Use of synergistic effects of the co-catalyst, p-n heterojunction, and porous structure for improvement of visible-light photocatalytic H2 evolution in porous Ni2O3/Mn0.2Cd0.8S/Cu3P@Cu2S

Article abstract of DOI:10.1016/j.jallcom.2020.155569

The efficient separation and transfer of photogenerated charge carriers play indispensable roles in improving the photocatalytic H₂ evolution activity. Herein, we designed a Ni₂O₃-modified Mn_0.2Cd_0.8S/Cu₃P@Cu₂S (MCS/CPS) p-n heterojunction structure with a porous morphology for efficient and stable photocatalytic H₂ evolution under visible-light irradiation. Novel porous Cu₃P@Cu₂S was obtained using a vulcanization method and Cu₂S nanoparticles were grown uniformly in situ on the surface of Cu₃P. Ni₂O₃ was adopted as a co-catalyst on the MCS/CPS p-n heterojunction surfaces to promote electrons transfer. Due to the synergistic effects of the co-catalyst, p-n heterojunction, and porous morphology, the as-synthesized Ni₂O₃/MCS/CPS composite with 9 wt% Ni₂O₃ and 2.5 wt% CPS exhibits an optimal photocatalytic H₂ evolution rate of 9.2 mmol g^?1 h^?1, which is 14.4 and 2.4 times higher than those of pure MCS and 9%Ni₂O₃/MCS, respectively. Meanwhile, the optimal sample exhibits an apparent quantum efficiency of 33.5% at 420 nm and an excellent stability under 20 h of irradiation. Moreover, a possible mechanism for the improved photoactivity of the as-synthesized Ni₂O₃/MCS/CPS composite has been discussed in this paper.

Full text of DOI:10.1016/j.jallcom.2020.155569

Journal Pre-proof
Use of synergistic effects of the co-catalyst, p-n heterojunction, and porous structure
for improvement of visible-light photocatalytic H evolution in porous Ni O /
2 3
2
Mn Cd S/Cu P@Cu S
.2 0.8
0
3
2
Dafeng Zhang, Yunxiang Tang, Xiaoxue Qiu, Jie Yin, Changhua Su, Xipeng Pu
PII:
S0925-8388(20)31933-2
DOI:
https://doi.org/10.1016/j.jallcom.2020.155569
JALCOM 155569
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 29 March 2020
Revised Date: 4 May 2020
Accepted Date: 8 May 2020
Please cite this article as: D. Zhang, Y. Tang, X. Qiu, J. Yin, C. Su, X. Pu, Use of synergistic effects of
the co-catalyst, p-n heterojunction, and porous structure for improvement of visible-light photocatalytic
H evolution in porous Ni O /Mn Cd S/Cu P@Cu S, Journal of Alloys and Compounds (2020),
2 3 0.2 0.8
2
3
2
doi: https://doi.org/10.1016/j.jallcom.2020.155569.
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CRediT author statement
Dafeng Zhang: Conceptualization, Writing - Original Draft,
Yunxiang Tang: Investigation, Validation
Xiaoxue Qiu: Investigation
Jie Yin: Resources, Formal analysis
Changhua Su: Resources, Formal analysis
Xipeng Pu: Conceptualization, Writing - Review & Editing

Use of synergistic effects of the co-catalyst, p-n heterojunction, and porous structure for
improvement of visible-light photocatalytic H evolution in porous Ni O /Mn Cd S/Cu P@Cu S
2
2
3
0.2
0.8
3
2
∗
Dafeng Zhang, Yunxiang Tang, Xiaoxue Qiu, Jie Yin, Changhua Su, Xipeng Pu
School of Materials Science and Engineering, Shandong Provincial Key Laboratory of Chemical Energy
Storage and Novel Cell Technology, Liaocheng University, Liaocheng 252000, P. R. China
Abstract
The efficient separation and transfer of photogenerated charge carriers play indispensable roles in
improving the photocatalytic H2 evolution activity. Herein, we designed
a Ni O -modified
2 3
Mn Cd S/Cu P@Cu S (MCS/CPS) p-n heterojunction structure with a porous morphology for efficient
0
.2
0.8
3
2
and stable photocatalytic H evolution under visible-light irradiation. Novel porous Cu P@Cu S was
2
3
2
obtained using a vulcanization method and Cu S nanoparticles were grown uniformly in situ on the surface
2
of Cu P. Ni O was adopted as a co-catalyst on the MCS/CPS p-n heterojunction surfaces to promote
3
2
3
electrons transfer. Due to the synergistic effects of the co-catalyst, p-n heterojunction, and porous
morphology, the as-synthesized Ni O /MCS/CPS composite with 9 wt% Ni O and 2.5 wt% CPS exhibits
2
3
2
3
−1
−1
an optimal photocatalytic H evolution rate of 9.2 mmol g h , which is 14.4 and 2.4 times higher than
2
those of pure MCS and 9%Ni O /MCS, respectively. Meanwhile, the optimal sample exhibits an apparent
2
3
quantum efficiency of 33.5% at 420 nm and an excellent stability under 20 h of irradiation. Moreover, a
possible mechanism for the improved photoactivity of the as-synthesized Ni O /MCS/CPS composite has
2
3
been discussed in this paper.
Keywords: Mn Cd S; Cu P@Cu S; Co-catalyst; p-n heterojunction; Photocatalytic H evolution
0.2
0.8
3
2
2
∗
Corresponding author: Tel: +86-635-8230915
E-mail address: xipengpu@hotmail.com (X. Pu)
1

1
. Introduction
Hydrogen (H ) as an ideal alternative to fossil fuels, has long been considered as one of the most
2
promising, renewable and environmentally friendly energy carriers [1, 2]. Traditionally, H was obtained
2
via the gasification of coal and the catalytic conversion of heavy oil or natural gas. However, these methods
are not only inefficient, but also require significant additional energy, rendering it impossible to produce H2
commercially on a large scale. Owing to the recent materials advancements with regard to semiconductors,
generation of H from water by using solar energy has attracted significant attention [3, 4]. The key to
2
photocatalytic H2 evolution is the development of cheap, highly efficient, and visible-light-driven
photocatalysts [5]. Thus far, various photocatalysts have been developed, including metal sulfides [6-8],
metal oxides [9], carbon nitride [10] and metal organic frameworks [11, 12]. Among these materials,
Mn Cd S solid solutions have attracted significant attention based on their tunable compositions as well as
x
1-x
a moderate band gap, and excellent visible light absorption capacity [13]. However, Mn Cd S alone does
x
1-x
not exhibit excellent photocatalytic activity due to the rapid charge carrier recombination and
photocorrosion. Several approaches, including bandgap engineering [14], co-catalyst loading [15, 16], and
the fabrication of heterojunction with other semiconductors [17-19], have been used to improve the
photocatalytic activity.
In general, co-catalysts not only capture charge carriers to improve separation efficiency but also reduce
the reaction energy barriers for H evolution and provide additional active sites [20]. Noble metals, such as
2
Pd [21], Pt [22], and Au [23], have been developed as efficient co-catalysts for photocatalytic H evolution;
2
however, those materials are too scarce and expensive to be applied on a large scale for photocatalytic
water splitting [24]. Therefore, developing highly efficient and low-cost co-catalyst is crucial. Recently,
nickel and its oxides, such NiO and Ni O , have been proved to be efficient electron traps that can improve
2
3
2

the charge separation of photocatalysis [25]. For example, Shangguan et al. reported that Ni O can act as
2
3
an electron trap and can promote charge separation to enhance photocatalytic performance [26]. Li’s group
reported that the photocatalytic activity of Cd Zn S/O-MoS heterojunction was improved by NiO
x
1-x
x
2
decoration [27]. Therefore, NiO can be used as an eminent co-catalyst for improving photocatalytic H2
x
evolution activity.
The photocorrosion of sulfide semiconductors is an unavoidable problem that reduces their photostability.
Constructing heterojunctions with other semiconductors for facilitating the separation and transfer of
photogenerated charge carriers has been proved to be an efficient approach to improve photocatalytic
activity and photostability [28]. Recently, Cu-based materials, such Cu P [29], CuS [30] and Cu S [6], have
3
2
been applied in the photocatalytic field based on their low cost and natural abundance. Thus far, several Cu-
based heterojunction photocatalysts, including Cu P/g-C N [29], CuS/ZnS [30], and Cu S/CdS [31], have
3
3
4
2
been reported. Additionally, the recent studies indicated that the synergy of P-S anions can provide a
powerful mechanism for catalyst production [32, 33]. For example, Fu and coworkers reported that
MoS @MoP core-shell heterojunctions can serve as high-performance catalysts for H evolution reactions
2
2
[
34]. Wang et al. reported that MoS -MoP heterojunctions on polymer-derived carbon substrates show
2
greater H evolution activity than that of pure MoS or MoP and that this improved catalytic performance
2
2
can be attributed to the electronical interactions between MoS and MoP [35].
2
Motivated by these reports, we loaded Cu S nanoparticles onto the surface of porous Cu P to form
2
3
Cu S@Cu P (CPS) heterojunctions using a vulcanization method. Both Cu P and Cu S are p-type
2
3
3
2
semiconductors and Mn Cd S (MCS) is a typical n-type semiconductor. MCS/CPS p-n heterojunctions
0.2
0.8
were synthesized by combining MCS and CPS. Additionally, Ni O nanoparticles were introduced as non-
2
3
noble metal co-catalysts, and a porous Ni O /MCS/CPS composite photocatalyst was obtained. As expected,
2
3
3

the Ni O /MCS/CPS composites exhibited improved photoactivity as compared to those of pure MCS and
2
3
Ni O /MCS. The enhanced photoactivity of Ni O /MCS/CPS can be attributed to the synergistic effects of
2
3
2
3
the co-catalyst, p-n junctions between CPS and MCS, and a porous morphology. Additionally, a possible
mechanism for charge transfer over the Ni O /MCS/CPS composite was discussed.
2
3
2
. Experimental section
.1. Synthesis of MCS and Ni O /MCS
2
2
3
MCS was prepared using a solvothermal method [36], as illustrated in Fig. 1. Typically, 1.1417 g of
thiourea (CH N S), 4.3495 g of Cd(Ac) ⋅4H O, and 0.2074 g of Mn(Ac) ⋅2H O were dissolved in 60 mL of
4
2
2
2
2
2
ethylenediamine under magnetic stirring for 1 h. This mixture was then transferred into a 100 mL stainless
steel autoclave and held at 160 °C for 24 h. After cooling to room temperature, the resulting precipitates
were collected and washed several times with deionized water and ethanol, and then dried at 80 °C for 12 h.
The Ni O /MCS was prepared using an in situ photo-deposition method. Specifically, 0.3 g of MCS and
2
3
a certain amount of NiCl ⋅6H O (containing 1 wt%, 5 wt%, 9 wt%, or 13 wt% of Ni.) were dispersed in 30
2
2
mL of ethanol and ultrasonicated for 30 min. Subsequently, the mixed solution was injected with nitrogen
for thorough degassing and then irradiated under a 300 W Xe lamp (λ > 400 nm) for 30 min with stirring.
These samples were denoted as x%Ni O /MCS (x = 1, 5, 9, and 13).
2
3
2
.2. Synthesis of Cu P and CPS composite
3
As illustrated in Fig. 1, Cu(OH) was first prepared using a simple precipitation method. Typically, 1.208
2
of g Cu(NO ) and 0.2 g of NaOH were dissolved in 100 mL and 20 mL of deionized water, respectively.
3
2
Next, the aqueous solution of NaOH was slowly poured into the aqueous solution of Cu(NO ) under
3
2
constant stirring. Cu(OH) was then obtained via filtration and dried at 60 °C for 8 h under vacuum. Next,
2
0
.3 g of Cu(OH) and 1.5 g of NaH PO were ground in an agate mortar and transferred into a porcelain
2 2 2
4

-1
boat. The mixture was then calcined at 300 °C for 2 h at a rate of 2 °C min in a N atmosphere. After
2
cooling, the black Cu P was washed several times with deionized water and ethanol, and then dried at 80
3
°C for 12 h.
To prepare the CPS composite, 0.15 g of Cu P and 1.5 g of CH N S were placed in square combustion
3
4
2
boats. Next, the two square combustion boats were transferred into a tube furnace with CH N S loaded
4
2
-1
upstream and calcined at 300 °C for 2 h at a rate of 2 °C min in a N atmosphere. Finally, after coolling to
2
room temperature, the CPS composite was obtained.
2
.3. Preparation of 9%Ni O /MCS/CPS composites
2 3
Ni O /MCS/CPS composites were synthesized as illustrated in Fig. 1. First, 0.1 g of 9%Ni O /MCS and
2
3
2
3
a certain amount of CPS (1.0, 2.5, or 5.0 mg) were dispersed in 30 mL of ethanol solution and subjected to
ultrasonication for 1 h (100 W, 40 KHz). After stirring for 4 h, the particles were centrifuged and dried at
6
0 °C for 8 h. Finally, the mixture was transferred into a muffle furnace and annealed in air at 100 °C for 2
h. The obtained composites were denoted as Ni O /MCS/CPS-x, where x = 1, 2.5, and 5. For comparison, a
2
3
Ni O /MCS/Cu P-2.5 composite was prepared using the same procedure.
2
3
3
The corresponding characterizations, photoelectrochemical measurements, and photocatalytic H2
evolution experiments have been presented in the Supporting Information.
Fig. 1. Schematic illustration of the Ni O /MCS/CPS preparation process.
2
3
5

3
3
. Results and discussion
.1. Characterization of the samples
The crystalline structures of the as-prepared photocatalysts were characterized by using X-ray powder
diffraction (XRD). The XRD patterns of the MCS and Ni O /MCS photocatalysts are presented in Fig. S1.
2
3
In the case of pure Cu P, three primary diffraction peaks at 36.0°, 45.1° and 46.1° are attributed to the (112),
3
(
300), and (113) reflections of hexagonal Cu P (JCPDS #71-2261), respectively [37]. After vulcanization of
3
Cu P, in addition to the typical diffraction peaks of Cu P, three new peaks at 37.3°, 45.8° and 48.2° are
3
3
attributed to the (102), (110), and (103) crystal planes of hexagonal Cu S (JCPDS #84-0206), respectively
2
[
38]. These results indicate that the coexistence of Cu P and Cu S phases in the material. Furthermore, the
3 2
diffraction profiles of the Ni O /MCS/CPS composites are similar to those of MCS due to the low Cu P and
2
3
3
Cu S content. With a high CPS content (Ni O /MCS/CPS-5), two additional humps can be observed at 45.1
2
2
3
°
and 46.1 °, which can be ascribed to the Cu P phase. These results indicate that the Ni O /MCS/CPS
3 2 3
composites were successfully synthesized.
Fig. 2. XRD patterns of as-prepared photocatalysts.
The morphologies of the as-synthesized samples were investigated using scanning electron microscopy
SEM). As shown in Fig. 3a, Cu P exhibits a porous structure. After vulcanization of Cu P, Cu S
(
3
3
2
nanoparticles were grown in situ and uniformly decorated onto the surface of Cu P (Fig. 3b), resulting in
3
6

porous CPS. In Fig. 3c, the MCS is composed of nanorods. In the case of Ni O /MCS/CPS-2.5 (Fig. 3d),
2
3
Ni O /MCS particles can be observed on the CPS surface. Notably, the as-synthesized Ni O /MCS/CPS
2
3
2
3
inherits a porous structure, which not only increases the number of active sites, but also enhances light
absorption through multiple reflections in the structure [39]. Ni O nanoparticles are difficult to observe,
2
3
but the presence of Ni O can be confirmed by the following transmission electron spectroscopy (TEM) and
2
3
X-ray photoelectron spectroscopy (XPS) results. Additionally, the energy-dispersive X-ray spectroscopy
(
(
EDS) spectrum confirms the presence of Cd, Mn, S, Ni, O, Cu, and P elements in Ni O /MCS/CPS-2.5
2 3
Fig. 3e). The EDS mapping images show the elemental distribution (Fig. 3f). The distribution patterns of P
and Cu confirm the porous structures of the samples.
7

Fig. 3. SEM images of (a) Cu P, (b) CPS, (c) MCS, and (d) Ni O /MCS/CPS-2.5. (e) EDS spectrum and (f)
3
2
3
EDS mapping images of Ni O /MCS/CPS-2.5.
2
3
Furthermore, high-resolution TEM (HRTEM) was used to investigate the microstructures of the samples
in greater detail, as shown in Fig. 4. For the pure MCS, the interplanar distance of 0.33 nm can be indexed
to the (002) plane (Fig. 4a). In Fig. 4b, the interplanar distance of 0.23 and 0.35 nm can be indexed to the
(
102) and (100) planes of Ni O3 (JCPDS #14-0481) and MCS, respectively. In the case of
2
Ni O /MCS/CPS-2.5 (Fig. 4c), several lattice fringes with withs of 0.33 and 0.35 nm (corresponding to the
2
3
(
002) and (100) planes of MCS, respectively), 0.28 nm ((002) plane of Ni O ) and 0.20 nm ((113) plane of
2 3
Cu P or (110) plane of Cu S) can be observed, confirming the coexistence of MCS, Ni O , Cu P, and Cu S
3
2
2
3
3
2
in the as-synthesized Ni O /MCS/CPS composites.
2
3
8

Fig. 4. HRTEM images of (a) MCS, (b) 9% Ni O /MCS and (c) Ni O /MCS/CPS-2.5.
2
3
2
3
To investigate the chemical compositions and valence states of elements in Cu P and Ni O /MCS/CPS-
3
2
3
2
.5 in greater detail, the samples were characterized using XPS. As shown in Fig. 5a, the XPS survey
spectrum of Ni O /MCS/CPS-2.5 indicates the presence of Cd, Mn, S, Ni, O, Cu, and P. In Fig. 5b, the
2
3
high-resolution Cd 3d spectrum contains two peaks at 404.8 and 411.5 eV, which are attributed to the Cd
2+
3
d5/2 and Cd 3d3/2 of Cd in the Ni O /MCS/CPS-2.5 composite, respectively [40]. The XPS peaks of Mn
2 3
2
+
at 641.5 and 652.1 eV can be attributed to the Mn 2p3/2 and Mn 2p1/2 of Mn , respectively [13]. Two peaks
2-
at 161.1 (S 2p ) and 162.3 eV (S 2p ) are assigned to the S species in the composite (Fig. 5d) [13]. As
3/2
1/2
shown in Fig. 5e, the Ni 2p spectrum can be deconvoluted into four peaks. The peaks at 855.8 and 873.4 eV
are attributed to Ni 2p3/2 and Ni 2p , respectively, which are close to the peaks of Ni O [27]. The two
1/2
2
3
peaks at 861.5 and 879.6 eV are satellite peaks. For the Cu 2p spectrum (Fig. 5f), the two peaks at 932.5
(
(
2p ) and 952.3 (2p ) eV correspond to the Cu P signal. The peaks at 934.3 eV (2p ) and 954.3 eV
3/2 1/2 3 3/2
2+
2p ), as well as the additional satellite peaks, correspond to oxidized copper (Cu ) generated by surface
1
/2
9

oxidation [41, 42]. Fig. 5g presents the P 2p XPS spectrum, where the peaks at 129.0 and 133.3 eV can be
attributed to phosphide species and oxidized species, respectively [43].
1
0

Fig. 5. (a) XPS survey spectrum and high-resolution XPS spectra of (b) Cd 3d, (c) Mn 2p, (d) S 2p, (e) Ni
p, (f) Cu 2p and (g) P 2p of Cu P and Ni O /MCS/CPS-2.5.
2
3
2
3
Fig. 6a shows the ultraviolet-visible (UV-vis) diffuse reflectance absorption spectra (DRS) of the as-
synthesized photocatalysts. Cu P and CPS exhibit strong absorption over the entire visible light range
3
owing to their narrow band gaps, whereas pure MCS exhibits an absorption edge at approximately 530 nm.
For the Ni O /MCS/CPS composites, the absorption edge shows an obvious red shift due to the strong
2
3
absorption of CPS. The enhanced visible light absorption of the Ni O /MCS/CPS composites has a
2
3
beneficial effect on the improved utilization of visible light photons. Additionally, the corresponding
energy band gaps (E ) of MCS and CPS can be calculated according to the Kubelka-Munk function [44], as
g
shown in Fig. 6b. The E values of Cu P, CPS and MCS are estimated to be 1.69, 1.67 and 2.38 eV,
g
3
respectively.
Fig. 6. (a) UV-vis DRS spectra and (b) corresponding Tauc plots of the as-synthesized samples.
Fig. 7 presents the Fourier transform infrared (FTIR) spectra of MCS, 9%Ni O /MCS, and
2
3
−1
−1
Ni O /MCS/CPS-2.5. The absorption peaks in the range of 3000–3500 cm and at 1647 cm are attributed
2
3
to the O-H stretching vibrations of adsorbed water on the samples [45]. The characteristic peaks of MCS at
−
1
6
1
11 cm can be attributed to the typical vibrations of Mn-S bonds and the absorption bands at 1100 and
−
1
383 cm correspond to the vibrations of Cd-S bonds [46-48]. For the 9%Ni O /MCS and
2
3
1
1

Ni O /MCS/CPS-2.5 composites, the absorption peaks are similar to those of pure MCS due to the low
2
3
contents of Ni O and CPS.
2
3
Fig. 7. FTIR spectra of MCS, 9%Ni O /MCS, and Ni O /MCS/CPS-2.5.
2
3
2
3
3
.2. Photocatalytic activity and stability of as-synthesized photocatalysts
The photocatalytic H evolution activities of the as-synthesized photocatalysts were tested under
2
irradiation of a 300 W Xe lamp with a filter (λ > 400 nm), using a 0.35 M Na S/0.25 M Na SO aqueous
2
2
3
solution as a sacrificial agent (Fig. 8 and S2). As shown in Fig. 8b, the pure MCS and CPS exhibit low
−1
−1
−1 −1
photocatalytic H evolution rates of 0.64 mmol g
h
and 0.15 mmol g h , respectively. After loading
2
Ni O as a co-catalyst, the Ni O /MCS composites exhibit enhanced H evolution activities, which can be
2
3
2
3
2
attributed to enhanced charge separation (Fig. S2). The optimal content of Ni O is 9 wt%, resulting in the
2
3
−1
−1
best H evolution rate of 3.8 mmol g h , which is 5.9 times greater than that of the pure MCS. The
2
photocatalytic activities of 9%Ni O /MCS hybridized with different amounts of CPS were investigated
2
3
further. It can be seen that the H evolution activity is further enhanced (Fig. 8a) due to the formation of
2
MCS/CPS p-n heterojunctions. With the increase in CPS content from 1 wt% to 5 wt%, the H evolution
2
−1
rate of Ni O /MCS/CPS first increases and then decreases. The maximum H evolution rate (9.2 mmol g
2
3
2
−1
h ) is obtained at 2.5 wt%. This rate is 14.4 and 2.4 times greater than those of pure MCS and
9
%Ni O /MCS, respectively. The corresponding apparent quantum efficiency (AQE) is 33.5% at 420 nm.
2
3
1
2

Increasing the amount of CPS to 5 wt% results in a significant decrease in the H evolution rate (6.3 mmol
2
−1
−1
g
h ), which can be attributed to excessive CPS hindering the light absorption of Ni O /MCS. For
2 3
comparison, the photocatalytic activity of Ni O /MCS/Cu P-2.5 was also evaluated. The H evolution rate
2
3
3
2
-1
-1
of Ni O /MCS/Cu P-2.5 (5.6 mmol g h ) is clearly lower than that of Ni O /MCS/CPS-2.5. This suggests
2
3
3
2
3
that the formation of a CPS composite has a beneficial effect on the photoactivity of Ni O /MCS/CPS.
2
3
Furthermore, some reported photoactivity of Mn Cd S (0≤x≤1) based photocatalysts for photocatalytic H
2
x
1-x
evolution are listed in Table S1. Apparently, our Ni O /MCS/CPS photocatalysts have a better
2
3
photocatalytic activity than most other reported materials.
To investigate the photostability and reusability of the synthesized photocatalysts, a typical cycle test of
H evolution was performed using a Ni O /MCS/CPS-2.5 sample. As shown in Fig. 8c, after 20 h of
2
2
3
irradiation under visible light, the Ni O /MCS/CPS-2.5 retains almost 100% of its photocatalytic activity.
2
3
In addition, no obvious changes in crystalline structure or morphology can be observed following
photocatalytic reaction (Figs. S3 and S4). This result suggests that the photostability of the
Ni O /MCS/CPS photocatalyst is acceptable.
2
3
1
3

Fig. 8. (a) Photocatalytic H evolution performance and (b) average rates of H evolution of the different
2
2
photocatalysts, as well as (c) cycling tests of the Ni O /MCS/CPS-2.5 composite.
2
3
3
.3. Photocatalytic mechanism
To investigate the separation of photogenerated charge carriers, photoluminescence (PL) and time-
resolved PL (TRPL) decay spectra were recorded. The results are plotted in Figs. 9a and 9b. The PL
intensity of Ni O /MCS/CPS-2.5 is clearly weaker than that of MCS, 9%Ni O /MCS, and
2
3
2
3
Ni O /MCS/Cu P-2.5 under 410 nm light excitation, indicating suppressed charge recombination in
2
3
3
Ni O /MCS/CPS-2.5 (Fig. 9a) [49]. The TRPL decay spectra reveal that the charge carrier lifetime of
2
3
Ni O /MCS/CPS-2.5 is longer than that of pure MCS (Fig. 9b). The long charge carrier lifetime of
2
3
Ni O /MCS/CPS-2.5 ensures that more electrons can participate in photocatalytic reaction [50].
2
3
To verify the improved separation efficiency of charge carriers in Ni O /MCS/CPS-2.5, electrochemical
2
3
characterizations were conducted. As shown in Fig. 9c, the transient photocurrent response of
Ni O /MCS/CPS-2.5 exhibits the largest current density. This enhanced photocurrent response can be
2
3
ascribed to the higher separation efficiency of charge carriers [51], which is consistent with the highest H2
evolution rate. Additionally, the electrochemical impedance spectroscopy (EIS) result for Ni O /MCS/CPS-
2
3
2
.5 reveals a smaller arc radius compared to MCS, 9%Ni O /MCS, and Ni O /MCS/Cu P-2.5 (Fig. 9d),
2 3 2 3 3
1
4

indicating reduced charge transport resistance in the Ni O /MCS/CPS ternary composites [52]. Fig. S5
2
3
presents the linear sweep voltammetry curves of MCS, 9%Ni O /MCS, and Ni O /MCS/CPS-2.5. One can
2
3
2
3
see that Ni O /MCS/CPS-2.5 has a much lower overpotential for H evolution compared to MCS and
2
3
2
9
%Ni O /MCS, indicating that loading Ni O and CPS can effectively increase H evolution reaction
2 3 2 3 2
activity [53, 54].
Fig. 9. (a) PL spectra, (b) TRPL decay spectra, (c) photocurrent densities and (d) EIS Nyquist plot of MCS,
%Ni O /MCS, Ni O /MCS/Cu P-2.5 and Ni O /MCS/CPS-2.5.
9
2
3
2
3
3
2
3
The surface photovoltage (SPV) technique is a powerful technique for understanding the dynamic
behavior of photogenerated charge carriers [5]. As shown in Fig. 10, all samples exhibit responses in the
wavelength range of 300–550 nm, which is consistent with the E values of samples. The as-synthesized
g
composites exhibit stronger SPV signals than pure MCS, indicating that the recombination of charge
1
5

carriers is effectively restrained. In particular, Ni O /MCS/CPS-2.5 exhibits the highest SPV, indicating the
2
3
highly efficient separation of charge carriers.
Fig. 10. SPV spectra of the as-synthesized photocatalysts.
Additionally, the semiconductor types and flat band potentials (E ) of MCS, CPS, and Cu P were
fb
3
measured based on Mott-Schottky plots. It is known that semiconductor types can be concluded from the
slopes of the Mott-Schottky plots. As shown in Fig. 11(a, b), MCS is an n-type semiconductor with a
positive slope, whereas both Cu P and CPS exhibit p-type characteristics with negative slopes. The E
3
fb
values of MCS, Cu P and CPS are approximately −1.08, 0.66 and 0.83 V versus a saturated calomel
3
electrode, respectively (−0.84, 0.90 and 1.08 V versus NHE, respectively). Generally, the conduction band
potentials (E ) of n-type semiconductors and valence band potentials (E ) of p-type semiconductors are
CB
VB
believed to be approximately 0.1 V above and below E , respectively [55]. Therefore, the E value of
fb
CB
MCS as well as EVB values of Cu P and CPS were estimated to be −0.94, 1.0 and 1.18 V vs. NHE,
3
respectively. Based on the E values, the E value of MCS was calculated to be 1.44 V and the ECB values
g
VB
of Cu P and CPS were calculated to be −0.69 and −0.49 V, respectively. According to our previous report,
3
the ECB and EVB values of Cu S are −0.22 and 1.06 V, respectively [38]. Furthermore, the Mott-Schottky
2
1
6

plot of Ni O /MCS/CPS-2.5 exhibits an inverted ‘V-shaped’ profile, indicating the existence of expected p-
2
3
n heterojunctions (Fig. 11c) [56].
Fig. 11. Mott-Schottky plots of (a) MCS, (b) CPS and Cu P (inset), and (c) Ni O /MCS/CPS-2.5.
3
2
3
Based on the analysis above, both the Ni O co-catalyst and formed p-n heterojunctions are responsible
2
3
for the enhanced photocatalytic H evolution of the Ni O /MCS/CPS composites. A possible mechanism
2
2
3
for photocatalytic H evolution is presented in Fig. 12. The energy band structures of the MCS, Cu S, and
2
2
Cu P prior to contact are presented in Fig. 12a. The Fermi levels (E ) of MCS and CPS are close to the
3
f
conduction band (CB) and valence band (VB), respectively. However, when the p-type CPS and n-type
MCS contact each other, a p-n heterojunction is formed at the phase interface (Fig. 12b). Furthermore,
electrons from the n-type region flow into the p-type region and holes migrate from the p-type region to the
1
7

n-type region owing to their different Fermi levels. Consequently, an equilibrium between the Fermi levels
of MCS and CPS is achieved. Under visible light irradiation, photogenerated electrons are generated in
Cu P, Cu S, and MCS, and then transfer from the CB of Cu P to the CB of MCS through the CB of Cu S
3
2
3
2
due to the ECB level differences. Furthermore, electrons migrate to the Ni O co-catalyst due to the
2
3
+
relatively lower overpotential of Ni O [27]. Finally, the accumulated electrons in Ni O can reduce H into
2
3
2
3
H . Conversely, the holes in the VB of the MCS transfer spontaneously to the VB of the CPS and are
2
consumed by sacrificial agents. Combined with the Ni O co-catalyst and formed p-n heterojunctions, the
2
3
synthesized porous structure can capture more incident light through multiple reflections and provide more
active sites, which is also beneficial to improved photoactivity.
Fig. 12. (a) Energy band diagrams of MCS, Cu S and Cu P. (b) Possible charge transfer mechanism for H
2
2
3
evolution over the Ni O /MCS/CPS-2.5 photocatalyst.
2
3
4
. Conclusions
In this study, we successfully synthesized a novel Ni O /MCS/CPS ternary photocatalyst by coupling
2
3
Ni O /MCS and CPS. Cu S nanoparticles were grown in situ and uniformly attached to the surface of Cu P
2
3
2
3
by using the vulcanization method. Ni O /MCS/CPS-2.5 exhibits an optimal photocatalytic H evolution
2
3
2
−1
−1
rate of 9.2 mmol g h , which is 14.4 and 2.4 times higher than those of pure MCS and 9%Ni O /MCS,
2
3
respectively. Additionally, Ni O /MCS/CPS-2.5 exhibits a high AQE of 33.5% at 420 nm and an excellent
2
3
1
8

photostability over 20 h in a Na S/Na SO solution. This improved photocatalytic activity and stability can
2
2
3
be attributed to synergistic effects offered by the p-n heterojunctions that are formed between MCS and
CPS, the favorable co-catalytic activity of Ni O , and a porous structure, which facilitates the separation
2
3
and transfer of photogenerated charge carriers and improves the light absorption. This work provides a new
avenue for the development and design of highly efficient visible-light-driven photocatalysts.
Acknowledgments
This work was supported by Shandong Province Natural Science Foundation, China (No.
ZR2018MEM019).
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[
1
2
6

Highlights
1
2
3
. Ni O /MCS/CPS composite show excellent photocatalytic activity and stability.
2 3
. Ni O as a cocatalyst favors the charge transfer and provides active sites.
2
3
. The p-n heterojunction between MCS and CPS boosts charge separation.

Declaration of interests
☐
The authors declare that they have no known competing financial interests or personal relationships that could
have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be considered as potential
competing interests:
Declarations of interest: none.