Light‐Induced Dynamic Activation of Copper/Silicon Interface for Highly Selective Carbon Dioxide Reduction

Numerous studies have shown a fact that phase transformation and/or reconstruction are likely to occur and play crucial roles in electrochemical scenarios. Nevertheless, a decisive factor behind the diverse photoelectrochemical activity and selectivity of various copper/silicon photoelectrodes is still largely debated and missing in the community, especially the possibly dynamic behaviors of metal catalyst/semiconductor interface. Herein, through in situ X‐ray absorption spectroscopy and transmission electron microscope, a model system of Cu nanocrystals with well‐defined facets on black p‐type silicon (BSi) is unprecedentedly demonstrated to reveal the dynamic phase transformation of forming irreversible silicide at Cu nanocrystal‐BSi interface during photoelectrocatalysis, which is validated to originate from the atomic interdiffusion between Cu and Si driven by light‐induced dynamic activation process. Significantly, the adaptive junction at Cu−Si interface is activated by an expansion of interatomic Cu−Cu distance for CO2 electroreduction, which efficiently restricts the C−C coupling pathway but strengthens the bonding with key intermediate of *CHO for CH4 yield, resulting in a remarkable 16‐fold improvement in the product ratio of CH4/C2 products and an intriguing selectivity switch. This work offers new insights into dynamic structural transformations of metal/semiconductor junction and design of highly efficient catalysts toward photosynthesis. This work unprecedentedly reveals the dynamic phase transformation of forming irreversible silicide at Cu/BSi interface during photoelectrochemical CO2 reduction reaction, which was validated to originate from the atomic interdiffusion between Cu and Si driven by a light‐induced dynamic activation process. The adaptive junction is activated by an expansion of interatomic Cu−Cu distance, resulting in a remarkable 16‐fold improvement in the product ratio of CH4/C2 products.