The effective utilization of solar beams is one of the most important challenges in chemistry today not only for the conversion and storage of solar energy but also for the reduction and elimination of environmental pollution such as toxic air and water. Strong contenders as environmentally-harmonious catalysts are photocatalysts which are able to work effectively under visible and/or solar light irradiation. In this article, an advanced metal ion-implantation method has been successfully applied to improve the photocatalytic properties of semiconducting titanium oxide photocatalysts. The catalysts enables the absorption of visible light even longer than 450 nm so that they are able to operate effectively as photocatalysts under visible or solar light irradiation. The development of such environmentally-friendly titanium oxide photocatalysts can be considered a breakthrough in the efficient and large scale utilization of solar energy to address urgent environmental and energy needs for the 21st century.
Recently, we have found that Cu2O powder suspended in distilled water by stirring decompose water catalytically into H2 and O2 under visible light irradiation. To our surprise, the experiment revealed that the catalytic decomposion of water by Cu2O is due to the mechanical energy produced in to stirring and the reaction under stirring was enhanced by the irradiation of visible light having photon energies more than the band gap energy of Cu2O(2 eV). The reaction is regarded as “mechano-catalytic overall splitting”; and is distinct from conventional photocatalytic water decomposition.
As generally recognized silica is an inert material and therefore chemically stable. Recently, however, there has been increased number of the reports that the chemical activity of silica generated when silica is treated by evacuation at a high temperature. With this regard, the present report is an overview of our work on photocatalysis by silica. The evacu-ation at a high temperature brings about a generation of an active site; possibly an abnormally coordinated silicon atom that is a photoluminescence center as well as a paramagnetic radical center. When the site is in contact with oxygen gas, an O2-anion radical forms by UV light irradiation, which can oxidize alkene molecules and carbon monoxide. In the absence of oxygen, by introduction of alkene molecules to the system involving UV-irradiated silica, alkene metathesis takes place. The metathesis reaction proceeds via. a metalacyclobutane intermediate although the presence of carbene species has not been confirmed yet.
The photo-corrosion of the TiO2 oxide film (9.2 nm thick) formed anodically on titanium was studied in 0.1 M sulfuric acid solution by in-situ ellipsometry during ultra-violet (uv) light irradiation of 325 nm wavelength. The uv light irradiation induces some changes in the oxide film as observed by ellipsometry. From the change of the ellipsometric parameters, reduction of the oxide film thickness was estimated. It was estimated that 5 % of photo-induced current is consumed for the photo-corrosion of the oxide film. The photo-corrosion rate is proportional to the anodic bias relative to the flat-band potential and to the uv light intensity. The mechanism of the photo-corrosion was discussed in terms of a photo-effect of n-type semiconductive oxide electrode; the photocorrosion is induced by an accumulation of photo-excited holes at the oxide surface, which may increase interfacial potential difference at the oxide surface and weaken the Ti-O bond of the oxide.
There exist two types of photo-effects on TiO2 surface. One is the well-known photocatalytic reaction proceeding on the TiO2 surface irradiated by UV light. Another is the photo-induced super-hydrophilic and simultaneously superoleophilic conversion of the TiO2 surface. In other words, both water and oil drops spread completely on the UV irradiated surface. This effect was found recently, and is attributed to the nanometer scale hydrophilic domain structure formed on an original oleophilic TiO2 surface by the UV irradiation. The results of recent detailed experiments suggest that the hydrophilic domains are formed by the surface reduction by the photo-produced electrons. The produced holes will be used by the oxidation of bridging oxygen ions of TiO2 surface, releasing molecular oxygen followed by the adsorption of molecular water. The detailed mechanism is explained in this review article. Moreover it is described that various materials coated with this type of TiO2 transparent film can show anti-fogging and self-cleaning effects. Their industrial applications are also described.
Photoirradiation onto semiconducting materials with light of energy greater than their band gap induces creation of pairs of photoexcited electron and positive hole. They react with the species adsorbed on the surface to give redox reactions, otherwise undergo mutual recombination without substantial chemical reactions. Such redox reactions, i.e., heterogeneous electron (or positive hole) transfer triggered by light, at solid/liquid or solid/gas interfaces are called “photoelectrochemical”; or “;photocatalytic”; reactions, and of great importance from both fundamental and applied standpoints. However, mechanism and/or dynamics of such reactions has been poorly understood, because of difficulties of spectroscopic measurement due to heterogeneity of the reaction systems including interfaces, as well as to the rate of electron transfer much faster than the detection limit of conventional time-resolving techniques. Recent progress in laser technology enabled us to measure directly the dynamics of heterogeneous electron transfer with the time resolution even faster than hundred femtosecond. In this article, we show principles of femtosecond pump-probe reflectance absorption spectroscopy and review examples of studies using this promising technique.
The flat-band potential (UFB) of HF-etched single-crystal n-Si(111) and (100) electrodes in concentrated hydrogen halide (HX) solutions shifts toward the negative in the order of HF, HCI, HBr, and HI, i.e., in the order of decreasing electronegativity of halogen atoms. The results can be explained by taking account of formation of Si-X (X=halogen) termination bonds through substitution reaction at Si-OH bonds on the HF-etched Si surfaces. The formation of Si-X bonds leads to changes in surface bond dipole and hence in surface potential. This finding makes it possible to obtain an n-Si chip having different surface band energies at the front and rear surfaces. Such an n-Si chip with surface-band asymmetry can be used to achieve efficient solar-to-chemical energy conversion. It is shown that a high conversion efficiency of 3.4 % was obtained by use of only one n-Si chip through decomposition of HI into H2 and I2 (or I3-) by simulated solar AM 1.5 G (100 mW/cm2) illumination.
Self-assembled monolayers (SAMs) of alkanethiols on gold have been one of the most well studied systems. The SAM with a photoactive group and an electron relay group formed on a metal electrode in a solution containing an electron acceptor and/or donor should be a good candidate for a stable, efficient photoconversion device where up-hill photoinduced electron transfer, i.e., photocurrent generation, takes place. We have achieved a control of a direction of photoinduced electron transfer and a very efficient photocurrent generation at the gold electrodes modified with the SAMs of porphyrin-quinone-thiol and porphyrin-ferrocene-thiol linked molecules.
Using X-ray diffraction and Fourier-transform infrared spectroscopy, we carried out a structure study of vacuum evaporated films and self-assembled monolayers (SAM) of terthiophene. It is shown with vacuum evaporated films that the long axis of terthiophene aligns normal to the surface. On the other hand, terthiophene in the SAM orients parallel to the gold substrate. Our results, which contradict the recent quantum chemical calculations predicting that thiophene would not interact with gold, experimentally demonstrated direct interaction between thiophene and gold.
Recent studies on the structures and reactivity of bimetallic surfaces by use of the well-defined single-crystals are reviewed. Attempts are made to extract basic rules which govern the complex systems.