PNNL-13501 - Pacific Northwest National Laboratory

Figure 1. (a) – (c) Successive magnification of carbon nanotubes grown on a stainless steel substrate
porous silica film. Additional studies are under way to
determine conditions required to grow crack-free carbon
nanotube films.
Two types of surface modification activities were
pursued: 1) decoration of carbon nanotubes with noble
metals (such as platinum), and 2) derivatization of carbon
nanotubes with polymers. Shown in Figure 2 is the
secondary electron image from the in-lens detector of a
carbon nanotube sample that has been decorated with
platinum. The platinum was deposited electrochemically
(5-second deposition period) using the electronically
conductive carbon nanotubes as electrodes. Thus, we
successfully demonstrated that carbon nanotube surfaces
can be electrochemically modified with metal
nanoparticles (approximately 10 nm platinum particles).
Figure 2. Secondary electron image from the in-lens
detector for a 5-second deposition time of platinum
Platinum deposits on carbon nanotubes are of interest to a
wide range of applications from catalysis in general to
hydrogen storage. The use of carbon nanotubes for
catalyst support is of interest because of the uniformity of
carbon nanotube surfaces (defect-free carbon nanotubes
are single crystals) and high thermal and electrical
conductivities. The possibility of storing hydrogen in the
hollow centers of carbon nanotubes has sparked interest
worldwide. Because noble metals (such as platinum) are
known to dissociate molecular hydrogen into atomic
304 FY 2000 Laboratory Directed Research and Development Annual Report
hydrogen, the kinetics of hydrogen uptake and release
may be improved.
The high surface area of carbon nanotube films makes
them viable support materials for electrodes. Commercial
polyvinylferrocene from Polysciences Inc. was coated on
carbon nanotube films using a solvent casting technique
where the polyvinylferrocene was dissolved in
dichloromethane. The cyclic voltammogram of the
polyvinylferrocene-coated carbon nanotube film was
obtained in a solution of 0.5 M NaNO3. The result was
then compared to the cyclic voltammogram of a
polyvinylferrocene-coated carbon cloth electrode (GC-14
from the Electrosynthesis Co.). The integrated charge
passed as a function of time for both electrodes is shown
in Figure 3a. The polyvinylferrocene-coated carbon
nanotube electrode exhibited a higher surface charge
density. Furthermore, no difference in electrode
accessibility (solution mass transport rates) was observed
between the two electrodes. This was evident from
Figure 3b, where required for passing the same relative
amount of charge. Therefore, carbon nanotube films
provide additional surface area with negligible mass
transfer limitations.
We have verified the feasibility of using a nuclear
reaction analysis to directly measure the amount of H2
absorbed in carbon nanotubes. Briefly, a beam of 19 F
with a specific energy is imparted on carbon nanotube
samples. The nuclear reaction, 19 F + 1 H → 16 O + 4 He + γ,
occurs only when the 19 F has a specific energy (i.e., the
resonant energy). Because the reaction is very specific to
19 F + 1 H, a measure of the γ yield provides a measure of
the concentration of hydrogen. In addition, variation of
the 19 F energy gives us the ability to obtain the
concentration of hydrogen as a function of depth (the 19 F
must expend sufficient energy before the resonant nuclear
reaction will take place). Figure 4 shows the hydrogen
content of a virgin carbon nanotube sample and the same
sample after it has been dosed with hydrogen at
1 atmosphere for 1 hour. The amount of hydrogen is
approximately 0.2 wt%. This is consistent with published