(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  or photocatalysis . Moreover, their ability to change conductivity when gaseous molecules are reacting with the surface makes them particularly applicable for chemoresistive portable gas sensors  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 . In fact, flame spray synthesis of the latter led to the formation of Sn1-xTixO2 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  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 Sn1-xTixO2 have been investigated. The Sn1-xTixO2 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 Sn1-xTixO2 nanoparticles at a total Ti content of 4.6 %mol . Furthermore, these findings give fundamental information for the design of high-performing photocatalysts independent of the relative humidity.
 J.F. Wager, Science 300 (2003), 1245.
 A. Fujishima, K. Honda, Nature 238 (1972), 37.
 A. Tricoli, M. Righettoni, S.E. Pratsinis, Nanotechnology 20 (2009), 315502.
 G. Lippert, J. Hutter, M. Parrinello, Mol. Phys. 92 (1997), 477.