Hydrothermal synthesis of WO3/CoS2n-n heterojunction for Z-scheme photocatalytic H2evolution

Article abstract of DOI:10.1039/d0nj04021e

One of the most effective methods to improve electron-transfer efficiency and photocatalytic reaction activity is to construct a photocatalyst with a heterojunction interface. Herein, a WO3/CoS2 n-n heterojunction composite catalyst was synthesized, and used in photocatalytic hydrogen evolution, and was optimized by adjusting the pH of the triethanolamine solution, the amount of Eosin Y added, and the content of WO3 in the composite catalyst. After 5 h of photocatalytic reaction, WO3/CoS2 (WCS-2) had a H2 production of 221.15 μmol, which was 80.41 times that of single WO3 and 2.17 times that of CoS2, and had good stability. XRD, SEM, TEM, and XPS characterization results indicated that WO3/CoS2 was successfully prepared. Fluorescence test and electrochemical test results showed that the WO3/CoS2 catalyst had a higher photoelectron transfer efficiency. Finally, it is pointed out that WO3/CoS2 enhanced the photocatalytic activity, which may be due to the electron migration of WO3 and CoS2 following the Z-scheme mechanism. This work presents a new idea for constructing n-n heterojunction to enhance the activity in photocatalytic hydrogen production.

Full text of DOI:10.1039/d0nj04021e

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DOI: 10.1039/D0NJ04021E
Hydrothermal synthesis of WO /CoS n-n heterojunction for Z-scheme
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photocatalytic H evolution
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Lijun Ma , Jing Xu* , Lingjiao Li , Min Mao , Sheng Zhao
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. School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan
50021, PR China.
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. Ningxia Key Laboratory of Solar Chemical Conversion Technology, North Minzu
University, Yinchuan 750021, PR China.
3. Key Laboratory of Chemical Engineering and Technology, State Ethnic Affairs
Commission, North Minzu University, Yinchuan 750021, PR China.
Corresponding author email: wgyxj2000@163.com
Abstract
One of the effective methods to improve electron transfer efficiency and photocatalytic reaction
activity is to construct a photocatalyst with a heterojunction interface. This article synthesized
WO /CoS n-n heterojunction composite catalyst, the photocatalytic hydrogen evolution
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experiment was optimized by adjusting the pH of the triethanolamine solution, the amount of
Eosin Y added and the content of WO in the composite catalyst. After five hours of
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photocatalytic reaction, WO /CoS (WCS-2) had a H production of 221.15 μmol, which was
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80.41 times that of single WO and 2.17 times that of CoS , and had good stability. XRD, SEM,
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TEM and XPS characterization results indicated that WO /CoS was successfully prepared.
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Fluorescence test and electrochemical test results showed that the WO /CoS catalyst had a higher
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photoelectron transfer efficiency. Finally, it is pointed out that the WO /CoS enhanced the
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photocatalytic activity, which may be due to the electron migration of WO and CoS following
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the Z-scheme mechanism. This work supplies a new idea for constructing n-n heterojunction to
enhance the activity of photocatalytic hydrogen production.
Keywords: Photocatalyst; WO /CoS ; n-n heterojunction; Z-scheme
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1
. Introduction

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Due to people's extensive use of non-renewable energy sources such as petroleum, it h
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a string of environmental contamination and energy shortage problems [1,2]. Hydrogen energy is
a clean and efficient new energy [3,4]. Therefore, the hydrogen production technology using
catalysts to decompose water under visible light has received extensive attention from researchers,
and research in this area has got tremendous achievements [5-7]. The catalyst plays a decisive role
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in the photocatalytic system [8]. Generally, semiconductor nanomaterials (such as TiO , CdS, etc.
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9,10]) are used as photocatalysts, they absorb photon energy under visible light and excite
photoelectrons to participate in the hydrogen evolution reduction reaction in water [11]. However,
since Honda and Fujishima observed photocatalysis in 1972 [12], there are still some problems in
the field of photocatalytic hydrogen evolution. For example, semiconductor materials have low
utilization of sunlight, and electrons and holes are easy to reorganize, limit the development of
photocatalysis [13].
Today, researchers in the field of photocatalysis have reported hundreds of semiconductor
materials that can decompose water under visible light [14,15], among them, TiO , ZnO, Fe O
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and other metal oxides [16-18] have received extensive attention in terms of photocatalytic
hydrogen production due to their low cost and good stability. As a typical metal oxide, WO is
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an ideal photocatalyst with a band gap (E ) of 2.4-2.8 eV [19]. However, because the conduction
g
band (CB) position of WO is relatively positive, it has a poor reducing ability in the
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photocatalytic water decomposition reaction [20]. The smaller band gap also makes it easier for
the electrons and holes inside to recombine, so pure WO has poor catalytic activity under visible
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light [21]. Therefore, some researchers combined WO with other materials to optimize the
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hydrogen production activity, such as WO /ZnCdS [22], WO /g-C N [23], WO /BiVO [24] and
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WO -CuS [25].
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In recent years, transition metal sulfide (WS , NiS, MoS , Co S , CoS , etc.) [26] have shown
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excellent hydrogen production performance in the field of photocatalysis, and such semiconductor
materials are cheap and readily available. The inherent metal conductivity of CoS can accelerate
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the transfer of electrons. Therefore, CoS has excellent catalytic activity in photocatalysis and
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electrocatalysis [27]. So far, CoS has been extensively studied in the field of electrocatalysis, but
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relatively little research has been done on the production of H by photocatalysis [28].
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0.1039/D0NJ04021E
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In this study, a simple hydrothermal method was used to load WO on the surface ofDO
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synthesize a new photocatalytic composite material, which was first applied to photocatalytic
hydrogen production. Experimental results illustrated that in the photocatalytic system with 10%
triethanolamine solution (10% TEOA solution, pH=9) as electronic sacrificial agent and Eosin Y
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(
EY) as sensitizer, the WCS-2 composite had higher H evolution activity than pure WO or CoS
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single catalyst.
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.Experimental section
2.1 Materials
NaCl (Sodium chloride, 99.5%), Na WO ·2H O (Sodium tungstate dihydrate, 99.5%), HCl
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hydrochloric acid, 36-38%), Co(NO ) ·6H O (cobalt nitrate hexahydrate, 99.99%),
3 2 2
Na S O ·5H O (sodium thiosulfate, 99.5%), anhydrous ethanol (99.7%), and deionized water
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(
DW) made by ultrapure water mahine.
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.2. Synthesis of WO3
According to previous reports [19], using hydrochloric acid as a surfactant, WO was prepared
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by hydrothermal method. Weigh 6.14 mmol Na WO ·2H O and 12.70 mmol NaCl dissolved in
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deionized water of 50 mL, then put it on a magnetic stirrer and stirred for 1 h. Concentrated HCl
(8 mL) was added dropwise during the stirring process, the solution gradually became
pale-yellow. Later, the mixture was moved to a 100 mL polytetrafluoroethylene (PTFE) reaction
still, kept at 180 ℃ for 24 h. After cooling, the products were washed 3 times with anhydrous
ethanol and DW, and then dried in an oven for 8 h at 60 ℃.
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.3. Synthesis of CoS2
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mmol Na S O ·5H O and 2 mmol Co(NO ) ·6H O were dissolved in a beaker containing 40
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mL DW, and then added 30 mL of anhydrous ethanol, continuously stirred for 1 h. Then, the
mixture was moved to a 100 mL PTFE reaction still and reacted for 12 h at 180 ℃. After cooling,
the washing and drying process of CoS was the same as that of WO .
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.4. Preparation of the WO /CoS
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Added 100 mg CoS and 55 mg, 58 mg, 60 mg, 62 mg or 65 mg WO (WCS-1,DO
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WCS-3, WCS-4, WCS-5, respectively named) to a beaker filled with 50 mL of deionized water,
sonicated for 20 min and stirred 1 h, transferred to 100 mL PTFE high-pressure reactor at 180 ℃
for 12 h. After natural cooling, washed and dried.
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.5. Characterizations
Advanced X-ray diffraction (Rigaku Smartlab, XRD) and Cu Kα radiation were used to
determine the crystal texture of photocatalysts. Obtained SEM (scanning electron microscope)
images of sample through ZEISS EVO 18 instrument. TEM and HRTEM (high-low magnification
transmission electron microscopy, JEOL JEM-2100) were used to study morphology and lattice.
X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) was used to study the chemical states
of the elements in WCS-2. The photoluminescence (PL) spectrum of the catalyst was obtained
using a fluorescence spectrophotometer (fluoromax-4). With BaSO as a background. UV-visible
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spectrophotometer (Lambda 750 S) was used to obtain the UV-visble diffuse reflection spectrum
(
UV-Vis DRS) of catalyst. A N adsorption-desorption instrument (ASAP 2020 M) was used to
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detect the BET specific surface area (SBET) and pore diameter distribution of the catalyst.
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.6. Photocatalytic activity assessment
The photocatalytic hydrogen production test was performed in a flat-bottomed quartz bottle (65
mL) with sealed rubber pad. A typical experimental procedure was to disperse the prepared 10 mg
of catalyst powder in a quartz bottle including 30 mL of TEOA solution (10%, v/v, pH=9), and
then added 16 mg EY, after stirred and sonicated to form a homogeneous solution. Filled with N2
in anaerobic environment for 20 min to discharge other gases in the reaction bottle. Then put the
reaction bottle into the multi-channel photocatalytic reaction system with 5W LED lamps for 5 h
of hydrogen evolution experiment, and used gas chromatograph (SP-2100, TCD, 13X column, N2
carrier) to detect the amount of H precipitation.
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.7. Photoelectrochemical measurement
A standard three-electrode electrochemical workstation (VersaSTA4-400, AMETEK) was used
for electrochemical testing of the photocatalyst. 0.2 mol/L Na SO solution was used as electrolyte
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solution, the counter electrode was Pt electrode and reference electrode was saturatedDO
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electrode (SCE). Working electrode: Dispersed 12 mg of sample powder in 1.5 mL of anhydrous
ethanol and stirred until thick, and evenly coated it on ITO (conductive glass). The effective area
was about one square centimeter. After drying, a working electrode was obtained. Under the
condition of 0.5 V bias and 300 W Xe lamp (CEL-HXF300, λ≥420 nm) as a stable light source,
electrochemical data were measured.
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.Results and discussion
3.1. XRD tests
Fig. 1 showed the XRD pictures of different photocatalysts synthesized. In Fig. 1a, the XRD
diffraction peak of pure WO was consistent with the data of WO standard card (PDF#83-950).
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The diffraction peaks at 27.9°, 32.3°, 36.2°, 39.7°, 46.2°, and 54.9° correspond to the (111), (200),
210), (211), (220) and (311) crystal planes of CoS (PDF#41-1471), respectively. Moreover, in
(
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the red curve (the XRD of WCS-2), both the diffraction peaks of WO and CoS existed and there
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was almost no change in the peak position. The above results indicated that pure WO , CoS and
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WCS-2 composite catalysts were successfully synthesized. The XRD diffraction peaks of the
WO /CoS with different proportions prepared by changing the amount of WO were shown in
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Fig. 1b, the difference in the intensity of diffraction peak may be due to the physical environment
in the synthesis process. In addition, the XRD patterns of the WO /CoS catalysts with different
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ratios had no other peaks except for the diffraction peaks of WO and CoS , showing that the
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synthesized WO /CoS had a relatively high purity.
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Fig. 1. XRD pattern: (a) WO , CoS , WCS-2 and (b) WO /CoS with different proportions.

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DOI: 10.1039/D0NJ04021E
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.2. Morphology and Structure Analysis
Fig. 2 was the SEM and TEM images of the sample. It could be found from Fig. 2a of SEM that
CoS existed as a spherical structure with irregular size, and the surface of the sphere was
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relatively smooth. The WO sample (Fig. 2b) consisted of an irregular nano-block structure.
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Combined with XRD, the main reason for the irregular structure may be that WO had poor
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crystallinity. As shown in SEM (Fig. 2c) and TEM (Fig. 2d) of the WCS-2 composite catalyst,
because a large amount of WO was uniformly and tightly dispersed on CoS sphere, compared
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with the single CoS , the surface of WCS-2 composite catalyst was relatively rough, provided
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abundant active sites, and the close contact between WO and CoS facilitated charge transfer. The
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HRTEM of the WCS-2 composite catalyst was shown in Fig. 2e, the lattice fringe spacing of
0.361 and 0.242 nm, belong to the (200) crystal plane of WO and the (210) plane of CoS ,
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respectively. The energy dispersive X-ray (EDX) spectrum (Fig. 2f) showed that the WCS-2
composite material was composed of Co, O, S, W elements, and the elements were evenly
distributed (Fig. 2g-k). This result indicated that WCS-2 was successfully synthesized.

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Fig. 2. SEM images of (a) WO
3 2
, (b) CoS
and (c) WCS-2. (d) TEM, (e) HRTEM, and (f) EDX spDeOcIt:r1u0m.10o3f9/D0NJ04021E
WCS-2. (g-k) elemental mapping of WCS-2.
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.3. XPS studies
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The elemental valence-state of WCS-2 composite catalyst was analyzed by XPS. In the full
spectrum (Fig. 3e), W, O, S and Co elements were included into the WCS-2. As shown in Fig. 3a,
the XPS of Co 2p can be fitted to four peaks, the two peaks located at the binding energy of 798.1
eV and 781.9 eV can be attributed to Co 2p1/2 and Co 2p , resprctively. And at the binding
3/2
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+
energy of 803.9 eV and 786.7 eV were satellite peaks of Co [29]. In Fig. 3b, the peaks of S 2p
spectrum at 168.6 eV and 169.7 eV corresponded to S 2p3/2 and S 2p1/2 [30]. In Fig. 3c, the peaks
at 37.8 eV and 35.6 eV corresponded to W 4f5/2 and W 4f7/2 respectively, indicated that element W
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in the composite catalyst existed as W [31]. Fig. 3d showed that O 1s had only one peak at 531.8
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eV, which can be attributed to the O ions in WO [32].
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Fig. 3. XPS analysis of WCS-2: (a)Co 2p, (b)S 2p, (c)W 4f, (d)O 1s and (e) Survey spectrum.

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.4. UV-Vis DRS
In order to analyze the optical performance of the catalyst, the UV-Vis DRS of the WO , CoS
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and WCS-X (X=1,2,3,4,5) samples were studied. The results were shown in Fig. 4a (the inset was
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a partial enlargement between 250nm and 500nm). The CoS sample had the highest light
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absorption intensity, and the light absorption edge of the WO sample appeared at about 460 nm,
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and the wavelength between 460-800 nm light response intensity was lower. Compared with the
pure WO sample, the light absorption intensity of the composite material was obviously
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increased, especially between 400-800nm, which further showed that the WO catalyst had been
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successfully loaded on the CoS2 surface. The light absorption intensity of WCS-2 was
significantly higher than other composite materials, and it had a more excellent light absorption
performance in the visible light range. When the load was too large, the agglomeration state of
WO will appear again, which was not conducive to the response to light. In addition, according to
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n
the equation: αhv=A(hv-Eg) (n is 2 or 1/2, hv is the photon energy, α is the absorption coefficient)
[33], the data of the UV-visible diffuse reflection can be processed to obtain Fig. 4b-d, where the
intersection of the tangent and the x-axis was the E of the catalyst. It could be seen that the E of
g
g
WO , CoS and WCS-2 were 2.41, 2.14 and 2.04 eV, respectively. Compared with the other two
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pure substances, the E of the WCS-2 photocatalyst was reduced. Moreover, the light absorption
g
edge of WCS-2 showed a clear red shift compared to WO . These results indicated that the
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synergy between WO and CoS can improve the utilization of visible light.
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1
1
1
1
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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
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4
4
4
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5
5
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5
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6
0
1
2
3
4
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9
0
1
2
3
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9
0
1
2
3
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
g 3 2
Fig. 4. (a) UV-Vis DRS of all samples; (b) E of WO , (c) CoS , and (d) WCS-2.
3
.5. BET Analysis
The N adsorption-desorption isotherms and pore diameter distributions of WO , CoS and
2
3
2
WCS-2 were shown in Fig. 5. As shown in Fig. 5a, three photocatalysts were type IV isotherms
with H3 hysteresis loop, which indicated that all the three samples were mesoporous materials. As
shown in Fig. 5b, the pore size of the three catalysts was concentrated at 2-50 nm, and also from
Table 1, the average pore size of WO , CoS and WCS-2 were 11, 12 and 15 nm respectively,
3
2
further illustrating the sample all were mesoporous materials. In addition, it can be found from
2
Table 1 that the SBET of WCS-2 was 14.83 m /g, which was significantly higher than that of WO
3
2
2
(
9.24 m /g) and CoS (7.54 m /g). This might be due to the fact that the agglomerated state of
2
WO was broken after being compounded with CoS and uniformly dispersed on the surface of the
3
2
CoS spherical particles. Similarly, WCS-2 had the largest pore volume. For WCS-2 complex
2
catalyst, a larger SBET could provide more active sites, which might be beneficial to photocatalytic
reactions.

New Journal of Chemistry
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9
0
1
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7
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9
0
1
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3
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 5. (a) N
2
3 2
adsorption-desorption isotherm, (b) pore diameter distribution curves of WO , CoS and WCS-2.
Table 1. The SBET, pore volume (P-V) and pore size (P-S) for three catalyst.
2
3
Catalyst
WO3
SBET(m /g)
P-V (cm /g)
P-S (nm)
11.013
15.014
12.103
9.238
7.540
14.828
0.025
0.026
0.048
CoS2
WCS-2
3
.6. PL analysis
The hole-electrons separation efficiency in the catalyst can be analyzed by the quenching of
fluorescence in the PL spectrum. Generally, the better the photocatalytic activity of catalyst, the
lower the steady-state fluorescence intensity and the higher the hole-electrons separation
efficiency. The PL spectra of all samples were shown in Fig. 6, in TEOA solution (10%, pH=9),
and under a 480 nm excitation wavelength, the highest emission peak with a peak intensity of 537
nm appeared in the presence of EY alone. After adding WO or CoS single catalyst to the
3
2
solution, the peak intensity of PL decreased slightly. However, when WO was introduced on the
3
surface of CoS catalyst, the peak intensity of various composite catalysts was obviously
2
weakened, indicating that the prepared composite material, especially WCS-2, had better
fluorescence quenching effect than single catalyst. However, the peak intensity of WCS-3, 4, and
5 was higher than that of WCS-2. This may be because after loading excessive WO , a new
3
electron-hole recombination center was formed between WO and CoS . The results showed that
3
2

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d
the formation of n-n heterojunction between WO and CoS effectively inhibited the elec
D
t
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n
10
a
n
3
2
hole recombination, promoted the photogenic electron transfer. Therefore, it was beneficial to
improve the H evolution activity of the photocatalyst.
2
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
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5
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0
1
2
3
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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
Fig. 6. Steady-state fluorescence of all samples.
3
.7. Hydrogen production experiment
For researching the photocatalytic activity of the prepared WO /CoS , in Fig. 7a-b, the H
2
3
2
evolution of different catalysts under the same conditions were compared. As shown in Fig. 7a,
hydrogen evolution of pure WO and CoS at the fifth hour was 2.75 μmol and 101.97 μmol,
3
2
respectively. The WCS-2 composite catalyst produced the highest amount of hydrogen, reaching
21.15 μmol in the fifth hour, which was 80.41 times that of WO and 2.17 times that of CoS
2
3
2
under the same conditions. The WCS-2 had excellent hydrogen evolution property, which was
related to a good synergy between WO and CoS . It can be seen from Fig. 7b that when the
3
2
amount of CoS was fixed, the loading of WO with different qualities had a significant influence
2
3
on the H evolution activity of the WO /CoS catalyst. Compared with pure CoS , with increasing
2
3
2
2
WO loadings, hydrogen evolution had a tendency to increase first and then decrease. This may be
3
because the surface of CoS changes from smooth to rough after loading an appropriate amount of
2
WO This defect could be used as a center for trapping electrons, which facilitated the separation
3
.
of electrons and holes. And excessive WO was loaded on the CoS surface, which masked the
3
2
active site of material, thereby reducing the hydrogen evolution activity. Compared with other
composite catalysts containing WO or CoS , WCS-2 had better H production performance. The
3
2
2

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DOI: 10.1039/D0NJ04021E
comparison results were shown in Table 2.
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1
2
3
4
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7
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0
3 2
Fig. 7. Photocatalytic hydrogen evolution comparison of (a)WO , CoS , WCS-2, and (b) WCS-X (X=1, 2, 3, 4, 5).
Table 2. Comparison of the hydrogen production activity of WO /CoS and similar photocatalysts
3
2
previously reported.
Photocatalyst
Light source
5 W LED
Sacrificial reagent
10 vol% TEOA
15 vol% TEOA
Activity (μmol/g·h) Ref.
This
WO /CoS
3
2
(
λ ≥ 420 nm)
00 W Xe-lamp
λ ≥ 420 nm)
05 W LED
λ ≥ 450 nm)
W LED
4423
3900
10990
2929
754
work
[21]
[22]
[34]
[35]
3
WO @ZnIn S
3
2
(
1
WO /ZnCdS
3
(
20 vol% lactic acid
15 vol% TEOA
5
NiS/WO /g-C N
4
3
3
(λ ≥ 420 nm)
300 W Xe-lamp
CdS/(WO ＆WS )
3
2
(
λ ≥ 400 nm)
00 W Xe-lamp
without filter)
10 vol% lactic acid
10 vol% lactic acid
3
WS /WO
2
3
(
680
129
[36]
[37]
4
5mL H O + 25mL
2
TiO /WO /Pt
2
3
300 W Xe-lamp
CH OH
3

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DOI: 10.1039/D0NJ04021E
(
without filter)
3
00 W Xe-lamp
CoS /g-C N
4
10 vol% TEOA
10 vol% TEOA
2
3
(λ ≥ 420 nm)
578
[38]
[39]
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
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4
4
4
4
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5
5
5
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1
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3
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0
1
2
3
4
5
6
7
8
9
0
5
W LED
CoS /FeLaO
2
3
(λ ≥ 420 nm)
5594
In previous reports, the hydrogen evolution activity of the semiconductor catalyst in
EY-sensitized system was affected by pH and the quality of EY. Therefore, the pH of 10 % TEOA
solution was adjusted or the mass of EY was changed during WCS-2 photocatalytic hydrogen
production experiment. As shown in Fig. 8a, when pH=9, the WCS-2 material had the best
hydrogen production performance. As the pH value increased, hydrogen production gradually
+
decreased. This was because in a more alkaline solution, the consistence of H was low, the
driving force for H production was suppressed, thereby reducing the H evolution activity of the
2
2
+
catalyst. And when the pH was relatively low, there were too many H , the solution may be
protonated, and reduced electronic supply capacity, thereby reducing the efficiency of exciting EY
molecules [40].
In Fig. 8b, the hydrogen production effect was the best when added 16 mg EY. The hydrogen
production of WCS-2 tended to increase first and then reduce as the mass of EY increases.
Because increasing the amount of EY was beneficial for the catalyst to adsorb dye on the surface
and accelerated electron transfer, but excessive addition of EY would cause free EY molecules to
absorb the incident light, reducing the utilization rate of the catalyst to the light source, thereby
reducing the photocatalytic performance [39].
Fig. 8c was a 20 h stability test of WCS-2 composite catalyst. The WCS-2 had the best
hydrogen evolution property, in the first cycle. In contrast, the H evolution of the second and
2
fourth cycles was markedly reduced, and before the third cycle, 3 mg EY was added, and the H2
evolution was the same as the second. Compared with the circulation, there was a recovery, which
may be caused by the loss of part of the catalyst during centrifugation, and the prolonged light

New Journal of Chemistry
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decomposition of EY. Fig. 8d tested the XRD patterns of the WCS-2 before and
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photocatalytic experiment. The XRD before and after hydrogen production did not change
significantly, showing that the crystalline phase structure of WCS-2 did not change after the
experiment. The results showed that the photocatalytic property of WCS-2 composite material was
comparatively stable.
0
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7
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9
0
Fig. 8. (a) Optimize PH (b) Adjust EY concentration (c) Stability test (d) XRD pattern before and after hydrogen
production by WCS-2.
3
.8. Photoelectrochemical experiment
To learn more about the electron transfer mechanism of WCS-2 composite material, the
following electrochemical tests were carried out. From the results of transient photocurrent of
WO , CoS and WCS-2 samples (Fig. 9a), it can be seen that under the action of four visible light
3
2
(
λ≥420nm) switching cycles, there were photocurrent response curves (I-T curve), and all the
photocurrent response were uniform. It explained that WO , CoS and WCS-X can all be used as
3
2
an excellent photocatalyst. Compared with the two single photocatalysts, the WCS-2 catalyst had
the highest photocurrent intensity, which indicated that under visible light, WCS-2 samples had
faster electron and hole transfer speeds and lower recombination rates. Moreover, the photocurrent

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intensity trend of other composite catalysts was consistent with the hydrogen productionDOacI:
t
1
i
0
v
.1
i
039/D0NJ04021E
ty
.
This showed that loading an appropriate amount of WO can effectively promote the separation of
3
photogenerated carriers and improve the activity of hydrogen production.
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
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0
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2
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2
3
4
5
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8
9
0
The photocatalytic activity depends highly on the over-potential of the photocatalytic reaction
[
41], so the linear scanning voltammetry (LSV) curves of WO , CoS and WCS-X samples were
3 2
tested. It can be found from Fig. 9b, under the same voltage conditions, the overpotential of each
composite catalyst was lower than that of two single catalysts. Among them, the over-potential of
the WCS-2 sample was the lowest, indicating that the photocatalytic activity of WCS-2 was the
best, which was consistent with the hydrogen evolution activity, and can indicate that WCS-2 was
an excellent photocatalytic material.
Fig. 9c showed the electrochemical impedance spectroscopy (EIS) of WO , CoS and WCS-X.
3
2
The radius of curvature reflected the magnitude of the resistance received during charge transfer.
A small arc radius indicated fast charge transfer. In Fig. 9c, the arc radius of each composite
catalyst was significantly reduced after WO was doped, indicating that the introduction of WO
3
3
could accelerate the transfer of electrons, thereby improving the photocatalytic activity. In
particular, WCS-2 had the smallest arc radius, which means that the electrons in WCS-2 had the
least resistance and the fastest electron transfer.
2 4
Fig. 9. (a) I-T, (b) LSV and (c) EIS curves of the prepared sample in 0.2 M Na SO solution.
The flat band potential (E ) of a single catalyst was tested using Mott-Schottky (M-S) curve
fb
[
42]. As shown in Fig. 10a and 10b, it can be seen from the positive slope of the curves that both
WO and CoS were n-type semiconductors [43], and measured against SCE, the E of WO and
3
2
fb
3
CoS were approximately 0.56 V and -0.53 V. Generally, the E of a n-type semiconductor is 0.2
2
fb

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V or 0.1 V lower than the ECB potential. Consequently, relative to SCE, the ECB of WO3
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can be estimated as 0.36 V and -0.73 V, then used ENHE=ESCE+0.241 V [44] formula to correct it
to the value under the NHE (normal hydrogen electrode), the ECB of WO was 0.60 V and the E
3
CB
of CoS was -0.49 V. Finally, used the formula E =E +E combined with E ( Fig. 4b and 4c,
2
VB
CB
g
g
0
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2
3
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2
3
4
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9
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WO was 2.41 eV, CoS was 2.14 eV), calculated the WO and CoS valence band (E ) were
3
2
3
2
VB
3
.01 V, 1.65 V.
Fig. 10. M-S plots of (a) WO
3
2
and (b) CoS .
3.9. Photocatalytic reaction mechanism
Based on the above results, it was speculated that the photocatalytic mechanism of WCS-2
material in the EY system. After the light irradiation (Fig. 11), the EY adsorbed on WCS-2 formed
1
1
the singlet excited state EY *. Then through a system transition, EY * became a triplet excited
3
3
-
-
state EY *. TEOA provided electrons to quench and reduce EY * to EY *. Electrons of EY * were
further moved to the surface of WCS-2 and restored to EY due to the loss of electrons. Electrons
+
on the surface of the WCS-2 sample reacted with H in TEOA solution to generate hydrogen.
Meanwhile, after absorbing photons, WO and CoS both generated electrons and removed from
3
2
the VB to the CB, leaving holes in the VB. At this time, if it was a traditional type II
heterojunction (Fig. 11a), electrons in the CB of CoS would moved to the CB of WO . However,
2
3
these electrons in the CB of WO were not conducive to the reduction of H O to H , the reason
3
2
2
+
was that the standard redox potential of H /H (0.00 eV vs. NHE) was more negative than the
2
conduction band potential of WO (0.60eV vs. NHE), which was in agreement with the previous
3
H evolution results of WO single catalyst. Therefore, the electron transfer mechanism of type II
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3

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heterojunction was negated. In addition, the Z-scheme electron transfer mechanism was
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Fig. 11b, electrons in the CB of WO were moved to the VB of CoS to combine with the holes,
3
2
and the electrons in the CB of CoS reduced protons to H , and the holes in the valence band of
2
2
WO were depleted by the TEOA. This mechanism of Z-scheme effectively promoted the
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
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6
0
1
2
3
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8
9
0
1
2
3
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5
6
7
8
9
0
1
2
3
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1
2
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5
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0
1
2
3
4
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8
9
0
photocarrier transfer of the WCS-2 and enhanced the photocatalytic hydrogen production activity,
which was in line with the experimental results of WCS-2 with higher hydrogen evolution activity
than the single photocatalyst. Therefore, it can be seen that WO /CoS n-n heterojunction may
3
2
follow the Z-scheme system.
Fig. 11. The mechanism of WO
3 2
/CoS n-n heterojunction decomposing water under visible light was speculated:
(a)Type II heterojunction, (b) Z-scheme system.
4
. Conclusion
In summary, it was proved that the hydrothermal method had successfully synthesized the
WO /CoS composite material. After WO was supported on the CoS surface, the catalyst surface
3
2
3
2
became rough and the specific surface area increased, which not only enhanced the adsorption
capacity of EY molecules but also provided multiple active sites. Compared with pure WO and
3
CoS , WO /CoS had excellent photocatalytic H evolution performance, and it can be known
2
3
2
2
3
from the hydrogen production experiment that the loading of WO had a large influence on the H2
evolution performance of the WO /CoS . Among them, the sample (WCS-2) synthesized with 58
3
2
mg WO and 100 mg CoS had the best hydrogen production performance. In addition, from
3
2
steady-state fluorescence and electrochemical tests (I-T, LSV, EIS), it was known that the

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heterojunction formed at the interface of the composite material suppresses the recombi
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photoelectrons and holes, and facilitated electron transfer. This work showed that the WO /CoS
3
2
composite material of the Z-scheme system was a stable new photocatalyst.
Conflicts of interest
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There are no conflicts to declare.
Acknowledgements
This work was financially supported by Xixia District Science and Technology Plan Project
XXKJ1901). This work also was supported by the New Catalytic Process in Clean Energy
(
Production (ZDZX201803), and the Natural Science Foundation of Ningxia Province (NZ17262).
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DOI: 10.1039/D0NJ04021E
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WO and CoS form n-n heterojunctions, and the synergy between them provides a new
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hydrogen-producing active center for each.