Tsutomu Minegishi

Vertically Aligned Ta3N5 Nanorod Arrays for Solar‐Driven Photoelectrochemical Water Splitting

A vertically aligned Ta(3)N(5) nanorod photoelectrode is fabricated by through-mask anodization and nitridation for water splitting. The Ta(3)N(5) nanorods, working as photoanodes of a photoelectrochemical cell, yield a high photocurrent density of 3.8 mA cm(-2) at 1.23 V versus a reversible hydrogen electrode under AM 1.5G simulated sunlight and an incident photon-to-current conversion efficiency of 41.3% at 440 nm, one of the highest activities reported for photoanodes so far.

Surface Modification of CoOx Loaded BiVO4 Photoanodes with Ultrathin p-Type NiO Layers for Improved Solar Water Oxidation

Photoelectrochemical (PEC) devices that use semiconductors to absorb solar light for water splitting offer a promising way toward the future scalable production of renewable hydrogen fuels. However, the charge recombination in the photoanode/electrolyte (solid/liquid) junction is a major energy loss and hampers the PEC performance from being efficient. Here, we show that this problem is addressed by the conformal deposition of an ultrathin p-type NiO layer on the photoanode to create a buried p/n junction as well as to reduce the charge recombination at the surface trapping states for the enlarged surface band bending. Further, the in situ formed hydroxyl-rich and hydroxyl-ion-permeable NiOOH enables the dual catalysts of CoO(x) and NiOOH for the improved water oxidation activity. Compared to the CoO(x) loaded BiVO4 (CoO(x)/BiVO4) photoanode, the ∼6 nm NiO deposited NiO/CoO(x)/BiVO4 photoanode triples the photocurrent density at 0.6 V(RHE) under AM 1.5G illumination and enables a 1.5% half-cell solar-to-hydrogen efficiency. Stoichiometric oxygen and hydrogen are generated with Faraday efficiency of unity over 12 h. This strategy could be applied to other narrow band gap semiconducting photoanodes toward the low-cost solar fuel generation devices.

Stable Hydrogen Evolution from CdS-Modified CuGaSe2 Photoelectrode under Visible-Light Irradiation

The photoelectrochemical properties of CuGaSe2 modified by deposition of a thin CdS layer were investigated. The CdS layer formed a p-n junction on the surface of the electrode, improving its photoelectrochemical properties. There was an optimal CdS thickness because of the balance between the charge separation effect and light absorption by CdS. CdS-deposited CuGaSe2 showed high stability under the observed reaction conditions and evolved hydrogen continuously for more than 10 days.

Pt/In2S3/CdS/Cu2ZnSnS4 Thin Film as an Efficient and Stable Photocathode for Water Reduction under Sunlight Radiation.

An electrodeposited Cu2ZnSnS4 (CZTS) compact thin film modified with an In2S3/CdS double layer and Pt deposits (Pt/In2S3/CdS/CZTS) was used as a photocathode for water splitting of hydrogen production under simulated sunlight (AM 1.5G) radiation. Compared to platinized electrodes based on a bare CZTS film (Pt/CZTS) and a CZTS film modified with a CdS single layer (Pt/CdS/CZTS), the Pt/In2S3/CdS/CZTS electrode exhibited a significantly high cathodic photocurrent. Moreover, the coverage of the In2S3 layer was found to be effective for stabilization against degradation induced by photocorrosion of the CdS layer. Bias-free water splitting with a power conversion efficiency of 0.28% was achieved by using a simple two-electrode cell consisting of the Pt/In2S3/CdS/CZTS photocathode and a BiVO4 photoanode.

Photoelectrochemical properties of LaTiO2N electrodes prepared by particle transfer for sunlight-driven water splitting

Photoelectrochemical (PEC) water splitting is a promising technology for the production of renewable solar hydrogen from water. A method of fabricating photoelectrodes from semiconductor particles for efficient water splitting was investigated using LaTiO2N as a test case. When a Ti conductor electrode with the back side covered with LaTiO2N particles was constructed by radio-frequency magnetron sputtering, the LaTiO2N photoelectrode generated a remarkable photoanodic current and evolved O2. The insertion of a Ta or Nb interlayer between the Ti conductor and the LaTiO2N particles enhanced the photocurrent. This method of fabricating photoelectrodes from semiconductor particles enables the use of simple powder semiconductors in solar energy conversion systems.

H2 Evolution from Water on Modified Cu2ZnSnS4 Photoelectrode under Solar Light

Cu2ZnSnS4 (CZTS) thin films were investigated as photoelectrodes for H2 evolution from water under solar light. Surface modifications with Pt, CdS, and TiO2 enhanced the incident photon-to-current conversion efficiency of CZTS thin film electrode for H2 production from water by three orders of magnitude to 40% (λ= 600 nm). The solar energy conversion efficiency in H2 production using modified CZTS reached 1.2%. Our results demonstrate that the photoelectrochemical properties of CZTS were significantly improved by surface modification, which suggests the feasibility of CZTS as a photocathode for a photoelectrochemical water-splitting system.

Enhancement of Solar Hydrogen Evolution from Water by Surface Modification with CdS and TiO2 on Porous CuInS2 Photocathodes Prepared by an Electrodeposition–Sulfurization Method

Porous films of p-type CuInS2, prepared by sulfurization of electrodeposited metals, are surface-modified with thin layers of CdS and TiO2. This specific porous electrode evolved H2 from photoelectrochemical water reduction under simulated sunlight. Modification with thin n-type CdS and TiO2 layers significantly increased the cathodic photocurrent and onset potential through the formation of a p-n junction on the surface. The modified photocathodes showed a relatively high efficiency and stable H2 production under the present reaction conditions.

Photoelectrochemical oxidation of water using BaTaO2N photoanodes prepared by particle transfer method.

A photoanode of particulate BaTaO2N fabricated by the particle transfer method and modified with a Co cocatalyst generated a photocurrent of 4.2 mA cm(-2) at 1.2 V(RHE) in the photoelectrochemical water oxidation reaction under simulated sunlight (AM1.5G). The half-cell solar-to-hydrogen conversion efficiency (HC-STH) of the photoanode reached 0.7% at 1.0 V(RHE), which was an order of magnitude higher than the previously reported photoanode made from the same material. The faradaic efficiency for oxygen evolution from water was virtually 100% during the reaction for 6 h, attesting to the robustness of the oxynitride.

Structural variation of cubic and hexagonal MgxZn1−xO layers grown on MgO(111)∕c-sapphire

We report on the structure study of MgxZn1−xO films and, in particular, we will focus on MgxZn1−xO layers with x=0.28 and 0.41 MgxZn1−xO layers with different crystal structures of cubic and wurtzite that have been grown by plasma-assisted molecular-beam epitaxy on MgO∕c-sapphire with Mg∕Zn flux ratio control. The MgxZn1−xO films have been characterized by high-resolution transmission electron microscopy (HRTEM) and high-resolution x-ray diffraction. The dependence of the cation-anion bond length to Mg content has been studied. A virtual crystal model of MgZnO has been applied to interpret the bond-length variation. HRTEM results indicate that the initial stage of the MgZnO growth on a MgO buffer layer starts with a cubic structure even in the case of a wurtzite structure at the end of growth.

Mg–Zr Cosubstituted Ta3N5 Photoanode for Lower-Onset-Potential Solar-Driven Photoelectrochemical Water Splitting

In p/n photoelectrochemical (PEC) cell systems, a low onset potential for the photoanode, as well as a high photocurrent, are critical for efficient water splitting. Here, we report a Mg-Zr cosubstituted Ta3N5 (Ta3N5:Mg+Zr) photoanode, designed to provide a more negative onset potential for PEC water splitting. The anodic photocurrent onset on Ta3N5:Mg+Zr was 0.55 V(RHE) under AM 1.5G-simulated sunlight, which represented a negative shift from the ca. 0.8 V(RHE) for pure Ta3N5. This negative shift in the onset potential of PEC water splitting was attributed to the change in the bandgap potential due to partial substitution by the foreign ions Mg(2+) and/or Zr(4+).