Shu Saeki

Optical bandgap widening of p-type Cu2O films by nitrogen doping

We have investigated the effect of N doping into Cu2O films deposited by reactive magnetron sputtering. With increasing N-doping concentration up to 3%, the optical bandgap energy is enlarged from ∼2.1 to ∼2.5 eV with retaining p-type conductivity as determined by optical absorption and Hall effect measurements. Additionally, photoelectron spectroscopy in air measurements shows an increase in the valence and conduction band shifts with N doping. These experimental results demonstrate possible optical bandgap widening of p-type N-doped Cu2O films, which is a phenomenon that is probably associated with significant structural changes induced by N doping, as suggested from x-ray diffraction measurements.

Visible light-sensitive mesoporous N-doped Ta2O5 spheres: synthesis and photocatalytic activity for hydrogen evolution and CO2 reduction

Crystallized mesoporous tantalum pentoxide spheres (CMTS) with particle diameters of ca. 100–500 nm and composed of Ta2O5 nanocrystals were synthesized for the first time by a combination of the sol–gel process and heat-treatment with the aid of carbon reinforcement. The specific surface area of the CMTS was up to 105 m2 g−1 and the pore diameter was controllable in the range of 5.6–17 nm by changing the crystallization temperature. Visible light-sensitive p-type N-doped Ta2O5 (N-CMTS) containing 5 at% N was successfully obtained by treatment of CMTS with ammonia, while retaining the mesoporosity and morphology of CMTS. N-CMTS exhibited excellent photocatalytic activity for hydrogen evolution and CO2 reduction (with ruthenium-complex) under visible light irradiation (≥410 nm) due to their larger surface area and controlled morphology compared with previously reported N-doped Ta2O5 fine particles.

Dual functional modification by N doping of Ta2O5: p-type conduction in visible-light-activated N-doped Ta2O5

We report dual functional modulation, both p-type conduction and band gap narrowing, of Ta2O5 semiconductor induced by heavy doping of nitrogen in films sputtered in N2/Ar mixture and ammonia-treated powders. The N doping induced a redshift in the optical absorption edge from 320 to 500 nm, resulting in the absorption of visible light. Simultaneously, the N doping caused a change in the conduction from n-type to p-type. As a result, the N–Ta2O5 photoelectrode containing 7.6 or 16.1 at. % of N exhibited a distinct cathodic photocurrent (due to p-type conduction) in solutions under visible light irradiation (>410 nm).

Application of dry grinding to reduction in transformation temperature of aluminum hydroxides

Abstract Dry grinding of gibbsite (α-Al(OH) 3 ) and pseudo (p)-boehmite (γ-AlO(OH)) powder samples separately, with and without fine α-alumina powder as a seed, was conducted using a planetary ball mill to investigate effects of grinding and addition of the seed on their transformation temperatures to α-phase during heating. The transformation temperatures of both samples reduced to below 1000°C with increasing grinding time as well as the amount of seed. The transformation temperatures depend on the grinding time and the amount of seed, and the minimum temperatures are about 910°C for gibbsite and about 950°C for p-boehmite when they are ground for more than 60 min with an equivalent amount of the seed powder. Combination of grinding with addition of the seed would be an effective operation for the reduction in the transformation temperature of both compounds to α-phase.

Evaluation of photocatalytic activities and characteristics of Cu- or Fe-modified nitrogen-doped titanium dioxides for applications in environmental purification

Photocatalytic activities, as manifested in gas decomposition rates of volatile organic compounds under visible light irradiation, are well known to be enhanced by Cu or Fe compounds loaded onto the surface of nitrogen-doped titanium dioxide (N-TiO2). Antimicrobial properties are also known to be improved by Cu compounds loaded onto N-TiO2. In this study, the acetaldehyde gas decomposition rates and antimicrobial properties achieved by loading Cu or Fe compounds onto N-TiO2 have been investigated from a practical viewpoint. Enhancements of the photocatalytic activities have been confirmed, and the durability of the Cu- or Fe-modified N-TiO2 in acetaldehyde gas decomposition has been demonstrated. The enhancements have been rationalized by considering the valence states of the loaded material. The enhancements were inferred to be a result of increased amounts of oxidative species arising from the photo-Fenton reaction at the surface of the loaded metal compounds. With a view to expanding the range of indoor applications of such visible-light-sensitive photocatalysts, preparation processes, i.e., nanosizing of the powder and improvement of the immobilization on various substrates, are also discussed.

Organic-inorganic hybrid transparent materials containing chromium ion-doped forsterite crystals as gain media

Organic-inorganic hybrid transparent materials containing chromium ion-doped forsterite (Cr:Mg2SiO4) crystals are investigated as gain media. Thin films and bulk samples are prepared by a sol-gel technique. Starting sols for host matrices, which are prepared from diphenyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, and titanium tetra-n-butoxide, are mixed with flame spray made Cr:Mg2SiO4 nanocrystals ultrasonically. The hybrid materials containing Cr:Mg2SiO4 with high transparency are realized by matching refractive indices of the host matrices with that of Cr:Mg2SiO4 by controlling starting sols’ composition. A broad emission centered at 1170nm is confirmed under the excitation at 976nm.

Nitrogen-Doping Induced Optical Bandgap Widening of P -Type Cu 2 O Flms

We have investigated the effect of N doping into Cu 2 O films deposited by reactive magnetron sputtering. With increasing N-doping concentration up to 3 at.%, the optical bandgap energy is enlarged from ˜2.1 to ˜2.5 eV with retaining p-type conductivity as determined by optical absorption and Hall-effect measurements. Additionally, photoelectron spectroscopy in air measurements shows an increase in the valence and conduction band shifts with N doping. These experimental results demonstrate possible optical bandgap widening of p -type N-doped Cu 2 O films, which is a phenomenon that is probably associated with significant structural changes induced by N doping, as suggested from x-ray diffraction measurements.