Origin of defect states on the surface ofTiO2

Abstract

We have investigated the electronic structure of oxygen vacancies on Ti02(110);including atomic relaxation. The study of different defect sites shows that the experimentally observed gap state at 0.7 eV below the conduction-band edge is indeed due to an oxygen vacancy. This state, however, is not due to the removal of an 0 atom from a surface bridging site, as frequently proposed, but results from a subsurface oxygen vacancy. In this configuration, there is a maximum reduction of the screening between surface cations.The knowledge of electronic properties of TiO2 surfaces is of fundamental importance for the understanding of the mechanisms taking place at an atomic scale in a number of practical applications of this material, such as heterogeneous catalysis, the photolysis of water, etc. ' In particular, the (110) face has been used as an ideal, thermodynamically stable prototype surface in the study of semiconductor-gas interactions. On these surfaces, point defects play a crucial role, as they act as active sites for chemisorption or molecular dissociation.Experimentally, no states are observed in the gap for defect-free Ti02 surfaces, but the creation of surface defects by Ar+-ion bombardment leads to a nonzero density of states in the 3-eV-wide gap.The energetic position of these states, as determined from. ultraviolet photoemission spectroscopy (UPS) and electron-energy-loss spectroscopy (EELS) measurements, is at 2.3 eV above the top of the valence band (i.e., 0.7 below the conduction-band edge E, ), with an experimental resolution of typically 0.2 eV. Auger electron analysis shows that the appearance of these states is associated with a reduction of the O/Ti ratio in the surface region, which suggests that they are due to oxygen surface vacancies. However, neither the precise atomic configuration nor the detailed electronic structure of these defects is presently known.In the present paper, we report the first theoretical study of the electronic structure of single point defects on otherwise perfect TiOq (110) surfaces. For the ideal (110) surface, we consider two different models: In the first one (I), the topmost atomic plane contains two Ti and two 0 atoms per surface unit cell and corresponds to the most compact atomic plane for this crystallographic orientation. In the second model (II), the semi-infinite solid is terminated by an additional 0 layer; this corresponds to the cutting of a minimum number of Ti -O bonds when creating the surface. These two models are displayed in Fig. 1 and their electronic structure has been studied previously. The creation of the surface leads essentially to the appearance of 0 p resonances in the conduction band and Tid resonances in the valence-band region. No occupied 0 p states appear in the gap, in accordance vvith the experimental results quoted above. Only for the type-I surface is there a Ti d state following closely the conduction-band edge. The existence of ideal bulk 0 t (00'I) I'. A WJ VJ o(~) ' o(4) c~0~õ {~) T~{~) o{z) 042, T I {p )' I r.Ṽ J 0 0 '@~. o(o) ...

Auger–Photoelectron Coincidence Spectroscopy (APECS) of Transition Metal Compounds

“…One can then consider the occupied valence density of states on the Ti site and then apply the self-convolution. The results of such a calculation, using the local density of states from the literature, [8][9][10][11] are shown as the dot-dashed curve in Figure 13.2(b). Clearly, this notion resolves many of the problems encountered by the previous approach.…”

Electrical and catalytic properties of SnO2/TiO2 measured in operando conditions

“…Two shallow donor levels found experimentally for SnO 2 (0.034 and 0.145 eV below the bottom of the conduction band [27]) were suggested to represent the neutral and mono-ionized oxygen vacancies; the deep donor levels in the case of TiO 2 were allocated either to subsurface oxygen vacancies or to cationic Ti 3? sites [28,29]. For polycrystalline n-type semiconductor powders, the conductivity is limited by the charge transfer across the intergrain barriers.…”

Reaction rate modeling in noncatalytic gas‐solid systems: Species transport and mechanical stress

“…They found that there are two processes present: charge transfer and chemisorption of oxygen molecules, and interaction of oxygen vacancies with chemisorbed species. Munnix and Schmeits (1985) reported that the origin of the surface defects in TiO, results from a subsurface oxygen vacancy. Recently, Zhong et al (1993) performed a comparative study on the local defect structure in vacuum-annealed and hydrogen-annealed rutile by means of scanning tunneling microscopy.…”