First Principles Analysis of H2O Adsorption on the (110) Surfaces of SnO2, TiO2 and Their Solid Solutions

Both associative and dissociative H2O adsorption on SnO2(110), TiO2(110), and Ti-enriched Sn1-xTixO2(110) surfaces have been investigated at low (1/12 monolayer (ML)) and high coverage (1 ML) by density functional theory calculations using the Gaussian and plane waves formalism. The use of a large supercell allowed the simulation at low symmetry levels. On SnO2(110), dissociative adsorption was favored at all coverages and was accompanied by stable associative H2O configurations. Increasing the coverage from 1/12 to 1 ML stabilized the (associatively or dissociatively) adsorbed H2O on SnO2(110) because of the formation of intermolecular H bonds. In contrast, on TiO2(110), the adsorption of isolated H2O groups (1/12 ML) was more stable than at high coverage, and the favored adsorption changed from dissociative to associative with increasing coverage. For dissociative H2O adsorption on Ti-enriched Sn1-xTixO2(110) surfaces with Ti atoms preferably located on 6-fold-coordinated surface sites, the analysis of the Wannier centers showed a polarization of electrons surrounding bridging O atoms that were bound simultaneously to 6-fold-coordinated Sn and Ti surface atoms. This polarization suggested the formation of an additional bond between the 6-fold-coordinated Ti-6c and bridging O atoms that had to be broken upon H2O adsorption. As a result, the H2O adsorption energy initially decreased, with increasing surface Ti content reaching a minimum at 25% Ti for 1/12 ML. This behavior was even more accentuated at high H2O coverage (1 ML) with the adsorption energy decreasing rapidly from 145.2 to 101.6 kJ/mol with the surface Ti content increasing from 0 to 33%. A global minimum of binding energies at both low and high coverage was found between 25 and 33% surface Ti content, which may explain the minimal cross-sensitivity to humidity previously reported for Sn1-xTixO2 gas sensors. Above 12.5% surface Ti content, the binding energy decreased with increasing coverage, suggesting that the partial desorption of H2O is facilitated at a high fractional coverage.