N, S-doped carbon quantum dots/TiO₂ nanocomposites for visible-light-driven photocatalytic degradation of water pollutants

Abstract

This thesis presents the synthesis and application of a novel carbon quantum dots (CQDs) sensitized TiO₂ composite in the treatment of water pollutants. Firstly, nitrogen and sulfur co-doped carbon quantum dots (N, S-CQDs) were synthesized from citric acid and thiourea through hydrothermal method. The as-prepared N, S-CQDs demonstrated strong visible-light absorption and green-light emission. By incorporating N, S-CQDs into TiO₂, a composite (N, S-CQDs/TiO₂) with great improvement in visible-light-harvesting and reduced band gap were obtained, which was subsequently used as a visible-light-responsive catalyst for the photocatalytic reduction of Cr (VI) from water. Experimental results from batch reaction processes indicated that the composite was able to completely reduce Cr (VI) to Cr (III) at pH2.0 after 3 hours’ exposure to visible light with the initial Cr (VI) concentration being 20.0 mg/L and a catalyst dosage of 1.0g/L. It was also found that lower pH and lower initial concentration of Cr (VI) led to high reduction efficiency. Compare with pure TiO₂, a four-fold increase in reduction efficiency of Cr (VI) was achieved by the N, S-CQDs/TiO₂ composite under the same reaction conditions. The N, S-CQDs/TiO₂ nanocomposite was also applied for synergistic photocatalytic reduction of Cr (VI) and degradation of erythromycin (ERY) from water under visible light irradiation. Because of the instant consumptions of photo-generated electrons and holes at conduction and valence bands, the composite exhibited improved Cr (VI) reduction efficiency in the binary Cr (VI)-ERY system compared to single Cr (VI) system, leading to complete reduction of Cr (VI) and 96.0% degradation of ERY with the increased initial concentration (c0 = 30.0 mg/L for each and catalyst dosage being 1.0 g/L). Moreover, the composite also exhibited very high chemical stability, and less than 10% decrease in Cr (VI) reduction rate and ERY degradation efficiency were achieved after four sequential runs. The results from radical trapping experiments proved that hole (h⁺) and hydroxyl radical (•OH) were the major active species for ERY degradation, whereas photo-generated electrons (e⁻) reduced Cr (VI) to Cr (III). Results from this study reveals that sensitization of TiO₂ with CQDs is efficient in broadening the light absorption range of TiO₂, expanding its application in visible-light-driven photocatalysis.