Yulin Huang with F. Deng-Dalton Trans. 2012. 41. 6914

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Synthesis of mesoporous silica nanobamboo with highly dispersed tungsten
carbide nanoparticles†‡
Yulin Huang,
a
Fei Deng,
b
Chaoying Ni,
b
Jingguang G. Chen*
a
and Dionisios G. Vlachos*
a
Received 2nd February 2012, Accepted 9th April 2012
DOI: 10.1039/c2dt30248a
By controlling the interaction between cationic surfactant
micelles and ammonium metatungstate during the formation
of mesoporous silica structure, highly dispersed tungsten
carbide (WC) nanoparticles of 2.0 nm in diameter on meso-
porous silica nanospheres were synthesized at lower concen-
tration of ammonium metatungstate. With additional
ammonium metatungstate, a novel mesoporous silica nano-
bamboo structure was formed with bimodal size-distributed
WC nanoparticles, in which 2.0 nm WC was homogeneously
distributed in nanobamboo’s mesoporous silica wall and
those with larger diameter (10.0–20.0 nm) were only formed
on the nanobamboo’s inner surface and at its internodes.
The mesoporous silica nanobamboo also had a very high
tensile strength due to its bamboo-like structure.
Mesoporous materials are important in catalysis, separation,
nanoreactors, sensors and emerging biomedical applications
because of their high surface area, tuneable pore size and con-
trollable functionalization.
1
Since the mesoporous molecular
sieves, such as the hexagonally ordered MCM-41, were discov-
ered in 1992,
2,3
surfactant-templated synthetic procedures have
been extended to synthesize a variety of mesoporous materials
with different pore size distributions, such as MCM-48,
3
SBA-15,
4
KIT-6
5,6
and periodic mesoporous organosilica
(PMO).
7
Typically, these highly ordered mesoporous materials
are formed due to the silica-surfactant self-assembly with simul-
taneous condensation of inorganic species. By exploiting the
structure-directing electrostatic, hydrogen-bonding, and van der
Waals interactions associated with amphiphilic surfactant mol-
ecules and silica precursors,
4,8–11
many mesoporous silica
materials with different controllable three-dimensional architec-
tures, including mesoporous films,
12
monoliths,
13
fibres,
14
rods,
15
spheres
16,17
and crystal morphologies,
18
have also been
synthesized using non-ionic block copolymers as supramolecular
templates
4,12
or co-condensation methods with cationic surfac-
tants as templates.
17
A chiral mesoporous silica with twisted
structure has also been synthesized with chiral surfactants
19
and
achiral surfactants
20,21
as templates. However, due to the metast-
ability of surfactant micelles through self-assembly and the weak
interaction between separated micelles, it is very difficult to turn
the micelles to different nanostructures via further self-assembly.
By using highly negatively charged anions, such as metatung-
state (H
2
W
12
O
40
6−
), that strongly interact with cationic surfactant
micelles, we anticipate that materials with new nanostructures
and applications would be achievable.
It is also well known that transition metal carbides, especially
tungsten carbide (WC), have enormous potential in practical
applications because of their superior properties, such as high
melting temperature, superior hardness, low friction coefficient
and good thermal and electrical conductivity. More importantly,
WC could be used as a low-cost Pt-like material with attractive
activity, stability, selectivity and resistance to poisoning in many
reactions,
22
including hydrodenitrogenation, hydrodesulfuriza-
tion, alkane isomerization, biomass conversion,
23
hydrogen evol-
ution reaction,
24
and anode or cathode reactions in fuel
cells.
22,25,26
However, pure WC materials produced from the
direct carburization of tungsten oxides at high temperatures
(1400–1600 °C) always show much inferior catalytic activity
compared to noble metals due to their low accessible active
surface atoms. To make them more applicable in catalysis, one
approach is to reduce WC to nanometer size combining with
different supports, for example carbon nanotubes and activated
carbon. However, it is difficult to control the size of the carbide
particle because of the harsh synthesis conditions and the
product is usually a mixture of WC, thermodynamically unstable
W
2
C, and mixed WO
x
phases.
27,28
Here, we present the synthesis of WC/mesoporous silica com-
posite with a novel nanobamboo structure. In the nanobamboo
material, the bamboo wall consists of a mesoporous silica wall
(about 30 nm in thickness) with uniform pore size (3.1 nm) and
well-defined WC nanoparticles (2 nm in diameter) embedded
within the mesoporous silica. The internodes of nanobamboos
consist mainly of pure WC nanoparticles of larger and fairly
narrow particle size distribution (10–20 nm). Due to the
bamboo-like structure, a very high tensile strength (2.0–4.0 GPa)
of this material was found. Our strategy for making WC/meso-
porous silica nanobamboo is to control first the interaction
†Electronic supplementary information (ESI) available: XRD diffraction
of materials, EDX analysis, nanobamboo wall thickness analysis and
additional SEM image of nanobamboo material. See DOI: 10.1039/
c2dt30248a
‡Dedicated to Professor David J. Cole-Hamilton on the occasion of his
retirement and for his outstanding contribution to transition metal
catalysis.
a
Catalysis Center for Energy Innovation, Center for Catalytic Science
and Technology, Department of Chemical and Biomolecular
Engineering, University of Delaware, Newark, DE 19716-3111, USA.
E-mail: vlachos@udel.edu, jgchen@udel.edu
b
Department of Materials and Engineering, University of Delaware,
Newark, DE 19716-3111, USA
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between the cationic surfactant cetyltrimethylammonium
bromide (CTA
+
Br
−
) and ammonium metatungstate
((NH
4
)
6
H
2
W
12
O
40
) and then the interaction between the self-
assembled CTA
+
/H
2
W
12
O
40
6−
composite and the additional
ammonium metatungstate.
Fig. 1 shows that the nitrogen adsorption–desorption of nano-
bamboo exhibits type IV isotherms with very clear H
1
-hysteresis
loops at relative high pressure, characteristic of mesoporous
materials with a BET (Brunauer–Emmett–Teller) surface area
over 296 m
2
g
−1
, a total mesopore volume of 0.3 cm
3
g
−1
, and a
very narrow BJH (Barrett–Joyner–Halenda) pore size distri-
bution centered at 3.1 nm. Small-angle X-ray diffraction
(Fig. S1†) confirms the existence of a less-ordered mesoporous
structure, compared to the highly ordered MCM-41,
2,3
with a
corresponding d
100
of 3.8 nm. The presence of the less-ordered
mesoporous structure was also confirmed by transmission elec-
tron microscopy (TEM). Large-angle X-ray diffraction in
Fig. S2†indicates a pure hexagonal WC phase in the nanobam-
boo. The 2θof 31.54, 34.84, 48.48, 64.16, 64.58, 73.28, 75.62
and 75.86° with dvalues of 0.2838, 0.2574, 0.1879, 0.1451,
0.1291, 0.1255, 0.1237 nm correspond to (001), (100), (101),
(110), (002), (111), (200) and (102) planes of the pure hexagonal
WC. There is an additional broad peak around 2θat 25–30°,
which is due to the amorphous silica in the nanobamboo.
The scanning electron microscopy (SEM) image in Fig. 2
shows that the nanobamboo material has an untwisted rod-like
morphology with mostly 150–300 nm outer diameter. The tube
length varies from one to a few microns (Fig. S3†).
The TEM and scanning transmission electron microscopy
(STEM) images in Fig. 3a and b reveal clear bamboo internodes
inside the bamboo tubes. Fig. 3c shows a higher magnification
of the nanobamboo tube, revealing a mesoporous wall of a
thickness around 30 ± 5 nm (Fig. S4†). Fig. 3d shows that WC
nanoparticles of 10–20 nm in diameter are only distributed on
the inner surface of nanobamboo and at its internodes.
The high resolution TEM (HRTEM) micrograph of an inter-
node nanoparticle in Fig. 4a clearly shows that the particle is
highly crystalline in nature with lattice fringes having d spacing
of 0.2907, 0.2854 and 0.1968 nm, which closely match the
(001), (100) and (101) planes of hexagonal WC and indicate that
Fig. 1 Nitrogen adsorption–desorption isotherm and BJH pore size
distribution of nanobamboo.
Fig. 2 SEM image and the outer diameter distribution of
nanobamboos.
Fig. 3 TEM (a) and STEM (b) images of nanobamboos, with higher
magnification TEM images of nanobamboo wall (c) and its internode
(d).
Fig. 4 HRTEM images of one internode particle (a) and the nanobam-
boo wall with nanoparticles (b). Magnified STEM images of one nano-
bamboo (c) and its wall (d).
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the internode is made of pure WC nanoparticles. The compo-
sition of the internode is further confirmed by energy-dispersive
X-ray spectroscopy (EDX) in Fig. S5,†in which silicon is below
the detection limit compared to tungsten. However, this does not
preclude the possibility of some silica being part of the inter-
node. In contrast, both silicon and tungsten are detected in the
nanobamboo wall, shown in Fig. S6,†indicating that the nano-
bamboo wall is a composite of WC and mesoporous silica.
The HRTEM image of the nanobamboo wall (Fig. 4b) and
STEM images of nanobamboo (Fig. 4c and 4d) also show that
WC nanoparticles with well-defined diameters (∼2.0 nm) are
homogeneously distributed throughout the nanobamboo wall.
Larger WC nanoparticles are at the internodes of nanobamboo or
on its inner surface. In the mesoporous wall of a nanobamboo,
the size of WC is controlled from the diameter of mesopores
(3.1 nm).
29
Interestingly, although the inner diameter of nano-
bamboo is larger than 150 nm, WC nanoparticles inside the
nanobamboo is relatively small (10–20 nm), which might be due
to the interaction between the WC nanoparticle and inner surface
of nanobamboo during the carburization of tungstate.
TEM images in Fig. 5 show that the morphology of the WC/
mesoporous silica composite can be controlled from a nano-
sphere (Fig. 5a), to a mixture of nanosphere and nanobamboo
(Fig. 5b), and to nanobamboo only (Fig. 3a) by increasing the
initial concentration of ammonium metatungstate. The higher
magnification TEM image of the nanospheres, which are syn-
thesized with lower concentration of initial ammonium meta-
tungstate (Fig. 5c), shows that WC nanoparticles are
homogeneously distributed in the entire mesoporous silica par-
ticle. Ordered lines of WC nanoparticles can be seen in the
STEM image of these nanospheres in Fig. 5d, which gives evi-
dence that the highly negatively charged metatungstate initially
interacts with CTAB on its micelle surface only before the for-
mation of the nanobamboo structure.
Although the synthetic mechanism of mesoporous nanobam-
boo is still under investigation, it is believed that the electrostatic
interaction between CTAB and the tungsten precursor plays a
very important role during the formation of the mesoporous
nanobamboo structure. In this process, the self-assembly of
CTAB forms a positively charged micelle in aqueous solution
that attracts electrostatically the negatively charged metatungstate
(H
2
W
12
O
40
6−
). With low concentration of added ammonium
metatungstate, the self-assembled micelle structure is retained
because the ammonium metatungstate could be distributed along
the surface of and stabilized by the CTAB micelle. Upon increas-
ing the amount of ammonium metatungstate, it cannot be further
accommodated on the CTAB micelle surface, leading to the
assembly of a nanobamboo structure resulting from the inter-
action between highly negatively charged metatungstate and the
positively charged CTAB micelles, in which metatungstate is
surrounded by the CTA
+
/H
2
W
12
O
40
6−
assembly due to their
electrostatic interactions. When TEOS is added in this basic
aqueous solution, a nanobamboo structure, consisting of a meso-
porous silica wall including CTAB micelles in the mesopores
and metatungstate inside the bamboo tube, is formed. After the
removal of surfactant via calcination in air and carburization of
tungstate in dry CH
4
and H
2
, a nanobamboo structure with meso-
porous silica wall and WC nanoparticles of bimodal size distri-
bution is achieved, in which the size of WC nanoparticles is
controlled by the pore size of mesoporous silica and the inner
diameter of the bamboo.
Because of the well-defined bimodal size distribution of WC
nanoparticles in the mesoporous silica, this nanobamboo
material can be a useful WC-based catalyst
22
with multi-func-
tionalities for several reactions, including biomass conversion,
23
hydrogen evolution reaction,
24
and anode or cathode reactions in
fuel cells.
22,25,26
Such a nanobamboo structure could also be
extended to other multifunctional inorganic materials or serve as
a new hard template to synthesize other nanomaterials.
Furthermore, the synthesized nanobamboo material should be
expected to have unique mechanical properties since the bamboo
structure in nature could provide very large tensile flex strength
due to its structural specificity.
30–34
The tensile strength of an
individual nanobamboo was measured within an in situ SEM
based on the force to fracture the nanobamboo as shown in
Fig. 6, in which nanobamboo on the work station was shown
before fracture (Fig. 6a), after fracture (Fig. 6b) and after fracture
with a higher resolution (Fig. 6c). The strength of nanobamboo
is in the range of 2.0–4.0 GPa, which is much higher than that of
high strength steel (0.2–0.5 GPa) and close to that of diamond
(∼1.0 GPa). Therefore this new nanobamboo material could also
be a potential nanomaterial with novel mechanical properties in
materials enforcement.
Conclusions
To the best of our knowledge, this is the first synthesis of meso-
porous nanobamboo structure using soft templates, resulting in
the synthesis of a WC/mesoporous silica composite with nano-
bamboo structure via the interaction between the structure-
Fig. 5 TEM and STEM images of WC/mesoporous silica composites
with nanosphere morphology (a, c, d) and the mixture of nanobamboo
and nanospheres (b).
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directing agent and the highly negatively charged inorganic ions.
In this mesoporous nanobamboo material, the WC particle size
within the mesoporous silica wall is controlled by the meso-
pore’s size, whereas along the inner surface and internodes they
are 10–20 nm in diameter, which is controlled by the nanobam-
boo tube. Because of the unique structure and the high tensile
fracture strength of mesoporous silica nanobamboo, it has a
potential application in materials enforcement.
Experimental
The nanobamboo material was synthesized using cetyltrimethyl-
ammonium bromide (CTAB) as a soft template and the dual-
interaction between CTAB micelles and ammonium metatung-
state. Typically, 500.0 mg of CTAB (1.37 mmol) and 1.8 mL of
aqueous sodium hydroxide (2.0 mol L
−1
) were dissolved into
230 mL of water at 80 °C with stirring ( pH = 11.8). 667.0 mg
(0.23 mmol) of ammonium metatungstate in 10.0 mL water and
2.5 mL of TEOS (11.0 mmol) were injected into the solution
with vigorous stirring. After 2 h of stirring, the solid was separ-
ated by hot filtration, washed with excess water and dried at
120 °C under vacuum overnight before the calcination at 550 °C
in air for 6 h, and carburization at 1000 °C with dry CH
4
(40.0 mL min
−1
) and H
2
(100.0 mL min
−1
)for1h.
WC/mesoporous silica nanospheres were synthesized under
the same conditions except that the amount of ammonium meta-
tungstate was decreased to 168.2 mg (0.06 mmol). By varying
the amount of ammonium metatungstate, between 0.06 mmol
and 0.23 mmol, a mixture of nanospheres and nanobamboo
structures could be synthesized.
The nitrogen adsorption–desorption isotherm was measured
with a Micromeritics ASAP 2020 system. The pore volume was
determined from the adsorption branch of the N
2
isotherm curve
at P/P
0
= 0.948. BJH analysis for the mesopore size distribution
was based on the desorption. XRD patterns were recorded on a
Phillips Norelco powder diffractometer using Cu Kαradiation
(40 kV, 40 mA). SEM images were measured using JSM-7400F
field emission scanning electron microscope. JOEL JEM-2010F
transmission electron microscope with a 200 kV acceleration
voltage was used to collect TEM and STEM images.
The tensile strength of individual nanobamboo was measured
within an in situ SEM with universal tensile testing stage. There
were two stages mounted on the original SEM ground plate: a
combined X–Ystage at one side and a Zstage at the opposite
side. The sample could be independently moved to X,Yor Z
direction. There were also coarse and fine motions for each
stage, and the motions were controlled by applying a direct
current voltage to a piezo element embedded in each stage. A
silicon cantilever tip (Spring constant, K=3Nm
−1
)was
mounted on the X–Ystage to detect the tensile force at nano
Newton level, and a silicon plate with dispersed nanobamboo on
the surface was mounted on the Zstage.
The tensile test was conducted following three steps: first, the
cantilever tip contacted with one of the exposed WC fibers dis-
persed on a silicon plate by moving the X–Ystage. Second, the
cantilever tip and the nanobamboo were bound together using
electron beam induced deposition (EBID) method within the
SEM.
35
Finally, the nanobamboo was bound on another clean
area of the plate. Nanobamboo was pulled away from the silicon
plate until it was fractured.
SEM images of nanobamboo on the work station before frac-
ture (Fig. 6a), after fracture (Fig. 6b) and a high resolution SEM
image of the WC fiber after fracture (Fig. 6c) were shown in
Fig. 6. During the observation of this tensile fracture process, the
fracture force (F) of the nanobamboo could be calculated based
on the equation F=KΔX(where Kis the spring constant of the
single crystal silicon cantilever tip, ΔXis the change in bending
displacement of the tip before and after the fracture of nanobam-
boo). Then, the tensile strength, σ=F/S(where Sis the cross-
section area of the tested nanobamboo), of the WC/mesoporous
silica nanobamboo could be obtained.
Acknowledgements
We gratefully acknowledge financial support from the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences under Award Number DE-SC0001004 for the Catalysis
Center for Energy Innovation, an Energy Frontier Research
Center.
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