Ferroelectric metal oxides are emerging as promising photocatalysts due to their efficient charge separation via a spontaneous polarization-induced built-in electric field. Such a spontaneous polarization is generally accompanied by some intrinsic defects, particularly oxygen vacancies (${\mathrm{O}}_{\mathrm{V}}$) confined on negatively polarized surfaces as active sites for the water oxidation reaction (WOR). However, the detailed WOR mechanism on the surface of ferroelectric metal oxides remains unclear; especially the impact of surface ${\mathrm{O}}_{\mathrm{V}}$ on WOR, which is far from fully understood. Herein, taking classic ferroelectric $\mathrm{PbTi}{\mathrm{O}}_3$ as an example, the WOR mechanism on a negatively polarized $\mathrm{PbTi}{\mathrm{O}}_3$ (001) surface has been comprehensively investigated by density functional theory (DFT) calculations. Our DFT results show that the WOR on a pristine surface follows the lattice oxygen oxidation mechanism (LOM) involving the participation of lattice oxygen species, and shifts to the adsorbate evolution mechanism (AEM) undergoing the evolution of a sequence of oxygen-containing intermediates (OH*, O*, and OOH*) on the defective surface with ${\mathrm{O}}_{\mathrm{V}}$. For both the LOM and AEM, the uphill energy demands for all potential determining steps can be overcome by photoexcited holes due to an extremely low-valence band edge (2.36 V vs NHE at $\mathrm{pH}=7$) of ferroelectric $\mathrm{PbTi}{\mathrm{O}}_3$. We have also found that surface ${\mathrm{O}}_{\mathrm{V}}$ can change the WOR from a $2{\mathrm{e}}^{-}$ path for yielding ${\mathrm{H}}_2{\mathrm{O}}_2$ to a $4{\mathrm{e}}^{-}$ path for ${\mathrm{O}}_2$ evolution, as a result of a stronger adsorption of key oxygen-containing intermediates (O*) on a defective surface with ${\mathrm{O}}_{\mathrm{V}}$. These results not only well explain the photocatalytic WOR activity of single-domain $\mathrm{PbTi}{\mathrm{O}}_3$ in the absence of cocatalyst, but also clearly uncover the essential role of surface ${\mathrm{O}}_{\mathrm{V}}$ in determining the reaction path of WOR, thus shedding light on developing other high-performance ferroelectric metal oxide photocatalysts.

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• Energy materials
• Ferroelectricity
• Hydrogen energy
• Surface & interfacial phenomena
• Ferroelectrics
• Semiconductors
• Density functional theory
• First-principles calculations