We describe the preparation of ε-Keggin-type cobaltomolybdate-based 3D frameworks with sodium cations, NaH[ε-CoMoMoOCo], and their characterization by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy. Atomic-scale HAADF-STEM images of ε-Keggin compounds were obtained for the first time, and positions of Mo and Co were confirmed. Furthermore, clear evidence of the presence of a CoO tetrahedron was obtained by X-ray absorption near-edge structure (XANES) analysis. Their characterization clearly revealed that ε-Keggin-type cobaltomolybdate units, [ε-CoMoO], constructed by a central CoO tetrahedron and 12 surrounding MoO octahedra, are linked with Co to form 3D frameworks.
A molecular-scale understanding of self-assembled monolayers (SAMs) of sulfonate-terminated alkanethiols is crucial for interfacial studies of functionalized SAMs and their various applications. However, such an understanding has been difficult to achieve because of the lack of direct information on these molecular-scale structures in real space. In this study, we investigated the structures of sulfonate SAMs of sodium 11-mercapto-1-undecanesulfonate (MUS) by frequency modulation atomic force microscopy (FM-AFM) in liquid. The subnanometer-resolution FM-AFM images showed that the single-component MUS SAM prepared in pure water had random surface structures. In contrast, the MUS SAM prepared in a water-ethanol mixed solvent showed periodic striped structures with a flat-lying conformation. The results suggest a significant solvent effect on molecular-scale structures of long-chain sulfonate SAMs. In addition, we investigated the molecular-scale structures of mixed SAMs of MUS and 11-mercapto-1-undecanol (MUO) with alkane chains of the same length. The FM-AFM images of the mixed SAMs showed clear phase separation between MUS SAM and MUO SAM domains. In the MUO SAM domains, the incorporated MUS molecules appeared as protrusions. The results obtained in this study provide direct structural information on long-chain sulfonate and mixed SAMs.
Small cantilevers with a megahertz-order resonance frequency provide excellent sensitivity and speed in liquid-environment atomic force microscopy (AFM). However, stable and accurate oscillation control of a small cantilever requires the photothermal excitation, which has hindered their applications to the studies on photo-sensitive materials. Here, we develop a magnetic excitation system with a bandwidth wider than 4 MHz, enabling a light-free excitation of small cantilevers. In the system, a cantilever with a magnetic bead is driven by a magnetic field generated by a coil. In the coil driver, a differentiation circuit is used for compensating the frequency dependence of the coil impedance and keeping the current constant. By implementing several differentiation circuits with different frequency ranges, we enable to drive various cantilevers having different resonance frequencies with sufficient excitation efficiency. In contrast to the conventional coil driver with a closed-loop circuit, the developed one consists of an open-loop circuit and hence can be stably operated regardless of the coil design. With the developed system, atomic-resolution imaging of mica in liquid using a small cantilever with a megahertz-order resonance frequency is demonstrated. This development should lead to the future applications of AFM with small cantilevers to the studies on various photo-sensitive materials and phenomena.
The brookite phase of TiO2 is much less explored than the other two polymorphs, rutile and anatase, despite its potential applications in photo-catalytic CO2 reduction and water splitting. The first hydration layer and surface hydroxyl groups on the brookite (210) surface and their structural changes under photo-irradiation have been considered to play significant roles in such applications. Hence, in this work, we focus on studying them at the atomic scale using a combination of liquid-environment frequency modulation atomic force microscopy (FM–AFM) and density functional theory (DFT) calculations. The striped feature found in AFM images and its photo-switching behavior accompanied by photo-increased surface hydrophilicity are revealed by ab initio molecular dynamics simulations to originate from photo-sensitive two-dimensional arrangements of molecular and dissociative H2O and proton-hopping behavior. The formation mechanism of the arrangements is further clarified by DFT static and nudged-elastic-band calculations.
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