Nano-zirconia supported by graphitic carbon nitride for enhanced visible light photocatalytic activity

Article abstract of DOI:10.1039/c9ra08540h

Graphitic carbon nitride (g-C₃N₄) was prepared by high-temperature calcination of urea. A mixture of g-C₃N₄ and nano-ZrO₂ precursor was directly calcined to prepare g-C₃N₄/ZrO₂ hybrid photocatalysts. The photocatalytic properties of the sample were characterized by degradation of rhodamine B (RhB) under visible light. The g-C₃N₄/ZrO₂ hybrid photocatalysts have better degradation performance than the pure g-C₃N₄ and ZrO₂. The prepared catalysts were characterized by various techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FT-IR), and photoluminescence spectroscopy (PL) and electrochemical tests. The reasons for the improvement of catalytic activity were investigated from the aspects of crystal structure, surface morphology and photoelectric properties, and the catalytic mechanism were studied. The results show that the ZrO₂ nanoparticles were coated with g-C₃N₄ to form a heterostructure. Compared with the pure g-C₃N₄ and ZrO₂, the g-C₃N₄/ZrO₂ hybrids reduce the charge transfer resistance and inhibit the recombination of electron-holes well. In addition, it affects the band structure and improves the absorption of visible-light. At the same time, the study found that the main active species in the catalytic process were h⁺ and ·O₂^-.

Full text of DOI:10.1039/c9ra08540h

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PAPER
Nano-zirconia supported by graphitic carbon
nitride for enhanced visible light photocatalytic
activity
Cite this: RSC Adv., 2020, 10, 524
*
Xiaojian Bi, Sirong Yu, Enyang Liu,
Xiaoli Yin, Yan Zhao and Wei Xiong
Graphitic carbon nitride (g-C3N4) was prepared by high-temperature calcination of urea. A mixture of g-
C3N4 and nano-ZrO2 precursor was directly calcined to prepare g-C3N4/ZrO2 hybrid photocatalysts. The
photocatalytic properties of the sample were characterized by degradation of rhodamine B (RhB) under
visible light. The g-C3N4/ZrO2 hybrid photocatalysts have better degradation performance than the pure
g-C3N4 and ZrO2. The prepared catalysts were characterized by various techniques including X-ray
diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis
diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FT-IR), and
photoluminescence spectroscopy (PL) and electrochemical tests. The reasons for the improvement of
catalytic activity were investigated from the aspects of crystal structure, surface morphology and
photoelectric properties, and the catalytic mechanism were studied. The results show that the ZrO2
nanoparticles were coated with g-C3N4 to form a heterostructure. Compared with the pure g-C3N4 and
ZrO2, the g-C3N4/ZrO2 hybrids reduce the charge transfer resistance and inhibit the recombination of
electron–holes well. In addition, it affects the band structure and improves the absorption of visible-
Received 18th October 2019
Accepted 17th December 2019
DOI: 10.1039/c9ra08540h
light. At the same time, the study found that the main active species in the catalytic process were h+
ꢀ
rsc.li/rsc-advances
and $O2
.
materials, such as urea, thiourea and melamine.17–19 The
forbidden band width of g-C3N4 is 2.7 eV, which is not only
excited by visible light, but also has high solar energy utiliza-
tion. In addition, g-C3N4 is a layered structure, and there is a gap
between layers, so the theoretical specic surface area should be
large.20 However, in actual performance studies, g-C3N4 was
found to exhibit signicant defects, and its photogenerated
carriers have extremely short lifetimes and are very easy to
recombine. In order to improve its photocatalytic activity,
various methods such as doping, noble metal loading and
construction of heterojunctions have been researched.21–28
Building a heterojunction is considered the most effective way.
Wang et al. synthesized Cd0.5Zn0.5S/g-C3N4 composite photo-
catalyst by two-step solvothermal method. The results showed
that the photocatalytic performance of g-C3N4 was greatly
improved by the loading of Cd0.5Zn0.5S.29 Lv et al. reported a g-
C3N4/BiOBr photocatalyst which greatly reduced the recombi-
nation rate of electron–hole pairs, thereby effectively improving
its catalytic activity.30 However, the preparation of g-C3N4/ZrO2
has rarely been reported. Since the conduction band potential
of g-C3N4 is ꢀ1.2 eV, ZrO2 can be sensitized to increase its
photocatalytic activity, it will be a very promising research
direction.
1. Introduction
With the development of the economy, the problem of envi-
ronmental pollution has become more and more prominent, so
increasingly scientic research workers are committed to the
treatment of environmental pollution.1,2 In recent years, the use
of solar energy for photolysis of water to produce hydrogen3–5
and photocatalytic degradation of pollutants6–8 has attracted
extensive attention. A large number of semiconductor catalysts
have been extensively studied, such as TiO2, ZnO, CeO2 and
WO3.9–12 However, their forbidden band widths are too large, so
they can only be excited under ultraviolet light, and the utili-
zation rate of solar energy is not more than 4%.13 In these
semiconductor photocatalysts, the conduction band potential
of ZrO2 is about ꢀ1.0 eV, which is much lower than some
semiconductors such as TiO2 (ꢀ0.12 eV) and CeO2 (ꢀ0.34 eV).12
Few semiconductors contain such negative CB that can be used
to sensitize ZrO2.
Senthil, Wang and Zhao reported a semiconductor photo-
catalyst, graphitic carbon nitride (g-C3N4), which is highly stable
and low price. It is highly efficient in catalyzing hydrogen
production and degrading organic pollutants under visible
light.14–16 It can be obtained by roasting inexpensive raw
In this paper, nano-ZrO2 with an average particle size of
15.4 nm was prepared by precipitation method. The g-C3N4/
ZrO2 composites were prepared by direct calcination. The
School of Materials Science and Engineering, China University of Petroleum (East
China), QingDao 266580, China. E-mail: enyangliu@126.com
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heterojunction structure of g-C3N4 with ZrO2 greatly improved glass was cut, then washed with acetone, boiling NaOH
the efficiency of photocatalytic degradation of rhodamine B, (0.1 mol Lꢀ1) and deionized water. Then weighed 0.018 g of
and the reasons for its photocatalytic activity and photocatalytic sample and 0.002 g of polyvinylidene uoride mixed and fully
mechanism were explored.
ground. Aer the grinding, 60 mL of 1-methyl-2-pyrrolidone was
added and ultrasonicated for 0.5 h, and then the resulting
suspension was applied onto an ITO glass substrate to a coating
area of 1 cm2. Finally, the coated ITO glass was dried at 50 ꢁC to
obtain a working electrode.31 An alternating current voltage of
10 mV was applied, and the initial potential was 0 V. The EIS
spectrum was made in 0.01 M Na2SO4 in the frequency range of
105 Hz to 10ꢀ2 Hz. The photocurrent response diagram was
measured using a 350 W Xe lamp as the light source and an
aqueous solution of Na2SO4 (0.01 M) as the electrolyte.
2. Experimental section
2.1 Catalysts preparation
All raw materials were of analytical grade and were not further
puried. They were purchased from Sinopharm Chemical
Reagent Network. Pure ZrO2 was prepared from ZrOCl2$8H2O
and stronger ammonia water. The method is as follows:
6.4450 g ZrOCl2$8H2O was added to 100 mL deionized water to
prepare 0.2 mol Lꢀ1 ZrOCl2 aqueous solution. Slowly added
stronger ammonia water to adjust pH to 10–11 and stirred for
1 h. Then, it was dried at 60 ꢁC for 10 h and thoroughly ground
to obtain a ZrO2 precursor powder. The precursor powder was
calcined at 500 ꢁC for 3 h in a tube furnace at a heating rate of
2.3 Photocatalytic reaction
Using photocatalytic degradation of rhodamine (18 mg Lꢀ1) as
a model reaction, the light source is a 250 W high-pressure
mercury lamp, and the wavelength below 420 nm is ltered by
a ltering device. Its light intensity is 7.0 mW cmꢀ2, the photo-
catalytic properties of different ratios of CN/ZrO2 composites and
pure g-C3N4 and ZrO2 were investigated. Added a certain amount
of photocatalytic materials (according to the amount of 0.08 g/
100 mL) in the congured rhodamine solution. The sample
was rst darkly adsorbed for 20 minutes, then the solution was
transferred to a light source for photodegradation reaction, and
samples were taken at the same time. All samples were centri-
fuged twice aer taking the supernatant. The absorbance was
measured with a 752 UV-vis spectrophotometer. The maximum
absorption wavelength of rhodamine was 554 nm. All samples
were repeated for the above steps. The above tests were per-
formed three times and averaged. The sample was recovered dry
and subjected to a cycle test to test its cycle performance.
5
ꢁC minꢀ1 to obtain a ZrO2 powder photocatalytic material.
Pure g-C3N4 was prepared as follows: 40 g of urea was added to
40 mL of deionized water, dissꢁolved by ultrasonic for 1 h, and
recrystallized by drying at 100 C for 12 h. The crystal product
was thoroughly ground, and the product was calcined at 550 ꢁC
for 4 h in a tube furnace at a heating rate of 2.3 ꢁC minꢀ1, to give
a pale yellow g-C3N4.
The g-C3N4/ZrO2 composites were prepared as follows: in the
process of preparing the ZrO2 precursor powder, aer adjusted
the pH to 10–11 and stirred uniformly, a certain amount of g-
C3N4 prepared in advance was added, and the mixture was
stirred for 1 h to make the mixture uniform, dried at 60 ꢁC,
ꢁ
ground for 30 min, and then heated at 500 C for 3 h in tube
furnace with a heating rate of 5 ꢁC minꢀ1, to obtain g-C3N4/ZrO2
composites. Among them, the g-C3N4/ZrO2 composites were
5 : 1, 4 : 1, 3 : 1 and 2 : 1 by mass ratio (respectively recorded as
5 : 1 CN/ZrO2, 4 : 1 CN/ZrO2, 3 : 1 CN/ZrO2 and 2 : 1 CN/ZrO2).
2.4 Mechanism exploration
In order to conrm the mechanism of this catalytic reaction,
benzophenone (BQ), triethanolamine (TEOA) and tert-butanol
(IPA) were used as scavengers for superoxide radicals ($O2ꢀ),
2.2 Catalyst characterizations
The crystal phase structure of the sample was measured using
a X' Pert PRD MPD type Cu target X-ray diffractometer from the
Dutch PANalytical B.V. at a scanning speed of 0.303841ꢁ sꢀ1
.
The SEM was performed using a Japanese JSM-7200 eld
emission scanning electron microscope. The TEM used a Japa-
nese JEM-2100 high-resolution transmission electron micro-
scope. The PL spectrum was measured with a F-7000 type
uorescence spectrophotometer, using a Xe lamp (excitation
wavelength is 365 nm) as light source. The DRS spectrum used
a U-3900 UV-vis spectrophotometer and BaSO4 as a reference
sample. The FT-IR spectrums were collected with a Tensor II
Fourier transform infrared spectrometer.
Electrochemical impedance spectroscopy (EIS) and photo-
current response (PR) tests were performed using the CS130
electrochemical workstation. A three-electrode system was
used, and the corresponding three electrodes are a working
electrode, a reference electrode (Ag/AgCl saturated KCl), and
a counter electrode (Pt rod). The working electrode was
Fig. 1 XRD patterns of CN/ZrO2 composites and pure g-C3N4 and
prepared as follows: a 2 ꢂ 1 cm piece of indium tin oxide (ITO) ZrO2.
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holes (h+) and hydroxyl radicals ($OH). These three kinds of using XRD. As shown in Fig. 1, the XRD patterns of g-C3N4
scavengers (0.01 mol Lꢀ1) were added to three same rhodamine corresponds to the strongest peak at 2q ¼ 27.4ꢁ, and its crystal
B solutions respectively before dark adsorption, and the rest of plane is (002), indicating periodic graphite deposition of
the process was consistent with the degradation experiment.32
conjugated aromatic CN units; at 2q ¼ 12.7ꢁ, the diffraction
peak corresponds to the (100) crystal plane, and is formed by
hybridization of the aromatic substance sp2 in the layer, which
has a 3 s-triazine structure.33 The pure phase ZrO2 is a cubic
phase, and the corresponding characteristic peaks are 2q ¼
30.22ꢁ, 35.27ꢁ, 50.73ꢁ and 60.20ꢁ, respectively. The CN/ZrO2
composites have only two peaks and no other phases, indicating
3. Results and discussion
3.1 XRD analysis
In order to determine the crystal phase structure of the semi-
conductor photocatalytic material, all samples were examined
Fig. 2 SEM and TEM spectra of ZrO2 (a and b), g-C3N4 (c and d), CN/ZrO2 composite (e and f) and EDS (g and h).
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that the two g-C3N4 and ZrO2 are well complexed, and as the
amount of g-C3N4 increases, the peak intensity increases and
the peak intensity of ZrO2 decreases.
3.2 Characterization of morphology and structure
SEM and TEM spectras of CN/ZrO2 composites and pure g-C3N4
and ZrO2 were observed in order to observe the morphology and
structure of the samples. It can be seen from the Fig. 2(a) and (b)
that the pure ZrO2 is an irregular particle and has a certain
agglomeration, and its average particle diameter is 15.4 nm. It
can be seen from Fig. 2(c) and (d) that the pure g-C3N4 is
a lamellar structure and exhibits multi-piece aggregation
phenomenon. Because the morphology of the two materials is
completely different, it is easy to distinguish in scanning elec-
tron microscopy. It can be clearly seen in Fig. 2(e) that the white
irregular mass is ZrO2 and is coated with aky g-C3N4, indi-
cating that the two are well compounded, and the TEM spec-
trum (Fig. 2(f)) can also be. It is seen that the darker color is
ZrO2, because it is covered by g-C3N4, so the graininess is not
obvious. To further conrm, the Fig. 2(e) EDS component
analysis is shown in Fig. 1(g) and (h). It is observed that the C, N,
O and Zr elements are all included. It shows that ZrO2 is coated
with g-C3N4 and is well compounded.
Fig. 4 PL spectrum of CN/ZrO2 composites and pure g-C3N4.
addition of ZrO2, it varies around 509 cmꢀ1 and 3373 cmꢀ1. It can
be seen that the FT-IR spectrum is consistent with the XRD
results, indicating the formation of a CN/ZrO2 heterostructure.
3.4 Photoluminescence spectroscopy
The photoluminescence spectroscopies of CN/ZrO2 composites
and pure g-C3N4 are shown in Fig. 4. The effect of CN/ZrO2
composites heterojunction on the separation efficiency of
electron–hole pairs was investigated. It can be seen from Fig. 4
that the pure g-C3N4 has a strong emission peak around
460 nm, which can be attributed to the band gap transition
emission, and the energy of the emitted light approximates the
band gap energy. The uorescence emission intensity of CN/
ZrO2 composites is greatly inhibited compared with that of pure
g-C3N4. With the increase of CN/ZrO2 ratio, the degree of uo-
rescence quenching decreases rst and then increases. It is
indicated that the electron–hole recombination rate decreases
rst and then increases, and the 4 : 1 CN/ZrO2 composite
recombination rate is the lowest.
3.3 Functional group studies
The FT-IR spectrum of CN/ZrO2 composite materials and pure g-
C3N4 and ZrO2 is shown in Fig. 3. The pure g-C3N4 has a sharp
absorption peak at 808 cmꢀ1, and a strong absorption peak in the
range of 1200–1700 cmꢀ1. This is caused by the bending vibration
of the 3 s-triazine C–N bond.34 From 3040 cmꢀ1 to 3302 cmꢀ1
corresponds to a broad absorption peak of g-C3N4, which is
caused by the group of the aromatic ring defect. Pure ZrO2 has
absorption peaks around 509 cmꢀ1 and 750 cmꢀ1 due to the
stretching vibration of Zr–O. The broad absorption peak is
around 3373 cmꢀ1, caused by hydroxyl groups on the surface of
the hydrated oxide and adsorbed water.35 The FT-IR of CN/ZrO2
composites is similar to that of pure phase g-C3N4. With the
3.5 Electrochemical test
In order to conrm the results obtained by the PL spectrum,
electrochemical impedance (EIS) and transient photocurrent
response experiments were further performed. The EIS spec-
trum is shown in Fig. 5(a). The diameter of the semi-arc is
equivalent to the charge transfer resistance of the catalyst. The
smaller the radius, the smaller the resistance, the higher the
electron–hole pair separation efficiency of the sample, and the
faster the interface charge transfer will be. It can be seen from
the Fig. 5(a) that the radius of the 4 : 1 CN/ZrO2 composite is the
smallest, that is, the charge transfer resistance is the smallest,
indicating that the addition of ZrO2 is benecial to the sepa-
ration of photogenerated electron–hole pairs and enhance the
photocatalytic activity of the sample. The transient photocur-
rent response of the semiconductor photocatalyst is also
directly related to the recombination rate of the electron–hole
Fig. 3 FT-IR spectra of CN/ZrO2 composites and pure g-C3N4, ZrO2. pair.36 Fig. 5(b) shows the transient photocurrent response of
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Fig. 5 EIS changes (a) and transient photocurrent responses (b) of 4 : 1 CN/ZrO2 and pure g-C3N4, ZrO2 electrodes.
4 : 1 CN/ZrO2 and pure phase g-C3N4 and ZrO2. As can be seen, absorption band can reach 465 nm, which indicates that the
both the 4 : 1 CN/ZrO2 composite and the pure g-C3N4 can be incorporation of ZrO2 improves the visible light absorption
excited under visible light and produce a stable photocurrent performance of g-C3N4, and 4 : 1 CN/ZrO2 is the best compound
response, indicating the presence of photogenerated carriers in ratio.
the sample. However, pure ZrO2 can hardly be excited under
Calculate the forbidden band width of a semiconductor
visible light, and only a very small amount of photogenerated according to the Kubelka–Munk formula:37,38
carriers exists. Under the illumination of the light source, the
photocurrent density of 4 : 1 CN/ZrO2 is greatly improved,
which is about 3 times that of g-C3N4. It is shown that the
(ahn)1/n ¼ A(hn – Eg)
(1)
where a is the absorption coefficient, h is the Planck constant,
and n is the frequency of light. A is a constant and Eg is the
forbidden band energy. The n value of ZrO2 and g-C3N4 is 1/
2.39,40 As shown in Fig. 6(b), the bandgaps of the samples were
estimated to be 2.68 eV, 2.82 eV and 4.91 eV corresponding to
4 : 1 CN/ZrO2, g-C3N4 and ZrO2, respectively, which were close to
the known values in previous reports.31 It also explained the
4 : 1 CN/ZrO2 composite has better light absorption properties
than g-C3N4, which may be due to the interaction of ZrO2 with g-
C3N4. Some chemical bonds formed between the two semi-
conductors may result in an increase in optical properties.
addition of ZrO2 effectively inhibits the recombination of g-
C3N4 electron–hole pairs, which also enhances its photo-
catalytic activity.
3.6 Optical performance studies
The UV-vis DRS of CN/ZrO2 and pure phase g-C3N4 and ZrO2 is
shown in Fig. 6. In order to detect the optical properties of the
sample, UV-vis DRS was performed on CN/ZrO2 and pure g-
C3N4, ZrO2. The cut line method is used to make the tangent
and the intersection with the abscissa to obtain the absorption
wavelength threshold. It can be seen from Fig. 6 that the
maximum absorption wavelength of ZrO2 for light is 247 nm.
The pure g-C3N4 can absorb light at a maximum wavelength of
3.7 Catalytic performance evaluation
428 nm, compared with the pure g-C3N4, the absorption band The photocatalytic activity of CN/ZrO2 and pure g-C3N4 and ZrO2
edge of CN/ZrO2 is red-shied, and the absorption of visible was evaluated by degrading RhB under visible-light. Fig. 7a–d
light is signicantly enhanced. The 4 : 1 CN/ZrO2 composite show the absorption spectra of RhB with different
Fig. 6 UV-vis spectra of CN/ZrO2 composites and pure g-C3N4, ZrO2 (a); the transformed diffuse reflectance spectra (b).
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Fig. 7 UV-vis absorption spectra of photocatalytic RhB degradation over (a) blank, (b) ZrO2, (c) g-C3N4 and (d) CN/ZrO2 under the light irra-
diation. (e) Photodegradation efficiency toward RhB measured at 554 nm and (f) the photocatalytic rate curves and corresponding fitted kinetics
curves of the blank experiment, ZrO2, g-C3N4, and CN/ZrO2 composites under the light irradiation.
concentrations during the photocatalytic reaction. For the photocatalytic efficiency as shown in Fig. 7(e) and (f). It can be
blank experiment, the absorption intensity at 554 nm remains seen from the Fig. 7(e) that the direct degradation of RhB can be
almost unchanged with the increasing irradiation time as neglected (<5%). The degradation of pure ZrO2 is very poor, with
shown in Fig. 7(a), indicating the photolysis of RhB molecules is only a small amount of adsorption and degradation, and the
almost negligible. Adding a photocatalyst prepared in advance nal degradation rate can only reach 20.2%. The degradation of
to the catalytic system, the RhB concentration gradually pure g-C3N4 is better than that of ZrO2, but the nal degradation
decreases with the extension of the catalytic time, as shown in rate can only reach 66%, and the effect is not satisfactory. Aer
Fig. 7(b)–(d). With the same catalytic time, the catalytic effi- the addition of ZrO2, the adsorption and degradation ability of
ciency CN/ZrO2 > g-C3N4 > ZrO2. Aer catalysis, the maximum g-C3N4 has been signicantly improved. With the increase of
absorption wavelength of RhB is still around 554 nm, indicating ZrO2 addition, the catalytic effect under visible light rst
that the sample has little effect on RhB. Test the specic becomes better and then deteriorates. 4 : 1 CN/ZrO2 is the best
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Table 1 Some g-C3N4 composite materials and degradation rate table
Sample
Degradation rate
SnO2–ZnO/g-C3N4 (ref. 41)
CeO2/P–C3N4 (ref. 42)
Ag/g-C3N4 (ref. 43)
0.01576 minꢀ1
BiV/g-C3N4 (ref. 44)
0.101 minꢀ1
0.0184 minꢀ1
0.0110 minꢀ1
compound ratio, and the nal catalytic degradation rate can composite cycle test was carried out, and the sample was
reach 98.5%. The catalyst with high catalytic efficiency also has recovered, dried, and ground at the end of each test, and then
good adsorption performance, but its catalytic activity is not subjected to the next round of tests, which were cycled ve
completely dependent on the adsorption capacity. The adsorp- times in total. The cycle performance is shown in Fig. 8(a). Aer
tion performance of 4 : 1 CN/ZrO2 is not optimal, but the pho- ve cycles of the sample circulation, the degradation effect is
tocatalytic degradation is the best. From Fig. 7(f) photocatalytic not signicantly reduced, indicating that the sample has good
degradation rate map, the degradation rate of pure g-C3N4 is stability and can be reused for degrading wastewater. The
0.0076 minꢀ1, which is ve times the degradation rate of ZrO2 photocatalytic mechanism was studied. As shown in Fig. 8(b),
(0.0015 minꢀ1). Aer g-C3N4 is combined with ZrO2, the elec- the photocatalytic effect of tert-butanol (TBA) and triethanol-
tron–hole separation efficiency and visible light absorption amine (TEOA) was not signicantly decreased in the pure g-
capacity can be signicantly improved. With the increase of C3N4 system. Aer the addition of benzoquinone (BQ), the
ZrO2 compounding amount, the degradation rate of g-C3N4/ degradation rate decreased from 66% to 21.3%, indicating that
ZrO2 composites increases rst and then decreases. 4 : 1 CN/ $O2ꢀ is the main active species. In the g-C3N4/ZrO2 system, the
ZrO2 composite have the highest degradation rate, up to photocatalytic effect was not signicantly reduced aer the
0.026 minꢀ1, which is 3.4 times that of pure g-C3N4.
addition of tert-butanol (TBA). Aer the addition of benzoqui-
The predecessors have also done a lot of modication of g- none (BQ), the degradation rate decreased from 98.5% to
C3N4. Some composite materials and degradation rates are 32.1%, and the degradation rate aer adding triethanolamine
ꢀ
shown in Table 1. Prabhakar vattikuti et al.41 have prepared reduced to 49%, indicating that $O2 and h+ work together in
SnO2–ZnO/g-C3N4 composites. The degradation rate is the reaction process in the g-C3N4/ZrO2 system.
0.0184 minꢀ1, which is 5 times that of pure g-C3N4. The
Further study on the changes of active species in the reaction
degradation rate of CeO2/P–C3N4 prepared by J. Luo et al.42 can process, in which the semiconductor VB and CB have the
reach 0.011 minꢀ1, which is 4 times higher than that of pure g- following formula:42
C3N4. Nagajyothi et al.43 doped g-C3N4 with Ag greatly increase
Eg
ECB ¼ X ꢀ Ee ꢀ
EVB ¼ ECB + Eg
(2)
(3)
the degradation rate, which can reach 0.01576 minꢀ1, which is
2.5 times higher than that of pure g-C3N4. Prabhakar vattikuti
et al.44 also prepared BiV/g-C3N4, the degradation rate can reach
0.101 minꢀ1, which is 2.5 times higher than BiV. Compared
with the composites prepared by the predecessors, the degra-
dation rate of the CN/ZrO2 composites prepared in this paper
increased greatly.
2
Ee is a constant relative to the standard H electrode, its value
is 4.5 eV, Eg is the band gap, X is the absolute electronegativity,
and X values of g-C3N4 and ZrO2 are 4.73 eV and 5.91 eV,
31,45,46
respectively.
E
values of g-C3N4 and ZrO2 were calculated
VB
3.8 Cycle performance and catalytic mechanism
to be 1.64 eV and 3.86 eV, respectively, and ECB values were
ꢀ1.18 eV and ꢀ1.05 eV. EVB values of g-C3N4 and ZrO2 are lower
than the potentials of $OH/H2O (+1.99 eV) and $OH/OHꢀ (+2.68
eV), so H2O and OHꢀ cannot be oxidized to $OH by valence
For photocatalytic materials, not only photocatalytic perfor-
mance is required, but also good stability is required, which can
be reused without losing activity. Therefore, 4 : 1 CN/ZrO2
Fig. 8 Photocatalytic cycle performance of 4 : 1 CN/ZrO2 (a) and reactant capture diagram (b).
530 | RSC Adv., 2020, 10, 524–532
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4. Conclusion
The high catalytic activity of CN/ZrO2 composite photocatalyst
was obtained by directly calcining the mixture of g-C3N4 and
ZrO2 precursor. Among them, 4 : 1 CN//ZrO2 exhibited the
highest degradation of RhB under visible light. The nal cata-
lytic efficiency can reach 98.5%. This is because the combina-
tion of g-C3N4 and ZrO2 reduces the charge transfer resistance
and effectively suppresses the recombination of electron–hole
pairs, thereby improving the photocatalytic activity. At the same
time, the cycle test shows that the prepared catalyst has good
stability and can be reused many times, so this catalyst has
broad application prospects in solving environmental pollution.
Fig. 9 Catalytic mechanism diagram of CN/ZrO2 composites.
Conflicts of interest
There are no conicts to declare.
band holes. Therefore, $OH isꢀnot an active species in the
system. The potential of O2/$O2 is ꢀ0.33 eV, which is higher
than ECB values of g-C3N4 and ZrO2, so O2 can be reduced to
$O2ꢀ, and the RhB is degraded by strong reduction of $O2ꢀ. In
the CN/ZrO2 system, electrons in g-C3N4 are excited from VB to
CB under visible light irradiation, thereby generating holes in
the VB of the semiconductor. Due to the difference in CB
potential, the photogenerated electrons on the surface of g-C3N4
easily migrate to the CB of ZrO2, while the holes remain on the
VB of g-C3N4. The forbidden band energy of ZrO2 is too large,
and the energy of visible light is not enough to excite electrons
on the valence band to the conduction band. The electrons
transferred from g-C3N4 on the ZrO2 conduction band will not
jump to the valence band. This can be considered as a single
transfer mechanism.47 The system can effectively improve the
separation of photogenerated electron–hole pairs, greatly
reducing the possibility of charge recombination, resulting in
high photocatalytic activity of CN/ZrO2 composites. It can be
seen from the above PL, photocurrent and catalytic perfor-
mance that the recombination of electron–hole pairs is sup-
pressed, and the direct oxidation ability of holes is greatly
Acknowledgements
This work was supported by the Natural Science Foundation of
Shandong Province of China (No. ZR2019MEM020, No.
ZR2017LEM004), the Fundamental Research Funds for the
Central Universities of China (No. 18CX02091A) and the Open
Fund (No. OGE201702-07) of Key Laboratory of Oil & Gas
Equipment, Ministry of Education (Southwest Petroleum
University).
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