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Electrochimica Acta 56 (2011) 3555–3561

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

oncatenation of electrochemical grafting with chemical or electrochemicalodification for preparing electrodes with specific surface functionality

allavi Verma, Pascal Maire1,2, Petr Novák ∗,1

aul Scherrer Institut, Electrochemistry Laboratory, Section Electrochemical Energy Storage, CH-5232 Villigen PSI, Switzerland

r t i c l e i n f o

rticle history:eceived 23 July 2010eceived in revised form6 November 2010ccepted 18 November 2010vailable online 25 November 2010

eywords:

a b s t r a c t

Surface modified electrodes are used in electro-analysis, electro-catalysis, sensors, biomedical applica-tions, etc. and could also be used in batteries. The properties of modified electrodes are determined bythe surface functionality. Therefore, the steps involved in the surface modification of the electrodes toobtain specific functionality are of prime importance. We illustrate here bridging of two routes of sur-face modifications namely electrochemical grafting, and chemical or electrochemical reduction. First,by electrochemical grafting an organic moiety is covalently immobilized on the surface. Then, eitherby chemical or by electrochemical route the terminal functional group of the grafted moiety is trans-

arbon electrodesurface modificationlectrochemical graftingunctional group transformation

formed. Using the former route we prepared lithium alkyl carbonate (–O(CH2)3OCO2Li) modified carbonwith potential applications in batteries, and employing the latter we prepared phenyl hydroxyl amine(–C6H4NHOH) modified carbon which may find application in biosensors. Benzyl alcohol (–C6H4CH2OH)modified carbon was prepared by both chemical as well as electrochemical route. We report combina-

wo stationing st

-ray photoelectron spectroscopy tions of conjugating the tfunctional group modificthe electrochemical graft

. Introduction

Chemically modified electrodes (CMEs) [1,2] have significantpplication in energy conversion and storage, and electro-analysis.odified electrodes are used to study charge transfer, double

ayer capacitance, surface reactions [3], etc. They are also used inlectrochemical/chemical/bio sensors, electro-organic syntheses,iquid chromatography detection [4], etc. Nature and applicationsf CMEs is very diverse. Substrate materials can be semi-conductors5], metals [6,7], carbons [8–11], silica [12], etc. The immobilizedpecies can be organic [13], inorganic [14,15], polymers, or enzymes16,17]. Properties of electrodes (such as wettability, adsorption,apacitance, catalytic behaviour, kinetics, and interface phenom-na) can be tuned by imparting it specific functionality via surfaceodifications.The most widely used method of covalent immobilization of

oieties onto electrode surfaces is electrochemical reduction of

iazonium salt precursor. The diazonium salt grafting is gener-lly accepted to occur via radical mechanism [18–27]. One electronransfer to the diazonium salt results in release of N2. The remain-ng radical molecule couples with a radical on the surface to form a

∗ Corresponding author. Tel.: +41 56 310 2457; fax: +41 56 310 4415.E-mail address: petr.novak@psi.ch (P. Novák).

1 ISE member.2 Present address: Renata AG, CH-4452 Itingen, Switzerland.

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.11.055

eps of surface modifications and show how the optimal route of terminaldepends on the chemical nature of the moiety attached to the surface inep.

© 2010 Elsevier Ltd. All rights reserved.

covalent bond. Diazonium salts can be grafted by electrochemicalgrafting [25] or by spontaneous method [22]. When the requireddiazonium salt is unstable or non-isolable, the diazonium salt issynthesized in situ followed by electrochemical reduction [28,29].The electrochemical method is faster, and more controllable; thushomogeneous monolayer or mixed component or two differentconsecutive layers can be prepared [24]. The spontaneous graftingroute is simple, slower, and scale-up is easier. In the electrochem-ical method due to high density of electrons on the electrode, thediazonium reduction is localized to the surface and the loss of dia-zonium salt by other parallel bulk reactions is minimal. This is notthe case for spontaneous grafting, where the probability of diazo-nium decay taking place on the surface is almost the same as itis in the bulk. Thus this normally results in lesser surface cover-age as compared to grafting by electrochemical means. This hasbeen proved by X-ray photoelectron spectroscopy (XPS) [20] aswell as by time of flight-secondary ion mass spectrometry (ToF-SIMS) [18]. Hence, for this work, we use electrochemical graftingroute. There are also other precursors which can be used for graft-ing organic moieties onto surfaces [30]. Alcohols [31–34], amines[35,36], carboxylates/acetates [37], etc. are grafted via oxidation;and iodonium salts [38–43], and sulfonium salts [44] are grafted

reductively. In this work we use alcohol grafting and diazoniumsalt grafting methods.

Due to the limited options of the precursors, it is rarely the casethat one obtains the desired functionality, after the step of covalentimmobilization on the electrode surface. For example, diazonium

3 mica Acta 56 (2011) 3555–3561

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alts of electron donating groups are not stable; hence normallyhe surface functional groups obtained by diazonium grafting arelectron withdrawing like –NO2 or –COOH. But for applications likeatteries [45], biosensors [16], protein adsorption [11], complexa-ion [46], composites [47], etc. one would require surface modifiedith electron donating groups like –CH2OH, –NH2, etc. It is thusecessary to introduce the second step of transforming the func-ionality of the immobilized moiety. Here there are two options. Wean either transform/reduce the grafted moiety electrochemicallyby applying negative potentials) or we can do it chemically (bysing reducing agents). For the chemical route wettability, reagentiffusion, interfacial reactivity, etc. are the critical factors. However,ith the electrochemical route the critical issues are the conduc-

ivity of the grafted layer, stability of the bonds in the potentialindow, suitable solvent, etc. By using the electrochemical routee assist the reduction by providing abundant electrons from the

xternal circuit to the surface, i.e. the site of reduction. With thehemical route the advantage is that there are innumerous chemi-al synthesis and transformation routes available in literature [48]hich can be employed; however it should be noted that the reac-

ion mechanisms may be different on molecules attached on a solidubstrate than on the molecules in the bulk. One route or the otheray be advantageous for a specific modification.The bare electrode surface and the electrode surface modified by

he two steps of covalent immobilization (linker attachment) andunctionality transformation are schematically shown in Fig. 1a.he steps of surface modification with the possible pathways areummarized in the flowchart in Fig. 1b.

In this paper we report preparation of three model modi-ed glassy carbon (GC) electrodes namely GC–O–(CH2)3–OCO2Li,C–C6H4–CH2OH and GC–C6H4–NHOH by different combinationsf the two steps. GC–O–(CH2)3–OCO2Li is prepared by electrochem-cal oxidative grafting of 1,3-propanediol followed by chemicalransformation, GC–C6H4–CH2OH is prepared by electrochemicaleductive grafting of p-carboxylicbenzenediazonium tetrafluoroorate salt (PCBD) followed by chemical as well as electrochemicaleduction, and the GC–C6H4–NHOH is prepared by electrochem-cal reductive grafting of p-nitrobenzene diazonium tetrafluoroorate salt (PNBD) followed by electrochemical reduction. Whichhemical groups are likely to prefer one or the other method ofunctionality transformation is discussed.

. Experimental

The electrode used for this study is glassy carbon (HTW,IGRADUR-G disc, 15 mm × 1 mm). GC is used as a model electrodeurface for graphite powders to simplify the surface modifica-ion and analyses procedure. For all electrochemical experimentsspecially designed three-electrode cell was used. This cell allows

omplete immersion of the GC samples in the electrolyte and cane hermetically sealed. The working electrode was GC, the counterlectrode used was Pt wire, and the reference electrode was Agire. Pristine GC sample is labeled A. Potential of Ag wire was cali-

rated from Fe(CN)63−/Fe(CN)6

4− aqueous system, it was found toe 0.14 V vs. SHE. However, the potential of Ag wire was not cali-rated for non-aqueous electrolytes. All the potentials in this paperre vs. Ag wire, unless specified otherwise.

Chemical analyses of the modified GC electrodes were per-ormed by X-ray photoelectron spectroscopy (XPS) with ESCALAB20iXL (Thermo Scientific, USA, formerly V.G. Scientific) instru-

ent using Al K� (1486.6 eV) radiation. Apart from the ultra sonicashing in acetone, no further sample preparation was performedrior to the XPS measurements.

FTIR spectra were recorded with Perkin–Elmer system 2000quipped with MIRTGS (mid infrared triglycine sulphate) detector

Fig. 1. (a) Illustration of the pristine and modified GC surface. (b) Flowchart of thesteps involved in surface modification with possible routes.

(4000–600 cm−1, resolution = 4 cm−1, 30 scans). Before recordingthe spectra the sample chamber was purged with N2.

All the chemicals used were purchased from Sigma–Aldrich andused without any further purification, unless specified otherwise.

2.1. Covalent immobilization

Alcohol grafting of 1,3-propanediol (scheme shown in Fig. 2a):the electrolyte was the parent alcohol and the supporting salt was0.1 M LiClO4. Cyclic voltammetry (CV) (5 scans) were run from 0.0 Vto 2.2 V vs. Ag wire at a scan rate of 5 mV/s. After grafting the GCsample, it was washed ultrasonically in acetone. The GC samplegrafted with 1,3-propanediol is called B.

Diazonium grafting of PNBD and PCBD (scheme shown inFig. 2b and c respectively): PNBD salt was purchased from

Sigma–Aldrich (stored below 0 ◦C), PCBD was synthesized fromp-amino benzoic acid [49] and characterized by FTIR (attenuatedtotal reflectance mode). It showed complete transformation ofNH2 (3458, 3360 cm−1), and presence of N N+ (2304 cm−1) and

P. Verma et al. / Electrochimica Acta 56 (2011) 3555–3561 3557

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Fig. 3. (a) Scheme of the procedure for chemical transformation of the alcoholgrafted GC (B) to carbonate containing GC (E). (b) Scheme of the procedure for elec-trochemical reduction of the alcohol grafted GC (B). (c) Scheme of the procedurefor chemical reduction of the PNBD grafted GC (C). (d) Scheme of the procedurefor electrochemical reduction of the PNBD grafted GC (C) to phenyl hydroxylamine

ig. 2. (a) Scheme of electrochemical oxidative grafting of 1,3-propanediol. (b)cheme of electrochemical reductive grafting of PNBD. (c) Scheme of electrochem-cal reductive grafting of PCBD.

F4− (1037 cm−1). The electrolyte was 0.1 M tetraethylammonium

etrafluoroborate (TEAB) in CH3CN (stored over molecular sievesA, H2O < 50 ppm, analyzed by Karl Fischer titration), containingmM diazonium salt (PNBD or PCBD). CV was scanned from −1.6 V

o 0.5 V vs. Ag wire for PNBD (scheme shown in Fig. 2b) and from2.0 V to 0.5 V vs. Ag wire for PCBD (scheme shown in Fig. 2c) at a

ate of 5 mV/s. The modified GC was then sonicated in acetone formin. GC modified with PNBD is called C, and GC grafted with PCBD

s called D. Sample C showed strong vibrations of –NO2 stretching1513, 1341 cm−1) and absence of N N vibration (2306 cm−1) inhe FTIR spectrum (reflective mode) (spectrum not shown). Theharacteristic CV response of the nitro group was also observed byycling C in 0.1 M TEAB in CH3CN (CV not shown), this is consistentith the 4 electron redox process described in the literature [23].

Samples B, C and D were then subjected to XPS analyses, CVn 5 mM K3Fe(CN)6 + 5 mM K4Fe(CN)6 in 1 M KCl aqueous solution5 mV/s), and further functionality transformation steps describedn Section 2.2.

.2. Functionality transformation

The sample B was chemically reduced by stirring in a solutionf acetonitrile with excess of LiH overnight in an inert atmosphere.ithout exposing the resulting alkoxide to air, CO2 was bubbled

hrough the solution for 4–5 h (Fig. 3a); the resulting sample isabeled E. The sample B was reduced electrochemically by CV in.1 M TEAB in CH3CN from 0.0 V to −3.0 V (Fig. 3b). Cycled sampleas sonicated in acetone; the resulting sample is labeled F. All the

teps of functionality modification were monitored by XPS.For the chemical reduction the sample C was reduced with Zn,

aCl2 [50] in aqueous ethanol under reflux for 2 h (Fig. 3c). Theesulting sample was sonicated in acetone and labeled G. For thelectrochemical reduction, the sample C was cycled in 0.1 M TEABn CH3CN from 0.0 V to −2.0 V at 5 mV/s for 10 cycles (Fig. 3d). The

esulting sample was sonicated in acetone and labeled H.

For the chemical reduction of sample D, it was stirred in 1 MH3 in THF overnight under Ar atmosphere (Fig. 3e). It was thenonicated in CH3CN. The resulting sample was labeled I. For elec-rochemical reduction of sample D, it was cycled in 0.1 M TEAB in

containing GC (H). (e) Scheme of the procedure for chemical reduction of the PCBDgrafted GC (D) to benzyl alcohol containing GC (I). (f) Scheme of the procedure forelectrochemical reduction of the PCBD grafted GC (D) to benzyl alcohol containingGC (J).

CH3CN from 0.0 V to −3.0 V at 5 mV/s for 10 cycles (Fig. 3f). It wasthen sonicated in acetone. The resulting sample was labeled J.

3. Results and discussion

3.1. Properties of the electrochemically grafted surfaces

Oxidative grafting of alcohol on GC was performed as shown inFig. 2a. The CV of the grafting illustrated in Fig. 4a, shows that it isan irreversible oxidation which results in the modification of theelectrochemical properties of the surface. This grafting is almostcomplete in the first run, after which there is minimal graftingcurrent in the consecutive cycles. The reductive grafting of the dia-zonium salts was performed by the procedure illustrated in Fig. 2band c. The CV of the grafting of PNBD is shown in Fig. 4b. It is evi-dent that the grafting is an irreversible reduction, which is almostcomplete in the first cycle. The current density in the consecu-tive cycles is diminished drastically. A similar trend is observed inthe PCBD grafting CV (voltammogram not shown). These are direct

electrochemical proofs of the modification of the electrochemicalproperties of the surface after grafting of the organic species.

To further study the electrochemical properties of the modifiedsurface, CV of the modified GC electrodes was performed in 5 mMK3Fe(CN)6 + 5 mM K4Fe(CN)6 in 1 M KCl aqueous solution. The CVs

3558 P. Verma et al. / Electrochimica Acta 56 (2011) 3555–3561

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0.5

1.0

1.5

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05

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b

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-2

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-1.5

-1.0

-0.5 0.0

0.5

-75

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ig. 4. (a) CV of electrochemical oxidative grafting of 1,3-propanediol. 1, 2, 3 are theonsecutive cycles. (b) CV of electrochemical reductive grafting of PNBD. 1, 2 are therst and second cycles.

f pristine and modified GC electrodes are shown in Fig. 5. PristineC is a good electronic conductor and hence gives a characteristicoltammogram of Fe2+/Fe3+ redox system. However after the cova-ent immobilization step the charge transfer peaks of the systemre diminished for alcohol grafted GC (B) and for PNBD grafted GCC). The same is observed for the GC grafted with PCBD (D) (CV nothown). This proves that the surface electronic conductivity of therafted GC electrodes is significantly diminished and thus a layerxists on the electrode surface. We assume that grafted species hin-er the approaching Fe(CN)6

4− or Fe(CN)63− ions from undergoing

harge transfer. The grafted GC samples were analyzed by XPS, toetermine the chemical nature of the grafted species. The quantita-ive and qualitative results are summarized in Table 1. The bindingnergies from the deconvoluted high resolution spectra of the func-ional groups of the grafted samples prove the existence of –COORR = C, H) terminal group on B, –NO2 group on C and –COOH groupn D. The quantitative analysis reveals that the alcohol graftedample is more heavily loaded compared to the diazonium graftedamples. This is in line with the expectation because the alcoholrafting is performed in the parent alcohol as electrolyte, whereashe diazonium salts are present in millimolar concentration in therafting bath. Loading during the diazonium salt grafting can bearied by varying the time of grafting and the concentration of theiazonium salt in the electrolyte, but this work is not the focus of

his paper. An additional proof of the alcohol grafting was that theristine surface could be redeemed by treating the sample B withorane, which results in the cleavage of the ether bond and hencehe molecule is cleaved off the surface.

Fig. 5. CV of pristine GC (A) (�), alcohol grafted GC (B) ( ) and PNBD grafted GC (C)( ) in 5 mM K3Fe(CN)6 + 5 mM K4Fe(CN)6 in 1 M KCl aqueous solution at 5 mV/s.

3.2. Monitoring the functional group modifications

Each of the grafted samples B, C and D was subjected to chemi-cal and electrochemical methods of end functional group reduction.These experiments and results are summarized in Table 2. The XPSresults are shown in Fig. 6. All the XPS spectra have been normalizedwith respect to intensity of the peak with highest counts per sec-ond. We observe that the alcohol grafted sample can be reducedto lithium alkoxide (evidenced in the further transformation tolithium alkyl carbonate) via chemical method. The evolution ofthe C 1s spectra of the pristine GC surface after the grafting andthe chemical treatment is shown in Fig. 6a. The signals at higherbinding energies in the grafted sample B are attributed to the–C–O–C– (286.5 eV) and C O (289.1 eV) carbons. After the treat-ment however it is seen that the C 1s signal attributed to C O isdiminished and a signal at lower binding energy (–CH2O–) appears.Another signal appears at high binding energy at 291.2 eV, this isattributed to the carbonate carbon (–CO3Li). Lithium alkoxides arecommon precursors for lithium alkyl carbonate synthesis [51,52],hence observation of the carbonate signal on the sample after expo-sure to CO2 can be worked back to be an evidence of the existenceof lithium alkoxide (ROLi) after the LiH reduction step. But B (alco-hol grafted GC) could not be reduced electrochemically. This maybe attributed to the hindered transference of electrons through thesaturated alkyl chain during the electro-reduction of the graftedsample B. The electrons supplied by the external circuit could notreach the –COOH end group, due to the absence of a conductionpathway for electrons such as a delocalized pi-cloud (or a conju-gated system) in the grafted layer. On the other hand, the chemicalreduction is feasible because the alkyl chain can be wetted by thereagents to undergo reaction on the surface. Additionally, it wasalso observed that B does not form amide with 1-octyl amine,

and does not form ester with 4-bromobutyryl chloride (results notshown). This may be attributed to two reasons, first that there isa possibility that the grafted sample has ring closures (cycliza-tion), which are not re-opened by these reagents, but are re-opened

P. Verma et al. / Electrochimica Acta 56 (2011) 3555–3561 3559

Table 1Qualitative and quantitative XPS analyses of pristine GC (A) and grafted GC surfaces (B, C, and D). The binding energies reported are for the deconvoluted component of thesignal which is assigned to the corresponding functional group.

Sample Structure Binding energy/eV of terminal group Functional group assignment Quantitative analysis (y)

A 284.3 C(sp2) –

BO

COOH 289.2 –COOR (R = H, C) 0.17 (±0.03)

C NO2 407.8 –NO2 0.11 (±0.03)

D COOH 289.1 –COOH 0.10 (±0.03)

y atom

btmatdto

TS

= atomic percentage of the deconvoluted component of the functional group/total

y the LiH. Second explanation could be that the mechanism ofhese condensation reactions is difficult to realize on a supported

olecule. Note that, carbonate modified carbons have a potentialpplication in Li-ion batteries. Graphites containing carbonate on

he surface are expected to have a lower irreversible charge lossue to suppression of the electrolyte reduction in the first lithia-ion of the graphite. Further work in this direction is underway inur laboratory.

able 2ummary of the experiments, outcome, and results of the chemical and electrochemical f

Grafted sample Functionality transformation

Method Experiment

B

OCOOH

Chemical Fig. 3a

Electrochemical Fig. 3b

C

NO2

Chemical Fig. 3c

Electrochemical Fig. 3d

D

COOH

Chemical Fig. 3e

Electrochemical Fig. 3f

ic percentage of carbon.

The PNBD grafted sample C could not be reduced by chemicalmethod using Zn, CaCl2 (procedure described in Fig. 3c, resultssummarized in Table 2). This illustrates the hindered reactivityof the surface groups towards the reagents. Since this reaction

requires reflux conditions, it may also be that the heat transfer isnot homogeneous when the molecule is attached to a substrate ascompared to when it is a bulk pot synthesis. However, the grafted–NO2 group could be relatively easily reduced by electrochemical

unctional group transformations of the grafted samples B, C, and D.

Success XPS results Modified sample

Yes Fig. 6a

E

O OCO2Li

No Not shown –No Not shown –

Yes Fig. 6b

H

NHOH

Yes Fig. 6c

I

CH2OH

Yes Fig. 6d

J

CH2OH

3560 P. Verma et al. / Electrochimica Acta 56 (2011) 3555–3561

294

292

290

288

286

284

282

280

050

0010

000

1500

020

000

2500

030

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ity/a

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Binding energy/eV

A = GC pristine B = GC RCOOH E = GC ROCO2Li

C=O

OCO2Li

a

412

410

408

406

404

402

400

398

A = GC pristine C = GC PhNO2

H = GC PhNHOH

Inte

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.

Binding energy/eV

NO2 HNOH

b

294

292

290

288

286

284

282

280

020

4060

8010

0

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nsity

/a.u

.

Binding energy/eV

A = GC pristine D = GC PhCOOH I = GC PhCH2OH

c

COOH

294

292

290

288

286

284

282

280

Inte

nsity

/a.u

.

Binding energy/eV

A = GC pristine D = GC PhCOOH J = GC PhCH2OH

d

COOH

Fig. 6. (a) C 1s high resolution spectra of pristine GC (A) (�), alcohol grafted GC (B) ( ), and chemically modified B containing carbonate (E) ( ). (b) N 1s high resolution

s duced

p enzyl

G

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pectra of pristine GC (A) (�), PNBD grafted GC (C) ( ), and electrochemically re

ristine GC (A) (�), PCBD grafted GC (D) ( ), and chemically reduced D containing b

C (D) ( ), and electrochemically reduced D containing benzyl alcohol (I) ( ).

ethod. As per the 4-electron reduction the –NO2 is reduced toNHOH [23]. This is evident in the XPS spectra of the samples illus-rated in Fig. 6b. Sample C has two main N 1s peaks, the one at highinding energy of 407.8 eV is assigned to NO2 whereas the one at01.9 eV is assigned to reduced nitrogen species. This peak at lowerinding energy has been observed for grafted NO2 diazonium saltsy many research groups and has been attributed to the reducedNO2 species [19,20,24,26,27,29,53,54]. After the electrochemicaleduction the peak at higher binding energy is almost completelyepressed at the expense of the N 1s peak at lower binding energyttributed to NHOH. Even the oxygen content of that sample (eval-ation not shown) is halved after reduction. This is a direct prooff the reduction of the –NO2 group to –NHOH.

PCBD grafted sample D was chemically reduced by borane (pro-edure described in Fig. 3e, results summarized in Table 2), the XPSesults of the surface modification are shown in Fig. 6c. The graftedample D shows C 1s peak at 289.4 eV which is attributed to car-

oxylic acid (–COOH). After the chemical reduction step sample Ihows complete absence of peak at 289.4 eV with a concomitantecrease of the quantity of the oxygen on the sample. This proveshat the carbonyl carbon has been reduced. However there stillemains a shoulder on the C 1s spectrum of the sample I which is

C containing phenyl hydroxylamine (H) ( ). (c) C 1s high resolution spectra of

alcohol (I) ( ). (d) C 1s high resolution spectra of pristine GC (A) (�), PCBD grafted

attributed to the C–O carbons of the reduced species. PCBD graftedsample D was also reduced electrochemically (procedure describedin Fig. 3f, results summarized in Table 2). The XPS results are shownin Fig. 6d. The signal at 289.4 eV attributed to –COOH is completelydepressed after reduction (sample J), with a concomitant decreaseof quantity of oxygen. The shape of the C 1s spectrum of sample Iand sample J is not completely identical, this is due to the differentmechanism employed for the reduction procedure, which resultsin slightly different surface chemistry. The apparent differences inthe shape of the C 1s spectrum of sample D in Fig. 6c and d are due tothe normalization performed on results which had different max-imum intensity during individual XPS measurement. There couldalso be slight variation in the different grafted samples of the samekind due to slight variation in the grafting time.

Thus from the results of the electrochemical reduction of thediazonium grafted samples (C and D) it is observed that the graftedsamples which have a conjugated system between the surface

and the end functional group can be reduced by electrochemicalmeans. This is attributed to the fact that the electrons can trans-port via the pi-electron cloud from the bulk electrode through theorganic grafted layer to the end group. The conjugated system ofthe phenyl ring serves as a convenient pathway for the electrons to

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ravel during the electrochemical reduction to reach the end group.his observation can be used to electrochemically reduce surfacesrafted with the aryl moieties. This is especially a useful pathway toeduce the species which are otherwise difficult to reduce at ambi-nt temperature like –NO2. Even for the PCBD grafted sample whichs reducible by chemical means, the electrochemical method is aetter choice as it avoids the use of expensive and toxic chemicals

ike borane.

. Conclusions

GC electrode with covalently attached lithium alkyl carbonate–O(CH2)3OCO2Li) surface layer was prepared by electrochemicalxidative grafting of alcohol followed by chemical transforma-ion steps. Phenyl hydroxylamine (–C6H4NHOH) modified GC wasrepared by electrochemical reductive grafting followed by elec-rochemical reduction. Benzyl alcohol (–C6H4CH2OH) modified GCas prepared by electrochemical reductive grafting followed by

oth chemical as well as electrochemical reduction. The methodol-gy of using the second step of transformation on grafted electrodeso obtain desired functionality can be extended in many dimen-ions. If the grafted moiety contains a conjugated system then itslectrochemical reduction is feasible. This is because the electronsan transport through the delocalized pi-clouds to the end groupo be reduced. However, electrochemical reduction of a saturatedrafted moiety is difficult due to unavailability of electron con-uction pathway across the layer. Thus it can be concluded thatlectrochemical reduction is a feasible method to reduce surfacesrafted by aryl species. In short, we developed an approach thatan be used to modify surface of the electrode materials for variouspplications.

Once developed on model surface such as glassy carbon, theseunctionalization methods can be transferred to other sp2 car-on materials, like fullerenes (0 dimension), carbon nanotubes (1imension), graphenes (2 dimension) or graphite (3 dimension).ome of these methods of surface modifications are currently beingpplied to graphite powders for applications in Li-ion batteries inur laboratory. Graphites with tailor made surface organic func-ional groups are expected to impart passivity towards electrolyteegradation and hence reduce the irreversible charge loss duringrst lithiation. The covalent bonds are expected to avert organic

ayer detachment from graphite surface during cycling. The func-ional groups intended to be immobilized for this application arehemically similar to the solvent systems in the Li-ion battery elec-rolytes. Thus they are expected to be stable in the potential andemperature window of Li-ion battery operation.

cknowledgement

We thank Swiss National Science Foundation for funding.

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[

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