Masao Katayama

Semiconductor electrodes can catalyze photo-induced redox reactions with light illumination. Photoexcitation produces excited carriers that subsequently transfer to the front and back contacts as determined by the bulk and surface properties of the photoelectrodes. This transfer defines the resultant quasi-Fermi levels of the photo-generated carriers at the photoelectrode surface, which, in turn, impacts the efficiency of surface photoelectrochemical reactions. However, determining such quasi-Fermi levels is not a simple task. In this study, we introduce a method for estimating the quasi-Fermi level of holes using outer-sphere electron transfer reactions. The quasi-Fermi level of holes is estimated by linking the oxidation photocurrent on photoanodes to the separately measured electrode potential on a stable metal electrode. Using this method, the quasi-Fermi level of holes at the surface is monitored in response to variations in applied potential and light intensity. This approach effectively separates the photocurrents of the CdS model electrode between surface redox reaction and photocorrosion, while concurrently quantifying the dynamic quasi-Fermi level at the surface. This work facilitates quantitative understanding of photoelectrochemical reactions on semiconductor electrodes to design green chemical transformation systems.

Valence band maximum (VBM) position does not necessarily indicate the driving force for oxidation reactions on photoanodes because it simply reflects an electrostatic potential. Rather, the quasi-Fermi level of the hole at the surface of photoanodes must be considered. Here, we report a protocol to quantitatively evaluate the quasi-Fermi level of the holes at the surface of photoanodes using redox species with various redox potentials. The quasi-Fermi level of these holes is estimated by correlating the photocurrent derived from redox species oxidation to the separately measured electrode potential on a stable model glassy carbon electrode. Using this protocol, the quasi-Fermi levels of holes on the surface of CdS and CdSe model photoanodes were found to be limited to merely 0.5 V vs. standard hydrogen electrode; this is fairly negative from the VBM position. The estimation of a quasi-Fermi level of the hole and the partial photocurrent density of the CdS and CdSe photocorrosion further enabled predictions of the photocorrosion kinetics. The predicted kinetics of the photocorrosion showed that the quasi-Fermi levels of hole at the surface of these photoanodes are pinned by the facile kinetics of the photocorrosion. This methodology offers a quantitative understanding of the oxidation capability of photoanodes.

Semiconductor photocatalysts for water splitting is considered one of the most promising technologies for solar energy conversion. SrTaO₂N has a band gap of 2.1 eV and is a candidate for achieving visible‐light‐driven water splitting. However, its poor charge separation efficiency limits its water splitting efficiency, which can be attributed to the charge recombination induced by reduced Ta⁴⁺ sites, which act as recombination centers. This study focuses on the improvement of photocatalytic activity of SrTaO₂N by introducing foreign metal cations with a low oxidation state to suppress the formation of reduced Ta⁴⁺ sites. Among the investigated samples, 1.0 mol % Ga³⁺‐doped SrTaO₂N was found to improve the photocatalytic activity significantly. X‐ray photoelectron spectroscopy (XPS) data show that the formation of Ta⁴⁺ species is suppressed after Ga doping, which might be responsible for the photocatalytic activity improvement of SrTaO₂N.

Zinc telluride (ZnTe)-based photocathodes with appropriate modifications showed incident-photon-to-current conversion efficiencies (IPCEs) higher than 30% at 420–520 nm, reached 40% IPCE at 460 nm, and were not zero up to 710 nm because of photoexcitation related to the intermediate-band. ZnTe films were prepared by productive close-spaced sublimation. After surface modification with CdS and Pt, the photocathode showed clear cathodic photoresponse in the phosphate buffer solution. The modulation of substrate temperature, incorporation of Cu, and further surface modification with very thin Mo and Ti layers significantly enhanced cathodic photocurrent which contributes to hydrogen evolution from water. Through the measurement of IPCEs, photoelectrochemical hydrogen evolution from water utilizing photoexcitation related to the intermediate-band was first confirmed. The photocathode showed advantageous durability under simulated sunlight: during irradiation of about 50 min, the stabilized photocurrent slightly decreased from 1.6 mA cm⁻² to 1.5 mA cm⁻².

SnNb₂O₆ is one of the promising photoanode materials for photoelectrochemical water splitting. We prepared the SnNb₂O₆ thin-film photoanode with various Sn/Nb ratios by a pulsed-laser deposition method and investigated impacts of lattice defects in the SnNb₂O₆ photoanode on the water oxidation property. Photoelectrochemical measurements and Raman spectroscopy analyses for the SnNb₂O₆ photoanodes presented that the water oxidation property can be improved by suppressing the formation of the acceptor site caused by the substitution of Sn⁴⁺ for the Nb⁵⁺ site in the crystal lattice of SnNb₂O₆. This study provides material’s design strategy based on defect chemistry to realize an efficient SnNb₂O₆ photoanode used for the photoelectrochemical cell, which can operate without an external bias.

Photoelectrochemical water splitting is regarded as a promising approach to the production of hydrogen, and the development of efficient photoelectrodes is one aspect of realizing practical systems. In this work, transparent Ta₃N₅ photoanodes were fabricated on n‐type GaN/sapphire substrates to promote O₂ evolution in tandem with a photocathode, to realize overall water splitting. Following the incorporation of an underlying GaN layer, a photocurrent of 6.3 mA cm⁻² was achieved at 1.23 V vs. a reversible hydrogen electrode. The transparency of Ta₃N₅ to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe₂ (CIS), which absorbs up to 1100 nm. A stand‐alone tandem cell with a serially‐connected dual‐CIS unit terminated with a Pt/Ni electrode was thus constructed for H₂ evolution. This tandem cell exhibited a solar‐to‐hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.

Photoelectrochemical water splitting is regarded as a promising approach to the production of hydrogen, and the development of efficient photoelectrodes is one aspect of realizing practical systems. In this work, transparent Ta₃N₅ photoanodes were fabricated on n‐type GaN/sapphire substrates to promote O₂ evolution in tandem with a photocathode, to realize overall water splitting. Following the incorporation of an underlying GaN layer, a photocurrent of 6.3 mA cm⁻² was achieved at 1.23 V vs. a reversible hydrogen electrode. The transparency of Ta₃N₅ to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe₂ (CIS), which absorbs up to 1100 nm. A stand‐alone tandem cell with a serially‐connected dual‐CIS unit terminated with a Pt/Ni electrode was thus constructed for H₂ evolution. This tandem cell exhibited a solar‐to‐hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.

The effects of altered morphology and size on the visible light-induced water oxidation activity and photoelectrochemical performance of BaTaO₂N crystal structures were studied.

The processes involved in the nitridation of Sr₂Nb₂O₇ and SrNbO₃ to SrNbO₂N were assessed by varying the nitridation time, and the related effects on the physical and photoelectrochemical properties of the nitrided products were investigated. In the case of the layered perovskite-type oxide Sr₂Nb₂O₇, the introduction of nitrogen and the extraction of oxygen took place concurrently, leading to lattice shrinkage and a porous structure. In contrast, during nitridation of the perovskite-type oxide SrNbO₃, nitrogen was initially introduced without any loss of oxygen, which caused phase separation as a result of a lattice expansion and a charge compensation. The photoelectrochemical properties of obtained SrNbO₂N under simulated sunlight were found to vary with the oxide precursor used and with the nitridation process.