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.