Synthesis of Cu₃P/SnO₂ composites for degradation of tetracycline hydrochloride in wastewater

Abstract

Antibiotic drugs have become dominating organic pollutants in water resources, and efficient removal of antibiotic drugs is the priority task to protect the water environment. Cu₃P/SnO₂ photocatalysts of various Cu₃P loadings (10–40 wt% Cu₃P) were synthesized using a combination of hydrothermal synthesis and a partial annealing method. Their photocatalytic activity was tested for tetracycline hydrochloride (TC-HCl) degradation under visible light irradiation. Cu₃P/SnO₂ samples were characterized by X-ray diffraction (XRD), N₂-adsorption, ultraviolet-visible diffuse reflectance spectra (UV-vis DRS), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The results showed that the p–n type heterostructure between Cu₃P and SnO₂ was successfully constructed, and addition of Cu₃P to SnO₂ could improve its photocatalytic activity at an optimized loading of 30 wt% Cu₃P. In photocatalytic degradation studies, removal rates of around 80% were found in 30 minutes of dark reaction and 140 min of photodegradation. The removal rate was superior to that of Cu₃P and SnO₂ alone under the same experimental conditions. According to trapping experiments and electron spin resonance (ESR) measurements, photogenerated holes (h⁺) and superoxide radicals ˙O₂⁻ were considered as the main oxidation species in the present system. Finally, the reuse experiments showed high stability of Cu₃P/SnO₂. This study reports Cu₃P as a cocatalyst combined with semiconductor SnO₂ to form a highly efficient heterogeneous photocatalyst for the degradation of tetracycline hydrochloride for the first time.

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1. Introduction

Recently, antibiotics have been widely misused in medical treatment as antimicrobial drugs. Due to their coupling with the food chain, they bioaccumulate at lower and higher nutrient levels, resulting in a long-term negative impact on the environment.^1,2 Tetracycline hydrochloride (TC-HCl) is a broad-spectrum antibiotic of tetracycline, which is widely used medically to prevent bacterial diseases of livestock, poultry and aquatic products.³ Its worldwide extensive misuse accelerates it accumulation in the environment.⁴ Because of its obvious mutagenic and teratogenic effects, it has drawn extensive attention.^5,6

Therefore, it is particularly important to remove TC-HCl in wastewater. Compared with advanced oxidation methods, biological treatment methods, membrane filtration methods and adsorption methods,^7–12 the photocatalytic degradation method is commonly used in the treatment of refractory organic wastewater due to its advantages of high efficiency, low energy consumption, and green environmental protection.¹³ The photocatalyst generates free radicals under light conditions and effectively decomposes organic chemicals in wastewater.^14–17 Many semiconductor photocatalysts have been developed for decades. For example, Shen et al. prepared hierarchical carbon nitride photocatalyst,¹⁸ Song et al. constructed KCl/NH₄Cl/g-C₃N₄ composite photocatalyst,¹⁹ Mao et al. prepared Bi₂WO₆/CuS composite photocatalyst,²⁰ these catalysts have been used to degrade TC-HCl, and have achieved good degradation effects.

From the reports of photocatalytic materials in last decades, SnO₂ as a photocatalyst has drawn extensive attention due to its excellent optical properties,²¹ electrical properties,²² and stable gas–sensitive properties.²³ As an n-type semiconductor, SnO₂ has a quite wide band gap of about 3.6 eV,²⁴ which can only be excited by ultraviolet light with a wavelength of less than 400 nm, and its absorption efficiency is low under sunlight.²⁵ The recombination rate of photogenerated electron–hole is high, which restricts its direct application in photocatalysis. Most researches of SnO₂ as a photocatalytic material involved coupling SnO₂ with other narrow-band semiconductors, or adding dopants to expand the light absorption to the visible light range.^26,27 Therefore, there were multiple researches reported the photocatalytic properties of SnO₂ doped metallic elements such as cobalt-doped SnO₂ nanoparticles, Au/SnO₂ composites and Ag/SnO₂ composites^28–30 or non-metallic elements such as carbon-doped SnO₂ nanostructures and Cl-doped SnO₂ photocatalysts.^31,32 Some publications reported SnO₂ coupled with other narrow-band semiconductors such as AgBr/SnO₂ composites and Co₃O₄/SnO₂ composites,^24,33,34 and SnO₂ coupled with wideband semiconductors such as TiO₂ (ref. 35) or ZnO.^36,37

Recently, studies have found that materials such as transition metal phosphides (MP) and their composites have excellent catalytic performance for hydrogen evolution reactions. For example, Gai et al. used Ni₂P³⁸ and Cao et al. used CoP³⁹ to improve the hydrogen evolution efficiency of CdS. In addition, Sun et al. prepared a Cu₃P/g-C₃N₄ heterostructure⁴⁰ for photocatalytic hydrogen production. As a p-type semiconductor, Cu₃P has a band gap of about 1.5 eV.⁴¹ If the p-type semiconductor is combined with the n-type semiconductor, an internal electric field is generated in the p–n junction area, which can efficiently facilitate the separation of photogenerated electrons and holes.⁴² Ioannidi et al. synthesized Cu₃P/BiVO₄ composite materials⁴³ to degrade sulfamethoxazole in aqueous media as similar study. It is worth noting that, Cu₃P-based composite materials have not been used as a photocatalyst to degrade TC-HCl yet based on literature study. This study filled the gap into the area.

The purpose of this work is to synthesize Cu₃P/SnO₂ heterostructure with different Cu₃P loading in order to explore its catalysis and characteristics as a heterogeneous photocatalytic composite material in area of TC-HCl decomposition. We evaluated the photocatalytic activity of Cu₃P/SnO₂ sample for degradation of TC-HCl under visible light irradiation. Finally, the photocatalytic activity results of Cu₃P/SnO₂ composite material were compared with that of the self-made parent material of Cu₃P or SnO₂ under the same experimental conditions. The results may provide an alternative method for preparation of photocatalysts, under the case of wide band gap SnO₂ semiconductors used.

2. Experiment section

2.1. Photocatalyst preparation

2.1.1. Synthesis of SnO₂ hollow microspheres. Hollow SnO₂ microspheres were prepared by hydrothermal method.⁴⁴ SnCl₂·2H₂O of 0.4890 g and a certain amount of Na₃C₆H₅O₇·2H₂O were dissolved in distilled water of 12.5 mL at different molar ratios of 1 : x (x from 3 to 6) to generate homogeneous solution A. NaOH of 0.30 g was dissolved in 12.5 mL distilled water to generate homogeneous solution B. Solution B was slowly added to solution A, with the mixture continuously stirred for 24 hours. The transparent solution stirred was transferred to a 50 mL reaction kettle and placed in an oven at 180 °C for 12 hours. After cooling of the reaction kettle, the white precipitate at the bottom was alternately washed with ethanol and distilled water repeatedly. The collected samples were dried overnight in an oven at 60 °C to produce white SnO₂ sample. The SnO₂ samples prepared with different ratios of raw materials were labeled as SnO₂-1 : 3, SnO₂-1 : 4, SnO₂-1 : 5 and SnO₂-1 : 6 respectively.

2.1.2. Synthesis of Cu₃P. The 0.189 g (0.003 mol) copper powder and 0.3097 g (0.01 mol) red phosphorus were thoroughly ground evenly with an agate mortar. The mixed powder was collected in a beaker, and added with 80 mL deionized water. After the solution was continuously stirred for 0.5 h, the solution was transferred to a 100 mL reaction kettle and kept in an oven at 200 °C for 24 h. After cooling of the reaction kettle, the precipitate were centrifugally washed with carbon disulfide (CS₂), ethanol and deionized water one by one, and the resulting products were dried overnight in the oven at 60 °C.

2.1.3. Synthesis of Cu₃P/SnO₂ heterostructure. 0.03 g Cu₃P and 0.30 g SnO₂ were sufficiently ground, and the mixture was transferred to 30 mL deionized water containing 0.25 g polyvinyl pyrrolidone (PVP). After stirring with ultrasonic for 24 hours, the precipitate was rinsed for many times with deionized water. Then it was dried, ground, and put into an atmosphere tube furnace at 300 °C first, and then annealed for 3 hours by introducing N₂ to produce Cu₃P/SnO₂ sample with content of 10%. In addition, by varying the loading of Cu₃P, the Cu₃P/SnO₂ heterostructure composite catalysts of 20%, 30% and 40% wt load were synthesized by the same preparation method, labeled as Cu₃P/SnO₂-1, Cu₃P/SnO₂-2, Cu₃P/SnO₂-3 and Cu₃P/SnO₂-4 respectively.

2.2. Characterization methods

The characterization of samples was carried out to understand their structure, surface, morphology, optical and photocatalytic properties. X-ray diffraction XRD was applied to analyze the phase composition of the prepared samples, with D/MAX-2500 diffractometer (Bruker test in Germany). The morphological characteristics were studied by scanning electron microscope (SEM), from Hitachi (S-4800). Ultraviolet-visible diffuse reflectance spectrometer of Japan Shimadzu Company's UV 2600 model was used to analyze the absorption properties of the prepared samples with BaSO₄ as a reference in the wavelength range of 250 nm to 800 nm. The N₂ adsorption and desorption isotherm test uses the Quantachrome company (NOVA 2200e) pore size analyzer. The specific surface area and pore size characteristics of the sample are calculated using Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) theoretical models, respectively.

2.3. Electrochemical test

The electrochemical impedance spectroscopy (EIS) test was performed on the electrochemical workstation of Shanghai Chenghua Instrument Co., Ltd. (CHI660E), China. Calomel electrode and platinum electrode were used for reference electrode and auxiliary electrode respectively. 0.5 M H₂SO₄ aqueous solution is used as the electrolyte, and copper electrode sample is used as the working electrode.

The electrode sample preparation process is as follows: 0.16 g polyvinylidene fluoride (PVDF) binder were mixed with 0.02 g catalyst sample and 0.02 g carbon black into a small glass jar. Then we add 500 μL N-methyl pyrrolidone to the mixture, and stir. After a few hours, the paste liquid is consistently coated on a copper sheet of 10 × 20 mm in size, and the other side of the copper sheet is pasted with insulating glue. The specific process of the electrochemical workstation was as follows: the three-electrode system and the electrochemical workstation were connected, then the open circuit potential of the electrode was tested, and later the initial voltage is set according to the open circuit potential. The SnO₂ open circuit potential tested in this experiment is 0.006 V. The measured open circuit potentials of Cu₃P/SnO₂-1, 2, 3 and 4 samples are 0.010 V, 0.049 V, 0.055 V, and 0.058 V in sequence. The initial voltage E was set according to open circuit potential value of tested sample, with other parameters remain unchanged.

2.4. Photocatalytic activity experiments

The photocatalytic activity of the samples was systematically evaluated by visible light-driven degradation of TC-HCl. The photocatalytic experiments were performed under a 1000 W Xenon lamp at constant temperature 25 °C. The photocatalytic degradation activity of Cu₃P/SnO₂-1–4 were evaluated. First, 50 mg L⁻¹ TC-HCl solution was prepared, with ultrasonic treatment for 20 minutes to ensure uniform dispersion of TC-HCl molecules. Photocatalyst Cu₃P/SnO₂ (50 mg) was uniformly dispersed in 40 mL TC-HCl solution. Then, stirring was continued for 30 minutes in the dark to make the TC and Cu₃P/SnO₂ sample reach the equilibrium of adsorption and desorption. After turning on the light source, 3 mL sample was taken every 20 minutes, and diluted with 3 mL distilled water. The diluted sample is centrifuged twice at 10 000 rpm to remove the sediment. The concentration of supernatant was determined by an UV-visible spectrophotometer at 360 nm, which is the typical absorption wavelength of TC-HCl. The photocatalytic process took for 140 minutes for several samples.

3. Result and discussion

3.1. Characterization analysis

SEM images (Fig. 1) were collected to observe the morphologies of SnO₂. The SEM of SnO₂ samples synthesized by hydrothermal method at different ration from 1 : 3 to 1 : 6 were displayed in Fig. 1a–d. The results showed that the SnO₂-1 : 4 sample (Fig. 1b) exhibited the best morphology, consistent with the conclusions in previous work⁴⁴ and the SnO₂-1 : 4 sample possessed good photocatalytic activity. Additionally, average diameter is about 1 μm of the SnO₂-1 : 4 sample, and each SnO₂ 1-X microsphere exhibit a hollow structure with pore sizes ranging from 200 nm to 500 nm. The morphology of SnO₂-1 : 5 samples (Fig. 1c) showed a hollow structure to some extent with different sizes. For the other two samples of SnO₂-1 : 3 (Fig. 1a) and SnO₂-1 : 6 (Fig. 1d), the hollow structure was obviously broken. The arrangement of SnO₂ was irregular with small porosity, and the specific surface area and additional active sites increased and turned conducive to photocatalytic degradation.

Fig. 1 SEM images of the prepared catalysts of SnO₂-1 : 3 (a), SnO₂-1 : 4 (b) hollow sphere, SnO₂-1 : 5 (c) hollow sphere, SnO₂-1 : 6 (d).

As shown in Fig. S1 in ESI,† Cu₃P is very small in size, similar in shape to cotton mass with a diameter of about 250 nm. Fig. 2a demonstrated the morphology of Cu₃P/SnO₂ composites, SnO₂ was a sphere with much larger size than that of Cu₃P, with a diameter of about 1 μm. Cu₃P nanoparticles were clustered on the surface of SnO₂ microspheres, with a solid–solid interface formed between the two chemicals. These results demonstrated that Cu₃P had been successfully installed on the surface of SnO₂.

Fig. 2 SEM images of Cu₃P/SnO₂ heterostructure (a) and XRD patterns of the prepared catalysts (b).

XRD patterns in Fig. 2b showed crystalline structures of the synthesized parent materials of Cu₃P and SnO₂, and the Cu₃P/SnO₂-3 heterostructure. From XRD pattern of SnO₂, the weaker diffraction peaks located at 37.949°, 54.757° and 71.276° which belonged to the (200), (220) and (202) lattice planes of SnO₂, while the stronger diffraction peaks located at 26.611°,33.893° and 51.780°, which is attributed to the (110), (101) and (211) lattice planes of SnO₂ respectively. XRD pattern of SnO₂ in Fig. 2 were keeping up with the standard card (JPDF no. 41-1445).

XRD pattern of Cu₃P in Fig. 2 was consistent with the standard card (JPDF no. 71-2261), and the diffraction peaks were at 36.005°, 39.078°, 41.577°, 45.091° and 46.158°, which was corresponded to (112), (202), (211), (300) and (113) lattice planes of Cu₃P, respectively. Besides, the purity of Cu₃P was very high since there is no extra impurity peak. It is worth noting that both diffraction peak of Cu₃P and diffraction peak of SnO₂ were plotted in the XRD diffraction pattern (Fig. 2) of the Cu₃P/SnO₂ heterostructure, indicating the successful combination of Cu₃P and SnO₂.

Compared with the XRD peaks on the (110), (101) and (211) lattice planes (Fig. 2) of SnO₂, the peak width of the Cu₃P/SnO₂ heterostructure became smaller, reflecting bigger average crystallite size. From Table 1, the average crystallite size of the (110), (101) and (211) lattice planes of SnO₂ was 97, 130 and 121 nm. The average crystallite size of Cu₃P and SnO₂ in the composite increased to some extent. The average crystallite size of the (110), (101) and (211) lattice planes of Cu₃P/SnO₂ heterostructure was to 143, 168 and 138 nm.

Table 1 Average crystallite size of (110), (101) and (211) lattice planes of sample

Sample	Average crystallite size of (110) lattice plane (nm)	Average crystallite size of (101) lattice plane (nm)	Average crystallite size of (211) lattice plane (nm)
SnO2	97	130	121
Cu3P/SnO2	143	138	138

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The pore size and BET specific surface area were determined by N₂ adsorption of the synthesized samples (Fig. 3). Fig. 3a showed the N₂ adsorption and desorption isotherms of SnO₂ and Cu₃P/SnO₂-1, 2, 3, and 4. According to the classification standard, the isotherm of the composite Cu₃P/SnO₂ at a relative pressure (p/p₀) of 0.5–1.0 belonged to the IV type isotherm, and a typical H₃ hysteresis loop. This verified that composite Cu₃P/SnO₂ heterostructure was a mesoporous material. Compared with pure SnO₂, the BET specific surface area of the heterostructure Cu₃P/SnO₂-3 increased significantly. According to the BET theoretical calculation model, Table 2 was categorized with the specific surface area of pure SnO₂ was 3.7892 m² g⁻¹, while the specific surface area of Cu₃P/SnO₂-3 was 53.1470 m² g⁻¹, which was about 14 times that of SnO₂. Fig. 3b showed the pore size distribution curves of Cu₃P/SnO₂ samples. The pore size of Cu₃P/SnO₂ heterostructure was distributed around 5–20 nm. Based on BJH theoretical calculation model, the pore volume of SnO₂ was 0.003904 cm³ g⁻¹, and the pore volume of Cu₃P/SnO₂-3 heterostructure was 0.046550 cm³ g⁻¹, with a tremendous increase of 12 times (Table 2). From Table 2, the specific surface area of Cu₃P/SnO₂-3 was the largest (53.1470 m² g⁻¹) while the average pore size was the smallest (3.8213 nm). The increase in the surface area were attributed to the surface modification of Cu₃P nanoclusters on SnO₂. The larger the specific surface area of the sample, the easier of the sample to expose active sites on the surface. As a photocatalyst, it increased the photocatalytic activity of the pollutant tetracycline (TC-HCl).

Fig. 3 Nitrogen adsorption and desorption isotherms (a) and pore diameter distribution curves (b) of SnO₂ and Cu₃P/SnO₂-1, 2, 3 and 4 composites.

Table 2 Specific surface area and pore parameters of different samples

Sample	Specific surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore size (nm)
SnO2	3.7892	0.003904	5.4221
Cu3P/SnO2-1	20.2940	0.023992	4.7529
Cu3P/SnO2-2	24.3244	0.032098	4.5883
Cu3P/SnO2-3	53.1470	0.046650	3.8213
Cu3P/SnO2-4	33.9863	0.039631	4.2267

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In order to study the modification effect of mesoporous Cu₃P nanoclusters on SnO₂ microspheres to improve its photocatalytic performance, UV-vis DRS was applied to analyze absorption properties of SnO₂, Cu₃P and Cu₃P/SnO₂-1–4 heterostructure. Fig. 4a showed the UV-vis absorption spectrum of SnO₂, Cu₃P and heterostructure Cu₃P/SnO₂-1, 2, 3, and 4 samples. Cu₃P has strong absorption strength in the whole wavelength range, especially in the visible light range. The SnO₂ has a strong absorption intensity in range of 200–360 nm (UV), while there was negligible absorption intensity above 400 nm (visible). This indicated that SnO₂ can only be excited by ultraviolet photons to undergo electronic transitions, without any optical response in the visible wavelength range. Compared with bare SnO₂, the absorption intensity of Cu₃P/SnO₂ heterostructure was quite different, with a higher absorption intensity in the visible range of 400–800 nm, which was related to the modification effect of mesoporous Cu₃P nanoclusters on the surface of SnO₂ nanosheets. This indicated a strong interaction between Cu₃P and SnO₂, which was beneficial to improving the migration efficiency of electron–hole pairs and inhibited their rapid recombination. Furthermore, the band gap values were estimated by Tauc plots and plotted in Fig. 4b. From the Fig. 4b the E_g for Cu₃P and SnO₂ samples are about 1.31 eV and 3.54 eV, respectively.

Fig. 4 Ultraviolet-visible absorption spectra of SnO₂, Cu₃P and heterostructure Cu₃P/SnO₂-1, 2, 3 and 4 samples (a); the gap width of SnO₂ and Cu₃P samples (b).

Fig. 5 showed the EIS measurements of pure SnO₂ and Cu₃P/SnO₂ samples. The arc size of the Nyquist plot of SnO₂ were large due to its large energy gap. In the EIS Nyquist diagram, the smaller the arc size, the smaller the charge transfer resistance and the higher the efficiency of electron–hole separation. Based on previous publications of SnO₂ materials,⁴⁴ different studies reported different results, from 20 to 1250 ohms.^45,46 From Fig. 5 the arc diameter of SnO₂ was about 120, reflecting the charge transfer resistance Rct of 120 ohms. The Rct of Cu₃P/SnO₂-3 sample was the smallest (40 ohms), followed by the order of Cu₃P/SnO₂-1 (80 ohm) > Cu₃P/SnO₂-2 (60 ohm) and Cu₃P/SnO₂-4 (50 ohm), respectively. The results indicated that the optimal Cu₃P loading is 30% wt. The tight solid–solid interface generated between the two semiconductors of Cu₃P and SnO₂ turned into a channel for charge transfer, which reduced the charge transfer resistance in interface and improved the migration rate of electric load current carrier.

Fig. 5 The Nyquist plots of SnO₂ and heterostructure Cu₃P/SnO₂-1, 2, 3 and 4 samples.

3.2. Adsorption and photocatalytic activity evaluation

From the photocatalytic experiment, the photocatalytic activity was explored of different samples under visible light radiation. Fig. 6a showed the dynamic curves of SnO₂, Cu₃P and composite material heterostructure Cu₃P/SnO₂-1, 2, 3 and 4 in the photocatalytic degradation of tetracycline. The samples underwent dark reaction adsorption for 30 minutes, and then was irradiated under visible light for 140 minutes. The results showed that after adsorption equilibrium was achieved in the dark, the maximum adsorption capacity was 10% of the heterostructure Cu₃P/SnO₂-3, and the adsorption capacity of SnO₂ sample was 5%, which indicated that the increase of physical adsorption capacity. Such difference was due to the morphological structure of Cu₃P/SnO₂ composite system increased their specific surface area.

Fig. 6 Photocatalytic degradation dynamic curves of SnO₂, Cu₃P and composite Cu₃P/SnO₂-1–4 under visible light radiation (a); first-order dynamics fitting curve(b); degradation rate constants of different samples (c); absorption spectra of tetracycline by Cu₃P/SnO₂-3 at different time points (d).

In the 140 minutes visible light irradiation stage, the degradation effect on tetracycline was obvious. The degradation effects were different of heterostructure materials Cu₃P/SnO₂-1, 2, 3 and 4 with different composite ratios. The degradation efficiency SnO₂ samples was only 35%, since SnO₂ cannot fully absorb visible light, and ultraviolet light accounts for a small proportion of visible light. When Cu₃P was introduced into the SnO₂ material and Cu₃P/SnO₂ was generated, the photocatalytic degradation effect was significantly enhanced. The photocatalytic degradation effect of Cu₃P/SnO₂-3 became the best among rest samples and the degradation efficiency reaches 80%. From Fig. 6a, the order of degradation efficiency was: Cu₃P/SnO₂-3 (80%) > Cu₃P/SnO₂-4 (70%) > Cu₃P/SnO₂-2 (64%) > Cu₃P/SnO₂-1 (53%) > Cu₃P(45%) > SnO₂ (34%), the higher the better.

Fig. 6b showed the kinetic curves of Fig. 6a, which fitted to the first-order kinetic model ln(C₀/C) = kt. The photo degradation process was first order reaction. Fig. 6c was a histogram of the rate constant K of degradation of tetracycline by SnO₂, Cu₃P and composite heterostructure Cu₃P/SnO₂-1, 2, 3 and 4. The rate constant k of pure SnO₂ and Cu₃P were 0.00228 and 0.00353 min⁻¹, while the rate constant k of the sample Cu₃P/SnO₂-3 was the maximum (k = 0.00940 min⁻¹), which was 4.1 times and 2.7 times than that of SnO₂ (k = 0.00228 min⁻¹) and Cu₃P (k = 0.00353 min⁻¹), indicating the Cu₃P/SnO₂ effectively enhanced the photocatalytic degradation efficiency. The rate constant k of the other samples Cu₃P/SnO₂-1, 2 and 4 were 0.00429, 0.00570 and 0.00697 min⁻¹, respectively. These results verified that the optimal composite percent was 30% wt of Cu₃P/SnO₂ composite, with the best photocatalytic performance for tetracycline. In addition, Fig. 6d showed the absorption spectra of Cu₃P/SnO₂-3 for tetracycline at different times. With the gradual extension time of visible light radiation, the characteristic absorption wavelength of 360 nm of tetracycline absorption showed a significant downward trend. This indirectly confirmed that with increased period of visible light radiation, tetracycline gradually was decomposed and opens the ring to breakdown into small molecules or ions.

Table 3 categorized several photocatalysts that had been reported in literature. The results showed that Cu₃P/SnO₂-3 contained 80% degradation rates, which was comparable with different reported catalysts, such as FeOOH/FeS₂ of 90%,⁴⁷ Ag/Bi₃TaO₇ of 85% (ref. 48) and Bi₁₂O₁₅C_l6/Bi₂WO₆ of 81%.⁴⁹ This work is better than BiFeO₃/TiO₂ of 72% (ref. 50) and In₂S₃/InVO₄ of 71%.⁵¹ Besides, the dosage (W_cat/TC) of Cu₃P/SnO₂-3 was 25 mg mg⁻¹, which was the minimum around the rest 5 types of catalysts around 50–100 mg mg⁻¹. The amount of catalysts Cu₃P/SnO₂-3 used in photocatalytic experiment was much less with less pollution and lower cost. This indicates that the catalyst is more favorable to treat the wastewater containing relatively high tetracycline.

Table 3 Photo degradation effects of TC among different catalysts

Photocatalysts	Catalyst dosage (mg L^−1)	TC concentration (mg L^−1)	Wcat/TC (mg mg^−1)	Dark (min)	Illumination (min)	Removal rate	Reference
FeOOH/FeS2	500	10	50	30	150	90%	Guo et al.^47
Ag/Bi3TaO7	1000	10	100	60	120	85%	Luo et al.^48
Bi12O15Cl6/Bi2WO6	500	10	50	30	60	81%	Wu et al.^49
BiFeO3/TiO2	1000	10	100	60	180	72%	Liao et al.^50
In2S3/InVO4	500	10	50	60	120	71%	Yuan et al.^51
Cu3P/SnO2	1250	50	25	30	180	80%	This study

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3.3. Stability and degradation mechanism studies 6tb

To evaluate the stability and reusability of the Cu₃P/SnO₂ heterostructure as a photocatalyst, we conducted five cycles of photodegradation experiments on the Cu₃P/SnO₂-3 catalyst. After completing a cycle of photodegradation experiment, the catalyst was collected by simple centrifugation, and treated with rinse, drying and other operations. The collected catalyst was prepared for the next cycle experiment. Fig. 7a and b showed the degradation curves and removal efficiency of tetracycline after five cycles. After multiple photodegradation cycles, the degradation efficiency of tetracycline changed a little, from 80% to 75%. After 5 consecutive cycles, it only dropped by 5%. These results proved stability of Cu₃P/SnO₂-3 as a photocatalyst, and it can be recycled with treatment, which effectively reduced the cost of catalyst production.

Fig. 7 The cyclic degradation curve of tetracycline over heterogeneous Cu₃P/SnO₂-3 composite catalyst (a); column comparison of degradation rates (b); degradation dynamic curve after the addition of capturing agent (c); histogram of degradation efficiency of different capture agents (d).

The reaction mechanism of tetracycline breakdown was studied by adding active oxidant species capture agent in the photocatalytic reaction stage. Isopropyl alcohol (IPA), benzoquinone (BQ) and ethylene diamine tetra acetic acid disodium salt (EDTA-2Na) were used to capture hydroxyl radical (˙OH), photogenerated holes (h⁺) and superoxide radicals (˙O₂⁻) of active oxidizing species, respectively. From the kinetic degradation curve (Fig. 7c) and the histogram (Fig. 7d) of degradation efficiency, the degradation of tetracycline by Cu₃P/SnO₂-3 catalysts was greatly different with different capture reagents. Results showed that the addition of active species capture agent did take into effect. After adding EDTA-2Na and BQ as capturing reagents for h⁺ and ˙O₂⁻ to the photocatalytic reaction pathway, the degradation efficiency of TC-HCl is dropped down to 19% and 24%, much lower than the blank test of non-capturing reagents (80%). This strong inhibitory effect indicated that h⁺ and ˙O₂⁻ played a key role in the degradation of tetracycline, and the effect of h⁺ was slightly higher than that of ˙O₂⁻. However, if IPA was used as a capture agent for ˙OH, the degradation efficiency of tetracycline drops from 80% to 57%, indicating that ˙OH plays a minor role in the degradation of TC-HCl. In summary, in the process of tetracycline degradation by Cu₃P/SnO₂ composite, the role of h⁺, ˙O₂⁻ and ˙OH follows h⁺ > ˙O₂⁻ > ˙OH of the degradation reaction.

The ESR measurements were further used to identify ˙O₂⁻ in Cu₃P/SnO₂-3 composite catalysts. As shown in Fig. S2 in ESI,† no signal was detected under dark conditions. Under visible light irradiation, ˙O₂⁻ appeared obviously, which proved that the production of the active substance was consistent with the free radical trapping experiment. Therefore, Cu₃P/SnO₂ photocatalyst showed good performance with the help of reactive oxygen species.

Fig. 8 showed the possible mechanism of the degradation of TC by heterostructure Cu₃P/SnO₂ catalyst under visible light. In the previous calculation, the band gap values of Cu₃P and SnO₂ were 1.31 eV and 3.54 eV, respectively. Under the visible light radiation, SnO₂ and Cu₃P are respectively excited, and the electrons in the valence band (VB) migrate to the conduction band (CB), leaving the hole h⁺ in the VB. Part of the electrons in the CB of Cu₃P were transferred to the CB of SnO₂ through the heterostructure interface. H₂O molecules trap holes h⁺ in the VB and convert them into ˙OH, and part of the oxygen O₂ molecules adsorbed on the catalyst surface trap electrons e⁻ in the CB and converted them into superoxide radical ˙O₂⁻. Finally, the holes h⁺, ˙O₂⁻ and ˙OH directly oxidize the tetracycline TC molecules. This was the general degradation principles of tetracycline TC by the studied Cu₃P/SnO₂ heterostructure. The specific and detailed degradation mechanism of tetracycline molecules await future research.

Fig. 8 Mechanism diagram of photocatalytic degradation of TC by heterogeneous Cu₃P/SnO₂.

4. Conclusion

Cu₃P/SnO₂, the p–n type heterostructure, was proved to be an efficient photocatalyst for the degradation of antibiotic drug pollutants like TC-HCl. Among different Cu₃P loaded samples, Cu₃P/SnO₂-3 (30% load) after 30 minutes of dark reaction adsorption and 140 minutes of photodegradation, the removal efficiency reached 80%, which is 2.3 times of the removal efficiency of SnO₂. The improved photocatalytic activity was benefited from the heterogeneous interface between SnO₂ and Cu₃P, which effectively enhances the efficient charge transfer and retarders the recombination of electron hole pairs. Furthermore, the cyclic photodegradation experiments verified that the heterojunction Cu₃P/SnO₂-3 was stable and the efficiency dropped 5% after 5 runs. From mechanistic study, the active oxidant species as h⁺, ˙O₂⁻, and ˙OH were all involved in photocatalytic system. It seems that h⁺ and ˙O₂⁻ played the major role in the photocatalytic degradation of tetracycline by Cu₃P/SnO₂ composites. Further work may involve the improvement of catalyst with better degradation activity.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra05905j

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