# Apparent Enhanced Solubility of Single-Wall Carbon Nanotubes in a Deuterated Acid Mixture

Authors: T. Ramanathan; Frank T. Fisher; Rodney S. Ruoff; L. Catherine Brinson
Journal: Research Letters in Nanotechnology
 (2008)

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## Abstract

An apparent enhanced solubility of single-wall carbon nanotubes (SWNTs) in the deuterated form of the standard 3 : 1 sulfuric (H2SO4) to nitric (HNO3) acid mixture treatment is reported and attributed to the stronger interaction of deuterium bonds with the single-wall carbon nanotube surface. UV-Visible spectroscopy was used to characterize the apparent enhanced solubility of the SWNTs treated in deuterated forms of the acid mixture in comparison to the standard acid mix, while FTIR was used to analyze the nature of the functional groups generated on the SWNTs as a result of the different acid treatments. The apparent enhanced solubility reported here is consistent with the limited number of computational and experimental results published in the literature regarding the interaction of carbon nanotubes with deuterated solvents; however, a detailed understanding of the underlying mechanism responsible for this observation is currently lacking. The apparent increased solubility observed here could potentially be utilized in many applications where carbon nanotube dispersion is required.


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## Body

## 1. Introduction

Owing to their excellent electrical and mechanical
properties, single-wall carbon nanotubes (SWNTs) have been an area of intense research since their discovery in 1991 [1]
and a variety of potential applications have been proposed [2, 3]. Many of these applications will likely require chemicalmodification (functionalization) of the nanotube surface tofacilitate their integration
into more complex assemblies, and thus solubilization of SWNTs in a variety of
solvent systems has been a considerable research pursuit [4].
One common functionalization technique is treatment of the SWNTs in a 3 : 1 acid
mixture of concentrated sulfuric and nitric acid (which we will refer to as the H-acid mixture), which creates
defects on the nanotube surface from which carboxylic acid groups are attached [5, 6].
The solubility of thenanotubes after this standard acid treatment is typically
much greater than that for the pristine nanotubes. However, we have recently
observed that a similar treatment with the deuterated forms of this acid
mixture (3 : 1 ratio of D2SO4 : DNO3, which we will refer to as the D-acid mixture)
results in an ap-parent significant enhancement in the solubility of the SWNTs. This solubility enhancement is attributed to the strong affinity for deuterium interactions with the nanotube surface.


## 2. Experimental

The purified (>90%) SWNTs used for this study wereBuckyPearls (Carbon Nanotechnologies, Inc., Houston, TX). As previously
reported in the literature, the diameters of these SWNTs are in the range of
0.8–1.3 nm [7].
The sur-face properties of the as-received SWNTs have been studied using X-ray photoelectron
spectroscopy (XPS) and the re-sults were discussed elsewhere [8].
Deuterated forms (99% of isotopes) of sulfuric acid (D2SO4:
catalog number DLM-33-50 with concentration of 98% D2SO4 in
D2O) and nitric acid (DNO3: catalog number DLM-33-50 with
concentration of 65% DNO3 in D2O) were used in thisstudy. Additional chemicals used in the experiments were heavy water (D2O)
from Cambridge Isotope Laboratories (Andover, MA); H2SO4 (SA 123 with
concentration of 98% H2SO4 in H2O); and HNO3 (SA 95 with concentration of 65% HNO3 in H2O) from Fisher
Scientific (Hanover Park, IL). In thisstudy the acids were used as received
without furthermodification. The D-acid mixture (4 ml) was prepared in a vial
with a 3 : 1 ratio of D2SO4 to DNO3 to which 2 mg SWNTs (sample I) was added. A similar solution was prepared using H2SO4 and HNO3 (sample II) to which 2 mg of SWNTs were added and treated
as a control. In each case the vial was capped and sonicated in a bath sonicator at 350 W for 4 hours at 40°C.
As described below, the apparent enhanced solubility of SWNTs in the D-acid
mixture was highly re-producible and confirmed by repeating the experiment three
times on three separate occasions using the procedure de-scribed above.


## 3. Results and Discussion

After completing the sonication process, the solubility of
the SWNTs was found to be significantly enhanced in the D-acid mixture compared
to the solubility obtained from the standard (undeuterated acid) H-acid
mixture. This observed apparent enhancement of the solubility of SWNTs in the
D-acid mixture was highly reproducible and confirmed by repeating the
experiment three times. For further characterization of the enhanced solubility
of the D-acid mixture, the concentrated sample I was diluted to 10% solution
using heavy water and the D-acid mixture, while sample II was diluted in a
similar manner using both distilled water and the H-acid mixture. A photograph
of the resulting solutions is shown in Figure1, where vials A and D contain the D-acid mixture treated SWNTs (sample I) in heavy water (vial A) and in the D-acid mixture (vial D); vials B and C (sample II)
contain the H-acid mixture-treated nanotubes in distilled water (vial B) and in the H-acid mixture (vial C). It is clear that vials A and D (sample I) are more transparent when compared to the
corresponding vials containing sample II. Also noticeable is the lack of discernible aggregates in the vials prepared using sample I.Figure 1
Different SWNT treatments diluted to 10% solutions. (a) D-acid mixture inD2O; (b) H-acid mixture in distilled water; (c) H-acid mixture in H-acid mixture; (d) D-acid mixture in D-acid mixture.Each of the 10% solutions shown in Figure1 was then
centrifuged (IEC Medispin, Needham Heights, MA) at 12000 rpm for 30 minutes.
Significant SWNT precipitation collected at the bottom of the centrifuge
microtubes (0.6 ml, SciMart, St. Louis, MO) for the H-acid mixture-treatedsolutions (Vials B and C) as shown in Figure 2. However, for
the case of SWNTs treated with the D-acid mixture significantly less precipitate
was found for the solution diluted in D2O (Vial A), and no precipitate is observed for the solution diluted in the
deuterated acid mixture (Vial D).
Figures 1 and 2 demonstrate the apparent increased solubilization of SWNTs
treated with the D-acid mixture.Figure 2
Precipitate after centrifugation for different SWNT
treatments dispersed in solution. (a)
D-acid mixture inD2O; (b)
H-acid mixture in distilled water; (c)
H-acid mixture in H-acid mixture; (d)
D-acid mixture in D-acid mixture.Further confirmation of this apparent increased
solubility was sought via analysis of the UV-Visible spectra for sample I
(Figure3(a)) and sample II (Figure 3(b)) using a double-beam UV-Vis-NIR
spectrophotometer (Cary 500, Varian, Palo Alto, CA). Figure 3(a) shows the
UV-Vis spectra obtained for the 10% solution of sample I in the D-acid mixture
(Figure 1, Vial D), the supernatant
after the centrifuge step (Figure 2, Vial D),
and the pure D-acid mixture; corresponding data is shown for the H-acid mixture-treated
SWNTs (sample II) in Figure 3(b). Figure 3(a) shows strong absorption between
325 and 400 nm for both the 10% solution and supernatant for the case of the
D-acid mixture treatment, with no noticeable absorption for the pure D-acid
mixture. However, for the case of sample II (Figure 3(b)), only a weak peak is
shown within this wavelength range for the 10% solution in the H-acid mixture,
with no detectable absorption observed for the supernatant. This demonstrates
that after sonication, the D-acid mixture-treated SWNTs are very finely
dispersed [9].
For comparison, the dispersion of sample II is not as fine, such that the
conglomeration of the SWNTs within the H-acid solution is reflected by the poor
absorption of these samples.UV-Vis spectrum of (a) D-acid mixture sample, (b) H-acid mixture sample.
(a)(b)Finally, FTIR spectra (BIO-RAD FTIR FTS 60, Hercules,
CA) were collected for the sample I precipitate diluted inD2O and
the sample II precipitate diluted in distilled water after each sample was
rinsed well with D2O and distilled water, respectively, and
centrifuged at 12000 rpm for 30 minutes. The precipitates were collected and
dried at 80°C under vacuum overnight prior to the FTIR analysis. For FTIR, the
SWNT samples were grounded well with KBr and the sample was made into a pellet
form using a hydraulic press. The FTIR spectrum was recorded using these
pellets. The FTIR spectrum for the D-acid mixture-treated sample I precipitate
(Figure 4(a)) displays bands at ~1732 cm−1 that may be due to the
presence of C=O from carboxylic acid groups, various bands between 1000 and
1185 cm−1 consistent with C–O–C bonds and C–O stretching
frequencies, and bands between 400 and 900 cm−1 which likely
represent aromatic rings [10].
The intermediate bands between 1000 and 1185 cm−1 are also in the
range of the C–D bending mode [11].
For the H-acid samples shown in Figure 4(b), the band at 3430 cm−1 represents the stretching frequency of –O–H groups, while
the bands at 1732 and 1650 cm−1 are attributed to the C=O bonds in
saturated and aromatic carboxylic acid. As expected, when sample I was diluted
with D2O, the –O–H stretching was
not present at ~3400 cm−1.FTIR spectra of (a) D-acid mixture sample, (b) H-acid mixture sample.
(a)(b)The strong presence of C–O, C–O–C and aromatic carbon
in the sample I spectra may be caused by a larger number of defects on the
nanotube sidewall resulting from the stronger interaction of heavy water (D2O)
in the D-acid mixture with the SWNTs. Such a hypothesis is consistent with ab initio results discussed in the
literature which suggest that a D-bond is stronger than an H-bond based on
their respective binding energies [12].
In addition, a recent computational study of water and single-layer graphite
found sufficiently large binding energies that are believed to be important in
the interaction of water with carbon nanotubes [13, 14].
We hypothesize that water (D2O and H2O, resp.) in
the D-acid and H-acid mixtures interacts with the carbon nanotube walls. In the
case of the D-acid mixture, a combination of the higher binding energy of (i)
DOD in D-acid mixture and (ii) further dilution of the concentrated solution
into heavy water and D-acid mixture (also containing D2O) results in
a stronger interaction with the SWNTs, resulting in the apparent increased
solubility for the D-acid treatment observed here. Other researchers have also
reported that strong C–D interactions are responsible for deuterium attachment
to carbon nanotubes [11]. Thus our preliminary conclusion is that the
apparent enhanced solubility of the SWNTs subjected to the D-acid treatment is
due to stronger interaction of D2O with the SWNTs in comparison to H2O
available in the H-acid treatment.In summary,
we have found that the treatment of as-received single-wall carbon nanotubes
with the deuterated form of the 3 : 1 sulfuric to nitric acid treatment results in
an apparent solubility enhancement of the nanotubes. This observation is
consistent with the limited number of computational and experimental results
published in the literature regarding the interaction of carbon nanotubes with
deuterated solvents; however, a detailed understanding of the underlying
mechanism responsible for this observation is currently lacking. This reported experimental observation regarding the interaction
of carbon nanotubes with different forms of chemical treatments should be
further investigated both experimentally and theoretically. In particular, at
the moment one cannot rule out the possibility that the D-acid treatment
results in significantly greater levels of damage to the structure of the SWNT,
which in effect could lead to fragments of oxygenated polyaromatic hydrocarbons
having similar UV-Vis and IR spectra as the oxidized SWNTs. In any case, the
apparent increased solubility of the D-acid treated SWNTs (or, alternatively,
the large-scale structural damage resulting from the D-acid treatment) is of
both scientific and technological interest. The increased solubility observed
here could be utilized in many applications where carbon nanotube dispersion is
required.



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*Source: 296928-2008-04-30.xml*