Vacancy Engineering in Semiconductor Photocatalysts: Implications in Hydrogen Evolution and Nitrogen Fixation Applications

It is a well-known fact that the pronounced photogenerated charge recombination and poor light absorption are the main bottlenecks of photocatalysis applications. The conventional approaches to address these problems involve bandgap engineering and suppression of charge recombination after light irradiation, which results in an enhancement in the photocatalytic performance of the materials. However, the most essential aspect of surface modification to engineer active sites on the catalyst surface is generally not given much importance. Contrary to this, defect engineering is another approach by which the optical, charge separation, and surface properties of the photocatalytic materials can be tuned. In this review article, the effect of the introduction of vacancies on the photocatalytic properties of selected semiconductor materials, viz., metal oxides, perovskite oxides, metal sulfides, oxyhalides, and nitrides is comprehensively summarized. The engineering of vacancies in these materials not only improves their optical and charge transfer properties but also affects the surface properties, which are helpful in the adsorption of the reactants on catalyst surface. Herein, photocatalytic hydrogen evolution and nitrogen fixation applications of vacancy engineered materials are discussed in detail along with the current trends, scalability requirements, and rigorous experimental protocols.