Metal sulfide photocatalytic materials have emerged as promising candidates for hydrogen evolution due to their narrow bandgaps and visible-light absorption capabilities. These materials, including cadmium sulfide (CdS) and zinc indium sulfide (ZnIn2S4), exhibit favorable electronic properties that enable efficient solar-driven water splitting. Their ability to harness a significant portion of the solar spectrum makes them attractive for sustainable hydrogen production, addressing the limitations of wide-bandgap semiconductors that only absorb ultraviolet light. However, challenges such as photocorrosion and rapid charge recombination hinder their practical application. Recent advances in nanostructuring, heterojunction design, and cocatalyst integration have significantly improved their performance and stability.

The narrow bandgaps of metal sulfides, typically ranging from 2.0 to 2.4 eV for CdS and 2.1 to 2.8 eV for ZnIn2S4, allow them to absorb visible light efficiently. This property is critical for maximizing solar energy utilization, as visible light constitutes a substantial portion of the solar spectrum. The electronic structure of these materials facilitates the generation of electron-hole pairs upon photoexcitation. In CdS, the conduction band minimum is positioned more negatively than the hydrogen evolution potential, enabling spontaneous proton reduction to hydrogen. Similarly, ZnIn2S4 possesses a suitable band alignment for both water reduction and oxidation, making it a versatile photocatalyst. The charge carrier dynamics in these materials, however, are complicated by rapid recombination losses and surface trapping states, which reduce the overall quantum efficiency.

Photocorrosion is a major limitation of metal sulfide photocatalysts, particularly for CdS, where the photoexcited holes oxidize sulfide ions in the lattice, leading to material degradation. This phenomenon is exacerbated under prolonged irradiation, resulting in the loss of catalytic activity. Several strategies have been developed to mitigate photocorrosion, including the application of protective coatings and the formation of heterojunctions. Protective coatings, such as carbon layers or metal oxides, act as physical barriers to prevent direct contact between the photocatalyst and the electrolyte, thereby reducing sulfide oxidation. For example, a thin TiO2 layer on CdS has been shown to enhance stability while maintaining photocatalytic activity. Heterojunction formation with other semiconductors, such as combining CdS with MoS2 or ZnIn2S4 with graphitic carbon nitride (g-C3N4), improves charge separation and reduces hole accumulation at the sulfide surface. These composite systems not only suppress photocorrosion but also enhance light absorption and charge transfer efficiency.

Nanostructuring has played a pivotal role in advancing metal sulfide photocatalysts by increasing surface area, improving light absorption, and shortening charge carrier diffusion paths. Hollow spherical structures of CdS and ZnIn2S4 have been engineered to enhance light scattering and provide abundant active sites for hydrogen evolution. These architectures also facilitate mass transport and reactant diffusion, further boosting catalytic performance. Two-dimensional nanosheets of ZnIn2S4 exhibit high crystallinity and exposed active facets, which promote efficient charge separation and reduce recombination losses. The ultrathin nature of these nanosheets allows for rapid migration of photogenerated carriers to the surface, where redox reactions occur. Additionally, hierarchical nanostructures, such as flower-like ZnIn2S4 microspheres, combine the benefits of high surface area and efficient light harvesting, leading to superior photocatalytic activity.

The integration of cocatalysts has been instrumental in enhancing the hydrogen evolution performance of metal sulfide photocatalysts. Noble metals like platinum and palladium are highly effective cocatalysts due to their low overpotentials for proton reduction. However, their high cost has driven research into earth-abundant alternatives such as nickel, cobalt, and molybdenum sulfides. These materials provide active sites for hydrogen adsorption and desorption, lowering the energy barrier for the reaction. For instance, NiS nanoparticles anchored on CdS surfaces have demonstrated comparable activity to platinum while offering better cost-effectiveness. The synergistic interaction between the cocatalyst and the photocatalyst improves charge separation and accelerates surface reaction kinetics. Furthermore, dual cocatalyst systems, combining reduction and oxidation cocatalysts, have been developed to simultaneously enhance hydrogen and oxygen evolution, enabling overall water splitting.

Recent studies have explored the role of defect engineering in optimizing the electronic properties of metal sulfide photocatalysts. Sulfur vacancies in CdS and ZnIn2S4 can act as electron traps, prolonging carrier lifetimes and improving photocatalytic efficiency. However, excessive defects may serve as recombination centers, underscoring the need for precise control over defect concentrations. Doping with transition metals or nonmetals has also been employed to modify band structures and enhance visible-light absorption. For example, manganese-doped CdS exhibits extended light absorption into the red region due to intermediate energy levels introduced by the dopant. These tailored electronic properties contribute to higher hydrogen evolution rates under simulated solar irradiation.

Despite significant progress, challenges remain in scaling up metal sulfide-based photocatalytic systems for industrial applications. Long-term stability under continuous operation needs further improvement, particularly in harsh reaction conditions. The development of scalable synthesis methods for nanostructured photocatalysts and cocatalysts is essential to ensure reproducibility and cost-effectiveness. Additionally, the environmental impact of cadmium-containing materials necessitates the exploration of less toxic alternatives, such as ZnIn2S4 or other ternary metal sulfides. Future research directions may focus on advanced characterization techniques to elucidate reaction mechanisms at the atomic level and machine learning approaches to optimize material compositions and architectures.

In summary, metal sulfide photocatalytic materials offer a compelling pathway for solar-driven hydrogen production, leveraging their visible-light activity and tunable electronic properties. Advances in nanostructuring, heterojunction design, and cocatalyst integration have addressed key challenges related to charge carrier dynamics and photocorrosion. Continued innovation in material engineering and process optimization will be critical to realizing the full potential of these photocatalysts in sustainable energy systems.