Effect of Sintering Temperature on Zn0.94Cr0.03Fe0 ... - Science Direct

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ScienceDirect Procedia Materials Science 10 (2015) 97 – 102

2nd International Conference on Nanomaterials and Technologies (CNT 2014)

Effect of Sintering Temperature on Zn0.94Cr0.03Fe0.03O Nanostructures Prakash Chand*, Anurag Gaur and Ashavani Kumar Department of Physics, National Institute of Technology, Kurukshetra, India

Abstract In present work, we report structural, morphological and optical properties of Zn0.94Cr0.03Fe0.03O nanostructures synthesized by hydrothermal method. The as prepared Zn0.94Cr0.03Fe0.03O samples were sintered in air at different temperatures (RT- 400 0C) to understand the effect of sintering temperature on the structural, morphological and optical properties of Zn0.94Cr0.03Fe0.03O nanostructures. The samples are characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) and UV-visible spectroscopy. The XRD patterns show a pure phase corresponding to ZnO without any other phases, indicating that Cr and Fe atoms are successfully doped into the lattice of ZnO. Moreover, PL spectrum also confirms the wurtzite phase formation of ZnO. Furthermore, the optical band gaps calculated through UV-visible spectroscopy is found to be decreasing as increasing the sintering temperature. © TheAuthors. Authors.Published Published Elsevier © 2015 2015 The by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the International Conference on Nanomaterials and Technologies (CNT 2014). (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Conference on Nanomaterials and Technologies (CNT 2014) Keywords : ZnO nanostructures, Hydrothermal method, Optical properties

1. Introduction Over the past few decades, nanostructured materials due to their unique optical, electrical and magnetic properties have stimulated considerable interests for scientific community due to their significance in fundamental physics studies and potential applications in different areas of science and technology [Alver et al. (2012) and Sahoo et al. (2010)]. * Corresponding author. Tel.: +91-1744 233549; fax: +91-1744 238050. E-mail address: [email protected]

2211-8128 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Conference on Nanomaterials and Technologies (CNT 2014) doi:10.1016/j.mspro.2015.06.030

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Prakash Chand et al. / Procedia Materials Science 10 (2015) 97 – 102

Among the various materials, group II–IV semiconductor material such as zinc oxide (ZnO) exhibit a wide range of optical and electrical properties that depend on both shape and size of nanostructure. Moreover, the optical properties of ZnO nanoparticles play a very important role in photochemical, catalytic and optoelectronic properties. Due to excellent thermal and chemical stabilities, and wide direct band gap (3.37 eV), large exciton binding energy (60 meV), zinc oxide have emerged as potentially prominent semiconductor materials for a variety of technological applications such as white-lighting devices, transparent conducting electrodes, magnetic storage media, field emission devices, chemical gas sensors, catalysis, optoelectronic devices, piezoelectric devices, electrical and optical switching devices, drug delivery, flat screen displays, sun screens, photodiodes and spintronics devices [Look et al., 2001, Bojorge et al. (2012), Jin et al. (2007), Zhu et al. (2003), Gupta et al. (2014), Moezzi et al. (2012), Gledhil et al. (2009), Major et al. (1988) and Alivov et al. (2003)]. With broad band gap semiconductor and enormous exciton binding energy, ZnO has also been of keen interest in low-voltage and short-wavelength electro-optical device applications [Cao et al. (2000) and Huang et al. (2001)]. Furthermore, the transparency characteristic of ZnO also makes it suitable material for the manufacture of transparent ohmic contacts and solar cells applications. Now days, the key challenges in optimizing the optical properties of ZnO is the inclusion of doping ions into the ZnO lattice. Doping with suitable elements bid an effective technique to a significant improvement and control of the structure, electrical, magnetic and optical properties of ZnO nanostructures without any change in their crystal structure. Especially, the doping of transition metal elements in ZnO has a profound effect on the materials characteristics and tuning the optical energy band gap to make useful for light emitters and UV detector applications [Senthilkumaar et al. (2008)]. In the recent past, researchers have doped ZnO with different transition metal like Cr, Fe, Co, Ni or Cu and observed variations across all types of physical properties [Yılmaz et al. (2011), Soumahoro et al. (2010) and Ghotbi et al. (2012)]. In light of the above cited work, we study the effect of sintering temperature on structural, morphological and optical properties of Zn0.94Cr0.03Fe0.03O. In the present work, the Zn0.94Cr0.03Fe0.03O nanostructures were prepared by hydrothermal method and studied their structural, morphological and optical properties. 2. Experimental Details All the reagents used in the present work for the chemical synthesis were of analytical grade and used without any further purification. Zn0.94Cr0.03Fe0.03O nanostructures have been synthesized by hydrothermal method at 3 % doping concentrations. A precursor solution for Zn0.94Cr0.03Fe0.03O was prepared by dissolving high purity zinc acetate (Zn(CH3COO)2), chromium nitrate (Cr(NO3)3.9H2O) and ferric nitrate (Fe(NO3)3.9H2O) in de-ionized water under vigorous magnetic stirring for one hour to get a clear solution. Then a 3M sodium hydroxide (NaOH) solution was added drop wise with constant stirring. After that the solution was transferred to a 100 ml Teflon-lined container and the Teflon container was tightly closed in a stainless steel autoclave. The autoclave was placed in an electric oven and heated at 140 0C for a period of one hour. On completion of reaction the autoclave was cooled naturally to room temperature. To remove the impurities, the resultant solution was filtered and washed several times with ethanol and deionized water. Further, the as prepared sample was centrifuged and dried at room temperature. The as prepared sample is sintered in air at different temperature from room temperature to 400 0C, to study the effect of sintering temperature on the morphology as well as particle size. Then as prepared ZnO samples were characterized by X-ray diffractometer (XRD; Rigaku Japan) with Cu-Kα radiation source (λ=1.54 Å) for structural and phase analysis in the 2θ range from 20 to 80 0. The surface morphologies were observed using a scanning electron microscope (SEM). Photoluminescence measurement was carried out at room temperature at excitation wavelength 220 nm. UV-visible spectrophotometer (Camspec M550 Double Beam Scanning) was used to measure the optical band gap ZnO samples. 3. Results and Discussion 3.1 X-Ray diffraction studies


X-ray diffraction (XRD) studies are carried out to study the crystallographic properties of Zn0.94Cr0.03Fe0.03O nanostructures sintered at room temperature to 400 0C. Figure 1(a) present the XRD spectra of Zn0.94Cr0.03Fe0.03O powder sintered at room temperature to 400 0C, respectively. The characteristic peaks with high intensities corresponding to the planes (100), (002) and (101) and lower intensities peaks at (102), (110), (103), (200), (112), (201), (004) and (2 02) planes confirm the formation of high-purity hexagonal (Wurtzite) structure of ZnO for all the

Fig. 1 (a) Room temperature XRD patterns of Zn0.94Cr0.03Fe0.03O nanostructures sintered at different temperatures; (b) Variation of crystallite size with sintering temperatures.

samples sintered at room temperature to 400 0C, respectively. It is obvious from the XRD spectra that there are no impurities peaks in the as- synthesized samples which indicate the pure phase formation. Furthermore, the observed line broadening in the peaks of XRD spectra (fig.1) indicate that the synthesized particles are in nanometer range. The average crystallite size (D) of Zn0.94Cr0.03Fe0.03O nanostructures are evaluated from the peak broadening of XRD spectra using Scherrer formula: D = K λ / β cosθB, where k = 0.89 is the shape factor, λ is wavelength of CuKα (1.54 Å) radiation, β is the full width at half maximum (FWHM) and θ B is the angle of Bragg diffraction. The average crystallite size (D) of Zn0.94Cr0.03Fe0.03O nanostructures, calculated through Debye Scherrer formula, increases from 13 to 25 nm as the sintering temperature increases from room temperature to 400 0C, respectively. Figure 1(b) shows the variation of crystallite size with sintering temperatures. It is clear from figure 1 (b) that the average crystallite size (D) increased with increasing sintering temperature. The increase in crystallite size is due to sintering temperature effect and hence the crystal structure of the Zn0.94Cr0.03Fe0.03O nanostructures is vigorously depends on the sintering temperature [Vishwas, M., et al., 2010]. 3.2 SEM studies SEM investigations carried out at room temperature to observe the surface morphology and size of the synthesized Zn0.94Cr0.03Fe0.03O nanostructures. Figure 2 shows the SEM images of Zn0.94Cr0.03Fe0.03O nanostructures synthesized at room temperature. It is clear from SEM images that the average particles size of Zn0.94Cr0.03Fe0.03O nanostructures is ~30 nm. The SEM images shows the agglomeration of Zn0.94Cr0.03Fe0.03O nanostructures and indicates the porosity exist in this sample 3.3 Photoluminescence studies

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In order to investigate the effect of sintering temperature on the photoluminescence (PL) property of Zn0.94Cr0.03Fe0.03O nanostructures, we measured the PL spectra of Zn0.94Cr0.03Fe0.03O nanostructures sintered at room temperature to 400 0C, respectively which is shown in figure 3.

Fig .2 SEM images of Zn0.94Cr0.03Fe0.03O nanostructures synthesized at room temperature.

Fig .3 Room temperature PL spectra of Zn0.94Cr0.03Fe0.03O nanostructures sintered at different temperatures.

PL spectra were carried out using Xe light under the excitation of 220 nm for all the samples. The emission spectrum of the excitation at 220 nm gives five peaks at 299, 381, 453, 586 and 675 nm, respectively. The peaks centred at 299 and 381 nm corresponds UV emission which is attributed to a near band edge (NBE) emission of ZnO. The characteristic UV band edge PL peak of the as-prepared Zn0.94Cr0.03Fe0.03O nanostructures originates by the recombination of the free exciton transition from the localized level below the conduction band to the valence band [Ng et al. (2003)]. The intensity of the UV emission varied with the sintering temperature due to structural variation within the samples. The luminescence blue bands at 453, 586 and 675 nm are due to transition vacancy of oxygen, zinc interstitials and interstitial oxygen [Ozgur et al. (2005) and Vanhusden et al. (1995)]. The yellow emission in ZnO is due to the recombination of singly ionized oxygen vacancy defects where as the red emission is due to the doubly ionized oxygen vacancy defects. The appearance of sharp and strong UV emission and a broad blue band emission in the PL spectra indicates that the synthesized ZnO nanostructures have good crystallization quality with excellent optical properties. The room temperature PL spectra of Zn0.94Cr0.03Fe0.03O nanostructures indicate that the blue-red emission decrease with increasing sintering temperature. 3.4 UV-visible studies The optical energy band gap of Zn0.94Cr0.03Fe0.03O nanostructures sintered at room temperature to 400 0C are calculated by using UV-visible spectrophotometer absorption data. Figure 4 shows the graph of (αhv)2 versus photon energy (hv) for Zn0.94Cr0.03Fe0.03O nanostructures sintered at different temperatures and insets show the variation of Eg versus sintering temperature. For the direct transitions, the absorption coefficient is given by the equation: (α hv) = A (hv- Eg)1/2, where Eg is the optical energy band gap, ν is the frequency of the incident radiation and h is Planck’s constant, A is a constant, α = absorption coefficient and n = ½ for the allowed direct energy band and n = 3/2 for a forbidden direct energy gap semiconductor.


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Fig. 4 (αhv) 2 versus photon energy (hv) plot of Zn0.94Cr0.03Fe 0.03O nanostructures sintered at different temperatures. Insets show the variation of Eg versus sintering temperature.

The absorption coefficient (α) is evaluated using the relation: α = 4πk/λ; k is the absorption index or absorbance, λ is the wavelength in nm. The optical energy band gap (E g) is calculated by plotting Tauc’s graphs between (αhv) 2 versus photon energy (hv) and the intercept of this linear region on the energy axis at (αhv) 2 equal to zero gives the energy band gap. The value of optical energy band gap of ZnO nanoparticles are 3.42, 3.39, 3.31, 3.27and 3.25 eV for samples sintered at room temperature, 100, 200, 300 and 400 0C, respectively. The optical band gaps calculated through UV-visible spectroscopy are found to be decreasing from 3.42 to 3.25 eV as sintering temperature increases from room temperature to 400 0C. The decrease in optical band gap (Eg) with sintering temperature is due to increase in particle size [Sengupta et al. (2011), Sedky et al. (2012) and Sendi et al. (2013)]. Hence, the variation in optical band gap energy (Eg), reveals the impact of sintering temperature on the optical properties of the Zn0.94Cr0.03Fe0.03O nanostructures. 4. Conclusion In summary, we studied the effect of sintering temperature on the structural, morphological and optical properties in Zn0.94Cr0.03Fe0.03O nanostructures prepared via hydrothermal method. The XRD analysis revealed that all samples sintered at different temperature (room temperature to 400 0C) have the pure hexagonal (wurtzite) ZnO phase. The average crystallites sizes as measured through XRD data are found to be in the range of 13 -25 nm. PL study also confirms the phase formation of ZnO and reveals that yellow and red emissions in ZnO originate from the singly ionized and doubly ionized oxygen vacancy defects. It is observed that optical energy band gap varies from 3.42 to 3.25 eV as we increase the sintering temperature from room to 400 0C, respectively. The present study indicates that sintering temperature plays a significant role in tailoring the optical band gap of Zn0.94Cr0.03Fe0.03O nanostructures.

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