PREPARATION OF SOME FUNCTIONAL MATERIALS
NANOPARTICLES SOL IN AQUEOUS MEDIA FOR SIZE
MEASUREMENTS BY DYNAMIC LIGHT SCATTERING
S. Estemirova, N. Pechishcheva, G. Kozhina, K. Vorontsova,
N. Nemytova
Institute of metallurgy of UD RAS, Yekaterinburg, Russian Federation
Introduction
Currently, development of nanoscale functional materials
production technologies is a promising direction. Such materials include
rare earth manganites and based on them substitutional solid solutions.
Over the past 20 years, they have attracted much attention of researchers
because of their unique electrical, magnetic, and magnetoresistive
properties [1, 2]. Owing to the correlation of the structural and
physicochemical properties manganites have become a model object for
theoretical concepts. Apart from fundamental research interests such
materials can be used in many areas of industry. These include, first of
all, microelectronics, Environmentally Friendly Energy (replacement of
primary energy sources by renewable ones - solid oxide fuel cells [3, 4]).
Nanoscale lanthanide manganites are actively developing for the
production of catalysts, sensors of carbon monoxide et al. [5].
Most of the practical applications require the development of the
manganites fine powders production technology [6, 7]. Physicochemical properties of ultrafine materials may differ significantly from
the properties of the bulk samples. Intense mechanical activation by
grinding, large specific surface energy, the small size of the
morphological elements contributes to the formation of non-equilibrium
states. Because due to not compensated bonds of a large proportion of
atoms in the surface layers having mainly neighbors on one side, broken
symmetry in the distribution of forces acting on each of them. To
achieve stability, technologies for producing materials with specified
properties would involve the control both the composition and grain
size. Rapid techniques of determining the size of the grains are very
important. One of the problems of analyzing nanometer-sized particles is
their tendency to agglomerate [8].
Another promising nanoscale material is silver nanoparticles (AgNPs). They are used in power industry, as catalyst, as antimicrobial
agent. Been stabilized in aqueous media they can be used particularly for
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certified reference material (CRM) making for quality control of
nanoproducts by light scattering methods. The hydrosols have good
stability; they typically may have such certified characteristics as
particles size, silver concentration, maxima location and intensity of the
plasmon resonance band. In the paper [9] stabilization of Ag-NPs in cyclodextrin (-CD) aqueous solution for CRM preparation have been
described, but they haven’t particles size as certified characterictic.
The aim of the present work is to develop methods of express
estimation of grain size of the pure and substituted rare earth elements
manganites solid solutions and Ag-NPs/-CD hydrosol by dynamic light
scattering (DLS) method. Much attention has been paid to the methods
of sample preparation and dispersion liquid pre-selection.
Experimental
In the present work we used Zetasizer Nano S90 (Malvern)
dynamic light scattering analyzer. Structurally DLS analyser consists of
a measuring unit with the sample compartment, a set of cuvettes and
computer. Scattered light registered angle is 90º, laser power is 50 mW
at a wavelength of 532 nm. Metrological characteristics of the analyzer:
measurement range of particle size - from 10 to 5000 nm, the limits of
permissible relative error of particle size measurement - ± 10%. The
DLS method consists in determining the size of the dispersed particles in
liquid based on diffusion coefficient determined by the analysis of the
characteristic intensity fluctuations time of the light scattered by the
particles. Particles in suspension, in probing them with a laser beam
scatter light, which is going at a certain angle and registered by the
photodetector. Fluctuations of the scattering intensity arising from the
Brownian motion of the particles are analyzed by a correlator. On the
basis of the correlation function containing information about the
diffusion coefficient particle size is calculated by software. The
measurement results are presented on the screen in both numerical and
graphical form.
Measurement of the particles size using analyzer Zetasizer Nano
S90 requires a knowledge of the refractive index (n), dynamic viscosity
(), specific density () of the dispersion liquid. Measurement of the
refractive index was performed using a refractometer IRF-454-B-2M,
kinematic viscosity - using a capillary viscometer VPZh-1m; to measure
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the density we used pycnometry method. The dynamic viscosity is
calculated as =.. For example 0.2% solution of benzalkonium
chloride (cationic surfactant) has following characteristics at 25 ºC: n =
1.3325,  = 997 kg / m 3,  = 0.001014 Pa·s.
Results and discussion
Manganites. As objects of study we used samples of rare earth
elements manganites solid solutions Nd(Gd)xCa1-xMnO3± (х=0.00,
0.10). Samples were synthesized by ceramic technology from rare earth
oxides Nd2O3 (Nd-E brand), Gd2O3 (GdO-D brand), Mn2O3 (SigmaAldrich, > 99% of the main substance) and CaCO3 (reagent grade).
According to X-ray data (Shimadzu XRD-7000 diffractometer, CuKradiation), all the samples were single-phase and had a perovskite
orthorhombic structure (space group Pnma). To obtain ultrafine powders
all the synthesized samples were subjected to mechanical activation by
dry grinding in high-energy planetary mill Pulverizette 7 premium line
(Fritsch) with a tungsten carbide headset, the rotational speed of the
grinding jars was 800 rev/min. Then, the powder was frayed under
ethanol during 10 min.
At first we used solution of the anionic surfactant as dispersion
liquid. It was assumed that during the mechanical action of oxygen
leaves the sites of the crystal lattice of the surface layers, after which the
surface may have a slight positive charge. For measurement 2% aqueous
solution of sodium dodecyl sulfate (C12H25SO4Na) was prepared,
manganite samples were added and mixed. Large scatter in the data after
the first measurements could be the result of the presence of particle
agglomerates. Ultrasonic treatment of the suspension for 1 hour
(Sonopuls mini20 device, Bandelin, ultrasonic frequency 30 kHz,
ultrasonic sensor MS 2.5) reduced the scatter of the three measurements
results. However (see Fig. 1a) it was still unsatisfactorily large.
Further measurements were carried out in 0.2% solution of
benzalkonium chloride. In three parallel experiments for each
composition (see Fig. 1b) the results have a good reproducibility.
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Fig. 1. Results of measurements of Nd(Gd)xCa1-xMnO3± (х=0.00, 0.10) solid
solution samples particles size (3 parallel measurements): а – dispersed
in anionic surfactant (C21H38ClN); b – dispersed in cationic surfactant
(C21H38ClN).
From this it follows that the surface of the grain after mechanical
activation has a slight negative charge. Possible cause of the charge may
be adsorption of air gazes and moisture during mechanical activation,
which, after their interaction with the elements of the main phase,
formed a new metastable phase on the surface of the grains. Small
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amounts of these phases as well as the small size of the crystallites of
both surface and the main phase do not allow detecting the impurities by
X-ray diffraction method.
The reason for this is that a small grain size results in strong
broadening of the diffraction maxima at small angles and in their
complete disappearance at large angles.
In the table 1 results of DLS measurements of the particles size
with benzalkonium chloride as dispersant and CSR size of the
investigated manganites are presented. One can see that CSR data in ~ 4
times less than size from DLS measurements.
Table 1. Microstructure parameters of the
Nd(Gd)xCa1-xMnO3± (х=0.00, 0.10) solid solution powders
Composition
NdMnO3±
Nd0.9Ca0.1MnO3±
GdMnO3±
Gd0.9Ca0.1MnO3±
Particles sizes in maxima of
distribution, nm
148
138
158
148
CSR size, nm
30
36
53
30
Probable cause of the difference between the two different
methods results may not only be large defective surface of the grains,
but also the existence of small agglomerates in the aqueous media,
which can not be destroyed by sonication. In addition, the lack of DLS
particles size measurement is the formation of a shell of the stabilizer on
the particle surface, which also increases its size. Shed light on the true
size of the powder particles can further studies, for example, by electron
microscopy, which, however, is rather time-consuming. However,
studies show sufficiently acceptable results, especially taking into
account simplicity and rapidity of the DLS procedure. It is possible that
results could be improved, for example, increasing the time of
sonication, and conducting a search for the more suitable dispersion
liquid.
Ag-NPs/-CD. Although hydrosols of Ag-Nps (1.310-4 М of
silver) in alkaline (pH=10.5) solution of -CD after microwave
treatment is stable enough (for 6 month) to be silver concentration and
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plasmon resonance band certified reference material, measurements of
the sol particles size by DLS method did not give satisfactory results.
One can see in Fig. 2, that particles size measured 3 times in 3 min are
very different.
Fig.2. Results of measurements of Ag-Nps in -CD not filtered sol samples
particles size (3 parallel measurements).
It has been suggested that this may be caused by agglomeration of
cyclodextrin molecules. Therefore, filtration of cyclodextrin solution
was conducted using Millipore membrane filters (mixed cellulose esters,
Hydrophilic, 0.05 µm) before synthesis and after synthesis of Ag-NPs/CD.
In addition, we carried out multifactorial experiment, varying pH,
silver concentration, ratio Ag/-CD concentration, time of the
microwave treatment, stabilization time after preparation. Optimal
parameters have been found: pH=10.0, 1.0 M Ag, Ag/-CD =10, MW
treatment time – 80 s, stabilization time – 4 days. As a result of the
research we have achieved stability of the studied hydrosols for 4
months. Fig.3 is demonstrating results of 3 consecutive DLS
measurements of Ag-NPs/-CD optimal composition hydrosol.
Together with measurements of our sol particle size we have
reproduced a certified Ag-NPs size of Russian CRM 9967 and
satisfactory results have been obtained: certified value of hydrodynamic
diameter is 69.5  3 nm, found value is 72.1  0.07 nm.
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Fig.3. Results of measurements of Ag-Nps in -CD filtered sol samples
particles size (3 parallel measurements).
Thus, we have obtained a hydrosol having a mean particle size
18.6 ± 0.6 nm (n = 10) which can be used as material for CRM
preparation. It should be noted that DLS size in this case also is greater
than gives another method, namely, scanning microscopy (10 nm is
obtained for dried in vacuum particles on carbon adhesive tape with
Auriga Carl Zeiss microscope) Probably microscopy gives only the size
of the silver core composite particles.
Conclusion
The possibility of DLS using in aqueous media was shown to
determine the particle size of powders of perovskite manganite
Nd(Gd)xCa1-xMnO3± , treated by grinding in a high-energy planetary
ball mill. As the dispersion liquid 0.2% aqueous solution of
benzalkonium chloride (cationic surfactant) was used. Sonication of the
suspension was used to break up agglomerates. Method of pretreatment
of the Ag-NPs/-CD sol also was developed for reproducible DLS
measurements. The results of measurements of size distribution for
manganites and Ag-NPs are compared with those obtained by the other
methods. DLS gives higher results, probably caused by stabilizer layers,
but it is good for quick size estimation. Ag-NPs/-CD sol obtained in the
present work may be used as source for stable certified reference
material preparation.
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Acknowledgments
This work was financially supported by Presidium of UD RAS,
project № 14-3-NP-143.
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