Highly Efficient Photocatalytic Degradation of Hydrogen Sulfide in the Gas Phase Using Anatase/TiO2(B) Nanotubes.

Hydrogen sulfide (H2S) is a highly toxic and corrosive gas that causes a foul odor even at very low concentrations [several parts per billion (ppb)]. However, industrially discharged H2S has a concentration range of several tens of ppb to several parts per million (ppm), which conventional methods are unable to process. Therefore, advanced and sustainable methods for treating very low concentrations of H2S are urgently needed. TiO2-based photocatalysts are eco-friendly and have the ability to treat environmental pollutants, such as low-concentration gases, using light energy. However, there are no reports on H2S decomposition or oxidation at concentrations below several ppb. Therefore, in this study, we employed anatase/TiO2(B) nanotubes, which have a high specific surface area and an efficient charge-transfer interface, to treat H2S. We successfully reduced 10 ppm of H2S to 1 ppb or less at a kinetic rate of 75 µmol h-1 g-1. The suitability of our method was further demonstrated by the generation of sulfate ions and sulfur (as detected by X-ray photoelectron spectroscopy and ion chromatography), which are industrially useful as oxidation products, whereas sulfur dioxide, a harmful substance, was not produced. This is the first study to report H2S decomposition down to the ppb level, providing meaningful solutions for malodor problems and potential health hazards associated with H2S.

Photoreforming of Organic Waste into Hydrogen Using a Thermally Radiative CdOx/CdS/SiC Photocatalyst.

To achieve superior efficiency for photocatalytic reactions, it is necessary to utilize visible light, which accounts for most of the solar energy. Herein, by applying a photocatalytic reaction, we aimed to develop a method for generating hydrogen by reforming organic waste, which is discharged as part of domestic, agricultural, forestry, and industrial practice. In the prepared CdS/SiC composite photocatalyst, etching of the oxide film of SiC and oxidation of the atomic-level surface of CdS proceeded in an alkaline reaction solution to form a CdOx/CdS/SiC composite. This composite is stable under light irradiation in a high-temperature alkaline reaction solution and can steadily promote hydrogen production. CdOx/CdS/SiC exhibits absorption in the entire ultraviolet and visible light region. In particular, the visible light region on the long-wavelength side, which is derived from the crystal defect of SiC, was used for heat radiation, and it was effective in increasing the temperature of the reaction solution. The high-temperature alkaline reaction solution promoted the hydrolysis of organic wastes with high molecular weight. Elution of small organic molecules by this process facilitated the progress of photocatalytic reactions and improved the rate of hydrogen production. Furthermore, in the absorption region derived from the interband transition below 580 nm, electron transfer between SiC and CdS suppressed recombination and improved the photocatalytic activity. Particularly, we achieved a high quantum yield of almost 20% in the ultraviolet region of 380 nm, where electron transfer from SiC was remarkable. Even in the visible light region, 2.0% was achieved at 420 nm, indicating an activity superior to that of conventional photoreforming systems. Using the developed photocatalytic system, we succeeded in producing hydrogen by photoreforming organic waste, such as cellulosic biomass, animal biomass, and plastic, under sunlight. Therefore, it is possible to solve waste disposal, environmental, and energy problems using sustainable photocatalytic processes.

Elucidating the Factors Affecting Hydrogen Production Activity Using a CdS/TiO2 Type-II Composite Photocatalyst.

CdS/TiO2 is a composite photocatalyst that has been studied over many years and in which electron transfer from CdS to TiO2 is believed to lead to high photocatalytic activity. However, most reports on improved activity involve hydrogen production in the presence of a sulfide reducing agent. In this study, we comprehensively examined the effects of electron transfer, hydrogen overvoltage, substrate adsorption, and the cocatalyst from relationships between hydrogen production ability and the total number of trapped electrons in the presence of various reducing agents. As a result, we clarified that the electron transfer between CdS and TiO2 does not necessarily lead to high activity. We showed that the composite photocatalyst needs to be designed for the intended purpose and that evaluating the hydrogen production ability using sacrificial reagents provides insufficient information for use in an actual environment.

Photocurrent generation by a photosystem I-NiO photocathode for a p-type biophotovoltaic tandem cell.

Photosynthesis is a process used by algae and plants to convert light energy into chemical energy. Due to their uniquely natural and environmentally friendly nature, photosynthetic proteins have attracted attention for use in a variety of artificial applications. Among the various types, biophotovoltaics based on dye-sensitized solar cells have been demonstrated in many studies. Although most related works have used n-type semiconductors, a p-type semiconductor is also a significant potential component for tandem cells. In this work, we used mesoporous NiO as a p-type semiconductor substrate for Photosystem I (PSI) and demonstrated a p-type PSI-biophotovoltaic and tandem cell based on dye-sensitized solar cells. Under visible light illumination, the PSI-adsorbed NiO electrode generated a cathodic photocurrent. The p-type biophotovoltaic cell using the PSI-adsorbed NiO electrode generated electricity, and the IPCE spectrum was consistent with the absorption spectrum of PSI. These results indicate that the PSI-adsorbed NiO electrode acts as a photocathode. Moreover, a tandem cell consisting of the PSI-NiO photocathode and a PSI-TiO2 photoanode showed a high open-circuit voltage of over 0.7 V under illumination to the TiO2 side. Thus, the tandem strategy can be utilized for biophotovoltaics, and the use of other biomaterials that match the solar spectrum will lead to further progress in photovoltaic performance.

Elucidation of the electron energy structure of TiO2(B) and anatase photocatalysts through analysis of electron trap density.

A clear understanding of the electron energy structure of TiO2(B)/anatase is needed to study the related catalytic reactions and design new composite photocatalysts. In this study, the electron energy structures of TiO2(B) and anatase were estimated by analyzing the energy-resolved distribution of electron traps measured by reversed double-beam photoacoustic spectroscopy. In the mixture of TiO2(B) and anatase, interfacial charge-transfer excitation from anatase to electron traps of TiO2(B) was suggested. By analyzing this for TiO2(B), the electron level with a relatively high density of states was found to be located ∼0.07 eV deeper than that for anatase. Furthermore, a similar electron energy structure was suggested for a composite photocatalyst having a mixed phase of TiO2(B) and anatase.

Integrated Photon Upconversion Dye-Sensitized Solar Cell by Co-adsorption with Derivative of Pt-Porphyrin and Anthracene on Mesoporous TiO2.

Photon upconversion via triplet-triplet annihilation (TTA-UC) is a process that converts two lower-energy photons to a higher-energy photon, which is expected to increase the maximum solar cell efficiency beyond the Shockley-Queisser limit. To incorporate TTA-UC into a dye-sensitized solar cell (DSSC), we used a co-adsorption approach, in which both a TTA-UC donor with an alkyl carboxylic acid chain and a TTA-UC acceptor dye were adsorbed onto mesoporous TiO2. Incident photon-to-current conversion efficiency spectra and excitation intensity dependence indicated that a photocurrent was generated under irradiation at wavelengths above 490 nm by the TTA-UC mechanism. The power-conversion efficiency of the DSSC was increased to 0.72%, and the photocurrent contributed by TTA-UC was 0.036 mA cm-2 under 1 sun irradiation.

Efficient hydrogen production using photosystem I enhanced by artificial light harvesting dye.

In this study, we improved the hydrogen production efficiency by combining photosystem I with an artificial light harvesting dye, Lumogen Red. In the reaction system, Lumogen Red allows light absorption and energy transfer to photosystem I by Förster resonance energy transfer; therefore, the Pt nanoparticles act as active sites for hydrogen generation.

In situ synthesis of CdS/CdWO4 nanorods core-shell composite via acid dissolution.

The prevention of photocorrosion in photocatalysts allows for the use of a wide variety of visible-light-responsive photocatalysts, leading to highly efficient photocatalytic reactions. This study aimed to avoid the photocorrosion issues associated with pure CdS, a known photocorrosive photocatalyst, by forming a stable CdWO4 shell on the surface of a CdS core. The CdS/CdWO4 core-shell composite was formed using a unique method based on CdS elution under acidic conditions. An optimal CdWO4 nanorod shell was formed at a pH of 0.8, a reaction time of 30 min, and a calcination temperature of 400 °C, where the core remained intact and was sufficiently coated. The prepared CdS/CdWO4 core-shell composite was shown to be stable when exposed to light irradiation in pure water. Furthermore, it was successfully used in water splitting with an oxidation reaction side photocatalyst. This core-shell synthesis method based on core dissolution was easily and highly controlled, and is suitable for use in other similar core-shell composite applications.

Effective Photocatalytic Hydrogen Evolution by Cascadal Carrier Transfer in the Reverse Direction.

Visible-light-responsive photocatalysts used in the highly efficient hydrogen production exhibit several disadvantages such as photocorrosion and fast recombination. Because of the potential important applications of such catalysts, it is crucial that a simple, effective solution is developed. In this respect, in this study, we combined SiC (ß modification) and TiO2 with CdS to overcome the challenges of photocorrosion and fast recombination of CdS. Notably, we found that when irradiated with visible light, CdS was excited, and the excited electrons moved to the conduction band of TiO2, thereby increasing the efficiency of charge separation. In addition, by moving the holes generated on CdS to the valence band of SiC, in the opposite direction of TiO2, photocorrosion and fast recombination were prevented. As a result, in the sulfide solution, the CdS/SiC composite catalyst exhibited 4.3 times higher hydrogen generation ability than pure CdS. Moreover, this effect was enhanced with the addition of TiO2, giving 10.8 times higher hydrogen generation ability for the CdS/SiC/TiO2 catalyst. Notably, the most efficient catalyst, which was obtained by depositing Pt as a cocatalyst, exhibited 1.09 mmol g-1 h-1 hydrogen generation ability and an apparent quantum yield of 24.8%. Because water reduction proceeded on the TiO2 surface and oxidative sulfide decomposition proceeded on the SiC surface, the exposure of CdS to the solution was unnecessary, and X-ray photoelectron spectroscopy confirmed that photocorrosion was successfully suppressed. Thus, we believe that the effective composite photocatalyst construction method presented herein can also be applied to other visible-light-responsive powder photocatalysts having the same disadvantages as CdS, thereby improving the efficiency of such catalysts.

Charge-transfer complex versus σ-complex formed between TiO2 and bis(dicyanomethylene) electron acceptors.

A novel group of organic-inorganic hybrid materials is created by the combination of titanium dioxide (TiO2) nanoparticles with bis(dicyanomethylene) (TCNX) electron acceptors. The TiO2-TCNX complex is produced by the nucleophilic addition reaction between a hydroxy group on the TiO2 surface and TCNX, with the formation of a σ-bond between them. The nucleophilic addition reaction generates a negatively-charged diamagnetic TCNX adsorbate that serves as an electron donor. The σ-bonded complex characteristically shows visible-light absorption due to interfacial charge-transfer (ICT) transitions. In this paper, we report on another kind of complex formation between TiO2 and TCNX. We have systematically studied the structures and visible-light absorption properties of the TiO2-TCNX complexes, with changing the electron affinity of TCNX. We found that TCNX acceptors with lower electron affinities form charge-transfer complexes with TiO2 without the σ-bond formation. The charge-transfer complexes show strong visible-light absorption due to interfacial electronic transitions with little charge-transfer nature, which are different from the ICT transitions in the σ-bond complexes. The charge-transfer complexes induce efficient light-to-current conversions due to the interfacial electronic transitions, revealing the high potential for applications to light-energy conversions. Furthermore, we demonstrate that the formation of the two kinds of complexes is selectively controlled by the electron affinity of TCNX.

Uncovering the mechanism for selective control of the visible and near-IR absorption bands in bacteriochlorophylls a, b and g.

Bacteriochlorophylls (BChls) play an important role as light harvesters in photosynthetic bacteria. Interestingly, bacteriochlorophylls (BChls) a, b, and g selectively tune their visible (Qx) and near IR (Qy) absorption bands by the substituent changes. In this paper, we theoretically study the mechanism for the selective control of the absorption bands. Density functional theory (DFT) and time-dependent DFT (TD-DFT) and four-orbital model analyses reveal that the selective red-shift of the Qy band with the substituent change from BChl a to b occurs with the lower-energy shift of the (HOMO, LUMO) excited state directly induced by the molecular-orbital energy changes. In contrast, the Qx band hardly shifts by the cancellation between the higher- and lower-energy shifts of the (HOMO-1, LUMO) excited state directly induced by the molecular-orbital energy changes and configuration interaction, respectively. On the other hand, with the substituent changes from BChl a to g, the Qx band selectively blue-shifts by the larger higher-energy shift of the (HOMO-1, LUMO) excited state directly induced by the molecular-orbital energy shifts than the lower-energy shift due to the configuration interaction. In contrast, the Qy band hardly shifts by the cancellation between the higher- and lower-energy shifts of the (HOMO, LUMO) excited state directly induced by the molecular-orbital energy changes and configuration interaction, respectively. Our work provides the important knowledge for understanding how nature controls the light-absorption properties of the BChl dyes, which might be also useful for design of porphyrinoid chromophores.

Two-Dimensional Molecular Assembly of Bacteriochlorophyll a Derivatives Using Synthetic Poly(ethylene glycol)-Linked Light-Harvesting Model Polypeptides on a Gold Electrode Modified with Supported Lipid Bilayers.

The two-dimensional molecular assembly was accomplished of bacteriochlorophyll a (BChl a) and zinc-substituted BChl a (Zn-BChl a) together with synthetic poly(ethylene glycol)(PEG)-linked light-harvesting (LH) model polypeptides on a gold Au(111) electrode modified with supported lipid bilayers. Model polypeptides for LH1-α from Rhodospirillum (Rs.) rubrum were successfully synthesized and stably assembled with Zn-BChl a in 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1'-glycerol)] (DOPG) lipid bilayers on an electrode at room temperature, as well as in liposomal solution, in which the Zn-BChl a complex unlike BChl a, was stably assembled. The PEG moiety of the model polypeptide assisted the stable assembly with an α-helical conformation of the LH1-α model peptides together with these pigments onto the gold electrode with defined orientation. The photocurrent response depended on the combinations of the pigments and synthetic LH model polypeptides. The results presented herein will be useful for the self-assembly of these complexes on electrodes to construct efficient energy-transfer and electron-transfer reactions between individual pigments in lipid bilayers.

Immobilization and Photocurrent Activity of a Light-Harvesting Antenna Complex II, LHCII, Isolated from a Plant on Electrodes.

A light-harvesting (LH) antenna complex II, LHCII, isolated from spinach was immobilized onto an indium tin oxide (ITO) electrode with dot patterning of 3-aminopropyltriethoxysilane (APS) by utilizing electrostatic interactions between the cationic surface of the electrode and the anionic surface of stromal side of the LHCII polypeptide. Interestingly, the illumination of LHCII assembled onto the ITO electrode produced a photocurrent response that depends on the wavelength of the excitation light. Further, LHCII was immobilized onto a TiO2 nanostructured film to extend for the development of a dye-sensitized biosolar cell system. The photocurrent measured in the iodide/tri-iodide redox system of an ionic liquid based electrolyte on the TiO2 system showed remarkable enhancement of the conversion efficiency, as compared to that on the ITO electrode.

Immobilization of porphyrin derivatives with a defined distance and orientation onto a gold electrode using synthetic light-harvesting α-helix hydrophobic polypeptides.

Molecular assembly of Zn-porphyrin pigments on a gold electrode using synthetic 1α-helix hydrophobic polypeptides which have similar amino acid sequences to the hydrophobic core in the native photosynthetic light-harvesting (LH) 1-ß polypeptide from Rhodobacter sphaeroides has been achieved. This method is clearly successful in allowing assembly of porphyrins together with LH1 type functional complexes with a defined distance and orientation on the electrode. In this case, the photocurrent direction and the distance of electron transfer of pigments could be controlled by these synthetic LH1 model polypeptides. This method will be useful for the self-assembly of these pigment and protein complexes in order to study the energy transfer and electron transfer reactions between individual pigments in the supramolecular complexes on the electrode, as well as to provide insight into the effect of the distance and orientation of pigments and the effect of the structure of 1α-helix hydrophobic polypeptide on the energy transfer and electron transfer reactions.

Enhancement of incident photon-to-current conversion efficiency for phthalocyanine-sensitized solar cells by 3D molecular structuralization.

Enlarging the molecular size of zinc phthalocyanine (Pc) dyes three dimensionally with 2,6-diphenylphenoxy substituents significantly reduced the aggregation of the dyes on a TiO(2) surface. As a result, the incident photon-to-current conversion efficiency was improved not only in the Q band but over the whole absorption range, achieving 4.6% energy conversion efficiency under one-sun conditions. Electron lifetime measurements indicated that these Pc dyes do not enhance charge recombination, encouraging further development of Pc.

Electron transfer of quinone self-assembled monolayers on a gold electrode.

Dialkyl disulfide-linked naphthoquinone, (NQ-Cn-S)2, and anthraquinone, (AQ-Cn-S)2, derivatives with different spacer alkyl chains (Cn: n=2, 6, 12) were synthesized and these quinone derivatives were self-assembled on a gold electrode. The formation of self-assembled monolayers (SAMs) of these derivatives on a gold electrode was confirmed by infrared reflection-absorption spectroscopy (IR-RAS). Electron transfer between the derivatives and the gold electrode was studied by cyclic voltammetry. On the cyclic voltammogram a reversible redox reaction between quinone (Q) and hydroquinone (QH2) was clearly observed under an aqueous condition. The formal potentials for NQ and AQ derivatives were -0.48 and -0.58 V, respectively, that did not depend on the spacer length. The oxidation and reduction peak currents were strongly dependent on the spacer alkyl chain length. The redox behavior of quinone derivatives depended on the pH condition of the buffer solution. The pH dependence was in agreement with a theoretical value of E 1/2 (mV)=E'-59pH for 2H+/2e(-) process in the pH range 3-11. In the range higher than pH 11, the value was estimated with E 1/2 (mV)=E'-30pH , which may correspond to H+/2e(-) process. The tunneling barrier coefficients (beta) for NQ and AQ SAMs were determined to be 0.12 and 0.73 per methylene group (CH2), respectively. Comparison of the structures and the alkyl chain length of quinones derivatives on these electron transfers on the electrode is made.

Phospholipid-linked quinones-mediated electron transfer on an electrode modified with lipid bilayers.

Phospholipid-linked naphthoquinones separated by spacer methylene groups (C(n)), PE-C(n)-NQ (n=0, 5, 11), were synthesized to investigate the quinone-mediated electron transfers on a glassy carbon (GC) electrode covered with phospholipids membrane. The PE-C(n)-NQ could be incorporated in lipid bilayer composed of phosphatidylcholine and exhibited characteristic absorption spectral change corresponding to their redox state, quinone/hydroquinone. The cyclic voltammogram of PE-C(n)-NQ-containing lipid bilayer modified on a GC electrode indicated a set of waves corresponding to the consecutive two-electron and two-proton transfer reduction of the quinone moiety. The peak currents of PE-C(n)-NQ as a function of temperature showed a sharp break point in the current-temperature behavior, reflecting the gel-fluid phase transition. The shape of the cyclic voltammograms changed with the pH of the buffer solution. Below pH 6 the first step of the reduction of quinone was a monoprotonation of quinone, whereas above pH 10 the first step of the oxidation was a monodeprotonation of hydroquinone. This indicates that reaction sequences of quinone/hydroquinone were different with the change of the pH. These results showed that the PE-C(n)-NQ exhibited electron transfer associated with proton transfer in the lipid membranes, depending on the diffusivity of the redox species in the membrane and pH. Interestingly, less effect of the number of methylene of the spacer group on the peak currents was observed. Comparison of manganese porphyrin-mediated electron transfer that depends on the spacer methylene lengths [M. Nango, T. Hikita, T. Nakano, T. Yamada, M. Nagata, Y. Kurono, T. Ohtsuka, Langmuir 14 (1998) 407] is made.

Self-assembled monolayer of light-harvesting core complexes from photosynthetic bacteria on a gold electrode modified with alkanethiols.

Light-harvesting antenna core (LH1-RC) complexes isolated from Rhodoseudomonas palustris were self-assembled on a gold electrode modified with self-assembled monolayers (SAMs) of the alkanethiols NH2(CH2)nSH, n = 2, 6, 8, 11; HOOC(CH2)7SH; and CH3(CH2)7SH, respectively. Adsorption of the LH1-RC complexes on the SAMs depended on the terminating group of the alkanethiols, where the adsoption increased in the following order for the terminating groups: amino groups > carboxylic acid groups > methyl groups. Further, the adsorption on a gold electrode modified with SAMs of NH2(CH2)nSH, n = 2, 6, 8, 11, depended on the methylene chain length, where the adsorption increased with increasing the methylene chain length. The presence of the well-known light-harvesting and reaction center peaks of the near infrared (NIR) absorption spectra of the LH1-RC complexes indicated that these complexes were only fully stable on the SAM gold electrodes modified with the amino group. In the case of modification with the carboxyl group, the complexes were partially stable, while in the presence of the terminal methyl group the complexes were extensively denatured. An efficient photocurrent response of these complexes on the SAMs of NH2(CH2)nSH, n = 2, 6, 8, 11, was observed upon illumination at 880 nm. The photocurrent depended on the methylene chain length (n), where the maximum photocurrent response was observed at n = 6, which corresponds to a distance between the amino terminal group in NH2(CH2)6SH and the gold surface of 1.0 nm.

Molecular assembly of artificial photosynthetic antenna core complex on an amino-terminated ITO electrode.

Bacterial photosynthetic membrane proteins, light-harvesting antenna complex (LH1), reaction center (RC), and their combined 'core' complex (LH1-RC) are functional elements in the primary photosynthetic events, i.e., capturing and transferring light energy and subsequent charge separation. These photosynthetic units (PSUs) isolated from Rhodospirillum rubrum (Rs. rubrum) were assembled onto an ITO electrode modified with 3-aminopropyltriethoxysilane (APS-ITO). The near IR absorption spectra of PSUs on the assembled electrodes were identical to those of solutions, indicating that the LH1 and LH1-RC core complexes were native on the electrode. Photocurrent response of PSUs on the electrode was examined upon illumination of the LH1 complex at 880 nm. The LH1-RC and a mixed assembly of LH1 and RC exhibited photocurrent response, but not LH1 only, consistent with the function of these PSUs, capturing light energy and transferring electron. This result provides useful methodology for building an artificial fabrication of PSUs on the electrode.

Self-assembled monolayer of light-harvesting core complexes of photosynthetic bacteria on an amino-terminated ITO electrode.

Light-harvesting antenna core (LH1-RC) complexes isolated from Rhodospirillum rubrum and Rhodopseudomonas palustris were successfully self-assembled on an ITO electrode modified with 3-aminopropyltriethoxysilane. Near infra-red (NIR) absorption, fluorescence, and IR spectra of these LH1-RC complexes indicated that these LH1-RC complexes on the electrode were stable on the electrode. An efficient energy transfer and photocurrent responses of these LH1-RC complexes on the electrode were observed upon illumination of the LH1 complex at 880 nm.