Draft:Scanning Electrochemical Cell Microscopy

Scanning Electrochemical Cell Microscopy (SECCM) employs an electrolyte filled micropipette or nanopipettes a probe to measure spatially resolved electrochemical processes. SECCM was developed by Dr. Allen Bard and Co-Workers at the University of Texas in 1989. Thanks to this discovery, break throughs in chemistry, biology, and materials sciences have been found. This technique is often used along side other surface characterization methods, such as Atomic Force Microscopy (AFM), Raman Spectroscopy, and Photoluminescence Spectroscopy.

Operation
A theta pipet, both compartments equipped with a quasi-reference counter electrode (QRCE) and electrolyte solution, produces a meniscus at the pointed end of the probe that contacts the surface/working electrode. The QRCE is typically Ag/AgCl wire with Pd-H2 being used as well. Conventional three electrode set up is also used with QRCE split into the auxiliary and reference electrodes when using Cl- free electrolyte at potentials for reducing carbon dioxide. The probe or surface/working electrode is used on a automated positioner for even analysis across the surface, similar to atomic force microscopy (AFM), using voltametric experiments. After analysis, specific locations on the surface can be selected and further analyzed via microscopy or spectroscopy.

Scientific Basis
The double barrel nature of the theta pipet allows for the meniscus to transport a current between both barrels. When contact occurs with the substrate/surface that acts as the working electrode, an alternating conductance (AC) current occurs and is detected. The AC current is observed during an SECCM experiment and is used as a feedback signal.

SECCM is effective at measuring substrates of conducting or semi-conductive nature as there is a generated ultramicroelectrode as one of the QRCE electrodes act as a counter electrode for voltametric and amperometry measurements in direct current. Due to the current made (ion-conductance), voltametric and amperometry analysis can be conducted simultaneously. The dc current can be predicted for a fixed probe by solving the Nernst–Planck equation, and the ac response can also be derived from it.

Substances each have a unique electrochemical potential that produces a characteristic energy output when the circuit between the surface and SECCM tip is completed, allowing for the analysis of elemental composition of the surface. This potential measurement can also change based on the pH of the system, charge of the particular analyte, or oxidation state of a metal being analyzed. Local electrochemical cells, which are essential for SECCM operation, allow surface reactions to be studied on a microscopic scale. This helps to explore heterogeneous catalysis and electrocatalysis phenomena with unprecedented spatial precision, enhancing our understanding of electrode surface reaction mechanisms.

The geometry of the pipet significantly influences the dc conductance current and can be easily measured using microscopy. One of the key features of SECCM is its control over mass transport to the substrate surface, primarily through diffusion and ion migration, especially for charged analytes. This control allows for the transport of charged redox-active analytes to the electrode/solution interface in a well-defined and controllable manner, facilitating the study of heterogeneous electron transfer kinetics. The voltametric response in SECCM is influenced by various factors, which have been determined through simulation and experiment. The technique enables simultaneous quantitative measurements of voltammetry and ion conductance, with the general rule of thumb indicating that the surface contacted by electrolyte is of the order of the pipet probe dimensions.

Modes of operation
Single Nanoparticle Electrochemical Impact (SNEI): Single Nanoparticle Electrochemical Impact (SNEI) measures the current produced by the stochastic collisions of colloidal nanoparticles with the working electrode surface. When nanoparticles interact with the working electrode, a heterogenous electron transfer occurs on its surface, producing a current. The probe measures statically or at a particular point.

Local Ensemble: In local ensemble, the probe encloses individual nanoparticles onto an inert working electrode surface. The size of the ensemble, or number of nanoparticles, is affected by the dimensions of the probe. In order to fully enclose the nanoparticles, the probe should be larger than the nanoparticles being studied. Fully enclosing the nanoparticles allows for the electrochemical surface area to be measured, which can be used for further quantitative analysis. Like SNEI, the probe measures statically.

Electrochemical Imaging: SECCM imaging has two modes: scanning and hopping. Both modes are measured with a dynamic or moving probe unlike SNEI and local ensemble measurements.

Scanning-Mode: The dual channel probe continuously scans across the working electrode surface at a fixed speed with a constant contact distance. The ion conductance current feedback from contact is used to maintain a constant distance between the surface and the tip. Both a lower speed linear scanning pattern and a higher speed spiral scanning pattern have been introduced for SECCM. This measurement is comparable to the contact mode of atomic force microscopy (AFM). Scanning Mode is ideal for acquiring a detailed electrochemical activity map, pivotal for understanding reactions over large surface areas. Hopping Mode, in contrast, is optimized for pinpointing localized phenomena, critical when investigating single catalytic processes or interactions at specific nanoparticle sites. These modes equip researchers with flexible tools to tailor their approach to the unique requirements of their investigative focus, whether broad or highly localized.

Hopping-Mode: The probe moves across the working electrode surface in a stepwise fashion. At each step, contact between the tip and surface is broken and reestablished and an electrochemical measurement, such as voltammetry, amperometry, and potentiometry, is taken. This process both offers more chemical information and a simpler experimental setup than scanning-mode. Hopping-mode is comparable to AFM tapping mode.

Applications
Electrochemical studies: SECCM enables the investigation of electrochemical reactions and processes occurring at solid-liquid interfaces with high spatial resolution. It can be used to study phenomena such as electrodeposition, corrosion, and electrocatalysis at the nanoscale.

Surface characterization: SECCM can be employed for surface imaging and characterization, allowing researchers to visualize surface morphology, topography, and chemical composition with nanometer-scale resolution. This makes it valuable for studying surface modifications, adsorption processes, and surface reactions.

Biological and Biomedical research: SECCM has applications in biological and biomedical research for studying cellular processes, biomolecular interactions, and biological interfaces. It can provide insights into redox reactions in biological systems, neurotransmitter release, and cellular signaling at the nanoscale.

Two-dimensional materials: Two-dimensional materials have been the focus of recent research efforts, with studies delving into the activity found at both the edge and basal surfaces of various substances, such as hexagonal boron nitride (h-BN) and transition metal dichalcogenides like MoS2 and WS2.

Single-particle studies: SECCM can be used for single nanoparticle investigations regarding their individual structure and morphology within an ensemble of them. This is important as individual nanoparticles can show subtle structural differences within a synthesized batch.

Complex electrodes: As electrodes increase in complexity, investigating their electrochemical properties is vital. SECCM is able to probe these properties.

Benefits
SECCM contains a unique meniscus-cell design that provides many benefits over similar instrumentation methods. SECCM offers quantitative electrochemical analysis on a scale that is on a much smaller scale and with the ability to move. The small size of the probe also provides minimal noise, improving signal to noise ratio. Additionally, the makeup of the working electrode such as its material components and size are not limited with SECCM. This allows for materials to be used as working electrodes that might not normally be able to be converted into an electrode format. Another benefit is the ability to control the probe position to target specific sites on the surface. Having a small enough probe, on the nano-scale, allows for measurement of that size on sample. This eliminates the need for macroscopic techniques which average the activity of an entire sample surface. One limitation of SECCM however, is imaging depth. The probe can only go so deep into a surface so other techniques may need to be incorporated to get a full image. In addition, SECCM's precise analytical capabilities help drive the development of energy-related technologies, and understanding the electrochemical properties of the interface can improve battery design and increase fuel cell efficiency.