Acidic Site Determination

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Determination of the acidic sites of puried single-walled

carbon nanotubes by acidbase titration

H. Hu, P. Bhowmik, B. Zhao, M.A. Hamon, M.E. Itkis, R.C. Haddon *

Departments of Chemistry and Chemical and Environmental Engineering, University of California, Riverside, CA 92521-0403, USA

Received 17 March 2001; in nal form 13 July 2001

Abstract

We report the measurement of the acidic sites in three dierent samples of commercially available full-length puried

single-walled carbon nanotubes (SWNTs) as obtained from CarboLex (CLI), Carbon Solutions (CSI) and

Tubes@Rice (TAR) by simple acidbase titration methods. Titration of the puried SWNTs with NaOH and

NaHCO3 solutions was used to determine the total percentage of acidic sites and carboxylic acid groups, respectively.

The total percentage of acidic sites in full length puried SWNTs from TAR, CLI and CSI are about 13%. 2001

Published by Elsevier Science B.V.

1. Introduction

Single-walled carbon nanotubes (SWNTs) have

attracted attention due to their unique electronic

and mechanical properties [1]. To fully realize

these properties it is necessary to chemically pro-

cess the SWNTs in order to purify current prepa-

rations, and to bring about appropriate

functionalization. Most purications of SWNTs

make use of oxidative methods that typically in-

volve treatment with nitric acid [24]. These re-

agents remove the caps of the SWNTs and

introduce defects involving oxygen functionalities

mainly in the forms of carboxylic acid groups on

the surface and at the ends of the SWNTs [57]. It

is important to determine the concentration of

these defects, since they not only change the

properties of SWNTs, but also serve as reactive

sites for SWNT dissolution chemistry [8,9]. Previ-ous determinations of the density of the surface

defect sites of SWNTs have been reported. The

evolution of CO2 (g) and CO (g) from puried

SWNTs heated above 1200 K has been measured

[7], and used to estimate a concentration of $5at.% defective carbon atoms in puried SWNTs.

Calibrated energy dispersive X-ray spectroscopy

indicated $4 at.% oxygen atoms in puriedSWNTs [10].

Acidbase titration methods have been used to

determine the acidic and basic characters of the

surfaces of a wide variety of carbon materials [11

14], including multi-walled carbon nanotubes

(MWNTs) [1517]. The reported density of surface

acidic groups in the MWNTs treated with nitric

acid is in the range 0.20.5 at.% [15,17]. The total

acidic sites (including carboxylic acids, lactones

and phenols), can be determined by titration with

NaOH, while the carboxylic acid groups can be

individually determined by titration with

NaHCO3. In this Letter we report the measure-

7 September 2001

Chemical Physics Letters 345 (2001) 2528

www.elsevier.com/locate/cplett

* Corresponding author.

E-mail address: [email protected] (R.C. Haddon).

0009-2614/01/$ - see front matter

2001 Published by Elsevier Science B.V.PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 8 5 1 - X


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ment of the acidic sites in full-length nitric acid

puried SWNTs, by use of acidbase titration

methods.

2. Experimental

We examined three dierent kinds of full-length

puried (P) SWNT samples by titiration: Carbo-

Lex, Inc. (CLI)-P, Carbon Solutions, Inc (CSI)-P,

and Tubes @Rice (TAR)-P. According to com-

pany literature, all of the samples were rst sub-

jected to a nitric acid treatment to remove catalyst,

amorphous carbon and to begin to break up the

nanoparticulate impurities. TAR-P SWNTs (pu-

rity $90%) are from laser-produced SWNTs, [18]while the CLI-P (purity 6080%) and CSI-P

SWNTs were produced in a modied electric arc

method [19]. In addition, we studied a low func-

tionality form of full-length puried SWNTs (CSI-

P-LF), in which CSI-P material had been further

processed to reduce the oxygen functionality.

The SWNT samples were rst washed with de-

ionized water to remove any acidic residues left in

the samples. The air-dried samples were then he-

ated at 100 C for 0.5 h under vacuum to degas

absorbed carbon dioxide and water. After thispretreatment, the SWNT samples were ready for

the titration experiments. The titration procedure

is exemplied below for the sodium bicarbonate

analysis of carboxylic groups in the SWNT

(SWNTCOOH).

Forward titration

SWNTCOOH NaHCO3 excess

3 SWNTCOONa

NaHCO3 determined by titration

Backward titration

SWNTCOONa HCl excess

3 SWNTCOOH

HCl determined by titration

In a typical experiment, 98.86 mg of TAR-P

SWNTs was stirred in 50.00 ml of 0.05 N NaHCO3aqueous solution under argon for 48 h. Prolonged

stirring was required to allow the solid SWNTs to

equilibrate with the NaHCO3 solution. The mix-

ture was then ltered through a membrane (pore

size of 1.2 lm). The SWNTs collected on the

membrane were washed with de-ionized water to

remove all of the NaHCO3 residues. The combined

ltrate and washings were added to 50.00 ml of

0.05 N aqueous HCl solution and boiled for 20

min to degas the CO2 from the solution. After

cooling to room temperature, the excess HCl in the

solution was titrated with 2.20 ml of 0.05 N

aqueous NaOH solution to reach the neutral point

(pH 7.00), as monitored by a pH meter (Corning

pH meter 445). By calculation, the amount of

SWNTCOOH groups in the SWNTs is 0.143

mmol. The amount of carbon in the SWNTs is

estimated to be 8.231 mmol by assuming that theSWNTs are solely composed of carbon. The mole

percentage of the SWNTCOOH groups in the

TAR-P SWNT sample is 1.7%.

We also back titrated, in order to ensure the

reliability of the results (see above). From the for-

ward titration we collected 60.92 mg of the

NaHCO3-treated SWNTs, now in the form of the

conjugate base, SWNTCOONa. After drying

and weighing the sample, it was soaked in 50.00 ml

of 0.05 N aqueous HCl solution for 48 h with

stirring under argon to re-acidify the conjugatebase of the SWNTCOOH. The mixture was l-

tered through a membrane and the solid washed

with de-ionized water to remove residual HCl from

the sample. The combined ltrate and washings

were neutralized (pH 7.00), with 48.50 ml of 0.05

N NaOH. By a similar calculation to that given

above, the percentage of the SWNTCOONa

groups to the total amount of carbon is 1.5% in the

NaHCO3-treated SWNTs, which is roughly con-

sistent with the forward titration.

To determine the total percentage of acidic sites

in the SWNTs, 0.05 N NaOH was used instead of

NaHCO3 in the forward titrations.

3. Results and discussion

The mole percentages of acidic sites in the

original puried SWNT samples together with the

mole percentages of basic sites in the conjugate

base SWNT samples are summarized in Table 1.

26 H. Hu et al. / Chemical Physics Letters 345 (2001) 2528


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The dierent commercial samples give fairly con-

sistent results: the SWNTCOOH functionality is

about 12% and the total SWNT acid functional-

ity is about 13%.

The TAR-P SWNTs gave very close values forthe total acidic sites and for the carboxylic acid

groups; thus virtually all of the acidic sites in this

sample are carboxylic acid groups.

A number of the samples gave a higher acidity

count in the forward direction (leading to, for

example, SWNTCOONa), than in the back-

ward step (to produce SWNTCOOH). We at-

tribute this to a partial purication of the SWNTs

by basic extraction, in the forward titration. We

experience mechanical losses in each step, due to

the ltration process and re-isolation of theSWNTs, so this cannot be quantied but it seems

reasonable that some of the carboxylated carbons

[3] are taken into aqueous solution and removed

by base in the forward titrations. This is particu-

larly obvious with the CLI-P sample, in which the

percentage of total acid sites was rst measured to

be 6.9%. During the backward titration, the total

basic site percentage of the NaOH treated SWNTs

decreased to 2.0%. When we repeated the forward

titration of the total acidic sites in this re-acidied

sample, the total acidity remained constant at

2.0%.

4. Conclusion

We report a quantitative study of the acidic

sites in full-length puried SWNTs from the three

dierent commercial suppliers using acidbase ti-

tration methods. This approach provides a simple

and inexpensive method for the routine evaluation

of the acidic surface defects in puried SWNT

samples. The dierent commercial samples give

fairly consistent results: the SWNTCOOH func-

tionality is about 12% and the total SWNT acid

functionality is about 13%.

Acknowledgements

This work was supported by the MRSEC Pro-

gram of the National Science Foundation under

Award Number DMR-9809686 and by the oce

of Naval Research under Award Number N00014-

99-1-0770.

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Table 1

Acidity of puried SWNTs

Sample SWNTCOOH titration

(%)

Total SWNTacid titration

(%)

Forward Backward Forward Backward

TAR-P 1.7 1.5 1.5 1.5

CLI-P 2.0 1.6 6.9 (2.0)a 2.0

CSI-P 2.2 2.1 3.2 2.7

CSI-P-LF 0.7 0.8 2.2 1.0

a Repeat result after recycling sample.

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Acidic Site Determination