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Mixed anatase/rutile TiO2 exhibits high photocatalytic activity; however, the mechanism underlying the high performance of the mixed phases is not fully understood. We have performed time-domain ab initio calculations to study the exited state dynamics in mixed phase TiO2 and to investigate the impact of an oxygen vacancy on the dynamics. The anatase(100)/rutile(001) heterostructures with and without an oxygen vacancy used in this work exhibit type II band alignment with the conduction band of rutile residing above that of anatase. The oxygen vacancy introduces a hole trap state inside the bandgap. Owing to a strong coupling between the donor and acceptor states, the electron and hole transfers across the anatase/rutile interface occur on an ultrafast 100 fs timescale in both systems. The decoupling of electron and hole favors a long-lived charge separated state. The electron–hole recombination across the pristine anatase/rutile interface takes 6.6 ns and is significantly slower than that in the pure anatase and rutile phases, showing good agreement with experiments. The electron transfer dynamics is independent of the oxygen vacancy, which has some influence on the hole transfer and a strong effect on carrier recombination. By creating a hole trap state, the vacancy accelerates carrier losses by over an order of magnitude. The fast charge separation and the long lifetime of the charge separated state rationalize the enhanced photocatalytic performance of mixed phase TiO2 compared to the pure phases. [ABSTRACT FROM AUTHOR]

Recently, we reported enhanced H 2 evolution on TiO 2 -passivated GaAs. Based on density functional theory (DFT) calculations, this enhancement was attributed to Ti 3+ states, which bind reactant species and increase charge transfer across the semiconductor–liquid interface. Here, we provide a quantitative correlation between Ti 3+ 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 TiO 2 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 TiO 2 produces the best conditions for photocatalysis. XPS spectra show that thinner TiO 2 films (1 nm) have a higher density of Ti 3+ states than thicker films (5 nm), which have lower photocatalytic performance. PL spectroscopy provides further evidence for these Ti 3+ surface states, which cause increased surface recombination. While it is apparent that the TiO 2 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 TiO 2 thicknesses above 10 nm, which are crystalline and, therefore, considerably more insulating than thinner amorphous TiO 2 films. [ABSTRACT FROM AUTHOR]