Journal of Analytical and Applied Pyrolysis 104 (2013) 461–469
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Journal of Analytical and Applied Pyrolysis
journal homepage: www.elsevier.com/locate/jaap
Synthesis of ceramic nanoparticles by laser pyrolysis: From research
to applications
Rosaria D’Amato a,∗ , Mauro Falconieri b , Serena Gagliardi b , Ernest Popovici c ,
Emanuele Serra b , Gaetano Terranova a , Elisabetta Borsella a
a
ENEA-UTAPRAD, C.R. Frascati, via E. Fermi 45, 00044 Frascati, Roma, Italy
ENEA-UTTMAT, C.R. Casaccia, via Anguillarese 301, 00123 Roma, Italy
c
NILPRP, Lasers Department, 409 Atomistilor, 077125 Magurele-IF, Bucharest, Romania
b
a r t i c l e
i n f o
Article history:
Received 20 March 2013
Accepted 28 May 2013
Available online 3 June 2013
Keywords:
CO2 laser pyrolysis
Silicon carbide nanoparticles
Silica nanoparticles
Titania nanoparticles
Nanoparticles synthesis
Nanomaterials applications
a b s t r a c t
Nanoparticles are the building blocks of many approaches for realizing nanostructured materials and
devices. The technique of CO2 laser pyrolysis of gas- or vapour-phase precursors for the synthesis of
nanoparticles has proven to be a very flexible and versatile technique which permits to cope with several
challenges in different sectors of nanotechnology. Different kinds of pyrolytic nanopowders with average
size in the range 5–60 nm and a narrow size distribution, have been produced and tested for the fabrication
of structural materials and/or for functional applications in various fields. Here we report new results
on the synthesis of SiC, TiO2 and SiO2 nanoparticles for energetic applications and cultural heritage
preservation.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Nanoparticles (NPs) are particles having at least one dimension that is less than 100 nm in size. In the 90s, scientific evidence
was found that materials on the nanoscale may possess properties (optical/electronic/magnetic) that are dramatically and even
entirely different from the bulk [1]. After this discovery, the development of nanoscience and nanotechnology has created a whole
new “nanoworld”. NPs can be regarded as one of the fundamental building blocks of nanotechnology since they are the starting
point of several “bottom up” approaches for fabricating nanomaterials used, or being evaluated for use, in many fields ranging from
catalysis to photonics and opto-electronics, from energy production to thermal management [2]. Moreover, biological systems are
considered as the ideal playground for NPs applications. In fact, due
to their very small size, NPs are able to gain access and even operate
within cells. Promising applications in bio-medicine span from drug
delivery to cancer therapy by hyperthermia and to bio-imaging [3].
Nowadays, NPs of a wide range of chemical compositions and
phases can be prepared by a variety of methods; however the production of large amounts of pure, non-agglomerated NPs, with
desired size and narrow size distribution, still results to be an
∗ Corresponding author. Tel.: +39 069 400 5469; fax: +39 069 400 5312.
E-mail address: rosaria.damato@enea.it (R. D’Amato).
0165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jaap.2013.05.026
extremely difficult task [4]. To this respect, the technique of CO2
laser pyrolysis of gas- and vapour-phase reactants appears as a
scalable synthesis route for preparing NPs with controllable morphology and in quantities sufficient to be tested for structural and
functional applications [5]. This technique has been applied to
the synthesis of a large variety of oxide and non-oxide nanopowders. Si-based nanoparticles (SiC, Si3 N4 , Si/C/N, Si/Ti/C) were the
first to be produced by laser pyrolysis for the development of
nanocomposite ceramic materials for high temperature structural
applications [5–9]. Even if the results were interesting, it was found
that the presence of critical defects in the structural materials based
on nanophase constituents [9,10] severely affects the final performance and, consequently, the attention was shifted towards
the development of nanocomposites for applications where the
sensitivity to defect population is not very critical (like abrasion
resistance [11], polymer reinforcement by addition of nanoparticles
etc.) or specific functions are required (optical [12–14], magnetic
[15,16], thermal [17] etc.).
A great effort was made for the development and exploitation of
the optical and electronic properties of Si and Si-based nanopowders prepared by laser pyrolysis for applications in photonics
and optoelectronics [12,18,19]. In this framework, it was demonstrated that Si-based nanopowders can be effectively excited at
all the photon energies above the band gap and subsequently
transfer their energy to nearby rare-earth ions which decay emitting photons at their characteristic transition energies [18]. This
462
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functionality was tested on pyrolytic Si-based nanopowders and Er
co-doped sol–gel silica glasses for the realization of optical amplifiers emitting at 1.54 ␮m (i.e. the telecommunication wavelength)
[18] and, afterwards, for the realization of fluorescent bio-markers
by exploitation of the aptitude of Yb ions to form complexes with
amino groups which are easily linkable to the surface of pyrolytic
Si nanoparticles [20].
A great attention was indeed devoted to the development of
optical and magnetic probes for biomedical imaging applications
[3]. In this context, several procedures were followed to exploit the
optical properties of Si nanoparticles prepared by laser pyrolysis for
the visualization of medically relevant targets [20–23]. Moreover,
the laser pyrolysis technique was employed for the production
of magnetic Fe2 O3 nanoparticles to be tested as contrast agents
for magnetic resonance imaging [15,16,24]. Multimodal bioimaging with nanoparticles has also been addressed by several routes,
including the synthesis of a nano-complex incorporating pyrolytic
Si nanoparticles and gadolinium ions [25]. More recently, laser
pyrolysis has been applied to the synthesis of TiO2 nanoparticles
for the realization of UV protection products [26] and inks for ink
jet printing techniques [27].
These results prove the great flexibility of the method of
laser pyrolysis for preparing prototypal nanoparticles for various applications and leave room for further developments in
both the synthesis process and the use of the produced nanoparticles for quite different functions. Here we will report new
results on the synthesis of SiC, TiO2 and SiO2 nanopowders,
prompted by a surge of renovated interest for these materials for
applications in the energetic sector and cultural heritage preservation.
2. Laser pyrolysis for nanopowder production
The CO2 laser pyrolysis technique was first developed by Haggerty and co-workers at the beginning of 80s [6] and it is usually
classified as a vapour-phase synthesis process for the production of
NPs. In this class of synthesis routes, NP formation starts abruptly
when a sufficient degree of supersaturation of condensable products is reached in the vapour phase [28]. Once nucleation occurs,
fast particle growth takes place by coalescence/coagulation rather
than further nucleation. At sufficiently high temperatures, particle coalescence is faster than coagulation and spherical particles
are formed. At lower temperatures, coalescence slows down and
partially sintered, non-spherical particles and/or loose agglomerates of particles are formed [28]. It follows that, to prepare
small spherical particles, it is necessary to create a high degree
of supersaturation for inducing the formation of a high density
of nuclei and then quickly quench the particle growth either by
removing the source of supersaturation or by slowing down the
kinetics.
In the process of CO2 laser pyrolysis, the condensable products result from laser induced chemical reactions at the crossing
point of the laser beam with the molecular flow of gas- or vapourphase precursors (see Fig. 1). The pre-requisite for energy coupling
into the system, leading to molecular decomposition, is that at
least one of the precursors absorbs through a resonant vibrational mode the infrared (IR) CO2 laser radiation tuned at about
10 ␮m [9]. Alternatively, an inert photo-sensitizer is added to the
vapour phase mixture. Ethylene gas (C2 H4 ), sulphur hexafluoride
(SF6 ) or ammonia (NH3 ) are the most widely employed sensitizers due to their resonant absorption at about 10 ␮m and to the
relatively high dissociation energy (7.2 eV for C2 H4 , 3.95 eV for
SF6 , 3.91 eV for NH3 ). The high power of the CO2 laser induces
the sequential absorption of several IR photons in the same
molecule, followed by collision assisted energy pooling, leading to
Fig. 1. Schematic of the set-up for laser synthesis of nanoparticles from gas-phase
reactants.
a rapid increase of the average temperature in the gas through V-T
(vibration-translation) energy transfer processes, often accompanied by the appearance of a flame in the interaction volume [29].
If molecules are excited above the dissociation threshold, molecular decomposition, eventually followed by chemical reactions,
occurs with the formation of condensable and/or volatile products.
Compared with other vapour-phase synthesis methods
[1,3,4,28], it is fairly evident that laser pyrolysis permits highly
localized and fast heating (leading to rapid nucleation) in a
volume that can be limited to a few hundred mm3 , followed by
fast quenching of the particle growth (in a few ms). As a result,
NPs with average size ranging from 5 to 60 nm and narrow size
distribution are formed in the hot region. The mean NP size can
be varied by acting on the process parameters like the residence
time of the particles in the flame (tres ), the total pressure in the
reaction chamber and the reaction temperature. This explains the
great potentiality of laser pyrolysis for the preparation of ad-hoc
nanopowders for different applications. However, as in other
vapour-phase synthesis methods, one of the major obstacles to
the widespread use of pyrolytic NPs is the formation of chained
agglomerates of small particles. Agglomeration occurs when the
NPs leave the high temperature region and coalescence becomes
much slower than coagulation. High-power ultrasonic treatment
and/or ball milling are effective in reducing the nanoaggregate
mean size by disaggregation of soft-agglomerates, whereas partially sintered aggregates can only be eliminated by centrifugation
or filtration, with significant material loss. Stabilization of the NPs
dispersed in liquids is usually achieved by use of surfactants, that is
surface active agents adsorbed on the surface of the NPs. Repulsive
inter-particle forces are required to overcome the attractive Van
der Waals forces between the particles. The two most common
methods used are based either on electrostatic repulsion or on
steric forces [1].
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463
a removable bag, located in a tank between the reaction chamber
and the vacuum pump.
3.2. Characterization techniques
Fig. 2. Picture of the ENEA set-up for laser synthesis of nanoparticles from gas-phase
reactants.
Nanopowders prepared by laser pyrolysis are characterized by
several complementary techniques.
IR absorption spectra are recorded on a Perkin-Elmer Spectrum100 FTIR spectrophotometer, while the UV-VIS and near IR
spectra are recorded on a Perkin-Elmer 330 spectrophotometer
equipped with a 60 mm integrating sphere for diffuse reflectance
measurements.
Scanning Electron Microscopy (SEM) micrographs are collected
by a FEG-SEM LEO 1530 (Zeiss, Obercoken-Germany) equipped
with in-lens secondary electron detector, conventional secondary
electron detector and scintillation detector for backscattered electrons (Centaurus) to investigate the nanoparticles diameters and
their polydispersion.
Structural investigation is performed by Raman spectroscopy.
Raman measurements were performed using a modular HORIBAJobin-Yvon spectrometer, consisting in a fibre-coupled 550 mm
grating monochromator and a liquid-nitrogen cooled CCD detector.
A home-made sampling head equipped with a notch filter for rejection of elastically scattered light was used to deliver the 532 nm
excitation beam on to the sample by a 32X long-working-distance
objective, and to collect the Raman signal in backscattering geometry.
Dynamic light scattering (DLS) measurements by a Malvern PCS
4800 apparatus allows to estimate the hard-bound aggregate size
on nanopowders samples dispersed in water or ethanol. Data analysis was based on the model-independent Zave parameter. Powders
are dispersed using an high-power ultrasonic probe (BRANSON 450
sonicator equipped with 3 mm micro-tip) for 30 min to promote
partial disaggregation whenever necessary.
4. Nanopowder preparation and characterization
3. Experimental
3.1. Experimental set-up
The set-up for the production of nanopowders by CO2 laser
pyrolysis is shown in Fig. 2. The CW CO2 laser beam ( = 10.6 ␮m)
is focused by a spherical lens (focal length F.L. = 12 or 19 cm) at the
centre of the reaction chamber (volume V ∼6.8 × 10−3 m3 ) where it
orthogonally intersects the reactant gas flow. The maximum laser
power is 1.2 kW and the power density in the focal region can
be varied up to 275 kW/cm2 . Reactant gases enter the chamber
through the inner tube of a coaxial stainless steel nozzle. An inert
gas (He or Ar) flows through the outer tube with the purpose of
confining and cooling the particles. The pressure in the reaction
chamber is kept constant and measured by a pressure control unit.
Typical pressures are in the range 6.7–80 × 103 Pa.
In the laser pyrolysis process, the reactants are most often in the
gas-phase, however in some cases, liquid precursors are either the
only choice or the most advantageous from an economical point
of view. The use of liquid precursors is made possible by an evaporating system installed at the laser pyrolysis set-up: the liquid
precursor is extracted from the reservoir by bubbling the inert gas,
or one of the gas-phase reactants, and subsequently introduced
in the evaporator unit and maintained at proper temperature by
a heater; at the exit of the evaporator the vapour precursor and
gas mixture are carried out through a heated flexible tube to the
reaction chamber.
At the exit of the reaction region, marked by a flame, a strong
quenching effect stops the particle growth at nanometric size. The
produced powder are driven by the gas flow through a chimney into
Here we report on the latest developments in the synthesis of
SiC, SiO2 and TiO2 nanoparticles in terms of controlling the particlegrowth mechanism and tuning the final structure and composition
of these nanoparticles for various applications in the energetic sectors and cultural heritage preservation, as described in Paragraph
5.
4.1. SiC-NPs synthesis and characterization
Si and Si-based nanoparticles are probably the most widely
investigated for two main reasons: (i) silane (SiH4 ) gas, a common
precursor for Si radical formation, has a very high absorptivity for
the emission line of an untuned, multimodal CO2 laser (at 10.6 ␮m);
(ii) the enormous importance of Si and Si-based nanoparticles for
a multitude of applications, as reviewed in Section 1.
Silicon carbide is preferably formed by laser induced reactions of
silane with acetylene, since the global process is strongly exothermic [5,6]. The reactive gases are injected into the chamber in molar
proportion 2:1 through the use of mass flow controllers, to produce powders whose stoichiometry comes close to that of pure SiC.
The mean nanoparticle size can be varied in the range 20–50 nm by
acting on the process parameters, in particular the laser power density, the reagent flows, the total pressure in the reaction chamber
and the inner nozzle diameter. Details on process parameters are
reported in Table 1.
Although changes in the SiC nanopowder final properties as
a function of process parameters have been studied by several
authors [5,6,30], the details of the ignition process and of the laser
induced reactions are more difficult to characterize. To this respect,
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Table 1
Experimental parameters, productivities and mean diameters (d) of SiC-NPs.
Powder
Lens F.L. (cm)
Nozzle diameter (mm)
˚SiH4 (sccm)
Laser power (W)
Productivity (g/h)
d (nm)
SiC-15
SiC-16
SiC-17
SiC-20
SiC-21
SiC-22
SiC-23
SiC-24
SiC-26
12
12
12
19
19
19
19
19
19
3
3
4
3
4
4
4
4
6
500
300
500
500
300
500
500
500
500
630
630
630
630
630
630
400
900
630
50
15
50
55
30
55
50
50
50
25
30
30
35
45
40
35
50
50
±
±
±
±
±
±
±
±
±
2
2
3
13
18
13
3
18
22
Fig. 3. Left side: 3D – view of the interaction region between the laser beam and the molecular gas flow in the focusing geometry with F.L. (focal length) = 19 cm. Right side:
flame luminosity map.
the map of flame temperatures can be used to gain insights into the
burning process itself and the subsequent evolution in space (and
time) of the nanoparticle growth.
In the following, the effect of two different types of laser focusing geometries on the final characteristics of SiC nanopowders will
be analyzed and discussed. The most common optical configuration
consists in matching the CO2 laser beam waist in the focal region
with the reactant flow cross-section in the interaction volume. To
get this condition, in our set-up the CO2 laser beam was focused by
a spherical lens (F.L. = 19 cm) down to a spot of about 5 mm in diameter on the reactant flow, a few mm below the exit of the gas inlet
nozzle having an internal diameter of 4 mm (see Fig. 3 left side). The
laser power was varied in the range 400–900 W (cfr SiC 22–23–24
in Table 1), all the other process parameters being kept constant:
the pressure in the reaction chamber was set at 8.0 × 104 Pa, the
acetylene flow was set at 250 sccm and the Ar flow, which ensures
the confinement of the reaction, was set at 5 slm. Typical productivities ranged between 30–50 g/h and reaction yields were near to
100%.
The temperature distribution in the reaction zone is visualized
by the flame luminosity map in the picture shown in Fig. 3 right
side. The observed flame asymmetry is due to the Ar flow at the
optical window to avoid powder deposition and accumulation on
the inner surface. It is evident that in this focusing configuration the
temperature distribution is rather uniform in the reaction volume
(≥3.2 × 102 mm3 ), which is laterally limited by the confining Ar gas
flow. However, the temperature appears lower at the edge of the
flame where the reactant molecules are diluted (and cooled) by the
Ar flow. The maximum temperature in the flame depends on the
laser power density that is varied from 2.0 to 4.6 kW/cm2 . The high
productivities (and reaction yields) observed in the whole range of
laser power densities ensure that all the reactant molecules are dissociated. SEM analysis of the produced SiC particles shows a broad
size distribution (Fig. 4) that can be explained by the temperature
gradient at the edge of the flame. At the flame boundaries, in fact,
the reactants are more strongly diluted with Ar and smaller particles are formed as a consequence of the lower radical concentration
and lower reaction temperature T [28–30]. On the other hand, the
results reported in Table 1 confirm that the peak of the size distribution depends on the laser power when all the other process
parameters are kept constant (cfr SiC 22–23–24).
In order to narrow the particle size distribution, a different
focusing geometry was designed to change the temperature profile
of the flame and minimize the particle formation at the flame edge.
To this purpose, tight focusing of the laser beam on the reactant
flow was realized by use of a shorter focal length, spherical lens
(F.L. = 12 cm) (see Fig. 5a). After SiH4 ignition in the laser focus,
the flame propagates in the reaction volume and the temperature
Fig. 4. SEM image of SiC nanopowder sample (SiC-24).
R. D’Amato et al. / Journal of Analytical and Applied Pyrolysis 104 (2013) 461–469
465
Fig. 5. Left side: 3D – view of the interaction region between the laser beam and the molecular gas flow in the focusing geometry with F.L. (focal length) = 12 cm. Right side:
flame image and luminosity map.
distribution in the gas mixture was established as visualized by the
flame luminosity map in the Fig. 5b. It is evident that in this configuration the reaction volume at the highest temperature is smaller
(≤1.8 × 102 mm3 ) and more confined reactions are expected to
occur in the central part of the flame, with a lower effect of the dilution (and cooling) of the reactants at the outer edge of the flame. A
narrower size distribution of the produced SiC nanoparticles was
indeed observed in this configuration (cfr. SiC-15 with SiC-20 and
SiC-17 with SiC-22), as shown by the SEM pictures in Fig. 6.
In selected laser focusing conditions, the final mean size of the
nanoparticles can also be varied by acting on the residence time of
the nanoparticles in the reaction volume, that is by changing the
velocity of the reactants. The speed of the reagents can be decreased
by decreasing the molecular flow or by changing the inner nozzle
diameter. The effect of a change in these parameters on the final
SiC nanoparticle size is shown in Table 1. Bigger particles can be
prepared by decreasing the molecular flow as it is evidenced by
the comparison between SiC-15 and SiC-16 and between SiC-21
and SiC-22. Also the use of a larger nozzle diameter leads to larger
particles (compare SiC-15 with SiC17 and SiC-20 with SiC-22 and
SiC-26).
All the nanopowders present the typical infrared feature of SiC
at 830 cm−1 and the X-ray diffraction patterns of crystalline ˇ-SiC
(Fig. 7).
4.2. SiO2 -NPs synthesis and characterization
Amorphous, nanosized Si/C/O powders were synthesized using
alkoxysilane as liquid precursor [31]. In order to obtain pure silica
nanoparticles, we chose a different precursor, i.e. tetraethoxysilane
(TEOS) Si(OC2 H5 )4 , a liquid at room temperature, with a vapour
pressure of about 130 Pa, in which Si atoms are already oxidized.
TEOS is not a strong absorber of laser radiation at 10 ␮m, thus ethylene gas (C2 H4 ) was added as a reaction sensitizer to increase
the coupling of the laser energy in the system, due to its resonant
absorption at about 10 ␮m and to the relatively high dissociation
energy. The aerosol was produced by bubbling Ar into the TEOS bottle and carried to the reaction chamber by Ar flow through a liquid
evaporator. The temperature of the evaporator was settled at either
Tev = 140 ◦ C, 160 ◦ C, 170 ◦ C or 180 ◦ C. Ar gas can also be used as a
diluent for the reagents. C2 H4 and TEOS aerosol were mixed just
before the exit of the inlet nozzle (6 mm in diameter). The TEOS
mass flow resulted to be of about 50 g/h (aerosol flow = 400 slm)
and the reactor pressure was fixed at 5.3 × 104 Pa. Other experimental parameters such as ethylene flow and laser power were
varied according to data reported in Table 2.
The as-synthesized powders generally appeared dark for contamination with free-carbon coming from TEOS and/or unwanted
ethylene decomposition. In order to remove these contaminants
it was necessary to perform a thermal treatment at T = 600 ◦ C for
6 h. Unfortunately, the DLS measurements on thermally treated
nanopowders revealed a high degree of aggregation, that is undesirable for most applications. Consequently, carbon contamination
was decreased to less than 6%, w/w (as shown by thermal
Silica NPs formation needs a precursor different from silane,
that reacts violently with oxygen and can cause an explosion.
Fig. 6. SEM image of SiC nanopowder sample (SiC-17).
Fig. 7. Typical IR spectrum and X-ray diffraction pattern (inset) of laser synthesized
SiC nanopowders.
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Table 2
Effect of synthesis parameters: experimental parameters, carbon contamination, mean diameters (d, as obtained by SEM analysis) and Zave (as obtained by DLS) of SiO2 -NPs.
Powder
Tev (◦ C)
C2 H4 (sccm)
TEOS-1
TEOS-2
TEOS-3
TEOS-4
TEOS-8
TEOS-9
TEOS-7
TEOS-14
TEOS-12
TEOS-10
TEOS-15
TEOS-11
TEOS-16
TEOS-17
140
140
140
140
160
160
160
170
170
170
170
170
180
180
400
0
400
600
400
400
400
0
200
400
600
400
600
400
Laser (W)
800
1200
1200
1200
800
1000
1200
700
700
700
700
900
700
800
gravimetric analysis) by a fine tuning of the process parameters (see
Table 2) such as evaporator temperature, laser power and ethylene
flow.
By comparing carbon contamination values reported in Table 2,
first of all we can observe that an increase in the evaporator temperature improves the nanopowder purity, since it favours the
complete dissociation of TEOS precursor and its conversion to
silica. Another important parameter is the reaction (and flame)
temperature which depends on both the laser power and ethylene flow. If the laser power is high, TEOS dissociation takes
place with a limited, or even without, addition of ethylene whose
unwanted decomposition is the main contamination source (see
TEOS-2–3–4 in Table 2). On the other hand, when the laser power
is low, the presence of ethylene increases the reaction temperature, consequently TEOS conversion to SiO2 is complete and
carbon contamination is decreased as found in TEOS 10–12–14–15.
When the reaction temperature is too high (high laser power and
high ethylene flow, cfr TEOS 7–8–9) the carbon contamination
increases again, probably for side reactions of decomposition. The
best results were obtained in case of TEOS-10, TEOS-15 and TEOS16, in which carbon contamination is less than 6%, the as-prepared
powder colour was almost white and no thermal treatment was
needed.
A control on NPs dimensions was achieved and nanopowders
with mean size ranging from 10 to 40 nm were produced (see
Fig. 8). An increase in the evaporator temperature and ethylene
flow resulted in the synthesis of bigger particles, because in these
conditions there is a high concentration of radicals in the reaction
zone due to almost complete dissociation of TEOS.
Fig. 8. SEM image of the nanosilica sample TEOS-7.
%C (w/w)
10
10
20
18
8
10
13
15
9
6
6
9
5
9
d (nm)
Zave (DLS)
12
15
20
18
28
25
22
18
15
35
25
35
28
10
221
248
241
235
150
158
190
204
180
157
186
183
195
204
The nanopowders were found to be slightly aggregated, as
evidenced by Zave values reported in Table 2. The evaporator temperature (Tev ) has an influence on the aggregate size: at Tev = 140 ◦ C,
Zave is of about 240 nm, while at higher evaporator temperatures
Zave is less than 200 nm. Aggregate mean size depends also on the
reaction temperature: in fact at high temperature coalescence is
the most important process and the NP mean size increases, while
at low temperature coagulation is favoured and there is a tendency
towards aggregation rather than to NP size growth.
The infrared absorption features of the as-formed nanosilica
powders are very similar. A typical IR spectrum is shown in Fig. 9. A
strong absorption band appears at 1100 cm−1 with a weaker bands
at 809 cm−1 which correspond to Si O stretching of the opaline
silica type.
4.3. TiO2 -NPs synthesis and characterization
Titania NPs were synthesized by laser pyrolysis of an aerosol
of titanium (IV) isopropoxide (TTIP) Ti[OCH(CH3 )2 ]4 mixed with
either ethylene or ammonia (NH3 ) as reaction sensitizer. The
aerosol was produced as described above. Typical reaction
parameters were: evaporator temperature Tev = 180 ◦ C, aerosol
flow = 400 sccm, laser power = 1100 W. The gas flows and the total
pressure in the chamber used in different runs are reported in
Table 3. The as-prepared powders are black or blue for the presence of C contamination that was subsequently eliminated by soft
thermal treatment in air at T = 500 ◦ C for 3 h.
It was found by XRD and TEM analysis that the average crystallite size of sample TiO2 -1, produced with ethylene as sensitizer,
Fig. 9. IR spectrum of the nanosilica sample TEOS-10.
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467
Table 3
Experimental parameters for the synthesis of pure and N-doped TiO2 -NPs.
Powder
Sensitizer
˚TTIP (g/h)
˚sensitizer (sccm)
Pressure (Pa)
TiO2 -1
N-TiO2 -2
N-TiO2 -3
N-TiO2 -4
C2 H4
NH3
NH3
NH3
50
40
40
50
400
600
600
600
5.3 × 104
8.7 × 104
4.7 × 104
5.7 × 104
was of about 10 nm, the crystalline phase being approximately 30%
rutile and 70% anatase (data not shown here). When NH3 was used
as sensitizer, the average crystallite size resulted to be of about
15 nm and the crystalline phase was mostly anatase. The crystalline phase of all these samples was also determined by means
of Raman spectroscopy. Data reported in Fig. 10 show the presence
of a small percentage of rutile in sample TiO2 -1, and a practically
pure anatase phase in the other samples [32]. From the structural
point of view, rutile is known to be the TiO2 high-temperature stable phase and anatase the low-temperature one. Throughout the
different synthesis runs, remarkable changes in the pyrolysis flame
could be seen with the naked eye. During the synthesis of samples
N-TiO2 -2–3–4 the flame intensity was lower than in the presence of
ethylene (TiO2 -1), probably for the cooling effect of NH3 . As a result
of this lower flame temperature, the rutile phase is not present in
these samples.
Moreover, as a consequence of partial dissociation of NH3 , nitrogen can be incorporated in the NPs. N-doping has the effect to
shift the optical absorption of titania towards the visible range
as evidenced by the different colour of the nanopowders; in fact
after the thermal treatment, the powders colour changes from
white in case of pure TiO2 to yellow in case of N-doped TiO2
samples. A semi-quantitative estimation of the absorption coefficient of the produced nanopowders (Table 3) can be obtained from
Kubelka–Munk analysis of their diffuse reflectance (DR) spectra
[33] reported in Fig. 11. An absorption feature at about 430 nm
is observed in the spectra taken on samples synthesized in the
presence of ammonia and confirms the nitrogen-doping.
It is important to note that the level of N concentration in the
samples can be varied by changing the process parameters. In particular, a decrease of the TTIP flow and an increase of the pressure
in the chamber led to increased N concentration in the produced
powders due to the higher relative concentration of N radicals in
the reaction zone.
Fig. 10. Raman spectra of TiO2 samples and attribution of peaks to anatase () and
rutile (•) phase on the basis of their intensity and position, showing the prominence
of anatase phase.
Fig. 11. Absorption coefficient of TiO2 nanopowders obtained from Kubelka–Munk
analysis of diffuse reflectance spectra. Note the logarithmic scale used to evidence
the low-intensity absorption at wavelengths longer than 400 nm, induced by nitrogen doping.
5. Recent applications of nanopowders prepared by laser
pyrolysis
The laser pyrolysis method for the synthesis of nanoparticles
has proven to be a very flexible and versatile technique which
permits to finely tune NP properties for a variety of applications
in challenging research fields and competitive industrial sectors.
More recently, applications in critical sectors like energy generation
and saving as well as in new fields like cultural heritage preservation have been started. Preliminary results are presented in the
following.
5.1. Development of nanofluids for enhanced heat exchange
The need for more efficient and better performing coolants
has lately generated considerable interest for the development of
nanofluids, that are dilute and stable suspensions of nanoparticles
in base fluids [34,35]. In principle, if compared to conventional
solid–liquid suspensions, nanofluids should offer several advantages like higher specific surface areas for heat transfer between
particles and fluids, reduced clogging when passing through
microchannels, better thermal properties. A firm assessment of
enhanced heat transfer coefficient in nanofluids with respect
to conventional coolants would push towards their industrial
exploitation for a variety of applications ranging from cooling
of electronic components, transformer oil and heat-exchange
devices to the development of better performing chillers, domestic
refrigerators–freezers and more [34].
In this framework, pyrolytic nanoparticles of TiO2 , SiO2 and SiC
were used for the preparation and testing of selected nanofluids. An
enhancement in the thermal conductivity (with respect to distilled
water) was observed in all these nanofluids at different concentrations in water. The thermal conductivity of the nanofluids was
determined either with conventional methodologies [36] or with a
technique based on optical, laser-induced grating, which provides
the possibility for in situ monitoring of coolant performances [37].
The relative thermal conductivity of the water based nanofluids was
found in substantial agreement with the prediction of the Maxwell
model [35].
Erosion tests of nanofluids flowing on different targets were
conducted by use of an experimental facility designed to allow
for direct comparison with distilled water [38]. It was found that
SiC water-based nanofluids have a negligible effect, if compared
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R. D’Amato et al. / Journal of Analytical and Applied Pyrolysis 104 (2013) 461–469
to distilled water, when impact on Al, Cu and stainless steel targets. Moreover, evaluation tests of the heat transfer performance
of water-based nanofluids containing either TiO2 or SiC pyrolytic
nanoparticles (in different concentrations) were carried out in the
same dual test rig [39]. Using the Reynolds number as a reference
parameter, it has been found that the heat transfer coefficient for
the nanofluids is higher than for the basic fluid. This result has
been attributed to the higher viscosity of the nanofluids, which,
for the same Reynolds number, corresponds to a higher velocity,
thus allowing for a better heat transfer performance [39]. However, due to the higher velocity for the same Reynolds number,
pressure drops associated with nanofluids are higher than those
typical of the basic fluid. It follows that higher pumping power values are required. These conclusions have to be taken into account
when considering the perspectives for exploitation of nanofluids in
practical applications.
5.2. Development of TiO2 -based porous semiconductor electrodes
TiO2 nanopowders are considered for applications in many hot
areas as photocatalysis, functional surface coating and working
electrodes for dye-sensitized solar cells (DSSC) just to name a few.
While traditional synthesis methods like flame pyrolysis or sol–gel
approaches are the most investigated, laser-assisted pyrolysis can
provide advantageous material characteristics related to the morphology of aggregates, and to the possibility of efficient doping.
Presently, we are exploring nitrogen-doped titania for applications in photocatalysis and DSSC electrodes. In the first case,
the visible absorption band produced by doping is expected to
extend the photocatalytic activity of the material beyond the UV
range proper of intrinsic titania [40], with obvious energy efficiency gain. For DSSC applications, the N-doped working electrodes
are reported to improve the electron collection efficiency [41]. Our
preliminary results show that the conversion efficiency of a nonoptimised DSSC device using N-doped titania compare well with
those of a standard device. Work is in progress to study the transport properties of doped and undoped electrodes, in order to assess
their respective electronic performances also in different applications requiring porous semiconductor electrodes as hydrogen
generation, sensors, and others.
5.3. Development of nanocomposites for cultural heritage
preservation
In the last few years, nanoparticles and nanostructured materials have been applied to restoration and conservation of artworks.
The modulation of physical and chemical properties of a protective
coating can be obtained by a proper blending of the coating material with suitably chosen nanoparticles. In particular, nanosilica and
nanotitania were selected for their physical properties, such as the
improved water repellence.
For that reason, SiO2 and TiO2 nanoparticles with mean size
of about 15 nm, as produced by CO2 laser pyrolysis, were added
as nanometric fillers to two commercial products, largely used
for protective coatings, i.e. an acrylic resin and a silicon-based
polymer. The purity of the nanopowders and their low aggregation are considered important items to obtain homogeneous
nanocomposites that do not alter the colour of the artworks. In
fact the synthesis of nanocomposites to get a final homogenous
dispersion is a tricky step, depending on the chosen preparation
method, solvent and particle concentrations. Different solutions of
the polymers with dispersed silica and titania nanoparticles were
prepared and applied on the surface of two different litotypes,
very common in outdoor cultural heritage: white marble (statuary
and veined Carrara) and travertine [42]. Artificial ageing processes,
both in climatic chamber and in solar box, were carried out to
simulate real degradation processes in terms of photo-thermal
effects and mechanical damage.
The performances of the different nanocomposites were evaluated comparatively by means of several diagnostic techniques.
The results demonstrate that nanoparticles enhance the efficacy of
consolidating and protective materials because they induce substantial changes in the surface morphology of the coating layer and
hinder the physical damage observed during artificial weathering,
especially in alkylsiloxane products [42].
6. Conclusions and perspectives
We have shown the capability of laser pyrolysis to produce
nanoparticles with the composition and morphology adequate to
cope with challenging applications in different sectors of nanotechnology. In the past years, attention was mainly focused on the use
of nanoparticles in the field of structural nanoceramics, wear resistant coatings and functional nanomaterials for opto-electronics,
photonics and bio-imaging. More recently interest was attracted
by applications in the energetic sector. In this perspective, new
results have been reported here showing the possibility to tune
the final properties of SiC nanoparticles for the preparation of
nanofluids with improved thermal conductivity for enhanced thermal exchange. Moreover, we have addressed the development of
doped and pure TiO2 NPs for applications in photocatalysis and
DSSC electrodes fabrication. Finally, we have described preliminary
tests showing the promising performances of SiO2 nanoparticles prepared by laser pyrolysis for the realization of protective
nanocomposite coatings for cultural heritage preservation. This is
an emerging and important application field in which nanotechnology is expected to give a significant contribution.
Acknowledgments
We wish to thank Dr. R. Fantoni for helpful and valuable
discussions on the process of nanoparticle formation; Dr. F. Fabbri and Dr. F. Rondino for their contribution in different phases
of the experimental activity on the synthesis, processing and
characterization of nanoparticles; Dr. M. Carewska for thermal
gravimetric analysis and thermal processing; Dr. L. Caneve and
Dr. F. Persia for collaboration on the development of nanocomposites for cultural heritage preservation; Dr. F. D’Annibale
for cooperation in the research activities on nanofluid properties. Funding for our research came from EC funded Projects:
FP5-IST-SINERGIA, FP6-Life-Science-BONSAI and FP7-Large-ScaleCollaborative-HENIX (NanoHex) Project. We thank all the partners
involved in these Projects for their precious contribution to the
development of our activities.
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