Z-scheme and S-scheme heterostructured photocatalyst for photocatalytic water splitting: A review towards efficient H2 generation

Global warming due to greenhouse gas emissions couple with the limited supply of fossil fuels are in the fore front of the problems facing humanity. Splitting H₂O to get hydrogen is among the most innovative alternative to replace fossil fuels and reduce greenhouse gas emission. Photocatalytic water splitting reaction depends on the performance and efficiency of semiconductor photocatalysts to absorb photons from sunlight and generates $e^{-}$ and $h^{+}$ pairs. One of the main challenges experienced in development of novel photocatalysts that could be used to efficient split water is “recombination of $e^{-}$ and $h^{+}$ pairs”, often called charge carrier recombination. Of all the alternatives to resolve this challenge via heterojunction formation, the Z $-$ scheme and S$-$ scheme heterojunctions approach shows the highest promising results. This review attempts to overhaul the latest achievements made in development of Z$-$ scheme and S$-$ scheme heterojunction photocatalysts. Additionally, the review discussed and evaluate the role of sacrificial agent, defect engineering, doping effect, along with effect of built $-$ in electric field (BIEF) as strategies to enhance photocatalytic water splitting. The review concludes that increasing the efficiency and stability of photocatalysts is necessary to comprehend the mechanism of charge transfer in Z$-$ scheme together with S $-$ scheme heterojunctions as well as obtaining further progress in photocatalytic water splitting.