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CHAPTER 3: SYNTHESIS OF ZINC OXIDE NANOPARTICLES
3.1 INTRODUCTION
Nanomaterials find wide range of applications owing to their exciting physical, chemical
and catalytic properties [1]. The two approaches for nanomaterials synthesis are (i) top
down and (ii) bottom up approaches (Figure 3.1). Top down approach involves synthesis
of nanomaterials from bulk materials through size reduction using mechanical methods
such as mechanical milling [2,3] (Figure 3.1). Bottom up approach involves synthesis of
nanomaterials from the self-assembly of their constituent atoms into larger, organized
systems using chemical or physical methods (Figure 3.1). Bottom up approaches have
been widely used by the researchers due to advantages like reproducibility, relative ease
of scale up and high degree of control on morphology.
Nanomaterial synthesis by chemical methods such as precipitation, sol-gel, hydrothermal,
solvothermal techniques have been widely practiced. In the present work, room
temperature chemical precipitation method has been adopted for synthesis of ZnO
nanoparticles. This method has its own advantages such as
(i) Low processing cost
(ii) Isothermal & ambient temperature reaction
(iii) High yield
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Figure 3.1. Strategies of nanomaterial synthesis
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3.2 MATERIALS AND METHODS
3.2.1 Materials
Zinc nitrate hexahydrate (Laboratory grade) and ammonium carbonate (Laboratory
grade) were purchased from Merck, India and used as such without any further
purification. Deionized water has been used for the experiments.
3.2.2 Synthesis of ZnO nanoparticles
Zinc oxide nanoparticles were synthesized by chemical precipitation method using zinc
nitrate hexahydrate as precursor and ammonium carbonate as the reaction partner.
Equimolar solutions of the reactants were used. Drop-wise addition of zinc nitrate
solution to ammonium carbonate solution under continuous stirring resulted in
precipitation of white solid mass.
The white precipitate was collected by filtration and washed using deionized water
several times in order to remove the unreacted soluble reactants. The filtrate was then
washed with methanol to remove any organic impurities, if present and dried at 80 °C for
5 h [4].
The dried powder was annealed at high temperature in a muffled furnace for 2 hours in
order to facilitate the conversion of Zinc carbonate to Zinc oxide and improve the
crystallinity of ZnO powder. The optimum annealing temperature was identified by
investigating the morphology of ZnO powders annealed at different temperatures over a
temperature range of 100-800 °C.
Effective mixing of reactants is a requirement to achieve uniformity in morphology of
nanomaterials prepared through chemical precipitation method. Any unmixed areas in the
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reaction vessel is likely to result in formation of non-uniform structures. Hence efforts
were made to identify simple methods that promote uniform mixing.
Figure 3.2 shows the schematic representation of the ZnO synthesis and characterization
procedures.
Figure 3.2. Overview of ZnO synthesis and characterization procedures.
3.2.3 Characterization of synthesized ZnO nanoparticles
3.2.3.1 Thermal analysis
Thermal analysis of the as-synthesized powder was carried out using a thermogravimetric
analyzer (SDT Q600, TA instruments, USA). About 2 mg of the sample was placed in an
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alumina pan and subjected to controlled temperature increase from ambient temperature
to 1000 °C at a heating rate of 10 °C/min and the weight loss was recorded.
3.2.3.2 Morphology
The morphology of the synthesized ZnO powder was analyzed using scanning electron
microscopy and transmission electron microscopy.
(i)
Scanning electron microscopy
The morphology of the as-synthesized powder and that annealed at different calcination
temperatures were studied using a cold field emission electron microscope (JSM 6701F,
JEOL, Japan). The powder sample was sprinkled over a carbon tape affixed over a brass
stub. An ultra-thin film of gold was coated over the sample using sputter coating in order
to improve the conductivity of the sample. The gold coated sample was introduced into
the specimen chamber. The samples were imaged at an acceleration voltage of 3 kV and a
working distance of about 5.5 mm.
(ii)
Transmission electron microscopy
The morphology of the synthesized powder (annealed at optimum calcination
temperature) was studied using a transmission electron microscope (JEM 2100F, JEOL,
Japan). The powder sample was dispersed in water using bath ultrasonication for 10 min.
A drop of the dispersion was placed over a copper grid and dried. The sample was
imaged at an acceleration voltage of 200 kV.
3.2.3.3 Crystallographic information
Crystalline nature of the as-synthesized powder and powder annealed at optimum
calcination temperature was studied using an X-ray diffractometer (D8 Focus, Bruker,
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Germany) with Cu-Kradiation of wavelength 1.5418 Å. The analysis was performed
for 2 values from 10° to 60° with a step size of 0.01°/min.
3.2.3.4 Spectroscopic analysis
Optical absorption spectra of the as-synthesized powder and powder annealed at optimum
calcination temperature were recorded using a UV-visible spectrophotometer (Lamda
750, Perkin Elmer, USA). The samples were scanned over the  range from 250 to 800
nm.
3.2.3.5 BET surface area analysis
The surface area of ZnO nanopowder was determined from BET isotherms recorded
using a surface area analyzer (ASAP 2020, Micromeritics, USA).
3.3 RESULTS AND DISCUSSION
3.3.1 Thermal analysis
The chemical reaction between Zinc nitrate and ammonium carbonate forms zinc
carbonate. Thermal decomposition of the precursor of ZnO, Zinc carbonate hydroxide
hydrate (hydrozincite) yields ZnO and the decomposition is significant above 200 °C [5].
It has been reported that calcination at higher temperatures results in one-step
decomposition of Zinc carbonate to ZnO, whereas calcination at lower temperatures
results in incomplete decomposition and hence a mixture of ZnO and hydrozincite [5].
Figure 3.3 shows the change in weight of the as-synthesized powders subjected to the
controlled temperature rise. The onset of a rapid decrease in weight was observed at 125
°C which progressed upto 250 °C indicating the conversion of zinc carbonate hydroxide
to ZnO, which was inline with the observations of Kanari et al. [5]. The percentage loss
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in weight from 125 to 250 °C is 20.3 %. The weight loss progressed upto about 550 °C,
after which there was no significant loss of weight.
Figure 3.3. Thermogram of as-synthesized powders.
3.3.2 Morphology
3.3.2.1 Influence of calcination temperature on morphology
The as-synthesized powder (Zinc carbonate hydroxide hydrate) was irregular in shape
(Figure 3.4), which on calcination yielded regular spherical shaped ZnO particles. Since
the Zinc carbonate hydroxide hydrate to ZnO conversion occurs above 200 °C and
progresses upto 400 °C, there was an persistent increase in the fraction of particles with
regular morphology with increase in calcination temperature upto 550 °C (Figure 3.4).
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These results are inline with the thermal analysis of as-synthesized powders (Figure
3.3).The primary particle size of synthesized ZnO ranged between 25 and 40 nm.
Figure 3.4. Influence of calcination temperature on morphology of synthesized ZnO
nanoparticles.
However, the primary particle sizes of ZnO powders were larger for samples annealed at
calcination temperatures of 600 °C, 700 °C and 800 °C. There was no conversion of Zinc
carbonate to ZnO beyond 550 °C as evident from thermal analysis (Figure 3.3). The
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larger primary particle size at higher calcination temperatures (below melting point) can
be attributed to growth of grains. At higher temperatures, fusion of more than one particle
due to melting of particle surfaces resulted in larger particle sizes [6]. With these
observations, 550 °C was chosen as the optimum calcination temperature due to the
uniform spherical morphology of particles and complete conversion of hydrozincite to
ZnO.
Figure 3.5. Transmission electron micrograph of ZnO nanoparticles.
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The transmission electron micrograph of ZnO nanoparticles (Figure 3.5) shows the
spherical morphology of the particles and the size ranging between 25-40 nm
3.3.2.2 Influence of mixing conditions on morphology
In order to investigate the role of mixing conditions on the morphology of the ZnO
nanoparticles, three different mixing arrangements were used as follows:
(i) Mechanical stirring using overhead stirrer
(ii) Magnetic stirring
(iii)Magnetic stirring in a baffled vessel (The reaction vessel was equipped with four
baffles with a width of about 1/10th of the diameter of the reaction vessel)
ZnO nanoparticles prepared using overhead stirrer (Mechanical stirring) were irregular in
shape with elongated aggregates (Figure 3.6 (a)). The use of magnetic stirring for mixing
of reactants yielded spherical particles. Though magnetic stirring without baffles yielded
spherical particles, the particle sizes were not uniform (Figure 3.6 (b)) probably due to
vortices. The use of baffles in the reaction vessel improved the uniformity of the shape of
the particles (Figure 3.6 (c)). This can be attributed to the more effective mixing of the
reactants in the baffled reaction vessel in which the detrimental effects of vortices are
minimal.
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Figure 3.6. Influence of mixing conditions on the morphology of ZnO nanoparticles
(annealed at 550 °C).
3.3.3 Crystallographic information
Figure 3.7 shows the X-ray diffraction patterns of as-synthesized powder and ZnO
annealed at 550 °C. X-ray diffraction pattern of as-synthesized powder was well in
correspondence with that of Zinc carbonate hydroxide hydrate (Zn4CO3(OH)6.H2OJCPDS card no: 11-0287).
The annealing of the synthesized powders at 550 °C resulted in the formation of highly
crystalline ZnO nanoparticles as evident from the X-ray diffractogram (Figure 3.7). The
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diffraction peaks at 2 of 31.72°, 34.37°, 36.2°, 47.48°, 56.54° correspond to the planes
(100), (002), (101), (102), (110) with hexagonal wurtzite structure (JCPDS Card No. 89–
1397) with strong diffraction at 2 of 36.2°. The broadening of peaks shows the
nanocrystalline nature of the synthesized powder.
Figure 3.7. X-ray diffraction patterns of as-synthesized powders and ZnO calcined at 550
°C.
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The crystallite size of the ZnO nanoparticles was calculated using the scherrer formula
(Eq. (1))
D
0.9
 cos 
(3.1)
In Eq. (3.1), ‘D’ is crystallite size, ‘’ is wavelength of incident X-ray (nm), ‘’ is full
width at half maximum intensity (radians), ‘’ is Bragg diffraction angle. Crystallite size
of ZnO nanoparticles calculated using Scherrer formula was found to be 25.78 nm, which
was well in accordance with electron microscopy results.
3.3.4 Optical absorption
The absorption spectra of as-synthesized ZnO and ZnO calcined at 550 °C are shown
in Figure 3.8. The as-synthesized powder showed no specific peak between 250 and 800
nm, whereas ZnO nanoparticles calcined at 550°C showed a peak at 360 nm. The
electronic spectra (Figure 3.8) of the annealed sample was in good agreement with those
reported by Chang and Tsai [7]. The absence of any characteristic absorption in the
electronic spectra of as-synthesized sample indicates that the as-synthesized dried powder
did not contain ZnO nanoparticles. The thermal decomposition of the as-synthesized
dried powder (hydrozincite) yielded ZnO nanopowder, as evident from the characteristic
max of the electronic spectrum of calcined powder.
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Figure 3.8. Electronic spectra of as-synthesized powder and ZnO annealed at 550 °C.
3.3.5 Surface area
The surface area of ZnO nanoparticles annealed at 550 °C was found to be 29.58 m2/g
using BET technique.
3.4 CONCLUSIONS
ZnO nanoparticles were synthesized using room temperature chemical precipitation
method. The influence of calcination temperature on the morphology of ZnO
nanoparticles has been investigated, from which 550 °C was identified as the optimum
calcination temperature due to the uniform spherical morphology and complete
conversion of hydorzincite to ZnO at this temperature. The particles annealed at 550 °C
were spherical with a size range of 25-40 nm as confirmed using electron microscopy. X-
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ray diffraction pattern of synthesized ZnO showed that the particles were highly
crystalline and were of hexagonal wurtzite phase. Further, electronic spectrum of
synthesized ZnO showed a characteristic max at 360 nm.
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