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. 2022 Dec 23;8(12):e12644. doi: 10.1016/j.heliyon.2022.e12644

ZnO/Cu2O/g-C3N4 heterojunctions with enhanced photocatalytic activity for removal of hazardous antibiotics

Yujie Zhu a, Ling Wang b, Wentao Xu c, Zehai Xu a,∗, Junsheng Yuan c, Guoliang Zhang a,c,∗∗
PMCID: PMC9834774  PMID: 36643305

Abstract

In view of the environmental pollution caused by antibiotics, the creation of an efficient photocatalytic material is an effectual way to carry out water remediation. Herein, we developed a smart strategy to synthesize ZnO/Cu2O/g-C3N4 heterojunction photocatalysts for the photodegradation of hazardous antibiotics by one-pot synthesis method. In this system, the Cu2O nanoparticles with electrons reducing capacity were coupled with g-C3N4 composites. The photocarriers were generated from the electric field of type Ⅰ heterojunction between ZnO and g-C3N4 and type Ⅱ heterojunction between Cu2O and g-C3N4. ZnO as a co-catalyst was doped to Cu2O/g-C3N4 catalyst system for removal of broad-spectrum antibiotics with the condition of visible light to protect Cu2O from photocorrosion, which thereby accelerated photocatalytic reactivity. Benefiting by new p-n-n heterojunction, the resulting ZnO/Cu2O/g-C3N4 composites had an excellent degradation performance of broad-spectrum antibiotics such as tetracycline (TC), chlortetracycline (CTC), oxytetracycline (OTC) and ciprofloxacin (CIP), the degradation of which were 98.79%, 99.5%, 95.35% and 73.53%. In particular, ZnO/Cu2O/g-C3N4 photocatalysts showed a very high degradation rate of 98.79% for TC in first 30 min under visible light, which was 1.35 and 10.62 times higher than that of Cu2O/g-C3N4 and g-C3N4, respectively. This work gives a fresh visual aspect for simultaneously solving the instability deficiencies of traditional photocatalysts and improving photocatalytic performance.

Keywords: ZnO/Cu2O/g-C3N4 heterojunctions, Co-catalyst, Photocorrosion, Photocatalytic, Degradation of antibiotics

ZnO/Cu2O/g-C3N4 heterojunctions; Co-catalyst; Photocorrosion; Photocatalytic; Degradation of antibiotics.

1. Introduction

Over last few decades, abuse use of antibiotics have led to a dramatical increase in antibiotic-resistant, which is a threat to people and animals [1, 2]. The traditional wastewater treatment can not meet the needs for the degradation of antibiotics. Therefore, a large amount of works to develop the new treatment technologies like biological method, Fenton reaction, electrodeposition and photocatalysis to degrade pollutants [3, 4]. In antibiotics disposal, photocatalytic process as an energy-saving and high efficiency technology has attracted a lot of attentions. Among numerous photocatalysts such as graphite carbon nitride (g-C3N4), titanium dioxide and bismuth(III) oxide [5, 6, 7], g-C3N4 as one of the most popular photocatalysts is used in the removing of broad-spectrum antibiotics owing to the unique structure of n-type nonmetallic semiconductor, thermodynamic stability and visible light absorption [8, 9]. However, the high recombination between electrons and holes and low visible light adsorption ability decrease the efficiency of the antibiotics elimination [10, 11].

In order to overcome this obstacle, many researchers are devoted to surface assembly by constructing heterojunction systems [12, 13]. According to transfer mechanism of g-C3N4 base heterostructure in photogenerated charge carriers, it can be grouped into: type-I heterojunction, type-II heterojunction, S-scheme heterojunction and Z-type heterojunction [14, 15, 16]. The narrow band gap about g-C3N4 would be improved through introducing another semiconductor photocatalysts and the visible light would be made the most of so as to achieve the high redox capability, fast photocarriers migration and space separation. Various metal oxides (TiO2 [17], WO3 [18], CeO2 [19], In2O3 [20], MoO3 [21], SnO2 [22], Fe2O3 [23]), metal sulfides (CdS [24], ZnS [25], MoS2 [26]), halides (BiOI [27], BiOCl [28], BiOBr [29], AgI [30], AgBr [31]), revised g-C3N4 and other semiconductors (such as Bi2WO6 [32], BiPO4 [33], Ag3PO4 [34], SiC [35]) have been applied in forming customary type II heterojunction systems based on g-C3N4. Among all metal oxide semiconductors, Cu2O has drawn attention owing to its proper band position and good visible-light photocatalytic activity, when it acted as a typical p-type semiconductor to couple with a n-type semiconductor, formed heterojunction can specifically to broaden the photoabsorption area, improve photocatalysis performance and accelerate the valid separations of photocarriers [36, 37, 38]. The corresponding research confirmed that the progress on the production of hydrogen through combining Cu2O with g-C3N4 to form type II heterojunction [39]. But this kind of photocatalytic agent are easily oxidized when produced electrons fail to remove them, so that it would be lose its chemical activity in the process of eliminating antibiotics. Therefore, a more smart strategy is needed to get much wider applications and have a more chemical stability after continual operation [40, 41, 42]. The three-phase heterojunction changes the rate-determining step and increases the catalytic rate. The synergy between carbon dots and heterojunctions could not only enhance light absorption range of semiconductor particles, but support separation of photogenerated charge carriers [43, 44]. Besides, Surface functionalization by incorporating some co-catalysts could facilitate the charge separation. Due to the energy level and electronic structure can match up with Cu2O, ZnO is often used as co-catalysts [45, 46, 47]. Inspired by these concepts, we put forward an idea where the combination of ZnO co-catalyst and Cu2O/g-C3N4 catalyst could be a feasible strategy to prepare high active heterojunction photocatalyst. There is no report on the preparation of highly active ZnO/Cu2O/g-C3N4 heterojunction photocatalytic heterojunction for antibiotics removal so far.

In this work, we invented a facile way to produce ZnO/Cu2O/g-C3N4 heterojunction photocatalysts by one-pot synthesis approach for the photodegradation of tetracycline. In this system, Cu2O nanoparticles with electrons reducing capacity were connected with g-C3N4 composites. The photocarriers were generated from the electric field of type Ⅰ heterojunction between ZnO and g-C3N4 and type Ⅱ heterojunction between g-C3N4 and Cu2O. Meanwhile, the ZnO particles as a co-catalyst became a transition place to accept more electrons, so that the photocorrrosion of Cu2O was inhibited, and thereby accelerating photocatalytic reactivity. Moreover, ZnO as a co-catalyst to protect Cu2O from photocorrosion. The photocatalytic activity, stability and possible mechanism of prepared ZnO/Cu2O/g-C3N4 heterojunction were evaluated. The prepared ZnO/Cu2O/g-C3N4 heterojunction exhibited good feasibility in absorbing visible light and could be applied in a variety of broad-spectrum antibiotics.

2. Experimental section

2.1. Materials

Copper chloride (CuCl2·2H2O), citric acid (C6H8O7), zinc nitrate (Zn(NO3)2·6H2O), sodium hydroxide (NaOH), ethanol, cetyltrimethy-lammonium bromide, polyvinylpyrrolidone (PVP), urea and ascorbic acid (AA) were bought from Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China) and can be directly operated.

2.2. Preparation of g-C3N4 nanosheet

An alumina crucible with a lid filled with the urea as a precursor, and citric acid was added to promote polycondensation reaction. Then, mixed substance was heated to 500 °C for 1 h in the air, and the subsequent gray solid powder was g-C3N4 nanosheets [48, 49].

2.3. Preparation of ZnO nanorods

ZnO nanorods were synthesized by hydrothermal methods. In general, 0.079 g cetyltrimethylammonium bromide and 1.92 g NaOH were dissolved in 20 mL of deionized water, which was stirred to make a transparent solution. 2.32 g Zn(NO3)2·6H2O was then introduced to above solution and vigorously stirred for 1h. Subsequently, the solution were transferred to reaction caldron under 90 °C for heating for 15 h. Finally, the ZnO was collected after centrifugation and drying.

2.4. Preparation of Cu2O/g-C3N4 composites

Preparation of binary Cu2O/g-C3N4 photocatalysts were made by a hydrothermal approach. The calculated amount of CuCl2·2H2O and NaOH aqueous solutions were first magnetic stirred at room temperature. After that, 0.2 g g-C3N4 powder was dispersed in the above mixed solution by rapid agitation. After stirring for 30 min, 0.01 M of ascorbic acid (AA) was put in the resultant above mixture and was stirred for 1 h. Eventually, powder was washed with ethanol before dried up in vacuum drying chamber at 30 °C for 24 h, so the collected ultimate production was labeled as Cu2O/g-C3N4.

2.5. Preparation of ZnO/Cu2O/g-C3N4 composite

Preparation of ternary ZnO/Cu2O/g-C3N4 composites were made by a hydrothermal strategy (Figure 1). The preparation of ZnO/Cu2O/g-C3N4 compound was same with the prepared Cu2O/g-C3N4 except the addition of ZnO when the g-C3N4 composites were added. The resulting precipitate was centrifuged after the reaction, and was washed by ethanol and was added in a vacuum drying oven and then at 30 °C for 24 h. The final obtained product was called as ZnO/Cu2O/g-C3N4 powder. For the convenience of description, the amount of ZnO added to obtain ZnO/Cu2O/g-C3N4 composites were 5:1, 1:1 and 1:5, referred to as ZnO/Cu2O/g–C3N4–1, ZnO/Cu2O/g–C3N4–2, and ZnO/Cu2O/g–C3N4–3, respectively.

Figure 1.

Figure 1

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The process flow diagram of the preparation of g-C3N4/ZnO/Cu2O photocatalysts.

2.6. Characterization

The chemical properties of the materials were observed by Fourier transform infrared (FT-IR) spectroscopy (Nicolet 6700, Thermo Scientific, USA). Scanning electron microscopy (SEM, TM-1000, Hitachi, Japan) characterizes the EDS elements and morphology of materials. At room temperature, the X'Pert PRO diffraction (Panalytical, Netherlands) observes the X-ray diffraction (XRD) patterns with Cu Ka radiation (40 kV, 40 mA, λ = 0.154056 nm). The X-ray photoelectron spectroscopy (XPS) was operated by PHI 5000C ESCA type and the X-ray source was Al Kα ray (hv = 1486.6 eV). The UV-vis DRS were performed on the Varian Cary 500 UV-Visible Diffuse Reflectometer of the American Company with high purity BaSO4 as the standard reagent, with a scanning range of 200–800 nm and a scanning speed of 40 nm min−1. TECNAI G2 F30 S-TWIN transmission electron microscope (TEM) was applied in studying surface morphology and particle size of the catalyst, and accelerating voltage was 300 kV. Bruker EMXPLUZ paramagnetic resonance spectrometer (ESR/EPR) was used to detect oxygen vacancies of free radicals and catalysts in photocatalytic degradation, and superoxide radicals (•O2-) and hydroxyl radicals (•OH) were captured with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The fluorescence intensity of catalyst was analyzed using the Edinburgh FLS1000 steady-state/transient fluorescence spectrometer (PL) and electrochemical impedance spectroscopy (EIS). Liquid chromatography Agilent 1290UPLC and mass spectrometry Agilent QTOF6550 were used to set up high performance liquid chromatography-tandem mass spectrometry (HPLC-MS) to analyze intermediates.

3. Results and discussion

X-ray diffraction (XRD) patterns of g-C3N4, Cu2O/g-C3N4, and ZnO/Cu2O/g-C3N4 are exhibited in Figure 2a. Representative peaks of Cu2O at 2θ = 36.5°, 42.4°, 61.5° and 73.7° are matched with (111), (200), (220), and (311) crystal surface indexes of Cu2O plane (JCPDS No. 78-2076), respectively [50]. g-C3N4 reveals two characteristic peaks (100) and (002) at 2θ = 13.1° and 27.4°, individually, which is according to the simple aromatic ring and triazine stacking between layers [51]. In addition, the ZnO pattern shows three feature peaks at 31.8° (100), 34.4° (002) and 36.3° (101), in line with JCPDS No. 65-3411 [52,53]. Apparently, ZnO (★) and Cu2O (●) coexist in the ZnO/Cu2O/g-C3N4 sample, and the size of Cu2O (14.5 nm) in the ZnO/Cu2O/g-C3N4 photocatalyst was higher than the crystal size of Cu2O (7.9 nm) in the Cu2O/g-C3N4 catalyst through calculation of Scherrer formula. The addition of ZnO has an specific effect on the crystal size of Cu2O. It demonstrates that Cu2O in the ZnO/Cu2O/g-C3N4 photocatalyst has better crystalline phase than Cu2O in the Cu2O/g-C3N4 photocatalyst, which reduces photocorrosion rate [54]. The Fourier Transform Infrared (FT-IR) spectroscopy of original g-C3N4, Cu2O/g-C3N4, and ZnO/Cu2O/g-C3N4 compounds was observed in Figure 2b. Typical absorption peaks at 807 cm−1 and 890 cm−1 were connected with tri-s-triazine ring systems and the bending vibrations of N–H [55]. 1240 cm−1, 1322 cm−1, 1410 cm−1 and 1638 cm−1 characteristic peaks could affirm the presence of tensile vibrations of C–N and C=N, and the broader absorbing band of nearby 3167 cm−1 is connected with N–H and O–H vibrations [56]. Besides, the FT-IR spectrum of ternary ZnO/Cu2O/g-C3N4 composite showed no obvious difference in the characteristic skeleton of the g-C3N4, confirming that ZnO and Cu2O which were brought into is uninfluential in the initial structure of the g-C3N4 nanosheet, which is the same as the above XRD analysis outcomes.

Figure 2.

Figure 2

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(a) XRD patterns and (b) FT-IR spectra of g-C3N4, g-C3N4/Cu2O and g-C3N4/ZnO/Cu2O photocatalysts.

The SEM and TEM results of the resulting ZnO/Cu2O/g-C3N4 photocatalysts is observed in Figure 3. The microscopic test of g-C3N4 (Figure 3a) exhibits a uniform nanosheet two-dimensional flake structure. The appearance in Figure 3b, Figures 3d and Figure 3g exhibits a large number of Cu2O nanoparticles with good adhesion stocking on the layer of g-C3N4. Meanwhile, as is vividly exhibited in Figure 3e and and Figure 3f, the pure ZnO prepared by hydrothermal method has a glossy nanorod surface topography. The EDS spectra of ZnO/Cu2O/g-C3N4 is provided in Figure 3c, it can be obviously noticed that Zn, O, Cu, C and N elements consist of ZnO/Cu2O/g-C3N4 compound catalyst. The atomic weight of Cu and Zn elements are measured by the peak area, which is a value of 12.26% and 6.30% respectively. It demonstrates the successful preparation of a ZnO/Cu2O/g-C3N4 compound.

Figure 3.

Figure 3

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SEM images of (a, b) ZnO/Cu2O/g-C3N4, (c) The EDS spectrum of ZnO/Cu2O/g-C3N4, SEM images of (d, e) ZnO and Cu2O and TEM images of (g, h) ZnO/Cu2O/g-C3N4.

The X-ray photoelectron spectroscopy (XPS) is utilized to analyze and study composition of ZnO/Cu2O/g-C3N4 in Figure 4a. The XPS spectrum of Cu 2p reveals two typical peaks in Figure 4b, which the Cu 2p3/2 and Cu 2p1/2 are found in the 932.05 eV and 952.0 eV, separately [57]. Furthermore, Figure 4c displays the two typical peaks of C 1s, the peaks at 284.35 eV and 287.55eV are related to the C=C sp2 hybridized carbon in the structure of g-C3N4 and N–C=N sp2 hybridization in the aromatic ring [58, 59]. In Figure 4d, binding energy of Zn 2p are detected in the 1020.65 eV and 1044.6 eV, which agrees with the Zn 2p3/2 and Zn 2p1/2, and is related to Zn2+ in ZnO [60, 61]. What's more, the spectrum of N 1s is separated into four peaks in Figure 4e. The peaks at about 398 eV, 398.8 eV, 400.3 eV, 404.2 eV are consistence with different combinations of N elements which includes C=N–C, N-(C)3 groups, C–N(H)–C, Zn–N and π-excitation in the structure of g-C3N4 composites and ZnO composites [58, 62]. Finally, Figure 4f shows three peaks of O 1s at 530.5 eV, 531.46 eV and 532.83 eV, which are linked with the existence of the weak binding of –OH, and the combination between O2- and the coactions of Cu/Zn [59, 63].

Figure 4.

Figure 4

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XPS spectra of (a) survey spectrum, (b) Cu 2p, (c) C 1s, (d) Zn 2p (e) N 1s (f) O 1s of ZnO/Cu2O/g-C3N4 composites.

In addition, specific surface area and aperture distribution of photocatalysts are analyzed by using the BET surface area with N2 adsorption-desorption isotherms which provides a more detailed basis for further analysis of the relation between the structure and properties of mesoporous material [64]. The calculation values of specific surface area Cu2O/g-C3N4 and ZnO/Cu2O/g-C3N4 ternary nanocomposites were 19.76 m2/g and 93.372 m2/g, individually (Figure 5). The BET measurement results of ZnO/Cu2O/g-C3N4 nanocomposites was 4.73 times the height of Cu2O/g-C3N4 samples. The porous Cu2O/g-C3N4 composites exhibited a small specific surface area because the Cu2O composites coat on the pore canal of g-C3N4 surface. The consequences show the higher surface area may promote more reaction sites to adsorb active substance and capture the charge carriers on its surface, which could be conducive to improving photocatalytic capability of ZnO/Cu2O/g-C3N4 photocatalysts.

Figure 5.

Figure 5

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The N2 adsorption-desorption isotherms and pore diameter of g-C3N4, ZnO, Cu2O/g-C3N4 and ZnO/Cu2O/g-C3N4 composite samples.

To study the optical properties and electrons transfer of photocatalytic materials, UV-vis absorption spectroscopy was utilized ranging 200–800 nm. As exhibited in Figure 6a, spectrographic absorption of original Cu2O, g-C3N4, Cu2O/g-C3N4 and ZnO/Cu2O/g-C3N4 photocatalysts were collected. The g-C3N4 has an absorption edge near 200–410 nm due to electronic transition from N 2p to the C 2p orbit [65, 66]. Besides, the band energy (Eg) was measured by the method of Kubelka–Munk formula, and the value of Cu2O, g-C3N4 and ZnO in Figures 6b, 6c and 6d were approximately 2.25 eV, 2.44 eV and 3.15 eV, which was consistence with preceding reports [36, 48, 66]. In addition, with the addition of ZnO and Cu2O composites, absorption intensity of ZnO/Cu2O/g-C3N4 heterostructure was expanded with the strongest visible light absorption zone of about 568 nm. These results showed that ZnO/Cu2O/g-C3N4 ternary composites have higher visible light activity and much broader scale.

Figure 6.

Figure 6

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(a) UV-Vis DRS absorption spectra of g-C3N4, Cu2O, Cu2O/g-C3N4 and major ZnO/Cu2O/g-C3N4 heterostructures.(b) band gap energy of g-C3N4, Cu2O, ZnO.

The photocatalytic activity of the prepared materials were measured by using visible light illumination toward target pollutants TC. Figure 7a shows the degradation dynamic curves of g-C3N4, Cu2O/g-C3N4, and ZnO/Cu2O/g-C3N4 composites. Before catalytic reaction, all the samples were subjected to dark adsorption experiments, and the absorbent equilibrium could be achieved after 30 min. Then results exhibited that ternary ZnO/Cu2O/g-C3N4 photocatalyst had best photocatalytic performance (TC degradation rate of 98.79%) than that of original g-C3N4, Cu2O, Cu2O/g-C3N4. As depicted in the graph, pure g-C3N4 displayed the lowest absorption about visible light and separation speed about photogenerated carrier, leading to the lowest degradation performance of TC (only 9.24%). In addition, the degradation rate of the original Cu2O was 58.36%, whose degradation was needed to improve. It is noteworthy that when Cu2O/g-C3N4 is coupled, photocatalytic activity is increased and TC degradation efficiency of Cu2O/g-C3N4 is 98.1% in first 1 h. Strikingly, when the ZnO co-catalyst was introduced into Cu2O/g-C3N4 heterojunction systems, the photocatalytic ability of ternary ZnO/Cu2O/g-C3N4 heterojunctions was significantly enhanced, all things being equal. Amazingly, ZnO/Cu2O/g-C3N4 degrades TC at 30 min in visible light at a rate of 98.79%. The increase of photocatalytic efficiency of ternary ZnO/Cu2O/g-C3N4 composites is primarily due to the construction of p-n-n ZnO/Cu2O/g-C3N4 ternary heterojunctions and the synergy between components. With the gradual increase of ZnO loading, photocatalytic efficiency of ternary composites diminish and the light shading effect is major reason.

Figure 7.

Figure 7

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(a) Under visible light exposure (λ > 420 nm), TC is photodegraded on the prepared photocatalyst. (b) First-order dynamic plot. (c) The photodegradation test of the prepared photocatalyst was performed under visible light irradiation Antibiotics: over-the-counter, carbon tetrachloride and CIP. (d) Cyclic operation of photocatalytic TC degradation in the presence of ZnO/Cu2O/g-C3N4 before and after 4 runs.

The primary reaction kinetic equation was utilized to study the degradation law of the above photocatalysts [67], and thus the related ln(C0/C) curve shows a good linear relationship (Figure 7b). What's more, the values of kinetics rate constant about all catalysts are revealed in a more visualized clearer line diagram. It is easy to see that the maximum rate constant of ZnO/Cu2O/g-C3N4 is 0.0737 min−1, which is about 49.13, 4.45 and 2.56 times the size of original g-C3N4, Cu2O and Cu2O/g-C3N4, respectively. It is worth noting that the comparison between this work and other previously reported ternary photocatalysts were shown in Table 1, and ZnO/Cu2O/g-C3N4 photocatalytic materials displayed strong photocatalytic performance against degradation of pollutant TC together with visible light. Subsequently, OTC, CTC and CIP as other broad-spectrum antibiotics were selected to make further studies of prepared materials, as exhibited in Figure 7c. The photocatalytic degraded trend of antibiotics was similar to that of the above TC. Among them, the ternary ZnO/Cu2O/g-C3N4 compound photocatalyst still shown best photocatalytic performance as a whole, which vividly indicated that the ternary ZnO/Cu2O/g-C3N4 photocatalyst consisted of g-C3N4, ZnO and Cu2O has much wider applications than others. Finally, for the sake of study the stability of the ZnO/Cu2O/g-C3N4 photocatalyst, it was explored again using a cyclic experiment under the same circumstances, as exhibited in Figure 7d. After four cycles, performance of photocatalytic degradation of TC almost did not decrease, which confirmed its superior stability. Meanwhile, the TEM images in Fig. S1 showed that no obvious change existed in the topography and size of the sample before and after the reaction. Besides, for further test structural stability after the reaction, XRD was carried out, Fig. S2 showed that the material remained structural integrity after cycling.

Table 1.

Comparison of photocatalytic efficiencies of other previously reported ternary composite photocatalysts for degradation TC in recent years.

Samples Catalyst gL−1 CTC mgL−1 Light source Xe lamp/W DR% Rate constant min−1 References
N-GNDs/Ag/BiVO4 0.5 20 300（λ > 420 nm） 85.4（80 min) 0.02433 [76]
Ag3PO4/Co3(PO4)2/g-C3N4 0.5 10 300（λ > 420 nm） 88（120 min) 0.0159 [77]
RGO/CdIn2S4/g-C3N4 1.0 10 500（λ > 420 nm） 74.02（180 min) 0.00766 [78]
BiOI@UIO-66(NH2)@g-C3N4 0.2 20 300（λ > 420 nm） 80（180 min) 0.00851 [79]
Bi2O3/Bi2S3/BaFe12O19 1.0 5 300（λ > 420 nm） 80（15 min) 0.0264 [80]
Ag@g-C3N4@BiVO4 0.3 20 300（λ > 420 nm） 82.75（60 min) — [81]
Cu3P/ZnSnO3/g-C3N4 0.5 10 500（λ > 420 nm） 98.45% (60 min) 0.0543 [82]
BiOI/g-C3N4/CeO2 0.5 20 300（λ > 420 nm） 91.6（120 min) 0.0205 [83]
Ag/g-C3N4/SnS2 0.2 15 500（λ > 420 nm） 94.9（150 min) 0.0201 [84]
Cu2O/ZnO/g-C3N4 0.5 20 350（λ > 420 nm） 98.7（30 min) 0.07372 This work
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The major active substances in the degradation course of TC were determined by the capture experiment. In this process, triethanolamine, isopropanol (IPA) and p-benzoquinone were added to play a role as h+, •OH, and •O2- scavengers in the degradation of TC. As can be known from Figures 8a and 8b, the degradation of TC was importantly inhibited with the appropriate addition of triethanolamine. What's more, introduction of p-benzoquinone had a considerable effect on inactivation of the ZnO/Cu2O/g-C3N4 photocatalysts. The results of experiment showed h+ and •O2- are main factor with the equal circumstances in the degradation of ZnO/Cu2O/g-C3N4 composites.

Figure 8.

Figure 8

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Photocatalytic activity of ZnO/Cu2O/g-C3N4 composites on TC degradation under different quenching conditions.

Tetracycline hydrochlorides are a kind of amphoteric compounds, which consists of phenolic diketone moiety, dimethylammonium group and tricarbonyl system [68, 69]. Consequently, three structures are found including positively-charged ions (TCH3+) which exist in below 3.3, zwitter-ions existing between 3.3 and 7.7 (TCH2) and negatively-charged ions (TCH− or TCH2-) in the existence of above 7.7, as shown in Figure 9a [69]. Therefore, pH factor were operated by changing different pH values, and the degradation performance of TC consequently were discussed. Admittedly, •OH and •O2- were generated separately from holes and electrons [70, 71, 72]. However, the leading factors including •O2- and h+ was confirmed by the scavenger photodegradation experiments in Figure 8a. Therefore, the detailed reactions would be shown in the following equation:

ZnO/Cu2O/g-C3N4 + hν→ e− + h+ (1)
e− + O2 → •O2- (2)
•O2- + H+ → •HO2→ H2O2 (3)
h+/•O2-/H2O2 + TC→ CO2 + H2O (4)

Figure 9.

Figure 9

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(a) Molecular structure of TC under different pH. The photodegradation test of the prepared photocatalyst was performed under visible light irradiation Antibiotics: (b) The effect of variable pH values. (c) The effect of different positive ions. (d) The effect of different negative ions.

As is vividly exhibited in Figure 9b and Table 2, degradation performance in the alkaline environment was stronger than that in acidity solution. Owing to the existence of •O2-, rate constant was decreased from 0.0737 min−1 to 0.0295 min−1 by changing pH from 4 to 7. These results was demonstrated that •O2- really stimulated a good photodegradation response.

Table 2.

The kinetic parameter of different photocatalysts in the degradation of TC.

Samples TC（mgL−1） K(min−1) R2
g-C3N4 40 0.0015 0.96
Cu2O 40 0.0166 0.90
Cu2O/g-C3N4 40 0.0288 0.90
ZnO/Cu2O/g–C3N4–1 40 0.0737 0.91
ZnO/Cu2O/g–C3N4–2 40 0.0435 0.94
ZnO/Cu2O/g–C3N4–3 40 0.0176 0.94
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What's more, the existence of co-existing ions were researched to get better evaluations as a result of the complexity of wastewater. In general, the positive ions like Na+、Mg2+、K+、Zn2+ have a little impact on the degradation performance in Figure 9c because these ions are fail to compete with catalysts and even enhance the conductivity. Meanwhile, the introduction of NO2- was different from other negative ions including Cl−、NO3-、SO42- in Figure 9d. It has a significantly lower efficiency due to the NO2- ions suffering competition with tetracycline hydrochlorides. NO2- ions tended to be oxidized by the •O2- and the photodegradation consequently was lower.

High performance liquid chromatography (HPLC)-mass spectrometry (MS) was applied in revealing for complete pathway of TC photocatalytic oxidation. Photocatalytic oxidation ·O2- free radicals attack the double bonds, phenol groups and amine groups of TC, and generate different intermediates m/z = 476.3, 396.8 and 229.2 in 4.496min, 5.770min and 6.843min, and finally generate CO2 and H2O in Figure 10. Based on detected intermediates, the degradation pathway associated with TC is inferred as shown in Figure 11 [73].

Figure 10.

Figure 10

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HPLC-MS of the degradation of TC with ZnO/Cu2O/g-C3N4 catalyst at different times.

Figure 11.

Figure 11

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Possible TC degradation pathways.

According to test data of valence band X-ray photoelectron spectroscopy (VB XPS), obtained pattern is extrapolated to the intersection of horizontal extension line around 0 eV, and the intersection point is Ev. As shown in Figure 12, according to the VB XPS maps of g-C3N4, Cu2O and ZnO, a straight line is found to be extended near 0 eV, and the intersection point is the valence band position energy obtained by extending the horizontal part less than 0 eV. Therefore, valence band positions of g-C3N4, Cu2O and ZnO correspond to energies of 1.6 eV, 1 eV and 2 eV, respectively. Therefore, the values of Ev and Ec of g-C3N4, Cu2O and ZnO composites was deduced and photocatalytic mechanism of ZnO/Cu2O/g-C3N4 heterojunctions are shown in Figure 14 [39].

Figure 12.

Figure 12

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Valence band X-ray photoelectron spectroscopy: (a) g-C3N4, (b) ZnO and Cu2O.

Figure 14.

Figure 14

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Photocatalytic mechanism of ZnO/Cu2O/g-C3N4 p-n-n heterojunctions.

Electrochemical impedance spectroscopy tests were applied in knowing charge transfer behavior, and results were shown in Figure 13a. ZnO/Cu2O/g-C3N4 had the smallest radius of arc and resistance, denoting the preparation of ZnO/Cu2O/g-C3N4 composites could effectually cut the transfer resistance of interfacial charge from electrode to electrolyte molecule, promoting the efficient transport and separation between photogenerated carriers [75]. Besides, the electron-hole pairs of ZnO, g-C3N4, Cu2O and ZnO/Cu2O/g-C3N4 in the separation efficiency was analyzed by photoluminescence spectroscopy in Figure 13b. Admittedly, higher fluorescence intensity shows greater recombination of electron-hole. Due to electron-hole pairs excited by light absorption are recombined in large quantities, g-C3N4 had the highest the PL peak intensity. ZnO/Cu2O/g-C3N4 had much lower PL peak intensities than other catalysts, denoting that hole and electron recombination was inhibited. Therefore, the photoelectric performance about ZnO/Cu2O/g-C3N4 was noticeably superior than that about single-phase materials, denoting the prominent photocatalytic performance.

Figure 13.

Figure 13

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(a) EIS response and (b) photoluminescence spectra of g-C3N4, Cu2O, ZnO and ZnO/Cu2O/g-C3N4 materials.

In order to confirm the mechanism of electron transport between three materials among Cu2O, g-C3N4 and ZnO, DMPO was used as the self-selected trapping agent of active free radicals in the electron spin resonance spectroscopy (ESR) to determine free radical generation. Under the condition of dark, ZnO/Cu2O/g-C3N4 didn't produce any free radical signals, while after visible light irradiation DMPO ·OH appeared in Figure 15a. The OH signal was particularly weak because the ZnO with VB of 2 eV was dispersed to VB of 1.6 eV of g-C3N4, which was inferior to the generation potential of hydroxyl radicals(the value of H2O/·OH is 1.99 eV) [74]. As exhibited in Figure 15b, ZnO/Cu2O/g-C3N4 did not produce any free radical signals under dark conditions, and a strong DMPO ·O2- appeared after visible light irradiation, which was due to the CB values of g-C3N4 (−0.84 eV) and Cu2O (−1.25 eV) were higher than the potential for superoxide radical generation (O2/·O2- = -0.33 eV） [75].

Figure 15.

Figure 15

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ESR spectra of (a) DMPO ·OH of ZnO/Cu2O/g-C3N4. (b) DMPO ·O2- of ZnO/Cu2O/g-C3N4.

The Fermi level (EF) of n-type ZnO and p-type Cu2O semiconductors is close to that of Electronic Valence Band, whereas the EF of n-type ZnO and g-C3N4 semiconductors is near Electronic Conduct Band. Strikingly, when the photogenerated carriers of ZnO/Cu2O/g-C3N4 photocatalyst are transferred with using visible light due to typical heterojunction mechanism, photogenerated electrons on the CB of ZnO and Cu2O are transported to the conduct band of g-C3N4, and the holes are generated on the ZnO and g-C3N4's VB and which are transported on the valence band of Cu2O. The VB of g-C3N4 in visible light is used to catch and restore O2 to •O2- to minish the concentration of antibiotics, when h+ on the VB of Cu2O will directly break down the antibiotics into other products, which is type Ⅱ heterojunction between g-C3N4 and Cu2O and type Ⅰ heterojunction between g-C3N4 and ZnO. The mechanism of band gap is logical, which is correspond with the results of the experimental catch, further demonstrating the reasonability of the p-n-n heterojunction theory. Based on above results, the ZnO/Cu2O/g-C3N4 heterojunction composite photocatalyst successfully realized high rate of photogenerated electrons separation on the contact of interface, and thus material removal efficiency of pollutants was improved in water.

4. Conclusion

In summary, ZnO/Cu2O/g-C3N4 p-n-n type heterogeneous composite was successfully synthesized by an uncomplicated and effortless hydrothermal approach. The photocarriers were generated from the electric field of type Ⅰ heterojunction between g-C3N4 and ZnO and type Ⅱ heterojunction between g-C3N4 and Cu2O. Meanwhile, the ZnO particles as a co-catalyst became a transition place to accept more electrons, so that the photocorrrosion of Cu2O was inhibited, thus accelerating photocatalytic reactivity. On account of the advantages of the internal electric field derived from heterojunction system, photogenerated electron-hole pairs could be quickly separated and transported in ternary ZnO/Cu2O/g-C3N4 heterojunction. Besides, compared to pure catalysts and binary composites, photocatalytic antibiotic activity was largely enhanced with the condition of visible light. As a result, ZnO/Cu2O/g-C3N4 revealed a very high degradation rate of 98.79% for TC in first 30 min under visible light, which was 1.35 and 10.62 times the height of Cu2O/g-C3N4 and g-C3N4, respectively. The produced p-n-n heterojunction photocatalyst may give a fresh viewpoint in the area of designing photocatalysts with progressive structures and high performance in the water remediation.

Declarations

Author contribution statement

Yujie Zhu: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Ling Wang, Wentao Xu, Junsheng Yuan: Analyzed and interpreted the data.

Zehai Xu: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Guoliang Zhang: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data.

Funding statement

Dr. Zehai Xu and Guoliang Zhang National Natural Science Foundation of China (21808202 & 21736009).

Data availability statement

Data will be made available on request.

Declaration of interests statement

The authors declare no competing interests.

Additional information

No additional information is available for this paper.

Contributor Information

Zehai Xu, Email: xuzehai520@163.com.

Guoliang Zhang, Email: guoliangz@zjut.edu.cn.

Appendix A. Supplementary data

The following is the supplementary data related to this article:

20221027 Heliyon Supporting Information
mmc1.doc (193.5KB, doc)

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