Nanostructured semiconductor photoanodes play an important role in solar fuel generation, and the design of the semiconductor aqueous electrolyte interface can be crucial in enhancing the energy conversion efficiency. We have investigated the effects on photoelectrochemical oxygen evolution for monoclinic nanostructured BiVO4 films uncoated and coated with microporous sol-gel Al2O3 "over layers". Variation of the thickness of the Al2O3 coating (formed by surface sol-gel deposition and annealing at 435 degrees C) led to a reduction of pseudo-capacitance and allowed optimization of the quantum efficiency. Exploration of the photocurrent enhancement as a function of applied potential reveals two distinct potential domains/mechanisms: (i) a low bias region enhancement effect (assigned to a lowering of the rate of external recombination of electrons with oxygen) and (ii) a high bias region of enhancement (assigned to higher charge carrier mobility due to less trapping in surface states). (C) 2016 Elsevier B.V. All rights reserved.