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Fabrication and characterization of electrospun orthorhombic InVO4 nanofibers
Applied Surface Science 258 (2012) 3789–3794
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication and characterization of electrospun orthorhombic InVO4 nanofibers Lingjun Song, Suwen Liu ∗ , Qifang Lu, Gang Zhao Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics, Shandong Polytechnic University, Jinan 250353, PR China
a r t i c l e
i n f o
Article history: Received 10 September 2011 Received in revised form 20 November 2011 Accepted 6 December 2011 Available online 13 December 2011 Keywords: Orthorhombic InVO4 Electrospinning technique Sintering Nanofibers
a b s t r a c t The novel orthorhombic InVO4 nanofibers have been successfully synthesized by annealing electrospun precursor fibers. Citric acid was used as a ligand for it could react with metal salts to get a transparent homogeneous precursor solution and homogeneous precursor sol for electrospining. Polyvinyl pyrrolidone (PVP, K-30) was used as a binder and a structure guide reagent because it was one kind of water-soluble polymers. It is easy to gain one-dimensional materials while the viscosity of the citrate/PVP sol was suitable. The structure, morphology and photocatalytic properties of the nanofibers were characterized by X-ray diffraction (XRD), thermogravimetry analysis (TGA), scanning electron microscopy (SEM) analysis, UV–vis spectrophotometer and fluorescence spectrophotometer. The nanofibers calcined at 700 ◦ C were orthorhombic InVO4 with a width in the range of 30–100 nm and length in micron-grade. This one-dimensional pure orthorhombic InVO4 had the higher photocatalytic activity under visible light irradiation. The photo-degradation rate of nitrobenzene aqueous solution under visible light reached 69% after 6 h. It is obvious that the orthorhombic InVO4 nanofibers have a potential application in wastewater-treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, InVO4 has become a promising photo-catalyst with a narrow band gap (Eg = 2.0 eV), which is able to induce hydrolysis of water molecules under visible-light irradiation [1–3]. InVO4 belongs to a large family of orthovanadate compounds with a general formula M3+ VO4 (M3+ = In, Fe, Cr, Al, rare earths) [4,5] which has two phases: the stable high-temperature orthorhombic InVO4 -III (Cmcm) phase and low-temperature metastable monoclinic InVO4 I phase [6,7]. The structure of InVO4 is composed of chains of the InO6 octahedral linked together by the VO4 tetrahedral [8], and the only difference is that the structure of monoclinic InVO4 -I phase consists of compact In4 O16 which groups of four edge-shared InO6 octahedra linked to each other by VO4 tetrahedra and the orthorhombic InVO4 is composed of chains of InO6 octahedra which are linked together by VO4 tetrahedra [9]. As a new type of semiconductor, orthorhombic InVO4 has also attracted considerable interests for its special photocatalytic properties [4,10]. One-dimensional nanostructures, such as nanowires, nanofibers, and nanobelts are expected to play an important role due to their potential applications in nanodevices. Various one-dimensional nanostructured materials have been fabricated by a variety of methods, including templating direction [11], solid-state reaction method [12], hydrothermal treatment [13],
∗ Corresponding author. Tel.: +86 0531 89631632; fax: +86 0531 89631227. E-mail address: [email protected] (S. Liu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.12.029
etc. Besides, electrospinning is a unique fiber spinning process because it can make fibers with a diameter of 50–500 nm [14]. Single-crystalline InVO4 nanotubes produced by annealing electrospun precursor fibers have been first reported by Yi and Li [15]. To the best of our knowledge, other morphological InVO4 nano-products synthesized by electrospinning technology have not been reported. So, it is very interesting and essential to prepare the InVO4 nanoscale one-dimensional materials via the electrospinning route. In the present work, the electrospinning technique was used to prepare citrate/PVP composite fibers. Orthorhombic InVO4 nanofibers were obtained by calcining the precursor nanofibers above 700 ◦ C. 2. Experiment 2.1. Synthesis procedure NH4 VO3 (99.0%) was bought from Shanghai Chemical Reagent Co. Ltd., ethanol of analytical grade was bought from Tianjin Guangcheng Chemical Reagent Co. Ltd. In(NO3 )3 ·4.5H2 O with a purity of 99.5% and Polyvinyl pyrrolidone (PVP (K30), Mw = 4.0 × 104 ) were purchased from Sinopharm Chemical Reagent Co. Ltd. A schematic of the electrospinning process is shown in Fig. 1. In a typical experiment for the preparation of the electrospinning solution, 2 mmol In(NO3 )3 ·4.5H2 O was dissolved into 10 mL distilled water under magnetic stirring to form a colorless transparent
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Fig. 1. The preparation process of InVO4 samples.
solution. Then 2 mmol NH4 VO3 was added into the above solution to form an orange suspension. 2 mmol citric acid used as a chelating agent was gradually added into the beaker under continuous stirring until the suspension turned into a yellow green transparent precursor solution (recorded as precursor A). 2 g PVP (K-30) was dissolved in 5 mL ethanol (recorded as precursor B). Then 3 mL precursor solution A was transferred to precursor B and the mixture was further stirred for 10 min to form a viscous hydrosol for electrospinning. Here, PVP (K-30), one kind of watersoluble polymers, was used as a binder and a structure guiding reagent. Besides, another precursor A without citric acid was prepared by adding In(NO3 )3 solution into hot NH4 VO3 solution directly while all other things being equal. The precursor sol was drawn into a syringe connected with a stainless steel capillary with inner diameter of 0.40 mm and outer diameter of 0.60 mm. The positive terminal of a variable highvoltage power supply (BGG-200 kV/20 mA) was connected to the needle tip of the capillary while the other was connected to the collector plate. During the electrospinning, the applied voltage was kept at 25 kV and the distance between spinneret and collector was optimized and around 28 cm. The feeding rate of the solution was kept at 0.5 mL/h at room temperature. When the spinning was completed, the as-prepared precursor composite fibers were collected by a forcep, dried at 80 ◦ C for 12 h. And then all gel fibers were put into an air-atmosphere programmable tube furnace for heat treatment according to the TG result. The fibers were fired from room temperature to desired temperatures at a rate of 1 ◦ C/min with a hold time of 1 h. The products were naturally cooled to room temperature in the furnace to obtain the resulting InVO4 nanofibers. Then the calcined nanofibers were given characterization.
characterized with a field emission scanning electron microscopy (Hitachi FESEM-4800, Japan, working voltage of 10 kV and operating distance of 7.7 mm). TG analysis of composite fibers was measured with a thermal analyzer (TGA/SDTA 851, Mettler) at an O2 flow rate of 20 mL/min, using a heating rate of 20 ◦ C/min in air. The purity of O2 was 99.5%. UV–vis diffuse reflectance spectra and the photodegradation decoloring rate of the samples were measured using a Shimadzu UV-2550PC spectrophotometer. The photocatalytic activities of the as-synthesized samples were determined through the degradation of nitrobenzene (NB). The formation of hydroxyl radicals (• OH) was detected by the photoluminescence (PL) spectra which were measured on a Hitachi F-4500 fluorescence spectrophotometer. 0.12 g photocatalysts were added in 30 mL NB solution with a concentration of 20 mg/L under stirring, using a 500 W Xenon lamp with a UV-cutoff filter ( ≤ 400 nm) as the light source. Took 4 mL solution out once every one hour, centrifuged and measured the absorbance at the maximum absorption wavelength ( = 268 nm) of the NB. The decoloring ratio of the NB was estimated according to the equation: = [(A0 –At )/A0 ] × 100%, where is the degradation ratio of NB, A0 is the initial absorbency, and A is the absorbency at a certain time. Additionally, the photodegradation ratio of NB solution without photocatalyst was also measured under the same condition to estimate the influence of the photodegradation over the organic compounds. The detection experiment process of the • OH is similar to the photodegradation experiment, with the exception of a basic terephthalic acid solution instead of NB solution. Terephthalic acid (5 × 10−4 M) was dissolved in NaOH solution (2 × 10−3 M). The sampling was carried out every 10 min, and the withdrawn solution was measured after centrifugation. The product of terephthalic acid hydroxylation can give a characteristic fluorescence peak at about 425 nm by excitation with the wavelength of 315 nm. 3. Results and discussion 3.1. TG analysis Fig. 2 shows the typical thermal behavior of the precursor composite nanofibers. The TG curve indicates four different weight loss steps with the increase of temperature in the whole combustion process. The first weight loss (∼16.48%) occurred before 190 ◦ C was mostly attributed to the gradual evaporation of free
2.2. Characterization The phase and crystallinity of the InVO4 products were examined by X-ray diffraction (XRD, Rigaku D/Max 2200PC, Germany) ˚ at a scanning rate of 0.02◦ /s with Cu K␣ radiation ( = 1.5148 A) in the range of 20–60◦ , and the accelerating voltage and the applied current were 28 kV and 20 mA, respectively. The morphology and microstructure of the orthorhombic InVO4 samples were
Fig. 2. The TG curve of the precursor composite fibers.
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49.05%. These weight losses were caused by the decomposition of the citric acid complex [16] and polyvinyl pyrrolidone, the oxidation of carbon and carbon monoxide released by the decomposition of PVP [17,18]. All organic components could have been eliminated at 550 ◦ C because there was no further weight loss.
3.2. XRD patterns analysis
Fig. 3. The XRD patterns of the samples calcined at different temperatures: (a) 550 ◦ C, (b) 600 ◦ C, (c) 700 ◦ C for 1 h.
water and crystal water. The region from 190 to 265 ◦ C with a continuously small weight loss of 5.27% could be ascribed to the decomposition and the complete combustion of nitrate radical. The sharp decrease of the weight was separated into two parts: the region from 265 to 350 ◦ C with weight loss of 25.49% and the region from 350 to 550 ◦ C with weight loss of
According to the TG results, all organic components had been eliminated before 550 ◦ C, so the dry composite fibers were calcined above 550 ◦ C. Fig. 3 presents the XRD patterns of the InVO4 calcined at various temperatures. XRD pattern of the product calcined at 550 ◦ C for 1 h is shown in Fig. 3a. All the diffraction peaks can be indexed to InVO4 with a monoclinic phase (JCPDS card number 481016, a = 1.0271 nm, b = 0.9403 nm, c = 0.7038 nm). Combined with the TG analysis, it can be inferred that the liberated heat resulted from the combustion of the citric acid complex and polyvinyl pyrrolidone provided energy for the crystallinity phase transformation from amorphous to monoclinic. As shown in Fig. 3b, it is obvious that the intense peaks at 2 = 31.0◦ , 33.0◦ and 35.1◦ belong to the orthorhombic phase. The orthorhombic InVO4 phase coexists with the monoclinic phase when the products calcined at 600 ◦ C. This monoclinic phase converted completely to the orthorhombic phase InVO4 (JCPDS card no.48-0898, as shown in Fig. 3c) when the annealing temperature reached 700 ◦ C, which is almost in agreement with Touboul and Popot’s results [19]. The average crystallite size of the as-prepared pure orthorhombic InVO4 nanofibers estimated by the Scherrer formula is about 61.97 nm.
Fig. 4. SEM images of the electrospun citrate/PVP composite nanofibers (a and b), novel orthorhombic InVO4 nanofibers sintered at 700 ◦ C for 1 h (c) and orthorhombic InVO4 nanoparticles obtained by annealing precursor composite without citric acid at 700 ◦ C (d) for 1 h.
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reagent is extremely effective. During the calcining process, an amorphous InVO4 would be formed when the contiguous ions of indium and vanadium in the precursor reacted with each other. Followed by the continual calcination, the amorphous InVO4 particles started to crystallize and grow. Known from the XRD results, the phase of the InVO4 nanofibers in Fig. 4c was orthorhombic phase. Even though PVP (K-30) in the fiber was decomposed with the diameter reduction of the fiber, the continuous microstructure of the fiber was still maintained. As shown in Fig. 4c, the size of the nanofibers is uniform. The average size was about 62 nm according to the XRD result calculated by Debye–Scherrer formula. As shown in Fig. 4d, the orthorhombic InVO4 obtained without citric acid are not continual fibers but particles larger than 200 nm without well dispersity. It is obvious that the homogeneous InVO4 precursor solution using citric acid as chelant has more beneficial properties to get one-dimensional structure.
Fig. 5. UV–vis diffuse reflectance spectra of the obtained InVO4 nanofibers (a) and nanoparticles (b) calcined at 700 ◦ C for 1 h.
3.3. Morphologies of the samples SEM images of precursor composite fibers and orthorhombic InVO4 products are given in Fig. 4. The individual and ideal onedimensional composite fibers with width in a range of 80–200 nm are shown in Fig. 4a and b. The surface of composite fibers is very smooth. It is obvious that the using of PVP as a structure guiding
3.4. UV–vis diffuse reflectance spectra and photodegradation property Fig. 5a and b show the UV–vis diffuse reflection spectra of the novel InVO4 nanofibers and InVO4 nanoparticles calcined at 700 ◦ C, respectively. Both of the InVO4 products showed photoresponse in visible light region. The novel orthorhombic InVO4 fibers (as shown in Fig. 5a) had the higher absorbance under visible light. As a semiconductor photocatalyst, InVO4 absorbs a photon with energy equal to or larger than the bandgap of the catalysts directly and generates electron–hole pairs in the body. The free electron
Fig. 6. UV–vis diffuse reflectance spectra of nitrobenzene over novel InVO4 nanofibers (A) and nanoparticles (B) under visible light irradiation; (C) the photodegradation ratio of the novel InVO4 nanofibers (a), the InVO4 nanoparticles (b) and the photodegradation of the NB.
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in conduction band which consists of mainly d orbital of the transition metal moves along electric field and can be reduced by the oxygen molecule when it reaches to the surface. And the hole in valence band which is spanned dominantly by 2p orbital of oxygen may oxidate the OH− , H2 O or organic dyes absorbed on the surface of the fibers. Based on the above consideration, in the present study, the photodegradation of an aqueous solution of NB using InVO4 samples under visible light irradiation was studied in order to verify their practical values. The typical UV spectra of the photodegradation of NB for the InVO4 products at different times are shown in Fig. 6. As can be seen, the absorption band at 268 nm decreased with time of photocatalytic reaction increased, which demonstrated the gradual photodegradation of NB. After irradiation for 6 h, the photocatalytic activity of InVO4 nanofibers (69%) was higher than that of InVO4 nanoparticles (about 43.1%). This confirms that this one-dimensional nanomaterial indeed is an excellent candidate of photocatalyst for treating organic contamination in wastewaters. 3.5. PL spectra In order to further understand the roles of active species in the photocatalytic process occurring on the samples, the formation of hydroxyl radicals (• OH) on the surface of the photocatalysts was detected by the PL technique using terephthalic acid as a probe molecule. Terephthalic acid, which reacts readily with • OH to produce only one kind of highly fluorescent product, 2hydroxyterephthalic acid, was employed as a probe molecule [20–22]. With an excitation wavelength at 315 nm, the PL spectra of InVO4 nanofibers and nanoparticles respectively fixed at 10 min and 40 min (inset in Fig. 7) reveal that InVO4 nanofibers always display a stronger emission peak. In fact, the fluorescence intensity of 2-hydroxyterephthalic acid around 425 nm is proportional to the amount of • OH formed in solution [23]. So it indicates that the quantities of • OH radicals on the surface of InVO4 nanofibers are more than those of the InVO4 nanoparticles. The fluorescence intensity increases along with the irradiation time as shown in Fig. 7, which demonstrates that • OH radicals on the InVO4 nanofibers are really produced under visible light irradiation but not simple photogenerated holes. The excited holes from the conduction band reacted with H2 O or OH− on the surface of the catalysts and formed • OH radicals with strong oxidizing property. Then the terephthalic acid is oxidized by • OH rather than photogenerated holes in catalysis
Fig. 8. The XRD patterns of the InVO4 nanofibers pre- and post-photocatalysis.
because aromatic compounds can be oxidized much faster by the former [20]. This suggests that the dominant active species in the photocatalytic process are • OH radicals which are highly oxidative active species to many toxic organic pollutants. It can be in good agreement with the results of the photodegradation measurements and proved the obvious superiority of the InVO4 nanofibers. 3.6. The stability of the photocatalysts In order to explore the stability of the orthorhombic InVO4 nanofibers, XRD patterns of the photocatalysts before and after photocatalysis were measured together for comparison (as shown in Fig. 8). All the diffraction peaks can be indexed as the orthorhombic InVO4 which was consistent with the JCPDS 48-0898. The only difference is that the full width at half maximum (FWHM, radian) decrease from 0.114 to 0.098. And the average grain size increases to 71.9 nm after photocatalysis resulted from the organic molecules adsorbed on the surface of catalysts. The results indicate that the photodegradation process does not destroy the lattice structure of the catalysts and declare that InVO4 nanofibers are stable under visible light irradiation. 4. Conclusion In summary, the electrospinning method combining with annealing treatment was found to be an effective method for synthesizing InVO4 nanofibers in this work. After calcined the electrospun PVP-precursor nanofibers at 700 ◦ C, orthorhombic InVO4 fibers were obtained. The low ordering ability of the fibers could be attributed to the random stacking of the fiber. The PL spectra proved that the active species in the photocatalytic process were • OH radicals. The UV–vis diffuse reflection spectra demonstrated that the absorption property of the pure orthorhombic InVO4 nanofibers was in visible light region. Compared with general InVO4 nanoparticles, the nanofibers possessed higher photocatalysis. Acknowledgments
Fig. 7. Fluorescence spectra varies with visible light irradiation time on the InVO4 nanofibers in a 5 × 10−4 M basic solution terephthalic acid and fluorescence spectra of different orthorhombic samples during visible light irradiation in a 5 × 10−4 M basic solution terephthalic acid fixed at 10 min and 40 min (inset).
This work was supported by the National Natural Science Foundation of China (Grant No. 50872076 and No. 51172113), the Ministry of Education of Shandong Province (Grant No. J09LD23) and the Key Project of Chinese Ministry of Education (Grant No. 211098). The authors also thank the Analytical Center of Shandong Polytechnic University for the technological support.
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[12] R.J. Lundgren, L.M.D. Cranswick, M. Bieringer, J. Solid State Chem. 179 (2006) 3599–3606. [13] L.M. Chen, Y.N. Liu, Z.G. Lu, D.M. Zeng, J. Coll. Sci. Imp. Univ. Tokyo 295 (2006) 440–444. [14] J.M. Deitzel, J. Kleinmeyer, D. Harris, N.C. Beck Tan, Polymer 42 (2001) 261–272. [15] X. Yi, J.L. Li, J. Am. Ceram. Soc. 93 (2010) 596–600. [16] S.C. Zhang, C. Zhang, H.P. Yang, Y.F. Zhu, J. Solid State Chem. 179 (2006) 873–882. [17] Y.K. Du, P. Yang, Z.G. Mou, N.P. Hua, L. Jiang, J. Appl. Polym. Sci. 99 (2006) 23–26. [18] Y.K. Kim, H.B. Park, Y.M. Lee, J. Membr. Sci. 251 (2005) 159–167. [19] M. Touboul, A. Popot, J. Therm. Anal. 31 (1986) 117–124. [20] K. Ishibashi, A. Fujishima, T. Watanabe, K. Hashimoto, Electrochem. Commun. 2 (2000) 207–210. [21] Q. Xiao, Z.C. Si, J. Zhang, C. Xiao, X.K. Tan, J. Hazard. Mater. 150 (2008) 62–67. [22] X.X. Yu, J.G. Yu, B. Cheng, Huang BB, Chem. Eur. J. 15 (2009) 6731–6739. [23] F.B. Li, X.Z. Li, Chemosphere 48 (2002) 1103–1111.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication and characterization of electrospun orthorhombic InVO4 nanofibers Lingjun Song, Suwen Liu ∗ , Qifang Lu, Gang Zhao Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics, Shandong Polytechnic University, Jinan 250353, PR China
a r t i c l e
i n f o
Article history: Received 10 September 2011 Received in revised form 20 November 2011 Accepted 6 December 2011 Available online 13 December 2011 Keywords: Orthorhombic InVO4 Electrospinning technique Sintering Nanofibers
a b s t r a c t The novel orthorhombic InVO4 nanofibers have been successfully synthesized by annealing electrospun precursor fibers. Citric acid was used as a ligand for it could react with metal salts to get a transparent homogeneous precursor solution and homogeneous precursor sol for electrospining. Polyvinyl pyrrolidone (PVP, K-30) was used as a binder and a structure guide reagent because it was one kind of water-soluble polymers. It is easy to gain one-dimensional materials while the viscosity of the citrate/PVP sol was suitable. The structure, morphology and photocatalytic properties of the nanofibers were characterized by X-ray diffraction (XRD), thermogravimetry analysis (TGA), scanning electron microscopy (SEM) analysis, UV–vis spectrophotometer and fluorescence spectrophotometer. The nanofibers calcined at 700 ◦ C were orthorhombic InVO4 with a width in the range of 30–100 nm and length in micron-grade. This one-dimensional pure orthorhombic InVO4 had the higher photocatalytic activity under visible light irradiation. The photo-degradation rate of nitrobenzene aqueous solution under visible light reached 69% after 6 h. It is obvious that the orthorhombic InVO4 nanofibers have a potential application in wastewater-treatment. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, InVO4 has become a promising photo-catalyst with a narrow band gap (Eg = 2.0 eV), which is able to induce hydrolysis of water molecules under visible-light irradiation [1–3]. InVO4 belongs to a large family of orthovanadate compounds with a general formula M3+ VO4 (M3+ = In, Fe, Cr, Al, rare earths) [4,5] which has two phases: the stable high-temperature orthorhombic InVO4 -III (Cmcm) phase and low-temperature metastable monoclinic InVO4 I phase [6,7]. The structure of InVO4 is composed of chains of the InO6 octahedral linked together by the VO4 tetrahedral [8], and the only difference is that the structure of monoclinic InVO4 -I phase consists of compact In4 O16 which groups of four edge-shared InO6 octahedra linked to each other by VO4 tetrahedra and the orthorhombic InVO4 is composed of chains of InO6 octahedra which are linked together by VO4 tetrahedra [9]. As a new type of semiconductor, orthorhombic InVO4 has also attracted considerable interests for its special photocatalytic properties [4,10]. One-dimensional nanostructures, such as nanowires, nanofibers, and nanobelts are expected to play an important role due to their potential applications in nanodevices. Various one-dimensional nanostructured materials have been fabricated by a variety of methods, including templating direction [11], solid-state reaction method [12], hydrothermal treatment [13],
∗ Corresponding author. Tel.: +86 0531 89631632; fax: +86 0531 89631227. E-mail address: [email protected] (S. Liu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.12.029
etc. Besides, electrospinning is a unique fiber spinning process because it can make fibers with a diameter of 50–500 nm [14]. Single-crystalline InVO4 nanotubes produced by annealing electrospun precursor fibers have been first reported by Yi and Li [15]. To the best of our knowledge, other morphological InVO4 nano-products synthesized by electrospinning technology have not been reported. So, it is very interesting and essential to prepare the InVO4 nanoscale one-dimensional materials via the electrospinning route. In the present work, the electrospinning technique was used to prepare citrate/PVP composite fibers. Orthorhombic InVO4 nanofibers were obtained by calcining the precursor nanofibers above 700 ◦ C. 2. Experiment 2.1. Synthesis procedure NH4 VO3 (99.0%) was bought from Shanghai Chemical Reagent Co. Ltd., ethanol of analytical grade was bought from Tianjin Guangcheng Chemical Reagent Co. Ltd. In(NO3 )3 ·4.5H2 O with a purity of 99.5% and Polyvinyl pyrrolidone (PVP (K30), Mw = 4.0 × 104 ) were purchased from Sinopharm Chemical Reagent Co. Ltd. A schematic of the electrospinning process is shown in Fig. 1. In a typical experiment for the preparation of the electrospinning solution, 2 mmol In(NO3 )3 ·4.5H2 O was dissolved into 10 mL distilled water under magnetic stirring to form a colorless transparent
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Fig. 1. The preparation process of InVO4 samples.
solution. Then 2 mmol NH4 VO3 was added into the above solution to form an orange suspension. 2 mmol citric acid used as a chelating agent was gradually added into the beaker under continuous stirring until the suspension turned into a yellow green transparent precursor solution (recorded as precursor A). 2 g PVP (K-30) was dissolved in 5 mL ethanol (recorded as precursor B). Then 3 mL precursor solution A was transferred to precursor B and the mixture was further stirred for 10 min to form a viscous hydrosol for electrospinning. Here, PVP (K-30), one kind of watersoluble polymers, was used as a binder and a structure guiding reagent. Besides, another precursor A without citric acid was prepared by adding In(NO3 )3 solution into hot NH4 VO3 solution directly while all other things being equal. The precursor sol was drawn into a syringe connected with a stainless steel capillary with inner diameter of 0.40 mm and outer diameter of 0.60 mm. The positive terminal of a variable highvoltage power supply (BGG-200 kV/20 mA) was connected to the needle tip of the capillary while the other was connected to the collector plate. During the electrospinning, the applied voltage was kept at 25 kV and the distance between spinneret and collector was optimized and around 28 cm. The feeding rate of the solution was kept at 0.5 mL/h at room temperature. When the spinning was completed, the as-prepared precursor composite fibers were collected by a forcep, dried at 80 ◦ C for 12 h. And then all gel fibers were put into an air-atmosphere programmable tube furnace for heat treatment according to the TG result. The fibers were fired from room temperature to desired temperatures at a rate of 1 ◦ C/min with a hold time of 1 h. The products were naturally cooled to room temperature in the furnace to obtain the resulting InVO4 nanofibers. Then the calcined nanofibers were given characterization.
characterized with a field emission scanning electron microscopy (Hitachi FESEM-4800, Japan, working voltage of 10 kV and operating distance of 7.7 mm). TG analysis of composite fibers was measured with a thermal analyzer (TGA/SDTA 851, Mettler) at an O2 flow rate of 20 mL/min, using a heating rate of 20 ◦ C/min in air. The purity of O2 was 99.5%. UV–vis diffuse reflectance spectra and the photodegradation decoloring rate of the samples were measured using a Shimadzu UV-2550PC spectrophotometer. The photocatalytic activities of the as-synthesized samples were determined through the degradation of nitrobenzene (NB). The formation of hydroxyl radicals (• OH) was detected by the photoluminescence (PL) spectra which were measured on a Hitachi F-4500 fluorescence spectrophotometer. 0.12 g photocatalysts were added in 30 mL NB solution with a concentration of 20 mg/L under stirring, using a 500 W Xenon lamp with a UV-cutoff filter ( ≤ 400 nm) as the light source. Took 4 mL solution out once every one hour, centrifuged and measured the absorbance at the maximum absorption wavelength ( = 268 nm) of the NB. The decoloring ratio of the NB was estimated according to the equation: = [(A0 –At )/A0 ] × 100%, where is the degradation ratio of NB, A0 is the initial absorbency, and A is the absorbency at a certain time. Additionally, the photodegradation ratio of NB solution without photocatalyst was also measured under the same condition to estimate the influence of the photodegradation over the organic compounds. The detection experiment process of the • OH is similar to the photodegradation experiment, with the exception of a basic terephthalic acid solution instead of NB solution. Terephthalic acid (5 × 10−4 M) was dissolved in NaOH solution (2 × 10−3 M). The sampling was carried out every 10 min, and the withdrawn solution was measured after centrifugation. The product of terephthalic acid hydroxylation can give a characteristic fluorescence peak at about 425 nm by excitation with the wavelength of 315 nm. 3. Results and discussion 3.1. TG analysis Fig. 2 shows the typical thermal behavior of the precursor composite nanofibers. The TG curve indicates four different weight loss steps with the increase of temperature in the whole combustion process. The first weight loss (∼16.48%) occurred before 190 ◦ C was mostly attributed to the gradual evaporation of free
2.2. Characterization The phase and crystallinity of the InVO4 products were examined by X-ray diffraction (XRD, Rigaku D/Max 2200PC, Germany) ˚ at a scanning rate of 0.02◦ /s with Cu K␣ radiation ( = 1.5148 A) in the range of 20–60◦ , and the accelerating voltage and the applied current were 28 kV and 20 mA, respectively. The morphology and microstructure of the orthorhombic InVO4 samples were
Fig. 2. The TG curve of the precursor composite fibers.
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49.05%. These weight losses were caused by the decomposition of the citric acid complex [16] and polyvinyl pyrrolidone, the oxidation of carbon and carbon monoxide released by the decomposition of PVP [17,18]. All organic components could have been eliminated at 550 ◦ C because there was no further weight loss.
3.2. XRD patterns analysis
Fig. 3. The XRD patterns of the samples calcined at different temperatures: (a) 550 ◦ C, (b) 600 ◦ C, (c) 700 ◦ C for 1 h.
water and crystal water. The region from 190 to 265 ◦ C with a continuously small weight loss of 5.27% could be ascribed to the decomposition and the complete combustion of nitrate radical. The sharp decrease of the weight was separated into two parts: the region from 265 to 350 ◦ C with weight loss of 25.49% and the region from 350 to 550 ◦ C with weight loss of
According to the TG results, all organic components had been eliminated before 550 ◦ C, so the dry composite fibers were calcined above 550 ◦ C. Fig. 3 presents the XRD patterns of the InVO4 calcined at various temperatures. XRD pattern of the product calcined at 550 ◦ C for 1 h is shown in Fig. 3a. All the diffraction peaks can be indexed to InVO4 with a monoclinic phase (JCPDS card number 481016, a = 1.0271 nm, b = 0.9403 nm, c = 0.7038 nm). Combined with the TG analysis, it can be inferred that the liberated heat resulted from the combustion of the citric acid complex and polyvinyl pyrrolidone provided energy for the crystallinity phase transformation from amorphous to monoclinic. As shown in Fig. 3b, it is obvious that the intense peaks at 2 = 31.0◦ , 33.0◦ and 35.1◦ belong to the orthorhombic phase. The orthorhombic InVO4 phase coexists with the monoclinic phase when the products calcined at 600 ◦ C. This monoclinic phase converted completely to the orthorhombic phase InVO4 (JCPDS card no.48-0898, as shown in Fig. 3c) when the annealing temperature reached 700 ◦ C, which is almost in agreement with Touboul and Popot’s results [19]. The average crystallite size of the as-prepared pure orthorhombic InVO4 nanofibers estimated by the Scherrer formula is about 61.97 nm.
Fig. 4. SEM images of the electrospun citrate/PVP composite nanofibers (a and b), novel orthorhombic InVO4 nanofibers sintered at 700 ◦ C for 1 h (c) and orthorhombic InVO4 nanoparticles obtained by annealing precursor composite without citric acid at 700 ◦ C (d) for 1 h.
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reagent is extremely effective. During the calcining process, an amorphous InVO4 would be formed when the contiguous ions of indium and vanadium in the precursor reacted with each other. Followed by the continual calcination, the amorphous InVO4 particles started to crystallize and grow. Known from the XRD results, the phase of the InVO4 nanofibers in Fig. 4c was orthorhombic phase. Even though PVP (K-30) in the fiber was decomposed with the diameter reduction of the fiber, the continuous microstructure of the fiber was still maintained. As shown in Fig. 4c, the size of the nanofibers is uniform. The average size was about 62 nm according to the XRD result calculated by Debye–Scherrer formula. As shown in Fig. 4d, the orthorhombic InVO4 obtained without citric acid are not continual fibers but particles larger than 200 nm without well dispersity. It is obvious that the homogeneous InVO4 precursor solution using citric acid as chelant has more beneficial properties to get one-dimensional structure.
Fig. 5. UV–vis diffuse reflectance spectra of the obtained InVO4 nanofibers (a) and nanoparticles (b) calcined at 700 ◦ C for 1 h.
3.3. Morphologies of the samples SEM images of precursor composite fibers and orthorhombic InVO4 products are given in Fig. 4. The individual and ideal onedimensional composite fibers with width in a range of 80–200 nm are shown in Fig. 4a and b. The surface of composite fibers is very smooth. It is obvious that the using of PVP as a structure guiding
3.4. UV–vis diffuse reflectance spectra and photodegradation property Fig. 5a and b show the UV–vis diffuse reflection spectra of the novel InVO4 nanofibers and InVO4 nanoparticles calcined at 700 ◦ C, respectively. Both of the InVO4 products showed photoresponse in visible light region. The novel orthorhombic InVO4 fibers (as shown in Fig. 5a) had the higher absorbance under visible light. As a semiconductor photocatalyst, InVO4 absorbs a photon with energy equal to or larger than the bandgap of the catalysts directly and generates electron–hole pairs in the body. The free electron
Fig. 6. UV–vis diffuse reflectance spectra of nitrobenzene over novel InVO4 nanofibers (A) and nanoparticles (B) under visible light irradiation; (C) the photodegradation ratio of the novel InVO4 nanofibers (a), the InVO4 nanoparticles (b) and the photodegradation of the NB.
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in conduction band which consists of mainly d orbital of the transition metal moves along electric field and can be reduced by the oxygen molecule when it reaches to the surface. And the hole in valence band which is spanned dominantly by 2p orbital of oxygen may oxidate the OH− , H2 O or organic dyes absorbed on the surface of the fibers. Based on the above consideration, in the present study, the photodegradation of an aqueous solution of NB using InVO4 samples under visible light irradiation was studied in order to verify their practical values. The typical UV spectra of the photodegradation of NB for the InVO4 products at different times are shown in Fig. 6. As can be seen, the absorption band at 268 nm decreased with time of photocatalytic reaction increased, which demonstrated the gradual photodegradation of NB. After irradiation for 6 h, the photocatalytic activity of InVO4 nanofibers (69%) was higher than that of InVO4 nanoparticles (about 43.1%). This confirms that this one-dimensional nanomaterial indeed is an excellent candidate of photocatalyst for treating organic contamination in wastewaters. 3.5. PL spectra In order to further understand the roles of active species in the photocatalytic process occurring on the samples, the formation of hydroxyl radicals (• OH) on the surface of the photocatalysts was detected by the PL technique using terephthalic acid as a probe molecule. Terephthalic acid, which reacts readily with • OH to produce only one kind of highly fluorescent product, 2hydroxyterephthalic acid, was employed as a probe molecule [20–22]. With an excitation wavelength at 315 nm, the PL spectra of InVO4 nanofibers and nanoparticles respectively fixed at 10 min and 40 min (inset in Fig. 7) reveal that InVO4 nanofibers always display a stronger emission peak. In fact, the fluorescence intensity of 2-hydroxyterephthalic acid around 425 nm is proportional to the amount of • OH formed in solution [23]. So it indicates that the quantities of • OH radicals on the surface of InVO4 nanofibers are more than those of the InVO4 nanoparticles. The fluorescence intensity increases along with the irradiation time as shown in Fig. 7, which demonstrates that • OH radicals on the InVO4 nanofibers are really produced under visible light irradiation but not simple photogenerated holes. The excited holes from the conduction band reacted with H2 O or OH− on the surface of the catalysts and formed • OH radicals with strong oxidizing property. Then the terephthalic acid is oxidized by • OH rather than photogenerated holes in catalysis
Fig. 8. The XRD patterns of the InVO4 nanofibers pre- and post-photocatalysis.
because aromatic compounds can be oxidized much faster by the former [20]. This suggests that the dominant active species in the photocatalytic process are • OH radicals which are highly oxidative active species to many toxic organic pollutants. It can be in good agreement with the results of the photodegradation measurements and proved the obvious superiority of the InVO4 nanofibers. 3.6. The stability of the photocatalysts In order to explore the stability of the orthorhombic InVO4 nanofibers, XRD patterns of the photocatalysts before and after photocatalysis were measured together for comparison (as shown in Fig. 8). All the diffraction peaks can be indexed as the orthorhombic InVO4 which was consistent with the JCPDS 48-0898. The only difference is that the full width at half maximum (FWHM, radian) decrease from 0.114 to 0.098. And the average grain size increases to 71.9 nm after photocatalysis resulted from the organic molecules adsorbed on the surface of catalysts. The results indicate that the photodegradation process does not destroy the lattice structure of the catalysts and declare that InVO4 nanofibers are stable under visible light irradiation. 4. Conclusion In summary, the electrospinning method combining with annealing treatment was found to be an effective method for synthesizing InVO4 nanofibers in this work. After calcined the electrospun PVP-precursor nanofibers at 700 ◦ C, orthorhombic InVO4 fibers were obtained. The low ordering ability of the fibers could be attributed to the random stacking of the fiber. The PL spectra proved that the active species in the photocatalytic process were • OH radicals. The UV–vis diffuse reflection spectra demonstrated that the absorption property of the pure orthorhombic InVO4 nanofibers was in visible light region. Compared with general InVO4 nanoparticles, the nanofibers possessed higher photocatalysis. Acknowledgments
Fig. 7. Fluorescence spectra varies with visible light irradiation time on the InVO4 nanofibers in a 5 × 10−4 M basic solution terephthalic acid and fluorescence spectra of different orthorhombic samples during visible light irradiation in a 5 × 10−4 M basic solution terephthalic acid fixed at 10 min and 40 min (inset).
This work was supported by the National Natural Science Foundation of China (Grant No. 50872076 and No. 51172113), the Ministry of Education of Shandong Province (Grant No. J09LD23) and the Key Project of Chinese Ministry of Education (Grant No. 211098). The authors also thank the Analytical Center of Shandong Polytechnic University for the technological support.
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