Recent advances in semiconductor heterojunctions: a detailed review of the fundamentals of photocatalysis, charge transfer mechanism and materials

Photocatalysis, due to its operability under sustainable and green energy conditions, is one of the cardinal branches of the environmental remediation domain. To date, a significant amount of work has been carried out in the design and development of various photocatalysts for applications such as dye degradation, CO2 and NOx reduction, organic transformation and hydrogen generation. Among several factors leading to enhancement of the photocatalytic activity, decreasing the electron–hole (exciton) recombination is regarded as one of the prime factors. Typically, the lifetime of the excitons can be increased by combining two or more semiconductors via forming a heterojunction. Various types of heterojunctions, such as the Schottky barrier, p–n (or non-p–n), van der Waals and facet heterojunctions, can be fabricated depending on specific applications. Each type of heterojunction has its advantages and limitations; hence, proper choice of heterojunction is essential. Almost all classes of semiconductor materials, for instance, metal oxide, perovskites, chalcogenides, metal–organic frameworks (MOFs), covalent organic frameworks (COFs) and MXenes, with a suitable band gap, have been studied for photocatalysis. This review details different classes of materials and types of heterojunctions from the recent literature to provide the reader with a deeper understanding of the same. Initially, the fundamentals of photocatalysis and its basic mechanism are discussed, followed by a detailed discussion on the various types of heterojunctions based on the charge transfer mechanism, such as types I, II and III, with representative examples from recent reports. This panoramic review attempts to encourage a rational design of heterojunctions by choosing the proper candidates to push the process efficiency to its limit.