Along with the rapid development of domestic and foreign industries that engender impressive economic benefits, egregious environmental pollutants have been introduced to the planet, e.g., heavy metal ions, pesticides, and antibiotics. These pollutants not only affect the water and air quality but also seriously threaten human health [1, 2]. Within the past several decades, a large number of antibiotics have emerged and are still being accumulated globally. They are randomly discharged into natural environments, including surface water, groundwater, and sediments [2, 3]. Tetracycline hydrochloride (TCH) is one of them; it is common and widely used in pharmaceutical care and veterinary fields [4, 5]. The TCH residues in aquatic environment can induce the development of antibiotic-resistant pathogens and have a potential long-term detrimental effect on human health [6-8]. Moreover, TCH is difficult to eliminate [3, 9]; thus, an increasing number of studies is centered on its complete removal [10].
However, generally, the application of conventional physical adsorption and biodegradation methods to realize this objective is considered difficult owing to the cumbersome post-treatment challenges associated with adsorbents, high processing costs, and antibacterial properties[8]. In this case, advanced oxidation treatment has been considered in view of the possibility to generate superoxide free radicals (•O2–) and hydroxyl radicals with strong oxidative abilities (•OH, +2.4 eV) to decompose many recalcitrant organic contaminants [8, 11].
•O2– + organic contaminants→ Degradation products,
•OH + h+ + organic contaminants → egradation products.
Thus far, conventionally, the degradation of TCH has been carried out using the Fenton reagent [8, 12] and photocatalytic oxidation [8, 13-15]. Photocatalytic technology is considered as a practical method for degrading TCH due to its high efficiency, simple operation, and low cost [8]. Most photocatalytic processes employ TiO2–based photocatalysts; however, TiO2 only responds to ultraviolet light, and its wide bandgap limits further practical application [13, 14, 16, 17]. Therefore, exploring new semiconductor catalysts with visible-light-harvesting ability, high efficiency, and easy recycling is an effective way of effecting the degradation of tetracycline [18]. In recent years, bismuth oxyhalide (BiOX, X = F, Cl, Br, I) has attracted wide attention from researchers due to its unique layered crystal structure and optical properties [19-22]. BiOX (X = Cl, Br, and I) is a semiconductor with an indirect bandgap, and the recombination probability of the photogenerated carriers is relatively low [19-21, 23, 24]. These inherent structural advantages determine the intensive employment of BiOX in photocatalytic application [25-30].
Conversely, β-Bi2O3 is also a common germanium-containing semiconductor. Similarly, BiOX (X = Cl, Br and I) exhibits good absorption of visible light; thus, it can significantly facilitate the production of superoxide and hydroxide species under exposure to sunlight [31]. Therefore, in this work, it is chosen for combination with BiOCl0.9I0.1, which has an indirect bandgap, to form a composite catalyst [32]. Due to the internal electrostatic field enrichment, BiOClxI1–x exhibits a relatively high photocatalytic activity [19]. After compounding with β-Bi2O3, the photocatalytic properties of the composite are evaluated by degrading 20 mg·L–1 of TCH under light irradiation. The relative valence band (VB) and conduction band (CB) edge positions and element valence states of the prepared samples were analyzed by analyzing the UV-vis diffuse reflectance spectra, Mott-Schottky plots, and XPS spectra. The possible mechanism of the photocatalytic activity of the BiOCl0.9I0.1/15%β-Bi2O3 complex in the degradation of TCH was discussed using energy band evaluation.
Bi(NO3)3·5H2O, KCl, KI, tetracycline hydrochloride, benzoquinone (BQ), triethanolamine (TEOA), isopropyl alcohol (IPA), and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. All the reagents used were of analytical grade and were used without further purification.
In a typical synthesis, the Bi(NO3)3·5H2O salt was first heated at 200 ℃ for 1 h to evaporate the water molecules contained therein. Subsequently, the temperature was increased to 300 ℃ for 2 h, after which it was maintained at 425 ℃ for 2 h; the sample was naturally cooled in the furnace after the completion of the calcination.
BiOCl0.9I0.1/15%β-Bi2O3 was synthesized by an oil bath method using ethylene glycol as a solvent. In a typical synthesis, 6 mmol of Bi(NO3)3·5H2O was dissolved in 45 mL of ethylene glycol (EG) under magnetic stirring until the solid was completely dissolved. Thereafter, 0.6 mmol of KI and 5.4 mmol of KCl were added to 45 mL of deionized water, followed by stirring for 30 min under ultrasonic agitation. After the solid was dissolved, the Bi(NO3)3·5H2O solution was transferred to a 90 ℃ water bath and stirred for 10 min; subsequently, 0.285 g of β-Bi2O3 was added thereto and stirred for 10 min. Next, a mixed solution of KI and KCl was added dropwise to a mixed solution of Bi(NO3)3·5H2O and β-Bi2O3, and further stirred at 90 ℃ for 1 h. Finally, the prepared solution was filtered and dried at 80 ℃ for 12 h to obtain a BiOCl0.9I0.1/15%β-Bi2O3 composite. The preparation procedure for the BiOCl0.9I0.1/5%β-Bi2O3, BiOCl0.9I0.1/10%β-Bi2O3, and BiOCl0.9I0.1/20%β-Bi2O3 catalysts was the same except that different amounts of β-Bi2O3 were employed.
Photocatalytic degradation experiments were carried out with a photocatalytic reaction device (XPA-G6) using a 350 W Xe lamp as the light source. The reactor was placed horizontally and surrounded by water to cool. A portion (20 mg) of the photocatalyst was dispersed in an aqueous solution of TCH (50 mL, 20 mg·L–1). The suspension was magnetically stirred under dark condition for 1 h to fully reach the adsorption/desorption equilibrium of the TCH molecules on the surface of the photocatalyst. Afterward, light was irradiated onto the tube containing the mixed solution, and about 5 mL of the suspension was continuously withdrawn from the reaction cell at intervals of 30 min for concentration analysis after centrifugation, and sampling was completed after 2 h of light irradiation. The degradation efficiency of the photocatalyst to TCH was evaluated by measuring the concentration of the centrifuged TCH solution using a UV-vis spectrophotometer (TU-1901, Beijing General).
Fig. 1(a) and Fig. 1(b) show the morphologies of the BiOCl0.9I0.1 and BiOCl0.9I0.1/15%β-Bi2O3 composites by SEM analysis, respectively. In Fig. 1(a), it is indicated that the pure BiOCl0.9I0.1 samples have a square nanosheet structure with a size of about 100 nm, which was regularly shaped and laminated. The basic morphology of the prepared BiOCl0.9I0.1/15%β-Bi2O3 composite was compared with those of the pure BiOCl0.9I0.1 and pure β-Bi2O3 catalysts, and the pure β-Bi2O3 exhibited a massive structure (Fig. S1). After forming the composite, the BiOCl0.9I0.1 nanosheets were embedded as a component in the β-Bi2O3 block that was exposed and combined with β-Bi2O3 blocks. At the same time, BiOCl0.9I0.1 nanosheets partially aggregated on the surface of the β-Bi2O3 into small flower-like microstructures covering the entire surface eventually. The presence of BiOCl0.9I0.1 nanosheets greatly increased the specific surface area of the complex and provided more active sites for the reaction.
To further demonstrate that BiOCl0.9I0.1 and β-Bi2O3 are well combined, the TEM and HRTEM results were analyzed. As shown in Fig. 2(a), the obtained BiOCl0.9I0.1 nanosheets were squares having a particle diameter of approximately 100 nm, and the distribution was uniform with a regular shape. As can be seen in Fig. 2(b), the BiOCl0.9I0.1 nanosheets were partially embedded in β-Bi2O3. Further, the lattice fringes observed in Fig. 2(c) correspond to the (110) crystal plane of BiOCl0.9I0.1 (d = 0.28 nm). Similarly, the lattice fringes in Fig. 2(d) correspond to the (110) crystal faces of BiOCl0.9I0.1 (d = 0.28 nm), while d= 0.33 nm corresponds to the (113) crystal faces of β-Bi2O3. Evidently, BiOCl0.9I0.1 and β-Bi2O3 have been successfully combined.
Fig. 3(a) is the XRD patterns of the samples. The XRD diffraction peaks of BiOCl0.9I0.1, β-Bi2O3, and BiOCl0.9I0.1/x%β-Bi2O3 can be seen from Fig. 3(a). The 11.59°, 25.72°, 32.35°, and 33.33° degrees represent the planes of (001), (101), (110), and (102), respectively (Fig.S3). As compared with the peaks of BiOCl and BiOCl0.9I0.1/x%β-Bi2O3, the peaks of BiOCl0.9I0.1/x%β-Bi2O3 shift slightly, which can be ascribed to the introduced I in BiOCl that forms the solid solution. The peaks of the samples are strong, indicating that these substances are crystalline. From the XRD diffraction peaks of the composite catalyst, only the diffraction peak of BiOCl0.9I0.1 is observed, and the diffraction peak of β-Bi2O3 is not observed, from which the evidence of the combination is insignificant. In this case, to explicitly prove the incorporation, SEM, TEM, and XPS analyses were additionally conducted. Fig. 3(b) shows the diffuse reflectance spectra of the prepared pure BiOCl0.9I0.1, β-Bi2O3, and their complexes. Both BiOCl0.9I0.1 and β-Bi2O3 exhibit strong absorptions in the visible light region. It is worth noting that a broad absorption peak near 310 nm can be observed in both BiOCl0.9I0.1 and its composite, and the light absorption intensity of β-Bi2O3 is higher than that of BiOCl0.9I0.1. After the combination of the two semiconductors, the absorption intensity of the BiOCl0.9I0.1/x%β-Bi2O3 composite increases with the β-Bi2O3 content, and the light absorption intensity is between those of B-iOCl0.9I0.1 and β-Bi2O3. This phenomenon could be attributed to the coexistence of the two components: BiOCl0.9I0.1 and β-Bi2O3[33]. The bandgap of BiOCl0.9I0.1/x%β-Bi2O3 was estimated, as shown in Fig. 3(c), and the BiOCl0.9I0.1/15%β-Bi2O3 composite exhibited a narrow bandgap of 2.31 eV compared to 2.90 eV for β-Bi2O3 and 2.84 eV for BiOCl0.9I0.1, suggesting a more efficient light absorption [34]. The lower bandgap promoted the photon excitation from its VB to its CB, thereby effectively accelerating the migration process of the photogenerated carriers and greatly promoting the photocatalytic activity. Fig. 3(d) shows the PL of BiOCl0.9I0.1, β-Bi2O3 , and BiOCl0.9I0.1/15%β-Bi2O3. It can be clearly seen that the strength of BiOCl0.9I0.1/15%β-Bi2O3 is lower than those of the single components, BiOCl0.9I0.1 and β-Bi2O3, implying that BiOCl0.9I0.1/x%β-Bi2O3 can effectively inhibit the recombination of the photogenerated electron pairs[32].
To further study the electron transport of the samples during the photocatalytic reaction, the transient photocurrent responses and EIS Nyquist curves of each sample under illumination conditions were analyzed. Fig. 4(a) shows the transient photocurrent responses of the samples. It can be found that the transient photocurrent intensity of the pure BiOCl0.9I0.1 is higher than that of the pure β-Bi2O3. This is because BiOCl0.9I0.1 is more suitable as a catalyst for photocatalytic degradation, while β-Bi2O3 plays the role of a co-catalyst during the experiment. It can be seen from Fig. 4(a), that the photocurrent intensities of the composite catalysts are significantly higher than those of the pure BiOCl0.9I0.1 and β-Bi2O3 catalysts, indicating that the separation efficiency of the photogenerated electrons and holes in the reaction process increases after the compounding. When the amount of β-Bi2O3 support increases, the transient photocurrent also increases. As the β-Bi2O3 loading reaches 15%, the transient photocurrent of the composite catalyst is the highest, which is highly favored by the reaction [32, 35]. Fig. 4(b) shows the EIS Nyquist curves of the samples. It can be seen from Fig. 4(b), that the impedance of each sample is exactly one-to-one, corresponding to the transient photocurrent. A lower medium impedance implies that the transmission of photogenerated electrons is significantly favorable. It can be seen from Fig. 4(b), that the impedance of the BiOCl0.9I0.1/x%β-Bi2O3 composite catalyst is significantly smaller than that of the pure BiOCl0.9I0.1 and β-Bi2O3 catalysts. In addition, with the increase in the β-Bi2O3 loading, the impedance of the composite catalyst declines. When the β-Bi2O3 loading reaches 15%, the impedance of the composite catalyst is the lowest, further confirming that this proportion is optimal for photocatalytic reactions [36, 37].
The complex was measured by testing the degradation of TCH in water under simulated sunlight. As shown in Fig. 5(a), the sample was observed to reach adsorption equilibrium after stirring for 30 min in the dark. After 120 min of illumination, the degradation rate of TCH in the samples containing the BiOCl0.9I0.1/15%β-Bi2O3 composite reached 82.4%, while the degradation rate of TCH in the samples containing BiOCl0.9I0.1, β-Bi2O3, BiOCl0.9I0.1/5%β-Bi2O3, BiOCl0.9I0.1/10%β-Bi2O3, and BiOCl0.9I0.1/20%β-Bi2O3 was only 54.8%, 21.0%, 70.0%, 73.2%, and 79.9%, respectively. The degradation of TCH can be described by the simplified first-order equation of the Langmuir-Hinshelwood model: ln(C0/Ct) = kt, where C0 is the initial concentration of TCH; Ct, the concentration of TCH; k, the apparent first-order rate constant; and t, the light time. As shown in Fig. 5(b), the apparent rate constants of BiOCl0.9I0.1, β-Bi2O3, BiOCl0.9I0.1/5%β-Bi2O3, BiOCl0.9I0.1/10%β-Bi2O3, BiOCl0.9I0.1/15%β-Bi2O3, and BiOCl0.9I0.1/20%β-Bi2O3 are 0.4×10–2, 0.9×10–3, 0.62×10–2, 0.79×10–2, 0.91×10–3, and 0.96×10–3 min–1, respectively. Therefore, the photocatalytic activity of the BiOCl0.9I0.1 nanosheet was effectively improved by combining it with β-Bi2O3[33].
To explore the active substances involved in the TCH photodegradation, series scavenge trials were executed. As shown in Fig. 5(c), 1, 4-benzoquinone (BQ, •O2− scavenger) and triethanolamine (TEOA, h+ scavenger) greatly inhibited the photodegradation efficiency of TCH. In contrast, the addition of isopropanol (IPA, •OH scavenger) only had a slight effect on the TCH photodegradation. Based on these results, we concluded that the main active species of the TCH photodegradation in BiOCl0.9I0.1/x%β-Bi2O3 are •O2− and •OH [38]. To investigate the recyclability of the BiOCl0.9I0.1/x%β-Bi2O3 composite catalyst, the sample powder, after the photocatalytic reaction, was collected and reused three times in the photocatalytic reaction, under the same conditions. As shown in Fig. 5(d), the photocatalytic activity of BiOCl0.9I0.1/x%β-Bi2O3 did not decrease significantly during the three degradation cycles, indicating that BiOCl0.9I0.1/15%β-Bi2O3 exhibits excellent stability and recyclability [18].
Fig. 6 shows the nitrogen adsorption-desorption isotherms for the BiOCl0.9I0.1 nanosheets and BiOCl0.9I0.1/15%β-Bi2O3 samples. It can be seen that sample exhibits a type IV isotherm, and the specific surface area measured of BiOCl0.9I0.1/15%β-Bi2O3 is 139.09 m2·g–1, which is higher than that of the pure BiOCl0.9I0.1 (63.01 m2·g–1). The relatively large specific surface area can provide the BiOCl0.9I0.1/15%β-Bi2O3 sample with more active sites, which are beneficial to the photocatalytic reaction [39].
To further analyze the electronic state of the elements in the BiOCl0.9I0.1/15%β-Bi2O3 catalyst, XPS was conducted. Fig. 7 show the XPS spectra of Bi 4f, O 1s, Cl 2p, and I 3d. The peak value of C 1s at the binding energy of 284.6 eV was employed as the reference. It can be seen from Fig. 7(a) that the Bi 4f5/2 and Bi 4f7/2 orbitals can fit three peaks, and the two main peaks at the binding energies of 159.54 and 164.81 eV correspond to Bi 3+. The fitted peaks at 160.80 and 165.85 eV correspond to Bi5+ [40]. The fitted peak at the binding energies of 158.41 and 163.83 eV correspond to Bi(3–x)+, where the peaks can symbolize oxygen vacancies [19]. Fig. 7(b) shows the XPS spectrum of O 1s. It can be seen that there are three fitted peaks at 529.0, 530.35, and 533.32 eV, which are ascribed to Bi5+-O, Bi3+-O, and Bi(3–x)+-O, respectively. A slight shift in the position was observed during the evaluation of the O species in BiOCl0.9I0.1/15%β-Bi2O3 compared to the case with BiOCl, which might be attributed to a small amount of I added to BiOCl [19]. It can be seen from Fig. 7(c) that Cl 2p had two fitted peaks of Cl 2p3/2 and Cl 2p1/2 at binding energies of 198.07 and 199.5 eV, respectively [41]. Fig. 7(d) shows the XPS spectrum of I 3d. It was found that the peaks at both I 3d3/2 and I 3d3/2 can be fitted into three peaks. The binding energies are as follows: I− at 618.63 and 629.83 eV, I3− at 619.60 and 630.9 eV, and I5+ at 622.23 and 633.83 eV [32].
The mechanism of the electron transfer in the photocatalytic reaction is predicted. When visible light is irradiated onto the surface of the catalyst, BiOCl0.9I0.1 can generate photogenerated electrons (e–). The photogenerated electrons can migrate from the VB to the CB of the catalyst, while an equal amount of photogenerated holes (h+) can be generated on the VB. According to the model Schottky curve (Fig. S2), the β-Bi2O3 catalyst is also in a similar situation. Specifically, the CB potential of β-Bi2O3 (–0.4 eV) is more negative than that of BiOCl0.9I0.1 (1.2 eV), while the VB potential of β-Bi2O3 (2.5 eV) is smaller than that of BiOCl0.9I0.1 (3.04 eV). When the catalyst and the cocatalyst are combined to form a composite catalyst, the CB and the VB positions between those of β-Bi2O3 and BiOCl0.9I0.1 can form a ladder structure. This structure facilitates the formation of a current loop by the charge transfer, which can effectively improve the separation efficiency of the photogenerated electrons and holes. In addition, the photogenerated electron, in the process of transport, can combine with the dissolved oxygen on the surface of the composite to produce superoxide radicals •O2−, which are active substances that can degrade TCH and combine with both H+ to form –OOH and H2O2, and –OOH to form hydroxyl radicals (•OH) that are capable of completely oxidizing the pollutant. Moreover, since the VB potential of the catalyst is higher than the standard oxidation-reduction potential (•OH/OH–) (+2.4 eV) [42], it is suggested that the OH− species in the reaction system can also transform into hydroxyl radicals •OH; •O2− and •OH can directly participate in the degradation reaction of TCH [43]. The reaction process can be generalized as follows:
In summary, the BiOCl0.9I0.1/15%β-Bi2O3 composite was successfully synthesized through a simple oil bath method at 90℃. The TEM and XPS images could prove that BiOCl0.9I0.1 and β-Bi2O3 were successfully combined, and the (110) and (113) crystal lattice fringes of BiOCl0.9I0.1 and β-Bi2O3 were clearly observed in the HRTEM image. The BET analysis showed that the BiOCl0.9I0.1/15%β-Bi2O3 composite had a large specific surface area, providing more active sites for the reaction. The BiOCl0.9I0.1/15%β-Bi2O3 composite had a significantly reduced bandgap compared to those of the pure BiOCl0.9I0.1 and pure β-Bi2O3 catalysts. The degradation rate by the BiOCl0.9I0.1/15%β-Bi2O3 composite material of TCH in water was 82.4%, which was higher by 27.6% and 61.4% than those of the pure BiOCl0.9I0.1 and pure β-Bi2O3 catalysts, respectively. Cyclic experiments showed that the BiOCl0.9I0.1/15%β-Bi2O3 composites exhibited good cycle stability.
2020,
Vol. 41 Issue (10): 1535-1543 DOI: S1872-2067(19)63486-8 
PDF
2020,
Vol. 41 Issue (10): 1535-1543 DOI: S1872-2067(19)63486-8 PDF