Facet-Controlled Synthesis of Mn3O4 Nanorods for Photothermal Synergistic Catalytic Oxidation of Carcinogenic Airborne Formaldehyde

Crystal facet engineering, the selective exposure of reactive crystal facets, has emerged as an important technique for the design of efficient catalysts. However, in the facet-controlled synthesis of nanocrystals by either traditional top-down or bottom-up routes, the selection of capping agents is crucial and challenging. Herein, a phase transition strategy that does not require the assistance of capping/etching agents was developed to achieve the selective exposure of {103}, {101}, and {112} facets in Mn3O4. Facet-dependent activity in the photothermal decomposition of the carcinogen formaldehyde was investigated. The resulting Mn3O4 with exposed {103} facets showed the best photothermal catalytic activity, achieving the complete mineralization of formaldehyde at ambient temperature. The synergistic mechanism of the photothermal catalysis of Mn3O4 was discovered to be a thermal-assisted photocatalytic process rather than solar-light-driven thermal catalysis. In addition, the photothermal synergistic decomposition process of HCHO was also revealed. The photothermal decomposition of HCHO undergoes the processes of HCHO → DOM (dioxymethylene) → formates → carbonates → CO2. The findings proposed in this work not only broaden the study of crystal facet engineering but also provide suggestions for the purification of volatile organic compounds (VOCs) in indoor air.