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

[Image: see text] 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-TiO(2) to prepare CQDs/TiO(2) 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/TiO(2) composite with 1:1 molar ratio of l-Glu to TiO(2), 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-TiO(2) 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 TiO(2). 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 TiO(2).