Promoting the Photo-Fenton catalytic activity with carbon dots: Broadening light absorption, higher applicable pH and better reuse performance

A new visible and near-infrared light-responsive heterogeneous Photo-Fenton catalyst, Fe3O4@Cu2O/carbon quantum dots (CQDs)/nitrogen-doped carbon quantum dots (N-CQDs) (FCCN) was prepared under mild conditions with the structure of double quantum dots growing on the surface of Fe3O4@Cu2O nanoparticles. The prepared FCCN could be used for the degradation of azo compounds, such as methyl orange, acid orange II and mordant yellow 10, even at neutral and alkaline pH (pH: 7-12) with a shortest complete degradation time of 15 min. During the Photo-Fenton process, CQDs and N-CQDs acting as up-converting photo-sensitizers improved the utilization of the light, in which the contribution of N-CQDs was greater than that of CQDs. Besides, CQDs and N-CQDs could transfer photo-generated charges between Cu2O and Fe3O4 for the rapid decomposition of H2O2 to (OH)-O-center dot. CQDs as solid light-sensitive acid sites could consume OH- and enable the efficient decomposition of H2O2 under alkaline conditions. The magnetism of Fe3O4 made this catalyst easily separated and the insoluble layers of CQDs and N-CQDs allowed it to be repeatedly used without activity change even after 10 cycles. The degradation constant of methyl orange in FCCN/H2O2/light system is 5.4 times higher than that of the Fe3O4@Cu2O/H2O2/light system, indicating that the loaded quantum dots greatly enhanced the photocatalytic activity. The degradation constant in FCCN/H2O2/light system was 2.5 times higher than that of the simple mixture (Fe3O4@Cu2O + CQDs + N-CQDs)/H2O2/light system, which indicated these excellent catalytic performances should come from the synergism effect of all components in FCCN.