Selective insulation with poly(tetrafluoroethylene) of substrate electrodes for electrochemical background reduction in scanning electrochemical microscopy

We describe a wet process for the fabrication of poly(tetrafluoroethylene) (PTFE)-covered electrodes in which arrays of holes ( approximately 200 microm) are formed. The PTFE coating provides electrical insulation of most of the electrode surface with selected regions exposed for electrochemical experiments. The arrays of microholes can be controllably patterned and filled with precursor solutions using a piezoelectric dispenser. A micrometer spot of electrocatalyst is produced after reduction of the precursor. The application is tested for scanning electrochemical microscopy (SECM) in the tip generation-substrate collection (TG-SC) studies of electrocatalysts. The method is shown to reduce the substrate background currents that are included in the electrochemical signal read from the local perturbation induced with the SECM tip to the substrate in the TG-SC mode of SECM. This background current reduction is consistent with the decrease in the exposed area of the electrode. The general methodology for the fabrication of the substrate electrodes and two proof-of-concept applications in the TG-SC SECM modality are described.     

Scanning electrochemical microscopy. 55. Fabrication and characterization of micropipet probes

The fabrication and characterization of novel micropipet probes for use in scanning electrochemical microscopy (SECM) are described. These can be used to dispense small (pL) amounts of a solution while monitoring the electrochemical response at a substrate and at a ring electrode tip on the micropipet probe. The probes were constructed by insulating gold-coated borosilicate micropipets with electrophoretic paint and exposing a ring electrode at the tip by heat treatment. Characterization of the probes was performed using scanning electron microscopy, cyclic voltammetry, and SECM approach curve experiments. Routine construction of tips with diameters of the order of 3 microm was possible using this technique. The probes exhibited stable steady-state currents and positive and negative feedback approach curves that agreed with those predicted by theory. Demonstrative SECM imaging experiments were performed using a picodispenser to continuously dispense an electroactive solution (ferrocenemethanol) to the SECM cell while the probe was located within a few micrometers of a Pt substrate surface. Oxidation of the dispensed electroactive solution was performed at the substrate, and feedback currents were measured at the probe tip by holding the gold ring at a reducing potential. This mode of tip-dispensing SECM was used to obtain images of a platinum substrate electrode while monitoring both the substrate current and the feedback current at the probe.     

Scanning electrochemical microscopy, 52. Bipolar conductance technique at ultramicroelectrodes for resistance measurements

The bipolar conductance, BICON, technique for the measurement of solution resistance, based on the application of microsecond current pulses, as originally described by Enke and co-workers for measurements with conventional electrodes, was extended for use with ultramicroelectrodes, with a focus on its application in scanning electrochemical microscopy (SECM). When the plateau time used to make the measurement lies within the BICON conditions, the solution conductance can be obtained directly from the output without the need for calibration curves. However, decreasing the size of the ultramicroelectrode decreases the range of values that satisfy these conditions, and one must resort to calibration curves to obtain solution conductance from the measured current, which was nevertheless found to be proportional to electrolyte concentration with electrodes as small as 5 mum in diameter. BICON/SECM approach curves over insulating substrates followed SECM negative feedback theory and approach curves in the presence of low (micromolar) or no added electrolyte are possible once the background conductivity is taken into account. Approach curves to a conducting substrate at open circuit potential are influenced by the solution time constant (solution resistance at the electrode tip x electrode double layer capacitance), which is a function of the tip/substrate distance, as well as the substrate size.     

Carbon nanofiber electrodes and controlled nanogaps for scanning electrochemical microscopy experiments

The electrochemical behavior of electrodes made by sealing carbon nanofibers in glass or with electrophoretic paint has been studied by scanning electrochemical microscopy (SECM). Because of their small electroactive surface area, conical geometry with a low aspect ratio and high overpotential for proton and oxygen reduction, carbon nanofiber (CNF) electrodes are promising candidates for producing electrode nanogaps, imaging with high spatial resolution and for the electrodeposition of single metal nanoparticles (e.g., Pt, Pd) for studies as electrocatalysts. By using the feedback mode of the SECM, a CNF tip can produce a gap that is smaller than 20 nm from a platinum disk. Similarly, the SECM used in a tip-collection substrate-generation mode, which subsequently shows a feedback interaction at short distances, makes it possible to detect a single CNF by another CNF and then to form a nanometer gap between the two electrodes. This approach was used to image vertically aligned CNF arrays. This method is useful in the detection in a homogeneous solution of short-lifetime intermediates, which can be electrochemically generated at one electrode and collected at the second at distances that are equivalent to a nanosecond time scale.     

Scanning electrochemical microscopy. 60. Quantitative calibration of the SECM substrate generation/tip collection mode and its use for the study of the oxygen reduction mechanism

The substrate generation/tip collection (SG/TC) mode of scanning electrochemical microscopy (SECM) coupled with linear voltammetry is proposed as a way to quantify reaction intermediates generated in the solution at small substrates (100 mum diameter). The collection efficiency (CE) for SG/TC mode depends on the collector tip radius (a), the tip/substrate distance (d), and the size of the insulating glass sheath surrounding the collector tip (RG). In this work, we present experimental and simulated calibration CE values for different SG/TC geometries. Results of digital simulations in axial 2-D symmetry with the tip approaching a planar substrate are shown and fit experimental results obtained using ferrocenemethanol as a redox mediator very well. This model assumes that the mediator reacts under stationary-state conditions and undergoes diffusion-controlled electron transfer without any heterogeneous or homogeneous kinetic complications. Empirical equations for all SG/TC geometries reported here are provided as a convenient way to predict the maximum CE value for any given distance within the calibration range. Hydrogen peroxide quantification during the oxygen reduction reaction (ORR) at a Hg on Au electrode in acid pH was carried out using the SG/TC mode of SECM to demonstrate the utility of this technique in determining the number of electrons transferred (n) in the ORR. The results (n = 2.12-2.19) clearly point out the predominance of the two-electron pathway over the four-electron pathway when ORR takes place at this electrode material. Therefore, this work presents a powerful alternative to the rotating ring-disk electrode (RRDE) as means of obtaining mechanistic information by calculating the number of electrons transferred during an electrochemical reaction.     

Scanning electrochemical microscopy. 46. Shielding effects on reversible and quasireversible reactions

An approximate theory for the feedback mode of the scanning electrochemical microscope (SECM) is developed to interpret the effects of substrate shielding on an ultramicroelectrode tip during a recording of iT versus d curves (approach curves) for reversible and quasireversible kinetics at a substrate surface. The resulting expressions for the tip current, iT, show a good fit to more accurate SECM simulations as well as to the experimental response of a reversible and quasireversible reaction. SECM shielding experiments thus give an interesting new insight into SECM approach curves over electrodes at different potentials, which suggest possible applications to measuring heterogeneous kinetics for fast reactions and diffusion coefficient determination.     

Screening of oxygen evolution electrocatalysts by scanning electrochemical microscopy using a shielded tip approach

Oxygen evolution electrocatalysts in acidic media were studied by scanning electrochemical microscopy (SECM) in the substrate generation-tip collection (SG-TC) imaging mode with a 100 microm diam tip. Pure IrO2 and Sn(1-x)Ir(x)O2 combinatorial mixtures were prepared by a sol-gel route to form arrays of electrocatalyst spots. The experimental setup has been developed to optimize screening of electrocatalyst libraries under conditions where the entire array is capable of the oxygen evolution reaction (OER). The activity of individual spots was determined by reducing the interference from the reaction products of neighboring spots diffusing to the tip over the spot of interest. A gold layer deposited on the external wall of the SECM tip was used as a tip shield. In this study the shield was kept at a constant potential to reduce oxygen under mass transfer controlled conditions. The tip shield consumes oxygen coming from the neighbor spots in the array and enables the tip to correctly detect the activity of the spot below the tip. Simulations and experimental results are shown, demonstrating the effectiveness of the tip shield with the SG-TC setup in determining the properties of the composite materials and imaging arrays.     

Scanning electrochemical microscopy 50. Kinetic study of electrode reactions by the tip generation-substrate collection mode

A scanning electrochemical microscopy (SECM) methodology for localized quantitative kinetic studies of electrode reactions based on the tip generation-substrate collection (TG-SC) operation mode is presented. This approach does not use the mediator feedback required in typical kinetic SECM experiments. The reactant is galvanostatically electrogenerated on a tip placed in proximity to the substrate. It diffuses through the tip-substrate gap and undergoes the reaction of interest on the substrate surface. The substrate current is monitored with time until it reaches an apparent steady-state value. The process was digitally simulated using an explicit finite difference method, for an irreversible first-order electrode reaction at the substrate. Transient responses, steady-state polarization curves, and TG-SC approach curves can be used to obtain substrate kinetics. The effects of the experimental parameters were analyzed. The possibility of easily changing the experimental conditions with the SECM is an attractive approach to obtain independent evidence that can be used for a strict test of reaction mechanisms. The technique was applied for a preliminary simplified kinetic examination of the oxygen reduction reaction in phosphoric acid.     

Local feedback mode of scanning electrochemical microscopy for electrochemical characterization of one-dimensional nanostructure: theory and experiment with nanoband electrode as model substrate

Local feedback mode is introduced as a novel operation mode of scanning electrochemical microscopy (SECM) for electrochemical characterization of a single one-dimensional (1D) nanostructure, for example, a wire, rod, band, and tube with 1-100-nm width and micrometer to centimeter length. To demonstrate the principle, SECM feedback effects under diffusion limitation were studied theoretically and experimentally with a disk probe brought near a semi-infinitely long band electrode as a geometrical model for a conductive 1D nanostructure. As the band becomes narrower than the disk diameter, the feedback mechanism for tip current enhancement is predicted to change from standard positive feedback mode, to positive local feedback mode, and then to negative local feedback mode. The negative local feedback effect is the only feedback effect that allows observation of a 1D nanostructure without serious limitations due to small lateral dimension, available tip size, or finite electron-transfer rate. In line-scan and approach-curve experiments, an unbiased Pt band electrode with 100-nm width and 2.6-cm length was detectable in negative local feedback mode, even using a 25-microm-diameter disk Pt electrode. Using a 2-microm-diameter probe, both well-defined and defected sites were observed in SECM imaging on the basis of local electrochemical activity of the nanoband electrode. Noncontact and spatially resolved measurement is an advantage of this novel SECM approach over standard electrochemical approaches using electrodes based on 1D nanostructure.     

Probing heterogeneous electron transfer at an unbiased conductor by scanning electrochemical microscopy in the feedback mode

The theory of the feedback mode of scanning electrochemical microscopy is extended for probing heterogeneous electron transfer at an unbiased conductor. A steady-state SECM diffusion problem with a pair of disk ultramicroelectrodes as a tip and a substrate is solved numerically. The potential of the unbiased substrate is such that the net current flow across the substrate/solution interface is zero. For a reversible substrate reaction, the potential and the corresponding tip current depend on SECM geometries with respective to the tip radius including not only the tip-substrate distance and the substrate radius but also the thickness of the insulating sheath surrounding the tip. A larger feedback current is obtained using a probe with a thinner insulating sheath, enabling identification of a smaller unbiased substrate with a radius that is approximately as small as the tip radius. An intrinsically slow reaction at an unbiased substrate as driven by a SECM probe can be quasi-reversible. The standard rate constant of the substrate reaction can be determined from the feedback tip current when the SECM geometries are known. The numerical simulations are extended to an SECM line scan above an unbiased substrate to demonstrate a "dip" in the steady-state tip current above the substrate center. The theoretical predictions are confirmed experimentally for reversible and quasi-reversible reactions at an unbiased disk substrate using disk probes with different tip radii and outer radii.     

Combined scanning electrochemical/optical microscopy with shear force and current feedback

A technique that combines scanning electrochemical microscopy (SECM) and scanning optical microscopy (OM) was developed. Simultaneous scanning electrochemical/optical microscopy (SECM/OM) was performed by a special probe tip, which consists of an optical fiber core for light passage, surrounded by a gold ring electrode, and an outermost electrophoretic insulating sheath, with the tip attached to a tuning fork. To regulate the tip-substrate distance, either the shear force or the SECM tip current was employed as the feedback signal. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force allowed simultaneous topographic, along with SECM and optical imaging in a constant-force mode. The capability of this technique was confirmed by obtaining simultaneously, for the first time, topographic, electrochemical, and optical images of an interdigitated array electrode. Current feedback from SECM also provided simultaneous electrochemical and optical images of relatively soft samples, such as a polycarbonate membrane filter and living diatoms in a constant-current mode. This mode should be useful in mapping the biochemical activity of a living cell.     

Scanning electrochemical microscopy of model neurons: constant distance imaging

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.     

Visualization of chlorine evolution at dimensionally stable anodes by means of scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) has been used to detect and visualize the local electrocatalytic activity of dimensionally stable anodes (DSA) for Cl(2) evolution from brine. The sample generation-tip collection (SG-TC) mode of SECM shows limitations arising from complications connected with the reduction of Cl(2) at the SECM tip due to the presence of a significant amount of nondissolved Cl(2) gas. Because only dissolved Cl(2) can be electrochemically reduced at the tip, a large amount of the Cl(2) gas which is produced at active spots of the DSA is not detected. Additionally, a decrease of the cathodic current at the tip may occur owing to the adhesion of gas bubbles and blocking of the electrode surface. To overcome this limitation, the redox competition mode of SECM was extended and applied to the local visualization of Cl(2) evolution from highly concentrated brine solutions. High concentrations of Cl(2) produced at the sample can cause inhibition of the same reaction at the tip by accumulation of Cl(2) in the proximity of the SECM tip. In this way the tip current is decreased, which can be used as a measure for the catalytic activity of the sample underneath the tip.     

纤维状多孔钙锰氧化物制备及其氧还原反应电催化活性

首次以海藻酸钙纤维为载体制得层状-钙钛矿结构的Ca₂MnO₄钙锰氧化物(Ca-Mn-O), 并以尖晶石结构的CaMn₃O₆为对比样, 测试了样品作为电化学氧还原催化剂(ORR)的性能。通过XRD、FE-SEM、TEM和BET对材料进行表征, 结果表明该层状-钙钛矿结构的氧化物具有内部相互交联的多孔网状结构和大的比表面积。电催化测试结果表明: Ca₂MnO₄氧化物具有明显的催化活性, 高的极限扩散电流密度、接近四电子氧化还原以及在碱性溶液中较低产率的过氧化氢, 说明这种廉价而丰富的ORR催化剂具有潜在的应用前景。Ca-Mn-O氧化物的催化活性(初始还原电位、极限电流密度以及电子转移数)很大程度依赖于表面Mn的氧化态和晶体结构。此外, 大的比表面积和多孔网状结构, 增多了氧还原反应的电催化活性位; 高的氧空位浓度有利于氧还原反应的进行; 合适的晶体结构具有开放的空间, 有利于氧的吸附。     

Scanning electrochemical microscopy with slightly recessed nanotips

Slightly recessed nanoelectrodes were prepared by controlled etching of nanometer-sized, flat Pt electrodes. By using high-frequency (e.g., 2 MHz) ac voltage, the layer of Pt as thin as greater, approximately >3 nm was removed to produce a cylindrical cavity inside the insulating glass sheath. The etched electrodes were characterized by combination of voltammetry and scanning electrochemical microscopy (SECM) to determine the radius and the effective depth of the recess. The theory was developed for current versus distance curves obtained with a recessed tip approaching either a conductive or an insulating substrate. Good agreement between the theoretical and experimental approach curves indicated that recessed nanotips are suitable for quantitative feedback mode SECM experiments.     

Noncontact electrochemical imaging with combined scanning electrochemical atomic force microscopy

Combined scanning electrochemical atomic force microscopy (SECM-AFM) is a recently introduced scanned probe microscopy technique where the probe, which consists of a tip electrode and integrated cantilever, is capable of functioning as both a force sensor, for topographical imaging, and an ultramicroelectrode for electrochemical imaging. To extend the capabilities of the technique, two strategies for noncontact amperometric imaging-in conjunction with contact mode topographical imaging-have been developed for the investigation of solid-liquid interfaces. First, SECM-AFM can be used to image an area of the surface of interest, in contact mode, to deduce the topography. The feedback loop of the AFM is then disengaged and the stepper motor employed to retract the tip a specified distance from the sample, to record a current image over the same area, but with the tip held in a fixed x-y plane above the surface. Second, Lift Mode can be employed, where a line scan of topographical AFM data is first acquired in contact mode, and the line is then rescanned to record SECM current data, with the tip maintained at a constant distance from the target interface, effectively following the contours of the surface. Both approaches are exemplified with SECM feedback and substrate generation-tip collection measurements, with a 10-microm-diameter Pt disk UME serving as a model substrate. The approaches described allow electrochemical images, acquired with the tip above the surface, to be closely correlated with the underlying topography, recorded with the tip in intimate contact with the surface.     

Quasi-steady-state voltammetry of rapid electron transfer reactions at the macroscopic substrate of the scanning electrochemical microscope

We report on a novel theory and experiment for scanning electrochemical microscopy (SECM) to enable quasi-steady-state voltammetry of rapid electron transfer (ET) reactions at macroscopic substrates. With this powerful approach, the substrate potential is cycled widely across the formal potential of a redox couple while the reactant or product of a substrate reaction is amperometrically detected at the tip in the feedback or substrate generation/tip collection mode, respectively. The plot of tip current versus substrate potential features the retraceable sigmoidal shape of a quasi-steady-state voltammogram although a transient voltammogram is obtained at the macroscopic substrate. Finite element simulations reveal that a short tip-substrate distance and a reversible substrate reaction (except under the tip) are required for quasi-steady-state voltammetry. Advantageously, a pair of quasi-steady-state voltammograms is obtained by employing both operation modes to reliably determine all transport, thermodynamic, and kinetic parameters as confirmed experimentally for rapid ET reactions of ferrocenemethanol and 7,7,8,8-tetracyanoquinodimethane at a Pt substrate with ～0.5 μm-radius Pt tips positioned at 90 nm-1 μm distances. Standard ET rate constants of ～7 cm/s were obtained for the latter mediator as the largest determined for a substrate reaction by SECM. Various potential applications of quasi-steady-state voltammetry are also proposed.     

Bifunctional Transition Metal Hydroxysulfides: Room‐Temperature Sulfurization and Their Applications in Zn–Air Batteries

Bifunctional electrocatalysis for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) constitutes the bottleneck of various sustainable energy devices and systems like rechargeable metal–air batteries. Emerging catalyst materials are strongly requested toward superior electrocatalytic activities and practical applications. In this study, transition metal hydroxysulfides are presented as bifunctional OER/ORR electrocatalysts for Zn–air batteries. By simply immersing Co‐based hydroxide precursor into solution with high‐concentration S^2?, transition metal hydroxides convert to hydroxysulfides with excellent morphology preservation at room temperature. The as‐obtained Co‐based metal hydroxysulfides are with high intrinsic reactivity and electrical conductivity. The electron structure of the active sites is adjusted by anion modulation. The potential for 10 mA cm^?2 OER current density is 1.588 V versus reversible hydrogen electrode (RHE), and the ORR half‐wave potential is 0.721 V versus RHE, with a potential gap of 0.867 V for bifunctional oxygen electrocatalysis. The Co₃FeS_1.5(OH)₆ hydroxysulfides are employed in the air electrode for a rechargeable Zn–air battery with a small overpotential of 0.86 V at 20.0 mA cm^?2, a high specific capacity of 898 mAh g^?1, and a long cycling life, which is much better than Pt and Ir‐based electrocatalyst in Zn–air batteries.     

A positionable microcell for electrochemistry and scanning electrochemical microscopy in subnanoliter volumes

Positionable voltammetric cells with tip diameters of < 50 microm were constructed from theta glass capillaries. One channel of the pulled glass capillary contains a carbon fiber microelectrode sealed in epoxy while the other houses a Ag/AgCl reference electrode that makes electrical contact to the analyte solution via a salt bridge at the tip. The device can be operated as a two-electrode cell and can therefore make measurements in droplets of solution that are similar in size to the tip. Alternatively, if the droplet of solution is larger than the tip, spatially resolved measurements of a substrate in solution can be made. Voltammetric experiments and feedback imaging with the scanning electrochemical microscope (SECM) were accomplished in microdroplets with solution volumes of less than 1 nL. pH images of a substrate immersed in 70-microL-thick films of solution were obtained in the generator-collector mode of SECM using an iridium oxide-modified microcell. This type of microcell is particularly useful for making electrochemical measurements in very small droplets of solution where a mobile working electrode could easily collide with a separately positioned reference electrode.     

Fabrication and characterization of a nanometer-sized optical fiber electrode based on selective chemical etching for scanning electrochemical/optical microscopy

We have already reported a method for fabricating ultramicroelectrodes (Suzuki, K. JP Patent, 2004-45394, 2004). This method is based on the selective chemical etching of optical fibers. In this work, we undertake a detailed investigation involving a combination of etched optical fibers with various types of tapered tip (protruding-shape, double- (or pencil-) shape and triple-tapered electrode) and insulation with electrophoretic paint. Our goal is to establish a method for fabricating nanometer-sized optical fiber electrodes with high reproducibility. As a result, we realized pencil-shaped and triple-tapered electrodes that had radii in the nanometer range with high reproducibility. These nanometer-sized electrodes showed well-defined sigmoidal curves and stable diffusion-limited responses with cyclic voltammetry. The pencil-shaped optical fiber, which has a conical tip with a cone angle of 20 degrees , was effective for controlling the electrode radius. The pencil-shaped electrodes had higher reproducibility and smaller electrode radii (r(app) < 1.0 nm) than those of other etched optical fiber electrodes. By using a pencil-shaped electrode with a 105-nm radius as a probe, we obtained simultaneous electrochemical and optical images of an implantable interdigitated array electrode. We achieved nanometer-scale resolution with a combination of scanning electrochemical microscopy SECM and optical microscopy. The resolution of the electrochemical and optical images indicated sizes of 300 and 930 nm, respectively. The neurites of living PC12 cells were also successfully imaged on a 1.6-microm scale by using the negative feedback mode of an SECM.