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Photoexcited charge manipulation was demonstrated by molecular engineering in conjugated polymers, bearing a Ru(ii) complex as the CO2 reduction photocatalyst.

An efficient spatial separation of photoexcited carriers is demonstrated in a layered-oxyhalide nanoplate by controlling the direction of carrier flows upon Rh cocatalyst loading, which leads to drastic photocatalytic H2 evolution activity.

Bismuth-based oxyhalides with layered Sillén(−Aurivillius) structures have attracted significant attention as photocatalysts. Recent studies have unveiled a part of the structure-property relationship of this material; however, it has not been...

Photocatalytic reduction of CO2 using water as a reductant and visible light as the energy source is an important target in artificial photosynthesis. In this study, we report a highly stable photoelectrochemical CO2 reduction involving a new molecular photocathode comprising a NiO electrode and polymerized complexes of the Ru(II) photosensitizer and Ru(II) catalyst (NiO/PRu-poly-Ru-RuCAT1). CO and HCOOH were stably and selectively produced for over 100 h under visible light irradiation and a low bias. The turnover number of CO2 reduction products exceeded 1200, which represents the highest durability reported for photoelectrochemical reactions using molecular photocathodes. Moreover, a connected system of the NiO/PRu-poly-Ru-RuCAT1 molecular photocathode and a CoOx/BiVO4 photoanode suitable for water oxidation facilitated stable CO2 reduction with water as an electron donor and visible light as the energy source, with no bias for over 24 h. An energy conversion efficiency of 1.7 × 10−2% was obtained, which is the highest value reported for visible light driven systems utilizing a molecular photocatalyst for CO2 reduction with water as the reductant.

Sillen–Aurivillius layered oxyhalides with single-, double- and triple-perovskite slabs (n = 1–3) are promising visible-light-driven photocatalysts for water splitting with different behaviors observed depending on n. However, there was no report on the photocatalytic activity of n ≥ 4 phases. Here, we report three new oxychlorides BaBi5Ti3O14Cl, Ba2Bi5Ti4O17Cl and Ba3Bi5Ti5O20Cl, respectively, with triple-, quadruple- and quintuple-perovskite layers. The synthesis of these phases involves a “bricklaying” synthesis, where a plain perovskite, BaTiO3, a Sillen phase, BaBiO2Cl, and Aurivillius phases Bi4Ti3O12, BaBi4Ti4O15, and Ba2Bi4Ti5O18 were used as building blocks. The present Sillen–Aurivillius oxychlorides have appropriate valence and conduction band levels for visible-light-induced water splitting. DFT calculation for BaBi5Ti3O14Cl (n = 3) and Ba3Bi5Ti5O20Cl (n = 5) indicates that their valence and conduction bands are separated spatially on the perovskite and fluorite layers, respectively. The photocatalytic activity of the three Sillen–Aurivillius oxychlorides is enhanced with increasing the number of the perovskite layers.

Z-scheme water splitting has exhibited significant potential for practical water splitting under visible light irradiation. This system comprises two photocatalysts for O2 and H2 evolution, along with a redox shuttle for transporting electrons between these photocatalysts. Therefore, the promotion of redox reaction of the shuttle play a crucial role in efficient water splitting. In particular, for multielectron redox reactions (e.g., IO3−/I−), a cocatalyst is indispensable, where only noble metals may contribute thus far. Herein, we demonstrate that FeOx catalyzes the multielectron IO3− reduction on the Bi4TaO8Cl photocatalyst. While bare Bi4TaO8Cl does not show O2 evolution from an aqueous IO3− solution because of the lack of multielectron reduction ability, FeOx loading enables O2 evolution. Based on a series of experimental investigations, such as time-resolved spectroscopy, we elucidated that the O2 evolution enhancement stems from the promotion of multielectron reduction of IO3− by FeOx. The FeIII/FeII redox couple in the loaded FeOx facilitates both electron carrier capture from Bi4TaO8Cl and reduction of the redox mediator. In addition, FeOx is effective for other photocatalysts and redox mediators. To the best of our knowledge, the developed cocatalysts is the first earth-abundant cocatalyst for multielectron redox mediators.

We show that oxyhalide Bi5PbTi3O14Cl is a potential photocatalyst for visible-light-induced water splitting owing to the appropriate band levels for both water reduction and oxidation. The present ...

Owing to the rapid advancement in computer hardware and quantum chemical software, chemists can relatively easily calculate the electronic properties of semiconducting photocatalysts. However, thes...

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.

The development of semiconductors with narrow band gap and high stability is crucial for achieving solar to chemical energy conversion. Compounds with iodine, which has a high polarizability, have attracted attention because of their narrow band gap and long carrier lifetime, as typified by halide perovskite solar cells; however, they have been regarded as unsuitable for harsh photocatalytic water splitting because iodine is prone to self-oxidation. Here, we demonstrate that Ba2Bi3Nb2O11I, a layered Sillen-Aurivillius oxyiodide, not only has access to a wider range of visible light than its chloride and bromide counterparts, but also functions as a stable photocatalyst, efficiently oxidizing water. Density functional theory calculations reveal that the oxygen 2p orbitals in the perovskite block, rather than the fluorite Bi2O2 block as previously pointed out, anomalously push up the valence band maximum, which can be explained by a modified Madelung potential analysis that takes into account the high polarizability of iodine. In addition, the highly polarizable iodide contributes to longer carrier lifetime of Ba2Bi3Nb2O11I, allowing for a significantly higher quantum efficiency than its chloride and bromide counterparts. Visible-light-driven Z-scheme water splitting was achieved for the first time in an iodine-based system using Ba2Bi3Nb2O11I as an oxygen-evolution photocatalyst. The present study provides a novel approach for incorporating polarizable "soft" anions into building blocks of layered materials to manipulate the band structure and improve the carrier dynamics for visible-light responsive functions.

In the present study, a conductive carbon microfiber felt (CMF) with three-dimensional structure of carbon fibers (CFs) was employed as a conductive substrate of visible light responsive WO3 photoanodes for oxidative decomposition of organic contaminants and oxidation of water to molecular oxygen (O2). Since the surface of original CFs in CMF are highly hydrophobic, the pretreatment of CMF and the composition of W-precursor solution significantly affected the morphology and the resulting performance of photoanodes. The preheating of CMF at optimal temperature (640 °C) in air make the surface of CFs hydrophilic substantially while maintaining the mechanical strength of CMF structure, enabling the better impregnation of a water-soluble tungsten precursor, ammonium metatungstate (AMT) solution. The addition of polyethylene glycol 300 (PEG) to the precursor solution further improved the affinity to the surface of CFs, resulting in homogeneous loading of fine WO3 particles preferentially on the surface of CFs as well as in the voids of CFs to some extent. The results on optimization of loaded amount of WO3 of CMF from various precursor solutions indicated that both the WO3 particles loaded on the surface of CFs and those in the voids contribute to the photocurrent generation, while the efficiency of electron injection from WO3 on the CFs seems higher. Consequently, the optimized WO3/CMF photoanode exhibited higher photocurrent compared to conventional porous WO3/FTO photoanodes. It was demonstrated that a WO3 photoanode prepared on CMF could efficiently oxidize water to O2 with considerably high stability under visible light, not only for oxidation of 2-propanol.

Recently, Bi4MO8X (M = Nb and Ta; X = Cl and Br), Sillen–Aurivillius-type single-layered perovskite oxyhalides, have been shown to be promising visible-light-responsive photocatalysts with unique v...

A recent study on Bi4NbO8Cl and related Bi-based layered oxyhalide photocatalysts using a layer-by-layer Madelung potential (L2MP) analysis with a sphere model revealed that oxide anions in the fluorite-type slab of Bi4NbO8Cl are electrostatically destabilized by the 2nd nearest sublayer composed of the apical oxygen of the perovskite slab. Here we provide an extended L2MP analysis of Bi-based layered oxyhalides using a modified (square-prism) model and apply it to other layered compounds. It is found that consideration of up to the 3rd nearest sublayer is necessary to account for the nearly equal valence band maximum between Bi2GdO4X and Bi4NbO8Cl. Additionally, BiOX and Bi2GdO4X show a distinct X-dependence of the Madelung site potential of oxide anions. The L2MP analysis of Bi2WO6, Sr3Sc2Cu2S2O5 and Sr2ScCuSO3 revealed a sizable contribution of distant layers to their electrostatic potentials. This study demonstrates that this method can be applied to a wide range of layered materials and serves as a powerful guide toward finding composition and layer stacking sequences suitable for photocatalysis and other functions.

The development of systems for photocatalytic CO2 reduction with water as a reductant and solar light as an energy source is one of the most important milestones on the way to artificial photosynthesis. Although such reduction can be performed using dye-sensitized molecular photocathodes comprising metal complexes as redox photosensitizers and catalyst units fixed on a p-type semiconductor electrode, the performance of the corresponding photoelectrochemical cells remains low, e.g., their highest incident photon-to-current conversion efficiency (IPCE) equals 1.2%. Herein, we report a novel dye-sensitized molecular photocathode for photocatalytic CO2 reduction in water featuring a polypyrrole layer, [Ru(diimine)3]2+ as a redox photosensitizer unit, and Ru(diimine)(CO)2Cl2 as the catalyst unit and reveal that the incorporation of the polypyrrole network significantly improves reactivity and durability relative to those of previously reported dye-sensitized molecular photocathodes. The irradiation of the novel photocathode with visible light under low applied bias stably induces the photocatalytic reduction of CO2 to CO and HCOOH with high faradaic efficiency and selectivity (even in aqueous solution), and the highest IPCE is determined as 4.7%. The novel photocathode is coupled with n-type semiconductor photoanodes (CoOx/BiVO4 and RhOx/TaON) to construct full cells that photocatalytically reduce CO2 using water as the reductant upon visible light irradiation as the only energy input at zero bias. The artificial Z-scheme photoelectrochemical cell with the dye-sensitized molecular photocathode achieves the highest energy conversion efficiency of 8.3 × 10−2% under the irradiation of both electrodes with visible light, while a solar to chemical conversion efficiency of 4.2 × 10−2% is achieved for a tandem-type cell using a solar light simulator (AM 1.5, 100 mW cm−2).

We show that layered oxyhalides PbBiO2X (X = Cl, Br, I) with a Sillen-type structure possess band levels appropriate for visible-light-induced water splitting. Under visible light, PbBiO2Cl and PbBiO2Br with band gap (BG) of 2.51 and 2.48 eV, respectively, stably oxidized water to O2 in the presence of an Fe3+ electron acceptor. A comparison with structurally related SrBiO2Cl and BaBiO2Cl (BG = 3.55 and 3.54 eV) combined with DFT calculations revealed a significant interaction between O 2p and Pb 6s orbitals leading to the upward shift of the valence band maximum in PbBiO2X as compared with (Sr,Ba)BiO2Cl. Z-scheme water splitting into H2 and O2 has been demonstrated using PbBiO2Cl as an O2-evolving photocatalyst, coupled with an appropriate H2-evolving photocatalyst in the presence of an Fe3+/Fe2+ redox mediator.

Addition of sodium salts in hydrothermal synthesis was revealed to significantly influence the morphology and particle size of WO3 particles due to oriented growth. The plate-like shaped WO3 particles prepared with appropriate amounts of alkaline salts such as NaHCO3 and NaOH exhibited much higher activity for O2 evolution in the presence of Fe3+ electron acceptor, compared to that prepared without any additives and a highly-active commercial one.

The surface modification with zinc or indium cyanoferrate species was revealed to significantly increase the rate of H2 evolution on Pt-loaded ZnIn2S4 photocatalyst under visible light in the presence of [Fe(CN)6]4− as an electron donor basically due to the stable redox behavior, in addition to the previously reported cadmium cyanoferrate species. A stepwise modification was found to enable the formation of thin layers of such cyanoferrate on the photocatalyst surface and thereby improve further the rate of H2 evolution.

In this study, a Mo-substituted Keggin-type polyoxometalate [SiW11MoVIO40]4− was examined as a shuttle redox mediator in Z-scheme water splitting, and its electrochemical and chemical stability were also investigated. The redox cycle of [SiW11MoVIO40]4−/[SiW11MoVO40]5−, which corresponds to the valence change between the incorporated MoVI/MoV species, was confirmed to possess appropriate redox potential (+0.58 V vs. SHE at pH 2.7) for use as a shuttle redox (i.e. between water reduction and oxidation potentials). A sufficiently stable redox cycle in acidic (pH 2.7) aqueous solutions was indeed exhibited, whereas slow but obvious degradation was observed under mildly acidic conditions (pH 4.5). Under visible light, the oxidized form, [SiW11MoVIO40]4−, was found to function as an electron acceptor over WO3-based photocatalysts (i.e., an O2-evolving system), and the reduced form, [SiW11MoVO40]5−, was found to function as an electron donor over Ru-loaded SrTiO3 doped with Rh species (i.e., a H2-evolving system). Finally, two-step water-splitting into H2 and O2 was demonstrated under visible light using [SiW11MoVIO40]4−/[SiW11MoVO40]5− couples as a shuttle redox mediator between Ru/SrTiO3:Rh and PtOx/WO3 photocatalyst particles, under acidic conditions with pH 2.7.

A two-step synthesis via the polymerized complex method (2PC) was developed to improve the photocatalytic activity of a Sillen–Aurivillius oxychloride Bi4TaO8Cl and related oxychlorides for O2 evolution (i.e., water oxidation) in Z-scheme water splitting under visible light. This method uses the polymerized complex reaction to prepare a precursor oxide (e.g., Bi3TaO7), which is subsequently calcined with BiOCl to yield a pure Bi4TaO8Cl phase with smaller particle sizes than those obtained via a conventional single-step solid-state reaction (1SSR). Furthermore, time-resolved microwave conductivity (TRMC) measurements revealed that the Bi4TaO8Cl sample prepared by the 2PC method at 973 K (2PC_973) achieved more than five times longer-lived charge separation than that by the 1SSR at 973 K (1SSR_973), which probably arises from lower numbers of charge-recombination centers produced in the 2PC synthesis. Thus, the synthesized Bi4TaO8Cl samples exhibited a higher rate of O2 evolution (e.g., 20 μmol h−1 for 2PC_973 vs. 4 μmol h−1 for 1SSR_973). Overall water splitting into stoichiometric H2 and O2 was demonstrated by constructing a Z-scheme photocatalytic system consisting of 2PC_973, Ru-modified SrTiO3:Rh, and an Fe3+/Fe2+ shuttle redox mediator, with an external quantum efficiency of 0.9% at 420 nm, which was much higher than that using the sample derived from the optimized 1SSR method at 1173 K (0.4%). The 2PC synthesis was successfully extended to other Sillen–Aurivillius type oxychlorides, Bi4NbO8Cl, Bi6NbWO14Cl and Sr2Bi3Ta2O11Cl, all of which exhibited superior water splitting activity compared to those prepared through the 1SSR.

Bi4NbO8Cl with a Sillen–Aurivillius type perovskite structure has recently been demonstrated to stably and efficiently oxidize water under visible light, possibly related to its unique valence band with O-2p orbitals located at unusually high potentials compared with conventional oxides. Here we study a series of isostructural oxyhalides, Bi4MO8X (M = Nb, Ta; X = Cl, Br), to examine how the cation and anion substitution affects the band structure and the resultant photocatalytic activity. We found experimentally and theoretically that both M and X substitutions have little influence on the electronic structures, providing similar valence band maximums (VBMs) and band gaps to those of Bi4NbO8Cl. They all functioned as stable O2-evolving photocatalysts under visible light without suffering from self-oxidative deactivation, as opposed to BiOBr. DFT calculations further revealed a fairly strong hybridization between the Bi-6s orbitals and the O-2p orbitals, which is interpreted using a revised lone pair (RLP) model, thus explaining at least partly why the O-2p orbitals are elevated in energy.

Re(VII)-based M3ReO8 (M = Y, La, Nd, Sm, Eu, Gd, Dy, Yb) semiconductors have been found to be visible-light-responsive photocatalysts for water oxidation. The low conduction band minimum was mainly composed of Re 5d orbitals, which allowed the semiconductors to utilize visible light up to 500–550 nm.

A layered oxychloride Bi4NbO8Cl is a visible-light responsive catalyst for water splitting, with its remarkable stability ascribed to the highly dispersive O-2p orbitals in the valence band, the origin of which, however, remains unclear. Here, we systematically investigate four series of layered bismuth oxyhalides, BiOX (X = Cl, Br, I), Bi4NbO8X (X = Cl, Br), Bi2GdO4X (X = Cl, Br), and SrBiO2X (X = Cl, Br, I), and found that Madelung site potentials of anions capture essential features of the valence band structures of these materials. The oxide anion in fluorite-like blocks (e.g., [Bi2O2] slab in Bi4NbO8Cl) is responsible for the upward shift of the valence band, and the degree of electrostatic destabilization changes depending on building layers and their stacking sequence. This study suggests that the Madelung analysis enables a prediction and design of the valence band structures of bismuth and other layered oxyhalides and is applicable even to a compound where DFT calculation is difficult to perform.

Because the oxidation of water is both the key and a common process in artificial photosynthesis systems, coupled with the reduction of water or carbon dioxide, the development of photocatalysts or photoelectrodes that can efficiently oxidize water to oxygen molecules (O2) under a wide range of conditions (e.g., pH) is crucial. Although the loading of CoOx cocatalysts on oxynitride (e.g., TaON) semiconductor photoanodes has been proven to boost O2 evolution significantly under basic conditions (pH > 8), their performance has been found to decrease significantly with decreasing pH, mainly owing to the dissolution of CoOx; thus, their use in CO2 reduction systems that are generally carried out under acidic solutions has been limited. Here, we demonstrate that nanoparticulate RhOx species can function as an effective cocatalyst to boost O2 evolution on TaON photoanodes under visible-light irradiation, affording not only superior activity to previous CoOx systems but also versatile performance even under acidic conditions. Although the photocurrent on the RhOx/TaON photoanode under acidic conditions (1.5

Metal mixed-anion compounds (e.g., metal oxyhalides and sulfahalides) have recently emerged as promising photo-absorber materials that harvest a wide range of solar spectrum, whereas fabrication of their porous-structured photoelectrodes for efficient wet-type photoelectrochemical (PEC) reactions remains a challenge. This prime issue is that although partial sintering between the particles in the porous electrodes is required for efficient charge transport, conventional (air-)calcinations for such sintering often cause damage to mixed-anion materials. We herein applied pressure assisted treatment (PAT) to fabrication of porous-structured photoelectrodes based on BiOI and BiSX (X=Br, I). PAT for BiSX electrodes resulted in forming particle layer of densely-packed arraignment accompanied by an obvious decrease in thickness of the particle layers, along with partial fusion of BiSX particles only around the electrode surfaces. The resulting decreased thickness was greatly larger than the theoretical one estimated as a fully filled film, supporting the presence of substantial pores to which liquid electrolytes and reactants can access. In terms of PEC performance, PAT even at 323 K, close to room temperature, for BiSX electrodes led to an apparent increase in the anodic photocurrent, strongly suggesting that PAT enhanced the electron transport through the sintered and enlarged contacts between the particles. On the other hand, PAT for BiOI electrodes did not show any improvement of the PEC performance. This stark contrast suggests that softer nature of BiSX originated from the composition of S2– compared to BiOI is one of the keys to produce an effect on fabricating intended porous structure by the pressure-assisted sintering at low temperature.

A CuGaO2 p-type semiconductor electrode was successfully employed for constructing a new hybrid photocathode with a Ru(II)–Re(I) supramolecular photocatalyst (RuRe/CuGaO2). The RuRe/CuGaO2 photocathode displayed photoelectrochemical activity for the conversion of CO2 to CO in an aqueous electrolyte solution with a positive onset potential of +0.3 V vs. Ag/AgCl, which is 0.4 V more positive in comparison to a previously reported hybrid photocathode that used a NiO electrode instead of CuGaO2. A photoelectrochemical cell comprising this RuRe/CuGaO2 photocathode and a CoOx/TaON photoanode enabled the visible-light-driven catalytic reduction of CO2 using water as a reductant to give CO and O2 without applying any external bias. This is the first self-driven photoelectrochemical cell constructed with the molecular photocatalyst to achieve the reduction of CO2 by only using visible light as the energy source and water as a reductant.

Following the recent discovery of a Sillen–Aurivillius perovskite Bi4NbO8Cl with a stable photocatalytic activity for water oxidation under visible light, we studied a structurally related intergrowth compound Bi6NbWO14Cl. It also works as a stable O2-evolving photocatalyst in a Z-scheme water splitting system, but has a slightly wider band gap than that in Bi4NbO8Cl. These results suggest the importance of the Sillen–Aurivillius framework, offering possible band gap tuning by varying structural motifs and their sequence in the Sillen–Aurivillius-related systems.

Rectangular WO3 particles with selectively exposed faces were prepared via hydrothermal reaction and employed as O2-evolving photocatalyst in the presence of Fe3+ electron acceptor. The prepared WO3 photocatalyst showed a higher rate and selectivity for O2 evolution than other commercial ones even with decreased Fe3+ and increased Fe2+ concentrations, certainly attributable to the unique adsorption property of the exposed faces.

The development of new photocatalytic materials that can stably reduce and/or oxidize water by harvesting a wider range of visible light is indispensable to achieve high practical efficiency in artificial photosynthesis. Although we have recently found that tungstic acid monohydrate (H2WO4) can stably oxidize water to O2 by harvesting a wider range of visible light (∼500 nm) than WO3 (∼460 nm), the origin of its narrower bandgap has not been clarified. Herein, we studied H2WO4 in detail, along with the analogous tungstic acid dihydrate (H4WO5), especially focusing on their band structures and their photocatalytic activities for water oxidation in the presence of different electron acceptors (Ag+ and Fe3+). It was revealed that the conduction band minimum (CBM) of H2WO4 is considerably lower than that of conventional monoclinic WO3, whereas both the band gap and level of H4WO5 are similar to those of WO3. The lower CBM of H2WO4 can probably be explained by a more dispersed conduction band caused by greater orbital overlap between W-5d and O-2p orbitals than in monoclinic WO3. Both tungstic acids stably generate O2 from aqueous solutions containing Fe3+ corresponding to each of their photoabsorption properties (∼460 and ∼500 nm), while they showed negligible activity from sacrificial electron acceptor Ag+. Visible-light-driven Z-scheme water splitting proceeded using both tungstic acids as O2-evolving photocatalysts.

By using a high-pressure reaction, we prepared a new oxynitride ZnTaO2N that crystallizes in a centrosymmetric (R (3) over barc) high-temperature LiNbO3-type structure (HTLN-type). The stabilization of the HTLN-type structure down to low temperatures (at least 20 K) makes it possible to investigate not only the stability of this phase, but also the phase transition to a noncentrosymmetric (R3c) LiNbO3-type structure (LN-type) which is yet to be clarified. Synchrotron and neutron diffraction studies in combination with transmission electron microscopy show that Zn is located at a disordered 12c site instead of 6a, implying an order disorder mechanism of the phase transition. It is found that the dosed d-shell of Zn2+, as well as the high-valent Ta5+ ion, is responsible for the stabilization of the HTLN-type structure, affording a novel quasitriangular ZnO2N coordination. Interestingly, only 3% Zn substitution for MnTaO2N induces a phase transition from LN- to HTLN-type structure, implying the proximity in energy between the two structural types, which is supported by the first-principles calculations.

International audience; A push-pull organic dye and a cobaloxime catalyst were successfully co-grafted on NiO and CuGaO$_2$ to form efficient molecular photocathodes for H$_2$ production with >80% Faradaic efficiency. CuGaO$_2$ is emerging as a more effective p-type semiconductor in photoelectrochemical cells and yields a photocathode with four-fold higher photocurrent densities and 400 mV more positive onset photocurrent potential compared to the one based on NiO. Such an optimized CuGaO$_2$ photocathode was combined with a TaONǀCoO$_x$ photoanode in a photoelectrochemical cell. Operated in this Z-scheme configuration, the two photoelectrodes produced H$_2$ and O$_2$ from water with 87% and 88% Faradaic efficiency, respectively, at pH 7 under visible light and in the absence of an applied bias, equating to a solar to hydrogen conversion efficiency of 5.4×10$^{−3}$%. This is, to the best of our knowledge, the highest efficiency reported so far for a molecular-based noble metal-free water splitting Z-scheme.

A series of Sillen–Aurivillius layered oxyhalides with a single- and double-perovskite layer, Bi4MO8X (M = Nb, Ta; X = Cl, Br) and AA′Bi3M2O11Cl (A, A′ = Sr, Ba, Pb, Bi; M = Ti, Nb, Ta), have been recently demonstrated as promising photocatalysts for visible-light-induced water splitting due to relatively high energy valence bands arising from strong interactions between Bi-6s and O-2p orbitals. Herein, we report the synthesis, characterization and photocatalytic property of Bi5SrTi3O14Cl with triple-perovskite layers. The combined use of synchrotron X-ray diffraction and scanning transmission electron microscopy allowed the precise determination of the crystal structure (space group: P2an), including site occupancies. The experimental results revealed that Bi5SrTi3O14Cl possesses a similar bandgap and valence band level as the previously reported Bi5PbTi3O14Cl, indicating that the presence of Pb-6s is not essential for the formation of a higher energy valence band maximum. Partial density of states for both materials obtained by DFT calculations suggested considerable interactions of the Bi-6s orbitals with the O-2p orbitals, in particular those in the perovskite layers, which is the key to the formation of the higher energy valence band. Bi5SrTi3O14Cl exhibited higher activity for O2 evolution than Bi5PbTi3O14Cl, suggesting the detrimental effect of Pb2+ on the photocatalytic activity, thereby allowing the construction of a Z-scheme water splitting system workable under visible light.

Photoelectrochemical water splitting into H2 and O2 over a semiconductor-based photocatalyst offers a promising way to achieve the sustainable harvesting and storage of solar energy. However, short diffusion lengths and inefficient separation of the charge carriers in the semiconductors following light absorption result in fast recombination of holes and electrons and eventually poor performance. Herein, we address this problem by integrating an efficient and robust water oxidation catalyst, cobalt oxide (CoOx), into screen-printed TaON photoanodes premodified with TiO2 coatings for better stability. SEM, TEM, and ICP-MS analysis of the Co deposits and electrochemical techniques were used to demonstrate the advantages provided by the photoassisted CoOx electrodeposition method. Specifically, this method allows the selective and facile functionalization of the TiO2-TaON surface with a uniform layer of near-(hemi)spherical CoOx particles having a diameter of 5–15 nm. In comparison to the TiO2-TaON photoanod...

Porous photoanodes for visible-light-induced water oxidation were prepared via simple electrophoretic deposition of TaON particles, preliminarily modified with CoO x nanoparticles, on a Ti substrate. Post-necking process involving methanolic TaCl5 solution and subsequent heating in NH3 stream formed bridges between TaON particles, which facilitated electron transport within the porous electrode and thereby increased the photocurrent significantly. The temperature of the NH3 treatment in the post-necking process significantly influenced both the charge transport through the bridges and the activity of the CoO x cocatalyst for water oxidation, thus producing maximum photocurrent after heating at 723 K. The highly dispersed CoO x nanoparticles considerably improved the stability of the photocurrent due to efficient capture of photogenerated holes and consequent reduction of the probability of self-oxidative deactivation of the TaON surface. The combination of phosphate buffer solutions with a highly dispersed CoO x cocatalyst on the TaON surface significantly increased the photocurrent due to the in situ photoelectrochemical production of an amorphous cobalt/phosphate (Co–Pi) phase, which covers the TaON surface almost entirely, and consequently improved the its stability in long-term photoirradiation.

Mixed anion compounds are expected to be a photocatalyst for visible light-induced water splitting, but the available materials have been almost limited to oxynitrides. Here, we show that an oxychrolide Bi4NbO8Cl, a single layer Sillen-Aurivillius perovskite, is a stable and efficient O2-evolving photocatalyst under visible light, enabling a Z-scheme overall water splitting by coupling with a H2-evolving photocatalyst (Rh-doped SrTiO3). It is found that the valence band maximum of Bi4NbO8Cl is unusually high owing to highly dispersive O-2p orbitals (not Cl-3p orbitals), affording the narrow band gap and possibly the stability against water oxidation. This study suggests that a family of Sillen-Aurivillius perovskite oxyhalides is a promising system to allow a versatile band level tuning for establishing efficient and stable water-splitting under visible light.

Particles of tungsten oxide loaded with a palladium oxide cocatalyst (PdOx/WO3) exhibit higher selectivity in comparison to other photocatalysts, such as Pt/WO3, Pd/TiO2, and Pt/TiO2, in the partial oxidation of alcohols such as 2-propanol to the corresponding aldehydes or ketones (e.g., 80% selectivity for acetone production with ca. 96% conversion of 2-propanol) in water containing molecular O2. A detailed investigation of 2-propanol oxidation as a model reaction revealed significant differences between the reactivities of the WO3 and TiO2 systems. On TiO2 photocatalysts, complete decomposition to CO2 proceeded readily, due to the occurrence of direct oxidation of 2-propanol and acetone adsorbed by holes, resulting in significantly low selectivity for partial oxidation. On the other hand, the rates of acetone peroxidation on WO3 photocatalysts were much lower than those on TiO2 due to the low affinity of the WO3 surface to the substrates, particularly acetone. The low affinity of the WO3 surface also en...

Nitrogen doping into a series of layered tantalates (ALaTa2O7, where A = Li, Na, K, Rb, or Cs) was attempted in order to produce materials capable of catalyzing non-sacrificial and endergonic water reduction under visible light. Heating of KLaTa2O7 and RbLaTa2O7 in an NH3 stream at 1073 K led to successful nitrogen doping, accompanied by a significant shift in the absorption edge of the material toward the visible-light region, while similar treatment of the other tantalates resulted in the collapse of the layered structure or partial anion substitution at the surface. Although the NH3 heating of a conventional RbLaTa2O7 precursor prepared with nearly stoichiometric Rb (Rb/La = 1.2) resulted in the formation of impurities such as Ta3N5 and amorphous tantalum nitrides, the use of a Rb-rich precursor prepared with excess Rb (Rb/La = 2.4) effectively suppressed this impurity formation. The Rb+ cations in the prepared pure nitrogen-doped sample were exchanged with H+ to facilitate the intercalation of water, and a cationic Pt precursor was then selectively introduced into the interlayers and photocatalytically reduced to Pt metal particles. The internally platinized H+/RbLaTa2O7−xNy showed stable H2 evolution in the presence of I− as an electron donor under visible light, accompanied by the generation of I3−. Although the externally platinized H+/RbLaTa2O7−xNy sample and other bulk-type photocatalysts such as Ta3N5 generated H2 in the presence of a sacrificial electron donor, H2 evolution was negligible in the presence of I−. The stable H2 evolution over the internally platinized H+/RbLaTa2O7−xNy sample is due to the suppressed backward reduction of I3− to I− at selective reduction sites in the interlayer spaces, which are accessible only to cationic species and water.

A porous ZnRh2O4 electrode was fabricated by an electrophoretic deposition method on a fluorine-doped tin oxide substrate, and photoelectrochemical water splitting under visible light irradiation (λ > 420 nm) was performed. The porous ZnRh2O4 electrode exhibited a cathodic photocurrent under visible light irradiation and an extremely positive onset potential at +1.2 V vs. reversible hydrogen electrode (RHE) in aqueous Na2SO4 solution. ZnO necking treatment, by which effective contact between ZnRh2O4 particles is formed, afforded a significant increase in the photocurrent. The incident photon to current efficiencies (IPCEs) of the ZnRh2O4 and ZnO/ZnRh2O4 photocathodes were calculated to be ca. 8% and ca. 13% at 400 nm, respectively, at 0 V vs. RHE in aqueous Na2SO4 solution. H2 evolution under visible light (λ > 420 nm) was demonstrated using the ZnRh2O4 and ZnO/ZnRh2O4 photocathodes combined with a Pt electrode under an applied bias (0 V vs. RHE).

In this paper, we introduce a new synthesis method to prepare fine particles of BiVO4 with a scheelite-monoclinic (s-m) phase, which is known as the most favorable crystal phase for photocatalytic water oxidation (O2 evolution) under visible light irradiation, based on a coordination chemistry approach in water. Stable aqueous solutions that contain both Bi3+ and V5+ complexes were prepared by simply mixing two aqueous solutions in which each cation was stabilized with an appropriate chelating agent. The use of chelating agents (glycolic acid (gly), L(+)-tartaric acid (tart), citric acid (cit), or ethylenediamine tetraacetic acid (edta)) was effective to form stable V5+ complexes from NH4VO3. On the other hand, only the use of two equivalents of edta with Bi(NO3)3·5H2O was effective to stabilize the Bi3+ complex in water, while the use of other ligands resulted in precipitations. Evaporation of the aqueous solution containing the stable Bi3+ and V5+ complexes and subsequent calcination in air at 500 °C yielded s-m BiVO4 particles smaller than 300 nm, which were much smaller than BiVO4 particles prepared via conventional solid-state reactions (1–10 μm). In particular, the BiVO4 particles that were prepared with the tart ligand for V5+ stabilization possessed the smallest size (∼80 nm) and exhibited the highest photocatalytic activity for O2 evolution from an aqueous solution containing an electron acceptor (Ag+ or Fe3+) under visible-light irradiation. These results strongly suggested that the tart ligand effectively suppresses particle growth during the crystallization process and thereby affords small BiVO4 particles with high crystallinity, both of which are necessary to achieve highly efficient photocatalysis.

We investigated the structural and optical properties of (A0.6X)Bi4(Nb0.6W0.4)O8X (A = K, Rb, Cs and X = Cl, Br). We found that these Sillen–Aurivillius-related oxyhalides have band gaps of ca. 3.1 eV (X = Cl) and ca. 2.8 eV (X = Br), which are surprisingly larger than 2.4 eV for the archetypal Bi4NbO8Cl, meaning a substantial stabilization of O-2p orbitals upon A0.6X layer insertion. The present study suggests versatile possibilities of band gap engineering in Sillen–Aurivillius type compounds.

RHfO2N perovskites with R = La, Nd and Sm show a GdFeO3-type structure and are semiconductors with band gaps of 3.35, 3.40 and 2.85 eV and relative dielectric constants of 30, 16 and 28 respectively. These compounds have adequate reduction and oxidation potentials to conduct the overall water splitting reaction, and the analogous perovskite LaZrO2N with a band gap of 2.8 eV shows photocatalytic activity under visible light irradiation for O2 evolution.

A photocathode for CO2 reduction was successfully developed using a hybrid electrode comprising a Ru(II)-Re(I) supramolecular photocatalyst and a NiO electrode. Selective photoexcitation of the Ru photosensitizer unit of the photocatalyst at -1.2 V vs. Ag/AgNO3 selectively afforded CO with high faradaic efficiency.

Oxide-supported rhodium catalysts with excellent activity in silylative coupling reactions have been developed. Reductive pretreatment of the catalysts in the presence of 0.5 equiv triphenylphosphine under a hydrogen atmosphere enhanced and stabilized the catalytic activity. Of the catalysts examined, ceria-supported rhodium had the highest activity in the homo-coupling of vinylsilanes to bis(silyl)ethenes at 170 °C. A zirconia-supported catalyst selectively gave E-1-aryl-2-silylethenes by cross-coupling of vinylsilanes with styrenes at 130 °C, and a high turnover frequency of >8200 h−1 was achieved at 170 °C. Spectroscopic studies revealed that well-dispersed surface rhodium(I) species predominantly formed on ceria or zirconia were transformed into rhodium hydride species, which are believed to be responsible for the high activity. These catalysts were recyclable without loss of activity, and leaching of rhodium species from the catalysts was not observed.

Fine particles of Rh doped SrTiO3 (SrTiO3:Rh) were prepared via a newly developed facile water-based process; stable aqueous precursor solutions were prepared by simple mixing of stable aqueous titania sol with Sr and Rh salts in the presence of an acrylic emulsion, then dried and finally calcined in air at 900–1050 °C. The SrTiO3:Rh particles prepared at 1000 °C were smaller than 50 nm in diameter and exhibited much higher efficiency for H2 evolution from methanol aqueous solution under visible light (e.g. 13.2% of quantum yield at 420 nm) than those prepared by conventional solid state reaction (∼5%). On the other hand, the SrTiO3:Rh particles prepared from the aqueous titania sol without the acrylic emulsion or from other aqueous titania precursors with the acrylic emulsion were found to have an increased particle size up to 100 nm, and exhibited lower photocatalytic activity, indicating that the combination of the aqueous titania sol and the acrylic emulsion effectively suppresses the particle growth, and consequently yields SrTiO3:Rh particles having a small particle size and high crystallinity, both of which are necessary to achieve highly efficient photocatalysis.