Hollow Cobalt-Based Bimetallic Sulfide Polyhedra for Efficient All-pH-Value Electrochemical and Photocatalytic Hydrogen Evolution

Article abstract of DOI:10.1021/jacs.5b11986

The development of highly active, universal, and stable inexpensive electrocatalysts/cocatalysts for hydrogen evolution reaction (HER) by morphology and structure modulations remains a great challenge. Herein, a simple self-template strategy was developed to synthesize hollow Co-based bimetallic sulfide (M_xCo_3-xS₄, M = Zn, Ni, and Cu) polyhedra with superior HER activity and stability. Homogenous bimetallic metal-organic frameworks are transformed to hollow bimetallic sulfides by solvothermal sulfidation and thermal annealing. Electrochemical measurements and density functional theory computations show that the combination of hollow structure and homoincorporation of a second metal significantly enhances the HER activity of Co₃S₄. Specifically, the homogeneous doping in Co₃S₄ lattice optimizes the Gibbs free energy for H? adsorption and improves the electrical conductivity. Impressively, hollow Zn_0.30Co_2.70S₄ exhibits electrocatalytic HER activity better than most of the reported nobel-metal-free electrocatalysts over a wide pH range, with overpotentials of 80, 90, and 85 mV at 10 mA cm^-2 and 129, 144, and 136 mV at 100 mA cm^-2 in 0.5 M H₂SO₄, 0.1 M phosphate buffer, and 1 M KOH, respectively. It also exhibits photocatalytic HER activity comparable to that of Pt cocatalyst when working with organic photosensitizer (Eosin Y) or semiconductors (TiO₂ and C₃N₄). Furthermore, this catalyst shows excellent stability in the electrochemical and photocatalytic reactions. The strategy developed here, i.e., homogeneous doping and self-templated hollow structure, provides a way to synthesize transition metal sulfides for catalysis and energy conversion.

Full text of DOI:10.1021/jacs.5b11986

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Article
Hollow Cobalt-based Bimetallic Sulfide Polyhedra for Efficient All-
pH-Value Electrochemical and Photocatalytic Hydrogen Evolution
Zhen-Feng Huang, Jiajia Song, Ke Li, Muhammad Tahir, Yu-
Tong Wang, Lun Pan, Li Wang, Xiangwen Zhang, and Ji-Jun Zou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b11986 • Publication Date (Web): 16 Jan 2016
Downloaded from http://pubs.acs.org on January 16, 2016
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Page 1 of 9
Journal of the American Chemical Society
Hollow Cobaltꢀbased Bimetallic Sulfide Polyhedra for Efficient AllꢀpHꢀValue Electrochemical and Photocatalytic
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Hydrogen Evolution
ZhenꢀFeng Huang, Jiajia Song, Ke Li, Muhammad Tahir, YuꢀTong Wang, Lun Pan, Li Wang, Xiangwen Zhang, and JiꢀJun Zou*
Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology,
Tianjin University, Tianjin 300072, China; Collaborative Innovative Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China.
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Abstract: The development of highly active, universal and stable inexpensive electrocatalysts/cocatalysts for hydrogen evolution
reaction (HER) by morphology and structure modulations remains a great challenge. Herein, a simple selfꢀtemplate strategy was
x 4
developed to synthesize hollow Coꢀbased bimetallic sulfide (M Co3ꢀxS , M=Zn, Ni, Cu) polyhedra with superior HER activity and
stability. Homogenous bimetallic metalꢀorganic frameworks are transformed to hollow bimetallic sulfides by solvothermal
sulfidation and thermal annealing. Electrochemical measurements and density functional theory computations show that the
combination of hollow structure and homoꢀincorporation of second metal significantly enhances the HER activity of Co
Specifically, the homogenous doping in Co lattice optimizes the Gibbs free energy for H* adsorption and improves the
electrical conductivity. Impressively, hollow Zn0.30Co2.70 exhibits electrocatalytic HER activity better than most of the reported
nobelꢀmetalꢀfree electrocatalysts over a wide pH range, with overpotentials of 80 mV, 90 mV and 85 mV at 10 mA cm , 129 mV,
3 4
S .
3 4
S
S
4
ꢀ
2
ꢀ
2
1
44 mV and 136 mV at 100 mA cm in 0.5 M H SO , 0.1 M phosphate buffer and 1 M KOH, respectively. It also exhibits
2 4
photocatalytic HER activity comparable to Pt cocatalyst when working with organic photosensitizer (Eosin Y) or semiconductors
TiO , C ). Furthermore, this catalyst shows excellent stability in the electrochemical and photocatalytic reactions. The strategy
(
2
3 4
N
developed here, i.e. homogenous doping and selfꢀtemplated hollow structure, provides a way to synthesize transition metal
sulfides for catalysis and energy conversion.
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1
. INTRODUCTION
demonstrates excellent photocatalytic HER activity that is
comparable to Pt. Note that pristine Co S is not a good HER
catalyst compared with other sulfides. However, the activity is
improved to a superior level due to the synergetic effect
between homogenous incorporation of second metals in the
lattice and formation of hollow structure.
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Hydrogen, as a clean renewable energy source of fuel, is
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4
one of the most promising energy carriers for replacing
traditional fossil fuels. Water electrolysis driven by renewable
resourceꢀderived electricity and water splitting driven by
solarꢀenergy photocatalysis are promising pathways for
sustainable hydrogen production. Hydrogen evolution
reaction
electrocatalyst/cocatalyst to promote the sluggish kinetics. Pt
is the most active HER catalyst, but the concerns related to its
higher cost and rarity are big hurdles for practical
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(HER)
requires
highꢀperformance
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c,d,f
application.
Therefore, numerous efforts have been
devoted to exploring stable and nobleꢀmetalꢀfree HER catalyst
with good efficiency. In this regard, transition metal sulfides
have been recently suggested as promising catalyst for HER,
inspired by the structure/composition of nitrogenase and
hydrogenase. But their activity is far from satisfied compared
with noble Pt catalyst, and most of them work only in acidic
media. Enhancing the electrical conductivity and increasing
the number of exposed surface active sites are effective ways
2
Scheme 1. Schematic illustration of fabrication of hollow
Coꢀbased bimetallic sulfides.
3
2. EXPERIMENTAL SECTION
2.1. Materials. Eosin
Y
(EY), Co(NO ) ·6H O,
3 2 2
Zn(NO ) ·6H O, Ni(NO ) ·6H O, Cu(NO ) ·3H O and
3
2
2
3 2
2
3 2
2
thioacetamide (TAA) were obtained from J&K Chemical.
Methanol, absolute ethanol and isopropanol were purchased
from Tianjin Guangfu Fine Chemical Research Institute.
Nafion solution (5 wt%) and 2ꢀmethylimidazole were obtained
from Sigma Aldrich. All the chemicals are reagent grade and
used as received. High purity water (18.25 Mꢁ·cm) supplied
by a UP Water Purification System was used in all the
experiments.
4
to improve the activity. Especially, homoꢀbimetallic sulfides,
different from surface doping and heterojunction at
surface/interface, are believed to manipulate the inherent
electronic and/or surface structures of the host material toward
better activity more effectively. Moreover, very recent work
shows that doping metal sulfides with secondary metal ions
may produce catalyst that can work in alkaline and neutral
medium. However, such homogenous composition is hard to
achieve mainly due to the difficulty in preparing
homoꢀbimetallic precursors.
Porous/hollow structures have attracted many attentions in
catalysis due to the intriguing features such as large surface
area, low density, kinetically favorable open structure and
2c,5
6
2
.2. Synthesis of Hollow M Co S Polyhedra. The
x 3-x 4
synthesis of MCoꢀMOFs is similar to literatures that can be
9d,e
seen in supporting information (SI).
Then 80 mg of
MCoꢀMOFs was reꢀdispersed into 40 mL of ethanol, followed
by the addition of 0.12 g of TAA. Thereafter, the mixture was
transferred into a Teflonꢀlined stainless steel autoclave and
heated at 120 ºC for 4 h. The formed M Co S hollow
7
surface permeability. But fabrication of hollow nanomaterials
x
3ꢀx 4
with delicately controlled composition and morphology is still
a challenge. Hollow microꢀ/nanostructures can be produced
using template method including hard, sacrificial and soft
templates. Specifically, metalꢀorganic frameworks (MOFs)
are promising sacrificial template or selfꢀtemplate to construct
polyhedra were centrifuged and washed with ethanol for
several times. Finally they were annealed under N2
atmosphere at 350 ºC for 2 h.
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2.3. Characterization. Xꢀray diffraction (XRD) was
recorded with a Bruker D8 Focus operating at 40 kV and 40
mA equipped with a nickelꢀfiltered Cu Kα radiation (λ =
1.54056 Å). Field emissionꢀscanning electron microscopy
9
hollow or porous structures.
Moreover, the good
compatibility of organic frameworks with various metal ions
makes it easy to synthesize homogenous multiꢀmetallic MOFs.
With them as precursor, it is possible to fabricate homogenous
multiꢀmetallic materials with controllable composition.
Here we report a generalized method to synthesize hollow
bimetallic sulfides (M Co S ) with homogenous composition
(
FEꢀSEM) images were obtained using a Hitachi Sꢀ4800 SEM.
TEM images and corresponding electron energy loss
spectroscopy elemental mapping data (EELS) were obtained
using a JEMꢀ2100F transmission electron microscope at 200
kV. Element composition was analyzed using a VISTAꢀMPX
EL02115765 inductive coupled plasma atomic emission
spectrometer (ICP AES). BET surface area was determined
using N2 adsorption/desorption isotherm measurements at
x
3ꢀx 4
(
Scheme 1). Homogenous bimetallic MOFs are used as
selfꢀtemplates and precursors to synthesize bimetallic sulfides
via solvothermal sulfidation reaction and thermal annealing.
Importantly, we also report M Co S , especially
Zn0.30Co2.70S4, shows excellent electrochemical HER activity
better than most of the reported nonꢀnoble metal catalysts over
a wide pH range (pH=0ꢀ14). Moreover, the material also
x
3ꢀx 4
ꢀ
196 ºC on a Micrometrics TriStar 3000 equipment. UVꢀvis
diffuse reflectance spectra (UVꢀvis DRS) were recorded with
a Hitachi Uꢀ3010 spectrometer using BaSO as the reference.
4
Xꢀray photoelectron spectroscopy (XPS) data were obtained
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0a,b
on a PHIꢀ1600 instrument equipped with Al Kα radiation.
.4. Electrocatalytic HER. All the electrochemical
model (DFT+U).
The projector augmented wave (PAW)
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model with PerdewꢀBurkeꢀErnzerhof (PBE) function was
employed to describe the interactions between core and
electrons, and the value of the correlation energy (U) was
fixed at 5.9, 7.5, 6.4 and 4.0 eV for the 3d orbits of Co, Zn, Ni
measurements were conducted using an IVIUMSTAT
working station (Ivium Technologies BV, Netherlands) in a
typical threeꢀelectrode setup with an electrolyte solution, a
graphite rod as the counter electrode and a saturated calomel
electrode (SCE) as the reference electrode. The reference
1
0c,d
and Cu, respectively.
An energy cutoff of 550 eV was used
for the planeꢀwave expansion of the electronic wave function.
The Brillouin zones of all systems were sampled with
Gammaꢀpoint centered MonkhorstꢀPack grids. A 3×3×3 and
3×3×1 Monkhorst Pack kꢀpoint setup were used for bulk and
slab geometry optimization, while 9×9×9 for the density of
states and electronic band structure. The force and energy
convergence criterion were set to 0.02 eV Åꢃ and 10ꢃ eV,
respectively. Molecular modelꢀbuilding details and
freeꢀenergy calculations are given in the SI.
electrode is previously calibrated in H saturated electrolyte
2
with respect to an in situ reverse hydrogen electrode (RHE),
by using platinum wires as working electrodes, which yields
the relation E(RHE) = E(SCE) + 0.279 V at 0.5 M H SO
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f,2c
solution.
A glassy carbon electrode with a diameter of 3
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5
mm covered by a thin catalyst film was used as the working
electrode. Typically, 4 mg catalyst was suspended in 1 mL
waterꢀisopropanol solution with volume ratio of 3:1 with 50
ꢂL Nafion solution to form a homogeneous ink assisted by
ultrasound for 3 h. Then 5 ꢂL of the ink was spread onto the
3. RESULTS AND DISCUSSION
ꢀ2
surface of glassy carbon electrode (loading 0.285 mg cm ) by
3.1. Formation of Homogenous Bimetallic MOFs.
Bimetallic MOFs (designated as MCoꢀMOFs, M=Zn, Ni and
Cu) were synthesized via roomꢀtemperature coꢀprecipitation
reaction of Co(NO
2ꢀmethylimidazole in methanol (see Experimental for details
a micropipette and dried under room temperature. Then, 3 ꢂL
of 5 wt% Nafion solution was dropꢀcast on top to protect the
film. Electrochemical measurements were conducted in 0.5 M
H SO , 0.1 M phosphate buffer and 1 M KOH, respectively.
)
and M(NO
)
mixtures with
3
2
3
2
2
4
2
+
For Linear Sweep Voltammetry (LSV) measurements, the
in SI). Theoretically, M can be homogenously introduced in
CoꢀMOFs framework by substituting Co sites due to their
ꢀ1
2+
scan rate was set to be 5 mV s . The polarization curves were
plotted as overpotential (η) vs log current (log J) to get Tafel
plots for assessing the HER kinetics. The Tafel slope (b) was
obtained by fitting the linear portion of Tafel plots to the Tafel
equation (η= b log (J) + a). Primary test shows that data with
and without iR compensation are identical (Figure S1), so all
similar ionic radius and electronegativity (Figure 1a, Table S1
9d,e
and Table S2).
XRD patterns of all MCoꢀMOFs samples
exhibit strong diffraction peaks at identical positions similar to
CoꢀMOF (Figure 1b and Figure S2). The atomic ratios of
M/Co detected by ICP in asꢀsynthesized samples are very
close to those in the starting materials (Table S3), indicating
the effective coꢀprecipitation of all metal ions with
2ꢀmethylimidazole. Additionally, the color of both
1
g,2c,g
data were reported without iR compensation.
.5. Photocatalytic HER. The photocatalytic hydrogen
2
production experiments were performed in a closed 50 mL
flask vertically irradiated by a 300 W highꢀpressure xenon
lamp (PLSꢀSXE300UV, Beijing Trusttech. Co. Ltd.). UV light
2+
2+
ZnCoꢀMOFs (even at molar ratio of Zn /Co =1) and
CoꢀMOF are purple, very different from the white color of
ZnꢀMOFs (Figure S3). This phenomenon visually confirms
that Zn ions are incorporated in the frame of CoꢀMOF instead
of separate ZnꢀMOF phase. SEM images (Figure 1cꢀf) show
that the asꢀsynthesized MOFs are highly uniform polyhedra
with size of ca. 600 nm. Different view positions of a single
particle (Figure 1d, inset) reveal that it has a wellꢀdefined
rhombic dodecahedral morphology.
−
2
(
220ꢀ400 nm, 36.5 mW cm at 365 nm) were separated by
−2
UVꢀref optical filter. Visible light (>400 nm, 43.0 mW cm at
420 nm) were separated by UVꢀIRꢀcut filter. For
EYꢀsensitized reactions under visible light, 1.0 mg of sulfides
or Pt/C (20 wt%) and 3.5 mg of EY dye were stirred in 20 mL
of TEOAꢀH O solution (5%, v/v). For semiconductor catalytic
2
1
c,d
reactions, 0.2 mg of sulfides or inꢀsitu photodeposited Pt,
0 mg of TiO2 (C N ), 20 mL of CH OH(TEOA)ꢀH O
2
3
4
3
2
solution (5%, v/v) and UV(visible) light irradiation were used.
Prior to the irradiation, the reactor was deaerated by bubbling
Ar for 30 min to remove air. The amount of evolved H was
2
monitored by a gas chromatograph (Bruker, GCꢀ450, TCD, Ar
carrier). TiO was from Degussa: Hulls Corporation. C N was
2
3
4
synthesized by calcining urea at 600 ºC for 4 h according to
6
d
literature.
.6. Density Functional Theory Calculations. The
2
electronic structures and adsorption free energy were
computed by Vienna ab initio Simulation package (VASP)
using spinꢀpolarized density functional with the Hubbard
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Figure 1. (a) Unit of bimetallic MCoꢀMOF (Green color
indicates the sites of Co atom, whereas pink shows the sites of
substitutional M atom.). (b) XRD patterns of M0.30Co2.70ꢀMOF
and CoꢀMOF. (cꢀf) SEM images of M0.30Co2.70ꢀMOF (M = Zn,
Ni, Cu).
Figure 2. (a, b) SEM and (c, d) TEM images of Zn0.30Co2.70S4,
and (e) EELS element mapping of zone shown in (d).
XRD pattern (Figure 3a) shows that the sulfides can be
indexed as cubic Co S (JCPDS No. 47ꢀ1738). Particularly,
3.2. Formation of Homogenous Bimetallic Cobalt
Sulfides. The formation of homoꢀbimetallic MOF provides a
chance to fabricate homoꢀbimetallic sulfides. In detail, the
bimetallic sulfides can be produced by inꢀsitu topotactic
transformation of MOFs in TAA solution, as follows:
3
4
the incorporation of second metals (Zn, Cu and Ni) do not
affect the spinel structure, and there is no new peak
originating from other metal sulfides, indicating the
homogenous incorporation of metal ions into the lattice of
CH CSNH + C H OH → CH (NH )C(OC H )ꢀSH
(1)
3
2
2
5
3
2
2
5
Co S by taking the sites of Co ions (Figure 3b, c). Moreover,
3 4
CH (NH )C(OC H )ꢀSH + C H OH → CH (NH )C(OC H ) + H S (2)
3
2
2
5
2
5
3
2
2
5
2
2
the peaks of doped Co S do not show any shift compared to
3
4
4
H S + M Co (C H N ) → M Co S +8C H N
(3)
2
x
3ꢀx
4
5
2
8
x
3ꢀx
4
4
6
2
those of pristine Co S , suggesting the doping almost does not
3 4
Upon further thermal annealing in N atmosphere, the rhombic
2
influence the cell parameters due to the very similar ionic
radius of guest and host metal ions. The uniform distribution
of metal and S species is also confirmed by EELS element
mapping (Figure 2e), and the M/Co ratio in sulfides is very
close to that of bimetallic MCoꢀMOF precursors (Table S3).
XPS analysis also excludes the existence of carbon and
nitrogen residues in the sulfides (Figure S6).
dodecahedral morphology of MOF precursors is well
maintained but the crystallinity is greatly improved (Figure 2a,
Figure S4 and Figure S5).
The formation of hollow structure can be discerned
unambiguously from some cracked particles (Figure 2b). TEM
image provides further insight into the hollow interior and
detailed structure (Figure 2c). The wellꢀdefined inner cavities
inside uniform polyhedra are clearly elucidated by the sharp
contrast between the center and the edge. The shell has a
thickness of ca. 30ꢀ50 nm and is robust enough to endure the
sulfidation reaction and subsequent thermal annealing.
Highꢀresolution TEM images (Figure 2d and Figure S5b)
reveal that the shell is composed of nanoparticles of ca. 20 nm.
The hollow polyhedrals afford high BET specific surface area
b)
a)
Co S
3
4
Zn0.30Co2.70S4
Co S4
3
Ni0.30Co2.70S4
c)
Cu0.30Co2.70S4
2
ꢀ1
(
125, 109, 121 and 147 m g ) and large pore volume (0.91,
JCPDS No. 47-1738
3
1
0.81, 0.87 and 1.17 cm gꢃ for Zn0.30Co2.70S4, Ni0.30Co2.70S ,
4
20
30
40
50
60
70
Cu0.30Co2.70S4 and Co S , respectively).
M Co3-xS4
3
4
x
2
Theta (degree)
Figure 3. (a) Powder XRD patterns and (b, c) cell units of
Co S and M Co S . (Blue, yellow and purple balls represent
3
4
x
3ꢀx 4
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Figure 4. (a) Timeꢀdependent formation of hollow
Co, S and M atoms, respectively; M atom dopant is introduced
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to substitute Co .)
Zn0.30Co2.70S4 polyhedra. (b) EELS element mapping of
ZnCoꢀMOF after 2h sulfidation reaction. (c, d) SEM and TEM
images of Ni0.30Co2.70S4, Cu0.30Co2.70S4 and Co S ,
Timeꢀdependent experiments were carried out to monitor
the formation process of hollow sulfides (Figure 4a). At the
3
4
respectively.
2
ꢀ
beginning of sulfidation procedure, S ions released from
TAA upon hydrolyzation quickly react with metal ions on
MOFs surface to form a thin layer of sulfides, which acts as a
physical barrier to prevent the direct contact and chemical
reaction between outside sulfide ions and inner metal ions.
EELS element mapping (Figure 4b) confirms the formation of
MOF@metal sufide core/shell structure. Then, metal and/or
sulfide ions have to diffuse through this newly formed sulfide
layer to continue the sulfidation reaction. Since the ionic
radius of metal ions (72~74 pm) is significantly smaller than
that of S ions (184 pm) (Table S1), the outward diffusion of
metal ions is much easier than the inward diffusion of S ions.
Therefore the inner organic framwork is gradually dissolved
and the released metal ions penetrate the preꢀformed shell to
react with S ions on the outer surface, which finally produces
a hollow void inside the shell. In this case, the MOF precusors
serve as a selfꢀtemplate to produce hollow polyhedra
morphology. Importantly, this strategy can be extended to
fabricate other hollow binaryꢀ and monoꢀmetal sulfides with
tunable compostion using corresponding MOF as the
precursor (Figure 3a, Figure 4c, d, Figure S4, Figure S7b and
Table S3).
3.3. Superior Electrocatalytic Performance in Acidic
Media. The homogenous incorporation of second metal in
2
c,5a,11a
Co S , different from the surface doping/heterojunction,
3
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
provides a good platform to investigate the relationship
between activity and composition. The electrocatalytic HER
activity of M Co S in acidic media (pH=0) was evaluated
x
3ꢀx 4
with commercially available Pt/C catalyst (20 wt%) as
reference. The bimetallic sulfides show obviously enhanced
performance compared with Co
S
, with the overpotential at
10 mA cm (η10) decreasing from 380 mV for pristine Co S
3 4
3
4
2
ꢀ
ꢀ2
2ꢀ
to 243 mV for Cu0.30Co2.70
80 mV for Zn0.30Co2.70
S
, 186 mV for Ni0.30Co2.70
Co3ꢀxS
4
S
, and
with
4
4
S
4
, (Figure 5a). For Zn
x
different Zn/Co molar ratio, an optimal HER catalytic activity
presents when x = 0.3, with the lowest onset overpotential of
35 mV (Figure S8) and a rapid cathodic current increases at
2
ꢀ
higher bias along with vigorous evolution of H
bubbles from
electrode also
4
2
the electrode surface (Video. SI). Zn0.30Co2.70
S
ꢀ2
produces a catalytic current density of 100 mA cm with a
low overpotential of 129 mV. By plotting overpotential (η)
against log current (log J), the kinetic parameters of HER
were calculated. The linear part of Tafel plots (Figure 5b)
ꢀ1
reveals Tafel slopes of ∼30.4, ∼85.3 and ∼47.5 mV dec for
Pt/C, Co S and Zn0.30Co2.70S4, respectively. This suggests that
3
4
hydrogen evolution over Zn0.30Co2.70S4 proceeds via the
VolmerꢀHeyrovsky mechanism and the electrochemical
desorption step is rateꢀlimiting. By further applying
extrapolation method to the Tafel plots (Figure S9), the
ꢀ2
exchange current density (J ) of 0.15 mA cm was obtained
0
for Zn0.30Co2.70S4, which is 6 times larger than that of pristine
ꢀ2
Co S (0.025 mA cm ). It is worth noting that such high
3
4
catalytic activity, ie, low overpotential, low Tafel slope, and
high exchange current density of hollow Zn0.30Co2.70S4
polyhedra is much better than most of the reported
nobleꢀmetalꢀfree HER catalysts (with similar catalyst loading)
including transition metal sulfides, selenides, phosphides,
carbides and nitrides, and metalꢀfree carbonꢀbased materials
(
Table S4).
Figure 5c compares the amount of H2 generated over
hollow Zn0.30Co2.70S4 versus theoretically possible H amount
2
that can be evolved from electrochemical HER. Zn0.30Co2.70S4
shows not only a stable HER performance but also a rate (49.7
ꢀ1
µmol h ) very close to the theoretically achievable value. In
other words, Zn0.30Co2.70S4 gives about 100% Faradaic yield
and has excellent stability in electrochemical HER. Actually it
retains the activity and morphology even after 60 hours testing
(Figures 5d and Figure S10), with no change in the
polarization curve after 1000 cycles (Figures 5d, inset). In
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Journal of the American Chemical Society
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2
+
2+
contrast, Co S shows evident decreased HER activity after 10
introducing other metal ions such as Ni and Cu . The
reduced bandgap is attributed to the enhanced hybridization
3
4
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
h, with the retention of current density about 75% (Figure
S11). Therefore, Zn doping not only enhances HER activity
but also significantly promotes the stability of Co S .
2b
between Co dꢀorbital and S pꢀorbital, as illustrated by charge
density distribution (Figure 6b and Figure S14).
3
4
When Zn0.30Co2.70S4 hollow polyhedra are converted into
powders by ball milling (Figure S12), the activity is decreased
evidently with η10 of 125 mV (Figure 5a), indicative of the
advantage of hollow structure. Even though, the Zn0.30Co2.70S4
powders still show much better activity than hollow Co S ,
DFT calculations were also carried out to gain further
insights into the nature of the catalytic process. The HER can
be summarized in a threeꢀstate diagram, consisting of an initial
+
H state, an intermediate H* state and 1/2H as the final
2
12a
product (Figure 6c). A good catalyst for HER should have a
3
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Cu0.30Co2.70S4 and Ni0.30Co2.70S4. Furthermore, electrochemical
double layer capacitance (Figure S13a) reveals a higher
electrochemical active surface area (ECSA) of bimetallic
moderate free energy for H adsorption (ꢄG *) to compromise
H
12a
the reaction barriers of the adsorption and desorption steps.
The |ꢄG *| value of the most efficient catalyst, Pt, is nearly
H
ꢀ2
sulfides (46.9, 43.1, 41.5 and 41.3 mF cm for Zn0.30Co2.70S4,
milled Zn0.30Co2.70S4, Ni0.30Co2.70S4 and Cu0.30Co2.70S4,
0.09 eV, which is very close to the thermoneutral. Our
calculations show that the second metal doping can
significantly influence the |ꢄG *| value of Co S . For instance,
ꢀ2
respectively) than Co S (32.5 mF cm ). Note that pure ZnS
3
4
H
3 4
exhibits negligible HER activity (Figure S7c). Therefore,
Co S is the major catalyst and the homogenous doping of Zn
the |ꢄG *| value of Znꢀdoped Co S is 0.32 eV, comparable to
H 3 4
12
many active nonꢀnoble metal catalysts. In addition, the
3
4
ions contributes to the catalytic performance. It is also noted
that the catalyst without thermal annealing has a low activity
due to the poor crystallinity (Figure S1).
|ꢄG *| value of Niꢀdoped Co S is 0.39 eV, also significantly
H
3 4
lower than that of Co S (0.53 eV). Contrarily, Cuꢀdoped
3
4
Co S shows a very strong hydrogen adsorption (|ꢄG *|=0.57
3
4
H
0
.3 b)
a)
eV), which counteracts the enhanced electrical conductivity.
The collaborative effect of electrical conductivity and
hydrogen adsorption explains why Znꢀdoping has excellent
HER activity.
0
20
40
60
-1
8
5.3 mV dec
no catalyst
Co S
-
-
-
3
4
0.2
Cu0.30Co270S4
Zn0.30Co2.70S4
Ni0.30Co2.70S4
Co S4
-1
3
47.5 mV dec
Zn0.30Co2.70S4
Pt/C
0.1
Pt/C
milled Zn0.30Co2.70S4
-1
3
0.4 mV dec
-80
0.0
-
100
-
0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0
0.0
0.5
1.0
1.5
2.0
2.5
-2
Potential (V)
log (J (mA cm ))
180
0
0
c)
d)
η
= -100 mV
150
-10
-10
0
120
-20
-20
90
60
30
0
-20
-40
-60
1th
1000th
-
30
-80
-30
measured value
theoretical value
-40
-50
-100
-0.15
-0.10
-0.05
Potential (V)
0.00
-40
0
30
60
90
120
150
180
210
0
10
20
30
40
50
60
Time (min)
Time (h)
Figure 5. (a) Polarization data of M Co S , Co S and Pt/C
x
3ꢀx
4
3 4
electrodes in 0.5 M H SO . (b) Tafel plots. (c) Amount of H
2
2
4
production over Zn0.30Co2.70S4 at a current density of ca. 25
ꢀ2
mAcm . (d) Timeꢀdependent current density curve of HER
over Zn0.30Co2.70S4 at η=ꢀ100 mV over 60 h (Inset:
polarization curve after 1000 cycles).
3.4. Optimized Electronic Structure and H*
Adsorption. Nyquist plots (Figure S13b) reveals
a
Figure 6. (a) Projected density of states of pristine and Zn (Ni,
dramatically decreased charge transfer resistance (R ) for
ct
Cu)ꢀdoped Co S . (b) Difference charge density of valence
3
4
bimetallic sulfides (3.7, 10.5 and 25.1 ꢁ for Zn0.30Co2.70S4,
Ni0.30Co2.70S4 and Cu0.30Co2.70S4, respectively, compared to
band (left) and conduction band (right) in pristine and
Znꢀdoped Co3S4 (Red and blue color represents electron
accumulation and electron depletion.) (c) Calculated
freeꢀenergy diagram of HER at equilibrium potential on
pristine and Zn (Ni, Cu)ꢀdoped Co S .
6
6.1 ꢁ for Co S ). Theoretical calculation was further used to
3 4
verify the enhanced electrical conductivity. As shown in
calculated density of states (Figure 6a), the incorporation of
2
+
3 4
Zn into Co S can narrow the bandgap from 0.82 to 0.68 eV.
3
4
This bandgap narrowing leads to the enhanced excitation of
charge carriers to the conduction band, which is beneficial for
the electrical conductivity and the electrochemical
3.5. Universal Electrocatalyst over a Wide pH Range.
Until now most nonꢀnobleꢀmetal HER electrocatalysts work
+
well only in acidic medium to catalyze the conversion of H O ,
3
2
b,11b,c
performance.
The bandgap can be further reduced by
ACS Paragon Plus Environment

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Journal of the American Chemical Society
300
0
0
and the HER activities in alkaline and neutral media to
c)
200 d)
-
10
-
10
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
240
catalyze the conversion of H O are usually about two to three
pH=7
-20
-30
-40
-50
160 pH=14
2
-20
6a,13
180
orders of magnitude lower than in acidic medium.
1
20
-30
-40
-50
-60
However, many waterꢀalkali and chloroꢀalkali HER
electrolyzers need active and durable catalysts that can work
in alkaline medium, and many microbial electrolysis cells
typically need electrocatalysts that can function well in neutral
medium. As shown in Figure 5a and Figure 7a, b, Co3S4
exhibits very similar activity when the pH value changes from
120
60
0
80
-60
measured value
theoretical value
40
0
measured value
theoretical value
-70
-80
0
30
60
90
120
150
180
210
0
30
60
90
120
150
180
210
Time (min)
Time (min)
1
81.7
1
80 e)
1200.6
1200 f)
1000
800
155.2
1
005.3
1
20
60
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
to 14, which suggests that Co S ꢀbased material may be a
3 4
600
545.1
581.3
good candidate to work at allꢀpHꢀvalue. Actually, it is
400
reported that the HER activity of amorphous CoS is almost
x
200
0
0
6
a
independent of pH value.
10.3
0
0
Upon further incorporation of Zn, Zn0.30Co2.70S4 exhibits
remarkably improved HER activity both in neutral and
alkaline media than Co S , affording a current density of 10
Figure 7. (a, b) Polarization data of Zn0.30Co2.70S4, Co S and
Pt/C electrodes at pH=7 and pH=14. (c, d) Electrocatalytic
3
4
3
4
ꢀ2
mA cm at overpotential of 90 and 85 mV, respectively,
which is better than most of the reported nonꢀnobleꢀmetal
based HER electrocatalysts (with similar catalyst loading)
hydrogen production over Zn0.30Co2.70S4 at pH=7 and pH=14.
(e) Photocatalytic HER rate over EYꢀsensitized Zn0.30Co2.70S4
under visible light. (f) Photocatalytic HER rate over
Zn0.30Co2.70S4ꢀTiO under UV light and Zn0.30Co2.70S4ꢀC N
4
(
Table S4). Additionally, Zn0.30Co2.70S4 shows excellent
2
3
stability and about 100% Faradaic yield in both media (Figure
7c, d). These results clearly indicate that Zn0.30Co2.70S4 is a
universal and efficient HER electrocatalyst over a wide pH
under visible light.
4
. CONCLUSIONS
A general MOF selfꢀtemplated method to synthesize
hollow bimetallic sulfide (M Co S ) polyhedra has been
range (pH=0-14). Recent work also shows that MoS doped
x
with Co or Ni ions can be operated in alkaline and neutral
x
3ꢀx 4
6
media with considerable HER activity and stability. The
established. The second metal ions are homogenously
dispersed in the lattice of Co S due to the homogenous
secondary metal incorporation is very likely to promote the
water dissociation that is the first step for HER in alkaline and
3
4
composition of MOF precursor. The incorporation of second
metal and the formation of hollow structure improve the
catalytic active sites, optimize the electrical conductivity and
hydrogen adsorption of Co S . The bimetallic sulfides exhibit
significantly improved electrochemical and photocatalytic
HER activity compared with pristine Co S . Particularly,
Zn0.30Co2.70S4 shows electrochemical HER activity over a
wide pH range (pH=0ꢀ14) better than most of the reported
nonꢀnoble metal catalysts and photocatalytic HER activity
comparable to Pt. The strategy developed here, i.e.
homogenous doping and selfꢀtemplated hollow structure,
provides a way to synthesize transition metal sulfides for
catalysis and energy conversion.
6
a
neutral media. Then the optimized H adsorption (ꢄG * in
H
Figure 6c) by doping facilitates the formation of H from the
2
1
3c
H intermediates of water dissociation.
.6. Excellent Photocatalytic HER Performance.
3
4
3
Hollow Zn0.30Co2.70S4 also works very well as cocatalyst in
photocatalytic HER. In photocatalysis, dyes and
semiconductors can absorb light, generate and transfer
3
4
electrons to cocatalyst for H evolution (Figure S15). Figure
2
−1
7e shows a promising H evolution rate of 155.2 µmol h
2
over Zn0.30Co2.70S4 when sensitized by Eosin Y dye under
visible light, which is comparable to the activity of Pt/C
−1
(
188.7 µmol h ). Furthermore, the H evolution rate is very
2
stable during longꢀterm test (Figure S16). TiO and C N are
2
3
4
1
h,i,7d
promising UVꢀ and visibleꢀresponse semiconductors,
ASSOCIATED CONTENT
Supporting Information.
respectively (Figure S17). When combined with them,
Zn0.30Co2.70S4 shows HER activity very close to Pt (Figure 7f).
Detailed experimental and computation procedures, results of
characterizations.
0
20
40
60
80
a)
0
-20
b)
-
-
-
-
AUTHOR INFORMATION
Corresponding Author
pH=7
pH=14
-40
3
Co S4
3
Co S4
Zn0.30Co2.70S4
Zn0.30Co2.70S4
-60
*
Eꢀmail: jj_zou@tju.edu.cn (J.ꢀJ. Zou)
Pt/C
Pt/C
-80
Notes
-100
0.6
-100
-0.6
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors appreciate the support from the National Natural
Science Foundation of China (U1462119, 21506156,
-
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
Potential (V)
Potential (V)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 9
U1463205), and the Tianjin Municipal Natural Science
Foundation (15JCZDJC37300).
Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304,
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Zhang, X.; Wang, L. Chem. Commun. 2014, 50, 10959; b) Lou, X.
W.; Archer, L. A.; Yang, Z. Adv. Mater. 2008, 20, 3987.
1
2
3
4
5
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8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
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3
3
3
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