Synthesis of α-Fe₂O₃/Bi₂WO₆ layered heterojunctions by in situ growth strategy with enhanced visible-light photocatalytic activity

Abstract

Layered heterojunction structure with larger interface region for electron migration has attracted much attention in recent years. In this work, layered α-Fe₂O₃/Bi₂WO₆ heterojunctions with strong interlayer interaction were successfully synthesized through a facile in situ growth method. The strong interaction between α-Fe₂O₃ and Bi₂WO₆ had resulted in excellent photoelectrochemical performance. It was found that such structure promoted the interfacial photogenerated charges separation according to EIS and Tafel analysis, except for the expansion of visible-light absorption range. PL and TRPL characterizations further demonstrated that the recombination ratio of photoexcited electron-hole pairs was greatly reduced. The toluene photocatalytic degradation tests had showed that α-Fe₂O₃/Bi₂WO₆ composites exhibited much well activity under visible-light irradiation. Especially, 4%-Fe₂O₃/Bi₂WO₆ sample displayed the highest photocatalytic activity, which was around 3 and 4 times higher than that of pure Bi₂WO₆ and α-Fe₂O₃. Based on ESR results and free radical trapping experiments, hydroxyl radicals (·OH) and holes (h⁺) were regarded as the main active species. The establishment of Fe₂O₃/Bi₂WO₆ with layered heterojunctions could provide new insights into the construction of novel photocatalysts.

Introduction

In recent years, indoor air pollution caused by volatile organic pollutants (VOCs) has attracted lots of public attentions. Photocatalytic oxidation was considered as an environmental-friendly technology for indoor VOCs purification^1,2,3,4,5,6. Therefore, many kinds of photocatalysts have been extensively investigated, such as TiO₂^7,8,9, ZnO^10,11, SnO₂¹², SrTiO₃¹³ and so on. However, these semiconductors still have some common shortcomings, such as narrow light absorption range and high recombination ratio of photogenerated charges¹⁴. Thus, developing visible-light driven and highly active photocatalysts is one of the most urgent topics.

N-type Bi₂WO₆ has widely regarded as a promising photocatalyst for its outstanding photooxidation ability, nontoxicity, well thermal and chemical stability^15,16. Nevertheless, pure Bi₂WO₆ can’t efficaciously utilize visible light due to the fact that it can only be driven by light shorter than 450 nm^17,18. Moreover, low separation ratio of photogenerated electrons and holes also impedes its application. Semiconductor heterojunction has been demonstrated to be an effective way to solve this issue above, such as the coupling of MoS₂/Bi₂WO₆¹⁹, g-C₃N₄/Bi₂WO₆²⁰, CeO₂/Bi₂WO₆²¹, BiOBr/Bi₂WO₆²² and so on.

Layered heterojunction photocatalyst possesses larger interfacial area compared to line contact and point contact heterojunction photocatalysts, which benefits the transfer of photogenerated electron-hole pairs²³. By taking this advantage into consideration, many photocatalysts with layered heterostructure have been fabricated, such as SnS₂/g-C₃N₄²³, g-C₃N₄/Bi₂WO₆²⁴, g-C₃N₄/Bi₂₀TiO₃₂²⁵, α-Fe₂O₃/graphene²⁶ and so on. Motivated by the above work, Bi₂WO₆ coupled with α-Fe₂O₃ (a low-price and narrow band gap semiconductor) nanosheets as layered heterojunction photocatalyst may exhibit significantly enhanced photoinduced interfacial charge transfer rate, which could effectively promote the photocatalytic activity.

In this work, layered α-Fe₂O₃/Bi₂WO₆ heterojunctions were fabricated via a facile in situ growth hydrothermal method. And toluene was chosen as a typical kind of indoor VOCs in the experiment. Then, the photodegradation efficiency of gaseous toluene under visible light irradiation was tested. After that, the inherent structure-performance relationship was then disclosed based on the physiochemical and photo-electrochemical characterizations. The main purpose of our work was to shed new light on the synthesis of layered heterojunctions and reveal the role of such structure in photocatalytic process.

Experimental

Chemicals

Bi(NO₃)₃·5H₂O, Na₂WO₄·2H₂O, FeCl₃·6H₂O, Na₂SO₄, NH₃·H₂O, Na₂C₂O₄, salicylic acid, benzoquinone, sodium acetate and ethanol were purchased from Sigma-Aldrich. All chemical reagents were of analytical grade and without any further purification.

Synthesis of α-Fe₂O₃/Bi₂WO₆ composites

Bi₂WO₆ was synthesized via the same method as reported in our previous work¹⁵. Layered α-Fe₂O₃/Bi₂WO₆ heterojunctions were synthesized by in situ growth method. Typically, 2 g Bi₂WO₆ was ultrasonic dispersed in 120 mL ethanol for 30 minutes. Then, appropriate amount of FeCl₃·6H₂O and sodium acetate were added into the above solution. After vigorous stirring for about 2 h, the mixture was transformed to a 200 mL Teflon-lined autoclave and then heated in an oven at 180 °C for 24 h. The obtained participates were collected by vacuum filtration and washed with deionized water and ethanol several times. Finally, these samples were dried in air at 70 °C before being used. Composites of 2, 4 and 6% α-Fe₂O₃/Bi₂WO₆ samples were prepared, respectively. Pure hexagonal nanoplates of α-Fe₂O₃ were synthesized without the addition of Bi₂WO₆ via the method reported in the reference²⁶.

Characterization of samples

The crystal phase and composition of these as-prepared catalysts were obtained using an X-ray diffraction (XRD, model D/max RA, Rigaku Co., Japan with Cu Kα radiation). Raman measurement was performed using a LABRAM-HR Ramas-cope fitted with a spectra physics argon ion laser. Laser radiation (λ = 514 nm) was used as excitation source at 20 mW. The surface properties were performed using X-ray photoelectron spectroscopy (XPS) measurement (Thermo, ESCALAB 250). The standard binding energy of 284.8 eV from C1s value was chosen as a reference. The morphology and microstructure information of the catalysts were obtained by Scanning electron microscopy (SEM, FEI-quanta 200F, USA) and transmission electron microscopy (TEM, H-600, Hitachi, Ltd., Japan). The specific surface area of catalysts was ascertained by using a nitrogen adsorption apparatus (Beijing JWGB Sci. & Tech. Co., Ltd). The light adsorption ability of these samples was obtained by using a Scan UV-visible spectrophotometer (UV-visible DRS: TU-1901, China) equipped with an integrating sphere assembly. The spectra were recorded at room temperature in air, ranging from 230 to 850 nm. Photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra were recorded using a fluorospectrophotometer (PL: RAMANLOG 6, USA) with a 390 nm Ar⁺ laser as excitation source. All these photoelectrochemical properties of the samples were measured on an electrochemical system (CHI 660B, Shanghai, China) using a three-electrode photo electrochemical cell. Platinum wire and saturated Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively. The working electrode was composed of indium tin oxide (ITO, 20 × 30 × 1.1 mm, 15 Ω) with an area of about 1 cm², glass coated with the prepared samples. The details of preparing working electrode was reported in our previous work¹⁵. The electrolyte was 0.2 M Na₂SO₄ solution. Electron spin resonance (ESR) signals of radical species trapped by 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) were detected on a JES FA200 spectrometer.

Photocatalytic activity tests

Photocatalytic activity of as-prepared samples was evaluated via photodegradation of toluene. Experiment was performed in a 1.5 L batch reactor sealed with quartz plate. Circulating cooling water in the jacket around the reactor was used to control the reaction temperature. For each test, 0.1 g photocatalyst was uniformly dispersed onto a glass dish with a diameter about 10 cm. After that, the catalyst-coated dish was played on the bottom of the reactor. An appropriate amount of toluene was injected into the reactor with a micro-springe. Before each test, the system was maintained in the darkness to achieve adsorption-desorption equilibrium. The initial concentration of toluene was controlled at about 25 ppm. A 300 W Xe lamp (Celhx300UV, Ceaulight, China) equipped with two optical glass filters (420 nm < λ < 780 nm) was used as the light source. At given intervals (every 30 minutes), the concentration of toluene in the reactor was measured with a GC-FID (FULI 9790, China). The schematic of the photocatalytic reactor was provided in the supporting information (see Fig. S1).

Results and Discussion

Structure and morphology

The crystal structure and phase composition of these samples were detected by XRD analysis (Fig. 1a). For pure Bi₂WO₆, distinct diffraction peaks at 28.5^°, 32.8^°, 47.1^°, 55.7^°, 58.5^° and 75.8^° were perfectly corresponding to (131), (200), (202), (133), (262) and (391) crystallographic planes (PDF#39-0256)²⁷, respectively. In addition, all the diffraction peaks of pure α-Fe₂O₃ (see Fig. S2) were in good agreement with those for hematite (PDF#33-0664)²⁸. The intensity of the diffraction peaks of α-Fe₂O₃/Bi₂WO₆ composites was stronger than that of pure Bi₂WO₆ (see Fig. 1a), which could be ascribed to the growth of crystals during the hydrothermal process. It was noteworthy that no peaks of α-Fe₂O₃ were observed for α-Fe₂O₃/Bi₂WO₆ composite photocatalysts, which may be ascribed to the high dispersion and low content of α-Fe₂O₃²⁹. Moreover, a shift to lower 2θ value of the band related to (131) lattice plane for α-Fe₂O₃/Bi₂WO₆ could be observed (Fig. 1b). Based on Bragg’s law, this fact verified the slight expansion of the interplanar spacing related to Bi₂WO₆³⁰. According to the reference³¹, the increase in d spacing of Bi₂WO₆ could be attributed to the partial substitution of Bi sites by Fe ions. From this viewpoint, it was confirmed that there existed strong interaction between α-Fe₂O₃ and Bi₂WO₆. Similar results were also reported by the reference²⁵.

Figure 1

(a) XRD patterns of pure Bi₂WO₆ and Fe₂O₃/Bi₂WO₆ composites. (b) Enlargement of XRD peaks in lattice (131) plane.

Raman spectra were also collected to further analyze the phase structure of photocatalysts. In Fig. 2, the peaks in the 60–160 cm⁻¹, 200–400 cm⁻¹ and 600–1000 cm⁻¹ regions could be assigned to translational motions of Bi³⁺ and W⁶⁺, WO₆ bending modes and Bi-O stretching and bending modes, and W–O bands stretching modes³², respectively. In detail, the bands at about 95 cm⁻¹ and 305 cm⁻¹ were associated with translational modes involving simultaneous motions of Bi³⁺ and WO₆^{6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33}. The peak at about 710 cm⁻¹ could be ascribed to an antisymmetric bridging mode of the tungstate chain^33,34. Two bands at about 790 cm⁻¹ and 820 cm⁻¹ matched well with antisymmetric and symmetric A_g modes of terminal O–W–O groups^33,35. No Raman vibrational peaks of α-Fe₂O₃ were detected, which may be due to a lower α-Fe₂O₃ loading²⁹. As illustrated in Fig. 2a,b, three bands at 95 cm⁻¹, 305 cm⁻¹ and 790 cm⁻¹ of the α-Fe₂O₃/Bi₂WO₆ composites migrated to higher wave numbers, indicating the in situ growth of α-Fe₂O₃ on Bi₂WO₆ crystal had an influence on the phase structure of Bi₂WO₆. This finding fitted well with the results obtained from XRD characterization above.

Figure 2

Raman spectra for pure Bi₂WO₆ and Fe₂O₃/Bi₂WO₆ composites in the range of (a) 50–500 cm⁻¹ and (b) 500–1000 cm⁻¹.

The microstructure and morphology of the obtained samples were visualized by SEM, TEM and HR-TEM measurements. Figure 3 showed the SEM images of pure Bi₂WO₆, pure α-Fe₂O₃ and 4%-Fe₂O₃/Bi₂WO₆ composite, respectively. It could be clearly seen that the geometric shape of pure Bi₂WO₆ was a laminated structure (Fig. 3a,b). Pure α-Fe₂O₃ had a uniform and hexagonal nanoplate structure (Fig. 3c,d). No significant change could be observed in the SEM images of 4%-Fe₂O₃/Bi₂WO₆ composite (Fig. 3e,f) compared with that of pure Bi₂WO₆, indicating the prepared process hardly damaged the origin layered structure of pure Bi₂WO₆. Additionally, no hexagonal nanoplate structure of α-Fe₂O₃ could be found as well, which may be ascribed to the low content of α-Fe₂O₃²⁹. The TEM images of pure Bi₂WO₆ (Fig. 4a,b) and pure α-Fe₂O₃ (Fig. 4c,d) showed layered structure as well. Figure 4e displayed that α-Fe₂O₃ nanosheets randomly grew on the laminated structure of Bi₂WO₆. A typical HR-TEM image of α-Fe₂O₃/Bi₂WO₆ composite was given in Fig. 4f. The lattice with interplanar distances of 0.314 nm and 0.251 nm attached to (131) lattice plane of Bi₂WO₆³⁰ and (110) lattice plane of Fe₂O₃²⁶, respectively. In view of long-time ultrasonication pretreatment in the TEM process, it could be concluded that there existed strong interlayer interaction between α-Fe₂O₃ and Bi₂WO₆ nanoplates³⁶.

Figure 3

SEM images of (a,b) pure Bi₂WO₆, (c,d) pure α-Fe₂O₃ and (e,f) 4%-Fe₂O₃/Bi₂WO₆ composite.

Figure 4

TEM images of pure (a,b) Bi₂WO₆, (c,d) pure α-Fe₂O₃, (e) TEM and (f) HR-TEM images of 4%-Fe₂O₃/Bi₂WO₆ composite.

Surface composition analysis

X-ray photoelectron spectroscopy (XPS) measurement was used to identify the oxidation state and surface composition of α-Fe₂O₃/Bi₂WO₆ composites. For pure Bi₂WO₆, two peaks with binding energy of 164.60 eV and 159.25 eV were the split signals of Bi 4f (Fig. 5a), which could be assigned to the Bi³⁺ species in the sample³⁷. Two characteristic peaks in the W 4f spectrum (Fig. 5b) at 37.70 eV and 35.50 eV were ascribed to W 4f_5/2 and W 4f_7/2³⁷, respectively. As displayed in Fig. S3, the peaks at 724.58 eV and 710.68 eV belonged to Fe 2p_1/2 and Fe2p_3/2³⁸, suggesting the presence of Fe³⁺. The peaks of Bi³⁺ 4f and W⁶⁺ 4f both showed a slightly positive shift (see Fig. 5), which indicated the surface electron density for Bi and W had changed^39,40. This result indicated that the heterostructure interface between Fe₂O₃ and Bi₂WO₆ could have been formed^15,36. It agreed well with the abovementioned XRD, Raman and HR-TEM results.

Figure 5

High-resolution XPS spectra of (a) Bi 4f (b) W 4f for different photocatalysts.

Optical properties

Expanding the visible light absorption range played a crucial role for the modification of photocatalysts towards improving the visible-light photocatalytic activity. Optical diffuse reaction spectra (DRS) was used to estimate the visible light adsorption property of as-obtained photocatalysts. As shown in Fig. 6, pure α-Fe₂O₃ displayed an excellent light absorption ability among almost the whole visible light range due to its narrow band gap⁴¹. The spectrum of α-Fe₂O₃/Bi₂WO₆ composites showed obvious red-shift compared with pure Bi₂WO₆, implying the improvement of the utilization of visible light⁴². Furthermore, the band gap of pure Bi₂WO₆ and pure Fe₂O₃ was calculated based on Kubelka–Munk theory to be about 2.65 eV and 1.95 eV, respectively (see Fig. S4). These results agreed well with the references^43,44.

Figure 6

UV−vis absorption spectra of pure Bi₂WO₆, pure α-Fe₂O₃ and α-Fe₂O₃/Bi₂WO₆ composites.

Photoelectrochemical performances

Electrochemical impedance spectroscopy (EIS) was applied to shed light on the interface charge separation and transfer efficiency of different photocatalysts under visible light irradiation. The arc radius of Fe₂O₃/Bi₂WO₆ composites was smaller than that of pure Bi₂WO₆ and Fe₂O₃(see Fig. 7a), revealing that the photoexcited eletrons could easily transfer across the interface and efficiently migrate to the surface due to the Fe₂O₃/Bi₂WO₆ layered heteojunction^39,45. Especially, the 4%-Fe₂O₃/Bi₂WO₆ composite had the smallest arc radius, indicating it possessed the least electrons transfer resistance. Tafel analysis was performed to get a vivid view of the current density values of the photocatalysts investigated under visible light irradiation⁴⁶. Generally, larger values of corrosion current density and anodic Tafel slope meant more photogenerated pairs and faster electron transfer rate¹², which benefitting the photocatalytic process. As displayed in Fig. 7b, 4%-Fe₂O₃/Bi₂WO₆ composite exhibited larger J_corr value and anodic slope as compared with pure Bi₂WO₆. Consequently, the results further demonstrated that the heterojunction between Fe₂O₃ and Bi₂WO₆ could improve charge transfer rate and efficiently separate photoexcited electrons and holes. Moreover, as shown in Fig. S5, the photocurrent of Fe₂O₃/Bi₂WO₆ composite was much higher than that of pure Bi₂WO₆, suggesting the separation ratio of photoexcited charges enhanced. Photoluminescence (PL) spectra was further used to demonstrate the recombination ratio of photoinduced pairs. As displayed in Fig. 7c, the intensity of 4%-Fe₂O₃/Bi₂WO₆ was much lower than that of pure Bi₂WO₆, indicating that the recombination of photogenerated charge carriers was effectively inhibited^15,47. Time-resolved photoluminescence (TRPL) spectra (Fig. 7d) was also used to assess recombination kinetics of photoinduced electron-hole pairs. The TRPL decay spectrum waves were fitted by exponential decay kinetics function displayed as Eq. (1) ⁴⁸:

$$I(t)={{\rm{A}}}_{1}\,\exp (\,-\,{\rm{t}}/{{\rm{\tau }}}_{1})+{{\rm{A}}}_{2}\,\exp (\,-\,{\rm{t}}/{{\rm{\tau }}}_{2})$$

(1)

The average emission time was calculated based on Eq. (2) ⁴⁸:

$${{\rm{\tau }}}_{{\rm{av}}}=\sum {{\rm{A}}}_{{\rm{i}}}{{{\rm{\tau }}}_{{\rm{i}}}}^{2}/\sum {{\rm{A}}}_{{\rm{i}}}{{\rm{\tau }}}_{{\rm{i}}}$$

(2)

where τ₁ and τ₂ were lifetimes, A₁ and A₂ were the corresponding weighting factors. Based on the fitted results listed in Table 1, the average lifetime of 4%-Fe₂O₃/Bi₂WO₆ composite was longer than that of pure Bi₂WO₆, enhancing the likelihood of photoinduce pairs to participate in the reaction of gas-phase toluene photodegradation³⁹, thereby implying higher photocatalytic activity for Fe₂O₃/Bi₂WO₆ composites.

Figure 7

(a) Electrochemical impedance spectroscopy spectra of different catalysts, (b) Tafel polarization curves of the various electrodes under visible light irradiation, (c) Photoluminescence (PL) spectra and (d) time-resolved photoluminescence (TRPL) spectra of different samples.

Table 1 Kinetic parameters of the emission Decay of different samples.

Photocatalytic performances

To evaluate the photocatalytic efficiency of as-prepared photocatalysts, gaseous toluene was chosen as a probe indoor air contaminant. The toluene degradation efficiency for the samples investigated was displayed in Fig. 8 and the relative apparent rate constant (k) based on pseudo-first-order kinetic model was obtained (see Fig. S6). Pure Bi₂WO₆ and α-Fe₂O₃ showed rather poor photocatalytic activity under three-hour visible light irradiation. In stark contrast, the gaseous toluene removal efficiency was remarkedly enhanced over Fe₂O₃/Bi₂WO₆ composites owing to their well photoelectrochemical property as discussed above. Among all the samples, 4%-Fe₂O₃/Bi₂WO₆ catalyst showed the highest photocatalytic activity, whose k value (0.3469 h⁻¹) was much higher than those of pure Bi₂WO₆ (0.0749 h⁻¹) and pure α-Fe₂O₃ (0.0649 h⁻¹). Besides, the surface area didn’t play an important role in enhancing the photocatalytic activity, as the surface area of 4%-Fe₂O₃/Bi₂WO₆ sample was modest (see Table S1). Based on the above analysis, it could be concluded that the layered heterojunctions between Fe₂O₃ and Bi₂WO₆ palyed a key role in promoting the photocatalytic process.

Figure 8

Photocatalytic degradation of gaseous toluene in the presence of different samples under visible light irradiation.

Possible photocatalytic mechanism

DMPO spin-trapping ESR technique was employed to verified the active radicals produced in the photocatalytic system. As presented in Fig. 9, no peaks could be observed in the darkness. Clearly, characteristic peaks for ·OH and ·O₂⁻ emerged once the as-obtained samples were irradiated by visible light. Both intensity for ·OH and ·O₂⁻ became stronger with increasing irradiation time. Notably, the 4%-Fe₂O₃/Bi₂WO₆ exhibited stronger intensity for ·OH and ·O₂⁻ than that of pure Bi₂WO₆ under the same condition, implying the oxidation ability of the composite photocatalyst had been effectively promoted⁴⁹. As reported in the references^50,51,52, the involvement of active radical species (such as ·O₂⁻, ·OH and h⁺) was very important in the photocatalytic process. Thus, free radicals trapping experiment was performed to further figure out the role of the radical species during the photocatalytic reaction. In a typical experiment, Na₂C₂O₄⁵³, salicylic acid (SA)⁵⁴ and benzoquinone (BQ)⁵⁵ were applied as scavengers of h⁺, ·OH and ·O₂⁻, respectively. As illustrated in Fig. 10, the addition of BQ had little effect on the photocatalytic activity. However, the toluene removal efficiency obviously decreased in the presence of Na₂C₂O₄ and SA. In particular, the degradation efficiency of toluene was whittled down into about 5% when adding appropriate amount of SA. Therefore, it could be inferred that h⁺ and ·OH played dominant roles in the toluene photodegradation process.

Figure 9

DMPO spin-trapping ESR spectra in Bi₂WO₆ and 4%-Fe₂O₃/Bi₂WO₆ composite in drak or under visible light irradiation: (a) in aqueous dispersion for DMPO-·O₂⁻ and (b) in methanol dispersion for DMPO-·OH.

Figure 10

Photocatalytic activities of 4%-Fe₂O₃/Bi₂WO₆ composite for the photooxidation of toluene in the presence of different quenchers.

Based on the Mott-Schottky analysis (see Fig. S7), the value of Fermi energy level was −0.30 eV and −0.60 eV (vs NHE) for Bi₂WO₆ and α-Fe₂O₃⁵⁶, respectively. For n-type semiconductor, the conduction band (CB) edge was more negative (about 0–0.2 eV) than Fermi level⁵⁷. Herein, the difference value was set to 0.1 eV. Therefore, as illustrated in Fig. 11, the conduction band (CB) bottom and valence band (VB) top values of pure Bi₂WO₆ and pure α-Fe₂O₃ could be obtained. The in situ growth strategy could guarantee the intimate contact between Bi₂WO₆ and α-Fe₂O₃ with stronger interfacial interaction. Once the heterojunction was irradiated by visible light, both Bi₂WO₆ and α-Fe₂O₃ could generate electron-hole pairs. Due to the type-II heterojunction and intimate contact between Bi₂WO₆ and α-Fe₂O₃, the photoexcited electrons on the CB of α-Fe₂O₃ had the tendency to transfer to that of Bi₂WO₆, whereas photoinduced holes spontaneously moved to the VB of α-Fe₂O₃ (see Fig. 11). Therefore, the photogenerated electrons and holes could be separated efficiently, which greatly enhanced the photocatalytic activity. The CB bottom level of Bi₂WO₆ was more negative than that of O₂/·O₂⁻ (−0.33 eV)⁵⁸, thus the photoexcited electrons could reduce oxygen molecules absorbed on the surface to ·O₂⁻ species. What’s more, part of O₂ could be reduced to H₂O₂ by photo-generated electrons based on the fact that the redox potential of O₂/H₂O₂ was 0.695 eV⁵⁹, then the formed H₂O₂ could produce ·OH by capture the photo-generated electrons^60,61. Although the VB top value of α-Fe₂O₃ was less positive than the potential of ·OH/OH⁻ (1.99 eV)⁵⁸, characteristic signals of ·OH (1:2:2:1 quartet pattern) could still be obviously observed in the ESR spectrum (see Fig. 9). This may be ascribed to the fact that part of the photogenerated holes remaining in the VB top of Bi₂WO₆ could also oxidize absorbed H₂O into ·OH. Similar phenomenon was also founded by Li. et al.³⁹. Hence, these radical species had strong oxidation ability to destroy toluene molecule.

Figure 11

Diagram of the energy band levels of Fe₂O₃/Bi₂WO₆ composite and proposed possible photodegradation of toluene process.

Conclusions

In summary, α-Fe₂O₃/Bi₂WO₆ layered heterojunctions were successfully synthesized via a simple in situ growth method in this work. XRD, Raman, HR-TEM and XPS results demonstrated there existed strong interaction between Bi₂WO₆ and α-Fe₂O₃. The α-Fe₂O₃/Bi₂WO₆ layered heterojunctions could effectively broaden visible light absorption range and improve photoexcited charges separation efficiency according to the characterization results of UV-vis, EIS, Tafel curve, PL and TRPL. Especially, 4%-Fe₂O₃/Bi₂WO₆ sample with strong interlayer interaction exhibited the highest photocatalytic activity. Given the results of ESR and trapping experiments, h⁺ and ·OH species played a crucial role during the photocatalytic process of toluene removal. Such layered heterojunction photocatalyst had potential applications for indoor air purification.