High visible light photocatalytic activity of nitrogen-doped ZnO thin films deposited by HiPIMS

Reactive High Power Impulse Magnetron Sputtering (HiPIMS) of a pure Zn target in Ar/N2/O2 gas mixture was used to synthesize ZnOxNy thin films with nitrogen content and optical band-gap energy values ranging over 0-6.2 at.% and 3.34-1.67 eV, respectively. The fine control of the nitrogen content in the deposited ZnOxNy thin films composition was possible through the stabilization of the reactive HiPIMS discharge in the transition region, between the metallic and compound target sputtering modes. Various analytical techniques such as AFM, XPS, XRD, UV-Vis and Raman spectroscopy have been employed to characterize the properties of the deposited thin films. The photocatalytic activity, light excitation efficiency and life time of photo-generated charge carriers in the ZnOxNy films were investigated by photo-electrochemical and photo-current measurements during visible light on/off irradiation cycles. The as-deposited films showed poor visible-light photocatalytic activity and photo-current response. Post-deposition annealing of the films in nitrogen atmosphere resulted in a slight enhancement of crystalline order. However, the thermal treatment improved considerably the film photocatalytic activity and stability for water splitting under visible light irradiation. The optimum photo-current response and photocatalytic activity have been obtained for the annealed ZnOxNy films with a nitrogen content of 3.4 at.% (photon-to-current efficiency up to 33% at λ = 370 nm and 0.5 V biasing potential vs. Ag/AgCl). Increasing the nitrogen content above this value, in spite of lowering the energy band-gap, worsened the visible light photocatalytic activity of the films due to deterioration of the crystalline order.