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Article

BiPO4/Ov-BiOBr High-Low Junctions for Efficient Visible Light Photocatalytic Performance for Tetracycline Degradation and H2O2 Production

by
Minghui Tang
1,
Xibao Li
1,2,*,
Fang Deng
2,
Lu Han
3,
Yu Xie
2,
Juntong Huang
1,
Zhi Chen
1,
Zhijun Feng
1 and
Yingtang Zhou
4,*
1
School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China
2
National-Local Joint Engineering Research Center of Heavy Metal Pollutants Control and Resource Utilization, Nanchang Hangkong University, Nanchang 330063, China
3
School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China
4
National Engineering Research Center for Marine Aquaculture, Marine Science and Technology College, Zhejiang Ocean University, Zhoushan 316004, China
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(3), 634; https://doi.org/10.3390/catal13030634
Submission received: 4 March 2023 / Revised: 19 March 2023 / Accepted: 20 March 2023 / Published: 22 March 2023
(This article belongs to the Special Issue Photocatalytic Nanomaterials for Environmental Purification)
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Abstract

:
Through a two-step solvothermal method, different molar ratios of BiPO4 were grown in situ on the surface of oxygen-vacancy-rich BiOBr (Ov-BiOBr), successfully constructing a BiPO4/Ov-BiOBr heterojunction composite material. By constructing a novel type I high-low junction between the semiconductor BiPO4 and Ov-BiOBr, stronger oxidative holes or reductive electrons were retained, thereby improving the redox performance of the photocatalyst. The composite catalyst with a 10% molar content of BiPO4 demonstrated the highest degradation rate of tetracycline (TC), degrading over 95% within 90 min, with a rate constant of 0.02534 min−1, which is 2.3 times that of Ov-BiOBr and 22 times that of BiPO4. The 10% BiPO4/Ov-BiOBr sample displayed the best photocatalytic activity, producing 139 μmol·L−1 H2O2 in 120 min, which is 3.6 times the efficiency of Ov-BiOBr and 19 times that of BiPO4. This was due to the appropriate bandgap matching between BiPO4 and Ov-BiOBr, the photo-generated electron transfer channel via Bi-bridge, and efficient charge separation. It was inferred that the free radical species ·OH and ·O2− played the dominant role in the photocatalytic process. Based on experimental and theoretical results, a possible photocatalytic mechanism was proposed.

1. Introduction

Since the acceleration of urbanization and rapid industrial development in modern times, environmental pollution has created increasingly serious problems, among which organic pollution has gradually become one of the major issues faced by human society in the 21st century. Tetracycline (TC), a widely used antibiotic, is present in numerous wastewater streams and environmental samples. Its excessive use and discharge have led to environmental pollution issues, such as the spread of antibiotic resistance genes. Therefore, studying the photocatalytic degradation of tetracycline is of significant environmental importance. As a pollutant with representative antibiotic structural features, tetracycline’s photocatalytic degradation helps in better understanding and evaluating the treatment effects of different photocatalysts on similar contaminants. The high stability and low natural degradation of tetracycline in water present challenges for the design and optimization of photocatalysts. The aim is to discover novel and efficient photocatalysts through the study of tetracycline’s photocatalytic degradation process. Various methods have been employed for tetracycline degradation, including biodegradation, photochemical degradation, advanced oxidation processes (AOPs), membrane filtration technology and adsorption. However, these methods have drawbacks such as slow degradation rates, high energy consumption, susceptibility to environmental conditions, generation of secondary pollutants, stringent operating conditions, only achieving separation and concentration rather than true degradation of pollutants, reaching adsorption saturation easily, and the potential for membrane fouling. The photocatalytic method is chosen for tetracycline degradation in this research due to its environmental friendliness, low energy consumption, and lack of secondary pollution. By designing and optimizing photocatalysts, the goal is to improve degradation efficiency and reduce energy consumption. In terms of environmental purification, semiconductor catalysis technology has become a promising green energy utilization technology, which has significant advantages over traditional organic treatment methods, such as no secondary pollution, low cost, and strong oxidation, and has received widespread attention. The research focus of photocatalytic semiconductor catalysis technology is the selection, synthesis, and performance improvement of semiconductors [1,2,3,4].
The photocatalytic performance of heterojunctions mainly depends on the generation of electron-hole pairs in the conduction and valence bands after absorbing light energy and maintaining spatial separation to participate in redox reactions, thus achieving photocatalytic effects [5,6,7]. However, due to the structural characteristics of the semiconductor material itself, the recombination rate of photo-generated charge carriers is usually greater than the migration rate. In addition, the bandgap width of the semiconductor material itself is too large, and the wavelength range of light absorption is too narrow, resulting in a decrease in the absorption and utilization efficiency of visible light, which leads to the excited electrons and holes not being able to participate in redox reactions in a timely manner; instead, they are lost in the form of thermal energy or light energy [8]. Therefore, the recombination of electron-hole pairs and the wavelength range of light absorption by semiconductor materials are the two main factors affecting the photocatalytic performance of composite photocatalysts [9,10].
BiOBr is a ternary bismuth-based layered semiconductor photocatalyst. The staggered arrangement of its layered structure leads to an inherent electric field between the Br- layer and the [Bi2O2]2+ layer, facilitating better separation of electrons and holes [11,12,13,14]. Moreover, BiOBr has oxygen vacancy defect sites, which can capture excited-state electrons and create spatial separation between different photogenerated charge carriers, preventing recombination and improving the efficiency of redox reactions. The layered structure of BiOBr can effectively reduce the probability of recombination of photogenerated charge carriers; moreover, BiOBr has good electrical, optical, and magnetic properties. Density functional theory calculations show that under illumination, electrons in Br 4p of BiOBr are excited and pass through the K layer to reach the conduction band, making it difficult for holes and electrons to recombine. Wu et al. prepared a new photocatalyst, p-n heterojunction BiOBr/La2Ti2O7, and used in situ FT-IR dynamic monitoring of the adsorption-reaction process of visible light oxidation of NO [15]. They found that the built-in electric field could accelerate the separation of e−-h+ pairs, thus improving the photocatalytic efficiency. In addition, Ye et al. anchored Fe3+ sites on oxygen-deficient BiOBr nanosheets, effectively regulating charge/energy transfer and adsorption of reactants, thereby enhancing the dual functions of photocatalytic reaction and highly selective activation of O2 to 1O2 [16]. BiPO4 is an n-type semiconductor with three crystal structures: low-temperature monoclinic phase, high-temperature monoclinic phase, and hexagonal phase. The low-temperature monoclinic phase has the highest photocatalytic efficiency, especially when combined with a staggered heterojunction formed with BiOBr, which promotes the separation of electron-hole pairs. Despite its small bandgap width of only 3.8–4.2 eV, BiPO4 has a narrow absorption range and can only absorb UV light [17,18]. However, BiPO4 contains the non-metallic oxide PO43−, which induces the separation of photoinduced electron-hole pairs and the abundance of negatively charged PO43− facilitates this separation, thereby enhancing photocatalytic activity [19]. BiPO4 nanoparticles are actually aggregates of BiPO4 quantum dots with an average diameter of about 5 nm [20]. Therefore, ultrathin Bi nanosheets can increase edge sites and promote the conversion of CO2 to carboxylates, increasing the redox reaction sites and improving photocatalytic efficiency. Currently, research mainly focuses on adjusting the composite structure of BiPO4 to improve its solar energy conversion efficiency and quantum yield [21]. Lu et al. used ion exchange to form in-situ Bi2S3 nanoparticles on the surface of BiPO4 rods, achieving an 80% degradation rate of methylene blue under visible light irradiation within 3 h [22]. Zhang et al. prepared Ag/BiPO4 composite photocatalysts using a combination of hydrothermal synthesis and impregnation methods, which showed much higher efficiency for the degradation of methylene blue under UV light irradiation compared to BiPO4 alone [23]. Phosphates also provide clear directions and strategies for designing new photocatalysts. Oxygen vacancy defects are one of the important factors in improving photocatalytic efficiency [24,25,26]. In BiOBr, oxygen defect sites can capture photoexcited electrons, thereby reducing the recombination of electron-hole pairs and achieving spatial separation of photo-generated charge carriers. Introducing high-concentration oxygen vacancy defects can extend the range of the light response wavelength and increase photocatalytic activity. Research by Ren et al. showed that BiOX with oxygen vacancy defects can photocatalytically reduce CO2 under simulated solar light irradiation, and oxygen vacancy defects can promote the adsorption and activation of CO2 [27]. Chen et al. introduced oxygen vacancy (Ov) defects on the exposed surface of BiOBr nanosheets, modifying the band structure of BiOBr, enhancing its adsorption capacity, and lengthening the N≡N bond of adsorbed N2 molecules [28].
This study aimed to prepare BiPO4/Ov-BiOBr Type I heterojunction composites using a two-step solvothermal method to achieve exceptional visible-light photocatalytic performance for TC degradation and H2O2 generation [29,30,31]. The preparation parameters were adjusted to obtain optimal morphology, structure, and optical properties. The resulting composites were comprehensively characterized using various techniques. The photocatalytic degradation performance of the composites under visible-light irradiation was evaluated and compared with pure-phase BiPO4 and Ov-BiOBr materials, revealing the advantages of heterojunction structures. The photocatalytic H2O2 generation performance of the composites under visible-light irradiation was investigated, and efforts were made to optimize photocatalytic efficiency by adjusting the composition and structure of the materials. The mechanisms of photocatalytic degradation and H2O2 generation were studied, with a focus on the generation, separation, migration, and consumption of photogenerated electron-hole pairs. Finally, the stability and reusability of the composites in photocatalytic processes were assessed through continuous cycling experiments, verifying their practicality and potential applications.

2. Results and Discussion

2.1. Crystal Structure Analysis

Figure 1 presents the X-ray diffraction (XRD) patterns of BiPO4, Ov-BiOBr, and BiPO4/Ov-BiOBr. The diffraction peaks of pure Ov-BiOBr and BiPO4 in the XRD patterns match well with the standard data of BiOBr and BiPO4 (BiOBr: PDF#73-2061; BiPO4: PDF#15-0767). For BiPO4/Ov-BiOBr composite, the diffraction peaks appearing at 2θ values of 21.4°, 25.3°, 29.1°, 31.2°, 38.6°, 41.6°, and 46.3° correspond to the (101), (111), (120), (012), (022), (031), and (310) crystal planes of BiPO4, respectively, indicating the presence of BiPO4 in the composite material [32,33]. The diffraction peaks appearing at 2θ values of 25.3°, 27.1°, 32.3°, 38.6°, and 46.3° correspond to the (111), (200), (110), (022), and (310) crystal planes of BiOBr, respectively, indicating the presence of BiOBr in the composite material [16,34]. Due to the effects of high temperature and ethylene glycol, the Bi-O bond on the [Bi2O2]2+ layer breaks leading to the precipitation of some Bi atoms from the BiOBr lattice [34]. The diffraction peak at 2θ of 27.1° corresponds to the (012) crystal plane of Bi phase (PDF#85-1329). Furthermore, the significant decrease in the intensity of the (120) diffraction peak of BiPO4 in the BiPO4/Ov-BiOBr composite is likely due to the formation of a heterojunction structure between the (110) plane of BiPO4 and BiOBr, thereby reducing its diffraction peak intensity. In Figure 1, it can be observed that the diffraction peaks of the composite materials are significantly shifted to the right, indicating the strong interaction between the two components in the composite.

2.2. TEM Analysis

Figure 2a shows the TEM image of the high-low heterojunction of BiPO4/Ov-BiOBr, which reveals the nano-rod structure of BiPO4 which is deposited on the spherical Ov-BiOBr. To confirm the nanorods as BiPO4, the HRTEM images were analyzed, as shown in Figure 2b, which displays clear lattice fringes of BiPO4 and good contact at the interface between BiPO4 and Ov-BiOBr. The interplanar spacing of the (110) crystal plane in Ov-BiOBr was found to be 0.227 nm, while that of the (200) crystal plane in BiPO4 was 0.327 nm [14,17]. At the interface of BiPO4 and Ov-BiOBr, the main crystal plane of Bi was found to be (110), corresponding to interplanar spacing of 0.227 nm [13,34]. It is likely that the heterojunction was formed due to the in-situ deposition of BiPO4 on the Bi sites in Ov-BiOBr [35,36,37,38]. The elemental mapping of Bi, Br, O, and P, as shown in Figure 2c–f, demonstrated that the O, Bi, Br, and P elements were evenly distributed on the BiPO4/Ov-BiOBr composite photocatalyst [39]. These results indicate that the two-step hydrothermal method can effectively promote the in-situ deposition of BiPO4 on Ov-BiOBr nanospheres, eventually forming the BiPO4/Ov-BiOBr heterojunction.

2.3. XPS Analysis

The change in binding energy of elements can directly reflect the transfer of electrons and charge transfer processes in heterogeneous catalysts [40,41]. Therefore, monitoring the changes in binding energy can be used to study the direction of electron transfer in heterojunction photocatalysts [42]. In this study, in situ irradiation XPS analysis was performed on the BiPO4/Ov-BiOBr composite before and after irradiation with a 300 W xenon lamp (420 nm < λ < 780 nm), and changes in the binding energies of Bi 4f, Br 3d, O 1s, and P 2p were observed. In the Bi 4f spectrum, the binding energy of Bi 4f5/2 decreased by 0.12 eV after irradiation, possibly due to electron transfer from PO43− in BiPO4 to BiOBr (Figure 3a) [17,43]. In the Br 3d spectrum, the binding energy of Br 3d5/2 decreased by 0.1 eV after irradiation, indicating a decrease in the electron density of BiOBr. These results indicate an increase in the electron density on BiOBr in the irradiated composite (Figure 3b). In the O 1s spectrum, the characteristic peak binding energies of lattice oxygen and surface hydroxyls at 533 eV and 529.9 eV increased by 0.1 eV after irradiation, possibly due to a decrease in the binding energy of OVs in the composite [44,45]. These changes indicate that the O in PO43− provides electrons and transfers to BiOBr, and the O in the surface hydroxyls of BiPO4 also provides electrons and participates in electron transfer (Figure 3c). In the P 2p spectrum, the binding energy of P 2p increased by 0.1 eV after irradiation, indicating an increase in the electron density of BiPO4. This further confirms that electrons transfer from PO43− in BiPO4 to BiOBr (Figure 3d) [46]. Taken together, these results demonstrate the existence of the BiPO4/Ov-BiOBr heterojunction and electron transfer between the heterojunctions.
Based on the experimental results, under visible light irradiation, the photogenerated electrons in the BiPO4/Ov-BiOBr sample show a transfer direction from BiPO4 to Ov-BiOBr. This finding provides strong evidence for the transfer direction of photogenerated electrons in BiPO4/Ov-BiOBr heterojunction.

2.4. Raman Analysis

To verify the presence of precipitated metallic elemental Bi in Ov-BiOBr, Raman spectra of Ov-BiOBr and BiPO4/Ov-BiOBr samples were analyzed (Figure 4). In the Raman spectrum of BiOBr, three distinct bands can be observed at 56 cm−1, 112 cm−1, and 374 cm−1, respectively. The 56 cm−1 and 112 cm−1 Raman peaks correspond to the A1g external and Eg internal Bi-O stretching modes of BiOBr, while the relatively weaker peak at 374 cm−1 can be attributed to Eg and B1g bands caused by oxygen atom vibration [47]. For the BiPO4/Ov-BiOBr composite, the band at 56 cm−1 can be attributed to the first-order scattering A1g mode of elemental Bi, the 112 cm−1 band corresponds to the A1g internal Bi-O stretching mode, and the 397 cm−1 band corresponds to Bi-O stretching [48,49]. These results further confirm the presence of precipitated metallic elemental Bi in Ov-BiOBr and the distinct Raman spectral features of the BiPO4/Ov-BiOBr composite as compared to those of BiOBr.

2.5. UV–Vis Diffuse Reflectance Spectra

To investigate the photoresponsive properties of BiPO4/Ov-BiOBr photocatalyst, the band structure analysis is essential. The band structure of a photocatalyst is closely related to its photoresponsive properties. UV-Vis diffuse reflectance spectroscopy was used to analyze BiPO4, Ov-BiOBr, and BiPO4/Ov-BiOBr composite materials [50]. As shown in Figure 5a, the light absorption edge of BiPO4 is between 300–400 nm, indicating that it is mainly responsive to UV light [51,52]. The light absorption edge of Ov-BiOBr is between 400–800 nm, indicating it has good visible light responsiveness. BiPO4/Ov-BiOBr composite material has an absorption edge larger than 500 nm and strong visible light absorption characteristics. By converting the diffuse reflectance spectrum data, the band gap width diagrams of BiPO4 and Ov-BiOBr can be obtained, as shown in Figure 5b. Extending the linear part of the curve to the intersection with the abscissa axis gives a band gap width of 4.2 eV and 2.7 eV for BiPO4 and Ov-BiOBr, respectively. According to the formula Eg = E(VB) − E(CB), the valence band of BiPO4 is 3.6 eV and the valence band of Ov-BiOBr is 2.22 eV. From the perspective of the band structure, n-type semiconductors with staggered band structures such as BiPO4 and Ov-BiOBr can form effective heterojunctions.

2.6. Photoelectrochemical Properties

Band structure and light responsiveness are critical factors influencing the performance of photocatalysts. Therefore, in the photoelectrochemical experiments, we also detected and analyzed the Mott–Schottky (MS) curves of BiPO4 and Ov-BiOBr [53]. Figure 6a,b show the MS curves at different frequencies, and the Erof values were calculated to be approximately −0.60 V and −0.38 V (vs. NHE, pH = 7) for BiPO4 and Ov-BiOBr, respectively. In addition, we observed that the MS curves of BiPO4 and Ov-BiOBr had a positive slope in the linear part, indicating that both materials have n-type semiconductor properties. According to the rule that the conduction band bottom potential of n-type semiconductors is approximately equal to the flat band potential, the conduction band potential (CB) of BiPO4 was calculated to be −0.60 eV and that of Ov-BiOBr was −0.38 eV.
Figure 6c,d show the VB-XPS spectra of Ov-BiOBr and BiPO4. By reading the position of the intersection of the image, Evb values of 2.21 and 3.49 eV for Ov-BiOBr and BiPO4 were obtained, respectively. Considering the possibility of a potential difference between the sample and the instrument, we calculated the VB potential of Ov-BiOBr and BiPO4 using the formula EVB-NHE = φ + EVB-XPS − 4.44, where φ is the instrument work function of the XPS analyzer (4.55 eV). Therefore, the EVB-NHE values of Ov-BiOBr and BiPO4 are 2.32 and 3.6 eV, respectively. This is consistent with the results of the MS measurement of the valence band position. In conclusion, we have successfully determined the band structure of the BiPO4/Ov-BiOBr heterojunction and demonstrated the successful formation of the BiPO4/Ov-BiOBr composite with a type-I band alignment.
The photoelectrochemical measurements on BiPO4, Ov-BiOBr, and 10% BiPO4/Ov-BiOBr composite photocatalysts were performed. The photocurrent (Figure 7a) of pure BiPO4 and pure Ov-BiOBr under visible light irradiation is 2 μA and 12 μA, respectively. Among all the composite catalysts, 10% BiPO4/Ov-BiOBr displays the best photocurrent response (39 μA), which is three times that of Ov-BiOBr. In addition, the electrochemical impedance spectroscopy (EIS) diagram (Figure 7b) shows that 10% BiPO4/Ov-BiOBr has smaller charge transfer resistance and larger transient photocurrent response. This indicates that the excellent photoelectrochemical performance of the 10% BiPO4/Ov-BiOBr heterojunction can be attributed to its strong interface interaction, resulting in smaller photo-generated electron transfer resistance.

2.7. Photocatalytic Activity

The photocatalytic performance of samples was evaluated by the photocatalytic degradation experiment of tetracycline (TC). Compared to pure Ov-BiOBr, the composite material BiPO4/Ov-BiOBr shows a significant enhancement in photocatalytic activity, with the 10% mol BiPO4/Ov-BiOBr sample exhibiting the best degradation efficiency, degrading over 80% of TC under visible light irradiation in 30 min, far higher than other samples (Figure 8a). The photocatalytic degradation of TC by BiPO4/Ov-BiOBr follows first-order reaction kinetics. As observed from Figure 8b, the reaction rate constants (K) of pure Ov-BiOBr, 10% BiPO4/Ov-BiOBr, 20% BiPO4/Ov-BiOBr, 30% BiPO4/Ov-BiOBr, 40% BiPO4/Ov-BiOBr, and pure BiPO4 are 0.00944, 0.02534, 0.01376, 0.01043, 0.00991, and 0.00114 min−1, respectively. The rate constant of 10% BiPO4/Ov-BiOBr is 22, 2.68, 2.56, 2.42, and 1.84 times that of pure BiPO4, pure Ov-BiOBr, 40% BiPO4/Ov-BiOBr, 30% BiPO4/Ov-BiOBr, and 20% BiPO4/Ov-BiOBr, respectively. To detect the stability of BiPO4/Ov-BiOBr, cyclic tests of TC photocatalytic degradation and XRD detection before and after illumination were carried out (Figure 8c,d). The experimental results show that after four cycles of photocatalytic degradation, the degradation rate of TC by 10% BiPO4/Ov-BiOBr still reached 90%. Subsequently, 39 more cycles were conducted, and after 3.25 days, the degradation efficiency decreased to 45%. The experiments indicate that the synthesized catalyst has relatively high stability. Meanwhile, the XRD pattern remained unchanged, indicating the relatively stable photocatalytic activity and structural stability of 10% BiPO4/Ov-BiOBr during the photocatalytic process. The comparison of the degradation performance of the samples prepared in this study with other reported Bi-based photocatalysts is shown in Table 1.
The study also investigated the photocatalytic activity of samples prepared under visible light irradiation (λ > 420 nm) for the in-situ production of H2O2 [59,60]. By substituting the absorbance peak values at different time points into the equation, the change in H2O2 concentration was calculated and a curve was plotted as shown in Figure 9. The results show that 10%BiPO4/Ov-BiOBr has the best photocatalytic activity for producing H2O2, and after 120 min of visible light irradiation, the H2O2 concentration in the reactor reaches 139 μmol·L−1, which is 3.6 times that of BiOBr and 19 times that of BiPO4. The free radical reaction process of this photocatalytic reaction includes O2 + 2e− + 2H+ → H2O2 (0.69 V vs. NHE) and h+ + OH− → ·OH + ·OH → H2O2 (2.29 V vs. NHE). Therefore, 10% BiPO4/Ov-BiOBr obtained by solvothermal treatment may generate more photogenerated electrons and transfer them to the molecular surface for reacting with dissolved O2. The successful construction of BiPO4/Ov-BiOBr type I high-low junction retains the h+ on the deep valence band of BiPO4, thereby oxidizing OH− to produce H2O2, which generates more H2O2 during the photocatalytic reaction. These results indicate that the nanoheterojunction photocatalyst with a more compact interface formed by solvothermal treatment is beneficial for improving the photocatalytic efficiency.

2.8. Free Radical Capture

To further determine the main active species of BiPO4/Ov-BiOBr in photocatalytic reactions, ESR and free radical capture experiments were conducted. The results are shown in Figure 10. IPA, BQ, and AO were used as free radical capture agents for ·OH, ·O2−, and h+, respectively [18]. The experimental results show that ·OH and ·O2− are the main active species in the TC photocatalytic degradation process. In addition, Figure 10b,c show the characteristic peaks of DMPO-·OH and DMPO-·O2− observed under visible light irradiation, while no signal was detected in the dark state. This indicates that ·OH and ·O2− play a crucial role in the photocatalytic process of BiPO4/Ov-BiOBr. The ESR free radical detection experiment further confirms the results of the free radical capture experiment, thereby determining that ·OH and ·O2− are the main active species in the photocatalytic degradation reaction of BiPO4/Ov-BiOBr.

2.9. Photocatalytic Mechanism

The difference in work function (Φ) or Fermi level (Ef) of semiconductor photocatalysts in heterojunction nanocomposites is considered an intrinsic driving force based on the VB-XPS measurement of the work function of Ov-BiOBr, BiPO4, and BiPO4/Ov-BiOBr; it can be observed that the material with a higher work function attracts electrons while the material with a lower work function loses electrons until the Fermi levels of both materials reach equilibrium. Consequently, an internal electric field (IEF) is formed at the interface, with the material with the higher work function being negatively charged and the material with the lower work function being positively charged [61,62]. Using the formula Φ = ΔV + φ (where ΔV is the contact potential difference between the sample and the instrument sample holder, φ is the work function of the XPS analyzer, taken as 4.55 eV, and Φ is the work function of the sample) [30,63], the work functions of Ov-BiOBr, BiPO4, and BiPO4/Ov-BiOBr are calculated to be 6.18, 6.96, and 5.99 eV, respectively, and the distances between their two IP points are 1.63, 2.41, and 1.44 eV, respectively (Figure 11a–c). Since the Fermi level (Ef1) of BiPO4 is lower than that of Ov-BiOBr (Ef2) (Figure 11d), when BiPO4 comes into close contact with Ov-BiOBr, electrons will spontaneously diffuse from the material with the lower work function (Ov-BiOBr) to the material with the higher work function (BiPO4) until the Fermi levels reach a new equilibrium (Ef3). After reaching equilibrium, an electron accumulation layer will form on the surface of BiPO4, and an electron depletion layer will form on the surface of Ov-BiOBr, resulting in BiPO4 being negatively charged and Ov-BiOBr being positively charged, thereby creating an IEF pointing from Ov-BiOBr to BiPO4 (Figure 11e). This IEF provides a driving force for the directed movement of photogenerated charge carriers between different semiconductors. After photoexcitation, photogenerated electrons transfer from the conduction band of BiPO4 to the conduction band of Ov-BiOBr, consistent with the results of in situ XPS measurement of electron transfer. Upon contact between the two semiconductors, the energy band of BiPO4 and Ov-BiOBr will bend, leading to the migration of electrons from the conduction band of BiPO4 to that of Ov-BiOBr. On the other hand, the photogenerated holes on the valence band of BiPO4 will stay in place due to the band bending effect and Coulomb repulsion (Figure 11f). In summary, by matching the energy band structure, IEF, and energy band bending, the charge carriers with strong redox ability can be preserved, which creates conditions for improving photocatalytic activity.
According to the experimental results, a possible reaction mechanism of the BiPO4/Ov-BiOBr heterojunction photocatalyst is proposed (Figure 12). After illumination, the photogenerated electrons in BiPO4 flow to Ov-BiOBr via the metallic Bi bridge, while the defect energy levels between the conduction and valence bands in Ov-BiOBr are excited [48,64,65,66,67]. Due to the effect of IEF and band bending, the holes in BiPO4 cannot transfer to Ov-BiOBr. Therefore, the strongly oxidizing holes in BiPO4 are retained. Since the conduction band potential of Ov-BiOBr is lower than that of dissolved oxygen, the photogenerated electrons on Ov-BiOBr can react with dissolved oxygen to produce ·O2−. In addition, the holes in BiPO4 and Ov-BiOBr can effectively oxidize H2O to generate ·OH. Since the ·OH has strong oxidizing properties, it can participate in the degradation of pollutants. Therefore, ·O2− and ·OH are the main active species in the process of pollutant degradation. Based on the analysis of the energy band structure, electron transfer, work function, and active species detection results of the BiPO4/Ov-BiOBr heterojunction, the possible charge transfer mechanism and reaction mechanism of the BiPO4/Ov-BiOBr type I high-low junctions are proposed.

3. Experimental Section

3.1. Chemicals

Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 99%), potassium bromide (KBr, 99%), sodium dihydrogen phosphate (NaH2PO4, 99%), isopropanol (C3H8O), were purchased from Shanghai Tikem Industrial Co. Ethylene glycol ((CH2OH)2) (Shanghai, China) was purchased from Xilong Chemical Co. Tetracycline (C22H24N2O8) (Guangdong, China) was purchased from Shanghai Maclean Biochemical Technology Co. Anhydrous ethanol (CH3CH2OH) (Shanghai, China) was purchased from Xilong Science Co.

3.2. The Preparation of Ov-BiOBr

1 mmol Bi(NO3)3·5H2O was added into 15 mL of ethylene glycol and stirred thoroughly to dissolve Bi(NO3)3·5H2O completely in ethylene glycol, obtaining solution A [26]. Next, 1 mmol KBr was dissolved in 15 mL of ethylene glycol using the same method as for solution A, obtaining solution B. Solutions A and B were placed on a magnetic stirrer and stirred for 30 min at room temperature. Then, solution B was added dropwise into solution A and stirring was continued for 1 h. The mixed solution was transferred to a Teflon-lined autoclave and heated at 180 °C for 24 h. After cooling, the mixture was centrifuged at 9000 r/min for 5 min to ensure complete separation of the supernatant and the precipitate. The precipitate was repeatedly washed with ethanol and distilled water, and dried at 80 °C for 12 h. Finally, the sample was collected and ground as the precursor for the synthesis of the composite material [28,34,37].

3.3. The Preparation of BiPO4 and BiPO4/Ov-BiOBr

To prepare the BiPO4/Ov-BiOBr heterojunction, a suitable amount of Bi(NO3)3·5H2O was dispersed in 20 mL of distilled water. In another container, 50 mmol of Ov-BiOBr was dispersed in 20 mL of distilled water and sonicated for 30 min before being mixed with the Bi(NO3)3·5H2O solution. Then, under vigorous stirring, an equimolar amount of NaH2PO4 compared to Bi(NO3)3·5H2O was added dropwise into the mixed solution. The resulting mixture was transferred to a Teflon-lined high-pressure reaction vessel and heated at 170 °C for 12 h. After natural cooling, the sample was centrifuged to remove the supernatant and repeatedly washed with ethanol and distilled water. Finally, the sample was dried at 60 °C for 12 h. When preparing BiPO4/Ov-BiOBr photocatalysts, different proportions of BiPO4 (10%, 20%, 30%, and 40%) can be used to obtain various ratios. If Ov-BiOBr is not added, the resulting sample will be pure BiPO4. The specific preparation process is shown in Figure 13.

3.4. Chracterization

The crystal structures were analyzed using a D8 ADVANCE X-ray diffractometer (XRD, Bruker, Billerica, MA, USA). The morphology of BiPO4/Ov-BiOBr was observed using a FEI Talos F200X transmission electron microscope (TEM, Waltham, MA, USA). Elemental analysis was performed using an Axis Ultra DLD X-ray photoelectron spectrometer (XPS, Kratos analytical, Manchester, UK). The Bi-O bond in Ov-BiOBr and BiPO4/Ov-BiOBr was detected using Portman polarized Raman spectroscopy (e.g., Hengyuan, Shanghai, China). The UV-visible diffuse reflectance spectra (DRS) of Ov-BiOBr, BiPO4, and BiPO4/Ov-BiOBr were obtained using a UV-9000 H spectrophotometer (Metash, Shanghai, China) equipped with an integrating sphere. The photoelectrochemical properties of Ov-BiOBr, BiPO4, and BiPO4/Ov-BiOBr were analyzed using a CHI660E electrochemical analyzer (Chenhua, Shanghai, China) equipped with a three-electrode system. The degradation pathway of TC was analyzed using an LC-20A liquid chromatograph (Shimadzu, Kyoto, Japan). The detection of free radicals and characterization of defects in BiPO4/Ov-BiOBr were tested using a JES-FA200 ESR spectrometer (JEOL, Tokyo, Japan).

3.5. Assessment of Photocatalytic Activity

In this experiment, the photocatalytic degradation performance of BiPO4/Ov-BiOBr catalyst under visible light irradiation was investigated. Firstly, a 300 W xenon arc lamp (PLS-SXE 300, Beijing Perfectlight Technology Co., Ltd., Beijing, China) was fixed approximately 15 cm above a double-walled cylindrical quartz reactor. Then, 100 mg of photocatalyst powder with different ratios was mixed with 100 mL of tetracycline solution at a concentration of 50 mg·L−1. With the light source turned off or blocked, the suspension underwent 30 min of dark adsorption and was stirred in the dark with a magnetic stirrer to establish a dynamic equilibrium between adsorption and desorption prior to illumination. To verify whether tetracycline would decompose under light, a control experiment was conducted. In this experiment, no catalyst was added, and the degradation of tetracycline (TC) was directly analyzed using a UV-visible spectrophotometer. During the experiment, approximately 4 mL of the suspension was withdrawn with a pipette every 30 min and transferred to a test tube. The suspension was centrifuged twice at 9000 r/min for 5 min each time to ensure complete separation of the supernatant and precipitate. Then, the supernatant was transferred with a plastic dropper to a colorimetric dish for absorbance and concentration measurements of degradation products. The qualitative detection of the target compound concentration was performed by detecting the specific absorption spectrum generated by the sample molecule upon excitation by UV or visible light. Quantitative analysis was carried out by determining the specific concentration of the target compound through the posiive correlation between the concentration and absorbance intensity of the same substance at the maximum absorption peak, as described by the Lambert–Beer law.

3.6. Active Species Trapping Experiments

Under the conditions of isopropyl alcohol (IPA, 10 mmol/L) and ammonium oxalate (AO, 10 mmol/L), the BiPO4/Ov-BiOBr heterojunction was used for photocatalytic degradation of TC, while detecting the generation of hydroxyl radicals (·OH) and holes (h+). To determine the presence of superoxide radicals (·O2−), an appropriate amount of photocatalyst was added to benzenequinone (BQ, 0.1 mmol/L), and the same experimental procedure for evaluating photocatalytic activity was performed. Finally, according to the Lambert–Beer law, the concentration of TC solution was analyzed at 423 nm using an UV-visible spectrophotometer equipped with an integrating sphere. ESR testing was conducted to further verify free radicals.

4. Conclusions

In summary, this study successfully synthesized a BiPO4/Ov-BiOBr type-I high-low junction photocatalyst using a two-step solvothermal method, which exhibited higher photocatalytic activity compared to single BiPO4 and Ov-BiOBr. The experimental results demonstrated that the BiPO4 content had a significant impact on the catalyst performance, with the optimal addition amount being 10 mol%. The prepared sample displayed the best catalytic activity, with a reaction rate constant of 0.02534 min−1, which was 2.3 times that of pure Ov-BiOBr. This was attributed to the synergistic effect of the bandgap matching between BiPO4 and Ov-BiOBr, including factors such as charge transfer channels and efficient charge separation. Through free radical scavenging experiments, it was found that ·OH and ·O2− free radicals were the main active species in the photocatalytic process. This study provides a new approach for the development of efficient visible-light type-I high-low junction photocatalysts.

Author Contributions

Conceptualization, X.L. and Y.Z.; methodology, J.H.; software, Z.F.; validation, X.L., Y.Z. and J.H.; formal analysis, F.D.; investigation, Z.C.; resources, Z.F.; data curation, Z.C.; writing—original draft preparation, M.T.; writing—review and editing, X.L.; visualization, L.H.; supervision, F.D.; project administration, Y.X.; funding acquisition, X.L. All authors have read and agreed to the published version of the manuscript.

Funding

The research was funded by the National Natural Science Foundation of China (Grant No. 22262024, 51962023, 51862024, 51978324).

Data Availability Statement

The data generated during and/or analysed during the study are available from the corresponding author on reasonable request.

Acknowledgments

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 22262024, 51962023, 51862024, 51978324), Zhejiang Province Key Research and Development Project (2023 C01191), the Double Thousand Talent Plan of Jiangxi Province, the Jiangxi Academic and Technical Leader of Major Disciplines (20213BCJL22053), the Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycling (Nanchang Hangkong University) (Grant No. ES202002077), Postgraduate Innovation Special Fund of NCHU (Nanchang Hangkong University), China (YC2021-003).

Conflicts of Interest

The authors declare no conflict of interest.

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  67. Hao, W.; Teng, F.; Liu, Z.; Yang, Z.; Liu, Z.; Gu, W. Synergistic Effect of Mott-Schottky Junction with Oxygen Defect and Dramatically Improved Charge Transfer and Separation of Bi@Bi-Doped TiO2. Sol. Energy Mater. Sol. Cells 2019, 203, 110198. [Google Scholar] [CrossRef]
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Figure 1. XRD patterns of Ov-BiOBr, BiPO4 and BiPO4/Ov-BiOBr composites.
Figure 1. XRD patterns of Ov-BiOBr, BiPO4 and BiPO4/Ov-BiOBr composites.
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Figure 2. TEM image (a), HRTEM image (b), and elemental mapping of Bi (c), Br (d), O (e), and P (f) of BiPO4/Ov-BiOBr.
Figure 2. TEM image (a), HRTEM image (b), and elemental mapping of Bi (c), Br (d), O (e), and P (f) of BiPO4/Ov-BiOBr.
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Figure 3. In situ irradiation XPS spectra of BiPO4/Ov-BiOBr (a); high-resolution XPS maps of Bi 4f (b), Br 3d (c), O 1s (d) and P 2p (e).
Figure 3. In situ irradiation XPS spectra of BiPO4/Ov-BiOBr (a); high-resolution XPS maps of Bi 4f (b), Br 3d (c), O 1s (d) and P 2p (e).
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Figure 4. Raman spectra of Ov-BiOBr (a) and BiPO4/Ov-BiOBr (b).
Figure 4. Raman spectra of Ov-BiOBr (a) and BiPO4/Ov-BiOBr (b).
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Figure 5. UV-Vis DRS (a) and forbidden band width (b) plots of the prepared samples.
Figure 5. UV-Vis DRS (a) and forbidden band width (b) plots of the prepared samples.
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Figure 6. M-S plots (a,b) and VB-XPS (c,d) of Ov-BiOBr, BiPO4.
Figure 6. M-S plots (a,b) and VB-XPS (c,d) of Ov-BiOBr, BiPO4.
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Figure 7. Photocurrent profiles (a) and impedance profiles (b) of the samples.
Figure 7. Photocurrent profiles (a) and impedance profiles (b) of the samples.
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Figure 8. Photodegradation curve (a) and photodegradation kinetic curve (b) of TC; photodegradation cycle test curve (c) and XRD detection before and after illumination of BiPO4/Ov-BiOBr (d).
Figure 8. Photodegradation curve (a) and photodegradation kinetic curve (b) of TC; photodegradation cycle test curve (c) and XRD detection before and after illumination of BiPO4/Ov-BiOBr (d).
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Figure 9. Concentration-absorbance curve of H2O2 (a); time-concentration curve of H2O2 produced under visible light irradiation (b).
Figure 9. Concentration-absorbance curve of H2O2 (a); time-concentration curve of H2O2 produced under visible light irradiation (b).
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Figure 10. Experiments of radical trapping (a) ESR detection for 10% BiPO4/Ov-BiOBr under dark and visible light irradiation: DMPO-·OH (b) and DMPO-·O2− (c).
Figure 10. Experiments of radical trapping (a) ESR detection for 10% BiPO4/Ov-BiOBr under dark and visible light irradiation: DMPO-·OH (b) and DMPO-·O2− (c).
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Figure 11. Work function of Ov-BiOBr, BiPO4 and BiPO4/Ov-BiOBr (a–c), carrier transfer mechanism of BiPO4/Ov-BiOBr (d–f).
Figure 11. Work function of Ov-BiOBr, BiPO4 and BiPO4/Ov-BiOBr (a–c), carrier transfer mechanism of BiPO4/Ov-BiOBr (d–f).
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Figure 12. Photocatalytic mechanism of BiPO4/Ov-BiOBr high-low junction.
Figure 12. Photocatalytic mechanism of BiPO4/Ov-BiOBr high-low junction.
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Figure 13. Preparation process of BiPO4/Ov-BiOBr composite photocatalyst.
Figure 13. Preparation process of BiPO4/Ov-BiOBr composite photocatalyst.
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Table
Table 1. Recent work of BiOBr-based heterojunctions in degradation of TC.
Table 1. Recent work of BiOBr-based heterojunctions in degradation of TC.
CatalystsLight Source (λ > 420 nm)Concentration (g·L−1)Pollutant (mg·L−1)k (min−1)References
BiPO4/Ov-BiOBr300 W Xe lamp1 50(TC)0.02534This work
W18O49/BiOBr300 W Xe lamp120(TC)0.014[54]
UiO-66-NH2/Bi/BiOBr350 W Xe lamp130(TC)0.00838[55]
CN@BOB-CQDsLED light irradiation120(TC)0.221[56]
BiOBr/Bi2WO6500 W Xe lamp110(TC)0.0229[57]
BiOBr/Bi3O4Br500 W Xe lamp110(TC)0.02013[58]
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Tang, M.; Li, X.; Deng, F.; Han, L.; Xie, Y.; Huang, J.; Chen, Z.; Feng, Z.; Zhou, Y. BiPO4/Ov-BiOBr High-Low Junctions for Efficient Visible Light Photocatalytic Performance for Tetracycline Degradation and H2O2 Production. Catalysts 2023, 13, 634. https://doi.org/10.3390/catal13030634

AMA Style

Tang M, Li X, Deng F, Han L, Xie Y, Huang J, Chen Z, Feng Z, Zhou Y. BiPO4/Ov-BiOBr High-Low Junctions for Efficient Visible Light Photocatalytic Performance for Tetracycline Degradation and H2O2 Production. Catalysts. 2023; 13(3):634. https://doi.org/10.3390/catal13030634

Chicago/Turabian Style

Tang, Minghui, Xibao Li, Fang Deng, Lu Han, Yu Xie, Juntong Huang, Zhi Chen, Zhijun Feng, and Yingtang Zhou. 2023. "BiPO4/Ov-BiOBr High-Low Junctions for Efficient Visible Light Photocatalytic Performance for Tetracycline Degradation and H2O2 Production" Catalysts 13, no. 3: 634. https://doi.org/10.3390/catal13030634

APA Style

Tang, M., Li, X., Deng, F., Han, L., Xie, Y., Huang, J., Chen, Z., Feng, Z., & Zhou, Y. (2023). BiPO4/Ov-BiOBr High-Low Junctions for Efficient Visible Light Photocatalytic Performance for Tetracycline Degradation and H2O2 Production. Catalysts, 13(3), 634. https://doi.org/10.3390/catal13030634

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