Phase-transfer synthesis of amorphous palladium nanoparticle-functionalized 3D helical carbon nanofibers and its highly catalytic performance towards hydrazine oxidation

Article abstract of DOI:10.1016/j.cplett.2012.05.042

Amorphous palladium nanoparticles functionalized helical carbon nanofibers (ApPd-HCNFs) were synthesized using a phase-transfer method. Palladium nanoparticles (Pd-NP) were first prepared using n-dodecyl sulfide as reducing agent and stabilizing ligands in ethanol. The Pd-NPs were then modified with benzyl mercaptan and transferred into a toluene solution with HCNFs which were decorated with amorphous palladium. The materials were characterized with high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and cyclic voltammetry showing that amorphous palladium nanoparticles were uniformly anchored at the HCNFs surface and that the ApPd-HCNFs exhibit high electrocatalytic activity towards hydrazine oxidation.

Full text of DOI:10.1016/j.cplett.2012.05.042

Chemical Physics Letters 543 (2012) 96–100
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Phase-transfer synthesis of amorphous palladium nanoparticle-functionalized 3D
helical carbon nanofibers and its highly catalytic performance towards
hydrazine oxidation
a
a
a
a
b
a,
⇑
Guangzhi Hu , Tiva Sharifi , Florian Nitze , Hamid Reza Barzegar , Cheuk-Wai Tai , Thomas Wågberg
a
Department of Physics, Umeå University, S-901 87 Umeå, Sweden
Department of Materials and Environmental Chemistry and Berzelii Center EXSELENT on Porous Materials, Arrhenius Laboratory, Stockholm University, Stockholm S-106 91, Sweden
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Amorphous palladium nanoparticles functionalized helical carbon nanofibers (ApPd–HCNFs) were syn-
thesized using a phase-transfer method. Palladium nanoparticles (Pd-NP) were first prepared using n-
dodecyl sulfide as reducing agent and stabilizing ligands in ethanol. The Pd-NPs were then modified with
benzyl mercaptan and transferred into a toluene solution with HCNFs which were decorated with amor-
phous palladium. The materials were characterized with high-resolution transmission electron micros-
copy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and
cyclic voltammetry showing that amorphous palladium nanoparticles were uniformly anchored at the
HCNFs surface and that the ApPd–HCNFs exhibit high electrocatalytic activity towards hydrazine
oxidation.
Received 15 January 2012
In final form 21 May 2012
Available online 7 June 2012
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
Direct fuel cells have attracted considerable attention as ad-
performance. To prevent this mechanism different kinds of support
materials have been used to immobilize noble metal particles to
retain their electrocatalytic activity [12–14]. Carbon nanomaterials
are especially attractive as support due to their excellent conduc-
tivity, good chemical inertness and large surface area for the
immobilization of metal particles [15–20].
vanced power sources for portable applications due to advantages
such as, high efficiency, low environmental impacts, and easy stor-
age of raw materials [1–3]. Although hydrazine exhibits some
drawbacks regarding toxicity it also has significant advantages
over traditional organic fuels such as methanol, ethanol and formic
acid, since it easily can be used in neutral aqueous solutions and
since the electrochemical oxidation process of hydrazine does
not produce any poisoning decomposition species such as carbon
monoxide and sulfur composites [4,5], which might seriously affect
the efficiency of the expensive anode catalysts in fuel cell systems.
Noble metals, such as Pt and Rh show good electrocatalytic
activity towards hydrazine [6,7]. However, these noble metals
are extremely rare in the Earth’s crust (5 and 1 ppb in weight,
respectively) and hence quite expensive, limiting their industrial
and practical applications in commercial fuel cell systems.
Compared to traditional bulk material, nanoscale materials can
evidently minimize the needed amount of precious catalyst mate-
rial but also they possess some unique physical and chemical prop-
erties [8] such as a significantly enhanced electrocatalytic activity
towards hydrazine oxidation [9–11]. However, small Pd nanoparti-
cles with high surface energy quite easily accumulate to form
large and irregular Pd particles, seriously limiting their catalytic
Compared to other methods for preparing noble metal/carbon
nanocomposites such as electroless deposition [21], microemul-
sion [22], and chemical reduction [23], phase transfer synthesis
is a more simple, rapid and straightforward method for the prepa-
ration of noble metal nanoparticles. This method not only avoids
the accumulation of noble metal nanoparticles in the polarized
water phase but also easily allows control of size, monodispersity
and surface chemical properties of the nanoparticle in the organic
phase [24]. Chen et al. reported an efficient anchorage of Pd nano-
particles on carbon nanotubes using phase transfer method and
applied it as catalyst for hydrazine oxidation [25], and this has also
been demonstrated for synthesizing high quality gold/carbon
nanotube hybrids in toluene and water phases [26].
In this Letter, we prepared amorphous Pd nanoparticle-func-
tionalized helical carbon nanofibers (ApPd–HCNFs) using a novel
phase transfer method. Pd nanoparticles were first prepared in eth-
anol containing n-dodecyl sulfide as reduction reagent and stabi-
lizing ligands. Then the as-prepared nanoparticles were modified
with benzyl mercaptan and transferred into the toluene phase.
The benzyl ring functionalized Pd nanoparticles were easily fixed
on the helical carbon nanofiber surface via the
tween benzyl ring groups and graphite surface of HCNFs. We show
p–p interaction be-
⇑
Corresponding author.
E-mail address: Thomas.wagberg@physics.umu.se (T. Wågberg).
0
009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cplett.2012.05.042

G. Hu et al. / Chemical Physics Letters 543 (2012) 96–100
97
that the as-prepared material significantly decreases the oxidation
potential of hydrazine and thereby increases the oxidation current
in neutral buffer solution. The excellent electrocatalytic activity to-
wards hydrazine oxidation shows that the ApPd–HCNF composite
has potential as high performance catalyst in a direct hydrazine
fuel cell.
in Scheme 1. The preparation of Pd–HCNFs composite synthesized
without using the phase transfer step is the same as that of ApPd–
HCNFs but by ignoring step (ii) in Scheme 1. This material is la-
beled as Pd–HCNFs.
The morphology and structure of ApPd–HCNFs was carried on a
transmission electron microscopy (TEM, JEOL-1230) with an accel-
eration voltage of 80 kV. The high-resolution TEM images were ob-
tained with a JEOL-2100F at 200 keV. X-ray diffraction was
2
. Experimental
performed on a Siemen’s D5000 using Cu Ka (a = 1.5406) radiation.
X-ray photoelectron spectrum of the products was carried on an
AXIS Ultra DLD (Kratos Analytical Ltd., UK).
Sodium tetrachloropalladate (99.998%), benzyl mercaptan
99%), n-dodecyl sulfide (99%) and Nafion solution (5%) were pur-
chased from Ion Power Inc. Hydrazine hydrate (80% in water) is
from Merck Corporation in Germany. Ethanol was supplied by
Eka Chemicals AB in Sweden. Other chemicals are analytical grade.
High quality helical carbon nanofibers were synthesized accord-
ing to our latest report using Pd C60 as catalyst [27]. The Pd nano-
2
particles synthesis is similar to our former report and can be
described as follows [28,29]: (i) 21 mg palladium acetate and
70 mg n-dodecyl sulfide were carefully dissolved in 50 mL etha-
nol solution. The yellow and transparent solution was heated to
33 K and kept for about 1 h; (ii) then 25 mL of toluene containing
.25 mL phenyl mercaptan were added into the above-mentioned
mixture and kept stirring for 1 h; (iii) a large volume (about 5-fold)
water was added into the mixture to remove the miscible ethanol
from toluene, and Pd nanoparticles were successfully transferred
into the toluene solution from the water phase containing ethanol;
iv) the black up-layer solution (toluene phase) was transferred
into another beaker using a separating funnel and then 33 mg
HCNFs were added and stirred at 700 rpm overnight. Due to the
(
The electrocatalytic performance of the as-prepared catalysts
was measured on Autolab PGSTAT30 with a three-electrode cell
at room temperature (22 ± 1 °C). As counter and reference elec-
trodes a Pt wire and a saturated calomel electrode were used and
as working electrode a glassy carbon electrode coated with the cat-
alyst was used. The working electrode was prepared as follows: (i)
a glassy carbon electrode was first polished with 0.05 lm alumi-
num oxide paste on a chamois and then washed in acetone, ethanol
and water under a ultrasonication for 5 min, successively; (ii)
1
2 lL ApPd–HCNF (or HCNFs or Pd–HCNFs) suspension mixed
.5
with DMF (2 mg/mL) was carefully dropped onto the upward sur-
face of the electrode and dried at room temperature; (iii) 5 L of
3
0
l
Nafion solution (0.05%) was covered on the surface of the cata-
lyst-covered electrode and dried in air without any heating pro-
cess. Phosphate buffer solution (concentration: 0.1 M, pH = 6.8)
was used as the supporting electrolyte during the electrochemical
measurement. The electrolyte solution was bubbled with argon for
at least 30 min to completely remove dissolved oxygen before elec-
trochemical measurement.
(
p–p interaction between Pd nanoparticles and HCNFs the benzyl
mercaptan-functionalized Pd nanoparticles were attached at the
HCNF surface; (v) the as-prepared suspension was carefully filtered
and washed three times with ethanol and water, respectively, and
then dried at 80 °C in air overnight. The as prepared composite was
labeled as ApPd–HCNFs. The detailed synthesis process is listed as
3. Results and discussion
The TEM image of the HCNFs is shown in Figure 1A. The mor-
phology of HCNFs is a classic helix structure with a diameter about
Scheme 1. Preparation procedure for ApPd–HCNFs.

9
8
G. Hu et al. / Chemical Physics Letters 543 (2012) 96–100
Figure 1. Bright-field TEM images of HCNFs (A) and ApPd–HCNFs (B); (C and D) are HR TEM of ApPd–HCNFs. Inset of (B) is the TEM image of Pd–HCNFs that synthesized
without using phase transfer method. Inset of (D) shows a zoom of the area indicated by the red dotted square. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
8
0–100 nm, which is in good agreement with our former reports
66.09°, 79.53° and 83.84° These are assigned to the (111), (200),
(220), (311) and (222) crystalline planes of the face centered cu-
bic structure of Pd, respectively. However all these peaks have
been shown in earlier reports to originate from the intrinsic Pd-
nanoparticle that are used in the synthesis of the helical fibers
[30] and that is visible for example in Figure 1B (highlighted by
an arrow). This is further supported by an estimation of the parti-
cles size from the Scherrer’s fomula [35], which reveals a size of
35.6 nm for the crystalline Pd nanoparticles which is in good agree-
ment with the size of the encapsulated Pd particles but quite dif-
ferent from the statistical result (10 nm) of TEM analysis of the
externally anchored Pd-NPs. From our earlier studies we also know
that it is relatively easy to distinguish between diffraction peaks
from externally anchored small Pd-NPs and encapsulated larger
Pd particles since the smaller particles normally show a different
lattice constant leading to broadened (due to the smaller size)
[
27,30]. The unique helical structure of the HCNFs makes them to
excellent candidates as catalyst support like coiled and bamboo-
like carbon nanotubes [31–34]. Figure 1B presents a TEM image
of ApPd–HCNF nanocomposites. Pd nanoparticles are uniformly at-
tached at the surface of HCNFs without a significant accumulation.
Also, although the TEM grids were prepared by ultrasonication of
the samples no palladium nanoparticles can be found on the car-
bon film indicating a strong interaction between the nanoparticles
and the nanofiber surface. The TEM image of the sample prepared
without phase transfer method, shows that by this method only
very few Pd particles were fixed on the HCNFs surface, and that
these were consistently smaller with a diameter about 1–2 nm
(Figure 1B inset). This observation points to the fact that when
not using the phase transfer method the larger Pd nanoparticles
are not attached at the HCNF surface during the synthesis process
and therefore lost in the filtration process. Further magnification of
ApPd–HCNFs in the high-resolution TEM image (Figure 1C) shows
that the Pd nanoparticles have an unusual form compared to our
earlier reports [28–30]. The Pd nanoparticles (Pd-NPs) in this Letter
are more oval in shape and seem to be much more rigidly attached
to the HCNF surface so that the interaction between the HCNF sur-
face and the Pd-NP hinders the Pd-NPs to form standard spherical
shapes. The form of these Pd-NPs rather resembles freshly splashed
mud at a brick wall. The Pd-NPs are homogenously distributed at
the HCNF surface with a diameter range from 3 to 15 nm. The aver-
age size obtained by measuring 170 randomly selected Pd-NP in
TEM micrographs show an average diameter of 8.2 ± 2.2 nm. Fur-
ther magnification of the selected area (Figure 1D) shows that
these nanoparticles are not crystalline but have completely amor-
phous structure in contrast to earlier reports using phase transfer
methods with other types of short linker molecules [28,29]. A pos-
sible explanation for the amorphous character of the Pd-NPs is that
long-chain n-dodecyl sulfide attaches so strongly at the surface of
Pd nanoclusters that it effectively hinders the ‘free’ reorganization
of the Pd-atoms into ordered crystalline structures.
Pd(111)
Pd(200)
Pd(220)
Pd(311)
Pd(222)
30
35 40 45 50 55 60 65 70 75 80 85 90
To complement the structural analysis of the Pd-NPs the sam-
ples were characterized by X-ray diffraction. The X-ray diffraction
pattern shown in Figure 2 displays five sharp at 2h = 39.04°, 45.35°,
2
θ (degree)
Figure 2. XRD pattern of the Pd/HCNFs.

G. Hu et al. / Chemical Physics Letters 543 (2012) 96–100
99
and shifted (due to the difference in lattice constant) side peaks.
The lack of separated side peaks in the XRD spectrum gives further
support to the amorphous character of the Pd-NPs.
assigned to Pd 3d5/2 and Pd 3d3/2 at 335.6 and 340.9 eV, respec-
tively, which is in good agreement with the reported values
(335.5 and 341.1 eV) of bulk Pd(0) [40]. The second pair of peaks
appears at 336.8 and 342.2 eV, respectively, and can be assigned
to Pd(II)-type species such as oxides and sulfides [41]. The third
pair of peaks 338.3 and 343.6 eV are attributed to Pd(IV) [42]. Fig-
ure 3D shows the high resolution core level spectra of S 2p. Three
pairs of doublet peaks for S 2p3/2 and S 2p1/2 can be deconvoluted
by a typical 2:1 peak area ratio with a 1.2 eV peak separation. The
peak at 162.5 eV is unequivocally attributed to Pd–S bonds fixed
on the Pd nanoparticles at the HCNFs surface [43]. The peak at
To prove the formation mechanism of the Pd/HCNFs, energy-
dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spec-
troscopy (XPS) were further used for the characterization of the as-
prepared materials. The EDX spectrum of the ApPd–HCNFs shown
in Figure S1 (Supporting information) confirms the presence of C, S,
O and Pd in the Pd/HCNF composite. Si, Cu and Cr are originating
from the copper grid and the electron microscopy setup. The atom-
ic ratio of Pd to S on the functionalized materials is about 3.9,
which is significantly lower than that of Pd–HCNFs synthesized
using a self-assembly method [28], indicating that the surface of
the Ap-NPs in this Letter have a much higher coverage of sulfur.
This supports our proposed model for explaining their amorphous
character observed by TEM and XRD.
3
167.9 V can also be assigned to oxidized sulfur (–SO –) groups
originating from the air exposure of the as-prepared materials
during storage [44] while peak at 164.6 eV most probably repre-
sents sulfur-containing molecules (n-dodecyl sulfide and benzyl
mercaptan) adsorbed on the nanocomposite [45]. The EDX and
XPS results give strong support that the synthesis of the Pd nano-
particles and their anchoring onto the HCNFs occur by the pro-
posed self-assembly process.
It is well known that sulfur compounds can have a negative ef-
fect on the electrocatalytic oxidation of small organic molecules
such as methanol and ethanol, except for Pd nanocrystals with
low loading of short chain sulfur compounds which in some cases
have been shown to exhibit very high electrocatalytic activity to
methanol, ethanol and formic acid in alkaline solutions [28,29].
We have tested our materials catalytic activity to several different
organic solvents and find that the ApPd–HCNFs have very low elec-
trocatalytic activity to ethanol, methanol and formic acid. In sharp
contrast to this we find that our material exhibits very high activity
to the oxidation of hydrazine which has received considerable
attention for use in direct fuel cells applied to high-value systems
such as satellites and submarines.
The XPS spectrum of Pd/HCNFs in Figure 3A, shows the O1s
peak at 531.6 eV, the C1s peak at 284.3 eV, as well as peaks of S
2
p and Pd 3d. Further analysis of the XPS data shows that the
C1s peak can be deconvoluted into five peaks at 284.3, 285.6,
86.9, 287.9 and 290.5 eV (I–V) in Figure 3B. As well known, the
2
peaks at 286.9, 287.9 and 290.5 eV (C1s III–V) are mainly attrib-
uted to the carbon atoms bound to oxygen atoms in the HCNFs
(
C–OH, C@O and COOH) and
p–p transition loss [36,37]. The C1s
(
II) peak at 285.6 eV is assigned to the defect carbon atoms in
the graphite structure while the main peak at 284.3 eV originates
2
from the sp hybridized graphite carbon C@C bond [38]. The shift
of the C1s peak of HCNFs from 284.8 to 284.3 eV confirms the
bonding of benzyl mercaptan on the HCNFs achieved through the
typical p–p interaction [33,39].
The Pd 3d spectrum in Figure 3C shows a doublet structure,
Pd 3d5/2,3/2 which contains three pairs of peaks: the first pair is
C1s
A
C(1s)
B
Pd 3d
Pd3p+O1s
O KLL
Pd 3s
S2s S2p
1
000
800
600
400
200
0
295
290
285
280
Binding Energy (eV)
Binding Energy (eV)
Pd(3d)
C
D
S(2p)
3
46 344 342 340 338 336 334 332
Binding Energy (eV)
158 160 162 164 166 168 170 172
Binding Energy (eV)
Figure 3. (A) XPS spectra of ApPd–HCNFs and XPS patterns of C1s (B), Pd3d (C) and S2p (D) of Pd/HCNFs.

1
00
G. Hu et al. / Chemical Physics Letters 543 (2012) 96–100
The electrocatalytic activity of the ApPd–HCNF nanocomposite
Acknowledgements
towards hydrazine oxidation was tested in 0.1 M phosphate buffer
solution (PBS, pH = 6.8). Figure 4 shows the cyclic voltammetric
This work has been supported by Vetenskapsrådet (dnr-2010
3973), Ångpanneföreningen, Gustaf Richerts stiftelse. T. Wågberg
thanks Magnus Bergvalls stiftelse, and Kempestiftelsen for gener-
ous support. T. Wågberg and G. Z Hu thanks Wenner-Gren stiftel-
sen for support. T. Sharifi thanks Kempestiftelsen for generous
support. F. Nitze thanks Gustaf Richerts stiftelse. The Knut and
Alice Wallenberg Foundations are acknowledged for an equipment
grant for the electron microscopy facilities in Stockholm University.
(
CV) curves of different electrodes in argon-saturated 0.1 M PBS
solution containing 0.01 M hydrazine. The curve in Figure 4 shows
a weak electrocatalytic oxidation for the bare glassy carbon elec-
trode with no peaks of hydrazine oxidation in the potential
window from À0.2 to 0.6 V. This is attributed to the slow heteroge-
neous electron-transfer process of hydrazine at traditional bulk
carbon materials. The HCNF-modified electrode (Figure 4, curve
b) shows a limited improvement towards hydrazine oxidation
compared to the bare GC electrode. This is ascribed to a number
of defects at the HCNFs surface. For the electrode modified with
Pd–HCNFs synthesized without the phase transfer method a weak
and broad oxidation peak (the peak potential is at 0.474 V with a
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cplett.2012.05.042.
2
current density about 1.88 mA/cm ) for the oxidation of hydrazine
can be seen (Figure 4, curve c). However, at the electrode modified
with ApPd–HCNFs that was synthesized with the phase transfer
method (Figure 4, curve d), the oxidation peak potential of hydra-
zine significantly decreases to 0.068 V and shows a very high cur-
rent density of 15.6 mA/cm2 (8 times higher than Pd–HCNFs
synthesized without phase transfer method), showing that the
ApPd–HCNF composite can effectively catalyze the electro-oxida-
tion of hydrazine. The above experimental data shows that amor-
phous ApPd–NPs on the HCNFs surface have a particularly good
electrocatalytic activity towards oxidation of hydrazine, which is
in accordance with the previous literature [25]. Additionally, the
current density of hydrazine for our material is higher than for
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Figure 4. Cyclic voltammograms of hydrazine oxidation on pristine HCNFs (b, red
line), Pd–HCNFs without using phase transfer method (c, green line) and ApPd–
HCNFs using phase transfer method (d, blue line) and bare GC (a, black line)
electrodes in 10 mM hydrazine + 0.1 M PBS solution (pH = 6.8) in room tempera-
ture. Scanning rate: 50 mV/s. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
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