Operando liquid-cell electron microscopy of the electrochemical polymerization of beam-sensitive conjugated polymers
Date
2020
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
The conjugated polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives have received widespread recent interest due to their high chemical stability, biocompatibility, low oxidation potential and high conductivity. Their application in interfacing ionically conducting living tissue with electronically conducting metallic or semiconducting biomedical devices has been of particular interest due to their ability to provide a mechanically compliant interface and transport charge both ionically and electronically. PEDOT-coated electrodes typically have impedances around two to three orders of magnitude lower than the uncoated metal electrodes in the low-frequency region (<1 kHz) of particular interest for biological signals. ☐ Electrochemically deposited conjugated polymer films can show dramatic variations in morphology. Typically, the films have a somewhat lumpy surface structure, with details that depend on the deposition conditions and choice of solvent. They can also form relatively fibrillar, low density, open structures. They can also deposit around dissolvable templates, or onto gels or even living tissues if they are present in the reaction medium. However, the factors that determine the development of these structures during electrodeposition are not yet well established. This makes it difficult to design new systems and optimize device performance. Considering that the morphology of electrochemically-polymerized thin-films can be fine-tuned by controlling the early stage nucleation and growth of the oligomeric clusters, we have used operando liquid-cell low-dose Transmission Electron Microscopy to image and obtain a detailed understanding of the fundamental processes occurring at the electrode-solution interface, especially the evolution of the mobile oligomeric clusters that precede solid polymer film formation. ☐ During the deposition, we clearly see the early stages of the electrodeposition where the liquid-like oligomers (dark in contrast due to higher mass thickness) initially nucleate from the glassy carbon working electrode (lighter shade of grey compared to the oligomers) then merge, coalesce and increase in size and thickness before finally depositing onto the working electrode as a solid, stable and dark conjugated polymer film. Further, we used transmitted light optical microscopy to correlatively study this process which revealed the change in color of the translucent clusters to the dark polymer film caused due to the increase in conjugation length. ☐ Furthermore, using a macromolecular counter-ion, poly(acrylic acid) (PAA) during the electrodeposition facilitates the formation of highly anisotropic nano-fibrils of PEDOT. The nanofibrillar structure thereby reduces the overall impedances due to increase in the effective surface area available for charge transport. Operando liquid-cell electron microscopy has helped us observe their early stage growth in an oriented, dendritic manner at the solution-electrode interface, allowing us to quantitatively understand the nano-fibril formation process. Finally, we have also found substantial variations when the chemistry of the monomer is changed, including a dramatic increase in the nucleation density when a more hydrophilic, crystalline, carboxylic acid substituted EDOT (EDOT-acid) was used as a comonomer. These insights have proven to be of utmost importance while understanding the polymerization process thus helping us to design better systems and optimize device performance.
Description
Keywords
Conjugated polymers, Electrochemistry, Electron microscopy, In-situ liquid-cell TEM, Nucleation and growth, Operando microscopy