This article shows the potential of FlexAFM to study charged solid-liquid interfaces. A conductive specimen was placed in an electrochemical liquid cell to undertake the electrochemical (in-situ) AFM tests. A lab-built bipotentiostat was then connected to the specimen.
A Clavilier-type Au(111) single crystal bead crystal electrodes with facets of micrometer-wide terraces were used. The growth and dissolution, and surface oxidation of copper clusters in sulphuric acid solution, and the lifting of the Au(111)-(px√3) reconstruction were explored as examples (Figure 1).
Figure 1. Experimental Setup. (A) Overview showing the Nanosurf EasyScan2 controller, the FlexAFM scan head as positioned on an isoStage table and the lab-build potentiostat. (B) Electrochemical liquid cell mounted below the FlexAFM scan head. (C) AFM sample: A bead single crystal attached to supporting gold plate. (D) Facets of the bead crystal as observed with an optical microscope. (E) AFM cantilever aligned on the upper Au(111) facet of the bead crystal as seen through the camera integrated into the FlexAFM scan head.
An Au(111) bead single crystal, submerged in 0.1 M H2SO4 solution in the liquid cell, and NCHAuD cantilevers (Nanosensors) were used to perform the AFM imaging in the dynamic mode. The copper deposit experiments were carried out by adding 1 mM CuSO4.
Platinum wires submerged in the liquid cell acted both as a reference electrode in copper-free solution and as a counter electrode, while the bead crystal specimen was attached to the potentiostat to function as a working electrode. For the solution containing copper, a copper wire was used as a reference electrode.
Au(111) in Sulfuric Acid Solution
A freshly annealed Au(111)-(px√3) surface was placed under potential control at E=–0.6 V vs. platinum at the start of the EC-AFM experiment. The reconstructed surface, which is formed at the time of the flame annealing, is preserved at this potential. Big, atomically smooth terraces were found (Figure 2B).
The reconstruction was removed when the potential was increased to 0 V. Two big gold clusters as well as numerous small clusters are created on the surface (Figure 2C). When the potential is increased to E>0.6 V, a rough surface oxide was formed. This, however, was reduced when the potential was reduced to E<0.2 V (Figure 2D).
Figure 2. Au(111) in 0.1M H2SO4. (A) Cyclic voltammogram of the bead crystal as attached to the gold sheet in 0.1M H2SO4. AFM images: (B) atomically-flatter races, E=–0.6V; (C) same area after lifting of the reconstruction, E=0V; (D) roughening and smoothing of the gold surface upon several subsequent oxidation and reduction cycles.
3D Copper Deposition on Au(111)
Employing a copper reference electrode, the EC- AFM experiment in copper-containing solution was conducted on Au(111) in 0.1 M H2SO4 containing 0.1 mM Cu2+. At this potential, E=0 V matches the equilibrium between dissolved Cu2+ ions and metallic copper.
Over potential (3D) copper-deposition happens at negative substrate potentials. The substrate potential strongly influences the nucleation of the new phase and its development.
The formation of a nucleus on an atomically flat Au(111) surface, when the potential is reduced from 0 to –50 mV, is shown in Figure 3. Figure 3 also shows the subsequent development of the nucleus after a nucleation period of 2 seconds at –10 mV.
This was followed by the development of the copper cluster in real-time. Figure 3 shows some images of this formation. When the potential was finally increased to 15 mV, the cluster dissolved rapidly.
Figure 3. 3D deposition and dissolution of copper on Au(111). 3D AFM images (time series) showing the growth and dissolution of a single copper cluster. All images have a lateral size of 2µm×2µm and a normalized vertical scale of 250nm.
This information has been sourced, reviewed and adapted from materials provided by Nanosurf AG.
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