Tunable photocurrent spectrum in well-oriented zinc oxide nanorod arrays with enhanced photocatalytic activity

Well-oriented ZnO nanorod arrays were successfully prepared using a seed growth method based on catalyst-free hydrothermal synthesis and characterized by field emission scanning electron microscopy, optical spectroscopy, photoelectrochemistry, and X-ray diffraction techniques. Compared to ZnO nanoparticles or randomly distributed ZnO nanorods, the well-oriented ZnO nanorod arrays exhibited significantly different electronic and photoelectrochemical properties. The generated photocurrent of the well-oriented ZnO nanorod array electrode was enhanced with several orders of magnitude higher intensities than that of the ZnO nanoparticle or the randomly distributed ZnO nanorod electrodes with the maximum wavelength of the photocurrent spectrum tunable from 345 to 385 nm. The photocatalytic activity of the well-oriented ZnO nanorod arrays, evaluated using the photodegradation of Rhodamine B as a probe reaction, was also significantly enhanced in comparison to that of ZnO nanoparticles or TiO2 P25 nanoparticles. The enhancement of the photocurrent and photocatalytic activity is suggested to arise from the tunable photoresponse that enhances the light harvesting and thereby generation of a larger number of photoinduced electron-hole pairs at active sites. Other possible causes include favorable electron transfer and electronic properties of the well-oriented ZnO nanorod arrays, for example, high densities of surface states and electron donor states as well as negative shift of the conduction band. The results are important for understanding the unique electronic and photoelectrochemical properties of well-oriented arrayed nanostructures and for designing novel arrayed nanostructures with advanced functionalities for photocatalytic and photoelectrochemical applications.