Masatoshi Osawa

Surface-Enhanced Infrared Spectroscopy: The Origin of the Absorption Enhancement and Band Selection Rule in the Infrared Spectra of Molecules Adsorbed on Fine Metal Particles

Infrared transmission spectra of molecules adsorbed on silver island films evaporated on CaF2 have been investigated. The spectra are remarkably simple compared with those of the molecules in the solid state (KBr pellets). Only the vibrational modes which give dipole changes perpendicular to the metal surface are infrared active. In addition, their intensities are about 200 times larger than those of the free molecule. These results can be fully accounted for if the electric field which excites the surface molecule is perpendicular to the local surface of the metal islands and is stronger than the incident electric field. The origin of the absorption enhancement and the surface selection rule is discussed theoretically by using a classical electromagnetic model.

Surface-Enhanced Infrared Absorption

Molecules adsorbed on metal island films or particles exhibit 10-1000 times more intense infrared absorption than would be expected from conventional measurements without the metal. This effect is referred to as surface-enhanced infrared absorption (SEIRA) to emphasize the similarities with surface-enhanced Raman scattering (SERS). The electromagnetic interactions of the incident photon field with the metal and molecules play predominant roles in this effect. The chemical interactions of the molecules with the surface can give additional enhancement. The enhancement mechanisms and some applications of SEIRA.

Electroless deposition of gold thin films on silicon for surface-enhanced infrared spectroelectrochemistry

Electroless (or chemical) deposition technique has been used in preparing Au island film electrodes on Si for in situ infrared spectroscopic studies of the electrochemical interface in attenuated total reflection mode. Owing to surface-enhanced infrared absorption (SEIRA) effect, absorption bands of molecules adsorbed on the chemically deposited films were one order of magnitude as large as those observed on smooth Au electrode surfaces. Although the enhancement factor was identical to that observed on vacuum evaporated Au island films, this simple method is superior to vacuum evaporation method with respect to the adhesion of the film, surface contamination, reproducibility, and cost.

Visible light responsive pristine metal oxide photocatalyst: enhancement of activity by crystallization under hydrothermal treatment.

Photocatalytic activities of amorphous and crystal bismuth tungstate (Bi(2)WO(6)) were investigated using oxidative decomposition of gaseous acetaldehyde under visible light irradiation (>400 nm). Here, for the first time, negligible photocatalytic activity of amorphous Bi(2)WO(6) owing to the fast recombination of electron-hole pairs and the high quantum efficiency of Bi(2)WO(6) crystallites under visible light were demonstrated by action spectrum analysis and time-resolved infrared absorption measurements. Crystallization of the amorphous phase provided a red shift of the photoabsorption edge and marked increase in the lifetime of photoexcited electrons, resulting in an increase of photocatalytic activity.

The Role of Bridge‐Bonded Adsorbed Formate in the Electrocatalytic Oxidation of Formic Acid on Platinum

The oxidation of formic acid (HCOOH) on platinum electrodes has been extensively investigated as a model electrocatalytic reaction. It is generally accepted that HCOOH is oxidized to CO2 through a dual-pathway mechanism: one pathway (the main pathway) involves a fast reaction via a reactive intermediate and the second pathway includes a step in which a poisoning species is formed. This species, which is oxidized to CO2 at high potentials, has been identified as adsorbed CO, which is formed by dehydration of HCOOH. Adsorbed hydroxycarbonyl (COOHads) has long been assumed to be the reactive intermediate in the main pathway, but the spectroscopic detection of this species has not been reported to date. By using surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATRSEIRAS), Miki et al. observed that formate is adsorbed in a bridge-bonded configuration on Pt electrodes during HCOOH oxidation. On the basis of systematic time-resolved ATR-SEIRAS analysis of the oxidation dynamics, Samjesk et al. suggested that adsorbed formate (HCOOads) is a reactive intermediate in the main pathway and its decomposition to CO2 is the rate-determining step (rds). The adsorbed formate is in equilibrium with HCOOH in the bulk solution and the reaction pathway (formate pathway) can be represented by Equation (1)

Importance of Acid–Base Equilibrium in Electrocatalytic Oxidation of Formic Acid on Platinum

Electro-oxidation of formic acid on Pt in acid is one of the most fundamental model reactions in electrocatalysis. However, its reaction mechanism is still a matter of strong debate. Two different mechanisms, bridge-bonded adsorbed formate mechanism and direct HCOOH oxidation mechanism, have been proposed by assuming a priori that formic acid is the major reactant. Through systematic examination of the reaction over a wide pH range (0-12) by cyclic voltammetry and surface-enhanced infrared spectroscopy, we show that the formate ion is the major reactant over the whole pH range examined, even in strong acid. The performance of the reaction is maximal at a pH close to the pKa of formic acid. The experimental results are reasonably explained by a new mechanism in which formate ion is directly oxidized via a weakly adsorbed formate precursor. The reaction serves as a generic example illustrating the importance of pH variation in catalytic proton-coupled electron-transfer reactions.

ATR-SEIRAS Investigation of the Fermi Level of Pt Cocatalyst on a GaN Photocatalyst for Hydrogen Evolution under Irradiation

The interaction of photogenerated carries in GaN photocatalyst with Pt cocatalyst for hydrogen evolution under irradiation was investigated from in situ ATR-SEIRAS measurement by following the CO vibrational frequency. After irradiation, the CO frequency shifted higher, indicating that the Fermi level of Pt particles was positively shifted by the photogenerated holes. Thereafter, a lower frequency peak appeared, indicating that the Fermi level of some Pt particles was negatively shifted to the hydrogen evolution potential by photogenerated electrons, which is the essential function of cocatalysis for hydrogen evolution.

Surface-enhanced infrared ATR spectroscopy for in situ studies of electrode/electrolyte interfaces

Abstract Infrared (IR) spectra of molecules adsorbed on a silver electrode surface have been investigated by using the Kretschmann attenuated-total-reflection (ATR) method, where a thin metal film evaporated on an ATR prism was used as the electrode. The sensitivity of this technique is ca. 50 times higher than that of reflection-absorption spectroscopy (RAS) technique due to a surface-enhanced IR absorption phenomenon associated with surface roughness of the evaporated metal film. The advantage of the ATR spectroscopy is discussed in comparison with RAS. The mechanism of the surface-enhanced IR absorption phenomenon is also discussed theoretically.

Silver island films for surface-enhanced infrared absorption spectroscopy: effect of island morphology on the absorption enhancement

Surface-enhanced infrared absorption (SEIRA) spectra of p-nitrobenzoic acid adsorbed on vacuum evaporated silver island films were investigated and the experimental conditions under which the maximum absorption enhancement is observed were explored. The absorption intensity depends on the substrate on which the metal film is evaporated. Low-refractive substrates are optically favorable as the substrate for the SEIRA experiments. The absorption intensity depends also greatly on metal deposition conditions, thickness and evaporation rate of the metal, and temperature and surface properties of the substrate. This is explained by the difference in morphology of the metal islands. By closely observing the metal films by using a scanning electron microscope, it was found that a large enhancement could be obtained when the metal islands are aggregated very densely but are not connected to each other. The largest enhancement factor (peak intensity ratio with and without a metal island film) of about 1000 was observed with a 20 nm thick silver film evaporated on BaF2 at room temperature and at an evaporation rate of 0.01 nm s−1.

Adsorbed formate: the key intermediate in the oxidation of formic acid on platinum electrodes.

The electrooxidation of formic acid on Pt and other noble metal electrodes proceeds through a dual-path mechanism, composed of a direct path and an indirect path through adsorbed carbon monoxide, a poisoning intermediate. Adsorbed formate had been identified as the reactive intermediate in the direct path. Here we show that actually it is also the intermediate in the indirect path and is, hence, the key reaction intermediate, common to both the direct and indirect paths. Furthermore, it is confirmed that the dehydration of formic acid on Pt electrodes requires adjacent empty sites, and it is demonstrated that the reaction follows an apparently paradoxical electrochemical mechanism, in which an oxidation is immediately followed by a reduction.