Spinel-Type Mixed Metal Sulfide NiCo2S4 for Efficient Photocatalytic Reduction of CO2 with Visible Light

Article abstract of DOI:10.1002/cctc.201901379

Mixed metal oxides with a spinel structure have exhibited great opportunities for photocatalytic CO₂ reduction; however, the abilities of their sulfide counterparts in this promising area are much less reported. Herein, we demonstrate the synthesis of a ternary metal sulfide, namely NiCo₂S₄, and its first application to catalyze the CO₂ photoreduction reaction under visible light irradiation. The NiCo₂S₄ material is prepared through a coupled solvothermal-ion exchange strategy, and is fully checked by diverse characterizations, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX) and N₂ sorption measurements. Under visible light irradiation in a classic tandem photochemical system, this non-noble-metal NiCo₂S₄ catalyst affords a considerable activity and high stability for CO₂ deoxygenative reduction, delivering a high CO-liberating rate of 43.5 μmol mg^?1 h^?1. The in-situ PL and transient photocurrent measurements reveal that the spinel catalyst can hamper recombination and accelerate transfer of light-excited charge carriers, and thus boosting the CO₂ reduction reaction. At last, a possible mechanism of the NiCo₂S₄-catalyzed CO₂ photoreduction reaction is proposed.

Full text of DOI:10.1002/cctc.201901379

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DOI: 10.1002/cctc.201901379
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Spinel-Type Mixed Metal Sulfide NiCo2S4 for Efficient
Photocatalytic Reduction of CO2 with Visible Light
Zhuang Xiong,[a] Lijuan Huang,[a] Junwen Peng,[a] Yidong Hou,[a] Zhengxin Ding,*[a] and
Sibo Wang[a]
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Mixed metal oxides with a spinel structure have exhibited great
opportunities for photocatalytic CO2 reduction; however, the
abilities of their sulfide counterparts in this promising area are
much less reported. Herein, we demonstrate the synthesis of a
ternary metal sulfide, namely NiCo2S4, and its first application to
catalyze the CO2 photoreduction reaction under visible light
irradiation. The NiCo2S4 material is prepared through a coupled
solvothermal-ion exchange strategy, and is fully checked by
diverse characterizations, including X-ray diffraction (XRD),
scanning electron microscopy (SEM), transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy (XPS),
energy-dispersive X-ray spectroscopy (EDX) and N2 sorption
measurements. Under visible light irradiation in a classic
tandem photochemical system, this non-noble-metal NiCo2S4
catalyst affords a considerable activity and high stability for CO2
deoxygenative reduction, delivering a high CO-liberating rate of
43.5 μmol mgÀ 1 hÀ 1. The in-situ PL and transient photocurrent
measurements reveal that the spinel catalyst can hamper
recombination and accelerate transfer of light-excited charge
carriers, and thus boosting the CO2 reduction reaction. At last, a
possible mechanism of the NiCo2S4-catalyzed CO2 photoreduc-
tion reaction is proposed.
Introduction
to be active sites of photocatalysts, which can powerfully
promote the separation and transfer of light-excited charges of
CO2 photoconversion catalysis.[18,26–31] When the reduction
reaction is further coupled with the protons, reinforced
performance of CO2 photoreduction can be achieved.
Generation of valuable carbon fuels with copious CO2 as the
feedstock by renewable energy is an ideal strategy to cope with
the universal crisis on energy and environment.[1–4] Among the
proposed strategies, photocatalytic CO2 reduction is deemed as
a promising technology to realize the direct conversion of
sunlight irradiation into energy-rich chemicals (e.g., CO) for
energy supply, and simultaneously, reduce atmospheric CO2
level to alleviate the climate changes.[5–12] Despite the attractive
prospect, the extreme stability of linear CO2 molecules imposes
great energy barrier to the conversion reaction. Besides, the
multistep-involved photocatalytic processes usually exhibit low
efficiencies, particularly the separation and migration of photo-
induced charges.[13–14] These two factors greatly hamper the
performance of semiconductor photocatalysts to reduce CO2.
Indeed, a huge number of catalysts have been developed to
work CO2 photoreduction with considerable progress
achieved,[15–23] the exploitation of new alternatives with high-
efficiency and stability, especially the candidates made of
affordable elements, is still highly desirable.
Recently, mixed metal oxides with a spinel structure, for
example, ZnCo2O4,[32] NiCo2O4,[33–34] CuCo2O4,[35] and MnCo2O4,[36]
have been developed as active catalysts for photochemical CO2
reduction by the cooperation of appropriate photosensitizes,
due to their plentiful redox functions, high chemical stability,
and rich optical and electronic features.[37] In comparison,
ternary metal sulfides possess the advantages of high electrical
conductivity, low electronegativity, rich redox properties and
suitable band structures, which are favorable features to photo-
catalytic CO2 reduction.[38–40] Based on all these considerations
aforementioned, it may be highly desirable to exploit the
functions of spinel metal sulfides for photocatalytic CO2
reduction reactions.
In this work, the spinel-type ternary metal sulfide NiCo2S4
(Figure 1a) is demonstrated to be an efficient and noble-metal-
free catalyst for CO2 photoreduction with visible light irradi-
The efficient CO2 photoreduction systems are typically
composed of several modules, such as light harvesters, electron
mediator, CO2 activators and their desired integration, to
operate the reaction through a tandem manner.[24–27] Nickel and
cobalt species with rich redox states have been demonstrated
[a] Z. Xiong, L. Huang, J. Peng, Prof. Y. Hou, Prof. Z. Ding, Dr. S. Wang
State Key Laboratory of Photocatalysis on Energy and Environment
College of Chemistry
Fuzhou University
Fujian, Fuzhou, 350002 (China)
E-mail: zxding@fzu.edu.cn
Figure 1. (a) Crystal structure of cubic spinel NiCo2S4. (b) XRD pattern of
NiCo2S4.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201901379
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ation. The NiCo2S4 material is synthesized by a coupled
solvothermal-ion exchange method and is fully studied by
diverse physiochemical characterizations, including XRD, XPS,
SEM, TEM, EDX, and N2 sorption. Performance of the NiCo2S4
catalyst is evaluated in the classic CO2 photoreduction system
formation of highly crystallized spinel NiCo2S4 material. Elemen-
tal mappings of Ni, Co and S indicate that these elements are
uniformly distributed in the whole NiCo2S4 microspheres (Fig-
ure S1, Supporting Information).
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The chemical composition of the NiCo2S4 material is first
checked by EDX. As shown in Figure 3a, only signals of Ni, Co
and S elements are detected. The mass ratio of S in the NiCo2S4
sample is 41.95% determined by ICP-OES tests, which is nearly
identical to the theoretical value of 42.08%. The XPS survey
spectrum also confirms the elemental composition of as-
prepared NiCo2S4 solid (Figure S2a, Supporting Information).
Only one O species is detected in the survey spectrum, which is
attributed to the physically adsorbed oxygen (Figure S2b,
Supporting Information). All these results further indicate the
formation of pure phase NiCo2S4 material. The high resolution
XPS spectra of Ni 2p (Figure 3b) and Co 2p (Figure 3c) can be
well fitted into two spin-orbit doublets together with two
shakeup satellites (denoted as “Sat.”). In the high-resolution Ni
2p XPS spectrum, the two peaks with binding energies of 856.4
2+
with Ru(bpy)3 (shortened as Ru) as a photosensitizer and
triethanolamine (TEOA) as an electron donor. Under visible light
irradiation (λ�420 nm), NiCo2S4 shows a considerable CO2
reduction
activity
with
a
CO-releasing
rate
of
43.5 μmol mgÀ 1 hÀ 1. Importantly, the spinel material is stable to
preserve its intrinsic catalytic activity for 5 cycles. PL and
transient photocurrent analyses indicate that the NiCo2S4
catalyst can efficiently impede recombination and promote
transfer of photoexcited charges. Moreover, a possible mecha-
nism of the photocatalytic CO2 reduction catalysis is also
proposed.
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Results and Discussions
and 873.9 eV correspond to Ni 2p3/2 and Ni 2p1/2
,
To study the crystallographic structure and phase purity of the
NiCo2S4 material, powder XRD characterization is carried out. As
presented in Figure 1b, all the diffraction peaks in the XRD
pattern of NiCo2S4 can be indexed to the standard pattern of
the cubic phase (JCPDS card No. 43-1477). The sharp XRD peaks
suggest the high crystallinity of the sample. Besides, no
impurity peaks are detected, indicating the preparation of
single phase NiCo2S4 material through the developed recipe.
SEM and TEM analyses are conducted to discern the
morphology and structure of as-prepared NiCo2S4. The SEM
images exhibit that the NiCo2S4 particles display a microsized
quasi-spherical morphology with a rough surface. (Figure 2a, b).
The TEM image demonstrates the solid structure of the material
(Figure 2c). The high-resolution TEM (HRTEM) image gives clear
lattice fringes with the d-spacings of 0.34 and 0.23 nm (Fig-
ure 2d–h), corresponding to the (220) and (400) crystal planes
of cubic NiCo2S4, respectively. The HRTEM results reaffirm the
respectively,[33,41] which indicates the existence of both Ni2+ and
Ni3+ in the spinel material. The strong peaks at around 779.78
and 794.84 eV in Co 2p high-resolution XPS spectrum represent
the Co 2p3/2 and Co 2p1/2 of NiCo2S4, pointing to the presence of
Co2+ and Co3+ couple in the structure.[36,42] The S 2p spectrum
displays two peaks located at 161.3 and 162.5 eV, attributing to
S 2p3/2 and S 2p1/2, respectively (Figure 3d), which reveals the
valance of S2À in the sample.[43–44] Another peak with the
binding energy of 168.2 eV represents the SÀ O bond, implying
that some of the surface sulfide ions are oxidized during the
XPS tests.[45] These observations display the numerous solid
redox species (e.g., Ni2+, Ni3+, Co2+, and Co3+) in spinel NiCo2S4
structure, which may offer rich catalytic-active locations to
promote heterogeneous photocatalytic CO2 reduction reactions.
Figure 2. (a,b) SEM images, (c) TEM image, (d) HRTEM image, (e,g) the
inverse fast Fourier transformation (IFFT) images, (f,h) the corresponding line
scan of the corresponding dotted regions of the NiCo2S4 material.
Figure 3. (a) EDX spectrum of NiCo2S4. High-resolution XPS spectra of
NiCo2S4: (b) Ni 2p, (c) Co 2p, and (d) S 2p.
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Figure 4. N2 sorption isotherms of the NiCo2S4 sample. Inset is the
corresponding pore size distribution curve.
Figure 5. (a) Time courses of photocatalytic CO evolutions, (b) Production of
CO and H2 in the photo-chemical CO2 reduction system under various
conditions, (c) Results of GC-MS analysis for CO produced from 13CO2 isotope
experiment, (d) Wavelength dependence of yields of CO and H2, and the
light absorption spectrum of Ru photosensitizer.
The textural characteristics of the NiCo2S4 material are
investigated by N2 sorption measurements. As shown in
Figure 4, the N2 adsorption-desorption isotherms of NiCo2S4 are
categorized as the type IV fashion with a type H3 hysteresis
loop. Such results indicate that the material possesses a lot of
mesopores. The presence of mesopores in NiCo2S4 is further
identified by the corresponding pore size distribution plot
(Inset, Figure 4). The specific BET surface area and pore volume
of NiCo2S4 are calculated to be of 13 m2 gÀ 1 and 0.04 cm3 gÀ 1,
respectively. In general, the mesoporous structure is favorable
to unveil more catalytically active sites and accelerate mass/
charge transport for heterogeneous CO2 photoreduction
reactions.[46–47]
420 nm. The photocatalytic CO2-to-CO reduction efficiency is
comparable to that of many reported works in similar systems
(Table S2). Thereafter, the generation rates of both CO and H2
reduce clearly, which is ascribed to the photo-bleaching effect
of the ruthenium complex after long-time catalytic
operation.[25,50–51] The degradation of the Ru photosensitizer is
further revealed by the UV-visible spectrum of the reaction
solution after the CO2 reduction reaction for 1 h (Figure S3,
Supporting Information).
A series of control experiments are designed to investigate
the functions of the CO2 photoreduction system. First, the high-
efficiency of the NiCo2S4 catalyst is demonstrated by the
observation that no efficient CO2 reduction reaction happens
with its absence in the system (column 2, Figure 5b). When
conducting the reaction without the presence of visible light
irradiation (column 3) or the Ru photosensitizer (column 4), no
any products can be detected. These results suggest that the
CO2 reduction reaction is triggered by visible light excitation of
the photosensitizer. Previous works have demonstrated that
Co2+ ion is an active homogeneous for CO2 photoreduction in
the current system.[52] The control experiment of using Co2+
(i.e., CoCl2·6H2O) to replace NiCo2S4 only produces much
weakened CO2 reduction performance (column 5), highlighting
the high performance of the NiCo2S4 particulate catalyst in the
system. After the photoreaction for 30 min, the NiCo2S4 catalyst
is removed for the system by filtration, and the resultant
supernatant is further applied to re-start the CO2 reaction
catalysis. No apparent formation of CO/H2 is observed (column
6), indicating that the leaching of Co2+/Ni2+ is negligible.
Moreover, this finding also confirms the heterogeneous nature
of the CO2 photoreduction reaction.
The CO2 photoreduction performance of the NiCo2S4 catalyst
is studied in the tandem catalytic system that works in mixed
solvents of H2O and acetonitrile under mild conditions (e.g.,
°
30 C, 1 atm CO2) and visible light illumination, employing Ru
and TEOA as
a photosensitizer and an electron donor,
respectively.[24] CO is detected as the sole CO2 reduction
product, coupled with a small amount of H2. No any other CO2
reduction products (e.g., HCOOH, CHOH, CH3OH and CH4) are
detected in the system, which is in agreement with the results
of previous works.[18,26,33,36] Such a product distribution may be
attributed to lower conduction band potential of NiCo2S4. To be
specific, the formation of CH4 and other hydrocarbon products
typically proceeds via a series of elementary steps with CO as
the intermediate, involving transfer of an electron in most of
the steps.[48] When the reduction potential of the excited
electron is lower, the reaction rate of the elementary steps
becomes slower kinetically. Therefore, the successive reaction
steps after CO formation cannot proceed fast enough before
CO desorbs from the surface of the catalyst,[49] thus leading to
CO as the main product. The time-yield plots (Figure 5a) of the
products indicate that in the initial reaction period of 1 h, the
catalytic system displays a high efficiency, giving a CO evolution
rate of 43.5 μmol mgÀ 1 hÀ 1 together with a CO selectivity of
70.2% in the products, which affords an apparent quantum
yield (AQY) of 1.45% under monochromatic light irradiation of
To research the carbon origin of produced CO, a reference
reaction utilizing Ar as the gas feedstock is conducted (column
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7), which only generates H2 but not any CO. Also, when the
reaction is performed in a 13CO2 isotope atmosphere (Figure 5c),
13CO with the m/z value of 29 is observed as the sole product.
During the GC-MS analysis, some 13CO can be cracked into 13C
and 16O fragments with m/z=13 and 16, respectively, leading
to the presence of these two signals.[53] Such discoveries firmly
corroborate that the CO formation indeed comes from the CO2
gas source. The photocatalytic CO2 conversion reactions are
further assessed under light illumination of various wavelengths
controlled by applying suitable long-pass cutoff filters (Fig-
ure 5d). As can be seen, the variation tendency in CO/H2
generation rate is consistent with the optical harvesting
spectrum of the Ru photosensitizer. This observation further
reflects that the CO2 reduction catalysis is evoked by optical
absorption of the photosensitizer.
Stability of the NiCo2S4 catalyst is checked by the cycling
experiments. As presented in Figure 6a, when using the catalyst
for successive CO2 reduction reaction for 5 cycles, there is no
noticeable decrease in the generation rates of CO and H2,
revealing the high stability and reusability of the spinel catalyst
in the reaction system. In addition, by comparing the XRD
patterns (Figure 6b) and XPS spectra (Figure S4, Supporting
Information) of the fresh and used NiCo2S4 samples, its high
stability in the reaction system of the catalyst is further
supported.
In situ steady-state photoluminescence (PL) and transient
photocurrent generation spectra of the systems are collected to
explore the separation and transport dynamics of photoexcited
charges during the CO2 reduction catalysis. The apparent PL
quenching of the system with the presence of NiCo2S4 indicates
the reduced recombination of photo-stimulated charges (Fig-
ure 7a).[54–56] Meanwhile, the system displays an enhanced
transient photo-current response compared to the situation of
removing the NiCo2S4 catalyst, reflecting the spinel catalyst can
assist transport of photoexcited charges (Figure 7b). The
corresponding findings of PL and photocurrent analyses
indicate that the spinel NiCo2S4 catalyst can efficiently expedite
the separation and transfers of light-induced charges, and
therefore, improving the CO2 photoreduction performance.
To determine the band structures of the NiCo2S4 catalyst,
ultraviolet photoelectron spectroscopy (UPS) tests are carried
out. The valance band (VB) position of NiCo2S4 is determined to
be of 0.67 V (vs. normal hydrogen electrode, NHE) (Figure S5a,
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Figure 7. (a) PL spectra and (b) Transient photocurrent responses of the
reaction systems with and without the NiCo2S4 catalyst.
Supporting Information). The band gap energy of NiCo2S4 is
about 1.45 eV estimated by the Tauc plot (Figure S5b, Support-
ing Information). Therefore, the conduction band of NiCo2S4 is
calculated about À 0.78 V (vs. NHE), which is more negative
than the redox potential of CO2/CO (i.e., À 0.53 V vs. NHE) and
more positive than the redox potential of Ru(bpy)32+/Ru(bpy)3
+
(i.e., À 1.09 V vs. NHE). As a result, the NiCo2S4 catalyst holds the
suitable redox potential to accept the photoexcited electron
from the reduced photosensitizer to operate the CO2-to-CO
reduction catalysis. It should be noted that NiCo2S4 serves as an
active “co-catalyst” for the CO2 reduction reaction and itself
alone cannot catalyze the reaction. This may be because that
the valance band position of NiCo2S4 is too lower and the
photogenerated holes therefore only has poor oxidation ability,
which is not enough to work the oxidation reaction. As a result,
the photogenerated electrons and holes will recombine imme-
diately and do not have the chance to run the redox reactions.
Based on these results, the possible process of the CO2
photoreduction reaction catalyzed by NiCo2S4 is proposed. As
illustrated in Scheme 1, being irradiated by visible light, the Ru
2+
(bpy)3 complex will be motivated to the excited state of Ru
2+* [57]
2+*
(bpy)3
.
The Ru(bpy)3
species would then experience a
reductive quenching by the electron donor TEOA to generate
the reduced state [Ru(bpy)3]+.[58–63] Afterward, the excited
electrons of the reduced ruthenium complex will delocalize and
migrate to the NiCo2S4 catalyst to reduce the CO2 molecules
Figure 6. (a) Generation rates of CO and H2 in stability tests and (b) XRD
patterns of the fresh and used NiCo2S4 samples.
Scheme 1. Possible reaction processes of the CO2 photoreduction catalyzed
by the NiCo2S4 catalyst under visible light irradiation.
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In summary, demonstrated in this work are the facile synthesis
of the spinel mixed metal sulfide NiCo2S4 and its first utilization
as an efficient and stable catalyst for photocatalytic CO2
reduction under visible light. The NiCo2S4 material is studied by
various physicochemical techniques. In a typical photo-chemical
system, the spinel catalyst exhibits considerable performance
for CO2 deoxygenation with a high CO evolution rate of
43.5 μmol mgÀ 1 hÀ 1, under the assistance of a ruthenium light
harvester. Notably, the NiCo2S4 catalyst also manifests high
stability and reusability for the CO2 conversion reaction in the
constructed system. The in-situ PL and photocurrent response
characterizations reveal the NiCo2S4 catalyst can effectively
prohibit recombination and facilitate transportation of photo-
induced charges. This work may encourage further studies on
spinel-type mixed metal sulfides and their hybrids of semi-
conductors for photocatalytic CO2 reduction toward the
sustained solar fuel generation.
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Acknowledgements
This work was supported financially by the National Natural
Science Foundation of China (21872033), the Open Project
Program of the State Key Laboratory of Photocatalysis on Energy
and Environment (Grant No. SKLPEE-KF201715), and the State Key
Laboratory of Photocatalysis on Energy and Environment (SKLPEE-
2017B02).
Conflict of Interest
The authors declare no conflict of interest.
Keywords: CO2 reduction,
photocatalysis, · spinel
· metal sulfides, · NiCo2S4, ·
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Manuscript received: July 29, 2019
Revised manuscript received: August 28, 2019
Accepted manuscript online: September 1, 2019
Version of record online: ■■■, ■■■■
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It is light to do it! Spinel-type mixed
metal sulfide NiCo2S4 is prepared
through a coupled solvothermal-ion
exchange method. Under the assis-
tance of a ruthenium photosensitizer,
the NiCo2S4 catalyst is demonstrated
to be of high-efficiency for photoca-
talytic CO2 reduction with visible
light, affording a notable CO
Z. Xiong, L. Huang, J. Peng, Prof. Y.
Hou, Prof. Z. Ding*, Dr. S. Wang
1 – 7
Spinel-Type Mixed Metal Sulfide
NiCo2S4 for Efficient Photocatalytic
Reduction of CO2 with Visible Light
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evolution rate of 43.5 μmol mgÀ 1 hÀ 1.
Moreover, the NiCo2S4 catalyst also
manifests high stability and reusabil-
ity for the CO2 reduction reaction in
the tandem system.