Carbon 43 (2005) 153–161

www.elsevier.com/locate/carbon

High resolution XPS characterization of chemical functionalisedMWCNTs and SWCNTs

T.I.T. Okpalugo *, P. Papakonstantinou, H. Murphy, J. McLaughlin, N.M.D. Brown

NIBEC, University of Ulster, Shore Road, Belfast BT37 0QB, Northern Ireland, UK

Received 5 March 2004; accepted 29 August 2004

Available online 18 October 2004

Abstract

High resolution XPS analysis of chemical functionalised multi-wall carbon nanotubes (MWCNTs) and single wall carbon nano-

tubes (SWCNT) was done with ESCA300 (overall instrument resolution of 0.35eV). Information to the degree of functionalisation

was ascertained by argon ion bombardment of the samples followed by XPS analysis to detect the functional groups, the percentage

atomic concentration of various elements present and whether or not the detected functional groups imposed a chemical shift on the

CNT atoms. The results show that true chemical functionalisation was achieved and by argon ion bombardment these functional

groups can be altered relative to the C1s carbon atoms of the CNT. The choice of chemicals used for functionalisation, the tech-

niques employed and the types of nanotubes treated are important factors in chemical characterisation. The carbon atom on the

nanotube ring to which the functional group (atom) is bonded, the chirality of the CNT, the electronegativity of the functional

group, the bond type and whether the CNT is single-wall or multi-wall, or cut (short) could play a role in determining the chemical

shift on the CNTs atoms. These investigations are relevant to chemical functionalisation of carbon nanotubes for various applica-

tions for example DNA sensors and other biomedical sensors.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: A. Carbon nanotubes, Chemically modified carbon; B. Heat treatment; C. X-ray photoelectron spectroscopy; D. Functional groups

1. Introduction

Carbon nanotubes are structural carbons which

forms very small tubes (single wall tubes, SWCNTs ortubes within tubes, multi-wall, MWCNTs) of very small

diameter (hence nanotubes). SWCNT has approxi-

mately, 1.2–1.4nm diameter, 2.5–2.8eV C–C tight bond-

ing overlap energy [1,2], and the lattice vector

parameters defining the various types of nanotubes con-

formation as armchair (n,n), zig-zag (n, 0) or chiral

0008-6223/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2004.08.033

* Corresponding author. Tel.: +44 2890368997/2890368663; fax:

+44 2890366863.

E-mail addresses: tit_okpalugo@yahoo.com, thomas@nibec-

s1.nibec.ulst.ac.uk (T.I.T. Okpalugo).

(m,n). The values of m and n determine the chirality

(twist) of the nanotube which in turn determines the

nanotube properties. Carbon nanotubes can be thought

of as rolled up graphene sheets held together by van derWaals bonds. Like graphite CNT is relatively non-reac-

tive, except at the nanotube caps which are more reac-

tive due to the presence of dangling bonds. The

reactivity of the side walls of the CNT p-system can also

be influenced by the tube curvature or chirality (twist).

The acid washings/sonication commonly utilised to pur-

ify the nanotubes (separate the nanotubes from amor-

phous carbons and metallic catalysts impurities) createan open end termini in the structure, which are stabilised

by –COOH and –OH groups [3,4] left bonded to the

nanotubes at the end termini and/or the sidewall defect

sites. The –COOH are expected to covalently bond to

154 T.I.T. Okpalugo et al. / Carbon 43 (2005) 153–161

CNT due to the strong interaction of CNT carbon

atoms and the concentrated H2SO4 and/or HNO3 used

for the CNT purification. Very reactive elements like flu-

orine can also chemically bond to or functionalise nano-

tubes at room temperature and can lead to severe

modification at higher temperatures of about 500–600 �C [5]. Also at the CNT end termini, the –COOH

group can be coupled to various chemical groups

depending on the choice of chemical, to further func-

tionalise the nanotube. These chemicals can form cova-

lent (or irreversible van der Waals) bonds with the

nanotubes which could alter the sp2 hybridisation of

CNTs to sp3 hybridisation. Strong and covalently

bound atoms/molecules are expected to lead to a chem-ical shift in the CNT environment. In this study we sus-

pended the nanotubes in either methanol or DMF and

non-covalently functionalised the nanotubes with a bi-

functional molecule, 1-pyrenebutanoic acid, succinyl es-

ter, PBASE [6]. PBASE irreversibly adsorb on to the

hydrophobic surface created by the organic solvents

of dimethylformamide, DMF or methanol. Glucose

oxidase for example can be immobilised onto carbonnanotubes by the linking molecule 1-ethyl-3-(3-dimeth-

ylaminopropyl carbodiimide), (EDC) through covalent

immobilisation [7,8]. The –COOH group was therefore

also activated with 1-ethyl-3-(3-dimethylaminopropyl

carbodiimide (EDC). DNA biomolecules can be linked

to CNT by following similar non-covalent and/or cova-

lent interactions [9].

2. Experimental details

2.1. CNT preparation

The MWNTs were prepared by CVD process. Acety-

lene was used as carbon source and Fe and Ni as cata-

lyst. The reactive temperature was 700–800 �C. The asdeposited MWNTs were rinsed with HCl and distilled

water and were 85% pure, 10% amorphous carbon, 5%

NiO, FeO and Al2O3, 1–3nm in diameter and 1–10lmlong. The single-wall carbon nanotubes, SWCNT were

purchased from Sigma-aldrich (50% purity). Pyrenebut-

anoic acid, succinyl ester (PBASE) and EDC were used

for chemical modification. The functionalised CNT were

either dispersed in DMF or methanol and the drip drytechniques were used to deposit the CNT on the

substrates.

2.2. CNT functionalisation

The nanotubes were purified by heating at reflux with

concentrated nitric acid (HNO3) for 4–5h at 80–90 �Ctemperature. After cooling to room temperature, themixture was diluted with distilled water and decanted

until a PH of above 3 was achieved. A filter system

(diameter of 0.2lm) and vacuum pump was erected in

which the nanotube mixture was washed and poured

through. They were dried on filter paper in the oven at

100 �C overnight to produce bucky paper. Eighty milli-

grams of these carboxylated carbon nanotubes were dis-

persed in 25ml of the solvent dimethylformamide(DMF) and another 80mg of the nanotubes were dis-

persed in 25ml of the solvent methanol. The solutions

were sonicated for 2–3h to allow even dispersion.

2.2.1. Short/cut CNT

0.1g of multi-walled nanotubes was cut into short

tubes by chemical oxidation in 98% concentrated

H2SO4 and 70% HNO3 in a 3:1 ratio (75ml:25ml) fol-lowed by sonication for 8h in a fume cupboard. The

nanotubes were cooled to room temperature and diluted

using distilled water to achieve a PH of above 3. After

many litres of distilled water were used and decanted

off, the nanotubes were poured into the filter, vacuum

system, where they were retrieved on filter paper (dia-

meter 0.2lm). The filter paper was then dried in the

oven at 100 �C overnight. The nanotubes were placedin a clean beaker and then dispersed in methanol.

2.2.2. PBASE and EDC

Fifty milligrams of non-functionalised single-walled

nanotubes were added to 23mg of 1-pyrenebutanoic

acid succinimidyl ester and then dispersed in 10ml of

dimethylformamide. The solution was sonicated for 1–

2h. This method was repeated for multi-walled carbonnanotubes. Twenty-three milligrams of EDC was added

to 50mg of functionalised multi-walled nanotubes and

then dispersed in 10ml of dimethylformamide. The solu-

tion was sonicated for 1–2h.

2.3. The XPS experimental set up

The ESCA300 XPS machine equipped with the highresolution Scienta-ESCA300 spectrometer was used for

the analysis. The instrument employs a high power

(18kW) rotating anode, monochromatised AlKa/CrKb(hv = 1486.7/5946.7eV) x-ray source, a large, seven crys-

tal, double focusing monochromator which focuses the

X-rays to a line image, 6 by 0.5mm on the sample, a

multi-element electron lens and a hemispherical electron

energy analyser and a multi-channel detector. Pressureranges are as follows: 2 · 10�6mbar (fast entry cham-

ber), 4 · 10�8mbar (preparation chamber) and

4 · 10�9mbar (sample analysis chamber). This setting:

high transmission FAT mode, 14.12keV, 25mA, AlKa(1486.7eV) was used for the analysis at 90� electron take

off angle for normal non-charging samples (45� for the

charging samples). The analyser slit width was set for

0.8mm and the resulting overall energy resolution was0.35eV. The SCIENTA software was used for data

acquisition and data analysis. The binding energy of

T.I.T. Okpalugo et al. / Carbon 43 (2005) 153–161 155

the C1s of graphite, 284.5eV (±0.35eV energy resolu-

tion of the spectrometer at the settings employed) was

taken as the reference. Prior to individual elemental

scans a survey scan was taken for all the samples in

order to detect the elements present.

2.3.1. The electron flood gun

The electron flood gun setting for the charging sam-

ples are: 10% emission, KE varied from 0 to 5eV

depending on the degree of charging, 35V deflection

and maximum filament current of 35%.

2.3.2. The Ar+ ion bombardment

Ion gun control AGS2 was used for argon sputteringin the preparation chamber. The ion gun setting was:

2kV, 18lA, gun angle of 45� and chamber pressure

(Ar) = 2.9 · 10�8mbar.

3. Results and discussion

The C1s core level peak positions of the carbonatoms are approximately at 284.5eV (Tables 1–5). The

peak position for oxygen is centred at around

532.5eV. In non-functionalised multi-wall nanotube dis-

persed in DMF there appeared somewhat like a shake

up satellite peak at around 291.27eV. However, the car-

Table 1

XPS analysis results of CNTs dispersed in different chemical solutions

Sample Element Peak position

NF MWNT in DMF on Si wafer C1s 284.59

O1s 532.82

NF MWNT in methanol on Si wafer C1s 284.61

O1s 533.64

NF MWNT, PBASE in DMF on Si wafer C1s 284.87

O1s 532.91

N1s 401.14

F1s 688.25

NF SWNT, PBASE in DMF on Si wafer C1s 284.92

O1s 532.83

N1s 400.91

F1s 687.77

F short MWNT in methanol on Si wafer C1s 285.07*

O1s 533.07

F MWNT in DMF on Si Wafer C1s 284.68

O1s 532.62

N1s 400.33

F1s 687.6

F MWNT in methanol on Si wafer C1s 284.59

O1s 532.99

F MWNT, EDC in DMF on Si wafer C1s 284.59

O1s 532.52

N1s 400.38

* True and significant chemical shift in functionalised short MWCNT.

bon peak of non-functionalised multi-walled nanotubes

in methanol and functionalised multi-walled nanotubes

in methanol does not change significantly.

A slight change in binding energy <0.35eV is consid-

ered insignificant based on the energy resolution of the

XPS instrument used. The functionalised multi-wallednanotubes in methanol show a greater percentage of

oxygen, 14.315%, compared to the functionalised mul-

ti-walled nanotubes in DMF (8.156% O). However,

functionalised short MWCNTs in methanol present

the highest amount of oxygen (16.324–19.273at.%). This

is obviously due to increased functionalisation from in-

creased surface area and/or open end termini from the

cut tubes since functionalisation of the sidewalls is hard-er to realise. This is also associated with a chemical shift

(0.11–0.2eV) of the carbon atom binding energy to a

higher energy (+0.46�0.55eV), which may imply that

the open end structures created by oxidizing acids in

the short MWCNT in methanol are equally stabilised

by the –COOH and –OH [3,4] functional group bonded

to the nanotubes end loops. This chemical shift was not

observed in the uncut functionalised MWCNT in meth-anol. Core-hole screening/loss of electronic screening of

the CNT carbon atom due to the electronegative oxygen

atomic polarisation of the surrounding ions could lead

to the observed chemical shift. Since the inner-shell elec-

trons of an atom are less sensitive to their environment

(eV) Sensitivity Area Atomic % concentration

1 5.284 · 103 96.701

2.8 5.048 · 102 3.299

1 4.984 · 103 94.485

2.8 8.146 · 102 5.515

1 2.946 · 103 89.274

2.8 7.459 · 102 8.073

1.73 7.273 · 101 1.274

5.1 2.320 · 102 1.379

1 3.444 · 103 91.261

2.8 7.613 · 102 7.206

1.73 6.556 · 101 1.004

5.1 1.017 · 102 0.528

1 3.243 · 103 83.676

2.8 1.771 · 103 16.324

1 4.732 · 103 90.3

2.8 1.197 · 103 8.156

1.73 7.391 · 101 0.815

5.1 1.945 · 102 0.728

1 3.293 · 103 85.685

2.8 1.540 · 103 14.315

1 4.438 · 103 90.55

2.8 8.227 · 102 6

1.73 2.930 · 102 3.45

Table 2

XPS analysis results of CNTs on different substrates

Sample Element Peak position

(eV)

Sensitivity Area Atomic %

concentration

F MWNT from bucky paper of Carbon nanotubes

purified in HNO3 (flood gun employed)

C1s 284.42 1 2.892 · 103 92.013

O1s 532.54 2.8 7.028 · 102 7.987

F short MWNT treated/cut-HNO3 and H2SO4

on bucky paper from solution

C1s 283.11a 1 3.856 · 103 80.824

O1s 531.42 2.8 2.562 · 103 19.176

F short MWNT treated/cut-HNO3 and H2SO4

on bucky paper (from sediments)

C1s 282.48a 1 3.260 · 103 80.727

O1s 530.89 2.8 2.179 · 103 19.273

F short MWNT treated/cut-HNO3 and H2SO4

on bucky Paper (from sediment) after 5min argon sputtering

C1s 283.36a 1 3.661 · 103 93.262

O1s 531.7 2.8 7.406 · 102 6.738

Raw single wall nanotubes on thin double sided tape,

using no flood gun

C1s 284.58 1 3.163 · 103 94.899

O1s 532.62 2.8 4.760 · 102 5.101

Raw single wall nanotubes on thin double sided tape

after 5min Argon sputtering

C1s 284.72 1 3.061 · 103 98.042

O1s 532.52 2.8 1.712 · 102 1.958

Raw multi-wall carbon nanotubes on thin double sided

tape (without electron flood gun)

C1s 284.45 1 3.297 · 103 97.843

O1s 532.43 2.8 2.036 · 102 2.157

a BE shift to lower energy, the reason for this is not obvious, but could be substrate related. Flood gun was used.

156 T.I.T. Okpalugo et al. / Carbon 43 (2005) 153–161

than the valence electrons, the charge potential (point

charge) model and/or the sudden (frozen orbital)

approximation are usually employed to explain this

interaction. Thus charge transfer from CNT C-atom to

oxygen atom due to oxidation by oxygen atom reduced

the �screening effect� on the C-hole thereby decreasing

the CNT C-atom kinetic energy, and subsequently in-

creased the CNT C-atom binding energy. A puzzlingquestion however arises, that is which of the carbon

atoms on the ring structure of CNTs are bonded to

the functional groups/atoms; on which of the walls of

the MWCNTs is the functional group and finally what

is the �primary� and/or �secondary steric effect� existingin the near neighbour atomic environment? The bond

location of the functional group/atom (the carbon

atom(s) on the ring bonded to the functional group),the chirality of the CNT, the electronegativity of the

functional group/atom, the bond type (whether covalent

or non-covalent) and whether the CNT is single or mul-

ti-wall could play a role in determining the chemical

shift on the CNTs atoms. In Table 2 below, the effects

of different substrates are shown. The functionalised

short CNTs on �bucky papers� seem to show a significant

shift to lower energy even though the electron flood gunwas employed. The reason for this is not understood and

requires further investigation.

Highly oxidative environment are associated with

chemical shift. Calculations have also suggested that

the oxidation rates of CNTs depend on the helical con-

formation of the nanotubes [12]. Moreover, surface to

core level shift (SCLS) [14] phenomena is likely in

functionalised multi-wall nanotubes but this was not

determined at the routine settings used in this study.

This phenomenon could be determined in nanotubes

by employing high resolution angle resolved XPS

techniques and/or by ab initio (eg Self Consistent

Field/Hartree-Fock, Density function theory) and/orsemi-empirical Tight Binding calculations.

The purified multi-walled nanotubes in DMF show

traces of nitrogen and fluorine. Nitrogen may present

from the dimethylformamide and the fluorine may be

an impurity if the nanotubes had come in contact with

water through washing when they were produced. For

non-functionalised multi-walled and functionalised mul-

ti-walled nanotubes in DMF and methanol, results showthat the carbon percentage for non-functionalised is

greater than the functionalised nanotubes and that the

functionalised nanotubes have a greater percentage of

oxygen. This is due to the fact that the functionalised

nanotubes were oxidised in an oxidative acid and there-

fore have oxide functional groups attached to them.

Voigt function was used for XPS peak deconvolution

of the C1s (Fig. 1), O1s (of samples dispersed inDMF) and N1s (of EDC molecular linked, samples

dispersed in DMF) peaks. The following bonds were

assigned: sp2 C@C (284.38–284.53eV), sp3 C–C

(285.11–285.5eV), C–O (286.21–287.53eV), C–NHx(286–288.5eV), >C@O (286.45–287.92eV), –COO

Table 3

CNTs XPS results from the literature as reported by various authors

XPS Model CNT type Functionalisation and

further treatment

Peak (eV) and bond-assigned Chemical shift/comments Ref.

VG Esca-lab 200R SWNT 3M HNO3; 45h C1s: 284.6 C-graphite; 286.3 (–C–O) H–O–H band slight increase with

acid-treatment; C@O, C–O and

H–O–H decr./ann.

[15]

Arc-d Air[O] @300�C; 1h 287.6 (>C@O); 288.8 (–COO); 291

AT/950�C; 10h (p–p*); O1s: 531.6 (O@C); 533.3

(O–C); 534.7(H–O–H)

Not given SWNT Carbolex 1. 3M HNO3 C1s: 284.2 (C-gra.); 285.9(C–O/CNT

sat.); 288.2 (–COO)

+1.7eV [9]

2. SOCl2 + H2NC2H4NH2 C1s: 284.2 (C-bulk), 285.5 (C-shlder) +4.0eV

3. SMCC; DNA50thiol 286–286.5 (NH2); N1s: 400.2 (NH2) +1.3eV

COOH at 288.2 was removed

Not given Not specified 1. gas-phase [O]; C1s: 284.6 (C-gra.); 285 (CNT) [16]

2. HNO3; 286.3 (C–O), 4%; 287.6(C@O), 2%

3. H2SO4; 4. (1 + 2); 288.8 (C@O),9%

5. KHMNO4

VG Microtech

spectrometer

MWC nanoparticle;

Arc-dischar.

BrF3 C1s (ref. 284.4); 285.3 +0.9 [17]

288.6

PHI5300 XPS

(PE Corp.)

MWCNTs Bezene/ferrocene

catalyst, H2 carrier gas,

1150 �C

Br-water@90�C, 3hHeat/air, 520�C, 45min

5M/l HCl DI-wa,

dried/150�C, 12h

As prepared C1s: 284.6 (C);

289–289.4(COO) Purified +

heat; C1s: 285

Bromine was physisorbed and

not Chemisorbed

[18]

ESCA microsc.

ELETTRA syn

(XPEM)

Purified SWCNT/Carbole x Annealed in UHV at 500K C1s: 284.2 (C); 284.42 (C) +0.22 C chemical shift [19]

Kratos XSAM 800 SWCNT, arc-discharge 1. Dichlorocarbene Ar+

sputtering (3.5keV, 5.5mA)

C1s: 285(sp2C, sp3C) 286.4

(C–O; C–NH2) 287.7 (C–Cl; –C@O);

288.9 (O–C@O) Cl2p: 201.3 (C–Cl)

FWHM +2.2eV Ar-sput.: C% incr.;

Cl% = 65% decr.; O at.% decr.

(from 20.7 to 5.5)

[11]

Not given SWCNT, laser ablation./C;

225C/18h, 325C/1.5h,

350C/1h

Heat[O] + sonication in

HCl = –COOH; SOCl2;

H2N–R–NH2; TPA/PDA +

tributylamine; DMF/Isopropanol

C1s: 284.38 (sp2C);

284.0 (NT-Cl) Cl2p:

201.4 (C–Cl)

�0.4 in Cl sample (�0.4eV suggested

acceptor behaviour of Cl atoms to

C of CNT)

[20]

ESCALAB

220I-XL inst.

C60 + P heated at

650-950�C, SWT

C1s: 281 (C-P); 284.3;285.5

O1s: 531 (C–O/O-P)

�3.3

1.1–4.0

[21]

SMCC = Succinyl imidyl 4-(N-maleimidomethyl)-cyclohexane-1-carboxylate; TPA = Tripropylenetetramine; PDA = phenylenediamine.

T.I.T

.Okpalugoetal./Carbon43(2005)153–161

157

Table 4

XPS analysis of argon sputtered FMWCNT in DMF and FMWCNT, EDC in DMF

Sample Element Peak position

(eV)

Sensitivity Area Atomic %

concentration

F MWNT in DMF on Si wafer after 5min argon sputtering C1s 284.92 1 4.849 · 103 96.384

O1s 532.72 2.8 5.094 · 102 3.616

F MWNT in DMF on Si wafer after 10min argon sputtering C1s 284.85 1 4.579 · 103 97.216

O1s 532.72 2.8 3.672 · 102 2.784

F MWNT in DMF on Si wafer after 15min argon sputtering C1s 284.79 1 4.877 · 103 97.668

O1s 532.62 2.8 3.260 · 102 2.332

F MWNT in DMF on Si wafer after 20min argon sputtering C1s 284.92 1 4.694 · 103 98.077

O1s 532.57 2.8 2.576 · 102 1.923

F MWNT in DMF on Si wafer after 25min argon sputtering C1s 284.79 1 4.872 · 103 98.187

O1s 532.72 2.8 2.519 · 102 1.813

F MWNT in DMF on Si wafer after 30min argon sputtering C1s 284.58 1 4.491 · 103 97.035

O1s 532.57 2.8 3.843 · 102 2.965

F MWNT, EDC in DMF Si Wafer after 5min argon sputtering C1s 284.55 1 4.047 · 103 94.685

O1s 532.52 2.8 3.607 · 102 3.014

N1s 399.9 1.73 1.701 · 102 2.301

F MWNT, EDC in DMF on SiWafer, after 10min argon sputtering C1s 284.85 1 3.941 · 103 93.447

O1s 532.48 2.8 3.331 · 102 2.82

N1s 399.86 1.73 2.724 · 102 3.733

F MWNT, EDC in DMF on Si Wafer, after 15min argon sputtering C1s 284.92 1 4.553 · 103 95.333

O1s 532.57 2.8 2.420 · 102 1.809

N1s 399.75 1.73 2.361 · 102 2.858

F MWNT, EDC in DMF on Si Wafer, after 25min argon sputtering C1s 284.85 1 4.124 · 103 94.866

O1s 532.52 2.8 1.880 · 102 1.544

N1s 399.71 1.73 2.700 · 102 3.59

F MWNT, EDC in DMF on Si wafer, after 35min argon sputtering C1s 284.92 1 4.125 · 103 94.913

O1s 532.52 2.8 1.507 · 102 1.239

N1s 399.63 1.73 2.894 · 102 3.849

Table 5

XPS analysis of FMWCNT, EDC, in DMF thermally annealed at �200�C

Sample Element Peak position

(eV)

Sensitivity Area Atomic %

concentration

F MWNT, EDC in DMF on thin Si after 35min thermal annealing C1s 284.45 1 3.272 · 103 89.168

O1s 532.77 2.8 9.207 · 102 8.962

N1s 400.29 1.73 1.187 · 102 1.87

F MWNT, EDC in DMF on thin Si after 70min thermal annealing C1s 284.45 1 3.333 · 103 88.985

O1s 532.82 2.8 9.397 · 102 8.96

N1s 400.21 1.73 1.331 · 102 2.054

F MWNT, EDC in DMF on thin Si after 100min thermal annealing C1s 284.51 1 3.232 · 103 89.687

O1s 532.96 2.8 8.654 · 102 8.576

N1s 400.29 1.73 1.083 · 102 1.737

F MWNT, EDC in DMF on thin Si after 135min thermal annealing C1s 284.51 1 1.579 · 103 80.117

O1s 532.96 2.8 8.756 · 102 15.868

N1s 400.37 1.73 1.369 · 102 4.016

158 T.I.T. Okpalugo et al. / Carbon 43 (2005) 153–161

(288.39–289.54eV), O–COO (289–291.6eV), CHF

(287.8–290.2eV); O@C (531.65–531.94eV), O–C

(533.30–533.80eV); NH2 (400.23–400. 44eV). These

bonds and their binding energy; the functionalisation

techniques employed, the resulting chemical shift, as

well as the XPS spectrometer details are compared with

those reported in the literature (Table 3 [9,11,15–21]).

Our results are consistent with these publications inthe literature.

Fig. 2a below is an example of the XPS survey scan to

show elements present in the CNT sample. This was ta-

ken for all the samples prior to individual elemental

scans (C1s, O1s, N1s, etc.). The detected elements are

labelled as shown and the impurities are insignificant

and barely above noise level. Fig. 2b shows that the

C1s peak for the functionalised short multi-walled

nanotubes in methanol shows a prominent raised bumpat 289.26eV similar to that reported in the literature

86

88

90

92

94

96

98

100

0 5 10 15 20 25 30

Argon sputter time (minutes)

C %

at.

conc

. (%

)

0

1

2

3

4

5

6

7

8

9

O %

at.

conc

. (%

)C1s

O1s

Fig. 3. XPS C1s and O1s atomic % composition of functionalised

(–COOH) MWNT in DMF at different argon sputter time.

280 285 290 295 3000

2000

4000

6000

8000

Cou

nts

(/s)

Binding energy (eV)

sp2 C=C/sp3 C- C

-C-O

>C=O

p-p*-COO

Fig. 1. An example of XPS C1s peak deconvolution of MWCNT and

the assigned bonds.

0 200 400 600 800 1000 1200 1400

Cou

nts

(/s)

Binding energy (eV)

F-MWNT in methanolC1S

O1S O-augerC-auger

NF-MWNT in methanol

C1S

O1S O-augerC-auger

280 290 300

Cou

nts

(/s)

Binding energy (eV)

Functionalised MWNT in methanol

Functionalised short MWNT in methanol

(a)

(b)

Fig. 2. (a) Survey scan of non-functionalised (NF) and functionalised

(F) MWCNT in methanol. (b) XPS C1s peak of short MWCNT

treated in H2SO4:HNO3 and the functionalised MWCNT treated in

HNO3.

T.I.T. Okpalugo et al. / Carbon 43 (2005) 153–161 159

[10]. This is likely a carboxylate, O–COO– functional

group which is likely to be responsible for the discussed

chemical shift associated with the short/cut functional-

ised samples. However, this does not seem to be the case

with the C1s spectra for the uncut functionalised multi-

walled nanotubes in methanol (Fig. 2b).

Lee et al. [11] reported that the C1s peak for un-

treated SWCNTs and treated SWCNTs with dichloro-carbene lies at 285eV with a higher value for the full

width at half maximum (FWHM) for the functionalised

nanotubes compared to the non-functionalised nano-

tubes. Increased oxidation and/or functionalisation

could be associated with increased disorder in the

CNT structure and thus increased spectral line width.

Increased FWHM with increased functionalisationwere not consistently observed to be the case in this

XPS analysis as this may be more relevant to a chemical

shift than disorder in an XPS analysis. However, our

preliminary Raman analysis seems to indicate an in-

creased disorder, a pronouncement of the second order

dispersive disorder peaks (2D) and its third order com-

bination frequency (2D + E2g) following functionalisa-

tion with nitric acid.

3.1. Ar+ ions bombardment

Argon sputtering was carried out on the functional-

ised MWCNTs, EDC in DMF (Table 4, Fig. 4). It

was observed that the carbon percentage increased as

the samples were sputtered at 5min intervals. The most

noticeable change was the oxygen percentage decreasefrom 6% to 1.239% after 35min of sputtering (Fig.

4a). The N1s peak generally increased in atomic per-

centage over the 35min (Fig. 4b), though it seemed to

fluctuate, which may suggest that as the sputtering time

increased the nitrogen atoms were being released gradu-

ally from the bulk of the sample. The functionalised

multi-walled nanotubes in DMF sample was also sput-

tered for 30min at 5min intervals (Fig. 3, Table 4).The increase in C1s was more noticeable (Fig. 3) than

the above sputtered sample. It increased from 90.3%

to 97.035%. The O1s peak showed a noticeable decrease

also (Fig. 3, Table 4). It changed from 8.156% to 1.813%

at 25min and increased to 2.965% at 30 min sputtering.

The reason for the increase at the end could be that the

sputtering had invoked some of the silicon oxide (SiO2)

layer of the silicon substrate below the layer of function-alised nanotubes and this increase in oxygen could

be due to the oxygen layer on the substrates surface.

Thus an accurate measurement of the sputtering rate

88

89

90

91

92

93

94

95

96

0 5 10 15 25 35Ar sputtering time (s)

C a

tom

ic %

(%)

0

1

2

3

4

5

6

7

O a

tom

ic %

(%)C

O

88

89

90

91

92

93

94

95

96

0 5 10 15 25 35

Ar sputtering time (minutes)

C a

tom

ic %

(%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

N a

tom

ic %

(%)

C

N

0

1

2

3

4

5

6

7

0 5 10 15 25 35

Ar sputter time (minutes)

O a

tom

ic %

(%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

N a

tom

ic %

(%)

O

N

(a)

(b)

(c)

Fig. 4. XPS C1s and O1s atomic % composition (a), C1s and N1s (b),

O1s and N1s (c) of F MWNT, EDC in DMF on Si wafer at different

argon sputter time.

78

80

82

84

86

88

90

92

0 35 70 100 135

Thermal annealing time (minutes)

C (A

t. %

)

4

6

8

10

12

14

16

18

O (A

t. %

)

80

82

84

86

88

90

92

C (A

t. %

)

1.5

2

2.5

3

3.5

4

4.5

N (A

t. %

)

C

O

C

N

(a)

160 T.I.T. Okpalugo et al. / Carbon 43 (2005) 153–161

calculated over the time period in this way could indi-

cate the thickness of the deposited CNTs.

The argon sputtering results detailed above show the

same trend as those results discussed in the literature

[11]. The C1s peak increased at every 5min interval of

sputtering and the oxygen peak decreased at every5min interval. These results give evidence to the fact

that oxygen atoms binds to the nanotubes at functional-

isation process. A lower energy of 2keV used during the

argon sputtering in this study led to a systematic change

in the functional groups compared to the use of an inci-

dent ion gun at 3.5keV where the percentage of C1s and

O1s decreased dramatically [11].

780 35 70 100 135

Thermal annealing time (minutes)

1

(b)

Fig. 5. XPS of functionalised MWCNT, EDC in DMF before and

after thermal annealing (�200�C), C1s and O1s atomic % composi-

tion (a), and C1s and N1s atomic % (b).

3.2. Comparison of results of CNT dispersed in DMF

and methanol

It would appear that the solvent in which the nano-

tubes are dispersed has an influence on the percentage

composition of elements present in the sample. The

functionalised multi-walled nanotubes treated in

HNO3 on the bucky paper (Table 2) show a carbon per-

centage of 92.013% and an oxygen percentage of

7.987%. For the functionalised multi-walled nanotube

in methanol, the carbon percentage is 85.685% and theoxygen percentage is 14.315%, which is different from

the original sample of carbon nanotubes on bucky paper

before being dispersed in methanol. The functionalised

multi-walled nanotubes in DMF samples have a carbon

percentage of 90.3% and an oxygen percentage of

8.156%. N1s and F1s are also present in this sample

with percentages of 0.815% and 0.728% respectively.

These differences seem to suggest that the solvent inwhich the nanotubes are dispersed could affect the atom-

ic percent of the detected elements.

3.3. Effect of low temperature thermal annealing

The results for the functionalised multi-walled nano-

tubes, EDC in DMF un-annealed and annealed at

35min intervals at about 200 �C (Table 5, Fig. 5) showthat the C1s peak shifted from 284.59eV to a lower en-

ergy of 284.45 after 35min thermal annealing and to

284.51eV after 135min thermal annealing in total (Table

5, Fig. 5). This energy shift is considered insignificant

since the total energy resolution of the instrument at

the settings used is 0.35eV. The nitrogen atomic percent-

ages concentration fluctuated (Table 5, Fig. 5b).

T.I.T. Okpalugo et al. / Carbon 43 (2005) 153–161 161

Compared to the un-annealed sample the percentage

atomic concentration of oxygen seems to suddenly rise

after 135min of interrupted annealing (Table 5, Fig.

5a). The reason for this is not clear since the annealing

was carried out under vacuum, but it is possible that

thermal annealing at low temperature(s) (�200 �C, inthis study) can increase chemical reactions and re-organ-

ization leading to formation of more stable bonds. This

is compared to the fact that thermal annealing (UV irra-

diation from high pressure mercury lamp) could break

C–C, C–H bond to form C–O bonds in DLC [13],

though evolution of CO2, CHx and H2 is normally ex-

pected with annealing generally. It is also possible that

this could result from the thermal effusion of oxygenfrom the SiO2 layer on the silicon wafer substrate after

135min of thermal annealing. The annealing carried

out upon this sample was at intervals and the tempera-

ture could not be monitored accurately. It could be more

appropriate to carry out the thermal annealing under a

controlled steady temperature without interruption.

4. Conclusion

Non-functionalised CNTs, covalently bonded and

non-covalently bonded chemical functionalised

MWCNTs, SWCNTs, short CNTs dispersed in different

chemical solutions; argon sputtered and thermally an-

nealed have been extensively investigated with a high res-

olution XPS. Significant chemical shift was observed inshort CNTs where strong covalently bonded chemical

functional groups were detected. Non-covalent bonded

and weak covalent functional groups do not seem to lead

to the spectrometer resolvable chemical shift (±0.35eV).

The exact degree and nature of the functional groups

presented have further been expounded through selective

argon sputtering over different depths in the CNTs which

also revealed true and differential functionalisation pre-sented. Low temperature thermal annealing seems not

to lead to any significant change in the elemental atomic

percentage concentration or chemical shift. Moreover,

the method chosen for functionalisation and the chemi-

cal media in which the CNT is dispersed seems to affect

the ESCA results. An ab initio and semi-empirical com-

putational quantum chemical approach is underway and

could help to further tackle the raised questions.

Acknowledgments

We wish to acknowledge CCLRC for permission to

use the ESCA300 instrument and Dr. Daniel Law for

technical support.

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