Sayaka Suzuki

Highly Active GaN‐Stabilized Ta3N5 Thin‐Film Photoanode for Solar Water Oxidation

Ta3 N5 is a very promising photocatalyst for solar water splitting because of its wide spectrum solar energy utilization up to 600 nm and suitable energy band position straddling the water splitting redox reactions. However, its development has long been impeded by poor compatibility with electrolytes. Herein, we demonstrate a simple sputtering-nitridation process to fabricate high-performance Ta3 N5 film photoanodes owing to successful synthesis of the vital TaOδ precursors. An effective GaN coating strategy is developed to remarkably stabilize Ta3 N5 by forming a crystalline nitride-on-nitride structure with an improved nitride/electrolyte interface. A stable, high photocurrent density of 8 mA cm-2 was obtained with a CoPi/GaN/Ta3 N5 photoanode at 1.2 VRHE under simulated sunlight, with O2 and H2 generated at a Faraday efficiency of unity over 12 h. Our vapor-phase deposition method can be used to fabricate high-performance (oxy)nitrides for practical photoelectrochemical applications.

Fabrication and photocatalytic performance of highly crystalline nanosheets derived from flux-grown KNb3O8 crystals

Potassium triniobate (KNb3O8), which is an oxide semiconductor photocatalyst, has a layered structure consisting of negatively charged sheets of linked NbO6 octahedral units and K+ ions between the sheets. We report the flux growth of KNb3O8 crystals and their application for the photocatalytic decomposition of organic thin films. First, high quality, idiomorphic KNb3O8 crystals were successfully grown by cooling a KCl flux. Transparent-colorless KNb3O8 crystals had relatively uniform sizes and shapes. The size, morphology and phase of the grown crystals were dependent on the holding temperature and solute concentration. Next, highly crystalline NbOx nanosheets were successfully prepared via a two-step process, that is, proton exchange and subsequent exfoliation of the KNb3O8 crystals. Finally, the nanosheet layer spin-coated on a silica glass was used for photodegradation of hydrophobic organosilane thin films. The fabricated layer was colorless and transparent, and it absorbed ultraviolet (UV) light with a wavelength less than 350 nm. When organosilane thin films were placed in contact with the nanosheet layer and UV light was irradiated to the organosilane thin films through the transparent nanosheet layer, the wettability of organosilane layers was drastically converted from hydrophobic to ultrahydrophilic. The highly crystalline nanosheet layer was found to exhibit excellent photocatalytic properties.

Direct fabrication and nitridation of a high-quality NaTaO3 crystal layer onto a tantalum substrate

High-quality nano-/microtextured NaTaO3 crystal layers were successfully fabricated on Ta substrates at a relatively low temperature using molten NaNO3 as the starting material. Ta substrates coated with an aqueous NaNO3 solution were heated at 500 °C in an infrared heating furnace, whereupon Ta reacted with NaNO3 (which acted as both the Na source and the flux) to afford a layer of densely packed cubic NaTaO3 crystals that adhered readily onto the substrate. Nitridation of the NaTaO3 crystal layer by heating at 850 °C under an NH3 flow yielded a Ta3N5 crystal layer. The crystals retained their original size and shape but became highly porous after nitridation. TEM observations clearly indicated that the porous cubic crystals consisted of highly crystalline nanoparticles.

Novel fabrication of NIR-vis upconversion NaYF4:Ln (Ln = Yb, Er, Tm) crystal layers by a flux coating method

High quality upconverting NaYF4:Ln (Ln = Yb, Er, Tm) crystal layers were successfully fabricated directly onto glass substrates at a relatively low temperature of 350 °C by a novel two-step flux coating method. First, mixed pastes of solute (NaF and YF3) and dopants (LnF3) were coated on glass substrates by simple bar-coating, and then flux pastes (NaNO3 or NaNO3–NaF) were coated onto the dopant-containing solute layer. After the two-step coating, raw material-coated substrates were heated at 350 or 400 °C in an electric furnace. To remove the remaining flux, the crystal layers were washed with warm water. Finally, high quality, nanotextured NaYF4:Ln crystal layers with good adhesion were directly grown onto the glass substrates. These crystal layers consisted of densely packed idiomorphic NaYF4:Ln crystals. In particular, individual NaYF4:Ln crystals were typically shaped as hexagonal prisms. The NaYF4:Ln crystal phases depended strongly on the heating conditions, i.e., temperature and time. The NaNO3 and NaNO3–NaF fluxes were effective for the growth of high-quality idiomorphic NaYF4:Ln crystals. Additionally, the upconversion fluorescence properties of NaYF4:Ln crystal layers were controlled by changing dopant types and ratios. The NaYF4:10%Yb,1%Er, NaYF4:50%Yb,1%Er, NaYF4:20%Yb,1%Tm and NaYbF4:1%Er crystal layers emitted green, orange, blue and red fluorescences, respectively, under 980 nm laser irradiation.

Highly crystalline niobium oxide converted from flux-grown K4Nb6O17 crystals

Highly crystalline niobium oxide (Nb2O5) nanotubes without defects such as bent and node were successfully prepared by a two-step process. The first step entails making high quality, layered K4Nb6O17 crystals as a precursor material. In this study, well-developed, highly crystalline, layered K4Nb6O17 crystals were readily grown by the rapid cooling of a KCl flux at a holding temperature of 800 °C and a cooling rate of 300 °C h−1. The grown layered crystals of K4Nb6O17 were transparent-colorless and had a median diameter of 530 nm. They were plate-like with well-developed faces. The second step is to transform the layered K4Nb6O17 crystals into highly crystalline Nb2O5nanotubes. In order to make the nanotubes, an intercalation–exfoliation process using tetra(n-butyl)ammonium hydroxide (TBA+OH−) aqueous solution was carried out, and highly crystalline Nb2O5nanotubes having a uniform diameter were successfully fabricated in this medhod. The crystallinity, uniformity and size (diameter and length) of nanotubes were significantly dependent on those of the precursor crystals. The flux-grown crystals, therefore, played a very important role in the nanotube fabrication. The average length and outer diameter were, respectively, about 100–500 nm and 15–25 nm. The photocatalytic properties of the layered K4Nb6O17 crystals and the Nb2O5nanotubes were basically almost the same, although their Brunauer–Emmett–Teller (BET) surface areas were quite different from each other. The BET surface area of the Nb2O5nanotubes (108.71 m2 g−1) was ca 20 times larger than that of the layered K4Nb6O17 crystals (5.14 m2 g−1). As compared with the flux-grown K4Nb6O17 crystals, the Nb2O5nanotubes exhibited high photocatalytic activity for the photodegradation of trichloroethylene. The grown layered K4Nb6O17 crystals and Nb2O5nanotubes were investigated thoroughly by means of field emission scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction analysis, energy-dispersive X-ray spectrometry, BET surface area and pore size distribution analysis, and spectrophotometry.

A novel flux coating method for the fabrication of layers of visible-light-responsive Ta3N5 crystals on tantalum substrates

Layers of well-developed crystals of Ta3N5 were successfully fabricated on Ta substrates by using a novel flux coating method in a flow of NH3. The flux coating method is a simple one: the Ta substrates were coated with aqueous solutions of sodium compounds (= fluxes) and subsequently heated in a flow of NH3, whereupon the surfaces of the Ta substrates were dissolved in the flux, resulting in Ta3N5 crystal layers. The Ta in Ta3N5 was provided by the substrate. Therefore, crystal layers with good adhesion could be grown directly on the substrates. The shape of the individual crystals as well as the surface morphology of the layers formed was determined by the flux used. The crystals fabricated using NaCl–Na2CO3 as the flux were prismatic and had relatively smooth faces, covering the surface of the Ta substrate uniformly. The crystal growth field resulting from the use of this method yielded well-formed crystals, which presumably grew from a solution. Finally, it was confirmed that a thus-synthesized Ta3N5 crystal layer modified using Co-Pi as the co-catalyst generated a photoanodic current under visible-light irradiation.

Growth of ultralong potassium titanate whiskers by the KCl flux method with metallic titanium materials

Well-formed, highly crystalline potassium titanate whiskers were successfully grown by the KCl flux cooling method at a holding temperature of 800 °C using metallic titanium materials. Of primary importance were the metallic Ti spheres and potassium chloride powders that were used as the starting materials for growth of the titanate whiskers. Ultralong K2Ti6O13 whiskers grew radially from the center of the Ti spheres by our flux technique. Cross-sectional SEM images demonstrate that the ultralong whiskers were grown on the TiO2 crystal layer, and the whisker spheres had a bilayer structure that was hollow in the center. When Ti powders were used as the starting material, spherical aggregation of K2Ti6O13 and K2Ti4O9 whiskers occurred. The product structures were different from those obtained from metallic Ti spheres. Based on SEM, EDS and XRD, the formation mechanism of the ultralong whiskers was also discussed. Furthermore, the photocatalytic activity of the whiskers was confirmed by trichloroethylene degradation under ultraviolet light irradiation.

The growth of highly crystalline, idiomorphic potassium titanoniobate crystals by the cooling of a potassium chloride flux

Highly crystalline, well-developed potassium titanoniobate (KTiNbO5 and KTi3NbO9) crystals were successfully grown at temperatures ranging from 700 to 1000 °C by a KCl flux cooling method. Colourless and transparent KTiNbO5 crystals with a smallest average size of 750 nm were grown at a relatively low temperature of 700 °C. The sizes of grown crystals were observed to greatly dependent on the holding temperature and solute concentration. HRTEM observations indicated that the flux-grown KTiNbO5 crystals were of high quality. The band gap energies of the fabricated KTiNbO5 and KTi3NbO9 crystals, as estimated from their absorption edges in UV-vis diffuse reflectance spectra, were approximately 3.54 and 3.35 eV, respectively. Furthermore, these crystals exhibited good photocatalytic activities under UV light irradiation. The photodecomposition of TCE gas occurred via a photocatalytic process under UV light irradiation. This study demonstrates that KCl flux cooling is an environmentally friendly and nature-mimetic process for growing high-quality, well-developed, and photocatalytic KTiNbO5 and KTi3NbO9 crystals.

Epitaxial growth of orthorhombic NaTaO3 crystals on SrTiO3 (100) surface by flux coating

Orthorhombic NaTaO3 crystals were epitaxially grown on a SrTiO3 (100) surface using flux coating. The selected-area electron diffraction results revealed that the well-oriented orthorhombic NaTaO3 crystals and SrTiO3 had an orientation relationship of (10−1)[010] NaTaO3//(100)[010] SrTiO3.

In-situ observations of flux growth of NaTaO3 crystals on tantalum substrate

The growth of NaTaO3 crystals through the reaction between NaNO3 and a Ta substrate was directly observed using in-situ laser microscopy and in-situ X-ray diffraction. Voids and creases were first produced at ∼430 °C and then NaTaO3 crystals grew at ∼440 °C in a moment. Because the NaTaO3 crystals had formed during heating, the driving force for crystal growth was concluded to be the evaporation of NaNO3.