High-surface-area plasmonic MoO3-: X: Rational synthesis and enhanced ammonia borane dehydrogenation activity

Article abstract of DOI:10.1039/c7ta01217a

Well-crystallized, high-surface-area plasmonic MoO_3-x was synthesized by combining an evaporation induced self-assembly (EISA) process and a subsequent hydrogen reduction at a certain temperature. The intrinsic anisotropic crystal growth process of tiny MoO₃ nuclei to MoO₃ nanosheets was successfully inhibited. Detailed characterization by means of XRD, TEM, N₂ physisorption, and XPS measurements revealed that the synthesized MoO_3-x not only showed a strong localized surface plasmon resonance (LSPR) under incident light but also had a relatively large specific surface area. The specific surface area (S_BET) of MoO_3-x after reduction at 200 °C was 30.0 m² g^-1, which was 22.7 and 9.1 times higher than those of commercially available MoO₃ and our previously reported MoO_3-x nanosheets, respectively. We also demonstrate that such a semiconductor with a large surface area could be used as a highly efficient catalyst that dramatically enhances the dehydrogenation activity for ammonia borane (NH₃BH₃; AB) under visible light irradiation.

Full text of DOI:10.1039/c7ta01217a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C7TA01217A
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ARTICLE
High-surface-area plasmonic MoO3-x: rational synthesis and
enhanced ammonia borane dehydrogenation activity
Received 00th January 20xx,
Accepted 00th January 20xx
Haibo Yin,a Yasutaka Kuwahara,ab Kohsuke Mori,abc Hefeng Cheng,a Meicheng Wena and Hiromi
Yamashitaab
DOI: 10.1039/x0xx00000x
The well-crystallized, high-surface-area plasmonic MoO3-x was synthesized by combining evaporation induced self-
assembly (EISA) process and the following hydrogen reduction in certain temperature. The intrinsic anisotropic crystal
growth process of tiny MoO3 nuclei to MoO3 nanosheets was successfully inhibited. Detailed characterization by means of
XRD, TEM, N2 physisorption, and XPS measurements revealed that the synthesized MoO3-x not only showed a strong
localized surface plasmon resonance (LSPR) upon incident light but also had a relatively large specific surface area. The
specific surface area (SBET) of MoO3-x post-reduced at 200 oC was 30.0 m2/g, which was 22.7 and 9.1 times higher than
those of commercially available MoO3 and our previously reported MoO3-x nanosheets, respectively. We also demonstrate
that such semiconductor with a large surface area could be used as a highly efficient catalyst that dramatically enhance the
dehydrogenation activity from ammonia borane (NH3BH3; AB) under visible light irradiation.
www.rsc.org/
more concern recently.8,9 For example, Alivisatos’s group
reported that the high energy of the WO2.83 LSPR has high
charge carrier density of N = 6.3 × 1021 cm-3.2 Our group has
Introduction
Localized surface plasmon resonances (LSPRs) typically
originate from nanostructures of noble metals leading to
enhanced and geometrically tuneable absorption and
scattering.1 The overwhelming majority of research
achievements of LSPRs have been focused on noble metals,
such as Au and Ag, as they are stable under several conditions,
have high charge carrier densities and exhibit apparent LSPRs
from the ultraviolet (UV) region to the near infrared (NIR).2,3
However, it is difficult to get the desired plasmonic noble
metals because plasmonic frequencies of noble metals are
extremely relied on their shapes and size.4 Moreover, the
amount of noble metals on the earth is scare and they usually
have high resistive loss, which greatly limits their practical
application.5 Thus, the exploration of the qualified candidates
is a challenging task.
already synthesized plasmonic molybdenum oxide nanosheets
by a facile non-aqueous procedure without using any surface-
capping agents, which displayed strong LSPR under visible light
irradiation.3,10 However, heavily doped semiconductor oxides,
such as WO3-x and MoO3-x, usually exhibit very small specific
surface area, which frequently results in relatively low catalytic
activities.11,12 Some researchers have already reported
importance of increasing in specific surface area of plasmonic
transition-metal oxides. Zhao and co-workers successfully
prepared highly ordered mesoporous WO3 with a large surface
area by using self-made template (amphiphilic poly (ethylene
oxide)-b-polystyrene (PEO-b-PS) diblock copolymers) as a
structure-directing agent.13,14 However, it is still a challenge to
synthesize heavily doped transition-metal oxides with a strong
LSPR and large surface area at the same time, especially for
LSPRs are not primarily confined to the noble metals, but
happen in conducting metal oxides.6,7 Some heavily doped
semiconductor nanocrystals with appreciable free carrier
densities, including copper chalcogenides, indium tin oxides
(ITO), and transition-metal oxides have attracted more and
11,12,15
MoO3-x
.
Take MoO3 nanosheets (SBET = 3.3 m2/g) as an
example, the intrinsic anisotropic crystal growth tends to
result in the preferential formation of 2D MoO3 nanosheet,16-18
because the crystal structure plays
a crucial role in
determining the morphology of the samples and the layered
crystal structure of orthorhombic MoO3 is composed of MoO6
octahedra by sharing edges and corners.19 Therefore, it is of
great difficulty, but important to exploit the synthetic
methodology for fabrication of plsamonic MoO3-x with large
specific surface area.
a.Division of Materials and Manufacturing Science, Graduate School of Engineering,
Osaka University, Osaka, 565-0871, Japan.
b.Unit of Elements Strategy Initiative for Catalyst & Batteries (ESICB), Kyoto
University, Kyoto, 615-8245, Japan.
c. JST, PRESTO, 4-1-8 HonCho, Kawaguchi, Saitama, 332-0012, Japan.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Herein, we report MoO3-x nanostructures with relatively
high surface area of 30.0 m2/g and a strong LSPRs in visible
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region by combining evaporation induced self-assembly (EISA) 1.5406 Å). Transmission electron microscopy (TEM) images
View Article Online
process20-30 and the following hydrogen reduction at a certain were measured with a Hitachi H-800 electron microscope
DOI: 10.1039/C7TA01217A
temperature. In this study, F127 was used as an organic equipped with an energy dispersive X-ray detector operated at
template, which efficiently inhibited intrinsic growth process 200 kV. UV-Vis-NIR diffuse reflectance spectra were collected
of tiny MoO3 nuclei to MoO3 nanosheets as illustrated in on
a Shimadzu UV 2600 recording spectrophotometer
Scheme 1. It was found that the experimental parameters, equipped with an integrating sphere at room temperature. The
such as hydrogen reduction temperature, were found to play a reference was BaSO4 and the absorption spectra were
crucial role in LSPRs response intensity of MoO3-x from the obtained using the Kubelka-Munk function. XPS measurements
visible region to the NIR. Eventually, it was further certified were carried out using a Shimadzu ESCA 3200 photoelectron
that such hydrogen-reduced MoO3-x showed greatly improved spectrometer with Mg kα radiation, and C 1s (284.6 eV) was
activity in the hydrogen (H2) production from ammonia borane used to calibrate the peak positions of the elements. Nitrogen
(NH3BH3; AB) compared with MoO3-x nanosheets (SBET = 3.3 adsorption measurements (BET) were performed using BEL
m2/g) and commercial MoO3 (SBET = 1.3 m2/g), which may SORP-max system (MicrotracBEL Corp.) at -196 oC. Samples
o
provide a new strategy for highly efficient dehydrogenation were degassed under vacuum at 200 C for 24 h prior to data
from hydrogen storage materials. To the best of our collection. Thermogravimetric (TG) analysis was acquired on a
knowledge, this should be the first report that definitely Bruker TG-DTA2010SA system from room temperature to 800
improves specific surface area of plasmonic MoO3-x
.
oC at a heating rate of 10 oC/min under air flow.
Catalytic Activity Measurements
The catalytic performances of the molybdenum oxide
nanostructures were measured by dehydrogenation of NH3BH3
in aqueous suspensions at room temperature (25 C). Typically,
Experimental Section
Chemicals
o
MoO3-x-T sample (20 mg) was suspended in 5 mL distilled
Molybdenum metal powder, hydrogen peroxide (H2O2, 30
wt%), nitric acid (HNO3, 69.2 wt%), acetic acid (CH3COOH),
ethanol, Pluronic F127 (PEO106 PPO70PEO106, Mw = 12600 g
mol-1), were purchased from Nacalai Tesque Inc. Commercial
MoO3 was purchased from Wako Pure Chemical Industries.
Ammonia borane (NH3BH3) was obtained from Sigma-Aldrich
Co. All chemical reagents were used without any further
purification.
water in a test tube. 200 μL of NH3BH3 (0.1 M, 20
mol)
solution was injected into the above solution through a rubber
septum after bubbling with Argon gas for 30 min. The reaction
was conducted with magnetically stirred in the dark condition
and under visible light irradiation (λ > 420 nm) with cutoff filter.
The amount of H2 in gas phase was determined by a Shimadzu
GC-8A gas chromatograph with MS-5A column by TCD detector.
Catalyst Synthesis
Synthesis of Plasmonic MoO3-x-T
Results and discussion
The well-crystallized, high-surface-area MoO3-x was
synthesized by EISA process and the following hydrogen
reduction at a certain temperature. In a typical synthetic
procedure, 2 mmol molybdenum metal powder was added to
a flask (100 mL) containing 15 ml ethanol under magnetically
stirred. Then 3 mL of H2O2 was injected and continued stirring
for 30 min to acquire the transparent yellow solution (solution
A). Next, 2.0 g of Pluronic F127 was added into 10.0 mL of
Route 1 and Route 2 in Scheme 1 show the designed synthetic
routes of our previously reported MoO3-x nanosheets and
MoO3-x-T developed in this study, respectively. Firstly, pure Mo
powder was added to the ethanol solution containing 3 mL of
H2O2 (30 wt%). The Mo powder was dissolved and present as a
transparent yellow solution after magnetic stirring for 30 min.
In this process, the solution-soluble precursor compound
MoO2(OH)(OOH) could form easily.31,32 In the subsequent
growth stage, tiny MoO3 nuclei was generated by dehydration
of the dissolved yellow Mo-complex. The crystal structure
plays a very important role in determining the morphology of
the sample.17-19
o
ethanol at 40 C to obtain a colorless solution with vigorous
stirring. After that, 4.5 mL HNO3 and 2.4 g CH3COOH were
added to the above solution dropwise (solution B). In the
subsequent steps, solution A was added into solution B with
o
vigorous stirring at 40 C for 2 h to get a transparent yellow
solution. This solution was cast in petri dish to form a thin gel
layer and then aged at 40 oC for 24 h and 100 oC for another 24
o
h. The resulting film was calcined at 400 C for 4 h in air with a
heating rate of 1 oC/min to obtain MoO3. Finally, hydrogen
reduction of MoO3 was carried out with pure H2 gas at
ambient pressure (1 bar). The products obtained at different
o
reduction temperature (100 oC, 200 C, and 300 oC) for 1 h
with 5 oC/min of heating rate were defined as MoO3-x–T (T
means the reduction temperature).
Catalyst Characterizations
Scheme 1. Schematic illustration for the plausible formation process of MoO3-x
nanosheets (Route 1) and high-surface-area MoO3-x-T (Route 2).
The X-ray diffraction (XRD) patterns were recorded using a
Rigaku Ultima IV diffractometer with Cu kα radiation (λ =
2 | J. Name., 2012, 00, 1-3
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polyether polyol chemical structure of F127 might effectively
View Article Online
DOI: 10.1039/C7TA01217A
bind with dissolved Mo complex or MoO3 nuclei, thereby
reducing the interaction between nucleus and then efficiently
inhibiting the crystal growth of MoO3-x (Route 2).21-23
MoO3-x-300 oC
(a)
According to TG/DTA analysis of (Figure S1b), the mass of
F127 decreases to 0 wt% from 100 wt% after temperature
o
MoO3-x-200 oC
MoO3-x-100 oC
increased to 400 C. So surfactant was completely removed by
o
calcination at 400 C for 4 h. As shown in Figure S1a, the mass
o
of MoO3-x-200 C doesn’t have any change with the increasing
of temperature (from 0 oC to 700 oC), which means that
catalyst is successfully prepared without any impurity.
As-synthesized
MoO3
XRD patterns of commercial MoO3, as-synthesized MoO3,
MoO3-x-100 oC and MoO3-x-200 oC are shown in Figure 1a,
which clearly show that all samples consist of orthorhombic
MoO3 phase (JCPDS NO.5-0508). The three-dimensional
structure framework of MoO3-x-T (100 oC and 200 oC) was
retained to a large extent in MoO3, which demonstrates that
hydrogen reduction did not destroy the pristine crystal
structure. When hydrogen reduction is performed at 300 oC,
the intensity of some characteristic peaks of MoO3 was
significantly weakened, which is presumably because
orthorhombic phase of MoO3-x has been transformed into
amorphous MoO2.5 Figure 1b is the magnified patterns
corresponding to the diffraction peaks of (040), (060), and
(002) in Figure 1(a), respectively. Careful comparison between
two patterns indicates that the 2θ values decrease to a certain
degree from as-synthesized MoO3 to MoO3-x-200 oC. According
to Bragg Formula 2 dsinθ = λ, the smaller 2θ value means
larger interplane distance (d value).33 Therefore, MoO3-x-200 oC
shows larger distance (d values) between crystalline planes of
(004), (060), and (002) compared with as-synthesized MoO3,
indicating that the lattice unit expands after hydrogen atom
are inserted into MoO3, as reflected on the calculated lattice
constants that are listed in Table 1.
As shown in Figure S2a, N2 adsorption-desorption isotherms
did not apparently show type IV isotherms according to the
IUPAC classification, which is closely related to the capillary
condensation taking place in mesoporous solids. Meanwhile,
Figure S2b shows the corresponding pore size distribution
curves, which is calculated from the BJH method by using the
adsorption branches. As shown in Figure S2b, the pore size of
as-synthesized MoO3 and MoO3-x-T (100 oC, 200 oC, and 300 oC)
is about 2.18 nm, 2.13 nm, 2.18 nm, and 2.13 nm, respectively,
which fall within the scope of mesoporous. Structural
parameters containing specific surface area (SBET), pore
diameter (d), pore volume, are shown in Table 2. Specific
surface area and pore diameter of as-synthesized MoO3 and
MoO3-x-T display similar volumes, indicating that sample
structures were not destroyed after hydrogen reduction
process. The specific surface areas of as-synthesized MoO3 and
MoO3-x-T are larger than commercial MoO3 and MoO3-x
nanosheets in Figure S2c (Route 1), and the resulted SBET is
much higher than those of reported work.3,5,7,8,16, 32,33
Commercial MoO3
10
20
30
40
50
60
70
2 Theta (degree)
MoO3-x-200 oC
(b)
As-synthesized
MoO3
20
25
30
35
40
45
50
55
60
2 Theta (degree)
Figure 1. (a) Wide-angle XRD patterns of MoO3-x-T samples hydrogen-treated at
different temperatures and commercial MoO3. (b) Magnified XRD patterns of as-
synthesized MoO3 and MoO3-x-200 oC corresponding to the diffraction peaks of (040),
(060), and (002) in Figure 1a.
Table 1. Lattice constants calculated by XRD data.
Sample
Interplane distance (Å)
Lattice constants (Å)
d(060)
d(002)
1.84
d(200)
a
b
c
as-synthesized
MoO3
MoO3-x-200 oC
2.32
2.33
1.98
1.99
3.96 13.9 3.69
1.86
3.97 14.0 3.72
Because of the layered crystal structure of orthorhombic
MoO3 composed of MoO6 octahedra by sharing edges and
corners, the intrinsic anisotropic crystal growth led to the
preferential formation of 2 D MoO3-x nanosheets with very
small specific area under solvothermal treatments at a certain
TEM images of hydrogen-treated MoO3-x-T samples at
temperature (Route 1).2,3 In order to prevent the unfavorable different temperatures are shown in Figure 2. MoO3-x
synthesized via Route 1 is composed of smooth 2D nanosheets
(Figure 2a), which was deemed to be related to the intrinsic
growth of MoO3 nuclei because of its layered crystal
structure.31,32 As shown in Figure 2b-e, as-synthesized MoO3
and MoO3-x-T synthesized via Route 2 are also made up of
nanosheets, but the nanosheets size of these samples is much
smaller than that of our previously reported plasmonic MoO3-x
Route 1 and increase surface area of plasmonic MoO3-x, we
designed the Route 2. It is widely accepted that surfactants
can efficiently control the crystal growth rate and crystal habit
of a variety of materials.20,21 F127 was used as a surfactant to
control growth process of MoO3 nuclei to modify the final size
of MoO3-x during EISA process. Possible process is as follow,
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nanosheets in Figure 2a. This should be the main reason for
increasing the specific surface area of as-synthesized MoO3
and MoO3-x, which is consistent with Route 2 of Scheme 1.
Meantime, it is clearly found that the surface of as-synthesized
MoO3 and MoO3-x-T (red circle in Figure 2b-e) become more
rough than the smooth surface of MoO3-x nanosheets (white
circle in Figure 2a), which is also beneficial to improve surface
area.
View Article Online
DOI: 10.1039/C7TA01217A
Mo6+ 3d3/2
Mo6+ 3d5/2
MoO3-x-300oC
Mo4+ 3d3/2
Mo4+ 3d5/2
MoO3-x-200oC
Mo5+ 3d3/2
Mo5+ 3d5/2
Table 2. Structural parameters of commercial MoO3, as-synthesized MoO3 and MoO3-x
T samples hydrogen-treated at different temperatures.
-
MoO3-x-100oC
Samples
Specific surface
area (m2/g)
1.32
Pore diameter Pore volume
As-synthesized MoO3
(nm)
-
(cm3/g)
0.002
Commercial
MoO3
as-synthesized
MoO3
25.1
2.18
0.040
Commercial MoO3
MoO3-x-100oC
MoO3-x-200 oC
MoO3-x-300 oC
27.7
30.0
27.4
2.13
2.18
2.13
0.065
0.067
0.066
240 238 236 234 232 230 228 226
Binding energy (eV)
Figure 4. Mo 3d XPS spectra of commercial MoO3, as-synthesized MoO3 and MoO3-x-T
samples.
Table 3. Mo chemical state of different samples determined by XPS data.
Samples
Mo6+
Mo5+
Mo4+
Mo average
state
Commercial MoO3
as-synthesized
MoO3
100%
97.3%
0%
2.70%
0%
0%
6.00
5.97
MoO3-x-100 oC
MoO3-x-200 oC
MoO3-x-300 oC
93.1%
90.5%
92.5%
6.90%
9.50%
0%
0%
0%
7.5%
5.93
5.91
5.85
Figure 3 shows the UV/Vis-NIR spectra of commercial MoO3,
as-synthesized MoO3 and MoO3-x-T. Three strong absorption
peaks located at approximately 623 nm, 756 nm, and 920 nm
connected with LSPRs are clearly observed in the spectra of
MoO3-x-T (T = 300 oC, 200 oC, and 100 oC). With the decrease of
reduction temperature, plasmonic wavelength of MoO3-x-T
shifts to longer wavelength due to the decrease of free
electrons concentration.5 As-synthesized MoO3 shows a weak
absorption peak at 953 nm associated with the reduction
ability of polyhydric alcohols structure of F127 or ethanol
during the preparation process. On the contrary, commercial
MoO3 doesn’t exhibit any absorption peak in visible light range
and only show a peak in UV-light range with the absorption
edge at about 400 nm, which corresponds to its wide band gap
(ca. 3.1 eV).
Figure 2. TEM images of different samples: (a) MoO3-x nanosheets, (b) as-synthesized
MoO3, (c) MoO3-x-100 oC, (d) MoO3-x-200 oC, (e) MoO3-x-300 oC.
(e)
(d)
(c)
(b)
(a)
In order to investigate the chemical state of Mo in the
plasmonic MoO3-x-T, X-ray photoelectron spectroscopy (XPS)
measurement were implemented. Mo 3d XPS core spectra of
commercial MoO3, as-synthesized MoO3 and MoO3-x-T are
displayed in the Figure 4. For the commercial MoO3, only Mo6+
is detected and two peaks (236.15 and 233.05 eV) are assigned
to the 3d3/2 and 3d5/2 of Mo6+, respectively. On the contrary,
the binding energies of as-synthesized MoO3 and MoO3-x-T
shift to the lower energy position with the increase of
200
400
600
800
1000
1200
1400
Wavelength (nm)
Figure 3. UV/Vis-NIR diffuse reflectance spectra of different samples. (a) Commercial
MoO3, (b) as-synthesized MoO3, (c) MoO3-x-100 oC, (d) MoO3-x-200 oC, and (e) MoO3-x
300 oC.
-
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Almost 45.8, 57.5, and 65.6 mol% conversion of AB was
reduction temperature compared with commercial MoO3.
According to Table 3, Mo5+ cation ratio increases gradually in
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obtained by MoO3 and MoO3-x-T (100 oC, 200 oC) after reaction
DOI: 10.1039/C7TA01217A
o
the as-synthesized MoO3, MoO3-x-100 C and MoO3-x-200 oC,
for 60 min. The highest H2 production activity is due to the
largest surface area and relatively strong LSPRs. In contrary,
only 30.5 mol% conversion of AB was yielded by commercial
MoO3. Interestingly, when the dehydrogenation reaction was
carried out under visible light irradiation (Figure 5b),
productive rate of hydrogen was dramatically improved.
and the average Mo oxidation states of as-synthesized MoO3,
o
o
MoO3-x-100 C and MoO3-x-200 C are determined to be 5.97,
5.93 and 5.91, respectively. Mo4+ state emerges for only MoO3-
o
o
x-300 C because of phase transformation at 300 C, which is
consistent with XRD spectra.3,5
o
MoO3-x-200 C could give 64.7 mol% conversion of AB in the
Ammonia-borane (NH3BH3; AB) has emerged as a candidate
of hydrogen storage materials because of its low molecular
weight (30.87 g mol-1), nontoxicity, high stability in solid form
under ambient conditions and higher theoretical hydrogen
gravimetric capacity (19.6 wt%). The hydrolysis of AB can be
attained under mild reaction condition with suitable catalysts,
as illustrated in Equation 1.
first 10 min, whose initial reaction rate (3.88 μmol/min) was
4.21 times higher that of commercial MoO3 (0.92 μmol/min).
o
o
After reaction for 60 min, MoO3-x-100 C and MoO3-x-200 C
each obtained almost 92.9, 100 mol% conversion of AB, which
were also much higher than commercial MoO3. For as-
synthesized MoO3 and commercial MoO3 sample, they could
response slightly to visible light due to in situ partial reduction
of MoO3 by H2 produced from AB. Comparing commercial
o
o
MoO3 (SBET = 1.32 m2/g) with MoO3-x-T (100 C, 200 C), having
a strong LSPRs and a large specific surface area were beneficial
to product hydrogen from AB. The larger surface area of MoO3-
Catalyst
NH3BH3 + 2 H2O
NH4+ + BO2- + 3H2
(1)
Recently, many noble metal nanostructures, such as Pd, Au, Ru
and Rh, have been already demonstrated to exhibit high
activity for dehydrogenation of AB solutions.34-36 Considering
o
o
x-T (100 C, 200 C) usually means the more oxygen vacancy,
which is beneficial to increase the dehydrogenation
performance of AB. Despite of its strong LSPR peak in the
the
practical
application,
we
firstly
investigated
o
dehydrogenation activities of samples in the dark condition. As
shown in Figure 5a, it is easy to obtain H2 over as-synthesized
visible range and a relatively large surface area, MoO3-x-300 C
did not show good hydrogen production activity both in the
dark and under visible light irradiation because of phase
transition from MoO3-x to amorphous MoO2 at around 300 oC.
o
o
MoO3 and MoO3-x-T (100 C, 200 C). Initial reaction rates of
as-synthesized MoO3 and MoO3-x-T (100 oC, 200 oC) with
relatively large surface area was 1.70, 2.11 and 2.21 μmol/min,
respectively. This was much faster than the rate of commercial
MoO3 (0.440 μmol/min).
70
(a)
60
50
40
30
20
10
0
60
Commercial MoO3
(a)
As-synthesized MoO3
MoO3-x-100 oC
MoO3-x-200 oC
In dark condition
50
40
30
20
10
0
MoO3-x-300 oC
> 420 nm
> 450 nm
(b)
1.6
1.4
1.2
1.0
0.8
0.6
0
10
20
30
40
50
60
Time (min)
60
50
40
30
20
10
0
(b)
 > 420 nm
Commercial MoO3
As-synthesized MoO3
MoO3-x-100 oC
MoO3-x-200 oC
200
400
600
800
1000
1200
1400
MoO3-x-300 oC
Wavelength (nm)
0
10
20
30
40
50
60
Figure 6. (a) The comparison of H2 evolution from NH3BH3 aqueous solution at room
temperature over MoO3-x-200 oC sample under different visible light irradiation (λ > 420
nm and λ > 450 nm) using Y45 cutoff filter (AGC Asahi Glass Co., Ltd., Japan). (b)
Wavelength-dependent initial H2 yield rate enhancement of MoO3-x-200 oC samples
upon irradiation by LED light. (blue light λ = 470 nm, 33.3 mW cm-2, green light λ = 530
nm, 17.0 mW cm-2, red light λ = 650 nm, 23.5 mW cm-2).
Time (min)
Figure 5. Time course of H2 evolution from AB at room temperature for different
samples: (a) in dark and (b) under visible light irradiation (λ > 420 nm). 100 mol%
conversion of 200 μL of NH3BH3 (0.1 M, 20 μmol) could generate 60 μmol of hydrogen.
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Journal of Materials Chemistry A
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Journal Name
μmol/min) was 1.32 times higher than that in the dark
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condition.
DOI: 10.1039/C7TA01217A
NaHCO3 (as a positive charge scavenger), 2-propanol (as
hydroxyl radicals (.OH) scavengers), NaNO3 and Na2S2O8 (as
negative charge scavengers) were added to the suspension of
MoO3-x-200 oC sample in the dark and under visible light
irradiation condition for dehydrogenation of NH3BH3 to
investigate whether H2 production was connected with the
o
strong light-harvesting property of MoO3-x-200 C induced by
LSPR. As shown in Figure S3, catalytic activity was dramatically
decreased in the dark and under visible light in the presence of
NaHCO3 and 2-propanol. MoO3-x-200 oC could be excited by
visible light irradiation and generate electron-hole, and then
Scheme 2. Proposed reaction mechanism of hydrolysis of aqueous AB solution over
plasmonic MoO3-x-T under visible light.
-
HCO3 and 2-proponol can easily react with a positive charge
and .OH, respectively. So the catalytic performance of samples
was greatly retarded under visible light condition. However,
catalytic activity has only decreased marginally (7.30 μmol or
10.0 μmol) after adding NaNO3 or Na2S2O8 under visible light
condition, which means only a small of electrons participate in
dehydrogenation reaction of AB. The reasons for the
nonstoichiometric ratio of the dehydrogenation activity of AB
after adding NaHCO3, NaNO3 and Na2S2O8 or what kinds of
roles the electrons play are not clear at present. Additional
studies on the detailed mechanism of dehydrogenation of AB
with MoO3-x-T and our previously reported MoO3-x nanosheets
are underway in this laboratory.
100
80
60
40
20
0
(a)
(b)
Reused
Reused
Fresh
10
20
30
40
50
60
70
80
1
2
3
4
5
2 Theta (degree)
Recycle times
(d)
(c)
Fresh
Reused
Fresh (SBET = 30.0 m2/g)
Reused (SBET = 26.0 m2/g)
It has been reported that a plausible mechanism for
catalytic dehydrogenation reaction of AB involves the
following three steps: (1) an activated complex species forms
between the metal particle surface and AB, (2) the B-N bond is
dissociated after attack by a H2O molecule, and (3) the
resulting BH3 intermediate hydrolyzes to produce H2 along
with the formation of a BO2- ion.38-39 According to above study,
we point out the possible dehydrogenation mechanism of AB
molecules (Scheme 2). Firstly, MoO3-x-T effectively adsorbs of
200
400
600
800
1000
1200
1400
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
Relative pressure (P/P0)
Figure 7. (a) The five recycling experiments for NH3BH3 dehydrogenation under visible
light irradiation. (b) XRD patterns, (c) UV-Vis-NIR diffuse reflectance spectra and (d) N2
adsorption-desorption isotherms of fresh prepared and reused MoO3-x-200 oC samples
after the recycling NH3BH3 dehydrogenation experiments.
AB molecule because of its large surface area to form MoO3-x
-
T-AB complex species. Then, plasmonic MoO3-x-T can be
excited due to the strong LSPRs under visible light irradiation
and generate electron-hole pairs. Meanwhile, a new band
below the bottom of the original conduction band may be
formed due to the presence of defective sites (Mo5+ center
and oxygen vacancies), which decreases recombination of the
photogenerated electrons and holes.7 From the difference of
catalytic activity after adding NaHCO3 (2.49 μmol of H2) and 2-
propanol (22.35 μmol of H2) (as shown in Figure S3), Holes as
main active species can react with absorbed AB directly, on the
other hand, holes can also react with the absorbed H2O to
generate hydroxyl radicals (.OH), which will further interact
with absorbed AB. Meanwhile, a small amount of electrons
might gather on the surface of catalyst and react with
dissolved oxygen molecules to produce the superoxide anions
Our previously reported MoO3-x nanosheets have more Mo5+
cations (22.7% of the total Mo) and a lower absorption peak
position (680 nm associated with LSPR)3 in comparison with
o
the MoO3-x-200 C developed in this study (9.5% of Mo5+ and
absorption peak position at 756 nm). Nevertheless, the MoO3-
o
x-200 C (100 mol% conversion of AB at 60 min) exhibited a
higher dehydrogenation activity than the conventional MoO3-x
nanosheets (75.4 mol% conversion under the identical
conditions) owing to its high surface area.
In order to exclude the possible effect of band-gap
excitation on the photocatalytic activity of MoO3-x-T,
dehydrogenation experiment of AB was carried out under
visible-light irradiation with a longer wavelength (λ > 450 nm)
(Figure 6a). Compared to activity under the irradiation with λ >
-
(O2 ) (secondary-reaction).40 Subsequently, not only H2O
420 nm (60.0 μmol), 53.7 μmol of H2 was yielded for MoO3-x
-
.
-
o
molecule but also photogenerated OH and O2 attack the
MoO3-x-T-AB complex species to dissociate the B-N bond.
Finally, the formed BH3 will hydrolyze to generate H2 and form
200 C under the irradiation with λ > 450 nm, indicating that
the contribution by LSPR is greater than that given by band-
gap excitation. By using LED lamp (λ = 470, 530, and 650 nm),
the wavelength-dependence for H2 production enhancement
was also surveyed (Figure 6b). The enhancement of
dehydrogenation rate was in line with the light absorption of
MoO3-x-200 oC, indicating that the enhancement of reaction
activity can be attributed to the LSPRs effect of MoO3-x-200
oC.37 Red LED light irradiation (λ = 650 nm) was achieved the
maximum performance and its initial H2 production rate (2.91
-
a BO2 ion.
o
The catalytic stability of MoO3-x-200 C was also inspected
by repeated use. MoO3-x-200 C still showed a similar catalytic
o
activity over five repeated cycle (Figure 7a). Furthermore, the
structure, optical stability and surface adsorption capacity of
MoO3-x-200 oC was further ascertained by XRD spectra, UV/Vis-
NIR diffuse spectra and N2 adsorption-desorption isotherms
6 | J. Name., 2012, 00, 1-3
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Page 7 of 8
Journal of Materials Chemistry A
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15 Y. Li, D. Wang, Q. An, B. Ren, Y. Rong and Y. Yao, J. Mater.
(Figure 7(b-d)) even after recycling experiment, which means
high potential application prospect of plasmonic MoO3-x-T with
efficient and stable property.
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In summary, plasmonic MoO3-x-T was successfully obtained by
EISA approach and the following hydrogen reduction in certain
temperature. The prepared MoO3-x with high specific surface
area exhibits strong LSPR in the visible region, which can be
tuned by varying the experimental parameter. Under visible
light irradiation, such plasmonic MoO3-x-T showed higher H2
evolution from NH3BH3 solutions compared with the MoO3-x
nanosheet catalyst obtained by the conventional approach
because of larger specific surface area. A relatively high
NH3BH3 dehydrogenation activity was achieved under visible
light irradiation, which exceeded the MoO3-x nanosheet
catalyst. This study offers a promising strategy in exploring
stable and efficient plasmonic semiconductor photocatalysts
with a large surface area for saving hydrogen evolution
problems of new energy resources and energy carriers.
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Acknowledgements
The present work was supported by the Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of Japan (26620194
and 26220911). Part of this work was performed under a
management of “Elements Strategy Initiative for Catalysts and
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA01217A
Graphical abstract
Plasmonic MoO3-x with high surface area efficiently enhances ammonia borane dehydrogenation
activity.