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Hot-Tip Scanning Electrochemical Microscopy: Theory and Experiments Under Positive and Negative Feedback Conditions Zhiling Zhao, Kevin C. Leonard, and Aliaksei Boika Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05192 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019
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Analytical Chemistry
Hot-Tip
Scanning
Electrochemical
Microscopy:
Theory and Experiments Under Positive and Negative Feedback Conditions Zhiling Zhao,a Kevin C. Leonardb and Aliaksei Boikaa,* a: Department of Chemistry, The University of Akron, Akron, OH 44325. b: Department of Chemical & Petroleum Engineering, Center for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence KS 66045. * Corresponding Author, E-mail:
[email protected] Abstract Hot-tip scanning electrochemical microscopy (HT-SECM) is a novel surface characterization technique utilizing an alternating current (ac) polarized disk microelectrode as a probe. High frequency (ca. 100 MHz) ac waveform applied between the tip and a counter electrode causes the resistive heating of the surrounding electrolyte solution that leads also to the electrothermal fluid flow (ETF). The effects of the temperature and the convection driven by the ETF result in the increased rate of mass transfer of the redox species. In this paper, HT-SECM was studied in positive and negative feedback modes, for which approach curves and cyclic voltammograms were recorded. The experimental data showed that the use of a hot tip leads to a more pronounced feedback compared to that at room temperature. Numerical simulations performed in COMSOL Multiphysics supported the experimental findings. Additional analytical approximations were developed that could be used to predict the faradaic response in HT-SECM experiments. Finally, a possible contribution to the current from the Soret effect was studied theoretically. A good understanding of HT-SECM was achieved, both experimentally and theoretically, suggesting that this methodology could be applied to investigate electrode kinetics under the conditions of the elevated temperature and increased rate of mass transfer.
Introduction As an electrochemical probe technique, Scanning Electrochemical Microscopy (SECM)
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is a powerful tool for studying solution-phase morphology and solution-phase chemical processes in micrometer- and nanometer- sized systems.1 It can probe electron, ion, and molecule transfers, in addition to reactions at solid-liquid, liquid–liquid, and liquid-air interfaces.2 SECM has been used to study the kinetics of several fast electron-transfer reactions (e.g., the oxidation of ferrocenemethanol and ferrocene in water and acetonitrile, respectively, and the reduction of TCNQ in acetonitrile) at solid-liquid interfaces.3 Moreover, SECM can provide local quantitative electrochemical information about catalytic mixtures and materials.4 It was also used to study the redox activity of individual mammalian cells, and the imaging of the living cells was carried out, such as the human breast cells, single protoplast and PC12 neuron cells.5,6,7 Most recently, the resolution of SECM has been improved by using smaller electrodes.8 Nanoelectrodes were used to image individual metal nanoparticles with the size of 1050 nm in a tunneling mode.9 Single solid-state nanopores were imaged by using a nanopipet-supported interfaces between two immiscible electrolyte solutions in an SECM tip.10 As another scanning electrochemical probe microscopy, scanning ion conductance microscopy (SICM) also gives good topographical imaging resolution and can be used to detect surface charge.11 However, SICM is insensitive to electrochemical properties; in combination with SECM, a multifunctional dual-channel scanning probe nanopipette was used to spatially map the uptake of molecules of interest at living cells.12 In SECM, there are two modes of operation: a feedback mode and a collectiongeneration mode.2 In the feedback mode, the current measured at the tip is caused by the electrochemical reaction taking place at the surface of the electrode. As the biased tip approaches a substrate, the redox mediator in solution is either reduced or oxidized, which is determined by both the topography and the electrochemical activity of the substrate electrode. When the tip approaches an insulator, a negative feedback is observed since the diffusion of the redox species to the tip is hindered by the presence of the substrate. However, in the case of a conducting substrate, the tip reaction product can be regenerated at the substrate surface leading to an increased flux and, therefore, an increase in the current; thus, a positive feedback is observed. For collection-
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Analytical Chemistry
generation (GC) modes, apart from the working electrode, the substrate is also held at a certain potential sufficient for the tip reaction product to react, and the current on the substrate electrode is recorded as well. Based on where the electroactive species is generated, there are two types of GC experiments, substrate generation/tip collection (SG/TC) and tip generation/substrate collection (TG/SC) modes. Unlike the feedback mode where experiments are mostly performed in amperometric mode, GC mode can also be carried out in a potentiometric mode and, in this way, mainly used to investigate corrosion process.13,14 Due to a number of positive effects of localized heating on electrode processes, such as faster mass transfer and reaction kinetics, and the electrode ‘cleaning’ effect, the use of heated microelectrodes has been shown to be useful in many areas of electroanalysis.15,16,17 Electrodes can be heated directly by passing alternating current (ac) through their body,18 or indirectly by applying microwave radiation,19 or a high frequency ac waveform.20,21 Depending on the way the heating is achieved, the electrode temperature can be measured either directly or indirectly. Indirect determination involves recording of the shift in the half-wave potential determined from the cyclic voltammetry experiments; the value of the electrode temperature is then calculated using the temperature coefficient of the electrode reaction standard potential.15 The temperature can also be determined directly using thermocouple electrodes,15,22 and calculated based on the theoretical considerations.21,23 In recent years, SECM measurements at elevated temperatures have attracted considerable attention.
22,24,25
Schäfer et al. described a setup which used a Peltier
element as the heating device for the whole solution in the SECM cell. However, the heating pulses applied to the Peltier element led to a low signal-to-noise ratio if they were not synchronized with the current acquisition.25 In addition, heating of the whole cell induced evaporation of water at high temperatures, which could lead to the uncertainty in the solute concentrations. Pan et al. used a thermocouple microelectrode as the tip for the SECM.22 The heating in their setup was caused by the dc power applied to an enameled copper wire that was wrapped around a copper substrate. As a result, the temperature gradient was established in the solution. Similarly to the Schäfer’s
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approach, evaporation of water at high temperatures would happen if a relatively large substrate is used. In addition, Pan et al. used the temperature value measured at the tip thermocouple electrode to evaluate the activation energy of the reaction happening at the substrate. However, because of the temperature gradient, this could introduce systematic errors to their kinetic measurements. We recently reported on the use of microelectrodes polarized by a high frequency ac waveform (hot microelectrodes) as tips for the novel technique called hot-tip SECM (HT-SECM). We described that by using HT-SECM one is capable of distinguishing substrate materials, such as alumina and polystyrene, based on their thermal conductivities. Thus, the technique was applied to the negative feedback mode and the experimental results were compared to the simulations using COMSOL.26 With respect to the other approaches,25,22 HT-SECM generates heat in a small region of the solution that is near the tip surface, without any effect on the bulk. Based on the COMSOL model developed in our previous paper, we achieved a good understanding of the temperature distribution and the electrothermal force in the solution.26 In this paper, we investigated the behavior of HT-SECM in the positive feedback mode when a hot tip is positioned above an electrically conductive substrate such as platinum. The theory of HT-SECM for positive feedback conditions was studied as well, based on the model built in COMSOL and the experimental data. The goal here was to combine the theory for SECM at room temperature with the theory for hot microelectrodes.27,28,29 The theory for SECM feedback at room temperature was originally developed by Kwak and Bard, which applied to tips with thick insulating sheathes (RG≥10).30 Analytical approximations for tips with thinner insulating sheathes (RG
