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A high degree of cocatalyst dispersion and intimate contact with the photocatalyst are vital for improving the activity of narrow bandgap photocatalytic materials. In this work, the addition of a small number of Na ions to a Pt cocatalyst deposited on a semiconducting BaTaO2N photocatalyst (with an absorption edge of 650 nm) enhanced photocatalytic H2 evolution under visible light irradiation. The addition of Na ions to the Pt-loaded BaTaO2N (Pt/BaTaO2N) caused more efficient evolution of H2 than Na-free Pt/BaTaO2N during Z-scheme overall water splitting. These results should assist in the fabrication of photocatalysts modified with highly dispersed cocatalysts for efficient water splitting.

Here we show that nanoparticulate CoAl2O4 spinel serves as an efficient cocatalyst for photocatalytic oxidation of water. The O2 evolution activity of CoAl2O4/g-C3N4 from an aqueous solution of AgN...

Undoped layered oxynitrides have not been considered as promising H2 -evolution photocatalysts because of the low chemical stability of oxynitrides in aqueous solution. Here, we demonstrate the synthesis of a new layered perovskite oxynitride, K2 LaTa2 O6 N, as an exceptional example of a water-tolerant photocatalyst for H2 evolution under visible light. The material underwent in-situ H+ /K+ exchange in aqueous solution while keeping its visible-light-absorption capability. Protonated K2 LaTa2 O6 N, modified with an Ir cocatalyst, exhibited excellent catalytic activity toward H2 evolution in the presence of I- as an electron donor and under visible light; the activity was six times higher than Pt/ZrO2 /TaON, one of the best-performing oxynitride photocatalysts for H2 evolution. Overall water splitting was also achieved using the Ir-loaded, protonated K2 LaTa2 O6 N in combination with Cs-modified Pt/WO3 as an O2 evolution photocatalyst in the presence of an I3 - /I- shuttle redox couple.

In this study, graphitic carbon nitride (g-C3N4, CN) was decorated with polyaniline (PANI), which was incorporated with inorganic (H2SO4) and organic (5-sulfosalicylic acid, SSA) acids for photocatalytic H2O2 production under simulated solar irradiation. The PANI/CN sample exhibited H2O2 production of approximately 1.5 mM within 3 h, which is much higher than that of either CN or PANI alone in the presence of AgNO3 solution. The increased photocatalytic activity for H2O2 production could be ascribed to the extended visible-light absorption, large specific surface area, and reduced electrical resistance as well as the appropriate redox potential. It should be noted that doping of H2SO4 on PANI could lead to a high oxidation level, which can increase the electrical conductivity of PANI/CN, while the doping of SSA on PANI may induce delocalization and, hence, the enlargement of the specific surface area of PANI. Additionally, CN could afford electron transit through π–π* between the conjugate units of PANI and its tri-s-triazine units, facilitating effective separation for photoexcited electron–hole pairs. This study demonstrated that the doping of PANI with H2SO4 and SSA and coupling with g-C3N4 is a promising technique for photocatalytic H2O2 production utilizing solar energy.

Rutile TiO2 codoped with tantalum and nitrogen (TiO2:Ta,N) was assessed as a water oxidation photocatalyst for Z-scheme water splitting driven by visible light. This material was prepared by thermal ammonolysis of TiO2:Ta with dry NH3 at 773 K, while samples of the oxide precursor were synthesized using a microwave-assisted solvothermal technique, applying various conditions. The photocatalytic activity of the TiO2:Ta,N during water oxidation to O2 from an aqueous FeCl3 solution was found to be greatly affected by the synthesis parameters. The rate of O2 evolution was increased upon increasing the level of Ta doping in conjunction with a highly-crystalized TiO2:Ta precursor resulting from applying a higher temperature during synthesis of the oxide. IrO2 loading of the TiO2:Ta,N photocatalyst also improved the O2 evolution activity. The optimized IrO2/TiO2:Ta,N photocatalyst was applied to a Z-scheme water splitting system in combination with Ru/SrTiO3:Rh and in the presence of redox mediators (Fe3+/2+ or [Co(bpy)3]3+/2+). Under AM1.5G simulated sunlight, this system exhibited a maximum solar-to-hydrogen energy conversion efficiency of 0.039%, which was nearly twice as high as the previously reported system with the use of RuO2/TiO2:Ta,N (0.021%).

Pb2Ti2O5.4F1.2 is a new photocatalyst that has recently been reported to function under visible light with suitable stability, albeit with insufficient photocatalytic activity. The present work demonstrates that the photocatalytic activity of Pb2Ti2O5.4F1.2 during visible-light-driven H2 evolution is improved following post-treatment with alkaline chlorides. The photocatalytic activity of the post-treated Pb2Ti2O5.4F1.2 was found to be greatly affected by both the type of alkaline chloride and the treatment temperature. Among the alkaline chlorides examined, a eutectic NaCl–CsCl mixture (35 : 65 molar ratio) was found to be the most effective. The optimized material, further modified with a Pt cocatalyst using an in situ photodeposition method with a H2PtCl6 precursor, showed approximately 10 times higher activity than that of an untreated analogue. The optimally modified Pb2Ti2O5.4F1.2 was also found to outperform Ru-loaded, Rh-doped SrTiO3, which is one of the most active metal oxide photocatalysts workable under visible light. Transient absorption spectroscopy indicated that the reactivity of photogenerated free and/or shallowly trapped electrons in the Pt-loaded, NaCl–CsCl-treated Pb2Ti2O5.4F1.2 with H2O was higher than that in an analogue not treated with the NaCl–CsCl mixture. Based on various physicochemical analyses, it is evident that the NaCl–CsCl treatment creates ion-exchangeable alkaline-titanate species on the catalyst surface that undergo proton-exchange during the photodeposition of Pt, leading to improved affinity for H2O. This effect explains the high activity of the NaCl–CsCl-treated Pb2Ti2O5.4F1.2 during H2 evolution under visible light.

Promotional effects of co-existing ions on overall water splitting into H2 and O2 have been studied in bulk-type semiconductor photocatalysts (e.g., TiO2), but such an effect remains unexplored in two-dimensional nanosheet photocatalysts. Here we examined the effect of co-existing ions on the photocatalytic water splitting activity of Pt nanocluster-intercalated KCa2Nb3O10 nanosheets. Interestingly, not only anions, as usually observed in bulk-type photocatalysts, but also cations had a significant influence on the photocatalytic performance. The rates of H2 and O2 evolution over Pt/KCa2Nb3O10 as well as the product stoichiometry were improved in the presence of NaI. I− ions were found to effectively suppress undesirable backward reactions, consistent with the previous work by Abe et al. (Chem. Phys. Lett., 2003, 371, 360–364). On the other hand, Na+ ions in the reaction solution were exchanged for K+ in the interlayer space of KCa2Nb3O10 during the water splitting reaction, which promoted interlayer hydration and consequently improved photocatalytic performance.

The Z-scheme CO2 reduction activity of metal complex–semiconductor hybrid photocatalysts was investigated in detail with a focus on the interfacial electron transfer process. Semiconductors of GaN:...

Graphitic carbon nitride (g-C3N4) was synthesized by heating urea at different temperatures (773–923 K) in air, and was examined as a photocatalyst for CO2 reduction. With increasing synthesis temperature, the conversion of urea into g-C3N4 was facilitated, as confirmed by X-ray diffraction, FTIR spectroscopy and elemental analysis. The as-synthesized g-C3N4 samples, further modified with Ag nanoparticles, were capable of reducing CO2 into formate under visible light (λ > 400 nm) in the presence of triethanolamine as an electron donor, with the aid of a molecular Ru(II) cocatalyst (RuP). The CO2 reduction activity was improved by increasing the synthesis temperature of g-C3N4, with the maximum activity obtained at 873–923 K. This trend was also consistent with that observed in photocatalytic H2 evolution using Pt-loaded g-C3N4. The photocatalytic activities of RuP/g-C3N4 for CO2 reduction and H2 evolution were thus shown to be strongly associated with the generation of the crystallized g-C3N4 phase.

Oxynitrides are promising visible-light-responsive photocatalysts, but their structures are almost confined with three-dimensional (3D) structures such as perovskites. A phase-pure Li2 LaTa2 O6 N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium-rich oxide precursor. Li2 LaTa2 O6 N exhibited high crystallinity and visible-light absorption up to 500 nm. As opposed to well-known 3D oxynitride perovskites, Li2 LaTa2 O6 N supported by a binuclear RuII complex was capable of stably and selectively converting CO2 into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li2 LaTa2 O6 N possesses a lower density of mid-gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride.

Nitrogen doping into a metal oxide is a conventional method to prepare a visible-light-responsive photocatalyst. However, the charge imbalance that results from aliovalent anion substitution (i.e., O2–/N3– exchange) generally limits the concentration of nitrogen that can be introduced into a metal oxide, which leads to insufficient visible-light absorption capability. Here we report an effective route to synthesize nitrogen-doped metal oxide using KTiNbO5, which is a compositionally flexible layered oxide and can be exfoliated into nanoscale sheets. KTiNbO5 has a unique layered structure, in which Ti4+ and Nb5+ coexist in the same two-dimensional sheet, and controllable Ti4+/Nb5+ ratios while maintaining the original KTiNbO5-type structure. The use of a Nb-rich oxide precursor could allow for the improvement in the introduction of nitrogen compared with stoichiometric KTiNbO5 during thermal ammonolysis with ammonia gas. Reassembled KTiNbO5 nanosheets with a larger surface area were found to be more useful...

Hybrid photocatalysts constructed with a mononuclear Ru(II)-complex (RuP), silver nanoparticles, and carbon nitride nanosheets (NS-C3N4) photocatalyze CO2 reduction to selectively form formate under visible light. The structure of the nanoparticulate silver species, which worked as promoters for the reaction, was characterized by X-ray diffraction, UV-VIS diffuse reflectance spectroscopy, high-resolution transmission microscopy, and X-ray absorption fine-structure spectroscopy. The silver promoters were loaded on the surface of NS-C3N4 by an impregnation method from an aqueous solution containing AgNO3 or an in situ photodeposition method. Impregnation of NS-C3N4 with 2.0 wt% Ag followed by reduction with H2 at 473 K (further modified with RuP) resulted in the highest photocatalytic activity, giving a turnover number of 5700 (based on RuP), which was the greatest value among the formate-generating hybrid systems with a mononuclear complex. While the optimized photocatalyst contained highly dispersed Ag2O-like nanoclusters as the major silver species, experimental results suggested that highly dispersed Ag0 species are more important for enhancing CO2 reduction activity, that is, the obtained experimental results led us to conclude that there are two major factors affecting activity: one is the feature size of silver species (smaller is better), and the other is the oxidation state of silver (metallic is better).

Co3O4-loaded TiO2 is a photocatalyst capable of oxidizing water into O2 by absorbing an entire range of visible light (400 < λ < 850 nm). In this work, the photocatalytic activity for water oxidation was investigated with respect to crystal phase, specific surface area, and surface morphology of TiO2 support. Results of photocatalytic reactions using six different TiO2 samples that possessed single-phase anatase or rutile structure indicated that the activity could be improved by applying a TiO2 support that had larger specific surface area, because it could accommodate larger amount of Co3O4 with minimal impact of undesirable aggregation. It was also suggested that when the specific surface area is similar, the activity is largely insensitive to crystal phase of TiO2, but is influenced by the surface morphology of TiO2, which can affect the dispersion of Co3O4.

Carbon nitride nanosheets (NS-C3N4) were found to undergo robust binding with a binuclear ruthenium(II) complex (RuRu′) even in basic aqueous solution. A hybrid material consisting of NS-C3N4 (further modified with nanoparticulate Ag) and RuRu′ promoted the photocatalytic reduction of CO2 to formate in aqueous media, in conjunction with high selectivity (approximately 98 %) and a good turnover number (>2000 with respect to the loaded Ru complex). These represent the highest values yet reported for a powder-based photocatalytic system during CO2 reduction under visible light in an aqueous environment. We also assessed the desorption of RuRu′ from the Ag/C3N4 surface, a factor that can contribute to a loss of activity. It was determined that desorption is not induced by salt additives, pH changes, or photoirradiation, which partly explains the high photocatalytic performance of this material.

The photocatalytic activity of SrTiO3 modified with Co3O4 nanoparticles for water oxidation under visible light (λ > 480 nm) was investigated with respect to the physicochemical properties of the SrTiO3 support. SrTiO3 was synthesized by a polymerized complex method or a hydrothermal method, followed by calcination in air at different temperatures in order to obtain SrTiO3 particles having different sizes. Co3O4 nanoparticles, which provide both visible light absorption and water oxidation centers, were loaded on the as-prepared SrTiO3 by an impregnation method using Co(NO3)2 as the precursor, followed by heating at 423 K in air. Decreasing the SrTiO3 particle size (that is, improving the crystallinity) enhanced the photocatalytic activity by promoting the formation of Co3O4 nanoparticles that provided optimal light absorption and catalytic sites. However, Co3O4 aggregation occurred on overly large SrTiO3 particles, leading to a decrease in activity. This study demonstrates the possibility of tuning the photocatalytic activity of a Co3O4-loaded wide-gap semiconductor for visible light water oxidation through the appropriate selection of the support material.

A hybrid photocatalyst constructed with Ta3N5 and a binuclear ruthenium(II) complex had the ability to reduce CO2 into formate with >99% selectivity under visible light (λ > 480 nm), presenting the...

In this study, graphitic carbon nitride (g-C

In this study, graphitic carbon nitride (g-C3N4) nanosheets (CNns) were modified using polyaniline (PANI) codoped with an inorganic (hydrochloric acid, HCl) and an organic (phytic acid, PA) acid. O...

Tantalum/nitrogen codoped titanium dioxide (TiO2:Ta,N) having a rutile structure has recently been reported to be a good photocatalyst for visible-light water oxidation. In this work, three differe...

Cobalt-based compounds, such as cobalt(II) hydroxide, are known to be good catalysts for water oxidation. Herein, we report that such cobalt species can also activate wide-band-gap semiconductors towards visible-light water oxidation. Rutile TiO2 powder, a well-known wide-band-gap semiconductor, was capable of harvesting visible light with wavelengths of up to 850 nm, and thus catalyzed water oxidation to produce molecular oxygen, when decorated with cobalt(II) hydroxide nanoclusters. To the best of our knowledge, this system constitutes the first example that a particulate photocatalytic material that is capable of water oxidation upon excitation by visible light can also operate at such long wavelengths, even when it is based on earth-abundant elements only.

Mixed anion compounds such as oxynitrides and oxychalcogenides are recognized as potential candidates of visible-light-driven photocatalysts since, as compared with oxygen 2p orbitals, p orbitals of less electronegative anion (e.g., N3–, S2–) can form a valence band that has more negative potential. In this regard, oxyfluorides appear unsuitable because of the higher electronegativity of fluorine. Here we show an exceptional case, an anion-ordered pyrochlore oxyfluoride Pb2Ti2O5.4F1.2 that has a small band gap (ca. 2.4 eV). With suitable modification of Pb2Ti2O5.4F1.2 by promoters such as platinum nanoparticles and a binuclear ruthenium(II) complex, Pb2Ti2O5.4F1.2 worked as a stable photocatalyst for visible-light-driven H2 evolution and CO2 reduction. Density functional theory calculations have revealed that the unprecedented visible-light-response of Pb2Ti2O5.4F1.2 arises from strong interaction between Pb-6s and O-2p orbitals, which is enabled by a short Pb–O bond in the pyrochlore lattice due to the f...

Metal nanoclusters (involving metals such as platinum) with a diameter smaller than 1 nm were deposited on the interlayer nanospace of KCa2 Nb3 O10 using the electrostatic attraction between a cationic metal complex (e.g., [Pt(NH3 )4 ]Cl2 ) and a negatively charged two-dimensional Ca2 Nb3 O10 (-) sheet, without the aid of any additional reagent. The material obtained possessed eight-fold greater photocatalytic activity for water splitting into H2 and O2 under band-gap irradiation than the previously reported analog using a RuO2 promoter. This study highlighted the superior functionality of Pt nanoclusters with diameters smaller than 1 nm for photocatalytic overall water splitting. This material shows the greatest efficiency among nanosheet-based photocatalysts reported to date.

Amorphous Fe(III)–Cr(III) mixed oxide nanoparticles were deposited on the surface of powdered SrTiO3 from an aqueous solution containing FeCl3 and K2CrO4, using a photochemical method and without the aid of a heat treatment. The mixed oxide was found to function as a cocatalyst for water oxidation.

The photocatalytic activity of SrTiO

Carbon nitride nanosheets (NS-C

The structure of cobalt oxide (CoOx) nanoparticles dispersed on rutile TiO2 (R-TiO2) was characterized by X-ray diffraction, UV–vis–NIR diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray absorption fine-structure spectroscopy, and X-ray photoelectron spectroscopy. The CoOx nanoparticles were loaded onto R-TiO2 by an impregnation method from an aqueous solution containing Co(NO3)2·6H2O followed by heating in air. Modification of the R-TiO2 with 2.0 wt % Co followed by heating at 423 K for 1 h resulted in the highest photocatalytic activity with good reproducibility. Structural analyses revealed that the activity of this photocatalyst depended strongly on the generation of Co3O4 nanoclusters with an optimal distribution. These nanoclusters are thought to interact with the R-TiO2 surface, resulting in visible light absorption and active sites for water oxidation.

Tantalum oxynitrides with various nitrogen contents were synthesized from Ta2O5 powder by nitridation under a flow of ammonia at 1123 K for various durations. X-ray powder diffraction, transmission electron microscopy, Ultraviolet-visible spectroscopy, energy-dispersive spectroscopy, elemental analysis and photocatalytic reaction were performed to investigate these samples. Selected-area electron diffraction analysis of the mixed crystalline phases of powder samples revealed that each particle had only one crystalline phase. This indicates that entire particles underwent a rapid structural transformation once their nitrogen content reached a critical value. We discovered a new intermediate crystalline phase of tantalum oxynitride, TaO(a1)N(b1), appeared before the generation of the β-TaON phase. The crystal structure of TaO(a1)N(b1) is suggested to be monoclinic, with unit cell parameters of a = 5.1 Å, b = 35.6 Å, c = 5.4 Å and β = 93.5°. The ratio of nitrogen to oxygen in the samples increased with increasing nitridation duration. The increasing rate is different in the different nitridation stage due to the different structure of the samples. Nitrogen entered the samples quickly during the initial 5 h of nitridation, and a monoclinic β-TaON phase was formed. A mesoporous structure emerged in the nitrided particles during the phase transition, greatly increasing the surface area of the samples. The more the nitrogen entered one sample, the darker the color of it due to the narrower the band gap. H2 and O2 evolved by water splitting from the nitrided samples irradiated with visible light. Change in the evolution rate of H2 and O2 had a relation with the structure of the samples.

The mechanism of PdCrOx multi-component cocatalyst formation on SrTiO3 was investigated using transmission electron microscopy, X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy. The PdCrOx/SrTiO3 samples were synthesized by a photodeposition method under UV light irradiation (λ &gt, 300 nm) for various time periods (0–5 h). The fine structure and valence state of the Pd species of PdCrOx nanoparticles were varied from Pd oxide to a mixture of metallic Pd and oxidized Pd species with an increase in the irradiation time. The overall water-splitting activity of PdCrOx was strongly dependent on the photoirradiation time during deposition. Although longer photoirradiation time during preparation did not influence the H2 evolution activity of PdCrOx/SrTiO3 from aqueous methanol solution, it was effective in suppressing the O2 photoreduction activity, which is one of the backward reactions during overall water splitting.

Lanthanoid oxide modifier layers were applied to Rh metal nanoparticles on (Ga1–xZnx)(N1–xOx) photocatalyst for overall water splitting under visible-light irradiation. Structural analysis by transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy revealed that lanthanoid(III) oxide layers coated the Rh nanoparticles on (Ga1–xZnx)(N1–xOx), although they were also deposited directly on the (Ga1–xZnx)(N1–xOx) surface. Certain lanthanoid oxide layers (La, Pr, Sm, Gd, and Dy) functioned as modifiers for Rh-metal/(Ga1–xZnx)(N1–xOx) to produce H2 and O2 gases, although the Rh-metal/(Ga1–xZnx)(N1–xOx) photocatalyst exhibited little activity for overall water splitting due to rapid water formation from H2 and O2 on Rh. Ce and Eu oxide layers showed no photocatalytic activity, most likely due to their ability to capture photoexcited electrons from Rh-metal/(Ga1–xZnx)(N1–xOx). The enhancement of photocatalytic activity by lanthanoid...

A two-step photocatalytic water splitting (Z-scheme) system consisting of a modified BaZrO3–BaTaO2N solid solution, a paired photocatalyst, and a reversible donor/acceptor pair (i.e., redox mediator) was constructed. BaZrO3–BaTaO2N (Zr/Ta = 0.05) having a band gap of 1.8 eV has sufficient potential to reduce and oxidize water by absorbing visible photons of up to 660 nm. Upon suitable modification of this material with nanoparticulate cocatalysts, both H2 and O2 were evolved individually in the presence of reversible redox couples (viz., IO3–/I– and Fe3+/Fe2+), although O2 evolution from aqueous solution containing Fe3+ was negligible. Among various combinations tested, BaZrO3–BaTaO2N modified with Pt nanoparticles was the most suitable for H2 evolution in the presence of an IO3–/I– pair, achieving Z-scheme water splitting into H2 and O2 in combination with either PtOx/WO3 or TiO2 rutile as an O2 evolution photocatalyst even under simulated sunlight.

The synthesis of a transition metal nitride, Ta3N5, was studied. Morphological changes of Ta3N5 particles, depending on the synthetic method, starting materials, and nitridation conditions, were examined in detail. Flux-assisted nitridation was found to be a facile one-step route to a metal nitride with morphological control of the resulting particles. Scanning electron microscopy (SEM) observation revealed that morphologically defined particles of Ta3N5 were obtained using Zn, NaCl, or Na2CO3 as a flux. Flux-assisted nitridation of TaCl5 with Zn or NaCl produced nearly monodisperse fine particles larger than a few tens of nanometers. The size of these particles could be controlled by variation of the nitridation temperature. Highly crystallized Ta3N5 particles with a rectangular parallelepiped shape were produced from TaCl5−NaCl or Ta2O5−Na2CO3 mixtures by nitridation above 1123 K, because of the annealing effect of the flux.

Chiral mesoporous silica (CMS) has been successfully synthesized in the presence of basic amino acids; the use of basic amino acids in combination with the chiral anionic surfactant is advantageous for the formation of CMS in terms of uniformity in the twisted morphology. We first demonstrate that thus obtained chiral mesoporous silicas can be used for the enantioselective separation of racemic compounds; the helical rod-shaped CMS is found to be capable of asymmetric separation of racemic N-trifluoroacetylalanine ethyl ester (CF3CO-Ala-OEt). The left handedness-rich CMS shows asymmetric preferential adsorption of the L isomer and vice versa.

Ordered mesoporous Ta3N5 with crystalline thin-wall structures was successfully obtained from an amorphous mesoporous Ta2O5 by nitridation at 1073 K under flowing NH3 (500 mL min−1). The silica lay...

Modification of tantalum nitride (Ta(3)N(5)), which has a band gap of 2.1 eV, with nanoparticulate iridium (Ir) and rutile titania (R-TiO(2)) achieved functionality as an O(2) evolution photocatalyst in a two-step water-splitting system with an IO(3)(-)/I(-) shuttle redox mediator under visible light (lambda420 nm) in combination with a Pt/ZrO(2)/TaON H(2) evolution photocatalyst. The loaded Ir nanoparticles acted as active sites to reduce IO(3)(-) to I(-), while the R-TiO(2) modifier suppressed the adsorption of I(-) on Ta(3)N(5), allowing Ta(3)N(5) to evolve O(2) in the two-step water-splitting system.

Simultaneous photodeposition of rhodium (Rh) and chromium (Cr) nanoparticles on a semiconductor powder was examined as a means of constructing active sites for hydrogen evolution in photocatalytic overall water splitting. A solid solution of gallium nitride and zinc oxide (GaN:ZnO) powder that catalyzes overall water splitting with visible light was employed as a semiconductor support. The photodeposition was carried out in aqueous suspension containing GaN:ZnO, (NH4)3RhCl6, and K2CrO4, and used H2O as an electron donor. With increasing concentration of K2CrO4, the valence state of the codeposited Rh species varied from metallic to trivalent, while that of Cr remained trivalent. At intermediate concentrations of K2CrO4, the photodeposits were core/shell-like crystalline nanoparticles consisting of a metallic Rh core and an Rh(III)–Cr(III) mixed-oxide shell. The photocatalytic activity for visible-light-driven overall water splitting (λ > 400 nm) was strongly dependent on the structure of the photodeposits. Comparative experiments using an analogue, modified with core/shell-structured Rh/Cr2O3 nanoparticles, revealed that core/shell-structured nanoparticles consisting of a metallic Rh core were better than Rh–Cr trivalent mixed oxides for enhancing the photocatalytic activity.

A two-step photocatalytic water splitting (Z-scheme) system consisting of a modified ZrO(2)/TaON species (H(2) evolution photocatalyst), an O(2) evolution photocatalyst, and a reversible donor/acceptor pair (i.e., redox mediator) was investigated. Among the O(2) evolution photocatalysts and redox mediators examined, Pt-loaded WO(3) (Pt/WO(3)) and the IO(3)(-)/I(-) pair were respectively found to be the most active components. Combining these two components with Pt-loaded ZrO(2)/TaON achieved stoichiometric water splitting into H(2) and O(2) 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 (IO(3)(-) ions) on the Pt/ZrO(2)/TaON and Pt/WO(3) 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 ZrO(2)/TaON, which results in a lower probability of undesirable electron-hole recombination in ZrO(2)/TaON than in TaON.

A new method is introduced to synthesize Ta3N5 crystal by reaction of TaCl5 and NH4Cl powders in vacuo at 850 °C. The synthesized Ta3N5 has a needlelike morphology with preferential growth in the [100] direction. It is possible that adsorption of chloride on the surfaces of the growing crystal inhibits growth in all but this preferential direction. A low photochemical reactivity for water splitting on the needlelike Ta3N5 suggests photochemical reductive sites on (100) surfaces of the Ta3N5 crystal.