Facial fabrication of carbon quantum dots (CDs)-modified N-TiO_2-x nanocomposite for the efficient photoreduction of Cr(VI) under visible light

In this study, for the first time, a novel CDs-N-TiO_2-x nanocomposite was successfully fabricated by decorating the carbon quantum dots (CDs) on the nitrogen (N) and Ti³⁺ codoped TiO₂ nanoparticles using a facile hydrothermal-calcination method. X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS), electron paramagnetic resonance (EPR) spectra, and X-ray photoelectron spectroscopy (XPS), etc., were utilized to investigate the physicochemical properties of the catalyst and to confirm the successful introduction of CDs and the formation of N/Ti³⁺ codoping levels in the nanocomposite. Owing to the synergy between the modification of CDs and codoping of N and Ti³⁺, the as-prepared CDs-N-TiO_2-x nanocomposite exhibited enhanced visible light absorption and more efficient charge separation compared with its counterparts, such as the pristine TiO₂, N-doped TiO₂ (N-TiO₂), N and Ti³⁺ codoped TiO₂ (N-TiO_2-x), and carbon quantum dots-TiO₂ (CDs-TiO₂), etc. As a result, the CDs-N-TiO_2-x nanocomposite displayed a remarkable performance for Cr(VI) reduction under visible light irradiation. Moreover, adding the appropriate amount of citric acid as the hole scavenger significantly increased the reduction rate of Cr(VI). In addition, the reduction efficiency was better under acidic conditions and considerably affected by the initial Cr(VI) concentration and catalyst dosage. Thermogravimetric analysis and cyclic runs demonstrated a unique thermal and chemical stability of the CDs-N-TiO_2-x nanocomposite. The mechanism of the enhanced photoreduction of Cr(VI) by the CDs-N-TiO_2-x nanocomposite was discussed based on all the characterization and experimental results. Moreover, the developed CDs-N-TiO_2-x nanocomposite also exhibited efficient photocatalytic performance to reduce the Cr(VI) contained in real laboratory wastewater.