Supercritical carbon dioxide-assisted TiO₂/g-C₃N₄ heterostructures tuning for efficient interfacial charge transfer and formaldehyde photo-degradation

Long exposure to formaldehyde has been a concern for causing respiratory problems and TiO2/g-C3N4 composite is a potential choice to address this issue. This study developed a supercritical carbon dioxide (ScCO2)-assisted method to fabricate TiO2/g-C3N4 composites with two distinct heterostructures. The g-C3N4 nanosheet/TiO2 nanoparticle (denoted as type 1 heterostructure) heterojunction could accelerate electron migration because of its large interfacial surface contact. The ternary TiO2{101}/g-C3N4/TiO2{001} heterojunction (denoted as type 2 heterostructure) was synthesized for the first time, further promoting the separation of photogenerated electron/hole pairs under full-spectrum irradiation because it provided an additional migration channel for photocarriers. By loading different amounts of TiO2, the heterostructures were modulated, thereby influencing the photoactivity of the composites. The TiO2/g-C3N4 composite with 58% TiO2 loading possessed the best photocatalytic performance, achieving the highest rate constant for formaldehyde degradation, approximately 7 times higher than pure bulk g-C3N4 under either visible or full-spectrum light. Under full-spectrum irradiation, the maximum decomposition rate approached nearly 100% within 90 min. The tuned heterostructures facilitated the generation of O-2(center dot-) and center dot OH, which were dominant active oxygen radicals responsible for formaldehyde removal under light irradiation. Thus, this study developed a green route to synthesize composites with tunable heterojunctions and enhanced photocatalytic activity that can be used for indoor air pollutant degradation.