J Nanopart Res (2016) 18:236 DOI 10.1007/s11051-016-3555-2
RESEARCH PAPER
Electrospinning synthesis of InVO4/BiVO4 heterostructured nanobelts and their enhanced photocatalytic performance Zhendong Liu . Qifang Lu . Enyan Guo . Suwen Liu
Received: 21 April 2016 / Accepted: 3 August 2016 Ó Springer Science+Business Media Dordrecht 2016
Abstract In the present work, one-dimensional InVO4/BiVO4 heterostructured nanobelts with the width of about 800 nm have been successfully prepared by a simple electrospinning technique followed by the subsequent calcination process. The prepared products were characterized by thermogravimetry, fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, UV–Vis absorbance spectroscopy, highperformance liquid chromatography, and photoluminescence spectroscopy. The obtained InVO4/BiVO4 heterostructured nanobelts presented an admirable morphology and excellent photocatalytic properties for the degradation of methylene blue solution under visible-light irradiation. Keywords Electrospinning InVO4/BiVO4 Heterostructure Nanobelt Heterojunctions
Electronic supplementary material The online version of this article (doi:10.1007/s11051-016-3555-2) contains supplementary material, which is available to authorized users. Z. Liu Q. Lu (&) E. Guo S. Liu Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass and Functional Ceramics, School of Material Science and Engineering, Qilu University of Technology, Jinan 250353, People’s Republic of China e-mail:
[email protected]
Introduction As the energy shortages and environmental crises become the greatest challenges in the twenty-first century, the semiconductor photocatalysis has become a focused research due to its potential applications and advantages, such as the transformation from solar energy into electrical energy, photochemical water splitting, and degradation of organic pollutants (Abdi et al. 2013; Bach et al. 1998; Fujishima and Honda 1972; Liu et al. 2015; Yue et al. 2015). As a new type of semiconductor photocatalyst, monoclinic BiVO4 has attracted extensive interests due to its narrow band gap (2.4 eV) and excellent photocatalytic efficiency (Sun et al. 2014). However, there is still plenty of room for improving the photocatalytic efficiency of BiVO4-based photocatalysts (Cao et al. 2012). It is plausible to construct the heterojunctions in order to reduce the recombination of photoexcited electron–hole pairs and improve the photocatalytic properties (Liu and Kang 2016). The combination of two semiconductors with different energy levels may form an ideal system with rapid photoinduced charge separation and decrease the recombination rate of electron–hole pairs by the synergetic effect (Liu and Kang 2016). Therefore, many BiVO4-based heterojunctions have been prepared, such as g-C3N4/BiVO4 (Zhang et al. 2015a), BiOCl/BiVO4 (He et al. 2014), WO3/BiVO4 (Su et al. 2011), and TiO2/BiVO4 (Zhang et al. 2013).
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InVO4, an important functional material, has received great interest because of its potential applications in various fields (Orel et al. 2001; Wang and Cao 2007). There are two polymorphic forms of InVO4: monoclinic and orthorhombic phases, and between them, the orthorhombic InVO4 with a narrow band gap (Eg = 2.0 eV) owns the better visible-light-driven photodegradation of organic pollutants (Song et al. 2012). At the same time, the energy levels of InVO4 and BiVO4 are well matched, which makes InVO4 another suitable material for constructing the InVO4/BiVO4 heterojunctions (Guo et al. 2015; Lin et al. 2015). As is known to all, the morphology and structure of the semiconductor play an important role in the physical and chemical properties of the photocatalysts (Yu and Kudo 2006). Among the various morphologies, onedimensional (1D) nanostructures with long axial ratio and high specific surface area have drawn much attention due to their improved performance in photocatalysis (Jung et al. 2016), and the electrospinning technique is precisely a new cheap and simple method to obtain 1D nanomaterials (Peng et al. 2016). In the present paper, InVO4/BiVO4 heterostructured nanobelts have been synthesized by a simple electrospinning method, and the photocatalytic activity tests for methylene blue (MB) degradation have also been performed in detail.
Experimental section Preparation of the spinnable precursor sols All chemicals and reagents used were of analytical reagent (AR) grade and were used without further purification. In a typical experiment, 0.097 g (0.2 mmol) Bi(NO3)35H2O, 0.0764 g (0.2 mmol) In(NO3)34.5H2O, 0.0468 g (0.4 mmol) NH4VO3, and 0.4 g citrate acid were dissolved in 3 mL deionized water in a 20-mL glass vessel, and then 0.5 mL HCl (conc. 37.5 wt%) was added dropwise, accompanied by vigorous magnetic stirring for 20 min at room temperature to obtain solution A. Meanwhile, solution B was prepared by mixing 1 g polyvinylpyrrolidone (PVP, K-90) with 10 mL anhydrous ethanol. The PVP–Bi(NO3)35H2O–In(NO3)34.5H2O–NH4VO3 precursor sols were obtained by mixing the transparent solution A with solution B and then stirring for 6 h at room temperature.
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Electrospinning and calcination process of the samples The precursor sols were loaded into a plastic syringe with a stainless steel nozzle (0.5 mm diameter). The solution feed rate was set at 2.26 mL/h, which was controlled by a syringe pump. The metallic needle clamped with an electrode was connected to a variable high-voltage power supply, and an aluminum foil collector was used as a grounded counter electrode 30 cm away from the needle tip. The PVP–Bi(NO3)35H2O–In(NO3)34.5H2O–NH4VO3 composite microbelts were formed at a high voltage of 20 kV. The entire process proceeded at room temperature and room humidity. For the following thermolysis process, the electrospun samples were heated to 550 °C at the rate of 1 °C/min and then kept in a muffle furnace for a soaking time of 2 h. The final products were obtained after cooling to room temperature naturally. Characterization Thermogravimetry (TG) analysis was performed using a simultaneous thermal analyzer (Labsys evo STA, France) under the air atmosphere in the range of 45–800 °C. Fourier transform infrared spectroscopy (FT-IR) spectra were measured with a FT-IR spectrometer (Bruker Vertex 70, Germany) in the range of 400–4000 cm-1. The X-ray diffraction (XRD) patterns of the samples were measured on an X-ray diffractometer (Bruker D8 Advance, Germany), using monochromatized Cu-Ka (k = 0.15418 nm) radiation with a scan range from 10° to 70°. The morphologies and microstructures of the obtained nanobelts were analyzed by field emission scanning electron microscope (SEM, Hitachi S-4800, Japan) and transmission electron microscope (TEM, JEOL JEM-2010F, Japan). The UV–Vis spectra of the samples were recorded on a UV–Vis spectrophotometer (Shimadzu UV-2550, Japan) in the wavelength range of 200–800 nm. High-performance liquid chromatography (HPLC) curves were recorded using C18 column equipped with a dual-wavelength PDA detector (Shimadzu, Japan). The flow rate was set at 1 mL/min and the injection volume was 15 lL. The detection wavelength was set at 650 nm. Photoluminescence (PL) spectra were collected on an FLS-920 spectrometer (Edinburgh Instrument) using a Xe lamp (excitation at 350 nm) as the light source.
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Photocatalytic degradation of MB under visiblelight irradiation As an example, the photocatalytic activities of the products were evaluated by the photodegradation of MB solution under visible-light irradiation using a 500 W Xe lamp equipped with cutoff filters (k C 400 nm) at room temperature. The experiments were performed in a sealed block box and Xe lamp was placed in a quartzose cold hydrazine with a circulating water system to cool down the MB solution and prevent the thermal catalytic effects. The distance between Xe lamp and MB solution was 100 mm. 0.1 g of photocatalysts were added into 100 mL MB solution (20 mg/L). Prior to irradiation, the suspension was magnetically stirred in the dark for 30 min to achieve the adsorption–desorption equilibrium under room conditions, and then the photodegradation experiment was started. 2 mL mixed solution was sampled at a given irradiation time interval (30 min) and centrifuged to remove the photocatalysts for analysis. The filtrates were analyzed by UV–Vis absorbance spectroscopy and HPLC.
Results and discussion TG curve of the electrospun precursor sample was recorded in the temperature range of 45–800 °C, and there exist four weight loss steps as shown in Fig. 1a. The first stage (ca. 5 %) below 100 °C can be mainly attributed to the evaporation of trapped ethanol and absorbed water (Du et al. 2006). The second weight loss step (ca. 25 %) from 100 to 280 °C is due to the removal of crystal water of the nitrates and decomposition of citric acid. The weight loss of approximately 49 % from 280 to 440 °C is ascribed to the decomposition of nitrate and PVP side chains (Du et al. 2006; Liu et al. 2015). The final weight loss of ca. 15 % in the range of 440–530 °C results from the release of the oxidation of carbon and carbon monoxide from the thorough decomposition of the main polymer chain of PVP (Du et al. 2006). The total weight loss amounts to 94 %. Figure 1b shows the FT-IR spectra of the electrospun precursors and different samples calcined at 550 °C for 2 h. For the electrospun precursor samples (gel), the peaks from 950 to 3100 cm-1 are down to the characteristic vibration of the organics (PVP and
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citrate acid). The peaks at 2960, 1660, 1460, and 1240 cm-1 are due to the vibration of C–H, C=O, C=C, and C–N bonds, respectively (Peuravuori et al. 2005). As is shown in the FT-IR spectrum of InVO4/ BiVO4 heterostructured nanobelts, the organics disappeared from the calcined samples. The peak at about 460 cm-1 is assigned to the V–O–V band and the strong peak at 731 cm-1 indicates the existence of VO43- groups (Liu et al. 2014; Zhang et al. 2015b). The vibration at 845 cm-1 is due to Bi–O bond, while the vibration peak at about 415 cm-1 can be attributed to the overlap of Bi–O cm-1 at 410 and In–O at 420 cm-1 (Liu et al. 2015). And doublet peaks at 900 and 950 cm-1 correspond to the characteristic vibration of V–O–In and V–O, respectively (Yao et al. 2009). XRD patterns of BiVO4 microbelts and InVO4/ BiVO4 heterostructured nanobelts are provided in Fig. 2. The characteristic peaks at 19.0°, 29.0°, 30.6°, 42.5°, and 53.3° are attributed to (011), (121), (040), (051), and (161) planes of monoclinic BiVO4 (JCPDS No: 14-0688), respectively (Fig. 2a). The peaks of InVO4 (JCPDS No: 48-0898) are also marked and assigned to (020), (200), (112), (130), (222), and (150) planes as shown in Fig. 2b. It is obvious from the XRD pattern that the InVO4/BiVO4 heterostructured nanobelts are composed of monoclinic BiVO4 and orthorhombic InVO4 and no diffraction peaks from the other impurities are observed. XPS spectra of the samples are presented to determine the oxidation state and elemental composition for each member of the heterojunctions as shown in Fig. 3. It is indicated that InVO4/BiVO4 nanobelts are composed of Bi, In, V, and O elements as illustrated by the full-scale XPS spectrum of Fig. 3a (Ai et al. 2010; Li et al. 2015; Yang et al. 2015). As shown in Fig. 3b, the XPS signals of Bi 4f observed at 164.4 (Bi 4f5/2) and 158.9 eV (Bi 4f7/2) for InVO4/ BiVO4 nanobelts are slightly shifted to high binding energy, compared to the Bi 4f in BiVO4 peak positions, indicating the strong chemical bonds of Bi3? in InVO4/BiVO4 nanobelts (Lamdab et al. 2015). The In 3d peaks (Fig. 3c) are centered at 452.1 and 444.1 eV, which correspond to In 3d3/2 and 3d5/2 signals of In3? species in InVO4/BiVO4 nanobelts, respectively (Ai et al. 2010). The doublet peaks of V 2p1/2 and V 2p3/2 in InVO4/BiVO4 nanobelts are observed at 524.5 and 517 eV in Fig. 3d, respectively (Li et al. 2015). The asymmetric XPS lines of O 1s
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Fig. 1 TG curve of the gel sample (a), FT-IR spectra (b), and the magnified sections from 400 to 1100 cm-1 (c) of the electrospun precursor and different samples calcined at 550 °C for 2 h
Fig. 2 XRD patterns of BiVO4 microbelts (a) and InVO4/ BiVO4 heterostructured nanobelts at 550 °C for 2 h (b)
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(Fig. 3e) are fitted with the lattice oxygen (BE = 530 eV) and adsorbed oxygen (BE = 530.4 eV) (Lamdab et al. 2015; Li et al. 2015). The above results demonstrate that the heterojunctions of BiVO4 and InVO4 can be formed via the electrospinning process. The morphological features and the microstructure of InVO4/BiVO4 heterostructured nanobelts are shown in Fig. 4. Figure 4a, b shows the typical SEM images of the electrospun precursor samples at different magnifications. It is clearly seen that the samples have the well-defined 1D belt structure with a smooth surface due to the amorphous nature of the precursors and are relatively uniform with the width of about 1 lm. After calcined at 550 °C for 2 h, as shown in Fig. 4c, d, the samples can retain well the 1D morphology with the width of approximately 800 nm
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Fig. 3 XPS survey spectra of the samples (a), Bi 4f (b), In 3d (c), V 2p (d), and O 1 s (e)
and the thickness of about 200 nm, respectively. Furthermore, the surface of the nanobelts becomes quite rough because of the decomposition of organics and crystallization of InVO4/BiVO4 nanobelts (Cheng et al. 2013), and a porous structure can be observed in Fig. 4d. The microstructure of the InVO4/BiVO4 nanobelts was further investigated by TEM. As illustrated in Fig. 4e, f, InVO4/BiVO4 nanobelts calcined at 550 °C for 2 h display an obviously porous structure, which may be due to the release of more gas during the removal of organics and the decomposition of inorganic salts (Cheng et al. 2013). The representative N2 adsorption and desorption isotherms and the
corresponding BJH pore size distribution curve of the InVO4/BiVO4 nanobelts calcined at 550 °C for 2 h are displayed in Fig. S2, and the most probable pore size is about 60 nm. This kind of porous structure could be advantageous to improve the specific surface area, and a high specific surface area can provide more reactionactive sites which could be more beneficial to improve the photocatalytic reactivity due to the facilitating molecular transport of reactants and products (Cheng et al. 2013). In order to further investigate the composition of InVO4/BiVO4 heterostructures, the EDS mappings which can indicate intuitively the corresponding elements’ distribution in the
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Fig. 4 SEM images of electrospun precursor nanobelts (a, b) and InVO4/BiVO4 nanobelts calcined at 550 °C for 2 h (c, d). TEM images of InVO4/BiVO4 nanobelts calcined at 550 °C for 2 h (e, f)
heterostructured nanobelts are shown in Fig. 5. As shown in Fig. 5b–f, the elements such as Bi, In, V, and O are uniformly embedded in the nanobelts, and high degree of combination of each other. UV–Vis diffuse reflectance spectra of BiVO4 microbelts and InVO4/BiVO4 nanobelts are displayed in Fig. 6. The samples exhibited the photoresponse property in the visible-light region as can be clearly observed, indicating that it is quite easy to stimulate the photocatalyst to generate the electron–hole pairs under visible-light irradiation and consequently degrade the organic contaminant. Based on UV–Vis diffuse reflectance spectra, the band gap can be calculated by a classical Tauc approach according to the following equation: Eg = 1240/k, where Eg and k are the band gap energy and the absorption edge, respectively (Xu et al. 2016). As can be seen in Fig. 6, the absorption edge of BiVO4 microbelts and InVO4/
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BiVO4 nanobelts is 519 and 525 nm, and their corresponding Eg can be calculated as 2.39 and 2.36 eV, respectively. The photocatalytic activities and degradation abilities of the samples were evaluated by the degradation of MB solution under visible-light irradiation. Figure 7a, b reveals the temporal evolution of the absorption spectra at a time interval of 0.5 h using BiVO4 microbelts and InVO4/BiVO4 nanobelts as the photocatalysts, respectively. As time goes on, MB concentration is reduced gradually and an obvious blue shift can be observed in the spectra which may result from N-demethylation of MB, and oxidative degradation occurred concomitantly (Ferreira et al. 2000; Zhang et al. 2001). Figure 7c intuitively demonstrates the photodegradation efficiency of the MB. Here, C0 and C are the concentrations of MB initially and at each time interval during irradiation,
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Fig. 5 SEM image (a) and EDS mapping (b–f) of InVO4/BiVO4 heterostructured nanobelts
Fig. 6 UV–visible diffuse reflectance spectra of BiVO4 microbelts and InVO4/BiVO4 heterostructured nanobelts
respectively. As shown in Fig. 7c, the corresponding decomposition rate of InVO4/BiVO4 nanobelts is 85.6 % under visible-light irradiation for 3.5 h which is much higher than that of BiVO4 microbelts (73.9 %). The photodegradation reaction of MB can be described by the first-order reaction kinetics: ln(C0/ C) = kt ? a, where k is the first-order rate constant, t is the irradiation time, and a is a constant (Cheng et al. 2013). A high k value usually implies that the reaction rate of photocatalysts is fast. The calculated k values are 0.40 and 0.57 h-1 for the BiVO4 microbelts and InVO4/BiVO4 nanobelts, respectively, as displayed in Fig. 7d. The results indicate that the InVO4/BiVO4 heterostructured nanobelts exhibit the higher photocatalytic activity than BiVO4 microbelts. Figure 8a shows five-cycle photocatalytic degradation of MB
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Fig. 7 Temporal evolution of the spectra during photodegradation of MB mediated by BiVO4 microbelts (a) and InVO4/ heterostructured nanobelts (b). Different BiVO4
photodegradation processes (c) and kinetic linear simulation curves (d) of MB photocatalytic degradation
Fig. 8 Photocatalytic reduction of MB solution using InVO4/ BiVO4 nanobelts calcined at 550 °C for 2 h as the photocatalyst under visible-light illumination for five cycles (a), a typical
SEM image (b), and XRD pattern (c) of the InVO4/BiVO4 nanobelts after five cycles for the degradation of MB
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Fig. 9 HPLC curves of the corresponding irradiation time using InVO4/BiVO4 nanobelts as a photocatalyst under visiblelight irradiation
Fig. 10 PL spectra of BiVO4 microbelts and InVO4/BiVO4 nanobelts
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solution in the presence of InVO4/BiVO4 nanobelts calcined at 550 °C for 2 h under visible-light illumination, and the photodegradation ratio of the five runs reaches 85.6, 85.2, 84.5, 84.4, and 84.1 %, respectively. In addition, the typical SEM image (Fig. 8b) and XRD pattern (Fig. 8c) of the sample after five runs of photocatalytic degradation of cycling experiments are shown. The results demonstrate that the photocatalysts can retain the 1D structures and phase composition after the degradation of MB, suggesting that the InVO4/BiVO4 photocatalysts are reused for long-term application. The HPLC experiment was performed to further study the degradation of MB solution. As shown in Fig. 9, MB solution shows a single peak at 4.718 min, initially. The area of the peaks decreases with increasing the visible-light irradiation time. In addition, a new peak appears at 4.253 min, which indicates that MB is gradually decomposed during the photocatalytic degradation process and an intermediate product arised from the N-demethylation of MB and oxidative degradation (Ferreira et al. 2000; Wang et al. 2011). It is the reason why UV–Vis absorption spectrum of MB solution presents the blue shift during the photocatalytic degradation process (Ferreira et al. 2000; Zhang et al. 2001). PL spectroscopy measurements of BiVO4 microbelts and InVO4/BiVO4 nanobelts were carried out to explain the effective separation of photogenerated electron–hole pairs (Guo et al. 2015). As displayed in Fig. 10, BiVO4 microbelts exhibit a strong mission at about 530 nm, which is consistent with the prvious reports (Lamdab et al. 2015). However, the InVO4/
Fig. 11 Photocatalytic mechanism description of InVO4/BiVO4 heterostructured nanobelts
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BiVO4 nanobelts present much lower PL intensity than BiVO4 microbelts, which implied that InVO4/ BiVO4 nanobelts suppress the recombination of electron–hole pairs (Lamdab et al. 2015; Nong et al. 2015). A photocatalytic mechanism of InVO4/BiVO4 heterostructured nanobelts is shown in Fig. 11. Under visible-light irradiation, both BiVO4 and InVO4 could be excited to generate the electrons and holes. The CB of BiVO4 is more positive than that of InVO4, while the VB of InVO4 is more negative than that of BiVO4 (Lamdab et al. 2015). Hence, there is a greater tendency to transfer electrons from InVO4 to BiVO4 and holes from BiVO4 to InVO4. Thus, the successful construction of 1D InVO4/BiVO4 heterostructures inhibits the recombination of electron–hole pairs and then enhances the photocatalytic activity (Guo et al. 2015; Lamdab et al. 2015).
Conclusion In conclusion, 1D InVO4/BiVO4 heterostructured nanobelts were successfully fabricated by a simple electrospinning process. The construction of heterostructures and porous structure of the nanobelts were in favor of hindering the recombination of photogenerated electron–hole pairs, so InVO4/BiVO4 heterostructured nanobelts showed the enhanced photocatalytic efficiency to degrade the MB solution under visible-light irradiation. This work could exploit a simple method to synthesize high-efficiency heterostructured photocatalysts. Acknowledgments This work was supported by Project of Independent Innovation of University Institute of Jinan (Grant No. 201311034) and Science and Technology Development Plan Project of Shandong Province (2014GGX102039).
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