Adsorption properties versus oxidation states of rutile TiO2(110)

Using density functional theory we have studied the adsorption properties of different atoms and molecules deposited on a stoichiometric, reduced, and oxidized rutile TiO(2)(110) surface. Depending on the oxidation state of the surface, electrons can flow from or to the substrate and, therefore, negatively or positively charged species are expected. In particular, we have found that a charge transfer process from or to the surface always occurs for highly electronegative or highly electropositive species, respectively. For atoms or molecules with intermediate electron affinity, the direction of the charge flow depends on the oxidation state of the rutile surface and on the adsorption site. Generally, the charging effect leads to more stable complexes. However, the increase in the binding energy of the adsorbates is highly dependent on the electronic states of the surface prior to the adsorption event. In this work we have analyzed in details these mechanisms and we have also established a direct correlation between the enhanced binding energy of the adsorbates and the induced gap states.