently at high
rates for thousands of charge/discharge cycles. Logically,
a low cost per kWh of energy storage capacity, which is
highly dependent on manufacturing, is also a key
requirement [119]. In brief, GNCs are attracting a lot of
research attention recently due to their promising ability
as SEs in SSBs [120‒127].
2.5
Catalysis
Catalysis is an essential technology to accelerate and
direct chemical transformations [128‒129]. Heterogeneous catalysts have several advantages over homogeneous catalysts: easy catalyst separation, reuse, minimal
product contamination, and high stability [130‒131].
Heterogeneous catalytic processes constitute 90% of all
chemical production [132]. Specifically, supported metal
NPs are the most common type of heterogeneous catalyst
in industry [132]. Metal NPs are excellent catalysts in
many industrially important chemical reactions
[133‒134]. The size, shape, and surface functionalities of
metal NPs control the activity of the metal NP-based
catalysts [128,135]. Metal NPs, which exhibit extremely
high surface energy, tend to aggregate and, thus, reduce
their catalytic performance. Therefore, one of the critical
functions of the support is to stabilize and immobilize the
metal NPs. The support can also affect the catalytic
efficiency of the metal NP and alter the reaction
mechanism [128]. Glasses, which are inexpensive,
environmentally friendly, durable, reusable, and inert, are
candidates to replace conventional catalyst supports. The
main drawback of glasses to be used as metal NP supports
is their lack of porosity. It is worth mentioning that highsurface-area porous supports allow the diffusion of
reagents, products, unreacted precursor molecules and
byproducts. Consequently, porous glasses are gaining
attention as support materials of metal NP catalysts. In
other words, GNCs are attracting interest as catalyst
[136‒140].
2.6
Biomedicine
Since their development in the late 1960s, bioactive
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Front. Mater. Sci. 2022, 16(3): 220607
glasses (BGs) have been applied in many biomedical
practices such as bone regeneration, soft tissue repair,
dental grafting, and antibacterial treatment [141‒145]. It is
worth mentioning that Bioglass® 45S5 (Na2O‒CaO‒
SiO2‒P2O5), which was first reported by Hench et al. in
1971 [146], has revolutionized tissue engineering. During
the last decade, bioactive glass nanoparticles (BGNs) have
gained attention due to their small size, large specific
surface area, and large surface to volume ratio [147].
Specifically, the porosity and surface area of BGNs favor
their applicability as carriers of drugs and other
biomolecules. BGNs exhibit obvious advantages over
BGs in drug delivery [148].
The incorporation of NPs into BGs and BGNs has been
found to improve their healing qualities, their bioadhesive
strength, and their anticancer potential. The treatment of
malignant bone tumors has attracted the attention of many
research groups in the field of bone tissue engineering
[149‒150]. As innovative anticancer agents, NPs with
anticancer potential are embedded in BGN. These NPs are
released by the BGN and taken up by bone cancer cells.
Ideally, NPs for this application have high toxicity to bone
cancer cells and very low toxicity to healthy cells,
promoting rapid bone healing [151]. BGs with embedded
iron oxide NPs have also been developed for bone
regeneration and anticancer purposes. It should be
mentioned that, when exposed to a magnetic field,
magnetic NPs generate a local temperature rise, which
favors hyperthermia therapy [152‒153].
NPs have also been embedded in BGs and BGNs for
antibacterial purposes [154]. A composite prepared by
incorporating Cu NPs into BGNs (Cu NPs@BGNs) and
an eggshell membrane (ESM) has shown great efficiency
in wound healing. Cu NPs@BGN significantly improved
the surface hydrophilicity and surface hardness of ESM.
Moreover, Cu NPs@BGN stimulated angiogenesis,
inhibited the growth of bacteria, and improved healing
quality and formation of continuous and uniform
epidermis layers [155].
2.7
Art
GNCs have been used for decorative purposes since
ancient times. The Romans used Au NPs to color glasses
as early as the 4th century BC [156]. Silver stain
decoration, which is based on incorporating a surface
layer of Ag NPs within a glass, was developed in
Mesopotamia in the 8th century AD. This technique
evolved towards the luster decoration consisting of a thin
glass film, composed of a heterogeneous distribution of
Ag and Cu NPs, deposited on a ceramic substrate [157].
The luster decoration exhibits peculiar optical properties,
producing brilliant reflections of different colors and
iridescence. Silver stain and luster decorations were
carried over to the West through Spain during the 13th
century [158‒159]. These decorative techniques began to
be applied to color window glasses during the 13th
century and became widespread in the 14th, 15th and 16th
centuries. Thus, the glass windows of cathedrals were
colored with metal NPs in Europe during the Middle
Ages. In Italy, luster decoration was used to produce the
well-known polychrome luster Renaissance pottery of
Deruta and Gubbio [157]. Historical GNCs may suffer
surface degradation due to centuries of exposure to
weathering. Nowadays, studies on the degradation of
GNCs are promoted to facilitate preservation and
restoration of the Historical Heritage [160‒162].
3
Synthesis methods of GNCs
The most important synthesis methods of GNCs (meltquenching, sol-gel, ion implantation, ion-exchange,
staining process, spark plasma sintering, radio frequency
sputtering, spray pyrolysis, and chemical vapor deposition
techniques) are described in detail in this section. Table 1
compares the different GNC synthesis methods. The
mechanisms of GNC formation and the kinetics growth of
NPs within glass matrices are reviewed.
3.1
3.1.1
Melt-quenching
Introduction
Melt-quenching technique is a long-standing, wellestablished and simple method to produce GNCs (Fig. 3).
In fact, most GNCs have been prepared by this technique.
In this process, an appropriate mixture of raw materials is
prepared. The mixture is generally milled in a wet
medium, such as acetone, to obtain a uniform powder.
Then, it is dried in the air to evaporate the wet medium
and calcined at a temperature below the melting point to
release out the gaseous substances (moisture + gas). The
calcined homogeneous mixture of raw materials is held at
a certain temperature for a certain duration depending on
the melting point of its components. In doing so, a
Javier FONSECA. NPs embedded into glass matrices: GNCs
Table 1
9
Comparison of the different GNC synthesis methods
Merits
GNC synthesis methods
Meltquenching
Advantage
Disadvantage
Sol-gel
Ion
implantation
Ion-exchange
Staining
process
Spark plasma
sintering
Radio
frequency
sputtering
Short
processing
times, low
temperature
processing,
control of NP
size, control of
NP size
distribution
Long-standing, Inexpensive,
Short
Inexpensive, Long-standing,
Short
inexpensive,
low
processing control of NP inexpensive,
processing
simple, short temperature
times, low location within
does not
times,
processing
processing
temperature the glass, high damage glass, continuous
times,
processing, concentration co-embedding processing
continuous
control of NP of NPs within various NPs
processing
location within the glass, does
the glass, high not damage
concentration
glass, coof NPs within embedding
the glass
various NPs
High
Complex, long Expensive, no Limited to
Limited to
Expensive,
Expensive,
temperature
processing control of NP some species some species complex, high complex, no
processing, no
times,
size
of NPs
of NPs
temperature control of NP
control of NP discontinuous distribution,
processing location within
size, no control processing, no may damage
the glass
of NP size control of NP glass, may
distribution, no location within
generate
control of NP
the glass
impurities
location within
the glass
Fig. 3
Spray
pyrolysis
CVD
Inexpensive, Simple, short
short
processing
processing
times,
times,
continuous
continuous
processing
processing
High
temperature
processing,
low
production
rate
High
temperature
processing,
high purity
precursors
Schematic illustration of GNC preparation using melt-quenching technique.
uniform fusion of molten materials is achieved. The
homogenization of the molten GNC can be achieved by
stirring. The molten GNC is poured in a preheated mold.
The GNCs are generally quenched in air or in liquid such
as liquid nitrogen or mercury. The GNC can also be
quenched in contact with a solid material with high
thermal conductivity to achieve ultrafast cooling rates.
The obtained GNC is annealed near the glass transition
temperature (Tg) to remove residual thermal stresses
generated during quenching. Finally, the as-prepared
GNCs are usually sawed, grinded, and polished for
subsequent characterization or application. The formation
of NPs and their size distribution within the glass matrices
can only be partially controlled with this method.
Therefore, when GNCs are prepared by melt-quenching
technique, their properties are only slightly tailorable
[11,163].
Three melt-quenching approaches are distinguished for
embedding NPs in a glass matrix: (1) concomitant
synthesis of glass matrix and NPs; (2) precipitation of
NPs; and (3) addition of NPs to the glass precursors
before the melting process. Table 2 shows examples of
GNCs prepared by melt quenching techniques
[75,97,164‒189]. These methods are reviewed in the
following sections paying particular attention to the
synthesis protocol.
3.1.2 Concomitant synthesis of glass matrix and
nanoparticles
Glass matrix and NPs can be prepared concomitantly by
melt-quenching. Controlling the size of NPs is the great
challenge of this approach. The size of NPs is often highly
dependent on melting temperature and time. It should be
Oxyfluoride glass (36.10 wt.% SiO2,
19.61 wt.% Al2O3, 14.91 wt.% ZnF2,
21.14 wt.% SrF2, 5.70 wt.% B2O3,
and 2.54 wt.% Na2O)
Oxyfluoride glass (35.06 wt.% SiO2,
19.04 wt.% Al2O3, 14.48 wt.% ZnF2,
20.53 wt.% SrF2, 5.54 wt.% B2O3,
2.47 wt.% Na2O, and 2.89 wt.% YbF3)
Tellurite glass (85.70 wt.% TeO2,
11.58 wt.% ZnO, and 2.72 wt.% Er2O3)
2.49
0.007
0.03
2.37
(7.5)
5‒15
10‒35
(5)
0.38
2.49
(12)
Ag NPs
5.14
5.23
‒
0.001
0.001
1.10
0.11
0.24
0.23
0.5‒1
Loading of
NPs/wt.% in
excess
(3.5)
(< 5)
(< 5)
3‒9
(40)
(40)
30‒40
30‒40
(20)
5‒20
(4.8)
Size of NPs
(min‒max
(mean))/nm
Ag NPs
Ag NPs
Au NPs
Au NPs
Au NPs
Au NPs
Au NPs
Cu NPs
Sodium borosilicate glass (62.48 wt.%
Ag NPs
SiO2, 8.78 wt.% B2O3, 11.72 wt.% Na2O,
3.97 wt.% NaF, and 13.06 wt.% SrO)
Sodium borosilicate glass (62.48 wt.%
Ag NPs
Ag NPs@SmF3-doped
sodium borosilicate
SiO2, 8.78 wt.% B2O3, 11.72 wt.% Na2O,
glass
3.97 wt.% NaF, 13.06 wt.% SrO, and
2.49 wt.% SmF3)
Bi-coated Ag
Bismuthate glass (79.13 wt.% Bi2O3, Ag NPs coated
NPs@bismuthate glass
12 wt.% K2O, and 8.87 wt.% B2O3)
with Bi
Bi-coated Ag
Bismuthate glass (79.13 wt.% Bi2O3, Ag NPs coated
NPs@bismuthate glass
12 wt.% K2O, and 8.87 wt.% B2O3)
with Bi
Ag NPs@calcium
Calcium fluorophosphate glass
Ag NPs
fluorophosphate glass
(12.63 wt.% CaO, 4.40 wt.% CaF2,
79.94 wt.% P2O5, and 3.03 wt.% SnO)
Ag NPs@sodium
borosilicate glass
Ag NPs@tellurite glass
Ag NPs@oxyfluoride
glass
Ag NPs@oxyfluoride
glass
Au NPs@bismuthate
glass
Au NPs@lanthanum
borate glass
Au NPs@lanthanum
borate glass
Au NPs@lanthanum
borate glass
Au NPs@lanthanum
borate glass
Cu NPs@tellurite glass
Cu NPs@tellurite glass
Lanthanum borate glass (23.05 wt.%
La2O3, 47.38 wt.% PbO,
and 29.56 wt.% B2O3)
Lanthanum borate glass (23.05 wt.%
La2O3, 47.38 wt.% PbO,
and 29.56 wt.% B2O3)
Lanthanum borate glass (23.05 wt.%
La2O3, 47.38 wt.% PbO,
and 29.56 wt.% B2O3)
Lanthanum borate glass (23.05 wt.%
La2O3, 47.38 wt.% PbO,
and 29.56 wt.% B2O3)
Bismuthate glass (91.22 wt.% Bi2O3,
6.82 wt.% B2O3, and 1.96 wt.% SiO2)
CdSe QDs
Silicate glass (56 wt.% SiO2,
8 wt.% B2O3, 24 wt.% K2O,
3 wt.% CaO, and 9 wt.% BaO)
Tellurite glass (87.81 wt.% TeO2, 7.76
wt.% Sb2O3, 2.33 wt.% Yb2O3, 0.97
wt.% Ce2O3, and 1.13 wt.% Er2O3)
Tellurite glass (90.94 wt.% TeO2,
7.91 wt.% Sb2O3, and 1.15 wt.% Er2O3)
Cu NPs
Reinforcement
Host matrix
Composite
CdSe QDs@silicate
glass
GNCs prepared by melt-quenching techniques [75,97,164–189]
Table 2
‒
Applications
Heat-assisted precipitation of NPs: meltquenching for glass preparation (melting at
1200 °C for 15 min); heat-assisted NP
precipitation (at 550 °C for 10 h)
Concomitant synthesis of glass matrix and NPs:
melt-quenching (melting at 1100 °C for
10 min); annealing (at 350 °C for 2 h)
Concomitant synthesis of glass matrix and NPs:
melt-quenching (melting at 800 °C for 15 min);
annealing (At 350 °C for 8 h)
Concomitant synthesis of glass matrix and NPs:
melt-quenching (melting at 1450 °C for 1 h);
annealing (at 450 °C for 2 h)
Concomitant synthesis of glass matrix and NPs:
melt-quenching (melting at 1200 °C for
30 min); annealing (at 390 °C for 2 h)
Concomitant synthesis of glass matrix and NPs:
melt-quenching (melting at 1450 °C for 45 min)
Concomitant synthesis of glass matrix and NPs:
melt-quenching (melting at 1100 °C for
30 min); annealing (at 500 °C for 2 h)
‒
Nanophotonics and
optoelectronics
Solid-state lasers
Nanophotonics
LED illumination and
solar cells
NLO devices: χ(3)(800 nm) =
4.88 × 10−10 esu
NLO devices: n2(532 nm) =
5.6 × 10−14 m2·W−1
NLO devices: n2(532 nm) =
6.7 × 10−14 m2·W−1
Concomitant synthesis of glass matrix and NPs:
IR optical devices
melt-quenching (melting at 800 °C for 3 min),
Sb2O3 redox route
Concomitant synthesis of glass matrix and NPs:
Lasers (biomedical and
melt-quenching (melting at 800 °C for 3 min),
bioimaging fields);
Sb2O3 redox route
thermophotovoltaic conversion
of thermal radiation
Concomitant synthesis of glass matrix and NPs: NLO devices: β(532 nm) =
melt-quenching (melting at 1100 °C for
2 × 10−11 m·W−1
30 min); annealing (at 500 °C for 2 h)
NLO devices: β(532 nm) =
1.4 × 10−12 m·W−1
Concomitant synthesis of glass matrix and NPs:
melt-quenching (melting at 1400 °C for 1.5 h)
Melt-quenching technique
[173]
[172]
[171]
[97]
[170]
[169]
[168]
[167]
[166]
[165]
[164]
Ref.
10
Front. Mater. Sci. 2022, 16(3): 220607
Au NPs
Silicate glass (70.02 wt.% SiO2,
9.34 wt.% CaO, and 20.64 wt.% Na2O)
Silicate glass (70.02 wt.% SiO2,
Ag NPs and Au
9.34 wt.% CaO, and 20.64 wt.% Na2O)
NPs
Ag NPs
Silicate glass (70.02 wt.% SiO2,
9.34 wt.% CaO, and 20.64 wt.% Na2O)
Borate glass (6.78 wt.% K2O, 4.04 wt.%
CaO, and 89.19 wt.% B2O3)
Soda-lime silicate glass (46.62 wt.%
SiO2, 18.37 wt.% B2O3, 7.19 wt.% MgO,
9.62 wt.% Na2O, and 18.20 wt.% Al2O3)
Borosilicate glass (75.22 wt.% SiO2,
16.98 wt.% Na2O, 3.01 wt.% Al2O3, and
4.79 wt.% CaO)
Borosilicate glass (72.68 wt.% SiO2,
15.67 wt.% Na2O, 2.19 wt.% Al2O3, and
9.45 wt.% CaO)
Ag NPs@silicate glass
Au NPs@silicate glass
Ag NPs/Au
NPs@silicate glass
Cu NPs@borate glass
Cu NPs@soda-lime
silicate glass
Ag NPs@borosilicate
glass
Au NPs@borosilicate
glass
(9)
Cu NPs
(1.8)
1‒5.6 (2.8)
3‒5 (4)
‒
6‒8
1‒8
Cu NPs
Au NPs
Ag NPs
Ag NPs and Au
NPs
Phosphate glass (82.58 wt.% P2O5,
3.69 wt.% MgO, 11.10 wt.% ZnSO4,
and 2.63 wt.% Eu2O3)
Ag NPs/Au
NPs@phosphate glass
(27.65)
(6.40)
Ag NPs@lanthanum
sodium borate glass
Ag NPs
Lanthanum sodium borate glass
(67.23 wt.% B2O3, 11.14 wt.% Na2O,
19.52 wt.% La2O3, and 2.11 wt.% Eu2O3)
Ag NPs@lanthanum
sodium borate glass
(2.72)
Ag NPs
Lanthanum sodium borate glass
(67.23 wt.% B2O3, 11.14 wt.% Na2O,
19.52 wt.% La2O3, and 2.11 wt.% Eu2O3)
Lanthanum sodium borate glass
(67.23 wt.% B2O3, 11.14 wt.% Na2O,
19.52 wt.% La2O3, and 2.11 wt.% Eu2O3)
Ag NPs@lanthanum
sodium borate glass
(42)
Ag NPs
Size of NPs
(min‒max
(mean))/nm
(15)
(4.64)
Calcium fluorophosphate glass
(12.63 wt.% CaO, 4.40 wt.% CaF2,
79.94 wt.% P2O5, and 3.03 wt.% SnO)
Ag NPs@calcium
fluorophosphate glass
Ag NPs
Reinforcement
Ag NPs
Calcium fluorophosphate glass
(12.63 wt.% CaO, 4.40 wt.% CaF2,
79.94 wt.% P2O5, and 3.03 wt.% SnO)
Host matrix
Ag NPs@calcium
fluorophosphate glass
Composite
Melt-quenching technique
Heat-assisted precipitation of NPs: meltquenching for glass preparation (melting at
1200 °C for 15 min); heat-assisted NP
precipitation (at 550 °C for 30 h)
2.37
Heat-assisted precipitation of NPs: meltquenching for glass preparation (melting at
1200 °C for 15 min); heat-assisted NP
precipitation (at 550 °C for 50 h)
0.17
Heat-assisted precipitation of NPs: meltquenching for glass preparation (melting at
1050 °C for 2 h); heat-assisted NP precipitation
(at 450 °C for 10 h)
0.17
Heat-assisted precipitation of NPs: meltquenching for glass preparation (melting at
1050 °C for 2 h); heat-assisted NP precipitation
(at 450 °C for 15 h)
0.17
Heat-assisted precipitation of NPs: meltquenching for glass preparation (melting at
1050 °C for 2 h); heat-assisted NP precipitation
(at 450 °C for 20 h)
Heat-assisted precipitation of NPs: melt0.44 wt.% Ag,
quenching for glass preparation (melting at
1.5 wt.% Au
1100 °C for 1.5 h); heat-assisted NP
precipitation (at 300 °C for 3 h)
Irradiation-assisted precipitation of NPs: melt‒
quenching for glass preparation (melting at
1550 °C for 1 h); femtosecond laser irradiation;
annealing (at 550 °C for 10 min)
Irradiation-assisted precipitation of NPs: melt‒
quenching for glass preparation (melting at
1550 °C for 1 h); femtosecond laser irradiation;
annealing (at 550 °C for 30 min)
Irradiation-assisted precipitation of NPs: melt‒
quenching for glass preparation (melting at
1550 °C for 1 h); femtosecond laser irradiation;
annealing
Irradiation-assisted precipitation of NPs: melt‒
quenching for glass preparation (melting at
1000 °C for 30 min); annealing (at 300 °C for
2 h); femtosecond laser irradiation
Irradiation-assisted precipitation of NPs: melt‒
quenching for glass preparation (melting at
1400 °C for 1 h); annealing (at 400 °C for
12 h); femtosecond laser irradiation; heat
treatment (at 600 °C for 1 h)
Irradiation-assisted precipitation of NPs: melt‒
quenching for glass preparation (melting at
1450 °C for 4 h); X-ray irradiation (34 min);
heat treatment (at 400 °C for 15 min)
Irradiation-assisted precipitation of NPs: melt‒
quenching for glass preparation (melting at
1400 °C for 3 h); annealing (at 550 °C for
15 min); X-ray irradiation (5 min); heat
treatment (at 600 °C for 20 min or for 3 h)
Loading of
NPs/wt.% in
excess
2.37
Art, optical memory, and alloptical switching devices
Art, optical memory, and alloptical switching devices
Art, optical memory, and alloptical switching devices
Art, optical memory, and alloptical switching devices
Art, optical memory, and alloptical switching devices
Art, optical memory, and alloptical switching devices
Art, optical memory, and alloptical switching devices
Solid-state lasers
Solid-state lasers
Applications
[182]
[181]
[180]
[179]
[178]
[177]
[176]
[175]
[174]
Ref.
(continued)
Javier FONSECA. NPs embedded into glass matrices: GNCs
11
Borate glass (58.10 wt.% B2O3,
31.12 wt.% ZnO, and 10.78 wt.% Na2O)
Bio-borosilicate glass (10.17 wt.% SiO2,
14.15 wt.% B2O3, and 75.68 wt.% TeO2)
Antimony phosphate glass
(68.47 wt.% SbPO4, 16.47 wt.% ZnO,
and 15.06 wt.% PbO)
NiO NPs@borate glass
Sm2O3 NPs@bioborosilicate glass
CdFe2O4/SiO2
NPs@antimony
phosphate glass
Tellurite glass (86.97 wt.% TeO2,
10.17 wt.% ZnO, 0.21 wt.% LiO, and
2.66 wt.% Er2O3)
Phosphate glass (77.24 wt.% P2O5,
19.68 wt.% ZnO, and 3.08 wt.% Er2O3)
Phosphate glass (77.10 wt.% P2O5,
19.79 wt.% ZnO, and 3.10 wt.% Er2O3)
Fe3O4 NPs@tellurite
glass
Fe3O4 NPs@phosphate
glass
Fe3O4 NPs@phosphate
glass
a) Size of NPs before being incorporated into the glass.
Tellurite glass (88.44 wt.% TeO2,
and 11.56 wt.% ZnO)
Fe3O4 NPs@tellurite
glass
(26)
Fe3O4 NPs
2.35
18‒70(a)
3.6222
2.72
3.18
18‒70(a)
(31)
2.5
(3.5)
8.37
0.52
(70)a)
(71.74)
0.4
‒
Loading of
NPs/wt.% in
excess
(4.93)
2‒8
Size of NPs
(min‒max
(mean))/nm
Fe3O4 NPs
Fe3O4 NPs
Fe3O4 NPs
CdFe2O4/SiO2
NPs
Sm2O3 NPs
NiO NPs
Ag NPs
Cu NPs
Silicate glass (75.32 wt.% SiO2,
16.8 wt.% Na2O, 7.6 wt.% CaO,
and 0.28 wt.% SnO2)
Phosphate glass (77.46 wt.% P2O5,
3.79 wt.% MgO, 15.19 wt.% ZnSO4,
and 3.56 wt.% Ho2O3)
Cu NPs@silicate glass
Ag NPs@phosphate
glass
Reinforcement
Host matrix
Composite
Applications
Irradiation-assisted precipitation of NPs: melt‒
quenching for glass preparation; ion irradiation;
heat treatment (at 400 °C for 10 h)
NPs added before glass formation: meltSolid-state lasers
quenching for glass preparation (melting at
1100 °C for 1.5 h); annealing
(at 300 °C for 3.5 h)
NPs added before glass formation: meltSemiconductor applications
quenching for glass preparation (melting at
1200 °C for 2 h); annealing (at 400 °C for 1 h)
NPs added before glass formation: meltNon-linear optics
quenching for glass preparation (melting at
1050 °C for 3 h); annealing (at 400 °C for 2 h)
NPs added to a molten glass: melt-quenching
Magneto-optical devices
for glass preparation (melting at 1100 °C for
30 min); NPs added to a molten glass; meltquenching for embedding CdFe2O4/SiO2 NPs
(melting at 1100 °C for 7 min)
NPs added before glass formation: meltMagneto-optical devices
quenching for glass preparation (melting at
850 °C for 20 min); annealing
(at 300 °C for 3 h)
NPs added before glass formation: meltMagneto-optical devices
quenching for glass preparation (melting at
850 °C for 20 min); annealing
(at 300 °C for 3 h)
NPs added before glass formation: meltMagneto-optical devices
quenching for glass preparation (melting at
950 °C for 1 h); annealing (at 300 °C for 3 h)
Melt-quenching technique
[189]
[188]
[187]
[186]
[185]
[184]
[75]
[183]
Ref.
(continued)
12
Front. Mater. Sci. 2022, 16(3): 220607
Javier FONSECA. NPs embedded into glass matrices: GNCs
noted that the high melting temperatures used to obtain
the glasses can dissolve NPs.
Franco et al. have developed a melt-quenching
approach, referred to as the Sb2O3 redox route, to obtain
Cu NPs embedded in a glassy matrix (NaPO3‒Sb2O3‒
CuO) [190]. The precursors were melted at 1000 °C under
atmospheric conditions. Subsequently, the melt was
quenched. Both Cu2+ and Cu+ ions, as well as Cu0, were
present in the glass. The reducing properties of Sb2O3
during melting were used to decrease the oxidation
number of copper ions in the glassy matrix. In other
words, the oxidation reaction Sb3+ → Sb5+ + 2e− during
the melting process induced the reduction of Cu2+ ions.
Thus, the presence of Cu NPs was favored by high Sb2O3
concentration in the glasses. Moreover, the average
particle size was found to increase with increasing
reducing agent content. Therefore, control of the Cu NP
size was possible through composition adjustments. The
presence of Cu+ can be explained by an incomplete
reduction process from Cu2+ to Cu0 species, leading to the
formation of Cu2O, or by a partial oxidation process of Cu
NPs, forming Cu‒Cu2O core‒shell NPs near to the surface
of the glass. The GNCs with different contents of Sb2O3
were doped with Er2O3. Cu NPs enhanced the
fluorescence of the Er3+ ions [190]. The Sb2O3 redox
route has also been applied to incorporate Cu NPs into a
tellurite glass (TeO2‒Sb2O3‒CuO) [165]. The glasses
were prepared by melting the precursors at 800 °C for 3 or
20 min and then quenching. The formation of Cu NPs
embedded in tellurite glasses was found to depend on the
melting time. Cu NPs with dimensions ranging from 5 to
20 nm were observed when the melting time was 3 min.
However, when the melting time increased, Cu NPs
solubilized leading to the formation of Cu2+. The tellurite
glass (obtained through 20 min of melting) and the Cu
NPs@tellurite glass (prepared through 3 min of melting)
were doped with Er3+, Yb3+ and Ce3+, which produced the
composites called TeSbCuYCE and TeSbCuYCE + Cu
NP, respectively. The influence of Cu NPs in the infrared
emission Er3+: 4I13/2 → 4I15/2 at 1.55 µm in TeSbCuYCE +
Cu NP was investigated. It is worth mentioning that the
Yb3+ ion is an activator for this erbium transition and the
Ce3+ ion increases the 1.55 µm transition lifetime and
emission [191‒194]. The LSPR of the Cu NPs enhanced
the 1.55 µm Er3+ emission by 47%. Moreover, the
lifetime of the Er3+: 4I13/2 → 4I15/2 transition increased by
about 50% in the presence of Cu NPs. TeSbCuYCE + Cu
NP can be used in infrared optical devices [165].
13
Er3+-doped TeO2‒Sb2O3‒CuO glass with embedded Cu
NPs, which has also been prepared by the Sb2O3 redox
route, has exhibited vibronic transitions of Er3+ ions at
room temperature. This unprecedented behavior can be
applied in biomedical and bioimaging fields and in
thermophotovoltaic conversion of thermal radiation [166].
Rajaramakrishna et al. have embedded Au NPs in a
lanthanum borate glass (La2O3‒PbO‒B2O3) by meltquenching technique [167‒168]. A mixture of raw
materials was melted at 1100 °C for 30 min. The molted
GNC was quenched at 200 °C and subsequently annealed
at 500 °C for 2 h. All Au3+ ions were found to be reduced
to Au0 after melt-quenching. The size of the Au NPs in
the as-prepared GNCs ranged between 2 and 9 nm. These
NPs grew at 40 nm when annealed. At 532 nm, β of the
composites with 1.1 and 0.11 wt.% Au NPs was
1.4 × 10−12 and 2 × 10−11 m·W−1, respectively [167]. At
532 nm, n2 of the composites with 0.001 wt.% Au NPs
annealed at 500 °C for 2 and 4 h was 6.7 × 10−14 and
5.6 × 10−14 m2·W−1, respectively [168]. Similarly, Au
NPs have also been embedded into a bismuthate glass
(Bi2O3‒B2O3‒SiO2) using melt-quenching approach
[169]. Raw materials were melted at 1200 °C for 30 min
and subsequently quenched. The quenched composite was
annealed at 390 °C for 2 h. The size of the Au NPs in the
glass ranged between 3 and 9 nm (Fig. 4). This GNC
showed the χ(3) of 4.88 × 10−10 esu at 800 nm, which was
of two orders of magnitude higher than that of the pristine
glass [169]. Therefore, these GNCs (Au NPs@lanthanum
borate glass and Au NPs@bismuthate glass) are
potentially useful for NLO devices.
Ag NPs have been formed within an Yb3+-doped
oxyfluoride glass by a melt-quenching method [170]. The
precursors were mixed, and then melted at 1450 °C for
45 min. The molted GNC was quenched at room
temperature. Ag+ ions were reduced to Ag0 atoms during
the melt-quenching process. Ag nanoclusters (in the
Agmn+ form) smaller than 5 nm were suggested to be
homogeneously distributed within the glass matrix. The
GNC exhibited broad absorption and emission bands in
the ultraviolet and visible spectral range, respectively. The
GNC displayed a luminescence quantum yield (QY) of
96.75%. Due to the energy transfer from Ag NPs to Yb3+,
the NIR emission of this GNC (Ag NPs embedded in an
Yb3+-doped oxyfluoride glass) was four times stronger
than that of Yb3+-doped oxyfluoride glass. This GNC is a
promising candidate for LED illumination and solar cells
[170]. In a similar study, Dousti et al. have embedded Ag
14
Front. Mater. Sci. 2022, 16(3): 220607
Fig. 4 (a) TEM image of the GNC (Au NPs@bismuthate
glass). (b) HRTEM image of Au NP embedded into the glass
matrix. Reproduced with permission from Ref. [169]
(Copyright 2011 Elsevier).
NPs in an Er3+-doped tellurite glass (TeO2‒ZnO‒Er2O3)
using melt-quenching method (Fig. 5) [97]. The precursor
mixture was melted at 800 °C for 15 min and
subsequently quenched and annealed at 350 °C for 8 h.
Ag NPs were formed and grown during melting and
annealing, respectively. The average size of Ag NPs
embedded in the glass matrix was approximately 12 nm.
The Ag NPs showed a non-spherical shape. Ag NPs
enhanced the Raman and PL intensities (of Er3+) eight and
five times, respectively. It was suggested that high
concentration of Ag NPs improves Raman and PL
intensities in three ways: (1) providing the “hot-spots”,
(2) increasing the coalescence of the NPs, and
(3) favoring the energy transfer from NP to fluorophore.
The authors also observed that quenching of PL emission
in the visible range occurred after further annealing due to
dissolution of aggregated NPs. In brief, improving the upconversion luminescence and Raman intensity in RE
doped glasses contributes to the development of
nanophotonics [97].
Guo et al. have investigated the effects of SmF3 doping
on the formation and growth of Ag species (including Ag+
ions, Ag nanoclusters and Ag NPs) in a sodium
borosilicate glass (SiO2‒B2O3‒Na2O‒NaF‒SrO) [171].
Ag species embedded in the SmF3-doped glass were
prepared by melt-quenching. A mixture of raw materials
was melted at 1450 °C for 1 h and subsequently quenched
and annealed at 450 °C for 2 h. It should be noted that
while the average size of Ag NPs in the undoped glass
was around 3.5 nm, the mean size of Ag NPs in the SmF3doped glass increased to 7.5 nm. Sm3+ and Sm2+ ions
were suggested to accelerate the formation and growth of
Ag NPs as follows:
Sm3+ + e− → Sm2+
(12)
Fig. 5 (a) UV-vis spectra of GNCs. Notation: A05H8 =
Composite made from Ag NPs (0.5 mol.% AgCl precursor)
embedded into an Er3+-doped tellurite glass; A10H8 =
Composite made from Ag NPs (1 mol.% AgCl precursor)
embedded into an Er3+-doped tellurite glass. E1A05H8
revealed three prominent plasmon peaks at 562, 598, and
628 nm. E0A10H8 showed two peaks at 550 and 578 nm.
This difference between E1A05H8 and E0A10H8 was
attributed to the non-spherical Ag NPs. (b) Size distribution of
Ag NPs (0.5 mol.% AgCl precursor) embedded into an Er3+doped tellurite glass. (c) TEM image of Ag NPs (0.5 mol.%
AgCl precursor) embedded into an Er3+-doped tellurite glass.
(d) HRTEM micrograph of Ag NPs (0.5 mol.% AgCl
precursor) embedded into an Er3+-doped tellurite glass. The
image shows a 0.125 nm fringe spacing corresponding to the
(3 1 1) Ag lattice spacing. Reproduced with permission from
Ref. [97] (Copyright 2013 Elsevier).
Sm2+ + Ag+ → Sm3+ + Ag0
(13)
Ag0 + Ag+ → Ag+2
(14)
Ag0 + Ag+2 → Ag+3
(15)
Similarly, the self-reduction of Eu3+ to Eu2+ in a glass
matrix has also been reported to promote the formation
and growth of Ag NPs within the glass [83,195].
Furthermore, the F− vacancies within the glass matrix
were attractors of Ag+ ions, which benefited the formation
of bigger Ag NPs. The annealing also favored the growth
of Ag NPs in bigger ones. The luminescence properties of
the composite were found to vary with the growth of Ag
NPs [171].
Complex NPs have also been embedded within glass
matrices, thus forming GNCs. Worsch et al. have prepared
multicore magnetite NPs within a glass matrix
(K2O‒Al2O3‒B2O3‒SiO2) by melt-quenching [196]. A
Javier FONSECA. NPs embedded into glass matrices: GNCs
mixture of raw materials was melted at 1400 °C for 2 h
and subsequently at 1480 °C for 20 min. Finally, the
molten glass was quenched. In doing so, GNC droplets
with diameters between 100 and 150 nm were formed.
Magnetite crystals ranging in size from 10 to 30 nm were
prepared within the glass matrix. In this work, multicore
magnetite NPs were extracted from the glassy matrix by
boiling the pulverized composite in a concentrated
aqueous solution of NaOH. The size of the multicore
magnetite NPs was reported to range from 80 to 250 nm.
The NPs exhibited superparamagnetic behavior. In
addition, they showed a narrow hysteresis loop and a ratio
of remanent versus saturation magnetization that was not
high enough for uniaxial anisotropy. The temperaturedependent magneto-relaxometry (TMRX) measurements
showed peaks at 13 and 39 K in the distribution of the
relaxation of magnetic moments [196]. In brief, the
crystallization of a glass containing Fe2O3−x allows the
preparation of magnetite within the glass with a narrow
size distribution and a large magnetic moment [197‒198].
Magnetite NPs can be subsequently extracted by
dissolving or boiling the glass matrix [199].
Singh et al. have embedded Bi-coated Ag NPs in a
bismuthate glass (K2O‒Bi2O3‒B2O3) using meltquenching technique [172]. The raw materials were mixed
and subsequently melted at 1100 °C for 10 min. During
the melting, the Bi3+ ions of Bi2O3 were thermally
reduced as follows:
Bi3+ → Bi2+ → Bi+ → Bi0
(16)
In the presence of Bi2+ or Bi+, the Ag+ ions were reduced
to Ag0:
Bi2+ /Bi+ + Ag+ → Bi3+ /Bi2+ + Ag0
(17)
Therefore, Ag+ ions were first reduced to Ag0 and then
Bi3+/Bi2+/Bi+ ions were reduced to Bi0. In brief, Ag NPs
were initially generated and later Bi0 coated the Ag NPs.
Finally, the molten composite was quenched and then
annealed at 350 °C for 2 h. Ag NPs were found to be
spherical with a size ranging from 5 to 15 nm when the
composite was doped with 0.007 wt.% Ag NPs. However,
when the composite was doped with 0.03 wt.% Ag NPs,
the Ag NPs were reported to be hexagonal with a size
between 10 to 35 nm. The size of spherical Ag NPs
increased with heat treatment at 360 °C. On the contrary,
the size of the hexagonal Ag NPs remained constant after
such treatment for up to 65 h. It is worth mentioning that
15
the GNCs were dichroic. The different orientations of NPs
were suggested to be responsible for the dichroic nature of
the composite. These GNCs have potential applications in
nanophotonics and optoelectronics [172].
3.1.3
Precipitation of nanoparticles
After glass formation by melt-quenching, NPs can be
precipitated within the as-prepared glass matrix by
applying an external stimulus. Two principal precipitation
techniques are distinguished: (1) heat-assisted NP
precipitation; and (2) irradiation-assisted NP precipitation.
3.1.3.1
Heat-assisted nanoparticle precipitation
Metal ions (Mn+) are present in a glass matrix at
temperatures as high as those reached when it melts.
Therefore, when a molten glass is quenched, the glass
matrix becomes supersaturated with respect to Mn+. The
heat treatment favors the diffusion and aggregation of Mn+
within the glass matrix, which leads to the formation of
metal NPs. Besides metal NPs, other NPs such as QDs
and nanocrystals (NCs) can also be precipitated within a
glass matrix. In these cases, nuclei are generally produced
during quenching, and as with metal NPs, the heat
treatment facilitates coalescence and growth of these
nuclei.
Ag NPs have been embedded in calcium fluorophosphate glass (CaO‒CaF2‒P2O5) via melt-quenching
technique and heat treatment [173]. A mixture of raw
materials was melted at 1200 °C for 15 min and then
quenched at room temperature. To precipitate Ag NPs, the
quenched glasses were heated at 550 °C for 10, 20, 30, 40,
and 50 h. The growth of Ag NPs in the glass matrix was
found to be time dependent. Nucleation and growth theory
explained the growth mechanism of Ag NPs within the
glass matrix:
• Ag+ and Sn2+ ions were present in the glass matrix at
high temperature during melting. SnO was added to
prevent nucleation and crystal growth of unwanted Ag2O
[200].
• The glass system became supersaturated with respect
to Ag+ when the molten glass was rapidly quenched.
• Ag+ ions were reduced to Ag0 when the composite
was heated at 550 °C. In other words, the quenched
composites were heated to precipitate Ag NPs [201].
The average size of the NPs within a GNC heated for
10, 30, and 50 h was approximately 5, 15, and 42 nm,
16
Front. Mater. Sci. 2022, 16(3): 220607
respectively (Fig. 6). As the heating duration increased:
(1) the GNC became compact; (2) Vickers microhardness
and fracture toughness increased while brittleness
decreased; (3) the full width at half maxima (FWHM) of
the SPR in the visible region of the composite was
broadened; and (4) higher dielectric constant and lower
dielectric loss were observed for the GNC [173].
Ag NPs have also been embedded in Eu3+-doped
lanthanum sodium borate glass (B2O3‒Na2O‒La2O3‒Eu2O3)
by melt-quenching and heat-assisted precipitation [174].
A mixture of precursors was melted at 1050 °C for 2 h
and subsequently quenched at 150 °C. The melt-quench
material was annealed at 250 °C for 4 h. Finally, the
glasses were heated at 450 °C for different times (5, 10,
15, and 20 h) allowing the nucleation and growth of Ag
NPs. Ag NPs were found to be homogeneously distributed
`within the glass matrix. The average size of the Ag NPs
within the GNCs prepared with a heating time of 10, 15,
and 20 h was approximately 2.72, 4.64, and 6.40 nm,
respectively. Ag NPs enhanced the Eu3+ ion luminescence
properties. Specifically, the optimum PL was obtained by
the composite heated for 10 h. PL decreased in those
Fig. 6 (a) TEM image of Ag NPs embedded in calcium
fluorophosphate glass (heated at 550 °C for 10 h). Inset: the
size distribution of Ag NPs. (b) HRTEM of Ag NPs@calcium
fluorophosphate glass (heated at 550 °C for 10 h). (c) TEM
image of Ag NPs embedded in calcium fluorophosphate glass
(heated at 550 °C for 30 h). Inset: the size distribution of Ag
NPs. (d) Selected area electron diffraction (SAED) image of
Ag NPs embedded in calcium fluorophosphate glass (heated at
550 °C for 30 h). Reproduced with permission from Ref.
[173] (Copyright 2015 Elsevier).
composites heated more than 10 h. In other words, the PL
quenching was more significant for the composites with
longer heating time. The radiative parameters, which were
calculated applying the Judd‒Ofelt theory, and the
luminescence results suggest that the composite heated for
10 h is an appropriate material for the preparation of highefficiency LEDs and solid-state lasers [174]. In a similar
study, Ag and Au NPs have been co-embedded in a Eu3+doped phosphate glass (P2O5‒MgO‒ZnSO4‒Eu2O3) by
melt-quenching and precipitation [175]. Firstly, the
precursors were melted at 1100 °C for 1.5 h. Then, the
molten glass was quenched and finally heated at 300 °C
for 3 h, which favored the precipitation of NPs. The
average diameter of NPs within the glass matrix was
reported to be 27.65 nm. The stability of the glass matrix
decreased as the concentration of Au NPs increased. It
was attributed to the breaking of many bridging oxygen
(BO) linkages and the generation of more non-bridging
oxygens (NBOs). The high quantum efficiency, the
excellent CIE color purity, and the PL intensity for the red
peak suggested that this composite is a great red laser
prospect [175].
Zhang et al. have embedded Ag NPs and CsPbBr3 QDs
in a sodium borosilicate glass by melt-quenching and
precipitation approach [202]. The mixture of glass
precursors was melted in an air atmosphere at 1200 °C for
15 min and subsequently quenched. To release thermal
stresses, the glass was annealed at 350 °C for 10 h.
Finally, the annealed glass was heated at 475 °C for 5 h to
precipitate Ag NPs and CsPbBr3 QDs. Both Ag NPs and
CsPbBr3 QDs coexisted in the glass matrix. They were
reported to be almost uniformly distributed in the sodium
borosilicate glass. The size of the QDs decreased as the
Ag NPs content increased, which in turn depended on the
initial amount of Ag2O precursor. The average size of
CsPbBr3 QDs was around 4.7, 4.1, 3.4, and 3.3 nm when
the initial amount of Ag2O precursor was 0, 0.1, 0.2, and
0.4 mol.%, respectively. The mean size of Ag NPs was
10 nm when the initial amount of Ag2O precursor was
0.1 mol.% (Fig. 7). Notably, the PL of CsPbBr3 QDs/Ag
NPs@sodium borosilicate glass (0.1 mol.% of Ag2O
precursor) was 2.37 times higher than that of CsPbBr3
QDs@sodium borosilicate glass (0 mol.% of Ag2O
precursor). This PL enhancement was facilitated by LSPR
in Ag NPs. PL was found to decrease as the amount of Ag
NPs increased due to self-adsorption of LSPR. Moreover,
the GNC exhibited stability in air, water, and high
temperature (Fig. 8) [202].
Javier FONSECA. NPs embedded into glass matrices: GNCs
17
Irradiation
Glass matrix −−−−−−−→ h+ + e−
(18)
Mn+ + ne− → M0
(19)
Heat treatment
zM0 −−−−−−−−−−→ Mz
Fig. 7 (a) TEM image of CsPbBr3 QDs@sodium
borosilicate glass. (b) TEM image of CsPbBr3 QDs/Ag
NPs@sodium borosilicate glass (0.1 mol.% of Ag2O
precursor). Notation: red spheres denote the CsPbBr3 QDs.
Reproduced with permission from Ref. [202] (Copyright 2019
John Wiley & Sons).
Fig. 8 CsPbBr3 QDs/Ag NPs@sodium borosilicate glass
(0.1 mol.% of Ag2O precursor) in water under the irradiation
of a 400 nm lamp. Reproduced with permission from Ref.
[202] (Copyright 2019 John Wiley & Sons).
Metals other than Ag have also been precipitated within
glasses. Manzani et al. have precipitated Cu NPs in a glass
(Pb2P2O7‒WO3) by conventional melt-quenching method
and heat treatment [203]. A mixture of raw materials was
melted at 980 °C for 1 h. The molten glass was quenched
and annealed at 80 °C for 2 h. To obtain Cu NPs within
the glass matrix, the melt-quenched glass was treated at
410 °C during 5, 20, 60, and 120 min. The authors suggest
that the Cu NP nuclei were produced during quenching.
By heat treatment, the glass allowed the diffusion and
aggregation of nuclei which led to the formation of NPs.
Therefore, Cu NPs grew by heat treatment at 410 °C for
5 min or more. Specifically, the size of Cu NPs prepared
by heat treatment for 120 min was reported to range from
5 to 15 nm. This GNC exhibited high n2, low β, and
ultrafast response time [203].
3.1.3.2 Irradiation-assisted nanoparticle precipitation
The mechanism of precipitation of metal NPs by
irradiation (and heat treatment) can be expressed as
follows:
(20)
A Mn+-doped glass matrix is irradiated with a
femtosecond laser, a nanosecond laser, or X-ray.
Electrons (e−) and holes (h+) are generated from the glass
matrix due to multiphoton absorption (Eq. (18)). In the
area near the focal point, the metal ions (Mn+) are reduced
to metal atoms (M0) by trapping the electrons (Eq. (19)).
Driven by heat treatment, the M0 atoms diffuse and
aggregate to form metal NPs (Mz, where z is the number
of M0 aggregated into metal NPs) within the glass matrix
(Eq. (20)). The heat treatment refers to the thermal effect
of irradiation and/or any other additional thermal
approach. Therefore, the precipitation of metal NPs is due
to a mixed effect of light and heat. It is worth mentioning
that femtosecond lasers generally do not cause heat
diffusion in a composite due to the ultrashort time of
light-matter interaction. Logically, heat accumulation
occurs when femtosecond laser pulses are applied with a
high repetition rate. Other NPs such as QDs and NCs tend
to precipitate following heat-assisted NPs precipitation
and therefore irradiation-assisted NPs precipitation
technique is preferably applied to prepare metal NPs.
Qiu’s research group has reported the space-selective
precipitation of metal NPs by irradiating glass matrices
with a focused infrared femtosecond laser [204]. Ag NPs
have been precipitated in an Ag+-doped silicate glass
(SiO2‒CaO‒Na2O). The glass was prepared by melting
the raw materials at 1550 °C for 1 h and subsequently
quenching. Ag+-doped silicate glass was irradiated with
400 mW femtosecond laser pulses. Electrons were driven
out from the 2p orbital of the NBOs in the SiO4 by the
multiphoton absorption after femtosecond laser
irradiation. In the area near the focal point, Ag+ ions were
reduced to Ag0 by trapping the electrons. The Ag0 atoms
aggregated to form NPs after further annealing at 550 °C
for 10 min. Therefore, the diffusion of Ag0 was driven by
the heat treatment. The focused area turned gray after
laser irradiation and yellow after annealing. The size of
the Ag NPs was found to range from 1 to 8 nm [176]. Au
NPs have also been precipitated inside a Au3+-doped
silicate glass (SiO2‒CaO‒Na2O) using femtosecond laser
irradiation [177]. A mixture of raw materials was melted
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Front. Mater. Sci. 2022, 16(3): 220607
at 1550 °C for 1 h and subsequently quenched, forming
the Au3+-doped silicate glass. The composite was then
irradiated using a femtosecond laser and finally annealed
at 550 °C for 30 min. In the focused area, gray spots
appeared after irradiation. These turned red after
annealing. The photoreduction of Au3+ to Au0 was
induced by multiphoton process. Driven by heat
treatment, these Au0 atoms aggregated and precipitated
into Au NPs. The size of the Au NPs was found to range
from 6 to 8 nm. The laser irradiation conditions allowed
controlling the size and spatial distribution of the NPs. It
should be mentioned that the NPs could also be spaceselectively “dissolved” by femtosecond laser irradiation,
and reprecipitated by annealing [177]. Ag and Au NPs
have also been co-precipitated within a silicate glass by
irradiating femtosecond laser pulses and subsequent
annealing at high temperatures [178]. Firstly, a raw
material mixture was melted at 1550 °C for 1 h and
quenched to prepare the Ag+/Au3+ co-doped silicate glass.
The composite was irradiated with a femtosecond laser,
releasing electrons. By trapping these electrons, the Ag+
and Au3+ ions were reduced to Ag0 and Au0 atoms,
respectively. The subsequent annealing promoted the
migration and aggregation of Ag0 and Au0 atoms, thus
forming NPs in the glass matrix. The irradiated area
exhibited different colors depending on the annealing
temperature. While it was yellow after annealing at
400 °C for 30 min, it became red after annealing at 570 °C
for 30 min (Fig. 9). Ag NPs were found to emerge earlier
than Au NPs during annealing. It is worth mentioning that
the precipitation rate of metal NPs is determined by the
diffusion rate of metal atoms. Since the radius of the Au
atom is bigger than that of the Ag atoms, the Au atom
moves more slowly than the Ag atom in the glass
Fig. 9 (a) Photograph of the composite after femtosecond
laser irradiation and annealing at 400 °C for 30 min.
(b) Photograph of the composite after femtosecond laser
irradiation and annealing at 570 °C for 30 min. The average
laser power was 10 mW. Reproduced with permission from
Ref. [178] (Copyright 2006 Elsevier).
network. Therefore, the Ag atoms aggregated into the Ag
NPs earlier than the Au atoms [178]. The same research
group has also precipitated Cu NPs in a borate glass
(K2O‒CaO‒B2O3‒CuO) by irradiation with femtosecond
laser pulses (Fig. 10) [179]. A mixture of raw materials
was melted at 1000 °C for 30 min and subsequently
annealed at 300 °C for 2 h. Cu NPs with a size between
3 and 5 nm were precipitated from the CuO-doped borate
glass after irradiation with 830 mW femtosecond laser
pulses (Fig. 11). A non-linear interaction between the
femtosecond laser and the glass sample generated free
electrons and holes. These free electrons were trapped by
Cu2+ or Cu+ ions, forming Cu0 atoms. Due to the thermal
effect of the femtosecond laser, the Cu0 atoms diffused
and aggregated to create Cu NPs. Therefore, the
Fig. 10 Schematic illustration of femtosecond laser inducing
the precipitation of Cu NPs. Reproduced with permission
from Ref. [179] (Copyright 2010 Elsevier).
Fig. 11 (a) Photograph of the glass sample with a grating
pattern after irradiation with a femtosecond laser. (b) Optical
microscope photograph of the enlarged grating pattern.
(c)(d) Photographs of the diffraction patterns of glass samples
before irradiation (panel (c); no diffraction pattern was
observed in glass) and after irradiation (panel (d); a diffraction
pattern was observed when the grating pattern was irradiated
by a 532 nm laser) with a femtosecond laser. Reproduced with
permission from Ref. [179] (Copyright 2010 Elsevier).
Javier FONSECA. NPs embedded into glass matrices: GNCs
precipitation of Cu NPs was due to a mixed effect of light
and heat. The area in the vicinity of the laser focal point
within the glass sample turned red due to the precipitation
of Cu NPs. The red color of the focal regions became
deeper and deeper as the irradiation time increased. It was
found that femtosecond laser power less than 760 mW did
not produce precipitates [179].
Cu NPs have been precipitated within a borosilicate
glass by a combination of femtosecond laser irradiation
and heat treatment [180]. A Cu2+-doped borosilicate glass
(SiO2‒B2O3‒MgO‒Na2O‒Al2O3) was prepared by
melting raw materials at 1400 °C for 1 h. The molten
composite was quenched and annealed at 400 °C for 12 h.
The Cu2+-doped borosilicate glass was irradiated with a
femtosecond laser. In doing so, Cu2+ ions were
photoreduced to Cu0. The free electrons, which were
generated by the non-linear interaction, favored this
photoreduction. Subsequently, the composite was treated
at 600 °C for 1 h, promoting the aggregation of Cu0 and,
consequently, the formation of Cu NPs. These Cu NPs
were only observed in the irradiated areas after heat
treatment. The mean size of the Cu NPs was 9 nm. A
preferential growth of Cu NPs was observed at the bottom
of the irradiated region, which was attributed to a selffocusing effect as the beam propagated through the
composite [180].
Chen et al. have precipitated Ag NPs in an Ag+-doped
soda-lime silicate glass (SiO2‒Na2O‒Al2O3‒CaO) by a
combination of X-ray radiation and heat treatment [181].
The glass matrix was prepared by melting a mixture of
raw materials at 1450 °C for 4 h. The composite was
irradiated with X-ray radiation for 34 min, generating
electrons (Eq. (21)). These electrons were trapped by Ag+
ions, producing Ag0 (Eq. (22)). The subsequent heat
treatment at 400 °C for 15 min favored the aggregation of
Ag0 towards Ag NPs within the glass matrix (Eq. (23)).
The size of the Ag NPs ranged between 1 and 5.6 nm with
a mean size of (2.8 ± 0.93) nm.
X-ray
Glass −−−−→ h+ + e−
(21)
Ag+ + e− → Ag0
(22)
zAg0 → Agz
(23)
The Ag NPs were dissolved by further annealing at
500 °C for 15 min, generating electrons (Eq. (24)), and
these electrons recombined with the holes in the glass
19
matrix (Eq. (25)):
Agz → zAg+ + ze−
(24)
h+ + e− → Recombination
(25)
Therefore, the generation and dissolution of Ag NPs can
be controlled by a combination of X-ray radiation and
heat treatment [181]. Similarly, Shenget et al. have
precipitated Au NPs within an Au3+-doped soda-lime
silicate glass (SiO2‒Na2O‒Al2O3‒CaO) by a combination
of X-ray radiation and heat treatment [182].
Au NPs have been precipitated in an Au3+-doped
silicate glass (SiO2‒Na2O‒CaO) by irradiation (with
femtosecond laser, nanosecond laser or X-ray) and heat
treatment [205]. A mixture of raw materials was melted
at 1550 °C for 4 h and subsequently quenched to prepare
the Au3+-doped silicate glass. The composite was
irradiated with a femtosecond laser, a nanosecond laser
for 4 min, or with X-ray for 4 h. The irradiated composites
were then heat treated at 550 °C. The inner electrons of
atoms in glass became excited when the composite was
irradiated with X-ray. Thus, Au+ ions were reduced to
Au0 atoms. Subsequently, the heat treatment promoted the
diffusion of Au0 and, therefore, the formation of Au NPs.
When the composite was irradiated with a nanosecond
laser, the active electrons and holes of NBOs of the glass
were excited. Au+ ions captured these free electrons
forming Au0 and later NPs in the heat treatment process.
Similarly, in the case of femtosecond laser irradiation, the
laser beam ionized the glass matrix. Then, Au NPs were
created during the heat treatment process. Due to the
extremely short time of femtosecond laser irradiation, this
approach allowed highly localized laser photons in both
time and spatial domains eliminating thermal effects,
and non-linear processes. Therefore, femtosecond laser
can be more effective than nanosecond laser or X-ray
techniques in microprocessing inside glasses. Irradiation
was found to reduce the heat treatment temperature
required for the precipitation of Au NPs. Whereas Au NPs
appeared after heat treatment at 550 °C in irradiated
composites, Au NPs only formed at 580 °C in nonirradiated samples. It was found that the mean diameter of
the precipitated Au NPs depended on the irradiation step
(size of Au NPfemtosecond laser < size of Au NPX-ray < size of
Au NPnanosecond laser) [205].
Ion irradiation with very high fluences of protons or
He+ ions at energies between 0.1 and 1.5 MeV can also be
20
Front. Mater. Sci. 2022, 16(3): 220607
used to irradiate glass composites and precipitate metal
NPs [206‒207]. Herein, NP precipitation occurs during
irradiation due to interactions between the metal and
irradiation-induced defects. Alternatively, the irradiation
of heavy ions (Br−, Si4+ and O2− ions at energies between
6 and 12 MeV) allows the nucleation and growth
processes to be completely decoupled, which minimizes
the size distribution of the prepared NPs. Valentin et al.
have precipitated Cu NPs in a Cu+-doped silicate glass
(SiO2‒Na2O‒CaO‒SnO2) by heavy ion irradiation and
heat treatment [183]. Ion irradiation initiated the
nucleation of Cu0. Cu NPs appeared after post-irradiation
heat treatment at 400 °C for 10 min. The growth of NPs
was reported to follow the Lifshitz‒Slyozov‒Wagner
(LSW) theory of diffusion-limited precipitate ripening
[208]. The average size of Cu NPs was found to range
from 2 to 8 nm [183].
3.1.4 Nanoparticles added to glass precursors before
melting process
NPs can be added to glass precursors before the melting
process. This approach allows any type of NP to be
incorporated into a glass. However, the NPs can aggregate
and dissolve during the melting process to form glass. The
melting time and temperature during glass formation are
important factors that must be controlled to preserve the
structure of the incorporated NPs. The process of
dissolution and growth of NPs is called Ostwald ripening.
Ostwald ripening is based on the migration of adatoms or
mobile species, driven by differences in free energy and
local concentrations of adatom [10]. These processes can
be thwarted by applying surface engineering to NPs, such
as silica coating. Ag NPs have been embedded in Ho3+doped phosphate glass (P2O5‒MgO‒ZnSO4‒Ho2O3) by
applying this method [75]. The mixed precursors,
including Ag NPs (size < 100 nm), were melted at
1100 °C for 1.5 h and subsequently annealed at 300 °C for
3.5 h. The Ag NPs were homogeneously distributed in the
host matrix. The size of the Ag NPs ranged from 2.70 to
9.84 nm with an average diameter of approximately
4.93 nm. The absorption spectra of the composite
exhibited 12 bands related to the transitions in the Ho3+
ions. The intensity of PL was influenced by the
concentration of Ag NPs. Specifically, the intensity of the
green and red emission from the Ho3+ was enhanced by
the LSPR of Ag NPs. The effect of LSPR of Ag NPs on
the Ho3+ emission was more notable in the composite
prepared with low amount of Ho3+ ions. This GNC
(AgNPs@Ho3+-doped phosphate glass) is a promising
candidate for the development of an efficient solid-state
laser medium [75]. Similarly, a borate glass
(B2O3‒ZnO‒Na2O) has been doped with NiO NPs by the
conventional melt-quenching method [184]. A mixture of
raw materials, including NiO NPs with average size of
70 nm, was melted at 1200 °C for 2 h. The molten glass
was quenched and subsequently annealed at 400 °C 1 h.
This GNC can be used in semiconductor applications
[184].
Sm2O3 NPs have been incorporated into a bioborosilicate glass (SiO2‒B2O3‒TeO2) [185]. The silica
precursor (98.13 wt.% SiO2 and 1.87 wt.% impurities)
was obtained from the waste rice husk. A homogeneous
mixture of precursors, which included Sm2O3 NPs, was
pre-heated at 400 °C for 1 h and then melted at 1050 °C
for 3 h. The molten glass was quenched and annealed at
400 °C for 2 h. While the average size of raw Sm2O3 NPs
was around 15‒30 nm, the mean size of embedded Sm2O3
NPs was found to be 71.74 nm. Ostwald ripening can
explain the increase in the size of NPs [209]. This
composite was suggested to have potential in non-linear
optics [185]. In a similar work, Azlina et al. have
homogeneously embedded Nd2O3 NPs in a tellurite glass
(TeO2‒B2O3‒ZnO). Nd2O3 NPs, TeO2, B2O3, and ZnO
were mixed and subsequently melted at 900 °C. The mean
size of the raw Nd2O3 NPs was reported to range from 15
to 30 nm. The molted glass was quenched at room
temperature producing the GNC (Nd2O3 NPs@tellurite
glass) [210]. Likewise, Noorazlan et al. have embedded
Er3O2 NPs in the tellurite glass (TeO2‒B2O3‒ZnO) [211].
The GNC was prepared from a mixture of Er2O3 NPs,
TeO2, ZnO, and B2O3. The mixture was heated at 400 °C
for 30 min, melted at 900 °C for 2 h and finally quenched
and annealed at 400 °C for 2 h. The incorporation of
Er2O3 NPs within the glass matrix was reported to favor
the formation of NBO [211]. Unfortunately, neither the
Azlina nor the Noorazlan studies explored the size of the
embedded NPs. Halimah et al. have also incorporated
Nd2O3 NPs into a tellurite glass (TeO2‒ZnO) [212]. A
mixture of precursors, which included Nd2O3 NPs, was
heated at 400 °C for 1 h, melted at 830 °C for 1.5 h, and
quenched and annealed at 400 °C for 2 h. The average
size of embedded Nd2O3 NPs was reported to be
approximately 74 nm. Thus, the NPs were suggested to
aggregate within the glass matrix [212]. The composite
was proposed to be used for linear and NLO applications.
Javier FONSECA. NPs embedded into glass matrices: GNCs
Nalin’s group has incorporated CdFe2O4 NPs protected
with a silica layer into a phosphate glass [213]. First,
CdFe2O4 NPs were synthesized by a coprecipitation
method. The NPs were coated with a silica layer to protect
them during a subsequent melt-quenching process.
CdFe2O4 NPs were added to a coacervate, which was
previously prepared by mixing solutions of Na(PO3)n and
CaCl2. The new mixture was heated to 600 °C for 15 min
and then to 1000 °C for 15 min. The CdFe2O4@phosphate
glass composite was prepared by quenching the molten
glass. Finally, the composite was annealed at 340 °C for
2 h. The embedded CdFe2O4 NPs exhibited a size
distribution ranging from 2.1 to 4.2 nm and an average
size of 2.8 nm. Considering that the mean size of CdFe2O4
NPs before incorporation was 3.9 nm, the size of the NPs
underwent a 29% reduction during melting. The 31P MAS
NMR and Raman results showed that the polyphosphate
chains were broken and new P−O−Si bonds were formed
in the composite [213]. CdFe2O4 NPs coated with a silica
layer have also been incorporated in a 60SbPO4‒30ZnO‒
10PbO glass [186]. The precursors were melted at
1100 °C for 30 min and subsequently quenched,
producing the glass 60SbPO4‒30PbO‒10ZnO. The glass
was mixed with CdFe2O4 NPs, which were prepared by
the coprecipitation method and protected with a silica
layer. The mixture was melted at 1100 °C for 7 min and
then annealed at 340 °C for 2 h. The CdFe2O4 NPs
exhibited an average diameter of 3.5 nm. Considering that
the mean size of CdFe2O4 NPs before incorporation was
(3.9 ± 0.1) nm, the size of the NPs underwent a 10%
reduction during melting. Therefore, the silica layer was
21
able to protect NPs during melting at high temperatures.
The matrix acquired thermal stability by the inclusion of
CdFe2O4 NPs coated with a silica layer. However, the
dispersion of NPs in the glass was not homogeneous
[186]. Applying this synthesis approach, the same
research group has embedded Fe3−δO4 NPs coated with
silica into a phosphate glass (Fig. 12) [214]. Fe3−δO4 NPs
were synthesized by thermal decomposition. The NPs
were coated by a layer of silica to protect them during the
melting. The coacervate gel, which was prepared by
mixing Na(PO3)n and CaCl2 solutions, and Fe3−δO4 NPs
were melted together at 1000 °C during different times
(15, 45, and 75 min). After melting for 15 min, the size
distribution of Fe3−δO4 NPs was (19.7 ± 0.6) nm, which
was close to that of the as-prepared Fe3−δO4 NPs (20 nm).
Silica successfully protected Fe3−δO4 NPs during melting
for 15 min. However, the average size of embedded
Fe3−δO4 NPs was (9.4 ± 0.6) and (4.3 ± 0.5) nm after
melting for 45 and 75 min, respectively. Therefore, the
melting time is an important factor to preserve the
structure of the NPs embedded in the glass matrix. The
incorporation of Fe3−δO4 NPs coated with silica in the
phosphate glass favored the formation of P−O−Si bonds,
which provided high thermal stability to the GNC. These
composites can be used in magneto-optical or ultrasensitive magnetic sensors [214].
Sahar’s research group has prepared GNCs by
incorporating nano ferrite oxide (Fe3O4) in glass matrices.
This group has incorporated Fe3O4 with a size ranging
from 18 to 70 nm in tellurite glass (TeO2‒ZnO) by the
melt-quenching technique [187]. A mixture of raw
Fig. 12 Schematic illustration of (a) the synthesis of Fe3−δO4@SiO2 NPs and (b) the preparation of the phosphate glasses containing
monodisperse Fe3−δO4@SiO2 NPs. Reproduced with permission from Ref. [214] (Copyright 2020 Elsevier).
22
Front. Mater. Sci. 2022, 16(3): 220607
materials, including Fe3O4 NPs, was melted at 850 °C for
20 min and subsequently quenched. Finally, the composite
was annealed at 300 °C for 3 h. The GNC containing
Fe3O4 NPs exhibited higher thermal stability than the
glass without Fe3O4. The GNC was found to be a
paramagnetic material. The composite with 2 mol.%
Fe3O4 showed a magnetization (M‒H) curve with high
saturation magnetic field and a very small hysteresis
(Mr = 7.0 × 10−3 emu·g−1, Hc = 1.7 × 10−2 T) [187]. The
Fe3O4 NPs have also been incorporated into an Er3+doped tellurite glass (TeO2‒ZnO‒Li2O‒Er2O3) by meltquenching [188]. Again, the mixture of raw materials,
which included Fe3O4 NPs with a size ranging from 18 to
70 nm, was melted at 850 °C for 20 min. After quenching,
the composite was annealed at 300 °C for 3 h. The Fe3O4
NPs increased the thermal stability of glass. Furthermore,
according to the upconversion luminescence spectra,
Fe3O4 NPs were found to quench the emission bands of
the Er3+ ion [188]. This group has also embedded Fe3O4
NPs into an Er3+-doped phosphate glass (P2O5‒ZnO‒Er2O3)
by melt-quenching method [189]. The mixture of raw
materials, including Fe3O4 NPs, was melted at 950 °C for
1 h. The molten composite was quenched and annealed at
300 °C for 3 h. Fe3O4 NPs were homogenously distributed
within the glass matrix. In the GNC with 1.5 mol.% Fe3O4
NPs, the mean size of the NPs was approximately 31 nm.
The average size of NPs in the GNC with 2 mol.% Fe3O4
NPs was approximately 26 nm. The optical bandgap
energy for direct and indirect transitions was found to
decrease as the content of Fe3O4 NPs increased. It was
ascribed to the creation of NBO ions in the glass network.
The luminescence was quenched by increasing Fe3O4 NPs
concentration. This was attributed to the transfer of energy
from the Er3+ ion to the NPs. The GNCs with 1.5‒
2 mol.% Fe3O4 NPs showed a ferrimagnetic behavior
(Fig. 13) [189]. These Fe3O4 NPs-doped composites are
promising candidates for magneto-optic devices.
3.2
3.2.1
Sol-gel technique
Introduction
The sol-gel method is an important wet-chemical
approach to synthesize inorganic materials and organic‒
inorganic hybrids from liquid sources. Sol-gel processing
of glasses began in the mid-19th century with the studies
of Ebelman and Graham [215‒216]. Since then, it has
been widely used to prepare glasses [217‒219]. In sol-gel
Fig. 13 M‒H curves at room temperature for (a) Fe3O4 NPs
and (b) GNCs (Fe3O4 NPs@phosphate glasses) with different
concentrations of embedded Fe3O4 NPs. Reproduced with
permission from Ref. [189] (Copyright 2015 Elsevier).
method, firstly, a suspension of colloidal particles, called
sol, is prepared by mixing them in a solvent, generally
water, at a pH that prevents precipitation (Fig. 14). Sol
particles can also be formed by hydrolyzing a liquid
alkoxide precursor in water [220]. The size of the sol
particles strongly depends on the pH. It is worth
mentioning that the sol, which is a low-viscosity liquid,
can be cast into a mold. A gel may be formed by growing
an interconnected three-dimensional rigid network from a
sol. The gel network may be also formed by the
simultaneous hydrolysis and polycondensation of an
organometallic precursor [9]. The kinetics of these
simultaneous processes depends on the pH, the
composition and the concentration of the reagents, the
temperature, and pressure [221]. The size of particles and
the degree of cross-linking before gelation strongly
influence the physical characteristics of the interconnected
gel network. Viscosity increases sharply due to gelation. It
Javier FONSECA. NPs embedded into glass matrices: GNCs
23
Eq. (26) for a cylindrical pore with radius (r) [223‒227]:
pc = −
Fig. 14 Schematic illustration of the sol-gel process
sequence to prepare NPs embedded into glass matrices.
should be noted that the gel must have sufficient strength
to resist cracking during thermal treatment. Therefore,
before drying, the gel can be maintained immersed in a
liquid for a period of time (from hours to days). This
aging, also referred to as syneresis, increases the strength
of the gel through polycondensation and reprecipitation of
the gel network. The pore liquid is generally removed
from the interconnected solid gel network by drying
between 100 and 180 °C at or near ambient pressure,
obtaining a monolith termed as xerogel. When the pore
liquid is primarily alcohol-based, the monolith is often
referred to as alcogel. The pore liquid can also be
evacuated as a gas phase under hypercritical conditions
producing a low-density aerogel. The generic term gel is
generally applied to xerogels or alcogels, while aerogels
are generally referred to as such. A gel is considered as
dry when the physisorbed water is completely removed. A
dry gel still contains chemisorbed hydroxyls. Heat
treatment between 500 and 800 °C desorbs the hydroxyls,
resulting in a stabilized gel. The heat treatment of a gel
reduces the number of pores and their connectivity,
thereby increasing the density of the monolith. The gel is
transformed to a dense glass when all pores are
eliminated. This process is called densification [11]. The
densification temperature ranges from 1000 to 1500 °C,
depending considerably on the dimensions of the pore
network, the connectivity of the pores, and surface area
[222].
As mentioned previously, the drying process must be
controlled to avoid the gel to crack. As drying proceeds,
capillary pressure (pc) is induced in the pores due to stress
arising from solvent evaporation, and pc is expressed by
(
)
2 γ −γ
2γLV cos θ
= − SV SL
r
r
(26)
where θ is the contact angle, γLV is the interfacial tension
between the liquid and the vapor, γSV is the interfacial
tension between the solid and the vapor, and γSL is the
interfacial tension between the solid and the liquid.
Therefore, methods for overcoming stresses during
drying include (1) decreasing γLV, (2) increasing r, and
(3) increasing θ (or decreasing γSV − γSL ).
(1) Methods to decrease γLV. Additives such as
surfactants and drying control chemical additives
(DCCAs) can be incorporated to lower the γLV of the
solvent phase [228‒229]. It is worth mentioning that
supercritical drying allows highly porous dry gels to be
obtained without fracture [230].
(2) Methods to increase r. Aging can increase the pore
size of the wet gels [231]. In addition, this phenomenon
strengthens the gel framework, preventing fracture during
drying and bloating during sintering [232‒233]. It is
worth mentioning that gels with small pores (typically
around 5 nm or so) can be slowly dried without fracture
[234‒237]. Interestingly, particles can be incorporated
into precursor solution to reduce the pc [227].
(3) Methods to increase θ (or decrease γSV − γSL ). The
surface of the wet gel can be modified to form a hydrophobic surface [238].
The sol-gel approach is a melt-free process. Therefore, it
requires lower processing temperature than traditional
glass melting methods. This method is versatile and
flexible, specifically, it has potential to increase the
concentration of reinforcements and broaden the
compositional range of glasses. In addition, this technique
provides materials of high purity and homogeneity.
Glasses with high melting temperatures, high crystallization tendencies, and compositions within the stable or
metastable liquid‒liquid immiscibility region are preferably prepared by the sol-gel method [239‒240]. Thus,
this approach is particularly attractive for the development
of GNCs that are difficult to prepare by melt-quenching
and vapor phase methods. The NP precursors can be
initially mixed with glass precursors. Thus, glass matrix
and NPs can be synthesized concomitantly. In doing so,
the reinforcements are usually homogeneously distributed
within the resulting glass matrix. An alternative to such an
approach is a sol-gel method for glass synthesis combined
with impregnation with the NP or NP precursor, generally
24
Front. Mater. Sci. 2022, 16(3): 220607
followed by heat treatment. The following sections of this
review are intended to provide an overview of the main
sol-gel approaches for GNC synthesis. Table 3 shows
examples of GNCs prepared by sol-gel techniques
[14‒15,241‒257].
3.2.2 Concomitant synthesis of glass matrix and
nanoparticles
Schubert’s research group have prepared composites
containing uniform metal NPs, homogeneously dispersed
in SiO2 matrices by a three-step procedure [241,258]:
(1) An alkoxysilane of the type (RO)3Si(CH2)nA (A =
coordinating organic group = NH2, NHCH2CH2NH2, CN),
a metal salt (AgNO3, AgOAc, Cd(NO3)2, Co(OAc)2,
Cu(OAc)2, Ni(OAc)2, Pd(acac)2, or Pt(acac)2), and,
optionally, Si(OR)4 were processed by the sol-gel method,
producing a gel with composition [O3/2Si(CH2)nA]yMXm·
xSiO2. The metal loading was adjusted by adding the
Si(OR)4.
(2) The polycondensates were calcined in air, and
thereby the metal oxide@SiO2 composites (MOz@(x +
y)SiO2) (MOz = Ag2O, CdO, CoO, CuO, NiO, PdO or
PtO) were formed. The oxidation temperature ensured the
complete oxidation of all organic components, without
causing excessive sintering of NPs.
(3) The metal oxide particles were reduced by H2,
generating the nanocomposites M@(x + y)SiO2 (M = Ag,
Co, Cu, Ni, Pd, or Pt). Therefore, reduction of the metal
precursor with H2 at high temperature was required to
create the NPs.
The metal particle size did not change significantly
during the reduction step [241,258]. The size of the
embedded metal NPs was found to depend on the metal,
the metal loading, the chemical composition of both the
metal precursor and the organo(alkoxy)silane, and the
conditions for calcination [242,259‒260]. Moreover, the
textural properties (surface area, porosity, etc.) of the
prepared composites have been reported to depend on the
metal precursor [243,261]. PdNi alloy@silica glass and
CuNi alloy@silica glass composites have also been
prepared by this approach [262‒263]. In brief, Schubert’s
approach tethers metal precursors to the matrix during solgel processing to embed metal NPs into silica glass. This
technique makes possible to prepare MOx@SiO2 or
M@SiO2 nanocomposites with a controllable size of NPs.
In addition, the method has been extended to more
complex systems such as complex hybrid organic —
“multi inorganic” particles SiO2@PDCL-ZrO2@ (Ag@
SiO2) (PDCL = poly(N-dicarbazolyl-lysine)) [264].
Crystalline nanoclusters of Fe2P, RuP, Co2P, Rh2P,
Ni2P, Pd5P2, and PtP2 have been formed in silica xerogel
by a similar approach [244]. The general synthesis of
metal phosphide@silica xerogel nanocomposites also
required three steps:
(1) Preparation of single-source transition metal
complexes that served as precursors to metal phosphide
nanoclusters. The metal complexes contained bifunctional
phosphine ligands possessing alkoxysilyl (Si-(OR)3)
functional groups.
(2) Covalent incorporation of the precursors into the
silica xerogel matrixes as they were being formed. The
silica xerogel host matrix was formed by sol-gel approach.
Therefore, the precursors exhibited adequate solubility
and chemical stability to survive sol-gel process without
undergoing chemical degradation.
(3) Heat treatment under reducing conditions to convert
the doped xerogel host matrix to a metal phosphide@
silica xerogel nanocomposite.
The nanoclusters of Fe2P, RuP, Co2P, Rh2P, Ni2P, Pd5P2,
or PtP2 were highly dispersed throughout the xerogel
matrix. The size of the NPs ranged from 2.0 to 11.3 nm
[244]. Nucleation and growth of NPs were suggested to be
driven by diffusion, similar to the processes observed in
the preparation of the GNCs (CdS@silica or
CdSe@silica) by melt-quenching from supersaturated
glasses at high temperature [164]. The synthesis
procedure described above, based on the covalent
incorporation of suitable precursors into silica xerogel
matrices and reduction at high temperature, has also been
followed in the preparation of Co3C@silica xerogel,
Ge@silica xerogel, Os@silica xerogel, and PtSn@silica
xerogel composites [245‒247].
CdS QDs have been embedded into a sodium
borosilicate glass matrix via sol-gel method [248]. A
cadmium-doped sodium borosilicate gel was prepared by
mixing the precursors (sodium silicate (Na2SiO3), boric
acid (H3BO3), and cadmium sulfate (CdSO4)) in an
aqueous solution at low temperature (60‒80 °C). Sulfurcontaining complexes were dissolved in the gel to perform
Cd2+ sulphidation during gel stabilization at temperatures
ranging from 300 to 500 °C. After quenching the gel from
750 °C to room temperature, the crystalline quality of CdS
QDs was improved by annealing the glass. The size of
CdS QDs was reported to range from 4 to 20 nm
depending on the annealing temperature and the initial
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Ag NPs@silica glass (Ag·4SiO2)
Ag NPs@silica glass (Ag·12SiO2)
Co NPs@silica glass (Co·SiO2)
Cu NPs@silica glass (Cu·SiO2)
)a)
Silica glass
Silica glass
Silica glass
Silica glass
Ni NPs@silica glass (Ni·3SiO2)a)
Ni NPs@silica glass (Ni·5.5SiO2)a)
Ni NPs@silica glass (Ni·10SiO2)a)
)a)
RuP NCs
Co2P NCs
Silica xerogel
Silica xerogel
Silica xerogel
Silica xerogel
RuP NCs@silica xerogel
Co2P NCs@silica xerogel
Rh2P NCs@silica xerogel
Ni2P NCs@silica xerogel
Ni2P NCs
Rh2P NCs
Fe2P NCs
Ni NPs
Ni NPs
Ni NPs
Silica glass
Ni NPs@silica glassb)g)
Silica xerogel
Silica glass
Ni NPs@silica
Ni NPs
Fe2P NCs@silica xerogel
Silica glass
glassb)f)
Ni NPs@silica
Pt NPs
Pt NPs
Pt NPs
Pt NPs
Pt NPs
Pd NPs
Pd NPs
Pd NPs
Pd NPs
Pd NPs
Pd NPs
Pd NPs
Ni NPs
Ni NPs
Ni NPs
Ni NPs
Ni NPs
Ni NPs
Cu NPs
Co NPs
Ag NPs
Ag NPs
Reinforcement
Ni NPs@silica glassb)h)
Silica glass
glassb)e)
Pt NPs@silica
Silica glass
Silica glass
Pt NPs@silica glass (Pt·32SiO2)
Silica glass
Silica glass
Pd NPs@silica glass (Pd·15SiO2)
Pt NPs@silica glassd)
Silica glass
Pd NPs@silica glass (Pd·32SiO2)
Pt NPs@silica glassc)
Silica glass
Pd NPs@silica glass (Pd·22SiO2)
Silica glass
Silica glass
Pd NPs@silica glass (Pd·12SiO2)
Silica glass
Silica glass
Pd NPs@silica glass (Pd·7SiO2)
Pt NPs@silica glassb)
Silica glass
Pd NPs@silica glass (Pd·4.5SiO2)
glassa)
Silica glass
Pd NPs@silica glass (Pd·2SiO2)
Ni NPs@silica glass (Ni·33SiO2
Silica glass
Ni NPs@silica glass (Ni·2SiO2)a)
Ni NPs@silica glass (Ni·SiO2
Host matrix
GNCs prepared by sol-gel techniques [14,15,241–257]
Composite
Table 3
(2.6)
(2.0)
(5.0)
(4.7)
(4.6)
(8.5)
(8.2)
(8.5)
(8.5)
(5.0)
(4.0)
(3.5)
(7.1)
0.8‒4.2 (2.5)
2.8‒5.2 (3.8)
1.3‒3.7 (2.4)
‒
‒
1.8‒4.2 (2.8)
‒
1.8‒4.2 (3.0)
2.5‒12.4 (5.9)
Bimodal particle distribution:
7.5‒12.4, 27.5‒72.4
Bimodal particle distribution:
2.5‒22.4, 62.5‒102.4
Bimodal particle distribution:
2.5‒12.4, 32.5‒57.4
12.5‒47.4 (22.9)
37.5‒62.4 (50.3)
1.5‒7.4 (3.9)
11.0‒24.9 (17.4)
9.0‒30.9 (19.5)
‒
Size of NPs (min‒max
(mean))/nm
6.83
8.79
4.96
8.23
6.64
10
24
9
4
21.5
24.3
24.8
17
7.93
9.36
5.52
7.18
10.84
18.12
26.03
44.81
2.65
6.99
14.01
20.05
31.08
43.7
46.17
47.51
28.7
10.22
Loading of
NPs/wt.%
Concomitant synthesis of glass matrix
and NPs: tethering metal precursors to
the matrix during sol-gel processing
Concomitant synthesis of glass matrix
and NPs: tethering metal precursors to
the matrix during sol-gel processing
Concomitant synthesis of glass matrix
and NPs: tethering metal precursors to
the matrix during sol-gel processing
Concomitant synthesis of glass matrix
and NPs: tethering metal precursors to
the matrix during sol-gel processing
Sol-gel technique
‒
‒
‒
‒
Application
[244]
[243]
[242]
[241]
Ref.
Javier FONSECA. NPs embedded into glass matrices: GNCs
25
(32.63)
1.5‒5 (2.7)
(30.28)
(23)
Sb NPs
Cu NPs
Cu2In NPs
In2O3 NCs
In2O3 NCs
In2O3 NCs@silica glass
Au NPs@sodium borosilicate glass Sodium borosilicate glass
(75 wt.% SiO2, 20 wt.%
B2O3, and 5 wt.% Na2O)
Cu3.8Ni@sodium borosilicate glass Sodium borosilicate glass
(72.88 wt.% SiO2,
21.38 wt.% B2O3, and
5.74 wt.% Na2O)
Au NP/Ni NP@sodium
Sodium borosilicate glass
borosilicate glass
(75 wt.% SiO2, 20 wt.%
B2O3, and 5 wt.% Na2O)
2.82‒9.97 (5.48)
15‒40 (27.5)
8 nm Au NPs,
2 nm Ni NPs
Au NPs
Cu3.8Ni alloy
NPs
Au NPs and
Ni NPs
(54)
(4.3)
CdS QDs
Silica glass
(5.7)
2‒14 (6)
CdS QDs
PtSn alloy NPs
1‒7 (2.1)
(15.4)
Silica xerogel
PtSn@silica xerogel
Os NPs
2.5‒14.5 (6.7)
CdS QDs
Silica xerogel
Os NPs@silica xerogel
Ge NCs
10‒46 (25)
(18.4)
Silica xerogel
Ge NCs@silica xerogel
Co3C NCs
(11.3)
(4)
Size of NPs (min‒max
(mean))/nm
CdS QDs
Silica xerogel
Co3C NCs@silica xerogel
Pd5P2 NCs
PtP2 NCs
Reinforcement
Sodium borosilicate glass
(33.33 wt.% SiO2,
33.33 wt.% B2O3, and
33.33 wt.% Na2O)
CdS QDs@sodium borosilicate
Sodium borosilicate glass
j)
glass
(33.33 wt.% SiO2,
33.33 wt.% B2O3, and
33.33 wt.% Na2O)
CdS QDs@sodium borosilicate
Sodium borosilicate glass
k)
glass
(33.33 wt.% SiO2,
33.33 wt.% B2O3, and
33.33 wt.% Na2O)
CdS QDs@sodium borosilicate
Sodium borosilicate glass
glassl)
(33.33 wt.% SiO2,
33.33 wt.% B2O3, and
33.33 wt.% Na2O)
Sb NPs@sodium borosilicate glass Sodium borosilicate glass
(72.88 wt.% SiO2,
21.38 wt.% B2O3, and
5.74 wt.% Na2O)
Cu NPs@sodium borosilicate glass Sodium borosilicate glass
(72.88 wt.% SiO2,
21.38 wt.% B2O3, and
5.74 wt.% Na2O)
Cu2In NPs@sodium borosilicate
Sodium borosilicate glass
glass
(72.88 wt.% SiO2,
21.38 wt.% B2O3, and
5.74 wt.% Na2O)
In2O3 NCs@silica glass
Silica glass
Silica xerogel
Silica xerogel
Pd5P2 NCs@silica xerogel
PtP2 NCs@silica xerogel
CdS QDs@sodium borosilicate
glassi)
Host matrix
Composite
2 wt.% Au,
1 wt.% Ni
1.5
0.25
3
1
1.5
1.25
1.5
‒
‒
‒
‒
‒
0.2‒1
‒
‒
25.84
2.77
Loading of
NPs/wt.%
‒
‒
‒
Application
NLO devices:
χ(3)(800 nm) =
3.32 × 10−10 esu
NLO devices:
χ(3)(800 nm) =
2.41 × 10−11 esu
NLO devices:
χ(3)(800 nm) =
4.85 × 10−11 esu
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
NLO devices:
χ(3)(800 nm) =
2.31 × 10−12 esu
NLO devices:
χ(3)(800 nm) =
1.7 × 10−14 esu
NLO devices:
χ(3)(800 nm) =
4.92 × 10−11 esu
Concomitant synthesis of glass matrix
Photocatalytic
and NPs: sol-gel method combined with
degradation of
heat treatment in a suitable atmosphere organic pollutants
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix Solar concentration;
and NPs
lasers
Concomitant synthesis of glass matrix
and NPs: tethering metal precursors to
the matrix during sol-gel processing
Concomitant synthesis of glass matrix
and NPs: tethering metal precursors to
the matrix during sol-gel processing
Concomitant synthesis of glass matrix
and NPs: tethering metal precursors to
the matrix during sol-gel processing
Sol-gel technique
[254]
[253]
[252]
[251]
[15]
[250]
[249]
[248]
[247]
[246]
[245]
Ref.
(continued)
26
Front. Mater. Sci. 2022, 16(3): 220607
Sm2O3 NPs
Sm2O3 NPs
Au NPs
AuAg alloy NPs
Au NPs and Ag
NPs
CuO NCs
Cu2O NCs
(4.1)
(2.7)
10‒30
5.6‒51.9 (16)
3.7‒63 (22)
1‒5 (2.24)
1‒6 (3.12)
9‒34 (18.8)
22.3 nm Au NPs,
22.3 nm Cu NPs
Au NPs and Cu
NPs
Cu NPs
18.08 nm Au NPs,
18.08 nm Cu NPs
Au NPs and Cu
NPs
12.89 nm Au NPs,
12.89 nm Cu NPs
7 nm Au NPs,
7 nm NiO NPs
Au NPs and
NiO NPs
Au NPs and Cu
NPs
Size of NPs (min‒max
(mean))/nm
Reinforcement
4.44
5.12
‒
‒
‒
2
2
2
0.25 wt.% Au,
0.75 wt.% Cu
0.25 wt.% Au,
0.75 wt.% Cu
0.25 wt.% Au,
0.75 wt.% Cu
2 wt.% Au,
1 wt.% NiO
Loading of
NPs/wt.%
Art
NLO devices:
χ(3)(800 nm) =
1.81 × 10−13 esu
NLO devices:
χ(3)(800 nm) =
3.1 × 10−12 esu
NLO devices:
χ(3)(800 nm) =
5.4 × 10−12 esu
NLO devices:
χ(3)(800 nm) =
4.4 × 10−12 esu
NLO devices:
χ(3)(800 nm) =
6.4 × 10−14 esu
NLO devices:
χ(3)(800 nm) =
1.6 × 10−14 esu
NLO devices:
χ(3)(800 nm) =
2.6 × 10−14 esu
Art
Application
NLO devices:
β(532 nm) =
−1.37 × 10−10
m·W−1
Sol-gel technique combined with NP or High-density optical
NP precursors impregnation: sol-gel
storage, undersea
method and loading of NP precursors communication, and
color displays
Sol-gel technique combined with NP or
NP precursors impregnation: sol-gel
method and loading of NP precursors
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Concomitant synthesis of glass matrix
and NPs: sol-gel method combined with
heat treatment in a suitable atmosphere
Sol-gel technique combined with NP or
NP precursors impregnation: sol-gel
method and loading of NPs
Sol-gel technique combined with NP or
NP precursors impregnation: sol-gel
method and loading of NPs
Sol-gel technique combined with NP or
NP precursors impregnation: sol-gel
method and loading of NPs
Sol-gel technique
[257]
[256]
[156]
[14]
[255]
Ref.
a) A = 3-(2-aminoethylamino)propyltriethoxysilane (AEAPTS); b) A = aminopropyltriethoxysilane (APS); c) A = (3-trimethoxysilylpropyl) diethylenetriamine (TAS); d) A = 2-(trimethoxysilyl-ethyl)pyridine (PyS); e)
nickel precursor = Ni(NO3)2; f) nickel precursor = Ni(OAc)2; g) nickel precursor = NiCl2; h) nickel precursor = Ni(acac)2; i) 0.3 mol.% Cd precursor; j) 0.03 mol.% Cd precursor; k) 0.017 mol.% Cd precursor, 4.5 h at
540 °C; l) 0.010 mol.% Cd precursor, 4.5 h at 540 °C; m) red; n) green; o) blue.
Sm2O3 NPs@sodium borosilicate
glass
Borosilicate glass
(85.17 wt.% SiO2,
14.83 wt.% B2O3)
Sodium borosilicate glass
(77.80 wt.% SiO2,
13.54 wt.% B2O3, and
8.65 wt.% Na2O)
Silica glass
Au NPs@silica glass
Sm2O3 NPs@borosilicate glass
Silica glass
Sodium borosilicate glass
(75 wt.% SiO2, 20 wt.%
B2O3, and 5 wt.% Na2O)
Sodium borosilicate glass
(96.7 wt.% SiO2, 2.7 wt.%
B2O3, and 0.6 wt.% Na2O)
Sodium borosilicate glass
(96.7 wt.% SiO2, 2.7 wt.%
B2O3, and 0.6 wt.% Na2O)
Sodium borosilicate glass
(96.7 wt.% SiO2, 2.7 wt.%
B2O3, and 0.6 wt.% Na2O)
Sodium borosilicate glass
(75 wt.% SiO2, 20 wt.%
B2O3, and 5 wt.% Na2O)
Sodium borosilicate glass
(75 wt.% SiO2, 20 wt.%
B2O3, and 5 wt.% Na2O)
Sodium borosilicate glass
(75 wt.% SiO2, 20 wt.%
B2O3, and 5 wt.% Na2O)
Silica glass
Host matrix
AuAg@silica glass
Au NPs/Ag NPs@silica glass
CuO NCs@borosilicate glasso)
Cu2O NCs@borosilicate glassn)
Cu NPs@borosilicate glassm)
Au NPs/Cu NPs@sodium
borosilicate glass (3)
Au NPs/Cu NPs@sodium
borosilicate glass (2)
Au NPs/Cu NPs@sodium
borosilicate glass (1)
Au NP/NiO NP@sodium
borosilicate glass
Composite
(continued)
Javier FONSECA. NPs embedded into glass matrices: GNCs
27
28
Front. Mater. Sci. 2022, 16(3): 220607
concentration of cadmium precursor. Alternatively, H2S
gas flow was also used to achieve Cd2+ sulphidation. In
this case, the size of CdS QDs depended on the
temperature and time of exposure to H2S and the
concentration of Cd2+ [248]. The most immediate
applications of this composite are as solar concentrators
and as an active medium in tunable lasers [265].
Xiang’s group has combined the sol-gel method with
heat treatment in suitable atmospheres, which allows
uniform distribution of reinforcements and controls the
reinforcement concentrations. In this approach, the dry gel
is generally treated in three atmospheres: (1) an oxidizing
atmosphere, eliminating the organic residues of the
precursors and producing an aerogel; (2) a reducing
atmosphere, forming the NPs; and (3) an inert
atmosphere, densifying the stabilized gel and producing
the final composite (NPs@glass). It is worth mentioning
that this method requires low temperature and, therefore,
is a low-cost synthesis route. Many GNCs have been
prepared following this approach [266]:
(a) Sb NPs have been embedded in a sodium
borosilicate glass matrix (Fig. 15). Antimony trichloride
(SbCl3), tetraethoxysilane (TEOS), H3BO3, and metallic
sodium were used as Sb ion source, SiO2 precursor, boron
source, and sodium source, respectively. Gelation and
Fig. 15 (a) TEM image of Sb NPs embedded in sodium
borosilicate glass. (b) Size distribution of Sb NPs in the glass
matrix. (c) HRTEM of Sb NPs@sodium borosilicate glass.
(d) SAED image of Sb NPs@sodium borosilicate glass.
Reproduced with permission from Ref. [249] (Copyright 2014
Elsevier).
aging were performed at 80 °C for two weeks.
Subsequently, the dry gel was heated in three
atmospheres: (1) O2 at 450 °C for 10 h, producing an
aerogel; (2) dry H2 at 450 °C for 10 h, obtaining the Sb
NPs; and (3) N2 at 600 °C for 10 h, creating the final
composite (Sb NPs@sodium borosilicate glass). Sb NPs
were reported to have a spherical shape and an average
size of approximately 32.63 nm. At the wavelength of
800 nm, β, n2, and χ(3) of this GNC were −1.71 ×
10−10 m·W−1, 8.15 × 10−16 m2·W−1, and 4.85 × 10−11 esu,
respectively. Therefore, this Sb NPs@sodium borosilicate
glass composite has potential for NLO devices [249].
(b) Sodium borosilicate glass has been doped with Cu
NPs. Copper nitrate (Cu(NO3)2), TEOS, H3BO3, and
metallic sodium were used as Cu ion source, SiO2
precursor, boron source, and sodium source, respectively.
The sol was gelled and aged at 80 °C for 2 weeks. To
remove the organic substances and decompose Cu(NO3)2,
the wet gel was dried in O2 at 450 °C for 10 h. Then, the
dry gel was exposed to dry H2 at 450 °C for 10 h in order
to convert CuO to Cu. Finally, the stabilized gel was
densified in N2 at 600 °C for 10 h. The size of the Cu NPs
ranged from 1.5 to 5 nm with a mean particle size of
2.7 nm. At the wavelength of 800 nm, β, n2, and χ(3) were
found to be 2.10 × 10−11 m·W−1, 6.42 × 10−17 m2·W−1,
and 2.41 × 10−11 esu, respectively [250].
(c) Cu2In NPs has been incorporated into a sodium
borosilicate glass. Cu(NO3)2, indium nitrate (In(NO3)3),
TEOS, H3BO3, and metallic sodium were used as Cu ion
source, In ion source, SiO2 precursor, boron source, and
sodium source, respectively. The precursors were mixed,
gelled for 2‒3 d, and dried at 80 °C for 2‒3 weeks. The
dry gel was heated in O2 atmosphere at 450 °C for 10 h.
Then, the aerogel was treated with dry H2 at 450 °C for
10 h. Finally, the aerogel was heated in N2 atmosphere at
600 °C for 10 h. The size distribution of Cu2In NPs was
reported to range from 15 to 50 nm with an average size
of approximately 30.28 nm. Cu2In NPs exhibited
hexagonal structure and spherical shape. At the
wavelength of 800 nm, β, n2, and χ(3) of these NPs
embedded in sodium borosilicate glass were 6.96 ×
10−10 m·W−1, −5.99 × 10−16 m2·W−1, and 3.32 × 10−10
esu, respectively [15].
(d) In2O3 semiconductor NCs have been embedded into
silica glass. In(NO3)3, and TEOS were used as In ion
source, and SiO2 precursor, respectively. The precursor
solution gelled at room temperature for 7 d. The gel was
aged at 150 °C for 3 to 4 weeks. To eliminate organic
Javier FONSECA. NPs embedded into glass matrices: GNCs
residues, the gel was heated in O2 atmosphere at 600 °C
for 5 h, obtaining the In2O3@SiO2 composite. The mean
size of the obtained In2O3 NCs was found to increase
from 23 to 54 nm when the doping of the glass matrix
increased from 1 to 3 wt.% of In2O3. The photodegradations of rhodamine (RhB) under UV irradiation in the
presence of In2O3 (1 wt.%)@SiO2 and In2O3 (3 wt.%)@
SiO2 composites for 120 min were ~35.5% and ~61.6%,
respectively. The photodegradation efficiency of silica
glass was significantly improved by embedding In2O3
NCs. Therefore, In2O3@SiO2 composites are promising
photocatalysts for the degradation of organic pollutants
[251].
(e) Sodium borosilicate glass has been doped with Au
NPs (Fig. 16). Hydrogen tetrachloroaurate (HAuCl4),
TEOS, H3BO3, and metallic sodium were used as Au ion
source, SiO2 precursor, boron source, and sodium source,
respectively. Within a week, a wet gel was formed from
the mixed solution. The wet gel was dried at 80 °C for
about 3 weeks to obtain the dry gel. To remove the
residual organic solvent and to decompose HAuCl4, the
dry gel was heated in O2 atmosphere at 450 °C for 5 h.
Then, it was exposed to H2 at 450 °C for 5 h, forming
metallic Au. Finally, the stabilized gel was densified in N2
atmosphere at 600 °C for 2 h, producing the
Fig. 16 (a) TEM image of Au NPs embedded in sodium
borosilicate glass. (b) Size distribution of Au NPs in the glass
matrix. (c) HRTEM of Au NPs@sodium borosilicate glass.
(d) SAED image of Au NPs@sodium borosilicate glass.
Reproduced with permission from Ref. [252] (Copyright 2015
Springer Nature).
29
AuNPs@sodium borosilicate glass composite. The size of
embedded Au NPs was found to range from 2.82 to
9.97 nm, with an average size of 5.48 nm. At the
wavelength of 800 nm, β, n2, and χ(3) were −6.5 ×
10−14 m·W−1, 3.0 × 10−20 m2·W−1, and 1.7 × 10−14 esu,
respectively [252].
(f) Cu3.8Ni NCs have been embedded in a sodium
borosilicate glass matrix. Nickel nitrate (Ni(NO3)2),
Cu(NO3)2, TEOS, H3BO3, and metallic sodium were used
as Ni ion source, Cu ion source, SiO2 precursor, boron
source, and sodium source, respectively. After preparation
and gelation of the sol, the gel was heated in H2
atmosphere at 600 °C, producing the composite. XPS
analysis indicated that the obtained nanocrystal was
Cu3.8Ni alloy. The size of Cu3.8Ni NCs was found to range
from 15 to 40 nm with an average size of 27.5 nm. At the
wavelength of 800 nm, β, n2, and χ(3) were (3.73 ±
0.02) × 10−10 m·W−1, (1.07 ± 0.08) × 10−15 m2·W−1, and
(4.92 ± 0.05) × 10−11 esu, respectively. χ(3) was larger
than those of the corresponding monometallic
NPs [253].
(g) Au and NiO NPs or Au and Ni NPs have been
incorporated in sodium borosilicate glass. HAuCl4 and
Ni(CH3COO)2 were used as Au ion source and Ni ion
source, respectively. The Au NPs/NiO NPs@sodium
borosilicate glass composite was synthesized by treating
the gel in O2 atmosphere at 450 °C for 10 h and N2
atmosphere at 600 °C for 2 h. The Au NPs/Ni
NPs@sodium borosilicate glass composite was fabricated
when the gel was exposed to O2 at 450 °C for 10 h, H2
atmosphere at 450 °C for 10 h, and finally, N2 atmosphere
at 600 °C for 2 h. The NPs of the Au NPs/NiO
NPs@sodium borosilicate glass composite exhibited a
size of 2 and 8 nm. The smallest and largest NPs were
suggested to be NiO and Au, respectively. In Au NPs/Ni
NPs@sodium borosilicate glass composite, the size of
both NPs was found to be around 7 nm. They exhibited
superior optical non-linearities compared to Au
NPs@sodium borosilicate glass composite. At the
wavelength of 800 nm, χ(3) of Au NPs/NiO NPs@sodium
borosilicate glass composite and Au NPs/Ni NPs@sodium
borosilicate composite was reported to be 1.81 × 10−13
and 2.31 × 10−12 esu, respectively [254].
(h) Au and Cu NPs have been embedded in sodium
borosilicate glass. Cu(NO3)2, HAuCl4, TEOS, H3BO3, and
metallic sodium were used as Cu ion source, Au ion
source, SiO2 precursor, boron source, and sodium source,
respectively. Gelation, aging and drying of the mixture
30
Front. Mater. Sci. 2022, 16(3): 220607
was carried out by heating at 100 °C for 3 weeks. Then,
the dry gel was exposed to a H2 atmosphere at 450 °C for
10, 20, and 30 h, producing stabilized gels referred to as
(1), (2), and (3), respectively. Finally, the stabilized gels
were densified in a N2 atmosphere at 600 °C. No AuNi
alloy was observed. It was suggested that the Au NPs
migrated toward the core of the matrix and the Cu NPs
coated the surface of the matrix. The mean size of the NPs
embedded in the glass matrix was found to be 12.89,
18.08, and 22.30 nm for GNC (1), GNC (2), and GNC (3),
respectively. At the wavelength of 800 nm, χ(3) values of
GNC (1), GNC (2), and GNC (3) were 3.1 × 10−12,
5.4 × 10−12, and 4.4 × 10−12 esu, respectively. Thus, the
NPs improved the optical non-linearities of the glass
matrix [255].
(i) Red, green and blue glass composites have been
prepared by embedding Cu NPs, Cu2O NCs and CuO NCs
into sodium borosilicate glass, respectively (Fig. 17).
Cu(NO3)3, TEOS, H3BO3, and metallic sodium were used
as Cu ion source, SiO2 precursor, boron source, and
sodium source, respectively. The mixture solution gelled
at 80 °C for 3 weeks. The wet gel was exposed to an O2
atmosphere at 400 °C for 10 h. In doing so, the organic
matter was removed, and the copper nitrate decomposed.
The dry gel was heated in H2 atmosphere at 600 °C for 10,
5, and 0 h, forming Cu NPs, Cu2O NCs, and CuO NCs,
respectively. Therefore, depending on exposure time in a
reducing atmosphere, the valence state of copper was 0
(Cu0), +1 (Cu1+) or +2 (Cu2+). Finally, the stabilized gel
was densified in N2 atmosphere at 600 °C for 5 h,
producing the composites. The red, green, and blue colors
of the composites were found to correspond to Cu (Cu0),
Cu2O (Cu1+), and CuO (Cu2+) NCs embedded in the glass
matrix, respectively. The size of these three different
crystals in the glass matrices ranged from 9 to 34 nm, 1 to
6 nm, and 1 to 5 nm, respectively. The average size of the
crystals was found to be 18.8, 3.12, and 2.24 nm,
respectively. At the wavelength of 800 nm, χ(3) of the red,
green, and blue composites was 6.4 × 10−14, 1.6 × 10−14,
and 2.6×10−14 esu, respectively [14].
Yanes et al. have prepared an YF3 NC@Ln3+-doped
silica glass composite, where Ln3+ = Eu3+ or Sm3+, and an
YF3 NC@Yb3+, Tm3+ co-doped silica glass composite by
sol-gel method [267]. An initial homogeneous solution
containing yttrium acetate (Y(CH3COO)3), TEOS and
europium acetate (Eu(CH3COO)3) or samarium nitrate
(Sm(NO3)3) gelled at 35 °C for several days. The gel was
treated in air at 675 °C to precipitate YF3 NCs, thus
forming the YF3 NC@Ln3+-doped silica glass composite.
The same procedure was used to prepare YF3 NC@Yb3+,
Tm3+ co-doped silica glass composite. In this case, the
source of Yb3+ was ytterbium acetate (Yb(CH3COO)3).
YF3 NCs of 11 nm were found to precipitate in the glass
matrices. YF3 is a quantum efficient host lattice for RE
ions due to its wide band gap and the possibility of
replacing Y3+ sites by trivalent RE ions with similar ionic
radii and valence state [268‒269]. YF3 NC@Ln3+-doped
silica glass composite spectra revealed a significant
partition of Ln3+ into the precipitated YF3 NCs.
Moreover, YF3 NC@Yb3+, Tm3+ co-doped silica glass
composite achieved bright and efficient infrared to UV
and visible up-conversion. It is worth mentioning that the
UV up-conversion emissions improved with increasing
Yb3+ concentration. YF3 NC@Yb3+, Tm3+ co-doped silica
glass composite opens the door to develop shortwavelength solid-state lasers for applications in integrated
photonic devices [267].
Recently, Kalwarczyk et al. have reported the synthesis
of Au NPs@silica glass composite by sol-gel method.
HAuCl4, TEOS, and L-ascorbic acid were used as Au ion
source, SiO2 precursor, and reducing agent, respectively.
L-ascorbic acid was applied in an amount sufficient for
partial reduction of Au3+ ions to Au+. The Au+ ions were
suggested to be encapsulated within micelles, preventing
their reduction to Au0 and facilitating their homogeneous
distribution in the gel. The micelles were made of cationic
surfactants (benzyldimethylhexadecylammonium chloride
(BDAC) or/and cetyltrimethylammonium bromide
(CTAB)). The reduction of Au+ to Au0 (Au NPs) was
proposed to occur when micelles break during the
hydrolysis of TEOS. Therefore, Au NPs were formed
through in situ nucleation and growth during the gelation
and/or drying steps. It should be noted that the growth of
the Au NPs within the silica matrix occurred at only 35 °C
for 7‒8 d. Moreover, the Au NP@silica glass composite
did not require any post-synthetic treatment such as heat.
This GNC was found to contain 0.027 wt.% of Au. The
average size of the embedded Au NPs was (6.9 ± 0.3) nm.
Au NP@silica glass composite was very stable.
Embedded Au NPs did not aggregate or disintegrate for
one year. Despite small amount of Au NPs, the composite
exhibited strong NLO properties, and was capable of
effective third-harmonic generation of ultrashort near-IR
laser pulse. Therefore, this GNC can be potentially
employed as an IR-UV converter [270].
Javier FONSECA. NPs embedded into glass matrices: GNCs
31
Fig. 17 TEM images of (a) red GNC, (b) green GNC, and (c) blue GNC. HRTEM images of (d) red GNC, (e) green GNC, and
(f) blue GNC. Size distribution images of (g) red GNC, (h) green GNC, and (i) blue GNC. Photographs of as-obtained (j) red GNC,
(k) green GNC, and (l) blue GNC. Reproduced with permission from Ref. [14] (Copyright 2015 American Chemical Society).
3.2.3 Sol-gel technique combined with nanoparticle or
nanoparticle precursors impregnation
Ventura et al. have prepared GNCs made from Au and/or
Ag NPs embedded in silica glass matrix [156]. Au and Ag
NPs were synthesized by reduction of HAuCl4 and silver
nitrate (AgNO3). These NPs were incorporated into the
sol solution containing TMOS, which was the precursor to
silica glass. The colloidal solutions containing Ag and Au
NPs showed a characteristic UV-vis absorption peak at
393 and 520 nm, respectively. Conversely, the colloidal
solution with a mixture of NPs exhibited two separate
UV-vis absorption peaks corresponding to the absorbance
of pure Ag and pure Au NPs. Logically, the absorbance of
one component increased as the component concentration
increased. The colors of the gels ranged from yellow to
32
Front. Mater. Sci. 2022, 16(3): 220607
deep red with increasing gold concentration. The gels
were dried to obtain the colored transparent glasses. AgAu
alloys were formed by thermal treatment at temperatures
above 200 °C. Both the glasses containing alloys and the
glasses containing a mixture of NPs exhibited the orange
tones between the ruby red color of Au NPs@silica glass
composite and the yellow color of Ag NPs@silica glass
composite. Glass composites with pure Ag NPs were
bleached at temperatures higher than 300 °C. A small
amount of Au NPs was found to avoid the bleaching
process [156].
Rouge et al. have embedded zirconia-coated gold NPs
into silica glass matrix [256]. The precursors of SiO2,
ZrO2, and Au NPs were tetramethyl orthosilicate (TMOS),
zirconium(IV) propoxide, and HAuCl4, respectively.
Firstly, previously synthesized Au NPs were embedded in
a zirconia sol and subsequently incorporated to a TMOS
solution. This new sol was gelled and dried at 120 °C and
then annealed in air at 850 °C. Finally, the dry gel was
densified in air at 1100 °C, which led to the formation of
the composite. The size of Au NPs in annealed gel ranged
from 8 and 18 nm. In addition, the zirconia phase was
dispersed in the silica matrix. In the densified doped glass,
the size of Au NPs was between 10 and 30 nm. Particles
of ZrO2 appeared in the silica matrix after annealing at
1100 °C. Zirconia was suggested to protect Au NPs
during annealing and densification. At the wavelength of
532 nm, β of zirconia-coated Au NPs@silica composite
was −1.37 × 10−10 m·W−1 [256].
Sm2O3 NPs have been embedded in the reticular
structure of a borosilicate glass by sol-gel route and
impregnation [257]. TEOS and trimethyl borate
(C3H9BO3) were used as SiO2 and boron precursors,
respectively. The gelation of the mixture of precursors
(sol) was carried out at 50 °C for 6 h. The gel was aged
for 1 week. Then, the gel was dried at 50 °C for 1
additional week. The dry gel was doped with Sm(NO3)3.
This doped dry gel was heated in air and N2:H2
(95:5 vol.%) at 450 °C during 5 and 12 h, respectively.
Sm(NO3)3 was transformed to Sm2O3 [271]:
2Sm(NO3 )3 → Sm2 O3 + 6NO2 + 1.5O2
(27)
Thus, Sm2O3 NPs@borosilicate glass composite was
prepared. In brief, the oxidation and reduction treatments
were suggested to remove the organic substance, oxidize
Sm(NO3)3, and promote the luminescence of Sm2O3.
Embedded Sm2O3 behaved like network modifier. It
produced NBOs [272]. Therefore, BO3 was transformed
to BO4 by taking NBO from Sm2O3, generating the boric
anomaly [273]. Similarly, Sm(NO3)3 was also embedded
into a sodium borosilicate glass. The composite was
synthesized by doping the dry gel with NaNO3 and
subsequently with Sm(NO3)3. Then, the doped dry gel
was also exposed to air and N2:H2 atmospheres to produce
a Sm2O3 NPs@sodium borosilicate glass composite. It is
worth mentioning that the glass transition temperature and
the melting temperature of the glass are often reduced by
adding network modifiers such as the sodium oxide. In
addition, network modifier oxides break and open the
reticular structure of glasses, which results in loss of
connectivity and, therefore, favors doping [273]. In this
study, Sm2O3 and Na2O increased the glass transition
temperature due to the boric anomaly. The size of the
embedded Sm2O3 was 2.7 and 4.1 nm for the Sm2O3
NPs@borosilicate glass and Sm2O3 NPs@sodium
borosilicate glass composites, respectively. The optical
band gap of Sm2O3 NPs@borosilicate glass and Sm2O3
NPs@sodium borosilicate glass composites was
3.65‒3.86 and 3.39‒3.57 eV, respectively. Therefore, the
optical band gap decreased with increasing the amount of
Sm2O3 and Na2O. The observed green luminescence
(545 nm) under UV light in Sm3+ doped glasses, which
was found to be stable for 6 months, has been associated
to the 4F3/2 → 6H7/2 electronic transition of Sm3+ ions
[257]. A good dispersion of RE NPs in the glass matrix is
necessary to increase the PL efficiency. It should be noted
that sol-gel methods allow incorporating a higher
concentration of reinforcements than conventional
methods. Up to 5‒10 wt.% of RE NPs can be embedded
into glasses using sol-gel approaches [274‒279]. The
aggregation of RE NPs and the devitrification become
significant above these concentrations [280].
3.3
3.3.1
Ion implantation
Introduction
Ion implantation is an effective tool for introducing
reinforcements into the surface layer of a substrate to a
depth of several micrometers [281]. Specifically, ion
implantation of metals is a well-established technique to
embed metal NPs into glass matrices. Table 4 shows
examples of GNCs prepared by ion implantation
techniques [282‒293]. This method requires lowtemperature processing, controls the concentration of NPs,
and overcomes solubility limits of conventional
Ag NPs
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Chalcohalide glass
(48.80 wt.% GeS2, 36.04 wt.%
Ga2S3, and 15.16 wt.% KBr)
Chalcohalide glass
(48.80 wt.% GeS2, 36.04 wt.%
Ga2S3, and 15.16 wt.% KBr)
Chalcohalide glass
(48.80 wt.% GeS2, 36.04 wt.%
Ga2S3, and 15.16 wt.% KBr)
Chalcohalide glass
(48.80 wt.% GeS2, 36.04 wt.%
Ga2S3, and 15.16 wt.% KBr)
Silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Ag NPs@silica glass
Ag NPs@silica glass
Ag NPs@chalcohalide
glass
Silica glass
Silica glass
Silica glas
Silica glas
Silica glas
Silica glas
Silica glas
Silica glas
Silica glas
Borate glass (10.66 wt.%
B2O3, 67.12 wt.% PbO,
9.62 wt.% GeO2, 10.71 wt.%
Bi2O3, and 1.89 wt.% Pr2O3)
Ag NPs@silica glass
Ag NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@borate glass
Ag NPs@silica glass
Ag NPs@chalcohalide
glass
Ag NPs@chalcohalide
glass
Ag NPs@chalcohalide
glass
Ag NPs
Host matrix
Composite
(11.68)
(2.8)
Au NPs
(9.98)
(9.14)
(3.02)
(3.18)
(2.66)
(2.84)
(1)
(1)
(1.2)
Au NPs
Au NPs
Au NPs
Au NPs
Au NPs
Au NPs
Au NPs
Ag NPs
Ag NPs
Ag NPs
(300)
(200)
Ag NPs
Ag NPs
(150)
(100)
(2.7)
(7.7)
(12.6)
(5.6)
(3.6)
Size of NPs (min‒max
(mean))/nm
(2)
Ag NPs
Ag NPs
Au NPs
Au NPs
Au NPs
Au NPs
Reinforcement
GNCs prepared by ion implantation techniques [282‒293]
Table 4
NLO devices: χ(3)(800 nm) =
1.4 × 10−13 esu
NLO devices: χ(3)(800 nm) =
3.3 × 10−13 esu
NLO devices: χ(3)(800 nm) =
2.8 × 10−13 esu
NLO devices: χ(3)(800 nm) =
2.6 × 10−13 esu
‒
‒
Application
Single implantation: ion implantation — Ag ions of 330 keV,
‒
ion fluence (1 × 1016 ion·cm−2); annealing (at 600 °C for 1 h)
Single implantation: ion implantation — Ag ions of 1.2 MeV,
ion fluence (1 × 1016 ion·cm−2); annealing (at 600 °C for 1 h)
Single implantation: ion implantation — Ag ions of 1.7 MeV,
ion fluence (1 × 1016 ion·cm−2); annealing (at 600 °C for 1 h)
Single implantation: ion implantation — Au ions of 60 keV,
Optoelectronics
ion fluence (3 × 1016 ion·cm−2), flux (1.5 µA·cm−2)
Single implantation: ion implantation — Au ions of 60 keV,
ion fluence (4 × 1016 ion·cm−2), flux (1.5 µA·cm−2)
Single implantation: ion implantation — Au ions of 60 keV,
ion fluence (5 × 1016 ion·cm−2), flux (1.5 µA·cm−2)
Single implantation: ion implantation — Au ions of 60 keV,
ion fluence (6 × 1016 ion·cm−2), flux (1.5 µA·cm−2)
Single implantation: ion implantation — Au ions of 60 keV,
ion fluence (7 × 1016 ion·cm−2); flux (1.5 µA·cm−2)
Single implantation: ion implantation — Au ions of 60 keV,
ion fluence (8 × 1016 ion·cm−2), flux (1.5 µA·cm−2)
Single implantation: ion implantation — Au ions of 60 keV,
ion fluence (9 × 1016 ion·cm−2), flux (1.5 µA·cm−2)
Single implantation: Melt-quenching for glass preparation
Gain media for amplifiers in
(melting at 1250 °C for 1 h); annealing (at 330 °C for 2 h); ion optical telecommunication
implantation — Au ions of 300 keV, ion fluence
windows
(1 × 1016 ion·cm−2); annealing (at 330 °C for 12 h)
Single implantation: ion implantation — Ag ions of 200 MeV,
ion fluence (2 × 1017 ion·cm−2)
Single implantation: ion implantation — Ag ions of 200 MeV,
ion fluence (1 × 1017 ion·cm−2)
Single implantation: ion implantation — Ag ions of 200 MeV,
ion fluence (5 × 1016 ion·cm−2)
Single implantation: ion implantation — Au ions of 190 keV,
ion fluence (4 × 1016 ion·cm−2), flux (2 µA·cm−2)
Single implantation: ion implantation — Au ions of 190 keV,
ion fluence (4 × 1016 ion·cm−2), flux (2 µA·cm−2); annealing
(at 900 °C for 1 h)
Single implantation: ion implantation — Au ions of 190 keV,
16
ion fluence (4 × 10 ion·cm−2), flux (2 µA·cm−2); annealing
(at 900 °C for 3 h)
Single implantation: ion implantation — Au ions of 190 keV,
16
ion fluence (4 × 10 ion·cm−2), flux (2 µA·cm−2); annealing
(at 900 °C for 12 h)
Single implantation: ion implantation — Ag ions of 1 MeV,
ion fluence (5 × 1016 ion·cm−2), flux (1 µA·cm−2)
Single implantation: ion implantation — Ag ions of 1 MeV,
ion fluence (5 × 1016 ion·cm−2), flux (1 µA·cm−2); ion
implantation — Si ions of 9.4 MeV, ion fluence
(1 × 1014‒8 × 1015 ion·cm−2)
Single implantation: ion implantation — Ag ions of 200 MeV,
ion fluence (1 × 1016 ion·cm−2)
Ion implantation technique
[287]
[286]
[285]
[284]
[283]
[282]
Ref.
Javier FONSECA. NPs embedded into glass matrices: GNCs
33
Host matrix
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Composite
Cu NPs@silica glass
Cu NPs@silica glass
Au NPs@silica glass
Cu NPs@silica glass
Ag NPs@silica glass
Ag NPs/Ni NPs@silica
glass
Ag NPs@silica glass
Ag NPs@silica glass
Ag NPs/Zn NPs/AgZn
NPs@silica glass
Ag NPs, Zn NPs
and AgZn alloy
NPs
Ag NPs
Ag NPs
Ag NPs and Ni
NPs
Ag NPs
Cu NPs
Au NPs
Cu NPs
Cu NPs
Reinforcement
2‒12
4‒8
3‒17
0.5‒2 nm Ag NPs,
0.5‒4 nm Ni NPs
35‒48
1‒15 (5.6)
6‒10
6‒10
Size of NPs (min‒max
(mean))/nm
(20)
Application
Single implantation: ion implantation — Cu ions of 40 keV,
‒
ion fluence (5 × 1016 ion·cm−2), flux (5 µA·cm−2)
Single implantation: ion implantation — Cu ions of 200 keV, NLO devices: χ(3)(532 nm) =
ion fluence (3 × 1016 ion·cm−2); annealing (at 400 °C for 1 h)
1.5 × 10−8 esu
Single implantation: ion implantation — Au ions of 1.5 MeV, NLO devices: χ(3)(532 nm) =
ion fluence (1 × 1017 ion·cm−2); annealing (at 400 °C for 1 h)
−3.7 × 10−12 esu
Single implantation: ion implantation — Cu ions of 180 keV, NLO devices: χ(3)(532 nm) =
ion fluence (1 × 1017 ion·cm−2), flux (1.5 µA·cm−2)
−2.1 × 10−7 esu, χ(3)(1064 nm) =
−1.2 × 10−7 esu
Multiple implantation: ion implantation — Ag ions of
‒
200 keV, ion fluence (5 × 1016 ion·cm−2), flux (1 µA·cm−2);
ion implantation — Cu ions of 110 keV, ion fluence
(5 × 1016 ion·cm−2), flux (1.5 µA·cm−2)
Multiple implantation:ion on implantation — Ni ions of
Optoelectronics, optical
60 keV, ion fluence (5 × 1016 ion·cm−2), flux (4 µA·cm−2);
sensors, and antibacterial
ion implantation — Ag ions of 70 keV, ion fluence
materials
(5 × 1016 ion·cm−2), flux (4 µA·cm−2); annealing
(at 600 °C for 1 h)
Single implantation: ion implantation — Ag ions of 35 keV,
Optoelectronics, optical
ion fluence (5 × 1016 ion·cm−2), flux (4 µA·cm−2)
sensors, and antibacterial
materials
Multiple implantation: ion implantation — Zn ions of 50 keV,
ion fluence (5 × 1016 ion·cm−2), flux (4 µA·cm−2); ion
implantation — Ag ions of 35 keV, ion fluence
(5 × 1016 ion·cm−2), flux (4 µA·cm−2)
Multiple implantation: ion implantation — Ag ions of 35 keV,
ion fluence (5 × 1016 ion·cm−2), flux (4 µA·cm−2); ion
implantation — Zn ions of 50 keV, ion fluence
(5 × 1016 ion·cm−2), flux (4 µA·cm−2)
Ion implantation technique
[293]
[292]
[291]
[290]
[289]
[288]
Ref.
(continued)
34
Front. Mater. Sci. 2022, 16(3): 220607
Javier FONSECA. NPs embedded into glass matrices: GNCs
techniques [163]. Ion implantation reaches a high metal
filling (beyond the equilibrium limit of metal solubility) in
an irradiated glass matrix. It should be noted that high
filling factors favor a high χ(3), when the other GNC
parameters are maintained. However, ion implantation
does not allow an effective control on the size distribution
of NPs within the matrix. In addition, this technique does
not enable uniform depth of penetration of implanted ions
into the matrix [281,294]. Post-implantation thermal or
laser annealing are often applied to control the size
distribution and penetration of NPs embedded in glass
matrices.
Embedded metal ions tend to form neutrally charged
metal atoms. Metal ions can be neutralized by forming
chemical bonds with radicals and ions of the glass or by
participating in oxidation processes. Considering the
difference in the Gibbs free energies, metal-metal bonds
are predominantly created.
In other words, clusters of metal atoms are mainly
formed. These clusters serve as nuclei of metal NPs. The
growth of the metal NPs depends on both the diffusion
coefficient and the local concentration of metal atoms.
Since diffusion can occur through defects, their
production under irradiation is crucial for the formation of
NPs. Under diffusion-limited conditions, NPs grow
predominantly by joining new implanted metal ions.
Moreover, under these conditions, large NPs grow while
smaller NPs dissolve. This growth mechanism is known
as coarsening and is governed by the Gibbs‒Thomson
effect, where the chemical potential is inversely
proportional to the particle size. When growth occurs
under diffusion-limited and mass conservation conditions,
the coarsening mechanism is called Ostwald ripening
[282].
The energy of the irradiation is transferred to the glass
matrix by electron shell excitation (ionization) and nuclear
collisions. Therefore, ion implantation may induce
modifications of the glass matrix such as extended and
point defects, amorphization and local crystallization,
precipitation of a new phase made up of host atoms or
implanted ions, etc. The degree of glass modification
induced by ion implantation depends on the chemical and
structural composition of the glass, on variations in
implantation parameters such as type, fluence and energy
of ion species, and on process temperature [281].
The implant amount is determined by ion dose (F0). The
ions implanted through low-dose irradiation (F0 ≤
5.0 × 1014 ions·cm−2) are usually well-dispersed
35
throughout the glass matrix and well separated from each
other. These ions remain isolated within the glass matrix.
When the irradiation ranges from 1015 to 1017 ions·cm−2,
the implant concentration often exceeds the solubility
limit of metal atoms in glass matrix. Consequently, the
reinforcements tend to relax by nucleation and growth.
This causes the formation of isolated NPs. At high-dose
irradiation (F0 ≥ 1017 ions·cm−2), metal NPs coalescence
and form either aggregates or thin quasi-continuous films
at the glass surface [281].
The size of metal NPs formed at different depths is
proportional to the metal filling factor at the specific
depth, which, in turn, depends on the ion irradiation.
Generally, different concentrations of embedded atoms
are found from layer to layer in an irradiated glass matrix.
Large NPs are usually close to the surface of the glass
matrix, while subnanometric metal clusters and small NPs
are found within the glass matrix but far from the
irradiated surface. When applying high dose ion
irradiation, the largest NPs are found on the surface due to
surface sputtering (Fig. 18) [295]. This size distribution of
NPs has a decisive effect on the properties of the GNC.
Therefore, to control the properties of GNCs for specific
application purposes, the material designer must carefully
manipulate the synthesis of such GNCs by ion irradiation.
The fluence-dependent depth distribution function
(G(z)) for implanted species can be expressed according
to Eq. (28) if the implantation probability function is
Gaussian-like [296]:
N
G(z) =
2Y
 



  z − Rp + ΦY/ N 
 z − Rp 
erf  √
 − erf  √
 (28)
2 · ∆Rp
2 · ∆Rp
where z is the depth coordinate with respect to the surface,
N is the atomic density of the substrate, Y is the sputtering
yield, Φ is the ion fluence, Rp is the projected range of
ions, and ΔRp is the range straggling related to Rp.
3.3.2
Single implantation
In an early study, Stepanov et al. embedded Ag NPs in
silica glass by ion implantation [295]. 30 keV Ag+ ions
with a dose of 5×1016 ions·cm−2 were implanted in silica
glass at ion current densities ranging from 4 to 15
µA·cm−2. The size of the embedded NPs within the glass
matrix was found to increase as the ion current density
increased. An increase in the ion current density caused a
temperature rise in the glass matrix that elevated the
36
Front. Mater. Sci. 2022, 16(3): 220607
Fig. 18 Physical phenomena involved in the formation of metal NPs by ion implantation. Reproduced with permission from Ref.
[295] (Copyright 2004 Elsevier).
mobility of the atoms, thus favoring the growth of NPs
[295]. Srivastava et al. have also incorporated Ag NPs
into a silica glass by ion implantation of Ag+ ions [283].
Ag+ ions of 1 MeV were implanted in the silica matrix at
a fluence of 5 × 1016 ions·cm−2. The Ag+ current density
was kept around 1 µA·cm−2. Ag NPs with an average size
of 7.7 nm were formed within the silica glass (Fig. 19(a)).
The as-implanted composite was bombarded with Si5+
ions of 9.4 MeV at fluences ranging from 1 × 1014 to
8 × 1015 ions·cm−2. Ag NPs were found to dissolve by
irradiation of Si ions. The average size of the Ag NPs
reduced to 2.7 nm (Fig. 19(b)). The UV-vis optical
absorption of the GNC was found to change drastically
when the material was irradiated with high-energy Si ions
[283].
Metal ions have also been implanted in glasses other
than silica glass. Liu et al. have embedded Ag NPs in a
chalcohalide glass (GeS2‒Ga2S3‒KBr) by ion implantation [284]. This glass, which was prepared by meltquenching method, was bombarded with Ag+ ions of
200 keV at various fluences (1 × 1016, 5 × 1016, 1×1017,
and 2 × 1017 ions·cm−2), thus forming Ag NPs within the
glass matrix. The average size of the Ag NPs was 100,
150, 200, and 300 nm within the GNCs prepared with a
fluence of 1 × 1016, 5 × 1016, 1 × 1017, and 2 × 1017
ions·cm−2, respectively. Therefore, the size of the NPs
increased as the fluence increased. Furthermore, χ(3) of the
GNCs improved as the Ag NPs embedded became larger.
Thus, at 800 nm, χ(3) values were reported to be
2.6 × 10−13, 2.8 × 10−13, 3.3 × 10−13, and 1.4 × 10−13 esu
for the GNCs prepared with fluence values of 1 × 1016,
5 × 1016, 1 × 1017, and 2 × 1017 ions·cm−2, respectively.
χ(3) of the GNC prepared with a fluence of 2 × 1017
ions·cm−2 was lower than that of the GNC prepared with a
fluence of 1 × 1017 ions·cm−2 due to the greater
Fig. 19 TEM images of Ag NPs within silica glass
(a) before and (b) after Si ion-irradiation. NP size distribution
histograms are shown as insets. Reproduced with permission
from Ref. [283] (Copyright 2014 Elsevier).
interaction of Ag NPs in the first mentioned GNC [284].
The effect of ion implantation energy on the nucleation
of Ag NPs within different glasses (silica glass, BK7
(SiO2‒B2O3‒Na2O‒K2O‒As2O3‒BaO), GIL49 (SiO2‒
Na2O‒K2O‒Al2O3‒CaO‒MgO), and Glass B (SiO2‒Na2O‒
Al2O3)) have been explored [285]. These glasses were
implanted with Ag+ ions of 0.33, 1.2, and 1.7 MeV at a
constant fluence of 1 × 1016 ions·cm−2. The as-implanted
GNCs were annealed at 600 °C for 1 h in air. In silica
glass, the average size of the embedded Ag NPs was
approximately 1 nm for 1.2 and 1.7 MeV Ag+
implantations. The average size of the Ag NPs within the
same glass was around 1.2 nm for 330 keV Ag+
implantation. In those glasses containing Na+ (BK7,
GIL49 and Glass B), Ag NPs were prepared at Ag+
energies of 1.2 and 1.7 MeV. However, the subsequent
annealing of these GNCs dissolved the NPs already
created. In other words, no Ag NPs were found after the
annealing procedure in glasses containing Na+ ions. This
was explained by an easy migration of Ag+ ions due to
Javier FONSECA. NPs embedded into glass matrices: GNCs
Ag+‒Na+ ion-exchange in these glasses. The location of
the Ag NPs was found to be deeper within the glasses as
the ion implantation energy increased. In addition, the
location of the Ag NPs was also found to depend on the
density of the glasses. The location of the Ag NPs was
deeper within a glass as the density of the glasses
decreased [285]. In a very similar study, Stanek et al. have
embedded Ag NPs in Er3+-doped silicate glasses
(SiO2‒Na2O‒ZnO‒Al2O3) by Ag+ ion implantation and
subsequent annealing [297]. The glasses with different
Er3+ content (0.1, 0.2, and 0.25 mol.% of Er) were
previously prepared by melt-quenching. The glasses were
bombarded with Ag+ ions of 1.2 and 1.7 MeV at the
fluence of 1 × 1016 ions·cm−2. In doing so, Ag NPs with a
size of approximately 1‒2 nm were embedded within the
glasses. The GNCs were annealed at 600 °C for 1 h in an
air atmosphere. This post-implantation annealing
dissolved the Ag NPs. It should be noted that regardless
of the ion implantation energy, the as-implanted samples
exhibited the PL intensity almost three times that of nonimplanted glasses. The PL intensity remained the same in
the annealed samples although the Ag NPs were no longer
present [297].
Au NPs have also been incorporated into silica glass by
ion implantation [286]. Au ions of 60 keV were implanted
into the glass matrix. The ion fluence was varied from
3 × 1016 to 9 × 1016 ions·cm−2 with a flux of 1.5 µA·cm−2.
The Au NP size was found to be control by such variation
of the total fluence. The average size was 2.84, 2.66, 3.18,
37
and 3.02 nm for an ion fluence of 3 × 1016, 4 × 1016,
5 × 1016, and 6 × 1016 ions·cm−2, respectively. The
amount of Au NPs increased as ion fluence increased, but
without an increase in size. However, the average size of
the Au NPs was 9.14, 9.98, and 11.68 nm for an ion
fluence of 7 × 1016, 8 × 1016, and 9 × 1016 ions·cm−2,
respectively. Therefore, the NPs grew with these higher
implantation fluences. It is worth noting that only a part of
the NPs grew, while the rest maintained their size. In
other words, for these three ion fluences, the larger
particles were accompanied by smaller ones. The Ostwald
ripening mechanism was suggested in view of this
inhomogeneous size distribution [298]. The Au NPs were
found to be located near the upper surface of the matrix.
In this study, the NLO properties of the GNC prepared
with an ion fluence of 6 × 1016 ions·cm−2 were also
explored [286].
Marchi et al. have investigated the growth kinetics of
Au NPs embedded in silica glass [282]. The GNC
(AuNPs/silica glass) was prepared by Au+ ion implantation. 190 keV Au+ ions with a fluence of 4 ×
1016 ions·cm−2 and a current density of less than
2 µA·cm−2 were implanted in silica glass. Subsequently,
the as-implanted GNC was annealed in an air atmosphere
at 900 °C for different times (1, 3, and 12 h). The mean
size of the as-implanted GNC and the GNCs annealed
during 1, 3, and 12 h was 2, 3.6, 5.6, and 12.6 nm,
respectively. Therefore, the mean size of the NPs
increased as the annealing time increased (Fig. 20). The
Fig. 20 Cross-sectional bright-field TEM images of (a) the as-implanted and the annealed for (c) 1 h, (e) 3 h, and (g) 12 h GNCs. Au
NPs size distribution in (b) the as-implanted and the annealed for (d) 1 h, (f) 3 h, and (h) 12 h GNCs. Reproduced with permission
from Ref. [282] (Copyright 2002 AIP Publishing).
38
Front. Mater. Sci. 2022, 16(3): 220607
Au NPs in the GNCs were present at a depth of
approximately 120 nm below the surface. The growth
kinetics of NPs was found to change with annealing times.
NPs grew mainly following the diffusion limited
mechanism when the heating time was less than 4 h.
However, when the annealing time exceeded 5 h, the NPs
grew mainly following the Ostwald ripening mechanism.
Therefore, there was a transition from a diffusion-limited
mechanism to the Ostwald ripening mechanism in the
growth kinetics of the Au NPs during annealing time
between 4 and 5 h [282].
Au NPs have been implanted into a Pr3+-doped
B2O3‒PbO‒Bi2O3‒GeO2 glass [287]. The Pr3+-doped
glass was prepared by melt-quenching technique. Au+
ions of 300 keV were implanted into the glass matrix. The
ion fluence was 1 × 1016 ions·cm−2. This composite was
annealed at 330 °C for 4 and 12 h to form the Au NPs.
The average size of Au NPs was 2.8 nm in the GNC
obtained after annealing for 12 h. The annealing was
found to crystallize a nanometer thick layer of Pb3Ge2O7,
Bi4Ge3O12, and Bi2GeO5 at the composite surface due to
the defects created by ion implantation. It should be
mentioned that defects created during ion implantation
can act as nucleation centers promoting crystallization
under heat treatment [299]. The luminescent properties of
this GNC were also investigated in this study [287].
Beyond Ag and Au NPs, many other metal NPs have
been incorporated into glass matrices by ion implantation
[288,300]. For instance, the implantation of Cu+ ions
within silica glass has been extensively studied
[289‒290]. Ghosh et al. have incorporated Cu NPs in
silica glass by Cu+ ion implantation [289]. Cu+ ions of
200 keV at a dose of 3 × 1016 ions·cm−2 were implanted
in the glass. The as-implanted glass was subsequently
annealed in Ar atmosphere at 400 °C for 1 h, thus
generating Cu NPs within the glass matrix. In the same
study, Au NPs were also obtained within a silica glass by
1.5 MeV Au+ ion implantation with a dose of 1 × 1017
ions·cm−2 and subsequent annealing in Ar atmosphere at
400 °C for 1 h. The size of the NPs prepared in both
composites (Cu NPs@silica glass and Au NPs@silica
glass) was found to range between 6 and 10 nm. At the
wavelength of 532 nm, the χ(3) was 1.456 × 10−8 and
3.66 × 10−12 esu at 532.8 nm for Cu NPs@silica glass and
Au NPs@silica glass, respectively [289]. Similarly, Wang
et al. have prepared Cu NPs in silica glass by implantation
of Cu+ ions [290]. The glass matrix was bombarded with
180 keV Cu+ ions with a fluence of 1×1017 ions·cm−2 and
a current density of 1.5 µA·cm−2. Thus, Cu NPs with a
size ranging from 1 to 15 nm and an average size of
5.6 nm were formed within the glass matrix. The χ(3) of
this GNC was −2.1 × 10−7 and −1.2 × 10−7 esu at 532 and
1064 nm, respectively [290].
3.3.3
Multiple implantation
The preparation of GNCs with different metals or metal
alloys through multiple implantation experiments has
attracted much attention. In an early study, Gonella et al.
have prepared AuCu alloy NPs embedded in silica glass
by dual ion implantation and heat treatment [301]. Au+
ions of 190 keV (3 × 1016 ions·cm−2) and subsequently
Cu+ ions of 90 keV (3 × 1016 ions·cm−2) were implanted
in silica glass. Current densities were kept below 2
µA·cm−2. Thus, AuCu alloy NPs were formed in the asimplanted GNC. Larger AuCu alloy NPs were found
when the as-implanted GNC was annealed in an H2
atmosphere at 900 °C for 1 h. The as-implanted GNC was
also annealed in an air atmosphere at 900 °C for 1 h. In
doing so, Cu migrated to the surface of the glass matrix
where it was oxidized to form CuO and Cu2O, while Au+
ions diffused deeper into the matrix [301]. Therefore, the
GNC was modified by selective annealing in a reducing or
oxidizing atmosphere. In another study, AuAg alloy NPs
and nanoplanets (NPLs) have been embedded in silica
glass by dual ion implantation [302]. It should be
mentioned that NPLs consist of a central NP surrounded
by small satellite NPs [303]. 190 keV Au+ ions were
implanted into the glass matrix with a fluence 3 × 1016
ions·cm−2. The current density was 2 µA·cm−2. Then,
130 keV Ag+ ions were implanted into the matrix with a
fluence of 3 × 1016 ions·cm−2. The current density was
kept at 2 µA·cm−2. The as-implanted sample was
annealed at 800 °C for 1 h, forming AuAg alloy NPs with
an average size of 12 nm. This GNC (AuAg alloy
NPs@silica glass) was irradiated with Ar at an energy of
190 keV, a current density of 0.2 µA·cm−2 and a fluence
of 2.5 × 1016 ions·cm−2. In doing so, NPs became NPLs.
Au-rich satellite NPs were formed with a mean size of
2 nm at 2‒3 nm from the original NPs, thus developing a
NPL structure [302].
Multiple implantations can also be employed to
synthesize monometallic NPs without any further
treatment. These monometallic NPs can be quite
distinctive in shape, size, and spatial distributions due to
the influence of the previous or subsequent implantation
Javier FONSECA. NPs embedded into glass matrices: GNCs
of the other ions. For example, Xiao et al. have
incorporated Ag NPs into a silica glass by Ag+ ion
implantation and Cu+ ion post-implantation [291]. The
implantation fluences of Ag+ and Cu+ ions were 5 ×
1016 ions·cm−2. The flux density of Ag was 1 µA·cm−2
with an energy of 200 keV. The flux density of Cu was
1.5 µA·cm−2 with an energy of 110 keV. Spherical Ag
NPs with a size ranging from 35 to 48 nm were found to
be aligned at the same depth in the silica glass. Cu NPs
were not detected. However, they may have existed but
with a much smaller size than the Ag NPs. The Ag NPs
formed were bombarded by Cu+ ions that induced thermal
diffusion, which caused Ag+ to exceed the solubility limit
and form large Ag NP. The thermal diffusion was also
suggested to increase the diffusion capacity of Cu ions.
Therefore, dual ions implantation offered some control of
the size and position of the NP in layered structures. In
contrast, a wide Ag NP size distribution was found when
silica glass was only bombarded with Ag ions (200 keV at
fluences of 5 × 1016 and 1.0 × 1017 ions·cm−2, and a flux
of 1 µA·cm−2). Large NPs were found near the surface
and small NPs were found deep within the glass matrix
[291]. This distribution of NPs may be explained by the
following guidelines:
(1) Small NPs near the surface of the glass matrix are
sputtered by the following implanted ions. It should be
remembered that the sputtered effect is proportional to
fluence.
(2) Defects in the glass matrix occur during
implantation. These defects are close to the surface of the
matrix. Implanted ions are easily trapped in these defects
to form numerous metal nuclei. Therefore, larger NPs are
formed near the surface of the matrix.
(3) Metal ions penetrate deep into the matrix due to
sputtering.
Silica glass was implanted with Ni2+ ions of 60 keV at
fluences of 1 × 1016, 5 × 1016, and 1 × 1017 ions·cm−2 and
then with Ag+ ions of 70 keV at the fluence of 5.0 × 1016
ions·cm−2 [292]. The implantation was carried out at a
beam current density of 4 µA·cm−2. Subsequently, the
composites were annealed at 600 °C for 1 h in Ar
atmosphere. The pre-implanted Ni2+ ions produced
nucleation sites that affected post-implanted Ag+ ions, Ag
nucleation, and NP growth. Specifically, the preimplantation of Ni improved the migration and nucleation
process of the Ag implants. Furthermore, this preimplantation decreased the sputtering rate of Ag postimplantation. At a low fluence of 1.0 × 1016 Ni2+
39
ions·cm−2, a small number of Ni2+ ions were implanted
and therefore the quantity of defects produced to provide
nucleation sites was low. After the annealing at 600 °C,
the Ni nuclei formed were tiny and highly dispersed.
Furthermore, Ag NPs with a wide size distribution
appeared on the surface of the glass matrix. The
concentration of Ag increased when applying a preimplantation of Ni2+ ions at a fluence of 5.0 ×
1016 ions·cm−2. This GNC exhibited the highest Ag
concentration. During Ag+ ion implantation, Ni NPs near
the surface of the composite were removed by sputtering.
In this GNC, while the size of Ni NPs ranged between 0.5
and 4 nm, the size of the Ag NPs ranged between 0.5 and
2 nm. It was also suggested that AgNi alloy NPs were
formed. At the high fluence of 1.0 × 1017 Ni2+ ions·cm−2,
many Ni0 atoms were implanted near the glass surface,
which trapped Ag0 atoms during the Ag postimplantation. The Ag0 atoms were trapped near the
surface, favoring heavy sputtering effects [292]. It is
worth noting that the sputtered thickness is often
proportional to the ion fluence [304]. To synthesize this
composite without annealing, Yamada et al. have
performed a dual implantation with 380 keV Ag+ ions and
then with 200 keV Ni2+ ions on silica glass [305]. The ion
fluences were 3 × 1016 and 6 × 1016 ions·cm−2 for Ag+
and Ni2+ ions, respectively. Ag NPs were found within the
glass after Ag+ ion implantation. Then Ni2+ ions were
implanted at almost the same depth. Ni2+ ion implantation
was found to affect the SPR peak of Ag NPs. The authors
suggested three possible reasons for the change in Ag NPs
after Ni2+ ion implantation: (1) synthesis of composite
NPs that include not only Ag0 atoms but also Ni0 atoms;
(2) change in Ag NPs morphology; and/or (3) change in
n2 of the glass matrix due to implantation-induced lattice
defects.
Liu’s research group has prepared different GNCs by
dual implantation of Zn2+ and Ag+ ions in different
sequences (Fig. 21) [293]. Spherical and ellipsoidal Ag
NPs were embedded in silica glass by Ag+ ion
implantation (Ag ions of 35 keV; 5 × 1016 Ag ions·cm−2;
4 µA·cm−2). The layer containing NPs was about 40 nm
thick. The size of the Ag NPs ranged between 3 and
17 nm. Large NPs were found near the surface, while
small NPs were found in a deeper region. When the silica
glass was bombarded with Zn2+ ions and then Ag+ ions
(Zn2+ ions of 50 keV; 5 × 1016 Zn2+ ions·cm−2; Ag+ ions
of 35 keV; 5 × 1016 Ag+ ions·cm−2; 4 µA·cm−2), spherical
NPs with a size ranging from 4 to 8 nm were formed in a
40
Front. Mater. Sci. 2022, 16(3): 220607
Fig. 21 (a) Cross-sectional TEM image, (b) SAED result, and (c) Ag NP size distribution of the GNC prepared by Ag+ ion
implantation. (d) Cross-sectional TEM image, (e) SAED result, and (f) Ag NP size distribution of the GNC prepared by Zn2+ ion
implantation and Ag+ ion post-implantation. (g) Cross-sectional TEM image, (h) SAED result, and (i) Ag NP size distribution of the
GNC prepared by Ag+ ion implantation and Zn2+ ion post-implantation. Reproduced with permission from Ref. [293] (Copyright 2013
AIP Publishing).
35 nm thick region. Most NPs were reported to be
metallic Ag. Therefore, this dual implantation provided a
smaller NP size and narrower size distribution. The Zn2+
ion implantation formed Zn-related compounds in the
matrix and also generated defects. These products and
defects favored the subsequent nucleation of Ag and
promoted the diffusion of Ag0 atoms [306‒307]. An
inverse implantation sequence (first Ag+ ion implantation
and then Zn2+ ion implantation) generated NPs from 2 to
12 nm. The layer containing NPs was about 53 nm thick.
These NPs were suggested to be Ag NPs, Zn NPs, and
AgZn alloy NPs [293]. This group has also explored the
effects of Zn2+ ion post-implantation on Cu NPs
embedded in silica glass [308‒309]. Cu NPs were
incorporated into silica glass by 45 keV Cu+ ion
implantation at a fluence of 1.0 × 1017 ions·cm−2 and a
flux density of 4 µA·cm−2. Spherical Cu NPs were formed
and distributed in a depth range of 4 to 55 nm. The Cu
NPs exhibited a size distribution of 2 to 17 nm with a
mean size of (6.9 ± 3.1) nm. This GNC was subsequently
bombarded with 50 keV Zn2+ ions at fluences of 1 × 1016,
5 × 1016, and 1 × 1017 ions·cm−2. The flux density was
kept at 4 µA·cm−2. At the low fluence of 1.0 × 1016
ions·cm−2, the loss of Cu by sputtering was negligible.
The NPs were still mainly Cu NPs. At the fluence of
1.0 × 1017 ions·cm−2, most of the pre-implanted Cu atoms
were sputtered. It resulted in the formation of Zn NPs. At
the intermediate fluence of 5.0 × 1016 ions·cm−2, the
sputtering effect induced a significant loss of Cu. Large
Cu NPs were found near the surface of the glass matrix
and CuZn alloy NPs were found in the depth range of
about 7‒28 nm. In the deepest region, there were small Zn
NPs and/or Zn-rich alloy NPs. The GNC prepared by
applying a post-implantation with intermediate fluence
(5.0 × 1016 ions·cm−2) was annealed in a N2 atmosphere.
Zn NPs grew by Ostwald ripening during low temperature
annealing [310]. Zn-rich CuZn alloy NPs were formed by
annealing at temperatures of 300 to 350 °C (Fig. 22(b)).
There was an increase in the Zn content in the CuZn alloy
NPs due to a thermally driven diffusion of Zn0 atoms
toward these NPs. This process was referred to as
realloying of the NPs. Core‒shell (Cu-rich core with a Znrich shell) NPs were formed at annealing temperatures in
the range of 400 to 450 °C (Fig. 22(c)). This thermal
treatment decomposed the CuZn alloy NPs. Moreover, the
Zn0 atoms diffused towards the surface of the CuZn alloy
NPs according to the Kirkendall effect [311‒313]. This
process was called dealloying of the NPs. The
decomposition of the CuZn alloy NPs due to Zn diffusion
was completed at 500 °C. Thus, the Cu0 atoms formed a
core of the new NPs, while the shell of these NPs was
made of Zn or Zn-related compounds (Fig. 22(d))
[308,314‒315]. In a similar study, Au core‒Ag shell
bimetallic NPs have been prepared into a silica glass by
dual Ag+ and Au+ ions implantation and subsequent
annealing [316]. Silica glass was bombarded with
1.8 MeV Ag+ ions and 3.7 MeV Au+ ions at the same
fluence of 5 × 1016 ions·cm−2. There was a mixture of
Javier FONSECA. NPs embedded into glass matrices: GNCs
41
Fig. 22 Schematic illustration of the annealing processes in a GNC formed by Cu+ ion implantation and Zn2+ ion post-implantation
(fluence of 5 × 1016 ions·cm−2): (a) CuZn alloy NP; (b) Zn diffusion-driven realloying process; (c) Zn diffusion-driven formation of
the Cu-rich core and Zn-rich shell structure; (d) Cu core and Zn shell NP. Reproduced with permission from Ref. [308] (Copyright
2013 American Chemical Society).
small Ag NPs and Au atoms in the glass matrix prior to
any annealing. The as-prepared GNC were annealed at
900 °C for 1 h in air. In doing so, monometallic Au NPs
and Au‒Ag core‒shell NPs with homogeneous distribution appeared in the GNC. The Au NPs acted as
nucleation centers. Thus, Ag atoms and NPs formed a
continuous shell around the Au NPs through an Ostwald
ripening mechanism [316]. In brief, multiple implantations can be used to fabricate core‒shell bimetallic NPs.
The sequential ion implantation adds a further degree of
freedom to the engineering of new materials.
In addition, nonmetals can also be implanted into
glasses. Torres‒Torres et al. have incorporated Si QDs
and Au NPs into silica glass by ion implantation [317].
The glass was exposed to Si2+ ions of 1.5 MeV at the
fluence of 2.5 × 1017 ions·cm−2. Subsequently, the asimplanted glass was annealed in a reducing atmosphere at
1100 °C for 1.5 h. This heat treatment promoted the
nucleation and growth of Si QDs within the glass matrix.
Au2+ ions of 1.5 MeV at the fluence of 8.5 × 1016
ions·cm−2 were implanted in this GNC (Si QDs@silica
glass). A new annealing was carried out in an oxidizing
atmosphere at 1000 °C for 1 h to precipitate the Au NPs.
Si2+ and Au2+ ions were found to reach 2 and 0.66 µm
depth in the glass matrix, respectively. In other words,
both ion distributions did not overlap. Therefore, it was
shown that it is possible to control the position of the ion
distributions by adjusting the energy and the dose of the
ions to be implanted [317].
3.4 Ion-exchange
3.4.1 Introduction
The ion-exchange process has been widely used to dope
glasses with metal ions and, after promoting the
precipitation of these ions, prepare GNCs [157,318‒319].
Ion-exchange is a simple and reproducible technique that
allows incorporating up to 1021 atoms·cm−3 in a glass
matrix, co-doping the glass matrix with different metal
species, and space-selectively precipitating NPs within the
glass matrix. Additionally, the ion-exchange process is
inexpensive in terms of materials, equipment, and
manufacturing process. This technology depends on the
ion-exchange parameters such as temperature, glass
composition and molten salt solution, and on the post ionexchange treatments such as thermal annealing and laser
irradiation techniques. Consequently, these parameters
and treatments have been and still are intensively studied
to achieve control of the preparation of GNCs through
ion-exchange technology. Therefore, ion-exchange is a
promising technique for NP formation within a glass
matrix. Unfortunately, this process is limited to the
incorporation of some metal species and alkali-containing
glasses [163].
Ion-exchange is typically carried out by replacing
monovalent cations, such as alkali cations (Li+, Na+, K+,
etc.), present in a glass with other monovalent cations
from a molten salt solution (Fig. 23). The overall chemical
reaction of the ion-exchange process is [320]:
A+B ↔ A+B
(29)
where A is the monovalent cation initially contained in the
molten salt solution and B is the monovalent cation
initially located in the glass matrix. When ion-exchange
occurs, A and B become A and B, respectively.
Therefore, A and B are the monovalent cations situated in
the glass matrix and in the molten salt solution,
respectively. In other words, the bar denotes the cations in
the glass phase. The equilibrium constant (K) can be
42
Front. Mater. Sci. 2022, 16(3): 220607
Fig. 23 (a) Schematic illustration of thermally assisted ion-exchange. (b) The monovalent cation A is introduced into the glass
matrix, which contains the monovalent cation B, by an interphase chemical potential gradient. To maintain charge neutrality, the
monovalent cation B is released into the molten salt solution. (c) Schematic illustration of electric field-assisted ion-exchange.
expressed as follows:
K=
aA · aB
aB · aA
(30)
where aA , aA , aB, and aB are the thermodynamic
activities of cations A and B in the corresponding phases.
A high K value indicates a large uptake of A by the glass
matrix [321].
Ion-exchange can be limited by (1) the mass transport
of cations in the molten salt solution; (2) the kinetics of
adsorption and desorption at the interface; and (3) the
mass transport of cations in the glass matrix (Fig. 23)
[321].
(1) The mass transport of cations in the molten salt
solution. The mass transfer of A from the molten salt
solution to the glass-solution interface occurs by
convection and diffusion. Although convection can be
forced, there is a stagnant boundary near the glasssolution interface where convective mixing does not
occur. Here, the mass transfer of A to the glass-solution
interface processes through diffusion. Conversely, the
mass transfer of B from the glass-solution interface to the
molten salt solution occurs first by diffusion through the
stagnant boundary and then by convection through the
molten salt solution. The parameter α informs about the
ease with which diffusion happens:


1/2
 N A   DA 




α=

NA DA
where NA and N A are the mole fraction of A in the molten
salt solution and in the glass matrix, respectively. DA and
DA are the diffusion coefficient of A in the molten salt
solution and in the glass matrix, respectively. When α ≥
10, the ion-exchange is not limited by mass transfer due to
diffusion in the molten salt solution [322].
(2) The kinetics of adsorption and desorption at the
interface. Surface kinetics are not likely to be a limiting
factor in the ion-exchange process. This action is much
faster than mass transport processes in molten salt
solution and glass matrix.
(3) The mass transport of cations in the glass matrix. In
the glass matrix, the mass transport of cations is carried
out entirely by diffusion. Therefore, this is a relatively
slow action. The cation diffusion depends on the surface
boundary condition (Eq. (29)) and the diffusion properties
in the glass.
According to the Regular Solution Theory [323], the
relationship between NA and N A can be expressed as
follows [320]:


)
 N A 
NA
H
 − ln K

(1 − 2NA ) = n ln 
ln
+
1 − NA
RT
1 − NA
(
where n is given by
n=
(31)
(32)
∂ (ln aA )
lnC A
(33)
H is the interaction energy of the two cations in the
Javier FONSECA. NPs embedded into glass matrices: GNCs
43
molten salt solution, R is the gas constant, T is the
temperature, and C A is the concentration of A in the glass
matrix. It should be mentioned that n = 1 when the glass
has an ideal behavior. As mentioned above, a high K
value indicates a large uptake of A by the glass matrix.
Once the metal ions have been incorporated into the
glass matrix through the ion-exchange process, they can
be converted into metal NPs by the process itself or by
applying thermal annealing or laser irradiation techniques.
Table 5 shows examples of GNCs prepared by ionexchange techniques [79,324‒336].
3.4.2 Direct formation of NPs
Ag NPs have been embedded in a borosilicate glass
(SiO2‒B2O3‒Na2O‒Al2O3‒NaF) applying only Ag+‒Na+
ion-exchange [324]. The glass was ion-exchanged in a
molten bath of AgNO3:NaNO3 with different molar ratios
(1:10, 1:50, 1:200, and 1:1000) at 310 °C for 1 h. In the
ion-exchange process, smaller Na+ ions were replaced by
larger Ag+ ions, leading to the breaking of Si−O bonds
and the generation of structural defects and NBO anions
[337]. The state of Ag within the glass matrix was ionic
Ag+ when the molar ratio of the molten mixture of
AgNO3:NaNO3 was 1:1000 (Fig. 24(a)). However, when
the AgNO3:NaNO3 molar ratio increased, Ag NPs
appeared within the glass (Fig. 24). The size of the Ag
NPs increased as the AgNO3:NaNO3 molar ratio
increased. The size of Ag NPs within the glass after ionexchange with an AgNO3:NaNO3 molar ratio of 1:10,
1:50, and 1:200 was (6.2 ± 2.5), (4.2 ± 2.2), and
(2.0 ± 1.0) nm, respectively. Similarly, Ag NPs were
embedded within a Sm3+-doped borosilicate glass
(SiO2‒B2O3‒Na2O‒Al2O3‒NaF‒Sm2O3) by Ag+‒Na+ ionexchange. It is worth mentioning that the Ag NPs
precipitated within the Sm3+-doped borosilicate glass
when the AgNO3:NaNO3 molar ratio was 1:10. Upon
excitation at 260 nm, the PL of Sm3+ was enhanced by the
energy transfer from Ag+ ions to Sm3+ ions. However,
upon excitation at 337 nm, the PL of Sm3+ was enhanced
by the energy transfer from Ag NPs to Sm3+ ions [324].
Quaranta et al. have explored in detail the rearrangement
of a sodium borosilicate glass after Ag+‒Na+ ionexchange [338]. In their study, the glass was immersed in
a molten mixture of AgNO3:NaNO3 with different molar
fractions (0.1:99.9, 0.5:99.5, 1:99, 2:98, 20:80, and
40:60 mol.%) at 330 °C for 1 h. There was a significant
decrease in Q3 content and an increase in Q2 content
Fig. 24 TEM images of GNCs prepared by ion-exchange in
a molten bath with the AgNO3:NaNO3 molar ratio of
(a) 1:1000, (b) 1:200, (c) 1:50, and (d) 1:10. The insets are the
Ag NP size distribution histograms. Reproduced with
permission from Ref. [324] (Copyright 2019 John Wiley and
Sons).
relative to the pristine glass when the Ag content
increased. It is worth mentioning that the Q4 correspond
to the fully polymerized structural species (SiO2); the Q3
species are those in which the number of NBO per silicon
(NBO/Si) is 1 (Si2O5); and the Q2 species are those in
which NBO/Si is 2 (SiO3) [339‒340]. Therefore, a
progressive depolymerization of the silica network was
found as the Ag content increased. The depolymerization
was favored by the self-coordination tendency of Ag+ ions
and by the release of the compressive stress energy with
the breaking of Si−O bonds [341‒342]. Furthermore, a
progressive aggregation of Ag NPs was also found as the
Ag content increased. The size of the Ag NPs embedded
within the glass increased slightly as the molar ratio of the
molten mixture for ion-exchange increased. The mean
size of the Ag NPs within those glasses that were ionexchanged
with
0.1:99.9,
20:80,
and
40:60
AgNO3:NaNO3 mol.% was 1.2, 1.5, and 1.5 nm,
respectively. The reduction of Ag+ ions during the ionexchange was related to the depolymerization of the glass
network. This correlation has been evidenced by Araujo
et al. [343]. Furthermore, the presence of SiO2 in
phosphate glasses has also been shown to promote the
reduction of Ag+ ions due to the increase in the basicity of
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@borosilicate glass
Ag NPs@borosilicate glass
Ag NPs
Borosilicate glass (52.70 wt.% SiO2,
11.11 wt.% B2O3, 24.71 wt.% Na2O,
8.13 wt.% Al2O3, and 3.35 wt.% NaF)
Borosilicate glass (52.70 wt.% SiO2,
11.11 wt.% B2O3, 24.71 wt.% Na2O,
8.13 wt.% Al2O3, and 3.35 wt.% NaF)
Borosilicate glass (52.70 wt.% SiO2,
11.11 wt.% B2O3, 24.71 wt.% Na2O,
8.13 wt.% Al2O3, and 3.35 wt.% NaF)
Silicate glass (40.45 wt.% SiO2,
15.26 wt.% Al2O3, 13.91 wt.% Na2O,
6.03 wt.% MgO, and 24.35 wt.% ZnO)
Silicate glass (40.45 wt.% SiO2,
15.26 wt.% Al2O3, 13.91 wt.% Na2O,
6.03 wt.% MgO, and 24.35 wt.% ZnO)
Silicate glass (40.45 wt.% SiO2,
15.26 wt.% Al2O3, 13.91 wt.% Na2O,
6.03 wt.% MgO, and 24.35 wt.% ZnO)
Silicate glass (40.45 wt.% SiO2,
15.26 wt.% Al2O3, 13.91 wt.% Na2O,
6.03 wt.% MgO, and 24.35 wt.% ZnO)
Silicate glass (40.45 wt.% SiO2,
15.26 wt.% Al2O3, 13.91 wt.% Na2O,
6.03 wt.% MgO, and 24.35 wt.% ZnO)
Silicate glass (40.45 wt.% SiO2,
15.26 wt.% Al2O3, 13.91 wt.% Na2O,
6.03 wt.% MgO, and 24.35 wt.% ZnO)
Silicate glass (40.45 wt.% SiO2,
15.26 wt.% Al2O3, 13.91 wt.% Na2O,
6.03 wt.% MgO, and 24.35 wt.% ZnO)
Silicate glass (40.45 wt.% SiO2,
15.26 wt.% Al2O3, 13.91 wt.% Na2O,
6.03 wt.% MgO, and 24.35 wt.% ZnO)
Silicate glass (40.45 wt.% SiO2,
15.26 wt.% Al2O3, 13.91 wt.% Na2O,
6.03 wt.% MgO, and 24.35 wt.% ZnO)
Silicate glass (39.41 wt.% SiO2, 14.86 wt.%
Al2O3, 13.55 wt.% Na2O, 5.88 wt.% MgO,
23.73 wt.% ZnO, and 2.57 wt.% Eu2O3)
Silicate glass (39.41 wt.% SiO2, 14.86 wt.%
Al2O3, 13.55 wt.% Na2O, 5.88 wt.% MgO,
23.73 wt.% ZnO, and 2.57 wt.% Eu2O3)
Silicate glass (39.41 wt.% SiO2, 14.86 wt.%
Al2O3, 13.55 wt.% Na2O, 5.88 wt.% MgO,
23.73 wt.% ZnO, and 2.57 wt.% Eu2O3)
Silicate glass (39.41 wt.% SiO2, 14.86 wt.%
Al2O3, 13.55 wt.% Na2O, 5.88 wt.% MgO,
23.73 wt.% ZnO, and 2.57 wt.% Eu2O3)
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
(3.6 ± 1.4)
(3.4 ± 1.5)
(3.3 ± 1.2)
(3.0 ± 1.6)
(2.4 ± 1.2)
(1.8 ± 1.2)
(2.5 ± 1.2)
(2.0 ± 1.0)
(1.6 ± 1.0)
(2.3 ± 1.8)
Ag NPs
Ag NPs
(2.2 ± 1.1)
(2.0 ± 1.1)
(1.5 ± 1)
(6.2 ± 2.5)
(4.2 ± 2.2)
(2.0 ± 1.00)
Size of NPs (minmax (mean))/nm
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Reinforcement
Host matrix
GNCs prepared by ion-exchange techniques [79,324–336]
Ag NPs@borosilicate glass
Composite
Table 5
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(0.99:99.01 wt.%), 310 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(3.84:96.16 wt.%), 310 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(16.66:83.34 wt.%), 310 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(1.98:98.02 wt.%), 350 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 350 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(33.32:66.68 wt.%), 350 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(66.65:33.35 wt.%), 350 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 380 °C for 15 min
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 380 °C for 1.5 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 380 °C for 4 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 300 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 400 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(1.98:98.02 wt.%), 380 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 380 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(33.32:66.68 wt.%), 380 °C for 1 h
Direct formation of NPs: ion-exchange
— AgNO3:NaNO3 melt
(66.65:33.35 wt.%), 380 °C for 1 h
Ion-exchange technique
Photoluminescence
‒
‒
Photoluminescence
Application
[325]
[325]
[325]
[324]
Ref.
44
Front. Mater. Sci. 2022, 16(3): 220607
Silicate glass (71.58 wt.% SiO2, 14.36 wt.%
Na2O, 6.59 wt.% CaO, 2.67 wt.% MgO,
3.21 wt.% Al2O3, 0.94 wt.% K2O, 0.4 wt.% SO3,
and 0.26 wt.% Fe2O3)
Silicate glass (71.58 wt.% SiO2, 14.36 wt.%
Na2O, 6.59 wt.% CaO, 2.67 wt.% MgO,
3.21 wt.% Al2O3, 0.94 wt.% K2O, 0.4 wt.% SO3,
and 0.26 wt.% Fe2O3)
Silicate glass (71.58 wt.% SiO2, 14.36 wt.%
Na2O, 6.59 wt.% CaO, 2.67 wt.% MgO,
3.21 wt.% Al2O3, 0.94 wt.% K2O, 0.4 wt.% SO3,
and 0.26 wt.% Fe2O3)
Silicate glass (71.58 wt.% SiO2, 14.36 wt.%
Na2O, 6.59 wt.% CaO, 2.67 wt.% MgO,
3.21 wt.% Al2O3, 0.94 wt.% K2O, 0.4 wt.% SO3,
and 0.26 wt.% Fe2O3)
Silicate glass (71.58 wt.% SiO2, 14.36 wt.%
Na2O, 6.59 wt.% CaO, 2.67 wt.% MgO,
3.21 wt.% Al2O3, 0.94 wt.% K2O, 0.4 wt.% SO3,
and 0.26 wt.% Fe2O3)
Silicate glass (71.58 wt.% SiO2, 14.36 wt.%
Na2O, 6.59 wt.% CaO, 2.67 wt.% MgO,
3.21 wt.% Al2O3, 0.94 wt.% K2O, 0.4 wt.% SO3,
and 0.26 wt.% Fe2O3)
Soda-lime silicate glass (71.63 wt.% SiO2,
2.08 wt.% Al2O3, 0.5 wt.% Fe2O3, 0.19 wt.%
TiO2, 0.59 wt.% SO3, 6.27 wt.% CaO, 2.8 wt.%
MgO, 15.4 wt.% Na2O, and 0.53 wt.% K2O)
Soda-lime silicate glass (71.63 wt.% SiO2,
2.08 wt.% Al2O3, 0.5 wt.% Fe2O3, 0.19 wt.%
TiO2, 0.59 wt.% SO3, 6.27 wt.% CaO, 2.8 wt.%
MgO, 15.4 wt.% Na2O, and 0.53 wt.% K2O)
Soda-lime silicate glass (71.63 wt.% SiO2,
2.08 wt.% Al2O3, 0.5 wt.% Fe2O3, 0.19 wt.%
TiO2, 0.59 wt.% SO3, 6.27 wt.% CaO, 2.8 wt.%
MgO, 15.4 wt.% Na2O, and 0.53 wt.% K2O)
Soda-lime silicate glass (73.65 wt.% SiO2,
14.64 wt.% Na2O, 3.2 wt.% Al2O3, 7.5 wt.%
CaO, and 1.0 wt.% MgO)
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@soda-lime silicate
glass
Ag NPs@soda-lime silicate
glass
Ag NPs@soda-lime silicate
glass
Ag NPs@soda-lime silicate
glass
Corning 0211 glass
Host matrix
Ag NPs@silicate glass
Ag NPs@Corning 0211 glass
Composite
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Reinforcement
(2)
(2.2 ± 0.6)
(1.9 ± 1.2)
(1.8 ± 1.1)
(7.2)
(4.4)
(2.9)
(4.0 ± 1.5)
(3.3 ± 1.2)
(2.8 ± 1)
(10)
Size of NPs (minmax (mean))/nm
Application
Direct formation of NPs: ion-exchange NLO devices: β(800 nm) =
— AgNO3:NaNO3:KNO3 melt
1.4 × 10−14 cm2·W−1
(8.77:41.66:49.57 wt.%), 300 °C for 6 h;
Al film evaporation; immersion in a
KNO3 salt at 400 °C for 2 h
Thermal annealing in air: ion-exchange
Photoluminescence
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 370 °C for few min;
post ion-exchange annealing — air
atmosphere (450 °C for 1 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 370 °C for few min;
post ion-exchange annealing — air
atmosphere (500 °C for 1 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 370 °C for few min;
post ion-exchange annealing — air
atmosphere (550 °C for 1 h)
Thermal annealing in air: ion-exchange
Photoluminescence
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 370 °C for 2 min;
post ion-exchange annealing — air
atmosphere (500 °C for 1 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 370 °C for 2 min;
post ion-exchange annealing — air
atmosphere (550 °C for 1 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(13.08:86.92 wt.%), 370 °C for 2 min;
post ion-exchange annealing — air
atmosphere (600 °C for 1 h)
Thermal annealing in air: ion-exchange
‒
— AgNO3:NaNO3 melt
(33.33:66.66 wt.%), 360 °C for 1 h; post
ion-exchange annealing — air
atmosphere (450 °C for 30 min)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(33.33:66.66 wt.%), 360 °C for 1 h; post
ion-exchange annealing — air
atmosphere (500 °C for 30 min)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(33.33:66.66 wt.%), 360 °C for 1 h; post
ion-exchange annealing — air
atmosphere (550 °C for 30 min)
Thermal annealing in air: ion-exchange
‒
— AgNO3:NaNO3 melt
(3.92:96.08 wt.%), 350 °C for 10 min;
post ion-exchange annealing — air
atmosphere (600 °C for 45 h)
Ion-exchange technique
[330]
[329]
[328]
[327]
[326]
Ref.
(continued)
Javier FONSECA. NPs embedded into glass matrices: GNCs
45
Borosilicate glass (72.0 wt.% SiO2, 14.0 wt.%
Na2O, 0.6 wt.% K2O, 7.1 wt.% CaO, 4.0 wt.%
MgO, 1.9 wt.% Al2O3, 0.1 wt.% Fe2O3, and
0.3 wt.% SO3)
Borosilicate glass (72.0 wt.% SiO2, 14.0 wt.%
Na2O, 0.6 wt.% K2O, 7.1 wt.% CaO, 4.0 wt.%
MgO, 1.9 wt.% Al2O3, 0.1 wt.% Fe2O3, and
0.3 wt.% SO3)
Borosilicate glass (72.0 wt.% SiO2, 14.0 wt.%
Na2O, 0.6 wt.% K2O, 7.1 wt.% CaO, 4.0 wt.%
MgO, 1.9 wt.% Al2O3, 0.1 wt.% Fe2O3, and
0.3 wt.% SO3)
Borosilicate glass (72.0 wt.% SiO2, 14.0 wt.%
Na2O, 0.6 wt.% K2O, 7.1 wt.% CaO, 4.0 wt.%
MgO, 1.9 wt.% Al2O3, 0.1 wt.% Fe2O3, and
0.3 wt.% SO3)
Cu NPs@silicate glass
Cu NPs@silicate glass
Cu NPs@silicate glass
Cu NPs@silicate glass
Ag NPs
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs
Ag NPs
Soda-lime silicate glass (69.36 wt.% SiO2,
15.63 wt.% Na2O, 3.04 wt.% Al2O3, 6.05 wt.%
CaO, 3.41 wt.% MgO, 1.72 wt.% K2O, 0.53 wt.%
SO3, and 0.27 wt.% TiO2)
Ag NPs@soda-lime silicate
glass
Ag NPs
Soda-lime silicate glass (69.36 wt.% SiO2,
15.63 wt.% Na2O, 3.04 wt.% Al2O3, 6.05 wt.%
CaO, 3.41 wt.% MgO, 1.72 wt.% K2O, 0.53 wt.%
SO3, and 0.27 wt.% TiO2)
Cu NPs
Cu NPs
Cu NPs
Cu NPs
Cu NPs
Ag NPs
Ag NPs@soda-lime silicate
glass
Cu NPs@silicate glass
Ag NPs@silicate glass
Ag NPs
Ag NPs
Silicate glass (5.73 wt.% Na2O, 7.52 wt.% ZnO,
9.42 wt.% Al2O3, 5.55 wt.% SiO2, 36.42 wt.%
Yb2O3, and 35.36 wt.% Er2O3)
Silicate glass (5.75 wt.% Na2O, 7.55 wt.% ZnO,
9.46 wt.% Al2O3, 5.58 wt.% SiO2, 36.58 wt.%
Yb2O3, and 35.07 wt.% Ho2O3)
Silicate glass (5.71 wt.% Na2O, 7.50 wt.% ZnO,
9.39 wt.% Al2O3, 5.54 wt.% SiO2, 36.31 wt.%
Yb2O3, and 35.55 wt.% Tm2O3)
Borosilicate glass (72.0 wt.% SiO2, 14.0 wt.%
Na2O, 0.6 wt.% K2O, 7.1 wt.% CaO, 4.0 wt.%
MgO, 1.9 wt.% Al2O3, 0.1 wt.% Fe2O3, and
0.3 wt.% SO3)
Ag NPs@silicate glass
Ag NPs@silicate glass
Reinforcement
Host matrix
Composite
(2.0)
(1.6)
4‒6
2‒4
(10)
(9.5)
(8)
(7)
(7)
10‒40
10‒60
3‒10
Size of NPs (minmax (mean))/nm
Thermal annealing in air: ion-exchange
— CuSO4·5H2O:Na2SO4 melt
(67.36:32.64 wt.%), 590 °C for 2 min;
post ion-exchange annealing — air
atmosphere (450 °C for 1 h)
Thermal annealing in air: ion-exchange
— CuSO4·5H2O:Na2SO4 melt
(67.36:32.64 wt.%), 590 °C for 2 min;
post ion-exchange annealing — air
atmosphere (500 °C for 1 h)
Thermal annealing in air: ion-exchange
— CuSO4·5H2O:Na2SO4 melt
(67.36:32.64 wt.%), 590 °C for 2 min;
post ion-exchange annealing — air
atmosphere (550 °C for 1 h)
Thermal annealing in air: ion-exchange
— CuSO4·5H2O:Na2SO4 melt
(67.36:32.64 wt.%), 590 °C for 2 min;
post ion-exchange annealing — air
atmosphere (600 °C for 1 h)
Thermal annealing in air: ion-exchange
— CuSO4·5H2O:Na2SO4 melt
(67.36:32.64 wt.%), 590 °C for 2 min;
post ion-exchange annealing — air
atmosphere (650 °C for 1 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt (0.2:99.8 wt.%),
320 °C for 30 min; post ion-exchange
annealing — H2 atmosphere
(180 °C for 12 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt (0.2:99.8 wt.%),
320 °C for 30 min; post ion-exchange
annealing — H2 atmosphere
(250 °C for 5 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(1.98:98.02 wt.%), 320 °C for 20 min;
post ion-exchange annealing — air
atmosphere (450 °C for 1 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(1.98:98.02 wt.%), 320 °C for 20 min;
post ion-exchange annealing — air
atmosphere (500 °C for 1 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3:KNO3 melt
(14:45:41 wt.%), 280 °C for 50 min
Ion-exchange technique
‒
NLO devices
NLO devices
Photoluminescence
Application
[333]
[332]
[331]
[79]
Ref.
(continued)
46
Front. Mater. Sci. 2022, 16(3): 220607
Ag NPs
Ag NPs
Ag NPs
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Silicate glass (69.59 wt.% SiO2, 15.17 wt.%
Na2O, 5.08 wt.% MgO, 6.52 wt.% CaO,
1.73 wt.% Al2O3, 1.14 wt.% K2O,
and 0.78 wt.% SO3)
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs@silicate glass
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Reinforcement
Host matrix
Composite
(1.9)
(2.2)
(2.6)
(2.4)
(2.6)
(2.7)
(2.7)
(2.6)
Size of NPs (minmax (mean))/nm
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(1.98:98.02 wt.%), 320 °C for 20 min;
post ion-exchange annealing — air
atmosphere (550 °C for 1 h)
Irradiation: ion-exchange —
AgNO3:NaNO3 melt (1.98:98.02 wt.%),
320 °C for 20 min; post ion-exchange
annealing — air atmosphere (550 °C for
1 h); post ion-exchange irradiation (λ =
266 nm, number of pulses = 50, laser
fluence = 18 J·cm−2)
Irradiation: ion-exchange —
AgNO3:NaNO3 melt (1.98:98.02 wt.%),
320 °C for 20 min; post ion-exchange
annealing — air atmosphere (550 °C for
1 h); post ion-exchange irradiation (λ =
266 nm, number of pulses = 200, laser
fluence = 72 J·cm−2)
Irradiation: ion-exchange —
AgNO3:NaNO3 melt (1.98:98.02 wt.%),
320 °C for 20 min; post ion-exchange
annealing — air atmosphere (550 °C for
1 h); post ion-exchange irradiation (λ =
355 nm, number of pulses = 40, laser
fluence = 14.4 J·cm−2)
Irradiation: ion-exchange —
AgNO3:NaNO3 melt (1.98:98.02 wt.%),
320 °C for 20 min; post ion-exchange
annealing — air atmosphere (550 °C for
1 h); post ion-exchange irradiation (λ =
355 nm, number of pulses = 200, laser
fluence = 72 J·cm−2)
Irradiation: ion-exchange —
AgNO3:NaNO3 melt (1.98:98.02 wt.%),
320 °C for 20 min; post ion-exchange
annealing — air atmosphere (550 °C for
1 h); post ion-exchange irradiation (λ =
532 nm, number of pulses = 10, laser
fluence = 3.6 J·cm−2)
Irradiation: ion-exchange —
AgNO3:NaNO3 melt (1.98:98.02 wt.%),
320 °C for 20 min; post ion-exchange
annealing — air atmosphere (550 °C for
1 h); post ion-exchange irradiation (λ =
532 nm, number of pulses = 50, laser
fluence = 18 J·cm−2)
Irradiation: ion-exchange —
AgNO3:NaNO3 melt (1.98:98.02 wt.%),
320 °C for 20 min; post ion-exchange
annealing — air atmosphere (550 °C for
1 h); post ion-exchange irradiation (λ =
532 nm, number of pulses = 200, laser
fluence = 72 J·cm−2)
Ion-exchange technique
‒
‒
Application
[333]
[333]
Ref.
(continued)
Javier FONSECA. NPs embedded into glass matrices: GNCs
47
Ag NPs
Soda-lime silicate glass (74.2 wt.% SiO2,
14.3 wt.% Na2O, 1.9 wt.% Al2O3, 8.1 wt.% CaO,
and 1.5 wt.% MgO)
Silicate glass (71.92 wt.% SiO2, 13.31 wt.%
Na2O, 4.15 wt.% MgO, 8.70 wt.% CaO, and
1.92 wt.% Al2O3)
Ag NPs@soda-lime silicate
glass
Ag NPs@soda-lime silicate
glass
Soda-lime silicate glass
Ag NPs
Soda-lime silicate glass (74.2 wt.% SiO2,
14.3 wt.% Na2O, 1.9 wt.% Al2O3, 8.1 wt.% CaO,
and 1.5 wt.% MgO)
Ag NPs@soda-lime silicate
glass
Ag NPs@silicate glass
Ag NPs
Soda-lime silicate glass (74.2 wt.% SiO2,
14.3 wt.% Na2O, 1.9 wt.% Al2O3, 8.1 wt.% CaO,
and 1.5 wt.% MgO)
Ag NPs@soda-lime silicate
glass
Ag NPs
Ag NPs
Reinforcement
Host matrix
Composite
(4.2)
(4.93 ± 1.88)
(2)
(1)
3‒8 (7)
Size of NPs (minmax (mean))/nm
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(3.92:96.08 wt.%), 320 °C for 10 min;
post ion-exchange annealing—air
atmosphere (600 °C for 45 h)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(3.92:96.08 wt.%), 320 °C for 10 min;
post ion-exchange annealing — UV-laser
irradiation, λ = 193 nm, pulse energy
density (30 mJ∙cm−2), repetition
frequency (10 Hz), pulse duration
(20 ns), irradiation time (5 s)
Thermal annealing in air: ion-exchange
— AgNO3:NaNO3 melt
(3.92:96.08 wt.%), 320 °C for 10 min;
post ion-exchange annealing — UV-laser
irradiation, λ =193 nm, pulse energy
density (30 mJ∙cm-2), repetition
frequency (10 Hz), pulse duration
(20 ns), irradiation time (30 min)
Irradiation: ion-exchange
— AgNO3:NaNO3 melt (20:80 wt.%),
350 °C for 30 s; post ion-exchange
irradiation — Ar+ ions of 200 keV, ion
fluence (5 × 1015 ion·cm−2), flux
(2 µA·cm−2)
Irradiation: ion-exchange
— AgNO3:NaNO3 melt (2:98 wt.%),
400 °C for 2h; post ion-exchange
irradiation — 100 kV electrons, flux
(6.4 A·cm−2)
Ion-exchange technique
‒
Optoelectronics
‒
‒
Application
[336]
[335]
[334]
[334]
Ref.
(continued)
48
Front. Mater. Sci. 2022, 16(3): 220607
Javier FONSECA. NPs embedded into glass matrices: GNCs
the glass and the electron donor affinity of the O2− ions
[344].
Ag NPs have also been incorporated in a silicate glass
(SiO2‒Al2O3‒Na2O‒MgO‒ZnO) through the ionexchange process [325]. The Ag+ ions replaced the
smaller the Na+ ions in the glass host during the Ag+‒Na+
ion-exchange. Before the formation of NPs, a portion of
Ag+ ions was reduced to Ag0 atoms as follows:
2(≡SiO− ) + Ag+ ↔ ≡Si − O − O − Si≡ +Ag0
(34)
The subsequent formation of Ag NPs was suggested to
proceed via Ostwald ripening [345‒347]. The influence of
ion-exchange parameters on the size of the Ag NPs was
investigated (Fig. 25):
(1) The silicate glass was subjected to Ag+‒Na+ ionexchange using a molten mixture of AgNO3 and NaNO3
with different molar ratios at 350 °C for 1 h. The size and
quantity of Ag NPs were found to increase as the
AgNO3:NaNO3 ratio increased. When the AgNO3:NaNO3
ratio was 1:99, 7:93, 20:80, and 50:50 mol.%, the mean
size of the Ag NPs was 1.5, 2.0, 2.2, and 2.3 nm,
respectively.
(2) The silicate glass was subjected to Ag+‒Na+ ionexchange at 380 °C for 15, 90, and 240 min. The
AgNO3:NaNO3 ratio was fixed at 7:93 mol.%. The
average size of the Ag NPs was approximately 1.6, 2.0,
and 2.5 nm when the ion-exchange time was 15, 90, and
240 min, respectively. Therefore, the size of the Ag NPs
increased as the ion-exchange time increased.
(3) The silicate glass was subjected to Ag+‒Na+ ionexchange at different temperatures (300, 350, and 400 °C)
for 1 h. The AgNO3:NaNO3 ratio was also fixed at
7:93 mol.%. The mean size of the Ag NPs was 1.8, 2.3,
and 2.4 nm when the ion-exchange temperature was 300,
49
350, and 400 °C, respectively. Therefore, increasing the
ion-exchange temperature resulted in the formation of
larger Ag NPs.
(4) The glass was doped with Eu3+ ions. The Eu3+doped glass was subjected to Ag+‒Na+ ion-exchange
using a variable AgNO3:NaNO3 ratio at 380 °C for 1 h.
The average size of the Ag NPs was 3.0, 3.3, 3.4, and
3.6 nm when the AgNO3:NaNO3 ratio was 1:99, 7:93,
20:80, and 50:50 mol.%, respectively. Therefore, under
the same preparation conditions, the Ag NPs within the
Eu3+-doped glass were larger than those embedded in the
glass without Eu3+ ions. The formation of larger Ag NPs
was attributed to the self-reduction of Eu3+ ions to Eu2+
ions which proceeded as follows [195,348]:
Eu3+ + e− → Eu2+
(35)
Eu2+ + Ag+ → Eu3+ + Ag0
(36)
zAg0 → Agz
(37)
It is worth mentioning that the Ag NPs enhanced the
luminescence of the embedded Eu3+ ions. The energy
efficient transfer from Ag NPs to Eu3+ ions was suggested
to be the reason for the luminescence enhancement of
Eu3+ [325].
Chen et al. and Karvonen et al. have embedded Ag NPs
into a commercial glass (Corning 0211) by an
unconventional two-step ion-exchange method [326,349].
Corning 0211 glass was placed in a molten mixture of
AgNO3:NaNO3:KNO3 (5:47.5:47.5 mol.%) at 300 °C for
6 h. In doing so, ion-exchange occurred. Then, an Al film
was evaporated on the ion-exchanged glass using an
electron-beam evaporator. The Al film provided electrons
Fig. 25 Schematic illustration of the formation and growth of Ag NPs. Reproduced with permission from Ref. [325] (Copyright 2018
Elsevier).
50
Front. Mater. Sci. 2022, 16(3): 220607
to reduce Ag+ ions to Ag0 atoms. The GNC was immersed
in a KNO3 salt at 400 °C for 2 h to promote the formation
of Ag NPs. The KNO3 salt favored the flow of Ag+ ions
during the electrolytic deposition. The average size of the
embedded Ag NPs was found to be about 10 nm.
Moreover, the size distribution of Ag NPs was small.
At the wavelength of 800 nm, this GNC exhibited n2
and β of 1.4 × 10−10 cm2·W−1 and −1.9 × 10−5 cm·W−1,
respectively [326].
3.4.3
Thermal annealing in air
After ion-exchange process, the most common and simple
strategy for precipitating metal NPs within a glass matrix
is thermal annealing performed in air at a temperature
close to the glass transition temperature. Thermal
annealing produces free electrons from NBOs in the glass
matrix. These electrons are captured by metal cations to
form metal atoms. The formation and growth of metal
NPs generally increases as the annealing time and
temperature increase (as long as the temperature remains
below the glass transition temperature) [350].
Mathpal research’s group has explored the
incorporation Ag NPs into a soda-lime silicate glass
(SiO2‒Na2O‒CaO‒MgO‒Al2O3‒K2O‒SO3‒Fe2O3)
by
Ag+‒Na+ ion-exchange followed of thermal annealing in
an air atmosphere [327‒328]. The glass was immersed in
a molten salt bath of AgNO3:NaNO3 (7:93 mol.%) at
370 °C for 2 min. Under these conditions, the Ag+ ions
diffused into the glass matrix and replaced the Na+ ions in
the glass. Then, the ion-exchanged composite was
annealed in an oxidizing atmosphere at 450, 500, and
550 °C for 1 h. The ion-exchange generated stress in the
system due to the difference in size of the Ag+ and Na+
ions. This stress was relaxed by the diffusion of Ag+ ions
near the surface of the soda-lime glass during the thermal
annealing. Thus, the total energy of the system was
minimized. Annealing also resulted in the reduction of
Ag+ ions to Ag0 atoms and the subsequent formation of
spherical Ag NPs. Consequently, most of the Ag NPs
were located near the surface of the soda-lime glass. The
average size of the embedded Ag NPs was (2.8 ± 1),
(3.3 ± 1.2), and (4.0 ± 1.5) nm for the GNCs annealed at
450, 500, and 550 °C, respectively [327]. The soda-lime
silicate glass was also immersed in a mixture of
AgNO3:NaNO3 (20:80 mol.%) at 370 °C for 2 min [328].
This ion-exchanged composite was annealed at 500, 550,
and 600 °C for 1 h. The size of the Ag NPs was found to
be 2.9, 4.4, and 7.2 nm when the annealing temperature
was 500, 550, and 600 °C, respectively. The Ag NPs also
accumulated near the surface of the soda-lime glass
during thermal annealing. The largest accumulation of Ag
NPs near the glass surface was found in the GNC
annealed at 550 °C. The near surface accumulation in the
GNC annealed at 600 °C was between that of the GNC
annealed at 500 °C and that of the GNC annealed at
550 °C. Therefore, when the annealing temperature was
600 °C, the Ag0 atoms diffused deep into the glass matrix
before aggregation to form Ag NPs [328]. Regardless of
the AgNO3:NaNO3 molar ratio of the molten bath, the size
of the NPs always increased as the annealing temperature
increased [327‒328]. The PL intensity of these GNCs was
reported to decrease as the temperature and time of the
post ion-exchange annealing treatment increased.
Therefore, the growth of Ag NPs resulted in the
quenching of PL intensity [327‒328].
The formation mechanism of Ag NPs within the sodalime silicate glass (SiO2‒Al2O3‒Fe2O3‒TiO2‒SO3‒CaO‒
MgO‒Na2O‒K2O) was explored by Simo et al. [329]. Ag
NPs were embedded within this glass by combining ionexchange and subsequent annealing in air (Fig. 26). The
ion-exchange was carried out by immersing the glass in a
mixture of AgNO3:NaNO3 (33.33:66.66 mol.%) at 360 °C
for 1 h. Na+ ions from the surface and the subsurface
region of the glass were replaced by Ag+ ions. Ag clusters
with a size less than 1 nm in diameter (approximately
0.6 nm) were generated during ion-exchange. The
formation of NPs was initiated by heat treatment. This
post ion-exchange treatment was carried out at different
temperatures and times:
(1) The ion-exchanged glass was annealed in air at
different temperatures (between 100 and 600 °C) for
30 min. Ag clusters (d < 1 nm) persisted at annealing
temperatures below 410 °C. Ag NPs were formed above
410 °C. It was suggested that Ag+ ions were reduced by
electrons extracted directly from NBOs as follows
[351‒352]:
2(≡Si−O− Ag+ ) ↔ ≡Si−O−Si≡ +O− + (Ag2 )+
O− + (Ag2 )+ → 2Ag0 + 0.5O2
(38)
(39)
The size of the Ag NPs was found to increase slightly as
the annealing temperature increased. The average size of
the Ag NPs formed was (1.8 ± 1.1), (1.9 ± 1.2), and
(2.2 ± 0.6) nm at 450, 500, and 600 °C, respectively.
Javier FONSECA. NPs embedded into glass matrices: GNCs
51
Fig. 26 Schematic illustration of the preparation of Ag NPs within a soda-lime silicate glass by ion-exchange and subsequent
annealing. Reproduced with permission from Ref. [329] (Copyright 2012 American Chemical Society).
(2) Annealing was performed at 400 °C for times
ranging from 1 min to 13 h. Ag NPs were not formed
under these conditions. Only the number of Ag cluster
increased as the annealing time increased. The composite
annealed for 13 h at 400 °C contained exclusively Ag
clusters. Moreover, the Ag NPs were also not formed
when this annealed composite was subsequently treated at
500 °C. In other words, the Ag clusters were stable at high
temperatures. Therefore, the Ostwald ripening mechanism
did not happen.
(3) The ion-exchanged glass was annealed at 550 °C for
times ranging from 1 min to 16 h. The Ag clusters became
Ag NP. Although the size of the Ag NPs increased, their
number remained constant. Therefore, in contrast to the
widely accepted Ostwald ripening processes of particle
growth in this GNCs, the particle growth was produced by
the addition of Ag monomers due to the decomposition of
)
(
≡ Si − O− Ag+ .
In summary, the size of the Ag NPs depended on the
concentration of the Ag cluster, which served as nuclei for
the formation of Ag NPs, and on the duration of
annealing. However, the size was limited by the number
of reducing agents (NBOs). The ion-insertion process and
the growth of NPs were separated in this process which is
essential to control the size of the NPs [329].
In a very similar study, Sheng et al. have formed Ag
NPs within a soda-lime silicate glass (SiO2‒Na2O‒Al2O3‒
CaO‒MgO) by Ag+‒Na+ ion-exchange and thermal
annealing in air [330]. Firstly, the glass was dipped in a
molten mixture of AgNO3:NaNO3 (2:98) at different
temperatures (320 or 350 °C) for 10 min. This ionexchange did not generate Ag NPs but Ag clusters
(< 1 nm). The ion-exchanged glasses were then annealed
in air at 570 or 600 °C for up to 100 h. The formation of
Ag NPs occurred after heating at 570 or 600 °C for more
than 25 h. The mean size of the Ag NPs was
approximately 2 nm after annealing at 600 °C for 45 h.
The Ag NPs grew more as the temperature and time of
annealing increased. After ion-exchange, Ag+ ions were
mainly attached to NBO atoms. During annealing, the
Ag−O bonds were broken. The resulting Ag+ ions
diffused to the glass surface to minimize the surface
tensile stress introduced by the size difference between
Ag+ and Na+ during the ion-exchange process. Finally,
Ag+ ions were reduced to Ag0 atoms by capturing
electrons from the glass structure and defects during
annealing [330].
Ag NPs have also been incorporated in a photo-thermorefractive (PTR) glass (SiO2‒Na2O‒ZnO‒Al2O3‒F‒Sb2O3)
through ion-exchange and subsequent heat treatment in air
[88,353]. It should be mentioned that PTR glasses are
multicomponent glasses whose n2 can vary with UV
irradiation and heat treatment due to the growth of NaF
NCs in the UV irradiated areas [354]. These glasses are
widely used in photonics [355]. The glass was immersed
in a melt bath of AgNO3:NaNO3 (5:95 mol.%) at 320 °C
for different durations (from 5 min to 21 h). The ionexchanged glass was treated at different temperatures and
durations. The heat treatment of the glass at 450 °С
induced: (i) diffusion of Ag+ ions towards the glass
surface, (ii) reduction of Ag+ ions to Ag0 atoms, and (iii)
aggregation of isolated Ag0 atoms in NPs. The formation
of Ag clusters and NPs was reported to be adjusted by
controlling the temperature and time of the post ionexchange treatment. The Sb3+ ions, which were contained
in the PTR glass, were found to act as electron donors for
the Ag+ ions. It was also observed that an increase in Sb3+
ions in the PTR glass increased the rate of reduction of
Ag+ ions to Ag0 atoms, which is consistent with the law of
mass action:
52
Front. Mater. Sci. 2022, 16(3): 220607
2Ag+ + Sb3+ ↔ 2Ag0 + Sb5+
(40)
Therefore, the formation of Ag NPs was highly dependent
on the concentration of Sb3+ ions in the initial PTR glass.
In fact, Ag+ ions remained in ionic form within a PTR
glass without Sb3+ ions regardless of the post ionexchange treatment temperature [88,353].
Varak et al. have incorporated Ag NPs into RE-doped
silicate glasses by ion-exchange and subsequent heat
treatment in air [79]. First, three silicate glasses
(Na2O‒ZnO‒Al2O3‒SiO2‒Yb2O3‒RE2O3, where RE = Er,
Ho, Tm) were prepared by melt-quenching technique.
Then, Ag+ ions were incorporated into the glasses by ionexchange. The AgNO3:NaNO3:KNO3 melt (14:45:41 wt.%)
was used for ion-exchange at 280 °C for 50 min. Finally,
the ion-exchanged glasses were annealed in air at 590 °C
for 1 h. Before annealing, the Ag+ ions remained in a
surface layer. However, when annealing was applied, the
Ag+ ions were reduced and formed Ag dimers and NPs
that migrated deeper into the glasses. These Ag species
were found in a layer 8 to 15 µm deep. The largest
amount of Ag NPs was present in the Ho-doped glass.
The size of these Ag NPs ranged between 10 and 60 nm.
The size of Ag NPs embedded within Er and Tm doped
glasses was found to vary from 3 to 10 nm and 10 to
40 nm, respectively. It is worth mentioning that Ag
species enhanced the PL of the RE ions embedded within
the glass matrices by various mechanisms. Upon
excitation in the 200 to 450 nm range, Ag dimers
increased the PL of RE-doped glasses by the energytransfer mechanism. After excitation at 975 nm, the PL of
the RE-doped glasses increased due to the SPR effect of
Ag NPs and due to structural changes of the silicate
matrices, which were generated by Na+ ↔ Ag+
substitution during ion-exchange [79].
Ag NPs have been embedded in a Eu3+-doped
sodium‒aluminosilicate glass (SiO2‒Al2O3‒Na2O‒MgO‒
ZnO‒Eu2O3) by Ag+‒Na+ ion-exchange and subsequent
heat treatment [195]. The glass was immersed in a molten
mixture of AgNO3:NaNO3 (5:95 mol.%) at 360 °C for
2 h. Then, the ion-exchanged glass was annealed at
450 °C for 10 h. Eu3+ ions spontaneously reduced to Eu2+
in the glass matrix [356‒357]. As the standard reduction
potential (E0) of Ag+/Ag0 is higher (0.7996 eV) than that
of Eu3+/Eu2+ (−0.35 eV), it was assumed that reaction
(Eq. (41)) might have occurred in the Ag+ ion-exchange
process:
Eu2+ + Ag+ → Eu3+ + Ag0
(41)
Subsequently, the following reactions might have taken
place:
Ag0 + Ag+ → Ag+2
(42)
zAg0 → Agz
(43)
This can explain that the concentration of Ag NPs and
ionic Ag species was found to increase as the
concentration of Eu3+ ions increased [195].
Beyond Ag NPs, other metal NPs have also been
incorporated into glass matrices by ion-exchange and
post-treatment. For instance, Cu NPs have been
precipitated in a soda-lime silicate glass (SiO2‒Na2O‒
K2O‒CaO‒MgO‒Al2O3‒Fe2O3‒SO3) [331]. The glass
was immersed in a mixture of CuSO4·5H2O:Na2SO4
(54:46 mol.%) at 590 °C for 2 min to perform Cu+‒Na+
ion-exchange. The embedded Cu+ ions were subsequently
reduced to Cu0 atoms by heat treatment in air at 450, 500,
550, 600, and 650 °C for 1 h. These Cu0 atoms aggregated
near the surface of the glass to form evenly distributed Cu
NPs. The average size of the Cu NPs within the GNC
treated at 450, 500, 550, 600, and 650 °C was (7 ± 1),
(7 ± 2), (8 ± 3), (9.5 ± 1), and (10 ± 2) nm, respectively
(Fig. 27). In other words, the size of the Cu NPs increased
as the temperature for heat treatment increased. Therefore,
the nucleation and growth of Cu NPs were controlled by
post ion-exchange heat treatment. These GNC, regardless
of the heat treatment temperature, exhibited good NLO
behavior with possible applications in the field of laser
technology and all-optical switching devices [331].
Different metal NPs have been embedded together in a
glass matrix. Cu NPs and Ag NPs have been incorporated
in a same soda-lime glass (SiO2‒Na2O‒CaO‒MgO‒
Al2O3‒SO3‒Fe2O3) by sequential Cu+‒Na+ and Ag+‒Na+
ion-exchange and subsequent heat treatment in air [358].
The glass was dipped in a molten salt bath of
CuSO4·5H2O:Na2SO4 for about 1 min and then in a
molten salt bath of AgNO3:NaNO3 also for also about
1 min. Annealing was performed in air at different
temperatures ranging from 250 to 650 °C for 1 h. Cu NPs
and Ag NPs were formed within the glass matrix. At high
annealing temperatures, Cu NPs were found to dissolve
during Ag clusterization. When the glass matrix was first
Ag+‒Na+ ion-exchanged, then Cu+‒Na+ ion-exchanged,
and finally annealed, only the Ag NPs precipitated within
the glass matrix [358].
Javier FONSECA. NPs embedded into glass matrices: GNCs
53
Fig. 27 TEM images and corresponding Ag NP size distributions of the GNC treated (after ion-exchange) at (a)(b) 450 °C, (c)(d) 500 °C,
(e)(f) 550 °C, (g)(h) 600 °C, and (i)(j) 650 °C. Reproduced with permission from Ref. [331] (Copyright 2019 Elsevier).
3.4.4 Thermal annealing in H2 atmosphere
Another post ion-exchange treatment of a glass is thermal
annealing in a controlled H2 atmosphere. This post ionexchange treatment not only favors the formation of NPs
within the glass but also the formation of metal island
films (MIFs) on the glass surface. MIFs are generally
produced on the glass surface by out-diffusion of metal
atoms due to the hydrogen penetration into the glass
[332,359]. This technique requires safety precautions.
In an early example, Ag NPs have been embedded into
a soda-lime silicate glass (SiO2‒Na2O‒Al2O3‒CaO‒
MgO‒K2O‒SO3‒TiO2) by Ag+‒Na+ ion-exchange and
subsequent annealing in a H2 atmosphere [332]. The glass
was immersed in a molten mixture of AgNO3:NaNO3
(0.1:99.9 mol.%) at 320 °C for 30 min. About 20% of the
Na+ ions were replaced by Ag+ ions. The ion-exchanged
glasses were treated in H2 at different temperatures (120,
140, 160, 180, 200, and 250 °C) for 5 h. The ionexchanged glasses were also annealed in H2 at 180 °C for
different times (2, 4, 8, and 12 h). The annealing in H2
atmosphere was found to cause the diffusion of Ag+ ions
towards the glass surface and also the near-surface
precipitating of Ag NPs. The permeation of H2 in the
glass was accompanied by the Na+‒H+ ion-exchange. It is
worth mentioning that some Na+ ions remained in the
glass matrix after the Ag+‒Na+ ion-exchange. The
migration of Ag+ ions towards the glass surface was
attributed to a direct interaction with H2, but also to a
charge balance during Na+‒H+ ion-exchange. The size of
the Ag NPs within the glasses annealed in H2 at 180 °C
for 12 h and at 250 °C for 5 h ranged between 2 and 4 nm
and between 4 and 6 nm, respectively. The size of the Ag
NPs was reported to increase as the annealing time and
temperature increased. Furthermore, to obtain a direct
comparison of the effect of the annealing atmosphere, an
ion-exchanged glass was treated in air at 200 °C for 5 h.
Ag NPs were not found within this composite. Therefore,
H2 annealing was carried out at temperatures low enough
to avoid NP formation from thermal effects alone [332].
3.4.5
Laser irradiation techniques
Laser irradiation can also be used as post ion-exchange
54
Front. Mater. Sci. 2022, 16(3): 220607
treatment of glass matrices for the precipitation of metal
NPs within such glasses. Continuous [360‒363] and/or
pulsed lasers [364‒366] working at different wavelengths
of the spectrum can reduce metal ions to metal NPs. The
formation of NPs depends on the laser power at the
surface of the ion-exchanged glass matrix. When the laser
power ensures that the temperature of the irradiated area
remains close to the glass transition temperature
throughout the process, the interaction of the laser beam
with the sample is thermal in nature. In other words, the
reduction of metal cations follows a similar trend to that
of thermal annealing. However, this post treatment is
faster in terms of NP formation [350]. At higher laser
powers, the glass surface temperature is higher than the
glass transition temperature. Consequently, the irradiated
region of the ion-exchanged glass melts. In this liquid
state, NPs generally form in a deep region of the irradiated
area and migrate towards the edges of the laser beam
where they aggregate to form larger NPs (Fig. 28)
[360‒362].
Rahman et al. have studied the precipitation of Ag NPs
in Ag+‒Na+ ion-exchanged soda-lime silicate glass
(SiO2‒Na2O‒MgO‒CaO‒Al2O3‒K2O‒SO3) by different
post ion-exchange treatments [333]. The glass was
immersed in a molten salt bath of AgNO3:NaNO3
(1:99 mol.%) at 320 °C for 20 min to perform Ag+‒Na+
ion-exchange. Subsequently, the ion-exchanged sample
was annealed at different temperatures (450, 500, and
550 °C) for 1 h and irradiated at different wavelengths
(266, 355 and 532 nm) and different number of
consecutive laser pulses. Ag+ ions were reduced to Ag
NPs during annealing in air. The Ag NPs were
progressively larger as the annealing temperature
increased. A strong interaction between the light and the
Fig. 28 Formation and growth of metal NPs by high power
laser irradiation.
Ag NPs appeared when the 550 °C annealed composite
was irradiated at 532 nm. Consequently, ionized
multimeric structures, such as (Ag3)2+, were formed from
the Ag NPs. Thus, the size of Ag NPs decreased.
Similarly, the size of the Ag NPs within the 550 °C
annealed composite was reduced by irradiation with many
laser shots (200 pulses) at 355 nm. Ag+ ions were released
from the Ag NPs into the glass matrix. This resulted in the
depolymerization of the silica network. Under 266 nm
laser irradiation on the 550 °C annealed sample, the
temperature gradient between the Ag NPs and the
surrounding glass matrix remained low. There was no size
reduction of Ag NPs. Laser irradiation at this wavelength
was found to promote the formation of Ag NPs with very
narrow size distribution. In brief, the size and size
distribution of Ag NPs embedded in annealed glass
matrices can be adjusted by selecting laser irradiation
wavelength and power density (Fig. 29). Furthermore,
Rahman et al. also studied the formation and precipitation
of Ag NPs within soda-lime silicate glass solely by laser
irradiation. This is considered a multiphoton process
[367]. Valence band electrons are excited to the
conduction band by absorbing photons. Free electrons in
the conduction band react with Ag+ ions, reducing them to
Ag0 atoms. However, according to the results of Rahman
et al., irradiation with a 532 nm (2.32 eV) laser on
Ag+‒Na+ ion-exchanged soda-lime silicate glass did not
generate free charge carriers in SiO2 (bandgap ≈ 8.9 eV).
The reduction of Ag+ and the subsequent precipitation of
Ag NPs under laser irradiation with 532 nm on Ag+‒Na+
ion-exchanged soda-lime silicate glass were described as
follows:
≡Si − O − Ag+ + hv → ≡Si − O − h+ + (e− + Ag+ )
(44)
e− + Ag+ → Ag0
(45)
zAg0 → Agz
(46)
Electrons were driven out from the 2p orbital of NBO in
the glass and captured by nearby Ag+ ions to form Ag0
atoms, which then precipitated Agz. When Ag+‒Na+ ionexchanged soda-lime silicate glass was treated with
266 nm (4.66 eV) laser irradiation, the free electrons
could have been driven out by both mechanisms: the
multiphoton process and the 2p orbital of NBO.
Therefore, the 266 nm laser irradiation is a more efficient
irradiation than other wavelengths in reducing Ag+ ions
Javier FONSECA. NPs embedded into glass matrices: GNCs
55
Fig. 29 Schematic illustration of the evolution of Ag species as a function of the different post ion-exchange treatments. Reproduced
with permission from Ref. [333] (Copyright 2021 Elsevier).
and promoting Ag NP precipitation [333].
Zhang et al. have also explored different post-treatments
to precipitate Ag NPs in an ion-exchanged soda-lime
silicate glass (SiO2‒Na2O‒Al2O3‒CaO‒MgO) [334]. The
glass was immersed in a molten salt bath of
AgNO3:NaNO3 (2:98 mol.%) at 320 °C for 10 min. The
ion-exchanged glass was subject to (1) thermal annealing,
or (2) UV-laser irradiation, or (3) X-ray irradiation to
promote the formation of Ag NPs.
(1) Thermal annealing was carried out in air at
temperatures ranging between 500 and 600 °C for
different durations (2‒45 h). Through annealing, Ag+ ions
first diffused to the glass surface trying to minimize the
surface tensile stress introduced during the ion-exchange
process. Then, the Ag+ ions were reduced to Ag0 atoms by
capturing the electron from the glass structure or
impurities. Finally, the Ag0 atoms aggregated and became
Ag NPs. When the ion-exchanged glass was annealed at
600 °C for 45 h, the size of the Ag NPs was found to vary
between 3 and 8 nm with an average size of 7 nm.
(2) The ion-exchanged glass was irradiated with an UVlaser (λ = 193 nm) with a pulse energy density of
30 mJ·cm−2. The UV-laser irradiation time ranged from
5 s to 30 min. The repetition frequency and pulse duration
were 10 Hz and 20 ns, respectively. Electrons were driven
out from the NBOs after laser irradiation.
Laser
Glass −−−−→ h+ + e−
Ag+ ions captured those electrons to form Ag0 atoms.
(47)
56
Front. Mater. Sci. 2022, 16(3): 220607
Ag+ + e− → Ag0
(48)
The aggregation of Ag0 atoms was suggested to occur due
to the high temperature of the glass. It is worth noting that
UV-laser irradiation was responsible for the increase in
temperature of the glass. Therefore, this irradiation
promoted the formation of Ag NPs. After 5 s and after
30 min of irradiation, the average size of the Ag NPs was
1.0 and 2.0 nm, respectively. The size of the NPs
increased as the UV-laser irradiation time increased.
(3) The ion-exchanged glass was also treated with X-ray
irradiation (Rh Kα, λ = 0.061 nm, 50 kV, 50 mA) at room
temperature for 30 min. X-ray irradiation caused defects
and induced the reduction of Ag0 atoms. However, it did
not promote the formation of Ag NPs [334].
Ag NPs have also been precipitated within an Ag+‒Na+
ion-exchanged soda-lime silicate glass (SiO2‒Na2O‒
MgO‒CaO‒Al2O3) by irradiation with Ar at an incidence
angle of 40° [335,368]. The soda-lime silicate glass was
immersed in a molten mixture of AgNO3:NaNO3
(20:80 wt.%) at 350 °C for 30 s. In doing so, some Na+
ions within the glass were replaced by Ag+ ions contained
in the molten mixture. To form and grow the Ag NPs,
Ag+‒Na+ ion-exchanged soda-lime silicate glass was
bombarded by 200 keV Ar+ ions (ion fluences = 5 ×
1015 ions·cm−2; flux = 2 µA·cm−2) at an angle of
incidence of 40° (Fig. 30). The 200 keV Ar+ ionized the
Ag+‒Na+ ion-exchanged glass, thus providing electrons.
Ag+ was reduced to Ag0 atoms by capturing those
electrons. In addition, the energetic beam also created
NBO defect centers [369‒370]. Ag0 atoms diffused to
these NBO sites, where they accumulated and formed Ag
NPs [328,370]. The Ag NPs were well distributed within
the Ag+‒Na+ ion-exchanged glass matrix. The size of the
Ag NPs ranged between 2 and 10 nm. The average size
was found to be (4.93 ± 1.88) nm. Increasing Ar+ fluence
from 5 × 1015 to 1 × 1016, and 5 × 1016 ions·cm−2 was
found to convert more Ag+ ions to Ag0 atoms, which in
turn diffused and agglomerated to form more Ag NPs.
This increase in the concentration of Ag NPs decreased
the intensity of PL and increased the electrical
conductivity of the GNC [335].
Hofmeister et al. have embedded Ag NPs into the sodalime silicate glass by ion-exchange and subsequent
electron beam irradiation [336]. Ion-exchange was carried
out by immersing the glass in a molten mixture of
AgNO3:NaNO3 (2:98 wt.%) at 400 °C for 2 h.
Subsequently, the ion-exchanged glass was irradiated with
Fig. 30 Schematic illustration of the preparation of Ag NPs
within an Ag+‒Na+ ion-exchanged soda-lime silicate glass.
Reproduced with permission from Ref. [335] (Copyright
2021 Elsevier).
100 kV electrons with a beam current density of
6.4 A·cm−2. During irradiation, the temperature of the
glass was kept below 300 °C. The diffusivity of Ag+ ions
achieved by irradiation was larger than that achieved by
heat treatment. This, together with the bond breaking and
the creation of defects within the glass network, favored
the precipitation of high concentration of Ag NPs. The
size distribution of the NPs was narrow. Ag NPs with a
mean size of 4.2 nm were found to be homogeneously
distributed within the glass matrix [336]. Therefore, the
electron beam irradiation post-treatment may be superior
to the heat post-treatment in the concentration of NPs
produced, the homogeneous distribution of NPs within the
glass matrix, the narrow size distribution of NPs, the
speed of NPs formation and the reproducibility of the
process.
3.5
Less conventional techniques
Table 6 shows examples of GNCs prepared by staining
process, spark plasma sintering, radio frequency
sputtering, spray pyrolysis, and chemical vapor deposition
[371‒376].
3.5.1
Staining process
The staining process is an especial ion-exchange method
widely used in the fields of art and craft for coloring
glasses. This method consists of enameling a glass surface
with a mixture of a metal salt and a pigment. A
subsequent annealing favors the ion-exchange process
(metal cations — K+ or Na+). The metal cations are
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
Silica glass
GaAs NPs@silica glass
GaAs NPs@silica glass
GaAs NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Au NPs@silica glass
Cu NPs@silica glass
Au NPs@silica glass
Cu NPs
Au NPs
Au NPs
Au NPs
Au NPs
Au NPs
GaAs NPs
GaAs NPs
GaAs NPs
Au NPs
Cu NPs
Ag NPs
Aluminosilicate glass (75.63 wt.% SiO2,
8.83 wt.% Na2O, 7.92 wt.% CaO,
4.22 wt.% MgO, 1.81 wt.% SnO2,
0.74 wt.% Al2O3, 0.43 wt.% Fe2O3,
0.27 wt.% SO3, and 0.14 wt.% K2O)
Aluminosilicate glass (77.82 wt.% SiO2,
6.52 wt.% Na2O, 7.50 wt.% CaO,
4.24 wt.% MgO, 1.88 wt.% SnO2,
0.76 wt.% Al2O3, 0.03 wt.% Fe2O3,
0.25 wt.% SO3, and 1.00 wt.% K2O)
Silica glass
Ag NPs@
aluminosilicate glass
Cu NPs@
aluminosilicate glass
Reinforcement
14
47.08
17.5
8.09
3.64
2.43
7.7
4.4
2.7
3‒10
25‒40
2.5‒6
Size of NPs
(min‒max
(mean))/nm
‒
‒
‒
‒
‒
‒
‒
‒
‒
0.05
‒
‒
Loading of NPs
(wt.% in excess)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 4.7 mTorr Ar atmosphere
at room temperature, radio frequency power
(15 W), sputtering time (30 s)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 4.7 mTorr Ar atmosphere
at room temperature, radio frequency power
(15 W), sputtering time (60 s)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 4.7 mTorr Ar atmosphere
at room temperature, radio frequency power
(15 W), sputtering time (90 s)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 4.7 mTorr Ar atmosphere
at room temperature, radio frequency power
(10 W), sputtering time (30 s)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 4.7 mTorr Ar atmosphere
at room temperature, radio frequency power
(10 W), sputtering time (60 s)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 4.7 mTorr Ar atmosphere
at room temperature, radio frequency power
(10 W), sputtering time (120 s)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 4.7 mTorr Ar atmosphere
at room temperature, radio frequency power
(10 W), sputtering time (300 s)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 4.7 mTorr Ar atmosphere
at room temperature, radio frequency power
(10 W), sputtering time (600 s)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 0.003‒0.004 mTorr Ar
atmosphere at room temperature, radio
frequency power, silica target (250 W), radio
frequency power, Cu target (25 W), cosputtering time (105 min)
Spark plasma sintering: uniaxial pressure
(50 MPa), in vacuum at 1020 °C for 3 min
Staining process: pigment (red ochre);
annealing (T < 660 °C for 1 h and 40 min)
Staining process: pigment (red ochre);
annealing (T < 660 °C for 1 h and 40 min)
Ion-exchange technique
GNCs prepared by staining process, spark plasma sintering, radio frequency sputtering, spray pyrolysis, and CVD techniques [371–376]
Host matrix
Composite
Table 6
‒
‒
NLO devices
β(720 nm) =
5.72 × 10−12 m·W−1
‒
Art
Application
[374]
[373]
[373]
[372]
[371]
Ref.
Javier FONSECA. NPs embedded into glass matrices: GNCs
57
Ag NPs
Ag NPs
Ag NPs
Ag NPs
Mesoporous bioactive glassb)
Mesoporous bioactive glassa)
Mesoporous bioactive glassa)
Mesoporous bioactive glassa)
Silica glass
Ag NPs@mesoporous
bioactive glasses
Ag NPs@Mesoporous
bioactive glasses
Ag NPs@mesoporous
bioactive glasses
Ag NPs@mesoporous
bioactive glasses
Au NPs@silica glass
a) Silver acetate as precursor; b) silver nitrate as precursor.
Ag NPs
Mesoporous bioactive glassa)
Ag NPs@mesoporous
bioactive glasses
Ag NPs
Cu NPs
Silica glass
Cu NPs@silica glass
Reinforcement
Host matrix
Composite
(50)
(9 ± 3)
(8 ± 3)
(8 ± 2)
(7 ± 3)
(9 ± 4)
Size of NPs
(min‒max
(mean))/nm
12
‒
6.56 mol.%
3.28 mol.%
1.64 mol.%
‒
‒
‒
Loading of NPs
(wt.% in excess)
Radio frequency sputtering: 13.56 MHz radio
frequency sputtering, 0.003‒0.004 mTorr Ar
atmosphere at room temperature, radio
frequency power, silica target (250 W), radio
frequency power, Cu target (40 W), cosputtering time (90 min)
Spray pyrolysis: dispersion of precursors into
droplets, preheating at 400 °C, calcination at
700 °C, annealing 500 °C
Spray pyrolysis: dispersion of precursors into
droplets, preheating at 400 °C, calcination at
700 °C, annealing 500 °C
Spray pyrolysis: dispersion of precursors into
droplets, preheating at 400 °C, calcination at
700 °C, annealing 500 °C
Spray pyrolysis: dispersion of precursors into
droplets, preheating at 400 °C, calcination at
700 °C, annealing 500 °C
Spray pyrolysis: dispersion of precursors into
droplets, preheating at 400 °C, calcination at
700 °C, annealing 500 °C
Chemical vapor deposition: aerosol assisted
chemical vapor deposition, Au NPs in toluene,
aerosol generation (ultrasonic humidification),
N2 flow rate (1 L·min−1),
glass temperature 450 °C
Ion-exchange technique
‒
Antibacterial activity
Antibacterial activity
Application
[376]
[375]
[154]
Ref.
(continued)
58
Front. Mater. Sci. 2022, 16(3): 220607
Javier FONSECA. NPs embedded into glass matrices: GNCs
reduced to metal atoms and these grow and coalesce into
metal NPs within the glass matrix. The metal NPs
together with the pigment interact with light and,
therefore, color the glass [377].
The staining process has been applied to an
aluminosilicate glass (SiO2‒Al2O3‒Na2O‒CaO‒MgO) to
study the formation of NPs, the evolution of the glass
matrix and the final color of the prepared GNC [371]. The
glass was enameled with a mixture of red ochre and
Ag2SO4 (to obtain a yellow GNC) or a mixture of red
ochre and Cu2SO4 (to obtain a red GNC). Subsequently,
the glass was treated at a temperature up to 660 °C for 1 h
and 40 min. This annealing favored the Ag+ (or Cu+)‒Na+
ion-exchange, the neutralization of those Ag+ (or Cu+)
ions and, finally, the precipitation of Ag (or Cu) NPs
within the matrix. It was suggested that Sn2+ ions can
favor the precipitation of Ag (or Cu) NPs as follows
[378]:
Sn2+ → Sn4+ + 2e−
(49)
Ag+ + e− → Ag0
(50)
Cu+ + e− → Cu0
(51)
Cu2+ + 2e− → Cu0
(52)
The size of the embedded Ag NPs was reported to range
from 2.5 to 6 nm. The size of the Cu NPs ranged between
25 and 40 nm. These NPs were a core‒shell system where
the core was Ag (or Cu) NPs, and the shell was probably
an oxidized form of Ag (or Cu). The Ag NPs were
homogeneously distributed on the glass surface. However,
the Cu NPs were accumulated in Cu-rich regions. There
was a decrease in Q4 species and an increased in Q2 and
Q3 species due to the staining process. Therefore, this
process caused the depolymerization of the glass surface
as a classical ion-exchange process does. The
depolymerization originated by ion-exchange:
Na−O−Si≡ ↔ M−O−Si≡
3.5.2
Spark plasma sintering
Spark plasma sintering (SPS), also known as pulsed
electric current sintering (PECS) [379], field-assisted
sintering technique (FAST) [380‒381], plasma-activated
sintering (PAS) [382] and current-activated pressureassisted densification (CAPAD) [383‒384], has gained
importance in the last 20 years. SPS is a technique that
allows materials to be densified at low temperatures in a
short time [385‒386]. This technique is based on the
simultaneous application of pulsed direct current, heat,
and uniaxial pressure on a sample (as far as this review is
concerned, a mixture of GNC precursors) loaded into a
graphite die (Fig. 31) [387]. The process can be carried
out in different environments such as vacuum, argon,
hydrogen, air, etc. It is generally claimed that the high
densification rates come primarily from the use of highenergy pulsed direct current. Pulsed direct current is often
achieved by applying a voltage of about 30 V and a
current of 600‒1000 A. The duration of each pulse varies
between 1 and 300 ms [387]. The electrical discharges
across the sample can generate microcosmic spark
discharge and plasma. There is a debate about the
existence of plasma and spark discharge [388‒389]. It is
primarily believed that the spark discharges appear in the
gaps among the sintered particles under appropriate
conditions in SPS [390]. After the initial spark discharge
stage, spark plasma would be generated [391]. The high
local temperature, which can be generated momentarily in
the initial spark discharge stage, can modify the GNC
precursors. The total synthesis time depends on the
material to be prepared [387].
Zhang et al. have embedded Au NPs within a silica
glass matrix by applying SPS to SBA-15 with confined
(53)
where M is the metal ion. However, the glass surface was
also affected by a flow of oxygen at the interface.
≡Si−O−Si≡ + 2M+ + (1/2)O2 ↔ 2(M−O−Si≡)
(54)
Therefore, ion-exchange and oxygen flow at the interface
were responsible of the depolymerization [371].
59
Fig. 31 Schematic illustration of the features of a spark
plasma sintering apparatus.
60
Front. Mater. Sci. 2022, 16(3): 220607
Au NPs [372]. It is worth mentioning that SBA-15 is a
well-ordered hexagonal mesoporous silica with uniform
pore sizes [392]. First, Au NPs with a size of 3 to 10 nm
were encapsulated within the pores of SBA-15 by postsynthetic functionalization. This encapsulation did not
modify the ordered hexagonal mesostructure of SBA-15.
Then, the composite (SBA-15 with confined Au NPs) was
loaded into a graphite die and sintered using SPS under a
uniaxial pressure of 50 MPa in vacuum at 1020 °C for
3 min. Thus, SBA-15 with confined Au NPs became glass
silica with embedded Au NPs. The Au NPs were found to
be homogenously distributed within the glass matrix. The
size of these NPs also ranged from 3 to 10 nm. Therefore,
the size of the NPs was preserved. At the wavelength of
720 nm, n2 and β of this GNC were 1.74 × 10−18 m2·W−1
and 5.72 × 10−12 m·W−1, respectively [372]. This method
has also been applied to SBA-15 with confined Ag NPs or
Pt NPs to obtain Ag NPs or Pt NPs embedded within
silica glass [393].
3.5.3
Radio frequency sputtering
by sputtering; convection of the gas phase target to the
substrate; and the condensation, nucleation and growth of
the target on the substrate. Therefore, the process is a
solid‒gas‒solid reaction [394‒395].
Gallium arsenide (GaAs) NPs and Au NPs have been
embedded in silica glasses using radio frequency
sputtering (13.56 MHz) [373]. Radio frequency sputtering
was performed in a 4.7 mTorr Ar atmosphere at room
temperature. The radio frequency power supplied to the
GaAs and Au targets was 15 and 10 W, respectively. First,
the GaAs target was sputtered onto a silica glass. To form
GaAs NPs, the GaAs deposition time was shorter than the
time required in forming a continuous layer. Then, SiO2
was sputtered onto the GaAs NPs supported on silica
glass. Thus, the GaAs NPs were embedded within a silica
glass. The average size of the GaAs NPs was 2.7, 4.4, and
7.7 nm for sputtering times of 30, 60, and 90 s,
respectively (Fig. 33). Therefore, the size of the GaAs
NPs increased as the sputtering time increased. The GaAs
NPs that were close to each other coalesced as they grew.
Most of the GaAs NPs were found to be connected to each
other during a sputtering time of 240 s. The Au NPs were
Radio frequency sputtering is a high-energy vaporization
technique. It is based on generating an energetic wave
through an inert gas in a vacuum chamber (Fig. 32). Thus,
the inert gas ionizes. These high-energy ions bombard the
target material, sputtering off the target atoms to inject
them into a substrate. As far as this review is concerned,
the substrate is a glass matrix. A surface atom of the target
is only sputtered if it receives enough energy to exceed its
binding energy. High sputtering rates are obtained for
frequencies in the MHz region. In brief, the sputterdeposition process comprises vaporizing a target material
Fig. 32 Schematic illustration of the radio frequency
sputtering process.
Fig. 33 TEM images of GaAs NPs embedded in silica glass
prepared by radio frequency sputtering for (a) 30 s, (b) 60 s,
(c) 90 s, (d) 120 s, and (e) 240 s. Reproduced with permission
from Ref. [373] (Copyright 1997 AIP Publishing).
Javier FONSECA. NPs embedded into glass matrices: GNCs
similarly embedded in a silica glass. First, Au was
deposited on the silica glass. Subsequently, SiO2 was
sputtered to bury the Au NPs. The average diameter of Au
NPs for sputtering times of 30, 60, 120, 300, and 600 s
was 2.43, 3.64, 8.09, 17.5, and 47.08 nm, respectively
(Fig. 34). The growth kinetics of GaAs and Au NPs were
different. While the number of Au NPs was maintained as
the sputtering time increased, the number of GaAs
decreased under the same conditions. This was attributed
to a larger mobility of the GaAs precursors and NPs than
those of Au [373].
Cattaruzza et al. have embedded Co NPs in silica glass
by 13.56 MHz radio frequency sputtering [374]. The
depositions were performed in an atmosphere of Ar at a
pressure ranging from 30 × 10−4 to 40 × 10−4 mbar and at
room temperature. Silica and Co were co-sputtered on
silica glass for 120, 105, and 90 min. The radio frequency
power supplied to the silica target was 250 W. The radio
frequency power supplied to the Co target was 10, 25, and
40 W when co-sputtering was 120, 105, and 90 min,
respectively. After co-sputtering, silica was sputtered for
5 min. Both Co0 atoms and Co2+ ions were found within
the silica glasses. Finally, the samples were treated to
promote the precipitation of Co0 atoms to Co NPs within
the glass matrix. Annealing was carried out in a H2
atmosphere at 900 °C for 2 h, in air at 700 °C for 5 h, or in
air at 700 °C for 5 h followed by annealing in a H2
atmosphere at 900 °C for 2 h. Thermal annealing in a H2
atmosphere promoted the precipitation of Co0 atoms to
form Co NPs. Thermal treatment in air favored the
formation of Co2+ ions within the silica glass. Subsequent
reducing treatment did not promote Co NPs formation.
The average size of the Co NPs obtained after annealing
in H2 atmosphere the samples prepared with a radio
frequency power of 25 and 40 W was approximately 14
and 12 nm, respectively. These results were attributed to
larger nucleation sites in the GNC with the higher
concentration of Co. In conclusion, radio frequency cosputtering together with heat treatments allows achieving
different GNCs [374]. This research group has also
incorporated AuCu alloy NPs into silica glass using
13.56 MHz radio frequency co-sputtering [396‒397]. The
depositions were carried out in an Ar atmosphere at a
pressure of 35×10−2 Pa. Whereas the radio frequency
power supplied to the silica target was set at 250 W, the
powers supplied to the Au and Cu targets changed from 5
to 13 W and from 5 to 17 W, respectively. The radio
frequency power was established to obtain Au:Cu ratios
within the glass matrices close to 0, 1, 2, 3, and 5. Silica,
Au and Cu were co-deposited on a silica glass for 50 min.
After co-sputtering, silica was sputtered for 5 min.
Finally, the composites were annealed in a reducing
atmosphere at 900 °C for 2 h. The Cu atoms, which were
mostly in oxidized forms, were reduced during annealing.
After reduction of the Cu atoms, the already formed Aurich nanoclusters were converted into AuCu alloy NPs.
Therefore, annealing favored the formation of AuCu alloy
NPs of different composition, depending on the relative
amount of Au and Cu within the glass matrix. The average
size of the embedded alloy NPs, regardless of the alloy
composition, was found to range from 4 to 10 nm
(Fig. 35) [396‒397].
3.5.4
Fig. 34 TEM images of Au NPs embedded in silica glass
prepared by radio frequency sputtering for (a) 30 s, (b) 60 s,
(c) 120 s, (d) 300 s, and (e) 600 s. Reproduced with
permission from Ref. [373] (Copyright 1997 AIP Publishing).
61
Spray pyrolysis
Spray pyrolysis (SP) is a versatile and flexible technique
for producing materials with a wide range of composition,
size, and morphology. SP includes all synthesis processes
in which a solution is atomized and thermolyzed to obtain
a suitable phase. The main mechanisms of particle
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Front. Mater. Sci. 2022, 16(3): 220607
Fig. 35 Cross-sectional TEM images of the GNC made of
Au47Cu53 alloy embedded within a glass matrix: (a) lowmagnification bright-field image (the inset is the sizehistogram); (b) high-resolution image of an Au47Cu53 alloy
embedded within a glass matrix. Reproduced with permission
from Ref. [397] (Copyright 2007 Elsevier).
formation in SP are one-particle-per-drop and gas-toparticle conversion. In the later, the particles are created in
the gas phase reactions. The reaction products can
precipitate on the existing particle seeds producing
nanostructured particles or nucleate to form new particles.
In this review, we will focus on the one-particle-per-drop
mechanism, which is the dominant formation procedure of
GNCs by SP [398].
As mentioned above, SP can be applied to synthesize
GNCs through the mechanism of one-particle-per-drop.
Solutions, colloidal dispersions, emulsions, and sols can
be used as precursors. These precursors are atomized by
various techniques such as pneumatic (pressure, two-fluid,
nebulizers), ultrasonic, and electrostatic approaches. For a
specific atomizer, the physicochemical characteristics of
the drops are determined by the density, viscosity, and
surface tension of the precursor. Furthermore, the
atomizers affect the rate of atomization, the drop size, and
the drop velocity. The atomized precursors are driven into
a series of reactors (Fig. 36). Many physical phenomena
occur simultaneously in the diffusion drier (first step of
SP): the solvent evaporates from the surface of the drop,
the solvent vapors diffuse away from the drop into the gas
phase, the solute diffuses towards the center of the drop,
and the drop shrinks. It should be mentioned that the
drops can coagulate before and during this first step. In
doing so, two or more drops collide and coalesce into a
larger drop. This can only happen if there is a liquid
phase. In the calcination reactor (second step of SP), the
precursor is completely decomposed and the solute
precipitates. Therefore, the GNC is formed during this
step. The particles of GNC are sintered and annealed in
the annealing furnace (third and last step of SP) [399].
The production of GNCs with suitable characteristics
requires control of the atomization, coagulation,
evaporation, precipitation, decomposition, and sintering
processes. The calcination atmosphere and the precursors
also affect the chemistry of the GNC. Precursors with
high solubility precipitate homogenously and tend to form
solid spherical particles in a process named volume
precipitation. In contrast, precursors with low solubility
precipitate earlier at the surface rather than at the center of
the particle. This is known as surface precipitation.
The scaling up of SP processes is limited by the drop
size and the coagulation process. Drops smaller than 5 µm
in diameter are required to achieve an optimal production
rate. In addition, drop coagulation should be avoided.
Therefore, further developments in atomization, and
coagulation control are needed to commercialize SP
processes. Insufficient information on the fundamental
characteristics of the precursor systems also limited the
industrial applicability of this method. However, SP offers
the opportunity to produce new materials.
Shih et al. have developed GNCs by SP [154]. The
composites were mesoporous bioactive glasses (MBGs)
with embedded Ag NPs. The precursors of the MBGs
were tetraethyl orthosilicate (TEOS), calcium nitrate
(Ca(NO3)2), and triethyl phosphate (TEP). A solution of
MBG precursors and surfactant (Pluronic F-127) in
ethanol was mixed with a solution of Ag precursors (silver
acetate (AgA) or silver nitrate (AgN) in DI water). The
mixture was dispersed into fine droplets using an
ultrasonic nebulizer. The droplets were preheated to
evaporate the solvent, calcined to precipitate the solute
and decompose the precursors, and annealed, at 400, 700,
Javier FONSECA. NPs embedded into glass matrices: GNCs
Fig. 36
63
Schematic illustration of the steps to prepare GNCs by spray pyrolysis.
and 500 °C, respectively. The particle size of pure MBG,
GNC prepared using AgA as a precursor, and GNC
prepared using AgN as a precursor was (536 ± 174),
(500 ± 158), and (494 ± 166) nm, respectively. Ag NPs
were found on the surface of the composite prepared from
AgA. The composite prepared with the AgN precursor
contained the Ag NPs homogenously distributed within
the glass matrix. It should be mentioned that AgN has
greater solubility in water than AgA, which was suggested
to facilitate the distribution of AgN before calcination.
The average size of Ag NPs was (9 ± 4) and (7 ± 3) nm
for the GNC prepared from AgA, and the GNC prepared
from AgN, respectively. It was suggested that the Ag NPs
grew larger in the composite prepared from AgA than in
the composite prepared from AgN due to the higher
relative concentration of Ag on the surface of the former.
The BET surface area was reported to be (116 ± 1),
(148 ± 6), and (202 ± 3) m2·g−1 for pure MBG, GNC
prepared from AgA, and GNC prepared from AgN,
respectively. Therefore, the incorporation of Ag NPs
increased the surface area of the glass matrix. The GNCs
showed bioactivity, which is defined as the ability to form
hydroxyapatite (HA) layers after immersion in human
body fluid. Furthermore, both composites exhibited
antibacterial activity whereas pure MBG did not [154].
To determine the role of Ag NPs on the bioactivity of a
MBG, the same research group has incorporated different
concentrations (1.64, 3.28, and 6.56 mol.%) of this
reinforcement in the glass using the SP technique [375].
The glass precursors were TEOS, Ca(NO3)2, TEP and a
surfactant (Pluronic F-127). A solution of these precursors
was mixed with a specific amount of AgA. This new
mixture was dispersed into fine droplets with a nebulizer.
The droplets were preheated, calcined, and annealed at
400, 700, and 500 °C, respectively. In other words, they
were converted into GNC particles via SP. The size of the
Ag NPs embedded in a MBG was found to be (8 ± 2),
(8 ± 3), and (9 ± 3) nm for those GNCs that contained
1.64, 3.28, and 6.56 mol.% Ag, respectively. In these
composites, most of the Ag NPs were located on the
surface of the MBG. The BET surface area was (192.6 ±
0.4), (174.6 ± 10.5), (164.6 ± 22.1), and (148.2 ±
6.4) m2·g−1 for the MBG and the GNCs that contained
1.64, 3.28, and 6.56 mol.% Ag, respectively. While MBG
did not show inhibition of bacterial growth, the
antibacterial activity of the GNCs was found to increase
as the concentration of embedded Ag NPs increased.
Moreover, the Ag NPs also enhanced bioactivity. It
should be mentioned that the Ag NPs embedded in a glass
decrease the bioactivity of such glass if the reinforcement
is located within the glass matrix [400]. Therefore, it was
concluded that the influence of Ag NPs on the bioactivity
of MBG depends on the location of these NPs (within the
glass structure or on the glass surface) [375].
3.5.5
Chemical vapor deposition
Chemical vapor deposition (CVD) is a well-studied
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Front. Mater. Sci. 2022, 16(3): 220607
technique for producing high quality 2D films on
substrate surfaces at a reasonable cost [401‒403]. A film
structure is not favorable when the attractive forces
between the deposited materials are stronger than the
forces between the substrate and the deposited materials.
Under these conditions, the growth of the deposited
material follows an island formation scheme [404‒405].
Moreover, the substrate surface can be functionalized to
create areas with and without nucleation sites. Therefore,
NPs can be controllably grown on substrates using the
CVD technique [406].
The conversion of precursors into the desired products
by CVD involves thermal decomposition, transport, and
deposition [402]. A carrier gas facilitates the introduction
of chemical precursor gas or gases into a reaction system
containing a heated substrate to be coated. The deposition
occurs on and near a hot surface substrate. The flow rate
and partial pressure of each precursor can be varied to
control the deposition. CVD can be accompanied by the
production of chemical by-products. These are expelled
from the chamber together with the unreacted precursor
gases. High purity precursors are often required to avoid
unwanted side reactions and by-products [407].
Ertorer et al. have embedded Au NPs in BK7 glass by
CVD [406]. The BK7 glass was silanized with hexamethyldisilazane. This functionalization was carried out to
create nucleation sites. An organometallic Au precursor
((trimethylphospine)methylgold = [(CH3)3P]AuCH3) was
used in this CVD. The deposition of [(CH3)3P]AuCH3 on
the functionalized substrate was performed in an Ar
atmosphere at 65 °C and 5 Pa for 13, 15, 18, and 23 min.
The −NH-terminated substrate surface was assumed to
interact with the Au in the precursor. The phosphorous
group of the precursor (−P−(CH3)3) was proposed to be
displaced by the nitrogen group (−NH) on the substrate
surface. Thus, Au was immobilized on the glass surface
and the phosphorous group was released. Then, the Au
from a second precursor molecule bound to the already
immobilized gold species. The average size of the Au NPs
Fig. 37
was 11 and 17 nm when the CVD time was 13 and
23 min, respectively. Therefore, the size of the Au NPs
increased as the CVD time increased. Furthermore,
increasing CVD time also resulted in NPs with less
uniform size distribution and different morphologies, such
as spherical, star-shaped, etc. For short CVD times, the
only morphology of the Au NPs was spherical. These
GNCs (Au NPs@BK7 glass) were found to be suitable for
biomolecule-sensing applications [406].
As in the above example, conventional thermal CVD
requires volatile precursors. Organometallic precursors
are effective. However, they have several drawbacks:
(1) they are often sensitive to air and humidity, (2) their
synthesis can be difficult, and (3) they can incorporate
impurities into the final composite. Inorganic precursors
are generally more stable and give a cleaner decomposition. However, they are normally less volatile and
require high temperatures and low pressures [408].
Aerosol-assisted chemical vapor deposition (AACVD) is
a variant of the CVD process. AACVD uses liquid‒gas
aerosols to transport soluble precursors to a heated
substrate (Fig. 37). Therefore, the limitations of thermal
stability and volatility are eliminated by designing
precursors specifically for AACVD [409].
Au NPs have been deposited on silica glass using
AACVD [376]. First, Au colloids were synthesized in
toluene. In this solution, the average size of the Au NPs
was 10 nm. Then, an aerosol was generated from this
solution using an ultrasonic humidifier with an operating
frequency of 40 kHz. The aerosol was transported to a
horizontal-bed CVD reactor by N2 gas. The gas flow was
continued until all the precursors passed through the
reactor. The deposition was carried out at a glass
temperature of 450 °C. The size of the Au NPs deposited
on silica glass was around 50 nm in diameter. The Au NPs
in the GNC were larger than those in the precursor
solution. Therefore, the Au NPs agglomerated in the gas
phase or on the surface of the substrate. Gas-phase
particles are subject to a thermophoretic force when
Schematic illustration of the AACVD system.
Javier FONSECA. NPs embedded into glass matrices: GNCs
exposed to a temperature gradient [410]. This force is
directed away from the hot surface. Larger particles are
repelled by a hot surface and attracted to a cold surface.
Thus, the larger particles cannot diffuse through the
thermal boundary layer on the surface of the hot substrate.
Thermophoresis was the dominant force determining the
location of Au NPs deposition. Therefore, the Au NPs
were obtained exclusively on the substrate surface [376].
Although this is beyond the scope of this review, it is
worth mentioning that the AACVD technique has been
used extensively to deposit nanocomposite films on
substrates [411‒413].
4 Conclusions
Research on GNCs has been very active and productive in
the past decades. GNCs have played — and still do — an
important role in the fields of optoelectronics, photonics,
sensing, electrochemistry, catalysis, biomedicine, and art.
Regarding optoelectronics, all-optical switching has been
extensively investigated for decades because it can
potentially overcome the speed limitation of electrical
switching devices [30,414‒416]. However, all-optical
switching devices require a large driving energy, which is
a fundamental limitation for most applications. There is a
trade-off between speed and energy. In other words,
energy can be reduced by sacrificing speed. The large
energy required is attributed to the inherently small
optical non-linearity in existing materials. Therefore,
future opportunities in all-optical switching will depend
on the preparation of materials with enhanced Kerr
susceptibilities, as these materials are still at a relatively
early stage of development. Particularly, GNCs are
expected to break this trade-off.
In the field of photonics, the addition of noble metal
NPs to REs-doped glasses further enhances their
efficiency as solid-state light sources. Noble metal NPs
improve the emission intensity of RE ions. These GNCs
are also promising photovoltaic (PV) cells. Silicon-based
PV systems cannot efficiently convert a large part of the
solar radiation in the UV spectrum into electricity.
However, GNCs can overcome this limitation. GNCs have
potential as low-cost photonic systems with improved
characteristics, but further research is required.
GNCs can be ideal SERS platforms. Low-cost sensors
and biosensors with ultra-sensitive detection limits can be
made of GNCs [360,364,417‒418]. They can provide high
65
efficiency (in terms of signal enhancement) and reuse.
However, NPs are prone to oxidation and sulfidation
phenomena. Therefore, ensuring the long-term stability of
metal NPs within glass matrices is an indispensable
condition for developing GNCs-based sensors and
biosensors. The preparation of GNCs as SERS substrates
remains a challenging task.
Extensive work and research have been done to develop
solid materials capable of replacing liquid electrolytes.
SEs can improve the performance, safety, and durability
of batteries [419]. The performance of SSBs is highly
dependent on ion diffusion within the electrolyte. SEs
must exhibit high ionic conductivity and very low
electronic conductivity [420]. An ideal SE is a pure ion
conductor. Thermal and chemical stabilities are also
important factors of SEs [421]. Therefore, GNCs made of
extremely high concentrations of superionic metastable
NPs embedded within stable glasses are promising
candidates for SEs applicable to SSBs [422].
Inexpensive, environmentally friendly, durable,
reusable, and inert glasses are attractive supports for
catalytically active metal NPs. The main drawback of
these glasses is their lack of porosity. Therefore, further
research is expected for the development of porous
glasses. GNCs made of porous glasses and metal NPs can
become competitive catalysts.
Recent advances in the fields of bone regeneration and
cancer therapy focus on the development of MBGs
[423‒424]. These glasses also have potential for targeted
drug delivery [425]. The incorporation of NPs within
MBGs can improve the mechanical and antibacterial
properties and the biodegradation rates of the pristine
MBGs [426]. However, many questions about the
usability of MBG-based GNCs for biomedicine have yet
to be addressed: optimal dose of NPs and drugs to
incorporate into MBG, fate of MBG in vivo, long-term
side effects, etc.
The conservation–restoration of GNCs-based Historical
Heritage consists of preventing their deterioration and
applying the appropriate treatments to restore the
masterpieces to their original state without permanently
altering them. Best practices employ the protocol of
“avoiding, blocking, detecting, and responding” to any
threat. It is also important to assess risks and needs based
on the main agents of deterioration. Cleaning methods
have evolved from washing with water to using special
chemical cleaners made especially for GNCs. In addition,
cleaning methods also differ if the GNCs are already
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Front. Mater. Sci. 2022, 16(3): 220607
damaged, fragile, or very old. It should be noted that
today the conservation-restoration of the GNCs-based
Historical Heritage is strongly promoted.
The most important synthesis methods of GNCs (meltquenching, sol-gel, ion implantation, ion-exchange,
staining process, spark plasma sintering, radio-frequency
sputtering, spray pyrolysis, and CVD techniques) are
extensively described in this review. Considering that
future directions of GNC synthesis appear to lie in the
combination of various GNC synthesis approaches, such
as ion implantation and radio frequency sputtering or laser
irradiation [35], a thorough understanding of GNC
synthesis methods is the first step towards unprecedented
GNCs development. Furthermore, in-depth knowledge of
GNC synthesis techniques may facilitate innovative uses
of such techniques [427]. In this review, we have also
explored the growth of NPs within glass matrices. The
size-controlled preparation of NPs within glass matrices,
which remains a challenge, is essential for advanced
applications.
The field of GNCs will continue to provide opportunities for those seeking new engineering materials. In
addition, despite all the advances in our knowledge about
GNCs, there are still fundamental questions that deserve
to be investigated. For instance, the relationships between
the chemical composition, structure, and properties of
GNCs are still far from scientific understanding. There is
no doubt that the field of GNCs will continue to grow and
flourish. These are exciting times ahead.
Acknowledgements The Chemical Engineering
Northeastern University supported this work.
Department
at
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