Dual Effect of Oxygen Vacancy‐Enriched TiO ₂ Interlayer in Si Photocathode for Enhanced Photoelectrochemical CO ₂ Reduction to HCOOH

Integrating nanostructured catalysts with semiconductors is a prevalent strategy for the design of photoelectrochemical (PEC) photocathodes toward CO₂ reduction reaction (CO₂RR). However, it is still a challenge to achieve high efficiency and selectivity due to the incompatible catalyst/semiconductor heterogeneous interface. Here, it is proposed that engineering oxygen vacancy in the TiO₂ interlayer plays a multifunctional role in boosting the PEC activity and selectivity for the CO₂RR on a Bi catalyst modified Si photocathode (denoted as Si/dT/Bi). It is discovered that oxygen vacancy in the TiO₂ interlayer accelerates the carrier transport. These oxygen vacancies also promote the growth of the Bi-based catalysts as sponge-like nanostructures during the photoelectro-deposition process. Numerous PEC experimental results combined with in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy reveal that these sponge-like Bi nano-catalysts on Si/dT/Bi photocathode provide a high density of active sites for CO₂ adsorption and promote the kinetics for HCOOH production by accelerating the formation of the key intermediate of *OCHO. This oxygen vacancy engineering in interlayer provides a unique route for future advancements in CO₂ reduction technologies.