Ab initio electronic structure calculation of oxygen vacancies in rutile titanium dioxide Faruque M. Hossain a,b , G. E. Murch a, ⁎ , L. Sheppard b , J. Nowotny b a Diffusion in Solids Group, School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia b Centre for Materials Research in Energy Conversion, School of Materials Science and Engineering, The University of New South Wales, NSW 2052, Australia Received 2 February 2006; received in revised form 28 November 2006; accepted 21 December 2006 Abstract The electronic structure of rutile TiO 2 - x is studied using first-principles density functional theory (DFT) calculations. Nonstoichiometry in rutile TiO 2 due to defects in the form of oxygen vacancies leads to a considerable change in the electronic structure. In this paper, we calculate the band structure, density of states, and orbital energy distribution in a reduced (oxygen deficient) TiO 2 - x for different concentrations of oxygen vacancies (x). Energy levels are found to appear inside the forbidden energy region either as an isolated form of bands at different energy levels or merged with the conduction band depending on the value of x and the size of the super cells. © 2007 Elsevier B.V. All rights reserved. Keywords: Electronic structure; Titanium dioxide; Nonstoichiometry; Point defects 1. Introduction 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]. TiO 2 holds considerable promise due to its chemical stability in aqueous environments and under high energy illumination [3]. However, due to its large band gap, ∼ 3.0–3.2 eV (for rutile and anatase phases respectively), titanium dioxide lacks sensitivity to visible light which is necessary for high performance under solar illumination. This is especially the case for the generation of hydrogen from water to reach commercial efficiencies [3]. Accordingly, efforts have been made to increase the visible light sensitivity through band gap reduction [3,4]. However, an understanding of how the band gap can be manipulated in practice is limited, and made more difficult by discrepancies in the reported band gap literature. In terms of band gap, the rutile phase is preferable due to the lower band gap energy (3.05 eV) than the anatase (3.2 eV) phase. The rutile phase is also the more extensively studied phase of TiO 2 . There are many possibilities of band gap reduction using extrinsic doping with lattice matched foreign atoms, but these doped materials suffer from thermal instability and are associated with higher carrier recombination centers. Such extrinsic doping techniques cannot adequately increase the photo-electrochemical water-splitting efficiency and the effi- ciency of other photo-catalytic devices. It is instructive in this context to note that intrinsic defects such as oxygen vacancies, titanium interstitials, and titanium vacancies in the bulk or on the surface of TiO 2 may introduce defect energy levels inside the band gap. Larger aggregates of such point defects may form energy bands inside the band gap, which in turn may enhance the quantum efficiency of photo-catalytic devices. It is also a matter of concern that the quantum efficiency alone cannot increase the overall efficiency of these devices, unless the electrical conductivity of the electrode material fabricated of TiO 2 is also increased. Hence, to achieve better perfor- mance of such devices both the quantum efficiency and the electrical conductivity should increase concurrently. Pure TiO 2 at room temperature is an electrical insulator. It is well known Solid State Ionics 178 (2007) 319 – 325 www.elsevier.com/locate/ssi ⁎ Corresponding author. E-mail addresses: Mdfaruque.Hossain@newcastle.edu.au (F.M. Hossain), Graeme.Murch@newcastle.edu.au (G.E. Murch). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.12.015