Tracking Electrodeposition of Polyaniline Using eQCM-I

Fig. 1. QCM-I frequency-shift response during polyaniline electrodeposition at constant current.

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 13^th overtone was ~19 Hz smaller than the 3^rd 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.

Fig. 2. QCM-I FWHM response during polyaniline electrodeposition at constant current corresponding to Fig. 1. After a certain time (marked with a red arrow), bandwidth increases.

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.

Fig. 3. FWHM plotted against frequency shift for an electrodeposited copper film and the electrodeposited polyaniline film (Fig. 1).

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.

Fig. 4. The quartz crystal (circular disk held in the QCM-I support) after polyaniline electrodeposition. Areas where trapped gas bubbles prevented film formation are outlined in red.

Some tips:

1. Avoid using the QCM-I to heat chilled solutions (outgassing can occur).
2. Set the QCM-I temperature slightly below room temperature, if no temperature-control is needed.
3. Use an inline bubble trap.
4. Wet the surface with a small volume of “blank” solution before attaching the lid of the cell and inserting into the QCM-I.
5. Increase flow rate, when first introducing solution to an empty cell, to dislodge bubbles.
6. 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).

Conclusion

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.