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Au–TiO2 photocatalysts were prepared using electrons trapped in oxygen vacancies of TiO2. Their Au dispersibility and hydrogen production ability are higher than those prepared using a conventional photodeposition method.

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.

We have established a highly efficient protocol for hydrogen production from lignocellulosic biomass at high temperatures in an alkaline solution using a stable CdS/SiC composite. The composite abs...

In this study, we fabricated a device that can be used for water purification in dark areas by combining a light-leakage type plastic optical fiber with a photocatalyst. The light-leakage type fibe...

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.

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.

Photosynthetic pigment–protein-based biophotovoltaic devices are attracting interest as environmentally friendly energy sources. Photosystem I (PSI), a photosynthetic pigment–protein, is a proven biophotovoltaic material because of its abundance and high charge separation quantum efficiency. However, the photocurrent of these biophotovoltaic devices is not high because of their low spectral response. We have integrated an artificial light-harvesting antenna into a PSI-based biophotovoltaic device to expand the spectral response. To fabricate the device, a perylene di-imide derivative (PTCDI) was introduced onto a TiO2 surface as an artificial antenna. In the photovoltaic cells formed by the PTCDI/PSI-assembled TiO2 electrode, the magnitude of the incident photon-to-current conversion efficiency spectrum was significantly enhanced in the range 450–750 nm, and the photocurrent increased to 0.47 mA/cm2. The result indicates that the photons absorbed by PTCDI transfer to PSI via Forster resonance energy transfer.

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 Forster resonance energy transfer; therefore, the Pt nanoparticles act as active sites for hydrogen generation.

A clear understanding of the electron energy structure of TiO

Photocatalytic water splitting under visible light has attracted attention as a possible solution to the energy exhaustion problem. Hitherto, water splitting has been generally achieved using several oxynitrides, oxysulfides, and nitrides, and only a few studies report water splitting using cadmium sulfide (CdS) as a photocatalyst. A major reason for this is that CdS undergoes photocorrosion. In this study, we achieved an overall water splitting under visible light using a CdS/WO3/CdWO4 tricomposite photocatalyst. In the process, photocorrosion of CdS was suppressed by covering it with WO3 and CdWO4, and the oxidation reaction progressed in WO3 by the Z-scheme type photocatalytic reaction.

In this work, CdS/SiC/TiO2 tri-composite photocatalysts that exploit electron- and hole-transfer processes were fabricated using an easy two-step method in the liquid phase. The photocatalyst with a 1:1:1 M ratio of CdS/SiC/TiO2 exhibited a rate of hydrogen evolution from an aqueous solution of sodium sulfite and sodium sulfide under visible light of 137 μmol h−1 g−1, which is 9.5 times that of pure CdS. β-SiC can act as a sink for the photogenerated holes because the valence band level of β-SiC is higher than the corresponding bands in CdS and TiO2. In addition, the level of the conduction band of TiO2 is lower than those of CdS and β-SiC, so TiO2 can act as the acceptor of the photogenerated electrons. Our results demonstrate that hole transfer and absorption in the visible light region lead to an effective hydrogen-production scheme.

We herein report the oxidation efficiency of different concentration ammonia waters via a complex photocatalyst of zeolite and TiO2 under UV light. The generated NO2− and NO3− concentrations were h...

Some of the most significant challenges of the 21st century are related to energy exhaustion and global warming. In order to overcome these issues, it is necessary to produce sustainable energy without carbon dioxide emissions. Since the discovery of the Honda-Fujishima effect (H2O + 2h ν → 1/2 O2 + H2) in 1972, photocatalytic water splitting has attracted considerable attention as a solution to energy exhaustion. Since hydrogen is a clean source of energy that does not generate carbon dioxide when employed, water splitting has been considered an effective measure against global warming. The most commonly studied photocatalyst is TiO2. However, hydrogen production from water using TiO2 requires ultraviolet light, which deems it inefficient. In order to achieve a high conversion efficiency, which is considered suitable for practical applications, it is necessary to achieve a minimal absorption of visible light. To satisfy this requirement, a visible-light-responsive photocatalyst should be employed in the water splitting process. However, many of the photocatalysts with high visible light responsiveness, such as sulfide semiconductors and nitride semiconductors, have photocorrosive issues. This is an issue where hole carriers generated by photo-excitation cause the oxidative decomposition of the photocatalyst itself. To date, two approaches have been reported to suppress photocorrosion. The first is to transfer photocorrosive holes to different photocatalysts. The transfer of holes to the valence band of another photocatalyst has been reported. (H. Nagakawa et al., ACS Omega, 2018, 3, 10, 12770-12777.) In the previous study, the hydrogen generation efficiency was increased by transporting the excited electrons and holes from CdS to SiC. This approach successfully suppressed the photocorrosion of CdS. The second photocorrosion suppression method is to protect the photocorrosion-susceptible photocatalyst with different stable photocatalysts, thereby reducing exposure to the solution and preventing elution. The current authors reported that this method promoted the water splitting reaction in a CdS composite photocatalyst. (H. Nagakawa et al., ACS Applied Energy Materials, 2018, 1, 12, 6730-6735.) In the study, we achieved an overall water splitting under visible light using a home-synthesized tri-composite, in which CdS was covered with a photocatalyst on both the oxidation and reduction sides. Previously, the use of CdS/SiC/TiO2 tri-composite photocatalysts was found to promote the consumption of holes on CdS and reduce the exposure of CdS to the solution, thereby suppressing photocorrosion. However, since oxidation of water cannot be achieved with the oxidizing power of SiC, in this study, we employed WO3 instead of SiC and realized stronger oxidizing power by constructing a Z-scheme. In addition, we employed CdWO4 as a photocatalyst on the reduction side in order to enable the transport of electrons excited on CdS to the conduction band of CdWO4. A highly controlled CdS/CdWO4 core–shell composite was synthesized by a simple method via the dissolution of CdS under acidic conditions. The eluted Cd2+ reacted with the dissolved WO4 2- in solution in situ, allowing selective deposition on the CdS surface for uniform shell synthesis. (H. Nagakawa et al., RSC Advances, 2020, 10, 1, 105-111.) As a result, recombination was suppressed and the hydrogen production ability was improved compared to that of the dispersion system of CdS and WO3. Furthermore, water splitting was achieved by reducing the CdS exposure and suppressing photocorrosion. Hydrogen evolution experiments were conducted in vial bottles under Ar flow. First, 1 wt % Pt was deposited on the prepared photocatalyst using H2PtCl6·6H2O through photo-deposition under visible light (λ > 420 nm) in 30 vol% methanol aqueous solution. The synthesized CdS/WO3/CdWO4-Pt composite photocatalyst (0.03 g) and pure water (15 mL) were prepared in the reaction cell. A Xe lamp with a UV cutoff filter (λ > 420 nm) was used as a visible-light source. A 0.1 mL aliquot of the evolved gas was collected using a gastight syringe, and the sampled gas was injected into the gas chromatograph to determine the amounts of evolved hydrogen and oxygen. As a result, hydrogen and oxygen were constantly produced at a ratio of 2 : 1 while suppressing photocorrosion. This is the first report of water splitting by a powder-based photocatalyst using CdS under visible light. Figure 1

The photocatalytic reaction uses light energy to advance various chemical reactions, so they are attracting attention as a clean process. The range of photocatalysis applications is wide, and it has been put into practical use and researched in many fields such as water splitting for energy production, self-cleaning, and environmental purification. We focused on the field of water purification using photocatalysts. Hydroxyl radicals generated by electrons excited on the photocatalyst by light irradiation have strong oxidation. It removes harmful compounds such as organic compounds and bacteria. When decomposing harmful substances using a photocatalyst in water purification, the surface of the material coated with the photocatalyst is irradiated with light. However, photocatalysis requires the introduction of light, which limits its use in dark places such as suspended water, water pipes, and water tanks. One way to address these issues is to provide the photocatalyst with light from the coating material itself. Optical fiber composites have been devised as one such method. Devices reported to date have consisted of silica fibers coated with a photocatalyst. However, silica fiber has limitations such as weak bending, small core diameter, and material cost. To overcome these shortcomings, we combined plastic optical fibers and photocatalysts. Plastic optical fiber（POF）was selected because it is more bendable than silica optical fiber, has a larger core diameter, is lower cost, and is easier to process. Next, we examined the light leakage method. In previous studies, removal of the fiber cladding by heat or physical damage was common. However, it is not a preferable method for increasing the number of manufacturing processes and for uniform light leakage. This time, during the fiber manufacturing process, an optical fiber that leaks light was added by adding a light scattering agent during core manufacturing. The light Leakage-type plastic optical fiber（POF）were made using previously reported methods. The preform was made by polymerizing methyl methacrylate (179 g, MMA) into a tubular shape with closed ends. (inner diameter / outer diameter 14.7 / 22.0 mm, length 600 mm).3.87 mg of titanium dioxide (TiO2) powder (P25), which is a material with a high refractive index, was added as a scattering agent when manufacturing the core. Then, the obtained preform was heat-drawn to produce L-POF having a diameter of 1000 μm. The photocatalyst layer was formed by three kinds of coating methods. In the dip coating method, the photocatalyst layer was formed by immersing the fiber in a suspension of TiO2 dispersed in ethanol. In the paste coating method, commercially available TiO2 paste Was applied to the fiber. In the two-layer coating method, an adhesive solvent (NRC-300A,) was dip coated, and then an adhesive solvent (NRC-300C) in which TiO2 was similarly dispersed was dip coated. The fabricated fiber was evaluated by near-field pattern measurement. This is to measure the light intensity at the fiber end face by observing the laser light incident from the end face of the optical fiber with a CCD camera installed at the other end face. The surface of each coated fiber was observed with a scanning electron microscope (SEM, S4800). The photocatalytic performance was evaluated by a methylene blue decomposition experiment. In this study, we succeeded in producing L-POF by adding a light scattering agent to the core of POF. In addition, a TiO2 photocatalyst layer was formed on the surface, enabling decomposition of organic substances in the dark. Light incident from the end face of L-POF leaks into the clad layer due to the scattering agent inside the core and excites the photocatalyst. Electron-hole carriers excited on the photocatalyst generate reactive oxygen species that decompose methylene blue. Since this method uses POF, which is inexpensive and easy to process, it can realize various photocatalytic reactions not only in the purification of contaminated water but also in dark places. By changing the incident light and coating the photocatalyst, devices suitable for various reactions can be made. From a practical point of view, by connecting a POF and photocatalytic L-POF, light could be transported to target areas for photocatalytic application with reduced light loss. Figure 1

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.

2021