Tracking Electrodeposition of Polyaniline Using eQCM-I
An experiment using a quartz-crystal microbalance (QCM) with impedance (QCM-I) is a unique technique popular in characterizing biological films that are structurally complex. While basic QCM measurements track only the shift in the resonant frequency upon film deposition or growth, QCM-I provides quantitative information on a film’s viscoelastic properties. In this Technical Note, we take a look at example QCM-I data using electrodeposited polyaniline films.
Polyaniline is a great starter film to learn how electrochemistry can be used to form complex films. The monomer is available as aniline∙HCl and is ready-to-use as is without a complex preparative procedure. Simply dissolve the white-colored crystalline solid in de-ionized water (be sure to read the safety data sheet!). Solubility in water is about 107 g/100 mL at 20°C. In our case, we started with a 100 mM solution of aniline∙HCl in water. This solution was loaded into the QCM-I mini EC-flow cell. We removed trapped bubbles and allowed time for the solution to settle. We then applied a constant current of 20 µA. A leakless Ag|AgCl reference electrode was used as reference. The counter electrode was a Pt disk positioned above the electrode.
The electrode potential response and normalized frequency shift at multiple overtones are shown in Fig. 1. In this case, normalized frequency shift is the measured frequency variation divided by the harmonic number, n. For example, n = 1 for the fundamental frequency, which is 5 MHz for the crystal sensors used here. The third overtone of this frequency has n = 3; the fifth overtone corresponds to n = 5, etc.
In the first minute of polymerization, the frequency shift at all overtones overlapped, because negligible film was deposited. When the voltage reached a critical threshold, deposition began and mass was detected by a decreasing frequency shift. After ~24 min of deposition, the frequency shift reached values around –260 Hz. Notably, the frequency shift for the 13th overtone was ~19 Hz smaller than the 3rd overtone and was consistent with viscoelastic film behaviorThis is one of the reasons that multiple-overtone analysis is useful for detecting viscoelastic behavior.
The secondary y-axis plots the electrode potential over time as 20 µA is passed. This anodic current drives the electrode potential positive as electrons are removed from the electrode. The initial rise in the electrode potential is controlled by the electrode capacitance. At some point, the electrode potential reaches a potential which generates the active monomer and deposition begins. Typically, this occurs at 0.6 V to 0.65 V vs Ag|AgCl although the exact value depends on many experimental factors. During electrodeposition, the electrode potential is controlled by the electrochemical reaction (as opposed to the electrode capacitance). Over time, the electrode potential may vary depending on whether the deposited film promotes or inhibits further film deposition. In our case, the electrode potential rises as it becomes increasingly difficult to deposit more film. In standard nomenclature, it is said that the overpotential of the electrochemical reaction increases with time.
The bandwidth (Full Width at Half Maximum, FWHM) of the quartz-crystal sensor increases when a viscoelastic film is deposited (see Application Note “Basics of a Quartz Crystal Microbalance”). This behavior is captured in Figure 2 where bandwidth only increases after electrodeposition begins (arrow). The bandwidth response of the polyaniline film is complex; we cannot go into detail here.
However, to put the response into perspective, we plot the FWHM against the frequency shift for a copper film, polyaniline film, and a theoretical viscous solution in Figure 3. As expected for a thin, rigid film, the copper response is a near-horizontal line—practically no change in dissipation is detected for copper. A viscous solution would exhibit a slope of –2 (see the Technical Note “The Principles of QCM-I”). Viscoelastic films such as polyaniline exhibit a response that varies between a viscous solution and rigid film.
On Trapped Gas Bubbles
Degassing solutions before QCM-I measurements is important to reduce the detrimental effect of gas bubbles on your experiment. After every experiment, we advise you to inspect the quartz-crystal surface for areas of film exclusion caused by a trapped gas bubble. These areas are outlined in red in Figure 4.
- Avoid using the QCM-I to heat chilled solutions (outgassing can occur).
- Set the QCM-I temperature slightly below room temperature, if no temperature-control is needed.
- Use an inline bubble trap.
- Wet the surface with a small volume of “blank” solution before attaching the lid of the cell and inserting into the QCM-I.
- Increase flow rate, when first introducing solution to an empty cell, to dislodge bubbles.
- Use an in-line valve to switch between solutions instead of manually disconnecting/reconnecting the flow inlet port (or moving a tube from one well to another).
This Technical Note demonstrates the use of QCM-I for studying the deposition of polyaniline. The principles outlined herein can be applied to all sorts of viscoelastic films and solutions. They also offer a way to demonstrate the rigidity of a film when applying the Sauerbrey equation.
Technical Note Electrodeposition of Polyaniline Rev. 1.0 5/9/2019© Copyright 2019 Gamry Instruments, Inc. Interface, Reference, and Framework are trademarks of Gamry Instruments, Inc.