- Email: [email protected]
Processing, device fabrication and electrical characterization of LaMnO3 nanofibers
Materials Science in Semiconductor Processing 41 (2016) 364–369
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Processing, device fabrication and electrical characterization of LaMnO3 nanofibers Khizar Hayat a,b,1, S. Shaheen Shah a,1, M. Yousaf a,1, M. Javid Iqbal a,1, Muhammad Ali a,1, S. Ali a,1, Muhammad Ajmal c, Yaseen Iqbal a,n,1 a
Materials Research Laboratory, Department of Physics, University of Peshawar, 25120, Pakistan Department of Physics, Abdul Wali Khan University Mardan (AWKUM), 23200 Mardan, Pakistan c Department of Physics, Islamia College University, Peshawar 25120, Pakistan b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 September 2015 Received in revised form 5 October 2015 Accepted 8 October 2015
Lanthanum manganite (LaMnO3) nanofibers were synthesized using electrospinning technique. The size and uniformity of these nanofibers were optimized by varying PVP concentration. X-ray diffraction analysis revealed the formation of single phase LaMnO3 nanofibers (average diameter 400 nm) when the composite nanofibers were calcined at 600 °C. M″ and Z″ spectroscopic plots of impedance spectroscopy data confirmed the presence of two distinct electro-active regions referred to as the grain and grain boundary regions. The activation energies of the grain and grain boundary regions were 0.27 eV and 0.41 eV, respectively; which suggested two different transport mechanisms in these fibers. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Electrospinning Nanofibers Electrical microstructure Electro-active regions Impedance spectroscopy
1. Introduction Complex oxides with perovskite or perovskite-related structure have attracted increased research attention due to their diverse technological applications such as electrode materials in solid oxide fuel cells (SOFCs) [1], nano-filtration membranes and gas sensors [2–4]. Lanthanum manganite (LaMnO3) crystallizes into perovskite structure and is known for its interesting electric, magnetic and structural properties [5–9]; therefore, its in-depth study is important for understanding both the basic physics and related technology [2,10]. Furthermore, it is considered as a potential candidate material for device fabrication for applications in the field of colossal magneto-resistance [11] and SOFCs [12,13]. Nano-structured materials may be in the form of nanoparticles, nanorods, nanowires, nanofibers, nanoplates, or nanowhiskers. These materials exhibit unique physical and chemical properties due to their small size and large surface area in comparison to their bulk counterparts [14]. Consequently, controlling the size, shape and structure of these materials is vital for their applications in nanotechnology. Recently, one dimensional (1D) organic [15– 17], inorganic [2,18–20] and composite nanofibers [21,22] were
investigated because of their relatively larger surface area, fibrous morphology, flexible nature and small diameter [23]. 1D nanofibers are used as metal oxide electrodes to increase the energy conversion efficiency of fuel cells and dye-sensitized solar cells [14,16,17]. Furthermore, these nanofibers are also used in tissue engineering, nano-filtration, sensors, nano-electronics, photovoltaics and ultra-light weight spacecraft materials [16]. A number of techniques such as phase separation [24], self-assembly [25], drawing [26] and template synthesis [27], can be used for synthesis of 1D nanofibers. Electrospinning is relatively less laborious and more cost-effective and was, therefore, used in the present study. Furthermore, studies regarding the synthesis of LaMnO3 nanofibers via electrospinning technique are relatively limited while the electrical characterization results of these nanofibers have not been reported so far. In this paper, we report the processing, device fabrication and electrical microstructure of LaMnO3 nanofibers investigated via temperature-dependent impedance spectroscopy.
2. Experimental details n
Corresponding author. E-mail address: [email protected] (Y. Iqbal). 1 Fax: þ92 91 9216473.
http://dx.doi.org/10.1016/j.mssp.2015.10.009 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
Lanthanum nitrate hexahydrate (La(NO3)3 6H2O), manganese acetate tetrahydrate (Mn(CH3COO)2 4H2O), polyvinyl pyrrolidone
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Fig. 1. (a) A schematic showing the preparation of the homogenous viscous solution and La(NO3)3/PVP/Mn(CH3COO)2 composite fibers via ES technique, (b, c) variation in the surface smoothness of the composite fibers as a function of PVP concentration, and (d, e) SEIs of samples heat treated at 100 and 600 °C, respectively. The inset image in Fig. 1(e) shows the high magnification image of the same micro-region. EDX spectrum of the micro-region of the sample heat treated at 600 °C is superimposed on Fig. 1(e).
(PVP, Mw E1,300,000) and ethanol were used as starting materials to synthesize lanthanum manganite (LaMnO3) nanofibers. Fig. 1 (a) shows a schematic of the preparation of a homogenous viscous solution and the process of formation of fibers using electrospinning (ES) technique. La(NO3)3 6H2O and Mn(CH3COO)2 4H2O (0.5 g each) were dissolved separately in 3 ml ethanol and thoroughly stirred for 20 min. 0.67 g PVP was dissolved in 6 ml ethanol followed by stirring for 20 min. Lanthanum nitrate and manganese acetate solutions were poured into the PVP solution and the final mixture was stirred for 2 h at 50 °C in order to prepare a homogeneous viscous solution. The final solution was loaded into a 10 cc surgical plastic syringe of the ES setup for the production of La(NO3)3/PVP/Mn(CH3COO)2 composite fibers. ES setup consisted of an injection pump, fiber collector and a high voltage DC power supply. The distance ( 14 cm) between the tip of the stainless steel (SS) needle of the loaded syringe and the collector was kept constant. The collector was covered by aluminum (Al) foil to avoid any damage in the collection process. A high voltage DC power supply was employed to apply a uniform strong electric field between the needle and the collector. At a voltage of 17 kV, a continuous jet of La(NO3)3/PVP/Mn(CH3COO)2 composite nanofibers was produced and deposited over the Al-foil. These composite fibers were initially heat treated in a drying oven at 100 °C for 3 h. The processing conditions were optimized for the production of uniform and homogeneous composite nanofibers by varying PVP concentration. The microstructure of the fabricated nanofibers was examined using a JSM-5910, JEOL (Japan) scanning electron microscope (SEM) equipped with an INCA 200 (Oxford Instruments) energy dispersive X-ray electron spectroscopy (EDS) detector for chemical (elemental) analysis of the dried and calcined nanofibers. Thermo-gravimetric analysis (TGA) was performed using a Perkin Elmer (USA) Diamond series TG/DTA unit, to investigate the temperature of evaporation of volatile constituents and evolution of PVP from nanofibers. A Rigaku Geiger flux X-ray Diffractometer with CuKα1 (λ ¼1.504 Å) source was used for phase analysis. Fourier Transform Infra Red (FTIR) spectroscopy (Nicolet 6700) was employed to investigate chemical changes in the nanofibers as a function of heat treatment temperature. An Agilent E4980A LCR meter (20 Hz to 2 MHz) was used to perform temperature
dependent (298–378 K) impedance spectroscopy to distinguish different electro-active regions and determine the corresponding activation energies.
3. Results and discussion Fig. 1(b) is a secondary electron SEM image (SEI) recorded for the as-dried composite fibers obtained at 1 g PVP concentration. A close inspection of these fibers indicated that these fiber were non-uniform and their diameter varied from 1 to 2 μm. When the PVP concentration was decreased to 0.67 g, the formation of uniform and homogenous composite nanofibers with an average diameter of 600 nm was observed (Fig. 1(c)). It can be seen in the inset of Fig. 1(c) that the surface of nanofibers fabricated at the optimum PVP concentration was almost smooth. Fig. 1(d) is the SEI recorded for the composite nanofibers heat treated at 100 °C while Fig. 1(e) is the SEI recorded for the LaMnO3 nanofibers heat treated at 600 °C. These images demonstrated the variation in surface smoothness of nanofibers with an increase in heat treatment temperature. At 100 °C, the surface of the composite nanofibers was smooth with an average diameter of 600 nm; however, as the temperature was raised to 600 °C, the surface of these nanofibers appeared relatively coarser or granular (see inset in Fig. 1e). The average diameter of nanofibers treated at 600 °C decreased to 400 nm. The observed decrease in the diameter of nanofibers may be due to the decomposition, subsequent removal of the constituent PVP and shrinkage. The removal of PVP, enabled the La- and Mn- ions to react with each other in the presence of environmental oxygen, and formed crystalline LaMnO3. Fig. 1(e) also shows an EDS spectrum (superimposed over the corresponding SEI) recorded for the sample heat treated at 600 °C which confirmed the presence of La, Mn and O in these nanofibers, and no organic materials. The peaks due to gold (Au) were due to the Au coating of the sample, commonly used to avoid charging in SEM. Fig. 2(a) shows the TGA curve of the La(NO3)3/PVP/Mn(CH3COO)2 composite fibers, recorded in air atmosphere in the temperature range 30–800 °C. The first downwards slop of the TGA curve observed at 50–90 °C with a
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Fig. 2. (a) TGA curve of PVP/La(NO3)3/Mn(CH3COO)2 composite fibers heat treated at 100 °C, (b) FT-IR spectra of pure PVP and the samples heat treated at 100, 300 and 600 °C, (c) XRD patterns of samples heat treated at 100, 300 and 600 °C along with the standard XRD pattern of single phase LaMnO3 ceramics (JCPDS card# 75–440), and (d) schematic of the device fabrication of LaMnO3 nanofibers.
corresponding weight loss of 11% may be due to the loss of moisture i.e. dehydration of the sample. The second weight loss region was observed at 250–300 °C equivalent to 21% weight loss which may be associated with the decomposition of the nitrate, acetate and PVP chains. The third downwards slope with a corresponding weight loss of 26% was observed at 390–440 °C, probably due to the oxidation combustion of PVP main chain [28]. Fig. 2(a) demonstrated that the composite nanofibers had to be heated at temperatures higher than at least 450 °C to ensure the removal of all the volatile constituents such as nitrides, main chains of PVP and consequent formation of LaMnO3 nanofibers. Fig. 2(b) shows the FTIR spectra of pure PVP and the samples heat treated at 100, 300 and 600 °C. Five dominant absorption peaks were observed on the FTIR spectrum for pure PVP. The two broad peaks at 3450 cm 1 and 2950 cm 1 may be due to the stretching vibrations of the hydroxyl group (νO–H) and C–H bond (νC–H) respectively [29]. The other three sharp peaks at 1650, 1421, and 1270 cm 1 may be associated with the stretching of νC ¼ O, νC–H, and νC–N (or νC–O) bonds, respectively [29]. In the case of PVP/La(NO3)3/Mn(CH3COO)2 composite nanofibers heat treated at 100 °C, all the observed FTIR peaks were still matching those from pure PVP but with relatively lower intensities. When the heat treatment temperature was increased to 300 °C, the intensity of the PVP peaks decreased further and two new peaks emerged at 595 cm 1 and 478 cm 1 which could be clearly identified on the FTIR spectrum recorded for the sample heat treated at 600 °C. No peaks corresponding to PVP could be seen at 600°C, indicating its complete removal. The two new prominent peaks observed at 595 cm 1 and 478 cm 1 are due to the stretching and
bending modes (associated with internal phonons) of MnO6 octahedra in LaMnO3 crystal structure. The peak observed at 595 cm 1 may be due to the Mn–O bond vibration and the peak observed at 478 cm 1 may be associated with the change of bond angle of Mn–O–Mn in MnO6 octahedra [30]. Fig. 2(c) shows the XRD patterns of the composite nanofibers heat treated at 100, 300 and 600 °C. The standard XRD pattern of single phase LaMnO3 ceramics (JCPDS card# 75–440) is shown on the top of Fig. 2(c) for comparison. The XRD patterns recorded for the samples heat treated at 100 and 300 °C were typical of amorphous materials as no crystalline peaks could be observed on these patterns. Six (06) broad diffraction peaks were observed on the XRD pattern of the sample heat treated at 600 °C which matched the (100), (110), (111), (200), (210) and (211) reflections of the standard XRD pattern (JCPDS card# 75–440) for single phase LaMnO3 ceramics. This demonstrated the formation of single phase LaMnO3 nanofibers at 600 °C. Fig. 2(d) shows the schematic of the device fabrication of LaMnO3 nanofibers. A ceramic substrate, containing highly conductive copper (Cu) layer followed by a photo-resistive layer, was used for the fabrication of the inter-digitated electrodes. The photo-resistive layer was masked by an inter-digitated electrodetype pattern and exposed to UV-light for 2–3 min. In order to soften the exposed area, the ceramic substrate was dipped for 5– 10 min in a NaOH solution prepared in distilled water, followed by thorough washing to remove the soft photo-resistive layer. For etching the exposed Cu layer, the ceramic substrate was dipped and shacked for 5–10 min in FeCl3 solution. In order to remove the photo-resistive layer followed by the pattern, the substrate was
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Fig. 3. (a, b). Complex impedance plane plots at selected temperatures, and (c, d) spectroscopic plots of Z″ at selected temperatures.
dipped in Na2CO3 solution. Finally, the substrate was thoroughly washed with water and then ethanol. Each electrode contained five combs 8 mm in length and the average distance between two consecutive combs was 20 mm. The combs were then covered by a paste of LaMnO3 nanofibers prepared in ethanol. Finally, the device was heat treated in an oven at 150 °C to establish good electrical contacts. Fig. 3(a) and (b) shows the complex impedance plane plots (Z′ vs. Z″) at temperatures ranging from 298 K (room temperature) to 378 K. These complex impedance plane plots give information in the form of semicircles or arcs and each arc represents an electroactive region with an associated relaxation time, τ ¼ RC, where τ, R and C are the relaxation time, resistance and capacitance of the charge carriers, respectively. The arrow direction (Fig. 3(a) and (b)) shows the increase in angular frequency (ω). At low frequencies, the intercept of the impedance arc with Z′-axis gives the total resistance “R” of these nanofibers. The radius of each arc showed a decreasing trend with increasing temperature, illustrating a decrease in the resistivity of nanofibers with an increase in temperature, a typical characteristic of semiconducting materials. These arcs appeared overlapped and depressed with their centers lying below the Z′-axis at all the employed temperatures which suggested the presence of more than one electro-active region in these materials. A justification of more than one electro-active region is shown in the (M″, Z″) spectroscopic plots. Fig. 3(c) and (d) shows the spectroscopic plots of the imaginary component of the complex impedance Z″ i.e. log f vs. Z″. These plots are useful in investigating the behavior of the most resistive region in a sample. From these spectroscopic plots, the R and C values were calculated at 298 K, using the peak height and associated relaxation frequency of the Z″ peak i.e. Rgb ¼2Z″max and the relationship (2πfmax) RC¼1 at the peak maximum, respectively. Similarly, the resistance and capacitance values of the relatively
higher temperatures can be calculated using the aforementioned equations. On the basis of the magnitude of the capacitance values, the electro-active region at 298 K was identified as the grain boundary region consistent with the criterion proposed by Irvine et al. [31]. The shift and broadening of the peaks are associated with temperature variation. Upon a decrease in temperature from 378 K to 298 K, the peaks shifted from higher to lower frequencies, indicating a decrease in the mobility of the charge carriers in the nanofibers. Also with decreasing temperature, broadening of the peaks was observed which suggested the occurrence of more than one relaxation phenomenon in these nanofibers and this may be due to the relatively shorter relaxation time [32]. Complex impedance plane plot is considered as an appropriate method for presenting such results; however, the alternative formalism can give additional information that is not easily obtainable from the complex impedance plane plot. The single semicircle (Fig. 4(a)) observed in the complex impedance plane plot showed a subtle deviation from ideality at high frequencies. When the same data (i.e. complex impedance plane plot at 298 K) was reprocessed and presented in the form of the complex electric modulus, M*, then two semicircles were observed, shown in Fig. 4 (b). In the modulus plot, the arrow direction shows the increase in angular frequency (ω). This phenomenon was demonstrated more clearly in the form of spectroscopic plots of the imaginary components M″ and Z″, shown in Fig. 4(c). Presenting data as a combined (M″, Z″) spectroscopic plot gives different weightings to the data, showing different features of the investigated materials [33]. The Z″ spectroscopic plots identified the most resistive region in the investigated nanofibers as the Zmax ‵‵ peak height is equal to R/2. Similarly the M″ spectroscopic plots recognized the regions with the smallest capacitance as the modulus peak height is equal to ɛo/2C for that particular region [34]. In Fig. 4(c), the Z″ spectroscopic plot contains a single peak which has already been
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Fig. 4. (a) Impedance complex plane plot at 298 K, (b) modulus complex plane plot at 298 K, (c) combined (M″, Z″) spectroscopic plots at 298 K, (d, e) C′ spectroscopic plots at 378 and 298 K, respectively, and (f) Arrhenius plots of bulk/grain (extracted from M″ plots) and grain boundary (extracted from Z″ plots).
assigned to the grain boundary region, discussed with reference to Fig. 3(c). The M″ spectroscopic plots gave two relaxations, the high and the low frequency relaxation peaks. The M″ spectrum has the associated capacitance values of 5.46 10 12 F and 2.30 10 11 F for the peaks at high and low frequencies, respectively, derived from the height and associated fmax values of M″ peak i.e., C ¼1/ (2 Mmax ‵‵ ). These high and low frequency relaxation regions are referred to as the grain and grain boundary regions proposed by Irvine et al. [31]. Fig. 4(d) and (e) shows the capacitance C′ spectroscopic plots at the selected temperatures. At 378 K, the capacitance was dominated by the grain boundary response up to 102 kHz and the grain response emerged at higher frequencies. As the temperature was decreased to 298 K, the grain boundary region shifted to low frequency values i.e., 101 kHz and the grain response was more pronounced at higher frequencies. Arrhenius plots of 1/Rg (extracted from the M″ spectroscopic plots) and 1/Rgb (extracted from the Z″ spectroscopic plots) are shown in Fig. 4(f). The grain and grain boundary conduction processes in LaMnO3 nanofibers were found to have activation energies (Ea) of 0.27 eV and 0.41 eV, respectively. The lower value of the activation energy for the grain (bulk) showed its more conducting nature than the grain boundary region. These two electro-active regions having different thermal activation energies suggested two different transport mechanisms in the investigated LaMnO3 nanofibers.
4. Conclusions Lanthanum manganite nanofibers were synthesized via electrospinning technique. It was evident from the SEM micrographs that the morphology of nanofibers varied with heat treatment temperature. At 100 °C, the surface of the composite nanofibers was smooth with an average diameter of 600 nm; however, as the heat treatment temperature was raised to 600 °C, the surface of the nanofibers became relatively coarser i.e. granular and their average diameter decreased to 400 nm. TGA revealed that these
fibers required calcination at temperatures above 450 °C for the formation of LaMnO3 nanofibers and decomposition of all the volatile entities, nitrides and main chains of PVP. The FTIR spectroscopy and X-ray diffraction confirmed the removal of volatiles and the formation of LaMnO3 phase at 600 °C. The temperaturedependent impedance spectroscopy results showed that LaMnO3 nanofibers behaved as a semiconductor material. The two electroactive regions associated with the grain and grain boundary were identified using (M″, Z″) spectroscopic plots. These grain and grain boundary regions had different activation energies i.e. 0.27 eV and 0.41 eV, respectively; which suggested two different transport mechanisms in the investigated nanofibers.
Acknowledgments The authors acknowledge the financial support (ADP no. 130314) extended by the Khyber Pakhtunkhwa Government through the Directorate of S&T, Peshawar for the upgradation of Materials Research Laboratory, University of Peshawar.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Processing, device fabrication and electrical characterization of LaMnO3 nanofibers Khizar Hayat a,b,1, S. Shaheen Shah a,1, M. Yousaf a,1, M. Javid Iqbal a,1, Muhammad Ali a,1, S. Ali a,1, Muhammad Ajmal c, Yaseen Iqbal a,n,1 a
Materials Research Laboratory, Department of Physics, University of Peshawar, 25120, Pakistan Department of Physics, Abdul Wali Khan University Mardan (AWKUM), 23200 Mardan, Pakistan c Department of Physics, Islamia College University, Peshawar 25120, Pakistan b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 September 2015 Received in revised form 5 October 2015 Accepted 8 October 2015
Lanthanum manganite (LaMnO3) nanofibers were synthesized using electrospinning technique. The size and uniformity of these nanofibers were optimized by varying PVP concentration. X-ray diffraction analysis revealed the formation of single phase LaMnO3 nanofibers (average diameter 400 nm) when the composite nanofibers were calcined at 600 °C. M″ and Z″ spectroscopic plots of impedance spectroscopy data confirmed the presence of two distinct electro-active regions referred to as the grain and grain boundary regions. The activation energies of the grain and grain boundary regions were 0.27 eV and 0.41 eV, respectively; which suggested two different transport mechanisms in these fibers. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Electrospinning Nanofibers Electrical microstructure Electro-active regions Impedance spectroscopy
1. Introduction Complex oxides with perovskite or perovskite-related structure have attracted increased research attention due to their diverse technological applications such as electrode materials in solid oxide fuel cells (SOFCs) [1], nano-filtration membranes and gas sensors [2–4]. Lanthanum manganite (LaMnO3) crystallizes into perovskite structure and is known for its interesting electric, magnetic and structural properties [5–9]; therefore, its in-depth study is important for understanding both the basic physics and related technology [2,10]. Furthermore, it is considered as a potential candidate material for device fabrication for applications in the field of colossal magneto-resistance [11] and SOFCs [12,13]. Nano-structured materials may be in the form of nanoparticles, nanorods, nanowires, nanofibers, nanoplates, or nanowhiskers. These materials exhibit unique physical and chemical properties due to their small size and large surface area in comparison to their bulk counterparts [14]. Consequently, controlling the size, shape and structure of these materials is vital for their applications in nanotechnology. Recently, one dimensional (1D) organic [15– 17], inorganic [2,18–20] and composite nanofibers [21,22] were
investigated because of their relatively larger surface area, fibrous morphology, flexible nature and small diameter [23]. 1D nanofibers are used as metal oxide electrodes to increase the energy conversion efficiency of fuel cells and dye-sensitized solar cells [14,16,17]. Furthermore, these nanofibers are also used in tissue engineering, nano-filtration, sensors, nano-electronics, photovoltaics and ultra-light weight spacecraft materials [16]. A number of techniques such as phase separation [24], self-assembly [25], drawing [26] and template synthesis [27], can be used for synthesis of 1D nanofibers. Electrospinning is relatively less laborious and more cost-effective and was, therefore, used in the present study. Furthermore, studies regarding the synthesis of LaMnO3 nanofibers via electrospinning technique are relatively limited while the electrical characterization results of these nanofibers have not been reported so far. In this paper, we report the processing, device fabrication and electrical microstructure of LaMnO3 nanofibers investigated via temperature-dependent impedance spectroscopy.
2. Experimental details n
Corresponding author. E-mail address: [email protected] (Y. Iqbal). 1 Fax: þ92 91 9216473.
http://dx.doi.org/10.1016/j.mssp.2015.10.009 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
Lanthanum nitrate hexahydrate (La(NO3)3 6H2O), manganese acetate tetrahydrate (Mn(CH3COO)2 4H2O), polyvinyl pyrrolidone
K. Hayat et al. / Materials Science in Semiconductor Processing 41 (2016) 364–369
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Fig. 1. (a) A schematic showing the preparation of the homogenous viscous solution and La(NO3)3/PVP/Mn(CH3COO)2 composite fibers via ES technique, (b, c) variation in the surface smoothness of the composite fibers as a function of PVP concentration, and (d, e) SEIs of samples heat treated at 100 and 600 °C, respectively. The inset image in Fig. 1(e) shows the high magnification image of the same micro-region. EDX spectrum of the micro-region of the sample heat treated at 600 °C is superimposed on Fig. 1(e).
(PVP, Mw E1,300,000) and ethanol were used as starting materials to synthesize lanthanum manganite (LaMnO3) nanofibers. Fig. 1 (a) shows a schematic of the preparation of a homogenous viscous solution and the process of formation of fibers using electrospinning (ES) technique. La(NO3)3 6H2O and Mn(CH3COO)2 4H2O (0.5 g each) were dissolved separately in 3 ml ethanol and thoroughly stirred for 20 min. 0.67 g PVP was dissolved in 6 ml ethanol followed by stirring for 20 min. Lanthanum nitrate and manganese acetate solutions were poured into the PVP solution and the final mixture was stirred for 2 h at 50 °C in order to prepare a homogeneous viscous solution. The final solution was loaded into a 10 cc surgical plastic syringe of the ES setup for the production of La(NO3)3/PVP/Mn(CH3COO)2 composite fibers. ES setup consisted of an injection pump, fiber collector and a high voltage DC power supply. The distance ( 14 cm) between the tip of the stainless steel (SS) needle of the loaded syringe and the collector was kept constant. The collector was covered by aluminum (Al) foil to avoid any damage in the collection process. A high voltage DC power supply was employed to apply a uniform strong electric field between the needle and the collector. At a voltage of 17 kV, a continuous jet of La(NO3)3/PVP/Mn(CH3COO)2 composite nanofibers was produced and deposited over the Al-foil. These composite fibers were initially heat treated in a drying oven at 100 °C for 3 h. The processing conditions were optimized for the production of uniform and homogeneous composite nanofibers by varying PVP concentration. The microstructure of the fabricated nanofibers was examined using a JSM-5910, JEOL (Japan) scanning electron microscope (SEM) equipped with an INCA 200 (Oxford Instruments) energy dispersive X-ray electron spectroscopy (EDS) detector for chemical (elemental) analysis of the dried and calcined nanofibers. Thermo-gravimetric analysis (TGA) was performed using a Perkin Elmer (USA) Diamond series TG/DTA unit, to investigate the temperature of evaporation of volatile constituents and evolution of PVP from nanofibers. A Rigaku Geiger flux X-ray Diffractometer with CuKα1 (λ ¼1.504 Å) source was used for phase analysis. Fourier Transform Infra Red (FTIR) spectroscopy (Nicolet 6700) was employed to investigate chemical changes in the nanofibers as a function of heat treatment temperature. An Agilent E4980A LCR meter (20 Hz to 2 MHz) was used to perform temperature
dependent (298–378 K) impedance spectroscopy to distinguish different electro-active regions and determine the corresponding activation energies.
3. Results and discussion Fig. 1(b) is a secondary electron SEM image (SEI) recorded for the as-dried composite fibers obtained at 1 g PVP concentration. A close inspection of these fibers indicated that these fiber were non-uniform and their diameter varied from 1 to 2 μm. When the PVP concentration was decreased to 0.67 g, the formation of uniform and homogenous composite nanofibers with an average diameter of 600 nm was observed (Fig. 1(c)). It can be seen in the inset of Fig. 1(c) that the surface of nanofibers fabricated at the optimum PVP concentration was almost smooth. Fig. 1(d) is the SEI recorded for the composite nanofibers heat treated at 100 °C while Fig. 1(e) is the SEI recorded for the LaMnO3 nanofibers heat treated at 600 °C. These images demonstrated the variation in surface smoothness of nanofibers with an increase in heat treatment temperature. At 100 °C, the surface of the composite nanofibers was smooth with an average diameter of 600 nm; however, as the temperature was raised to 600 °C, the surface of these nanofibers appeared relatively coarser or granular (see inset in Fig. 1e). The average diameter of nanofibers treated at 600 °C decreased to 400 nm. The observed decrease in the diameter of nanofibers may be due to the decomposition, subsequent removal of the constituent PVP and shrinkage. The removal of PVP, enabled the La- and Mn- ions to react with each other in the presence of environmental oxygen, and formed crystalline LaMnO3. Fig. 1(e) also shows an EDS spectrum (superimposed over the corresponding SEI) recorded for the sample heat treated at 600 °C which confirmed the presence of La, Mn and O in these nanofibers, and no organic materials. The peaks due to gold (Au) were due to the Au coating of the sample, commonly used to avoid charging in SEM. Fig. 2(a) shows the TGA curve of the La(NO3)3/PVP/Mn(CH3COO)2 composite fibers, recorded in air atmosphere in the temperature range 30–800 °C. The first downwards slop of the TGA curve observed at 50–90 °C with a
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Fig. 2. (a) TGA curve of PVP/La(NO3)3/Mn(CH3COO)2 composite fibers heat treated at 100 °C, (b) FT-IR spectra of pure PVP and the samples heat treated at 100, 300 and 600 °C, (c) XRD patterns of samples heat treated at 100, 300 and 600 °C along with the standard XRD pattern of single phase LaMnO3 ceramics (JCPDS card# 75–440), and (d) schematic of the device fabrication of LaMnO3 nanofibers.
corresponding weight loss of 11% may be due to the loss of moisture i.e. dehydration of the sample. The second weight loss region was observed at 250–300 °C equivalent to 21% weight loss which may be associated with the decomposition of the nitrate, acetate and PVP chains. The third downwards slope with a corresponding weight loss of 26% was observed at 390–440 °C, probably due to the oxidation combustion of PVP main chain [28]. Fig. 2(a) demonstrated that the composite nanofibers had to be heated at temperatures higher than at least 450 °C to ensure the removal of all the volatile constituents such as nitrides, main chains of PVP and consequent formation of LaMnO3 nanofibers. Fig. 2(b) shows the FTIR spectra of pure PVP and the samples heat treated at 100, 300 and 600 °C. Five dominant absorption peaks were observed on the FTIR spectrum for pure PVP. The two broad peaks at 3450 cm 1 and 2950 cm 1 may be due to the stretching vibrations of the hydroxyl group (νO–H) and C–H bond (νC–H) respectively [29]. The other three sharp peaks at 1650, 1421, and 1270 cm 1 may be associated with the stretching of νC ¼ O, νC–H, and νC–N (or νC–O) bonds, respectively [29]. In the case of PVP/La(NO3)3/Mn(CH3COO)2 composite nanofibers heat treated at 100 °C, all the observed FTIR peaks were still matching those from pure PVP but with relatively lower intensities. When the heat treatment temperature was increased to 300 °C, the intensity of the PVP peaks decreased further and two new peaks emerged at 595 cm 1 and 478 cm 1 which could be clearly identified on the FTIR spectrum recorded for the sample heat treated at 600 °C. No peaks corresponding to PVP could be seen at 600°C, indicating its complete removal. The two new prominent peaks observed at 595 cm 1 and 478 cm 1 are due to the stretching and
bending modes (associated with internal phonons) of MnO6 octahedra in LaMnO3 crystal structure. The peak observed at 595 cm 1 may be due to the Mn–O bond vibration and the peak observed at 478 cm 1 may be associated with the change of bond angle of Mn–O–Mn in MnO6 octahedra [30]. Fig. 2(c) shows the XRD patterns of the composite nanofibers heat treated at 100, 300 and 600 °C. The standard XRD pattern of single phase LaMnO3 ceramics (JCPDS card# 75–440) is shown on the top of Fig. 2(c) for comparison. The XRD patterns recorded for the samples heat treated at 100 and 300 °C were typical of amorphous materials as no crystalline peaks could be observed on these patterns. Six (06) broad diffraction peaks were observed on the XRD pattern of the sample heat treated at 600 °C which matched the (100), (110), (111), (200), (210) and (211) reflections of the standard XRD pattern (JCPDS card# 75–440) for single phase LaMnO3 ceramics. This demonstrated the formation of single phase LaMnO3 nanofibers at 600 °C. Fig. 2(d) shows the schematic of the device fabrication of LaMnO3 nanofibers. A ceramic substrate, containing highly conductive copper (Cu) layer followed by a photo-resistive layer, was used for the fabrication of the inter-digitated electrodes. The photo-resistive layer was masked by an inter-digitated electrodetype pattern and exposed to UV-light for 2–3 min. In order to soften the exposed area, the ceramic substrate was dipped for 5– 10 min in a NaOH solution prepared in distilled water, followed by thorough washing to remove the soft photo-resistive layer. For etching the exposed Cu layer, the ceramic substrate was dipped and shacked for 5–10 min in FeCl3 solution. In order to remove the photo-resistive layer followed by the pattern, the substrate was
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Fig. 3. (a, b). Complex impedance plane plots at selected temperatures, and (c, d) spectroscopic plots of Z″ at selected temperatures.
dipped in Na2CO3 solution. Finally, the substrate was thoroughly washed with water and then ethanol. Each electrode contained five combs 8 mm in length and the average distance between two consecutive combs was 20 mm. The combs were then covered by a paste of LaMnO3 nanofibers prepared in ethanol. Finally, the device was heat treated in an oven at 150 °C to establish good electrical contacts. Fig. 3(a) and (b) shows the complex impedance plane plots (Z′ vs. Z″) at temperatures ranging from 298 K (room temperature) to 378 K. These complex impedance plane plots give information in the form of semicircles or arcs and each arc represents an electroactive region with an associated relaxation time, τ ¼ RC, where τ, R and C are the relaxation time, resistance and capacitance of the charge carriers, respectively. The arrow direction (Fig. 3(a) and (b)) shows the increase in angular frequency (ω). At low frequencies, the intercept of the impedance arc with Z′-axis gives the total resistance “R” of these nanofibers. The radius of each arc showed a decreasing trend with increasing temperature, illustrating a decrease in the resistivity of nanofibers with an increase in temperature, a typical characteristic of semiconducting materials. These arcs appeared overlapped and depressed with their centers lying below the Z′-axis at all the employed temperatures which suggested the presence of more than one electro-active region in these materials. A justification of more than one electro-active region is shown in the (M″, Z″) spectroscopic plots. Fig. 3(c) and (d) shows the spectroscopic plots of the imaginary component of the complex impedance Z″ i.e. log f vs. Z″. These plots are useful in investigating the behavior of the most resistive region in a sample. From these spectroscopic plots, the R and C values were calculated at 298 K, using the peak height and associated relaxation frequency of the Z″ peak i.e. Rgb ¼2Z″max and the relationship (2πfmax) RC¼1 at the peak maximum, respectively. Similarly, the resistance and capacitance values of the relatively
higher temperatures can be calculated using the aforementioned equations. On the basis of the magnitude of the capacitance values, the electro-active region at 298 K was identified as the grain boundary region consistent with the criterion proposed by Irvine et al. [31]. The shift and broadening of the peaks are associated with temperature variation. Upon a decrease in temperature from 378 K to 298 K, the peaks shifted from higher to lower frequencies, indicating a decrease in the mobility of the charge carriers in the nanofibers. Also with decreasing temperature, broadening of the peaks was observed which suggested the occurrence of more than one relaxation phenomenon in these nanofibers and this may be due to the relatively shorter relaxation time [32]. Complex impedance plane plot is considered as an appropriate method for presenting such results; however, the alternative formalism can give additional information that is not easily obtainable from the complex impedance plane plot. The single semicircle (Fig. 4(a)) observed in the complex impedance plane plot showed a subtle deviation from ideality at high frequencies. When the same data (i.e. complex impedance plane plot at 298 K) was reprocessed and presented in the form of the complex electric modulus, M*, then two semicircles were observed, shown in Fig. 4 (b). In the modulus plot, the arrow direction shows the increase in angular frequency (ω). This phenomenon was demonstrated more clearly in the form of spectroscopic plots of the imaginary components M″ and Z″, shown in Fig. 4(c). Presenting data as a combined (M″, Z″) spectroscopic plot gives different weightings to the data, showing different features of the investigated materials [33]. The Z″ spectroscopic plots identified the most resistive region in the investigated nanofibers as the Zmax ‵‵ peak height is equal to R/2. Similarly the M″ spectroscopic plots recognized the regions with the smallest capacitance as the modulus peak height is equal to ɛo/2C for that particular region [34]. In Fig. 4(c), the Z″ spectroscopic plot contains a single peak which has already been
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Fig. 4. (a) Impedance complex plane plot at 298 K, (b) modulus complex plane plot at 298 K, (c) combined (M″, Z″) spectroscopic plots at 298 K, (d, e) C′ spectroscopic plots at 378 and 298 K, respectively, and (f) Arrhenius plots of bulk/grain (extracted from M″ plots) and grain boundary (extracted from Z″ plots).
assigned to the grain boundary region, discussed with reference to Fig. 3(c). The M″ spectroscopic plots gave two relaxations, the high and the low frequency relaxation peaks. The M″ spectrum has the associated capacitance values of 5.46 10 12 F and 2.30 10 11 F for the peaks at high and low frequencies, respectively, derived from the height and associated fmax values of M″ peak i.e., C ¼1/ (2 Mmax ‵‵ ). These high and low frequency relaxation regions are referred to as the grain and grain boundary regions proposed by Irvine et al. [31]. Fig. 4(d) and (e) shows the capacitance C′ spectroscopic plots at the selected temperatures. At 378 K, the capacitance was dominated by the grain boundary response up to 102 kHz and the grain response emerged at higher frequencies. As the temperature was decreased to 298 K, the grain boundary region shifted to low frequency values i.e., 101 kHz and the grain response was more pronounced at higher frequencies. Arrhenius plots of 1/Rg (extracted from the M″ spectroscopic plots) and 1/Rgb (extracted from the Z″ spectroscopic plots) are shown in Fig. 4(f). The grain and grain boundary conduction processes in LaMnO3 nanofibers were found to have activation energies (Ea) of 0.27 eV and 0.41 eV, respectively. The lower value of the activation energy for the grain (bulk) showed its more conducting nature than the grain boundary region. These two electro-active regions having different thermal activation energies suggested two different transport mechanisms in the investigated LaMnO3 nanofibers.
4. Conclusions Lanthanum manganite nanofibers were synthesized via electrospinning technique. It was evident from the SEM micrographs that the morphology of nanofibers varied with heat treatment temperature. At 100 °C, the surface of the composite nanofibers was smooth with an average diameter of 600 nm; however, as the heat treatment temperature was raised to 600 °C, the surface of the nanofibers became relatively coarser i.e. granular and their average diameter decreased to 400 nm. TGA revealed that these
fibers required calcination at temperatures above 450 °C for the formation of LaMnO3 nanofibers and decomposition of all the volatile entities, nitrides and main chains of PVP. The FTIR spectroscopy and X-ray diffraction confirmed the removal of volatiles and the formation of LaMnO3 phase at 600 °C. The temperaturedependent impedance spectroscopy results showed that LaMnO3 nanofibers behaved as a semiconductor material. The two electroactive regions associated with the grain and grain boundary were identified using (M″, Z″) spectroscopic plots. These grain and grain boundary regions had different activation energies i.e. 0.27 eV and 0.41 eV, respectively; which suggested two different transport mechanisms in the investigated nanofibers.
Acknowledgments The authors acknowledge the financial support (ADP no. 130314) extended by the Khyber Pakhtunkhwa Government through the Directorate of S&T, Peshawar for the upgradation of Materials Research Laboratory, University of Peshawar.
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