Electron–Hole Recombination Time at TiO₂ Single-Crystal Surfaces: Influence of Surface Band Bending

Photocatalytic activity is determined by the transport property of photoexcited carriers from the interior to the surface of photocatalysts. Because the carrier dynamics is influenced by a space charge layer (SCL) in the subsurface region, an understanding of the effect of the potential barrier of the SCL on the carrier behavior is essential. Here we have investigated the relaxation time of the photoexcited carriers on single-crystal anatase and rutile TiO₂ surfaces by time-resolved photoelectron spectroscopy and found that carrier recombination, taking a nanosecond time scale at room temperature, is strongly influenced by the barrier height of the SCL. Under the flat-band condition, which is realized in nanometer-sized photocatalysts, the carriers have a longer lifetime on the anatase surface than the rutile one, naturally explaining the higher photocatalytic activity for anatase than rutile

Strong Hydrogen Bonds at the Interface between Proton-Donating and -Accepting Self-Assembled Monolayers on Au(111)

Hydrogen-bonding heterogeneous bilayers on substrates have been studied as a base for new functions of molecular adlayers by means of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), and density functional theory (DFT) calculations. Here, we report the formation of the catechol-fused bis­(methyl­thio)­tetra­thia­ful­valene (H₂Cat-BMT-TTF) adlayer hydrogen bonding with an imidazole-terminated alkanethiolate self-assembled monolayer (Im-SAM) on Au(111). The heterogeneous bilayer is realized by sequential two-step immersions in solutions for the individual Im-SAM and H₂Cat-BMT-TTF adlayer formations. In the measurements by AFM, a grained H₂Cat-BMT-TTF adlayer on Im-SAM is revealed. The coverage and the chemical states of H₂Cat-BMT-TTF on Im-SAM are specified by XPS. On the vibrational spectrum measured by IRAS, the strong hydrogen bonds between H₂Cat-BMT-TTF and Im-SAM are characterized by the remarkably red-shifted OH stretching mode at 3140 cm^–1, which is much lower than that for hydrogen-bonding water (typically ∼3300 cm^–1). The OH stretching mode frequency and the adsorption strength for the H₂Cat-BMT-TTF molecule hydrogen bonding with imidazole groups are quantitatively examined on the basis of DFT calculations

Correlation between Photocatalytic Activity and Carrier Lifetime: Acetic Acid on Single-Crystal Surfaces of Anatase and Rutile TiO₂

Photocatalytic activity and lifetime of photoexcited carriers on well-defined single-crystalline anatase and rutile TiO₂ surfaces with different surface orientation have been systematically studied by photoelectron spectroscopy. Photocatalytic activity, evaluated with reference to the photocatalytic degradation of acetic acid, has a positive and linear correlation with carrier lifetime at the crystal surface, which was determined by following the time evolution of the ultraviolet-induced surface photovoltage. This indicates that the carrier lifetime is a prime factor for the photocatalytic activity so that it can be viewed as the origin of the crystal-surface-orientation dependence of the photocatalytic activity

What Determines the Lifetime of Photoexcited Carriers on TiO₂ Surfaces?

Pump–probe time-resolved X-ray photoelectron spectroscopy measurements have been carried out to comparatively assess the relaxation process of the photoexcited states on pristine and Ar⁺-sputtered TiO₂(110) surfaces and a TiO₂(011)-2 × 1 surface, on which the accumulation-type space charge layers are developed. Ultraviolet laser irradiation induces a surface photovoltage (SPV) of around 0.1 eV. The SPV relaxation time on pristine TiO₂(110) is determined to be approximately 100 ns and is doubled on the sputtered surface. In contrast, a much shorter time of 1 ns is observed on TiO₂(011)-2 × 1. The difference in the relaxation time on the two TiO₂(110) surfaces is explained by differences in the O vacancy density on the surface as well as the barrier height of the surface potential for the photoexcited holes. A large hole capture cross section of a state characteristic of TiO₂(011)-2 × 1 is, on the other hand, responsible for the fast SPV relaxation on this surface

Adsorption of CO₂ on Graphene: A Combined TPD, XPS, and vdW-DF Study

The adsorption of CO₂ molecules on monolayer epitaxial graphene on a SiC(0001) surface at 30 K was investigated by temperature-programmed desorption and X-ray photoelectron spectroscopy. The desorption energy of CO₂ on graphene was determined to be (30.1–25.1) ± 1.5 kJ/mol at low coverages and approached the sublimation energy of dry ice (27–25 kJ/mol) with increasing the coverage. The adsorption of CO₂ on graphene was thus categorized into physisorption, which was further supported by the binding energies of CO₂ in core-level spectra. The adsorption states of CO₂ on graphene were theoretically examined by means of the van der Waals density functional (vdW-DF) method that includes nonlocal correlation. The experimental desorption energy was successfully reproduced with high accuracy using vdW-DF calculations; the optB86b-vdW functional was found to be most appropriate to reproduce the desorption energy in the present system

Elucidation of Rh-Induced In-Gap States of Rh:SrTiO₃ Visible-Light-Driven Photocatalyst by Soft X‑ray Spectroscopy and First-Principles Calculations

The occupied and unoccupied in-gap electronic states of a Rh-doped SrTiO₃ photocatalyst were investigated by X-ray emission spectroscopy and X-ray absorption spectroscopy for different Rh impurity valence states and doping levels. An unoccupied midgap Rh⁴⁺ acceptor state was found 1.5 eV below the SrTiO₃ conduction band minimum. Both Rh⁴⁺ and Rh³⁺ dopants were found to have an occupied donor level close to the valence band maximum of SrTiO₃. The density of states obtained from first-principles calculations show that all observed spectral features can be assigned to electronic states of substitutional Rh at the Ti site and that Rh:SrTiO₃ is an unusual titanate compound with a characteristic p-type electronic structure. The Rh doping results in a large decrease of the bandgap energy, making Rh:SrTiO₃ an attractive material for use as a visible-light-driven H₂-evolving photocatalyst in a solar water splitting reaction