Preparing Ti₃C₂-modified ZnFe₂O₄ photocatalytic materials and evaluating their performance in degrading tetracycline in water

Abstract

To address the increasingly severe problem of water pollution, investigating novel, highly efficient, water treatment materials has become the current research hotspot. Herein, metal–oxygen-rich Ti₃C₂-modified ZnFe₂O₄ photocatalytic materials have been designed under alkaline conditions using the solvothermal method to achieve high-efficiency degradation of tetracycline hydrochloride (TCH) in water. Experimental results reveal that Ti₃C₂/ZnFe₂O₄ couples well and exhibits excellent optoelectronic and carrier-separating properties. Under visible light, 0.25/1 Ti₃C₂/ZnFe₂O₄ exhibited the best photocatalytic performance with a TCH removal efficiency of 87.10%; its reaction rate was 2.0 and 35.2 times higher than those of ZnFe₂O₄ and Ti₃C₂, respectively. Using the electron spin resonance technique, ·O₂⁻, ·OH and h⁺ were determined to be the active substances involved in the photocatalytic degradation of TCH; on this basis, a reaction mechanism was proposed. The intermediates of TCH generated during photocatalytic degradation were identified using high-performance liquid chromatography–mass spectrometry, and possible degradation pathways were analysed. In addition, the potential toxicity of the intermediates in the environment was calculated and analysed using Ecological Structure Activity Relationships (ECOSAR) simulations, and the responsiveness of the photocatalytic materials to other pollutants in water was assessed in terms of universality. Experimental results reveal that Ti₃C₂ and ZnFe₂O₄ coupled photocatalytic materials exhibit excellent performance when degrading TCH, as well as high environmental friendliness, broadening their potential application in the field of water environment remediation.

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1. Introduction

Antibiotics are widely used in human, animal and plant medicines. Among them, broad-spectrum, highly effective tetracycline hydrochloride (TCH) has received considerable attention; however, despite its positive effects, its resistance to degradation in the natural environment can cause considerable accumulation in aquatic settings, posing a serious threat to human health and ecosystems.^1–3 In recent years, technologies such as adsorption, flocculation and biochemical treatment have been used to treat TCH in the environment; however, their application has been limited by long cycle times, incomplete degradation, high costs and the tendency to cause secondary pollution of the aquatic environment. Therefore, there is an urgent need to develop an efficient and environmentally friendly strategy for the removal of TCH compared to the existing pollutant treatment methods.⁴ Photocatalysis is a safe, efficient, economical and environmentally friendly method whose sustainable use has aroused widespread interest among researchers.⁵

In recent years, zinc ferrate (ZnFe₂O₄), a spinel oxide with a tunable bandgap (1.9–2.1 eV), has been demonstrated to be an excellent photocatalyst.⁶ As an n-type semiconductor, ZnFe₂O₄ contains Zn in tetracoordination and Fe in hexacoordination, occupying the tetrahedral and octahedral gaps, respectively, in O₂-dense stacking (see the crystal structure in Fig. S1†⁷). Its characteristics offer broad possibilities for optimising the structure and physicochemical properties of photocatalysts. Nonetheless, ZnFe₂O₄, as a pure n-type semiconductor, suffers from a fast rate of photogenerated electron–hole (e⁻–h⁺) pair complexation and has not been widely employed in photocatalysis.^8,9 To overcome this challenge, researchers have proposed an effective strategy that involves forming stable chemical bonds between different phases to construct photocatalysts with high-quality heterojunction structures, thereby improving the photocatalytic performance of ZnFe₂O₄. Two-dimensional (2D) layered Ti₃C₂ is characterised by good photoelectrochemical properties,¹⁰ tunable surface functional groups (exposed surfaces terminated by –OH, –O and –F after HF etch stripping)¹¹ and a large specific surface area,¹² promoting the directional transport of photogenerated carriers¹³ and enhancing interfacial interactions.^14,15 Notably, theoretical studies^16,17 have revealed that the Fermi energy level of layered Ti₃C₂ depends on its surface end groups such that Ti₃C₂ terminated by –OH has an extremely low work function (∼2.0 eV), making it easy to build a tight heterojunction structure.¹⁸ In addition, according to previous studies,¹⁹ alkaline conditions can further modulate the chemical composition and electronic structure, allowing end-group conversion to improve hydrophilicity. Furthermore, the ultra-thin 2D structure of layered Ti₃C₂ exposes more internal atoms [Ti atoms (low coordination)],²⁰ leading to the formation of various defects, thereby providing more surface active sites for improved adsorption performance.²¹

Ti₃C₂ exhibits a tunable end-group functional group and good adsorption capacity and is capable of forming tight contacts with several semiconductors to create built-in electric fields, thereby modulating carrier migration paths. Consequently, heterojunctions can be constructed by adjusting the surface chemical properties of Ti₃C₂ and ZnFe₂O₄ to achieve the effective transfer of photogenerated charges and enhanced redox capacity required to remove pollutants from water bodies. Inspired by the literature referred to above, the alkaline solvothermal method is used herein to transform Ti₃C₂ surface groups into metal–oxygen-rich hydrophilic capping groups. They are coupled with ZnFe₂O₄ to provide a more stable heterojunction interface, forming a composite heterojunction photocatalyst with excellent photoelectrochemical performance in response to visible light. The surface–interface structure and bonding of the synthesised composites were analysed using serial characterisation, and their photoelectrochemical properties and the degradation of TCH under visible-light irradiation were evaluated. The mechanism by which the composite photocatalytic material Ti₃C₂/ZnFe₂O₄ exhibits considerably enhanced photocatalytic activity for TCH degradation was systematically demonstrated.

2. Experimental

All chemical reagents used in the study are listed in section S1† and all reagents were of analytical grade. In this paper, we first provide a scheme for preparing Ti₃C₂, ZnFe₂O₄ and Ti₃C₂/ZnFe₂O₄ nanocomposite photocatalytic materials (Ti₃C₂/ZnFe₂O₄ mass ratios of 1.5/1, 1/1, 0.5/1, 0.25/1 and 0.2/1), as shown in Fig. 1(a) and (b). Using ethylene glycol as a solvent and adjusting the pH using NaOH, while synthesising ZnFe₂O₄, the capping groups of Ti₃C₂ undergo alcoholisation and alkalinisation under solvent heating and alkaline conditions, forming materials rich in metal–oxygen groups (Ti–O/OH). Experimental operation details are provided in the ESI† (sections S2 and S3). To investigate the TCH degradation performance of the prepared photocatalytic materials, a 300-W xenon lamp (section S4†) equipped with a universal filter (λ > 420 nm) was employed as a visible-light source and experiments on TCH degradation were performed in a quartz photocatalytic reactor. The specific characterisation conditions of the material are specified in section S5.† Simultaneously, the photocatalytic degradation of TCH was evaluated using high-performance liquid chromatography–mass spectrometry (HPLC–MS) (section S6†) to identify possible reaction pathways.

Fig. 1 Schematic of (a) Ti₃C₂ and (b) synthesis of the Ti₃C₂/ZnFe₂O₄ heterostructure.

3. Results and discussion

3.1. Characterisation of catalysts

3.1.1. Analysis of the crystal structure and composition. The crystal structures of the single substances Ti₃C₂ and ZnFe₂O₄ and the composite 0.25/1 Ti₃C₂/ZnFe₂O₄ were analysed using X-ray diffraction (XRD). As shown in Fig. 2(a), the characteristic diffraction peaks appearing in the XRD pattern of the single substance Ti₃C₂ are consistent with those reported in the literature,²² confirming the successful etching of Ti₃AlC₂ to Ti₃C₂.²³ The characteristic peak of anatase (TiO₂) (JCPDS: 21-1272)²⁴ appeared at 24.96° due to certain oxidation during the Ti₃C₂ preparation. The characteristic peaks corresponding to every diffraction angle appearing in the single substance ZnFe₂O₄ are attributed to the standard cubic phase of spinel ZnFe₂O₄ (JCPDS: 22-1012). From the XRD pattern of the composite material 0.25/1 Ti₃C₂/ZnFe₂O₄, it can be observed that adding Ti₃C₂ does not affect the crystal structure of ZnFe₂O₄, while the characteristic peaks of Ti₃C₂ and ZnFe₂O₄ appear in the composite structure, confirming the heterojunction formation. Furthermore, the intensity of the diffraction peak at 24.96° did not decrease in the composite, indicating that some of the Ti³⁺ in the composite was converted to Ti⁴⁺ and formed TiO₂ due to in situ growth. Notably, some variations in the peak shape (widening) and positions (6.58°, 18.1°, 35.2°, 62.1°) are slightly shifted in the direction of the decreasing diffraction angle; such phenomena are related to the grain size, lattice distortion and residual stresses (microscopic and macroscopic).^25–27 This is because of the influence of different ion/composition changes on the crystalline phase. Ions with larger radii replace those with smaller radii in the structure, resulting in an increase in the lattice spacing, causing the peak shape and position to accordingly change. Furthermore, this confirms the existence of strong interactions between semiconductors. Meanwhile, the peak position of Ti₃C₂ decreases (6.58°) because of the peak shift caused by the occurrence of ethylene glycol alcoholisation and alkalisation reactions under solvothermal conditions, resulting in the enrichment of –OH and –O groups²⁸ and promoting close binding between substances. The functional groups of the prepared samples and the functional group interaction states between the substances were investigated using Fourier transform infrared (FTIR) spectroscopy. Fig. 2(b) presents the FTIR spectra of Ti₃C₂, ZnFe₂O₄ and 0.25/1 Ti₃C₂/ZnFe₂O₄. The vibrational peak at 477 cm⁻¹ in the Ti₃C₂ spectrum is attributed to the Ti–C telescopic vibration and the peaks at 610 and 761 cm⁻¹ are attributed to the Ti–O telescopic vibration.²⁹ In the FTIR spectrum of ZnFe₂O₄, the two absorption peaks between 400 and 600 cm⁻¹ are related to the bonding of metal cations and oxygen anions at tetrahedral and octahedral sites; i.e. they are related to the inherent telescopic vibration of metallic tetrahedral Fe–O (500–600 cm⁻¹) and octahedral Zn–O (400–480 cm⁻¹),³⁰ respectively. Furthermore, the vibration peaks at 1079, 1618 and 3413 cm⁻¹ are attributed to O–H bonds or the stretching and bending vibrations of adsorbed H₂O molecules.³¹ This indicates that the characteristic structure of the precursor in the synthesised sample remains unchanged.

Fig. 2 (a) X-ray diffraction (XRD) patterns and (b) Fourier transform infrared spectra of the raw Ti₃AlC₂ and Ti₃C₂, pure ZnFe₂O₄ and the 0.25/1 Ti₃C₂/ZnFe₂O₄ composite.

Furthermore, the changes in the surface elemental composition and chemical bonding state observed in Ti₃C₂, ZnFe₂O₄ and the 0.25/1 Ti₃C₂/ZnFe₂O₄ composite were analysed using X-ray photoelectron spectroscopy (XPS). The XPS spectrum presented in Fig. 3(a) indicates the presence of characteristic peaks for C 1s, Ti 2p, O 1s, Fe 2p and Zn 2p in the 0.25/1 Ti₃C₂/ZnFe₂O₄ composite, confirming the successful construction of a heterojunction, which is consistent with the conclusions of XRD analyses. The five anti-convolution peaks of the C 1s spectrum occur at 281.78 (C–Ti), 284.62 (C–C), 286.29 (C–O), 288.84 (C=O, alkyl carbon on the benzene ring) and 291.75 eV (C–F) [Fig. 3(b)].³² After binding with ZnFe₂O₄, the intensity of the C–Ti peak at 281.78 eV became considerably weak, indicating that the C–Ti bond is broken because of the shearing action of alkali during the synthesis process, promoting the exposure of Ti species and producing more active sites at the surface. Simultaneously, following alcoholisation and alkali treatment, the F content of the surface is considerably reduced, indicating the substitution of the end group. The XPS fine spectrum of Ti 2p [Fig. 3(c)] reveals characteristic peaks for Ti 2p_3/2 (455.18, 456.39 and 457.58 eV) and Ti 2p_1/2 (461.20 and 462.19 eV) corresponding to Ti–C, Ti²⁺, Ti³⁺, Ti²⁺ and Ti³⁺, respectively. In the high-resolution Ti 2p XPS spectrum of the composite, the peaks at 464.08 and 458.27 eV are attributed to Ti–O (Ti⁴⁺ 2p_1/2)³³ and Ti–O bonds at the end groups of Ti₃C₂, respectively; the latter occurred primarily due to the presence of Ti on the surface in the form of Ti⁴⁺ under alkaline conditions.¹⁹ This is in agreement with the C 1s analysis, favouring the formation of strong connections between substances to promote the formation of heterojunctions.³⁴ Compared with that of the original Ti₃C₂, the 2p spectrum of low-valency Ti is weaker, indicating that low-valency Ti is consumed during the synthesis of the composite.³⁵ Notably, the glycol solvent heat treatment under alkaline conditions, which is similar to those in saponification and alcohol halogenation, causes an increase in the density of –O terminations at the surface of the pristine HF-etched Ti₃C₂ and the conversion of the –F group to enrich the hydrophilic –OH group,²⁸ thereby obtaining abundant metal–oxygen bonds (Ti–O) that help in promoting the bonding of Ti₃C₂ and ZnFe₂O₄ [Fig. 3(d)]. The spectral peak at 529.73 eV in O 1s is characteristic of lattice oxygen in metal oxides. The characteristic peak at 531.21 eV occurs due to the lack of lattice oxygen during the solvothermal process, and the increase in the relative intensity of complex oxygen vacancies at the corresponding peak positions indicates an increase in the content of oxygen vacancies, indicating the accumulation of numerous photocatalytically active centres.³⁶ Furthermore, electron paramagnetic resonance characterisation demonstrates the formation of oxygen vacancies in the synthesised samples (Fig. S2†). In addition, the peak at 532.49 eV denotes oxygen adsorption and the corresponding binding energy of the composite is observed to slightly increase because of the reduction of the adsorbed oxygen content, enhancing the lattice vibration and promoting the generation of e⁻ or h⁺, thereby improving the photocatalytic performance of the material.³⁷ As shown in Fig. 3(e), the high-resolution spectrum of Fe 2p can be divided into three types of peaks, which are characterised by two spin–orbit splitting peaks (Fe 2p_3/2: 710.18 and 712.37 eV, corresponding to the octahedral and tetrahedral positions of spinel ferrite, respectively; Fe 2p_1/2: 723.79 and 726.08 eV) and a satellite peak (718.29 and 732.31 eV). The characteristic peaks at 710.18 and 723.79 eV indicate the presence of Fe²⁺ (charge carrier) due to e⁻ hopping at the octahedral position. The characteristic peaks at the binding energies of 712.37 and 726.08 eV indicate the presence of Fe³⁺ in the synthetic material, whereas the satellite peaks at 718.29 and 732.31 eV indicate the presence of Fe³⁺ with the corresponding binding energy in the composites. For the characteristic spectrum of Zn 2p [Fig. 3(f)], 1021.10 (Zn 2p_3/2) and 1044.11 eV (Zn 2p_1/2) correspond to Zn²⁺ occupying the tetrahedral and octahedral positions, respectively.³⁸ For nano-scale ZnFe₂O₄, the cations that originally occupied the above two positions in the spinel structure are reassigned and these movements exhibit a considerable effect on the electronic, magnetic and catalytic properties of the material.³¹ Notably, the spectral peaks of the individual elements of the composite material shift in the direction of high or low binding energy compared to those of the single material. This indicates the formation of heterojunctions in which the strong interactions among the elements cause changes in the e⁻ density in the interfacial region,³⁹ providing favourable evidence for the interactions between charge transfer and degradation target products.

Fig. 3 (a) XPS survey spectra of Ti₃C₂, ZnFe₂O₄ and 0.25/1 Ti₃C₂/ZnFe₂O₄, and (b) C 1s, (c) Ti 2p, (d) O 1s, (e) Fe 2p, and (f) Zn 2p spectra of the synthesised materials.

3.1.2. Morphological characterisation. The morphologies of pure substances Ti₃C₂ and ZnFe₂O₄ and the 0.25/1 Ti₃C₂/ZnFe₂O₄ nano-composite were investigated using field-emission scanning electron microscopy. Fig. 4(a) and (b) reveal the successful preparation of a typical MXene multilayer accordion-like material Ti₃C₂, and it can be observed that the flakes are thin layers. 0.25/1 Ti₃C₂/ZnFe₂O₄ is a heterojunction synthesised through loading several ZnFe₂O₄ nanoparticles onto the surface and partial interlayer of 2D layered Ti₃C₂ [Fig. 4(c) and (d)]. The surface particles of ZnFe₂O₄ and the composite photocatalyst present irregular shapes, surface roughness and agglomeration phenomena, which may be caused by the electrostatic interaction, polarity and high surface energy between zinc ferrite particles.⁴⁰ Furthermore, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images reveal the microstructure of the 0.25/1 Ti₃C₂/ZnFe₂O₄ nano-composite. The formation of a tight heterojunction structure between the ZnFe₂O₄ nanoparticles (red circles) and Ti₃C₂ can be clearly observed [Fig. 4(e)]. The yellow circle in Fig. 4(e) corresponds to the partial enlargement shown in Fig. 4(f), where the lattice stripes characterised by crystal plane spacings (d) of 0.209 and 0.252 nm correspond to the (105) and (311) crystal planes of Ti₃C₂ and ZnFe₂O₄, respectively. Furthermore, the lattice of the catalytic material is twisted, which is caused by the destruction of the equilibrium state among the atoms due to alkaline conditions and different occupations between ions. The elemental diagram (Fig. S3†) and energy-dispersive X-ray spectroscopy (EDX) analysis of the Ti₃C₂/ZnFe₂O₄ binary system [Fig. 4(g)–(l)] indicate that Ti, C, Zn, Fe and O in the 0.25/1 Ti₃C₂/ZnFe₂O₄ composite are uniformly distributed throughout the region. The structural characteristics of the materials (specific surface area and pore structure) were analysed using N₂ adsorption–desorption isotherms. Hysteresis loops of different sizes are observed for isotherms under different relative pressures [Fig. S4(a)†], indicating the presence of mesoporous structures in all the samples. The BET values of Ti₃C₂, ZnFe₂O₄ and 0.25/1 Ti₃C₂/ZnFe₂O₄ were 40.534, 238.159 and 130.858 m² g⁻¹, respectively. BJH analysis indicated that the average pore sizes of the abovementioned samples were 3.704, 7.726 and 5.589 nm, respectively [Fig. S4(b)†]; detailed data are presented in Table S1.† The coupled specific surface area, total pore volume and pore size of Ti₃C₂ and ZnFe₂O₄ were observed to increase relative to those of Ti₃C₂, providing more active sites and enhancing the adsorption capacity of TCH. Unlike that for ZnFe₂O₄, its monolayer adsorption cannot completely explain the TCH adsorption.⁴¹

Fig. 4 FESEM patterns of (a and b) Ti₃C₂, (c) ZnFe₂O₄, and (d) 0.25/1 Ti₃C₂/ZnFe₂O₄, (e) TEM image of 0.25/1 Ti₃C₂/ZnFe₂O₄, (f) HRTEM images of 0.25/1 Ti₃C₂/ZnFe₂O₄ [partial enlarged image corresponding to the yellow circle area in Fig. 4(e)] and (g–l) elemental mapping images of 0.25/1 Ti₃C₂/ZnFe₂O₄.

3.1.3. Optical and electrochemical analysis. To study the optical and electrochemical properties of the prepared photocatalytic materials, ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), photoluminescence (PL) spectroscopy, time-resolved photoluminescence (TR-PL) spectroscopy, photocurrent (PC) analysis and electrochemical impedance spectroscopy (EIS) were performed. UV–vis DRS spectra [Fig. 5(a)] reveal the light absorption characteristics of the photocatalysts, indicating no obvious absorption edge in the absorption curve for Ti₃C₂; this is related to the particular energy band structure and metallic properties of Ti₃C₂.⁴² Compared to pure ZnFe₂O₄, the visible light absorption of the 0.25/1 Ti₃C₂/ZnFe₂O₄ composite is enhanced and the absorption edge is red-shifted. This is caused by the strong interactions during the group transition, leapfrogging of orbital e⁻ within tetrahedra and octahedra or the bandgap mixing effect that introduces defects.⁴³ The bandgap energy was calculated using the Kubelka–Munk equation [Fig. 5(b)]; the Ti₃C₂, ZnFe₂O₄ and 0.25/1 Ti₃C₂/ZnFe₂O₄ bandgap values were 1.19, 2.04 and 1.90 eV, respectively. The bandgap energy and width of ZnFe₂O₄ were reduced owing to local defects or exchanges between local and band e⁻ (charge neutralisation creates some oxygen vacancies) and interaction after successful loading of the group,⁴⁴ resulting in a composite material with improved visible light absorption. The efficient separation of photogenerated carriers is one of the key factors required for the improvement of photocatalytic activity, and PL spectroscopy can be used to analyse the separation efficiency capacity of charge trapping and e⁻–h⁺ pairs in semiconductors. Fig. 5(c) depicts the PL spectra of the sample at an excitation wavelength of 386 nm, where Ti₃C₂ acts as an e⁻ acceptor, trapping e⁻ and quenching several steady-state PL spectra of ZnFe₂O₄ (ref. 45) due to the formation of heterojunctions between substances that promote interfacial charge transport. In addition, the optical behaviour of the e⁻–h⁺ pairs of the photocatalytic materials was investigated using TR-PL spectroscopy [Fig. 5(d); trapped e⁻ lifetimes were fitted and analysed, and specific parameters and calculations are presented in Table S2†]. The average decay lifetimes of the prepared materials were calculated, indicating that the heterojunction 0.25/1 Ti₃C₂/ZnFe₂O₄ (1.724 ns) exhibits a longer carrier lifetime than Ti₃C₂ (1.353 ns) and ZnFe₂O₄ (1.450 ns), thereby facilitating the photocatalytic reaction. The transient photocurrent behaviour of the photocatalytic materials Ti₃C₂, ZnFe₂O₄ and 0.25/1 Ti₃C₂/ZnFe₂O₄ was investigated to evaluate the e⁻–h⁺ separation rate. The photocurrent of the 0.25/1 Ti₃C₂/ZnFe₂O₄ composite considerably increases compared to that of a single substance, implying high carrier separation efficiency and explaining the longer lifetime of the composite [Fig. 5(e)]. To verify the charge transfer capability of 0.25/1 Ti₃C₂/ZnFe₂O₄, the EIS Nyquist plots of each substance were determined and analysed [Fig. 5(f)]. The radius of the Nyquist semicircle located in the high frequency region is related to the charge transfer process at the photoelectrode interface such that the smaller the radius, the higher the charge transfer efficiency. This indicates that alcoholisation and alkalinisation under solvent heating enrich Ti₃C₂ with hydrophilic groups (–OH and –O), forming a tight heterojunction with ZnFe₂O₄, reducing resistance to e⁻ transport, enhancing responses to visible light, promoting efficient carrier separation and interfacial charge transfer rate, prolonging the carrier lifetime and improving the photocatalytic performance.

Fig. 5 (a) UV–vis DRS spectra, (b) Tauc plots, (c) PL spectra, (d) TR-PL spectra, (e) transient photocurrent responses, and (f) EIS Nyquist plots of Ti₃C₂, ZnFe₂O₄ and 0.25/1 Ti₃C₂/ZnFe₂O₄.

3.2. Photocatalytic performance test

Photocatalytic degradation tests performed on TCH under visible light irradiation allowed the photocatalytic activity of the sample to be investigated. Fig. 6(a) and (b) show the removal efficiency of TCH with and without a photocatalyst; TCH undergoes little photolysis in the absence of a catalyst owing to its high chemical stability. Under visible light irradiation, 0.25/1 Ti₃C₂/ZnFe₂O₄ exhibited the highest total removal of TCH (87.10%) owing to the synergistic effect, adsorption and photocatalysis. Its removal rate was 1.4 and 11.3 times higher than those of ZnFe₂O₄ (62.66%) and Ti₃C₂ (7.73%). The oxygen-containing groups (–OH and –O) and oxygen-active sites of Ti₃C₂ increase following the alcoholisation and alkalinisation reactions performed under solvent heating, during which the heterojunction formed between Ti₃C₂ and ZnFe₂O₄ enhances the absorption of visible light and carrier separation efficiency, improving the photocatalytic degradation performance. According to the kinetics [Fig. 6(c)], light conditions with −ln(C_t/C₀) are consistent with the first-order kinetics. Fig. 6(d) depicts the rate constant k, with 0.25/1 Ti₃C₂/ZnFe₂O₄ exhibiting the highest reaction kinetic constant (0.01197 min⁻¹) (i.e. 2.0 and 35.2 times higher than those of Ti₃C₂ and ZnFe₂O₄, respectively). The effects of different catalysts on the apparent rate of decline, kinetic constants and correlation coefficients for TCH degradation are summarised in Table S3.† Additionally, the photodegradation efficiency of 0.25/1 Ti₃C₂/ZnFe₂O₄ and other types of photocatalysts on TCH was compared (Table S4†), revealing that the 0.25/1 Ti₃C₂/ZnFe₂O₄ composite catalyst exhibits a higher efficiency for the removal of tetracycline from the aqueous environment and realistic significance for practical applications. To study the stability of the catalyst, a cycling experiment [Fig. S5(a)†] was performed, revealing that the catalytic efficiency was reduced because the e⁻ accumulated by Ti₃C₂ is consumed by the reduction reaction of Fe while participating in the photocatalytic degradation. The TCH or intermediate on the catalyst surface is not completely resolved such that some active sites are occupied, influencing the photocatalytic efficiency during the cycle. Moreover, the structures of the recovered samples were analysed [Fig. S5(b)†]. XRD characterisation indicated that the corresponding peak intensity of the recovered sample lightly changed compared to the fresh sample. This is related to the partial shedding of the surface-loaded ZnFe₂O₄. However, no considerable difference was observed, indicating that the sample retained high recyclability and structural integrity after the cycling experiment. Adsorption was investigated to exclude the effect of dark adsorption on the subsequent photodegradation (Fig. S6†), indicating that the removal of TCH via dark adsorption was not considerable; the subsequent target pollutant (i.e. TCH) still needed to be degraded under light.

Fig. 6 (a) Degradation curves of the photocatalytic degradation of tetracycline by composite materials, (b) histogram of the photocatalytic degradation efficiency of tetracycline by different materials, (c) fitting curves of the pseudo-first-order kinetics and (d) degradation rate constant K.

3.3. Analysis of intermediate products and degradation pathways

To explore the intermediates of composite photocatalysts in the photocatalytic degradation of TCH, the intermediate product (Fig. S7†) was identified using HPLC–MS. The intensity of the TCH signal at m/z = 445.16 was observed to gradually decrease with the progressing reaction, generating different mass spectral peaks, demonstrating that TCH was decomposed into different intermediates (Table S5† presents molecular formulas and details). Based on the detected intermediates and their analysis (Fig. S7†), three possible pathways by which TCH could be degraded into smaller molecules by active radical (·O₂⁻, h⁺ and ·OH) attack when exposed to visible light may be proposed (Fig. 7 and Table S5†). In pathway I, because of the attack of h⁺, the intermediate P1 (m/z = 417.16) was formed by the demethylation of TCH. This was further converted to the intermediate P4 (m/z = 433.28) via hydroxylation. The hydroxyl, double bond and amine in TCH are relatively susceptible to the attack by active free radicals in the photocatalytic process;⁴⁶ here, P4 (m/z = 433.28) can be degraded into smaller molecules by two approaches – deamidation and carboxylation – to produce the by-product P8 (m/z = 282.20). Furthermore, it can be gradually cleaved by a ring-opening reaction and ketonised to form a low-molecular-weight ketone compound P9 (m/z = 268.19), which can then be carboxylated to form the carboxylic acid compound P10 (m/z = 119.08).⁴⁷ In pathway II, the double bond of the TCH molecule can be hydroxylated (addition) to form intermediate P2 (m/z = 461.15). Furthermore, removal of the –N(CH₃)₂ functional group and radical oxidation of the double bond provide intermediate P5 (m/z = 432.24),⁴⁸ and the subsequent removal of enol, amide and H₂O produces the intermediate P6 (m/z = 318.30).⁴⁹ In addition, P5 (m/z = 432.24) can be generated by the cleavage of the double bond (ring-opening reaction) to form intermediate P7 (m/z = 439.15). P5 and P7 can then be sequentially cleaved via the ring-opening reactions to form intermediate P11 (m/z = 274.27). In pathway III, TCH (m/z = 445.16) underwent demethylation and deformylation to generate intermediate P3 (m/z = 388.25),⁵⁰ which was further decomposed via the ring-opening, H₂O removal and deamination reactions to produce intermediate P12 (m/z = 227.17).⁵¹ Eventually, these organic compounds continue to be mineralised to CO₂, H₂O and other small molecules under attack by free radicals, as demonstrated by TOC analysis (59.75% TOC degradation after 2 h of light; Fig. S8†). By introducing Ti₃C₂, an association of ZnFe₂O₄ grows in situ, i.e. a heterojunction is constructed between the two. This heterojunction produces active substances under visible-light conditions, destroying the structure of the target pollutant and generating several intermediates that ultimately degrade TCH into small molecules (CO₂ and H₂O) that are not toxic to the environment.

Fig. 7 Possible degradation pathway of TCH under visible-light irradiation.

3.4. Exploration of active substances and photocatalytic mechanisms

Catalyst photodegradation reactions generally involve the participation of ·OH, h⁺ and ·O₂⁻. To better identify the main active substances and the charge transfer mechanisms involved during TCH degradation, IPA, EDTA-2Na and P-BQ were introduced into the reaction system as trapping agents for ·OH, h⁺ and ·O₂⁻, respectively.⁵² The TCH removal rate decreased to 25.30% following the addition of EDTA-2Na, which was considerably lower than the TCH removal rate of 0.25/1 Ti₃C₂/ZnFe₂O₄ (87.10%) [Fig. 8(a)]. The P-BQ addition reduced the removal rate to 47.18%. In the presence of IPA, the removal rate of TCH was 72.61%, i.e. little effect on the degradation was observed. The above results indicate that ·O₂⁻ and h⁺ are the main active components involved in the photocatalytic degradation. On this basis, the active free radicals in the photoreaction process were verified using electron spin resonance (ESR) spectroscopy. As shown in Fig. 8(b) and (c), under visible light irradiation, the intensity ratio of the four peaks attributed to the characteristic signals of DMPO–·OH and DMPO–·O₂⁻ is 1 : 2 : 2 : 1 and the four strong group peaks of 1 : 1 : 1 : 1 (ref. 53) are considerably enhanced compared to those under dark conditions, verifying that the active substances ·OH and ·O₂⁻ generated under illumination accelerate the separation and transfer of space charges. The increase in the illumination time enhances the relative strength of the active group, indicating a corresponding increase in the yield. Although 2,2,6,6-tetramethylpiperidine oxide (TEMPO) reacts with h⁺ to form spin adducts, the ESR signal gradually weakens with increasing illumination time, verifying the presence of h⁺ [Fig. 8(d)].⁵⁴ This indicates that the photogenerated carriers in the 0.25/1 Ti₃C₂/ZnFe₂O₄ composite exhibit superior e⁻–h⁺ pair separation properties under light irradiation.

Fig. 8 (a) Effect of radical scavengers on the removal of TCH and ESR spectra of (b) DMPO–·O₂⁻, (c) DMPO–·OH and (d) TEMPO–h⁺ for 0.25/1 Ti₃C₂/ZnFe₂O₄ under visible light irradiation.

In addition, the photocatalytic activity of the system depends on the positions of the conduction band (CB) and valence band (VB). The VB-XPS spectrum (Fig. S9†) was used to clarify the CB and VB potential of the photocatalytic material and the e⁻ migration paths of Ti₃C₂ and ZnFe₂O₄ during the interaction at the heterojunction interface. According to the formula E_VB = E_CB + Eg, the E_VB values of Ti₃C₂, ZnFe₂O₄ and 0.25/1 Ti₃C₂/ZnFe₂O₄ are approximately 0.65, 1.03 and 1.09 eV and the corresponding E_CB values are −0.54, −1.01 and −0.81 eV, respectively. Because Ti₃C₂ acts as an ‘e⁻ trap’, the e⁻ density at the Fermi energy level is high, further changing the forbidden band width and enhancing its response to visible light. Based on the above analysis, a possible mechanism for photocatalytic degradation is proposed (Fig. 9). During the process of the photoinduced generation of photogenerated e⁻ and h⁺, the terminal group of Ti₃C₂ promotes the accumulation of photogenerated e⁻ owing to the loading of –OH and –O hydrophilic groups, forming a space charge layer (built-in electric field) near the interface of Ti₃C₂/ZnFe₂O₄. The metal–oxygen clusters in the n-type semiconductor ZnFe₂O₄ play the role of exciting photogenerated e⁻, and as its Fermi level (E_f ≈ E_CB) is higher than that of Ti₃C₂ (E_f = −0.60 V),⁵⁵ e⁻ is driven through the contact interface between the two to transfer to the Ti₃C₂ CB until a uniform Fermi level is formed. Meanwhile, the h⁺ is stranded on the VB, forming a Schottky barrier at the interface between the two, in which excited e⁻ crosses the interface and migrates in the form of a drift current beam and participates in the reaction such that no accumulation or recombination occurs.⁵⁶ Simultaneously, because the reduction potential of Fe²⁺/Fe³⁺ (Fe²⁺/Fe³⁺ = −0.77 V)⁵⁷ is lower than the E_CB of ZnFe₂O₄, Fe captures the photogenerated e⁻ of ZnFe₂O₄ and inhibits the recombination of e⁻–h⁺ pairs,⁵⁵ effectively separating the carriers and improving the photocatalytic efficiency. For Ti₃C₂, the CB (E_CB = −0.54 eV) is more negative than the potential of O₂/·O₂⁻ (−0.33 eV vs. NHE), implying that its enrichment and the photoactivated e⁻ trapped due to the presence of oxygen vacancies will be consumed to activate the dissolved O₂ adsorbed onto the surface, reducing O₂ to strongly oxidising superoxide radicals (·O₂⁻).⁵⁸ Subsequently, the reaction of ·O₂⁻ and H⁺ in water can produce secondary hydroxyl radicals (·OH),⁵⁹ reacting with e⁻ to produce H₂O₂, while Fe(II) can be converted into Fe(III) in the presence of H₂O₂ while producing ·OH. Because ZnFe₂O₄ exhibits E_VB = 1.03 eV, which is more negative than the ·OH/OH⁻ potential (+1.99 eV vs. NHE), the local water molecule cannot be oxidised to OH⁻ by h⁺ at the VB but its h⁺ at the VB can directly oxidise the TCH molecule in water.⁵³ This is in agreement with the results of ESR and active substance capture experiments. The specific reaction equation is presented as eqn (S1).†

Fig. 9 Schematic of the photocatalytic mechanism of tetracycline hydrochloride (TCH) degradation by the 0.25/1 Ti₃C₂/ZnFe₂O₄ composite photocatalyst.

3.5. Ecological toxicity detection

To explore whether the intermediates formed during the 0.25/1 Ti₃C₂/ZnFe₂O₄ photocatalytic degradation of TCH are harmful to the aquatic environment, the toxicity of a specific case of 50% lethal concentrations (fish: 96 h, Daphnia: 48 h and green algae: 96 h) was modelled using the ECOSAR simulation. Lower acute toxicity levels (LC₅₀) and chronic toxicity (ChV) values correspond to higher toxicity. The effects of TCH and its degradation products on fish, Daphnia and green algae, with the exception of individual substances [i.e. P3 and P5 in Fig. 10(e)], are essentially harmless to their development and growth (Fig. 10). In the aquatic environment, toxicity reduction or even non-toxicity is essential. Based on the above analysis, the intermediate formed is slightly toxic. Under visible light, TCH and its intermediates can be completely converted into small molecules, such as CO₂ and H₂O, as the mineralisation process proceeds, to attain completely non-toxic conditions.

Fig. 10 Ecotoxicity of TCH and its by-products: (a)–(c) acute toxicity and (d)–(f) chronic toxicity.

4. Universality of the photocatalyst

Based on the investigation of the degradation efficiency of TCH (Fig. 6) to explore the universality of the composite photocatalyst (0.25/1 Ti₃C₂/ZnFe₂O₄), malachite green (MG), xylenol orange (XO) and chlortetracycline (CTC) were used as target pollutants. Their UV–vis spectra were monitored at absorption wavelengths (λ) of 618, 460 and 275 nm, respectively, to evaluate the photocatalytic degradation efficiency [Fig. 11(a) and (b)]. Furthermore, their reaction kinetics [Fig. 11(c)] were analysed. Ti₃C₂ and ZnFe₂O₄ control experiments for the removal of the above pollutants are shown in Fig. S10.† The removal efficiencies of Ti₃C₂ for TCH, MG, XO and CTC were 7.29%, 7.73%, 3.36% and 2.41%, respectively, and those of ZnFe₂O₄ for TCH, MG, XO and CTC were 62.66%, 62.12%, 36.41% and 16.00%, respectively. However, the composite photocatalyst exhibited higher removal efficiencies for TCH (87.10%), MG (76.10%), XO (68.16%) and CTC (56.54%) [Fig. 11(a–c)]. In addition, by investigating the reaction kinetics of the photocatalytic degradation of the target pollutants by the composite photocatalyst [Fig. 11(c) and S11†], it was observed that TCH, MG, XO and CTC more closely followed the pseudo-second-order kinetics; the maximum value of the rate constants was 0.08035 (TCH), followed by 0.02470 (MG), 0.00794 (XO) and 0.00243 (CTC). The above analysis determined that the composite photocatalyst 0.25/1 Ti₃C₂/ZnFe₂O₄ exhibits high adaptability to pollutants in water, which is attributed to the active free radicals generated by the photocatalyst under light conditions, accompanied by a series of reactions, thereby further degrading the target pollutants.

Fig. 11 (a) Degradation curves of water pollutants malachite green, xylenol orange and chlortetracycline hydrochloride under visible light and (b) the corresponding degradation efficiency histogram and (c) pseudo-second-order kinetics analysis using 0.25/1 Ti₃C₂/ZnFe₂O₄ as the photocatalyst.

5. Conclusions

A photocatalytic material with a stable interface and high catalytic efficiency was prepared via simultaneously performing alcoholisation and alkalisation using a simple solvothermal method. The heterojunction constructed is coupled from an n-type semiconductor (ZnFe₂O₄) and a metal semiconductor rich in metal–oxygen groups owing to alkalisation and ethylene glycol alcoholisation (Ti₃C₂), exhibiting a close surface–interface contact, a strong visible-light response, a high photogenerated e⁻–h⁺ pair separation efficiency, a satisfactory electron transfer rate and high photocatalytic activity. Under visible-light (λ > 420 nm) irradiation, the removal rate of TCH was 87.10%, and its mineralisation rate was 59.75%. ·OH, h⁺ and ·O₂⁻ were identified as the active components in the degradation of TCH, and based on this a possible pathway for TCH degradation was proposed. Toxicity risk assessment revealed that in the presence of 0.25/1 Ti₃C₂/ZnFe₂O₄, the photodegradation of TCH reduces its toxicity and potential risk to the aquatic environment. Universality exploration demonstrated that the photocatalyst exhibits a good response capability to other pollutants in water bodies. Consequently, this study fully confirms the photocatalytic mechanism of Ti₃C₂/ZnFe₂O₄ heterojunctions through experimental and theoretical analyses, yielding several new ZnFe₂O₄-based heterojunction photocatalysts, describing their construction and providing a new approach to the photocatalytic degradation of water pollutants.

Author contributions

Hongqing He: conceptualisation, data curation, formal analysis, investigation, methodology, writing – original draft, writing – review & editing. Yu Fang: validation. Xinhao Sun: formal analysis. Xianbin Li: formal analysis. Shunzhi Li: formal analysis. Yang Cao: conceptualisation, funding acquisition, resources, writing – review & editing.

Conflicts of interest

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

Acknowledgements

This work was supported by the National Natural Science Foundation of China [No. 52161029], the Guizhou Science and Technology Cooperation Foundation-ZK[2021] General 243, and the Research Project of Introducing Talents of Guizhou University: No. 60, Renjihe Zi (2020).

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Footnote

† Electronic supplementary information (ESI) available: Chemicals and reagents, specific experimental operations for sample synthesis, evaluation of photocatalytic activity and characterisation methods/detection conditions. The spinel crystal structure, ESR spectrum, EDS spectrum, N₂ adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution, performance diagram of the photocatalytic degradation cycle, mass spectrometry, photocatalytic degradation concentration of TCH at different time points and the corresponding TOC values as well as the VB-XPS spectrum. A comparative table of specific surface area, pore volume and pore size distribution and the parameters and calculation formulas of decay life, degradation rate, kinetic coefficient and correlation coefficient table as well as the performance comparison table of different catalysts for TCH degradation. The possible intermediate products and specific reaction equations of photocatalytic degradation. See DOI: https://doi.org/10.1039/d3re00374d

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