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 × 10⁵ M^–1 s^–1. The present methodology and the achievement could be widely applicable to various analytical techniques using chemiluminescence

OH Radical Formation at Distinct Faces of Rutile TiO₂ Crystal in the Procedure of Photoelectrochemical Water Oxidation

It has been believed that the photogenerated OH radicals are major active species which cause photocatalytic oxidation of water. To investigate the actual contribution of OH radicals to the photocatalytic O₂ generation, the amount of the OH radicals was measured for the three kinds of rutile TiO₂ electrodes having (100), (110), and (001) crystalline surfaces. The current efficiencies for O₂ generation measured with an oxygen sensor were almost 100% for all electrodes. However, the current efficiencies for OH radical formation estimated by means of a coumarin fluorescence probe method were less than 1%. Thus, it was experimentally elucidated that the contribution of OH radicals to the O₂ production is negligibly small. The amount of OH radical production decreased in the order of (100) > (110) > (001), along with the increase in the efficiency of the O₂ production. A plausible mechanism of OH radical formation as a byproduct in the O₂ generation process was proposed

Quantitative Detection of OH Radicals for Investigating the Reaction Mechanism of Various Visible-Light TiO₂ Photocatalysts in Aqueous Suspension

The reaction mechanism of visible-light responsive photocatalysts was explored by analyzing OH radicals (^•OH) quantitatively by means of a coumarin fluorescence probe method. The photocatalysts investigated were various modified TiO₂, i.e., nitrogen-doped, Pt-complex-deposited, Fe­(III)-grafted, and Fe­(III)-grafted Ru-doped TiO₂. The formation rate of ^•OH was measured to calculate the ^•OH quantum yield from the absorbed intensity of 470 nm LED light. The highest quantum yield was obtained for Fe­(III)-grafted Ru-doped TiO₂. The ^•OH yield was increased on the addition of H₂O₂ for the Fe­(III)-grafted TiO₂, indicating that H₂O₂ is supposedly a reaction intermediate for producing ^•OH. The photocatalytic activity for each sample was obtained by measuring CO₂ generation rate on the acetaldehyde decomposition in an aqueous suspension system and then it was compared with the ^•OH formation rate. Although the CO₂ generation rate is positively correlated with the ^•OH formation rate for each photocatalyst, the values of CO₂ generation were extremely larger than those of ^•OH. This finding indicates that the oxidation reaction takes place dominantly with surface trapped holes which probably exchange with the ^•OH in solution

Mechanism of Singlet Oxygen Generation in Visible-Light-Induced Photocatalysis of Gold-Nanoparticle-Deposited Titanium Dioxide

Superoxide radical (O₂^–) and singlet molecular oxygen (¹O₂) were successfully detected on gold-nanoparticle-deposited titanium dioxide (AuNP/TiO₂) under visible-light irradiation for the first time. For the samples prepared with 13 commercially available TiO₂ powders, the generations of O₂^– and ¹O₂ in the AuNP/TiO₂ aqueous suspensions were measured by chemiluminescence photometry and near-infrared emission, respectively. By investigating the effects of the particle size, the crystalline phase of TiO₂, the solution pH, and the light intensity on the ¹O₂ generation, the following generation process was proposed. Under the plasmon resonance excitation, an electron in the AuNP transfers to the conduction band of TiO₂ to reduce O₂ to O₂^– at the TiO₂ surface. The produced O₂^– is oxidized by the positive hole remained in the AuNP to generate ¹O₂. This mechanism could clearly explain the good correlation between the O₂^– and ¹O₂ generations and the mixed crystalline phase effect of TiO₂, which could not be explained by the other proposed mechanisms such as energy transfer and two-photon excitation

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 TiO₂ 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 Pt³⁺ and Cl radical by the excitation of the ligand-to-metal charge transfer. The Pt³⁺ gives an electron to the conduction band of TiO₂, which has Pt³⁺ return to Pt⁴⁺. The electron in the conduction band reduces the oxygen molecule into O₂^–. The presence of Pt³⁺ and O₂^– has been elucidated in the present study. Moreover, valence band holes of TiO₂ 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 TiO₂ 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 TiO₂ under visible light irradiation

Photocatalytic Reaction Mechanism of Fe(III)-Grafted TiO₂ Studied by Means of ESR Spectroscopy and Chemiluminescence Photometry

We successfully clarified the mechanisms of visible-light-driven photocatalytic reactions of Fe­(III)-grafted TiO₂ (Fe/TiO₂) and Fe­(III)-grafted Ru-doped TiO₂ (Fe/Ru:TiO₂). ESR spectroscopy revealed that the visible-light response of the Fe/TiO₂ photocatalyst resulted in the direct charge transfer from the valence band of TiO₂ to the grafted Fe ions. For the Fe/Ru:TiO₂ photocatalyst, acceptor levels were formed by doping Ru ions in the lattice of TiO₂, and the electrons at the acceptor levels excited on visible-light irradiation readily transfer to Fe ions. Since a longer wavelength light generated the conduction band electrons, we also proposed a two-step electron excitation from valence band to the conduction band through defect levels such as oxygen vacancy. As a result, a part of photogenerated electrons in the conduction band transfer to the grafted Fe ions. Therefore, the Fe/Ru:TiO₂ photocatalyst showed a higher activity because such two kinds of indirect charge transfer to the grafted Fe ions occurred in addition to the direct interfacial charge transfer observed for Fe/TiO₂. Moreover, chemiluminescence photometry confirmed that the grafted Fe ions function as a promoter to reduce O₂ into H₂O₂ via two-electron reduction. Therefore, the acceleration in the reduction of O₂ with doping Ru and grafting Fe ions allows a larger number of holes to oxidize organic compounds, resulting in the higher photocatalytic activity

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

Bifunctionality of Rh³⁺ Modifier on TiO₂ and Working Mechanism of Rh³⁺/TiO₂ Photocatalyst under Irradiation of Visible Light

A rhodium­(III) ion (Rh³⁺)-modified TiO₂ (Rh³⁺/TiO₂) photocatalyst, prepared by a simple adsorption method and exhibiting high levels of photocatalytic activity in degradation of organic compounds, was investigated by using X-ray absorption fine structure (XAFS) measurements, (photo)­electrochemical measurements, double-beam photoacoustic (DB-PA) spectroscopic measurements, and photoluminescence measurements. Based on the results, the features of the Rh³⁺ modifier and the working mechanism of the Rh³⁺/TiO₂ photocatalyst are discussed. XAFS measurements revealed that the Rh³⁺ species were highly dispersed and almost atomically isolated on TiO₂. The (photo)­electrochemical measurements, DB-PA spectroscopic measurements, and photoluminescence showed a unique bifunction of the Rh³⁺ modifier as a promoter for O₂ reductions and an electron injector to the conduction band of TiO₂ for response to visible light. The reasons for the Rh³⁺/TiO₂ photocatalyst exhibiting higher levels of photocatalytic activity than those of TiO₂ photocatalysts modified with other metal ions are also discussed on the basis of obtained results