P Doped MoO3? x Nanosheets as Efficient and Stable Electrocatalysts for Hydrogen Evolution

Article abstract of DOI:10.1002/smll.201700441

A P doped MoO_{3? x} nanocomposite material with rich oxygen vacancies is successfully fabricated by a two-step intercalation method, which presents superior activity for the hydrogen evolution reaction with low overpotential and fast electron transfer. In 0.5 m H₂SO₄, it displays an overpotential of 166 mV for driving the current density of 10 mA cm^?2. Moreover, it also shows a good catalytic stability in the electrolytes with different pH, 0.5 m H₂SO₄ (strong acid), 0.5 m Na₂SO₄ (neutral solution), and 0.1 m NaOH (strong base). The superior catalytic activity and stability are due to to the synergistic effect between the P element doping and the oxygen vacancies.

Full text of DOI:10.1002/smll.201700441

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Electrocatalysis
P Doped MoO Nanosheets as Efficient and Stable
3
−x
Electrocatalysts for Hydrogen Evolution
Ling Li, Ting Zhang, Junqing Yan,* Xuediao Cai,* and Shengzhong (Frank) Liu
A
P doped MoO3−x nanocomposite material with rich oxygen vacancies is successfully
fabricated by a two-step intercalation method, which presents superior activity for
the hydrogen evolution reaction with low overpotential and fast electron transfer. In
0
1
.5 m H SO , it displays an overpotential of 166 mV for driving the current density of
0 mA cm . Moreover, it also shows a good catalytic stability in the electrolytes with
2 4
−
2
different pH, 0.5 m H SO (strong acid), 0.5 m Na SO (neutral solution), and 0.1 m
2
4
2
4
NaOH (strong base). The superior catalytic activity and stability are due to to the
synergistic effect between the P element doping and the oxygen vacancies.
Electrocatalytic water splitting for hydrogen generation has have been reported for the efficient HER as the identification
[
8,9]
been considered as a promising approach to provide the effi- of MoS or WS (XS ).
Much of the acceptable viewpoints
2
2
2
[
1–3]
cient and sustainable clean fuels. Noble metal platinum have suggested that the metallic edges of the XS crystals are
Pt) owns the low overpotential of the hydrogen evolution attributed to considerable electrocatalytic activity.
reaction (HER; 2H + 2e → H ) with the electrochemical certain extent, it can be said that the electrical conductivity
2
[
10–13]
(
To a
+
−
2
[
4–7]
activity and stability.
However, Pt’s high cost and scarcity of an electrocatalyst determines the final HER efficiency.
limit its large-scale applications and commercial potential And so, considerable activity and improvement of electro-
for such catalysts. Therefore, developing robust and low- catalytic HER activity by changing the conductivity of XS2
cost alternatives for noble metal-free Pt-like HER catalysts or other oxide catalysts have opened up an exciting new
is crucial to the future of hydrogen economy. To this aim, path for hydrogen evolution. Not like XS , some traditional
2
numerous efforts have been devoted to the search of nonpre- oxides, such as molybdenum oxide and tungsten oxide always
[
14–16]
cious metal-based HER catalysts. Recently, promising results give the poor HER activity.
To obtain the enhanced
hydrogen generation rate and enlarge the family of Pt-free
electrocatalyst, oxygen vacancy preparation or anion doping
[
14,16–18]
has been developed in the recent reports.
In contrast
L. Li, T. Zhang, Dr. X. Cai
Key Laboratory of Macromolecular Science
of Shaanxi Province
Shaanxi Engineering Lab for Advanced
Energy Technology
School of Chemistry and Chemical Engineering
Shaanxi Normal University
Xi’an 710119, China
to XS , the traditional oxides are easy to achieve the mass
synthesis, making them attractive for a wide application of
HER but the excellent catalytic properties remain rare.
Molybdenum oxide (α-MoO ) is a low-cost and envi-
ronmentally benign compound and has been reported to be
Its orthorhombic phase has the layered
structure with relative thermodynamically stable. To date,
the van der Waals between the layered structures of α-MoO3
2
3
[
19,20]
applied in HER.
E-mail: xdcai@snnu.edu.cn
Dr. J. Yan, Prof. S. (Frank) Liu
Key Laboratory of Applied Surface and Colloid Chemistry
National Ministry of Education
can be destroyed through intercalating foreign molecules or
[
21–23]
ions for expanding the interlayer distance.
The previous
Shaanxi Engineering Lab for Advanced Energy Technology
School of Materials Science and Engineering
Shaanxi Normal University
Xi’an 710119, China
E-mail: junqingyan@snnu.edu.cn
reports suggest that the presence of oxygen vacancies pro-
[3,17,18]
duces greater electrical conductivity
and, in the case
of α-MoO , an expanded interlayer distance. Both of these
3
features are likely to lead to improved electrochemical per-
[
24,25]
formance.
The expanded layered α-MoO3 has been
DOI: 10.1002/smll.201700441
applied into the 2D material field such as supercapacitors
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and Li-ion batteries through some certain posttreatment 4-BBPA intercalates into the interlayers of MoO . The XRD
3
[
26]
means.
The layered materials with expanded structure pattern of MoO /4-BBPA suggests successful intercalation
3
and stable oxygen vacancies can also be used in the electro- as shown in Figure S1 (Supporting Information). In contrast
catalytic process, theoretically. Meanwhile, the cation doping to the MoO /DDA, the narrowed interlayer spacing can be
3
strategy may be tried through the expanding of the layered concluded for MoO /4-BBPA. The IR spectra of the studied
3
structure. However, the relative literature about the two samples were shown in Figure S2 (Supporting Information).
above aspects are not massive. On the other hand, for doped According to the IR results, the MoO /DDA and MoO /4-
3
3
5+
α-MoO , the element with the valence state of +5 (M ) may BBPA samples give no correlative signals, suggesting the suc-
3
help the stable existence of oxygen vacancies; that is, the cessful exchange process. The Brunner–Emmet–Teller (BET)
replacement of Mo6 by M in α-MoO will be accompa- values were detected as 38.1, 41.2, and 45.3 m g for MoO3,
+
5+
2
−1
3
nied by oxygen vacancies. The high concentration of oxygen MoO /DDA, and MoO /4-BBPA, respectively (Figure S3,
3
3
vacancies has been confirmed to improve the electrical con- Supporting Information). The little change of the BET may
[
27–31]
ductivity and provide more active sites for HER.
Herein, we report a facile and controllable two-step
suggest the successful intercalation and replacement.
Previous studies showed that the oxygen vacancies of
intercalation method for the synthesis of P-MoO3−x com- oxides can be obtained through calcination under O -free
2
[
17,27,28]
posite materials (Scheme 1 shows the detailed fabrication atmosphere.
For MoO /4-BBPA, to obtain the MoO
3 3
process) with the P5 ions doping into the lattices of α-MoO3 with lots of oxygen vacancies and P element doping, the ben-
nanosheet. Our experimental results show that P-MoO3−x zene ring should be removed first. Figure S4 (Supporting
hybrid materials exhibit high electrocatalytic activity for Information) shows the thermogravimetric (TG) results of
HER under acidic, neutral, or alkaline conditions. The rela- bare MoO and MoO /4-BBPA under N atmosphere. As for
+
3
3
2
tive low electronegativity of P element may help the adsorp- bare MoO , the process of sublimation happens at the tem-
3
tion and desorption of protons and then enhance the activity perature of higher than 700 °C. But for MoO /4-BBPA, three
3
of the catalyst for HER. To the best of our knowledge, the TG inflection points can be found. The first peak loaded at
synergistic effect between oxygen vacancies and the nearby ≈200 °C maybe assigned to the adsorbed organic impurities
5
+
P
elements on HER was first reported. This is also the first and the breakdown of carbon–Br bond. The second peak
time that a P-MoO3−x material is shown to serve as an effi- loaded at ≈400 °C belongs to the pyrolysis of intercalated
cient HER electrocatalyst over a wide pH range. The element organics. And the last one loaded at ≈700 °C is assigned to
doping strategy may provide us the new way for designing sublimation of MoO . Based on the TG result, the precursor
3
materials, and the synergistic effect of foreign species and of MoO /4-BBPA was further treated from 400 to 700 °C
3
defect sites help better understanding of the HER.
under the N condition. The synthesized final products were
named as P-MoO3−x-n, where n represents the calcination
2
+
The previous work had suggested that CH (CH ) NH
3
2 11
3
ions could intercalated into the interlayers of MoO to expand temperature (in hundreds). Figure S5 (Supporting Informa-
3
[
32,33]
the interlamellar spacing.
Figure S1 (Supporting Infor- tion) gives the corresponding XRD patterns of the samples.
mation) shows the X-ray powder diffraction (XRD) patterns Upon calcination of MoO /4-BBPA, the interlayered 4-BBPA
3
of MoO , MoO /dodecylamine (DDA), and MoO /4-bromine will pyrolyze under the following process: MoO /4-BBPA →
3
3
3
3
benzyl phosphoric acid (4-BBPA). Obviously, the typical lay- P-MoO3−x + HBr↑ + CH ↑ + H O↑. The laminated structure
x
2
ered MoO structure can be detected with the three XRD will close, that is, the interplanar spacing will diminish. Spe-
3
peaks of (020), (040), and (060). As for the intercalated MoO3 cially, all the four samples held the layered structure; and
(
MoO /DDA), five sharp (0 k 0) peaks loaded at the 2θ range with the increasing calcination temperature, the main peak at
3
from 2° to 15° dominate the diffraction pattern, indicating ≈10° shifted to the high diffraction angle as shown from the
[
31,32]
the successful intercalation of DDA into MoO3.
The results of the enlarged XRD results of 2θ ranging from 10° to
MoO /DDA owns the enlarged interlayer spacing, which can 14° in Figure S5b (Supporting Information). Table S1 (Sup-
3
ensure the replacement of DDA ions by 4-BBPA, that is, the porting Information) listed the detailed values with the cor-
responding interlayer spacings (d). Clearly,
the d values of P-MoO3−x-n are smaller
than MoO /4-BBPA; and with increasing
3
of the calcination temperature, the d value
is close to bare MoO . Moreover, the
3
d values of MoO /DDA and MoO /4-
3
3
BBPA are 3.073 and 1.438 nm, larger than
the 0.692 nm, indirectly suggesting the suc-
cessful replacement of DDA by 4-BBPA
and the necessity of the insertion of DDA
first for the 4-BBPA insertion.
A high-resolution X-ray photoelec-
tron spectroscopy (XPS) measurement
was further studied for investigating the
individual element valence states and
then giving the chemical composition of
Scheme 1. Schematic illustration of the preparation process for P-MoO3−x nanosheet.
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Figure 1. XPS spectra of a) Mo 3d, b) O 1s, and c) P 2p of MoO /4-BBPA and P-MoO3−x-n samples.
3
samples. The wide XPS spectra of the samples are given 533.4 eV can be detected for P-MoO3−x-n, and the peak can
[
34]
in Figure S6 (Supporting Information), which suggests correspond to surface adsorbed species.
For P 2p spectra
no impurity elements except the C, O, Mo, and P. Figure 1 in Figure 1c, the main peak of 133.6 eV represents the typical
[
36]
shows the Mo 3d, O 1s, and P 2p XPS spectra of MoO /4- peak of phosphorus oxide.
The P 2p peak broadens for
3
BBPA and P-MoO3−x-n samples. For Mo 3d spectra, two the P-MoO3−x-n, suggesting the generation of MoP bond.
main peaks loaded at 232.4 and 235.6 eV, which are attrib- Figure S8 (Supporting Information) shows the Br 3d XPS
6
+
uted to 3d and 3d3/2 of Mo , can be detected for the five result of studied samples with the result of MoO /4-BBPA
5
/2
3
[
14,15]
samples.
Upon calcination of MoO /4-BBPA, two new showing the strong signal and the P-MoO3−x-n nothing. The
3
peaks of Mo 3d XPS curve appear at the binding energy of signal loss of Br 3d XPS suggests the volatilization of HBr
5
+
[15]
2
31.1 and 234.3 eV, belonging to Mo species. The relative during the pyrolysis of 4-BBPA.
5+
6+
content of the two signals of Mo and Mo can be calcu-
The structure of P-MoO3−x-n was studied in detail
lated through the integration of the XPS peaks. We also give by electron microscope. Figure 2 and Figures S9 and S10
the C 1s XPS spectra of the P-MoO3−x samples (Figure S7, (Supporting Information) show the results. Clearly, the
Supporting Information). Two peaks loaded at the binding MoO /4-BBPA and P-MoO3−x-n give the typical layer-by-
3
energy of 284.8 and 289.1 eV can be detected, the 284.8 eV layer structure without any visible changes. Typically, the
peak can be assigned to the CH and the other is attributed scanning electron microscopy (SEM) image of MoO /4-
3
[
22]
to CO. Specially, the C 1s peaks of the four samples pre- BBPA in Figure S9 (Supporting Information) shows a little
sent no obvious difference, which suggests that the carbon thicker than the treated samples, suggesting the pyrolysis
signals can originate from foreign or remaining organic of 4-BBPA under calcination and agreeing with the above
carbon. Table S2 (Supporting Information) gives the corre- XRD result. The interplanar spacings were measured to
5+
6+
sponding peak area ratios of the Mo and Mo . The rela- be 0.18 nm in Figure 2c, corresponding to the spacing of
5
+
[21]
tive ratios of Mo in Mo element are 8.2%, 15.5%, 35.6%, orthorhombic MoO3 (002) plane.
No carbon layer on
and 30.2% for P-MoO3−x-4, P-MoO3−x-5, P-MoO3−x-6, and MoO nanosheet can be detected. Figure 2d shows the ele-
3
5+
P-MoO3−x-7, respectively. That is, the percentage of Mo is mental mapping characterization and the result suggests that
increasing with the increasing calcination temperature. The the nanosheet was evenly composed of molybdenum, oxygen,
5
+
formation of Mo may be derived from the reduction effect and phosphorus. It should be noted that the element ratio
by CH during the calcination process. High temperature of of P-MoO3−x-n samples may have the non-negligible effect
x
7
00 °C may induce the part sublimation of MoO (TG result on the final electroconductivity and the HER performance.
3
in Figure S4 in the Supporting Information) and then the The relative atom ratios of P/Mo (%) obtained from three
5
+
decreased Mo content. For O 1s spectra, one main peak methods, i.e., inductively coupled plasma (ICP), SEM energy
located at 532.0 eV can be found for the samples and this dispersive spectrometer (SEM–EDS), and XPS are shown in
2
− [34,35]
peak is attributed to lattice oxygen (O ).
However, for Table S3 (Supporting Information). The number of P element
the P-MoO3−x-n samples, the binding energy of O2 shifts to is similar to Mo in the synthesized samples, suggesting the
a lower energy level; and with the increasing calcination tem- existence of lots of oxygen vacancies indirectly, that is, the P
perature, the shift turns more. This result indicates a coordi- sites ensure the stable existence of oxygen vacancies.
−
5
+
nation change of Mo with O and suggests the formation of
Based on the above experimental discussion, the detail
[
28]
oxygen vacancies.
Moreover, one more peak located at formation mechanism of P-MoO3−x-n nanosheet should be
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Figure 2. a) SEM image of P-MoO3−x-6. b) TEM and c) HRTEM images of P-MoO3−x-6, the scale bars are 100 and 2 nm, respectively. d) SEM–EDX
elemental mapping of P-MoO3−x-6, and the scale bar is 100 nm. e–g) Corresponding Mo, O, and P elements, respectively.
further clear. The 4-BBPA molecules are easy to intercalate and good HER activation. To obtain more information
the layered structures of MoO /DDA, in which the interla- about the HER performance, the Tafel plots for the various
3
mellar spacing of MoO had been expanded (Figure S1 and catalysts derived from polarization curves were calculated
3
Table S1, Supporting Information). Upon calcination of and the linear portions were fitted using the Tafel equation
MoO /4-BBPA, the 4-BBPA was resolved into gaseous CH , (η = blogj + a, where j is the current density and b is the
3
x
H O, and solid PO during the temperature range from 400 Tafel slope). As shown in Figure 3b, the Pt/C catalyst gives
2
x
to 600 °C; that is, the interlayered 4-BBPA can be decom- the Tafel slope of 32 mV per decade, in agreement with the
[
27,28]
posed completely when calcined at 600 °C (TG results in reports of literature.
The P-MoO3−x-n samples present
Figure S4 in the Supporting Information). When the inter- the higher Tafel slopes of 42, 55, 46, 65, and 98 mV per decade
layered sample was treated under 600 °C, the Mo5 content for P-MoO3−x-6, P-MoO3−x-7, P-MoO3−x-5, P-MoO3−x-4, and
+
reached the maximum (XPS discussion in Figure 1). And MoO /4-BBPA, respectively. The Tafel slopes of P-MoO3−x-n
3
when the calcination temperature was up to 700 °C, the suggest the Volmer–Heyrovsky process for HER pathway,
MoO presented the weightless appearance. The P-MoO3−x-6 referring to the following steps: electrochemical hydrogen
3
5+
+
−
nanosheet owns lots of Mo (oxygen vacancies sites) and adsorption: H O + e → H + H O; electrochemical chem-
should present the considerable electrocatalytic ability.
3
ads
2
+
ical desorption: Hads + H O → H ↑ + H O. The oxygen
3 2 2
To examine the electrocatalytic HER activity, the vacancies may act as the Hads sites and the neighboring
P-MoO3−x-n samples were loaded on the glassy carbon P doping sites may induce the protons to approach the surface
−2
(
GC; ≈0.2 mg cm ) and the measurement was carried out of catalyst, the two effects promote the HER. To illustrate
in 0.5 m N -saturated H SO using the standard three-elec- the excellence of P-MoO3−x samples for HER, we also tested
2
2
4
trode electrochemical configuration. The commercially avail- the HER performance of MoO /DDA samples. The MoO /
3
3
able 20 wt% Pt/C was also tested for a direct comparison. DDA was calcined under 400, 500, 600, and 700 °C to form
Figure 3a shows the polarization curves (iR corrected) on the the MoO -D-4, MoO -D-5, MoO -D-6, and MoO -D-7. The
3
3
3
3
reversible hydrogen electrode (RHE) scale. The Pt/C elec- obtained samples showed poor HER as given in Figure S11
trode shows the excellent HER activity with a low overpo- (Supporting Information). All the samples give the over-
tential (η) of 0 mV for the HER. The electrodes coated with potential of more than 400 mV at the current density of
−
2
P-MoO3−x-n samples exhibited considerable HER activity. 10 mA cm .
Specifically, the P-MoO3−x-6 shows the η of 161 mV at the The stability of a catalyst is a crucial part for electro-
−2
current density of 10 mA cm , followed by P-MoO3−x-7, catalytic application. The long-term HER performance of
P-MoO3−x-5, and P-MoO3−x-4 (Table S4, Supporting Infor- P-MoO3−x-6 sample under static overpotential of 166 mV
mation). In contrast, the MoO /4-BBPA catalyst exhibits for 11 h was assayed. As shown in Figure 3c, the current
3
the lowest HER performance. Bare MoO3 owns the poor density of P-MoO3−x-6 electrode in 0.5 m H SO holds
2
4
−
2
electrical conductivity and can be used in the photocatalysis the value of 10 mA cm steadily over 11 h of continuous
field. The treatment of intercalation by 4-BBPA does not operation. No decrease can be detected, suggesting the sta-
[
37]
change the situation. However, upon calcination of MoO /4- bility of the reaction sites for the adsorption and transfer of
3
BBPA, the interlayered organic molecule was removed with protons. To further elucidate the synergistic effect between
the formation of P element doping and the oxygen vacancies. P element doping and oxygen vacancies for the inter-
The P doping/oxygen vacancies improve the electroconduc- esting stability, the MoO3−x nanosheet was prepared and
tivity of the materials, which provide fast electron transfer Figure S12 (Supporting Information) gives the result. The
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Figure 3. a) Polarization curves of 20% Pt/C, MoO /4-BBPA, P-MoO -n samples. b) Tafel plots of 20% Pt/C, MoO /4-BBPA, P-MoO3−x-n samples
3
3−x
3
obtained from the polarization curves. c) The time-dependent current density curve for P-MoO3−x-6 sample under static overpotential of 166 mV
for 11 h. d) Nyquist plots of the MoO /4-BBPA and P-MoO3−x-n samples.
3
HER process decreased gradually during the continuous nanosheet image under transmission electron microscope
catalytic reaction. The oxygen vacancies may adsorb the (TEM) observation after continuous linear potential
−
OH group owing to the need of saturation coordination sweeps in the three solutions. The corresponding results
6
+
of the Mo element and then reduce the
reaction sites. The existence of nearby P5+
maintains the stability of oxygen vacan-
cies and thus holds the stability of HER
process. Moreover, a good electrocatalyst
with the potential commercial applica-
tion should also own considerable sta-
bility in the electrolyte with different pH.
We chose three electrolytes: 0.5 m H SO4
2
(
strong acid), 0.5 m Na SO (neutral solu-
2 4
tion), and 0.1 m NaOH (strong base)
for this purpose. As shown in Figure 4,
the P-MoO3−x-6 sample gives three dif-
ferent onset overpotentials of 82, 92, and
1
00 mV for acid, neutral, and base solu-
tions, respectively. After 1000 cycles, the
three polarization curves recorded at the
−
1
continuous potential sweeps at 50 mV s
exhibit a continuous without obvious
loss of activity, suggesting the negligible
corrosion of electrocatalyst in the basic
electrolyte and featuring a robust sta-
bility of the mentioned reaction sites.
Figure 4. Polarization curves recorded for P-MoO3−x-6 sample before and after 1000 cycles
with the potential sweeps from −0.45 to 0.15 V versus RHE under three different electrolytes:
0
.5 m H SO4, 0.5 m Na SO , and 0.1 m NaOH. The insets are the TEM images of P-MoO3−x-6:
2 2 4
a) before the polarization test; b) after the test in 0.5 m H SO ; c) after the test in 0.5 m Na SO ;
2
4
2
4
We have also examined the P-MoO3−x-6 (d) after the test in 0.1 m NaOH. The scale bar is 50 nm.
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are also shown in Figure 4 (inset). Clearly, the nanostruc- anhydrous acetic (4:1 in volume ratio) was ultrasound for 30 min.
ture and morphology are well retained after the continuous Subsequently, 3 g 4-bromobenzyl bromide phosphoric acid was
measurements, further corroborating the good stability of added in the above solution and the mixture solution was refluxed
the catalyst. In line with the above studies, the P doping at 70 °C for 120 h, and then the system was permitted to cool
electrocatalyst with lots of oxygen vacancies owns fea- down to the room temperature. The white precipitate was collected
tures, such as the low overpotential for HER, the fast HER by centrifugation, washed with anhydrous ethanol, and dried at
kinetics, and the excellent stability in both acid and base, 70 °C overnight in vacuum. The obtained intercalation products
which portend the further applied room. Also, the prepa- MoO /4-BBPA were heated to various temperatures (400, 500,
3
ration process for the doped materials with lots of stable 600, and 700 °C) for 2 h with a heating rate of 5 °C min− and N2
oxygen vacancies may give us a direction for the synthesis flow at ambient to prepare the final products. These resulted sam-
of other catalysts. Table S5 (Supporting Information) gives ples at four different temperatures were correspondingly named
the comparison of HER activity of the relative materials as were designed as P-MoO3−x-4, P-MoO3−x-5, P-MoO3−x-6, and
and our studied P-MoO3−x samples. Obviously, the electro- P-MoO3−x-7, respectively.
1
catalytic performance of P-MoO3−x is superior to most of
reported Mo-based catalysts.
Material Characterizations: The crystal structure of the sam-
ples was characterized using XRD, Its measurements were carried
In summary, the present study illustrates a new strategy out on a D/Max 2550 VB/PC (Rigaku, Japan) X-ray diffractometer
o
for the element doping of layered nanomaterials and the operated at 40 kV and 100 mA in the range of 2°–70 with Cu-Kα
presented method is facile and easily reproducible. The two- radiation (λ = 1.5406 Å). The morphologies and microstructure of
step process of intercalation for element well-distributed the samples were examined using SEM and TEM. The SEM images
doping into layered MoO guarantees the integrality of the were taken on environmental scanning electron microscope (FEI,
3
2
D nanosheet structure and also the generation of lots of Quanta 2000); the TEM study was performed on high-resolu-
stable oxygen vacancies. The synthesized P doped MoO3−x tion transmission electron microscopy (HRTEM, JEM-2100), and
nanosheet shows a considerable HER performance with the field transmittance electron microscopy (FEI Tecnai G2 F20). XPS
low overpotential and fast electron transfer for hydrogen experiments were conducted using an AXIS ULTRA spectrometer
evolution. We believe that such a strategy could be appli- (Kratos Analytical Ltd.) with an Al Kα (1486.6 eV) X-ray source
cable for designing other layered materials, thus opening operated at 15 kV and 150 mW. Raman spectra were recorded on
new opportunities to synthesize high-performance functional a in Via-Reflex microscope (Renishaw, United Kingdom) with set-
materials for HER or other applications.
ting 10 s exposure time, 5 mW power, 20 accumulations, and a
5
32 nm excitation laser. Thermal analyses (TG/DTA) were per-
formed on thermal analysis system (TA, SDT-Q600, Version 20.9,
Build 20) in N from room temperature to 1000 °C at a heating rate
2
1
Experimental Section
of 20 °C min−
.
Chemicals: All chemical reagents were used as received ones
Electrochemical Measurements: Electrochemical measure-
without further purification.
ments were carried out with a computer-controlled IM6ex (Zahner,
Physical data should be quoted with decimal points and nega- Germany) in a standard three-electrode cell using Ag/AgCl (in
tive exponents.
3.5 m KCl, aq) reference electrode and a platinum foil counter
Synthesis of Bromobenzylphosphonic Acid (BBPA): The pre- electrode that were used together with the GC working electrode
cursor of 4-BBPA was synthesized according to the literature.[30] to form a standard three-electrode cell. The working electrode
A mixture of 4-bromobenzyl bromide (4.99 g, 20 mmol) and was fabricated as follows: 4 mg of catalyst and 80 µL of 5 wt%
triethyl phosphite (6.89 mL, 40 mmol) was heated and stirred Nafion solution were dispersed in 1 mL of a solution of deionized
under Ar at 160 °C for 6 h to give the diethylphosphonate ester water and ethanol (v/v = 4:1). After stirring by ultrasonication for
and then removed the excess triethyl phosphite and byprod- 2 h, 5 µL of the resulting solution was dropcast onto the top of a
ucts by vacuum distillation with heating at 100 °C. After that, glassy carbon electrode with 3 mm diameter. The catalyst-coated
the diethylphosphonate ester was hydrolyzed to give BBPA by GC electrode was dried at 80 °C for 2 h to yield a catalyst loading
addition of excess concentrated aqueous HCl and was heated of 0.285 mg cm− . Cyclic voltammetry, with scan rate of 10 mV s
2
−1
at 100 °C overnight. Cooling to room temperature, the product was conducted in 0.1 m NaOH, 0.5 m Na SO , and 0.5 m H SO ,
2
4
2
4
was recrystallized using ethyl acetate to obtain the pure respectively. The electrochemical impedance spectroscopy was
product.
recorded in the same configuration at the corresponding voltage
Preparation of MoO /DDA Intercalation Complex: The interca- from 10− to 106 Hz with an AC voltage amplitude of 5 mV. All the
1
3
lation of DDA into layered host MoO3 was carried out referring to potentials were calibrated to an RHE according to Nernst equation
the previous literature.[31] 1.0 g of α-MoO (commercial bulk MoO
ERHE = EAg/AgCl + 0.0592 × pH + 0.197V.
3
3
(
Aladdin, 99.95%) was annealed at 600 °C for 12 h in a muffle fur-
nace) and 6.5 g of DDA were mixed in 100 mL anhydrous ethanol
and ultrasound for 30 min. Thereafter, the mixture was refluxed at
7
0 °C for 72 h. The MoO /DDA products were obtained by centrifu-
3
gation, washed several times with anhydrous ethanol, and dried at Supporting Information
70 °C overnight in vacuum.
Preparation of P-MoO3−x Composite Materials: 0.6 g MoO3/ Supporting Information is available from the Wiley Online Library
DDA with 60 mL of solution containing anhydrous ethanol and or from the author.
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Acknowledgements
The authors thank the support from the National Science Basic
Research Plan in Shaanxi Province of China (2016JM2019),
the National University Research Fund (GK261001009), the
National Key Research Program of China (2016YFA0202403), the
Natural Science Foundation of China (No. 21603136), and the
Changjiang Scholar and Innovative Research Team (IRT_14R33).
The 111 Project (B14041), the Fundamental Research Funds for
the Central Universities (GK201602007), and the Chinese National
1000-Talent-Plan program are also acknowledged.
Conflict of Interest
The authors declare no conflict of interest.
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Received: February 8, 2017
Revised: March 24, 2017
Published online:
2
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