The contrasting effect of the Ta/Nb ratio in (111)-layered B-site deficient hexagonal perovskite Ba5Nb4-xTaxO15 crystals on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The effect of the Ta/Nb ratio in the (111)-layered B-site deficient hexagonal perovskite Ba5Nb4-xTaxO15 (0 <= x <= 4) crystals grown by a KCl flux method on visible-light-induced photocatalytic water oxidation activity of their oxynitride derivatives BaNb1-xTaxO2N (0 <= x <= 1) was investigated. The Rietveld refinement of X-ray data revealed that all Ba5Nb4-xTaxO15 samples were well crystallized in the space group P (3) over bar m1 (no. 164). Phase-pure BaNb1-xTaxO2N (0 <= x <= 1) porous structures were obtained by nitridation of the flux-grown oxide crystals at 950 degrees C for 20, 25, 30, 35, and 40 h, respectively. The absorption edge of BaNb1-xTaxO2N (0 <= x <= 1) was slightly shifted from 720 to 690 nm with the increasing Ta/Nb ratio. The O-2 evolution rate gradually progressed and reached the highest value (127.24 mu mol in the first 2 h) with the Ta content up to 50 mol% but decreased at 75 and 100 mol% presumably due to the reduced specific surface area and high density of structural defects, such as grain boundaries acting as recombination centers, originated from high-temperature nitridation for prolonged periods. Transient absorption spectroscopy provided evidence for the effect of the Ta/Nb ratio on the behavior and energy states of photogenerated charge carriers, indicating a direct correlation with photocatalytic water oxidation activity of BaNb1-xTaxO2N

Chelation ability of spironaphthoxazine with metal ions in silica gel

Spironaphthoxazine (SNO) and three metal ions, Mg2+, Zn2+, and Al3+, 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 Al3+, Zn2+, and Mg2+ 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.ArticlePHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES. 11(7):1164-1173 (2012)journal articl

Photoinduced electron transfer in rhodamine B-containing amorphous titania gels

Amorphous titania gel films containing the dye rhodamine B (RhB) were prepared by a sol-gel method, without heating. The RhB existed in the films as the RhB cation, which was transformed into the rhodamine 110 (Rh110) cation; its xanthene skeleton then decomposed during irradiation with visible light. This process was induced by electron transfer from the RhB cation to the titania gel matrix, because formation of the RhB radical on loss of an electron caused sequential dissociation of the N-ethyl groups. During irradiation with visible light a photocurrent was observed in electrodes coated with the RhB-containing titania gel and immersed in an I-2-LiI electrolyte. This photocurrent resulted from electron injection from the LUMO level of the dye into conduction band-like states of the titania gel. Because of its effective electron supply the electrolyte inhibited dissociation of the N-ethyl groups of RhB.ArticleRESEARCH ON CHEMICAL INTERMEDIATES. 41(6):3803-3816 (2015)journal articl

Activation routes for high surface area graphene monoliths from graphene oxide colloids

Graphene monoliths made from graphene oxide colloids by unidirectional freeze-drying method were activated by typical activation processes of CO2 activation, chemical activation using ZnCl2 or H3PO4, and KOH activation. The porosity development of graphene monolith markedly depends on the activation method. The monoliths with highest surface area are obtained by the KOH activation method; only the KOH activation is effective for production of the graphene monolith of which surface area is in the range of 1760–2150 m2 g−1. The mechanism of the porosity development by KOH activation method is proposed. This work provides a promising route for the bottom-up design of pore width-tunable nanoporous carbons.K.K., F.T., T.F., R.C-S, M.T. and M.E. were supported by Exotic Nanocarbons, Japan Regional Innovation Strategy Program by the Excellence, JST. This work was supported by Grant-in-Aid for Scientific Research (A) (24241038) and Concert-Japan project: Efficient Energy Storage and Distribution, JST. D. M., and this study were partially supported by JST CREST “Creation of Innovative Functional Materials with Advanced Properties by Hyper-nano-space Design”

Hydrogen evolution and electric power generation through photoelectrochemical oxidation of cellulose dissolved in aqueous solution

Photoelectrochemical (PEC) cells capable of generating electricity or hydrogen through the oxidation of cellulose dissolved in an aqueous solution were constructed. As distinct from solid cellulose, the cellulose liquid fuel was beneficial for enabling a continuous reactant supply. Product analyses revealed that the cellulose macromolecules were cleaved to small organic acids and finally decomposed to CO2. During the electricity generation, the maximum power density of the PEC cell reached 1.13 mW cm−2. The influence of the photoelectrode structure on the PEC performance was elucidated. The monolithic structure of photoanodes consisting of an approximate monolayer of SrTiO3 particles anchored onto the backside metal layer provided a high photocurrent during oxygen evolution. The porous photoanode composed of heavily stacked TiO2 nanoparticles was found to be suitable for cellulose decomposition because of the high specific surface area. The present study should serve as a foundation for the direct energy utilization of waste biomass.journal articl

Photoinduced cooling effect of water-retentive composite of TiO2 and saponite

TiO2 coating of a building envelope is expected to contribute to the mitigation of the heat-island effect because of the photoinduced cooling effect. However, the limited water adsorbability and retentivity of the TiO2 thin film can require successive sprinkling of water onto a building to achieve the sufficient cooling effect. In this study, a thin film consisting of a mixture of TiO2 and saponite was prepared through a sol–gel method. The high water retentivity of saponite was found to enhance the photoinduced cooling effect of the TiO2-based thin film. The photoinduced temporal change in the water adsorbability and retentivity of the highly hydrophilic composite was successfully monitored by using a hydrophilic xanthene dye adsorbed onto the coating as a probe. The effect of environmental humidity on the photoinduced cooling effect was also investigated. We found that higher humidity in the surrounding environment triggered stronger photoinduced cooling effects of the composite film. Especially during summer in Japan, when humidity is high, the TiO2–saponite composite film did not require intentional water sprinkling to achieve cooling effects.journal articl

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.ArticlePHOTOCHEMISTRY AND PHOTOBIOLOGY. 90(4):747-759 (2014)journal articl

Influence of dye dispersion on photoelectric conversion properties of dye-containing titania electrodes

Dye-dispersing titania electrodes were prepared from the dye-containing titanium alkoxide sols by a room temperature sol-gel process and steam treatment at 110 degrees C. The spectroscopic and photoelectric conversion properties of the electrodes were investigated in order to clarify the influences of the dye dispersion and the co-dispersion of the two dyes on the electron transfer process. The fluorescein and eosin Y molecules were dispersed into the titania as their monomers. The shapes of the photocurrent action spectra of the fluorescein and/or eosin Y-dispersing titania electrodes corresponded well to those of their absorption spectra because the excited electrons in the dyes were directly injected into the titania conduction band without any interaction between the dye molecules, such as energy transfer. This result indicated that the dye molecules were separately encapsulated in the pores between the titania nanoparticles and tightly adsorbed or bonded to the titania particle surface. The internal quantum efficiency of the photoelectric conversion was higher than that of the conventional dye-adsorbing titania electrodes in which the dye molecules were easily aggregated and thus deactivated by the energy transfer. The co-dispersion of the two dyes on the titania surface allowed effective extension of the visible light region for the photoelectric conversion.ArticleCATALYSIS SCIENCE & TECHNOLOGY. 3(6):1512-1519 (2013)journal articl