Chemisorption and charge transfer at ionic semiconductor surfaces: Implications in designing gas sensors
A detailed atomistic understanding of charge transfer reactions between semiconductor surfaces and adsorbing particles is essential for designing gas sensors or metal-oxide catalysts. This will be demonstrated in a discussion of thermodynamically or kinetically controlled solid/gas interactions at extensively investigated “prototype surfaces”, such as ZnO (1010) and TiO 2 (110). Interaction steps discussed are physisorption, chemisorption, surface and volume reactions of small molecules. The discussion is based upon results from (PAR) UPS, XPS, BELS, LEED, AES, EPR, TDS and measurements of conductivities and work functions. Chemisorption steps and reactions involving surface as well as bulk defects of the substrate are of particular importance for sensor applications. Both types of interaction generally involve localized charge redistribution in the valence-band range and delocalized charge tranfer of electrons in the conduction band. In this context, quantum chemical cluster calculations are particularly useful in interpreting and generalizing experimental data.