Carbon quantum dots: Synthesis via hydrothermal processing, doping strategies, integration with photocatalysts, and their application in photocatalytic hydrogen production

Carbon quantum dots (CQDs), a quasi-spherical carbon-based nanomaterial, have attracted great attention in photocatalysis due to their unique optical and electrochemical properties, such as tunable fluorescence emission, up-conversion property, and the ability to accelerate charge separation. Significant efforts have been made to synthesize, dope, and surface-functionalize CQDs to tune their photo-physical/chemical properties. Hydrothermal (HT) processes provide a clean, cost-effective, and efficient synthesis method to prepare CQDs and their derivatives, through hydrolysis, polymerization, and carbonization reactions. This review summarizes the synthesis of HT-CQDs using various precursors, followed by evaluating the CQDs doping and heterostructure formation strategies based on their fundamental characteristics (e.g., morphology, crystallinity, absorption ability, and photoluminescent properties). Doped CQDs exhibit enhanced optical properties, improved charge transfer efficiency, better electron mobility, and increased photocatalytic activity through introducing new surface states and active sites. Decorating traditional photocatalysts with CQDs improves light absorption and charge separation, significantly boosting the overall photocatalytic activity. The performance of CQD-based photocatalysts in hydrogen (H 2 ) evolution is systematically evaluated as well. CQDs enhance photocatalytic H 2 generation by acting as photosensitizers and/or electron mediators, accelerating the separation of e − -h + pairs and reducing recombination rates. This review highlights the significant advancement in CQDs synthesis, and doping and decorating strategies, showcasing their pivotal role in improving the photocatalytic efficiency of H 2 production. However, limited understanding of mechanisms, precise control over doping and surface functionalization, and scalability of synthesis methods remain key challenges for CQD-based photocatalysts. In the future, integrating artificial intelligence (AI) and machine learning (ML) tools with advanced characterization techniques might help the development of CQD-based photocatalysts, enabling precise property tuning and scalable synthesis approaches.