Effective Visible Light-Driven Photocatalytic Degradation of Ciprofloxacin over Flower-like Fe3O4/Bi2WO6 Composites.

Photocatalytic degradation of organic pollution is a vital path to deal with environmental problems. Here, a direct Z-scheme 2D/2D heterojunction of a Fe3O4/Bi2WO6 photocatalyst is fabricated for the degradation of ciprofloxacin by a self-assembly strategy. Furthermore, to characterize the morphology of the obtained composite photocatalysts, various kinds of characterization methods were employed like XRD, XPS, SEM, and TEM. It is indicated that the flower-like photocatalyst is composed of nanosheets. Comparable photocatalysts were prepared by controlling the hydrothermal temperature and the iron content. In the photocatalytic degradation of ciprofloxacin (CIP) in water, under visible light irradiation, FB-180 (synthesized at 180 °C with 4% iron content) presents approximately 99.7% degradation efficiency in only 15 min. Meanwhile, during photocatalytic degradation reactions, the Fe3O4/Bi2WO6 heterojunction also displayed excellent stability, which still kept above 90% degradation efficiency after five consecutive cycles. UV-Vis DRS and M-S analyses showed that the Fe3O4/Bi2WO6 catalyst has a strong visible light absorption capacity and the transfer pathway of photo-induced charge carriers. PL, EIS, and TPR showed that Fe3O4/Bi2WO6 has an efficient separation and transfer rate of the photo-generated carriers. ESR analysis proved that the superoxide radical (•O2 -) and hydroxyl radical (•OH) play a major role in the Fe3O4/Bi2WO6 photocatalytic system. This special 2D/2D heterojunction we proposed may have huge potential for marine pollution treatment by photocatalysis degradation with dramatically boosted activities.

Introduction

Excessive antibiotic content in water threatens the sustainable development of the environment and human health.[1−3] CIP, one of the widely used antibiotics in the world, has been applied to human therapy. Because of its broad spectrum antibacterial activity, it is used in the treatment of human diseases.[4] However, the drug abuse seriously threatened the living environment of organisms. CIP has been detected in rivers, lakes, farmland soils, and groundwater in many countries. Human beings living in such an environment for a long time are still compromised. Currently, there are many physical and chemical methods to treat CIP in water, including adsorption, biodegradation, and flocculation. Unfortunately, because of some defects such as complicated procedures, high cost, long cycle, and incomplete removal, the application of these treatment methods is hampered. Nowadays, photocatalysis has been considered one of the most effective technologies for antibiotics degradation.[5,6] To apply photocatalytic technology more widely, it is a core issue to develop a photocatalyst with a wide range of light source absorption wavelengths and superior charge separation ability.

Over the past few years, many new photocatalytic materials have been developed such as g-C3N4,[7] Ti3C2,[8] and Bi2MoO6.[9] However, it is generally believed that the original photocatalyst still has many shortcomings, such as a large energy band gap and photogenerated current, high recombination rates of photogenerated carriers, lack of visible light response, and so forth.[10] Bismuth tungstate (Bi2WO6, band gap: 2.60–2.80 eV), a typical bismuth-based semiconductor photocatalyst made of [Bi2O2]2+ and perovskite-like [WO4]2–, is recognized as an effective photocatalyst for degrading pollutants because of its nontoxicity, simple preparation process, and excellent chemical stability.[11] However, the limited light absorption range and high electron–hole recombination rate seriously limit the practicability of the photocatalyst.[12] Accordingly, many modifications of the catalyst have been developed, for example, doping metals,[13] heterojunction construction,[14] controlling morphologies,[15,16] defect introduction, and so on.[17] Among the above methods, the fabrication of Bi2WO6-based heterojunction photocatalysts is demonstrated to be an effective strategy to improve photocatalytic degradation efficiency. Although the traditional Bi2WO6-based heterojunction photocatalyst can enhance the separation ability of photogenic carriers, the oxidation ability of holes, and the reduced ability of electrons will decrease because of the recombination.[18] Therefore, the traditional heterojunction-type (e.g., n–n heterojunction and p–n heterojunction) photocatalytic system has only a low charge carrier separation efficiency and weak redox ability.[19]

However, the Z-scheme heterojunction shows outstanding advantages owing to the efficient separation of photogenerated carriers and the excellent redox ability.[20,21] Constructing a direct Z-scheme heterojunction photocatalyst with a suitable conduction band and valence band potential is better for improving light absorption capacity and redox ability.[18] Thus far, some signs of progress on the exploration of Z-scheme heterojunctions have been reported. It is still a huge challenge to further develop Bi2WO6-based Z-scheme heterojunction photocatalysts with great performance and excellent stability.

A ferroferric oxide nanosheet (Fe3O4, Eg ≈ 1.69 eV), as a novel type of photocatalyst, has become the vital photocatalyst for contaminant removal because of its broad high super-magnetism, suitable energy band structure, and excellent photochemical stability.[22] Nevertheless, the catalytic activity of Fe3O4 is restrained by the rapid recombination of photogenerated carriers. Inspiringly, the Fe3O4 nanosheet, as a reduction photocatalyst, has a high conduction potential and reduction ability. Bi2WO6 has a low valence potential and strong oxidation ability. These two materials are conducive to form the Z-scheme heterojunction owing to their band structure.[23] The Z-scheme heterojunction is endowed with higher light absorption capacity and outstanding separation efficiency of charge carriers, thus having excellent photocatalytic activity. It is worth noting that the introduction of magnetic nanomaterials is a feasible way to make the catalyst easy to recover.[24,25] Many researchers have prepared a composite photocatalyst combining Fe3O4 with Bi2WO6.[26,27] Fe3O4 and Bi2WO6 combine to form many different types of core–shell photocatalysts. Generally, the surface coating of the Fe3O4 core is necessary to adhere to the semiconductor material.[26] The Fe3O4 nanosheet contains a lot of carbon on its surface because of its unique preparation method.[23] Therefore, our self-made Fe3O4 nanosheet can directly contact Bi2WO6 to form heterojunctions. Because of the presence of carbon, the electron transfer rate between heterojunctions is greatly increased, which greatly improves the photocatalytic activity. The combination of Fe3O4 (2D) and Bi2WO6 nanosheets greatly increases the contact area between the two substances, which is beneficial to the transfer of photogenic carriers. This 2D/2D structure effectively enhances the photocatalytic activity.[28,29]

Up to now, Z-scheme Fe3O4/Bi2WO6 (2D/2D) heterojunction photocatalysts for the degradation of organic pollutants have not been reported. This may be because of the challenges in either the synthesis of Fe3O4 nanosheets or the selection of suitable semiconductors, which can form a suitable band gap structure for photocatalysis degradation. Therefore, it is promising to design and construct a novel Fe3O4/Bi2WO6 Z-scheme heterojunction photocatalyst for the removal of organic pollutants.

Herein, an original Fe3O4/Bi2WO6 Z-scheme heterostructure was synthesized by a simple hydrothermal method. The Z-scheme heterojunction not only improves the light-harvesting ability but also inhibits the rapid recombination of photogenic carriers. It is applied to degrade harmful pharmaceutical pollutants under visible light illumination. Photocatalytic experimental results show Fe3O4/Bi2WO6 has high photocatalytic properties and excellent stability for the degradation of CIP. Therefore, the flower-like Fe3O4/Bi2WO6 catalyst opens new opportunities for environmental pollution treatment.

Results and Discussion

Scheme shows the synthesis schematic diagram of the Z-scheme heterojunction Fe3O4/Bi2WO6. The Fe3O4/Bi2WO6 photocatalyst was constructed by a hydrothermal method on a self-assembly Fe3O4 nanosheet. XRD was conducted to further confirm the crystalline structure of the as-prepared samples. The XRD patterns of Bi2WO6, Fe3O4, and binary composites were described in Figure a. These distinct diffraction peaks of Bi2WO6 at 2θ = 28.42, 33.01, 47.21, and 55.86° correspond to the (113), (020), (220), and (313) crystal planes of orthorhombic of Bi2WO6 (JCPDS 73-1126).[30] It means that Bi2WO6 are successfully synthesized. The XRD spectra of Fe3O4 nanosheets (JCPDS 19-0629) shows four peaks at 2θ = 18.26, 30.09, 35.42, and 62.51° belonged to the (111), (220), (311), and (440) crystal planes (Figure S2). Besides, the diffraction peak of Fe3O4 nanosheets is almost invisible in FB-180 in the XRD pattern of FB-180 owing to the low crystallinity and less content of Fe3O4. Also, the diffraction peak of Fe3O4 nanosheets is almost invisible in the XRD pattern of FB-180 owing to the low crystallinity and less content of Fe3O4. It indicates that the addition of Fe3O4 nanosheets does not affect the crystallinity of Bi2WO6.

Scheme 1

Synthetic Strategy for Fe3O4/Bi2WO6 Preparation

Figure 1

(a) XRD patterns of Fe3O4 nanosheets, Bi2WO6, and FB-180; (b) XPS survey spectra of the FB-180; high-resolution spectra of (c) Fe 2p and (d) Bi 4f.

The surface elemental composites and chemical states of the FB-180 were showed by XPS characterization. The XPS spectra of the FB-180 exhibited the peaks of Fe, W, O, and Bi elements (Figure b). The peak corresponding to the Bi element in FB-180 is located at 159.2 and 164.5 eV, respectively. It confirmed the existence of Bi3+ in the composite material (Figure c).[31] Through curve fitting, both feeble bands at 710.5 and 725.3 eV, as shown in Figure d, can be discriminated against and attributed to Fe 2p3/2 and Fe 2p1/2 of Fe species, which confirmed the existence of Fe2+ and Fe3+.[32] The existence of Fe3+ and Fe2+ proves that the iron-based two-dimensional nanosheets used in the synthesis of composite nanomaterials are Fe3O4. As shown in Figure S3, the broad spectrum diagram shows three signals, two of them at 530.8 and 532.0 eV of O 1s were ascribed to the chemical bonds of Bi–O and W–O in the form of [WO4]2– and [Bi2O2]2+, respectively.[33,34] The last one peak centered at 529.9 eV is indexed to Fe–O bonds. Fe3O4 was closely contacted with Bi2WO6, which is conducive to the separation of electrons and holes in the process of photocatalytic oxidation. The above analysis shows that the photocatalyst Fe3O4/Bi2WO6 has been successfully synthesized.

The morphology and microstructure of the as-prepared samples (FB-160, FB-180, and FB-200) by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and element mapping were characterized. The SEM and TEM images of Fe3O4 prove that the structure of Fe3O4 is two-dimensional nanosheets (Figure S4a,b). From Figure a, the flower size of the catalyst FB-180 is relatively uniform, and each flower is relatively complete with fewer stacked. Catalysts prepared at different temperatures have different morphologies and structures. The FB-160 sample with a synthesis temperature of 160 °C formed an irregular block structure (Figure S5a,b). When the sample synthesis temperature reaches 180 °C, it had an about 3 μm diameter flower-like microsphere assembled by numerous two-dimensional nanosheets (Figure b). The thickness of the two-dimensional nanosheets that make up the flower-shaped microspheres is about 20–40 nm. The electronegativity of the Fe3O4 nanosheet surface leads to the uniform adsorption of Bi3+ on the nanosheet surface. Then, Bi3+ could react with WO42- to generate Bi2WO6 nanosheets. Bi2WO6 nanosheets formed three-dimensional flower-like microspheres under the influence of the Fe3O4 template base. Such a flower-like heterostructure composed of nanosheets facilitates the separation of photo-induced carriers. Upon further elevating the Fe3O4/Bi2WO6 synthesis temperature to 200 °C, the sample formed sheet-like structures of different shapes and agglomerates, resulting in a reduction in the surface area of the photocatalyst and the contact area of pollutants (Figure S4c,d). Therefore, it is speculated that the flower-like structure of the FB-180 sample was formed through the process of self-aggregation and self-assembly.

Figure 2

(a,b) SEM images of the FB-180; (c,d) TEM images of FB-180; (e) HRTEM images of the FB-180 (Inset: FFT of the region marked in the red square); and (f) EDX mapping profiles of Fe, Bi, O, and W.

To further illustrate the microstructure of the catalyst FB-180, TEM, and HR-TEM analyses were performed and displayed. Figure c proves the existence of a flake structure in the FB-180 photocatalyst. It can be seen that the blue arrow could find the sheet-like structure. Two different nanosheets can be seen in Figure d, and they form a 2D (dimensional)-2D heterostructure, which demonstrated the formation of Fe3O4/Bi2WO6 heterojunctions. This result also showed that FB-180 was a heterojunction in the structure rather than a physical mixture of two materials of Fe3O4 nanosheets and Bi2WO6. It enables Bi2WO6 to grow along the lateral direction of Fe3O4 nanosheets and to obtain a three-dimensional rose-like structure by self-assembly.[35]

HRTEM was used to characterize the lattice fringes of FB-180 (Figure e). The lattice fringe in the red rectangle corresponded to the (020) plane of Bi2WO6 [Figure e (inset)]. The observed tight bonding of the interface between Fe3O4 and Bi2WO6 may facilitate the transfer of photogenerated carriers and promote the separation of photogenerated electron–hole pairs. Furthermore, element mapping images (Figure f) indicates Fe, Bi, O, and W has a good distribution. The result provides evidence of a great dispersity of Bi2WO6 in FB-180, which verifies that the Bi2WO6 nanosheets are uniformly covered Fe3O4 during the hydrothermal reaction process. According to these characterizations, we could conclude that the flower-like two-dimensional Fe3O4/Bi2WO6 heterostructure has been successfully constructed by a simple method.

To explore the photocatalytic performance of the samples prepared under different conditions, we carried out a series of control experiments (Figure ). In the experiment, one group of degradation experiments was performed for 30 min of dark adsorption, and the other group was directly subjected to the photocatalytic degradation (Figure a).[36] It can be seen that the concentration of CIP in water decreased rapidly during dark adsorption. The concentration of CIP reached the equilibrium of adsorption and desorption after 10 min. After 15 min of visible light irradiation, the concentration of CIP in the two control experiments gradually balanced and approached the same. Hence, the dark adsorption has little influence on the final degradation efficiency of CIP in water. The dark adsorption data will not be analyzed in the following experiments. Moreover, the activities of Fe3O4/Bi2WO6 heterojunctions are closely related to the hydrothermal synthesis temperature (Figure b). Through adjusting hydrothermal synthesis temperature, the photocatalytic performance of Bi2WO6/Fe3O4 heterojunctions could be regulated. The CIP solution with an initial concentration of 10 mg L–1 was degraded by the FB-X (X = 120, 140, 160, 180, and 200) photocatalysts. Under visible light radiation, FB-180 has the highest degradation efficiency of CIP in water. After the photocatalysis of FB-180, the degradation rate of CIP reaches 99.7%. The reason may be that the photocatalyst FB-180 has excellent separation ability of photogenic carriers and a high charge transfer efficiency. Remarkably, the photocatalytic degradation ability of FB-180 was superior to that of many reported materials (Table ). Because most of CIP is degraded within 5 min, the kinetics simulation of the reaction process in the first 5 min is only carried out (Figure S6). In addition, the k values for the photocatalysts in degrading CIP are displayed in Table S1. The k values of FB-X (X = 120, 140, 160, 180, and 200) follow the order of FB-180 (0.55709 min–1) > FB-200 (0.25459 min–1) > FB-160 (0.22173 min–1) > FB-140 (0.11725 min–1) > FB-120 (0.04140 min–1). It is demonstrated that the catalyst synthesized at 180 °C has the best photogenic carrier separation ability and efficiency.

Figure 3

(a) Effect of adsorption time on the degradation efficiency; (b) photodegradation of CIP (10 mg L–1, 100 mL) by FB-X (X = 120, 140, 160, 180, and 200); (c) photodegradation of CIP by catalysts with different iron content; (d) and cycling tests for CIP decomposition by FB-180.

Table 1

Photocatalytic Degradation of CIP by Various Photocatalysts

photocatalysts	light range (nm)	dosage (mg)	volume and concentration of CIP	degradation efficiency	refs
CDs/Bi4O5Br2	λ > 400	10	50 mL, 10 mg/L	98% (120 min)	(37)
g-C3N4/Bi4O5Br2	λ > 400	50	100 mL, 10 mg/L	90% (75 min)	(38)
Fe2O3/Bi2WO6	λ > 400	100	100 mL, 15 mg/L	65% (120 min)	(39)
P–Bi2WO6	λ > 400	100	100 mL, 20 mg/L	60% (140 min)	(40)
Bi2WO6/GNPs	λ > 420	10	20 mL, 10 mg/L	96.7% (60 min)	(41)
Bi2WO6/Ta3N5	λ > 420	40	100 mL, 20 mg/L	81.1% (150 min)	(42)
Fe3O4/Bi2WO6	λ > 420	30	100 mL, 10 mg/L	99.7% (25 min)	This work

Subsequently, the impact of the iron content on the photocatalytic activity of the catalyst was investigated (Figure c). The degradation rates of pure Bi2WO6 and Fe3O4 are only 51.8 and 5.0%, respectively. The degradation efficiencies of samples with Fe content of 0.5, 2, 4, and 8% are 93.5, 97.4, 99.7, and 96.5%, respectively. Of note, when the Fe content is 4%, the photocatalyst shows the highest activity. This could be ascribed to the most effective separation of charge carriers in FB-180 (4% Fe). The iron content of the catalyst used in the experiment is 4% by default.

In addition, the photocatalytic stability of the FB-180 composite was evaluated by a cycling experiment.[43] As shown in Figure d, the degradation efficiency of FB-180 decreased slightly after five consecutive cycles, but the degradation rate was still higher than 90%. It indicates that the FB-180 possesses excellent stability in water pollution treatment.[44] Moreover, as presented in Figure S7a,b, there is no change in the state of Bi2WO6 as soon as the external magnetic field is available. As shown in Figure S7c,d, the composite photocatalyst FB-180 can be adsorbed in the presence of an external magnetic field. It exposes that the composite photocatalyst can be simply separated and effectively recycled in wastewater treatment.

The UV–visible diffuse-reflectance spectrum (UV–vis DRS) was used to study the light absorption capacity and band gap width of Fe3O4, Bi2WO6, and FB-180, as shown in Figure a. It proved that the visible light absorption is greatly enhanced by the addition of trace Fe3O4. The bare Bi2WO6 is sensitive to ultraviolet light and partially visible light region until 500 nm with an absorption edge of 480 nm.[45] Amazingly, the flower-like Fe3O4/Bi2WO6 catalyst can strongly absorb the visible light with a wavelength as long as 750 nm. It evidenced that the Fe3O4/Bi2WO6 heterojunction has a strong light absorptive ability and can be expected to possess robust photogenic carrier transferability.

Figure 4

(a) UV–vis DRS of Fe3O4/Bi2WO6(FB-180), Bi2WO6, Fe3O4, and Tauc’s plots of Bi2WO6; (b) TPR spectra; (c) EIS spectra of FB-x (x =120, 140, 160, 180, and 200), Bi2WO6, and Fe3O4; and (d) PL spectra of Bi2WO6 and FB-180.

In order to further explain the photogenic carrier transfer, the band positions of Bi2WO6 and Fe3O4 were determined by UV–vis DRS and Mott–Schottky curves. Generally, band gaps (Eg) of as-synthesized photocatalysts can be projected according to the Kubelka–Munk method, (αhν) = A(hν–Eg), where α, h, ν, and A are explained as absorption coefficient, Planck constant, light frequency, and a constant, respectively.[46] Besides, the value of n depends on the transition characteristics of the semiconductor. Because Bi2WO6 is of direct transition, the value of n is adopted as 1/2. By plotting (αhν)1/2 versus hν, as shown in Figure a1, the Eg of Bi2WO6 can be projected to be about 2.70 eV, which is quite similar to those shown in other literature studies.[47−49] Fe3O4 is a metal oxide, so the value of n is adopted as 2.[50] By plotting the relationship between (αhν)2 and hν, it can be estimated that the Eg value of Fe3O4 is about 1.69 eV(Figure S8). The flat band potential of Bi2WO6 and Fe3O4 was analyzed by Mott–Schottky measurements.[51] The slopes of the Mott–Schottky curve of Bi2WO6 and Fe3O4 are negative and positive, respectively. Therefore, Bi2WO6 and Fe3O4 are n-type semiconductors and p-type semiconductors, respectively. Figure S9 presents the Mott–Schottky curve of Bi2WO6, where the flat-band potential (Ef) of Bi2WO6 is determined to be 0.4 eV (vs NHE). Figure S10 shows the Mott–Schottky curve of Fe3O4, where the flat-band potential (Ef) of Fe3O4 is determined to be 1.07 eV (vs NHE). However, the Ef of the n-type semiconductor of Bi2WO6 is located 0.2 eV below the ECB potential.[52] The Ef of the p-type semiconductor of Fe3O4 is located 0.2 eV above the EVB potential. Therefore, the ECB of Bi2WO6 and the EVB of Fe3O4 was calculated to be 0.2 and 1.27 eV, respectively. According to the equation ECB = EVB – Eg, The EVB of Bi2WO6 and the ECB of Fe3O4 are estimated as 3.0 eV and −0.42 eV, respectively.

The transient photocurrent responses (TPR) were analyzed to study the photoelectric response characteristics of photocatalysts (Figure b).[53] The transient photocurrent response of FB-180 is much stronger than that of Fe3O4 and Bi2WO6, which shows that FB-180 has the best separation of the electron–hole pair under the test conditions. The trend of the current density curves remains unchanged after five cycles of the photocurrent test. It can be seen that the current density of the composite material is very stable under the test conditions. This shows that the separation of photogenerated carriers is relatively stable.

Electrochemical impedance spectroscopy (EIS) provides favorable evidence for exploring the interfacial charge transfer property of composite photocatalysts (Figure c). Generally, the charge transfer resistance (Rct) between the working electrode and the electrolyte is represented by the second semicircle of the EIS spectrum. The smaller the arc of the curve, the lower the impedance of the photocatalyst. Photocatalysts with lower impedance have higher efficient charge transfer capabilities.[54] FB-180 has the lowest impedance of these composite photocatalysts. The flower-like structure of FB-180 increases the contact area, which facilitates the charge transfer. FB-180 presents optimum electrical conductivity. According to the results of EIS, it demonstrates that the FB-180 possessed the highly efficient separation and transfer capability of photogenerated carriers. It demonstrates that FB-180 has an excellent photogenic electron–hole pair separation ability.

The photoluminescence (PL) spectrum further proves the transfer and separation rate of photo-excited carriers because photoluminescence is caused by the recombination of electrons and holes.[55] The PL of Bi2WO6 and FB-180 was obtained at an excitation wavelength of 440 nm. Bi2WO6 shows a significant PL emission peak at ∼468 nm because of excessive photogenic carrier recombination and the band edge transition (Figure d). FB-180 exhibited much lower PL intensity than that of pristine Bi2WO6. The PL measurement results further confirm that FB-180 has a better photogenerated carrier separation ability than pristine Bi2WO6.

It is indispensable to understand the reasonable photocatalysis mechanism. The proposal of a reasonable photocatalytic degradation mechanism is conducive to improving the performance of photocatalysis and constructing novel heterojunction.[56,57] Therefore, the electron spin resonance (ESR) technique is used to confirm the main active material produced by FB-180 and pure Bi2WO6 under visible light irradiation (Figure a,b). As shown in Figure , signals of DMPO-•OH and DMPO-•O2– are photoexcited by FB-180 under visible light irradiation. However, only the representative peaks of DMPO-•OH are spotted by Bi2WO6 under visible light irradiation. In addition, the DMPO-•OH signal intensity of FB-180 is stronger than that of Bi2WO6 (Figure b). It indicates that more DMPO-•OH are produced in the FB-180 system. The produced •O2– and increased •OH facilitates the degradation of antibiotics over FB-180. Therefore, the result of ESR measurement shows that both •O2– and •OH active species play a decisive role in the photocatalytic degradation of CIP.

Figure 5

ESR signals of Bi2WO6 and FB-180 for (a) DMPO-•O2– and (b) DMPO-•OH.

The interface charge transfer between Bi2WO6 and Fe3O4 affects the photocatalytic activity of the composite photocatalyst FB-180. According to the band structure of Bi2WO6 and Fe3O4, the mechanism of FB-180 photocatalytic degradation of CIP is proposed (Figure ). For FB-180 composites, there are two possible ways of charge transfer.[58] The CB (0.3 eV) and VB (3.0 eV) of Bi2WO6 are more positive than the CB (−0.42 eV) and VB (1.27 eV) of Fe3O4. According to the principle of energy band arrangement, FB-180 may have established a typical type-II heterojunction (Figure S11). Under visible light irradiation, electrons (e–) and holes (h+) are formed and separated in Bi2WO6 and Fe3O4. The photoinduced electron in the CB of Fe3O4 would migrate to the CB of Bi2WO6. The photogenerated hole in the VB of Bi2WO6 would transfer to the VB of Fe3O4, resulting in the separation of e– and h+. Because the reduction potential of O2/•O2–(−0.33 eV) is more positive than the CB potential of Bi2WO6, the electrons on the conduction band of Bi2WO6 could not reduce O2 to generate •O2–. Besides, the h+ amassed in the VB of Fe3O4 could not oxidize OH– to produce •OH because the oxidation potential of ·OH/–OH (2.40 eV) is more negative than the VB potential of Fe3O4. This was not in line with ESR analysis. Hence, the transfer path of photogenerated carriers in FB-180 may be different from the transfer mode of photogenerated carriers in a typical type-II heterojunction.[59,60] The transfer path of e– and h+ in FB-180 conforms to the direct Z-scheme mechanism (Figure ). When FB-180 is irradiated with visible light, the photogenerated electrons accumulated on the CB of Bi2WO6 are transferred. The migrated e– recombine with the h+ in the VB of Fe3O4. This mechanism promotes the efficient separation of electron–hole pairs. The photo-excited electrons were stored in the CB of Fe3O4 and the holes were accumulated at the VB of Bi2WO6. Because the CB potential of Fe3O4 (−0.42 eV) is more negative than that of O2/•O2–(−0.33 eV vs NHE), photogenerated electrons can reduce O2 to •O2– in the CB of Fe3O4. Because of the potential of VB of Bi2WO6 was more positive than that of •OH/–OH (2.40 eV vs NHE). The h+ in the VB of Bi2WO6 (3.147 eV) could activate −OH to form •OH and further oxidize organic pollutants.[61] The test results of ESR are consistent with the above results and analysis. In summary, the charge transfer system of the direct Z-scheme heterojunction conforms to the catalytic reaction of the composite photocatalyst FB-180. The system enhances the separation and effective transfer of photogenerated carriers and further improves the redox capability for the exceedingly efficient degradation of CIP.

Figure 6

Schematic illustration of the proposed photocatalysis mechanism of the direct Z-scheme Fe3O4/Bi2WO6 heterojunction.

Conclusions

In conclusion, we have successfully constructed a photocatalyst with a direct Z-scheme heterostructure through a hydrothermal reaction. The Z-scheme heterojunction charge transfer pathways facilitate the separation of photo-generated carriers. SEM and TEM showed that Bi2WO6 and Fe3O4 nanosheets formed three-dimensional flower-like photocatalysts. Under visible light irradiation, the FB-180 displayed the optimal efficiency of CIP degradation, which reached about 99.7% within 25 min. The degradation efficiency of CIP by the FB-180 composite increased by 47.9% compared with pure Bi2WO6. Recycling experiments demonstrate that FB-180 has excellent reusability and stability. The TPRs indicate that the FB-180 heterojunction has excellent photogenic carrier’s separation efficiency. The electron spin resonance measurements suggested that •OH and •O2– were the primary active species in the CIP degradation. Thus, this work provides a unique insight for the reasonable design of 2D/2D Z-scheme heterojunction photocatalysts with effective charge separation for environmental pollution treatment.

Experimental Section

Materials

All relevant chemicals and reagents in the current study were directly used without any further depuration. Iron nitrate nonahydrate [Fe(NO3)3·9H2O, 98.5%], citric acid (CA, 99.5%), ethylene glycol (EG, 99.5%), ammonium hydroxide (NH3·H2O, 25.0–28.0%), nitric acid (HNO3, 65.0–68.0%), sodium hydroxide (NaOH, 96.0%), and sodium sulfate (Na2SO4, 99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium tungstate dihydrate (Na2WO4·2H2O, 99.5%), Ciprofloxacin (CIP, 98.0%), and bismuth nitrate pentahydrate [Bi(NO3)3·5H2O, 99.0%] were supplied by Shanghai Macklin Biochemical Co., Ltd.

Synthesis of Fe3O4 Nanosheets

Fe3O4 nanosheets were fabricated through fast calcination reported in previous research.[23] First, 67.5 mmol Fe(NO3)3·9H2O and 108 mmol CA were dissolved in 25 mL of the deionized water. Then, 24 mL of EG was put into the solution with vigorous agitation for 5 min. It was adjusted to pH 7.0 with the addition of NH3·H2O. Afterward, the solution was heated in a water bath at 80 °C for 4 h. The heated solution was quickly transferred to an oil bath at 120 °C for 6 h. After heating, a gel-like liquid could be obtained. N2 was filled as the protective gas. When the temperature reached 600 °C, the gel was roasted for 1 min. After cooling, a two-dimensional Fe3O4 nanosheet could be obtained.

Synthesis of the Fe3O4/Bi2WO6 Heterojunction

Fe3O4/Bi2WO6 heterojunctions were fabricated by a hydrothermal reaction. First, a certain proportion (0.5, 2, 4, 8%) of Fe3O4 nanosheets was dissolved in 30 mL of deionized water for 30 min. After that, a certain amount of Bi(NO3)3·5H2O and Na2WO4·2H2O was dissolved into 20 mL of 1 mol L–1 HNO3 and 20 mL of 1 mol·L–1 NaOH solution, respectively. The Bi(NO3)3·5H2O solution and Na2WO4·2H2O solution were added in turn to the Fe3O4 suspension. After being vigorously stirred for 30 min, the solution was poured into a 100 mL Teflon-lined autoclave and treated at different temperatures (120, 140, 160, 180, and 200 °C) for 8 h. The solid products were washed by centrifugation with the deionized water. The sample was dried at 60 °C for 12 h. The final product Fe3O4/Bi2WO6 was obtained. The synthesized samples at different hydrothermal temperatures were named FB-120, FB-140, FB-160, FB-180, and FB-200, respectively (Figure S1).