title: The structure of the ionic liquid / electrode interface and its possible influence on electrochemical materials synthesis: an in situ STM and AFM Study
Creators: name: Borisenko, Natalia
Publisher: name: MyCoRe Community
Year: 2023
Subjects: subject: Ionic liquids
subject: in situ STM and AFM
subject: IL/Electrode interface
subject: ionische Flüssigkeiten
subject: IL/Elektrode Grenzfläche
subject: 540

description: Ionic liquids (ILs) have become one of the most popular material classes over the past twenty years. They have been considered as alternative materials to molecular solvents and molten salts in various applications (e.g. batteries, capacitors, lubricants, solvents for electrodeposition purposes). A detailed understanding of the IL/solid interfacial nanostructure and of the spatial distribution of the electroactive species at the interface is important for the development of various technical processes using ILs. 
In the present work the structure of the electrified IL/solid interface was investigated using in situ scanning tunnelling microscopy (STM) and atomic force microscopy 
(AFM). Various ILs, namely [Py1,4]TFSA, [Py1,4]FAP, [Py1,4]FSA, [Py1,4]TfO, [EMIm]TFSA, [OMIm]TFSA, [EMIm]FAP, [EMIm]TfO and [HMIm]FAP were applied for this study. In situ AFM reveals that multiple interfacial layers are present at the IL/electrode interface. Furthermore, the applied potential determines whether cations or anions are preferably adsorbed to the substrate with stronger near surface layering detected at higher positive or negative surface potentials. Both the cation and the anion of the IL have a strong influence on the structure and composition of the interface. In situ STM shows that the appearance of the Au(111) and H-terminated p-Si(111) surfaces is different in various ILs, due to specific ion/surface and ion/ion interactions, which are dependent on the type of the functional groups (pyrrolidinium or imidazolium ring and the length of the alkyl chains), resulting in a different interfacial structure for various cations. With the same anion, the Au(111) surface undergoes the (22 x √3) reconstruction with [Py1,4]+ during cathodic polarization, but with [EMIm]+, [HMIm]+ and [OMIm]+ the herringbone superstructure has not been obtained. Furthermore, different [OMIm]TFSA superstructures with the lateral size between 1.2 and 1.7 nm are probed with in situ STM on highly ordered pyrolytic graphite (HOPG) at high negative electrode potentials, as a result of several cation and/or anion layers probed at the same time by the STM tip. AFM measurements show that the imidazolium cation is adsorbed at the H-Si(111)/[EMIm]TFSA interface leading to an ordered clustered facet structure of ~3.8 nm in size. In comparison, the Si(111)/[Py1,4]TFSA interface appears the same as the native surface under argon. 
The interfacial structure is sensitive to metal and semiconductor salts dissolved in the IL. In general the ion layering at the IL/solid interface is markedly different in the presence of dissolved solutes than for the pure IL systems, as the presence of salt ions alters both the IL-surface and IL-IL interactions at the interface. For instance, AFM measurements reveal that interfacial layering is markedly weaker in the presence of LiCl and SiCl4 in [Py1,4]FAP. The presence of Li+ and Si(IV) affects interactions between [Py1,4]+ and the gold surface hindering the (22 x √3) Au(111) reconstruction. Marked changes in the IL/Si(111) interfacial structure are obtained on addition of LiTFSA in [Py1,4]TFSA and [EMIm]TFSA. The concentration of the solute has also a significant effect on the structure of the IL/electrode interface. Thus, considerable changes are obtained on addition of high concentrations of NaFSA and LiTFSA in [Py1,4]FSA and [Py1,4]TFSA, respectively. For the [EMIm]TfO/Au(111) interface the structure of the innermost layer also depends on the amount of added water. A transition from a multilayered structure to a classical double layer structure occurs at -1.0 V vs. Pt on changing the water concentration from 30 to 50 vol%. The formation of a double layer structure at the IL/Au(111) interface is also obtained on increasing the concentration of SiCl4 in [Py1,4]FAP. In situ AFM force-separation measurements confirm that the dissolved solute is present within the innermost (Stern) layer. The altered interfacial structures represent the best compromise between the IL ion and solute surface affinities, packing constraints, and charge localisations. Thus, the IL EDL is highly complex both in the pure ILs and in the presence of solutes and often leads to an unpredictable electrochemical behaviour. This in turn should affect the reactions that occur at the IL/electrode interface (e.g. metal deposition). Thus, in [Py1,4]TFSA, TaF5 can be reduced to elemental Ta, while in [Py1,4]FAP the electroreduction processes are practically inhibited. The difference for Si deposition from SiCl4 in [Py1,4]TFSA and [Py1,4]FAP is roughly 1 V. The crystal sizes of the electrodeposited Ga varies upon changing the ILs. The deposit made from [Py1,4]TFSA consists of spherical structures of 60-260 nm in diameter, while the crystal sizes of a Ga deposit obtained from [Py1,4]TfO are between 15 and 110 nm. In the case of [Py1,4]FSA a nanocrystalline Ga deposit with a crystal size of less than 50 nm is obtained. Furthermore, GaSb obtained in [EMIm]TFSA and [Py1,4]TFSA exhibits a band gap of about 1.2 eV and 0.9 eV, respectively. 
The present work shows that the electrodeposition of metals and semiconductors in ILs is complicated. On the one hand, the (chemical) structure of the IL has an influence on the EDL structure of the Au(111)/IL interface. On the other hand, the addition of precursors, which are required for the deposition processes, can also affect the structure of the electrode/IL interface. Furthermore, the nanostructure of interfacial layers can vary if the concentration of the precursor is increased, which might facilitate the deposition processes. Such interfacial effects have to be considered for all applications that involve reactions at the IL/solid interface.