Ultra‐Long Bi Nanowires Coupled With Tapered Si Microwires for Selective Photoelectrochemical CO ₂ ‐to‐Formate Conversion

Photoelectrochemical CO₂ reduction offers a promising route to valorize CO₂ into value-added chemicals; however, achieving both high efficiency and selectivity remains challenging. The synthesis of ultra-long Bi₂O₃ nanowires that are electrochemically transformed into metallic Bi nanowires is reported, which exhibit highly efficient and stable CO₂ reduction when integrated into electrochemical and photoelectrochemical systems. Rietveld refinement and Halder-Wagner analyses quantify oxygen vacancy formation during the reduction of Bi₂O₃ to Bi, and theoretical mechanistic studies reveal that these vacancies stabilize the key OCHO intermediate, playing a crucial role in attaining high CO₂-to-formate selectivity. The ultra-high aspect ratio of Bi₂O₃-derived Bi nanowires maximizes the density of electrochemically active sites and facilitates rapid electron transport, collectively contributing to superior CO₂ reduction performance. Consequently, the Bi nanowires achieve Faradaic efficiencies above 95% for electrochemical formate production across a wide potential window, with hydrogen evolution effectively suppressed. Motivated by a leaf inspired network, the coupling of Si microwires with Bi nanowires enables efficient charge transfer while preserving light harvesting, functioning as a co-catalyst without blocking incident photons. As a result, the integrated Bi nanowire-tapered Si microwire photoelectrode demonstrates efficient and selective solar-driven CO₂-to-formate conversion with outstanding activity and long-term stability under simulated sunlight irradiation.