Upon cathodizing a conducting substrate in O₂ saturated aqueous mixed solution of ZnCl₂ and eosinY,ZnO/eosinY nano-hybrid films with high crystallinity, high transparency and grain boundary-free nano-porous structure are deposited. By soaking the films in soft alkaline solution, eosinY is completely removed without any change in film thickness. By re-loading various dyes onto the surface of these films, highly transparent ZnO films with various colors are easily obtained. The ZnO films thus loaded with various dyes are effectively applied to flexible and colorful dye-sensitized solar cells. Owing to superior properties of the films obtained under optimized conditions, the incident photon to current conversion efficiencies of the solar cells re-loaded with some kinds of organic dyes amount to 90%. Solar-to-electrical energy conversion efficiency of the DSSC employing D149 dye (Mitsubishi Paper Mills Limited) amount to 5.6% under illumination of AM1.5 simulated sun light (100 mW cm⁻²). Efficient flexible colorful solar cells by using ITO-coated PET film substrates have been fabricated. Novel application of our plastic solar cells such as wearable solar cells and the solar powered functional traffic signs has been proposed.

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A titanium(IV) oxide (TiO₂) sample having a large surface area and showing a large visible light absorption was prepared by modification of commercial TiO₂ with a silane coupling reagent and subsequent nitrogen doping with ammonia at 500°C. The thus-prepared visible light-responding TiO₂ sample was used photoinduced removal of gaseous nitrogen oxides (NO_x) under irradiation from blue light-emitting diodes or white fluorescent lamp and exhibited higher level of NO_x removal than nitrogen-doped unmodified TiO₂. A remarkable improvement in NO_x removal was achieved by removing silica species with sodium hydroxide solution presumably due to the large surface area for effective adsorption of NO_x and storage of nitric acid, and sufficient efficiency for both visible light and UV light.

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We investigate the influence of illumination on morphology of less-noble metal electrodeposits on p-type Si. Compared to the case of Pt or Cu systems, the particle size and density did not change so much with light intensity in Ni and Co systems, while the cathodic current drastically changed with light intensity. The hexagonal shape of Co particles, which were formed under illumination of low intensity, indicated that the growth of particles was relatively preferred under the low carrier flux condition. The current transient follows progressive nucleation according to the Scharifker-Hills model, as can be seen in Pt or Cu.

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Photocatalytic activity of Pt-loaded metal sulfide with organic dye for the H₂ production from aqueous solution containing Na₂S–K₂SO₃ as a sacrificial reagent was investigated. Hydrogen formation rate was improved by the dye-adsorption onto SnS photocatalyst by using Cr-tetraphenylporphyrin (Cr-TPP). SnS loaded with Pt exhibits H₂ formation up to 800 nm when Na₂S and K₂SO₃ is used as sacrificial reagent, and it was found that the H₂ formation rate is much improved by the adsorption of Cr-TPP. In particular, the dye-adsorption effect was significantly appeared in the wavelength range less than 500 nm, which is in good agreement with the Soret band absorption of Cr-TPP.

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WO₃ is a visible-light-responsive photocatalyst, but the potential of its conduction band is not negative enough to produce H₂ from water or to reduce O₂ in the photodegradation reaction of various waste organic materials, environmental pollutants, and biomass. The activity of WO₃ for the photodegradation of various organic substances into CO₂ was markedly enhanced by the addition of Fe³⁺, mainly due to the electron-accepting effect of the reduction of Fe³⁺ to Fe²⁺. WO₃ in the presence of Fe³⁺ was more active than TiO₂ even under ultraviolet-visible light for the degradation of lower-molecular-weight organic substances. The Fe²⁺ generated in the photocatalytic reaction could be oxidized to Fe³⁺ by means of a hybrid system of electrolysis and photocatalysis accompanied by the production of H₂ from water at a low external bias (<1 V).

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Tin oxide, SnO₂, transparent conducting oxide (TCO) films for dye-sensitized solar cells (DSSCs) was investigated. We found that an electrical conductivity is not the only parameter that governs conversion efficiency of DSSC. The conversion efficiency was strongly influenced by haze ratio that is easily tuned by controlling the surface morphology of TCO layer by spray pyrolysis deposition (SPD) technique. The conversion efficiency was enhanced as high as 7.6%, attributing to an effective incident light harvest of a working electrode in the consequence of the confinement of the incident light within the cell.

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By monitoring the time profiles of ¹O₂ at the 1270-nm band, the reaction of singlet molecular oxygen (¹O₂) with four kinds of organic molecules, methionine, pyrrole, collagen, and folic acid was examined in photocatalytic systems of TiO₂ aqueous suspension. A fast decay observed in 1.5 µs after the pulse excitation attributable to the fast reaction of formed ¹O₂ with these molecules. From the analysis of the decay profiles, it is concluded that the organic molecules should be adsorbed on the TiO₂ surface to react with ¹O₂.

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Chlorin e6, a metal-free hydrophilic derivative of chlorophyll, was used as a sensitizer of mesoporous TiO₂ film to construct a high-efficiency solar cell as a model for artificial photosynthesis. The cell exhibited wide spectral responsivity in the visible light region that resembles action spectrum of photosynthesis. Optimization of dye adsorption method by using co-adsorbing surfactant agents that suppress intermolecular aggregation of the dye achieved high energy conversion efficiency up to 4.3% under simulated sunlight of 100 mW cm⁻² intensity.

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Pd-particle-modified n-Si can be etched at a high rate in a hydrofluoric acid solution without a particular oxidizing agent under dissolved oxygen free and dark conditions. In this study, Pd thin film patterned n-Si is used. The etching is localized at the boundary between the 29-nm-thick-Pd film and the non-Pd-deposited area of the n-Si surface at the initial stage. Then Pd particles form on the non-Pd-deposited area toward which the etched area extends. Thin (4.3 nm) Pd films localize the etching under the films.

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The role of molecular oxygen (O₂) in the photocatalytic decomposition of a model compound, acetic acid, was studied using suspensions and electrodes of metal oxide particles, titanium(IV) oxide (TiO₂) or tungsten(VI) oxide (WO₃). Results for photocurrent efficiency in photoelectrochemical measurements revealed that reaction of O₂ with radicals liberated by positive holes increases the overall rate of photocatalytic oxidative decomposition of acetic acid.

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The electrochromic cell, which consisted of transparent conductive substrate, electrolyte and nano-crystalline titania thin film modified with viologen was assembled. The solvent dependency on kinetic of these cells was mainly investigated. When polyethylene glycol (PEG) was used as a solvent, the cell clearly indicated fast electrochromic reaction under an applied potential of 3.0 V. The polyethylene glycol (molecular weight: 200) electrolyte was showed the best performance for keeping a strong coloration state for more than one month without any applied potential, thus making it an attractive candidate for paper-like displays with low energy consumptions.

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The effect of photoetching (photoelectrochemical etching) on photoinduced hydrophilic conversion of titanium dioxide (TiO₂) film electrodes prepared by thermal oxidation of Ti plate has been studied. We have found that photoetching of TiO₂ film results in the formation of nano-porous structure depending upon preparation condition (oxidation temperature), photoetching condition (electrode potential) and crystallographic orientation of the TiO₂ grain. Photoetching of the film oxidized at temperature higher than 900°C makes nano-porous structure consisting of (100) plane of rutile structure of TiO₂ and improves the photoinduced hydrophilic conversion property. Photoetching under anodic polarization increases the rate of photoinduced hydrophilic conversion, while the film photoetched under weak anodic polarization does not show the photoinduced hydrophilic conversion property.

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Photoelectric performance of plastic dye-sensitized solar cell (PDSC) using indoline dye D149 as an organic sensitizer was improved by introduction of a thin titania buffer layer on ITO-PEN (indium tin oxide coated polyethylenenaphthalate) film. With prominent increase in photocurrent density, open circuit voltage, and fill factor, the energy conversion efficiency of PDSC using D149 achieved 3.7%.

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Plasmon resonance-based solid-state photovoltaic cells were fabricated with the TiO₂ electron transport layer, gold nanoparticles as the visible light sensitizer and an organic or inorganic hole transport layer. The devices showed prompt, reproducible photocurrent and photovoltage responses during periodical visible light illumination. The action spectra for the photocurrent and photovoltage closely matched the absorption spectrum of the gold nanoparticles, indicating that the observed photoresponses arose from light absorption of the gold nanoparticles. The cell performances depended on the hole mobility of the hole transport layer. The largest photocurrent and photovoltage were recorded when CuI was used as the hole transport material.

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The absorption spectra of five β-diketonato ruthenium tricarboxyterpyridyl dyes in ethanol have been calculated by the Time Dependent Density Functional Theory, and studied by the Transition-Component Analysis (TCA). The computed absorption spectra well reproduced the experimental ones based on the shape for the entire visible light range. The TCA quantitatively explained the contents of the absorption spectrum in detail, and, by comparing to the results for Black Dye, found that the β-diketonato ligands, by enhancing the electron donor ability, could contribute to the absorption spectra in the longer wavelength region, and made the β-diketonato ruthenium tricarboxyterpyridyl dyes work well as photo-sensitizers in dye-sensitized solar cells.

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Photoelectrochemical etching was applied to (0001), (000-1), and (10-10) surfaces of ZnO single crystals to produce unique surface structures, which depend on the crystallographic face. These surfaces turned hydrophilic with a water contact angle of 5–10° under ultraviolet light irradiation, but the hydrophilicity gradually became small in the dark. Among them, we successfully generated a hydrophobic surface (contact angle >120°) stored in a dark under ambient conditions on the etched (000-1) face with hexagonal holes that had a diameter of approximately 100 nm along the c-axis. In addition, storing at 75°C in a dry oven greatly shortened the period of the hydrophobic conversion.

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