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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]

A robust and reliable method for improving the photocatalytic performance of InP, which is one of the best known materials for solar photoconversion (i.e., solar cells). In this article, we report substantial improvements (up to 18×) in the photocatalytic yields for CO2 reduction to CO through the surface passivation of InP with TiO2 deposited by atomic layer deposition (ALD). Here, the main mechanisms of enhancement are the introduction of catalytically active sites and the formation of a pn-junction. Photoelectrochemical reactions were carried out in a nonaqueous solution consisting of ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4), dissolved in acetonitrile, which enables CO2 reduction with a Faradaic efficiency of 99 % at an underpotential of +0.78 V. While the photocatalytic yield increases with the addition of the TiO2 layer, a corresponding drop in the photoluminescence intensity indicates the presence of catalytically active sites, which cause an increase in the electron-hole pair recombination rate. NMR spectra show that the [EMIM]+ ions in solution form an intermediate complex with CO2−, thus lowering the energy barrier of this reaction. [ABSTRACT FROM AUTHOR]

Green Chemistry. Photocatalytic reduction of CO2 is an exciting reaction system with the ability to convert an abundant greenhouse gas to combustible hydrocarbon fuels using sunlight. In their Communication on page 13502 ff., S. Cronin et al. describe a robust and reliable method for improving the photocatalytic performance of InP, which is one of the best known materials for solar photoconversion. Substantial improvements (up to 18×) in the photocatalytic yields for CO2 reduction to CO through the surface passivation of InP with TiO2 deposited by atomic layer deposition (ALD) are reported. [ABSTRACT FROM AUTHOR]