Synthesis, characterization and studies on optical properties of indium doped ZnO nanoparticles

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1 Indian Journal of Chemistry Vol. 53A, April-May 2014, pp Synthesis, characterization and studies on optical properties of indium doped ZnO nanoparticles Manu Sharma & P Jeevanandam* Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee , India jeevafcy@iitr.ac.in Received 26 December 2013; revised and accepted 17 February 2014 Indium doped zinc oxide hierarchical nanostructures have been synthesized using a simple aqueous solution based method. A change in morphology from hierarchical nanostructures in ZnO to nanorods after doping with indium has been observed. The optical properties of nanostructures have been investigated using diffuse reflectance and photoluminescence spectroscopy. The band gap of zinc oxide nanoparticles changes after the incorporation of indium and a blue shift of band gap is observed. Keywords: Nanostructures, Nanorods, Doping, Optical properties, Photoluminescence, Diffuse reflectance spectroscopy, Zinc oxide, Indium Semiconductor nanoparticles such as ZnO, TiO 2 and SnO 2 have become the centre of attention due to their unique optical, electronic, structural and catalytic properties. Zinc oxide is a semiconductor with a wide direct band gap (3.37 ev) and a large excitonic binding energy (60 mev) 1. Owing to their interesting properties, zinc oxide nanoparticles possess interesting applications in the fields of electronics, optoelectronics and UV laser devices 2 4. Recently, change in band gap of ZnO nanoparticles when doped with different metal ions such as Mg 2+, Cd 2+, Fe 2+, Mn 2+ and Co 2+ has been reported 5 8. Ga, In, Sn, and Al doped ZnO nanostructures have also been synthesized to enhance their electrical, and optical properties 9. ZnO nanowires doped with a high concentration of Ga, In, and Sn have been synthesized via thermal evaporation method 10. Indium doped ZnO nanostructures with different morphologies have been synthesized using solvothermal method in the presence of various cationic, anionic and non-ionic surfactants 11. ZnO is an interesting semiconductor material having hexagonal close packed (hcp) lattice with empty octahedral sites 12. There are plenty of sites available in ZnO that can be accommodated by intrinsic defects and extrinsic dopants. Doping of selective elements in ZnO nanoparticles is an effective way to modify/tune the optical, electrical, magnetic and catalytic properties of ZnO 11. The optical properties of ZnO nanoparticles can be tuned by incorporating various ions such as alkaline earth metal ions, transition metal ions and rare earth metal ions into the crystal lattice. It has been reported that indium doped ZnO nanoparticles as well as thin layers possess excellent conductivity, high transparency and piezoelectricity and are useful in fabricating solar cells, electrodes, and sensors Wide band gap semiconductors such as ZnO undergo dramatic changes in their optical and electrical properties on doping with various metal ions. According to the theory of semiconductor metal transition, the band gap decreases when the amount of impurity is more than the Mott critical density 16,17. It has also been reported that the optical band gap of indium doped ZnO nanoparticles is larger than that of pure ZnO due to Burstein-Moss effect and there are more electrons contributed by indium that make up the energy levels located at the bottom of the conduction band 18. Currently, various methods have been used for the synthesis of indium doped ZnO nanostructures, viz., hydrothermal 19, thermal evaporation 20, pulsed laser deposition 21, spray pyrolysis 22, electrostatic spray deposition 23, chemical spray deposition 24,25 and Rf-magnetron sputtering 26,27. In the present study, indium doped zinc oxide nanostructures have been successfully synthesized using co-precipitation method at low temperature. The major goal was to synthesize indium doped zinc oxide nanoparticles

2 562 INDIAN J CHEM, SEC A, APRIL-MAY 2014 and to investigate changes in the optical properties of ZnO nanoparticles with the incorporation of indium ions. Materials and Methods The preparation of hierarchical ZnO nanostructures was based on a previously reported method 28. For the synthesis of indium doped zinc oxide, 2.1 g of zinc acetate was dissolved in 200 ml of distilled water along with varying amounts of indium acetate (1 50 mol %). After 10 minutes of stirring, about 1.5 g of trisodium citrate in 10 ml water and 4.2 ml of 25% ammonia solution were added. Then, 20 ml of 2 M NaOH aqueous solution was added drop wise with vigorous stirring. The temperature of the contents was raised to 80 o C and kept at this temperature for about 12 hours. The contents were centrifuged, washed with water and dried at 60 o C to get indium doped ZnO powder samples. Characterization of In doped ZnO Powder XRD patterns were recorded using a Brucker AXS-D8 diffractometer operating with Cu-Kα radiation (λ = Å) with a scanning speed, 2 o /min. Field emission scanning electron microscope (FE-SEM) images were obtained using a FEI Quanta 200F electron microscope operating at an accelerating voltage of 20 kv. For the study of optical properties of indium doped zinc oxide nanoparticles, a Shimadzu UV-3600 UV-visible NIR spectrophotometer was used in the wavelength range nm along with a diffuse reflectance accessory. BaSO 4 was used as the reference. Photoluminescence studies were carried out using a Varian Cary Eclipse fluorescence spectrometer at an excitation wavelength of 350 nm. About 10 mg each of the powder samples was suspended in ethanol, sonicated for about 10 minutes and then the fluorescence spectra were recorded for the suspensions. powder X-ray diffraction patterns of indium doped zinc oxide nanoparticles show a shift of (002) reflection to higher angle (see the inset in Fig. 1) on increasing the indium acetate concentration used during the synthesis of indium doped ZnO nanoparticles. The shift of (002) reflection is clear evidence for the doping of indium in zinc oxide nanoparticles. The crystallite size of pure zinc oxide was found to be smaller in comparison with that of indium doped zinc oxide nanostructures. At higher indium acetate concentration used during the synthesis, the peaks due to In 2 O 3 are also observed. Figure 2 shows the FE-SEM images of pure zinc oxide and indium doped zinc oxide samples. The images show change in morphology of particles from hierarchical nanostructures in pure ZnO to distorted hierarchical nanostructures ([In 3+ ] = 12.5 %) and then to rods ([In 3+ ] = 50 %). Pure zinc oxide shows hierarchical structures with flower-like morphology with a diameter of 2.9 µm. The hierarchical structures are further made up of small nanorods with mean length 1.1 µm and width 70 nm. When the amount of indium acetate used during the synthesis of indium doped zinc oxide nanoparticles was increased to 50 mol %, rods were formed. The average length of rods observed is ~4 µm and the width is ~0.25 µm. Recently, Yang et al. 29 have reported a change in morphology of ZnO nanoparticles to tetrapods and nanorods with magnesium doping. EDX data of pure zinc oxide and indium doped zinc oxide hierarchical nanostructures confirm the Results and Discussion Characterization of ZnO and In doped ZnO XRD patterns of pure zinc oxide and indium doped zinc oxide samples are shown in Fig. 1. The XRD patterns of pure ZnO and indium doped zinc oxide samples show wurtzite structure. All the reflections match with that of ZnO (JCPDS file No ). Indium doped ZnO nanoparticles also possess the wurtzite structure similar to ZnO, due to the similar ionic radii of In 3+ (0.62 Å) and Zn 2+ (0.74 Å). The Fig. 1 XRD patterns of pure ZnO and indium doped ZnO hierarchical nanostructures. [Inset: XRD in the range of indicating shift in (002) reflection].

3 SHARMA & JEEVANANDAM: SYNTHESIS & OPTICAL PROPERTIES OF In DOPED ZnO NANOPARTICLES 563 Fig. 2 FESEM images of pure ZnO and indium doped ZnO samples. [(a) pure zinc oxide; (b) [In 3+ ] = 5%; (c) [In 3+ ] = 12.5%; (d) [In 3+ ] = 50%]. presence of indium in doped zinc oxide hierarchical nanostructures (Supplementary Data, Fig. S1). IR spectra of indium doped ZnO nanostructures show bands at ~ cm -1 and 1600 cm -1 which are attributed to hydroxyl stretching and bending, respectively 5 (Supplementary Data, Fig. S2). The IR spectra also show bands in the region cm -1, attributed to C=O, C O and C H groups due to acetate groups 30. The intensity of the bands in the region cm -1 increases with In 3+ doping due to the presence of more acetate groups on the surface of ZnO. The band at ~ cm -1 is attributed to the formation of ZnO nanoparticles 31. Optical properties UV-visible diffuse reflectance spectra of indium doped zinc oxide hierarchical nanostructures show blue shift of band gap with respect to pure ZnO after the incorporation of indium ions (Fig. 3). The band gap increases from 3.38 ev (pure ZnO) to 3.58 ev Fig. 3 Diffuse reflectance spectra of pure and indium doped zinc oxide hierarchical nanostructures.

4 564 INDIAN J CHEM, SEC A, APRIL-MAY 2014 of visible emission. The visible emission band at ~ 550 nm is due to doubly charged oxygen vacancy (V ++ O ). On the other hand, the singly charged oxygen vacancy (V + O ) is responsible for the emission band at about 500 nm 37. Jhu et al. 38 have also reported that the photoluminescence peak intensity of zinc oxide nanoparticles decreases after indium doping, as observed in the present study. Fig. 4 Photoluminescence spectra of pure and indium doped zinc oxide hierarchical nanostructures. on indium doping. A plot of band gap versus concentration of indium ions (mol%) used during the preparation of indium doped ZnO shows the maximum band gap at [In 3+ ] = 12.5 %. In the present study, the change in band gap (blue shift) of ZnO is attributed to indium doping and not to the change in particle size. Figure 4 shows the photoluminescence spectra of indium doped zinc oxide hierarchical nanostructures. The photoluminescence spectra show two emission bands; one due to the near band edge emission of ZnO and the other due to oxygen vacancies. The UV emission band at about 385 nm, corresponding to the near band edge emission, is due to excitonic recombination in ZnO. The intensity of near band emission band is lower in indium doped ZnO samples at all concentrations of indium as compared to that of pure ZnO. The decrease of the UV emission intensity after indium doping is due to weak excitons (Coulomb interaction effect) 18. The emission band observed at ~ 550 nm corresponds to oxygen vacancies and the intensity of this band decreases after the doping of indium in the zinc oxide hierarchical nanostructures. This is attributed to the reduction in the number of oxygen vacancies. Visible emission of zinc oxide nanoparticles is usually observed as a broad band at room temperature. The visible emission from ZnO in the blue-green region has been reported as related to surface defects, such as oxygen vacancies The ionization state of the vacancies has a strong impact on the intensity Conclusions In the present study, indium doped zinc oxide nanoparticles have been successfully synthesised by an aqueous solution based method. Change in morphology of ZnO hierarchical nanostructures to rods is observed on doping with indium. The optical properties of indium doped ZnO hierarchical nanostructures depend on the concentration of indium ions and show a blue shift of band gap after the incorporation of indium ions into pure zinc oxide. The present method is one of the simplest methods for the synthesis of indium doped ZnO hierarchical nanostructures. Supplementary Data The supplementary data associated with this article i.e., Figs S1 and S2, are available in the electronic form at ijca/ijca_53a(4-5) _suppldata.pdf Acknowledgement Generous funding from the Department of Science and Technology, Government of India in the form of a research grant (Project No. SR/S1/PC-06/2007) is gratefully acknowledged. References 1 Lu X, Liu Z, Zhu Y & Jiang L, Mater Res Bull, 46 (2011) Kumpika T, Thongsuwan W & Singjai P, Thin Solid Films, 516 (2008) Djurišić A B, Ng A M C & Chen X Y, Prog Quan Electron, 34 (2010) Zhang C, Zhang F, Xia T, Kumar N, Hahm J, Liu J, Wang Z L & Xu J, Opt Express, 17 (2009) Qiu X, Li L, Zheng J, Liu J, Sun X, & Li G, J Phys Chem C, 112 (2008) Li G, Bu Q, Zheng F, Su C & Tong Y, Cryst Growth Des, 9 (2009) Luo J T, Yang Y C, Zhu X Y, Chen G, Zeng F & Pan F, Phys Rev B, 82 (2010) Wang H B,Wang H, Zhang C, Yang F J, Duan J X, Yang C P, Gu H S, Zhou M J & Li Q, J Nanosci Nanotechnol, 9 (2009) 3308.

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