Visible–Ultraviolet Upconversion Carbon Quantum Dots for Enhancement of the Photocatalytic Activity of Titanium Dioxide

Visible-ultraviolet upconversion carbon quantum dots (CQDs) are synthesized with a hydrothermal method using l-glutamic acid (l-Glu) and m-phenylenediamine (MPD) and then combined with commercial nano-TiO2 to prepare CQDs/TiO2 composites. The fluorescence spectra prove that the prepared CQDs can convert approximately 600 nm visible light into 350 nm ultraviolet light. In photocatalysis experiments, CT-1, a CQDs/TiO2 composite with 1:1 molar ratio of l-Glu to TiO2, has the best degradation efficiency for methyl orange (MO). Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) experiments confirm that CT-1 is composed of quasi-spherical nano-TiO2 and CQDs with a crystal plane of graphitic carbon. CT-1 can degrade 70.56% of MO (40 ppm) within 6 h under the irradiation of a 600 nm light source, which is close to its degradation rate of 78.75% under 365 nm ultraviolet light. The apparent rate constant of CT-1 degradation equation is 12.7 times that of TiO2. Free radical scavenging experiments and electron spin resonance (ESR) tests show that the degradation ability should be attributed to the existence of h+ and •OH under visible light. Therefore, we provide a simple and low-cost solution with heavy-metal-free products to improve the photocatalytic performance of TiO2.