Kohei Uosaki

Photocatalytic activity of transition-metal-loaded titanium(IV) oxide powders suspended in aqueous solutions : Correlation with electron-hole recombination kinetics

Photocatalytic reactions by transition-metal (V, Cr, Fe, Co, Cu, Mo, or W) loaded TiO2 (M-TiO2) powders suspended in aqueous solutions of methanol, (S)-lysine (Lys), or acetic acid were investigated. The photoactivities of various samples were compared with the rate constant (kr) of recombination of photoexcited electrons and positive holes determined by femtosecond pump–probe diffuse reflection spectroscopy (PP-DRS). As a general trend, increased loading decreased the rate of formation of the main products (H2 , pipecolinic acid (PCA), and CO2) under UV (>300 nm) irradiation, and the effect became more intense on increasing the loading. In PP-DRS, these M-TiO2 gave similar decays of absorption at 620 nm arising from excitation by a 310 nm pulse (<100 fs). The second-order rate constant (kr) markedly increased with loading, even at a low level (0.3%) and further increased with an increase in loading up to 5%. The photocatalytic activity of platinized M-TiO2 for H2 and PCA production under deaerated conditions depended strongly on kr, but the relation between kr and the rate of CO2 production by unplatinized M-TiO2 under aerated conditions was ambiguous; other factor(s) might control the rate of the latter. These different kr dependences of photoactivity on the reaction kinetics governed by e−–h+ recombination were attributed to the presence of O2 and Pt deposits. A simple kinetic model to explain the overall rate of these photocatalytic reactions is proposed, and the effect of recombination kinetics on photoactivity is discussed.

Quantitative analysis of defective sites in titanium(IV) oxide photocatalyst powders

The molar amounts of defective sites (Md) in several titanium(IV) oxide (TiO2) powders were determined using photoinduced reactions of electron accumulation in deaerated aqueous solutions containing sacrificial hole scavengers and subsequent reduction of methylviologen to its cation radical. Measurements of pH dependence of typical anatase and rutile TiO2 powders showed that these defective sites were of electronic energy just below the conduction band edge of TiO2 in ranges of 0–0.35 V for anatase and 0–0.25 V for rutile. A linear relation of Md with the rate constant of electron-hole recombination determined by femtosecond pump-probe diffuse reflection spectroscopy revealed that Md could be a quantitative parameter of recombination between a photoexcited electron and a positive hole. The fact that there was no linear relation between Md and the specific surface area suggests that the surface area was not directly reflected on Md. A reciprocal correlation between photocatalytic activity for water oxidation in aqueous silver sulfate solution and Md revealed that the rate of recombination is one of the predominant physical properties governing the activities of TiO2 powders in this reaction system.

Boron Nitride Nanosheet on Gold as an Electrocatalyst for Oxygen Reduction Reaction: Theoretical Suggestion and Experimental Proof

Boron nitride (BN), which is an insulator with a wide band gap, supported on Au is theoretically suggested and experimentally proved to act as an electrocatalyst for oxygen reduction reaction (ORR). Density-functional theory calculations show that the band gap of a free h-BN monolayer is 4.6 eV but a slight protrusion of the unoccupied BN states toward the Fermi level is observed if BN is supported on Au(111) due to the BN-Au interaction. A theoretically predicted metastable configuration of O2 on h-BN/Au(111), which can serve as precursors for ORR, and free energy diagrams for ORR on h-BN/Au(111) via two- and four-electron pathways show that ORR to H2O2 is possible at this electrode. It is experimentally proved that overpotential for ORR at the gold electrode is significantly reduced by depositing BN nanosheets. No such effect is observed at the glassy carbon electrode, demonstrating the importance of BN-substrate interaction for h-BN to act as the ORR electrocatalyst. A possible role of the edge of the BN islands for ORR is also discussed.

Layered perovskite oxide: a reversible air electrode for oxygen evolution/reduction in rechargeable metal-air batteries.

For the development of a rechargeable metal-air battery, which is expected to become one of the most widely used batteries in the future, slow kinetics of discharging and charging reactions at the air electrode, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively, are the most critical problems. Here we report that Ruddlesden-Popper-type layered perovskite, RP-LaSr3Fe3O10 (n = 3), functions as a reversible air electrode catalyst for both ORR and OER at an equilibrium potential of 1.23 V with almost no overpotentials. The function of RP-LaSr3Fe3O10 as an ORR catalyst was confirmed by using an alkaline fuel cell composed of Pd/LaSr3Fe3O10-2x(OH)2x·H2O/RP-LaSr3Fe3O10 as an open circuit voltage (OCV) of 1.23 V was obtained. RP-LaSr3Fe3O10 also catalyzed OER at an equilibrium potential of 1.23 V with almost no overpotentials. Reversible ORR and OER are achieved because of the easily removable oxygen present in RP-LaSr3Fe3O10. Thus, RP-LaSr3Fe3O10 minimizes efficiency losses caused by reactions during charging and discharging at the air electrode and can be considered to be the ORR/OER electrocatalyst for rechargeable metal-air batteries.

Electrocatalytic reactivity for oxygen reduction at epitaxially grown Pd thin layers of various thickness on Au(111) and Au(100)

Abstract Electrochemical reduction of oxygen was studied at the epitaxially grown ultra thin Pd layers of various thicknesses on Au(111) and Au(100) single crystal surfaces in 50 mM HClO 4 solution saturated by oxygen. The oxygen reduction at the Pd thin layers on the gold substrate started at more positive potential than that at the gold single crystal surfaces even only a submonolayer of Pd was deposited. The potentials where cathodic current of oxygen reduction started to increase were dependent on the potentials where the oxide of the ultra thin Pd layers was reduced.

The Rate of the Photoelectrochemical Generation of Hydrogen at p‐Type Semiconductors

The current-potential relations with and without illumination, quantum efficiency-wavelength relations at several potentials, the flatband potentials, the transient behavior, and the stability of seven p-type semiconductors, i.e., ZnTe, CdTe, GaAs, InP, GaP, SiC, and Si, have been measured in 1N NaOH and 1N H/sub 2/SO/sub 4/. The position of the photocurrent-potential relations are related to the flatband potential and the energy gap of the semiconductor. The existence of the maximum in quantum efficiency-wavelength relation is analyzed by considering surface recombination. The stability and the transient behavior are analyzed.

Sum frequency generation (SFG) study of the pH-dependent water structure on a fused quartz surface modified by an octadecyltrichlorosilane (OTS) monolayer

The interface-sensitive spectroscopic method, sum frequency generation (SFG), has been used to investigate the interfacial water structure on a fused quartz surface modified by an octadecyltrichlorosilane (OTS) self-assembled monolayer in phosphate buffered solutions at various pHs. The experimental results demonstrate that the water molecules at the quartz/OTS surface flip while the water molecules at the OTS surface maintain their orientation when the solution pH is changed from neutral to acidic. The results show that most of the silanol groups still exist on the fused quartz surface even after a silane coupling reaction of OTS under the reported experimental conditions.

Electrochemical and electrogenerated chemiluminescence properties of tris(2,2'-bipyridine)ruthenium(II)-tridecanethiol derivative on ITO and gold electrodes

Abstract The electrochemical and electrogenerated chemiluminescence (ECL) properties of gold and indium tin oxide (ITO) electrodes modified by a tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)2+3) derivative with two tridecanethiol groups were studied. Although the electrochemical response of the gold electrode modified by the Ru(bpy)2+3-tridecanethiol derivative was irreversible and unstable because oxidation of the gold surface and the thiolate monolayer itself started before oxidation of the Ru(bpy)2+3 head group, the modified ITO electrode showed the stable redox of Ru(bpy)2+3 head group. Both the modified gold and modified ITO electrodes showed ECL properties in solution containing C2O2−4. We also found that a monolayer of the Ru(bpy)2+3-tridecanethiol derivative on ITO acts as a mediator for the oxidation of C2O2−4 in solution. Transient responses of current and ECL intensity of ITO and gold electrodes modified with the Ru(bpy)2+3-tridecanethiol derivative were also studied to elucidate the ECL mechanism.

Thickness dependent electrochemical reactivity of epitaxially electrodeposited palladium thin layers on Au(111) and Au(100) surfaces

Abstract Electrochemical characteristics of ultra thin Pd epitaxial layers deposited electrochemically on Au(111) and Au(100) surfaces were found to be strongly dependent on the surface structure and the thickness of the Pd thin layers. The electrochemical characterizations demonstrate that the Pd/Au(111) and Pd/Au(100) surfaces behave essentially like Pd(111) and Pd(100) surfaces, respectively. Formation and reduction of oxide on the Pd sub-monolayer surfaces took place at more and less positive potentials, respectively, than those on the surfaces of the Pd multilayer. The Pd/Au(100) electrode shows a much higher electrocatalytic activity for the oxidation of formaldehyde than that at the Pd/Au(111) electrode. Furthermore, the highest activity for the electro-oxidation of formaldehyde was observed at the Pd/Au(100) electrode when the thickness of the Pd thin layers was less than a monolayer. This behavior was discussed in terms of the potential- and thickness-dependent oxide formation–reduction on the Pd thin layers.

A self-assembled monolayer of ferrocenylalkane thiols on gold as an electron mediator for the reduction of Fe(III)-EDTA in solution

Abstract Self-assembled monolayers of ferrocenylalkane thiols were formed on gold substrate. It is demonstrated that a ferrocene group attached to the electrode acts as an electron mediator for the reduction of Fe(III)-EDTA in solution.