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Article

Fabrication of Two-Dimensional Bi2MoO6 Nanosheet-Decorated Bi2MoO6/Bi4O5Br2 Type II Heterojunction and the Enhanced Photocatalytic Degradation of Antibiotics

by
Fengshu Kang
,
Gaidong Sheng
,
Xiaolong Yang
and
Yan Zhang
*
College of Chemistry and Chemical Engineering, Qingdao University, No. 308 Ning-Xia Road, Qingdao 266071, China
*
Author to whom correspondence should be addressed.
Inorganics 2024, 12(11), 289; https://doi.org/10.3390/inorganics12110289
Submission received: 3 October 2024 / Revised: 29 October 2024 / Accepted: 31 October 2024 / Published: 4 November 2024
(This article belongs to the Special Issue Development of Nanocomposite Materials for Environmental Remediation and Biomedical Application)
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Abstract

:
This article successfully synthesized a series of Bi2MoO6/Bi4O5Br2 heterojunctions using a two-step solvothermal method followed by calcination, and the photocatalytic activity by degradation of tetracycline hydrochloride (TC) was investigated. Compared with pure Bi4O5Br2 and Bi2MoO6, a series of Bi2MoO6/Bi4O5Br2 heterojunctions exhibit higher photocatalytic activity, which can be attributed to the heterostructures with strong interfacial interaction, improving the charge separation. The 2% Bi2MoO6/Bi4O5Br2 heterojunction shows the best photocatalytic activity under visible light irradiation, which is 1.9 times and 1.8 times that of Bi2MoO6 and Bi4O5Br2, respectively. In addition, cyclic experiments have shown that 2% Bi2MoO6/Bi4O5Br2 heterojunction has high stability, with a degradation efficiency only decreasing by 3% after 5 cycles. From the capture agent experiment and ESR test, it can be seen that ·O2− and h+ are the main active species. A possible photocatalytic mechanism of 2% Bi2MoO6/Bi4O5Br2 heterojunction under visible light irradiation was proposed.

1. Introduction

In recent years, healthcare has made significant contribution to meeting people’s growing needs. According to the survey, the market size of the global pharmaceutical industry is expected to increase from 15,850.5 billion USD in 2022 to 24,012.2 billion USD in 2029 [1]. As an antibiotic, tetracycline is produced and used on a large scale because of its versatility. However, due to the complex molecular structure of tetracycline, it is difficult to degrade naturally, which makes tetracycline a major cause of water pollution. Therefore, the research and development of effective technologies to remove antibiotics has become an urgent task in the future [2,3]. Photocatalytic technology has been widely studied by researchers as an efficient, clean, and environmentally friendly technology. Among them, Peng et al. reported the facile synthesis of porous TiO2 for the degradation of tetracycline under visible light [4]. Han et al. reported the use of coal gangue to optimize the morphology of carbon nitride for photocatalytic degradation of tetracycline [5]. Photocatalytic degradation of pollutants has been verified on various semiconductor catalysts, such as TiO2 [6], ZnO [7], g-C3N4 [8], Bi2WO6 [9], and BiOBr [10].
As a Bi-rich layered semiconductor material, Bi4O5Br2 (BOB) has recently attracted wide attention [11,12]. Due to its conduction band (CB), Bi4O5Br2 is beneficial to the photocatalytic activation of O2 [13], which has broad application prospects in the fields of photocatalytic hydrogen production [14], photocatalytic degradation [15], organic light reaction and CO2 reduction [16,17], and photocatalytic nitrogen fixation [18]. Nevertheless, pure Bi4O5Br2 suffers from a relatively fast photoexcited carrier recombination, which hinders its application as a photocatalyst [19]. In order to solve these shortcomings, various modification strategies have been developed to accelerate the separation of photogenerated carriers and further enhance the catalytic activity [20]. Notably, the unique ‘nanosheet-to-nanosheet’ heterostructure presents larger coupled heterogeneous surfaces, facilitating the construction of built-in electric fields and thereby improving photocatalytic efficiency [21,22]. In the heterojunction-based photocatalytic system, for the type I heterojunction, the band energy level of one semiconductor is contained within that of another semiconductor, allowing electrons and holes to migrate in the same direction [23,24]. In contrast, due to the existence of interlaced bands in type II heterojunctions, electrons and holes can be transferred to different semiconductors, thereby realizing charge separation in space [25,26]. Therefore, the preparation of efficient ‘nanosheet-to-nanosheet’ type II heterojunctions is more attractive for improving photocatalytic activity.
Bi2MoO6 (BMO) is a typical Aurivillius material [27,28], which has attracted extensive attention due to its excellent physical and chemical properties [29,30]. Bi2MoO6 features a special perovskite-type layered structure [31], which promotes the transfer of photogenerated electrons and holes [32]. The band structure and crystal structure characteristics make it have broad application prospects in the photocatalytic fields [33,34,35,36]. Although Bi2MoO6 has its effectiveness as a photocatalyst, its response in the visible light range is limited, and there is still a problem that the recombination rate of photogenerated carriers is fast, resulting in lower catalytic performance and light quantum efficiency. In fact, several effective methods have been proposed to overcome the inherent limitations of Bi2MoO6, including ion doping [37,38], creating heterojunctions [39,40], etc. According to these methods, the development of type II heterojunctions has attracted much attention because it makes full use of the advantages of each component and overcomes their respective shortcomings. The creation of the nanosheet-to-nanosheet heterojunction allows the formation of face-to-face contact, resulting in a large interface region that accelerates the transfer of photogenerated carriers. Therefore, the photocatalytic efficiency of Bi2MoO6 can be further improved by forming type II heterojunctions.
The ultrathin nanosheets offer abundant active sites, while the heterojunction structure significantly improves the separation of carriers. The synergistic effect of these two factors significantly boosts the catalyst performance. In this study, type II Bi2MoO6/Bi4O5Br2 heterojunctions were synthesized using a two-step solvothermal method. Thus, controllable modification of Bi2MoO6 on Bi4O5Br2 can be realized. Afterwards, a strong interaction was formed between Bi4O5Br2 and Bi2MoO6 in the composite by calcination, in favor of carrier transfer. Based on the experimental results, the mechanism of photocatalytic degradation of tetracycline was discussed.

2. Results and Discussion

The crystal purity and phase composition of the samples were determined by X-ray diffraction (XRD). The XRD patterns of pure Bi4O5Br2, Bi2MoO6, 1% BMO–BOB, 2% BMO–BOB, 5% BMO–BOB, and 7% BMO–BOB are shown in Figure 1a. The crystal planes of (112), (11-3), (020), (105), and (422) can be assigned to 24.2°, 29.2°, 31.80°, 42.9°, and 45.6° (JCPDS 37-0699) of Bi4O5Br2, respectively [16]. In addition, according to the standard card of Bi2MoO6 (JCPDS 84-0787) [41,42], pure Bi2MoO6 corresponds to 28.14°, 32.42°, 46.62°, and 55.42° crystal planes of (131), (002), (202), and (331), respectively. XRD patterns of Bi4O5Br2 and Bi2MoO6 are highly crystallized without impurities, which are consistent with the result reported previously [26]. This is enough to show that Bi4O5Br2 and Bi2MoO6 were successfully synthesized by the first preparation scheme. Due to lower Bi2MoO6 contents, typical signals of Bi2MoO6 in BMO–BOB composites are not particularly evident in the figure. However, with the increase in Bi2MoO6 loading, the diffraction peak intensity of Bi2MoO6 gradually increased, particularly in 7% BMO–BOB. The combination of Bi4O5Br2 and Bi2MoO6 was further confirmed by HRTEM analysis.
In order to further study the chemical composition and elemental state of Bi4O5Br2 and 2% BMO–BOB samples, X-ray photoelectron spectroscopy (XPS) characterization was carried out. Figure 1b shows the full energy spectrum of 2% BMO–BOB, where obvious Bi, O, Br, and Mo element symbols are collected, indicating the presence of these elements on the surface of the material. The binding energy peaks of Bi 4f7/2 and Bi 4f5/2 in Bi4O5Br2 are located at 159.2 eV and 164.5 eV in Figure 1c, indicating that Bi element exists in the form of Bi3+ [43,44]. The Bi 4f peaks of 2% BMO–BOB shifted downward to 159.06 eV and 164.36 eV, respectively, indicating the electron cloud density increased, which demonstrates the strong interaction formed between Bi4O5Br2 and Bi2MoO6 [45]. Figure 1d shows that the 3d signal peak convolution of Br at 68.4 eV and 69.4 eV, belonging to Br 3d5/2 and Br 3d3/2 [46], respectively. In the 2% BMO–BOB material, both Br 3d5/2 and Br 3d3/2 shift to low binding energy by 0.2 eV, which are still in the form of Br− [47]. The high-resolution XPS spectra (O1s) of BOB, BMO and, 2% BMO–BOB materials are shown in Figure 1e. The peaks at 529.98, 529.8, and 529.85 eV are assigned to lattice oxygen; the peaks at 531.1, 531, and 531.1 eV are assigned to surface hydroxyl groups, and the peaks at 532.46, 532.37 and 532.5 eV are assigned to oxygen vacancies [48], belonging to BMO, 2% BMO–BOB, and BOB materials, respectively. Figure 1f shows the high-resolution spectra of Mo, and the peaks at 235.6 eV and 232.45 eV are attributed to Mo 3d3/2 and Mo 3d5/2, respectively, indicating that the Mo element exists in the form of Mo6+. It is worth noting that the two weak peaks at 234.3 eV and 231.4 eV are assigned to Mo5+ cations [49,50]. Only two strong peaks at 235.5 eV and 232.35 eV were found in 2% BMO–BOB, which were attributed to Mo 3d3/2 and Mo 3d5/2, respectively, belonging to Mo6+ cations. The low-valent Mo ions (Mo5+) are replaced by high-valent Mo6+, indicating that electron transfer occurs during the process of the composite, causing Mo5+ to be oxidized to high-valent Mo6+ [49]. Compared to pure BMO and BOB, it can be inferred that there is a strong chemical interaction between Bi2MoO6 and Bi4O5Br2 in the composite.
Scanning electron microscopy was used to observe the morphology structure of the pure Bi4O5Br2 and the 2% BMO–BOB composite. As shown in Figure 2a, the morphology of Bi4O5Br2 is a typical 2D nanosheet structure. Compared with pure Bi4O5Br2, it is clear that the composite 2% BMO–BOB nanosheets become thinner and more uniform in Figure 2b. So, the construction of the BMO–BOB heterojunction not only reduces the agglomeration of Bi4O5Br2, but also produces more coupling heterointerfaces.
In order to further study and determine the successful synthesis of BMO–BOB composite materials, HRTEM analysis and element mapping images were carried out. As shown in Figure 2c,e, both pure Bi4O5Br2 and the 2% BMO–BOB composite have a clear lattice structure. In Figure 2d,g, the lattice spacing of Bi4O5Br2 is shown to be 0.281 nm, which is consistent with the (11-3) crystal plane spacing. Meanwhile, the lattice spacing in Figure 2f is shown to be 0.314 nm, which matches the (131) crystal plane of Bi2MoO6. In addition, the element mapping analysis was carried out, shown in Figure 2h. The results indicate the elements of Bi, Mo, O, and Br were uniformly dispersed on the surface of 2% BMO–BOB. These results further confirmed the successful synthesis of BMO–BOB composites.
The optical properties of the catalyst can reflect its photocatalytic performance. Figure 3a shows the optical properties of BOB, BMO, and 2% BMO–BOB samples. The band gap of pure BOB and BMO were calculated by the Tauc method, following the equation (αhν) = A(hν − Eg) n/2, where α, ν, Eg, and A represents absorption coefficient, light frequency, band gap, and a constant, respectively. For Bi2MoO6, which is a direct band gap, n = 1 [51]. For Bi4O5Br2, which is an indirect band gap, n = 4 [52]. The Eg values are shown to be 2.51 and 2.33 eV for BMO and BOB in Figure 3b, respectively. The flat band potential (Efb) of the material was measured by the electrochemical test method and recorded in the Mott–Schottky diagram. As shown in Figure 3c, the flat band potentials (Efb) of Bi4O5Br2 and Bi2MoO6 are −0.42 eV and −0.38 eV (vs. Ag/AgCl), respectively. For n-type semiconductors, the CB position is minus 0.1~0.2 eV compared to the flat band potential [53]. Therefore, the CB of pure Bi4O5Br2 and Bi2MoO6 are −0.62 and −0.58 eV, respectively (vs. Ag/AgCl). According to ENHE = EAg/AgCl + E0Ag/AgCl; E0Ag/AgCl = 0.1976V vs. NHE (25 °C) [54], the CB of pure Bi4O5Br2 and Bi2MoO6 are about −0.42 and −0.38 eV, respectively (vs. NHE). The EVB is calculated by Eg and ECB (EVB = Eg + ECB), as shown in Figure 3d, the EVB of Bi4O5Br2 and Bi2MoO6 are 1.91 eV and 2.13 eV, respectively.
Photoelectrochemical techniques and fluorescence spectroscopy were used to analyze the separation behavior of carriers. Figure 4a shows the transient photocurrent response of Bi4O5Br2, Bi2MoO6, and the 2% BMO–BOB catalyst under intermittent visible light irradiation. It is clearly observed that the 2% BMO–BOB material produced a high photocurrent response, indicating that the 2% BMO–BOB heterojunction effectively separates the photogenerated carriers [55]. The EIS was used to study the impedance of the catalyst, as shown in Figure 4b. In the EIS Nyquist diagram, the smaller the semicircle radius, the smaller the corresponding charge resistance and the higher the charge transfer efficiency [56]. Obviously, the semicircle radius of 2% BMO–BOB is the smallest, and the resistance of the composite catalyst is lower than that of pure Bi4O5Br2 and Bi2MoO6. This proves that the composite catalyst can effectively reduce the resistance and increase the photogenerated charge transfer rate [57]. In addition, the steady-state fluorescence intensity of the catalyst was tested at an excitation wavelength of 380 nm. As shown in Figure 4c, the peak intensity of Bi4O5Br2 and Bi2MoO6 are both higher, indicating that the recombination behavior of photogenerated carriers is serious. The peak intensity of the 2% BMO–BOB catalyst is lower than that of Bi4O5Br2 and Bi2MoO6, which proves that the photogenerated carriers achieve efficient separation in the synthesized 2% BMO–BOB heterojunction. In the time-resolved transient fluorescence spectrum (Figure 4d), the average fluorescence lifetime of 2% BMO–BOB (6.1342 ns) is longer than that of Bi2MoO6 (1.8986 ns) and Bi4O5Br2 (5.0402 ns). It shows that the heterojunction between Bi2MoO6 and Bi4O5Br2 inhibits the recombination behavior and makes the charge transfer rapidly and, thus, prolongs the lifetime of photogenerated carriers.
In this study, the photocatalytic degradation of tetracycline over different catalysts is used to evaluate the performance of the catalyst. As shown in Figure 5a, the catalyst reaches adsorption and desorption equilibrium within one hour under dark conditions. Under two hours of illumination, the degradation effect of BMO–BOB composite catalysts was better than that of single catalyst. The degradation effect of 2% BMO–BOB was the best, with a degradation rate of 76% (Figure 5b). The 5% BMO–BOB catalyst was followed by it, and the degradation rate reached 68%, which was consistent with the results of photoelectrochemical analysis. This is because the photogenerated carriers are efficiently separated and quickly transferred. In order to study the stability of the catalyst, cycle experiments were carried out to evaluate the performance of 2% BMO–BOB. As shown in Figure 5c, the performance of the catalyst only decreased by 3% after five cycles of experiments. Furthermore, the recycled 2% BMO–BOB catalyst was characterized by XRD, as shown in Figure 5d. The XRD analysis indicates that the recycled catalyst maintains the same crystal structure as the original material, underscoring the stability of the catalyst. The effects of pH value and temperature on the photocatalytic degradation of tetracycline were investigated. As shown in Figure S1, the TC degradation efficiency of 2% BMO–BOB reached 78%, 79%, 76%, 74%, and 59%, respectively, across different pH (3, 5, 7, 9, 11) conditions. Therefore, the 2% BMO–BOB catalyst maintains a high degradation efficiency under acidic to mildly alkaline environments. As the reaction temperature rose from 15 °C to 25 °C in Figure S2, the degradation rate increased by 3%. Further increasing the temperature from 25 °C to 35 °C, the degradation rate was stable (76.4%). Thus, the 2% BMO–BOB photocatalyst shows good activity, stability, and adaptability in the photocatalytic degradation of tetracycline.
The active species of 2% BMO–BOB photocatalytic degradation of tetracycline were investigated by capture experiments. P-benzoquinone (BQ), ethylenediaminetetraacetic acid (EDTA), and tert-butanol (TBA) were introduced into the system to quench the ·O2−, h+ and ·OH active species in the process of photocatalytic degradation of tetracycline. When ·O2− and h+ were captured, the degradation rate decreased from 76% to 15% (BQ) and 34% (EDTA). This indicates that·O2− and h+ produced by photocatalysis play a major role in this system. In addition, combined with the energy band structure, the ECB of Bi4O5Br2 and Bi2MoO6 is more negative than the redox potential of ·O2− (E [O2/·O2−] = −0.33 eV vs. NHE) [58], so it can drive the redox reaction of O2 to form ·O2−. Meanwhile, ·OH radicals can still play an inhibitory role for the degradation of tetracycline, and the degradation rate is reduced from 76% to 56%. The EVB of Bi4O5Br2 and Bi2MoO6 was more negative than the redox potential of ·OH (E[H2O/·OH] = 2.34 eV vs. NHE) [59]. Therefore, ·OH radicals cannot be produced during the photocatalytic process, which was converted from ·O2−. The ·OH radical plays a role in assisting the oxidation of tetracycline.
ESR technology was used to further explore the ability of the material to produce active species by photocatalysis. DMPO was used to capture superoxide radicals. As shown in Figure 6b,c, under dark conditions, there is no signal peak in pure Bi4O5Br2 and 2% BMO–BOB. After 10 min of illumination, DMPO–·O2− signal peaks can be observed for both materials. Compared with pure Bi4O5Br2, the DMPO–·O2− signal peak of the 2% BMO–BOB heterojunction is more obvious, indicating that 2% BMO–BOB produces more superoxide radicals during the degradation process. As shown in Figure 6d,e, after 10 min of illumination, the intensity of the triple TEMPO–h+ peak collected by the 2% BMO–BOB heterojunction decreased more significantly than that of Bi4O5Br2, indicating that more photogenerated holes can be separated to participate in the reaction after the formation of the heterojunction. As shown in Figure 6f,g, after 10 min of illumination, DMPO–·OH (peak intensity ratio 1:2:2:1) signal peaks were observed for both Bi4O5Br2 and 2% BMO–BOB, and the signal peak of 2% BMO–BOB was stronger than that of Bi4O5Br2. This is due to the fact that 2% BMO–BOB produces more superoxide radicals, which leads to increased conversion of these radicals into hydroxyl radicals. This result is consistent with the results of free radical capture experiments.
Based on the above results, a possible type II approach was adopted to explain the charge separation and transfer process on the 2% BMO–BOB heterojunction, as shown in Figure 7. Under visible light irradiation, electrons of both Bi4O5Br2 and Bi2MoO6 are excited from VB to CB. The CB potential of Bi4O5Br2 (−0.42 eV) is more negative than that of Bi2MoO6 (−0.38 eV), while the VB of Bi2MoO6 (2.13 eV) is more positive than that of Bi4O5Br2 (1.91 eV). Bi4O5Br2 and Bi2MoO6 are in close contact to construct a heterojunction; photogenerated electrons can be transferred from Bi4O5Br2 to the CB of the Bi2MoO6. Meanwhile, photogenerated holes can be transferred from Bi2MoO6 to the VB of the Bi4O5Br2. Therefore, the effective separation of holes and electrons can be realized. The photoexcited h+ in the VB of Bi4O5Br2 can directly participate in the oxidative degradation of TC. Electrons on the CB of Bi2MoO6 have a more negative reduction potential than the O2/·O2− (−0.33 eV) potential, which can effectively generate active ·O2− by reacting with dissolved O2 in the solution. In addition, since the VB potential is lower than H2O/·OH (2.34 eV), it is difficult to generate ·OH radicals by direct oxidation of H2O. Instead, the photoexcitation of e− induced O2 to produce ·O2−, which was further converted into H2O2 and subsequently into ·OH radicals. [60,61]. The ESR spectrum of DMPO–·OH also confirmed the formation of ·OH. In summary, ·O2− and photogenerated h+ are considered to be the key active species to promote TC degradation and the ·OH radical-assisted oxidation reaction. These discussions are in good agreement with the results of free radical trapping experiments. The 2% BMO–BOB type II heterojunction achieves a higher separation rate of the photogenerated electron-hole pairs, allowing for more efficient utilization of photogenerated carriers in the degradation process. Thus, the reaction rate of tetracycline degradation was accelerated.

3. Materials and Methods

3.1. Preparation of Photocatalysts

The agents used in the synthesis of the photocatalysts include: Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), potassium bromide (KBr), Sodium molybdate (Na2MoO4·2H2O), ethylene glycol (CH2OH)2, sodium hydroxide (NaOH), and ethanol (C2H6O) are analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Tetracycline was purchased from Dalian Ronghai Biotechnology Co., Ltd. (Dalian, China). Solutions were prepared with deionized water.
For the preparation of pure Bi4O5Br2, a one-step solvothermal process was used. In a typical synthesis, 970 mg Bi(NO3)3·5H2O was dispersed in 25 mL (CH2OH)2, stirring for 30 min. Meanwhile, 238 mg KBr was dispersed in 10 mL (CH2OH)2, stirring until the solution becomes clear. The above two solutions were uniformly mixed. The pH of the mixed solution was adjusted to 10.5–11.5 using 2.0 mol/L NaOH solution. Finally, the mixture was loaded into a 100 mL Teflon-lined autoclave and maintained at 160 °C for 12 h. The product was washed three times with deionized water and anhydrous ethanol, and then dried at 60 °C. The obtained product was labeled as Bi4O5Br2 (BOB).
The synthesized route of Bi2MoO6/Bi4O5Br2 is illustrated in Scheme 1. Briefly, a certain amount of Bi4O5Br2 was dissolved in ethylene glycol. Then, 9.8994 mg of Bi (NO3)3·5H2O and 2.4689 mg of Na2MoO4·2H2O were added to the above solution and stirred for 2 h. Next, 45 mL ethanol was added and stirred for 1 h. The mixed solution was poured into a 100 mL autoclave and self-assembled at 160 °C for 24 h. The product was washed and dried. The resulting products were placed in a porcelain boat with a lid and heated to 350 °C at 5 °C/min in air atmosphere for 2 h. The resulting sample is marked as X% Bi2MoO6/Bi4O5Br2 (X% BMO–BOB). The amount of Bi4O5Br2 remains the same, by changing the molar number of Bi (NO3)3·5H2O and Na2MoO4·2H2O, and 1% BMO–BOB, 5% BMO–BOB and 7% BMO–BOB were prepared. Pure Bi2MoO6 was prepared without Bi4O5Br2.

3.2. Characterizations

The crystal structures of catalysts were detected by an X-ray diffractometer (XRD, Rigaku, Tokyo, Japan), and the range of diffraction angles was at 5–85°. In order to observe the morphology and structure of the photocatalyst, Regulus 8100 SEM (JOEL, Tokyo, Japan) and 2100F transmission electron microscopy (HRTEM, JOEL, Tokyo, Japan) were used. The chemical composition and state of the sample were identified using an Escalab Xi+ XPS spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Diffuse reflection UV-vis spectra were obtained on UV-2700 (Shimadzu, Tokyo, Japan) with BaSO4 as the background. PL spectroscopy was performed at room temperature (293 K) using a xenon lamp with an excitation wavelength of 375 nm on FLS 980 (Ediburgh, Livingston, Scotland, UK). Photoelectrochemical characterization is performed on the CHI660E electrochemical workstation. The working electrodes are, respectively, loaded onto the ITO electrodes. The counter electrode is a Pt plate, and the reference electrode is an Ag/AgCl electrode. Using liquid chromatography–mass spectrometry technology ESR signals were collected on EMX-plus (Bruker, Berlin, Germany) under visible light irradiation (λ ≥ 420 nm).

3.3. Photocatalytic Degradation of TC

The investigation of the obtained samples for tetracycline degradation was evaluated under visible light irradiation (λ ≥ 420 nm). In the typical experiment, 15 mg of the photocatalyst was added into 30 mL of tetracycline hydrochloride solution (20 mg/L). Prior to illumination, the suspensions were magnetically stirred in the dark for 60 min to achieve an absorption–desorption equilibrium between the photocatalysts and tetracycline. Once equilibrium was reached, 4 mL samples of the reaction solution were taken at 30-min intervals, filtered with a 0.22 μm filter membrane, and the concentration of tetracycline was determined by measuring the absorbance at 355 nm using a UV-Vis spectrophotometer (Meipuda, Shanghai, China).

4. Conclusions

In summary, a series of type II Bi2MoO6/Bi4O5Br2 heterojunctions were successfully synthesized by a two-step solvothermal method followed by calcination for the degradation of tetracycline hydrochloride. The results show that the obtained 2% Bi2MoO6/Bi4O5Br2 catalyst exhibits excellent photocatalytic degradation performance (76%) for tetracycline hydrochloride degradation under visible light irradiation, which is 1.9 times that of pure Bi2MoO6 (39%) and 1.8 times that of Bi4O5Br2 (43%), respectively. Capture experiments and ESR analysis showed that superoxide radicals and photogenerated holes were the main active substances for TC degradation. After five cycles, the prepared BMO–BOB photocatalyst still has good stability. The improved photocatalytic efficiency of Bi2MoO6/Bi4O5Br2 can be attributed to the formation of a type II heterojunction, which improves the charge transfer efficiency. This study opens up a new way for the development and design of efficient environmental remediation photocatalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics12110289/s1, Figure S1: Effect of pH on photocatalytic degradation rate of 2% BMO–BOB; Figure S2: Effect of temperature on photocatalytic degradation rate of 2% BMO–BOB.

Author Contributions

Conceptualization, Y.Z.; methodology, F.K.; software, F.K.; formal analysis, G.S. and X.Y.; investigation, F.K. and G.S.; writing—original draft preparation, F.K.; writing—review and editing, F.K. and Y.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 12 00289 g001
Figure 1. (a) XRD patterns; (b) High-resolution XPS; (c,d) Bi 4f and Br 3d spectra of 2% BMO–BOB and BOB; (e) O1s spectrum of 2% BMO–BOB, BMO, and BOB; (f) Mo6+ and Mo5+ 3d spectra of 2% BMO–BOB and BOB.
Figure 1. (a) XRD patterns; (b) High-resolution XPS; (c,d) Bi 4f and Br 3d spectra of 2% BMO–BOB and BOB; (e) O1s spectrum of 2% BMO–BOB, BMO, and BOB; (f) Mo6+ and Mo5+ 3d spectra of 2% BMO–BOB and BOB.
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Figure 2. FESEM of (a) BOB, (b) 2% BMO–BOB; HRTEM of (c,d) BOB, (e–g) 2% BMO–BOB; (h) Element mapping image of 2% BMO–BOB.
Figure 2. FESEM of (a) BOB, (b) 2% BMO–BOB; HRTEM of (c,d) BOB, (e–g) 2% BMO–BOB; (h) Element mapping image of 2% BMO–BOB.
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Inorganics 12 00289 g003
Figure 3. (a) UV–vis DRS spectra; (b) Tauc plots; (c) Mott–Schottky plots of BOB and 2% BMO–BOB; (d) Schematic illustration of band structure over BOB and 2% BOB–BOB.
Figure 3. (a) UV–vis DRS spectra; (b) Tauc plots; (c) Mott–Schottky plots of BOB and 2% BMO–BOB; (d) Schematic illustration of band structure over BOB and 2% BOB–BOB.
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Figure 4. (a) Transient photocurrent response test of BMO, BOB, and 2% BMO–BOB; (b) Impedance test of the sample; (c) The steady-state fluorescence spectra; and (d) time-resolved transient fluorescence spectra of BMO, BOB, and 2% BMO–BOB.
Figure 4. (a) Transient photocurrent response test of BMO, BOB, and 2% BMO–BOB; (b) Impedance test of the sample; (c) The steady-state fluorescence spectra; and (d) time-resolved transient fluorescence spectra of BMO, BOB, and 2% BMO–BOB.
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Inorganics 12 00289 g005
Figure 5. (a) Photocatalytic activity of samples for TC degradation, (b) Degradation rate of TC, (c) Cyclic experiments of 2% BMO–BOB for the degradation of TC, (d) The XRD patterns of the recycled and original 2% BMO–BOB.
Figure 5. (a) Photocatalytic activity of samples for TC degradation, (b) Degradation rate of TC, (c) Cyclic experiments of 2% BMO–BOB for the degradation of TC, (d) The XRD patterns of the recycled and original 2% BMO–BOB.
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Figure 6. (a) Free radical capture experiment; (b–g) The ESR spectra of BOB and 2% BMO–BOB; (b,c) ·O2− spectra of BOB and 2% BMO–BOB; (d,e) h+ spectra of BOB and 2% BMO–BOB; (f,g) ·OH spectra of BOB and 2% BMO–BOB.
Figure 6. (a) Free radical capture experiment; (b–g) The ESR spectra of BOB and 2% BMO–BOB; (b,c) ·O2− spectra of BOB and 2% BMO–BOB; (d,e) h+ spectra of BOB and 2% BMO–BOB; (f,g) ·OH spectra of BOB and 2% BMO–BOB.
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Inorganics 12 00289 g007
Figure 7. Mechanism of dgradation tetracycline by 2% BMO–BOB.
Figure 7. Mechanism of dgradation tetracycline by 2% BMO–BOB.
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Scheme 1. Schematic illustration of the BMO–BOB preparation processes.
Scheme 1. Schematic illustration of the BMO–BOB preparation processes.
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Kang, F.; Sheng, G.; Yang, X.; Zhang, Y. Fabrication of Two-Dimensional Bi2MoO6 Nanosheet-Decorated Bi2MoO6/Bi4O5Br2 Type II Heterojunction and the Enhanced Photocatalytic Degradation of Antibiotics. Inorganics 2024, 12, 289. https://doi.org/10.3390/inorganics12110289

AMA Style

Kang F, Sheng G, Yang X, Zhang Y. Fabrication of Two-Dimensional Bi2MoO6 Nanosheet-Decorated Bi2MoO6/Bi4O5Br2 Type II Heterojunction and the Enhanced Photocatalytic Degradation of Antibiotics. Inorganics. 2024; 12(11):289. https://doi.org/10.3390/inorganics12110289

Chicago/Turabian Style

Kang, Fengshu, Gaidong Sheng, Xiaolong Yang, and Yan Zhang. 2024. "Fabrication of Two-Dimensional Bi2MoO6 Nanosheet-Decorated Bi2MoO6/Bi4O5Br2 Type II Heterojunction and the Enhanced Photocatalytic Degradation of Antibiotics" Inorganics 12, no. 11: 289. https://doi.org/10.3390/inorganics12110289

APA Style

Kang, F., Sheng, G., Yang, X., & Zhang, Y. (2024). Fabrication of Two-Dimensional Bi2MoO6 Nanosheet-Decorated Bi2MoO6/Bi4O5Br2 Type II Heterojunction and the Enhanced Photocatalytic Degradation of Antibiotics. Inorganics, 12(11), 289. https://doi.org/10.3390/inorganics12110289

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