Nickel sulfide nanocrystals for electrochemical and photoelectrochemical hydrogen generation

Article abstract of DOI:10.1039/c9tc05703j

Photoelectrochemical water splitting has been considered as the most promising technology for generating hydrogen energy. Herein, we report nickel sulfide nanocrystals (NCs) as excellent catalysts for electrochemical and solar-driven photoelectrochemical

Full text of DOI:10.1039/c9tc05703j

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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
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DOI: 10.1039/C9TC05703J
ARTICLE
Nickel Sulfide Nanocrystals for Electrochemical and
Photoelectrochemical Hydrogen Generation
a
a
a
a
a
a
Jisun Yoo,‡ In Hye Kwak,‡ Ik Seon Kwon,‡ Kidong Park, Doyeon Kim, Jong Hyun Lee, , Soo
Received 00th January 20xx,
Accepted 00th January 20xx
b
a
a
A Lim, Eun Hee Cha,* and Jeunghee Park*
DOI: 10.1039/x0xx00000x
Photoelectrochemical water splitting has been considered as the most promising technology for
generating hydrogen energy. Herein, we report nickel sulfide nanocrystals (NCs) as excellent catalysts for
electrochemical and solar-driven photoelectrochemical (PEC) hydrogen evolution. Two polymorphic NiS
and NiS1.97 NCs were synthesized with a controlled composition, and the NiS NCs showed higher
electrocatalytic activity than the NiS1.97 NCs toward the hydrogen/oxygen evolution reactions in 0.5 M
www.rsc.org/
2 4
H SO and 1 M KOH. Photocathodes were fabricated by growing the NiS and NiS1.97 NCs directly onto Si
nanowire (NW) arrays. The deposition of NiS NCs results in excellent PEC performance in both acid and
alkaline electroytes; the photocurrent is 10 mA cm-2 (at 0 V vs. RHE) and the onset potential is 0.2 V. In
contrast, the Si-NiS1.97 photocathode exhibit much lower photocurrent and less stability. Detailed
structure analysis using X-ray photoelectron spectroscopy reveals that the NiS is more metallic than the
NiS1.97 and retains the same electronic structures during the catalytic reaction. The high catalytic activity
of metallic NiS NCs should be the main cause of this enhanced PEC performance, promising efficient
water-splitting Si-based PEC cells.
nanostructures.5,13-25 It is well known that the application of Si-
based photoelectrodes has been seriously limited by their poor
intrinsic instability, which is due to the sluggish water-splitting
reaction that causes surface oxidation. Decorating the Si surface
with electrocatalysts for HER or oxygen evolution reaction (OER)
can accelerate the reaction rate and improve the PEC cell
performance by retarding the surface oxidation. Since most OER
catalysts only work sustainably in alkaline electrolytes, there is a
1
. Introduction
The steeply growing need for energy has encouraged scientists
to develop renewable and clean energy sources to replace fossil
fuels. Solar-driven photoelectrochemical (PEC) water splitting for
hydrogen production is currently an important research topic,
since the solar energy is virtually inexhaustible while hydrogen is
stable and storable. To achieve high performance, the
photoelectrodes in the PEC cell devices must supply
thermodynamic potential of 1.23 V for overall water splitting
after absorbing the solar spectrum. Following the report of the
great need to develop Si photocathodes working in alkaline
a
22,25
conditions.
However, the majority of reported Si
photocathodes were only tested in acidic electrolytes.
Recently, earth-abundant metal chalcogenide (sulfide or
selenide) nanomaterials have been considered as top candidates
for HER/OER electrocatalysts. In particular, a large body of work
has reported excellent electrocatalytic activity of various nickel
first PEC cells using TiO
2
photocatalysts in 1972, there has been
1
-
tremendous effort to develop more efficient and stable PEC cells.
1
2
Nevertheless, the energy conversion efficiency, durability, and
cost of materials remain insufficient for high-performance solar-
to-fuel conversion.
Silicon (Si) is the most widely used photovoltaic material due to
sulfide (e.g., NiS, Ni
3
S
2
, NiS
2
) nanocrystals (NCs) in acidic and
26-42
alkaline solutions.
effectively enhance the catalytic activity.
example, Ni exhibits better HER and OER electrocatalytic
performance than NiS and NiS
Notably, the appropriate crystal phase can
28,29,35,40,42-44
For
g
its earth abundance, low cost, and narrow band gap (E = 1.18 eV).
3 2
S
Furthermore, it has a suitable conduction band edge level with
respect to the hydrogen evolution reaction (HER). These
properties make Si promising for photoelectrodes in solar-driven
water-splitting PEC cells.
absorption capacity and increase the charge collection efficiency,
there has been much interest in using low dimensional
28,40
2
.
However, few researchers
have tried to apply nickel sulfide NCs as catalysts in Si-based PEC
cells. It is important to identify the best crystal phase of catalysts
7
-25
In order to improve the light
23,45
for the Si-based PEC cells.
In the present study, two polymorphic nickel sulfide (hexagonal
phase NiS and cubic phase NiS1.97) NCs were synthesized directly
on carbon cloth as well as pre-grown Si nanowire (NW) array with
controlled composition. They were for use as bifunctional
HER/OER electrocatalysts and as photocathodes in solar-driven
water-splitting PEC cells, respectively. The selective growth of NiS
and NiS1.97 NCs was achieved by controlling the growth condition,
and the (photo)electrocatalytic performance of the Si-NiS and Si-
NiS1.97 NCs was evaluated. Until now, the catalytic activity of
NiS1.97 has been rarely investigated, although its superior
a. Department of Advanced Materials Chemistry, Korea University, Sejong 339-700,
Republic of Korea; E-mail address: chaeunhee@korea.ac.kr, parkjh@korea.ac.kr
Department of Nanoscience and Nanotechnology, Jeonbuk National University
b.
Chonju, Chonbuk 560-756, Republic of Korea
†
Electronic Supplementary Information (ESI) available: additional data. See
DOI: 10.1039/x0xx00000x
J.Y., I.H.K., and I.S.K equally contribute.
‡
This journal is © The Royal Society of Chemistry 20xx
J. Mater. Chem. C, 2019, 00, 1-3 | 1
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photoanodic performance to NiS
Olge’s group. Our results showed that the Si-NiS photocathode
exhibits better performance than the Si-NiS1.97 one in both acidic
2
(at pH 7.4) was reported by phase (JCPDS No. 77-1624, P6
3
/mmc, a = 3.439 and c = 5.324 Å)
View Article Online
_
4
6
DOI: 10.1039/C9TC05703J
and cubic phase (JCPDS No. 83-0574, Pa , a = 5.686 Å),
respectively, without any impurity phases. These samples were
referred to as CC-NiS and CC-NiS1.97, respectively.
3
(
2 4
0.5 M H SO ) and alkaline (1 M KOH) electrolytes by increasing
the photocurrents and the stability. Furthermore, we
demonstrated the relatively higher electrocatalytic activity of NiS
toward the HER, which is consistent with the higher PEC
performance observed in Si-NiS. To the best of our knowledge,
this is the first time that the electrochemical and PEC catalytic
efficiencies are compared between the NiS and NiS1.97 NCs.
2. Experimental Section
Synthesis. For the synthesis of NiS and NiS1.97 NCs, we prepared
Ni(OH) nanosheets as a first step. In a typical preparation, nickel
nitrate hexahydrate (Ni(NO ·6H O, 2 mmol, 581.58 mg), urea (>
9% pure, 8 mmol, 480.48 mg), and ethylene glycol (28.0 mL)
were dispersed in deionized water (4 mL) by ultrasonication. The
solution was transferred to Teflon-lined stainless-steel
2
3
)
2
2
9
a
autoclave and then heated at 120 °C for 12 h in an oven. Finally,
the obtained precipitate was collected as powder and rinsed with
ethanol and deionized water several times. Then, the powder was
dried on the hot plate at 100 °C, producing the Ni(OH)
nanosheets.
Ni(OH) (20 mg) was placed in a quartz tube inside a chemical
2
2
Fig. 1 (a) SEM and HRTEM images of NCs synthesized on CC. (b)
_
vapor deposition (CVD) reactor (electrical furnace), and S powder Lattice-resolved TEM images of the (b) NiS (zone axis = [1101])
200 mg) was placed in a quartz boat 3 cm away from the Si and (c) NiS1.97 NCs (zone axis = [110]) and the corresponding FFT
substrate. Argon gas was continuously flowed with a rate of 100 images. (d, e) HAADF STEM image; EDX elemental mapping of Ni,
(
-1
sccm (mL min ) during the whole process. The temperature of S, and O; and the corresponding EDX spectrum for (d) NiS and (e)
the S powders and the substrate was 200 and 350-550 °C during NiS1.97 NCs.
the growth of NCs and the reaction time was 30 min. For the
deposited form of NCs on carbon cloth, Ni(OH)
2
nanosheets (2.5
Based on the scanning electron microscopy (SEM) images in Fig.
mg) were loaded on the hydrophilic/water proof carbon cloth 1a, the NiS NCs were fully covered on the CC, with an average size
(
WIZMAC Co., thickness = 0.35 mm, through-plane resistance = 1 of 100 nm according to the high-resolution transmission electron
2
m) that was cut with a size of 11 cm . The Ni(OH)
2
nanosheets microscopy (HRTEM) image. Lattice-resolved TEM and the
on carbon cloth were sulfurized in the CVD reactor using the same corresponding fast Fourier-transform (FFT) images (zone axis =
_
condition as described above.
The synthesis of Si NW array using Ag-assisted chemical etching
has been described elsewhere. We used p-type (100) Si wafers
[
1 01]) revealed that the NiS NCs consisted of hexagonal phase,
1
with a (101) d-spacing of 2.6 Å that is consistent with the
reference value (d101 = 2.5992 Å) (Fig. 1b). Fig. 1c shows the
2
3
that were lightly doped with boron (R = 1–10  cm). The pieces lattice-resolved TEM and corresponding FFT images of NiS1.97 NCs
2
of Si wafer (area = 2 cm ) were placed in a mixed aqueous solution at the zone axis of [110]. The d-spacing of (001) planes is 5.9 Å,
3 2 2
of 4.8 M HF, 0.005 M AgNO , and 0.4 M H O . After 5 min etching, which is consistent with the reference value d001 = 5.686 Å. The
the Si pieces were washed repeatedly with deionized (DI) water high-angle annular dark-field scanning TEM (HAADF STEM)
and then immersed in 30% HNO aqueous solution to dissolve the image and energy-dispersive X-ray spectroscopy (EDX) elemental
Ag nanoparticles. The etched Si pieces were washed with 5% HF mapping (using Ni L-shell,
L-shell, and K-shell
solution to remove the oxide layer and then cleaned with DI peaks)/spectrum revealed that the Ni:S atomic ratio is about 1:1
water, producing the Si NWs. The synthesis of NiS (or NiS1.97
3
S
O
)
for NiS NCs and 1:2 for NiS1.97 NCs (Fig. 1d). The oxide shell is
nanocrystals (NCs) onto the Si NW array was carried out as more clearly detectable for the NiS NCs, compared to the NiS1.97
follows. A substrate of Si NW array was immersed in 0.4 M NCs.
3 2
Ni(NO ) ethanol solution for 5 min, and then rinsed with ethanol.
The NiS and NiS1.97 NCs were synthesized on the Si NWs, which
were referred to as Si-NiS and Si-NiS1.97, respectively. The
selective synthesis of hexagonal phase NiS and cubic phase NiS1.97
NCs was also controlled using the sulfurization temperature (450
vs. 550 °C), as confirmed by the XRD data (Fig. S3†). The SEM
images in Fig. 2a show that the NCs were grown homogeneously
The Si substrates were then sulfurized using the same condition
as described above. The characterization method and
(
photo)electrochemical measurement are described in the ESI†.
3
. Results and Discussion
In order to examine the electrocatalytic activity, the NiS and on the Si NWs, the latter had a length of about 4 μm and were
NiS1.97 NCs were synthesized on carbon cloth (CC) as well as in prepared by the Ag-assisted etching of Si (100) wafer. HRTEM
free-standing forms by the sulfurization of Ni(OH)
prepared by a hydrothermal reaction. The TEM images and XRD hexagonal plate-type NiS NCs (Fig. 2b). The size of the
patterns of Ni(OH)
nanosheets are shown in Fig. S1†. The nanoparticles is in the range of 10-30 nm. Lattice-resolved TEM
sulfurization at 450 and 550 °C produced selectively the NiS and and the corresponding FFT images at the zone axis of [2 0]
2
nanosheets images show that the surface of Si NW was decorated with the
2
_
_
11
NiS1.97 NCs, respectively. Below 450 C, no crystal synthesized. At revealed that the plate-shaped NiS NCs were attached on the Si
the higher temperature than 550 C, the oxide form appears. X- NW via (001) planes (Fig. 2c). The d-spacing of (001) planes in
ray diffraction (XRD) patterns of the NC samples are shown in Fig. hexagonal phase NiS was identified as 5.3 Å, which is consistent
S2†. The phase of NiS and NiS1.97 was identified as hexagonal
2
| J. Mater. Chem. C, 2019, 00, 1-3
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ARTICLE
with the reference value of 5.324 Å. The (001) planes of vertically
0
0
(
a) Ni 2p
Ni (b) S 2p
S
View Article3/2Online
2p
2
p3/2
N2
N2
aligned cubic phase Si NWs were exposed, with a d-spacing of 5.4
DOI: 10.1039/C9TC05703J
_
2p1/2
Å (according to the reference value: Card No. 80-0018, F 3m, a =
2p1/2
Si-NiS1.97
4
N1
5
.392 Å). Therefore, the (001) planes of NiS and Si have good
S1
lattice matching along the [001] axis of cubic phase Si NW,
suggesting that the hexagonal-shaped NiS NCs were grown
epitaxially on the etched lateral side of the Si NWs.
N3
Si-NiS
N1
S3
N3
S1
S2
8
80
870
860
850 170 168 166 164 162 160
Binding Energy (eV)
Binding Energy (eV)
(
c)
(d)
8
337.9
8
339.4
340.8
8
335.8
8
8
333.1
Si-NiS
Ni S
Si-NiS1.97
NiO
3
2
Ni
8
320 8340 8360 8380 8400
8330
8340
8350
Photon Energy (eV)
Photon Energy (eV)
Fig. 3 Fine-scanned XPS data of (a) Ni 2p and (b) S 2p peaks for
Si-NiS and Si-NiS1.97. The data points (open circles) are fitted by
Voigt functions, and the sum of the resolved bands is represented
0
0
by black lines. The position of the neutral peak (Ni and S ) is
marked by a vertical dotted line to delineate the blueshift or
redshift. (c) XANES spectra at the Ni K edge for Si-NiS, Si-NiS1.97
NiO powder, Ni powder, and Ni foil. (d) First derivative of
absorption curve in the onset region.
,
3 2
S
The electronic structures of the samples were analyzed using X-
X-ray photoelectron spectroscopy (XPS). The fine-scanned Ni
2p3/2 and 2p1/2 peaks of the Si-NiS and Si-NiS1.97, separated by 17.3
eV, are displayed in Fig. 3a. The Voigt function was used to resolve
each peak into three bands: N1, N2, and N3 in red, green, and blue,
respectively. The Voigt function was used to resolve each peak
into three bands: N1, N2, and N3 in red, green, and blue,
respectively. The 2p3/2 peak of Si-NiS consisted of the N1 band at
8
52.6 (-0.1), the N2 band at 855.2 (2.5), and the N3 band at 860.4
Fig. 2 (a) SEM images and (b) HRTEM images of Si NW array
(
7.7) eV. The number in parentheses represents the blue shift
relative to neutral Ni (Ni ) at 852.7 eV (marked by magenta dotted
deposited homogeneously with the NiS NCs. Lattice-resolved TEM
0
_
_
and corresponding FFT images of (c) NiS (zone axis = [2 0]) and line). For the band assignment, we measured the XPS spectra of
1
1
_
3 2
commercially available NiO and Ni S powders (bulk) as
references (Fig. S4†). The bulk NiO shows two separated bands of
Ni 2p3/2 at 853.7 and 855.4 eV due to the presence of Ni(II)-O and
(
d) NiS1.97 (zone axis = [1 2]) NCs. (e, f) The HAADF STEM image;
1
EDX elemental mapping of Si, Ni, S, and O elements; and
corresponding EDX spectrum for (e) Si-NiS and (f) Si-NiS1.97
.
3 2
Ni(III)-O bonds, respectively. The bulk Ni S shows the bands at
8
52.5 and 855.4 eV, similar to those of NiS. Therefore, the N1 band,
Fig. 2d shows the lattice-resolved TEM and corresponding FFT
0
_
whose position is the same as that of Ni , is assigned to the Ni-S.
The N2 band at 855.2 eV is assigned to the Ni-S defects forming
the Ni(III)-O bonds at surface. The N3 band is a shake-up satellite
peak due to the highly oxidized states such as Ni , and its
intensity is proportional to that of the N2 band.
images of cubic phase NiS1.97 NCs at the zone axis of [1 2]. The d-
1
spacings of (111) and (110) planes are 3.28 and 4.0 Å, which are
consistent with the respective reference values of d111 = 3.2831 Å
and d110 = 4.0208 Å. The (110) planes of Si NW, which have a d-
spacing of 1.9 Å, can have 5% lattice mismatching with the (220)
planes of NiS1.97. The HAADF STEM image (left) and EDX
elemental mapping using Si K-shell, Ni L-shell, S L-shell, and O K-
shell peaks and the corresponding EDX spectrum are shown for
the Si-NiS (Fig. 2e) and Si-NiS1.97 (Fig. 2f). Only the Ni and S
elements are present in the NCs, while Si is localized in the Si NWs.
According to the EDX spectrum, the Ni:S atomic ratio is about 1:1
3
+
Si-NiS1.97 exhibits the N1 band at 853.2 (0.5), N2 band at 855.3
2.3), and N3 band at 859.3 eV (6.6), which are assigned to the Ni-
(
S, Ni-O, and satellite peaks, respectively. The lower binding
energy of Ni-S bonds in the Si-NiS compared to the Si-NiS1.97
implies a more metallic nature of NiS than NiS1.97. NiS has stronger
peak intensities in the N2 and N3 bands compared to the Si-NiS1.97
indicating that it contains a higher fraction of surface oxide layers.
,
for NiS and 1:2 for NiS1.97
.
Fig. 3b shows the fine-scanned S 2p3/2 and S 2p1/2 peaks, which
are separated by about 1.2 eV. In Si-NiS, these peaks are located
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3 2
at 160.7 and 161.8 eV, respectively, which are 3.3 eV red-shifted of Si-NiS and Si-NiS1.97 is closer to that of Ni S and NiO,
View Article Online
0
with respect to those of S (2p3/2 at 164.0 eV, marked by magenta respectively. The first derivative of absorpDtiOoIn: 1c0u.1r0v3e9/iCn9tThCe0o5n70s3eJt
dotted line, and 2p1/2 at 165.2 eV). These peaks were resolved into region shows peak at 8333.1, 8335.8, and 8340.8 eV,
four bands, two each for the Ni-S (S1 bands at 161.0 and 162.1 eV) respectively, for the Ni, Ni , and NiO (Fig. 3d). Meanwhile, Si-NiS
anions, S2 bands at 163 and and Si-NiS1.97 show this peak at 8337.9 and 8339.4 eV,
64.2 eV). The blue-shifted peak (S3 bands) at 168 eV is assigned corresponding to a 1.5-eV red shift of NiS from NiS1.97, also
a
3 2
S
2
-
and the S vacancies/defects (e.g., S
2
1
mainly to the S-O bond of surface oxide. For Si-NiS1.97, the peaks indicating the more metallic nature of NiS compared to the NiS1.97
.
appear at 162.0 and 163.2 eV, which are 2.0 eV red-shifted with
respect to the signal of S , corresponding to the S anions bonded
Based on the XPS and XANES analyses, we conclude that the NiS
NCs (on Si NWs) are more metallic than the NiS1.97 NCs, and have
more surface oxide layers. Previous works showed consistently
that NiS has metallic band gap structures and electrical
0
with the Ni cations. No S-O peak indicates insignificant amount of
the surface oxide compared to that of NiS. The larger red shift in
NiS than in NiS1.97 (3.3 vs. 2.0 eV), corresponding to more
negatively charged S atoms, is probably related to its stronger
metallic nature. The XPS data consistently that the CC-NiS NCs are
more metallic than the CC-NiS1.97 NCs and have more surface
oxide layers (Fig. S5†).
40,47,48
properties.
Meanwhile, experimental and theoretical
studies demonstrated that the (optical) band gap of
30,40,49
semiconductor NiS
the electrical property of NiS
semiconductor and close to that of NiS
2
is close to zero.
Krill et al. reported that
x
(x = 1.93–1.98) is that of a
50
2
. These previous studies
The local crystal structure of Ni was probed using Ni K-edge X- support the more metallic properties of NiS NCs than that of
ray absorption near edge spectra (XANES) analysis (Fig. 3c). The NiS1.97 NCs. We measured the valence band spectra and UV-
reference samples were the Ni foil, Ni S , and NiO. The profiles of visible-infrared diffuse reflectance spectra, showing that the
3 2
the Si-NiS and Si-NiS1.97 are significantly different, suggesting a optical band gap of both NCs (synthesized as free-standing form)
remarkable change in the local atomic arrangements of Ni. The is below 0.5 eV, and that NiS has more surface oxide than NiS1.97
absorption edge shows a red shift following the order of (Fig. S6†).
metallicity: Ni foil > Ni
3
S
2
 NiS > NiS1.97 > NiO. The peak feature
0
(
a)
(b)
0
-5
(c)
0.5 M H SO4 (0.215 V)
2
0
0
0
.4
.3
.2
NiS1.97
-
-
-
20
40
60
)
6
0
1
.5 M H SO4
2
-10
2
1
(
)
4
8
NiS
7(
.97
NiS
i
S
1
-15
0
iS 1.9
N
NiS1.97
M KOH
NiS
N
1 M KOH (0.202 V)
6)
(6
6)
iS
N
(9
-5
NiS1.97
iS
N
-
10
NiS1.97
0.1
NiS
-
80
-
-15
0.5
-0.4
-0.3
-0.2
-0.1
0.0
-0.5 0.0
0.5
1.0- 1.5
2.0
0
20
40
60
80 100 120
2
Potential (V vs. RHE)
log J (mA cm )
Time (min)
1
00
2
2
5
(
d)
(
e)
(f)
1 M KOH (1.6 V)
NiS
8
6
4
2
0
0
0
0
0
0.6
0
-
1
)
dec
0
0
0
.5
.4
.3
15
10
5
NiS
NiS1.97
V
m
2
4
7(1
-1
)
1.9
dec
NiS1.97
i
S
N
V
14 m
(
1
iS
N
0
1
.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
Potential (V vs. RHE)
0
.5
1.0
1.5 -2
2.0
0
20
40
60
80 100 120
log J (mA cm )
Time (min)
Fig. 4 (a) Linear sweep voltammetry (LSV) curves (scan rate: 5 mV s–1) of NiS and NiS1.97 as catalysts toward HER in H
M H SO and 1 M KOH. (b) Tafel plots derived from the LSV curves for HER, where the Tafel slope is indicated in parentheses. (c)
Chronoamperometric (CA) responses of NiS and NiS1.97 at 0.215 V (in 0.5 M H SO ) and 0.202V (in 1 M KOH) for a total of 2 h. (d) LSV
-saturated 1.0 M KOH. (e) Tafel plots derived from the LSV
2
-saturated 0.5
2
4
2
4
–
1
curves (scan rate: 5 mV s ) of NiS and NiS1.97 as catalysts for OER in O
2
curves for OER, where the Tafel slope is indicated in parentheses. (f) The corresponding CA responses at 1.6 V.
The electrocatalytic HER activity of NiS and NiS1.97 on CC was NiS and NiS1.97, respectively. The kinetics of the catalytic reaction
investigated by linear sweep voltammetry (LSV), using the was examined using the Tafel equation η = b log(J/J
0
), where η is
samples as working electrodes in a typical three-electrode setup. the overpotential, b is the Tafel slope (mV dec ), J is the measured
Fig. 4a shows the LSV curves obtained using a scan rate of 2 mV current density, and J is the exchange current density. The Tafel
and 1 M KOH. The overpotentials plots at low potentials, which correspond to the activation-
-1
0
-1
s
(
in H
) required to deliver a current density of 10 mA cm (J=10
were 0.215 and 0.256 V (pH 0) and 0.202 and 0.259 V (pH 14) for yield a Tafel slope of 66 and 84 mV dec at pH 0, and 96 and 126
2 2 4
-saturated 0.5 M H SO
-2
)
controlled region, are presented in Fig. 4b. The NiS and NiS1.97 NCs
-1
4
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ARTICLE
mV dec-1 at pH 14, respectively. The chronoamperometric (CA)
In the case of OER in alkaline solution (4OH → O
-
+ 2H
O + 4e ),
−
2
2
View Article Online
-
-
response at η = 0.215 and 0.202 V (at pH 0 and 14, respectively) the possible mechanism was proposed as ODOHI:→10.O10H3*9/+C9eT;CO05H7*03→J
+
-
-
confirms that the NiS and NiS1.97 NCs generated the stable current O* + H + e is followed by either 2O* → O
2
or O* + OH → *OOH +
O + e . The surface oxide sheathing the
NCs would play an important role in enhancing the OER kinetics,
/CoS catalysts by Yin et al. The formation
2 2
of *OOH (Ni-OOH) occurs efficiently on the Ni sites of surface
-
-
-
(
Fig. 4c).
2 2
e , *OOH + OH → O + H
2
Fig. 4d shows the LSV curves obtained in O -saturated 1.0 M KOH
5
5
as suggested for NiS
(
pH 14). The NiS1.97 and NiS NCs required potentials of 1.568 and
3
+
-2
1
.613 V to afford a current density of 10 mA cm , respectively,
5
6,57
oxide during the reaction.
The more surface oxide layers of
corresponding to J=10 = 0.339 and 0.384 V, where η is defined as
the applied potential (vs. RHE) subtracted by 1.229 V. Fig. 4e
displays the Tafel plots at low potentials. The Tafel slopes are 114
NiS than that of NiS1.97 would facilitate greatly the OER and ensure
the stability. For all processes, the high electrical conductivity of
metallic phase NiS crucially enhances the catalytic performance.
Recently, Shi et al. showed consistently that nickel sulfide with
surface enriched Ni sites exhibits enhanced catalytic activity
toward the OER/HER in alkaline solution. Our results are also
consistent with the previous works for the enhanced HER/OER
-1
and 142 mV dec for NiS and NiS1.97, respectively. The CA
response at η = 0.370 V (corresponding to 1.6 V) also indicates the
much more stable current of NiS than that of NiS1.97 (Fig. 4f). The
increase of current density observed in the NiS1.97 could be due to
the progressive surface oxidation that enhances the OER kinetics.
Table S1† and Table S2† summarize the HER (in acid/alkaline
3
+
4
2
2
8,40
electrocatalytic performance of the metallic Ni
3
S
2
.
Therefore,
we ascribe the higher catalytic activity of NiS NCs to the more
metallic NiS phase and the more surface oxide layers. We expect
that the higher catalytic activity of NiS NCs would make a major
contribution to the higher PEC performance of Si-NiS than that of
electrolytes) and OER data (in alkaline electrolyte) of NiS, NiS
2
,
and Ni , respectively, reported by other groups. The catalytic
3 2
S
performance of our samples is comparable to those of many
previous works, and the higher HER/OER catalytic activity of NiS
Si-NiS1.97
.
2
vs. NiS1.97 (in pH 14) is coherent with that of NiS vs. NiS reported
4
0
by Zheng et al.
0
-2
0
EIS measurements provided the Rct values for all HER and OER
as shown in Fig. S7†. NiS always exhibits a smaller Rct value than
NiS1.97, consistent with its higher electrocatalytic activity toward
the HER/OER. We examined the structure of the samples after the
(b) Si-NiS at 0 V (pH 0)
(a)
-
5
-4
-6
-
-
-
-
-
-
10
15
20
25
30
35
-8
-
-
10
12
2 4
CA experiments of HER in 0.5 M H SO . The HAADF-STEM images
and EDX data showed persistent composition and morphology
before and after, as shown in Fig. S8†. The XRD patterns also
confirmed that the crystal phase of NiS and NiS1.97 persisted after
HER (see Fig. S2†). Finally, the XPS data show that the electronic
structures of metallic NiS and semiconducting NiS1.97 remained
the same after HER (see Fig. S5†).
1
00 (c)
8
0
st
nd
1
and 2 scan
60
40
20
0
Si-NiS at pH 0
Si-NiS at pH 14
Si-NiS1.97 at pH 0
Experiment
FE = 100%
0
.0
0.5
1.0
1.5
2.0
-
0.6
-0.4
-0.2
0.0
0.2
The typical process of HER (2H+ + 2e → H
−
2
) is a two-step
Potential (V vs. RHE)
Time (h)
electrochemical reaction that takes place at the catalytic active
sites and ends with H generation. In acidic solution, the first step
is the electrochemical hydrogen adsorption (Volmer step, H + e
H*, where H* represents H atom adsorbed on the catalyst),
followed by the electrochemical desorption (Heyrovsky step, H*
-
2
Fig. 5 (a) Current density (mA cm ) vs. potential (V vs. RHE) for
Si-NiS and Si-NiS1.97 photocathodes, measured in 0.5 M H SO (pH
) or 1 M KOH (pH 14) under AM1.5G irradiation (100 mW cm
2
+
-
2
4
–
2
0
)
→
–
1
and with on/off cycles. The scan rate is 20 mV s . (b)
Chronoamperometric response of photocurrent for Si-NiS at pH
, and (c) H evolution vs. time (h) under an applied potential of 0
2
V vs. RHE. The dotted line represents the calculated H
from current generation with FE = 100%.
+
-
+
H + e → H
2 2
) or chemical desorption (Tafel step, H* + H* → H ).
0
-1
The Tafel slope of NiS (66 mV dec ) is closer to that of Volmer-
Heyrovsky reaction pathway (40 mV dec ). The S anion sites
could be active sites for the adsorption of H and promote its
subsequent reduction into H
2
evolution
-1
+
2
7,52
2
.
Wang et al. reported the
Now we discuss the I-V curves measured for the water-splitting
PEC cells that consisted of Si-NiS and Si-NiS1.97 as photocathodes
SO (pH 0) or 1 M KOH (pH 14) under solar irradiation
2 4
AM1.5G, 100 mW cm ). The current density (mA cm ) versus
applied potential (V vs. RHE) is plotted in Fig. 5a. The quick
response of photocurrents is observed during light irradiation in
s on/off cycles. For the Si-NiS photocathode, in both electrolytes
the onset potential is about 0.2 V and the photocurrent at 0 V vs.
RHE (JSC, short circuit current) is about 10 mA cm The I-V curves
are almost identical between the first and second scans. The
calculation showing that the exposed S atoms of NiS surface are
5
3
the most favorable sites for H adsorption. The proton-accepting
S sites and hydride-accepting Ni sites can also work in a
in 0.5 M H
(
–
2
–2
2
7
cooperative way. So, the more anionic S atoms in NiS act as
+
excellent active sites to adsorb H . The more cationic Ni atoms at
the surface oxide layers would contribute in increasing the H
2
2
evolution kinetics. Furthermore, the metallic NiS phase
underneath the surface oxide layers also provides good electrical
conduction between the active sites and CC electrode.
–
2
-
-
In alkaline condition, the two step reactions are H
2
O + e → OH +
+ OH (Heyrovsky step)
(Tafel step). Since water as a reactant is very
–2
maximum current reaches 30 mA cm at -0.25 V. The Si-NiS1.97
photocathode at pH 0 produces JSC = 5.3 mA cm and the onset
-
-
H* (Volmer step) and H* + H
or H* + H* → H
2
O + e → H
2
–2
2
potential of 0.2 V for the first scan, which is much lower than that
of the Si-NiS photocathode. The second scan shows a dramatic
degradation of photocurrent (JSC = 0). At pH = 14, the Si-NiS1.97
shows JSC = 0 for the first scan (not shown here).
Fig. 5b displays the chronoamperometric response of
photocurrent (at 0 V vs. RHE) for Si-NiS (at pH 0), showing a
degradation of 3% over 2 h. At pH 14, the stability of Si-NiS is
much lower than that of pH 0, so the photocurrent gradually
stable, its dissociation kinetics in the Volmer step, namely the
cleavage of O–H bond in water, is usually the rate-determining
step for the overall reaction. The Ni cation sites become active for
adsorbing water and promoting its subsequent dissociation. The
H* formed from water dissociation can move to the adjacent
5
4
anionic S atoms that actively reduce it into H
2
.
So, the more
anionic S atoms and more cationic Ni atoms in NiS act as excellent
active sites to enhance the HER kinetics in alkaline electrolytes.
This journal is © The Royal Society of Chemistry 20xx
J. Mater. Chem. C, 2019, 00, 1-3 | 5
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Journal Name
decreases to zero after 20 min. In order to confirm water splitting (450 and 550 C, respectively). XPS and XANES analVyieswesArrtieclveeOanlleinde
in the PEC cell at pH 0, we monitored the evolution of hydrogen that the NiS NCs are more metallic than thDeONI:i1S0.103N9C/Cs9aTnCd05h7a0v3eJ
1
.97
(
2
H ) gas at 0 V (vs. RHE). Fig. 5c displays the H
2
evolution data (in more surface oxide layers. We evaluate the electrocatalytic

mol) of Si-NiS. The Faradic efficiency (FE) for the production of activity toward the HER/OER. The NiS NCs exhibited higher
+
-
H
2
(2H + 2e  H
2
(g)) was calculated by the equation: FE =
catalytic performance for both HER and OER than the NiS1.97 NCs,
because of the more metallic NiS phase and the more surface
oxide layers. The Si-NiS photocathodes demonstrated excellent
2
× 푁H × 96485
2
,
where NH2 is the amount (mol) of H , and Q is the
2
푄
total amount of generated charge in coulomb (photocurrent  performance in solar-driven PEC cells in both 0.5 M H SO and 1
time). The average FE toward the H
line represents the calculated
2
4
–
2
2
evolution is 90% (the dotted M KOH, in terms of photocurrents (10 cm at 0 V vs. RHE) and the
evolution from the onset potential (0.2 V vs. RHE). The PEC performance of Si-NiS is
H
2
photocurrents with FE = 100%). Table S3† compares the PEC superior to that of Si-NiS1.97 in terms of the photocurrent and the
performance with the previously reported data of Si-based PEC stability. The Faradaic efficiency of Si-NiS PEC cell for water-
cells, showing that the performance of Si-NiS is superior to that of splitting hydrogen generation is 90%. XPS data collected after the
2
3
Si-NiSe
We examined the structure and morphology of the samples after reaction. The higher PEC performance of the Si-NiS is ascribed to
the PEC H evolution for 2 h. The XRD patterns showed that the the higher catalytic activity of NiS NCs. Overall, the NiS NCs on Si
2
.
test showed that the metallic NiS phase persisted during the PEC
2
NiS and NiS1.97 retained their crystal phases (Fig S3†). The XPS NWs are very promising catalysts for the photocathode of water-
data indicate that the electronic structures of metallic NiS and splitting PEC cells.
semiconducting NiS1.97 also remained the same afterwards,
despite the occurrence of surface oxidization (Fig. S9†). The
HAADF-STEM images and EDX data in Fig. S10† confirmed that
the NCs remained on the Si NWs with similar composition and
morphology after the PEC. The NiS NCs have uniformly the oxide
Conflicts of interest
There are no conflicts to declare.
shell, and the core-shell structures are persistent during the PEC
reaction. In contrast, the NiS1.97 NCs have less oxide layers
compared to the NiS NCs.
Acknowledgements
This study was supported by 2019R1D1A3A03019576,
Electrochemical impedance spectroscopy (EIS) measurements
provided the charge transfer resistance (Rct) under dark and light
irradiation, as shown in Fig. S11†. In the dark, the Rct value of Si-
NiS and Si-NiS1.97 at 0 V (vs. RHE) is 294.7 and 498.2 ,
respectively, while that under light irradiation is 45.2 and 168.9
2
018R1A2B6003634, and 2014R1A6A1030732, funded by
Korean Ministry of Science and ICT. The HVEM measurements
were supported by the KBSI under the R&D program (D38700).

. The smaller Rct value of the former implies more facile
(
photo)electron transfer kinetics to increase the current, which
Notes and references
mainly accounts for the higher PEC performance obtained for the
Si-NiS.
The flat-band potential (Efb) of Si-NiSand Si-NiS1.97 was obtained
by using Mott–Schottky (MS) plots to be 0.4 and 0.16 eV,
respectively (Fig. S12†). The carrier concentrations estimated
using the MS plots is much larger in Si-NiS than in Si-NiS1.97. It is
1
2
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| J. Mater. Chem. C, 2019, 00, 1-3
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TOC Figure
Nickel sulfide nanocrystals act as excellent catalysts for electrocatalytic and photoelectrochemical hydrogen evolution.
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