Double heterojunction photocatalysts: A strategic fabrication, and mechanistic insight towards sustainable fuel production

Excessive energy crisis has triggered the transformation of solar energy into chemical energy via photocatalysis to establish a sustainable and carbon-neutral society. In this regard, the fabrication of visible-light-active photocatalysts with favourable band edge positions is preferred for achieving maximum solar energy conversion efficiency. However, a single semiconductor suffers from several disadvantages, such as rapid electron-hole recombination, inefficient electron-hole separation and sluggish charge migration dynamics. To improve photocatalytic performance, constructing heterostructures using two semiconductors has been considered an effective strategy. Nonetheless, these binary heterostructures also present several challenges, which can be addressed by combining three semiconductors to form double heterojunctions. The formation of double heterojunctions enhances visible light absorption, increases charge carrier concentration and facilitates superior charge separation owing to the presence of in-built electric fields, thereby ameliorating the photocatalytic efficacy of these heterostructures compared to binary ones. This review article provides a deep insight into the charge transfer mechanisms that occur in different types of double heterojunctions. Moreover, it highlights the applications of these heterostructures in various fields of photocatalysis, such as water splitting, CO2 reduction, N2 fixation and pollutant degradation.