Mitsutake Oshikiri

Nano‐photocatalytic Materials: Possibilities and Challenges

Semiconductor photocatalysis has received much attention as a potential solution to the worldwide energy shortage and for counteracting environmental degradation. This article reviews state-of-the-art research activities in the field, focusing on the scientific and technological possibilities offered by photocatalytic materials. We begin with a survey of efforts to explore suitable materials and to optimize their energy band configurations for specific applications. We then examine the design and fabrication of advanced photocatalytic materials in the framework of nanotechnology. Many of the most recent advances in photocatalysis have been realized by selective control of the morphology of nanomaterials or by utilizing the collective properties of nano-assembly systems. Finally, we discuss the current theoretical understanding of key aspects of photocatalytic materials. This review also highlights crucial issues that should be addressed in future research activities.

A novel hydrogen-evolving photocatalyst InVO4 active under visible light irradiation

Abstract InVO4, with band gap of about 2.0 eV, was found to be a new visible light responding photocatalyst for water decomposition. The photocatalyst showed activity to visible light in a wide wavelength range up to 600 nm. Although the native photocatalyst could evolve H2 from pure water under visible light irradiation (λ>420 nm ) , the photocatalytic activity increases significantly by loading NiO as a co-catalyst. Correlation of the photocatalytic properties with crystal and electronic structure of the compound is discussed in connection with the recently reported 4d and 5d transition metal photocatalysts InNbO4 and InTaO4.

Electronic structures of promising photocatalysts InMO4 (M=V, Nb, Ta) and BiVO4 for water decomposition in the visible wavelength region

The compounds InMO4 (M=V, Nb, Ta) and BiVO4 are promising photocatalysts which are able to induce hydrolysis of water molecules under visible light irradiation. By first principles calculations, supported by experiments, we inspect their peculiar electronic structure in an attempt to rationalize the link between the bulk crystal architecture of the materials and the related electronic properties. We find that the bottom of the conduction band of InMO4 systems consists of a large contribution (about 20%) due to 5s orbitals of In atoms. Another dominant component comes from d orbitals of V, Nb, and Ta. On the other hand, the top of the valence band of the BiVO4 shows a contribution from 6s orbitals of Bi of about 18% as well as a dominant component due to 2p states of O. We can infer that the photocatalytic activity could be improved by the large mobility coming from the s orbital component as well as by tuning the electron affinity (position of the bottom of the conduction band) and ionization potential (t...

Cyclotron resonance in n-type ZnO

Abstract The magnetic field dependence and the temperature dependence of the cyclotron resonance were investigated in n-type ZnO. The observed resonance was assigned as not a conduction electron cyclotron resonance but trapped electron cyclotron resonance from the magnetic field dependence of the resonant energy. The effective mass and the electron–phonon coupling constant were calculated as m * =0.23 m 0 and α =1.2 from the obtained polaron mass, m * pol =0.29 m 0 .

Quasiparticle Energy Calculations on II(Zn)-VI(O, S, Se) and III(Al,Ga)-V(N) Semiconductors in the Wurtzite Structure

Quasiparticle energies of AlN, GaN, ZnO, ZnS and ZnSe in the wurtzite structure have been calculated by the GW approximation with a full random phase approximation dielectric matrix instead of using plasmon pole approximation. The linear muffin tin orbital basis was used for this work and the d orbitals of the Zn and Ga atoms were treated as valence state explicitly in every case. The calculated quasiparticle band gaps and the semicore levels by the GW method are in good agreements with the experimental values. The electronic energy levels of these five systems calculated using the local density, GW and Hartree-Fock approximations were compared systematically.

Water adsorption onto Y and V sites at the surface of the YVO4 photocatalyst and related electronic properties.

The dynamics of water molecules and the adsorption properties at the V and Y sites on the surface of the photocatalyst YVO(4) have been investigated by first principles molecular dynamics. This system has shown an excellent performance in the production of both hydrogen and oxygen in the ultraviolet region. Yet, its catalytic properties, related to the electronic structure, are poorly understood. Here we show that imperfectly oxygen coordinated V sites (i.e., not fourfold oxygen coordinated vanadium but threefold oxygen coordinated vanadium) exposed on the catalyst surface play a central role in the dissociation of water molecules. By simulating the H(2)O adsorption process and by performing an analysis of the electronic structure of the unoccupied orbitals corresponding to the lowest unoccupied energy level of the system, we can infer that the dissociation of water at these imperfectly oxygen coordinated V sites can promote the proton reduction and is expected to trigger the H(2) generation.

Far-infrared cyclotron resonance of wide-gap semiconductors using pulsed high magnetic fields

Abstract A cyclotron resonance experiment for n-ZnO and the ‘camels back’ n-GaP was carried out in pulsed magnetic fields using an optically pumped far-infrared laser as a radiation source. The cyclotron resonance transitions of n-GaP for 0+ → 1−, 0− → 2+ and 0+ → 2− were observed for the first time as well as the usual cyclotron transitions associated with the transverse effective mass mt. The longitudinal effective mass m1 was determined to be ∼ 10mo by the angular dependence of cyclotron resonance at 433 μm. The electron effective mass of n-ZnO was estimated to be approximately 0.3m0 at 394 μm for B ⊥ ( 10 1 ¯ 0 ) surface.

Magnetostriction and Thermal Expansion Measurements of CuGeO3

The magnetostriction and thermal expansion of CuGeO 3 have been measured in magnetic fields of up to 14 T using a capacitance method. An anomalous increase and hysteresis were observed in the magnetostriction at the transition between the spin-Peierls (SP) phase and the magnetic (M) phase, which show that the strain introduced at the transition from the uniform (U) phase to the SP phase is partially removed at the transition from the SP phase to the M phase.

Far-infrared spectroscopy in high magnetic fields

Abstract A far-infrared spectrometer. which can be used in the energy range between 3 and 40 cm −1 , has been constructed to measure magneto-optical spectra with the Tsukuba Magnet Laboratories hybrid magnet system. The spectrometer was examined by measuring ESR of the inorganic spin-Peierls (SP) compound CuGeO 3 with a superconducting magnet instead of the hybrid magnet. ESR lines were observed at steady fields for both the SP phase and the field-induced incommensurate M phase.

Cyclotron Resonance of n-GaP in a Wide Far Infrared Region

The cyclotron resonance of n-GaP, which has the camels back structure in the conduction band around the X point, has been studied at a very wide far infrared region (119, 215, 433, 690, 1500 µm) up to 30 T on an epitaxially grown single crystal. The values of the effective mass band parameter m t , m l , and the camels back height Δ E have been determined by fitting cyclotron resonances around the magnetic break down point as well as in the high energy region to the Landau level energy structure. The k -linear term was added as a perturbation in the calculation. The results show that m t =(0.251±0.005)m 0 , m l =(0.90±0.04)m 0 and Δ E =2.7±0.1 meV.