Competitive Adsorption of O 2 and H 2 O at the Neutral and Defective SnO 2 (110) Surface
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Competitive Adsorption of O2 and H2O at the Neutral and Defective SnO2 (110) Surface Ben Slater, C. Richard A. Catlow, David E. Williams1 and A. Marshall Stoneham2 Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle St., London, W1S 4BS, U.K. 1 Department of Chemistry, University College London, 20 Gordon St, London, WC1H OAJ, U.K. 2 Department of Physics and Astronomy, University College London, Gower St, London, WC1E 6BT, U.K. ABSTRACT Using first principles techniques we have examined the relative physisorption/chemisorption energetics of neutral molecular water and oxygen at the most thermodynamically stable surface (110) of tin dioxide. We find that water binds more strongly to the perfect surface at 5 coordinate tin sites than oxygen. However, binding of both water and oxygen at bridging oxygen vacancies in the defective surface is comparable. In the context of gas-sensing behaviour at moderate temperatures (~300K), we propose that the Mars and van Krevelen [1] re-oxidation reaction will slow when the partial pressure of water is high, since the number of favourable adsorption sites will effectively decrease. In addition, one would expect that the surface conductivity will increase, since the re-oxidation reaction will be hindered. INTRODUCTION Understanding the chemistry which governs the electrical conductivity of surfaces remains a challenge in the context of gas sensors. Although materials such as tin dioxide and tungsten oxide are widely used commercially, their effective exploitation is based on largely empirical findings. The aim of our calculations is to assist in the clarification of elementary reaction mechanisms which control the sensor response, specifically for SnO2. Recently, Williams and Pratt [2] identified distinct sites at the (110) surface which were proposed as possible sites for adsorption of neutral or charged molecular or monoatomic oxygen species. Furthermore, Williams [3] has suggested a possible reaction mechanism between oxygen and SnO2 which accounts for the observed decrease in conductivity upon re-oxidation. Tin dioxide has been widely studied at the first principles level, in particular by Goniakowski et al.[4,5,6]. Recently, Oveido and Gillan [7] reported a systematic study of defect chemistry on the SnO2 surface, which explored the energetics of line and point oxygen defect formation. Lindan [8] has also reported a comparative study of water adsorption characteristics at both SnO2 and TiO2 (110) surfaces. Additionally, we have recently reported [9] viable mechanisms for oxygen dissociation at the SnO2 (110) surface. Clearly, it is desirable for these findings to be rationalised in the context of competitive adsorption characteristics of these simple molecules to elucidate the gas sensing mechanism. We have therefore carried out a combined study of oxygen and water adsorption using consistent, identical simulation conditions. THEORY
GG9.33.1
Density functional, plane-wave based methods (CASTEP [10]) have been used throughout the following st
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