Significance of the Different Exposed Surfaces of ZnO Single Crystals and Nanowires on the Photocatalytic Activity and Processes

The heterogeneous photocatalysis of organic dyes using ZnO nanowires (NWs) is of high interest to face the challenge of eco-efficient water remediation. However, the effects of the wurtzite structure of ZnO and hence of the shape of nanostructures on the photocatalytic processes are still under debate. Herein, it is shown that the photocatalytic activity of ZnO single crystals with five different orientations follows a pseudo-first-order kinetics as: (0001 ) < {1010 } < {2021} < {1010} < (0001). The photocatalytic processes are independent of the nature of the crystallographic planes, apart from the semipolar {2021} orientation. Interestingly, ZnO NWs exhibit a photocatalytic activity that is relatively independent of their length, which is neither due to the penetration of organic dyes nor to the penetration of UV light. Instead, the sidewalls of ZnO NWs are much less efficient than the ZnO single crystal with the same nonpolar m-plane orientation, indicating that the structural morphology and chemical composition of the surface, as well as their much higher doping level, govern the photocatalytic activity and processes. These findings indicate that the increase in the photocatalytic activity of ZnO NWs should be addressed by designing more active surfaces rather than simply increasing their total surface area.