N-doped carbon quantum dots for TiO2-based photocatalysts and dye-sensitized solar cells

N-doped nanostructured carbon materials have attracted much attention because the doping can induce new unique physical and chemical properties of carbon. In this study, we report, for the first time, that N-doped carbon quantum dots (NCQDs) can combine with rutile TiO 2 and form NCQDs/TiO 2 hierarchical microspheres assembled by nanorods with enhanced photocatalytic activities under visible light irradiation. As a result, the pseudo-first-order reaction constant for the degradation of rhodamine B (RhB) on NCQDs/TiO 2 (0.11   min −1 ) is 215.7, 7.3 and 1.3 times much larger than that of NCQDs (0.00051   min −1 ), TiO 2 (0.015   min −1 ), and CQDs/TiO 2 (0.086   min −1 ), respectively. The functions of NCQDs were discussed and a new synergistic photosensitized mechanism was proposed for the degradation of RhB on the as-synthesized NCQDs/TiO 2 nanocomposites based on the experimental results. NCQDs were also applied as metal-free sensitizers in dye-sensitized solar cells (DSSCs) for the first time. A preliminary study shows that under one sun illumination (AM 1.5), the open circuit voltage and fill factor values reach 0.46   V and 43%, respectively, achieving a power conversion efficiency of 0.13% in the proof-of-concept TiO 2 -based DSSCs device. The performance of NCQDs sensitized solar cells surpasses that of N-free counterpart as well. N-doping lowered the work function of CQDs, which was responsible for enhanced photocatalytic activity of NCQDs/TiO 2 and better NCQDs sensitized TiO 2 -based solar cell performance.