Ionic Porous Aromatic Framework as a Self-Degraded Template for the Synthesis of a Magnetic γ-Fe2O3/WO3·0.5H2O Hybrid Nanostructure with Enhanced Photocatalytic Property

An ionic porous aromatic framework is developed as a self-degraded template to synthesize the magnetic heterostructure of γ-Fe2O3/WO3·0.5H2O. The Fe3O4 polyhedron was obtained with the two-phase method first and then reacted with sodium tungstate to form the γ-Fe2O3/WO3·0.5H2O hybrid nanostructure. Under the induction effect of the ionic porous network, the Fe3O4 phase transformed to the γ-Fe2O3 state and complexed with WO3·0.5H2O to form the n-n heterostructure with the n-type WO3·0.5H2O on the surface of n-type γ-Fe2O3. Based on a UV-Visible analysis, the magnetic photocatalyst was shown to have a suitable band gap for the catalytic degradation of organic pollutants. Under irradiation, the resulting γ-Fe2O3/WO3·0.5H2O sample exhibited a removal efficiency of 95% for RhB in 100 min. The charge transfer mechanism was also studied. After the degradation process, the dispersed powder can be easily separated from the suspension by applying an external magnetic field. The catalytic activity displayed no significant decrease after five recycles. The results present new insights for preparing a hybrid nanostructure photocatalyst and its potential application in harmful pollutant degradation.


Introduction
The construction of a heterostructure catalyst with different semiconductor constituents has become more popular in recent years [1][2][3][4][5][6]. This combination can improve the efficiency of photocatalytic reaction activity by building an inner electric field to separate charge carriers. It is extremely significant to utilize heterostructure photocatalysts to purify water polluted by dyes because almost 20% of the world's water pollution is caused by dyes [7,8]. Additionally, heterogeneous photocatalysis shows effectiveness in degrading a wide range of dyes into readily biodegradable compounds and eventually mineralizes them into innocuous carbon dioxide and water [9][10][11][12][13]. Tungsten oxide (WO 3 ), with a band gap of 2.5-2.8 eV, is considered to be a possible catalyst due to its suitable response to the solar spectrum in the near ultraviolet and blue regions, stable chemical properties in aqueous solution, good oxidizing ability of the holes in the valence band, high resistance against anodic photo-corrosion, and long-term stability during irradiation [14][15][16]. However, the conduction band (0.5 eV vs. NHE, normal hydrogen electrode) is not negative enough to consume photogenerated electrons for the oxygen reduction, leading to low photocatalytic activity [17]. For the purpose of separating photogenerated electron-hole pairs to improve the catalytic performance, several approaches have been developed in recent years, including surface modification with noble metals, special morphologies/phase control, and the formation of nanomaterial composites [18][19][20]. However, most strategies encounter high costs and uncontrollable morphology with limited improvement of photocatalysis performance, which hinders the practical application of WO 3 -based composites.
γ-Fe 2 O 3 is a stable form of iron oxide with a cubic structure, which has excellent properties of strong magnetism; it is photocatalytic and has low toxicity and good biocompatibility. It can be widely used in the fields of electromagnetics, biology, water treatment, and medicine [21][22][23][24]. It is a potential semiconductor that can be coupled with WO 3 . In the past decade, efforts have been made to prepare γ-Fe 2 O 3 under high-temperature pyrolysis [25], sol-gel [26], gas phase deposition [27], combustion synthesis [28], and so on. However, these processes are too complex and require too much energy. In recent years, some researchers have utilized the method of phase transition to prepare compounds because of high raw material utilization and the mild reaction conditions [29,30]. Fe 3 O 4 is a kind of common and cheap iron oxide, which has almost the same structure as that of γ-Fe 2 O 3 . If we can use an in situ phase transition method to prepare γ-Fe 2 O 3 from Fe 3 O 4 , it will be cost effective. When the phase transition happens, Fe 2+ occupies the octahedral B position of cubic Fe 3 O 4 oxidized to Fe 3+ [31] The procedure leads to the formation of oxygen vacancy, and so the new cubic γ-Fe 2 O 3 is not stable. Then, it may need to be connected with another compound such as n-type WO 3 [34]. Although these composites showed better relevant chemical properties, the synthetic procedure of these composites is always too complex to be extended to practical application.
In this work, a quaternary pyridinium-type PAF sample (PAF-50) was adopted as a self-degraded template [51,52]. Due to the highly charged network, WO 4 2− anions concentrated around the porous network and γ-Fe 2 O 3 /WO 3 ·0.5H 2 O hybrid composites were successfully prepared through a two-step hydrothermal method. This is the first time utilizing phase transition to prepare γ-Fe 2 O 3 /WO 3 ·0.5H 2 O with the help of PAF-50. The morphologies and crystalline structure of γ-Fe 2 O 3 /WO 3 ·0.5H 2 O were investigated using SEM, TEM, and XRD, respectively. The samples were examined by using UV-Vis diffuse reflectance spectra. Meanwhile, the photocatalytic property of the catalysts was evaluated by decomposing RhB in aqueous solution under visible light irradiation. Remarkably, the γ-Fe 2 O 3 /WO 3 ·0.5H 2 O heterojunction was demonstrated to have remarkable photocatalytic characteristics and good reusability compared with that of pure WO 3 .

Chemicals
The chemical and reagents were of analytical grade and used as such without any further purification. All the chemicals viz. sodium tungstate (Na 2 WO 4 ·2H 2 O), ferric acetylacetonate (C 15  The synthesis of Fe 3 O 4 was based on the literature [53] using a low-temperature hydrothermal method in a Teflon-lined stainless steel autoclave with a capacity of 50 mL: 0.035 mol/L ferric acetylacetonate in a mixture of toluene, and oleic acid (volume ratio of 20:10 mL) was added into 10 mL of hydrazine hydrate inside the Teflon-lined stainless steel autoclave to form a two-phase reaction system. The obtained mixed solution was sealed and then kept at 130 • C for 24 h. After reaction, the mixture was poured out by adding ethanol and sonicated with several minutes, washed with ethanol with a magnet to reduce the possibility of impurities in the products, and then dried in air.

Synthesis of γ-Fe
Hybrid photocatalysts were fabricated via a hydrothermal method using PAF-50 as a self-degraded template. Briefly, 3 mmol Na 2 WO 4 ·2H 2 O and 100 mg PAF-50 (precise type: Cl-PAF-50) were dispersed in 30 mL deionized water; the pH value was adjusted between 4 to 5 by drop-wise addition of concentrated HCl solution under continuous stirring; after adding the Fe 3 O 4 obtained above, the mixture was transferred to autoclave and maintained at 160 • C for 6 h. The collected sample was rinsed with deionized water and dried in air.

Materials Characterization
XRD (X-ray diffraction) patterns give information on the phase and crystallinity of the as-prepared materials, which were collected on a Rigaku D/Max-2550 diffractometer (Tokyo, Japan) equipped with Cu-Ka radiation (λ = 0.15418 nm) at a scanning range of 20-80 • and scanning speed of 5 • /min. HRTEM images were obtained using a Tecnai G220S-Twin transmission electron microscope (Hillsboro, OR, USA) at an accelerating voltage of 120 kV, and the images were observed at 200 kV instead of 120 kV. XPS spectra were performed on a Thermo ESCALAB 250 (Waltham, MA, USA) with Al Kα radiation at θ = 90 • for the X-ray sources; the binding energies were calibrated using the C 1s peak at 284.8 eV. UV-Visible solid absorbances of the samples were obtained using a PerkinElmer Lambda950 UV-Visible solid spectrometer (Waltham, MA, USA) using BaSO 4 as a reference. The magnetic properties were investigated with a Quantum Design SQUID-MPMS-XL (San Diego, CA, USA). Magnetic hysteresis loops were measured at 300 K under a magnetic field up to 2 T.

Measurement of Photocatalytic Activity
The photocatalytic activity of the hybrid photocatalysts was evaluated by examining the degradation of Rhodamine B (RhB) in aqueous solution. In a typical reaction procedure, 20 mg of the prepared sample is dispersed into 50 mL dye solution (20 mg/L) and continuously stirred in dark overnight to allow comprehensive adsorption-desorption equilibrium. The sample solutions are irradiated with 500 W xenon lamp, and then withdrawn at regular time intervals followed by separation with a magnet to remove the catalyst. Any change in concentration of RhB was monitored using UV-Vis spectrophotometer during the photoreaction process.  . No additional peaks were observed in the XRD pattern, confirming the purity of the products. In order to explore the effect of template, we prepared γ-Fe 2 O 3 /WO 3 ·0.5H 2 O in the absence of PAF-50. The results ( Figure 1c) showed that in the products Fe 3 O 4 did not turn out to be γ-Fe 2 O 3 . We inferred that PAF-50 played a role helping to oxidize Fe 3 O 4 to γ-Fe 2 O 3 and in accumulating metal ions to form a complex [26,45,48]. Figure 1 shows the XRD patterns of pure Fe3O4 and γ-Fe2O3/WO3·0.5H2O samples, respectively. As shown in Figure 1a, diffraction peaks (marked as #) are indexed to cubic Fe3O4 (JCPDS No.: 65-3107). In Figure 1b, the peaks marked with the sample after being loaded with Fe3O4 show diffraction peaks at (111), (311), (222), (400), (331), (422), (511), (440), (531), (533), and (622), which matched well with WO3·0.5H2O (JCPDS No.:84-1851), whereas the peaks at (220), (311), (400), (422), and (511) (Marked as *) show diffraction peaks located at γ-Fe2O3 (JCPDS No.: . No additional peaks were observed in the XRD pattern, confirming the purity of the products. In order to explore the effect of template, we prepared γ-Fe2O3/WO3·0.5H2O in the absence of PAF-50. The results (Figure 1c) showed that in the products Fe3O4 did not turn out to be γ-Fe2O3. We inferred that PAF-50 played a role helping to oxidize Fe3O4 to γ-Fe2O3 and in accumulating metal ions to form a complex [26,45,48].

Surface and Structure Characterization of Samples
Two Theta (degree) 20 40 60 Intensity (a.u.) 20 40 60 Two Theta (degree) XPS spectra were performed to determine the valence state of the elements. Figure  2b shows the spectrum of Fe 2p; the locations at 711.1 and 724.8 eV correspond to Fe 2p3/2 and Fe 2p1/2 [54,55]. The satellite peak of Fe 2p3/2 for Fe2O3 is located approximately 8 eV higher than the main Fe 2p3/2 peak. In addition, there appears to be another satellite peak at 733.5 eV, which may be a satellite peak for Fe 2p1/2 [56]. The corresponding satellite peak around 719.1 eV confirms the γ-Fe2O3 phase, which is consistent with the results of XRD [57]. During this hydrothermal reaction, Fe3O4 has a phase transformation to Fe2O3. In Figure 2c, the W 4f binding energies of two peaks located at 35.7 eV and 37.8 eV correspond to +6 valence of W. The whole spectra (Figure 2a) show that content of W is much higher than that of Fe. We can infer that WO3 covers the surface of FexOy during the synthesis of hybrid nanostructure. Additionally, XPS is a surface-sensitive analytical technique [58]. Thus, the intensity of Fe 2p peak is weak. XPS spectra were performed to determine the valence state of the elements. Figure 2b shows the spectrum of Fe 2p; the locations at 711.1 and 724.8 eV correspond to Fe 2p 3/2 and Fe 2p 1/2 [54,55]. The satellite peak of Fe 2p 3/2 for Fe 2 O 3 is located approximately 8 eV higher than the main Fe 2p 3/2 peak. In addition, there appears to be another satellite peak at 733.5 eV, which may be a satellite peak for Fe 2p 1/2 [56]. The corresponding satellite peak around 719.1 eV confirms the γ-Fe 2 O 3 phase, which is consistent with the results of XRD [57]. During this hydrothermal reaction, Fe 3 O 4 has a phase transformation to Fe 2 O 3 . In Figure 2c, the W 4f binding energies of two peaks located at 35.7 eV and 37.8 eV correspond to +6 valence of W. The whole spectra (Figure 2a) show that content of W is much higher than that of Fe. We can infer that WO 3 covers the surface of Fe x O y during the synthesis of hybrid nanostructure. Additionally, XPS is a surface-sensitive analytical technique [58]. Thus, the intensity of Fe 2p peak is weak. Two Theta (degree) XPS spectra were performed to determine the valence state of the elements. Figure  2b shows the spectrum of Fe 2p; the locations at 711.1 and 724.8 eV correspond to Fe 2p3/2 and Fe 2p1/2 [54,55]. The satellite peak of Fe 2p3/2 for Fe2O3 is located approximately 8 eV higher than the main Fe 2p3/2 peak. In addition, there appears to be another satellite peak at 733.5 eV, which may be a satellite peak for Fe 2p1/2 [56]. The corresponding satellite peak around 719.1 eV confirms the γ-Fe2O3 phase, which is consistent with the results of XRD [57]. During this hydrothermal reaction, Fe3O4 has a phase transformation to Fe2O3. In Figure 2c, the W 4f binding energies of two peaks located at 35.7 eV and 37.8 eV correspond to +6 valence of W. The whole spectra (Figure 2a) show that content of W is much higher than that of Fe. We can infer that WO3 covers the surface of FexOy during the synthesis of hybrid nanostructure. Additionally, XPS is a surface-sensitive analytical technique [58]. Thus, the intensity of Fe 2p peak is weak.     Figure 3a that Fe3O4 shows uniform and regular twelve faces with a size range from 80 to 100 nm. There is no significant change after composed with tungsten. A single particle was selected, and the relevant HRTEM image is shown in Figure 3c. The edge coupled with the center presents a strong contrast, which further proves the formation of well-defined products. There is more than one lattice overlap in the polyhedron, with a lattice spacing of 0.297 nm corresponding to WO3·0.5H2O (JCPDS No.: 84-1851). In   Figure 3a that Fe 3 O 4 shows uniform and regular twelve faces with a size range from 80 to 100 nm. There is no significant change after composed with tungsten. A single particle was selected, and the relevant HRTEM image is shown in Figure 3c. The edge coupled with the center presents a strong contrast, which further proves the formation of well-defined products. There is more than one lattice overlap in the polyhedron, with a lattice spacing of 0.297 nm corresponding to WO 3 ·0.5H 2 O (JCPDS No.: 84-1851). In addition, the obvious lattice spacing in the HRTEM image further confirms that the sample addition, the obvious lattice spacing in the HRTEM image further confirms that the sample comprises two phases of γ-Fe2O3 and WO3 to form a hybrid structure, and the morphology has no significant change after composition.  Figure 4 shows the hysteresis loops of γ-Fe2O3/WO3·0.5H2O at 300 K with the applied field ± 2T. The saturation magnetization of the heterostructure increases with the field and tends to slow until ± 2000 Oe. From the enlarged part of the illustration, we can see that the product is ferromagnetic with a residual magnetization (Mr) of 2.3 emu/g and a coercivity (Hc) of 61.2 Oe [59]. The saturation magnetization (Ms) of 20 emu/g is less than that of the bulk γ-Fe2O3 of 73.5 emu/g [60]. This is mainly due to the surface of the hybrid structure being covered with hydrated tungsten oxide, which affects the morphology and the spin density, which in turn affects the saturation magnetization.  Figure 4 shows the hysteresis loops of γ-Fe 2 O 3 /WO 3 ·0.5H 2 O at 300 K with the applied field ±2T. The saturation magnetization of the heterostructure increases with the field and tends to slow until ±2000 Oe. From the enlarged part of the illustration, we can see that the product is ferromagnetic with a residual magnetization (Mr) of 2.3 emu/g and a coercivity (Hc) of 61.2 Oe [59]. The saturation magnetization (Ms) of 20 emu/g is less than that of the bulk γ-Fe 2 O 3 of 73.5 emu/g [60]. This is mainly due to the surface of the hybrid structure being covered with hydrated tungsten oxide, which affects the morphology and the spin density, which in turn affects the saturation magnetization.

Photocatalytic Activity
Both WO 3 and γ-Fe 2 O 3 are typical n-type semiconductors, and the n-n-type heterostructure has absorption in the ultraviolet region and visible region [61]. UV-Visible spectrum analysis was used to explore the light response. As shown in Figure 5a, compared with Fe 3 O 4 , the complex can absorb light in a larger wavelength range. The adsorption band is at 300 to 600 nm, which can absorb ultraviolet and visible light. The light density of the xenon lamp and solar light density is 120 mW/cm 2 and 54 mW/cm 2 , respectively; therefore, the 500 W xenon lamp was used as the light source in this experiment [62]. The band gap of γ-Fe 2 O 3 /WO 3 ·0.5H 2 O can be obtained from the plot of (αhν) 2 versus hν by extrapolating the strait portion of (αhν) 2 to zero, as shown in Figure 5b. After calculation, the value was determined to be 1.8 eV, which is narrower than 2.64 eV of Fe 3 O 4 . A narrow band gap is beneficial for the efficient utilization of visible light for photocatalysis [63].

Photocatalytic Activity
Both WO3 and γ-Fe2O3 are typical n-type semiconductors, and the n-n-type heterostructure has absorption in the ultraviolet region and visible region [61]. UV-Visible spectrum analysis was used to explore the light response. As shown in Figure 5a, compared with Fe3O4, the complex can absorb light in a larger wavelength range. The adsorption band is at 300 to 600 nm, which can absorb ultraviolet and visible light. The light density of the xenon lamp and solar light density is 120 mW/cm 2 and 54 mW/cm 2 , respectively; therefore, the 500 W xenon lamp was used as the light source in this experiment [62]. The band gap of γ-Fe2O3/WO3·0.5H2O can be obtained from the plot of (αhν) 2 versus hν by extrapolating the strait portion of (αhν) 2 to zero, as shown in Figure 5b. After calculation, the value was determined to be 1.8 eV, which is narrower than 2.64 eV of Fe3O4. A narrow band gap is beneficial for the efficient utilization of visible light for photocatalysis [63]. Rhodamine B (RhB) was used to simulate pollutants in water under UV-Visible light at room temperature. γ-Fe 2 O 3 /WO 3 ·0.5H 2 O was dispersed, and the absorption spectrum of the solution was tested. The curve of absorbance wavelength versus time is shown in Figure 6a. The intensity of the absorption peak at 553 nm [64] gradually decreased with the increase in time, and the absorption peak was blue shifted, which indicated that the ethyl on RHB molecule was removed. The characteristic absorption peak of Rhodamine B could hardly be seen at 100 min, and Rhodamine B was completely degraded.
From Figure 6b, the photocatalytic reactions over both Fe 3 O 4 and γ-Fe 2 O 3 /WO 3 ·0.5H 2 O obey first-order kinetics. The degradation percentages of RhB only reached 62% and 1% after photodegradation for 100 min under UV irradiation in the case of the presence of pure Fe 3 O 4 and in the absence of any catalysts, respectively, which are too low compared to that of γ-Fe 2 O 3 /WO 3 ·0.5H 2 O heterostructures (94.9%), revealing the significantly enhanced photocatalytic properties of the γ-Fe 2 O 3 /WO 3 ·0.5H 2 O heterostructures. This removal efficiency is more than that of pure WO 3 [65] or γ-Fe 2 O 3 [66] according to relevant studies. However, γ-Fe 2 O 3 /WO 3 ·0.5H 2 O takes more time than Fe 2 O 3 /WO 3 in the degradation of RhB with the same concentration [32].
The composite catalyst is the key factor for this degradation reaction, which is mainly due to the synergistic and coupling effects of Fe 2 O 3 and WO 3 . When γ-Fe 2 O 3 excited by visible ultraviolet, the electron flow to the valence band of WO 3 , and the hole in the valence band migrates from WO 3 to γ-Fe 2 O 3 . At the same time, the internal electric field also promotes the electron hole migration, and the formation of the heterostructure promotes the separation of photo-generated electrons and hole pairs [67]. This can reduce the recombination probability and reduce the energy needed for the transition, so it has a better photocatalytic effect than single WO 3 or γ-Fe 2 O 3 . However, the lower removal efficiency as compared to the reported Fe 2 O 3 /WO 3 may due to the water molecules in the structure of γ-Fe 2 O 3 /WO 3 ·0.5H 2 O. Rhodamine B (RhB) was used to simulate pollutants in water under UV-Visible light at room temperature. γ-Fe2O3/WO3·0.5H2O was dispersed, and the absorption spectrum of the solution was tested. The curve of absorbance wavelength versus time is shown in Figure 6a. The intensity of the absorption peak at 553 nm [64] gradually decreased with the increase in time, and the absorption peak was blue shifted, which indicated that the ethyl on RHB molecule was removed. The characteristic absorption peak of Rhodamine B could hardly be seen at 100 min, and Rhodamine B was completely degraded. The catalytic activities of the complex were measured by the photodegradation of RhB for five recycles. After each cycle, fresh RhB solution was used for the next photocatalytic experiment. Additionally, the photocatalyst was collected from the previous experiment followed by washing and drying. It is shown in Figure 7 that the catalytic activity displayed no significant decrease after five recycles with a period of 100 min. This indicates that the stability of γ-Fe 2 O 3 /WO 3 ·0.5H 2 O is excellent, and it can keep its high photocatalytic activity after the photocatalytic reaction and recycling process [32,68]. the separation of photo-generated electrons and hole pairs [67]. This can reduce the recombination probability and reduce the energy needed for the transition, so it has a better photocatalytic effect than single WO3 or γ-Fe2O3. However, the lower removal efficiency as compared to the reported Fe2O3/WO3 may due to the water molecules in the structure of γ-Fe2O3/WO3·0.5H2O. In conventional photocatalysts, the photoinduced electrons and holes migrate randomly, and the recombination of the charge carriers reduces the quantum yield in the catalytic process. We can see in Figure 8 that when γ-Fe 2 O 3 (as the primary light absorber) forms a heterojunction with WO 3 (as an electron acceptor), the band bending formed at the interface between γ-Fe 2 O 3 and WO 3 impels the carriers to diffuse in opposite directions until their Fermi levels reach equivalence [69]. As other studies have claimed that the CB edge potential of γ-Fe 2 O 3 (0.29 eV) is lower than that of WO 3 (0.79 eV) [32,70,71], upon irradiation, the ground-state γ-Fe 2 O 3 and WO 3 go to an exited state to produce some electron-hole pairs because of their narrow band gaps. Therefore, the photo-excited electrons on the CB of γ-Fe 2 O 3 transferred to the CB of WO 3 . Additionally, the VB edge potential of WO 3 (2.60 eV) was found to be larger than that of the γ-Fe 2 O 3 (2.20 eV), which helps the photo-excited holes on the VB of WO 3 transfer to the VB of γ-Fe 2 O 3 . The electrons and holes transfer rapidly in the opposite direction due to the heterojunction-generated inner electric field [72]. In this regard, the recombination rate of charge carriers is remarkably reduced, and the γ-Fe 2 O 3 /WO 3 ·0.5H 2 O shows enhanced photoactivity for organic pollution degradation under light.
The catalytic activities of the complex were measured by the photodegradation of RhB for five recycles. After each cycle, fresh RhB solution was used for the next photocatalytic experiment. Additionally, the photocatalyst was collected from the previous experiment followed by washing and drying. It is shown in Figure 7 that the catalytic activity displayed no significant decrease after five recycles with a period of 100 min. This indicates that the stability of γ-Fe2O3/WO3·0.5H2O is excellent, and it can keep its high photocatalytic activity after the photocatalytic reaction and recycling process [32,68]. In conventional photocatalysts, the photoinduced electrons and holes migrate randomly, and the recombination of the charge carriers reduces the quantum yield in the catalytic process. We can see in Figure 8 that when γ-Fe2O3 (as the primary light absorber) forms a heterojunction with WO3 (as an electron acceptor), the band bending formed at the interface between γ-Fe2O3 and WO3 impels the carriers to diffuse in opposite directions until their Fermi levels reach equivalence [69]. As other studies have claimed that the CB edge potential of γ-Fe2O3 (0.29 eV) is lower than that of WO3 (0.79 eV) [32,70,71], upon irradiation, the ground-state γ-Fe2O3 and WO3 go to an exited state to produce some electron-hole pairs because of their narrow band gaps. Therefore, the photo-excited electrons on the CB of γ-Fe2O3 transferred to the CB of WO3. Additionally, the VB edge potential of WO3 (2.60 eV) was found to be larger than that of the γ-Fe2O3 (2.20 eV), which helps the photo-excited holes on the VB of WO3 transfer to the VB of γ-Fe2O3. The electrons and holes transfer rapidly in the opposite direction due to the heterojunction-generated inner electric field [72]. In this regard, the recombination rate of charge carriers is remarkably reduced, and the γ-Fe2O3/WO3·0.5H2O shows enhanced photoactivity for organic pollution degradation under light.

Conclusions
In conclusion, γ-Fe2O3/WO3·0.5H2O heterostructures were synthesized using an ionic porous aromatic framework as self-degraded template and a facile low-temperature hydrothermal growth method. The experimental results show that the structures are composed of γ-Fe3O4 and WO3. The role of PAF-50 is to accumulate metal ion and oxidize Fe3O4 to Fe2O3. Compared with Fe3O4, the complex can absorb light in a larger wavelength range. The results of the degradation experiment revealed that product loading with WO3 exhibited higher photocatalytic activity than with pure Fe3O4, and the degradation efficiency reached 95% for the RhB solution after 100 min. After the degradation process, the dispersed powder can be easily separated from the suspension by applying an external magnetic field. The charge transfer mechanism in γ-Fe2O3/WO3·0.5H2O shows that the enhanced photocatalytic properties of the heterostructures are attributed to the larger spectral range, a narrower band gap, and a lower recombination rate of electrons and holes.

Conclusions
In conclusion, γ-Fe 2 O 3 /WO 3 ·0.5H 2 O heterostructures were synthesized using an ionic porous aromatic framework as self-degraded template and a facile low-temperature hydrothermal growth method. The experimental results show that the structures are composed of γ-Fe 3 O 4 and WO 3 . The role of PAF-50 is to accumulate metal ion and oxidize Fe 3 O 4 to Fe 2 O 3 . Compared with Fe 3 O 4 , the complex can absorb light in a larger wavelength range. The results of the degradation experiment revealed that product loading with WO 3 exhibited higher photocatalytic activity than with pure Fe 3 O 4 , and the degradation efficiency reached 95% for the RhB solution after 100 min. After the degradation process, the dispersed powder can be easily separated from the suspension by applying an external magnetic field. The charge transfer mechanism in γ-Fe 2 O 3 /WO 3 ·0.5H 2 O shows that the enhanced photocatalytic properties of the heterostructures are attributed to the larger spectral range, a narrower band gap, and a lower recombination rate of electrons and holes.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.