effect of diameter of zno

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Effect of the diameter on the band gap of ZnO Nanorods

Journal: Green Chemistry Letters and Reviews

Manuscript ID: Draft

Manuscript Type: Research Letter

Date Submitted by the Author: n/a

Complete List of Authors: Kasim, Muhd; University of Technology MARA, Faculty of Applied ScienceKamarulzaman, Norlida; University of technology MARA, Centre forNanomaterials Research, Institute of ScienceRusdi, Roshidah; University of Technology MARA, Centre for NanomaterialsResearch, Institute of Science

Keywords: ZnO nanorod, Band gap, lattice parameters

URL: http:/mc.manuscriptcentral.com/tgcl Email: [email protected]

Green Chemistry Letters and Reviews


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widening of nanostructured ZnO, MgO and doped metal oxides compounds. Her effort in

research is to contribute towards the better understanding of phenomena observed in

nanostructured materials. She has published more than 80 papers to major international journals

such asJournal of Power Sources,Powder Technology,J. Cryst. Growth, Advanced Materials

Research,and etc.

Roshidah Rusdi is a Ph.D student at the Universiti Teknologi MARA (UiTM). She has her

bachelors degree in chemistry and is currently working on complex metal oxides for

application in Li-ion batteries. She did her M.Sc on Materials Science (ZnO nanostructured

materials) and has published results on the band gap widening of high aspect ratio ZnO

nanorods. She has published more than 30 papers to major international journals such as

Powder Technology,Advanced Materials Research, J. Cryst. Growth, and etc. She hopes to be

able to make new findings on Li-ion battery materials and fabricate thin film Li-ion batteries.

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Effect of the diameter on the band gap of ZnO Nanorods

ZnO is considered one of the safest metal oxides for consumer. There are many

techniques that can be used to synthesize ZnO nanostructures. In this study, the

sol-gel synthesis method is used. The effect of a gelling agent is investigated in

terms of the ZnO nanostructures obtained. One sample is done using a sol-gel

synthesis route using tartaric acid and the other sample is synthesized without

using a gelling agent at all. Results showed that the samples are pure, however

the sample prepared without using the gelling agent were rods with long aspect

ratios while the one obtained using the gelling agent were rods with lower aspect

ratios. It was found that the band gap of the high aspect ratio ZnO is smaller than

the one with low aspect ratio. This is attributable to the lattice expansion of thelower aspect ratio ZnO nanorods as obtained from quantitative X-Ray diffraction.

Keywords: ZnO nanorod; band gap; lattice parameters

Introduction

Over the past years, several oxide nanoparticles have become important applications

and one of them is ZnO which is considered as the best to be exploited at nano

dimensions. Zinc oxide is considered non-toxic and therefore is environmentally

friendly, suitable for safe use in many applications. Zinc oxide (ZnO) have been

actively studied because of their potential applications in solar-cells, optoelectronic

(UV) devices, gas sensors, and UV light emitting/detecting technology [1-4] and has

motivated many researchers to study them. This is due to their unique properties of

which are wide and direct band gap (3.37eV) and large exciton binding energy (60

meV) [3-5]. The unique properties of ZnO have triggered researchers to develop many

simpler and low cost techniques to produce ZnO nanostructures.The dependence of

properties on the size and shape of ZnO particles has led to many interesting

applications [6, 7], one of them is the band gap tuning of ZnO semiconductors. It has

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been found that the band gap of ZnO is dependent on size and shape of the

nanoparticles and it may be possible to tune band gaps of ZnO by controlling their

dimensions.

A wide variety of synthesis methods and techniques have been used to

synthesize ZnO nanostructures. They include hydrothermal methods [3, 8], pulsed laser

deposition (PLD) [5], vapour transport process [9], metal organic vapour phase epitaxial

growth [10], etc. In this work, the sol-gel method has been used to synthesize ZnO

nanostructures. The sol-gel method is popular due to their simple, low cost and ease of

synthesis. This work explores the effect of using a gelling agent on the ZnO

nanostructures and comparing it with a synthesis route that does not use a gelling agent.

The functional property of band gap is investigated in relation to the thermal properties

of the materials.

Experimental

ZnO nanostructures have been synthesized using a sol-gel method. The starting material

used for both samples is Zinc acetate dehydrate (99.5% purity). Zinc acetate dihydrate

was first dissolved in absolute ethanol and then was stirred for about 1 hour. While

stirring, the gelling agent, tartaric acid (TA) (Fluka, 99.5% purity, 1 M solution) was

added to one of the mixture until a thick, white gel was obtained. The pH for both

solutions was 5. Then, the samples were grinded using an agate mortar to obtain fine

powders. The precursors were annealed at 300 C for 3h. The samples were named as

Z1 for ZnO synthesized using tartaric acid and Z2 for ZnO synthesized without using

tartaric acid. For the ZnO without using the chelating agent, no thick gel was observed

to have been formed even during the drying process. The annealed samples were

characterized by using X-Ray diffraction (XRD) using the PANalytical Xpert Pro MPD

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diffractometer. The morphologies and physical dimensions of the materials were then

examined by Field Emission Scanning Electron Microscopy (FESEM), the JEOL JSM-

7600F. Band gap energies of the materials were evaluated by using a UV-Vis

spectrophotometer, the Perkin Elmer Lambda 950 UV-Vis-NIR. The measurements

were done in reflection mode.

Results and discussions

Thermal studies of the precursors of ZnO for both samples were carried out by using

simultaneous thermogravimetric analysis (STA). The results are shown in Figure 1b and

1c with and without chelating agents.

(Figure 1)

The result in Figure 1a of the starting material shows that it decomposes at about

280oC. From the result in Fig 1b, the ZnO precursor with chelating agent has three

mass losses. The first mass loss is observed starting from 90oC in Figure 1b and it is

attributed to the residual water loss in the precursor. This first mass loss is about 11.3

%. The second mass loss which is a major mass loss of the ZnO precursor is about 54.%

and it started at about of 250oC. This mass loss is believed to be the decomposition of

the zinc intermediates formed by the fixation of the metal compound to the organic

polymeric matrix. The third mass loss which is about 5.7 % is due to a second

intermediate formed during the reaction which decomposes at a higher temperature of

380oC. The material is stable after about 390

oC. For the precursor of sample Z2, the

first and major mass loss occurring at 250oC which is about 65% is due to the

decomposition of the complex organometallic compound formed during the reaction. In

support of this, only one endothermic peak is observed during this mass loss. Another

exothermic peak is observed during the end of this mass loss attributed to the release of

stress when the crystal structure changes its morphology from the long nanorod to the

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spherical shaped crystals. From the STA results, 300oC is an interesting temperature to

use for investigating ZnO nanostructures that will be formed because at this temperature

a very big difference in mass loss is observed for both ZnO synthesized without

chelating agent and with chelating agent, that is, about 8% and 65% respectively. This

different mass losses indicates that the ZnO synthesized without chelating agent has

higher thermal stability than with chelating agent. These facts can be correlated to the

rate of crystal growth of ZnO discussed later.

For structural studies, a Rietveld refinement of the XRD patterns of ZnO

material is done using the PanAlytical Xpert Highscore Plus software. All data taken

are of high quality with the highest peak at around 10 000 counts satisfying the statistics

needed for quantitative analysis. The reference structure used in the refinement is the

ZnO hexagonal crystal structure with space group P63mc (ICSD 67454). The XRD

patterns of the ZnO samples annealed at 300 C for 3h are shown in Figure 2 and the

structural parameters obtained are displayed in Table 1. The diffraction patterns of each

sample were in good agreement with the ICDD reference number 01-089-0511 as

indexed in Figure 2. They indicate that the synthesized samples were single phase of the

hexagonal crystal structure and space group P63mc. This is the stable phase of ZnO. It

is observed that the (002) peak is relatively high compared to the (100) peak. This

means that there is preferred orientation in the direction of this crystal plane. The ratio

of (002)/ (100) peaks for Z1 and Z2 are 0.9945 and 1.0389 respectively. These results

indicate that the Z2 material has a high growth rate in the [002] crystal plane direction

as compared to Z1 material. It is also observed that Z2 material has higher intensity than

Z1 material. This means that Z2 material is more crystalline than Z1 material as can be

seen by the higher counts of the peaks exhibited by Z2 material compared to Z1.

(Figure 2)

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(Table 1)

The SEM images of the ZnO samples annealed at 300 C for 3h are shown in

Figure 3. It can be seen that both samples have nanorod shaped morphology. However,

the Z2 sample possesses longer and thinner rods than Z1 sample. The average aspect

ratio (length/diameter) for Z1 and Z2 are 19.47 and 27.66 respectively. It is a known

characteristic of ZnO to have a preferred growth direction in the c-axis of the crystal

orientation [1]. That is in the [002] crystal plane direction (as can be seen by the

relatively higher (002) peak compared to standard XRD patterns). Therefore, SEM

results supported the results of XRD.

(Figure 3)

Absorption edges are shown in the reflectance spectra of the UV-Vis results

shown in Figure 4. The UV-Vis spectroscopic measurements were carried out at room

temperature in the wavelength range of 330-800 nm. It can be seen that the absorption

edge of the Z2 material is slightly shifted to the right as compared to Z1 material. This

indicates that light is absorbed at a higher wavelength for Z2 material. Higher

crystallinity Z1 material, seem to affect light absorption properties of the material. So, it

can be said that the light absorption characteristics of the materials is influenced by the

physical dimensions of the material which in this case refers to the length and diameter

of the ZnO nanorod. Band gaps can be evaluated from the absorption edges. The band

gap energies of ZnO materials are determined by using Tauc plots. Equation (1) below

is used

(h)2= C (h -Eg) (1)

where is the absorption coefficient of the material, is the wavelength, h is Planck's

constant, C is the proportionality constant, is the frequency of light and Eg is the

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band gap energy. The graph plotted is (h)2vs h and from equation (1), extrapolating

the linear part of the graph until it meets the x-axis will give the value of the band gap

[1]. The band gap energies of the materials are determined from the Tauc plots shown in

Figure 5. It is observed that the band gap energy for Z1 material is higher than Z2

material. Looking at the cell parameters

(Figure 4)

(Figure 5)

(Table 1), it can be seen that Z1 which has a lower aspect ratio, has larger cell

parameters. The material with the larger cell parameter shows a higher band gap.

Therefore, the band gap expansion of Z1 can be attributed to the lattice parameter

expansion of the nano crystals. The mean diameter of Z1 and Z2 sample is 44 and 61

nm respectively while the length is 812 and 1611 nm respectively. It is obvious here

that the diameter plays a part in the band gap expansion of sample Z1. The diameter of

the nanorods are in the nano range while the length is no longer considered nano

dimensional. Band gap change is a phenomenon attributed to the quantum particle

effects of nanstructures.

Conclusion

This work shows that band gap changes can occur in nanostructured materials. The cell

dimension, which is in this case the diameter, is responsible for the observed

phenomenon. The nanostructured ZnO with a smaller diameter (Z1) experienced an

increase in lattice parameter resulting in the expansion of the band gap.

Acknowledgement

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The authors would like to thank the Faculty of Applied Sciences UiTM Shah Alam,

Malaysia for funding of the project and the Institute of Science, UiTM Shah Alam,

Malaysia for internal funding and equipment support.

References

1. Rusdi, R.; Rahman, A.A.; Mohamed, N.S.; Kamarudin, N.; Kamarulzaman, N.;

Powder Technol.2011,210, 1822.

2. Waclawik, E. R.; Chang, J.; Ponzoni, A.; Concina, I.; Zappa, D.; Comini, E.; Motta,

N.; Faglia, G.; Sberveglieri, G.;Beilstein J. Nanotechnol. 2012, 3, 368377.

3. Anandan, S.; Ohashi, N.; Miyauchi, M.;Appl. Catal., B2010, 100, 502509.

4. Luo, L.; Zhang, Y.; Mao, S.S.; Lin, L.; Sens. Actuators, A 2006, 127, 201206.

5. Shan, F.K.; Liu, G.X.; Lee, W.J.; Shin, B.C.;J. Cryst. Growth2006, 291,328333.

6. Xu, H.; Liu, X.; Cui, D.; Li, M.; Jiang, M.; Sens. Actuators, B 2006, 114,301307.

7. Sonmezoglu, S.; Eskizeybek, V.; Toumiat, A.; Avc, A.; J. Alloys Compd. 2014,

586, 593599.

8. Ni, Y.; Wu, G.; Zhang, X.; Cao, X.; Hu, G.; Tao, A.; Yang, Z.; Wei, X.; Mater. Res.

Bull. 2008, 43, 29192928.

9. Yu, D.; Trad, T.; McLeskey Jr., J.T.; Craciun, V.; Taylor, C.R.; Nanoscale Res Lett.

2010, 5, 13331339.

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Table 1. Crystallographic parameters of the ZnO sample from the Rietveld refinement

of the XRD data sets (s.o.f.= site occupancy factor)

Figure 1. STA results of (a) zinc acetate, (b) ZnO precursors with chelating agent (c)

ZnO precursors without chelating agent.

Figure 2. The XRD pattern of (a) Z1 and (b) Z2 sample

Figure 3.The SEM images of (a) Z1 and (b) Z2 sample

Figure 4. The UV-Vis results of (a) Z1 and (b) Z2 sample showing the absorption edge

Figure 5. The Tauc plot graph of (a) Z1 and (b) Z2 sample

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Table 1. Crystallographic parameters of the ZnO sample from the Rietveld refinement of the XRD datasets (s.o.f.= site occupancy factor)

Sample a=b() c () V (

3) c/a Rw s.o.f ofZn s.o.f ofO

Z1 3.2497 5.2067 47.6205 1.6022 6.4759 0.9455 1

Z2 3.2493 5.2063 47.6036 1.6023 10.1919 0.9442 1

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STA results of (a) zinc acetate, (b) ZnO precursors with chelating agent (c) ZnO precursors without chelatingagent.

80x156mm (600 x 600 DPI)

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The XRD pattern of (a) Z1 and (b) Z2 sample39x39mm (600 x 600 DPI)

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The SEM images of (a) Z1 and (b) Z2 sample16x6mm (300 x 300 DPI)

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The UV-Vis results of (a) Z1 and (b) Z2 sample showing the absorption edge45x29mm (600 x 600 DPI)

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The Tauc plot graph of (a) Z1 and (b) Z2 sample45x29mm (600 x 600 DPI)

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effect of diameter of zno

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