Scanning electrochemical microscopy (SECM) with a scanning optical fiber was applied to rapidly identify potential Zn0.3Cd0.7S-based photocatalysts for water oxidation. Among the photocatalysts studied, the spot with the precursor composition Ag-0.3-(Zn0.3Cd0.7S)(0.7) shows the highest photocurrent in 0.1 M Na2SO4/Na2SO3 solution under both UV-visible and visible light irradiation. The X-ray diffraction and X-ray photoelectron spectrometric analyses reveal that the Ag-0.3-(Zn0.3Cd0.7S)(0.7) photocatalyst is comprised of monoclinic Ag2S and solid solution Zn0.3Cd0.7S phases with average crystallite size in the range 18-28 nm. The addition of 30% Ag2S to Zn0.3Cd0.7S photocatalysts enhances light absorption in the visible region. The SECM screening results are also confirmed with bulk film studies. The Ag-0.3-(Zn0.3Cd0.7S)(0.7) electrode exhibits highly efficient visible-light photocatalytic activity with an IPCE value above 20% for a light wavelength of 500 nm. (C) 2014 Elsevier Ltd. All rights reserved.