Daling Lu

Efficient Nonsacrificial Water Splitting through Two-Step Photoexcitation by Visible Light using a Modified Oxynitride as a Hydrogen Evolution Photocatalyst

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.

Photocatalytic Overall Water Splitting Promoted by Two Different Cocatalysts for Hydrogen and Oxygen Evolution under Visible Light

Overall water splitting using a particulate photocatalyst and solar energy has attracted significant attention as a potential means of large-scale H2 production from renewable resources without carbon dioxide emission. 2] The reaction occurs in three steps: 1) the photocatalyst absorbs photon energy greater than the band-gap energy of the material and generates photoexcited electron–hole pairs in the bulk, 2) the photoexcited carriers separate and migrate to the surface without recombination, and 3) adsorbed species are reduced and oxidized by the photogenerated electrons and holes to produce H2 and O2, respectively. The first two steps are strongly dependent on the structural and electronic properties of the photocatalyst, while the third step is promoted by an additional catalyst (called cocatalyst). Therefore, it is important to develop a photocatalyst and a cocatalyst in harmony. Recently, our group has focused on active sites for H2 evolution on the surface of a photocatalyst, because most photocatalysts lack surface H2 evolution sites. [2b] Using a solid solution of GaN and ZnO (abbreviated GaN:ZnO hereafter) that can harvest visible photons up to ca. 500 nm, chromium-containing transition-metal oxides or noble-metal/ chromia (core/shell) nanoparticles (NPs) have been shown to function as H2 evolution cocatalysts, resulting in efficient water splitting under visible light. Meanwhile, also several sulfides were proposed as efficient catalysts for H2 evolution, and the role of H2 evolution cocatalysts has been explored by spectroscopic and electrochemical techniques. It would be natural to expect that loading both H2 and O2 evolution cocatalysts onto the same photocatalyst would improve water-splitting activity, compared to photocatalysts modified with either an H2 or O2 evolution cocatalyst. [8] It is easy to imagine how these two different cocatalysts would separately facilitate H2 and O2 evolution, thereby promoting overall water splitting in harmony. Unfortunately, no successful and reliable example of this has been reported since the initial reports on photocatalytic water splitting in the 1980s. The actual demonstration of the concept remains a major challenge. Herein, we show a proof-of-concept using GaN:ZnO loaded with Rh/Cr2O3 (core/shell) and Mn3O4 NPs as H2 and O2 evolution promoters, respectively, under irradiation with visible light (l> 420 nm). First, Mn oxide was introduced onto GaN:ZnO, prepared by our previous method, as O2 evolution cocatalyst. Some Mn oxides have been reported to act as O2 evolution promoters, and it is well known that a Mn complex is the O2 evolution center in the photosynthesis of green plants. MnO NPs with a mean size of (9.2 0.4) nm (Figure S1 in the Supporting Information) were adsorbed onto GaN:ZnO. It was revealed by UV/vis spectroscopy that the introduced MnO NPs (ca. 1.0 wt %) were almost quantitatively anchored on the GaN:ZnO surface, based on the change in the absorption band of the MnO NPs (Figure S2 in the Supporting Information). The as-prepared MnO/GaN:ZnO sample was then calcined in air at 673 K for 3 h to remove organic residues. Separate experiments with thermogravimetry, differential thermal analysis (TG-DTA), and X-ray diffraction (XRD) showed that the organic ligands stabilizing the MnO NPs were completely burned off by calcination in air at 673 K, and that calcination of dried MnO NP powder under the above conditions resulted in phase transformation of the MnO into Mn3O4 (Figure S3 in the Supporting Information). Transmission electron microscopy (TEM) observation revealed that the particle size of the Mn oxide was maintained, even after calcination (Figure S1 in the Supporting Information). Thus, GaN:ZnO particles were successfully decorated with Mn3O4 NPs which were expected to act as water oxidation cocatalysts. Because GaN:ZnO is an n-type semiconductor, it is possible to monitor the photooxidation reaction occurring on its surface using an electrochemical technique. Under [*] Dr. K. Maeda, A. Xiong, N. Sakamoto, Dr. T. Hisatomi, Prof. Dr. K. Domen Department of Chemical System Engineering The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan) Fax: (+ 81)3-5841-8838 E-mail: domen@chemsys.t.u-tokyo.ac.jp Homepage: http://www.domen.t.u-tokyo.ac.jp/

Modified Ta3N5 Powder as a Photocatalyst for O2 Evolution in a Two-Step Water Splitting System with an Iodate/Iodide Shuttle Redox Mediator under Visible Light

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 (lambda > 420 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.

Preparation of Core-Shell-Structured Nanoparticles (with a Noble-Metal or Metal Oxide Core and a Chromia Shell) and Their Application in Water Splitting by Means of Visible Light

Core-shell-structured nanoparticles, consisting of a noble metal or metal oxide core and a chromia (Cr(2)O(3)) shell, were studied as promoters for photocatalytic water splitting under visible light. Core nanoparticles were loaded by impregnation, adsorption or photodeposition onto a solid solution of gallium nitride and zinc oxide (abbreviated GaN:ZnO), which is a particulate semiconductor photocatalyst with a band gap of approximately 2.7 eV, and a Cr(2)O(3) shell was formed by photodeposition using a K(2)CrO(4) precursor. Photodeposition of Cr(2)O(3) on GaN:ZnO modified with a noble metal (Rh, Pd and Pt) or metal oxide (NiO(x), RuO(2) and Rh(2)O(3)) co-catalyst resulted in enhanced photocatalytic activity for overall water splitting under visible light (lambda>400 nm). This enhancement in activity was primarily due to the suppression of undesirable reverse reactions (H(2)-O(2) recombination and/or O(2) photoreduction) and/or protection of the core component from chemical corrosion, depending on the core type. Among the core materials examined, Rh species exhibited relatively high performance for this application. The activity for visible-light water splitting on GaN:ZnO modified with an Rh/Cr(2)O(3) core-shell configuration was dependent on both the dispersion of Rh nanoparticles and the valence state. In addition, the morphology of the Cr(2)O(3) photodeposits was significantly affected by the valence state of Rh and the pH at which the photoreduction of K(2)CrO(4) was conducted. When a sufficient amount of K(2)CrO(4) was used as the precursor and the solution pH ranged from 3 to 7.5, Cr(2)O(3) was successfully formed with a constant shell thickness (approximately 2 nm) on metallic Rh nanoparticles, which resulted in an effective promoter for overall water splitting.

Nano-sized TiN on carbon black as an efficient electrocatalyst for the oxygen reduction reaction prepared using an mpg-C3N4 template.

The direct synthesis of TiN nanoparticles on carbon black (CB) was achieved using an mpg-C(3)N(4)/CB composite as a template. The obtained TiN/CB composites ensured improved contact between TiN and CB, functioning as an efficient cathode catalyst for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). The preparation procedure developed in this study is applicable for the synthesis of a variety of supported nano-nitride catalysts.

Intercalation of Highly Dispersed Metal Nanoclusters into a Layered Metal Oxide for Photocatalytic Overall Water Splitting

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.

Oxidation of Water under Visible‐Light Irradiation over Modified BaTaO2N Photocatalysts Promoted by Tungsten Species

In recent years, photocatalytic water splitting on illuminated semiconductor powder has attracted considerable attention as a potential way to produce the solar fuel H2 from renewable resources. Water oxidation is a particularly important step in artificial photosynthesis for solar fuel production not only for water splitting but also for CO2 reduction. [2] Because the main spectral component of sunlight is visible light (400< l< 800 nm), the development of heterogeneous water oxidation systems that operate under a wide range of visible light is currently a hot topic in chemistry. To effectively use solar energy, a narrow-gap semiconductor like (oxy)nitrides and oxysulfides is highly desirable. 3] However, narrowing the band gap of a photocatalyst decreases the driving force for redox reactions. This would become a more serious concern in water oxidation than in water reduction in terms of kinetics, because water oxidation involves a complicated four-electron process. It will also become difficult for a semiconductor to meet the thermodynamic requirement for water splitting; that is, the valence band maximum and the conduction band minimum should straddle the water-splitting potential, when the band gap energy decreases. While a photocatalyst having a band gap smaller than 2 eV and the ability to reduce and oxidize water is highly desirable for efficient solar energy conversion, however, such a photocatalyst had not been developed until very recently. We have experimentally demonstrated that the reduction and oxidation of water are both possible using perovskite BaTaO2N and the solid-solution materials with BaZrO3 (0 Zr/Ta 0.1) that have band gaps of 1.7–1.8 eV. This is the first example of producing H2 and O2 from water under visible-light irradiation using a semiconductor having a band gap lower than 2 eV. However, the BaZrO3-BaTaO2N solid solutions have a drawback in that the water oxidation activity is very low (about 0.03% apparent quantum yield, AQY, at 420 nm). The cause of the low activity for water oxidation is most likely due to the valence band potential being close to the water oxidation potential, as suggested by photoelectrochemical measurements. In such a situation, one may think that it is possible to tune the band-edge positions of a semiconductor by replacing the original constituent element with another. Taking BaTaO2N for example, it would be easy to expect that the valence band maximum shifts to more positive potential if the concentration of nitrogen, which forms the upper part of the valence band, is reduced. This is actually achievable by making a solid solution with a wide-gap oxide semiconductor, which is in general known as “band-gap enerineering”. From the viewpoint of efficient solar energy use, however, this may not be preferable because the reduction of the nitrogen concentration in BaTaO2N results in a blue shift of the absorption edge, downgrading the good light absorption capability. Thus, the ordinary strategy has a limitation, and a new way to enhance photocatalytic activity needs to be developed. Here we report that the activity of water oxidation of BaTaO2N photocatalyst can be enhanced by the introduction of pentavalent tungsten species, while maintaining the small band gap. In general, doping transition-metal cations having partly filled d orbitals into semiconductor photocatalysts contributes to a significant drop in photocatalytic activity. Such doped elements form a donor or accepter level in the forbidden band of the material, which may case by case act as a center for absorption at visible wavelengths. However, doping also obstructs prompt migration of photogenerated electrons or holes at the surface and in the material bulk, since the dopant frequently provides a discreet energy level rather than an energy band. Nevertheless, introducing pentavalent W species having a [Xe]4f5d electron configuration into BaTaO2N is exceptionally effective to enhancing the oxidation activity of water. [*] Prof. Dr. K. Maeda Department of Chemistry Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-1-NE-2 Ookayama, Meguro-ku, Tokyo 152-8550 (Japan) and Precursory Research for Embryonic Science and Technology Science and Technology Agency (JST) 4-1-8 Honcho Kawaguchi, Saitama 332-0012 (Japan) E-mail: maedak@chem.titech.ac.jp

Simultaneous photodeposition of rhodium–chromium nanoparticles on a semiconductor powder: structural characterization and application to photocatalytic overall water splitting

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.

The Effects of Starting Materials in the Synthesis of (Ga1−xZnx)(N1−xOx) Solid Solution on Its Photocatalytic Activity for Overall Water Splitting under Visible Light

The influence of starting materials on the physicochemical and photocatalytic properties of (Ga(1-x)Zn(x))(N(1-x)O(x)) were investigated in an attempt to optimize the preparation conditions. The catalyst was successfully prepared by nitriding a starting mixture of ZnO and Ga2O3. A mixture of metallic zinc and GaN, however, did not afford the desired compound. The crystallinity, surface area, composition, and absorption characteristics of the resultant (Ga(1-x)Zn(x))(N(1-x)O(x)) solid solution are found to be dependent on the morphology of ZnO but largely insensitive to the choice of Ga2O3 polymorph. The use of coarser-grained ZnO results in a coarser-grained catalyst with elevated zinc and oxygen content and reduced uniformity in composition and crystallinity. The results demonstrate the importance of selecting appropriate ZnO and Ga2O3 starting materials for maximizing the photocatalytic activity of (Ga(1-x)Zn(x))(N(1-x)O(x)) for overall water splitting under visible light.

Preparation and crystallization characteristics of mesoporous TiO2 and mixed oxides

Mesoporous TiO2 and mixed Ti oxides are prepared by a modified sol-gel method using a block copolymer as a mesoporous template. Calcination of the material at 400 °C for template removal is shown to induce crystallization of pure TiO2, resulting in the destruction of the original mesoporous structure and the development of larger pores (7.5 nm) as inter-particle voids. The addition of tri-, tetra- and penta-valent metals (Al, Zr and Nb) to Ti oxide in the amorphous phase in the ratio Ti : M = 1 : 2 (M = Al, Zr or Nb) increases the thermal stability of the material by elevating the crystallization temperature above that for pure TiO2. The formation of Al2TiO5, TiZr2O6 and TiNb2O7 crystal phases without phase separation is confirmed in these materials, demonstrating the homogeneous mixing of TiO2 and the additive element at the atomic level. The mixed mesoporous oxides are thermally stable at 400 °C, although the porous structure is still destroyed due to crystallization. Thus, a homogeneous mixture of Ti with another element (Al, Zr or Nb) in mesoporous amorphous Ti-mixed oxide is observed.