Applications of some EPR methods to the investigation of the radical species produced by the reactions of hydroxyl radicals with PEFC-related fluorinated organic acids.

Fundamental information on the reactions of ˙OH radicals with perfluoroalkyl sulfonic acids and carboxylic acids is important for understanding the degradation of polymer electrolyte fuel cells (PEFCs). In the present research, the intermediate radicals produced by these reactions were detected and analyzed by means of three methods of electron paramagnetic resonance (EPR) spectroscopy. The conventional CW-EPR technique was applied to both frozen and flowing aqueous solution systems for detecting the reaction intermediates, while the time-resolved (TR) EPR technique was applied to the flowing solution system for analyzing spin dynamics parameters. The reactants tested were CF3SO3H, CHF2CF2SO3H, CH3SO3H, CF3COOH, CHF2COOH, etc., and the ˙OH radical was generated from H2O2 by the irradiation of a UV laser. The radicals detected were ˙SO3-, ˙CO2-, ˙CF3, ˙CF2CF2SO3H, ˙CF2COOH, etc. Based on the measurements of TR-EPR spectra, the dependences of the signal intensity on the time and magnetic field were analyzed, and then the longitudinal relaxation time (T1) and the lifetime of these radical species were evaluated. The three EPR methods for detecting the intermediate radicals were compared to show the limitations of these techniques. Based on the detected radicals, the degradation mechanism reported for perfluoro acids was discussed.

Spin Dynamics of the Radicals Produced from Methane Sulfonic Acid and Acetic Acid by Hydroxyl Radicals in Aqueous Solution Studied by Means of Time-Resolved Electron Spin Resonance Spectroscopy.

Fundamental information on the reactions of ·OH radical with alkyl sulfonic acid and carboxylic acid is important for understanding the degradation of the polymer electrolytes for fuel cells. In the present research, the spin dynamics of the organic radicals generated by the ·OH radical was investigated by means of a time-resolved electron spin resonance (TR-ESR) method with the pulsed laser irradiation on H2O2 in aqueous solution. The time profiles of the ESR signals of ·SO3-, ·CH2SO3-, ·CH2COOH, and ·CH2COO- have been analyzed along with the evaluation of characteristics of electron spins such as the g factor, hyperfine coupling constant A, lifetime of radicals, and electron spin relaxation time T1. The effects of the laser repetition frequency on the TR-ESR spectra, the pH dependence on the kinetic parameters, and the mechanism of chemically induced dynamic electron polarization (CIDEP) were discussed. The radical pair mechanism was suggested for the CIDEP, where the electron spins of these radicals are polarized by forming a free radical pair with another radical or the·OH radical, which have remained in the vicinity of photon absorption.

Intrinsic nature of photocatalysis by comparing with electrochemistry.

Photocatalysis has been gathering much attention because of the unique applications of photoenergy for environmental cleaning and solar fuel production. Electron transfer (ET) at the solid-liquid interface, which initiates photocatalytic reactions, has been the subject of electrochemistry, and hence the reactions are often analyzed in terms of electrochemistry. However, how extensively the concept of electrochemistry can be incorporated has not been discussed so far. In this report, by comparing with electrochemistry, the intrinsic nature of photocatalysis is disclosed and the limitation of the use of the concept of electrochemistry was pointed out. The electric potential near the photocatalyst surface was calculated and visualized, showing a potential gradient similar to that at the electrode surface but localized near the positive hole. Since the frequency of the ET at the photocatalyst surface is limited by the photon absorption, the investigation of photocatalysis in terms of energy states and kinetics should be different from those for electrochemistry. Since semiconductor photocatalysts are not wired to the electric source, the estimation of energy band positions may be altered, which was actually discussed in terms of the band alignments of anatase and rutile TiO2 crystals.

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.

The pH dependence of OH radical formation in photo-electrochemical water oxidation with rutile TiO2 single crystals.

It has been believed that photocatalytic oxidation in water proceeds with the reaction of OH radicals generated on the photocatalysts. To explore the actual contribution of OH radicals to photocatalytic oxidation, OH radicals were detected by fluorescence probe method in photoelectrolysis with rutile TiO2 of (100) and (110) facets. The effect of hydrogen peroxide on the OH radical formation at pH 6.7 was investigated to confirm the relevant intermediate which was suggested in our previous report for water oxidation. In alkaline solutions at pH 9.6 and 12.5, the current efficiencies of OH radical formation were 0.01-0.05%, which are far smaller than those at pH 6.7 (0.2-0.6%) due to the deprotonation of the reaction intermediate as confirmed by FT-IR measurements. These experimental results support a plausible reaction mechanism that the surface Ti-O-O-Ti structure is an intermediate of the water oxidation process, by which mechanism the O2 production becomes favorable in alkaline solution.

Difference in TiO2 photocatalytic mechanism between rutile and anatase studied by the detection of active oxygen and surface species in water.

Various kinds of TiO2 photocatalysts have been practically applied in various fields. Knowing the exact surface properties is a prerequisite to develop further and efficient applications. However, the cause of the essential difference in the activities of the two polymorphs of TiO2, rutile and anatase, has not been clearly elucidated yet. We tried to clarify the cause in terms of active oxygen species (˙OH, ˙O2(-), and H2O2) photogenerated on the surfaces, which are considered practically involved in the photocatalytic reactions. It was revealed that for anatase the rate of ˙OH generation was high, but it decreased in the presence of H2O2. On the other hand, for rutile, ˙OH generation was very low but it increased in the presence of H2O2. The formation rate of ˙O2(-) for rutile was higher than that for anatase, suggesting that the photoinduced reduction process should not be accountable for the higher photocatalytic activity of anatase. Since the Ti-Ti distance on a rutile surface is smaller than that for anatase, rutile is capable of forming a surface structure such as Ti-OO-Ti, leading to readily form O2. The mechanism of fast coupling of two photoinduced conduction band holes to form Ti-OO-Ti was proposed, which is accountable for the lower reactivity of rutile. This mechanism was verified by the analysis of surface species with ATR-IR spectroscopy.

Auger ionization beats photo-oxidation of semiconductor quantum dots: extended stability of single-molecule photoluminescence.

Despite the bright and tuneable photoluminescence (PL) of semiconductor quantum dots (QDs), the PL instability induced by Auger recombination and oxidation poses a major challenge in single-molecule applications of QDs. The incomplete information about Auger recombination and oxidation is an obstacle in the resolution of this challenge. Here, we report for the first time that Auger-ionized QDs beat self-sensitized oxidation and the non-digitized PL intensity loss. Although high-intensity photoactivation insistently induces PL blinking, the transient escape of QDs into the ultrafast Auger recombination cycle prevents generation of singlet oxygen ((1) O2 ) and preserves the PL intensity. By the detection of the NIR phosphorescence of (1) O2 and evaluation of the photostability of single QDs in aerobic, anaerobic, and (1) O2 scavenger-enriched environments, we disclose relations of Auger ionization and (1) O2 -mediated oxidation to the PL stability of single QDs, which will be useful during the formulation of QD-based single-molecule imaging tools and single-photon devices.

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.

Kinetics simulation of luminol chemiluminescence based on quantitative analysis of photons generated in electrochemical oxidation.

The kinetics of electrogenerated chemiluminescence (ECL) of luminol at a gold electrode in alkaline solution was investigated by measuring the absolute number of photons emitted in an integrating sphere. The ECL efficiency as the ratio of photon to electric charge was 0.0004 in cyclic voltammography and 0.0005 in chronoamperometry. By numerically solving the rate equations based on a diffusion layer model, the observed time profile of the luminescence intensity could be successfully simulated from the oxidation current of luminol in the chronoamperometry. In the simulation, the rate constant for the oxidation of luminol by superoxide radicals in alkaline solution was determined to be 6 × 10(5) M(-1) s(-1). The present methodology and the achievement could be widely applicable to various analytical techniques using chemiluminescence.

Singlet-oxygen-sensitizing near-infrared-fluorescent multimodal nanoparticles.

Nanoprobes based on quantum clusters (QC) with near-infrared fluorescence, magnetic-resonance-imaging contrast, and singlet-oxygen-sensitized intracellular fluorescence are studied. The generation of singlet oxygen and singlet-oxygen-sensitized fluorescence uncaging by magnetic and NIR-emitting nanoparticles are exploited for multimodal bioimaging in vitro.

Reinvestigation of the photocatalytic reaction mechanism for Pt-complex-modified TiO2 under visible light irradiation by means of ESR spectroscopy and chemiluminescence photometry.

A plausible reaction mechanism for a visible light photocatalyst of TiO(2) modified with platinum(IV) chloride (PtCl) was proposed on the basis of the measurements with electron spin resonance (ESR) spectroscopy and chemiluminescence photometry. Under visible light (λ > 500 nm) irradiation, the deposited Pt(IV) chloride is charge-separated into Pt(3+) and Cl radical by the excitation of the ligand-to-metal charge transfer. The Pt(3+) gives an electron to the conduction band of TiO(2), which has Pt(3+) return to Pt(4+). The electron in the conduction band reduces the oxygen molecule into O(2)(-). The presence of Pt(3+) and O(2)(-) has been elucidated in the present study. Moreover, valence band holes of TiO(2) were detected by ESR spectroscopy under visible light irradiation. Therefore, besides being used to oxidize organic compounds, the photogenerated Cl radicals likely receive electrons from the TiO(2) valence band by visible light excitation, producing the valence band holes. Because the valence band holes have a stronger oxidation power than Cl radicals, the excitation of valence band electrons to Cl radicals would be the origin of the high photocatalytic activity of the PtCl-modified TiO(2) under visible light irradiation.

Density functional study of the high-temperature oxidation of o-, m- and p-xylyl radicals.

Theoretical calculations at the CBS-QB3 level of theory have been performed to investigate the potential energy surface for the reaction of o-, m- and p-xylyl with molecular oxygen. The differences of the relative potential energies for the products and the transition states of o-, m- and p-xylyl with molecular oxygen were found to be within 8.5 kJ/mol at the CBS-QB3 level of theory. Although the reaction of m- and p-xylyl radicals with molecular oxygen have the same reaction pathways and also the same reaction thermochemistry as that of benzyl radicals, the o-xylylperoxy radicals formed by the reaction of o-xylyl + O(2) had an additional intramolecular isomerization pathway to form the o-xylyl hydroperoxy radicals. The rate constants and the product branching ratios for the o-xylyl + O(2) and its subsequent reactions were evaluated by the RRKM and master equation analysis. Possible roles for these reaction pathways on the combustion of o-xylenes are discussed.

Adsorption and photocatalytic decomposition of amino acids in TiO2 photocatalytic systems.

The adsorption and photodecomposition of seven kinds of amino acids on a TiO2 surface were investigated by zeta potential measurements and 1H NMR spectroscopy in TiO2 aqueous suspension systems. The decomposition rates increased in the order of Phe < Ala < Asp < Trp < Asn < His < Ser. For Phe, Trp, Asn, His, and Ser, the isoelectric point (IEP) of TiO2 shifted to a lower pH with increasing decomposition rates upon adsorption on TiO2, suggesting that the effective adsorption and photocatalytic sites for these amino acids should be the basic terminal OH on the solid surface. Since the amino acids that decomposed faster than the others contain -OH (Ser), -NH (Trp, His), or -NH2 (Asn) in their side chain, they are considered to interact with the basic terminal OH groups more preferably by the side chain and are vulnerable to photocatalytic oxidation. On the other hand, Ala interacts with the acidic bridged OH on TiO2 to cause an IEP shift to a higher pH. The correlation of the surface hydroxyl groups with the photocatalysis of amino acids was verified by the use of calcined TiO2 without surface hydroxyl groups.

Electron spin resonance studies on the oxidation mechanism of sterically hindered cyclic amines in TiO2 photocatalytic systems.

A sterically hindered cyclic amine, 4-hydroxy-2,2,6,6-tetramethylpiperidine (HTMP), is converted to the corresponding aminoxyl radical (nitroxide radical), 4-hydroxy-2,2,6,6-tetramethyl piperidine 1-oxyl (TEMPOL radical) as a result of a photocatalytic reaction in TiO2 aqueous suspension. The time profile of the radical formation and the effect of additives, such as SCN-, I-, methanol, and H2O2, on the initial formation rate were measured in order to elucidate the reaction mechanism. The experimental observations indicated that the direct photocatalytic oxidation of HTMP followed by reaction with O2 is the dominant process in the formation of TEMPOL radicals. Electrochemical measurements showed that HTMP is oxidized at 0.7 V (vs NHE), which is consistent with the proposed mechanism. The possibility of other processes, involving reactions with singlet molecular oxygen, superoxide radical, and hydroxyl radical, were excluded from the reaction mechanism.

Direct detection of OH radicals diffused to the gas phase from the UV-irradiated photocatalytic TiO2 surfaces by means of laser-induced fluorescence spectroscopy.

The important roles of OH radicals for remote oxidation using TiO(2) photocatalysts were evidenced by the in situ detection of OH radicals in the gas phase using the laser-induced fluorescence (LIF) technique. The appearance of OD-LIF intensities after the exposure of D(2)O vapors over TiO(2) powders and the decrease of the time-resolved signals of OH-LIF intensities with increasing calcined temperatures of TiO(2) powders suggested that the exchangeable water at the TiO(2) surface is the origin of the diffused OH radicals.

Effects of thermal treatments on the recovery of adsorbed water and photocatalytic activities of TiO2 photocatalytic systems.

The effects of thermal treatments on the rehydration process and photocatalytic activity were investigated by 1H NMR spectroscopy for six anatase abundant TiO2 photocatalysts with different properties. Acetic acid and benzoic acid were employed for photodecomposition in aqueous suspension. After the calcinations at 973 K, physisorbed water layers recovered relatively fast for P25, F4, and AMT-600 (shorter than 24 h) with no significant enhancement of the photocatalytic decomposition. On the other hand, for ST-01, UV-100, and AMT-100, the recovery was very slow (longer than 1 week) and only partially reversible, and the photocatalytic decomposition was considerably enhanced but retarded with rehydration. In the presence of adsorbed water, the binding of a carboxyl group of the molecules with adsorbed water is considered to compete with the direct adsorption on the surface, which reduces the amount of the direct adsorption and results in the reduction in the photocatalytic efficiency. In addition, the photocatalytic decomposition of benzoic acid with an aromatic ring was much faster in all of the TiO2 aqueous suspensions and more enhanced for the fully dehydroxylated TiO2 than that of acetic acid. These results suggest that the most efficient photocatalytic sites should be the hydrophobic sites on the TiO2 surface. The difference among the rehydration rates of different TiO2 is discussed in terms of thermally induced changes of surface morphology.

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.

Novel method for linking CdS nanoparticles at liquid-liquid interface.

Liquid-liquid interface of water-hexane provides a unique reaction environment in which CdS nanoparticles capped with mercaptoethylamine could be linked together to form a homodimer with a divalent acid chloride, sebacoyl chloride. Prior to the reaction, mercaptoethylamine-capped CdS in aqueous solution was purified by dialysis and freeze-drying. The observation with a transmission electron microscope suggested the formation of a homodimer of CdS nanoparticles.

Study by Use of 1H NMR Spectroscopy of the Adsorption and Decomposition of Glycine, Leucine, and Derivatives in TiO2 Photocatalysis.

The photocatalytic decomposition and adsorption of glycine (Gly), Gly-Gly, and Gly-Gly-Gly, and leucine (Leu), Leu-Gly, Gly-Leu, and Leu-Gly-Gly, in TiO2 (100% anatase crystal form) aqueous suspension were investigated by 1H NMR spectroscopy. The side chain of Leu, the carboxylic group, and the peptide bond were recognized as the adsorptive sites of the peptides on the surface of TiO2. For Gly-Leu and Leu-Gly-Gly, the photocatalytic decomposition that took place under UV irradiation resulted from the preferable adsorption of the hydrophobic side chain of Leu on the TiO2 surface, while for Gly-Gly and Gly-Gly-Gly, the photodecomposition proceeded by weak adsorption of the peptide bonds on the surface of TiO2.

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.