Chapter 6

213

Chapter-6

Synthesis, Characterization and Studies of Zinc

Orthophosphate & Cobalt Orthophosphate Nanoplatelets

A part of this work has been presented:

1. T. George, S. Joseph, S. Mathew, National Symposium on Current Trends in

Inorganic Chemistry, CTIC-04, CUSAT, Cochin, 15-17 March, 2004

2. T. George, S. Joseph, S. Mathew, Indo–Australian International Symposium

on Nanoscience and Nanotechnology, IISc, Bangalore, 31March–1April,

2006

3. T. George, S. Joseph, S. Mathew, International Symposium on Materials

Chemistry, ISMC-06, BARC, Mumbai, 4 –8 December, 2006

Chapter 6

214

6.1. Introduction

Phosphate ceramics are getting attention due to their variety of

applications in optical, electrical, prosthetics and structural fields as

fluorescent materials, dielectric substances, dental cements, metal surface

coatings, fuel cells, pigments [1-3]. They are also used in catalysis, ion

exchangers and in low thermal expansion ceramic materials [4-6].

Zinc phosphate is a non-toxic white inorganic pigment featuring

corrosion protection and adhesion capability. It is used as a flame retardant

and also as a chemically bonded ceramic material (CBC). Zinc phosphate is

the first bioceramic to be proposed in dental applications [7, 8]. In the color

televisions different phosphors are used, for their emission in frequency ranges

corresponding to each of the primary colors. The decay time of the phosphor is

vital in these applications, with the relevant time scale imposed for the

electron beam to sweep the phase of the tube. Zinc phosphate doped with

manganese is a well-known phosphor used in cathode tubes [9].

Generally violet colored cobalt pigments are called cobalt violet. First

developed in the early 19th century, cobalt violet was the primary permanent

violet pigment available. Cobalt violets range from deep to pale shades with

either a pink or blue hue. The first cobalt violets used were composed of

cobalt arsenate. This highly toxic compound is now rarely used. Instead most

current cobalt violets are non-toxic and are made from either cobalt phosphate,

or cobalt ammonium phosphate. Cobalt violets are used in paints [10].

To prepare metal phosphates, methods like molten salt flux synthesis

[11], sol-gel synthesis [12], thermolysis/flame pyrolysis of polymer matrix

based precursor solution synthesis [13], boron phosphate method [14],

chemical precipitation [15, 16], solid- state reaction [17], had been developed.

Chapter 6

215

One of the oldest techniques for the synthesis of nanoparticles is the

precipitation of products from bulk solutions [18-20]. In this investigation

simple, inexpensive and highly reproducible aqueous precipitation method

was used for the preparation of zinc orthophosphate and cobalt

orthophosphate. This pollution free method has the advantage of good

stoichiometric control and the production of ultra fine particles with high

purity in a relatively short processing time at lower temperatures with

improved compositional homogeneity. The concentration of the metal species

present in the initial reaction mixture has the largest effect on the overall

nanoparticle size.

6.2. Synthesis and characterization of Zn3 (PO4) 2. 4H2O

nanoplatelets

Nanosized powders of Zn3(PO4)2.4H2O were prepared by reacting

stoichiometric amounts of AR grade zinc nitrate (Zn(NO3)2.6H2O, 0.005M)

and disodium hydrogen phosphate (Na2HPO4.2H2O, 0.003M) in distilled

water. The precipitate was filtered, washed a number of times using distilled

water and dried in an oven at 100°C for 3 hours to get fine, soft and white

powders of zinc orthophosphate. Powder X-ray diffraction (XRD) patterns

were recorded on a RINT 2100 X-ray diffractometer with Ni filtered Cu K

radiation in the range 5o-120

o in 2θ steps of 0.02

o. This recording procedure

enabled us to detect even the weak peaks in our XRD pattern. Phases in the

sample were determined using quantitative phase analysis employing

Powdercell 2.4 program [21].

The specific surface area of zinc orthophosphate nanoparticles was

determined by the BET (Brunauer, Emmett and Teller) method with nitrogen

adsorption using Gemini Micromeritics surface area analyzer. The size and

Chapter 6

216

morphology of the nanoparticles were determined by Transmission Electron

Microscope (TEM) using a JEOL model 1200EX instrument operated at an

accelerating voltage of 120 kV. The sample was ultrasonically dispersed in

ethanol for several minutes prior to depositing onto the grid and allowed the

solvent to evaporate. The surface features were studied using Scanning

electron microscope (SEM Hitachi S -520). The chemical composition of the

prepared sample was studied using energy dispersive X-ray analysis system

attached to the scanning electron microscope. Thermogravimetric analysis

(TGA) and Differential thermal analysis (DTA) of the sample was carried out

in the temperature range from room temperature to 800°C using Schimadzu

DTG 60. The photoluminescence spectrum of pure zinc orthophosphate and

Mn doped zinc orthophosphates were taken using a Perkin Elmer LS-55

luminescence spectrometer (slit width 10 nm).

6.3. Results and discussion

6.3.1. Powder XRD studies of Zn3 (PO4)2. 4H2O

Figure 6.1 shows the powder X-ray diffraction patterns of zinc

orthophosphate nanopowders prepared by aqueous precipitation method. The

pattern reveals the formation of polycrystalline Zn3(PO4)2.4H2O and the phase

confirmed by comparing with JCPDS file [22].

Chapter 6

217

Figure 6.1 XRD pattern of Zn3(PO4)2.4H2O

The structure resembles that of the mineral ‘hopeite’. The unit cell of

the prepared Zn3(PO4)2 4H2O is having orthorhombic crystal structure with

a = 10.5749; b = 18.3768; c = 5.0398 and = = = 90° and the space group

symbol is Pnma (# 62) The average particle size of the nanocrystalline

samples, calculated from the full width at half maximum (FWHM) of major

peaks of the XRD patterns using Scherrer formula [23] is 30 nm.

6.3.2. Electron Microscopic Analysis of Zn3(PO4)2. 4H2O

The SEM micrograph (figure 6.2) and the TEM micrograph in (figure

6.3) shows the morphological features of zinc orthophosphate nanoparticles.

Chapter 6

218

Figure 6.2 SEM images of zinc orthophosphate nanoplatelets

It is clear from the SEM and TEM images that the particles are

aggregated in a peculiar way to form clusters of thin, transparent, platelets.

Figure 6.3. TEM images of zinc orthophosphate nanoplatelets

Chapter 6

219

The energy dispersive X-ray analysis spectrum of Zn3(PO4)2. 4H2O is shown

in figure 6.4.

Figure 6.4 EDX spectrum of Zn3(PO4)2. 4H2O

The EDX analysis (figure 6.4) reveals the presence of Zn, P and O in

appropriate ratios. Trace amount of Na from a secondary phase can also be

seen in the EDX spectrum even after repeated purification of the sample. In

order to quantify the secondary phase, we tried a quantitative phase analysis

by Reitveld refinement.

The Rietveld method is a powerful and relatively new method for

extracting detailed crystal structural information from X-ray and neutron

powder diffraction data since such structural details dictate many of the

physical and chemical attributes of materials. Since most materials of

technological interest are not available as single crystals but often only in

polycrystalline or powder form, the Rietveld method has become very

Chapter 6

220

important and is now widely used in all branches of science that deal with

materials at the atomic level [24]. The Rietveld analysis [25] was carried out

on crystalline (Zn3PO4)2.4H2O heated at 150 °C for 2 h (figure 6.5). The

structure of the mineral hopeite was first described by Liebau in 1965 [26].

This analysis was performed using the fullprof package assuming

Pnma space group for a ‘hopeite’ type orthorhombic structure. The tick marks

indicate the position of the all possible Bragg reflections from the structural

model shown in table 6. 1.

Figure 6. 5. Rietveld plot for crystalline Zn3PO4)2.4H2O powders heat treated

at 150 °C for 2 h in air atmosphere

The experimental and simulated intensity data (Ie and Is) are plotted as

open circles (o) and solid lines, respectively and I = Ie-Is. Zoomed 6-15° 2

region of the Rietveld plot for crystalline Zn3(PO4)2.4H2O powders heat

treated at 150 °C is shown in figure 6.6.

Chapter 6

221

Figure. 6.6. Zoomed 6-15° 2 region of the Rietveld plot for crystalline

Zn3PO4)2.4H2O

Circles are experimental points. Solid lines are sum due to zinc

orthophosphate and the secondary phase. Dotted lines in the zoomed 6-15° 2

region (figure 6.6) of refined plot correspond to the secondary phase identified

as sodium di zinc triphosphate nonahydrate. Here the precursors used were

zinc nitrate and disodium hydrogen phosphate in aqueous medium at room

temperature. The secondary phase sodium di zinc triphosphate nonahydrate

may also be formed along with zinc orthophosphate. The amount of secondary

phase calculated by quantitative phase estimation is 6 %.

Chapter 6

222

Table 6.1. Atomic positions and isotropic thermal parameters of atoms in zinc

orthophosphate.

Atomic positions and isotropic thermal parameters of atoms in zinc

orthophosphate obtained by the Rietveld refinement are shown in table 6.1.

The occupancy factor for Zn, P and O atoms obtained by this refinement are

full. Figure 6.7 illustrates the orthorhombic Zn3(PO4)2.4H2O.unit cell with

space group Pnma.

Atom X y z Occupancy Isotropic thermal

parameter (Å2)

O1 0.3046 0.0418 0.1348 1 1.3

O2 0.5307 0.0792 0.1471 1 0.9

O3 0.3678 0.1707 0.2237 1 1.8

O4 0.401 0.0865 0.5795 1 1.9

O5 0.1549 0.1674 0.6558 1 1.7

O6 0.8937 0.25 0.7615 1 0.5

O7 0.6121 0.25 0.3762 1 1

Zn1 0.7651 0.25 0.0681 1 1.7

Zn2 0.1433 0 0.2938 1 2

P 0.3955 0.0929 0.2673 1 1.2

Chapter 6

223

a

b

c

Zn2 P

O1

O2

O1

Zn1

O3

Zn2

P Zn2

O3 O3

P

O7 O2

O4

O7

O4

Zn2

O2

O3

O6

O1

Zn2

Zn1

O5

P

O1

O5 O6

O5

O5

O4

O4

Zn2

O2

O2

Zn2

O4

O4

O5

O5

O6 O5

O1

P

O5

Zn1

Zn2

O1

O6

O3

O2

Zn2

O4

O7

O4

O2 O7

P

O3 O3

Zn2 P

Zn2

O3

Zn1

O1

O2

O1

P Zn2

PowderCell 2 .0

Figure 6.7. Crystal structure of Zn3 (PO4)2. 4H2O

Red circles are oxygen atoms, pink circles are phosphorous atoms, and the

blue circles are Zn. Also note that water basis is replaced by oxygen atoms

because of the poor X-ray scattering power of hydrogen atom. The

phosphorous atoms are coordinated to the four oxygen atoms in a [PO4]3−

tetrahedral configuration (see the polyhedra). The Six-coordinate Zn2+

cations

in this unit cell is cis-bonded to two phosphate groups and to four O atoms of

four water molecules (two of which are located on mirror planes), forming a

framework structure. In addition, hydrogen bonds of the type O—H----O are

present throughout the crystal structure.

Chapter 6

224

6.3.3. Thermal analysis of Zn3 (PO4)2. 4H2O

The TGA and DTA curves are shown in figure 6.8. The TGA cuive

shows two distinct weight loss steps and the DTA curve shows two

Figure 6.8. TGA and DTA curves of the zinc phosphate nanoclusters.

endothermic peaks. The first weight loss step in the temperature range

33–215°C which was accompanied by an endothermic peak at 119.25°C and

the second weight loss step in the temperature range 219–440°C accompanied

by an endothermic peak at 300.81°C. The total weight loss in the two stages

was 14.65 %. Calculations show that the weight loss is equivalent to 4 units of

water of crystallization. The final formula of the prepared sample is assumed

to be Zn3(PO4)2 .4H2O.

Chapter 6

225

6.3.4. Photoluminescent study of Zn3 (PO4)2. 4H2O

Figure 6.9 shows the excitation and emission spectrum of pure zinc

orthophosphate and zinc orthophosphate doped with 3% of Mn2+

. Figure 6.9.

(a) is the excitation & emission spectrum of pure zinc orthophosphate.

Figure 6.9 (a) Excitation & Emission spectrum of pure zinc orthophosphate.

(b) Excitation & Emission Spectrum of Mn2+

doped zinc

orthophosphate

Chapter 6

226

Figure 6.9. (b) is the excitation & emission spectrum of Mn2+

doped

zinc orthophosphate. The room temperature photoluminescence studies show a

strong intense emission around 570 nm. It is seen that there is an increase in

the intensity of emission peak (PL) of zinc orthophosphate, when it is doped

with Mn2+

.

The experiment is performed with 1, 2 and 3% of Mn2+

doped samples.

But it is noticed that there is no substantial enhancement in the intensity of the

emission peaks when the Mn2+

concentration is increased further. Also there is

no shift in the peak position. The enhancement in the intensity of the emission

peak may be due to the 3d-3d transition of Mn2+

. The PLE

(photoluminescence excitation) spectra of pure and 3% Mn doped zinc

orthophosphate nanoparticles are also shown in figure 6.9. In doped

semiconducting materials the possible paths for the luminescence excitation of

the impurity ions include indirect excitation of the host lattice and direct

excitation of impurity ions. The maximum excitation peak is centered at 390

nm and it can be attributed to the transitions of the 3d5 multiplet states of

Mn2+

. Hence it is evident from the photoluminescence studies that Mn2+

ions

can effectively improve the luminescence of zinc phosphate [27, 28].

6.4. Synthesis and characterization of Co3 (PO4)2. 8 H2O

nanoplatelets

Fine powders of cobalt orthophosphate were synthesized by chemical

reaction between AR grade cobalt nitrate and disodium hydrogen phosphate.

The precipitates were centrifuged, filtered, washed a number of times using

distilled water followed by drying in a vacuum desiccator at room

temperature. The crystal structure and particle size were determined by

Chapter 6

227

powder X-ray diffraction using Cu Kα radiations and the surface morphology

by scanning electron microscope (SEM). The thermal behavior of the sample

was studied by thermogravimetric analysis (TGA) and differential thermal

analysis (DTA). The magnetic behavior of the sample was studied at various

temperatures from 296 K to 573 K using vibration sample magnetometer

(VSM). A typical sample was consolidated into pellet of diameter 10 mm and

thickness 1.9 mm by applying pressure using a hydraulic press. Dielectric

constant (ε') and ac electrical conductivity (σac) were obtained using HIOKI

3532 LCR Hitester, impedance analyzer at different frequencies from 100 Hz

to 1400 KHz over the temperature range from 303 K to 353 K.

6.5. Results and discussion

6.5.1. XRD studies of Co3 (PO4)2. 8 H2O

Figure 6.10 shows the XRD patterns of the as prepared powder

samples of nanocrystalline cobalt orthophosphate with reactant concentrations

Chapter 6

228

Figure 6.10. XRD patterns of cobalt phosphate nanoparticles

(a) 0.0015 M cobalt nitrate and 0.001 M disodium hydrogen phosphate, (b)

0.003 M cobalt nitrate and 0.002M disodium hydrogen phosphate, (c) 0.006 M

cobalt nitrate and 0.004 M disodium hydrogen phosphate and the samples are

named as C1, C2 and C3 respectively. X-raydiffraction pattern reveals that the

powder samples are well crystallized. The pattern agrees well with that of

Co3(PO4)2.8 H2O in the JCPDS card [29]. The crystals are monoclinic in

nature with space group C2/m. The average particle size of the samples

Chapter 6

229

calculated from the full width at half maximum (FWHM) of major peaks using

Scherrer formula is 24 nm for sample C1, 27 nm for sample C2, 30 nm for

sample C3 which shows an increase of particle size with increase of

concentration of the reactants.

6.5.2. Electron microscopic analysis of Co3 (PO4)2. 8 H2O

The SEM images of Co3 (PO4)2. 8 H2O (sample C3) in figure 6.11.

Figure 6.11. SEM image of cobalt orthophosphate nanoparticles

The SEM image shows that the flat crystals are in the form of plate-lets. The

thickness of the plate-lets is estimated to be of the order of 30 nm.

6.5.3. Thermal studies of Co3 (PO4)2. 8 H2O

Figure 6.12 show the TGA and DTA curve of the as synthesized cobalt

phosphate sample C1. The TGA curve shows a major weight loss in the

temperature range 125-180oC followed by a slow weight loss continuously till

Chapter 6

230

up to 500oC. This weight loss is due to the drying of the sample and also due

to the loss of water of crystallization. The DTA curve shows two endothermic

peaks at 162oC and 215

oC, which are attributed to the loss of the water of

crystallization in the sample. Calculations using the loss of weight from the

TGA curve agree with the presence of 8 units of water of crystallization in the

basic formula unit.

Figure 6.12. TGA and DTA curve of the as synthesized cobalt orthophosphate

6.5.4. Magnetic studies of Co3 (PO4)2. 8 H2O

Figure 6.13 indicates the variation of the molar magnetic susceptibility

(χ) with temperature of Co3 (PO4)2. 8 H2O (sample C1) from 296 K to 573 K

using VSM. The curve shows the nature of a paramagnetic sample displaying

the Curie-Weiss behavior [30]. At 296 K the value of molar susceptibility is

Chapter 6

231

0.0273 cgs units and at 473 K it is 0.012 cgs units. The literature value of

molar susceptibility of anhydrous Co3 (PO4)2 at 291 K is 0.02811 cgs units

which closely agrees to the above result [31].

280 300 320 340 360 380 400 420 440 460 480

0.010

0.012

0.014

0.016

0.018

0.020

0.022

0.024

0.026

0.028

Mol

ar s

usce

ptib

ility

(cgs

uni

ts)

Temperature K

Figure 6.13 Variation of the molar magnetic susceptibility (χ) with of

Co3 (PO4)2. 8H2O from 296 K to 573 K using VSM

6. 5.5. Dielectric studies of Co3 (PO4)2. 8 H2O

The variation of dielectric constant (ε') with log of frequency of the

applied field for temperatures from 303 K to 353 K of Co3 (PO4)2. 8 H2O

(sample C1) is shown in figure 6.14. It is found that the dielectric constant, for

Chapter 6

232

all temperatures, has high value at low frequencies, which decreases

2 3 4 5 6

-200

0

200

400

600

800

1000

1200

1400

1600

1800

Die

lect

ric c

onst

ant '

Log f

303 K

323 K

353 K

Figure 6.14 Variation of dielectric constant (ε') with log of frequency

of the applied field for temperatures from 303 K to 353 K of

Co3 (PO4)2. 8 H2O

continuously as frequency increases. For 303 K the value of dielectric

constant, ε' at 100 Hz is 861 which decreases continuously to 0.075 at 1400

KHz. For 323 K the corresponding values of ε' are 1224 and 1.4, and for 353

K the values are 1680 and 2.78 respectively. The variation of tan δ of

Co3 (PO4)2.8 H2O as a function of frequency and temperature is shown in

figure 6.15. At 303 K the value of tan δ is 4.15 at 100 Hz and 49.4 at 1400

KHz.

Chapter 6

233

2 3 4 5 6

0

10

20

30

40

50

tan

Log f

303 K

323 K

353 K

Figure 6.15 Variation of dielectric loss (tan δ) as a function of frequency of

Co3 (PO4)2. 8 H2O

At 323 K the corresponding values are 2.5 and 4.8 but at 353 K the values are

2 and 3.4 respectively. The dielectric behaviour of nanomaterials is primarily

due to different types of polarizations present in the material. Nanocrystalline

materials possess enormous number of interfaces and large number of defects

such as dangling bonds, vacancies and micropores present in these interfaces

can cause a change of positive or negative space charge distribution [32].

When an electric field is applied these space charges move and are trapped by

these defects resulting in the formation of dipole moments which is called

space charge polarization. Interfaces in nanostructured materials posses many

oxygen or nitrogen ion vacancies, which are equivalent to positive charges

giving dipole moments, which in an electric field will rotate giving a resultant

dipole moment in the direction of the applied field. This is called rotation

direction polarization. The high value of ε' at low frequencies is due to space

Chapter 6

234

charge polarization and rotation direction polarization. [33, 34]. As

temperature increases more and more dipoles will be oriented in the field

direction, which results in the high value of the dielectric constant. When

frequency increases the electron exchange will not follow the external field,

which lowers the values of dielectric constant. In nanophase materials, the

inhomogeneities present at the interfaces produces an absorption current

resulting in the dielectric loss [35].

2 3 4 5 6

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

ac c

ondu

ctiv

ity x

10

-4

Sm

-1

Log f

303 K

323 K

353 K

Figure 6.16 Variation of ac electrical conductivity (σac) as a function of

frequency and temperature of a typical sample (b).

The variation of ac electrical conductivity (σac) as a function of

frequency and temperature is shown in figure 6.16. At low frequencies σac

have lower values, which increase continuously up to 1400 KHz. The values

are shifted upwards as temperature increases. The maximum value of σac at

303 K is 2.9 x 10-4

Siemens.m-1

, while the corresponding value of σac at 353 K

Chapter 6

235

is 7.3 x 10-4

Siemens.m-1

. The low temperature electrical conductivity in

cobalt phosphate can be explained in terms of electron hopping and at high

temperature σac is controlled by thermally activated polaron hopping. Electron

hopping occurs between Co2+

and Co3+

sites. Small polaron formation takes

place in those materials whose conduction band belong to incomplete‘d’ or ‘f’

orbital [33]. In the present case cobalt being a transition metal with

incomplete‘d’ orbital, which is responsible for the formation of small

polarons. The sudden release of space charges accumulated at the grain

boundaries increases the conductivity at higher temperatures.

6.6. Conclusion

Nanoplatelets of zinc orthophosphate and cobalt orthophosphate are

prepared by aqueous precipitation method and characterized. Rietveld analysis

and TGA/DTA studies of zinc orthophosphate confirmed the formula as

Zn3 (PO4)2. 4H2O. The SEM and TEM images show that the particles are thin

transparent platelets with an average particle size of 30 nm. The specific

surface area for the sample calculated by the BET method is 90 m2g

-1 and

the pore volume of the sample is 0.026 cm3g

-1. Quantitative phase analysis

reveals the presence of 6 secondary phase consisting of sodium

di-zinc-nonahydrate. The photoluminescence spectrum shows an increase in

the intensity of emission peak of zinc phosphate when it is doped with Mn2+

and hence it can be used as a phosphor in cathode tubes.

The average particle size of cobalt orthophosphate estimated from

XRD results ranges from 24-30 nm. It is seen that the particle size increases

with increase of concentration of the reactants. The SEM image shows that the

particles are thin transparent platelets. The VSM study reveals that the molar

Chapter 6

236

magnetic susceptibility of cobalt orthophosphate has a decreasing trend with

increase of temperature. The dielectric constant shows a decreasing trend as

frequency increases whereas the ac conductivity increases with temperature

and frequency.

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