► Water splitting systems workable under visible light is reviewed. ► Two photocatalysts combined with a redox mediator split water under visible light. ► A solid solution photocatalyst split water ...with one step under visible light. ► New types of cocatalysts effectively enhance water reduction on photocatalysts. ► Various visible light-responsive photoelectrodes show high IPCE values.
Photocatalytic and photoelectrochemical (PEC) water splitting using semiconductor materials has attracted considerable interest due to its potential to cleanly produce H
2 from water by utilizing abundant solar light. Since Fujishima and Honda used a TiO
2 photoanode in 1972 to split water, researchers have been attempting to develop water-splitting systems that can efficiently use visible light (which accounts for almost half of the solar spectrum on the Earth's surface) in order to realize efficient conversion of solar light. In this report, we review recent progress in this field by focusing on strategies that utilize visible light. Such strategies include two-step photoexcitation systems that were inspired by photosynthesis in nature, band engineering for producing novel photocatalysts that have both a high visible light absorption and suitable energy levels for water splitting, the development of new cocatalysts for efficient H
2 or O
2 production, fabrication of efficient photoelectrodes based on visible-light-responsive semiconductors, and the construction of tandem-type PEC water-splitting systems.
Solar energy is attractive because it is free, renewable, abundant and sustainable. Photocatalysis is one of the feasible routes to utilize solar energy for the degradation of pollutants and the ...production of fuel. Perovskites and their derivatives have received substantial attention in both photocatalytic wastewater treatment and energy production because of their highly tailorable structural and physicochemical properties. This review illustrates the basic principles of photocatalytic reactions and the application of these principles to the design of robust and sustainable perovskite photocatalysts. It details the structures of the perovskites and the physics and chemistry behind photocatalytic reactions and describes the advantages and limitations of popular strategies for the design of photoactive perovskites. This is followed by examples of how these strategies are applied to enhance the photocatalytic efficiency of oxide, halide and oxyhalide perovskites, with a focus on materials with potential for practical application, that is, not containing scarce or toxic elements. It is expected that this overview of the development of photocatalysts and deeper understanding of photocatalytic principles will accelerate the exploitation of efficient perovskite photocatalysts and bring about effective solutions to the energy and environmental crisis.
Highly stable photoelectrochemical water splitting is demonstrated for the first time on a tantalum oxynitride (TaON) photoanode under visible light irradiation. Highly dispersed CoO x nanoparticles ...on the TaON photoanode efficiently scavenge photogenerated holes and effectively suppress self-oxidative deactivation of the TaON surface, resulting in a stable photocurrent. The use of highly dispersed CoO x cocatalyst on TaON together with phosphate solutions significantly increased the photocurrent due to the formation of a cobalt/phosphate phase. This enabled us to stably split water into H2 and O2 under visible light irradiation at a relatively low applied bias (0.6 V vs Pt counter electrode).
The band structure offers fundamental information on electronic properties of solid state materials, and hence it is crucial for solid state chemists to understand and predict the relationship ...between the band structure and electronic structure to design chemical and physical properties. Here, we review layered oxyhalide photocatalysts for water splitting with a particular emphasis on band structure control. The unique feature of these materials including Sillén and Sillén–Aurivillius oxyhalides lies in their band structure including a remarkably high oxygen band, allowing them to exhibit both visible light responsiveness and photocatalytic stability unlike conventional mixed anion compounds, which show good light absorption, but frequently encounter stability issues. For band structure control, simple strategies effective in mixed-anion compounds, such as anion substitution forming high energy p orbitals in accordance with its electronegativity, is not effective for oxyhalides with high oxygen bands. We overview key concepts for band structure control of oxyhalide photocatalysts such as lone-pair interactions and electrostatic interactions. The control of the band structure of inorganic solid materials is a crucial challenge across a wide range of materials chemistry fields, and the insights obtained by the development of oxyhalide photocatalysts are expected to provide knowledge for diverse materials chemistry.
Photoanodes made from BaTaO2N that can harvest visible light up to 660 nm wavelength were fabricated on Ti substrates for achieving efficient water splitting. Both pre-treatment of BaTaO2N particles ...with an H2 stream and post-necking treatment with TaCl5 effectively increased the photocurrent due to the decreased electrical resistance in the porous BaTaO2N photoanode. A combination of pre-loading of CoO x on the BaTaO2N particles and post-loading of RhO x significantly improved both the photocurrent and stability under visible light irradiation, along with an obvious negative shift (ca. 300 mV) of the onset potential for water oxidation, while sole loading resulted in a lower photocurrent or insufficient stability. The IPCE value was estimated to be ca. 10% at 1.2 V vs RHE under 600 nm, which is the highest among photoanode materials that can harvest light beyond 600 nm for water oxidation. Photoelectrochemical water splitting into H2 and O2 under visible light was demonstrated using RhO x /CoO x /BaTaO2N/Ti photoanodes under an externally applied bias larger than 0.7 V to a Pt counter electrode.
A two‐step, photocatalytic water splitting system consisting of Pt‐loaded TaON (a H2 evolution photocatalyst), Pt‐loaded WO3 (an O2 evolution photocatalyst), and an iodate–iodide (IO3−/I−) shuttle ...redox mediator is investigated under visible light irradiation. Photocatalytic oxidation of water to O2 and reduction of IO3− to I− proceeded with good selectivity over the Pt–WO3 photocatalyst, even in the presence of a considerable amount of I− anions in the solution. The key difference between the adsorption properties of IO3− and I− anions on WO3 strongly suggested that the photoexcited electrons could react efficiently with IO3− adsorbed on WO3, whereas the photogenerated holes selectively reacted with water molecules owing to the low adsorptivity of I− on WO3. Photocatalytic H2 evolution on Pt–TaON proceeded efficiently, accompanied by I− oxidation to IO3− due to a substantial amount of adsorption of I− anions on the surface, whereas H2 evolution was significantly inhibited by the competitive adsorption of IO3−, which consumes photoexcited electrons. It was also found that WO3 photocatalysts loaded with platinum oxide (PtO) showed a much higher activity for O2 evolution in the presence of the electron acceptor IO3−, compared to those loaded with Pt metal. Overall water splitting at a steady rate was demonstrated using a combination of Pt–TaON and Pt(PtO)–WO3 in an aqueous NaI solution with neutral or weakly acidic pH values, where the concentration of NaI significantly affected the efficiency.
Overall water splitting under visible light is demonstrated through a two‐step photoexcitation between Pt–TaON and Pt(PtO)–WO3 in the presence of an iodate–iodide (IO3−/I−) shuttle redox mediator. The efficiency is significantly affected by the concentration of iodide ions (I−), the pH of the solution, and the physicochemical properties of the platinum cocatalyst.
Visible-light-driven hydrogen (H2) production from water is a promising strategy to convert and store solar energy as chemical energy. Covalent organic frameworks (COFs) are front runners among ...different classes of organic photocatalysts. The photocatalytic activity of COFs depends on numerous factors such as the electronic band gap, crystallinity, surface area, exciton migration, stability of transient species, charge separation and transport, etc. However, it is challenging to fine tune all of these factors simultaneously to enhance the photocatalytic activity. Hence, in this report, an effort has been made to understand the interplay of these factors and identify the key factors for efficient photocatalytic H2 production through a structure–property–activity relationship. Careful molecular engineering allowed us to optimize all of the above plausible factors impacting the overall catalytic activities of a series of isoreticular COFs. The present study determines three prime factors: light absorption, charge carrier generation, and its transport, which influence the photocatalytic H2 production of COFs to a much greater extent than the other factors.
Tungsten oxide loaded with nanoparticulate platinum is demonstrated to exhibit high activity for the decomposition of organic compounds both in liquid and gas phases; the activity was almost ...comparable to that of TiO2 under UV light irradiation and much higher than that of nitrogen-doped TiO2 under visible irradiation.
A two-step photocatalytic water splitting (Z-scheme) system consisting of a modified ZrO2/TaON species (H2 evolution photocatalyst), an O2 evolution photocatalyst, and a reversible donor/acceptor ...pair (i.e., redox mediator) was investigated. Among the O2 evolution photocatalysts and redox mediators examined, Pt-loaded WO3 (Pt/WO3) and the IO3 −/I− pair were respectively found to be the most active components. Combining these two components with Pt-loaded ZrO2/TaON achieved stoichiometric water splitting into H2 and O2 under visible light, achieving an apparent quantum yield of 6.3% under irradiation by 420.5 nm monochromatic light under optimal conditions, 6 times greater than the yield achieved using a TaON analogue. To the best of our knowledge, this is the highest reported value to date for a nonsacrificial visible-light-driven water splitting system. The high activity of this system is due to the efficient reaction of electron donors (I− ions) and acceptors (IO3 − ions) on the Pt/ZrO2/TaON and Pt/WO3 photocatalysts, respectively, which suppresses undesirable reverse reactions involving the redox couple that would otherwise occur on the photocatalysts. Photoluminescence and photoelectrochemical measurements indicated that the high activity of this Z-scheme system results from the moderated n-type semiconducting character of ZrO2/TaON, which results in a lower probability of undesirable electron−hole recombination in ZrO2/TaON than in TaON.
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