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Bi2MoO6 heterostructures by electrospinning process with enhanced photocatalytic activity
Journal of Alloys and Compounds 646 (2015) 417e424
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of one-dimensional a-Fe2O3/Bi2MoO6 heterostructures by electrospinning process with enhanced photocatalytic activity Jie Zhao a, Qifang Lu a, *, Mingzhi Wei a, Cuiqing Wang b a
Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics, School of Material Science and Engineering, Qilu University of Technology, Jinan 250353, PR China b Lunan Research Institute of Coal Chemistry, Jining 272000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 April 2015 Received in revised form 27 May 2015 Accepted 29 May 2015 Available online 10 June 2015
One-dimensional (1D) a-Fe2O3/Bi2MoO6 heterostructures have been prepared by the electrospinning in combination with the calcination process. The length of a-Fe2O3/Bi2MoO6 heterostructures calcined at 500 C for 2 h was up to several millimeters, and the diameter was approximately 100e150 nm. The asprepared samples were characterized by thermogravimetric and differential scanning calorimetry (TGDSC), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UVevis diffuse reflectance spectrum (UVevis DRS). The photocatalytic degradation tests reveal that the obtained a-Fe2O3/Bi2MoO6 heterostructures exhibit the higher degradation rate of methylene blue (MB) than the pure Bi2MoO6 nanofibers and TiO2 (Degussa P25) under the simulated sunlight irradiation. The construction of a-Fe2O3/Bi2MoO6 heterostructures can effectively impede the recombination of photoelectrons and holes and the possible photocatalytic mechanism has also been discussed in details. © 2015 Elsevier B.V. All rights reserved.
Keywords: One-dimensional a-Fe2O3/Bi2MoO6 heterostructures Electrospinning Photocatalysis
1. Introduction As a green chemical technology, the semiconductor photocatalysis is a potentially promising approach to take advantage of the solar energy to decompose the harmful organic pollutants present in air and aqueous systems [1e4]. The photocatalytic activity of semiconductor photocatalysts often goes hand in hand with their morphologies, structures and specific surface areas. Onedimensional (1D) nanomaterials are suitable for applying to the photocatalysis due to possessing the large specific surface area, distinctive geometric structure and unique optical properties [5]. As is known to all, the electrospinning technique is capable of fabricating the 1D nanomaterials which have been employed in many applications [6]. Bi-based semiconductors have attracted much attention because of the widespread availability and low cost of their component materials [7]. Bismuth molybdate (Bi2MoO6) with a small band gap (2.5e2.8 eV) possesses the visible-light-driven photocatalytic activity for water splitting and degradation of organic contaminants [8]. Up to now, 1D Bi2MoO6 nanomaterials, * Corresponding author. E-mail address: [email protected] (Q. Lu). http://dx.doi.org/10.1016/j.jallcom.2015.05.191 0925-8388/© 2015 Elsevier B.V. All rights reserved.
such as microtubes [9] and microbelts [10], have been successfully fabricated by the electrospinning method. However, the application of single Bi2MoO6 photocatalyst meeting the practical requirements encountered the bottleneck of the poor quantum yield, which is due to the rapid recombination of photoinduced electrons and holes [11,12]. To overcome the drawbacks of the single semiconductor, 1D heterostructure photocatalysts consisting of two or three components have recently attracted considerable attention for their great potentials to efficiently degrade the pollutants [13,14]. Nowadays, Bi2MoO6-based nanofibers, for example Bi2MoO6/Bi4Ti3O12 [15], Bi2MoO6/carbon [16] and Bi2MoO6/TiO2 [17] have been reported and the separation rate of photoinduced surface and volume charge carriers can be significantly increased in these composites. Iron oxide (a-Fe2O3, hematite) with the narrow band gap at about 2.2 eV, absorbing the light with 600 nm wavelength and collecting about 40% of the solar spectrum energy, is one of the promising materials for photocatalytic application [18]. Unfortunately, the extremely short electronehole diffusion length hampers the photogenerated electrons and holes to separate from each other and increases the probability of charge recombination [19]. One of the possible solutions for improving the photocatalytic efficiency is to combine a-Fe2O3 with another semiconductor which possessed
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the suitable electronic band structure to form a composite photocatalysts [20]. In view of the band edge of a-Fe2O3 and Bi2MoO6, the efficient heterostructures can be formed for the separation of photogenerated electron-holes, when coupling them together. To the best of our knowledge, no investigations on the synthesis of 1D aFe2O3/Bi2MoO6 heterostructured photocatalysts were reported by the electrospinning technique. In the present paper, 1D a-Fe2O3/ Bi2MoO6 heterostructures were successfully synthesized by combination of electrospinning and calcination process. The asprepared heterostructures exhibited an excellent photocatalytic activity in the simulated sunlight irradiation due to the good light absorption capability of Bi2MoO6 and a-Fe2O3 and excellent charge separation characteristics of the formed heterojunction between Bi2MoO6 and a-Fe2O3.
SDTA 851e Mettler). The phase of a-Fe2O3/Bi2MoO6 heterostructures was identified by X-ray diffraction (XRD) analysis using an X-ray diffractometer (Rigaku D/Max 2200PC) with a graphite monochromater and Cu-Ka radiation (l ¼ 0.15418 nm) in the range of 10e70 at room temperature while the voltage and electric current were held at 40 kV and 20 mA, respectively. X-ray photoelectron spectroscopy (XPS) was performed on a Phi 5300 ESCA system with Mg Ka radiation (photoelectron energy 1253.6 eV). The C1s peak at 284.6 eV was used to calibrate peak positions. The morphology and microstructure of the samples were characterized via a scanning electron microscopy (SEM, Hitachi S-520, JXA-840) and transmission electron microscopy (TEM, JEM 100-CXII). UVeVisible diffuse reflectance spectra or absorption spectra of the samples were performed using a UV-2550 spectro-photometer (Shimadzu) in the wavelength range of 200e800 nm.
2. Experimental
2.4. Photocatalytic test
2.1. Preparation of the precursor sols
The photocatalytic activity of a-Fe2O3/Bi2MoO6 heterostructures was evaluated by the degradation of methylene blue (MB) under the simulated sunlight irradiation by using a 500 W Xe lamp. For the photodegradation test, 0.1 g Degussa P25, g-Bi2MoO6 and aFe2O3/Bi2MoO6 photocatalysts were suspended in 40 ml MB solution with the initial concentration of 20 mg/L, respectively. The experiments were carried out in a sealed black box and the Xe lamp was placed in a quartz photocatalytic reactor with a circulating water system to cool down the MB solution and prevent thermal catalytic effects. The solution was stirred in the dark for 30 min to obtain a good dispersion and reach the adsorptionedesorption equilibrium between MB and the catalyst surface. At given intervals of 0.5 h, 4 ml reacting solutions were taken out continuously and analyzed. The concentrations of MB in the reacting solutions were analyzed by UV-2550 spectrophotometer.
In a typical experimental process, 2.500 g (11.9 mmol) citric acid was dissolved into 20 ml deionized water. Then, 0.442 g (0.357 mmol) (NH4)6Mo7O24·4H2O, 2.425 g (5 mmol) Bi(NO3)3·5H2O and 4 ml concentrated nitric acid (15 M) were added to the above mentioned solution, respectively. After stirring for 1 h, a homogeneous and clear solution was obtained which was marked as solution 1. Then, 0.505 g (1.25 mmol) Fe(NO3)3·9H2O was dissolved solution 1 and the mixture was marked as solution 2.2 ml solution 2 and 1 ml concentrated HNO3 solution were added to 10 ml absolute ethanol, and this resultant mixture was labeled as solution 3. The molar ratio of Bi3þ and Fe3þ is 4:1. After mixing evenly, 0.8 g PVP (K-90) was dissolved in the solution 3. Then the transparent and homogeneous precursor sols were obtained after stirring for 10 h. 2.2. Fabrication of 1D a-Fe2O3/Bi2MoO6 heterostructures by electrospinning The electrospun precursor sols were transferred into a 20 ml plastic syringe which was connected to a stainless steel needle. The stainless needle with 1 mm inner diameter was connected to 18 kV power supply and the injector was propelled with the speed of 2.27 ml/h. The atmospheric humidity was about 30% and the distance between the needle tip and collector was 15 cm. The ascollected nanofibers were dried in an oven at 70 C for 12 h, and then calcined at 500 C for 2 h to form 1D a-Fe2O3/Bi2MoO6 heterostructures. As a contrast, g-Bi2MoO6 nanofibers could be fabricated as follows: 2.500 g (11.9 mmol) citric acid was dissolved into 20 ml deionized water. Then, 0.442 g (0.357 mmol) (NH4)6Mo7O24·4H2O, 2.425 g (5 mmol) Bi(NO3)3·5H2O and 4 ml concentrated nitric acid (15 M) were added to the above mentioned solution, respectively. After stirring for 1 h, a homogeneous and clear solution was obtained which was marked as solution 1.2 ml solution 1 and 1 ml concentrated HNO3 solution were added to 10 ml absolute ethanol, and then 0.8 g PVP (K-90) was dissolved in the solution to get the Bi2MoO6 precursor sols. The other steps of fabricated g-Bi2MoO6 nanofibers by electrospinning and calcining process were in accordance with those of the preparation of a-Fe2O3/Bi2MoO6 heterostructures.
3. Results and discussion 3.1. TG-DSC curves TG-DSC curves of the electrospun gel nanofibers are displayed in Fig. 1. The TG curve is performed in the temperature range of 30e800 C to investigate the amounts of organics remaining in the precursor nanofibers. It is clear from the TG curve that all the volatile substances (H2O, ethanol), organic components (PVP, citric acid), and NO-3 groups are removed completely below 550 C, which results in the formation of a-Fe2O3/Bi2MoO6 heterojunctions. An initial weight loss (about 15%) step at around 160 C results from
2.3. Characterization The thermogravimetric and differential scanning calorimetry (TG-DSC) curves were obtained by using a thermal analyzer (TGA/
Fig. 1. TG-DSC curves of the gel nanofibers.
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the evaporation of water, residual ethanol and absorbed moisture. The removal of the coordinated water molecules of the nitrates takes place at the range of 160e260 C, which is a slow and small weight loss (~10%) step. The significant weight loss of approximately 38% between 280 and 400 C was attributed to the complete decomposition of nitrates and the degradation of PVP, which involves both intra- and intermolecular transfer reactions [21,22]. The two exothermic peaks at around 313 and 382 C on the DSC curve correspond to the decomposition of citric acid and the degradation of PVP, respectively. The last process from 400 to 550 C accompanied by the exothermic peak at about 464 C on the DSC curve is considered to be due to the continuous decomposition of nitrate. After that the weight of the sample remains constant. The total weight loss amounts to 83%. 3.2. XRD patterns
Fig. 2. XRD patterns of pure g-Bi2MoO6 nanofibers (a) and 1D a-Fe2O3/Bi2MoO6 heterostructures (b) calcined at 500 C for 2 h.
XRD patterns were employed to investigate the crystal structure and the phase composition of the samples. As can be seen in Fig. 2, the gel nanofibers calcined at 500 C for 2 h could get the pure g-
Fig. 3. XPS survey spectra (a) and high-resolution XPS spectra of the Bi 4f (b), Mo 3d (c), Fe 2p (d) and O 1s (e) regions for the a-Fe2O3/Bi2MoO6 heterojunctions.
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Bi2MoO6 phase and the diffraction peaks at 2q ¼ 10.9 , 23.6 , 27.0 , 28.3 , 32.5 , 36.1, 46.7, 47.1, 55.7, 56.3 and 58.4 could be perfectly indexed to (020), (111), (140), (131), (002), (151), (202), (062), (133), (191) and (262) crystal faces of koechlinite Bi2MoO6 (JCPDS 21e0102), respectively. As for the pattern of a-Fe2O3/ Bi2MoO6 heterostructures calcined at 500 C for 2 h (Fig. 2b), the additional diffraction peaks with 2q values of 33.1, 35.5 , 38.8 and 63.1 is corresponding to (103), (110), (113) and (214) crystal planes of orthorhombic phase a-Fe2O3 (JCPDS 33e0664), respectively. It is obvious that a-Fe2O3/Bi2MoO6 heterojunctions can be successfully achieved which integrated the hematite phase a-Fe2O3 nanostructures with the koechlinite Bi2MoO6.
3.3. X-ray photoelectron spectroscopy (XPS) spectra More detailed information regarding the chemical and bonding environment of 1D a-Fe2O3/Bi2MoO6 heterojunctions was ascertained using X-ray photoelectron spectroscopy (XPS). Fig. 3a shows the survey scan spectra of a-Fe2O3/Bi2MoO6 at 0e1100 eV which indicates that C, O, Bi, Mo and Fe elements exist in 1D a-Fe2O3/ Bi2MoO6 heterojunctions. The high resolution XPS spectrum of aFe2O3/Bi2MoO6 heterojunctions in the Bi 4f region is shown in Fig. 3b. The peaks around 159.45 and 164.75 eV are attributed to Bi 4f7/2 and Bi 4f5/2 of Bi3þ, respectively [23]. Fig. 3c shows that the binding energies for Mo 3d5/2 and Mo 3d3/2 of Mo6þ are around
Fig. 4. SEM images: low-magnification (a) and high-magnification (b) of gel nanofibers; low-magnification (c) and high-magnification (d) of 1D a-Fe2O3/Bi2MoO6 heterostructures calcined at 500 C for 2 h and EDS spectra (e) of a-Fe2O3/Bi2MoO6 heterojunctions.
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232.3 and 235.35 eV, respectively [24]. An obvious peak centered at 711 eV is the characteristic peak of Fe3þ shown in Fig. 3d [25]. From Fig. 3e, the O1s peak around 530.1 eV can be attributed to BieO (529.3 eV) and MoeO (530.4 eV) in Bi2MoO6, FeeO (529.9 eV) in Fe2O3 and surface hydroxyl groups (531.1 eV). Moreover, the percentage of Bi, Mo, Fe and O was estimated by XPS to 18.9%, 9.1%, 4.5% and 67.5%, respectively. The ratio of Bi and Fe was about 4:1 in keeping with the original data. The above XRD and XPS results confirmed the coexistence of a-Fe2O3 and Bi2MoO6 in a-Fe2O3/ Bi2MoO6 heterojunctions. 3.4. SEM images of the samples The typical SEM images of the gel nanofibers are shown in Fig. 4. As can be seen from SEM results, the gel nanofibers show the well defined 1D nanostructure with the random distribution. The length of the gel fibers is up to the tens of millimeters, and the diameter is approximately 300e400 nm. The high-magnification SEM image as shown in Fig. 4b indicates that the appearance of the surface of the as-prepared nanofibers is smooth and homogeneous resulting from the existence of PVP. Fig. 4c and d shows SEM images of the heterostructured fibers calcined at 500 C for 2 h. After calcination, the 1D nanostructures still preserve except for a lot of spherical particles existed in the fibers and the diameter of the fibers is around 100e150 nm. The energy-dispersive-spectroscopy (EDS) spectra of the sample given in Fig. 4e show that O, Fe, Bi and Mo elements exist in 1D a-Fe2O3/Bi2MoO6 heterojunctions. 3.5. TEM image of the samples To further investigate the microstructure of 1D a-Fe2O3/ Bi2MoO6 heterojunctions, TEM measurement was performed on the samples. The typical TEM image of nanofibers calcined at 500 C for 2 h is shown in Fig. 5. It can be clearly observed that the surface of the nanofibers is irregular, and bulges are a-Fe2O3 particles. Furthermore, the diameter of the heterostructure nanofibers is about 100e150 nm which is coinciding with the results of the SEM observation. Fig. 6 displays that the formation process of 1D a-Fe2O3/ Bi2MoO6 heterostructures goes through two steps. Firstly, the composite nanofibers are fabricated by the electrospinning process. Then the gel nanofibers are calcined at 500 C for 2 h to form 1D aFe2O3/Bi2MoO6 heterostructures.
Fig. 5. TEM image of a-Fe2O3/Bi2MoO6 samples calcined at 500 C for 2 h.
Fig. 6. Schematic diagrams of electrospinning fabrication of nanofibers and formation a-Fe2O3/Bi2MoO6 composites.
3.6. UVevis diffuse reflectance spectra of samples Fig. 7 shows UVevis diffuse reflectance spectra of the Bi2MoO6 nanofibers and a-Fe2O3/Bi2MoO6 heterojunctions calcined at 500 C for 2 h. From the typical diffuse absorption spectra, the pure Bi2MoO6 sample displays the photoabsorption property from the UV light to visible light region which is shorter than 480 nm, and the band gap of the Bi2MoO6 is estimated to be 2.78 eV (seen the inset of Fig. 7). However, the obtained a-Fe2O3/Bi2MoO6 heterojunction exhibits a wide visible-light absorption in the range of 480e800 nm. The increased light absorption ability of the a-Fe2O3/ Bi2MoO6 heterojunctions can be attributed to the interaction of aFe2O3 and Bi2MoO6 [26]. 3.7. Photocatalytic activity of a-Fe2O3/Bi2MoO6 samples In the present study, MB is chosen as a model contaminant to evaluate the photocatalytic activity of the samples of a-Fe2O3/ Bi2MoO6 heterojunctions, Bi2MoO6 nanofibers and P25, respectively (Fig. 8). The decolorization of MB solution was measured at the wavelength of 664.0 nm. After stirring for 30 min in the dark, the solution reached the adsorptionedesorption equilibrium between MB and the catalyst surface (seen in Fig. 8a) and there was no perceptible degradation of MB after 4 h in the absence of the photocatalysts. As shown in Fig. 8b, after the simulated sunlight
Fig. 7. UVevis diffuse reflectance spectra of Bi2MoO6 (a) and a-Fe2O3/Bi2MoO6 (b) calcined at 500 C for 2 h.
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Fig. 8. (a) Degradation curves of MB in the presence of different photocatalysts in the dark (a) and under the simulated sunlight irradiation (b). The temporal evolution of the spectra during the photodegradation of MB mediated by the Bi2MoO6 nanofibers (c) and a-Fe2O3/Bi2MoO6 heterostructures (d) calcined at 500 C for 2 h.
irradiation for 4 h, the degradation efficiency rate of MB is about 37%, 65.6%, 90.7% and 3.8% for P25, Bi2MoO6, a-Fe2O3/Bi2MoO6 and no photocatalyst, respectively. Obviously, a-Fe2O3/Bi2MoO6 heterojunctions show the highest photocatalytic activity toward degradation of MB under the simulated sunlight irradiation. Based on the above results of the experiment, the photocatalytic mechanism of a-Fe2O3/Bi2MoO6 heterojunctions for the degradation of MB was supposed as illustrated in Fig. 9. The conduction band (CB) and the valence band (VB) of the semiconductor at the point of zero charge can be calculated by the following empirical equation [27]:
ECB ¼ X EC 0:5Eg ; EVB ¼ ECB þ Eg ;
where EC is the energy of free electron on the hydrogen scale (about 4.5 eV), Eg is the band gap of the semiconductor and X is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of all atoms in compounds, which is defined as the arithmetic mean of the first ionization energy and the atomic electron affinity (for Bi2MoO6 and a-Fe2O3, X is 6.13 and 4.78 eV, respectively) [28,29]. So ECB and EVB of g-Bi2MoO6 were calculated to be 0.24 and 3.02 eV, respectively. Similarly, ECB and EVB of a-Fe2O3 with band gap energy of 2.2 eV were 0.82 and 1.38 eV, respectively. The CB edge potential of aFe2O3 is more active than that of g-Bi2MoO6, so a-Fe2O3 could be easily excited by the sunlight and produced photogenerated electronehole pairs. The photogenerated electrons are easy to migrate to the CB of Bi2MoO6 at the interface of the heterostructures. In such a way, the photoinduced electronehole pairs could be effectively separated and could further prevent the recombination of photoelectrons and holes. The efficient charge separation could improve the photocatalytic activity. The processes can be summarized by the following equations:
a-Fe2O3 þ hn / (e) þ a-Fe2O3(hþ)
(1)
(e) þ Bi2MoO6 / Bi2MoO6(e)
(2)
Fig. 9. Postulated photodegradation mechanism of 1D a-Fe2O3/Bi2MoO6 heterostructures under the simulated sunlight irradiation.
Bi2 MoO6 e þ O2 /Bi2 MoO6 þ $O 2
(3)
2$O 2 þ 2H2 O/H2 O2 þ O2 þ 2OH
(4)
OH þ a-Fe2O3 (hþ) / $OH þ a-Fe2O3
(5)
$OH þ MB / degradation products
(6)
When 1D a-Fe2O3/Bi2MoO6 heterojunctions are irradiated by the simulated sunlight, electrons on the CB of a-Fe2O3 could be excited and generated electronehole pairs. Then the excited
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electrons from the CB of a-Fe2O3 could easily immigrate to the CB of Bi2MoO6 because of the lower CB, while the photogenerated holes remain on the VB of a-Fe2O3. Subsequently, the electrons will further react with the O2 in the solution to generate ·O 2 and then react with the H2O to generate OH. Last, the holes on the VB of aFe2O3 will react with OH to produce reactive ·OH radicals which is a strong oxidizing agent to decompose the organic dye. The enhanced photocatalytic performance of a-Fe2O3/Bi2MoO6 is due to the following factors: Firstly, according to UVeVis diffuse reflectance spectra analysis, a-Fe2O3/Bi2MoO6 exhibits the enhanced visible light absorption in the region of 480e800 nm and could absorb more visible light than pure Bi2MoO6 and P25, and thus heterostructures show the better photocatalytic activity. Secondly, the formed heterojunction between a-Fe2O3 and Bi2MoO6 in the heterostructure photocatalysts could further restrain the recombination between photoelectrons and photoholes. As the observation results of SEM and TEM images, there is a close contact of a-Fe2O3 nanoparticles with Bi2MoO6 nanoparticles in the aFe2O3/Bi2MoO6 heterojunctions. It is more effective in constraint of the electronehole recombination to the close contact. Furthermore, Bi2MoO6 could act as the electron traps to facilitate the separation of photogenerated electronehole pairs and promotes interfacial electron transfer process [30]. Therefore, the photoelectrons on the Bi2MoO6 surface and photoholes on the a-Fe2O3 surface could take part in the photocatalytic oxidation reactions to degrade organic contaminants, resulting in the enhanced photocatalytic activity of a-Fe2O3/Bi2MoO6 photocatalysts. 4. Conclusions 1D a-Fe2O3/Bi2MoO6 heterostructures were prepared via the electrospinning and calcination process. The UVevis absorption spectra of the a-Fe2O3/Bi2MoO6 heterostructures took on a wide visible-light absorption and narrow band gap compared with the pure Bi2MoO6. The investigation of the photocatalytic ability indicated that 1D a-Fe2O3/Bi2MoO6 nanofibers showed the higher photocatalytic activity toward degradation of MB than Bi2MoO6 nanofibers and P25 due to the construction of a-Fe2O3/Bi2MoO6 heterojunctions which was in favor of hindering the recombination of electrons and holes under the simulated sunlight irradiation.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
Acknowledgments This work was supported by the Natural Science Foundation of Shandong Province (Grant No. ZR2013BQ001), Project of Independent Innovation of University Institute of Jinan (Grant No. 201311034), Science and Technology Development Plan Project of Shandong Province (2014GGX102039) and Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J13LA01). References [1] M. Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338e344, http:// dx.doi.org/10.1038/35104607. [2] X.B. Chen, L. Liu, P.Y. Yu, S.S. Mao, Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals, Science 331 (2011) 746e750, http://dx.doi.org/10.1126/science. 1200448. [3] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Efficient photochemical water splitting by a chemically modified n-TiO2, Science 297 (2002) 2243e2245, http:// dx.doi.org/10.1126/science. 1075035. [4] S.Y. Reece, J.A. Hamel, K. Sung, T.D. Jarvi, A.J. Esswein, J.J. Pijpers, D.G. Nocera, Wireless solar water splitting using silicon-based semiconductors and earthabundant catalysts, Science 334 (2011) 645e648, http://dx.doi.org/10.1126/ science. 1209816. [5] J.T. Huang, Z.H. Huang, S. Yi, Y.G. Liu, M.H. Fang, S.W. Zhang, Fe-catalyzed growth of one-dimensional a-Si3N4 nanostructures and their
[20]
[21]
[22]
[23]
[24]
[25]
[26]
423
cathodoluminescence properties, Sci. Rep. 3 (2013) 1e9, http://dx.doi.org/ 10.1038/srep03504. X.F. Lu, W. Ce, Y. Wei, One-dimensional composite nanomaterials: synthesis by electrospinning and their applications, Small 5 (2009) 2349e2370, http:// dx.doi.org/10.1002/smll.200900445. M. Shang, W. Wang, J. Ren, S. Sun, L. Zhang, Nanoscale Kirkendall effect for the synthesis of Bi2MoO6 boxes via a facile solution-phase method, Nanoscale 3 (2011) 1474e1476, http://dx.doi.org/10.1039/C0NR00974A. Y. Zhang, Y.K. Zhu, J.Q. Yu, D.J. Yang, T.W. Ng, P.K. Wong, J.C. Yu, Enhanced photocatalytic water disinfection properties of Bi2MoO6-RGO nanocomposites under visible light irradiation, Nanoscale 5 (2013) 6307e6310, http:// dx.doi.org/10.1039/c3nr01338c. M.Y. Zhang, C.L. Shao, P. Zhang, C.Y. Su, X. Zhang, Y.Y. Sun, Y.C. Liu, Bi2MoO6 microtubes: controlled fabrication by using electrospun polyacrylonitrile microfibers as template and their enhanced visible light photocatalytic activity, J. Hazard Mater. 225e226 (2012) 155e163, http://dx.doi.org/10.1016/ j.jhazmat. 2012.05.006. Y.Y. Sun, W.Z. Wang, S.M. Sun, L. Zhang, A general synthesis strategy for onedimensional Bi2MO6 (M¼Mo, W) photocatalysts using an electrospinning method, CrystEngComm 15 (2013) 7959e7964, http://dx.doi.org/10.1039/ C3CE41347K. W.Z. Yin, W.Z. Wang, S.M. Sun, Photocatalytic degradation of phenol over cage-like Bi2MoO6 hollow spheres under visible-light irradiation, Catal. Commun. 11 (2010) 647e650, http://dx.doi.org/10.1016/ j.catcom.2010.01.014. G.H. Tian, Y.J. Chen, W. Zhou, K. Pan, Y.Z. Dong, C.G. Tian, H.G. Fu, Facile solvothermal synthesis of hierarchical flower-like Bi2MoO6 hollow spheres as high performance visible-light driven photocatalysts, J. Mater. Chem. 21 (2011) 887e892, http://dx.doi.org/10.1039/C0JM03040F. T. Zhang, J.F. Huang, S. Zhou, H.B. Ouyang, L.Y. Cao, A.T. Li, Microwave hydrothermal synthesis and optical properties of flower-like Bi2MoO6 crystallites, Ceram. Intern. 39 (2013) 7391e7394, http://dx.doi.org/10.1016/ j.ceramint.2013.02.079. G.P. Li, L.Q. Mao, Magnetically separable Fe3O4-Ag3PO4 sub-micrometre composite: facile synthesis, high visible light-driven photocatalytic efficiency, and good recyclability, RSC Adv. 2 (2012) 5108e5111, http:// dx.doi.org/10.1039/C2RA20504A. Y. Liu, M.Y. Zhang, L. Li, X.T. Zhang, One-dimensional visible-light-driven bifunctional photocatalysts based on Bi4Ti3O12 nanofiber frameworks and Bi2MoO6(X¼Mo, W) nanosheets, Appl. Catal. B: Environ. 160e161 (2014) 757e766, http://dx.doi.org/10.1016/j.apcatb.2014.06.023. M.Y. Zhang, C.L. Shao, J.B. Mu, X.M. Huang, Z.Y. Zhang, Z.C. Guo, P. Zhang, Y.C. Liu, Hierarchical heterostructures of Bi2MoO6 on carbon nanofibers: controllable solvothermal fabrication and enhanced visible photocatalytic properties, J. Mater. Chem. 22 (2012) 577e584, http://dx.doi.org/10.1039/ C1JM13470A. M.Y. Zhang, C.L. Shao, J.B. Mu, Z.Y. Zhang, Z.C. Guo, P. Zhang, Y.C. Liu, Onedimensional Bi2MoO6/TiO2 hierarchical heterostructures with enhanced photocatalytic activity, CrystEngComm 14 (2012) 605e612, http://dx.doi.org/ 10.1039/c1ce05974b. Z.H. Zhang, M.F. Hossain, T. Takahashi, Fabrication of shape-controlled aFe2O3 nanostructures by sonoelectrochemical anodization for visible light photocatalytic application, Mater. Lett. 64 (2010) 435e438, http://dx.doi.org/ 10.1016/j.matlet.2009.10.07. Y.S. Hu, A. Kleiman-Shwarsctein, G.D. Stucky, E.W. McFarland, Improved photoelectrochemical performance of Ti-doped alpha-Fe2O3 thin films by surface modification with fluoride, Chem. Commun. 21 (2009) 2652e2654, http://dx.doi.org/10.1039/b901135h. A. Biabani-Ravandi, M. Rezaei, Z. Fattah, Catalytic performance of Ag/Fe2O3 for the low temperature oxidation of carbon monoxide, Chem. Eng. J. 219 (2013) 124e130, http://dx.doi.org/10.1016/j.cej.2012.12.094. M. Shamshi Hassan, T. Amna, M.S. Khil, Synthesis of High aspect ratio CdTiO3 nanofibers via electrospinning: characterization and photocatalytic activity, Ceram. Intern. 40 (2014) 423e427, http://dx.doi.org/10.1016/ j.ceramint.2013.06.018. Y.C. Miao, G.F. Pan, Y.N. Huo, H.X. Li, Aerosol-spraying preparation of Bi2MoO6: A visible photocatalyst in hollow microspheres with a porous outer shell and enhanced activity, Dyes Pigments 99 (2013) 382e389, http:// dx.doi.org/10.1016/j.dyepig. 2013.05.005. Y.S. Xu, W.D. Zhang, Monodispersed Ag3PO4 nanocrystals loaded on the surface of spherical Bi2MoO6 with enhanced photocatalytic performance, Dalton Trans. 42 (2013) 1094e1101, http://dx.doi.org/10.1039/c2dt31634j. H.P. Li, Q.H. Deng, J.Y. Liu, W.G. Hou, N. Du, R.J. Zhang, X.T. Tao, Synthesis, characterization and enhanced visible light photocatalytic activity of Bi2MoO6/Zn-Al layered double hydroxide hierarchical heterostructures, Catal. Sci. Technol. 4 (2014) 1028e1037, http://dx.doi.org/10.1039/ c3cy00940h. D.D. Hu, K.Y. Zhang, Q. Yang, M.J. Wang, Y. Xi, C.G. Hu, Super-high photocatalytic activity of Fe2O3 nanoparticles anchored on Bi2O2CO3 nanosheets with exposed {001} active facets, Appl. Surf. Sci. 316 (2014) 93e101, http:// dx.doi.org/10.1016/j.apsusc.2014.07.185. G.K. Mor, H.E. Prakasam, O.K. Varghese, K. Shankar, C.A. Grimes, Vertically oriented Ti-Fe-O nanotube array films: toward a useful material architecture for solar spectrum water photoelectrolysis, Nano Lett. 7 (2007) 2356e2364, http://dx.doi.org/10.1021/nl0710046.
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[27] H.W. Huang, L.Y. Liu, Y.H. Zhang, N. Tian, One pot hydrothermal synthesis of a novel BiIO4/Bi2MoO6 heterojunction photocatalyst with enhanced visiblelight-driven photocatalytic activity for rhodamine B degradation and photocurrent generation, J. Alloy Compd. 619 (2015) 807e811, http://dx.doi.org/ 10.1016/j.jallcom.2014.08.262. [28] Y.L. Tian, F.X. Cheng, X. Zhang, F. Yan, B.C. Zhou, Z. Chen, J.Y. Liu, F.N. Xi, X.P. Dong, Solvothermal synthesis and enhanced visible light photocatalytic activity of novel graphitic carbon nitride-Bi2MoO6 heterojunctions, Powder Technol. 267 (2014) 126e133, http://dx.doi.org/10.1016/ j.powtec.2014.07.021.
[29] Y.D. Guo, G.K. Zhang, J. Liu, Y.L. Zhang, Hierarchically structured a-Fe2O3/ Bi2WO6 composite for photocatalytic degradation of organic contaminants under visible light irradiation, RSC Adv. 3 (2013) 2963e2970, http:// dx.doi.org/10.1039/c2ra22741j. [30] B. Yuan, C.H. Wang, Y. Qi, X.L. Song, K. Mu, P. Guo, L.T. Meng, H.M. Xi, Decorating hierarchical Bi2MoO6 microspheres with uniformly dispersed ultrafine Ag nanoparticles by an in situ reduction process for enhanced visible lightinduced photocatalysis, Colloids Surf. A: Physicochem. Eng. Asp. 425 (2013) 99e107, http://dx.doi.org/10.1016/j.colsurfa.2013.02.058.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of one-dimensional a-Fe2O3/Bi2MoO6 heterostructures by electrospinning process with enhanced photocatalytic activity Jie Zhao a, Qifang Lu a, *, Mingzhi Wei a, Cuiqing Wang b a
Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics, School of Material Science and Engineering, Qilu University of Technology, Jinan 250353, PR China b Lunan Research Institute of Coal Chemistry, Jining 272000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 April 2015 Received in revised form 27 May 2015 Accepted 29 May 2015 Available online 10 June 2015
One-dimensional (1D) a-Fe2O3/Bi2MoO6 heterostructures have been prepared by the electrospinning in combination with the calcination process. The length of a-Fe2O3/Bi2MoO6 heterostructures calcined at 500 C for 2 h was up to several millimeters, and the diameter was approximately 100e150 nm. The asprepared samples were characterized by thermogravimetric and differential scanning calorimetry (TGDSC), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UVevis diffuse reflectance spectrum (UVevis DRS). The photocatalytic degradation tests reveal that the obtained a-Fe2O3/Bi2MoO6 heterostructures exhibit the higher degradation rate of methylene blue (MB) than the pure Bi2MoO6 nanofibers and TiO2 (Degussa P25) under the simulated sunlight irradiation. The construction of a-Fe2O3/Bi2MoO6 heterostructures can effectively impede the recombination of photoelectrons and holes and the possible photocatalytic mechanism has also been discussed in details. © 2015 Elsevier B.V. All rights reserved.
Keywords: One-dimensional a-Fe2O3/Bi2MoO6 heterostructures Electrospinning Photocatalysis
1. Introduction As a green chemical technology, the semiconductor photocatalysis is a potentially promising approach to take advantage of the solar energy to decompose the harmful organic pollutants present in air and aqueous systems [1e4]. The photocatalytic activity of semiconductor photocatalysts often goes hand in hand with their morphologies, structures and specific surface areas. Onedimensional (1D) nanomaterials are suitable for applying to the photocatalysis due to possessing the large specific surface area, distinctive geometric structure and unique optical properties [5]. As is known to all, the electrospinning technique is capable of fabricating the 1D nanomaterials which have been employed in many applications [6]. Bi-based semiconductors have attracted much attention because of the widespread availability and low cost of their component materials [7]. Bismuth molybdate (Bi2MoO6) with a small band gap (2.5e2.8 eV) possesses the visible-light-driven photocatalytic activity for water splitting and degradation of organic contaminants [8]. Up to now, 1D Bi2MoO6 nanomaterials, * Corresponding author. E-mail address: [email protected] (Q. Lu). http://dx.doi.org/10.1016/j.jallcom.2015.05.191 0925-8388/© 2015 Elsevier B.V. All rights reserved.
such as microtubes [9] and microbelts [10], have been successfully fabricated by the electrospinning method. However, the application of single Bi2MoO6 photocatalyst meeting the practical requirements encountered the bottleneck of the poor quantum yield, which is due to the rapid recombination of photoinduced electrons and holes [11,12]. To overcome the drawbacks of the single semiconductor, 1D heterostructure photocatalysts consisting of two or three components have recently attracted considerable attention for their great potentials to efficiently degrade the pollutants [13,14]. Nowadays, Bi2MoO6-based nanofibers, for example Bi2MoO6/Bi4Ti3O12 [15], Bi2MoO6/carbon [16] and Bi2MoO6/TiO2 [17] have been reported and the separation rate of photoinduced surface and volume charge carriers can be significantly increased in these composites. Iron oxide (a-Fe2O3, hematite) with the narrow band gap at about 2.2 eV, absorbing the light with 600 nm wavelength and collecting about 40% of the solar spectrum energy, is one of the promising materials for photocatalytic application [18]. Unfortunately, the extremely short electronehole diffusion length hampers the photogenerated electrons and holes to separate from each other and increases the probability of charge recombination [19]. One of the possible solutions for improving the photocatalytic efficiency is to combine a-Fe2O3 with another semiconductor which possessed
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the suitable electronic band structure to form a composite photocatalysts [20]. In view of the band edge of a-Fe2O3 and Bi2MoO6, the efficient heterostructures can be formed for the separation of photogenerated electron-holes, when coupling them together. To the best of our knowledge, no investigations on the synthesis of 1D aFe2O3/Bi2MoO6 heterostructured photocatalysts were reported by the electrospinning technique. In the present paper, 1D a-Fe2O3/ Bi2MoO6 heterostructures were successfully synthesized by combination of electrospinning and calcination process. The asprepared heterostructures exhibited an excellent photocatalytic activity in the simulated sunlight irradiation due to the good light absorption capability of Bi2MoO6 and a-Fe2O3 and excellent charge separation characteristics of the formed heterojunction between Bi2MoO6 and a-Fe2O3.
SDTA 851e Mettler). The phase of a-Fe2O3/Bi2MoO6 heterostructures was identified by X-ray diffraction (XRD) analysis using an X-ray diffractometer (Rigaku D/Max 2200PC) with a graphite monochromater and Cu-Ka radiation (l ¼ 0.15418 nm) in the range of 10e70 at room temperature while the voltage and electric current were held at 40 kV and 20 mA, respectively. X-ray photoelectron spectroscopy (XPS) was performed on a Phi 5300 ESCA system with Mg Ka radiation (photoelectron energy 1253.6 eV). The C1s peak at 284.6 eV was used to calibrate peak positions. The morphology and microstructure of the samples were characterized via a scanning electron microscopy (SEM, Hitachi S-520, JXA-840) and transmission electron microscopy (TEM, JEM 100-CXII). UVeVisible diffuse reflectance spectra or absorption spectra of the samples were performed using a UV-2550 spectro-photometer (Shimadzu) in the wavelength range of 200e800 nm.
2. Experimental
2.4. Photocatalytic test
2.1. Preparation of the precursor sols
The photocatalytic activity of a-Fe2O3/Bi2MoO6 heterostructures was evaluated by the degradation of methylene blue (MB) under the simulated sunlight irradiation by using a 500 W Xe lamp. For the photodegradation test, 0.1 g Degussa P25, g-Bi2MoO6 and aFe2O3/Bi2MoO6 photocatalysts were suspended in 40 ml MB solution with the initial concentration of 20 mg/L, respectively. The experiments were carried out in a sealed black box and the Xe lamp was placed in a quartz photocatalytic reactor with a circulating water system to cool down the MB solution and prevent thermal catalytic effects. The solution was stirred in the dark for 30 min to obtain a good dispersion and reach the adsorptionedesorption equilibrium between MB and the catalyst surface. At given intervals of 0.5 h, 4 ml reacting solutions were taken out continuously and analyzed. The concentrations of MB in the reacting solutions were analyzed by UV-2550 spectrophotometer.
In a typical experimental process, 2.500 g (11.9 mmol) citric acid was dissolved into 20 ml deionized water. Then, 0.442 g (0.357 mmol) (NH4)6Mo7O24·4H2O, 2.425 g (5 mmol) Bi(NO3)3·5H2O and 4 ml concentrated nitric acid (15 M) were added to the above mentioned solution, respectively. After stirring for 1 h, a homogeneous and clear solution was obtained which was marked as solution 1. Then, 0.505 g (1.25 mmol) Fe(NO3)3·9H2O was dissolved solution 1 and the mixture was marked as solution 2.2 ml solution 2 and 1 ml concentrated HNO3 solution were added to 10 ml absolute ethanol, and this resultant mixture was labeled as solution 3. The molar ratio of Bi3þ and Fe3þ is 4:1. After mixing evenly, 0.8 g PVP (K-90) was dissolved in the solution 3. Then the transparent and homogeneous precursor sols were obtained after stirring for 10 h. 2.2. Fabrication of 1D a-Fe2O3/Bi2MoO6 heterostructures by electrospinning The electrospun precursor sols were transferred into a 20 ml plastic syringe which was connected to a stainless steel needle. The stainless needle with 1 mm inner diameter was connected to 18 kV power supply and the injector was propelled with the speed of 2.27 ml/h. The atmospheric humidity was about 30% and the distance between the needle tip and collector was 15 cm. The ascollected nanofibers were dried in an oven at 70 C for 12 h, and then calcined at 500 C for 2 h to form 1D a-Fe2O3/Bi2MoO6 heterostructures. As a contrast, g-Bi2MoO6 nanofibers could be fabricated as follows: 2.500 g (11.9 mmol) citric acid was dissolved into 20 ml deionized water. Then, 0.442 g (0.357 mmol) (NH4)6Mo7O24·4H2O, 2.425 g (5 mmol) Bi(NO3)3·5H2O and 4 ml concentrated nitric acid (15 M) were added to the above mentioned solution, respectively. After stirring for 1 h, a homogeneous and clear solution was obtained which was marked as solution 1.2 ml solution 1 and 1 ml concentrated HNO3 solution were added to 10 ml absolute ethanol, and then 0.8 g PVP (K-90) was dissolved in the solution to get the Bi2MoO6 precursor sols. The other steps of fabricated g-Bi2MoO6 nanofibers by electrospinning and calcining process were in accordance with those of the preparation of a-Fe2O3/Bi2MoO6 heterostructures.
3. Results and discussion 3.1. TG-DSC curves TG-DSC curves of the electrospun gel nanofibers are displayed in Fig. 1. The TG curve is performed in the temperature range of 30e800 C to investigate the amounts of organics remaining in the precursor nanofibers. It is clear from the TG curve that all the volatile substances (H2O, ethanol), organic components (PVP, citric acid), and NO-3 groups are removed completely below 550 C, which results in the formation of a-Fe2O3/Bi2MoO6 heterojunctions. An initial weight loss (about 15%) step at around 160 C results from
2.3. Characterization The thermogravimetric and differential scanning calorimetry (TG-DSC) curves were obtained by using a thermal analyzer (TGA/
Fig. 1. TG-DSC curves of the gel nanofibers.
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the evaporation of water, residual ethanol and absorbed moisture. The removal of the coordinated water molecules of the nitrates takes place at the range of 160e260 C, which is a slow and small weight loss (~10%) step. The significant weight loss of approximately 38% between 280 and 400 C was attributed to the complete decomposition of nitrates and the degradation of PVP, which involves both intra- and intermolecular transfer reactions [21,22]. The two exothermic peaks at around 313 and 382 C on the DSC curve correspond to the decomposition of citric acid and the degradation of PVP, respectively. The last process from 400 to 550 C accompanied by the exothermic peak at about 464 C on the DSC curve is considered to be due to the continuous decomposition of nitrate. After that the weight of the sample remains constant. The total weight loss amounts to 83%. 3.2. XRD patterns
Fig. 2. XRD patterns of pure g-Bi2MoO6 nanofibers (a) and 1D a-Fe2O3/Bi2MoO6 heterostructures (b) calcined at 500 C for 2 h.
XRD patterns were employed to investigate the crystal structure and the phase composition of the samples. As can be seen in Fig. 2, the gel nanofibers calcined at 500 C for 2 h could get the pure g-
Fig. 3. XPS survey spectra (a) and high-resolution XPS spectra of the Bi 4f (b), Mo 3d (c), Fe 2p (d) and O 1s (e) regions for the a-Fe2O3/Bi2MoO6 heterojunctions.
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Bi2MoO6 phase and the diffraction peaks at 2q ¼ 10.9 , 23.6 , 27.0 , 28.3 , 32.5 , 36.1, 46.7, 47.1, 55.7, 56.3 and 58.4 could be perfectly indexed to (020), (111), (140), (131), (002), (151), (202), (062), (133), (191) and (262) crystal faces of koechlinite Bi2MoO6 (JCPDS 21e0102), respectively. As for the pattern of a-Fe2O3/ Bi2MoO6 heterostructures calcined at 500 C for 2 h (Fig. 2b), the additional diffraction peaks with 2q values of 33.1, 35.5 , 38.8 and 63.1 is corresponding to (103), (110), (113) and (214) crystal planes of orthorhombic phase a-Fe2O3 (JCPDS 33e0664), respectively. It is obvious that a-Fe2O3/Bi2MoO6 heterojunctions can be successfully achieved which integrated the hematite phase a-Fe2O3 nanostructures with the koechlinite Bi2MoO6.
3.3. X-ray photoelectron spectroscopy (XPS) spectra More detailed information regarding the chemical and bonding environment of 1D a-Fe2O3/Bi2MoO6 heterojunctions was ascertained using X-ray photoelectron spectroscopy (XPS). Fig. 3a shows the survey scan spectra of a-Fe2O3/Bi2MoO6 at 0e1100 eV which indicates that C, O, Bi, Mo and Fe elements exist in 1D a-Fe2O3/ Bi2MoO6 heterojunctions. The high resolution XPS spectrum of aFe2O3/Bi2MoO6 heterojunctions in the Bi 4f region is shown in Fig. 3b. The peaks around 159.45 and 164.75 eV are attributed to Bi 4f7/2 and Bi 4f5/2 of Bi3þ, respectively [23]. Fig. 3c shows that the binding energies for Mo 3d5/2 and Mo 3d3/2 of Mo6þ are around
Fig. 4. SEM images: low-magnification (a) and high-magnification (b) of gel nanofibers; low-magnification (c) and high-magnification (d) of 1D a-Fe2O3/Bi2MoO6 heterostructures calcined at 500 C for 2 h and EDS spectra (e) of a-Fe2O3/Bi2MoO6 heterojunctions.
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232.3 and 235.35 eV, respectively [24]. An obvious peak centered at 711 eV is the characteristic peak of Fe3þ shown in Fig. 3d [25]. From Fig. 3e, the O1s peak around 530.1 eV can be attributed to BieO (529.3 eV) and MoeO (530.4 eV) in Bi2MoO6, FeeO (529.9 eV) in Fe2O3 and surface hydroxyl groups (531.1 eV). Moreover, the percentage of Bi, Mo, Fe and O was estimated by XPS to 18.9%, 9.1%, 4.5% and 67.5%, respectively. The ratio of Bi and Fe was about 4:1 in keeping with the original data. The above XRD and XPS results confirmed the coexistence of a-Fe2O3 and Bi2MoO6 in a-Fe2O3/ Bi2MoO6 heterojunctions. 3.4. SEM images of the samples The typical SEM images of the gel nanofibers are shown in Fig. 4. As can be seen from SEM results, the gel nanofibers show the well defined 1D nanostructure with the random distribution. The length of the gel fibers is up to the tens of millimeters, and the diameter is approximately 300e400 nm. The high-magnification SEM image as shown in Fig. 4b indicates that the appearance of the surface of the as-prepared nanofibers is smooth and homogeneous resulting from the existence of PVP. Fig. 4c and d shows SEM images of the heterostructured fibers calcined at 500 C for 2 h. After calcination, the 1D nanostructures still preserve except for a lot of spherical particles existed in the fibers and the diameter of the fibers is around 100e150 nm. The energy-dispersive-spectroscopy (EDS) spectra of the sample given in Fig. 4e show that O, Fe, Bi and Mo elements exist in 1D a-Fe2O3/Bi2MoO6 heterojunctions. 3.5. TEM image of the samples To further investigate the microstructure of 1D a-Fe2O3/ Bi2MoO6 heterojunctions, TEM measurement was performed on the samples. The typical TEM image of nanofibers calcined at 500 C for 2 h is shown in Fig. 5. It can be clearly observed that the surface of the nanofibers is irregular, and bulges are a-Fe2O3 particles. Furthermore, the diameter of the heterostructure nanofibers is about 100e150 nm which is coinciding with the results of the SEM observation. Fig. 6 displays that the formation process of 1D a-Fe2O3/ Bi2MoO6 heterostructures goes through two steps. Firstly, the composite nanofibers are fabricated by the electrospinning process. Then the gel nanofibers are calcined at 500 C for 2 h to form 1D aFe2O3/Bi2MoO6 heterostructures.
Fig. 5. TEM image of a-Fe2O3/Bi2MoO6 samples calcined at 500 C for 2 h.
Fig. 6. Schematic diagrams of electrospinning fabrication of nanofibers and formation a-Fe2O3/Bi2MoO6 composites.
3.6. UVevis diffuse reflectance spectra of samples Fig. 7 shows UVevis diffuse reflectance spectra of the Bi2MoO6 nanofibers and a-Fe2O3/Bi2MoO6 heterojunctions calcined at 500 C for 2 h. From the typical diffuse absorption spectra, the pure Bi2MoO6 sample displays the photoabsorption property from the UV light to visible light region which is shorter than 480 nm, and the band gap of the Bi2MoO6 is estimated to be 2.78 eV (seen the inset of Fig. 7). However, the obtained a-Fe2O3/Bi2MoO6 heterojunction exhibits a wide visible-light absorption in the range of 480e800 nm. The increased light absorption ability of the a-Fe2O3/ Bi2MoO6 heterojunctions can be attributed to the interaction of aFe2O3 and Bi2MoO6 [26]. 3.7. Photocatalytic activity of a-Fe2O3/Bi2MoO6 samples In the present study, MB is chosen as a model contaminant to evaluate the photocatalytic activity of the samples of a-Fe2O3/ Bi2MoO6 heterojunctions, Bi2MoO6 nanofibers and P25, respectively (Fig. 8). The decolorization of MB solution was measured at the wavelength of 664.0 nm. After stirring for 30 min in the dark, the solution reached the adsorptionedesorption equilibrium between MB and the catalyst surface (seen in Fig. 8a) and there was no perceptible degradation of MB after 4 h in the absence of the photocatalysts. As shown in Fig. 8b, after the simulated sunlight
Fig. 7. UVevis diffuse reflectance spectra of Bi2MoO6 (a) and a-Fe2O3/Bi2MoO6 (b) calcined at 500 C for 2 h.
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Fig. 8. (a) Degradation curves of MB in the presence of different photocatalysts in the dark (a) and under the simulated sunlight irradiation (b). The temporal evolution of the spectra during the photodegradation of MB mediated by the Bi2MoO6 nanofibers (c) and a-Fe2O3/Bi2MoO6 heterostructures (d) calcined at 500 C for 2 h.
irradiation for 4 h, the degradation efficiency rate of MB is about 37%, 65.6%, 90.7% and 3.8% for P25, Bi2MoO6, a-Fe2O3/Bi2MoO6 and no photocatalyst, respectively. Obviously, a-Fe2O3/Bi2MoO6 heterojunctions show the highest photocatalytic activity toward degradation of MB under the simulated sunlight irradiation. Based on the above results of the experiment, the photocatalytic mechanism of a-Fe2O3/Bi2MoO6 heterojunctions for the degradation of MB was supposed as illustrated in Fig. 9. The conduction band (CB) and the valence band (VB) of the semiconductor at the point of zero charge can be calculated by the following empirical equation [27]:
ECB ¼ X EC 0:5Eg ; EVB ¼ ECB þ Eg ;
where EC is the energy of free electron on the hydrogen scale (about 4.5 eV), Eg is the band gap of the semiconductor and X is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of all atoms in compounds, which is defined as the arithmetic mean of the first ionization energy and the atomic electron affinity (for Bi2MoO6 and a-Fe2O3, X is 6.13 and 4.78 eV, respectively) [28,29]. So ECB and EVB of g-Bi2MoO6 were calculated to be 0.24 and 3.02 eV, respectively. Similarly, ECB and EVB of a-Fe2O3 with band gap energy of 2.2 eV were 0.82 and 1.38 eV, respectively. The CB edge potential of aFe2O3 is more active than that of g-Bi2MoO6, so a-Fe2O3 could be easily excited by the sunlight and produced photogenerated electronehole pairs. The photogenerated electrons are easy to migrate to the CB of Bi2MoO6 at the interface of the heterostructures. In such a way, the photoinduced electronehole pairs could be effectively separated and could further prevent the recombination of photoelectrons and holes. The efficient charge separation could improve the photocatalytic activity. The processes can be summarized by the following equations:
a-Fe2O3 þ hn / (e) þ a-Fe2O3(hþ)
(1)
(e) þ Bi2MoO6 / Bi2MoO6(e)
(2)
Fig. 9. Postulated photodegradation mechanism of 1D a-Fe2O3/Bi2MoO6 heterostructures under the simulated sunlight irradiation.
Bi2 MoO6 e þ O2 /Bi2 MoO6 þ $O 2
(3)
2$O 2 þ 2H2 O/H2 O2 þ O2 þ 2OH
(4)
OH þ a-Fe2O3 (hþ) / $OH þ a-Fe2O3
(5)
$OH þ MB / degradation products
(6)
When 1D a-Fe2O3/Bi2MoO6 heterojunctions are irradiated by the simulated sunlight, electrons on the CB of a-Fe2O3 could be excited and generated electronehole pairs. Then the excited
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electrons from the CB of a-Fe2O3 could easily immigrate to the CB of Bi2MoO6 because of the lower CB, while the photogenerated holes remain on the VB of a-Fe2O3. Subsequently, the electrons will further react with the O2 in the solution to generate ·O 2 and then react with the H2O to generate OH. Last, the holes on the VB of aFe2O3 will react with OH to produce reactive ·OH radicals which is a strong oxidizing agent to decompose the organic dye. The enhanced photocatalytic performance of a-Fe2O3/Bi2MoO6 is due to the following factors: Firstly, according to UVeVis diffuse reflectance spectra analysis, a-Fe2O3/Bi2MoO6 exhibits the enhanced visible light absorption in the region of 480e800 nm and could absorb more visible light than pure Bi2MoO6 and P25, and thus heterostructures show the better photocatalytic activity. Secondly, the formed heterojunction between a-Fe2O3 and Bi2MoO6 in the heterostructure photocatalysts could further restrain the recombination between photoelectrons and photoholes. As the observation results of SEM and TEM images, there is a close contact of a-Fe2O3 nanoparticles with Bi2MoO6 nanoparticles in the aFe2O3/Bi2MoO6 heterojunctions. It is more effective in constraint of the electronehole recombination to the close contact. Furthermore, Bi2MoO6 could act as the electron traps to facilitate the separation of photogenerated electronehole pairs and promotes interfacial electron transfer process [30]. Therefore, the photoelectrons on the Bi2MoO6 surface and photoholes on the a-Fe2O3 surface could take part in the photocatalytic oxidation reactions to degrade organic contaminants, resulting in the enhanced photocatalytic activity of a-Fe2O3/Bi2MoO6 photocatalysts. 4. Conclusions 1D a-Fe2O3/Bi2MoO6 heterostructures were prepared via the electrospinning and calcination process. The UVevis absorption spectra of the a-Fe2O3/Bi2MoO6 heterostructures took on a wide visible-light absorption and narrow band gap compared with the pure Bi2MoO6. The investigation of the photocatalytic ability indicated that 1D a-Fe2O3/Bi2MoO6 nanofibers showed the higher photocatalytic activity toward degradation of MB than Bi2MoO6 nanofibers and P25 due to the construction of a-Fe2O3/Bi2MoO6 heterojunctions which was in favor of hindering the recombination of electrons and holes under the simulated sunlight irradiation.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
Acknowledgments This work was supported by the Natural Science Foundation of Shandong Province (Grant No. ZR2013BQ001), Project of Independent Innovation of University Institute of Jinan (Grant No. 201311034), Science and Technology Development Plan Project of Shandong Province (2014GGX102039) and Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J13LA01). References [1] M. Gratzel, Photoelectrochemical cells, Nature 414 (2001) 338e344, http:// dx.doi.org/10.1038/35104607. [2] X.B. Chen, L. Liu, P.Y. Yu, S.S. Mao, Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals, Science 331 (2011) 746e750, http://dx.doi.org/10.1126/science. 1200448. [3] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Efficient photochemical water splitting by a chemically modified n-TiO2, Science 297 (2002) 2243e2245, http:// dx.doi.org/10.1126/science. 1075035. [4] S.Y. Reece, J.A. Hamel, K. Sung, T.D. Jarvi, A.J. Esswein, J.J. Pijpers, D.G. Nocera, Wireless solar water splitting using silicon-based semiconductors and earthabundant catalysts, Science 334 (2011) 645e648, http://dx.doi.org/10.1126/ science. 1209816. [5] J.T. Huang, Z.H. Huang, S. Yi, Y.G. Liu, M.H. Fang, S.W. Zhang, Fe-catalyzed growth of one-dimensional a-Si3N4 nanostructures and their
[20]
[21]
[22]
[23]
[24]
[25]
[26]
423
cathodoluminescence properties, Sci. Rep. 3 (2013) 1e9, http://dx.doi.org/ 10.1038/srep03504. X.F. Lu, W. Ce, Y. Wei, One-dimensional composite nanomaterials: synthesis by electrospinning and their applications, Small 5 (2009) 2349e2370, http:// dx.doi.org/10.1002/smll.200900445. M. Shang, W. Wang, J. Ren, S. Sun, L. Zhang, Nanoscale Kirkendall effect for the synthesis of Bi2MoO6 boxes via a facile solution-phase method, Nanoscale 3 (2011) 1474e1476, http://dx.doi.org/10.1039/C0NR00974A. Y. Zhang, Y.K. Zhu, J.Q. Yu, D.J. Yang, T.W. Ng, P.K. Wong, J.C. Yu, Enhanced photocatalytic water disinfection properties of Bi2MoO6-RGO nanocomposites under visible light irradiation, Nanoscale 5 (2013) 6307e6310, http:// dx.doi.org/10.1039/c3nr01338c. M.Y. Zhang, C.L. Shao, P. Zhang, C.Y. Su, X. Zhang, Y.Y. Sun, Y.C. Liu, Bi2MoO6 microtubes: controlled fabrication by using electrospun polyacrylonitrile microfibers as template and their enhanced visible light photocatalytic activity, J. Hazard Mater. 225e226 (2012) 155e163, http://dx.doi.org/10.1016/ j.jhazmat. 2012.05.006. Y.Y. Sun, W.Z. Wang, S.M. Sun, L. Zhang, A general synthesis strategy for onedimensional Bi2MO6 (M¼Mo, W) photocatalysts using an electrospinning method, CrystEngComm 15 (2013) 7959e7964, http://dx.doi.org/10.1039/ C3CE41347K. W.Z. Yin, W.Z. Wang, S.M. Sun, Photocatalytic degradation of phenol over cage-like Bi2MoO6 hollow spheres under visible-light irradiation, Catal. Commun. 11 (2010) 647e650, http://dx.doi.org/10.1016/ j.catcom.2010.01.014. G.H. Tian, Y.J. Chen, W. Zhou, K. Pan, Y.Z. Dong, C.G. Tian, H.G. Fu, Facile solvothermal synthesis of hierarchical flower-like Bi2MoO6 hollow spheres as high performance visible-light driven photocatalysts, J. Mater. Chem. 21 (2011) 887e892, http://dx.doi.org/10.1039/C0JM03040F. T. Zhang, J.F. Huang, S. Zhou, H.B. Ouyang, L.Y. Cao, A.T. Li, Microwave hydrothermal synthesis and optical properties of flower-like Bi2MoO6 crystallites, Ceram. Intern. 39 (2013) 7391e7394, http://dx.doi.org/10.1016/ j.ceramint.2013.02.079. G.P. Li, L.Q. Mao, Magnetically separable Fe3O4-Ag3PO4 sub-micrometre composite: facile synthesis, high visible light-driven photocatalytic efficiency, and good recyclability, RSC Adv. 2 (2012) 5108e5111, http:// dx.doi.org/10.1039/C2RA20504A. Y. Liu, M.Y. Zhang, L. Li, X.T. Zhang, One-dimensional visible-light-driven bifunctional photocatalysts based on Bi4Ti3O12 nanofiber frameworks and Bi2MoO6(X¼Mo, W) nanosheets, Appl. Catal. B: Environ. 160e161 (2014) 757e766, http://dx.doi.org/10.1016/j.apcatb.2014.06.023. M.Y. Zhang, C.L. Shao, J.B. Mu, X.M. Huang, Z.Y. Zhang, Z.C. Guo, P. Zhang, Y.C. Liu, Hierarchical heterostructures of Bi2MoO6 on carbon nanofibers: controllable solvothermal fabrication and enhanced visible photocatalytic properties, J. Mater. Chem. 22 (2012) 577e584, http://dx.doi.org/10.1039/ C1JM13470A. M.Y. Zhang, C.L. Shao, J.B. Mu, Z.Y. Zhang, Z.C. Guo, P. Zhang, Y.C. Liu, Onedimensional Bi2MoO6/TiO2 hierarchical heterostructures with enhanced photocatalytic activity, CrystEngComm 14 (2012) 605e612, http://dx.doi.org/ 10.1039/c1ce05974b. Z.H. Zhang, M.F. Hossain, T. Takahashi, Fabrication of shape-controlled aFe2O3 nanostructures by sonoelectrochemical anodization for visible light photocatalytic application, Mater. Lett. 64 (2010) 435e438, http://dx.doi.org/ 10.1016/j.matlet.2009.10.07. Y.S. Hu, A. Kleiman-Shwarsctein, G.D. Stucky, E.W. McFarland, Improved photoelectrochemical performance of Ti-doped alpha-Fe2O3 thin films by surface modification with fluoride, Chem. Commun. 21 (2009) 2652e2654, http://dx.doi.org/10.1039/b901135h. A. Biabani-Ravandi, M. Rezaei, Z. Fattah, Catalytic performance of Ag/Fe2O3 for the low temperature oxidation of carbon monoxide, Chem. Eng. J. 219 (2013) 124e130, http://dx.doi.org/10.1016/j.cej.2012.12.094. M. Shamshi Hassan, T. Amna, M.S. Khil, Synthesis of High aspect ratio CdTiO3 nanofibers via electrospinning: characterization and photocatalytic activity, Ceram. Intern. 40 (2014) 423e427, http://dx.doi.org/10.1016/ j.ceramint.2013.06.018. Y.C. Miao, G.F. Pan, Y.N. Huo, H.X. Li, Aerosol-spraying preparation of Bi2MoO6: A visible photocatalyst in hollow microspheres with a porous outer shell and enhanced activity, Dyes Pigments 99 (2013) 382e389, http:// dx.doi.org/10.1016/j.dyepig. 2013.05.005. Y.S. Xu, W.D. Zhang, Monodispersed Ag3PO4 nanocrystals loaded on the surface of spherical Bi2MoO6 with enhanced photocatalytic performance, Dalton Trans. 42 (2013) 1094e1101, http://dx.doi.org/10.1039/c2dt31634j. H.P. Li, Q.H. Deng, J.Y. Liu, W.G. Hou, N. Du, R.J. Zhang, X.T. Tao, Synthesis, characterization and enhanced visible light photocatalytic activity of Bi2MoO6/Zn-Al layered double hydroxide hierarchical heterostructures, Catal. Sci. Technol. 4 (2014) 1028e1037, http://dx.doi.org/10.1039/ c3cy00940h. D.D. Hu, K.Y. Zhang, Q. Yang, M.J. Wang, Y. Xi, C.G. Hu, Super-high photocatalytic activity of Fe2O3 nanoparticles anchored on Bi2O2CO3 nanosheets with exposed {001} active facets, Appl. Surf. Sci. 316 (2014) 93e101, http:// dx.doi.org/10.1016/j.apsusc.2014.07.185. G.K. Mor, H.E. Prakasam, O.K. Varghese, K. Shankar, C.A. Grimes, Vertically oriented Ti-Fe-O nanotube array films: toward a useful material architecture for solar spectrum water photoelectrolysis, Nano Lett. 7 (2007) 2356e2364, http://dx.doi.org/10.1021/nl0710046.
424
J. Zhao et al. / Journal of Alloys and Compounds 646 (2015) 417e424
[27] H.W. Huang, L.Y. Liu, Y.H. Zhang, N. Tian, One pot hydrothermal synthesis of a novel BiIO4/Bi2MoO6 heterojunction photocatalyst with enhanced visiblelight-driven photocatalytic activity for rhodamine B degradation and photocurrent generation, J. Alloy Compd. 619 (2015) 807e811, http://dx.doi.org/ 10.1016/j.jallcom.2014.08.262. [28] Y.L. Tian, F.X. Cheng, X. Zhang, F. Yan, B.C. Zhou, Z. Chen, J.Y. Liu, F.N. Xi, X.P. Dong, Solvothermal synthesis and enhanced visible light photocatalytic activity of novel graphitic carbon nitride-Bi2MoO6 heterojunctions, Powder Technol. 267 (2014) 126e133, http://dx.doi.org/10.1016/ j.powtec.2014.07.021.
[29] Y.D. Guo, G.K. Zhang, J. Liu, Y.L. Zhang, Hierarchically structured a-Fe2O3/ Bi2WO6 composite for photocatalytic degradation of organic contaminants under visible light irradiation, RSC Adv. 3 (2013) 2963e2970, http:// dx.doi.org/10.1039/c2ra22741j. [30] B. Yuan, C.H. Wang, Y. Qi, X.L. Song, K. Mu, P. Guo, L.T. Meng, H.M. Xi, Decorating hierarchical Bi2MoO6 microspheres with uniformly dispersed ultrafine Ag nanoparticles by an in situ reduction process for enhanced visible lightinduced photocatalysis, Colloids Surf. A: Physicochem. Eng. Asp. 425 (2013) 99e107, http://dx.doi.org/10.1016/j.colsurfa.2013.02.058.