Photocatalytic transition-metal-oxides-based p–n heterojunction materials: synthesis, sustainable energy and environmental applications, and perspectives

In recent years, photocatalysis has gained particular attention due to its crucial potential applications in addressing many essential energy and environmental challenges. Considerable efforts have been devoted to developing photocatalysts to understand the fundamental processes and enhance photocatalytic efficiencies. The rate of photoinduced e−–h+ reassembly is one of the difficulties encountered in semiconductor photocatalysis. Various alternative photosystems were designed to overcome this problem and thereby improve the efficiency of the heterojunction photocatalyst. Among the explored methods, the charge carrier separation using a built-in electric field attracts considerable attention as a new concept. The present review highlights the development of p–n heterojunctions to overcome the existing challenges in rigorously explored type-I, II, and III heterojunctions. Herein, reports on widely explored TiO2, ZnO, and various other transition metal oxides based p–n heterojunctions are extensively deliberated. This review pinpoints the benefits of constructing p–n junctions, including their impact on optical absorption, physical, and chemical properties over other n–n and p–p heterojunctions. The mechanistic route followed to construct effective p–n heterojunction and practical work carried out by generated internal electric field in isolating the charge carriers is also highlighted. Transition-metal-oxides based p–n heterojunction shows promising practical applications in various fields, including H2 evolution, CO2 reduction, overall water splitting, photo-reforming, and photodegradation of harmful pollutants. The various challenges and future perspectives for developing metal oxides-based p–n heterojunction materials are also summarized.