In situ scanning probe microscopy investigations of electroactive films

Abstract

Dimensional changes in electroactive films of nickel hydroxide, iridium oxide and polyaniline were investigated in situ in the electrochemical environment by scanning tunneling microscopy (STM), atomic force microscopy (AFM) and intermittent contact atomic force microscopy (ICAFM) during electrochemical oxidation and reduction. The advantages and disadvantages of the three measuring techniques are demonstrated with specific examples.While STM measurements are ambiguous due to the changing conductance of the sample during the redox process, AFM measurements on soft films such as anodic iridium oxide (AIROF) may result in surface modifications. Intermittent contact AFM (ICAFM) appears to be best suited for most samples.Shrinking of up to 40% could be observed on nickel oxide films during oxidation, which depended upon the location on the sample investigated and on the measuring technique employed. For a polyaniline film with a reversible charge capacity of 4.3 mC/cm 2 corresponding to 75 ± 7 nm film thickness, a thickness increase of 10 ± 3 nm was observed during oxidation.Electroactive materials like conducting polymers and metal oxides are of interest to be used as electrode materials in charge storage devices, electrochromic displays, sensors and actuators [1][2][3][4][5]. During electrochemical oxidation and reduction, electroactive materials reveal pronounced changes in their chemical and physical properties. Dimensional and morphological changes of electroactive films may cause mechanical stress and pressure within a device, and may affect the adhesion of an electroactive film to its substrate as well as its long-term stability during repetitive electrochemical cycling.Electrochemical processes are conventionally investigated by a variety of techniques that involve measurement of the current response following a perturbation of the electrode potential (e.g. cyclic voltammetry, linear sweep voltammetry, potential or current step techniques). However, these approaches generally provide a macroscopic picture of behavior which may be interpreted in terms of a statistical ensemble of atomic-or molecular-level events. While spectroscopic methods and scanning electron microscopy, as well as scanning tunneling microscopy (STM) and atomic force microscopy (AFM), can be used for ex situ characterization of electrodes, scanning probe microscopy (SPM) is uniquely capable of direct in situ characterization of the electrode-electrolyte interface. STM allows simultaneous measurement of the electrode topography and the electric current between a sharp tip and the electrode, while AFM permits the simultaneous measurement of the electrode topography and the interaction force between a small tip integrated in a cantilever and the electrode. Therefore, SPM techniques provide an opportunity to characterize the surface properties and their changes in situ at the nanometer scale [6][7][8]. Furthermore, it offers the advantage of studying material changes (crack formation, swelling, shrinking) directly in the electrolytic environmen...

EC-AFM investigation of reversible volume changes with electrode potential in polyaniline

“…Increased PANI film thickness during oxidation was observed in these experiments also. This was also supported by a similar report from the same group few years later [25]. However, in this case the RMS roughness value was more at À0.2 V (2.7 nm) than that at +0.2 V (2.3 nm).…”

“…In this case 52% change in the volume of this island was observed from initial value. Swelling and shrinking of the PANI thin film (prepared electrochemically on HOPG) has been observed earlier during repetitive potential cycling in 1 M H 2 SO 4 [25]. From our measurements we could get quantitative details of volume changes with electrode potential in large single island of PANI-SO 2À 4 from the EC-AFM images obtained at constant potentials.…”

“…Such volume changes have been observed in several conducting polymers by different methods [17,18]. Volume changes during oxidation of leucoemeraldine (LM) to emeraldine (EM) form of PANI have been studied using electrogravimetry [19], ellipsometry [20,21], scanning probe microscopy [22][23][24][25] and video microscopic observation [26][27][28]. As shown, there are very few studies of volume changes in PANI using scanning probe microscopy.…”

Nanoscale Inhomogeneity of Conducting‐Polymer‐Based Materials

“…Häring et al reported the comparison of in situ imaging of PANI films during electrochemical doping-undoping processes using EC-STM, EC-AFM, and EC-intermittent contact-mode atomic force microscopy (IC-AFM) [141]. As Nyffenegger et al a few years earlier, they concluded that EC-STM is not well-adapted to the imaging of swelling/ deswelling of electronically conducting polymers, as the severe drop in the tunneling current expected to occur during the transition from the doped to the undoped state of the polymer will definitely lead either to artifacts or to damages to the polymer film during the imaging procedure.…”

Morphological changes with electrode potential in microtubules and nanowires of Polyaniline: An in-situ EC-AFM study

“…Siegenthaler and co-workers studied the in-situ AFM of PANI synthesized on HOPG with repetitive potential cycling in sulphate medium [9]. Recently, Szklarczyk and co-workers have done the in-situ AFM study of effect of monomer concentration on the morphology of poly (omethoxyaniline) [10].…”