The surface and materials science of tin oxide
The study of tin oxide is motivated by its applications as a solid state gas sensor material, oxida- tion catalyst, and transparent conductor. This review describes the physical and chemical properties that make tin oxide a suitable material for these purposes. The emphasis is on surface science studies of single crystal surfaces, but selected studies on powder and polycrystalline films are also incorpo- rated in order to provide connectingpoints between surface science studies with the broader field of materials science of tin oxide. The key for understandingmany aspects of SnO2 surface properties is the dual valency of Sn. The dual valency facilitates a reversible transformation of the surface com- position from stoichiometric surfaces with Sn4+ surface cations into a reduced surface with Sn2+ sur- face cations dependingon the oxygen chemical potential of the system. Reduction of the surface modifies the surface electronic structure by formation of Sn5s derived surface states that lie deep within the band gap and also cause a lowering of the work function. The gas sensing mechanism appears, however, only to be indirectly influenced by the surface composition of SnO2. Critical for triggering a gas response are not the lattice oxygen concentration but chemisorbed (or iono- sorbed) oxygen and other molecules with a net electric charge. Band bending induced by charged molecules cause the increase or decrease in surface conductivity responsible for the gas response sig- nal. In most applications tin oxide is modified by additives to either increase the charge carrier con- centration by donor atoms, or to increase the gas sensitivity or the catalytic activity by metal additives. Some of the basic concepts by which additives modify the gas sensing and catalytic prop- erties of SnO2 are discussed and the few surface science studies of doped SnO2 are reviewed. Epitax- ial SnO2 films may facilitate the surface science studies of doped films in the future. To this end film growth on titania, alumina, and Pt(111) is reviewed. Thin films on alumina also make promising test systems for probinggas sensingbehavior. Molecular adsorption and reaction studies on SnO2 sur- faces have been hampered by the challenges of preparing well-characterized surfaces. Nevertheless some experimental and theoretical studies have been performed and are reviewed. Of particular interest in these studies was the influence of the surface composition on its chemical properties. Finally, the variety of recently synthesized tin oxide nanoscopic materials is summarized.