Spindle-like MIL101(Fe) decorated with Bi2O3 nanoparticles for enhanced degradation of chlortetracycline under visible-light irradiation

Characterization

Figure 1 shows the XRD pattern of the synthesized MIL101(Fe), Bi2O3, and BOM-20 composite.

[2190-4286-13-91-1]

Figure 1: XRD patterns of MIL101, Bi2O3, and BOM-20.

The characteristic diffraction peaks of MIL101(Fe) at 8.9°, 10.2°, 10.6° and 16.4° were observed, which is in good accordance with previous reports [37,51], indicating that MIL101(Fe) was successfully prepared. In the XRD pattern of the prepared Bi2O3 sample, peaks at 2θ = 25.88°, 26.87°, 27.37°, 33.15°, 35.12°, 46.52°, and 52.56° correspond to the (002), (111), (120), (122), (212), (041), and (321) plane of monoclinic α-Bi2O3, which matches well with the standard card α-Bi2O3 (PDF#71-2274) [52], suggesting good crystallization of the prepared Bi2O3. The characteristic diffraction peaks of MIL101(Fe) and Bi2O3 are all found in the XRD pattern of BOM-20, indicating the existence of MIL101(Fe) and Bi2O3 in BOM-20. Additionally, the presence of characteristic peaks of MIL101(Fe) in the BOM-20 composite demonstrates that the framework of MIL101(Fe) is unchanged after loading the Bi2O3 nanoparticles.

The morphology and microstructure of Bi2O3, MIL101(Fe), and BOM-20 were observed by SEM, TEM, and HRTEM. Figure 2 shows SEM images of Bi2O3, MIL101(Fe), and BOM-20. Figure 2a reveals that MIL101(Fe) appears as an octahedron with a smooth surface and size of approx. 1–2 μm, which is consistent with a previous report [53]. As shown in Figure 2b, pristine Bi2O3 shows nanoparticles with diameters of approx. 10–20 nm. In the SEM image of the composite BOM-20 (Figure 2c), Bi2O3 nanoparticles are tightly attached on the surface of MIL101(Fe). Note that MIL101(Fe) in BOM-20 presents a spindle-like shape instead of an octahedron shape, and its size increases compared with that of pristine MIL101(Fe). In previous reports, graphene quantum dots changed the balance of organic ligands, which led to the change of MIL101(Fe) size [38]. In addition, Liao et al. reported that morphology or size of MOFs are dependent on nucleation rate, (i.e., slow nucleation could increase crystal growth and as a result produce large-sized particles [54]). As a result, Bi2O3 nanoparticles affect the shape and size of MIL101(Fe) and this may be due to the addition of Bi2O3 to the precursor, altering the balance of ligands and simultaneously slowing the nucleation rate of MIL101(Fe).

[2190-4286-13-91-2]

Figure 2: SEM images of (a) MIL101(Fe), (b) Bi2O3, and (c,d) BOM-20.

To verify the formation of a heterojunction between MIL101(Fe) and Bi2O3, TEM and HRTEM images were obtained. As seen in Figure 3b and Figure 3c, TEM images of BOM-20 confirm that tiny Bi2O3 nanoparticles closely and uniformly adhere to the surface of MIL101(Fe). Figure 3d shows an HRTEM image of BOM-20 composites. The lattice fringes of 0.37 nm correspond to the (120) facet of α-Bi2O3[55]. Meanwhile, the tight contact interface between Bi2O3 and MIL101(Fe) can be clearly observed. Furthermore, the elemental mapping images of BOM-20 (Figure 3e–h) reveal the homogeneous distribution of Bi in the entire BOM-20 composite, which further confirms that Bi2O3 nanoparticles are uniformly loaded on the surface of MIL101(Fe). Results of SEM and TEM provided convincing evidence for the successful construction of heterojunctions with a 0D/3D structure between Bi2O3 nanoparticles and tridimensional MIL101(Fe). This 0D/3D heterojunction is conducive to fast separation and migration of the photoproduced electron–hole pairs due to more contact areas between Bi2O3 and MIL101(Fe) and shorter transfer distances of photoinduced charge carriers [8], which results in enhanced photocatalytic activity of the Bi2O3/MIL101(Fe) composite.

[2190-4286-13-91-3]

Figure 3: TEM image of (a) MIL101(Fe) and (b, c) BOM-20. (d) HRTEM image of BOM-20. (e) HAADF-STEM image of BOM-20 and the corresponding elemental maps of (f) Fe, (g) O, and (h) Bi.

X-ray photoelectron spectroscopy was performed to explore the elemental compositions and the surface chemical states of BOM-20 and the results are presented in Figure 4. The XPS survey spectrum (Figure 4a) indicates the existence of Fe, Bi, O, and C in BOM-20 composites. In the high-resolution spectrum of Fe 2p (Figure 4b), peaks at 725.2 and 711.4 eV are ascribed to Fe 2p1/2 and 2p3/2, respectively, implying the existence of Fe–O bonds [37]. The difference of binding energy between these two peaks is 13.9 eV, suggesting the presence of Fe3+ in BOM-20 [56]. Furthermore, the satellite peak of Fe(III) at 714.5 eV is observed, further indicating the presence of Fe3+ in BOM-20 [38]. However, the characteristic peak at 714.5 eV of Fe3+ in the BOM-20 composite was shifted toward lower binding energy relative to MIL101(Fe) (717.1eV), indicating an increased electron density on Fe3+[42,57]. As shown in Figure 4c, the Bi 4f spectra of BOM-20 were fitted by two peaks at 159.5 and 164.9 eV, which are assigned to Bi 4f7/2 and Bi 4f5/2, respectively, corroborating the presence of Bi3+[52,57]. The binding energies of Bi 4f at 159.5 and 164.9 eV of BOM-20 are higher than those of pristine MIL101(Fe), which indicates a decreased electron density on Bi. In BOM-20, the increased electron density on Fe3+ and the decreased electron density on Bi may be attributed to the interaction between Bi2O3 and MIL101(Fe), which further confirms the formation of Bi2O3/MIL101(Fe) heterojunction [57]. In the C 1s spectrum (Figure 4d), peaks at 284.8, 285.6, and 288.8 eV can be assigned to the carbon atom bond (C–C) in the benzoic rings and carbon organic linkers (C–O and O–C=O) of the H2BDC ligand, respectively [37].

[2190-4286-13-91-4]

Figure 4: (a) XPS survey spectra of BOM-20 and high-resolution XPS spectra of (b) Fe 2p; (c) Bi 4f; and (d) C 1s.

Optical and electronic properties

To investigate the optical response and bandgap of the prepared samples, UV–vis diffuse reflectance (UV–vis DRS) spectra were recorded and given in Figure 5.

[2190-4286-13-91-5]

Figure 5: (a) UV–vis spectra, (b) PL spectra, (c) transient photocurrent responses, and (d) EIS spectra of MIL101(Fe), Bi2O3, and BOM-20.

As seen from Figure 5a, both Bi2O3 and MIL101(Fe) show strong visible-light response with absorption edge of 460 nm and 510 nm, respectively. Compared with MIL101(Fe) and Bi2O3, the absorption edge of BOM-20 significantly red shifts to approx. 620 nm. MIL101(Fe) and Bi2O3 show a synergistic effect in improving visible-light absorption of the BOM-20 composite, which improves the photocatalytic activity due to the generation of more photoinduced electrons and holes. The bandgap (Eg) of a semiconductor is usually estimated by the Tauc formula (αhν) = A(hv − Eg)n/2, where α is the absorbance, hν is the photon energy, A is a constant, Eg is the bandgap , and n is a constant. For Bi2O3 and MIL101(Fe), as direct bandgap semiconductors, the value of n is 1 [58]. The plots of (αhν)2 as a function of (hν) for Bi2O3 and MIL101(Fe) are shown in the insert of Figure 5a. According to that, the bandgap of Bi2O3 and MIL101(Fe) are 2.78 and 2.75 eV, respectively.

In order to explore the separation of photoproduced electron–hole pairs in BOM-20, PL spectra were measured at an excitation wavelength of 464 nm. Generally, the weaker PL emission, the lower the recombination efficiency of electron–hole pairs [8]. As displayed in Figure 5b, Bi2O3 possesses a strong PL emission peak at approximately 550 nm, which is due to the rapid recombination of photogenerated electron–hole pairs in Bi2O3. MIL101(Fe) and BOM-20 composite show similar emission peaks at 546 nm. Moreover, BOM-20 peak intensity is obviously weaker than that of MIL101(Fe) and Bi2O3, which indicates that the recombination of photogenerated electron–hole pairs was effectively inhibited in BOM-20.

To further elucidate the transfer behavior of the charge carriers in BOM-20, photocurrent responses were carried out under visible light (λ > 420 nm) and were displayed in Figure 5c. The photocurrent density of the BOM-20 composite is significantly higher than that of pristine Bi2O3 and MIL101(Fe), which suggests that the formation of the heterojunction can promote charge transfer on the interface between MIL101(Fe) and Bi2O3. Furthermore, the interfacial charge transfer behavior of the BOM-20 composite is also confirmed by EIS. In Figure 5d, BOM-20 exhibits the smallest arc radius, meaning that BOM-20 possesses the lowest resistance and the highest separation rate of electron–hole pairs on the interface of MIL101(Fe) and Bi2O3[57], which is consistent with the results from PL spectra and photocurrent analysis. In summary, PL spectra, photocurrent responses, and EIS analysis all indicated that Bi2O3/MIL101(Fe) heterojunctions can result in more effective electron–hole pair separation, higher interfacial electron transfer rate, and thus weakened charge transfer resistance, which may facilitate the improvement of Bi2O3/MIL101(Fe) photocatalytic activity.

Photocatalytic degradation of chlortetracycline

To evaluate the photocatalytic activity of BOM-20, the photodegradation of chlortetracycline (CTC) under visible-light irradiation was conducted. It can be seen from Figure 6a that Bi2O3 shows poor adsorption and photocatalytic activity, and the CTC removal rate is only 40% within 120 min. MIL101(Fe) shows strong adsorption capacity in the dark and a poorer photocatalytic activity. With the photocatalysis of MIL101(Fe), the photodegradation efficiency of CTC only could reach 64.5% after irradiation for 120 min. Compared with MIL101(Fe), Bi2O3/MIL101(Fe) composites exhibit a slightly weakened adsorption efficiency for CTC, which is perhaps due to decreased specific surface areas relative to MIL101(Fe) (Supporting Information File 1, Figure S1, Table S1). However, the photocatalytic activity of the Bi2O3/MIL101(Fe) composite is substantially higher than that of pristine Bi2O3 and MIL101(Fe). Among various composites, BOM-20 shows the highest activity with a CTC removal rate up to 88.2%. Additionally, Figure 6b demonstrates that the photocatalytic degradation of CTC accords with the quasi-first-order kinetic model. Figure 6c shows the apparent kinetic constant (k) of CTC photodegradation by the as-prepared catalysts. The k value of BOM-20 (0.01348 min−1) is the largest, which is 5.9 and 4.3 times higher than that of Bi2O3 (0.00226 min−1) and MIL101(Fe) (0.00309 min−1), respectively. The enhanced photocatalytic activity is attributed to the effective separation and migration of the photoinduced charge carriers due to the formation of the Bi2O3/MIL101(Fe) heterojunction and the enhanced visible-light absorption. In addition, some recently reported research results of photocatalytic CTC degradation are shown in Supporting Information File 1, Table S2. The CTC degradation efficiency of the as-prepared BOM-20 composite in this work is significantly satisfactory.

[2190-4286-13-91-6]

Figure 6: (a) Photodegradation of CTC over as-prepared photocatalysts under visible irradiation (λ > 420 nm). (b) Quasi-first-order kinetic curve. (c) Apparent rate constants (k) and (d) degradation rate of CTC in a four-cycle test of the BOM-20 photocatalyst.

The stability of BOM-20 for the degradation of CTC was examined via cycling degradation tests. As depicted in Figure 6d, BOM-20 exhibits no obvious loss of activity after four consecutive cycles. A slight decrease in CTC degradation rate is perhaps caused by an inevitable loss of the photocatalyst during recycling. As a result, the BOM-20 heterojunction possesses excellent stability in the photocatalytic reaction. Moreover, in the XRD patterns of fresh BOM-20 and BOM-20 after the photocatalytic cyclic test (Supporting Information File 1, Figure S2), no obvious change was found, which suggests that the crystal structure of BOM-20 remained unchanged after the photocatalytic reaction. This result further confirms the stability of the BOM-20 heterojunction as a photocatalyst.

To explore the CTC degradation pathway with the existence of a BOM-20 heterojunction, the intermediate products in CTC degradation were detected by liquid chromatography–mass spectrometry (LC–MS), and the mass spectra are showed in Supporting Information File 1, Figure S3. Based on analysis of intermediates, the possible degradation pathways were speculated and shown in Figure 7. Two possible degradation pathways were deduced through hydroxylation, dehydration, and ring opening [59]. First, dechlorination of CTC due to the attack of radicals resulted in the formation of P1 intermediates with m/z of 444. Then P2 (m/z 432) was obtained from P1 through the loss of an N-methyl group and hydroxylation [4]. The P2 was in turn decomposed into possible intermediates (P3, P4, P5, P6) via ring opening, ketonization/dehydroxylation, deamidation, and dialdehyde [11,60]. The second pathway was initiated from deamidation or deamination of CTC. The P7 (m/z 486) was obtained from deamidation and hydroxylation of CTC. Meanwhile, CTC was decomposed into P8 (m/z 453) through deamination and hydroxylation. Then, the produced P7 and P8 were converted to P9 (m/z 362) through deamidation, ketonization/dihydroxylation, and demethylation [3]. The P10 (m/z 318) was derived from P9 due to the loss of hydroxyl and ketone groups and was further decomposed into P11 (m/z 294) or P12 (m/z 274). The P11 and P12 were further decomposed into P13 (m/z 208) by double-bond break and ring-opening reaction [1]. Subsequently, with the attack of active species, intermediates (P6, P13) are further oxidized and eventually evolve into possible small molecular intermediates such as (P14 m/z 116), Cl−, CO2, and H2O [60].

[2190-4286-13-91-7]

Figure 7: Possible degradation pathways of CTC catalyzed by BOM-20.

Proposed charge-transfer mechanism

To explore charge-transfer mechanisms, the main active species involved in degradation of CTC catalyzed by BOM-20 was investigated by radical trapping test. Ethylenediaminetetraacetic acid disodium (EDTA-2Na), isopropanol (IPA), and vitamin C (VC) were used as scavengers for holes (h+), hydroxyl radicals (•OH), and superoxide radicals (•O2−), respectively.

Figure 8a shows the degradation rate of CTC in the presence of three scavengers. As shown in Figure 8a, when VC was added to the photocatalytic reaction system, the degradation efficiency of CTC significantly decreased from 88.2% to 12.5%, which revealed that •O2− is main active species in the degradation of CTC. In addition, after the addition of IPA and EDTA-2Na, the degradation efficiency of CTC decreased to 73.6% and 65.1%, respectively, indicating that h+ and •OH also played a role in the photocatalytic process to some extent. To further confirm the free radicals produced in the photocatalytic degradation of CTC, ESR spectra of the samples were measured. As given in Figure 8b,c, no signals of •O2− and •OH were found in dark conditions. However, the strong characteristic signals of •O2− and •OH were detected after visible-light irradiation for 10 min, which demonstrated that •O2− and •OH were produced during the photodegradation process. Figure 8d shows that the signal of h+ in the dark is stronger than that under visible-light irradiation, which indicates that h+ plays a role in photocatalytic reaction progress. This result is in accordance with that of the radical trapping test.

[2190-4286-13-91-8]

Figure 8: (a) The degradation efficiency of CTC in the presence of different scavengers. ESR spectra of (b) DMPO •O2−, (c) DMPO •OH, and (d) TEMPO •h+ for the BOM-20 photocatalyst.

In order to account for the formation of the active species and reveal the photocatalytic mechanism, the band structure was measured by UV–vis DRS and valence band X-ray photoelectron spectroscopy (VB-XPS). UV–vis DRS spectra and the bandgap (Eg) of Bi2O3 and MIL101(Fe) were discussed in Figure 5a, and the obtained bandgap of Bi2O3 and MIL101(Fe) are 2.78 and 2.75 eV, respectively. Figure 9a shows the VB-XPS spectra of Bi2O3 and MIL101(Fe). As seen in Figure 9a, the valence band potential (EVB) of Bi2O3 and the highest occupied molecular orbital (HOMO) potential of MIL101(Fe) are 3.12 and 1.92 eV, respectively. According to the equation: Eg = EVB − ECB, the conduction band potential (ECB) of Bi2O3 and the lowest unoccupied molecular orbital (LUMO) potential of MIL101(Fe) are calculated as 0.34 and −0.83 eV, respectively. The energy band position of Bi2O3 and MIL101(Fe) are shown in Figure 9b. According to Figure 9b, ECB and EVB of Bi2O3 are more positive than the LUMO and HOMO potentials of MIL101(Fe), respectively, which suggests that possible charge-transfer mechanisms on the interface of Bi2O3/MIL101(Fe) heterojunction may be conventional type-II charge transfer or Z-scheme charge transfer. For the type-II charge-transfer mechanism, photogenerated electrons (e−) tend to transfer from the LUMO of MIL101(Fe) to the conduction band (CB) of Bi2O3, while h+ in the valence band (VB) of Bi2O3 will migrate to the HOMO of MIL101(Fe). Resultantly, photogenerated e− mainly accumulate in the CB of Bi2O3. Nevertheless, the ECB (0.34 eV) of Bi2O3 is more positive than the potential of O2/•O2− (−0.33 eV), so •O2− cannot be produced. Furthermore, the HOMO potential (1.92 eV) of MIL101(Fe) is more negative than the potential of H2O/•OH (2.4 eV), indicating that the generation of •OH is unlikely. Obviously, the hypothesis of type-II charge transfer is contradictory to the results from radical trapping and ESR experiments. Thus, the Z-scheme charge-transfer model is expected to clarify the photocatalytic mechanism of BOM-20 for degradation of CTC. As illustrated in Figure 10, under visible-light irradiation, MIL101(Fe) and Bi2O3 can be excited to produce e− and corresponding h+, respectively. Due to the internal field at the interface of the heterojunction, the photogenerated electrons in the CB of Bi2O3 transfer to the VB of MIL101(Fe) and quickly recombine with the holes of MIL101(Fe), which boosts interfacial separation of charge carriers. As a result, e− and h+ are reserved in the CB of MIL101(Fe) and in the VB of Bi2O3, respectively. Since the ECB (−0.83 eV) of MIL101(Fe) is more negative than the reduction potential of O2/•O2− (−0.33 eV), the e− in LUMO of MIL101(Fe) can capture O2 in solution to generate •O2−. Similarly, the h+ in the VB of Bi2O3 may oxidize H2O to produce •OH due to the fact that the VB potential (3.12 eV) of Bi2O3 is more positive than the redox potential of H2O/•OH (2.4 eV). The produced •O2− and •OH decompose CTC. Additionally, the h+ in the VB of Bi2O3 can directly participate in CTC degradation. These results are in good agreement with the analysis of the trapping test and ESR spectra. Therefore, the Z-scheme charge-transfer pathway is confirmed in the Bi2O3/MIL101(Fe) heterojunction. Apparently, the Z-scheme heterojunction is conducive to the effective separation of photogenerated carriers. Furthermore, h+ with higher oxidizing ability and e− with stronger reducing ability participate in the redox reaction and production of •OH and •O2−. Accordingly, the significant improvement of the photocatalytic activity of Bi2O3/MIL101(Fe) can be attributed to the formation of a Z-scheme heterojunction.

[2190-4286-13-91-9]

Figure 9: (a) The VB-XPS spectra and (b) the energy band position of Bi2O3 and MIL101(Fe).

[2190-4286-13-91-10]

Figure 10: Z-Scheme charge-transfer mechanism of a BOM-20 heterojunction.

留言 (0)

沒有登入
gif