Delivered by Ingenta toSCD Universite Bordeaux 1

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RESEARCHARTICLE

Copyright copy 2009 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 9 6113ndash6119 2009

Comparison Between Early Stage OxygenationBehavior of Fullerenes and Carbon Nanotubes

Gregory Van Lier1lowast Christopher P Ewels2 Montserrat Cases-Amat13 daggerIrene Suarez-Martinez2 and Paul Geerlings1

1Research Group of General Chemistry (ALGC) Vrije Universiteit Brussel Pleinlaan 2 B-1050 Brussels Belgium2Institut des Mateacuteriaux Jean Rouxel CNRS UMR6502 2 Rue de la Houssiniegravere 44322 Nantes France

3Institut de Quimica Computacional and Departament de Quimica Universitat de Girona E-17071 Girona Catalonia Spain

We explore early stage oxygen addition to C60 buckminsterfullerene and compare its oxygenationbehavior to that of both pristine and defective metallic carbon nanotubes using ab initio theoreticalmodeling For fullerene oxygen addition up to C60O4 in general oxygenation preferentially occursat the pentagonndashhexagon bonds (56 type addition) leading to open annulene structures asopposed to the closed 66 epoxide isomers For carbon nanotubes the preference for annulenestructures is significantly more pronounced as all epoxide addition is endothermic Higher reactionenthalpies are found for oxidation in the proximity of defects as compared to the pristine sidewallsIn most cases higher reaction enthalpies are found for fullerene oxygenation as compared to carbonnanotubes

Keywords Fullerene Carbon Nanotube Oxygenation Ab Initio Theoretical Modeling

1 INTRODUCTION

Fullerene oxides were the first fullerene derivatives dis-covered and have been extensively studied since12 show-ing a rich chemistry in reactions both with fullerenesand with themselves3ndash5 They are thermally labile andcan liberate the attached oxygen upon heating C60 mul-tiply oxidizes readily explaining the difficulty to isolatethe low degree of oxygenation derivatives of C606 C60On

with n = 2ndash7 have been observed in the mass spectra offullerene oxides478 Highly oxygenated C60 can also beenobtained by corona discharge ionization in the gas phase9

Nucleophilic substitution of fluorine in fluorofullerenes byOH with subsequent elimination of HF results in epox-ide fullerenes such as C60O9 and C60O18 isostructural withC60F18 and C60F36 respectively1011 as well as numer-ous fluorinated compounds such as C60F16O5 C60F18O5C60F10O9 and species containing up to 18 oxygen atomsplus 8 or 10 fluorine atoms12ndash16

Single oxygenation of C60 can result in either a 66-closed epoxide isomer17 or a 56-open oxidoannu-lene (an ether structure oxa-homo[60]fullerene)18 Thefullerene epoxide 66-closed C60O is the key fullerene

lowastAuthor to whom correspondence should be addresseddaggerPresent address Chemogenomics Laboratory Institut Municipal

drsquoInvestigacioacute Megravedica Parc de Recerca Biomegravedica Doctor Aiguader 88E-08003 Barcelona Spain

oxygenation product playing an important role in thesynthesis of various organic organometallic and polymericfullerene derivatives since it provides easy access tofurther functionalisation3ndash5 Previous theoretical studiesof fullerene oxygenation showed that the reactivity islocal and that 66-double bonds adjacent to an exist-ing epoxide functionality are more easily oxidized619

Recent modeling of heterofullerene oxygenation showsthat azafullerenes oxidize similarly to pristine fullereneswhile phosphofullerenes preferentially form phospheneoxides with drastic effects on the heterofullerene electronicbehavior (turning the phosphofullerene radical C59Pbull intoan acceptor)20

Oxygenation studies of nanotubes typically focus onmore aggressive oxygenation involving nanotube dam-age and carbon loss such as tube tip opening removalof unwanted amorphous impurities and catalyst21 Cut-ting of carbon nanotubes has also been achieved throughozonolysis22 The introduction of oxidized surface defectscan improve nanotube reactivity for example through oxy-gen plasma treatments23 with possible applications for gassensing devices2425 Defects are typically assumed to berandomly distributed over the tube surfaces26 and in caseof a single vacancy the defect has been predicted to forma vacancy-oxygen complex upon reaction with an oxygenmolecule (VO2)23 Although different oxygenation tech-niques exist the literature is sparse on lsquogentlersquo oxygenatom addition routes to carbon nanotubes despite the

J Nanosci Nanotechnol 2009 Vol 9 No 10 1533-4880200996113007 doi101166jnn20091577 6113

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RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

apparent superficial structural similarities between nano-tubes and fullerenes27

Some theoretical investigations of carbon nanotube sin-gle oxygen addition have already been performed forpristine side-wall oxygenation It was predicted that forboth zig-zag and armchair carbon nanotubes a preferenceexists towards annulene addition at circumferential bondswhereas epoxides form preferentially at axial bonds28ndash30

A recent theoretical study considered capped ultra-narrownanotubes (C80 and C84) but unsurprisingly obtained pref-erential bonding to the fullerene caps31

Here we perform a comparative study where systematicoxygen atom addition to the surface of carbon nanomate-rials is explored through the use of ab initio theoreticalmodeling In particular early stage oxygen addition to theC60 fullerene is analyzed and compared with addition to a55 carbon nanotube both to the pristine side wall andto a single vacancy

2 METHOD

Since the number of possible isomers to consider isextremely large we use an automated process to gener-ate optimize and characterize the isomers of a given car-bon system XOn where X is either the fullerene C60 ora section of a carbon nanotube using a recently devel-oped code SACHA3233 In this case we have analyzed apristine 55 carbon nanotube section C240H20 as wellas the same nanotube containing a vacancy-O2 defectC239H20O2 In brief given a starting structure we add anadditional oxygen atom at all possible candidate bondingsites (ie above all carbonndashcarbon bonds) one at a timeEach of these possible structures is first geometrically opti-mized using the semi-empirical AM1 level of theory withGaussian0334 and the resultant system energies tabulatedSecond stage full geometry optimizations are performed at

Table I Calculated most stable isomers and corresponding reaction enthalpy ER for addition to the carbon atoms numbered in bold and the structuraldata for the C60On under consideration References are indicated in the final column if the system has been experimentally isolated and characterized

CndashO bond length Underlying CndashC distance

Isomer ER (Aring) (Aring) CndashOndashC bond angle Characterized experimentally

66 type addition12-C60O minus16 1457 1522 6298 [7 17]

1234-C60O2 minus70 14601451 1512 6261 [3]121011-C60O2 minus182lowast 13951454 22141558 105076483

123456-C60O3 minus77 1453 1512 6272 [40]1234910-C60O3 minus64 14581451 15131501 62666218 [39]12341112-C60O3 minus57 14451461 15071498 58416232 [39]

56 type addition19-C60O minus109 1398 2193 10338 [18]

19212-C60O2 minus377 13981400 2251 10712 [8]1967-C60O2 minus367 14011405 2280 10867

192121011-C60O3 minus554 1403 2262 1074019212101167-C60O4 minus280 1400 2259 10782

lowastER =minus89 kcalmol for when formed from 19-C60O

the HF3-21G level of theory for all isomers within 05 eVof the most stable at AM1 level The most stable struc-ture at HF3-21G is then taken as our starting point forthe addition of the following oxygen atom In this waywe can systematically increase the quantity of oxygen inthe system reflecting the thermodynamic addition route ofoxygenation We only considered structures with oxygenin CndashC bond bridging sites (epoxide or annulene) ie weare not concerned here with structures such as ozonidesndashOndashOndashOndash 2 or molecular oxygen doping of fullerites35

The automated approach using the SACHA code pro-vides great computational saving compared to lsquobrute forcersquoinvestigation of all possible isomers rendering such stud-ies of nanotube oxygenation feasible Further discussionof the approach is given in Refs [32 33] and has alreadybeen applied for the analysis of fullerene fluorination36

Reaction enthalpies are approximated as the energy differ-ences [ER = E(C60OnminusE(C60Onminus1minus 12E(O2)] con-sidering the triplet state of the O2 molecule thus providinga measure for the exothermicity of oxygenation

In the current study we have restricted ourselves to earlystage oxygenation ie up to a few oxygen atoms sincethe majority of experimental characterization studies offullerene oxides are restricted to this range We note thatgiven sufficient computing time there is no reason in prin-ciple that such an approach could not consider higher orderoxygenation

21 Oxygenation of Buckminsterfullerene C60

We first consider oxygenation of C60 and compare withexisting literature data Two isomers exist for C60Onamely to the bond between two hexagons (66 typeaddition) and between a pentagon and a hexagon (56type addition) When only 66 type addition is con-sidered there are only eight possible regioisomers of

6114 J Nanosci Nanotechnol 9 6113ndash6119 2009

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RESEARCHARTICLE

Van Lier et al Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes

C60O2 but already 46 regioisomers exist when both56 and 66 bonds can be oxygenated Both di-and trioxides of C60 are quite unstable71737 Experimen-tally six C60O2 dioxides have been prepared The mostreadily formed is 1234-C60O2338 but 121330-C60O2

has also been identified38 with possibly one other dioe-poxide identified and the remainder either diannulenes orepoxyannulenes28

Three C60O3 triepoxides have been experimentallyreported namely 1234910-C60O3 and 12341112-C60O339 and more recently 123456-C60O340 Theoret-ical calculations comparing isomer stabilities for C60On

(n = 2 3) have also been reported primarily on epoxidestructures61941 but also annulene and di-ketones41 (wehave not considered di-ketones here since Ref [41] foundthem to be significantly less stable than epoxide or annu-lene forms for C60O2) Curry et al showed that the reac-tivity of C60 with oxygen is local and 66-double bondsadjacent to an existing epoxide functionality are more eas-ily oxidized than the others6

In Table I reaction enthalpies ER and structural detailsare given for the most stable C60On structures For theisomers that have been isolated and fully characterizedin the literature the corresponding references are alsolisted in Table I although other isomers listed might havebeen synthesized2 The fully optimized geometries forthe isomers under consideration are depicted in Figure 1with numbering used throughout this work depicted inthe insert of Figure 242 where an overview of the fol-lowing results is presented Consistent with the litera-ture we find the epoxy- and annulene forms of C60Oclose in energy with slightly higher stability of the annu-lene (+92 kcalmol)619 The 12-C60O epoxide isomerhas a reaction enthalpy of minus16 kcalmol and maintainsthe underlying 66 hexagonndashhexagon bond dilating itto 1522 Aring with CndashO bond lengths of 1457 Aring andCndashOndashC bond angle of 6298 (see Fig 1(a)) For theannulene 19-C60O isomer the underlying 56 pentagonndashhexagon CndashC bond is broken with a distance of 2193 Aringgiving CndashO bond lengths of 1398 Aring and CndashOndashC bondangle of 10338 (see Fig 1(g)) This isomer has a reactionenthalpy of minus109 kcalmol

The most stable C60O2 with only 66 addition is the1234-diepoxy [60] fullerene with a reaction enthalpy ofminus70 kcalmol (see Fig 1(b)) The oxygen atoms forma pair of epoxides around the same hexagon with CndashObond lengths of 1460 Aring (atoms 1 and 4)1451 Aring (atoms 2and 3) and CndashOndashC bond angles of 6261 The underly-ing CndashC bonds are dilated to 1512 Aring with the remain-ing 66 bond of the hexagon changing from 1367 Aringin C60 to 1354 Aring in C60O2 Further oxygenation of1234-C60O2 leads to three triepoxy isomers close inenergy consistent with previous theoretical studies19 Themost stable isomer is 123456-C60O3 with a reactionenthalpy of minus77 kcalmol (see Fig 1(d)) Three epox-ide units form around the same hexagon with CndashO bond

[66] type addition

(a) (b)

(c) (d)

(e)

[56] type addition

12-C60O 1234-C60O2

121011-C60O2

1234910-C60O3

19-C60O

1967-C60O2

19212101167-C60O4

19212-C60O2

192121011-C60O3

12341112-C60O3

123456-C60O3

(f)

(g)

(i) (j)

(h)

(k)

Fig 1 Optimized structures for C60 oxygenation with 66 (andashf) and56 type addition (gndashk) Structural and energetic data given in Table I

J Nanosci Nanotechnol 9 6113ndash6119 2009 6115

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RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

Fig 2 Overview of the reaction enthalpies (kcalmol) with the Schlegeldiagrams of the corresponding structures for C60 oxygenation with label-ing according to Figure 1 Insert showing a Schlegel diagram with sitenumbering for C60 as used throughout this work

lengths 1453 Aring CndashOndashC bond angles of 6272 andunderlying CndashC bonds now dilated to 1512 Aring The nextmost stable isomers are 1234910-C60O3 (S structureFig 1(e)) and 12341112-C60O3 (T structure Fig 1(f))(13 and 20 kcalmol less stable than 123456-C60O3

respectively) These three epoxides correspond to the onescharacterized experimentally3940

However a more stable isomer can be formed from 12-C60O by oxygenation of a 56 bond to form 121011-C60O2 with a reaction enthalpy of minus182 kcalmol (seeFig 1(c)) This isomer has both a 56 and a 66bond oxygenated opposite the same hexagon and is+112 kcalmol more stable than 1234-C60O2

From further oxygenation of 19-C60O with only 56addition 19212-C60O2 is the most stable isomer pre-dicted with a reaction enthalpy of minus377 kcalmol (seeFig 1(h)) This isomer forms a diannulene structurearound the same hexagon with the CndashOndashC bond anglefurther expanded to 10712 The second most stable iso-mer is 1967-C60O2 which is only 1 kcalmol less sta-ble where the diannulene is formed around the samepentagon (see Fig 1(i)) From 19-C60O the above-mentioned 121011-C60O2 can also be formed but

with a lower reaction enthalpy of minus89 kcalmol Uponfurther oxygenation of 19212-C60O2 the most stableisomers are 192121011-C60O3 and 19212101167-C60O4 respectively with reaction enthalpies of minus554 andminus280 kcalmol (see Figs 1(j and k))

Overall we see a clear thermodynamic tendency towardsopen annulene structures resulting from the oxygenationof 56 bonds giving more stable reaction products ascompared to the closed epoxide products This can beunderstood from the CndashOndashC angle which is much smallerin the case of the epoxides as compared to the angle ineg H2O In the case of the annulenes the angle aroundthe oxygen atom approaches the one in water resulting inless strain on the structure and more stable isomers How-ever epoxide structures are typically favored on kineticgrounds since for annulene addition it is first necessaryto overcome an appreciable energy barrier associated withbreaking the underlying CndashC bond

22 Oxygenation of Pristine Metallic 55Carbon Nanotubes

We next consider bridging oxygen addition to the surfaceof pristine metallic 55 carbon nanotubes The activesites for the fully optimized geometries of the isomersunder consideration are depicted in Figure 3 (the activesite is indicated in blue on the insert) where an overviewof the following results is presented The correspondingreaction enthalpies ER and structural details are listed inTable II Unlike fullerenes only hexagonndashhexagon bondsare present however the curvature of the tube breaks sys-tem symmetry resulting in bonds with different length andsingledouble character In case of a 55 armchair carbonnanotube only two different bond types exist orientatedeither perpendicular (circumferential bond with length of1432 Aring) or along (axial with a bond length of 1396 Aring)the nanotube axis For our nanotube test system C240H20

(see insert Fig 3) oxygenation is endothermic for axialaddition (reaction enthalpy of +191 kcalmol NT-O1 ax)whereas circumferential addition has a reaction enthalpy ofminus222 kcalmol (NT-O1 circ) This big difference stems fromthe difference in addition type namely the axial additionresults in epoxide formation whereas the circumferentialaddition forms an annulene the structures consistent withprevious literature results28ndash30 Since the epoxide addition ispredicted to be endothermic the annulene form is the onlyisolated oxygen form expected to be present on a pristinenanotube surface upon oxygenation provided the systemcan overcome the energy barrier associated with opening upthe underlying CndashC bond much as annulene oxygen addi-tion to fullerenes is kinetically inhibited Assuming annu-lene addition has a high reaction barrier like for fullerenesthis can explain the relatively high resistance of pristinenanotube surfaces to oxygenation

Addition of a second oxygen to the unstable epoxide iso-mer can be exothermic for circumferential addition of an

6116 J Nanosci Nanotechnol 9 6113ndash6119 2009

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RESEARCHARTICLE

Van Lier et al Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes

Fig 3 Overview of the reaction enthalpies (kcalmol) with the corresponding active site of the optimized structures for carbon nanotube oxygenationInsert showing the 55 armchair C240H20 carbon nanotube test system with the positions of the atoms of the active site indicated in blue and theposition of the vacancy indicated in green Structural and energetic data given in Table II

annulene with a reaction enthalpy of minus246 kcalmol (NT-O1 ax circ) whereas addition of a second epoxide is againendothermic with a reaction enthalpy of +284 kcalmol(NT-O2 ax) We note that when the NT-O1 ax circ sys-tem is formed from the annulene structure NT-O1 circa reaction enthalpy of +167 kcalmol is found ieepoxide addition next to pre-existing annulene oxy-gen is approximately as unfavorable as to the pristinenanotube surface Thus these results confirm there is

Table II Calculated most stable isomers and corresponding reaction enthalpy ER for the addition indicated in bold and structural data for oxygenationof a pristine nanotube and a nanotube with an oxygenated vacancy (VO2-NT)

Isomer Addition type ER CndashO bond length (Aring) Underlying CndashC distance (Aring) CndashOndashC bond angle

NT-O1 ax Epoxide +191 1482 1494 6051NT-O1 circ Annulene minus222 1403 2184 10224

NT-O2 ax Diepoxide +284 1481 1490 6044NT-O2 ax-circ Epoxideannulene minus246 14791403 14952185 605710218NT-O2 circ Diannulene minus380 1409 2252 10612

VO2-NT Annulene+ketone minus2063lowast 13751212 2440na 12564naVO2-NTO1 circ Annulene minus499 1403 2263 10736VO2-NTO1 circprime Annulene minus218 1403 2199 10329VO2-NTO1 circprimeprime Epoxide +12 1491 1502 6174

lowastas compared to V-NT

no thermodynamic driving force for epoxide formationeven next to pre-existing annulene oxygenated additionsites

Addition of a second oxygen to the annulene isomervia formation of a second annulene on the same hexagongives the most stable isomer with a reaction enthalpy ofminus380 kcalmol (NT-O2 circ) Thus further annulene forma-tion in the presence of pre-existing annulene oxygen isthermodynamically favored

J Nanosci Nanotechnol 9 6113ndash6119 2009 6117

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RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

We note that the reaction enthalpies can be influencedby the nanotube diameter as previously predicted for oxy-gen addition to pristine 60 carbon nanotubes43 In gen-eral the reaction enthalpies appear lower as compared tofullerenes

23 Oxygenation of Defective Metallic 55Carbon Nanotubes

In reality the majority of nanotube surfaces are defectiveand these defects are known to be much more reactiveTherefore we will next consider oxygen addition around asingle vacancy The reaction enthalpies ER and structuraldetails are again listed in Table II and an overview of thefollowing results is presented in Figure 3 together with theactive sites for the fully optimized geometries (the activesite is indicated in blue on the insert with the position ofthe vacancy indicated in green)

In the case of a single vacancy (V-NT) it has beendemonstrated that an O2 oxygen molecule will dissoci-ate upon addition giving rise to an ether and a ketonefunctional group (VO2-NT)23 This type of functionalisa-tion will also occur when nanotubes are oxygenated usingRF-plasma functionalisation23 The reaction enthalpy foraddition of two oxygen atoms to a vacancy is predictedto be minus2063 kcalmol In the case of further oxygena-tion of this type of oxygenated defects our results pre-dict that functionalisation will preferentially occur in thevicinity of the defect The most stable structure forms anannulene on the ring next to the vacancy with a reac-tion enthalpy of minus499 kcal mol (VO2-NT-O1 circ) Thenext most stable isomer also forms an annulene but onehexagon away from the vacancy with a reaction enthalpyof minus218 kcalmol (VO2-NTO1 circprime ) which is comparableto circumferential addition to a pristine sidewall Thus thispredicts that the increased tube reactivity to oxygenationin the vicinity of a vacancy is a very local effect The moststable epoxide isomer also on a circumferential bondis formed next to the ether function and is once againendothermic with a reaction enthalpy of +12 kcalmol(VO2-NTO1 circprimeprime ) This is sufficiently close to zero as tolie within our error bounds and it is possible that epoxideoxygen addition in this site may be very weakly stablepossibly with a short lifetime particularly if oxygen addi-tion is coming from less stable sources such as ozone If anoxygen treatment method is used which traditionally pro-duces only epoxide surface-bonded oxygen (such as roomtemperature reaction with ozone in toluene) such epoxidegroups may therefore form in the vicinity of pre-existingsurface defects while the rest of the nanotube sidewallwill remain oxygen free However once annulene addi-tion is possible with its higher reaction barriers this willoccur everywhere notably preferentially near to defectsIncreased reactivity towards oxygenation around defectsalso suggests that they play an important role in eg

cutting experiments where extensive oxygenation resultsin opening and cutting of the nanotubes22 Co-operativeannulene addition which serves to open circumferentialbonds in the vicinity of other oxygenated bonds whileleaving axial bonds un-reacted is consistent with such amechanism

3 GENERAL CONCLUSIONS

Oxygenation of C60 has been analyzed and a number ofC60O2 and C60O3 stable isomers have been identified andare in agreement with experimental assignments whereavailable Our results indicate that for fullerene additionoxygenation preferentially occurs at the 56 bonds lead-ing to open annulene structures These open structureshave larger CndashOndashC angles giving less strain on the struc-ture as compared to the closed epoxide functions resultingin more stable isomers The same is observed for car-bon nanotube oxygenation where the annulene isomers aremuch lower in energy and indeed epoxide-type oxygena-tion is endothermic both on pristine nanotube sidewalls asin the vicinity of defects

We note that the reaction enthalpies quoted aboveare based on oxygen molecules in their ground statetriplet state (the energy difference with the singlet stateis minus257 kcalmol at our level of theory so reactionenthalpies are shifted down by the same amount for addi-tion of singlet oxygen) If the oxygen is in a singlet statethen epoxide addition becomes mildly exothermic on thepristine tube and strongly exothermic next to defects Thissuggests that UV excited oxygen should show qualitativelydifferent oxygenation behavior on carbon nanotubes

In general higher reaction enthalpies are found forfullerenes as compared to carbon nanotubes (except for thefunctionalisation of the under-coordinated carbon atoms ofthe vacancy) Thus oxygenation will occur preferentiallyat fullerene-like nanotube tips before sidewall functional-isation This also suggests that fullerenes could be usedto lsquotraprsquo oxygen atoms more readily as compared to thepristine carbon nanotube sidewall

Acknowledgments Gregory Van Lier acknowledgesthe Research FoundationndashFlanders (FWO) for financialsupport as a Postdoctoral Research Fellow ChristopherP Ewels and Irene Suarez-Martinez acknowledge theEU-STREP project 003311 ldquonano2hybridsrdquo for funding

References and Notes

1 R Taylor Proc Electrochem Soc 97 281 (1997)2 D Heymann and R B Weisman C R Chimie 9 1107 (2006)3 A L Balch C A Costa B C Noll and M M Olmstread J Am

Chem Soc 117 8926 (1995)4 M P Barrow N J Tower R Taylor and T Drewello Chem Phys

Lett 293 302 (1998)5 S Lebedkin S Ballenweg J Gross R Taylor and W Kraumltschmer

Tetrahedron Lett 36 4971 (1995)

6118 J Nanosci Nanotechnol 9 6113ndash6119 2009

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IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

apparent superficial structural similarities between nano-tubes and fullerenes27

Some theoretical investigations of carbon nanotube sin-gle oxygen addition have already been performed forpristine side-wall oxygenation It was predicted that forboth zig-zag and armchair carbon nanotubes a preferenceexists towards annulene addition at circumferential bondswhereas epoxides form preferentially at axial bonds28ndash30

A recent theoretical study considered capped ultra-narrownanotubes (C80 and C84) but unsurprisingly obtained pref-erential bonding to the fullerene caps31

Here we perform a comparative study where systematicoxygen atom addition to the surface of carbon nanomate-rials is explored through the use of ab initio theoreticalmodeling In particular early stage oxygen addition to theC60 fullerene is analyzed and compared with addition to a55 carbon nanotube both to the pristine side wall andto a single vacancy

2 METHOD

Since the number of possible isomers to consider isextremely large we use an automated process to gener-ate optimize and characterize the isomers of a given car-bon system XOn where X is either the fullerene C60 ora section of a carbon nanotube using a recently devel-oped code SACHA3233 In this case we have analyzed apristine 55 carbon nanotube section C240H20 as wellas the same nanotube containing a vacancy-O2 defectC239H20O2 In brief given a starting structure we add anadditional oxygen atom at all possible candidate bondingsites (ie above all carbonndashcarbon bonds) one at a timeEach of these possible structures is first geometrically opti-mized using the semi-empirical AM1 level of theory withGaussian0334 and the resultant system energies tabulatedSecond stage full geometry optimizations are performed at

Table I Calculated most stable isomers and corresponding reaction enthalpy ER for addition to the carbon atoms numbered in bold and the structuraldata for the C60On under consideration References are indicated in the final column if the system has been experimentally isolated and characterized

CndashO bond length Underlying CndashC distance

Isomer ER (Aring) (Aring) CndashOndashC bond angle Characterized experimentally

66 type addition12-C60O minus16 1457 1522 6298 [7 17]

1234-C60O2 minus70 14601451 1512 6261 [3]121011-C60O2 minus182lowast 13951454 22141558 105076483

123456-C60O3 minus77 1453 1512 6272 [40]1234910-C60O3 minus64 14581451 15131501 62666218 [39]12341112-C60O3 minus57 14451461 15071498 58416232 [39]

56 type addition19-C60O minus109 1398 2193 10338 [18]

19212-C60O2 minus377 13981400 2251 10712 [8]1967-C60O2 minus367 14011405 2280 10867

192121011-C60O3 minus554 1403 2262 1074019212101167-C60O4 minus280 1400 2259 10782

lowastER =minus89 kcalmol for when formed from 19-C60O

the HF3-21G level of theory for all isomers within 05 eVof the most stable at AM1 level The most stable struc-ture at HF3-21G is then taken as our starting point forthe addition of the following oxygen atom In this waywe can systematically increase the quantity of oxygen inthe system reflecting the thermodynamic addition route ofoxygenation We only considered structures with oxygenin CndashC bond bridging sites (epoxide or annulene) ie weare not concerned here with structures such as ozonidesndashOndashOndashOndash 2 or molecular oxygen doping of fullerites35

The automated approach using the SACHA code pro-vides great computational saving compared to lsquobrute forcersquoinvestigation of all possible isomers rendering such stud-ies of nanotube oxygenation feasible Further discussionof the approach is given in Refs [32 33] and has alreadybeen applied for the analysis of fullerene fluorination36

Reaction enthalpies are approximated as the energy differ-ences [ER = E(C60OnminusE(C60Onminus1minus 12E(O2)] con-sidering the triplet state of the O2 molecule thus providinga measure for the exothermicity of oxygenation

In the current study we have restricted ourselves to earlystage oxygenation ie up to a few oxygen atoms sincethe majority of experimental characterization studies offullerene oxides are restricted to this range We note thatgiven sufficient computing time there is no reason in prin-ciple that such an approach could not consider higher orderoxygenation

21 Oxygenation of Buckminsterfullerene C60

We first consider oxygenation of C60 and compare withexisting literature data Two isomers exist for C60Onamely to the bond between two hexagons (66 typeaddition) and between a pentagon and a hexagon (56type addition) When only 66 type addition is con-sidered there are only eight possible regioisomers of

6114 J Nanosci Nanotechnol 9 6113ndash6119 2009

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Van Lier et al Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes

C60O2 but already 46 regioisomers exist when both56 and 66 bonds can be oxygenated Both di-and trioxides of C60 are quite unstable71737 Experimen-tally six C60O2 dioxides have been prepared The mostreadily formed is 1234-C60O2338 but 121330-C60O2

has also been identified38 with possibly one other dioe-poxide identified and the remainder either diannulenes orepoxyannulenes28

Three C60O3 triepoxides have been experimentallyreported namely 1234910-C60O3 and 12341112-C60O339 and more recently 123456-C60O340 Theoret-ical calculations comparing isomer stabilities for C60On

(n = 2 3) have also been reported primarily on epoxidestructures61941 but also annulene and di-ketones41 (wehave not considered di-ketones here since Ref [41] foundthem to be significantly less stable than epoxide or annu-lene forms for C60O2) Curry et al showed that the reac-tivity of C60 with oxygen is local and 66-double bondsadjacent to an existing epoxide functionality are more eas-ily oxidized than the others6

In Table I reaction enthalpies ER and structural detailsare given for the most stable C60On structures For theisomers that have been isolated and fully characterizedin the literature the corresponding references are alsolisted in Table I although other isomers listed might havebeen synthesized2 The fully optimized geometries forthe isomers under consideration are depicted in Figure 1with numbering used throughout this work depicted inthe insert of Figure 242 where an overview of the fol-lowing results is presented Consistent with the litera-ture we find the epoxy- and annulene forms of C60Oclose in energy with slightly higher stability of the annu-lene (+92 kcalmol)619 The 12-C60O epoxide isomerhas a reaction enthalpy of minus16 kcalmol and maintainsthe underlying 66 hexagonndashhexagon bond dilating itto 1522 Aring with CndashO bond lengths of 1457 Aring andCndashOndashC bond angle of 6298 (see Fig 1(a)) For theannulene 19-C60O isomer the underlying 56 pentagonndashhexagon CndashC bond is broken with a distance of 2193 Aringgiving CndashO bond lengths of 1398 Aring and CndashOndashC bondangle of 10338 (see Fig 1(g)) This isomer has a reactionenthalpy of minus109 kcalmol

The most stable C60O2 with only 66 addition is the1234-diepoxy [60] fullerene with a reaction enthalpy ofminus70 kcalmol (see Fig 1(b)) The oxygen atoms forma pair of epoxides around the same hexagon with CndashObond lengths of 1460 Aring (atoms 1 and 4)1451 Aring (atoms 2and 3) and CndashOndashC bond angles of 6261 The underly-ing CndashC bonds are dilated to 1512 Aring with the remain-ing 66 bond of the hexagon changing from 1367 Aringin C60 to 1354 Aring in C60O2 Further oxygenation of1234-C60O2 leads to three triepoxy isomers close inenergy consistent with previous theoretical studies19 Themost stable isomer is 123456-C60O3 with a reactionenthalpy of minus77 kcalmol (see Fig 1(d)) Three epox-ide units form around the same hexagon with CndashO bond

[66] type addition

(a) (b)

(c) (d)

(e)

[56] type addition

12-C60O 1234-C60O2

121011-C60O2

1234910-C60O3

19-C60O

1967-C60O2

19212101167-C60O4

19212-C60O2

192121011-C60O3

12341112-C60O3

123456-C60O3

(f)

(g)

(i) (j)

(h)

(k)

Fig 1 Optimized structures for C60 oxygenation with 66 (andashf) and56 type addition (gndashk) Structural and energetic data given in Table I

J Nanosci Nanotechnol 9 6113ndash6119 2009 6115

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RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

Fig 2 Overview of the reaction enthalpies (kcalmol) with the Schlegeldiagrams of the corresponding structures for C60 oxygenation with label-ing according to Figure 1 Insert showing a Schlegel diagram with sitenumbering for C60 as used throughout this work

lengths 1453 Aring CndashOndashC bond angles of 6272 andunderlying CndashC bonds now dilated to 1512 Aring The nextmost stable isomers are 1234910-C60O3 (S structureFig 1(e)) and 12341112-C60O3 (T structure Fig 1(f))(13 and 20 kcalmol less stable than 123456-C60O3

respectively) These three epoxides correspond to the onescharacterized experimentally3940

However a more stable isomer can be formed from 12-C60O by oxygenation of a 56 bond to form 121011-C60O2 with a reaction enthalpy of minus182 kcalmol (seeFig 1(c)) This isomer has both a 56 and a 66bond oxygenated opposite the same hexagon and is+112 kcalmol more stable than 1234-C60O2

From further oxygenation of 19-C60O with only 56addition 19212-C60O2 is the most stable isomer pre-dicted with a reaction enthalpy of minus377 kcalmol (seeFig 1(h)) This isomer forms a diannulene structurearound the same hexagon with the CndashOndashC bond anglefurther expanded to 10712 The second most stable iso-mer is 1967-C60O2 which is only 1 kcalmol less sta-ble where the diannulene is formed around the samepentagon (see Fig 1(i)) From 19-C60O the above-mentioned 121011-C60O2 can also be formed but

with a lower reaction enthalpy of minus89 kcalmol Uponfurther oxygenation of 19212-C60O2 the most stableisomers are 192121011-C60O3 and 19212101167-C60O4 respectively with reaction enthalpies of minus554 andminus280 kcalmol (see Figs 1(j and k))

Overall we see a clear thermodynamic tendency towardsopen annulene structures resulting from the oxygenationof 56 bonds giving more stable reaction products ascompared to the closed epoxide products This can beunderstood from the CndashOndashC angle which is much smallerin the case of the epoxides as compared to the angle ineg H2O In the case of the annulenes the angle aroundthe oxygen atom approaches the one in water resulting inless strain on the structure and more stable isomers How-ever epoxide structures are typically favored on kineticgrounds since for annulene addition it is first necessaryto overcome an appreciable energy barrier associated withbreaking the underlying CndashC bond

22 Oxygenation of Pristine Metallic 55Carbon Nanotubes

We next consider bridging oxygen addition to the surfaceof pristine metallic 55 carbon nanotubes The activesites for the fully optimized geometries of the isomersunder consideration are depicted in Figure 3 (the activesite is indicated in blue on the insert) where an overviewof the following results is presented The correspondingreaction enthalpies ER and structural details are listed inTable II Unlike fullerenes only hexagonndashhexagon bondsare present however the curvature of the tube breaks sys-tem symmetry resulting in bonds with different length andsingledouble character In case of a 55 armchair carbonnanotube only two different bond types exist orientatedeither perpendicular (circumferential bond with length of1432 Aring) or along (axial with a bond length of 1396 Aring)the nanotube axis For our nanotube test system C240H20

(see insert Fig 3) oxygenation is endothermic for axialaddition (reaction enthalpy of +191 kcalmol NT-O1 ax)whereas circumferential addition has a reaction enthalpy ofminus222 kcalmol (NT-O1 circ) This big difference stems fromthe difference in addition type namely the axial additionresults in epoxide formation whereas the circumferentialaddition forms an annulene the structures consistent withprevious literature results28ndash30 Since the epoxide addition ispredicted to be endothermic the annulene form is the onlyisolated oxygen form expected to be present on a pristinenanotube surface upon oxygenation provided the systemcan overcome the energy barrier associated with opening upthe underlying CndashC bond much as annulene oxygen addi-tion to fullerenes is kinetically inhibited Assuming annu-lene addition has a high reaction barrier like for fullerenesthis can explain the relatively high resistance of pristinenanotube surfaces to oxygenation

Addition of a second oxygen to the unstable epoxide iso-mer can be exothermic for circumferential addition of an

6116 J Nanosci Nanotechnol 9 6113ndash6119 2009

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Van Lier et al Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes

Fig 3 Overview of the reaction enthalpies (kcalmol) with the corresponding active site of the optimized structures for carbon nanotube oxygenationInsert showing the 55 armchair C240H20 carbon nanotube test system with the positions of the atoms of the active site indicated in blue and theposition of the vacancy indicated in green Structural and energetic data given in Table II

annulene with a reaction enthalpy of minus246 kcalmol (NT-O1 ax circ) whereas addition of a second epoxide is againendothermic with a reaction enthalpy of +284 kcalmol(NT-O2 ax) We note that when the NT-O1 ax circ sys-tem is formed from the annulene structure NT-O1 circa reaction enthalpy of +167 kcalmol is found ieepoxide addition next to pre-existing annulene oxy-gen is approximately as unfavorable as to the pristinenanotube surface Thus these results confirm there is

Table II Calculated most stable isomers and corresponding reaction enthalpy ER for the addition indicated in bold and structural data for oxygenationof a pristine nanotube and a nanotube with an oxygenated vacancy (VO2-NT)

Isomer Addition type ER CndashO bond length (Aring) Underlying CndashC distance (Aring) CndashOndashC bond angle

NT-O1 ax Epoxide +191 1482 1494 6051NT-O1 circ Annulene minus222 1403 2184 10224

NT-O2 ax Diepoxide +284 1481 1490 6044NT-O2 ax-circ Epoxideannulene minus246 14791403 14952185 605710218NT-O2 circ Diannulene minus380 1409 2252 10612

VO2-NT Annulene+ketone minus2063lowast 13751212 2440na 12564naVO2-NTO1 circ Annulene minus499 1403 2263 10736VO2-NTO1 circprime Annulene minus218 1403 2199 10329VO2-NTO1 circprimeprime Epoxide +12 1491 1502 6174

lowastas compared to V-NT

no thermodynamic driving force for epoxide formationeven next to pre-existing annulene oxygenated additionsites

Addition of a second oxygen to the annulene isomervia formation of a second annulene on the same hexagongives the most stable isomer with a reaction enthalpy ofminus380 kcalmol (NT-O2 circ) Thus further annulene forma-tion in the presence of pre-existing annulene oxygen isthermodynamically favored

J Nanosci Nanotechnol 9 6113ndash6119 2009 6117

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RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

We note that the reaction enthalpies can be influencedby the nanotube diameter as previously predicted for oxy-gen addition to pristine 60 carbon nanotubes43 In gen-eral the reaction enthalpies appear lower as compared tofullerenes

23 Oxygenation of Defective Metallic 55Carbon Nanotubes

In reality the majority of nanotube surfaces are defectiveand these defects are known to be much more reactiveTherefore we will next consider oxygen addition around asingle vacancy The reaction enthalpies ER and structuraldetails are again listed in Table II and an overview of thefollowing results is presented in Figure 3 together with theactive sites for the fully optimized geometries (the activesite is indicated in blue on the insert with the position ofthe vacancy indicated in green)

In the case of a single vacancy (V-NT) it has beendemonstrated that an O2 oxygen molecule will dissoci-ate upon addition giving rise to an ether and a ketonefunctional group (VO2-NT)23 This type of functionalisa-tion will also occur when nanotubes are oxygenated usingRF-plasma functionalisation23 The reaction enthalpy foraddition of two oxygen atoms to a vacancy is predictedto be minus2063 kcalmol In the case of further oxygena-tion of this type of oxygenated defects our results pre-dict that functionalisation will preferentially occur in thevicinity of the defect The most stable structure forms anannulene on the ring next to the vacancy with a reac-tion enthalpy of minus499 kcal mol (VO2-NT-O1 circ) Thenext most stable isomer also forms an annulene but onehexagon away from the vacancy with a reaction enthalpyof minus218 kcalmol (VO2-NTO1 circprime ) which is comparableto circumferential addition to a pristine sidewall Thus thispredicts that the increased tube reactivity to oxygenationin the vicinity of a vacancy is a very local effect The moststable epoxide isomer also on a circumferential bondis formed next to the ether function and is once againendothermic with a reaction enthalpy of +12 kcalmol(VO2-NTO1 circprimeprime ) This is sufficiently close to zero as tolie within our error bounds and it is possible that epoxideoxygen addition in this site may be very weakly stablepossibly with a short lifetime particularly if oxygen addi-tion is coming from less stable sources such as ozone If anoxygen treatment method is used which traditionally pro-duces only epoxide surface-bonded oxygen (such as roomtemperature reaction with ozone in toluene) such epoxidegroups may therefore form in the vicinity of pre-existingsurface defects while the rest of the nanotube sidewallwill remain oxygen free However once annulene addi-tion is possible with its higher reaction barriers this willoccur everywhere notably preferentially near to defectsIncreased reactivity towards oxygenation around defectsalso suggests that they play an important role in eg

cutting experiments where extensive oxygenation resultsin opening and cutting of the nanotubes22 Co-operativeannulene addition which serves to open circumferentialbonds in the vicinity of other oxygenated bonds whileleaving axial bonds un-reacted is consistent with such amechanism

3 GENERAL CONCLUSIONS

Oxygenation of C60 has been analyzed and a number ofC60O2 and C60O3 stable isomers have been identified andare in agreement with experimental assignments whereavailable Our results indicate that for fullerene additionoxygenation preferentially occurs at the 56 bonds lead-ing to open annulene structures These open structureshave larger CndashOndashC angles giving less strain on the struc-ture as compared to the closed epoxide functions resultingin more stable isomers The same is observed for car-bon nanotube oxygenation where the annulene isomers aremuch lower in energy and indeed epoxide-type oxygena-tion is endothermic both on pristine nanotube sidewalls asin the vicinity of defects

We note that the reaction enthalpies quoted aboveare based on oxygen molecules in their ground statetriplet state (the energy difference with the singlet stateis minus257 kcalmol at our level of theory so reactionenthalpies are shifted down by the same amount for addi-tion of singlet oxygen) If the oxygen is in a singlet statethen epoxide addition becomes mildly exothermic on thepristine tube and strongly exothermic next to defects Thissuggests that UV excited oxygen should show qualitativelydifferent oxygenation behavior on carbon nanotubes

In general higher reaction enthalpies are found forfullerenes as compared to carbon nanotubes (except for thefunctionalisation of the under-coordinated carbon atoms ofthe vacancy) Thus oxygenation will occur preferentiallyat fullerene-like nanotube tips before sidewall functional-isation This also suggests that fullerenes could be usedto lsquotraprsquo oxygen atoms more readily as compared to thepristine carbon nanotube sidewall

Acknowledgments Gregory Van Lier acknowledgesthe Research FoundationndashFlanders (FWO) for financialsupport as a Postdoctoral Research Fellow ChristopherP Ewels and Irene Suarez-Martinez acknowledge theEU-STREP project 003311 ldquonano2hybridsrdquo for funding

References and Notes

1 R Taylor Proc Electrochem Soc 97 281 (1997)2 D Heymann and R B Weisman C R Chimie 9 1107 (2006)3 A L Balch C A Costa B C Noll and M M Olmstread J Am

Chem Soc 117 8926 (1995)4 M P Barrow N J Tower R Taylor and T Drewello Chem Phys

Lett 293 302 (1998)5 S Lebedkin S Ballenweg J Gross R Taylor and W Kraumltschmer

Tetrahedron Lett 36 4971 (1995)

6118 J Nanosci Nanotechnol 9 6113ndash6119 2009

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RESEARCHARTICLE

Van Lier et al Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes

C60O2 but already 46 regioisomers exist when both56 and 66 bonds can be oxygenated Both di-and trioxides of C60 are quite unstable71737 Experimen-tally six C60O2 dioxides have been prepared The mostreadily formed is 1234-C60O2338 but 121330-C60O2

has also been identified38 with possibly one other dioe-poxide identified and the remainder either diannulenes orepoxyannulenes28

Three C60O3 triepoxides have been experimentallyreported namely 1234910-C60O3 and 12341112-C60O339 and more recently 123456-C60O340 Theoret-ical calculations comparing isomer stabilities for C60On

(n = 2 3) have also been reported primarily on epoxidestructures61941 but also annulene and di-ketones41 (wehave not considered di-ketones here since Ref [41] foundthem to be significantly less stable than epoxide or annu-lene forms for C60O2) Curry et al showed that the reac-tivity of C60 with oxygen is local and 66-double bondsadjacent to an existing epoxide functionality are more eas-ily oxidized than the others6

In Table I reaction enthalpies ER and structural detailsare given for the most stable C60On structures For theisomers that have been isolated and fully characterizedin the literature the corresponding references are alsolisted in Table I although other isomers listed might havebeen synthesized2 The fully optimized geometries forthe isomers under consideration are depicted in Figure 1with numbering used throughout this work depicted inthe insert of Figure 242 where an overview of the fol-lowing results is presented Consistent with the litera-ture we find the epoxy- and annulene forms of C60Oclose in energy with slightly higher stability of the annu-lene (+92 kcalmol)619 The 12-C60O epoxide isomerhas a reaction enthalpy of minus16 kcalmol and maintainsthe underlying 66 hexagonndashhexagon bond dilating itto 1522 Aring with CndashO bond lengths of 1457 Aring andCndashOndashC bond angle of 6298 (see Fig 1(a)) For theannulene 19-C60O isomer the underlying 56 pentagonndashhexagon CndashC bond is broken with a distance of 2193 Aringgiving CndashO bond lengths of 1398 Aring and CndashOndashC bondangle of 10338 (see Fig 1(g)) This isomer has a reactionenthalpy of minus109 kcalmol

The most stable C60O2 with only 66 addition is the1234-diepoxy [60] fullerene with a reaction enthalpy ofminus70 kcalmol (see Fig 1(b)) The oxygen atoms forma pair of epoxides around the same hexagon with CndashObond lengths of 1460 Aring (atoms 1 and 4)1451 Aring (atoms 2and 3) and CndashOndashC bond angles of 6261 The underly-ing CndashC bonds are dilated to 1512 Aring with the remain-ing 66 bond of the hexagon changing from 1367 Aringin C60 to 1354 Aring in C60O2 Further oxygenation of1234-C60O2 leads to three triepoxy isomers close inenergy consistent with previous theoretical studies19 Themost stable isomer is 123456-C60O3 with a reactionenthalpy of minus77 kcalmol (see Fig 1(d)) Three epox-ide units form around the same hexagon with CndashO bond

[66] type addition

(a) (b)

(c) (d)

(e)

[56] type addition

12-C60O 1234-C60O2

121011-C60O2

1234910-C60O3

19-C60O

1967-C60O2

19212101167-C60O4

19212-C60O2

192121011-C60O3

12341112-C60O3

123456-C60O3

(f)

(g)

(i) (j)

(h)

(k)

Fig 1 Optimized structures for C60 oxygenation with 66 (andashf) and56 type addition (gndashk) Structural and energetic data given in Table I

J Nanosci Nanotechnol 9 6113ndash6119 2009 6115

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IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

Fig 2 Overview of the reaction enthalpies (kcalmol) with the Schlegeldiagrams of the corresponding structures for C60 oxygenation with label-ing according to Figure 1 Insert showing a Schlegel diagram with sitenumbering for C60 as used throughout this work

lengths 1453 Aring CndashOndashC bond angles of 6272 andunderlying CndashC bonds now dilated to 1512 Aring The nextmost stable isomers are 1234910-C60O3 (S structureFig 1(e)) and 12341112-C60O3 (T structure Fig 1(f))(13 and 20 kcalmol less stable than 123456-C60O3

respectively) These three epoxides correspond to the onescharacterized experimentally3940

However a more stable isomer can be formed from 12-C60O by oxygenation of a 56 bond to form 121011-C60O2 with a reaction enthalpy of minus182 kcalmol (seeFig 1(c)) This isomer has both a 56 and a 66bond oxygenated opposite the same hexagon and is+112 kcalmol more stable than 1234-C60O2

From further oxygenation of 19-C60O with only 56addition 19212-C60O2 is the most stable isomer pre-dicted with a reaction enthalpy of minus377 kcalmol (seeFig 1(h)) This isomer forms a diannulene structurearound the same hexagon with the CndashOndashC bond anglefurther expanded to 10712 The second most stable iso-mer is 1967-C60O2 which is only 1 kcalmol less sta-ble where the diannulene is formed around the samepentagon (see Fig 1(i)) From 19-C60O the above-mentioned 121011-C60O2 can also be formed but

with a lower reaction enthalpy of minus89 kcalmol Uponfurther oxygenation of 19212-C60O2 the most stableisomers are 192121011-C60O3 and 19212101167-C60O4 respectively with reaction enthalpies of minus554 andminus280 kcalmol (see Figs 1(j and k))

Overall we see a clear thermodynamic tendency towardsopen annulene structures resulting from the oxygenationof 56 bonds giving more stable reaction products ascompared to the closed epoxide products This can beunderstood from the CndashOndashC angle which is much smallerin the case of the epoxides as compared to the angle ineg H2O In the case of the annulenes the angle aroundthe oxygen atom approaches the one in water resulting inless strain on the structure and more stable isomers How-ever epoxide structures are typically favored on kineticgrounds since for annulene addition it is first necessaryto overcome an appreciable energy barrier associated withbreaking the underlying CndashC bond

22 Oxygenation of Pristine Metallic 55Carbon Nanotubes

We next consider bridging oxygen addition to the surfaceof pristine metallic 55 carbon nanotubes The activesites for the fully optimized geometries of the isomersunder consideration are depicted in Figure 3 (the activesite is indicated in blue on the insert) where an overviewof the following results is presented The correspondingreaction enthalpies ER and structural details are listed inTable II Unlike fullerenes only hexagonndashhexagon bondsare present however the curvature of the tube breaks sys-tem symmetry resulting in bonds with different length andsingledouble character In case of a 55 armchair carbonnanotube only two different bond types exist orientatedeither perpendicular (circumferential bond with length of1432 Aring) or along (axial with a bond length of 1396 Aring)the nanotube axis For our nanotube test system C240H20

(see insert Fig 3) oxygenation is endothermic for axialaddition (reaction enthalpy of +191 kcalmol NT-O1 ax)whereas circumferential addition has a reaction enthalpy ofminus222 kcalmol (NT-O1 circ) This big difference stems fromthe difference in addition type namely the axial additionresults in epoxide formation whereas the circumferentialaddition forms an annulene the structures consistent withprevious literature results28ndash30 Since the epoxide addition ispredicted to be endothermic the annulene form is the onlyisolated oxygen form expected to be present on a pristinenanotube surface upon oxygenation provided the systemcan overcome the energy barrier associated with opening upthe underlying CndashC bond much as annulene oxygen addi-tion to fullerenes is kinetically inhibited Assuming annu-lene addition has a high reaction barrier like for fullerenesthis can explain the relatively high resistance of pristinenanotube surfaces to oxygenation

Addition of a second oxygen to the unstable epoxide iso-mer can be exothermic for circumferential addition of an

6116 J Nanosci Nanotechnol 9 6113ndash6119 2009

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Van Lier et al Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes

Fig 3 Overview of the reaction enthalpies (kcalmol) with the corresponding active site of the optimized structures for carbon nanotube oxygenationInsert showing the 55 armchair C240H20 carbon nanotube test system with the positions of the atoms of the active site indicated in blue and theposition of the vacancy indicated in green Structural and energetic data given in Table II

annulene with a reaction enthalpy of minus246 kcalmol (NT-O1 ax circ) whereas addition of a second epoxide is againendothermic with a reaction enthalpy of +284 kcalmol(NT-O2 ax) We note that when the NT-O1 ax circ sys-tem is formed from the annulene structure NT-O1 circa reaction enthalpy of +167 kcalmol is found ieepoxide addition next to pre-existing annulene oxy-gen is approximately as unfavorable as to the pristinenanotube surface Thus these results confirm there is

Table II Calculated most stable isomers and corresponding reaction enthalpy ER for the addition indicated in bold and structural data for oxygenationof a pristine nanotube and a nanotube with an oxygenated vacancy (VO2-NT)

Isomer Addition type ER CndashO bond length (Aring) Underlying CndashC distance (Aring) CndashOndashC bond angle

NT-O1 ax Epoxide +191 1482 1494 6051NT-O1 circ Annulene minus222 1403 2184 10224

NT-O2 ax Diepoxide +284 1481 1490 6044NT-O2 ax-circ Epoxideannulene minus246 14791403 14952185 605710218NT-O2 circ Diannulene minus380 1409 2252 10612

VO2-NT Annulene+ketone minus2063lowast 13751212 2440na 12564naVO2-NTO1 circ Annulene minus499 1403 2263 10736VO2-NTO1 circprime Annulene minus218 1403 2199 10329VO2-NTO1 circprimeprime Epoxide +12 1491 1502 6174

lowastas compared to V-NT

no thermodynamic driving force for epoxide formationeven next to pre-existing annulene oxygenated additionsites

Addition of a second oxygen to the annulene isomervia formation of a second annulene on the same hexagongives the most stable isomer with a reaction enthalpy ofminus380 kcalmol (NT-O2 circ) Thus further annulene forma-tion in the presence of pre-existing annulene oxygen isthermodynamically favored

J Nanosci Nanotechnol 9 6113ndash6119 2009 6117

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

We note that the reaction enthalpies can be influencedby the nanotube diameter as previously predicted for oxy-gen addition to pristine 60 carbon nanotubes43 In gen-eral the reaction enthalpies appear lower as compared tofullerenes

23 Oxygenation of Defective Metallic 55Carbon Nanotubes

In reality the majority of nanotube surfaces are defectiveand these defects are known to be much more reactiveTherefore we will next consider oxygen addition around asingle vacancy The reaction enthalpies ER and structuraldetails are again listed in Table II and an overview of thefollowing results is presented in Figure 3 together with theactive sites for the fully optimized geometries (the activesite is indicated in blue on the insert with the position ofthe vacancy indicated in green)

In the case of a single vacancy (V-NT) it has beendemonstrated that an O2 oxygen molecule will dissoci-ate upon addition giving rise to an ether and a ketonefunctional group (VO2-NT)23 This type of functionalisa-tion will also occur when nanotubes are oxygenated usingRF-plasma functionalisation23 The reaction enthalpy foraddition of two oxygen atoms to a vacancy is predictedto be minus2063 kcalmol In the case of further oxygena-tion of this type of oxygenated defects our results pre-dict that functionalisation will preferentially occur in thevicinity of the defect The most stable structure forms anannulene on the ring next to the vacancy with a reac-tion enthalpy of minus499 kcal mol (VO2-NT-O1 circ) Thenext most stable isomer also forms an annulene but onehexagon away from the vacancy with a reaction enthalpyof minus218 kcalmol (VO2-NTO1 circprime ) which is comparableto circumferential addition to a pristine sidewall Thus thispredicts that the increased tube reactivity to oxygenationin the vicinity of a vacancy is a very local effect The moststable epoxide isomer also on a circumferential bondis formed next to the ether function and is once againendothermic with a reaction enthalpy of +12 kcalmol(VO2-NTO1 circprimeprime ) This is sufficiently close to zero as tolie within our error bounds and it is possible that epoxideoxygen addition in this site may be very weakly stablepossibly with a short lifetime particularly if oxygen addi-tion is coming from less stable sources such as ozone If anoxygen treatment method is used which traditionally pro-duces only epoxide surface-bonded oxygen (such as roomtemperature reaction with ozone in toluene) such epoxidegroups may therefore form in the vicinity of pre-existingsurface defects while the rest of the nanotube sidewallwill remain oxygen free However once annulene addi-tion is possible with its higher reaction barriers this willoccur everywhere notably preferentially near to defectsIncreased reactivity towards oxygenation around defectsalso suggests that they play an important role in eg

cutting experiments where extensive oxygenation resultsin opening and cutting of the nanotubes22 Co-operativeannulene addition which serves to open circumferentialbonds in the vicinity of other oxygenated bonds whileleaving axial bonds un-reacted is consistent with such amechanism

3 GENERAL CONCLUSIONS

Oxygenation of C60 has been analyzed and a number ofC60O2 and C60O3 stable isomers have been identified andare in agreement with experimental assignments whereavailable Our results indicate that for fullerene additionoxygenation preferentially occurs at the 56 bonds lead-ing to open annulene structures These open structureshave larger CndashOndashC angles giving less strain on the struc-ture as compared to the closed epoxide functions resultingin more stable isomers The same is observed for car-bon nanotube oxygenation where the annulene isomers aremuch lower in energy and indeed epoxide-type oxygena-tion is endothermic both on pristine nanotube sidewalls asin the vicinity of defects

We note that the reaction enthalpies quoted aboveare based on oxygen molecules in their ground statetriplet state (the energy difference with the singlet stateis minus257 kcalmol at our level of theory so reactionenthalpies are shifted down by the same amount for addi-tion of singlet oxygen) If the oxygen is in a singlet statethen epoxide addition becomes mildly exothermic on thepristine tube and strongly exothermic next to defects Thissuggests that UV excited oxygen should show qualitativelydifferent oxygenation behavior on carbon nanotubes

In general higher reaction enthalpies are found forfullerenes as compared to carbon nanotubes (except for thefunctionalisation of the under-coordinated carbon atoms ofthe vacancy) Thus oxygenation will occur preferentiallyat fullerene-like nanotube tips before sidewall functional-isation This also suggests that fullerenes could be usedto lsquotraprsquo oxygen atoms more readily as compared to thepristine carbon nanotube sidewall

Acknowledgments Gregory Van Lier acknowledgesthe Research FoundationndashFlanders (FWO) for financialsupport as a Postdoctoral Research Fellow ChristopherP Ewels and Irene Suarez-Martinez acknowledge theEU-STREP project 003311 ldquonano2hybridsrdquo for funding

References and Notes

1 R Taylor Proc Electrochem Soc 97 281 (1997)2 D Heymann and R B Weisman C R Chimie 9 1107 (2006)3 A L Balch C A Costa B C Noll and M M Olmstread J Am

Chem Soc 117 8926 (1995)4 M P Barrow N J Tower R Taylor and T Drewello Chem Phys

Lett 293 302 (1998)5 S Lebedkin S Ballenweg J Gross R Taylor and W Kraumltschmer

Tetrahedron Lett 36 4971 (1995)

6118 J Nanosci Nanotechnol 9 6113ndash6119 2009

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

Fig 2 Overview of the reaction enthalpies (kcalmol) with the Schlegeldiagrams of the corresponding structures for C60 oxygenation with label-ing according to Figure 1 Insert showing a Schlegel diagram with sitenumbering for C60 as used throughout this work

lengths 1453 Aring CndashOndashC bond angles of 6272 andunderlying CndashC bonds now dilated to 1512 Aring The nextmost stable isomers are 1234910-C60O3 (S structureFig 1(e)) and 12341112-C60O3 (T structure Fig 1(f))(13 and 20 kcalmol less stable than 123456-C60O3

respectively) These three epoxides correspond to the onescharacterized experimentally3940

However a more stable isomer can be formed from 12-C60O by oxygenation of a 56 bond to form 121011-C60O2 with a reaction enthalpy of minus182 kcalmol (seeFig 1(c)) This isomer has both a 56 and a 66bond oxygenated opposite the same hexagon and is+112 kcalmol more stable than 1234-C60O2

From further oxygenation of 19-C60O with only 56addition 19212-C60O2 is the most stable isomer pre-dicted with a reaction enthalpy of minus377 kcalmol (seeFig 1(h)) This isomer forms a diannulene structurearound the same hexagon with the CndashOndashC bond anglefurther expanded to 10712 The second most stable iso-mer is 1967-C60O2 which is only 1 kcalmol less sta-ble where the diannulene is formed around the samepentagon (see Fig 1(i)) From 19-C60O the above-mentioned 121011-C60O2 can also be formed but

with a lower reaction enthalpy of minus89 kcalmol Uponfurther oxygenation of 19212-C60O2 the most stableisomers are 192121011-C60O3 and 19212101167-C60O4 respectively with reaction enthalpies of minus554 andminus280 kcalmol (see Figs 1(j and k))

Overall we see a clear thermodynamic tendency towardsopen annulene structures resulting from the oxygenationof 56 bonds giving more stable reaction products ascompared to the closed epoxide products This can beunderstood from the CndashOndashC angle which is much smallerin the case of the epoxides as compared to the angle ineg H2O In the case of the annulenes the angle aroundthe oxygen atom approaches the one in water resulting inless strain on the structure and more stable isomers How-ever epoxide structures are typically favored on kineticgrounds since for annulene addition it is first necessaryto overcome an appreciable energy barrier associated withbreaking the underlying CndashC bond

22 Oxygenation of Pristine Metallic 55Carbon Nanotubes

We next consider bridging oxygen addition to the surfaceof pristine metallic 55 carbon nanotubes The activesites for the fully optimized geometries of the isomersunder consideration are depicted in Figure 3 (the activesite is indicated in blue on the insert) where an overviewof the following results is presented The correspondingreaction enthalpies ER and structural details are listed inTable II Unlike fullerenes only hexagonndashhexagon bondsare present however the curvature of the tube breaks sys-tem symmetry resulting in bonds with different length andsingledouble character In case of a 55 armchair carbonnanotube only two different bond types exist orientatedeither perpendicular (circumferential bond with length of1432 Aring) or along (axial with a bond length of 1396 Aring)the nanotube axis For our nanotube test system C240H20

(see insert Fig 3) oxygenation is endothermic for axialaddition (reaction enthalpy of +191 kcalmol NT-O1 ax)whereas circumferential addition has a reaction enthalpy ofminus222 kcalmol (NT-O1 circ) This big difference stems fromthe difference in addition type namely the axial additionresults in epoxide formation whereas the circumferentialaddition forms an annulene the structures consistent withprevious literature results28ndash30 Since the epoxide addition ispredicted to be endothermic the annulene form is the onlyisolated oxygen form expected to be present on a pristinenanotube surface upon oxygenation provided the systemcan overcome the energy barrier associated with opening upthe underlying CndashC bond much as annulene oxygen addi-tion to fullerenes is kinetically inhibited Assuming annu-lene addition has a high reaction barrier like for fullerenesthis can explain the relatively high resistance of pristinenanotube surfaces to oxygenation

Addition of a second oxygen to the unstable epoxide iso-mer can be exothermic for circumferential addition of an

6116 J Nanosci Nanotechnol 9 6113ndash6119 2009

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Van Lier et al Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes

Fig 3 Overview of the reaction enthalpies (kcalmol) with the corresponding active site of the optimized structures for carbon nanotube oxygenationInsert showing the 55 armchair C240H20 carbon nanotube test system with the positions of the atoms of the active site indicated in blue and theposition of the vacancy indicated in green Structural and energetic data given in Table II

annulene with a reaction enthalpy of minus246 kcalmol (NT-O1 ax circ) whereas addition of a second epoxide is againendothermic with a reaction enthalpy of +284 kcalmol(NT-O2 ax) We note that when the NT-O1 ax circ sys-tem is formed from the annulene structure NT-O1 circa reaction enthalpy of +167 kcalmol is found ieepoxide addition next to pre-existing annulene oxy-gen is approximately as unfavorable as to the pristinenanotube surface Thus these results confirm there is

Table II Calculated most stable isomers and corresponding reaction enthalpy ER for the addition indicated in bold and structural data for oxygenationof a pristine nanotube and a nanotube with an oxygenated vacancy (VO2-NT)

Isomer Addition type ER CndashO bond length (Aring) Underlying CndashC distance (Aring) CndashOndashC bond angle

NT-O1 ax Epoxide +191 1482 1494 6051NT-O1 circ Annulene minus222 1403 2184 10224

NT-O2 ax Diepoxide +284 1481 1490 6044NT-O2 ax-circ Epoxideannulene minus246 14791403 14952185 605710218NT-O2 circ Diannulene minus380 1409 2252 10612

VO2-NT Annulene+ketone minus2063lowast 13751212 2440na 12564naVO2-NTO1 circ Annulene minus499 1403 2263 10736VO2-NTO1 circprime Annulene minus218 1403 2199 10329VO2-NTO1 circprimeprime Epoxide +12 1491 1502 6174

lowastas compared to V-NT

no thermodynamic driving force for epoxide formationeven next to pre-existing annulene oxygenated additionsites

Addition of a second oxygen to the annulene isomervia formation of a second annulene on the same hexagongives the most stable isomer with a reaction enthalpy ofminus380 kcalmol (NT-O2 circ) Thus further annulene forma-tion in the presence of pre-existing annulene oxygen isthermodynamically favored

J Nanosci Nanotechnol 9 6113ndash6119 2009 6117

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

We note that the reaction enthalpies can be influencedby the nanotube diameter as previously predicted for oxy-gen addition to pristine 60 carbon nanotubes43 In gen-eral the reaction enthalpies appear lower as compared tofullerenes

23 Oxygenation of Defective Metallic 55Carbon Nanotubes

In reality the majority of nanotube surfaces are defectiveand these defects are known to be much more reactiveTherefore we will next consider oxygen addition around asingle vacancy The reaction enthalpies ER and structuraldetails are again listed in Table II and an overview of thefollowing results is presented in Figure 3 together with theactive sites for the fully optimized geometries (the activesite is indicated in blue on the insert with the position ofthe vacancy indicated in green)

In the case of a single vacancy (V-NT) it has beendemonstrated that an O2 oxygen molecule will dissoci-ate upon addition giving rise to an ether and a ketonefunctional group (VO2-NT)23 This type of functionalisa-tion will also occur when nanotubes are oxygenated usingRF-plasma functionalisation23 The reaction enthalpy foraddition of two oxygen atoms to a vacancy is predictedto be minus2063 kcalmol In the case of further oxygena-tion of this type of oxygenated defects our results pre-dict that functionalisation will preferentially occur in thevicinity of the defect The most stable structure forms anannulene on the ring next to the vacancy with a reac-tion enthalpy of minus499 kcal mol (VO2-NT-O1 circ) Thenext most stable isomer also forms an annulene but onehexagon away from the vacancy with a reaction enthalpyof minus218 kcalmol (VO2-NTO1 circprime ) which is comparableto circumferential addition to a pristine sidewall Thus thispredicts that the increased tube reactivity to oxygenationin the vicinity of a vacancy is a very local effect The moststable epoxide isomer also on a circumferential bondis formed next to the ether function and is once againendothermic with a reaction enthalpy of +12 kcalmol(VO2-NTO1 circprimeprime ) This is sufficiently close to zero as tolie within our error bounds and it is possible that epoxideoxygen addition in this site may be very weakly stablepossibly with a short lifetime particularly if oxygen addi-tion is coming from less stable sources such as ozone If anoxygen treatment method is used which traditionally pro-duces only epoxide surface-bonded oxygen (such as roomtemperature reaction with ozone in toluene) such epoxidegroups may therefore form in the vicinity of pre-existingsurface defects while the rest of the nanotube sidewallwill remain oxygen free However once annulene addi-tion is possible with its higher reaction barriers this willoccur everywhere notably preferentially near to defectsIncreased reactivity towards oxygenation around defectsalso suggests that they play an important role in eg

cutting experiments where extensive oxygenation resultsin opening and cutting of the nanotubes22 Co-operativeannulene addition which serves to open circumferentialbonds in the vicinity of other oxygenated bonds whileleaving axial bonds un-reacted is consistent with such amechanism

3 GENERAL CONCLUSIONS

Oxygenation of C60 has been analyzed and a number ofC60O2 and C60O3 stable isomers have been identified andare in agreement with experimental assignments whereavailable Our results indicate that for fullerene additionoxygenation preferentially occurs at the 56 bonds lead-ing to open annulene structures These open structureshave larger CndashOndashC angles giving less strain on the struc-ture as compared to the closed epoxide functions resultingin more stable isomers The same is observed for car-bon nanotube oxygenation where the annulene isomers aremuch lower in energy and indeed epoxide-type oxygena-tion is endothermic both on pristine nanotube sidewalls asin the vicinity of defects

We note that the reaction enthalpies quoted aboveare based on oxygen molecules in their ground statetriplet state (the energy difference with the singlet stateis minus257 kcalmol at our level of theory so reactionenthalpies are shifted down by the same amount for addi-tion of singlet oxygen) If the oxygen is in a singlet statethen epoxide addition becomes mildly exothermic on thepristine tube and strongly exothermic next to defects Thissuggests that UV excited oxygen should show qualitativelydifferent oxygenation behavior on carbon nanotubes

In general higher reaction enthalpies are found forfullerenes as compared to carbon nanotubes (except for thefunctionalisation of the under-coordinated carbon atoms ofthe vacancy) Thus oxygenation will occur preferentiallyat fullerene-like nanotube tips before sidewall functional-isation This also suggests that fullerenes could be usedto lsquotraprsquo oxygen atoms more readily as compared to thepristine carbon nanotube sidewall

Acknowledgments Gregory Van Lier acknowledgesthe Research FoundationndashFlanders (FWO) for financialsupport as a Postdoctoral Research Fellow ChristopherP Ewels and Irene Suarez-Martinez acknowledge theEU-STREP project 003311 ldquonano2hybridsrdquo for funding

References and Notes

1 R Taylor Proc Electrochem Soc 97 281 (1997)2 D Heymann and R B Weisman C R Chimie 9 1107 (2006)3 A L Balch C A Costa B C Noll and M M Olmstread J Am

Chem Soc 117 8926 (1995)4 M P Barrow N J Tower R Taylor and T Drewello Chem Phys

Lett 293 302 (1998)5 S Lebedkin S Ballenweg J Gross R Taylor and W Kraumltschmer

Tetrahedron Lett 36 4971 (1995)

6118 J Nanosci Nanotechnol 9 6113ndash6119 2009

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Van Lier et al Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes

Fig 3 Overview of the reaction enthalpies (kcalmol) with the corresponding active site of the optimized structures for carbon nanotube oxygenationInsert showing the 55 armchair C240H20 carbon nanotube test system with the positions of the atoms of the active site indicated in blue and theposition of the vacancy indicated in green Structural and energetic data given in Table II

annulene with a reaction enthalpy of minus246 kcalmol (NT-O1 ax circ) whereas addition of a second epoxide is againendothermic with a reaction enthalpy of +284 kcalmol(NT-O2 ax) We note that when the NT-O1 ax circ sys-tem is formed from the annulene structure NT-O1 circa reaction enthalpy of +167 kcalmol is found ieepoxide addition next to pre-existing annulene oxy-gen is approximately as unfavorable as to the pristinenanotube surface Thus these results confirm there is

Table II Calculated most stable isomers and corresponding reaction enthalpy ER for the addition indicated in bold and structural data for oxygenationof a pristine nanotube and a nanotube with an oxygenated vacancy (VO2-NT)

Isomer Addition type ER CndashO bond length (Aring) Underlying CndashC distance (Aring) CndashOndashC bond angle

NT-O1 ax Epoxide +191 1482 1494 6051NT-O1 circ Annulene minus222 1403 2184 10224

NT-O2 ax Diepoxide +284 1481 1490 6044NT-O2 ax-circ Epoxideannulene minus246 14791403 14952185 605710218NT-O2 circ Diannulene minus380 1409 2252 10612

VO2-NT Annulene+ketone minus2063lowast 13751212 2440na 12564naVO2-NTO1 circ Annulene minus499 1403 2263 10736VO2-NTO1 circprime Annulene minus218 1403 2199 10329VO2-NTO1 circprimeprime Epoxide +12 1491 1502 6174

lowastas compared to V-NT

no thermodynamic driving force for epoxide formationeven next to pre-existing annulene oxygenated additionsites

Addition of a second oxygen to the annulene isomervia formation of a second annulene on the same hexagongives the most stable isomer with a reaction enthalpy ofminus380 kcalmol (NT-O2 circ) Thus further annulene forma-tion in the presence of pre-existing annulene oxygen isthermodynamically favored

J Nanosci Nanotechnol 9 6113ndash6119 2009 6117

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

We note that the reaction enthalpies can be influencedby the nanotube diameter as previously predicted for oxy-gen addition to pristine 60 carbon nanotubes43 In gen-eral the reaction enthalpies appear lower as compared tofullerenes

23 Oxygenation of Defective Metallic 55Carbon Nanotubes

In reality the majority of nanotube surfaces are defectiveand these defects are known to be much more reactiveTherefore we will next consider oxygen addition around asingle vacancy The reaction enthalpies ER and structuraldetails are again listed in Table II and an overview of thefollowing results is presented in Figure 3 together with theactive sites for the fully optimized geometries (the activesite is indicated in blue on the insert with the position ofthe vacancy indicated in green)

In the case of a single vacancy (V-NT) it has beendemonstrated that an O2 oxygen molecule will dissoci-ate upon addition giving rise to an ether and a ketonefunctional group (VO2-NT)23 This type of functionalisa-tion will also occur when nanotubes are oxygenated usingRF-plasma functionalisation23 The reaction enthalpy foraddition of two oxygen atoms to a vacancy is predictedto be minus2063 kcalmol In the case of further oxygena-tion of this type of oxygenated defects our results pre-dict that functionalisation will preferentially occur in thevicinity of the defect The most stable structure forms anannulene on the ring next to the vacancy with a reac-tion enthalpy of minus499 kcal mol (VO2-NT-O1 circ) Thenext most stable isomer also forms an annulene but onehexagon away from the vacancy with a reaction enthalpyof minus218 kcalmol (VO2-NTO1 circprime ) which is comparableto circumferential addition to a pristine sidewall Thus thispredicts that the increased tube reactivity to oxygenationin the vicinity of a vacancy is a very local effect The moststable epoxide isomer also on a circumferential bondis formed next to the ether function and is once againendothermic with a reaction enthalpy of +12 kcalmol(VO2-NTO1 circprimeprime ) This is sufficiently close to zero as tolie within our error bounds and it is possible that epoxideoxygen addition in this site may be very weakly stablepossibly with a short lifetime particularly if oxygen addi-tion is coming from less stable sources such as ozone If anoxygen treatment method is used which traditionally pro-duces only epoxide surface-bonded oxygen (such as roomtemperature reaction with ozone in toluene) such epoxidegroups may therefore form in the vicinity of pre-existingsurface defects while the rest of the nanotube sidewallwill remain oxygen free However once annulene addi-tion is possible with its higher reaction barriers this willoccur everywhere notably preferentially near to defectsIncreased reactivity towards oxygenation around defectsalso suggests that they play an important role in eg

cutting experiments where extensive oxygenation resultsin opening and cutting of the nanotubes22 Co-operativeannulene addition which serves to open circumferentialbonds in the vicinity of other oxygenated bonds whileleaving axial bonds un-reacted is consistent with such amechanism

3 GENERAL CONCLUSIONS

Oxygenation of C60 has been analyzed and a number ofC60O2 and C60O3 stable isomers have been identified andare in agreement with experimental assignments whereavailable Our results indicate that for fullerene additionoxygenation preferentially occurs at the 56 bonds lead-ing to open annulene structures These open structureshave larger CndashOndashC angles giving less strain on the struc-ture as compared to the closed epoxide functions resultingin more stable isomers The same is observed for car-bon nanotube oxygenation where the annulene isomers aremuch lower in energy and indeed epoxide-type oxygena-tion is endothermic both on pristine nanotube sidewalls asin the vicinity of defects

We note that the reaction enthalpies quoted aboveare based on oxygen molecules in their ground statetriplet state (the energy difference with the singlet stateis minus257 kcalmol at our level of theory so reactionenthalpies are shifted down by the same amount for addi-tion of singlet oxygen) If the oxygen is in a singlet statethen epoxide addition becomes mildly exothermic on thepristine tube and strongly exothermic next to defects Thissuggests that UV excited oxygen should show qualitativelydifferent oxygenation behavior on carbon nanotubes

In general higher reaction enthalpies are found forfullerenes as compared to carbon nanotubes (except for thefunctionalisation of the under-coordinated carbon atoms ofthe vacancy) Thus oxygenation will occur preferentiallyat fullerene-like nanotube tips before sidewall functional-isation This also suggests that fullerenes could be usedto lsquotraprsquo oxygen atoms more readily as compared to thepristine carbon nanotube sidewall

Acknowledgments Gregory Van Lier acknowledgesthe Research FoundationndashFlanders (FWO) for financialsupport as a Postdoctoral Research Fellow ChristopherP Ewels and Irene Suarez-Martinez acknowledge theEU-STREP project 003311 ldquonano2hybridsrdquo for funding

References and Notes

1 R Taylor Proc Electrochem Soc 97 281 (1997)2 D Heymann and R B Weisman C R Chimie 9 1107 (2006)3 A L Balch C A Costa B C Noll and M M Olmstread J Am

Chem Soc 117 8926 (1995)4 M P Barrow N J Tower R Taylor and T Drewello Chem Phys

Lett 293 302 (1998)5 S Lebedkin S Ballenweg J Gross R Taylor and W Kraumltschmer

Tetrahedron Lett 36 4971 (1995)

6118 J Nanosci Nanotechnol 9 6113ndash6119 2009

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes Van Lier et al

We note that the reaction enthalpies can be influencedby the nanotube diameter as previously predicted for oxy-gen addition to pristine 60 carbon nanotubes43 In gen-eral the reaction enthalpies appear lower as compared tofullerenes

23 Oxygenation of Defective Metallic 55Carbon Nanotubes

In reality the majority of nanotube surfaces are defectiveand these defects are known to be much more reactiveTherefore we will next consider oxygen addition around asingle vacancy The reaction enthalpies ER and structuraldetails are again listed in Table II and an overview of thefollowing results is presented in Figure 3 together with theactive sites for the fully optimized geometries (the activesite is indicated in blue on the insert with the position ofthe vacancy indicated in green)

In the case of a single vacancy (V-NT) it has beendemonstrated that an O2 oxygen molecule will dissoci-ate upon addition giving rise to an ether and a ketonefunctional group (VO2-NT)23 This type of functionalisa-tion will also occur when nanotubes are oxygenated usingRF-plasma functionalisation23 The reaction enthalpy foraddition of two oxygen atoms to a vacancy is predictedto be minus2063 kcalmol In the case of further oxygena-tion of this type of oxygenated defects our results pre-dict that functionalisation will preferentially occur in thevicinity of the defect The most stable structure forms anannulene on the ring next to the vacancy with a reac-tion enthalpy of minus499 kcal mol (VO2-NT-O1 circ) Thenext most stable isomer also forms an annulene but onehexagon away from the vacancy with a reaction enthalpyof minus218 kcalmol (VO2-NTO1 circprime ) which is comparableto circumferential addition to a pristine sidewall Thus thispredicts that the increased tube reactivity to oxygenationin the vicinity of a vacancy is a very local effect The moststable epoxide isomer also on a circumferential bondis formed next to the ether function and is once againendothermic with a reaction enthalpy of +12 kcalmol(VO2-NTO1 circprimeprime ) This is sufficiently close to zero as tolie within our error bounds and it is possible that epoxideoxygen addition in this site may be very weakly stablepossibly with a short lifetime particularly if oxygen addi-tion is coming from less stable sources such as ozone If anoxygen treatment method is used which traditionally pro-duces only epoxide surface-bonded oxygen (such as roomtemperature reaction with ozone in toluene) such epoxidegroups may therefore form in the vicinity of pre-existingsurface defects while the rest of the nanotube sidewallwill remain oxygen free However once annulene addi-tion is possible with its higher reaction barriers this willoccur everywhere notably preferentially near to defectsIncreased reactivity towards oxygenation around defectsalso suggests that they play an important role in eg

cutting experiments where extensive oxygenation resultsin opening and cutting of the nanotubes22 Co-operativeannulene addition which serves to open circumferentialbonds in the vicinity of other oxygenated bonds whileleaving axial bonds un-reacted is consistent with such amechanism

3 GENERAL CONCLUSIONS

Oxygenation of C60 has been analyzed and a number ofC60O2 and C60O3 stable isomers have been identified andare in agreement with experimental assignments whereavailable Our results indicate that for fullerene additionoxygenation preferentially occurs at the 56 bonds lead-ing to open annulene structures These open structureshave larger CndashOndashC angles giving less strain on the struc-ture as compared to the closed epoxide functions resultingin more stable isomers The same is observed for car-bon nanotube oxygenation where the annulene isomers aremuch lower in energy and indeed epoxide-type oxygena-tion is endothermic both on pristine nanotube sidewalls asin the vicinity of defects

We note that the reaction enthalpies quoted aboveare based on oxygen molecules in their ground statetriplet state (the energy difference with the singlet stateis minus257 kcalmol at our level of theory so reactionenthalpies are shifted down by the same amount for addi-tion of singlet oxygen) If the oxygen is in a singlet statethen epoxide addition becomes mildly exothermic on thepristine tube and strongly exothermic next to defects Thissuggests that UV excited oxygen should show qualitativelydifferent oxygenation behavior on carbon nanotubes

In general higher reaction enthalpies are found forfullerenes as compared to carbon nanotubes (except for thefunctionalisation of the under-coordinated carbon atoms ofthe vacancy) Thus oxygenation will occur preferentiallyat fullerene-like nanotube tips before sidewall functional-isation This also suggests that fullerenes could be usedto lsquotraprsquo oxygen atoms more readily as compared to thepristine carbon nanotube sidewall

Acknowledgments Gregory Van Lier acknowledgesthe Research FoundationndashFlanders (FWO) for financialsupport as a Postdoctoral Research Fellow ChristopherP Ewels and Irene Suarez-Martinez acknowledge theEU-STREP project 003311 ldquonano2hybridsrdquo for funding

References and Notes

1 R Taylor Proc Electrochem Soc 97 281 (1997)2 D Heymann and R B Weisman C R Chimie 9 1107 (2006)3 A L Balch C A Costa B C Noll and M M Olmstread J Am

Chem Soc 117 8926 (1995)4 M P Barrow N J Tower R Taylor and T Drewello Chem Phys

Lett 293 302 (1998)5 S Lebedkin S Ballenweg J Gross R Taylor and W Kraumltschmer

Tetrahedron Lett 36 4971 (1995)

6118 J Nanosci Nanotechnol 9 6113ndash6119 2009

Delivered by Ingenta toSCD Universite Bordeaux 1

IP 14721082188Thu 10 Sep 2009 142013

RESEARCHARTICLE

Van Lier et al Comparison Between Early Stage Oxygenation Behavior of Fullerenes and Carbon Nanotubes

6 N P Curry B Doust and D A Jelski J Cluster Sci 12 385(2000)

7 J M Wood B Kahr S H Hoke L Dejarme R G Cooks andD Ben-Amotz J Am Chem Soc 113 5908 (1991)

8 C Taliani G Ruani R Zamboni R Danieli S Rossini V NDenisov V M Burlakov F Negri G Orlandi and F ZerbettoJ Chem Soc Chem Commun 220 (1993)

9 H Tanaka K Takeuchi Y Negishi and T Tsukuda Chem PhysLett 384 283 (2004)

10 A D M Darwish A G Avent R Taylor and S R M WaltonJ Chem Soc Perkin Trans 2 2051 (1996)

11 S Jenkins M I Heggie and R Taylor J Chem Soc Perkin Trans2 2415 (2000)

12 R Taylor G J Langley A K Brisdon J H Holloway E G HopeH W Kroto and D R M Walton J Chem Soc Chem Commun875 (1993)

13 O V Boltalina J H Holloway E G Hope J M Street andR Taylor J Chem Soc Perkin Trans 2 1845 (1998)

14 O V Boltalina B de La Vaissiegravere P W Fowler A Y LukoninA K Abdul-Sada J M Street and R Taylor J Chem Soc PerkinTrans 2 2212 (2000)

15 R Taylor Chem Eur J 19 4074 (2001)16 O V Boltalina A D Darwish J M Street R Taylor and X-W

Wei J Chem Soc Perkin Trans 2 251 (2002)17 K M Creegan J L Robbins W K Robbins J Millar R D

Sherwood P J Tindall and D M Cox J Am Chem Soc 1141103 (1992)

18 R B Weisman D Heymann and S M Bachilo J Am Chem Soc123 9720 (2001)

19 D L Kepert and B W Clare Inorg Chim Acta 327 41 (2002)20 C P Ewels H El Cheikh I Suarez-Martinez and G Van Lier

Phys Chem Chem Phys 10 2145 (2008)21 E Dujardin T W Ebbesen A Krishnan and M M J Treacy Adv

Mater 10 611 (1998)22 Z Chen K J Ziegler J Shaver R H Hauge and R E Smalley

J Phys Chem B 110 11624 (2006)23 A Felten C Bittencourt J-J Pireaux G Van Lier and J-C

Charlier J Appl Phys 98 074308 (2005)24 R Ionescu E H Espinosa E Sotter E Llobet X Vilanova

X Correig A Felten C Bittencourt G Van Lier J-C Charlierand J-J Pireaux Sens amp Act B 113 36 (2006)

25 P C P Watts N Mureau Z N Tang Y Miyajima J D Careyand S R P Silva Nanotechnology 18 175701 (2007)

26 J-C Charlier Acc Chem Res 35 1063 (2002)27 M Burghard Surf Sci Rep 58 1 (2005)

28 V Barone J Heyd and G E Scuseria Chem Phys Lett 389 289(2004)

29 Z F Chen S Nagase A Hirsch R C Haddon W Thiel andP v R Schleyer Ang Chem Int Ed 43 1552 (2004)

30 S Dag O Gulseren T Yildirim and S Ciraci Phys Rev B 67165424 (2003)

31 Z Slanina L Stobinski P Tomasik H-M Lin and L AdamowiczJ Nanosci Nanotechnol 3 193 (2003)

32 G Van Lier C P Ewels and P Geerlings Comp Phys Commun179 165 (2008)

33 C P Ewels G Van Lier P Geerlings and J-C Charlier J ChemInf Model 47 2208 (2007)

34 M J Frisch G W Trucks H B Schlegel G E Scuseria M ARobb J R Cheeseman J A Montgomery Jr T Vreven K NKudin J C Burant J M Millam S S Iyengar J TomasiV Barone B Mennucci M Cossi G Scalmani N Rega G APetersson H Nakatsuji M Hada M Ehara K Toyota R FukudaJ Hasegawa M Ishida T Nakajima Y Honda O Kitao H NakaiM Klene X Li J E Knox H P Hratchian J B CrossV Bakken C Adamo J Jaramillo R Gomperts R E StratmannO Yazyev A J Austin R Cammi C Pomelli J W OchterskiP Y Ayala K Morokuma G A Voth P Salvador J J DannenbergG Zakrzewski S Dapprich A D Daniels M C Strain O FarkasD K Malick A D Rabuck K Raghavachari J B ForesmanJ V Ortiz Q Cui A G Baboul S Clifford J CioslowskiB B Stefanov G Liu A Liashenko P Piskorz I KomaromiR L Martin D J Fox T Keith M A Al-Laham C Y PengA Nanayakkara M Challacombe P M W Gill B JohnsonW Chen M W Wong C Gonzalez and J A Pople Gaussian 03Gaussian Inc Pittsburgh PA (2003)

35 Y M Shulrsquoga V M Martynenko A F Shestakov S A BashkakovS V Kulikov V N Vasilets T L Makarova and Y G MorozovRussian Chem Bulletin Int Ed 55 687 (2006)

36 G Van Lier C P Ewels F Zuliani A De Vita and J-C CharlierJ Phys Chem B 109 6153 (2005)

37 D Heymann and L P F Chibante Chem Phys Lett 207 339(1993)

38 J P Deng C Mou and C C Han J Phys Chem 99 14907(1995)

39 C Fusco R Seraglia and R Curci J Org Chem 64 8363 (1999)40 Y Tajima and K Takeuchi J Org Chem 67 1696 (2002)41 M Manoharan J Org Chem 65 1093 (2000)42 R Taylor J Chem Soc Perkin Trans 2 813 (1993)43 Y F Zhang and Z F Liu J Phys Chem B 108 11435 (2004)

Received 16 June 2008 Accepted 31 December 2008

J Nanosci Nanotechnol 9 6113ndash6119 2009 6119