Hydrothermal Synthesis of Mn3O4/CoS2 as a Promising Photocatalytic Material for Boosting Visible-Light Photocatalytic Hydrogen Production

Article abstract of DOI:10.1002/pssa.202100025

Photocatalytic hydrogen evolution has received extensive attention for energy conversion and storage of clean energy. Herein, the composite catalyst Mn₃O₄/CoS₂ is successfully prepared by a hydrothermal method. Photocatalytic hydrogen evolution experiments are conducted by adjusting the amount of Mn₃O₄. The results show that the composite photocatalyst Mn₃O₄/CoS₂ has higher photocatalytic hydrogen evolution performance. The hydrogen production of the 50 mg Mn₃O₄/CoS₂ composite catalyst at 5 h is 14.95 times and 1.60 times that of pure Mn₃O₄ and CoS₂, respectively, indicating that the 50 mg Mn₃O₄/CoS₂ composite catalyst has good photocatalytic stability. In addition, the structure, morphology, and composition of the prepared catalysts are characterized by scanning electron microcopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), electrochemical and PL techniques. Compared with Mn₃O₄ and CoS₂, the photocatalytic response of the 50 mg Mn₃O₄/CoS₂ composite catalyst is significantly enhanced, the current density is increased, the fluorescence quenching efficiency is accelerated, and the pore volume and pore size are increased. Therefore, the composite catalyst can accelerate the separation and transfer of photogenerated electrons and holes and improve the photocatalytic efficiency.

Full text of DOI:10.1002/pssa.202100025

RESEARCH ARTICLE
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Hydrothermal Synthesis of Mn3O4/CoS2 as a Promising
Photocatalytic Material for Boosting Visible-Light
Photocatalytic Hydrogen Production
Jing Xu,* Ye Liu, Xuanhao Li, and Yanru Li
1. Introduction
Photocatalytic hydrogen evolution has received extensive attention for energy
Hydrogen (H2) is a renewable energy
source, and it has been widely regarded
for its advantages of nonpollutant emission
and high energy density.[1–3] Solar energy is
a kind of inexhaustible energy, which also
attracts attention. In the past, semiconduc-
tor photocatalysts have been used to con-
vert solar energy into chemical energy,
through solar energy to produce hydro-
gen.[2,4,5] Because this technology neither
depends on fossil fuels nor emits pollu-
tants, it is considered to be an attractive
and promising way to solve global energy
problems.[6] However, the development of
highly efficient and inexpensive photocata-
lysts for photocatalytic hydrogen evolu-
tion remains a great challenge.[7] Initially,
it was the first water splitting system
TiO2 reported by Fujishima and Honda
in 1972,[8] and since then, as photocatalysts,
many materials have made great efforts in
photocatalytic hydrogen evolution. Among
all kinds of photocatalytic materials, Mn3O4
is an environmentally friendly material
conversion and storage of clean energy. Herein, the composite catalyst Mn3O4/
CoS2 is successfully prepared by a hydrothermal method. Photocatalytic
hydrogen evolution experiments are conducted by adjusting the amount of
Mn3O4. The results show that the composite photocatalyst Mn3O4/CoS2 has
higher photocatalytic hydrogen evolution performance. The hydrogen production
of the 50 mg Mn3O4/CoS2 composite catalyst at 5 h is 14.95 times and 1.60 times
that of pure Mn3O4 and CoS2, respectively, indicating that the 50 mg Mn3O4/
CoS2 composite catalyst has good photocatalytic stability. In addition, the
structure, morphology, and composition of the prepared catalysts are charac-
terized by scanning electron microcopy (SEM), transmission electron microscopy
(TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS),
Brunauer–Emmett–Teller (BET), electrochemical and PL techniques. Compared
with Mn3O4 and CoS2, the photocatalytic response of the 50 mg Mn3O4/CoS2
composite catalyst is significantly enhanced, the current density is increased, the
fluorescence quenching efficiency is accelerated, and the pore volume and pore
size are increased. Therefore, the composite catalyst can accelerate the sepa-
ration and transfer of photogenerated electrons and holes and improve the
photocatalytic efficiency.
because of its abundant resources, affordable price, and no pol-
lution to the environment.[9] Furthermore, manganese oxides
have been extensively studied due to their novel structures
and excellent physical and chemical applications.[10] Among
these manganese oxides, Mn3O4 is an interesting material for
researchers because of its remarkable surface area and unique
electronic structure at nanoscale, which is suitable for wide appli-
cations, such as its use as a thermochemical energy storage mate-
rial[11] and many applications in the field of biomedicine.[12]
Mn3O4 is a stable oxide with spinel structure. It also has catalytic
and electrochemical properties.[13,14] For example, it is used for
electrochemical water splitting to produce hydrogen,[15] photode-
gradation of organic pollutants in water,[16] preparation of photo-
electrochemical oxygen evolution reaction,[17] using Mn3O4 as
cocatalyst, etc. In this study, we choose Mn3O4 as a photocatalyst
for hydrogen production from water decomposition. The
improper optical bandgap and easy recombination of photogener-
ated carriers lead to unfavorable photocatalytic performance, so it
is necessary to modify Mn3O4. Recently, transition metal sulfides
(Ni, Co, Fe, and Mn) have attracted wide attention due to their
abundance in the earth and environmental friendliness.[18–22]
Among them, cobalt sulfide compounds have great attraction in
Prof. J. Xu, Y. Liu, X. Li, Y. Li
School of Chemistry and Chemical Engineering
North Minzu University
Yinchuan 750021, P. R. China
E-mail: xujing@nmu.edu.cn, wgyxj2000@163.com
Prof. J. Xu
State Key Laboratory of High-efficiency Utilization of Coal and Green
Chemical Engineering
Ningxia University
Yinchuan 750021, P. R. China
Prof. J. Xu
Ningxia Key Laboratory of Solar Chemical Conversion Technology
North Minzu University
Yinchuan 750021, P. R. China
Prof. J. Xu
Key Laboratory for Chemical Engineering and Technology
State Ethnic Affairs Commission
North Minzu University
Yinchuan 750021, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/pssa.202100025.
DOI: 10.1002/pssa.202100025
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thermal stability, energy storage, and conductivity.[23] There are
cobalt sulfide compounds with different chemical formulas
(Co3S4, CoS, CoS2, and Co9S8) in lithium ion batteries, electroca-
talysis,[24] degradation of organic matter,[25] and dye-sensitized
solar cells (DSSCs) to construct cobalt sulfide/carbon hybrid mate-
rials doped with heteroatoms.[26] They have all been studied.
Among them, CoS2 is considered to be a good catalyst because
of its high electrical conductivity and high activity.[27,28] Here,
we report a novel composite photocatalyst, Mn3O4/CoS2, which
has not been reported before. The composite catalyst Mn3O4/
CoS2 was successfully prepared by a hydrothermal method. In this
experiment, the amount of Mn3O4 added was investigated
in detail; compared with pure Mn3O4 and pure Mn3O4, the com-
posite catalyst 50 mg Mn3O4/CoS2 effectively inhibited the
photoelectron–hole pair recombination and exposed more active
sites, thereby exhibiting good photocatalytic hydrogen evolution
activity and good photocatalytic stability. Therefore, the obtained
Mn3O4/CoS2 composite material has a photocatalytic application
prospect and is a low-cost noble metal–free catalytic material.
2.2.2. Preparation of Composite Photocatalyst Mn3O4/CoS2
The composite catalyst Mn3O4/CoS2 was synthesized by the sec-
ond step hydrothermal method. In the experiment, the afore-
mentioned prepared 20 mg Mn3O4 was weighed and stirred in
a beaker containing 60 mL deionized water. Then, the following
raw materials were weighed: 5 mmol cobalt nitrate hexahydrate
(Co(NO3)2·6H2O), 1.2409 g sodium thiosulfate pentahydrate
(Na2S2O3·5H2O), and 500 mg PVP. These three chemicals were
added into the stirring solution, respectively. The mixed solution
was stirred for 2 h and transferred to a 100 mL autoclave. After
treatment at 160 ꢀC for 24 h, the solution was filtered, washed,
and dried at room temperature. Finally, the Mn3O4/CoS2 com-
posite catalyst was prepared. CoS2 was obtained by the same
method without adding Mn3O4 (Scheme 1).
2.3. Characterization
X-ray diffraction (XRD) was detected with a Rigaku RINT-2000 to
analyze the crystalline structure of the studied photocatalyst
using Cu Kα radiation. The chemical valence and chemical com-
ponents were performed by X-ray photoelectron spectroscopy
(XPS) using ultrahigh vacuum VG ESCALAB 250 electron
spectrometers. The morphology of the as-prepared materials
was obtained by scanning electron microscopy (SEM, Japan
JEOL, JSM-7500 F) operated at 20.0 kV voltage. The photolumi-
nescence spectrum (PL) of the materials was investigated by
2. Experimental Section
2.1. Materials
All materials were analytical grade and there was no purifi-
cation in this work: manganese acetate tetrahydrate
(CH3COO)2·4H2O), sodium hydroxide (NaOH), cobaltous
nitrate hexahydrate (Co(NO3)2·6H2O), sodium thiosulfate
pentahydrate (Na2S2O3·5H2O), Eosin Y (EY, C20H6Br4Na2O5),
triethanolamine (TEOA), polyvinyl pyrrolidone (PVP), and
deionized water was prepared by an ultrapure water machine.
a
FLUOROMAX-4 spectrophotometer (HORIBA Scientific,
France). The physical adsorption values of the catalyst were
obtained at 77 K with an ASAP 2020 M instrument. The UV–vis
diffuse-reflectance spectrum was obtained by a UV–vis spectro-
meter (UV-2500, SHIMADZU).
2.2. Catalyst
2.4. H2 Evolution Experiment
2.2.1. Synthesis of Mn3O4 Photocatalysts
Photocatalytic hydrogen evolution was conducted on a Perfect
Light (PCX50B, λ ≥ 420 nm) equipped with a magnetic stirring
device. The light source was a 5 W LED lamp. In this program,
the 10 mg catalyst powders were placed in 30 mL triethanol-
amine solution (TEOA, 10%, pH ¼ 10). After mixing evenly,
20 mg EY was added to the solution. Then, the mixture was uni-
formly dispersed by magnetic stirring. The process described
earlier was measured in a 65 ml quartz bottle. Before the light
experiment, nitrogen needed to be injected into the quartz bot-
tle to remove oxygen from the bottle. The hydrogen production
A
total of 0.742 g manganese acetate tetrahydrate (Mn
(CH3COO)2·4H2O) was accurately weighed and poured into a bea-
ker filled with 60 mL deionized water, and then stirred on a mag-
netic stirrer. A total of 0.4 g NaOH was taken and slowly poured
into the beaker to form a mixed solution. After mixing for 1 h, the
homogeneous solution was poured into a 100 mL autoclave. Then
the autoclave was put into the oven at 180 ꢀC for 11 h by hydro-
thermal treatment. After, the solution was cooled to room temper-
ature and filtered. Finally, the product was dried in a drying box
for 4 h, and the product Mn3O4 was obtained (Scheme 1).
Scheme 1. Synthesis process diagram of Mn3O4/CoS2 photocatalyst.
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amount was obtained by gas chromatography (SP-2100, 13X
column, TCD, N2 carrier).
3. Results and Discussion
3.1. SEM and TEM Analysis
2.5. Photoelectrochemical Experiments
The morphologies of Mn3O4, CoS2, and 50 mg Mn3O4/CoS2
were observed by scanning electron microscopy (SEM,
FESEM), transmission electron microscope (TEM, HRTEM).
Figure 1a clearly shows that the prepared Mn3O4 displays a par-
ticle size of ꢁ100 nm. The morphology of CoS2 and 50 Mg
Mn3O4/CoS2 composites was basically the same; the large spher-
ical particles were covered by a few small spherical particles
(Figure 1b,c). In the composite catalyst, the characteristic struc-
ture of Mn3O4 may not be observed because of the small amount
of Mn3O4 added.[29] It may also be that the morphology of Mn3O4
is consistent with the larger spherical morphology of CoS2, which
makes it difficult to distinguish. Figure 1d shows a transmission
electron microscopy (TEM) image of a composite sample. The
light gray sheet-like structure in the figure is CoS2, that is, the
area marked by the red line, and the black circular sheet on
the light gray line is Mn3O4, which is the area marked by yellow.
The high-resolution transmission of the composite catalyst is
shown in Figure 1e. The lattice spacing of CoS2 is 0.28 nm, which
Electrochemical experiments were conducted by electrochemical
instruments (CHI 660E, Shanghai Chen Hua Instrument
Company). A Pt electrode was used as the counter electrode,
and a saturated calomel electrode (SCE) as the reference elec-
trode, and the working electrode was obtained on indium tin
oxide (ITO) glass. The ITO was washed with absolute ethanol
and then the ITO was naturally dried. 12 mg sample powder
was placed in absolute ethanol and dispersed by sonication until
the absolute ethanol evaporated and became a viscous liquid at
room temperature. Subsequently, the viscous liquid was uni-
formly coated on the cleaned ITO by a pipette. The obtained
working electrode was dried at room temperature. The support-
ing electrolyte was 0.2
M Na2SO4 aqueous solution. The sample
area of the obtained working electrode was ꢁ1 cm2. The light
source was a 300 W xenon lamp (CEL-HXF300) with a 420 nm
UV cut-off filter.
Figure 1. SEM images of a) Mn3O4, b) CoS2, and c) 50 mg Mn3O4/CoS2 samples. d) TEM images, e) HRTEM images, and f) elemental mapping
micrographs of 50 mg Mn3O4/CoS2 samples, showing the presence of g) Co, h) S, i) O, and j) Mn elements.
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is consistent with the (200) crystal plane of CoS2. The lattice spac-
ing of Mn3O4 is 0.25 nm, which corresponds to the (211) crystal
plane of Mn3O4. To understand the distribution uniformity of
each element in the composite sample, the elemental mapping
micrographs of the composite catalyst are shown in Figure 1g–j,
and there are four elements Mn, O, S, and Co, and each ele-
ment’s distribution is relatively uniform. The content of the
Mn element is small, which is also consistent with the SEM.
778.23 and 793.56 eV are related to Co 2p3/2 and Co 2p1/2 from
CoS2, mainly in the form of Co2þ [32,33]
The binding energies at
.
780.9 and 797.2 eV are in accordance with Co 2p3/2 and Co2p1/2
from Co—S, which may be due to the sample being easily acces-
sible to air. In addition, two peaks locating at 785.2 and 802.3 eV
are the satellite peaks of Co 2p.[34,35] Figure 3c shows that the XPS
peaks of S 2p at 161.37 and 162.62 eV correspond to S 2p3/2 and
S 2p1/2, respectively, confirming the presence of the predominant
form of S2ꢂ in the 50 mg Mn3O4/CoS2 complex.[36] In addition,
two peaks at 167.75 and 168.82 eV of S 2p correspond to sulfide
oxide due to the sulfides being easily affected by contact with
air.[37] The XPS spectrum of O 1s for the composite sample is
shown in Figure 3d. Located at 529.8, 530.44, 531.18, and
532.14 eV are O 1s peaks, corresponding to Mn—O—Mn, lattice
oxygen, surface hydroxyl, and absorbed oxygen.[38,39] In the XPS
spectrum of Mn 2p (Figure 3e), it can be divided into two peaks.
The peaks with binding energies of 641.36 and 652.93 eV were
assigned to Mn 2p1/2 and Mn 2p3/2, respectively, and the differ-
ence in binding energy between the two was 11.57, which is con-
sistent with the literature.[40] As the content of Mn3O4 in the
composite catalyst is relatively small, the peak of Mn is relatively
weak, and the result corresponds to the SEM characterization. All
XPS results further confirmed the successful synthesis of the
composite catalytic material Mn3O4/CoS2.
3.2. Phase Structure Analysis (XRD)
The phase structure of the prepared materials was studied by
XRD, as shown in Figure 2. The Mn3O4 catalyst has several main
diffraction peaks. At 2θ ¼ 18.00ꢀ, 28.88ꢀ, 31.02ꢀ, 32.32ꢀ, 36.08ꢀ,
37.98ꢀ, 44.44ꢀ, 50.70ꢀ, 58.51ꢀ, 59.84ꢀ, and 64.65ꢀ, which can be
well indexed to (101), (112), (200), (103), (211), (004), (220), (105),
(321), (224), and (400) crystal faces of Mn3O4 (JCPDS#24-0734),
respectively. The 2θ of CoS2 samples is 27.8ꢀ, 32.3ꢀ, 36.2ꢀ, 39.8ꢀ,
46.3ꢀ, and 54.9ꢀ, the diffraction peaks of which match well with
(111), (200), (210), (211), (220), and (311) crystal planes of CoS2
(JCPDS 41-1471), respectively. It can be seen that the measured
diffraction peaks of the pure catalyst Mn3O4 and pure catalyst
CoS2 correspond to those on standard cards. In the composite
catalyst 50 mg Mn3O4/CoS2, there are fewer characteristic peaks
of Mn3O4 in the XRD spectrum because the content of Mn3O4 is
relatively low and the dispersion degree is high.[30] Compared
with the original CoS2 and Mn3O4 diffraction peaks, the diffrac-
tion peak intensity of the composite catalysts decreased and the
peaks were relatively broad, which indicated that CoS2 was suc-
cessfully introduced into the mixed photocatalyst.[31]
3.4. Brunauer–Emmett–Teller (BET) Analysis
The BET value of photocatalytic materials was explored by nitro-
gen adsorption–desorption isotherms and pore size distribution,
as shown in Figure 4. As Figure 4a, photocatalysts are shown as
type IV adsorption and desorption curve.[41] At high relative pres-
sure (P/P0 ¼ 0.9–1), the 50 mg Mn3O4/CoS2 composite catalyst
exhibited relatively high adsorption performance, revealing the
existence of accumulated pore.[42] In Figure 4b, the pore size dis-
tribution shows that the photocatalyst mainly exists in the form
of mesoporous structure (2–50 nm), and the pore size distribu-
tion is concentrated. The specific surface area and pore size of the
catalysts are shown in Table 1. The measured specific surface
areas of pure Mn3O4, pure CoS2, and 50 mg Mn3O4/CoS2 cata-
lysts are 24.13, 9.81, and 22.68 m2 gꢂ1, respectively. These
parameters confirm that the introduction of CoS2 has no obvious
effect on the surface area of Mn3O4/CoS2 nanocomposites. It is
further indicated that the surface area may not be an important
factor to enhance the hydrogen production performance of
binary composites Mn3O4/CoS2.[43] The average pore diameters
of pure Mn3O4, pure CoS2, and 50 mg Mn3O4/CoS2 catalysts are
22.68, 26.43, and 33.74 nm, respectively. After introducing CoS2,
the composite catalysts show large pore diameters, which may be
helpful to improve the hydrogen production performance. The
composite catalyst 50 mg Mn3O4/CoS2 has a larger pore volume
and pore size than pure Mn3O4 and pure CoS2. This is because
the load of metallic sulfides facilitates electron transport.
3.3. XPS
XPS is used to characterize the presence and chemical valence of
elements in composite materials. As shown in Figure 3a, the full
spectrum shows the presence of Mn, O, Co, and S elements in
the 50 mg Mn3O4/CoS2 composite. Figure 3b shows the XPS
spectrum of Co 2p in 50 mg Mn3O4/CoS2, which can be decon-
voluted into six peaks with binding energies at 778.23, 780.90,
785.20, 793.56, 797.20, and 802.36 eV. Two apparent peaks at
3.5. Photocatalytic Hydrogen Evolution Activity Analysis
The photocatalytic H2 production performance of the prepared
material was studied. Figure 5a measured the photocatalytic
hydrogen evolution amount of CoS2 with different Mn3O4
Figure 2. XRD patterns of Mn3O4, CoS2, and 50 mg Mn3O4/CoS2
materials.
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Figure 3. XPS patterns of 50 mg Mn3O4/CoS2 materials. a) Survey spectrum, b) Co 2p, c) S 2p, d) O 1s, and e) Mn 2p of 50 mg Mn3O4/CoS2 composite.
contents. As shown in the figure, with the increase of Mn3O4
content from 10 to 50 mg, its hydrogen production activity
increases obviously. When Mn3O4 is 50 mg, its hydrogen produc-
tion reaches the maximum. When Mn3O4 is further increased,
its hydrogen production activity decreases. The 50 mg Mn3O4/
CoS2 material has the highest H2 production activity. A possible
reason for this is that the best proportion of catalysts can expose
more active sites.[44] However, excessive Mn3O4 may cover up the
active site of the catalyst, restrict the absorption and utilization of
light, and thus reduce the amount of hydrogen evolution.[29] In
Figure 5b, the photocatalytic hydrogen production amount of
Mn3O4 and CoS2 was 21.27 and 197.57 μmol in 5 h, respectively.
It can be seen that 50 mg Mn3O4/CoS2 enhanced the photocata-
lytic H2 production performance of CoS2. The hydrogen produc-
tion of Mn3O4 was 318 μmol, which was ꢁ14.95 times higher
than that of Mn3O4. For the EY-sensitized system, the EY content
also plays a vital role on the photocatalytic hydrogen evolution. As
the EY concentration increases, the hydrogen production amount
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Figure 4. a) N2 adsorption–desorption isotherms; b) pore size distribution plot of Mn3O4, CoS2, and 50 mg Mn3O4/CoS2 materials.
Table 1. Parameter obtained from physical adsorption instrument.
of the CoS2/Mn3O4 sample increases (Figure 5c). When EY
reaches 20 mg, the H2 evolution amount of 50 mg CoS2/
Mn3O4 materials reaches the maximum value. The main reason
is that the catalyst-adsorbed EY can absorb more photons and
become the most significant active center. When continuing
to increase the EY concentration, the H2 production capacity
decreases. The reason is that the absorbed EY is saturated. If
Sample
SBET [m2 gꢂ1] Pore volume [cm3 gꢂ1] Average pore size [nm]
Mn3O4
24.13
9.81
0.14
0.07
0.19
22.68
26.43
33.74
CoS2
50 mg Mn3O4/CoS2
22.68
Figure 5. a) H2 evolution amount of the different Mn3O4 in CoS2 catalyst (TEOA, pH ¼ 10); b) the H2 evolution performance of Mn3O4, CoS2,
Mn3O4/CoS2 samples; c) hydrogen production performance with different EY contents; and d) the photocatalytic stability for H2 production of
50 mg Mn3O4/CoS2 sample.
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the EY content continues to increase, the high concentration (EY)
will hinder the absorption of light, thus reducing the light
utilization efficiency and significantly reducing the photocatalytic
performance.[45] The photocatalytic stability of the 50 mg Mn3O4/
CoS2 photocatalyst was also studied. As shown in Figure 5d, the
photocatalytic reaction was conducted for 20 h and detected inter-
mittently every 5 h. During the third and fourth cycles, the hydro-
gen production of the composite catalyst decreased slightly,
which may be due to the partial loss of the sample during cen-
trifugation. The aforementioned results show that our photoca-
talyst 50 mg Mn3O4/CoS2 has stable photocatalytic activity.
immediately and reaches a constant value when illuminated.
When the illumination is stopped, the photocurrent intensity
of the sample drops rapidly to zero. Under visible-light irradia-
tion, the maximum photocurrent density of the 50 mg Mn3O4/
CoS2 composite catalyst electrode is ꢁ15 μA cmꢂ2; compared
with the pure catalysts Mn3O4 and CoS2, the photocurrent inten-
sity of the composite catalyst is significantly enhanced. This indi-
cates that the composite catalyst 50 mg Mn3O4/CoS2 is more
effective in improving the separation efficiency of charge carriers
and is helpful in inhibiting the combination of photogenerated
electrons and holes.[46]
In addition, the electrochemical hydrogen production activi-
ties of EY-sensitized Mn3O4, CoS2 and 50 mg Mn3O4/ CoS2 were
further studied by LSV. As shown in Figure 7b, there is a poor
current response on the ITO electrode, whether at low or high
potential. Mn3O4, CoS2, and 50 mg Mn3O4/CoS2 electrodes have
little difference in current density and low current intensity in
the range of 0.2 to ꢂ0.28 V potential, which is mainly due to
the cathodic current generated when Hþ is reduced to H2
(ꢂ0.5 to 0.6 V).[47] Subsequently, in the range ꢂ0.5 to ꢂ0.6 V volt-
age, it can be observed that the composite catalyst 50 mg Mn3O4/
CoS2 has the highest current density compared with pure Mn3O4
and pure CoS2. This clearly shows that 50 mg Mn3O4/CoS2 can
effectively accelerate the migration of photogenerated electrons,
and the composite can effectively improve the activity of hydro-
gen production.[48]
Electrochemical impedance spectroscopy (EIS) impedance
maps of Mn3O4, CoS2, and 50 mg Mn3O4/ CoS2, are shown
in Figure 7c. The diameter of the semicircle in the curve reflects
the impedance of the interfacial charge transfer. As shown in
Figure 7c, the semicircle diameter of the sample CoS2,
Mn3O4, and 50 mg Mn3O4/ CoS2 decreases once. The results
show that the interface resistance of the single CoS2
electrode is the largest, which is not conducive to the efficient
transfer of charge.[49–52] It can be observed that the order of
resistance of electron transmission is CoS2 > Mn3O4 > 50 mg
Mn3O4/ CoS2, which demonstrates that the electrode impedance
of 50 mg Mn3O4/CoS2 is the smallest. And 50 mg Mn3O4/ CoS2
can improve the ability of internal charge separation and transfer.
3.6. UV–Vis Diffuse Reflectance Spectra
Figure 6a shows the UV–vis diffuse reflectance spectra of the
Mn3O4, CoS2, and 50 mg Mn3O4/CoS2. The absorption edge of
Mn3O4 is around 620 nm. Pure Mn3O4 is unresponsive in the
visible region. The response range of Mn3O4 in the visible-light
region is expanded by combining with CoS2. Strong light
absorption could improve the hydrogen evolution activity of
the catalyst, and the composite catalyst 50 mg Mn3O4/CoS2
shows the strongest light absorption, which is consistent with
the experiment results of hydrogen production. Figure 6b
shows the bandgap values of Mn3O4, CoS2, and 50 mg
Mn3O4/CoS2 calculated by the Kubelka–Munk function, which
are 0.89, 0.75, and 0.55 eV, respectively. The bandgap of the
composite catalyst 50 mg Mn3O4/CoS2 is smaller than that
of the pure Mn3O4 and pure CoS2, which indicates that the
composite catalyst 50 mg Mn3O4/CoS2 is more conducive to
electron transfer and thus improves the performance of
photocatalytic hydrogen evolution.
3.7. Photoelectrochemical Measurement Analysis
To explore the transport and separation of charge carriers in sam-
ples, the time-varying photocurrent response (it) and linear
sweep voltammetry (LSV) were measured. Figure 7a is the pho-
tocurrent response curve of the catalyst. It can be seen from it
that the photocurrent response of the catalyst increases
Figure 6. a) The UV–vis DRS of Mn3O4, CoS2, and 50 mg Mn3O4/CoS2; b) the bandgap of Mn3O4, CoS2, and 50 mg Mn3O4/CoS2.
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Figure 7. a) IT curves; b) LSV curves of Mn3O4, CoS2, and 50 mg Mn3O4/CoS2 samples in Na2SO4 (0.2 mol Lꢂ1). The scan rate was 0.5 mV sꢂ1. c) EIS of
Mn3O4, CoS2, and 50 mg Mn3O4/CoS2.
were conducted at room temperature. Figure 8a shows PL spec-
tra of EY, EY-Mn3O4, EY-CoS2, and EY-50 mg Mn3O4/CoS2
samples with excitation wavelength of 480 nm. The PL spec-
trum of pure EY has a strong emission peak centered at
535 nm, which indicates that photogenerated electrons and
holes are easily recombined in EY.[53] Obviously, compared
with pure EY, the PL emission intensity of EY-Mn3O4 and
EY-CoS2 decreases slightly, and the recombination rate of pho-
togenerated electron holes decreases slightly. When both
Mn3O4 and CoS2 were added into EY, the fluorescence intensity
decreased significantly and a slight redshift occurred. This phe-
nomenon originates from the transfer of photogenerated elec-
trons from Mn3O4 to the CoS2 interface. The fluorescence
quenching effect of the composite catalyst EY-50 mg Mn3O4/
CoS2 is the most significant. The results show that an effective
electron transfer process is conducive to the separation of pho-
togenerated electrons and holes, which plays a significant role
in improving the photocatalytic performance.
3.8. Photoluminescence Properties
Photoluminescence is a powerful characterization of photogen-
erated charge transfer. To understand the migration and separa-
tion of photogenerated carriers in samples, fluorescence studies
of catalysts EY, EY-Mn3O4, EY-CoS2, and EY-50 mg Mn3O4/CoS2
3.9. Photocatalytic H2 Evolution Schematic
According to the previous expression, the photocatalytic reaction
process and the possibility principle of electron transfer in this
article are shown in Figure 9. In Figure 9, EY is used as a
Figure 8. Steady-state PL spectra of the EY, EY-Mn3O4, EY-CoS2, and EY-
50 mg Mn3O4/CoS2 system in TEOA aqueous solution (10%, pH ¼ 10,
EY ¼ 1 ꢃ 10ꢂ6).
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In this study, a highly efficient and inexpensive composite photo-
catalyst was constructed by improving the transfer and separation
of photogenerated electrons.
Acknowledgements
This work was supported by the Natural Science Foundation of Ningxia
Province (NZ17262). This work was financially supported by the Open
Project of State Key Laboratory of High-efficiency Utilization of Coal
and Green Chemical Engineering, Ningxia University (2019-KF-36).
Figure 9. Photocatalytic mechanism for H2 production in the EY-sensitized
Mn3O4/CoS2 system photocatalysts under visible-light irradiation.
Conflict of Interest
The authors declare no conflict of interest.
sensitizer, TEOA is used as a sacrificial agent, and catalysts
Mn3O4 and CoS2 constitute heterojunction structures.
Data Availability Statement
EY and Mn3O4, can be excited by light. The detailed steps are
as follows:[54] 1) The EY molecule absorbs the photon energy and
becomes a single excited state (EY1*). Then, the singlet excited
(EY1*) state quickly turns to be the triplet excited state (EY3*)
by a valid intersystem crossing (ISC). The triplet excited state
EY3* is reduced to EYꢂ· by the sacrificial agent TEOA and pro-
duces an oxidative electron donor (TEOAþ) at the same moment.
Subsequently, a part of the electrons of EYꢂ· are transferred to
the reaction site of CoS2, at which time the protons are electron-
reduced to obtain H2. Another part of the electrons of EYꢂ is
transferred to the conduction band (CB) of Mn3O4 and further
transferred to CoS2. Finally, the EY molecule returns to the
ground state. 2) For Mn3O4, when the energy absorbed by
Mn3O4 is greater than the forbidden bandgap, its electrons tran-
sition from the valence band (VB) to the CB, and Mn3O4 simul-
taneously leaves holes in the VB. These electrons are then further
transferred to the reaction site of CoS2. At this time, protons are
reduced to obtain H2, and holes of Mn3O4 in the VB are reduced
by a sacrificial agent (TEOA).[55] In summary, this heterojunction
structure promotes the efficient separation and transfer of photo-
generated carriers, thereby improving the activity of photocata-
lytic H2 production.
Research data are not shared.
Keywords
CoS2/Mn3O4, high efficiency, hydrogen production, photocatalytic activity
Received: January 15, 2021
Revised: March 23, 2021
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