Detailed analyses on the role of nanobranches in photoelectrochemical performances of multiscale ZnO nanobrushes

In this study, brush-like hierarchical ZnO microrod/nanobranche photoanodes are fabricated via a facile two-step hydrothermal method. The application of the multiscale ZnO microrod backbone formed on a conductive glass substrate allows the growth of ZnO nanobrushes with controllable branch length, which enables detailed studies on the distinctive working mechanism of integrated ZnO microrod backbone and nanobranch structures. The growth and optimization of the uniformly formed secondary nanobranches significantly enhances PEC performance. In addition, the variations in photopotential and charge-carrier transfer are identified through open-circuit potential measurement and electrochemical impedance spectroscopy, to understand the electron–hole transfer kinetics of the ZnO nanobrushes. The results reveal that the distinctive performance of the ZnO nanobrushes originates from the synergetic effect of the ZnO microrod backbone/nanobranches, the subsequent effective charge separation, and enhanced carrier generation in the dominant space-charge region. In particular, the optimum photoelectrode is determined to be that containing the ZnO nanobrushes with nanobranches grown for 5 h, and the corresponding photocurrent density is measured to be 1.68 mA/cm2, which is 136.3% higher than that of the pristine ZnO microrod electrodes.