Bismuth oxyiodide has attracted strong interest as a nano-engineered material for high-performance electrodes in photoelectrochemical (PEC) cells. Herein, we demonstrate the visible-light-sensitive PEC photocathodes based on nano-engineered bismuth oxyiodide (BiOI). The photocathodes are comprised of plasmonic Au nanoparticles (NPs) on hydrogen-reduced BiOI (H-BiOI) nanoflake arrays. The BiOI-based photocathodes exhibit an AM 1.5 photocurrent of 3.6 + 0.1 mA/cm(2) at 0 V vs reversible hydrogen electrode. We attribute the enhanced photoreduction performance of our photocathodes to three factors that increase the formation and separation rates of electron-hole pairs for photocatalysis. Firstly, the small thickness of the nanoflakes facilitates the diffusion of minority carriers to the surfaces before their recombination. Secondly, the hydrogenation-induced oxygen vacancies in H-BiOI lead to increased acceptor density. Lastly, plasmonic effects in Au NPs enhance the light absorption, hot-carrier injection, and resonant energy transfer in the composite electrodes. Our experiments are coupled to optical simulations. This work demonstrates the tremendous potential of coupling plasmonic effects with nano-engineering to innovate visible-light-sensitive photocathodes for the realization of efficient PEC cells. (C) 2016 Elsevier Ltd. All rights reserved.