Effect of the mediator in feedback mode-based SECM interrogation of indium tin-oxide and boron-doped diamond electrodes

Nanoscale Reactivity Mapping of a Single-Crystal Boron-Doped Diamond Particle

Boron-doped diamond (BDD) is most often grown by chemical vapor deposition (CVD) in polycrystalline form, where the electrochemical response is averaged over the whole surface. Deconvoluting the impact of crystal orientation, surface termination, and boron-doped concentration on the electrochemical response is extremely challenging. To tackle this problem, we use CVD to grow isolated single-crystal microparticles of BDD with the crystal facets (100, square-shaped) and (111, triangle-shaped) exposed and combine with hopping mode scanning electrochemical cell microscopy (HM-SECCM) for electrochemical interrogation of the individual crystal faces (planar and nonplanar). Measurements are made on both hydrogen- (H-) and oxygen (O-)-terminated single-crystal facets with two different redox mediators, [Ru(NH₃)₆]^3+/2+ and Fe(CN)₆^4-/3-. Extraction of the half-wave potential from linear sweep and cyclic voltammetric experiments at all measurement (pixel) points shows unequivocally that electron transfer is faster at the H-terminated (111) surface than at the H-terminated (100) face, attributed to boron dopant differences. The most dramatic differences were seen for [Ru(NH₃)₆]^3+/2+ when comparing the O-terminated (100) surface to the H-terminated (100) face. Removal of the H-surface conductivity layer and a potential-dependent density of states were thought to be responsible for the behavior observed. Finally, a bimodal distribution in the electrochemical activity on the as-grown H-terminated polycrystalline BDD electrode is attributed to the dominance of differently doped (100) and (111) facets in the material.

Nanoscale redox mapping at the MoS2-liquid interface

Layered MoS₂ is considered as one of the most promising two-dimensional photocatalytic materials for hydrogen evolution and water splitting; however, the electronic structure at the MoS₂-liquid interface is so far insufficiently resolved. Measuring and understanding the band offset at the surfaces of MoS₂ are crucial for understanding catalytic reactions and to achieve further improvements in performance. Herein, the heterogeneous charge transfer behavior of MoS₂ flakes of various layer numbers and sizes is addressed with high spatial resolution in organic solutions using the ferrocene/ferrocenium (Fc/Fc⁺) redox pair as a probe in near-field scanning electrochemical microscopy, i.e. in close nm probe-sample proximity. Redox mapping reveals an area and layer dependent reactivity for MoS₂ with a detailed insight into the local processes as band offset and confinement of the faradaic current obtained. In combination with additional characterization methods, we deduce a band alignment occurring at the liquid-solid interface.

Here, high-resolution atomic force microscopy and scanning electrochemical microscopy are used to investigate the electron transfer behaviour of layered MoS₂ flakes in organic solutions, offering insights on the electronic band alignment at the solid-liquid interface.

A cost-efficient approach for simultaneous scanning electrochemical microscopy and scanning ion conductance microscopy

A novel and cost-efficient probe fabrication method yielding probes for performing simultaneous scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM) is presented. Coupling both techniques allows distinguishing topographical and electrochemical activity information obtained by SECM. Probes were prepared by deposition of photoresist onto platinum-coated, pulled fused silica capillaries, which resulted in a pipette probe with an integrated ring ultramicroelectrode. The fabricated probes were characterized by means of cyclic voltammetry and scanning electron microscopy. The applicability of probes was demonstrated by measuring and distinguishing topography and electrochemical activity of a model substrate. In addition, porous boron-doped diamond samples were investigated via simultaneously performed SECM and SICM.

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Combined scanning electrochemical and fluorescence microscopies using a tetrazine as a single redox and luminescent (electrofluorochromic) probe

The possibility of using a single electroactive and luminescent molecule both as a redox mediator and as a fluorophore in an experiment combining in situ Scanning Electrochemical Microscopy (SECM) and epifluorescence microscopy was validated. The usual working modes of SECM, namely positive and negative feedback as well as generation-collection, were used and the fluorescence images, intensity and spectra were recorded for each configuration. The tip potential, tip-substrate distance and, in the case of a conducting substrate, the substrate potential are the parameters that are likely to control the fluorescence. It is shown that the tip can be used to switch on and off the luminescence and that the modulation amplitude maximum is sensitive to the nature of the substrate. Approach curves based on this fluorescence modulation amplitude can be obtained showing a higher sensitivity than the classical electrochemical ones.

Comparison of fast electron transfer kinetics at platinum, gold, glassy carbon and diamond electrodes using Fourier-transformed AC voltammetry and scanning electrochemical microscopy

Complementary techniques reveal new insights on electron transfer rates at different electrode materials.

Scanning electrochemical cell microscopy: a versatile technique for nanoscale electrochemistry and functional imaging

Scanning electrochemical cell microscopy (SECCM) is a new pipette-based imaging technique purposely designed to allow simultaneous electrochemical, conductance, and topographical visualization of surfaces and interfaces. SECCM uses a tiny meniscus or droplet, at the end of a double-barreled (theta) pipette, for high-resolution functional imaging and nanoscale electrochemical measurements. Here we introduce this technique and provide an overview of its principles, instrumentation, and theory. We discuss the power of SECCM in resolving complex structure-activity problems and provide considerable new information on electrode processes by referring to key example systems, including graphene, graphite, carbon nanotubes, nanoparticles, and conducting diamond. The many longstanding questions that SECCM has been able to answer during its short existence demonstrate its potential to become a major technique in electrochemistry and interfacial science.

Active sites for outer-sphere, inner-sphere, and complex multistage electrochemical reactions at polycrystalline boron-doped diamond electrodes (pBDD) revealed with scanning electrochemical cell microscopy (SECCM)

The local rate of heterogeneous electron transfer (HET) at polycrystalline boron-doped diamond (pBDD) electrodes has been visualized at high spatial resolution for various aqueous electrochemical reactions, using scanning electrochemical cell microscopy (SECCM), which is a technique that uses a mobile pipet-based electrochemical cell as an imaging probe. As exemplar systems, three important classes of electrode reactions have been investigated: outer-sphere (one-electron oxidation of ferrocenylmethyltrimethylammonium (FcTMA(+))), inner-sphere (one-electron oxidation of Fe(2+)), and complex processes with coupled electron transfer and chemical reactions (oxidation of serotonin). In all cases, the pattern of reactivity is similar: the entire pBDD surface is electroactive, but there are variations in activity between different crystal facets which correlate directly with differences in the local dopant level, as visualized qualitatively by field-emission scanning electron microscopy (FE-SEM). No evidence was found for enhanced activity at grain boundaries for any of the reactions. The case of serotonin oxidation is particularly interesting, as this process is known to lead to deterioration of the electrodes, because of blocking by reaction products, and therefore cannot be studied with conventional scanning electrochemical probe microscopy (SEPM) techniques. Yet, we have found this system nonproblematic to study, because the meniscus of the scanning pipet is only in contact with the surface investigated for a brief time and any blocking product is left behind as the pipet moves to a new location. Thus, SECCM opens up the possibility of investigating and visualizing much more complex heterogeneous electrode reactions than possible presently with other SEPM techniques.

SECM investigations of immobilized porphyrins films

Electronic properties of electrogenerated Zn-porphyrin layers linked by an electroactive linker and immobilized on a semitransparent ITO electrode were investigated by steady-state SECM in unbiased conditions in view of the numerous possible applications of such surface. This SECM strategy took advantage of the variations of the charge transfer kinetics of the organic redox couple (the mediator used in SECM) on ITO surface with the standard potential of the mediator. After preliminary characterization of nonmodified ITO, analysis of the SECM approach curves recorded with a series of redox mediators allows the characterizations of both film permeability and charge transport inside the organic film in conditions close to a "real optoelectronic device". Two types of porphyrin films were considered. In the first one, the film was produced by electropolymerization of a modified zinc-β-octaethylporphyrin in which the bipyridinium pendant substituent is first introduced. The second type of film was prepared directly from an in situ electropolymerization method in which the Zn porphyrin is simply oxidized in the presence of 4,4'-bipyridine. Experiments show the occurrence of efficient charge transport inside both films after initial reduction of the electroactive linker. However, the first preparation method leads to films with stronger blocking character versus organic molecules and higher charge injection rates.

Electro-oxidative nanopatterning of silane monolayers on boron-doped diamond electrodes

Oxidized boron-doped diamond (BDD) electrodes were coated with a monolayer of n-octadecyltrichlorosilane as well as of other silanes. Scanning force microscopy was applied to pattern these monolayers, utilizing doped diamond-coated conductive probes. Patterns were generated on the nanometer scale, and conditions for the patterning process were quantified with regard to humidity and potential bias. It was observed that a sample bias of 3-3.5 V and a relative humidity >70% are necessary to generate reproducible and stable patterns. At potentials and relative humidities below these values, no or incomplete removal of the monolayer occurred. The results show that electro-oxidative patterning is an expedient way for the generation of nanostructures on chemically modified BDD.