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CHAPTER 3
SYNTHESIS, SIZE, MORPHOLOGY AND STRUCTURE
3.1 Introduction
There are various methods which have been used to produce
nanoparticles. Chemical method is one of the widely used techniques due to
the advantage of improved compositional homogeneity since the reactant
constituents are mixed at a molecular level. Moreover, it is a simple method.
Nanophase materials synthesised have to be characterised by the particle size,
morphology and structure with which it can be distinguished from bulk
crystalline materials. Transmission electron microscopy and X-ray diffraction
are generally used to determine the size and structure of nanoparticles.
Morphology of the particles can be studied using scanning electron microscopy.
The author used chemical methods for the synthesis of nanoparticles and TEM,
SEM, XRD techniques for the characterisation. This chapter deals with the
preparation and characterisation of nanoparticles used in the present study.
3.2 Synthesis of Nanoparticles
Nanoparticles of Ag3P04, FeP04 and ZnFe204 for three reactant
concentrations each were prepared. Ag3P04 was prepared by chemical
precipitation method.',2 Fel'04 was prepared from a polymer matrix based
precursor solution.' ZnFe204 was prepared by co-precipitation
All chemicals used were of ianalytical grade and no capping agents were used
for the synthesis.
3.2.1 Synthesis of ADPO~ Nanoparticles
For the synthesis of Ag3P04 nanoparticles the materials used were
AgN03 and Na2HP04. For colloidal precipitation, the concentration of the
reactants should be less than 0.05M. Nanoparticles of Ag3P04 were prepared
for three reactant concentrations (0.015M, 0.01M and 0.005M). For the
preparation of 0.015 M Ag3P04, lOml aqueous solution of 0.15M Na2HP04
and 30ml 0.15 M AgN03 is dropped to 60ml distilled water in to a conical flask
stirring vigorously using a magnetic stirrer at room temperature.
The equation of the reaction is Na2HP04 + 3 AgN03 + Ag3P04 +
2NaN03 + HN03. Similarly the other two concentrations of Ag3P04
nanoparticles were prepared. The yellow precipitate of Ag3P04 was separated,
washed repeatedly using distilled water, filtered, dried in an oven at 1 0 0 ' ~ for
2 hours. Ag3P04 is photoserisitive and hence the preparation was done in
darkness.
3.2.2 Synthesis of FeP04 Nanoparticles
The precursor solution constituted of Ammonium dihydrogen phosphate
(lmolel500ml H20) and Fe(N03)3.9H20 (lmolelllitre H20) are mixed together
and HN03 is added drop by drop so that pH = 1. Then to this solution 3 moles
of sucrose of fonnula weight 342 gm and a very small quantity of PVA 2.2 gm
in water (aqueous) are added and mixed together so that the total volume of the
solution is 3 litre. 300ml total volume solution was prepared by taking 10%
weight of each reacting compc~nents for making 1M FePO,. This solution was
then evaporated to a viscous liquid with the evolution of brown fumes of the
decomposed nitrates. After complete evaporation of the precursor solution, a
fluffy voluminous carbonaceous dry mass is left behind. This was crushed to
make a powder called precursor powder and ground well and was thermolysed
in the furnace upto a maximum temperature of 6 5 0 " ~ , so that a pinkish white
powder of FeP04 was formed. Similarly 0.1M and 0.02M FeP04 were
prepared by taking the molarity of reacting components 0.1M and 0.02M. The
role of each reacting component is given in reference 3.
3.2.3 Synthesis of ZnFe204 Nanoparticles
Zinc ferrite nanoparticles were prepared for 3 reactant concentrations
(O.IM, 0.01M and 0.002M). 0.1M ZnFe204 was prepared by adding 100ml 0.1M
aqueous solution of Zn(N03)2 to lOOml 0.2M aqueous solution of Fe(NO,), in a
conical flask under constant stining using a magnetic stirrer. While stining this
mixture, 25% liquor ammonia (NH40H) was added until the pH was in between
9 and 11 at room temperature. 'The precipitate formed was washed several times
w~th distilled water, filtered, dried in an oven at 9 0 ' ~ for 3 hours. The powder was
ground well and annealed at different temperatures (150°c, 300°c, 500°c, 700°c,
850 '~) and used for different studies. Similarly 0.01M ZnFe204 was prepared by
taking 0.01M aqueous solutiori of Zn(NO,), and 0.02M aqueous solution of
FeWO,),. For 0.002M ZnFe204, 0.002M aqueous solution of Zn(N03)2 and
0.004M aqueous solution of Fe(NO,), was used. The equation of reaction is
Zn(N0,)2 + 2Fe(NO& + 8NH4OH+ZnFe2O4+8NH4NO, + 4H20.
3.3 Preparation of Pellets for the Present Study
Pellets of the nanoparticles were made using a die and by applying a
pressure of O.5GPa in a hydraulic press. The pellets were 12mm in diameter and
1-2mm in thickness. For electrical measurements, both faces of the pellets were
coated with air-drymg silver paste and test leads were attached to each face.
3.4 TEM Study of Nanoparticles
TEM imaging of the powder samples is the most direct and convenient
method to see and analyse the structure of aggregates and to determine the size
of particles. TEM imaging was carried out in a Philips CM-200-Analytical
transmission electron microscope working at 120kV. The powder samples were
supported on conventional carl~on-coated film on copper grid. Particle size of
the samples was directly found out and structure of aggregates was analysed
using TEM image, in the present study.
3.4.1 TEM Study of Ag3P04 Nanoparticles
TEM image of Ag,P04 nanoparticles of O.Ol5M and O.OO5M-reactant
concentration and the corresponding electron diffraction pattern are shown in
fig. 3.1 and 3.2 respectively. The figures show that the clusters are in the form
of fractal aggregates, which may be fonned due to diffusion-limited
aggregation (DLA) or reaction-limited aggregation (RLA).' The fractal
aggregates of 0.015M (Fig. 3.la) is more open and less dense falling in the
DLA regime whereas 0.005M (Fig. 3.2a) is denser falling in the RLA
In the DLA regime, the aggregation rate is maximum and the reaction rate is
solely determined by the time needed for the clusters to encounter each other by
diffusion."ence, in the case of 0.015M Ag3P04 sample, which is more
concentrated, the clusters find it easier to come close to each other take less
time and the DLA regime results. For 0.005M sample (fig. 3.2) which is less
concentrated, the slow process, RLA regime - in which the cluster-cluster
repulsion has to be overcome by thermal activation process
(a) (b) *
Fig. 3.1: TEM image of 0.015M Ag,P04(a) and the corresponding electron diffractioil pattern (b).
Fig. 3.2: TEM image of 0.005 M Ag3P04 (a) and the corresponding electron diffraction pattern (b).
From TEM image, the particle size of low concentrated sample
(0.005M) is - 15nm whereas that of high concentrated sample (0.01 5M) is - 20nm. A significant feature noticed in the TEM image analysis of the two
sa~nples is that the reactant concentration shows considerable influence on the
morphology and size of nanoparticles of Ag3P04. The well defined selected
area electron diffraction (SAED) pattern shows spotty rings characteristic of
polycrystalline pattern, suggesting that the as prepared Ag3P04 powder is
nano~r~sta1line.l Discontinuous rings with spots indicate that the particles are
made of rather bigger crystallites.
3.4.2 TEM Study of FeP04 Nanoparticles
TEM images of FePO, nanoparticles of reactant concentration 1M and
0.02M are shown in fig. 3.3 and 3.4 respectively. From fig. 3.3, the particle
* size observed is - 90nm and froin fig. 3.4, the particle size is - 80nm. The
particles are almost spherical but highly agglomerated. It is reported that for
some ferric phosphate, the particles may fuse under an intense electron beam in
transmission electron m i c r o s ~ o ~ e . ~ This inay be a reason for the agglomeration
of the particles. The method employed for the preparation of nanophase FeP04
may also have a role in aggregation of particles.
Fig. 3.3: TEM image of FeP04 nanoparticles (TM)
Fig. 3.4: TEM image of FeP04 nanoparticles (0.02M)
3.4.3 TEM Study of ZnFe204 Na~loparticles
Fig. 3.5 shows the TEM image of 0.002M ZiiFe204 annealed at 150" for
2 hours and the corresponding electroil diffraction pattern. Fig 3.6 shows the TEM
image of 0.1 M ZnFe204 annealed at 1 5 0 ' ~ for 2 hours and the correspondiilg
elect~on diffraction pattern. Both figure shows that the particles are not aggregated
(dispersed), having almost uniform size distribution and are spherical. From TEM
image, the particle size of O.1M reactant concentration sample is 5nm and that of
0.002M sample is 4nm. Electron diffraction pattern shows circular rings which are
characteristic of nanocrystalline materials. Discontinuous rings with spots indicate
that the particles are made of bigger crystallites (fig 3.6b). We can observe five * clear rings in Fig. 3.6b SAED pattern, which are attributed to reflections fiom five
hkl planes of the spinel structure of Z I I F ~ ~ O ~ . ~ This shows that ZnFe204 particles
in the as prepared form (annealed only at 150'~) are crystalline. With a reduction
in the grain size, the nunlber of the particles illuminated by the electron beam
increases. The number of diffraction spots then increases correspondingly. Below a
critical size, the pafierns form a series of rings, veiy different from those of the
bulk state. This is one of the unique features of nanophase materials.I0
Fig. 3.5: TEM image and the corresponding electron diffraction pattern of 0.002M ZnFe204 (in the as prepared form)
Fig. 3.6: TEM image and the corresponding electron diffraction pattern of 0.1 M ZnFe204 (in the as prepared form)
3.5 SEM Study of Nanoparticles
The morphology and the random network connection of ,nanoparticle
aggregates have a considerable role in the transport propel-ties and mechanical
restonng force.6 SEM micrographs were taken to study the morphological features
of powdered samples of Ag3P04 (0.0 1 M) and FeP04 (0.1M). The micrographs
were taken using Philips electron scanning electron microscope XL 30.
Fig. 3.7 sl~ows the SEM image of Ag3P04 (0.01M) nanopaticles. From the
figure it is observed that the clusters are in the form of fractal agg-egatesl or in the
form of a coral. Each particle is oval shaped and the aggregate as a whole has a
fluffy nature with coilsiderable porosity. Since the material is porous, having large
volurne of interface, that will affcct the dielectric behaviour of nanophase Ag3P04.
Fig. 3.7: SEM image of 0.005 M Ag3P04
3.5.2 FeP04 Nanoparticles
Fig. 3.8 shows the SEM image of FcPO, (O.1M) nanoparticles. The
particles are highly aggregatcd, almost spherical but with non-uniform size
distribution. High temperature method of preparation (650 '~) may be a reason *
for the agglomeration of the particles. Compared with Ag3P04 SEM image, it is
more dense with less porosity.
Fig. 3.8: SEM image of IM FeP04
3.6 X-ray Diffraction Studies of Nanoparticles
X-ray diffraction studies were extensively carried out to determine the
size and structure of nanoparticles of Ag3P04, FeP04 and ZnFe204. XRD
profiles were taken by Philips 1710 PW powder X-ray diffractometer using Cu
K, radiation over a wide range of Bragg angles fitted with nickel filter. The
diffraction peaks so obtained are compared wit11 the X-ray powder data file
published by the joint committee on powder diffraction standards (JCPDS).
The particle size was calculated from the line broadening of the diffraction lines
using Scherrer fonnula. ' '
3.6.1 Ag3P04 Nanoparticles
Nanoparticles of Ag3P04 for three reactant concentrations (O.OlSM,
0.01M and O.OO5M) were prepared in the powder form and the corresponding
XRD patterns are shown in fig. 3.9, 3.10 and 3.1 1 respectively. To understand
the crystal structure of Ag3P04 nanoparticles, 'd' values were calculated for al l
three samples and are presented in table 3.1 along with the standard JCPDS
data (Card No: 6-505) of cubic Ag3P04. The peaks corresponding to different
crystallographic planes against an almost flat base line suggest the formation of
polycrystalline compounds.'2 When the molar concentration is changed, there is
no significant change in the crystalline nature as is evident from XRD patterns.
But 'd ' values of nanoparticles are found to be smaller than the bulk values. It
indicates a contraction of the lattice for nanoparticles of Ag,PO, compared to
its bulk. Lattice expansion ",I4 and lattice contraction for the nanoparticles
have been reported by many authors.
Table 3.1: Peaks observed in the XRD patterns of nanoparticle Ag3P04 (Cubic)
Fig. 3.9: XRD pattern of 0.015M Ag3P04
h k l
1 1 0
d values
JCPDS Observed
' 3.85 3.0016 1 3.0031
0.015M
3.0036
2.6896
2.4545
2.124
1.902 1
1.5037
1.3442
2.73
2.326
1 4.45
2 0 0
2 2 0
3 1 1
2 2 2
4 0 0
4 2 2
4 4 0
0.01M
4.256 4.2499
2.6884
2.4545
0.005M
4.2469
2.6916
2.4591
2.227
1.931
1.576
1.364
2.1275
1.9015
1.5028
1.344
2.126
1.9026
1.5035
1.3414
Fig. 3.10: XRD pattem of 0.01M Ag3P04
Fig. 3.11: pattern of 0.005M Ag3P0,
Lattice contraction in sniall particles is a real physical phenomenon
associated with surface properties of the clusters. The surface atoms will be
in a strained condition due to the extra surface energy they possess." This
may cause a contraction of the lattice without drastic change in the crystal
structure. For Ag3P04 nanoparticles, the line broadening in XRD pattern is
found to be small. The particle size determined by Schemer method is found
to be in the range of 40-60nm for the three samples studied. From XRD
pattern, the particle size variation with reactant concentration is not so
significant as was in TEM.
3.6.2 FeP04 Nanoparticles
XRD patterns for nanoparticle FeP04 for three reactant concentration
(IM, 0.1M and 0.02M) are shown in fig. 3.12. The patterns suggest the
formation of single-phase crystalline compounds (JCPDS, Card No. 29-715).
The patterns appear to be the same for all molarities and the crystallite size
calculated using Schemer formula is found to be in between 40 and 50nm.
There is only slight variation in crystalline size with molarity. It is found
that as reactant concentration increases the crystallite size increases by 2-
3rim. From XRD pattern, it is observed that the peak intensity and the
crystallinity increase with reactant concentration. The 'd' values were
calculated for two samples anti are presented in table 3.2 along with the
standard JCPDS data of hexagonal FeP04. The 'd' values of 0.02M sample
are greater than 0.1M sample indicating a lattice expansion when the particle
size decreases.
Table 3.2: Peaks observed in the XRD patterns of nanoparticle FeP04 (Hexagonal)
d values
Fig. 3.12: XRD pattern of FeP04 nanoparticles for lM, 0.1M and 0.02M concentrations
/ JCI'DS h k l Observed
0.1M 0.02M
3.6.3 ZnFe204 Nanoparticles
Fig. 3.13 shows the XRD pattern of ZnFe204 nanoparticles (0.1M)
annealed at different temperatures. The formation of single phase cubic spinel
ZnFe204 was confinned by the XRD pattern (JCPDS, Card No: 22-1012). It
reveals that the increase of annealing temperature yields the sharpness of the
peaks verifying the increase of the grain size with temperature. Table 3.3 gives
the diameters of the grains calculated using Scherrer method. The particle size
of 0.1M sample annealed at 1 5 0 ' ~ is found to be 4nm whereas that with TEM
imaging is 5nm. Fig. 3.14 shows the XRD pattern of ZnFe204 nanoparticles
annealed at 3 0 0 ' ~ for three reactant concentrations (O.lM, 0.01M and 0.002M).
Table 3.4 shows the variation of particle size with reactant concentration using
XRD. It is found that the reactant concentration has only a minor role in
varying the particle size compared with the effect of annealing temperature.
Clear XRD peaks are found to be absent in ZnFe20s particles annealed at low
temperatures. But TEM image shows diffraction rings characteristic of
nanocrystallites. That means as prepared particles consist of microcrystallites
that were not detected in X-ray diffraction study.
Table3.3: Particle size determined for 0.1M ZnFe204 particles annealed at different temperatures using Schemer formula
Particle size from XRD (nm)
4
6
500°C 12
700°C 2 1
8.50'~ 28
Table 3.4: Particle size determined for ZnFe204particles annealed at 3 0 0 ' ~ using Schemer formula
Fig. 3.13: XRD pattern of %nFe204 nanoparticles (0.1M) annealed at different temperatures
Reactant concentration (M)
0.002
Particle size from XRD (nm)
5.4
5.5
6
Fig. 3.14: XRD pattern of Znl:e204 nanoparticles (O.lM, 0.01M and 0.002M) annealed at 3 0 0 ~ ~ .
Variation of Particle Size with Reactant Concentration
Precipitation of a solid [tom a solution is a common technique for the
synthesis of fine particles. The general procedure involves reactions in aqueous
or non-aqueous solutions containing the soluble or suspended salts. Once the
solution becomes supersaturated with the product, the precipitate is formed by
either homogeneous of heterogeneous nucleation. The formation of a stable
material without the presence of foreign species is referred to as homogeneous
nucleation. The growth of the nuclei alter formation usually proceeds by
diffusion, in which case concentration gradients and reaction temperatures are
very important in determining the growth rate of the particles, for example, to
form monodispersed particles. For instance, to prepare unagglomerated
particles with a very narrow size distribution all the nuclei must form at nearly
the same time and subsequent growth must occur without further nucleation or
agglomeration. la
In general, the particle size and particle size distribution, the physical
properties such as crystallinity and crystal structure, and the degree of
dispersion can be affected by reaction kinetics. In addition, the concentration
of reactants, the reaction temperature, the pH and the order of addition of
reactants to the solution are also important. Thus control of chemical
homogeneity and stoichiometry requires a very careful control of reaction
conditions.'*
In the case of colloidal metal phosphates, it was reported that depending
on the concentration of the reacting components, duration and temperature of
heating, the precipitated particles varied in size, shape and uniformity.'9
Based on the above references, it was felt that it would be interesting to
study the effect of reactant concentration on particle size and hence on physical
properties. So studies on nanoparticles of Ag,P04, FeP04 and ZnFe204 were
carried out for three reactant concentrations each. TEM images have shown
variation in particle size with reactant concentration. But from XRD pattern,
the variation in particle size with reactant concentration was not so significant
as expected. Under experimental accuracy the variation may not be noticeable.
Though the variation in partic.le size is negligible or small from XRD, we can
expect large variation in physical properties since lnm size change may
introduce a considerable change in the number of surface atoms with lower
coordination and broken exchange bonds.
3.7 Conclusion
Nanoparticle Ag3P04, FeP04 and ZnFe204 were prepared by chemical
methods for three reactant concentrations each. The size and crystal structure
of these particles were studied using TEM and XRD. Morphology was studied
using SEM. TEM image has shown variation in particle size with reactant
concentration whereas with XllD the variation was negligible or very small.
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