Cadmium- and indium-doped zinc oxide by combustion synthesis using dopant chloride precursors
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C.R. Chenthamarakshan, N.R. de Tacconi, and K. Rajeshwara) Center for Renewable Energy Science and Technology, Department of Chemistry and Biochemistry, The University of Texas, Arlington, Texas 76019-0065 (Received 21 January 2006; accepted 19 September 2006)
Cadmium-doped ZnO was prepared for the first time by combustion synthesis using CdCl2 as a dopant precursor, with zinc nitrate and urea as the combustion mixture. Unlike previous studies of combustion synthesis of ZnO in the presence of an indium nitrate precursor, which resulted in (ZnO)mIn2O3 (m ⳱ 3 or 4) compound formation, In-doped ZnO was prepared by combustion synthesis in this study using an InCl3 precursor. The doped samples were compared and contrasted with undoped ZnO using scanning electron microscopy, x-ray powder diffraction, energy-dispersive x-ray analyses, and x-ray photoelectron spectroscopy. Diffuse reflectance spectroscopy showed the optical band gap of ZnO to shrink from 3.14 to 3.07 eV and 3.02 eV on Cd and In doping, respectively. Finally, the doped samples showed an improved photoelectrochemical response relative to undoped ZnO over the wavelength range from ∼300 to ∼450 nm.
I. INTRODUCTION
Zinc oxide (ZnO) is an important material for many applications, including varistors, transistors, piezoelectric devices, solar cell windows, UV and blue lightemitting devices, and the like. For many of these applications, it is important to control the physical, electronic, and optical properties of ZnO by doping. In this article, the preparation and characterization of Cd- and In-doped ZnO by combustion synthesis are described. The primary motivation for doping of ZnO is wavelength tunability for optoelectronic applications, although doping also results in marked improvement in the photoelectrochemical properties of ZnO as demonstrated in this work. Since the first report in 1996,1 a raft of studies have begun to appear on the Zn–Cd–O material system.2–12 These interesting materials have been prepared by the sol-gel process,1 spray pyrolysis,2,3,5 pulsed laser deposition,4 molecular beam epitaxy,6 DC reactive magnetron sputtering,8 coevaporation,9–11 plasma-enhanced metal organic chemical vapor deposition,10 and electrodeposition.12 Conspicuously absent from this array of preparation techniques is combustion synthesis.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0399 3234 J. Mater. Res., Vol. 21, No. 12, Dec 2006 http://journals.cambridge.org Downloaded: 24 Dec 2014
A pioneering study of the Zn–In–O material system appeared in 1995.13 This study, using pulsed laser deposition, showed that Zn–In–O films had similar electrical conductivity but better transparency in the visible and infrared spectral regions compared with Sn-doped In2O3 (ITO). Subsequent studies on this system have used simultaneous RF and DC magnetron sputtering,14 electrodeposition,15 and spray pyrolysis16 as the preparation techniques. Very recent studies by another group have described the preparation of (ZnO)mIn2O3 (m ⳱ 3 or
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