Green Synthesis of carbon quantum dots for enhancing photocatalytic activity: Hydrogen/oxygen evolution and dye photodegradation

Carbon quantum dots (CQDs) have recently attracted attention across various fields due to their small size, high conductivity, fluorescence emission, low toxicity, and other desirable characteristics. In this study, highly fluorescent CQDs with an average diameter of 3.7 nm were prepared via microwave irradiation using a standard commercial microwave oven and glycerol as solvent. Several cation promoters were examined for CQD synthesis, with copper ions ultimately chosen for comprehensive characterization and application. The CQDs were impregnated onto both commercially available and microwave-synthesized TiO₂ nanoparticles. The photocatalytic activity was evaluated with respect to the hydrogen and oxygen generation. Under the employed conditions, the oxygen evolution reaction (OER) exhibited over 12 times higher efficiency than the hydrogen evolution reaction (HER). The enhanced OER activity is attributed to the high electronic conductivity of the small Cu doped CQDs@TiO₂ (Cu-CQDs@TiO₂) facilitating an efficient electron transfer for the OER . Visible light activity (λ ≥ 400 nm) was demonstrated by photodegradation of the indigo carmine (IC) solution used as a model pollutant. Irradiation in the presence of the Cu-CQDs@TiO₂ photocatalyst resulted in complete degradation of the dye in less than 3 hours. The results presented here provide a promising methodology for designing high-performance photocatalysts based on environmentally friendly CQD syntheses. Crucial applications, from renewable energy production to environmental remediation, will benefit from strategies using the carbon abundance on Earth.