Visible-light photocatalytic reduction of Cr(VI) via carbon quantum dots-decorated TiO2 nanocomposites

Carbon quantum dots (CQDs) having sizes of 5–10 nm have been prepared facilely by a hydrothermal method and incorporated with TiO2 nanoparticles to produce CQDs-decorated TiO2 (C/TiO2) nanocomposites with various dosages of CQDs. Under visible-light irradiation, the photocatalytic activities of Cr(VI) reduction via C/TiO2 nanocomposites have been monitored and found to depend highly on the dosage of CQDs. In particular, the photocatalytic rate constant (0.0449 min−1) of C/TiO2 nanocomposites with 3 wt% CQDs is 8.6 times higher than that of pure TiO2 nanoparticles. When CQDs are excited by visible light, photo-generated electrons transfer from CQDs into the conduction band of TiO2 on the time scale of 1500 ps. Transferred electrons are subsequently captured by Cr(VI) ions, reducing Cr(VI) into Cr(III). Thus, π-conjugated CQDs act as sensitizers to provide visible-light response to the TiO2 nanoparticles of C/TiO2 dyade structures, leading to the highly efficient photocatalytic reduction of Cr(VI). Our as-prepared C/TiO2 nanocomposites are applicable in an inexpensive way to treat wastewater by utilizing solar energy without employing electron donors.