Advanced NiMoO4/g-C3N4 Z-scheme heterojunction photocatalyst for efficient conversion of CO2 to valuable products

Article abstract of DOI:10.1016/j.jallcom.2020.155860

Herein, g-C₃N₄ and NiMoO₄, which are moderate energy band gap semiconductors, have been effectively hybridized to create Z scheme heterojunction for successful visible-light photocatalytic converting CO₂ into va

Full text of DOI:10.1016/j.jallcom.2020.155860

Journal Pre-proof
Advanced NiMoO /g-C N Z-scheme heterojunction photocatalyst for efficient
3 4
4
conversion of CO to valuable products
2
Nguyen Thi Thanh Truc, Thanh-Dong Pham, Minh Viet Nguyen, Doan Van Thuan, Do
Quang Trung, Phuong Thao, Hoang Thu Trang, Van Noi Nguyen, Dinh Trinh Tran,
Dang Nhat Minh, Nguyen Thi Hanh, Ha Minh Ngoc
PII:
S0925-8388(20)32224-6
DOI:
https://doi.org/10.1016/j.jallcom.2020.155860
JALCOM 155860
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 7 January 2020
Revised Date: 27 May 2020
Accepted Date: 29 May 2020
Please cite this article as: N.T. Thanh Truc, T.-D. Pham, M.V. Nguyen, D. Van Thuan, D.Q. Trung,
P. Thao, H.T. Trang, V.N. Nguyen, D.T. Tran, D.N. Minh, N.T. Hanh, H.M. Ngoc, Advanced NiMoO /
4
g-C N Z-scheme heterojunction photocatalyst for efficient conversion of CO to valuable products,
3
4
2
Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.155860.
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2020 Published by Elsevier B.V.

Contributions of authors
Nguyen Thi Thanh Truc: Performed experiments, analyzed data and wrote the paper.
Minh Viet Nguyen and Thanh-Dong Pham: Supervised the research, analyzed data and revised the paper
Doan Van Thuan: Synthesized materials, analyzed data and co-wrote the paper.
Do Quang Trung and Phuong Thao: Synthesized materials, conducted conversion experiments
Hoang Thu Trang and Van Noi Nguyen: Synthesized materials and analyzed data
Dinh Trinh Tran and Dang Nhat Minh: Analyzed data and conducted conversion experiments
Nguyen Thi Hanh and Ha Minh Ngoc: Conducted recycling experiments

1
2
Advanced NiMoO /g-C N Z-scheme heterojunction photocatalyst for efficient
4
3
4
conversion of CO to valuable products
2
1
2,3,*
2,3,*
4,5
3
4
Nguyen Thi Thanh Truc , Thanh-Dong Pham , Minh Viet Nguyen , Doan Van Thuan , Do Quang
3
3
3
2,3
2,3
2,3
Trung , Phuong Thao , Hoang Thu Trang , Van Noi Nguyen , Dinh Trinh Tran , Dang Nhat Minh ,
6
2,3
5
6
7
8
9
0
1
2
3
4
5
6
Nguyen Thi Hanh , Ha Minh Ngoc
1
Faculty of Environment and Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
2
VNU Key Laboratory of Advanced Materials for Green Growth, University of Science, Vietnam National University,
Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
3
Faculty of Chemistry, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan,
1
1
1
1
1
1
1
Hanoi, Vietnam
4
NTT Hi-Tech Institute, Nguyen Tat Thanh University, Ho Chi Minh City 700000, Vietnam
5
Center of Excellence for Green Energy and Environmental Nanomaterials (CE@GrEEN), Nguyen Tat Thanh
6
Faculty of Environmental Sciences, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai,
Thanh Xuan, Hanoi, Vietnam
*Corresponding Authors:
1
7
8
E-mail addresses: nguyenthithanhtruc@tdtu.edu.vn (Nguyen Thi Thanh Truc); dong2802@vnu.edu.vn
(Thanh-Dong Pham); nguyenminhviet@hus.edu.vn (Minh Viet Nguyen)
1
1
9
0
2
ABSTRACT
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Herein, g-C N and NiMoO , which are moderate energy band gap semiconductors, have been
3
4
4
effectively hybridized to create Z scheme heterojunction for successful visible-light
photocatalytic converting CO into valuable products including CH , CO, O and HCOOH.
2
4
2
Ni(NO ) ·6H O and (NH ) Mo O ·4H O were used as precursors to synthesize
3
2
2
4 6
7
24
2
NiMoO photocatalyst, which was continuously mixed with melamine before calcinating at 520
4
o
C for 6 h to get NiMoO
4
/g-C
3
N
4
Z scheme heterojunction. We explored that NiMoO
4
intimately
contacted with g-C
3
N
4
. These band positions of the NiMoO were also perfectly matched with
4
-
those of the g-C N . Therefore, these photo-induced e on conduction band of the NiMoO could
3
4
4
+
+
-
easily travel to h on valence band of the g-C N (recombination); thereby, minimize h and e
3
4
1

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recombination in each material. Therefore, the NiMoO /g-C N direct Z-scheme heterojunctions
4 3 4
+
-
could produce significant available h on the valence band of the NiMoO and e on the
4
-
+
conduction band of the g-C N . These e /h have suitable redox potential to effectively convert
3
4
CO . Finally, the optimized g-C N mole ratio for maximum enhancing photocatalytic efficiency
2
3
4
of the NiMoO /g-C N heterojunction was 60 %. When the g-C N content increased to 70 %,
4
3
4
3
4
the excess g-C
3
N
4
amount would entirely cover NiMoO
4
surface leaded to form dense and closed
and CO as well as the
shell. The formed closed shell decreased contact between NiMoO
4
2
-
+
interface charge transfer, which reduced the e and h separation and transfer leading to decrease
in photocatalytic conversion efficiency.
Keywords: Z scheme heterojunction; NiMoO ; g-C N ; Photocatalytic CO conversion;
4
3
4
2
Valuable products
4
3
4
1. Introduction
4
Currently, CO has been considered as one of the main pollutants contributing to the climate
2
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53
change and greenhouse effects, which responds for various global problems [1]. Development of
suitable method utilizing CO
2
as an efficient source to form valuable products, including formic
OH) and methane (CH ), etc., has recently received extensive
acid (HCOOH), methanol (CH
3
4
attention in scientists. The utilization not only aims to reduce atmospheric CO levels but also
2
extend available carbon resources for chemical industry production chain [2, 3]. Various
techniques, such as chemical reduction (involving photo- and electro- reduction), thermocatalytic
hydrogenation and biological conversion, have been conducted for the purpose [4]. Among them,
photocatalysis for CO conversion has received great consideration because of its environmental
2
2
friendliness and its potential in utilizing solar energy [5-9]. In the photocatalytic CO conversion,
2

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the used photocatalyst could utilize solar or artificial light as sources of photon energy to
accelerate photo-reactions to convert CO into carbohydrates. Various photocatalysts, such as
2
metal oxides (ZnO, TiO , WO ), metal phosphides, nitrides or sulfides (Ta N , CdS, ZnS, GaP)
2
3
3
5
and binary-metal oxides (Bi WO , ZnGa O , Cu V O , NiWO , Zn SnO and Zn GeO ), have
2
6
2
4
2
2
7
4
2
4
2
4
been successfully applied for photocatalytic CO conversion [10-13]. Nevertheless, most of them
2
could be used only under ultraviolet irradiation and suffered from high photogenerated charge
recombination, low conversion efficiency and photocorrosion during photocatalytic reactions,
which are huge challenges hindering commercial applications in this field [14].
Recently, several transition metal molybdates including NiMoO , FeMoO , Bi MoO ,
4
4
2
6
CoMoO and CuMoO have significantly attracted scientists’ attention to apply them in many
4
4
fields including photocatalysis, dehydrogenation, and energy storage/conversion [15, 16].
Among them, NiMoO4 has been used as catalyst for oxidative dehydrogenation
of propane and n-butane [17]. More recently, NiMoO has been considered as an emerging
4
visible light photocatalyst because of its moderate energy band gap (~2.4 eV) and good electrical
conductivity [18]. The synthesized NiMoO
4
materials have been successfully used for
photocatalytic organic pollutant removal as well as bacterial inactivation [19, 20]. However, the
application of NiMoO photocatalyst for CO conversion is limited because its conduction band
4
2
-
+
potential is quite low and rapid recombination of their photo-excited e and h . Another emerging
photocatalyst, which has recently gained significant attention of global scientists, is g-C N [21-
3
4
23]. The photocatalyst also has moderate band gap energy and high stability. The conduction
-
band potential of the g-C
conduction band of the g-C
hydrocarbon products [24-26]. Nevertheless, the g-C
3
N
4
is around -1.20 V. Therefore, the generated e existing on the
3
N
4
would have high potential to reduce CO
2
into many valuable
3
N
4
photocatalytic efficiency is still away
from satisfactory due to the high recombination rate of photo-excited charges and low surface
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area, which are usually considered as major barriers of the single component semiconductor
photocatalyst [27]. Recently, Z-scheme heterojunctions, which have been formed from
combination of g-C N with another narrow band gap photocatalyst has been widely studied [28-
3
4
31]. In the formed Z scheme heterojunction, the g-C N and the combined photocatalyst could be
3
4
-
simultaneously excited by suitable incident light. Then, the photo-excited e of the combined
+
component could transfer to the g-C
3
N
4
to combine with h on its valence band (recombination).
-
Therefore, residual photo-excited e of the g-C
3
N
4
would exhibit strong reduction capacity while
+
the h in coupled photocatalyst present strong oxidation capacity. Consequently, the formed Z-
scheme heterojunction could show great photocatalytic performance [32, 33]. To effectively
form a Z-scheme heterojunction, the conduction band potential of the combined photocatalyst
should be more positive than the conduction band potential of the g-C N while the valence band
3
4
potential of the g-C N should be more negative than that of the coupled photocatalyst. Thus, the
3
4
NiMoO is considered as perfectly suitable photocatalyst for combination with g-C N to create
4
3
4
Z-scheme heterojunction. However, until now, there was no reported study combining g-C
and NiMoO to form Z-scheme heterojunction (NiMoO /g-C ). Thus, the study aims to
combine NiMoO and g-C N to create NiMoO /g-C N Z-scheme heterojunction. The formed
3 4
N
4
4
3 4
N
4
3
4
4
3
4
heterojunction is expected to greatly prevent disadvantages of each single component to exhibit
novel photocatalytic performance for efficiency CO conversion to valuable products.
2
2. Experiments
2.1. Material preparation
Ni(NO
3
)
2
·6H
2
O
and (NH
4
)
6
Mo
7
O24·4H
2
O
were precursors for synthesis of
1
1
00
01
NiMoO photocatalyst. In a typical experiment, these precursors were dissolved in deionized
4
water to obtain 1 M solutions. Then, these obtained solutions were mixed by drop wise under
4

2
+
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
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23
24
25
magnetic stirring condition. Citric acid (0.1 M) with double molar weight of Ni used as
chelating agent. Then, the suspension was neutralized by ammonium hydroxide (0.1 M). The
obtained greenish yellow precipitate was centrifuged to collect and washed adequately with
water/ethanol. Then, the washed precipitate was dried at 80ꢀ°C for a half day. The obtained dried
greenish yellow powder was mixed with melamine. The obtained mixture was robustly mixed in
order to achieve a homogeneous mixture, which was continuously transported to a crucible. The
−
1
crucible was continuously calcinated at 520ꢀ°C for 6ꢀh with 5ꢀ°Cꢀmin heating rate to achieve
NiMoO /g-C Z-scheme heterojunction. The melamine amounts were calculated to synthesize
series of heterojunction photocatalysts, which were labeled as NiMoO /g-C N -X (X represented
4
3 4
N
4
3
4
the mole ratios of g-C N in the heterojunction photocatalysts, Xꢀ=ꢀ50, 60, and 70ꢀ%). A parallel
3
4
experiment without adding melamine was also carried out to synthesize NiMoO . Melamine was
4
o
also calcinated at 520 C for 6 h to obtain g-C N .
3
4
2.2. Conversion experiments
These photocatalytic CO
2
conversion tests have been carried out in a continuous model
containing a reactor with volume of 40 mL (Fig. S1 in the supplementary material). Quartz was
used to make bottom and top of the reactor placing in the center of a 24 L cask. The cask has
dark cover to use as a reaction chamber. A 30 W Duhalled (SBNL-830) bulb was hanged from
the ceiling and another was put on the bottom of the reaction chamber were used as light sources
generating visible light in range from 400 to 700 nm. The calculated power density of the
2
provided visible light was approximately 0.15 W/cm . 0.1 g synthesized materials was uniformly
2 2
spread onto floor of the reactor to photocatalyze for CO conversion. For photocatalytic CO
conversion, high purity CO
2
gas (99.9 %) was constantly supplied with flow rate of 30 mL/min.
o
Before reaching to the reactor, the CO flow was oriented passing water (25 C) to produce a
2
5

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1
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1
1
1
1
1
1
1
1
1
1
1
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36
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38
39
40
41
42
43
44
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46
47
48
49
mixture of CO and H O. By this condition, relative humidity of inlet gas reaching to
2 2
photocatalyst was around 60 %. Finally, the products of conversion processes were determined
by a gas chromatography (GC) system (Agilent 7890 B), which equipped with both a flame
ionization detector (FID) and a thermal conductivity detector (TCD). 100 μL gaseous products
was automatically injected into the GC system at intervals of 20 min. Ar gas was used as a
4 2
carrier gas. Standard curves of CO, CH HCOOH and O have been built to calculate the
concentration of produced gases.
3. Results and discussion
3.1. Material characteristics
3.1.1. Morphology, microstructure and surface areas
A Tecnai G2 F30 ransmission electron microscopy (TEM) has been used to determine
microstructures and morphologies of these synthesized NiMoO , g-C N and NiMoO /g-C N
4
t
4
3
4
4
3
materials. As presented in the Fig. 1A, the synthesized g-C N existed as lamella. The
3
4
synthesized NiMoO were nano-particles, which average size was approximately 170ꢀnm (Fig.
4
1B). In case of the synthesized NiMoO /g-C N material, the g-C N lamellar entirely twined
4
3
4
3
4
these NiMoO
been additional provided in the supplementary material to clearly show the distribution of the
NiMoO on the g-C lamella (Fig. S2 in the supplementary material). The obtained high
4 4 3 4
particles (the dark part). More TEM images of the NiMoO /g-C N materials have
4
3 4
N
resolution TEM image presented a lattice fringe of 0.397ꢀnm, which corresponded to the (220)
plane of the NiMoO , while another lattice plane space of 0.32ꢀnm was compatible with (002)
4
planes of the g-C N [34-36]. The obtained high resolution TEM image proved strong
3
4
hybridizing between g-C N and NiMoO . Nitrogen adsorption desorption isotherms combining
3
4
4
with BET equations were used to determine surface areas of the synthesized NiMoO , g-C N
4
4
3
6

150
151
152
153
154
155
156
and NiMoO /g-C N heterojunctions. These obtained results shown in the Table 1 indicated that
4 3 4
the surface areas of these NiMoO /g-C N heterojunctions were greater than those of NiMoO
4
4
3
4
and g-C N single components. The distribution of NiMoO on the g-C N diversified pore size
3
4
4
3
4
4 3 4
and increased pore number leading to increase in the surface area of the NiMoO /g-C N as
compared to those of both NiMoO
4
and g-C
/g-C
3
N
4
(Fig. S3 in the supplementary material). The
increase in surface areas of the NiMoO
4
3
N
4
heterojunction would effectively enhance their
CO absorption leading to increase in conversion efficiency (if any).
2
1
57
58
1
Fig. 1. TEM images of the g-C
3
N
4
(A), NiMoO
4
(B), NiMoO
4
/g-C
3
N
4
heterojunctions (C) and
/g-C (C1).
159
HRTEM of the selected area in the TEM image of the NiMoO
4
3 4
N
7

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161
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168
169
170
171
172
3.1.2. Phase structure
A Bruker AXS D8 Advance diffractometer was used to obtain X-ray diffraction (XRD)
patterns of these synthesized materials. These XRD patterns of these NiMoO /g-
4
C
3
N
4
heterojunctions with different g-C
these patterns of the pristine NiMoO and g-C
NiMoO XRD pattern were well indexed to monoclinic structure of α-NiMoO
86-0361) [37, 38]. The g-C N XRD pattern presented two typical peaks appearing at 13.2° and
3
N
4
contents have been shown in Fig. 2 together with
. The major diffraction peaks observed from the
(JCPDS Card No.
4
3 4
N
4
4
3
4
27.4°, which corresponded to (100) and (002) lattice planes of the g-C N , respectively [39].
3
4
These diffraction peaks related to the g-C N as well as the monoclinic structure NiMoO could
3
4
4
be seen from XRD patterns of these synthesized NiMoO /g-C N heterojunction. No impurity
4
3
4
diffraction peaks and noticeable peak shift were detected in the XRD patterns of these
synthesized NiMoO /g-C N heterojunction indicating that the distribution of NiMoO on the g-
4
3
4
4
3 4 4 3 4
C N has not affected crystallinity of both NiMoO and g-C N .
1
73
74
1
Fig. 2. The XRD patterns of NiMoO , g-C N and NiMoO /g-C N heterojunctions.
4
3
4
4
3
4
8

1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
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3.1.3. Elemental states
The elemental status of nickel and nitrogen in these g-C N , NiMoO and NiMoO /g-C N
4
3
4
4
4
3
were determined using an X-ray photoelectron spectrometer (XPS). First, the Gaussian fitted
XPS spectrum of N of g-C N presented three peaks at 398.4, 399.8 and 401.1 eV (Fig. 3). The
C while the obtained peak at
(bridging N atoms) building the fundamental g-
structure [40, 41]. The peak at 401.1 eV could be corresponded to N positive charge of
heptazine rings resulted from incomplete thermal polycondensation process [42]. In contrast, N
peaks of the NiMoO /g-C N heterojunctions shifted to 398.6, 400.1 and 401.6 eV. The 401.6
peak intensity of the NiMoO /g-C N heterojunction also significantly decreased as compared to
ꢀ
3
4
fitted peak at 398.4
ꢀeV can be attributed to nitrogen in C–Nꢀ=ꢀ
399.8 eV corresponded to N in N-(C)
ꢀ
3
C
3
N
4
ꢀ
ꢀ
ꢀ
4
3
4
4
3
4
the peak of the g-C N . The results indicated that chemical bonds occurred between the
3
4
NiMoO and g-C N [43-47].
4
3
4
The XPS spectrum of Ni of the NiMoO presented four peaks, which included two major
4
peaks accompanying with their satellite peaks (Fig. 3). The major peak (at 854.7
shakeup type peak (at 862.1 eV) corresponded to Ni 2p3/2 orbital while the major peak (at
872.5 eV) and its shakeup type peak (at 880.3 eV) corresponded to Ni 2p1/2 orbital [48-51]. The
Ni 2p3/2 and Ni 2p1/2 peaks were separated by 17.8 eV showing the nickel state in the NiMoO4
was Ni . However, these Ni 2p and Ni 2p3/2 peaks of the NiMoO /g-C N heterojunction
ꢀeV) and its
ꢀ
ꢀ
ꢀ
ꢀ
2
+
1
/2
4
3
4
shifted a little. The Gaussian fitting analysis deconvoluted the 2p spectrum into peaks indicating
2
+
3+
the co-existence of both Ni and Ni in the synthesized NiMoO /g-C N heterojunction [51-53].
4
3
4
3
+
3+
In detail, these peaks at 856.3 and 874.4
ꢀ
eV could be appointed to Ni 2p3/2 and Ni 2p1/2 status,
2
+
respectively; while these peaks at 854.7 and 872.5
ꢀ
eV still corresponded to Ni 2p3/2 and
2
+
3+
4 3 4
Ni 2p1/2 status, respectively. The new occurrence of Ni state in the NiMoO /g-C N
heterojunction confirmed the existence of chemical bonding (Ni and N) between NiMoO and g-
4
9

199
200
201
C N . The XPS analysis of C also confirmed that NiMoO and g-C N were not just connected
3 4 4 3 4
physically, but forming a tight interface via chemical bonding (C-O) between C of the g-C N
3
4
and O of the NiMoO (Fig. S4 in the supplementary material).
4
2
02
03
2
Fig. 3. XPS spectra of N 1s in g-C N and NiMoO /g-C N (A) and XPS spectra of Ni 2p of
3
4
4
3
4
204
205
206
207
208
209
NiMoO and NiMoO /g-C N (B).
4 4 3 4
3.1.4. Optical properties
The UV–Vis absorbance of these synthesized g-C N , NiMoO and NiMoO /g-C N
4
3
4
4
4
3
heterojunctions were presented in Fig. 4. In pristine NiMoO , the coordination environments of
4
2
+
6+
Ni and Mo responded for its optical absorption [54]. First, the absorption in region of 205 to
400ꢀnm was due to the electron transfers from nonbonding ligand orbitals to antibonding metal
1
0

2
−
6+
3
3
2
2
2
2
2
10
11
12
13
14
orbitals (from O 2p to Mo 5d) [55]. Secondly, the d-d transitions A (F)ꢀ→ꢀ T (P) of the
2g 1g
nickel in octahedral symmetry and charge transfers from these occupied Ni 3d orbitals to these
unoccupied anti-bonding Mo 5d orbitals responded for absorption in region of 400 to 550ꢀnm
[56]. Finally, the optical absorption in region of 600–800 nm centered at 700 nm was attributed
2
+
to Ni (O ) species [57].
h
2
2
15
16
Fig. 4. UV–Vis absorption of these synthesized g-C N , NiMoO and NiMoO /g-C N
4
3
4
4
4
3
2
2
2
2
2
2
2
17
18
19
20
21
22
23
heterojunctions.
presented an absorption edge at 460 nm. As compared
/g-C heterojunctions
presented great increase in visible-light absorbance. The energy band gap (E ) of g-C N and
3 4
The UV–Vis absorption of the g-C N
to these NiMoO
4
and g-C
3
N
4
samples, these synthesized NiMoO
4
3 4
N
bg
3
4
NiMoO , which were estimated via Kubelka-Munk functions, were 2.68 and 2.45ꢀeV,
4
respectively (Fig. S6 in the supplementary material). The estimated Ebg of these synthesized
NiMoO /g-C N heterojunctions were great lower than those of single components (Table 1).
4
3
4
1
1

-
+
2
2
24
25
Table 1. Surface areas, E , production rates and efficient used charges (e or h ) of the NiMoO ,
bg
4
g-C N and NiMoO /g-C N heterojunction.
3
4
4
3
4
Production rates
Efficient used
-1
-1
Surface areas
E
bg
(eV)
(µmol. g cat. h )
-
+
2
charges (e or h )
(
m /g)
-1 -1
(
µmol. g cat. h )
CH
4
CO
O
2
HCOOH
g-C N
68.5
10.4
97.5
98.7
96.2
2.68
2.45
2.37
2.31
2.34
0
0
0
0
0
0
3
4
NiMoO
4
0
0
0
0
NiMoO /g-C N - 50
512 349 1449
635 432 1853
567 414 1637
578
647
625
5950
7238
6614
4
3
4
NiMoO /g-C N - 60
4
3
4
NiMoO
4
/g-C
3
N
4
- 70
2
2
2
2
2
2
2
2
2
2
2
2
2
2
26
27
28
29
30
31
32
33
34
35
36
37
38
39
The visible-light absorption increase and the Ebg decrease of these synthesized NiMoO /g-
4
C
3
N
4
heterojunctions as compared to single components were attributed to direct Z scheme
effects. The Mott-Schottky plots were applied to determine these maximum valence bands
(MVB) and minimum conduction bands (MCB) of these g-C and NiMoO in order to clearly
explain the direct Z-scheme mechanism of these NiMoO /g-C N heterojunctions (Fig. S7 in the
3
N
4
4
4
3
4
supplementary material). These determined MVB and MCB of the g-C N were 1.64 and
3
4
−1.04 V, respectively; while these determined MVB and MCB of the NiMoO were 2.57 and
4
0.12 V, respectively. In addition, g-C N and NiMoO was intimately contacted (see the obtained
3
4
4
HRTEM results). Therefore, g-C N and NiMoO have been suitable hybridized to form a novel
3
4
4
Z direct scheme (Fig. 5). Electrochemical impedance spectra (EIS) were also carried out to
confirm electron transfer between NiMoO and g-C . The obtained EIS Nyquist plots, which
4
3 4
N
was provided in the supplementary data (Fig. S8), showed that the charge transfer resistance of
the NiMoO /g-C N was much lower than those of single NiMoO and NiMoO /g-C N . This
4
3
4
4
4
3
4
1
2

2
2
40
41
verified these findings indicating that the Z scheme has been successfully established to enhance
electron transfer between NiMoO and g-C N .
4
3
4
2
2
42
43
Fig. 5. Mechanism for Z direct scheme heterojunction.
2
2
2
2
2
2
2
2
2
2
2
2
44
45
46
47
48
49
50
51
52
53
54
55
In the formed direct Z-scheme, narrow E g-C N and NiMoO photocatalysts could absorb
bg
3
4
4
-
+
visible light to excite e from valence band to conduction band leaving h on the valence band.
-
Then, these excited e on the conduction band of the NiMoO could easily transfer (migrate) to
4
+
the valence band of the g-C N to fill these h there. This effectively minimized recombination of
3
4
photo-excited charges in each material. Thus, the incident visible light was efficiently used for
-
+
4 3 4
separation of e and h or visible-light absorption of these synthesized NiMoO /g-C N was
greater than those of single components. The photoluminescence (PL) characterization was also
used to determine recombination status of photo-excited charges in the synthesized samples (Fig.
6). It is known that the higher the PL peak intensity indicates faster recombination. Fig. 6
presented that these PL peaks of these synthesized NiMoO /g-C N heterojunction were
4
3
4
obviously lower than those of the g-C N and NiMoO meaning that the direct Z-scheme
3
4
4
-
+
heterojunction remarkably restrained e and h recombination.
1
3

2
2
56
57
3 4 4 4 3 4
Fig. 6. Photoluminescence spectra of these synthesized g-C N , NiMoO and NiMoO /g-C N
2
2
2
2
2
2
2
2
2
2
58
59
60
61
62
63
64
65
66
67
heterojunctions.
3.2. CO conversion
2
No carbon-containing products was detected in blank experiments, which used a mixture of
N and H O vapor as inlet gas to feed into the irradiated reactor involving synthesized g-C N ,
2
2
3
4
NiMoO and NiMoO /g-C N heterojunctions. This observation verified that carbon products (if
4
4
3
4
any) could be generated from CO precursor rather than from carbon components (residues) of
2
these synthesized photocatalysts. After blank experiments, CO
2
and H
2
O vapor mixtures were
3 4
and NiMoO /g-C N ,
fed to the reactor, which contained these synthesized g-C , NiMoO
3
N
4
4
4
under both visible light and dark conditions. There was no converted product was noticed when
tests were conducted under darkness. This indicated the necessary of incident visible light as
1
4

2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
excitation source for photocatalytic performance. The single component photocatalysts (NiMoO4
or g-C N ) could not also exhibit any activity for conversion of CO when they were excited by
3
4
2
+
-
visible light. This result was due to rapid recombination of photo-excited h and e , which was
one of major disadvantages of single component photocatalyst. Interestingly, these NiMoO /g-
4
C N heterojunctions effectively converted CO into various carbon-containing products
3
4
2
including CO, CH
photocatalytic CO
in Table 1. The CO
4
and HCOOH. Huge O
2
gas have been also found as by-product from
2
conversion. The production rates of CO, CH
3 4
conversion efficiency of the NiMoO /g-C N
4
2
, HCOOH and O were shown
2
4
photocatalyst was much
higher than those of recent reported materials (Table S1 in the supplementary material). When
NiMoO was coupled with g-C N to create NiMoO /g-C N heterojunctions, both NiMoO and
4
3
4
4
3
4
4
g-C N could absorb large amount of incident visible light (because of their narrow E ) to excite
3
4
bg
-
+
of e from the valence band to the conduction band of each material leaving h at the valence
band. Because of suitable potential energy level and intimate contact between NiMoO and g-
4
-
+
C
3
N
4
, the photo-excited e on the conduction band of the NiMoO
band of the g-C (to recombine); thereby, effectively minimize charge recombination in each
material. Therefore, the NiMoO /g-C N direct Z scheme heterojunctions could produce
4
can travel to h on the valence
3
N
4
4
3
4
-
+
significant available e at the conduction band of the g-C N and h at the valence band of the
3
4
NiMoO . These produced charges presented suitable redox potential to convert CO describing
4
2
by following equations [32, 58, 59].
Visible light
-
+
(1)
NiMoO
4
/g-C
3
N
4
    → e + h
+
+
2
H O + 4h
4H + O2
(2)
(3)
(4)
2
+
-
0
CO
2
+ 8H + 8e
CH
4
+ 2H
2
O (E redox = - 0.24 V)
+
-
0
CO + 2H + 2e
CO + H O (E
= - 0.48 V)
redox
2
2
1
5

+
-
0
CO + 2H + 2e
HCOOH (E
= - 0.61 V)
redox
(5)
2
2
87
Based on the equations 2, 3, 4 and 5, it can be seen that the [produced O ] = 2
×
[produced
2
-
1
-1
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
88
89
90
91
92
93
94
95
96
97
98
99
00
01
02
03
04
05
06
07
08
09
CH ] +1/2
×
[produced HCOOH] + 1/2
×
[produced CO] = 1,809 (µmol. g cat. h ). The
4
calculated O products mostly matched with the detected produced O (Table 1). Therefore, the
2
2
proposed mechanism for photocatalytic CO conversion following equations (1 to 5) was
2
scientifically correct. In addition, equations 2, 3, 4 and 5 also indicated that these efficient used
-
+
charges (e or h ) = 4
[produced HCOOH]. These efficient used charges (e or h ) of these NiMoO /g-C N
4
× [produced O ] = 8 × [produced CH ] + 2 × [produced CO] + 2 ×
2 4
-
+
4
3
heterojunctions were also shown in Table 1. The efficient used charges of the NiMoO /g-C N -
4
3
4
-1
-1
60 was approximately 7,238 (µmol. g cat. h ), which was the highest among those of these
-1
-1
synthesized NiMoO
4
/g-C
3
N
4
heterojunctions (5,950 (µmol. g cat. h ) for NiMoO
4
/g-C
-70). The observation greatly matched with
these characterized results. Thus, the g-C N contents significantly affected photocatalytic
3 4
N -50
-1
-1
4 3 4
and 6.614 (µmol. g cat. h ) for NiMoO /g-C N
3
4
efficiency and the optimal g-C N molar ratio was 60 %. When g-C N content was increased to
3
4
3
4
70 %, the excess g-C N amount would entirely cover NiMoO surface leaded to form dense and
3
4
4
closed shell [41, 42]. The formed closed shell decreased contact between NiMoO and CO as
4
2
-
+
well as the heterojunction interface effects, which reduced e and h separation and transfer
efficiency leading to decrease in photocatalytic conversion efficiency.
Recycling experiments were conducted to determine the stability of the synthesized
4 3 4 2 4 3 4
NiMoO /g-C N during the CO conversion processes. The NiMoO /g-C N , which was used for
CO conversion for 4 h, was collected and vacuum dried for 24 h under dark condition and then
2
continuously applied for next cycle. The CO conversion efficiency of the NiMoO /g-C N was
2
4
3
4
constantly over five cycles (Table 2). The results indicated the high stability of the NiMoO /g-
4
C N material in the CO conversion process.
3
4
2
1
6

3
3
10
11
Table 2. Production rates and efficient used charges from CO conversion of the recycled
2
NiMoO /g-C N .
4
3
4
Production rates
Efficient used charges
-1
-1
-1
-1
(
µmol. g cat. h )
(µmol. g cat. h )
CH
4
CO
432
430
414
407
397
O
2
HCOOH
647
Cycle #1
Cycle #2
Cycle #3
Cycle #4
Cycle #5
635
617
625
608
612
1853
1802
1705
1754
1673
7238
7156
7088
6914
6880
680
630
618
595
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
4. Conclusions
We successfully combined g-C N and NiMoO , which are moderate E materials, to create Z
3
4
4
bg
scheme heterojunction to effectively use for visible-light photocatalytic CO conversion to
2
valuable products involving CH , HCOOH, CO and O . The intimate contact between NiMoO
4
4
2
with g-C N in NiMoO /g-C N direct Z scheme heterojunctions have been well proved. The
3
4
4
3
4
material characterization also indicated that these band position of NiMoO was also perfectly
4
-
matched with those of g-C
NiMoO
3
N
4
. Thus, these photo-excited e at the conduction band of the
+
4
could easily recombine with h at the valence band of the g-C
3
N
4
to preserve significant
-
+
available amounts of e at the conduction band of the g-C N and h at the valence band of the
3
4
+
-
NiMoO . These preserved h and e presented suitable redox potential to effectively convert CO
4
2
to produce CH , HCOOH, CO and O . Finally, we determined that the optimized g-C N molar
4
2
3
4
ratio in the NiMoO /g-C N heterojunction for the highest CO conversion efficiency was 60 %.
4
3
4
2
This was because the excess g-C N amount (when the g-C N content increased to 70 %) would
3
4
3
4
4
entirely cover NiMoO surface leaded to form dense and closed shell decreasing contact between
1
7

-
+
3
3
3
3
3
3
3
27
28
29
30
31
32
33
NiMoO and CO as well as the heterojunction interface effects, which reduced e and h
4 2
separation and transfer efficiency resulted in decrease in photocatalytic conversion efficiency.
Acknowledgements
This research is funded by Vietnam National Foundation for Science and Technology
Development (NAFOSTED) under grant number 104.05-2018.354
3
3
34
35
References
[1] S. Chaemchuen, X. Xiao, M. Ghadamyari, B. Mousavi, N. Klomkliang, Y. Yuan, F.
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
Verpoort, Robust and efficient catalyst derived from bimetallic Zn/Co zeolitic imidazolate
2
frameworks for CO conversion, J. Catal. 370 (2019) 38-45.
[2] T.-D. Pham, B.-K. Lee, Novel capture and photocatalytic conversion of CO2 into solar fuels
by metals co-doped TiO deposited on PU under visible light, Appl. Catal., A 529 (2017) 40-
2
48.
[3] W. Shi, X. Guo, J.-C. Wang, Y. Li, L. Liu, Y. Hou, Y. Li, H. Lou, Enhanced photocatalytic
2
3D/2D architecture for CO reduction over cuprous oxide octahedrons supported on
hexagonal phase tungsten oxide nanoflakes, J. Alloys Compd. 830 (2020) 154683.
[4] N.T.P. Le Chi, N.T. Dieu Cam, D. Van Thuan, T.T. Truong, N.T. Thanh Truc, C. Van
Hoang, T.T. Thu Phuong, T.-D. Pham, M.H. Thanh Tung, N.T. Minh Thu, N.M. Phuong,
V.N. Nguyen, Synthesis of vanadium doped tantalum oxy-nitride for photocatalytic reduction
of carbon dioxide under visible light, Appl. Surf. Sci. 467-468 (2019) 1249-1255.
[5] M. Xu, X. Hu, S. Wang, J. Yu, D. Zhu, J. Wang, Photothermal effect promoting CO2
conversion over composite photocatalyst with high graphene content, J. Catal. 377 (2019)
652-661.
1
8

3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
[6] Y. Wang, Z. Zhang, L. Zhang, Z. Luo, J. Shen, H. Lin, J. Long, J.C.S. Wu, X. Fu, X. Wang,
C. Li, Visible-Light Driven Overall Conversion of CO and H O to CH and O on 3D-
2
2
4
2
SiC@2D-MoS Heterostructure, J. Am. Chem. Soc. 140 (2018) 14595-14598.
2
[7] J. Jin, J. Yu, D. Guo, C. Cui, W. Ho, A Hierarchical Z-Scheme CdS–WO Photocatalyst with
3
Enhanced CO Reduction Activity, Small 11 (2015) 5262-5271.
2
[8] W. Zhong, R. Sa, L. Li, Y. He, L. Li, J. Bi, Z. Zhuang, Y. Yu, Z. Zou, A Covalent Organic
Framework Bearing Single Ni Sites as a Synergistic Photocatalyst for Selective
2
Photoreduction of CO to CO, J. Am. Chem. Soc. 141 (2019) 7615-7621.
[9] Y. Zhao, W. Cai, M. Chen, Y. Bu, Turning the activity of Cr–Ce mixed oxide towards
thermocatalytic NO oxidation and photocatalytic CO reduction via the formation of yolk
2
shell structure hollow microspheres, J. Alloys Compd. 829 (2020) 154508.
[10] Y. Chai, L. Li, J. Lu, D. Li, J. Shen, Y. Zhang, J. Liang, X. Wang, Germanium-substituted
Zn2TiO4 solid solution photocatalyst for conversion of CO into fuels, J. Catal. 371 (2019)
2
144-152.
[11] N.T. Thanh Truc, N.T. Hanh, M.V. Nguyen, N.T.P. Le Chi, N. Van Noi, D.T. Tran, M.N.
Ha, D.Q. Trung, T.-D. Pham, Novel direct Z-scheme Cu V O /g-C N for visible light
2
2
7
3
4
photocatalytic conversion of CO into valuable fuels, Appl. Surf. Sci. 457 (2018) 968-974.
2
[12] T.-D. Pham, B.-K. Lee, Novel photocatalytic activity of Cu@V co-doped TiO /PU for CO
2
2
reduction with H2O vapor to produce solar fuels under visible light, J. Catal. 345 (2017) 87-
95.
[13] N.L.M. Tri, D.S. Duc, D. Van Thuan, T.A. Tahtamouni, T.-D. Pham, D.T. Tran, N. Thi
4
Phuong Le Chi, V.N. Nguyen, Superior photocatalytic activity of Cu doped NiWO for
efficient degradation of benzene in air even under visible radiation, Chem. Phys. 525 (2019)
110411.
1
9

3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
[14] C. Ding, Z. Ma, C. Han, X. Liu, Z. Jia, H. Xie, K. Bao, L. Ye, Large-scale preparation of
BiOX (Xꢀ=ꢀCl, Br) ultrathin nanosheets for efficient photocatalytic CO conversion, J. Taiwan
2
Inst. Chem. Eng. 78 (2017) 395-400.
[15] T.K. Ghorai, D. Dhak, S.K. Biswas, S. Dalai, P. Pramanik, Photocatalytic oxidation of
organic dyes by nano-sized metal molybdate incorporated titanium dioxide (M Mo Ti O )
x
x
1−x
6
(M=Ni, Cu, Zn) photocatalysts, J. Mol. Catal. A Chem. 273 (2007) 224-229.
[16] J. Bi, L. Wu, J. Li, Z. Li, X. Wang, X. Fu, Simple solvothermal routes to synthesize
nanocrystalline Bi
4699-4705.
2 6
MoO photocatalysts with different morphologies, Acta Mater. 55 (2007)
[17] W. Hu, J. Yu, X. Jiang, X. Liu, R. Jin, Y. Lu, L. Zhao, Y. Wu, Y. He, Enhanced
photocatalytic activity of g-C N via modification of NiMoO nanorods, Colloids Surf. A
3
4
4
Physicochem. Eng. Asp. 514 (2017) 98-106.
[18] J. Haetge, I. Djerdj, T. Brezesinski, Nanocrystalline NiMoO with an ordered mesoporous
4
morphology as potential material for rechargeable thin film lithium batteries, ChemComm. 48
(2012) 6726-6728.
[19] M. Dhanasekar, S. Ratha, C.S. Rout, S.V. Bhat, Efficient sono-photocatalytic degradation of
methylene blue using nickel molybdate nanosheets under diffused sunlight, J. Environ. Chem.
Eng. 5 (2017) 2997-3004.
[20] S.K. Ray, D. Dhakal, C. Regmi, T. Yamaguchui, S.W. Lee, Inactivation of Staphylococcus
aureus in visible light by morphology tuned α-NiMoO , J. Photochem. Photobio. A 350
4
(2018) 59-68.
[21] A. Morozan, P. Jegou, S. Campidelli, S. Palacin, B. Jousselme, Relationship between
polypyrrole morphology and electrochemical activity towards oxygen reduction reaction,
ChemComm. 48 (2012) 4627-4629.
2
0

3
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
99
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
[22] N.T. Thanh Truc, T.-D. Pham, D. Van Thuan, L.T. Son, D.T. Tran, M.V. Nguyen, V.N.
Nguyen, N.M. Dang, H.T. Trang, Superior activity of Cu-NiWO /g-C N Z direct system for
4
3
4
photocatalytic decomposition of VOCs in aerosol under visible light, J. Alloys Compd. 798
(2019) 12-18.
[23] N.T.T. Truc, D.S. Duc, D. Van Thuan, T.A. Tahtamouni, T.-D. Pham, N.T. Hanh, D.T.
Tran, M.V. Nguyen, N.M. Dang, N.T.P. Le Chi, V.N. Nguyen, The advanced photocatalytic
degradation of atrazine by direct Z-scheme Cu doped ZnO/g-C
(2019) 875-882.
3 4
N , Appl. Surf. Sci. 489
[24] R. Pang, K. Teramura, H. Asakura, S. Hosokawa, T. Tanaka, Highly selective
photocatalytic conversion of CO by water over Ag-loaded SrNb O nanorods, Appl. Catal. B
2
2
6
Environ. 218 (2017) 770-778.
[25] F. Wang, Y. Ye, Y. Cao, Y. Zhou, The favorable surface properties of heptazine based g-
C3N4 (001) in promoting the catalytic performance towards CO conversion, Appl. Surf. Sci.
2
481 (2019) 604-610.
[26] F. Yi, J. Ma, C. Lin, L. Wang, H. Zhang, Y. Qian, K. Zhang, Insights into the enhanced
adsorption/photocatalysis mechanism of a Bi O Br /g-C N nanosheet, J. Alloys Compd. 821
4
5
2
3
4
(2020) 153557.
[27] V.-D. Dao, L.T. Son, T.D. Nguyen, N. Van Noi, N.M. Ngoc, T.-D. Pham, P. Van Quan,
H.T. Trang, Superior visible light photocatalytic activity of g-C N /NiWO direct Z system
3
4
4
for degradation of gaseous toluene, J. Solid State Chem. 272 (2019) 62-68.
[28] A. Kumar, A. Kumar, G. Sharma, A.a.H. Al-Muhtaseb, M. Naushad, A.A. Ghfar, C. Guo,
F.J. Stadler, Biochar-templated g-C /Bi CO /CoFe nano-assembly for visible and
3
N
4
2
O
2
3
2 4
O
solar assisted photo-degradation of paraquat, nitrophenol reduction and CO2 conversion,
Chem. Eng. J. 339 (2018) 393-410.
2
1

4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
[29] P. Zhou, J. Yu, M. Jaroniec, All-Solid-State Z-Scheme Photocatalytic Systems, Adv. Mater.
26 (2014) 4920-4935.
[30] M.H. Thanh Tung, N.T. Dieu Cam, D. Van Thuan, P. Van Quan, C. Van Hoang, T.T. Thu
Phuong, N.T. Lam, T.T. Tam, N.T. Phuong Le Chi, N.T. Lan, D.N. Thoại, T.-D. Pham, Novel
direct Z-scheme AgI/N–TiO photocatalyst for removal of polluted tetracycline under visible
2
irradiation, Ceram. Int. 46 (2020) 6012-6021.
[31] G. Sui, J. Li, L. Du, Y. Zhuang, Y. Zhang, Y. Zou, B. Li, Preparation and characterization
3 4 2
of g-C N /Ag–TiO ternary hollowsphere nanoheterojunction catalyst with high visible light
photocatalytic performance, J. Alloys Compd. 823 (2020) 153851.
[32] J. Wang, C. Qin, H. Wang, M. Chu, A. Zada, X. Zhang, J. Li, F. Raziq, Y. Qu, L. Jing,
Exceptional photocatalytic activities for CO conversion on AlO bridged g-C N /α-Fe2O z-
2
3
4
3
scheme nanocomposites and mechanism insight with isotopesZ, Appl. Catal. B Environ. 221
(2018) 459-466.
[33] J. Jia, X. Zhang, C. Jiang, W. Huang, Y. Wang, Visible-light-driven nitrogen-doped carbon
3 4 2 6
quantum dots decorated g-C N /Bi WO Z-scheme composite with enhanced photocatalytic
activity and mechanism insight, J. Alloys Compd. 835 (2020) 155180.
[34] L. Yang, L. Zeng, H. Liu, Y. Deng, Z. Zhou, J. Yu, H. Liu, W. Zhou, Hierarchical
microsphere of MoNi porous nanosheets as electrocatalyst and cocatalyst for hydrogen
evolution reaction, Appl. Catal. B Environ. 249 (2019) 98-105.
[35] S. Chen, Z. Zhang, W. Zeng, J. Chen, L. Deng, Construction of NiCo S @NiMoO Core-
2
4
4
Shell Nanosheet Arrays with Superior Electrochemical Performance for Asymmetric
Supercapacitors, ChemElectroChem 6 (2019) 590-597.
2
2

4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
[36] J. Wang, L. Zhang, X. Liu, X. Zhang, Y. Tian, X. Liu, J. Zhao, Y. Li, Assembly of flexible
CoMoO @NiMoO ·xH O and Fe O electrodes for solid-state asymmetric supercapacitors,
4
4
2
2
3
Sci. Rep. 7 (2017) 41088.
[37] Y. Zhang, C.-r. Chang, X.-d. Jia, Q.-y. Huo, H.-l. Gao, J. Yan, A.-q. Zhang, Y. Ru, H.-x.
Mei, K.-z. Gao, L.-z. Wang, Morphology-dependent NiMoO /carbon composites for high
4
performance supercapacitors, Inorg. Chem. Commun. 111 (2020) 107631.
[38] Y. Hao, H. Huang, Q. Wang, Q. Wang, G. Zhou, Nitrogen-doped carbon/NiMoO
4
nanospheres assembled by nanosheets and ultrasmall nanoparticles for supercapacitors, Chem.
Phys. Lett. 728 (2019) 215-223.
[39] W. An, K. Sun, J. Hu, W. Cui, L. Liu, The Z-scheme Ag CO @g-C N core-shell structure
2
3
3
4
for increased photoinduced charge separation and stable photocatalytic degradation, Appl.
Surf. Sci. 504 (2020) 144345.
[40] X. Bai, L. Wang, Y. Wang, W. Yao, Y. Zhu, Enhanced oxidation ability of g-C N
3
4
photocatalyst via C60 modification, Appl. Catal. B Environ. 152-153 (2014) 262-270.
2 3
[41] H. Guo, M. Chen, Q. Zhong, Y. Wang, W. Ma, J. Ding, Synthesis of Z-scheme α-Fe O /g-
C N composite with enhanced visible-light photocatalytic reduction of CO to CH OH, J.
3
4
2
3
CO Util. 33 (2019) 233-241.
2
[42] G. Li, B. Wang, J. Zhang, R. Wang, H. Liu, Rational construction of a direct Z-scheme g-
C N /CdS photocatalyst with enhanced visible light photocatalytic activity and degradation of
3
4
erythromycin and tetracycline, Appl. Surf. Sci. 478 (2019) 1056-1064.
[43] D. Xu, B. Cheng, W. Wang, C. Jiang, J. Yu, Ag CrO /g-C /graphene oxide ternary
reduction activity, Appl. Catal. B
2
4
3 4
N
nanocomposite Z-scheme photocatalyst with enhanced CO
2
Environ. 231 (2018) 368-380.
2
3

4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
[44] M. Ren, Y. Ao, P. Wang, C. Wang, Construction of silver/graphitic-C N /bismuth tantalate
3 4
Z-scheme photocatalyst with enhanced visible-light-driven performance for sulfamethoxazole
degradation, Chem. Eng. J. 378 (2019) 122122.
[45] Z. Zhang, Z. Pan, Y. Guo, P.K. Wong, X. Zhou, R. Bai, In-situ growth of all-solid Z-scheme
heterojunction photocatalyst of Bi O I /g-C N and high efficient degradation of antibiotic
7
9 3
3
4
under visible light, Appl. Catal. B Environ. 261 (2020) 118212.
[46] Z. Jin, R. Hu, H. Wang, J. Hu, T. Ren, One-step impregnation method to prepare direct Z-
3 4
scheme LaCoO /g-C3N heterojunction photocatalysts for phenol degradation under visible
light, Appl. Surf. Sci. 491 (2019) 432-442.
[47] Y. Tan, Z. Shu, J. Zhou, T. Li, W. Wang, Z. Zhao, One-step synthesis of nanostructured g-
C N /TiO composite for highly enhanced visible-light photocatalytic H evolution, Appl.
3
4
2
2
Catal. B Environ. 230 (2018) 260-268.
[48] H. Xuan, Y. Xu, Y. Zhang, H. Li, P. Han, Y. Du, One-step combustion synthesis of porous
CNTs/C/NiMoO composites for high-performance asymmetric supercapacitors, J. Alloys
4
Compd. 745 (2018) 135-146.
[49] X. Zhang, L. Wei, X. Guo, Ultrathin mesoporous NiMoO -modified MoO core/shell
4
3
nanostructures: Enhanced capacitive storage and cycling performance for supercapacitors,
Chem. Eng. J. 353 (2018) 615-625.
[50] M. Cao, Y. Bu, X. Lv, X. Jiang, L. Wang, S. Dai, M. Wang, Y. Shen, Three-dimensional
TiO nanowire@NiMoO ultrathin nanosheet core-shell arrays for lithium ion batteries, Appl.
2
4
Surf. Sci. 435 (2018) 641-648.
[51] S. Yue, H. Tong, L. Lu, W. Tang, W. Bai, F. Jin, Q. Han, J. He, J. Liu, X. Zhang,
2 4
Hierarchical NiCo O nanosheets/nitrogen doped graphene/carbon nanotube film with
2
4

4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
91
92
93
94
95
96
97
98
99
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
ultrahigh capacitance and long cycle stability as a flexible binder-free electrode for
supercapacitors, J. Mater. Chem. A 5 (2017) 689-698.
[52] C. Chen, S. Wang, X. Luo, W. Gao, G. Huang, Y. Zeng, Z. Zhu, Reduced
ZnCo O @NiMoO ·H O heterostructure electrodes with modulating oxygen vacancies for
2
4
4
2
enhanced aqueous asymmetric supercapacitors, J. Power Sources, 409 (2019) 112-122.
[53] S. Hussain, M.S. Javed, S. Asim, A. Shaheen, A.J. Khan, Y. Abbas, N. Ullah, A. Iqbal, M.
4
Wang, G. Qiao, S. Yun, Novel gravel-like NiMoO nanoparticles on carbon cloth for
outstanding supercapacitor applications, Ceram. Int. (2019).
[54] S.K. Ray, D. Dhakal, S.W. Lee, Rapid degradation of naproxen by AgBr-α-NiMoO4
composite photocatalyst in visible light: Mechanism and pathways, Chem. Eng. J. 347 (2018)
836-848.
[55] S.K. Ray, D. Dhakal, J.K. Sohng, S.-Y. Kim, S.W. Lee, Efficient inactivation of
Pseudomonas aeruginosa by Cu/Co-α-NiMoO in visible light, Chem. Eng. J. 347 (2018) 366-
4
378.
[56] S.K. Ray, R.P. Pandey, S. Jeong, S.W. Lee, Rapid Escherichia coli inactivation in visible
light by Fe/Zn-α-NiMoO nanorod, J. Photochem. Photobio. A Chem. 367 (2018) 162-170.
4
[57] S.K. Ray, D. Dhakal, Y.K. Kshetri, S.W. Lee, Cu-α-NiMoO photocatalyst for degradation
4
of Methylene blue with pathways and antibacterial performance, J. Photochem. Photobio. A
Chem. 348 (2017) 18-32.
[58] Y.Y. Lee, H.S. Jung, J.M. Kim, Y.T. Kang, Photocatalytic CO conversion on highly
2
ordered mesoporous materials: Comparisons of metal oxides and compound semiconductors,
Appl. Catal. B Environ. 224 (2018) 594-601.
[59] A. Halder, M. Kilianová, B. Yang, E.C. Tyo, S. Seifert, R. Prucek, A. Panáček, P.
Suchomel, O. Tomanec, D.J. Gosztola, D. Milde, H.-H. Wang, L. Kvítek, R. Zbořil, S. Vajda,
2
5

5
5
15
16
Highly efficient Cu-decorated iron oxide nanocatalyst for low pressure CO conversion, Appl.
2
Catal. B Environ. 225 (2018) 128-138.
2
6

Highlights
−
−
−
Successfully created NiMoO /g-C N Z scheme heterojunction for CO conversion
4
3
4
2
-
+
Hybridization of NiMoO
4
and g-C
3
N
4
prevented recombination of e /h in each material
The created NiMoO
4
/g-C
3
N
4
converted CO
2
into HCOOH, CH
4
and CO even under visible
light
−
The optimal molar ratio of g-C N /NiMoO for the best photocatalytic conversion was 60%
3 4 4