Electrospun nanofibers of p-type BiFeO₃/n-type TiO₂ hetero-junctions with enhanced visible-light photocatalytic activity

Abstract

One-dimensional BiFeO₃/TiO₂ heterostructure nanofibers with high visible-light photocatalytic activity have been successfully obtained via a facile hydrothermal process followed by an electrospinning technique. The results show that the BiFeO₃/TiO₂ nanofibers are as long as dozens of micrometers with the diameters of about 100–300 nm, where BiFeO₃ nanoparticles are surrounded by anatase-type TiO₂ nanocrystals. Compared with the corresponding pure BiFeO₃ nanoparticles, and TiO₂ nanofibers, the as-prepared BiFeO₃/TiO₂ nanofibers exhibit a markedly enhanced photocatalytic activity in the degradation of methyl blue under visible light irradiation. The enhanced photocatalytic activity is attributed to the formed p–n heterojunction between BiFeO₃ and TiO₂, which results in synergistic enhancement. Notably, the BiFeO₃/TiO₂ nanofibers could be easily recycled without the decrease in the photocatalytic activity because of their one-dimensional nanostructural property. With their high degradation efficiency and fine recyclability, the BiFeO₃/TiO₂ heterostructure nanofibers will have wide application in photodegradation of various organic pollutants.

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Introduction

During the previous decades, photocatalysis, as a “green” technique, has been extensively used in the area of environmental remediation because various kinds of pollutants can be decomposed completely by photocatalytically active semiconductors under light irradiation.^1–3 Among those nanostructural semiconductor metal oxides, Titania (TiO₂) nanomaterials, naturally n-type semiconductors with a wide bandgap (E_g = 3.2 eV for anatase), have been extensively studied owing to their excellent photo-chemical stability, low cost and non-toxicity.^4,5 Nevertheless, the currently available TiO₂ photocatalysts generally suffer from large band gap energy and rapid recombination of photo-induced carrier, which significantly deteriorate the photocatalytic activity. As a result, many methods have been made to extend the photo-response of the TiO₂ to visible region and suppress the recombination of the photo-induced carriers. Research found that TiO₂ doped with nonmetal (such as N, S, C, F, B, etc.^6–9) and metal (such as Ag, Au, Cu, Pt, etc.^10–14) could obviously extend the photo-response of the TiO₂ to visible region and corresponding results in higher photocatalytic activity when it is illuminated under visible light. However, although the above modifications could partly improve the photocatalytic activity of TiO₂, some key problems remain unresolved. For example, doped materials suffer from thermal instability, photocorrosion, lattice distortion, and an increase in the carrier recombination probability.^15,16

Another strategy to increase the visible light activity of titanium is to couple TiO₂ with other narrow bandgap semiconductors. Compared to a single semiconductor, coupled semiconductors forms a hetero-junction structure which can transfer electrons from an excited small band gap semiconductor into another attached one in the case of proper band potentials. Such as In₂O₃,¹⁷ Bi₂MoO₆,¹⁸ Bi₂WO₆,¹⁹ CoFe₂O₄ sensitized²⁰ or Ag–CdS²¹ co-sensitized TiO₂ heterostructures have been reported to favor for the separation of photoinduced electrons and holes and thus dramatically improve the photocatalytic efficiency of the semiconductor heterostructures.

Multiferric BiFeO₃ has been widely investigated due to its potential applications in information storage, spintronics, sensors and photocatalyst because of its narrow bandgap (∼2.0–2.8 eV).^21–24 Recently, Li et al. reported that BiFeO₃/TiO₂ core–shell nanocrystals exhibited a high activity for photodegradation of Congo red because of the enhancement of the quantum efficiency by effectively promotion of electron and hole separation.²⁵ However, the disadvantage of the structure is that the suspended particulates are easily lost in the process of photocatalytic reaction and separation, which may re-pollute the treated environment. Rohrer et al.²⁶ have fabricated heterostructures of thin TiO₂ film on BiFeO₃ substrates by pulsed laser deposition, which can efficiently photo-reduce aqueous silver cations into silver nanoparticles under visible-light irradiation, but its photocatalytic activity is limited because of the low surface area.

As a contrast to random nanoparticles or thin films, the one-dimensional nanostructure such as nanowire, nanorod or nanofiber always show superior optoelectrical and photocatalytic property, and better dispersion owing to their excellent mobility of charge carriers or higher surface areas. For instance, the TiO₂ nanofibers or modified TiO₂ nanotubes have been of immense interest in recent years due to their enhanced photocatalytic activity and favorable recycling characteristics.^27–30

Electrospinning, as an economical and simple method that is capable of fabricating nanofibers with high specific surface area and porous structure on a large scale, has been employed in many applications.^31,32 To the best of our knowledge, the photocatalysis performances based on the p-type BiFeO₃/n-type TiO₂ nanofibers heterojunctions have not been explored until now. Herein, motivated by the above concerns, we fabricate one-dimensional BiFeO₃/TiO₂ nanofiber photocatalysts by combining the electrospinning technique with the hydrothermal method. The enhanced photocatalytic performance of BiFeO₃/TiO₂ nanofibers is attributed to fine light absorption capability of BiFeO₃ and excellent charge separation characteristics of the formed heterojunction between BiFeO₃ and TiO₂. Besides, the BiFeO₃/TiO₂ nanofibers could be easily recycled without the decrease in the photocatalytic activity because of the large length to diameter ratio of nanofibers.

Experimental

Synthesis of BiFeO₃/TiO₂ heterostructure nanofibers

The composites were fabricated via a three-step process: (1) the BiFeO₃ nanoparticles were prepared via the combining of coprecipitation method and hydrothermal method. A typical process as following: at first, 0.01 mol Fe (NO₃)₃·9H₂O and 0.01 mol Bi (NO₃)₃·5H₂O were dissolved in 50 mL of distilled water, then, 2 M KOH was added dropwise into the mixture to adjust the pH value of the solution and also serve as a mineralizer. The mixture was then aged at room temperature for 2 h. After that, 2 mL H₂O₂ was added into the solution and it was sealed in the autoclave as quickly as possible. Finally, it was heated at 200 °C for 24 h before it was furnace-cooled to room temperature. A gray powder obtained after filtration was washed with distilled water and dried in air. (2) BiFeO₃/TiO₂ heterostructure nanofibers were synthesized by an electrospinning technology. Typically, a quantity of tetra-butyl titanate (TBOT) and 1 g BiFeO₃ seeds were added into a mixture of 40 mL ethanol and 10 mL acetic acid by magnetic stirring to form a biphase precursor. Then, 4 g of polyvinyl-pyrrolidone (PVP) was dissolved into the mixture by slowly stirring 6 h at room temperature. As followed, the mixture precursor solution was loaded into the 5 mL plastic syringe equipped with a 0.4 mm inner-diameter stainless steel, which was connected with an accelerating voltage of around 12 kV. The ultrafine nanofibers were collected in tinfoil substrate, with feeding rate of 30 μL min⁻¹ and needle tip-to-collector distance of 10 cm. (3) Finally, the obtained composite nanofibers were annealed at 450 °C for 2 h in air atmosphere with the heating rate of 10°C min⁻¹. The BiFeO₃-doping concentration (X) was chosen as 5, 8, 10, 15, 20, which was the mole percentage of BiFeO₃ in the theoretical TiO₂ powders. The corresponding concentrations in the as-obtained photocatalysts were denoted as BTX. Unless otherwise specified, BiFeO₃–TiO₂ in the work refers to BT10. Pure TiO₂ nanofibers were similarly prepared but without the addition of BiFeO₃ seeds.

Characterization

The powder X-ray diffraction (XRD) patterns of the samples were recorded on a D8 advance X-ray powder diffractometer (Germany) using CuKα radiation (λ = 0.1542 nm) in the 2θ range from 20° to 80°. The surface composition of the nanoparticles was evaluated by X-ray photoelectron spectra (XPS, ESCA-3400). Scanning electron microscope (SEM) images and selected-area electron diffraction (SAED) analyses were performed with a Sirion 200 field emission scanning electron microscope system at the accelerating voltage of 200 kV. Transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM) images were performed on a JEOL JEM-2010 transmission electron microscopy (Japan) at the accelerating voltage of 200 kV equipped with energy dispersive X-ray spectrometer (EDX). UV-vis diffuse reflectance spectra (DRS) of the samples were obtained using an UV-vis spectrophotometer (Cary 5000 UV-vis-NIR, Australia) with an Integrated Sphere Attachment. All the measurements were performed at room temperature. The photocatalytic activities were evaluated by the degradation of MB in aqueous solution under visible-light irradiation (Xe lamp, 500 W; visible cutoff filter >420 nm). The reaction temperature was kept at room temperature to prevent any thermal catalytic effect. The photocurrents were measured by an electrochemical analyzer (CHI660A, CH Instruments Co.) in a standard three-electrode system using as-prepared samples as the working electrode, a saturated calomel electrode as reference, and a platinum wire parallel to the working electrode as a counter electrode. Photocurrent as a function of time was measured in aqueous NaSO₄ (0.5 M) solution under visible light irradiation (λ> 420 nm). Electrochemical impedance spectroscopy (EIS) measurements were carried out in above configuration of cell under irradiation (λ> 420 nm) in a frequency range between 1 MHz and 0.1 Hz at applied biases is equal to −0.5 V. All measurements were conducted at room temperature and N₂ atmosphere to obtain highly reproducible data.

Photodegradation measurement

A self-made cylindrical glass vessel with a water-cooling jacket was used as a photochemical reactor. The photocatalytic activities of the as-prepared samples were evaluated by the degradation of a model pollutant dye solution, methylene blue (MB). The optical system for the photocatalytic reactions was composed of a 500 W xenon arc lamp and a cutoff filter (λ > 420 nm) at room temperature. Where, a 100 mL of the MB solution with an initial concentration of 10 mg L⁻¹ in the presence of solid catalyst (50 mg) was stirred in the dark for 30 min to obtain a good dispersion and adsorption equilibrium. And then the mixed solution was illuminated under visible light in the photochemical reactor. At 30 min intervals of illumination, MB after photodegradation was analyzed at 660 nm as a function of time on a UV-vis spectrophotometer (Model: Shimadzu 2450/2550 PC). The BT10 samples were repeatedly used for five times with same experiment conditions to test the photocatalytic stability.

Results and discussion

The structures of the as-obtained products were examined by X-ray powder diffraction (XRD), as shown in Fig. 1. It is revealed that the BiFeO₃ nanoparticles are highly crystallized and exhibit a single-phase rhombohedral structure (Fig. 1a), all the peaks can be well indexed to a pure hexagonal structure with R3c space group for bismuth ferrite (JCPDS no. 86-1518). No other phases such as Bi₂Fe₄O₉ and Bi₂O₃/Fe₂O₃ are detected in XRD spectra. Fig. 1b represents the X-ray diffraction pattern of as-obtained TiO₂ sample. All diffraction peaks of TiO₂ can be perfectly indexed by an anatase phase (JCPDS card no. 21-1272). No characteristic peaks of other impurities such as rutile and brookite were observed. The crystallite size of BiFeO₃ and TiO₂ were calculated to be 25.7 nm and 21.2 nm according to the broadening of corresponding X-ray spectral peaks by Scherrer formula: d = kλ/β cos θ. Here, d is the crystallite size, λ is the wavelength of the X-ray radiation (CuKα = 0.1542 nm), k is Scherrer constant, which is usually taken as 0.89, β is the line width at half-maximum height, and θ is the incidence angle. Fig. 1c represents the X-ray diffraction pattern of as-obtained BiFeO₃/TiO₂ sample (BT10). Both of the characteristic peaks of BiFeO₃ (JCPDS no. 86-1518, hexagonal phase) and TiO₂ (JCPDS card no. 21-1272, anatase) can be found in the pattern. This result confirmed the coexistence of BiFeO₃ and TiO₂ in the BiFeO₃/TiO₂ nanocomposites.

Fig. 1 XRD patterns of (a) BiFeO₃, (b) TiO₂, (c) BT10.

XPS studies were further conducted to investigate the surface composition and chemical state of the prepared heteroarchitectures. Fig. 2 shows the fully scanned spectra of TiO₂ and BT10 samples. The C element could be ascribed to the adventitious carbon-based contaminant, and the binding energy for C 1s peak at 284.6 eV was used as the reference for calibration. The survey XPS spectrum demonstrates the presence of Bi, O, Ti and Fe elements in the BT10 sample, suggesting a binary coexistence of TiO₂ and BiFeO₃ in the BT nanofibers. This result agrees well with XRD analysis. The typical high-resolution XPS spectra of O1s can be found in Fig. 2b. The peak at about 529.5 eV is due to oxygen in the TiO₂ crystal lattice and the peak at about 531.0 eV is due to hydroxyl oxygen. The hydroxyl oxygen peak of the BT10 is more intensive than that of the undoped TiO₂ and BiFeO₃ samples, which means that BT10 sample maybe has better photocatalytic performance.

Fig. 2 (a) Survey XPS spectra of the TiO₂ and BiFeO₃/TiO₂ nanofibers; (b) the typical high-resolution XPS spectra of O1s.

The morphology and microstructure of the samples were investigated by TEM and HRTEM. For BiFeO₃ nanoparticles, the images in Fig. 3a and b demonstrate that large-scale rust-red BiFeO₃ nanoparticles (inset of Fig. 3a) possess uniform size and well monodisperse with a diameter of about 25 nm. They are identified as polycrystalline quasi hexahedron structure from SAED pattern (inset of Fig. 3b). Fig. 3c shows a typical SEM image of BT10 nanofibers. These light-brown BT10 nanofibers (inset of Fig. 3c) were as long as dozens of micrometers or even at the millimeter scale with the diameters of about 100–300 nm. They are aligned in random orientation because of the bending instability associated with the spinning jet. The magnified TEM image of BT10 nanofiber (Fig. 3d) clearly shows that a single nanofiber was compactly packed with nanoparticles, which agglomerated to form BiFeO₃/TiO₂ nanofibers. To better investigate the crystal structure and micro-composition of the TiO₂ nanocomposite, HRTEM of as-obtained BT10 sample was performed and shown in Fig. 3e and f. The lattice fringes of the (110) planes with interplanar spacing of approximately 0.357 nm correspond to TiO₂, whereas the fringes of the (024) planes with interplanar spacings of approximately 0.192 nm correspond to BiFeO₃, indicating that TiO₂ and BiFeO₃ coexist in the microspheres. Obviously, BiFeO₃ nanoparticles are surrounded by anatase-type TiO₂ nanocrystals. The SAED pattern further confirms its polycrystalline property (inset of Fig. 3e).

Fig. 3 TEM images of (a) BiFeO₃ seeds at low-magnification, the inset is the corresponding powder picture; (b) BiFeO₃ nanoparticles at high-magnification, the inset is the corresponding SAED pattern; (c) SEM image of BiFeO₃/TiO₂ nanofibers, the inset is the corresponding powder picture; (d) TEM image of a single BiFeO₃/TiO₂ nanofiber, the inset is the corresponding SAED pattern; (e and f) HRTEM image of BiFeO₃/TiO₂ nanofiber.

The optical properties of samples were studied by UV-vis DRS, which is helpful for characterizing the optical absorption property and band gap energy of the semiconductor. In the study, the UV-vis diffuse reflectance spectra of the as-prepared samples were measured and converted into absorption spectra according to the Kubelka–Munk method. The corresponding band gaps are calculated from the plots of E_g = 1240/λ by extrapolating the linear portion of absorbance to the wavelength axis at absorbance = 0. As shown in Fig. 4, The TiO₂ nanofibers exhibit the characteristic spectra of TiO₂ with a steep absorption edge located at 380 nm (curve a). By fitting the plots, the band gap of the TiO₂ nanofiber is evaluated to be about 3.2 eV. BiFeO₃ nanoparticles display obvious absorption up to 600 nm (curve g), the corresponding bandgap is calculated to be 2.09 eV, which is in good agreement on the previously reported value.^33,34 Hence, BiFeO₃ has high absorption either in the UV range or in the visible light region. Meanwhile, the absorbance spectra reveal that all of these BiFeO₃/TiO₂ nanofibers possess absorption edges extending into the visible-light region, and the absorption edges show continuous red-shift with the increasing of BiFeO₃ content.

Fig. 4 UV-vis spectra of (a) TiO₂, (b) BT5, (c) BT8, (d) BT10, (e) BT15, (f) BT20, (g) BiFeO₃.

The photocatalytic activities of the above samples on the degradation of MB under visible light irradiation were shown in Fig. 5. It could be seen that the pure TiO₂ nanofibers exhibited poor photocatalytic activity, its photodegradation efficiency of MB only reached about 3% within 150 min under visible light irradiation. The activity of pure BiFeO₃ sample is much higher than that of TiO₂, its photodegradation efficiency of MB reached about 20% within 150 min. The BiFeO₃/TiO₂ heterojunctions showed better photocatalytic activity, the photodegradation efficiency of MB reached about 25% after 150 min of reaction for BT20 sample. In comparison, nearly 100% of MB is photocatalytically degraded by BT10 and BT5 within 150 min under visible light irradiation. Even the BT15 sample, only with a small loading amount of BiFeO₃, still showed higher photocatalytic efficiency than that of pure TiO₂ and BiFeO₃ samples. Obviously, the photocatalytic activity of BiFeO₃/TiO₂ did not strictly increasing with the increasing of BiFeO₃ content. The superior reactivity of the BiFeO₃/TiO₂ heterojunctions was observed at BT10 ([BiFeO₃] : [TiO₂] = 1 : 9). The possible reason maybe when the molar ratio of BiFeO₃ to TiO₂ is below the critical ratio, less BiFeO₃ decreased the visible-light absorption and correspondingly reduced the photocatalytic activity. When it is above the critical ratio, excessive BiFeO₃ covered the active sites of TiO₂ and hindered the electron transfer on the interfaces of BiFeO₃/TiO₂, correspondingly resulting in poor photocatalytic activity.

Fig. 5 The visible-induced photocatalytical activity of (a) TiO₂, (b) BiFeO₃, (c) BT20, (d) BT15, (e) BT8, (f) BT5, (g) BT10.

To understanding the heterojunction effect on the enhanced photocatalytic activity of BiFeO₃/TiO₂ heterostructures, the transient photocurrent responses of all samples (photocurrent (I)–time (t) response) were measured. Fig. 6 shows the typical I–t response curves for different samples with several on–off cycles of intermittent visible light irradiation. The photocurrent of the BiFeO₃/TiO₂ nanofibers are much higher than that of TiO₂ except for BT20 sample, the current value of the undoped TiO₂, BT5, BT8, BT10, BT15 and BT20 electrodes were 2.74, 16.85, 18.56, 7.72 and 1.75 μA cm⁻², respectively. Obviously, the BT10 sample shows a highest photocurrent, which is about 6.77 times as high as those of the TiO₂ electrode. The obvious enhancement of BiFeO₃/TiO₂ series samples in photocurrent indicated smaller recombination and more efficient separation of photogenerated electron–hole pairs at their interface.

Fig. 6 Photocurrent generation on the catalyst electrodes coated with TiO₂ and BiFeO₃/TiO₂ samples; [Na₂SO₄] = 0.5 M; λ > 420 nm, continuously N₂ purged.

To further investigate the charge-transfer behaviors, we employed the EIS, which is a powerful technique to probe the charge transfer mechanisms at the photocatalyst/solution interface.^35–37 In the Nyquist diagrams, the semicircle diameter of EIS is equal to surface charge-transfer resistance (R_ct). For the photocatalytic reaction, a decrease in the radius of resistance always means a quicker charge transfer and a slower recombination of photoelectron and holes occurred, which is beneficial to relevant photocatalytic activity. From Fig. 7, we can see that the semicircle radius decreases in the order of BiFeO₃ > TiO₂> BT20 > BT15 > BT8 > BT5 > BT10. For TiO₂ sample, a semicircle with R_ct of 3500 Ω was obtained, which is much larger than the other samples, suggesting that the TiO₂ nanofibers under visible light irradiation need to overcome a great energy barrier to take place photocatalytic reaction. For BiFeO₃ nanoparticles, a semicircle with R_ct is 2100 Ω, suggesting that it has less energy barrier than that of TiO₂ under the same condition. For BiFeO₃/TiO₂ samples (BT10), it's semicircle with R_ct was sharply reduced to 600 Ω, suggesting that the close contact of BiFeO₃ with TiO₂ was rather effective in suppression of the electron–hole recombination and acceleration of photocatalytic reaction. As a consequence, more long-lived photogenerated charges contribute to the photodegradation organic pollants.

Fig. 7 EIS Nyquist plots of different samples under an applied voltage of – 0.5 (vs. Ag/AgCl) in aqueous Na₂SO₄ (0.5 M) solution. The solution was continuously N₂ purged and was irradiated under λ > 420 nm.

The band structures of BiFeO₃ and TiO₂ were further estimated by the electronegativity of the oxide. The valence band (VB) and conduction band (CB) position of BiFeO₃ and TiO₂ at the point of zero charge can be calculated by the following empirical equation:^38,39

E_VB = X − E^e + 0.5E_g (1)

E_CB = E_VB − E_g (2)

where E_VB is the VB edge potential, X is the electronegativity of the semiconductor, E^e is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), E_g is the band gap energy of the semiconductor, the conduction band position can be calculated by eqn (2). The calculated energy level of TiO₂ and BiFeO₃ are shown in Scheme 1a.

Scheme 1 (a) the band energy schematic diagram of BiFeO₃ and TiO₂ before contace. (b) Postulate of the visible light-induced photo-degradation mechanism of BiFeO₃/TiO₂ nanofibers.

In general, BiFeO₃ is reported to be a p-type semiconductor,^40,41 whose Fermi energy level lies close to the valence band and its band gap can be changed from 2.0 to 2.7 eV. Here, we will assume a value of 2.09 eV according to the UV-vis analysis. Undoped TiO₂ is typically oxygen-deficit and thus is considered n-type with a band gap of 3.2 eV (for anatase)^42,43 whose Fermi energy level lies close to the conduction band (CB) (see Scheme 1a). When TiO₂ is coupled with BiFeO₃ to form the p–n junction, the Fermi energy level of BiFeO₃ and TiO₂ tend to descend and rise up. Meanwhile, an inner electric field will be formed in the interface of p–n heterojunctions. At equilibrium, the inner electric field makes the p-type semiconductor region have a negative charge, while the n-type semiconductor region has a positive charge. As a result, the region of TiO₂ is positively charged, and its CB position is more positive than that of BiFeO₃.^26,44,45 Based on the above analysis, a plausible energy level diagram for the BiFeO₃/TiO₂ heterostructures (see Scheme 1b) was constructed.

When the BiFeO₃/TiO₂ p–n heterojunction is radiated by visible light with the photon energy higher or equal to the band gap of p-type or n-type semiconductors, the photo-generated electrons can move to the CB of n-type semiconductors and the photo-generated holes to the VB of p-type semiconductors due to the formation of an inner electric field. Thus, the formation of the p–n heterojunction effectively hinders the recombination of photogenerated electron and hole and the photocatalytic efficiency is much enhanced.

For the practical applications, it is necessary to investigate the long-term stability of a photocatalyst during photocatalytic reaction. The conversions of MB obtained in five successive reaction cycles on the BiFeO₃/TiO₂ p–n junction nanofibers are shown in Fig. 8. The activity of BiFeO₃/TiO₂ catalysts decreased about 10% upon 5-recycling tests. So, The BiFeO₃/TiO₂ p–n junction nanofibers can be used as high-performance visible-light photocatalysts and for potential applications in environmental protection.

Fig. 8 Recyclability of the BiFeO₃/TiO₂ nanofibers for the degradation of MB under visible light irradiation.

Conclusions

In conclusion, the novel BiFeO₃/TiO₂ p–n junction nanofibers have been prepared for the first time by hydrothermal process combined with electrospinning technique, which was successful to realize a close contact of p-type BiFeO₃ nanoparticles with n-type TiO₂ nanoparticles in the BiFeO₃/TiO₂ nanofibers, as evidenced by TEM and HRTEM observations. Such close contact was effective in suppression of the electron–hole recombination. The composite showed high visible light responding ability, excellent electron–hole mobility, and low electron–hole recombination, and therefore demonstrated superior photocatalytic activity and stability.

Acknowledgements

The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (Grant no. 51272185, 51172166 & 61205166), the National Program on key Basic Research Project (973 Grant no. 2012CB821404), and Subsidy of Large-scale Instrument & equipment of Wuhan University (Grant no. LF20130288)

Notes and references

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