CoP nanoparticles as cocatalyst modified the CdS/NiWO4 p-n heterojunction to produce hydrogen efficiently

Article abstract of DOI:10.1039/c9nj05977f

Generally, cocatalyst modification is an efficient way to enhance the hydrogen evolution performance of visible light irradiation. In this work, CoP nanoparticles acted as a cocatalyst to modify the CdS/NiWO₄ p-n heterojunction. The composite catalyst CdS/NiWO₄/CoP was successfully prepared by a co-precipitation and physical mixing method. The maximum H₂ production rate of the composite catalyst was 47.7 mmol h^-1 g^-1, which was 11.9 times and 3.3 times that of pure CdS and CdS/NiWO₄ (50 wt%), respectively, and the apparent quantum efficiency was 9.7% in a 10% lactic acid system when the wavelength was 475 nm. This excellent performance is attributed to the CoP nanoparticles, which act as electron traps to provide more active sites and benefit electron transport. Photoluminescence spectroscopy and electrochemical performance analysis demonstrated that the composite catalyst had a better electron separation ability and light absorption conversion ability. Mott-Schottky and UV-visible diffuse reflectance testing indicated that the excellent electron and hole separation was due to the p-n junction formed between CdS and NiWO₄, and the modification of the CoP cocatalyst further enhanced the charge-separation efficiency, thereby improving the hydrogen evolution activity.

Full text of DOI:10.1039/c9nj05977f

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DOI: 10.1039/C9NJ05977F
CoP nanoparticles as cocatalyst modified the CdS/NiWO of p-n
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heterojunction to produce hydrogen efficiently
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Hua Liu 1,3,4, Teng Yan1,3,4, Zhiliang Jin1,2,3,4*, Qingxiang Ma2*
1. School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021,
P.R.China
2. State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering,
Ningxia University, Yinchuan,750021, PR China
3
. Ningxia Key Laboratory of Solar Chemical Conversion Technology, North Minzu University,
Yinchuan 750021, P.R.China
4
. Key Laboratory for Chemical Engineering and Technology, State Ethnic Affairs Commission,
North Minzu University, Yinchuan 750021, P.R.China
Corresponding author: zl-jin@nun.edu.cn (Z. L. Jin); maqx@nxu.edu.cn (Q. X. Ma)
Abstract: Generally, co-catalyst modification is an efficient way to enhance the hydrogen
evolution performance of visible light irradiation. In this work, CoP nanoparticles act co-catalyst
to modify the CdS/NiWO p-n heterojunction. The composite catalyst CdS/NiWO /CoP was
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successfully prepared by co-precipitation and physical mixing method. The maximum H2
-1
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production rate of composite catalyst is 47.7 mmol h g , which are 11.9 times and 3.3 times that
of pure CdS and CdS/NiWO (50%wt), respectively, and the apparent quantum efficiency is 9.7%
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in the 10% lactic acid system when the wavelength is 475 nm. This excellent performance is
attributed to CoP nanoparticles, which act as electron traps to provide more active sites and benefit
electron transport. The photoluminescence spectroscopy and electrochemical performance
analysis demonstrated that the composite catalyst has better electron separation ability and light
absorption conversion ability. The Mott-Schottky and UV-visible diffuse reflectance testing
indicated that the excellent electron and holes separation was due to the p-n junction formed
between CdS and NiWO , and the modification of CoP co-catalyst further enhanced the charge
4
separation efficiency, thereby improving hydrogen evolution activity.
Key words: cocatalyst, CoP nanoparticles, hydrogen evolution, charge separation.

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DOI: 10.1039/C9NJ05977F
1
. Introduction
The development of society cannot be separated from the use of fossil fuels. Nowadays,
fossil fuels are not only shortage, but also cause a series of serious environmental pollution
problems [1-3]. The solar energy and hydrogen energy make great concern to scientists as clean
and renewable energy [4-6]. Photocatalytic hydrogen production which is considered to be one of
the effective ways to solve the current problems can make good use of solar energy as well as
produce hydrogen [7]. Platinum has been widely used as catalyst for photocatalytic hydrogen
production, however, its high cost limit its further development [8]. Finding for cheap and
efficient catalysts instead of platinum is great significance for promoting photocatalytic hydrogen
production to fulfill large-scale industrial production [9-11].
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Over the past few decades, many non-precious metal catalysts have been studied, such as
carbon-based materials [13, 14], transition metal oxides [15, 16], metal sulfide [17, 18],
hydroxides [19] and so on. Among of these catalyst, metal sulfide have narrow band gap and
strong absorption ability of visible light, such as CdS [20-22], ZnIn S [23], and CdZnS [24].
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Especially CdS is used to enhance the absorption of visible light and crack water due to its
appropriate band gap (about 2.4 eV) as well as used to improve the edge potential due to its
appropriate conduction band [25, 26]. Unfortunately, the H production activity of pure CdS is not
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ideal due to the rapid recombination of photocarrier and the easy occurrence of photocorrosion
[
27]. The performance of CdS used as catalyst was improved by morphology regulation, element
doping, catalyst modification and heterogeneous junction construction [28, 29].
It is a common method to improve hydrogen production efficiency by loading co-catalyst on
catalyst surface [30]. The main functions of the co-catalyst include: effectively promoting the
separation of photogenic carriers; Reduce the semiconductor over potential or activation energy to
enhance the reduction ability of the catalyst [31]. Transition metal phosphide is a common
co-catalyst with low over potential, good stability and friendly to the environmental. Especially，
the stability of CoP in acid-based media is comparable to that of zero-valence metal [32]. In the
aspect of hydrogen production, the research on transition metal phosphates as a cocatalyst began
around 2014 [33]. Tian et al. [34] prepared CoP/WS by combination of vulcanized and
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phosphatize, they proved that CoP is an efficient catalyst for hydrogen production. Du's group [35]
reported that by attaching Ni P to the surface of in situ, hydrogen production performance can be
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significantly improved. Wang et al. [36] confirmed the hydrogen production efficien
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using lamellar Ni P/g-C N as catalyst is 22 times than use pure g-C N . However, there are very
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few reports concerned the CoP nanoparticles as catalyst to modify heterojunction.
In this article, CdS/NiWO /CoP composite catalyst was successfully prepared according to
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the follow steps. First, the NiWO loaded on CdS by the co-precipitation, from the p-n
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heterojunction CdS/NiWO . Second, the CoP nanoparticles are used as catalyst to modify the
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CdS/NiWO p-n heterojunction by the physical mixing. The successful preparation of ternary
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composites material CdS/NiWO /CoP has significantly increased hydrogen production activity
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and light absorption property. The CSNP-7 ternary composites improved photo catalytic H2
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conversion rate to 47.7 mmol h g which are 11.9 and 3.3 times higher than that of CSNW-5 and
bare CdS, respectively. This excellent performance is attributed to the modification of CoP
nanoparticles, acting as electron traps to provide more active sites and benefit electron transport
and improving the photo absorption ability of catalyst. This report provides a new research idea
for the decomposition of water into hydrogen by multi-component catalysts.
2. Experimental Section
All reagents are with analytical grade and used without more settlement.
2.1 Preparation of CdS nanorods, CdS/NiWO and CoP nanoparticles
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The preparation of CdS nanorods: 2 g of C H CdO ∙2H O and 1.75 g of H NCSNH were
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equably dispersed in 50 mL of ethylenediamine with uniform stirring for 1 h. Subsequently, the
above solution was added into 80 mL Teflon-lined, then the autoclave was sealed and reacted
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carried out at 160 C for 24 h. After that, the mixtures was centrifuged and washed with deionized
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water and ethanol, respectively. The final CdS was obtained by drying at 60 C for 8 h.
The preparation of CdS/NiWO (CSNW): The composite catalyst CSNW was synthesized by
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the co-precipitation [37]. Taking the preparation of CSNW-5 (m(NiWO )/m(CdS)×100%=50,
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named as CSNW-5) as an example. First, 0.2 g of CdS was added in 60 mL of deionized water by
ultrasound for 10 min, and then 0.077 g NiCl ∙6H O and 0.107 g Na WO ∙6H O were added to
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above the solution. After stirring for 2 h, the compound was centrifuged and washed with
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deionized water and alcohol, respectively. The CSNW was obtained by drying at 60 C for 10 h.
CSNW-2, CSNW-3, CSNW-4 and CSNW-6 were prepared by the same method. The synthesis

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DOI: 10.1039/C9NJ05977F
process of pure NiWO was similar without CdS under the same method.
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.2 Preparation of CoP nanoparticles, CdS/NiWO /CoP and CdS/CoP.
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The preparation of CoP nanoparticles: 1 g of Co(NO ) ∙6H O and 0.25 g sodium citrate were
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dispersed in 35 mL of deionized water. Then, 10 mL (7 M) NaOH solution was tardily added and
stirred at room temperature for 1 h. After centrifugation, they were washed three times with
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deionized water and alcohol, respectively. The reaction precursor was obtained by drying in an
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oven at 60 C overnight. 1 g of reaction precursor was dissolved in 20 mL of H O with 2 g of
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NaPO H ∙H O. The mixture was ground to a powder after evaporation. It was calcined with N at
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00 C for 4 h. Composite was obtained after centrifuge drying. 0.1 g of composite was steeped in
0 mL (3 M) HCl for 30 min, CoP nanoparticles were grinded after centrifuge drying.
The preparation of CdS/NiWO /CoP (CSNP): CSNP were compounded by the physical
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mixing. Taking the preparation of CSNP-5 (m(CoP)/m(CSNW)×100%=5, named as CSNP-5) as
an example. As prepared 0.5 g of CSNW and 0.025 g of CoP nanoparticles were added to the 30
mL anhydrous ethanol with ultrasound for 20 min to form a homogeneous solution. The ethyl
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alcohol was evaporated at 80 C and the last dried at 60 C. It was simply labeled CSNP-5. The
CSNP-1, CSNP-3, CSNP-7 and CSNP-9 were prepared in the same way.
The preparation method of CdS/CoP (CSCP): CSNP were compounded by the physical
mixing. First, 0.5 of pure CdS and 0.35 g of CoP nanoparticles were dispersed in 30 mL
anhydrous ethanol by ultrasound for 20 min. The uniformly stirring for 2 h to homogeneous
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solution, then steam dry at 60 C water bath. The CSCP was obtained by drying at 60 C for 10 h,
It was simply labeled CSCP-7.
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.3 Characterization of samples
All kinds of morphology and structure of CdS, CSNW and CSNP were detected by taking the
scanning electron microscope (SEM: Hitachi S-4800) with an accelerating voltage of 5.0 kV and a
transmission electron microscopy (TEM: Tecnai G2-TF30), energy dispersive X-ray (EDX) under
the acceleration voltage of 300 kV. The crystalline structure was analyzed by X-ray diffraction
(
XRD: Rigaku RINT-2000) patterns. X-ray photoelectron (XPS: ESCALAB 250Xi) was
introduced to record the element composition. UV-vis DRS was obtained by UV-2550 (shimazu)
spectrometer with using BaSO as a reference. The ASAP2020M instrument was employed
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measure the adsorption-desorption isotherms of nitrogen by analyzing the result with the

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Brunaure-Emmett-Teller (BET) equation. The FlUOROMAX-4 spectrophotometer (HDO
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Scientific, France) was engage to test photoluminescence (PL) emission and time-resolved spectra.
An electrochemical workstation (VersaSTA4-400, AMETEK) is used to detection the
photoelectrochemistry quizzes containing photocurrent density-time (I-T), electrochemical
impedance spectrum (EIS) and Mott-Schottky measurements, there into 0.2 mol/L Na SO
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aqueous solution as a electrolyte. The standard three-electrode cell consisted of Pt sheet as the
counter electrode, the reference electrode is saturated calomel electrode (SCE), and homemade
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surface of work electrodes was 1 cm . Furthermore, the test of I-T curve is performed in the light
source is 300 W xenon lamp cases at a fixed bias of 0.3 V vs. SCE. When the forward bias is 0.2
V, the samples of EIS are measured are measure at AC amplitude of 10 mV and a frequency scope
is 10000-1 Hz. Meanwhile, the AC amplitude of 10 mV, the Mott-Schottky of CdS and NiWO is
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tested from -1 to 1 Vs with at a frequency of 1000 HZ.
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.4 Photocatalytic H measurements
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Photocatalytic hydrogen production activity of all as-prepared samples is carried out in a 62
mL reaction bottle. First, the 0.01 g ternary composite catalysts were uniformly dispersed by
ultrasound in 30 mL of 10% lactic acid solution. N was purged into the reaction bottle 10 min to
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keep the reaction system in an anaerobic atmosphere by filling the N about 10 min to replace O .
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The gas chromatography (Tianmei GC7900, TCD, 13X column, N as the carrier) system was
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used to analyze extract 0.5 mL of H evolved every one hour. In addition, stability experiments are
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also carried out at room temperature and each cycle of 5 h each, the total amount of hydrogen
produced is calculate by the external standard method.
Further, the apparent quantum efficiency (AQE) of the CSNP-7 was tested out using xenon
lamps at 300 W. The PL-MW200 radiometer (Perfect Light) was used to test photon fluxes before
and after illumination. The experimental condition was basically the same as the hydrogen
production procedure except for the light source. The monochromatic wavelength is 420, 450, 475,
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00, 520 and 550 nm. Calculating the apparent quantum efficiencies by Eq (1) at different
wavelengths
2the number of evolved hydrogen molecules
AQE 
100% （1）
the number of incident photon

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. Results and discussion
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.1 XRD analysis
Fig. 1 shows the X-ray diffraction patterns of bare CdS, NiWO , CoP, CSNW and CSNP
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composites with different ratios. In Fig. 1A, seven distinct characteristic diffraction peaks at 24.9°,
6.6°, 28.3°, 43.7°, 47.9°, and 51.9° can be assigned to the (100), (002), (101), (102), (110), (103)
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and (112) crystal plane of CdS (PDF#41-1049), respectively [6, 18]. However, the XRD patterns
of CSNW-5 have not discovered corresponding peak of NiWO , which is probably because
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NiWO is amorphous. In Fig. 1B, CoP has five characteristic peaks of 30.1°, 30.5°, 32.5°, 33.6°
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and 36.8° corresponding to (011), (111), (112), (211) and (301) crystal plane of CoP
(
PDF#89-4862) [8], which prove the successful preparation of CoP. Nevertheless, no obvious CoP
characteristic diffraction peaks in the CSNP-7 composite catalyst, which may because content of
CoP is relatively low. As shown in Fig. 1C, pure NiWO4 has two scattered and widened peaks
located at 30.88°, 54.71° can be indexed to NiWO (PDF#72-480), may be caused by amorphous
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NiWO4.
Fig. 1 XRD patterns of (A) Pure CdS, NiWO
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and different proportions of CSNW, (B) CoP and CSNP composites
with different CoP content, (C) NiWO samples.
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.2 SEM and TEM analysis
The morphology of CdS and CSNP-7 samples are investigated by SEM and TEM, which are
utilized to offer more intuitive information about the sample distribution, size and space shape of
samples and so on. The bar shape CdS with smooth surface has a very uniform shape size in Fig.
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A. As depicted in Fig. 2B, the composite catalyst CSNP-7 has coarser surface compared with
that of the pure CdS nanorods, the aggregation state of NiWO is close contact with CdS nanorods,
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at the same time, ultra small of CoP nanoparticles are equably loaded on the surface of CdS and
NiWO , this spatial distribution of CoP nanoparticles can expose more active sites, immensely
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.1039/C9NJ05977F
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improving the hydrogen production performance of composite catalyst. Fig. 2C shows
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so the successful preparation of CSNP-7 may be proved. Further, Cu element is detected in the
spectrums, which principally come from Cu carriers [40]. In Fig. 2 D, CdS nanorods has sloppy
surface and the size is about the same, which is in agreement with the SEM consequence. It can be
seen from Fig. 2F that the outer surface of CSNP-7 becomes rough the when the CoP and NiWO4
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were loaded on the surface of CdS nanorods, which proves that NiWO and CoP are successfully
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loaded on the surface of CdS nanorods. More importantly, there are a lot of tiny CoP nanoparticles
loaded on the surface of CSNW-5. As observed in Fig. 2E, the lattice fringes of the composite
catalyst CSNF-7 are obviously observe by high resolution transmission electron microscopy
(
HRTEM), which it can be seen that the lattice fringe spacing of 0.336 nm and 0.247 nm are
corresponded to the (002) planes of CdS (PDF#41-1049) and (111) planes of CoP (PDF#89-4862).
Simultaneously, lattice fringes of NiWO are absent, once again shows that NiWO is amorphous,
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which is in agreement with the XRD results. In the next place, the NiWO covers the lattice
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fringes of CdS, which proves the NiWO exists on the surface of CdS in a deposited manner. Fig.
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2G and 2H shows the TEM images of the CSNP-7 samples obtained after the recycled tests. It can
be easily found that the fresh CSNP-7 and recycled CSNP-7 samples have the same lattice fringes,
indicating the compound catalyst has reusability and excellent stability.

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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 2 (A) SEM images of pure CdS and (B) CSNP-7; (C) the EDX spectrum of CSNP-7; TEM images (D) pure
CdS ; (E) TEM images of CSNP-7 and (F) HR-TEM images of CSNP-7; TEM images after cyclic stability tests
for (G) CSNP-7 and (H) HR-TEM images of CSNP-7.
3
.3 XPS analysis
The valence state and chemical composition of CSNP-7 samples before and after hydrogen
production are investigated by X-ray photoelectron spectra (XPS) as in Fig. 3. Fig. 3A represents
the XPS spectra of Cd 3d, S 2p, Ni 2p, W 4f, O 1s, Co 2p and P 2p for CSNP-7, and the peaks
corresponding to the elemental composition are in good agreement with the reported literature [10,
2
1, 39, 40, 41, 42]. Fig. 3B shows that the peaks of Cd 3d located at 405.1 and 411.8 eV, which
2+
are belong to Cd 3d5/2 and Cd 3d3/2 respectively for Cd state. As shown in Fig. 3C, along with

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2
-
0
the peaks of S 2p are observed at 161.5 and 162.7 eV, belonging to S 2p3/2 and S 2p1/2 of
D
S
OI: 1
[
2
.1
1
039/C9NJ05977F
].
Fig. 3D shows the Ni 2p3/2 and Ni 2p1/2 binding energies are noted at 855.8 and 873.5 eV,
respectively, together with satellite peaks assured at 862.0 and 879.4 eV. The distance in binding
energy (17.7 eV) between the two peaks of Ni 2p1/2 and Ni 2p3/2 verifier the divalent oxidation
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
state of Ni atoms exist in NiWO nanostructures [10, 39]. As shown in Fig. 3E and 3F, the W 4f
4
6
+
spectrum has two peaks located at 35.5 and 37.7 eV belonging to W 4f7/2 and W 4f , W
5/2
oxidation state, respectively [40]. At the same time, the two peaks around 530.9 and 532.1 eV can
be instructed to O 1s, which are communicated to O-Ni bond and lattice oxygen in the NiWO4
phase, respectively [41], all of these proves that NiWO exists in ternary compound. Additionally,
4
in the Fig. 3G, the Co 2p spectrum exhibits three binding energy at about 777.8 eV, 781.0 eV and
7
86.1 eV (satellite), which are typical characteristic peaks of CoP [20, 42]. It can be seen from Fig.
H, P 2p spectrum has two peaks, and the peaks locate at 129.2 and 133.3 eV indicate phosphate
3
species due to the surface oxidation [10, 22]. A series of indicators above prove the success load
of NiWO and CoP on CdS. Comparing to XPS spectra of CSNP-7 before and after hydrogen
4
production, it can be easily found that the valence state and chemical composition of composite
catalyst did not change after the reaction.
Fig. 3 XPS spectra of (A) sample: survey for CSNP-7, (B) Cd 3d, (C) S 2p, (D) Ni 2p, (E) W 4f, (F) O 1s, (G) Co
2p and (H) P 2p.
3.4 BET analysis

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1
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2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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For purpose of studying the effect of CoP nanoparticles loading on CdS/ NiWO4
D
ca
O
t
I
a
: 1
l
0
y
.
s
10ts
39/C9NJ05977F
,
such as size distribution, specific surface area (SBET) and pore volume are investigated in detail.
The nitrogen adsorption and desorption process for CdS, CSNW-5, CSCP-7 and CSNP-7 are
performed at 77K. As seen from Fig. 4A, the curves for all samples belong to type-IV isotherms,
with H3 hysteresis loop [6]. It confirms that all samples have a mesoporous structures, caused by
the flock together of nanometer materials [43]. Fig. 4B is a schematic diagram of aperture
distribution, the pore sizes of four catalysts are concentrated at 2-50 nm and their average pore
size is between 18 to 23 nm in Table 1, which further proves mesoporous structure of samples.
Further analysis of the data was showed in Table 1, the SBET of CdS and CSNW are 35.4 and 35.6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
-1
m g . However, when CoP nanoparticles are supported on two catalysts, the S
of CSCP and
BET
2
-1
CSNP are 30.2 and 32.4 m g , the S
of composite obviously decreased. Meanwhile pore
BET
volume and average bore size of latter is lower than that of former, which probably because some
of the pores of CdS are blocked or filled by CoP nanoparticles [21], this conclusion is consistent
with the results of TEM and SEM analysis. The changes in performance parameters testify that
CoP nanoparticles are successfully loaded onto the surface of the CdS and CSNW-5. Generally
speaking, the active sites will increase with the increase of SBET, the SBET of CSCP-7 and CSNP-7
are both increase after adding CoP into the system. Hence, it is demonstrated that the enhanced
HER activity of CSNP has nothing to do with the change of SBET
.
Fig. 4 (A) The N
2
absorption-desorption isotherms and (B) corresponding pore size distribution of the all samples.
Table 1 The physical adsorption performance parameters.
2
-1 a
3
-1 b
Average pore size (nm)b
21
Samples
CdS
S
BET (m g )
Pore volume (cm g )
35.4
0.18

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23 DOI: 10.1039/C9NJ05977F
CSNW-5
CSCP-7
CSNP-7
35.6
30.2
32.4
0.20
0.13
0.17
18
22
a Obtained from BET method;
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
b Relative pressure (P/P0) was 0.99.
3
.5 Photocatalytic H evolution performances
2
Fig. 5 shows the H evolution activities of diverse catalysts with 10% lactic acid solution as a
2
sacrificial agent under visible light irradiation (λ> 400 nm). In Fig. 5A, evidently, the pristine
NiWO has barely any H production activity for 5 h under visible light. At the same time,
4
2
because CdS is prone to photocorrosion and recombination rate of electron-hole is fast, so H2
production rate of pure CdS is relatively low. However, hydrogen evolution activity of composite
catalyst has been enhanced remarkably due to the addition of NiWO and CoP nanoparticles to
4
pure CdS. Fig. 5B exhibits the influence of different content the NiWO on CSNW activity. With
4
the increase the proportion of NiWO loading, the H production rate of CSNW first increase and
4
2
then decrease. The amount of H production is gradually increased at the beginning, which may
2
attributed to the role of p-n type heterojunction, and then the production of H is decreased
2
because excessive NiWO can capture a part of the active sites of CdS [33].
4
The debugging of CoP nanoparticles is carried out on the optimum ratio of binary catalyst
and the results are shown in Fig. 5C. The CoP nanoparticles loaded on CSNW-5 can distinctly
boost photocatalytic H evolution activity, proving that the CoP nanoparticles are high-efficiency
2
electron trapper. Specially, when the CoP nanoparticle content is 7%, the CSNP-7 composite
catalyst shows the highest hydrogen production activity (47.7 mmol h-1 g-1). However, the H2
production began to decrease with the content of CoP nanoparticles increased from 7% to 9%,
which may be caused by the shielding effect of the mixed catalyst and the excessive CoP blocking
the holes scavengers. In the Fig. 5D, the hydrogen production rate over CSNP-7 has 47.4 mmol h-1
-1
g , which is 11.9 and 3.3 times higher than that of CdS and CSNW-5. In addition, the H
2
-1
-1
evolution rate of CSCP-7 up to 27 mmol h g under the identical experimental conditions, which
is higher than that of CdS and CSNW-5 but far less than that of CSNP-7. In short, a series of
comparison experiments prove that the CoP nanoparticles are a co-catalyst with excellent

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performance. Further, we provide many H production parameters for the similar poto
D
ca
O
t
I
a
: 1
l
0
y
.
s
10ts
39/C9NJ05977F
,
2
the results are shown in Table 2.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig.5 H
2
evolution amount of (A) pure CdS, CSNW-5, CSCP-7 and CSNP-7; (B) hydrogen production rate of
CSNW-X (X=2, 3, 4, 5, 6), (C) CSNP-X (X=1, 3, 5, 7, 9) and (D) different catalysts.
Besides, hydrogen evolution stability and apparent quantum efficiency (AQE) of CSNP-7 are
important indicators to measure the practical application of catalysts. Fig. 6A shows
three-dimensional diagram of the H production of CSNP-7 in five consecutive circulation of 5 h
2
illumination, after the fifth cycle, the amount of H is accounts for 75% of the first cycle. The
2
reason for this phenomenon may be increased in reaction time, which would further exacerbate
consumption of lactic acid. Meanwhile, it also indicates that the CSNP-7 as a kind of composite
catalyst shows relatively excellent stability. Fig. 6B displays the AQE for CSNP-7 under different
incident wavelengths. The AQE value is approximate 8.1%, 5.7%, 9.7%, 4.6%, 0.4% and 0.04%
for CSNP-7 at a light source with of 420, 450, 475, 500, 520 and 550 nm, respectively. When the
wavelength of the light sources is adjusted to 550 nm, the minimum AQE is only 0.04%,

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manifesting that this wavelength is unfavorable to hydrogen generation efficiencyDO
o
I:
f
10.1th03
e
9/C9NJ05977F
composite catalyst; the highest AQE of CSNP-7 is 9.7% when the monochromatic wavelength is
75 nm.
4
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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5
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5
6
0
1
2
3
4
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 6(A) The stability test of hydrogen production cycle of CSNP-7, (B) AQE of CSNP-7 in the 420-550 nm
wavelength range.
Table 2 Comparison of catalytic activities of other similar photocatalytic decomposition and hydrogen evolution
systems
Sacrificial
reagent
Activity
Photocatalyst
Co-catalyst
Light source
Ref.
-1
-1
(μmol·h ·g )
5
W LED
λ≥420nm)
W LED
10 vol% lactic
acid
CdS
CdS
WPS
47700
This work
(
5
10 vol% lactic
acid
NiWO
CoP
4
15140
34490
15028
12000
362
6
8
(λ≥420nm)
4
3
00 W Xe-lamp
TEOA
3 4
Co O
(λ≥420nm)
(pH=9)
5 W LED
10 vol% lactic
acid
Zn
x
Cd1-x
S
WP
9
(λ≥420nm)
00 W Xe-lamp
0.1M Na
2
S
CdS
MnO
x
@ CoP
20
32
(λ≥420nm)
0.1M Na SO
2
3
3
00 W LED
g-C
3
N
4
NiP
TEOA
(λ≥420nm)
5W LED
10 vol% lactic
acid
Zn Cd S
NiWO
4
15950
838
38
40
0.7
0.3
(
λ≥420nm))
300WXe-lamp
λ≥420nm)
H
2
O(artificial
gill)
CdS
2
Ni P
(
3.6 Photoluminescence analysis
The PL spectrum of pure CdS, CSNW-5, CSCP-7, CSNP-7 are shown in Fig. 7A. Generally

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2
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2
2
2
2
3
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speaking, the intensity of the fluorescent signals and recombination of electrons and h
D
o
O
l
I:
e
1
s
0.1
a
039/C9NJ05977F
re
considered to be closely related. Emission wavelength of pure CdS is about 520 nm, it shows a
relatively strong PL emission intensity, and this indicates that it has a serious recombination rate
of electron-hole pairs. The PL intensity of CSNW-5 is lower than that of CdS, the main reason is
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
that CdS and NiWO formed p-n type heterojunction, thereby improving the separation effect of
4
electrons and holes [6]. It is interesting to note that PL intensity of CSCP-7 show significant
decrease and slight blue shift. It could be because CoP nanoparticles are uniformly distributed
character, it can promote electrons transfer by tightly contact with the outer surface of CdS [22],
which contributes to control the recombination of electron-hole pairs [47]. The PL intensity of
CSNP-7 is the lowest in all the samples due to the CSNP-7 has serious phenomenon of quenching,
which indicates that the CSNP-7 has excellent charge separation capacity in photocatalysis system
[
22]. This excellent performance is attributed to CoP nanoparticles, acting as electron traps to
provide more active sites and benefit electron transport. On the other hand, it has the synergistic
effects between the CdS, NiWO and CoP nanoparticles, which greatly improved the hydrogen
4
performance of composite catalyst. The variation tendency presented by hydrogen production
performance of catalysts is well consistent with the photoluminescence, meaning the positive
correlation between catalyst performance and degree of charge separation.
Fig. 7 (A) The PL spectra and (B) the time-resolved photoluminescence spectra of CdS, CSNW-5, CSCP-7,
CSNP-7.
Fig. 7B displays the kinetics studies of charge transfer coming from the time-resolved
photoluminescence (TRPL). The time-resolved photoluminescence of CdS, CSNW-5, CSCP-7

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039/C9NJ05977F
y
and CSNP-7 samples were tested. The decay curves were achieved by a fitted triexponent
D
ia
O
l
I:
d
1
e
0.
c
1
a
using the Eq. (2).
I(t) 
B exp(t/τ )
(2)

i
i
i1,2,3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
I is the normalised emission intensity; t is the time after the pulsed laser excitation; τ
i
are the respective decay
lifetimes; B are the corresponding weight factors.
i
The average life span of the catalyst is obtained by Eq. (3). Table 3 shows the dynamics
parameters of electron transfer for the samples.
2

τ 
Bi
/
Bi
（3）


τ
i
τ
i
i 1,2,3
i 1,2,3
<τ> is the average lifetime; τ
i
is the respective decay times; B
i
is the corresponding weight factors.
Usually, electron transfer rate is negatively correlated with the electron lifetime. The average
lifetime of CSNP-7 (1.16 ns) is significantly lower than that of CdS (2.27 ns), CSNW-5 (2.23 ns),
CSCP-7 (2.19 ns). The results show that CSNP-7 composite material has shorter time of electron
transport [46]. This is because CoP can effectively capture the electrons on the outer surface of
CdS, which creates additional decay channel to the excited states of the CSNW-5 [21]. In addition,
we also analyzed the charge migration of the catalyst by calculating the values of electron transfer
rate constant (K ) and the electron injection efficiency (η ), they can be calculated by Eq (4) and
ET
inj
(5) respectively:
1
1
KET


（4）
（5）
τ （sample） τ （CS）
ave
ave
τav（e sample）
τav（e CS）
ηinj  1
Because of the KET and ηinj value of CSNP-7 are larger than that CSCP-7 and CSNW-5,
which manifest that CoP nanoparticles as charge collector can effectively accept electrons
attached on the surface of CdS and NiWO [21]. Once again, it proves that CoP nanoparticles play
4
a huge role in improving H evolution performance of composite catalyst. When the amount of
2
CoP nanoparticles is 7% to CSNW-5, the photocatalytic performance of composite catalyst is
more excellent. On the one hand this is because the CoP as a catalyst exposes more active sites
and effectively inhibits the recombine of electrons and holes; On the other hand the synergistic

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1
1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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action of the three substances also has obvious effect on the improvement of catalyst perfo
D
r
O
m
I:
a
10
n
.
c
1039/C9NJ05977F
e
.
Table 3 The dynamics parameters of electron transfer for samples.
τ
1
τ
2
τ
3
τ
ave
K
ET
η
(inj)
Samples
9
-1
[
ns]
.47
27.83%)
.10
28.65%)
.17
28.82%)
.75
28.08%)
[ns]
[ns]
[ns]
[10 s ]
[%]
-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
0.78
(24.35%)
0.63
151.27
(47.81%)
149.15
CdS
2.72
-
(
(
(
(
5
CSNW-5
CSCP-7
CSNP-7
2.23
2.19
1.16
0.08
0.09
0.49
18.0
19.5
57.5
(46.96%)
0.63
(24.39%)
147.28
5
(24.93%)
0.36
(46.25%)
144.04
4
(29.13%)
(42.79%)
3
.7 Photoelectrochemical properties
In order to further investigate the electron separation efficiency, electrochemical analyses are
executed on the catalysts. In a general way, strength of instantaneous photocurrent response is
positively correlated with the separation effect of electrons-holes. Fig. 8A shows photocurrent
density-time curves of CdS, CSNW-5, CSCP-7 and CSNP-7, the photocurrent is generated when
light is irradiated the surface of catalyst, the curves of photocurrent response are accomplished
under discontinuous light. From the overall analysis, the photocurrent density of CdS and
CSNW-5 remains almost unchanged, CSCP-7 and CSNP-7 shows a decreasing trend with the
extension of illumination time. The photocurrent density of CdS is lowest compared, which may
be due to photocorrosion. In contrast, the transient photocurrent response of CSNP-7 is higher
than the rest of the catalysts, indicating that the CSNP-7 holds stronger ability in generating and
transferring the photoexcited charge carrier [8]. This could be because the modification of CoP
nanoparticles increases the separation efficiency of photogenerated electrons and holes [23].
In addition, Fig. 8B shows the AC impedance spectrum of CdS, CSNW-5, CSCP-7 and
CSNP-7 in the absence of light. Compared with other catalysts, CSNP-7 has the smallest arc
radius, indicating that it’s small charge migrating resistance, and enhanced charge transfer rate [11,

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2
3]. The inset of Fig. 8B shows the equivalent circuit diagram, and Rct is an electronicDOtrI:
a
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r
resistor; Rs is the solution resistance. The analysis of EIS and IT together demonstrates that the
CoP nanoparticles have greatly improved the electron transfer performance and attested the
strengthened effect of it on the segregation [11]. Meanwhile, this result is basically consistent with
the hydrogen production activity of the catalyst.
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Rct
Rs
CPE
Fig. 8 (A) Transient photocurrent responses of CdS, NiWO
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, CSNW-5, CSCP-7 and CSNP-7. (B) Electrochemical
impedance spectroscopy (EIS) of CdS, NiWO , CSNW-5, CSCP-7, CSNP-7.
4
3
.8 UV-vis diffuse reflectance spectra and Mott-Schottky analysis
In order to research optical natures of catalysts the UV-vis diffuse reflectance spectra is used
to analyze the samples as shown in Fig. 9A. Compared with the spectrum of CdS, both the
absorption range and the absorption intensity of CSNW-5 have some improvement and the red
shift occurs. This is due to a heterostructure was formed between CdS and NiWO in the prepared
4
CSNW composites, lending to the redshift of the light absorption of the composite samples [23].
For CSCP-7 and CSNP-7 catalysts, the light absorption range of the composite catalyst was
visibly expanded and the light absorption intensity was significantly increased between 510-800
nm. The hydrogen production performance of catalyst was greatly improved. This is because P
element has strong light capture ability [23], which expands the absorption range of the catalyst.
And by increasing the utilization of sunlight, more photogenerated electron and holes pairs can be
generated [48]. On the other hand, the enhancement of light absorption intensity is related to the
deepening of catalyst color [6].
The band gap of CdS and NiWO samples can be calculated by the following Eq. (6):
4

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DOI: 10.1039/C9NJ05977F
n/ 2
hv  A(hv  Eg )
(6)
α represents absorption coefficient, h represents Planck's constant, ʋ represents light frequency, E
g
represents
band-gap energy and A represents a constant, respectively.
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Fig. 9B presents energy band gap (E ) of samples, the E of CdS, NiWO , CSNW-5, CSCP-7
g
g
4
and CSNP-7 are2.33, 2.24, 2.29, 2.23 and 2.18 eV, respectively. All catalysts have E less than 3.0
g
ev, indicating that they are capable of generating photogenerated electrons and holes through
photoexcitation. Further analysis finds that the decrease of band gap was due to the addition of
CdS nanorods and NiWO . Catalysts with reduced band gap are more easily excited [48], thereby
4
improving hydrogen production performance of catalysts.
Fig. 9C, D exhibits the Mott-Schottky plots of CdS nanorods and NiWO . Since CdS
4
nanorods have a positive slope and NiWO has a negative slope, CdS nanorods is a characteristic
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n-type semiconductor and NiWO belongs to p-type semiconductor. The flat-band potential (E )
4
fb
of CdS and NiWO can be computed to be -0.35 and 1.65 V versus. SCE, respectively. On the
4
basis of previous literature, the ECB of n-type semiconductor is more negative 0.1 V or 0.2 V than
the E ; the E of p-type semiconductor is more positive about 0.1 V or 0.2 V than its E . And
fb
CB
fb
then from the formula of Eq: ENHE = ESCE + 0.24 V. The ECB of CdS nanorods and EVB of NiWO4
are calculated as -0.31 and 2.09 V versus NHE. According to the formula used to estimate the VB
positions of CdS nanorods and CB positions of NiWO : E = EVB - E . The final calculation
4
CB
g
results are shown in Table 4.

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Fig. 9 (A) The UV-vis DRS of CdS, NiWO
NiWO , CSNW-5, CSCP-7, CSNP-7. Mott-Schottky plots of (C) n-type CdS and (D) p-type NiWO
Table 4 Band gap energy, valence and conduction band edge potential of CdS and NiWO
4
, CSNW-5, CSCP-7, CSNP-7; (B) The forbidden band width of CdS,
4
4
.
4
.
Conduction band ECB
eV)
Semiconductor
CdS (n-type)
Band energy Eg (eV)
Valence band EVB (eV)
(
2.30
2.07
1.94
2.09
-0.31
0.02
4
NiWO (p-type)
3
.9 Proposed Reaction Mechanism
Firstly, the band gap positions of CdS and NiWO4 are drawn according to Table 4. The
energy band structure of composite catalyst CSNP-7 before and after touch is shown in Fig. 10.
Generally, CdS as n-type semiconductor, its Fermi level draw near to CB position. On the contrary,
for the p-type semiconductor, Fermi level of NiWO is approach to VB position [41]. When CdS
4
nanorods is close contact with NiWO , because Fermi level of CdS is lower than that NiWO ,
4
4
which affords a thermodynamic access to speed up charge transfer [6]. The two Fermi levels
gradually approach equilibrium. Finally, CdS and NiWO form p-n heterojunction and constructs
4
internal electric field from CdS to NiWO . The CdS and NiWO are excited by visible light
4
4
irradiation to produce electrons and holes, where both electrons move from VB to CB and holes
remain in their VB. Because of existence of potential difference and the internal electric field, the
electrons of the CB of NiWO can be rapidly transferred to CB of CdS, and further shifted to
4
surface of CdS. CoP nanoparticles as a competent electron trapping agent, it can capture and
accelerate electron transfer on the surface of CdS. Subsequently, the electron reacts with protons

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in the water to form H . In the meanwhile, the photogenerated holes on VB of CdS trans
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VB of NiWO , the lots of holes assembled on the surface of NiWO are expended by lactic acid,
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+
while lactic acid is oxidized to produce CO and H O . In addition, there is also a portion of CoP
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nanoparticles are adhered onto the surface of NiWO , part of electron can directly transfer from
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the CB of NiWO to CoP nanoparticles, and then they react with the protons to form H . In a word,
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taking advantage of CoP as a cocatalyst is used to modify CSNW hetero-junction, it can increase
separation efficiency of electrons and holes to provide more active sites, and enhance the
absorbance of the catalyst, thereby improving of H production activity of the composite catalyst.
2
Fig.10 Schematic illustration of charge transfer for CSNP-7
4
. Conclusion
In this work, CdS/NiWO p-n hetero-junction is successfully modified by CoP nanoparticles.
4
When 10% lactic acid used as a sacrificial agent, the composite catalyst CSNP-7 shows the
-1
-1
highest hydrogen production rate (47.7 mmol h g ), which is 11.9 times and 3.3 times that of
CdS (4.0 mmol h-1 g-1) and CSNW-5 (14.4 mmol h-1 g-1). The remarkable improvement in
hydrogen production performance can be ascribed to the CoP nanoparticles as an electron trapping
agent, it can accelerate the transfer of electrons and provide more active site. In addition, the
composite catalyst CSNP-7 has remarkable optical and electrical properties, which have been

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investigated by UV-visible diffuse reflectance, test photoluminescence and photoelectro
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detection. The present study is expected to provide novel insights for using co-catalyst modified
the heterojunction to produce hydrogen efficiently.
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Author contributions
Hua Liu and Teng Yan conceived and designed the experiments; Hua Liu and Teng Yan
performed the experiments; Zhiliang Jin, Qingxiang Ma contributed reagents/materials and
analysis tools; and Hua Liu and Teng Yan wrote the paper.
Conflicts of interest
The authors declare that they have no competing interests.
Acknowledgements
This work was financially supported by the Chinese National Natural Science Foundation
(21862002, 41663012), the Open Project of State Key Laboratory of High-efficiency Utilization
of Coal and Green Chemical Engineering, Ningxia University (2019-KF-36), the new technology
and system for clean energy catalytic production, Major scientific project of North Minzu
University (ZDZX201803) and the Ningxia low-grade resource high value utilization and
environmental chemical integration technology innovation team project of North Minzu
University.
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