Effect of Diluents on Crystallite Size and Electronic Band Gap of ZnO nanoparticles synthesized by Mechanochemical Processing

Mahesh Kumar Talari1,a, Mohd Salleh Mohd Deni1, Nursyahadah Mohd Zor1, Venugopal Thota2, Azlan Zakaria1

1Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia

2 Welding Alloys (Far East), Malaysia

a talari@gmail.com

Keywords: ZnO nanoparticles; Optical properties; Mechanochemical processing

Abstract.

This paper presents the characterization results of Zinc Oxide (ZnO) nanoparticles prepared by

mechanochemical processing using different moles of diluents. ZnO nanoparticles of different

crystallite size were synthesized by milling the precursor powders for 5 hours in a high energy ball

mill with Zirconia media. NaCl was added as process control agent (PCA) to control the reaction

kinetics, as final particle size of nano ZnO is influenced by the reaction rate. X-ray Diffraction

(XRD) data was used to compute and analyze the crystallite size of nanoparticles and also to

analyze the progress of reaction during milling process. Field Emission Scanning Electron

Microscope was employed to analyze the particle morphology and size distribution of ZnO

nanoparticles. Ultraviolet –Visible (Uv-Vis) spectroscope was employed to analyze the optical

absorption of ZnO nanoparticles. Tauc plots were used to determine the energy gap of the ZnO

nanoparticles. Crystallite size values of ZnO nanoparticles are seen to be influenced by the amount

of PCA and heat treatment. ZnO nanoparticles with a range of Eg (3.1 to 3.14 eV) were obtained

depending on process parameters and an inverse relationship was observed between the crystallite

size and the energy gap of the ZnO nanoparticles.

Introduction

The unique properties of nanoparticles are due to decreased size, quantum confinement of electrons

and the high surface area compared to the bulk materials [1]. ZnO is an important semiconductor

material, with a wide band gap of 3.37 eV and large exciton binding energy (60 meV), which

gained a lot of attention from researchers in the last decade [2-3]. ZnO nanoparticles display

modified optical and electronic properties depending on processing technique, particle size,

morphology, doping and surface modifications [4 - 5].

There are several methods to synthesize the nanoparticles viz., pyrolysis, thermal decomposition,

mechanical alloying, sol gel process and others [6]. Mechanochemical processing is becoming

popular for the synthesis of nanoparticles due to its versatility of process parameters [7]. Junmin

Xue et al. have synthesized perovskite BaTiO3 in an oxide matrix with a crystallite size of ∼14 nm

by mechanical activation of precursor powders, without any additional heat treatment in a nitrogen

atmosphere, which otherwise require a heat treatment of 800-10000C via solid state synthesis

route. [8]. As suggested by Suryanarayana, mechanochemical method when employed with suitable

process control agents (PCA), for the synthesis of oxide nanoparticles can avoid the agglomeration

of nanoparticles [7]. More recently, Sabri, et al milled the precursor powders in a 250 ml Zirconium

oxide vial along with ten Zirconium oxide balls of 20 mm diameter employing a NaCl/ZnCl2 molar

ratio of six, reported that XRD patterns of as milled powders showed diffraction peaks from ZnO

phase, which shows the calcination of the ZnCO3 product phase has progressed during milling itself

[9].

In the present work, ZnO nanoparticles were synthesized in a Zirconia vial (250ml) and balls (20

mm Φ) with different NaCl/ZnCl2 molar ratio. By adopting suitable heat treatment techniques, ZnO

nanoparticles with a range of crystallite sizes were synthesized to investigate the effect of heat

Advanced Materials Research Vol. 626 (2013) pp 786-790Online available since 2012/Dec/27 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.626.786

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 202.58.85.15, Universiti Teknologi Mara (UiTM), Shah Alam, Malaysia-14/03/13,06:49:54)

treatment on crystal growth and energy band gap (Eg). Optical properties characterization of the

‘Milled & Leached’ and ‘Milled-Heat treated & Leached’ ZnO nanoparticles with different

crystallite sizes, were carried out and variation of band gap (Eg) with crystallite size were discussed.

Experimental Procedure

High energy planetary ball mill (Fritsch Pulverisette 6) was employed for the synthesis of Zinc

Oxide (ZnO) nanoparticles. Anhydrous Zinc Chloride (ZnCl2), anhydrous Sodium Carbonate

(Na2CO3) and Sodium Chloride (NaCl) were milled in Zirconia vial (250ml) and balls (20 mm Φ)

for 5 hours at 500rpm. A ball to powder ratio of 10:1 was employed during the milling process.

NaCl were added as PCA to control the reaction kinetics during high energy ball milling. Chemical

reactions involved in the synthesis of ZnO nanoparticles synthesis can be seen in Table 1.

Table1 : Reactions during different stages of synthesis

Processes Reactions

Milling NaClxZnCOxNaClCONaZnCl )2(3322 ++→++ (1)

Calcination NaClxCOZnONaClxZnCO )2()2( 23 +++→++ (2)

Washing with H2O )(8 WaterindissolvedNaClZnONaClZnO +→+ (3)

Different amounts of NaCl (x = 4, 6, 8 moles) were added to the precursor powders in order to

study the effect of PCA on crystallite size and optical properties. After milling the precursor

powders for 5h, part of the milled powder was heated to a temperature of 6000C and held for 2

hours before leaching the PCA (NaCl); whereas the other part of the powder was leached directly

after milling. X-Ray diffraction (XRD) data was obtained by employing a ‘Panalytical X’Pert’ X-

Ray diffractometer. The samples were also analyzed using the Field Emission Scanning Electron

Microscopy (FESEM, Zeiss SUPRA VP 40) to examine the microstructure of ZnO nanoparticles.

Ultraviolet – Visible (UV-Vis, Perkin Elmer) spectroscope was employed to study absorption

characteristics of the ZnO nanoparticles in the range of 200-1000nm wavelength.

Results and Discussion

Mechanochemical Synthesis of ZnO Nano Particles: Figure 1 shows the XRD patterns of as

milled powders with four moles of NaCl as PCA which were ball milled at 500rpm for different

periods of time. It can be observed from the figure that the intensity of ZnCl2 peaks decrease with

the progress of milling and with simultaneous evolution of ZnO peaks. The ZnO peak is seen to

appear after milling the precursor powders for 1h and continuously grow with the progress of

milling. From these observations it can be confirmed that the reaction between ZnCl2 and Na2CO3

resulted in formation of ZnO without any intermediate products such as ZnCO3. The impact energy

supplied by the balls during milling not only causes the reaction to progress but also activates the

calcination of ZnCO3 simultaneously at low temperatures. The XRD patterns of as milled powders

ball milled at 500rpm with eight moles of NaCl as PCA are presented in figure 2. Though a similar

observation of reaction trend is observed, a clear ZnO peak was observed after three hours of

milling. From these observations it can be inferred that the reaction kinetics of formation of ZnO is

slower with an increase in amount of PCA. During the chemical reaction between the two species,

the product phase forms at the interface of reactants. With the increase in the amount of PCA, the

probability of reactants getting together is decreased and hence the reaction rate for the formation of

ZnO decreased with increase in amount of NaCl.

Advanced Materials Research Vol. 626 787

Figure 1: XRD patterns of as-milled powders

ball milled with zirconium oxide milling media

and four moles of NaCl as PCA

Figure 2: XRD patterns of as-milled powders

ball milled with zirconium oxide milling media

and eight moles of NaCl as PCA

After five hours of high energy ball milling, part of the product mixture is subjected for leaching

treatment in distilled water and ZnO nanoparticles are extracted. The XRD patterns of ‘milled &

leached’ ZnO nanoparticles are presented in figure 3. From this XRD pattern it is evident that single

phase crystalline ZnO nanoparticles are obtained as a result of mechanochemical reaction and

leaching. The other part of the product mixture is heated to a temperature of 600°C and held for two

hours before subjecting to a leaching treatment in distilled water. The XRD patterns of ‘milled, heat

treated & leached’ ZnO nanoparticles are presented in figure 4. Further, the crystallite size of

‘milled & leached’ as well as ‘milled, heat treated & leached’ ZnO nanoparticles, calculated from

XRD data are presented in figure 5. It is interesting to note that the crystallite size of ZnO

nanoparticles decreased with increased amount of PCA from four moles to six moles and there after

that slight increment was observed until eight moles of PCA. This can be attributed to the dynamics

of reactants during high energy ball milling.

Figure 3: XRD pattern of ‘milled and leached’

ZnO nanoparticles with different amounts of

PCA (NaCl)

Figure 4: XRD pattern of ‘milled, heat treated

and leached’ ZnO nanoparticles with different

amounts of PCA (NaCl)

High energy ball milling is a process of repeated comminution and cold welding [7]. The

mechanical properties of the material being milled will dominate one of these processes. In the

present case, the inorganic components such as ZnCl2, Na2CO3 and NaCl do not possess ductility

and hence comminution is the dominant process. As the milling progresses the material is crushed

into finer particles and simultaneously the surface energy of the particles increases. At a certain

stage, when the surface energy exceeds the threshold and two reactants namely ZnCl2 and Na2CO3

788 Advanced Materials Engineering and Technology

are in close proximity, the solid state reaction between them excites and results in the formation of

ZnO without any intermediate products. With a small amount of PCA, the probability of obtaining

reactant particles in close proximity is high and resulted in the formation of ZnO with larger

crystallite size. With increasing the amount of PCA, the probability of obtaining reactants in close

proximity decreases. This phenomenon decreases the reaction rate but aids in obtaining the product,

ZnO at a smaller crystallite size. From these observations, it is evident that six moles of NaCl is the

optimum PCA. As the amount of PCA decreased the crystallite size increased and as the amount of

PCA increased the reaction rate decreased without much affecting the crystallite size.

Figure 5: Crystallite size of ‘milled & leached’

and ‘milled, heat treated & leached’ ZnO

nanoparticles

Figure 6: SEM micrographs of ‘milled, heat

treated & leached’ ZnO nanoparticles with 4

mol NaCl as PCA

During heat treatment, elevated temperature assists the process of diffusion and movement of

grain boundaries in ZnO agglomerates. Consequently this resulted in the growth of certain crystals

of ZnO at the expense of neighboring crystals. However, it is worthwhile to note that the presence

of NaCl as PCA inhibited the ZnO particles to be obtained in close proximity and there by the

uncontrolled grain growth of nanocrystalline ZnO is avoided. SEM micrographs of ‘milled, heat

treated & leached’ ZnO nano particles with 4 mol NaCl as PCA are presented in figure 6. The

nanocrystalline ZnO particles are nearly spherical in shape with a minimal agglomeration. Further

the ZnO particle size is in close agreement with the crystallite size calculations of the ‘milled, heat

treated & leached’ ZnO nano particles. Hence it can be inferred that each ZnO nanoparticle is a

single crystal.

Ultraviolet –Visible (Uv-Vis) spectroscopy: Optical absorption spectra were collected from the

‘milled & leached’ and ‘milled, heat treated & leached’ ZnO nanoparticles in the UV and Visible

range. Tauc plots were used to evaluate optical band gap (Eg) of the ZnO nano particles from UV-

Vis absorption spectra. Optical band gap (Eg) of ‘milled & leached’ and ‘milled, heat treated &

leached’ ZnO nano particles with different amounts of PCA were shown in Figure 7. For ‘milled &

leached’ ZnO nanoparticles, Eg is seen to be higher compared to ‘milled, heat treated & leached’

ZnO nanoparticles. For ‘milled and leached’ ZnO nanoparticles, the Eg is seen to be highest (3.24

eV) for samples prepared with 6 moles of PCA. For ‘milled & leached’ as well as ‘milled, heat

treated & leached’ ZnO nanoparticles the Eg was increased from 4 moles to 6 moles PCA and then

decreased for 8 moles of PCA. Plot of variation of Eg with the crystallite size was shown in figure 8.

It can be observed from the figure that Eg increased with the decrease in the crystallite size.

Advanced Materials Research Vol. 626 789

Figure 7: Energy gap of ‘milled & leached’

and ‘milled, heat treated & leached’ ZnO

nanoparticles synthesized with different

amounts of PCA (NaCl)

Figure 8: Variation of energy gap with

Crystallite size for ‘milled & leached’ and

‘milled, heat treated & leached’ ZnO

nanoparticles

In general, quantum confinement shifts the energy levels of the conduction and valence bands

apart, giving rise to a blue shift in the transition energy as the particle size decreases. This relation

also supports the increase in Eg with decrease in crystallite size of ZnO nanoparticles, as observed

in our investigation.

Conclusions

High energy ball milling process is successfully used as a tool to activate mechanochemical reaction

of precursor powders and ZnO nanoparticles were successfully synthesized. The agglomeration of

product phase is controlled and optimized by means of process control agent (NaCl). The smallest

crystallite size of ZnO nano particle was obtained at six moles of NaCl in ‘milled and leached’

condition. It is also evident that the ZnO nanoparticles synthesized with six moles of NaCl exhibited

higher grain growth (from 18 nm to 42 nm) during heat treatment. Higher surface energy in as-

milled condition acted as driving force for grain growth. The energy gap (Eg) is also varied along

with ZnO crystallite size. Due to quantum confinement effects, Eg is seen to increase with the

decrease in crystallite size. It is also proved that the Eg of the ZnO nanoparticles can be tuned by

controlling processing parameters during ball milling such as PCA and heat treatment.

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