Sub-1.1 nm ultrathin porous CoP nanosheets with
dominant reactive {200} facets: a high mass activity
and efficient electrocatalyst for the hydrogen
evolution reaction†
Chao Zhang, Yi Huang, Yifu Yu, Jingfang Zhang, Sifei Zhuo and Bin Zhang
*
The exploration of a facile strategy to synthesize porous ultrathin nanosheets of non-layered materials,
especially with exposed reactive facets, as highly efficient electrocatalysts for the hydrogen evolution
reaction (HER), remains challenging. Herein we demonstrate a chemical transformation strategy to
synthesize porous CoP ultrathin nanosheets with sub-1.1 nm thickness and exposed {200} facets via
phosphidation of Co
3
O
4
precursors. The resultant samples exhibit outstanding electrochemical HER
performance: a low overpotential (only 56 and 131 mV are required for current densities of 10 and
100 mA cm
2
, respectively), a small Tafel slope of 44 mV per decade, a good stability of over 20 h, and
a high mass activity of 151 A g
1
at an overpotential of 100 mV. The latter is about 80 times higher than
that of CoP nanoparticles. Experimental data and density functional theory calculations reveal that a high
proportion of reactive {200} facets, high utilization efficiency of active sites, metallic nature, appropriate
structural disorder, facile electron/mass transfer and rich active sites benefiting from the unique ultrathin
and porous structure are the key factors for the greatly improved activity. Additionally, this facile
chemical conversion strategy can be developed to a generalized method for preparing porous ultrathin
nanosheets of CoSe
2
and CoS that cannot be obtained using other methods.
1. Introduction
Hydrogen produced from water electrolysis is considered
to be a promising alternative energy source to fossil fuels
by virtue of its environmental benignity and sustainable
features.
1– 3
Pt is an excellent electrocatalyst for the hydrogen
evolution reaction (HER),
4
but its practical use is hindered by
its high price and rarity. Since the pioneering report of MoS
2
as an electrocatalyst for HER,
5,6
low-cost promising c andi-
dates composed of earth-abundant elements including metal
chalcogenides,
7– 10
carbides,
11–15
phosphides,
16–22
phosphosul-
phides,
23
and oxides
24,25
have attracted increasing attention.
Although fascinating advances have been made in the search
for novel alternatives and the improvement of their HER
performances via various methods, the materials are mainly
restricted to nanoparticles, polycrystalline one-dimensional
nanomaterials and thick sheets. Thus, the relatively large
sizes and low numbers of active sites of some current catalysts
mean that improvement of HER activity, especially the mass
activity, is needed. In addition, the catalytic performance of
amaterialismainlydependentonitsexposedcrystalfacets.
26
However, th e devel opment of a type of elec trocatalyst with
a large proporti on of exposed active crystal planes with hi gh
mass activity for HER is still highly desirable.
Two-dimensional (2D) ultrathin nanosheets with thick-
nesses of several nanometers have been extensively studied as
ideal materials for both the fundamental understanding of
structure–activity relationships and their promising applica-
tions in various elds because of their large specic surface
areas, richness of active sites, short electron/carrier transfer
distance, structural defects and predominantly exposed crystal
facets.
27–36
For example, greatly enhanced catalytic perfor-
mances have been achieved by the pioneering 2D ultrathin
nanosheets made by the Xie,
27–30
Wei,
27,28,31
Yang
32
and
Zhang
33,34
groups. A huge mass activity for the oxygen evolution
reaction has been achieved by well-designed ultrathin CoOOH
solid nanosheets.
31
Thus, some rationally-designed methods
including liquid exfoliation,
27–30
graphene oxide-assisted
growth,
34
topotactic reduction
35
and conversion
33
have been
successfully developed to produce inorganic ultrathin nano-
sheets. However, the products are mainly solid, rather than in
porous single-crystalline form. In addition, compared to solid
materials, porous nanostructures can possess many more active
sites and exhibit more facile mass transfer, therefore exhibiting
Department of Chemistry, School of Science, Tianjin Key Laboratory of Molecular
Optoelectronic Science, Tianjin University and Collaborative Innovation Center of
Chemical Science and Engineering (Tianjin), Tianjin 300072, China. E-mail:
bzhang@tju.edu.cn
† Electronic supplementary information (ESI) available: Fig. S1–S14 and Tables
S1–S3. See DOI: 10.1039/c6sc05687c
Cite this: Chem. Sci.,2017,8,2769
Received 28th December 2016
Accepted 24th January 2017
DOI: 10.1039/c6sc05687c
rsc.li/chemical-science
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improved chemical and physical properties.
37,38
More impor-
tantly, making ultrathin 2D sheets porous can not only generate
more coordinated-unsaturated active atoms, but also allows
easy electrolyte inltration into the inside of the catalysts,
39
which contributes to providing more active sites to participate
in the catalytic reactions and hence ensures energy conversion
operating with high-efficiency. However, until now, the devel-
opment of a facile chemical conversion route to prepare single-
crystalline 2D non-layered nanosheets as highly active HER
materials, especially endowing them with both ultrathin and
porous characteristics, is still a big challenge.
Herein, by choosing CoP, one of the most efficient electro-
catalysts for HER and other applications,
40–42
as the model
target, we present a convenient chemical transformation
approach to synthesize ultrathin porous CoP nanosheets
(CoP UPNSs) with a high proportion of exposed {200} facets, two
unit cell-thin thickness and modest distorted atomic structures
via low-temperature phosphidation of Co
3
O
4
precursors. Our
theoretical calculations and experimental results demonstrate
that CoP UPNSs are highly efficient catalysts for HER with
a huge mass activity of 151 A g
1
at an overpotential of 100 mV.
The 2D ultrathin structure with abundant pores and active sites,
high fraction of exposed active facets, modest structural
disorder of CoP NSs and facile ion/electron transfer are the key
factors for the superior catalytic performance. Furthermore,
this facile chemical conversion method can be extended to
prepare UPNSs of CoSe
2
and CoS.
2. Experimental
2.1 Material synthesis
2.1.1 Preparation of ultrathin porous Co
3
O
4
nanosheets.
Co
3
O
4
nanosheets were synthesized according to the reported
literature.
43
Co(acac)
3
(100 mg) was dispersed into a mixed
solution of 20 mL of ethylene glycol and 4 mL of distilled water
under vigorous stirring for 12 h in a 50 mL Teon-lined stain-
less-steel autoclave. Then the mixture was treated at 190
C for
48 h and cooled down naturally. The blue products were the
CoO nanosheets and were collected by centrifuging the mixture,
washed with ethanol and water many times and then dried
under vacuum overnight. The as-prepared CoO nanosheets were
heated to 400
C at a rate of 5
C min
1
, and kept at 400
C for
3 h. They were then cooled to room temperature and the ob-
tained powders were the ultrathin porous Co
3
O
4
nanosheets.
Aer ultrasonic treatment, the powders were dried via vacuum
freeze-drying for phosphidation.
2.1.2 Synthesis of CoP ultrathin porous nanosheets (CoP
UPNSs). To obtain CoP UPNSs, Co
3
O
4
(10 mg) and NaH
2
PO
2
$2H
2
O
(2 g) were put in two separate quartz boats with NaH
2
PO
2
$2H
2
O
at the upstream side of the furnace. Subsequently, the samples
were heated to 300
C for 120 min in a static Ar atmosphere at
a rate of 2
C min
1
.Aer cooling to room temperature, the
sample was washed with water and ethanol several times and
nally dried at 40
C overnight.
2.1.3 Synthesis of CoP nanoparticles (CoP NPs). CoP NPs
were synthesized according to the reported literature.
44
10 mmol of CoCl
2
$6H
2
O and 40 mmol of NaH
2
PO
2
$2H
2
O were
mixed together in an agate mortar and ground to a ne mixture.
The mixture was transferred to a ceramic boat and heated
to 400
C for 2 h at a heating rate of 2
C min
1
under an Ar
atmosphere. Then the sample was naturally cooled to ambient
temperature, washed with water and ethanol several times and
nally dried at 40
C overnight.
2.1.4 Synthesis of ultrathin porous CoSe
2
nanosheets
(CoSe
2
UPNSs). To obtain CoSe
2
UPNSs, 5 mg of Co
3
O
4
was
added into 15 mL of ethylene glycol containing Na
2
SeO
3
(0.625 mmol) under continuous stirring. Aer 60 min of
vigorous agitation, the dispersion was transferred into a 20 mL
Teon-lined autoclave and maintained at 180
C for 24 h. The
samples were collected and washed three times with ethanol
and water, respectively, and then dried at 60
C for 6 h.
2.1.5 Synthesis of ultrathin porous CoS nanosheets (CoS
UPNSs). To obtain CoS UPNSs, 5 mg of Co
3
O
4
was added into
15 mL of ethylene glycol containing thioacetamide (40 mg)
under continuous stirring. A er 60 min of vigorous agitation,
the dispersion was transferred into a 20 mL Teon-lined auto-
clave and maintained at 120
C for 5 h. The samples were
collected and washed three times with ethanol and water,
respectively, and then dried at 60
C for 6 h.
2.2 Material characterization
The structures of the samples were determined using a Hitachi
S-4800 scanning electron microscope (SEM, 3 kV). Powder X-ray
diffraction (XRD) patterns were collected using a Bruker D8
Focus Diffraction System using a Cu Ka source (l ¼ 0.154178 nm).
Transmission electron microscopy (TEM), higher-magnication
transmission electron microscopy (HRTEM) and elemental
distribution mapping images were taken on a JEOL-2100F
system equipped with EDAX Genesis XM2. The thickness of the
nanosheets was determined using atomic force microscopy
(AFM) (Bruker multimode 8). X-ray photoelectron spectroscopy
(XPS) measurements were conducted with a PHI-1600 X-ray
photoelectron spectrometer equipped with Al Ka radiation.
All binding energies were referenced to the C 1s peak at
284.8 eV.
2.3 Electrochemical measurements
Electrochemical measurements were performed with a CHI
660D electrochemical workstation (CH Instruments, Austin, TX)
and a typical one-component three-electrode cell was used,
including a working electrode, a saturated calomel electrode
(SCE) as the reference electrode, and a glassy carbon counter
electrode in the presence of 0.5 M H
2
SO
4
as the electrolyte. The
reference electrode was calibrated with respect to an in situ
reverse hydrogen electrode (RHE), by using two platinum wire
electrodes as the working and counter electrodes, which yields
the relation E (V vs. RHE) ¼ E (V vs. SCE) + 0.245 V. A glassy
carbon electrode decorated with catalyst samples was used as
the working electrode. In a typical procedure for the fabrication
of the working electrode, 4 mg of CoP catalyst and 20 mLof5%
Naon solution were dispersed in 1 mL of de-ionized water by
sonication to generate a homogeneous ink. Then 5 mL of the
dispersion (containing 20 mg catalyst) was transferred onto
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a glassy carbon electrode with a diameter of 3 mm (loading
amount: 0.28 mg cm
2
). The as-prepared catalyst lm was dried
at room temperature. Polarization data were collected at
a sweep rate of 2 mV s
1
. Electrochemical impedance spec-
troscopy (EIS) measurements were carried out in the same
conguration at h ¼ 56 mV or j ¼ 10 mA cm
2
from 100 kHz to
0.1 Hz.
2.3.1 Mass activity. The mass activity (A g
1
) values, as
shown in Fig. 2c, of different samples were calculated from the
electrocatalyst loading m (0.28 mg cm
2
) and the measured
current density j (mA cm
2
)ath ¼ 50 mV, 75 mV, 100 mV and
125 mV:
Mass activity ¼
j
m
2.3.2 Double-layer capacitance values. Electrochemical
capacitance measurements were used to demonstrate the active
surface area of the material. The potential was swept between
0.05 to 0.15 V vs. RHE ve times at each of the given scan rates
(10, 20, 40, 60, 80, 100, 120, 160, 200, 250 and 300 mV s
1
)to
obtain the electrochemical capacitance. The cyclic voltammo-
grams for the CoP UPNSs and CoP NPs can be seen in Fig. 3a
and b. We chose the capacitive currents at 0.10 V vs. RHE,
where faradic processes could not be observed in the potential
range. The obtained capacitive currents are plotted as a func-
tion of scan rate in Fig. 3c and a linear t measured the specic
capacitance to be 7.87 mF cm
2
for the CoP UPNSs and
0.358 mF cm
2
for the CoP NPs. The specic capacitance for
a at surface is generally found to be in the range of 20–60 mF
cm
2
. We used a value of 40 mFcm
2
in the following calcula-
tions of the electrochemical active surface area.
45
A
CoP UPNSs
ESCA
¼
7:87 mF cm
2
40 mFcm
2
cm
ESCA
2
¼ 196:75 cm
ESCA
2
A
CoP NPs
ESCA
¼
0:358 mF cm
2
40 mFcm
2
cm
ESCA
2
¼ 8:95 cm
ESCA
2
2.4 Theoretical calculations
Density functional theory (DFT) calculations were computed by
the Vienna Ab initio Simulation Package (VASP). In the DFT
calculations, the (100) surface was obtained by cutting bulk CoP
along the [100] direction. The thickness of the surface slab was
chosen to be that of a two-layer slab of the CoP unit. A vacuum
layer as large as 12
˚
A was used along the c direction normal to
the surface to avoid periodic interactions. A (2 2) supercell
was used. The Gibbs free-energy change (DG
ads
) of H on CoP
(100) is dened as follows:
DG
ads
¼ DE
ads
+ DE
ZPE
TDS
where DE
ads
is the adsorption energy of the atomic H on the CoP
(100) surface, DE
ZPE
is the difference in zero-point energy (ZPE)
between the adsorbed hydrogen and hydrogen in the gas phase,
and DS is the entropy change of one H atom from the absorbed
state to the gas phase. Since the H atom is binding on the
surface, the entropy of the adsorbed hydrogen can be assumed
to be negligible. Therefore, D S can be estimated by 1/2 S
0
,in
which S
0
is the standard entropy of H
2
in the gas phase at
a pressure of 1 bar and pH ¼ 0 at 300 K. In summary, the Gibbs
free-energy change (DG
ads
) of H can be described as
DG
ads
¼ DE
ads
+ 0.24 eV
DE
ads
is dened as follows:
DE
ads
¼ E
H
E
slab
þ
1
2
E
H
2
where E
H/slab
is the total energy of the H atom on the CoP (100)
surface, E
slab
is the total energy of the CoP (100) surface and
E
H
is the energy of the H atom referenced to gas H
2
. The rst
two terms are calculated with the same parameters. The third
term is calculated by setting the isolated H
2
in a box of 12
˚
A
12
˚
A 12
˚
A.
Since there are two surface structures of CoP (100), i.e. Co or
P terminated, we have therefore calculated the surface energy of
both surfaces by the following formula:
46
E
surf
¼
E
slab
nE
bulk
2A
where E
slab
is the total energy of th e surface slab, E
bulk
is the
total energy of the bulk CoP, A is the surface area with a factor
of 2 due to each slab containing two surfa ces, and n is the
number of CoP formula units in the slab. A small E
surf
means
that the surface is more stable. Thus, the calculated E
surf
values
of 1.73 eV
˚
A
2
for P term inated (200 ) and 1.78 e V
˚
A
2
for Co
terminated (200) indicate that CoP (100) with a P termi nated
surface is much more stable than that with the Co termin ated
surface. Thus, we study the P terminated CoP (100) surface in
this work.
For the P terminated CoP (100) surface, only P atoms are
exposed on the surface. Thus, H atoms will only locate at the top
of the P atoms. In our model, there are eight surface P atoms
available for H adsorption. The adsorption energy of one H
atom (12.5% H coverage, Fig. S11b†)is0.32 eV, and the
adsorption energy will be further reduced to 0.11 eV as the
coverage of H is above 75%.
3. Results and discussion
Co
3
O
4
was selected as the initial material because of its good
thermal stability. Firstly, atomically-thick porous Co
3
O
4
precursor sheets (Fig. S1, ESI†) were synthesized according to
a rationally-designed scalable fast-heating strategy developed by
the Xie group.
43
Then the Co
3
O
4
nanosheets were transformed
into CoP through low temperature gas-phase phosphodation
using NaH
2
PO
2
as the phosphorus source. Transmission elec-
tron microscopy (TEM) images (Fig. 1a and b) demonstrate that
the ultrathin 2D porous structure can be successfully prepared
on a large scale, which is of great importance for potential
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catalytic applications. Moreover, a typical high-resolution TEM
(HRTEM) image (Fig. 1c) shows that the nanosheets possess
mesopores with diameters of several nanometers. Lattice
spacings of 0.280 and 0.283 nm can be attributed to the (002)
and (011) crystallographic planes of orthorhombic CoP,
respectively (Fig. 1c). A close-up view (Fig. 1d and S2a†) reveals
that a mixture of some atomic disorder structures and amor-
phous areas can be observed clearly. The appearance of struc-
tural distortion and the amorphous phase may be associated
with strain release due to lattice mismatches of Co
3
O
4
and
CoP.
47,48
Such structural defects should be conducive to
decreasing the surface energy in order to improve the stability of
ultrathin 2D sheets.
29
The associated Fast Fourier Transform
(FFT) pattern of the HRTEM image (inset in Fig. 1c) discloses
that the porous nanosheet is in single crystalline form with
a preferential [100] orientation. The atomic force microscopy
(AFM) image and the corresponding height conguration
(Fig. 1e and f) indicate that CoP UPNSs possess uniform
thickness of about 1.01 nm. This value corresponds to the
thickness of two unit cells along the [100] direction of
orthorhombic CoP (a ¼ 5.076
˚
A, b ¼ 3.279
˚
A, c ¼ 5.599
˚
A, in
JCPDS no. 29-0497), further illustrating that the as-transformed
nanosheets possess a preferentially exposed {200} crystal facet
with a two unit cell-thin thickness. An obvious Tyndall light-
scattering effect is observed by side-illuminating lighting (inset
in Fig. 1e), suggesting the formation of a well-dispersed ultra-
thin 2D sheet colloid. The diffraction peaks in the X-ray
diffraction (XRD) pattern (Fig. 1g) can be indexed as ortho-
rhombic CoP (JCPDS no. 29-0497), thus demonstrating the
successful conversion from Co
3
O
4
into CoP. Point-scan energy
dispersive X-ray spectroscopy (EDS) analysis (Fig. S3†) and the
scanning transmission electron microscopy EDS (STEM-EDS)
mapping images (Fig. 1h) indicate the existence and uniform
distribution of Co and P. In addition, the specic surface area of
UPNSs is 92.23 m
2
g
1
(Fig. S4†). All these results imply that CoP
UPNSs with a high proportion of exposed {200} facets and with
sub-1.1 nm thickness have been successfully fabricated through
the convenient chemical transformation route.
The electrocatalytic HER activity of CoP UPNSs was rstly
examined by linear scan voltammetry (LSV) in 0. 5 M H
2
-satu-
rated H
2
SO
4
solut ion. For comparison, CoP nanoparti cles (NPs)
(Fig. S5†) and commercial 20 wt% Pt/ C deposited on glassy
carbon (GC) electrodes with the same amount were also tested
under the same conditions. As shown in th e I–R corrected LSV
polarization curves (Fig. 2a and S6†), Pt/C unquestionably
exhibits the highest performance with negligible overpotential
Fig. 1 (a, b) TEM images, (c, d) HRTEM images and the associated FFT
pattern (inset c) of CoP UPNSs. (e) AFM image and the side-illuminating
lighting photo (inset e). (f) The corresponding height profiles of the
nanosheets. (g) XRD pattern and (h) STEM-EDS elemental mapping
images of CoP UPNSs.
Fig. 2 (a) I–R corrected polarization curves of CoP UPNSs, CoP NPs,
bare GC and 20% Pt/C and (b) corresponding Tafel plots of CoP
UPNSs, CoP NPs and 20% Pt/C in 0.5 M H
2
SO
4
at a scan rate of 2 mV
s
1
. (c) Mass activity as a function of the overpotential for CoP UPNSs
and NPs. (d) Electrochemical impedance spectra of CoP UPNSs and
NPs. (e) Polarization curves of CoP UPNSs initially and after 2000 CV
scans. (f) Time-dependent current density curve.
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and bare GC is totally inactive towar ds HER. Surprisingly, for
the as-obtained CoP UPNSs, the current densities of 10, and
100 mA cm
2
only require overpotentials of 56 mV an d 131 mV,
respectively, which are much lower than those requir ed for
CoP NPs and most reported TMPs under similar conditions
(Table S1†). This p erformance is far superior to most ot her
non-noble me tal HER catalysts (Table S2†), indicating the high
activity of CoP UPNSs . To probe the HER kinetics, Tafel slopes
were calculated. As depicted in Fig. 2b, the Taf el slope for Pt/C
is 32 mV per decade, which is consistent with the literature
values.
1–3,16–21
The Tafel slope for CoP UPNSs is calc ulated to be
44 mV per decade, i ndicating a rst-class electrocatalytic
activity towards HER with the Volmer– Heyrovsky m echanism.
21
This value is smaller than that o bserved for CoP NPs (81 mV per
decade) and those of other TMPs (Table S1†). Meanwhile, an
extrapolation method applied to the T afel plot reveals that the
excha nge cu rrent density (j
0
)is0.61mAcm
2
, which is the best
value f or TMP electrocatalysts (Table S1†). Very surprisingly,
CoP UPNSs possess a huge mass activity towards HER. For
instance, a low overpotential of 100 mV ca n deliver a mass
activity o f 151 A g
1
, which is over 80 times hig her than that of
CoP NPs (Fig. 2c). All these results demonstrate that CoP UPNSs
are highly active for HER with an extremely large mass activity.
Electroche mical i mpeda nce spectroscopy (EIS) results (Fig. 2d
and S7†) demons trate a smaller interfacial charge-transfer
resistance of CoP UPNSs than th at of CoP NPs. This greatly
accelerati ng interfacial ch arge transfer can be ascribed to the
improved conductivity
29
and efficient interfacial contact of 2D
porous materials with electrolyte. To evaluate the stability in
a strong acid environ ment, a long-term cycling test was adop-
ted by comparing the polar ization curves before and aer
2000 CV cycles. The na l polarization curv e of CoP UPNSs still
overlaps with the or iginal one (Fig. 2e). Fig. 2f shows that the
catalytic performance remains unchanged f or at least 24 h.
Additional characterizations clear ly show that the original
ultrathin porous 2D architecture and composition can be
maintained aer long-term measurements (Fig. S8 and S9†),
revealing that CoP UPNSs ar e highly stable for HER. Impor-
tantly, the CoP UPNSs exhibit almost 100% faradic efficiency
for HER (Fig. S10†).
To elucidate the origins of the outstanding performance of
CoP UPNSs, we perfor med a series of experimental charac-
teriza tions. The electrochemical active surface areas (ECS A)
are usually evaluated by their electro chemical double layer
capacitances (C
dl
) because of their positive proport ion re la-
tionship.
49
As shown in Fig. 3c, CoP UPNSs display a C
dl
value
of 7.87 mF cm
2
, which is 22 times higher than that of CoP NPs
(0.358 mF cm
2
), suggesting a m uch larger ECSA of CoP UPNSs
over the corresponding NPs. Thus, the large ECSA that origi-
nates from both the ultrathin and porou s chara cteristics of
CoP UPNSs can play an im portant role on the high activity
of the a s-converted UPNSs. In addition, the structural distor-
tions and amorphous areas observed in Fig. 1d should make
signicant contributions to the high activity.
29
To de ter mine
the facet effect and exclude the inuence of ECSA on the
electrochemical activ ity of CoP towards HER, the currents were
normalized to the relative ECSA. The normalization curves
(Fig. 3d) reveal that CoP UPNSs still exhibit a slight ly lower
onset potential and smaller Tafel slope than CoP NPs. We
speculate that the im provement of the normalized activity may
be associated with a high proportion of exposed {200} facets in
CoP UPNSs.
Next, density functional theory (DFT) calculations were
adopted to fundamentally understand the role of the exposed
{200} facets. For most electrocatalysts, DG
H*
and its coverage
dependence are the key descriptors for HER activity.
1–3,45,50–52
It
is believed that the optimal value of |DG
H*
| is zero.
45,50–52
For
example, the best catalyst, Pt, possesses a DG
H*
value of about
0.09 eV.
45,50–52
Surface formation energy calculations reveal
that the stable plane for the (100) facet, one typical plane of
{200} facets, is the P terminated CoP (100) surface (Fig. 4a).
Fig. 3 CV curves of CoP UPNSs (a) and CoP NPs (b) with various scan
rates, (c) charging current density differences plotted against scan
rates. The capacitive currents were measured at 0.10 V vs. RHE. (d) The
LSV curves from Fig. 2a normalized to the electrochemical active
surface area (ECSA).
Fig. 4 (a) Simulated structure and (b) the dependence of DG
H*
on
hydrogen coverage q
H*
. (c) Projected density of states of the P
terminated CoP (100) facet. Co atoms: blue, P atoms: purple and
hydrogen atoms: red.
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