Efficient hydrogen generation over (100)-oriented ZnO nanostructured photoanodes under solar light†
For the first time, ZnO photoanodes with the (100) preferential orientation have been shown to generate hydrogen under solar photon illumination. These films fabricated by the spray pyrolysis technique showed distinctly oriented and stacked layers with an average thickness of ~50 nm as observed in the cross-sectional studies. They exhibited a high texture coefficient (>2) for the (100) plane, mainly indicating an a-axis preferred oriented film. Implicitly, these films exhibited an improved absorption capability thereby yielding a high photocurrent density of ~149 μA and a solar-to-hydrogen efficiency of 0.12%. An evolved hydrogen of 260 μmoles h−1 under solar light was found to be maximum for the best film. The high donor density (1016 cm−3) and most suited flat band potential (−0.518 V vs. NHE) of the films as deduced from the electrochemical investigations seem to be a major factor responsible for such photoelectrochemical behavior of the films. The surface morphology of these films containing nanoprisms forming a “Y-shaped” network with uniform fibers of ~500 nm thickness seems to be additionally contributing to the displayed PEC performance. The photoanode was found to be 83.9% efficient at 360 nm wavelength as recorded from incident-photon-current conversion efficiency measurements. Unlike past reports on (002) oriented films, the present work demonstrates that even an (100)-oriented ZnO film shows a significantly high photocurrent generation, as well as a significant solar-to-hydrogen production yielding high evolved hydrogen over these films.