NANOscientific

Introduction

Interest in surface investigations and the study of chemical properties has been steadily increasing. Electrochemical property research, in particular, holds significant importance in various academic and industrial applications. These applications include studying ion transfer distribution across electrodes [1,2], characterizing anode/cathode materials for battery design [3], modifying substrates for surface enhancement [4,5], assessing metal-oxide corrosion [6,7], and studying single-cell electrochemical activity [8]. While numerous measurement techniques have been developed, pipette-based scanning probe microscopy (SPM) methods offer novel insights into exploring the chemical properties of nanoscale samples. Scanning electrochemical microscopy (SECM) [9], one of the first-generation pipette-based SPMs for electrochemical measurements, greatly contributed to visualizing chemical reactions at interfaces. However, SECM has limitations, such as challenges in controlling the distance between the probe and the sample surface and the need to immerse the sample in a liquid medium.

In contrast, scanning electrochemical cell microscopy (SECCM) [10-14] uses an electrolyte-filled nano-pipette to create a meniscus contact at the sample surface, forming an electrochemical cell at the tip of the pipette. Since its inception, SECCM has proven valuable in various research fields due to its ability to provide localized measurements at the nanometer scale, intuitive pipette control, and the capability to operate under ambient conditions.

In this primer, we introduce SECCM, including its principles, setup, and diverse applications. Additionally, we explain the interpretation of the cyclic voltammetry (CV) curve recorded by SECCM and the CV mapping image for few-layer graphene deposited on conductive substrates. We discuss the broad utility of SECCM in various fields, demonstrating its potential to significantly advance nanoscale electrochemical research.

SECCM Set-up and Electrochemical Measurements

SECCM utilizes a glass pipette with an inner diameter in the range of hundreds of nanometers for local electrochemical property measurements. To prepare it, a glass pipette (G100F3, Warner Instruments, U.S.A.) with a 1 mm outer diameter (O.D.) and a 0.58 mm inner diameter (I.D.) is pulled using a commercial CO¡ laser pipette puller (P2000, Sutter Instruments, U.S.A.) to fabricate nano-pipettes with an I.D. of approximately 200 to 350 nm (Fig. 1A). An Ag/AgCl electrode, serving as a quasi-reference counter electrode (QRCE), is inserted into the pipette, which is then filled with an electrolyte solution containing 10 mM Ru(NH½)¾Cl½ and 100 mM KCl. A voltage is applied between the conductive substrate and the pipette electrode filled with the electrolyte solution (Fig. 1B). The probe’s approach is controlled using the ionic current: a sharp increase in the ionic current indicates the formation of a meniscus (electrochemical cell). The distance between the pipette end and the sample is determined by the meniscus formed between them. As the pipette, with a hemisphere-shaped bubble of solution at the end, approaches the sample vertically, the meniscus forms between the pipette end and the sample surface, creating a closed electrical circuit that causes a sharp ionic current spike. This signal is used to halt the pipette’s approach and maintain a constant distance between the pipette and the sample [10], [12].

To visualize the electrochemical signal, approach-retract scanning (ARS, also known as hopping mode) is utilized. This method involves the approach of the nano-pipette, followed by CV curve determination, retraction of the nano-pipette, and its lateral movement. At each point, an independent CV curve is measured. CV mapping uses a series of predefined locations in a grid (Fig. 2) to build its images.

SECCM CV mapping process. The EC response contrast of the sample surface is displayed as an image with a cycle of repeated approach-CV curve measurement-retract-lateral movement.

Cyclic Voltammetry

Cyclic voltammetry (CV) is a widely used electrochemical technique that involves applying a varying potential to an electrochemical cell while measuring the resulting current. It provides information about the redox behavior and electrochemical properties of a system, such as the oxidation and reduction potentials, electron transfer kinetics, and di’usion coe’icients of electroactive species [15].

In a typical CV experiment, a working electrode is immersed in an electrolyte solution and connected to a potentiostat, which applies a potential ramp to the working electrode. The potential is scanned linearly with respect to time, and the resulting faradaic current is measured. The potential ramp can be either positive or negative, and the direction of the scan is reversed periodically to create a "cyclic" pattern. The measured current is plotted against the applied potential to generate a cyclic voltammogram, which is a graphical representation of the electrochemical behavior of the system under study. The resulting voltammogram typically exhibits peaks or waves corresponding to the oxidation and reduction of electroactive species present in the electrolyte solution.

CV curves obtained in SECCM can exhibit di’erent types, each carrying specific meanings and providing insights into the electrochemical behavior of the sample. For example, CV curves on highly oriented pyrolytic graphite (HOPG) with 10 mM Ru(NH½)¾Cl½ and 100 mM KCl were recorded.

Steady-state CV curve (Fig. 3A): The current response at the working electrode stabilizes at a constant or near-constant value a¼er several scan cycles. This indicates that the electrochemical system has reached a steady state, where the rates of oxidation and reduction reactions are balanced, and the current response has achieved stable, reproducible behavior. In a steady-state CV curve, species concentrations remain constant over time at any point along a curve and only vary with distance from the electrode. Reversible (Quasi-Reversible) CV curve (Fig. 3B): This type of CV curve typically exhibits well-defined anodic and cathodic peaks of equal height and shape, and the current response returns to the baseline between the peaks. This indicates a reversible or quasi-reversible redox process, where electron transfer is rapid, and the species undergoing oxidation and reduction have similar kinetics.

Irreversible CV curve (Fig. 3C): In this type of CV curve, the anodic and cathodic peaks are asymmetric. The current response does not return to the baseline between the peaks, indicating that the electron transfer is slow or involves chemical reactions. The peak potentials provide information about the redox potentials of the species, but due to the slow electron transfer or involvement of chemical reactions, the peak currents may not accurately reflect the concentration of the electroactive species.

The voltage scan rate plays a crucial role in the shape and characteristics of the CV curve. Changes in the scan rate can result in changes in the shape, size, and peak potential of the CV curve. The scan rate affects the rate of electron transfer reactions that occur at the electrode surface. A higher scan rate typically leads to faster electron transfer kinetics, resulting in changes in the shape and magnitude of the CV curve. Fig. 4 shows how the current signal was affected by the scan rate of the CV curves. To evaluate the effect of scan rate, CV curves on HOPG with 10 mM Ru(NH½)¾Cl½ and 100 mM KCl and a ~340 nm I.D. nano-pipette were performed using a range of scan rates (0.1 to 10 V/s). As shown in Fig. 4, the CV curve peak current increased as a function of scan rate. Theoretically, as the scan rate increases, the peak current increases, and the peak-to-peak potential also increases [12], [16].

At scan rates below 1 V/s, the peak current values decreased slightly and ranged from -65 pA to -72 pA. In contrast, at scan rates above 1 V/s, especially from 3 to 10 V/s, the response began to show the peak expected for linear diffusion, and the peak current increased to -130 pA. Although the CV curve displayed a somewhat peak-shaped form, the steady-state component still dominated as indicated by the current value at the switching potential and the small peak current ratio. Next, we obtained AFM images to determine surface morphologies and CV mapping to estimate EC signal contrasts on few layers graphene deposited on an indium tin oxide (ITO) substrate. All CV mapping images were obtained using a ~340 nm I.D. nano-pipette, 10 mM Ru(NH½)¾Cl½, 100 mM KCl supporting electrolyte, and a 1 V/s scan rate. Using confined meniscus contact, nano-scale imaging can distinguish the EC signal of the graphene basal plane, multi-layered graphene, and graphene with a conductive substrate. Heights from 50 to 100 nm were found at graphene multi-layers by AFM and a significantly clear EC signal contrast was recognized by CV mapping. Higher EC response can be attributed to the ITO substrate, so graphene flakes showed lower EC currents compared to the ITO substrate.

Conclusion

In this primer, we have highlighted the successful application of SECCM by Park Systems in measuring faradaic current signals and accurately reading potential in the presented data set. The accuracy of the potential reading is critical in electrochemistry, as it determines reaction efficiency, which is essential for practical applications such as catalyst development, even in fundamental research. SECCM is a powerful technique for studying local electrochemical properties at the nanoscale using pipette-based scanning electrochemical microscopy. It offers advantages such as intuitive pipette control, ambient condition measurement, and the ability to perform local measurements with high spatial resolution. By interpreting CV curves and CV mapping images, SECCM provides valuable insights into the electrochemical behavior of samples. This technique has been widely used in various research fields, including the study of ion transfer distribution, battery materials, surface modification, metal-oxide corrosion, and single-cell electrochemical activity. With its unique capabilities, SECCM continues to contribute to the growing interest in surface investigations and chemical properties, opening new possibilities for nanoscale electrochemical research.

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