Semiconductor photocatalysts for hydrogen evolution: critical role of cocatalysts in enhancing performance

The conversion of abundant solar energy into green H₂ fuel from photocatalytic water splitting has been extensively studied to address the growing energy demand. In this context, various semiconductors have been employed as photocatalysts; however, insufficient active sites for the redox reactions limit H₂ evolution posing a significant challenge in semiconductor photocatalysis. Often, the use of a tiny amount of an additional constituent, known as a 'cocatalyst', can solve the issue by synergistically enhancing the performance of the semiconductor. Over the past few decades, noble metals and their derivatives have attracted huge research interest as cocatalysts; nevertheless, their limited availability and high cost significantly hinder large-scale applications. Hence, tremendous research attention has been devoted recently to design and develop inexpensive, earth-abundant, and highly stable materials which can be used as H₂ evolution cocatalysts. This review article gives an outline of the recent advances in the high performance materials that have been used as efficient H₂ evolution cocatalysts alongside photoactive semiconductors. Specifically, the H₂ evolution cocatalysts summarized in this article are classified under metal nanoparticles, single atoms, bimetallic alloys, metal oxides, transition metal dichalcogenides, metal phosphides, metal carbides, metal borides and carbon based cocatalysts. Finally, the article discusses the pivotal challenges that restrict future advancements and provide potential research directions in the field of cocatalyst-driven photocatalytic H₂ evolution using active semiconductors.