Detailing Ionosorption over TiO2, ZrO2, and HfO2 from First Principles

Identifying the principles of surface to adsorbate charge transfer is key to a better understanding of metal oxide materials as both catalysts and gas sensors. The mechanism responsible for gas sensing effects is not fully understood, but is associated with electron transfer to adsorbates, forming negatively charged anions, or ionosorption. Catalytic surface reactions may also involve electron transfer from the oxide to the adsorbates. Using density functional theory, we modeled the adsorption of small molecules over stoichiometric and reduced metal oxide surfaces of group IV metals and quantify the effect of electron transfer upon adsorption. Surface reduction was accomplished through creation of oxygen vacancies, which lead to unpaired electrons within the oxide lattice, and which may eventually transfer to adsorbates. We examined the TiO2 anatase (101), tetragonal HfO2 (101), and tetragonal ZrO2 (101) surfaces. We first focused on O-2 (a known electron scavenger) adsorption at surface cation sites and observed formation of anionic O-2 species, stabilizing O-2 on the surface. The ability of O-2 to scavenge electrons was found to be geometry-dependent, as electron transfer only occurred for a specific O-2 configuration, O-2 lying flat on the surface. We found a correlation between the work function of the metal oxide, and the ionic adsorption of the oxygen molecule; surfaces with smaller work function values have larger adsorption energies for O-2. The ionic character of a surface, as measured by vacancy formation energy, also correlates well with the O-2 adsorption energy. Thus, if the work function or vacancy formation energy of a metal oxide surface is known, it may be possible to predict when electron transfer occurs and to what degree during adsorption. By examining several other adsorbates (such as H2O or CO), we found that charge transfer only occurs during the adsorption process of an adsorbate more electronegative than the surface, in agreement with previous work (Deskins et al. J. Phys, Chem. C 2010, 114, 5891-5897). Our results therefore do show that electron transfer does not occur with all adsorbates (i.e., those molecules with low electronegativity), but any studies involving these metal oxides should take into account the possibility of ionosorption due to unpaired electrons resulting from surface reduction (defects) to correctly describe the surface chemistry involving many typical compounds.