Thermally oxidized TiO2 ( rutile ) films on titanium showed about an order of magnitude higher photoelectrochemical energy conversion efficiency for water splitting ( similar to 0.2%, simulated AM1.5 sunlight ) when the substrate had been etched in acid prior to oxidation. We investigated the cause of the improved photocurrents. Film morphology is insufficient to explain the variation in photoelectrochemical response. Charge-carrier profiles from capacitance-voltage measurements using Schottky contacts indicated a gradual transition from near-stoichiometric oxide at the surface to highly substoichiometric deeper in the film approaching the titanium substrate for oxide grown on etched substrates. In contrast, the oxide layer on unetched samples showed higher carrier densities and a more abrupt transition to substoichiometric or metallic. This interpretation is supported by varying incidence-angle X-ray diffraction measurements suggesting more fully oxidized surface regions with a more gradual transition to suboxides for etched samples. Impedance measurements also suggested that the surface region of oxide layers grown on acid-etched substrates have fewer short-time-constant electronic defects (intraband gap states ) than the surface of layers grown on more disordered metallic substrates. The improved photoelectrochemical performance probably results from higher charge-carrier mobilities, reduced recombination, and higher photoconductivity due to long time constants of trapping states. (c) 2007 The Electrochemical Society. All rights reserved.