Scanning electrochemical probe microscopy investigation of two-dimensional materials

Research interests in two-dimensional (2D) materials have seen exponential growth owing to their unique and fascinating properties. The highly exposed lattice planes coupled with tunable electronic states of 2D materials have created manifold opportunities in the design of new platforms for energy conversion and sensing applications. Still, challenges in understanding the electrochemical (EC) characteristics of these materials arise from the complexity of both intrinsic and extrinsic heterogeneities that can obscure structure–activity correlations. Scanning EC probe microscopic investigations offer unique benefits in disclosing local EC reactivities at the nanoscale level that are otherwise inaccessible with macroscale methods. This review summarizes recent progress in applying techniques of scanning EC microscopy (SECM) and scanning EC cell microscopy (SECCM) to obtain distinctive insights into the fundamentals of 2D electrodes. We showcase the capabilities of EC microscopies in addressing the roles of defects, thickness, environments, strain, phase, stacking, and many other aspects in the heterogeneous electron transfer, ion transport, electrocatalysis, and photoelectrochemistry of representative 2D materials and their derivatives. Perspectives for the advantages, challenges, and future opportunities of scanning EC probe microscopy investigation of 2D structures are discussed.


Introduction
The discovery and development of atomically thin crystals (so-called two-dimensional, 2D, materials) has been a breakthrough for material science.Isolating single/few atomic layers from bulk layered materials has led to the discovery of a wide range of exotic phenomena [1,2].Interests in exploring the fascinating behavior of 2D materials have seen rapid growth across multiple disciplines including electronics [3,4], mechanics [5], photonics [6], and chemistry [7,8].Two-dimensional (2D) materials have shown great promise in supporting the interconversion of electrical energy and chemical energy due to their large surface area-to-volume ratio and highly tunable physicochemical properties [8].
Further development of 2D materials in the field of electrochemistry will rely on a deeper understanding of the fundamental factors that underpin interfacial charge transfer and chemical reactivity associated with these physicochemical properties.Importantly, heterogeneity arising from the presence of structural modifications and defects of the 2D crystals plays a significant role in determining the material's overall electrochemical (EC) behavior.Advanced characterization of the EC behavior of any 2D material, and mechanistic insights into the structure-function relationship of these systems is vital for identifying appropriate fundamental and/or engineering strategies that may optimize the system's potential in energy conversion and storage applications.
Several advanced techniques have been developed to characterize the properties of 2D materials that are critical for their EC performance.For instance, detailed morphology and crystal orientation of 2D flakes can be readily visualized by high-resolution transmission electron microscopy combined with electron diffraction [9,10].X-ray characterization techniques such as x-ray absorption spectroscopy can provide quantitative structural information such as the oxidation state, coordination number and interatomic distances [11,12].Raman spectroscopy is especially powerful in probing the evolution of structure and surface chemistry of 2D materials due to its sensitivity to the layer number, defects, strain, and doping level [13][14][15].However, with regard to EC characterization, conventional electroanalytical techniques suffer from their inability to isolate or resolve the contributions of heterogeneities at electrode surfaces.An alternative EC testing platform involves the fabrication of individual 2D flakes into on-chip devices [16,17], where local measurement may be performed by selectively exposing 2D flakes using lithography.This approach has provided a more direct strategy to spatially resolve EC activity and identify potential active sites.However, the dimensions of on-chip devices in the current stage are limited to micrometer scales, while higher spatial resolution is required to reveal the behaviors of (sub)nanoscale heterogeneities, which may include atomic defects and step edges.The throughput of this technique is also limited by the fact that only a small portion of the 2D material can be probed at a time.
Scanning electrochemical microscopy (SECM) is an electroanalytical scanning probe technique capable of obtaining the EC activity map of a sample with high spatial resolution.Engstrom pioneered the technique by employing ultramicroelectrodes to map concentration profiles within the diffusion layer at a macroelectrode surface [18].This technique was later introduced as SECM by Bard [19].In an SECM experiment, an ultramicroelectrode is scanned over the sample to build an EC map that depends on both the topography and the EC activity of the substrate.Scanning electrochemical cell microscopy (SECCM) was derived from SECM and introduced by Unwin in 2010 [20,21].In an SECCM experiment, an electrolyte-filled nanopipette is brought into contact with the sample to form a miniaturized liquid cell and perform localized EC measurements.Both techniques are suitable for carrying out spatially resolved, localized EC characterization with their resolution defined by the probe size.The success of applying scanning EC probe techniques for localized measurement stems from the exceptional spatial resolution realized by miniaturization of the EC probes (i.e.nanoelectrodes or nanopipette) that may attain spatial resolutions ⩽10 nm [22,23].Shrinking the physical size of the probes down to the nanometer range also permits steady-state response readily attained at the nanogap/nanocell.Well-established theories in combination with finiteelement simulations for various regimes of measurements have granted SECM/SECCM the capabilities to quantitatively interpret the EC data [22,24].
In comparison with many widely employed material characterization techniques, SECM/SECCM are uniquely fitted to provide direct insights into the EC behavior of the samples at the nanoscale level.In this context, scanning EC probe techniques are of increasing interest as platforms to investigate interfacial charge transfer at 2D material electrodes [25,26].The effects of structural heterogeneities (e.g.step edges, terraces, stacking variants) as well as external engineering (e.g.defects, substrate, strain) on the electron transfer kinetics and electrocatalytic activities have been clearly revealed with these spatially resolved EC characterization approaches.In this review, we present the basic principles of the SECMs, followed by a comprehensive summary of the present state-of-the-art SECM/SECCM work on 2D materials covering their research background, key data, and valuable insights obtained from the study.Finally, we discuss the current challenges and limitations of these methods, and signpost new avenues and future opportunities in the development of scanning EC probe microscopy to understand the chemistry of 2D materials.

Principles of SECM and SECCM
In this section we provide a concise overview of the working principles of SECM/SECCM.While this review places an emphasis on the studies of 2D materials, the readers are encouraged to refer to many comprehensive reviews of scanning EC probe methods [21][22][23][27][28][29][30] applied in a wide range of disciplines.

SECM
A schematic of a typical SECM instrument is shown in figure 1(a).The positioning system generally consists of three-dimensional stepper motors coupled with piezoelectric actuators for precise positioning of the probe relative to the substrate.A low current bipotentiostat is used to independently modulate and measure the potential and the current at the probe and substrate, respectively.A data acquisition and a control system are required to synchronize data and coordinate the bipotentiostat and the positioning system.An SECM probe (generally referred to as an SECM tip) is a critical component that defines the spatial resolution of the measurement.While a substantial number of reports have focused on the design and fabrication of various types of probes [31][32][33], diskshaped micro(nano)electrodes are generally the most common geometry (figure 1(b)).Other important accessories of an SECM include EC cells, vibration isolation stages, optical microscopes, and complementary spectrometers [22].

Modes of operation
In an SECM experiment, the scanning of the tip over the substrate surface is generally operated in two complementary modes.In the feedback mode experiment (figure 1(d)), the SECM tip is brought within a short distance (comparable to the probe size) of the substrate surface.With a suitable electrical potential applied to the tip, the redox active reactant (B) is oxidized/reduced to the product (A) at the tip surface.The regeneration of B at an electroactive substrate and the redox cycling of A/B within the tip-substrate gap result in an increasing tip current with decreasing distance (positive feedback).If the substrate is inert, the tip current decreases with decreasing distance due to the hindered diffusion of B to the tip electrode (negative feedback).The measurement of the current response as a function of the tip-to-substrate distance (generally referred to as an 'approach curve') has a strong dependence on the apparent reaction kinetics at the substrate.The approach curve can be described using analytical approximations [34,35] that allow charge transfer kinetics to be extracted quantitatively.The feedback mapping experiments are carried out by raster scanning the tip over the sample while maintaining the tip-substrate distance in the feedback region.The obtained map depends on both the topography and reactivity of the substrate material.
In substrate generation/tip collection mode (SG/TC, figure 1(e)), the tip collects the redox species generated at the substrate surface (e.g.B in figure 1(e)).The collection efficiency depends on the separation distance and relative sizes of the tip and the substrate.A lateral scan of the tip over the surface is used to identify 'hot spots' where reactions occur at a higher rate which and thus more products are collected at the tip.Tip generation/substrate collection mode (TG/SC) is operated where the redox species are generated at the tip and subsequently collected by the substrate, and is predominantly used for performing local modification of substrate [36].In additional to feedback and generation/collection (GC) modes, competition mode has developed for determining surface activities.In this mode, tip and substrate compete for the same redox process, and the consumption of redox species at substrate leads to the decrease in tip current [22].The attenuation of tip current is then used as an indirect report of the substrate catalytic activity.
The combination of SECM with atomic force microscopy (SECM-AFM) offers a promising approach to correlating EC activities with the nanoscopic topography of a sample [37,38].The integration of a nanoelectrode to the apex of an AFM probe allows local EC measurements taken at a controllable distance to the substrate.The ability to independently obtain topographic and EC information permits a reliable evaluation of the EC activity.Similarly, a hybrid system of SECM and scanning ion conductance microscopy (SICM) has been developed for simultaneous imaging of noncontact topography and local EC species [39,40].While this technique is particularly useful for probing soft samples such as living cells, it is not addressed in this review as detailed SICM studies of 2D materials remain few.

Types of reactions
Interfacial EC processes are generally classified as either outer-sphere or inner-sphere redox processes [41,42].In outer-sphere redox reactions, there are minimal bonding interactions between the redox species and electrode surface in interfacial electron transfer.These molecules undergo oxidation and reduction reversibly with simple reaction mechanisms.They are ideal mediators used in the feedback mode due to the EC reversibility and fast heterogeneous kinetics to achieve redox cycling between the tip and substrate.Combined with mass transport modeling, the reaction kinetic parameters are readily extracted from the feedback current collected at the tip.These properties are often correlated with the surface conductivity and electronic density of the studied 2D materials.
Inner-sphere redox (catalytic) processes are associated with bonding or adsorption of reactants, intermediates, and/or products to the electrode surface.The reactions involve two or more electrons, multiple elementary steps, and even coupled homogeneous chemical reactions.GC mode is particularly useful in collecting and quantifying intermediates that are otherwise inaccessible with other methods.Competition mode can be employed to indirectly probe the consumption rate of the reactants at the sample.The obtained reactivity is dictated by the ability to stabilize/destabilize key catalytic intermediates of the probed 2D surface.

SECCM
SECCM was derived from SECM and has developed into a new versatile scanning probe microscopy.The SECCM instrument has similar components to SECM but operates on a distinct principle: direct EC measurement is carried out at the cell formed between the sample and the nanopipette probe.

Dual channel setup
A double barrel (theta) nanopipette was originally used as the SECCM probe [21].Two quasi-reference counter electrodes (QRCEs) are placed in each channel filled with electrolyte solution (figure 1(f)).The EC cell consists of a meniscus droplet formed at the end of the pipette.A potential (V 1 ) is applied between the QRCEs to induce an ionic conductance current through the hanging meniscus.The change of ion current upon meniscus contact is detected with high sensitivity, allowing any surface to be probed regardless of its conductivity.The probe moves laterally while the pipette height is adjusted to maintain a constant meniscus shape and resolve topographical features (constant-distance mode).An additional voltage (V 2 ) between the substrate and QRCE is applied to implement voltametric and chronoamperometric experiments at the substrate.

Single channel setup
A simpler SECCM configuration based on single barrel nanopipettes (figure 1(c)) has been developed for synchronous EC/topographical imaging with high spatial and temporal resolution [30,43].This approach was reported first as scanning micropipet contact method [44] and later renamed single channel SECCM.The QRCE is kept at a desired voltage relative to the conductive substrate so that a measurable current will flow upon meniscus contact with the substrate, which causes the instrument to halt the vertical approach.Imaging is usually operated in a 'hopping mode' where the nanopipette lands onto the substrate at a series of predefined locations.Local voltammetric experiments are carried out to construct an EC activity map.Due to the relatively smaller size of a single channel nanopipette, direct topographical/EC imaging is achieved with much higher spatial resolution.
In comparison with SECM, SECCM is advantageous for being easier to implement.The pipette probes used in SECCM are easily fabricated at sizes smaller than typical microelectrodes.Since the electroactive area is dictated by the meniscus contact, the spatial resolution is determined by the nanopipette size and surface wetting, and is free from interference of diffusional broadening [21].The modes of operation are straightforward, as EC current associated with either outer-sphere or inner-sphere reactions is directly measured.

Graphene and its derivatives
The isolation of graphene from graphite by mechanical exfoliation from graphite and the discovery of its exotic electronic properties [45,46] ignited fundamental and applied research on graphene.Graphene and its derivatives have been of significant interest as electrode materials due to their high conductivity and tunable chemical/physical properties [8,47].The EC performance of these materials can be controlled by a range of methods that include stacking type, structural defects, chemical doping, and surface modification.Understanding how these fundamental factors influence interfacial activity is critical for exploring new directions in sensing and energy conversion/storage.In this section, we provide an overview of prior work on graphene and its derivatives, and discuss the unique benefits offered by spatially resolved EC information in elucidating the structure-function relationship of graphene-based electrodes.

Heterogeneous electron transfer (HET)
The kinetics of HET reactions are strongly influenced by the electronic structure, the energy level of the redox molecule, and electrolyte properties [48].Pristine (undoped) graphene is a zero-bandgap Dirac semimetal with its conduction and valence bands meeting at the charge-neutrality/Dirac point and a vanishing electronic density of states (DOS) at the Fermi level.The basal plane of graphene has been reported to allow rapid HET of a variety of redox mediators both in aqueous and organic media [49,50], and structural defects such as vacancies, adatoms, and grain boundaries have also been shown to strongly influence the electronic characteristics of graphene and thus impacts its HET behavior.

Defects and edges
SECM has demonstrated its powerful capability to interrogate the roles of defects at electrode surfaces.Tan et al examined the HET kinetics at the defect of CVD graphene and its surrounding areas were using SECM feedback imaging [51] as shown in figures 2(a) and (b).Much higher feedback currents were observed at the defect edges than over the surfaces far from the defects indicating more facile electron transfer kinetics at the defective sites.
The higher activities were attributed to the exposed edges and the chemical oxidation of sp 2 carbon centers in an aqueous environment.A more quantitative SECM study of the defect effects on HET kinetics was reported by Zhong et al [52].Raman spectroscopy was used to determine the defect density from the intensity of the D peak activated by structural defects (figure 2(c)).The SECM feedback current images (figure 2(d)) are well correlated with the Raman D-band mapping in indicating that HET kinetics (characterized by the heterogeneous ET rate constant, k 0 ) have a strong dependence on the defect density.The enhanced HET kinetics at relatively low defect density regions (<10 13 cm 2 s −1 ) was explained as resulting from the formation localized electronic states associated with the point defects producing high local DOS that increases the overlap between the electronic states of graphene and redox molecules.The correlation between Raman spectra and SECM was developed further by the work of Schorr et al in which the EC effects of graphene oxidation were assessed in real time [53].By correlating the Raman signatures of the graphene surface to the SECM Reproduced from [63], with permission from Springer Nature.response, the effects of exfoliation, defect density, and surface modification/passivation on EC response were isolated.
A comparison of HET kinetics at graphene edge sites and basal planes has been extensively investigated for both natural graphite and highly oriented pyrolytic graphite (HOPG) [54][55][56].The transition from bulk scale to mono/few-layer graphene has been largely explored by SECCM and microdroplet techniques [57][58][59][60].To probe the behaviors of graphene edges, Güell et al applied SECCM to image exfoliated graphene on SiO 2 -coated Si wafers using Ru(NH 3 ) 6 3+/2+ couple whose redox potential lies close to the intrinsic Fermi level of graphene [58].The AFM image and correlated SECCM map of the exfoliated graphene (figures 2(e) and (f)) evidently show that the regions with the highest HET activities are step edges.This dependence was attributed to the overlap in the DOS of the edge (defect) and basal graphene with the states of the Ru(NH 3 ) 6 3+/2+ redox couple.

Thickness
The thickness dependence of graphene HET was studied by Güell et al with SECCM [57].Dual channel pipettes filled with FcTMA + solution served as probes to obtain local voltammograms at the graphene surface.Comparison between the topography revealed by optical microscopy and the SECCM current map (figure 3(a)) showed a clear correlation between the HET activity and the number of graphene layers.The local EC current as a function of the layer number (figure 3(b)) clearly shows a systematic increase of the HET kinetics with the number of layers until the point where HET becomes reversible, consistent with the evolution of increasing electronic DOS through single layer, bilayer, and multilayer graphene.

Substrates
The substrate under graphene layers plays an important role in modulating the HET kinetics owing to the effects of carrier doping and electrostatic interactions [61].Hui et al reported on the modulation of the HET kinetics of outer-sphere redox mediators by metal electrodes buried in the sub-surface of continuous double layer graphene electrodes [62].Figures 3(c) and (d) shows an increase in feedback current and rate constant (k) of Fe(CN) 6 3−/4− at the graphene-covered Au area compared with bare graphene, although this enhancement is smaller than exposed Au.Liu et al examined the EC kinetics at Cu-supported graphene layers using SECCM and co-located structural microscopy [63].The SECCM 3+/2+ couple reveals that the HET kinetics follow the trend monolayer > bilayer > multilayer graphene.Varying the number of graphene layers modifies the electrostatic potential felt by the redox couple, which, in turn, changes the activation barrier for the reaction.In additional to these imaging-based studies, Chen et al investigated the effect of the poly(methyl methacrylate) (PMMA) supports on the HET behavior of FcMeOH 0/+ couple at CVD-grown graphene with SECM nanogap voltammetry [64].The electrostatic charges of the underlying PMMA film were shown to impact the access of charged redox molecules to graphene, yielding varying transfer coefficients (α) and standard rate constant.

Twistronics
The idea of twisting stacked 2D layers to manipulate their electronic properties gained rapid popularity in 2018 when it was discovered that bilayer graphene with a twist angle of ∼1.1 • (the so-called magic angle) behaves as a superconductor [65].Recently, the concept of 'twistronics' has been extended to control the HET at graphene materials by the interlayer twist angle.Yu et al employed SECCM (figure 4(a)) to selectively probe the EC kinetics of Ru(NH 3 ) 6 3+/2+ at the basal planes of twisted bilayer graphene (TBG) [66] stacked on boron nitride layers (figure 4(b)).Scanning tunneling microscopy (STM) and SECCM (figures 4(c) and (d)) were used to visualize the moiré patterns [67] formed by twisting layers and obtain local steady-state voltammograms, respectively.The intriguing angle dependence of the HET kinetics (figure 4(e)) was attributed to spatial variation in the TBG reactivity due to localization effects of the flat band and the associated lattice reconstruction [67,68] at angles <1.5 • .Zhang et al extended the study to twisted trilayer graphene (TTG) and discovered that HET rates are strikingly dependent on electronic localization in each atomic layer [69].Figure 4(f) shows the strong, nonmonotonic variation in k 0 of Ru(NH 3 ) 6 3+/2+ at TTG similar to the behavior of TBG.Importantly, a rotationally misaligned monolayer on a Bernal stacked bilayer (M-t-B structure) shows the greatest HET kinetics, suggesting the DOS enhancements are distinctly localized on the top two layers of M-t-B structures.In both studies, SECCM was leveraged to selectively probe twisted basal planes to exclude the effects from defective edges.
In summary, the simple reaction mechanisms of outer-sphere HET allow SECM feedback imaging and SECCM mapping to be employed for effectively visualizing the spatial variations of HET kinetics at graphene electrodes.Coupled with other  2022), with permission from Elsevier.(f) High-resolution electrochemical activity (i) and co-located quasi-topographical maps of a graphene|Nafion membrane, using SECCM in the voltammetric hopping mode configuration (right).Reprinted with permission from [77].Copyright (2022) American Chemical Society.(g) SECCM current map of proton transport and (h) AFM force map of graphene suspended over micrometer-sized holes etched into silicon nitride (SiNx) substrates.Reproduced from [78], with permission from Springer Nature.(i) SECM feedback images of TMPD on fresh patterned graphene at substrate potentials representing conditions before, during, and after (top to bottom) SEI formation.(j) CV-SECM image of patterned graphene sample denoting Li + flux (left) and SEM image of a similar region (right).Reprinted with permission from [81].Copyright (2016) American Chemical Society.characterization techniques such as Raman spectroscopy and AFM [70], factors including defects, thickness, and substrate were better understood using correlative studies.In additional to these mapping experiments, nanogap voltammetry realized in SECM was shown to be effective in determining rapid kinetics, while local voltammetry in SECCM allowed researchers to obtain EC measurements exclusively at the basal plane of the constructed graphene heterostructures.

Electronic and ionic transport
The electronic properties of graphene and its derivatives result in distinctive inter-or intra-layer transport properties, which can manifest in EC measurements [1].The reversible intercalation of mobile ions into the interlayer gaps of layered materials plays a key role in EC energy storage applications [71,72].SECM and SECCM have been employed to acquire spatially resolved EC data that shed light on the transport of charge and ions.

In-plane electron transport
The effects of in-plane electron transport on graphene electrochemistry have been mainly investigated using SECM feedback mode.Azevedo et al used SECM to obtain surface conductivity maps of heterogeneous graphene oxide (GO) thin films deposited on glass [73].The conductivity was approximated as a function of the feedback current, providing a general and simplified framework for quantitative conductivity mapping.Bourgeteau et al used SECM to investigate the electronic conduction of individual and interconnected reduced graphene oxide (r-GO) flakes [74], using SECM feedback measurements to map the EC activities of interconnected and isolated r-GO flakes deposited on Si/SiO 2 substrates (figure 5(b)).The redox cycling reaction at the substrate and positive feedback is necessarily associated with a counter reaction occurring elsewhere as depicted in figure 5(c).This indicates that the inter-flake contact resistance impacts the transport of electrons in r-GO-based materials.A similar SECM approach was used to characterize the role of contact resistance in the electrochemistry of MoS 2 flakes [75].A solid-state SECCM was created using polyacrylamide as a solid electrolyte to probe the electrochemistry of wrinkled graphene [76].Correlated SEM (figure 5(d)) and SECCM (figure 5(e)) images revealed lower current at the two sidewalls of the wrinkles compared to center of the wrinkle and the planar surface of graphene, suggesting a slower electron transfer rate at the wrinkle.This is because electrons need to climb over the nanoscale wrinkle, which restricts electron transport and leads to a low EC activity.This high spatial SECCM study provided the first and direct evidence of the electron transfer mechanism at the wrinkles.

Interlayer ion permeability
Graphene has been increasingly explored as an ionselective membrane for diverse applications.Bentley et al captured ion-flux images at a graphene|Nafion membrane using an SECCM [77].The SECCM configuration represented as an EC ion (proton) pump cell was employed to detect local proton transport across the graphene film in one direction in response to the proton-consuming reactions at the underlying Pt electrode.SECCM scanning of ion transport (figure 5(f)) revealed that the majority of the CVD graphene overlayer does not conduct protons, while proton transmission was noted to be highly localized in areas of the graphene film that contained atomic defects.Wahab et al later demonstrated that proton permeation through defect-free graphene and hexagonal boron nitride is greatly facilitated by nanoscale non-flatness where strain is accumulated [78].Figures 5(g) and (h) show clear wrinkles and edges in the AFM maps that correlate well with high-conductivity areas in the SECCM maps.The nanoscale ripples that are ubiquitous in 2D membranes result in considerable strain, accelerating proton transport due to lowered energy barrier for proton permeation.The discovery that strain and curvature can amplify proton conductivity in 2D crystals holds promise for diverse applications.This work further reinforce SECCM as an effective tool for resolving ion transport through 2D materials.

Ion insertion
Significant research interest related to energy storage has focused on exploring the intercalation dynamics and the evolution of electrode structures [79,80].Importantly, SECM has been shown to be a potentially powerful tool to characterize the surface conductivity and ion flux distribution during ion intercalation at graphite and few layers of graphene.
Hui et al used SECM to demonstrate changes in the surface conductivity throughout the evolution of the solid-electrolyte interphase (SEI) upon Li + intercalation in a few-layer graphene [81].SECM operated in feedback mode was used to image the spatially resolved HET activity of TMPD redox reaction at a patterned multilayer graphene electrode at various stages of SEI formation (figure 5(i)).The substrate potential at 2.6, 1.2, and 0.07 V vs Li/Li + denotes the condition before, during, and after SEI formation, respectively.The SECM image at 2.6 V shows good contrast between patterned holes and the graphene surface, demonstrating fast electron transfer kinetics at pristine graphene.As the potential is ramped negatively to induce SEI formation, a reduction in tip feedback response is noted, attributed to diminished substrate kinetics.At 0.07 V, the pattern became indistinguishable, providing solely negative feedback to the tip signal.This series of SECM images clearly reveal that stable and condensed SEI layer blocks electron transfer.The spatial variation of Li + conductivity was investigated by employing an Hg-capped ultramicroelectrode [82] as a selective Li-ion sensor to quantify Li + uptake into multilayer graphene with a formed SEI layer.The blue spots in the SECM image (figure 5(j)) represent areas of lower Li + concentration (therefore a larger inward Li + flux), matching well with the spatial distribution of the etched openings.These observations demonstrate that Li ions migrate into graphene interlayers more efficiently through the edge planes.This work was the first time that SECM is used to visualize ionic fluxes through an SEI on a battery material in real time.
Similar SECM approaches were developed to map Li + flux and electron transfer reactivity during SEI formation at a graphene electrode [83,84].The correlated maps revealed that location-specific uptake of Li + was closely associated with heterogeneous substrate distribution in feedback mapping.By developing a model of the intercalation process, the authors determined localized kinetic information of reversible Li + (de)intercalation on edge planes.SECM also allowed in-situ interfacial analysis of incipient SEIs in various alkali-ion electrolytes formed on multilayer graphene [85].By correlating SECCM measurements with XPS results, the authors observed substantial formation of metal fluorides in the Li + and Na + electrolytes, whereas there was no appreciable formation in the K + electrolyte.Sarbapalli et al further presented SECM experimental evidence of Na + intercalation in fluorinated few-layer graphene [86].It was discovered that reversible Na + intercalation necessitates a pre-existing Li-based SEI alongside surface fluorination.
In summary, these studies have underscored the promise of SECM/SECCM as tools for characterizing nanoscale electronic and ion flux at graphene and related materials.Importantly, the dynamics of inplane electron transport, ion transport across layers, and surface conductivity modulation were translated to the HET responses that were readily analyzed by SECM/SECCM.

Electrocatalysis
A fundamental challenge in developing metalfree electrocatalysts (e.g.graphene-based materials) for sustainable energy conversion chemistry lies in the low intrinsic catalytic activity of graphene [87].General strategies to improve the reactivity of pristine graphene basal planes include heterostructure construction [88] and heteroatom doping [89], since these routes can be used to tune electronic states and optimize the adsorption energies of key intermediates.Scanning EC probe techniques have been employed to mechanistically investigate the catalytic performance of graphene-based electrocatalysts optimized by these strategies.

Substrates
Electroactive materials adjacent to graphene layers have a strong impact on the electrocatalytic activity of graphene.The carrier doping effects induced by underlying metallic materials are well known to modulate the electronic structure and DOS of the top graphene layer [90,91].
Hui et al demonstrated the SECM study on the electronic effects induced by an underlying Au substrate on the graphene reactivity toward oxygen reduction reaction (ORR) [92].The H 2 O 2 generation from a two-electron reduction of O 2 at CVD-grown graphene was spatially investigated via an SECM SG/TC mode (figure 6(a)).Higher collection current of H 2 O 2 over the graphene/p-Au spot (figure 6(b)) reveals ORR kinetics enhanced by underlying metal.This aligns with their calculated electronic structure, where the electronic DOS of the heterostructures is dominated by the metal, particularly the d-subshell of Au.These findings underscore the significant catalytic benefits attained through the incorporation of underlayer metal supports for graphene.SECM competition mode was then used to explore the charge coupling effects between metal underlayers and adsorbed molecular catalysts (FeOEP) on graphene (figure 6(c)).Higher consumption rate of O 2 by the substrate is reflected by decreased tip current.The agreement between the current distribution and the known metal underlayer geometry supports the conclusion that enhanced ORR is attributable specifically to electronic donation from the Pt underlayer to the molecular overlayer through the electronically 'semi-transparent' graphene interface (figure 6(d)).
Schorr et al investigated the effects of plasmonic Au nanoparticles as an underlying substrate on the ORR reactivity of graphene with SECM [93].The ORR kinetics obtained from H 2 O 2 collection by SECM tip followed the temperature dependent Arrenhius behavior, suggesting that the photothermal effects other than hot carriers are primarily responsible for the enhanced EC reactivity.Substrate effects on the electrocatalytic hydrogen evolution reaction (HER) activity of a hexagonal boron nitride [94] (hBN, also known as 'white graphene') layer was studied using the SECCM technique [95].Local voltammetry and Tafel analysis reveal that Ausupported hBN exhibiting significantly enhanced HER kinetics compared to h-BN/Cu.The authors postulated that this is due to the modulation of hydrogen adsorption energy caused by the electronic interaction of hBN with the underlying metal substrate.

Doping
Heteroatom doping is an effective approach to tailor the electronic properties of graphene for efficient surface chemistry.Introducing high electronegativity atoms such as N, B, and P in the graphene lattice forms new active sites [89].Multi-doped materials show even higher electrocatalytic activities [96].New Insights into the doping effects were gained through the following SECCM studies.
Kumatani et al investigated the synergetic effects of the edge structure and N/P doping on the HER activity of a graphene-based electrocatalyst with SECCM [97].They synthesized edge-enriched graphene with N and P dopants that predominately reside near the graphene edges.Figures 6(e) and (f) show the SECCM topology and on-site HER current map for the NP-doped graphene samples.Comparison between the topology data and the current mapping reveal that the edge region is more active in HER.Specifically, the turnover frequency values for chemically doped graphene were observed to be approximately 100-200 times greater at the edges compared to the planar regions.The presence of edges introduces geometric frustration to the graphene lattice, facilitating the accumulation of chemical dopants, whereas defect-free regions promote electron transfer to the edge sites.Consequently, these regions synergistically contribute to the enhancement of reaction kinetics.This study directly demonstrates that the combination of chemical doping and edge engineering drastically boosts the HER activity.
SECCM was used to investigate the origins of enhanced EC activity at edges of N-doped reduced graphene oxide (N-rGO) [98,99].Local EC impedance spectroscopy coupled with SECCM revealed larger double-layer capacitance and smaller interfacial resistance in the edges [99].Jin et al extended the SECCM study to the HER reaction at N-rGO microsheets [98].Non-faradic and faradic currents from the total currents were isolated to map the charge accumulation and EC activity, respectively.The HER current map (figure 6(g)) and charge density map (figure 6(h)) provided a direct correlation between the activity and the accumulated charge at the active sites, showing that charge accumulation at edges has a great effect on promoting the catalytic performance.This direct evidence will be significant for a deep understanding of the mechanism of the HER process at highly catalytic materials.
Dechant et al applied SECCM to investigate the link between the geometry and EC activity of nitrogen and sulfur (NS) doped 3D curved graphene [100].The elemental mappings demonstrated the distribution of NS dopant atoms at the curved regions rather than the flat regions.SECCM mapping of the on-site HER current clearly demonstrates the dependence of activity on morphology, i.e. higher current was observed at highly curved topographies.The curved graphene induced topological defects and enhanced the electronic DOS, resulting in Gibbs free energy of hydrogen adsorption be nearly zero owing to the cooperation of the dopants near the topological defect sites.
Indeed, heteroatom doping introduces supplementary parameters for fine-tuning catalytic activity.Spatially resolved measurements with SECCM are deemed indispensable for elucidating the synergistic interplay between dopants and structural heterogeneities.

Surface functionalization
Modified graphene is often used for enhancing the specific absorption of analytes and anchoring molecules/nanoparticles.While this topic has been extensively reviewed [101,102], this section focuses on the applications of SECM in local EC functionalization of graphene surface and characterization of functionalized molecules.
SECM as lithographic tools are useful for local surface EC modifications at different scales by tuning the probe size and scanning range [103][104][105].Generally operated in TG/SC mode (or direct mode), the reactions controlled by the tip electrode induced local chemical events that modify the underlying substrate.Azevedo et al introduced an SECM approach for local functionalization of thin GO films [36].The in-situ generation of naphthalene radical anions by an SECM tip electrode placed in the vicinity of the GO substrate resulted in a localized reduction of GO associated with the formation of conductive r-GO patterns.Smaller tip size and higher displacement speed lead to higher spatial resolution and more confined reduction spot.The local transformation of GO into r-GO permitted a selective electro-grafting of diazonium layer which was used to immobilize Au nanoparticles.Torbensen et al modified the graphene surface with carboxylate groups via CO 2 reduction by SECM [106].The process was carried out with spatial control together with the degree of carboxylation manipulated by the EC potential and reaction time.
SECM has been employed to characterize the functionalized molecules and their effects on the EC behavior.Pham-Truong et al modified graphene with p-nitrophenyl diazonium and used SECM to probe the changes introduced in the HET kinetics [107].The kinetic parameters obtained from approach curves demonstrate that the attached nitrophenyl layer accelerates the HET kinetics.Rodríguez-López et al used SECM to investigate the activity of a tripodal molecular motif adsorbed on a monolayer graphene and quantify its surface diffusion [108].Using the SG/TC mode of SECM, they concluded that the catalytic tripod produced H 2 O 2 more rapidly than the bare graphene surface in the ORR.The evolution of the SECM images suggested that the activity at the tripodal spots decreased over time.This behavior was attributed to decreased surface concentration of the molecular catalyst due to diffusion, demonstrating how the SECM technique is an important approach for understanding the effects of mass transport in graphene functionalization as well as molecular electrocatalysis.
In table 1 we summarize SECM/SECCM work on graphene and its derivatives.A wide of range of outersphere redox reactions and catalytic reactions demonstrate the versatility of graphene electrode.In addition to elucidating intrinsic factors influencing HET kinetics, SECM feedback experiments are proved powerful in addressing electronic and ionic transport.SECM GC mode was employed to detect intermediates in complex catalytic reactions and perform local modification with high precision.SECCM served as a powerful tool to directly map surface activity variation in both HET and catalytic reactions.

Transition metal dichalcogenides (TMDs)
Two-dimensional transition metal chalcogenides (TMDs) are a class of van der Waals materials with structural and electronic properties that hold promise for a variety of applications in energy conversion and storage [109].As electrocatalysts, some TMDs have been touted as candidates for low-cost and highefficiency energy conversion [8,110].Layered TMDs can be described with the general chemical formula MCh 2 , where M is a transition metal element, and Ch is a chalcogen.The structural polytypes (phases), step edges, and other effects such as strain and the presence of atomic vacancies are considered key factors in determining the overall activity.In particular, the semiconducting 2H phases of group VI TMDs (M = Mo, W) with a direct bandgap in the monolayer limit can absorb light to generate photoexcited charge carriers for photoelectrochemical reactions.In this section, we summarize recent progress in investigating the rich (photo)electrochemistry of TMDs using scanning EC probe techniques.

Heterogeneous electron transfer (HET) 4.1.1. Defects
Like sp 2 carbon materials, HET reactions on MoS 2 proceed more facilely at edge sites than at basal planes due to higher electron density [111].Ritzert et al have demonstrated the spatial inhomogeneity in HET behavior of MoS 2 with submicron-resolution SECM [112].The current at the SECM tip associated with the collection of electrogenerated Ru(NH 3 ) 6

2+
was measured as a function of lateral tip position (figure 7(a)).Figures 7(b) and (c) show lowest reactivity on basal plane areas (region E), whereas regions possessing a trench (region C) and macrosteps (region A) consisting of a high density of edge sites were more active.Cabré et al further used the hopping mode of SECCM to map the EC reduction of Ru(NH 3 ) 6 3+ on a 2D MoS 2 sample immobilized on Au substrate to detect nanoscale defects [113].These defects give rise to EC responses that are equivalent to disk-shaped defects with radii of tens of nanometers in size, or to one-dimensional defects with nanometer to sub-nanometer widths.Importantly, only low densities of defects are

Structural polytypes
The metallic 1T phase and the related 1T' phase of MoS 2 display dramatically distinct EC properties in comparison with the semiconducting 2H phase.Converting 2H to 1T or 1T' via phase engineering has been shown to be effective in enhancing the EC activities [114,115].Due to the metastable nature of the T phases, these polytypes transform back to the 2H phase over time.Simultaneous nanoscale resolution of morphology and activity via SECM is a viable approach to characterize the electrochemistry of a complex system containing mixed phases.Sun et al used SECM feedback mode (figure 7(d)) to spatially probe 1T and 2H domains within a mesoscopic MoS 2 crystal using ferrocenemethanol as the redox mediator [116].A negative feedback response was obtained at 2H phase, while significant positive feedback was observed at the 1T phase.The activity map of the mixed-phase MoS 2 nanosheets shows that the 1T phase was sandwiched by the 2H phase with an abrupt boundary (figure 7(e)).This map suggested that the phase conversion appears to proceed from the outside of the flake inwards and along straight lines, which was likely caused by a sliding of the S atomic plane.Zhang et al demonstrated the stabilization of heavily n-type doped 2H and 1T MoS 2 monolayers with a low reversion to the initial phase [117].The nbutyl lithium immersion treatment converts the 2H phase to n-type 2H/1T', while surface functionalization stabilizes the phase.SECM images showed higher tip currents over the surface-functionalized monolayer (figure 7(g)) than 2H MoS 2 (figure 7(f)), suggesting that the entire MoS 2 monolayer is homogeneously n-type doped and the converted 1T' phase was stable in the air.

Thickness
SECCM was leveraged to quantify the effect of the thickness of bottom-contacted 2D TMDs (MoS 2 , MoSe 2 , WS 2 , WSe 2 ) on the EC response of the Ru(NH 3 ) 6 3+/2+ redox couple [118].The responses on all four materials were similar and showed a decrease in the electron transfer rate with an increase in layer thickness.This dependence was explained as resulting from by the electron transport process through the TMD layers-thicker layers resulting in less frequent electron tunneling and net slower transport from the bottom contact across the TMD layer to the TMD-electrolyte interface.
Another study by Du et al demonstrated that the HET behavior of unbiased MoS 2 flakes is affected by both layer numbers and redox mediator [119].Ferrocene (Fc) and decamethylferrocene (DmFc) redox probes in organic solutions were used to elucidate the central relevance of the mediator on the band alignment and the localization effects in SECM feedback experiments.A strong bilayer contrast was detected in SECM maps with a quantitative feedback enhancement of the order of 30% on top of the bilayer islands (figure 7(h)) when Fc was used as the redox mediator.Figure 7(j) demonstrates a reversed SECM feedback contrast when a more negative redox potential mediator (DmFc) was used for the otherwise identical sample system.The distinct behaviors were attributed to the work function differences (figure 7(i)) and how band offsets were aligned with unoccupied states of Fc + and DmFc + .
In contrast to graphene electrode, the HET at TMDs is predominately dictated by the carrier density and charge transport within the layer.The published work exploited the advantages of scanning probe electrochemistry to resolve the influence of defects, phase, thickness, and external electrostatic manipulation [120] to the charge carriers and the resultant HET activities.

Hydrogen evolution reaction (HER)
Two-dimensional (2D) TMDs have stimulated much interest in them from researchers in the field of HER electrocatalysis [121].The overall reaction kinetics of the HER are largely dictated by the Gibbs free energy associated with hydrogen adsorption, ∆G H * .The adsorption energy is determined by the electronic structure of the catalyst surface [122], which can be tuned by doping, defect engineering, phase engineering, and strain engineering [123].Spatially resolving HER activity across TMD layers with scanning EC probe techniques has been very useful for providing a mechanistic understanding of how these parameters impact electrocatalytic behavior.

Edges
The edges of MoS 2 have been proposed to be the active sites for HER from theory/computations [124] and experimental measurements [125].The sitespecific HER activities at MoS 2 surfaces have been extensively investigated by SECCM [126,127].For instance, Bentley et al carried out pixel-resolved linear-sweep voltammetry (LSV) at MoS 2 surfaces in acidic media using single channel nanopipettes to achieve a high spatial resolution [43].The topographic map (figure 8(a)) and the HER current map (figure 8(b)) show relatively low yet uniform activity across the basal plane and enhanced HER kinetics at the edge planes.Local LSVs obtained on different spots shown in figure 8(c) distinctly manifest the variations in activity across the SECCM map.Important HER kinetic parameters such as exchange current densities and Tafel slopes, were evaluated separately on the basal planes and edge sites, a distinction that is unachievable with any bulk measurement approach.Tao et al expanded the SECCM study to WS 2 , showing similar activity variation between basal and edge planes [128].The effects of surface aging were found to deteriorate catalytic activity, due to the build-up Reproduced from [116], with permission from the Royal Society of Chemistry.
of adsorbates and oxidation products, particularly at active edges.

Atomic vacancies
Introducing S vacancies to the MoS 2 lattice leads to the emergence of localized electronic states (associated with these S vacancies) within the bandgap.These S vacancy sites favor hydrogen adsorption on the TMD basal plane [129,130].Takahashi et al carried out SECCM measurements on the effects of defect engineering and TMD heterostructures [131].The S vacancies on a 1H-MoS 2 monolayer were generated by EC desulfurization and controlled by the applied potential.The SECCM current map in figure 8(d) shows that the highest HER activity was observed in the area activated by a bias of −1.40 V vs. RHE, whereas no significant improvement was seen with less positive biases.The results suggest that a threshold voltage is required to generate S vacancies.The SECCM current map (figure 8(e)) shows superior activity of cracked regions similar to the edge planes.In addition, inhomogeneous HER activity was imaged at the surface of MoS 2 /WS 2 heterostructures.In the SECCM current map shown in figure 8(f), the highest activity was observed at MoS 2 edges.The MoS 2 basal planes are slightly more active than WS 2 basal planes, but no distinctive catalytic activity was observed at the heterojunction.It has also been reported that the activation of the basal plane of MoS 2 by creating S vacancies can be further enhanced by an elastic strain that moves the midgap states closer to the Fermi level [132].Li et al employed SECM SG/TC mode to determine the kinetic information on both unstrained and strained S vacancies on the basal plane of MoS 2 monolayers [133].The rate constant of the sample with 2% uniaxial tensile strain was found to be enhanced by almost four-fold, confirming that strain indeed accelerates the HER kinetics at MoS 2 with S vacancies.

Structural polytypes
The activation of the TMD basal planes via phase engineering (i.e. from 2H to 1T or 1T') is a known route for enhancing charge transfer kinetics and improving the HER performance of TMDs [114].Sun et al have investigated the HER performance of MoS 2 nanosheets with mixed phases [116] using SECM SG/TC mode (figure 8(g)).The high-resolution EC maps in figures 8(h) and (i) demonstrate that the HER activity of the 1T phase is more active compared to the 2H basal plane, while 2H-MoS 2 edges also exhibit considerable activity.
While MoS 2 is the most studied compound, other non-precious metal TMDs have also been explored.Jasion et al synthesized 2D FeS 2 with controlled morphology [134].The SECM GC experiments suggested that 2D FeS 2 discs exhibit excellent HER activity and stability that is comparable to Pt.

Photoelectrochemistry
Isolating single or a few layers of TMDs from their bulk form results in fundamentally distinctive physical properties that in turn impact the photoelectrochemical behavior [135].Both light absorption and diffusion mechanisms of charge carriers are strongly dependent on the number of layers [136].Reproduced from [145], with permission from the Royal Society of Chemistry.(g) An optical image of a WSe2/WS2 heterostructure.(h) PL and (i) photocurrent maps of the sample in (g).Reproduced from [147].© IOP Publishing Ltd.All rights reserved.
The steps/edges at the 2D TMDs in the photoelectrochemical reactions can be both beneficial and detrimental since they concurrently serve as catalytic active sites and charge recombination sites [137].To obtain direct experimental insights into these effects requires spatially resolved characterization techniques.Scanning photocurrent microscopy using a near-filed laser scanned over the sample surface while recording the photocurrent has been employed to investigate TMDs [138][139][140].However, the collected signals are inevitably affected by non-illuminated spots, and the spatial resolution is fundamentally limited by optical diffraction.Scanning EC probe techniques are free from these limitations and have seen a growing number of applications to investigate several key factors of the 2D photoelectrochemistry.

Defects
Hill et al have investigated the photoelectrochemical behavior of individual p-type WSe 2 nanosheets using SECCM [141], highlighting the effects of layer thickness and geometric defect.SECCM data of HER at p-type WSe 2 nanosheets consisting of a topographical map and photocurrent map (figures 9(a) and (b)) suggest a strong correlation between photoelectrochemical activity and structural features.Small steps were found to enhance HER photocurrents due to their high catalytic activities (spot 2 in figure 9(b)), whereas large steps were generally found to be detrimental to HER (spot 1 in figure 9(a)).This behavior was explained as due to increased charge recombination at defect-rich sites, which degrades the photochemical response, unless the features are much smaller than the optical penetration depth [142].
SECCM was further employed to create local hole-like defects at the basal planes of individual p-type WSe 2 nanosheets by controlled anodization of the WSe 2 [143].The defect density was mapped by AFM (figure 9(c) i, ii) and compared to correlated SECCM images (figure 9(c) iii, iv).These results provide direct evidence supporting that the increased density of monolayer-high step-like features enhances the photoelectrochemical activities of TMDs.Strange et al investigated the modulation of the photoluminescence (PL) of 2D MoS 2 upon EC anodization [144].The enhancement and red-shift of the PL were originated from Mo oxidation that hinders nonradiative decay of excitons.SECCM mapping was used to reveal that the MoS 2 photooxidation is strongly localized at defective edge sites containing an abundance of Mo-SH functional groups.

Carrier transport
Understanding the factors governing carrier generation (CG) and transport within 2D TMDs during photoelectrochemical reactions is needed to guide the rational design of improved devices.Hill et al applied CG-TC SECCM to visualize carrier transport [145].In this approach, carriers are locally generated using a focused light source and detected as they drive photoelectrochemical reactions at a spatially offset electrolyte interface (figure 9(d)).Photocurrent images of an n-WSe 2 nanosheet (figure 9(e)) showed photocurrents increased in magnitude and widened spatially with increasing potential.Photocurrents at the edge (indicated by the dashed line) were significantly reduced, providing a clear, unambiguous visualization of carrier recombination at nanoscale defects (figure 9(f)).Tolbert et al extended the CG-TC SECCM approach to probe exciton transport down to monolayer limit [146].Photogenerated excitons in monolayer WSe 2 were found to drive reactions across distances in excess of 20 µm, suggesting the existence of long-lived charge transfer states.[149].SECCM maps showed higher photocurrent at strained heterostructures, in which strain was applied by controlling the height of the Cu 2 O nanorods.The results indicated that a more efficient separation of photogenerated carriers induced by the strain can effectively enhance HER activity.
In table 2 we summarize SECM/SECCM work on 2D TMD related materials.In analogous to graphene, the structural effects such as edge, defects, and thickness on HET and electrocatalytic reactions at TMDs have been shown to be impactful.In addition, spatially resolved measurements have elucidated the roles of phase, atomic vacancies, and strains in regulating the reactivity of TMD layers.Coupled with electromagnetic excitation, insights into the dynamics of carrier transport and the implications in

Transition metal oxides and hydroxides (TMOs and TMHs)
Most pristine TMOs and TMHs exhibit unsatisfactory performance in their bulk forms due to their poor activity and conductivity.The electrocatalytic activity of TMOs and TMHs can be significantly improved by shrinking their size and reducing the thickness towards the atomic scale [8].Two-dimensional (2D) TMOs and TMHs are especially active for electrocatalytic reactions involving the activation of water such as the oxygen evolution reaction (OER) [152,153].Nanoscale SECM and SECCM have been shown to be powerful operando techniques for accurately correlating the electrocatalytic OER activities with the local structures.

Defects
Through defect engineering, the OER activity of faceted nickel oxide (NiO) nanostructures was enhanced due to the exposure of nanoscale edge sites that significantly alter the electronic structure of Ni 2+ centers and promoted Ni 3+ states [154].Sun et al employed SECM to probe 2D NiO nanosheets containing defect holes with well-defined edges, and directly correlated the electrocatalytic OER activities with the local structural defects [155].The tip electrode collects the oxygen generated at the substrate in SG/TC mode (figure 10(a)).The SECM image in figure 10(b) shows more efficient O 2 generation at the NiO surface compared to the inactive HOPG.The map with even higher resolution shows increased OER current at the NiO/HOPG boundary (figure 10(c)), consistent with a significantly higher activity of the NiO edge.Atomic-resolution structural measurements of the edges using electron tomography showed that the edges are terminated with ( 100) and ( 111) facets, which were responsible for ∼200-fold enhancement of activity.

Ion insertion
Ion insertion redox reactions convert inactive materials into active electrocatalysts during operation.Single-crystalline β-Co(OH) 2 catalyzed OER is accompanied by hydroxide, water, and proton (de)intercalation as well as the change in the oxidation state of cobalt.Mefford et al used a suite of correlative SECCM and x-ray microscopy techniques [156] to establish a link between the OER activity and the local structure of β-Co(OH) 2 .Importantly, direct mapping of the OER current with SECCM (figure 10(d)) revealed that edge facets are the active surfaces.In both scanning mode (figure 10(e)) and hopping mode (figure 10(f)), the SECCM images revealed that the edge facets have high EC activity compared with the low activity of the basal planes.The difference of these facets was rationalized by the ion (de)intercalation characteristics of the system, in which ion (de)intercalation is facilitated at the edge facets, while ion movement is restricted in the absence of extended defects, which prevents the basal planes from serving as reaction sites.

MXenes
MXenes are a class of 2D materials consisting of layers of transition metal carbides, nitrides, or carbonitrides [157].The versatile chemistry of MXenes leads to tunable properties for applications including energy storage, sensors, and catalysis [158,159].The general formula of MXenes is M n+1 X n T x , where M is an early transition metal, X is carbon and/or nitrogen, and T is a functional group.Metallic MXenes that possess inherently active basal planes have been investigated via scanning EC approaches.

Conductivity
Gupta et al employed SECM to quantify the HET kinetics of Fe(CN) 6 4−/3− at Ti 3 C 2 T x MXenes and discovered a strong dependence on flake thickness and the type of electrolyte [160].Djire et al have developed mixed transition metal nitride MXenes and investigated their HER catalytic activity with SECM [161].Specifically, SECM was employed to determine the conductivity and catalytic activity of individual V-Ti 4 N 3 T x nanoflakes.The feedback map shown in figure 11(a) using Fc as the redox mediator exhibits no measurable redox regeneration, while the SG-TC map in an acidic solution shows measurable activity toward HER (figure 11(b)).The SECM results were explained by proposing that the basal planes of these V-Ti 4 N 3 T x are catalytically active despite their limited conductivity, due to the large exposed metallic sites available for proton adsorption.However, we note that modification of the conductivity and EC behavior by H + adsorption or intercalation [162] could be an alternative explanation for this apparent contradictory behavior.By increasing the V loading, enhanced conductivity and metallic character were observed in the feedback and SG-TC maps (figures 11(c) and (d)).

Charge storage mechanism
Titanium carbide MXenes exhibit excellent performance as supercapacitors, with a charge storage mechanism associated with fast ion intercalation into the interlayer space.Cabre et al conducted an SECCM study [163] to analyze a small subregion of a monolayer Ti 3 C 2 T x flake where contributions from ion-intercalation processes were eliminated, allowing them to isolate surface-dependent processes that contribute to MXene pseudocapacitive response.Using an SECCM approach, cyclic voltammograms (figures 11(e) and (f)) and quantitative analysis of  the pseudocapacitive response revealed that entire MXene flakes were charged through EC contact of only a small basal plane subregion, corresponding to as little as 3% of the surface area.These results suggest that the proton transport across the surface acts as a complementary mechanism during the fast charging/discharging of MXene-based supercapacitors.
In table 3  through SECM/SECCM and correlative characterization techniques.We also the benefits of the versatile SECM in understanding the correlation between conductivity and reactivity, as well as the capability of SECCM in selectively probing specific sites to clarify charge storage mechanisms.

Conclusions and perspectives
In this review, we have highlighted the development of scanning EC probe approaches to address questions related to the (photo)electrochemistry of 2D materials that are inscrutable using conventional methods.Experimental conditions of these research efforts have been outlined in tables 1-3.Versatile scanning EC platforms have proven to be capable of resolving EC information with a high spatial resolution to understand the structure-function relationships of the electroactive 2D materials.

Advantages and limitations
The most advantageous qualities of SECM/SECCM are originated from the miniaturization of the EC probes.Specifically, the high-resolution EC mapping has been shown to be effective in identifying heterogeneities that are buried in ensemble measurements, providing a general strategy to visualize the activity variation across the surface of a 2D material.As a result, the effects of morphology, defects, dopants, strain, substrate, and other factors are readily examined.Other benefits include (1) these nondestructive approaches allow the materials to be studied without altering their intrinsic properties; (2) the 2D material samples are not required to be manufactured into a device format; (3) the (micro)nano scale measurements reduce the background and improve the signal-to-noise ratio; (4) the ability to detect and quantify intermediates provides unique insights into reaction mechanisms; (5) both in-situ and ex-situ spectroscopic/microscopic analysis can be coupled to correlate activities with physical properties.In comparison to other EC techniques, SECM/SECCM presents distinct advantages.Ensemble measurements using a conventional threeelectrode setup fall short in resolving local variations.While integrating individual 2D flakes into on-chip devices [16,164] allows for local EC interrogation, its spatial resolution typically remains confined to micrometer scales.It is important to note that polymer contamination from the photoresist often proves unavoidable, undermining measurement reliability.Microdroplet techniques [60,111] bear conceptual similarities to SECCM, yet their throughput is constrained as only a small fraction of the 2D material can be probed at any given time.EC-STM [165] or EC-AFM [166] enables real-time observation of nanoscale surface morphology changes but is constrained by its inability to capture potentialdependent images.Moreover, the close proximity of the tip may interfere with EC processes at the 2D layers, potentially inducing shielding effects and introducing inaccuracies in measurements.
Despite the high merit of SECM/SECCM in investigating the electrochemistry of 2D materials, some limitations and challenges need to be overcome to push their applications to the next level.(1) The spatial resolution of SECM is influenced by both the size of nanoelectrode tip and diffusional broadening [21].The resolution of SECCM is determined by the size of nanopipette probe and wetting property of the sample.Strategies to push their resolution to a few nanometers or atomic scale are yet to be developed.
(2) The fabrication of SECM tips can be complex and often irreproducible, while the knowledge of geometry and dimensions of the probes is crucial for reliable quantitative data analysis.(3) Both highresolution SECCM and SECCM mapping experiments are time-consuming.This limitation demands high instrument stability that needs to be engineered through environmental control (e.g.vibrational isolation and humidity control).The poor temporal resolution limits studies of rapid dynamic EC processes.(4) Electroanalytical techniques (e.g.voltammetry, amperometry, and impedance spectroscopy) integrated in SECM/SECCM alone are incapable of providing comprehensive chemical information in complex catalytic reactions, limiting the ability to characterize intermediates/products distribution and reaction mechanisms.

Perspectives on technique development
The intuitive approaches to improve the spatial resolution of SECM/SECCM are probe miniaturization down to nanometers.While this is attractive, one should account for the changes of the fundamental behavior of nanoprobes associated with the shrink of size.Drastic double layer effects can impact the migration of redox ions that leads to either enhancement or inhibition to faradaic current [167].As a result, factors including electrolyte concentration, solvent dielectric property, and surface charge needs to be considered.Careful characterization of nanoelectrodes and nanopipettes is critical because unambiguous geometric parameters of the probes are necessary to interpret data.In SECCM, the true electroactive area is estimated by characterizing the meniscus 'footprint' with other microscopies [29].Consequently, the surface chemistry of probed materials and the polarity of the solvent should be considered to control the meniscus contact.Further discussions on probe fabrication and characterization can be found elsewhere [22,32,33,168].Alternative approaches to improve the spatial resolution other than probe engineering have been developed based on data postprocessing [27,169].For instance, algorithms to produce point spread function corrected SECM image have been developed to relax the requirement of small probes [170].
Reproducibility in SECM and SECCM may be impacted by fluctuations in probe geometry and surface hydrophobicity.As discussed earlier, meticulous characterization of probes is essential for ensuring dependable measurements.Surface modification of an SECCM probe, often achieved through silanization [171] of the glass nanopipette's outer walls with hydrophobic or hydrophilic groups, can bolster the stability of meniscus contact.An alternative approach, oil-immersed scanning micropipette contact method [172] improves droplet stability, particularly beneficial for aqueous solutions susceptible to evaporation in uncontrolled humidity environments.
Environmental variables like temperature and humidity can introduce variability in repeated measurements.Maintaining stable humidity levels is crucial for consistent SECCM measurements, achievable through humidified gas streams or humidifiers.Recent advancements involve integrating SECCM setups into glovebox environments [173], heightening precision when examining moisture and oxygensensitive materials.
Slow imaging speed remains as main challenge in SECM/SECCM mapping measurements.Developing new scanning patterns [174] and fluid dynamic simulations [175] to aid high-speed scanning have shown immense potential for applications in large data acquisition.We also envision that implementing artificial intelligence driven automation [176,177] will find a niche in scanning probe electrochemistry.In methods as such, self-driving experiments enabled by artificial intelligence are performed to identify and measure representative data in lieu of full dataset [176], therefore promoting the efficiency of measurements.However, this algorithm may not apply to complex EC systems with heterogeneities, and careful verification of the data fidelity is required.
Integrating SECM/SECCM with complementary techniques [178] is another promising avenue to pursue the comprehensive characterization of 2D materials.Combining in-situ Raman spectroscopy [15] and SECM/SECCM is expected to provide vital information on the evolution of structure, charge density, and surface chemistry of the 2D electrode during a reaction, although matching the optical resolution with that of SECM/SECCM and eliminating the interference from probe tips remain challenging.Coupling scanning EC probes to mass spectrometers [179] would allow researcher to extract spatially resolved product distribution in complex reactions such as CO 2 catalysis and EC organic synthesis.Challenges in achieving sufficient sensitivity to detect chemistry taking place at the nano-or micron-sized interfaces should be noted.Fabricating 2D materials in a field-effect transistor configuration [120,180] is particularly promising in measuring/controlling the electronic transport in a 2D layer while it is interrogated with SECM/SECCM.Multifunctional analytical platforms such as these would tremendously expand the versatility of scanning EC probe techniques and make them even more suited to investigating the rich physics and chemistry of 2D materials.

Perspectives on future objectives
There remains a wide space to expand the availability of SECM/SECCM to explore the untapped domains of 2D materials.While the studies described in this review are mostly devoted to revealing the activities of structural features, a wide variety of additional strategies to activate the inert pristine surfaces of 2D layers remain to be explored.For instance, the modulation of electronic structure by means of ion intercalation [181,182], external field effects [183][184][185] and other 'quantum' engineering [186] approaches are expected to impact the HET kinetics and catalytic activities.In addition to the reactions covered in this review, 2D materials have been shown to support other important reactions such as CO 2 reduction and ammonia synthesis.The complicated reaction mechanisms of CO 2 reduction [187,188] and nitrogen fixation [189] are largely hypothesized without direct experimental detection of intermediates.The opportunities for SECM/SECCM include using high-resolution mapping to identify catalytic active sites and adopting the nanogap methods [190,191] to detect intermediates that can elucidate the complex mechanisms of these multi-electron, multiproton reactions at the 2D catalysts.The chemical stability of the materials themselves requires attention as 2D catalysts undergo different degrees of corrosion [192] while mediating interfacial chemical transformations.
Many more materials and derivatives in the existing library of 2D crystals deserve attention.For instance, black phosphorus [193] is a layered semiconductor with distinctive properties for applications in energy storage and catalysis [194].Twodimensional (2D) analogues of metal-organic frameworks possess a high degree of exposed catalytic active sites and tunable structures that are also suitable for the synthesis of single-atom catalysts with high reported electrocatalytic activities [195].The versatility and tunability of 2D materials may be further enhanced by fabricating vdW heterostructures [25,[196][197][198][199] formed with dissimilar 2D layers.The new chemistry produced by constructing distinctive vdW heterointerfaces awaits rigorous investigation with nanoscale EC microscopy.
The capability of SECM and SECCM renders them indispensable in sensor technology and energy storage.SECM/SECCM enhances sensor design by enabling precise mapping of EC activity on sensor surfaces, crucial for crafting highly sensitive and selective sensing platforms catering to a myriad of analytes, from small molecules to biomolecules [200,201].The growing application of 2D materials in sensors is propelled by their distinctive attributes including a high surface-area-to-volume ratio, abundant reactive sites, and mechanical resilience and flexibility, aligning seamlessly with cutting-edge technologies like wearable electronics [202].We foresee exciting prospects in amalgamating SECM/SECCM with the developmental trajectory of 2D materials in sensor innovation.Both SECM and SECCM offer avenues to scrutinize the formation of SEIs on electrodes-a pivotal facet of battery research [203].Furthermore, by unraveling multiple charge transfer and mass transfer mechanisms in the complex EC systems, SECM/SECCM can directly impact the evolution of more efficient metal-ion batteries [203,204].The suitability of 2D materials as advanced battery electrode materials stems from their expansive specific surface area, minimized ion diffusion distances, and capacity to mitigate volume fluctuations during repeated charge/discharge cycles [205,206].The convergence of SECM/SECCM with the evolution of 2D materials for energy storage applications holds great promise.
In summary, scanning EC probe techniques have shown immense potential to become some of the most powerful tools for the characterization of 2D materials.Continued collaborations among chemists, physicists, and materials scientists will realize the next generation of high throughput EC microscopy and accelerate the discovery of exciting phenomena in this unique family of materials.

Figure 2 .
Figure 2. (a) Schematic of a mechanically induced defect on the graphene electrode.(b) SECM image of the mechanically induced defect.Reprinted with permission from [51].Copyright (2012) American Chemical Society.(c) Raman mapping of the D band of the defective graphene patterns.(d) SECM image of the defective graphene patterns.Reprinted with permission from [52].Copyright (2014) American Chemical Society.(e), (f) AFM topographical image (e) and SECCM current map (f) for the reduction of reduction of Ru(NH3)6 3+ .Reprinted with permission from [58].Copyright (2015) American Chemical Society.

Figure 3 .
Figure 3. (a) Optical image of CVD graphene with four different flakes labeled as A1 (monolayer), A2 (bilayer), A3 (trilayer), and A4 (multilayer) and the corresponding SECCM map of FcTMA + .(b) Correlation between the electrochemical current at a kinetic-controlled potential and the number of graphene layers.Reprinted with permission from [57].Copyright (2012) American Chemical Society.(c) SECM feedback image of Au patterned graphene spot and exposed Au spot and (d) calculated k mapping of ferrocyanide oxidation kinetics at graphene/Au area of an exfoliated graphene on a Si/SiO2 substrate.Reprinted from [62], Copyright (2016), with permission from Elsevier.(e) SECCM images of [Ru(NH3)6] 3+ reduction (top) and schematic diagram of the 2D heterostructure stacking (bottom).(f) Histogram of E1/2 values measured on ML graphene and BL graphene.Reproduced from[63], with permission from Springer Nature.

Figure 4 .
Figure 4. (a) Schematic of local voltammetric measurement at a TBG surface in an SECCM setup equipped with a single channel nanopipette probe; (b) optical image of a TBG/hBN heterostructure connected to a graphite contact; (c) constant current STM image of 1.15 • TBG acquired from the region marked with a yellow dot in (b); (d) representative steady-state voltammograms of 2 mM [Ru(NH3)6] 3+ solution obtained at graphene monolayer (blue), Bernal stacked bilayer (grey), 10 nm thick graphite (orange), and 1.15 • TBG (purple).(e) Standard rate constants (k • ) extracted from the experimental voltammograms as a function of twist angle compared to the theoretical values.Reproduced from [66], with permission from Springer Nature.(f) Dependence of the ET rate constant, k 0 , on the trilayer graphene stacking type (ABA, ABC) and θm for M-t-B TTG.Reprinted with permission from [69].Copyright (2023) American Chemical Society.

Figure 5 .
Figure 5. (a) SEM image and (b) SECM feedback image of an agglomerate of r-GO flakes deposited on Si-SiO2 substrate.(c) Schematic representation of the electronic pathway in an SECM feedback experiment.Reprinted with permission from [74].Copyright (2014) American Chemical Society.(d) SEM and (e) SECCM images of the graphene with the wrinkle.Reprinted from[76], Copyright (2022), with permission from Elsevier.(f) High-resolution electrochemical activity (i) and co-located quasi-topographical maps of a graphene|Nafion membrane, using SECCM in the voltammetric hopping mode configuration (right).Reprinted with permission from[77].Copyright (2022) American Chemical Society.(g) SECCM current map of proton transport and (h) AFM force map of graphene suspended over micrometer-sized holes etched into silicon nitride (SiNx) substrates.Reproduced from[78], with permission from Springer Nature.(i) SECM feedback images of TMPD on fresh patterned graphene at substrate potentials representing conditions before, during, and after (top to bottom) SEI formation.(j) CV-SECM image of patterned graphene sample denoting Li + flux (left) and SEM image of a similar region (right).Reprinted with permission from[81].Copyright (2016) American Chemical Society.

Figure 6 .
Figure 6.(a) Schematic of SECM SG/TC mode.(b) SECM SG/TC maps of H2O2 production over graphene/p-Au.(c) Schematic of SECM redox competition mode with the substrate and Hg-capped Pt UME simultaneously biased to reduce O2.(d) SECM map of electronic coupling with an asymmetric toroidal Pt underlayer increases the local competitive consumption of O2 by FeOEP adsorbed on graphene.Reprinted with permission from [92].Copyright (2018) American Chemical Society.(e) SECCM topography and (f) HER current on NP-doped graphene.[97] John Wiley & Sons.© 2019 The Authors.(g) Current and (h) charge density distribution of different sites using SECCM.Reprinted with permission from [98].Copyright (2021) American Chemical Society.

Figure 7 .
Figure 7. (a) Diagram showing the feedback between the tip and substrate.(b) AFM image of a MoS2 flake after SECM measurements.(c) SECM image of MoS2 in Ru(NH3)6 3+ .Reprinted with permission from [112].Copyright (2018) American Chemical Society.(d) Schematic representation of SECM positive feedback produced by oxidation/reduction of Fc.(e) Feedback mode image of a mixed-phase MoS2 flake on ITO.Reproduced from [116], with permission from the Royal Society of Chemistry.(f) Feedback mode SECM images of 2H MoS2 and (g) n-type 2H Et2N-MoS2 on ITO with 1 mM Fc. Reproduced from [117].© IOP Publishing Ltd.All rights reserved.(h) SECM map of high density of bilayer flakes on monolayer MoS2 using Fc mediator, white bar indicates 3 µm.(i) Local variation of the work function as recorded in KPFM.(j) SECM feedback for a bilayer within a monolayer MoS2 flake using DmFc mediator.Reproduced from [119], with permission from Springer Nature.

2D Mater. 11 (
2024) 032001 P Adanigbo et al needed to dominate the EC response of the entire surface area.

Figure 9 .
Figure 9. (a), (b) Photoelectrochemical imaging of hydrogen evolution at individual p-type WSe2 nanosheets.Correlated topographies (left) and photocurrent images are displayed.Reprinted with permission from [141].Copyright (2019) American Chemical Society.(c) Binary AFM image depicting edge features (i), edge density map (ii), SECCM image depicting HER photocurrent (iii), and image depicting potentials necessary to reach HER currents of 20 pA (iv).Reprinted with permission from [143].Copyright (2020) American Chemical Society.(d) Schematic of carrier generation-tip collection scanning electrochemical cell microscopy (CG-TC SECCM) used to map the minority carriers diffused within the material.(e) Optical transmission image of n-WSe2 nanosheet with the area imaged via SECCM.(f) Photocurrent images at a series of different applied potentials.Reproduced from[145], with permission from the Royal Society of Chemistry.(g) An optical image of a WSe2/WS2 heterostructure.(h) PL and (i) photocurrent maps of the sample in (g).Reproduced from[147].© IOP Publishing Ltd.All rights reserved.

Figure 10 .
Figure 10.(a) Schematic representations of substrate generation/tip collection of dioxygen at NiO nanosheet.(b) SG/TC mode SECM images of the NiO nanosheet with defect holes exposing the underlying HOPG and (c) high resolution SG/TC image of the NiO edge.Reproduced with permission from [155].© 2019.Published under the PNAS license.(d) Schematic demonstrating the SECCM technique probing a glassy carbon (GC) support with dispersed isolated β-Co(OH)2 particles.(e) Tip current density as a function of position in scanning constant height mode for the particles shown in the FE-SEM images on the right.(f) Topography and local current density maps at increasing applied voltage.Scale bar, 5 µm.Reproduced from [156], with permission from Springer Nature.

Figure 11 .
Figure 11.(a) SECM feedback mode and (b) SG-TC mode image of a V-Ti4N3Tx MXene sample.(c) SECM feedback mode and (d) SG-TC mode image of a V-Ti4N3Tx MXene sample prepared from higher V loading.[161] John Wiley & Sons.© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(e) Schematic of end of SECCM probe, highlighting the nanoscale electrochemical droplet cell at the end of the SECCM probe and the two-electrode electrochemical cell configuration.(f)Representative cyclic voltammograms over a carbon surface (black) and a single monolayer MXene flake (orange) at a scan rate of 0.5 V s −1 in 20 mM HClO4.Reproduced from[163], with permission from Springer Nature.

Table 1 .
Summary of scanning electrochemical studies of graphene and its derivatives.

Table 2 .
Summary of scanning electrochemical studies of TMDs.

Table 3 .
Summary of scanning electrochemical studies of TMOs and MXenes.
we summarize SECM/SECCM work on 2D TMO, TMH, and MXenes.Unique insights into the roles of structural heterogeneities were gained