Local structures and dynamics of interfacial imidazolium-based ionic liquid depending on the electrode potential using electrochemical attenuated total reflectance ultraviolet spectroscopy.

Recently, ionic liquids (ILs) have attracted attention as prospective electrolytes for Li-ion batteries, with safe performance. Herein, the dynamics of the IL at the electrochemical interface, which is the key to the electrochemical reaction, was monitored using attenuated total reflectance far- and deep-ultraviolet (ATR-FUV-DUV) spectroscopy. An original measurement system, which combined an ATR-FUV-DUV spectrometer with a Kretschmann type (fully metal-coated prism) electrochemical setup, was assembled. Spectral measurements and assignments were performed for the 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIM][TFSI])/Pt electrode (∼7 nm) interface. The incident light in the FUV and DUV regions entered a measurement system comprising an [EMIM][TFSI]/Pt electrode/ATR sapphire prism, and the potential-dependent absorption spectra were measured in the 180-450 nm range. This in-situ spectroscopic technique is unique in that the electronic transition spectra of the interfacial IL can be obtained. By switching the applied potentials, temporal spectral changes (i.e. relaxation signals) were tracked at wavelengths of 450 nm and 221 nm, where the direct electronic absorption of the IL was active and inactive, respectively. Comparing these relaxation times, it was revealed that the absorption signal at 221 nm changed more slowly than that at 450 nm. This indicated that the molecular conformations that affected the electronic absorption of the interfacial ILs changed slowly. Considering the surface-normal dipole selection rule for molecules on a metal surface, it is suggested that the slow changes in the molecular conformations can be ascribed to the potential-dependent interfacial orientations of [EMIM]+.

Electronic excitation spectra of organic semiconductor/ionic liquid interface by electrochemical attenuated total reflectance spectroscopy.

The interface of organic semiconductor films is of particular importance with respect to various electrochemical devices such as transistors and solar cells. In this study, we developed a new spectroscopic system, namely electrochemical attenuated total reflectance ultraviolet (EC-ATR-UV) spectroscopy, which can access the interfacial area. Ionic liquid-gated organic field-effect transistors (IL-gated OFETs) were successfully fabricated on the ATR prism. Spectral changes of the organic semiconductor were then investigated in relation to the gate voltage application and IL species, and the magnitude of spectral changes was found to correlate positively with the drain current. Additionally, the Stark shifts of not only the organic semiconductor, but also of the IL on the organic semiconductor films were detected. This new method can be applied to other electrochemical devices such as organic thin film solar cells, in which the interfacial region is crucial to their functioning.

Attenuated total reflectance far-ultraviolet and deep-ultraviolet spectroscopy analysis of the electronic structure of a dicyanamide-based ionic liquid with Li.

The electronic states of N-butyl-N-methylpyrrolidinium dicyanamide ([BMP][DCA]), a solvated ionic liquid, around Li+ were investigated using attenuated total reflectance far-ultraviolet and deep-ultraviolet (ATR-FUV-DUV) spectroscopy. The absorption bands ascribed to the [DCA]- were blue-shifted as the Li+ concentration increased, and the origin of the shift was explained by the energetic destabilization of the final (excited) molecular orbital using time-dependent density functional theory (TD-DFT) calculations. Using the multivariate curve resolution-alternating least squares (MCR-ALS) algorithm, the obtained spectra were decomposed into two types of [DCA]- at electronic state level, which were categorised as pure [BMP][DCA] and [DCA]- affected by Li+. Our results revealed that the number of [DCA]- with electronic states affected by a Li+, which was termed the electronic coordination number, was ∼5. This value was different from the coordination number within the first solvation layer, which was ∼4. Combining the TD-DFT with molecular dynamics simulations, we demonstrated that one [DCA]- outside the first solvation layer had a different electronic state from that of pure [BMP][DCA]. This is the first successful study that combines ATR-FUV-DUV spectroscopy with MCR-ALS calculations to build a solvation model that describes the electronic states.

Correlation between mobility and the hydrogen bonding network of water at an electrified-graphite electrode using molecular dynamics simulation.

Focusing on the electric double layer formed at aqueous solution/graphite electrode interfaces, we investigated the relationship between the mobility of interfacial water and its hydrogen bonding networks by using molecular dynamics simulations. We focused on the mobility of the first hydration layer constructed nearest to the electrode. The mobility was determined by calculating the diffusion coefficient which showed an opposite trend to that of the applied potential polarity. The mobility decreased upon positive potentials while showing an increase upon negative potentials, which is rationalized by the strength of the interfacial hydrogen bonding networks.