One-step construction of core/shell nanoarrays with a holey shell and exposed interfaces for overall water splitting

Article abstract of DOI:10.1039/c8ta10686j

Developing earth-abundant highly efficient water-splitting electrocatalysts has become of great importance for sustainable energy conversion processes. Herein, we construct a unique Ni foam-supported Ni₃S₂/VO₂ core/shell nanoarray (CSN) with exposed interfaces via facile one-step growth of a holey ultrathin VO₂ shell on Ni₃S₂ 1D nanowires for highly efficient water splitting. Systematic experiments and density functional theory calculations reveal that interfaces in situ generated between the Ni₃S₂ core and the VO₂ shell possess a low energy level d band center and less Gibbs free energy of reaction intermediates and further act as catalytic sites with enhanced intrinsic activity. Moreover, their 1D morphology, the holey shell, and the conductive substrate are beneficial for enhanced surface area and interfaces, efficient charge and mass transfer, good conductivity and structural stability. The obtained Ni₃S₂/VO₂ CSN exhibited extremely high electrocatalytic activity and excellent durability towards electrocatalysis for the OER, HER and overall water splitting. Indeed, it requires only a very low cell voltage of 1.42, 1.45 and 1.65 V to afford current densities of 10, 20 and 100 mA cm^-2, respectively, in alkaline electrolyzers, serving as one of the best water-splitting electrocatalysts to date. This work not only presents deeper understanding of intrinsic electrocatalytic properties of heterostructured catalysts with exposed interfaces but also opens new opportunities for better design of advanced electrocatalysts for water splitting.

Full text of DOI:10.1039/c8ta10686j

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
PAPER
One-step construction of core/shell nanoarrays
with a holey shell and exposed interfaces for overall
water splitting†
Cite this: J. Mater. Chem. A, 2019, 7,
196
1
a
a
ab
a
a
a
Qingliang Lv, Lei Yang, Wei Wang,
Siqi Lu, Tianen Wang, Lixin Cao*
a
and Bohua Dong
*
Developing earth-abundant highly efficient water-splitting electrocatalysts has become of great
importance for sustainable energy conversion processes. Herein, we construct a unique Ni foam-
supported Ni
a holey ultrathin VO
experiments and density functional theory calculations reveal that interfaces in situ generated between
the Ni core and the VO shell possess a low energy level d band center and less Gibbs free energy of
reaction intermediates and further act as catalytic sites with enhanced intrinsic activity. Moreover, their
D morphology, the holey shell, and the conductive substrate are beneficial for enhanced surface area
and interfaces, efficient charge and mass transfer, good conductivity and structural stability. The
obtained Ni /VO CSN exhibited extremely high electrocatalytic activity and excellent durability
towards electrocatalysis for the OER, HER and overall water splitting. Indeed, it requires only a very low
3
S
2
/VO
2
core/shell nanoarray (CSN) with exposed interfaces via facile one-step growth of
2
shell on Ni 1D nanowires for highly efficient water splitting. Systematic
3 2
S
3
S
2
2
1
3
S
2
2
ꢀ2
cell voltage of 1.42, 1.45 and 1.65 V to afford current densities of 10, 20 and 100 mA cm , respectively,
in alkaline electrolyzers, serving as one of the best water-splitting electrocatalysts to date. This work not
only presents deeper understanding of intrinsic electrocatalytic properties of heterostructured catalysts
with exposed interfaces but also opens new opportunities for better design of advanced electrocatalysts
for water splitting.
Received 6th November 2018
Accepted 6th December 2018
DOI: 10.1039/c8ta10686j
rsc.li/materials-a
30–34
other hybrids
have been developed, the catalytic efficiency
1
. Introduction
and overpotential of these catalysts are still less competitive
Electrochemical splitting of water into hydrogen and oxygen via than those of noble-metal-based catalysts. To boost the catalytic
the oxygen evolution reaction (OER) and hydrogen evolution performance, efforts are being focused in four main direc-
35–40
reaction (HER) has been attracting great attention for various tions:
(1) enhancing the intrinsic activity of catalytic sites;
energy storage and conversion devices, such as fuel cells, metal– (2) increasing the density of active sites; (3) improving the
1–6
air batteries and water splitting devices. However, the slug- conductivity of the electrode and (4) optimizing the transport of
gish kinetics of the HER/OER and their dependency on expen- reaction relevant species.
sive noble metals and their oxides (e.g., Pt, RuO2 and IrO2)
Guided by the above design principles, constructing a one-
hinder their large-scale industrial applications. Thus, design dimensional (1D) heterostructure nanoarray on a conductive
and synthesis of highly active and low-cost bifunctional elec- support has been proven to be an efficient strategy to improve
trocatalysts toward both the HER and OER at low overpotentials the catalytic performance of materials by precisely controlling
are quite necessary to make water splitting a practical tech- their composition, morphologies, interfaces and conductive
42
5
,7
41
nology. Although various bifunctional electrocatalysts based supports. For example, Yu et al. synthesized the Cu@NiFe
8–12
13–17
43
on inexpensive transition metal oxides,
suldes,
hydroxides,
and Cu@CoFe core–shell nanowire arrays on Cu foam as
phosphates and bifunctional electrocatalysts which exhibited outstanding
performance for overall water splitting owing to their smart
18–20
21–25
26–28
29
selenides,
phosphides,
structure and hybrid composition. Wang et al. reported
School of Materials Science and Engineering, Ocean University of China, 238 Songling systematic design of hybrid nanotube arrays on Ni foam
a
Road, Qingdao, 266100 P. R. China. E-mail: dongbohua@ouc.edu.cn; caolixin@ouc.
comprising transition-metal suldes for overall water split-
edu.cn
b
32
ting. Thanks to the optimization of chemical composition and
Aramco Research Center Boston, Aramco Services Company, Cambridge, MA 02139,
geometric structure principally, the designed heterostructure
nanoarrays show prominent performance and give current
USA
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ta10686j
ꢀ
2
densities of 10 and 100 mA cm at voltages of 1.429 and 1.72 V,
1
1196 | J. Mater. Chem. A, 2019, 7, 1196–1205
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
32
respectively. Recently, Xia et al. have reported that the is worth noting that the VO
2
peaks are relatively weak and broad
designed N–MoSe /TiC–C arrays showed superior HER perfor- due to the ultrathin shell layer and low crystallinity. The Ni S /
2
3 2
mance because of the synergistic effect of N doped MoSe and VO CSN on the Ni foam is further characterized by scanning
2
2
44
the highly conductive support of TiC–C arrays. Although many electron microscopy (SEM). As shown in Fig. 1c, the Ni foam is
specically designed 1D nanoarrays have been achieved, there uniformly covered with quasi-vertically aligned nanoarrays,
are still some major challenges and underlying problems such with an average diameter of ꢁ100 nm. The surface of the
as determination of active sites in the heterostructure, complex Ni
3
S
2
/VO
2
HN is observed to be rough due to the irregular
layer on the Ni nanowires
and multi-step synthesis, and undesirable overpotential at large deposition of the ultrathin VO
2
3 2
S
45
current density. Therefore, rationally constructing 1D hetero- (Fig. 1d). The quasi core/shell structure can also be deter-
structure nanoarrays as bifunctional electrocatalysts via mined from transmission electron microscopy (TEM) images
a simple method and identifying their catalytic sites are still with low (Fig. 1e) and high (Fig. 1f) magnication. The thick-
urgently needed, which is of both fundamental and pivotal ness of the VO shell deposited on Ni S nanowires is about
2
3 2
2
8,32,46
importance.
Herein, for the rst time, we construct a unique Ni foam- the VO
supported Ni /VO
1D core/shell nanoarray with exposed sponds to the (1 ꢀ2 1) plane, and the Ni
interfaces via facile one-step growth of a holey ultrathin VO
interplanar distance of 0.242 nm correspond to the (1 ꢀ2 1)
shell on Ni 1D nanowires as a bifunctional electrocatalyst for plane (Fig. 1g). Meanwhile, a clear interface between the Ni
5 nm. The high-resolution TEM (HRTEM) image reveals that
shell with an interplanar distance of 0.163 nm corre-
nanowires with an
2
3
S
2
2
3 2
S
2
3
S
2
3 2
S
highly efficient water splitting. Systematic experiments and core and the VO shell except numerous pores with a 3–10 nm
2
density functional theory calculations show that interfaces size on the VO2 shell is distinctly observed (Fig. 1g), which
possessing a low energy level d band center and less Gibbs free increases the active surface area and exposes more interfaces.
energy of H and O-containing intermediates are catalytic sites In addition, the high angle annular dark eld scanning TEM
and benecial for the OER and HER. Moreover, the holey (HAADF-STEM) and elemental mapping images in Fig. 1h
channels on the outer VO
but also facilitate transport of reaction relevant species. The homogeneously distributed on the whole Ni
D nanoarrays on Ni foam provide good conductivity and high nanowire.
2
shell not only expose more interfaces provide clear visual evidence that Ni, S, V and O elements are
20
3 2 2
S /VO core/shell
1
32
surface area to maximize the catalytic site density. The afore-
To investigate the composition of the obtained Ni S /VO
3 2 2
mentioned synergistic effects contribute to the remarkable CSN, X-ray photoelectron spectroscopy (XPS) was carried out.
electrocatalytic activity of the Ni S /VO CSN toward the OER Fig. 2a shows two strong peaks located at binding energies of
3
2
2
and HER. In particular, the OER current density reaches 10 mA 855.9 and 873.5 eV which correspond to Ni 2p3/2 and Ni 2p1/2,
ꢀ
2
cm at an overpotential of only about 150 mV, which is supe- respectively, as well as their two satellite peaks, which is
rior to most of the OER electrocatalysts. Moreover, the two- consistent with the previously reported Ni grown on Ni
CSN as foam. In the V 2p region (Fig. 2b), the V peaks of 524.13 eV and
both anode and cathode electrodes gives current densities of 10, 516.8 eV are attributed to V 2p1/2 and V 2p3/2, respectively,
3 2
S
48
3 2 2
electrode electrolyzer assembled using the Ni S /VO
ꢀ2
4+
47,49
2
0 and 100 mA cm at voltages of 1.42, 1.45 and 1.65 V, indicating the V oxidation state.
In the S 2p region, the two
respectively, which are comparable to and even better than main peaks observed at 162.2 eV and 163.8 eV can be ascribed to
those of the best overall water-splitting electrocatalysts to date. S 2p3/2 and S 2p1/2, respectively (Fig. 2c). In addition, the
2
ꢀ
Our work provides useful insights into identication of inter- binding energy of 162.2 eV is attributed to S in low coordi-
faces as active sites in the heterostructure and offers new nation at the surface; the peak at 163.8 eV originates from the
avenues for construction of advanced electrocatalysts for water sulfur–metal bonds. The O 1s spectrum is deconvoluted into
splitting.
two peaks at binding energies of 530.6 eV and 531.8 eV (Fig. 2d),
which can be indexed to the metal–oxygen bonds (V–O) and
47,49
physicochemical water on the surface of the VO2 shell.
Further XPS comparative analysis indicates the coupling inter-
CSN on Ni foam was synthesized via faces between the VO and Ni . As shown in Fig. S1,† for bare
a one-step hydrothermal method, as shown by the schematic VO , the peaks of V 2p3/2 and V 2p 1/2 are located at 516.5 and
illustration in Fig. 1a. During the reaction, Ni3S2 nanowires 523.9 eV, respectively. However, for VO /Ni , the binding
were rst grown on Ni foam, and then an ultrathin VO shell energies of V 2p3/2 and V 2p1/2 shi to 516.8 and 524.13 eV,
2. Results and discussion
In this work, a Ni
3
S
2
/VO
2
2
S
3 2
2
2
3 2
S
2
was slowly formed on the skeleton of Ni
subsequent oxidation of V (see the details of the reactions in Ni
the ESI†). Structural and morphological analysis of the nal that of the bare Ni
material was further performed by various methods. As pre- tence of strong electronic interactions between Ni
3
S
2
nanowires via respectively. Simultaneously, the Ni 2p3/2 signal of the VO
also exhibits a positive shi of about 0.33 eV relative to
. These results strongly suggest the exis-
and VO in
2
/
2
O
3
3 2
S
S
3 2
S
3
2
2
50
sented in the X-ray diffraction (XRD) pattern in Fig. 1b, besides VO
the XRD peaks of rhombohedral Ni (JCPDS 73-0698), a set of
XRD peaks that are characteristic of VO with a monoclinic step growth of the Ni
/Ni
On the basis of the above results, we have succeeded in one-
/VO CSN with a holey and ultrathin
structure (JCPDS 43-1051) can be identied. Furthermore, shell on Ni foam. Next, the electrocatalytic activity of the Ni
there are no impurity peaks except Ni peaks from Ni foam in foam-supported Ni /VO CSN for both the OER and HER is
XRD patterns, which indicates the pure phase of the sample. It investigated. First, the OER electrocatalytic properties of the as-
S .
2
3 2
3 2
S
3
S
2
2
2
47
3
S
2
2
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 1196–1205 | 1197

View Article Online
Journal of Materials Chemistry A
Paper
Fig. 1 (a) Schematic illustration of synthesis of the Ni
3
S
2
/VO
2
CSN on Ni foam. (b) XRD patterns of Ni
3
S
2
/VO
2
and bare Ni foam. (c) Low- and (d)
high-magnification SEM images of the Ni
image of a Ni /VO single nanowire. (h) HAADF-STEM image of a single Ni
Scale bar in (c), (d), (e), (f), (g) and (h) is 2 mm, 100 nm, 100 nm, 10 nm, 5 nm and 100 nm, respectively.
3
S
2
/VO
2
CSN. (e) Low- and (f) high-resolution TEM images of a Ni
3 2 2
S /VO single nanowire. (g) HRTEM
S
3 2
2
3
S
2
/VO nanowire and elemental mapping images of Ni, S, V and O.
2
ꢀ
2
3 2 2 3 2 2
prepared Ni S /VO CSN as the working electrode were evalu- mA cm . In sharp contrast, the Ni S /VO HN exhibits remark-
ated in 1 M KOH solution using a typical three-electrode cell ably enhanced catalytic activity compared to other employed
conguration. For comparison, the OER properties of bare Ni samples and just needs a much smaller overpotential of 150 mV to
ꢀ
2
foam, commercial IrO , VO powder and Ni S nanoarrays on Ni reach 10 mA cm (Fig. 3c). The reaction kinetics is further
2
2
3 2
foam with the same loading amount were also evaluated to studied using Tafel plots as a key kinetic parameter (Fig. 3b). As
highlight the superiority of the Ni /VO HN. Fig. 3a shows their shown in Fig. 3b and c, the slope value of the Tafel curve for the
linear-sweep voltammetry (LSV) curves on the reversible hydrogen Ni
electrode (RHE) scale. It can be observed that IrO /NF shows (73 mV dec ), bare Ni foam (221 mV dec ), VO
excellent OER activity with a low overpotential, whereas bare Ni and Ni
foam has poor OER activity. Ni and VO require overpotentials increasing applied potential. These results strongly demonstrate
of 340 and 360 mV, respectively, to afford a current density of 10
S
3 2
2
ꢀ
1
3 2 2 2
S /VO CSN is 47 mV dec , much smaller than that of IrO
ꢀ
1
ꢀ1
ꢀ1
2
2
(90 mV dec
)
ꢀ1
3 2
S (89 mV dec ), indicating rapid OER catalytic rate with
S
3 2
2
1198 | J. Mater. Chem. A, 2019, 7, 1196–1205
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
overpotentials are required for VO
2
3 2
, Ni S and bare Ni foam
(
1
Fig. 3g). The Ni S /VO CSN exhibits a smaller Tafel slope of
14 mV dec in the linear region (Fig. 3h), which shows the
3
2
2
ꢀ1
consistent enhancement of the HER kinetics. Also, the Ni S /
3
2
2
VO HN has good HER durability and could maintain its activity
ꢀ
2
for 15 h for a current density of around 10 mA cm with slight
variations (Fig. 3i).
We now return to the interfaces and the holes in the single
Ni S /VO core/shell nanowire to discuss their positive role in
3 2 2
boosting the catalytic activity. In order to visualize the atomic-
scale structure and interface of Ni S /VO , atomic-resolution
3
2
2
Z-contrast STEM-HAADF analysis was carried out. The
brighter spots in Ni (bottom layer in Fig. 4a) represent Ni and
3 2
S
S atoms due to their large and small atomic numbers. As STEM-
HAADF is sensitive only to heavy atoms, only the V atom can be
detected in the VO
2 2
shell (top layer in Fig. 4a); all V atoms in VO
nanodomains are aligned and arranged to form a monoclinic
47
structure. Based on atomic-resolution Z-contrast STEM-
HAADF analysis, the intimate contact of Ni S and VO is veri-
3
2
2

ed, in which abundant interfaces can be clearly identied.
Further STEM-HAADF analysis demonstrated that the VO layer
on the Ni core is not continuous, and many holes are clearly
Fig. 2 XPS spectra of (a) Ni 2p, (b) S 2p, (c) V 2p and (d) O 1s.
2
3 2
S
observed (Fig. 4b, S3 and S4†), which guarantees that the
abundant interfaces can be exposed (Fig. 4c and S3†). The
diameter of holes is about 2–10 nm which is consistent with the
result of the BJH pore-size distribution (Fig. S5†). Importantly,
that the OER activity of our Ni S /VO HN is much better than that
of any reported catalysts as compared in Table S1.†
Double-layer capacitance (Cdl) was employed to calculate the
value of electrochemically active surface area (ECSA) (Fig. S2†).
3
2
2
numerous holes on the VO shell could enhance the electrolyte
2
As shown in Fig. 3d, the Ni
value of 24.09 mF cm among the ve samples, suggesting the
presence of the most catalytically active sites for the OER.
Particularly, the high value of the Ni S /VO CSN further
3
S
2
/VO
2
CSN has the largest ECSA
ꢀ2
diffusion and augment the contact degree between reactants
20
32,50
and interfaces. Additionally, this unique structure of 1D core/
shell nanowires with a porous shell (Fig. 4c) facilitates electro-
3
2
2
lyte penetration and facile release of evolved H
2
and O
2
conrms the intrinsic optimization and enriched exposure of
32,46,48
48,51,52
bubbles.
Besides the pores and interfaces, from the STEM-
active sites due to the electron congured interfaces
and
20
HAADF image, we also observe that the layer with a several atom
numerous pores, which will be discussed further in the next
part. Electrochemical impedance spectroscopy tests were also
conducted to further investigate the electrode kinetics during
the OER process. The Nyquist plots (Fig. 3e) reveal that the
resistance of the Ni S /VO HN is much smaller than that of
thickness is on the surface of VO (Fig. S6†). Interestingly, the
2
interplanar distance of this layer neither belongs to VO nor to
2
VS . We assume that the layer is a composition of VS O
2
x
2ꢀx
2 3 2
because a small amount of the VO shell on Ni S could react
3
2
2
3
2,46
with H
In order to gain deep insight into the origin of the prominent
electrocatalytic activity and identify the active sites of Ni /VO
we performed density functional theory (DFT) calculations to
study the OER activity as a representative case for Ni /VO
The models of Ni S (Fig. S7†) and VO (Fig. S8†) are also built,
2 x
S to form VS O2ꢀx in the initial synthesis.
other contrastive samples and Ni foam,
electron transport in 1D core/shell nanoarrays directly anchored
on conducting Ni foam. The stability of the Ni /VO CSN
electrode for the OER was tested at a constant current density of
suggesting rapid
3
S
2
2
,
3
S
2
2
ꢀ2
3
S
2
.
2
1
0 mA cm , demonstrating that this catalyst can maintain its
3
2
2
catalytic activity for at least 15 h (Fig. 3f). It has to be noted that
the slight variation observed in Fig. 3f is reasonable because the
long-term stability test is a dynamic process, and many factors
including bubble absorption on the electrode surface will cause
their surfaces are cut along the (1 ꢀ2 1) and (ꢀ2 1 1) directions,
respectively, and the vacuum space along the z direction is set to
˚
be 15 A, which is enough to avoid interactions between the two
28
neighboring images. Six different interfaces (Fig. S9 and Table
S2†) between Ni S and VO are built considering the relative
3 2 2
slight variations of current density.
Aer studying the exceptional OER performance, we further
evaluated the HER performance of the Ni /VO CSN. The
Ni /VO CSN also exhibits outstanding HER activity in alka-
line electrolyte. Fig. 3g shows the LSV curves of Ni /VO , VO
Ni , bare NF and commercial Pt/C supported on Ni foam
locations and the most stable interface is used in the next
model. Then, O*, OH* and OOH* groups have been absorbed
on the substrate surfaces and interfaces (Fig. S7, S8 and S10†).
Recent reports have demonstrated that HER or OER activity is
usually determined by the different binding strengths of H- or
O-containing intermediates on the surfaces of different cata-
3
S
2
2
3
S
2
2
3
S
2
2
2
,
3 2
S
measured in 1.0 M KOH solution. As expected, the Pt/C catalyst
on Ni foam shows the best HER activity. The Ni S /VO CSN
3
2
2
41,53
lysts.
d
The d band center (3 ), which has been widely used to
exhibits outstanding HER activity, reaching a current density of
ꢀ
2
describe the binding strength of adsorbates on different
1
0 mA cm at a low overpotential of 100 mV, whereas higher
13,54
surfaces,
is thus rst computed. As shown in Fig. S11,†
J. Mater. Chem. A, 2019, 7, 1196–1205 | 1199
This journal is © The Royal Society of Chemistry 2019

View Article Online
Journal of Materials Chemistry A
Paper
Fig. 3 Electrochemical OER and HER performance of the Ni
3 2
S
/VO
2
CSN. (a) OER polarization curves of different electrodes with a three-
electrode configuration in 1.0 M KOH with 90% iR compensation. (b) The corresponding Tafel plots of samples studied in (a). (c) The comparison
ꢀ2
of the overpotential at 10 mA cm and Tafel slopes for different electrodes. (d) Cdl values of different catalysts. (e) Nyquist plots of different
electrodes. The inset is the equivalent circuit model which contains the electrolyte resistance (R ), a constant phase element (CPE) and a charge-
transfer resistance (Rct). (f) Chronoamperometric measurement of the OER at a current density of 10 mA cm for the Ni
without iR compensation. (g) HER polarization curves recorded for different electrodes in 1 M KOH electrolyte with 90% iR compensation. (h) The
s
ꢀ2
3 2 2
S /VO CSN electrode
ꢀ2
corresponding Tafel plots of samples studied in (g). (i) Chronoamperometric measurement of the HER at a current density of 10 mA cm for the
ꢀ2
3 2 2
Ni S /VO CSN electrode without iR compensation. The amount of the electrocatalyst is 8.0 mg cm in all cases.
+
ꢀ
compared with that of Ni
of the interface in Ni
ꢀ2.17 eV), implying electronic interactions between Ni
VO on their interfaces. The lower 3 within a moderate range
3
S
2
(ꢀ1.907 eV) and VO
/VO shis to a low-energy level
and
2
(ꢀ1.84 eV), the
* + H O / OH* + (H + e )
(1)
(2)
(3)
(4)
2
3
d
3
S
2
2
+
ꢀ
(
3
S
2
OH* / O* + (H + e )
2
d
+
ꢀ
O* + H
2
O / OOH* + (H + e )
means that the binding strength of intermediates will decrease
accordingly but not be too weak to carry out the subsequent
+
ꢀ
OOH* / * + O + (H + e )
13,54,55
2
reactions, which are benecial to facilitate the OER.
Next, DFT+U calculations are conducted to study the ther-
modynamic process in the OER. The OER performance of an
electrocatalyst has been shown to be closely associated with the
change of the Gibbs free energies (DG) of four one-electron
The corresponding DG for steps 1–4 can be expressed as
follows:
55–60
DG
1
¼ GOH* ꢀ GH O ꢀ eU + DG
H
+
(pH)
(pH)
+(pH)
(5)
(6)
(7)
2
55–59
transfer steps.
Thus, the DG for each step in the OER and
theoretical over-potential htheor obtained from Gibbs free energy
differences were calculated. Under acidic conditions, the four
DG ¼ G ꢀ GHO* ꢀ eU + DGH
+
2
O*
55–60
one-electron transfer steps of the OER proceed as follows:
DG
3
¼ GOOH* ꢀ GO* ꢀ eU + DG
H
1200 | J. Mater. Chem. A, 2019, 7, 1196–1205
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
Fig. 4 Atomic-scale characterization and DFT calculations of interfaces. HAADF-STEM images of interfaces (a) and holes (b) in a single Ni
nanowire. The dashed yellow lines in (b) represent interfaces. (c) Illustration of unique structures in a single Ni /VO nanowire including a VO
shell, numerous pore channels on the VO shell, exposed interfaces and a Ni
groups absorbed on the interface (top view) of Ni /VO . The inset of (d) shows the free-energy landscape compared to an ideal catalyst
, reaction (3) is the potential-limiting step. For U > 1.62 V, all steps are thermodynamically accessible.
e) The summarization of htheor and the corresponding models (side view) of Ni /VO , Ni and VO calculated from the DG (DG
¼ DGOOH
).
3 2
S
/VO
2
3
S
2
2
2
2
3 2
S core. (d) Schematic of the OER process with O*, OH* and OOH*
S
3 2
2
(
3 2 2
dashed line). As shown in (d), for Ni S /VO
(
3
S
2
2
3
S
2
2
3
3
ꢀ
DG
O
DG
4
¼ GO2 ꢀ GOOH* ꢀ eU + DG
H
+
(pH)
(8)
The calculated DG1–4 on the interface between Ni
is shown in Fig. 4d. Obviously, the PRDS on the interface is the
3 2 2
S and VO
where * refers to a surface active site, O*, OH* and OOH* are third step – OOH* formation (DG ¼ 1.62 eV). We also calcu-
3
adsorbed OER intermediates, and the term eU accounts for the lated the DG on the surfaces of VO and Ni S , which indi-
1
–4
2
3 2
applied potential. U is the potential measured against the cates that the third step – OOH* formation – on their surface is
normal hydrogen electrode (NHE) under standard conditions. the same PRDS (Fig. S12 and S13†). When DG < 0, all steps in the
The free energy change of the protons relative to the above OER are thermodynamically accessible (Fig. 4d). The theoretical
specied electrode at non-zero pH is represented by the Nernst results (Fig. 4e) indicate that the interfaces in Ni S /VO can
3
2
2
equation as DG
H
+
(pH) ¼ ꢀk
B
T ln(10) ꢂ pH. From the above catalyze the OER with a theoretical overpotential of 0.39 V,
relative energies, the potential rate-determining step (PRDS), which is lower than that of VO (0.92 V) and Ni S (0.79 V). This
2
3 2
named the thermodynamically most energy demanding demonstrates that interfaces possessing a low energy level
5
7
process, could be dened as
d band center and less Gibbs free energy of O-containing
intermediates are catalytic sites and benecial for oxygen
evolution, which greatly matches the experimental results.
The above results validate that the Ni S /VO CSN is an active
bifunctional electrocatalyst toward both the HER and OER in
alkaline media. It also exhibits outstanding catalytic stability
during the long-term test (Fig. 3f and i). Based on that, we
G
PRDS ¼ max[DG
1
, DG
2
, DG
3
, DG
4
].
(9)
3
2
2
The theoretical overpotential can then be calculated as
follows:
h
theor ¼ (GPRDS/e) ꢀ 1.23 V ¼ max[DG
1 2 3 4
, DG , DG , DG ]/e
3 2 2
constructed a two-electrode cell using the Ni S /VO CSN as
both the anode and cathode to demonstrate its real applications
ꢀ
1.23 [V].
(10)
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 1196–1205 | 1201

View Article Online
Journal of Materials Chemistry A
Paper
Fig. 5 (a) Schematic illustration of the Ni
3
S
2
/VO
, Ni
with 90% iR compensation. (c) Chronoamperometric measurement of water splitting at a current density of 10 mA cm
compensation. The inset is the photograph of the Ni /VO CSN–Ni /VO CSN two-electrode system. (d) Multistep chronopotentiometric
curve for the Ni /VO CSN–Ni /VO CSN couple without iR compensation. (e) Comparison of overpotential of different two electrodes at
current densities of 10, 20 and 100 mA cm
2
CSN couple for water splitting in 1 M KOH. (b) Two-electrode polarization curves of the Ni
3 2
S /
VO
2
3
HN–Ni S
2
/VO
2
CSN couple, VO
2
–VO
2
3
S
2
–Ni , Pt/C–IrO couple, and nickel foam-nickel foam couple for water splitting in 1 M KOH
3
S
2
2
ꢀ2
without iR
S
3 2
2
S
3 2
2
S
3 2
2
3
S
2
2
ꢀ
2
.
(
Fig. 5a). The water-splitting performance of the electrolyzer the reported Ni S /VO bifunctional electrocatalyst for overall
3 2 2
based on the Ni S /VO CSN is much better than that of elec- water splitting (Fig. 5e and Table S3†). Importantly, no obvious
3
2
2
trolytic cells containing VO , Ni S and Pt/C–IrO2 couples change in terms of the morphology and chemical element
2
3 2
(
Fig. 5b). Importantly, the electrolytic cell affords current distribution (Fig. S14–S16†) of the Ni
3
S
2
/VO
densities of 10, 20 and 100 mA cm at voltages of 1.42, 1.45 and for water splitting is observed aer the stability tests, demon-
.65 V, respectively, which are comparable to and even better strating its excellent stability. XPS measurements were also
than those of the reported bifunctional electrocatalysts for carried out to study the composition changes of the Ni /VO
2
CSN as the anode
ꢀ
2
1
3
S
2
2
overall water splitting (Fig. 5e and Table S3†). Additionally, the CSN as the anode before and aer water splitting. As shown in
electrolyzer based on the Ni S /VO CSN exhibits excellent Fig. S17,† there are no obvious changes in the binding energies
3
2
2
stability and mechanical robustness (Fig. 5c and d). Moreover, of Ni 2p and V 2p aer the water splitting test, further indicating
a battery with a working voltage of 1.5 V can drive the water- the stability of the Ni S /VO CSN. We assume that the excellent
3
2
2
splitting device with obvious and stable gas formation and stability of the Ni
3
S
2
/VO
2
x 2ꢀx
CSN is rendered by the outer VS O
release (Video S1, ESI†), again revealing the high efficiency of layer (Fig. S6†), which could act as a passivation layer to improve
1202 | J. Mater. Chem. A, 2019, 7, 1196–1205
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
the stability of the Ni
3
S
2
/VO
2
CSN in KOH solution. Exact prepared powder was heated in a tube furnace with a heating
ꢃ ꢃ
ꢀ1
determination of VS O
and deeper understanding of its rate of 5 C min at 550 C for 2 h in an argon gas atmosphere
x
2ꢀx
effects for stability in alkaline solution will be executed in future and nally cooled to room temperature to form VO2.
work. We further used the water drainage method to measure
2 2
the amount of H and O during one-hour processes of chro-
4.2 Materials characterization
noamperometric measurements at a constant current density.
As shown in Fig. S18,† the cathode/anode electrolyzer based on
the Ni S /VO CSN exhibits stable hydrogen/oxygen evolution
3 2 2
rates that match well with the theoretical values, implying
SEM measurements were carried out on a SU8010 eld emission
scanning electron microscope at an accelerating voltage of 3 kV.
The TEM and HRTEM measurements were performed using an
FEI Tecnai G2F20 eld emission electron microscope. High
resolution STEM-HAADF images were obtained on an aberra-
tion corrected transmission electron microscope JEM-ARM200F
equipped with a cold eld emission gun with an acceleration
voltage of 200 kV. XRD patterns were obtained on a Bruker D8
Advance X-ray diffractometer with CuKa radiation. XPS
measurements were performed on a Kratos Amicus X-ray
photoelectron spectrometer with an excitation source of Mg
with a working power of 180 W. The pore size distribution of the
sample was determined by the Barrett–Joyner–Halenda method
a nearly 100% faradaic efficiency for the HER and OER.
3. Conclusions
In summary, we have developed a facile approach to construct
Ni /VO core/shell nanoarrays with exposed interfaces for
highly efficient overall water splitting. The obtained Ni /VO
2
3
S
2
2
3
S
2
CSN exhibited extremely high electrocatalytic activity and
excellent durability towards electrocatalysis for the OER, HER
and overall water splitting. Indeed, it requires a very low cell
voltage of 1.42, 1.45 and 1.65 V to afford current densities of 10,
(TriStar 300-BET/BJH).
ꢀ
2
2
0 and 100 mA cm in alkaline electrolyzers, respectively. The
4.3 Electrochemical measurements
excellent catalytic performance for overall water splitting can be
mainly attributed to the synergistic effect of the 1D morphology, The electrochemical measurements of the HER and OER were
enriched holes, exposed interfaces, and the conductive performed using an electrochemical workstation (CHI760, CH
substrate. Furthermore, the DFT calculations demonstrated Instruments, Inc., Shanghai, China) in a three-electrode system.
3 2 2 3 2 2
that interfaces generated between Ni S and VO with a low- The Ni S /VO HN on Ni foam can be used directly as a working
2
energy level d band center and less Gibbs free energy are the electrode, with dimensions of 1 ꢂ 1 cm . As for VO
2
, Pt/C, and
origin of the prominent electrocatalytic activity. These ndings IrO /C, the catalysts were rst dispersed in 1 mL of a 4 : 1 (v/v)
2
provide further avenues to electrocatalytic optimization and water/ethanol mixture and 10 mL of a 5% PVDF solution prior to
hold promise for large-scale hydrogen generation.
a $30 min sonication to form a homogeneous ink and then
loaded onto the pre-processed 1 cm NF by drop-coating. A two-
2
electrode system was employed to detect the overall water
splitting performance, in which the as-prepared catalysts were
used as both the anode and cathode (for noble metal based
4
. Experimental
4
.1 Materials preparation
Synthesis of a Ni /VO
cm ) was washed with HCl, ethanol and deionized water anode). Polarization curves were recorded by using linear sweep
3
S
2
2
CSN on Ni foam. A piece of NF (3 ꢂ catalysts, Pt/C was used as the cathode and IrO
2
was used as the
2
3
several times to ensure that the surface of the NF was well voltammetry (LSV) in 1.0 M KOH (pH ¼ 14.0) aqueous solution
ꢀ
1
cleaned before use. The Ni S /VO CSN was synthesized using with a scan rate of 1 mV s . All potentials measured were
3
2
2
a one-step solvothermal method. In a typical procedure, 0.4 g of calibrated to RHE according to E(RHE) ¼ E(Hg/HgO) + 0.098 V +
Na VO $12H O and 0.35 g of CH CSNH (TAA) were dissolved 0.059pH, and the polarization curves were corrected with 90%
3
4
2
3
2
into 60 mL deionized water with magnetic stirring, transferred iR compensation within the electrolyte. The Nyquist plots of
into a 100 mL Teon-lined autoclave with a piece of the electrochemical impedance spectroscopy (EIS) were recorded in
ꢃ
prepared NF, sealed, and heated in an oven at 160 C for 184 h. the frequency range of 10 kHz–0.01 Hz at the open circuit
Aer the autoclave cooled down slowly at room temperature, the voltage of the HER and OER, respectively. Besides three-
sample was collected, washed with water and ethanol several electrode measurements in a single cell, overall water splitting
times and then dried in vacuum at room temperature.
was measured in a typical two-electrode system utilizing the as-
Synthesis of Ni S on Ni foam. The Ni S /Ni was synthesized prepared electrodes as the cathode and anode simultaneously.
3
2
3 2
by the same method as followed for the Ni
3
S
2
/VO
2
HN on Ni Cyclic voltammetry (CV) was performed in 1.0 M KOH (pH ¼
foam, except for adding Na
autoclave.
2 3 2
VO $12H O in the Teon-lined 14.0) aqueous solution with scan rates of 20, 40, 60, 80, 100, 120,
ꢀ
1
140, 160, 180 and 200 mV s to assess the electrochemical
Synthesis of VO
2 5
.728 g of V O was dissolved in 80 mL of deionized water. Aer times. The obtained current densities at the selected potential
2
. A mixture of 1.008 g of C
2
H
2
O
4
$2H
2
O and double-layer capacitance (Cdl) with no faradaic processes for ten
0
stirring for one hour, the obtained yellow-brown suspension had a linear relationship with the scan rates and the corre-
was transferred into a 100 mL Teon-lined autoclave. Aer sponding slopes of tting curves were considered as the C .
dl
ꢀ2
ꢃ
ꢀ2
reacting at 180 C for 24 h, the product was treated repeatedly Herein, a specic capacitance (20–60 mF cm ) of 40 mF cm
with ultrapure water and ethanol solution and subsequently was used to calculate the ECSA values according to the following
ꢃ
dried under vacuum conditions at 60 C for 12 h. The as- equation:
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 1196–1205 | 1203

View Article Online
Journal of Materials Chemistry A
Paper
C
dl
12 Y. P. Zhu, T. Y. Ma, M. Jaroniec and S. Z. Qiao, Angew. Chem.,
Int. Ed., 2017, 56, 1324–1328.
2
ECSA ¼
cmECSA
2
4
0 mF cm
13 T. Tang, W. J. Jiang, S. Niu, N. Liu, H. Luo, Y. Y. Chen,
S. F. Jin, F. Gao, L. J. Wan and J. S. Hu, J. Am. Chem. Soc.,
2017, 139, 8320–8328.
4.4 DFT calculations
14 X. Zou, Y. P. Liu, G. D. Li, Y. Y. Wu, D. P. Liu, W. Li, H. W. Li,
D. J. Wang, Y. Zhang and X. X. Zou, Adv. Mater., 2017, 29,
The rst principles calculations in the framework of density
functional theory, including structural and electronic perfor-
mances, were carried out based on the Cambridge Sequential
1700404.
1
5 Y. Jia, L. Z. Zhang, G. P. Gao, H. Chen, B. Wang, J. Z. Zhou,
M. T. Soo, M. Hong, X. C. Yan, G. R. Qian, J. Zou, A. J. Du and
X. D. Yao, Adv. Mater., 2017, 29, 1700017.
16 B. Liu, Y. Wang, H.-Q. Peng, R. Yang, Z. Jiang, X. Zhou,
C.-S. Lee, H. Zhao and W. Zhang, Adv. Mater., 2018,
1803144, DOI: 10.1002/adma.201803144.
7 S. C. Huang, Y. Y. Meng, S. M. He, A. Goswami, Q. L. Wu,
J. H. Li, S. F. Tong, T. Asefa and M. M. Wu, Adv. Funct.
Mater., 2017, 27, 1606585.
61
Total Energy Package known as CASTEP. The exchange–
correlation functional under the generalized gradient approxi-
62
mation (GGA) with norm-conserving pseudopotentials and the
Perdew–Burke–Ernzerhof functional was adopted to describe
63
the electron–electron interaction. An energy cut off of 750 eV
was used and a k-point sampling set of 5 ꢂ 5 ꢂ 1 was tested to
1
ꢀ
1
˚
be converged. A force tolerance of 0.01 eV A , an energy
ꢀ7
tolerance of 5.0 ꢂ 10 eV per atom and a maximum displace-
ꢀ
4
18 Y. P. Liu, Q. J. Li, R. Si, G. D. Li, W. Li, D. P. Liu, D. J. Wang,
˚
ment of 5.0 ꢂ 10 A were considered. For more details see the
ESI.†
L. Sun, Y. Zhang and X. X. Zou, Adv. Mater., 2017, 29,
1606200.
1
9 Y. Y. Wu, Y. P. Liu, G. D. Li, X. Zou, X. R. Lian, D. J. Wang,
L. Sun, T. Asefa and X. X. Zou, Nano Energy, 2017, 35, 161–
Conflicts of interest
170.
There are no conicts to declare.
2
0 Z. W. Fang, L. L. Peng, Y. M. Qian, X. Zhang, Y. J. Xie, J. J. Cha
and G. H. Yu, J. Am. Chem. Soc., 2018, 140, 5241–5247.
Acknowledgements
21 Z. W. Fang, L. L. Peng, H. F. Lv, Y. Zhu, C. S. Yan, S. Q. Wang,
P. Kalyani, X. J. Wu and G. H. Yu, ACS Nano, 2017, 11, 9550–
This work is supported by the Shandong Provincial Natural
Science Foundation, China (Grants ZR2018QB001), and
Fundamental Research Funds for the Central Universities
9
557.
2
2 C. Tang, N. Y. Cheng, Z. H. Pu, W. Xing and X. P. Sun, Angew.
Chem., Int. Ed., 2015, 54, 9351–9355.
3 Y. Q. Sun, K. Xu, Z. G. Wei, H. L. Li, T. Zhang, X. Y. li,
R. W. P. Cai, J. M. Ma, H. J. Fan and Y. Li, Adv. Mater.,
(Grant 201564001).
2
2
018, 30, 1802121.
Notes and references
2
4 L. Fang, W. X. Li, Y. X. Guan, Y. Y. Feng, H. J. Zhang,
S. L. Wang and Y. Wang, Adv. Funct. Mater., 2017, 27,
1701008.
1
B. Y. Xia, Y. Yan, N. Li, H. B. Wu, X. W. Lou and X. Wang, Nat.
Energy, 2016, 1, 15006.
2
H. Jin, C. Guo, X. Liu, J. Liu, A. Vasileff, Y. Jiao, Y. Zheng and 25 Y. Hou, M. Qiu, T. Zhang, X. Zhuang, C.-S. Kim, C. Yuan and
S. Z. Qiao, Chem. Rev., 2018, 118, 6337–6408.
S. Chu, Y. Cui and N. Liu, Nat. Mater., 2016, 16, 16–22.
I. Roger, M. A. Shipman and M. D. Symes, Nat. Rev. Chem.,
X. Feng, Adv. Mater., 2017, 29, 1701589.
26 L. A. Stern, L. G. Feng, F. Song and X. L. Hu, Energy Environ.
Sci., 2015, 8, 2347–2351.
3
4
2
017, 1, 0003.
27 N. Jiang, B. You, M. L. Sheng and Y. J. Sun, Angew. Chem., Int.
Ed., 2015, 54, 6251–6254.
28 F. Yu, H. Zhou, Y. Huang, J. Sun, F. Qin, J. Bao,
W. A. Goddard, S. Chen and Z. Ren, Nat. Commun., 2018,
9, 2551.
5
6
Y. Jiao, Y. Zheng, M. T. Jaroniec and S. Z. Qiao, Chem. Soc.
Rev., 2015, 44, 2060–2086.
I. Ledezma-Yanez, W. D. Z. Wallace, P. Sebastian-Pascual,
V. Climent, J. M. Feliu and M. T. M. Koper, Nat. Energy,
2
017, 2, 17031.
29 L. Yang, Z. L. Guo, J. Huang, Y. N. Xi, R. J. Gao, G. Su,
W. Wang, L. X. Cao and B. H. Dong, Adv. Mater., 2017, 29,
1704574.
30 L. Hui, Y. Xue, D. Jia, H. Yu, C. Zhang and Y. Li, Adv. Energy
Mater., 2018, 8, 1800175.
7
8
X. X. Zou and Y. Zhang, Chem. Soc. Rev., 2015, 44, 5148–5180.
J. Hwang, R. R. Rao, L. Giordano, Y. Katayama, Y. Yu and
Y. Shao-Horn, Science, 2017, 358, 751–756.
9
Z. H. Xiao, Y. Wang, Y. C. Huang, Z. X. Wei, C. L. Dong,
J. M. Ma, S. H. Shen, Y. F. Li and S. Y. Wang, Energy 31 Y. Zhang, X. Xia, X. Cao, B. Zhang, N. H. Tiep, H. He, S. Chen,
Environ. Sci., 2017, 10, 2563–2569.
Y. Huang and H. J. Fan, Adv. Energy Mater., 2017, 7, 1700220.
0 Q. Zhao, Z. H. Yan, C. C. Chen and J. Chen, Chem. Rev., 2017, 32 H. Li, S. Chen, Y. Zhang, Q. Zhang, X. Jia, Q. Zhang, L. Gu,
17, 10121–10211. X. Sun, L. Song and X. Wang, Nat. Commun., 2018, 9, 2452.
1 Y. L. Zhu, W. Zhou, Y. J. Zhong, Y. F. Bu, X. Y. Chen, 33 H. Y. Li, S. M. Chen, X. F. Jia, B. Xu, H. F. Lin, H. Z. Yang,
1
1
1
Q. Zhong, M. L. Liu and Z. P. Shao, Adv. Energy Mater.,
017, 7, 1602122.
L. Song and X. Wang, Nat. Commun., 2017, 8, 15377.
2
1204 | J. Mater. Chem. A, 2019, 7, 1196–1205
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
34 X. Fan, Y. Liu, S. Chen, J. Shi, J. Wang, A. Fan, W. Zan, S. Li, 51 T. Y. Ma, S. Dai, M. Jaroniec and S. Z. Qiao, J. Am. Chem. Soc.,
W. A. Goddard and X.-M. Zhang, Nat. Commun., 2018, 9,
809.
5 M. H. Sun, S. Z. Huang, L. H. Chen, Y. Li, X. Y. Yang,
2014, 136, 13925–13931.
1
52 J. Yin, Y. Li, F. Lv, M. Lu, K. Sun, W. Wang, L. Wang,
F. Cheng, Y. Li, P. Xi and S. Guo, Adv. Mater., 2017, 29,
1704681.
3
3
3
3
3
4
4
4
4
Z. Y. Yuan and B. L. Su, Chem. Soc. Rev., 2016, 45, 3479–3563.
6 F. Song, L. C. Bai, A. Moysiadou, S. Lee, C. Hu, L. Liardet and 53 V. Stamenkovic, B. S. Mun, K. J. J. Mayrhofer, P. N. Ross,
X. L. Hu, J. Am. Chem. Soc., 2018, 140, 7748–7759.
7 Y. Zheng, Y. Jiao, A. Vasileff and S.-Z. Qiao, Angew. Chem., Int.
Ed., 2018, 57, 7568–7579.
N. M. Markovic, J. Rossmeisl, J. Greeley and J. K. Norskov,
Angew. Chem., Int. Ed., 2006, 45, 2897–2901.
54 C. Ling, L. Shi, Y. Ouyang, X. C. Zeng and J. Wang, Nano Lett.,
2017, 17, 5133–5139.
8 Y. P. Zhu, C. X. Guo, Y. Zheng and S. Z. Qiao, Acc. Chem. Res.,
2
017, 50, 915–923.
9 C. Tang, H. F. Wang and Q. Zhang, Acc. Chem. Res., 2018, 51,
81–889.
55 D. Voiry, H. Yamaguchi, J. Li, R. Silva, D. C. B. Alves,
T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda and
M. Chhowalla, Nat. Mater., 2013, 12, 850–855.
8
0 Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. B. Chorkendorff, 56 D. Friebel, M. W. Louie, M. Bajdich, K. E. Sanwald, Y. Cai,
J. K. Norskov and T. F. Jaramillo, Science, 2017, 352, 333–337.
1 J. Liu, D. Zhu, Y. Zheng, A. Vasileff and S.-Z. Qiao, ACS Catal.,
A. M. Wise, M.-J. Cheng, D. Sokaras, T.-C. Weng,
R. Alonso-Mori, R. C. Davis, J. R. Bargar, J. K. Nørskov,
A. Nilsson and A. T. Bell, J. Am. Chem. Soc., 2015, 137,
1305–1313.
2018, 8, 6707–6732.
2 L. Yu, H. Zhou, J. Sun, F. Qin, F. Yu, J. Bao, Y. Yu, S. Chen and
Z. Ren, Energy Environ. Sci., 2017, 10, 1820–1827.
3 L. Yu, H. Zhou, J. Sun, F. Qin, D. Luo, L. Xie, F. Yu, J. Bao,
57 J. Soriano-L ´o pez, W. Schmitt and M. Garc ´ı a-Melchor, Curr.
Opin. Electrochem., 2018, 7, 22–30.
Y. Li, Y. Yu, S. Chen and Z. Ren, Nano Energy, 2017, 41, 58 M. Bajdich, M. Garc ´ı a-Mota, A. Vojvodic, J. K. Nørskov and
27–336. A. T. Bell, J. Am. Chem. Soc., 2013, 135, 13521–13530.
4 S. Deng, F. Yang, Q. Zhang, Y. Zhong, Y. Zeng, S. Lin, 59 I. C. Man, H.-Y. Su, F. Calle-Vallejo, H. A. Hansen,
3
4
X. Wang, X. Lu, C.-Z. Wang, L. Gu, X. Xia and J. Tu, Adv.
Mater., 2018, 1802223.
5 H. Zhou, F. Yu, Q. Zhu, J. Sun, F. Qin, L. Yu, J. Bao, Y. Yu,
J. I. Mart ´ı nez, N. G. Inoglu, J. Kitchin, T. F. Jaramillo,
J. K. Nørskov and J. Rossmeisl, ChemCatChem, 2011, 3,
1159–1165.
4
S. Chen and Z. Ren, Energy Environ. Sci., 2018, 11, 2858–2864. 60 B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich,
4
6 Y. Wu, X. Liu, D. Han, X. Song, L. Shi, Y. Song, S. Niu, Y. Xie,
J. Cai, S. Wu, J. Kang, J. Zhou, Z. Chen, X. Zheng, X. Xiao and
G. Wang, Nat. Commun., 2018, 9, 1425.
M. Garcia-Melchor, L. Han, J. Xu, M. Liu, L. Zheng,
F. P. Garcia de Arquer, C. T. Dinh, F. Fan, M. Yuan,
E. Yassitepe, N. Chen, T. Regier, P. Liu, Y. Li, P. De Luna,
A. Janmohamed, H. L. Xin, H. Yang, A. Vojvodic and
E. H. Sargent, Science, 2016, 352, 333–337.
4
4
4
7 B.-T. Liu, X.-M. Shi, X.-Y. Lang, L. Gu, Z. Wen, M. Zhao and
Q. Jiang, Nat. Commun., 2018, 9, 1375.
8 Y. Yang, K. Zhang, H. Lin, X. Li, H. C. Chan, L. Yang and 61 M. D. Segall, P. J. D. Lindan, M. J. Probert, C. J. Pickard,
Q. Gao, ACS Catal., 2017, 7, 2357–2366.
P. J. Hasnip, S. J. Clark and M. C. Payne, J. Phys.: Condens.
Matter, 2002, 14, 2717–2744.
9 Y. Song, W. Zhao, L. Kong, L. Zhang, X. Zhu, Y. Shao, F. Ding,
Q. Zhang, J. Sun and Z. Liu, Energy Environ. Sci., 2018, 11, 62 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett.,
620–2630. 1996, 77, 3865–3868.
0 J. Zhang, T. Wang, D. Pohl, B. Rellinghaus, R. Dong, S. Liu, 63 D. R. Hamann, M. Schl u¨ ter and C. Chiang, Phys. Rev. Lett.,
X. Zhuang and X. Feng, Angew. Chem., Int. Ed., 2016, 55, 1979, 43, 1494–1497.
702–6707.
2
5
6
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 1196–1205 | 1205