Water Adsorption and Dissociation on Calcium Copper Titanate (CaCu3Ti4O12) Photocatalyst: A First-Principles Density Functional Theory Investigation

Calcium copper titanate (CaCu3Ti4O12, CCTO) has emerged as a promising photocatalyst for solar-driven water splitting owing to its structural stability, visible-light response, and favorable electronic properties. However, the atomic-scale mechanisms governing water activation on different CCTO surface terminations remain poorly understood. In this work, first-principles density functional theory (DFT) calculations were employed to investigate the structural, electronic, and catalytic properties of the low-index (001), (011), and (111) surfaces of CCTO for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). We then performed surface-stability analysis, followed by molecular and dissociative water adsorption, to predict the reaction energetics and kinetics. We calculated the free energies of H+ and OH− species to assess HER activity, while the free energies of oxygen-containing intermediates (*OH, *O, and *OOH) informed OER activity. The results reveal strong facet-dependent catalytic behavior. The Ti-terminated (001) surface exhibits the strongest dissociative water adsorption and the most pronounced charge redistribution, whereas the Ti-terminated (011) surface promotes barrierless water dissociation. The Ti-terminated (111) surface displays a low activation barrier with strongly exothermic dissociation, while the Cu-terminated (011) surface lowers the activation barrier but weakly stabilizes the dissociation products. Hydrogen adsorption thermodynamics reveal a strong dependence on surface orientation and adsorption site, with the Cu site on the (011) facet exhibiting the most favorable free energy. OER free-energy analysis further reveals pronounced facet- and site-dependent activity, with the Cu and Ti sites on the (011) surface exhibiting the most favorable OER thermodynamics. When considered alongside the band-edge alignment, the (011) facet emerges as the most promising candidate for overall photocatalytic water splitting, as its band edges straddle both the H+/H2 and O2/H2O redox potentials while exhibiting comparatively favorable HER and OER thermodynamics. Nevertheless, the calculated OER limiting potential exceeds the corresponding valence-band maximum potentials, suggesting that additional driving force or catalytic optimization may be required for efficient oxygen evolution. These findings establish clear structure–activity relationships governing water activation, hydrogen evolution, and oxygen evolution on CCTO, providing mechanistic insights for facet engineering and the rational design of CCTO-based photocatalysts for overall water splitting.