Self-catalyzed hydrogenolysis of nickelocene: Functional metal coating of three-dimensional nanosystems at low temperature

Article abstract of DOI:10.1149/1.2946723

In this paper we demonstrate the hydrogenolysis of nickelocene can take place through a self-catalyzed process at low temperature (<70°C) in supercritical carbon dioxide to generate relatively uniform dispersed Ni (nickel) metal particles onto carbon nanotubes forests and Ni films on flat surfaces. The ability to form metal particles or films without a local catalyst at this low temperature is significant because it provides further insight into mechanisms for Ni deposition reactions, and it enables a new route for low-temperature metal coating on a range of nonmetal substrate materials with complex topographies and nanostructures, for example, to form Ni/polymer magnetic nanocomposites.

Full text of DOI:10.1149/1.2946723

D580
Journal of The Electrochemical Society, 155 ͑9͒ D580-D582 ͑2008͒
0
013-4651/2008/155͑9͒/D580/3/$23.00 © The Electrochemical Society
Self-Catalyzed Hydrogenolysis of Nickelocene: Functional
Metal Coating of Three-Dimensional Nanosystems
at Low Temperature
a
b
a,z
Qing Peng, Joseph C. Spagnola, and Gregory N. Parsons
a
b
Department of Chemical and Biomolecular Engineering, and Department of Materials Science and
Engineering, North Carolina State University, Raleigh, North Carolina 27695, USA
In this paper we demonstrate the hydrogenolysis of nickelocene can take place through a self-catalyzed process at low temperature
͑
Ͻ70°C͒ in supercritical carbon dioxide to generate relatively uniform dispersed Ni ͑nickel͒ metal particles onto carbon nanotubes
forests and Ni films on flat surfaces. The ability to form metal particles or films without a local catalyst at this low temperature is
significant because it provides further insight into mechanisms for Ni deposition reactions, and it enables a new route for
low-temperature metal coating on a range of nonmetal substrate materials with complex topographies and nanostructures, for
example, to form Ni/polymer magnetic nanocomposites.
©
2008 The Electrochemical Society. ͓DOI: 10.1149/1.2946723͔ All rights reserved.
Manuscript submitted February 18, 2008; revised manuscript received May 18, 2008. Published July 11, 2008.
The ability to fabricate unique three-dimensional ͑3D͒ macro-
structures consisting of nanostructures will lead to new materials
and constructs with unique electronic, optical, sensing, and protec-
windows on both sides, with a silicon wafer having MWCNT forests
grown on it. Following precursor loading, low-pressure fresh CO2
was used to purge the system for 10 min at ϳ70°C in order to purge
1
-3
tive capabilities. Forests and sheets of carbon nanotubes ͑CNTs͒,
air out of the reactor. After purging, high-pressure CO was fed into
2
4
,5
6
electrospun nanofibers, or nanostructured aerogels, for example,
the reactor through a high-pressure syringe pump ͑model 260D,
7
,8
could be used as platforms for advanced catalytic systems, bio-
medical systems, and energy storage or conversion systems
Isco, Inc.͒. The temperature of the scCO solution was stabilized by
2
9
10,11
a heating tape at the dissolving condition ͑T = 70°C, P = 17 MPa͒
for 4 h to form a uniform solution. During Ni͑Cp͒ dissolution, H
where the surface functionality of the nanostructures provides
unique performance capabilities. Many studies have addressed the
issues of surface coating and surface modification of nanostructures,
2
2
was fed into another clean, air-free high-pressure manifold vessel
͑
43 mL͒, at a pressure of ϳ3.5 MPa at 60°C. The vessel was then
6,12,13
including 3D surface in particular.
As a specific example,
further charged with fresh CO using the high-pressure syringe
2
nickel is an important catalyst, magnetic material and promising
pump to a pressure of ϳ34.5 MPa. This H /scCO solution was
3
2
2
energy storage material, and nickel ͑Ni͒-CNT nanocomposites have
kept stable at this condition for more than 2 h before being injected
1
4,15
been synthesized by chemical vapor deposition ͑CVD͒,
wet
into the high-pressure reactor. Upon H /scCO injection, the pres-
11,16
17,18
2
2
chemical processing,
electroless plating,
and by direct incor-
poration of Ni nanoparticle catalysts into as-grown CNTs. Re-
sure in the vessel dropped from ϳ34.5 to ϳ19.3 MPa, allowing the
1
4
amount of H fed into the reactor to be quantified. Normally the H
2
2
cently, supercritical carbon dioxide ͑scCO ͒ has been successfully
2
injection process is repeated to obtain a 50–100ϫ molar excess of
hydrogen relative to nickelocene in the reactor system. Upon the
addition of H , the scCO solution containing Ni͑Cp͒ maintained a
used as a green medium for chemical reaction because of its liquid-
like high density, gaslike high diffusivity, low viscosity, and zero
2
,19
2
2
2
surface tension. A number of different metal thin films, including
green color and the reaction system was left undisturbed at 70°C,
17–18 MPa for several hours. After 7–8 h substantial Ni film and
particles deposition were observed ͑as described in the Experimental
section͒. Following each experiment, the reactor was prepared for
another reaction sequence by removing the Ni deposition on the
Pt, Pd, Ru, Au, Ni, and Cu, have been deposited by using the scCO
2
8,19-24
process at modest temperatures and pressures.
For example, Ni
films were deposited onto silicon wafers in scCO solvent using
2
19
catalyst seed layers such as Pt, Pd, and Ni. This previous work
found this catalyst layer is critically important at temperatures less
19
than 120°C to initiate deposition. The requirement of a local sur-
face catalyst to initiate deposition is expected to significantly limit
the formation of film coating in small features and uniform coatings
on complex 3D surface topologies.
This article describes a new phenomenon of self-catalyzed hy-
drogenolysis of Ni͑Cp͒2 in scCO2 wherein Ni metal ͑including
nanoparticles and films͒ can be generated from direct hydrogenoly-
sis of nickelocene at 70°C without addition of any catalysts. Using
this method, dense Ni nanoparticles were anchored onto multiwalled
CNTs ͑MWCNTs͒ throughout the MWCNTs forest without destroy-
ing the original structure of the carbon nanotube forest.
Experimental
Nickelocene ͓Ni͑Cp͒ ͔ ͑Strem Chemicals͒ was used as the pre-
2
cursor and H was used as the reducing agent. Coleman-grade CO
2
2
͑
99.99%͒ and high-purity H ͑99.999%͒ ͑National Welders/Supply
2
Co.͒ were used without further purification. The experiment was
carried out in a 110 mL homemade high-pressure reactor as shown
in Fig. 1. In a typical experiment, 70–90 mg Ni͑Cp͒ was loaded
2
into the high-pressure reactor, which was then sealed by sapphire
Figure 1. Diagram of the reaction system. H2 is mixed with scCO in the
2
manifold to form a uniform solution. The temperature of the reactor and
manifold are controlled by feedback thermal couples individually. The reac-
tion system can be observed through the sapphire windows.
z
E-mail: parsons@ncsu.edu
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Journal of The Electrochemical Society, 155 ͑9͒ D580-D582 ͑2008͒
D581
Figure 2. ͑Color online͒ ͑a͒ A photo image of stable Ni͑Cp͒2 in scCO2
solution ͑green͒ at 70°C. ͑b͒ A photo image of the solution after 7–8 h
incubation time ͑orange͒.
vessel walls. The reactor was scrubbed, then sonicated in water,
followed by a deionized water flush and N flow dry.
2
As substrates for deposition, MWCNT forests were grown by a
catalytic CVD process in a homemade quartz reactor at 600°C with
ethanol as the carbon source. As the carrier gas, Ar was bubbled
through ethanol at room temperature with a flow rate of 1.5 slpm ͑at
Figure 3. ͑a͒ FESEM image of pristine MWCNTs. ͑b͒ FESEM image of the
Ni nanoparticle decorated MWCNT which was treated in the scCO -based
2
direct hydrogenolysis reaction between Ni͑Cp͒ and H at 70°C. ͑c͒ A higher
2
2
resolution SEM image of the Ni nanoparticle anchored MWCNTs. ͑d͒ A
0
°C, 101.3 kPa͒. The catalyst layer of Fe ͑0.5 nm͒ on Al O ͑10
typical TEM image of Ni nanoparticle decorated MWCNTs.
2
3
nm͒ was fabricated on the thermal oxide silicon wafer
͓
SiO ͑500 nm͒/Si͔ by sputtering Al O and E-beam evaporation of
2
2
3
Fe sequentially. No further processes were taken to purify or open
the end of the as-grown MWCNTs.
nanoparticles. However, once the Ni nuclei were formed and
Imaging of the samples was performed using field-emission
scanning electron microscopy ͑FESEM͒ on a JEOL 6400F instru-
ment at 5 kV. Transmission electron microscopy ͑TEM͒ images
were taken on a Hitachi HF-2000 operated at 200 kV. Energy-
dispersive X-ray spectroscopy ͑EDS͒ was carried out on an Oxford
Instruments Inca Energy 100 system attached onto the TEM to de-
termine the chemical composition of the Ni nanoparticle decorated
MWCNTs. For TEM measurement, Ni nanoparticle decorated
MWCNTs were dispersed into ethanol and sonicated for 1 min.
Then several droplets of the dispersion were pipetted onto Lacey
silicon TEM grids ͑Ted Pella, Inc.͒. The prepared samples were
dried at room temperature overnight before the TEM measurement.
reached
dissociation
a critical size, they acted to catalyze hydrogen
19,25
and promote rapid heterogeneous hydrogenation of
nickelocene, resulting in Ni particle formation, as shown by the
reaction, Eq. 2
͓2͔
On the surface of the MWCNTs where no catalysts such as Ni, Pt,
Pd are present, a large amount of Ni nanoparticles is attached to the
MWCNTs. Therefore, the Ni nanoparticle formation likely proceeds
through an initial homogeneous reaction ͑Eq. 1͒ in the supercritical
carbon dioxide solvent, followed by a faster heterogeneous catalytic
reaction ͑Eq. 2͒ on the as-formed Ni nanoparticles.
Results and Discussion
The deposition reaction was observed to be strongly time- and
temperature-dependent. As shown in Fig. 2a, the starting homoge-
neous solution of Ni͑Cp͒ in scCO is green in color. Without addi-
2
2
An FESEM image of an untreated pristine nanotube substrate
sample is shown in Fig. 3a. The sample is a dense forest of aligned
tion of H , the solution is stable and no reaction was observed after
2
more than 24 h. With injection of H , no noticeable color change
2
CNTs. The scCO -based Ni deposition process at 70°C using 78 mg
2
was observed, and no deposition reaction occurred for several hours.
However, after 7–8 h incubation time at 70°C, the solution color
changed very rapidly ͑within 1–2 min͒ from green to orange, as
shown in Fig. 2b, and then became transparent. The transition from
orange color to transparent occurred over ϳ20 min and corre-
sponded to the completion of the deposition reaction. The orange
color is ascribed to the reaction intermediates present during the
reaction process. The reaction scheme for the homogeneous reaction
is shown in Eq. 1
of Ni͑Cp͒ results in dense and dispersed Ni nanoparticles which are
2
anchored on the surface of the MWCNTs, as shown in a represen-
tative FESEM image in Fig. 3b. Figure 3b also shows that after the
deposition process the morphology of the nanotube structure main-
tains its original structure, with no undesired collapse or tube aggre-
gation observed. Figure 3c shows a typical high-resolution scanning
electron microscopy ͑SEM͒ image of resulting MWCNTs decorated
with a number of Ni particles. The same morphology was observed
2
across the entire sample surface with an area of 1 ϫ 1.5 cm . After
Ni deposition, the samples shown in Fig. 3b and c were exposed to
a sonication treatment ͑1 min in ethanol͒ to remove loosely bound
particulates. Figure 3d shows a typical TEM image of Ni particle
decorated MWCNTs after the sonication dispersal process. The di-
ameters of Ni particles are in the range of 10–60 nm.
͓
1͔
Figure 4 shows the EDS spectrum of the MWCNT sample after
Ni deposition, showing clear evidence for the presence of nickel.
The O peak probably results from the oxidation of Ni nanoparticles
and MWCNTs with exposure to ambient air before EDS measure-
ment. The C peak is ascribed to MWCNTs, and the Cu and Si peak
are the consequence of the Lacey Silicon TEM grid. The EDS spec-
trum of Ni-decorated MWCNTs verified the chemical composition
of Ni/MWCNT nanocomposites.
This incubation time for the reaction decreased as the reaction
temperature increased. At 130°C the reaction initiated immediately
to form Ni metal and the solution color disappeared shortly after
injection of H , which is in agreement with previous
2
1
9,25
observations.
At low reaction temperature ͑such as 70°C͒, it
takes a long period for H to reduce Ni͑Cp͒ to form intermediate
2
2
products ͑orange color͒, which are subsequently converted to Ni
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D582
Journal of The Electrochemical Society, 155 ͑9͒ D580-D582 ͑2008͒
contrast, on substrates such as planar SiO and Al O , which have
2
2
3
very high density of reactive sites, we found the Ni hydrogenation
process resulted in thin-film metal coatings as shown in Fig. 5. This
coating was also formed from the self-catalyzed heterogeneous
growth mode. However, we find that the complex nucleation process
for the Ni film on the SiO and Al O surfaces is relatively difficult
2
2
3
to control.
Conclusions
Results clearly indicate that the nickelocene hydrogenolysis pro-
cess can proceed through a self-catalyzed process at low tempera-
ture ͑Ͻ70°C͒ in scCO to generate relatively uniform dispersed Ni
2
metal particles onto CNT forests and Ni films on flat surfaces. The
ability to form metal particles or films without a local catalyst at this
low temperature is significant because it provides further insight into
mechanisms for deposition reactions, and it enables a new route for
low-temperature metal coating on a range of nonmetal substrate ma-
terials with complex topographies and nanostructures. The low-
Figure 4. EDS spectrum of Ni nanoparticle decorated MWCNTs.
temperature scCO -based Ni deposition process described here
2
could also have other applications, including metal deposition onto
thermally sensitive substrates to form, for example, Ni/polymer
magnetic nanocomposites.
Acknowledgments
This work was supported by the STC program of the National
Science Foundation under agreement no. CHE-9876674. Support is
also acknowledged from NSF CTS-0626256.
North Carolina State University assisted in meeting the publication costs
of this article.
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