Synthesis, characterization, and photocatalytic activity of stannum-doped MgIn2S4 microspheres

Article abstract of DOI:10.1016/j.jallcom.2020.158446

A series of Sn²⁺ doped MgIn₂S₄ photocatalysts were prepared via a facial hydrothermal method. The Sn dopants substitute the sites of Mg atom in MgIn₂S₄ unit cell, but not alter the crystal structure, demonstrated by the results of XRD and XPS. Compared to pristine MgIn₂S₄, Sn-doped MgIn₂S₄ samples exhibit significantly enhanced photocatalytic CO₂ reduction activity. With increasing the Sn dopant content, the CO₂ conversion rate first ascends, achieving the maximum rate at Sn-MgIn₂S₄-2 sample, and then decreases. After illumination for 4 h, the highest yield of CO and CH₄ for Sn-MgIn₂S₄-2 sample reaches about 3.35 and 3.33 times higher than that of pristine MgIn₂S₄. The theoretical results based on density functional theory calculations reveal that Sn doping in MgIn₂S₄ tunes the band structure from the direct-transition of MgIn₂S₄ to indirect-transition, diminishes band gap and extends the light absorption range, reduces the effective masses of holes and promotes the migration of photoinduced carriers. The experimental results also demonstrate the positive role of Sn dopant in accelerating the separation and transportation of charges, and improving CO₂ adsorption ability. This work systematically investigates and discusses the Sn²⁺ doping effect in MgIn₂S₄ on crystal structure, lattice variations, electronic band structures, CO₂ adsorption ability, and photocatalytic CO₂ reduction activity, which can provide a new hint for the fabrication of efficient photocatalyst by metal ion doping.

Full text of DOI:10.1016/j.jallcom.2020.158446

Journal of Alloys and Compounds 860 (2021) 158446
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Synthesis, characterization, and photocatalytic activity of stannum-doped
MgIn2S4 microspheres
Wenhong Yanga, Yujing Dongb, Zhipeng Wanga, Yuqin Lia, Chunhui Daic, Dongwei Mab,⁎
Yu Jiab, Zhen Yanga,⁎, Chao Zenga,⁎
,
a Institute of Advanced Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, 99 Ziyang Avenue, Nanchang, Jiangxi Province
330022, China
b Key Laboratory for Special Functional Materials of Ministry of Education, and School of Materials Science and Engineering, Henan University, Kaifeng 475004,
China
c Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China University of Technology, Nanchang 330013, China
a r t i c l e i n f o
a b s t r a c t
A series of Sn2+ doped MgIn2S4 photocatalysts were prepared via a facial hydrothermal method. The Sn
dopants substitute the sites of Mg atom in MgIn2S4 unit cell, but not alter the crystal structure, demon-
strated by the results of XRD and XPS. Compared to pristine MgIn2S4, Sn-doped MgIn2S4 samples exhibit
significantly enhanced photocatalytic CO2 reduction activity. With increasing the Sn dopant content, the
CO2 conversion rate first ascends, achieving the maximum rate at Sn-MgIn2S4-2 sample, and then decreases.
After illumination for 4 h, the highest yield of CO and CH4 for Sn-MgIn2S4-2 sample reaches about 3.35 and
3.33 times higher than that of pristine MgIn2S4. The theoretical results based on density functional theory
calculations reveal that Sn doping in MgIn2S4 tunes the band structure from the direct-transition of MgIn2S4
to indirect-transition, diminishes band gap and extends the light absorption range, reduces the effective
masses of holes and promotes the migration of photoinduced carriers. The experimental results also de-
monstrate the positive role of Sn dopant in accelerating the separation and transportation of charges, and
improving CO2 adsorption ability. This work systematically investigates and discusses the Sn2+ doping effect
in MgIn2S4 on crystal structure, lattice variations, electronic band structures, CO2 adsorption ability, and
photocatalytic CO2 reduction activity, which can provide a new hint for the fabrication of efficient photo-
catalyst by metal ion doping.
Article history:
Received 13 October 2020
Received in revised form 19 December 2020
Accepted 21 December 2020
Available online 24 December 2020
Keywords:
Photocatalysis
CO2 reduction
Sn
Doping
MgIn2S4
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
scope of light absorption, high electron-hole recombination rate,
sluggish kinetics for hydrocarbons evolution and so on [5–7]. Thus,
Simultaneous solar energy conversion and storage via artificial
photocatalytic process has arrested intensive interests in recent
years [1]. The photocatalytic conversion of CO2 into useful hydro-
carbons is regarded as an eco-friendly strategy to settle the crisis of
environmental issues and energy shortage [2]. However, this pro-
mising technology is hampered at the proof-of-concept stage be-
cause of the low CO2 photoconversion efficiency [3]. The typical CO2
photoreduction reaction generally involves three processes: photo-
absorption, photoinduced charge carriers’ separation and CO2 re-
duction by photogenerated electrons [4]. The low photocatalytic CO2
reduction activity can be ascribed to the following reasons: narrow
developing visible-light-active photocatalyst with superior photo-
conversion efficiency is tempting and essential.
Ternary chalcogenide AB2X4 (A = Zn, Cd, Ca, Cu; B = In, Ga, Al;
X = S, Se, Te) semiconductors, as potential visible-light-driven pho-
tocatalysts applied in various photocatalytic fields, has attracted
intensive attention because of diverse outstanding characters, such
as, unique and tunable electronic structure, excellent light-har-
vesting properties, and suitable energy level [8]. However, the iso-
lated AB2X4 photocatalysts still suffer from the relatively high
recombination rate of photoexcited carriers, resulting into the un-
satisfactory photocatalytic performance [8–10]. To ameliorate the
activity of photocatalysts, multifarious strategies have been
exploited, such as morphology regulation, element doping, cocata-
lysts loading, utilizing exposed crystal facets, and heterogeneous
coupling, and so on [11,12]. It is well-known that the carrier’s
⁎
Corresponding authors.
E-mail addresses: madw@henu.edu.cn (D. Ma), yangzhen@jxnu.edu.cn (Z. Yang),
czeng@jxnu.edu.cn (C. Zeng).
https://doi.org/10.1016/j.jallcom.2020.158446
0925-8388/© 2020 Elsevier B.V. All rights reserved.

W. Yang, Y. Dong, Z. Wang et al.
Journal of Alloys and Compounds 860 (2021) 158446
transportation from bulk to surface reactive sites needs hundreds of
picoseconds, while the recombination of electron-hole in the bulk of
photocatalysts just takes several picoseconds, revealing that charge
carriers are inclined to recombine rather than transfer to surface
reactive sites [13,14]. The high bulk-charge recombination rate
contributes significantly to the inefficient artificial photocatalysis
system. Among the above-motioned modification methods of pho-
tocatalyst, it is the element doping that can boost the carrier se-
paration and migration in the bulk, whilst the rest strategies just
affect surface carrier transportation [15]. Zhao et al. reported that
Cu-doped ZnIn2S4 exhibited enhanced photocatalytic hydrogen
evolution under visible light irradiation [16]. Wang et al. synthesized
Sn2+-doped ZnWO4 nanocrystals, which showed improved visible
photocatalytic efficiency for methylene orange (MO) degradation
[17]. Zhu et al. found that Sm-doped ZnIn2S4 microspheres would
efficiently improve the photoactivity in the removal of methyl or-
ange (MO) and Rhodamine B (RhB) [18]. In addition, other metal
dopants, such as K+ [19], Cu2+ [20], Y3+ [21], Ce3+ [22], and Mo5+ [23],
doping in semiconductor photocatalysts, would also promote the
photocatalytic properties.
2.2. Sample characterization
X-ray powder diffraction (XRD) were recorded on a Bruker D8
focus with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS)
was performed on an ESCALAB 250xi (ThermoFsher, England) elec-
tron spectrometer. Field emission scanning electron microscopy (FE-
SEM) and transmission electron microscopy (TEM) was conducted
on Hitachi SU8020 and JEM-2100, respectively. UV–vis diffuse re-
flectance spectra (DRS) were obtained using a Hitachi U-3310 UV–vis
spectrophotometer. The Brunauer-Emmett-Teller (BET) specific sur-
face area and pore diameter distribution of samples were measured
through nitrogen adsorption-desorption on Micromeritics ASAP
2460 (USA). The CO2 adsorption ability of as-obtained samples was
investigated by An ASAP 2020 analyzer (Micromeritics, USA). The
photoluminescence emission (PL) spectra were measured at a
fluorescence spectrophotometer (Edinburgh FLS980) using xenon
lamp as an excitation source. All the measurements were conducted
at room temperature.
2.3. Photocatalytic CO2 reduction test
MgIn2S4, a typical AB2X4 metal chalcogenide, has been reported
and exhibited the photocatalytic activity in the photo-degradation of
carbamazepine [24], methyl orange (MO) and 4-nitroaniline (4-NA)
[8], the photoreduction of Cr (VI) and photocatalytic water splitting
to hydrogen [25]. As mentioned earlier, element doping is an effi-
cient strategy to enhance photocatalytic activity. Our group prepared
carbon coating and doping MgIn2S4 with greatly enhanced photo-
catalytic CO2 reduction activity, and the function of doped carbon
and coated carbon in photocatalytic activity enhancement were in-
vestigated and discussed systematically [26]. Moreover, we also re-
ported that Co surface gradient diffused doping in MgIn2S4 created
an oriented built-in electric field, which could effectively extract
photoinduced carriers from inner to surface, so inhibited charge
recombination and enhanced photocatalysis performance for CO2
reduction to CO [27]. However, the influence of metal ion homo-
geneous doping on the catalytic activity over MgIn2S4 might hold
paramount importance in the understanding and designing of effi-
cient photocatalysts, which has never been reported so far. In this
work, a series of MgIn2S4 uniformly doped with different amounts of
Sn2+ ions were synthesized via a facial hydrothermal method. The
doping effect on crystal structure, lattice variations, electronic and
band structures, CO2 adsorption ability, and photocatalytic CO2 re-
duction activity were systematically investigated and discussed.
The photocatalytic performance of as-prepared samples were
evaluated by the photocatalytic CO2 reduction experiment. Typically,
50 mg photocatalyst was ultrasonically dispersed in 90 mL ultrapure
water and 10 mL TEOA (triethanolamine) used as sacrificial agent,
and then the suspension was transferred into a 300 mL photoreactor
sealed with an optical quartz window at the top. Before photoreac-
tion, the reactor was first vacuumed, and then was continuously
bubbled with high purity CO2 gas (99.999%) for 30 min to get the
saturated dissolution of CO2 and adsorption-desorption equilibrium.
The reactor internal pressure was 1 atm and the temperature was
kept at room temperature by being cooled with circulating water to
increase the solubility of CO2. A 300 W Xe lamp was used as the light
source. At certain period time, 0.2 mL resulting gas was collected by
gas sampling needle and then qualitatively analyzed by a GC A60 gas
chromatograph (Panna, Changzhou, TCD detector, FID detector,
argon carrier gas). Blank test was conducted without photocatalyst,
or without irradiation, or substitute CO2 gas with nitrogen gas. The
isotopic experiment in the presence of Sn-MgIn2S4-2 sample was
performed under the same conditions with using 13CO2 instead of
12CO2. To assess the stability, Sn-MgIn2S4-2 sample taken as a model
was reevaluated in the light of the aforementioned procedure.
2.4. Photoelectrochemical measurement
2. Experimental section
The photoelectrochemical measurements, including photo-
current response, electrochemical impedance spectra (EIS) and
Mott-Schottky curve, were carried out on an electrochemical ana-
lyzer (CS315H, Wuhan) equipped with a standard three-electrode
system. Platinum (Pt) wire and Ag/AgCl (saturated KCl) were used as
the counter electrode and reference electrode, respectively. A 300 W
Xe arc lamp was taken as light source. The electrolyte solution was
0.1 M Na2SO4 aqueous solution. The working electrodes was pre-
pared as follows. 10 mg sample was ultrasonically dispersed in
800 μl ethanol. Then, the suspension liquid was drop-coated on an
ITO glass with size of 20 mm × 40 mm, and dried at 60 ℃ for 10 h in
the air.
2.1. Preparation of the photocatalyst
The reagents, In(NO3)3·xH2O, thioacetamide (TAA), SnCl2·xH2O
were purchased from Shanghai Aladdin Bio-Chem Technology Co.,
LTD (Shanghai, China), and MgIn2S4·6H2O was purchased from
Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), all of which
were of analytical grade and used without further purification. The
Sn-doped MgIn2S4 microspheres were prepared by a simple hydro-
thermal method. Typically, MgIn2S4·6H2O, SnCl2·xH2O, In
(NO3)3·xH2O, and TAA were dissolved in 30 mL ultrapure water. After
30 min magnetic stirring, the resulting heterogeneous solution was
transferred into a 50 mL Teflon-lined stainless-steel autoclave and
maintained at 120 °C for 24 h in an oven. After natural cooling, the
product was collected by centrifugation, washed alternately with
distilled water and ethanol, and then dried at 60 °C for 10 h. For the
convenient reason, the as-synthesized samples with SnCl2·xH2O/
MgIn2S4·6H2O molar ratios of 10%, 20%, and 30% were named as Sn-
MgIn2S4-1, Sn-MgIn2S4-2, and Sn-MgIn2S4-3, respectively. Moreover,
the pure MgIn2S4 was synthesized by the same procedure without
adding SnCl2·xH2O.
2.5. Computational details
All the theoretical calculations were performed within the fra-
mework of density functional theory (DFT) using the plane-wave
pseudopotential approach as implemented in the VASP code [28].
The generalized gradient approximation formulated by Perdew,
Burke, and Ernzerhof (PBE) [29] as the exchange-correlation func-
tional has been used for all the structure relaxations. According to
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W. Yang, Y. Dong, Z. Wang et al.
Journal of Alloys and Compounds 860 (2021) 158446
where k represents the wave-vector and Ek represents the energy
corresponding to the wave vector k.
3. Results and discussion
The XRD patterns of MgIn2S4 and Sn-MgIn2S4-2 samples are
displayed in Fig. 1. The main peaks of MgIn2S4 at 14.0°, 23.2°, 27.4°,
28.2°, 33.3°, 36.5°, 43.5°, 47.8°, 50.0°, 56.4°, 57.5°, 59.5°, 61.8°, 66.7°,
70.1°, 77.1°, and 77.5° coincide with (111), (022), (113), (222), (004),
(133), (115), (044), (135), (335), (226), (444), (117), (137), (008),
(555), and (266) planes of cubic MgIn2S4 (ICSD＃53096). No other
impurities are detected, demonstrating the single-phase of MgIn2S4.
All of the diffraction peaks for Sn-doped MgIn2S4 samples corre-
spond well to the standard data of MgIn2S4, and no characteristic
peaks of any impurities are observed in the patterns, reflecting that
Sn dopant does not induce the alteration of crystal phase.
Fig. 1. XRD patterns of MgIn2S4 and Sn-MgIn2S4-2 samples.
To investigate the chemical composition and chemical situation of
the as-synthesized MgIn2S4 and Sn-MgIn2S4-2 samples, X-ray photo-
electron spectroscopy (XPS) are carried out. The C 1s peak at 284.8 eV
coming from adventitious carbon of the XPS instrument is employed to
calibrate all binding energies. As shown in Fig. 2a, the MgIn2S4 is
composed of Mg, In, S elements, and Sn-MgIn2S4-2 consists of Mg, In, S,
Sn elements. The O 1s peak with bind energy of 529.3 eV is ascribed to
the sample surface absorbed O2 and H2O [31]. For MgIn2S4, the peaks at
445.1 and 452.7 eV are ascribed to the In 3d5/2 and In 3d3/2 states of In3+
[32], while the In 3d peaks for Sn-MgIn2S4-2 sample are almost the
same as MgIn2S4 (Fig. 2b). The binding energies of S2- ions are 162.8
and 161.7 eV for MgIn2S4, 162.6 and 161.5 eV for Sn-MgIn2S4-2, which
can be indexed to S 2p1/2 and S 2p3/2 (Fig. 2c), respectively [33]. It can
be found from the crystal structure of MgIn2S4 in Fig. S7a and b that the
S element is only bonded to Mg element. For Sn-MgIn2S4-2 sample, the
negative shift of S 2p peaks and immobile In 3d peaks, in relative to
that of MgIn2S4, demonstrate that the Sn dopants substitute the sites of
Mg atoms. If the doped Sn exists in an interstitial configuration, the
the experimental results, the Sn-doped MgIn2S4 (Sn-MgIn2S4) has
been modelled by replacing one Mg atom with one Sn atom in the
unit cell, giving a doping concentration of 12.5% close to the ex-
perimental cases. The equilibrium structural parameters (including
both lattice parameters and internal atomic coordinates) of each
candidate material were obtained by the total energy minimization
through the conjugate-gradient algorithm for MgIn2S4 and Sn-
MgIn2S4. The kinetic energy cutoffs of 450 eV for the plane-wave
basis set are used to ensure the residual forces on atoms converged
to below 0.001 eV/Å. The 4 × 4 × 4 Monkhorst-Pack grids are adopted
for structural optimization and densities of states (DOS) calculation.
The effective masses of electrons (me*) and holes (mh*) were cal-
culated on the basis of the parabolic approximation of the conduc-
tion band minimum (CBM) and valence band maximum (VBM) [30]:
1
d2Ek
2
m* = ±
dk2
Fig. 2. Typical XPS survey spectra (a), high-resolution XPS spectra of In (b) and S (c) for MgIn2S4 and Sn-MgIn2S4-2 samples. Time-dependent high-resolution Sn 3d XPS spectra
upon Ar+ sputtering for Sn-MgIn2S4-2 sample (d).
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W. Yang, Y. Dong, Z. Wang et al.
Journal of Alloys and Compounds 860 (2021) 158446
Fig. 3. TEM image (a-b), HRTEM image (c), and EDX spectrum (d) for Sn-MgIn2S4-2 sample.
electron density of both In atoms and S atoms would be changed, and
the bind energy of In element would be changed [34]. To investigate
the distribution of Sn dopants in the bulk and subsurface of Sn-
MgIn2S4-2 sample, time-dependent high-resolution Sn 3d XPS spectra
upon Ar+ sputtering are performed. After Ar+ sputtering for 100 s, the
peak intensity of Sn 3d remains unchanged with prolong the sputtering
time [35]. Therefore, the Sn2+ ions are successfully introduced into the
unit cell of MgIn2S4 rather than absorbed on the surface or forms
surface gradient diffused doping.
The morphology and microstructure of the as-synthesized sam-
ples are investigated by SEM, TEM, HRTEM, and TEM-EDX, as ex-
hibited in Fig. 3 and Figs. S1–2. From Fig. S1, it can be seen that the
MgIn2S4 is characterized by microsphere composed of nanosheets.
The introduction of Sn dopant has not made an obvious difference to
the morphology of MgIn2S4. The TEM images in Fig. 3a and b and
Fig. S2 further confirm that the microsphere of as-prepared sample
is an aggregation of nanosheets. For the HRTEM image in Fig. 3c, Sn-
MgIn2S4-2 sample displays clear lattice fringes with lattice plane
spacing of 0.321 nm and 0.318 nm, which can be contributed to the
typical (113) facets. The result of TEM-EDX reflects the existence of
Mg, Sn, In, and S atoms (Fig. 3d).
Sn-MgIn2S4-2 sample, and then decrease because excess metal ion
dopant would act as the center of electron-hole recombination [27].
After illumination for 4 h, the highest yield of CO and CH4 is 1.70 and
0.13 umol/g, which is about 3.35 and 3.33 times higher than that of
pristine MgIn2S4 (the yield of CO and CH4 is 0.39 and 0.03 umol/g),
respectively. The Table S1 portrays the performance comparison of
different photocatalysts for photocatalytic CO2 reduction. To verify
the selectivity of product formation from CO2 reduction over Sn-
MgIn2S4-2 sample by using other scavenger, we conduct the pho-
tocatalytic CO2 reduction experiment under the same conditions just
substituting triethylamine for TEOA, and the time dependence of
CH4 and CO yields for photo-reduction of CO2 over Sn-MgIn2S4-2
sample is shown in Fig. S4. The products using triethylamine as
scavenger are CO and CH4, while the yield of CO is lower than that
using TEOA as scavenger, but the yield of CH4 is higher than that
using TEOA as scavenger. Particularly, in the blank experiments,
neither CO or CH4 is produced and detected without photocatalyst,
or without irradiation, or substitute CO2 gas with nitrogen gas.
Furthermore, as corroborated by the isotopic experiment using 13CO2
instead of 12CO2 (Fig. 4b), the m/z at 45, 17, and 29 on the GC-MS
spectra can be attributed to 13CO2, 13CH4, and 13CO, respectively.
Thus, it can be confirmed that the CO and CH4 are derived from the
photocatalytic CO2 reduction reaction over Sn-MgIn2S4-2 sample,
rather than the decomposition of any organic impurity.
The photocatalytic activities of as-prepared samples were eval-
uated by the photocatalytic CO2 reduction experiment. As shown in
Fig. 4a, CO2 can be photo-reduced to CO and CH4 in the presence of
MgIn2S4 and Sn-MgIn2S4 samples. The Sn-MgIn2S4 samples exhibit
markedly enhanced photocatalytic CO2 reduction activity compared
with pristine MgIn2S4. With enlarging the content of Sn dopant, the
CO2 conversion rate first ascends, achieving the maximum rate at
Besides the photocatalytic CO2 reduction activity of semiconductor
photocatalysts, the stability and recyclability are also pivotal for prac-
tical applications, which is evaluated by conducting three-run cycling
CO2 photoreduction experiments. After three successive cycles, as
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W. Yang, Y. Dong, Z. Wang et al.
Journal of Alloys and Compounds 860 (2021) 158446
Fig. 4. (a) Time dependence of CH4 and CO yields for photo-reduction of CO2 over MgIn2S4 and Sn-MgIn2S4 samples. (b) Mass spectra for CO2 reduction over Sn-MgIn2S4-2 sample
under 13CO2 atmosphere. Cycling runs of CO2 reduction over Sn-MgIn2S4-2 sample (c). XRD pattern of Sn-MgIn2S4-2 sample before and after photocatalytic reaction (d).
shown in Fig. 4c, the Sn-MgIn2S4-2 sample maintains over 60% of its
photocatalytic activity in photo-reducing CO2 to generate both CO and
CH4. In addition, it can be found from Fig. 4d and Fig. S5 that no obvious
variations of XRD pattern and SEM image for Sn-MgIn2S4-2 sample
between before and after photoreaction can be detected. These results
imply the favorable photostability of Sn-MgIn2S4-2 sample, which is
beneficial for its potential practical application.
the higher charge transfer efficiency is [38]. The Sn-MgIn2S4-2 sample
exhibits a remarkable enhanced current density in relative to that of
MgIn2S4 (Fig. 5c), while the higher current density represents a better
charge separation efficiency [39]. The photoluminescence emission (PL)
intensity of Sn-MgIn2S4-2 is lower than that of MgIn2S4 (Fig. 5d), de-
monstrating a lower charge recombination rate over Sn-MgIn2S4-2
[40]. According to the above measurement results, it can be inferred
that Sn2+ doping in MgIn2S4 can effectively promote the separation and
transportation efficiency of photogenerated carriers, so enhancing
photocatalytic CO2 reduction activity.
The specific surface areas (SBET) of MgIn2S4 and Sn-MgIn2S4-2
samples are determined to be 73.9 and 40.1 m2/g based on the
Brunauer-Emmett-Teller (BET) method (Fig. S6). Generally, a higher
surface area of semiconductor photocatalysts implies the more active
sites exposed for CO2 adsorption and photocatalytic CO2 reduction
reaction, which is be advantageous for the photocatalysis perfor-
mance [36]. Although the SBET of Sn-MgIn2S4-2 is smaller than that of
MgIn2S4, the enhanced photocatalytic activity of Sn-MgIn2S4-2 do
reveal the important positive effect of Sn dopant. We also measure the
CO2 adsorption ability of MgIn2S4 and Sn-MgIn2S4-2 samples, which is
crucial for the photocatalytic CO2 reduction to hydrocarbons [26]. As
shown in Fig. 5a, the introduction of Sn dopant would strengthen the
CO2 adsorption capacity. Moreover, during the photocatalytic CO2
reduction reaction, water serve as the electron donor and a proton
source, suggesting that the contact degree between photocatalyst and
water has an impact on the CO2 photoconversion efficiency [37]. The
static water contact-angle measurement of MgIn2S4 and Sn-MgIn2S4-
2 samples are examined, as exhibited in Fig. S7. It can be found that
the contact angle of these two samples is not significantly different, so
the contact degree is not the cause of the enhancement for photo-
catalytic activity.
DFT calculations are carried out to try to understand how the Sn
dopants alter the photocatalytic performance of MgIn2S4. In the light
of the XPS results (Fig. 2), the Sn-doped MgIn2S4 is modelled by re-
placing one Mg atom with one Sn atom in the unit cell, as shown in
Fig. 6a and b. Table. S2 exhibits that Sn doping has little effect on the
structure of MgIn2S4, and the lattice parameter increases slightly from
10.90 Å for MgIn2S4 to 10.97 Å for Sn-MgIn2S4, which is due to the
larger atomic radius of Sn elements. The main factors affecting the
photocatalytic conversion efficiency are the light absorption range of
photocatalytic materials and the separation and utilization efficiency
of photogenerated carriers. The band structure results exhibit that Sn-
MgIn2S4 has a narrower bandgap (0.69 eV) than MgIn2S4 (1.74 eV)
(Fig. 6c and Fig. S8c), and noticeably, the band structure changes from
the direct band gap to indirect band gap. For indirect band materials,
the momentum between the CBM and VBM is mismatch. The
recombination between electrons and holes must conserve mo-
mentum, so, for semiconductor photocatalyst, the indirect-transition
of band structure can significantly reduce charge recombination rate
compared with the direct-transition of band structure [41]. Sn doping
increase the dispersion of the contour distribution of VBM and CBM,
suggesting Sn doping promotes the charge mobility [42]. A narrow
band gap can improve the light absorption and affect both the elec-
tron delocalization and mobility of the photoexcited charge, which
significantly affect the photocatalytic efficiency [43]. Comparing the
DOS of the simulated MgIn2S4 and Sn-MgIn2S4 systems, for MgIn2S4,
The separation and migration efficiency of photo-generate charges
are critical to the photocatalysis performance over semiconductor
photocatalysts. The EIS Nynquist plots, transient photocurrent re-
sponses, and PL spectra of MgIn2S4 and Sn-MgIn2S4-2 samples are
measured and displayed in Fig. 5b-d. For EIS Nynquist plots (Fig. 5b),
the sharply reduced arc radius of Sn-MgIn2S4-2 compared with
MgIn2S4 can be detected. In general, the smaller EIS Nynquist plots is,
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W. Yang, Y. Dong, Z. Wang et al.
Journal of Alloys and Compounds 860 (2021) 158446
Fig. 5. CO2 adsorption curves (a), EIS Nynquist plots (b), transient photocurrent responses (c), and PL spectra (d) of MgIn2S4 and Sn-MgIn2S4-2 samples.
the CB and VB are mainly composed of In (s) + S (p), and In (p) + S (p),
respectively. For Sn-MgIn2S4, the CB and VB are mainly composed of
In (s) + S (p), and Sn (s) + S (p), respectively (Fig. 6d and Fig. S8d).
The indirect bandgap can reduce the recombination of electrons
and holes and make Sn-MgIn2S4 samples show the improved
photocatalytic activity. In order to further study the charge separa-
tion and transport characteristics, the effective masses of electrons
and holes of MgIn2S4 and Sn-MgIn2S4 are calculated and analyzed.
The effective masses of electrons and holes for MgIn2S4 are
m*e = 0.48 m0 and m*h = 6.87 m0, respectively, and those for
Fig. 6. Side (a) and top (b) views of model structure for Sn-MgIn2S4. Electronic band structure (c) and density of state (d) of the simulated Sn-MgIn2S4 system. The green, brown,
yellow and grey atoms represented Mg, In, S, and Sn atoms, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
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W. Yang, Y. Dong, Z. Wang et al.
Journal of Alloys and Compounds 860 (2021) 158446
band spectra of MgIn2S4 and Sn-MgIn2S4-2 samples are conducted to
investigate the band edges, and the VB XPS values are determined to
be 1.67 and 1.59 eV, the order of which coincides with the VB potential
calculated above. Both the CB potentials of MgIn2S4 and Sn-MgIn2S4-2
samples meet the thermodynamic requirements of the CO2 reduction
into CH4 and CO reaction: CO2 + 8H+ + 8e- → CH4 + 2H2O (the reaction
potential is −0.24 V VS NHE), CO2 + 2H+ + 2e- → CO + H2O (the reaction
potential is −0.53 V VS NHE), respectively. The CO evolution during the
photocatalytic CO2 reduction reaction over MgIn2S4 and Sn-MgIn2S4-2
samples are higher than CH4 evolution, which may be ascribed to the
easily occur for double electron reaction, and the same phenomenon
has also been reported [46–48]. Electrons and holes would be pro-
duced with irradiation and transfer to the photocatalytic surface re-
active site. CO2 would be reduced by electrons to be CH4 and CO, while
the TEOA would be oxidized by holes to generates C1 products as well
as H2 [49].
4. Conclusions
In summary, we confirm that metal ion doping is a feasible
strategy to alter the electronic and band structures for semi-
conductor photocatalysts and thus remarkedly promotes their pho-
tocatalysis performance with taking MgIn2S4 as the model. Sn2+
doped MgIn2S4 photocatalysts exhibits greatly enhanced photo-
catalytic CO2 reduction activity in relative to pristine MgIn2S4. Sn-
MgIn2S4-2 sample realize the highest CO2 photoconversion rate
among all of the samples. With further enlarging the content of Sn
dopant, the photocatalytic activities of Sn-doped MgIn2S4 samples
declines, revealing the existence of optimal amount for metal ion
doping. The theoretical calculation and experimental results
demonstrate that the enhancement of photocatalytic activity for
Sn-doped MgIn2S4 photocatalysts is ascribed to the promoted
separation and transportation efficiency of photoinduced charges,
strengthened CO2 adsorption ability, and expanded light absorption
range resulting from Sn doping. This study supports that metal ion
doping strategy is attractive for the designing and fabrication of
efficient semiconductor photocatalyst.
Fig. 7. (a) DRS spectra and digital photo of suspensions in the inset of MgIn2S4 and
Sn-MgIn2S4-2 samples. (b) Schematic band structures for MgIn2S4 and Sn-MgIn2S4-2
samples. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Sn-MgIn2S4 are m*e = 0.68 m0 and m*h = 5.09 m0, respectively. Large
m* reflects poor mobility of photoinduced carriers [44]. The effective
masse of holes is much larger than those of electrons for both the
pristine and the doped samples, and thus the mobility of photo-
induced carriers is primarily determined by the mobility of holes.
The m*h of Sn-MgIn2S4 is significantly reduced compared to that of
MgIn2S4, so the migration of photoinduced carriers is greatly pro-
moted.
CRediT authorship contribution statement
Wenhong Yang: Investigation, Writing. Yujing Dong: Software.
Zhipeng Wang: Data curation. Yuqin Li: Visualization. Chunhui Dai:
Methodology. Dongwei Ma: Investigation, Software. Yu Jia:
Software. Zhen Yang: Data curation, Resources. Chao Zeng:
Conceptualization, Methodology, Software, Writing
editing.
- review &
Fig. 7a shows the DRS spectra and digital photo of suspensions of
MgIn2S4 and Sn-MgIn2S4 sample. The introduction of Sn dopant can
effectively extend the light absorption range, which is consistent to the
color change from red (color of pristine MgIn2S4) to dark brown (color
of Sn-MgIn2S4), and induce the generation of a tail in the edge, which
keeps in line with the result of theoretical results in Fig. 6d. Because Sn
doping in MgIn2S4 causes the band structure changes from the direct
band gap to indirect band gap, the energy band gap (Eg) of MgIn2S4
and Sn-MgIn2S4-2 are determined to be 2.10 and 1.65 eV (Fig. S9) on
the basis of Kubelka-Munk equation, respectively. The experimental Eg
calculated from DRS spectra is a bit bigger than that of theoretical
value, the error of which is normal because of the limitation for the
exchange-correlation potentials [45]. On the basis of Mott-Schottky
curve in Fig. S10, the CB potential of MgIn2S4 and Sn-MgIn2S4-2
samples are calculated to be − 1.67 and − 1.38 eV versus Ag/AgCl, which
are equal to − 1.57, and − 1.28 V versus the normal hydrogen electrode
(NHE), so the VB potential can be determined to be 0.53 and 0.37 eV,
respectively, as portrayed in Fig. 7b. As shown in Fig. S11, XPS valence
Declaration of competing interest
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Natural Science
Foundation of China (No. 51872130, 52063016, 52002158,
2652015296, 21968009), the Natural Science Foundation of Jiangxi
Province (No. 20202BAB214009, 20202BAB203007), and the
Education Department of Jiangxi Province (No. GJJ191684). No con-
flict of interest exists in this work.
7

W. Yang, Y. Dong, Z. Wang et al.
Journal of Alloys and Compounds 860 (2021) 158446
Appendix A. Supporting information
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and bulk doped carbon on enhancing photocatalytic CO2 reduction for MgIn2S4
microflowers, Appl. Surf. Sci. 542 (2021) 148686.
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jallcom.2020.158446.
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8