Yosuke Goto

Flux-mediated doping of SrTiO3 photocatalysts for efficient overall water splitting

SrTiO3 is a photocatalyst that is well known for its activity for the overall water splitting reaction under UV light irradiation. In this study, the effects of SrCl2 flux treatments and Al doping on the photocatalytic properties of SrTiO3 were investigated. The SrTiO3, which showed an apparent quantum efficiency of 30% at 360 nm in the overall water splitting reaction, the highest value reported so far, was prepared by SrCl2 flux treatments in alumina crucibles. Scanning electron microscopy and X-ray diffractometry revealed that the flux-treated SrTiO3 consisted of well-crystalline particles with a cubic shape reflecting the perovskite-type structure. Inductively coupled plasma optical emission spectroscopy revealed that Al ions from the alumina crucibles were incorporated into the SrTiO3 samples. The SrTiO3 that was treated with SrCl2 flux in Al-free conditions showed a marginal improvement in photocatalytic activity despite the high crystallinity and the clear crystal habit. Doping SrTiO3 with Al improved the photocatalytic activity even without SrCl2 treatment. These results suggested that Al doping was a principal factor in the dramatic improvement in the water splitting activity of the flux-treated SrTiO3. The effects of flux treatments and Al doping on the morphology and water splitting activity of SrTiO3 were discussed separately.

From kesterite to stannite photovoltaics: Stability and band gaps of the Cu2(Zn,Fe)SnS4 alloy

Kesterite semiconductors, particularly Cu2ZnSnS4 (CZTS), have attracted attention for thin-film solar cells. We investigate the incorporation of Fe into CZTS to form the Cu2(Zn,Fe)SnS4 solid-solution for tuning the lattice spacing and band gap. First-principles calculations confirm a phase transition from kesterite (Zn-rich) to stannite (Fe-rich) at Fe/Zn ∼ 0.4. The exothermic enthalpy of mixing is consistent with the high solubility of Fe in the lattice. There is a linear band-gap bowing for each phase, which results in a blue-shift of photo-absorption for Fe-rich alloys due to the confinement of the conduction states. We propose compositions optimal for Si tandem cells.

New On-Chip De-Embedding for Accurate Evaluation of Symmetric Devices

For the millimeter-wave wireless transceivers, miniaturized on-chip passive devices are employed to increase wireless communication speed. For using miniaturized devices, it is necessary to evaluate test vehicles in advance, in which de-embedding is applied to on-chip evaluation. Although open-short de-embedding is currently most popular, accurate de-embedding is difficult because the ground plane in a short dummy pattern is not ideal in practice. To overcome this problem, we have proposed a new de-embedding method using only a through dummy pattern, called the through-only de-embedding method. By this through-only de-embedding method, we show that a small on-chip inductor with more than 100 pico-henries can be evaluated within 1.18% error.

Enhancement of thermoelectric properties by Se substitution in layered bismuth-chalcogenide LaOBiS2-xSex

We have investigated the thermoelectric properties of the novel layered bismuth chalcogenides LaOBiS2-xSex. The partial substitution of S by Se produced the enhancement of electrical conductivity (metallic characteristics) in LaOBiS2-xSex. The power factor largely increased with increasing Se concentration. The highest power factor was 4.5 μW/cmK2 at around 470 °C for LaOBiS1.2Se0.8. The obtained dimensionless figure-of-merit (ZT) was 0.17 at around 470 °C in LaOBiS1.2Se0.8.

Effect of Sn-Substitution on Thermoelectric Properties of Copper-Based Sulfide, Famatinite Cu3SbS4

Copper-based sulfide is an attractive material for Earth-abundant thermoelectrics. In this study, we demonstrate the effect of Sn-substitution on the electrical and thermal transport properties of fematinite Cu3SbS4 from 300 to 573 K. The carrier concentration is controlled in the range from 4 × 1018 to 8 × 1020 cm−3 by Sn-substitution. The density-of-states effective mass is found to be ∼3.0 me, assuming the single parabolic band model. The direct-type optical band gap is ∼0.9 eV, which is consistent with the density functional theory calculation. The dimensionless figure of merit reaches 0.1 for Sn-doped samples at 573 K.

Crystal Structure, Electronic Structure, and Photocatalytic Activity of Oxysulfides: La2Ta2ZrS2O8, La2Ta2TiS2O8, and La2Nb2TiS2O8

The novel oxysulfides La2Ta2ZrS2O8 (LTZSO), La2Ta2TiS2O8 (LTTSO), and La2Nb2TiS2O8 (LNTSO) were synthesized, and their crystal structures, electronic structures, and photocatalytic activities for water splitting under visible light were investigated. Density functional theory calculations showed that these compounds are direct-band-gap semiconductors. Close to the conduction band minimum, the main contribution to the band structure comes from the d orbitals of Zr or Ti ions, while the region near the valence band maximum is associated with the 3p orbitals of S ions. The absorption-edge wavelength was determined to be 540 nm for LTZSO and 700 nm for LTTSO and LNTSO. An analysis of the crystal structure using synchrotron X-ray diffraction revealed that these compounds contained antisite defects at transition metal ion sites, and these were considered to be the origin of the broad absorption at wavelengths longer than that corresponding to band-gap excitation. LTZSO was revealed to be active in the oxygen evolution reaction from aqueous solution containing a sacrificial electron acceptor under visible-light illumination. This result was supported by the band alignment and flat-band potential determined by photoelectron spectroscopy and Mott-Schottky plots.

Electrical and thermal transport of layered bismuth-sulfide EuBiS 2 F at temperatures between 300 and 623 K

We demonstrate the electrical and thermal transport of the layered bismuth-based sulfide EuBiS2F from 300 to 623 K. Although significant hybridization between Eu 4f and Bi 6p electrons was reported previously, the carrier transport of the compound is similar to that of F-doped LaBiS2O, at least above 300 K. The lattice thermal conductivity is lower than that of isostructural SrBiS2F, which is attributed to the heavier atomic mass of Eu ions than that of Sr ions.

The Electronic Structure of Structurally Strained Mn3O4 Postspinel and the Relationship with Mn3O4 Spinel

We report the electronic structure of Mn3O4 postspinel for the first time. In contrast with Mn3O4 spinel, Mn3O4 postspinel adopts a CaMn2O4-type structure with a built-in strain. We conducted both optical measurements and ab initio calculations, and systematically studied the electronic band structures of both the postspinel and spinel Mn3O4 phases. The theoretical electronic structure of Mn3O4 postspinel is consistent with the optical absorption spectra, and both Mn3O4 postspinel and spinel display characteristic band-splitting of the conduction band. The band gap of the postspinel phase is 0.9–1.3 eV smaller than that of the spinel phase. This difference can be explained by the lowering of Mn3+ 3d eg level related to the built-in strain of the postspinel structure. The Mn 3d \(t_{2g}\) and O 2p form antibonding orbitals situated at the conduction band with higher energy.

A particulate (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 photocathode modified with CdS and ZnS for sunlight-driven overall water splitting

A photocathode prepared using the photocatalyst (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 in powder form, and modified with CdS/ZnS overlayers and a Pt catalyst, exhibits a photocurrent of 4.3 mA cm−2 at 0 VRHE under AM 1.5G simulated sunlight. The photocathode was capable of utilizing photons up to 900 nm, while the onset potential was evaluated to be as high as 0.8 VRHE. Modification with thin CdS and ZnS layers significantly increases the cathodic photocurrent through the formation of a surface p–n junction. The half-cell solar-to-hydrogen conversion efficiency of 0.83% at 0.33 VRHE obtained from this device is among the highest yet reported for a photocathode fabricated from a particulate photocatalyst. A solar to hydrogen conversion efficiency of 0.60% was observed during bias-free overall water splitting using this photocathode together with a BiVO4 photoanode. The present work clearly demonstrates the possibility of efficient hydrogen generation using durable particulate semiconductor photoelectrodes.

Electronic structure and transport properties of Cu-deficient kuramite Cu3−xSnS4

Electrical and thermal transport properties of Cu-deficient kuramite Cu3−xSnS4 (CTS) was examined as a possible earth-abundant thermoelectric material. Crystallographic structure of CTS was characterized by partial disorder between Cu and Sn. In contrast to semiconducting electrical transport of related compounds, such as Cu2ZnSnS4 and Cu3SbS4, metallic conduction with an electrical resistivity of 0.4 mΩ cm and a carrier concentration of 3 × 1021 cm−3 was observed at 300 K. Lattice thermal conductivity was calculated at ~2.6 W m−1 K−1, which was probably reduced by Cu-deficiency and/or partial cation disorder. Density functional theory calculation indicates valence band was composed of hybridization between Cu 3d orbitals and S 3p orbitals.