- Email: [email protected]
Effect of polymer concentration, needle diameter and annealing temperature on TiO2-PVP composite nanofibers synthesized by electrospinning technique
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of polymer concentration, needle diameter and annealing temperature on TiO2-PVP composite nanofibers synthesized by electrospinning technique Charan Kuchi, G.S. Harish, P. Sreedhara Reddy
⁎
Department of Physics, S.V. University, Tirupati 517502, A.P., India
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrospinning TiO2-PVP nanofibers SEM EDS XRD Raman
In this study, TiO2-PVP nanofibers were successfully synthesized on an aluminium collector by using cost-effective electrospinning technique. The nanofibers were prepared at different polymer concentrations, needle diameters and annealing temperatures and properties were studied by various characterizations. The structural properties were studied by X-ray diffraction (XRD) and Raman spectroscopy techniques. Surface morphology and elemental analysis of the samples were investigated by scanning electron microscopy (SEM) attached with energy dispersive spectroscopy (EDS). The optical properties were carried out by UV–Visible absorption spectroscopy (UV–Vis). By varying the polymer concentration and needle diameter, the effect of viscosity and surface tension on the formation of TiO2-PVP nanofibers was clearly observed by SEM micro images. EDS spectrum shows effective composition of pure TiO2 nanofibers. XRD peaks observed at temperatures 500 °C, 700 °C and 900 °C confirmed the anatase, mixed and rutile phases of TiO2 nanofibers respectively. Raman studies also confirmed these phases of TiO2 nanofibers. The optical band-gap values calculated using Kubelka-Munk function lies in the range of 3.02–3.22 eV.
1. Introduction
semiconductor because of its high efficiency, excellent chemical stability and cost-effectiveness. TiO2 has three different well known phases such as anatase, rutile and brookite. These phases of TiO2 nanofibers have interesting applications in the current research work. In the recent studies, different functionalities of anatase, rutile and mixed phases in different fields such as photocatalysts [11], DSSC's [12], gas sensors [13], Li-ion batteries [14] and super-capacitors [15] have been presented. In the present study, TiO2-PVP conducting polymer solution was prepared with PVP. Bead-less and controllable TiO2 -PVP composite nanofibers with anatase, rutile and mixed phases were prepared at low cost. In the present work, we have studied in depth analysis of the morphologies of the prepared nanofibers at different polymer concentrations, different needle diameters and different annealing temperatures. The effect of these parameters were not yet well defined in the previous reports. The key parameters such as viscosity and surface tension of the liquid are responsible for phase change. They can be varied by varying the polymer concentration and needle diameter respectively. Optical bandgap of the prepared sample at different annealing temperatures was studied.
One-dimensional nanomaterials such as nanorods, nanotubes and nanowires have many potential applications over the past decade. Recently nanofibers are finding numerous applications due to their special properties [1]. Electrospinning is a simple and versatile technique to fabricate one-dimensional nanofibers. This technique is useful for fabricating nanofibers of polymers, ceramics and semiconductorpolymer composites [2]. Compared to the other synthesis techniques such as hydrothermal [3], template synthesis [4], phase separation [5], etc., the electrospinning can produce long and continuous nano range fibers at low cost. The electrospinning setup consists of a high voltage DC power supply in the range of few kV, a syringe to hold polymer solution and two electrodes attached to the collector. The polymer solution ejected from the tip of the needle is attracted by the collector and deposited as a nano-fiber mat. In this technique, uniform beadless nanofibers can be obtained by varying the parameters like viscosity, surface tension, distance between needle to collector, potential difference, solution flow rate, etc. [6]. Metal oxide semiconducting nanofibers, such as TiO2, SnO2, ZnO, NiO, etc., have been broadly investigated and finds many potential applications in the area of electrochemical, photochemical and electronic devices [7–10]. Among them, TiO2 is one of the best metal oxide
⁎
2. Experimental Titanium tetraisopropoxide (Ti(OiPr)4 – Aldrich − 97%), Poly-vinyl
Corresponding author. E-mail address: [email protected] (P.S. Reddy).
https://doi.org/10.1016/j.ceramint.2017.12.138 Received 7 December 2017; Received in revised form 18 December 2017; Accepted 19 December 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Kuchi, C., Ceramics International (2017), https://doi.org/10.1016/j.ceramint.2017.12.138
Ceramics International xxx (xxxx) xxx–xxx
C. Kuchi et al.
Fig. 1. (a) Self-equipped electrospinning unit and (b) Schematic diagram of the unit.
Fig. 2. SEM images of as-spun TiO2 nanofibers at different PVP concentrations (a) 7 wt%, (b) 8 wt%, (c) 9 wt% and (d) 10 wt%.
pyrrolidone (PVP) (Mw ̴ 1300000 - Aldrich), acetic acid, and ethanol were used as starting chemicals for preparing TiO2-PVP solution. In a typical procedure, the PVP (7, 8, 9 and 10 wt%) solution was prepared using ethanol as the solvent. 1.5 g of Ti(OiPr)4 was dissolved in 3 ml of acetic acid and then this solution was subsequently added to PVP solution. These solutions were mixed using a magnetic stirrer for 3 h continuously. Then the solution appeared as transparent yellowish colour and solution was immediately loaded in a 10 ml plastic syringe with stainless steel needles of different diameters (330 µm, 500 µm, 570 µm and 720 µm). The positive electrode of the high voltage source was connected to the needle and the negative electrode was connected
to an aluminium collector. The distance between the needle tip and the collector was maintained at a distance of 10 cm and a DC voltage of 10 kV was applied using the high voltage source. The solution flow rate of 1 ml/h was maintained using the precise syringe pump equipment. The time of the spinning was maintained at 20 min for a sample. Fig. 1(a) & (b) represents, the self-equipped electrospinning unit and schematic diagram of the electrospinning system. The resulting TiO2 nanofibers were collected on an aluminium collector as a mat. To remove the residual polymer solvent on the prepared mats, these fibers were taken from collector and heated at 50 °C for 3 h. Then the sample is allowed to cool for minimum 3hrs to get the TiO2 nanosample. To 2
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Fig. 3. SEM images of as-spun TiO2 nanofibers at different needle internal diameters (a) 330 µm, (b)500 µm, (c) 570 µm and (d) 720 µm (inset: SEM images of internal diameter of needles).
percentage of PVP affects the viscosity of the precursor solution [16,17]. The entanglement of polymer chain in a solution is totally related to the viscosity of a solution. Low viscous solution has low viscoelasticity and low electrostatic force, which is not suitable for electrospinning process. Visco-elasticity can be improved by increasing polymer concentration and this leads to the formation of uniform and smooth nanofibers without any bead structures. Further increasing the polymer concentration high viscous solution can be obtained and this creates the instability of the nanofiber jet. And hence the solution possess high electrostatic forces during the electrospinning [18]. The approximate of the thickness and the effective area of the TiO2-PVP nanofiber mat was observed about 2 µm and 25 cm2 respectively. The thickness of the nanofiber mat depends on flow rate of the solution and time of the deposition. The effective are of deposition depends on the distance between needle tip and collector. If we increase the distance, the effective area of deposition increases [19]. The effect of needle diameter on the morphology of nanofibers was analyzed using needles of internal diameter 330 µm, 500 µm, 570 µm and 720 µm. The micrographs of the TiO2 nanofibers with different needle diameters are shown in Fig. 3. The bead-less nanofibers fabricated at 9 wt% PVP concentration were used for all further analysis. From Fig. 3, it is clearly observed that the diameter of the nanofibers is strongly depending on the needle diameter. Fig. 3(a), (b), (c) & (d) shows the TiO2 nanofibers of average diameter around 150 ± 20 nm, 200 ± 20 nm, 250 ± 20 nm and 350 ± 20 nm respectively and were obtained using the needles of diameters 330 µm, 500 µm, 570 µm and 720 µm respectively. We know that when the radius of a droplet is
analyse the phase transition of as-prepared TiO2 nanofibers, the samples were annealed at 500 °C, 700 °C and 900 °C respectively for 2 h in air. The surface morphology and elemental composition of TiO2-PVP nanofibers were analyzed using SEM attachment with EDS (Carl Zeiss EVO-MA15 with Oxford Instruments INCA Penta FET x3). Structural properties of the prepared samples were characterized by X-ray diffraction (XRD) using Philips PW 1050 instrument,with Ni filtered Cu tube and Kα radiation. Raman spectra of the prepared nanofibers were recorded using Confocal Raman spectrometer (Lab RamHR800). The UV–visible diffuse reflectance spectra (DRS) of the samples were measured using JASCO V570 UV–Vis–NIR spectrometer.
3. Results and discussion 3.1. Morphological analysis The influence of PVP concentration on the morphology of TiO2 nanofibers were analyzed by SEM and shown in Fig. 2. The PVP precursor solution was prepared at 7 wt%, 8 wt%, 9 wt% and 10 wt% concentrations. From Fig. 2(a), it was observed that 7 wt% of PVP causes lot of elliptical beads on the as-spun nanofiber mats. With increasing the PVP concentration to 8 wt%, the reduced bead structures were obtained (Fig. 2b). The uniform and smooth nanofibers without any bead formation were observed at 9 wt% of PVP (Fig. 2c). At 10 wt% of PVP, the nanofiber jet causes instability and the nanofibers were formed with uneven diameter (Fig. 2d). The variation in the weight 3
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Fig. 4. SEM images of TiO2 nanofibers prepared at (a) as-spun (30 °C),and annealed at (b)500 °C, (c) 700 °C and (d) 900 °C.
3.2. Elemental composition analysis
decreased, the surface tension of the droplet increases [20]. The size of the droplet at the needle tip can be deceased by using the smaller needle diameter and hence the surface tension of the droplet increases. Small diameter of the nanofibers can obtain from the lower diameter of the needles [21]. However, the increasing diameter of the nanofibers was noticed from the figures with increasing the diameter of the needle. During the electrospinning process, the columbic force decreases the acceleration of jet and hence the solution will move slowly. This leads to the solution become more stretchable. The SEM images of TiO2 nanofibers observed at room temperature (as-spun 30 °C), 500 °C, 700 °C and 900 °C are shown in Fig. 4. These nanofibers were fabricated by using the needle of diameter 500 µm, which gives the nanofibers with uniform diameter. The nanofibers fabricated at room temperature are shown in Fig. 4(a) and are smooth with an average diameter of 200 ± 20 nm. As in Fig. 4(b), the samples annealed at 500 °C shows the nanofibers with an average diameter of 150 ± 20 nm and exhibits the shrinkage of the diameter. At 700 °C, the resulting nanofibers undergoes microstructural changes with an average diameter of 160 ± 20 nm as in Fig. 4(c). Further increasing the annealing temperature to 900 °C, the nanofibers were transformed to curled surface like largely linked poly crystalline particles with an average diameter of 100 ± 20 nm. However the micrographs explained that, at higher annealing temperature, the contraction of the nanofibers is high. Also, the higher annealing temperature, leads to removal of organic components from PVP, ethanol and acetic acid [22]. And also the high temperature did not affect the fibrous nature. But it was observed that the average diameter of the nanofibers was restrained while increasing the temperature from 500 °C to 700 °C.
The elemental composition of TiO2 nanofibers were studied by EDS spectroscopy. Fig. 5 shows the EDS spectrum of the samples prepared at room temperature (30 °C), 500 °C, 700 °C and 900 °C. Fig. 5(a) explains the presence of organic compound in the form of carbon in the TiO2 nanofibers at room temperature. At the annealing temperatures 500 °C and above, no residual contents were found and it shows pure elemental composition of TiO2 nanofibers. However, we observed the increment in the titanium weight percentage with increasing the temperature from 500 °C to 900 °C. This might be due to the transition from anatase to rutile crystal phase.
3.3. XRD and Raman spectroscopy analysis The XRD profiles of the as-spun TiO2 nanofibers along with miller indices are shown in Fig. 6(a). The peak positions (2θ) along with Bragg's planes [25.38° (101), 37.98° (004), 48.18° (200), 54.02° (105) and 62.87° (204)] observed from the diffraction patterns of the nanofibers annealed at 500 °C confirms the anatase phase with tetragonal structure according to JCPDS card no. 21-1272 [23]. For the sample annealed at 900 °C XRD peaks at 27.48° (110), 36.07° (101), 39.26° (200), 41.27° (111), 44.05° (210), 54.34° (211), 56.63° (220), 62.78° (002), 64.07° (310), 69.03° (301) and 69.82° (112) confirms the tetragonal rutile phase of the TiO2 nanofibers according to JCPDS card no. 21-1276 [24] and for the sample annealed at 700 °C, a mixed phase of the TiO2 was observed with the peak positions 25.38°, 27.53°, 36.16°, 37.88°, 41.34°, 48.12°, 54.36°, 55.10°, 56.70°, 62.82° and 69.03°. The 4
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Fig. 5. EDS spectra of TiO2 nanofibers prepared at (a) as-spun (30 °C), (b) 500 °C, (c) 700 °C and (d) 900 °C.
indicated by Raman studies. All the described Raman vibration modes were collected from the previously reported data [26–29].
average crystallite sizes of the anatase TiO2 at (101) plane, rutile TiO2 at (110) plane and the mixed phase at the average of (101) and (110) planes calculated from the Scherer's equation (D = 0.9λ / β cosθ) are 18 nm, 47 nm and 27 nm [25]. The widened XRD peak pattern confirms the nanoscale distribution of the prepared TiO2 nanofibers. The Raman Spectra of the annealed TiO2 nanostructures are shown in Fig. 6(b). The raman peaks observed at 143 cm−1, 396 cm−1, 515 cm−1 and 638 cm−1 confirms the anatase phase of TiO2 annealed at 500 °C. Among them, the peaks 143 cm−1(strong) and 638 cm−1 belongs to Eg vibration mode, 396 cm−1 and 515 cm−1 belongs to B1 g and A1 g modes respectively. The bending type vibrations (O-Ti-O) are involved at 143 cm−1 and 396 cm−1. However, the stretching type vibrations (Ti-O) are involved at 515 cm−1 and 638 cm−1. The Raman peaks of the samples prepared at 900 °C confirms the rutile phase of TiO2 nanofibers and the peaks observed at 142 cm−1, 235 cm−1, 447 cm−1 and 610 cm−1 belongs to B1g, Eg, Eg and A1g vibration modes respectively. The frequencies 142 cm−1, 235 cm−1, 447 cm−1, 517 cm−1 and 609 cm−1 confirmed the mixed phase of TiO2. Hence, the transition from the pure anatase to rutile phase occurred at 900 °C is clearly
3.4. Optical properties The UV–vis absorption spectra were taken to analyse the optical properties of as prepared TiO2 nanofibers. Fig. 7(a) & (b) shows the absorption spectra and the optical band-gap of TiO2 nanofibers annealed at temperatures 500 °C, 700 °C and 900 °C. For all the anatase, rutile and mixed phases of TiO2, the absorption edge was observed in the UV and visible region. As in Fig. 7(a), the optical absorption from UV to visible indicates a red shift towards the visible region [30,31]. The band-gap of an indirect crystalline semiconductor could be calculated by using the Kubelka-Munk standard formula (αhν)1/2 [32]. The graph plotted between (αhν)1/2 vs hν, gives the absorbed light energy. The calculated optical band-gap values of TiO2 are 3.22 eV, 3.10 eV and 3.02 eV for 500 °C, 700 °C and 900 °C respectively. However, it is wellknown that the band-gap of a semiconductor decreases with increasing crystallite size [33]. Here it is clearly noticed that the band-gap of TiO2
Fig. 6. (a) XRD pattern (b) Raman spectra of TiO2 nanofibers at annealing temperatures 500 °C, 700 °C and 900 °C.
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Fig. 7. (a) UV–vis absorption spectra and (b) calculated optical band-gap of TiO2 nanofibers.
nanofibers decreases due to the increasing crystallite size at higher annealing temperatures.
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4. Conclusion The TiO2-PVP composite nanofiber mats were successfully synthesized by the self-equipped electrospinning unit. This study was attempted to synthesize and analyse the morphology & structural variation of the nanofibers with the parameters like viscosity, surface tension and annealing temperature. By varying the polymer concentration, SEM images confirmed formation of the bead-less and continuous nanofibers at certain viscosity. The change of the needle diameters influences the average diameter of the nanofibers. It was observed that the annealing temperature influences the morphology and structural properties of the nanofibers. From the XRD pattern and Raman spectra it was clearly observed the transition from anatase to rutile phase of TiO2 nanofibers occurred at the annealing temperatures 500 °C and 900 °C respectively. The optical absorption studies, shows the absorption band edges in UV–vis region and the band-gap is decreasing with increasing annealing temperature. Hence, this study suggests that the TiO2 nanofibers are useful for finding photovoltaics, photocatalysis and optoelectronic device applications. Acknowledgement The authors would like to thank University Grants Commission (UGC), New Delhi for the financial support under UGC-BSR-RFSMS Fellowship and Mid-Career Award (No.F-19-207/2017(BSR)) schemes. References [1] S. Thenmozhi, N. Dharmaraj, K. Kadirvelu, H.Y. Kim, Electrospun nanofibers: new generation materials for advanced applications, J. Mater. Sci. Eng. B 217 (2017) 36–48. [2] A. Haider, S. Haider, Inn-Kyu Kang, A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology, J. Arab. J. Chem. (2015). [3] Y. Wen, M. Jiang, C.L. Kitchens, G. Chumanov, Synthesis of carbon nanofibers via hydrothermal conversion of cellulose nanocrystals, J. Cell. (2017) 1–6. [4] J. Martín, J. Maiz, J. Sacristan, C. Mijangos, Tailored polymer-based nanorods and nanotubes by "template synthesis": from preparation to applications, J. Polym. 53 (2012) 1149e1166. [5] L. Li, J. Ge, L. Wang, B. Guo, P.X. Ma, Electroactive nanofibrous biomimetic scaffolds by thermally induced phase separation, J. Mater. Chem. B 2 (2014) 6119. [6] N. Bhardwaj, S.C. Kundu, Electrospinning: a fascinating fiber fabrication technique, J. Biotechnol. Adv. 28 (2010) 325–347. [7] Su-jin Kim, Yu.Kyung Cho, Chongmok Lee, MyungHwa Kim n, Youngmi Lee, Realtime direct electrochemical sensing of ascorbic acid over rat liver tissues using RuO2 nanowires on electrospun TiO2 nanofibers, J. Biosens. Bioelectron. 77 (2016) 1144–1152. [8] K. Ahn, De Pham-Cong, H.S. Choi, S. Jeong, J.H. Cho, J. Kim, J. Kim, J. Bae, C. Cho, Bandgap-designed TiO2/SnO2 hollow hierarchical nanofibers: synthesis, properties, and their photocatalytic mechanism, J. Curr. Appl. Phys. 16 (2016) 251–260.
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Technol. 61 (2012) 1–7. [33] M. Stefan, O. Pana, C. Leostean, C. Bele, D. Silipas, M. Senila, E. Gautron, Synthesis and characterization of Fe3O4–TiO2 core-shell nanoparticles, J. Appl. Phys. 116 (2014) 114312.
[31] Y. Huang, Y. Wei, J. Wang, D. Luo, L. Fan, J. Wu, Controllable fabrication of Bi2O3/ TiO2 heterojunction with excellent visible-light responsive photocatalytic performance, J. Appl. Surf. Sci. 423 (2017) 119–130. [32] R. L.´pez, R. G.´mez, Band-gap energy estimation from diffuse reflectance measurements on sol–gel and commercial TiO2: a comparative study, J. Sol-Gel Sci.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of polymer concentration, needle diameter and annealing temperature on TiO2-PVP composite nanofibers synthesized by electrospinning technique Charan Kuchi, G.S. Harish, P. Sreedhara Reddy
⁎
Department of Physics, S.V. University, Tirupati 517502, A.P., India
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrospinning TiO2-PVP nanofibers SEM EDS XRD Raman
In this study, TiO2-PVP nanofibers were successfully synthesized on an aluminium collector by using cost-effective electrospinning technique. The nanofibers were prepared at different polymer concentrations, needle diameters and annealing temperatures and properties were studied by various characterizations. The structural properties were studied by X-ray diffraction (XRD) and Raman spectroscopy techniques. Surface morphology and elemental analysis of the samples were investigated by scanning electron microscopy (SEM) attached with energy dispersive spectroscopy (EDS). The optical properties were carried out by UV–Visible absorption spectroscopy (UV–Vis). By varying the polymer concentration and needle diameter, the effect of viscosity and surface tension on the formation of TiO2-PVP nanofibers was clearly observed by SEM micro images. EDS spectrum shows effective composition of pure TiO2 nanofibers. XRD peaks observed at temperatures 500 °C, 700 °C and 900 °C confirmed the anatase, mixed and rutile phases of TiO2 nanofibers respectively. Raman studies also confirmed these phases of TiO2 nanofibers. The optical band-gap values calculated using Kubelka-Munk function lies in the range of 3.02–3.22 eV.
1. Introduction
semiconductor because of its high efficiency, excellent chemical stability and cost-effectiveness. TiO2 has three different well known phases such as anatase, rutile and brookite. These phases of TiO2 nanofibers have interesting applications in the current research work. In the recent studies, different functionalities of anatase, rutile and mixed phases in different fields such as photocatalysts [11], DSSC's [12], gas sensors [13], Li-ion batteries [14] and super-capacitors [15] have been presented. In the present study, TiO2-PVP conducting polymer solution was prepared with PVP. Bead-less and controllable TiO2 -PVP composite nanofibers with anatase, rutile and mixed phases were prepared at low cost. In the present work, we have studied in depth analysis of the morphologies of the prepared nanofibers at different polymer concentrations, different needle diameters and different annealing temperatures. The effect of these parameters were not yet well defined in the previous reports. The key parameters such as viscosity and surface tension of the liquid are responsible for phase change. They can be varied by varying the polymer concentration and needle diameter respectively. Optical bandgap of the prepared sample at different annealing temperatures was studied.
One-dimensional nanomaterials such as nanorods, nanotubes and nanowires have many potential applications over the past decade. Recently nanofibers are finding numerous applications due to their special properties [1]. Electrospinning is a simple and versatile technique to fabricate one-dimensional nanofibers. This technique is useful for fabricating nanofibers of polymers, ceramics and semiconductorpolymer composites [2]. Compared to the other synthesis techniques such as hydrothermal [3], template synthesis [4], phase separation [5], etc., the electrospinning can produce long and continuous nano range fibers at low cost. The electrospinning setup consists of a high voltage DC power supply in the range of few kV, a syringe to hold polymer solution and two electrodes attached to the collector. The polymer solution ejected from the tip of the needle is attracted by the collector and deposited as a nano-fiber mat. In this technique, uniform beadless nanofibers can be obtained by varying the parameters like viscosity, surface tension, distance between needle to collector, potential difference, solution flow rate, etc. [6]. Metal oxide semiconducting nanofibers, such as TiO2, SnO2, ZnO, NiO, etc., have been broadly investigated and finds many potential applications in the area of electrochemical, photochemical and electronic devices [7–10]. Among them, TiO2 is one of the best metal oxide
⁎
2. Experimental Titanium tetraisopropoxide (Ti(OiPr)4 – Aldrich − 97%), Poly-vinyl
Corresponding author. E-mail address: [email protected] (P.S. Reddy).
https://doi.org/10.1016/j.ceramint.2017.12.138 Received 7 December 2017; Received in revised form 18 December 2017; Accepted 19 December 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Kuchi, C., Ceramics International (2017), https://doi.org/10.1016/j.ceramint.2017.12.138
Ceramics International xxx (xxxx) xxx–xxx
C. Kuchi et al.
Fig. 1. (a) Self-equipped electrospinning unit and (b) Schematic diagram of the unit.
Fig. 2. SEM images of as-spun TiO2 nanofibers at different PVP concentrations (a) 7 wt%, (b) 8 wt%, (c) 9 wt% and (d) 10 wt%.
pyrrolidone (PVP) (Mw ̴ 1300000 - Aldrich), acetic acid, and ethanol were used as starting chemicals for preparing TiO2-PVP solution. In a typical procedure, the PVP (7, 8, 9 and 10 wt%) solution was prepared using ethanol as the solvent. 1.5 g of Ti(OiPr)4 was dissolved in 3 ml of acetic acid and then this solution was subsequently added to PVP solution. These solutions were mixed using a magnetic stirrer for 3 h continuously. Then the solution appeared as transparent yellowish colour and solution was immediately loaded in a 10 ml plastic syringe with stainless steel needles of different diameters (330 µm, 500 µm, 570 µm and 720 µm). The positive electrode of the high voltage source was connected to the needle and the negative electrode was connected
to an aluminium collector. The distance between the needle tip and the collector was maintained at a distance of 10 cm and a DC voltage of 10 kV was applied using the high voltage source. The solution flow rate of 1 ml/h was maintained using the precise syringe pump equipment. The time of the spinning was maintained at 20 min for a sample. Fig. 1(a) & (b) represents, the self-equipped electrospinning unit and schematic diagram of the electrospinning system. The resulting TiO2 nanofibers were collected on an aluminium collector as a mat. To remove the residual polymer solvent on the prepared mats, these fibers were taken from collector and heated at 50 °C for 3 h. Then the sample is allowed to cool for minimum 3hrs to get the TiO2 nanosample. To 2
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Fig. 3. SEM images of as-spun TiO2 nanofibers at different needle internal diameters (a) 330 µm, (b)500 µm, (c) 570 µm and (d) 720 µm (inset: SEM images of internal diameter of needles).
percentage of PVP affects the viscosity of the precursor solution [16,17]. The entanglement of polymer chain in a solution is totally related to the viscosity of a solution. Low viscous solution has low viscoelasticity and low electrostatic force, which is not suitable for electrospinning process. Visco-elasticity can be improved by increasing polymer concentration and this leads to the formation of uniform and smooth nanofibers without any bead structures. Further increasing the polymer concentration high viscous solution can be obtained and this creates the instability of the nanofiber jet. And hence the solution possess high electrostatic forces during the electrospinning [18]. The approximate of the thickness and the effective area of the TiO2-PVP nanofiber mat was observed about 2 µm and 25 cm2 respectively. The thickness of the nanofiber mat depends on flow rate of the solution and time of the deposition. The effective are of deposition depends on the distance between needle tip and collector. If we increase the distance, the effective area of deposition increases [19]. The effect of needle diameter on the morphology of nanofibers was analyzed using needles of internal diameter 330 µm, 500 µm, 570 µm and 720 µm. The micrographs of the TiO2 nanofibers with different needle diameters are shown in Fig. 3. The bead-less nanofibers fabricated at 9 wt% PVP concentration were used for all further analysis. From Fig. 3, it is clearly observed that the diameter of the nanofibers is strongly depending on the needle diameter. Fig. 3(a), (b), (c) & (d) shows the TiO2 nanofibers of average diameter around 150 ± 20 nm, 200 ± 20 nm, 250 ± 20 nm and 350 ± 20 nm respectively and were obtained using the needles of diameters 330 µm, 500 µm, 570 µm and 720 µm respectively. We know that when the radius of a droplet is
analyse the phase transition of as-prepared TiO2 nanofibers, the samples were annealed at 500 °C, 700 °C and 900 °C respectively for 2 h in air. The surface morphology and elemental composition of TiO2-PVP nanofibers were analyzed using SEM attachment with EDS (Carl Zeiss EVO-MA15 with Oxford Instruments INCA Penta FET x3). Structural properties of the prepared samples were characterized by X-ray diffraction (XRD) using Philips PW 1050 instrument,with Ni filtered Cu tube and Kα radiation. Raman spectra of the prepared nanofibers were recorded using Confocal Raman spectrometer (Lab RamHR800). The UV–visible diffuse reflectance spectra (DRS) of the samples were measured using JASCO V570 UV–Vis–NIR spectrometer.
3. Results and discussion 3.1. Morphological analysis The influence of PVP concentration on the morphology of TiO2 nanofibers were analyzed by SEM and shown in Fig. 2. The PVP precursor solution was prepared at 7 wt%, 8 wt%, 9 wt% and 10 wt% concentrations. From Fig. 2(a), it was observed that 7 wt% of PVP causes lot of elliptical beads on the as-spun nanofiber mats. With increasing the PVP concentration to 8 wt%, the reduced bead structures were obtained (Fig. 2b). The uniform and smooth nanofibers without any bead formation were observed at 9 wt% of PVP (Fig. 2c). At 10 wt% of PVP, the nanofiber jet causes instability and the nanofibers were formed with uneven diameter (Fig. 2d). The variation in the weight 3
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Fig. 4. SEM images of TiO2 nanofibers prepared at (a) as-spun (30 °C),and annealed at (b)500 °C, (c) 700 °C and (d) 900 °C.
3.2. Elemental composition analysis
decreased, the surface tension of the droplet increases [20]. The size of the droplet at the needle tip can be deceased by using the smaller needle diameter and hence the surface tension of the droplet increases. Small diameter of the nanofibers can obtain from the lower diameter of the needles [21]. However, the increasing diameter of the nanofibers was noticed from the figures with increasing the diameter of the needle. During the electrospinning process, the columbic force decreases the acceleration of jet and hence the solution will move slowly. This leads to the solution become more stretchable. The SEM images of TiO2 nanofibers observed at room temperature (as-spun 30 °C), 500 °C, 700 °C and 900 °C are shown in Fig. 4. These nanofibers were fabricated by using the needle of diameter 500 µm, which gives the nanofibers with uniform diameter. The nanofibers fabricated at room temperature are shown in Fig. 4(a) and are smooth with an average diameter of 200 ± 20 nm. As in Fig. 4(b), the samples annealed at 500 °C shows the nanofibers with an average diameter of 150 ± 20 nm and exhibits the shrinkage of the diameter. At 700 °C, the resulting nanofibers undergoes microstructural changes with an average diameter of 160 ± 20 nm as in Fig. 4(c). Further increasing the annealing temperature to 900 °C, the nanofibers were transformed to curled surface like largely linked poly crystalline particles with an average diameter of 100 ± 20 nm. However the micrographs explained that, at higher annealing temperature, the contraction of the nanofibers is high. Also, the higher annealing temperature, leads to removal of organic components from PVP, ethanol and acetic acid [22]. And also the high temperature did not affect the fibrous nature. But it was observed that the average diameter of the nanofibers was restrained while increasing the temperature from 500 °C to 700 °C.
The elemental composition of TiO2 nanofibers were studied by EDS spectroscopy. Fig. 5 shows the EDS spectrum of the samples prepared at room temperature (30 °C), 500 °C, 700 °C and 900 °C. Fig. 5(a) explains the presence of organic compound in the form of carbon in the TiO2 nanofibers at room temperature. At the annealing temperatures 500 °C and above, no residual contents were found and it shows pure elemental composition of TiO2 nanofibers. However, we observed the increment in the titanium weight percentage with increasing the temperature from 500 °C to 900 °C. This might be due to the transition from anatase to rutile crystal phase.
3.3. XRD and Raman spectroscopy analysis The XRD profiles of the as-spun TiO2 nanofibers along with miller indices are shown in Fig. 6(a). The peak positions (2θ) along with Bragg's planes [25.38° (101), 37.98° (004), 48.18° (200), 54.02° (105) and 62.87° (204)] observed from the diffraction patterns of the nanofibers annealed at 500 °C confirms the anatase phase with tetragonal structure according to JCPDS card no. 21-1272 [23]. For the sample annealed at 900 °C XRD peaks at 27.48° (110), 36.07° (101), 39.26° (200), 41.27° (111), 44.05° (210), 54.34° (211), 56.63° (220), 62.78° (002), 64.07° (310), 69.03° (301) and 69.82° (112) confirms the tetragonal rutile phase of the TiO2 nanofibers according to JCPDS card no. 21-1276 [24] and for the sample annealed at 700 °C, a mixed phase of the TiO2 was observed with the peak positions 25.38°, 27.53°, 36.16°, 37.88°, 41.34°, 48.12°, 54.36°, 55.10°, 56.70°, 62.82° and 69.03°. The 4
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Fig. 5. EDS spectra of TiO2 nanofibers prepared at (a) as-spun (30 °C), (b) 500 °C, (c) 700 °C and (d) 900 °C.
indicated by Raman studies. All the described Raman vibration modes were collected from the previously reported data [26–29].
average crystallite sizes of the anatase TiO2 at (101) plane, rutile TiO2 at (110) plane and the mixed phase at the average of (101) and (110) planes calculated from the Scherer's equation (D = 0.9λ / β cosθ) are 18 nm, 47 nm and 27 nm [25]. The widened XRD peak pattern confirms the nanoscale distribution of the prepared TiO2 nanofibers. The Raman Spectra of the annealed TiO2 nanostructures are shown in Fig. 6(b). The raman peaks observed at 143 cm−1, 396 cm−1, 515 cm−1 and 638 cm−1 confirms the anatase phase of TiO2 annealed at 500 °C. Among them, the peaks 143 cm−1(strong) and 638 cm−1 belongs to Eg vibration mode, 396 cm−1 and 515 cm−1 belongs to B1 g and A1 g modes respectively. The bending type vibrations (O-Ti-O) are involved at 143 cm−1 and 396 cm−1. However, the stretching type vibrations (Ti-O) are involved at 515 cm−1 and 638 cm−1. The Raman peaks of the samples prepared at 900 °C confirms the rutile phase of TiO2 nanofibers and the peaks observed at 142 cm−1, 235 cm−1, 447 cm−1 and 610 cm−1 belongs to B1g, Eg, Eg and A1g vibration modes respectively. The frequencies 142 cm−1, 235 cm−1, 447 cm−1, 517 cm−1 and 609 cm−1 confirmed the mixed phase of TiO2. Hence, the transition from the pure anatase to rutile phase occurred at 900 °C is clearly
3.4. Optical properties The UV–vis absorption spectra were taken to analyse the optical properties of as prepared TiO2 nanofibers. Fig. 7(a) & (b) shows the absorption spectra and the optical band-gap of TiO2 nanofibers annealed at temperatures 500 °C, 700 °C and 900 °C. For all the anatase, rutile and mixed phases of TiO2, the absorption edge was observed in the UV and visible region. As in Fig. 7(a), the optical absorption from UV to visible indicates a red shift towards the visible region [30,31]. The band-gap of an indirect crystalline semiconductor could be calculated by using the Kubelka-Munk standard formula (αhν)1/2 [32]. The graph plotted between (αhν)1/2 vs hν, gives the absorbed light energy. The calculated optical band-gap values of TiO2 are 3.22 eV, 3.10 eV and 3.02 eV for 500 °C, 700 °C and 900 °C respectively. However, it is wellknown that the band-gap of a semiconductor decreases with increasing crystallite size [33]. Here it is clearly noticed that the band-gap of TiO2
Fig. 6. (a) XRD pattern (b) Raman spectra of TiO2 nanofibers at annealing temperatures 500 °C, 700 °C and 900 °C.
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Fig. 7. (a) UV–vis absorption spectra and (b) calculated optical band-gap of TiO2 nanofibers.
nanofibers decreases due to the increasing crystallite size at higher annealing temperatures.
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4. Conclusion The TiO2-PVP composite nanofiber mats were successfully synthesized by the self-equipped electrospinning unit. This study was attempted to synthesize and analyse the morphology & structural variation of the nanofibers with the parameters like viscosity, surface tension and annealing temperature. By varying the polymer concentration, SEM images confirmed formation of the bead-less and continuous nanofibers at certain viscosity. The change of the needle diameters influences the average diameter of the nanofibers. It was observed that the annealing temperature influences the morphology and structural properties of the nanofibers. From the XRD pattern and Raman spectra it was clearly observed the transition from anatase to rutile phase of TiO2 nanofibers occurred at the annealing temperatures 500 °C and 900 °C respectively. The optical absorption studies, shows the absorption band edges in UV–vis region and the band-gap is decreasing with increasing annealing temperature. Hence, this study suggests that the TiO2 nanofibers are useful for finding photovoltaics, photocatalysis and optoelectronic device applications. Acknowledgement The authors would like to thank University Grants Commission (UGC), New Delhi for the financial support under UGC-BSR-RFSMS Fellowship and Mid-Career Award (No.F-19-207/2017(BSR)) schemes. References [1] S. Thenmozhi, N. Dharmaraj, K. Kadirvelu, H.Y. Kim, Electrospun nanofibers: new generation materials for advanced applications, J. Mater. Sci. Eng. B 217 (2017) 36–48. [2] A. Haider, S. Haider, Inn-Kyu Kang, A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology, J. Arab. J. Chem. (2015). [3] Y. Wen, M. Jiang, C.L. Kitchens, G. Chumanov, Synthesis of carbon nanofibers via hydrothermal conversion of cellulose nanocrystals, J. Cell. (2017) 1–6. [4] J. Martín, J. Maiz, J. Sacristan, C. Mijangos, Tailored polymer-based nanorods and nanotubes by "template synthesis": from preparation to applications, J. Polym. 53 (2012) 1149e1166. [5] L. Li, J. Ge, L. Wang, B. Guo, P.X. Ma, Electroactive nanofibrous biomimetic scaffolds by thermally induced phase separation, J. Mater. Chem. B 2 (2014) 6119. [6] N. Bhardwaj, S.C. Kundu, Electrospinning: a fascinating fiber fabrication technique, J. Biotechnol. Adv. 28 (2010) 325–347. [7] Su-jin Kim, Yu.Kyung Cho, Chongmok Lee, MyungHwa Kim n, Youngmi Lee, Realtime direct electrochemical sensing of ascorbic acid over rat liver tissues using RuO2 nanowires on electrospun TiO2 nanofibers, J. Biosens. Bioelectron. 77 (2016) 1144–1152. [8] K. Ahn, De Pham-Cong, H.S. Choi, S. Jeong, J.H. Cho, J. Kim, J. Kim, J. Bae, C. Cho, Bandgap-designed TiO2/SnO2 hollow hierarchical nanofibers: synthesis, properties, and their photocatalytic mechanism, J. Curr. Appl. Phys. 16 (2016) 251–260.
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