Prereduction of Metal Oxides via Carbon Plasma Treatment for Efficient and Stable Electrocatalytic Hydrogen Evolution

Article abstract of DOI:10.1002/smll.201800340

Prereduction of transition metal oxides is a feasible and efficient strategy to enhance their catalytic activity for hydrogen evolution. Unfortunately, the prereduction via the common H₂ annealing method is unstable for nanomaterials during the hydrogen evolution process. Here, using NiMoO₄ nanowire arrays as the example, it is demonstrated that carbon plasma (C-plasma) treatment can greatly enhance both the catalytic activity and the long-term stability of transition metal oxides for hydrogen evolution. The C-plasma treatment has two functions at the same time: it induces partial surface reduction of the NiMoO₄ nanowire to form Ni₄Mo nanoclusters, and simultaneously deposits a thin graphitic carbon shell. As a result, the C-plasma treated NiMoO₄ can maintain its array morphology, chemical composition, and catalytic activity during long-term intermittent hydrogen evolution process. This work may pave a new way for simultaneous activation and stabilization of transition metal oxide-based electrocatalysts.

Full text of DOI:10.1002/smll.201800340

CommuniCation
Electrocatalysis
www.small-journal.com
Prereduction of Metal Oxides via Carbon Plasma Treatment
for Efficient and Stable Electrocatalytic Hydrogen Evolution
Yongqi Zhang, Bo Ouyang, Kun Xu, Xinhui Xia, Zheng Zhang, Rajdeep Singh Rawat,
and Hong Jin Fan*
industrial H2 production.[5–7] Therefore,
Prereduction of transition metal oxides is a feasible and efficient strategy
to enhance their catalytic activity for hydrogen evolution. Unfortunately,
the realization of hydrogen economy
relies largely on the development of high-
efficiency, cost-effective and durable alter-
natives as hydrogen evolution reaction
the prereduction via the common H annealing method is unstable for
2
nanomaterials during the hydrogen evolution process. Here, using NiMoO4
nanowire arrays as the example, it is demonstrated that carbon plasma
(
HER) electrocatalysts.
In the past few decades, intensive
(C-plasma) treatment can greatly enhance both the catalytic activity and the
research efforts have been focused on
transition metal-based materials, such as
long-term stability of transition metal oxides for hydrogen evolution. The
C-plasma treatment has two functions at the same time: it induces partial
surface reduction of the NiMoO nanowire to form Ni Mo nanoclusters,
[
8,9]
[10,11]
[12–14]
nitrides,
phosphides,
carbides,
sulfides,
[
15–17]
as efficient candidates
4
4
for HER catalysts due to their low cost,
earth abundance, and optimal strength of
metal hydride bond (M-H). Nevertheless,
most of the above catalysts are fabricated
from high-temperature annealing in harsh
environments that involve poisonous
feedstock and byproduct. Even worse, the
indispensible high temperature annealing
and simultaneously deposits a thin graphitic carbon shell. As a result, the
C-plasma treated NiMoO can maintain its array morphology, chemical
4
composition, and catalytic activity during long-term intermittent hydrogen
evolution process. This work may pave a new way for simultaneous activation
and stabilization of transition metal oxide-based electrocatalysts.
Hydrogen evolution from water splitting offers a sustainable
process makes it nearly impossible to maintain the high-
specific-area nanostructures, which is important for an efficient
catalyst. By contrast, transitional metal oxides are easily avail-
able compound materials with a wide variety of nanostructures
with high specific surface area. However, most of them fail to
play an important role in hydrogen evolution due to their low
electrical conductivity, sluggish catalytic kinetics, and poor long-
term stability. In this regard, it is highly desirable to improve
the HER performance of transition metal oxides.
method to acquire molecular hydrogen (H ) fuel, an ideal alter-
2
[1–4]
native to fossil fuels without CO emission.
To make this
2
technology viable, the catalyst in an electrolysis device should
exhibit both high electrocatalytic activity and long-term dura-
bility. In this regard, catalysts based on precious metals, such
as Pt, meet the above-mentioned characteristics, but their scar-
city, high cost and poor stability are hindrance to any large-scale
[
18]
Recently, there has been a few reports that modulation
of the local atomic structure via introducing oxygen vacancy
could improve the HER activity for certain metal oxides.
Y. Q. Zhang, Dr. K. Xu, Prof. H. J. Fan
School of Physical and Mathematical Sciences
Nanyang Technological University
[19,20]
6
37371, Singapore
For example, Yang and co-workers reported that the commer-
E-mail: fanhj@ntu.edu.sg
cial WO nanoparticles can show enhanced HER activity after
3
B. Ouyang, Prof. R. S. Rawat
[
20]
annealing under H2 atmosphere.
In addition, we found
Natural Sciences and Science Education
National Institute of Education
Nanyang Technological University
that metal oxides (herein, NiMoO4, Co3O4, and NiO) show a
self-activation effect during the continuous hydrogen evolu-
tion process (Figure S1, Supporting Information). The activity
improvement is probably due to the surface reduction of metal
oxides by the highly active hydrogen atoms that are produced
on their surface during the HER process. This phenomenon
is similar to the surface oxidation of some nonoxide catalysts
6
37616, Singapore
Prof. X. Xia
State Key Laboratory of Silicon Materials and Department
of Materials Science and Engineering
Zhejiang University
Hangzhou 310027, P. R. China
[
21,22]
Dr. Z. Zhang
during oxygen evolution reaction process.
However, this in
Institute of Materials Research and Engineering
A*STAR (Agency for Science, Technology and Research)
situ improvement by self-activation is slow (takes a few hours),
transient, and extremely unstable; the catalytic activity degrades
#
08-03, 2 Fusionopolis Way, Innovis 138634, Singapore
quickly when the H evolution is interrupted (Figure S1d, Sup-
2
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201800340.
porting Information). So prereduction of metal oxides should
be a more feasible and efficient strategy to improve their HER
catalytic activity. H2 annealing is a commonly employed for
DOI: 10.1002/smll.201800340
Small 2018, 1800340
1800340 (1 of 7)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
Scheme 1. The preparation of C-plasma treated and H annealed NiMoO .
2
4
prereduction of metal oxides. Unfortunately, the high tempera-
ture and long annealing time may destroy the nanostructure.
Also the reduction is still unstable and obvious degradation of
HER catalytic activity is observed for the H -annealed NiMoO ,
morphology of nanowire arrays was preserved and many nano-
particles with the diameter of 5–10 nm were separated out on
the surface of nanowires (Figure 1a,b). In the high magnified
transmission electron microscopy (TEM) images (Figure 1c,d),
d-spacing of 0.208 and 0.180 nm were measured for the nano-
particles, which corresponds to the (121) and (310) plane of the
Ni Mo allay (PDF# 65–5480). This indicates that Ni Mo alloy
2
4
Co O , and NiO (see Figure S1d, Supporting Information).
3
4
[23]
[24]
Zhang et al. and Chen et al. treated NiMoO nanorods by
4
reducing under an H atmosphere. While the electrodes after
2
4
4
H reduction show good catalytic performance in both studies,
could be separated under the reductive atmosphere, which is
2
[
23,31]
the nanorod surfaces were directly exposed and might be sub-
ject to oxidation during operation in the KOH electrolyte. This
has been demonstrated in our experiment (Figure S1d, Sup-
porting Information). Therefore, it becomes necessary that
a surface modification to the transition metal oxides should
simultaneously improve the catalytic activity and stability, and
maintain the nanostructures.
in agreement with previous reports.
About 1–2 layers of
carbon were coated on the surface of nanowire. By their nature
of catalytic decomposition of ethanol, Ni Mo nanoparticles have
4
a thicker carbon shell, which is around 2 nm, corresponding
to four graphitic layers. Furthermore, a high-angle annular
dark-filed scanning transmission electron microscopy (STEM)
image further confirmed that the metal nanoparticles decorated
nanowires were formed (Figure 1e). As presented in the corre-
sponding energy-dispersive X-ray (EDX) mappings, the Mo, O,
and C elements were homogeneously distributed throughout
the whole nanowire, while the Ni element was distributed gran-
ularly on the surface with distinguishable particle boundaries.
The STEM-EDX line analysis of the C-30s along the noted path
in Figure S3 of the Supporting Information further reveals the
enrichment of Ni element. The energy dispersive spectroscopy
confirms that the molar ratio of Ni to Mo is ≈1.05:1 (Figure S3c,
Supporting Information). The X-ray diffraction (XRD) pat-
terns in Figure S4 of the Supporting Information confirm
that NiMoO was converted into Ni Mo and MoO after 30 s
Herein, we report a more efficient method to boost the HER
catalytic activity of metal oxides with more stable performance
via carbon plasma modification. Plasma treatment has been
reported recently as a powerful approach for surface conver-
[
25–30]
sion reaction and modification of the electrode materials.
In this work, we take NiMoO nanowire arrays on carbon cloth
4
as a case study. As show in the Scheme 1, after the C-plasma
treatment, active species of Ni Mo nanoparticles are produced
4
due to reduction of the NiMoO . And the nanowire array struc-
4
ture is well preserved due to a very short process time (30 s). In
the meantime a thin outer layer of carbon shell is deposited on
the nanowire surface, which assures the chemical stability by
preventing oxidation. Benefiting from the high exposed surface
active sites (nanowire array), the generated active Ni–Mo alloy
nanoparticles and the protection of carbon shell, the C-plasma
4
4
2
C-plasma treatment.
The surface chemical composition and valence states of sam-
ples were further characterized by X-ray photoelectron spectros-
copy (XPS; Figure 2; Figure S5, Supporting Information). Four
elements including Ni, Mo, O, and C were identified from the
XPS spectra (Figure S5a, Supporting Information), consistent
with EDX mapping results. With the increasing treatment
duration, the peak intensities of Ni, Mo, and O became weaker,
while the peak intensities of C became stronger. For C-60s, the
layer of carbon was too thick to detect the signal of Ni, Mo, and
O. For the high resolution spectra of Ni 2p (Figure 2a), the peak
at 855.6 eV and its broad satellite speak at 861.8 eV belongs
treated NiMoO nanowire electrode displayed a low overpoten-
4
−
2
tial of 76 mV to reach 10 mA cm in 1 m KOH and superior
duration in intermittent operation condition. This strategy is
generic and also effective for Co O nanowire arrays. Our work
3
4
offers a feasible plasma-assisted strategy toward activation and
stabilization of transition metal oxide-based electrocatalysts.
To prepare the C-plasma treated NiMoO4 electrocatalyst,
the common NiMoO nanowire arrays were fabricated via a
4
solvethermal method. And then the obtained NiMoO4 was
treated under C-plasma atmosphere at different plasma dis-
charge duration: 15, 30, and 60 s, corresponding to the prod-
ucts of C-15s, C-30s, and C-60s, respectively, and presented
in Figure S2 of the Supporting Information. Figure 1 shows
the detailed morphological structures of the C-30s. The
2+
to Ni . After 30 s C-plasma treatment, the new peak at 852.4
[
32]
is indexed to metallic Ni. The partial reduction of NiMoO4
under C-plasma treatment could also be seen from the O 1s
spectra (Figure 2b). As exhibited, the intensity of the peak at
530.7 eV, corresponding to the O2 ions in the crystal line of
−
Small 2018, 1800340
1800340 (2 of 7)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
Figure 1. Electron microscopy images of C-30s: a) SEM, b–d) TEM, and e) STEM image and EDX elemental mapping.
NiMoO , decreased for the C-30s sample.[21] A new peak from
The inferior HER activity of C-60s attributes to the thicker and
hollow carbon shell, which blocks the active sites from electro-
lyte. In addition, the overpotential of C-30s is only 287 mV to
reach a high current density of 300 mA cm (Figure S6, Sup-
porting Information). The Tafel slope, as one of the experi-
mental descriptor to study the HER kinetics, were obtained by
plotting the overpotential versus the logarithm of the absolute
value of current density (Figure 3b). The Tafel slope of C-30s
4
the surface groups of carbon shell appeared at 531.8 eV. The
Mo 3d spectra in Figure 2c displays a new peak at low binding
energy at 229.4 eV after C-plasma treatment, which attributes
−
2
δ+
[33]
to low valence Mo species (δ = 0, 1, 2, 3). The percentage
δ+
of Mo in C-30s is 40.2%, which is higher than that of C-15s
(12.7%) as compared in Table S1 (Figure S5b, Supporting
Information). The ratio of Ni:Mo is around 1.1:1 based on the
XPS spectra of Ni 2p and Mo 3d. The slight higher ratio of Ni
element is probably due to the enrichment of Ni on the sur-
face. The C 1s spectra in Figure 2d only manifest the graphitic
carbon at 284.3 eV, and no carbide peak (lower binding energy,
−
1
−1
(78 mV dec ) is smaller than those of NiMoO (130 mV dec ),
4
−1
−
1
C-15s (106 mV dec ), and C-60s (103 mV dec ). The compar-
ison of Tafel slope versus η10 value for all of the studied cata-
lysts in Figure 3c has clearly shown that the C-plasma treated
≈
282.7 eV) was observed.
The electrocatalytic activity of all samples for HER were eval-
NiMoO had significantly improve the HER catalytic activity
4
(larger current density and faster HER rates). The C-30s elec-
trode exhibits comparable HER catalytic activity to those transi-
tion metal-based materials reported in the literature (Table S2,
Supporting Information). To better understand the superior
uated by a three-electrode system in 1 m KOH (Figure 3) and
compared to the state-of-the-art Pt/C electrocatlysts. Carbon rod
was used as the counter electrode to avoid the possible contami-
−
1
nation of Pt. The polarization curves at a sweep rate of 5 mV s
HER activity of C-plasma treated NiMoO , their electrochemical
4
after iR-correction show an increase in the HER activity in the
impedance spectroscopies (EIS) were carried out to study the
kinetics occurring at the electrode/electrolyte surface under
HER conditions. The typical Nyquist plots in Figure 3d depict
the diameter of the semicircles decreased in the following
following order: NiMoO , C-15s, C-60s, C-30s, and Pt/C. The
4
overpotential of only 76 mV is required for C-30s sample to
reach a current density of 10 mA cm− (η ), which is slightly
2
10
higher than that of Pt/C (45 mV) and obviously lower than that
order: NiMoO , C-15s, C-60s, and C-30s, indicating that C-30s
4
of NiMoO (406 mV), C-15s (135 mV), and C-60s (152 mV).
has the fastest electron transfer and HER activation.
4
Small 2018, 1800340
1800340 (3 of 7)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
Figure 2. High-resolution XPS for NiMoO and C-30s a) Ni 2p3/2, b) O 1s, c) Mo 3d, and d) C 1s.
4
The C-30s sample also exhibits an excellent long-term electro-
In order to verify the contribution of C shell, we inten-
chemical stability. As shown in Figure 4, Electrolysis at three con-
tionally synthesized a hydrogen-reduced NiMoO nanowires
4
−2
stant current densities (10, 30, and 50 mA cm ) was carried out
sequentially over 36 h (Figure 4a). The overpotenitals at different
current densities show a relatively small fluctuation after each
film in gas ambient (H -1h) and investigated its catalytic
2
enhancement and stability. Surface treatment of the NiMoO4
nanowires in an H gas ambient can also improve the catalyst
2
−
2
1
2 h electrolysis (−4, 8, and 17 mV at 10, 30, and 50 mA cm ,
activity, but cannot solve instability problem. The synthetic
−
2
respectively). The slight decrease of overpotential at 10 mA cm
and structural details of H -1h were presented in the Experi-
2
was probably due to the penetration of electrolyte to expose more
active sites. The degradation at 30 and 50 mA cm− was partially
due to reduced active sites caused by the physical adsorption
mental Section and Figure S7 of the Supporting Information.
The SEM image (Figure S7a, Supporting Information) and
XRD pattern (Figure S7b, Supporting Information) confirmed
2
of the generated H . As shown in Figure 4b, the linear sweep
that indeed Ni Mo nanoparticles were formed on the surface
2
4
voltammetry (LSV) curves after electrolysis at different current
densities almost overlap with the original one. In addition, a
multistep chronoamperometric curve was recorded in a wide
overpotential range (from 50 to 500 mV) with an increment of
of nanowires, similar to carbon-plasma reduction. Further-
more, the surface chemical composition and valence states of
H -1h were the same as those of C-30s (XPS of Ni and Mo in
2
Figure S8d,e, Supporting Information). A current density of
−
2
50 mV (Figure 4c). The response current densities remain con-
10 mA cm was achieved with an overpotential of only 60 mV
in 1 m KOH, which was better than that of C-30s (76 mV)
(Figure S7c, Supporting Information). However, it is quite
stant at each step, implying the excellent mass transportation,
conductivity, and mechanical robustness of the C-30s electrode.
The electrode also maintains its morphology and chem-
ical composition after the 36 h stability test (Figure 5). The
nanowire array morphology and the layer of carbon shell were
well preserved, as shown by scanning electron microscopy
−
2
unstable. After 1–2 h stability test at 10 mA cm , the over-
potential of H -1h was higher than that of C-30s (Figure 4a).
2
If soaking the H -1h in electrolyte for only 30 min, the η
2
10
would sharply increase to ≈223 mV (Figure S7c, Supporting
Information). The XPS spectra of Ni and Mo, after the 30 min
soaking (Figure S8d,e, Supporting Information), showed that
(
SEM) and TEM images in Figure 5a,b. The Ni Mo nanopar-
4
ticles are clearly observed in nanowires, owing to the protec-
tion provided by the carbon shell. We also compare the XPS
spectra of Ni and Mo before and after stability test (Figure 5c,d).
0
δ+
both peaks for Ni and Mo disappeared. So the active sites
δ+ [34]
are most likely Ni Mo and low valence (Mo ). To further
4
0
δ+
The peaks due to both Ni and Mo remain except for slight
decrease in their intensities. This verifies that the C-30s sample
is not only a highly active but also a very stable electrocatalyst
for HER in alkaline medium.
check their chemical stability, the NiMoO , H -1h, and C-30s
4
2
samples were immersed in 1 m KOH solution for 12 h (without
electrochemical measurement). We found the nanowire arrays
of the NiMoO and H -1h samples almost disappeared, but
4
2
Small 2018, 1800340
1800340 (4 of 7)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
Figure 3. Electrochemical test of all samples: a) LSV curves, b) Tafel slopes. c) Comparison of h values (potentials required to reach 10 mA cm− and
Tafel slopes for all catalysts. d) Nyquist plots at 10 mA cm .
2
1
0
−
2
not the C-30s (Figure S8, Supporting Information). This indi-
cates that the carbon shell effectively protects the active mate-
rials from dissolution in KOH solution.
Finally, to further demonstrate the versatility of our fabrica-
tion strategy, we also used C-plasma to treat Co O nanowire
arrays (data presented in Figure S9, Supporting Information).
3
4
Figure 4. Stability test. a) Pristine NiMoO nanowire, and C-30s sample, as well as another sample after H treatment. b) LSV curves of the C-30s
4
2
electrode under different HER test stage. c) C-30s at different overpotentials (without iR-correction).
Small 2018, 1800340
1800340 (5 of 7)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
Figure 5. C-30s sample after stability test. a) SEM, b) TEM, c) Ni 2p3/2 XPS spectra, and d) Mo 3d XPS spectra.
After the C-plasma treatment, the nanowire morphology was
retained and lower valence CoO phase was generated. Simi-
larly, a thin layer of graphitic carbon shell formed on the CoO
transferred into a Teflon-lined stainless steel autoclave liner. A piece of
clean carbon cloth was immersed into the reaction solution. The liner
was sealed in a stainless steel autoclave and maintained at 140 °C for
1
0 h. After cooling to room temperature, samples were washed with
nanowire surface. Compared with H annealing, the prereduc-
2
DI water and ethanol in sequence, dried in oven at 60 °C and then
annealed in furnace at 450 °C under Ar atmosphere for 30 min to
tion of Co O via C-plasma treatment not only improve the
3
4
−2
HER catalytic activity but also the longtime stability.
remove moisture. The average mass was about 1.2 mg cm . Finally,
the NiMoO4 was directly subjected to C-plasma to be modified. After
NiMoO4 was inserted into the reactor chamber, the temperature of
chamber was increased to 450 °C and the pressure was pumped down
to 0.2 mbar. Subsequently, a feedstock of ethanol was introduced. The
RF-plasma discharge was conducted at 300 W and 13.56 MHz for
different treatment time, i.e., 15, 30, and 60 s and the resulting products
were denoted as C-15s, C-30s, and C-60s, respectively.
In summary, we have demonstrated that prereduction of
transition metal oxides is an efficient method to greatly improve
their catalytic activity for hydrogen evolution. Compared with
the common H annealing, the C-plasma treatment not only
2
has a reduction reaction function, but also leads to a concurrent
encapsulation of the whole nanowire with a thin layer of gra-
phitic carbon. Benefiting from the protection of carbon shell,
the modified metal oxide exhibits outstanding physical and
chemical stability under longtime and intermittent operation
for hydrogen evolution. We show that this strategy is generic
and also effective to other metal oxides (such as Co O ), which
For comparison, the obtained NiMoO4 was partially reduced via
heating under H2/Ar (2:98) atmosphere at 450 °C for 1 h with 100 sccm.
Co O nanowire arrays grown on Ni foam were fabricated as previous
3
4
21]
[
report. And the plasma parameters are the same as that for NiMoO4
and the treatment time was 30 s.
3
4
Sample Characterization: The morphology and phase of samples
were characterized by field emission SEM (FEI SIRION), XRD (6000
Shimadzu), high-resolution transmission electron microscopy (FEI
Tecnai G2 20), and XPS (Thermo Fisher Scientific Theta Probe).
Electrochemical Characterization: All electrochemical measurements
were performed on an electrochemical workstation (Zahner zennuim,
Germany) in a glass cell with a three-electrode setup. The samples were
used as the working electrode; carbon rod (D = 8 mm) and Ag/AgCl
paves a new way to exploit transition metal oxide materials as
highly activity and stable HER catalysts to compete against pre-
cious metals.
Experimental Section
(3 m KCl) were used as the counter electrode and reference electrode,
Material Synthesis: Nickel acetate (NiAc ·4H O, 1.24 g) and
respectively. 1 m KOH solution (pH = 13.6) was used as the electrolyte.
All potential were calculated with respect to reversible hydrogen electrode
(RHE) via the following equation: E(RHE) = E(Ag/AgCl) + 0.059 pH +
0.210 V. All samples were first conducted the cyclic voltammetry test to
2
2
ammonium molybdate ((NH ) Mo O ·4H O, 1 g) were added to a
4
6
7
24
2
mixed solution (30 mL deionized (DI) water and 30 mL ethylene glycol)
under magnetic stirring to give a green solution. Then, the solution was
Small 2018, 1800340
1800340 (6 of 7)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com
www.small-journal.com
−
1
get a stable curve at 50 mV s . LSV and chronoamperometric tests were
conducted to evaluate the HER catalytic activity of the samples. The LSV
[8] Y. Zhang, B. Ouyang, J. Xu, S. Chen, R. S. Rawat, H. J. Fan, Adv.
Energy Mater. 2016, 6, 1600221.
[9] B. Cao, G. M. Veith, J. C. Neuefeind, R. R. Adzic, P. G. Khalifah,
J. Am. Chem. Soc. 2013, 135, 19186.
10] Y.-Y. Chen, Y. Zhang, W.-J. Jiang, X. Zhang, Z. Dai, L.-J. Wan,
J.-S. Hu, ACS Nano 2016, 10, 8851.
11] H. Ang, H. Wang, B. Li, Y. Zong, X. Wang, Q. Yan, Small 2016, 12,
−
1
tests were carried out at a scan rate of 5 mV s . The Tafel plots were
derived from LSV curves which were tested at the scan rate of 1 mV s .
The EIS were conducted at the current density of 10 mA cm , in a
frequency from 100 kHz to 50 mHz with an AC amplitude of 5 mV.
−
1
−
2
[
[
[
2
859.
12] H. Li, C. Tsai, A. L. Koh, L. Cai, A. W. Contryman, A. H. Fragapane,
J. Zhao, H. S. Han, H. C. Manoharan, F. Abild-Pedersen, Nat.
Mater. 2016, 15, 48.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
[13] Q. Ji, Y. Zhang, J. Shi, J. Sun, Y. Zhang, Z. Liu, Adv. Mater. 2016, 28,
207.
6
[
[
14] Y.-R. An, X.-L. Fan, Z.-F. Luo, W.-M. Lau, Nano Lett.. 2017, 17, 368.
15] Y. Tan, H. Wang, P. Liu, C. Cheng, F. Zhu, A. Hirata, M. Chen, Adv.
Mater. 2016, 28, 2951.
Acknowledgements
[
16] J. Tian, Q. Liu, A. M. Asiri, X. Sun, J. Am. Chem. Soc. 2014, 136,
Y.Q.Z. and B.O. contributed equally to this work. This work was
supported by the National Natural Science Foundation of China (Grant
No. 51728204). It was also supported by the Singapore MOE AcRF Tier
7
587.
[17] H. Liang, A. N. Gandi, D. H. Anjum, X. Wang, U. Schwingenschlögl,
H. N. Alshareef, Nano Lett. 2016, 16, 7718.
1
grant (RG98/15) and Tier 2 grant (MOE2017-T2-1-073). The authors
thank Qiaobao Zhang for the STEM image and EDS elemental mapping
measurement (Xiamen University).
[18] X. Xia, Y. Zhang, D. Chao, C. Guan, Y. Zhang, L. Li, X. Ge,
I. M. Bacho, J. Tu, H. J. Fan, Nanoscale 2014, 6, 5008.
[
[
19] J. Swaminathan, R. Subbiah, V. Singaram, ACS Catal. 2016, 6, 2222.
20] Y. H. Li, P. F. Liu, L. F. Pan, H. F. Wang, Z. Z. Yang, L. R. Zheng,
P. Hu, H. J. Zhao, L. Gu, H. G. Yang, Nat. Commun. 2015, 6, 8064.
21] Y. Zhang, B. Ouyang, J. Xu, G. Jia, S. Chen, R. S. Rawat, H. J. Fan,
Angew. Chem., Int. Ed. 2016, 55, 8670.
Conflict of Interest
[
[
[
[
The authors declare no conflict of interest.
22] K. Xu, H. Ding, H. Lv, S. Tao, P. Chen, X. Wu, W. Chu, C. Wu, Y. Xie,
ACS Catal. 2017, 7, 310.
23] J. Zhang, T. Wang, P. Liu, Z. Liao, S. Liu, X. Zhuang, M. Chen,
E. Zschech, X. Feng, Nat. Commun. 2017, 8, 15437.
24] Y.-Y. Chen, Y. Zhang, X. Zhang, T. Tang, H. Luo, S. Niu, Z.-H. Dai,
L.-J. Wan, J.-S. Hu, Adv. Mater. 2017, 29, 1703311.
Keywords
carbon shells, electrocatalysis, hydrogen evolution reaction, metal oxide,
plasma treatment
[
[
25] Y. Zhang, R. S. Rawat, H. J. Fan, Small Methods 2017, 1, 1700164.
26] L. Xu, Q. Jiang, Z. Xiao, X. Li, J. Huo, S. Wang, L. Dai, Angew.
Chem., Int. Ed. 2016, 55, 5277.
27] S. Dou, C. L. Dong, Z. Hu, Y. C. Huang, J. l. Chen, L. Tao, D. Yan,
D. Chen, S. Shen, S. Chou, Adv. Funct. Mater. 2017, 27, 1702546.
28] Z. Xiao, Y. Wang, Y.-C. Huang, Z. Wei, C.-L. Dong, J. Ma, S. Shen,
Y. Li, S. Wang, Energy Environ. Sci. 2017, 10, 2563.
Received: January 25, 2018
Revised: March 4, 2018
Published online:
[
[
[
[
1] M. A. Pellow, C. J. M. Emmott, C. J. Barnhart, S. M. Benson, Energy
Environ. Sci. 2015, 8, 1938.
2] Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev. 2015, 44,
[29] H. Liang, H. N. Alshareef, Small Methods 2017, 1, 1700111.
[30] L. Tao, X. Duan, C. Wang, X. Duan, S. Wang, Chem. Commun. 2015,
51, 7470.
2
060.
3] C. G. Morales-Guio, L.-A. Stern, X. Hu, Chem. Soc. Rev. 2014, 43, 6555.
4] J. Wang, H. Zhang, X. Wang, Small Methods 2017, 1, 1700118.
[31] L. M. Madeira, M. F. Portela, C. Mazzocchia, A. Kaddouri,
R. Anouchinsky, Catal. Today 1998, 40, 229.
[32] M. Gong, W. Zhou, M.-C. Tsai, J. Zhou, M. Guan, M.-C. Lin,
B. Zhang, Y. Hu, D.-Y. Wang, J. Yang, S. J. Pennycook, B.-J. Hwang,
H. Dai, Nat. Commun. 2014, 5, 4695.
[
[
[
[
5] F. Tao, Angew. Chem., Int. Ed. 2016, 55, 15212.
6] X. Wang, X. Gan, T. Hu, K. Fujisawa, Y. Lei, Z. Lin, B. Xu,
Z.-H. Huang, F. Kang, M. Terrones, R. Lv, Adv. Mater. 2016, 19,
[33] P. Reyes, I. Concha, G. Pecchi, J. L. G. Fierro, J. Mol. Catal. A: Chem.
1998, 129, 269.
1603617.
[
7] Y. Wang, Y. Zhang, Z. Liu, C. Xie, S. Feng, D. Liu, M. Shao, S. Wang,
Angew. Chem., Int. Ed. 2017, 56, 5867.
[34] H. Lin, Z. Shi, S. He, X. Yu, S. Wang, Q. Gao, Y. Tang, Chem. Sci.
2016, 7, 3399.
Small 2018, 1800340
1800340 (7 of 7)
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim