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

Abstract

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⁻.

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1. Introduction

With the development of the economy, the problem of environmental pollution has become more and more prominent, so increasingly scientific 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 hydrogen^3–5 and photocatalytic degradation of pollutants^6–8 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 utilization 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 photocatalyst, 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 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 utilization. In addition, g-C₃N₄ is a layered structure, and there is a gap between layers, so the theoretical specific surface area should be large.²⁰ However, in actual performance studies, g-C₃N₄ was found to exhibit significant 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 Cd_0.5Zn_0.5S/g-C₃N₄ composite photocatalyst by two-step solvothermal method. The results showed that the photocatalytic performance of g-C₃N₄ was greatly improved by the loading of Cd_0.5Zn_0.5S.²⁹ Lv et al. reported a g-C₃N₄/BiOBr photocatalyst which greatly reduced the recombination 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.

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 heterojunction structure of g-C₃N₄ with ZrO₂ greatly improved the efficiency of photocatalytic degradation of rhodamine B, and the reasons for its photocatalytic activity and photocatalytic mechanism were explored.

2. Experimental section

2.1 Catalysts preparation

All raw materials were of analytical grade and were not further purified. 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 L⁻¹ 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 5 °C min⁻¹ 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, dissolved 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 min⁻¹, 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, after 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 min⁻¹, 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).

2.2 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 0.303841° s⁻¹. The SEM was performed using a Japanese JSM-7200 field emission scanning electron microscope. The TEM used a Japanese JEM-2100 high-resolution transmission electron microscope. The PL spectrum was measured with a F-7000 type fluorescence 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 photocurrent 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 prepared as follows: a 2 × 1 cm piece of indium tin oxide (ITO) glass was cut, then washed with acetone, boiling NaOH (0.1 mol L⁻¹) and deionized water. Then weighed 0.018 g of sample and 0.002 g of polyvinylidene fluoride mixed and fully ground. After the grinding, 60 μL 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 10⁻² 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.

2.3 Photocatalytic reaction

Using photocatalytic degradation of rhodamine (18 mg L⁻¹) as a model reaction, the light source is a 250 W high-pressure mercury lamp, and the wavelength below 420 nm is filtered by a filtering device. Its light intensity is 7.0 mW cm⁻², the photocatalytic 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 configured rhodamine solution. The sample was first 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 centrifuged twice after 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 performed three times and averaged. The sample was recovered dry and subjected to a cycle test to test its cycle performance.

2.4 Mechanism exploration

In order to confirm the mechanism of this catalytic reaction, benzophenone (BQ), triethanolamine (TEOA) and tert-butanol (IPA) were used as scavengers for superoxide radicals (·O₂⁻), holes (h⁺) and hydroxyl radicals (·OH). These three kinds of scavengers (0.01 mol L⁻¹) were added to three same rhodamine B solutions respectively before dark adsorption, and the rest of the process was consistent with the degradation experiment.³²

3. Results and discussion

3.1 XRD analysis

In order to determine the crystal phase structure of the semiconductor photocatalytic material, all samples were examined using XRD. As shown in Fig. 1, the XRD patterns of g-C₃N₄ corresponds to the strongest peak at 2θ = 27.4°, and its crystal plane is (002), indicating periodic graphite deposition of conjugated aromatic CN units; at 2θ = 12.7°, 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 2θ = 30.22°, 35.27°, 50.73° and 60.20°, respectively. The CN/ZrO₂ composites have only two peaks and no other phases, indicating 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.

Fig. 1 XRD patterns of CN/ZrO₂ composites and pure g-C₃N₄ and ZrO₂.

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 electron microscopy. It can be clearly seen in Fig. 2(e) that the white irregular mass is ZrO₂ and is coated with flaky g-C₃N₄, indicating that the two are well compounded, and the TEM spectrum (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 confirm, 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. 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).

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 cm⁻¹, and a strong absorption peak in the range of 1200–1700 cm⁻¹. This is caused by the bending vibration of the 3 s-triazine C–N bond.³⁴ From 3040 cm⁻¹ to 3302 cm⁻¹ 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 cm⁻¹ and 750 cm⁻¹ due to the stretching vibration of Zr–O. The broad absorption peak is around 3373 cm⁻¹, 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 addition of ZrO₂, it varies around 509 cm⁻¹ and 3373 cm⁻¹. It can be seen that the FT-IR spectrum is consistent with the XRD results, indicating the formation of a CN/ZrO₂ heterostructure.

Fig. 3 FT-IR spectra of CN/ZrO₂ composites and pure g-C₃N₄, ZrO₂.

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 fluorescence 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 fluorescence quenching decreases first and then increases. It is indicated that the electron–hole recombination rate decreases first and then increases, and the 4 : 1 CN/ZrO₂ composite recombination rate is the lowest.

Fig. 4 PL spectrum of CN/ZrO₂ composites and pure g-C₃N₄.

3.5 Electrochemical test

In order to confirm the results obtained by the PL spectrum, electrochemical impedance (EIS) and transient photocurrent response experiments were further performed. The EIS spectrum 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 beneficial to the separation of photogenerated electron–hole pairs and enhance the photocatalytic activity of the sample. The transient photocurrent response of the semiconductor photocatalyst is also directly related to the recombination rate of the electron–hole pair.³⁶Fig. 5(b) shows the transient photocurrent response of 4 : 1 CN/ZrO₂ and pure phase g-C₃N₄ and ZrO₂. As can be seen, both the 4 : 1 CN/ZrO₂ composite and the pure g-C₃N₄ can be excited under visible light and produce a stable photocurrent response, indicating the presence of photogenerated carriers in the sample. However, pure ZrO₂ can hardly be excited under visible light, and only a very small amount of photogenerated 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 addition of ZrO₂ effectively inhibits the recombination of g-C₃N₄ electron–hole pairs, which also enhances its photocatalytic activity.

Fig. 5 EIS changes (a) and transient photocurrent responses (b) of 4 : 1 CN/ZrO₂ and pure g-C₃N₄, ZrO₂ electrodes.

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 428 nm, compared with the pure g-C₃N₄, the absorption band edge of CN/ZrO₂ is red-shifted, and the absorption of visible light is significantly enhanced. The 4 : 1 CN/ZrO₂ composite absorption band can reach 465 nm, which indicates that the incorporation of ZrO₂ improves the visible light absorption performance of g-C₃N_4, and 4 : 1 CN/ZrO₂ is the best compound ratio.

Fig. 6 UV-vis spectra of CN/ZrO₂ composites and pure g-C₃N₄, ZrO₂ (a); the transformed diffuse reflectance spectra (b).

Calculate the forbidden band width of a semiconductor according to the Kubelka–Munk formula:^37,38

(αhν)^1/n = A(hν – E_g) (1)

where α is the absorption coefficient, h is the Planck constant, and ν is the frequency of light. A is a constant and E_g 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 semiconductors may result in an increase in optical properties.

3.7 Catalytic performance evaluation

The photocatalytic activity of CN/ZrO₂ and pure g-C₃N₄ and ZrO₂ was evaluated by degrading RhB under visible-light. Fig. 7a–d show the absorption spectra of RhB with different concentrations during the photocatalytic reaction. For the blank experiment, the absorption intensity at 554 nm remains almost unchanged with the increasing irradiation time as shown in Fig. 7(a), indicating the photolysis of RhB molecules is almost negligible. Adding a photocatalyst prepared in advance to the catalytic system, the RhB concentration gradually decreases with the extension of the catalytic time, as shown in Fig. 7(b)–(d). With the same catalytic time, the catalytic efficiency CN/ZrO₂ > g-C₃N₄ > ZrO₂. After catalysis, the maximum absorption wavelength of RhB is still around 554 nm, indicating that the sample has little effect on RhB. Test the specific photocatalytic efficiency as shown in Fig. 7(e) and (f). It can be seen from the Fig. 7(e) that the direct degradation of RhB can be neglected (<5%). The degradation of pure ZrO₂ is very poor, with only a small amount of adsorption and degradation, and the final degradation rate can only reach 20.2%. The degradation of pure g-C₃N₄ is better than that of ZrO₂, but the final degradation rate can only reach 66%, and the effect is not satisfactory. After the addition of ZrO₂, the adsorption and degradation ability of g-C₃N₄ has been significantly improved. With the increase of ZrO₂ addition, the catalytic effect under visible light first becomes better and then deteriorates. 4 : 1 CN/ZrO₂ is the best compound ratio, and the final catalytic degradation rate can reach 98.5%. The catalyst with high catalytic efficiency also has good adsorption performance, but its catalytic activity is not completely dependent on the adsorption capacity. The adsorption performance of 4 : 1 CN/ZrO₂ is not optimal, but the photocatalytic degradation is the best. From Fig. 7(f) photocatalytic degradation rate map, the degradation rate of pure g-C₃N₄ is 0.0076 min⁻¹, which is five times the degradation rate of ZrO₂ (0.0015 min⁻¹). After g-C₃N₄ is combined with ZrO₂, the electron–hole separation efficiency and visible light absorption capacity can be significantly improved. With the increase of ZrO₂ compounding amount, the degradation rate of g-C₃N₄/ZrO₂ composites increases first and then decreases. 4 : 1 CN/ZrO₂ composite have the highest degradation rate, up to 0.026 min⁻¹, which is 3.4 times that of pure g-C₃N₄.

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 irradiation. (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.

The predecessors have also done a lot of modification of g-C₃N₄. Some composite materials and degradation rates are shown in Table 1. Prabhakar vattikuti et al.⁴¹ have prepared SnO₂–ZnO/g-C₃N₄ composites. The degradation rate is 0.0184 min⁻¹, which is 5 times that of pure g-C₃N₄. The degradation rate of CeO₂/P–C₃N₄ prepared by J. Luo et al.⁴² can reach 0.011 min⁻¹, which is 4 times higher than that of pure g-C₃N₄. Nagajyothi et al.⁴³ doped g-C₃N₄ with Ag greatly increase the degradation rate, which can reach 0.01576 min⁻¹, 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 min⁻¹, which is 2.5 times higher than BiV. Compared with the composites prepared by the predecessors, the degradation rate of the CN/ZrO₂ composites prepared in this paper increased greatly.

Table 1 Some g-C₃N₄ composite materials and degradation rate table

Sample	SnO2–ZnO/g-C3N4 (ref. 41)	CeO2/P–C3N4 (ref. 42)	Ag/g-C3N4 (ref. 43)	BiV/g-C3N4 (ref. 44)
Degradation rate	0.0184 min^−1	0.0110 min^−1	0.01576 min^−1	0.101 min^−1

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3.8 Cycle performance and catalytic mechanism

For photocatalytic materials, not only photocatalytic performance is required, but also good stability is required, which can be reused without losing activity. Therefore, 4 : 1 CN/ZrO₂ composite cycle test was carried out, and the sample was recovered, dried, and ground at the end of each test, and then subjected to the next round of tests, which were cycled five times in total. The cycle performance is shown in Fig. 8(a). After five cycles of the sample circulation, the degradation effect is not significantly reduced, indicating that the sample has good stability and can be reused for degrading wastewater. The photocatalytic mechanism was studied. As shown in Fig. 8(b), the photocatalytic effect of tert-butanol (TBA) and triethanolamine (TEOA) was not significantly decreased in the pure g-C₃N₄ system. After the addition of benzoquinone (BQ), the degradation rate decreased from 66% to 21.3%, indicating that ·O₂⁻ is the main active species. In the g-C₃N₄/ZrO₂ system, the photocatalytic effect was not significantly reduced after the addition of tert-butanol (TBA). After the addition of benzoquinone (BQ), the degradation rate decreased from 98.5% to 32.1%, and the degradation rate after adding triethanolamine reduced to 49%, indicating that ·O₂⁻ and h⁺ work together in the reaction process in the g-C₃N₄/ZrO₂ system.

Fig. 8 Photocatalytic cycle performance of 4 : 1 CN/ZrO₂ (a) and reactant capture diagram (b).

Further study on the changes of active species in the reaction process, in which the semiconductor VB and CB have the following formula:⁴²

E_VB = E_CB + E_g (3)

E ^e is a constant relative to the standard H electrode, its value is 4.5 eV, E_g 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, respectively.^31,45,46E_VB values of g-C₃N₄ and ZrO₂ were calculated to be 1.64 eV and 3.86 eV, respectively, and E_CB values were −1.18 eV and −1.05 eV. E_VB values of g-C₃N₄ and ZrO₂ are lower than the potentials of ·OH/H₂O (+1.99 eV) and ·OH/OH⁻ (+2.68 eV), so H₂O and OH⁻ cannot be oxidized to ·OH by valence band holes. Therefore, ·OH is not an active species in the system. The potential of O₂/·O₂⁻ is −0.33 eV, which is higher than E_CB values of g-C₃N₄ and ZrO₂, so O₂ can be reduced to ·O₂⁻, and the RhB is degraded by strong reduction of ·O₂⁻. 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 performance that the recombination of electron–hole pairs is suppressed, and the direct oxidation ability of holes is greatly improved, so that h⁺ and ·O₂⁻ are collectively active species in the CN/ZrO₂ system. From the mechanism diagram (Fig. 9), the interface between g-C₃N₄ and ZrO₂ greatly affects the separation efficiency of electron–hole pairs. If a large amount of ZrO₂ is present, the contact area is reduced, thereby reducing the separation efficiency and further reducing the photocatalytic performance. Therefore, the optimal ratio of CN/ZrO₂ composite material exists. The specific process of catalytic degradation of RhB by CN/ZrO₂ composites is as follows:

g-C₃N₄(e⁻ + h⁺) → ZrO₂(e⁻ + h⁺)/g-C₃N₄(e⁻ + h⁺) (5)

O₂ + e⁻ → ·O⁻₂ (6)

·O⁻₂/h⁺ + RhB → degradation products (7)

Fig. 9 Catalytic mechanism diagram of CN/ZrO₂ composites.

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 final catalytic efficiency can reach 98.5%. This is because the combination 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.

Conflicts of interest

There are no conflicts to declare.

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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