Electron transport mechanism in nano-structured TiO₂ electrodes are disclosed and discussed in view of improvement of dye-sensitized solar cells (DSC). Effective electron transport in the TiO₂ electrodes is well explained by ambipolar diffusion of electrons between trap-sites on TiO₂. The specific interaction of cationic species with TiO₂ interfaces plays an important role in charge screening of photo-injected electrons. The effects of shallow trapping sites introduced by fluorine-ion doping are discussed.

Unique titanium oxide photocatalysts incorporated within the cavities and frameworks of zeolites and mesoporous molecular sieves were prepared and their reactivity for the photocatalytic reduction of CO₂ with H₂O were investigated. UV-irradiation of these titanium oxide photocatalysts in the presence of CO₂ and H₂O at 323 K led to the formation of CH₄ and CH₃OH along with a minor formation of CO and O₂. The isolated tetrahedrally coordinated titanium oxide highly dispersed within the zeolite frameworks exhibited a characteristic reactivity for the selective formation of CH₃OH. The photocatalytic reactivity for the formation of CH₃OH was found to be closely associated with the concentration of charge transfer excited triplet state, (Ti³⁺—O⁻)*, of the highly dispersed tetrahedrally coordinated titanium oxide species and the selectivity for the photocatalytic formation of CH₃OH was controlled by the hydrophobic and hydrophilic properties of these photocatalysts.

Photoelectrochemical behavior of triarylmethylium salts ([Φ₃C]ClO₄ and [Φ′Φ^b₂C] ClO₄ [Φ and Φ′ represent 2,4,6-trimethoxyphenyl, 2,6-dimethoxyphenyl, 2-methoxyphenyl, 4-methoxyphenyl, and phenyl groups]), expected as a novel photosensitizer, was studied at an ITO electrode in 1,2-dichloroethane under potentiostatic conditions. The slow increase in a cathodic current was observed upon illumination. The observed photocurrent depended on the applied potential, and the half-wave potential was close to the redox potential based on methylium-methyl radical redox couples. The photocurrent also depended on the concentration of the salts, and the illuminance of the incident light.

The photo-assisted catalytic degradations of trichloroethylene (TCE) and tetrachloroethylene (PCE) were studied in a tubular photoreactor packed with TiO₂ pellets prepared by sol-gel method. The experiments were performed at 32°C in a non-circulating mode. Comparison with the ethylene degradation reveals that TCE and PCE were degraded more efficiently than ethylene. However, only 48% of TCE or 57% of PCE was converted to HCl and CO₂ whereas 95% of ethylene was mineralized. Phosgene, CHCl₃ and dichloroacetic acid were detected from the photodegradation of TCE while that of PCE yielded phosgene, CCl₄ and trichlroacetic acid. Phosgene in the effluent gas stream was completely removed by passing through water. The reaction mechanism to form these chlorinated products is discussed.

A photochemical new oxygen sensor using continuous wave Nd-YAG laser (CW laser) as an excitation light has been developed, which is based on the stationary state quenching of triplet-triplet (T-T) absorption of zinc 5,10,15,20-tetrakis-(pentafluorophenyl)-porphyrin (ZnTFPP) by oxygen. The intensity of ZnTFPP T-T absorption decreases with oxygen concentration, indicating the applicability for oxygen sensor.

Titania nano tube was successfully obtained by hydrothermal treatment of nano size TiO₂ fine powder in 10 M NaOH solution for 20 h at 110°C. The morphology of product was 8-10 nm in width and about 100 nm in length with tubular shape. The specific surface area was 270 m² g⁻¹, much higher than that of starting material titania powder with 50 m² g⁻¹. Photovoltaic properties of these titania film was also examined. The results were V_OC = 0.704, I_SC = 1.26 mA, η = 2.9%, FF = 0.66 for titania nano tube, and V_OC = 0.767, I_SC = 1.20 mA, η = 3.0%, FF = 0.72 for titania nano powder (P-25).

The gas-phase photocatalytic decomposition of individual- and two-component mixtures of chloroethylenes using near-UV-illuminated TiO₂ was studied. The apparent quantum efficiencies for the degradation of cis-1,2-dichloro- (0.42), trichloro- (9.6), and tetrachloroethylene (10.6) were found to increase with the number of chlorine substituents. cis-1,2-Dichloro- and trichloroethylene in the presence of tetrachloroethylene decomposed with higher quantum efficiency, specifically, 1.6 and 11.7, respectively. The decomposition of cis-1,2-dichloroethylene in the mixtures was remarkably promoted, while the apparent quantum efficiency of tetrachloroethylene to function as an accelerator showed a decrease for the decomposition with cis-1,2-dichloroethylene (3.6) or with trichloroethylene (9.1). These findings were explained in terms of the presence of chlorine atoms in photochemical and photocatalytic reactions.

The effect of Cl radical on direct photodegradation rates of phosgene was investigated using the circulation-type apparatus. Cl radical was hardly formed from molecular chlorine using a low-pressure mercury lamp, which had been employed for direct photodegradation of phosgene, because the spectrum of low-pressure mercury lamp is hardly absorbed by molecular chlorine. It was observed that Cl radical was formed from molecular chlorine by the irradiation from black-lights. Under presence of molecular chlorine and the irradiation from black-lights, significant increase in the rate constant of photodegradation of phosgene using low-pressure mercury lamp was observed. In addition small effect of increase of residence time in the reactor was observed.

TiO₂ thin films on borosilicate glass and quartz glass substrates were prepared in order to investigate the influence of the substrate on photocatalytic activity. The preparation of TiO₂ thin film was performed by sol-gel method. The influence of the substrate on TiO₂ crystallization was examined at different TiO₂ thin film sintering temperatures. The TiO₂ thin film was irradiated with black-light fluorescent lamps. Photocatalytic activity was compared using trichloroethylene (TCE). The photocatalytic activity of the TiO₂ thin film was confirmed to vary with the substrate. The crystal structures of the TiO₂ thin films prepared on the glass substrates were examined using X-ray diffraction analysis, and the UV absorption of the thin film was measured by means of a spectrophotometer.

A promising and cheaper alternative to the silicon solar cell, is the dye-sensitized photoelectrochemical cell and a solid-state dye-sensitized cells using p-CuI as the hole-conductor has advantages to conventional wet-type cells for practical use. However, this n-type semiconductor/Dye/p-type semiconductor device has problem of the long-term photostability. In this paper, the results of addition of several l-ethyl-3-methylimidazolium (Im) salts and the effect of their counter anions on the performance of the cell are described. We improved the performance and stability of the solid-state dye-sensitized solar cell using CuI with ImSCN whereas other Im salts showed only slight effect except for ImI.

This paper deals with solid-solid photocatalysis; i.e. photocatalytic reaction for a solid substrate using Pt/TiO₂ powdered photocatalyst (solid) under illumination. Malonic acid reacts to CO₂ and a small amount of other minor products in an aqueous suspension of powdered photocatalyst under Ar atmosphere. In the case of the solid-solid system, CH₄ was also produced from malonic acid as a major product together with CO₂. The change of products distribution was observed and the changing behavior of products distribution with an addition of water was examined. Our result showed that the allowed limit amount of water to change the selectivity could be determined. Scheme of product switching was also discussed.

Photoelectrochemical quartz crystal microbalance measurements were used for in situ monitoring of the photoelectrochemical deposition of PbO₂ onto a TiO₂ nanoparticulate film electrode. It was found that the photodeposition behavior of PbO₂ was influenced by pH of solution. At pH 3.8, the mass of the TiO₂ film electrode decreased in the initial stage of irradiation due to the desorption of Pb²⁺ from the TiO₂ surface and then linearly increased due to PbO₂ deposition with an increase in the charge flowing as a photocurrent, while the initial decrease of the mass was much smaller at pH 1.8.

The reaction between titanium(IV) oxide (TiO₂) powders and 1,1′-binaphthalene-2,2′-diol (bn(OH)₂), both of which are originally colorless, led to the formation of surface TiO₂-bn(OH)₂ complexes giving a broad absorption in the visible-light region below ca. 550 nm. When the surface complex was made on TiO₂ loaded with platinum deposits, the powder exhibited photocatalytic activity for H₂ evolution from deaerated triethanolamine solution under visible-light irradiation (>430 nm). On the basis of the experimental results, the reaction mechanism seems to involve visible-light excitation of the surface complexes, electron injection to TiO₂, and its migration to platinum deposits, where reduction of H⁺ takes place.

A liquid polyiodide, composed of 1-hexyl-3-methylimidazolium iodide and iodine without any solvent, was used as a redox mediator in cis-Ru(4,4′-dicalboxyl-2,2′-bipyridine)₂(thiocyanato)₂-sensitized porous TiO₂ solar cells with varying iodine mole fractions. For iodine mole fractions below 0.5, short-circuit anodic photocurrents were observed. However, for iodine mole fractions above 0.5, short-circuit cathodic photocurrents were observed. A mechanism of the photocurrents was discussed, and it is shown that a high concentration of I₃⁻ promotes the back reaction of electrons with I₃⁻ on transparent conducting oxide substrates under illumination, as well as acting as a filter of incident light into solar cells.

Over two times higher short-circuit photocurrent density was attained in the dye-sensitized solar cells by using titania nanotubes as a semiconductor thin film in the thin film thickness region in comparison with that made of P-25. Titania nanotubes were synthesized by a surfactant-assisted templating mechanism using a laurylamine hydrochloride/tetraisopropylorthotitanate with acetylacetone system.¹⁾ Nanotubes have a mono-crystalline structure of anatase and showed the highest photocatalytic activity among the commercially available nano-crystalline titania. The dye-sensitized solar cell system using the mono-crystalline titania nanotubes showed about 5% sunlight-electricity conversion.

Time- and frequency-resolved photoelectrochemical studies have been performed on nano-honeycomb porous TiO₂ electrode prepared by photoelectrochemical etching of single crystal TiO₂, both for its bare surface and when sensitized by cis-di(thiocyanato)-N,N-bis(2,2’-bipyridyl-4,4’-dicalboxylate)ruthenium(II). Since the remaining wall in the porous layer become as thin as the double thickness of the space charge layer by photoetching, electron diffusion comes to play the main role in the charge transport rather than field-driven migration. Intensity modulated photocurrent spectroscopy revealed much faster electron diffusion in the honeycomb electrode than that in colloid based porous electrode, because of the smaller electron trap density in the nano-honeycomb structure owing to the nonexistence of grain boundaries.

The surface structure of TiO₂ particles was changed by illumination in a sulfuric acid solution. Small dimples with a size less than a few tens of nm were formed when the surface of the particles was modified by platinum. Particles without the platinum modification did not show any change in their surface structure. A structural change was also observed when the particles were positively polarized in a sulfuric acid solution under illumination on an optically transparent glass electrode. The results suggest that the density of the photogenerated holes in the particles play an important role in inducing the photoetching reaction of TiO₂ particles.

SrTiO₃ photocatalyst was combined with an energy storage material, WO₃, to fabricate a new photocatalysis system, envisaging application to, e.g., protection of metals from corrosion. The SrTiO₃-WO₃ composite system stores reductive energy under UV irradiation, as does the TiO₂-WO₃ system. Since SrTiO₃ exhibits more negative photopotential (ca. −0.5 V vs Ag | AgCl) than TiO₂ (ca. −0.4 V), the SrTiO₃-WO₃ system can be charged more rapidly. In addition, self-discharging of the SrTiO₃-WO₃ system on the basis of aerobic re-oxidation of the system is slower than that of the TiO₂-WO₃ system.

A range of transition-metal oxynitrides and a nitride were examined as potential photocatalysts active under visible light. (Oxy)nitrides based on tantalum, niobium and titanium are found to absorb visible light (λ ≤ 600 nm), and some of them are found to evolve H₂ or O₂ from aqueous solutions containing a sacrificial electron donor (CH₃OH) or acceptor (Ag⁺), respectively, via band gap excitation without noticeable photo-degradation of the catalyst itself.

Preparation of titania/water paste for the electrode of dye-sensitized solar cell was examined. The dispersion mechanism of titania powder was related to both pH and viscosity of the paste in view of rheology. It can be concluded that lower pH than that of pzc (point zero charge) is necessary to keep on the highly dispersed state of titania powder in the paste. In the meanwhile, such a well-dispersed paste has very low viscosity, which are unsuitable for making a thick titania film. Therefore various kinds of additives as a thickener have also been studied.

A comprehensive study has been conducted on the cathodic electrodeposition of ZnO/eosinY hybrid thin films from aqueous mixed solutions containing Zn(NO₃)₂ and eosinY, as well as their photoelectrochemical characteristics. Adsorption of eosinY molecules on the growing surface of ZnO leads to the automatic formation of a nano-porous hybrid structure consisting of interconnected nano-sized ZnO crystals. The dye molecules are loaded into the film at a very high amount as orderly packed layer, being chemically attached to the internal surface of ZnO through their carboxylic acid group, and a major part of them could be desorbed by alkaline treatment without dissolving ZnO. Under the conditions of film preparation, electrochemical reduction of an eosinY molecule coupled with stable complex formation with a Zn²⁺ ion has been observed and is supposed to play a key role in the process of “self-assembly” of the three dimensional ZnO/eosinY hybrid structures. The electrodeposited ZnO/eosinY hybrid thin films exhibit a high performance as sensitized photoelectrodes. Intensity modulated photocurrent and photovoltage spectroscopies have shown a fast diffusion and a long lifetime of electrons injected from photoexcited eosinY into the conduction band of ZnO, indicating an almost barrier-free transport in the porous network of ZnO grown from solution. A high incident photon to current conversion efficiency of 47% was obtained in the absorption maximum of the loaded dye following a mild heat treatment of the film at 150°C, making the present material applicable to conductive plastic substrates for realization of flexible dye-sensitized solar cells.

The band gap energy, carrier concentration and resistivity of Hg_1−xCd_xSe vary with varying Hg: Cd ratio. We examined the characteristics of photoelectrochemical solar cell (PECSC) with the electrodeposited Hg_1−xCd_xSe film as a photoanode. Selection of deposition conditions, heat-treatment schedule, and electrolyte solution has resulted in power conversion efficiency of 5.86%, open circuit voltage of 0.661 V, short circuit current of 18.95 mA/cm² and fill factor of 0.42 under irradiation of air mass (AM) 2.0 (90 mW/cm²) solar simulator.