Yoshio Nosaka

ESR investigation into the effects of heat treatment and crystal structure on radicals produced over irradiated TiO2 powder

Abstract Electron spin resonance measurements were carried out at 77 K under irradiation for anatase TiO 2 powders treated by heating at various temperatures in the air. For the untreated powder photoproduced holes were trapped at the surface forming Ti 4+ O − Ti 4+ OH − radicals, while, for the heated powder, they were trapped as Ti 4+ O 2− Ti 4+ O − radicals at the surface. Photoproduced electrons were trapped as Ti 3+ at the desorption of surface hydroxyl groups and the change in the surface accompanied by the crystalline growth. No photoproduced radicals were detected for rutile TiO 2 powder, which may explain the low photocatalytic activity of this crystalline structure.

Nitrogen-doped titanium dioxide photocatalysts for visible response prepared by using organic compounds

Abstract In order to utilize visible light in photocatalytic reactions, nitrogen atoms were doped in commercially available photocatalytic TiO2 powders by using an organic compound such as urea and guanidine. Analysis by X-ray photoelectron spectroscopy (XPS) indicated that N atoms were incorporated into two different sites of the bulk phase of TiO2. A significant shift of the absorption edge to a lower energy and a higher absorption in the visible light region were observed. These N-doped TiO2 powders exhibited photocatalytic activity for the decomposition of 2-propanol in aqueous solution under visible light irradiation. The photocatalytic activity increased with the decrease of doped N atoms in O site, while decreased with decrease of the other sites. Degradation of photocatalytic activity based on the release of nitrogen atoms was observed for the reaction in the aqueous suspension system.

Photocatalytic ˙OH radical formation in TiO2 aqueous suspension studied by several detection methods

Photoinduced reaction with TiO2 semiconductor photocatalysts was investigated by using nitroxide radicals as spin probes for ˙OH radicals. The effects of some additives such as I−, Cl−, ClO4−, methanol and 2-propanol on the photocatalytic decay of nitroxide radicals, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidine-1-oxyl and 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, were investigated. Among the additives, only iodide ions that can be oxidized by trapped holes prohibited significantly the decay of the nitroxide radicals, indicating that trapped holes oxidize nitroxide probe radicals. For several TiO2 photocatalysts, the apparent quantum efficiency of ˙OH radical formation was calculated and compared with those obtained by the other detection methods, such as DMPO spin trapping and terephthalic acid fluorescence methods. The experimental observations suggest that the photocatalytic oxidation should be caused more preferably by trapped holes or adsorbed ˙OH radicals, rather than the photoinduced valence band holes in semiconductor and the free ˙OH radicals in solution.

Generation and Detection of Reactive Oxygen Species in Photocatalysis

The detection methods and generation mechanisms of the intrinsic reactive oxygen species (ROS), i.e., superoxide anion radical (•O2-), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (•OH) in photocatalysis, were surveyed comprehensively. Consequently, the major photocatalyst used in heterogeneous photocatalytic systems was found to be TiO2. However, besides TiO2 some representative photocatalysts were also involved in the discussion. Among the various issues we focused on the detection methods and generation reactions of ROS in the aqueous suspensions of photocatalysts. On the careful account of the experimental results presented so far, we proposed the following apprehension: adsorbed •OH could be regarded as trapped holes, which are involved in a rapid adsorption-desorption equilibrium at the TiO2-solution interface. Because the equilibrium shifts to the adsorption side, trapped holes must be actually the dominant oxidation species whereas •OH in solution would exert the reactivity mainly for nonadsorbed reactants. The most probable routes of generating intrinsic ROS at the surfaces of two polymorphs of TiO2, anatase and rutile, were discussed along with some plausible rational reaction processes. In addition to the four major ROS, three ROS, that is organic peroxides, ozone, and nitric oxide, which are less common in photocatalysis are also briefly reviewed.

Singlet oxygen formation in photocatalytic TiO2 aqueous suspension

The formation of reactive singlet oxygen molecule (1O2), the a1Δg state of excited O2, in the photocatalytic TiO2 aqueous suspension system was shown to be evident by detecting the near-infrared phosphorescence at 1270 nm by means of a gated photon counting method. The lifetimes of O2(1Δg) in three different media (water, ethanol, 1∶1 water–ethanol mixture) were elucidated and compared with those reported in corresponding homogeneous media. Based on the comparison of the lifetime with those of the other reactive species such as OH radicals and trapped holes, the contribution of 1O2 to the actual photocatalytic procedure was discussed.

Enhanced Photoactivity with Nanocluster-Grafted Titanium Dioxide Photocatalysts

Titanium dioxide (TiO2), as an excellent photocatalyst, has been intensively investigated and widely used in environmental purification. However, the wide band gap of TiO2 and rapid recombination of photogenerated charge carriers significantly limit its overall photocatalytic efficiency. Here, efficient visible-light-active photocatalysts were developed on the basis of TiO2 modified with two ubiquitous nanoclusters. In this photocatalytic system, amorphous Ti(IV) oxide nanoclusters were demonstrated to act as hole-trapping centers on the surface of TiO2 to efficiently oxidize organic contaminants, while amorphous Fe(III) or Cu(II) oxide nanoclusters mediate the reduction of oxygen molecules. Ti(IV) and Fe(III) nanoclusters-modified TiO2 exhibited the highest quantum efficiency (QE = 92.2%) and reaction rate (0.69 μmol/h) for 2-propanol decomposition among previously reported photocatalysts, even under visible-light irradiation (420-530 nm). The desirable properties of efficient photocatalytic performance with high stability under visible light with safe and ubiquitous elements composition enable these catalysts feasible for large-scale practical applications.

The function of metals in metal-compounded semiconductor photocatalysts

Abstract The effect of various metals on the photocatalytic activity of metal-compounded semiconductors was examined for the production of ammonia from azide ions and water. A correlation between the photocatalytic activity and the work function of the metals was found.

Photocatalytic carbon dioxide reduction by copper oxide nanocluster-grafted niobate nanosheets.

Amorphous copper oxide (Cu(II)) nanoclusters function as efficient electrocatalysts for the reduction of carbon dioxide (CO2) to carbon monoxide (CO). In addition to promoting electrocatalytic activity, Cu(II) nanoclusters act as efficient cocatalyts for CO2 photoreduction when grafted onto the surface of a semiconductor (light harvester), such as niobate (Nb3O8(-)) nanosheets. Here, the photocatalytic activity and reaction pathway of Cu(II)-grafted Nb3O8(-) nanosheets was investigated using electron spin resonance (ESR) analysis and isotope-labeled molecules (H2(18)O and (13)CO2). The results of the labeling experiments demonstrated that under UV irradiation, electrons are extracted from water to produce oxygen ((18)O2) and then reduce CO2 to produce (13)CO. ESR analysis confirmed that excited holes in the valence band of Nb3O8(-) nanosheets react with water, and that excited electrons in the conduction band of Nb3O8(-) nanosheets are injected into the Cu(II) nanoclusters through the interface and are involved in the reduction of CO2 into CO. The Cu(II) nanocluster-grafted Nb3O8(-) nanosheets are composed of nontoxic and abundant elements and can be facilely synthesized by a wet chemical method. The nanocluster grafting technique described here can be applied for the surface activation of various semiconductor light harvesters, such as metal oxide and/or metal chalcogenides, and is expected to aid in the development of efficient CO2 photoreduction systems.

Behavior of superoxide radicals formed on TiO2 powder photocatalysts studied by a chemiluminescent probe method

A luminol chemiluminescence (CL) probe method was successfully applied to the investigation of the superoxide radical (O2−˙) formed on photoirradiated TiO2 powders. For several kinds of commercially available TiO2 photocatalysts and their calcined samples, the amount of O2−˙ produced at the steady state was measured and found to increase with the secondary particle size or the degree of aggregation. The decay of O2−˙ for the non-calcined samples was as long as several hundred seconds and obeyed second order kinetics, indicating that disproportionation is the main deactivation pathway. For some TiO2 photocatalysts, oxidative species such as OH˙ radicals are suggested to exist for 1 s after irradiation. On the other hand, reductive photoinduced electrons may remain for several seconds after irradiation since the decay of O2−˙ starts after a delay of more than 1 s. On calcination at temperatures up to 1173 K, O2−˙ decays by a mechanism other than disproportionation, suggesting the prolonged lifetime of an oxidative species that can react with O2−˙.

FTO/SnO2/BiVO4 Composite Photoelectrode for Water Oxidation under Visible Light Irradiation

An SnO 2 layer prepared from ethanol solution was applied to fabricate an MO/SnO 2 /BiV0 4 composite photoelectrode. The layered composite films were characterized by X-ray diffraction, scanning electron microscope, and X-ray photoelectron spectroscopy techniques. The photocurrent efficiency of BiVO 4 increased remarkably when an SnO 2 film was layered between the BiVO 4 layer and the fluorine-doped tin oxide (FTO) conducting electrode, indicating the efficient charge separation via the electron transfer from BiVO 4 to SnO 2 and the blocking of the hole transfer from BiVO 4 to the FTO surface. It was also demonstrated that the sequence for depositing the individual metal oxide layer on the FTO substrate was an important factor for such efficient charge separation of the electrode.