Classification and catalytic mechanisms of heterojunction photocatalysts and the application of titanium dioxide (TiO2)-based heterojunctions in environmental remediation

Environmental pollution owing to modernization and industrialization causes severe harm to all the biodiversity on the earth. Therefore, for environmental remediation, nanosized TiO2-based heterojunction photocatalysts have been extensively investigated as effective and sustainable materials. Heterojunction technology employs two or more semiconductors for enhancing photocatalytic performance. However, we discovered that researchers tend to overlook the basic mechanism of heterojunctions, which hinders the in-depth understanding of heterojunction photocatalysts. In this review, heterojunctions are classified according to various criteria, and the similarity and differences between heterojunctions are presented for the first time. The photocatalytic mechanisms of various heterojunctions reported in recent studies, including Schottky, type-II, direct Z-schemes, and p-n junctions, are detailed. Furthermore, the construction and validation methods of heterojunctions are discussed, and nanosized TiO2-based heterojunction photocatalysts for environmental remediation are briefly analyzed. The findings of this review are expected to have strong implications for the future development of heterojunction photocatalysts.