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

ABSTRACT Photoelectrochemical CO ₂ reduction offers a promising route to valorize CO ₂ into value‐added chemicals; however, achieving both high efficiency and selectivity remains challenging. The synthesis of ultra‐long Bi ₂ O ₃ nanowires that are electrochemically transformed into metallic Bi nanowires is reported, which exhibit highly efficient and stable CO ₂ reduction when integrated into electrochemical and photoelectrochemical systems. Rietveld refinement and Halder–Wagner analyses quantify oxygen vacancy formation during the reduction of Bi ₂ O ₃ to Bi, and theoretical mechanistic studies reveal that these vacancies stabilize the key OCHO intermediate, playing a crucial role in attaining high CO ₂ ‐to‐formate selectivity. The ultra‐high aspect ratio of Bi ₂ O ₃ ‐derived Bi nanowires maximizes the density of electrochemically active sites and facilitates rapid electron transport, collectively contributing to superior CO ₂ 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 CO ₂ ‐to‐formate conversion with outstanding activity and long‐term stability under simulated sunlight irradiation.