Increase in Photocurrent Density of WO3 Photoanode by Placing a Layer of an Ordered Array of Mesoporous WO3 Micropillars on Top of a WO3 Sheet Layer.

A facile stamping method was developed to assemble ordered arrays of mesoporous WO3 micropillars with uniform sizes, shapes, and lengths on F-doped tin oxide glass. Using this method, a series of WO3 heterostructural bilayer photoanodes consisting of an array of m-µm long ordered mesoporous WO3 micropillars at the top and the n-µm thick mesoporous WO3 plain sheet layer at the bottom (denoted as m/n) were prepared. Among them 2.5/7.5 displayed a steady state photocurrent density of 3.6 mA cm-2 at 1.23 V (vs RHE) under AM 1.5 (1 Sun), which is much higher than that of the plain 10-µm thick WO3 sheet (2.5 mA cm-2). This phenomenon occurs owing to the following six benefits: increases in charge carrier density, number of photogenerated electron, charge collection rate, thermodynamic feasibility for the vectorial charge transport from the outermost layer of the photoanode to the inner layer, the surface hydrophilicity, and the decrease in charge transfer resistance.

Strain Development of Selective Adsorption of Hydrocarbons in a Cu-ZSM-5 Crystal.

Zeolites are 3D aluminosilicate materials having subnanometer pore channels. The Lewis basic pores have charge-balancing cations, easily tuned to metallic ions as more chemically active sites. Among the ion-exchanged zeolites, Cu2+ ion-exchanged ZSM-5 (Cu-ZSM-5) is one of the most active zeolites with chemical interactions of Lewis basic compounds. Even though the chemical interactions of hydrocarbons with Cu2+ sites in Cu-ZSM-5 have been tremendously studied in the category of zeolite catalysts, it is not yet thoroughly investigated how such interactions affect the structural lattice of the zeolite. Hydrocarbons with different chemical properties and their relative size can induce lattice strain by different chemical adsorption effects on the Cu2+ sites. In this work, we investigate the internal deformation of the Cu-ZSM-5 crystal using Bragg coherent X-ray diffraction imaging during the adsorption of four hydrocarbons depending on the alkyl chain length, the existence of a double bond in the molecule, linear structure versus benzene ring structure, and so forth. In the three-carbon system (propane and propene), relatively weak chemical adsorption occurred at room temperature and 100 °C, whereas strong adsorption was observed over 150 °C. For the six-carbon system (n-hexane and benzene), strong strains evolved in the crystal by active chemical adsorption from 150 °C. The observations suggest that propene and propane adsorb at the Cu2+ sites from the outer shell to the center with increasing temperature. In comparison, n-hexane and benzene adsorb at both parts at the same temperature. The results provide the internal structural information for the lattice with the chemical interactions of hydrocarbons in the Cu-ZSM-5 zeolite and help to understand zeolite-based chemisorption or catalysis research.

Time-resolved in situ visualization of the structural response of zeolites during catalysis.

Zeolites are three-dimensional aluminosilicates having unique properties from the size and connectivity of their sub-nanometer pores, the Si/Al ratio of the anionic framework, and the charge-balancing cations. The inhomogeneous distribution of the cations affects their catalytic performances because it influences the intra-crystalline diffusion rates of the reactants and products. However, the structural deformation regarding inhomogeneous active regions during the catalysis is not yet observed by conventional analytical tools. Here we employ in situ X-ray free electron laser-based time-resolved coherent X-ray diffraction imaging to investigate the internal deformations originating from the inhomogeneous Cu ion distributions in Cu-exchanged ZSM-5 zeolite crystals during the deoxygenation of nitrogen oxides with propene. We show that the interactions between the reactants and the active sites lead to an unusual strain distribution, confirmed by density functional theory simulations. These observations provide insights into the role of structural inhomogeneity in zeolites during catalysis and will assist the future design of zeolites for their applications.

Bipolar Membranes to Promote Formation of Tight Ice-Like Water for Efficient and Sustainable Water Splitting.

Bipolar membranes (BPMs) have recently received much attention for their potential to improve the water dissociation reaction (WDR) at their junction by utilizing catalysts. Herein, composite catalysts (Fe2 O3 @GO) comprising hematite nanoparticles (α-Fe2 O3 ) grown on 2D graphene oxide (GO) nanosheets are reported, which show unprecedentedly high water dissociation performance in the BPM. Furthermore, new catalytic roles in facilitating WDR at the catalyst-water interface are mechanistically elucidated. It is demonstrated that the partially dissociated bound water, formed by the strongly Lewis-acidic Fe atoms of the Fe2 O3 @GO catalyst, helps the "ice-like water" to become tighter, consequently resulting in weaker intramolecular OH bonds, which reduces activation barriers and thus significantly improves the WDR rate. Notably, Fe2 O3 @GO-incorporated BPM shows an extremely low water dissociation potential (0.89 V), compared to commercially available BPM (BP-1E, 1.13 V) at 100 mA cm-2 , and it is quite close to the theoretical potential required for WDR (0.83 V). This performance reduces the required electrical energy consumption for water splitting by ≈40%, as compared to monopolar (Nafion 212 and Selemion AMV) membranes. These results can provide a new approach for the development of water dissociation catalysts and BPMs for realizing highly efficient water splitting systems.

Long-Range π-Conjugation in Phenothiazine-containing Donor-Acceptor Dyes for Application in Dye-Sensitized Solar Cells.

Four organic donor-π-bridge-acceptor dyes containing phenothiazine as a spacer and cyanoacrylic acid as an acceptor were synthesized and tested as sensitizers in dye-sensitized solar cells (DSCs). The influence of iodide- and cobalt-based redox electrolytes on the photovoltaic device performance was investigated. In these new dyes, systematic π-conjugation was extended by inserting one or two phenothiazine moieties and investigated within the context of the resulting photoinduced charge-transfer properties. A detailed investigation, including transient absorption spectroscopy and quantum chemical methods, provided important information on the role of extended π-conjugation on the photophysical properties and photovoltaic device performance. Overall, the results showed that the extension of π-conjugation by one phenothiazine unit resulted in the best device performance owing to reduced recombination rates, whereas extension by two phenothiazine units reduced dye adsorption on TiO2 probably owing to the increase in molecular size. The performance of the dyes in DSCs was found to be a complex interaction between dye structure and size.

Formation of Frustrated Lewis Pairs in Ptx -Loaded Zeolite†NaY.

The formation of a frustrated Lewis pair consisting of sodium hydride (Na(+) H(-) ) and a framework-bound hydroxy proton O(H(+) ) is reported upon H2 treatment of zeolite NaY loaded with Pt nanoparticles (Ptx /NaY). Frustrated Lewis pair formation was confirmed using in situ neutron diffraction and spectroscopic measurements. The activity of the intrazeolite NaH as a size-selective catalyst was verified by the efficient esterification of acetaldehyde (a small aldehyde) to form the corresponding ester ethyl acetate, whereas esterification of the larger molecule benzaldehyde was unsuccessful. The frustrated Lewis pair (consisting of Na(+) H(-) and O(H(+) )) generated within zeolite NaY may be a useful catalyst for various catalytic reactions which require both H(-) and H(+) ions, such as catalytic hydrogenation or dehydrogenation of organic compounds and activation of small molecules.

CO2 capture from humid flue gases and humid atmosphere using a microporous coppersilicate.

Capturing CO2 from humid flue gases and atmosphere with porous materials remains costly because prior dehydration of the gases is required. A large number of microporous materials with physical adsorption capacity have been developed as CO2-capturing materials. However, most of them suffer from CO2 sorption capacity reduction or structure decomposition that is caused by co-adsorbed H2O when exposed to humid flue gases and atmosphere. We report a highly stable microporous coppersilicate. It has H2O-specific and CO2-specific adsorption sites but does not have H2O/CO2-sharing sites. Therefore, it readily adsorbs both H2O and CO2 from the humid flue gases and atmosphere, but the adsorbing H2O does not interfere with the adsorption of CO2. It is also highly stable after adsorption of H2O and CO2 because it was synthesized hydrothermally.

Oriented MFI Membranes by Gel-Less Secondary Growth of Sub-100 nm MFI-Nanosheet Seed Layers.

A zeolite membrane fabrication process combining 2D-zeolite nanosheet seeding and gel-free secondary growth is described. This process produces selective molecular sieve films that are as thin as 100 nm and exhibit record high permeances for xylene- and butane-isomers.

Manual assembly of nanocrystals for enhanced photoelectrochemical efficiency of hematite film.

To improve the optoelectronic properties of hematite film as a photoanode, hematite film was orientated on the (012) plane by the secondary growth of organized microcrystals. The resultant film showed promising photoelectrochemical effects compared with a randomly oriented one, with a maximum photocurrent of 0.8 mA cm(-2) at 1.23 V vs. RHE under the illumination of 1 sun.

Effect of metal doping, doped structure, and annealing under argon on the properties of 30 nm thick ultrathin hematite photoanodes.

The doping of the whole hematite layer with W (9.4%) and the additional doping of the bottom half of the W-doped hematite layer with Sn (8.6%), and the subsequent annealing under argon at 600 °C give rise to large increases in the Fe(2+) concentration (by >∼200 times), carrier density (Cd, by ∼48 times) and current density (i(d), by ∼8 times at 1.23 V vs. RHE, under 1 sun) with respect to those of bare hematite photoanodes. The measured i(d) (0.9 mA cm(-2)) is the highest among those of the ultrathin hematite photoanodes and the measured Cd (3.8 × 10(22) cm(-3)) is the highest among those ever observed for hematite.

A novel vanadosilicate with hexadeca-coordinated Cs(+) ions as a highly effective Cs(+) remover.

The effective removal of (137) Cs(+)  ions from contaminated groundwater and seawater and from radioactive nuclear waste solutions is crucial for public health and for the continuous operation of nuclear power plants. Various (137) Cs(+)  removers have been developed, but more effective (137) Cs(+)  removers are still needed. A novel microporous vanadosilicate with mixed-valence vanadium (V(4+) and V(5+) ) ions is now reported, which shows an excellent ability for Cs(+)  capture and immobilization from groundwater, seawater, and nuclear waste solutions. This material is superior to other known materials in terms of selectivity, capacity, and kinetics, and at very low Cs(+)  concentrations, it was found to be the most effective material for the removal of radioactive Cs(+)  ions under the test conditions. This novel vanadosilicate also contains hexadeca-coordinated Cs(+)  ions, which corresponds to the highest coordination number ever described.

Core-shell strain structure of zeolite microcrystals.

Zeolites are crystalline aluminosilicate minerals featuring a network of 0.3-1.5-nm-wide pores, used in industry as catalysts for hydrocarbon interconversion, ion exchangers, molecular sieves and adsorbents. For improved applications, it is highly useful to study the distribution of internal local strains because they sensitively affect the rates of adsorption and diffusion of guest molecules within zeolites. Here, we report the observation of an unusual triangular deformation field distribution in ZSM-5 zeolites by coherent X-ray diffraction imaging, showing the presence of a strain within the crystal arising from the heterogeneous core-shell structure, which is supported by finite element model calculation and confirmed by fluorescence measurement. The shell is composed of H-ZSM-5 with intrinsic negative thermal expansion whereas the core exhibits a different thermal expansion behaviour due to the presence of organic template residues, which usually remain when the starting materials are insufficiently calcined. Engineering such strain effects could have a major impact on the design of future catalysts.

Length-dependent change of optical, magnetic, and vibrational properties of vanadate (V(IV)O3(2-)) quantum wire embedded in AM-6 vanadosilicate.

AM-6 and VSH-1 are vanadosilicates containing VO(3)(2-) quantum wires and oxovanadate [O═VO(4)](2-) quantum dots, respectively. We developed methods to synthesize pure, highly crystalline, monodisperse, and all-V(IV) AM-6 and VSH-1 crystals with sizes between 0.2-0.3 and 10 µm. On the basis of their optical, magnetic susceptibility, vibrational, and electron spin resonance (ESR) properties, we have elucidated the following interesting phenomena. The length of the VO(3)(2-) quantum wire (l) linearly increases as the length along the [110] direction {L([110])} increases. The band gap energy (E(g)) of the VO(3)(2-) quantum wire progressively decreases with increasing l even when it reaches ~210 nm, indicating that the Bohr length (the length at which the quantum confinement effect no longer appears) is longer than 200 nm. The deduced µ(z) and µ(xy) are 0.0005m(e) and 15.7m(e), respectively. Per-V(IV)-ion oscillator strength of the d-d transition increases by 7-9 times and that of CT transition increases by 1.5-1.9 times with increasing l from ~50 to 210 nm (by ~4 times). The longitudinal vibration frequency ν of the VO(3)(2-) quantum wire decreases and the intensity of the vibrational band increases as l increases. The ESR intensity increases while the peak-to-peak width decreases as l increases, indicating that the spin-spin relaxation rate (R(ssr)) decreases as l increases. The magnetic susceptibility χ decreases as l increases, especially at T > 125 K, indicating that the tendency of the d(1) electron spins to orient to the external magnetic field decreases with increasing l.

Fluorescence characteristics of isolated dye molecules within silicalite-1 channels.

Fluorescence characteristics of hemicyanine dye molecules isolated from neighboring molecules and strongly restricted inside nanosized pores of zeolite (silicalite-1) crystal were investigated. For samples in which the molecules were sufficiently far away from the others, the fluorescence decay lifetime of the molecules was about 2.2 ns. As the intermolecular distance was reduced, the steady-state fluorescence peak shifted toward the longer wavelength and the fluorescence efficiency decreased markedly. The fluorescence decay lifetime also decreased to 0.8 ns for a sample with the smallest intermolecular distance of 2.1 nm. These results were explained in terms of a dipole-dipole interaction between pairs of dye molecules. From the relation between the intermolecular distances and the fluorescence decay lifetimes of the molecules, the radius of energy transfer of hemicyanine donor-acceptor pair in zeolite matrix was determined to be 2.2 nm, in fair agreement with the calculated Förster radius between dye molecules of the same species.

A novel class of nonlinear optical materials based on host-guest composites: zeolites as inorganic crystalline hosts.

The demand for nonlinear optical (NLO) materials with exceptional NLO properties is very large, and hence the search for such materials should be continued not only to enhance their functions in current applications but also to help expedite the materialization of photonics in which photons instead of electrons are used for signal processing, transmission, and storage. This article summarizes the preparation, characteristics, and the future perspectives of novel second order nonlinear optical (2NLO) materials prepared by orientation-controlled incorporation of 2NLO molecules into zeolite channels and third order nonlinear optical (3NLO) materials prepared by compartmentalization of very small (<1.3 nm) PbS QDs within zeolite nanopores under different environments, and the novel chemistry newly unveiled during the preparation of novel zeolite based NLO materials.

Synthesis of monodisperse mesoporous TiO2 spheres with tunable sizes between 0.6 and 3.1 µm and effects of reaction temperature, Ti source purity, and type of alkylamine on size and monodispersity.

We report a novel method for synthesizing monodisperse mesoporous TiO(2) spheres (sizes = 0.6-3.1 µm) by hydrolysis of titanium isopropoxide (TIP) in a mixture of C(8)-C(16)n-alkylamine, water, and ethanol. The size increases with decreasing temperature, TIP concentration, and water concentration, and upon purifying TIP. n-Dodecylamine gives the highest monodispersity.

Increase of third-order nonlinear optical activity of PbS quantum dots in zeolite Y by increasing cation size.

The third-order nonlinear optical (3NLO) activity of PbS quantum dots (QDs) encapsulated in zeolite Y has been expected to depend sensitively on the countercation of the zeolite host. However, ion exchange of the pristine countercation, H(+), with other cations has not been possible because the framework decomposes and the QDs aggregate immediately when the PbS QD-incorporating zeolite Y with H(+) as the countercation is exposed to the atmosphere. We now report that when H(+) is transformed to NH(4)(+), the framework of PbS QD-containing zeolite Y does not undergo decomposition and the PbS QDs do not undergo aggregation to form larger QDs during the aqueous ion exchange of NH(4)(+) with alkali-metal ions (M(A)(+) = Li, Na(+), K(+), Rb(+)). The 3NLO activity of the M(A)(+)-exchanged PbS QD-incorporating zeolite Y film increases with increasing size of M(A)(+). The stabilization of the surface-bound exciton by the electron-rich framework oxide and electron-poor cation is proposed to be responsible for the increase. This is the first example of a method for systematically increasing the 3NLO activity of QDs dispersed in a dielectric matrix by systematically changing its properties. These results will serve as a guideline for future research and also promote applications of QD-incorporating zeolites in various fields.

Growth of uniformly oriented silica MFI and BEA zeolite films on substrates.

Applications of zeolite films benefit from alignment of the integrated channels, but methods for film growth have nearly always introduced orientational randomization in the direction normal to the substrate. We now report facile methods to grow silicalite-1 films and pure silica beta zeolite films on substrates with straight or sinusoidal channels positioned uniformly upright at a thickness of up to 8 micrometers. Precise gel compositions and processing temperatures are critical to promote secondary growth on pre-formed oriented crystal monolayers while suppressing self-crystallization in the bulk medium. Preliminary results highlight the potential of these uniformly oriented films in the nonlinear optical response and separation of xylene isomers.