Heterogeneous photocatalytic water splitting under the irradiation of sunlight is an attractive method for generating hydrogen from water. While the photocatalytic mechanism has been extensively studied, most of the experimental studies have been performed with an ensemble of photocatalyst particles with various sizes, morphologies, and secondary structures. To gain a deeper understanding of the mechanism of photocatalysis, it is indispensable to clarify how the geometric structure of photocatalyst affects the kinetics of photogenerated carriers and redox reactions. In this study, the hole decay characteristics and photocatalytic activity of BiVO4, a promising photocatalyst for oxygen evolution with visible light, have been investigated with single-particle transient absorption microscopy. Upon irradiation with 527 nm light, well-faceted nonaggregated crystallites show fast hole decay and little reactivity for Fe3+ reduction. In contrast, aggregated particles with grain boundaries between small primary crystallites show slower hole decay and higher reactivity for Fe3+ reduction than the nonaggregated crystallites. This behavior is increasingly pronounced as the secondary particle size of aggregated crystallite increases. This indicates that grain boundaries in aggregated particles do not work as recombination centers but play an important role in elongation of carrier lifetime and thus in enhancing the reactivity of photocatalyst through trap–detrap processes.

Charge carrier trapping plays a vital role in heterogeneous photocatalytic water splitting because it strongly affects the dynamics of photogenerated charges and hence the photoconversion efficiency. Although hole trapping by water at water/photocatalyst interface is the first step of oxygen evolution in water splitting, little has been known on how water adsorbate itself is involved in hole trapping dynamics. To clarify this point, we have performed infrared transient and steady-state absorption spectroscopy of anatase TiO2 nanoparticles as a function of the number of water adsorbate layers. Here, we demonstrate that water molecules reversibly adsorbed in the first layer on TiO2 nanoparticles are capable to trap photogenerated holes, while water in the second layer hydrogen bonding to the first-layer water makes hole trapping less effective.

Photophysical properties of molecular aggregates are largely determined by exciton coherence size: a spatial extension of exciton delocalization. Increase in exciton coherence size can lead to fast energy transport as well as efficient charge separation. Here, we demonstrate that introducing alkyl chains to organic molecules can enhance the exciton coherence size significantly. Focusing on the thin films of excellent hole transport materials, dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene (DNTT) and its alkyl-substituted derivative, we analyze the steady-state and picosecond time-resolved photoluminescence spectra of the films to estimate exciton coherence sizes. The alkyl substitution enhances the coherence size by a factor of 2–3, indicating that a long-range ordering in the molecular aggregates is achieved with the additional van der Waals interaction between saturated alkyl chains. The coherence sizes of both the films decrease with increasing temperature owing to thermal populations within the vibronic exciton manifolds.

Cocatalysts are usually needed to improve photoconversion efficiency in water splitting with heterogeneous photocatalysts. Here, we show that NiOx (0 < x < 1) nanoparticles loaded on a layered perovskite, BaLa4Ti4O15 (BLT), serve as an effective electron sink. We have measured the time profiles of transient absorption (TA) of BLT with and without loading NiOx cocatalyst in air and in water in a wide wavelength range from 400 nm to 4 μm upon excitation with a femtosecond pulse at 266 nm. TA at 4 μm indicates that photogenerated electrons are rapidly transferred to cocatalyst within 1 ps. The time profile of TA at 402 nm from sub-μs to 10 ms contributed by surface photoholes is affected by loading of cocatalyst and redox reactions with water. Fittings of the decay profiles of TA at 402 nm with a trap–detrap kinetic model indicate that the oxidation of water appreciably starts at around 1 ms after the pump pulse, while the reduction of water takes place prior to oxidation in much early time domains. This implies that the redox reactions take place under substantial imbalance between electron and hole densities.

Bismuth vanadate (BiVO4) is an effective visible-light-driven photocatalyst for oxygen evolution from water. To understand the mechanism of photocatalytic oxidation of water, it is important to detect and characterize holes at the surfaces of powdered catalysts. Here, we report the transient absorption of BiVO4 in a wide time range from subpicosecond to 200 μs upon the excitation across the band gap with 400 nm femtosecond pulses. The effect of electron scavenger (Fe3+) on transient absorption decays indicates that the transitions at λ < 700 nm are mainly contributed by holes at the surfaces. While the transient absorption at λ > 700 nm rises almost instantaneously, the absorption λ < 700 nm has a slower rise component of τ ∼ 15 ps due to filling of surface traps with holes. Moreover, the rise component is modulated with strongly oscillating signals caused by coherent excitation of an external phonon mode between Bi3+ and VO43–. Thus, the transitions at λ < 700 nm associated with surface-trapped holes are strongly coupled to the external phonon mode. This study demonstrates that the time-domain spectroscopy is useful for characterizing the vibrational structure specific to the surface charge trap sites of powdered photocatalysts.

We have investigated the mechanism of enhanced absorption intensities of vibrational bands of adsorbates on copper meshes with subwavelength holes by measuring and simulating temporal profiles of infrared pulses transmitted through the meshes. As reported previously [Williams et al., J. Phys. Chem. B, 2003, 107, 11871], the absorption intensities of CH stretching bands of alkanethiolate adsorbed on the mesh increase substantially with decreasing hole size. The enhancements of absorption intensities are associated with temporal delays of infrared pulses transmitted through the mesh. Finite difference time domain calculations reproduce the observed pulse delays as a function of hole size. These facts indicate that the delays of transmitted pulses are not caused by coupling of infrared radiation to surface plasmon polaritons propagating on the front and rear surfaces of the mesh, but they are caused by the reduction in group velocity owing to coupling to waveguide modes of mesh holes. Consequently, the strong enhancements of the absorption intensities are attributed to adsorbates inside the holes rather than to those on the mesh surfaces that have been proposed previously.

Coherent Cs–Cu stretching vibration at a Cu(111) surface covered with a full monolayer of Cs is observed by using time-resolved second harmonic generation spectroscopy, and its generation mechanisms and dynamics are simulated theoretically. While the irradiation with ultrafast pulses at both 400 and 800 nm generate the coherent Cs–Cu stretching vibration at a frequency of 1.8 THz (60 cm–1), they lead to two distinctively different features: the initial phase and the pump fluence dependence of the initial amplitude of coherent oscillation. At 400 nm excitation, the coherent oscillation is nearly cosine-like with respect to the pump pulse and the initial amplitude increases linearly with pump fluence. In contrast, at 800 nm excitation, the coherent oscillation is sine-like and the amplitude is saturated at high fluence. These features are successfully simulated by assuming that the coherent vibration is generated by two different electronic transitions: substrate d-band excitation at 400 nm and the quasi-resonant excitation between adsorbate-localized bands at 800 nm, i.e., possibly from an alkali-induced quantum well state to an unoccupied state originating in Cs 5d bands or the third image potential state.

Ultrafast dynamics of vibrational energy transfer in overlayers of D2O and CO on Pt(111) have been investigated by femtosecond time-resolved (TR) IR−visible sum-frequency-generation (SFG) spectroscopy under ultrahigh-vacuum conditions. About 10 layers of D2O ice were epitaxially grown on c(4 × 2)-CO/Pt(111). The surface was excited by subpicosecond laser pulses, and subsequent energy transfer through low-frequency modes of adsorbates was monitored in terms of peak shifts and broadenings of C−O and O−D stretching bands in SFG spectra as a function of the pump−probe delay. Because D2O ice forms islands, there are two types of CO: one interacting with D2O and the other free from D2O. Simulations of the TR-SFG spectra by using a phenomenological model for the energy-transfer dynamics indicate that the coupling rate of perturbed CO is larger than that of free CO by a factor of 1.7; this is probably because CO 2π* states shift toward the Fermi level due to interaction with D2O. Two isolated bands at 2668 and 2713 cm−1 were assignable to the OD stretching bands of D2O directly interacting with CO at the D2O/CO interface and D2O at the vacuum/ice interface, respectively. Analysis of the temporal spectral changes of free D2O by using a diffusive thermal transport model indicates that heat transfer through low-frequency phonons of the ice layers occurs within 3 ps; this is substantially faster than the pulsed laser-induced melting of thin ice films reported previously.

Scanning tunnelling microscopy was used for studying surface structural evolution in the course of the hydrogen abstraction of H2O by O adatoms on oxidized Ag(110) surfaces where quasi-1D AgO chains form ordered structures. The reaction initially takes place slowly on Ag(110)–(5 × 1)O at the end of the AgO chain, whereas the reaction accelerates explosively upon the appearance of a reaction front that propagates along the direction perpendicular to the chain. The surface morphology of the region swept over by the reaction front completely changes from a (5 × 1)-O to a (2 × 3) structure with many rectangular islands possibly due to the formation of H2O(OH)2. The induction time and explosive acceleration with the propagating reaction front imply that the reaction proceeds autocatalytically. The water clusters hydrating OH adsorbates play likely a central role in accelerating the reaction by supplying H2O to the O adatoms in the AgO chains at the reaction front.

Deposition and fabrication of films of Au nanoclusters protected by alkanethiolate ligands are attempted on a TiO2(1 1 0) surface and the structures of films are observed by a scanning tunneling microscope (STM). Effects of oxygen and hydrogen-plasma etching in addition to UV irradiation on the structure and chemical composition of the films are also investigated by using STM and X-ray photoelectron spectroscopy. Alkanethiolate Au nanoclusters are produced using a modified Brust synthesis method and their LB films are dip-coated on TiO2(1 1 0). Alkanethiolate Au nanoclusters are weakly bound to the substrate and can be manipulated with an STM tip. Net-like structures of alkanethiolate Au nanoclusters are formed by a strong blast of air. Oxygen-plasma etching removes alkanethiolate ligands and simultaneously oxidizes Au clusters. At room temperature, prolonged oxygen-plasma etching causes agglomeration of Au nanoclusters. UV irradiation removes ligands partly, which makes Au nanoclusters less mobile. The net-like structure of alkanethiolate Au clusters produced by a blast of air is retained after oxygen and hydrogen-plasma etching.

Reduction of oxidized gold nanoclusters by exposures to foreign gases and irradiation of UV photons has been investigated using X-ray photoelectron spectroscopy. Gold nanoclusters with narrow size distributions protected by alkanethiolate ligands were deposited on a TiO2(1 1 0) surface with dip coating. Oxygen plasma etching was used for removal of alkanethiolate ligands and oxidization of gold clusters. The oxidized gold clusters were exposed to CO, C2H2, C2H4, H2, and hydrogen atoms. Although, C2H4 and H2 did not show any indications of reduction of oxidized gold clusters, CO, C2H2, and hydrogen atoms reduced the oxides on gold cluster surfaces. Among them, hydrogen atoms were most effective for reduction. Irradiation of UV photons around 400 nm could also reduce the oxidized gold clusters. The photochemical reduction mechanism was proposed as follows. The photo-reduction was initiated by electronic excitation of gold clusters and oxygen atoms activated reacted with carbon atoms at the surfaces of gold clusters. Carbon species were likely absorbed in gold clusters or remained at the boundaries between gold clusters when gold clusters agglomerated during oxygen plasma exposures. As the photochemical reduction progressed, carbon atoms segregated to the surfaces of gold clusters.

Scanning tunnelling microscopy was used for studying surface structural evolution in the course of the hydrogen abstraction of H2O by O adatoms on oxidized Ag(110) surfaces where quasi-1D AgO chains form ordered structures. The reaction initially takes place slowly on Ag(110)–(5 × 1)O at the end of the AgO chain, whereas the reaction accelerates explosively upon the appearance of a reaction front that propagates along the direction perpendicular to the chain. The surface morphology of the region swept over by the reaction front completely changes from a (5 × 1)-O to a (2 × 3) structure with many rectangular islands possibly due to the formation of H2O(OH)2. The induction time and explosive acceleration with the propagating reaction front imply that the reaction proceeds autocatalytically. The water clusters hydrating OH adsorbates play likely a central role in accelerating the reaction by supplying H2O to the O adatoms in the AgO chains at the reaction front.

We have investigated the mechanism of enhanced absorption intensities of vibrational bands of adsorbates on copper meshes with subwavelength holes by measuring and simulating temporal profiles of infrared pulses transmitted through the meshes. As reported previously [Williams et al., J. Phys. Chem. B, 2003, 107, 11871], the absorption intensities of CH stretching bands of alkanethiolate adsorbed on the mesh increase substantially with decreasing hole size. The enhancements of absorption intensities are associated with temporal delays of infrared pulses transmitted through the mesh. Finite difference time domain calculations reproduce the observed pulse delays as a function of hole size. These facts indicate that the delays of transmitted pulses are not caused by coupling of infrared radiation to surface plasmon polaritons propagating on the front and rear surfaces of the mesh, but they are caused by the reduction in group velocity owing to coupling to waveguide modes of mesh holes. Consequently, the strong enhancements of the absorption intensities are attributed to adsorbates inside the holes rather than to those on the mesh surfaces that have been proposed previously.