Surface photovoltage spectroscopy study of reduced and oxidized nanocrystalline TiO 2 films A. Rothschild a, * , A. Levakov b , Y. Shapira b , N. Ashkenasy b,1 , Y. Komem a a Faculty of Materials Engineering, Technion––Israel Institute of Technology, Technion City, Haifa 32000, Israel b Department of Electrical Engineering––Physical Electronics, Tel Aviv University, Tel Aviv 69978, Israel Abstract Nanocrystalline TiO 2 films used for gas sensors have been studied by means of surface photovoltage spectroscopy and other analytical tools to investigate the oxygen chemisorption effect on the electrical properties of the films. The results show that the surface (and intergranular interface) band bending increases with oxygen exposure due to electron trapping at midgap states induced by chemisorption. The surface electronic structure is revealed by the measurements, allowing determination of the sensing mechanism of these important films. In addition, a photoinduced chemisorption of oxygen at room temperature is observed. This has important implications for low-temperature gas sensors. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Titanium oxide; Polycrystalline thin films; Surface electronic phenomena (work function, surface potential, surface states, etc.); Surface photovoltage; Chemisorption; Interface states 1. Introduction Nanocrystalline titanium dioxide (TiO 2 ) has manyimportantapplicationssuchassolarcells[1], photocatalysts for water photolysis [2] and degra- dation of environmental pollutants in air and wastewaters [3,4], and as an oxygen- and gas-sen- sor material [5]. The key mechanisms in these ap- plications are based on surface redox reactions that lead to charge separation in the space-charge region adjacent to the surface and/or charge transfer between the surface and the bulk of the nanosized crystallites [6]. Thus, it is important to understand the impact of such reactions on the electronic properties of nanocrystalline TiO 2 . TiO 2 forms three different crystalline structures: rutile, anatase, and brookite. Rutile is the thermo- dynamically stable phase, while anatase and brookite are metastable polymorphs that irre- versibly transform to rutile upon heating [7]. The electronic structure and properties of single-crys- talline rutile have been studied quite extensively (see, e.g., Refs. [8–10]), but only little is known on nanocrystalline rutile [11], while most of the liter- ature on nanophase TiO 2 concerns ultra fine col- loids of the anatase modification (see, e.g., Refs. [12,13]). The interest in colloidal anatase stems from its high photocatalytic activity, which is considered by many to be superior to that of rutile [3,4]. However, the rutile phase is much more stable than anatase and easier to produce. In fact, * Corresponding author. Tel.: +972-4-8294598; fax: +972-4- 8321978. E-mail address: mtavner@tx.technion.ac.il (A. Rothschild). 1 Present address: The Scripps Research Institute, La Jolla, CA 92037, USA. 0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00154-7 Surface Science 532–535 (2003) 456–460 www.elsevier.com/locate/susc