Enhanced photocatalytic activity of α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers prepared by electrospinning technique

Abstract

One-dimensional α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers with the diameter of about 300–500 nm were successfully prepared by using a simple and straightforward protocol combining an electrospinning technique with a sintering process. The as-obtained products were characterized by SEM, TEM, XRD, FT-IR, XPS, BET and UV-vis spectroscopy. It was found that the construction of α-Fe₂O₃/Bi₂WO₆ heterostructures can effectively impede the recombination of photoelectrons and holes and α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers possess a superior photocatalytic activity compared to pure Bi₂WO₆ nanofibers for the degradation of methylene blue (MB) dye driven by visible light.

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1. Introduction

Since Fujishima and Honda reported the photocatalytic splitting of water on TiO₂ electrodes in 1972,¹ a great deal of research has been devoted to the semiconductor photocatalysts which are considered as an effective strategy for environment treatment and solar energy conversion.^2–4 However, the semiconductor TiO₂ with the large band gap can only absorb the ultraviolet light (λ < 400 nm) which accounts for only about 4% of the whole energy of incoming solar spectrum.^5–7 The considerable attention has therefore been paid to the development of the visible-light-driven semiconductor photocatalysts.

Bismuth tungstate (Bi₂WO₆), is composed of perovskite-like [WO₄]²⁻ layers sandwiched between bismuth oxide [Bi₂O₂]²⁺ layers.^8,9 The conduction band (CB) of Bi₂WO₆ is constituted of the W 5d orbital and its valence band (VB) is consisted by the hybridization of the O 2p and Bi 6s orbitals, which can not only make the potential of VB higher, but also lead to a narrowed band gap and make it have the intrinsic visible-light absorption ability.^10,11 Although the significant progress has been already made on the controllable preparation of Bi₂WO₆ photocatalysts with the different structures and morphologies,^12–14 the photocatalytic activity is restricted by the fast charge recombination. So it is significant to construct the photocatalytic heterojunctions to overcome the serious drawbacks of the fast charge recombination. Up to date, much research concerning the Bi₂WO₆-based heterostructures synthesized mainly by hydrothermal method has been performed, such as α-Fe₂O₃/Bi₂WO₆ microspheres,¹⁵ Bi₂O₃/Bi₂WO₆ rods,¹⁶ TiO₂–Bi₂WO₆ heterostructured tubes,¹⁷ BiOCl/Bi₂WO₆ nanoflowers,¹⁸ and BiOBr/Bi₂WO₆ spheres.¹⁹ However, few reports focus on the α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers prepared by the electrospinning technique which is a facile and convenient method to fabricate the one-dimensional nanomaterials.

α-Fe₂O₃ is a n-type semiconductor with a narrow band gap of 2.2 eV, which not only can promote the separation and migrate of photon-generated carriers, but also might contribute to form a higher conduction band position.¹⁵ Electrospinning is an efficient, well-known process producing the one-dimensional nanomaterials with a tunable diameter. This technique employs an electrified needle that ejects polymeric solutions toward a collector. In the presence of a large electric field, the droplet is ejected, stretched and forms the composite nanofibers when the electrostatic force exceeds the surface tension of the droplet at the tip of syringe needle, and ultimately fibers-film is obtained on the collector.²⁰ In the present work, α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers were successfully fabricated via the electrospinning method. The construction of α-Fe₂O₃/Bi₂WO₆ heterostructures can effectively impede the recombination of electrons and holes, and then migrate to the surface of the photocatalysts with an enhanced visible light photocatalytic activity.

2. Experimental section

2.1. Materials

Bismuth nitrate (Bi(NO₃)₃·5H₂O), citric acid (C₆H₈O₇·H₂O), ammonium tungstate ((NH₄)₁₀H₂(W₂O₇)₆), poly(vinyl pyrrolidone) (PVP), iron nitrate (Fe(NO₃)₃·9H₂O) and nitric acid (HNO₃) were purchased from Shanghai Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used as received without further purification.

2.2. Preparation of the precursor sols

In a typical experiment, 2.52 g (12 mmol) citric acid and 0.971 g (2 mmol) Bi(NO₃)₃·5H₂O were dissolved in 7 ml deionized water with the magnetic stirring for 15 min at room temperature, then 5 ml HNO₃ was added into the aforementioned solution and kept stirring for 30 min. This mixture was marked as solution 1. Meanwhile, 0.2552 g (0.083 mmol) (NH₄)₁₀H₂(W₂O₇)₆ was dissolved in 15 ml deionized water, and then was added dropwisely to the solution 1. The resultant solution marked as solution 2 was kept under the vigorous stirring for 30 min to ensure the thorough reaction. Subsequently, 0.8087 g (2 mmol) Fe(NO₃)₃ (1 : 1 in molar ratio with Bi³⁺) was dissolved in the solution 2. As a result, the yellow transparent precursor solution was obtained. 3.0 ml precursor solution was transferred to the mixture obtained by dissolving 1.0 g PVP-K90 in 10 ml absolute ethanol and the mixed solution was further stirred for 2 h to form a homogeneous and transparent precursor sols.

2.3. Electrospinning process and the calcination of samples

The precursor sols were subsequently placed in a 20 ml syringe attached to a stainless steel needle with the inner diameter of 0.6 mm, and then ejected from needle with a voltage of 20 kV. The tip-to-collector distance was set to 20 cm, and the aluminium foil was used to collect the electrospun fibers. The flow rate of the precursor sols was 2.26 ml h⁻¹ and the humidity level is maintained around 30% RH. The as-collected nanofibers were dried at 80 °C for 12 h.

According to the TG-DSC results, the organic components (PVP, citric acid) and NO₃⁻ groups were removed completely calcined at 500 °C. So, the dried nanofibers were put into an air-atmosphere programmable tube furnace for heat treatment and calcined from room temperature to 500 °C at a rate of 1 °C min⁻¹ and kept soaking time for 1 h. The samples were then naturally cooled to room temperature in the furnace.

2.4. Characterization of the nanofibers

The thermogravimetric and differential scanning calorimetric (TG-DSC) curves were determined by a thermal analyzer (TGA/SDTA 851e Mettler). X-ray diffraction (XRD) patterns were recorded on a Bruker D8 ADVANCE X-ray diffractometer with Cu Kα (λ = 0.15418 nm) radiation under 40 kV and 40 mA and scanning over the 2 theta range of 20–90°. The morphologies and microstructures of the α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers were analyzed by the Scanning Electron Microscope (SEM) (Hitachi S-520, JXA-840) and a High-Resolution Transmission Electron Microscope (HR-TEM, JEOL JEM-2100). The HRTEM specimens were prepared by dispersing the samples in the absolute ethanol which were picked up with holey carbon supporting films on copper grids. The copper grids with specimen must be dried for 30 min under the infrared lamp before tested. Nitrogen adsorption–desorption experiments were conducted at 77 K with a Micromeritics Tristar 3000 analyzer after the samples were degassed at 200 °C for 6 h. The Brunauer–Emmett–Teller (BET) surface areas of the products were estimated using the adsorption data. The surface compositions and chemical states were examined by X-ray photoelectron spectroscope (XPS) performed on an ESCALAB 250 photoelectron spectroscope using Mg Kα radiation. UV-vis diffuse reflectance spectra of the samples were obtained on a UV-2550 spectrophotometer (Shimaduz) with an integrating sphere attachment using BaSO₄ as the reference in the wavelength range of 200–800 nm.

2.5. Photocatalytic degradation of methylene blue (MB) under visible-light irradiation

Experiments on the photocatalytic activities were performed under the simulated solar light source by using a 500 W Xe lamp equipped with cutoff filters at room temperature and the wavelength range of the visible light is 400–760 nm. Methylene blue (MB) was used as a model chemical to evaluate the activity and properties of the α-Fe₂O₃/Bi₂WO₆ photocatalyst. The experiments were carried out in a sealed block box and the Xe lamp was placed in a quartzose cold hydrazine with a circulating water system to cool down the MB solution and prevent thermal catalytic effects. 40 ml aqueous suspension of MB (20 mg l⁻¹) and 60 mg samples calcined at 500 °C for 1 h were put into a 50 ml beaker. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to establish the adsorption–desorption equilibrium between the dye and the surface of the α-Fe₂O₃/Bi₂WO₆ photocatalyst under the room conditions. At given irradiation time intervals, 4 ml mixed solution was sampled and centrifuged to remove the catalyst particulates for analysis. The concentration of MB filtrates was detected with a UV-vis spectrophotometer (UV-2550). Meanwhile, the photocatalytic activity of Bi₂WO₆ nanofibers synthesized via the electrospinning method was tested.

3. Results and discussion

The TG-DSC curves of the as-spun α-Fe₂O₃/Bi₂WO₆ precursor nanofibers are shown in Fig. 1. The weight loss (ca. 20.71%) in the range of 30 to 250 °C can be ascribed to the evaporation of absorbed water, trapped ethanol and the removal of crystal water molecules of the nitrates.²¹ All these processes are accompanied by the occurrence of endothermic process, as evidenced by the DSC curve. The weight loss of approximately 23.45% from 250 to 400 °C was attributed to the oxidative decomposition of the side chains of PVP and the complete decomposition of citric acid, which is an exothermic reaction and present on the DSC curve.²¹ The significant weight loss of 41.86% in the range of 400–500 °C is attributed to the decomposition of nitrate and release of the oxidation of carbon and carbon monoxide from the thorough degradation of the main polymer chain of PVP,^22,23 corresponding to a dramatically exothermic peak the DSC curve, after which the weight of the samples remains constant. The total weight loss amounts to 86%.

Fig. 1 TG-DSC curves of the as-spun α-Fe₂O₃/Bi₂WO₆ precursor nanofibers.

XRD patterns of Bi₂WO₆ nanofibers and α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers calcined at 500 °C for 1 h are given in Fig. 2. All the diffraction peaks in Fig. 2a are in good agreement with the standard data of the pure orthorhombic Bi₂WO₆ phase (JCPDS no. 39-0256).²⁴ The peaks of α-Fe₂O₃/Bi₂WO₆ heterostructures shown in Fig. 2b can be well indexed as α-Fe₂O₃ (JCPDS no. 47-1409)¹⁵ and orthorhombic Bi₂WO₆ (JCPDS no. 39-0256).²⁴ No characteristic peaks of any impurities are detected and the sharp diffraction peaks imply the good crystallinity occurs. Based on the above XRD analysis, it is obvious that α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers were successfully achieved via the calcination at 500 °C for 1 h.

Fig. 2 XRD patterns of the nanofibers calcined at 500 °C for 1 h. (a) Bi₂WO₆ nanofibers; (b) α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers.

Fig. 3 shows the morphology and microstructural details of the samples. Fig. 3a and b show the typical SEM images of the as-prepared precursor nanofibers which have the well-defined one-dimensional structure and are relatively uniform with the diameter of about 0.5–1 μm, and length of up to tens of millimeters, respectively. After calcination at 500 °C for 1 h, the morphology of the α-Fe₂O₃/Bi₂WO₆ nanofibers can be well preserved with the diameter of about 300–500 nm and the surface of the nanofibers is rough (seen in Fig. 3c and d). This diameter reduction is due to the loss of the organic components of the precursor nanofibers and the crystallization of inorganic oxides during the calcination process. The morphology of the nanofibers cannot be destroyed during the removal of PVP and calcination at the high temperature. It is noteworthy that the α-Fe₂O₃/Bi₂WO₆ nanofibers are composed of Fe₂O₃ nanoparticles and Bi₂WO₆ nanosheets,²⁰ which is beneficial to enlarge the specific surface area and thus enhance the visible-light photocatalytic activity.^16,18,19 Energy Dispersive Spectroscopy (EDS) analysis (Fig. 3e) reveals that the nanofibers are only composed of W, O, Bi and Fe elements. The TEM images of α-Fe₂O₃/Bi₂WO₆ nanofibers calcined at 500 °C for 1 h are presented in Fig. 4a and b, which further exhibits that the microscopic structure of the resultant products is in correspondence with that of the SEM observations in Fig. 3c and d.

Fig. 3 Representative SEM images of the precursor nanofibers (a and b), α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers calcined at 500 °C for 1 h (c and d), and energy dispersive spectroscopy (EDS) spectra of α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers (e).

Fig. 4 TEM images of α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers calcined at 500 °C for 1 h.

In order to further investigate the microstructure of the α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers, high-resolution transmission electron microscope (HRTEM) images are displayed in Fig. 5. It is found that there are two sets of lattice fringes with interplanar spacing of 0.276 nm and 0.34 nm, which correspond to the (060) plane of orthorhombic Bi₂WO₆ and (002) plane of α-Fe₂O₃, respectively. The results further strongly demonstrate that the α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers are composed of Fe₂O₃ nanoparticles and Bi₂WO₆ nanosheets and in good consistent with the results of the XRD and SEM analysis.

Fig. 5 HRTEM images of α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers calcined at 500 °C for 1 h.

The XPS spectra were measured to determine the surface compositions and chemical states of α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers, as shown in Fig. 6. The binding energies obtained in the XPS analysis are corrected for specimen charging by reference to a C 1s value of 284.6 eV. The XPS spectrum of the Bi 4f region is displayed in Fig. 6a, consisting of two characteristic peaks with binding energies of 160.35 eV and 165.65 eV, which correspond to the signals from doublets of Bi 4f_7/2 and Bi 4f_5/2 in the trivalent oxidation state, respectively.²⁵ The peaks, located at 37.25 eV as shown in Fig. 6b, could be assigned to the +6 oxidation state of tungsten for the W 4f_5/2.²⁵ Fig. 6c provides the high-resolution XPS peaks of Fe element, exhibiting the Fe 2p region with two individual peaks at 711.4 eV and 724.6 eV, which can be assigned to Fe 2p_3/2 and Fe 2p_1/2 peaks in Fe₂O₃ phase, respectively.¹⁵ As shown in Fig. 6d, the characteristic peak of O 1s around 530.1 eV comes from the overlapping contributions of oxide ions Gaussian fitted into five peaks: at 529.2, 529.7, 530.35, 530.9 and 530.06 eV which were contributed to Fe–O, Bi–O, W–O lattice oxygen, –OH hydroxyl groups and chemisorbed water, respectively.^25,26 The XPS results demonstrate that the formation of Fe₂O₃ in the α-Fe₂O₃/Bi₂WO₆ composite occurs, which is in good accordance with the results of the HRTEM and XRD analysis.

Fig. 6 XPS spectra of (a) Bi 4f, (b) W 4f, (c) Fe 2p and (d) O 1s of α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers calcined at 500 °C for 1 h.

Fig. 7a shows the representative N₂ adsorption and desorption isotherms of α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers which belongs to the type IV. The corresponding BJH (Barret–Joyner–Halenda) pore size distribution (PSD) curve is displayed in Fig. 7b. The specific surface area of the α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers calculated by the multipoint Brunauer–Emmett–Teller (BET) method is 19.2 m² g⁻¹. In addition, the BJH analyses show that α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers with a sharp porous size distribution exhibit the pore size of about 6.5 nm.

Fig. 7 N₂ adsorption and desorption isotherms (a) and the corresponding BJH pore size distribution curve (b) measured at 77 K for the α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers obtained after calcination at 500 °C for 1 h. P is the pressure of nitrogen in the gas phase and P₀ is the saturation pressure. The unit of cm³ g⁻¹ refers to cubic centimeters of N₂ gas at standard temperature (0 °C) and pressure (1 atm) per gram of water.

Fig. 8 displays UV-visible absorption spectra and (αE_photon)² versus E_photon curves of Bi₂WO₆ nanofibers and α-Fe₂O₃/Bi₂WO₆ nanofibers [n(Fe³⁺/Bi³⁺) = 1] calcined at 500 °C for 1 h. It is clearly observed in Fig. 8a that α-Fe₂O₃/Bi₂WO₆ nanofibers exhibit much stronger photoresponse property than that of Bi₂WO₆ nanofibers, indicating that it is much easier to stimulate the α-Fe₂O₃/Bi₂WO₆ nanofibers photocatalyst to generate electron–hole pairs under UV-visible light irradiation, and consequently degrade organic contaminant. For the semiconductor materials, a classical Tauc approach is employed to estimate the E_g value of Bi₂WO₆ and α-Fe₂O₃/Bi₂WO₆ nanofibers according to the following equation: αE_photon = B(E_photon − E_g)^n/2, in which α, E_photon and E_g are absorption coefficient, the discrete photon energy, and band gap energy, respectively and B is a constant. Besides, n is determined by the type of optical transition of a semiconductor (i.e., n = 1 for direct transition and n = 4 for indirect transition).²⁷ As the Bi₂WO₆ nanofibers exhibit the characteristic of direct band transition, the value of n is 1.^28,29 The band gap of Bi₂WO₆ nanofibers and α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers is calculated to be 2.76 eV and 1.89 eV, respectively, as displayed in Fig. 8b.

Fig. 8 UV-vis diffuse reflectance spectra (a) and (αE_photon)² versus E_photon curves (b) of Bi₂WO₆ nanofibers and α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers.

On the basis of the above results, the photocatalytic activities of Bi₂WO₆ nanofibers and α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers were evaluated via the degradation of MB under the visible light irradiation. Temporal evolution of the absorbance changes at a time interval of 10 min is displayed in Fig. 9a and b. During the photodegradation reaction, MB concentration was gradually reduced at the wavelength of 664 nm and kept stable ultimately after 70 min. The photodegradation ratio reached 82.04% after irradiation for 70 min in the presence of the α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers, whereas that of pure Bi₂WO₆ nanofibers only reached 54.69% under the same conditions, which were intuitively displayed in Fig. 9c. Therefore, α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers exhibit the enhanced photocatalytic activity. The superior photocatalytic property of the α-Fe₂O₃/Bi₂WO₆ nanofiber photocatalysts could be attributed to the construction of α-Fe₂O₃/Bi₂WO₆ heterojunctions which can contribute to separation and migration of photon-generated carriers, and then react with the adsorbed reactants effectively under the visible light irradiation.^16,18,19

Fig. 9 The temporal evolution of the spectra during the photodegradation of MB mediated by the Bi₂WO₆ nanofibers (a) and α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers (b) under the visible light illumination (400 nm ≤ λ ≤ 760 nm). Different photodegradation of MB between Bi₂WO₆ nanofibers and α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers (c).

According to the above experimental results, the enhanced photocatalytic activity of α-Fe₂O₃/Bi₂WO₆ heterojunctions can be mainly attributed to the effective electron–hole separations at the interfaces of the two semiconductors. The photon-generated carriers could migrate to the surface to react with the adsorbed reactants, and the migration direction of the photogenerated electron–hole relays on the band edge positions of semiconductors.^10,24 The possible photo-induced electron–hole separation processes, calculated conduction band (CB) and valence band (VB) edge potentials of α-Fe₂O₃ and Bi₂WO₆ are shown in Fig. 10. As indicated in Fig. 10, photo-generated electrons on the surface of α-Fe₂O₃ could easily transfer into the conduction band of Bi₂WO₆ because the conduction band gap potential of α-Fe₂O₃ is more negative than that of Bi₂WO₆. On the other hand, holes on the valence band of Bi₂WO₆ can be transferred into that of α-Fe₂O₃ under the band energy potential difference. So, the photo-generated electrons and holes in the α-Fe₂O₃ and Bi₂WO₆ could be separated effectively in the α-Fe₂O₃/Bi₂WO₆ heterojunctions, and then migrate to the surface of the photocatalysts. Then the recombination of electron–hole pairs can be reduced, resulting in an enhanced visible light photocatalytic activity.

Fig. 10 Schematic description of the mechanism of the photocatalytic activity in α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers.

4. Conclusions

In summary, one-dimensional α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers with the diameter of about 300–500 nm were successfully fabricated by the electrospinning method. The excellent morphology of α-Fe₂O₃/Bi₂WO₆ heterostructured nanofibers was obtained through calcination at 500 °C for 1 h. The construction of α-Fe₂O₃/Bi₂WO₆ heterojunctions is in favor of hindering the recombination of electrons and holes, and thus improves the photocatalytic efficiency under the visible light irradiation.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant no. 51172133), Natural Science Foundation of Shandong Province (Grant no. ZR2013BQ001), Project of Independent Innovation of University Institute of Jinan (Grant no. 201311034) and Project of Shandong Province Higher Educational Science and Technology Program (Grant no. J13LA01). The authors also thank the Analytical Center of Qilu University of Technology for technological support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11613e

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