General Method for Fitting Kinetics from the SECM Images of Reactive Sites on Flat Surfaces.

Scanning electrochemical microscopy (SECM) is a technique for imaging electrochemical reactions at a surface. The interaction between electrochemical reactions occurring at the sample and scanning electrode tip is quite complicated and requires computer modeling to obtain quantitative information from SECM images. Often, existing computer models must be modified, or a new model must be created from scratch to fit kinetic parameters for different reactive features. This work presents a method that can simulate the SECM image of a reactive feature of any shape on a flat surface which is coupled to a computer program which effectuates the automated fitting of kinetic information from these images. This fitting program is evaluated along with several methods for estimating the shapes of reactive features from their SECM images. Estimates of the reactive feature shape from SECM images were not sufficiently accurate and produced median relative errors for the surface rate constant that were >50%. Fortunately, more precise techniques for imaging the reactive features such as optical microscopy can supply sufficiently accurate shapes for the fitting procedure to produce accurate results. Fits of simulated SECM images using the actual shape from the simulation produced median relative errors for the surface rate constant that were <10% for the smallest reactive features tested. This method was applied to the SECM images of aluminum alloy AA7075 which revealed diffusion-limited kinetics for ferrocene methanol reduction over inclusions in the surface of the alloy.

Effect of Redox Mediators on Corrosion Behavior and Scanning Electrochemical Microscopy Response.

Scanning electrochemical microscopy (SECM) is widely used to measure local electrochemical reactivity of corroding surfaces. A major criticism of using SECM in feedback mode for corrosion studies is the requirement of an external redox mediator (RM) as it could react with the metal and affect the Nernst potential at the metal-solution interface. Consequently, it becomes challenging to differentiate the interference caused by the RM from the local reactivity of the metal. Herein, a multiscale electrochemical approach is presented to investigate the effect of RM choice on the corroding substrate. Two common RMs, ferrocenemethanol and hexaammineruthenium(III) chloride, were used to perform SECM over copper and aluminum. It was found during macroscale electrochemical measurements that Ru(NH)63+ acted as an oxidant and promoted corrosion. The SECM feedback behavior varied for copper depending on the RM used, suggesting that the corrosion reactions controlled the negative feedback mechanism, not the formation of an insulating passive film. The passivated aluminum surface consistently exhibited negative feedback, regardless of the RM used. SECM approach curves also displayed a distortion in the steady state current, which was caused by the deposition of substrate-generated species on the microelectrode. These deviations in feedback response were accounted for during analysis through incorporation into a finite element model to accurately extract the RM kinetic rate constants. The importance of understanding these processes is highlighted to avoid misinterpretation of passive behavior and advances toward a more quantitative use of SECM for corrosion studies.

Spatially Resolved Optical Spectroscopic Measurements with Simultaneous Photoelectrochemical Mapping Using Scanning Electrochemical Probe Microscopy.

A deep understanding of the properties of semiconductor films at the micro-/nanoscale level is fundamental toward designing effective photoelectrocatalysts. Here, we integrated spatially resolved optical spectroscopy (SR-OS) with scanning photoelectrochemical microscopy (SPECM) to collect UV/vis spectra and quantify photocurrents of localized sites on a nanostructured BiVO4 thin film. Direct measurement of absorbance allowed for the determination of band gap energy at each location. Absorbance and photocurrent maps were obtained and used to investigate heterogeneities on the films. Scanning electrochemical cell microscopy (SECCM) was also coupled with SR-OS to acquire quantitative photoelectrochemical data at the FTO/BiVO4 film interface, revealing higher photocurrents at the boundary regions. As a droplet-based technique, SECCM was employed to estimate the wetted area by measuring the maximum height of droplet stretch at each point, allowing for the calculation of photocurrent density. This novel approach provides an advantageous mean to correlate localized photocatalytic activities and band gap energies.

Micro-Sized pH Sensors Based on Scanning Electrochemical Probe Microscopy.

Monitoring pH changes at the micro/nano scale is essential to gain a fundamental understanding of surface processes. Detection of local pH changes at the electrode/electrolyte interface can be achieved through the use of micro-/nano-sized pH sensors. When combined with scanning electrochemical microscopy (SECM), these sensors can provide measurements with high spatial resolution. This article reviews the state-of-the-art design and fabrication of micro-/nano-sized pH sensors, as well as their applications based on SECM. Considerations for selecting sensing probes for use in biological studies, corrosion science, in energy applications, and for environmental research are examined. Different types of pH sensitive probes are summarized and compared. Finally, future trends and emerging applications of micro-/nano-sized pH sensors are discussed.

Fitting Kinetics from Scanning Electrochemical Microscopy Images of Finite Circular Features.

Scanning electrochemical microscopy (SECM) is a powerful technique for imaging the electrochemical reactivity of a surface. Unfortunately, SECM images are mainly used qualitatively. Kinetics of reactions at the surface are almost exclusively obtained from the microelectrode current as it approaches the surface, called an approach curve. The approach curve method is excellent when the reaction at the surface has the same kinetics everywhere, but was not designed to fit the kinetics of finite-sized reactive features. We propose a method for extracting kinetics, feature area, and microelectrode tip-to-substrate distance from SECM images by fitting with simulated images of reactive discs using the Levenberg-Marquardt algorithm. The area of experimental reactive features can be fit to within 10% if the underlying feature is roughly disc-shaped. When the reaction at simulated reactive features is activation-limited, the rate constant can be fit to within 15% of the true value. This work heralds the beginning of quantifying kinetics from SECM images.