A series of ZnO-based photocatalyst arrays, composed of M-ZnO (M = Cd, Sn, Ag, Bi, Al, Fe, W, Pt, Ru and Ir) on fluorine-doped tin oxide (PTO) substrates, are fabricated using a picoliter dispensing system. Their photocatalytic activity for a water splitting reaction is rapidly determined by scanning electrochemical microscopy (SECM), using an optical fiber connected to a xenon lamp. The SECM screening results show that the Zn0.8Cd0.2O spot exhibits the highest photocurrent in 0.1 M Na2SO4/Na2SO3 solution under UV-visible light irradiation. The candidate is characterized by SEM, EDX, XRD, XPS and UV-vis spectroscopy analysis. The best photocatalyst is composed of a binary mixture of 78 mol% ZnO with a wurtzite structure and 22 mol% CdO with a cubic structure. The band gap energy is reduced to 2.99 eV when ZnO incorporates 22 mol% CdO. The photoelectrochemical (PEC) properties of the bulk film electrodes are also tested, to confirm the SECM results. The superior photocatalytic activity of the Zn0.8Cd0.2O composite is mainly attributed to the effective charge separation between the photo-excited electrons and holes. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.