Search

Ionic liquids (ILs) with their high dielectric constants (k) are promising dielectrics of organic field-effect transistors (OFETs). However, it is known that interfacial polarizations of dielectric...

The use of ionic liquid (IL)-based electrolytes is one of the methods for solving various problems that pertain to large-capacity anodes, such as Si and Li, for next-generation lithium ion batteries (LIBs). There is insufficient information about the combination of such anodes and IL electrolytes. In the present study, a Si thin flake, which is one of the promising Si anode active materials, and binary IL electrolytes, 83.3–16.7 mol% 1-ethyl-3-methylimidazolium (bis((fluorosulfonyl)amide)) ([C2mim][FSA])–lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), 83.3–16.7 mol% [C2mim][FSA]–lithium (bis((fluorosulfonyl)amide)) (Li[FSA]), and 50.0–50.0 mol% [C2mim][FSA]–Li[FSA], were used for collecting various information on the battery performance, morphology variation of the Si thin flake, and chemical species in the solid electrolyte interface (SEI) layer. Operando scanning electron microscopy (SEM) and ex situ X-ray photoelectron spectroscopy (XPS) revealed that the SEI layer composition depends on the Li salt species and that their molar fractions in the binary IL electrolytes strongly affect the morphology variation in the Si thin flakes during the charge process. When the favorable Li conductive components, e.g., LiF and Li3N, were abundantly contained in the SEI layer, the charge and discharge behavior of the Si thin flake anode was greatly improved. The best battery performance was obtained by the combination of the Si thin flake composite anode and the 50.0–50.0 mol% [C2mim][FSA]–Li[FSA]. Discharge capacity above 1100 mA h g−1 was achieved even after 500 cycles under the charge and discharge conditions at a 3C rate.

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.

Pt and PtNi alloy nanoparticle-supported multiwalled carbon nanotubes (Pt/MWCNTs and PtNi/MWCNTs) are prepared by a one-pot pyrolysis method with the N,N,N-trimethyl-N-propylammonium bis(trifluorom...

Ionic liquids (ILs) have attracted much attention as promising electrolytes for electrochemical devices due to their wide electrochemical window (stability) and negligible vaporization. For such applications, nanostructured electrodes have advantage on their large surface area leading to accumulation of large density energy at the interface. However, recent reports on IL/electrode interfaces revealed that quite unique structures can be formed, and it is not clear how the interface forms and how it changes by forming an electric double layer (EDL). In this study, we investigated the interfacial capacitance of nanostructured Au electrodes using electrochemical impedance spectroscopy (EIS) and IR measurements. Polystyrene beads were self-assembled in a close packed form on a gold substrate by dipping in polystyrene beads solution and rapid drying. Au electrodeposition on thus prepared surface followed by removal of the beads resulted in the fabrication of a nanostructured Au electrode with periodic dimples. Electrochemical behavior of a ferrocene dissolved ionic liquid (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide (BMI-TFSA)) at the nanostructured and flat Au electrodes were investigated by EIS. We found that the capacitance of the nanostructured electrode was smaller than that of the flat electrode in the whole potential range. This result suggests that the thickness of the electric double layer formed in the nano-sized dimple is thicker than that formed on the flat surface. In the case of nanostructured Au electrode having 160 nm diameter dimples, the large capacitance was observed comparing with other electrodes having the dimples of different sizes. In addition, on the negative-going scan, the capacitance takes maximum at 0.1 V vs. Fc/Fc+, whereas no peak was found on the reverse positive-going scan. This result indicates that the exchange of cation/anion layers smoothly proceeded on the negative-going scan, whereas such a smooth exchange was prevented due to strongly surface-adsorbed BMIM+ on the positive-going scan. We also found that the hysteresis behavior was relatively suppressed in the case of the electrodes having 70 nm and 120 nm diameter dimples. We carried out the In-situ ATR-IR measurement to investigate the structure of ionic liquid molecules at the ionic liquid/electrode interface. The details of the relationships between the arrangement of ionic liquid at the interface and the capacitance of the electrode will be discussed.

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.

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.

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.

We conducted the operando atomic force microscopy (AFM) of ionic liquid (IL)/rubrene single crystal interfaces under the operation of an electric double-layer (EDL)-gated field-effect transistor (FET). We developed a top-side-gated EDL-FET and performed the simultaneous measurement of device characteristics and frequency-modulation AFM in the IL droplet. The AFM images revealed microscopic and macroscopic changes in the rubrene single crystal surface upon carrier injection by applying a gate voltage.

Ionic liquids (ILs) have attracted keen attention due to its applicability to various kinds of electrochemical devices. However, we still lack enough knowledge of the interface structure and spatial distribution of chemical species during the electrochemical processes. In our previous study, we succeeded in the observation of the spatial distribution of Ag+ ions in ionic liquid solution close to the electrode under the applied potential using scanning electrochemical photoelectron spectroscopy (Scanning EC-PES) we originally developed. Interestingly, the diffusion layer where the concentration of metal ions is quite small (We named this region as “depletion layer”) was formed close to the electrode in a wide region. By a numerical simulation, we revealed that the metal ions diffused by the hopping-like mechanism, in which the apparent diffusion coefficient increased with increasing of the concentration of 'holes' (vacancies formed between IL molecules) acting as hopping sites. On the other hand, it can be anticipated that the local structure of ILs can be systematically controlled by changing of IL species (e.g. the alkyl chain length of imidazole cation, shape and size of anion), additive ions and/or temperature of solution. By using these techniques for controlling the local structure of ILs, we can strictly control the diffusion behavior of metal ions close to the electrode. In other words, we will be able to propose a new strategy for controlling the diffusion behavior of solutes in ionic liquid solutions close to the electrode. In this study, we investigated the factors for controlling the diffusion behavior of metal ion in IL electrolyte based on our proposing diffusion model, and revealed that the behavior of metal ions can be controlled by some factors. We investigated the additive effect using Li+ ion. In the case of the ~200 mM Ag+/BMI-TFSA((1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide) solution, the large depletion layer was formed close to the electrode, whereas the depletion layer was decreased when ~100 mM Li+ (as an electrochemically inactive additive) was added to the solution. This result indicates that the hopping-like diffusion was suppressed by the addition of the Li+ ion which efficiently occupied the holes (hopping sites) (Note that the depletion layer was formed by the fast hopping-like diffusion mechanism). On the other hand, when we added the excessive amount of additive (~500 mM Li+) to BMI-TFSA solution, a large depletion layer was formed again, indicating that the hopping diffusion was enhanced. This is due to that some Ag+ ions are drive out of the polar domain, and moved to the non-polar domain where the activation energy of hopping diffusion is relatively small. On the other hand, we carried out a similar experiment using HMI-TFSA instead of BMI-TFSA. We found that the hopping diffusion of Ag+ ion is enhanced compared with that in BMI-TFSA. This could also be explained by the local domain effect of ionic liquid. These results indicate that we can control the diffusion behavior of metal ions by adjusting the local structure of ionic liquid.

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.

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.

Polymer electrolyte fuel cells (PEFCs) are attractive as energy conversion systems with high conversion efficiency. Although theoretical energy conversion efficiency is high, 83% at 295 K, the actual efficiency is very low because of a large overpotential of oxygen reduction reaction (ORR). In addition, the durability of the catalysts becomes important for commercialization of PEFCs. Pt supported carbon cathode is easy to degrade at high potential region and potential fluctuation. Therefore, we have focused on group 4 and 5 oxide-based compounds as oxides with active sites and electro-conductive supports. We successfully demonstrated that the surface of the oxides had high quality active sites [1] and superior durability against start/stop and load cycle tests [2]. Recently, DFT calculation by Sugino’s group revealed that Pd or Rh doped TiO2 had possibility that these surfaces reached equilibrium potential for the ORR [3]. However, the DFT calculation was only based on the adsorption Gibbs energies of reaction intermediates. Considering the actual active sites, sufficient electron supply is necessary to proceed ORR fluently. Group 4 and 5 oxide-based compounds are essentially semi-conductors. Therefore, the semi-conducting physico-chemical properties such as carrier density and Flat-band potential affect the supply of electrons to the active sites. However, the effect of the semi-conducting properties of the oxide-based compounds on the oxygen reduction activity in acidic media has not been investigated yet. In this study, we attempted to reveal the effect of Flat-band potential and capacitance of space charge layer on the ORR activity. Thus, we focused on titanium oxides and prepared Nb, Ta, and Zr added titanium oxide to investigate the effect of foreign element doping. The oxide-coating film was prepared on a Ti plate (10 mm× 10 mm× 1 mm) substrate by the conventional dip-coating method using a 0.5 M butanolic solution of metal salt: titanium tetrabutoxide, zirconium tetrabutoxide, niobium pentaethoxide or tantalum pentaethoxide. The titanium substrate was etched with 10wt% oxalic acid at 80 °C for 1 h and then rinsed with deionized water before the dipping procedure. Calcination of the dip-coated salts was con-ducted in air at 350 °C. The dip-drying/calcination (alternating 10 min each) procedure was typically repeated 15 times. Electrochemical measurements were carried out using a three-electrode cell at 30 ± 1 °C in 0.1 M H2SO4. Cyclic voltammetry was performed with 5 mV sec-1 in the potential range from 0.2 to 1.2 V in an oxygen and nitrogen atmosphere to obtain oxygen reduction current. The oxygen reduction current density, i ORR, was normalized by the geometric area of the electrode. The ORR activity was evaluated by onset potential for the ORR, E onset, defined as the electrode potential at i ORR = 0.02 mA/cm2 and i ORR at 0.2 V, |i ORR@0.2V|. Electrochemical impedance measurements were performed with amplitude of 0.01 V and frequency of 1000 Hz to obtain capacitance of space charge layer, C scl, every 0.1 V below 1.1 V. In addition, the values of the (1/C scl)2 were plotted against the electrode potential, that is, Mott-Schottky plot, to obtain the flat-band potential, E fb. Figure 1 shows the relationship between E fb and E onset of Nb, Ta, or Zr doped TiO2 electrodes. The relation of E onset－E fb≒0.3 V was obtained and it was independent of the doped elements. The difference between the electrode potential and E fb corresponded to the barrier height of space charge layer. When the electrode potential is higher than the E onset, the electrons cannot pass through the barrier of the space charge layer, resulting that the ORR cannot proceed because the supply of electrons from bulk of semi-conductor to the active sites at the interface between oxide-electrolyte. When the electrode potential reaches at Eonset, the ORR starts to proceed due to the continuous supply of the electrons because the electrons can pass through the barrier. According to the relation of E onset－E fb≒0.3 V, the barrier height of the space charge layer, which the electrons can pass through, was estimated to be 0.3 V. Acknowledgement The authors wish to thank the support of the New Energy and Industrial Technology Development Organization (NEDO). References [1] S. Tominaka et al., ACS Omega, 2, 5209 (2017). [2] A. Ishihara et al., J. Electrochem. Soc., 163, F603 (2016). [3] Y. Yamamoto et al., J. Phys. Chem. C, 123, 19486 (2019). Figure 1

Ferrocene-terminated self-assembled monolayers (SAMs) have been widely studied in the past quarter century to reveal the electrochemical properties of chemically modified electrodes. It has been well-known that the formal potential of the system strongly depends on the local environment of the ferrocene moiety. Although electronic states of ferrocene-terminated SAMs should directly affect the electrochemical properties, knowledge concerning electronic structures with respect to different local environment is very limited. In this study, we performed density functional theory calculations of ferrocene-terminated SAMs with different coadsorbed species to reveal the relationship between the electronic structures and the local environment of ferrocene moieties. Depending on the local electrostatic potential, density of states derived from the highest-occupied molecular orbital (HOMO) and its vicinities (HOMO–1 and HOMO–2) of the ferrocene moiety were found to largely shift with respect to the electrode Fermi ...

Ionic liquids (ILs) have attracted keen attention due to its applicability to various kinds of electrochemical devices. However, we still lack enough knowledge on the interface structure and spatial distribution of chemical species during the electrochemical processes. In our previous study, we succeeded in the observation of the spatial distribution of XPS spectra close to the electrode under the applied potential using scanning electrochemical photoelectron spectroscopy (Scanning EC-PES) we originally developed. Interestingly, the shape of the diffusion layer (The spatial distribution of the concentration of the metal ions) was largely different from the concave function usually supposed in the case of aqueous solution. By a numerical simulation, we revealed that the characteristic shape of the diffusion layer of IL electrolyte was formed due to the fact that the metal ions diffused by the fast hopping-like mechanism, in which the apparent diffusion coefficient increased with increasing of the concentration of 'holes' (vacancies formed between IL molecules) acting as hopping sites. Based on these results, we proposed the new diffusion model in which hopping diffusion and conventional diffusion (which follows Stokes-Einstein equation) coexist. The most prominent feature of our diffusion model is that the local structure of ionic liquid (hole density, arrangement of the holes and/or IL molecules) strongly affect the diffusion behavior of metal ions. On the other hand, it can be anticipated that the local structure of ILs can be systematically controlled by the IL species (e.g. the alkyl chain length of imidazole cation, shape and size of anion), additive ions and/or temperature of solution. By using of these techniques for controlling the local structure of ILs, we can strictly control the diffusion behavior of metal ions close to the electrode. In other words, we will be able to propose the new strategy for controlling the diffusion behavior of solutes in ionic liquid solutions close to the electrode. In this study, we investigated the factors for controlling the diffusion behavior of metal ion in IL electrolyte based on our proposing diffusion model, and revealed that the behavior of metal ions can be controlled by some factors mentioned above.

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.

Ferrocene-terminated self-assembled monolayers (SAMs) have been studied in the last quarter-century to determine the relationship between their electrochemical properties and microscopic structures as a prototypical chemically modified electrode. Although electronic structures are directly related to the electrochemical properties, knowledge of such a relation concerning ferrocene-terminated SAMs is extremely limited. In this study, we deposited a small amount of ionic liquid onto three types of ferrocene-terminated SAMs possessing different ionization energies and performed X-ray and ultraviolet photoelectron spectroscopies to elucidate the changes in the electronic structure resulting from the adsorption of the ionic liquid. For a low-ionization-energy system, spontaneous oxidation of the ferrocene moieties occurred, while their electrochemical activities were retained; for other systems, only minor changes were observed. This result indicates that the interaction between the ionic liquid and ferrocene-...

A novel method for fabricating microsized and nanosized polymer structures from a room-temperature ionic liquid (RTIL) on a Si substrate was developed by the patterned irradiation of an electron beam (EB). An extremely low vapor pressure of the RTIL, 1-allyl-3-ethylimidazolium bis((trifluoromethane)sulfonyl)amide, allows it to be introduced into the high-vacuum chamber of an electron beam apparatus to conduct a radiation-induced polymerization in the nanoregion. We prepared various three-dimensional (3D) micro/nanopolymer structures having high aspect ratios of up to 5 with a resolution of sub-100 nm. In addition, the effects of the irradiation dose and beam current on the physicochemical properties of the deposited polymers were investigated by recording the FT-IR spectra and Young's modulus. Interestingly, the overall shapes of the obtained structures were different from those prepared in our recent study using a focused ion beam (FIB) even if the samples were irradiated in a similar manner. This may be due to the different transmission between the two types of beams as discussed on the basis of the theoretical calculations of the quantum beam trajectories. Perceptions obtained in this study provide facile preparation procedures for the micro/nanostructures.

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.

The relationship between the oxidation potential determined by electrochemical measurements and the ionization energies measured by gas-phase ultraviolet photoelectron spectroscopy (UPS) has long been a focus of research of various groups. The focus of this study is to reveal such a correlation for redox molecules, which are chemically attached to metal electrodes. X-ray photoelectron spectroscopy, UPS, and cyclic voltammetry were performed for three types of ferrocene-terminated self-assembled monolayers possessing different electron-donating abilities. The results of these experiments indicate a linear relation with a slope of ∼0.7 between the UPS-derived energy of the highest occupied molecular orbital (HOMO) and the electrochemical oxidation potential. This indicates that the HOMO energy can be used to determine the oxidation potential of chemically modified electrodes.

We previously found that a TiO2 surface was atomically roughened with the progress of the photo-oxidation reaction of water. Considering the fact that surface local structures affect the photocatalytic activity, the activity may be changed by the surface roughening. In this paper, the photocatalytic activities for oxygen evolution of the roughened and atomically flat TiO2 single-crystal electrodes in acidic and alkaline solutions were investigated. In the low pH (acidic condition) aqueous solution, the onset potential of the roughened TiO2 for oxygen photoevolution was shifted toward the negative side by 0.20–0.15 V from that of the atomically flat TiO2, whereas the shifts of the flat-band potential between them were much smaller than that of the onset potential. This result indicated that the activation energy for oxygen photoevolution of the roughened TiO2 was lower than that of the atomically flat TiO2. On the other hand, in the high pH (alkaline condition) aqueous solution, the onset potentials of bot...

Ionic liquids (ILs) have attracted keen attention due to its applicability to various kinds of electrochemical devices. However, we still lack enough knowledge on the interface structure and spatial distribution of chemical species during the electrochemical processes. In our previous study, we succeeded in the observation of the spatial distribution of XPS spectra close to the electrode under the applied potential using scanning electrochemical photoelectron spectroscopy (Scanning EC-PES) we originally developed. Interestingly, the shape of the diffusion layer (The spatial distribution of the concentration of the metal ions) was largely different from the concave function usually supposed in the case of aqueous solution. In the case of ionic liquid solution, the spatial distribution of concentration was convex and the depletion layer where the concentration of metal ions is quite small (We named this region as “depletion layer”) was formed close to the electrode in a wide region. By a numerical simulation, we revealed that the metal ions diffused by the hopping-like mechanism, in which the apparent diffusion coefficient increased with increasing of the concentration of 'holes' (vacancies formed between IL molecules) acting as hopping sites. In addition, we also revealed that the diffusion behavior (diffusion velocity) of metal ions can be controlled by controlling the concentration of the holes by using of the small amount of additive ions. These results indicate that we will be able to propose the new strategy for controlling the diffusion behavior of solutes in ionic liquid solutions close to the electrode. On the other hand, it is known that the diffusion behavior of metal ions affects the shape of the electrodeposits on the electrode. Therefore, the controlling of the diffusion behavior of metal ions close to the electrode enable us to control the shape of the electrodeposits. In this strudy, we investigated how we can control the diffuosion behavior of the metal ions based on the new diffusion model we previously proposed and we also investigated that how the diffusiton behavior of the metal ions in IL affects the shape of the electrodeposits on the surface. In the case of the ~200 mM Ag+/BMI-TFSA((1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide) solution, the large depletion layer was formed close to the electrode, whereas the depletion layer was decreased in the case that Li+ (as an electrochemically inactive additive) was added to the solution. This result indicates that the hopping-like diffusion was suppressed by the addition of the Li+ ion (Note that the depletion layer was formed by the fast hopping-like diffusion mechanism). On the other hand, the shape of the electrodeposit formed in the former solution is largely different from that formed in the latter solution. This is mainly due to the total diffusion velocity of Ag+ ion was largely decreased by the addition of Li+ ion. These result indicates that we can effectively control the shape of the electrodeposits by controlling the diffusion behavior of metal ions.

Physicochemical data of the novel low-melting-point melt systems combining urea with the 1-alkyl-3-methylimidazolium chloride ([R1MeIm]Cl) were examined in order to produce an alternative to room-temperature ionic liquid that is very expensive to prepare and purify compared to the conventional molecular solvents. [HexMeIm]Cl- and [MeOctIm]Cl-rich urea melts, which have almost the same pyrolysis temperature as neat [HexMeIm]Cl and [MeOctIm]Cl organic salts, showed a stable liquid phase at room temperature. The various physicochemical properties of the [MeOctIm]Cl-rich urea room-temperature melts were measured as a function of temperature. The VTF equation was fitted to the conductivity and viscosity data, and a simple polynomial was fitted to the density data. Interestingly, although the 75.0–25.0 mol% [BuMeIm]Cl–urea melt did not give a stable liquid phase at room temperature, the melt showed favorable conductivity that exceeds that for the [MeOctIm]Cl-rich urea melts at the temperature exceeding 323 K. Further investigation on the applications, e.g., Cu electroplating, SEM observation technique, and Au nanoparticle preparation, using the [R1MeIm]Cl–urea melt system revealed that the melt system has a great possibility as an alternative to the RTIL.

Bistable (hysteretic) behavior was observed in the current vs. potential relationship of p-type silicon grafted with copper nanoparticles in hydrofluoric acid. In-situ multiple-internal-reflectance FTIR spectroscopy indicated that the silicon surface is hydroxyl- (SiOH) or partially fluorine-terminated (SiF) in response to the bistability. It is considered that autocatalytic chemical reactions in which the shift of flat-band potential of silicon coupled with the change of the surface termination plays a key role in the observed bistability. Keywords: Surface dipole, Flatband potential, Bistable, Autocatalysis, Copper nanoparticles

The electrocatalytic activity of amorphous and crystalline RuO2 thin films for oxygen evolution in an aqueous solution was investigated. The RuO2 films were prepared on FTO substrates by electrodeposition or RF magnetron sputtering technique. The obtained films were annealed at various temperatures. In both cases, the as-prepared films or the 200 °C annealed film had an amorphous structure, whereas the films annealed at 300 °C and over were crystallized to rutile structure. The analysis of the Tafel slope indicated that the rate-determining step in the oxygen evolution reaction on the amorphous RuO2 was the combination of the adjacent Ru–OH groups, whereas that on crystalline RuO2 was the dissociation of O–H bond in Ru–OH group. The onset potentials of the amorphous RuO2 films for oxygen evolution were shifted toward the negative side by 0.06–0.03 V from those for the rutile crystalline samples. The shift of the onset potential is probably attributed to the structural flexibility which is characteristic of the amorphous surface. This result suggested that the electrocatalytic activity of amorphous RuO2 for oxygen evolution was higher than that of rutile crystalline RuO2.

Metal nanoparticles were prepared with γ-ray and accelerator electron beam irradiation in the 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (BuMeIm+Tf2N-) room-temperature ionic liquids. As for Au nanoparticles yielded in this investigation, the γ-ray irradiation made the particle size smaller compared to that by the accelerator electron beam irradiation. It is interesting to note that the nanoparticles prepared in this article were very stable although the surface was not covered with a stabilizing agent, which is usually employed so as to prevent aggregation. Similarly, other metal nanoparticle productions such as Ag, Cu, Mg, Al, Zn, and Fe were also investigated by means of the accelerator electron beam irradiation. In addition, some RTILs with Au nanoparticles yielded by the irradiation experiment showed fluorescent behavior under ultraviolet irradiation of 365 nm. It implies that the RTILs contain Au nanoparticles as well as Au clusters that cannot be observed by a common TEM system. In this study, we also found that the Si wafer, on which NaAuCl4⋅2H2O-added RTIL was spread, has distinctive two-dimensional Au microstructure crystals on the surface after the accelerator electron beam irradiation.

Photocurrents due to water oxidation at BiZn2VO6 (Eg 2.4 eV) particulate thin film electrodes were largely enhanced by pre-treatment with an aqueous TiCl4 solution. Photocurrents for BiZn2VO6 electrodes with no TiCl4 treatment were also enhanced by the addition of organic compounds such as methanol and trimethyl amine to the aqueous electrolyte. Interestingly, such enhanced photocurrents by organic compounds were further enhanced by the TiCl4 pre-treatment. EDAX and SEM investigations showed the formation of a flock-like TiO2 overlayer on BiZn2VO6 particles after the TiCl4 treatment. The photocurrent enhancement by the TiCl4 pre-treatment is thus mainly attributed to the necking effect of the flock-like TiO2 overlayer, which facilitates the transport of photogenerated electrons within the BiZn2VO6 particulate thin film electrode.

The oxygen photoevolution reaction on TiO2 and related metal oxides has attracted strong attention from the point of view of solar water splitting. In our previous study, we have proposed a new reaction model in which generated holes are consumed not only by the O2 evolution reaction but also by three competitive paths possibly occur at different surface sites; photoluminescence, surface roughening (i.e. photocorrosion), and nonradiative recombination [1,2]. Actually, we found that the branch ratio for these paths on TiO2 (110) changed by the density and the direction of steps (i.e. atomic leveled surface structure) [3]. On the other hand, it is expected that the nano-sized structure of the TiO2 surface also affect the branch ratio of 4 competitive photooxidation reactions. Thus, in this study, we investigated oxygen photoevolution efficiency on a nanostructured TiO2 electrode that exposes different facets. A Pt wire and an Ag/AgCl/sat.KCl electrode were used as a counter and a reference electrode, respectively. An n-type TiO2 (110) doped with 0.05wt% Nb was used as a working electrode. The j−U curves were measured in 0.1 M HClO4 (pH 1.1) under UV irradiation. Nanostructured TiO2(110) surfaces were prepared by photoetching in 0.05 M H2SO4 [4]. We obtained the nanostructured surfaces on which shallow grooves are regularly arranged. We should note that formed surface of the nanostructure is composed by (100) facet faces [4]. Photocurrent generated along with the O2 evolution from H2O on the nanostructured TiO2 (110) was apparently different from that on the flat TiO2 (100). The onset potential of nanostructured TiO2 electrode was shifted toward negative voltage, indicating that the activation energy of oxygen evolution reaction was smaller than that on the flat surface. In addition, the consumption ratio of the photogenerated holes by above mentioned 4 competitive reactions was largely different from that on the atomically flat (100) surface. Considering the fact that the photoetched TiO2(110) surface is composed by (100) microfacets[4], this result can be attributed to the nanostructuring effect. We also carried out the same experiment using the some other TiO2 electrode those have different nanostructure on their surfaces, and found that the nanostructuring effect (1. Branch ratio of 4 competitive photooxidation reactions, 2. Activation energy of O2 evolution reaction) depends on the nano-sized surface morphology of the TiO2 electrode. [1] A. Imanishi, T. Okamura, N. Ohashi, R. Nakamura, Y. Nakato, J. Am. Chem. Soc., 129 (2007) 11569. [2] A. Imanishi, K. Fukui, J. Phys. Chem. Lett., 5 (2014) 2108. [3] M. Kadono, Master’s thesis, Osaka Univ. (2013). [4] A. Imanishi, H. Suzuki, K. Murakoshi, Y. Nakato, J. Phys. Chem. B, 110 (2006) 21050.

The adsorption properties of n-TiO2 rutile (100) and (110) surfaces which were prepared by simply annealing, photoetching, and HF-annealing methods were investigated by using the C K-edge NEXAFS. It was revealed that the interaction between the substrate and the dye molecules on the simply annealed surface was stronger than that on the HF-annealed surface, whereas the net quantum efficiency of the dye-sensitized photocurrent on HF-annealed surface was larger than that of the simply annealed surface for both surface faces. These results indicated that the energy difference between the LUMO of Mc[18,1] and the conduction band of TiO2 was large enough to give a nearly 100% quantum efficiency of electron transfer from photoexcited dye to TiO2 even in the case of the simply annealed surface. The enhancement of photocurrent of the HF-annealed surface comparing with that of the simply annealed surface was explained by the assumption that HF-annealing process can remove inactive layers (or surface defects) on TiO...

A composite semiconductor electrode with the structure “n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ITO/RuO2” was fabricated for the purpose of achieving efficient solar water splitting. The electrode showed a stable photoanodic current due to oxygen evolution with a large negative photoshift (Vp) of about 2.2 V from an anodic current at a RuO2 electrode. The photoshift was large enough for full water splitting. A photoelectrochemical (PEC) cell, composed of the composite electrode, a Pt counter electrode, and 0.10 M Na2SO4 (pH 6.3), generated a photocurrent density of 1.88 mA cm−2 under simulated solar illumination (AM 1.5 G, 100 mW cm−2), yielding a solar to chemical conversion efficiency of 2.3% as calculated from the photocurrent value. The result has shown that the combination of “crystalline Si/a-Si/GaP” is suitable for efficient solar water splitting. It is shown that the efficiency can be increased by use of GaP with a well-regulated p-n junction.

The surface reactions of iodine ions on nearly atomically flat, H-terminated Si(111) surfaces, when immersed in 7.6 M HI in a temperature range of −20 to 63 °C, were investigated by atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, and core-level shift spectra. It was revealed that the following two reactions simultaneously occurred, which were competitive with each other: (1) substitution reaction of Si−H bonds with Si−I bonds and (2) cluster formation by the condensation of SiHxI4−x or their oligomer species, which were generated by surface etching reaction. The ratio of those competitive reactions was strongly dependent on the temperature of the HI solution. At the high temperature, the former reaction was enhanced, whereas the latter reaction was suppressed. Inversely, at the low temperature, the latter reaction was relatively enhanced, resulting in that the ordered rodlike clusters were formed on the Si(111) surface. These results were explained by the fact that the forme...

A coupled TiO2-BiVO4 system, Bix Tiy Vx O4x +2y (x ≈ y), with modified TiO2 and BiVO4 crystals through their interaction, was prepared in the form of powder by solid-state reaction under the concept of combining properties of active photocatalysts. Experiments of photocatalytic oxygen evolution from an aqueous powder suspension under visible-light irradiation (λ ≥ 420 nm) have shown that Bix Tiy Vx O4x +2y (x ≈ y) exhibits activity much higher than that of BiVO4, which is known as one of the most active visible-light-responsive photocatalysts, indicating the effectiveness of the interaction and the modification of TiO2 and BiVO4. XRD experiments showed that Bix Tiy Vx O4x +2y (x ≈ y) was composed of BiVO4 and TiO2 crystals slightly modified probably by partial exchange of V5+ and Ti4+ ions. Photocurrent vs. wavelength for a particulate Bix Tiy Vx O4x +2y (x ≈ y) film electrode demonstrated several components, in agreement with the conclusion that Bix Tiy Vx O4x +2y (x ≈ y) consisted of modified BiVO4 and TiO2 crystals. A possible band model was tentatively proposed to explain observed characteristics such as photocurrent density (j) vs. potential (U), j vs. illumination wavelength (λ), the U dependence of j vs. λ, and the effect of the addition of a reductant such as I−, MeOH, SCN− and Br– on j vs. U and j vs. λ. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Solar cells with a structure of p−n heterojunction, “ITO/p-CuI/n-Si” (ITO = indium tin oxide), were fabricated by depositing p-CuI on hydrogen (H)-terminated or methyl (CH 3)-terminated single-crystal n-Si (111) followed by the sputter deposition of ITO or by the press-contact of commercial conductive oxide glass with p-CuI. The solar cells generated very high open-circuit photovoltages ( V oc), reaching 0.617 V without surface texturing and back surface field treatments. Experiments have shown that a morphologically soft property of p-CuI as well as a low density of surface states at H-terminated or CH 3-terminated n-Si (111) is responsible for the generation of high V oc. Theoretical investigation has also shown that the p-CuI/n-Si contact forms an ideal minority-carrier controlled junction suitable for the generation of high V oc.

The crystal-face dependence of the dye-sensitized photocurrents and the adsorption properties of benzothiazole merocyanine (Mc[18,1]) dye molecules were investigated, using atomically flat (1 0 0) and (1 1 0) TiO 2 single crystal surfaces. From the estimation of the amount of the transferred charge from the TiO 2 surface to C O groups of dye molecules based on NEXAFS data, it was revealed that the interaction of the adsorbed molecules and the (1 1 0) surface was much stronger than that for the (1 0 0) surface. On the other hand, the absorbed photon to current conversion efficiency (APCE) value was almost the same for both surfaces. We suggested a possible explanation as follows: the energy difference between the LUMO of Mc[18,1] and the conduction band of TiO 2 was large enough to give a nearly 100% quantum efficiency of electron transfer from photoexcited dye to TiO 2 , which made the difference in the interaction between dye molecules and TiO 2 not apparent. The incident photon to current conversion efficiency (IPCE) for the (1 0 0) surface was much larger than that for the (1 1 0) surface, which was explained by the fact that the amount of the adsorbed dye molecules on the (1 0 0) surface was larger than on the (1 1 0) surface, probably due to the larger surface density of five-coordinated Ti sites in the former surface.

The formation of self-assembled monolayers (SAMs) of adsorbed cationic or anionic surfactant molecules on atomically flat H-terminated Si(111) surfaces in aqueous solutions was investigated by in situ AFM measurements, using octyl trimethylammonium chloride (C8TAC), dodecyl trimethylammonium chloride (C12TAC), octadecyl trimethylammonium chloride (C18TAC)) sodium dodecyl sulfate (STS), and sodium tetradecyl sulfate (SDS). The adsorbed surfactant layer with well-ordered molecular arrangement was formed when the Si(111) surface was in contact with 1.0x10(-4) M C18TAC, whereas a slightly roughened layer was formed for 1.0x10(-4) M C8TAC and C12TAC. On the other hand, the addition of alcohols to solutions of 1.0x10(-4) M C8TAC, C12TAC, or SDS improved the molecular arrangement in the adsorbed surfactant layer. Similarly, the addition of a salt, KCl, also improved the molecular arrangement for both the cationic and anionic surfactant layers. Moreover, the adsorbed surfactant layer with a well-ordered structure was formed in a solution of mixed cationic (C12TAC) and anionic (SDS) surfactants, though each surfactant alone did not form the well-ordered layer. These results were all explained by taking into account electrostatic repulsion between ionic head groups of adsorbed surfactant molecules as well as hydrophobic interaction between their alkyl chains, which increases with the increasing chain length, together with the increase in the hydrophobic interaction or the decrease in the electrostatic repulsion by incorporating alcohol molecules into the adsorbed surfactant layer, the decrease in the electrostatic repulsion by increasing the concentration of counterions, and the decrease in the electrostatic repulsion by alternate arrangement of cationic and anionic surfactant molecules. The present results have revealed various factors to form the well-ordered adsorbed surfactant layers on the H-Si(111) surface, which have a possibility of realizing the third generation surfaces with flexible structures and functions easily adaptable to circumstances.

The mechanism of water photooxidation reaction at atomically flat n-TiO2 (rutile) surfaces was investigated in aqueous solutions of various pH values, using photoluminescence (PL) measurements. The PL bands, which peaked at around 810 and 840 nm for the (110) and (100) surfaces, respectively, were assigned to radiative transitions between conduction-band electrons and surface-trapped holes (STH), [Ti−O=Ti2]s+, formed at triply coordinated (normal) O atoms at the surface lattice. The PL intensity (IPL) decreased stepwise with increasing solution pH, namely, it sharply decreased at around pH 4, near the point of zero charge of TiO2 (about 5), and then rapidly decreased to zero near pH 13. The first sharp decrease around pH 4 is attributed to the increased rate of nucleophilic attack of a water molecule to a hole at a site of surface bridging oxygen (Ti−O−Ti), the density of which increases with increasing pH. The nucleophilic attack is regarded as the main initiating step of the water oxidation reaction in ...

Photocatalytic activity of carbon-doped anatase TiO2 (C−TiO2) particles was investigated. Observation of a low anodic photocurrent for a C−TiO2 thin film electrode in 0.5 M Na2SO4 (pH = 6.4) under visible light irradiation suggested the occurrence of water oxidation by photogenerated holes in a C-induced midgap level of C−TiO2, though a C−TiO2 particulate suspension system in the presence of an electron acceptor (Fe3+) gave no detectable O2 evolution under the visible light irradiation. The photocurrent measurements also showed that the holes in the midgap level of C−TiO2 were able to oxidize efficiently methanol and formic acid. Interestingly, the oxidation of methanol was explained reasonably by a mechanism that it proceeded indirectly via a surface intermediate of the water oxidation such as Ti−O•, in harmony with the abovementioned assumption of the water photooxidation by the midgap-level holes, whereas the oxidation of formic acid was interpreted to occur by a direct reaction of the midgap-level hol...

Photoelectrochemical etching of n-TiO2 (rutile) single crystals produced ordered arrays of nanogrooves with the spacing of about 300 nm over a macroscopically wide area of 0.5 × 0.5 cm at the surface without any use of imprints or templates. Moreover, the photoetching by irradiation with patterned light intensities gave nanogrooves with a desired depth at a desired location, interesting from the point of view of nanoscience and nanotechnology. A plausible model is proposed for the formation of ordered arrays of nanogrooves in terms of self-organized photoetching in which autocatalytic accelerated photoetching at the bottom of the grooves, caused by an excess concentration of photogenerated holes due to a high potential gradient, is coupled with its retardation by a downward shift in the flat-band potential of n-TiO2 at the bottom of the grooves, induced by an increase in the solution pH due to accelerated photoreactions.