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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