Preparation of Cu₂O@TiOF₂/TiO₂ and its photocatalytic degradation of tetracycline hydrochloride wastewater

Abstract

A new high-efficiency photocatalyst Cu₂O@TiOF₂/TiO₂ was synthesized by a hydrothermal method and applied to the degradation of tetracycline hydrochloride (TTCH). The samples were analyzed by SEM, EDS, XRD, BET, UV-vis DRS, Raman, PL, FT-IR. The Cu : Ti = 1 : 8 catalyst showed a narrow band gap of 2.10 eV, indicating that it can degrade TTCH as a novel photocatalyst capable of responding to sunlight. The average particle diameter is (2–6) nm, and the particle size distribution is narrow. When the reaction was carried out under simulated solar light for 3 hours, the efficiency for degrading 10 mg L⁻¹ tetracycline hydrochloride was as high as 96.83% when the catalyst dosage was 40 mg. It is shown from the capture experiments that ·O₂⁻ and ·OH play a major role in this reaction. In addition, it was found that the degradation of TTCH conforms to the first-order kinetic model.

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Introduction

Organic pollutants with high biological toxicity have attracted the attention of researchers around the world.¹ Tetracycline hydrochloride (TTCH) is an antibacterial agent commonly used in human and veterinary medicine.^2,3 Tetracycline antibiotics are the second largest class of antibiotics in the world and are used in large quantities.⁴ Due to the biological toxicity of TC, its metabolites are difficult to degrade in the environment and organism.^5,6 The potential hazards of TC and its residues to the water environment have become a problem that cannot be ignored.⁷ Therefore, methods such as adsorption,⁸ advanced biological methods,⁹ membrane filtration¹⁰ and ultrasonic induction⁸ have been developed to solve this problem. However, these methods have disadvantages such as high cost, low efficiency or secondary pollution. Therefore, catalytic technology has become one of the most promising technologies for remediating water pollution and solving environmental problems.^11–13 At present, catalysts such as Bi₂₄O₃₁Br₁₀,⁶ g-C₃N₄/LaCoO₃ (ref. 14) and Pt–TiO₂ (ref. 15) have been used to degrade TTCH.

TiO₂ has good chemical and biological stability, but it has become a bottleneck for development due to its low light utilization and fast photocarriers recombination.^16–19 To change this situation, many studies have enhanced the response of TiO₂ in the visible region by doping TiO₂ with non-metals (C, N, F, S, etc.)^2,20–22 and semiconductors (Cu₂O, CdS, etc.).^17,23,24 The introduction of F ions can greatly change the performance of TiO₂, and titanium oxydifluoride can be formed when TiO₂ is in a high concentration of F ions. TiOF₂ as a semiconductor material has gradually appeared in the field of photocatalysis. Dong et al.²⁵ synthesized Ag₃PO₄/TiOF₂ by chemical precipitation and degraded methylene blue (MB) under visible light. It was found that TiOF₂ can improve the stability of the catalyst system. The novel TiOF₂ photocatalyst prepared by Wang et al.²⁶ exhibited certain activity and durability when degrading rhodamine B and 4-chlorophenol under visible light. Studies have shown that TiOF₂ can be converted to anatase TiO₂ to produce more carriers, and the synergistic effect of the two can improve its photocatalytic performance.^27,28 However, there are few studies on TiOF₂/TiO₂, and it mainly absorbs ultraviolet light.^27,29,30

Cu₂O is a typical P-type semiconductor with a narrow band gap and has unique visible light driveability,³¹ which has been extensively studied in the field of photocatalysis. In order to improve the photocatalytic performance, it is often combined with materials such as TiO₂,¹⁷ reduced graphene oxide,³² Cu₂S,³³ and CeO₂.³⁴ In this paper, the Cu₂O@TiOF₂/TiO₂ photocatalyst was prepared for the first time by hydrothermal method. The Cu₂O@TiOF₂/TiO₂ photocatalyst has a large specific surface area, pores volume and low cost. The catalyst is used for antibiotic wastewater treatment under simulating solar light irradiation by a good combination of adsorption performance and photocatalytic performance, and exhibits excellent degradation effect.

Experimental

Synthesis of the photocatalysts

All chemical reagents are of analytical grade and can be used without purification.

Titanium oxyfluoride (TiOF₂) crystals were synthesized by hydrothermal method. 12.5 mL of hydrofluoric acid (HF) was slowly dropped into 34 mL of butyl titanate (C₁₆H₃₆O₄Ti, TBT) with stirring, and then 60 mL of glacial acetic acid (C₂H₄O₂) was added dropwise. The mixture was stirred at room temperature for 30 min, transferred to a reaction kettle with a polytetrafluoroethylene liner for 15 h at 160 °C. After the reaction vessel was naturally cooled to room temperature, the solid product was collected by centrifugation. And then it was washed three times with absolute ethanol and pure water, dried under vacuum at 60 °C. The powder obtained was a sample of TiOF₂.

Cu₂O was prepared by hydrothermal method. 6.3 g of copper sulfate (CuSO₄) was dissolved in 100 mL of pure water, and stirred at room temperature for 10 min (solution A). 6.0 g of NaOH was dissolved in 20 mL of pure water. 6.3 g of glucose (C₆H₁₂O₆) was dissolved in 50 mL of pure water, the solution was heated to 34 °C. The NaOH solution was dropped into the solution A and stirred at a low speed for 5 min. The glucose solution was further added dropwise, and stirred at room temperature for 15 min at room temperature. The mixture was transferred to a reaction vessel of a polytetrafluoroethylene liner and reacted at 90 °C for 4 h. The sample was TiOF₂.

Cu₂O@TiOF₂/TiO₂ was prepared by hydrothermal method. 2.0 g of TiOF₂ and a certain amount of CuSO₄ were dissolved in 100 mL of pure water, and stirred at room temperature for 10 min (solution B). A certain amount of NaOH and glucose and dissolved in 20 mL and 50 mL of pure water, respectively. The glucose solution was heated to 34 °C. The NaOH solution was dropped into the solution B and stirred at a low speed for 5 min. The subsequent steps were the same as the preparation method of Cu₂O. According to the relative content of Cu and Ti (ratio of the amount of Cu/Ti substance), the obtained composite was named as Cu : Ti = 1 : 4, 1 : 8, 1 : 10.

Characterization of the samples

The surface morphology and elemental distribution of the samples were analyzed by SEM and corresponding energy dispersive X-ray spectroscopy (EDS) (JSM7500F, Japan). The catalyst crystal characteristics were analyzed using X-ray diffraction (XRD, ICP-XD-2, China) equipped with a Cu-Kα X-ray source (λ = 0.15418 nm). The N₂ adsorption–desorption specific surface area analyzer (BET, Micrometrics ASAP2020, USA) was used to analyze the specific surface area and porosity of the catalyst. Fourier transform infrared spectroscopy (FT-IR, Bruker-Tensor 27, Germany) was used to identify the surface functional groups of the catalysts. The absorption characteristics of the samples were measured by UV-visible diffuse absorption spectroscopy (UV-vis DRS, Shimadzu UV-2600, Japan). Raman studies were measured using Raman spectroscopy (Raman, HR800, France) with an excitation wavelength of 532 nm. The photoluminescence spectrum of the photocatalyst was measured by a fluorescence spectrometer (Shimadzu-RF-6000, Japan) with the excitation wavelength was 300 nm.

Photocatalytic activity measurement

Photocatalytic activity was measured by degradation of TTCH. A total of 30 mg of the catalyst was dispersed in a 150 mL double-layered quartz reactor containing 100 ml of a 10 mg L⁻¹ TTCH solution. Cooling water was introduced into the interlayer of the quartz reactor to maintain the solution at room temperature. A Jiguang-500 W Xe lamp (simulating solar light) was located 30 cm away from the TTCH solution. The solution was magnetically stirred for 0.5 h in the dark to obtain the adsorption–desorption equilibrium, before the Xe lamp was turned on to start the degradation. At time intervals of 0.5 h, about 5.0 ml of the solution was extracted and centrifuged at high-speed to remove catalysts. After this, the TTCH concentration was analysed with a Purkinje UV1901 UV-vis spectrophotometer at 355 nm. The photocatalyst was separated from the TTCH solution, before another run was started to investigate the durability of catalysts.

Results and discussion

Characterization results

Morphology and composition characterization. The morphology of Cu₂O (a), TiOF₂ (b) and Cu : Ti = 1 : 8 photocatalysts (c) was characterized by SEM. Cu₂O is a cubic particle with a diameter of 0.5–1.5 μm. TiOF₂ is a cubic structure with a diameter of 300–500 nm, and the surface of the particles is smooth. As can be seen from Fig. 1(c), Cu₂O covers the surface of the TiOF₂/TiO₂ particles. Compared with pure Cu₂O, the growth of Cu₂O in the composite is limited by the space of TiOF₂/TiO₂, and the particle size becomes smaller after compounding. Fig. 1(d) shows the EDS spectrum of Cu₂O@TiOF₂/TiO₂. It mainly contains four characteristic peaks of O, F, Cu and Ti. And its content (weight ratio) was 40.04%, 15.9%, 5.79% and 38.26%, respectively, and there were no other impurity peaks.

Fig. 1 SEM image of (a) Cu₂O, (b) TiOF₂, (c) Cu : Ti = 1 : 8 Cu₂O@TiOF₂/TiO₂; (d) EDS spectrum of Cu : Ti = 1 : 8.

Crystal structure. The XRD pattern of the sample is shown in Fig. 2. The prepared Cu₂O exhibits characteristic peaks at 2θ = 29.62°, 36.68°, 42.67°, 62.47°, 74.94°, and 78.34°, corresponding to the (110), (111), (200), (220), (311), (222) crystal faces of Cu₂O (JCPDS no. 65-3288).³⁵ Seven distinct characteristic peaks appear in TiOF₂, identical to the standard card of TiOF₂ (JCPDS: 08-0060).³⁶ After 4 h of composite reaction, a part of TiOF₂ is transformed into TiO₂, and there may be a large number of defects at the interface during this phase transition.^27,37 Cu : Ti = 1 : 8 group photocatalysts shows characteristic peaks of TiOF₂ at 2θ = 23.43°, 48.55°, 54.42°, 70.12°. Characteristic peaks of Cu₂O appear at 2θ = 36.43°, 42.52°, 62.48°, 75.06°, and 79.20°. Characteristic peaks corresponding to the anatase type TiO₂ (JCPDS no. 21-1272)³⁸ standard card appeared at 2θ = 25.28° and 38.16°. Therefore, it can be seen that Cu and Ti have been successfully compounded. It can be seen that as the Ti source increases, the characteristic peak of TiOF₂ at 2θ = 23.30° gradually appears in the composite catalyst. As the Cu content decreases, the characteristic peak of Cu₂O at 2θ = 36.61° is gradually weakened. Furthermore, there are no impurity peaks like copper oxide shown in the patterns, which illustrate the high purity of the prepared cuprous oxide.

Fig. 2 XRD patterns of Cu₂O@TiOF₂/TiO₂ photocatalysts with different Cu : Ti.

BET surface area analysis. Fig. 3 shows the N₂ adsorption–desorption curves for the different ratios of catalysts and the corresponding BJH pore size distribution. It can be seen from the Fig. 3(a) that the catalysts exhibit a typical type IV N₂ adsorption curve and a H3 hysteresis regression line, indicating that the catalyst has mesoporous formation.³⁹ According to the pore size distribution of Fig. 3(b), the main pore size of the composite catalyst is distributed at (2–6) nm, indicating that the catalyst has a narrow particle size distribution. The results of the surface area and pores of the catalyst calculated by the BJH method are shown in Table 1, respectively. The surface area and pore volume of Cu₂O and TiOF₂ are both small. However, the surface area and pore volume of Cu₂O@TiOF₂/TiO₂ increase sharply, and the opening of the hysteresis regression line is larger. It shows that the multi-layer adsorption of the catalyst is stronger and the pore structure is more developed.¹⁶ Combined with the SEM image, it is speculated that this is due to the fact that Cu₂O is covered on the surface of TiOF₂/TiO₂, and the particles are loosely packed with each other, forming more gaps and increasing the surface area of the catalyst. In the composite catalyst, the specific surface area and pore volume of Cu : Ti = 1 : 8 group are optimal, which is more favorable for the adsorption of macromolecular TTCH. Generally, a larger specific surface area, pore volume, and pore size promote absorption of degradants and enhance photocatalytic activity. Therefore, the composite catalyst has the highest photocatalytic activity.

Fig. 3 (a) Nitrogen adsorption/desorption isotherm; (b) pore size distribution.

Table 1 Characteristics of the prepared samples by N₂ adsorption–desorption tests

Sample	Surface area (m^2 g^−1)	Pore volume (cm^3(STP) g^−1)	Average pore size (nm)
Cu2O	0.63	0.002	11.64
TiOF2	4.80	0.02	18.74
Cu : Ti = 1 : 4	26.87	0.17	23.77
Cu : Ti = 1 : 8	42.31	0.24	19.80
Cu : Ti = 1 : 10	34.42	0.15	16.36

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UV-vis DRS analysis. The ultraviolet-visible diffuse reflectance absorptive spectra (UV-vis DRS) are used to characterize the optical absorbance of composites. As shownin Fig. 4, TiOF₂ shows an absorption threshold at 394 nm, and the corresponding band gap energy is 3.09 eV. The absorption threshold of Cu : Ti = 1 : 8 photocatalyst at 474 nm, the corresponding band gap energy is 2.10 eV. It can be seen that the utilization of simulating solar light by the composite catalyst is greatly improved. The composite catalyst shows different degrees of red shift, indicating that its has a significantly enhanced response to simulating solar light, which may be due to the formation of more electron–hole pairs.⁴⁰ With the decrease of Ti source, the absorption intensity of the composite catalyst increases gradually, indicating that the doping of Cu₂O is the key factor to improve the forbidden bandwidth of the catalyst. This is attributed to the synergistic light absorption of Cu₂O and TiO₂.²³ Additionally, the multiple reflections and scattering of light within the porous structure of Cu₂O@TiOF₂/TiO₂ is favorable to reinforce the Interaction between photons and catalyst, enhancing the light absorption of the catalyst. Therefore, the composite catalyst exhibits excellent photocatalytic activity upon degradation of TTCH.

Fig. 4 UV-vis DRS spectrum (a) and band gap energy (b) of Cu₂O, TiOF₂ and Cu : Ti = 1 : x (x = 4, 8, 10) with different Ti(x) molar ratios.

PL analysis. PL spectroscopy can be used to study electron–hole recombination in semiconductors. The PL spectra of the catalyst is shown in Fig. 5. Compared with TiOF₂ and Cu₂O, the PL strength of Cu₂O@TiOF₂/TiO₂ is greatly reduced and varies with the change of Ti source. This indicates that the introduction of Cu₂O effectively inhibits the recombination rate of electron–hole pairs.

Fig. 5 Photoluminescence (PL) spectra of Cu₂O, TiOF₂ and Cu : Ti = 1 : x (x = 4, 8, 10) with different Ti(x) molar ratios.

The coupling of TiOF₂ and TiO₂ in the heterostructures also effectively diminishes the recombination of photoinduced electron–hole pairs.²⁷ The broad emission band centered at 400 nm is ascribed to bound exciton emission due to the trapping of free excitons by titanate groups near defects.⁴¹ Therefore, it can be inferred that Cu₂O@TiOF₂/TiO₂ can improve the electron–hole recombination and exhibit high photocatalytic activity.

Raman analysis. Fig. 6 shows the Raman spectra of the catalyst prepared. The Raman peak corresponding to anatase are observed at (151 507) cm⁻¹ of the composite catalyst.⁴² The proportion of TiO₂ in the composite is large, which is consistent with the XRD results. The composite catalyst corresponds to the Raman peak of TiOF₂ at 394 cm⁻¹.⁴³ 620 cm⁻¹ corresponds to the characteristic peak of Cu₂O, which is an infrared activity mode and can be assigned to the Γ₁₅ mode excited by oxygen deficiency.⁴⁴ The change trend of the Raman peak of the composite catalyst is similar to that of the XRD pattern, and decreases with the increase of the Ti source. This result further confirmed the structure of Cu₂O@TiOF₂/TiO₂.

Fig. 6 Raman spectra of different proportions of photocatalysts.

FT-IR analysis. To further characterize the structure of Cu₂O@TiOF₂/TiO₂, analysis was performed using FT-IR. As shown in Fig. 7, the absorption peaks around 3716 cm⁻¹ and 1614 cm⁻¹ are attributed to the stretching vibration and bending vibration of O–H, and the water molecules absorbed by the catalyst surface.^45–47 The appearance of an absorption peak near 2961 cm⁻¹ is caused by a hydrogen bond formed by the intermolecular association of hydroxyl groups.⁴⁸ The absorption peaks around 1051 cm⁻¹ and 500 cm⁻¹ are due to the stretching vibration of the Ti–O group.⁴⁹ The absorption peak near 672 cm⁻¹ is due to the stretching vibration of the Cu–O bond,⁵⁰ thereby confirming the successful recombination of Cu and Ti.

Fig. 7 FT-IR spectra of Cu₂O, TiOF₂ and Cu : Ti = 1 : x (x = 4, 8, 10) with different Ti(x) molar ratios.

Photocatalytic performance

Photocatalytic degradability. Fig. 8 shows the degradation curve of TTCH under different conditions. It can be seen from Fig. 8(a) that Cu₂O@TiOF₂/TiO₂ exhibits good adsorption performance in a dark reaction within 0.5 h and changes with the change of Cu : Ti ratio. This is because random packing promotes the increase of surface area and pores during the deposition of Cu₂O on the surface of TiOF₂/TiO₂. However, when the ratio is too large, Cu₂O and TiOF₂/TiO₂ are excessively deposited, which hinders the contact of TTCH with the catalyst. Therefore, Cu : Ti = 1 : 8 shows the best adsorption effect, which is consistent with the BET result. The degradation of composite photocatalyst was significantly better than pure TiOF₂, pure Cu₂O and P25 during the whole degradation process. It can be seen from Fig. 8(b) that the concentration of TTCH does not change significantly when there is no catalyst under simulating solar light. This shows that TTCH exhibits good stability and light from xenon lamps can hardly induce degradation. This may be due to the stable naphthol ring structure of TTCH. Compared to the 40 mg matte curve in Fig. 8(b), the concentration of TTCH is further reduced when both the catalyst and the source are present. It is indicated that the degradation process after TTCH is saturated by adsorption is photocatalytic degradation. When the dosage of Cu : Ti = 1 : 8 catalyst is 30 mg, the degradation rate of TTCH is 89.14%. However, the addition of 40 mg of catalyst can be as high as 96.83%.

Fig. 8 (a) Photodegradation of TTCH over different samples under visible light (catalyst dosage is 30 mg); (b) Cu : Ti = 1 : 8 Cu₂O@TiOF₂/TiO₂ degraded TTCH under different conditions; (c) UV-vis absorption spectrum of Cu : Ti = 1 : 8 Cu₂O@TiOF₂/TiO₂.

It can be seen from Fig. 8(c) that TTCH has two main absorption peaks at 276 nm and 378 nm. The absorption peak at 272 nm may be related to the hydroxyl group and acylamino group produced during the reduction.^51,52 The absorption peak at 378 nm is due to the aromatic ring B–D, which may be caused by the cleavage of the phenol ring attached to the aromatic ring.⁵³ After the reaction is carried out for 0.5 h, the absorption peak decreased rapidly. It is indicated that the ring structure was destroyed after the addition of the light source, and the TTCH adsorbed on the catalyst could be further photocatalyzed. According to the study of TTCH degradation process by Peng⁵⁴ and Wang⁵⁵ et al., under the attack of active substances such as ·OH, functional groups such as amino group, hydroxyl group and methyl group are first separated from TTCH molecules. Upon further oxidation, the carbon chain is broken, forming an unstable ring opening product. Under the action of free radicals, the carbonyl group is separated from the ring opening. The intermediate is further oxidized to form a short chain carboxylic acid. Finally, the stable ring structure of the TTCH molecule is destroyed to achieve a degradation effect.

In order to further study and understand the kinetics of photocatalytic degradation of TTCH by Cu : Ti = 1 : 8 Cu₂O@TiOF₂/TiO₂, the analysis was carried out under optimal conditions. As shown in Fig. 9(a), the composite photocatalyst has a R² close to 1, consistent with the first order kinetic model. The repeated experimental results of the Cu : Ti = 1 : 8 composite material are shown in Fig. 9(b). After three repeated use of the catalyst, there is no significant reduction in the degradation of TTCH. This indicates that the catalyst has sufficient stability and repeatability.

Fig. 9 (a) The kinetic curves transform of TTCH degradation overdifferent samples; (b) the repetitive degradation diagram of Cu : Ti = 1 : 8.

Mechanism of photocatalytic activity

To determine the active substances in the degradation of TTCH, 1,4-benzoquinone (PBQ) was used as a scavenger for superoxideradicals (·O₂⁻).⁵⁶ 1,4-Terephthalic acid (PTA) was used as a scavenger for hydroxyl radicals (·OH).⁶ Methanol (MT) was used as a scavenger for the hole (h⁺). The capture experiments were identical to the photocatalytic experiments. As shown in Fig. 10, in the dark reaction stage, the scavenger has a slight effect on the adsorption of the system. It is speculated that the catalyst also has a small amount of adsorption on different scavengers, and the scavenger forms a competitive relationship with TTCH. The combination of different scavengers and catalyst surfaces may also be different, so different catalysts exhibit different adsorption effects in the dark reaction stage. When MT was added to the system, the degradation efficiency of the catalyst did not change much. When PBQ and PTA were added, the degradation rates were reduced by about 40% and 20%, respectively. It can be seen that ·O₂⁻ and ·OH determine the degradation activity of the catalyst on TTCH. The O₂ molecule is adsorbed on the surface of the catalyst by photoexcitation of electrons, and is reduced to form ·O₂⁻.⁵⁷

Fig. 10 Free scavenging experiment of Cu : Ti = 1 : 8 Cu₂O@TiOF₂/TiO₂ (catalyst dosage is 40 mg, and the concentration of scavenger 3 mmol L⁻¹).

Based on above experiments, a possible mechanism for the degradation of TTCH by Cu₂O@TiOF₂/TiO₂ under simulating solar light is proposed, as displayed in Fig. 11. Under simulating solar light (λ > 420 nm) irradiation, the Cu₂O is excited and generated photocarriers. Because the CB of Cu₂O is a little negative compared to TiO₂,⁵⁸ part of the photogenerated electrons on Cu₂O can migrate to the CB of TiO₂, and the other part reacts with dissolved oxygen O₂ to form ·O₂⁻. Similarly, photogenerated electrons on TiO₂ migrate to CB of TiOF₂.^30,37 On the other hand, holes are accumulated step by step onto the VB of Cu₂O, and the low-cost band of Cu₂O is advantageous for the transfer of effective holes. These holes can react directly with the TTCH adsorbed on the surface of the catalyst.

Fig. 11 Photocatalytic reaction mechanism of degradation of TTCH by Cu₂O@TiOF₂/TiO₂.

Conclusions

In summary, an easy hydrothermal route to synthesize Cu₂O@TiOF₂/TiO₂ (Cu : Ti = 1 : 4, 1 : 8, 1 : 10) hybrids has been demonstrated. The prepared Cu₂O@TiOF₂/TiO₂, especially the Cu : Ti = 1 : 8 nanocomposite, exhibited excellent activity towards the degradation of TTCH under simulating solar light irradiation. A lower electron–hole pairs recombination rate and larger specific surface area, pore diameter are the dominant factor that induces the photocatalytic performance enhancement of Cu₂O@TiOF₂/TiO₂ nanohybrids. The highest efficiency of TTCH removal reached 96.83% in 3 h. The degradation of TTCH is largely dependent on ·O₂⁻ and ·OH. Cu₂O@TiOF₂/TiO₂ has good repeatability, which is very important in practical applications. At present, for TTCH, there are few studies that can achieve high efficiency and economy. Therefore, this work opens an avenue for the removal of organic pollutants.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has been supported by the Shaanxi Province Education Department Science and Technology Research Plan (No. 15JK1460, No. 2019SF-250).

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