Contactless Surface Conductivity Mapping of Graphene Oxide ThinFilms Deposited on Glass with Scanning Electrochemical MicroscopyJoel Azevedo,† Celine Bourdillon,§ Vincent Derycke,† Stephane Campidelli,† Christine Lefrou,‡
and Renaud Cornut*,§
†CEA, IRAMIS, Service de Physique de l’Etat Condense, Laboratoire d’Electronique Moleculaire, F-91191 Gif sur Yvette, France‡Laboratoire d’Electrochimie et de Physico-chimie des Materiaux et des Interfaces, UMR 5279, CNRS-Grenoble-INP-UdS-UJF, 1130rue de la piscine, B.P. 75, Domaine Universitaire, 38402 Saint Martin d’Heres Cedex, France§CEA, IRAMIS, Service de Physique et de Chimie des Surfaces et Interfaces, Laboratoire de Chimie des Surfaces et Interfaces,F-91191 Gif sur Yvette, France
*S Supporting Information
ABSTRACT: The present article introduces a rapid, very sensitive, contactlessmethod to measure the local surface conductivity with Scanning ElectrochemicalMicroscopy (SECM) and obtain conductivity maps of heterogeneous substrates. Itis demonstrated through the study of Graphene Oxide (GO) thin films depositedon glass. The adopted substrate preparation method leads to conductivitydisparities randomly distributed over approximately 100 μm large zones. Datainterpretation is based on an equation system with the dimensionless conductivityas the only unknown parameter. A detailed prospection provides a consistenttheoretical framework for the reliable quantification of the conductivity of GO withSECM. Finally, an analytical approximation of the conductivity as a function of thefeedback current is proposed, making any further interpretation procedurestraightforward, as it does not require iterative numerical simulations any more.The present work thus provides not only valuable information on the kinetics ofGO reduction in mild conditions but also a general and simplified interpretation framework that can be extended to thequantitative conductivity mapping of other types of substrates.
Fullerene and carbon nanotubes have been the subject ofintense research in the past decade as promising carbon
nanomaterials. During the past few years, researchers have alsoincreased their attention to graphene, this one atom thickhoneycomb lattice of carbon atoms. Grabbing the scientificcuriosity and revealing its outstanding electrical and mechanicalproperties were not enough, and graphene is now promised torevolutionize a large panel of applications. However, materialavailability is still a key challenge to efficiently introduce thismaterial in practical devices.The oxidation and exfoliation of graphite into Graphene
Oxide (GO) is commonly accepted as one of the mostpromising scalable route to obtain large graphene flakes.1
However, this approach faces two crucial requirements: findingnot only an effective reduction step to convert GO into aconductive material2 but also an effective, reliable, andnondestructive method to measure its electrical properties. Inthe literature, among the numerous studies devoted to thereduction of GO (see ref 3 and references therein), very fewconcern the use of alkaline conditions.4 In the present study,the reduction of GO in such alkaline conditions but at roomtemperature is highlighted, and the reaction kinetics isinvestigated by measurement of the substrate’s conductivityafter different exposition times.
The conductivity of various thin films is generally evaluatedusing a four-point probe technique (see for example ref 5). Thisrequires putting metallic tips in contact with the substratewhich unavoidably damages the film, all the more when it isonly few nanometers thick. In addition, this procedure providesa conductivity evaluation that is averaged over large areas oftypically few millimeters. This is acceptable for highlyhomogeneous films but not suitable for very thin GO filmsfrom which it cannot reveal the local variability of conductivityat the micrometric scale. Measurements by conducting-AFMcould be a credible alternative as it is commonly used for theelectrical characterization of molecular materials,6 but thecomplex interactions between the probe and the substrate makedifficult the quantitative determination of the local conductivity.Most importantly, conventional conducting-AFM can only beperformed on a conductive underlying substrate and would notbe a simple alternative for the present study, where thegraphene oxide is deposited on an insulator.Scanning Electrochemical Microscopy (SECM) is an efficient
analytical method for the local characterization of interfaces at a
Received: October 31, 2012Accepted: December 22, 2012Published: December 22, 2012
micrometric scale.7 The method is based on the contactlesselectrochemical interaction of a microelectrode with theinterface to be analyzed, generally a solid−liquid interface.Until now, very few SECM studies have involved graphene.The pioneer works have investigated charge transfer8 andquantified the surface diffusion coefficient of a tripodaladsorbate on graphene,9 but, to the best of our knowledge,graphene oxide has never been investigated with SECM.In principle, SECM can be used to evaluate the local
conductivity of the substrate without any solid−solid contact, asit is a solution containing a redox species that transports thecurrent between the probe and the substrate. A model based ona first order kinetics of redox mediator regeneration at theelectrolyte/substrate interface combined with electron trans-port in the substrate material has been previously introduced.10
Such a model has been used in a few studies, for example forthe conductivity measurement of composite Langmuir-Shaeferfilms,11 polyaniline monolayers,12 gold nanocrystals ensem-bles,10c or porphyrin films.13 These studies are focused on theanalysis of homogeneous substrates, through the record of theprobe current during its approach to the substrate. On thecontrary, the present study deals with heterogeneous substratesthat are analyzed by mapping the feedback current at a givendistance from the substrate. Conductive spots of finite size arethus investigated theoretically in order to determine the mostappropriate conditions for quantitative conductivity mapping.In the present study, quantitative mapping of the electronic
conductivity of reduced GO layers is performed with SECM. Asverified, the charge transfer on reduced GO is not limitingwhen the usual redox mediator that is ferrocenedimethanol(FcMeOH) is used, so that it gets possible to simplify theoriginal equation system, with a feedback response thatdepends on only one parameter, i.e. the dimensionless substrateconductivity. A deep investigation of the model results isperformed as well as a discussion on the most appropriateconditions (microelectrode geometry, active spot size, redoxmediator concentrations) for the evaluation of the localconductivity from feedback current measurements. Addition-ally, an analytical approximation of the conductivity as afunction of the relevant parameters of the study is proposed.The present study thus provides a general and simplifiedinterpretation framework that can be extended to thequantitative conductivity mapping of other types of materialsthan GO.
■ EXPERIMENTAL AND TECHNICAL SECTIONMaterials. Electrochemical measurements were performed
in aqueous media (MiliQ water 18.2 MΩ),) and potassiumchloride (KCl) as supporting electrolyte. Electrode fabricationrequired 25 μm diameter Pt-wire (Goodfellow, purity 99.9%),glass Pasteur pipettes, silver conducting paint (Electrolube),and standard copper connection wires (diameter <0.3 mm).Standard procedures using a puller machine (PC10 Narishige)and abrasive disks (P4000 and P1000) were adopted. Theresulting electrodes had a surrounding glass thickness rangingapproximately from 30 to 100 μm, as observed with opticalmicroscope.Substrate Preparation. All standard chemicals and
solvents of research grade were purchased from Sigma Aldrichand used as received. 150 nm-SiO2 covered Si wafers weredegreased in acetone and ethanol by ultrasonication andcleaned in a mixture of concentrated hydrochloric and nitricacid (3:1 ratio) at 90 °C for 1 h. The graphene oxide film
formation through the bubble deposition method has alreadybeen described in a previous report.14 It leads to extremely thinfilms of about 3−5 nm thickness composed of percolating GOflakes. In a typical procedure, GO films are heated (near 100°C, during 1 min) and rinsed with ethanol and water in orderto remove surfactant residues. The reduced GO substrates areobtained by exposition to a pH 13 aqueous solution (KOH0.1M) at room temperature, for durations ranging from 30 s to2 min. A thermal annealing (1 h at 150 °C, under vacuum) wasperformed after each exposition.
Electrochemical Setup. SECM experiments were allperformed on the Princeton Applied Research 370 SECMWorkstation. A conventional three-electrode setup was used forthe voltammetry and SECM experiments. It involved aplatinum (Pt) microdisk working electrode, an Ag/AgClelectrode as reference, and a 0.5 mm diameter gold wireauxiliary electrode. 0.1 M KCl was used as supportingelectrolyte.A VSP Biologic potentiostat was used for the electrolysis of
the FcMeOH solutions in order to perform SECM experimentswith a mixture of reduced (Red) and oxidized (Ox) mediatorforms in solution. A Pt grid was used as working electrode,polarized at 0.4 V vs Ag/AgCl. It took about 1 h to transformhalf of the FcMeOH initially present in solution, as evidencedby cyclic voltamograms of the solution before and after theelectrolysis (see section 8 of the Supporting Information).
Numerical Calculations. All the numerical calculationshave been performed using Comsol Multiphysics 4.2, installedon an Optiplex 720 Dell workstation, with 8 GB RAM, i7-2600-CPU. See section 4 of the Supporting Information for moredetails on the calculation method.
■ RESULTS AND DISCUSSION
Model. In SECM, the most widespread operating mode isbased on the evaluation of the substrate regeneration rate of aredox mediator present in solution and consumed at themicroelectrode. This situation, generally called the feedbackmode,15 was considered in the present study. During anexperiment, if the electrons are not brought by an externalsource (i.e., in an unbiased situation), the regeneration of themediator underneath the tip is necessarily associated with acounter reaction at the same substrate. It occurs at a differentplace than that of the regeneration reaction, so that theelectrons need to be carried through the substrate.10c In thisway, this is similar to the galvanic coupling in corrosion,16 witha unique substrate that has anodic and cathodic zones.In the present study, the charge transfer at the film−solution
interface is assumed to be fast and has thus a reversible andNernstian behavior at any point of the interface
= ° +⎛⎝⎜
⎞⎠⎟E r E
R T
nFrr
( ) ln[Ox]( )
[Red]( )p
(1)
with Rp the perfect gas constant, T the temperature, n thenumber of electrons exchanged per molecule of redox mediator,and E the substrate electric potential at a radial coordinate r. Itmust be underlined that the presently considered equationsystem is independent of the exact electrochemical mechanism,provided that reversibility is fulfilled.The electron transport inside the thin film is assumed to
occur according to a local Ohm’s Law. Associated with thecurrent balance in cylindrical symmetry, this leads to
with r and z the radial and axial coordinates, and σ the surfaceconductivity. See the Supporting Information section of ref 10cfor more details. Using the usual dimensionless parameters forconcentration, length, and potential (see sections 2 and 3 of theSupporting Information), the equation system can be rewrittenand brings up the dimensionless conductivity σ*
σ σ* =*
eR T
nF nFD r1
[Red]p
tip (3)
with D the redox mediator diffusion coefficient, e the thicknessof the conducting film, and rtip the radius of the active part ofthe probe. Equation 3 is the same as that introduced in previousstudies of surface conductivity measurements with SECM.10c
The complete equation system is available in section 4 of theSupporting Information. As usual in SECM, the probe currentdata are converted into a normalized current
=Nii
iTT
T,sol (4)
where iT is the probe current, and iT,sol is the probe currentmeasured far from the substrate. In addition to thedimensionless conductivity σ*, NiT depends on severalparameters, such as the dimensionless probe substrate distance(L = d/rtip), the surrounding glass thickness of the probe (Rg),and the ratio of Red concentration divided by the sum of theconcentrations of Red and Ox species initially present insolution (N = [Red]*/([Red]*+[Ox]*)). According to Nernstlaw, this latter quantity is related to the equilibrium potential(Eeq) or the electrochemical equilibrium potential of electrons(μeq). Therefore it plays the same role as μeq in the previousstudies of surface conductivity measurement with SECM.10c
Among all the experimental parameters of the study, σ*remains as the only unknown quantity and can therefore bedetermined from one measure of NiT. Mapping theconductivity of a heterogeneous substrate is thus accessiblefrom the recording of one NiT map.Hypothesis Validation. Before any prospective analysis of
the model described above, it is necessary to verify that it isappropriate for the local characterization of reduced GO layersdeposited on glass. First, it must be verified that the
conductivity of reduced GO is effectively influencing themeasurement. Figure 1a presents experimental approach curvesof a reduced GO layer for different redox mediatorconcentrations. The feedback responses during the approachare very different: it goes from a slow decrease at highconcentration to a strong increase at low concentration. Thenormalized feedback current is thus clearly depending on theredox mediator concentration, contrary to what would havebeen obtained in the case of a perfect conductor with a limitedregeneration kinetics.17 These experiments prove that thesurface conductivity of the reduced GO is influencing thefeedback. Mathematically, this is related to the fact that whendecreasing the redox mediator concentration, the dimensionlessconductivity σ* is increased (see eq 3), while the dimensionlesskinetics parameter κ does not depend on the redoxconcentration (see section 3 of the Supporting Information).A model implementing the surface conductivity is thus
appropriate for the analysis of the experimental data. Besides,an originality of the present model lies in the fact that interfacialcharge transfer kinetics is considered as fast and therefore notlimiting, leading to eq 1. As a matter of fact, it is necessary tocheck that this hypothesis is appropriate for the study of asubstrate covered by reduced GO. Figure 1b presents thecalculated normalized current at L = 0.5, as a function of thedimensionless conductivity, for different interfacial kinetics. Itshows that the normalized current is limited to a value that issmaller than pure positive feedback if the interfacial kinetics isnot fast, even at small concentration (or large dimensionlessconductivity). In the case of the approach of reduced GO withFcMeOH as redox mediator, Figure 1a shows that the lowconcentration approach curve, with a maximal increase of thetip current by a factor of 6, is very close to pure positivefeedback (black continuous line). Therefore, the charge transferkinetics is large, which is in accordance with the outer-spherecharge transfer nature of this mediator. This validates thereversibility feature (eq 1) to interpret measurements withreduced GO layers deposited on glass and shows how changingthe redox mediator concentration can be used to check thereversibility hypothesis. It must be underlined that such averification procedure is not limited to the presentlyinvestigated substrates. The next parts detail the conditionsfor SECM to be particularly well-suited for analyzingheterogeneous substrates, through the study of conductivespots of limited size.
Figure 1. (a): Experimental approach curves above a reduced GO layer with FcMeOH at different concentrations (see figure), for a probe polarizedat 0.4 V vs Ag/AgCl, Rg = 10, rT = 12 μm, N = 1). Black continuous line and black dotted line: theoretical pure positive and negative feedbackapproach curves, respectively (Rg = 10) . (b): Normalized current as a function of the substrate conductivity for different charge transfer kineticconstants (see figure), for L = 0.5, Rg = 10, Rs = 100, and N = 1. For the same Rg, L, Rs, and N values, the normalized positive and negative feedbackcurrents are shown in the figure (respectively NiT,+ = 2.24 and NiT,‑ = 0.29).
Bipolar Behavior Limitations. In order to provide a deepunderstanding of the processes occurring during measurementsin the case of an unbiased conductive area of limited size, weconsider in this first step the simplified case of a perfectlyconducting spot. The impact of the bipolar nature of thesystem, i.e. the role of the mass transport associated with thecounter reaction, is investigated. As shown in Figure 2a, this
situation bears some resemblances to the one investigated byOleinick et al.18 In that study, the authors have specificallydesigned a substrate in order to separate the location of the tworeactions, so that the diffusion processes do not interact witheach other. The present part deals with a more conventionalsituation, where a circular conducting substrate is deposited onan insulator, as presented in Figure 2a. In this case, it is possibleto derive an analytical approximate expression of the current.The detail of the calculations, valid when one can consider thatthe probe hardly disturbs the counter reaction process, ispresented in section 5 of the Supporting Information. It leadsto
β=
++
+Ni
R Ni
Ni R(Rg)TS T,
T, S (5)
In this equation, β(Rg) is the well-known correcting factoraccounting for the impact of Rg on the current:19 iTsol =(4nFD[Red]*rtip)β(Rg), with iTsol the probe current measuredfar from the substrate.Figure 2b presents the comparison of the results obtained
with the formula and those obtained with the exact model,when the probe is approaching the center of the substrate, for L= 0.5, Rg = 2, and N = 1. It shows that eq 5 is accurate as longas Rs > 5 (this validity range strongly depends on Rg and to alower extent on L). This surprisingly large range of validity isrelated to the fact that the counter reaction current density isalmost entirely concentrated close to the edge of the activespot, so that the presence of the probe in its center is of littleconsequence (see more details in section 5 of the SupportingInformation). In addition, Figure 2b illustrates the specificity ofunbiased measurements: the feedback current dependssignificantly on the size of the active spot. The figure showsthat in the case of a biased substrate it is not the case anymore,except for particularly small active spots, in accordance with theliterature.20
In fact, the situation of an unbiased substrate is particularlyproblematic for conductivity imaging. Figure 2c showssimulated lateral scans of unbiased conductive spots of differentsizes having a limited conductivity (σ* = 5). In this case again,the current depends on the spot size, which is in accordancewith literature data.21 More importantly, it remains constantduring a scan. As a consequence, contrary to reactivityheterogeneities, in the case of conductivity heterogeneities, acurrent that does not depend on the position of the tip is not areliable indication that the response of the spot is the same asthe one of an homogeneous substrate. This shows that theinterpretation of SECM experiments for conductivity mappingis particularly problematic since in this case, the link betweenthe current and the local conductivity strongly depends on thesurrounding substrate properties, without any experimentalwarning such as a current variation during the scan. It is thusvery important to clarify when it is possible to rely on thecurrent measurements for the evaluation of the localconductivity. This is detailed in the next part.
Optimization of the Experimental Conditions for anAccurate Conductivity Mapping. In this section, the mostappropriate experimental conditions for the evaluation of theconductivity are reviewed in detail. We show how a judiciouschoice of the experimental conditions allows avoiding aninaccurate evaluation of the conductivity from the feedbackcurrent. For this, a current that depends as little as possible onthe size of the active spot is clearly preferable, as in this case thelink between the measured current and the deducedconductivity does not necessitate the precise consideration ofthe surrounding conductivity distribution. First of all, bychanging N, the fraction of species reacting at the tip versus thetotal concentration in redox mediator, it is possible to decreasethe impact of the spot size. Figure 3a presents the current as afunction of the spot size for different N values. It shows that inthe absence of Ox in solution, i.e. for N = 1, the feedbackcurrent strongly depends on Rs even for very large Rs values(Rs = 20): see for example the difference between the value forRs = 20 and the corresponding dotted line for Rs = 100,representative of an homogeneous substrate. This particularsituation comes from the fact that when the concentration at
Figure 2. (a): Schematic representation of the bipolar nature of thefeedback process above an unbiased substrate. (b): Normalized currentas a function of the active spot size, for a biased and unbiased substratewith infinite conductivity. The current as predicted by eq 5 is alsopresented (Rg = 2, L = 0.5, N = 1). (c): Normalized current during alateral scan above unbiased substrates of different sizes, for L = 0.5, Rg= 2, N = 1, and σ* = 5.
the substrate is close to 1, a very small variation of theconcentration induces a very large change in the electricalpotential, as predicted by eq 1 (see section 6 of the SupportingInformation for more details). Importantly, the impact of thespot size is much less important when equal amounts of Redand Ox are present in solution (N = 0.5): in this case, even forRs = 5, the current is hardly different from the one obtainedwith larger spots. In fact, when N is close to 0.5, the electricalpotential depends much less on the concentration variation,and the driving force for the regeneration process is then muchless sensitive to a local change in the concentration at thesubstrate. As a matter of fact, experimentally, it is much morepreferable to work with a solution that has a significant amountof both Red and Ox species, ideally in equal concentration.In addition to N, the impact of the active spot size on the
feedback current depends on the conductivity of the spot, aspresented in Figure 3b. It shows that when conductivity is large,the feedback current is importantly depending on Rs, even ifthis latter is large. This comes from the bipolar effect limitation,as discussed in the previous section. In addition, it must bementioned that extreme values (large or low) of theconductivity lead to a feedback current that hardly dependson the conductivity, implying that the evaluation of this latterfrom the measurement will be very imprecise (see section 7 ofthe Supporting Information for more details). One should alsoadd that measuring an exact small conductivity may be difficult
in real situations because the contribution of the surroundingsubstrate may lead to an overestimation of the localconductivity. In fact, the most favorable situation arises whenthe measured conductivity is approximately between σ* = 0.2and σ* = 2. It is then advised to adjust the redox mediatorconcentration and eventually the microelectrode radius (see eq3) in order to avoid extreme values of conductivities.In addition to σ* and N, the surrounding glass thickness of
the probe (Rg) impacts the applicability of the infinite substratemodel. Figure 3c shows the normalized current as a function ofRs for different Rg and highlights that the current is differentfrom the one that is obtained at large Rs as soon as Rs is smallerthan Rg + 4. For smaller conductive spots, the transport ofreactive species used for the counter reaction at the substrate ishindered by the insulating part of the probe. Moreover it mustbe mentioned that the locations of the reactions occurring atthe substrate (the feedback reaction and the counter reaction)are much more distant if Rg is large, as the separation distanceof the reactions is typically equal to Rg. The distance overwhich the substrate is solicited for the electronic transport isthus larger for large Rg. This leads to the important variationsof the current obtained at large Rs when different Rg are used(dotted lines of Figure 3c). These two effects, i.e. hindering ofcounter reaction and increase of the electron transport length,combine to give the very important effect of Rg as observed inFigure 3c. Experimentally, working with small Rg is then verypreferable.Finally, L has also an impact on the results: the closer the
probe, the higher the contrast between the zones. On the otherhand, a too small value of L increases the risk of damaging thesubstrate by a contact between the probe and the substrate aswell as the sensitivity of the measurements to any eventualsubstrate’s topological features. One can mention that L has avery limited impact on the conditions for which Rs impacts theresults. To sum up, evaluation of the microscopic scaleconductivity without having to consider the explicit size andgeometry of the active spot is guaranteed for Rs > Rg + 4 and0.2 < σ* < 2 if one uses a mix of Ox and Red species for themeasurements. In an experimental procedure, it is then advisedto adjust the microelectrode geometry and the redox mediatorconcentration in order to fulfill these conditions.To obtain a conductivity map from SECM measurements, an
analytical expression of the conductivity as a function of themeasured normalized current can be very helpful, as itsuppresses the necessity of iterative numerical simulations inorder to find the σ* value that leads to the appropriate probecurrent. For this purpose, using a step by step procedure similarto the one detailed in the literature,22 eq 6 has been constructedfrom the observation of numerous numerical simulation results(for N = 0.5).
σ* = +
+ +
− − −
+ +
−
L
Ni
L
(0.138 0.0858)
( 0.22ln(Rg) 0.4)
0.1152Rg 0.0087Rg 0.03
L LT
0.6 2.27 /Rg 3.28 2.3 /Rg
0.2 2 0.165ln(Rg)
0.146 1.54
(6)
It is valid with 7% accuracy (when compared to the numericalsimulation results) for 0.3 < L< 1.5, 2 < Rg < 20, and 0.1 < σ* <5.
Application to the Study of the Reduction of GO byExposition to an Alkaline Solution. The previouslyintroduced theoretical framework has been applied to theinvestigation of the reduction kinetics of GO layers deposited
Figure 3. Normalized current as a function of Rs for L = 0.5 and (a)different N, Rg = 2, σ* = 1. (b) N = 0.5, Rg = 2, different σ*. (c) N =0.5, different Rg, σ* = 1. For each curve, the dotted line is thecorresponding normalized current at Rs = 100 (representative of anhomogeneous substrate).
on glass, when exposed to an alkaline solution (KOH 0.1 M)and then annealed at 150 °C under vacuum. In the literature,heated (50−90 °C) alkaline solutions are already known toreduce GO, leading to an increase of its conductivity.4
Conversely, we performed the experiment at room temper-ature: Figure 4a and 4c show area scans before and after 30 s ofexposition to a KOH solution (0.1 M) at room temperature.For each scan, the chosen experimental conditions lead to anoptimal accuracy, in accordance with the previous theoreticalprospection. In particular, the solutions used for the measure-ments contained both FcMeOH and FcMeOH+ (FcMeOH wasproduced by electrolysis of FcMeOH), the microelectrode hada small Rg (Rg = 3), and a more concentrated solution wasused to map the more conductive situation in order to have σ*values that are mostly within the 0.2−2 range.Figure 4b and 4d present respectively the same data as Figure
4a and 4c, after conversion of the measured current intoconductivity using eq 6 and 3. In eq 3, the value of theexperimental current when the electrode is far from thesubstrate (iTsol) is used instead of the value of each individualparameter (i.e., σe = σ*iTsolnF/4βRpT). From the measure-ments, without the precise knowledge of the film thicknesses e,it is only possible to deduce the product of the conductivitywith the thickness, σe. First, the figure shows that the surface isnot perfectly homogeneous. This probably comes from aninhomogeneous distribution of the GO flakes over the surfaceduring the processing, leading to areas where there are moreoverlapping flakes. Figures 4a and 4b show that the studied GOfilms present a small conductivity, even before exposition. Anaverage value of 0.005 μS (or 2 × 108 Ohm/square) isobtained, with about 50% variations. One has to mention that
four-point probe measurements on similar substrates lead to avalue of 0.003 μS with 50% uncertainty, which is close to theaverage value presently obtained with SECM. The differencebetween the two methods probably comes from theinhomogeneity of the films that is unavoidable with theadopted substrate preparation procedure. This conductivityvalues for substrate that have not been exposed yet may comefrom a very slight reduction that occurred during the treatmentof the films after GO deposition. As a matter of fact, thisillustrates the sensitivity of the SECM for surface conductivitymeasurements. Figures 4c and 4d show that very mildconditions (pH 13, room temperature, followed by thermalannealing at 150 °C under vacuum) are sufficient tosignificantly increase the conductivity of the substrate, as theaverage σe after 30 s exposition is 0.03 μS, with about 50%variation over the substrate. The same substrate after 2 minexposure and annealing has a similar conductivity map (seesection 9 of the Supporting Information), with an average valueof 0.05 μS, showing that an important part of the reactionoccurred within the first step.One can mention that in the present study, the measured
conductivities (more precisely the product σe) were never closeto the extreme values achievable with the technique. On theone hand, lower σe values could be measured, as theconcentration of redox mediator could be further decreasedwithout reaching the sensitivity limit of the current measure-ment. On the other hand, higher value of σe would have beenaccessible by an increase of the redox mediator concentrationup to the solubility limit of the FcMeOH. The precise rangedepends on the experimental set up and conditions.
Figure 4. Area scans with a 12 μm radius probe (Rg = 3) and a probe-substrate distance during the scans of 12 μm (L = 1); before and afterexposure of GO substrates to a pH = 13 solution. (a) and (b): before exposure, (a) tip current and (b) corresponding σe according to eq 6 and 3(iT,sol = 0.43 nA). (c) and (d): after 30 s exposure, (c) tip current and (d) corresponding σe (iT,sol = 1.21 nA). The scanned zones are not the same inthe two data sets.
■ CONCLUSIONThe present article introduces a rapid, very sensitive,contactless method to measure the conductivity with SECM.The method is particularly appropriate for low conductivitymaterials as proven by the investigation of graphene oxide filmsdeposited on glass and reduced under very mild conditions (pH13, room temperature, followed by thermal annealing at 150 °Cunder vacuum). In addition, the present study provides aconsistent theoretical framework for the reliable quantificationof the local conductivity with SECM. An analytical approx-imation of the conductivity as a function of the feedbackcurrent is proposed (eq 6), making any further interpretationstep very straightforward, by avoiding the use of iterativenumerical simulations. The method can be applied to aprospective analysis of the different GO reduction procedures,as well as to the conductivity characterization of othersubstrates. Contrary to typical four-point probe conductivitymeasurements that only provide average conductivity values ata millimeter scale, SECM proves to be very powerful to revealmicrometric conductivity inhomogeneities, the impact of whichcan be of critical importance for an effective integration ofreduced GO films in practical devices.
■ ASSOCIATED CONTENT*S Supporting InformationAdditional information as noted in text. This material isavailable free of charge via the Internet at http://pubs.acs.org.
■ ACKNOWLEDGMENTSC. Zobrist is acknowledged for technical assistance with theconstruction of the TOC. J. Azevedo acknowledges fundingfrom the DGA. C. Bourdillon and R. Cornut acknowledgefunding from ANR-PDOC20011-Copel.
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