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Photocatalytic research performance of zinc oxide/graphite phase carbon nitride catalyst and its application in environment

  • Haiyang Liu , Zhe Wang , Heng Zhang , Lixia Jin EMAIL logo and Yuehui Zhao
Published/Copyright: September 8, 2023
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Abstract

In this article, ZnO quantum dots (QDs)-g-C3N4 complexes were prepared by a combined sol–gel method and ultrasound-assisted chemical method, and ZnO-g-C3N4 composites with different doping ratios were also prepared for photocatalytic degradation of dye wastewater. The composites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, Fourier transform infrared, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, and photoluminescence. The photocatalytic performance of the best ZnO QDs-g-C3N4 complexes with different g-C3N4 doping amounts was investigated, and the kinetics of their photocatalytic reactions were analyzed, and it was found that the best effect of ZnO-g-C3N4 10% could reach 89.08% and ZnO QDs-g-C3N4 could reach 91.53%. It was also demonstrated that ZnO-g-C3N4 10%, ZnO QDs-g-C3N4 cyclic stability is better, and the reaction mechanism of ZnO QDs-g-C3N4 was investigated. It can be used for the degradation of dyes in environmental wastewater and the removal of harmful substances from the natural environment.

1 Introduction

Rapid economic development from the past century to this century and the use of drugs and chemicals in agriculture have made water pollution more and more serious [1]. The deterioration of water quality in rivers and lakes and the scarcity of freshwater resources have seriously threatened the health of the water environment [2]. Especially in the era of rapid industrial development, printing and dyeing wastewater treatment has been a key issue. Dye wastewater with large amount, high organic content and high chroma is widely recognized as intractable industrial wastewater [3,4,5,6]. To solve these problems, numerous researchers have used various methods to treat dye wastewater [7].

Among the many water pollution treatment options that have been developed, photocatalysis is an effective method for solving water pollution problems because of its low operating costs, its ability to completely mineralize pollutants, and its ability to degrade organic pollutants to less toxic or non-toxic components [8]. Photocatalysis can use solar energy to degrade pollutants by exciting electron transfer in semiconductor photocatalysts [9]. The most important factor in this technology is the photocatalyst, which converts solar energy into chemical energy that destroys the pollutant. Currently, metals, metal oxides, nonmetals, semiconductors, metal–organic frameworks, and other materials have been extensively studied for the degradation of dye molecules in wastewater [10,11,12].

As a representative of photocatalytic materials, zinc oxide (ZnO) stands out in water treatment because of its unique attributes, such as low price, good stability, and obvious surface and interface effects [13,14,15]. However, ZnO photocatalyst itself also has some defects: its own light absorption capacity is very poor and electron-hole pair separation rate is very low [16,17,18,19]. Therefore, it has become a hot topic of research to improve the degradation efficiency under visible light through research on its own modification: doping modification, crystal appearance control, thin film loading, composite of different materials, etc. [20,21,22,23]. Also known as semiconductor nanoparticles or semiconductor nanocrystals, quantum dots (QDs) have a size-dependent electronic structure and excellent optoelectronic properties.

QDs have been regarded as a new generation of fluorescent biomarker probes that can replace traditional fluorescent organic molecules by researchers worldwide and have been widely used in various disciplines such as cell labeling [24,25], nano-drug delivery [26,27,28,29], and biosensors [30]. The main methods commonly used for the synthesis of ZnO QDs are hydrothermal method [31], sol–gel method [32], and microemulsion method [33].

The advantages of g-C3N4, such as high stability of physicochemical properties, diverse structures, simple preparation, and abundant and cheap preparation materials, make it widely used in photocatalytic carbon dioxide conversion [34,35,36,37,38], photocatalytic pollutant treatment [39,40,41], photocatalytic organic synthesis [42,43,44], and especially in photocatalytic decomposition of aquatic hydrogen.

Rhodamine B (RhB) is one of the main pollutants in dye wastewater. RhB is a bright-red synthetic onion quinone dye. Because of its bright color, it is widely used in dyeing and textile industry.

Commonly used water treatment methods have high treatment costs, low removal rate, secondary pollution, and other disadvantages, so it needs a water treatment technology with high efficiency, environmental protection, complete reaction, and no secondary pollution. Semiconductor photocatalysis technology has thus emerged. It is favoured by researchers in environmental wastewater treatment due to its conditions of high efficiency, non-pollution, low energy consumption, simple operation and easy control. It can also directly convert organic matter in water into non-toxic materials without causing secondary pollution [45,46,47,48].

Waseem et al. [49] greatly improved the degradation rate by doping neodymium and erbium on ZnO, where the neodymium-doped ZnO was essentially completely degraded for methylene blue within 10 min. Suwiwat et al. [50] used highly mesoporous carbon-ZnO nanocomposites and found that the adsorption of methylene blue and Congo red were 399 and 410 mg g−1, respectively, and the synergistic effect of adsorption and photocatalytic degradation resulted in a maximum removal of methylene blue of 614 mg g−1 and Congo red of 2,628 mg g−1. Huang et al. [51] prepared thin layers of porous amino-rich g-C3N4 by a precursor modification strategy using different acid-reformed dicyandiamide and applied them to the degradation of RhB.

In this article, ZnO-g-C3N4, ZnO QDs-g-C3N4 complexes with different doping ratios were synthesized, and a series of characterizations were performed to demonstrate their successful preparation. The photocatalytic degradation experiments were carried out using RhB solution as the simulated dye wastewater in the degradation experiments under visible light. The stability of the catalyst was investigated after five cycles using catalyst cycling stability experiments. The principle of photocatalytic degradation of ZnO QDs and its complexes was investigated using free radical capture experiments.

2 Material preparation

2.1 Preparation of ZnO-g-C3N4

One gram of ZnO was added to a beaker containing 60 mL of anhydrous ethanol, and the corresponding mass of g-C3N4 weighed in different ratios was added to the beaker and sonicated for 1 h. It was poured into the hydrothermal reactor and put into the high temperature drying oven at 140°C for 10 h. The purpose of fully combining the two powders under high temperature and pressure was achieved. The material was removed and centrifuged, the supernatant was poured off, and the remaining powder was placed in a vacuum drying oven at 60°C for 24 h to dry the sample sufficiently. Then the samples were put into a high temperature drying oven at 200°C and kept for 120 min, which can improve the crystallinity and remove the water from the photocatalyst at the same time. The products were labeled as ZC-3, ZC-5, ZC-7, ZC-10, and ZC-20 by adding g-C3N4 with different mass fractions of 3%, 5%, 7%, 10%, and 20%, respectively.

2.2 Preparation method of ZnO QDs-g-C3N4

One gram of ZnO QDs is weighed and added into a beaker containing 60 mL of anhydrous ethanol; then 0.01 g of g-C3N4 is added into the beaker and ultrasonication is performed for 1 h so that the mixed solution can be fully mixed. After the ultrasound is finished, the material in the beaker is poured into the hydrothermal reaction kettle and put into the high-temperature drying oven. The temperature of the high-temperature drying oven is set at 140℃ and the time is set at 10 h, to achieve the purpose of fully combining the two powders under high temperature and high pressure.

The material is taken out and centrifuged. The supernatant of the centrifuged material is poured out and the remaining powder was put into a vacuum drying oven at 60℃ for 24 h to dry the sample. To improve the crystallinity of the samples, the samples were then placed in a high-temperature drying oven set at 200°C for 120 min. At the same time, the water in the photocatalyst was removed and the product was labeled ZC QDs-10.

3 Results and discussion

3.1 Effect of different pH solutions on the degradation performance of RhB

A measure of 0.02 g of photocatalyst ZnO-g-C3N4 10% composite was taken, and 50 mL of 10 mg·L−1 of RhB solution with pH values of 1, 3, 5, 7, and 11 were prepared. The effect of pH of the solution on the degradation performance was discussed by dark reaction for 60 min and light reaction for 6 h at room temperature. The experimental results are shown in Figure 1.

Figure 1 
                  Effect of different pH solutions on the degradation performance of RhB.
Figure 1

Effect of different pH solutions on the degradation performance of RhB.

As can be seen from Figure 1, the degradation of catalyst is relatively good when the pH value is in the acidic range. When pH = 3, its degradation rate reaches the maximum, about 91%. The reason for this phenomenon is that the OH or H+ added to the solution changes the charge characteristics and interface properties of the surface of the photocatalyst of ZnO-g-C3N4 10% composite material, as well as the existence form of degradation substances, which further affects the catalytic activity of the photocatalyst. When there is a large amount of OH in the solution, the catalyst itself is negatively charged, while the analog dye RhB is positively charged. At this time, it will trap the resulting photogenic holes, thus reducing the hydroxyl radical. Therefore, in alkaline environment, its degradation ability is greatly inhibited. When a large amount of H+ exists in the solution, the situation is just the opposite. It can transfer the photogenerated electrons to itself, which is further conducive to the generation of oxidizing hydroxyl radical and improve the degradation performance of the photocatalyst. Therefore, in the acidic range, the degradation performance of the catalyst is very good, and the degradation rate of the simulated dye RhB is also high.

3.2 Field emission scanning electron microscopy (FESEM) (ZG)

The morphology of the nanoparticles can be directly observed by FESEM. Figure 2a and b shows that the morphology of ZnO nanoparticles shows a relatively regular granular structure and all particles have a relatively uniform size, so the preparation of the material is successful. As can be seen in Figure 2c, the prepared samples have a lamellar structure that is mostly broken, with a highly irregular surface and the appearance of holes, probably because of the unsatisfactory melamine condensation effect during firing. The structure of the lamellae was thin and relatively uniformly distributed. Figure 2d–f presents the SEM scans of ZnO-g-C3N4 with different mass percentages of doped g-C3N4; it can be seen that the surface morphology of the binary composite ZnO-g-C3N4 is also small particles and shows a uniform distribution, in which g-C3N4 is not clearly visible in the figure, indicating that ZnO is dispersed more uniformly on the surface of the flake g-C3N4, which also indicates that the two materials have good affinity when they are compounded. Moreover, the microscopic morphology of g-C3N4-ZnO did not change significantly with the increase of the mass percentage of g-C3N4.

Figure 2 
                  FESEM images of (a,b) ZnO, (c) g-C3N4, (d) ZnO-g-C3N4 3%, (e) ZnO-g-C3N4 10%, and (f) ZnO-g-C3N4 20% nanoparticles.
Figure 2

FESEM images of (a,b) ZnO, (c) g-C3N4, (d) ZnO-g-C3N4 3%, (e) ZnO-g-C3N4 10%, and (f) ZnO-g-C3N4 20% nanoparticles.

3.3 Transmission electron microscopy (TEM) (ZG)

To further observe the morphological characteristics inside the ZnO-g-C3N4 10% composite and the distribution of ZnO in g-C3N4, TEM characterization was performed. It can be seen from Figure 3a that ZnO is well dispersed, one by one in granular form with relatively uniform size. The spherical ZnO is attached to the g-C3N4 sheet in Figure 3b, verifying that the composite consists of ZnO and g-C3N4.

Figure 3 
                  TEM diagram of (a) ZnO and (b) ZnO-g-C3N4 10% nanoparticles.
Figure 3

TEM diagram of (a) ZnO and (b) ZnO-g-C3N4 10% nanoparticles.

3.4 High-resolution transmission electron microscopy (HRTEM) (ZG QDs)

In order to obtain the morphological characteristics of ZnO QDs, HRTEM analysis was performed using the ZnO QDs. It can be seen that the produced samples are well dispersed and have a relatively uniform particle size scale distribution. The dark-colored part with lattice display is the core of ZnO QDs. Figure 4a and b is the TEM image of ZnO QDs. As can be seen from the TEM image in Figure 4c, the sample prepared is the ZnO QDs-g-C3N4 composite structure.

Figure 4 
                  HRTEM images of (a,b) ZnO QDS and (c) ZnO QDS-g-C3N4 10% nanoparticles.
Figure 4

HRTEM images of (a,b) ZnO QDS and (c) ZnO QDS-g-C3N4 10% nanoparticles.

3.5 X-ray diffractometry (XRD)

Figure 5 shows the XRD plots of ZnO-g-C3N4 binary composites with different g-C3N4 contents. And it can be seen that the XRD plots of ZC binary composites with different g-C3N4 contents correspond well with the XRD peak positions of single ZnO. The peak of g-C3N4 appeared at 2θ of 27.8° in the binary composites 20% ZC and 10% ZC, indicating that g-C3N4 was successfully compounded with ZnO by hydrothermal method. g-C3N4 peaks did not appear in the XRD plots of 3% ZC, 5% ZC, and 7% ZC, probably due to the relatively low content of g-C3N4. 10% ZC had the sharpest peak pattern with more prominent and better recognizability.

Figure 5 
                  XRD diagram of composite samples with different g-C3N4 contents of ZnO-g-C3N4.
Figure 5

XRD diagram of composite samples with different g-C3N4 contents of ZnO-g-C3N4.

The data of the XRD patterns of ZnO QDs-g-C3N4 10%, ZnO-g-C3N4 10%, ZnO, and ZnO QD composite samples are displayed in Figure 6. It can be seen from the figure that compound ZnO QDs-g-C3N4 10% was compared with ZnO-g-C3N4 10%, and the peak area widened, which could improve the photocatalytic performance.

Figure 6 
                  XRD plots of ZnOQDs-g-C3N4 10%, ZnO-g-C3N4 10%, ZnO, and ZnO QD composite samples.
Figure 6

XRD plots of ZnOQDs-g-C3N4 10%, ZnO-g-C3N4 10%, ZnO, and ZnO QD composite samples.

3.6 Fourier transform infrared (FT-IR)

The complexes also have typical stretching vibrational absorption peaks of g-C3N4 around 1240–1640 cm−1 and 810 cm−1. This also indicates the successful complexation of ZnO, ZnO QDs with g-C3N4 in the complexes ZnO-g-C3N4 10%, ZnO QDs-g-C3N4 10%. The absorption peaks near 1,240–1,640 cm−1 and 810 cm−1 are attributed to the typical stretching vibration of g-C3N4, which also indicates the successful compounding of ZnO with g-C3N4 in the complex ZnO-g-C3N4.

Figure 7 
                  FT-IR spectrum map of the composite samples with different g-C3N4 contents of ZnO-g-C3N4.
Figure 7

FT-IR spectrum map of the composite samples with different g-C3N4 contents of ZnO-g-C3N4.

For the composites ZnO-g-C3N4 10%, ZnO QDs-g-C3N4 10%, it can be seen from Figure 8 that both have a wider typical absorption peak of ZnO in the range of 500 cm−1 vibrated by Zn-O stretching. The complexes also have typical stretching vibrational absorption peaks of g-C3N4 around 1240–1640 cm−1 and 810 cm−1. This also indicates the successful complexation of ZnO, ZnO QDs with g-C3N4 in the complexes ZnO-g-C3N4 10%, ZnO QDs-g-C3N4 10% in the successful complexation of ZnO, ZnO QDs, and g-C3N4.

Figure 8 
                  The FT-IR spectral map of ZnO-g-C3N4 10% and ZnO QDs-g-C3N4 10%.
Figure 8

The FT-IR spectral map of ZnO-g-C3N4 10% and ZnO QDs-g-C3N4 10%.

3.7 X-ray photoelectron spectroscopy (XPS)

Figure 9a–f presents the full spectrum analysis diagram of ZC QD composite sample and the XPS diagram of four elements. The presence of the four elements can be seen in the spectrogram of the XPS of the ZnOC QD composite in Figure 9a. The peak positions of C, N, O, and Zn in the binary composite can be determined from the figure, and there are no miscellaneous peaks in the figure, indicating the successful synthesis of ZC binary composite.

Figure 9 
                  The full-spectrum analysis diagram of the ZC composite samples and the XPS diagram of the four elements. (a) Full spectrum of ZC composite sample, (b) C spectrum, (c) 2p spectrum of Zn, (d) O spectrum, (e) N spectrum, (f) LM2 spectrum of Zn.
Figure 9

The full-spectrum analysis diagram of the ZC composite samples and the XPS diagram of the four elements. (a) Full spectrum of ZC composite sample, (b) C spectrum, (c) 2p spectrum of Zn, (d) O spectrum, (e) N spectrum, (f) LM2 spectrum of Zn.

Figure 9b shows that C1s has three feature peaks. C1s peak with peak position of 284.6 eV is used in the experiment to achieve calibration of peak position error. The C1s orbital in ZC sample can be divided into C–C, C–O, and N–C═N components, whose binding energies are located at 284.8, 286.4, and 288.7 eV, respectively [52,53]. There are two peaks at 284.8 and 288.7 eV, which correspond to C–C and N–C═N groups in g-C3N4.

Figure 9c shows that O1s has three feature peaks, namely Zn–O, C═O/–OH, and C–O, with binding energies at 530.3, 531.8, and 533.0 eV, respectively. The XPS spectra of O1s of ZC were obtained at high resolution. Through Gaussian fitting, O with two components of binding energies of 530.3 and 531.4 eV was found. The peak of 531.4 eV binding energy is generally considered to be OH adsorbed on the surface of the material, and the peak of 530.3 eV is O2− in ZnO [54,55,56].

As shown in Figure 9d and e, it can be seen that the energy difference of orbital spin splitting peaks (2p3/2 and 2p1/2) in Zn2p spectrum is about 23 eV, and two peaks (Zn2p3/2 and 2p1/2) appear at 1,021.7 and 1,044.8 eV, respectively, which is consistent with the XPS results of Zn2+ [57].

In Figure 9f, the 398.5 eV peak in N1s corresponds to the SP2 hybrid aromatic bonded to carbon atoms in g-C3N4, and the corresponding 399.8 eV peak tertiary N(N–(C)3/H–N–(C)2); the third peak with high binding energy at 400.9 eV is attributed to quaternary N in an aromatic ring [54,58]. Through element analysis, this is consistent with the data in the existing literature. It further proves that the ZC QD composite sample is successfully prepared.

3.8 Diffuse reflectance spectroscopy

The pure ZnO and ZnO QDs are white, and when combined with g-C3N4, the color of the sample changes from white to light yellow. Pure ZnO and ZnO QDs show strong intrinsic absorption in UV region. At the same time, due to the presence of surface defects (such as Zni, VZn, and VO), ZnO and ZnO QDs also show a certain light response in the visible region. Figure 10 shows the UV spectra of g-C3N4 with different contents of ZnO-g-C3N4. g-C3N4 has a band edge positioned at about 460 nm, corresponding to a band gap of 2.70 eV. However, the visible light absorption of bulk g-C3N4 is weak in the visible region from 500 to 800 nm. Compared with pure ZnO, the absorption edge of the ZnO-g-C3N4 nanocomposite photocatalyst is red-shifted with the increase of g-C3N4 content in the feedstock and the visible light absorption becomes stronger. It is noteworthy that the best visible light absorption is achieved when the mass proportion of g-C3N4 is 10%. The visible light absorption of the composite photocatalyst decreased as the amount of g-C3N4 continued to increase, probably because the excess coverage of the amorphous g-C3N4 layer on the ZnO surface affected the light absorption rate.

Figure 10 
                  UV spectrogram of g-C3N4 with different contents of ZnO-g-C3N4.
Figure 10

UV spectrogram of g-C3N4 with different contents of ZnO-g-C3N4.

According to Figure 11, the non-complex radiation centers are reduced, and the exciton absorption capacity is improved. ZnO QDs-g-C3N4 10% exciton absorption peak position and half peak width did not change significantly, indicating that the main body of the sample optical absorption is still ZnO core. The position of the 10% exciton absorption region of ZnO QDs-g-C3N4 has a slight red shift, because the particle size of QDs is changed due to the reaction conditions. In summary, ZnO QDs-g-C3N4 10% nanocomposite has the best photocatalytic performance.

Figure 11 
                  UV spectroscopy of ZnOQDs-g-C3N4 10%, ZnO-g-C3N4 10%, ZnOQDs, and ZnO.
Figure 11

UV spectroscopy of ZnOQDs-g-C3N4 10%, ZnO-g-C3N4 10%, ZnOQDs, and ZnO.

3.9 Photoluminescence (PL)

The recombination of photogenerated electron-hole pairs causes luminescence (PL); therefore, measuring the intensity of PL spectra helps us further understand the transport and compounding processes of photogenerated carriers in semiconductors. The emission of PL is caused by the recombination of photogenerated carriers [59]. In general, the weaker the fluorescence intensity, the better the photocatalytic activity. It can be seen from Figure 12 that ZnO-g-C3N4 10% photocatalytic effect is the best. The PL intensity is significantly reduced due to the carrier transfer between valence band (VB) of g-C3N4 and conduction band (CB) of ZnO, which inhibits, and limits, the recombination of photogenerated carriers [60,61].

Figure 12 
                  PL spectra of ZnO-g-C3N4 with different g-C3N4 contents.
Figure 12

PL spectra of ZnO-g-C3N4 with different g-C3N4 contents.

3.10 Photocatalytic performance of different g-C3N4 doping amount catalysts

The experimental conditions are as follows: 10 mg·L−1 RhB solution 50 mL, photocatalyst ZnO-g-C3N4 dosage 0.02 g, pH = 3, room temperature, dark adsorption for 60 min, and light reaction time 6 h.

Figure 13 shows the degradation curves of catalysts with different g-C3N4 doping levels. The photocatalytic efficiency of RhB solution was investigated by using ZnO and g-C3N4 ZC binary composite materials with different doping mass ratios (3%, 5%, 7%, 10%, 20%) under visible light. Efficiency of photocatalytic degradation of composite materials with different g-C3N4 doping mass ratios was investigated by comparing with single ZnO.

Figure 13 
                  Degradation plots of different g-C3N4 doping catalysts.
Figure 13

Degradation plots of different g-C3N4 doping catalysts.

As shown in Figure 13, the degradation rates of ZnO, ZC-3%, ZC-5%, ZC-7%, ZC-10%, and ZC-20% on RhB solution were 37.69%, 50.49%, 68.57%, 89.08%, 89.08%, and 80.96%, respectively, when visible light was irradiated for 360 min. It can be seen that in binary composite ZC, the higher the amount of g-C3N4 doping, the stronger the photocatalytic efficiency, and the visible light catalytic efficiency of ZC-10% is the highest. The main mechanism involves electron excitation from the VB of g-C3N4 to the CB under visible light irradiation, and then further transfer to the CB of ZnO. According to the aforementioned data, visible light catalytic efficiency is the best when the percentage of g-C3N4 is 10% in a certain range.

3.11 Photocatalytic activity of the optimum doping amount ZnO QDs-g-C3N4 catalyst

In the experiment, the photocatalytic ZnO-g-C3N4 10% degradation rate is the highest. Using ZnO-g-C3N4 10% as a reference to prepare the same proportion of ZnO QDs-g-C3N4 10% catalyst, its photocatalytic performance was explored. The experimental conditions are as follows: 50 mL, 10 mg·L−1 RhB solution, photocatalyst ZnO QDs-g-C3N4 0.02 g, pH = 3, room temperature, dark adsorption for 6 min, and light reaction time for 6 h.

Figure 14 is the degradation curve of ZnO QDs-g-C3N4 catalyst with the optimal doping amount. The graph shows that the adsorption energy of the photocatalyst doped with g-C3N4 was significantly enhanced during the dark adsorption process for the first 60 min. After 60 min, the degradation ability of the ZnO QDs-g-C3N4 composite photocatalyst doped with 10% g-C3N4 was significantly stronger than that of the pure ZnO-g-C3N4 powder catalyst.

Figure 14 
                  Degradation graph of optimal ZnOQDs-g-C3N4 catalyst.
Figure 14

Degradation graph of optimal ZnOQDs-g-C3N4 catalyst.

When the light was turned on for 6 h, the degradation rate of pure ZnO was only about 33%, while that of the ZnO-g-C3N4 composite photocatalyst achieved almost 89% degradation. Compared with ZnO-g-C3N4 10%, ZnO QDs-g-C3N4 10% composite has smaller particle size, which increases the visible light absorption contact area and improves the photocatalytic efficiency. The photocatalytic efficiency of ZnO QDs-g-C3N4 10% composite was about 91%; it is obvious that the photo-catalytic efficiency of ZnO QDs-g-C3N4 10% was higher.

3.12 Photocatalytic reaction kinetics analysis of different g-C3N4 doping amount catalysts

To reach the adsorption–desorption equilibrium, the composite samples were subjected to a dark reaction in RhB for 1 h. Subsequent photodegradation was performed, and it can be seen from Figure 15 that the addition of g-C3N4 to form a binary hetero-junction improved the degradation efficiency of RhB.

Figure 15 
                  ZC photocatalytic degradation of RhB primary reaction kinetic constants.
Figure 15

ZC photocatalytic degradation of RhB primary reaction kinetic constants.

The highest degradation efficiency was observed when the g-C3N4 doping was 10%, which may be related to the low efficiency of photo-generated electron-hole pair separation in the binary composite. It can be seen from Figure 15 that the photodegradation efficiency conforms to the first-order kinetic equation ln(C t /C o) = −kt, where t and k represent the time constant and the rate constant, respectively.

As shown in Table 1, ZC-10% has a higher k value, k = 0.002965, which is about twice that of ZnO. R 2 of each ZC sample is more than 0.9, and it is concluded that each sample conforms to the first-order reaction kinetic equation for D RhB.

Table 1

ZC photocatalytic degradation of RhB primary kinetic data

Sample name Regression equation K R 2
ZnO y = 0.0995x − 0.1077 0.00147 0.928
ZnO-g-C3N4 3% y = 0.1151x − 0.1327 0.00168 0.9746
ZnO-g-C3N4 5% y = 0.1177x − 0.0781 0.00182 0.9821
ZnO-g-C3N4 7% y = 0.1311x − 0.1575 0.00190 0.9843
ZnO-g-C3N4 10% y = 0.1998x − 0.2098 0.00296 0.9169
ZnO-g-C3N4 20% y = 0.1295x − 0.2182 0.00182 0.8533

3.13 Photocatalytic reaction kinetic analysis of optimum doping amount of ZnO-g-C3N4 and ZnO QDs-g-C3N4 catalysts

The starting time of the photoreaction was set to zero, and the ln(C 0/C t ) versus time t was analyzed for the ZC and ZC QD samples; as shown in Figure 16, a linear relationship exists for both samples, which is consistent with the primary reaction kinetics.

Figure 16 
                  Primary reaction constant for the photocatalyzed degradation of RhB in composite samples with optimal doped amounts of g-C3N4.
Figure 16

Primary reaction constant for the photocatalyzed degradation of RhB in composite samples with optimal doped amounts of g-C3N4.

As shown in Table 2, ZC QDs-10% has the highest catalytic efficiency compared to other material, k = 0.00323 min−1, about 2.2 times that of ZnO nanotubes. The correlation coefficient R 2 of ZC QD composite sample is more than 0.9, and it is concluded that the sample conforms to the first-order reaction kinetic equation for D RhB.

Table 2

Linear fit data for g-C3N4 optimal amount of composite material

Sample name Regression equation K R 2
ZnO-g-C3N4 10% y = 0.1998x – 0.2098 0.00296 0.9169
ZnO QDs-g-C3N4 10% y = 0.2174x – 0.2182 0.00323 0.9525

3.14 Cycle stability analysis

The stability and reusability of the catalysts were evaluated by cycling experiments of photocatalytic degradation of RhB. The ZG-10% sample with the best photocatalytic performance was selected to be washed with water and alcohol four times, dried, sonicated, and dried, respectively, for recycling. The control light time was at 180 min, and five replicate experiments were conducted, and the results are shown in Figure 17.

Figure 17 
                  Stability of ZnO-g-C3N4 10% photocatalytic degradation of RhB.
Figure 17

Stability of ZnO-g-C3N4 10% photocatalytic degradation of RhB.

It can be found that after repeating the experiment five times, the first degradation effect is the best, and after that the degradation rate decreases each time, but the change is not significant, and the catalyst still has a high degradation rate of about 83%. It showed that the catalysts were stable, regenerable, and reusable.

The 10% sample of ZnO QDs-g-C3N4 with the best photocatalytic performance was selected, filtered, sonicated, and dried, and then recovered for reuse. After repeating the experiment four times, the best degradation effect was achieved in the first time, and the degradation rate decreased in each subsequent time, but the change was not significant, and the catalyst still had a high degradation rate of about 85%. It showed that the catalysts were stable, regenerable, and reusable.

As can be seen from Figure 18, the photocatalytic degradation rate of the ZC QDs-10% photocatalyst decreased somewhat after four uses, but the decrease was not significant, which may be due to the photocorrosion phenomenon of ZnO QDs. Overall, the prepared photocatalysts have good photocatalytic stability and repeated use performance.

Figure 18 
                  Stability of ZnO QDs-g-C3N4 10% photocatalytic degradation of RhB.
Figure 18

Stability of ZnO QDs-g-C3N4 10% photocatalytic degradation of RhB.

3.15 Analysis of free radical capture experiment results

In order to elucidate the photocatalytic mechanism of ZC QD composite samples in more depth, different trapping agents were added in the photocatalytic process to identify the main active species. The photocatalytic activity was significantly reduced after the addition of the trapping agents, indicating that the active species is the main active species under this photocatalytic system. ZC-10% was selected as a control for RhB degradation, and the changes in photocatalytic degradation of RhB after the addition of the three trapping agents and the results are shown in Figure 19.

Figure 19 
                  Photocatalytic performance of RhB by ZCQDs with BQN, MT, and IPA.
Figure 19

Photocatalytic performance of RhB by ZCQDs with BQN, MT, and IPA.

As shown in Figure 19, the ZC QD sample D RhB = 91.53%. To reveal the main active species in the photocatalytic degradation of RhB in ZC QD composites, p-benzoquinone (BQN), isopropyl alcohol (IPA), and methanol (MT) were added as superoxide radical (˙O2−), hydroxyl radical (˙OH), and hole (h+) trapping agents. The photocatalytic degradation efficiency of the samples showed a significant decrease when all three trapping agents were added, which indicated that ˙O2−, ·˙OH, and h+ were the main active species in the photocatalytic process of the samples under visible light irradiation, where the effect size was ˙O2− > ˙OH > h+ in order.

3.16 ZnO QDs-g-C3N4 10% catalyst mechanistic analysis

Under UV irradiation, g-C3N4 is excited to produce photogenerated electrons and holes. Because the CB of g-C3N4 is higher than that of ZnO, the generated photogenerated electrons are rapidly transferred from CB of g-C3N4 to CB of ZnO, which effectively prevents the electron-hole pair recombination. At the same time, the electrons in ZnO CB can reduce O2 to form ˙O2−, while the holes in g-C3N4 VB can combine with water to form ˙OH. RhB can also be directly oxidized to CO2 and H2O, thus degrading organic pollutants efficiently. The photocatalytic mechanism diagram is shown in Figure 20.

(1) g-C 3 N 4 / ZnO + h v g-C 3 N 4 (e ) / ZnO(e ) + g-C 3 N 4 (h + ) / ZnO(e )

(2) h + + H 2 O ( ˙ OH ) + h +

(3) e + O 2 ( ˙ O 2 )

(4) ( ˙ O 2 ) + 2 h + 2 ( ˙ OH )

(5) RhB + h + / ( ( ˙ O 2 ) / ( ˙ OH ) ) CO 2 + H 2 O + other molecules

Figure 20 
                  Schematic diagram of the photocatalytic reaction mechanism.
Figure 20

Schematic diagram of the photocatalytic reaction mechanism.

4 Conclusion

In this article, a series of ZnO-g-C3N4 composites, ZnO QDs-g-C3N4 composite photocatalysts, were prepared. The degradation ability of the catalysts for RhB pollutants was tested, and it was found that ZnO-g-C3N4 10% worked best. ZnO-g-C3N4 10% and ZnO QDs-g-C3N4 degradation rates under visible light conditions were 89.08% and 91.53%, respectively. The ability of the catalysts to degrade RhB pollutants was tested, and the cyclic stability of the catalysts and the photocatalytic mechanism of the active components of the composite samples were analyzed. Therefore, this work is helpful to enrich the preparation method of heterojunction and promote the further development of environmental wastewater purification research.


# These authors contributed equally to this work.


  1. Funding information: This work was supported by the Natural Science Foundation of Heilongjiang Province (Grants No. LH2019H055), 2019 Heilongjiang University of Chinese Medicine “Outstanding Young Backbone Teachers Support Program” project, and Science and Technology Plan of Heilongjiang Provincial Health Commission (Grant No. 20222121 020963).

  2. Author contributions: Haiyang Liu and Zhe Wang: writing – original draft preparation, writing – review and editing, methodology, formal analysis; Heng Zhang and Lixia Jin: writing – original draft preparation, formal analysis, visualization, project administration; Haiyang Liu: data analysis; Lixia Jin: resources. All authors have read and agreed to the published version of the manuscript.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2023-04-03
Accepted: 2023-05-23
Published Online: 2023-09-08

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  72. Synthesis, characterization, and evaluation of nanoparticles of clodinofop propargyl and fenoxaprop-P-ethyl on weed control, growth, and yield of wheat (Triticum aestivum L.)
  73. The enhanced adsorption properties of phosphorus from aqueous solutions using lanthanum modified synthetic zeolites
  74. Separation of graphene oxides of different sizes by multi-layer dialysis and anti-friction and lubrication performance
  75. Visible-light-assisted base-catalyzed, one-pot synthesis of highly functionalized cinnolines
  76. The experimental study on the air oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid with Co–Mn–Br system
  77. Highly efficient removal of tetracycline and methyl violet 2B from aqueous solution using the bimetallic FeZn-ZIFs catalyst
  78. A thermo-tolerant cellulase enzyme produced by Bacillus amyloliquefaciens M7, an insight into synthesis, optimization, characterization, and bio-polishing activity
  79. Exploration of ketone derivatives of succinimide for their antidiabetic potential: In vitro and in vivo approaches
  80. Ultrasound-assisted green synthesis and in silico study of 6-(4-(butylamino)-6-(diethylamino)-1,3,5-triazin-2-yl)oxypyridazine derivatives
  81. A study of the anticancer potential of Pluronic F-127 encapsulated Fe2O3 nanoparticles derived from Berberis vulgaris extract
  82. Biogenic synthesis of silver nanoparticles using Consolida orientalis flowers: Identification, catalytic degradation, and biological effect
  83. Initial assessment of the presence of plastic waste in some coastal mangrove forests in Vietnam
  84. Adsorption synergy electrocatalytic degradation of phenol by active oxygen-containing species generated in Co-coal based cathode and graphite anode
  85. Antibacterial, antifungal, antioxidant, and cytotoxicity activities of the aqueous extract of Syzygium aromaticum-mediated synthesized novel silver nanoparticles
  86. Synthesis of a silica matrix with ZnO nanoparticles for the fabrication of a recyclable photodegradation system to eliminate methylene blue dye
  87. Natural polymer fillers instead of dye and pigments: Pumice and scoria in PDMS fluid and elastomer composites
  88. Study on the preparation of glycerylphosphorylcholine by transesterification under supported sodium methoxide
  89. Wireless network handheld terminal-based green ecological sustainable design evaluation system: Improved data communication and reduced packet loss rate
  90. The optimization of hydrogel strength from cassava starch using oxidized sucrose as a crosslinking agent
  91. Green synthesis of silver nanoparticles using Saccharum officinarum leaf extract for antiviral paint
  92. Study on the reliability of nano-silver-coated tin solder joints for flip chips
  93. Environmentally sustainable analytical quality by design aided RP-HPLC method for the estimation of brilliant blue in commercial food samples employing a green-ultrasound-assisted extraction technique
  94. Anticancer and antimicrobial potential of zinc/sodium alginate/polyethylene glycol/d-pinitol nanocomposites against osteosarcoma MG-63 cells
  95. Nanoporous carbon@CoFe2O4 nanocomposite as a green absorbent for the adsorptive removal of Hg(ii) from aqueous solutions
  96. Characterization of silver sulfide nanoparticles from actinobacterial strain (M10A62) and its toxicity against lepidopteran and dipterans insect species
  97. Phyto-fabrication and characterization of silver nanoparticles using Withania somnifera: Investigating antioxidant potential
  98. Effect of e-waste nanofillers on the mechanical, thermal, and wear properties of epoxy-blend sisal woven fiber-reinforced composites
  99. Magnesium nanohydroxide (2D brucite) as a host matrix for thymol and carvacrol: Synthesis, characterization, and inhibition of foodborne pathogens
  100. Synergistic inhibitive effect of a hybrid zinc oxide-benzalkonium chloride composite on the corrosion of carbon steel in a sulfuric acidic solution
  101. Review Articles
  102. Role and the importance of green approach in biosynthesis of nanopropolis and effectiveness of propolis in the treatment of COVID-19 pandemic
  103. Gum tragacanth-mediated synthesis of metal nanoparticles, characterization, and their applications as a bactericide, catalyst, antioxidant, and peroxidase mimic
  104. Green-processed nano-biocomposite (ZnO–TiO2): Potential candidates for biomedical applications
  105. Reaction mechanisms in microwave-assisted lignin depolymerisation in hydrogen-donating solvents
  106. Recent progress on non-noble metal catalysts for the deoxydehydration of biomass-derived oxygenates
  107. Rapid Communication
  108. Phosphorus removal by iron–carbon microelectrolysis: A new way to achieve phosphorus recovery
  109. Special Issue: Biomolecules-derived synthesis of nanomaterials for environmental and biological applications (Guest Editors: Arpita Roy and Fernanda Maria Policarpo Tonelli)
  110. Biomolecules-derived synthesis of nanomaterials for environmental and biological applications
  111. Nano-encapsulated tanshinone IIA in PLGA-PEG-COOH inhibits apoptosis and inflammation in cerebral ischemia/reperfusion injury
  112. Green fabrication of silver nanoparticles using Melia azedarach ripened fruit extract, their characterization, and biological properties
  113. Green-synthesized nanoparticles and their therapeutic applications: A review
  114. Antioxidant, antibacterial, and cytotoxicity potential of synthesized silver nanoparticles from the Cassia alata leaf aqueous extract
  115. Green synthesis of silver nanoparticles using Callisia fragrans leaf extract and its anticancer activity against MCF-7, HepG2, KB, LU-1, and MKN-7 cell lines
  116. Algae-based green AgNPs, AuNPs, and FeNPs as potential nanoremediators
  117. Green synthesis of Kickxia elatine-induced silver nanoparticles and their role as anti-acetylcholinesterase in the treatment of Alzheimer’s disease
  118. Phytocrystallization of silver nanoparticles using Cassia alata flower extract for effective control of fungal skin pathogens
  119. Antibacterial wound dressing with hydrogel from chitosan and polyvinyl alcohol from the red cabbage extract loaded with silver nanoparticles
  120. Leveraging of mycogenic copper oxide nanostructures for disease management of Alternaria blight of Brassica juncea
  121. Nanoscale molecular reactions in microbiological medicines in modern medical applications
  122. Synthesis and characterization of ZnO/β-cyclodextrin/nicotinic acid nanocomposite and its biological and environmental application
  123. Green synthesis of silver nanoparticles via Taxus wallichiana Zucc. plant-derived Taxol: Novel utilization as anticancer, antioxidation, anti-inflammation, and antiurolithic potential
  124. Recyclability and catalytic characteristics of copper oxide nanoparticles derived from bougainvillea plant flower extract for biomedical application
  125. Phytofabrication, characterization, and evaluation of novel bioinspired selenium–iron (Se–Fe) nanocomposites using Allium sativum extract for bio-potential applications
  126. Erratum
  127. Erratum to “Synthesis, characterization, and evaluation of nanoparticles of clodinofop propargyl and fenoxaprop-P-ethyl on weed control, growth, and yield of wheat (Triticum aestivum L.)”
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