Potential Dependence of the Buckling Structure of the Interfacial Water Bilayer on a Graphene Electrode

The interfacial structure between aqueous electrolytes and the epitaxial graphene on a SiC(0001) electrode has been determined using X-ray diffraction. The electrolyte and electrode potential dependences are investigated, and it is found that the water bilayer is stabilized on the graphene surface in a similar fashion to icelike structure. There are no specific adsorbed ions and no layer formation of electrolyte ions at the Helmholtz plane, which differs from the double-layer structure found on metal electrodes remarkably. The layer spacing of the water bilayer depends on the electrode potential, indicating that water reorientation occurs. The applied electrode potential is strongly related to the potential drop across the interface induced by the electric dipole field of the bilayer water. A small double-layer current results from non-faradaic charge by the reorientation of the bilayer water

Remarkable Lattice Shrinkage in Highly Oriented Crystalline Three-Dimensional Metal–Organic Framework Thin Films

Highly oriented crystalline thin films of metal–organic frameworks (MOFs) have promising practical applications, such as in gas separation, catalysis, and sensing. We report on the successful fabrication of highly oriented crystalline thin films of three-dimensional porous MOFs, Fe­(pz)­[M­(CN)₄] (M = Ni, Pd; pz = pyrazine). Synchrotron X-ray diffraction studies reveal not only the highly oriented crystalline nature but also the remarkable shrunken structure of the thin films (∼3–7% volume shrinkage) compared with bulk samples. Furthermore, because of lattice shrinkage, these films exhibit large lattice expansions upon guest adsorption, in marked contrast to the almost unchanged lattice in the bulk samples

Change in the Crystallite Orientation of Poly(ethylene oxide)/Cellulose Nanofiber Composite Films

The crystallite orientation and crystallographic domain structure of poly­(ethylene oxide) (PEO) in cellulose nanofiber-incorporated (CNF-incorporated) PEO films developed for packaging materials were observed using wide-angle X-ray diffraction for different CNF filling ratios. When a CNF filling ratio of <10 wt % was used, the molecular chains in the PEO crystallite region were oriented in a direction perpendicular to the surface of the film; however, when the ratio was >50 wt %, the PEO molecular chains were oriented in a direction parallel to the surface of the film. The fiber axis of the CNFs became parallel to the surface of the PEO/CNF composite film when the filling ratio was >25 wt %. The change in the orientation of the PEO crystals occurred because increasing the amount of CNF in the composite films decreased the space in which the PEO could be crystallized. Furthermore, the hydrogen bonds between the PEO and the CNF may behave as crystallization nuclei for the PEO. Our results thus pave the way toward the development of packaging materials that are more impermeable to gases than the current materials

Ethanol Oxidation on Well-Ordered PtSn Surface Alloy on Pt(111) Electrode

Surface and subsurface structures of PtSn surface alloy on Pt(111) were determined using in situ scanning tunneling microscopy (STM) and X-ray diffraction. Different ordered structures of the PtSn alloy layer were observed by STM in HClO₄ at coverage of θ_Sn ≤ 0.23. Superstructure of (√3 × √7)­R19.1° with small domain size was formed at θ_Sn = 0.23. This structure promoted the catalytic activity for the ethanol oxidation reaction with high durability. X-ray structural analysis showed that the ratio of Sn in the subsurface was below 3(2)%, The PtSn alloy layer was mainly formed at the surface of the Pt(111) electrode. The Sn atoms protruded by 0.02 nm from the Pt layer, which was similar to the surface structure of Pt₃Sn­(111). One Pt atom in the (√3 × √7)­R19.1° structure contacts to one or two surrounding Sn atoms, which lead to the highest activity for the EOR

Step-by-Step Fabrication of a Highly Oriented Crystalline Three-Dimensional Pillared-Layer-Type Metal–Organic Framework Thin Film Confirmed by Synchrotron X-ray Diffraction

Fabrication of a crystalline ordered thin film based on the porous metal–organic frameworks (MOFs) is one of the practical applications of the future functional nanomaterials. Here, we report the creation of a highly oriented three-dimensional (3-D) porous pillared-layer-type MOF thin film on a metal substrate using a step-by-step approach based on liquid-phase epitaxy. Synchrotron X-ray diffraction (XRD) study clearly indicates that the thin film is crystalline and its orientation is highly controlled in both horizontal and vertical directions relative to the substrate. This report provides the first confirmation of details of not only the crystallinity but also the orientation of 3-D MOF thin film using synchrotron XRD. Moreover, we also demonstrate its guest adsorption/desorption behavior by using <i>in situ</i> XRD measurements. The results presented here would promise useful insights for fabrication of MOF-based nanodevices in the future

Guest-Induced Two-Way Structural Transformation in a Layered Metal–Organic Framework Thin Film

Fabrication of thin films made of metal–organic frameworks (MOFs) has been intensively pursued for practical applications that use the structural response of MOFs. However, to date, only physisorption-induced structural response has been studied in these films. Chemisorption can be expected to provide a remarkable structural response because of the formation of bonds between guest molecules and reactive metal sites in host MOFs. Here, we report that chemisorption-induced two-way structural transformation in a nanometer-sized MOF thin film. We prepared a two-dimensional layered-type MOF Fe­[Pt­(CN)₄] thin film using a step-by-step approach. Although the as-synthesized film showed poor crystallinity, the dehydrated form of this thin film had a highly oriented crystalline nature (<b>Film-D</b>) as confirmed by synchrotron X-ray diffraction (XRD). Surprisingly, under water and pyridine vapors, <b>Film-D</b> showed chemisorption-induced dynamic structural transformations to Fe­(L)₂[Pt­(CN)₄] thin films [L = H₂O (<b>Film-H</b>), pyridine (<b>Film-P</b>)], where water and pyridine coordinated to the open Fe²⁺ site. Dynamic structural transformations were also confirmed by in situ XRD, sorption measurement, and infrared reflection absorption spectroscopy. This is the first report of chemisorption-induced dynamic structural response in a MOF thin film, and it provides useful insights, which would lead to future practical applications of MOFs utilizing chemisorption-induced structural responses

Change in the Crystallite Orientation of Poly(ethylene oxide)/Cellulose Nanofiber Composite Films

The crystallite orientation and crystallographic domain structure of poly­(ethylene oxide) (PEO) in cellulose nanofiber-incorporated (CNF-incorporated) PEO films developed for packaging materials were observed using wide-angle X-ray diffraction for different CNF filling ratios. When a CNF filling ratio of <10 wt % was used, the molecular chains in the PEO crystallite region were oriented in a direction perpendicular to the surface of the film; however, when the ratio was >50 wt %, the PEO molecular chains were oriented in a direction parallel to the surface of the film. The fiber axis of the CNFs became parallel to the surface of the PEO/CNF composite film when the filling ratio was >25 wt %. The change in the orientation of the PEO crystals occurred because increasing the amount of CNF in the composite films decreased the space in which the PEO could be crystallized. Furthermore, the hydrogen bonds between the PEO and the CNF may behave as crystallization nuclei for the PEO. Our results thus pave the way toward the development of packaging materials that are more impermeable to gases than the current materials

Structural Dynamics of the Electrical Double Layer during Capacitive Charging/Discharging Processes

Transitional structures of Cs⁺ at the outer Helmholtz plane (OHP) have been determined using time-resolved X-ray diffraction during the double-layer charging/discharging on the Ag(100) electrode in CsBr solution. At the double-layer potential region at which c(2 × 2)-Br is formed on Ag(100), the transient current comprises two exponential terms with different time scales: a rapid and a slow one are due to the dielectric polarization of water molecules and the transfer of Cs⁺, respectively. The slow term is composed of different dynamic processes of Cs⁺ during charging and discharging. When the potential is stepped in the positive direction, the coverage of Cs⁺ at the OHP decreases. In this step, the transient X-ray intensity at the (0 0 1) reflection, which is sensitive to the OHP structure, shows that Cs⁺ is released from the OHP according to exponential function of time. The decay of transient intensity of X-ray has a time scale similar to that of the current transient measurement. On the other hand, the accumulation process of Cs⁺ from the diffuse double layer to the OHP comprises two different kinetic processes after a potential step in the negative direction: a rapid one is the accumulation of Cs⁺ near the outer layer, and a slow one is the structural stabilization of the Cs⁺ layer

Crystal Isomers of ScFeO₃

In inorganic compounds, “crystal isomers”, which can exist in metastable phases, are obtained by various solution-processing techniques, high-pressure syntheses, as well as physical and chemical thin film fabrication techniques. The metastable phase depends on the processing, allowing the hierarchy of the Gibbs free energy to be controlled in a phase at a given temperature. In this study, we successfully stabilize five metastable phases, four phases of ScFeO₃ and one Sc_0.48Fe_1.52O₃, prepared from one ScFeO₃ target by the pulsed laser deposition technique. The crystal structures are identified by X-ray diffraction and high-angle annular dark field-scanning transmitted electron microscopy measurements. The relationship between the crystal structure of the film and the substrate is κ-Al₂O₃-type Sc_0.48Fe_1.52O₃ on SrTiO₃(111), spinel-type ScFeO₃ on MgO(001), corundum-type ScFeO₃ on Fe₂O₃/Al₂O₃(0001) and NdCaAlO₄(001), YMnO₃-type ScFeO₃ on Al₂O₃(0001), and bixbyite-type ScFeO₃ on YSZ(001). Four of these structures (all except the bixbyite structure) have not been reported by other processing techniques. These results suggest that the thin film growth technique is a strong tool for exploring novel functional materials and the metastable phases of oxide isomers

Fabrication and Structural Characterization of an Ultrathin Film of a Two-Dimensional-Layered Metal–Organic Framework, {Fe(py)₂[Ni(CN)₄]} (py = pyridine)

We report the fabrication and characterization of the first example of a tetracyanonickelate-based two-dimensional-layered metal–organic framework, {Fe­(py)₂Ni­(CN)₄} (py = pyridine), thin film. To fabricate a nanometer-sized thin film, we utilized the layer-by-layer method, whereby a substrate was alternately soaked in solutions of the structural components. Surface X-ray studies revealed that the fabricated film was crystalline with well-controlled growth directions both parallel and perpendicular to the substrate. In addition, lattice parameter analysis indicated that the crystal system is found to be close to higher symmetry by being downsized to a thin film