Chalcogenide semiconductors have emerged as promising candidates for photocatalytic applications, particularly in hydrogen evolution and carbon dioxide reduction. Their tunable bandgaps, efficient light absorption, and suitable band edge positions make them attractive alternatives to conventional oxide photocatalysts. This article examines the photocatalytic performance of chalcogenides, focusing on material design, co-catalyst integration, sacrificial agents, and stability challenges.
The electronic structure of chalcogenides enables efficient light harvesting across a broad spectrum. Cadmium sulfide (CdS), for instance, possesses a direct bandgap of approximately 2.4 eV, allowing visible light absorption up to 520 nm. Molybdenum disulfide (MoS2), a layered transition metal dichalcogenide, exhibits a tunable bandgap ranging from 1.2 eV in bulk to 1.9 eV in monolayers. These properties facilitate the generation of electron-hole pairs under solar irradiation, a prerequisite for photocatalytic reactions. The conduction band minimum of CdS lies at around -0.5 V versus the normal hydrogen electrode (NHE), sufficiently negative to drive proton reduction to hydrogen. Similarly, the valence band maximum of MoS2 can oxidize water or organic molecules, enabling both half-reactions in photocatalytic processes.
Co-catalyst integration significantly enhances the photocatalytic efficiency of chalcogenides by mitigating charge recombination and providing active sites. Noble metals such as platinum and palladium are commonly employed as reduction co-catalysts for hydrogen evolution. Studies show that Pt-loaded CdS can achieve hydrogen evolution rates exceeding 10 mmol g-1 h-1 under visible light. However, the high cost of noble metals has spurred research into earth-abundant alternatives. Cobalt phosphide (CoP) and nickel sulfide (NiS) have demonstrated comparable activity when coupled with CdS, with reported hydrogen evolution rates of 8.2 mmol g-1 h-1 and 6.7 mmol g-1 h-1, respectively. For CO2 reduction, copper-based co-catalysts facilitate multi-electron transfer processes necessary for hydrocarbon production. Copper-modified MoS2 has shown selectivity toward methanol with a yield of 45 μmol g-1 h-1 under simulated solar irradiation.
Sacrificial agents play a critical role in photocatalytic systems by consuming photogenerated holes and preventing charge recombination. Lactic acid and sodium sulfide/sulfite mixtures are widely used in conjunction with CdS, improving hydrogen evolution rates by up to 20-fold compared to pure water systems. The sulfide/sulfite system not only scavenges holes but also suppresses photocorrosion by maintaining a reducing environment. For CO2 reduction, triethanolamine and ascorbic acid serve as effective hole scavengers while minimizing competing hydrogen evolution. However, the use of sacrificial agents raises concerns about sustainability, as they are consumed during the reaction and require periodic replenishment.
Stability remains a significant challenge for chalcogenide photocatalysts, particularly in aqueous environments. CdS suffers from photocorrosion under prolonged illumination, where photogenerated holes oxidize sulfide ions in the lattice, leading to material degradation. Strategies to mitigate this include the formation of heterostructures with more stable materials. CdS-ZnS core-shell structures exhibit enhanced stability, with some systems maintaining 90% of initial activity after 20 hours of continuous operation. MoS2 demonstrates better chemical stability but faces limitations due to restacking of layers and poor interfacial charge transfer. Incorporating conductive scaffolds such as reduced graphene oxide or carbon nanotubes improves charge separation and structural integrity.
The photocatalytic mechanism of chalcogenides involves multiple steps that influence overall efficiency. Upon light absorption, excitons are generated and must dissociate into free carriers. The electrons migrate to the catalyst surface or co-catalyst sites, where they participate in reduction reactions. Simultaneously, holes drive oxidation reactions or are consumed by sacrificial agents. In CO2 reduction, the process is more complex, requiring multiple proton-coupled electron transfers to form hydrocarbons. The selectivity toward specific products depends on the catalyst's ability to stabilize intermediate species. CdS tends to favor carbon monoxide production, while MoS2-based systems show higher formate yields due to differences in surface binding energies.
Morphological control offers another avenue for optimizing chalcogenide photocatalysts. Nanostructuring increases surface area and reduces charge carrier diffusion lengths. Porous CdS architectures demonstrate hydrogen evolution rates 3 times higher than bulk counterparts due to enhanced light trapping and reactant accessibility. Edge-terminated MoS2 nanosheets exhibit superior activity compared to basal plane-dominated structures, as the edges provide abundant active sites for hydrogen adsorption. Dimensional engineering also plays a role, with quantum-confined CdSe nanoparticles showing improved charge separation efficiencies approaching 80%.
Doping and defect engineering can further tailor the electronic properties of chalcogenides. Nitrogen doping in CdS introduces mid-gap states that extend light absorption into the near-infrared region. Sulfur vacancies in MoS2 create localized electronic states that facilitate CO2 adsorption and activation. However, excessive defects can act as recombination centers, highlighting the need for precise control during synthesis. Recent advances in post-synthetic treatments, such as plasma etching and chemical vapor annealing, enable fine-tuning of defect concentrations with atomic precision.
Scalability and practical implementation present additional considerations for chalcogenide-based photocatalytic systems. Solution-processable synthesis routes, including hydrothermal and solvothermal methods, allow for large-scale production of nanostructured materials. Continuous-flow reactors have been tested with chalcogenide catalysts immobilized on substrates, achieving stable hydrogen production over 100 hours. For CO2 reduction, gas-phase systems utilizing fixed-bed reactors demonstrate better mass transfer compared to liquid-phase setups, though product separation remains challenging.
Environmental and toxicity concerns associated with heavy metal-containing chalcogenides like CdS have prompted research into greener alternatives. Zinc-based chalcogenides (ZnS, ZnSe) show promise but typically require ultraviolet activation due to their wider bandgaps. Ternary compounds such as CuInS2 and AgInS2 combine visible light responsiveness with reduced toxicity, though their complex synthesis often leads to phase impurities that hinder performance.
The future development of chalcogenide photocatalysts will likely focus on several key areas. Advanced characterization techniques, including in-situ spectroscopy and microscopy, can provide deeper insights into reaction mechanisms at the atomic scale. Machine learning approaches may accelerate the discovery of optimal material compositions and architectures. Hybrid systems combining chalcogenides with molecular catalysts or biological components could unlock new reaction pathways with improved selectivity. Ultimately, addressing the stability and scalability challenges will determine the viability of these materials for large-scale solar fuel production.
In summary, chalcogenide semiconductors offer unique advantages for photocatalytic hydrogen evolution and CO2 reduction, with performance metrics rivaling or exceeding conventional oxide materials. Through careful design of material composition, structure, and interfaces, these compounds can overcome current limitations and contribute to sustainable energy solutions. The field continues to evolve rapidly, with new discoveries expanding the potential applications of these versatile materials.