Electrochemical atomic force microscopy using a tip-attached redox mediator for topographic and functional imaging of nanosystems

The Influence of Cathode Degradation Products on the Anode Interface in Lithium-Ion Batteries

Degradation of cathode materials in lithium-ion batteries results in the presence of transition metal ions in the electrolyte, and these ions are known to play a major role in capacity fade and cell failure. Yet, while it is known that transition metal ions migrate from the metal oxide cathode and deposit on the graphite anode, their specific influence on anode reactions and structures, such as the solid electrolyte interphase (SEI), is still quite poorly understood due to the complexity in studying this interface in operational cells. In this work we combine operando electrochemical atomic force microscopy (EC-AFM), electrochemical quartz crystal microbalance (EQCM), and electrochemical impedance spectroscopy (EIS) measurements to probe the influence of a range of transition metal ions on the morphological, mechanical, chemical, and electrical properties of the SEI. By adding representative concentrations of Ni2+, Mn2+, and Co2+ ions into a commercially relevant battery electrolyte, the impacts of each on the formation and stability of the anode interface layer is revealed; all are shown to pose a threat to battery performance and stability. Mn2+, in particular, is shown to induce a thick, soft, and unstable SEI layer, which is known to cause severe degradation of batteries, while Co2+ and Ni2+ significantly impact interfacial conductivity. When transition metal ions are mixed, SEI degradation is amplified, suggesting a synergistic effect on the cell stability. Hence, by uncovering the roles these cathode degradation products play in operational batteries, we have provided a foundation upon which strategies to mitigate or eliminate these degradation products can be developed.

Nano-confined electrochemical reaction studied by electrochemical surface forces apparatus

Electrochemical reactions in a nano-space are different from those in bulk solutions due to structuring of the liquid molecules and peculiar ion behavior at the electric double layer and are important for applications involving sensors and energy devices. The electrochemical surface forces apparatus (EC-SFA) we developed enabled us to study the electrochemical reactions in a solution nano-confined between the electrodes with varying distance (D) at nm resolution. We recorded measurements of the current-distance profiles due to the electrochemical reaction of the redox couples in the electrolyte nano-confined between Pt electrodes using our EC-SFA. We observed a long-range feedback current due to redox cycling and the sudden current increase at a short distance, the latter for the first time. This sudden current increase was two orders greater than the conventional feedback current and was observed at D < 5 nm when the electrodes were approaching and D < 200 nm on separation. We simultaneously measured the electric double layer force and the current between the electrodes in the solution to study the mechanisms of this sudden current increase in the short distance range. The results revealed a molecular insight as to how the redox species affect the current between two electrodes under nano-confinement. This study demonstrated that EC-SFA is a powerful tool for obtaining fundamental knowledge about the nano-confined electrochemical reactions for nanoelectrodes which can be applied to sensors and energy devices.

Redox Switchable Polydopamine-Modified AFM-SECM Probes: A Probe for Electrochemical Force Spectroscopy

Polydopamine (PDA) has high potential in biorelevant applications as a versatile thin film material, e.g., as adhesive coating for cell immobilization or for sensing applications due to the plethora of functional groups. In this study we present the modification of conductive colloidal atomic force-scanning electrochemical microscopy (AFM-SECM) probes with electrochemically deposited PDA resulting in functional probes for quantitative electrochemical adhesion studies. Surface functionality of PDA can be altered by oxidation or reduction of functional groups applying an appropriate potential to the PDA-modified AFM-SECM probe, thereby enabling adhesion measurements under potential control. This facilitates probing specific interactions of surface groups present in PDA with various surfaces of different wettabilities. The versatility of such switchable AFM-SECM probes is demonstrated for electrochemical force spectroscopic studies at model samples such as plasma-treated gold substrates, hydrophobic or hydrophilic self-assembled monolayers, and for adhesion measurements of bacteria in dependence of altered surface charges of the colloidal probe. The maximum obtained adhesion force of a positively polarized PDA-modified AFM-SECM probe was 6.2 ± 2.2 nN, and it was about 50% less (i.e., 2.6 ± 1.1 nN) for a negatively polarized probe at a hydrophilic OH-terminated gold surface. In situ control of the active surface groups enabled investigations on the influence of surface charges on adhesion. Furthermore, plateaus of constant force were observed, which are a characteristic of polymer structures. Finally, electrochemical force measurements with switchable probes were used for the first time during adhesion studies of bacterial cells (i.e., Pseudomonas fluorescens). Positively biased PDA-coated colloidal probes revealed adhesion forces of 6.0 ± 1.1 nN, whereas significantly reduced adhesion forces 1.1 ± 0.7 nN were observed for negatively biased PDA-modified colloidal probes.

Multifunctional Boron-Doped Diamond Colloidal AFM Probes

Scanning probe microscopy techniques providing information on conductivity, chemical fluxes, and interfacial reactivity synchronized with topographical information have gained importance within the last decades. Herein, a novel colloidal atomic force microscopy (AFM) probe is presented using a spherical boron-doped diamond (BDD) electrode attached and electrically connected to a modified silicon nitride cantilever. These conductive spherical BDD-AFM probes allow for electrochemical force spectroscopy. The physical robustness of these bifunctional probes, and the excellent electrochemical properties of BDD renders this concept a unique multifunctional tool for a wide variety of scanning probe studies including conductive AFM, hybrid atomic force-scanning electrochemical microscopy, and tip-integrated chem/bio sensing.

Scanning electrochemical microscopy with conducting polymer probes: Validation and applications

Scanning electrochemical microscopy (SECM) allows spatially and temporally resolved measurements of a broad range of reactive surfaces and specimens, typically using electrochemically active metal probes. While conducting polymers (CPs) present several analytical properties of interest due to their chemical versatility, potentially enabling the measurement of ionic fluxes as well as redox processes, they have not been widely used as probe materials for SECM. CPs can be modified and fine-tuned to improve experimental parameters and they can be easily prepared by electrodeposition. In this paper, we show a new type of CP probe for SECM that retains the spatial resolution of conventional metal probes and introduces the possibility to exploit a wide range of ionic and redox systems. Poly-3,4-ethylenedioxythiophene (PEDOT) was electrochemically deposited on flat and recessed Pt microdisks to generate CP SECM probes. To demonstrate their usefulness, an insulating substrate with conducting features was imaged. Well-defined SECM feedback images were observed for both the CP well-probe and the Pt probe, proving the efficiency of the new electrode to image redox reactions. Additionally, an organosulfur compound was used as mediator taking advantage of the electrocatalytic effect PEDOT has on the molecule's kinetics. Finally, these probes were also used in a mediator-less fashion, taking advantage of the ion flux required to electrochemically oxidize the PEDOT deposit. We investigated the impact of anion size and concentration on current-distance relationships for SECM probe positioning. CP probes pose exciting prospects for the imaging and measurement of combined redox and ionic processes in energy materials.

(Multi)functional Atomic Force Microscopy Imaging

Incorporating functionality to atomic force microscopy (AFM) to obtain physical and chemical information has always been a strong focus in AFM research. Modifying AFM probes with specific molecules permits accessibility of chemical information via specific reactions and interactions. Fundamental understanding of molecular processes at the solid/liquid interface with high spatial resolution is essential to many emerging research areas. Nanoscale electrochemical imaging has emerged as a complementary technique to advanced AFM techniques, providing information on electrochemical interfacial processes. While this review presents a brief introduction to advanced AFM imaging modes, such as multiparametric AFM and topography recognition imaging, the main focus herein is on electrochemical imaging via hybrid AFM-scanning electrochemical microscopy. Recent applications and the challenges associated with such nanoelectrochemical imaging strategies are presented.

Estimation of π-π Electronic Couplings from Current Measurements

The π-π interactions between organic molecules are among the most important parameters for optimizing the transport and optical properties of organic transistors, light-emitting diodes, and (bio-) molecular devices. Despite substantial theoretical progress, direct experimental measurement of the π-π electronic coupling energy parameter t has remained an old challenge due to molecular structural variability and the large number of parameters that affect the charge transport. Here, we propose a study of π-π interactions from electrochemical and current measurements on a large array of ferrocene-thiolated gold nanocrystals. We confirm the theoretical prediction that t can be assessed from a statistical analysis of current histograms. The extracted value of t ≈35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the subangström level. The present work establishes a direct bridge between quantum chemistry, electrochemistry, organic electronics, and mesoscopic physics, all of which were used to discuss results and perspectives in a quantitative manner.

Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging

Multimodal nano-imaging in electrochemical environments is important across many areas of science and technology. Here, scanning electrochemical microscopy (SECM) using an atomic force microscope (AFM) platform with a nanoelectrode probe is reported. In combination with PeakForce tapping AFM mode, the simultaneous characterization of surface topography, quantitative nanomechanics, nanoelectronic properties, and electrochemical activity is demonstrated. The nanoelectrode probe is coated with dielectric materials and has an exposed conical Pt tip apex of ∼200 nm in height and of ∼25 nm in end-tip radius. These characteristic dimensions permit sub-100 nm spatial resolution for electrochemical imaging. With this nanoelectrode probe we have extended AFM-based nanoelectrical measurements to liquid environments. Experimental data and numerical simulations are used to understand the response of the nanoelectrode probe. With PeakForce SECM, we successfully characterized a surface defect on a highly-oriented pyrolytic graphite electrode showing correlated topographical, electrochemical and nanomechanical information at the highest AFM-SECM resolution. The SECM nanoelectrode also enabled the measurement of heterogeneous electrical conductivity of electrode surfaces in liquid. These studies extend the basic understanding of heterogeneity on graphite/graphene surfaces for electrochemical applications.

Fabrication of Scanning Electrochemical Microscopy-Atomic Force Microscopy Probes to Image Surface Topography and Reactivity at the Nanoscale

Concurrent mapping of chemical reactivity and morphology of heterogeneous electrocatalysts at the nanoscale allows identification of active areas (protrusions, flat film surface, or cracks) responsible for productive chemistry in these materials. Scanning electrochemical microscopy (SECM) can map surface characteristics, record catalyst activity, and identify chemical products at solid-liquid electrochemical interfaces. It lacks, however, the ability to distinguish topographic features where surface reactivity occurs. Here, we report the design and fabrication of scanning probe tips that combine SECM with atomic force microscopy (AFM) to perform measurements at the nanoscale. Our probes are fabricated by integrating nanoelectrodes with quartz tuning forks (QTFs). Using a calibration standard fabricated in our lab to test our probes, we obtain simultaneous topographic and electrochemical reactivity maps with a lateral resolution of 150 nm.

Nanoscale Electrochemistry of sp(2) Carbon Materials: From Graphite and Graphene to Carbon Nanotubes

Carbon materials have a long history of use as electrodes in electrochemistry, from (bio)electroanalysis to applications in energy technologies, such as batteries and fuel cells. With the advent of new forms of nanocarbon, particularly, carbon nanotubes and graphene, carbon electrode materials have taken on even greater significance for electrochemical studies, both in their own right and as components and supports in an array of functional composites. With the increasing prominence of carbon nanomaterials in electrochemistry comes a need to critically evaluate the experimental framework from which a microscopic understanding of electrochemical processes is best developed. This Account advocates the use of emerging electrochemical imaging techniques and confined electrochemical cell formats that have considerable potential to reveal major new perspectives on the intrinsic electrochemical activity of carbon materials, with unprecedented detail and spatial resolution. These techniques allow particular features on a surface to be targeted and models of structure-activity to be developed and tested on a wide range of length scales and time scales. When high resolution electrochemical imaging data are combined with information from other microscopy and spectroscopy techniques applied to the same area of an electrode surface, in a correlative-electrochemical microscopy approach, highly resolved and unambiguous pictures of electrode activity are revealed that provide new views of the electrochemical properties of carbon materials. With a focus on major sp(2) carbon materials, graphite, graphene, and single walled carbon nanotubes (SWNTs), this Account summarizes recent advances that have changed understanding of interfacial electrochemistry at carbon electrodes including: (i) Unequivocal evidence for the high activity of the basal surface of highly oriented pyrolytic graphite (HOPG), which is at least as active as noble metal electrodes (e.g., platinum) for outer-sphere redox processes. (ii) Demonstration of the high activity of basal plane HOPG toward other reactions, with no requirement for catalysis by step edges or defects, as exemplified by studies of proton-coupled electron transfer, redox transformations of adsorbed molecules, surface functionalization via diazonium electrochemistry, and metal electrodeposition. (iii) Rationalization of the complex interplay of different factors that determine electrochemistry at graphene, including the source (mechanical exfoliation from graphite vs chemical vapor deposition), number of graphene layers, edges, electronic structure, redox couple, and electrode history effects. (iv) New methodologies that allow nanoscale electrochemistry of 1D materials (SWNTs) to be related to their electronic characteristics (metallic vs semiconductor SWNTs), size, and quality, with high resolution imaging revealing the high activity of SWNT sidewalls and the importance of defects for some electrocatalytic reactions (e.g., the oxygen reduction reaction). The experimental approaches highlighted for carbon electrodes are generally applicable to other electrode materials and set a new framework and course for the study of electrochemical and interfacial processes.

Electrochemistry of Fe3+/2+ at highly oriented pyrolytic graphite (HOPG) electrodes: kinetics, identification of major electroactive sites and time effects on the response

Electron transfer kinetics of Fe^3+/2+ on HOPG is as fast as on metals, with the electroactivity dominated by basal plane.

Scanning Electrochemical Microscopy (SECM): Fundamentals and Applications in Life Sciences

In recent years, scanning electrochemical microscopy (SECM) has made significant contributions to the life sciences. Innovative developments focusing on high-resolution imaging, developing novel operation modes, and combining SECM with complementary optical or scanning probe techniques renders SECM an attractive analytical approach. This chapter gives an introduction to the essential instrumentation and operation principles of SECM for studying biologically-relevant systems. Particular emphasis is given to applications aimed at imaging the activity of biochemical constituents such as enzymes, antibodies, and DNA, which play a pivotal role in biomedical diagnostics. Furthermore, the unique advantages of SECM and combined techniques for studying live cells is highlighted by discussion of selected examples.

Face the Edges: Catalytic Active Sites of Nanomaterials

Edges are special sites in nanomaterials. The atoms residing on the edges have different environments compared to those in other parts of a nanomaterial and, therefore, they may have different properties. Here, recent progress in nanomaterial fields is summarized from the viewpoint of the edges. Typically, edge sites in MoS2 or metals, other than surface atoms, can perform as active centers for catalytic reactions, so the method to enhance performance lies in the optimization of the edge structures. The edges of multicomponent interfaces present even more possibilities to enhance the activities of nanomaterials. Nanoframes and ultrathin nanowires have similarities to conventional edges of nanoparticles, the application of which as catalysts can help to reduce the use of costly materials. Looking beyond this, the edge structures of graphene are also essential for their properties. In short, the edge structure can influence many properties of materials.

Electron transfer kinetics on natural crystals of MoS2 and graphite

Here, we evaluate the electrochemical performance of sparsely studied natural crystals of molybdenite and graphite, which have increasingly been used for fabrication of next generation monolayer molybdenum disulphide and graphene energy storage devices. Heterogeneous electron transfer kinetics of several redox mediators, including Fe(CN)6(3-/4-), Ru(NH3)6(3+/2+) and IrCl6(2-/3-) are determined using voltammetry in a micro-droplet cell. The kinetics on both materials are studied as a function of surface defectiveness, surface ageing, applied potential and illumination. We find that the basal planes of both natural MoS2 and graphite show significant electroactivity, but a large decrease in electron transfer kinetics is observed on atmosphere-aged surfaces in comparison to in situ freshly cleaved surfaces of both materials. This is attributed to surface oxidation and adsorption of airborne contaminants at the surface exposed to an ambient environment. In contrast to semimetallic graphite, the electrode kinetics on semiconducting MoS2 are strongly dependent on the surface illumination and applied potential. Furthermore, while visibly present defects/cracks do not significantly affect the response of graphite, the kinetics on MoS2 systematically accelerate with small increase in disorder. These findings have direct implications for use of MoS2 and graphene/graphite as electrode materials in electrochemistry-related applications.

Nucleation, aggregative growth and detachment of metal nanoparticles during electrodeposition at electrode surfaces

The nucleation and growth of metal nanoparticles (NPs) on surfaces is of considerable interest with regard to creating functional interfaces with myriad applications. Yet, key features of these processes remain elusive and are undergoing revision. Here, the mechanism of the electrodeposition of silver on basal plane highly oriented pyrolytic graphite (HOPG) is investigated as a model system at a wide range of length scales, spanning electrochemical measurements from the macroscale to the nanoscale using scanning electrochemical cell microscopy (SECCM), a pipette-based approach. The macroscale measurements show that the nucleation process cannot be modelled as either truly instantaneous or progressive, and that step edge sites of HOPG do not play a dominant role in nucleation events compared to the HOPG basal plane, as has been widely proposed. Moreover, nucleation numbers extracted from electrochemical analysis do not match those determined by atomic force microscopy (AFM). The high time and spatial resolution of the nanoscale pipette set-up reveals individual nucleation and growth events at the graphite basal surface that are resolved and analysed in detail. Based on these results, corroborated with complementary microscopy measurements, we propose that a nucleation-aggregative growth-detachment mechanism is an important feature of the electrodeposition of silver NPs on HOPG. These findings have major implications for NP electrodeposition and for understanding electrochemical processes at graphitic materials generally.

Electrochemistry at highly oriented pyrolytic graphite (HOPG): lower limit for the kinetics of outer-sphere redox processes and general implications for electron transfer models

Electron transfer kinetics for outer-sphere redox couples is fast on the basal surface of highly oriented pyrolytic graphite (HOPG).

Hyphenating atomic force microscopy

Atomic force microscopy can be readily combined with complementary instrumental techniques ranging from optical to mass-sensitive methods. This Feature highlights recent advances on hyphenated AFM technology, which enables localized studies and mapping of complementary information at surfaces and interfaces.

The edge- and basal-plane-specific electrochemistry of a single-layer graphene sheet

Graphene has a unique atom-thick two-dimensional structure and excellent properties, making it attractive for a variety of electrochemical applications, including electrosynthesis, electrochemical sensors or electrocatalysis, and energy conversion and storage. However, the electrochemistry of single-layer graphene has not yet been well understood, possibly due to the technical difficulties in handling individual graphene sheet. Here, we report the electrochemical behavior at single-layer graphene-based electrodes, comparing the basal plane of graphene to its edge. The graphene edge showed 4 orders of magnitude higher specific capacitance, much faster electron transfer rate and stronger electrocatalytic activity than those of graphene basal plane. A convergent diffusion effect was observed at the sub-nanometer thick graphene edge-electrode to accelerate the electrochemical reactions. Coupling with the high conductivity of a high-quality graphene basal plane, graphene edge is an ideal electrode for electrocatalysis and for the storage of capacitive charges.

Nanoscopic polypyrrole AFM-SECM probes enabling force measurements under potential control

Conductive polymers, and in particular polypyrrole, are frequently used as biomimetic interfaces facilitating growth and/or differentiation of cells and tissues. Hence, studying forces and local interactions between such polymer interfaces and cells at the nanoscale is of particular interest. Frequently, such force interactions are not directly accessible with high spatial resolution. Consequently, we have developed nanoscopic polypyrrole electrodes, which are integrated in AFM-SECM probes. Bifunctional AFM-SECM probes were modified via ion beam-induced deposition resulting in pyramidal conductive Pt-C composite electrodes. These nanoscopic electrodes then enabled localized polypyrrole deposition, thus resulting in polymer-modified AFM probes with a well-defined geometry. Furthermore, such probes may be reversibly switched from an insulating to a conductive state. In addition, the hydrophilicity of such polymer tips is dependent on the dopant, and hence, on the oxidation state. Force studies applying different tip potentials were performed at plasma-treated glass surfaces providing localized information on the associated force interactions, which are dependent on the applied potential and the dopant.

Heterogeneous electron transfer at nanoscopic electrodes: importance of electronic structures and electric double layers

Recent insights into the nanoscopic electrode size and structure effects on heterogeneous ET kinetics are presented.

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.

Parylene insulated probes for scanning electrochemical-atomic force microscopy

Scanning electrochemical-atomic force microscopy (SECM-AFM) is a powerful technique that can be used to obtain in situ information related to electrochemical phenomena at interfaces. Fabrication of probes to perform SECM-AFM experiments remains a challenge. Herein, we describe a method for formation of microelectrodes at the tip of commercial conductive AFM probes and demonstrate application of these probes to SECM-AFM. Probes were first insulated with a thin parylene layer, followed by subsequent exposure of active electrodes at the probe tips by mechanical abrasion of the insulating layer. Characterization of probes was performed by electron microscopy and cyclic voltammetry. In situ measurement of localized electrochemical activity with parylene-coated probes was demonstrated through measurement of the diffusion of Ru(NH)(6)(3+) across a porous membrane.

Direct measurement of electron transfer distance decay constants of single redox proteins by electrochemical tunneling spectroscopy

We present a method to measure directly and at the single-molecule level the distance decay constant that characterizes the rate of electron transfer (ET) in redox proteins. Using an electrochemical tunneling microscope under bipotentiostatic control, we obtained current−distance spectroscopic recordings of individual redox proteins confined within a nanometric tunneling gap at a well-defined molecular orientation. The tunneling current decays exponentially, and the corresponding decay constant (β) strongly supports a two-step tunneling ET mechanism. Statistical analysis of decay constant measurements reveals differences between the reduced and oxidized states that may be relevant to the control of ET rates in enzymes and biological electron transport chains.

Touching surface-attached molecules with a microelectrode: mapping the distribution of redox-labeled macromolecules by electrochemical-atomic force microscopy

We report on the development of a mediator-free electrochemical-atomic force microscopy (AFM-SECM) technique designed for high-resolution imaging of molecular layers of nanometer-sized redox-labeled (macro)molecules immobilized onto electrode surfaces. This new AFM-SECM imaging technique, we call molecule touching atomic force electrochemical microscopy (Mt/AFM-SECM), is based on the direct contact between surface-anchored molecules and an incoming microelectrode (tip). To validate the working-principle of this microscopy, we consider a model system consisting of a monolayer of nanometer long, flexible, polyethylene glycol (PEG) chains covalently attached by one extremity to a gold surface and bearing at their free end a ferrocene (Fc) redox tag. Using Mt/AFM-SECM in tapping mode, i.e., by oscillating the tip so that it comes in intermittent contact with the grafted chains, we show that the substrate topography and the distribution of the redox-tagged PEG chains immobilized on the gold surface can be simultaneously and independently imaged at the sub-100 nm scale. This novel type of SECM imaging may be found useful for characterizing the surface of advanced biosensors which use electrode-grafted, redox-tagged, linear biochains, such as peptides or DNA chains, as sensing elements. In principle, Mt/AFM-SECM should also permit in situ imaging of the distribution of any kind of macromolecules immobilized on electrode surfaces or simply conducting surfaces, provided they are labeled by a suitable redox tag.