Electronic Structure of Defects on Reduced TiO 2 (Rutile) Surfaces
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ELECTRONIC STRUCTURE OF DEFECTS ON REDUCED TiO 2 (RUTILE) SURFACES QIAN ZHONG*, JOHN M. VOHS**, AND DAWN A. BONNELL*
*Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104-6272 "**Department of Chemical Engineering, University of Pennsylvania, Philadelphia, PA 191046393
ABSTRACT Several TiO 2 (110) surfaces, reduced via different mechanisms, were investigated in ultra-high vacuum using Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) to determine the effect of reduction on the surface atomic and electronic structure. Defect chemistry and its relation to the change of property and structure are discussed. 1. INTRODUCTION A fundamental understanding of atomic and electronic structure of defects in titanium dioxide (TiO 2 , rutile) is crucial to practical applications such as catalysis, photoelectrolysis, and electronic ceramics. Moreover, TiO 2 serves as an excellent model system for empty d-band transition-metal oxides. Theoretical calculations of surface electronic structure and experiments using conventional surface-science techniques have been performed for both the stoichiometric surface and surfaces that are non-stoichiometric containing various types and densities of defects [1]. The use of Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS) provides a source of information that is complementary to existing techniques in that STM is capable of local imaging real-space atomic and geometric arrangements on the surface, STS probes both occupied and unoccupied states of surface electronic structure in a single experiment, and the combination of the two provides a spatial correlation between the geometric and electronic structure. STM, along with its spectroscopic applications, has developed significantly since its inception, both in the theoretical foundation and physical understanding of experimental results and in the practical aspects of data acquisition. Detailed discussions on the technique and recent progress in the applications can be found in a number of review papers [2]. It is worth noting, however, that the use of STM in ceramics, such as transition-metal oxides, is nontrivial; a large energy gap and low density of surface states often hamper tunneling. We have initiated a systematic study of various transition-metal oxides using STM and STS; a more detailed discussion of which has been published elsewhere [3]. Recently, we have examined several (110) surfaces of a rutile single crystal, reduced by various mechanisms. In this paper, evidence of differences in local electronic structure for defects produced via different reaction sequences is presented.
2. EXPERIMENTAL PROCEDURES A rutile single crystal (Atomergic Chemetals Corp.) with a (110) orientation was sliced into four pieces, each about 5 mm x 5 mm x 1 mm in size. Laue diffraction was used to verify the crystallinity and orientation. The four samples were prepared by the following processes: (1) hydrogen annealing (800 0 C, 2 hrs), (2) ion sputtering (2 kV, 1 hr)
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