Elucidating the barriers on direct water splitting: key role of oxygen vacancy density and coordination over PbTiO3 and TiO2

In this work, using the state-of-the-art first-principles calculations based on density functional theory, we found that the concentration and coordination of surface oxygen vacancies with respect to each other were critical for the direct water-splitting reaction on the (001) surfaces of PbTiO3 and TiO2. For the water-splitting reaction to happen on TiO2-terminated surfaces, it is necessary to have two neighboring O vacancies acting as active sites that host two adsorbing water molecules. However, eventual dissociation of O–H bonds is possible only in the presence of an additional nearest-neighbor O vacancy. Unfortunately, this necessary third vacancy inhibits the formation of molecular hydrogen by trapping the dissociated H atoms on TiO2-terminated surfaces. Formation of up to three O vacancies is energetically less costly on both terminations of PbTiO3(001) surfaces compared with those on TiO2; the presence of Pb leads to weaker O bonds over these surfaces. Molecular hydrogen formation is more favorable on the PbO-terminated surface of PbTiO3, requiring only two neighboring oxygen vacancies. However, the hydrogen molecule is retained near the surface by weak van der Waals forces. Our study indicates two barriers leading to low productivity of direct water-splitting processes. First and foremost, there is an entropic barrier imposed by the requirement of at least two nearest-neighbor O vacancies, sterically hindering the process. Furthermore, there are also enthalpic barriers of formation on TiO2-terminated surfaces or removal of H2 molecules from the PbO-terminated surface. The requirement dictating three nearest-neighbor oxygen vacancies for hydrogen evolution is also consistent with the chemical intuition: The nearest neighbor of the formed hydrogen should be reduced enough to inhibit spontaneous oxidation under ambient conditions.