Masahiro Tosa

Photocatalytic generation of hydrogen by core-shell WO 3 /BiVO 4 nanorods with ultimate water splitting efficiency

Efficient photocatalytic water splitting requires effective generation, separation and transfer of photo-induced charge carriers that can hardly be achieved simultaneously in a single material. Here we show that the effectiveness of each process can be separately maximized in a nanostructured heterojunction with extremely thin absorber layer. We demonstrate this concept on WO3/BiVO4+CoPi core-shell nanostructured photoanode that achieves near theoretical water splitting efficiency. BiVO4 is characterized by a high recombination rate of photogenerated carriers that have much shorter diffusion length than the thickness required for sufficient light absorption. This issue can be resolved by the combination of BiVO4 with more conductive WO3 nanorods in a form of core-shell heterojunction, where the BiVO4 absorber layer is thinner than the carrier diffusion length while it’s optical thickness is reestablished by light trapping in high aspect ratio nanostructures. Our photoanode demonstrates ultimate water splitting photocurrent of 6.72 mA cm−2 under 1 sun illumination at 1.23 VRHE that corresponds to ~90% of the theoretically possible value for BiVO4. We also demonstrate a self-biased operation of the photoanode in tandem with a double-junction GaAs/InGaAsP photovoltaic cell with stable water splitting photocurrent of 6.56 mA cm−2 that corresponds to the solar to hydrogen generation efficiency of 8.1%.

Nanostructured WO3/BiVO4 Photoanodes for Efficient Photoelectrochemical Water Splitting

Nanostructured photoanodes based on well-separated and vertically oriented WO3 nanorods capped with extremely thin BiVO4 absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO3 -NRs/BiVO4 photoanode modified with Co-Pi oxygen evolution co-catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm(2) at 1.23 V versus a reversible hydrogen electrode in a stable Na2 SO4 electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO4 layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO3 -NRs/BiVO4 photoanode effectively increases the optical thickness of the BiVO4 layer and results in efficient absorption of the incident light.

Highly packed InGaAs quantum dots on GaAs(311)B

We have fabricated highly packed and ordered In0.4Ga0.6As quantum dots (QDs) array on GaAs(311)B substrate without coalescence of QDs. Reflection high-energy electron diffraction and Auger spectra suggest the inhomogeneous distribution of In and Ga in QD. In concentration near the surface of QD is larger than that of the inside, and the inhomogeneous distribution of In and Ga in QDs prevents QDs from merging.

Photocatalytic Properties of TiO2 Nanostructures Fabricated by Means of Glancing Angle Deposition and Anodization

Structural, optical, and photocatalytic properties of various TiO 2 nanostructures prepared by glancing angle deposition (GLAD) and by electrochemical anodic oxidation of Ti have been studied. The TiO 2 nanorods were prepared on unheated glass substrates by using reactive sputtering of Ti in the GLAD regime. TiO 2 nanotubes and brush-type nanostructures were fabricated by anodic oxidation of flat Ti films and Ti nanorods prepared by GLAD, respectively. The optical studies revealed that the nanotubes and brush-type nanostructures possess antireflection properties. The photocatalytic activity of TiO 2 nanostructures was characterized by following decomposition of isopropanol under visible and UV light irradiation and found to be significantly higher in nanostructured samples than in their flat counterparts. Also, TiO 2 nanotubes and brush-type nanostructures showed superior photocatalytic activity in comparison with nanorods due to a significantly higher specific surface area.

Mechanical properties of hexagonal boron nitride synthesized from film of Cu/BN mixture by surface segregation

Hexagonal boron nitride (h-BN) has a low friction coefficient and weak surface attractive force similar to graphite. Furthermore, while graphite is conductive, BN is a good insulator. These properties make it suitable for application like lubricating coating or as an insulator/buffer layer in electronic devices. The synthesize of h-BN layer by surface segregation phenomena and mechanical properties of the h-BN surface segregated on Cu substrate have been investigated. During in situ annealing, the surface segregation of BN occurred on Cu/BN film deposited by deposition process with a rf magnetron co-sputtering system. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) analysis showed that though the h-BN layer synthesized was not covered whole area of substrate but the h-BN layers partially covered substrate. And the concentration of oxygen on the surface after exposure in air is decreased with increase of BN concentration. The topography of atomic forces microscopy (AFM) showed that h-BN phases surface segregated are discontinuous droplet shape. The force curves of AFM and friction force of lateral force microscopy (LFM) showed that the h-BN droplet surface segregated have very weak attractive force and low friction coefficient equal to h-BN sintered plate.

Surface precipitation of boron nitride on the surface of type 304 stainless steels doped with nitrogen, boron, and cerium

The surface compositions of two types of stainless steels in vacuum were observed with AES and XPS at 1000–1100 K; a 304 stainless steel doped with nitrogen and boron (304‐NB) and a 304 stainless steel doped with nitrogen, boron, and cerium (304‐NBCe). The changes of surface compositions of these steels were compared with that of commercial 304 stainless steel. A thin layer of boron nitride came out from grain boundaries and spread over the surface of the 304‐NB, but did not uniformly cover the surface. The spreading mechanism of the boron nitride layer at the initial stage of heating can be explained by Avrami’s model. In the case of 304‐NBCe, a boron nitride layer almost uniformly covered the surface at the early stage of heating, and rendered the surface inert to the adsorption of carbon and oxygen. Therefore, the 304‐NBCe stainless steel is a candidate material for vacuum vessels.

Hydrogen permeation properties and surface structure of BN-coated stainless steel membrane

Abstract We developed boron nitride-coated stainless steel for vacuum vessel materials, with low adsorption rate of oxygen and carbonates. In extremely high vacuum, of a range 10−10 Pa, the main component of the residual gas is hydrogen. We found a low adsorption rate of hydrogen on the surface of precipitated BN. A precipitated BN layer can also lower the hydrogen permeation rate of a stainless steel membrane. BN-coating has an effect on the reduction of outgassing due to, not only adsorbed hydrogen on the surface of vessel materials, but also contained hydrogen in vessel materials. We successfully coated BN on a stainless steel surface by using surface segregation from BN and the stainless steel co-deposited film. The RHEED pattern of BN showed the diffraction rings and spots of hexagonal-BN which has its C-axis orientated to the surface normal. The image using transmission electron microscopy showed a lattice of h-BN basal plane, and that the diameter of the BN crystals was 50–80 nm. This crystal structure inhibited hydrogen permeation.

Characteristics of thin films of hexagonal boron nitride mixed with copper controlled by a magnetron co-sputtering deposition technique

Thin film properties, such as surface morphology, internal stress and surface energy, of hexagonal boron nitride/copper mixtures, produced by a magnetron co-sputtering deposition technique, were investigated. Techniques included scanning electron microscopy, film deflection and contact angle measurements. It was possible to fabricate nano-scale islands on the surface of the film and to control the size of the islands by adjusting the sputter discharge conditions. The internal stress and surface energy can be significantly modified depending on these conditions indicating that they are dependent variables. This technique of co-sputtering deposition is a promising candidate for the control of nano-structures on a surface and for controlling internal stress and surface energy.

Interface reaction between sputter-deposited Al2O3 and stainless steels

Abstract Alumina films were deposited onto two types of stainless steels, type 304 and type 321, by the ion beam sputtering method. When the steels are heated in a vacuum, titanium carbide precipitates on the surface of type 321 steel whereas sulphur segregates to the surface of type 304. In this work, we studied the mechanism of adhesion of an alumina film to type 321 steel (adhered well) and to type 304 steel (adhered poorly) from the viewpoint of an interfacial reaction between Al2O3 and the stainless steels. X-ray photoelectron spectroscopy (XPS) measurements were made on type 304 and type 321 steels before and after deposition of Al2O3. Moreover changes in the XPS and Auger electron spectra were measured before and after heating the Al2O3-coated stainless steel. The results of Auger and XPS measurements suggest that the precipitated TiC forms Ti—O bonds at the Al2O3-type 321 interface and suppresses the segregation of sulphur which leads to Fe—O bond rupture and causes exfoliation of Al2O3 from stainless steels.

Optical near-field induced visible response photoelectrochemical water splitting on nanorod TiO2

Here we report a way to induce the visible response of non-doped TiO2 in the photocatalytic electrochemical water splitting, which is achieved by utilizing the optical near-field (ONF) generated on nanorod TiO2. The visible response is attributed to the ONF-induced phonon-assisted excitation process, in which TiO2 is excited by sub-bandgap photons via phonon energy. Our approach directly gets involved in the excitation process without chemical modification of materials; accordingly it is expected to have few drawbacks on the photocatalytic performance. This study may offer another perspective on the development of solar harvesting materials.