A paper recently published in the journal Applied Sciences reviewed the application of quantum dots for photocatalytic hydrogen production.
Schematic of water splitting over semiconductor photocatalysts. (1) the light absorption and the formation of the photogenerated electron–hole pairs; (2) the photogenerated carriers migrate to the surface of semiconductor; (3) the photogenerated carriers participate the surface reactions. Image Credit: https://www.mdpi.com/2076-3417/14/12/5333
Quantum Dots as Photocatalysts
The depletion of fossil fuels and their adverse environmental effects have led to a growing interest in converting solar energy into green chemical energy. One prominent eco-friendly strategy is the photocatalytic generation of hydrogen from water, which directly converts solar energy into hydrogen fuel. This approach holds significant potential for substituting fossil fuels and mitigating global environmental pollution and the energy crisis.
However, the technology currently faces challenges that limit its industrial applicability, primarily due to its lower photocatalytic efficiency compared to standard industrial applications. Issues such as low photostability, slow surface redox reaction kinetics, low light absorption efficiency, and rapid recombination of photogenerated holes and electrons adversely affect the photocatalytic production of hydrogen.
Therefore, the development of highly stable and efficient photocatalysts for hydrogen evolution is critical to achieving a sustainable energy future. Research in this field has focused on enhancing the stability, cost-effectiveness, and efficiency of photocatalytic hydrogen generation systems to make them scalable and commercially viable.
Quantum dots have garnered significant attention as photocatalysts and cocatalysts due to their exceptional light-harvesting ability, numerous surface active sites, and low recombination rates of photogenerated electrons and holes. This paper reviews the application of quantum dots in photocatalytic hydrogen production, highlighting their potential to improve the efficiency and scalability of these systems.
Carbon Quantum Dots-based Hybrid Photocatalyst
Carbon quantum dots possess unique electrical and optical properties, such as up-conversion of fluorescence emissions and broad optical absorption. In photocatalytic hydrogen generation using carbon quantum dot-based photocatalysts, these quantum dots serve as electron reservoirs to prevent charge carrier recombination and as photosensitizers to enhance solar light absorption, leading to improved photocatalytic performance.
Consequently, various semiconductor photocatalysts have been combined with carbon quantum dots to increase visible light absorption and promote the migration of photogenerated electron-hole pairs, thereby enhancing photocatalytic efficiency. Notably, carbon quantum dot/titanium dioxide (TiO2) nanosheets with (001) facets of TiO2-001 exhibit exceptional photocatalytic performance, long durability, and a significantly higher hydrogen generation rate.
Similarly, carbon quantum dot-doped iron, cobalt, and nickel phosphides with a unique open hollow structure have been synthesized. This strategy, involving both the open hollow structure and doping, increases the number of active centers, enhances the gas phase spillover rate, and significantly improves seawater electrolysis performance.
Single Semiconductor Quantum Dots as Photocatalyst
Semiconductor quantum dots can directly serve as photocatalysts in hydrogen evolution reactions and can also be combined with other photocatalysts or semiconductors to create hybrid materials for these reactions.
For example, cadmium sulfide (CdS) quantum dots, with their small particle sizes, offer unique advantages. These include an abundance of surface active sites, rapid photogenerated electron migration to the CdS surface, and the quantum confinement effect. These properties effectively promote the separation efficiency of photogenerated charge carriers, thereby enhancing photocatalytic performance.
Similarly, mono-metal sulfide (MS) quantum dots, where M represents cadmium or zinc, synthesized using a modified gel crystal growth method, exhibit higher hydrogen evolution rates compared to bulk MS. This improvement is due to the synergistic effects between the hydrogel and the quantum dots.
MS quantum dots exhibit unique light absorption properties due to their significantly smaller size compared to bulk MS photocatalysts. Their distinct up-conversion fluorescence effects maximize light absorption efficiency and extend the range of light absorption.
Additionally, the fluorescence efficiency of molybdenum disulfide (MoS2) quantum dots has been enhanced through monoatomic metal modification. Specifically, the fluorescence emission of MoS2 quantum dots increased fourfold under monoatomic gold (Au) modification.
Semiconductor Quantum Dots-based Hybrid Photocatalyst
The photocatalytic performance of single-semiconductor quantum dots is often limited because the short electron-hole diffusion distance in small-sized quantum dots leads to easy recombination of photogenerated charge carriers before they can migrate to surface active sites.
To overcome this limitation, hybrid photocatalysts combining semiconductor quantum dots with other chemical materials have been synthesized. These hybrid materials exhibit unique characteristics, such as rapid photogenerated charge carrier migration and efficient solar light utilization, resulting in enhanced photocatalytic hydrogen generation efficiency. For instance, CdS quantum dots combined with graphitic carbon nitride (g-C3N4) have demonstrated improved photocatalytic hydrogen evolution efficiency.
Similarly, a novel MoS2 quantum dot-modified g-C3N4 nanosheets/n-doped carbon dot heterojunction photocatalyst, prepared using thermal polymerization and solvothermal methods, has shown a photocatalytic hydrogen evolution rate of 212.41 µmol g⁻¹ h⁻¹. This rate is almost 53 times higher than that achieved with g-C3N4/MoS2-3 %.
In summary, quantum dots serve as efficient photocatalysts and cocatalysts for photocatalytic hydrogen production. However, due to the structure-activity relationship and up-conversion fluorescence effect of quantum dots, further research is needed to address the challenges in designing and studying quantum dot-based photocatalysts.
Journal Reference
Gui, X., Lu, Y., Wang, Q., Cai, M., & Sun, S. (2024). Application of Quantum Dots for Photocatalytic Hydrogen Evolution Reaction. Applied Sciences, 14(12), 5333. https://doi.org/10.3390/app14125333, https://www.mdpi.com/2076-3417/14/12/5333
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