Recently, we reported enhanced H-2 evolution on TiO2-passivated GaAs. Based on density functional theory (DFT) calculations, this enhancement was attributed to Ti3+ states, which bind reactant species and increase charge transfer across the semiconductor-liquid interface. Here, we provide a quantitative correlation between Ti3+ density, as measured by X-ray photoemission spectroscopy (XPS) and photoluminescence (PL) spectroscopy, and photocatalytic performance, which substantiates the hypothesis put forth previously. In the photo-I-V characteristics reported here, passivating GaAs with TiO2 produces a shift in the onset potential of +0.35 V at 1 mA/cm(2) and enhances the photocurrent by 32-fold over bare GaAs (at 0 V vs. RHE), resulting in a peak photoconversion efficiency of 1.5% under AM1.5 G illumination. We find that just 1 nm of TiO2 produces the best conditions for photocatalysis. XPS spectra show that thinner TiO2 films (1 nm) have a higher density of Ti3+ states than thicker films (5 nm), which have lower photocatalytic performance. PL spectroscopy provides further evidence for these Ti3+ surface states, which cause increased surface recombination. While it is apparent that the TiO2 films cause strong electron-hole recombination, the benefit that they provide by providing catalytically active sites outweighs their detriment associated with charge recombination. No enhancement is observed for TiO2 thicknesses above 10 nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO2 films. (C) 2016 Elsevier Inc. All rights reserved.