Low-temperature perovskite-type cadmium titanate thin films derived from a simple particulate sol–gel process M.R. Mohammadi a,b, * , D.J. Fray a a Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK b Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, Tehran, Iran Received 24 December 2007; received in revised form 29 September 2008; accepted 24 October 2008 Available online 4 December 2008 Abstract Low-temperature perovskite-type cadmium titanate (CdTiO 3 ) with a nanocrystalline and mesoporous structure was prepared at var- ious Ti:Cd molar ratios by a straightforward particulate sol–gel route. The prepared sols had a narrow particle size distribution, in the range 23–26 nm. X-ray diffraction and Fourier transform infrared spectroscopy revealed that the powders contained a mixture of ilmen- ite-CdTiO 3 , perovskite-CdTiO 3 , anatase and rutile phases, depending on the annealing temperature and the Ti:Cd molar ratio. Perov- skite-CdTiO 3 was the major type obtained from cadmium-prominent powders at low temperature, whereas ilmenite-CdTiO 3 was the major type obtained from titanium-prominent powders at high temperature. It was observed that the anatase-to-rutile phase transfor- mation accelerated with decreasing Ti:Cd molar ratio. Furthermore, the ilmenite-to-perovskite phase transformation accelerated with a decrease in both the Ti:Cd molar ratio and the annealing temperature. The crystallite sizes of the ilmenite- and perovskite-CdTiO 3 phases reduced with increasing the Ti:Cd molar ratio. Field emission scanning electron microscopic analysis revealed that the average grain size of the thin films decreased with an increase in the Ti:Cd molar ratio. Moreover, atomic force microscope images showed that CdTiO 3 thin films had a columnar-like morphology. Based on Brunauer–Emmett–Taylor analysis, cadmium titanate powder containing Ti:Cd = 75:25 showed the greatest surface area and roughness and the smallest pore size among all the powders annealed at 500 °C. This is one of the smallest crystallite sizes and largest surface areas reported in the literature, and can be used in many applications in areas from optical electronics to gas sensors. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Cadmium titanate; Perovskite; Low temperature; Sol–gel; Nanocrystalline 1. Introduction Nanocrystalline titanium dioxide has a wide range of applications, such as ultraviolet filters for optics and pack- ing materials [1], anti-reflection coatings for photovoltaic cells and passive solar collectors [2], photocatalysts for purification and treatment of water and air [3], anodes for lithium-ion batteries [4], electrochromic displays [5], transparent conductors, self-cleaning coatings of windows and tiles [6], humidity sensors [7] and gas sensors [8]. For gas-sensing applications, many efforts have been aimed at improving the performance by controlling the microstructure and doping with hetero-components (such as La, Cr, W, Fe, Nb, Ta, Ga or Mo), because active sites for particular gas species can be produced [9]. Further- more, it has been reported that the sensor performance can be modified by producing binary metal oxides (such as TiO 2 –Ga 2 O 3 , TiO 2 –MoO 3 , TiO 2 –WO 3 , TiO 2 –Cr 2 O 3 , TiO 2 –V 2 O 5 ) and varying the composition of its constitu- ents [10–14]. Interest in titanium-based oxides with the perovskite crystalline phase of the form ABO 3 for gas-sensing applica- 1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.10.040 * Corresponding author. Address: Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK. Tel.: +44 1223 334343/79 5416 4928. E-mail addresses: mrm41@cam.ac.uk, mohammadi@sharif.edu (M.R. Mohammadi). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 57 (2009) 1049–1059