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Oxygen-free solutions are often used for electrochemical experiments, as ambient oxygen readily dissolves in aqueous solutions and creates a reduction wave that interferes with many electrode reactions.
The deoxygenation in electrochemical SPM experiments poses a major issue, and research is often restricted to reactions that are slightly or not at all influenced by the oxygen-containing solution.
This article shows how to perform electrochemical AFM experiments in oxygen-free aqueous solutions using Nanosurf’s Electrochemistry Stage ECS 204 and FlexAFM.
In this analysis, the presence or absence of oxygen reduction wave helped to detect the presence or absence of dissolved oxygen. A commercial rod-like working electrode (CHI104, CH Instruments, ∅ 3 mm) was used as an AFM sample and working electrode.
This electrode is made from glassy carbon (GC) integrated into insulating Kel-F sheath. As an inert material, GC helped to observe oxygen reduction wave in a broad potential range. In addition, aqueous 0.1 M NaOH was used as an electrolyte to limit the evolution of hydrogen at negative potentials.
As shown in Figure 1, the GC electrode was placed in the ECS 204 stage. A commercial Ag/AgCl/3.4 M KCl electrode (ET072, eDAQ) was used as a reference electrode, and a platinum wire was used as a counter electrode.
Once the GC electrode was mounted, the EC-AFM cell was added with 1.5 ml of 0.1 M NaOH solution. During all of the experiments, the ECS 204 stage was placed on a vibration damping table.
Figure 1. ECS 204 with mounted rod-like electrode
The red line in Figure 2 shows the steady-state cyclic voltammogram quantified in the open cell in as-prepared solution, which was saturated by ambient oxygen. A cantilever holder suitable for making AFM measurements in liquid was integrated with an AFM probe of the type NCSTAuD (Nanosensors).
This holder was placed on the FlexAFM scan head, which was located onto the ECS 204 stage. The FlexAFM scan head was attached with a sealing membrane (Figure 3).
Figure 2. Steady-state cyclic voltammograms of glassy carbon electrode mounted in Electrochemistry Stage ECS 204 as measured in 0.1 M NaOH with the sweep rate 50 mV/s: (red) in an open cell with an as-prepared solution and (black) in deoxygenated solution. Current density was calculated using geometric area of the electrode.
Figure 3. Photo of FlexAFM scan head with a sealing membrane
Once the FlexAFM scan head is placed on the ECS 204 stage, the sealing membrane adheres to magnets which are placed on the top of ECS 204. This produces a tiny volume shielded from the ambient air surrounding the cantilever holder and the electrochemical cell.
Next, gas tubes are connected to the gas inlet and outlet. The free side of the outlet tube was submerged into a beaker with water to prevent any backflow of the ambient air. In this setup, the gas tightness was deemed adequate when a gas flow at the inlet allowed the observation of gas bubbling through the water at the outlet.
Figure 4 depicts an image of cantilever submerged in solution as visualized through the FlexAFM camera (side view) following laser alignment. A laser spot can be observed on the cantilever end (above) and its reflection in the electrode (below).
The AFM probe was placed over the GC surface using lateral translation mechanism incorporated into the ECS 204 stage. The GC surface is viewed as a lighter circle in the center enclosed by a darker Kel-F sheath.
Figure 4. Optical image of AFM cantilever above GC electrode as observed through the FlexAFM camera (side view).
In order to remove oxygen from the solution, nitrogen was purged via the protected volume above the solution, and a strong gas flow was used for 15 minutes to substitute air in the protected volume.
As soon as the atmosphere above the solution is free of oxygen, the rate of removing oxygen from the solution is established by its diffusion in the aqueous phase and is not influenced by the rate of gas flow.
After purging for 15 minutes, the rate of gas flow was reduced to a minimum value which is adequate enough to produce bubbles at the gas outlet. During the deoxygenation process, cyclic voltammograms were constantly recorded and after a period of 2 hours, they converged to a steady shape that remained unchanged for the next 10 minutes.
Comparison of this voltammogram (black line in Figure 2), one recorded prior to deoxygenation (red line in Figure 2) and intermediate ones enables the attribution of the reduction wave, reaching the highest at ca. -0.5 V to the oxygen reduction.
However, in the last voltammogram this reduction wave was entirely absent, leading to a conclusion that the oxygen was entirely removed from the solution, or its amount fell under the electrochemically detectable level.
The two hours required to remove oxygen from as-prepared solution indicate an upper estimate of the deoxygenation time. If an already deoxygenated solution is added to the cell, then the deoxygenation time can be considerably reduced.
After completing the deoxygenation, the AFM probe was brought to the sample and the sample was imaged in dynamic mode.
While there is an overpressure of gas at the inlet, stable AFM imaging can be carried out, but only if bubbles are not present at the outlet. This means that in this case, the gas was dissipated via the tiny holes.
When the gas overpressure is high enough to create bubbles at the outlet, periodic spikes were seen on AFM images, which frequency correlated with the bubbling and gas flow rates. These artifacts are due to outlet tube vibration and/or pressure shock waves within the gas that occur during the release of bubbles.
Figure 5 shows an image of bare GC surface acquired in 0.1 M NaOH under potential control. It reveals a grained surface obstructed by long lines produced during electrode polishing. Using the “Calculate Area Roughness” tool in the Nanosurf software, a root mean square roughness of 11.9 nm was measured from the image in Figure 5.
Figure 5. AFM image of GC surface (topography with “Line fit” filter) obtained in 0.1 M NaOH under potential control.
The results suggest the viability of oxygen-free in situ EC-AFM imaging experiments in Electrochemistry Stage ECS 204. Commercial rod-like electrodes, like the one used in this study, can be placed in the ECS 204 stage and electrochemically characterized using third-party instruments and by AFM using Nanosurf FlexAFM.
Combined in situ electrochemical, structural, and nanomechanical studies can be used to deal with different surface processes such as electrodeposition of corrosion or different materials. In addition, this type of electrodes can act as substrates for assembling active materials such as biomolecules, nanoparticles, organic layers, cells, etc. whose structure, electrocatalytic activity, and mechanical properties can be simultaneously characterized in an electrochemical AFM experiment.
This information has been sourced, reviewed and adapted from materials provided by Nanosurf AG.
For more information on this source, please visit Nanosurf AG.