Atsuko Y. Nosaka

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.

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.

Reconsideration of Intrinsic Band Alignments within Anatase and Rutile TiO2

Rutile TiO2 T dioxide (TiO2) is one of the most popular photocatalysts. In fact, based on an analysis with SciFinder of the American Chemical Society, 45% of documents concerning photocatalysis in the last two years deal with TiO2. There are two major polymorphs for TiO2: anatase and rutile with the band gap energy of 3.2 and 3.0 eV, respectively. Thus, it has been believed that the potential of the conductionband bottom (ECB) of anatase is 0.2 eV higher (more negative) than that of rutile, as illustrated in Figure 1A, based on which

Detection of OH Radicals Generated in Polymer Electrolyte Membranes of Fuel Cells

The long-term stability of the membrane of polymer electrolyte fuel cells (PEFCs) is an important factor limiting the lifetime of the PEFCs. In the present study, OH radicals (•OH) formed during the PEFC operation of a single cell were detected by means of a fluorescence probe method with coumarin. The coumarin molecules impregnated in the polymer electrolyte membrane (Nafion 115) react with •OH to form a highly fluorescent product, umbelliferone. After PEFC operation under various conditions, the cross section of the membrane for five positions was monitored by a fluorescence microscope and the distribution of the •OH density in the membrane was evaluated. [1] The effects of metal ion impurities and gas flow rate on the •OH radical formation during PEFC operation were also examined. The conditions for PEFC operation which generate a higher amount of •OH were as follows: 1. The higher the cell temperature, the more •OH is formed because Nafion becomes soft to increase gas crossleak 2. The lower the humidity of the gas, the higher amount of •OH is formed, because H2O2 remains in the membrane due to higher boiling point of H2O2. Moreover, the reactivity of •OH increases by decreasing the hydration of •OH. 3. The lower the operation current, the average amount of •OH increased. The anode side contains a slightly larger amount of •OH than the cathode side 4. Under OCV condition, larger amount of •OH is formed near O2 inlet because it is produced probably by O2 crossleak 5. Under higher loading current condition, larger amount of •OH is formed near H2 inlet because it is produced as a byproduct of O2 reduction. 6. The •OH densities largely depended on the position of the membrane in the cell, while the distribution through the membrane thickness is not as large. 7. Excess supply of H2 and O2 gases increase the •OH radical formation. 8. In the presence of Fe ions, the amount of •OH increases significantly under OCV condition, while the increase is slight under the operation with 200mA/cm. 9. In the presence of Ce ions, formation of •OH is accelerated slightly. 10. When Air is used in place of pure O2 gas, the amount of •OH is increased.

Spectroscopic Investigation of the Mechanism of Photocatalysis

Reaction mechanisms of various kinds of photocatalysts have been reviewed based on the recent reports, in which various spectroscopic techniques including luminol chemiluminescence photometry, fluorescence probe method, electron spin resonance (ESR), and nuclear magnetic resonance (NMR) spectroscopy were applied. The reaction mechanisms elucidated for bare and modified TiO2 were described individually. The modified visible light responsive TiO2 photocatalysts, i.e., Fe(III)-deposited metal-doped TiO2 and platinum complex-deposited TiO2, were studied by detecting paramagnetic species with ESR, •O2− (or H2O2) with chemiluminescence photometry, and OH radicals with a fluorescence probe method. For bare TiO2, the difference in the oxidation mechanism for the different crystalline form was investigated by the fluorescence probe method, while the adsorption and decomposition behaviors of several amino acids and peptides were investigated by 1H-NMR spectroscopy.

The behaviors of glutathione and related amino acids in the TiO2 photocatalytic system.

Glutathione is a tripeptide capable of diminishing active oxygen species in living cells. The photocatalytic decomposition of glutathione and related amino acids in TiO(2) suspension was investigated with (1)H NMR spectroscopy. The results suggest that both glutathione in reduced and oxidative forms is adsorbed on the TiO(2) surface by carboxyl or amino groups but not by the thiol group of the side chain which plays a crucial role in the glutathione cycle, to be degraded. This means that the function of glutathione cycle should be deteriorated in living cells by the adsorption. However, the decomposition rates are considerably low as compared with those of the constituent amino acids (Glu, Cys, and Gly), possibly reflecting the self-defensive property against active oxygen species.

Laser Ablation of the Photocatalytic BiVO4 and BiZn2VO6 Powders in Water and Their Photocurrent Properties

Thin-film electrodes were prepared with the laser-ablated BiVO 4 and BiZn 2 VO 6 photocatalytic particulates, and the photocurrent action spectra were measured from 350 to 600 nm. The photocurrent of the electrodes prepared from laser-ablated particulates was increased compared to that from nonablated particulates. It was suggested that the increase of the photocurrents for the laser-ablated photocatalytic thin electrode should be attributed to the increase in the surface defect sites on the photocatalytic surface rather than the reduction of the particle size.