Shunsuke Asahina

Large-Pore Apertures in a Series of Metal-Organic Frameworks

Maximizing Molecular Pore Diameters Amorphous materials, such as activated carbon, can have pore diameters of several nanometers, but the synthesis of ordered structures with very large pore diameters is often thwarted by the creation of interpenetrating networks or difficulties in removing guest molecules. Deng et al. (p. 1018) avoided these problems in the synthesis of metal-organic frameworks (MOFs) with very large diameters (some exceeding 3 nanometers) by using a combination of short and very long linking groups. The compounds formed channels almost 10 nanometers in diameter that could be visualized by electron microscopy and that were large enough to accommodate protein molecules. Metal-organic frameworks with hexagonal channel pores up to almost 100 angstroms in diameter have been synthesized. We report a strategy to expand the pore aperture of metal-organic frameworks (MOFs) into a previously unattained size regime (>32 angstroms). Specifically, the systematic expansion of a well-known MOF structure, MOF-74, from its original link of one phenylene ring (I) to two, three, four, five, six, seven, nine, and eleven (II to XI, respectively), afforded an isoreticular series of MOF-74 structures (termed IRMOF-74-I to XI) with pore apertures ranging from 14 to 98 angstroms. All members of this series have noninterpenetrating structures and exhibit robust architectures, as evidenced by their permanent porosity and high thermal stability (up to 300°C). The pore apertures of an oligoethylene glycol–functionalized IRMOF-74-VII and IRMOF-74-IX are large enough for natural proteins to enter the pores.

Synthesis of self-pillared zeolite nanosheets by repetitive branching

Go with the Flow Effective absorption or filtration can be achieved by having a material with multiple levels of porosity, so that the main flow can occur in the larger channels, while smaller passageways can be used to sequester a secondary material. It can be difficult to make these materials because the pores need to be different sizes, but still fully connected to each other. Zhang et al. (p. 1684) show that a hierarchical zeolite can be made through a simple process using a single structure-directing agent that causes repetitive branching. This leads to a material with improved transport and catalytic properties. Single-step synthesis of pillared zeolite nanosheets is achieved with a common structure-directing agent. Hierarchical zeolites are a class of microporous catalysts and adsorbents that also contain mesopores, which allow for fast transport of bulky molecules and thereby enable improved performance in petrochemical and biomass processing. We used repetitive branching during one-step hydrothermal crystal growth to synthesize a new hierarchical zeolite made of orthogonally connected microporous nanosheets. The nanosheets are 2 nanometers thick and contain a network of 0.5-nanometer micropores. The house-of-cards arrangement of the nanosheets creates a permanent network of 2- to 7-nanometer mesopores, which, along with the high external surface area and reduced micropore diffusion length, account for higher reaction rates for bulky molecules relative to those of other mesoporous and conventional MFI zeolites.

Cobalt phosphate-modified barium-doped tantalum nitride nanorod photoanode with 1.5% solar energy conversion efficiency

Spurred by the decreased availability of fossil fuels and global warming, the idea of converting solar energy into clean fuels has been widely recognized. Hydrogen produced by photoelectrochemical water splitting using sunlight could provide a carbon dioxide lean fuel as an alternative to fossil fuels. A major challenge in photoelectrochemical water splitting is to develop an efficient photoanode that can stably oxidize water into oxygen. Here we report an efficient and stable photoanode that couples an active barium-doped tantalum nitride nanostructure with a stable cobalt phosphate co-catalyst. The effect of barium doping on the photoelectrochemical activity of the photoanode is investigated. The photoanode yields a maximum solar energy conversion efficiency of 1.5%, which is more than three times higher than that of state-of-the-art single-photon photoanodes. Further, stoichiometric oxygen and hydrogen are stably produced on the photoanode and the counter electrode with Faraday efficiency of almost unity for 100 min.

Synthesis of chiral TiO 2 nanofibre with electron transition-based optical activity

The optical chirality induced at the absorption bands due to electronic exciton coupling of the transition dipole moments between chromophores in close proximity is ubiquitous in helical organic materials. However, inorganic materials with optical activity resulting from electronic transitions have not been explored. Here we report the synthesis of chiral TiO2 fibres via transcription of the helical structure of amino acid-derived amphiphile fibres through coordination bonding interactions between the organics and the TiO2 source. Upon calcination, the as-prepared amorphous TiO2 double-helical fibres with a pitch length of ~100 nm were converted to double-helical crystalline fibres with stacks of anatase nanocrystals in an epitaxial helical relationship. Both the amorphous and anatase crystalline helical TiO2 fibres exhibited optical response to circularly polarized light at the absorption edge around ~350 nm. This was attributed to the semiconductor TiO2-based electronic transitions from the valence band to the conduction band under an asymmetric electric field.

The Porosity, Acidity, and Reactivity of Dealuminated Zeolite ZSM-5 at the Single Particle Level: The Influence of the Zeolite Architecture

A combination of atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), focused-ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron-based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM-5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol-to-olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM-5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM-5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5-50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.

In situ growth-etching approach to the preparation of hierarchically macroporous zeolites with high MTO catalytic activity and selectivity

Silicoaluminophosphate zeolite SAPO-34 with CHA topology is known as one of the best catalysts for methanol-to-olefin (MTO) conversion. In this work, we demonstrate a facile one-step hydrothermal synthesis of hierarchically macroporous SAPO-34 through the etching effect of hydrofluoric acid. The highly crystalline hierarchically macroporous SAPO-34 is prepared as central-holed rhombohedral crystals with particle size of ca. 5–10 μm that comprise intracrystalline parallel macrochannels of ca. 100 nm. The formation of macroporous structures via in situ growth-etching can be directly imaged by SEM. Strikingly, a particular crystal configuration consisting of eight pyramidal parts has been first observed during the zeolite crystal growth process, which eventually grows to form a perfect rhombohedral shape. The HF etching effect is further elucidated by the analysis of changes of pH values as well as of solid and liquid compositions following the evolution of crystallization. The texture properties, chemical environments of framework atoms, and acidity of the synthesized SAPO-34 are characterized by N2 adsorption/desorption, MAS NMR and NH3-TPD measurements. The hierarchically macroporous SAPO-34 shows larger micropore volume, slightly stronger acid strength, and lower external surface acidity than its conventional counterpart synthesized without using HF. Consequently, the hierarchically macroporous SAPO-34 exhibits excellent MTO catalytic performance, showing much higher selectivity to ethylene and propylene as well as longer lifetime than the conventional counterpart. In comparison with previously reported methods for the generation of hierarchical porosity, this one-step HF-assisted in situ growth-etching synthetic route is simple, straightforward and cost-effective, which offers a new approach to prepare hierarchically porous zeolites with improved catalytic activity.

A review of fine structures of nanoporous materials as evidenced by microscopic methods

This paper reviews diverse capabilities offered by modern electron microscopy techniques in studying fine structures of nanoporous crystals such as zeolites, silica mesoporous crystals, metal organic frameworks and yolk-shell materials. For the case of silica mesoporous crystals, new approaches that have been developed recently to determine the three-dimensionally periodic average structure, e.g., through self-consistent analysis of electron microscope images or through consideration of accidental extinctions, are presented. Various structural deviations in nanoporous materials from their average structures including intergrowth, surface termination, incommensurate modulation, quasicrystal and defects are demonstrated. Ibidem observations of the scanning electron microscope and atomic force microscope give information about the zeolite-crystal-growth mechanism, and an energy for unstitching a building-unit from a crystal surface is directly observed by an anatomic force microscope. It is argued how these observations lead to a deeper understanding of the materials.

Optically Active Nanostructured ZnO Films

Inorganic nanomaterials endowed with hierarchical chirality could open new horizons in physical theory and applications because of their fascinating properties. Here, we report chiral ZnO films coated on quartz substrates with a hierarchical nanostructure ranging from atomic to micrometer scale. Three levels of hierarchical chirality exist in the ZnO films: helical ZnO crystalline structures that form primary helically coiled nanoplates, secondary helical stacking of these nanoplates, and tertiary nanoscale circinate aggregates formed by several stacked nanoplates. These films exhibited optical activity (OA) at 380 nm and in the range of 200-800 nm and created circularly polarized luminescence centered at 510 nm and Raman OA at 50-1400 cm(-1) , which was attributed to electronic transitions, scattering, photoluminescent emission, and Raman scattering in a dissymmetric electric field. The unprecedented strong OA could be attributed to multiple light scattering and absorption-enhanced light harvesting in the hierarchical structures.

Exploitation of Surface-Sensitive Electrons in Scanning Electron Microscopy Reveals the Formation Mechanism of New Cubic and Truncated Octahedral CeO2 Nanoparticles

Development of new analytical tools for nanostructures directly contributes to the study of catalysts. By using scanning electron microscopy (SEM) with a newly designed signal enhancer, we study cubic and truncated octahedral cerium oxide (CeO2) nanoparticles, which are composed of smaller primary octahedral CeO2 and are formed through bond formation with hexanedioic acid. The signal enhancer is designed to efficiently collect secondary electrons of kinetic energy less than 10 eV; thus, it greatly improves the S/N ratio. On the basis of the observed SEM images and electron backscattered diffraction patterns of the cross section of the nanoparticles, we discuss the formation mechanism of the nanoparticles and speculate that the primary CeO2 nanocrystals share their edges in the cubic nanoparticles and truncated octahedral nanoparticles. These results will contribute to the preparation of nanostructured metal oxide surfaces with controlled morphologies that could enhance catalytic activity.

New Methods for Cross-Section Sample Preparation Using Broad Argon Ion Beam

In 2006, we introduced a new specimen preparation apparatus, Cross-section Polisher (CP), which employs a broad argon ion beam to prepare cross-sections of specimens [1-2]. The principle of the CP is simple: a region of the specimen that is not covered by the masking plate is milled by an argon broad ion beam, as shown in Fig.1. The specimens with irregular shapes and rough surfaces that cannot be embedded prior to ion milling require additional care and consideration prior to ion-milling with CP.