Electrochemical surface plasmon resonance as a probe of redox reactions at the ionic liquid|gold interface

Electrochemical surface plasmon resonance (ESPR) has been employed as a probe of redox reactions at the interface between gold (Au) and ionic liquid (IL). Two redox couples, ferrocene (Fc)/Fc⁺ and (ferrocenylmethyl)trimethylammonium (FTA⁺)/FTA²⁺, and two ILs, trioctylmethylammonium bis(trifluoromethanesulfonyl)amide ([TOMA⁺][C₁C₁N⁻]) have been studied. In the ESPR measurements, the shift of the SPR angle has been recorded simultaneously with cyclic voltammogram (CV). It has been revealed that the SPR angle shift reproduces CV when the former is semi-differentiated or the latter is semi-integrated, and, therefore, that the ESPR response probes the surface concentration of redox couples at the IL|Au interface. Among the four combination of the two redox couples and the two ILs, the case for Fc/Fc⁺ in [TOMA⁺][C₁C₁N⁻] shows significantly greater ESPR response than the other three cases. A model has been established for the relationship between the SPR angle shift and the surface concentration of redox species. The model predicts that the SPR angle shift becomes pronounced with increasing DR/DO, the diffusion coefficient ratio of the reduced (R) and oxidized (O) species when the reduced species is initially dissolved in the IL as is the case for the present study. Significantly greater DR/DO for Fc/Fc⁺ in [TOMA⁺][C₁C₁N⁻] than the three other cases has been confirmed from the DR and DO measurements by CV with a microdisk electrode. The trend of the measured DR and DO values agrees with recent findings by researchers that small neutral solutes in ILs composed of large ions diffuse fast beyond the prediction of the Stokes-Einstein relation

Electrochemical surface plasmon resonance measurements of camel-shaped static capacitance and slow dynamics of electric double layer structure at the ionic liquid/electrode interface

Electrochemical surface plasmon resonance (ESPR) is applied to evaluate the relative static differential capacitance at the interface between 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquid (IL) and a gold electrode, based on the relationship between the SPR angle and surface charge density on the electrode. Potential-step and potential-scan ESPR measurements are used to probe the dynamics of the electric double layer (EDL) structure that exhibit anomalously slow and asymmetrical characteristics depending on the direction of potential perturbation. EDL dynamics respond at least 30 times more slowly to changes of potential in the positive direction than in the negative direction. ESPR experiments with the positive-going potential scan are significantly affected by the slow dynamics even at a slow scan. The surface charge density that reflects the relative static capacitance is obtained from the negative-going potential scans. The evaluated quasi-static differential capacitance exhibits a camel-shaped potential dependence, thereby agreeing with the prediction of the mean-field lattice gas model of the EDL in ILs. ESPR is shown to be an effective experimental method for determining relative values of the static differential capacitance

Template-Free and Spontaneous Formation of Vertically Aligned Pd Nanofiber Arrays at the Liquid-Liquid Interface between Redox-Active Ionic Liquid and Water

Vertically aligned Pd nanofiber arrays (NFAs) have been prepared at the liquid–liquid interface between redox-active ionic liquid (RAIL) and water (W) via a template-free manner. The RAIL with high hydrophobicity, (ferrocenylmethyl)dodecyldimethylammonium bis(nonafluorobutanesulfonyl)amide, plays dual roles of reducing agent for Pd precursor ions and the hydrophobic liquid phase simultaneously, and the RAIL|W interface has been utilized as the formation site for the spontaneous growth of Pd NFAs. The Pd NFAs consist of three parts: layers formed by partly connected particles on the top, NFAs in the middle, and firm sheetlike layers on the bottom. Because of the top and bottom supporting layers, the antideformation ability and durability of the Pd NFAs with a length reaching several micrometers are enhanced. A possible mechanism for the formation of the Pd NFAs has been discussed. The Pd NFAs show a good stability and a higher electrocatalytic activity toward the ethanol oxidation reaction than a commercial Pd/C catalyst. The present study provides a new strategy for the template-free and spontaneous formation of Pd NFAs

In Situ Surface Roughness Analysis of Electrodeposited Co Films in an Ionic Liquid Using Electrochemical Surface Plasmon Resonance: Effect of Leveling Additives

The effect of two leveling additives, thiourea (TU) and coumarin (CM), on cobalt electrodeposition process in an ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (C4mimTFSA), at the gold surface has been analyzed in situ using electrochemical surface plasmon resonance (ESPR), in which the SPR resonance angle (Δθ) has been recorded simultaneously with cyclic voltammograms. The two additives show Δθ behaviors different from the additive-free case and from each other. In the case of TU, the positive Δθ shift due to Co cathodic deposition is larger than the additive-free case. Even after the subsequent negative Δθ shift due to Co anodic dissolution, Δθ remains positive not returning to the original value. This indicates that the dissolution of gold facilitated by TU roughens the surface of the gold electrode. In contrast, in the case of CM, the positive Δθ shift due to Co cathodic deposition is smaller than the additive-free case. A model analysis using Fresnel reflectivity has revealed that CM actually smooths the surface of the Co film even in the initial process of the electrodeposition. These findings illustrate that ESPR can sense in situ the surface roughness change during electrodeposition on the order of Å

One-step fabrication of Au@Pd core-shell bimetallic nanofibers at the interface between water and redox-active ionic liquid

Au@Pd core-shell bimetallic nanofibers (BNFs) have been successfully prepared with a one-step fabrication method, where no additional preparation step using a template is required. The preparation is realized by the spontaneous growth of Au@Pd BNFs at the interface between water (W) and a hydrophobic redox-active ionic liquid (RAIL). The RAIL plays dual roles of reducing agent to reduce metal precursors dissolved in W, AuCl₄⁻ and PdCl₄²⁻, at the RAIL|W interface, as well as the hydrophobic liquid phase constituting the RAIL-W two-phase system. The reduction reactions of AuCl₄⁻ and PdCl₄²⁻ at the RAIL|W interface proceed sequentially; AuCl₄⁻ is reduced prior to PdCl₄²⁻ to produce Au nanofibers, which act as the core for the following deposition and growth of Pd shell on the Au surface. Thus, the reductive formation of Au as the core and that of Pd as the shell can be automatically completed in one step. The mechanisms for the interfacial charge transfer reactions and the growth of Au@Pd BNFs are discussed in detail. Control experiments have clearly confirmed that the RAIL greatly promotes the reduction of PdCl₄²⁻and prevents the agglomeration of Au@Pd BNFs. The prepared Au@Pd BNFs exhibit higher electrocatalytic performance towards ethanol oxidation reaction than the commercial Pd/C catalyst, and the catalytic activity is even improved after long-time cycles. TEM images reveal a structural transformation of the Pd shell in the Au@Pd BNFs after long-time cycles, which is responsible for the increased catalytic activity

Simultaneous Synthesis of One-and Two-Dimensional Gold Nanostructures/Reduced Graphene Oxide Composites in the Redox-Active Ionic Liquid/Water Interfacial System

In this study, one- and two-dimensional (1D and 2D) Au nanostructure/reduced graphene oxide (rGO) composites are simultaneously prepared via a redox-active ionic liquid (RAIL)|water (W) interfacial method without the use of a capping agent. The RAIL (ferrocenylmethyl)dodecyldimethylammonium tetrakis[3, 5-bis(trifluoromethyl)phenyl]borate ([FcMDDA]⁺][TFPB⁻]) plays the dual role of a hydrophobic liquid supporting the interfacial system as well as a reducing agent for the metal precursor and graphene oxide (GO). The RAIL|W interfacial system provides two kinds of composites in the growth regions with no interference between them, namely, Au nanofiber/rGO composites at the RAIL|W interface and Au nanosheet/rGO composites in the bulk of W, which can be easily separated after synthesis. The spontaneous formation mechanism of the two Au-rGO composites has been discussed in detail. Thus, an efficient strategy to prepare 1D and 2D Au-rGO composites simultaneously is developed by employing an interfacial system, which provides a new direction in the synthesis of metal–GO composites

Anion dependence of camel-shape capacitance at the interface between mercury and ionic liquids studied using pendant drop method

The electrocapillarity and zero-frequency differential capacitance, Cd, have been studied using pendant drop method, at the Hg interface of an ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [C2mim+][TFSA−], and have been compared with those of [C2mim+]BF, an IL with the common cation and a different anion, to focus on the anion dependence of zero-frequency Cd. The Hg interface of [C2mim+][TFSA−], the IL of the larger anion in the present study, exhibits greater zero-frequency Cd than that of [C2mim+]BF, the IL of the smaller anion. This behavior contradicts a simple expectation in which larger ion leads to smaller Cd. This apparent contradiction is explained by proximity of the charged moiety of TFSA− to the electrode surface compared with that of BF. The potential dependence of zero-frequency Cd for the two ILs both exhibits one-hump camel shape around the potential of zero charge (Epzc), which has been predicted to be specific behavior of the electrical double layer of ILs by theory and simulation. The humps are located at potentials more negative than Epzc. From a mean-field lattice-gas theory for the EDL in ILs, this negative shift can be interpreted that the charged moiety for C2mim+ is more easily condensed in the EDL than those for BF and TFSA−

Static capacitance at the electrochemical liquid-liquid interface between ionic liquids and eutectic Ga-In alloy measured using the pendant drop method

Static differential capacitance (Cdc) at the liquid-liquid interface between ionic liquids (ILs) and eutectic Ga-In alloy (EGaIn) has been measured using the pendant drop method for two ILs: 1-ethyl-3-methylimidazolium tetrafluoroborate ([C₂mim⁺]BF₄⁻) and 1-octyl-3-methylimidazolium bis(nonafluorobutanesulfonyl)amide ([C₈mim⁺][C₄C₄N⁻]). The potentials of zero charge for the IL|EGaIn interfaces are shifted compared with the IL|Hg interfaces with an amount that can be considered by the difference in the work functions of EGaIn and Hg. The measured Cdc at the [C₂mim⁺]BF₄⁻|EGaIn interface has well reproduced the camel-shape potential dependence of Cdc at the Hg interface of the same IL at the negatively charged potential region. This suggests that there are little specific interaction between the IL ions with EGaIn and Hg. The [C₈mim⁺][C₄C₄N⁻]|EGaIn has been compared with the [C₈mim⁺]BF₄⁻|Hg interface where IL-cation is the same but IL-anion is different. Also in that case, Cdc is similar to each other at the negatively charged potential region, which means that accumulated C₈mim⁺ ions at the interface mainly govern the Cdc behavior

Formation of Au Nanofiber/Fullerene Nanowhisker 1D/1D Composites via Reductive Deposition at the Interface between an Ionic Liquid and Water

Au nanofiber (NF)/fullerene nanowhisker (FNW) 1D/1D composites have been prepared at the liquid/liquid interface between an ionic liquid (IL) and water (W). Au NFs have been reductively deposited on the FNWs adsorbed at the IL/W interface via the electron transfer across the interface between AuCl₄⁻ in W and a reducing agent in IL, coupled with the ion transfer of AuCl₄⁻ from W to IL

One-dimensional Pt nanofibers formed by the redox reaction at the ionic liquid|water interface

Pt nanofibers have been prepared with a redox reaction occurring spacio-selectively at the liquid-liquid interface between a highly hydrophobic ionic liquid (IL) and water (W), which has been utilized as two-dimensional reaction field. The preparation is conducted based on the concept of the coupling between ion transfer (IT) and electron transfer (ET) across the IL|W interface, which previously enabled direct formation of dendritic Au nanofibers at the IL|W interface (Chem. Commun., 2015, 51, 13638). The successful preparation of Pt nanofibers has manifested the generality of the IL|W interface method for the formation of metal nanofibers. Electrochemical measurements at the IL|W interface have confirmed that IT of PtCl₄²⁻ from W to IL is actually coupled with ET between PtCl₄²⁻ initially dissolved in W and decamethylferrocene in IL, leading to the spontaneous formation of Pt nanofibers at the IL|W interface. Herein, IT of PtCl₄²⁻ ions from W to IL not merely neutralizes the excess charges generated in each liquid phase by ET, but also provides PtCl₄²⁻ into IL as reactants for the growth of Pt nanofibers that takes place at the IL side of the IL|W interface. The electrocatalytic activity of the Pt nanofibers in oxygen reduction reaction (ORR) has also been evaluated