The surface science of titanium dioxide

Titanium dioxide is the most investigated single-crystalline system in the surface science of metal oxides, and the literature on rutile (1 1 0), (1 0 0), (0 0 1), and anatase surfaces is reviewed. This paper starts with a summary of the wide variety of technical fields where TiO ₂ is of importance. The bulk structure and bulk defects (as far as relevant to the surface properties) are briefly reviewed. Rules to predict stable oxide surfaces are exemplified on rutile (1 1 0). The surface structure of rutile (1 1 0) is discussed in some detail. Theoretically predicted and experimentally determined relaxations of surface geometries are compared, and defects (step edge orientations, point and line defects, impurities, surface manifestations of crystallographic shear planes—CSPs) are discussed, as well as the image contrast in scanning tunneling microscopy (STM). The controversy about the correct model for the (1×2) reconstruction appears to be settled. Different surface preparation methods, such as reoxidation of reduced crystals, can cause a drastic effect on surface geometries and morphology, and recommendations for preparing different TiO ₂(1 1 0) surfaces are given. The structure of the TiO ₂(1 0 0)-(1×1) surface is discussed and the proposed models for the (1×3) reconstruction are critically reviewed. Very recent results on anatase (1 0 0) and (1 0 1) surfaces are included. The electronic structure of stoichiometric TiO ₂ surfaces is now well understood. Surface defects can be detected with a variety of surface spectroscopies. The vibrational structure is dominated by strong Fuchs-Kliewer phonons, and high-resolution electron energy loss spectra often need to be deconvoluted in order to render useful information about adsorbed molecules. The growth of metals (Li, Na, K, Cs, Ca, Al, Ti, V, Nb, Cr, Mo, Mn, Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) as well as some metal oxides on TiO ₂ is reviewed. The tendency to 'wet' the overlayer, the growth morphology, the epitaxial relationship, and the strength of the interfacial oxidation/reduction reaction all follow clear trends across the periodic table, with the reactivity of the overlayer metal towards oxygen being the most decisive factor. Alkali atoms form ordered superstructures at low coverages. Recent progress in understanding the surface structure of metals in the 'strong-metal support interaction' (SMSI) state is summarized. Literature is reviewed on the adsorption and reaction of a wide variety of inorganic molecules (H ₂, O ₂, H ₂O, CO, CO ₂, N ₂, NH ₃, NO _x, sulfur- and halogen-containing molecules, rare gases) as well as organic molecules (carboxylic acids, alcohols, aldehydes and ketones, alkynes, pyridine and its derivates, silanes, methyl halides). The application of TiO ₂-based systems in photo-active devices is discussed, and the results on UHV-based photocatalytic studies are summarized. The review ends with a brief conclusion and outlook of TiO ₂-based surface science for the future.