Photocatalytic and Photoelectrochemical Hydrogen Evolution from Water over Cu2SnxGe1-xS3 Particles.

Cu2SnxGe1-xS3 (CTGS) particles were synthesized via a solid-state reaction and assessed, for the first time, as both photocatalysts and photocathode materials for hydrogen evolution from water. Variations in the crystal and electronic structure with the Sn/Ge ratio were examined experimentally and theoretically. The incorporation of Ge was found to negatively shift the conduction band minimum, such that the bandgap energy could be tuned over the range 0.77-1.49 eV, and also increased the driving force for the photoexcited electrons involved in hydrogen evolution. The effects of the Sn/Ge ratio and of Cu deficiency on the photoelectrochemical performance of Cu2SnxGe1-xS3 and CuySn0.38Ge0.62S3 (1.86 < y < 2.1) based photocathodes were evaluated under simulated sunlight. Both variations in the band-edge position and the presence of a secondary impurity phase affected the performance, such that a particulate Cu1.9Sn0.38Ge0.62S3 photocathode was the highest performing specimen. This cathode gave a half-cell solar-to-hydrogen energy conversion efficiency of 0.56% at 0.18 V vs a reversible hydrogen electrode (RHE) and an incident-photon-to-current conversion efficiency of 18% in response to 550 nm monochromatic light at 0 VRHE. More importantly, these CTGS particles also demonstrated significant photocatalytic activity during hydrogen evolution and were responsive to radiation up to 1500 nm, representing infrared light. The chemical stability, lack of toxicity, and high activity during hydrogen evolution of the present CTGS particles suggest that they may be potential alternatives to visible/infrared light responsive Cu-chalcogenide photocatalysts and photocathode materials such as Cu(In,Ga)(S,Se)2 and Cu2ZnSnS4.

Photocatalytic oxygen evolution triggered by photon upconverted emission based on triplet-triplet annihilation.

A visible light responsive photocatalyst, Mo-doped BiVO4 (Mo:BVO), was shown to promote oxygen evolution from water in response to photon upconverted emission based on triplet-triplet annihilation (TTA) in the same aqueous dispersion. Composites comprising a triplet sensitizer (Pt(ii) octaethylporphyrin; PtOEP) and a singlet emitter (9,10-diphenylanthracene; DPA) intercalated in a layered clay compound (montmorillonite or saponite) were prepared using a facile but versatile solvothermal method. These composites were capable of converting green incident light (λ = 535 nm) to blue light (λ = 430 nm) even in air. The host layered clay as well as the co-intercalated surfactant evidently functioned as barriers against water and oxygen to prevent the quenching of the active compounds. The TTA upconversion driven photocatalytic oxygen evolution using the aqueous mixture of the dyes-clay composite and particulate photocatalysts can be a potential approach to eliminate the undesired optical losses and thus be a breakthrough for future industrial and large-scale installation in an inexpensive manner.

Boosted Hydrogen-Evolution Kinetics Over Particulate Lanthanum and Rhodium-Doped Strontium Titanate Photocatalysts Modified with Phosphonate Groups.

Phosphonate groups loaded on the surface of the visible-light-responsive photocatalyst Ru-loaded La,Rh-doped SrTiO3 (Ru/La,Rh:STO) via a silane-coupling treatment enhance the photocatalytic activity of this material during the hydrogen evolution reaction. Surface modification with an alkylsilane phosphonate accelerates the supply of reactants to active sites and is much more effective at improving the photocatalytic activity than the utilization of a phosphate-buffered electrolyte as a reaction solution. In contrast, the incorporation of amine, sulfonate, and propyl groups does not improve the activity. The effects of these functional groups introduced via silane coupling on the reaction kinetics of hydrogen evolution are evaluated separately from the oxidative reaction using electrochemical methods. It was also demonstrated that the present alkylsilane phosphonate modification increases the photocatalytic activity even under a low photon flux.

Titanium complex formation of organic ligands in titania gels.

Thin films of organic ligand-dispersing titania gels were prepared from titanium alkoxide sols containing ligand molecules by steam treatment without heating. The formation of the ligand-titanium complex and the photoinduced electron transfer process in the systems were investigated by photoelectrochemical measurements. The complex was formed between the 8-hydroxyquinoline (HQ) and titanium species, such as the titanium ion, on the titania nanoparticle surface through the oxygen and nitrogen atoms of the quinolate. A photocurrent was observed in the electrodes containing the complex due to the electron injection from the LUMO of the complex into the titania conduction band. A bidentate ligand, 2,3-dihydroxynaphthalene (DHN), formed the complex on the titania surface through dehydration between its two hydroxyl groups of DHN and two TiOH groups of the titania. The electron injection from the HOMO of DHN to the titania conduction band was observed during light irradiation. This direct electron injection was more effective than the two-step electron injection.

Precise analyses of photoelectrochemical reactions on particulate Zn0.25Cd0.75Se photoanodes in nonaqueous electrolytes using Ru bipyridyl complexes as a probe.

Recombination of photoexcited carriers at interface states is generally believed to strongly govern the photoelectrochemical (PEC) performance of semiconductors in electrolytes. Sacrificial reagents (e.g., methanol or Na2SO3) are often used to assess the ideal PEC performance of photoanodes in cases of minimised interfacial recombination kinetics as well as accelerated surface reaction kinetics. However, varying the sacrificial reagents in the electrolyte means simultaneously changing the equilibrium potential and the number of electrons required to perform the sacrificial reaction, and thus the thermodynamic and kinetic aspects of the PEC reactions cannot be distinguished. In the present study, we propose an alternative methodology to experimentally evaluate the energy levels of interfacial recombination centres that can reduce PEC performance. We prepare nonaqueous electrolytes containing three different Ru complexes with different bipyridyl ligands; redox reactions of Ru complexes represent one-electron processes with similar charge transfer rates and diffusion coefficients. Therefore, the Ru complexes can serve as a probe to isolate and evaluate only the thermodynamic aspects of PEC reactions. Recombination centres at the interface between a nonaqueous electrolyte and a Zn0.25Cd0.75Se particulate photoanode are elucidated using this method as a model case. The energy level at which photocorrosion proceeds is also determined.

Impact of Ball Milling on the Hydrogen Evolution Performance of Cu2Sn0.38Ge0.62S3 Photocatalytic Particles Synthesized via a Flux Method.

Ball milling has been shown empirically to produce fine photocatalytic particles from large bulky particles but to drastically reduce the photocatalytic activity of such material during water splitting due to mechanical damage to the photocatalyst surfaces. If the damaged photocatalyst surfaces could be removed or reconstructed, the reduced particle sizes resulting from milling would be expected to provide enhanced photocatalytic activity. In the present study, fine particles of crystalline Cu2Sn0.38Ge0.62S3 (CTGS), which is responsive to long wavelength light up to the near-infrared region, were synthesized by a flux method and subsequent ball milling. A photocathode made of such particles showed significantly enhanced photoelectrochemical (PEC) performance under simulated sunlight while the photocatalytic hydrogen evolution activity of a powder suspension system made from the same material exhibited a typical decrease. The CTGS crystalline particles synthesized using the flux method were found to be highly crystalline but to have relatively large micrometer-scale sizes. Ball milling reduced the particle size but produced an amorphous coating of oxidized species that lowered the photocatalytic activity of the powder suspension system. Typical surface modifications of a photocathode made from this material, consisting of wet chemical processes, also served as an etching treatment to successfully remove the minimally crystalline surface layer and provide greater PEC activity. These data suggest the benefits of combining flux crystal growth with ball milling and the appropriate chemical etching process to obtain high-crystallinity fine photocatalytic particles responsive to long wavelength light with improved PEC hydrogen evolution activity.

Chelation ability of spironaphthoxazine with metal ions in silica gel.

Spironaphthoxazine (SNO) and three metal ions, Mg(2+), Zn(2+), and Al(3+), were dispersed in silica gels by the sol-gel method. The chelation ability of SNO with the metal ions in silica gels was investigated by measuring the fluorescence spectra and was compared to that of 8-hydroxyquinoline (8-HQ) in ethanol and silica gels. A merocyanine-type isomer photoderived from SNO as well as 8-HQ easily formed complexes of the metal ions in the order of Al(3+), Zn(2+), and Mg(2+) because the coordination ability of the metal ions to such ligands depended on their electron affinity. The changes in the fluorescence spectra of the silica gel samples during light irradiation were also investigated. The relative band intensity due to the intermediate species between the original SNO and the merocyanine species decreased and that of the complex increased with the UV irradiation time. The reverse process was observed during visible irradiation. The UV irradiation effects on the chelation of SNO and its photochromic property also depended on the electron affinity of the metal ions.

In Situ Probing of Photoinduced Hydrophilicity on Titania Surface Using Dye Molecules.

The titania film surface exhibits superhydrophilicity after UV irradiation due to its photocatalytic function. This is caused by a change in the density of the surface hydroxyl groups, which affects the surface charge density. Titania films were prepared to observe the change in the surface charge during UV irradiation. The amounts of some xanthene dyes adsorbed and deposited on the titania films after UV irradiation were evaluated as a function of the irradiation time. The increase in the adsorption and deposition amounts of the more negative dye versus the UV irradiation time was greater. These results indicated that the titania surface charge became more positive by the UV irradiation. It was reported that basic hydroxyl groups formed on the titania particle surface during the UV irradiation. The surface dissociates hydroxyl ions into the water phase and has a positive charge, which was supported by the present study.

Photon Upconverted Emission Based on Dye-Sensitized Triplet-Triplet Annihilation in Silica Sol-Gel System.

Photon upconverted emission based on dye-sensitized triplet-triplet annihilation was observed in silica gel systems containing Pt(II) octaethylporphyrin (triplet sensitizer) and 9,10-diphenylanthracene (singlet emitter). The triplet sensitizer was encapsulated and highly dispersed in the silica gels prepared by the sol-gel method. The singlet emitter was adsorbed on the silica gel pores accessible to the outside. Phosphorescence of the triplet sensitizer was partially quenched, and the singlet emission was enhanced with an increase in the concentration of the singlet emitter. The emission intensity increased in proportion to the approximate square of the irradiation power. The triplet energy transfer from some of the encapsulated triplet sensitizer molecules to the adsorbed singlet emitter molecules was observed in the silica gels followed by the triplet-triplet annihilation and upconverted singlet emission. The phosphorescence quenching and upconverted singlet emission were more significantly observed in the gel dried at a lower temperature (wetter gel). The wetter gel contained higher amounts of solvent and water molecules, in which the triplet sensitizer and singlet emitter should collide and then the sensitized emitters should collide between themselves during their excited-state lifetime. The photon upconversion process required the triplet sensitizer and singlet emitter molecules to be in an environment similar to the solvents.

Dispersion of acid-treated carbon nanofibers into gel matrices prepared by the sol-gel method.

1-Naphthol has been used as an in-situ fluorescent probe to characterize the dispersibility of carbon nanofibers (CNFs) into the sol-gel matrix of silicon alkoxide. The ion-pair fluorescence of 1-naphthol was found in the gel dispersing acid-treated CNFs instead of 1Lb fluorescence, which was preferred in the low polar gel matrix. This indicates that 1-naphthol easily interacts with oxidized groups present on the surface of the acid-treated CNFs due to the high dispersibility of the CNFs into the gel matrix. The oxidized groups on the CNF surface are useful for preventing self-assembly and/or aggregation of the CNFs in the gel matrix.

Formation Process of Eosin Y-Adsorbing ZnO Particles by Electroless Deposition and Their Photoelectric Conversion Properties.

The thin films consisting of crystalline ZnO particles were prepared on fluorine-doped tin oxide electrodes by electroless deposition. The particles were deposited from an aqueous solution containing zinc nitrate, dimethyamine-borane, and eosin Y at 328 K. As the Pd particles were adsorbed on the substrate, not only the eosin Y monomer but also the dimer and debrominated species were rapidly adsorbed on the spherical ZnO particles, which were aggregated and formed secondary particles. On the other hand, in the absence of the Pd particles, the monomer was adsorbed on the flake-shaped ZnO particles, which vertically grew on the substrate surface and had a high crystallinity. The photoelectric conversion efficiency was higher for the ZnO electrodes containing a higher amount of the monomer during light irradiation.

Influence of dye content on the conduction band edge of titania in the steam-treated dye-dispersing titania electrodes.

The titania and dye-dispersing titania electrodes were prepared by a nitric acid-catalyzed sol-gel process. The dye-dispersing titania contains the dye molecules dispersed on the surface of the individual nanosized titania particles. The photo-cyclic voltammetry (Photo-CV) and photoelectric measurements of the dye-dispersing titania electrodes were conducted to clarify the factors changing the conduction band edge of the titania and the open-circuit voltage (Voc ) of the electrodes. The remaining nitrate ions caused a negative shift of conduction band edge of the titania of the dye-dispersing titania. The conduction band edge of the titania was shifted in a negative direction in the electrode containing a greater amount of the dye. These results are due to the adsorption of nitrate ions and the dye-titania complex formation on the titania particle surface. The effect of the dye-titania complex formation on the shift in the titania conduction band edge was greater than that of the adsorption of nitrate ions due to strong interaction between the dye and titania through the carboxylate and quinone-like groups of the dye. The shift in the titania conduction band edge corresponded to the change in the Voc value.

Influences of acid on molecular forms of fluorescein and photoinduced electron transfer in fluorescein-dispersing sol-gel titania films.

Fluorescein-dispersing titania gel films were prepared by the acid-catalyzed sol-gel reaction using a titanium alkoxide solution containing fluorescein. The molecular forms of fluorescein in the films, depending on its acid-base equilibria, and the complex formation and photoinduced electron transfer process between the dye and titania surface were investigated by fluorescence and photoelectric measurements. The titanium species were coordinated to the carboxylate and phenolate-like groups of the fluorescein species. The quantum efficiencies of the fluorescence quenching and photoelectric conversion were higher upon excitation of the dianion species interacting with the titania, i.e. the dye-titania complex. This result indicated that the dianion form was the most favorable for formation of the dye-titania complex exhibiting the highest electron transfer efficiency. Using nitric acid as the catalyst, the titania surface bonded to the fluorescein instead of the adsorbed nitrate ion during the steam treatment. The dye-titania complex formation played an important role in the electron injection from the dye to the titania conduction band.

Preparation of dye-adsorbing ZnO thin films by electroless deposition and their photoelectrochemical properties.

Dye-adsorbing ZnO thin films were prepared on ITO films by electroless deposition. The films were formed in an aqueous solution containing zinc nitrate, dimethylamine-borane, and eosin Y at 328 K. The film thickness was 1.2-2.0 µm. Thinner and larger-plane hexagonal columns were produced from the solution containing a higher concentration of eosin Y. A photocurrent was observed in the electrodes containing such ZnO films during light irradiation. The photoelectrochemical performance of the film was improved by increasing the concentration of eosin Y because of increases in the amount of absorbed photons and the electronic conductivity of ZnO.

Photocatalysis-induced selective decoration of semiconducting single walled carbon nanotubes: hole-doping effect.

We have examined the time-dependent effect of the titanium oxide photocatalysis on N-methyl-2-pyrrolidone individually dispersed single walled carbon nanotube (SWNT) suspensions. From optical spectroscopic studies, we found a selective decoration of the semiconducting tubes. Such selectivity is attributed to the preferential attack of the photogenerated active species on the hole-doped semiconducting SWNTs.

Zinc chelation and photofluorochromic behavior of spironaphthoxazine intercalated into hydrophobically modified montmorillonite.

Spironaphthoxazine (SNO) and Zn2+ were intercalated into montmorillonite interlayers hydrophobically modified by the alkyltrimethylammonium cation during UV light irradiation. The fluorescence spectra of the montmorillonite composites were observed to vary with an increase in the UV and visible light irradiation times. These composites exhibited two types of fluorescence emissions: F1, which originates from a new species, Xs, which is different from SNO (ring-closed form) and merocyanine (MC; ring-open form), and F2, which originates from the MC-Zn complex. With increasing UV light irradiation time, the F1 intensities decreased, whereas the F2 intensities increased. Xs, which is an intermediate species between SNO and MC, was transformed into MC and then coordinated with Zn2+ (i.e., MC-Zn complex) during the UV light irradiation. The reaction rate of the formation of the MC-Zn complex decreased for the hydrophobically modified montmorillonite due to a longer alkyl chain. The retrieval changes in the F1 and F2 intensities were observed with an increasing visible light irradiation time, implying the dissociation of the MC-Zn complex into Xs and Zn2+. The dissociation especially occurred for the hydrophobically modified montmorillonite with a longer alkyl chain. The formation and disappearance of Xs and the MC-Zn complex obeyed first-order kinetics, and therefore the interconversion between Xs and MC could be regarded as the rate-determining step of the whole reaction during the UV and visible light irradiations. The photoinduced reactions of the SNO species and Zn2+ were profoundly affected by the physicochemical environment provided by the clay interlayers. It is concluded that the present photoreactions can be controlled not only by the amounts of the intercalated SNO species and Zn2+, but also by the hydrophobic environment created by the surfactant molecules.