U.S. patent application number 10/523397 was filed with the patent office on 2005-07-28 for method for opening carbon nanotubes at the ends thereof and implementation.
Invention is credited to Beguin, Francois, Delpeux, Sandrine, Szostak, Katarzyna.
Application Number | 20050163697 10/523397 |
Document ID | / |
Family ID | 30471027 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050163697 |
Kind Code |
A1 |
Beguin, Francois ; et
al. |
July 28, 2005 |
Method for opening carbon nanotubes at the ends thereof and
implementation
Abstract
The invention relates to an efficient and non-damaging method
for opening carbon nanotubes, characterized in that it consists of
two oxidation stages, the first in liquid phase in a concentrated
acid, the second in gaseous phase.
Inventors: |
Beguin, Francois; (Orleans,
FR) ; Delpeux, Sandrine; (Chateauneuf/Loire, FR)
; Szostak, Katarzyna; (Poznan, PL) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Family ID: |
30471027 |
Appl. No.: |
10/523397 |
Filed: |
April 8, 2005 |
PCT Filed: |
August 8, 2003 |
PCT NO: |
PCT/FR03/02499 |
Current U.S.
Class: |
423/447.6 |
Current CPC
Class: |
B82Y 40/00 20130101;
B82Y 30/00 20130101; C01B 2202/36 20130101; C01B 32/178 20170801;
C01B 2202/06 20130101 |
Class at
Publication: |
423/447.6 |
International
Class: |
D01F 009/12 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 8, 2002 |
FR |
02101155 |
Claims
1. A method for opening carbon nanotubes comprising a first
oxidation stage of said nanotubes in a liquid phase in a
concentrated acid and a second oxidation stage of said nanotubes in
gaseous phase.
2. The method of claim 1, wherein the carbon nanotubes are
multiwall carbon nanotubes.
3. The method according to claim 2, wherein the concentrated acid
is nitric acid.
4. The method according to claim 3, wherein said first oxidation
comprises contacting said nanotubes and nitric acid in the
following proportions: 1 g of carbon nanotubes in 0.5 litres to 2
litres of concentrated nitric acid at 60-75% by weight.
5. The method according to claim 2 wherein said first oxidation
stage further comprises heating at reflux, under stirring.
6. The method according to claim 1, wherein said second oxidation
stage is performed in the presence of carbon dioxide at low
temperature.
7. The method according to claim 6, wherein said second oxidation
stage is performed in the presence of carbon dioxide at 500 to
600.degree. C., for 1 to 2 hours.
8. The method of claim 1, further comprising, between said first
oxidation stage and said second oxidation stage, an intermediate
stage of filtration and washing of said nanotubes.
9. (canceled)
10. Opened carbon nanotubes produced by the method of claim 1.
11. The method of claim 3 wherein said acid is combined with said
nanotubes in an excess amount.
12. The method of claim 4 wherein said nitric acid is combined in
amount of 1 litre at a concentration on the order of 68-70% by
weight.
13. The method of claim 7 wherein said second oxidation stage is
performed in the presence of carbon dioxide at 500 to 550.degree.
C., for 1 hour to 1 hour 40 minutes.
14. The method of claim 10 wherein said nanotubes comprise a
central channel which is filled with at least one conductive
species.
Description
[0001] The present invention relates, in a general way, to the post
treatment of carbon nanotubes and their applications. In
particular, the present invention is aimed at a method for opening
carbon nanotubes at the ends thereof and more specifically
multiwall carbon nanotubes.
[0002] Most methods of synthesis produce carbon nanotubes with
closed ends which can, for example, cause the inclusion of
impurities originating from the reaction medium in the central
channel of the nanotube. This occurs, in particular, during the
catalytic syntheses of carbon nanotubes. Moreover, when the
nanotubes are initially open, they can also close again during
post-treatments at high temperature.
[0003] The benefit of having open carbon nanotubes is firstly the
possibility of filling their central channel with numerous species
in particular conductive species (metals, conductive polymers etc.)
so as to produce conductive nanowires for applications in
nanoelectronics. The filled carbon nanotubes also prove to be of
increasing interest in catalytic applications, and for storing
energy. Moreover, hollow carbon nanotubes can prove to be excellent
reservoirs for gas, such as hydrogen, natural gas, etc.
[0004] It is now well known that the presence of topological faults
is necessary in order to close the graphene planes at the ends of
the carbon nanotubes. According to Euler's law, six pentagons are
necessary to ensure the closing of the carbon nanotubes at each
end. These regions of tension are of course the most useful sites
for the addition reactions, in particular on the doubles bonds
connecting a pair of pentagons.
[0005] Among the methods proposed for opening nanotubes, the
following are mentioned: chemical oxidation by strong oxidizing
agents in liquid phase (nitric acid, sulphuric acid or a mixture of
these two acids, potassium permanganate, etc.), the reactions in
gaseous phase under an air flow at temperatures varying from
500.degree. C. to 700.degree. C. and recently, impact grinding in
particular in order to cut and shorten the nanotubes or also
sonication.
[0006] Oxidation under air or oxygen is not as selective. These
treatments lead to a significant loss of material and the external
graphene planes are often seriously damaged due to the
uncontrollable nature of the reaction.
[0007] Other documents have recommended using CO.sub.2 at
850.degree. C. but at such temperatures, which are close to the
conditions generally used for activating carbonaceous materials,
the yields of open nanotubes are very low, the mass loss is very
significant and the outer layers of graphene are badly damaged.
[0008] The oxidation is much more homogeneous when the carbon
nanotubes are dispersed in an oxidizing solution. For example, the
carbon nanotubes obtained by decomposition of acetylene at
600.degree. C. on cobalt particles supported by zeolites often
contain carbonaceous impurities and have closed ends. It is thus
possible to carry out an attack with potassium permanganate both in
order to partially eliminate these impurities by oxidation and to
open a part of the ends of the carbon nanotubes.
[0009] However, once again, the results in terms of efficiency and
selectivity prove to be clearly inadequate.
[0010] The inventors have found that these drawbacks could be
overcome by subjecting nanotubes to two distinct oxidation stages,
carried out under specific conditions.
[0011] The purpose of the invention is thus to provide a method
allowing the opening of carbon nanotubes to be achieved rapidly and
effectively, while preserving their morphology, their quality, and
with reduced losses.
[0012] Thus, the method for opening carbon nanotubes according to
the invention, is characterized in that it comprises two oxidation
stages, the first in liquid phase in a concentrated acid, the
second in gaseous phase.
[0013] The oxidation stage in liquid phase thus allows open
nanotubes to be obtained directly. Moreover, this provides the
advantage of making most of the residual metal impurities trapped
at the ends accessible, for example following syntheses carried out
in the presence of a catalyst.
[0014] The random carbon appearing during the oxidation reaction in
liquid phase is eliminated during the course of the second stage in
gaseous phase.
[0015] Advantageously, the carbon nanotubes are multiwall carbon
nanotubes.
[0016] More particularly, the concentrated acid is nitric acid.
[0017] Preferably, the concentrated nitric acid is used in
excess.
[0018] Satisfactory results are thus obtained with 1 g of carbon
nanotubes in 0.5 litres to 2 litres of concentrated HNO.sub.3, in
particular HNO.sub.3 at 60%-75% by weight, in particular 1 litre of
nitric acid at a concentration of the order of 68-70% by
weight.
[0019] According to a particular embodiment of the invention, this
oxidation stage is carried out at reflux, under stirring.
[0020] Advantageously, the heating at reflux will last from 30 to
50 minutes, in particular approximately 35 minutes.
[0021] For the purposes of purification, an additional oxidation
stage in gaseous phase is carried out, at low temperature.
[0022] It is more particularly this stage which allows elimination
by controlled oxidation of the random carbonaceous structures
originating from the opening of the ends of the carbon nanotubes
during the stage of opening by oxidation in liquid phase.
[0023] Advantageously, a particular embodiment of this stage
consists of a treatment of approximately 1 to 2 hours, in
particular under CO.sub.2 at 500 to 600.degree. C., in particular
from 500 to 550.degree. C. and particularly 525.degree. C., for 1
hour to 1 hour 40 minutes.
[0024] More particularly also, the method according to the
invention is used with a linear velocity of said carbon dioxide of
40 to 100 cm/min, in particular of 50 to 70 cm/min, in particular
of the order of 60 cm/min.
[0025] Advantageously, the method according to the invention
comprises between said first oxidation stage in liquid phase and
said second oxidation stage in gaseous phase, an intermediate stage
of filtration and washing of the open nanotubes, in particular with
distilled water. The method according to the invention can comprise
an additional stage of treatment with hydrochloric acid in order to
eliminate any metallic particles, initially trapped in the central
channel, and released during the opening of the nanotubes.
[0026] The implementation of the above provisions, combining a
reaction in liquid phase followed of a reaction in gaseous phase,
allows yields of open nanotubes of at least 90% to be obtained,
without deterioration of the surface of the nanotubes and the
purity which remains at levels higher than 97%.
[0027] The effectiveness of the invention will be better understood
on reading the following detailed example with reference to the
figures in which:
[0028] FIG. 1 represents an image obtained by scanning electron
microscopy (SEM) of carbon nanotubes after a HNO.sub.3+CO.sub.2
treatment according to the invention,
[0029] FIG. 2 represents a picture obtained by transmission
electron microscopy (TEM) of carbon nanotubes after a
HNO.sub.3+CO.sub.2 treatment according to the invention,
[0030] FIG. 3 represents a TEM picture (fringe mode of a CO.sub.2
network) of an open end of a carbon nanotube after a treatment
according to the method of the invention, and
[0031] FIG. 4 represents adsorption-desorption isotherms of
nitrogen at 77K of the carbon nanotubes before (full thick curve)
and after implementation of the method according to the invention
(dotted curve).
[0032] The method of the invention was optimized on multiwall
carbon nanotubes synthesized by decomposition of acetylene at
600.degree. C. on solid solutions of Co.sub.xMg.sub.(1-x)O.
[0033] During a first stage, the carbon nanotubes are dispersed in
the concentrated nitric acid and oxidized at reflux (130.degree.
C.) for 35 minutes under continuous stirring (1 g of nanotubes in 1
litre of acid at 69% by weight). Then, the mixture is filtered,
then the solid is washed with distilled water until a neutral pH of
the filtrate is obtained. This first oxidation stage allows the
opening of the tubes.
[0034] Then a gentle oxidation is carried out using a current of
CO.sub.2 at low temperature. This reaction is based on the
Boudouard reaction (C+CO.sub.2.fwdarw.2CO (.DELTA.H=+159
kJ/mole).
[0035] Carbon nanotube powder is placed in a quartz crucible
equipped with a porous sintered glass disc allowing a rising flow
of CO.sub.2 to be introduced, at a linear velocity rate of 60
cm/min, at 525.degree. C.
[0036] The reaction is carried out for approximately 60 to 100
minutes. A selective oxidation of the random carbon nanostructures
which are produced during the first oxidation reaction is
obtained.
[0037] The accumulated mass loss remains less than 50%.
[0038] The use of a scanning electron microscope (Hitachi S 4200)
allows the quality of the samples of nanotubes to be evaluated
(FIG. 1).
[0039] Observation by TEM at 200 kV (Philips CM20) shows the
effectiveness of this method with regard to the opening of the
nanotubes at the ends (FIGS. 2 and 3). For this observation, the
samples are subjected to a sonication in anhydrous ethanol and a
droplet is placed on a copper grid covered with a carbon film.
[0040] The porous texture of the carbon nanotubes is characterized
by the adsorption of nitrogen at 77.degree. K (Micrometrics, ASAP
2000). Before the adsorption experiments, the samples are degassed
at 350.degree. C. (10.sup.-6 mbar) for 12 hours.
[0041] After the opening, another heat treatment can be carried out
at high temperature, at 1600-2800.degree. C., for several hours,
under nitrogen, in order to graphitize the aromatic layers of the
walls and to allow the sublimation of the metallic Co.
[0042] The diameter of the tubes reduces slightly after the
oxidation treatment and the rate of opening is greater than 90%
(FIG. 2; the arrows show open tubes). The quality of the samples is
not effected by the opening treatment and the content of nanotubes
is greater than 97%.
[0043] The TEM observations in fringe mode of a 002 network show
that the walls are not damaged (FIG. 3).
[0044] The carbon nanotubes used are very tangled. The adsorption
isotherm of nitrogen at 77K is of type IV, characteristic of a
mesoporous swelling solid (FIG. 4). Their BET surface is 220
m.sup.2/g and the mesoporous volume is very large (approximately 1
cm.sup.3/g), with a BJH diameter of the order of 15 nm which
corresponds to the menisci defined by the tangle of nanotubes.
After opening the ends according to the invention, the mesoporous
volume increases up to approximately 1.6 cm.sup.3/g. The BET
surface is then of the order of 300 m.sup.2/g, which demonstrates
the benefit of these nanotubes for the storage of energy or
gas.
[0045] The above method is applied to nanotubes having external
diameters of 7 to 25 mm approximately, but can be applied to
nanotubes of greater diameters by adjusting the treatment time with
nitric acid and with CO.sub.2.
[0046] This method can of course be used with carbon nanotubes
other than those obtained by catalytic methods.
[0047] The opening of carbon nanotubes with very high
cristallinity, in particular those which are synthesized by
vaporization of graphite, will require longer reaction times.
[0048] The method according to the invention is thus effective in
the context of opening carbon nanotubes. More particularly, the
method according to the invention is applied to the opening of
multiwall carbon nanotubes.
[0049] More particularly, the method according to the invention is
applied to multiwall carbon nanotubes with an external diameter
comprised between 7 and 25 nm.
[0050] Yet more particularly, the multiwall carbon nanotubes to
which the method according to the invention is applied are obtained
by the decomposition of acetylene at 600.degree. C. on a solid
solution Co.sub.xMg.sub.(1-x)O.
[0051] All the thus-treated and open carbon nanotubes will prove to
be of great economic and industrial benefit in particular in their
use for the production of conductive nanowires, for the storage of
energy, for the storage or filtration of gasses and/or for the
production of a catalyst support.
* * * * *