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

Abstract Integrating nanostructured catalysts with semiconductors is a prevalent strategy for the design of photoelectrochemical (PEC) photocathodes toward CO 2 reduction reaction (CO 2 RR). 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 TiO 2 interlayer plays a multifunctional role in boosting the PEC activity and selectivity for the CO 2 RR on a Bi catalyst modified Si photocathode (denoted as Si/dT/Bi). It is discovered that oxygen vacancy in the TiO 2 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 CO 2 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 CO 2 reduction technologies.