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Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties
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CERAMICS INTERNATIONAL
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Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties Gisele C. Leindeckern, Annelise K. Alves, Carlos P. Bergmann LACER, Federal University of Rio Grande do Sul, UFRGS Av. Osvaldo Aranha, 99/711, CEP 90035-190, Brazil Received 19 May 2014; received in revised form 1 July 2014; accepted 10 July 2014
Abstract In this work, nanofiber composites of niobium/ polyvinylpyrrolidone (Nb/PVP) were synthesized by electrospinning. Ceramic fibers of Nb2O5 were obtained by heat treatment. The heat treatment carried out at 600 1C and at 700 1C resulted in the formation of the hexagonal phase of Nb2O5 (TT–Nb2O5). The fibers were characterized by thermogravimetric analysis (TGA), infrared spectrophotometry by Fourier transform (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The average diameter and crystallite size, the specific surface area by BET (Brunnauer, Emmet and Teller method) and the band gap values by diffuse reflectance spectroscopy (DRS) were also determined. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Fibers; C. Optical properties; Electrospinning
1. Introduction Nanofibers are solid state linear nanomaterials with diameters ranging from 100 to 500 nm with good flexibility and lengths that exceed their diameter by one thousand times. The high specific surface area, combined with the good flexibility and high mechanical resistance make them suitable candidates for a variety of applications ranging from surface coatings to the strengthening of aerospace structures [1]. The nanofibers have been widely used in the field of catalysis. Catalysts in the form of fibers present several advantages in relation to catalysts supported on alumina or particulates, such as a high surface area, more intimate contact between reactants and the active sites, the absence of closed pores, while obviating the need to use a support reduces diffusion problems, especially reactions in liquid media [2]. Catalysts based on niobium compounds, mainly in the form of oxides, may be employed as active phase promoters or brackets in various reaction n
Corresponding author. E-mail address: [email protected] (G.C. Leindecker).
systems [3]. The niobium oxide has shown promising results in photocatalysis applications because it can be easily recovered and recycled at the end of the process [4]. Electrospinning is a method that uses very high voltages (kilo volts) at low currents to produce small diameter fibers, which involves accelerating and stretching a jet of fluid material in the presence of an external electric field [5–7]. The assembly of an electrospinning device consists basically a reservoir for the precursor solution, a high voltage source and a collector where the fibers are deposited. The polymeric solution contained in the reservoir, usually a syringe, is forced through a nozzle, normally with the aid of an infusion pump, in the presence of an external electric field until it reaches the collector tube. If the electric field is intense enough, a solution jet is formed and accelerated by the electric field in the direction of the ground electrode, which serves as a collector for the synthesized nanofibers [8]. In this paper, niobium oxide nanofibers were synthesized from metallic niobium by electrospinning. The fibers were characterized according to their thermal behavior, morphology,
http://dx.doi.org/10.1016/j.ceramint.2014.07.054 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: G.C. Leindecker, et al., Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.054
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crystallinity, presence of functional groups, specific surface area and optical properties. 2. Experimental 2.1. Electrospinning A solution was prepared by dissolving 0.3 g of metallic niobium powder in 2 mL of hydrofluoric acid and by the subsequent addition of 1 mL of acetic acid to catalyze hydrolysis [9]. The polymeric solution was prepared by dissolving 10 g of polyvinylpyrrolidone (PVP) in 100 mL of ethanol, maintained under magnetic stirring until complete dissolution of the polymer occurs. Finally, the electrospinning solution was formed by the addition of a 15 mL PVP (10 wt%) solution to the niobium solution, followed by magnetic stirring at room temperature for 1 h. This solution was transferred to a syringe positioned 130 mm away from the collector. A stainless steel hypodermic needle with an internal diameter of 1.2 mm served as a capillary. During the electrospinning process, the applied voltage was 15 kV and the solution flow rate was kept constant (1.5 mL/h). The fibers were collected on the aluminum foil wrapped around the rotating drum, which was changed every 30 min. After the electrospinning process, the collected material was stored in a desiccator for 12 h at most. The ceramic nanofibers of Nb2O5 were obtained by heat treatment of the as synthesized material at temperatures of 600–700 1C, thereby removing the polymer and other organic materials present in the fibers. 2.2. Characterization The thermal behavior of the synthesized composite fibers was evaluated by thermogravimetric analyzer (TGA) and Differential thermal analyzer (Metler Toledo SDTA 851e). The fibers were heated in the temperature range between 30 and 1100 1C, at a heating rate of 10 1C/min in a synthetic air atmosphere (flow rate of 10 mL/h). The presence of functional groups was analyzed with an infrared spectrometer (Shimadzu, IRAffinity) before and after heat treatment, using the method of diffuse reflectance and a standard of KBr. The crystalline phases were determined using an X-ray diffractometer (Phillips, X pert ´ MPD). The crystallite size of the fibers was calculated based on the Scherrer Formula [10]: DC ¼
0:89λ β cos θ
Where: DC ¼ crystallite size; Λ ¼ wavelength of X-rays; β¼ width of the diffraction peak of greater intensity, measured at mid-height; θ ¼ angle corresponding to the peak of greater intensity.
ð1Þ
The morphology of the Nb2O5 nanofibers was observed, before and after heat treatment, using a scanning electron microscope (SEM, JEOL KAL model 6060) and a transmission electron microscope (TEM, JEOL JEM model 1200Exll). For the SEM analysis, the samples were coated with Au before the measurements to make them conductive and to insure a good visualization of the fibers. For the TEM analysis, the samples were prepared by dispersing the fibers in acetone with the aid of ultrasound after depositing them on a copper grid coated with carbon film and then introduced into the microscope stand for the transmission of images. The images obtained by SEM were used to calculate the average diameter of the fibers with the aid of the Image Tool application, using the measured diameters of 30 different fibers. The specific surface area of the fibers was determined according to the Brunnauer, Emmet and Teller (BET) [11] method for nitrogen adsorption (Quantachrome Instrument, Autosorb-New 1000). The samples were dried in an oven at 100 1C for 96 h and then were subjected to BET analysis at temperature of 300 1C for 1 h, under vacuum. The optical energy gap was measured via diffuse reflectance spectroscopy (DRS) with an integrating sphere (Cary 5000 UV–vis–NIR, DRA-1800). The Kubelka–Munk method was used to determine this energy, which employs a simple expression to convert diffuse reflectance data into absorbance as shown in Eq. 2. k ð1 RÞ2 ¼ S 2R
ð2Þ
Where: R ¼ value of reflectance on the wavelength; k ¼ absorbance coefficient; S ¼ scattering coefficient.
The graph showing k/S versus the exciting light energy makes it possible to determine the energy gap of the produced fibers. A point of intersection where a straight line drawn on the curve would give an estimate of the band gap of the analyzed sample. 3. Results and discussion The thermogravimetric analysis curves of the Nb/PVP composite is shown in Fig. 1. Until the temperature reaches 300 1C, the loss of mass related to the elimination of residual moisture, acetic acid, hydrofluoric acid and ethyl alcohol used in the preparation of the fibers can be observed, which adds up to about 15% of the total mass loss. Beyond this temperature, up to just above 500 1C, an appreciable reduction in fiber's mass can be observed. Correlating this behavior with the DTA curve, we can conclude that the first peak at approximately 380 1C refers to the beginning of the crystallization of TT phase [12], and that the two smaller peaks, between 400 and 500 1C, correspond to the decomposition of the polymer.
Please cite this article as: G.C. Leindecker, et al., Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.054
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Fig. 1. Thermogravimetric analysis of the Nb/PVP fibers.
Fig. 3. XRD patterns of the Nb2O5 electrospun fibers after annealing at 600 1C and at 700 1C.
Fig. 2. FTIR spectra of PVP (10 wt%), as synthesized (as-spun) and heat treated fibers at 600 1C and 700 1C, respectively.
The IR spectrum can be seen Fig. 2. The bands in the range between 1000–2000 cm 1 correspond to the connections of PVP and can be observed in the as synthesized fibers, where there was no elimination of the polymer through heat treatment. The absorption band at approximately 1630 cm 1 can be attributed to the contribution from the C ¼ O group of the amide, the smaller peaks at 1420 and 1370 cm 1 are related to the C–H deformation of cyclic groups. The peaks at about 1100 and 1050 cm 1 observed in the PVP spectrum are related to the C–O bond of alcohol present in the polymeric solution. These bands from the polymer and alcohol become much less intense after heat treatment, but do not disappear completely, showing that there is a small amount of polymer that remains in the fibers after heat treatment. The characteristic bands of the binding energy of the Nb–O are in the range (500–1000 cm 1) [13], identified by the peaks in the spectrum at approximately 640 cm 1 and 680 cm 1. Stretches of these connections can be observed in the spectra
of both heat treatment temperatures, indicating that the niobium metal reacted with oxygen, thus forming niobium pentoxide fibers during heat treatment. The XRD analysis presented in Fig. 3 indicates the formation of the hexagonal phase of niobium pentoxide (TT–Nb2O5) after heat treatment of the fibers, with characteristic peaks of Nb2O5 appearing at approximately 2θ ¼ 22.61 and 28.61, respectively. The heat treatment at 600 1C resulted in the formation of the TT–Nb2O5 (JCPDS 28-0317). The diffraction pattern of the fibers subjected to heat treatment at 700 1C also corresponds to the hexagonal phase of Nb2O5 according to JCPDS 07-0061. The XRD patterns revealed an increase in the intensity of the diffraction peaks associated with increasing annealing temperature. The crystallite size obtained from the Scherrer equation for the TT–Nb2O5 fibers heat treated at 600 1C was 21.34 nm. The increase in annealing temperature caused an increase in crystallite size, rising to 37.08 nm for the fibers treated at 700 1C. Fig. 4 shows images obtained by scanning electron microscopy before and after heat treatment, as well as the histograms of the fiber diameter distribution for the as-spun and annealed Nb2O5 nanofibers. Note that the fibers have smooth surfaces and a variable cross section profile. The range of fiber diameters can be observed in the histograms, this variation in diameter occurs due to jet instability during the electrospinning process [14]. The average diameter of the as synthesized fibers was 0.34 mm, while the fibers annealed at 600 1C and at 700 1C had average diameters of 0.16 mm and 0.15 mm, respectively. Although the fiber diameter was reduced by more than half after heat treatment, the morphology of the fibers did not experience any significant change and remained dense and robust. The reduction in diameter of the fibers can be ascribed to the densification of Nb2O5 and the removal of polymers and other organic materials present in the as synthesized fibers [15]. In the
Please cite this article as: G.C. Leindecker, et al., Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.054
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Fig. 4. SEM images of the Nb2O5 nanofibers before (as-spun) and after the heat treatment (at 600 1C and at 700 1C), and corresponding diameter distribution histograms.
images obtained by TEM analysis (shown in Fig. 5), it can be seen that the fibers appear to be formed by interconnected particles or crystals. The darker region observed in the heat treated fibers at 700 1C represent an area of higher concentration of crystallites. The specific surface areas of the fibers were found to be 43.6 m2/g and 31.3 m2/g for those heat treated at 600 1C and 7001C, respectively. This decrease in surface area with increase in temperature may indicate the beginning of the sintering process of the particles, whose driving force is a decrease in
surface area. The optical properties of the produced material were studied by diffuse reflectance spectroscopy. Fig. 6 shows the graph of the Kubelka–Munk [16] equation (K/S) versus absorbed energy for the samples annealed at 600 1C and 700 1C. The results indicate that the optical gap decreases with increasing annealing temperature. The band gap value determined for the TT–Nb2O5 fibers heat treated at 600 1C was 3.47 eV and 3.32 eV for the fibers treated at 700 1C. Fig. 7 shows the absorbance spectrum of different fibers and reveals that the TT–Nb2O5 fibers that underwent heat treatment
Please cite this article as: G.C. Leindecker, et al., Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.054
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Fig. 5. TEM images of the nanofibers after heat treatment at 600 and at 700 1C.
treated at 600 1C present larger band gaps, which means it takes more energy to make the electrons leave the conduction band and to return to valence band. The band gap results obtained for the nanostructured Nb2O5 fibers are similar to those of a photocatalyst. High energy absorption indicates improved photonic efficiency, which usually favors photocatalytic activity. Furthermore, a wide band gap can provide strong oxidation capacity to the fibers and, consequently, increased degradation efficiency during a photocatalytic reaction [15]. 4. Conclusions
Fig. 6. Plots of (K/S) vs. energy of absorbed light.
In this work, nanostructured fibers Nb2O5 were successfully prepared by the electrospinning method. The fibers showed an average diameter of about 0.15 mm. It was observed that after heat treatment the fiber structure was maintained, but that the diameter was reduced to more than half as compared to that of as synthesized fibers. The heat treatment promoted the crystallization of the hexagonal phase of the niobium pentoxide (TT– Nb2O5). The specific surface area of the fibers was reduced with increasing annealing temperature, from 43.6 to 31.3 m2/g, for temperatures at 600 and 700 1C, respectively. Likewise, the optical energy gap reduced, from 3.47 eV to 3.32 eV. The TT– Nb2O5 fiber obtained with heat treatment at 600 1C absorbed more energy at a given wavelength, when compared with the TT–Nb2O5 sintered at 700 1C, in addition to presenting a greater specific surface area. Acknowledgment The authors acknowledge the financial support from Fapergs/Cnpq-Pronex. References
Fig. 7. UV–vis diffuse reflectance spectra of TT–Nb2O5 samples heat treatment at 600 and at 700 1C.
at 600 1C absorbed more energy at wavelengths shorter than 320 nm when compared to the TT–Nb2O5 fibers heat treated at 700 1C. This can be attributed to the fact that the fibers heat
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CERAMICS INTERNATIONAL
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Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties Gisele C. Leindeckern, Annelise K. Alves, Carlos P. Bergmann LACER, Federal University of Rio Grande do Sul, UFRGS Av. Osvaldo Aranha, 99/711, CEP 90035-190, Brazil Received 19 May 2014; received in revised form 1 July 2014; accepted 10 July 2014
Abstract In this work, nanofiber composites of niobium/ polyvinylpyrrolidone (Nb/PVP) were synthesized by electrospinning. Ceramic fibers of Nb2O5 were obtained by heat treatment. The heat treatment carried out at 600 1C and at 700 1C resulted in the formation of the hexagonal phase of Nb2O5 (TT–Nb2O5). The fibers were characterized by thermogravimetric analysis (TGA), infrared spectrophotometry by Fourier transform (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The average diameter and crystallite size, the specific surface area by BET (Brunnauer, Emmet and Teller method) and the band gap values by diffuse reflectance spectroscopy (DRS) were also determined. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Fibers; C. Optical properties; Electrospinning
1. Introduction Nanofibers are solid state linear nanomaterials with diameters ranging from 100 to 500 nm with good flexibility and lengths that exceed their diameter by one thousand times. The high specific surface area, combined with the good flexibility and high mechanical resistance make them suitable candidates for a variety of applications ranging from surface coatings to the strengthening of aerospace structures [1]. The nanofibers have been widely used in the field of catalysis. Catalysts in the form of fibers present several advantages in relation to catalysts supported on alumina or particulates, such as a high surface area, more intimate contact between reactants and the active sites, the absence of closed pores, while obviating the need to use a support reduces diffusion problems, especially reactions in liquid media [2]. Catalysts based on niobium compounds, mainly in the form of oxides, may be employed as active phase promoters or brackets in various reaction n
Corresponding author. E-mail address: [email protected] (G.C. Leindecker).
systems [3]. The niobium oxide has shown promising results in photocatalysis applications because it can be easily recovered and recycled at the end of the process [4]. Electrospinning is a method that uses very high voltages (kilo volts) at low currents to produce small diameter fibers, which involves accelerating and stretching a jet of fluid material in the presence of an external electric field [5–7]. The assembly of an electrospinning device consists basically a reservoir for the precursor solution, a high voltage source and a collector where the fibers are deposited. The polymeric solution contained in the reservoir, usually a syringe, is forced through a nozzle, normally with the aid of an infusion pump, in the presence of an external electric field until it reaches the collector tube. If the electric field is intense enough, a solution jet is formed and accelerated by the electric field in the direction of the ground electrode, which serves as a collector for the synthesized nanofibers [8]. In this paper, niobium oxide nanofibers were synthesized from metallic niobium by electrospinning. The fibers were characterized according to their thermal behavior, morphology,
http://dx.doi.org/10.1016/j.ceramint.2014.07.054 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: G.C. Leindecker, et al., Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.054
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crystallinity, presence of functional groups, specific surface area and optical properties. 2. Experimental 2.1. Electrospinning A solution was prepared by dissolving 0.3 g of metallic niobium powder in 2 mL of hydrofluoric acid and by the subsequent addition of 1 mL of acetic acid to catalyze hydrolysis [9]. The polymeric solution was prepared by dissolving 10 g of polyvinylpyrrolidone (PVP) in 100 mL of ethanol, maintained under magnetic stirring until complete dissolution of the polymer occurs. Finally, the electrospinning solution was formed by the addition of a 15 mL PVP (10 wt%) solution to the niobium solution, followed by magnetic stirring at room temperature for 1 h. This solution was transferred to a syringe positioned 130 mm away from the collector. A stainless steel hypodermic needle with an internal diameter of 1.2 mm served as a capillary. During the electrospinning process, the applied voltage was 15 kV and the solution flow rate was kept constant (1.5 mL/h). The fibers were collected on the aluminum foil wrapped around the rotating drum, which was changed every 30 min. After the electrospinning process, the collected material was stored in a desiccator for 12 h at most. The ceramic nanofibers of Nb2O5 were obtained by heat treatment of the as synthesized material at temperatures of 600–700 1C, thereby removing the polymer and other organic materials present in the fibers. 2.2. Characterization The thermal behavior of the synthesized composite fibers was evaluated by thermogravimetric analyzer (TGA) and Differential thermal analyzer (Metler Toledo SDTA 851e). The fibers were heated in the temperature range between 30 and 1100 1C, at a heating rate of 10 1C/min in a synthetic air atmosphere (flow rate of 10 mL/h). The presence of functional groups was analyzed with an infrared spectrometer (Shimadzu, IRAffinity) before and after heat treatment, using the method of diffuse reflectance and a standard of KBr. The crystalline phases were determined using an X-ray diffractometer (Phillips, X pert ´ MPD). The crystallite size of the fibers was calculated based on the Scherrer Formula [10]: DC ¼
0:89λ β cos θ
Where: DC ¼ crystallite size; Λ ¼ wavelength of X-rays; β¼ width of the diffraction peak of greater intensity, measured at mid-height; θ ¼ angle corresponding to the peak of greater intensity.
ð1Þ
The morphology of the Nb2O5 nanofibers was observed, before and after heat treatment, using a scanning electron microscope (SEM, JEOL KAL model 6060) and a transmission electron microscope (TEM, JEOL JEM model 1200Exll). For the SEM analysis, the samples were coated with Au before the measurements to make them conductive and to insure a good visualization of the fibers. For the TEM analysis, the samples were prepared by dispersing the fibers in acetone with the aid of ultrasound after depositing them on a copper grid coated with carbon film and then introduced into the microscope stand for the transmission of images. The images obtained by SEM were used to calculate the average diameter of the fibers with the aid of the Image Tool application, using the measured diameters of 30 different fibers. The specific surface area of the fibers was determined according to the Brunnauer, Emmet and Teller (BET) [11] method for nitrogen adsorption (Quantachrome Instrument, Autosorb-New 1000). The samples were dried in an oven at 100 1C for 96 h and then were subjected to BET analysis at temperature of 300 1C for 1 h, under vacuum. The optical energy gap was measured via diffuse reflectance spectroscopy (DRS) with an integrating sphere (Cary 5000 UV–vis–NIR, DRA-1800). The Kubelka–Munk method was used to determine this energy, which employs a simple expression to convert diffuse reflectance data into absorbance as shown in Eq. 2. k ð1 RÞ2 ¼ S 2R
ð2Þ
Where: R ¼ value of reflectance on the wavelength; k ¼ absorbance coefficient; S ¼ scattering coefficient.
The graph showing k/S versus the exciting light energy makes it possible to determine the energy gap of the produced fibers. A point of intersection where a straight line drawn on the curve would give an estimate of the band gap of the analyzed sample. 3. Results and discussion The thermogravimetric analysis curves of the Nb/PVP composite is shown in Fig. 1. Until the temperature reaches 300 1C, the loss of mass related to the elimination of residual moisture, acetic acid, hydrofluoric acid and ethyl alcohol used in the preparation of the fibers can be observed, which adds up to about 15% of the total mass loss. Beyond this temperature, up to just above 500 1C, an appreciable reduction in fiber's mass can be observed. Correlating this behavior with the DTA curve, we can conclude that the first peak at approximately 380 1C refers to the beginning of the crystallization of TT phase [12], and that the two smaller peaks, between 400 and 500 1C, correspond to the decomposition of the polymer.
Please cite this article as: G.C. Leindecker, et al., Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.054
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Fig. 1. Thermogravimetric analysis of the Nb/PVP fibers.
Fig. 3. XRD patterns of the Nb2O5 electrospun fibers after annealing at 600 1C and at 700 1C.
Fig. 2. FTIR spectra of PVP (10 wt%), as synthesized (as-spun) and heat treated fibers at 600 1C and 700 1C, respectively.
The IR spectrum can be seen Fig. 2. The bands in the range between 1000–2000 cm 1 correspond to the connections of PVP and can be observed in the as synthesized fibers, where there was no elimination of the polymer through heat treatment. The absorption band at approximately 1630 cm 1 can be attributed to the contribution from the C ¼ O group of the amide, the smaller peaks at 1420 and 1370 cm 1 are related to the C–H deformation of cyclic groups. The peaks at about 1100 and 1050 cm 1 observed in the PVP spectrum are related to the C–O bond of alcohol present in the polymeric solution. These bands from the polymer and alcohol become much less intense after heat treatment, but do not disappear completely, showing that there is a small amount of polymer that remains in the fibers after heat treatment. The characteristic bands of the binding energy of the Nb–O are in the range (500–1000 cm 1) [13], identified by the peaks in the spectrum at approximately 640 cm 1 and 680 cm 1. Stretches of these connections can be observed in the spectra
of both heat treatment temperatures, indicating that the niobium metal reacted with oxygen, thus forming niobium pentoxide fibers during heat treatment. The XRD analysis presented in Fig. 3 indicates the formation of the hexagonal phase of niobium pentoxide (TT–Nb2O5) after heat treatment of the fibers, with characteristic peaks of Nb2O5 appearing at approximately 2θ ¼ 22.61 and 28.61, respectively. The heat treatment at 600 1C resulted in the formation of the TT–Nb2O5 (JCPDS 28-0317). The diffraction pattern of the fibers subjected to heat treatment at 700 1C also corresponds to the hexagonal phase of Nb2O5 according to JCPDS 07-0061. The XRD patterns revealed an increase in the intensity of the diffraction peaks associated with increasing annealing temperature. The crystallite size obtained from the Scherrer equation for the TT–Nb2O5 fibers heat treated at 600 1C was 21.34 nm. The increase in annealing temperature caused an increase in crystallite size, rising to 37.08 nm for the fibers treated at 700 1C. Fig. 4 shows images obtained by scanning electron microscopy before and after heat treatment, as well as the histograms of the fiber diameter distribution for the as-spun and annealed Nb2O5 nanofibers. Note that the fibers have smooth surfaces and a variable cross section profile. The range of fiber diameters can be observed in the histograms, this variation in diameter occurs due to jet instability during the electrospinning process [14]. The average diameter of the as synthesized fibers was 0.34 mm, while the fibers annealed at 600 1C and at 700 1C had average diameters of 0.16 mm and 0.15 mm, respectively. Although the fiber diameter was reduced by more than half after heat treatment, the morphology of the fibers did not experience any significant change and remained dense and robust. The reduction in diameter of the fibers can be ascribed to the densification of Nb2O5 and the removal of polymers and other organic materials present in the as synthesized fibers [15]. In the
Please cite this article as: G.C. Leindecker, et al., Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.054
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Fig. 4. SEM images of the Nb2O5 nanofibers before (as-spun) and after the heat treatment (at 600 1C and at 700 1C), and corresponding diameter distribution histograms.
images obtained by TEM analysis (shown in Fig. 5), it can be seen that the fibers appear to be formed by interconnected particles or crystals. The darker region observed in the heat treated fibers at 700 1C represent an area of higher concentration of crystallites. The specific surface areas of the fibers were found to be 43.6 m2/g and 31.3 m2/g for those heat treated at 600 1C and 7001C, respectively. This decrease in surface area with increase in temperature may indicate the beginning of the sintering process of the particles, whose driving force is a decrease in
surface area. The optical properties of the produced material were studied by diffuse reflectance spectroscopy. Fig. 6 shows the graph of the Kubelka–Munk [16] equation (K/S) versus absorbed energy for the samples annealed at 600 1C and 700 1C. The results indicate that the optical gap decreases with increasing annealing temperature. The band gap value determined for the TT–Nb2O5 fibers heat treated at 600 1C was 3.47 eV and 3.32 eV for the fibers treated at 700 1C. Fig. 7 shows the absorbance spectrum of different fibers and reveals that the TT–Nb2O5 fibers that underwent heat treatment
Please cite this article as: G.C. Leindecker, et al., Synthesis of niobium oxide fibers by electrospinning and characterization of their morphology and optical properties, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.07.054
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Fig. 5. TEM images of the nanofibers after heat treatment at 600 and at 700 1C.
treated at 600 1C present larger band gaps, which means it takes more energy to make the electrons leave the conduction band and to return to valence band. The band gap results obtained for the nanostructured Nb2O5 fibers are similar to those of a photocatalyst. High energy absorption indicates improved photonic efficiency, which usually favors photocatalytic activity. Furthermore, a wide band gap can provide strong oxidation capacity to the fibers and, consequently, increased degradation efficiency during a photocatalytic reaction [15]. 4. Conclusions
Fig. 6. Plots of (K/S) vs. energy of absorbed light.
In this work, nanostructured fibers Nb2O5 were successfully prepared by the electrospinning method. The fibers showed an average diameter of about 0.15 mm. It was observed that after heat treatment the fiber structure was maintained, but that the diameter was reduced to more than half as compared to that of as synthesized fibers. The heat treatment promoted the crystallization of the hexagonal phase of the niobium pentoxide (TT– Nb2O5). The specific surface area of the fibers was reduced with increasing annealing temperature, from 43.6 to 31.3 m2/g, for temperatures at 600 and 700 1C, respectively. Likewise, the optical energy gap reduced, from 3.47 eV to 3.32 eV. The TT– Nb2O5 fiber obtained with heat treatment at 600 1C absorbed more energy at a given wavelength, when compared with the TT–Nb2O5 sintered at 700 1C, in addition to presenting a greater specific surface area. Acknowledgment The authors acknowledge the financial support from Fapergs/Cnpq-Pronex. References
Fig. 7. UV–vis diffuse reflectance spectra of TT–Nb2O5 samples heat treatment at 600 and at 700 1C.
at 600 1C absorbed more energy at wavelengths shorter than 320 nm when compared to the TT–Nb2O5 fibers heat treated at 700 1C. This can be attributed to the fact that the fibers heat
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