Revealing the structure-activity relationship of Mn₃O₄/TiO₂ via phase engineering for enhanced photocatalytic CO₂ reduction

The structure-activity relationship plays a crucial role for enhanced photocatalytic performance, nevertheless, precisely controlling the phase of transition metal oxides remains a significant challenge. Herein, we developed 2D-layered Mn₃O₄ decorated with TiO₂ catalyst to investigate the effects of manganese oxide phase on photocatalytic CO₂ reduction activity. We optimized the manganese oxide phase by varying the amount of the TiCl₄ precursor. During the synthesis reaction, TiCl₄ is a key role for determining the oxidation state of manganese oxide. We identified that Mn₃O₄ is the best phase for photocatalytic CO₂ reduction, and Mn₃O₄/TiO₂ exhibits 20.1-, 7.8-fold enhanced photocatalytic CO₂ reduction performance than pristine TiO₂ and Mn₃O₄, respectively. The improved photocatalytic activity is relevant to the phase of manganese oxide, the enhanced separation of charge carriers and prolonged lifetimes of charge carriers. In situ X-ray Absorption Near-Edge Structure (XANES) confirms the changes of oxidation state under photocatalytic CO₂ reduction conditions. Furthermore, the photocatalytic mechanism is proposed by in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. This study provides an effective design strategy for modulating oxidation state of transition metal oxide-based photocatalyst for enhanced solar fuels generation.