Interaction between dye and zinc in the dye-dispersing ZnO films prepared by a wet process

Dye-dispersing ZnO precursor gel films were prepared on indium tin oxide electrodes from a zinc acetate solution containing eosin Y by dip-coating, steam treatment, and then heating at a low temperature. The electronic interaction between the dye and zinc in the dye-dispersing gel films were investigated by spectroscopic and electrochemical measurements. A photocurrent was observed in the dye-dispersing gel electrodes before the steam treatment. The photocurrent value increased by the steam treatment and heating due to crystallization of the gel and removal of organic impurities. The dye molecules existed between the interlayers of the layered zinc hydroxide coexisting with the ZnO. The photoexcited electron in the dye should be injected into the ZnO conduction band via the layered zinc hydroxide. The value increased with an increase in the dye content even though the ZnO crystallinity decreased. The dye-zinc interaction, i.e., the complex formation and photoinduced electron injection, played an important role in the electron transport and photoelectric conversion.ArticleRESEARCH ON CHEMICAL INTERMEDIATES. 41(9):6559-6574 (2015)journal articl

Formation of ZnO thin films by photocatalytic reaction

Zinc oxide and layered zinc hydroxides were deposited from an aqueous solution of zinc nitrate at 323–358 K on a substrate plate with a very thin titanium dioxide film by a photocatalytic reaction. The amorphous or low crystalline zinc hydroxide aggregates were deposited at a low temperature. The zinc oxide crystals with about 1–2 μm-sized hexagonal columns and 10 nm-sized spheres were formed at 338–358 K. Nitrate ions in the solution were reduced to nitrite ions, and water was transformed into hydroxide ions by a photocatalytic reaction on the titanium dioxide film. The pH value increased on the substrate surface with the titanium dioxide film, which caused the zinc hydroxide formation on the film. The zinc hydroxides were then dehydrated and transformed into zinc oxide. The average crystallite size of the zinc oxide decreased with an increase in the reaction temperature because the reaction rates of the formation and dehydration of the zinc hydroxides increased which resulted in an increase in the formation rate of the crystal zinc oxide nuclei.ArticleAPPLIED CATALYSIS B-ENVIRONMENTAL. 160:651-657 (2014)journal articl

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

Formation of ZnO thin films by photocatalytic reaction

Zinc oxide and layered zinc hydroxides were deposited from an aqueous solution of zinc nitrate at 323–358 K on a substrate plate with a very thin titanium dioxide film by a photocatalytic reaction. The amorphous or low crystalline zinc hydroxide aggregates were deposited at a low temperature. The zinc oxide crystals with about 1–2 μm-sized hexagonal columns and 10 nm-sized spheres were formed at 338–358 K. Nitrate ions in the solution were reduced to nitrite ions, and water was transformed into hydroxide ions by a photocatalytic reaction on the titanium dioxide film. The pH value increased on the substrate surface with the titanium dioxide film, which caused the zinc hydroxide formation on the film. The zinc hydroxides were then dehydrated and transformed into zinc oxide. The average crystallite size of the zinc oxide decreased with an increase in the reaction temperature because the reaction rates of the formation and dehydration of the zinc hydroxides increased which resulted in an increase in the formation rate of the crystal zinc oxide nuclei.ArticleAPPLIED CATALYSIS B-ENVIRONMENTAL. 160:651-657 (2014)journal articl

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

Facile Morphological Modification of Ba₅Nb₄O₁₅ Crystals Using Chloride Flux and in Situ Growth Investigation

The cation-deficient layered perovskite oxide Ba₅Nb₄O₁₅ is one of the functional materials that exhibits a microwave-responsive dielectric property and an ultraviolet-active photocatalytic property. Although systematic control of the morphology of Ba₅Nb₄O₁₅ is beneficial for improving these properties, synthesized Ba₅Nb₄O₁₅ usually has a plate-like shape owing to its crystal structure, with a particle size less than 5 μm. For systematic morphological control of Ba₅Nb₄O₁₅, the crystal growth was studied by using a chloride-based flux method. Idiomorphic plate-like Ba₅Nb₄O₁₅ crystals up to 50 μm in size and polyhedron ones ∼10 μm in size were obtained using a BaCl₂ flux by changing the solute concentration to 5–20 mol % and 50 mol %, respectively. The growth of the Ba₅Nb₄O₁₅ crystals was investigated by thermogravimetric and differential thermal analysis and in situ X-ray diffraction analysis. These analyses revealed the flux-growth manner of Ba₅Nb₄O₁₅ as follows: (I) Ba₅Nb₄O₁₅ was formed by a solid-state reaction above ∼650 °C. (II) After the melting of BaCl₂ above ∼962 °C, the Ba₅Nb₄O₁₅ crystals became larger and assumed idiomorphic shapes, indicating that they were somewhat dissolved in the flux and that the crystal growth was promoted. Increasing the holding time yielded an increased number of crystals larger than 28 μm. This indicates that Ostwald ripening effectively yields Ba₅Nb₄O₁₅ crystals up to 50 μm in size. Chloride fluxes with different alkaline or alkaline earth cation fluxes did not produce such large crystals. It is assumed that the common ion effect of Ba²⁺ in the solute and flux provides an effective reaction field to facilitate Ostwald ripening

Template-Assisted Size Control of Polycrystalline BaNbO₂N Particles and Effects of Their Characteristics on Photocatalytic Water Oxidation Performances

The photocatalytic water oxidation using solar irradiation is a sustainable way to convert a natural source to energy. The perovskite-type oxynitride BaNbO₂N is a candidate photocatalyst for this process because its long-range light absorbance of up to ca. 740 nm leads to the high ability of energy conversion. However, it is necessary to improve its poor performance by optimizing the crystallographic characteristics, chemical formula, depositions of cocatalysts, and so on. In this study, we aimed to identify the dominant factors of the photocatalytic performance of BaNbO₂N. We controlled the particle characteristics by nitriding size-controlled Ba₅Nb₄O₁₅ crystals in sizes of 0.2–50 μm as sacrificial templates. Porous BaNbO₂N secondary particles of different sizes were achieved, and they exhibited distinctive photocatalytic performances for O₂ evolution with rates between 14.1 and 113.9 μmol·h^–1, depending on the precursor size and nitriding time. By correlating the performance with the basal particle characteristics, we assume that the crystallinity and anion deficiency are the two dominant factors that competitively affect the photocatalytic performance of BaNbO₂N