Titanium dioxide (TiO 2 ) has a number of uses in catalysis, photochemistry, and sensing that are linked to the reducibility of the oxide. Usually, bridging oxygen (O br ) vacancies are assumed to cause the Ti 3d defect state in the band gap of rutile TiO 2 (110). From high-resolution scanning tunneling microscopy and photoelectron spectroscopy measurements, we propose that Ti interstitials in the near-surface region may be largely responsible for the defect state in the band gap. We argue that these donor-specific sites play a key role in and may dictate the ensuing surface chemistry, such as providing the electronic charge required for O 2 adsorption and dissociation. Specifically, we identified a second O 2 dissociation channel that occurs within the Ti troughs in addition to the O 2 dissociation channel in O br vacancies. Comprehensive density functional theory calculations support these experimental observations.

The role of bulk defects in the oxygen chemistry on reduced rutile TiO(2)(110)-(1 × 1) has been studied by means of temperature-programmed desorption spectroscopy and scanning tunneling microscopy measurements. Following O(2) adsorption at 130 K, the amount of O(2) desorbing at ∼410 K initially increased with increasing density of surface oxygen vacancies but decreased after further reduction of the TiO(2)(110) crystal. We explain these results by withdrawal of excess charge (Ti(3+)) from the TiO(2)(110) lattice to oxygen species on the surface and by a reaction of Ti interstitials with O adatoms upon heating. Important consequences for the understanding of the O(2)-TiO(2) interaction are discussed.

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By focusing on the similarities between the three stages of sintering, a single equation is derived that quantifies sintering as a continuous process from beginning to end. The microstructure is characterized by two separate parameters representing geometry and scale. The dimensionless geometry parameter, denoted r, comprises five scaling factors that relate specific microstructural features (e.g., surface curvature) to the scale (grain diameter). Calculations of r from experimental data show (a) agreement with computer simulations of initial-stage sintering, (b) the effect of surface diffusion on I?, and (c) changes in r with microstructural evolution during sintering. Application of the model to the design of firing schedules and the study of microstructural geometry effects on sintering is discussed. [

By means of high-resolution scanning tunneling microscopy (STM), we have revealed unprecedented details about the intermediate steps for a surface-catalyzed reaction. Specifically, we studied the oxidation of H adatoms by O(2) molecules on the rutile TiO(2)(110) surface. O(2) adsorbs and successively reacts with the H adatoms, resulting in the formation of water species. Using time-lapsed STM imaging, we have unraveled the individual reaction intermediates of HO(2), H(2)O(2), and H(3)O(2) stoichiometry and the final reaction product-pairs of water molecules, [H(2)O](2). Because of their different appearance and mobility, these four species are discernible in the time-lapsed STM images. The interpretation of the STM results is corroborated by density functional theory calculations. The presented experimental and theoretical results are discussed with respect to previous reports where other reaction mechanisms have been put forward.

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in rutile TiO 2 (110) from the bulk to the surface has been studied utilizing two experimental techniques. Electron-stimulated desorption of O + ions was employed to kinetically monitor the reaction between oxygen adatoms with Ti i 3+ species at temperatures between 360 and 400 K. Scanning tunneling microscopy was also used to measure the Ti i 3+ diffusion rate. Both methods yield a rate constant k Ti i 3+ ) 5 × 10 -4 s -1 at 393 K. The activation energy as measured by the rate dependence on temperature is ∼1.0 eV. Titania (TiO 2 ) has wide applications in the fields of heterogeneous catalysis, photocatalysis, photovoltaic cells, and gas sensors. 1-4 To better understand the relationship between atomic and electronic structure and reactivity, the rutile TiO 2 (110)-(1 × 1) surface ( Figure 1a) has been extensively studied as a model oxide surface. 1-3,5,6 Surface as well as bulk defects have been postulated as active sites for the chemical and photochemical reactivity of the TiO 2 surface. 1,2,7,8 Since reduced TiO 2 crystals contain interstitial Ti i 3+ ions, 8-15 the diffusion of these ions to the TiO 2 surface may well be involved in a variety of surface processes. Noteworthy is the SMSI (strong metal-support interaction) effect, where TiO x layers, produced by Ti i 3+ diffusion to the surface, cover the surface of metal particles deposited on the TiO 2 surface, strongly affecting the rates of surface reaction on the particles. [16][17][18][19] Therefore, accurate measurements of the rate and activation energy for Ti i 3+ bulk diffusion are crucial for an improved understanding of surface processes driven by Ti i 3+ interstitial ions in the TiO 2 bulk. Because of a lack of sensitivity to buried species the study of interstitial species in the bulk and near surface region is a challenge to conventional surface science methods. Recently, however, it has been found from electron-stimulated desorption (ESD) studies that oxygen adatoms (O t ) on the TiO 2 (110)-(1 × 1) surface (Figure 1a), produced by dissociation of O 2 and chemisorption on the 5-fold-coordinated Ti (Ti 5c ) sites, 6,20-22 have a very high ionic cross section for O + production. 23 The high sensitivity of ESD to the O t adatoms therefore provides an excellent tool to measure the kinetics of the reaction between O t adatoms and out-diffusing Ti i 3+ interstitial species. In this letter we study the kinetics of interstitial Ti i 3+ species diffusion through rutile TiO 2 (110) crystals using both ESD and high-resolution scanning tunneling microscopy (STM). Monitoring the reaction between chemisorbed oxygen atoms with Ti i 3+ species at temperatures between 360 and 400 K by means of ESD allows us to deduce the rate and activation energy for the diffusion process. Excellent agreement is found for the Ti i 3+ diffusion rate constant at 393 K, which is ∼5 × 10 -4 s -1 as extracted independently from ESD and STM studies. In addition, an energy barrier of ∼1 eV was estimated for the diffusion of Ti i 3+ interstitials toward the surface.The ESD and STM expe...