Name: probing surface electrochemical activity of nanomaterials using a hybrid atomic force microscope-scanning electrochemical microscope (afm-secm)
Uploaded: 2021-02-10T12:30:00
Description: Watch this Scientific Journal Video about Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope AFM-SECM at JoVE.com

This work is funded by the national science foundation (Award Number: 1756444) via Biological &#38; 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 &#38; Phase-2 at New Jersey Institute of Technology.

1. Sample preparation
<ol>
	<li>Preparation of faceted Cu2O nanoparticles and deposition on silicon substrate
	<ol>
		<li>Dissolve 0.175 g of CuCl2&#8729;2H2O (99.9%) into 100 mL of deionized (DI) water to generate an aqueous solution of 10 mM CuCl2.</li>
		<li>Add 10.0 mL of 2.0 M NaOH and 10 mL of 0.6 M ascorbic acid dropwise into the CuCl2 solution.</li>
		<li>Heat the solution in a 250 mL round-bottom flask under constant stirring in a 55 &#176...

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</...

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 (&#181;m) 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 &#60;1 &#956;m (also known as ultrafine bubbles)43, and they elicit many intriguing properties44,45, including long residence times in the solutions46,47 and high&#160;efficiency of gas mass transfer46,48. Furthermore, the collapse of NBs creates shock waves and the formation of hydroxyl radicals (&#8226;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.

<table>
	<tbody>
		<tr>
			<td>Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Atomic force microsopy</td>
			<td>Bruker, CA</td>
			<td>Dimenison Icon</td>
			<td></td>
		</tr>
		<tr>
			<td>Bipotentiostat</td>
			<td>CH Instruments, Inc.</td>
			<td>CHI 700E</td>
			<td></td>
		</tr>
		<tr>
			<td>Materials</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Silicon wafer</td>
			<td>TED PELLA, Inc.</td>
			<td>16013</td>
			<td></td>
		</tr>
		<tr>
			<td>Fresh gold plates</td>
			<td>Bruker, CA</td>
			<td>model 119-017-307</td>
			<td></td>
		</tr>
		<tr>
			<td>PF-SECM-AFM probes</td>
			<td>Bruker, CA</td>
			<td>990-050138</td>
			<td></td>
		</tr>
		<tr>
			<td>PF-SECM strain-release module</td>
			<td>Bruker, CA</td>
			<td>840-012-724</td>
			<td></td>
		</tr>
		<tr>
			<td>PF-SECM Probe Holder</td>
			<td>Bruker, CA</td>
			<td>900-050121</td>
			<td></td>
		</tr>
		<tr>
			<td>PF-SECM Chuck</td>
			<td>Bruker, CA</td>
			<td>PF-SECM Chuck</td>
			<td></td>
		</tr>
		<tr>
			<td>PF-SECM O-ring</td>
			<td>Bruker, CA</td>
			<td>598-000-106</td>
			<td></td>
		</tr>
		<tr>
			<td>PF-SECM cover glass, SECM Cell</td>
			<td>Bruker, CA</td>
			<td>900-050137</td>
			<td></td>
		</tr>
		<tr>
			<td>EC Cell Assy</td>
			<td>Bruker, CA</td>
			<td>932-017-300</td>
			<td></td>
		</tr>
		<tr>
			<td>ESD Field Service</td>
			<td>Bruker, CA</td>
			<td>490-000-066</td>
			<td></td>
		</tr>
		<tr>
			<td>PF-SECM Boot</td>
			<td>Bruker, CA</td>
			<td>900-050136</td>
			<td></td>
		</tr>
		<tr>
			<td>Spring connector block</td>
			<td>Bruker, CA</td>
			<td>900-050524</td>
			<td></td>
		</tr>
		<tr>
			<td>PFSECM Tweezers</td>
			<td>Bruker, CA</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Cable, SECM Tip module</td>
			<td>Bruker, CA</td>
			<td>468-050171</td>
			<td></td>
		</tr>
		<tr>
			<td>Ag wire</td>
			<td>Bruker, CA</td>
			<td>249-000-056</td>
			<td></td>
		</tr>
		<tr>
			<td>Pt wire</td>
			<td>Bruker, CA</td>
			<td>248-000-004</td>
			<td></td>
		</tr>
		<tr>
			<td>Hard sharp wire</td>
			<td>Bruker, CA</td>
			<td>TT-ECM10</td>
			<td></td>
		</tr>
		<tr>
			<td>Tubular ceramic membrane</td>
			<td>Refracton</td>
			<td>WFA0.1</td>
			<td></td>
		</tr>
		<tr>
			<td>Chemicals</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Copper(II) chloride dihydrate</td>
			<td>ACROS Organics</td>
			<td>AC315281000</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Hydroxide</td>
			<td>Fisher Chemical</td>
			<td>S318-100</td>
			<td></td>
		</tr>
		<tr>
			<td>Ascorbic Acid</td>
			<td>Fisher Chemical</td>
			<td>A61-25</td>
			<td></td>
		</tr>
		<tr>
			<td>Epoxy</td>
			<td>Loctite</td>
			<td>Instant Mix</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium Chloride</td>
			<td>Fisher Chemical</td>
			<td>P217-500</td>
			<td></td>
		</tr>
		<tr>
			<td>Hexaammineruthenium(III) chloride</td>
			<td>ACROS Organics</td>
			<td>AC363342500</td>
			<td></td>
		</tr>
	</tbody>
</table>

probing surface electrochemical activity of nanomaterials using a hybrid atomic force microscope-scanning electrochemical microscope (afm-secm)

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.

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 identi&#64257;ed in Figure 9, which provides...

Watch this Scientific Journal Video about Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope AFM-SECM at JoVE.com

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

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.

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

Scanning electrochemical microscopy (SECM) is used to measure the local electrochemical behavior of liquid/solid, liquid/gas and liquid/liquid interfaces. ...

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