Carbon Nanotubes as a Solid-State Electron Mediator for Visible-Light-Driven Z-Scheme Overall Water Splitting.

So-called Z-scheme systems, which typically comprise an H2 evolution photocatalyst (HEP), an O2 evolution photocatalyst (OEP), and an electron mediator, represent a promising approach to solar hydrogen production via photocatalytic overall water splitting (OWS). The electron mediator transferring photogenerated charges between the HEP and OEP governs the performance of such systems. However, existing electron mediators suffer from low stability, corrosiveness to the photocatalysts, and parasitic light absorption. In the present work, carbon nanotubes (CNTs) were shown to function as an effective solid-state electron mediator in a Z-scheme OWS system. Based on the high stability and good charge transfer characteristics of CNTs, this system exhibited superior OWS performance compared with other systems using more common electron mediators. The as-constructed system evolved stoichiometric amounts of H2 and O2 at near-ambient pressure with a solar-to-hydrogen energy conversion efficiency of 0.15%. The OWS reaction was also promoted in the case that this CNT-based Z-scheme system was immobilized on a substrate. Hence, CNTs are a viable electron mediator material for large-scale Z-scheme OWS systems.

Overall Water Splitting by a SrTaO2N-Based Photocatalyst Decorated with an Ir-Promoted Ru-Based Cocatalyst.

The development of narrow-bandgap photocatalysts for one-step-excitation overall water splitting (OWS) remains a critical challenge in the field of solar hydrogen production. SrTaO2N is a photocatalytic material having a band structure suitable for OWS under visible light (λ ≤ 600 nm). However, the presence of defects in the oxynitride and the lack of cocatalysts to promote simultaneous hydrogen and oxygen evolution make it challenging to realize OWS using this material. The present work demonstrates a SrTaO2N-based particulate photocatalyst for OWS. This photocatalyst, which was composed of single crystals, was obtained by nitriding SrCl2 and Ta2O5 together with NaOH, with the latter added to control the formation of defects. The subsequent loading of bimetallic RuIrOx nanoparticles accelerated charge separation and allowed the SrTaO2N photocatalyst to exhibit superior OWS activity. This research presenting the strategies of controlling the oxygen sources and promoting the cocatalyst function is expected to expand the range of potential OWS-active oxynitride photocatalysts and permit the design of efficient cocatalysts for photocatalytic OWS.

Carbon Nitride Loaded with an Ultrafine, Monodisperse, Metallic Platinum-Cluster Cocatalyst for the Photocatalytic Hydrogen-Evolution Reaction.

For the realization of a next-generation energy society, further improvement in the activity of water-splitting photocatalysts is essential. Platinum (Pt) is predicted to be the most effective cocatalyst for hydrogen evolution from water. However, when the number of active sites is increased by decreasing the particle size, the Pt cocatalyst is easily oxidized and thereby loses its activity. In this study, a method to load ultrafine, monodisperse, metallic Pt nanoclusters (NCs) on graphitic carbon nitride is developed, which is a promising visible-light-driven photocatalyst. In this photocatalyst, a part of the surface of the Pt NCs is protected by sulfur atoms, preventing oxidation. Consequently, the hydrogen-evolution activity per loading weight of Pt cocatalyst is significantly improved, 53 times, compared with that of a Pt-cocatalyst loaded photocatalyst by the conventional method. The developed method is also effective to enhance the overall water-splitting activity of other advanced photocatalysts such as SrTiO3 and BaLa4 Ti4 O15 .

Identification of a Self-Photosensitizing Hydrogen Atom Transfer Organocatalyst System.

We developed organocatalyst systems to promote the cleavage of stable C-H bonds, such as formyl, α-hydroxy, and benzylic C-H bonds, through a hydrogen atom transfer (HAT) process without the use of exogenous photosensitizers. An electronically tuned thiophosphoric acid, 7,7'-OMe-TPA, was assembled with substrate or co-catalyst N-heteroaromatics through hydrogen bonding and π-π interactions to form electron donor-acceptor (EDA) complexes. Photoirradiation of the EDA complex induced stepwise, sequential single-electron transfer (SET) processes to generate a HAT-active thiyl radical. The first SET was from the electron-rich naphthyl group of 7,7'-OMe-TPA to the protonated N-heteroaromatics and the second proton-coupled SET (PCET) from the thiophosphoric acid moiety of 7,7'-OMe-TPA to the resulting naphthyl radical cation. Spectroscopic studies and theoretical calculations characterized the stepwise SET process mediated by short-lived intermediates. This organocatalytic HAT system was applied to four different carbon-hydrogen (C-H) functionalization reactions, hydroxyalkylation and alkylation of N-heteroaromatics, acceptorless dehydrogenation of alcohols, and benzylation of imines, with high functional group tolerance.

Improvement of Visible-Light H2 Evolution Activity of Pb2 Ti2 O5.4 F1.2 Photocatalyst by Coloading of Rh and Pd Cocatalysts.

Pb2 Ti2 O5.4 F1.2 modified with various metal cocatalysts was studied as a photocatalyst for visible-light H2 evolution. Although unmodified Pb2 Ti2 O5.4 F1.2 showed negligible activity, modification of its surface with Rh led to the best observed promotional effect among the Pb2 Ti2 O5.4 F1.2 samples modified with a single metal cocatalyst. The H2 evolution activity was further enhanced by coloading with Pd; the Rh-Pd/Pb2 Ti2 O5.4 F1.2 photocatalyst showed 3.2 times greater activity than the previously reported Pt/Pb2 Ti2 O5.4 F1.2 . X-ray absorption fine-structure spectroscopy, photoelectrochemical, and transient absorption spectroscopy measurements indicated that the coloaded Rh and Pd species, which were partially alloyed on the Pb2 Ti2 O5.4 F1.2 surface, improved the electron-capturing ability, thereby explaining the high activity of the coloaded Rh-Pd/Pb2 Ti2 O5.4 F1.2 catalyst toward H2 evolution.

Enhanced Overall Water Splitting by a Zirconium-Doped TaON-Based Photocatalyst.

Solar-powered one-step-excitation overall water splitting (OWS) using semiconducting materials is a simple means of achieving scalable and sustainable hydrogen production. While tantalum oxynitride (TaON) is one of the few photocatalysts capable of promoting OWS via single-step visible-light excitation, the efficiency of this process remains extremely poor. The present work employed 15 nm amorphous Ta2 O5 ⋅3.3 H2 O nanoparticles as a new precursor together with Zr doping and an optimized nitridation duration to synthesize a TaON-based photocatalyst with reduced particle sizes and low defect densities. Upon loading with Ru/Cr2 O3 /IrO2 cocatalysts, this material exhibited stoichiometric water splitting into hydrogen and oxygen, with an order of magnitude improvement in efficiency. Our findings demonstrate the importance of inventing/selecting the appropriate synthetic precursor and of defect control for fabricating active OWS photocatalysts.

Surface Modifications of (ZnSe)0.5(CuGa2.5Se4.25)0.5 to Promote Photocatalytic Z-Scheme Overall Water Splitting.

Charge separation is crucial for an efficient artificial photosynthetic process, especially for narrow-bandgap metal sulfides/selenides. The present study demonstrates the application of a p-n junction to particulate metal selenides to enhance photocatalytic Z-scheme overall water splitting (OWS). The constructed p-n junction of CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 significantly boosted charge separation. A thin TiO2 coating layer also was introduced to inhibit photocorrosion of CdS and suppress the backward reaction of water formation from hydrogen and oxygen. By employing Pt-loaded TiO2/CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 as a hydrogen evolution photocatalyst (HEP), we assembled a Z-scheme OWS system, together with BiVO4:Mo and Au as an oxygen evolution photocatalyst and electron mediator, respectively. An apparent quantum yield of 1.5% at 420 nm was achieved, which is by far the highest among reported particulate photocatalytic Z-scheme OWS systems with metal sulfides/selenides as HEPs. The present work demonstrates that a well-tailored p-n junction structure is effective for promoting charge separation in photocatalysis and opens new pathways for the development of efficient artificial photosynthesis systems involving narrow bandgap photocatalysts.

Simultaneously Tuning the Defects and Surface Properties of Ta3N5 Nanoparticles by Mg-Zr Codoping for Significantly Accelerated Photocatalytic H2 Evolution.

The simultaneous control of the defect species and surface properties of semiconducting materials is a crucial aspect of improving photocatalytic performance, yet it remains challenging. Here, we synthesized Mg-Zr-codoped single-crystalline Ta3N5 (Ta3N5:Mg+Zr) nanoparticles by a brief NH3 nitridation process, exhibiting photocatalytic water reduction activity 45 times greater than that of pristine Ta3N5 under visible light. A coherent picture of the relations between the defect species (comprising reduced Ta, nitrogen vacancies and oxygen impurities), surface properties (associated with dispersion of the Pt cocatalyst), charge carrier dynamics, and photocatalytic activities was drawn. The tuning of defects and simultaneous optimization of surface properties resulting from the codoping evidently resulted in the generation of high concentrations of long-lived electrons in this material as well as the efficient migration of these electrons to evenly distributed surface Pt sites. These effects greatly enhanced the photocatalytic activity. This work highlights the importance and feasibility of improving multiple properties of a catalytic material via a one-step strategy.

Synthesis of Three-Layer Perovskite Oxynitride K2Ca2Ta3O9N·2H2O and Photocatalytic Activity for H2 Evolution under Visible Light.

Substitution of oxide anions (O2-) in a metal oxide for nitrogen (N3-) results in reduction of the band gap, which is attractive in heterogeneous photocatalysis; however, only a handful of two-dimensional layered perovskite oxynitrides have been reported, and thus, the structural effects of layered oxynitrides on photocatalytic activity have not been sufficiently examined. This study reports the synthesis of a Ruddlesden-Popper phase three-layer oxynitride perovskite of K2Ca2Ta3O9N·2H2O, and the photocatalytic activity is compared with an analogous two-layer perovskite, K2LaTa2O6N·1.6H2O. Topochemical ammonolysis reaction of a Dion-Jacobson phase oxide KCa2Ta3O10 at 1173 K in the presence of K2CO3 resulted in a single-phase layered perovskite, K2Ca2Ta3O9N·2H2O, which belongs to the tetragonal P4/mmm space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized K2Ca2Ta3O9N·2H2O has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. K2Ca2Ta3O9N·2H2O modified with a Pt cocatalyst generated H2 from an aqueous solution containing a dissolved NaI as a reversible electron donor under visible light (λ > 400 nm) with no noticeable change in the crystal structure and light absorption properties. However, the H2 evolution activity of K2Ca2Ta3O9N·2H2O was an order of magnitude lower than that of K2LaTa2O6N·1.6H2O. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in K2Ca2Ta3O9N·2H2O was shorter than that in K2LaTa2O6N·1.6H2O, which could explain the low photocatalytic activity of K2Ca2Ta3O9N·2H2O.

How g-C3N4 Works and Is Different from TiO2 as an Environmental Photocatalyst: Mechanistic View.

Graphitic carbon nitride (CN) as a popular visible light photocatalyst needs to be better understood for environmental applications. The behaviors of CN as an environmental photocatalyst were systematically studied in comparison with a well-known TiO2 photocatalyst. The two photocatalysts exhibit different photocatalytic oxidation (PCO) behaviors and dependences on the experimental conditions (e.g., pH, Pt loading, and the kind of organic substrate and scavenger). The PCO of organic substrates was significantly enhanced by loading Pt on TiO2 under UV light (λ > 320 nm), whereas Pt-CN exhibited a lower PCO activity than bare CN under visible light (λ > 420 nm). While the presence of Pt enhances the charge separation in both TiO2/UV and CN/visible light systems (confirmed by transient IR absorption spectroscopic analysis), the opposite effects of Pt are ascribed to the different mechanisms of •OH generation in the two photocatalytic systems. The negative effect of Pt on CN is ascribed to the fact that Pt catalytically decomposes in situ-generated H2O2 (a main precursor of OH radical), which hinders •OH production. The production of OH radicals on CN is favored only at acidic pH but 1O2 generation is dominant in alkaline pH. The pH-dependent behaviors of reactive oxygen species generation on CN were confirmed by electron paramagnetic resonance spin trap measurements.

Enhancement of photoelectrochemical activity of TiO2 electrode by particulate/dense double-layer formation.

Photoelectrochemical water splitting has attracted significant attention as an environment-friendly method to generate H2 and O2. Electrodes composed of powders exhibit a large surface area and are favorable for surface reactions; however, their high resistance prevents charge transportation. In contrast, photoelectrodes composed of a dense film exhibit high conductivity; however, they constitute a small surface area. Therefore, a combination of these two films can lead to higher photoelectrochemical activities. Herein, a particulate/dense TiO2 hybrid electrode exhibited eightfold and twofold higher activities for water oxidation at sufficiently positive potentials as compared to those of the single particulate and dense film electrodes, respectively. Electrochemical impedance measurements and the light intensity dependence of the photocurrent suggest that the activity enhancement is responsible for the synergistic effects of effective charge separation in the highly conductive dense TiO2 film and effective hole-consuming reaction at the particulate TiO2 layer. However, the activity rather decreased near the onset potential of water oxidation (<0.3 V) under the illumination of light near the bandgap energy (375 nm). Such an activity decrease was not observed for the 340 nm illumination; hence, the recombination of charge carriers generated in dense and particulate layers is responsible: the charge transfer resistance at the particulate/dense interface prevents the effective charge separation. These results demonstrate that the combination of particulate matter and dense films can produce a synergistic effect; however, the resistance at the junction remains a significant bottleneck, rendering resistance reduction necessary to maximize the advantages of hybrid electrodes, especially under the illumination of longer wavelength light.

Heteroatom Dopants Promote Two-Electron O2 Reduction for Photocatalytic Production of H2 O2 on Polymeric Carbon Nitride.

Polymeric carbon nitride modified with selected heteroatom dopants was prepared and used as a model photocatalyst to identify and understand the key mechanisms required for efficient photoproduction of H2 O2 via selective oxygen reduction reaction (ORR). The photochemical production of H2 O2 was achieved at a millimolar level per hour under visible-light irradiation along with 100 % apparent quantum yield (in 360-450 nm region) and 96 % selectivity in an electrochemical system (0.1 V vs. RHE). Spectroscopic analysis in spatiotemporal resolution and theoretical calculations revealed that the synergistic association of alkali and sulfur dopants in the polymeric matrix promoted the interlayer charge separation and polarization of trapped electrons for preferable oxygen capture and reduction in ORR kinetics. This work highlights the key features that are responsible for controlling the photocatalytic activity and selectivity toward the two-electron ORR, which should be the basis of further development of solar H2 O2 production.

Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode.

Mixed-anion compounds (e.g., oxynitrides and oxysulfides) are potential candidates as photoanodes for visible-light water oxidation, but most of them suffer from oxidative degradation by photogenerated holes, leading to low stability. Here we show an exceptional example of a stable, mixed-anion water-oxidation photoanode that consists of an oxyfluoride, Pb2Ti2O5.4F1.2, having a band gap of ca. 2.4 eV. Pb2Ti2O5.4F1.2 particles, which were coated on a transparent conductive glass (FTO) support and were subject to postdeposition of a TiO2 overlayer, generated an anodic photocurrent upon band gap photoexcitation of Pb2Ti2O5.4F1.2 (λ <520 nm) with a rather negative photocurrent onset potential of ca. -0.6 V vs NHE, which was independent of the pH of the electrolyte solution. Stable photoanodic current was observed even without loading a water oxidation promoter such as CoOx. Nevertheless, loading CoOx onto the TiO2/Pb2Ti2O5.4F1.2/FTO electrode further improved the anodic photoresponse by a factor of 2-3. Under AM1.5G simulated sunlight (100 mW cm-2), stable water oxidation to form O2 was achieved using the optimized Pb2Ti2O5.4F1.2 photoanode in the presence of an applied potential smaller than 1.23 V, giving a Faradaic efficiency of 93% and almost no sign of deactivation during 4 h of operation. This study presents the first example of photoelectrochemical water splitting driven by visible-light excitation of an oxyfluoride that stably works, even without a water oxidation promoter, which is distinct from ordinary mixed-anion photoanodes that usually require a water oxidation promoter.

Plasmonic p-n Junction for Infrared Light to Chemical Energy Conversion.

Infrared (IR) light represents an untapped energy source accounting for almost half of all solar energy. Thus, there is a need to develop systems to convert IR light to fuel and make full use of this plentiful resource. Herein, we report photocatalytic H2 evolution driven by near- to shortwave-IR light (up to 2500 nm) irradiation, based on novel CdS/Cu7S4 heterostructured nanocrystals. The apparent quantum yield reached 3.8% at 1100 nm, which exceeds the highest efficiencies achieved by IR light energy conversion systems reported to date. Spectroscopic results revealed that plasmon-induced hot-electron injection at p-n heterojunctions realizes exceptionally long-lived charge separation (>273 µs), which results in efficient IR light to hydrogen conversion. These results pave the way for the exploration of undeveloped low-energy light for solar fuel generation.

Structural changes of water molecules during photoelectrochemical water oxidation on TiO2 thin film electrodes.

Behaviors of photogenerated charge carriers and structural changes of water molecules on TiO2 photoelectrodes were investigated by using time-resolved visible to mid-IR absorption spectroscopy. From the spectra measured in the visible to NIR region, it was shown that the lifetime of trapped electrons and holes becomes longer upon applying more positive potentials. This result was reasonably explained by the enhancement of the upward band bending at the water/TiO2 interface. On the other hand, from the spectra measured in the mid-IR region, structural changes of the water molecules were observed. When a TiO2 electrode was photoexcited at the potential where the water oxidation starts, a new absorption peak appeared at 3620 cm-1 with a slight decrease in the intensity of hydrogen-bonded water. This new peak was assigned to the isolated O-H band of water molecules. Usually, TiO2 surfaces exhibit super-hydrophilic properties with strong hydrogen-bonding; however, the obtained result was opposite. Therefore, the appearance of this isolated O-H band was ascribed to the cleavage of the hydrogen-bonding networks resulting from the production of reaction intermediates such as OH radicals or H2O2. The intensity of the isolated O-H decreases when applying more positive potentials, where the O2 evolution proceeds more efficiently. This could be ascribed to the rapid consumption of the reaction intermediates. At these potentials, the intensity of hydrogen-bonded water was also further decreased.

Oxygen induced enhancement of NIR emission in brookite TiO2 powders: comparison with rutile and anatase TiO2 powders.

Brookite TiO2 attracts considerable attention in photocatalysis owing to its superior performance in several photocatalytic reactions. In this work, we investigated the behavior of charge carriers in brookite, rutile, and anatase TiO2 by using photoluminescence (PL) and transient absorption (TA) spectroscopies. PL measurements revealed that brookite TiO2 exhibits a visible and a NIR emission at ∼520 nm and ∼860 nm, respectively. Addition of methanol vapor quenched both the visible and NIR emissions by the hole-consuming reaction of methanol. However, exposure to O2 shows curious behaviors: the visible emission was quenched but the NIR emission was enhanced. These results can be accounted for by the enhancement of upward band bending resulting in the effective separation of electrons and holes into the bulk and the surface, respectively. Furthermore, the shallowly trapped electrons, which are responsible for visible PL, are consumed by O2; hence, the visible emission is quenched. However, in the case of NIR emission, the deeply trapped electrons are responsible and they are mainly located at the surface defects. The O2 adsorption promotes the hole accumulation at the surface and then assists the recombination of these deeply trapped electrons, resulting in the enhancement of the NIR emission. We also found that the lifetime of NIR emission (τ1 = 43 ± 0 ns and τ2 = 589 ± 1 ns) was much longer than that of visible emission (τ1 = 15 ± 0 ns and τ2 = 23 ± 0 ns), since the mobility of these deeply trapped electrons to encounter with holes is lower than that of the shallowly trapped electrons. However, even for this slow NIR emission, the actual lifetime of the deeply trapped electrons estimated by TA (1.5 ± 0.0 µs and 17 ± 0 µs) was one or two orders of magnitude longer, confirming that non-radiative recombination is dominant and it is much slower than radiative recombination: TAS and PL provide detailed information on the radiative and non-radiative recombination processes. The PL of anatase and rutile TiO2 powders was also measured and the difference from brookite TiO2 was discussed.

Undoped Layered Perovskite Oxynitride Li2 LaTa2 O6 N for Photocatalytic CO2 Reduction with Visible Light.

Oxynitrides are promising visible-light-responsive photocatalysts, but their structures are almost confined with three-dimensional (3D) structures such as perovskites. A phase-pure Li2 LaTa2 O6 N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium-rich oxide precursor. Li2 LaTa2 O6 N exhibited high crystallinity and visible-light absorption up to 500 nm. As opposed to well-known 3D oxynitride perovskites, Li2 LaTa2 O6 N supported by a binuclear RuII complex was capable of stably and selectively converting CO2 into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li2 LaTa2 O6 N possesses a lower density of mid-gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride.

Elucidating the impact of A-site cation change on photocatalytic H2 and O2 evolution activities of perovskite-type LnTaON2 (Ln = La and Pr).

Transition metal (oxy)nitrides with perovskite-type structures have been regarded as one of the promising classes of inorganic semiconductor materials that can be used in solar water splitting systems for the production of hydrogen as a renewable and storable energy carrier. The performance of transition metal (oxy)nitrides in solar water splitting is strongly influenced by the crystal structure-related dynamics of photogenerated charge carriers. Here, we have systematically assessed the influence of A-site cation exchange on the visible-light-induced photocatalytic H2 and O2 evolution activities, photoanodic response, and dynamics of photogenerated charge carriers of perovskite-type LnTaON2 (Ln = La and Pr). The structural refinement results reveal the orthorhombic Imma and Pnma structures for LaTaON2 and PrTaON2, respectively; the latter has a more distorted crystal structure from the ideal cubic perovskite due to the smaller size of Pr3+ cations. Compared with LaTaON2, PrTaON2 exhibits lower photocatalytic H2 and O2 gas evolution activities and photoanodic response owing to an excessive amount of intrinsic defects associated with anionic vacancies and reduced tantalum species stemming from a long high-temperature nitridation process under reductive NH3 atmosphere. Transient absorption signals evidence the faster decay of photogenerated electrons (holes) in Pt (CoOx)-loaded LaTaON2 than that in Pt (CoOx)-loaded PrTaON2, consistent with the photocatalytic and photoelectrochemical performance of the two photocatalysts. This study suggests that in addition to selecting a suitable A-site cation, it is prerequisite to synthesize LnTaON2 (Ln = La and Pr) crystals with a low defect density to improve their photo-conversion efficiency for solar water splitting.

Nature-Inspired, Highly Durable CO2 Reduction System Consisting of a Binuclear Ruthenium(II) Complex and an Organic Semiconductor Using Visible Light.

A metal-free organic semiconductor of mesoporous graphitic carbon nitride (C3N4) coupled with a Ru(II) binuclear complex (RuRu') containing photosensitizer and catalytic units selectively reduced CO2 into HCOOH under visible light (λ > 400 nm) in the presence of a suitable electron donor with high durability, even in aqueous solution. Modification of C3N4 with Ag nanoparticles resulted in a RuRu'/Ag/C3N4 photocatalyst that exhibited a very high turnover number (>33000 with respect to the amount of RuRu'), while maintaining high selectivity for HCOOH production (87-99%). This turnover number was 30 times greater than that reported previously using C3N4 modified with a mononuclear Ru(II) complex, and by far the highest among the metal-complex/semiconductor hybrid systems reported to date. The results of photocatalytic reactions, emission decay measurements, and time-resolved infrared spectroscopy indicated that Ag nanoparticles on C3N4 collected electrons having lifetimes of several milliseconds from the conduction band of C3N4, which were transferred to the excited state of RuRu', thereby promoting photocatalytic CO2 reduction driven by two-step photoexcitation of C3N4 and RuRu'. This study also revealed that the RuRu'/Ag/C3N4 hybrid photocatalyst worked efficiently in water containing a proper electron donor, despite the intrinsic hydrophobic nature of C3N4 and low solubility of CO2 in an aqueous environment.

Surface-enhanced IR absorption spectroscopy of the KcsA potassium channel upon application of an electric field.

Surface-enhanced IR absorption spectroscopy (SEIRAS) is a powerful tool for studying the structure of molecules adsorbed on an electrode surface (ATR-SEIRA). Coupled with an electrochemical system, structural changes induced by changes in the electric field can be detected. All the membrane proteins are subjected to the effect of membrane electric field, but conformational changes at different membrane potentials and their functional relevance have not been studied extensively except for channel proteins. In this contribution, background information of potential-dependent functional and structural changes of a prototypical channel, the KcsA channel, is summarized, and SEIRAS applied to the KcsA channel under the application of the potential is shown. The potassium channels allow K(+) to permeate selectively through the structural part called the selectivity filter, in which dehydrated K(+) ions interact with backbone carbonyls. In the absence of K(+), the selectivity filter undergoes conformational changes to the non-conductive collapsed conformation. To apply the electric field, the KcsA channels were fixed on the gold surface in either upside or reverse orientation. The SEIRA spectrum in K(+) or Na(+) solution revealed both backbone structural changes and local changes in the OCO-carboxylate groups. Upon application of the negative electric field, the spectrum of OCO was enhanced only in the K(+) solution. These results indicate that the negative electric field accumulates local K(+) concentration, which turned the collapsed filter to the conductive conformation. ATR-SEIRA serves as an unprecedented experimental system for examining membrane proteins under an electric field.