Synergistic influence of FRET, bulk recombination centers, and charge separation in enhancing the visible-light-driven photocatalytic activity of Cu<sup>2+</sup>-ion-doped ZnO nanoflowers
Sathi Chatterjee, Asit Kumar Kar
Abstract
TEM. Absorption spectroscopy yields a reduction in the bandgap up to 6 wt%, after which it is increased for 8 wt%. An enhanced plasmon band in the spectra reveals the presence of CuO. The photoluminescence is quenched for doping up to 6 wt%, and with further doping the emission is enhanced. These observations are explained by the doping-concentration-dependent Förster resonance energy transfer (FRET) phenomenon between the ZnO (donor) and the CuO (acceptor). For the highest doping concentration, the emission profile shows a sudden enhancement resulting from the simultaneous competition of two FRET mechanisms (the intra-acceptor mechanism and the inter-donor-acceptor mechanism). By contrast, for other doped nanomaterials, the inter-donor-acceptor FRET mechanism with doping-concentration dependence is able to explain the suppression of the emission intensity. All doped nanomaterials show an improved visible-light-driven photocatalytic efficiency compared with pure ZnO for methylene blue, which results from the synergistic effects of a reduction in the concentration of bulk defects, enhanced charge separation, and FRET. The highest photocatalytic performance is demonstrated by the 6 wt% nanomaterial due to its optimum doping concentration. However, beyond this concentration, the formation of excessive CuO on the surface of ZnO increases the concentration of bulk defects, and the simultaneous occurrence of the inter-donor-acceptor FRET and intra-acceptor FRET mechanisms takes place leading to the rapid recombination of electron-hole pairs and reduced photocatalytic activity.