An oriented built-in electric field induced by cobalt surface gradient diffused doping in MgIn2S4for enhanced photocatalytic CH4evolution

Article abstract of DOI:10.1039/d0dt01686a

A gradient cobalt-doped MgIn2S4 (MgIn2S4-Co) homojunction photocatalyst was reported, creating an oriented built-in electric field for efficient extraction of photogenerated carriers from the inside to the surface of the photocatalyst. The MgIn2S4-Co photocatalysts showed remarkably enhanced photocatalytic CO2 reduction activity compared with pristine MgIn2S4.

Full text of DOI:10.1039/d0dt01686a

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An oriented built-in electric field induced by cobalt surface gradient
diffused doping in MgIn2S4 for enhanced photocatalytic CH4
evolution
Received 00th January 20xx,
Accepted 00th January 20xx
Chao Zeng,*a,b Qing Zeng,b Chunhui Dai,c and Yingmo Hu*b
DOI: 10.1039/x0xx00000x
A
gradient cobalt-doped MgIn2S4 (MgIn2S4-Co) homojunction
Some commonly used strategies to steer charge flow,
photocatalyst was reported, creating an oriented built-in electric such as, surface defect engineering, semiconductor
field for efficient extraction of photogenerated carriers from inner combinations, modification of size and morphology, cocatalyst
to surface of the photocatalyst. The MgIn2S4-Co photocatalysts loading and so on, although can influence charge separation on
showed remarkedly enhanced photocatalytic CO2 reduction activity the photocatalyst surface, cannot minimize charge
compared with pristine MgIn2S4.
recombination in the bulk.11 Surface doping is an effective
strategy to restrain charge recombination and promote charge
separation in the bulk of photocatalysts, enhancing their
photocatalytic activity.12-14 Element gradient doping causes
nonuniform charge distribution between different constituent
layers, then forming a multilayer core-shell structure with
peculiar “antiquantum well” band structure.15 Benefiting from
the nonuniform charge distribution in the material, an oriented
built-in electric field is developed, which can extensively
facilitate the charge extraction.16 MgIn2S4, a red semiconductor
material, is reported as an efficient semiconductor
photocatalyst in the removal of liquid pollutants, the
photoreduction of Cr (VI), and photocatalytic H2 production,17-
Fossil fuels, the major energy contributors for the whole world,
will be depleted soon. Moreover, the combustion of fossil fuel emits
massive carbon dioxide (CO2) into the atmosphere, triggering the
greenhouse effect which seriously damages the ecological
environment.1-2 Since Inoue et al. firstly reported the
photoelectrocatalytic reduction of CO2 by semiconductor in 1979,3
solar photocatalytic conversion of CO2 to value-added chemical
energy inspires the immense research interests, due to its potential
in settling the increasing energy crisis and environmental issues
synchronously.4-6 However, the conversion of solar energy to
chemical energy in hydrocarbons over semiconductor photocatalysts
suffers from low efficiency, far below to satisfy the needs of practical
applications.7 The pivotal obstacle limiting the efficiency is the high
electron-hole recombination rate in photocatalysts.8-9 As we known,
the journey for carriers from bulk to surface reactive sites takes
hundreds of picoseconds, but recombination inside photocatalyst
lattice takes several picoseconds.10 Most of charge carriers prefer to
recombine with each other in the photocatalyst bulk before reaching
the photocatalyst surface. In consequence, only a few carriers are
available for the ultimate surface catalytic reactions, so most
19
but have never be applied for photocatalytic CO2 reduction
into hydrocarbon fuel. In this work, we designed and prepared
gradient Co-doped MgIn2S4 microsphere by hydrothermal
reaction to realize efficient photocatalytic reduction of CO2. The
concentrate of Co became lower from surface to inner. A built-
in oriented electric field is formed from the nonuniform charge
distribution between inner and surface of photocatalyst, so the
photoinduced carriers are extracted from the core region to the
surface defect sites. MgIn2S4-Co homojunctions exhibit
remarkably enhanced activity for photocatalytic CO2 reduction
into CH4 under visible-light irradiation.
The hierarchical MgIn2S4 marigold microflowers with Co doped
was synthesized via a facile hydrothermal method. By varying the
molar ratio of Co(NO3)2·6H2O to MgIn2S4 from 10% to 20%, 40%, 60%
and 80%, five types of MgIn2S4-Co homojunction (named as MgIn2S4-
Co1 to MgIn2S4-Co5) with various compositions were obtained. XRD
of MgIn2S4 showed peaks at 27.7°, 33.4°, 43.7°, and 47.9° (Figure
S1a), which correspond to (113), (004), (115), and (044) planes of
cubic MgIn2S4 with Fd-3m space group (JCPDS No. 70-2893),
respectively. No other peak was detected, suggesting the high purity
of the MgIn2S4 sample. Co doping did not alter the crystal phase, but
the XRD peak of MgIn2S4-Co2 obviously shifted to a small angle
ascribing to the expansion of the lattice. Compared to MgIn2S4, the
semiconductor
photocatalysts
exhibit
unappealing
CO2
photoconversion efficiencies. Therefore, it is essential to develop a
facile strategy to inhibit charge recombination in the bulk of
photocatalysts and realize efficient photocatalytic CO2 conversion.
a
Institute of Advanced Materials (IAM), Jiangxi Normal University, Nanchang,
330022, P. R. China
b
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid
Wastes, National Laboratory of Mineral Materials, School of Materials Science and
Technology, China University of Geosciences, Beijing 100083, China
c
Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China
University of Technology, Nanchang 330013, P. R. China
*E-mail: czeng@jxnu.edu.cn, hyingmolunwen@163.com
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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light-harvesting ability for MgIn2S4-Co2 was markedly enhanced (b), and MgIn2S4-Co2 (c) samples. TEM image (d), HRTEM image (e),
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DOI: 10.1039/D0DT01686A
(Figure S1b), which was consistent with the color of the catalyst EDX spectrum (f), and EDX elemental mapping images (g) for
solutions changed from red (color of pristine MgIn2S4) to black (color MgIn2S4-Co2 sample.
of MgIn2S4-Co2).
Photocatalytic CO2 reduction experiment was performed under
The XPS survey spectra (Figure S2a) validated the existence of
visible light (λ > 420 nm) illumination in a gas-solid reaction system
Mg, In, S, and Co elements in MgIn2S4-Co2 sample. The C 1s peak at
without any co-catalysts or sacrificial agents (Figure 2a-b). The
284.2 eV could be assigned to the adventitious carbon of the XPS
photocatalytic performance of Co doped MgIn2S4 conspicuously
instrument. The O 1s peak at 529.3 eV resulted from O2 and H2O
outperformed that of nondoped photocatalyst. With increasing the
adsorbed on the surface of sample. The binding energies of In3+ 3d3/2
Co doped concentration, the CO2 conversion rate for Co doped
and In3+ 3d5/2 for MgIn2S4-Co2 were 452.5 and 445.0 eV (Figure
MgIn2S4 first ascended, reached the maximum at MgIn2S4-Co2, and
S2b),20 while the peaks of S2- 2p1/2 and S2- 2p3/2 located at 162.9 and
then declined because excess Co2+ ions would be the centre site
161.7 eV (Figure S2c),21 respectively. To further verify the
where photogenerated electrons and holes recombines.22 MgIn2S4-
distribution of Co dopants in the subsurface of MgIn2S4-Co2, time-
Co2 sample exhibited the highest CH4 evolution rate of 1.6 umol/g/h,
dependent high-resolution Co 2p XPS spectra upon Ar+ sputtering
which was about 8-fold higher than that of pristine MgIn2S4 (0.2
were conducted. With the time of Ar+ sputtering prolonging, the Co
umol/g/h). Noticeably, as corroborated by control experiments, no
2p peak intensity decreased, demonstrating that the concentrate of
CH4 could be detected without photocatalyst, light illumination, or
Co become lower from surface to inner and the surface gradient
substituting CO2 atmosphere with argon. This indicated that the CO2
distribution of Co element (Figure 1a). Particularly, the gradient
reduction into CH4 process takes place by our photocatalysts.
distribution of Co element provides basis for constructing a built-in
According to the result of isotopic experiment using 13CO2, the m/z
electric field.
at 17 on the GC-MS spectra can be ascribed to 13CH4, supporting CH4
The SEM image (Figure 1b) of the MgIn2S4 sample was
are formed from the photocatalytic CO2 reduction on MgIn2S4-Co2
characterized by marigold microflowers with a diameter of ～ 5 um,
(Figure S3).
which was composed of a large quantity of nanosheets. Co doping
The stability and recyclability of photocatalysts are crucial for
did not alter the morphology of MgIn2S4 (Figure 1c). TEM image
practical applications besides photocatalytic performance. The
(Figure 1d) also reflected the inerratic flower-like morphology of
cycling runs for the photocatalytic CO2 reduction with the MgIn2S4-
MgIn2S4-Co2. In the HRTEM of MgIn2S4-Co2 (Figure 1e), the lattice
Co2 sample were conducted. As exhibited in Figure S3, after
spacing was determined to be 0.632 nm, which could be attributed
undergoing 3 repeated runs, there was a slight shrinkage on the CH4
to the (111) planes. Some parts marked with yellow loop even
evolution rate (Figure S4). The MgIn2S4-Co2 sample after cycling runs
became amorphous while most areas exhibited ordered lattice
was collected and analyzed by XPS and SEM (Figure S5-6). Intuitively,
fringes. This might be due to the Co species anchored on the surface
XPS spectra and SEM image of MgIn2S4-Co2 sample exhibited
of MgIn2S4-Co2. The energy dispersive X-ray spectroscopy (EDX)
unconspicuous difference between before and after photoreaction.
spectrum confirmed the existence of Mg, In, S, and Co elements in
These results confirmed that MgIn2S4-Co2 sample is stable in activity
MgIn2S4-Co2 sample (Figure 1f). The TEM-EDX elemental mapping
and it can be utilised repeatedly.
(Figure 1g) manifested that the Mg, In, S, and Co elements uniformly
distributed throughout the MgIn2S4-Co2 sample.
Figure 2. Time dependence (a) and apparent rate constants (b) of CH4
yield for photo-reduction CO2 over as-obtained samples under visible
light irradiation (λ > 420 nm).
Figure 1. Time-dependent high-resolution Co 2p XPS spectra upon
Ar+ sputtering for MgIn2S4-Co2 sample (a). SEM images of MgIn2S4
2 | J. Name., 2020, 00, 1-3
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Upon realizing the improved photocatalytic activity by Co
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DOI: 10.1039/D0DT01686A
doping, a question naturally arises: what is the performance
enhancement mechanism? The separation and migration efficiency
of photo-induced charge carriers is pivotal for photocatalytic activity.
Photoluminescence decay curves and EIS Nynquist plots were
measured (Figure 3a-b). The short lifetime τ1 and long lifetime τ2 was
ascribed to the direct formation of self-trapped excitons and the
indirect formation of self-trapped excitons, respectively.23 Both the
short and long lifetime of MgIn2S4-Co2 were shorter than that of
MgIn2S4. Shorter lifetime represents the presence of a nonradiative
pathway, namely, the delocalization of electrons from MgIn2S4 to Co
and hence efficient carrier separation.24 In general, a smaller arc
radius of EIS Nynquist plots usually indicates a higher charge transfer
efficiency.25 The MgIn2S4-Co2 possessed a smaller arc radius
compared to MgIn2S4 whenever with or without light irradiation. It
could be deduced that Co doping MgIn2S4 can inhibit the
recombination and improve the separation of photogenerated
charge carriers, as a result, enhancing the photocatalytic activity.
To further gain insight into the intrinsic reason for the excellent
performance of Co doped MgIn2S4, we performed theoretical
calculations using density functional theory (DFT) coupled with a
plane-wave method. Presence of Co dopant could lower the CO2
adsorption energy (from -0.182 eV to -0.211 eV) and facilitate CO2
adsorption. It was theoretically possible that In atoms with charge
enrichment can act as active site to more efficiently stabilize the
COOH* intermediates of CO2 photocatalytic reduction reaction.26
After doping with Co, the bond length of C-In decreased (from 2.576
Å to 2.307 Å), reflecting that *COOH intermediate absorbed on the
doped catalyst surface was more stable. Charge difference
distribution of *COOH on the MgIn2S4 and Co-doped MgIn2S4 were
carried out to further verify the stability of COOH* absorbed on
catalyst surface (Figure 3c-f). Cyan and lilac sphere, represented
charge lost and equivalent charge gain. It could be detected that the
charge transfer between COOH* intermediates and catalyst become
more active after Co doping, which may be explained as follows. The
electronegativity of Co is larger than that of Mg, so S atoms originally
coordinated with Mg was hard to get electrons from Co, meanwhile,
In was caputured more electons by S and consequently exhibited
higher electropositivity. The Bader charge calculation comfirmed
that the valence state of Mg changed from +1.57 to 0.75, and the
valence state of In changed from +1.20 to 1.25. Moreover, the higher
chemical valence state of In (active site) was conducive to the
activation of CO2,27-28 which manifested the more stable adsorption
of COOH*. As the rate-determining step of CO2 photocatalytic
reduction reaction:26 ∗CO2 + H+ + e− → ∗COOH, the activation energy
barrier were calcululated to be 2.29 eV for MgIn2S4 and 2.00 eV for
Co-doped MgIn2S4, of which the decrease could be ascribed to
enhanced CO2 adsorption ability and the improved stability of
COOH* absorbed on catalyst surface.
Figure 3. Photoluminescence decay curves (insets are lifetime) (a)
and EIS Nynquist plots (b) for MgIn2S4 and MgIn2S4-Co2 samples.
Charge difference distribution of *COOH on the MgIn2S4 (c) and Co-
doped MgIn2S4 (e) from topview and their corresponding sideview
(d) and (f). The green, brown, yellow and blue atoms represented
Mg, In, S, and Co atoms, respectively. The asterisk (*) represented
the surface substrate active site.
The gradient distribution of Co element from inner to surface of
the catalyst would change its electronic band structure. The
conduction band (CB) potential of MgIn2S4, and MgIn2S4-Co2 samples
was measured to be -1.12 and -1.04 V versus the normal hydrogen
electrode (NHE) based on the Mott-Schottky curve (Figure S8a-b),
respectively, meeting the thermodynamic requirements of the CO2
reduction into CH4 reaction CO2 + 8H+ + 8e- → CH4 + 2H2O (the
reaction potential is -0.244 V VS NHE).29 After Co doping, as shown in
electronic band structure (Figure 5a-b) and Density of state (Figure
4c, Figure S9), an impurity energy level emerged above valence band
(VB) and CB edge down-shifted of Co-doped MgIn2S4 compared to
the pristine MgIn2S4. Therefore, the nonuniform charge distribution
between inner and surface of photocatalyst created a built-in
oriented electric field, which would extract and drive photoinduced
carriers migrate from inner to surface defect sites, where electrons
reduced CO2 into CH4 (Figure 4d). In a word, Co surface gradient
diffused doping MgIn2S4 microsphere created an oriented built-in
electric field, which could restrict charge recombination in the bulk,
meanwhile, Co doping also could facilitate CO2 adsorption and
decrease the activation energy barrier of the rate-determining step
for CO2 photocatalytic reduction reaction (*CO2 → *COOH), hence
enhancing photocatalytic performance.
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Figure 4. Electronic band structure of MgIn2S4 (a) and Co-doped
MgIn2S4 (b). Density of state of the simulated Co-doped MgIn2S4
system (c). Schematic energy level and charge carriers transfer
pathways in MgIn2S4-Co homojunction with a gradient distribution of
Co element from inner to surface of the catalyst (d).
In conclusion, we prepared gradient Co-doping MgIn2S4
(MgIn2S4-Co) homojunction photocatalyst via a facile hydrothermal
method. The combined analyze of experimental and theoretical
results elucidated that the surface gradient diffused of Co doping
created an oriented built-in electric field, which could efficiently
extract photogenerated carriers from inner to surface, so restricted
charge recombination in the bulk, meanwhile, Co doping also could
facilitate CO2 adsorption and decrease the activation energy barrier
of CO2 photocatalytic reduction reaction. The activity for
photocatalytic CH4 evolution realised conspicuous enhancement for
Co-doped MgIn2S4 samples compared with pristine MgIn2S4, which
was evaluated by a gas-solid reaction without any additional co-
catalysts or sacrificial agents. The highest CH4 evolution rate
achieved at MgIn2S4-Co2 sample (1.6 umol/g/h), reaching about 8-
fold higher than that of MgIn2S4 (0.2 umol/g/h). This work provides a
novel strategy for developing efficient and stable solar fuel catalysts
by surface gradient diffusion doping.
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This work was supported by the Fundamental Research Funds
for the Central Universities of China (Grant No. 2652015296) and the
Education Department of Jiangxi Province (Grant No. GJJ170216).
Conflicts of interest
There are no conflicts to declare.
Notes and references
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Co gradient doping in MgIn2S4, creates an oriented built-in electric field for efficiently
extracting carriers from inner to surface.