Osamu Tomita

Highly selective phenol production from benzene on a platinum-loaded tungsten oxide photocatalyst with water and molecular oxygen: selective oxidation of water by holes for generating hydroxyl radical as the predominant source of the hydroxyl group

Particles of tungsten oxide loaded with nanoparticulate platinum (Pt/WO3) photocatalytically produced phenol from benzene with high selectivity (e.g., 74% at 69% of benzene conversion) in water containing molecular O2; the selectivity for phenol was much higher than that on conventional titanium oxide (TiO2) photocatalysts (both the unmodified and Pt-loaded) that generated CO2 as a main product. Results confirmed that photoexcited electrons on the Pt/WO3 photocatalysts mainly generated H2O2 from molecular O2 through a two-electron reduction; the H2O2 generated did not significantly contribute to the undesirable peroxidation of the phenol produced. In contrast, the oxygen radical species, such as ˙O2− or ˙HO2, generated on TiO2 photocatalysts partially contributed to the successive oxidation of phenol and other intermediates to reduce the selectivity for phenol. More importantly, the reactions using 18O-labeled O2 and H2O clearly revealed that the holes generated on Pt/WO3 react primarily with H2O molecules, even in the presence of benzene in aqueous solution, selectively generating ˙OH radicals that subsequently react with benzene to produce phenol. In contrast, a portion of the holes generated on TiO2 photocatalysts reacts directly with benzene molecules, which are adsorbed on the surface of TiO2 by strong interaction with surface hydroxyl groups. This direct oxidation of substances by holes undoubtedly enhanced non-selective oxidation, consequently lowering the selectivity for phenol by TiO2. The two unique features of Pt/WO3, the absence of reactive oxygen radical species from O2 and the ability to selectively oxidize water to form ˙OH, are the most likely reasons for the highly selective phenol production.

Two-step photocatalytic water splitting into H2 and O2 using layered metal oxide KCa2Nb3O10 and its derivatives as O2-evolving photocatalysts with IO3−/I− or Fe3+/Fe2+ redox mediator

Two-step photoexcitation (Z-scheme) systems that can split water into H2 and O2 under UV light were constructed using a layered potassium calcium niobate (KCa2Nb3O10) and its derivatives as O2-evolving photocatalysts, combined with an appropriate H2-evolving photocatalyst in the presence of iodate/iodide (IO3−/I−) or iron(III)/(II) (Fe3+/Fe2+) as an electron mediator. The original KCa2Nb3O10 showed negligibly low activity for photocatalytic water oxidation to O2 in the presence of IO3− as an electron acceptor, since the anionic IO3− cannot penetrate into the interlayer spaces owing to the strong electrostatic repulsion between IO3− and the negatively charged niobate layers. The rate of O2 evolution was significantly increased after the exfoliation-restack process of KCa2Nb3O10, certainly due to much facilitated access of IO3− to the opened reduction sites of nanosheet surfaces. The loading of RuOx or PtOx cocatalysts on the samples significantly increased the rate of O2 evolution. The electrochemical analysis indicated that these cocatalysts effectively decreased the overpotential of IO3− reduction, which occurs through the 6-electron process. On the other hand, the original layered structure was effective for the photocatalytic O2 evolution in the presence of Fe3+ electron acceptor even without any cocatalysts, suggesting that the interlayer spaces of the layered niobate can work as effective reduction sites for cationic Fe3+. Finally, simultaneous evolution of H2 and O2 was attempted by using these KCa2Nb3O10-based materials as O2-evolving photocatalysts, combined with an appropriate H2-evolving photocatalyst. By employing the appropriate combination of KCa2Nb3O10-based materials and the redox couple, which was suggested by the result of half O2-evolution reactions, simultaneous evolution of H2 and O2 stably proceeded with higher rates.

Fabrication of cation-doped BaTaO2N photoanodes for efficient photoelectrochemical water splitting under visible light irradiation

A series of cation-doped BaTaO2N particle was synthesized to control the donor density in the bulk for improving the performance of photoelectrochemical water splitting on porous BaTaO2N photoanodes under visible light. Among the dopants (Mo6+, W6+, Zr4+, and Ti4+) examined, Mo6+ cations can be introduced into the Ta5+ site up to 5 mol. % without producing any impurity phases; the donor density of BaTaO2N was indeed increased significantly by introducing higher ratio of Mo6+ dopant. The porous photoanodes of Mo-doped BaTaO2N showed much higher photocurrent than others including undoped one and also exhibited much improved performance in photoelectrochemical water splitting into H2 and O2 after loaded with cobalt oxide cocatalyst and coupled with Pt counter electrode.

Manganese-Substituted Polyoxometalate as an Effective Shuttle Redox Mediator in Z-Scheme Water Splitting under Visible Light

In the present study, a polyoxometalate is for the first time applied as a shuttle redox in two-step (Z-Scheme) photocatalytic water splitting. Photocatalytic H2 evolution using a Mn-substituted polyoxometalate [SiW11 O39 Mn(II) (H2 O)](6-) as an electron donor proceeded over a Ru-loaded SrTiO3 :Rh photocatalyst under visible light with relatively high selectivity, accompanied by the stoichiometric production of its oxidized form [SiW11 O39 Mn(III) (H2 O)](5-) . Photocatalytic O2 evolution using the oxidized [SiW11 O39 Mn(III) (H2 O)](5-) as an electron acceptor proceeded over PtOx -loaded WO3 photocatalyst under visible light with relatively high quantum efficiency and selectivity, whereas the loading of effective PtOx cocatalyst was necessary to facilitate the reduction of polyoxometalate. Finally, a two-step water splitting into H2 and O2 was demonstrated under visible light using the couple of Mn-substituted polyoxometalate as shuttle redox between Ru/SrTiO3 :Rh and PtOx /WO3 photocatalysts, under mildly acidic conditions with pH≈4.5.

Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems

Visible light-driven water splitting using cheap and robust photocatalysts is one of the most exciting ways to produce clean and renewable energy for future generations. Cutting edge research within the field focuses on so-called “Z-scheme” systems, which are inspired by the photosystem II–photosystem I (PSII/PSI) coupling from natural photosynthesis. A Z-scheme system comprises two photocatalysts and generates two sets of charge carriers, splitting water into its constituent parts, hydrogen and oxygen, at separate locations. This is not only more efficient than using a single photocatalyst, but practically it could also be safer. Researchers within the field are constantly aiming to bring systems toward industrial level efficiencies by maximizing light absorption of the materials, engineering more stable redox couples, and also searching for new hydrogen and oxygen evolution cocatalysts. This review provides an in-depth survey of relevant Z-schemes from past to present, with particular focus on mechanistic breakthroughs, and highlights current state of the art systems which are at the forefront of the field.

Strong hybridization between Bi-6s and O-2p orbitals in Sillén–Aurivillius perovskite Bi4MO8X (M = Nb, Ta; X = Cl, Br), visible light photocatalysts enabling stable water oxidation

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.

Surface-modified metal sulfides as stable H2-evolving photocatalysts in Z-scheme water splitting with a [Fe(CN)6]3−/4− redox mediator under visible-light irradiation

Z-scheme splitting of water into H2 and O2 was achieved using K2[CdFe(CN)6]-modified metal sulfides as H2-evolving photocatalysts combined with an appropriate O2-evolution system (CoOx-loaded TaON photoanode) in the presence of [Fe(CN)6]3−/4− as an electron mediator. Pt/CdS showed high activity (67.7 μmol h−1) for H2 evolution in borate buffer solution containing [Fe(CN)6]4− as an electron donor under visible-light irradiation. A cyano complex containing [Fe(CN)6]4− and Cd2+, which efficiently scavenged photogenerated holes and effectively facilitated the oxidation of [Fe(CN)6]4− to [Fe(CN)6]3−, was formed on the CdS surface in the reaction. Modification of other metal sulfides, such as ZnIn2S4 and CdIn2S4, with the cyano complex K2[CdFe(CN)6] significantly improved their rates of H2 evolution (ZnIn2S4: 4.8 → 15 μmol h−1; CdIn2S4: 0.8 → 6 μmol h−1) in the presence of [Fe(CN)6]4− and an appropriate O2-evolution system (CoOx-loaded TaON photoanode), enabling Z-scheme water splitting.

Design of nitrogen-doped layered tantalates for non-sacrificial and selective hydrogen evolution from water under visible light

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.

Enhanced oxygen evolution on visible light responsive TaON photocatalysts co-loaded with highly active Ru species for IO3− reduction and Co species for water oxidation

Loading an appropriate cocatalyst significantly enhances the activity of semiconductor photocatalysts in both conventional one-step water splitting and Z-scheme-type water splitting with a redox couple. In the present study, we examined the catalytic activity of Ru-based cocatalysts for IO3− reduction to improve the efficiency of O2 evolution on a TaON photocatalyst under visible light with an IO3− acceptor, which is an important half component in Z-scheme water splitting systems. X-ray photoelectron spectroscopy analyses and electrochemical measurements revealed that the calcination temperature in the loading of Ru species from aqueous RuCl3 significantly affected the catalytic activity for the reduction of IO3−. Calcination at or below 200 °C produced Ru(OH)xCly species that drastically enhanced the reduction of IO3−, whereas that at 300 °C generated conventional RuO2 that showed lower activity for IO3− reduction than Ru(OH)xCly but considerably enhanced the water oxidation. Co-loading the Ru(OH)xCly and Co cation species as water oxidation cocatalysts significantly improved O2 evolution on the TaON photocatalyst by more than a factor of two compared to that loaded with only the conventional RuO2 cocatalyst. This suggests the effectiveness of co-loading two different cocatalysts that are independently optimized for reduction and oxidation.

Fabrication of CuInS2 photocathodes on carbon microfiber felt by arc plasma deposition for efficient water splitting under visible light

An efficient copper indium disulfide (CuInS2) photocathode was fabricated on conductive carbon microfiber felt (CMF), which comprises a three-dimensional (3D) network of carbon fibers (CFs), by the sequential deposition of metal precursors (Cu and In) and subsequent annealing under a stream of diluted H2S. Although the conventional electrochemical deposition method failed to deposit the metal precursors homogeneously on the CMF, the arc plasma deposition (APD) method did so successfully. The unique features of the APD method enabled superior homogeneity of the Cu/In ratio throughout the CMF, not only on the outer surface, but also inside the 3D network, under optimized conditions. Another unique feature of the APD method is its ability to deposit metal species on the back surface of the CFs, thus allowing an almost full-coverage coating of the 3D structure of the CMF substrate. The as-prepared CuInS2/CMF photocathode was further modified with a thin layer of CdS and Pt particles, and then used for photoelectrochemical (PEC) water reduction under visible light. The modified Pt–CdS/CuInS2/CMF photocathode exhibited relatively high incident photon-to-electron conversion efficiency (IPCE) values (ca. 40% at 0 V vs. the RHE under 600 nm) and higher cathodic photocurrent density under continuous visible-light irradiation (λ > 400 nm) than a conventional Pt–CdS/CuInS2 cathode fabricated on a two-dimensional molybdenum substrate. PEC water reduction proceeded stably over the Pt–CdS/CuInS2/CMF photocathode under visible light with almost 100% faradaic efficiency, indicating that CMF is a promising photocathode substrate for PEC water splitting.