Advances in Z-scheme semiconductor photocatalysts for the photoelectrochemical applications: A review

With continuous consumption of nonrenewable energy, solar energy has been predicted to play an essential role in meeting the energy demands and mitigating environmental issues in the future. Despite being green, clean and pollution-free energy, solar energy cannot be adopted directly as it cannot provide sufficiently high energy density to work in the absence of machinery. Thus, it is necessary to develop an effective strategy to convert and store solar energy into chemical energy to achieve social sustainable development using solar energy as the main power source. Photocatalysis, in which semiconductor photocatalysts play a key role, is one of the most promising candidates for realising the effective utilisation of sunlight in a green, low-cost and environmentally friendly method. The photocatalytic efficiency of photocatalysts is considerably influenced by their compositions. Among the various heterostructures, Z-scheme heterojunction is one of the most interesting architecture due to its outstanding performance and excellent artificial imitation of photosynthesis. Z-scheme photocatalysts have attracted considerable attention in the past few decades. Herein, we review contemporary Z-scheme systems, with a particular focus on mechanistic breakthroughs, and highlight current state-of-the-art systems. Z-type photocatalysts are classified as traditional, all-solid-state, direct Z-schemes and S-scheme photocatalysts. The morphology, characterisation and working mechanism of each type of Z-scheme are discussed in detail. Furthermore, the applications of Z-scheme in photoelectrochemical water splitting, nitrogen fixation, pollutant degradation and carbon dioxide reduction are illustrated. Finally, we outline the main challenges and potential advances in Z-scheme architectures, as well as their future development directions.