Assembly of Ultra-Thin NiO Layer Over Zn1?xCdxS for Stable Visible-Light Photocatalytic Overall Water Splitting

Article abstract of DOI:10.1002/cssc.201802926

Photocatalytic splitting of water into hydrogen and oxygen by using visible light is considered to be a clean, green, and renewable route for solar energy conversion and storage. Although the Zn_1?xCd_xS catalysts show comparatively higher activity for photocatalytic hydrogen generation under visible-light irradiation, they suffer from serious photocorrosion during the photocatalytic reaction. The deposition of a protective layer over the Zn_1?xCd_xS catalysts is believed to be an effective way to inhibit photocorrosion. However, only a few materials exhibit satisfactory catalytic properties for hydrogen evolution as well as a good protection ability. In this work, a new Zn_1?xCd_xS photocatalyst was developed for water splitting under visible-light illumination by assembling an ultrathin NiO layer over Zn_0.8Cd_0.2S through an in situ photodeposition method. The as-prepared NiO/Zn_0.8Cd_0.2S showed significantly higher activity for overall water splitting compared with Pt/Zn_0.8Cd_0.2S under the same conditions without photocorrosion. An apparent quantum efficiency of 0.66 % was achieved for hydrogen evolution at 430 nm with an accomplished multicycle stability for up to 12 h without any significant decay. The strong electronic coupling between the NiO layer and Zn_1?xCd_xS also promoted efficient charge separation and migration.

Full text of DOI:10.1002/cssc.201802926

Accepted Article
Title: Significant enhancement of stability for visible photocatalytic
overall water splitting by assembling ultra-thin layer of NiO over
Zn1-xCdxSX
Authors: Xiaofeng Ning, Wenlong Zhen, Xuqiang Zhang, Gongxuan
Lu, and Dear Editor
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To be cited as: ChemSusChem 10.1002/cssc.201802926
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10.1002/cssc.201802926
ChemSusChem
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Significant enhancement of stability for visible photocatalytic
overall water splitting by assembling ultra-thin layer of NiO over
Zn1-xCdxS
Xiaofeng Ning,[a, b] Wenlong Zhen,[a] Xuqiang Zhang,[a] and Gongxuan Lu*[a]
Abstract: Solar light driven water splitting into hydrogen and oxygen
using visible light active photocatalyst has been considered as a
clean, green, and renewable route to solar energy conversion and
storage. Although Zn1-xCdxS catalyst shows comparatively higher
activity for photocatalytic hydrogen generation under visible light
irradiation, it suffers serious photocorrosion during the photocatalytic
reaction. Deposition of protection layer over Zn1-xCdxS catalyst is
believed to be an effective way to inhibit such photocorrosion.
Nevertheless, seldom of protection layer exhibits satisfied catalytic
properties for hydrogen evolution while presents good protection
ability. In this work, a new Zn1-xCdxS photocatalyst has been
developed for water splitting under visible light illumination by
assembled an ultra-thin NiO layer over Zn0.8Cd0.2S via in-situ
photodeposition method. By this strategy, NiO/Zn0.8Cd0.2S showed
significant higher activity for overall water splitting than
Pt/Zn0.8Cd0.2S under same conditions without photocorrosion. The
AQE of 0.66% for hydrogen evolution at 430 nm has been achieved
and multi-cycle stability has been accomplished up to 12 hours
without significant decay. Moreover, the strong electronic coupling
between NiO layer and Zn1-xCdxS promoted efficient charge
separation and migration.
tunable absorption properties in the visible region of the solar
spectrum, excellent electrical conductivity, suitable band position
at relatively negative potentials for proton reduction, allowing
Zn1−xCdxS catalysts without noble metal loading possess
remarkable activity for visible-light driven photocatalytic
hydrogen production.[12-14] For example, Bard et al [15, 16] reported
the Zn1−xCdxS catalyst exhibited the excellent photocatalytic
activity in the presence of S2− (or S2- + SO32−) and good charge
separation efficiency. Guo et al. found that the synthesis
[18, 19]
method,[17] alkaline metals doping
and constructing special
nano-structure [20-22] promoted the activities of Zn1−xCdxS catalyst.
The Zn1−xCdxS solid solution catalyst even showed higher
photocatalytic activity than Pt/CdS.[23] However, Zn1−xCdxS solid
solution catalyst still suffers the photocorrosion during the
photocatalytic process. Some strategies have been developed to
overcome this drawback, such as fabricating heterojunction in
Zn1−xCdxS,[24-26] constructing Z-scheme structure,[27, 28] and using
hole scavengers, etc.[29, 30] It has been found that assembled a
protection layer over Zn1−xCdxS catalyst is an effective method to
resist photocorrosion, for instance, draping graphene layers over
a Zn1−xCdxS can accelerate charge transfer from the surface of
semiconductor to graphene, which impedes hole oxidation of
32]
catalyst,[31,
however, seldom of protection layer exhibits
satisfied catalytic properties for hydrogen evolution while shows
good protection ability. That is why seldom Zn1-xCdxS based
catalysts have been reported for photocatalytic overall water
splitting. More recently, using artificial gill (AG) and
perfluorodecalin (PFDL) removing or transferring nascent
formed oxygen from dispersion of metal sulfides were identified
to be an efficient way to retard the oxygen induced
photocorrosion,[33-38] thus that could enhance the stability of
Zn1−xCdxS as well, resulting in increased efficiency for
photocatalytic overall water splitting.
Introduction
Photocatalytic hydrogen generation from water splitting instead
of fossil resource is a fascinating process for solar energy
conversion and storage since it offers a way to generate
hydrogen without carbon dioxide emission.[1-5] There have been
many semiconductor photocatalysts reported to be active for
water splitting, however, only few of candidates can be used
under the visible light illumination.[6-8] The main drawbacks are
the photocorrosion of visible light active metal sulfide catalysts
and quite weak visible light adsorption nature of metal oxide
catalyst.[9-11]
NiO exhibits excellent catalytic activity, chemical stability,
good
conductive
properties
in
heterogeneous
and
electrochemical catalysis fields.[39-41] Herein, we developed a
new photocatalyst for water splitting by in-situ photodeposition of
ultra-thin layer NiO on Zn1-xCdxS surface. We found this catalyst
exhibited higher activity for water splitting than Pt/Zn1-xCdxS with
significant enhanced stability. The overall water splitting reaction
could be operated without photocorrosion. The AQE of 0.66%
for hydrogen evolution at 430 nm has been achieved and multi-
cycle stability has been accomplished up to 12 hours without
significant decay. Moreover, the strong electronic coupling
between NiO layer and Zn0.8Cd0.2S promoted efficient charge
separation and migration, therefore, the photo-excited electron
lifetime has been remarkable prolonged. This work presents a
practical way to design noble metal free sulfide semiconductor
photocatalyst for the solar energy conversion via water splitting.
Zn1−xCdxS, as the “alloy” of ZnS and CdS, possesses the
[a]
[b]
Dr. X. Ning, W. Zhen, X. Zhang, Prof. G. Lu
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences
Lanzhou 730000, China
E-mail: gxlu@lzb.ac.cn
Dr. X. Ning
University of Chinese Academy of Sciences
Beijing 100049, China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201802926
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Results and Discussion
NiO/Zn0.8Cd0.2S. No characteristic diffraction peaks of NiO
species are observed in NiO/Zn0.8Cd0.2S, this probably owing to
its high dispersion status. The results indicate that there is no
significant change in the phase structure of Zn0.8Cd0.2S after NiO
loading on its surface. Therefore, it is reasonable to speculate
that the improvement of the photocatalytic activity of
NiO/Zn0.8Cd0.2S might be not caused by any alteration in the
crystal structure of Zn0.8Cd0.2S.
The phase structures of the prepared samples were
characterized by XRD. Figure 1 depicts the XRD patterns of the
Zn1−xCdxS samples. It can be seen that the diffraction peaks of
the ZnS sample (x= 0) are readily indexed to a pure cubic phase
(JCPDS 65-0309), with the three prominent diffraction peaks
corresponding to (111), (220) and (311) planes. As compared to
the pure ZnS, the diffraction peaks of (111) plane for Zn1-xCdxS
nanoparticles shift toward lower angles with increasing Cd
content, i.e., from 28.8 to 28.4, 28.3, 27.6, 26.9 and 26.7°, as the
content of Cd increased (x = 0, 0.1, 0.2, 0.5, 0.9 and 1.0)
respectively, which is due to the replacement of some zinc
atoms in ZnS by cadmium atoms with larger radii.[29] The
occurrence of the successive peak shift clearly implies that the
obtained nanocrystals should be Zn1-xCdxS solid solutions,[13, 29]
rather than simple physical mixture of ZnS and CdS.
Figure 1. XRD patterns of Zn1-xCdxS solid solutions with different x values.
Figure 3. a) XPS survey spectra of Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S. High
resolution XPS spectra of b) Zn 2p, c) Cd 3d, d) S 2p, e) Ni 2p and f) O 1s of
Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S.
XPS was carried out to determine the surface chemical
compositions and electronic state of the as-prepared samples.
Therefore, the successful deposition of NiO on Zn0.8Cd0.2S can
be further confirmed by XPS. The binding energy data were
calibrated by setting the C 1 s peaks at 284.8 eV as a reference.
The survey spectra (Figure 3a) indicate the presence of Zn, Cd
and S elements in both Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S samples,
and the evident Ni 2p is detected in NiO/Zn0.8Cd0.2S sample. As
displayed in Figure 3b, the high resolution spectra of Zn 2p in
Zn0.8Cd0.2S show two characteristic peaks at 1021.9 and 1044.9
eV, which correspond to Zn 2p3/2 and Zn 2p1/2 of Zn2+,
respectively.[42] While for NiO/Zn0.8Cd0.2S, the Zn 2p3/2 and Zn
2p1/2 peaks are observed at 1022.1 and 1045.1 eV, respectively,
which shift 0.2 eV toward a higher binding energy (BE) side. And
the peaks centered at 405.2 and 412.0 eV (Figure 3c) can be
assigned to the Cd 3d5/2 and Cd 3d3/2 in Zn0.8Cd0.2S, suggesting
the valences of Cd is bivalent Cd2+.[43] As compared to
Figure 2. XRD patterns of pure Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S composite
photocatalysts.
Figure 2 displays the XRD diffraction pattern for Zn0.8Cd0.2S and
NiO/Zn0.8Cd0.2S samples. It is clearly that the synthesized
Zn0.8Cd0.2S is crystallized into a standard cubic structure. In
addition, compared to Zn0.8Cd0.2S, the characteristic peaks of
Zn0.8Cd0.2S exhibit no obvious changes in intensity and width for
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Zn0.8Cd0.2S, the BE of Cd 3d5/2 and Cd 3d3/2 of NiO/Zn0.8Cd0.2
S
detected in the EDX spectra on the NiO/Zn0.8Cd0.2S composite in
Figure S1. Based on above the TEM results, there is a closely
contacted interface between NiO and Zn0.8Cd0.2S components in
the composite sample, which could serve as an effective bridge
for the separation of photo-generated electrons and holes.
also shift to the positive side slightly. Meanwhile, in Figure 3d,
the peaks of S 2p are located at approximately 161.4 eV and
162.6 eV, which indicate that S2− is the dominant chemical state
in Zn0.8Cd0.2S,[44] although the BE of S 2p shifted positively 0.3
eV for NiO/Zn0.8Cd0.2S. Since the BE of S 2p, Zn 2p and Cd 3d
have systematically shifted towards high binding energy side
with 0.1-0.3 eV, the strong electronic coupling between
[45, 46]
Zn0.8Cd0.2S and loaded NiO may take place.
The shifts
might attribute to a partial electron transfer from the electron-rich
[24, 47]
structure of Zn0.8Cd0.2S to NiO.
Such electron transfer had
the potential to increase the electron density of NiO active sites,
which is believed to be beneficial to the hydrogen evolution in
water splitting.[30] As observed in Figure 3e, the Ni 2p signal of
NiO could be deconvoluted into four peaks, the BE at 856.3 and
861.9 eV are attributed to the main Ni 2p3/2 peak and its satellite
peak, and the peaks centered at the binding energy of 874.0 and
880.2 eV correspond to Ni 2p1/2 and its satellite peak within
[48, 49]
experimental error.
The Ni 2p peaks are assigned to
characteristic Ni2+ in the NiO/Zn0.8Cd0.2S sample, which are
consistent with the previously reported values.[40,
Figure 3f
50]
shows the O 1s spectrum, which can be deconvoluted by
Gaussian curves corresponding into two distinct peaks. The
peak centered at 529.5 eV confirms the Ni-O bonding in NiO.[48,
Figure 4. a-1) TEM, a-2) SAED, a-3) HRTEM images, and a-4) particle size
distribution of Zn0.8Cd0.2S, b-1) TEM, b-2) SAED, b-3) HRTEM images, and b-
4) particle size distribution, and c) The elemental mapping of NiO/Zn0.8Cd0.2S.
51]
The peak at 531.2 eV is attributed to irreversibly adsorbed
water molecules on the surface of the NiO/Zn0.8Cd0.2S sample.[52]
The morphology and micro-structure of the obtained
samples were characterized by TEM as depicted in Figure 4. It
is found that Zn0.8Cd0.2S nanoparticles are agglomerated (in
Figure 4a-1), which is likely due to the limited passivation of
Zn0.8Cd0.2S. Specifically, a statistical analysis on more than 50
nanoparticles exhibits that the Zn0.8Cd0.2S nanoparticles are
mono-dispersed with an average particle size of ≈ 4.53 nm
(Figure 4a-4). The selected area electron diffraction (SAED) also
reveal detail structure of the Zn0.8Cd0.2S, as displayed in Figure
4a-2, which can be indexed to cubic polycrystalline Zn0.8Cd0.2S.
The diffraction concentric rings observed in the SAED pattern
from inside to outside could be assigned to (111), (220), and
(311) planes of cubic phase of Zn0.8Cd0.2S. This is in good
agreement with the XRD results. And the HRTEM image (Figure
4a-3) displays the clear lattice spacing of 0.31 nm, which was
close to the theoretically predicted value for the (111) plane of
cubic Zn0.8Cd0.2S, well matched with the reported data.[31] After
photodeposition of NiO on Zn0.8Cd0.2S, there is no remarkable
morphology change, representing deposited NiO as ultra-thin-
layer on the Zn0.8Cd0.2S (Figure 4b-1). The average diameter of
NiO/Zn0.8Cd0.2S nanocomposite is ≈ 6.16 nm (see in Figure 4b-
4). The high resolution TEM image in Figure 4b-3 shows that
ultra-thin NiO nanolayers have been compactly attached on the
surfaces of Zn0.8Cd0.2S, moreover, the high resolution TEM
image clearly reveals a lattice spacing of 0.24 nm indexed to the
NiO (111) plane in the nanolayers[43], clearly demonstrating the
highly crystalline. The homogeneous element distribution is
further confirmed by the elemental mapping of selected area
NiO/Zn0.8Cd0.2S composite. It is found that Zn, Cd, S, Ni and O
elements distribute almost uniformly in the surface of
NiO/Zn0.8Cd0.2S (Figure 4c). Meanwhile, Ni peaks can be
It is well known that the optical absorption property of the
semiconductor is associated with the unbound electrons and
remarkably influence the optical properties and activity of the
53]
photocatalyst.[30,
A
comparison of UV-visible diffuse
absorption spectra Zn1-xCdxS (x = 0, 0.1, 0.2, 0.5, 0.9 and 1.0)
samples is displayed in Figure 5a. Obviously, there is a
continuous red shift of the absorption edges for the samples with
increasing Cd content in Zn1−xCdxS nanocrystals. As shown in
Figure 5b, NiO/Zn0.8Cd0.2
S
hybrids show an enhanced
absorption spectra in the visible light region compared to the
pure Zn0.8Cd0.2S. NiO is a high band-gap semiconductor with the
absorption edge in the UV region and no absorption in the
visible region (Figure S2 and S3). In addition, the band gap
energies (Eg) of Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S (in Figure 5c)
were determined from a plot of (ahν)2 versus energy (hν),[42, 54]
and estimated to be 2.62 and 2.68 eV, respectively.
The flat-band potential (Vfb) can be quantified by the Mott–
Schottky (M–S) plots. As the results shown in Figure 5d, the M-S
plots of both Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S showed apparent
typical n-type characteristic, due to the positive slope of the
linear plots.[55] The derived flat-band potential for Zn0.8Cd0.2S is
about −0.85 V vs. SCE (−0.61 V vs. NHE).[56] Compared with
Zn0.8Cd0.2S, the Vfb of NiO/Zn0.8Cd0.2S is upgraded to -0.93 V vs.
SCE (−0.69 V vs. NHE).[57] Given that the Vfb of sample is the
difference between Fermi level and water-reduction potential,[58]
it can be deduced that the Fermi level of NiO/Zn0.8Cd0.2S are
raised up 0.08 eV compared that of Zn0.8Cd0.2S.
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Table
1
Summary of textural properties and photocatalytic activities of
samples
SBET
(m2g-1)
PV[a]
(cm3g-1)
APS[b]
(nm)
Rate[c]
(μmol/m-2h-1)
Sample
Zn0.8Cd0.2
S
193.6
151.3
0.22
0.24
4.0
5.5
0.26
0.66
NiO/ Zn0.8Cd0.2
S
[a] PV: Pore volume. [b] APS: Average pore size. [c] Normalized photocatalytic
H2 production rate is acquired by dividing the photocatalytic H2 production rate
of the as-prepared samples by their BET surface area.
The obtained Zn1-xCdxS solid samples were systematically
evaluated for pure water splitting without using electron
sacrificial agents under visible-light irradiation. As shown in
Figure 7a, ZnS does not show H2 production activity. After the
introduction of Cd atoms, the activities of Zn1-xCdxS are
dramatically improved, in which the Zn0.8Cd0.2S shows the
highest catalytic activity with an average H2 production rate of
50.4 μmol h-1 g-1. Figure 7b displays the photocatalytic hydrogen
evolution rate of x%NiO/Zn0.8Cd0.2S samples with different
content of NiO, from 1% to 20%. When NiO concentration
increased from 1 to 10 wt%, the H2 evolution rate gradually
enhanced, and reached to the maximum value (99.9 μmol h-1 g-
1) with 10 wt% of NiO. Furthermore, 10%NiO/Zn0.8Cd0.2S showed
higher activity than Pt/Zn0.8Cd0.2S sample (78.7 μmol h-1 g-1,
Figure 7c). Since the photocatalytic activity of mechanical
mixture of NiO and Zn0.8Cd0.2S only showed 46.9 μmol h-1 g-1 of
hydrogen generation rate (seen in Figure 7c), the improved
photocatalytic performance of NiO/Zn0.8Cd0.2S may be attributed
to the strong interaction between NiO and Zn0.8Cd0.2S as shown
in XPS results, which facilitate the easy separation and transfer
of the photo-generated electron-hole pairs. However, further
loading of NiO leads to a drastical decrease in H2 evolution rate.
This decrease might due to the following factors: (i) excess NiO
covers Zn0.8Cd0.2S surface and decreases light adsorption; (ii)
aggregation of NiO under high NiO loading leads to less active
sites available for hydrogen evolution. Therefore, the optimized
loading amount of NiO for photocatalytic hydrogen evolution is
10 wt%. In addition, long-term recycle experiment was
performed to evaluate the photostability of the as-synthesized
catalysts. From Figure 7d, it can be seen that the activity of
Zn0.8Cd0.2S gradually decreases with irradiation time, probably
due to the catalyst photocorrosion. On the contrary,
NiO/Zn0.8Cd0.2S exhibits excellent stability for splitting pure water
during the four repeated runs over a total reaction time of 12 h.
These results proved the excellent anti-photocorrosion property
of NiO/Zn0.8Cd0.2S. To confirm this anti-photocorrosion property,
Cd2+ ions concentration in the catalyst dispersion was measured
Figure 5. a) UV−visible diffuse reflection spectra of the Zn1−xCdxS samples, b)
UV−visible diffuse reflection spectra of Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S, c)
band gap energies of the samples d) Mott-Schottky plots of Zn0.8Cd0.2S and
NiO/Zn0.8Cd0.2S in 0.1 M Na2SO4 solution.
To further study the surface area and pore structure of the
prepared samples, N2 adsorption–desorption measurements
were carried out. According to the results shown in Figure 6, two
nitrogen sorption isotherms are very similar and are type IV with
hysteresis loops (Brunauer–Deming–Deming–Teller (BDDT)
classification), which is typical mesoporous characteristic [59]. In
addition, the isotherm displays a narrow hysteresis loop in the
P/P0 range of about 0.4-1.0, indicating the presence of
mesopores and/or macropores, possibly formed by the stacking
of constituent nanoparticles.[30] Based on the desorption branch
of the isotherm, the pore-size distributions (inset in Figure 6) of
the Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S samples indicate a wide
pore-size distribution from 1.7 to 180 nm, further confirming the
existence of mesopores and macropores. The normalized
activity of the as-prepared catalysts by surface area are given in
Table 1.
by ICP method. As shown in Figure 7e, for the Zn0.8Cd0.2
S
sample, the concentration of Cd2+ apparently increased with
prolonged light irradiation, and reached to 8.4 ppm after the
fourth cycle. While for NiO/Zn0.8Cd0.2S, negligible Cd2+ was
found after several cycle runs up to 12 h. To further verify the
enhanced stability of NiO/Zn0.8Cd0.2S sample, XRD and XPS
analysis of the used catalyst have been performed. As shown in
Figure S4 and S5, no distinguish changes can be observed from
the XRD pattern and XPS spectrum.
Figure 6. Nitrogen adsorption–desorption isotherms and corresponding pore
size distribution curves (inset) of the Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S samples.
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Some reference ZnxCd1-xS-based photocatalyst activities for H2
production are summarized in Table S1. In the reference cases,
the sacrificial reagents were used. The comparison of
photocatalytic overall water splitting activities without sacrificial
whether there was a reverse reaction between evolved H2 and
O2 to recombine into H2O, the stoichiometric H2 and O2 was
injected into reactor. As shown in Figure 8e, the amount of O2
decreased significantly with the irradiation time in the sealed
Pyrex flask, indicating the occurrence of hydrogen and oxygen
recombination. Therefore, separating the generated H2 and O2
from the reaction system is one of the key points for high
efficient overall water splitting. When photocatalytic water
splitting reactions were performed in the presence of the oxygen
transfer reagents, perfluorodecalin (PFDL) and artificial gill (AG),
the H2 evolution rates were remarkably enhanced as shown in
Figure 8f. With the aid of PFDL or AG, the H2 production rate
reached to 227.3 μmol h-1 g-1. These findings imply that the
oxygen transfer reagents in the photocatalytic reaction can
inhibit the recombination of the evolved H2 and O2, and stabilize
the Zn0.8Cd0.2S catalyst and inhibit O2 or H2O2 oxidiation, leading
to high photocatalytic efficiency. In addition, the photocatalytic
water splitting stability tests in the oxygen transfer reagents were
tested (shown in Figure S6) and the Cd2+ concentration in the
reaction solutions for each run were measured in Figure S7. The
addition of AG or PFDL increased the H2 production activity and
stability of NiO/Zn0.8Cd0.2S catalyst.
reagent indicates the NiO/Zn0.8Cd0.2
S
photocatalyst is
outstanding catalyst for over-all photocatalytic water splitting.
Figure 7. a) Photocatalytic H2 evolution over Zn1-xCdxS, b) Rate of H2
evolution over x%NiO/Zn0.8Cd0.2S composite samples, c) The comparison the
H2 evolution rate over 10%NiO/Zn0.8Cd0.2S, 2%Pt/ Zn0.8Cd0.2
NiO+Zn0.8Cd0.2S (simple mechanical mixture NiO particles and Zn0.8Cd0.2S), d)
Cycling runs for the photocatalytic H2 evolution in pure water of Zn0.8Cd0.2
S
and
S
and 10 wt% NiO/Zn0.8Cd0.2S composite sample under visible light irradiation,
e) Cd2+ leakage profiles under visible light irradiation.
To further confirm the catalyst is able to drive photocatalytic
overall water splitting, isotopes tracer test was performed. As
displayed in Figure 8a, D2 (m/z=4) was detected when D2O was
used as a reactant, thus confirming that the hydrogen were
generated from D2O. In addition, according to the results shown
in Figure 8b, the signal of m/z=36 was identified, that proved
18O2 was formed from H218O, verifying that O2 was indeed
generated from the photocatalytic overall water splitting.
Since H2O2 formation might be the kinetically competitive
reaction with oxygen generation via a single electron pathway by
photo-generated holes oxidation and combination of ·OH,[57] the
H2O2 accumulation was evaluated through the UV–vis spectra of
reaction solutions containing 1% o-tolidine. As shown in Figure
8c, the absorption peak at 437 nm in the UV-vis spectrum
indicated the formation of H2O2, and a gradual increase of
absorbance intensity at 437 nm was observed with irradiation
time (see Figure 8d). Besides, the generated hydrogen and
oxygen can rapidly recombine back to water.[33-38] To verify
Figure 8. a) GC-MS signals of the gas generated over the NiO/Zn0.8Cd0.2
photocatalytic splitting D2O and b) H218O, c) UV–vis spectrophotometry of time
dependence of the absorption intensity of H2O2 over the NiO/Zn0.8Cd0.2
suspension during photocatalytic water splitting, d) The absorbance at 437 nm
against irradiation time for NiO/Zn0.8Cd0.2S, e) Time courses of the H2 and O2
S
S
recombination reaction over NiO/Zn0.8Cd0.2S in a sealed Pyrex flask in
irradiation at room temperature, and f) Photocatalytic H2 evolution rate over [A]
NiO/Zn0.8Cd0.2S, [B] NiO/Zn0.8Cd0.2S +1mL PFDL, and [C] NiO/Zn0.8Cd0.2S+AG.
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The AQEs of NiO/Zn0.8Cd0.2S at different light wavelengths by
using various band-pass filters without sacrificial agent were
measured. The tendency of AQE on wavelength closely followed
that of the absorbance presented in Figure 9, the
potential of the water splitting reaction.[63] Hence, the polarization
curves for hydrogen evolution reaction of photocatalysts were
measured. Figure10c displays the linear sweep voltammetry
curves of catalysts, it is clearly seen that NiO/Zn0.8Cd0.2
S
NiO/Zn0.8Cd0.2
S
achieved
a
high AQE of 0.66% at the
possesses a higher cathodic current density attributing to the
reduction of water to H2.[64] In other words, NiO/Zn0.8Cd0.2S has a
lower over-potential, which can accelerate the protonation and
subsequent H2 formation rate. Besides, cyclic voltammetric
wavelength of 430 nm. The AQE value decreased with increase
of wavelength of the incident light, which matched well with the
optical absorption spectrum. Furthermore, with the help of PFDL
or AG, the AQE could reach to 0.95 or 1.50% at 430 nm,
respectively.
technique
has
been
employed
to
explore
the
photoelectrochemical properties of the photocatalysts. As
displayed in Figure 10d, the cyclic voltammograms of the
photocatalyst electrodes present two reversible anodic and
cathodic peaks. NiO/Zn0.8Cd0.2
S electrode shows a larger
current density, further demonstrating a significantly enhanced
rate of electron transfer across the contact interface between
electrode and electrolyte solution over NiO/Zn0.8Cd0.2S.[65]
Lifetime of photoelectrons has been conducted by means of
open circuit photovoltage (Voc) decay (OCPD). It is well known
that OCPD technique can be utilized to estimate the
photoelectrons lifetime and to evaluate the separation efficiency
of the photo-generated electron-hole pairs.[66] Figure 10e shows
the curve Voc with time. The Voc decay rate is closely related with
the photoelectron lifetime. Based on the relevant formula,[34, 66]
the calculated electron lifetime as a function of Voc is shown in
Figure 10f. It is obviously that NiO/Zn0.8Cd0.2S exhibits prolonged
electron lifetime in comparison with bare Zn0.8Cd0.2S. Collectively,
these commendable photo-electrochemical results support that
loading NiO onto Zn0.8Cd0.2S can greatly reduce the resistance
of the charge transfer and separation and prolong the
photoelectron lifetime, which should be responsible for the
enhanced photocatalytic water splitting performances.
Figure 9. Wavelength-dependent AQE of NiO/Zn0.8Cd0.2S at various light
wavelength.
To gain more insight into the inherent mechanism of the
enhancement of the photocatalytic hydrogen production activity
over
NiO/Zn0.8Cd0.2S,
a
series
of
complementary
characterizations of the photoelectrochemical properties were
carried out. It is known that the charge separation and transfer
efficiency is an important factor influencing the photocatalytic
activity.[60] The charge separation behavior was first investigated
through the transient photocurrent measurement. Figure10a
shows the photoelectrochemical responses of the catalysts
under visible light irradiation. Not surprisingly, NiO/Zn0.8Cd0.2
S
exhibits enhanced photocurrent intensity. It is known that a
higher photocurrent corresponds to a more efficient separation,
faster transmission or a longer lifetime of the photogenerated
electron-hole pairs.[61] The electrochemical impedance
spectroscopy (EIS) Nyquist plot is displayed In Figure 10b. Such
a pattern of the EIS can be fitted by an equivalent circuit shown
in inset of Figure 10b. The RΩ is total ohmic resistance of the
electrolyte solution; Rsei and Csei are resistance and capacitance
of the solid-state interface layer formed on the surface of the
electrodes, Rct and Cd are faradic charge-transfer resistance and
its relative double-layer capacitance, ZW is the Warburg
impedance related to the diffusional effects.[62]. The charge-
transfer resistance Rct can be calculated according to the
equivalent circuit. The Rct of NiO/Zn0.8Cd0.2S is 137.9 Ω, which is
smaller than that of Zn0.8Cd0.2
S (150.7Ω), implying that
NiO/Zn0.8Cd0.2S was excellent conductor for accelerating the
electron transfer. Another significant factor that influences the
photocatalytic H2 evolution rate highly depends on the over-
Figure 10. a) Transient photocurrent responses under visible-light irradiation,
b) Electrochemical impedance spectroscopy (EIS) Nyquist plots, c) Linear
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10.1002/cssc.201802926
ChemSusChem
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sweep voltammetry (LSV) curves, d) Cyclic voltammograms of the as-
prepared thin films on ITO glass in 0.1 M KCl aqueous solution containing 0.01
M K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) e) Illuminated open circuit potential and f)
Photoelectron lifetime determined from the decay of open circuit potential of
bare Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S nanocomposites.
carriers in the defect states of Zn0.8Cd0.2S, whereas the longer
lifetime component of τ2 is caused by the recombination of free
excitons in the Zn0.8Cd0.2S. Compared with Zn0.8Cd0.2S (∼3.90
ns), NiO/Zn0.8Cd0.2S shows an increased intensity-average PL
lifetime (∼5.38 ns), the long-lived photo-generated electron
lifetime might be due to the presence of a more effective trap
level. That is to say, the excited electrons of Zn0.8Cd0.2S are
more captured by NiO as electron trap centers, thus, the decay
rate of electrons becomes slower, and the lifetime is prolonged.
The prolonged photo-generated electrons lifetime represent a
higher probability of their involvement in the photocatalytic
reaction before recombination. Taken together, the improvement
of the visible-light photocatalytic H2-generation activity of
NiO/Zn0.8Cd0.2S sample is ascribed partially to high separation
and transfer efficiency and prolonged electron lifetime.
The steady-state and time-resolved photoluminescence (PL)
spectra of bulk Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S samples were
also performed, which could elucidate the formation and
migration dynamics behavior of the specific charge carriers in
68]
photocatalysts.[67,
Figure S8 compares the steady-state PL
spectra for Zn0.8Cd0.2S and NiO/Zn0.8Cd0.2S under 380 nm light
excitation. The emission peaks exhibit no obvious shift after
introduction of NiO. It is well known that for most of
semiconductors, there are two kinds of emission band, one is
band-edge emission intrinsically originating from electronic
transitions across the band gap closely located at the absorption
edge, and the other is trap-state emission extrinsically arising
from defect states centered at longer wavelengths.[69] The two
emission bands centered at ∼468 and 623 nm, correspond to
the band to band transition and the trap-state emission,
respectively. NiO/Zn0.8Cd0.2S shows a much lower PL peak
intensity, suggesting a suppressed photo-generated charge
recombination. In addition, Figure 11 displays time-resolved
On the basis of the above experimental and theoretical results,
a photocatalytic water splitting mechanism of NiO/Zn0.8Cd0.2S is
proposed in Scheme 1. Due to larger Eg of NiO, only Zn0.8Cd0.2
S
can be excited by visible light to generate electrons and holes.
The excited electrons transfer from the CB of Zn0.8Cd0.2S to that
of NiO, reduce H+ to hydrogen. Since the CB of NiO is more
negative than the reduction potential of H+ to H2, while the
photo-induced holes in the VB of Zn0.8Cd0.2S will react with H2O
to generate O2. The nascent generated O2 will be combined by
PEDL and transformed out of the system by the AG timely, thus
the H2 and O2 recombination was inhibited and Zn0.8Cd0.2S was
pretced from photocorrosion.
photoluminescence (TRPL) spectra of Zn0.8Cd0.2
S
and
NiO/Zn0.8Cd0.2S monitored at 468 nm. The emission decay
curves of the samples are fitted with a biexponential kinetics
function (1),
-t/τ2
I(t) = A1 e-t/τ + A2 e
(1)
1
which generates two lifetime values, τ1 and τ2, and the
corresponding amplitudes, A1 and A2. Then, based on the fitted
data, the intensity-average exciton lifetime (τ), τ is calculated
and presented to make an overall comparison of the PL lifetimes
of all samples by the equation (2).[70]
2
(τ) = (A1τ12 + A2τ2 )/ (A1τ1 + A2τ2)
(2)
Scheme
1
Mechanism of overall water splitting over NiO/Zn0.8Cd0.2
S
composite with AG under visible illumination.
Conclusions
In summary, we have developed
a
convenient in-situ
photodeposition method to synthesize NiO ultra-thin layer
modified Zn0.8Cd0.2 photocatalyst. Photocatalytic hydrogen
S
Figure 11. Time-resolved PL decay curves of of Zn0.8Cd0.2
NiO/Zn0.8Cd0.2S nanocomposites.
S
and
evolution from pure water is successfully realized by using
NiO/Zn0.8Cd0.2S nanocomposite in the absence of any electron
sacrificial agents. The photocatalytic activity and stability on
Zn0.8Cd0.2S can be significantly improved by loading of ultra-thin
catalytic active NiO layer. The outstanding activity can be
ascribed to the synergetic effect of the highly efficient charge
separation and migration from Zn0.8Cd0.2S to NiO and the
The two decay components are derived (insets in Figure 11),
which contains a fast (τ1) and a slow (τ2) from the equation, τ1 is
originated from the nonradiative recombination of charge
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10.1002/cssc.201802926
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FULL PAPER
(Micromeritics apparatus ASAP 2020M) at 77K. Adsorption-desorption
isotherm was used to identify the pore size distribution via the
Barret−Joyner−Halender (BJH) method, assuming a cylindrical pore
model. Transmission electron microscopy (TEM) and HRTEM images
were taken with a Tecnai-G2-TF20 field emission transmission electron
microscope operating at accelerating voltage of 200 kV. Elemental
mapping was performed by using an energy-dispersive X-ray
spectrometer (EDS) attached to the TEM instrument. Ultraviolet-visible
(UV-vis) diffuse reflectance spectra (DRS) were obtained with an UV-
2550 (Shimadzu) spectrophotometer in which BaSO4 powder was used
as the internal standard to obtain the optical properties of the samples.
The photo-luminescence (PL) spectra for samples were obtained on an
Edinburgh FL/FS900 spectrophotometer with an excitation wavelength of
380 nm.
prolonged lifetime of photogenerated electrons. Efforts of
effective separation of the evolved H2 and O2 makes the
inhibition of Zn0.8Cd0.2S photocorrosion possible, leading to
achieve stable overall water splitting reaction. This result can
potentially provide a pathway to design efficient solar water-
splitting catalyst.
Experimental Section
Materials
All chemicals were analytical pure and used without further purification.
Cadmium chloride (CdCl2·2.5 H2O, Sinopharm Chemical Reagent Co.,
Ltd, AR, ≥99%), zinc acetate (Zn(CH3COO)2·2H2O, Tianjin Kemiou
Chemical Reagent Co., Ltd, AR, ≥99.0%), sodium sulfide (Na2S·9H2O,
Chengdu Kelong Chemical Reagent Co., Ltd, AR, ≥98.0%), nickel
acetate (Ni(CH3COO)2·4H2O, Sinopharm Chemical Reagent Co., Ltd, AR,
≥98.5%), anhydrous methanol (CH3OH, Sinopharm Chemical Reagent
Co., Ltd, AR, ≥99%), sodium sulfate (Na2SO4, Xilong Chemical Co., Ltd.,
AR, >99.5%) are used as provided. Deionized water was used in the
synthesis and reaction.
Photocatalytic H2 evolution activity tests
Photocatalytic experiments were performed at room temperature in a
sealed Pyrex flask (184 mL) with a flat window (an efficient irradiation
area of 9.8 cm2) and a silicone rubber septum for sampling. 100 mg of
catalyst was dispersed into 100 mL H2O under the ultrasound treatment
(25 kHz, 250 W) about 30 min. Prior to irradiation, the reactant mixture
was degassed by bubbling Ar gas for 30 min. The Xenon lamp (HSX-UV
300) with a 420 nm cut-off filter was used as a light source to trigger the
photocatalytic reaction. The amount or rate of hydrogen evolution was
measured using gas chromatograph (Agilent 6820, TCD, 13 X columns,
Ar carrier). A continuous magnetic stirrer was applied in order to keep the
photocatalyst in suspension status during the whole experiment.
Preparation of Zn1-xCdxS
Zn1-xCdxS was synthesized according to the previously reported
method.[29] In a typical synthesis, given amounts of Zn(CH3COO)2·2H2O
and CdCl2·2.5H2O, with a specified value in Zn1-xCdxS (x=0, 0.1, 0.2, 0.5,
0.9 and 1), were dissolved in 50 mL of distilled water and the total molar
amount was 6 mmol. Then, a certain amount of 0.2 M NaOH aqueous
solution was added to adjust the pH value of the aqueous solution to 7.3.
The resulting suspension was stirred for several minutes. Afterwards, 30
mL of aqueous solution containing excessive Na2S was added dropwise
into the suspension. The resulting precipitates were aged in the mother
liquors for 18 h under continuous stirring. Finally, the obtained products
were thoroughly washed with distilled water and dried in an oven
overnight at 80°C.
The apparent quantum efficiency (AQE) was measured under the same
photocatalytic reaction conditions with irradiation light through
a
bandpass filter (430, 460, 490, 520, and 550 nm). Photon flux of the
incident light was determined using a Ray virtual radiation actinometer
(FU 100, silicon ray detector, light spectrum, 400-700 nm; sensitivity, 10-
50 μV μmol−1 m−2 s−1). The reaction solutions were irradiated for 2 h with
bandpass filters for AQE tests on the H2 production. The following
equation (3) was used to calculate the AQE.
2 the number of evolved hydrogen molecules
AQE
100%
(3)
the number of incident photos
Photodeposition of NiO or Pt on Zn0.8Cd0.2
S
The recycling test of photocatalytic H2 evolution over the catalyst was
done as follows. Typically, after the photocatalytic reaction of the first run
under visible light irradiation, the photocatalytic system was thoroughly
degassed again, without the separation of photocatalysts. Subsequently,
the reactor was irradiated again by a 300 W Xe lamp with a 420 nm cut-
off filter. Analogously, the following runs of photocatalytic recycling tests
were performed.
An in-situ photodeposition method was used to prepared NiO/Zn0.8Cd0.2
S
catalyst. 0.15 g of Zn0.8Cd0.2S was suspended in 100 mL of aqueous
solution containing 30 mL methanol with calculated amount nickel
acetate. The suspension was then thoroughly degassed and irradiated by
a Xe lamp (300 W) equipped with an optical cutoff filter (≥ 420 nm). The
deposition amount of NiO was controlled by varying the dosage of
Ni(CH3COO)2 solution. After the photodeposition procedure, the resulted
particles were collected by centrifugation and then rinsed thoroughly with
deionized water and finally dried in vacuum oven. Pt/Zn0.8Cd0.2S was
prepared by a similar photodeposition procedure with aqueous H2PtCl6
as the Pt precursor.
Photocatalytic splitting D2O and H218O isotope-labeled experiments
The isotopes tracer experiments have been performed under downsized
reactor under same experimental conditions using D2O and H218O.
Typically, 10 mg catalyst dispersed 10 mL of D2O or 2 mL of H218O in the
sealed Pyrex flask. After ultrasonic treatment for 30 min, the suspended
aqueous solution is degassed by bubbling Ar gas for another 30 min.
After visible light irradiation for 4 h, the gas mixture in container was
detected by GC–MS (Agilent Technologies, GC 7890A with a PONA
capillary column (50 m × 0.20 mm × 0.50 μm), MS 5975C, the
temperature of sample injector was maintained at 240◦C).
Characterizations of the catalysts
X-ray diffraction (X’pert Philips PRO with
a Panalytical X’celerator
detector, Germany) was carried out using graphite monochromized Cu
Kα radiation (40 kV) in the 2θ ranging from 10 to 80°. Chemical state
characterization was carried out by X-ray photoelectron spectroscopy
(ESCALAB 250Xi, ThermoFisher Scientific). The binding energies were
calibrated by the C1s binding energy of 284.8 eV. The specific surface
areas of the catalysts were determined by N2 adsorption-desorption
measurements by employing the Brunauer-Emmet-Teller (BET) method
Photoelectrochemical performance
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10.1002/cssc.201802926
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Photoelectrochemical experiments were carried out on an
electrochemical workstation (CHI 660E). The electrochemical system
consists of three electrodes with Pt sheet as a counter electrode, a
saturated calomel electrode (SCE) as a reference electrode, and a
working electrode prepared with catalyst sample (1 cm×1 cm). 0.1 M
aqueous Na2SO4 electrolyte solution without any additives was used in
the test except cyclic voltammograms test in 0.1 M KCl aqueous solution
containing 0.01 M K3[Fe(CN)6]/K4[Fe(CN)6] (1:1). The Mott−Schottky
relationship is derived by solving Poisson’s equation for a depleted
semiconductor space-charge layer with the imposition of many
assumptions and simplifications, which can be expressed as C−2 (2/ε ε0
e N A2)·(E−Efb−kT/e),[71] where C is the spacecharge capacitance, ε is the
dielectric constant, ε0 is the permittivity of free space, e is the charge on
the electron, N is the carrier density, A is the sample area, E is the
impressed voltage, Efb is the flat band potential, T is the temperature, and
k is Boltzmann’s constant. A 300 W Xe arc lamp equipped with a band-
pass light filter (λ ≥ 420 nm) was applied as light source. The measured
potentials vs. SCE were converted to the normal hydrogen electrode
(NHE) scale by eqn. (4):
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10.1002/cssc.201802926
ChemSusChem
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Developed a convenient in-situ
X. Ning, W. Zhen, X. Zhang, G. Lu*
deposition method synthesize ultra-
  –  
thin NiO layer modified Zn0.8Cd0.2
S
photocatalyst. Photocatalytic HER is
successfully realized over
Significant enhancement of stability
for visible photocatalytic overall
water splitting by assembling ultra-
thin layer of NiO over Zn1-xCdxS
NiO/Zn0.8Cd0.2S without sacrificial
agents. Zn0.8Cd0.2S harvests visible
light, NiO serves catalytic sites for
HER and protection layer from
photocorrosion. Strong interaction
between NiO and Zn0.8Cd0.2
S
improved charge separation, transfer
and prolonged charge lifetime.
This article is protected by copyright. All rights reserved.