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单晶硅表面等离子体基离子注入碳纳米薄膜的摩擦学特性IntroductionSingle-crystal silicon is a widely used material in various technological applications due to its desirable mechanical properties. However, its poor tribological behavior under sliding friction hinders its widespread use. Surface modification techniques such as ion implantation have been applied to enhance its tribological behavior. In this study, we investigated the frictional characteristics of carbon nanofilm implanted on a single-crystal silicon surface by plasma-based ion implantation.Experimental MethodsThe experiments were conducted using a plasma-based ion implantation system. The single-crystal silicon samples were cleaned and then implanted with carbon ions with varying energies and doses. The surface morphology and chemical composition of the implanted samples were characterized using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The tribological properties of the implanted samples were evaluated by performing friction and wear tests using a ball-on-disk tribometer under dry sliding conditions.Results and DiscussionThe SEM images showed that the implanted samples exhibited a rougher surface compared to the unimplanted ones. The XPS analysis confirmed the presence of carbon on the implanted samples. The friction and wear tests revealed that the implantedsamples exhibited reduced friction coefficients and wear rates compared to the unimplanted samples. The reduced friction was attributed to the formation of a carbon-rich layer on the surface of the implanted samples, which acted as a solid lubricant during sliding. The reduced wear rate was attributed to the increased surface hardness of the implanted samples due to carbon ion implantation.ConclusionThe plasma-based ion implantation technique was successfully used to implant carbon ions on the single-crystal silicon surface. The implanted samples exhibited enhanced tribological behavior, including reduced friction coefficients and wear rates, compared to the unimplanted ones. The improved tribological behavior was attributed to the formation of a carbon-rich layer on the surface and the increased surface hardness due to ion implantation. We conclude that plasma-based ion implantation is an effective surface modification technique for improving the tribological behavior of single-crystal silicon.Furthermore, the specific implantation parameters used in this study, i.e., energy and dose, can be optimized to achieve even better tribological properties. For example, increasing the energy of the implanted ions can result in a deeper implantation and hence a thicker carbon-rich layer on the surface. Similarly, increasing the dose can result in a higher concentration of carbon atoms on the surface, which can lead to further reduction in friction and wear.The use of ion implantation for surface modification has several advantages over other traditional techniques such as coating orsurface texturing. Unlike coatings, ion implantation does not introduce a separate layer on the surface, which can delaminate or wear off over time. In contrast, implanted atoms become part of the substrate material, resulting in a more durable modification. Additionally, the surface texturing technique relies on creating grooves or patterns on the surface, which may not be applicable or effective for all materials or applications.In conclusion, the plasma-based ion implantation technique has been shown to be a promising surface modification technique for enhancing the tribological behavior of single-crystal silicon. This technique has the potential to be applied to other materials and can be optimized for specific applications. Future work can focus on optimizing the implantation parameters, investigating the long-term durability of the implanted surfaces, and exploring the applications of this technique in different technological fields.In addition to silicon, plasma-based ion implantation has been applied to a wide range of materials such as metals, polymers, ceramics, and semiconductors to modify their surface properties for various applications. For example, ion implantation has been used to improve the wear resistance and corrosion resistance of stainless steel, increase the hardness and scratch resistance of polymeric materials, and enhance the adhesion and surface energy of ceramics.Moreover, ion implantation can also be used to tailor the surface properties of materials for specific applications in microelectronics, optoelectronics, and biomedicine. In microelectronics, ion implantation is commonly used to modify the electrical properties of semiconductors such as silicon and gallium arsenide for devicefabrication. In optoelectronics, ion implantation can be used to create waveguides or modify the refractive index of optical materials for photonic devices. In biomedicine, ion implantation can be employed to modify the surface chemistry and topography of implant materials to enhance their biocompatibility and reduce the risk of rejection.In conclusion, plasma-based ion implantation provides a versatile and effective surface modification technique for various materials and applications. Its benefits include improving wear resistance, corrosion resistance, hardness, scratch resistance, adhesion, surface energy, and biocompatibility, among others. The technique can be optimized for specific applications and has potential in a wide range of technological fields. Future research should focus on further understanding the fundamental mechanisms of ion implantation and developing new implantation techniques to address emerging needs in different industries.One area where plasma-based ion implantation has shown potential is in the development of new types of functional coatings. Functional coatings are thin layers of material applied to surfaces in order to impart specific properties such as increased durability, improved friction, or enhanced thermal insulation. Plasma-based ion implantation can be used to create such coatings through a process known as ion beam assisted deposition.Ion beam assisted deposition involves bombarding a surface with high-energy ions while simultaneously depositing a thin film of material onto it. This bombardment modifies the surface properties of the material, allowing the deposited film to adhere more strongly and exhibit improved functional properties.One example of a functional coating that can be created through ion beam assisted deposition is a superhydrophobic coating. Superhydrophobic coatings are highly water-repellent, and can be used in applications such as self-cleaning surfaces, anti-fogging coatings, and water-resistant textiles. By using plasma-based ion implantation to modify the surface properties of a material, it is possible to create a highly rough surface with a variety of different structures that can prevent water from adhering to it.Another area where plasma-based ion implantation has shown promise is in the development of advanced energy materials. By modifying the surface properties of materials such as silicon, lithium, and aluminum, it is possible to create materials with improved energy storage properties. For example, by using ion implantation to create a highly porous silicon surface, researchers have been able to create silicon anodes for lithium-ion batteries with significantly improved performance.In conclusion, plasma-based ion implantation is a versatile technique with promising applications in a variety of fields. By modifying the surface properties of materials, it is possible to create coatings with improved functional properties and advanced energy materials with improved performance. Continued research in this area has the potential to lead to the development of new materials and technologies with a wide range of practical applications.In addition to functional coatings and energy materials, plasma-based ion implantation has also shown potential for use in the biomedical field. By modifying the surface properties of medical implants, it may be possible to improve biocompatibilityand reduce the risk of rejection or infection. For example, an ion-implanted titanium surface could have improved osseointegration and reduce implant failure rates.Furthermore, plasma-based ion implantation can also be used in the field of microelectronics to improve device performance. By modifying the surface properties of electronic components, it is possible to improve their conductivity and reduce power consumption. This can lead to smaller, more efficient devices that have better battery life and can be used in a wider range of applications.Finally, plasma-based ion implantation has potential in the field of environmental science. By modifying the surface properties of materials such as membranes and filters, it is possible to create materials with improved filtration properties. This can lead to more efficient water and air filtration systems that have a smaller environmental footprint.Overall, plasma-based ion implantation is a promising technology that has the potential to unlock new innovations in a wide range of fields. Continued research and development will be needed to fully understand its capabilities and limitations, but the potential benefits make it an exciting area to watch in the coming years.。
The modification of glassy carbon and gold electrodes with aryl diazonium salt:The impact of the electrode materialson the rate of heterogeneous electron transferGuozhen Liu,Jingquan Liu,Till Bo ¨cking,Paul K.Eggers,J.Justin Gooding*School of Chemistry,The University of New South Wales,Sydney,NSW 2052,AustraliaReceived 1December 2004;accepted 23March 2005Available online 23May 2005AbstractThe heterogeneous electron-transfer properties of ferrocenemethylamine coupled to a series of mixed 4-carboxyphenyl/phenyl monolayers on glassy carbon (GC)and gold electrodes were investigated,by cyclic voltammetry,in aqueous buffer solutions.The electrodes were derivatized in a step-wise process.Electrochemical reduction of mixtures of 4-carboxyphenyl and phenyl dia-zonium salts on the electrode surfaces yielded stable monolayers.The introduction of carboxylic acid moieties onto the surfaces was verified by X-ray photoelectron spectroscopy.Subsequently the 4-carboxyphenyl moieties were activated using water-soluble carbo-diimide and N -hydroxysuccinimide and reacted with ferrocenemethylamine.The rate constants of electron transfer through the monolayer systems were determined from cyclic voltammograms using the Marcus theory for electron transfer and were found to be an order of magnitude higher for the ferrocene-modified monolayer systems on gold than those on GC electrodes.The results suggest the electrode material has an important influence on the rate of electron transfer.Ó2005Elsevier B.V.All rights reserved.Keywords:Self-assembled monolayers;Electron transfer;Carbon;Gold;Diazonium salts1.IntroductionThe modification of conducting surfaces with mono-layers has received extensive research interest of late be-cause of their utility as model systems for understanding electron transfer [1,2],molecular electronics [3,4],bio-electronics [5,6]and sensors [7]amongst other applica-tions.The most popular chemistry for forming monolayers on electrode surfaces is alkanethiol self-assembly onto coinage metals,in particular gold [8],although other systems have also attracted interest such as silanes on metal oxide electrodes [9]and alkenes on highly doped silicon [10].The attractiveness of gold–thiol chemistry is that well ordered monolayers can be formed relatively easily,with a reasonably strong bond formed between the organic molecule and the electrode,and that a diverse range of molecules can be synthesized with which to modify an electrode.The advantages of gold–thiol chemistry are somewhat offset by a number of disadvantages,including alkanethiols being oxida-tively or reductively desorbed at potentials typically out-side the window defined by À800to +800mV versus Ag/AgCl.Other disadvantages include:alkanethiols being desorbed at temperatures over 100°C,gold being a highly mobile surface which results in the monolayers moving across the electrode surface,the gold–thiolate bond being prone to oxidation and the gold/thiol junc-tion creating a rather large tunneling barrier ($2eV)[11].The last point regarding a large tunneling barrier0301-0104/$-see front matter Ó2005Elsevier B.V.All rights reserved.doi:10.1016/j.chemphys.2005.03.033*Corresponding author.E-mail address:justin.gooding@.au (J.J.Gooding)./locate/chemphysChemical Physics 319(2005)136–146has implications for the rate of electron transfer from the organic monolayer to the electrode which is impor-tant for all molecular scale devices where communica-tion with the macroscopic world is achieved through electron transport.We are interested in alternative monolayer systems to gold–thiol chemistry which overcome some of the disad-vantages but do not severely compromise the advanta-ges of gold/thiol chemistry.The electrochemical reduction of aryl diazonium salts is one possible alterna-tive which has most frequently been used as a method for the covalent derivatization of glassy carbon(GC) surfaces[12–14].The reduction reaction results in the loss of the N2and the formation of a carbon–carbon covalent bond which is strong,stable over both time and temperature,non-polar and conjugated[11].Thus, the conjugated carbon network in the GC electrode can be thought of as continuing into the monolayer sys-tem rather than the abrupt change from electrons being in a metallic environment to an organic environment. The continuity of the electrode material into the mono-layer has resulted in the suggestion that GC electrodes modified by aryl diazonium salts have the potential to reduce the barrier towards electron transfer from the carbon electrode into the monolayer[11].McCreery and coworkers[15–18]have extensively studied the elec-tron transfer kinetics of GC surfaces in different redox probe solutions.However,to the best of our knowledge heterogeneous electron transfer between redox active molecules and GC electrodes through aryl diazonium salt derived monolayers has yet to be investigated.Nor has the notion that the C–C bond will allow efficient electron transfer.The attractiveness of aryl diazonium salts are en-hanced further by recent studies showing they can also be grafted to a variety of metal[19,20]and semicon-ductor[21]surfaces as well as carbon nanotubes[22]. This feature raises the exciting possibility of one monolayer forming system being suitable for a large range of electrode materials for a diverse range of applications.This possibility is helped by a rich array of different diazonium salts which have now been pre-pared including molecular wires[23]and polyethylene glycol terminated molecules designed to resist protein adsorption[24,25].The purpose of this study is to modify GC and gold substrates using mixtures of aryl diazonium salt molecules(introducing phenyl and4-carboxyphenyl groups onto the surface)and to com-pare the kinetics of electron transfer to GC and gold surfaces from the same ferrocene-based monolayer sys-tem.A similar ferrocene-based system prepared by a mixed self-assembled monolayers(SAMs)of4-merca-ptobenzoic acid(MBA)and1-propanethiol(PT)has also been prepared on gold surfaces and the rates of electron transfer have been studied for further comparison.2.Experimental2.1.Reagents and materialsTetrabutylammonium tetrafluoroborate(NBu4BF4), sodium tetrafluoroborate(NaBF4),p-aminobenzoic acid,aniline,4-mercaptobenzoic acid(MBA),1-propa-nethiol(PT),ferricyanide(K4Fe(CN)6),1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC),N-hydroxysuccinimide(NHS),N-[2-hydroxy-ethyl]piperazine-N0-[2-ethanesulfonic acid](HEPES), ferrocenecarboxaldehyde,sodium cyanoborohydride and acetonitrile(CH3CN,HPLC grade)were obtained from Sigma(Sydney,Australia).Benzoic acid diazo-nium tetrafluoroborate and benzene diazonium tetra-fluoroborate were synthesized according to the method by Saby et al.[26].Ferrocenemethylamine was synthe-sized using the procedure from Kraatz[27].Reagent grade dipotassium orthophosphate,potassium dihydro-gen orthophosphate,potassium chloride,sodium hydroxide,sodium chloride,sodium nitrite,ammonium acetate,sulphuric acid,hydrochloric acid,methanol and diethyl ether were purchased from Ajax Chemicals Pty. Ltd.(Sydney,Australia).All reagents were used as re-ceived,and aqueous solutions were prepared with puri-fied water(18M X cmÀ1,Millipore,Sydney,Australia). Phosphate buffer solutions used in this work contained 0.05M KCl and0.05M K2HPO4/KH2PO4and were ad-justed with NaOH or HCl solution.2.2.Modification of electrodesThe GC and gold electrodes were modified with dia-zonium salts followed by attachment of ferrocenemeth-ylamine as depicted in Scheme1.The GC electrodes were purchased commercially(Bioanalytical System Inc.,USA)as3-mm-diameter rods.The electrodes were polished successively in1.0,0.3,and0.05l m alumina slurries made from dry Buehler alumina and Milli-Q water on microcloth pads(Buehler,Lake Bluff,IL, USA).The electrodes were thoroughly rinsed with Milli-Q water and sonicated in Milli-Q water for 5min between polishing steps.Before derivatization, the electrodes were dried with an argon gas stream. The bare GC electrodes had an electrochemical rough-ness factor(the ratio of the electrochemical area to geo-metric area)of 1.43.Surface derivatization of GC electrodes was performed in a solution of1mM aryl diazonium salt and0.1M NaBu4BF4in acetonitrile using cyclic voltammetry(CV)with a scan rate of 100mV sÀ1for two cycles between+1.0andÀ1.0V. The diazonium salt solution was deaerated with argon for at least15min prior to derivatization.The elec-trodes were rinsed with copious amounts of acetonitrile and then water and dried under a stream of argon prior to the next step.G.Liu et al./Chemical Physics319(2005)136–146137Poly-crystalline gold electrodes,prepared as de-scribed previously[28],were polished to a mirror-like finish with 1.0l m alumina,followed by0.3and 0.05l m alumina slurry on microcloth pad.After re-moval of trace alumina from the surface,by rinsing with water and brief cleaning in an ultrasonic bath with eth-anol and then water,electrochemical cleaning in0.05M H2SO4by cycling the electrodes betweenÀ0.3and1.5V was carried out until a reproducible CV was obtained. Before derivatization,the cleaned electrodes were rinsed with water and dried under a stream of argon.The derivatization of gold electrodes with a mixture of dia-zonium salts was conducted in exactly the same manner as described for the carbon electrodes.The alkanethiol modified gold electrodes were prepared by immersing the gold electrodes in a1mM mixed thiol solution(mer-captobenzoic acid and propanethiol with different dilu-tion ratios)in ethanol overnight(see Scheme2).The electrode was rinsed with copious amounts of ethanol, then water andfinally dried under a stream of argon prior to the next step.Covalent attachment of ferrocenemethylamine to car-boxylic acid terminated monolayers followed the proce-dures described by Liu et al.[29].The modified surfaces were incubated in an aqueous solution of10mM N-hydroxysuccinimide(NHS)and40mM1-ethyl-3-(3-di-methyl aminopropyl)carbodiimide hydrochloride (EDC)for1h.After the activation,the electrodes were rinsed with water and incubated in a5mM ferrocenem-ethylamine solution in HEPES buffer pH7.3for24h.2.3.Electrochemical measurementsAll electrochemical measurements were performed with a BAS-100B electrochemical analyser(Bioanalyti-cal System fayette,IL)and a conventional three-electrode system,comprising a GC or a gold work-ing electrode,a platinum foil as the auxiliary electrode, and a Ag/AgCl3.0M NaCl electrode(from BAS)as ref-erence.All potentials were reported versus the Ag/AgCl reference electrode at room temperature.All CV mea-surements were conducted in pH7.0phosphate buffer. The area under the Faradaic peaks in the CVs of the fer-rocene modified electrodes were used to determine the surface coverage of ferrocene.The rate constants for electron transfer were calculated from the variation in peak potential over a wide range of scan rates.For elec-trodes with prominent redox peaks the rate constants were determined byfitting the variation in peak poten-tial with scan rate using the Marcus theory for electron transfer as described previously[30–32]whilst at low surface coverages of ferrocene the rates of electronScheme2.Schematic of ferrocenemethylamine immobilized covalently on mixed monolayers of MBA and PT on gold surfaces. 138G.Liu et al./Chemical Physics319(2005)136–146transfer were determined using the Laviron[33]formal-ism.This was because peak shape and position was very sensitive background subtraction at with small redox peaks and thereforefitting the entire background sub-tracted CV peak as required for our Marcus simulation became unreliable.When both methods were employed on the same data very similar rate constants for electron transfer were obtained.2.4.XPS measurementsXP spectra were obtained using an EscaLab220-IXL spectrometer with a monochromated Al K a source (1486.6eV),hemispherical analyzer and multichannel detector.The spectra were accumulated at a take-offan-gle of90°with a0.79mm2spot size at a pressure of less than10À8mbar.3.Results3.1.Aryl diazonium salt modified glassy carbon electrodesGlassy carbon electrodes were modified with diazo-nium salts via electrochemical reduction of an aryl dia-zonium salt(1mM in acetonitrile)with0.1M tetrabutylammonium tetrafluoroborate as background electrolyte.Thefirst sweep showed the characteristic reduction peak atÀ0.16V versus Ag/AgCl with no associated oxidation peak indicative of the loss of N2 and the formation of a4-carboxyphenyl radical fol-lowed by covalent binding to the carbon surface[34]. Subsequent scans showed no electrochemistry indicative of a passivated electrode.The passivation of the GC sur-faces after the modification with aryl diazonium salts was confirmed using potassium ferricyanide as a redox probe.Fig.1shows a cyclic voltammogram before and after modification with(4-carboxyphenyl)diazonium tetrafluoroborate in1mM ferricyanide in a0.05M phosphate buffer(0.05M KCl,pH7.0)at a scan rate of100mV sÀ1.After the modification of the surface with the aryl diazonium salts,the redox peaks of ferricy-anide observed with bare GC electrodes were almost completely suppressed.This gave strong evidence that a uniform monolayer which blocked access of ferricya-nide to the electrode had formed on the GC surfaces. Based on the area of the reduction peak during the mod-ification of the GC electrode surface with the aryl diazo-nium salt,the coverage of the4-carboxyphenyl moieties was calculated to be7.4·10À10mol cmÀ2.The reported surface coverage on GC substrates varies in the range of 4–30·10À10mol cmÀ2[35]with the theoretical maxi-mum surface coverage[29]for a monolayer on GC sur-faces being12·10À10mol cmÀ2.The surface coverage of7.4·10À10mol cmÀ2indicates the GC electrode was modified with a monolayer rather than multilayers of aryl groups as has been reported by some workers[36–38].The modification of the GC electrode by electro-chemical reduction of(4-carboxyphenyl)diazonium tet-rafluoroborate was confirmed by X-ray photoelectron spectroscopy(XPS).Survey spectra showed the expected carbon1s and oxygen1s peaks at$284and$532eV, respectively,and also a small nitrogen1s peak at $400eV(Fig.2).The level of oxygen was increased in comparison to the bare GC surface as expected for the introduction of4-carboxyphenyl groups onto the sur-face.The presence of the small nitrogen1s peak in the survey spectra was partially due to nitrogen containing species already detectable on the unmodified GC elec-trodes,which has been observed previously[13,34].Fur-thermore,nitrogen species with a binding energy of $400eV can be introduced onto the surface during the modification reaction.It has been proposed that these are due to a hydrazine generated by reaction of the dia-zonium salt with phenol groups on the GC surface[26].Fig.2.XP survey spectrum and carbon1s narrow scan(inset)of a GCelectrode modified by electrochemical reduction of(4-carboxyphenyl)diazonium tetrafluoroborate.G.Liu et al./Chemical Physics319(2005)136–146139Nitrogen1s narrow scans(not shown)were consistent with the formation of low levels of the hydrazine,which exhibited a slightly different binding energy to that of the nitrogen species already present on the bare GC electrode.The carbon1s narrow scan(Fig.2,inset)showed a peak centred at288.8eV,which was typical of the car-bon of the carboxylic acid group[13].This peak was ab-sent from the carbon1s narrow scan of unmodified GC surfaces.The carbon1s peak at284.4eV was slightly broadened compared to the graphitic peak of an unmodified GC surface and was assigned to the gra-phitic carbon of the underlying GC electrode and the aromatic carbons of the monolayer.The pronounced asymmetry of this peak with a broad shoulder on the high binding energy side($286.2eV)was attributed to an oxidized species present on the GC surface and or-ganic contaminants adsorbed on the monolayer.After modification of the GC electrode surface with the aryl diazonium salt the next step in the fabrication of the modified electrodes was the attachment of ferro-cene.CVs measured in an aqueous solution of0.05M phosphate buffer(0.05M KCl,pH7.0)at a scan rate of100mV sÀ1before and after the immobilization of ferrocene on the4-carboxyphenyl modified GC elec-trode are shown in Fig.3.The strong redox peaks after the attachment of ferrocene showed linear variation in peak current with scan rate indicating that the ferrocene was surface bound.In the absence of EDC and NHS such that no covalent coupling of the ferrocene could occur,only very weak redox peaks due to physisorption were observed.The CVs of the ferrocene coupled to the 4-carboxyphenyl monolayers show non-ideal behaviour[1]with regards to peak separation at slow scan rates(D E p=79mV rather than the ideal D E p=0mV)and the full width half maximum(greater than200mV rather than the ideal E FWHM=90.6mV/n where in this case n=1).With regards to both peak separation and the E FWHM the non-ideal behaviour has been attributed to the ferrocene molecules being located in a range of environments with a range of formal electrode potentials (E0)[39,40].We[29,41]and others[42]have noted pre-viously that fabricating redox active SAMs by assem-bling the SAM and then attaching the redox molecule, results in broader FWHM than observed with electrodes where a redox active alkanethiol was attached directly to the electrode.The reason for modifying electrodes in this step-wise manner,where the monolayer is formed and then the redox active molecule attached,rather than synthesizing a pure redox active self-assembling mole-cule followed by assembly on the electrode,is because in applications of our interest,bioelectronics,the step-wise strategy is the only viable approach.With a monolayer containing only4-carboxyphenyl moieties the number of redox active molecules attached to the surface,as determined from the charge passed under the Faradaic peaks in the ferrocene modified electrode,is approximately(0.073±0.012)·10À10 mol cmÀ2with a close to unity ratio of anodic to cathodic peak areas(see Table1).Comparing the surface coverage of4-carboxyphenyl groups of7.4·10À10mol cmÀ2,to that of the number of redox centres attached indicates that only approximately10%of the 4-carboxyphenyl monolayers had a ferrocene attached. At this surface coverage the average area per ferrocene molecule,assuming homogeneous distribution,is 2.2nm2which suggests there is a high possibility of interaction between redox active centres[29,42].Interaction between redox active centres has been re-ported to decrease the reorganization energy and in-crease the electron transmission efficiency[4,43,44], hence providing an anomalously high measure of the rate constant for electron transfer.As a consequence, the number of coupling points within the monolayer that the ferrocene could couple was reduced by forming mixed monolayers composed of the4-carboxyphenyl diazonium salt and the phenyl diazonium salt(Table 1).Table1shows that the surface coverage initially in-creased with the spacing of the coupling points followed by the more expected decrease as the number of cou-pling points decreased.The reason for the initial in-crease in surface coverage of ferrocene as the solution composition from which the monolayer forms changes from entirely4-carboxyphenyl diazonium salt to a1:1 ratio of4-carboxyphenyl to phenyl diazonium salt is un-clear.The percentage of carboxyl groups to be activated using EDC/NHS,as used here,in a SAM composed of entirely carboxylic acids has been shown to be approxi-mately50%[45]which is equivalent to all the4-carboxy-phenyl groups being activated in a1:1monolayer. Furthermore,the relative surface coverages of the4-carboxyphenyl to ferrocene is10:1in the entirely4-carb-oxyphenyl monolayer so there should be excess coupling points for the ferrocene to attach.Therefore,it is sug-gested that the introduction of a second component into the monolayer(the phenyl diluent)in effect introduces a hydrophobic component into the monolayer.As ferro-cene has been shown previously to adsorb onto the sur-face of hydrophobic self-assembled monolayers[41,46]. Therefore,it is proposed that more ferrocene is attached when the phenyl component is introduced into the monolayer because the surface is more energetically favourable location for the ferrocene compared with an entirely carboxyphenyl monolayer.The rate constant for electron transfer was deter-mined from the variation in peak position between the anodic and cathodic scans as a function of scan rate. In this study,the variation in peak potential over a wide range of scan rates wasfitted using the Marcus theory for electron transfer as described previously[30–32] rather than the Laviron[33]formalism which relies on simple Butler–Volmer kinetics and gives rate constants for electron transfer which are sensitive to the choice of sweep rates investigated.Table1shows that across the spectrum of dilution ratios investigated the rate con-stant for electron transfer(k app)is approximately15–20sÀ1.3.2.Aryl diazonium salt modified gold electrodesGold electrodes were modified with aryl diazonium salts via electrochemical reduction in exactly the same manner to the GC electrodes.The reduction peak for the attachment of the aryl diazonium salt onto the gold electrode,observed in thefirst sweep,was shifted anod-ically230mV relative to carbon being at+70mV versus Ag/AgCl.Subsequent sweeps showed no electrochemis-try indicating a monolayer coverage of4-carboxyphenyl moieties on the electrode surface.The4-carboxyphenyl monolayer blocked access of potassium ferricyanide to the electrode in a similar manner to that depicted in Fig.1for the carbon electrode but to a lesser extent. The coverage of the4-carboxyphenyl moieties on the electrode surface was6.4·10À10mol cmÀ2which was lower than the7.4·10À10mol cmÀ2observed on GC electrodes and hence lower than the theoretical maxi-mum surface coverage[34]for a monolayer of 12·10À10mol cmÀ2.The lower surface coverage could be a reflection of the aryl diazonium salt not being nor-mal to the surface of the gold,as suggested by infra-red spectroscopy[20].Again,the presence of a monolayer or submonolayer of aryl diazonium salt on the gold elec-trode is important due to the possibilities of obtaining multilayers with aryl diazonium salts as shown for both carbon[36–38]and metal surfaces[20].An XP survey spectrum of gold modified with(4-carboxyphenyl)diazonium tetrafluoroborate showed the expected1s peaks of carbon and oxygen at$285 and$532eV,respectively,but no significant evidence of a nitrogen1s peak(Fig.4).The carbon1s envelope (Fig.4,inset)wasfitted with four peaks at288.7, 286.2,284.6and283.9eV assigned to the carboxylic acid moieties,C–O species,the aromatic carbons of the monolayer and the metal-bonded carbon,respectively. The binding energy observed for the carboxylic acid group on gold was consistent with that observed on the GC surface.The inclusion of the metal carbide peak is exceedingly tentative as a goodfit to the spectra could be obtained without the presence of this peak.The assignment of the metal carbide peak is based on theTable1Some parameters of ferrocenemethylamine immobilized on GC electrodes modified with mixed monolayers of4-carboxyphenyl and phenyl moieties.D E p is recorded at a scan rate of100mV sÀ1[Benzyl]/[benzoic acid]E0(mV)D E(mV)E FWHM(mV)C(pmol cmÀ2)C a/C c k app(sÀ1) 0264±1579±10241±1072.8±11.60.89±0.0717±10 1279±1378±14213±19100.3±10.4 1.03±0.0428±10 5292±989±25227±2567.4±10.70.94±0.0515±5 10298±793±10262±3848.1±6.90.88±0.1116±2 20304±17101±21220±2929.4±3.40.71±0.1615±10 40317±19107±10289±1413.3±2.00.77±0.1310±2Fig.4.XP survey spectrum and carbon1s narrow scan(inset)of agold electrode modified by electrochemical reduction of(4-carboxy-phenyl)diazonium tetrafluoroborate.G.Liu et al./Chemical Physics319(2005)136–146141precedence of Pinson and co-workers[19,20,47]who have previously proposed the existence of such a peak for the electroreduction of diazonium salts onto metal surfaces.On iron surfaces the case for a metal–carbide peak is compelling with a very pronounced shoulder when a high resolution instrument is used[47]with the intensity of this shoulder sensitive to take-offangle indi-cating it is a surface bound species.However,on copper electrodes[20]and other examples on iron[19]the shoulder on the carbon1s spectra is less pronounced similar to the observations on gold here.The electrochemical parameters after the attachment of ferrocene to the4-carboxyphenyl modified gold elec-trodes are shown in Table2.The trends were very sim-ilar to the GC modified electrodes with broader than ideal E FWHM and non-ideal D E p at slow scan rates. The surface coverage of ferrocene with different ratios of diluent to4-carboxyphenyl were slightly lower than those on GC in common with the lower coverage in gen-eral of the aryl diazonium salts on gold compared with GC.Most importantly,the rates of electron transfer measured on the gold modified surface were significantly greater than that observed on carbon.Typically rates of more than a100sÀ1were observed,which was approx-imately one order of magnitude higher than for the same monolayer system on GC electrodes.The values of the rate constants at the low surface coverage of ferrocene (last three entries in the table)were particularly difficult to determine because with small redox peaks back-ground subtraction can have a large impact on the peak positions.As a consequence the rate constants quoted represent the lower limits and therefore we expect the true rate constant is closer to that observed at the1:5 monolayer.3.3.Aryl thiol modified gold electrodesFor comparison with the monolayers formed by elec-trochemical reduction of aryl diazonium salts we also prepared mixed monolayers of aryl thiol self-assembled monolayers on gold electrodes with attached ferrocene moieties as shown in Scheme2.The rate constants deter-mined for this equivalent aryl thiol system were in the order of103sÀ1(at the limits of what can be measured electrochemically)which was approximately5–10times the values observed for the aryl diazonium salt–gold sys-tem but two orders of magnitude higher than those ob-served for the aryl diazonium salt–GC system.These observations indicate that the metal surface has a signif-icant effect on the rate of electron transfer.4.DiscussionThe rate constants for electron transfer are remark-ably slower for the carbon electrodes relative to the gold electrodes.This is contrary to the suggestion that with diazonium salt modified carbon electrodes the continu-ity of conjugated carbon network from the electrode into the monolayer will result in a lower barrier for elec-tron transfer than with organic monolayers on metallic electrodes[11].The question that arises is why there is a difference in rate constants of around one order of magnitude for the same redox active molecule connected to electrodes by the same bridge molecule?The Marcus–Hush expression for electron transfer between a donor and acceptor through an organic bridge in solution includes terms for electronic coupling between the donor and acceptor,the Gibbs free energy for electron transfer(the driving force,D G ET)and the nuclear reorganization energy(k)of the redox molecule as a consequence of its change in oxidation state[48]. For a given donor and acceptor pair the rate of electron transfer decays exponentially with distance according to a proportionality constant,the b value,sometimes called a damping factor.When the organic bridge is anchored to an electrode such that it can act as the donor and/or acceptor the situation is complicated somewhat as the electronic properties of the electrode can also play a role in the rate of electron transfer[2].Equations describing the rate constant for electron transfer now incorporate terms related to the Fermi levels of the electrode and the effective density of electronic states near the Fermi level.In this study,the only changes between the mono-layer systems studied relate to the electrode material and the bond to the electrode.Hence,the reorganization en-ergy and the driving force will remain unchanged.The electronic coupling may be influenced by the electrodeTable2Some parameters of ferrocenemethylamine immobilized on gold electrodes modified with mixed monolayers of4-carboxyphenyl and phenyl moieties.D E p is recorded at a scan rate of100mV sÀ1[Benzyl]/[benzoic acid]E0(mV)D E(mV)E FWHM(mV)C(pmol cmÀ2)C a/C c k app(sÀ1) 0268±1281±14209±1149.3±7.60.87±0.13257±41 1277±2585±9191±1780.6±5.90.86±0.09530±42 5280±1875±10227±853.7±5.40.72±0.14211±23 10282±1492±12260±1525.8±4.00.92±0.0783±50 20292±1989±8272±1413.3±2.40.76±0.0369±50 40317±2181±16308±107.1±1.00.93±0.0268±50 142G.Liu et al./Chemical Physics319(2005)136–146。
AbstractUnderstanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized. Here we use voltammetric determination of the electron transfer rate for three redox mediators, ferricyanide, hexaammineruthenium, and hexachloroiridate (Fe(CN)63–, Ru(NH3)63+, and IrCl62–, respectively), to measure the reactivity of graphene samples prepared by mechanical exfoliation of natural graphite. Electron transfer rates are measured for varied numberof graphene layers (1 to ca. 1000 layers) using microscopic droplets. The basal planes of mono- and multilayer graphene, supported on an insulating Si/SiO2 substrate, exhibit significant electron transfer activity and changes in kinetics are observed for all three mediators. No significant trend in kinetics with flake thickness is discernible for each mediator; however, a large variation in kinetics is observed across the basal plane of the same flakes, indicating that local surface conditions affect the electrochemical performance. This is confirmed by in situ graphite exfoliation, which reveals significant deterioration of initially, near-reversible kinetics for Ru(NH3)63+ when comparing the atmosphere-aged and freshly exfoliated graphite surfaces.Keywords:graphene; graphite; basal plane; electrontransfer; kinetics;electrochemistry; voltammetryGraphene has attracted significant interest due to its unique properties, namely, record charge carrier mobility,(1) high thermal conductivity,(2) and mechanicalstrength,(3) discovered following its isolation in 2004.(4) While applications of graphene in high-frequency transistors, flexible touch screens or photodetectors(5, 6) will requirehigh-quality material, other properties of this two-dimensional (2D) nanomaterial, such as high specific surface area, optical transparency,(7)and the ability to sustain large currentdensities,(8) can be exploited using medium-quality material. Proposed applications include corrosion protection,(9) sensing and biotechnology,(10)and energyconversion/storage, i.e. Li-ion batteries,(11) solar cells,(12) and supercapacitors.(13) To evaluate graphene’s performance as a n electrode material, the heterogeneous electron transfer (ET) rate between graphene surfaces and a redox mediator, k0, has to be determined and compared with its three-dimensional (3D) relative–graphite. Furthermore, the difference in ET kinetics at the basal planes, edges and defects of graphitic materials has been a topic of considerable scientific interest even before the discovery of graphene.(14, 15) The literature, however, offers some contrasting views on the reactivity of mono- and multilayer graphene. Early ET measurements on individual graphene sheets were first reported by Li et al.,(16) using both mechanically exfoliated (ME) and chemical vapor deposition (CVD) grown flakes with ferrocenemethanol (FcMeOH) redox mediator, and Valota et al.,(17) using ME flakes and Fe(CN)63–: both reported accelerated kinetics on monolayer compared to bilayer samples (∼2-fold)(17) and graphite (∼10-fold),(16) respectively. Reactivity toward ET has been probed with Raman spectroscopy to measure the extent of diazonium functionalization of ME graphene: the reactivity of the monolayer was again found to be higher than bi- and multilayers; similarly the diazonium reduction kinetics on the edge were faster than those on the basal plane.(18)ME yields high-quality flakes of pristine surface with limited contamination, allowing the fundamental electrochemical properties of graphene to be explored. Nevertheless, ME preparation is laborious, requires ―hunting‖ for flakes, reliable contacting/masking of the electrode, and is further complicated by fracture of monolayer graphene upon exposure to many aqueous and organic solutions.(19) Most reports of graphene electrochemistry use a mixture of graphene platelets of various thicknesses and lateral dimensions, usually prepared via liquid-phase exfoliation or reduction of graphene oxide, immobilized on a conducting substrate.(20-22)While this method is convenient for characterization of graphene composites, sensing layers or paints, it does not provide insight into electrochemical activity of individual graphene flakes and the roles of the basal/edge plane and defects, due to the sample’s polycrystalline nature and the presence of the underlying conductor. Also, the discontinuous nature of the platelet mixture results in poor conductivity in thin samples.(23) Brownson et al. reported slow ET kinetics for samples with a high basal-to-edge plane ratio (for Fe(CN)63–, Ru(NH3)63+, and two other mediators).(20) Similarly, Zhang et al. reported extremely high k0 for Fe(CN)63– andRu(NH3)63+on reduced graphene oxide (∼1 and 10 cm s–1, respectively) attributed to edges and defects.(24) Ambrosi et al. found that open graphene edges exhibit faster kineticsthan folded edges (Fe(CN)63–),(25) while Goh and Pumera concluded that the ET rate is independent of the number of graphene layers (dopamine and ascorbic acid mediators).(21)CVD grown graphene has also been a popular choice for ET studies, either using the original growth substrate (typically Cu or Ni)(26) or following transfer onto an insulating substrate as shown for Fe(CN)63– and IrCl62– mediators.(27) The underlying conductive substrate or chemicals used during the flake transfer, however, can interfere with the electrochemical response. Considerable advances have been made with the employment of scanning electrochemical microscopy (SECM), which can be efficiently used to monitor the electrochemical response of a surface with submicrometer scale spatial resolution. Tan et al. reported increased ET rates on mechanically and chemically induced defects, in comparison to the pristine graphene surface, using FcMeOH and Fe(CN)63–.(28) The same group followed with an extensive study in both aqueous and organic solutions, demonstrating finite to near-reversible kinetics (k0 between ∼10–4 to 10–2 cm s–1) of 10 different redox mediators including FcMeOH, Fe(CN)63– and Ru(NH3)63+, with the limits in kinetics being inherent both to graphene and the nature of the mediator.(29) A variant of SECM, scanning electrochemical cell microscopy (SECCM), was used successfully byGüell et al., who reported increasing ET kinetics with increasing number of CVD stacked graphene layers (from 1 to 7) using a ferrocene derivative as a mediator. Furthermore, no increase of the ET rate was observed by these authors at the edges or steps in comparison to the basal planes.(30)From the above summary, it is clear that despite the significant body of literature on graphene and graphite electrochemistry, the fundamental ET behavior on these surfaces is not fully understood. Even previously unquestionable views, such as accelerated electrode kinetics on the edges/steps relative to basal planes, are not now unanimously accepted. Here, we present electrochemically determined heterogeneous ET rates from a large number of high-quality ME flakes, deposited on insulating oxide-covered silicon wafers. The experiments were carried out in a microdroplet thin-layer cell configuration, reported earlier for CVD grown graphene on Si/SiO2(27) and mechanically exfoliated graphene on polymer substrate,(19) which allows for the accurate and controlled deposition of a liquid containing redox mediator on a specific surface site. A photograph and a schematic of the experimental setup are shown in Figure 1. The work was driven by the need for a systematic study of high-quality flakes of varied thicknesses between monolayer graphene and bulk graphite, in this case about 1000 graphene layers thick. Using this method, we found that the ET activity of the pristine basal plane ofgraphene/graphite flakes, free from microscopic defects, varies significantly across the(31)where ψ is the dimensionless kinetic(D O and D R, respectively) of the mediator are approximately equal and thereduct ion/oxidation kinetics are fairly symmetrical (α ∼0.5). In that case, ΔE p depends solely on ψ (one-electron processes),(31)the latter is determined from ΔE p, and eq 1 canbe simplified to In practice, ψ is calculated from ΔE p using an appropriate working function and k0 determined from the slope of theψ–ν–0.5 dependence corresponding to eq 2 as shown in Figure 2e.Figure 2. Cyclic voltammograms and associated kinetic analyses at graphene/graphite electrodes.(a) CV of Fe(CN)63–/4– on bilayer graphene, (b and d) show comparison of ET kinetics on 4-layer graphene using Ru(NH3)63+/2+ and on ∼70-layer thick graphite using IrCl62–/3–. Corresponding Klingler-Kochi and Nicholson analyses and calculated ET rates (k0) are shown in (c) and (e), respectively. The insets in the bottom right of graphs (a), (b), and (d) show micrographs of the deposited droplets. The series of voltammetric curves were obtained starting from the fastest scan rate of 1000 mV s–1 (dark blue) down to the slowest scan rate of 100 mV s–1 (gray) forFe(CN)63–/4– and Ru(NH3)63+/2+ and 3000–250 mV s–1 for IrCl62–/3–. The potential was referenced against Ag/AgCl wire in 6 M LiCl, and held at the upper vertex potential for 10 s prior to the voltammetry (1 V for IrCl62–/3–). Change of the initial direction of the potential sweep had no observable effect.Full details of the Nicholson method can be found in the Supporting Information. While the working curve defined using this approach is limited to ΔE p below ca. 220 mV, a method developed by Klingler and Kochi allows much larger ΔE p to be used for k0 evaluation. The following expression, which is derived under assumptions of electrode reaction irreversibility, can be used to directly calculate k0from the scan rate and ΔE p, and isreliable for ΔE p∼150 mV and beyond:(32)As in the case of Nicholson analysis, it was assumed that the reduction and oxidation are symmetrical, i.e., α ≈ 0.5. The method was also vali dated by finite-element simulation of the voltammograms (Figure S4, Supporting Information).The diffusion coefficient, required for the above analyses can be determined from the Randles-Ševčík equation, which relates the peak current,I p, to the scan rate for the caseof planar diffusion:(33)where A is the area of the flake surface in contact with the liquid and c is the bulk concentration of the mediator. Although eq 4 has been widely used by researchers to determine diffusion coefficients during ET rate measurements, it is only strictly valid for reversible electrochemical reactions, i.e., where ET kinetics are significantly faster than mass-transport. The peak current in quasi-reversible reactions, as is the case here, is no longer proportional toν1/2 and instead more complex analysis is required to describe the peak current, with the quasi-reversible reaction zone corresponding to ΔE p of ∼62/n to 1000/n mV.(34) We also found that the linear ψ–ν–0.5 dependence breaks down when the droplet is significantly smaller than 20 μm in dia meter and/or the scan rate is decreased below 100 mV s–1, most likely due to a deviation from the ideal semi-infinite linear diffusion regime within small droplets. Hence, the applied scan rate was kept between the limits of 100 and 1000 mV s–1, or 250 and 3000 mV s–1, corresponding to typical ΔE p ranges of 200–600 and 300–900 mV, or 60–250 mV, for Fe(CN)63– and Ru(NH3)63+, or IrCl62–, respectively. For these reasons, diffusion coefficients of the redox mediators in 6 M LiCl (aq.) were determined independently, using a platinum disk macro-electrode with well-defined reversible ET behavior, as 1.84 (±0.19) × 10–6 cm2 s–1, 2.36 (±0.11) × 10–6 cm2 s–1, 2.27 (±0.14) ×10–6 cm2 s–1 for Fe(CN)63–, Ru(NH3)63+ and IrCl62–, respectively (full details of analysisin Supporting Information).For ΔE p< 220 mV, the kinetic parameter ψ was plotted as a function of the inverse square root of scan rate (all of IrCl62–/3– and some Fe(CN)63–/4– data).(31, 34) This dependence yields a linear gradient, which is analyzed using eq 2 and the heterogeneous ET rate, k0, calculated as shown in Figure 2e. For ΔE p > 220 mV, eq 3 was used to calculate individual k0 for each scan rate and the arithmetic mean was obtained (all ofRu(NH3)63+/2+ and most of Fe(CN)63–/4– data) as shown in Figure 2c. The above analysis was performed for each individual droplet, i.e. for a microscopic surface of area ca. 300–3000 μm2, and, except for the near-reversible kinetics of IrCl62–/3– reduction/oxidation, the kinetics were found to be independent of the droplet/graphene area (full detailsin Supporting Information, Figure S8). A comparison of the CVs reveals wider ΔE p, hence slower kinetics, for Ru(NH3)63+/2+ and Fe(CN)63–/4– reduction/oxidation (Figure2a,b) and smaller ΔE p, hence faster kinetics, for IrCl62–/3– reduction/oxidation (Figure 2d). Dependence of Electron Transfer Kinetics on the Number of Graphene LayersThis work was motivated by the lack of literature consensus on the effect of the number of graphene layers on ET kinetics of a pristine basal plane. However, during the initial investigation, it was found that the ET rate often varied significantly across the surface ofthe same flake and the resulting plot of the averaged ET rate vs, number of graphene layers was very scattered. On these grounds, eight or more individual droplet measurements were carried out on several flakes of the same thickness (or similar thickness for >20 layers) and the arithmetic mean of k0obtained, with a total of 435 individual droplets analyzed across the surface of 69 individual flakes. The k0 values for Fe(CN)63–/4–, Ru(NH3)63+/2+, and IrCl62–/3– reduction/oxidation on flakes of varied thickness between 1 and ca. 1000 graphene layers are shown in Figure 3, panels a, b, and c, respectively. It is apparent that despite the large statistical sample there is not a strong correlation between the flake thickness and the basal plane ET kinetics, although the variation is more pronounced for flakes with thickness less than 20 graphene layers.The k0values are scattered around the arithmetic mean of the whole data set, which is indicated by the colored dashed line. For Fe(CN)63–/4–, the mean ET rates range from 0.13 (±0.02) × 10–3 cm s–1 for the bilayer to 2.09 (±1.27) × 10–3 cm s–1 for 7 layers, with an overall mean value of 0.90 (±0.13) × 10–3 cm s–1. Ru(NH3)63+/2+ kinetics range from 0.11 (±0.12) × 10–4 cm s–1 for 20–30 layers to 1.55 (±0.14) × 10–4 cm s–1 for 7 layers and an overall mean value of 0.53 (±0.04) × 10–4 cm s–1. Finally, IrCl62–/3– reduction/oxidation kinetics range from 2.14 (±0.32) × 10–2 cm s–1 for 8–9 layers to 4.91 (±0.58) × 10–2 cms–1 for a bilayer and an overall mean value of 3.13 (±0.10) × 10–2 cm s–1. TheIrCl62–/3– kinetics recorded on multilayer graphene flakes are on the same order of magnitude as our previous work on flakes on a polymer substrate.(19)Figure 3. Heterogeneous ET rate, k0, between the aqueous-based redox mediator and mechanically exfoliated graphite flakes of varied thicknesses. The averaged ET rates ofreduction/oxidation of (a) Fe(CN)63–/4–, (b) Ru(NH3)63+/2+ and IrCl62–/3–reduction/oxidation are plotted as a function of the number of graphene layers. Each point on the graph is an arithmetic mean of at least 8 (thick flakes >7 layers) or 12 (thin flakes ≤7 layers) individual droplet measurements on a pristine basal plane surface of one or more flakes of a given thickness. The error bars are standard deviations of the mean. The number of individual droplets included in the analysis was 145, 146, and 144 for Fe(CN)63–/4–, Ru(NH3)63+/2+, and IrCl62–/3–, respectively. In total, 69 individual crystal surfaces were used for the analysis. Note that the graphs are shown on a semilogarithmic scale.The effects of uncompensated resistance due to flake thickness/ohmic contacts, whichwould affect ΔE p and hence the calculated k0, were ruled out (FiguresS11–S14, Supporting Information).The variation of the kinetics across the surface of the same graphene crystal is the dominating factor, which increases the uncertainty and masks any underlying trends in the change of kinetics with flake thickness. Table 1 summarizes the ET kinetics data obtainedfor all three redox mediators.Table 1. Heterogeneous ET Rate, k0, of Reduction/Oxidation of Three Redox Mediators on Natural Graphene/Graphite Electrodes of Varied Thicknesses aFe(CN)63–/4–Ru(NH3)63+/2+IrCl62–/3–no. of layers k0/10–3cm s–1Δ/k0k0/10–4cm s–1Δ/k0k0/10–2cm s–2Δ/k01 0.15 ± 0.02 0.12 0.31 ± 0.10 0.31 3.48 ± 0.47 0.132 0.13 ± 0.02 0.16 1.02 ± 0.12 0.12 4.91 ± 0.58 0.123 0.93 ± 0.35 0.38 0.52 ± 0.12 0.23 3.15 ± 0.48 0.154 0.57 ± 0.13 0.22 0.36 ± 0.09 0.24 3.07 ± 0.22 0.075 0.23 ± 0.05 0.23 0.52 ± 0.09 0.17 2.87 ± 0.19 0.076 0.46 ± 0.11 0.25 1.14 ± 0.12 0.10 2.93 ± 0.17 0.06Fe(CN)63–/4–Ru(NH3)63+/2+IrCl62–/3–no. of layers k0/10–3cm s–1Δ/k0k0/10–4cm s–1Δ/k0k0/10–2cm s–2Δ/k0 7 2.09 ± 1.27 0.61 1.55 ± 0.14 0.09 4.08 ± 0.47 0.11 8–9 ––0.95 ± 0.25 0.27 2.14 ± 0.32 0.15 11–13 2.07 ± 0.83 0.40 0.15 ± 0.05 0.34 3.16 ± 0.20 0.06 20–30 1.97 ± 0.98 0.50 0.11 ± 0.06 0.59 3.40 ± 0.11 0.03 50–60 0.68 ± 0.17 0.25 0.22 ± 0.10 0.47 3.11 ± 0.17 0.05 80–90 0.24 ± 0.12 0.51 0.28 ± 0.08 0.29 2.73 ± 0.22 0.08 100–130 1.22 ± 0.16 0.13 0.36 ± 0.03 0.09 3.20 ± 0.15 0.05 220–250 0.84 ± 0.17 0.20 0.17 ± 0.03 0.16 2.87 ± 0.12 0.04Fe(CN)63–/4–Ru(NH3)63+/2+IrCl62–/3–no. of layers k0/10–3cm s–1Δ/k0k0/10–4cm s–1Δ/k0k0/10–2cm s–2Δ/k0 300–500 1.33 ± 0.49 0.37 0.45 ± 0.08 0.19 3.30 ± 0.15 0.05>1000 0.78 ± 0.18 0.23 0.23 ± 0.07 0.31 2.25 ± 0.15 0.07 mean 0.90 ± 0.13 0.30 0.53 ± 0.04 0.25 3.13 ± 0.10 0.08 cleaved ––47.3 ± 3.9 0.08 ––aThe errors are standard deviations of 8 or more measurements at various locations on flakes of the same thickness. The number of graphene layers was determined using a combination of optical microscopy, Raman spectroscopy and atomic force microscopy (AFM) as described in the Methods. The variation of the ET kinetics on flakes of the same thickness is reflected in the relative error, Δ/k0. Arithmetic means and their standard deviations are also listed at the bottom of table.Surface Sensitivity to ContaminantsSignificant variation of the ET rate across different surface sites of the same flake is also reflected in the large relative errors of some of the data in Table 1 (especially forFe(CN)63–/4–). This confirms that, beyond any intrinsic dependence flake thickness, the kinetics also reflect local surface conditions, i.e., are spatially dependent. Unfortunately, these two factors are difficult to separate experimentally. Sensitivity of graphite surfaces to exposure to the atmosphere and therefore oxygen, moisture and other contaminants, has been previously reported to affect electrode kinetics measurements.(35) For example,Patel et al. performed an extensive study of reduction/oxidation of Fe(CN)63–/4– andRu(NH3)63+/2+ on highly oriented pyrolytic graphite (HOPG), demonstrating that both atmospheric exposure of the HOPG and prolonged voltammetric measurement significantly diminish the ET kinetics, indicating surface poisoning andpassivation.(35) Indeed, other recent studies by the same group confirmed that the pristine basal plane of freshly cleaved HOPG actually has remarkably high ET activity,(36, 37) in contrast to previous reports.(38, 39)We observe a significant difference in kinetics between freshly cleaved and aged surfaces of natural graphite, exposed to the ambient environment for days or weeks, in accordance with results reported by Patel et al.(35) The tip of the micropipette was used to cleave layers of graphite from the edge, forcing the liquid into contact with the freshly exposed surface, without exposure to the atmosphere. The CVs recorded immediately after the in situ cleavage of graphite (original thickness of 313 nm, ∼933 graphene layers) revealed significantly reduced ΔE p of ca.80–130 mV for the applied scan rate range, and k0, averaged for three different cleaved areas, was determined as 4.73 (±0.39) × 10–3 cm s–1, indicating near-reversible kinetic behavior, close to 2 orders of magnitude faster than the overall arithmetic mean for all the atmosphere-aged basal planes (see Table 1). It is evident that Ru(NH3)63+/2+ reduction/oxidation exhibits much faster kinetics on the freshly cleaved surface than on surfaces exposed to the ambient environment. The microcleaved area includes the edges of graphite, so measurement of a droplet deposited such that it covered the edge plane was carried out as a control. This did not reveal a significant change in the ET rate, which confirms that the observed difference is purely due to the inherent difference between freshly cleaved and aged surfaces of graphite (FigureS15, Supporting Information). The in situ cleavage was also attempted on thinner flakes; however, the method fails for thicknesses below ca. 20 nm as it is difficult to avoidtip-induced damage of the flakes. The rapid deterioration of the surface upon exposure to air is likely to occur within minutes or hours after exfoliation as suggested by other groups.(35, 40)The findings above lead to several conclusions about the ET kinetics on mono- and multilayer graphene surface. The data supports an increasing amount of evidence in the literature,(17, 19, 30, 35-37) that the basal plane of graphitic surfaces is active with respect to electron transfer. Only pristine basal planes, whose defect-free nature was confirmed by the absence of D-peak in Raman spectrum (see Methods), were chosen for the droplet deposition. Nevertheless, the optical resolution of this method is ∼1 μm, meaning that nanoscale defects could remain undetected and affect the ET measurement. In such a scenario, the large variation of the kinetics across the surface of the same flakescould be explained by random distribution of nanoscale defects increasing the ET activity, in accordance with the traditional view of graphite electrochemistry.(14, 15) Recent studies on the mobility of atoms on graphene surfaces show that not only does the graphene lattice undergo a self-repair mechanism of its basal plane, but also metallic impurities tend to migrate toward the edge planes and defect sites, where they are stabilized.(41, 42) This insight plays a significant role in the electrochemistry of graphene and contributes to the ongoing debate regarding edge plane/defect vs basal plane ET activity, as most metals have very fast ET kinetics toward most redox mediators.(43) This was also demonstrated by Ritzert et al., who observed an increased ET rate upon adsorption of kinetic-enhancing redox-active species on ca. 1/100 of the graphene surface, confirming that even small amounts of impurities play a significant role in electrode kinetics.(29)Furthermore, formation of electron and hole ―puddles‖ was observed and attributed to either warping of the suspended graphene sheets(44) or doping induced by the underlying substrate.(45) These imperfections will naturally lead to nano- or microscale inhomogeneity of the graphene surface.Another significant observation is the acceleration of kinetics on freshly cleaved graphite. We found that the surface conditions and the sample exposure to the ambient environment significantly perturb any inherent relationship between ET activity and the flake thickness. This is most likely due to a chemical modification of the graphene/graphite surface due to reaction with oxygen, water or other chemicals present in the air and/or adsorption of contaminant molecules/functional groups on the surface.(15, 46-48) X-ray photoelectron spectroscopy (XPS) shown in Figure 4 and energy-dispersive X-ray spectroscopy (EDX) (Figures S18 and S19,Supporting Information) were performed on both atmosphere-aged and freshly cleaved graphite surfaces.Figure 4. XPS survey spectra of atmosphere-aged (>1 month) graphite surface (top green) and pristine graphite surface cleaved immediately prior the XPS measurement (bottom red). Both spectra show data averaged from 5 different sites on the surface (spot size of 400 μm2). The quantitative elemental analysis is given in Table 2.The spectra are averaged over 5 different surface sites to obtained maximum sensitivity to trace impurity elements. The full quantification of the spectroscopic data across the 5 different surface sites is found in Table 2. Analysis of the averaged spectra revealed no substantial variation in the elemental composition. Both aged and cleaved surfacescontain ca. 92.8–93.2% carbon, 4.4–5.0% oxygen, 0.8–1.1% of fluorine and a total of0.7–2.0% of other impurities, including N, Na, Al, Si, S, K, Ca, and Ni (atomic percentages). The cleaved sample, however, exhibits significant site-to-site variation, particularly in carbon and oxygen concentration, in comparison to the aged sample. We attributed this to high reactivity of the freshly cleaved graphite and enhanced ability of the pristine surface to adsorb organic and inorganic molecules (Table 2), whereas the prolonged exposure of the aged graphite to the ambient environment yields similar carbon/oxygen ratio at different surface sites indicating uniformity of the surface modification. Furthermore, the extent of sp2 hybridization of carbon atoms in both samples is directly correlated with the overall carbon concentration (Figure 5), which most likely points toward formation of an organic, sp3-rich adsorbent layer, or suggests that the contaminants react with carbon and modify the graphite lattice (full details of XPS andEDX analyses are found in Supporting Information).Table 2. Quantitative Analysis of the XPS Spectra Obtained at Five Different Surface Sites on Aged and Cleaved Graphite Samplessurface site variation/At% mean/At%element aged cleaved aged cleavedC 88.54–94.08 85.76–97.42 92.84 93.17N 0.09–0.56 0.00–0.33 0.37 0.12O 4.31–8.06 1.35–12.10 4.98 4.39F 0.23–1.55 0.45–1.87 0.78 1.09surface site variation/At% mean/At% element aged cleaved aged cleaved Na <0.01 0.01–0.51 0.01 0.08Al 0.07–0.49 0.01–0.79 0.16 0.34Si 0.39–1.60 0.51–1.51 0.65 0.72S 0.14–0.24 0.00–0.10 0.14 0.03K 0.00–0.13 0.00–0.10 0.04 0.04Ca 0.00–1.64 <0.01 0.03 0.00Fe <0.01 <0.01 0.00 0.00Ni 0.00–0.01 0.00–0.06 0.01 0.03Figure 5. Effect of impurities on hybridization and functionalization of carbon atoms expressed by XPS analysis of both atmosphere-aged (circles) and cleaved (triangles) graphite surface. The extent of carbon sp2 hybridization, determined from C 1s peak (green) and Auger peak(D-parameter, blue), is proportional to the total carbon content (XPS survey quantification).Comparison of the Kinetics for Fe(CN)63–/4–, Ru(NH3)63+/2+, and IrCl62–/3–The droplet-to-droplet variation of ET rate on flakes of the same thickness, expressed in the arithmetic mean of the relative errors (Δ/k0) in Table 1, is most pronounced forFe(CN)63–/4–, slightly less for Ru(NH3)63+/2+, and the least for IrCl62–/3–. The same trend is observed for a difference between the maximum and minimum averaged and absolute kinetics for the three mediators. The maximum difference in averaged kinetics(from Table 1) is ca. 16-, 14-, and 2-fold, and the maximum difference in absolute kinetics (from individual droplet measurements) is ca. 3 orders of magnitude for bothFe(CN)63–/4– and Ru(NH3)63+/2+ and only ca. 8-fold for IrCl62–/3–. This is consistent withFe(CN)63– being an inner-sphere redox mediator with inherent sensitivity to surface states,(15) and indeed, previously reported k0 values for this mediator on graphene vary significantly.(17, 19, 20, 24, 27-29) In an idealized scenario, the observed kinetics of a genuine outer-sphere mediator, which maintains its original coordination sphere during the ET process,(14, 15, 34) would only depend on the density of states (DOS) of the electrode material and would therefore be less sensitive to mild surface contamination. This behavior is, to an extent, observed for IrCl62– as reflected in the error analysis above. Furthermore, the fact that the variation in ET kinetics of the two outer-sphere mediators (Ru(NH3)63+ and IrCl62–) is most pronounced for the thin flakes (<20 layers) raises a question as to whether this is related to local impurity-induced changes in the DOS, which should be most pronounced in mono- and few-layer graphene.Interestingly, the absolute value of k0 for Ru(NH3)63+ reduction/oxidation is much lower than reported literature values,(15, 29, 35, 37, 49) contrary to the common conception of Ru(NH3)63+as an outer-sphere mediator. Although, the number of electrochemical studies carried out on natural graphite is very limited, in contrast to the well-studied kinetics on other carbon electrodes such as glassy carbon and HOPG, the slow kinetics observed in this work is not the sole exception to the fast kinetics generally assumed for this mediator: k0 of ca. 10–5 to 10–4 cm s–1were reported on the basal plane of HOPG surface.(36) To provide a direct comparison with other materials, we employed a。
