Jiali Zhang and Byong Kim
Research Technical Services, Park Systems, Inc., USA
Figure 3. Single SECCM LSV acquired with a glass nanopipette filled with 5 mM [Ru(NH3)6]Cl3. The LSV is recorded at a sweep rate of 10 mV/s with an initial potential at 0 V.
The Holy Grail in electrocatalysis and energy storage is to correlate electrochemical activity with nanostructured electrochemical interfaces (electrodes) . However, it is challenging to quantify the heterogeneity of electrode structures or study the local structure-activity relationship for these interfaces using conventional macroscopic electrochemical techniques. This is because macroscopic electrochemical studies can only measure the total electron transfer on an entire sample. To solve this issue, a new strategy to characterize nanoscale electrochemical activity is needed.
Scanning electrochemical cell microscopy (SECCM) is a new pipette-based nanoelectrochemical scanning probe technique designed to investigate the local electrochemical properties of electrode surfaces [1-4]. A quasi-reference counter electrode (QRCE) is inserted into the nanopipette, which is filled with an electroactive species. Lowering the nanopipette using the AFM (atomic force microscope) Z scanner and creating a meniscus at the contact surface allows a tiny droplet, or nanoelectrochemical cell, to form. The electroactive species in the confined droplet undergo an electrochemical reaction when a bias is applied between the QRCE and the working electrode placed on the XY scanner. An electrochemical current mapping is extracted by collecting multiple cyclic voltammograms at various positions.
In SECCM, researchers can perform thousands of confined nanoelectrochemical measurements (droplet area ranges from nm2 to µm2) on a single surface . High-throughput experimentation is achievable. Researchers can easily alter the chemical systems by merely swapping a new pipette with another electroactive species, and there is little need for special preparation of samples. Pipette preparation is straightforward and cost-effective. The data is easy to interpret; a higher current represents a higher rate or electrochemical reaction in the probed region. All of the above advantages make SECCM the suitable solution for electrochemical interrogation of individual platinum nanoparticles  or for correlation of local electrocatalytic activities with the local structures on polycrystalline electrode surfaces [5-6].
In this study, the electrochemically reversible [Ru(NH3)6]3+/2+ redox process at a highly-ordered pyrolytic graphite (HOPG) surface is recorded using Park NX12 AFM system. All Park NX systems can be the platform for SECCM. A glass nanopipette with a Ag/AgCl QRCE is utilized. Using previous successful experience in commercialized pipette-based electrochemical microscopy , Park Systems’s hardware and software enable localized nanoscopic cyclic voltammetry measurements each time the meniscus contacts the surface. Thus, Park NX12 produces a spatially-resolved surface electroactivity mapping of HOPG with high-throughput at the micro- and nanoscale.
This work demonstrates the effectiveness of Park Systems’s commercial SECCM option for quantitative electroanalysis at the nanoscale. This capability could also facilitate the rational design of functional electromaterials with potential applications in energy storage (battery) studies and corrosion research.
Potassium chloride (KCl, Sigma-Aldrich) and hexaammineruthenium (III) chloride ([Ru(NH3)6]Cl3, Sigma-Aldrich) are used without modification. 0.0155 g [Ru(NH3)6]Cl3 and 0.037 g KCl are dissolved together in 10 mL deionized (DI) water to prepare the mixed electrolyte solution with a concentration of 5 mM [Ru(NH3)6]Cl3 and 50 mM KCl. The highly-ordered pyrolytic graphite (HOPG) sample is the working electrode that was cleaved using the “scotch tape method” before usage .
For all SECCM experiments in the Park NX12 system, decoupled XY and Z piezoelectric scanners control the pipette and sample movement. The SECCM experiments are conducted using Park Systems SmartScanTM software. Figure 1 depicts the schematic diagram of the SECCM. A glass nanopipette with a ∼100 nm-diameter tip opening, fabricated by pulling a borosilicate capillary, is first filled with an aqueous electrolyte solution (5 mM [Ru(NH3)6]Cl3 +50 mM KCl). Then an Ag/AgCl electrode is inserted in the pipette to serve as the QRCE. Next, the electrolyte-filled pipette is fixed on the Park SICM head and positioned above the HOPG electrode surface. The SICM head contains a current amplifier, which is assembled onto the Park NX12 system for current measurement.
The SECCM mode consists of five windows and panels that allow the operator to control the nanoelectrochemical process (Figure 2). In the first step, a liquid meniscus forms as the pipette approaches the sample, which behaves as a nanoscale electrochemical cell. Using the vision view, the operator can check the distance from the tip end to the sample surface (Figure 2A). During the approach, a potential bias of (-0.5V) is applied to the sample surface. The current log channel records the current across the meniscus (Figure 2B). Changes in this current serve as signals to control the Z scanner movement until a meniscus forms without contacting the substrate. When the probe meniscus contacts the surface, the Z scanner stops, and a reduction reaction happens within the confined droplet at the predesignated potential . Thus, the current at pA range is detected by comparing it to the background fA current in the air. The current log channel in Figure 2B indicated that this sudden current change generated a current spike. This refers to the change in current versus time in the current log file when the meniscus forms and a reduction reaction happens.
Figure 1. Schematic illustration of SECCM configuration with a single channel pipette.
After the pipette contacts the droplet, the operator can measure the electrochemical activity at the designated position using I/V spectroscopy mode to obtain a single linear, cyclic voltammogram. In the control panel (Figure 2C), the operator can input the desired experiment conditions, including sample bias voltage, sweep rate/speed, cycle repetitions, and output channels. The cyclic voltammograms (CVs) for the Ru (NH3)6]3+/2+ redox reaction are run on the HOPG surface at various scan rates by applying a sample bias voltage ranging from -0.5 V to 0 V. In the Position area (Figure 2D), the operator can use the “Point List” function to assign the location for the single CV curve to be obtained. The “Point Grid” function allows the operator to obtain the I/V spectroscopy repeatedly across a predetermined surface and create an image of the electrochemical activity. This function is referred to as the Approach-Retract-Scanning (ARS) mode . As the pipette lands at various predefined grid positions using AFM, linear CVs are recorded upon each meniscus forming. Finally, the obtained CV will appear in the “Data View” panel (Figure 2E).
Results and Discussion
In our SECCM experiments, a glass nanopipette with a ∼100 nm-diameter tip opening filled with 5 mM [Ru(NH3)6]Cl3 is mounted on the SICM head positioned above the HOPG electrode surface. Once the meniscus forms, a linear sweep voltammetry (LSV) is used to record the localized electrochemical activity across the HOPG surface. To acquire the microscopic understanding of the [Ru (NH3)6]3+/2+ electron transfer process on HOPG, an LSV is recorded in the range 0V → -0.5V → 0V. The bulk macroscopical Ru (NH3)6]3+/2+ redox reaction CV curve determined this bias range. Figure 3 shows a typical SECCM LSV curve for the reduction of (NH3)6]3+. The smooth sigmoidal wave shape observed is characteristic of the LSV acquired in SECCM format . This sigmoidal plot corresponds to a quasi-steady-state voltammogram, and the steady-state limit current is about -5.3 pA with a sample voltage of -0.5 V. The small current magnitude detected demonstrates the power of Park Systems’ low-noise current detector. The redox reaction is reversible. When the potential sweeps from 0V to – 0.5 V, the [Ru(NH3)6]3+ reduction occurs, and when the potential sweeps back to 0 V from -0.5 V, oxidation happens.
The SECCM LSV is highly reproducible and robust. Figure 4 shows a set of 4 typical LSV curves obtained on HOPG at an increased scan rate of 200 mV/s. There is little variation between the consecutive CVs.
Figure 2. SECCM Mode in Park SmartScanTM software. (A) Vision & Monitoring view. (B) Current monitor panel. (C) I/V spectroscopy parameter control panel. (D) Spectroscopy positions control panel for point list or point grid function. (E) Data viewing panel.
Figure 4. Four overlaid SECCM LSVs in 5mM [Ru(NH3)6]3+ and 50 mM KCl at a sweep rate of 200 mV/s