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J. Phys. Chem. C 2009, 113, 10747–10750
10747
Immobilization of RuO2 on Carbon Nanotube: An X-ray Absorption Near-Edge Structure
Study
J. G. Zhou,† H. T. Fang,‡ Y. F. Hu,† T. K. Sham,*,§ C. X. Wu,| M. Liu,⊥ and F. Li⊥
Canadian Light Source Inc, Saskatoon, Canada, School of Materials Science and Engineering, Harbin Institute
of Technology, Harbin, People’s Republic of China, Department of Chemistry, UniVersity of Western Ontario,
London, Canada, School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin,
People’s Republic of China, and Shenyang National Laboratory for Materials Science, Institute of Metal
Research, Chinese Academy of Sciences, Shenyang, China
ReceiVed: March 30, 2009; ReVised Manuscript ReceiVed: April 28, 2009
The electronic structures of carbon nanotube/RuO2 core/shell nanocomposite (RuO2 thin layer coated
multiwalled carbon nanotubes (MWNTs)) have been studied by X-ray absorption near-edge structures (XANES)
at C K-edge, O K-edge, and Ru M5,4- and L3-edges. The variation in white-line features of the XANES at
these edges supports strongly that RuO2 interacts with MWNTs through Ru-O-C bonding, which also results
in charge redistribution between C 2p-derived states in MWNT and the conduction band in RuO2. Such
chemical bonding is necessary to immobilize RuO2 on MWNT and ensures good conductivity of MWNT/
RuO2 core/shell nanocomposite.
Introduction
Ruthenium oxide (RuO2)-coated carbon nanotubes are useful
functional nanocomposites in many applications, including
supercapacitors, fuel cells, catalysts, biosensors, and field
emitters.1-8 In particular, this nanocomposite has exhibited
excellent performance in supercapacitors, which are the essential
device in electric vehicles. The information of its electronic
behavior and bonding is crucial in understanding and predicting
its properties. These properties strongly depend on the interaction between RuO2 and carbon nanotubes. As has been
recognized in conventional solid catalyst-support9 and nanoparticles (Pt or SnO2)-carbon nanotube composites,10-12 the
interaction between particles and the support is important in
achieving the desired functionality. Despite the significance to
the understanding of its functionality, studies of nanoparticlesubstrate interaction are still lacking, partly because of the
complexity of the problem and partly because of relatively
limited analysis methods.
X-ray absorption near-edge structures (XANES) spectroscopy
involves the measurement and interpretation of the photoexcitation cross-section across a particular core level (absorption
edge) of an atom in a chemical environment up to ∼50 eV above
the threshold. The absorption features in this region track bound
to bound and bound to quasi-bound (multiple scattering)
transitions. Thus this technique is element specific and it is very
sensitive to the local chemistry of the absorbing atom. XANES
has been successfully applied to investigate the chemical
bonding, electronic structure, and surface chemistry of many
nanomaterials.13-17 Specifically, the XANES of C K-edge and
Pt M3-edge, and C K-edge and Sn M5,4 edge, have been applied
* To whom correspondence should be addressed. Fax: +1-519-661-3022.
E-mail: [email protected].
†
Canadian Light Source Inc.
‡
School of Materials Science and Engineering, Harbin Institute of
Technology.
§
University of Western Ontario.
|
School of Chemical Engineering and Technology, Harbin Institute of
Technology.
⊥
Chinese Academy of Sciences.
to elucidate the synergic-bonding interactions in a Pt NPs-carbon
nanotubes composite system5 and in a SnO2 NPs-carbon
nanotubes composite,12 respectively. Ru K-edge X-ray absorption spectroscopy has also been applied to study the structure
of zeolite-confined nano-RuO2,21 especially the role of
RuO2 · xH2O. In this work, we report a study of RuO2-CNT
interactions in CNT/RuO2 core/shell nanocomposite using
XANES.
Experimental Section
RuO2-coated multiwalled carbon nanotubes (MWNTs) and
RuO2 nanoparticles were prepared by a sol-gel method.8 The
as-received MWNTs with diameters of 60-100 nm (purchased
from Shenzhen Nanotech Port Co. Ltd.) were first treated by
refluxing in nitric acid (40%) at 110 °C for 2 h to generate
oxygen-containing functional groups on the surface of MWNTs
and are henceforth denoted MWNTs. MWNTs (100 mg) were
then dispersed in a 0.1 M RuCl3 (30 mL) solution by ultrasonication for 5 min. A 0.3 M NaHCO3 aqueous solution was added
slowly to the above mixture under stirring until the pH of the
solution reached 7. After another 15 h of stirring, the sediments
were washed several times with distilled water, and then dried
in air flow at room temperature for 10 h. The black powder
thus obtained was annealed at 150 °C for 19 h in air and the
final product is henceforth denoted MWNT/RuO2. RuO2 nanoparticles were prepared following a similar sol-gel process in
0.1 M RuCl3 in the absence of MWNTs. The black powder was
also annealed at 150 °C for 19 h in air and is henceforth denoted
RuO2 NPs. The SEM image of MWNT/RuO2 in Figure 1a
exhibits that MWNTs are coated by RuO2 to form a core/shell
structure. The TEM image of CNT/RuO2 (inset) at the upper
corner of Figure 1a shows the RuO2 coating on MWNTs. The
SEM image of RuO2 NPs in Figure 1b shows that the diameter
is around 100 nm. The XANES at the C K-edge, Ru M5,4-edge,
and O K-edge were obtained on the spherical grating monochromator (SGM) beamline (∆E/E: ∼10-4) and Ru L3-edge on
the Soft X-ray Microcharacterization Beamline (SXRMB, ∆E/
E: ∼10-4) at the Canadian Light Source (CLS), a 2.9 GeV third
10.1021/jp902871b CCC: $40.75  2009 American Chemical Society
Published on Web 05/19/2009
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J. Phys. Chem. C, Vol. 113, No. 24, 2009
Zhou et al.
Figure 1. (a) SEM image of MWNT/RuO2 and TEM of MWNT/RuO2 (inset) and (b) SEM image of RuO2 NPs.
generation synchrotron source, using a Si(111) double crystal
monochromator. XANES were recorded in the surface sensitive
total electron yield (TEY) with use of specimen current and
the bulk sensitive total fluorescence yield (FLY) with a channel
plate detector. In soft X-ray XANES, FLY often suffers from
thickness effect and exhibits broadening except in very thin
specimens. The FLY, however, complements TEY in tracking
major surface contamination if it is present. Data were first
normalized to the incident photon flux I0 measured with a
refreshed gold mesh at SGM prior to the measurement and with
an ion chamber at SXRMB. After background correction, the
XANES are then normalized to the edge jump, the difference
in absorption coefficient just below and at a flat region above
the edge (297, 550, and 2920 eV for C, O, and Ru, respectively).
Results and Discussions
Figure 2a shows the C K-edge XANES of MWNTs and
MWNTs/RuO2 overlapping with the Ru M5,4-edge XANES of
RuO2 NPs in TEY. The photon energy is calibrated to the C 1s
to π* transition of CNTs at 285 eV.12The FLY (Figure 2b) is
consistent with that of the TEY (Figure 2a) in terms of peak
positions except for some broadening and variation in relative
intensities but clearly shows no noticeable surface contamination. Ru M5,4-edge in RuO2 occurs in the same energy range as
the C K-edge as seen in Figure 2. Since XANES follows the
dipole transition selection rule,18 the Ru M5,4-edge and C K-edge
are dominated by the electronic transitions from Ru 3d5/2,3/2 to
Ru 4p and C 1s to C 2p, respectively. The area under the
resonance in the vicinity of the threshold in XANES (also called
whiteline) is proportional to the unoccupied density of states
(DOS) for a randomly oriented sample (angular dependence
averages out). The resonances beyond whiteline involve transitions to the higher energy band/multiple scattering states which
are very sensitive to the local environment.
Let us first focus on the XANES of MWNTs collected at
TEY in Figure 2 a, where two main peaks are clearly displayed
at ∼285 and ∼291 eV attributable to C 1s to the graphitic C-C
π* and C-C σ* transitions,19 respectively. The transitions at
∼288 eV are characteristic of chemical defects and can be
attributed to carboxylic-type CdO groups resulting from the
oxidation of MWNTs.14 A weak pre-edge transition at 283.5
eV is also observed, which can be associated with disordered
carbon (e.g., amorphous carbon). The carbon-oxygen species
and disordered carbon in MWNTs result from the oxidation of
pristine MWNTs in HNO3. Introducing CdO function groups
Figure 2. C K-edge XANES of CNTs and CNT/RuO2 nanocomposite
along with Ru M5,4-edge XANES of RuO2 NPs recorded in (a) TEY
and (b) FLY.
in MWNTs has been identified as a crucial step for the
immobilization of RuO2 NP, forming a uniform coating on
carbon nanotubes.20
We next look at the spectrum of the RuO2 NP in Figure 2a.
The peaks at 283.5 and ∼288 eV correspond to the Ru M5and M4-edge (3d5/2, 3/2-p transitions), respectively, probing the
unoccupied 4p densities of state (DOS) in RuO2. The shoulder
just above the M5-edge and the weak feature at 286 eV are either
part of the RuO2 DOS or carbonaceous species on the RuO2
surface. There is no noticeable feature present at 285 eV
indicating that there is no detectable unsaturated carbon on the
RuO2 surface. The feature at 290.5 eV, which is also seen in
FLY, is a possible sign of surface impurity, but this feature is
also reproduced in band structure calculations of RuO2 rutile
Immobilization of RuO2 on Carbon Nanotube
Figure 3. O K-edge XANES of CNT/RuO2 nanocomposite and RuO2
NPs.
(to be published elsewhere). Fortunately, regardless of its origin,
it is weak and is not in the region of interest (286-289.5 eV)
where significant CNT-RuO2 interaction is observed.
We can now interpret the XANES of MWNT/RuO2. From
Figure 2a, we can clearly identify the Ru M5,4-edges from RuO2
NP and the MWNT π* and σ* transitions. The intensity of the
RuO2 features (Ru M5,4) is greatly reduced in the FLY in which
the underlying MWNT features are more apparent as expected
from a more bulk sensitive technique. Several interesting
features are noted from the TEY. First, π* transition around
285 eV in the nanocomposites is shifted by ∼0.3 eV to lower
photon energy from that of MWNTs. The presence of π*
transition in MWNT/RuO2 shows that the graphitic framework
of the MWNT remains intact upon the coating of RuO2. Thus
good electric conductivity in this nanocomposite can still be
expected. The energy shift infers interaction of RuO2 with
MWNT at the interface, which is required for the immobilization
of RuO2 on MWNT. Second, the π* transition intensity, which
reflects unoccupied DOS of π* character, is reduced in MWNT/
RuO2 compared to MWNTs. This provides direct evidence that
charge transfer from RuO2 to C 2p-derived π* states in MWNTs
has occurred at the interface. In the FLY XANES (Figure 2b),
which is more bulk sensitive, the Ru M5 is suppressed and the
π* intensity is nearly normal. It is conceivable that RuO2, which
is slightly metallic, donates electrons to MWNT resulting in
the reduction of unoccupied π* DOS in MWNT at the interface.
The third and perhaps the most interesting feature in MWNT/
RuO2 is the dramatic intensity increase of the 288 eV transitions,
indicating that there is a strongly oxidized C environment at
the interface, which withdraws a significant amount of charge
from C producing a localized, high density of unoccupied states
of C p character. It should be noted that Ru M4 transition (close
to 288 eV) from RuO2 coating alone cannot make such a sharp
increase because its contribution is proportional to Ru M54
transition, which is already weak. The presence of this sharp
peak at 288 eV, together with observation of the behavior of
the π* transition, must be interpreted as the result of strong
interaction of RuO2 with MWNT through a Ru-O-C bonding,
conceivably the carboxylic oxygens are bonded to the Ru. It
should be noted that this interaction is confined to the interface
since the more bulk-like FLY exhibits a normal MWNT XANES
and a considerably weaker 288 eV peak, as seen in Figure 2b.
The O K-edge XANES of the MWNT/RuO2 and RuO2 NPs
are shown in Figure 3. Those spectra again reflect the dipole
electron transitions from the core level O 1s into the unoccupied
O 2p projected states above the Fermi level in RuO2. Due to
hybridization between O 2p and Ru 4d and Ru 5sp,22 the
XANES features represent (1) transitions into the O 2p-Ru 4d
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10749
Figure 4. Ru L3-edge XANES of CNT/RuO2 nanocomposite, RuO2
NPs, and RuO2 powder (Aldrich).
hybridized bands which split into peaks a1 and a2 and (2)
transitions into O 2p-Ru 5sp hybridized bands (a broad peak
b). Since the transition maps the O 2p projected electronic states,
area under the whiteline is proportional to unoccupied DOS of
O 2p character. A close examination of the absorption whitelines
shown in the Figure 3 reveals an increase in intensity in the
MWNT/RuO2 XANES relative to that of RuO2 NPs. Pending
no countervailing symmetry (texture effect) and inhomogeneity
arguments, the increase in unoccupied DOS indicates charge
redistribution (depletion of O p electrons) between RuO2 and
MWNT, which has been observed by C K-edge XANES (C π*
states (transition at 285 eV) gains e- charge). Another interesting
feature is the lack of well-defined O 1s to σ* O-H transitions
in water. Active RuO2 catalysts often contain hydrated water.
Thus it is likely that water is present in a small amount and its
O K-edge resonance suffers from solid state broadening. We
will return to this in the following Ru L3,2-edge XANES
discussion.
In Figure 4, we show the Ru L3-edge XANES of MWNT/
RuO2, RuO2 NPs, and RuO2 powder (Aldrich). The similarity
between these spectra confirms the same valence state in these
substances. The weak peak at ∼2820 eV in sol-gel produced
MWNT/RuO2 and RuO2 NPs is the Cl K-edge from Cl-, which
is from the remaining precursor, RuCl3. The intense Ru L3
whiteline at ∼2841 eV arises from Ru 2p3/2 to 3d5/2,3/2 transition
in a tetragonally distorted Oh environment (rutile) and the broad
resonance at ∼50 eV above the whiteline is from the shape
resonance transition due to the multiple scattering.23,24 Close
examination of the Ru L3-edge whiteline in MWNT/RuO2, RuO2
NPs, and the standard, rutile RuO2 powder (microcrystals),
reveals that the whiteline is a doublet resulting from crystal field
splitting. Analysis of the doublet (second derivative) shows a
significant difference in the apparent crystal field splitting. Rutile
RuO2 microcrystal has the largest value of 2.64 eV compared
to the values of 2.22 and 2.03 eV for RuO2 NP and MWNT/
RuO2, respectively This difference is apparent from the inset
of Figure 4 where the 2p-t2 g transition is clearly visible for
rutile RuO2 but less obvious for RuO2 NP and MWNT/RuO2.
We attribute this reduction in crystal field splitting in the RuO2
NP and MWNT/RuO2 to a reduction in tetragonal distortion
and repulsion due to the presence of RuO2 · xH2O and
RuO2-MWNT interaction, respectively. This, to our knowledge,
is the first direct observation of a change in local environment
in these materials. More detailed discussion on the interplay of
theory and spectroscopy of this system will be described
elsewhere.
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Conclusions
In summary, XANES at the C K-edge (along with Ru M5,4),
O K-edges, and Ru L3-edge has been used to characterize the
electronic structure of RuO2-coated MWNTs. We found strong
evidence from C K-edge XANES, especially the sharp feature
at 288 eV, that RuO2 interacts with MWNT through Ru-O-C
bonding at the interface, which facilitates the immobilization
of RuO2 on MWNT and ensures good conductivity. We also
found noticeable changes in the local environment between
microcrystals of rutile RuO2, RuO2 NP, and MWNT/ RuO2.
Such interaction also alters the unoccupied electronic states in
MWNT and RuO2 and potentially can be used to track the
performance of these materials. The results also demonstrate
the advantage of XANES in clarifying the interaction in this
complex nanocomposite.
Acknowledgment. We thank Dr. R. Blyth and T. Regier of
the Canadian Light Source for their technical assistance on the
beamline. The work at the University of Western Ontario was
supported by NSERC, OIT, and CRC (T.K.S.). Research at HIT
and IMR was supported by the National Science Foundation of
China (Grant Nos. 50872026 and 5060211). Research at CLS
is supported by NSERC, NRC, CIHR, and the University of
Saskatchewan.
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