Construction of Z-scheme NiO/NiC/g-C3N4composites using NiC as novel cocatalysts for the efficient photocatalytic degradation

Article abstract of DOI:10.1039/d1ra03562b

A novel composite consisting of NiO/NiC/g-C₃N₄with excellent photocatalytic properties was successfully synthesized by the simple calcination of layered double metal hydroxide (LDH) and melamine. The color and chemical composition of the as-prepared composites could be tailored by changing the mass ratio of NiAl-LDH and g-C₃N₄. For the L4C composite at the ratio of 1?:?1, it showed the desired dark color due to the generated NiC. It also showed high photodegradation efficiency under visible light irradiation, reaching 97.5% toward Rhodamine B and 92.6% toward tetracycline. The high photodegradation efficiency could be mainly attributed to the unique formation of NiC cocatalysts coupled with g-C₃N₄and NiO semiconductors, which constructed a Z-scheme system and facilitated the efficient separation of the photogenerated electron-hole pairs. The present findings provide a promising approach for fabricating the new types of composite photocatalysts for pollutant degradation.

Full text of DOI:10.1039/d1ra03562b

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Construction of Z-scheme NiO/NiC/g-C N4
3
composites using NiC as novel cocatalysts for the
Cite this: RSC Adv., 2021, 11, 24822
efficient photocatalytic degradation†
Xiaojie Song,‡ Sisi Ye,‡ Xin Zhou, Wanrui Gui, Can Yang and Zhihong Yang
*
3 4
A novel composite consisting of NiO/NiC/g-C N with excellent photocatalytic properties was successfully
synthesized by the simple calcination of layered double metal hydroxide (LDH) and melamine. The color
and chemical composition of the as-prepared composites could be tailored by changing the mass ratio
3 4
of NiAl-LDH and g-C N . For the L4C composite at the ratio of 1 : 1, it showed the desired dark color
due to the generated NiC. It also showed high photodegradation efficiency under visible light irradiation,
reaching 97.5% toward Rhodamine B and 92.6% toward tetracycline. The high photodegradation
Received 7th May 2021
Accepted 24th June 2021
3 4
efficiency could be mainly attributed to the unique formation of NiC cocatalysts coupled with g-C N
and NiO semiconductors, which constructed a Z-scheme system and facilitated the efficient separation
of the photogenerated electron–hole pairs. The present findings provide a promising approach for
fabricating the new types of composite photocatalysts for pollutant degradation.
DOI: 10.1039/d1ra03562b
rsc.li/rsc-advances
behavior is mainly determined by the quality of the photo-
catalyst; hence, various semiconductor-based materials have
1
. Introduction
With the development of human society, the problem of envi- been explored for the photocatalytic degradation of organic
ronmental pollution has aroused extensive attention and pollutants.
concern. Large amounts of synthetic dyes are consumed and
discharged per year in various industries including textile, hydrotalcite-like materials and a family of anionic clays, are
Layered double metal hydroxides (LDH), which are
1–3
leather, paper, cosmetics and plastics. Similarly, antibiotics, among the most important photocatalysts intensively investi-
8,9
which are widely used to improve human and animal health gated for wastewater treatment. LDH, oen expressed by the
2
+
3+
x+ nꢀ
2 2
] (A )x/n$mH O, are char-
and treat infections, are frequently discharged into surface general formula [M1ꢀx
water, seawater and groundwater.
M
x
(OH)
4,5
The effluent organic acterized by positively charged metal hydroxide laminates and
wastewater without effective treatment severely pollutes water negatively charged balanced anions in the interlayer, with
resources and ecosystems, and even threatens human health. controllable composition and gallery spacing. LDH containing
So, the hazardous organic matter must be efficiently separated photo-active metal components (e.g., Zn, Ni, Cr, or Fe) show
from the water before its discharge and many treatment tech- photocatalytic properties and have been widely employed as
nologies have been developed in recent years, such as physical heterogeneous catalysts, catalyst precursors or catalyst supports
10,11
methods (adsorption, membrane separation and magnetic for environmental clean-up and energy production.
More-
separation), chemical methods (electrochemical process, pho- over, when LDH is calcined at intermediate temperatures, the
tocatalytic oxidation, Fenton and Fenton-like oxidation and calcination products consist of poorly crystallized mixed metal
ozone oxidation), and biological methods (anaerobic method, oxides (MMOs) with many advantages in comparison to
6
aerobic method, anaerobic–aerobic combination process).
common metal oxides and LDH, such as the reconstruction of
Among these wastewater treatment methods, photocatalytic the layered structure by rehydration, a uniform distribution of
degradation involving advanced oxidation processes is an metal cations at the atomic level and their adjustable compo-
12
energy-saving and highly effective technology using efficient nents in a controllable manner. However, LDH and LDH-
7
photocatalysts under light irradiation. The photocatalytic derived MMOs are not highly efficient in photocatalytic
processes due to the recombination of electron–hole pairs and
13
the low utilization of visible light.
Recently, 2D graphitic carbon nitride (g-C
signicant attention due to its simple synthesis, stable prop-
Faculty of Materials Science and Chemistry, Engineering Research Center of
Nano-Geomaterials of Ministry of Education, China University of Geosciences,
Wuhan, 430074, China. E-mail: yzhh05@126.com; Tel: +86-27-67884814
3 4
N ) has attracted
14,15
erties, suitable band energy, non-toxicity and high stability.
It also exhibits photocatalytic activities toward many organic
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra03562b
16,17
pollutants
because of its appropriate band structure and
Both authors contributed equally to this work.
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strong light absorption ability. However, its catalytic efficiency mild strategy for the design and preparation of fascinating NiC-
is quite low due to the inadequate utilization of visible light and containing photocatalysts that can serve as promising candi-
the rapid recombination of photogenerated electrons and dates in photodegradation and photocatalytic H evolution.
2
18,19
holes.
It has been identied that its catalytic efficiency can
be improved by coupling with other semiconductor photo-
catalysts via transferring the photogenerated electron–hole
2
. Experimental
pairs between g-C
charge recombination.
Recently, it was conrmed that the lifetimes of photo-
generated charge carriers increased and visible-light-induced
photocatalytic properties were enhanced when g-C N4 was
incorporated with LDH or LDH-derived MMOs. E.g., through
3 4
N
and other semiconductors to depress the 2.1 Preparation of photocatalysts
17
2.1.1 Materials. All materials used in this research were of
analytical grade and used as received. All chemicals were
purchased from Sinopharm Chemical Reagent Co. Ltd.,
3
including aluminum nitrate (Al(NO)
NO $6H O), urea (CH O), and melamine (C
.1.2 Synthesis of NiAl-LDH. NiAl-LDH was synthesized by
3
$9H
2
O), nickel nitrate (Ni
(
3
)
2
2
4
N
2
3
N
3
2 3
(NH ) ).
the formation of 2D/2D interface heterostructures, g-C
3
N
4
/NiAl-
/NiFe-
2
2
0–22
23
15
LDH,
g-C
3
N
4
/ZnTiLDH, g-C
N /CoFe-LDH composites were synthesized
4
3
N
4
/ZnAl-LDH, g-C
3
N
4
39
a hydrothermal method. Here, 0.75 g of Al(NO)
3 2
$9H O and
19
13
LDH and g-C
3
1
.75 g of Ni(NO $6H O were added to 40 mL of deionized
3
)
2
2
with improved visible-light-driven performance, much better
than that of either single-phase g-C or LDH. Moreover, when
the precursor of g-C N was mixed with LDH and then calcined,
water with stirring for 30 min until they dissolved completely.
Subsequently, 2.4 g of urea was added to the mixture with
stirring for 30 min. Then the mixture was transferred into
3 4
N
3
4
MMOs/g-C N composites were obtained and the photocatalytic
ꢁ
3
4
a 100 mL Teon-lined autoclave and heated at 120 C for 12 h.
24
efficiency was promoted. E.g., Di reported that the coupling of
ZnFe-MMOs (ZnO and ZnFe ) with g-C was benecial to
the degradation of ibuprofen (IBF) and sulfadiazine (SDZ). Lan
identied that the obtained nanocomposites of ZnIn-MMOs
In and ZnO) and g-C exhibited improved photo-
degradation activity toward Rhodamine B (RhB) under visible
Aer cooling to room temperature, the white solid was collected
by high-speed centrifugation and washed with deionized water
2
O
4
3 4
N
12
until the pH reached 7. Finally, the collected solid was dried at
ꢁ
6
0 C for 10 h in a vacuum drying oven to obtain NiAl-LDH.
(
2
O
3
3 4
N
2
.1.3 Synthesis of NiO/NiC/g-C N composites. The NiO/
3 4
NiC/g-C N composites were synthesized by calcination treat-
3
4
25
light irradiation. Patnaik found that the incorporation of
ZnCr-MMOs (ZnCr O and ZnO) and g-C N exhibited good H
ment. Melamine was mixed with the as-prepared NiAl-LDH in
ball milling for 2 h and then put in the crucible covered with
aluminum foil. Subsequently, the crucible was calcined at
2
4
3
4
2
and O2 production. From these reports, it can be seen that
MMOs and g-C products were detected from the calcination
of g-C precursor and LDH with the enhancement of photo-
N
4
ꢁ
ꢁ
ꢀ1
3
5
50 C for 4 h at a heating rate of 2 C min in the muffle
3 4
N
furnace to obtain the products. According to the mass propor-
tions of NiAl-LDH and melamine at 1 : 6, 1 : 5, 1 : 3, 1 : 1, 1 : 0.5,
the corresponding products were labeled as L1C, L2C, L3C, L4C,
L5C, respectively. L was for the NiAl-LDH, the C was for the g-
C N . For comparison, the sample produced from pure NiAl-
catalytic properties. However, whether metal carbides as the
possible derived products would present and how these metal
compounds affect the photocatalytic efficiency were unclear.
Some transition-metal carbides with the benets of relative
3
4
27,28
29
30
earth abundance, such as Ni C,
WC, Mo C and Ti C (ref.
2 2 3
3
LDH was labeled as NiAl-MMO.
3
1) have been identied to be potential photocatalysts or elec-
26
trocatalysts. Nickel carbides (NiC
x
) with superior electric
2
.2 Characterization
X-ray powder diffraction (XRD) patterns were examined on
and a Bruker D8-ADVANCE powder X-ray Diffractometer at 200 mA
conductivity provide the possibility to fabricate the catalysts or
32
cocatalysts with high performance and cost-effectiveness.
A
33
34,35
series of nickel carbides including NiC0.2
,
NiC,
36–38
Ni
3
C
with different crystal structures were synthesized for and 40 kV using Cu Ka radiation (l ¼ 0.15406 nm) over the 2q
ꢁ
water-splitting catalysts and showed good catalytic activity. range of 5–90 , and FT-IR spectra were recorded by using a Perkin-
ꢀ
1
However, to the best of our knowledge, most of these reports Elmer 100 spectrometer in the range of 4000–500 cm using the
were focused on their electrocatalytic performances for KBr disc technique. The morphologies of the samples were
hydrogen evolution applications, and very limited reports determined by scanning electron microscopy (Hitachi SU-70
addressed their applications as promising photocatalysts. SEM). The ultraviolet-visible diffuse reectance spectra (UV-vis
Therefore, exploring the photocatalytic activity of nickel DRS) of samples were recorded over the range of 200–800 nm
carbides is of great importance to seeking valuable alternative using a UV-2600. X-ray photoelectron spectroscopy (XPS) was
+
cocatalysts for high photocatalytic efficiency.
characterized with EscaLab Xi equipment with Al Ka radiation
In this paper, NiC, NiO, and g-C N products were success- (hv ¼ 1486.6 eV). The distribution characterization was further
3
4
fully synthesized by the simple calcination of a NiAl-LDH and analyzed by a transmission electron microscope (TEM, Talos
melamine mixture. This obtained novel composite combines F200X) equipped with energy dispersive X-ray (EDX). The Bru-
the unique properties of nanostructured conductors (NiC) and nauer–Emmett–Teller (BET) analysis was carried out via N2
3 4
semiconductors (NiO and g-C N ), showing signicantly adsorption using (SSA-7000), in which the Barrett–Joyner–
improved photocatalytic activity in degrading organic Rhoda- Halenda (BJH) method was followed to analyze the pore size
mine B (RhB) dye and tetracycline (TC) in comparison to pure distribution. Photoluminescence (PL) spectra of the samples were
NiAl-MMO and g-C
3
N
4
. This work provides a controllable and obtained
at
room temperature on
a
steady-state
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ꢁ
ꢁ
spectrouorometer (SHIMADZU RF-6000) with an excitation peaks at 13.1 and 27.6 , corresponding to the characteristic
wavelength of 330 nm. The electrochemical impedance spectros- interlayer stacking of the conjugated aromatic systems and in-
40–42
copy (EIS) measurements were performed in the presence of planar structure repeating unit, respectively.
For pure
a 0.1 M Na SO solution with the frequency range of 0.1–100 kHz NiAl-LDH, the characteristic diffraction peaks at 2q angles
2
4
ꢁ
43,44
under open-circuit potential conditions (CHI 760E). Pt and Ag/ below 30 correspond to the (003) and (006) planes,
indi-
AgCl electrodes were used as the counter electrode and refer- cating the regular two-dimensional hydrotalcite-like layered
ence electrode, respectively; the working electrode was prepared structure. When NiAl-LDH was calcinated into NiAl-MMO, the
by loading a catalyst on ITO glass. Here, 10 mg of the catalyst was characteristic diffraction peaks of (003) and (006) reecting the
dispersed in 300 mL ethanol and 100 mL Naon ionomer solution interlayer structure disappeared, while the diffraction peaks at
ꢁ ꢁ ꢁ
2 wt%), then ultrasonicated for 30 min to form a uniform 37.04 , 44.32 and 62.69 appeared, which are ascribed to the
(
45
suspension. Next, 100 mL of the suspension was dropped on the (111), (200) and (220) of NiO planes, respectively.
ITO glass to form a 0.5 cm ꢂ 0.5 cm square and dried at room When NiAl-LDH and melamine are mixed at suitable mass
temperature. The electron spin resonance (ESR) spectra were ratios, it is interesting that in addition to g-C , the generated
determined using a Bruker EMXPLUS spectrometer. products consist of NiO and NiC, and a small amount of Ni N;
the corresponding crystal plane peaks of NiC are the (111) and
200), and the peak of Ni N is (111), different from the only
3 4
N
3
(
3
2.3 Photocatalytic tests
46
metal oxide phase in other reports. Al-containing compounds
were not detected in all samples, which may be attributed to
their low content or amorphous structure. As the amount of
melamine decreased, the peak of g-C N (002) gradually
The photocatalytic degradations of organic dyes and an antibiotic
were performed under visible light irradiation. In the experiment,
50 mg of catalyst was dispersed in 40 mL aqueous solutions
3
4
containing the dye or tetracycline at different concentrations.
Prior to irradiation, the mixture dispersion was magnetically
stirred for 15 min in the dark to establish the adsorption–
desorption equilibrium. Under the irradiation from a 300 W Xe
arc lamp, 3 mL of the reaction solution was withdrawn at 30 min
intervals and ltered with a 0.45 mm microporous membrane to
remove the photocatalyst, then the absorbances of RhB, TC, and
methyl orange (MO) were measured by UV-Vis spectroscopy at
wavelengths of 554, 357 and 460 nm, respectively.
decreased; for the L1C samples with high melamine content
and L4C, L5C with high NiAl-LDH, the main phases are g-C
and NiAl-MMO, respectively.
3 4
N
Fig. 2 presents the FT-IR patterns of the synthesized g-C
NiAl-MMO, and NiO/NiC/g-C composites. For the pure g-
is attributed to the breathing
3 4
N ,
3 4
N
ꢀ1
C N the band at 811 cm
3
4,
47,48
vibration of the triazine heterocycle,
and the broad absorp-
ꢀ1
tion band at 3300 cm corresponds to the stretching vibration
of N–H. In addition, the bands between 1200 and 1700 cm are
ꢀ1
assigned to the typical C–N and C]N stretching modes of the
4
9
triazine ring. For the NiAl-MMO sample, the bands at 3450
3
Results and discussion
ꢀ
1
and 1636 cm can be assigned to the O–H stretching and
3.1 Structural characterization of NiO/NiC/g-C N
3 4
ꢀ1
bending vibrations, respectively. The band at 1415 cm may
correspond to the bending vibrations of NO ions intercalated
in the layer of NiAl-LDH. The other bands below 800 cm are
composites
Fig. 1 shows XRD patterns of all the samples. As shown in
Fig. 1a, the pure g-C exhibited (100) and (002) diffraction
ꢀ
3
50
ꢀ1
3 4
N
3 4 3 4
Fig. 1 XRD patterns of the synthesized samples, (a) g-C N , NiAl-LDH, NiAl-MMO, and (b) NiO/NiC/g-C N composites.
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ꢀ
1
composites showed a new weak peak at 1129 cm , which may
be related to the formation of NiC.
Fig. 3 shows SEM images of g-C N , NiAl-LDH, NiAl-MMO,
3
4
and NiO/NiC/g-C N samples. It can be seen that the bulk g-
3
4
3 4
C N sample exhibited serious aggregation. The NiAl-LDH
sample displayed a 3D aky morphology with plenty of thin
nanoakes, and the NiAl-MMO sample showed a similar ower-
like morphology with decreased particle size. For the L1C
sample, a large number of plate-like particles were observed,
which consist of g-C N lamellas based on the above XRD
3
4
analysis. For the L5C sample, more 2D akes were observed and
they showed the tendency to develop into ower-like shapes,
similar to the morphology of pure NiAl-MMO. For L2C, L3C, and
L4C samples, in addition to ake-like particles, more granular
particles were detected with the sharply decreased size. Partic-
ularly, the morphology of L4C samples at high magnication
showed that the lamellas were mixed with many small nano-
particles, suggesting that g-C N and NiO semiconductors with
two-dimensional lamella structure were well coupled with zero-
dimensional NiC conductors with the formation of the
heterojunction.
In Fig. 4, the TEM image of the L4C sample reveals that the
nanoparticles with diameters of 5–50 nm were distributed on
3 4 3 4
Fig. 2 FT-IR patterns of g-C N , NiAl-MMO and NiO/NiC/g-C N
composites.
3
4
attributed to the translational modes of metal–oxygen (Ni–O
51
and Al–O) and metal–oxygen–metal (Ni–O–Al) bands.
3 4
Compared with NiAl-MMO and g-C , the NiO/NiC/g-C N
3
N
4
3 4
Fig. 3 SEM image of the samples, (a) g-C N , (b) NiAl-LDH, (c) NiAl-MMO, (d) L1C, (e) L2C, (f) L3C, (g) L4C, (h) L5C.
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Fig. 4 Characterization of the L4C sample: (a) TEM image, (b) high-resolution TEM image, (c) HAADF-STEM images and the corresponding
element maps of N, O, Ni, Al, and C.
24
a cotton-like g-C N4 matrix. HRTEM images presented the
The XPS spectra were obtained to analyze the detailed
3
lattice fringes of these nanocrystals. The d-spacings of 0.2404 chemical status of the composite photocatalysts. In Fig. 5, the
and 0.2035 nm were indexed as the (111) facets in NiO and NiC XPS spectrum of the L4C sample exhibits peaks corresponding
crystals, respectively, and the uffy part is attributed to g-C
3
N
4
.
to Ni, Al, C, N, and O elements. The Ni 2p spectrum displays two
These different types of nanocrystals formed intertwined peaks with the binding energies of 853.9 and 872.7 eV, corre-
structures, identifying the formation of closely linked hetero- sponding to Ni 2p3/2 and Ni 2p1/2, respectively, which reects
2
+
junctions. The corresponding element maps show that the the typical binding energies of Ni in the L4C composite.
bright nanoparticles in the HAADF image were mainly Moreover, Ni 2p3/2 can be deconvoluted into three peaks at the
2
+
composed of nickel-containing compounds with a small binding energies of 854.2 and 856.6 eV, corresponding to Ni in
3
+
53,54
number of aluminum compounds. Moreover, C and N element NiO, and Ni in NiC compounds,
respectively, in agreement
maps indicated that the dark regions in the HAADF image were with the XRD pattern results as discussed above. The appear-
3
+
covered with g-C
3 4
N nanosheets, further indicating that nickel- ance of Ni is probably due to the existence of nickel vacancies
2
+
containing nanoparticles were homogeneously distributed on and oxygen vacancies along with the oxidation of surface Ni to
52
3+
the surface of g-C
3
N
4
sheets.
Ni
to achieve charge neutrality during the calcination
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Fig. 5 XPS profile spectra of the L4C sample: (a) survey spectrum, (b) Ni 2p, (c) Al 2p, (d) N 1s, (e) C 1s, (f) O 1s.
5
5
20,57,58
process. The N 1s spectrum can be t to four peaks. The one peaks located at 283.6, 285.1, 286.5, and 288.3 eV (Fig. 5e).
53,56
2
located at 397.1 eV may be associated with N–Ni,
other three at 398.9, 400.4, and 400.9 eV were assigned to sp - hybridized carbon atoms in C–C bonds, respectively. The
hybridized nitrogen in triazine rings (C–N]C), bridged tertiary peaks at 286.5 and 288.3 eV were assigned to the C–O functional
nitrogen N–(C) groups, and amino functional groups (N–H), group and the sp -hybridized carbon bonded to N inside the
3
respectively. The C 1s spectrum showed four deconvoluted triazine rings,
and the The peaks at 283.6 and 285.1 eV were assigned to NiC and sp -
2
59
59
2
20
57,60
respectively. The XPS spectrum of O 1s located
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3 4
Fig. 6 UV-Vis DRS of the synthesized samples (a), and the band gaps of the synthesized g-C N (b), NiAl-LDH (c), and NiAl-MMO (d).
61
at 531.65 eV is related to hydroxide ions and the Al 2p spec- including L2C, L3C, and L4C are exclusively black due to the
6
3
trum may be ascribed to Al O produced from the calcination of presence of black NiC and NiO nanoparticles, which would
2
3
62
NiAl-LDH.
Fig. S1† shows the colors of all the samples. Compared with
yellow g-C and green NiAl-MMO, the NiO/NiC/g-C
composites became much darker. Particularly, the samples g-C
signicantly promote the absorption of visible light.
The photoabsorption properties of g-C , NiAl-LDH, and
NiO/NiC/g-C were examined by UV-VIS DRS (Fig. 6). The pure
exhibited a typical semiconductor-like absorption in the
3 4
N
3
N
4
3
N
4
3 4
N
3 4
N
2 3 4
Fig. 7 (a) N adsorption–desorption isotherms and (b) pore size distribution curves of g-C N , NiAl-LDH, NiAl-MMO, and L4C.
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Table 1 BET surface areas and pore parameters of samples
L4C showed a small semicircle diameter, indicating its low
interfacial charge transfer resistance. This is attributed to the
presence of metallic NiC that rapidly transfers the photo-
generated electrons and hence accelerates the separation of
electrons and holes.
The separation of the photo-generated carriers for the
hybrids can be further analyzed by photoluminescence (PL).
Fig. 8b shows the photoluminescence spectra of the samples.
2
ꢀ1
3
ꢀ1
Samples
g-C
S
BET (m g
)
V
total (cm g
)
Daverage (nm)
3
N
4
5.09
0.06
0.52
0.80
0.23
25.03
8.88
9.56
8.59
NiAl-LDH
NiAl-MMO
L4C
118.21
167.86
82.55
3 4
Compared with other samples, the g-C N sample showed
a strong emission peak at 330 nm due to its terrible recombi-
nation of photogenerated electrons and holes. The inset graph
shows that the emission peak of L4C is weaker than those of
NiAl-LDH and NiAl-MMO, suggesting that the recombination of
photogenerated carriers can be effectively restrained through
blue region of the visible spectrum, and the absorption edge
was around 460 nm, which may be assigned to n-p* transitions
of the lone pairs on the edge of the N-atom of the triazine rings.
3 4 g
The bandgap of g-C N was calculated by using the equation E
¼
hc/l and was found to be approximately 2.61 eV. In addition,
3 4
the desired hybridization between g-C N , NiO, and NiC.
the pure NiAl-LDH mainly showed three characteristic absorp-
tion bands in the UV and visible regions, where the rst one is
located from 200 to 300 nm, the second one from 300 to 500 nm,
and the third one from 600 to 800 nm. For the synthesized NiAl-
MMO, three similar absorption bands were detected with
obvious optimal red-shis and enhanced absorption intensity.
It can be concluded that the bandgap of NiAl-MMO was about
3
.2 Photocatalytic activity of NiO/NiC/g-C
The photodegradation behavior of the as-prepared NiO/NiC/g-
photocatalysts was evaluated by Rhodamine B dye in the
3 4
N composites
3 4
C N
ꢀ
1
solution of concentration 10 mg L under visible light irradi-
ation. For comparison, the photocatalytic properties of pure g-
C N and NiAl-LDH were also tested. As shown in Fig. 9a and b,
the degradation efficiencies of g-C N , NiAl-LDH and NiAl-MMO
3 4
were 58%, 34.6% and 41.5% within 90 min, respectively.
Compared with these photocatalysts, the photocatalytic ratios
64,65
3
.27 eV, close to the bandgap of pure NiO in the literature,
further indicating that NiAl-MMO was mainly composed of NiO.
Among the as-prepared NiO/NiC/g-C samples, L1C and
3
4
3 4
N
L5C nanocomposites showed absorption features similar to g-
C N and NiAl-MMO, respectively. However, the absorption
3
4
3 4
of the NiO/NiC/g-C N composites were signicantly improved,
spectra of L2C, L3C, and L4C are signicantly different from
other samples, showing approximate straight lines with no
obvious absorption edges in the region ranging from 200 to
reaching 77.7%, 87.4% and 95.5% for L1C, L2C and L3C,
respectively. Particularly, the L4C sample showed complete
degradation activity with a degradation ratio close to 100%.
However, the photocatalytic activities of L5C greatly decreased,
which was attributed to the very low contents of NiC and g-C N
3 4
in the composites. Moreover, the reaction rate constant was
8
00 nm, which is attributed to their dark color as well as the
66
typical metallic character of NiC. Therefore, the formation of
NiC nanoparticles could effectively increase the absorbance of
visible light, and thus favors the photocatalytic properties of the
nanocomposites.
determined using the rst-order reaction kinetics equation by

0 t
tting the kinetic data via a linear plot of ln(C /C ) against time.
The BET surface area distribution, pore volume and average
As shown in Fig. 9c, the RhB photocatalytic degradation
behaviors well match the pseudo-rst-order reaction dynamic
model. Concretely, the photodegradation rate constant over
L4C was 0.06795 min , which is 7.3 and 12.7 times higher as
compared to g-C and NiAl-MMO, respectively.
Since RhB at a concentration of 10 mg mL was almost
completely degraded by the L4C sample, the solutions with
higher concentrations were prepared to further explore the
degradation performance. In Fig. 9d, for the solution at
pore size of g-C
were studied by N
3
N
4
, NiAl-LDH, NiAl-MMO and L4C materials
adsorption/desorption isotherms (Fig. 7),
2
and the results are summarized in Table 1. All the physical
adsorption isotherms showed typical type IV adsorption
ꢀ1
3 4
N
3
behavior with H hysteresis loops. The BET surface areas of g-
ꢀ
1
C N , NiAl-LDH, NiAl-MMO and L4C were 5.09, 118.21, 167.86
3
4
2
ꢀ1
ꢀ1
and 82.55 m g , respectively; the total pore volume of the g-
C N (0.06 cm g ) was lower than that of other samples. The
3
3
4
surface area and pore volume of NiAl-MMO were well above that
of NiAl-LDH, which could be due to the elimination of interlayer
water during the calcining process. In addition, the surface area
ꢀ1
concentrations of 30, 50, and 90 mg mL , the degradation
efficiencies of L4C were 97.5%, 76.6% and 46.6% within
120 min, respectively. Therefore, the degradation ratio
2
ꢀ1
of L4C (82.55 m g ) decreased as comparison with NiAl-MMO
decreased with the increase of the dye concentration and RhB
can be completely degraded in the solution with a concentra-
2
ꢀ1
(
3 4
167.86 m g ), this is due to g-C N having a higher propor-
tion, resulting in a smaller specic surface area.
ꢀ
1
tion of less than 30 mg mL . Compared with g-C
containing photocatalysts toward RhB in the literature
Table 2), the L4C composite showed good photodegradation
3 4
N and Ni-
EIS (electrochemical impedance spectroscopy) Nyquist
spectra at open-circuit voltage were employed to investigate the
charge transfer and recombination behavior of the nano-
(
performance in this study.
67
composites. The semicircle of the high-frequency region on
the EIS plots is related to the charge transfer resistance in the
electrode reaction. The smaller the semicircle, the faster the
charge transfer rate. In Fig. 8a, compared with other samples,
The photocatalyst may be deactivated due to the combina-
tion of secondary products or oxidized intermediates. Cycling
tests were operated to evaluate the stability and reusability of
the L4C sample under visible light irradiation. Aer 90 min of
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3 4
Fig. 8 (a) EIS Nyquist plots of all the samples. (b) PL spectra of the g-C N , NiAl-LDH, NiAl-MMO, and L4C samples.
irradiation in each cycle, the photocatalyst was separated from initial catalytic activity, indicating that the obtained photo-
the aqueous suspension by centrifugation and washing with catalyst has both good stability and a long lifetime. As shown in
deionized water. Fig. 10a shows that this photocatalyst was Fig. 10b, the XRD spectrum of the L4C sample aer six cycles
successfully cycled for 6 successive cycles with a small loss of its was almost the same as that of the sample before cycling tests,
ꢀ1
Fig. 9 (a and b) The photocatalytic degradation of 10 mg L RhB dye by using different samples (dosage: 50 mg); (c) the pseudo-first-order
kinetic fitting curve; (d) the degradation of RhB with different concentrations by using L4C.
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Table 2 Comparison of the degradation efficiencies of representative photocatalysts
ꢀ
1
Photocatalyst
Dye
Light source
Dye (mg L ) of solution
Time (min)
Rate of (%) degradation
References
g-C
g-C
3
N
N
4
4
RhB
RhB
RhB
RhB
RhB
RhB
Visible
Visible
Visible
Visible
Visible
Visible
10
20
30
2
5
10
120
240
20
30
60
90.08
99
75.2
99
99
99
57
53
54
55
3
-40@NiAl-LDH
NiO@g-C
Ni/NiO/TiO
Ag/NiAl-LDH/g-C
NiAl-MMO/g-C
3
N
4
2
3
N
4
56
3
N
4
90
This paper
ꢀ1
Fig. 10 (a) Recycled runs for the photocatalytic degradation of 10 mg L RhB by using the L4C composite. (b) XRD patterns of L4C before and
after recycling.
further indicating that the composite has good photostability NiAl-MMO, and g-C
3 4
N are negatively charged, but NiAl-LDH is
and chemical stability under the studied conditions. positively charged (3.61 mV) due to the positive charge of its
68,69
brucite-like layer.
surface of NiAl-LDH nanoparticles through electrostatic attrac-
tion and is rejected by L4C and g-C samples through elec-
Cationic MO is easily adsorbed on the
3.3 Photocatalytic activity of the composites toward MO and
3 4
N
TC
trostatic repulsion, so it is not efficient to employ these samples
as the photocatalyst to degrade anionic organic pollutants.
Fig. 11d and e show the photocatalytic ratios and reaction
rate constants toward antibiotic TC, respectively. In the solution
with a TC concentration of 30 mg L , L4C exhibited a high
photocatalytic ratio (92.6%) and reaction rate constant (0.0209)
To investigate the photodegradation behaviors of L4C toward
other organic pollutants, TC antibiotic and the anionic MO dye
ꢀ
1
were employed at the concentrations of 30 mg L
and
0 mg L , respectively. In Fig. 11a, the photodegradation ratio
of MO is low (13%) for L4C and almost no photodegradation
was detected for g-C and NiAl-MMO. Additionally, a higher
ꢀ1
1
ꢀ1
3 4
N
in comparison with NiAl-LDH and g-C N , reecting the good
3
4
absorption ratio of NiAl-LDH was detected in the adsorption
stage and then a small decrease was observed under the irra-
diation of visible light, indicating the signicant absorption
ability of NiAl-LDH with poor photocatalytic ability. For
comparison, the adsorption experiment without light irradia-
tion was carried out simultaneously and the results showed that
the adsorption ratio was almost the same as that from the
photodegradation experiment (Fig. 11b), further identifying the
good absorption and poor photodegradation ability of NiAl-
LDH. These results can be explained by the fact that the
absorbance and photodegradation performance of these
photodegradation due to the synergetic enhancement of NiO,
NiC, and C . Interestingly, NiAl-MMO also showed good
3 4
N
adsorption but poor photodegradation toward TC, which was
identied by the adsorption experiment in Fig. 11f. The good
adsorption of NiAl-MMO may be attributed to the abundant
surface hydroxyl groups of the mixed oxides, which form
hydrogen bonds with the hydroxyl of the TC molecule.
3.4 The photocatalytic mechanism of NiO/NiC/g-C
3 4
N
composites
samples are severally affected by the surface charge properties, As shown in Fig. 12, the heterogeneous catalysts including g-
which were evaluated via the zeta potentials. In Fig. 11c, L4C,
3 4
C N , NiO and NiC simultaneously appeared in the as-prepared
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ꢀ1
ꢀ1
Fig. 11 Photocatalytic degradation of (a) 10 mg L MO and (d) 30 mg L TC. Adsorption and photodegradation experiment of (b) MO over NiAl-
LDH and (f) TC over L4C samples. (c) Zeta potential. (e) The pseudo-first-order kinetic fitting curve of TC over the L4C sample.
3 4
Fig. 12 Diagram of the formation of NiO/NiC/g-C N nanocomposites.
composites produced by calcining the optimum mixture of NiAl- were determined to be 2.61 and 3.27 eV, respectively. According
LDH and melamine. NiO is an inorganic metal-semiconductor to the Butler and Ginley model, CB and VB energies of the
with the characteristics of high chemical stability, low price, semiconductors at the points of zero charge can be calculated
71
and good photocatalytic activity. Ni-metal carbides have the by the following equations:
synergic advantages of being cost-effective with superior electric
70
ECB ¼ EVB ꢀ Eg
(1)
(2)
conductivity due to their alloy properties, acting as highly effi-
cient cocatalysts to improve the catalytic activity of the photo-
catalysts and electrocatalysts.
e
EVB ¼ X ꢀ E + 0.5E
g
The nanocomposites consisting of two semiconductors (g-
C N and NiO) and one conductor (metallic NiC) are in keeping
3 4
where X is the absolute electronegativity of the semiconductor,
E is the energy of free electrons on the hydrogen scale (4.5 eV),
and E is the bandgap energy of the semiconductor.
g
e
with the Z-scheme photocatalytic system (Fig. 13a). From the
UV-Vis spectra in Fig. 6a, the band gap values of g-C N and NiO
3
4
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Fig. 13 (a) The mechanism of the photocatalytic degradation in NiO/NiC/g-C
N
3 4
, (b), (c) photocatalytic activities of L4C for the RhB and TC in
the presence of scavengers.
The X value of NiO was reported earlier as 5.75 eV in the degradation of the organic pollutants into harmless products
72,73
64,79
literature,
so the CB and VB energies were determined to be like CO and H O.
2 2
ꢀ
1.07 and 1.54 eV for g-C N , and -0.35 eV and 2.92 eV for NiO,
To further detect the main reactive oxidative species in the
3
4
respectively. The more positive VB potential of NiO and more photocatalytic process, isopropanol (IPA), triethanolamine
74,75
negative CB potential of g-C
3
N
4
indicated
the photooxida- (TEOA), and benzoquinone (BQ) were used as the scavengers
+
tion reaction via the VB holes of NiO and the photoreduction for hydroxyl radicals (cOH), holes (h ) and superoxide radicals
reaction via the CB electrons of g-C
metallic NiC acted as electron mediators and can rapidly as 10 mmol L in the RhB solution of 50 mg L and TC
transfer photogenerated electrons from the conduction band of solution of 30 mg L . As shown in Fig. 13b and c, the pho-
ꢀ
3
N
4
under irradiation. The (O
2
c ), respectively. The scavenger dosages were determined
ꢀ
1
ꢀ1
ꢀ1
76
NiO to the holes in the valence band of g-C N .
todegradation efficiency decreased in the presence of the
3
4
+
ꢀ
The photogenerated electrons in the CB of g-C N reacted scavengers, implying that h , O c and cOH species were
3
4
2
ꢀ
with the dissolved O molecules to generate O c because the generated for the degradation. Particularly, the introduction of
potential of O
potentials of the CB of the semiconductors. On the other hand, comparison with other scavengers, so O
2
2
ꢀ
2
/O
2
c
(ꢀ0.33 eV) is more positive than the BQ greatly decreased the photodegradation efficiency in
ꢀ
2
c
is the most
the potential of O
potentials of g-C
the CB can reduce O2 to H O and then form cOH radicals signal over the L4C sample was carried out and shown in
2 2 2
/H O (0.69 eV) is more positive than the CB important reactive species in this experiment. To further verify
ꢀ ꢀ
3 4 2 2
N and NiO, indicating that the electrons in the production of O c , the ESR analysis of the DMPO–O c
2
2
ꢀ
+
ꢀ
according to the following reactions: O + 2e + 2H ¼ H O , Fig. S2.† There were no DMPO–O c characteristic peaks in the
2
2
2
2
ꢀ
ꢀ
ꢀ
H O + e ¼ cOH + OH . Furthermore, the VB holes (2.92 eV) dark, while four strong DMPO–O2c characteristic peaks
2
2
ꢀ
from NiO can directly oxidize water and OH to form cOH appeared aer 20 min of visible-light illumination, demon-
ꢀ
radicals, because the VB oxidation potential is more positive strating that O
2
c can be produced with the sample L4C under
The visible light irradiation, which can be used for the photo-
ꢀ
77,78
than those of OH /cOH (1.99 eV) and H
2
O/
c
OH (2.37 eV).
ꢀ
superoxide radical anion O
2
c , hydroxyl radical cOH, and the catalytic degradation of RhB and TC.
holes are known as the reactive species that facilitate the
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T. W. Shaw, E. Abelev and A. B. Bocarsly, Chem. Rev., 2015,
4
. Conclusion
1
15, 12888–12935.
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components, offering great advantages for photodegradation
Eng. J., 2011, 168, 519–524.
3 4
The novel NiO/NiC/g-C N
3 4
N
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9
tunities to develop novel Ni-containing compounds for high-
performance catalysts.
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20 S. Tonda, S. Kumar, M. Bhardwaj, P. Yadav and S. Ogale, ACS
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2
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4 G. Di, Z. Zhu, Q. Huang, H. Zhang, J. Zhu, Y. Qiu, D. Yin and
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors gratefully acknowledge the nancial support by the
Open Research Program of Engineering Research Center of
Nano-Geomaterials of Ministry of Education (Grant No.
NGM2020KF017).
5 S. Patnaik, D. P. Sahoo, L. Mohapatra, S. Martha and
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