Scope and prospect of transition metal-based cocatalysts for visible light-driven photocatalytic hydrogen evolution with graphitic carbon nitride

The role of transition metal derived cocatalyst to improve the hydrogen evolution activity of graphitic carbon nitride has been discussed. • Improving photocatalytic hydrogen evolution of graphitic carbon nitride by cocatalysts loading. • Exploring transition metal derived cocatalysts for improved charge separation and transport. • The potential use of transition metal derived cocatalyst to replace noble metal Pt. • Modulating structural features of cocatalyst for improved hydrogen evolution. • Realizing the mechanism of charge separation, transfer and recombination. The photocatalytic production of H 2 using sunlight is considered a sustainable solution to fulfill the global energy demand and reduce the emission of greenhouse gases generated from the burning of fossil fuels. H 2 has the highest energy density (120–140 MJ kg −1 ) and produces only water as a combustion product when it reacts with O 2 . Therefore, it is regarded as one of the best possible contenders to meet the future energy demand. In this respect, developing efficient semiconductor-based photocatalysts is crucial to make the photocatalytic H 2 evolution commercially viable. Although a series of metal-based semiconductors have been explored for the photocatalytic H 2 evolution, photo-corrosion of these materials makes them impractical for long-term application. In contrast, utilization of the metal-free polymeric graphitic carbon nitride ( g -CN) was beneficial to attain excellent photocatalytic activity and long term-stability (even for months). However, the poor electronic conductivity of g -CN results in charge recombination and poor charge transport to make the overall photocatalytic process inefficient. Nevertheless, effective utilization of cocatalyst like Pt can significantly improve the charge separation and transport. The high cost and scarcity of Pt lead to finding out transition metal-based cocatalysts, and the challenge is to reach an excellent photocatalytic activity and stability with transition metal-based cocatalysts when combined with g -CN. Hence, a tremendous effort has been provided to integrate transition metal-based cocatalysts with g -CN to achieve excellent photocatalytic activity, high quantum efficiency (QE), and long-term stability. Although non-noble metal-based cocatalysts attain major attention for the photocatalytic H 2 evolution with g -CN, its role in improving the charge separation, recombination, and hence the H + reduction activity was never reviewed. This review focuses on designing transition metal-based cocatalysts and their application with g -CN to improve the H 2 evolution activity by enhancing the charge separation, transport, and minimizing the recombination of charge carriers. The basic principles of the cocatalyst design, their combination with g -CN, and the mechanism of electron-hole separation, charge transport, and recombination have been described. Besides, the challenges in this field have been addressed with a possible solution. Overall, this review deals with the fundamentals of photocatalytic hydrogen evolution with g -CN, recent progress in this field, and efficient utilization of the transition metal-based cocatalysts to boost photocatalytic activity.