Research Article
www.acsami.org
Wide Range pH-Tolerable Silicon@Pyrite Cobalt Dichalcogenide
Microwire Array Photoelectrodes for Solar Hydrogen Evolution
Chih-Jung Chen,† Kai-Chih Yang,‡ Mrinmoyee Basu,† Tzu-Hsiang Lu,† Ying-Rui Lu,⊥,∥
Chung-Li Dong,⊥,¶ Shu-Fen Hu,*,‡ and Ru-Shi Liu*,†,§
†
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan
§
Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of
Technology, Taipei 10608, Taiwan
⊥
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
¶
Department of Physics, Tamkang University, Tamsui 25137, Taiwan
∥
Program for Science and Technology of Accelerator Light Source, National Chiao Tung University, Hsinchu 30010, Taiwan
‡
S Supporting Information
*
ABSTRACT: This study employed silicon@cobalt dichalcogenide microwires (MWs) as wide range pH-tolerable photocathode material for solar
water splitting. Silicon microwire arrays were fabricated through
lithography and dry etching technologies. Si@Co(OH)2 MWs were
utilized as precursors to synthesize Si@CoX2 (X = S or Se)
photocathodes. Si@CoS2 and Si@CoSe2 MWs were subsequently
prepared by thermal sulfidation and hydrothermal selenization reaction
of Si@Co(OH)2, respectively. The CoX2 outer shell served as cocatalyst
to accelerate the kinetics of photogenerated electrons from the underlying
Si MWs and reduce the recombination. Moreover, the CoX2 layer
completely deposited on the Si surface functioned as a passivation layer by
decreasing the oxide formation on Si MWs during solar hydrogen
evolution. Si@CoS2 photocathode showed a photocurrent density of
−3.22 mA cm−2 at 0 V (vs RHE) in 0.5 M sulfuric acid electrolyte, and
Si@CoSe2 MWs revealed moderate photocurrent density of −2.55 mA cm−2. However, Si@CoSe2 presented high charge transfer
efficiency in neutral and alkaline electrolytes. Continuous chronoamperometry in acid, neutral, and alkaline solutions was
conducted at 0 V (vs RHE) to evaluate the photoelectrochemical durability of Si@CoX2 MWs. Si@CoS2 electrode showed no
photoresponse after the chronoamperometry test because it was etched through the electrolyte. By contrast, the photocurrent
density of Si@CoSe2 MWs gradually increased to −5 mA cm−2 after chronoamperometry characterization owing to the
amorphous structure generation.
KEYWORDS: cobalt dichalcogenide, silicon microwire arrays, wide range pH toleration, hydrogen evolution, solar water splitting,
co-catalyst
■
INTRODUCTION
reaction (HER). Noble metals that served as cocatalysts were
integrated on Si photoelectrode to reduce the recombination of
photogenerated electron−hole pairs.2−4 In our previous
research, Ag plasmonic particles were deposited on Si
photocathode through a facile chemical method. The photocurrent density of Ag−Si electrode was achieved −35 mA cm−2
at −1.0 V (vs Ag/AgCl), but expensive Ag particles reduced the
mass production possibility.5 Furthermore, noble metals were
unable to protect Si photoelectrode from oxidation or
corrosion during the photoelectrochemical (PEC) reaction
Depleted fossil fuels and increased environmental concerns
have triggered an urgent demand for clean and sustainable
alternative energy sources. Photoelectrolysis of water for
hydrogen generation was first demonstrated by Honda and
Fujishima in 1972.1 The use of hydrogen gas was a promising
method to substitute the utilization of fossil fuels because of the
former’s zero carbon emission. Among numerous semiconductor materials employed as photoelectrodes for solar
water splitting, silicon shows a highly negative conduction band
edge (−0.46 V vs NHE) and small band gap (1.1 eV) to absorb
visible illumination and reduce protons in the electrolyte.
However, low kinetics of photoinduced carriers on Si
photoelectrode limited its efficiency in solar hydrogen evolution
© 2016 American Chemical Society
Received: January 2, 2016
Accepted: February 9, 2016
Published: February 9, 2016
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and exposure of electrolyte, leading to a serious degradation or
even deactivation. Titanium oxide (TiO2)6,7 or aluminum oxide
(Al2O3)8,9 passivation layer was the commonly studied material
to modify Si photoelectrodes and consequently improve the
photocatalytic durability. However, Pt metal was applied to
decorate these passivation layer modified-Si electrodes and thus
accelerate the migration of photoinduced carriers to the
electrolyte and reduce the overpotential. Although Pt particles
modified on Si photoelectrode to dramatically improve the
water splitting performance, the high cost limited the real
applications.2−4 Therefore, Earth abundant Ni−Mo alloy
catalyst was prepared to enhance the activity of Si photoelectrode for solar fuel production in the studies of N. S. Lewis
and co-workers.10,11 Besides, robust electrocatalysts were more
attractive materials to deposit on Si photoelectrode because
they were simultaneously used as both active and protective
layers for improving the photoelectrochemical activity and
stability.9,12
Water electrolysis devices composed of proton exchange
membrane (PEM) technology were worked under extremely
acidic electrolyte conditions, and PEC cells of solar hydrogen
generation were designed to function under acidic conditions.
Recently, Earth-abundant transition metal dichalcogenide
materials (with the general formula MK2, where M = Fe, Co,
or Ni; and K = S or Se) showed promising electrolytic
hydrogen evolution efficiencies in acidic electrolyte.13−17
However, microbial electrolysis cell (MEC) was operated
under neutral wastewater to produce hydrogen from the
degradation of organic materials.18 Bacteria hybrid solar carbon
dioxide (CO2) fixation to valued-added chemicals was also
conducted in neutral condition.19 Moreover, water photoelectrolysis in alkaline media was significant for practical
applications. In a previous work, NiOx-Fe2O3-coated p-Si
photocathodes prepared for solar water splitting in neutral pH
water showed a moderate photoelectrochemical performance.20
The three-dimensional branched ZnO/Si heterojunction
photocathode was only adopted for solar hydrogen evolution
in neutral electrolyte owing to unstability of ZnO in the acidic
and alkaline solution.21−23 Therefore, photoconversion of solar
energy into chemical fuels under wide range pH conditions
represents a prospective study.
In our previous study, CoSe2 naonorods with a marcasite
phase were prepared through a hydrothermal reaction to
decorate on Si microwires (MWs), showing a promising
photoelectrochemical performance.24 In the presented work,
Si@CoX2 (X = S or Se) MW arrays were prepared as wide
range pH-tolerable photocathodes for solar water splitting, as
shown in Figure 1. As compared with marcasite CoSe2
materials, the CoX2 outer shell was pyrite phase to show a
more robust durability during water splitting reaction because
of the close packing characteristics. The CoX2 outer shell
functioned as a cocatalyst to enhance the charge transfer
efficiencies of photogenerated electrons from Si MWs and
reduce the recombination. Moreover, the CoX2 layer, which
was completely deposited on the Si surface, served as a
passivation layer by decreasing the oxide formation on Si MWs
during solar hydrogen evolution. Therefore, pyrite CoS2
decorated-Si MWs drove the water photoelectrolysis reaction
in acid electrolyte for 9 h without obvious degradation.25 In this
study, Si@CoS2 photocathode showed photocurrent density of
−3.22 mA cm−2 (at 0 V) in 0.5 M sulfuric acid solution, and
Si@CoSe2 MWs revealed moderate photocurrent density of
−2.55 mA cm−2. However, Si@CoSe2 presented higher charge
Figure 1. Schematic illustration of Si@CoX2 (X = S or Se) microwires
functioned as wide range pH-tolerable photocathodes for solar water
splitting.
transfer efficiency and stability in neutral and alkaline
electrolytes. Continuous chronoamperometry in acid, neutral,
and alkaline solutions was conducted at 0 V (vs RHE) to
evaluate the photoelectrochemcial duribility of Si@CoX2 MWs
in various pH electrolytes. Si@CoS2 electrode showed no
photoresponse after the chronoamperometry test because it
was etched through the electrolyte. By contrast, the photocurrent density of Si@CoSe2 MWs gradually increased to −5
mA cm−2 during chronoamperometry characterization due to
the amorphous structure generation.
■
EXPERIMENTAL SECTION
Fabrication of Si MWs. B-dope (100)-oriented silicon wafers (ptype Si, resistivity: 1−25 Ω cm) were used to produce Si MWs by
lithography and dry etching technologies. First, p-Si wafers were
cleansed by STD cleaning method with hydrofluoric acid. The SiO2
layer (500 nm) was deposited on the Si substrate through wetoxidation process of the horizontal furnace. The photoresist (800 nm)
was subsequently spread on an oxide layer by spin coating. The
photoresist was then exposed and developed by Tracker to form the
graph (1 cm × 1 cm) of rod array with pitch size of 850 nm. The hard
mask was prepared by etching the SiO2 layer under mixed atmosphere
with Ar, O2, CF4, and CHF3 by using TEL5000 and removing the
photoresist by Mattson and H2SO4. Afterward, Si MWs (∼10 μm)
were prepared under mixed atmosphere of O2, SF6, and Ar by using
ICP etcher, and Si MWs wafers were then sent to Yuanfa Electronics
Corporation to polish the back side for reducing the diffusion distance
of photogenerated carriers until the thickness was ∼350 μm. Finally,
the hard mask was removed by B.O.E, and the aluminum layer (500
nm) on the back side of Si MWs wafer was deposited by E-gun as the
current collector. The aluminum back electrode was annealed at 400
°C for 30 min to improve the attachment on Si MWs.
Fabrication of Si@Co(OH)2 MWs. Si@CoS2 and Si@CoSe2 MWs
were correspondingly fabricated by thermal sulfidation and hydrothermal selenization reaction of the precursor Si@Co(OH)2. The Si@
Co(OH)2 MWs were prepared through a facile chemical deposition
reaction. Before the reaction, the Al back electrode was protected by
Teflon tape from reacting with the precursor solution. Si MWs were
then fixed on a glass slide and reversed into the place Si MWs face
down in the Petri dish. The space between the glass slide and the
bottom of the Petri dish was approximately 2 mm. Precursor solution
about 10 mmol was subsequently prepared with 2.91 g of cobalt nitrate
hexahydrate [Co(NO)3·6(H2O)] dissolved in 10 mL of deionized
water in a beaker; the mixture was stirred at 200 rpm by using a
magnet stir bar. Afterward, 40 mL of ammonia solution (30% to 33%)
was slowly added in the solution. After the reaction solution was
stirred by 500 rpm for 30 min, it was decanted to the Petri dish and
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ultrasonicated to remove the residual gas between Si MWs for 10 min.
Another Petri dish covered its top and heated to 90 °C for 6 h in an
oven. Following this chemical deposition process, the residues on Si@
Co(OH)2 were cleansed by deionized water and dried by N2 flow.
Fabrication of Si@CoSe2 MWs. After the Si@Co(OH)2 MWs
were synthesized, the Teflon tape stuck on its Al back electrode was
kept for selenization through the hydrothermal reaction. The 0.5
mmol solution was prepared by dispersing 39.4 mg of Se powder in 12
mL of deionized water; the solution was then ultrasonicated in an
autoclave for 10 min. The Si@Co(OH)2 MWs were placed upside
down in the reaction solution, and 22.6 mg of NaBH4 was added.
Immediately, the solution was ultrasonicated for 5 min and sealed in a
stainless steel container. The Si@CoSe2 MW electrode was
successfully synthesized by hydrothermal reaction at 180 °C for 15 h.
Fabrication of Si@CoS2 MWs. Si@CoS2 MWs were fabricated by
thermal sulfidation of Si@Co(OH)2 MWs through the horizontal tube
furnace. The Si@CoS2 MWs were placed in alumina crucible, which
was placed at the middle of horizontal tube furnace. Up to 2 g of sulfur
powder was added uniformly to another alumina crucible and then
placed at the upstream position of the tube. Thermal sulfidation was
maintained at 500 °C for 1 h under Ar atmosphere with a flow rate of
45 sccm. After the reaction, the samples were naturally cooled down to
35 °C.
Photoelectrochemical Measurement of Si@CoX2 MWs. The
three-electrode system was adopted to measure the photoelectrochemical performance of Si@CoX2 MWs. The counter electrode was
the Pt plate, and the reference electrode was the saturated Ag/AgCl
electrode. The working electrode was Si@CoX2 MWs for photocatalytic water splitting characterization. The 0.5 M H2SO4, 1 M
phosphate-buffered saline (PBS), and 1 M KOH solution were utilized
as the acidic, neutral, and alkaline electrolytes. All photoelectrochemical characterizations were carried out under the illumination of a
xenon lamp equipped with an AM 1.5 filter, and the light intensity was
constant at 100 mW cm−2. The results were recorded through utilizing
a potentiostat (Eco Chemie AUTOLAB, The Netherlands) and
General Purpose Electrochemical System (GPES) software. Linear
sweep voltammetry was scanned from +0.55 V to −0.45 V (vs RHE)
with a scan rate of 20 mV s−1. Chronoamperometry and gas evolution
were conducted at 0 V (vs RHE).
Electrochemical Measurement of CoX2/Ti. CoX2 layer was
prepared on Ti foil to characterize the electrochemical performance
through the identical synthesis method of photoelectrode materials.
The three-electrode system was composed of Pt counter electrode,
Ag/AgCl reference electrode, and CoX2/Ti working electrode. All
electrochemical data were recorded using an electrochemical workstation (760D, CH Instruments). Linear sweep voltammetry was
scanned from 0 V to −0.30 V (vs RHE), and cyclic voltammetry was
measured at the nonfaradic voltage region between +0.10 V and +0.20
V.
Characterization of Materials. JEOL JSM-6700F field-emission
scanning electron microscopy (SEM) was applied to investigate the
morphologies of materials. The crystallization phase and stretching
modes of materials were characterized using a Bruker D2 PHASER Xray diffraction (XRD) analyzer with Cu Kα radiation (λ = 1.54178 Å)
and a Thermo DXR Raman microscope with a 532 nm laser. A
Thermo EVOLUTION 220 spectrometer was adopted to observe the
reflectance spectra of materials. Total reflectance spectra were
measured in this study. All X-ray absorption spectroscopy (XAS)
experiments were performed at the 20A1 beamline of the National
Synchrotron Radiation Research Center (NSRRC) in Hsinchu City,
Taiwan.
photocathode provided shorter diffusion lengths of minority
carriers and a single conducting direction of majority carriers to
decrease the recombination of electron−hole pairs. The specific
surface area of Si MWs was much higher than that of planar
structure. Si MWs with high surface area reacted with the
electrolyte to increase its photoresponse. SEM images showed
that the length and diameter of Si photoelectrodes were 10 μm
and 800 nm, respectively (Figure S1a,c). The Si@Co(OH)2
MWs fabricated through a chemical deposition method were
employed as precursors to prepare Si@CoX2 MWs. As shown
in Figure S1b,d, Si@Co(OH) 2 MWs were core−shell
structures, and the radius of single wire increased from 400
to 700 nm after the Co(OH)2 layer modification. The Si@CoS2
and Si@CoSe2 MWs were separately synthesized by the
thermal sulfidation and hydrothermal selenization reaction of
Si@Co(OH)2 (see the details in the Experimental Section).
The radii of Si@CoS2 and Si@CoSe2 were approximately 760
and 840 nm (Figure 2). The thickness of the CoX2 shell on Si
MWs increased from CoS2 to CoSe2 because the ionic radius of
S2− anion (184 pm) was smaller than that of Se2− (198 pm).
Figure 2. (a, b) Top view and (c, d) cross-sectional SEM images of
Si@CoS2 and Si@CoSe2 microwire arrays.
XRD spectra in Figure 3, panel a show that Si@CoS2 and
Si@CoSe2 MWs were consistent with the pyrite cubic structure.
The diffraction peaks of Si@CoS2 at 2-theta values of 27.8°,
32.3°, 36.2°, 39.8°, 46.4°, and 54.9° corresponded to the Miller
indices of (111), (200), (210), (211), (220), and (311). The
diffraction peaks of Si@CoSe2 at 2-theta values of 30.5°, 34.1°,
46.5°, and 51.8° were separately related to the Miller indices of
(200), (210), (221), and (311). Two diffraction peaks
contributed to Si MWs at approximately 33° and 62°. XRD
spectra showed no impurity byproducts in the Si@CoS2 and
Si@CoSe2 materials. The primary particle sizes of CoS2 and
CoSe2 outer shell were estimated to be 45.3 and 9.48 nm from
the diffraction peak with the strongest intensity, as calculated by
Scherrer equation. Raman spectrum of bare Si MWs at Raman
shift of 520.3 cm−1 is shown in Figure 3, panel b.26 After CoS2
and CoSe2 layers were deposited, the Raman peak of Si MWs
disappeared because the CoX2 outer shell completely covered
the surface of underlying Si MWs. Three Raman peaks were
observed at 288, 316, and 390 cm−1, which were associated with
Eg, Tg(1), and Ag of CoS2 Raman modes. One shoulder
appeared at ∼423 cm−1 and was correlated to Tg(2) stretching
mode.27 Eg and Ag modes represented the librational vibration
■
RESULTS AND DISCUSSION
In this study, Si MW arrays were sequentially prepared through
photolithography and dry etching technologies. The absorption
of Si MWs was enhanced because the incident illumination
proceeded with multiple paths in the microstructures of Si
photoelectrode. The scattered light was reabsorbed through Si
MWs to reduce the reflectance. One-dimensional structure of Si
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standard, CoCl2 and CoO commercial powders were used as
standards of Co2+ cations, and Co3O4 was the mixed valence
states of Co2+ and Co3+ ions. Two peaks observed in the Co Ledge spectra indicate L2 (∼797 eV) and L3 (∼782 eV) edge
(Figure 4a). The integrated L3-edge area of Si@CoX2 MWs
corresponded to Co2+ standards (Figure 4b).
CoX2 materials were synthesized on Ti foils through the
same procedure for Si photocathodes to analyze the electrochemical activities. Figure S2 reveals the XRD spectra of CoS2/
Ti, CoSe2/Ti, and Ti foils. No impurity side products were
detected, which further indicates the successful deposition of
CoX2 electrocatalysts on Ti foils. The three-electrode system
was utilized to perform linear sweep voltammetry (LSV) for
measuring HER activities of CoX2/Ti (X = S or Se) in 0.5 M
H2SO4 aqueous solution, as shown in Figure S3a. The
electrochemical results were presented after the iR compensation and background correction. The current densities of CoS2/
Ti and CoSe2/Ti achieved −41.5 and −32.0 mA cm−2 at −0.25
V (vs RHE). In this work, the onset potential was defined as the
voltage of the current density that reached −1.0 mA cm−2. The
inset of Figure S3a shows that the onset potentials of CoS2/Ti
and CoSe2/Ti were −0.139 and −0.176 V, correspondingly.
This result indicates that the CoS2 electrocatalyst overcame
lower overpotential to drive HER reaction, as compared with
the CoSe2 material. Tafel plot of CoX2/Ti was applied to
analyze the rate-determining step during the HER process
(Figure S3b). The classic two-electron HER reaction occurred
through two following mechanisms: a discharging step of the
Volmer reaction (H3O+ + e− → Hads + H2O) followed by a
desorption step of the Heyrovsky reaction (Hads + H3O+ + e−
→ H2 + H2O) or a recombination of the Tafel reaction (Hads +
Hads → H2), where Hads is the hydrogen atom adsorbed at the
active site on the catalyst surface. Besides, the electrocatalyst
with the Tafel slope of 116, 38, or 29 mV dec−1 revealed that
the rate-determining step in the HER process was the
discharging, desorption, or recombination step, respectively.
The Tafel slope of CoS2/Ti was 62.8 mV dec−1, indicating that
the HER rates were determined by the competition between
the Volmer and Heyrovsky reactions. The rate-determining
step of CoSe2/Ti during the HER process was the desorption
step of adsorbed hydrogen atoms owing to its Tafel slope at
39.5 mV dec−1.
Cyclic voltammetry (CV) was conducted to measure the
electrochemical double-layer capacitance of the CoX2/Ti
electrocatalyst in Figure S4. This characterization was
conducted by recording the capacitive current as a function
of potential sweep rate between a non-Faradaic voltage range
(0.1−0.2 V vs RHE). The result provides an approximation of
electrochemically active surface area on the electrocatalyst to
drive the HER reaction. Figure S4c reveals the electrochemical
double-layer capacitance (Cdl) of CoX2/Ti. The double-layer
capacitance of CoS2/Ti was approximately 0.442 mF cm−2,
which was higher than that of CoSe2/Ti (0.273 mF cm−2). This
result shows more revealed active sites on the surface of CoS2
electrocatalyst, leading to more positively shifted onset
potential, as compared with CoSe2 materials.
The LSV measurement for PEC efficiencies of Si@CoX2
photocathode materials was conducted in the three-electrode
cell under various pH ranges of the electrolyte under solar
simulation, as shown in Figure 5, panel a. The 0.5 M H2SO4, 1
M phosphate-buffered saline (PBS), and 1 M KOH solution
were utilized as the acidic, neutral, and alkaline electrolytes. The
bare Si MWs showed no photoresponse in various pH values of
Figure 3. (a) XRD spectra of Si@CoS2 and Si@CoSe2 microwire
arrays. (b) Raman spectra of Si@CoS2, Si@CoSe2, and bare Si
microwire arrays.
and the in-phase stretching vibration of sulfur dumbbells,
respectively. Tg(1) mode comprised the stretching and
librational vibration of sulfur atoms, whereas Tg(2) represented
the out-of-phase stretching vibration of sulfur dumbbells.28 The
broad Raman peak of Si@CoSe2 MWs at 184 cm−1 was
assigned to the Se−Se stretching mode of cubic CoSe2.16,28,29
Cobalt L-edge X-ray absorption near-edge structure
(XANES) spectra provided quantitative determination of
oxidation state, which is proportional to the area under the
Co L3-edge, as shown in Figure 4.30,31 The XANES pre- and
postedge Si@CoX2 MW absorption areas were normalized to 0
and 1, respectively. Co foil functioned as a Co zerovalent
Figure 4. (a) Normalized XANES Co L-edge of Si@CoS2, Si@CoSe2
microwire arrays, CoCl2, CoO, and Co3O4 standard materials. (b)
Integrated area under the Co L3-edge versus the oxidation states.
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Figure 6. Reflectance spectra of Si@CoS2, Si@CoSe2, and bare Si
microwire arrays.
amount of cocatalyst on Si plane resulted in reduced
photocurrent density due to decreasing the light harvest of Si
electrode. Furthermore, the CV characterization in the H2SO4
solution showed that the electrochemically active surface of
CoSe2 was less than that of CoS2 (Figure S4). With respect to
Si@CoS2 photocathode, less photogenerated carriers and
electrochemically active sites of Si@CoSe2 MWs resulted in
lower photocatalytic activity in acidic electrolyte. CoX 2
cocatalyst was also decorated on Si plane for the photocatalytic
activity comparison through the identical synthesis method of
Si@CoX2 MWs materials. The photoelectrochemical performances of CoX2/Si plane and pristine Si plane were conducted in
0.5 M H2SO4 electrolyte (Figure S5). Si plane showed the
photocurrent about −0.774 mA cm−2 until the applied bias was
−0.85 V (vs RHE). In our previous study, the current density
of bare Si MW arrays achieved −3.12 mA cm−2 at −0.85 V (vs
RHE).25 After CoX2 cocatalyst was decorated on Si plane, the
onset potentials of CoS2/Si plane and CoSe2/Si plane positively
shifted to −0.443 and −0.577 V, respectively. However, Si@
CoX2 MWs showed much better photoelectrolysis activity as
compared to CoX2/Si plane photocathodes. This result reveals
that Si MWs enhanced solar hydrogen generation efficiency
because of stronger absorption of incident light and higher
surface area for reacting with protons.
The photocatalytic activity of Si@CoS2 decayed seriously in
the neutral and alkaline electrolytes, as shown in Figure 5, panel
a. The photocurrent densities of Si@CoS2 MWs in PBS and
KOH at 0 V (vs RHE) were separately −0.876 and −1.07 mA
cm−2, and the onset potentials dropped to −0.0743 and 0.0264
V in Figure 5, panel b. Notably, Si@CoSe2 MWs retained
photocurrent density at −2.60 and −3.83 mA cm−2 in the
neutral and alkaline solutions as the applied bias was 0 V.
Besides, the onset potential of Si@CoSe2 photocathode
maintained at 0.137 and 0.218 V, respectively. Transient
photocurrent density of CoX2 MWs was measured at 0 V (vs
RHE) in various pH ranges of the electrolyte, as shown in
Figure S6. All Si@CoX2 photocathodes showed rapid on and
off characteristics under chopped illumination. The pulsed
photocurrent density of Si@CoS2 was approximately −3.2 mA
cm−2 in the acid electrolyte. The transient cathodic current of
Si@CoX2 MWs at 0 V (vs RHE) was normalized to analyze the
lifetime of photoinduced carriers in Figure 7. As the incident
light was activated, Si@CoS2 MWs revealed overshot photocurrent observed in PBS and KOH solution. This result
indicated that photoinduced carriers, accumulating on photocathode, cannot efficiently migrate to the interface between
CoS2 layer and electrolyte. The photogenerated electron−hole
Figure 5. (a) Photoelectrochemical linear sweep voltammograms of
Si@CoS2 and Si@CoSe2 photocathodes in various pH ranges of
electrolyte under solar illumination. (b) Onset potentials of Si@CoS2
and Si@CoSe2 photocathodes versus various pH ranges of electrolyte.
solutions at the potential range of +0.55 V to −0.45 V (vs
RHE). The photocurrent density of Si@CoS2 MWs reached
−3.22 mA cm−2 at 0 V (vs RHE) in 0.5 M H2SO4 aqueous
solution (pH = 0.3). Onset potentials of Si@CoX2 photocathodes versus various pH ranges of the electrolyte are
summarized in Figure 5, panel b. The onset potential of Si@
CoS2 photoelectrode was 0.248 V (vs RHE) in the acidic
solution. The moderate photocurrent density at 0 V (vs RHE)
and onset potential in H2SO4 solution achieved −2.55 mA
cm−2 and 0.137 V, respectively, through Si@CoSe2 MWs. The
reflectance spectra were applied to analyze the photoelectrochemical performance of Si@CoX2 photoelectrode, as
shown in Figure 6. The incident illumination with wavelength
from 450−1100 nm was absorbed by bare Si MWs. Moreover,
the reflectance was stronger as the wavelength was shorter than
450 nm. After depositing CoS2 layer, Si MWs retained its
absorption, and CoS2 outer shell also reduced the reflectance of
shorter wavelength irradiation. However, Si@CoSe2 photocathode showed an enhanced reflectance for the incident light
as compared with Si@CoS2 MWs. This result indicated that the
CoSe2 layer showed the inner-filter effect to block the incident
illumination and decreased the absorption of underlying Si
MWs. Therefore, the amount of photoinduced carriers on
CoSe2@Si MWs was reduced to drive solar water splitting
reaction.5,32 The inner-filter effect was also observed in previous
works, which applied cobalt sulfide or cobalt molybdenum
sulfide-deposited upon Si plane as photocathodes for photoelectrochemical hydrogen evolution.33,34 Excessive loading
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Figure 7. Normalized transient cathodic current of Si@CoS2 and Si@
CoSe2 microwire arrays at 0 V (vs RHE) in various pH ranges of
electrolyte under chopped illumination.
pairs recombined until a steady-state photocurrent was
achieved. As compared in the H2SO4 solution, the lifetime of
charge carrier was suppressed during the Si@CoS2 photocathode drove solar hydrogen evolution in neutral and alkaline
electrolytes because of the surface chemistry or bulk trapping
states.35 Besides, Figure S6 shows that the transient photocurrent of Si@CoS2 MWs in KOH electrolyte gradually
decayed within 200 s, and the overshot photocurrent
disappeared after 120 s of measurement. These results showed
that Si@CoS2 photocathode was unstable in alkaline solution,
possibly owing to the photo-oxidation or photocorrosion that
occurred on Si@CoS2 MWs through accumulated photoinduced holes. By contrast, Si@CoSe2 photocathode showed
conformed photocurrent to LSV results in Figure S6, and no
overshot current was observed in Figure 7. With respect to Si@
CoSe2 MWs, we proposed that more photoinduced carriers and
higher electrochemically active surface, achieved through Si@
CoS2 photocathode, were characterized by reflectance spectra
and CV measurement. However, lower separation of photogenerated electron−hole pairs on Si@CoS2 MWs in neutral and
alkaline solution was observed, leading to worse photoelectrochemical activity, as shown in Figure 5. Besides, Si@
CoSe2 MWs revealed a better photoelectrochemcial performance in alkaline solution than in acid or neutral electrolyte. This
result was also investigated from electrochemical hydrogen
evolution measurements of transition metal phosphide in
previous studies.36,37 Transition metal phosphide materials
showed significant performance of electrochemical hydrogen
generation over wide pH range of electrolyte.36−38 However,
the minimum overpotential of these electrocatalysts was
approximately 100 mV for hydrogen evolution reaction.
Furthermore, most research on photocathode devices was
conducted in the particular pH condition and limited the
practical application in the future. In this work, we suggested
using wide range pH-tolerable photocathode materials drove
solar water splitting for producing chemical fuels. Under the
solar irradiation, Si@CoX2 MWs photocathode enabled the
generation of cathodic current at 0 V (vs RHE) in various pH
solutions.
Bare Si and Si@CoX2 MWs photocathode were measured
the chronoamperometry at 0 V (vs RHE) under solar
simulation in various pH ranges of the electrolyte, as shown
in Figure 8, panel a. The photocurrent density of Si@CoS2
MWs in H2SO4 and PBS solution was respectively maintained
Figure 8. (a) Chronoamperometry of Si@CoS2 and Si@CoSe2 and
bare Si microwire arrays at 0 V (vs RHE) in various pH ranges of
electrolyte under solar illumination. (b) Photoelectrochemical hydrogen gas evolution of Si@CoS2 and Si@CoSe2 microwire arrays at 0 V
(vs RHE) during initial 50 min of chronoamperometry measurement
in different electrolytes.
at about −3 and −1 mA cm−2 after 100 min measurement. The
initial photocurrent current of Si@CoS2 electrode achieved
approximately −1 mA cm−2 in KOH electrolyte. However, its
current density gradually decayed and showed no photoresponse after about 75 min of chronoamperometry characterization in alkaline solution. SEM images were applied to
investigate the morphology variation of photoelectrode
materials after the chronoamperometry measurement. Figure
S7a,b shows that Si@CoS2 MWs was etched by the electrolyte
under time dependence of cathodic current density characterization, leading to the serious degradation and deactivation.
Besides, XRD and Raman spectra in Figure S8 reveal that only
the residual Si bottom plane was detectable. Gas evolution was
collected at 0 V (vs RHE) during initial 50 min of
chronoamperometry measurement in different electrolytes in
Figure 8, panel b. The hydrogen gas faradic efficiencies of Si@
CoS2 MWs were separately about 78.7%, 77.8%, and 67.5% in
H2SO4, PBS, and KOH solution, respectively.
The photocurrent density of Si@CoSe2 MWs in H2SO4 and
PBS solution was maintained at about −2 mA cm−2 after 100
min of measurement in Figure 8, panel a. The faradic
efficiencies of Si@CoSe2 photocathodes separately achieved
74.2% and 78.8% in acidic and neutral electrolyte, as shown in
Figure 8, panel b. The photocurrent density of Si@CoSe2 MWs
was approximately −3.5 mA cm−2 at 0 V (vs RHE) in KOH
solution, and its current increased to −5 mA cm−2 after 100
min of characterization. The improved cathodic current of
molybdenum disulfide/cobalt diselenide hybrid catalyst was
also observed during driving the HER reaction.39 The faradic
efficiency of Si@CoSe2 MWs in alkaline electrolyte was about
73.5%. SEM image shows that CoSe2 outer shell became
thinner after the continuous chronoamperometry measurement
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DOI: 10.1021/acsami.6b00027
ACS Appl. Mater. Interfaces 2016, 8, 5400−5407
Research Article
ACS Applied Materials & Interfaces
in various pH ranges of the electrolyte in Figure S7c. The radius
of Si@CoSe2 MWs was approximately 650 nm after the time
dependence of cathodic current density test. Figure S8a reveals
that there was only Si MWs diffraction peaks of Si@CoSe2
electrode observed in XRD spectrum after the chronoamperometry characterization. This result indicated that CoSe2
cocatalyst became an amorphous structure. In addition,
Raman spectrum of Si@CoSe2 electrode after chronoamperometry measurement showed a broad peak at about 186 cm−1,
which corresponded to the Se−Se stretching mode of cubic
CoSe2, as shown in Figure S8b. The Raman peak intensity of
CoSe2 was reduced because of the low crystallization. However,
there was an additional sharp Raman peak of Si MWs
investigated at 186 cm−1 because the incident laser beam
penetrated the thinner CoSe2 layer to detect the underlying Si
MWs. Here, the thinner CoSe2 cocatalyst modified Si MWs
(Si@CoSe2-4h) was prepared through shortening the chemical
deposition duration of Co(OH)2 to 4 h for the photoelectrochemical performance comparison. LSV measurement in
0.5 M H2SO4 electrolyte was conducted to evaluate the
photocurrent of Si@CoSe2-4h as compared to Si@ oSe2-6h in
Figure S9. The onset potential and current density of Si@
CoSe2-4h were worse than those of the Si@CoSe2-6h
photocathode. This result showed that the enhanced photocurrent of Si@CoSe2 MWs after the chronoamperometry test
in Figure 8 attributed to the amorphous structure instead of
thinner cocatalyst outer shell. The improved catalytic activity of
amorphous CoSe2 layer modified-Si MWs relative to crystalline
structure resulted from the high density of exposed active sites,
which was proposed by a previous work.31 From the above
results, we proposed that the CoSe2 layer was more stable to
protect underlying Si MWs to drive solar hydrogen evolution in
wide pH values of electrolyte.
XRD characterization revealed that CoSe2 outer shell became a
thinner and amorphous structure, resulting in a better
photoelectrochemical activity.
CONCLUSIONS
In summary, Si@cobalt dichalcogenide MWs were employed as
wide range pH-tolerable photocathode materials for solar water
splitting. Silicon MW arrays were fabricated through lithography and dry etching technologies. Si@CoS2 and Si@CoSe2
MWs were prepared by thermal sulfidation and hydrothermal
selenization reaction, respectively, of the precursor Si@
Co(OH)2. The photocurrent density of Si@CoS2 MWs
reached −3.22 mA cm−2 at 0 V (vs RHE) in H2SO4 aqueous
solution, but the Si@CoSe2 photocathode materials showed
moderate photocurrent density of −2.55 mA cm−2. This result
is attributed to the CoSe2 layer that showed the inner filter
effect to block the incident illumination and decreased the
absorption of underlying Si MWs. Besides, Si@CoS2 MWs
presented higher electrochemically active surface for reacting
with the electrolyte. However, Si@CoS2 photocathode revealed
low activity in neutral and alkaline electrolyte. By contrast, no
overshot current was observed on Si@CoSe2 MWs in transient
photocurrent measured in the PBS and KOH solution. This
result revealed that Si@CoSe2 with higher charge transfer
efficiencies stably drove solar hydrogen evolution in neutral and
alkaline electrolyte. Continuous chronoamperometry in acid,
neutral, and alkaline solutions was conducted at 0 V (vs RHE)
to evaluate the photocatalytic durability of Si@CoX2 MWs.
From SEM observation, Si@CoS2 electrode was etched
through the electrolyte after the chronoamperometry test,
leading to serious degradation and deactivation. The photocurrent density of Si@CoSe2 MWs gradually increased to −5
mA cm−2 after chronoamperometry characterization. SEM and
(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a
Semiconductor Electrode. Nature 1972, 238, 37−38.
(2) Boettcher, S. W.; Warren, E. L.; Putnam, M. C.; Santori, E. A.;
Turner-Evans, D.; Kelzenberg, M. D.; Walter, M. G.; McKone, J. R.;
Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S. Photoelectrochemical
Hydrogen Evolution Using Si Microwire Arrays. J. Am. Chem. Soc.
2011, 133, 1216−1219.
(3) Oh, I.; Kye, J.; Hwang, S. Enhanced Photoelectrochemical
Hydrogen Production from Silicon Nanowire Array Photocathode.
Nano Lett. 2012, 12, 298−302.
(4) Dai, P.; Xie, J.; Mayer, M. T.; Yang, X.; Zhan, J.; Wang, D. Solar
Hydrogen Generation by Silicon Nanowires Modified with Platinum
Nanoparticle Catalysts by Atomic Layer Deposition. Angew. Chem., Int.
Ed. 2013, 52, 11119−11123.
(5) Chen, C. J.; Chen, M. G.; Chen, C. K.; Wu, P. C.; Chen, P. T.;
Basu, M.; Hu, S. F.; Tsai, D. P.; Liu, R. S. Ag-Si Artificial Microflowers
for Plasmon-enhanced Solar Water Splitting. Chem. Commun. 2015,
51, 549−552.
(6) Seger, B.; Pedersen, T.; Laursen, A. B.; Vesborg, P. C. K.;
Hansen, O.; Chorkendorff, I. Using TiO2 as a Conductive Protective
Layer for Photocathodic H2 Evolution. J. Am. Chem. Soc. 2013, 135,
1057−1064.
(7) Li, S.; Zhang, P.; Song, X.; Gao, L. Photoelectrochemical
Hydrogen Production of TiO2 Passivated Pt/Si-Nanowire Composite
Photocathode. ACS Appl. Mater. Interfaces 2015, 7, 18560−18565.
(8) Fan, R.; Dong, W.; Fang, L.; Zheng, F.; Su, X.; Zou, S.; Huang, J.;
Wang, X.; Shen, M. Stable and Efficient Multi-crystalline n+p Silicon
Photocathode for H2 Production with Pyramid-like Surface Nanostructure and Thin Al2O3 Protective Layer. Appl. Phys. Lett. 2015, 106,
013902.
(9) Fan, R.; Min, J.; Li, Y.; Su, X.; Zou, S.; Wang, X.; Shen, M. n-type
Silicon Photocathodes with Al-doped Rear p+ Emitter and Al2O3-
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b00027.
SEM images of Si and Si@Co(OH)2 microwires; XRD
spectra, electrochemical linear sweep voltammograms,
and cyclic voltammograms of CoX2/Ti foil; photoelectrochemical linear sweep voltammograms of CoX2/Si
plane and Si@CoX2 microwires with various cocatalyst
thicknesses; transient photocurrent density of Si@CoX2
microwires; SEM images, XRD, and Raman spectra of
Si@CoX2 microwires after continuous chronoamperometry measurement (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:[email protected].
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors are grateful for the financial support of the
Ministry of Science, Technology of Taiwan (Contract Nos.
MOST 103-2112-M-003-009-MY3 and MOST 104-2113-M002-012-MY3), Academia Sinica (Contract No. AS-103-TPA06), and National Taiwan University (104R7563-3).
■
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REFERENCES
DOI: 10.1021/acsami.6b00027
ACS Appl. Mater. Interfaces 2016, 8, 5400−5407
Research Article
ACS Applied Materials & Interfaces
coated Front Surface for Efficient and Stable H2 Production. Appl.
Phys. Lett. 2015, 106, 213901.
(10) McKone, J. R.; Warren, E. L.; Bierman, M. J.; Boettcher, S. W.;
Brunschwig, B. S.; Lewis, N. S.; Gray, H. B. Evaluation of Pt, Ni, and
Ni-Mo Electrocatalysts for Hydrogen Evolution on Crystalline Si
electrodes. Energy Environ. Sci. 2011, 4, 3573−3583.
(11) Warren, E. L.; McKone, J. R.; Atwater, H. A.; Gray, H. B.; Lewis,
N. S. Hydrogen-evolution Characteristics of Ni-Mo-coated, Radial
Junction, n+p-silicon Microwire Array Photocathodes. Energy Environ.
Sci. 2012, 5, 9653−9661.
(12) Laursen, A. B.; Pedersen, T.; Malacrida, P.; Seger, B.; Hansen,
O.; Vesborg, P. C. K.; Chorkendorff, I. MoS2-an Integrated Protective
and Active Layer on n+p-Si for Solar H2 Evolution. Phys. Chem. Chem.
Phys. 2013, 15, 20000−20004.
(13) Kong, D.; Cha, J. J.; Wang, H.; Lee, H. R.; Cui, Y. First-row
Transition Metal Dichalcogenide Catalysts for Hydrogen Evolution
Reaction. Energy Environ. Sci. 2013, 6, 3553−3558.
(14) Ivanovskaya, A.; Singh, N.; Liu, R.-F.; Kreutzer, H.; Baltrusaitis,
J.; Van Nguyen, T.; Metiu, H.; McFarland, E. Transition Metal Sulfide
Hydrogen Evolution Catalysts for Hydrobromic Acid Electrolysis.
Langmuir 2013, 29, 480−492.
(15) Faber, M. S.; Dziedzic, R.; Lukowski, M. A.; Kaiser, N. S.; Ding,
Q.; Jin, S. High-Performance Electrocatalysis Using Metallic Cobalt
Pyrite (CoS2) Micro- and Nanostructures. J. Am. Chem. Soc. 2014, 136,
10053−10061.
(16) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 Nanoparticles Grown
on Carbon Fiber Paper: An Efficient and Stable Electrocatalyst for
Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 4897−
4900.
(17) Faber, M. S.; Lukowski, M. A.; Ding, Q.; Kaiser, N. S.; Jin, S.
Earth-Abundant Metal Pyrites (FeS2, CoS2, NiS2, and Their Alloys) for
Highly Efficient Hydrogen Evolution and Polysulfide Reduction
Electrocatalysis. J. Phys. Chem. C 2014, 118, 21347−21356.
(18) Kundu, A.; Sahu, J. N.; Redzwan, G.; Hashim, M. A. An
Overview of Cathode Material and Catalysts Suitable for Generating
Hydrogen in Microbial Electrolysis Cell. Int. J. Hydrogen Energy 2013,
38, 1745−1757.
(19) Liu, C.; Gallagher, J. J.; Sakimoto, K. K.; Nichols, E. M.; Chang,
C. J.; Chang, M. C. Y.; Yang, P. Nanowire−Bacteria Hybrids for
Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals.
Nano Lett. 2015, 15, 3634−3639.
(20) Kargar, A.; Cheung, J. S.; Liu, C.-H.; Kim, T. K.; Riley, C. T.;
Shen, S.; Liu, Z.; Sirbuly, D. J.; Wang, D.; Jin, S. NiOx-Fe2O3-coated pSi Photocathodes for Enhanced Solar Water Splitting in Neutral pH
Water. Nanoscale 2015, 7, 4900−4905.
(21) Sun, K.; Jing, Y.; Li, C.; Zhang, X.; Aguinaldo, R.; Kargar, A.;
Madsen, K.; Banu, K.; Zhou, Y.; Bando, Y.; Liu, Z.; Wang, D. 3D
Branched Nanowire Heterojunction Photoelectrodes for Highefficiency Solar Water Splitting and H2 Generation. Nanoscale 2012,
4, 1515−1521.
(22) Kargar, A.; Sun, K.; Jing, Y.; Choi, C.; Jeong, H.; Jung, G. Y.; Jin,
S.; Wang, D. 3D Branched Nanowire Photoelectrochemical Electrodes
for Efficient Solar Water Splitting. ACS Nano 2013, 7, 9407−9415.
(23) Kargar, A.; Sun, K.; Jing, Y.; Choi, C.; Jeong, H.; Zhou, Y.;
Madsen, K.; Naughton, P.; Jin, S.; Jung, G. Y.; Wang, D. Tailoring nZnO/p-Si Branched Nanowire Heterostructures for Selective Photoelectrochemical Water Oxidation or Reduction. Nano Lett. 2013, 13,
3017−3022.
(24) Basu, M.; Zhang, Z. W.; Chen, C. J.; Chen, P. T.; Yang, K. C.;
Ma, C.-G.; Lin, C. C.; Hu, S. F.; Liu, R. S. Heterostructure of Si and
CoSe2: A Promising Photocathode Based on a Non-noble Metal
Catalyst for Photoelectrochemical Hydrogen Evolution. Angew. Chem.,
Int. Ed. 2015, 54, 6211−6216.
(25) Chen, C. J.; Chen, P. T.; Basu, M.; Yang, K. C.; Lu, Y. R.; Dong,
C. L.; Ma, C.-G.; Shen, C. C.; Hu, S. F.; Liu, R. S. An Integrated
Cobalt Disulfide (CoS2) Co-catalyst Passivation Layer on Silicon
Microwires for Photoelectrochemical Hydrogen Evolution. J. Mater.
Chem. A 2015, 3, 23466−23476.
(26) Piscanec, S.; Ferrari, A. C.; Cantoro, M.; Hofmann, S.; Zapien, J.
A.; Lifshitz, Y.; Lee, S. T.; Robertson, J. Raman Spectrum of Silicon
Nanowires. Mater. Sci. Eng., C 2003, 23, 931−934.
(27) Lyapin, S. G.; Utyuzh, A. N.; Petrova, A. E.; Novikov, A. P.;
Lograsso, T. A.; Stishov, S. M. Raman Studies of Nearly Half-metallic
Ferromagnetic CoS2. J. Phys.: Condens. Matter 2014, 26, 396001.
(28) Zhu, L.; Teo, M.; Wong, P. C.; Wong, K. C.; Narita, I.; Ernst, F.;
Mitchell, K. A. R.; Campbell, S. A. Synthesis, Characterization of a
CoSe2 Catalyst for the Oxygen Reduction Reaction. Appl. Catal., A
2010, 386, 157−165.
(29) Zhang, H.; Lei, L.; Zhang, X. One-step Synthesis of Cubic
Pyrite-type CoSe2 at Low Temperature for Efficient Hydrogen
Evolution Reaction. RSC Adv. 2014, 4, 54344−54348.
(30) Liu, H.; Guo; Yin, Y.; Augustsson, A.; Dong, C.; Nordgren, J.;
Chang, C.; Alivisatos, P.; Thornton, G.; Ogletree, D. F.; Requejo, F.
G.; de Groot, F.; Salmeron, M. Electronic Structure of Cobalt
Nanocrystals Suspended in Liquid. Nano Lett. 2007, 7, 1919−1922.
(31) Kornienko, N.; Resasco, J.; Becknell, N.; Jiang, C.-M.; Liu, Y.-S.;
Nie, K.; Sun, X.; Guo, J.; Leone, S. R.; Yang, P. Operando
Spectroscopic Analysis of an Amorphous Cobalt Sulfide Hydrogen
Evolution Electrocatalyst. J. Am. Chem. Soc. 2015, 137, 7448−7455.
(32) Chen, H. M.; Chen, C. K.; Chen, C. J.; Cheng, L. C.; Wu, P. C.;
Cheng, B. H.; Ho, Y. Z.; Tseng, M. L.; Hsu, Y. Y.; Chan, T. S.; Lee, J.
F.; Liu, R. S.; Tsai, D. P. Plasmon Inducing Effects for Enhanced
Photoelectrochemical Water Splitting: X-ray Absorption Approach to
Electronic Structures. ACS Nano 2012, 6, 7362−7372.
(33) Sun, Y.; Liu, C.; Grauer, D. C.; Yano, J.; Long, J. R.; Yang, P.;
Chang, C. J. Electrodeposited Cobalt-Sulfide Catalyst for Electrochemical and Photoelectrochemical Hydrogen Generation from Water.
J. Am. Chem. Soc. 2013, 135, 17699−17702.
(34) Chen, Y.; Tran, P. D.; Boix, P.; Ren, Y.; Chiam, S. Y.; Li, Z.; Fu,
K.; Wong, L. H.; Barber, J. Silicon Decorated with Amorphous Cobalt
Molybdenum Sulfide Catalyst as an Efficient Photocathode for Solar
Hydrogen Generation. ACS Nano 2015, 9, 3829−3836.
(35) Li, J.; Cushing, S. K.; Zheng, P.; Meng, F.; Chu, D.; Wu, N.
Plasmon-induced Photonic and Energy-transfer Enhancement of Solar
Water Splitting by a Hematite Nanorod Array. Nat. Commun. 2013, 4,
2651.
(36) Pu, Z.; Liu, Q.; Asiri, A. M.; Sun, X. Tungsten Phosphide
Nanorod Arrays Directly Grown on Carbon Cloth: A Highly Efficient
and Stable Hydrogen Evolution Cathode at All pH Values. ACS Appl.
Mater. Interfaces 2014, 6, 21874−21879.
(37) Shi, Y.; Xu, Y.; Zhuo, S.; Zhang, J.; Zhang, B. Ni2P Nanosheets/
Ni Foam Composite Electrode for Long-Lived and pH-Tolerable
Electrochemical Hydrogen Generation. ACS Appl. Mater. Interfaces
2015, 7, 2376−2384.
(38) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-Supported
Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D
Hydrogen-Evolving Cathode over the Wide Range of pH 0−14. J. Am.
Chem. Soc. 2014, 136, 7587−7590.
(39) Gao, M.-R.; Liang, J.-X.; Zheng, Y.-R.; Xu, Y.-F.; Jiang, J.; Gao,
Q.; Li, J.; Yu, S.-H. An Efficient Molybdenum Disulfide/Cobalt
Diselenide Hybrid Catalyst for Electrochemical Hydrogen Generation.
Nat. Commun. 2015, 6, 5982.
5407
DOI: 10.1021/acsami.6b00027
ACS Appl. Mater. Interfaces 2016, 8, 5400−5407