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Scanning electrochemical microscopy is a scanning probe technique that is based on faradaic current changes as a small electrode is moved across the surface of a sample. The images obtained depend on the sample topography and surface reactivity. The response of the scanning electrochemical microscope is sensitive to the presence of conducting and electroactive species, which makes it useful for imaging heterogeneous surfaces. The principles and instrumentation used to obtain images and surface reaction-kinetic information are discussed, and examples of applications to the study of electrodes, minerals, and biological samples are given.

In 1958 he joined the faculty of The University of Texas at Austin, where he currently holds the Norman Hackerman Regents Chair in Chemistry. Guy Denuault received his DUT from The University of Reims, his MST from the University of Bordeaux, and his Ph.D. from the University of Southampton. His research has focused on the use of ultramicroelectrodes. Chongmok Lee received his B.S. from Seoul National Univeristy in 1981 and his M.S. from Korea Advanced Institute of Science and Technology In 1983. His doctoral research has focused on the use of SECM and ellipsometry In electroanalytical chemistry. Daniel Mandler received his B.Sc. in 1983 and his Ph.D. In 1988 from the Hebrew University of Jerusalem. His doctoral research was in the area of artificial models for photosynthesis. David 0. Wipf received his B.S. from the University of South Dakota in 1984. He received his Ph.D. In 1989 from Indiana University, where he worked on developing high-speed cyclic voltammetry at ultramlcroelectrodes. Guy Denuault, Daniel Mandler, and David 0. Wipf are postdoctoral fellows, and Chongmok Lee Is a doctoral candidate, currently working with Professor Bard at The University of Texas.

The use of fast-scan cyclic voltammetry for the measurement of heterogeneous electron-transfer kinetics has been examined. The distortions caused by the measurement Instrumentation, ohmic drop, and the cell time constant have been considered. These parameters combine to set an upper limit on the scan rate at which undlstorted data can be achieved. At higher scan rates, meaningful data can be obtained and the distortion can be accounted for qualitatively. With the exception of the Instrumental distortion, the upper limit Is Inversely proportional to the radius of a disk or hemispherical electrode. Several outer-sphere electron-transfer couples have been examined. The measured rate for Ru(NH3)e3+ reduction agrees with literature values. In contrast, much larger heterogeneous rates are measured for ferrocene oxidation and Ru(bpy)32+ reduction than have been previously reported. The ability to obtain electrochemical data at a scan rate of 0.5 MV s™1 Is demonstrated for ferrocene oxidation.

The dependence of the SECM feedback current on finite heterogeneous electron-transfer (et) kinetics at the substrate electrode was examined by experimental studies of the reduction of Fe(III) in 1M H2SO4 at a Pt tip over a biased glassycarbon substrate. At the extremes of very fast and very slow et rates, the feedback current was identical to the theoretical response for conducting and insulating substrates. At intermediate et rates, the feedback response depended on the et rate and the tip insulator dimensions. These results are presented as a three-dimensional calibration surface of the feedback current as a function of rate constant and tip-to-substrate distance. At very close tip-to-substrate spacings, heterogeneous rate constants greater than 1 cm s 1 could be distinguished. A model of the behavior of an unbiased substrate is also presented and is used to interpret experimental feedback current results.

Undifferentiated and differentiated PC12 cells were imaged with the constant-distance mode of scanning electrochemical microscopy (SECM) using carbon ring and carbon fiber tips. Two types of feedback signals were used for distance control: the electrolysis current of a mediator (constant-current mode) and the impedance measured by the SECM tip (constant-impedance mode). The highest resolution was achieved using carbon ring electrodes with the constant-current mode. However, the constant-impedance mode has the important advantages that topography and faradaic current can be measured simultaneously, and because no mediator is required, the imaging can take place directly in the cell growth media. It was found that vesicular release events do not measurably alter the impedance, but the depolarizing solution, 105 mM K+, produces a dramatic impedance change such that constant-distance imaging cannot be performed during application of the stimulus. However, by operating the tip in the constant-height mode, cell morphology (via a change in impedance) and vesicular release could be detected simultaneously while moving the tip across the cell. This work represents a significant improvement over previous SECM imaging of model neurons, and it demonstrates that the combination of amperometry and constant-impedance SECM has the potential to be a powerful tool for investigating the spatial distribution of neurotransmitter release in vitro.

A new constant-distance imaging method based on the relationship between tip impedance and tip-substrate separation has been developed for the scanning electrochemical microscope. The tip impedance is monitored by application of a high-frequency ac voltage bias between the tip and auxiliary electrode. The high-frequency ac current is easily separated from the dc-level faradaic electrochemistry with a simple RC filter, which allows impedance measurements during feedback or generation/collection experiments. By employing a piezo-based feedback controller, we are able to maintain the impedance at a constant value and, thus, maintain a constant tip-substrate separation. Application of the method to feedback and generation/collection experiments with tip electrodes as small as 2 microm is presented.

Developing highly efficient, cost effective, and environmentally friendly electrocatalysts for the oxygen reduction (OER), oxygen evolution (ORR), and hydrogen evolution (HER) reactions is of interest for sustainable and clean energy technologies, including metal-air batteries and fuel cells. In this work, the screening of electrocatalytic activities of a series of single metallic iron, cobalt, and nickel nanoparticles and their binary and ternary alloys encapsulated in a graphitic carbon shell towards the OER, ORR, and HER in alkaline media is reported. Synthesis of these compounds proceeds by a two-step sol-gel and carbothermal reduction procedure. Various ex situ characterizations show that with harsh electrochemical This article is protected by copyright. All rights reserved. 2 activation, the graphitic shell undergoes an electrochemical exfoliation. The modified electronic properties of the remaining graphene layers prevent their exfoliation, protect the bulk of the metallic cores, and participate in the electrocatalysis. The amount of near-surface, higher-oxidation-state metals in the as-prepared samples increased with electrochemical cycling, indicating that some metallic nanoparticles are not adequately encased within the graphite shell. Such surface oxide species provide secondary active sites for the electrocatalytic activities. The Ni-Fe binary system gives the most promising results for the OER, and the Co-Fe binary system shows the most promise for ORR and HER.