Defective TiO₂/MIL-88B(Fe) Photocatalyst for Tetracycline Degradation: Characterization and Augmented Photocatalytic Efficiency

Abstract

Photocatalysts, such as TiO₂, are widely used in photoreduction. However, drawbacks like their wide band gap and short carrier lifetime lead to lower efficiencies with their use. Introducing defects and forming heterostructures of TiO₂ could extend the carrier’s light-harvesting range from UV to visible light and enhance its lifetime. Herein, an electron-beam irradiation-defected TiO₂ was induced in MIL-88B(Fe). The structure of the material was characterized using XRD, FT-IR, TEM, HRTEM, and XPS techniques. Remarkably, TiO₂ under 300 kGy electron-beam irradiation performed the best with a series of 0, 100, 300, and 500 kGy irradiation ratios. PL and UV–vis DRS were utilized to measure the material’s optical properties. The introduction of MIL-88B(Fe) expanded the light response range, reduced the optical band gap, and lengthened the carrier lifetime of the defective TiO₂ composite photocatalysts, resulting in superior TC photoreduction capabilities of 88B5%300, which degraded 97% of tetracycline (10 mg/L) in water after 120 min.

1. Introduction

Owing to its excellent photocatalytic ability, high economic efficiency, and non-toxicity, TiO₂ is widely used as a semiconductor material for environmental purification [1,2,3], which involves the elimination of dyes [4,5], heavy metals [6], pesticides, and antibiotics [7] from the environment. The mechanism of photodegradation is relative to the separation of photocarriers (e⁻/h⁺ pairs). When TiO₂ nanoparticles absorb UV light (280–400 nm), an electron placed in the valence band (VB) transitions to the conduction band (CB), which leaves a hole in the original position, called an electron–hole pair, or collectively known as a photogenerated carrier. Charged species, with their strong oxidizing and reducing properties, contribute to the generation of free radicals in the system, which play a prominent role as the driving force of photocatalytic reactions. Currently, photocatalysis, as a typical advanced oxidation process, utilizes redox reactions to activate the oxidant, which produces reactive species with higher redox potentials, such as •OH/H₂O₂ [8,9], SO₄^•−/PMS [10,11]. Persulfate-based advanced oxidation processes are now drawing attention for the efficient generation of highly reactive species, such as SO₄^•− and ¹O₂ [12]. Compared with traditional free radical species, the terrific catalytic effects and low energy requirement lead to more benefits upon their application. However, the dormancy under visible light and speedy recombination rate of photogenerated carriers limit their practical utilization. Many attempts have been made to counteract these drawbacks of TiO₂. Measures such as inducing ion NPs, creating heterostructures, and introducing defects are under study. For instance, Nithyaa et al. [13] doped Pr into TiO₂ to induce a magnetic response. Kandasamy et al. [14] confirmed the possibility that including N-S into a TiO₂ lattice could cause defects. The addition of N-S led to the presence of unpaired electrons on the surface, resulting in an increase in oxygen vacancy, which leads to an effective enhancement in the material’s ferromagnetism and electrical conductivity [15]. In addition, defect engineering is recognized as a brand-new field of material modification, which is the construction of specific defects to enhance the properties of materials. Defect engineering can also optimize heterogeneous structures and modulate the direction of electron migration to enhance the catalytic performance [16]. Instead of focusing on introducing defects in completely coordinated materials, defects on fundamental parts of the original material have been suggested, which could be possible since defective sites contain more reactive species and active functions than the overall material [15]. In a recent study, Yong et al. used electron-beam irradiation to treat reduced graphene oxides to increase the gas sensing behavior, and first report the mechanism [17]. Electron-beam irradiation has been revealed to create defect sites, such as slits and vacancies. The surface and electronic properties of the catalyst are modified by the introduction of vacancies. Vacancies are formed due to the absence of oxygen atoms in the lattice, and these defect sites have a high electron affinity that attracts and traps free electrons, thus inhibiting the complexation of photogenerated carriers in photocatalysis. The advantage of the EBI approach to introducing vacancies is that it is highly efficient and controllable and does not require the use of chemical reagents or changes in environmental modulation. A highly controllable process of introducing defects can be achieved by the precise modulation of energy.

Meanwhile, growing photo-active materials on metal organic frameworks (MOFs) is also a promising approach to improving the catalytic performance. In this way, the organic skeleton of MOFs could be utilized to transfer electrons, which effectively inhibits the recombination of photogenerated carriers [18]. MIL-88B(Fe), as one of the typical Fe(III)-based MOFs, is visible-light-sensitive, environmentally friendly, non-toxic, and cost-effective. Moreover, in sulfate radical-based photocatalysis, MIL-88B(Fe) could cycle between Fe(III)-O and Fe(II)-O to generate persulfate radicals from persulfate efficiently. In general, the preparation of defective TiO₂ and the construction of heterojunctions with Fe-MOFs is an effective way to improve the catalytic performance.

In line with these concepts, an electron beam irradiation technique was used to construct defects on TiO₂. Electron-beam irradiation-defected TiO₂/MIL-88B(Fe) photocatalysts are synthesized through a facile solvothermal method. The photocatalytic performance of the prepared photocatalysts is investigated using a PDS/visible light/TC system. XRD, XPS, FT-IR, TEM, and HRTEM were used for the characterization of the materials.

2. Results

The defective TiO₂/MIL-88B(Fe) composite photocatalysts were synthesized by a simple one-step hydrothermal method, and MIL-88B(Fe) was able to grow during the process and allow TiO₂ to attach to its structure. The exact conditions of the process will be described in Section 3.2. The material was named 88B@X%YYY, according to the addition ratio (X%: 0%, 3%, 5%, 7%, 10% of the yield of MIL-88B(Fe)) of TiO₂ under an irradiation dose of YYY kGy.

2.1. Characterizations

X-ray diffraction (XRD) data were recorded to demonstrate the crystallinity of the samples. As displayed in Figure 1a, the XRD patterns of MIL-88B(Fe) are consistent with the simulated XRD pattern [19]. The intense diffraction peaks observed at 9.8, 17.0, 25.9, 30.9, and 37.7 degrees correspond to the (100), (102), (103), (202), and (211) crystallographic planes. In Figure 1b, the XRD spectra of the composite catalysts with different TiO₂ additions are shown. It can be observed that the characteristic peaks in the samples are consistent with the standard card for anatase TiO₂ (JCPDS card no. 21-1272), corresponding to the (101), (103), (104), (112), (200) crystal faces. Clearly, the characteristic peaks of anatase TiO₂ and MIL-88B(Fe) appeared in the composite catalyst and increased with the rising dosage.

Fourier transform infrared spectrometry (FT-IR) showed the surface functional groups of the as-prepared samples (Figure 1c,d). The symmetrical stretching vibration of the Fe-O bond caused a sharp peak at 555 cm⁻¹ [20]. The peaks at 1661, 1597, and 1388 cm⁻¹ represent the stretching, asymmetric, and symmetric vibration caused by the C=O functional group, respectively [21], and the peak at 750 cm⁻¹ represents the vibrations of -COOH on the organic bones of the MOFs [22,23].

The X-ray photoelectron spectroscopy (XPS) information is given as follows. Peaks representing C, O, Fe, and Ti appeared in the survey spectrum of 88B@5%300. In the C 1 s XPS spectrum of 88B@5%300 (Figure 2b), the peaks at 284.80 and 286.25 eV could be ascribed to C-C, C-H [24], and CH₂-O [25]. Moreover, the peak at 288.74 eV corresponds to O-C=O, belonging to the organic bones of MOFs. The O 1 s XPS spectrum (Figure 2c) showed an absorption peak at 531.85 and 533.15 eV, which can be attributed to the C=O and C-O group in the MOFs [26]. The BE at 530.41 eV corresponds to the oxygen vacancy, and the peak at 529.91 eV was due to the lattice oxygen in Ti-O [27]. In the narrow spectrum of Fe 2 p (Figure 2d), the peaks at binding energy of 711.7 (Fe 2p 3/2) and 725.7 eV (Fe 2p 1/2) with two satellite peaks at 718.5 and 730.2 eV represent Fe³⁺ [28,29]. Additionally, the peaks at 714.1 and 730.2 eV correspond to Fe 2p 3/2 and Fe 2p 1/2 corresponding to Fe²⁺. In the Ti 2 p spectrum (Figure 2e), the peaks at 458.78 eV(Ti 2 p3/2) and 464.52 eV(Ti 2 p1/2) correspond to Ti⁴⁺ [24].

In the TEM images in Figure 3a, the spun cone shape of MIL-88B(Fe) can be seen, as expected, with TiO₂ dispersed discretely on it. More detailed structural information on TiO₂ is given in Figure 3b. Crystal lattices with d-spaces equivalent to 0.35 and 0.32 nm corresponding to the {101} and {110} planes [30,31,32] of anatase TiO₂ were found in the majority of the individual crystallites throughout the area observed. Distortions in the TiO₂ lattice were clearly observed, and all the observed variations in the lattice striations could be attributed to the increase in lattice strain or crystal defects due to high-energy electron-beam irradiation [33]. In particular, in some regions, the lattice stripes were distorted or their regular arrangement was lost (Figure 3(b3,b4)), suggesting that the crystal structure underwent strain; then, oxygen vacancies were formed.

2.2. Photocatalysis Performance

The photocatalytic performance of 88B@X%YYY was evaluated in terms of the tetracycline degradation rate, under the assistance of visible light and PDS. The results of the photocatalytic experiments (Figure 4a,b) pointed to the negligible photo-bleaching of TC as well as the low activity of 300 kGy TiO₂, at 35%. It was easy to observe that the photodegradation effect of the catalyst was significantly enhanced with the assistance of visible light and PDS. The TC photocatalytic degradation efficiency of the TiO₂/MIL-88B(Fe) composites with different irradiation dosages was also tested in the study, and it was found that TiO₂ irradiated under 300 kGy displayed superior degradation abilities when compared to the others. TC removal substantially increased, reaching 78%, 85%, 81%, and 72.5% with 3%, 5%, 7%, and 10% 300kGy TiO₂/MIL-88B(Fe), respectively. The highest photodegradation rate was observed for 88B@5%300.

The optimal amount of catalyst and the optimal amount of PDS in the catalyst/PDS/visible light system were also investigated. As can be seen in Figure 4c, as the catalyst concentration increased from 2 to 5 mg, the TC degradation rate increased to 90%; however, it decreased to 82% when the concentration increased to 10 mg. It was evident that at a concentration of 5 mg, the catalyst was able to optimally promote photocatalytic degradation in the system. In the PDS concentration experiment (Figure 4d), the highest degradation rate of the system, 92%, was achieved by increasing the concentration to 500 ppm. Considering that PDS was the paramount source of reactive species, increasing PDS dramatically increased the yield of ROS, which ultimately improved photodegradation in the 88B@5%300/Vis/PDS system [19]. The joining of PDS was activated by Fe(II) to produce SO₄^•− (Equation (1)) [34] by electron transfer. Additionally, the reaction between PDS and H₂O (Equation (2)) [35], generating HSO₅⁻, could be activated by Fe(III) to produce SO₄^•− (Equation (3)) [36]. The system achieved the continuous transformation of Fe(II) and Fe(III), cyclically generating highly reactive free radicals. While the catalytic efficiency decreased significantly as the concentration increased to 1000 ppm, clearly, the excess catalyst and PDS concentration led to the production of additional SO₄^•−, and the self-quenching reaction of SO₄^•− produced a less reactive species (Equations (4) and (5)) [37], leading to the system being inefficient.

$$\mathrm{Fe}\left(\mathrm{II}\right)+{ \mathrm{S}}_2{\mathrm{O}}_8^{2-}\to \mathrm{Fe}\left(\mathrm{III}\right)+\mathrm{S}{\mathrm{O}}_{4 }^{•-}+\mathrm{S}{\mathrm{O}}_4^{2-}$$

(1)

$${\mathrm{S}}_2{\mathrm{O}}_8^{2-}+{\mathrm{H}}_2\mathrm{O} \to \mathrm{HS}{\mathrm{O}}_4^{-}+\mathrm{HS}{\mathrm{O}}_5^{-}$$

(2)

$$\mathrm{Fe}\left(\mathrm{III}\right)+\mathrm{HS}{\mathrm{O}}_5^{-}\to \mathrm{Fe}\left(\mathrm{II}\right)+\mathrm{S}{\mathrm{O}}_5^{•-}+{\mathrm{H}}^{+}$$

(3)

$$\mathrm{S}{\mathrm{O}}_4^{•-}+\mathrm{S}{\mathrm{O}}_4^{•-}\to {\mathrm{S}}_2{\mathrm{O}}_8^{2-}$$

(4)

$$\mathrm{S}{\mathrm{O}}_4^{•-}+{\mathrm{S}}_2{\mathrm{O}}_8^{2-}\to \mathrm{HS}{\mathrm{O}}_4+{\mathrm{S}}_2{\mathrm{O}}_8^{•-}$$

(5)

The influence of pH was depicted in Figure 5a. As illustrated, the acidic condition accelerated the TC degradation, which achieved a removal rate of 99% with an initial pH of 5.4. However, When PH increased to 9.8, the change in degradation was divided into two stages. During the dark stage, the TC degradation rate was significantly enhanced but gradually slowed down in the photocatalytic stage. Singlet oxygen(¹O₂) has also been considered as a potent active species in PDS-assisted degradation systems, as it has exotic activities under an alkaline nature. Therefore, a hypothesis has been raised to explain the increase that PDS has transferred to ¹O₂ during the dark-reaction stage (Equations (6)–(11)) [38].

Nevertheless, the TC removal was lower than the reaction in natural nature at the end. This might be caused by excessive OH⁻ scavenging the radical [39]. The result also indicated that the material is able to withstand a range of pH changes while preserving a high degree of degradation (>94%). Meanwhile, in the recycling experiment (Figure 5b), the material retains a favorable catalytic performance after three cycles.

$${\mathrm{S}}_2{\mathrm{O}}_8^{2-}+{\mathrm{H}}_2\mathrm{O} \to {\mathrm{H}}^{+}+2\mathrm{S}{\mathrm{O}}_4^{2-}+\mathrm{H}{\mathrm{O}}_2^{•}$$

(6)

$${\mathrm{S}}_2{\mathrm{O}}_8^{2-}+\mathrm{H}{\mathrm{O}}_2^{•}\to \mathrm{S}{\mathrm{O}}_4^{•-}+\mathrm{S}{\mathrm{O}}_4^{2-}+{\mathrm{O}}_2^{•-}+{\mathrm{H}}^{+}$$

(7)

$${2\mathrm{O}}_2^{•-}+2{\mathrm{H}}_2\mathrm{O}\to {}^1\mathrm{O}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(8)

$${\mathrm{O}}_2^{•-}+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{H}}^{+}+{}^1\mathrm{O}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(9)

$$\mathrm{H}{\mathrm{O}}_2^{•}+\mathrm{S}{\mathrm{O}}_4^{•-}\to {}^1\mathrm{O}_2+\mathrm{HS}{\mathrm{O}}_4^{-}$$

(10)

$${\mathrm{HO}}_2^{•}+•\mathrm{OH}\to {}^1\mathrm{O}_2+{\mathrm{H}}_2\mathrm{O}$$

(11)

2.3. Mechanism

In order to elucidate the active species that have a primary role in the system as well as the reaction mechanism, equal amounts of tert-butanol (TBA), 1, 4-benzoquinone (BQ), furfuryl alcohol (FAL), and methanol (MeOH) were added as quenching agents. TBA was able to quench SO₄^•−, while MeOH was able to simultaneously quench SO₄^•− and •OH. Therefore, a comparison of the two allowed for determining the contribution of SO₄^•− and •OH to the system. BQ and FAL were used as quenchers of the superoxide radical (O₂^•−) and the singlet oxygen (¹O₂). The role of ROS in the experiments can be determined by the change in the degradation rate. As the results show in Figure 6a, TBA and MeOH both suppressed the degradation of TC. Thus, both SO₄^•− and •OH were involved in the reaction, in which •OH played a major role. Furthermore, the inhibition of BQ was more pronounced compared with MeOH. The experimental results also demonstrate that the addition of FAL had an influence on the reaction efficiency, hypothesizing that ¹O₂ was greatly involved in the degradation of TC, resulting in a declining degradation rate of 59%.

The UV–vis DRS spectrum (Figure 6b) clearly shows that the material displays good absorption in the visible region; the Tauc-plot method (Figure 6c) calculated that the composite material is significantly narrower compared to TiO₂, and the composite material has a narrower energy band gap than that of the pure MIL-88B(Fe), which is 2.37 eV and 2.57 eV, respectively.

In the PL spectrum (Figure 6d), it was noticed that MIL-88B(Fe) had stronger peaks, which was related to the separation and recombination rate of the photogenerated carriers. This result indicated that the photogenerated carriers have a prolonged lifetime, which is favorable for the photocatalytic reaction. Based on the above discussion, the mechanism of 88B@5%300/Vis/PDS photocatalysis is shown in Figure 7.

3. Materials and Methods

3.1. Materials

The following chemicals were used directly without further purification. Titanium dioxide (commercial TiO₂, ≥99.5%) was purchased from Degussa AG Co., Ltd. (Essen, Germany). N, N-Dimethylformamide (DMF, 99.5%), and tetracycline was purchased from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Iron chloride hexahydrate (FeCl₃·6H₂O, 99.0%) and terephthalic acid (H₂BDC, 99.0%) were obtained from the Aladdin Chemistry Co., Ltd. (Shanghai, China). Deionized water was also used.

3.2. Catalyst Preparation

3.2.1. Electron-Beam Irradiation of TiO₂

The irradiation experiments were carried out using a GJ-II electron gas pedal from the Institute of Applied Radiation Research at Shanghai University, with the electron-beam energy set to 1.8 MeV and the beam current set to 1.0 mA. 1 g of TiO₂ was laid flat in a sealed sample bag of 1 mm thickness. Then, the sample was placed in a reciprocating table 30 cm from the titanium window to ensure that the samples were all irradiated. The intensity of the irradiation was set to 0 (no irradiation), 100, 300, and 500 kGy.

3.2.2. Synthesis of 88B@X%YYY Series Materials

A simple hydrothermal measure was used to create the catalyst (Figure 8). Firstly, 2.3 mmol FeCl₃·6H₂O and 2.3 mmol terephthalic acid (H₂BDC) were mixed in 50 mL DMF for 30 min. Then, an appropriate amount of treated TiO₂ powder was added to the above suspension and sonicated for 30 min. After stirring, the solution was heated at 145 °C for 12 h. The brown suspension was washed with MeOH and DMF three times, then vacuum-dried at 70 °C for 12 h to obtain the sample. TiO₂ was added at X% (X = 3, 5, 7, 10%) of the yield of MIL-88B(Fe), which was prepared in the same manner, and the sample was designated as 88B@X%YYY (Y = 0, 100, 300, 500 kGy is the irradiation dose of TiO₂).

3.3. Photocatalytic Experiments

Firstly, 10 mg of 88B@X%YYY was added to 50 mL of 10 mg/L TC and was stirred in the dark for 30 min. After achieving the adsorption–desorption balance, the light source was turned on to simulate visible light with a filter, and 200 ppm PDS was added at the same time. The sample was tested using a UV-vis spectroscopy(Model Evolution 60S, Thermo Fisher Scientific, Waltham, MA, USA) at a wavelength of 357 nm.

3.4. Characterization Techniques

X-ray diffraction (XRD, Ultima IV, Rigaku Corporation, Tokyo, Japan), fourier transform infrared spectrometer (FT-IR, Nicolet iS50, Thermofisher Scientific, Waltham, MA, USA), X-ray photoelectron spectrometry (XPS, ESCALAB 250Xi, Thermofisher Scientific, Waltham, MA, USA), and transmission electron microscope (TEM, Talos F200S G2, Thermo Fisher Scientific, Waltham, MA, USA) were used for recognizing the crystal structure, appearance, and surface functional groups and elements. To further understand the mechanism of photocatalytic enhancement, UV–vis DRS (UV2600, Shimadzu Corporation, Kyoto, Japan) and photoluminescence spectroscopy (PL, F-7000, Hitachi High-Tech Corporation, Tokyo, Japan) were also employed in the experiments. Photocatalytic experiments were carried out using a photocatalytic reaction apparatus (PLS-SXE 500, Beijing Perfectlight Technology Co., Ltd., Beijing, China) with filters to simulate visible light catalysis in a dark box.

4. Conclusions

In this study, 88B@5%300 composites were synthesized using the one-step hydrothermal method. When the electron-beam irradiation technique was applied to TiO₂, its lattice was observed to be distorted or disrupted on TEM, which contributed to the generation of oxygen vacancies. The photocatalysis performance was determined by the photocatalytic degradation of TC under the assistance of visible light and PDS. The main reactions involved were represented by the previous Equations (1)–(3), Equations (6)–(11) and subsequent Equations (12)–(14) [40]. Based on the results of the quenching experiments, the degradation of the present system consists of both the radical pathway and the non-radical pathway, where the impact of the non-radical ¹O₂ has more pronounced effects compared to the other active species.

$$88\mathrm{B}@5\%300+\mathrm{h}\upsilon \to {\mathrm{h}}^{+}+{\mathrm{e}}^{-}$$

(12)

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to { \mathrm{O}}_2^{•-}$$

(13)

$${}^1\mathrm{O}_2/{ \mathrm{O}}_2^{•-}/\mathrm{S}{\mathrm{O}}_4^{•-}/•\mathrm{OH}+\mathrm{TC} \to \mathrm{Product}$$

(14)

Under optimal experimental conditions, 88B@5%300 was able to degrade 97% of 10 mg/L TC at 120 min. The FT-IR results indicated that the high irradiation dose TiO₂ and the excess defective TiO₂ both could impact the crystal structure. Moreover, the improvement not only contributed to the loading on MIL-88B(Fe), but the synergistic effect of PDS and visible light. The result of the UV-vis DRS and the Tauc-plot demonstrated that the visible light absorption was enhanced, the bandwidth was effectively reduced by the composite, and the organic bones of MIL-88B(Fe) could be utilized to extend the photogenerated carriers’ lifetime, which also effectively improved the efficiency of the photocatalysis treatment. In summary, the idea of modifying the base material rather than the overall material to further enhance the existing material is feasible and provides theoretical support for subsequent modification and enhancement of the material.