Interfacial Design of Particulate Photocatalyst Materials for Green Hydrogen Production.

Green hydrogen production using particulate photocatalyst materials has attracted much attention in recent years because this process could potentially lead to inexpensive and scalable solar-to-chemical energy conversion systems. Although the development of efficient particulate photocatalysts enabling one-step overall water splitting (OWS) with solar-to-hydrogen efficiencies in excess of 10 % remains challenging, promising photocatalyst candidates exhibiting OWS activity have been demonstrated. This review provides a comprehensive introduction to the solar-to-hydrogen energy conversion process of semiconductor photocatalyst materials and highlights recent advances in photocatalytic OWS via both one-step and two-step photoexcitation processes. The review also covers recent developments in the photocatalytic OWS of SrTiO3, including the establishment of large-scale photocatalytic systems, interfacial design using cocatalysts to enhance water splitting activity, and its photoelectrochemical (PEC) properties at the electrified solid/liquid interface. In addition, there is a special focus on visible-light-absorbing oxynitride and oxysulfide particulate photocatalysts with absorption edges near 600 nm. Methods for photocatalyst preparation and surface modification, as well as PEC properties, are also discussed. The semiconductor properties of particulate photocatalysts obtained from photoelectroanalytical evaluations using particulate photoelectrodes are evaluated. This review is intended to provide guidelines for the future development of particulate photocatalysts capable of efficient and stable OWS.

Understanding the Activation Mechanism of RhCrOx Cocatalysts for Hydrogen Evolution with Nanoparticulate Electrodes.

Mixed oxides of Rh-Cr (RhCrOx), containing Rh3+ and Cr3+ cations, are commonly used as cocatalysts for the hydrogen evolution reaction (HER) on particulate photocatalysts. The precise physicochemical mechanisms of the HER at the catalytic sites of these oxides are not well understood. In this study, model cocatalyst electrodes, composed of nanoparticulate RhCrOx, were fabricated to investigate the physicochemical mechanisms of the HER. Electroanalytical and X-ray photoelectron spectroscopic measurements revealed that nanoparticulate RhCrOx produces reduced Rh (Rh0) species by maintaining an electrode potential more negative than 0.03 V versus the reversible hydrogen electrode (VRHE). This results in significant enhancement of the HER activity. The catalytic activity for the HER stems from the reduced Rh species, and the inclusion of Cr3+ (CrOx) aided in the electron transfer process at the solid/liquid interface, resulting in a higher current density during the HER. To achieve a solar-to-hydrogen efficiency of over 3%, the conduction band minimum of the particulate photocatalyst should be positioned more negatively than -0.10 VRHE. Moreover, the formation of electron trap states at potentials more positive than 0.03 VRHE should be avoided. This study highlights the importance of understanding the catalytic sites on metal oxide cocatalysts. Moreover, it offers a design strategy for enhancing the efficiency of photocatalytic water splitting.

Physicochemical insights into semiconductor properties of a semitransparent tantalum nitride photoanode for solar water splitting.

The self-conductivity of tantalum nitride (Ta3N5) thin film-based semitransparent photoanodes was found to promote the current originating from the photoelectrochemical oxygen evolution reaction (PEC OER) without a conducting substrate. With surface modification by the NiFeOx-electrocatalyst, an optimized Ta3N5 thin film fabricated directly on a transparent insulating quartz substrate generated a photocurrent density of ∼5.9 ± 0.1 mA cm-2 at 1.23 V vs. the reversible hydrogen electrode under simulated AM 1.5G solar illumination. The correlation between the PEC OER performance of NiFeOx-modified Ta3N5 photoanodes and the electrical properties of Ta3N5 thin films was investigated based on the Hall effect measurements. By changing the nitridation conditions, these properties can be tuned so that the higher the Hall mobility (0.2 to 1.7 cm2 V-1 s-1) and the lower the carrier concentration (1020 to 1019 cm-3). The surface chemical states of Ta3N5 thin films were investigated using X-ray photoelectron spectroscopy as a means of evaluating surface oxygen impurities and nitrogen vacancies, which may correlate with the PEC OER performance and the electrical properties of the material.

Co-doping of a La5Ti2Cu0.9Ag0.1O7S5 photocatalyst (λ < 700 nm) with Ga and Al to enhance photocatalytic H2 evolution.

La5Ti2Cu0.9Ag0.1O7S5 (LTCA) (λ < 700 nm) can function as a photocatalyst for H2 evolution. Co-doping LTCA with Ga3+ and Al3+ at Ti4+ sites effectively enhanced the H2 evolution activity of LTCA, yielding an apparent quantum efficiency of 18% at 420 nm. The activity of this material was greater than that previously reported for Ga-doped LTCA by a factor of 1.6. Such activity enhancement is attributed to increasing the population of long-lived photogenerated electrons and facilitating the electron transfer to the cocatalyst. This work significantly improved the LTCA-based photocatalyst for H2 evolution, making it a promising material for future application in non-sacrificial Z-scheme water splitting.

Mechanistic Insights into Enhanced Hydrogen Evolution of CrOx /Rh Nanoparticles for Photocatalytic Water Splitting.

The hydrogen evolution reaction (HER) of Rh nanoparticles (RhNP) coated with an ultrathin layer of Cr-oxides (CrOx ) was investigated as a model electrode for the Cr2 O3 /Rh-metal core-shell-type cocatalyst system for photocatalytic water splitting. The CrOx layer was electrodeposited over RhNP on a transparent conductive fluorine-doped tin oxide (FTO) substrate. The CrOx layer on RhNP facilitates the electron transfer process at the CrOx /RhNP interface, leading to the increased current density for the HER. Impedance spectroscopic analysis revealed that the CrOx layer transferred protons via the hopping mechanism to the RhNP surface for HER. In addition, CrOx restricted electron transfer from the FTO to the electrolyte and/or RhNP and suppressed the backward reaction by limiting oxygen migration. This study clarifies the crucial role of the ultrathin CrOx layer on nanoparticulate cocatalysts and provides a cocatalyst design strategy for realizing efficient photocatalytic water splitting.

Exploring the Photocorrosion Mechanism of a Photocatalyst.

Photoelectrochemical (PEC) water splitting using Ta3N5 anodes shows a high solar-to-hydrogen (STH) efficiency approaching 10%. However, the long-term stability of gas evolution should be improved for the commercial utilization of PEC water-splitting technology. Herein, we examined the photocurrent degradation of photoanodes prepared by uniformly loading a NiFeOx cocatalyst onto a Ta3N5 semiconductor. Although spectroscopic analysis showed that the degradation was attributable to the formation of an oxide layer, several oxide growth kinetic laws and mechanisms are known. We theoretically derived the photocurrent kinetic laws instead of the oxide growth kinetic laws by generalizing the Cabrera-Mott oxidation theory of metal oxidation in air to apply it to photocorrosion. The measured photocurrent kinetics are fully consistent with the theoretical kinetic laws. We show that ion drift due to charging of the oxide layer limits oxide growth even though uniform cocatalyst loading is designed to prevent self-oxidation of Ta3N5.

Synthesis of Metal Chalcogenide Semiconductors by Thermal Decomposition of Organosulfur and Organoselenium Compounds.

Metal chalcogenides - because of their excellent optical and electrical properties - are important semiconductor materials for optical devices, such as solar cells, sensors, and photocatalysts. The challenges associated with metal chalcogenides are the complexity of the conventional synthesis methods and the stringent synthesis conditions. In this study, the synthesis conditions were simplified in a solvent-free synthesis method using cadmium precursor, thiourea and selenium to synthesize metal chalcogenides, such as CdS and CdSe, which have particularly suitable band gaps for the optical devices. CdSx Se1-x solid solution was successfully synthesized under molten thiourea as the reactive reaction medium at relatively low temperatures, even at 180 °C, with residual melamine derivatives in the solid phase. The luminescence properties of CdSx Se1-x and the products in the gas and solid phases were investigated. Optimization of the synthesis conditions for solid solutions of CdSx Se1-x and the role of organic compounds in the formation of metal chalcogenides are discussed.

Maximizing Oxygen Evolution Performance on a Transparent NiFeOx/Ta3N5 Photoelectrode Fabricated on an Insulator.

A transparent Ta3N5 photoanode is a promising candidate for the front-side photoelectrode in a photoelectrochemical (PEC) cell with tandem configuration (tandem cell), which can potentially provide high solar-to-hydrogen (STH) energy conversion efficiency. This study focuses in particular on the semiconductor properties and interfacial design of transparent Ta3N5 photoanodes fabricated on insulating quartz substrates (Ta3N5/SiO2), typically the geometric area of 1 × 1 cm2 in contact with indium on its edge. This material utilizes the self-conductivity of Ta3N5 to make the PEC system operational, and the electrode would strongly reflect the intrinsic nature of Ta3N5 without a back contact that is commonly introduced. First, PEC measurements using acetonitrile (ACN)/H2O mixed solution were made to elucidate the intrinsic photoresponse in the presence of tris(2,2'-bipyridine)ruthenium(II) bis(hexafluorophosphate) (Ru(bpy)3(PF6)2) without water contact which avoids a multielectron-transfer oxygen evolution reaction (OER) and photoinduced self-oxidation. The potential difference between the onset potential of Ru2+ PEC oxidation by Ta3N5/SiO2 and the redox potential of Ru2+/3+ in the nonaqueous environment was about 0.7 V. While a stable photoanodic response was observed for Ta3N5/SiO2 in the nonaqueous phase, the addition of a small quantity of water into this nonaqueous system led to the immediate deactivation of Ta3N5/SiO2 photoanode under illumination by self-photooxidation to form TaOx at the solid/water interface. In aqueous phase, flatband potentials estimated from Mott-Schottky analysis varied with solution pH (constant potential against reversible hydrogen electrode (RHE)). Photoelectrode modification by a transparent NiFeOx layer was attempted. The complete coverage of the Ta3N5 surface with transparent NiFeOx electrocatalysts, achieved by an optimized spin-coating protocol with controlled Ni-Fe precursors, allowed for the successful protection of Ta3N5 and demonstrated an extremely stable photocurrent for hours without any additional protective layers. The stability of the resultant NiFeOx/Ta3N5/SiO2 was limited not by Ta3N5 but mainly by a NiFeOx electrocatalyst due to Fe dissolution with time.

Efficient Water Oxidation Using Ta3 N5 Thin Film Photoelectrodes Prepared on Insulating Transparent Substrates.

Photoelectrochemical (PEC) water splitting using visible-light-responsive photoelectrodes is the preferred approach to converting solar energy into hydrogen as a renewable energy source. A transparent Ta3 N5 photoanode embedded within a PEC cell having a tandem configuration is a promising configuration that may provide a high solar-to-hydrogen energy conversion efficiency. Ta3 N5 thin films are typically prepared by heating precursor films in an NH3 flow at high temperatures, which tends to degrade the transparent conductive layer, such that producing efficient Ta3 N5 transparent photoanodes is challenging. Herein, the direct preparation of transparent Ta3 N5 photoanodes on insulating quartz substrates was demonstrated without the insertion of a transparent conductive layer. The resulting devices generated a photocurrent of 6.0 mA cm-2 at 1.23 V vs. a reversible hydrogen electrode under simulated sunlight. This study provides a new strategy for the preparation of transparent photoelectrodes that mitigates current challenges.

An Al-doped SrTiO3 photocatalyst maintaining sunlight-driven overall water splitting activity for over 1000 h of constant illumination.

Photocatalytic water splitting is a viable approach to the large-scale production of renewable solar hydrogen. The apparent quantum yield for this reaction has been improved, but the lifespan of photocatalysts functioning under sunlight at ambient pressure have rarely been examined, despite the critical importance of this factor in practical applications. Herein, we show that Al-doped SrTiO3 (SrTiO3:Al) loaded with a RhCrO x (rhodium chromium oxide) cocatalyst splits water with an apparent quantum yield greater than 50% at 365 nm. Moreover, following the photodeposition of CoOOH and TiO2, this material maintains 80% of its initial activity and a solar-to-hydrogen energy conversion efficiency greater than or equal to 0.3% over a span of 1300 h under constant illumination by simulated sunlight at ambient pressure. This result is attributed to reduced dissolution of Cr in the cocatalyst following the oxidative photodeposition of CoOOH. The photodeposition of TiO2 further improves the durability of this photocatalyst. This work demonstrates a concept that could allow the design of long-term, large-scale photocatalyst systems for practical sunlight-driven water splitting.

Transparent Ta3 N5 Photoanodes for Efficient Oxygen Evolution toward the Development of Tandem Cells.

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 Ta3 N5 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 Ta3 N5 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.

CdTe-Based Photoanode for Oxygen Evolution from Water under Simulated Sunlight.

This study investigated the properties of a photoanode fabricated by depositing a p-type CdTe thin film on a CdS-coated FTO substrate (CdTe/CdS/FTO) via close-space sublimation. This CdTe/CdS/FTO electrode was found to work as a photoanode with a long absorption edge wavelength of 830 nm. In a CdTe-based photoanode such as this, the p-n junction formed at the CdTe/CdS interface promotes charge separation of photoexcited carriers and forces photogenerated holes to move toward the photoanode surface to promote oxidation reactions on the electrode surface. A MoOx buffer layer was also found to play a crucial role in facilitating the transfer of photogenerated holes to surface reaction sites through decreasing the energy barrier at the interface between the CdTe and a surface protective layer. A biphotoelectrode photoelectrochemical cell composed of a CdTe-based photoanode and a CdTe-based photocathode exhibited a solar-to-hydrogen conversion efficiency of 0.22% without an external voltage in response to illumination by AM 1.5G light.

Overall water splitting by photoelectrochemical cells consisting of (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 photocathodes and BiVO4 photoanodes.

The design of photoelectrochemical (PEC) cell structures that mitigate challenges related to mass transfer is the key to achieving efficient hydrogen production. A PEC cell based on an integrated photoelectrode incorporating (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 strips and BiVO4 strips exhibited 1.0% solar-to-hydrogen energy conversion efficiency even without stirring the electrolyte solution.

Enhancement of Charge Separation and Hydrogen Evolution on Particulate La5Ti2CuS5O7 Photocathodes by Surface Modification.

Particulate La5Ti2CuS5O7 (LTC) photocathodes prepared by particle transfer show a positive onset potential of 0.9 V vs RHE for the photocathodic current in photoelectrochemical (PEC) H2 evolution. However, the low photocathodic current imposes a ceiling on the solar-to-hydrogen energy conversion efficiency of PEC cells based on LTC photocathodes. To improve the photocurrent, in this work, the surface of Mg-doped LTC photocathodes was modified with TiO2, Nb2O5, and Ta2O5 by radio frequency reactive magnetron sputtering. The photocurrent of the modified Mg-doped LTC photocathodes was doubled because these oxides formed type-II heterojunctions and extended the lifetimes of photogenerated charge carriers. The enhanced photocathodic current was attributed to hydrogen evolution at a positive potential of +0.7 V vs RHE. This work opens up possibilities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment.

Diphenyl viologen on an HOPG electrode surface: less sharp redox wave than dibenzyl viologen.

Redox behavior of diphenyl viologen (dPhV) on a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode was described using the results of voltammetric and electroreflectance measurements. Its characteristics were compared to those of dibenzyl viologen (dBV), which undergoes the first-order faradaic phase transition. Unlike dBV, dPhV-dication (dPhV(2+)) was found to take a strongly adsorbed state on an HOPG surface. This is due to much stronger π-π interaction between phenyl rings of dPhV(2+) and HOPG surface than between benzyl groups of dBV(2+) and the surface. The participation of this strongly adsorbed dPhV(2+) in the redox process can be avoided by (1) a shorter than ∼3 min time period elapsing from touching a freshly cleaved HOPG surface to dPhV solution until the start of potential scan, (2) complete equilibration at the electrode potentials at which superficial dPhV molecules are fully reduced, or (3) multiple cyclic potential scanning to repeat oxidation-reduction of adsorbed species. Even in such conditions, although voltammograms of thin-layer electrochemistry for the surface-confined dPhV(•+)/dPhV(2+) couple are obtained with peak widths being as narrow as those of dBV, it is not the first-order phase transition. The participation of strongly adsorbed dPhV(2+) molecules results in another new voltammetric feature with a broader peak. The film formed by strongly adsorbed dPhV(2+) was hydrophilic, whereas dBV(2+) does not form such a film but only a gas-like layer. Measurements using X-ray photoelectron spectroscopy confirmed that the film consists of dPhV(2+) with coexistent water. These results reveal a typical case that delicate interaction balance among V(2+), V(•+), and electrode surface determines whether the two-dimensional first-order transition takes place or not.

Faradaic phase transition of dibenzyl viologen on an HOPG electrode surface studied by in situ electrochemical STM and electroreflectance spectroscopy.

Phase transitions of an adsorption layer of dibenzyl viologen (dBV) as a typical diaryl viologen on a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode are described using voltammetry, in situ electrochemical scanning tunneling microscopy (EC-STM), and electroreflectance (ER) spectroscopy. A monolayer redox process at less negative potential than the bulk redox process was found to be the first-order faradaic phase transition between a gaslike adsorption layer of dication (dBV(2+)) and a 2D condensed monolayer of radical cation (dBV(•+)). Comparison of the results of cyclic voltammetry and potential step chronoamperometry was made with those of heptyl viologen (HV), which also undergoes a faradaic phase transition of the first order. It suggested that the contribution of intermolecular π-π interaction between benzyl groups of dBV to the phase transition is minor and apparently equivalent to interchain interaction between the heptyl chains of HV. In situ EC-STM images of the 2D condensed monolayer demonstrated stripe patterns of the rows of dBV(•+) molecules forming 3-fold rotationally symmetric domains. The results of the ER measurements also revealed that the orientation of the longitudinal molecular axis of the bipyridinium moiety of dBV(•+) molecules lying flat on the HOPG electrode surface, most likely with a side-on configuration.

Non-pungent capsaicin analogs (capsinoids) increase metabolic rate and enhance thermogenesis via gastrointestinal TRPV1 in mice.

Capsinoids are non-pungent capsaicin analogs which increase energy expenditure like capsaicin. However, the mechanisms underlying the enhancement of their energy expenditure despite their non-pungency are poorly understood. We suggest here that capsinoids increase energy expenditure in wild-type mice, but not in transient receptor potential vanilloid 1 (TRPV1) knockout mice, implying that capsinoids increase energy expenditure via TRPV1. The jejunal administration of capsinoids to anesthetized mice raised the temperature of the colon and intrascapular brown adipose tissue. Denervation of the extrinsic nerves connected to the jejunum inhibited this temperature elevation. These findings suggest that capsinoids increase energy expenditure by activating the intestinal extrinsic nerves. Although the jejunal administration of capsinoids did not raise the tail skin temperature, an intravenous injection of capsinoids did, indicating that capsinoids could barely pass through the intestinal wall into the blood. Taken together, gastrointestinal TRPV1 may be a critical target for capsinoids to enhance energy expenditure.

Contribution of TRPV1 to the bradykinin-evoked nociceptive behavior and excitation of cutaneous sensory neurons.

Bradykinin (BK), a major inflammatory mediator, excites and sensitizes nociceptor neurons/fibers, thus evoking pain and hyperalgesia. The cellular signaling mechanisms underlying these actions have remained unsolved, especially in regard to the identity of channels that mediate acute excitation. Here, to clarify the contribution of transient receptor potential vanilloid 1 (TRPV1), a heat-sensitive ion channel, to the BK-evoked nociceptor excitation and pain, we examined the behavioral and physiological BK-responses in TRPV1-deficient (KO) mice. A nocifencive behavior after BK injection (100 pmol/site) into mouse sole was reduced in TRPV1-KO mice compared with wild-type (WT). A higher dose of BK (1 nmol/site), however, induced the response in TRPV1-KO mice indistinguishable from that in the WT. BK-evoked excitation of cutaneous C-fibers in TRPV1-KO mice was comparable to that in WT. BK clearly increased intracellular calcium in cultured dorsal root ganglion (DRG) neurons of TRPV1-KO mice, although the incidence of BK-sensitive neurons was reduced. BK has been reported to activate TRPA1 indirectly, yet a considerable part of BK-sensitive DRG neurons did not respond to a TRPA1 agonist, mustard oil. These results suggest that BK-evoked nociception/nociceptor response would not be simply explained by activation of TRPV1 and A1, and that BK-evoked nociceptor excitation would be mediated by several ionic mechanisms.

Sensitization of TRPA1 by PAR2 contributes to the sensation of inflammatory pain.

Proinflammatory agents trypsin and mast cell tryptase cleave and activate PAR2, which is expressed on sensory nerves to cause neurogenic inflammation. Transient receptor potential A1 (TRPA1) is an excitatory ion channel on primary sensory nerves of pain pathway. Here, we show that a functional interaction of PAR2 and TRPA1 in dorsal root ganglion (DRG) neurons could contribute to the sensation of inflammatory pain. Frequent colocalization of TRPA1 with PAR2 was found in rat DRG neurons. PAR2 activation increased the TRPA1 currents evoked by its agonists in HEK293 cells transfected with TRPA1, as well as DRG neurons. Application of phospholipase C (PLC) inhibitors or phosphatidylinositol-4,5-bisphosphate (PIP(2)) suppressed this potentiation. Decrease of plasma membrane PIP(2) levels through antibody sequestration or PLC-mediated hydrolysis mimicked the potentiating effects of PAR2 activation at the cellular level. Thus, the increased TRPA1 sensitivity may have been due to activation of PLC, which releases the inhibition of TRPA1 from plasma membrane PIP(2). These results identify for the first time to our knowledge a sensitization mechanism of TRPA1 and a novel mechanism through which trypsin or tryptase released in response to tissue inflammation might trigger the sensation of pain by TRPA1 activation.

Lipophilicity of capsaicinoids and capsinoids influences the multiple activation process of rat TRPV1.

Analogs of capsaicin, such as capsaicinoids and capsinoids, activate a cation channel, transient receptor potential cation channel vanilloid subfamily 1 (TRPV1), and then increase the intracellular calcium concentration ([Ca2+]i). These compounds would be expected to activate TRPV1 via different mechanism(s), depending on their properties. We synthesized several capsaicinoids and capsinoids that have variable lengths of acyl moiety. The activities of these compounds towards TRPV1 heterologously expressed in HEK293 cells were determined by measuring [Ca2+]i. When an extracellular or intracellular Ca2+ source was removed, some agonists such as capsaicin could increase [Ca2+]i. However, a highly lipophilic capsaicinoid containing C18:0 and capsinoids containing C14:0, C18:0, or C18:1 (the latter was named olvanilate) could not elicit a large increase in [Ca2+]i in the absence of an extracellular or intracellular Ca2+ source. These results suggest that highly lipophilic compounds cause only a slight Ca2+ influx, via TRPV1 in the plasma membrane, and are not able to activate TRPV1 in the endoplasmic reticulum.