Surface forces measurement is a powerful tool for molecular level of characterization of solid-liquid interfaces and complex systems. In this review, we report the development of novel nano-scale measurements based on surface forces apparatus (SFA): (1) Twin-path SFA, which enabled us to study wide variety of samples such as metals and ceramics. (2) The resonance shear measurement (RSM), which is a sensitive method for evaluating properties of confined liquids for nano-rheology and tribology. We also summarize some of our recent researches on (1) evaluation of a surface potential and charge density of the electrodes using electrochemical surface forces apparatus, (2) characterization of ionic liquids confined between silica surfaces.
Polycrystalline indium tin oxide (ITO) film has a rough enzyme-sized surface structure, which efficiently enhances the direct electron transfer (DET) caused by the electrocatalytic activity of cytochrome c (cyt c). It is explained that the heme iron of cyt c was sufficiently close to the polycrystalline ITO electrode to achieve DET. Polycrystalline ITO is a suitable material for the enzyme-modified electrode of an electrochemical biosensor.
The mechanism of water oxidation by MnO2 electrodes was investigated at various pH values, using in situ UV-vis absorption spectroscopy. The surface Mn3+ formed as an intermediate species during water oxidation showed a d-d transition band with a peak at 510 nm at pH 6, while the peak shifted to 470 nm at pH 13. This shift of the absorption peak is attributed to the deprotonation of hydroxyl ligands of Mn3+ from OH− to O2−. Interrelation between the protonation state of Mn3+ and the O2 evolution activity of MnO2 electrocatalysts is discussed.
In this paper, an evaluation method for measuring contact resistance of aluminum current collector surfaces in energy storage systems was established. Contact resistance was calculated by a combination of chronopotentiometry and AC impedance method. Also, we found that contact resistance is dependent on current density.
Nanoscale cell surface topography was visualized using a scanning ion conductance microscope (SICM), where a nanopipette was used as the scanning probe to detect ionic current as feedback signal. SICM was an effective tool for noncontact topographical imaging of live cells, because measurements were performed under physiological conditions. This breakthrough technique opens up a wealth of possible new experiments in membrane and cell biology.
Thin film structures of [Cu3(BTC)2(H2O)3]n (HKUST-1), which is one of Metal-organic frameworks (MOFs), prepared via step-by-step liquid phase epitaxy (LPE) method, on a TiO2(110) surface were investigated by polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). For the ultra thin film, PTRF-XAFS enlightened that HKUST-1 eventually formed even at 1 cycle LPE. For further growth, XRD and SEM verified well-oriented < 111 > HKUST-1 thin films on the surface. These results proposed that the adsorption structure of linkers on the surface plays an important role to control the orientation of MOF films.
A new method to visualize electrochemical reactions by fluorescence is demonstrated by using the photoluminescence quenching of semiconductor nanoparticles which changes according to the redox states of quenchers. The photoluminescence intensity of ZnS-AgInS2 solid solution semiconductor nanoparticles can be controlled by the electrochemical manipulation of quencher molecules. By using a fluorescence optical microscopy, the formation of Nernst diffusion layer is successfully observed as a photoluminescence intensity gradient.
The structure and photo-induced charge transfer time of pyridine molecules adsorbed on a rutile TiO2(110) surface have been studied by near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, density functional theory (DFT) calculations and core-hole-clock (CHC) spectroscopy. Polarization dependence of NEXAFS spectra and geometrical optimization by the DFT calculations revealed that the pyridine molecules are bound to the TiO2 surface with an upright configuration where the N atom binds to a surface Ti atom. The CHC results indicate that the charge transfer from the LUMO+2 orbital of pyridine with a π* symmetry to the conduction band of TiO2 is quite fast, where the timescale is less than 3 fs.
Flattening processes of a commercially available Au(111) single crystal surface were step-by-step investigated using atomic force microscopy (AFM), scanning tunneling microscopy (STM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS) from a micron scale to an atomic dimension. AFM images in a micron scale showed that there were many scratches with a few nm depth on the as-received Au(111) disk surface. SEM, EDX, and XPS results indicated that there were surface contaminations such as carbon and silicon species. As a first, these contaminations could be removed by dipping the disk in concentrated H2SO4 and then in boiling concentrated HF. The scratches described above were disappeared after annealing by a gas flame, which is a most popular method for Au(111) single crystal preparation. After the electrochemical polishing and annealing in an electric furnace under argon atmosphere, finally, atomically flat Au(111) surfaces with dimensions of more than 500 × 500 nm2 were obtained. With increasing of annealing period, larger terraces were obtained.
Real surface structures of Pd(110) = 2(111)-(111) and Pd(311) = 2(100)-(111) have been determined using surface X-ray scattering (SXS) at 0.5 V (RHE) in 0.1 M HClO4 saturated with Ar and O2. Both surfaces have unreconstructed (1 × 1) structures. These results differ from those of Pt(110) and Pt(311) of which surfaces are reconstructed to (1 × 2) in 0.1 M HClO4. Interlayer spacing between the first and the second layer d12 is expanded on Pd(110) in O2 saturated solution, whereas the spacing d12 on Pd(311) is larger than that of the bulk in both Ar and O2 saturated solutions.
The electrochromic transition of a nickel borate thin film between colorless and brown was examined by means of in situ XAFS and UV/vis spectroscopy. The XAFS spectra showed that the average valence state of the nickel species in the film changed from +2.1 to +3.8 following the application of an electrode potential. Additionally, a broad peak at 700 nm was observed during in situ UV/vis absorption measurements on the application of a positive potential. These results suggest that the nickel borate film reversibly forms a NiOOH structure with a domain size of several nanometers during the electrochromic reaction.
Direct observation of phenomena occurring under atmospheric conditions, especially at the nanometer scale, would offer a unique opportunity to understand the dynamics of various processes. A novel electron microscope, the atmospheric scanning electron microscope (ASEM), has recently been developed and allows for the observation of nanoscale objects under atmospheric conditions. In this paper, we present some examples of dynamic phenomena in polymer materials observed using ASEM. The first example is phase separation of a binary polymer blend upon solvent evaporation, a representative example of a non-linear non-equilibrium phenomenon in physics. Phase-separated structures were found to appear at the final stage of solvent evaporation. Also, we found that irradiation of organic liquids (e.g., dibenzyl ether) with the ASEM electron beam induced polymerization, and the resulting material showed interesting cathodoluminescence behavior. Thus, ASEM may be useful as a tool for simultaneous polymerization and fabrication, in addition to offering a means for direct nanoscale observation of materials under atmospheric conditions.
This paper studies the effect of current density on electrochemical Li deposition/dissolution at glassy solid electrolyte (LiPON) interfaces with a thin-film Cu current collector by in-situ scanning electron microscopy (SEM). The Li nucleation rate and the saturation density of Li nuclei increase with increasing current density. When the current density is smaller than 300 µA cm−2, Li islands continue to separately grow under a Cu film to the critical sizes to produce small cracks in the Cu film resulting in isolated Li rod growth from the cracks. On the other hand, a current density of 1.0 mA cm−2 provokes the nucleation of Li islands at a number of sites. They rapidly coalesce under a Cu film in all lateral directions before cracking the Cu film, whereby Li growth is prevented.
Proton conductive spots on the membrane surface of sulfonated poly(arylene ketone) multiblock copolymer were investigated by current-sensing atomic force microscopy (CS-AFM) under the hydrogen atmosphere with changing relative humidity, temperature, and bias voltage. The bright spots, where the hydrophilic clusters should be effectively connected inside the membrane, were distributed rather inhomogeneously on the surface at low temperature and humidity but became more homogeneous at higher temperature and humidity. The average diameter of the spots was approximately 10 nm at 40% RH, which increased to 13 nm at 70% RH. The total area of the proton conducting spots, as well as current at each spot, on the membrane surface increased at high humidity and temperature. In addition, the diameter of the proton-conductive spots and the ratio of proton-conductive area on the membrane surface continuously increased with increasing the bias voltage. This increase of the conducting area and the current should be related to the change of the bulk ionic conductivity.
Near-field fluorescence correlation spectroscopy measurements of 20 and 40 nm polystyrene nanoparticles were performed, using fiber probes with aperture diameters ranging from 100 to 250 nm. The experimental data were best fit by a two-dimensional diffusion model, implying that there was significant anisotropy associated with the movement of the particles, such that mobility normal to the wall was much lower than that parallel to the wall. Interestingly, the lateral diffusion coefficients were approximately ten times greater than predicted from the Stokes-Einstein relationship corrected by taking into account hydrodynamic drag forces due to particle-wall interactions. This discrepancy between experimental and theoretical results is possibly due to strong viscous forces as well as attractive forces at the interface.
Structural analysis of the interfaces between an ionic liquid (IL) and an organic monolayer was carried out by phase modulation atomic force microscopy (PM-AFM). A quartz tuning fork sensor with a sharpened tungsten tip was used as a force sensor instead of a Si cantilever. Topographic imaging of the monolayer-covered Si(111) substrate revealed that the PM-AFM is capable of imaging the atomic steps originating from the substrate in an IL. We also carried out force curve measurement using the PM-AFM in order to directly confirm the presence of solvation layers and revealed that at least 4 layers, each with a thickness of 0.77 nm, were formed on the interface. In addition, we obtained topographic images at different driving frequencies and indicated that it is possible to image not only the sample surface but also the solvation layers formed on the IL/monolayer interface.
Self-assembly process of a porphyrin derivative [5,10,15,20-tetra-(m-mercapto-p-methoxyphenyl) porphyrin; TMMPP], which was designed and synthesized to be flatly adsorbed on a gold substrate surface, on a Au(111) single crystal surface was investigated by ex situ scanning tunneling microscopy (STM) and electrochemical reductive desorption measurements. A small number of TMMPP molecules were randomly but flatly adsorbed on the Au(111) surface when the deposition time was less than 1 h as the initial stage of the self-assembly process. When the deposition time was more than 20 h as the final stage of the self-assembly process, the densely-packed and flatly adsorbed TMMPP SAM formed on the Au(111) surface via a phase transition from a random to an ordered structure. These results indicated that we succeeded in the formation of the flatly adsorbed and well-defined porphyrin self-assembled monolayer (SAM) on the Au(111) surface.
The effect of annealing on 4-PySH and 3-PySH SAMs on Au(111) and Au(100) substrates was examined using high-resolution electron energy loss spectroscopy (HREELS) for the first time. At 300 K, the surfaces of 4- and 3-PySH SAMs on Au(111) were partially covered with artifacts. However, after annealing at the appropriate temperature, clear HREEL spectra of 4-PySH and 3-PySH SAM on Au(111) were recorded, and both the 4-PySH and 3-PySH molecules adsorbed in a flat orientation. In contrast, on Au(100), the HREEL spectra of 4-PySH and 3-PySH SAMs prepared at 300 K revealed that the pyridine rings are oriented almost perpendicular to the surface. Annealing at an appropriate temperature had a dramatic effect on the surface structure of the 4-PySH and 3-PySH SAMs: Both the 4- and 3-PySH molecules adsorbed in a flat orientation.
Water libration on a silver electrode in various alkali hydroxides (MOH, M = Li, Na, K or Cs) aqueous solutions was examined through simultaneous experiments using surface-enhanced Raman scattering (SERS) and electrochemical analysis. This technique, referred to as SERS spectroelectrochemistry, reveals the Raman spectral band between 200 and 750 cm−1, assignable to water libration on the electrode surface, through electrode potential scanning. The Raman intensity in this spectral band changes with the electrode potential. The variation in Raman intensity of water libration observed when scanning the electrode potential is discussed in terms of water molecules at the electrode interface in various alkali hydroxide aqueous solutions.
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