Method Article

Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope (AFM-SECM)

DOI:

10.3791/61111

⸱

February 10th, 2021

 ,  ,  , 
1Department of Civil and Environmental Engineering, New Jersey Institute of Technology

In This Article

Summary

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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.

Abstract

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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.

Introduction

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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.

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Protocol

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1. Sample preparation

  1. Preparation of faceted Cu2O nanoparticles and deposition on silicon substrate
    1. Dissolve 0.175 g of CuCl2∙2H2O (99.9%) into 100 mL of deionized (DI) water to generate an aqueous solution of 10 mM CuCl2.
    2. Add 10.0 mL of 2.0 M NaOH and 10 mL of 0.6 M ascorbic acid dropwise into the CuCl2 solution.
    3. Heat the solution in a 250 mL round-bottom flask under constant stirring in a 55 °C water bath for 3 h.
    4. Collect the resulting precipitate by centrifugation (5,000 x g for 15 min), followed by washing with DI water 3 times and ethanol twice to remove the residual inorganic ions and polymers.
    5. Dry precipitate in vacuum at 60 °C for 5 h53.
    6. Use the prepared silicon wafer as the substrate to deposit Cu2O nanoparticles as illustrated in Figure 1A using epoxy to ensure the testing.
      Caution: The silicon wafer (Ø3” Silicon wafer, Type P/<111>) was cut into a single piece of 38 mm x 38 mm, followed by washing using ethanol, methanol and DI water to remove organic and inorganic contaminants.
    7. Directly deposit 10 µL of epoxy on the cleaned silicon wafer using a pipette tip and tile with a clean glass slide. After about 5 min, drop 10 µL of the nanoparticles/water suspension (10 mg L-1) on different epoxy-coated silicon wafer substrates, separately. The four different red spots shown in Figure 1B indicate the potential position of the deposited nanoparticles.
    8. Vacuum dry the substrate at 40 °C for 6 h.
    9. Place the sample substrate into the EC sample cell (Figure 4) to be filled with 1.8 mL of a 0.1 M KCl containing 10 mM Ru(NH3)6Cl3(98%).
  2. Preparation of NBs
    1. Generate oxygen nanobubbles by direct injection of compressed oxygen (purity 99.999%) through a tubular ceramic membrane (100 nm pore size, WFA0.1) into DI water.
      NOTE: The gas was injected continuously under a pressure of 414 kPa and a flow of 0.45 L·m-1 until reaching stable bubble size distribution as reported elsewhere54.
    2. Add 1.8 mL of the water suspension of NBs on a gold substrate in the EC sample cell and stabilize for 10 min.
      NOTE: Fresh 40 mm x 40 mm gold plates (Au on Si) were used as the substrate to immobilize NBs.
    3. Decant 0.9 mL of NB suspension and replace with 0.9 mL of a 10 mM Ru(NH3)6Cl3 solution in 0.1 M KCl.

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.

  1. Replace the existing sample chuck with SECM chuck and screw the chuck in place using two M3 x 6 mm socket head cap screws and a 2.5 mm hex wrench (Figure 3A).
  2. Connect the temperature control cable to the SECM chuck, and connect the low-noise SECM cables to the spring connector block (color to color) and switch block (Figure 3B).
    NOTE: The switch needs to be kept on the right side during SECM testing.
  3. Install the strain-release module onto the AFM scanner and also connect it to the working electrode connector on the spring connector block with extension cable (Figure 3C).
  4. Assemble the EC sample cell.
    1. Put the insert onto the top ring (Figure 4A).
    2. Assemble two O-rings onto the bottom groove and top groove of the insert, respectively (Figure 4B and Figure 4C).
    3. Put a glass cover onto the top ring top and then tighten by four screws lightly and diagonally (Figure 4D).
    4. Use a hard sharp wire with a 24 mm diameter (Figure 4E) to poke two holes in the O-ring through two channels of plastic part on top ring (Figure 4F).
    5. Insert Ag wire and Pt wire through the hole on the O-ring, and curve the Pt wire to a circle in EC sample cell as shown in Figure 4G.
    6. To seal the EC sample cell top part, press the assembled EC sample cell down on the EC sample cell bottom to make the O-ring fully contact the glass cover (Figure 4H).
    7. Place the top part of the EC sample cell upside-down and face the test sample (or substrate) downward so that the spring-loaded pins (pogo pins) touch the sample surface as shown in Figure 4I and Figure 4J. The test sample should be covering the O-ring to make EC sample cell bottom part seal.
    8. Put the EC sample cell bottom on and tighten diagonally with right length screw (Figure 4K).

3. Operation of AFM-SECM

  1. Initialization of the AFM and bipotentiostat instruments
    1. Double click the two software icons to initialize the AFM system and the bipotentiostat control interface.
  2. Loading SECM Probe
    1. Prepare the ESD field service package including antistatic pad, electrostatic discharge (ESD) protective probe stand, wearable anti-static gloves and wrist strap (Figure 5A). Figure 5B shows the connection of the ESD monitor with wrist strap.
      NOTE: The ESD monitor beeps when the red pad is connected with ground. The beep will stop when user wears the wrist strap.
    2. To prevent AFM scanner from exposure to liquid, use a protective boot (Figure 6A) during AFM-SECM testing. Put the probe holder onto the ESD protective probe stand (Figure 6B). Use a pair of plastic tweezers to attach the protective boot to the tip holder (Figure 6C). Then, align the small cut in the protective boot to the notch in the probe holder as illustrated in Figure 6D.
    3. Open the box of AFM-SECM probes (Figure 7A) using a tip tweezer (green color) to grab the probe from both sides of the grooves (Figure 7B). While using the disk gripper (silver color) to hold the probe holder on the stand, put the probe wire into the hole of the stand, and then slide the probe into the slot of the probe holder (Figure 7C). After the probe is inside the slot, use the flat end of the tweezer to push it in. Make sure the probe is completely in the tip holder (Figure 7D).
    4. As shown in Figure 8A, attach the whole probe holder (including the holder-boot) to the scanner.
    5. Use the Teflon tip tweezer to grab the wire right below the copper ring and connect it to the module (Figure 8B).
    6. Put the scanner back to the dovetail.
  3. Loading the sample cell
    1. After assembling the test sample (or substrate) in the EC sample cell, which was mentioned in Section 2.4, put the EC sample cell on the central point of the SECM chuck and the pseudo-reference electrode (Ag wire) and connect the counterelectrode (Pt wire) to the spring connector block (Figure 3). The EC sample cell is magnetically attached to the chuck.
  4. SECM software preparation before imaging
    1. In the AFM-SECM software, select SECM- PeakForce QNM to load the workspace (Figure S2).
    2. In Setup, load the SECM probe, and then align a laser on the tip using an alignment station.
    3. Go to Navigation (Figure S3). Move the scanner downwards slowly to focus on the sample surface. Adjust the position of EC sample cell slightly to make sure the scanner would not touch the glass cover of EC sample cell while moving. After focusing on the sample, click Update Blind Engage Position.
      Caution: Different samples have different heights, so it is necessary to update the blind engage position after changing a sample.
    4. Click Move to Add Fluid Position.
    5. Add ~1.8 mL of the buffer solution into the EC sample cell, to make sure the level of the solution is lower than the glass cover. If the water level is over the glass cover, water can creep to the scanner and cause an electric short and break the scanner. Wait for another 5 minutes and use a pipette to agitate the solution to remove bubbles.
      NOTE: The buffer solution (10 mM [Ru(NH3)6]3+ with supporting electrolyte of 0.1 M KCl) should be constantly stored in a refrigerator after preparation. Use a syringe with filter (no larger than 1 µm pore size) to filter the solution before using it.
    6. Click Move to Blind Engage Position. The tip will move back into the buffer solution. Adjust the laser slightly to make sure the laser is aligned on the tip.
    7. Open CHI software. As shown in Figure S4, click on the Technique command on the toolbar to open up the tech selector and select Open Circuit Potential – Time. Use the default setting (Run time as 400 s) for OCP measurement and run the OCP measurement.
      NOTE: The potential showed in OCP test should be near zero stably.
    8. Click the Technique command again and run Cyclic Voltammetry (CV), as shown in Figure S5 and Figure S6.
      NOTE: Set up the parameters as below. Set “sweep segments” to a larger number if needed. The “init E/Final E” should be as the same as the potential value from OCP measurement and “High E” and “Low E” could be +0.3 V or −0.3 V of “ init E/Final E”, respectively. Here we use 0 V as initial and high E and -0.4 V as Low and Final E. The scan rate was 0.05 V/s and the sensitivity was 1 e-009. Run the CV test, the highest current (i) measured here should be 0.3-1.2 nA for 10 mM [Ru(NH3)6]3+.
  5. SECM Imaging
    1. Go back to the AFM-SECM software. Since the tip is already in the liquid, click Engage.
    2. After scanning, turn on lift mode (Lift by Sensor) with a lift height of 100 nm and adjust the lift height based on the sample roughness.
    3. In CHI software, run a chronoamperometry with parameters shown in Figure S7. Set the initial E as -0.4 V, the pulse width as 1000 seconds (which is the maximum number accepted by the system), and the sensitivity the same with CV scan.
      NOTE: The chronoamperometry technique was chosen because of the absence of amperometric i-t technique in the presented bi-potentiostat.
    4. With the CHI program running, go back to AFM-SECM software, check the real-time reading on the strip chart and click on Start (Figure S8). The reading will be updated in real-time. Then both topography imaging and current imaging process will begin. Save images in the AFM-SECM software.
  6. Check approach curve
    1. Engage the tip on sample or substrate region with a scan size of 1 µm.
    2. Run the Chronoamperometry as mentioned in 3.5.3.
    3. Go back to AFM-SECM Software and select the command Go To Ramp.
    4. Click Ramp. An approach curve would be recorded in the AFM-SECM software.
  7. Tip cleaning
    1. Using the EC sample cell as a clean water container. Move the tip in and out of the liquid using blind engage functions in the navigation panel. Change the clean water three times. After this three-time cleaning, using clean wipes to carefully remove residual water from the probe holder and put the probe back in the probe box.
      Caution: After imaging, the AFM-SECM probe needs to be carefully cleaned. Never use water coming out from the wash bottle to clean the probe as the electrostatic charge might damage the probe.

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Results

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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 to...

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Discussion

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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

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Disclosures

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The authors have nothing to disclose.

Acknowledgements

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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.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Equipment
Atomic force microsopyBruker, CADimenison Icon
BipotentiostatCH Instruments, Inc.CHI 700E
Materials
Silicon waferTED PELLA, Inc.16013
Fresh gold platesBruker, CAmodel 119-017-307
PF-SECM-AFM probesBruker, CA990-050138
PF-SECM strain-release moduleBruker, CA840-012-724
PF-SECM Probe HolderBruker, CA900-050121
PF-SECM ChuckBruker, CAPF-SECM Chuck
PF-SECM O-ringBruker, CA598-000-106
PF-SECM cover glass, SECM CellBruker, CA900-050137
EC Cell AssyBruker, CA932-017-300
ESD Field ServiceBruker, CA490-000-066
PF-SECM BootBruker, CA900-050136
Spring connector blockBruker, CA900-050524
PFSECM TweezersBruker, CA
Cable, SECM Tip moduleBruker, CA468-050171
Ag wireBruker, CA249-000-056
Pt wireBruker, CA248-000-004
Hard sharp wireBruker, CATT-ECM10
Tubular ceramic membraneRefractonWFA0.1
Chemicals
Copper(II) chloride dihydrateACROS OrganicsAC315281000
Sodium HydroxideFisher ChemicalS318-100
Ascorbic AcidFisher ChemicalA61-25
EpoxyLoctiteInstant Mix
Potassium ChlorideFisher ChemicalP217-500
Hexaammineruthenium(III) chlorideACROS OrganicsAC363342500

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Scanning Electrochemical MicroscopyAtomic Force MicroscopyNanomaterial Surface AnalysisElectrochemical Activity MappingFaceted NanoparticlesOxygen NanobubblesTip Current ImagingCyclic VoltammetrySample Preparation

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