The Effect of Defect Disorder on the Electronic Structure of Rutile TiO 2-x Faruque M. Hossain 1),a) and G. E. Murch Diffusion in Solids Group, School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia. and L. Sheppard and J. Nowotny Centre for Materials Research in Energy Conversion, School of Materials Science and Engineering, The University of New South Wales, NSW 2052, Australia. Keywords: Electronic structure, titanium oxide, defect formation energies, nonstoichiometry Abstract. The purpose of this work is to study the effect of bulk point defects on the electronic structure of rutile TiO 2 . The paper is focused on the effect of oxygen nonstoichiometry in the form of oxygen vacancies, Ti interstitials and Ti vacancies and related defect disorder on the band gap width and on the local energy levels inside the band gap. Ab initio density functional theory is used to calculate the formation energies of such intrinsic defects and to detect the positions of these defect induced energy levels in order to visualize the tendency of forming local mid-gap bands. Apart from the formation energy of the Ti vacancies (where experimental data do not exist) our calculated results of the defect formation energies are in fair agreement with the experimental results and the defect energy levels consistently support the experimental observations. The calculated results indicate that the exact position of defect energy levels depends on the estimated band gap and also the charge state of the point defects of TiO 2 . Introduction The electronic structure of point defects in oxide semiconductors has been extensively studied in order to establish the relationship between defect chemistry and semi-conducting properties. In principle, a perfect stoichiometric metal oxide exhibits insulating properties. Preparation-induced intrinsic defects or structural changes can turn these oxide materials into semi- conducting or conducting depending on the concentration of defects introduced or the level of structural deformation. Point defects in oxide materials either form deep energy levels, which may act as trapping centers or shallow energy levels, which may act as donors. In order to understand the effect of defect disorder on the electronic properties, it is essential to know the energetic position of the various defects inside the band gap which can predominate. Titanium dioxide (TiO 2 ) is a wide-gap semiconductor and important oxide material because of its broad range of technological applications, long-term chemical stability, and non-toxicity. Since the pioneering work of Fujishima and Honda [1], TiO 2 has received special attention as a prime candidate material for photo-electrochemical water-splitting and other photo-catalytic applications [2-4]. Titanium dioxide is a promising semiconducting material on account of its chemical stability in aqueous environments and under high energy illumination [3]. However, due to its large band gap, ~3-3.2 eV (for rutile and anatase phases respectively), titanium dioxide lacks Defect and Diffusion Forum Vols. 251-252 (2006) pp 1-12 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland Online available since 2006/Mar/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 134.148.29.34-20/01/09,00:43:43)