Photocatalytic CO2 Reduction Using an Osmium Complex as a Panchromatic Self-Photosensitized Catalyst: Utilization of Blue, Green, and Red Light.

The photocatalytic reduction of carbon dioxide (CO2) represents an attractive approach for solar-energy storage and leads to the production of renewable fuels and valuable chemicals. Although some osmium (Os) photosensitizers absorb long wavelengths in the visible-light region, a self-photosensitized, mononuclear Os catalyst for red-light-driven CO2 reduction has not yet been exploited. Here, we discovered that the introduction of an Os metal to a PNNP-type tetradentate ligand resulted in the absorption of light with longer-wavelength (350-700 nm) and that can be applied to a panchromatic self-photosensitized catalyst for CO2 reduction to give mainly carbon monoxide (CO) with a total turnover number (TON) of 625 under photoirradiation (λ≥400 nm). CO2 photoreduction also proceeded under irradiation with blue (λ0=405 nm), green (λ0=525 nm), or red (λ0=630 nm) light to give CO with >90 % selectivity. The quantum efficiency using red light was determined to be 12 % for the generation of CO. A catalytic mechanism is proposed based on the detection of intermediates using various spectroscopic techniques, including transient absorption, electron paramagnetic resonance, and UV/Vis spectroscopy.

Contribution of RNA-RNA Interactions Mediated by the Genome Packaging Signals for the Selective Genome Packaging of Influenza A Virus.

The influenza A virus genome is composed of eight single-stranded negative-sense viral RNA segments (vRNAs). The eight vRNAs are selectively packaged into each progeny virion. This process likely involves specific interactions between the vRNAs via segment-specific packaging signals located in both the 3'- and 5'-terminal regions of the respective vRNAs. To assess the importance of vRNA-vRNA interactions via packaging signals for selective genome packaging, we generated mutant viruses possessing silent mutations in the packaging signal region of the hemagglutinin (HA) vRNA. A mutant virus possessing silent mutations in nucleotides (nt) 1664 to 1676 resulted in defects in HA vRNA incorporation and showed a reduction in viral growth. After serial passage, the mutant virus acquired additional mutations in the 5'-terminal packaging signal regions of both the HA and polymerase basic 2 (PB2) vRNAs. These mutations contributed to the recovery of viral growth and HA vRNA packaging efficiency. In addition, an RNA-RNA interaction between the 5' ends of HA and PB2 vRNAs was confirmed in vitro, and this interaction was disrupted following the introduction of silent mutations in the HA vRNA. Thus, our results demonstrated that RNA-RNA interactions between the packaging signal regions of HA vRNA and PB2 vRNA are important for selective genome packaging. IMPORTANCE While numerous viral genomes comprise a single genome segment, the influenza A virus possesses eight segmented genomes. Influenza A virus can benefit from having a segmented genome because the segments can reassort with other strains of the influenza virus to create new genetically distinct strains. The influenza A virus efficiently incorporates one copy of each of its eight genomic segments per viral particle. However, the mechanism by which each segment is specifically selected is poorly understood. The genome segments contain RNA signals that facilitate the incorporation of segments into virus particles. These regions may facilitate specific interactions between the genome segments, creating an eight-segment complex, which can then be packaged into individual particles. In this study, we provide evidence that RNA signals contribute to specific interactions between two of the influenza virus genome segments.

Solar-Driven CO2 Reduction Using a Semiconductor/Molecule Hybrid Photosystem: From Photocatalysts to a Monolithic Artificial Leaf.

The synthesis of organic chemicals from H2O and CO2 using solar energy is important for recycling CO2 through cyclical use of chemical ingredients produced from CO2 or molecular energy carriers based on CO2. Similar to photosynthesis in plants, the CO2 molecules are reduced by electrons and protons, which are extracted from H2O molecules, to produce O2. This reaction is uphill; therefore, the solar energy is stored as the chemical bonding energy in the organic molecules. This artificial photosynthetic technology mimicking green vegetation should be implemented as a self-standing system for on-site direct solar energy storage that supports CO2 recycling in a circular economy. Herein, we explain our interdisciplinary fusion methodology to develop hybrid photocatalysts and photoelectrodes for an artificial photosynthetic system for the CO2 reduction reaction (CO2RR) in aqueous solutions. The key factor for the system is the integration of uniquely different functions of molecular transition-metal complexes and solid semiconductors. A metal complex catalyst and a semiconductor appropriate for a CO2RR and visible-light absorption, respectively, are linked, and they function complementary way to catalyze CO2RR under visible-light irradiation as a particulate photocatalyst dispersion in solution. It has also been proven that Ru complexes with bipyridine ligands can catalyze a CO2RR as photocathodes when they are linked with various semiconductor surfaces, such as those of doped tantalum oxides, doped iron oxides, indium phosphides, copper-based sulfides, selenides, silicon, and others. These photocathodes can produce formate and carbon monoxide using electrons and protons extracted from water through potential-matched connections with photoanodes such as TiO2 or SrTiO3 for oxygen evolution reactions (OERs). Benefiting from the very low overpotential of an aqueous CO2RR at metal complexes approaching the theoretical lower limit, the semiconductor/molecule hybrid system demonstrates a single tablet-formed monolithic electrode called "artificial leaf." This single electrode device can generate formate (HCOO-) from H2O and CO2 in a water-filled single-compartment reactor without requiring a separation membrane under unassisted or bias-free conditions, either electrically or chemically. The reaction proceeds with a stoichiometric electron/hole ratio and stores solar energy with a solar-to-chemical energy conversion efficiency of 4.6%, which exceeds that of plants. In this Account, the key results that marked our milestones in technological progress of the semiconductor/molecule hybrid photosystem are concisely explained. These results include design, proof of the principle, and understanding of the phenomena by time-resolved spectroscopies, synchrotron radiation analyses, and DFT calculations. These results enable us to address challenges toward further scientific progress and the social implementation, including the use of earth-abundant elements and the scale-up of the solar-driven CO2RR system.

Photocatalytic CO2 Reduction Using Water as an Electron Donor under Visible Light Irradiation by Z-Scheme and Photoelectrochemical Systems over (CuGa)0.5ZnS2 in the Presence of Basic Additives.

We demonstrated photocatalytic CO2 reduction using water as an electron donor under visible light irradiation by a Z-scheme photocatalyst and a photoelectrochemical cell using bare (CuGa)0.5ZnS2 prepared by a flux method as a CO2-reducing photocatalyst. The Z-scheme system employing the bare (CuGa)0.5ZnS2 photocatalyst and RGO-(CoOx/BiVO4) as an O2-evolving photocatalyst produced CO of a CO2 reduction product accompanied by H2 and O2 in a simple suspension system without any additives under visible light irradiation and 1 atm of CO2. When a basic salt (i.e., NaHCO3, NaOH, etc.) was added into the reactant solution (H2O + CO2), the CO formation rate and the CO selectivity increased. The same effect of the basic salt was observed for sacrificial CO2 reduction using SO32- as an electron donor over the bare (CuGa)0.5ZnS2 photocatalyst. The selectivity for the CO formation of the Z-schematic CO2 reduction reached 10-20% in the presence of the basic salt even in an aqueous solution and without loading any cocatalysts on the (CuGa)0.5ZnS2 metal sulfide photocatalyst. It is notable that CO was obtained accompanied by reasonable O2 evolution, indicating that water was an electron donor for the CO2 reduction. Moreover, the present Z-scheme system also showed activity for solar CO2 reduction using water as an electron donor. The bare (CuGa)0.5ZnS2 powder loaded on an FTO glass was also used as a photocathode for CO2 reduction under visible light irradiation. CO and H2 were obtained on the photocathode with 20% and 80% Faradaic efficiencies at 0.1 V vs RHE, respectively.

Hot-carrier photocatalysts for artificial photosynthesis.

We applied hot-carrier extraction to particulate photocatalysts for artificial photosynthetic reactions including water splitting for H2 production and CO2 reduction to CO and HCOOH, and elucidated promising features of hot-carrier photocatalysts (HC-PCs). We designed a specific structure of the HC-PC; a semiconductor core in which thermalization of photo-generated carriers is significantly suppressed is surrounded by a shell whose bandgap is wider than that of the core. Among the photo-generated hot carriers in the core, only carriers whose energies are larger than the shell bandgap are extracted passing through the shell to the active sites on the shell surface. Thus, the shell functions as an energy-selective contact. We calculated the upper bounds of the rates of the carrier supply from the core to the active sites using a newly constructed detailed-balance model including partial thermalization and nonradiative recombination of the carriers. It has been revealed that the HC-PCs can yield higher carrier-supply rates and thus potentially higher solar-to-chemical energy conversion efficiencies for H2 and CO production than those of conventional photocatalysts with the assistance of intraband transition and Auger recombination/impact ionization. It should be noted, however, that one of the necessary conditions for efficient hot-carrier extraction is sufficiently large carrier density in the core, which, in turn, requires concentrated solar illumination by several hundreds. This would raise rate-limiting problems of activities of the chemical reactions induced by the photo-generated carriers and material-transfer properties.

Study of Excited States and Electron Transfer of Semiconductor-Metal-Complex Hybrid Photocatalysts for CO2 Reduction by Using Picosecond Time-Resolved Spectroscopies.

A semiconductor-metal-complex hybrid photocatalyst was previously reported for CO2 reduction; this photocatalyst is composed of nitrogen-doped Ta2 O5 as a semiconductor photosensitizer and a Ru complex as a CO2 reduction catalyst, operating under visible light (>400 nm), with high selectivity for HCOOH formation of more than 75 %. The electron transfer from a photoactive semiconductor to the metal-complex catalyst is a key process for photocatalytic CO2 reduction with hybrid photocatalysts. Herein, the excited-state dynamics of several hybrid photocatalysts are described by using time-resolved emission and infrared absorption spectroscopies to understand the mechanism of electron transfer from a semiconductor to the metal-complex catalyst. The results show that electron transfer from the semiconductor to the metal-complex catalyst does not occur directly upon photoexcitation, but that the photoexcited electron transfers to a new excited state. On the basis of the present results and previous reports, it is suggested that the excited state is a charge-transfer state located between shallow defects of the semiconductor and the metal-complex catalyst.

Particulate photocatalytic reactors with spectrum-splitting function for artificial photosynthesis.

We have applied spectrum splitting, which is the most reliable way for highly efficient solar energy utilization, to particulate photocatalytic reactors. We have elucidated that the spectrum splitting is feasible using plural cells/compartments, in which photocatalyst particles of different bandgaps are suspended respectively, arranged optically in series. When the particles are sufficiently small (≤20 nm in diameter), high-energy photons are absorbed in the wide-gap cell/compartment on the solar illumination side while low-energy photons reach the backside narrow-gap cell/compartment with being scarcely diffuse-reflected. We have proposed two concrete configurations of the reactors: wide-gap cell/narrow-gap Z-scheme cell (WG/Z), and wide-gap cell/two-compartment cell of middle-gap and narrow-gap (WG/MG-NG), based on the previous configuration of a two-compartment cell of wide-gap and narrow-gap (WG-NG). We have constructed a new model of the carrier supply process from the semiconductor photocatalysts to the active sites, and calculated the practical upper limits of the carrier supply rates and solar-to-chemical conversion efficiencies. The spectrum-splitting reactors can yield higher efficiencies of artificial photosynthetic H2 and CO production by up to 1.5-1.6 times than the conventional Z-scheme reactors. The newly proposed WG/Z reactor widens the room of the material developments and improves the robustness against solar spectrum variation, and hence would be a promising practical solution, although the efficiency is slightly lower than that for the ideal WG-NG reactor. The WG/MG-NG reactor yields the highest efficiency among the three configurations, with high spectral robustness.

Subjective Effects of Inhaling Kuromoji Tea Aroma.

Teas and various herbal teas are well-known beverages and are commonly consumed around the world. In this study, we focused on kuromoji tea. Kuromoji is a deciduous shrub of the Lauraceae family, and the plucked leaves and branches have been drunk as a tea in production areas for a long time. However, no studies have investigated the subjective and physiological effects of kuromoji tea. In this study, the effects of kuromoji tea were examined on the basis of the measurements of heart rate variability and cerebral blood flow, core body temperature and subjective assessments. Moreover, the results of this study showed that a pleasant subjective feeling could be obtained by sniffing the aroma of kuromoji teas, especially tea leaves. It was also found that the aroma of kuromoji teas has the potential to stimulate saliva secretion and increase subjective and physiological excitements in the oral cavity. 1,8-Cineole, linalool, terpinen-4-ol, carvone and geraniol were determined in both kuromoji leaves and branches. In this study, the beneficial effects of kuromoji teas when drunk conventionally were investigated.

Photocatalytic CO2 Reduction Using a Robust Multifunctional Iridium Complex toward the Selective Formation of Formic Acid.

A highly efficient tetradentate PNNP-type Ir photocatalyst, Mes-IrPCY2, was developed for the reduction of carbon dioxide. The photocatalyst furnished formic acid (HCO2H) with 87% selectivity together with carbon monoxide to achieve a turnover number of 2560, which is the highest among CO2 reduction photocatalysts without an additional photosensitizer. Mes-IrPCY2 exhibited outstanding photocatalytic CO2 reduction activity in the presence of the sacrificial electron source 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) in CO2-saturated N,N-dimethylacetamide under irradiation with visible light. The quantum yield was determined to be 49% for the generation of HCO2H and CO. Electron paramagnetic resonance and UV-vis spectroscopy studies of Mes-IrPCY2 with a sacrificial electron donor revealed that the one-electron-reduced species is the key intermediate for the selective formation of HCO2H.

Charge Trapping Process in Photoexcited Nitrogen-Doped Titanium Oxides.

We present a first-principles study on the structural changes induced by charge trapping that occurs after photoexcitation in nitrogen-doped titanium oxide (N-TiO2). The charge trapping site and the corresponding K edge EXAFS spectra of Ti atoms were predicted and compared with those obtained by an experiment under ultraviolet (UV) light excitation. The results indicate that charge trapping occurs in the neighborhood of the oxygen vacancy (O-vac) sites. Furthermore, our calculations show that the O-vac site significantly affects the EXAFS spectra, while substitutional nitrogen doping for an oxygen site in the vicinity of the O-vac site is insensitive in the EXAFS spectra. Based on this observation combined with the knowledge from previous experiments, we propose a charge trapping process where the UV light-excited electron migrates at the O-vac site in bulk (∼300 ps) while the visible light-excited electron (N 2p → Ti 3d) is immediately trapped at the O-vac site neighboring the N site (∼1 ps).

First principles calculations of surface dependent electronic structures: a study on ß-FeOOH and γ-FeOOH.

We report a theoretical study on iron oxyhydroxide (FeOOH). The FeOOH surface is expected to act as an efficient electrochemical catalyst for the oxygen evolution reaction (OER), because it is based on iron, an element of the fourth highest Clarke number. Experimentally, the OER activity of ß-FeOOH is known to be higher than that of γ-FeOOH. However, the details of the OER mechanism and the surface reactivities of the FeOOH polymorphs have not yet been fully understood. We performed first-principles calculations of bulk and surfaces of ß-FeOOH and γ-FeOOH using density functional theory, to investigate their electronic structures and catalytic activities. The calculations suggest that depending on the surface indices, several surfaces may be favored for catalytic activities.

Electrocatalytic CO2 reduction near the theoretical potential in water using Ru complex supported on carbon nanotubes.

We successfully developed a highly efficient electrode for CO2 reduction using a Ru-complex catalyst ([Ru]) supported on carbon paper coated with multi-walled carbon nanotubes (CPCNT/[Ru]). The CPCNT/[Ru] electrode promoted the CO2 reduction reaction in aqueous solution near the theoretical potential, and produced formate linearly with a current density of greater than 0.9 mA cm-2 at -0.15 V (versus RHE) for at least 24 h. Due to the outstandingly low overpotential, a monolithic tablet-shaped photo-device was realized by coupling the CPCNT/[Ru] catalyst with amorphous SiGe-jn as a light absorber and IrO x as a water oxidation catalyst, and the device produced formate from CO2 and water in a single-compartment reactor. The nanotubes enhanced the rate for CO2 reduction at [Ru], and accordingly a solar-to-chemical conversion efficiency of 4.3% for formate production was achieved when the CO2 reduction and H2O oxidation sites had the same area.

Band bending and dipole effect at interface of metal-nanoparticles and TiO2 directly observed by angular-resolved hard X-ray photoemission spectroscopy.

This paper describes the observation of band bending and band edge shifts at the interfaces between nanoscale metals and TiO2 film over a wide depth range by angular-resolved hard X-ray photoemission spectroscopy (HAXPES). The HAXPES results indicate strong electrostatic interactions between the TiO2 semiconductor and metal nanoparticles, while density functional theory (DFT) calculations suggest that these interactions are primarily associated with charge transfer leading to electric dipole moments at the interface in the ground state. The effects of these dipole moments are not limited to the surface but also occur deep in the bulk of the semiconductor, and are highly dependent on the coverage of the metal nanoparticles on the semiconductor species.

Toward Solar-Driven Photocatalytic CO2 Reduction Using Water as an Electron Donor.

Developing a system for the production of organic chemicals via CO2 reduction is an important area of research that has the potential to address global warming and fossil fuel consumption. In addition, CO2 reduction promotes carbon source recycling. Solar energy is the largest exploitable resource among renewable energy resources, providing more energy to Earth per hour than the total energy consumed by humans in 1 year. This report describes the advantages and disadvantages of the available CO2 reduction and H2O oxidation photocatalysts and the conjugation of photocatalytic CO2 reduction with H2O oxidation for the creation of an artificial photosynthesis system. In this system, CO2 photoreduction and H2O photooxidation proceeded simultaneously within one system under sunlight irradiation using a hybrid of semiconductors and molecular metal-complex catalysts.

Enhanced performance of molecular electrocatalysts for CO2 reduction in a flow cell following K+ addition.

Electrocatalytic CO2 reduction is a key aspect of artificial photosynthesis systems designed to produce fuels. Although some molecular catalysts have good performance for CO2 reduction, these compounds also suffer from poor durability and energy efficiency. The present work demonstrates the improved CO2 reduction activity exhibited by molecular catalysts in a flow cell. These catalysts were composed of a cobalt-tetrapyridino-porphyrazine complex supported on carbon black together with potassium salt and were both stable and efficient. These systems were found to promote electrocatalytic CO2 reduction with a current density of 100 mA/cm2 and generated CO over at least 1 week with a selectivity of approximately 95%. The optimal catalyst gave a turnover number of 3,800,000 and an energy conversion efficiency of more than 62% even at 200 mA/cm2.

Highly selective CO2 electrolysis in aqueous media by a water-soluble cobalt dimethyl-bipyridine complex.

A water-soluble Co complex with dimethyl-bipyridine ligands reduced CO2 to CO electrochemically with almost 100% selectivity at -0.80 V vs. NHE in an aqueous medium (pH 6.8) without an organic solvent. The reaction overpotential was 270 mV. A possible CO formation mechanism was discussed based on experiments and calculations.

Ni-modified ß-FeOOH nanorod cocatalysts for oxygen evolution utilising photoexcited holes on a N 2p level in a N-doped TiO2 electrode.

Traditionally, N-doped TiO2 (N-TiO2) has been regarded as unsuitable for the oxygen evolution reaction (OER) under visible light. Ni-modified ß-FeOOH nanorod cocatalysts enabled to use N 2p holes in the N-TiO2 photoanode induced by 400-500 nm visible light photons for the OER, enhancing anodic photocurrent of N-TiO2 with 13-fold with 100% faradaic efficiency.

Exploring the Physiological and Psychological Effects of Digital Shinrin-Yoku and Its Characteristics as a Restorative Environment.

This study investigated the physiological and psychological therapeutic effects of a digital Shinrin-yoku environment constructed indoors in an urban facility as well as the characteristics of the environment that contribute to restorativeness (restorative traits). We measured the fluctuations in the physical and mental states of 25 subjects by obtaining both before-after measurements and continuous measurements while exposed to a digital Shinrin-yoku environment that reproduced visual, auditory, and olfactory elements. The results demonstrated that the parasympathetic nerve activity was significantly increased and that the heart rate was significantly decreased during the exposure compared with that during the resting state. As for mood, five of the six Profile of Mood States (POMS) scales ("Tension-Anxiety," "Depression," "Anger-Hostility," "Fatigue," and "Confusion") were significantly decreased after the experience. In addition, psychological restorative effects were also confirmed, with a significant decrease in "negative affect" (measured using the Positive and Negative Affect Schedule (PANAS)) and a significant increase in the sense of restorativeness (Restorative Outcome Scale (ROS)) after the experience. In contrast, comparing the digital Shinrin-yoku environment with the actual forest environment and the urban environment using POMS, PANAS, ROS, and Perceived Restorativeness Scale (PRS), the psychological effects and environmental traits of the digital Shinrin-yoku were found to be considerably similar to those of the actual forest environment.

Importance of steric bulkiness of iridium photocatalysts with PNNP tetradentate ligands for CO2 reduction.

A series of Ir complexes has been developed as multifunctional photocatalysts for CO2 reduction to give HCO2H selectively. The catalytic activities and photophysical properties vary widely across the series, and the bulky group insertion resulted in the formation of HCO2H and CO with the catalyst turnover number of >10 400.

Selective CO2 conversion to formate conjugated with H2O oxidation utilizing semiconductor/complex hybrid photocatalysts.

Photoelectrochemical reduction of CO(2) to HCOO(-) (formate) over p-type InP/Ru complex polymer hybrid photocatalyst was highly enhanced by introducing an anchoring complex into the polymer. By functionally combining the hybrid photocatalyst with TiO(2) for water oxidation, selective photoreduction of CO(2) to HCOO(-) was achieved in aqueous media, in which H(2)O was used as both an electron donor and a proton source. The so-called Z-scheme (or two-step photoexcitation) system operated with no external electrical bias. The selectivity for HCOO(-) production was >70%, and the conversion efficiency of solar energy to chemical energy was 0.03-0.04%.