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.