Structure and properties of TiO2 surfaces:: a brief review

Titanium oxides are used in a wide variety of technological applications where surface properties play a role. TiO2 surfaces, especially the (110) face of rutile, have become prototypical model systems in the surface science of metal oxides. Reduced TiO2 single crystals are easy to work with experimentally, and their surfaces have been characterized with virtually all surface-science techniques. Recently, TiO2 has also been used to refine computational ab initio approaches and to calculate properties of adsorption systems. Scanning tunneling microscopy (STM) studies have shown that the surface structure of TiO2 (110) is more complex than originally anticipated. The reduction state of the sample, i.e. the number and type of bulk defects, as well as the surface treatment (annealing in vacuum vs. annealing in oxygen), can give rise to different structures, such as two different (1 x 2) reconstructions, a 'rosette' overlayer, and crystallographic shear planes. Single point defects can be identified with STM and influence the surface chemistry in a variety of ways; the adsorption of water is discussed as one example. The growth of a large number of different metal overlayers has been studied on TiO2(110)Some of these studies have been instrumental in furthering the understanding of the 'strong metal support interaction' between group-VIII metals and TiO2, as well as low-temperature oxidation reactions on TiO2-supported nanoscopic gold clusters. The growth morphology, interfacial oxidation/reduction reaction, thermal stability, and geometric structure of ultra-thin metal overlayers follow general trends where the most critical parameter is the reactivity of the overlayer metal towards oxygen. It has been shown recently that the technologically more relevant TiO2 anatase phase can also be made accessible to surface investigations.