Stable containment of radionuclides on the nanoscale by cut single-wall carbon nanotubes

Article abstract of DOI:10.1021/jp0456436

The physisorption of radiolabeled ¹²⁵I^- ions from aqueous solution and the Brunauer-Emmett-Teller (BET) surface areas of various carbonaceous materials [HiPco single-wall carbon nanotubes (SWNTs), F-SWNTs, cut SWNTs, charcoal, graphite, F-graphite and C₆₀] have been measured and compared. By far, cut SWNTs (mainly 20-50 nm lengths) displayed the largest surface area of the materials (1180 m²g^-1), being approximately double that of uncut SWNT and charcoal. At low concentrations of ¹²⁵I^-1, nearly all of the ¹²⁵I^- was adsorbed from aqueous solution within 1 min at room temperature by the cut SWNTs, uncut SWNTs, and charcoal; the other materials showed much less adsorption under the same conditions. Once adsorbed, the ¹²⁵I^- wash-off rate by pure water was highly variable but was especially slow for cut SWNTs (t1/2 a?? 2720 h) compared to the other materials; wash-off of ¹²⁵I^- by an aqueous H ₂U₂ solution (¹²⁵I^-H ₂O₂ ¹²⁵I₂) was even slower (t1/2 a?? 14 300 h). Taken together, these data demonstrate the greatly increased surface area and dramatically enhanced retention properties of cut SWNTs over uncut SWNTs. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0456436

5482
J. Phys. Chem. B 2005, 109, 5482-5484
Stable Containment of Radionuclides on the Nanoscale by Cut Single-Wall Carbon
Nanotubes
Yuri A. Mackeyev,† John W. Marks,‡ Michael G. Rosenblum,‡ and Lon J. Wilson*,†
Department of Chemistry, Center for Nanoscale Science and Technology, and Center for Biological and
EnVironmental Nanotechnology, MS-60, Rice UniVersity, 6100 Main Street, Houston, Texas 77251-1892, and
M. D. Anderson Cancer Center, The UniVersity of Texas, 1515 Holcombe BouleVard,
Houston, Texas 77030-4009
ReceiVed: September 24, 2004; In Final Form: NoVember 29, 2004
The physisorption of radiolabeled 125I- ions from aqueous solution and the Brunauer-Emmett-Teller (BET)
surface areas of various carbonaceous materials [HiPco single-wall carbon nanotubes (SWNTs), F-SWNTs,
cut SWNTs, charcoal, graphite, F-graphite and C60] have been measured and compared. By far, cut SWNTs
(mainly 20-50 nm lengths) displayed the largest surface area of the materials (1180 m2‚g-1), being
approximately double that of uncut SWNT and charcoal. At low concentrations of 125I-, nearly all of the
125I- was adsorbed from aqueous solution within 1 min at room temperature by the cut SWNTs, uncut SWNTs,
and charcoal; the other materials showed much less adsorption under the same conditions. Once adsorbed,
the 125I- wash-off rate by pure water was highly variable but was especially slow for cut SWNTs (t1/2 ≈ 2720
H2O2
h) compared to the other materials; wash-off of 125I- by an aqueous H2O2 solution (125I-
8 125I2) was even
slower (t1/2 ≈ 14 300 h). Taken together, these data demonstrate the greatly increased surface area and
dramatically enhanced retention properties of cut SWNTs over uncut SWNTs.
The surface area of single-wall carbon nanotubes (SWNTs)
has been of special interest recently because of the potential of
this material to store and transport H2 economically.1,2 In this
paper, we report that the surface area of pristine HiPco SWNTs
can be approximately doubled by “cutting” the material into
20-50 nm length via the fluorination/pyrolysis procedure
reported by Margrave and co-workers.3 In addition, the adsorp-
tion/desorption properties of these cut SWNTs and uncut
SWNTs have been further compared by use of adsorption/
desorption data for radiolabeled 125I- ions and 125I2 molecules
in aqueous solution.
The HiPco SWNTs used in this investigation were obtained
from Carbon Nanotechnologies, Inc. of Houston, Texas (iron
content ∼12%). Fluorination of the SWNTs was performed as
previously reported (on the same fluorination apparatus)3 at 100
°C for 2 h and at a He:F2 ratio of 99:1. The resulting fluorinated
nanotubes (F-SWNTs), with a composition between C8F and
C6F, were observed to slowly evolve HF (etching the sample
bottle) in air over several days, so only freshly prepared
F-SWNTs were used for the “cutting” procedure by pyrolysis
under argon at 1000 °C for 1 h. As reported by Margrave and
co-workers,3 this procedure results in cut SWNTs with lengths
ranging mainly between 20 and 50 nm.
average composition C2F) and C60 fullerene (MER Corporation,
99.5+ % purity). The results are summarized in Table 1.
As seen in Table 1, the measured BET surface area for cut
SWNTs of 1180 m2‚g-1 is approximately double that of uncut
SWNT (and charcoal). Our value of 675 m2‚g-1 for the surface
area of uncut SWNT is within the range of previous results in
the literature.4,5 It is tempting to speculate that the increase in
surface area from “cutting” SWNTs into shorter lengths arises
from greater access to the interior walls of the cut tubes because
of side-wall damage (and possibly end opening) during the
fluorination/pyrolysis procedure. Side-wall damage is clearly
evidenced by Raman spectroscopic data3, and it is reasonable
that such damage would create openings large enough for small
molecules (N2, F2, H2) to efficiently enter cut nanotubes.
Alternately, the cut SWNT may simply be debundled to a greater
extent than uncut nanotubes, and the increase in surface area
for the cut nanotubes is due to a larger available outside surface
area. However, we tend to favor the “side-wall damage”
explanation because (1) the 125I-/125I2 radiotracer study given
below demonstrates a greatly enhanced retention capability for
only the cut nanotube adsorbent in Table 1, (2) strongly
sonicated (at 500 W) and unsonicated samples gave identical
surface areas for cut and uncut SWNT samples, even though
sonication is known to debundle SWNTs,9,10 and (3) we have
been able to internally load I2 into cut SWNTs (up to 40% I2
by weight) either by sublimation or from a I2/CHCl3 solution.11,12
The surface areas of the cut and uncut SWNTs were then
measured at 77 K with a Micromeritics ASAP 2010 Brunauer-
Emmett-Teller (BET) surface analysis instrument that used N2
gas. For comparison, similar measurements were also performed
for F-SWNT, charcoal (Fisher Scientific), graphite (Aldrich
Chemical Co.), F-graphite (Aldrich Chemical Co.; white color,
An aqueous solution of Na125I (AmerSham) was used to
obtain a solution in the concentration range of 10-9-10-12
mol‚L-1. Each insoluble carbonaceous substance (1.0 mg) was
then added, with stirring, to 1 mL of HPLC-grade water
containing 2.79 × 10-14 mol of 125I-. In a typical adsorption
experiment, the carbonaceous material was removed by filtration
after 1 min and the amount of 125I- remaining in solution and
* Corresponding author: e-mail durango@rice.edu; phone 713-348-3268;
fax 713-348-5155.
† Rice University.
‡ The University of Texas.
10.1021/jp0456436 CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/05/2005

Stable Containment of Radionuclides by Cut SWNTs
J. Phys. Chem. B, Vol. 109, No. 12, 2005 5483
TABLE 1: BET Surface Areas for Carbonaceous Materials
adsorbent
surface area,a m2‚g-1
lit. value
ref
cut-SWNT
uncut SWNT
charcoal
F-SWNTc
F-graphite
graphite
1180
675
632
220
155
266-800
400-900b
4, 5
8
110
4.31-6.22
6
7
5.88
0.127
C60
a This work; max estimated error is 10%. b For most commercially
available brands. c Freshly prepared sample.
TABLE 2: Adsorption of 125I- from Aqueous Solution by
Carbonaceous Materials after 1 min at RT
adsorbent
125I- amount adsorbed,a %
cut-SWNT
charcoal
uncut SWNT
F-SWNTc
graphite
F-graphite
C60
99.7 ( 0.8b
98.4 ( 1.5b
95.2 ( 1.4b
67.4 ( 1.7b
6.51 ( 1.8d
1.96 ( 1.4d
1.05 ( 0.9d
Figure 2. Desorption rate of 125I- by pure water.
a Data for eight or more experiments for all except graphite,
F-graphite, and C60, which had three experiments. b Standard deviation
(σ) is 0.28% for cut SWNT, 0.49% for charcoal, and 0.46% for uncut
SWNT. The errors given are 3σ (95% confidence). c Freshly prepared
sample. d The errors given are the range of three experiments.
Figure 3. Desorption rate of 125I- by NaI solution.
Figure 1. Adsorption of 125I- from aqueous solution vs cut SWNT:
125I- molar ratio. To estimate the ratio, a cut SWNT length of 50 nm
with MW ) 115 000 was assumed.
the amount in the solid material were measured on a γ counter.
Cut SWNTs exhibited the greatest adsorption of 99.7% with
similar results for charcoal (98.4%) and uncut SWNTs (95.2%),
while the other carbonaceous materials adsorbed far less of the
radiotracer ion under the same conditions. The results are
summarized in Table 2. Adsorption levels were also found to
decrease as the ratio of adsorbent:125I- decreased, as displayed
in Figure 1 for the case of cut SWNTs.
After adsorption of 125I-, the carbonaceous materials (charcoal
and cut and uncut SWNTs) were placed on a 4 mm poly-
tetrafluoroethylene (PTFE) filter (Aldrich) and washed with
HPLC-grade water, an unlabeled aqueous NaI solution (10-2
mol‚L-1), or an aqueous H2O2 solution (10-3 mol‚L-1). In the
experiments, a 6 mL‚day-1 flow rate of each solution was used,
and the activities of the PTFE filters and portions of the wash
solutions were measured separately by a Packard Cobra-2, Auto-
Gamma counter at 20 and 40 min and 1, 2, 4, 24, and 48 h
intervals. From these data, desorption plots (time vs 125I- loss
Figure 4. Desorption rate of 125I- by H2O2 solution.
rate, which is calculated by dividing the washed-out fraction
of 125I- by time) were constructed as shown in Figures 2-4.
The lines in the figures are linear least-squares fits of the data
points and the error bars shown are the ranges obtained for four
data points taken at each time (days). The resulting half-lives,
t
1/2, for the desorption processes of Figures 2-4 are summarized
in Table 3.

5484 J. Phys. Chem. B, Vol. 109, No. 12, 2005
Mackeyev et al.
TABLE 3: Half-Lives for Desorption of 125I- by the
Acknowledgment. We thank Carbon Nanotechnologies, Inc.
for support of this work, and L.J.W. thanks the Robert A. Welch
Foundation for partial support as well. Finally, we dedicate this
contribution to the memory of Professor John L. Margrave who,
along with co-workers, first prepared cut SWNTs by the
fluorination procedure.
Aqueous Solutions at a 6 mL‚Day-1 Wash-off Rate
t1/2, ha,b
adsorbent
water
aq NaI soln
aqueous H2O2 soln
cut-SWNT
uncut SWNT
charcoal
2720
54
81
422
14
14
14300
147
309
References and Notes
a Each entry in Figures 2-4 is the average of four readings. b Max
estimated error is 8%: instrument error (5%) plus weighting error (3%).
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The rate of 125I- wash-off by the NaI solution was found to
be 4-7 times higher than for pure water, which can be explained
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in the desorption rate caused by a aqueous H2O2 solution is
attributed to the oxidation of I- to I2 by H2O2, with I2 having
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SWNT would be one of the first documented examples of the
chemical reaction occurring within the confines of a carbon
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materials. For example, charcoal lost 125I- approximately 30
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the desorption half-life of I2 in aqueous solution to 14 300 h or
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