Core-shell nanostructures have emerged as a promising class of photocatalysts for hydrogen production due to their ability to enhance light absorption, improve charge separation, and mitigate recombination losses. These materials consist of a core material surrounded by a shell of another semiconductor or metal, creating a heterojunction that optimizes photocatalytic activity. Common examples include TiO2@CdS and g-C3N4@metal oxide systems, which exhibit synergistic effects that boost hydrogen evolution rates.

The synthesis of core-shell photocatalysts involves precise control over morphology, composition, and interfacial properties. For TiO2@CdS, the core is typically synthesized first via sol-gel or hydrothermal methods, followed by deposition of the CdS shell using chemical bath deposition or successive ionic layer adsorption and reaction (SILAR). The thickness of the shell is critical, as it affects light penetration and charge transfer. In g-C3N4@metal oxide systems, g-C3N4 is often prepared by thermal polymerization of nitrogen-rich precursors, and metal oxide shells are deposited through sol-gel or atomic layer deposition. The core-shell structure ensures intimate contact between the components, facilitating efficient electron-hole separation.

Charge separation mechanisms in core-shell photocatalysts rely on the band alignment between the core and shell materials. In Type-II heterojunctions, such as TiO2@CdS, the conduction band of CdS is higher than that of TiO2, while the valence band is lower. Under irradiation, electrons migrate from CdS to TiO2, and holes move in the opposite direction, reducing recombination. In g-C3N4@metal oxide systems, the metal oxide often acts as an electron sink, extracting photogenerated electrons from g-C3N4 and leaving holes for oxidation reactions. Additionally, localized surface plasmon resonance (LSPR) effects in metal-decorated core-shell structures can further enhance light absorption and charge separation.

Efficiency metrics for hydrogen production are typically reported in terms of apparent quantum yield (AQY) and turnover frequency (TOF). For instance, TiO2@CdS systems have demonstrated AQY values exceeding 20% under visible light, while g-C3N4@CoOx composites achieve TOF values of up to 50 h⁻¹. These metrics depend on factors such as light intensity, wavelength, and sacrificial agent concentration. The use of hole scavengers like methanol or triethanolamine is common to suppress backward reactions and improve efficiency.

Despite their advantages, core-shell photocatalysts face challenges such as photocorrosion and scalability. CdS-based systems are prone to degradation under prolonged irradiation due to oxidation of sulfide ions, leading to loss of activity. Strategies to mitigate this include coating with protective layers like carbon or TiO2, or incorporating cocatalysts like Pt or NiS. Scalability is another hurdle, as many synthesis methods require precise conditions that are difficult to replicate on an industrial scale. Continuous flow reactors and scalable deposition techniques like spray pyrolysis are being explored to address this issue.

Future developments in core-shell photocatalysts may focus on optimizing interfacial engineering, exploring new material combinations, and integrating machine learning for predictive design. For example, dual-shell structures or gradient compositions could further enhance charge separation, while earth-abundant materials like Fe2O3 or Cu2O could reduce costs. Advances in operando characterization techniques will also provide deeper insights into reaction mechanisms, enabling rational design of more efficient systems.

In summary, core-shell photocatalysts represent a versatile platform for hydrogen production, offering tunable properties and improved performance over single-component systems. While challenges remain, ongoing research into material design and process engineering holds promise for overcoming these limitations and advancing toward practical applications.