1. No category

CARBON
8 5 ( 2 0 1 5 ) 1 6 8 –1 7 5
Available at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/carbon
Carbon nanotube hybrids with MoS2 and WS2
synthesized with control of crystal structure
and morphology
Xin Li, Zhenyu Wang, Jinying Zhang *, Chong Xie, Beibei Li, Rui Wang, Jun Li,
Chunming Niu
Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong
University, Xi’an 710054, Shaanxi, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Carbon nanotube hybrids with molybdenum and tungsten disulfides have attracted lots of
Received 23 October 2014
attentions due to their unique electronic and photonic properties. MoS2 and WS2 with dif-
Accepted 22 December 2014
ferent layers and morphology have been produced from homogeneous ultra-fine MoO3 and
Available online 30 December 2014
WO3 nanoparticles (1–2 nm) with different densities on multi-walled carbon nanotubes
(MWCNTs) in this work. The different MWCNT hybrid structures not only provide the
investigation feasibility of exciton transfer but also give potential applications of catalysts
and batteries. A facile method, ultra-sonication, has been adopted to produce MWCNT
hybrids with homogeneous ultra-dense and ultra-fine MoO3 and WO3 nanoparticles. The
as-produced molybdenum trioxide nanoparticles and free-standing molybdenum trioxide
nanowires from the same method with different reaction time have been found to crystallize in different crystal lattices. The nanoparticle morphology leads to the detachment of
H2O from molybdenum trioxide lattices, which was confirmed by thermodynamic analysis
based on density functional theory. MWCNT hybrids with layered MoS2 and WS2 are preferred after the sulfuration of MoO3 and WO3 nanoparticle-MWCNT hybrid structures.
The layers and morphology of MoS2 and WS2 have been controlled by the densities of
trioxide nanoparticle precursors on MWCNTs.
2014 Elsevier Ltd. All rights reserved.
1.
Introduction
MoS2 and WS2, graphite similar transition-metal dichalcogenides, have attracted lots of attentions owing to their bandgap
crossover from indirect in the bulk to direct in monolayer and
prominent potentials in electronic and photonic applications
[1–3]. Carbon nanotubes (CNTs) are appealing components for
various hybrid nanostructures due to their unique mechanical, electrical, and optoelectronic properties. Hybridization
* Corresponding author.
E-mail address: [email protected] (J. Zhang).
http://dx.doi.org/10.1016/j.carbon.2014.12.090
0008-6223/ 2014 Elsevier Ltd. All rights reserved.
of MoS2 and WS2 with CNTs provides fantastic mechanical,
electrical and photonic properties for various applications,
especially monolayer MoS2 and WS2 with single-walled carbon nanotubes (SWCNTs). The inner cavities of CNTs provide
effective chambers for hetero structure encapsulation [4–11].
The outer surfaces of CNTs provide effective supports for hetero structure attachment. Reducing particle sizes to increase
active surface areas is crucial for effective catalysts, battery
and supercapacitor electrode materials. However, aggregation
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8 5 (2 0 1 5) 1 6 8–17 5
of nanoparticles (NPs) is an inevitable challenge with particle
sizes decreasing. CNTs present effective supports for the
active NPs to prevent their aggregation and also serve as conductive and inert networks to afford rapid electron transport
for the active NPs. Up to date, most hybrid structures of NPs
with carbon nanomaterials were produced by post-treatments of carbon nanomaterials with the pre-synthesized
NPs [12–15]. The NP sizes and distributions are limited by
NP synthesis and hybridization methods. The adductive NPs
are around several nanometers to dozens of nanometers.
The distribution density and adhesion strength with CNTs
are also limited. Molybdenum and tungsten oxide NPs can
serve as effective anode materials for lithium ion batteries
[16,17]. Hybridization with CNTs provides effective conductive networks for charging and discharging of molybdenum
and tungsten oxide NPs. Molybdenum/tungsten sulfide/
nitride/carbide have also served as effective catalysts for
hydrogenation [18,19] and hydrodesulfurization [20], where
molybdenum and tungsten oxides serve as very important
precursors. Molybdenum/tungsten sulfide/nitride/carbide
can be produced from temperature programmed reaction of
molybdenum/tungsten oxides with H2S/NH3/CH4 + H2 [21–
23]. Monolayer, bilayer and trilayer molybdenum disulfides
display distinguishable features at photoluminescence due
to the bandgap structure variation [24]. CNT hybrids with different layer or even island MoS2 and WS2 provide ideal nanostructures for charge transfer studies between transitionmetal dichalcogenides and CNTs.
Herein, homogeneous ultra-fine molybdenum and tungsten trioxide NPs (1–2 nm) with different densities have been
in-situ produced and anchored into the defects of multiwalled carbon nanotubes (MWCNTs) to give NP-MWCNT
hybrid structures by a facile ultra-sonication method. The
NP-MWCNT hybrid structures have been further sulfurized
into MoS2 and WS2 MWCNT hybrid structures with different
layers and morphology. The structures and optical properties
of as-produced structures have been characterized by highresolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), selective area electron diffraction
(SAED), Raman spectroscopy, and photoluminescence (PL)
spectroscopy. The as-produced molybdenum oxide NPs
from the phosphomolybdic acid decomposition have been
observed to be crystallized in orthorhombic MoO3. However,
the free-standing molybdenum trioxide nanowires (NWs)
with prolonged reaction time have been found to be
crystallized in monoclinic MoO3Æ0.5H2O (m-MoO3Æ0.5H2O).
The MoO3, WO3, MoS2, and WS2 MWCNT hybrid nanostructures, with highly active surfaces and conductive networks,
have prominent potentials for catalysts, batteries, and other
applications.
2.
Experimental section
2.1.
Synthesis of MWCNTs
MWCNTs were produced by chemical vapor deposition (CVD)
using Co/Fe-Al2O3 as catalysts, ethylene as carbon source. The
pristine MWCNTs were oxidized in air at 793 K for 1 h to
remove amorphous carbon and to produce more defects.
169
2.2.
Synthesis and sulfuration of MWCNT hybrids with
homogeneous molybdenum oxide and tungsten oxide NPs
with different densities
0.5 mg MWCNTs were added into 25 mL H3PMo12O40 (or
H3PW12O40) (analytical reagent, Sinopharm) saturated aqueous solution. The solution was sonicated for 6 h at intervals
of 5 s at the power of 200 W (BILON92-IIL equipped with a
U6 mm ultrasonic horn, Xi’an Bilon Biotechnology), and then
heated under reflux for 2 h to give ultra-dense and ultra-fine
molybdenum oxide (tungsten oxide) NP-MWCNT hybrid
structures. The concentration of H3PMo12O40 (or H3PW12O40)
and MWCNTs and sonication time were adjusted to tune
the NP densities. The product mixture was then filtered by
0.45 lm membrane film and thoroughly rinsed by milli-Q
water. The byproducts and NP-MWCNT hybrid structures
were then extracted by toluene and water solution. NPMWCNT hybrid structures were transferred into their sulfide
counterparts by heating the NP-MWCNT hybrid structures
at 1073 K in an atmosphere of H2S/H2 for 2 h.
2.3.
Characterization
Raman and photoluminescence spectroscopies were taken in
a back-scattering geometry using a single monochromator
with a microscope (Reinishaw inVia) equipped with CCD array
detector (1024 · 256 pixels, cooled to 70 C) and an edge filter. The samples were excited by an Argon ion laser at
514 nm. The spectral resolution and reproducibility were
determined to be better than 0.1 cm1. HRTEM images were
acquired by JEOL JEM-2100F transmission electron microscopy
(TEM; acceleration voltage: 200 kV). XRD patterns were
obtained from a Rigaku SmartLab using Cu/Ka aradiation
˚ ) at 40 kV and 30 mA. The thermogravimetric
(k = 1.5418 A
analysis (TGA) was performed with METTLER TOLEDO thermal analysis TGA/DSC 1 Star system.
2.4.
Structural model creation
All the crystal models were based on the XRD data (MoO3: #050508, MoO3Æ0.5H2O: #49-0652). The initial structural clusters of
MoO3 and MoO3Æ0.5H2O were directly from their supercells
consisting of 36 Mo atoms, which were expressed as
(MoO3)36 and (MoO3Æ0.5H2O)36 with two Mo–O octahedra in
the center constrained to their bulk position during geometry
optimization. H atoms were used to saturate the dangling
bonds of Mo atoms. Thus the final clusters were expressed
as (MoO3)36-42H and (MoO3Æ0.5H2O)36-36H.
2.5.
Theoretical calculations
All calculations in this work were carried out by the Dmol3
code [25,26] using the density functional theory (DFT) within
the generalized gradient approximation (GGA) proposed by
Perdew, Burke, and Ernzerhof (PBE) [27] for exchange–correlation energy. The DFT semicore pseudopotentials (DSPP) [28]
were implemented for all electron calculations, where the
core electrons were replaced by a simple effective potential.
A double numerical plus polarization (DNP) was employed as
the basis set with higher accuracy than Gaussion 6–31** [29].
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3.
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8 5 ( 2 0 1 5 ) 1 6 8 –1 7 5
Results and discussion
The MWCNTs from CVD have uniform diameters of
10 ± 3 nm, as shown in Fig. S3a. Less than 0.9% catalysts in
the as-produced MWCNTs were detected from TGA measurements (Fig. S3b). A stronger D-band than G-band were
observed from the Raman spectrum (Fig. S3c), indicating a
highly defective structure. The local high pressure and high
temperature produced by ultra-sonication provide a facile
path to synthesize NP-MWCNT hybrid structures. The high
density of defects gives sufficient active sites for the tight
attachment of NPs produced from the decomposition of phosphomolybdic acid hydrate (PMA) and phosphotungstic acid
(PWA) under ultra-sonication. MoO3 and H3PO4 (WO3 and
H3PO4) were obtained from PMA (PWA) decomposition under
sonication. Ultra-dense MoO3 (or WO3) NPs with diameters
around 1–2 nm are uniformly distributed on the surface of
MWCNTs (Fig. 1a–d). Most of the NPs are tightly anchored into
the defects of MWCNTs (inset of Fig. 1a). The left H3PO4
solution was removed by filtration and thoroughly rinsed by
Fig. 1 – HRTEM images of (a) and (b) MoO3 NP-MWCNT
hybrid structures, inset shows the hetero-junction between
NP and MWCNT; (c) and (d) WO3 NP-MWCNT hybrid
structures; (e) MoO3Æ0.5H2O nanowire; (f) SAED pattern of
MoO3Æ0.5H2O nanowire along (2 2 1) zone axis; (g) SAED
pattern of WO3 NPs.
milli-Q water. Byproducts with colors from grey to blue, in
addition to NP-MWCNT hybrid structures, were formed from
PMA reaction with extended refluxing time to more than 10 h.
The light blue byproducts are free-standing molybdenum
oxide NWs with diameters of dozens to hundreds of nanometers (Fig. 1e), which were expected to have the same lattice
structures as their NP counterparts. However, there was no
NW structure observed from the PWA reaction even the reaction time was extended to be more than 2 days. The tungsten
oxide NP-MWCNT hybrid structures present even denser and
better NP distribution (Fig. 1c and d).
The structures of molybdenum and tungsten oxide have
been further characterized by XRD (Fig. 2a and b), SAED
(Fig. 1f and g), and Raman spectroscopy (Fig. 3a and c). The
free standing molybdenum oxide NWs have been demonstrated to be monoclinic MoO3Æ0.5H2O (space group P21/m
˚,
(11)) with crystallographic lattice constants of a = 9.676 A
˚ , c = 7.100 A
˚ , and b = 102.39. The lattice structure
b = 3.708 A
and structure details are shown in Figs. 2c, S1 and Table S1.
The XRD pattern of the molybdenum oxide NWs (Fig. 2a
green) agrees well with the corresponding standard
MoO3Æ0.5H2O pattern in JCPDS card No. #49-0652. The
interplanar crystal spacings and angles in the SAED pattern
provide further evidence for the MoO3Æ0.5H2O crystal structure. The electron diffraction spots in the SAED pattern are
assigned to the crystal planes of (1 0 2), (1 1 0) and (2 1 2)
(Figs. 1f and S2). The interplanar angles measured in Fig. 1f
are consisting well with calculated monoclinic interplanar
angles. The zone axis was determined to be (2 2 1). The
calculation details are demonstrated in the Supplementary
information. The crystal structure of the molybdenum oxide
NPs was expected to be monoclinic MoO3Æ0.5H2O. However,
the XRD pattern of the molybdenum oxide NP-MWCNT
hybrid structures (Fig. 2a black) presents strong signals
corresponding to (0 2 0), (0 4 0), (0 4 1) and (0 6 0) crystal planes
of the orthorhombic MoO3 (o-MoO3) structure (space group
Pbnm (62), JCPDS card No. #05-0508) with crystallographic
˚ , b = 13.858 A
˚ , c = 3.697 A
˚ . The
lattice constants of a = 3.962 A
corresponding structural model is shown in Fig. 2d.
The powder X-ray diffraction pattern of tungsten oxide
NP-MWCNT hybrid structures agrees well with standard
monoclinic WO3 (m-WO3) pattern in JCPDS card No. #54-0508
(Fig. 2b). The XRD pattern presents features of (0 1 1), (1 1 1),
(2 0 0), (0 0 2), (2 1 0), (1 2 0), (0 2 2)/(1 2 2), (3 1 2) planes from
monoclinic WO3 (space group P21/a (14)) with crystallographic
˚ , b = 4.570 A
˚ , c = 5.316 A
˚ , and
lattice constants of a = 6.160 A
b = 101.41 (Fig. 2e). The XRD pattern is consisting well with
the monoclinic WO3 film produced from hot-filament metal
oxide deposition [30]. The electron diffraction circles of the
SAED pattern of the m-WO3 NP-MWCNT hybrid structures
present features corresponding to crystal planes of (0 1 1),
(2 1 2)/(1 1 2) (Fig. 1g), consisting well with the XRD pattern.
The lattice structures of m-MoO3Æ0.5H2O NWs (Fig. 2c),
o-MoO3 (Fig. 2d), and m-WO3 (Fig. 2e) NP-MWCNT hybrid
structures were further confirmed by Raman spectroscopy.
The Raman features of MoO3Æ0.5H2O NWs are shown in Fig. 3a
(green). Raman shifts around 150–200 cm1 are deformation
modes of O(2)@Mo(1)@O(4) and O(7)@Mo(2). The Raman features
around 668 cm1 corresponding to the asymmetric stretching
of Mo–O–Mo bridge, 818 cm1 corresponding to the symmetric
20
Intensity /a.u.
818
907980
MoO3·0.5H2O NWs
404
378
MoS2-MWCNTs
MoO3 NP-MWCNTs
40
200
Pristine MWCNTs
400
600
800-1
Raman shift /cm
40
0 2 2/-1 2 2
-3 1 2
50
2θ / degree
1000
514nm Excitation
Intensity /a.u.
210
(b)
120
30
302
041
060
003
102
201
002
30
2θ / degree
002
-1 1 1
200
Intensity /a.u.
(b)
20
011
10
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8 5 (2 0 1 5) 1 6 8–17 5
(a) 514nm Excitation
040
101
200
020
Intensity /a.u.
(a)
001
CARBON
520
522
533
540
537 542
MoO3·0.5H2O NWs
525.5
525 MoS2-MWCNTs
MoO3 NP-MWCNTs
537
542
Pristine MWCNTs
520
530
540 550 600 650
60
Emission /nm
(c) 514nm Excitation
Intensity /a.u.
(c)
417
352
WS2-MWCNTs
801
263 WO NP-MWCNTs 690
3
200
(e)
Raman shift /cm
(d)
Intensity /a.u.
(d)
Pristine MWCNTs
600
800
-1
400
514nm Excitation
553
560 597
606
615 660
526
524 WS -MWCNTs
2
536
520
533
WO3 NP-MWCNTs
543
Pristine MWCNTs
520
530
540550
600
Emission /nm
Fig. 2 – The experimental XRD patterns of (a) MoO3Æ0.5H2O
NWs (green) and o-MoO3 NP-MWCNT hybrid structures
(black), drop lines corresponding to standard XRD patterns
in JCPDS card No. #49-0652 (green) and No. #05-0508 (Black);
(b) m-WO3 NP-MWCNT hybrid structures, drop lines
corresponding to standard XRD patterns in JCPDS card No.
#54-0508; structural models of (c) MoO3Æ0.5H2O, (d) o-MoO3
and (e) m-WO3. (A color version of this figure can be viewed
online.)
650
Fig. 3 – Raman spectra of molybdenum composites (a) and
tungsten composites (c); photoluminescence spectra of
molybdenum composites (b) and tungsten composites (d).
(A color version of this figure can be viewed online.)
stretching of terminal oxygen atoms, 907 cm1 corresponding
to the stretching vibration of Mo(2)–O(6), 925 cm1 corresponding to m3 mode of O(4)@Mo(1)@O(2), 980 cm1 corresponding to
the stretching vibration of Mo(1)–O(2) have been detected in
MoO3Æ0.5H2O NWs. The Raman features are consisting well
172
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8 5 ( 2 0 1 5 ) 1 6 8 –1 7 5
with the reported experimental and calculated data of
MoO3Æ0.5H2O [31]. o-MoO3 NP-MWCNT hybrid structures
(Fig. 3a black) present only slightly different Raman pattern
from MoO3Æ0.5H2O NWs since both Mo6+ positions keep
octahedral oxygen environments. A narrow line at 995 cm1,
˚)
15 cm1 shift from stretching vibration of Mo(1)@O(2) (1.72 A
in MoO3Æ0.5H2O NWs, is due to the vibration of Mo@O(1)
˚ ). A very strong and well defined line at 820 cm1 is
(1.67 A
from the symmetric stretching vibrations of Mo–O(3 0 )–Mo
[31]. The Raman features of o-MoO3 NP-MWCNT hybrid
structures agree well with the reported o-MoO3 data [31,32].
The absence of Raman features at 907 cm1 and 925 cm1 in
o-MoO3 NP-MWCNT hybrid structures further confirmed the
detachment of H2O in the molybdenum oxide structure,
giving a less distorted octahedral structure.
The lattice structures of PMA decomposition products are
depending on size and morphology. o-MoO3 lattice structure
is favored for NPs, while m-MoO3Æ0.5H2O lattice structure is
favored for NWs. The structure preference is consisting well
with the relative Gibbs free energy calculations. The relative
Gibbs free energy of bulk MoO3Æ0.5H2O (perfect crystal) was
calculated to be 0.1611 eV (per MoO3 unit) less than bulk oMoO3 (perfect crystal), indicating a more stable structure of
MoO3Æ0.5H2O in large scale. The NW structures are assumed
as perfect crystals compared to NP counterparts. The proportion of surface areas becomes more and more prominent with
decreasing size. The atoms in surface areas are relaxed to
energetic favored positions from previous lattice coordinates.
The water molecules in MoO3Æ0.5H2O lattice tend to dissociate
from near neighboring Mo and escape from the lattice constraint during simulation process in cluster size. The relative
Gibbs free energy of MoO3Æ0.5H2O cluster is 0.1390 eV (per
MoO3 unit) higher than o-MoO3 cluster, indicating a less stable
structure of MoO3Æ0.5H2O in cluster morphology. The calculation results are consisting well with our experimental data.
The Raman spectrum of m-WO3 NP-MWCNT hybrid structures presents strong bands centered at 801 cm1 and
690 cm1, corresponding to the stretching modes of W–O
bonds. A broad band around 100–400 cm1 was also detected.
The features below 200 cm1 and 200–400 cm1 are corresponding to lattice vibration and deformation modes, respectively. The Raman features agree with monoclinic WO3 better
than other WO3 lattice or its hydrates reported [33]. The band
at 801 cm1 agrees well with monoclinic WO3 produced from
hot-filament metal oxide deposition [30]. However, the
stretching mode at 690 cm1 is lower than reported
710 cm1 [30,33]. The W–O bond distance can be calculated
from empirical relationships between metal–oxygen bond
and Raman stretching mode wavenumbers (Eq. (1)) [34] to be
˚ and 1.904 A
˚ from Raman shifts of 801 cm1 and
1.826 A
1
690 cm , respectively. The bond valences calculated from
Eq. (2) [34] are 1.285 and 0.998, corresponding to W–O(2) and
W–O(1), respectively. W–O(1) bond of the m-WO3 NP-MWCNT
˚ longer than the reported bond of
hybrid structures is 0.019 A
monoclinic WO3 structures [30,33], which may due to the
nano-size or MWCNT hybrid structure effect. However,
W–O(2) is not distorted. The Raman shifts corresponding to
shorter W–O(3/4/5/6) bonds (more than 1000 cm1) were not
distinguished here since they are overlapping with Raman
features of MWCNTs.
v=cm1 ¼ 25823e1:902R
6:0
R
sðW–OÞ ¼
1:904
ð1Þ
ð2Þ
where v is the Raman stretching mode wavenumber (cm1)
and R is the W–O bond length.
A high specific capacity (748 mAh g1), volumetric capacity
(1500 mAh cm3), good rate capability, and high cyclability
have been demonstrated by mesoporous tungsten oxides
[17]. The battery capacity of MoO3 NPs with diameters around
5 lm has been demonstrated to be about half of that of MoO3
NPs with diameters around 30–60 nm at 0.5 C rate [16]. Here,
the ultra-dense and ultra-fine MoO3 and WO3 NP-MWCNT
hybrid structures with conductive network and tightly bonding with CNTs have prominent applications in lithium ion
batteries. MoS2 and WS2 can be produced from a gas-phase
reaction of MoO3x and WO3x with H2S, respectively [35].
MoS2 and WS2 have high melting points and low solubility
with good corrosion resistance, high stability and mechanical
strength. Their hybrid structures with CNTs not only serve as
excellent anode materials for lithium ion batteries [36], but
also serve as effective catalysts for hydrogenation [18,19]
and hydrodesulfurization [20]. Sulfuration of MoO3 and WO3
NP-MWCNT hybrid structures into MoS2 and WS2 MWCNT
hybrid structures provides practical feasibilities for various
applications.
MoS2 and WS2 NP-MWCNT hybrid structures were
expected to be obtained from the sulfuration of MoO3 and
WO3 NP-MWCNT hybrid structures. However, only layered
structures of MoS2 and WS2 (Fig. 4) along the walls of
MWCNTs have been achieved from the sulfuration of MoO3
and WO3 NP-MWCNT hybrid structures with H2S gas, respectively. The MWCNT hybrids with MoS2 (Fig. 4a) and WS2
(Fig. 4b) nanotubes (2–3 walled) were obtained from the
sulfuration of ultra-dense and ultra-fine MoO3 and WO3
NP-MWCNT hybrid structures, respectively. The layers and
morphology of MoS2 and WS2 have been found to depend
on the densities of MoO3 and WO3 NPs on MWCNTs. The
Fig. 4 – HRTEM images of MoS2-MWCNT hybrid structures
(a) and WS2-MWCNT hybrid structures (b) from ultra-dense
and ultra-fine MoO3 and WO3 NP-MWCNT hybrid structures
as shown in Fig. 1a–d; (c) structural models of MoS2 and
WS2. (A color version of this figure can be viewed online.)
173
8 5 (2 0 1 5) 1 6 8–17 5
well with reported MoS2 structures [36,37]. The XRD pattern
(Fig. 6a) presents features of (0 0 2), (1 0 0), (1 0 3), (0 0 6), (1 0 5),
and (1 1 0) planes of hexagonal MoS2 (2H-MoS2, JCPDS card
No. #37-1492), consisting well with reported XRD pattern of
MoS2 structures [38]. Raman spectrum of WS2-MWCNT hybrid
structures presents strong Raman features at 352 cm1 and
417 cm1 (Fig. 3c, blue) of WS2. Raman peak at 352 cm1 is
assigned to the longitudinal acoustic (2LA) and E2g modes,
while 417 cm1 is assigned to A1g mode of 2H-WS2 [39]. The
XRD pattern (Fig. 6b) gives features of (0 0 2), (1 0 0), (1 0 3),
(1 1 0) and (2 0 1) planes of hexagonal WS2 (2H-WS2, JCPDS card
No. #08-0237), consisting well with reported XRD pattern of
WS2 structures [39]. Both Raman and XRD patterns further
confirmed the successful synthesis of MoS2 and WS2 from
MoO3 and WO3, respectively.
It is well known that semi-conducting SWCNTs give different PL features depending on their chirality [40–42]. PL signals
at 533 nm, 560 nm, 597 nm, 606 nm, 615 nm, and 660 nm
(Fig. 3b and d red) with excitation laser at 514 nm were also
detected in the pristine MWCNTs. The PL features are not
affected by various treatments in this work. The origination
of the MWCNT PL will be further investigated in our future
work. Different PL features were obtained from MWCNT
hybrids with molybdenum oxide, tungsten oxide, molybdenum sulfide, and tungsten sulfide with excitation laser at
514 nm. o-MoO3 NP-MWCNT hybrid structures give PL peaks
at 537 nm and 542 nm in addition to the signals from pristine
MWCNTs. The PL pattern is slightly different from the mMoO3Æ0.5H2O NWs, where another strong peak at 540 nm
was observed. The m-WO3 NP-MWCNT hybrid structures give
strong PL signals at 536 nm with shoulder at 533 nm and two
other bands centered at 520 nm and 543 nm in addition to the
30
110
105
006
40
50
2θ /degree
60
201
110
Intensity /a.u.
10
Fig. 5 – HRTEM images of MoS2-MWCNT hybrid structures
(b), (d) and WS2-MWCNT hybrid structures (f), (h) from the
corresponding MoO3 NP-MWCNT hybrid structures (a), (c)
and WO3 NP-MWCNT hybrid structures (e), (g) with different
densities.
20
002
10
(b)
100
103
Intensity /a.u.
002
(a)
103
densities of MoO3 and WO3 NPs were controlled by sonication
time and reagent concentrations.
Homogeneous MoO3 (Fig. 5a) and WO3 (Fig. 5e) NP-MWCNT
hybrid structures with lower densities were produced when
the sonication of MWCNTs with saturated aqueous PMA
and PWA were reduced to 10 min without further reflux.
The MWCNT hybrids with single-walled MoS2 (Fig. 5b) and
WS2 (Fig. 5f) nanotubes were produced after sulfuration. With
increasing MWCNT concentration and decreasing PMA and
PWA concentration, the WO3 and MoO3 NPs become much
sparser (Fig. 5c and g). The MWCNT hybrids with MoS2
(Fig. 5d) and WS2 (Fig. 5h) islands were synthesized after sulfuration of the sparse MoO3 and WO3 NP-MWCNT hybrid
structures. Layered structures of MoS2 and WS2 were produced instead of NP structures on MWCNTs. The MoS2 and
WS2 structures are tightly attached the walls of MWCNTs,
as shown in Figs. 4 and 5. The structures of MoS2 and WS2
were further confirmed by Raman and XRD.
The MoS2-MWCNT hybrid structures give strong Raman
shifts at 378 cm1 and 404 cm1 (Fig. 3a blue), indicating the
emergence of MoS2 structures. The peak at 378 cm1 corresponds to the in-plane (E12g) mode, while the peak at
404 cm1 is attributed to the out-of-plane (A1g) mode of
MoS2. The Raman spectrum of MoS2 MWCNT products agrees
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30
40
50
60
2θ / degree
70
80
Fig. 6 – The experimental XRD patterns of (a) MoS2-MWCNT
hybrid structures and (b) WS2-MWCNT hybrid structures,
drop lines corresponding to standard XRD patterns in JCPDS
card No. #37-1492 (a) and No. #08-0237 (b).
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8 5 ( 2 0 1 5 ) 1 6 8 –1 7 5
signals from pristine MWCNTs. The PL features of both MoO3
and WO3 were not detected after sulfuration, further confirming the successful transformation of MoS2 and WS2 from
MoO3 and WO3 NPs. New PL features at 525 nm and
525.5 nm were detected in the as-produced MoS2-MWCNT
hybrid structures (Fig. 3b). The as-produced WS2-MWCNT
hybrid structures give analogous features at 524 nm and
526 nm (Fig. 3d). PL features of 525 nm and 525.5 nm instead
of 620 nm and 680 nm [37,43] were observed in MoS2-MWCNT
hybrid structures (Figs. 3b and S4). PL features from MoS2MWCNT hybrid structures were observed to be different from
free standing few layered MoS2 [37,43], indicating a strong
interaction between MoS2 and MWCNTs. 1–3 layer WS2 structures show PL peaks in the range between 630 and 640 nm
[44]. However, no PL peak around 630–640 nm was detected
in WS2-MWCNT hybrid structures. New PL features at
524 nm and 526 nm have been observed from WS2-MWCNT
hybrid structures, indicating a strong interaction between
WS2 and MWCNTs. Strong exciton interactions between transition-metal dichalcogenides and MWCNTs have been
observed from MWCNT hybrids with both MoS2 and WS2.
4.
Conclusions
The MWCNT hybrids with homogeneous ultra-dense and
ultra-fine MoO3 and WO3 NPs (1–2 nm) have been produced
by a facile ultra-sonication method. The NPs were tightly
attached to the defects of MWCNTs. The lattice structures
of MoO3 and WO3 NPs have been determined by HRTEM,
SAED, XRD, Raman and PL spectroscopy to be o-MoO3 and
m-WO3, respectively. The structures of molybdenum oxide
decomposed from PMA have also been found to depend on
morphology. The NPs on MWCNTs were found to crystallized
in o-MoO3, while the NWs were found to crystallized in
m-MoO3Æ0.5H2O. The structure selectivity was also proved
by thermodynamic analysis. The m-MoO3Æ0.5H2O lattice
structure is energetic favored in bulk morphology, while the
o-MoO3 lattice structure is energetic favored in cluster
morphology. The NP densities have also been tuned by controlling the sonication time and reagent concentrations. The
MWCNT hybrids with 2–3 layers of MoS2 and WS2 nanotubes
have been produced from ultra-dense and ultra-fine MoO3
and WO3 NP-MWCNT hybrid structures. The MWCNT hybrids
with single layer or island MoS2 and WS2 have been produced
from sparse MoO3 and WO3 NP-MWCNT hybrid structures.
The hybrid nanostructures of MoS2 and WS2 with MWCNTs
can be tuned by MoO3 and WO3 NP-MWCNT hybrid precursors, providing practical feasibilities of various photonic and
electronic investigations.
Acknowledgments
This research is supported by National Natural Science Foundation of China (51302210). We thank T. He from Center for
Materials Chemistry, Frontier institute of science and technology, for use of XRD. The TEM work was done at International
Center for Dielectric Research (ICDR). The authors also thank
C. Ma for his help in using TEM. We thank Prof. Y. Cheng for
providing SGI working station and CASTEP code.
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.carbon.
2014.12.090.
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