Atomic force microscopy (AFM) combined with scanning electrochemical microscopy (SECM), namely, AFM-SECM, can be used to simultaneously acquire high-resolution topographical and electrochemical information on material surfaces at nanoscale. Such information is critical to understanding heterogeneous properties (e.g., reactivity, defects, and reaction sites) on local surfaces of nanomaterials, electrodes and biomaterials.
Scanning electrochemical microscopy (SECM) is used to measure the local electrochemical behavior of liquid/solid, liquid/gas and liquid/liquid interfaces. Atomic force microscopy (AFM) is a versatile tool to characterize micro- and nanostructure in terms of topography and mechanical properties. However, conventional SECM or AFM provides limited laterally resolved information on electrical or electrochemical properties at nanoscale. For instance, the activity of a nanomaterial surface at crystal facet levels is difficult to resolve by conventional electrochemistry methods. This paper reports the application of a combination of AFM and SECM, namely, AFM-SECM, to probe nanoscale surface electrochemical activity while acquiring high-resolution topographical data. Such measurements are critical to understanding the relationship between nanostructure and reaction activity, which is relevant to a wide range of applications in material science, life science and chemical processes. The versatility of the combined AFM-SECM is demonstrated by mapping topographical and electrochemical properties of faceted nanoparticles (NPs) and nanobubbles (NBs), respectively. Compared to previously reported SECM imaging of nanostructures, this AFM-SECM enables quantitative assessment of local surface activity or reactivity with higher resolution of surface mapping.
Characterization of electrochemical (EC) behavior can provide critical insights into the kinetics and mechanisms of interfacial reactions in diverse fields, such as biology1,2, energy3,4, material synthesis5,6,7, and chemical process8,9. Traditional EC measurements including electrochemical impedance spectroscopy10, electrochemical noise methods11, galvanostatic intermittent titration12, and cyclic voltammetry13 are usually performed at macroscopic scale and provide a surface-average response. Thus, it is difficult to extract information on how electrochemical-activity is distributed across a surface, but local scale surface properties in nanoscale are especially important where nanomaterials are widely used. Therefore, new techniques capable of simultaneously capturing both nanoscale multidimensional information and electrochemistry are highly desirable.
Scanning electrochemical microscopy (SECM) is a widely used technique for measuring the localized electrochemical activity of materials at micro- and nanoscales14. Typically, SECM uses an ultra-microelectrode as a probe for detecting electroactive chemical species as it scans a sample surface to spatially resolve local electrochemical properties15. The measured current at the probe is produced by reduction (or oxidation) of the mediator species, and this current is an indicator of the electrochemical reactivity at the surface of the sample. SECM has evolved significantly after its first inception in 198916,17 but it is still challenged by two main limitations. Since EC signals are typically sensitive to tip-substrate interaction characteristics, one limitation of SECM is that keeping the probe at a constant height prevents a direct correlation of electrochemical activity with the surface landscape, due to the convolution of topography with the collected EC information18. Second, it is difficult for a commercial SECM system to obtain sub-micrometer (µm) image resolution as the spatial resolution is partially determined by the probe dimensions, which is on the micrometer scale19. Therefore, nanoelectrodes, the electrodes with a diameter in the nanometer range, are increasingly used in SECM to achieve a resolution below the sub-micrometer scale20,21,22,23.
To provide a constant tip-substrate distance control and obtain a higher spatial electrochemical resolution, several hybrid techniques of SECM have been used, such as ion conductance positioning24, shear force positioning25, alternating current SECM26, and atomic force microscopy (AFM) positioning. Among these instrumentations, SECM integrating AFM positioning (AFM-SECM) has become a highly promising approach. As AFM can provide fixed tip-substrate distances, the integrated AFM-SECM technique enables simultaneous acquisition of nanoscale surface structural and electrochemical information through mapping or sample sweeping with the sharp AFM tips. Since the first successful operation of AFM-SECM by MacPherson and Unwin in 199627, significant improvements have been achieved on probe design and fabrication, as well as its applications in various research fields such as electrochemistry in chemical and biological processes. For example, AFM-SECM has been implemented for imaging composite material surfaces, such as noble metal nanoparticles28, functionalized or dimensionally stable electrodes29,30, and electronic devices31. AFM-SECM can map the electrochemically active sites from the tip current image.
Simultaneous topographical and electrochemical measurements could also be achieved by other techniques such as conductive AFM32,33,34,35, electrochemical AFM (EC-AFM)36,37,38,39, scanning ion conductance microscopy-scanning electrochemical microscopy (SICM-SECM)24,40, and scanning electrochemical cell microscopy (SECCM)41,42 The comparison between these techniques has been discussed in a review paper1. The aim of the present work was to employ SECM-AFM to demonstrate the electrochemical mapping and measurement on faceted crystalline cuprous oxide nanomaterials and nanobubbles in water. Faceted nanomaterials are widely synthesized for metal oxide catalysts in clean energy applications because the facets with distinctive crystallographic features have distinctive surface atomic structures and further dominate their catalytic properties. Moreover, we also measured and compared the electrochemical behavior at the liquid/gas interfaces for surface nanobubbles (NBs) on gold substrates. NBs are bubbles with a diameter of <1 μm (also known as ultrafine bubbles)43, and they elicit many intriguing properties44,45, including long residence times in the solutions46,47 and high efficiency of gas mass transfer46,48. Furthermore, the collapse of NBs creates shock waves and the formation of hydroxyl radicals (•OH)49,50,51,52. We measured the electrochemical reactivity of oxygen NBs in the solution to better understand the fundamental chemical properties of NBs.
1. Sample preparation
2. Setup of AFM-SECM
NOTE: AFM was used in the presented AFM-SECM measurements. To perform the EC analyses, the AFM was equipped with a bipotentiostat and SECM accessories. As shown in Figure S1, the bipotentiostat was connected to the AFM controller and both the potentiostat and AFM were connected to the same computer. The accessories include an SECM chuck, an SECM probe holder with protective boot, and a strain-release module with a resistance selector (10 MΩ resistance was used) to limit the maximum current flow55. As shown in Figure 2, the AFM-SECM probes have a tip radius of 25 nm and a tip height of 215 nm. The sample acted as a working electrode, which shares the same pseudo-reference using the Ag wire electrode (25 mm diameter) and the counter electrode of a Pt wire (25 mm in diameter). The probe and the sample could be biased at different potentials (vs the Ag wire pseudo-reference electrode) to enable different redox reactions. In the presented work, the tip reduces the [Ru(NH3)6]3+ to [Ru(NH3)6]2+ at -400 mV versus an Ag wire pseudo-reference electrode.
3. Operation of AFM-SECM
Topography and current imaging of ONBs by AFM-SECM
Previous studies that characterized NBs with AFM only reported topography images to reveal the size and distribution of NBs immobilized on a solid substrate56,57. The experiments here revealed both morphological and electrochemical information. Individual oxygen nanobubbles (ONBs) can be clearly identified in Figure 9, which provides the topography as well as the tip current mapping or information. The tip current was generated by the redox reaction of [Ru(NH3)6]3+ that is reduced to [Ru(NH3)6]2+ at the tip under a bias potential at -0.4 V, as depicted in Figure 9C. A comparison of the topography and current image shows the good correlation between the locations of the NBs and the current spots. This result confirms that ONBs could facilitate the diffusion and mass transfer of [Ru(NH3)6]3+ from the bulk solution to the tip area58 and result in a higher current (relative to the substrate background current of 6 pA) when the AFM-SECM tip scanned over NBs59.
Topography and current imaging of Cu2O NPs by AFM-SECM
The topography and current images of Cu2O nanoparticles are presented in Figure 10. The tip current was generated due to the redox reaction of [Ru(NH3)6]3+, which are also reduced at the tip with a potential at -0.4 V, as depicted in Figure 10C. The nanoparticle is about 500-1000 nm in size. The presented topography image was processed with a 1st order flattening treatment. The particle size determined by AFM is comparable to that obtained from the SEM image. The length or width is slightly larger than the height of the nanoparticles (around 500 nm) due to the tip convolution effect, a well-known artifact in the AFM imaging process that causes the overestimation of the object dimension by a finite-sized AFM tip60. In this study, as the Cu2O nanoparticle has a sharp octahedron shape, the AFM tip may fail to touch the steep sidewall and bottom, and this convolution effect can account for many lateral broadening of the surface61. Figure 10B indicates that the nanoparticle visible in the topography image is associated with evident electric current “spot” in current image, whereas the background current (~10 pA) corresponds to the flat silicon substrate.
CV and Approach curves of Cu2O NPs
Figure 11A shows five representative CV curves of the AFM-SECM tip with the tip at around 1 mm away from the substrate in 10 mM [Ru(NH3)6]3+ and 0.1 M KCl. The diffusion-limited tip current (~1.2 nA) did not decrease with time. Figure 11A shows the CV curve at a scan rate as 50 mV s−1, which confirms the bias potential of -0.4 V vs Ag/AgCl led to the maximum plateau tip current due to the reduction reaction of [Ru(NH3)6]3+.
Figure 11B shows the changes of the tip current as the tip move towards the sample surface. The AFM-SECM tip approached the substrate surface in the Z direction until it reached a setpoint (5 nN in this work) that indicates the physical tip-substrate contact or bending as a result of the contact62,63. The current on the plots were normalized to i0 (i0=3.385 nA), which is defined as the tip current measured when the tip is 1 μm above the sample surface. The tip was biased at −0.4 V vs Ag/AgCl in electrolyte containing 10 mM [Ru(NH3)6]3+and 0.1 M KCl. The normalized tip current increased with the decreasing tip-sample distance. At <8 nm, the tip was in contact with the nanoparticle surface and the normalized tip current increased sharply, probably because the negatively charged Si surface would result in an increased local concentration of [Ru(NH3)6]3+ near the surface.
Figure 1: Deposition of Cu2O nanoparticles on a silicon wafer. Please click here to view a larger version of this figure.
Figure 2: Schematic of AFM-SECM system. Please click here to view a larger version of this figure.
Figure 3: Installation procedure for SECM chuck and other accessories. Please click here to view a larger version of this figure.
Figure 4: Assemblage procedure of the EC sample cell. Please click here to view a larger version of this figure.
Figure 5: The ESD field service package.
(A) Parts of ESD protective parts; (B) Connections of ESD monitor, wrist strap and ground wire. Please click here to view a larger version of this figure.
Figure 6: Attachment procedure for the protective boot onto the probe holder. Please click here to view a larger version of this figure.
Figure 7: Loading the SECM probe to the probe holder. Please click here to view a larger version of this figure.
Figure 8: The SECM Probe.
(A) Attach the probe-holder-boot assembly to the scanner; (B) Connection of probe to the strain released module. Please click here to view a larger version of this figure.
Figure 9: Simultaneously acquired topography (A) and tip current (B) images of oxygen NBs in electrolyte containing 10 mM [Ru(NH3)6]3+and 0.1 M KCl.
The tip (end tip radius is 25nm) was biased at -0.4V. (C) Schematic illustration of AFM-SECM measurement of NBs Please click here to view a larger version of this figure.
Figure 10: Simultaneously acquired topography (A) and tip current (B) images of Cu2O nanoparticles in electrolyte containing 10 mM [Ru(NH3)6]3+and 0.1 M KCl.
The tip (end tip radius is 25nm) was biased at -0.4V (C) Schematic illustration of AFM-SECM measurement of NPs. Please click here to view a larger version of this figure.
Figure 11: CV and Approach curves of Cu2O NPs.
(A) Five CV scan in 10 mM [Ru(NH3)6]3+and 0.1 M KCl. (B) Approach curves of nanoelectrode probe on Cu2O nanoparticle surface. Please click here to view a larger version of this figure.
Reaction | E0 / V | Concentration | Applied Potential | Ref |
2H+ + 2e− ![]() |
0 | |||
[Ru(NH3)6]3+ + e−![]() |
0.10 (NHE) | 10 mM | −0.4 V (Ag/AgCl) | 1 |
2NO2− + 3H2O + 4e− ![]() |
0.15(NHE) | 0.1 M | +0.95V (Ag/AgCl) | 2 |
[Fe(CN)6]3− + e−![]() |
0.358(NHE) | 2~5 mM | +0.0 ~ 0.5V(Ag/AgCl) | 3 |
ClO4− + H2O + 2e− ![]() |
0.36(NHE) | 0.1~1 M | +0.30 V(SCE) | 4 |
[IrCl6]3− + 3e− ![]() |
0.77(NHE) | 10 mM | +1.0 V(Ag/AgCl) | 5 |
SO42− + H2O + 2e−![]() |
-0.93 (NHE) | 10 mM | -0.45 V(Ag/AgCl) | 6 |
AgCl + e− ![]() |
0.22233(NHE) | |||
References: 1. Jiang, J. et al. Nanoelectrical and Nanoelectrochemical Imaging of Pt/p-Si and Pt/p+-Si Electrodes. ChemSusChem. 10 (22), 4657-4663, (2017). 2. Izquierdo, J., Eifert, A., Kranz, C. & Souto, R. M. In situ monitoring of pit nucleation and growth at an iron passive oxide layer by using combined atomic force and scanning electrochemical microscopy. ChemElectroChem. 2 (11), 1847-1856, (2015). 3. Jones, C. E., Unwin, P. R. & Macpherson, J. V. In Situ Observation of the Surface Processes Involved in Dissolution from the Cleavage Surface of Calcite in Aqueous Solution Using Combined Scanning Electrochemical–Atomic Force Microscopy (SECM–AFM). ChemPhysChem. 4 (2), 139-146, (2003). 4. Anne, A., Cambril, E., Chovin, A., Demaille, C. & Goyer, C. Electrochemical atomic force microscopy using a tip-attached redox mediator for topographic and functional imaging of nanosystems. ACS nano. 3 (10), 2927-2940, (2009). 5. Macpherson, J. V., Jones, C. E., Barker, A. L. & Unwin, P. R. Electrochemical imaging of diffusion through single nanoscale pores. Analytical chemistry. 74 (8), 1841-1848, (2002). 6. Izquierdo, J., Eifert, A., Kranz, C. & Souto, R. M. In situ investigation of copper corrosion in acidic chloride solution using atomic force—scanning electrochemical microscopy. Electrochimica Acta. 247 588-599, (2017). |
Table 1: Examples of redox mediators used in literature.
Figure S1: Photo showing connection between the bipotentiostat and the AFM controller. Please click here to download this figure.
Figure S2: Load the PeakForce SECM workspace in the software. Please click here to download this figure.
Figure S3: Navigation panel for SECM workspace. Please click here to download this figure.
Figure S4: Run Open Circuit Potential – Time. Please click here to download this figure.
Figure S5: Run Cyclic Voltammetry. Please click here to download this figure.
Figure S6: Parameter setting for cyclic voltammetry measurement. Please click here to download this figure.
Figure S7: Parameters for a Chronoamperometry measurement. Please click here to download this figure.
Figure S8: Start current reading in AFM-SECM software. Please click here to download this figure.
Figure S9: Parameters for Amperometric i-t technique. Please click here to download this figure.
A combined AFM-SECM technique that enables high-resolution multimodal imaging has been described in this protocol. This technique allows for topography to be mapped simultaneously with the SECM current collected or mapped on single nanoparticles or nanobubbles. Experiments were performed using commercial probes. These probes were designed to provide chemical compatibility with a wide range of electrochemical environments, electrochemical performance, mechanical stability, and multiple-cycle handling18. However, the stability and durability of the AFM-SECM probes are critical for the measurement of the electrochemical information with reliable and high resolution. As a result, the steps mentioned in steps 3.2 and 3.7 are critical to protecting the AFM-SECM tip from destroying by electrostatic discharge. Detailed discussion related to specific protocol steps are described as well.
In step 3.4.5, 10 mM [Ru(NH3)6]3+ with supporting electrolyte of 0.1 M KCl was used in the presented test. 5-10 mM is a commonly used concentration of [Ru(NH3)6]3+ in literature to obtain good current signals30. More examples of commonly used redox mediators in AFM-SECM measurements are summarized in the discussion (Table 1).
In step 3.4.6, the quality and stability of electrodes are confirmed with the OCP measurement. If the potential measured in OCP is not near zero or unstable, then the counter and pseudo-reference electrodes must be checked. The possible reasons for unstable OCP may be the attachment of bubbles on the electrodes or the electrodes not immersed in liquid.
In step 3.4.8, the potential range mentioned here “High E” and “Low E” could be +0.3 V or −0.3 V of “init E/Final E” is a safe choice to start the CV test. Then, the potential range could be adjusted based on the potential value that led to a plateau current in the CV curve. Scan rate could vary between 0.01 V/s to 0.1 V/s. A higher scan rate attributes to a higher sensibility, but the charging current would also increase. Also, at high scan rates the voltammograms presented distorted shapes64. A higher sensitivity value should be selected as long as CV test does not show “overflow”. If an “overflow” message showed, then the sensitivity should be decreased.
In step 3.5.2, for imaging, the AFM-SECM imaging process was performed using a lift scan mode with a lift height typically 40-150 nm. If a lower lift height was selected, then there may be a possibility for tip crashing onto the sample surface. If the lift height was too high, then it may decrease the current imaging resolution since the tip is far away from the sample surface.
In step 3.5.3 in the presented measurement protocol, -0.4 V versus Ag/AgCl (-0.18V versus NHE) was chosen to perform the reduction of [Ru(NH3)6]3+. The probe may reduce the [Ru(NH3)6]3+ to [Ru(NH3)6]2+ at -0.35 to -0.5 V vs Ag wire pseudo-reference electrode, while the sample maybe biased at 0 to -0.1 V for [Ru(NH3)6]3+ regeneration. This value depends on the plateau current measured in the CV scan. It will also vary with different redox mediators as summarized in Table 1.
Also, the chronoamperometry technique was chosen because of the absence of Amperometric i-t technique in the presented bi-potentiostat. If readers have a bi-potentiostat that supports Amperometric i-t technique, they can set the i-t technique as shown in Figure S9. The run time was selected as 2000 seconds to make sure it is enough for at least one current imaging process in AFM-SECM.
Moreover, sample preparation is very important as well since the solid particles must be immobilized on the substrate completely so that particles do not detach during the imaging process. Moreover, to scan or probe electrochemical or electrical properties of sample surfaces (e.g., electrode), the binding between samples and substrates needs to ensure the electrical conductivity. The sample preparation methods should be useful and referable to a wide range of applications, especially for nano-objects characterization; however, sample immobilization methods may vary with specific samples65,66. Overall, we demonstrated that AFM-SECM enables high-resolution imaging of oxygen NBs and Cu2O nanoparticles. Clearly, this AFM-SECM protocol is anticipated to play important roles in interfacial electrochemical analysis and will have broad applications in different research fields, such as material science, chemistry, and life science1,19.
The authors have nothing to disclose.
This work is funded by the national science foundation (Award Number: 1756444) via Biological & Environmental Interfaces of Nano Materials, the USDA National Institute of Food and Agriculture, AFRI project [2018-07549] and Assistance Agreement No. 83945101-0 awarded by the U.S. Environmental Protection Agency to New Jersey Institute of Technology. It has not been formally reviewed by EPA. The views expressed in this document are solely those of authors and do not necessarily reflect those of the Agency. EPA does not endorse any products or commercial services mentioned in this publication. The authors also thank Undergraduate Research and Innovation program (URI) Phase-1 & Phase-2 at New Jersey Institute of Technology.
Equipment | |||
Atomic force microsopy | Bruker, CA | Dimenison Icon | |
Bipotentiostat | CH Instruments, Inc. | CHI 700E | |
Materials | |||
Silicon wafer | TED PELLA, Inc. | 16013 | |
Fresh gold plates | Bruker, CA | model 119-017-307 | |
PF-SECM-AFM probes | Bruker, CA | 990-050138 | |
PF-SECM strain-release module | Bruker, CA | 840-012-724 | |
PF-SECM Probe Holder | Bruker, CA | 900-050121 | |
PF-SECM Chuck | Bruker, CA | PF-SECM Chuck | |
PF-SECM O-ring | Bruker, CA | 598-000-106 | |
PF-SECM cover glass, SECM Cell | Bruker, CA | 900-050137 | |
EC Cell Assy | Bruker, CA | 932-017-300 | |
ESD Field Service | Bruker, CA | 490-000-066 | |
PF-SECM Boot | Bruker, CA | 900-050136 | |
Spring connector block | Bruker, CA | 900-050524 | |
PFSECM Tweezers | Bruker, CA | ||
Cable, SECM Tip module | Bruker, CA | 468-050171 | |
Ag wire | Bruker, CA | 249-000-056 | |
Pt wire | Bruker, CA | 248-000-004 | |
Hard sharp wire | Bruker, CA | TT-ECM10 | |
Tubular ceramic membrane | Refracton | WFA0.1 | |
Chemicals | |||
Copper(II) chloride dihydrate | ACROS Organics | AC315281000 | |
Sodium Hydroxide | Fisher Chemical | S318-100 | |
Ascorbic Acid | Fisher Chemical | A61-25 | |
Epoxy | Loctite | Instant Mix | |
Potassium Chloride | Fisher Chemical | P217-500 | |
Hexaammineruthenium(III) chloride | ACROS Organics | AC363342500 |