Mechanisms of impurity-enhanced spatial carrier separation and photocatalytic activity of F-N codoped m-ZrO2 nanocrystals

Metal oxide photocatalysts have high chemical stability In aqueous solutions, but their photocatalytic activity is usually restricted by poor electron-hole separation. Although impurity doping has been demonstrated to be an effective way to tune photocatalytic properties, the mechanisms of enhanced photocatalytic activity remain ambiguous. Here we show that appropriately incorporated impurities can markedly improve the spatial carrier separation and photocatalytic activity. This concept is demonstrated by experimental and density functional theory studies on the effects of fluorine (F) and nitrogen (N) dopants on photocatalytic H-2 evolution performance of m-ZrO2 nanocrystals. We found that F readily enters the lattice of m-ZrO2 to form F substitutional defect (F-O) with low defect formation energy (DFE), but N has difficulty entering the lattice to form N substitutional defect (N-O) unless N is codoped with F. The F doping doubled the photocatalytic activity of m-ZrO2 without reducing its bandgap, which is attributed to the enhanced spatial carrier separation caused by defect F-O. The F-N codoping quadrupled the photocatalytic activity of m-ZrO2, with the enhanced photocatalytic activities attributed to the synergetic effect of the impurity-enhanced spatial carrier separation and the increased light absorption. We observed that F-N codoping enhances photocatalytic performance by affecting both separation and recombination of the carriers: (1) F-N codoping enhances spatiotemporal separation of electrons and holes, and (2) F-N codoping reduces spatial overlap between the electron and hole distributions and inhibits recombination of the carriers. Our findings may provide novel insights into designing high-performance photocatalysts.