Self-Derived Metal/Oxide Interface Engineering via Active Nitrogen Mediation

Self-derived phase engineering of metal oxides offers a powerful route to create interfaces without introducing foreign metallic elements, yet achieving systematic control over phase transition remains challenging. Here, active nitrogen-mediated phase engineering was demonstrated across various metal oxides (SnO 2 , ZnO, and TiO 2 ). As a model biphasic material, Sn/SnO 2 was investigated where urea thermolysis enabled the progressive formation of metallic Sn while largely preserving the host structure, allowing the Sn0 fraction to be systematically controlled. Temperature-programmed experiments using mass spectrometry and FT-IR revealed the involvement of nitrogen-containing intermediates during urea decomposition and supported their central role in driving phase engineering of SnO 2 . Density functional theory (DFT) based calculations on a Sn/SnO 2 model constructed using a machine learning interatomic potential (MLIP) revealed electron transfer from metallic Sn to SnO 2 and the resulting redistribution of surface electron density. The resulting self-derived Sn/SnO 2 biphasic material yields favorable surface chemistry by altered surface oxygen behavior and reactivity translating directly into two applications: enhanced chemoresistive formaldehyde gas sensing under humid conditions with ppb-level detection, as well as improved formate production in electrochemical CO 2 reduction reaction (CO 2 RR). Our results identify active nitrogen as a chemical handle for regulating oxide partial reduction and provide a general framework for the systematic engineering of metal/oxide interfaces.