Enhanced adsorption and visible-light photocatalysis on TiO₂ with in situ formed carbon quantum dots

Carbon quantum dots (CQDs) can be used to modify TiO₂ to extend the light absorption threshold and enhance its photocatalytic efficiency. In this study, different amounts of CQDs modified TiO₂ (CQDs-x/TiO₂) were synthesized by a facile, mild, and environmental friendly hydrothermal method at a low temperature. The physicochemical properties were investigated by a variety of advanced characterization techniques. It was found that the anchoring of CQDs endowed the CQDs-x/TiO₂ with a large specific surface area, which is beneficial to adsorb more organic pollutants and promote the rate of photocatalytic oxidation. The XRD results also showed that the in situ formation of CQDs on the surface of TiO₂ made the crystallinity of TiO₂ tend to be complete. Among these photocatalysts, CQDs-20/TiO₂ showed the highest pollutant removal efficiency under visible light irradiations. The classical quenching tests revealed that the O₂^•-, ^•OH, and hole (h⁺) were the oxidizing species. Among them, h⁺ was the primary factor contributing to the degradation. The electrochemical tests showed that the anchoring of CQDs on TiO₂ increased the photocurrent by about four times, as compared with the pure TiO₂. In particular, the cyclic voltammetry results showed that the photo-generated electrons of CQDs were freer to transfer to TiO₂ under visible light irradiations, promoting the separation of photo-generated electrons and holes. This study explains adequately why the CQDs/TiO₂ system has a good photocatalytic degradation of organic compounds.