Akimasa Tasaka

Electrochemically constructed p-Cu2O/n-ZnO heterojunction diode for photovoltaic device

Polycrystalline n-ZnO/p-Cu2O heterojunctions have been fabricated by low-temperature eletrodepositions of ZnO and Cu2O layers in aqueous solutions. The condition for forming the Cu2O layer significantly reflected the electrical rectification characteristic and the photovoltaic performance, and the heterojunction fabricated under optimized conditions showed an excellent electrical rectification characteristic and a photovoltaic performance of 1.28% in conversion efficiency under an AM 1.5 illumination.

Impedance Study on the Electrochemical Lithium Intercalation into Natural Graphite Powder

Electrochemical lithium intercalation into natural graphite powder of different sizes was studied by alternating current impedance spectroscopy. Impedance spectra at various potentials were fitted with a modified Randles equivalent circuit including a pseudocapacitance to express the observed finite diffusional behavior. The variations of electrochemical parameters with electrode potential, such as the charge-transfer resistance, the pseudocapacitance, the Warburg prefactor, and, finally, the chemical diffusion coefficient of lithium ion within graphite, were evaluated and discussed. It was shown that the charge-transfer reaction takes place on the whole surface of graphite particles, whereas lithium ion is intercalated from the edge plane and diffuses to the interior. The kinetics of the charge-transfer reaction was independent of the structure of the host. In contrast, the diffusivity of lithium ion within graphite was strongly dependent on the host structure, and the dependence was explained in terms of differences in in-plane and stacking order of lithium-graphite intercalation compounds formed by the intercalation.

Effect of Agglomeration of Pt/C Catalyst on Hydrogen Peroxide Formation

Various amounts of 20 wt % Pt/C catalysts (56.7-5.7 μg c a r b o n cm - 2 ) were loaded on glassy carbon (GC) disk electrode, and the effect of agglomeration on hydrogen peroxide formation in oxygen reduction was investigated by the rotating ring-disk electrode technique. The formation of H 2 O 2 was enhanced with a decrease in agglomeration of Pt/C. Even in the operating potential range of polymer electrolyte fuel cell cathodes (0.6-0.8 V), 10% hydrogen peroxide was formed at 5.7 μg c a r b o n cm - 2 Pt/C loaded on GC. These results revealed that series two-electron reduction pathway, which is negligible on clean bulk Pt surface, does exist on Pt particles supported on carbon.

Raman study of layered rock-salt LiCoO2 and its electrochemical lithium deintercalation

Unpolarized and polarized Raman spectra (200–800 cm-1) of LiCoO2 with a layered rock-salt structure were measured. The Raman-active lattice modes of LiCoO2 were assigned by polarized Raman measurements of a c-axis oriented thin film. The variation of the Raman spectra of Li1-xCoO2 powder prepared by electrochemical lithium deintercalation was investigated, and the spectral changes were well correlated with the structural changes determined by x-ray diffraction except that peak splitting by the distortion in the monoclinic phase was not observed. The observed line broadening of the second hexagonal phase and the monoclinic phase indicated that the lithium ions remaining in the lattice after deintercalation randomly occupy the available sites on the lithium planes in the lattice the layered rock-salt structure.

Structural and Electrical Characterizations of Electrodeposited p-Type Semiconductor Cu2O Films

The p-type semiconductor cuprous oxide (Cu 2 O) film has been of considerable interest as a component of solar cells and photodiodes due to its bandgap energy of 2.1 eV and high optical absorption coefficient. We prepared Cu 2 O films on a conductive substrate by electrodeposition at 318 K from an aqueous solution containing copper sulfate and lactic acid. The structural and electrical characterizations of the resulting films were examined by X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption measurements, and the Hall effect measurement, respectively. The resistivity varied from 2.7 × 10 4 to 3.3 X 10 6 Ω cm, while the carrier density was from 10 1 2 to 10 1 4 cm - 3 and the mobility from 0.4 to 1.8 cm 2 V - 1 s - 1 , depending on the preparation conditions, i.e., solution pH and deposition potential. The carrier density was sensitive to the atomic ratio of Cu to O in the films and the mobility to the grain size.

Photochemical Construction of Photovoltaic Device Composed of p-Copper(I) Oxide and n-Zinc Oxide

The photochemical deposition of a p-type semiconducting copper(I) oxide (Cu 2 O) layer by irradiating visible light in an aqueous solution containing a lactic acid and copper sulfate hydrate is presented. The p-Cu 2 O layer was photochemically stacked on a chemically deposited n-type semiconducting ZnO layer/quartz glass substrate, and the photovoltaic device was constructed by forming a Au top electrode using a sputtering technique. The Au/p-Cu 2 O/n-ZnO photovoltaic device showed an electrical rectification and the performance of 46 mV in open-circuit voltage (V oc ) and 75 μA cm -2 in short-circuit current density (J sc ) under an Air Mass 1.5 illumination.

Preparation of Core/Shell and Hollow Nanostructures of Cerium Oxide by Electrodeposition on a Polystyrene Sphere Template

Core/shell nanostructures of polystyrene (PS)/CeO2 have been prepared on conductive glass substrates by using a novel electrochemical route consisting of (i) the electrophoretic deposition of a PS sphere monolayer on the substrate and (ii) the following potentiostatic electrodeposition of CeO2 on the PS sphere template in Ce(NO3)3 aqueous solutions. The structural morphologies of the deposit changed drastically depending on the Ce(NO3)3 concentration; i.e., spherical and needlelike shells were deposited. The deposit was formed only on the PS sphere surface because of an interaction between cationic cerium species and a sulfate group that was immobilized on the PS sphere surface. The spherical shell layer was assigned as CeO2, and the needlelike shells were composed of Ce(OH)3 needles formed on the CeO2 layer surface, indicating that the deposit species changes from CeO2 to Ce(OH)3 during electrodeposition only in a 1 mM Ce3+ solution. Deposition of Ce(OH)3 would begin when electrogenerated hydrogen peroxide was consumed by decomposition under reductive conditions and could no longer oxidize Ce3+ ions. The corresponding CeO2 hollow shells were obtained by thermal elimination of the PS sphere core and transformation of Ce(OH)3 into CeO2 while keeping their original shapes.

Growth rate of yttria-stabilized zirconia thin films formed by electrochemical vapour-deposition using NiO as an oxygen source II. Effect of the porosity of NiO substrate

Abstract Yttria-stabilized zirconia (YSZ) thin films were formed at 1000°C by a modified electrochemical vapour-deposition (EVD) using NiO as an oxygen source, and ZrCl4 and YCl3 as metal sources. Growth rate kinetics were examined using NiO pellet substrates with different pore structures. The thickness of YSZ film increased linearly with deposition time, and the growth rate increased with increasing the porosity of the substrate. The pore size as well as the porosity affected the growth rate. In addition, the observed growth rate was much slower than the theoretical one assuming that the electrochemical transportation of the charged species across the growing film is rate limiting. From these results, it was concluded that the rate-determining step is not the bulk electrochemical transport, but the mass transport of dissociated oxygen in the substrate pore.

Hydrogen oxidation on partially immersed Nafion®-coated electrodes

The mass transport of hydrogen on partially immersed Nafion®-coated electrodes (Nafion® ¦Pt¦highly oriented pyrolytic graphite) was investigated as a model of the three-phase region of the gas-diffusion electrodes in proton-exchange membrane fuel cells. On raising the electrode from a position at which the Nafion® coating was fully immersed in the electrolyte solution (h = 0 mm), the dependence of the H2 oxidation current on h was similar to that reported for a partially immersed flat platinum electrode. In contrast, on raising from a partially immersed position (h = 3 mm), the current was independent of h, and higher than that when raising from h = 0 mm at a given Nafion® thickness. When raising from h = 3 mm, the Nafion® coating above the intrinsic meniscus was not covered with a thin liquid electrolyte film, but functioned solely as a supermeniscus. This made hydrogen diffusion easier, resulting in the observed higher current. From the above results, useful implications for the design of gas-diffusion electrodes in PEMFCs are demonstrated.

Plasma etching of SiC surface using NF3

NF3 was applied in the reactive ion etching of SiC. The effects of rf power and NF3 pressure on the etching rate and the surface morphology were investigated by means of scanning electron microscopy and atomic force microscopy. A procedure for getting the smooth and residue-free etched surface of SiC with a high etching rate of 87 nm/min was obtained under the conditions such as rf power of 100 W and NF3 pressure ranging from 0.5 to 1 Pa. A rough surface with spikes was obtained under the NF3 pressures higher than 3 Pa. It was found that the repetitive alternating treatment for the spike-formed and rough surface with the down flow etching using NF3 and Ar plasma sputtering enables us to obtain the smooth surface within the scale of ∼300 nm.