Defect chemistry and semiconducting properties of titanium dioxide: I. Intrinsic electronic equilibrium q T. Bak, J. Nowotny * , M. Rekas, C.C. Sorrell Centre for Materials Research in Energy Conversion, School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Received 11 October 2002; accepted 9 December 2002 Abstract The present work describes defect chemistry and semiconducting properties of TiO 2 within the n – p transition regime. Quantitative considerations on the relationships between the concentration of ionic and electronic defects at the minimum of electrical conductivity vs. oxygen partial pressure resulted in the derivation of a theoretical model. The model is based on the empirical data of electrical conductivity for Cr-doped TiO 2 (exhibiting n – p transition). This model was then applied for the determination of the intrinsic electronic equilibrium constant which is the following function of temperature: K i ¼ 3:74 £ 10 22 exp 2 3:039 ^ 0:053 eV kT   ð1Þ A good agreement between this equilibrium constant and the experimental data of electrical conductivity for TiO 2 doped with donors (Cr) was revealed. It was observed that the concentration of oxygen vacancies determined using the derived model is only slightly dependent on the value of the electronic intrinsic equilibrium constant. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: D. Electrical properties; D. Electrical conductivity; D. Transport properties; D. Defects; D. Semiconductivity; A. Electronic materials 1. Introduction The vast majority of the experimental material on electrical transport and defect chemistry of TiO 2 is limited to the range of p(O 2 ) at which this oxide material exhibits n- type properties. Then its conduction has been considered using the theoretical models assuming that electrons are the predominant electronic charge carriers. So far, presence of electron holes has been ignored in the consideration of defect chemistry of TiO 2 [1–6]. Consequently, little is known about the transport mechanism of electron holes and their impact on conduction. The assumption that the charge transport is determined by electrons is valid for reduced TiO 2 . The effect of minority charge carriers (electron holes) on conduction increases with the increase of the concentration of acceptor-type defects and, consequently, increase of p(O 2 ), and decrease of temperature [7]. Then the electrical conduction should be considered within a model adequate to the n – p transition regime rather then n-type regime. Then the charge transfer should be considered in terms of the conduction mechanism of both electronic charge carriers. The experimental and theoretical material on the transport of electron holes in TiO 2 [7,8] is very limited [7–10]. Therefore, the purpose of the present work is the determination of the impact of electron holes on electrical 0022-3697/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0022-3697(02)00479-1 Journal of Physics and Chemistry of Solids 64 (2003) 1043–1056 www.elsevier.com/locate/jpcs q This work is a part of a research program on titania-based photo- electrodes for solar hydrogen. * Corresponding author. Tel.: þ61-2-9385-6465; fax: þ 61-2- 9385-6467. E-mail address: j.nowotny@unsw.edu.au (J. Nowotny).