Design and Fabrication of Ag 2 O@CuFe 2 O 4 @g-C 3 N 4 Photocatalysts for the Degradation of Organic Dyes Under Visible Light

doi:10.21203/rs.3.rs-2348828/v1

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Design and Fabrication of Ag 2 O@CuFe 2 O 4 @g-C 3 N 4 Photocatalysts for the Degradation of Organic Dyes Under Visible Light

https://doi.org/10.21203/rs.3.rs-2348828/v1

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A new series of Ag₂O@CuFe₂O₄@g-C₃N₄ composite photocatalyst has been synthesized by a facile one pot hydrothermal reaction approach using urea, g-C₃N₄, FeCl₃ and Cu(AC)₂ as precursors. Urea was decomposed over 90^oC and adjusted the pH value of reaction solution over 10. In Ag₂O@CuFe₂O₄@g-C₃N₄−3%, the decomposition rate of MB was 99.93% under visible light within 35 minutes. In the composites, Ag₂O and CuFe₂O₄ have been uniformly dispersed in g-C₃N₄ matrix, inducing more enhanced efficiency of Ag₂O@CuFe₂O₄@g-C₃N₄ heterostructure in photo-degradation methylene blue (MB) under visible light. The photocatalytic activity of Ag₂O@CuFe₂O₄@g-C₃N₄ was enhanced based on the p-n structure between Ag₂O, CuFe₂O₄ and g-C₃N₄. The decomposition rate of MB was reduced from 99.08–93.59% for several recycling use, thus Ag₂O@CuFe₂O₄@g-C₃N₄ show high catalytic activity and excellent stability. The photocatalytic mechanism of Ag₂O@CuFe₂O₄@g-C₃N₄ was discussed based on the active species •OH during photocatalytic process.

Ag2O@CuFe2O4@g-C3N4

Heterostructure

Organic oxidation

Photocatalysis

With the rapid development of global population and economy, the crisis of energy and water has become severer. The surface water is contaminated by persistent organic substances such as dyes, drug metabolite or pesticide etc.. Water pollution has been a worldwide environmental problem. The degradation and effective remediation of organic compounds in wastewater has become an important subject. Organic dyes composed by heterocyclic romatic/aromatic-azo group are carcinogenic, pathogenic, toxic, stubborn etc.[1-5]. The treatment processes such as physical separation is inefficiency to degrade the dye organic molecules. Therefore, many treatment processes such as chemical oxidation, biological degradation and advanced oxidation processes (AOPs) have been widely employed for the removal of dyes from wastewater. The noble metal, metallic oxides and their heterocompound have been identified as a green photocatalyst to resolve the increasing energy and environmental crisis by using solar light sources special visible light for organic dye degradation and hydrogen generation from water[2-8]. The success of TiO₂ in photocatalysis has opened the prelude to the photoelectrochemical degradation of organic dye[9-15]. However, this classical photocatalyst with wide bandgap which locate the ultraviolet region and low solar energy utilization efficiency encounters the problem to adapt the demanded applications. Recently, the development of photocatalysts has been stimulated and accelerated by various strategies, such as band engineering by heterogeneous doping, sensitization by incorporating efficient visible-light converters (e.g. metal-organic dyes), invention/discovery of narrow band-gap semiconductors and so on. The ferrites were considered as novel and promising heterogeneous photosensitive Fenton catalysts due to their stable structure, low band gap energy and excellent magnetic properties[1-6]. The quantum yield and photocatalytic activity of single component ferrites are lower. In order to overcome the shortcomings, composite catalytic materials, especially those with p-n junction structure and photo-Fenton effect, are used to enhance the quantum yield and photocatalytic activity. The efficient Fenton reagent was generated by CuFe₂O₄ photocatalyst which was illuminated under solar light because coupling between Fe²⁺/Fe³⁺ and Cu⁺/Cu²⁺ cyclic redox[11-13]. The applicability of CuFe₂O₄photocatalyst is limitedin photo-Fenton-like reaction because the catalytic activity is affected for rapid recombination of electron-hole pairs, a high value of ion leaching and a low ratio of surface area to mass. To overcome shortcoming, CuFe₂O₄wasprepared by several methods which is constructing various structures, doping with different elements, coupling with different semiconductor and designing metal-organic frameworks to lower the recombination of electron-hole pairs. Ag₂O as an p-types semiconductor is widen studied for the efficient UV-Vis-NIR light absorption because of the narrow band gap(1.2eV)[7-11]. The overlap band structure of Ag₂O and CuFe₂O₄ can construct the p-n junction for enhancing the separation efficiency of photogenerated charges[9,11].

Graphite nitrogen carbide (g-C₃N₄) is a metal-free polymer semiconductor material. Sample g-C₃N₄ has emerged as favorable photocatalyst because of its band gap of 2.7 eV, it is able to capture visible light[3,5,8-11]. To enhance the photoactivity of g-C₃N₄, construct the heterostructure by introducing metal and metal oxides on the surface area. The bandgap of heterojunction is reduced because numerous active sites which supplied by 2-D morphology of g-C₃N₄ incorporated metal and metal oxides. The light absorption of designed g-C₃N₄ heterojunctions is improved. The separation of photo-generated electron-hole pair is enhanced in the process of photocatalysis.

In this work, we integrated Ag₂O and CuFe₂O₄ with g-C₃N₄ to form photocatalyst Ag₂O@CuFe₂O₄@g-C₃N₄. Subsequently, the Ag₂O@CuFe₂O₄@g-C₃N₄system under visible light was constructed to degrade organic dyes. The degradation performance of Ag₂O@CuFe₂O₄@g-C₃N₄ on organic dyes solution was studied. Ag₂O@CuFe₂O₄@g-C₃N₄was then used to investigate the effect of the CuFe₂O₄ and Ag₂O content on the efficacy of organic dyes. Results indicated that combining Ag₂O and CuFe₂O₄with g-C₃N₄ resulted in a system that compensated for their individual shortcomings. The highest organic dyes oxidation efficiency was 100%, twofold that of g-C₃N₄. The effect of active substance in Ag₂O@CuFe₂O₄@g-C₃N₄ system was certified to postulate the degradation mechanism.

1.1. Chemicals

Analytical grade urea, melamine, ferric chloride, cupric acetate, silver nitrate, sodium citrate, citric acid, methylene blue, hydrogen peroxide and ethylene glycol are purchased from sinopharm chemical reagent Co., Ltd. The deionized water is self made in laboratory.

1.2. Synthesis of samples

1.2.1synthesis of g-C₃N₄ sample

Graphitic carbon nitride (g-C₃N₄) were synthesized through pyrolysis from mixture of urea and melamine[10-11]. The mixture of urea and melamine(20 g ,weight ratio:1:1) were ground to powder. The white powder was used high temperature resistance crucible with muffle furnace uniformly heated maintenance 550 °C in 2°C/min for 4 h. Finally, the samples were cooled down to 25℃. The yellow bulk samples were ground to powder for further characterization and application.

1.2.2 synthesis of Ag₂O@CuFe₂O₄@g-C₃N₄ sample

The samples Ag₂O@CuFe₂O₄@g-C₃N₄ were synthesized by hydrothermal method[11,12,16,17]. Firstly, 0.5g g-C₃N₄ powder was dispersed to ethylene glycol(60ml) under sonication and continuously stirring process to form solution A . Secondly, at darkroom, different quality ferric chloride, cupric acetate and silver nitrate(total weight g-C₃N₄ 0, 1, 2, 3 and 4%; ferric chloride:cupric acetate:silver nitrate in mole ratio 2:1:0.5) was dissolved in deionized water(25ml) under stirring 1h to form solution B. The mixture(3g,weight ratio:1:1) composited by urea and sodium citrate were added to solution A. After stirred 30minutes, the solution B was dropwise add to solution A and continuously agitated for 60minutes. Finally, the primrose mixture was shifted into a PTFE bladder with 100 mL capacity, and then loaded to Teflon-lined stainless steel autoclave and heated to 180 °C for 12 h. After chemical reaction, the suspension was centrifuged with 7500rpm/min in 10minutes. The precipitate samples were washed three times by water and ethanol. The samples were transferred into vacuum oven and dried at 60 °C for 12h. The feeding ratio of Ag₂O@CuFe₂O₄@g-C₃N₄ composites as list in Table 1.

Table 1 The feeding ratio of Ag₂O@CuFe₂O₄@g-C₃N₄ composites
	g-C₃N₄/g	FeCl₃/g	Cu(AC)₂/g	AgNO₃/g	Sodium citrate and Urea/g	Water/ml	Ethylene Glycol/ml
Ag₂O@g-C₃N₄	0.50	0.00	0.00	0.005	1.00	25	60
CuFe₂O₄@g-C₃N₄	0.50	0.00	0.005	0.00	1.00	25	60
Ag₂O@CuFe₂O₄@g-C₃N₄ -0	0.50	0.00	0.00	0.00	1.00	25	60
Ag₂O@CuFe₂O₄@g-C₃N₄ -1	0.50	0.0028	0.0014	0.0008	1.00	25	60
Ag₂O@CuFe₂O₄@g-C₃N₄ -2	0.50	0.0056	0.0028	0.0016	1.00	25	60
Ag₂O@CuFe₂O₄@g-C₃N₄ -3	0.50	0.0086	0.0043	0.0021	1.00	25	60
Ag₂O@CuFe₂O₄@g-C₃N₄ -4	0.50	0.0115	0.0056	0.0029	1.00	25	60

1.3. Characterization

The phase structures of samples g-C₃N₄ and Ag₂O@CuFe₂O₄@g-C₃N₄ were confirmed by X-ray diffraction (XRD). The source of X-ray diffractometer(Rigaku Ultima IV) is Cu Kα1 which the wavelength is 0.15418nm. The scale 2θ angles of the scanning is from 5 to 90° with speed of 0.02°s^-1. The morphology and microstructures of the as-prepared samples were examined by a transmission electron microscope (TEM). The chemical valence of the element in the composites was confirmed by X-ray photoelectron spectroscopy (XPS). The source of the XPS is a K-Alpha which is constructed by Al Kα in energy 1486.68 eV. The changed concentration of methylene blue (MB) were characterized according light absorbance intensity at 664 nm by a PerkinElmer LAMBDA750 spectrophotometer.

1.4. Degradation performance

The activity property of Ag₂O@CuFe₂O₄@g-C₃N₄ was characterized according the decomposition of the wastewater which was composited by MB and freshwater. The mixture which were constructed by wastewater and photocatalysts was irradiated under visible-light at ambient temperature. The visible-light was conducted by a 300 W xenon arc lamp. Under darkroom, 0.05g Ag₂O@CuFe₂O₄@g-C₃N₄photocatalysts were distributed in 100 mL wastewater which is composited by methylene blue and water in concentration 30mg/L. The catalysis system was keep mechanical agitated for 60minutes to achieve adsorption of MB in the darkroom. To determined the MB concentration, 3ml tested liquid was drunk every 10 minutes during the photodegradation tests. For lower MB dense, the kinetics of photodegradation can be expressed as

2.1. Structure and morphological analysis

2.1.1 XRD analysis

The crystalline quality of the prepared samples g-C₃N₄，Ag₂O@g-C₃N₄, CuFe₂O₄@g-C₃N₄ and Ag₂O@CuFe₂O₄@g-C₃N₄ was characterized by XRD as illustrated in Fig. 1. The black curve is the diffraction of g-C₃N₄. The 2θ angles in 13.3° and 27.8° are belong to (100) and (002) diffraction planes compared with the standard card(JCPDS card No. 87-1526) in Fig. 1(a)[8,11,12]. In heterojunction photocatalyst CuFe₂O₄@g-C₃N₄，the diffraction angles of 2θ located at 24.18, 33.08, 35.9, 40.9, 43.64, 49.68, 54.22, 57.68, 62.46, 64.22, 71.98 and 75.64◦is ascribed to the crystal planes of (012), (104), (110), (113), (202), (024), (116),(122), (214), (300), (1010) and (220), respectively^{[11-13,16-17]}. The characteristic peak suggest that CuFe₂O₄ascribe to tetragonal spinel structure and were good agreement with the standard value(JCPDS #34-0425) in Fig. 1(b) . In photocatalyst Ag₂O@g-C₃N₄，the diffraction peaks of 2θ located at 38.0, 44.2, 64.5 and 77.3◦is belongs to the orientation planes of (200), (220), (311) and(222) respectively[8-10]. The results suggest that Ag₂O ascribe to cubic structure with the standard value(JCPDS #41-1104) in Fig. 1(c). As for Ag₂O@CuFe₂O₄@g-C₃N₄ samples, characteristic peaks of g-C₃N₄ were discovered due to its crystallization in Fig. 1(d). The impurity peaks was not detected in samples Ag₂O@CuFe₂O₄@g-C₃N₄. The results illuminated that the structure of g-C₃N₄ was not destroyed by the doped Ag₂O and CuFe₂O₄.

The heterojunction photocatalystswere well synthesized and crystallized in nanomaterials. The size of nanoparticles which loaded on the surface of the g-C₃N₄ nanosheets were calculated according the Scherrer equation:

where the parameter D, k, λ, β, and θ are defined as particle size, Scherrer constant,X-ray wavelength, Full Width at Half Maximum and angle at Maximum,respectively. The size of the Ag₂O were calculated as 51.88nm, 35.30 and 46.39nm according the characteristic peak which located at the crystal planes of (200), (220) and (331) in the diffraction curves. As a results, the average particle size of the Cu₂O is calculated as 44.52nm. The size of the CuFe₂O₄ were calculated as 51.15nm, 45.92nm, 46.65nm, 38.76nm and 49.07nm according the characteristic peak which located at the crystal planes of (104), (110), (113), (024) and (116)in the diffraction curves. As a results, the average particle size of the Cu₂O is calculated as 46.31nm. The size of the Ag₂O and CuFe₂O₄ were calculated as 51.45nm, 51.49nm, 47.54nm, 48.13nm and 51.56nm according the characteristic peak which located at the crystal planes of (104), (110), (202), (024) and (331) in the diffraction curves. As a results, the average particle size of the Ag₂O and CuFe₂O₄ is calculated as 50.03nm.

2.1.2 SEM/TEM analysis

The morphologies and microstructures of the pure g-C₃N₄, Ag₂O@g-C₃N_4,CuFe₂O₄@g-C₃N₄and Ag₂O@CuFe₂O₄@g-C₃N₄heterojunction photocatalyst were investigated by TEM as shown in Fig. 2. In TEM image of pure g-C₃N4, it seems to be wrinkle and curl consists of nanosheets with mesopore in Fig. 2(a). Ag₂O independent nanoparticles which were point out in black dots were uniformly located on the g-C₃N₄ surface in the Ag₂O@g-C₃N₄ sample as shown in Fig. 2(b). The CuFe₂O₄ individual particles were uniformly located on the g-C₃N₄ interface in the CuFe₂O₄@g-C₃N₄ sample as shown in Fig. 2(c). In the heterojunction Ag₂O@CuFe₂O₄@g-C₃N₄ photocatalyst, the particles Ag₂O and CuFe₂O₄ were loaded on the layer of g-C₃N₄. Ag₂O and CuFe₂O₄particles which coupled with g-C₃N₄in heterojunction ware formed in cubic structure. The diameter of the particles Ag₂O and CuFe₂O₄have been detected between 30 to 120nm, which is consistent with the results of XRD detection.

2.2. Chemical state analysis

The surface composition of Ag₂O@CuFe₂O₄@g-C₃N₄ heterostructures was detected by X-ray photoelectron spectroscopy (XPS). The full XPS spectrum of Ag₂O@CuFe₂O₄@g-C₃N₄ heterostructures that shows the characteristic peaks were ascribed to Cu, Fe, Ag, N, O and C elements in Fig. 3(a), which further confirms the incorporation of Ag₂O and CuFe₂O₄ into g-C₃N₄[5]. The corresponding high-resolution spectra of Cu 2p, Fe 2p, Ag3d ,C 1s, N 1s, and O 1s are given. The valence state of Cu element was distinguished in the form of Cu¹⁺ or Cu²⁺ by scanning of Cu2p, illustrated in Fig. 3(b). The characteristic peaks in the binding energy 932.4eV and 952.2eV are ascribed to Cu 2p_3/2 and Cu 2p_1/2 in Ag₂O@CuFe₂O₄@g-C₃N₄compound (Fig. 3b). The Cu 2p_3/2 peak can be divided into two peaks at 932.3 eV and 934.6eV, which may be belonged to Cu⁰ and Cu²⁺, respectively. The two peaks suggest the CuO and Cu are coupled in Ag₂O@CuFe₂O₄@g-C₃N₄compound. In Fe 2p scanning XPS spectrum, the characteristic peaks of Fe 2p_1/2 and Fe 2p_3/2 were detected for oxidized Fe species in Fig. 3c. The characteristic peak at 724.4 eV is ascribed to the binding energies of Fe2p_1/2 for Fe³⁺and Fe²⁺ ions. The peaks located at 722.7 eV is belonged to the binding energies of Fe2p_1/2 for Fe²⁺ ion. The characteristic peaks at 712.3 and 710.8 eV is ascribed to the Fe 2p_3/2 of the Fe³⁺and Fe²⁺ ions in Ag₂O@CuFe₂O₄@g-C₃N₄compound, respectively. The characteristic peaks of Ag 3d_5/2 and Ag 3d_3/2 are belonged to 368.4 and 374.4 eV. The characteristic doublet peaks are approaching the binding energy values of Ag⁺ to Ag₂O in Fig.3d. The scanning N 1s XPS spectrum can be divided into two obvious peaks at 398.6 and 400.1eV in Fig. 3e. The doublet peaks can be assigned to C=N-C and N-C₃group, respectively. In Fig.3f, two characteristic peaks of O 1s located at 530.2 and 531.9 eV, which are ascribed to the O^2- in Ag₂O@CuFe₂O₄@g-C₃N₄and surface absorbed hydroxyl group, respectively. The C 1s peak located at 288.5 eV is belonged to N–C=N and located at 284.6 eV can be assigned the amorphous carbon on the surface of Ag₂O@CuFe₂O₄@g-C₃N₄as shown in Fig.3g. As discussed in XPS curves and TEM photographs, the Ag, Fe and Cu element was successfully loaded on the g-C₃N₄nanosheets as Ag⁺, Fe²⁺, Fe³⁺, Cu⁰and Cu⁺in form of Ag₂O, FeO, Fe₂O₃, Cu⁰andCuO. The synthesized Ag₂O@CuFe₂O₄@g-C₃N₄was determined that its were constructed in heterostructure.

2.3 Photocatalytic activity[8]

The photocatalytic activities of the as-synthesized Ag₂O@CuFe₂O₄@g-C₃N₄photocatalysts was characterized by the degradation of wastewater which is aqueous solution of MB (30 mg / L) irradiated by visible lights. The visible light irradiation time was carried from 0 to 50 minutes and 3ml tested solution was sampled every 5 minutes during the photodegradation tests. The photo-degradation experiment was implemented in the photographic laboratory at room temperature. The density of the MB in the wastewater was determined by the change of the character peak 664 nm with increasing of the irradiation time as shown in Fig 4(a). For comparison, the activities of pure g-C₃N₄, Ag₂O@g-C₃N₄, CuFe₂O₄@g-C₃N₄and Ag₂O@CuFe₂O₄@g-C₃N₄ were also tested under the same conditions shown in Fig 4(b). The photo-degradation efficiency of MB is 82.41%, 71.56%, 47.55%, 95.17%, 93.65%, 99.94% and 99.91% belong to Ag₂O@g-C₃N₄, CuFe₂O₄@g-C₃N_4,Ag₂O@CuFe₂O₄@g-C₃N₄-0%, Ag₂O@CuFe₂O₄@g-C₃N₄-1%, Ag₂O@CuFe₂O₄@g-C₃N₄-2%, Ag₂O@CuFe₂O₄@g-C₃N₄-3% and Ag₂O@CuFe₂O₄@g-C₃N₄-4% irradiated by visible lights in 35minutes, respectively. The blank experiment indicates that the direct photolysis of MB is almost ignored in the absence of photocatalysts and the degradation of MB is resulted from the photocatalytic reaction. It can be clearly observed that the degradation of MB is increased compare Ag₂O@g-C₃N₄ and CuFe₂O₄@g-C₃N₄with pure g-C₃N₄because of the formed heterojunction. It can be obviously observed that when the weight ratio of Ag₂O@CuFe₂O₄ and g-C₃N₄ is higher than 0%, the as-synthesized photocatalysts exhibit enhancer photocatalytic activities than that of the bare g-C₃N₄, Ag₂O@g-C₃N₄ and CuFe₂O₄@g-C₃N₄. The reason is explained by the formed heterojunction between Ag₂O@CuFe₂O₄andg-C₃N₄. Besides, the enhanced dispersibility and the lowered particle size of Ag₂O and CuFe₂O₄ of the as-synthesized photocatalysts also act important factor to increase photocatalytic activity. About the Ag₂O@CuFe₂O₄@g-C₃N₄ photocatalysts, the photocatalytic activities is increased slowly with increasing the weight ratios of Ag₂O@ CuFe₂O₄ing-C₃N₄ from 0 to 3% and then decreased at 4%. When the weight ratio of Ag₂O@CuFe₂O₄andg-C₃N₄ is 3%, the photocatalyst represents the maximum of activity, which overs 2.10 times than pure g-C₃N₄, 1.21 times than Ag₂O@g-C₃N₄ and 1.40 times than CuFe₂O₄@g-C₃N₄. In this degradation system, nearly 50% of MB has been degraded during initial 5-7 min, and 90% of MB can be decomposed after 20-17 min. This conclusion can be clearly shown in Fig. 4. With extended visible light irradiation, the absorption intensity of the peak at 664 nm, which is represented to the characteristic absorption of MB molecule, decreases gradually and then undiscovered disappears after 25-30 min irradiated. When the weight ratio of Ag₂O@ CuFe₂O₄ and g-C₃N₄ is 4%, the slightly decreased photocatalytic activity was discovered. The reason was explained that the higher content of Ag₂O@CuFe₂O₄ resulted in the agglomerated larger Ag₂O@ CuFe₂O₄ particles causing a low distribution on the surface of g-C₃N₄. The larger Ag₂O@ CuFe₂O₄ particles limit the transfer of photogenerated charge carriers and then the recombination efficiency of the photoinduced electron−hole pairs were enhanced. As discussed above, the as-synthesized Ag₂O@CuFe₂O₄@g-C₃N₄ photocatalysts with mass ratio 3% was regard as an effective photocatalyst.

2.4 Stability experiment

The stability and recycling performance of Ag₂O@CuFe₂O₄@g-C₃N₄ samples were carried out with the same catalytic condition. The conclusions declare that among ten continuous operate in 35 minutes and no severe inactivation can be discovered as shown in Fig 5. The constructed heterostructure catalysts showed high stability for practical application in the wastewater treatment. Hence, it is proposed that the flexible Ag₂O@CuFe₂O₄@g-C₃N₄ heterostructure advantages for degrading toxic pollutants in wastewater. The photocatalytic activity of Ag₂O@CuFe₂O₄@g-C₃N₄was not sacrificed in a large-scale process for degrading organic pollutants by stably recycling used.

2.5 Active substances

The active substances were produced in the process of the visible light irradiation. The active substances were composited by photogenerated h⁺, ·OH and ·O^2-, etc. The MB was degraded by the hydroxyl radical (·OH) which is produced by Ag₂O@CuFe₂O₄@g-C₃N₄under the visible light irradiated. To illustrate the effect of hydroxyl radical (·OH) on the MB, the system which was composited by Ag₂O@CuFe₂O₄@g-C₃N₄, MB and tert butyl alcohol was irradiated by the visible light. The tert butyl alcohol is trapping agent for hydroxyl radical (·OH). The degradation rate was decreased in the system which contained tert butyl alcohol systems. As shown in Fig. 6, the curves of degraded MB in two systems Ag₂O@CuFe₂O₄@g-C₃N₄andAg₂O@CuFe₂O₄@g-C₃N₄+tert-butanol were illustrated in the irradiating time 50 minutes.The degradation rate was decreased from 99.93% to 26.68% for containing tert butyl alcohol during 25minuties. The results show that the hydroxyl radical (·OH) was trapped by tert butyl alcohol and the MB was not completely degraded for inadequate hydroxyl radical (·OH) to oxidizing the organic dyes. During 50minutes irradiating, the degradation rate of MB is creased with the irradiated time increasing in the Ag₂O@CuFe₂O₄@g-C₃N₄+tert-butanol system. As a results, the hydroxyl radical (·OH) is predominantly active substances in the degradation system.

2.6 Possible Photocatalytic Mechanism

As discussed above, a probable photocatalytic mechanism of Ag₂O@CuFe₂O₄@g-C₃N₄photocatalysts were illustrated when the photocatalysts were irradiated by visible lights[4,5,8]. As every one knows, the nanoparticles of Ag₂O, CuFe₂O₄and g-C₃N₄are defined as typical p-type, n-type and n-type semiconductor, respectively. The conduction band is located at -1.21eV, -0.43eV and 0.15eV for g-C₃N₄, ZnO andAg₂O, respectively. The conduction band of Ag₂Ois lesser to g-C₃N₄and CuFe₂O₄. The electrons-holes pair was generated in CuFe₂O₄, Ag₂O and g-C₃N₄excited under visible light. In the Ag₂O@CuFe₂O₄@g-C₃N₄ photocatalysts, the photoexcited electrons will have a trend to transfer from conductor band of CuFe₂O₄andg-C₃N₄ to Ag₂O. The valence band is located at 0.21eV, 2.46eV and 1.35eV for CuFe₂O₄,g-C₃N₄andAg₂O, respectively. The valence band of g-C₃N₄is larger than CuFe₂O₄ and Ag₂O. The holes have an negative transfer direction because of the inner electric field conducted in the p-n junctions and transferred to valence band of CuFe₂O₄and Ag₂O. The excited electron-hole pair was separated because of inner electric field and energy band structure. The transferred electrons between valence band of g-C₃N₄, CuFe₂O₄ andAg₂O is partly restricted. The motion of holes can be accelerated between the valence band of g-C₃N₄, CuFe₂O₄ andAg₂O. The recombination rate of the excited electron-hole pair was reduced for rapidly migration of charges for the p-n junctions. The photocatalytic activity is enhanced because of rapidly migration of the excited electron-hole. Thus, the charge carrier separation at Ag₂O@CuFe₂O₄@g-C₃N₄ heterostructure was rapidly separated and transferred. At the same time, the recombination rate of the excited electron-hole was diminished. The reactive species which were composited by •OH and •O²⁻ radicals were generated with high concentration in degradation system. Additionally, it is important to distinguish which active species is effect on the MB dye under sunlight irradiation. The active species is determined for •OH radicals in the degradation system, as discussed in part 2.5 Active substances and Fig.6. The charge carrier separation and active species production were illustrated in Fig.7. The reaction equation were shown by equation (1) to (5). As shown in Fig.7, the degradation reaction were took place in two different region which was divided to photo-Fenton reaction(Ⅰ) and advanced oxidation processes(Ⅱ). This result solemnly declared that the synergistic effects of generating •OH radicals, Fenton reagent and reduced charge recombination efficiency at Ag₂O@CuFe₂O₄@g-C₃N₄ heterojunction are suitable for enhancing degradation of organic pollutants.

The composites of Ag₂O@CuFe₂O₄@g-C₃N₄ heterojunction was successfully synthesized by a facile one pot hydrothermal reaction approach for degrading toxic organic pollutants in wastewater. The photocatalytic activity was enhanced with uniform dispersing of CuFe₂O₄ and Ag₂O on nanosheets in g-C₃N₄materials. The photoexcited electron hole pair was efficiently separated because of Ag₂O@CuFe₂O₄@g-C₃N₄ heterostructure which was composited by different bandgap architectures. The reactive radical species is hydroxyl radical (·OH) which was generated in Ag₂O@CuFe₂O₄@g-C₃N₄ heterostructures during the visible lights irradiating. The photocatalytic activity of Ag₂O@CuFe₂O₄@g-C₃N₄was not sacrificed in a large-scale process for degrading organic pollutants by stably recycling used. In this heterogeneous composite, CuFe₂O₄ and Ag₂O acts as a hole trap during the photocatalytic reaction. This work demonstrates a new costeffective approach to construct heterogeneous composite catalyst of CuFe₂O₄ and Ag₂O with g-C₃N₄ for degradation of organic pollution under visible light, and the synergetic catalysis effect of the composite may provide a new route to design photocatalysts for enhancer solar energy utilization.

Availability of data and materials

The datasets supporting the conclusions of this article are included within the article and its additional files.

Funding The authors were financially supported by Natural Science Foundation of Liaoning Province(Nos.2020-Ms-306).

Declaration of competing interest

The authors declare that they have no known competing for financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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No competing interests reported.

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Design and Fabrication of Ag 2 O@CuFe 2 O 4 @g-C 3 N 4 Photocatalysts for the Degradation of Organic Dyes Under Visible Light

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