Co-MOF as an electron donor for promoting visible-light photoactivities of g-C3N4 nanosheets for CO2 reduction

Article abstract of DOI:10.1016/S1872-2067(19)63497-2

A possible mechanism for boosting the visible-light photoactivities of graphitic carbon nitride (g-C₃N₄) nanosheets for CO₂ reduction via coupling with the electron donor Co-metal-organic framework (MOF) is proposed in this study. Specifically, Co-MOF as an electron donor is capable of transferring the photogenerated electrons in the lowest unoccupied molecular orbital (LUMO) to the conduction band of g-C₃N₄ to facilitate charge separation. As expected, the prepared Co-MOF/g-C₃N₄ nanocomposites display excellent visible-light-driven photocatalytic CO₂ reduction activities. The CO production rate of 6.75 μmol g^?1 h^?1 and CH₄ evolution rate of 5.47 μmol g^?1 h^?1 are obtained, which are approximately 2 times those obtained with the original g-C₃N₄ under the same conditions. Based on a series of analyses, it is shown that the introduction of Co-MOF not only broadens the range of visible-light absorption but also enhances the charge separation, which improves the photocatalytic activity of g-C₃N₄ to a higher level. In particular, the hydroxyl radical (?OH) experiment was operated under 590 nm (single-wavelength) irradiation, which further proved that the photogenerated electrons in the LUMO of Co-MOF can successfully migrate to g-C₃N₄. This work may provide an important strategy for the design of highly efficient g-C₃N₄-based photocatalysts for CO₂ reduction.

Full text of DOI:10.1016/S1872-2067(19)63497-2

Chinese Journal of Catalysis 41 (2020) 514–523
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Co-MOF as an electron donor for promoting visible-light
photoactivities of g-C3N4 nanosheets for CO2 reduction
Qiuyu Chen a, Sijia Li a, Hongyi Xua, Guofeng Wang a,*, Yang Qu a,#, Peifen Zhu b,$, Dingsheng Wang c
a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University,
Harbin 150080, Heilongjiang, China
b Department of Physics and Engineering Physics, The University of Tulsa, Tulsa, OK 74104, USA
c Department of Chemistry, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A possible mechanism for boosting the visible-light photoactivities of graphitic carbon nitride
(g-C3N4) nanosheets for CO2 reduction via coupling with the electron donor Co-metal-organic
framework (MOF) is proposed in this study. Specifically, Co-MOF as an electron donor is capable of
transferring the photogenerated electrons in the lowest unoccupied molecular orbital (LUMO) to
the conduction band of g-C3N4 to facilitate charge separation. As expected, the prepared
Co-MOF/g-C3N4 nanocomposites display excellent visible-light-driven photocatalytic CO2 reduction
activities. The CO production rate of 6.75 µmol g–1 h–1 and CH4 evolution rate of 5.47 µmol g–1 h–1 are
obtained, which are approximately 2 times those obtained with the original g-C3N4 under the same
conditions. Based on a series of analyses, it is shown that the introduction of Co-MOF not only
broadens the range of visible-light absorption but also enhances the charge separation, which im-
proves the photocatalytic activity of g-C3N4 to a higher level. In particular, the hydroxyl radical
(•OH) experiment was operated under 590 nm (single-wavelength) irradiation, which further
proved that the photogenerated electrons in the LUMO of Co-MOF can successfully migrate to
g-C3N4. This work may provide an important strategy for the design of highly efficient g-C3N4-based
photocatalysts for CO2 reduction.
Received 15 July 2019
Accepted 23 August 2019
Published 5 March 2020
Keywords:
Co-MOF
g-C3N4 nanosheets
Charge separation
Visible-light photoactivity
Photocatalytic CO2 conversion
© 2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
since the first report on the utilization of TiO2 for the photo-
catalytic reduction of CO2 into organic chemical products in
A continuous increase in the concentration of carbon diox-
ide (CO2) has led to enhanced greenhouse effect, which in turn
has resulted in global warming [1–3]. In order to address this
issue, photocatalytic reduction of CO2 into renewable fuels is
considered as an attractive and promising solution for directly
converting solar energy into chemical products [4–6]. In fact,
1979, tremendous efforts have been devoted to developing
high-efficiency catalysts for CO2 photoreduction [7]. In recent
years, various semiconductor photocatalysts, such as Zn2GeO4
[8], CdS [9], Fe3O4 [10], g-C3N4 [11], SrTiO3 [12], and TiO2
[13,14], have been researched for diverse photocatalytic appli-
cations, including CO2 reduction. However, these semiconduc-
* Corresponding author. E-mail: wanggf_w@163.com
# Corresponding author. E-mail: quyang@hlju.edu.cn
$ Corresponding author. E-mail: peifen-zhu@utulsa.edu
This work was supported by the National Natural Science Foundation of China (21871079,21501052), and the Outstanding Youth Project of Natural
Science Foundation of Heilongjiang Province (YQ2019B006).
DOI: S1872-2067(19)63497-2 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 3, March 2020

Qiuyu Chen et al. / Chinese Journal of Catalysis 41 (2020) 514–523
515
tors either display large bandgaps, which lead to quite low visi-
2.1. Synthesis of samples
ble-light-driven photoactivities, or are toxic, which causes addi-
tional environmental issues. Thus, seeking environmentally
benign semiconductors with moderate bandgaps is the key to
addressing the issue of global warming.
All the reagents used in this work were of analytical grade
and employed without further purification. 2-Methylimidazole
(2-MeIm), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), meth-
anol (CH3OH), urea, and deionized water were used in all the
reaction systems.
Recently, g-C3N4 has attracted much attention as one of the
most promising semiconductor materials for visi-
ble-light-driven CO2 reduction owing to its suitable bandgap
energy (2.7 eV), pollution-free property, and good chemical and
thermal stabilities [15]. However, the photocatalytic CO2 reduc-
tion performance of current g-C3N4 photocatalysts is still far
from that required for practical applications because of the
limited light absorption capability and high recombination rate
of the photogenerated electron-hole pairs, which severely af-
fect the photocatalytic CO2 reduction activity [16]. Thus, it is
necessary to increase the charge separation rate of g-C3N4 to
some extent in order to improve the visible-light-driven CO2
reduction activity. A variety of methods have been developed to
improve the charge separation efficiency and further enhance
the overall activities of g-C3N4-based photocatalysts [17].
Among them, coupling g-C3N4 with other materials with suita-
ble conduction band (CB) positions is one of the most promis-
ing solutions to improving the performance of the photocata-
lytic system, as it can increase the charge separation rate [18].
Cao et al. [19] reported much higher H2 and CH4 production
rates under visible-light illumination by employing
g-C3N4-In2O3 composite as a photocatalyst, than those obtained
with g-C3N4. Shi et al. [20] also revealed a high photocatalytic
CO2 reduction activity by combining g-C3N4 with NaNbO3 nan-
owires.
Extensive efforts have been taken to find a material that can
be combined with g-C3N4 to boost the charge separation effi-
ciency and thereby improve the photocatalytic CO2 reduction
performance. Recently, Co-metal-organic framework (MOF), a
typical MOF, has received significant attention owing to its ex-
cellent features that offer promise in important applications in
the photocatalytic field [21]. The narrower bandgap of Co-MOF
leads to a wider range of visible-light absorption. Furthermore,
its lowest unoccupied molecular orbital (LUMO) potential is
highly negative and matches with the CB level of g-C3N4.
Therefore, Co-MOF was selected for combining with g-C3N4 to
improve the charge separation efficiency and enhance the
photocatalytic activity of g-C3N4.
For the synthesis of g-C3N4, urea was heated at 550 °C for 4
h under N2 atmosphere to prepare g-C3N4 nanosheets. Finally,
the acquired light yellow product was ground for further ex-
perimental work, and the sample has been named CN here.
Co-based MOF materials were synthesized via the low-cost
and large-scale coprecipitation method. In a typical synthesis, 2
mmol Co(NO3)2·6H2O and 12 mmol 2-MeIm were first dis-
solved in 30 mL and 10 mL methanol, respectively. Then, the
solution of 2-MeIm was added slowly to the solution of
Co(NO3)2·6H2O with vigorous stirring. After the two solutions
were mixed thoroughly, the resulting solution was aged at
room temperature for 24 h. The purple products obtained were
centrifuged and washed with ethanol several times and then
dried at 60 °C. Herein, the material has been named Co-MOF.
The composites Co-MOF/g-C3N4 were synthesized through a
facile method. A certain amount of g-C3N4 was added to meth-
anol and placed in an ultrasonic bath for 3 h to exfoliate the
g-C3N4 completely. Then, Co-MOF of different masses was dis-
persed in the above solution with ultrasonication for 1 h. Final-
ly, the suspension was dried at 60 °C. Here, the obtained sam-
ples of Co-MOF/g-C3N4 composed of 1%, 2%, 3%, 5%, 10%,
20%, and 50% Co-MOF (wt%) have been named Co-CN1,
Co-CN2, Co-CN3, Co-CN5, Co-CN10, Co-CN20, and Co-CN50,
respectively.
2.2. Characterization
The morphologies and sizes of the samples were analyzed
by transmission electron microscopy (TEM; JEM-2100 electron
microscope, Japan) and scanning electron microscopy (SEM;
Hitachi, S-4800). X-ray diffraction (XRD) measurements were
performed with a Bruker D8 Advance instrument using Cu Kα
radiation (λ = 1.5406 Å). The Fourier transformed infrared
(FT-IR) spectra were measured on a Perkin-Elmer Spectrum
One spectrometer by using KBr pellets. The diffuse reflectance
spectra (DRS) of the photocatalysts were obtained using a
UV-visible spectrophotometer (SHIMADZU UV-2550) in the
wavelength range 200–800 nm. Thermogravimetric analysis
(TGA) was carried out from 20 °C to 800 °C in air atmosphere
by using a thermal analyzer (TGA-7, Perkin-Elmer, USA). The
N2 adsorption and desorption isotherms were analyzed with
Tristar II 3020. The pore size distribution charts were obtained
by the Barrett-Joyner-Halenda method. The elemental compo-
sitions of the catalysts were determined using X-ray photoelec-
tron spectroscopy (XPS; Kratos-AXIS ULTRA DLD apparatus
with Al (mono) X-ray source). The photoluminescence (PL)
spectra were measured by a Hitachi F-4600 fluorescence spec-
trophotometer loaded with a 150 W Xe lamp at the excitation
wavelength of 370 nm.
Based on the above discussion, a possible mechanism for
enhanced photocatalytic CO2 reduction of g-C3N4 nanosheets
via coupling with the electron donor Co-MOF was proposed. As
had been expected, a series of Co-MOF/g-C3N4 nanocomposites
were fabricated that displayed remarkable photocatalytic CO2
reduction activities that far exceeded that of pristine g-C3N4
nanosheets. Based on our analysis, the improved photocatalytic
activity is attributed to the widened visible-light absorption
range and to efficient charge separation between Co-MOF and
g-C3N4. This work may provide a new perspective for promot-
ing the photocatalytic performance of g-C3N4.
2. Experimental

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Qiuyu Chen et al. / Chinese Journal of Catalysis 41 (2020) 514–523
2.3. Hydroxyl radical (•OH) measurements
chromatograph (GC2002). All the photocatalytic reactions were
measured twice and the data reported are the average values to
ensure accuracy.
Coumarin fluorescence strategy was adopted to analyze the
number of •OH produced during the photocatalytic reaction
process. In this system, 50 mg of the product was dissolved in
40 mL aqueous coumarin solution (0.001 mol L–1), and then,
the solution was stirred adequately for 10 min and illuminated
with a 300 W Xe lamp for 1 h. The solution was separated by
centrifugation and poured into a Pyrex glass cell. The obtained
solution was analyzed through fluorescence measurement of
7-hydroxycoumarin by using a spectrofluorometer at the exci-
tation wavelength of 390 nm (Perkin-Elmer LS55).
3. Results and discussion
The morphological properties of the typical samples were
characterized by TEM and SEM (Fig. 1). The ultrathin
nanosheets observed for pure CN are shown in Fig. 1(a) and
Fig. 1(d). The high surface-to-volume ratio of these ultrathin
nanosheets significantly increased the number of reac-
tion-active sites, which facilitated the photocatalytic reduction
[22]. In addition, Fig. 1(b) and Fig. 1(e) show that the Co-MOF
structure was successfully deposited on the surface of CN while
the nanosheets structure remained intact. Fig. 1(c) and Fig. 1(f)
show the polyhedral structure and solid texture of pure
Co-MOF, respectively. The color of pure Co-MOF is bright pur-
ple. Therefore, with the increase in Co-MOF content of the
nanocomposite, the color of Co-MOF/g-C3N4 gradually changed
to purple.
The crystal structures and the phases of the products were
obtained by XRD. Fig. 2(a) shows the XRD patterns of the origi-
nal CN, Co-MOF, and Co-MOF/g-C3N4 nanocomposites. The
sharp peaks observed in the XRD pattern of Co-MOF indicate
that an advanced crystalline material was obtained. This is con-
sistent with the results in previous reports [23–25]. Thus, we
successfully synthesized Co-MOF. The two diffraction peaks
observed at 13.3° and 26.7° in the XRD pattern of pristine CN
are attributed to the in-plane trigonal N linkages of tri-triazine
units and to the stacking of conjugated aromatic systems, re-
spectively [26]. It is obvious that, with the increase in the
amount of Co-MOF, the peaks at 13.3° and 26.7° are weakened.
Moreover, the composites with higher Co-MOF contents
showed the diffraction peaks of CN and Co-MOF distinctly,
which indicated a two-phase composition of CN and Co-MOF in
these samples.
2.4. Photoelectrochemical measurements
Photoelectrochemical measurements were carried out by
using a computer-controlled CHI-660 electrochemical work-
station (Chenhua Instrument, Shanghai, China) with a 300 W Xe
arc lamp as the light source. The sample was transformed into a
film that served as the working electrode, and a Pt foil and
Ag/AgCl (with saturated KCl) were used as the counter and
reference electrodes, respectively. Aqueous 0.5 mol L–1 Na2SO4
solution was used as the electrolyte. Before the experiments,
high-purity N2 gas was passed through the electrolyte for 30
min. For preparing the working electrodes, conductive fluo-
rine-doped tin oxide (FTO)-coated glasses were used as sub-
strates after thoroughly washing them with ethanol and deion-
ized water. The working electrodes were prepared as follows.
0.1 g of the as-prepared sample was added to 1 mL isopropyl
alcohol under vigorous stirring. 0.05 g of Macrogol-2000 was
added to the above solution and thoroughly mixed using ultra-
sonic irradiation for 10 min. After that, the suspension was
subjected to vigorous stirring for 30 min, and then, 0.05 mL
acetylacetone was added to it. The obtained solution was kept
under adequate stirring for a week. The mixture was painted
onto a 1 cm  1 cm FTO glass electrode, which was the efficient
surface area, by doctor blade method. Finally, the electrode was
dried at room temperature and annealed at 150 °C for 2 h un-
der N2 atmosphere. The specific CO2 electrochemical reduction
step was procedurally the same as the photoelectrochemical
measurements mentioned above, except that the Na2SO4 elec-
trolyte was introduced into CO2 instead of N2 before the test.
The FT-IR spectra of the photocatalysts are shown in Fig.
2(b). For the Co-MOF samples, the bands observed between
600 and 1500 cm–1 are attributed to the stretching and bending
modes of the imidazole ring. The peak observed at 1580 cm–1 is
assigned to the stretching mode of the C=N bond in the ligand
2.5. Catalytic performance
The photocatalytic performance of the typical samples was
evaluated for visible-light-driven CO2 reduction. The entire
reaction took place in a 100 mL cylindrical steel reactor at
room temperature and the irradiation area was 3.5 cm2. 20 mg
of the powder sample was dispersed in 4 mL water under con-
stant stirring and then loaded into the reactor. High-purity CO2
was passed through the reaction system to remove air and
establish a balance between adsorption and desorption before
the irradiation. The samples were irradiated using a 300 W Xe
lamp with a 420 nm cutoff filter (PLSSXE300/300UV, Perfect-
light, Beijing), and the light intensity was 455 mW cm–1. Finally,
the generated gases were analyzed and quantified using a gas
Fig. 1. SEM images of (a) CN, (b) Co-CN2, and (c) Co-MOF, and TEM
images of (d) CN, (e) Co-CN2, and (f) Co-MOF.

Qiuyu Chen et al. / Chinese Journal of Catalysis 41 (2020) 514–523
517
Fig. 2. (a) XRD patterns; (b) FT-IR spectra; (c) UV-vis DRS; (d) Bandgap patterns; (e) N2 adsorption-desorption isotherms; (f) TGA curves.
2-MeIm. The bands at 2929 and 3135 cm−1 originated from the
stretching of the C–H bond of the aromatic ring and the ali-
phatic chain of 2-MeIm, respectively [27]. The bands of
1200–1700 cm−1 are primarily attributed to the typical
stretching vibrations of CN heterocycles. The peaks in the range
3000–3500 cm–1 correspond to the stretching vibration modes
of NH and NH2 groups. The peak observed at around 811 cm–1
in the spectrum of CN corresponds to the typical breathing
mode of the triazine unit. The characteristic peak intensity of
Co-MOF decreased gradually with the decrease in the amount
of Co-MOF, and the peak disappeared eventually. Notably, the
peak at 811 cm–1 for CN shifted to a higher wavenumber for
Co-CN50 composite, as shown in the inset of Fig. 2(b), which
indicated the existence of some interactions between CN and
Co-MOF; unfortunately, when the Co-MOF content was low, this
phenomenon was not obvious [28].
The optical absorptions of pure CN, Co-MOF, and
Co-MOF/g-C3N4 composite samples were analyzed using their
UV-vis DRS. As observed in Fig. 2(c), CN displayed only weak
visible-light absorption at ~460 nm, whereas Co-MOF exhibit-
ed an intense absorption band in the range 420–700 nm. Nota-
bly, compared with that of CN, the Co-MOF/g-C3N4 composites
showed increased absorptions (by different degrees) in the
visible-light region. Meanwhile, the absorption edges of the
composites slightly red-shifted with the increase in Co-MOF
content, which indicated increased absorption ability in the
visible region, compared to that of CN, thereby, more elec-
tron-hole pairs were generated in the photocatalytic process
[29]. It is expected that the composites will show improved
photocatalytic activity compared to that of CN because of the
higher number of photocarriers generated in the reaction sys-
tem [30]. As shown in Fig. 2(d), the bandgap energies (Eg) of
the photocatalysts were calculated from Tauc's plots using
1 n
h
 A h  E



g
 , where α, h, ν, Eg, and A are the absorption co-
efficient, Plank's constant, light frequency, bandgap energy, and
a constant, respectively [31]. Thus, the Eg of CN, Co-CN2,
Co-CN10, and Co-MOF are approximately 2.70, 2.67, 1.96, and
1.94 eV, respectively.
The BET surface areas of the samples were obtained from
the N2 adsorption-desorption isotherms. Fig. 2(e) shows that
the BET area of Co-MOF is 2149.13 m2 g–1, which is dueto the
unique structure of Co-MOF. After the Co-MOF modification, the
BET area of the Co-MOF/g-C3N4 nanocomposite increases,
which is beneficial for improving the photocatalytic activity
[32,33]. The CO2 adsorptions of CN and Co-CN2 in air and CO2
gas were separately investigated by infrared spectroscopy
(though the results are not shown in this article). In air, the CO2
adsorption peak area of Co-CN2 sample is 1.77 times that of
pure g-C3N4. Similarly, the CO2 adsorption peak area of Co-CN2
sample is 2.18 times that of pure g-C3N4 in CO2 gas. The photo-
catalysis of the samples will be discussed in detail later.
TGA was carried out to determine the CN and Co-MOF con-
tents of the composites and estimate their thermal stabilities.
As shown in Fig. 2(f), all the photocatalysts displayed good
thermal stability in air. The decomposition of pure Co-MOF
started at 330 °C and the weight loss was about 65.6% between
330 °C and 400 °C, which was related to the removal of guest
molecules [34]. The weight of residual Co-MOF remained stable
up to 800 °C. In addition, the pure CN sample showed a total
weight loss in the temperature range 500–680 °C, which might
be attributed to the collapse of the CN nanosheets [35]. Sur-
prisingly, the thermal stability of Co-CN2 was worse than that
of CN; the main weight loss was observed between 430 °C and
510 °C, however, this was higher than that of the Co-MOF sam-

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Qiuyu Chen et al. / Chinese Journal of Catalysis 41 (2020) 514–523
ple. It was understandable that the amount of Co-MOF in the
Co-CN2 was slightly less than the designed value, based on the
TG result.
main peaks are observed at 781.13 and 796.63 eV, which cor-
respond to Co 2p3/2 and Co 2p1/2, respectively [42]. In general,
the chemical states of N are similar for CN and Co-CN2, but the
chemical states of C and O are different. Similar phenomena
were reported for g-C3N4-based photocatalysts such as Bi-
PO4/g-C3N4, Ag2CrO4/g-C3N4, and Ag3VO4/g-C3N4 composites.
The shift in binding energy indicated the interaction between
Co-MOF nanoparticles and g-C3N4 sheets [43–45].
XPS measurements were carried out to explore the chemical
statuses of the elements, which are displayed in Fig. 3. The pat-
terns showed not only the existence of Co, C, N, and O elements
in Co-MOF and Co-CN2, but also the C and N of CN (Fig. 3(a)).
Fig. 3(b) presents the corresponding XPS VB spectra of CN and
Co-MOF. The VB positions of the samples were calculated to be
~1.58 and ~0.73 eV, respectively. In the N 1s spectra (Fig. 3(c)),
the four peaks observed at 398.4, 399.0, 400.6, and 404.2 eV
were ascribed to the sp2 C–N=C bonds, tertiary nitrogen N–(C)3
groups, amino groups (C–N–H), and charging effects in the het-
erocycles, respectively [36,37]. However, only two peaks at
399.4 and 398.4 eV were detected for the Co-MOF sample, and
they were assigned to pyrrolic-N and pyridinic-N, respectively
[38]. In the C 1s spectra shown in Fig. 3(d), three peaks at
284.5, 286.2, and 288.0 eV were detected. The first peak corre-
sponded to sp2 C=C bonds. The second one was assigned to the
sp2 C=N bonds. The last peak pertained to N=C–N coordination
[39]. It is worth noting that the binding energy of Co-CN2 pho-
tocatalyst revealed a slight red-shift compared to that of CN. At
the same time, the intensities of the peaks were also different
for the two samples, which proved the existence of interaction
in Co-CN2 composite [40].
The photocatalytic CO2 reduction performance of the dif-
ferent samples was evaluated by measuring the CO and CH4
production rates under visible-light irradiation. All the meas-
urements were carried out at room temperature for 6 h, and
the results are shown in Fig. 4(a). When the original CN was
used as a photocatalyst, the production rates of CO and CH4
were 3.41 and 2.11 μmol g–1 h–1, respectively. It is worth noting
that the highest production rates were obtained by employing
Co-CN2 as the photocatalyst; one-fold increases in the CO and
CH4 yields were observed, in comparison with those of pure CN.
Our findings indicate that the coupling of Co-MOF nanoparticles
with CN can greatly improve the photocatalytic activity, as ex-
pected. Interestingly, the production rates first increased and
then declined with an increase in Co-MOF content. This might
be attributed to the reduction in active sites on the CN surface
that resulted from excess Co-MOF coverage, which in turn re-
sulted in reduced efficiency of charge separation [46,47].
Moreover, the mechanical mixture was also prepared and test-
ed; it contained the same weight ratio of CN to Co-MOF as the
Co-CN2 nanocomposite. The result showed no obvious en-
hancement in the photoactivity, compared to that of pure CN.
Thus, the performance improvement of the composite sample
can be attributed to the interaction between CN and Co-MOF. At
the same time, it is crucial that Co-MOF content is appropriate.
In Fig. 3(e), the O 1s peak observed at 531.9 eV for Co-MOF
corresponds to its lattice O, which is shifted to 532.3 eV for
Co-CN2 composite, and the other peaks at ~533.3 and 533.8 eV
are assigned to the OH group. It is obvious that the two peaks of
Co-CN2 red-shift compared to that of Co-MOF, which indicates
interaction between the Co-MOF nanoparticles and the CN
sheets [41]. In the Co 2p spectrum of Co-MOF (Fig. 3(f)), two
Fig. 3. (a) XPS survey patterns; (b) Valence band (VB) XPS spectra; High-resolution (c) N 1s, (d) C 1s, (e) O 1s, and (f) Co 2p XPS spectra of the tested
samples.

Qiuyu Chen et al. / Chinese Journal of Catalysis 41 (2020) 514–523
519
Fig. 4. (a) Comparison of the production rates for the typical samples; (b) Evaluation of the stability of Co-CN2 photocatalyst.
The amounts of the products CO and CH4 increased linearly
with irradiation time, as shown in the inset of Fig. 4(a). The
stability of Co-CN2 sample was also evaluated, as revealed in
Fig. 4(b). No obvious decrease in the production rate was ob-
served after recycling 5 times, which indicated good stability of
Co-CN2 composite.
To illustrate the role of the composites in improving the
electron transfer in the reaction systems, photoelectrochemical
measurements were conducted. Fig. 5(a) shows the photocur-
rent generation behavior of pure CN nanosheets and Co-CN2
nanocomposite under visible-light illumination. The photocur-
rent density of Co-CN2 was higher than that of CN, which sug-
gested that much more photogenerated carriers were trans-
ferred and separated in the case of Co-CN2, which in turn in-
creased the photocatalytic activity [48–50]. EIS was employed
to further investigate the improvement in the charge transmis-
sion efficiency, and the results are shown in Fig. 5(b). The
equivalent circuit of the Nyquist plots is shown in the inset of
Fig. 5. (a) Photocurrent-time profiles; (b) Results of electrochemical impedance spectroscopy (EIS) obtained in the dark and under illumination; (c)
Results of electrochemical CO2 reduction; (d) Mott-Schottky plots.

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Qiuyu Chen et al. / Chinese Journal of Catalysis 41 (2020) 514–523
Fig. 5(b). The smaller the radius of the arc, the faster is the in-
terfacial charge transfer [51,52]. Fig. 5(b) reveals a smaller
radius for Co-CN2 than that for CN, which indicated faster
charge transfer in the case of the Co-CN2 sample, which in turn
improved the separation efficiency of the photogenerated elec-
tron-hole pairs. Moreover, electrochemical CO2 reduction ex-
periments were performed using the samples. The cathodic
current of Co-CN2 was obviously higher than that of CN, as
shown in Fig. 5(c). This provides strong evidence that Co-CN2
exhibits improved photocatalytic ability to reduce CO2. Fig. 5(d)
displays the Mott-Schottky plots for CN and Co-CN2. The posi-
tive slopes revealed that the materials prepared were n-type
semiconductors. A smaller slope of the Mott-Schottky curve
corresponds to a higher carrier density [53]. Compared with
that of CN, Co-CN2 revealed a smaller slope, which suggested a
higher carrier density and a smaller interface resistance for
Co-CN2, which resulted in improved electron-hole separation
and, thereby, enhanced photocatalytic reaction. All the electro-
chemical characterizations further proved our conclusions and
the results were in line with our expectations.
7-hydroxy-coumarin. Finally, the fluorescence spectra of
7-hydroxy-coumarin were measured under 390 nm light exci-
tation. The fluorescence intensity of 7-hydroxy-coumarin is
directly related to the amount of •OH, and thereby, to the pho-
toactivity of the photocatalyst. As shown in Fig. 6(a), the fluo-
rescence intensity of Co-CN2 is much higher than that of CN,
which indicates that the photoactivity of Co-CN2 is higher. In
order to further prove that electrons migrate from the LUMO of
Co-MOF to the CB of CN, the catalytic reaction in the presence
of coumarin under 590 nm light irradiation was carried out.
The
corresponding
fluorescence
spectra
of
7-hydroxy-coumarin are shown in Fig. 6(b). It is noted that the
590 nm light mainly excites Co-MOF, and thus, the electrons in
the LUMO of Co-MOF can migrate to the CB of CN, thereby en-
hancing the photocatalytic CO2 reduction activity.
Fig. 6(c) presents the PL spectra of the Co-CN2 and pure CN
photocatalysts obtained at the excitation wavelength of 370
nm. It is well known that PL is commonly used in the field of
photocatalysis to analyze charge separation. Here, the PL in-
tensity of g-C3N4 decreases dramatically after the Co-MOF mod-
ification. Therefore, a weaker PL peak intensity implies that
Co-MOF modification decreases the recombination rate of the
photogenerated electron-hole pairs, which in turn results in the
photoinduced charge separation efficiency being enhanced
[55–57]. The luminescence decay curves of CN and Co-CN2
were also measured to evaluate the photogenerated carrier
lifetimes. As shown in Fig. 6(d), both the attenuation curves
It is well known that •OH is produced as an intermediate
product during the photocatalytic reduction of CO2 [54]. Here,
the coumarin fluorescence method was employed to explore
the photocatalytic mechanism of the samples. First, the catalyt-
ic reaction was carried out in the presence of coumarin under
visible-light irradiation, and then, the •OH produced under
irradiation reacted with coumarin to form the luminescent
Fig. 6. Fluorescence spectra related to the hydroxyl radicals produced under 390 nm light excitation: (a) Catalytic reaction with visible-light irradia-
tion using Xe lamp as the light source; (b) Catalytic reaction with 590 nm light irradiation; (c) PL emission spectra obtained under 370 nm light exci-
tation; (d) Luminescence decay curves obtained under 370 nm light excitation.

Qiuyu Chen et al. / Chinese Journal of Catalysis 41 (2020) 514–523
521
were well-fitted with double exponential functions. The aver-
catalytic CO2 reduction catalyst based on a one-fold improve-
ment in the photoactivity. This work reveals that g-C3N4-based
composites offer promise for important applications in the field
of energy conversion and environmental purification.
age lifetime (τ) was determined using the formula
2
2
 
i1Ai2
/
i1Ai
[58]. Clearly, the carrier lifetime of


i
i
Co-CN2, which was 1759.7 ns, was much higher than that of CN
(1348.0 ns), which suggested more effective electron-hole pair
generation in the case of the former. The above analysis
showed that Co-CN2 photocatalyst is beneficial for charge
transfer and separation.
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Graphical Abstract
Chin. J. Catal., 2020, 41: 514–523 doi: S1872-2067(19)63497-2
Co-MOF as an electron donor for promoting visible-light photoactivities of g-C3N4 nanosheets for CO2 reduction
Qiuyu Chen, Sijia Li, Hongyi Xu, Guofeng Wang*, Yang Qu *, Peifen Zhu *, Dingsheng Wang
Heilongjiang University, China; The University of Tulsa, USA; Tsinghua University, China
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Co-MOF作为电子供体用于提高g-C3N4纳米片的可见光催化还原CO2性能
陈秋雨a, 李思佳a, 许宏一a, 王国凤a,*, 曲 阳a,#, 朱培芬b,$, 王定胜c
a黑龙江大学化学与材料科学学院, 功能无机材料化学教育部重点实验室, 黑龙江哈尔滨150080, 中国
b塔尔萨大学物理与工程物理系, 塔尔萨74104, 美国
c清华大学化学系, 北京100084, 中国
摘要: 作为温室效应的主要气体CO2浓度持续上升, 已经成为全球环境问题. 将CO2光催化还原成可再生能源不仅可以解
决CO2带来的温室效应, 而且可以将太阳能转化为燃料物质而取代传统意义上的化石能源. 实际上光催化的研究可以追溯
到1979年, 自从Inoue首次报道了光催化CO2和水制取甲酸、甲烷等有机物, 人们一直在努力开发高效的CO2转化光催化剂.
近年来, 随着光催化技术的快速稳定发展, 各种半导体光催化剂, 如Zn2GeO4, CdS, Fe3O4, g-C3N4和SrTiO3等, 已被开发用于
光催化还原二氧化碳. 在这些半导体中, 有的材料具有较大的带隙导致较低的可见光活性, 有的材料具有毒性引起额外的
环境问题. 因此, 寻求具有适度带隙且环境友好的半导体材料是解决全球变暖问题的关键.
近年来, g-C3N4因其带隙(约2.7 eV)较窄, 具有一定的可见光吸收性能, 无污染, 以及化学和热稳定性良好等特点, 被视
为理想的可见光响应光催化材料之一. 但是, g-C3N4光吸收有限、光生电子空穴复合率较高等缺点严重限制了其光催化活
性. 为了进一步提高g-C3N4的CO2可见光催化还原活性, 国内外研究者开发了许多方法来提高电荷分离效率, 进而提高
g-C3N4光催化剂的总体活性. 在这些策略中, 将g-C3N4与具有合适导带位置的其他材料偶联以促进电子空穴分离是提高光
催化性能的有效方法之一. 由于Co-MOF具有较窄的带隙且导带位置与g-C3N4匹配, 我们选择Co-MOF与g-C3N4复合来克
服g-C3N4的缺点, 进而达到提高其光催化活性的目的.
作为电子供体的Co-MOF能够将最低未占分子轨道(LUMO)上的光生电子转移到g-C3N4的导带以促进电荷分离, 同时
水被g-C3N4价带上的空穴氧化, 最终生成氧气, 从而提高光催化还原CO2的性能. 制备的Co-MOF/g-C3N4纳米复合材料在可
见光照射下具有优异的光催化还原CO2性能, 约为纯g-C3N4的光催化活性的2倍. 一系列分析表明, Co-MOF的引入不仅拓
宽了可见光的吸收范围, 而且促进了电荷分离, 有利于提高g-C3N4的光催化活性. 特别是在590 nm单波长照射下进行的羟
基自由基实验进一步证明了Co-MOF的LUMO上的光生电子可以转移到g-C3N4. 该研究结果为基于g-C3N4的光催化体系的
合理构建提供了新思路.
关键词: Co-MOF; g-C3N4纳米片; 电荷分离; 可见光活性; 光催化还原二氧化碳
收稿日期: 2019-07-15. 接受日期: 2019-08-23. 出版日期: 2020-03-05.
*通讯联系人. 电子信箱: wanggf_w@163.com
#通讯联系人. 电子信箱: quyang@hlju.edu.cn
$通讯联系人. 电子信箱: peifen-zhu@utulsa.edu
基金来源: 国家自然科学基金(21871079, 21501052); 黑龙江省自然科学基金优秀青年项目(YQ 2019B006).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).