(267f) First-Principles Investigation of H2O Adsorption On Ti-Doped SnO2(110) Surfaces

The semiconducting nature of many metal oxides makes them interesting materials for numerous industrially and economically important applications. Wide band gap semiconducting metal oxides such as SnO₂ and TiO₂ are used, for example, in solar cells [1] or photocatalysis [2]. Moreover, their ability to change conductivity when gaseous molecules are reacting with the surface makes them particularly applicable for chemoresistive portable gas sensors [3] used, for example, in medical diagnostics (breath analysis). A main drawback of metal oxide materials both in photocatalysis and as gas sensors is the reaction of their surfaces with water vapor. In other words, changes in the relative humidity of the environment can significantly influence the performance of the metal oxide device. Experimental studies have shown recently that this major drawback can be overcome by co-synthesis of SnO₂ with another oxide such as TiO₂ [3]. In fact, flame spray synthesis of the latter led to the formation of Sn_1-xTi_xO₂ solid solutions with controlled surface properties and high sensitivity (e.g. to EtOH). The understanding of the synthesis process and the resulting formation of the rutile lattice, however, is still in progress.

In this project, density functional theory within the Gaussian and plane waves formalism [4] has been used to study H₂O adsorption mechanisms on rutile (110) surfaces of Ti-doped SnO₂ in comparison to pure TiO₂ and SnO₂. The stability of surfaces with homogeneously distributed Ti atoms in the whole crystal was compared to that obtained with surface localized ones, showing that the localization at the surface is thermodynamically favored, in particular, when six-fold coordinated surface Sn atoms are substituted by Ti atoms. Furthermore, the adsorption properties of H₂O on the surface of SnO₂, TiO₂ and of Sn_1-xTi_xO₂ have been investigated. The Sn_1-xTi_xO₂ solid solutions have been studied for x values from 0 to 20% where Ti atoms were distributed homogeneously on the surface and specifically on six-fold coordinated sites. For high H₂O coverage (1 monolayer), the binding energy of dissociatively adsorbed H₂O decreased monotonously with increasing Ti-surface content. At low coverage (1/12 monolayer), however, the binding energy showed remarkable dependency on the distribution of Ti atoms on the surface. Substitution of six-fold coordinated surface Sn-atoms with Ti led to a remarkable decrease of the binding energy. This study indicates that the presence of Ti surface atoms greatly affects the adsorption of water on the oxide surface and gives a possible explanation for the experimentally observed minimum of the cross-sensitivity to humidity of Sn_1-xTi_xO₂ nanoparticles at a total Ti content of 4.6 %mol [3]. Furthermore, these findings give fundamental information for the design of high-performing photocatalysts independent of the relative humidity.

[1]   J.F. Wager, Science 300 (2003), 1245.

[2]   A. Fujishima, K. Honda,  Nature 238 (1972), 37.

[3]   A. Tricoli, M. Righettoni, S.E. Pratsinis, Nanotechnology 20 (2009), 315502.

[4]   G. Lippert, J. Hutter, M. Parrinello, Mol. Phys. 92 (1997), 477.