OER trends on TiO₂ anatase hydroxylated surfaces: a DFT study

As there are scarce studies on the effect of TiO₂ hydroxylated surfaces on the OER, this study provides a systematic investigation using density functional theory (DFT) calculations for the six degrees of hydroxylation and the oxygen coverage on each anatase (100) and (101) surface. Possible changes in mechanisms and active sites when transitioning from one degree of hydroxylation/oxygen coverage to the other were investigated using the thermodynamic model. The associative mechanisms were predicted on highly hydroxylated surfaces (TiO₂(100) and TiO₂(101): 100% and 86%, respectively), while the associative mechanism was competing with the binuclear one on moderately hydroxylated surfaces (TiO₂(100) and TiO₂(101): 71% and 57%, respectively). On the least hydroxylated surfaces, an associative mechanism occurred with the aid of bridging oxygen (TiO₂(100) and TiO₂(101): 43% and 29%, respectively). The analysis was performed from the perspective of balancing the number of electrons between the electron donor (H*-O_b) and electron acceptor (HO*, O*, and HOO*-Ti_cus) groups. This relation affected both the position of the Fermi level and the strength of adsorption of the OER intermediates. The data were also integrated into the scaling relationship regimes, with the trendlines of O* adsorption energies on titanium sites being very close either to the HOO* trendlines or to the HO* trendlines. The adsorption on the three-coordinated surface oxygen atom (O_3c) shifted the intercept closer to the mid of HO* and HOO* intercepts. This imprinted lower theoretical overpotentials. The donor-acceptor electron balance influenced the adsorption energies to a much larger extent than the coverage regimes, which had a negligible effect.