NANOscientific

Myung-Hoon Choi, ¹Hanaul Noh, ¹Lane A. Baker² and Stefan B Kaemmer¹

Park Systems Inc., Santa Clara, CA USA. Department of Chemistry, Texas A&M University, College Station, TX, USA

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ABSTRACT

 

Scanning Electrochemical Cell Microscopy (SECCM) offers enhanced spatial resolution for visualizing electroactivity on surfaces. This study reveals potential-dependent electroactivity trends at both step edges and the basal plane of HOPG from the concurrent acquisition of topographical information and corresponding faradaic current maps. Statistical analysis of step edges underscores the relationship between thickness and electroactivity, contributing insights into the heterogeneous electroactivity patterns. The applied methodology allows for simultaneous topography and electroactivity mapping, with minimal electrolyte spread during potential-based measurements. These findings advance our understanding of electrocatalysis and its relevance for catalyst design and energy conversion processes with high accuracy and reliability.

 

Introduction

 

Highly Oriented Pyrolytic Graphite (HOPG) is a popular substrate for Scanning Probe Microscopy (SPM) research due to its flatness, uniformity, and ordered layered structure. HOPG has been extensively used as a model electrode for developing and optimizing electrocatalytic materials and reactions crucial in energy conversion processes [1-4]. By studying electrochemical behavior, one can gain insights into the fundamental mechanisms of electrocatalysis, enabling the design of more efficient catalysts for a variety of applications. From a microscopic viewpoint, the basal plane and edge of the HOPG surface exhibit heterogeneous electro- and electrocatalytic characteristics due to differences in their electronic and structural properties [5]. Conventionally, Scanning Electrochemical Microscopy (SECM) is employed to study the electrochemical heterogeneity at surfaces allowing for the visualization of electrochemical processes [6]. By using a microelectrode probe, SECM measures the electrochemical activity of local areas, providing information on electrocatalytic properties. In comparison, SECCM offers higher spatial resolution measurements than SECM, thus providing more spatially localized information about the electrochemical activity. SECCM works by using a small meniscus formed at the pipette tip end as a probe and electrochemical cell, allowing for precise delivery of reactants to a specific location on the surface [7-10].

This study demonstrates the visualization of the heterogeneity in the electrochemical activities of HOPG using a customized Park Systems SECCM setup that allows for the simultaneous acquisition of topography and electrochemical activity maps. The grade 2 HOPG used in this study possesses an intermediate quality, making it suitable for both extensive use and high resolution imaging purposes. A series of electrochemical maps at different potentials applied to the HOPG substrate were acquired by operating SECCM in AC mode, which was previously demonstrated [11, 12].

 

Experimental Chemical and Materials

 

Chemical and Materials

All solutions were prepared at 25 °C using ultrapure H2O obtained from a Milli-Q water purification system (with a resistivity of 18.2 MΩ·cm, Millipore Corp.). The chemicals used, including potassium chloride (KCl, VWR Analytical), were used as received. HOPG grade 2 (SPI) was utilized as the substrate in the experiments. The ruthenium hexamine redox couple was purchased from Sigma-Aldrich and stored at room temperature before utilization.

 

Dual Barrel Probe Fabrication and Use

 

Theta borosilicate capillaries (O.D. 1.2 mm, I.D. 0.9 mm) were transformed into nanopipettes (I.D. 50-70 nm, O.D. 150-200 nm) using a CO₂ laser puller (P-2000, Sutter Instruments). The pipette dimensions were confirmed with SEM (FEI Quanta 600F). The pipettes were loaded with specific solutions, such as 100 mM KCl and 5 mM Ru(NH₃)₆Cl₃. Two Ag/AgCl wires were inserted into each barrel of the pipette, serving as quasireference counter electrodes (QRCEs).

 

SECCM Instrumentation

 

The SECCM system with a dual-barrel configuration was developed on a Park NX12 AFM platform (Park Systems) [11, 12], integrating a custom-built potentiostat. The current readings obtained from the potentiostat were fed into the Aux channel of the AFM controller. These current readings were crucial in maintaining a consistent distance between the probe tip and the HOPG sample surface throughout the SECCM measurements. For stable SECCM cyclic voltammetry (CV) mapping operations and measurements, a custom-built humidity chamber was employed to maintain humidity levels between 20%-40%. The SECCM measurement was carried out in collaboration with the Baker Lab at the Department of Chemistry in Texas A&M University.

 

Results and Discussion

 

The operational principles of the SECCM instrumentation have been thoroughly described in prior publications by the Unwin group [7-10]. The key element is a symmetrically fabricated dual-barrel probe. This importance stems from the fact that within an SECCM setup, the electrolyte meniscus functions as a miniature electrochemical cell. The size of this meniscus typically falls within the attoliter range, and its dimensions are intricately linked to those of the probe tip, which usually measures between 150-250 nanometers in outer diameter (Fig. 1). The vertical view revealed the true shape of the dual-barrel probe in an ecliptic
structure (Fig. 1A and 1B). The dual-barrel probes that were pulled demonstrated a linearohmic response between the two barrels used in SECCM measurements (Fig. 1C).

 

 

To ensure safe and consistent contact of the meniscus with the substrate, which serves as the working electrode, we meticulously monitored the ionic current in both DC and AC components. Once the meniscus contacted the substrate, we initiated a potential sweep (V¯) ranging from 0 to -0.5 V to acquire cyclic
voltammograms. Using the potentiostat, the desired potential difference was achieved by applying ± V_1/2 to two Ag/AgCl electrodes. As a result, the effective potential (V_we) at the substrate, relative to the Ag/AgCl electrodes, was approximately -V¯. The electrochemical current signal (mi_we) generated at the working electrode was accurately recorded (Fig. 2A) using a built-in current amplifier of the AFM. We compared two CVs obtained at different scan rates, 10 and 50 mV/s, using the Ru(NH₃)₆ ²⁺ /Ru(NH¨)©³⁺ redox couple through nanoscale cyclic voltammetry, facilitated by the dual barrel SECCM setup. The CVs exhibited a typical steady-state current pattern characteristic of ultramicroelectrodes, and notably, they appeared nearly identical, suggesting that the electrochemical response remains consistent regardless of the scan rate (Fig. 2B).

Simultaneous and correlative SECCM topography and electroactivity images of HOPG during reversible electron transfer measurements with ruthenium hexamine were successfully obtained (Fig. 3). The lateral image resolution obtained was equivalent to the diameter of the dual barrel probe tip. This suggests that the electrolyte meniscus did not spread out to the substrate surface during the measurement when the applied potential was present. To investigate the potential-dependent electroactivity, measurements were conducted at working electrode potentials of - 0.25, - 0.3, and - 0.5 V vs Ag/AgCl. The results revealed that higher applied potentials corresponded to higher electroactivity at both the step edges and the basal plane. Specifically, at the step edge, the electroactivity ranged from 8 to 22 pA (max.), while at the basal plane, it ranged from 5 pA to
15 pA (max.) from the line profile analysis (Fig.4). SECCM enabled correlative data analysis of the characterization of position (or thickness)- dependent electroactivity.

The in-depth analysis included comparing line profiles of thickness and faradaic current at 7 distinct step edges (Fig. 4). This approach allowed us to characterize the heterogeneous electroactivity, expressed as the faradaic current per single layer of graphene. The estimated number of single graphene layers ranged from 6 to 30. By directly comparing topography with faradaic current, we were able to assess the variation in electroactivity at individual step edges on the HOPG surface. Additionally, we observed clear potential-dependent effects in each local area(Fig. 4B).

At last, the electroactivity of step edges on HOPG was selectively investigated using statistical analysis. The analysis focused on step edges with a thickness ranging from 8 nm to 20 nm. The dataset consisted of 1727, 1902, and 1870 data points for the electroactivity map at potentials of - 0.25 V, - 0.3 V, and - 0.5 V, respectively (Fig. 5A). A box-and-whisker analysis revealed a clear trend: as the step edges became thicker, the electroactivity increased, and the variation in electroactivity decreased. This trend was observed consistently across all three potentials (Fig. 5B and Table 1).

 

Conclusions

In conclusion, the dual barrel SECCM system has proven to be a valuable tool for electrochemistry researchers by employing a custom-designed potentiostat. It allows for simultaneous high-resolution topography and electroactivity imaging while effectively containing the electrolyte during measurements with applied potential. On the model system used, our study revealed a clear relationship between higher applied potentials and increased electroactivity at both step edges and the basal plane of HOPG. Additionally, our analysis of thickness-dependent electroactivity offered valuableinformation, estimating the number of single graphene layers.We have demonstrated that dual barrel SECCM is a useful tool for electrochemistry researchers, facilitating a deeper understanding of electrocatalysis and its potential applications in catalyst development and energy conversion processes.

 

 

 

References

1. Banerjee, S.; Sardar, M.; Gayathri, N.; Tyagi, A.K.; and Raj, B., Phys. Rev. B, 2005, 72, 075418.

2. Ma, H.; Lee, L.; Brooksby, P.A.; Brown, S.A.; Fraser, S.J.; Gordon, K.C.; Leroux, Y.R.; Hapiot, P.; Downard, A. J., J. Phys. Chem. C, 2014, 118 (11), 5820-5826.

3. Pham, K. D.; Hieu, N.N.; Phuc, H.V.; Fedorov, I.A.; Duque, C.A.; Amin, B.; Nguyen, C.V., Appl. Phys. Lett. 2018, 113, 171605.

4. Tao, L.; Qiao,M.; Jin, R.; Li, Y.; Xiao, Z.; Wang, Y.; Zhang, N.; Xie, C.; He, Q.; Jiang, D.; Yu, G.; Li, Y.; Wang, Sh., Angew. Chem. Int. Ed. 2019, 58, 1-7.

5. Jaouen, K.; Henrotte, O.; Campidelli, S.; Jousselme, B.; Derycke, V.; Cornut, R., Appl. Mater. Today, 2017, 8, 116-124.

6. Nioradze, N.; Chen, R.; Kurapati, N.; Khvataeva-Domanov, A.; Mabic, S.; Amemiya, S., Anal. Chem., 2015, 87 (9), 4836-4843.

7. Snowden, M.E.; Güell, A.G.; Lai, S. C. S.; McKelvey, K.; Ebejer, N.; O’Connell, M.A.; Colburn, A.W.; Unwin, P.R.,Anal. Chem., 2012, 84 (5), 2483-2491.

8. Patel, A.N.; Collignon, M.G.; O’Connell, M.A.; Hung, W.O.Y.; McKelvey, K.; Macpherson, J.V.; Unwin, P.R., J. Am. Chem. Soc., 2012, 134 (49), 20117-20130.

9. Lai, S.C.S., Patel, A.N., McKelvey, K. and Unwin, P.R., Angew. Chem. Int. Ed., 2012, 51, 5405-5408.

10. Zhang, J. and Kim, B. Park AFM Application Note #48

11. Choi, M.-H.; Siepser, N.P.; Jeong, S.-J.; Wang, Y.; Jagdale, G.; Ye, X.; and Baker, L. A., Nano Lett. 2020, 20 (2), 1233-1239.

12. Jeong, S.‡; Choi, M.-H.‡; Jagdale, G.; Zhong Y.; Siepser, N.P.; Wang, Y.; Baker, L.A.; Ye, X. J. Am. Chem. Soc. 2022, 144, 28, 12673–12680