Preparation of Nanoparticles and Diffusion Behavior of Metal Ions in Ionic Liquids.

Ionic liquids (ILs) have attracted much attention as tunable liquids because of their unique structures and properties. However, the mechanisms of chemical reactions and solute diffusion in ionic liquids are still unknown. This article summarizes our previous studies and recent results on the mechanisms of metal particle formation and solute diffusion in ionic liquids, focusing on the local structure of ionic liquids. It was found that the shape and size of metal particles formed in ionic liquids using electron beams or X-rays are strongly influenced by the local structure. In the study of the diffusion behavior of metal ions in ionic liquids, we proposed a hopping-like diffusion model and proposed that this behavior could be strongly influenced by local structures such as hole concentration and/or domain structures.

Rapid improvements in charge carrier mobility at ionic liquid/pentacene single crystal interfaces by self-cleaning.

We report the rapid improvement in the carrier mobility of the electric double layer field-effect transistor based on the ionic liquid (IL)/pentacene single crystal interface. Generally, the surface oxidation of the pentacene single crystal is unavoidable, and the considerable degradation restricts the performance of the field-effect transistor. However, the formation of the IL/pentacene single crystal interface resolves this problem by increasing the carrier mobility by approximately twice the initial value within a few hours. Furthermore, frequency-modulation atomic force microscopy revealed that the aforementioned rapid improvement is attributed to the appearance of a clean and flat surface of the pentacene single crystal via the defect-induced spontaneous dissolution of pentacene molecules into the IL.

Structural and dynamic properties of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide/mica and graphite interfaces revealed by molecular dynamics simulation.

It has been observed that the properties of room temperature ionic liquids near solid substrates are different from those of bulk liquids, and these properties play an important role in the development of catalysts, lubricants, and electrochemical devices. In this paper, we report microscopic studies of ionic liquid/solid interfaces performed using molecular dynamics simulations. The structural and dynamic properties of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI) on mica and graphite interfaces were thoroughly investigated to elucidate the microscopic origins of the formation of layered structures at the interfaces. Our investigation included the observation of structural and orientational changes of ions as a function of distance from the surfaces, and contour mappings of ions parallel and perpendicular to the surfaces. By virtue of such detailed analyses, we found that, during the 5 ns simulation, the closest layer of BMIM-TFSI behaves as a two-dimensional ionic crystal on mica and as a liquid or liquid crystal on graphite.

Potential dependent changes in the structural and dynamical properties of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide on graphite electrodes revealed by molecular dynamics simulations.

An understanding of the characteristics of ionic liquid/graphite interfaces is highly important for electrochemical devices such as batteries and capacitors. In this paper, we report microscopic studies of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI) on charged graphite electrodes using molecular dynamics simulations to reveal the two-dimensional arrangement of the ions and their dynamics at the interfaces. Analyses of surface distribution and mobility of ions revealed that the ion arrangement changes from a bilayer type to a checkerboard type with increasing applied potential. Whereas the bilayer type arrangement increases the ionic mobility parallel to the interfaces with the negative potential, the ions arranged in the checkerboard type tend to localize because of the increased lateral electrostatic interactions. Furthermore, we revealed that the inhomogeneity of ionic distribution at the positive potential propagates up to a few nanometers from the interface.

Microscopic properties of ionic liquid/organic semiconductor interfaces revealed by molecular dynamics simulations.

Electric double-layer transistors based on ionic liquid/organic semiconductor interfaces have been extensively studied during the past decade because of their high carrier densities at low operation voltages. Microscopic structures and the dynamics of ionic liquids likely determine the device performance; however, knowledge of these is limited by a lack of appropriate experimental tools. In this study, we investigated ionic liquid/organic semiconductor interfaces using molecular dynamics to reveal the microscopic properties of ionic liquids. The organic semiconductors include pentacene, rubrene, fullerene, and 7,7,8,8-tetracyanoquinodimethane (TCNQ). While ionic liquids close to the substrate always form the specific layered structures, the surface properties of organic semiconductors drastically alter the ionic dynamics. Ionic liquids at the fullerene interface behave as a two-dimensional ionic crystal because of the energy gain derived from the favorable electrostatic interaction on the corrugated periodic substrate.

Computational investigations of electronic structure modifications of ferrocene-terminated self-assembled monolayers: effects of electron donating/withdrawing functional groups attached on the ferrocene moiety.

The electrochemical properties of chemically modified electrodes have long been a significant focus of research. Although the electronic states are directly related to the electrochemical properties, there have been only limited systematic efforts to reveal the electronic structures of adsorbed redox molecules with respect to the local environment of the redox center. In this study, density functional theory (DFT) calculations were performed for ferrocene-terminated self-assembled monolayers with different electron-donating abilities, which can be regarded as the simplest class of chemically modified electrodes. We revealed that the local electrostatic potentials, which are changed by the electron donating/withdrawing functional groups at the ferrocene moiety and the dipole field of coadsorbed inert molecules, practically determine the density of states derived from the highest occupied molecular orbital (HOMO) and its vicinities (HOMO-1 and HOMO-2) with respect to the electrode Fermi level. Therefore, to design new, sophisticated electrodes with chemical modification, one should consider not only the electronic properties of the constituent molecules, but also the local electrostatic potentials formed by these molecules and coadsorbed inert molecules.

A relationship between the force curve measured by atomic force microscopy in an ionic liquid and its density distribution on a substrate.

An ionic liquid forms a characteristic solvation structure on a substrate. For example, when the surface of the substrate is negatively or positively charged, cation and anion layers are alternately aligned on the surface. Such a solvation structure is closely related to slow diffusion, high electric capacity, and chemical reactions at the interface. To analyze the periodicity of the solvation structure, atomic force microscopy is often used. The measured force curve is generally oscillatory and its characteristic oscillation length corresponds not to the ionic diameter, but to the ion-pair diameter. However, the force curve is not the solvation structure. Hence, it is necessary to know the relationship between the force curve and the solvation structure. To find physical essence in the relationship, we have used statistical mechanics of a simple ionic liquid. We found that the basic relationship can be expressed by a simple equation and the reason why the oscillation length corresponds to the ion-pair diameter. Moreover, it is also found that Derjaguin approximation is applicable to the ionic liquid system.

Molecularly clean ionic liquid/rubrene single-crystal interfaces revealed by frequency modulation atomic force microscopy.

The structural properties of ionic liquid/rubrene single-crystal interfaces were investigated using frequency modulation atomic force microscopy. The spontaneous dissolution of rubrene molecules into the ionic liquid was triggered by surface defects such as rubrene oxide defects, and the dissolution rate strongly depended on the initial conditions of the rubrene surface. Dissolution of the second rubrene layer was slower due to the lower defect density, leading to the formation of a clean interface irrespective of the initial conditions. Molecular-resolution images were easily obtained at the interface, and their corrugation patterns changed with the applied force. Force curve measurements revealed that a few solvation layers of ionic liquid molecules formed at the interface, and the force needed to penetrate the solvation layers was an order of magnitude smaller than typical ionic liquid/inorganic solid interfaces. These specific properties are discussed with respect to electric double-layer transistors based on the ionic liquid/rubrene single-crystal interface.

Local analyses of ionic liquid/solid interfaces by frequency modulation atomic force microscopy and photoemission spectroscopy.

Local analyses of ionic liquid/solid electrode interfaces at a controlled electrode potential are of fundamental importance to understanding the origin and properties of the electric double layer at the interfaces, which is necessary for their application to electrochemical devices. This account summarizes our recent achievements of such analyses by using the novel analytical tools of electrochemical frequency modulation AFM (EC-FM-AFM) and electrochemical photoemission spectroscopy (EC-PES). Rather stable stepped structures composed of layers of ion pairs and softer solvation layers outside of the imaged layer were clearly visualized by FM-AFM depending on the substrates. An extremely extended diffusion layer was directly observed by EC-PES during the electrodeposition of metal ion solutes.

Size and shape of Au nanoparticles formed in ionic liquids by electron beam irradiation.

Au nano-particles were synthesized via a reductive reaction in ionic liquid solution containing Au(3+) ions using a low-energy electron irradiation technique. In this study, we focused on how the electron beam conditions (acceleration energy, beam current and irradiation time) and the kinds of ionic liquid affected the size and shape of the prepared Au particles. The sizes of the primary particles increased with higher acceleration energy of the electron beam, whereas they did not depend so much on the beam current. Although the amount of secondary particles increased with longer irradiation time, the sizes of the primary particles remained constant. The anion of the ionic liquid strongly affected the size and shape of the primary particles, which was due to the different local structure of the ionic liquid around the Au particles. When the thickness of the ionic liquid layer was smaller than the penetration length of the electron beam, the formation of secondary particles was suppressed. The present results gave an important knowledge for controlling the size and shape of the metal particles, which is important for application of various catalyst or devices.

Formation of Au nanoparticles in an ionic liquid by electron beam irradiation.

A novel method to synthesize Au nanoparticles via a reductive reaction in an ionic liquid containing Au3+ ions was demonstrated using a low-energy electron beam irradiation technique; Au nanoparticles (approximately 122 nm) formed by the incident electron beam were well dispersed and crystallized; this finding opens up the possibility that the use of electron beams and ionic liquids is of key importance in the development of new fabrication techniques for nanomaterials.

Structural Effects on the Incident Photon-to-Current Conversion Efficiency of Zn Porphyrin Dyes on the Low-Index Planes of TiO2.

The structural effects of substrates on the incident photon-to-current conversion efficiency (IPCE) of Zn porphyrin (ZnP) dyes (ZnP-ref, YD2, and ZnPBAT) have been studied on well-defined single-crystal surfaces of rutile TiO2 (TiO2(111), TiO2(100), and TiO2(110)). IPCE of ZnP-ref depends on the structure of the substrates remarkably: TiO2(100) < TiO2(110) < TiO2(111). IPCE of ZnP-ref/TiO2(111) is 13 times as high as that of ZnP-ref/TiO2(100) at 570 nm. YD2 and ZnPBAT also give the highest IPCE on TiO2(111). The relative coverages of the porphyrin dyes give the following order: TiO2(111) < TiO2(110) < TiO2(100). This order is opposite to that of IPCEs. The orientation of the dyes is predicted using density functional theory calculations on simplified models of TiO2 surfaces. The highest IPCE on TiO2(111) is attributed to the high rate of electron transfer through the space due to the fluctuation of the tilt angle of the adsorbed dyes.

Highly ordered chevron-shaped arrays of continuous copper nano-dot lines formed by electroless deposition on hydrogen-terminated Si(111) surfaces.

We have succeeded in forming highly ordered chevron-shaped arrays of continuous copper nano-dot lines by electroless deposition on hydrogen-terminated Si(111) (H-Si(111)) surfaces. Detailed investigations have shown that tiny Cu clusters are preferentially formed at step edges when the electroless deposition is carried out in a deoxygenated neutral aqueous solution of a low Cu2+ concentration (less than 10 microM) with pH approximately = 7. This finding was combined with highly ordered step-edge lines on H-Si(111) prepared by the previously reported method of Teflon scratching and NH4F etching, which has led to the above success. The present result indicates that designed ordered metal nanowires can be produced by the electroless deposition method, using H-Si(111) surfaces with well-regulated step lines as a substrate.

Crystal-face dependence and photoetching-induced increases of dye-sensitized photocurrents at single-crystal rutile TiO2 surfaces.

Dye-sensitized photocurrents at (100)-, (001)-, and (110)-cut TiO(2) rutile surfaces were increased by photoetching of TiO(2), but the increasing ratio strongly depended on the cut crystal faces and the illumination intensity for the photoetching. For the (110)-cut surface, the photocurrent increase was moderately large and in proportion to the increase in the surface area of TiO(2) induced by the photoetching, irrespective of the illumination intensity for the photoetching. On the other hand, the photocurrent increases for the (001)- and (100)-cut surfaces, especially that for the (001)-cut surface, were prominent and largely exceeded the increases in the surface area. The results were explained by taking into account the following factors: (1) The (001)- and (100)-cut surfaces were thermodynamically unstable in contrast to the (110)-cut surface and had thicker inactive surface layers (or higher densities of surface defects), produced by surface reconstruction during heat treatment of TiO(2) at 550 degrees C in a hydrogen atmosphere for getting n-type semiconductivity. (2) Photoetching not only increased the surface area through formation of nanoholes and grooves at the surface but also effectively removed the thin inactive surface layers (or surface defects).

Crystal-face dependences of surface band edges and hole reactivity, revealed by preparation of essentially atomically smooth and stable (110) and (100) n-TiO(2) (rutile) surfaces.

Essentially atomically smooth (100) and (110) n-TiO(2) (rutile) surfaces were prepared by immersion of commercially available single-crystal wafers in 20% HF, followed by annealing at 600 degrees C in air. The obtained surfaces were stable in aqueous solutions of pH 1-13, showing no change in the surface morphology on an atomic level, contrary to atomically flat surfaces prepared by ion sputtering and annealing under UHV. The success in preparation of the atomically smooth and stable n-TiO(2) surfaces enabled us to reveal clear crystal-face dependences of the surface band edges and hole reactivity in aqueous solutions.

Atomic-Scale Surface Local Structure of TiO2 and Its Influence on the Water Photooxidation Process.

The water photooxidation reaction on TiO2 and related metal oxides has been attracting strong attention from the point of view of solar water splitting. The water photooxidation reaction (i.e., oxygen evolution reaction) accompanies three other kinds of side reactions (photoluminescence (PL), surface roughening, and nonradiative recombination). These reactions are competitive with each other, and the ratio of their quantum efficiencies strongly depends on the atomic-scale surface local structure. This Perspective focuses on the atomic-scale surface local structure dependence of those four kinds of competitive reactions on a TiO2 (rutile) single-crystal electrode on which not only a terrace structure but also step structures were strictly controlled. The experimental results are discussed based on the reaction model of water photooxidation that we previously proposed. The photocatalytic activity of the TiO2 surface roughened by the photoinduced roughening process is also focused on.

Three-dimensional micro/nano-scale structure fabricated by combination of non-volatile polymerizable RTIL and FIB irradiation.

Room-temperature ionic liquid (RTIL) has been widely investigated as a nonvolatile solvent as well as a unique liquid material because of its interesting features, e.g., negligible vapor pressure and high thermal stability. Here we report that a non-volatile polymerizable RTIL is a useful starting material for the fabrication of micro/nano-scale polymer structures with a focused-ion-beam (FIB) system operated under high-vacuum condition. Gallium-ion beam irradiation to the polymerizable 1-allyl-3-ethylimidazolium bis((trifluoromethane)sulfonyl)amide RTIL layer spread on a Si wafer induced a polymerization reaction without difficulty. What is interesting to note is that we have succeeded in provoking the polymerization reaction anywhere on the Si wafer substrate by using FIB irradiation with a raster scanning mode. By this finding, two- and three-dimensional micro/nano-scale polymer structure fabrications were possible at the resolution of 500,000 dpi. Even intricate three-dimensional micro/nano-figures with overhang and hollow moieties could be constructed at the resolution of approximately 100 nm.

Structural investigation of ionic liquid/rubrene single crystal interfaces by using frequency-modulation atomic force microscopy.

Frequency-modulation atomic force microscopy (FM-AFM) was employed to reveal the structural properties of a rubrene single crystal immersed in an ionic liquid. We found large vacancies formed by the anisotropic dissolution of rubrene molecules. Molecular resolution imaging revealed that structures of FM-AFM images deviated from the bulk-terminated structure.

Various metal nanoparticles produced by accelerated electron beam irradiation of room-temperature ionic liquid.

Various metal nanoparticles including base metal were produced by a brief accelerated electron beam irradiation of 1-alkyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide room-temperature ionic liquid without a stabilizing agent, which is usually employed so as to prevent aggregation.

Si(111) surface modified with alpha,beta-unsaturated carboxyl groups studied by MIR-FTIR.

A Si(111) surface modified with alpha,beta-unsaturated carboxyl groups was fabricated using activated alkynes such as propiolic acid and propiolic acid methyl ester via hydrosilylation reaction. The obtained coverage of carboxyl groups was roughly estimated to be 55-60% in both cases from the Si-2p and C-1s X-ray photoelectron specroscopy (XPS) peak intensities. The detailed surface structures were investigated by multiple internal reflection Fourier transform infrared (MIR-FTIR) measurement. It was revealed that this reaction was promoted by visible light irradiation at room temperature. The Si surface modified with functional groups prepared under such a moderate condition is adaptable to functional devices which are easily damaged under UV irradiation or high temperature conditions.