The study investigates the interface-confined intermediate phase (MIS) in TiO₂, a prototypical photocatalyst, to explain the superior photocatalytic performance of P25 (a mixture of anatase and rutile TiO₂). Using first-principles calculations, the researchers uncover a metastable intermediate structure formed at the anatase/rutile interface due to confinement effects. This MIS has a high conduction-band minimum level, enhancing the overpotential for hydrogen evolution and improving electron-hole separation. The band alignment at the interface also leads to efficient separation of electrons and holes, and the interfacial confinement creates a wide distribution of the band gap, enhancing optical absorption. These factors collectively contribute to the enhanced photocatalytic efficiency of P25. The findings provide a rationale for the superior performance of P25 and offer a strategy for achieving highly efficient photocatalysis through interface engineering.The study investigates the interface-confined intermediate phase (MIS) in TiO₂, a prototypical photocatalyst, to explain the superior photocatalytic performance of P25 (a mixture of anatase and rutile TiO₂). Using first-principles calculations, the researchers uncover a metastable intermediate structure formed at the anatase/rutile interface due to confinement effects. This MIS has a high conduction-band minimum level, enhancing the overpotential for hydrogen evolution and improving electron-hole separation. The band alignment at the interface also leads to efficient separation of electrons and holes, and the interfacial confinement creates a wide distribution of the band gap, enhancing optical absorption. These factors collectively contribute to the enhanced photocatalytic efficiency of P25. The findings provide a rationale for the superior performance of P25 and offer a strategy for achieving highly efficient photocatalysis through interface engineering.