Successful electrospinning fabrication of ZrO2 nanofibers: A detailed physical – chemical characterization study

Journal of Alloys and Compounds 828 (2020) 154414

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Successful electrospinning fabrication of ZrO2 nanofibers: A detailed physical e chemical characterization study Saba Khalili , Hossein Mahmoudi Chenari * Department of Physics, Faculty of Science, University of Guilan, Namjoo Ave, Po Box 41335-1914, Rasht, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 November 2019 Received in revised form 1 February 2020 Accepted 17 February 2020 Available online xxx

Electrospinning fabrication of Zirconia nanofibers was successfully achieved, followed by different thermal treatment. A careful physical characterization study was performed based on the calcination temperature effect, which was confirmed by a wide range of analysis such as SEM, TG-DT analysis, XRD, FT-IR, DRS, VSM, and PL spectra. The morphological characterization of both polymer and ceramic nanofibers was achieved by SEM images through an image analyzing software. The average diameter of the produced nanofibers was estimated below 100 nm after rising thermal treatment temperature. The thermal behavior of the as-spun ZrO2 nanofibers was studied using thermogravimetric analyses (TGA). The whole powder pattern modeling (WPPM) approach using the PM2K package and the Rietveld refinement approach were applied on the X-ray diffraction pattern to get insight into the effect of calcination temperature on the microstructure parameters of nanostructured zirconia fibers. A comprehensive study was done on the chemical properties of the fabricated samples based on the absorption bands using FT-IR spectroscopy. Chemical study shows a good correlation between the XRD and FT-IR findings. The optical properties were investigated using diffuse reflectance spectroscopy and roomtemperature photoluminescence spectra. An enhancement of PL intensity of the ZrO2 calcined at 800
C could be due to its high crystal quality. The magnetic properties of zirconia nanofibers were investigated by a vibrating sample magnetometer (VSM). Room temperature magnetization results showed a hysteresis loop formation, indicating the ferromagnetic behavior of samples. © 2020 Elsevier B.V. All rights reserved.

Keywords: ZrO2 Nanofibers Electrospinning Characterization

1. Introduction Zirconium oxide is one of the most commonly used zirconium compounds in nature, which crystallizes in the three phases, monoclinic (m-ZrO2) [1], tetragonal (t-ZrO2) [2] and cubic (c-ZrO2) [3], at ambient pressure. Besides these three phases, the orthorhombic phase of ZrO2 is formed at high pressures [4e6]. The predominant phase depends severely on the parameters such as the sample preparation method, temperature, doping, oxygen vacancy, and crystallite size [7e11]. ZrO2 is a metal oxide semiconductor with a broad band gap [12]. The short-wavelength photoluminescence (PL) emission and broad band gap of zirconium dioxide can provide a suitable condition for light-emitting devices [13]. Due to high ionic conductivity, good mechanical strength, high

* Corresponding author. E-mail addresses: [email protected], [email protected] (H.M. Chenari). https://doi.org/10.1016/j.jallcom.2020.154414 0925-8388/© 2020 Elsevier B.V. All rights reserved.

melting point, excellent refractoriness, low thermal conductivity at high temperature, good thermal stability, and chemical resistance, ZrO2 includes a wide range of applications like sensors, air filtration, capacitors, thermal barrier and corrosion-resistant coatings, fuel cells, photocatalytic materials, medical field and ultraviolet light-emitting diode [14e22]. Also, ZrO2 nanofibers were added into TiO2 photoelectrode to improve efficiency of dye-sensitized solar cells [23]. Zirconia nanostructures have prepared in different methods, such as chemical vapor synthesis [24], sol-gel [25], pulsed laser deposition (PLD) [26], hydrothermal [27], and electrospinning [28,29]. Nanostructures in the form of nanofibers have gained lots of interest from both academia and industry due to their unique properties such as nano-scale diameter, high surface-tovolume ratio and porosity, flexibility and better mechanical performance. Among the various techniques to produce nanofibers, the electrospinning has advantages such as relatively simple procedure, cost-effective, continuity of operations, and production of fibers with complicated architectures [30e32]. Up

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to now, electrospinning synthesis of ZrO2 have been reported by some research group [28,29,33,34]. For example, Shao et al. prepared ZrOCl2/PVA composite fibres and studied morphological, and structural properties and chemical bond of samples [28]. The influence of calcination temperature and multistage heating regimes on crystal structure of ZrO2 nanofiber was shown by Davies et al. [29]. Also, ZrO2 nanofibers were fabricated from Zr(C5H7O2)4 and PAN by Rodaev et al. [33]. They revealed the stages of the transition of zirconium acetylacetonate to zirconia as followed: Zr(C5H7O2)4 / Zr(OH)(CH3COO)3 / ZrO(CH3COO)2 / t-ZrO2 / t-ZrO2 þ m-ZrO2 / m-ZrO2. Bezir et al. synthesized undoped and Dy-Eu-Ce co-doped ZrO2 nanofibers and studied their crystal structure, optical and electrical properties [34]. In view of optical properties, photoluminescence (PL) spectra of ZrO2 shows a broad emission band at around 410 nm, which attributed to the defects especially the singly ionized oxygen vacancy [13]. In addition, the effect of oxygen environment on the PL features of ZrO2 thin films was scrutinized [35]. To the best of our knowledge, there is not report focus on the photoluminescence emission study of ZrO2 nanofibers. Zirconia nanofiber mats have not been investigated in detail in view of the structural and optical properties. ZrO2 as one of the diluted magnetic semiconductors (DMS) has also engrossed the attention of many researchers in experimental study regarding the room temperature ferromagnetism (RTFM). Magnetic property of porous ZrO2 thin films prepared in different conditions was measured with the out-of-plane and in-plane applied field [36]. Also, Wang et al. found ferromagnetic behavior for ZrO2 nanocrystals annealed up to 1000
C, that the result showed descending magnetization with increment annealing temperature [37]. In another study, the effect of oxygen partial pressure during deposition on the room temperature ferromagnetism of un-doped zirconia thin films was investigated [38]. Up to our knowledge, the magnetic properties of zirconia nanofibers have not been studied. In this work, the zirconia nanofibers were successfully fabricated, aiming to get an insight into the calcination temperature effect on the physical properties. Hence, scanning electron microscopy (SEM), X-ray diffraction (XRD), UVeVis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL) and vibrating-sample magnetometer (VSM) analyses are accomplished to investigate morphological, structural, optical and magnetic properties, respectively. In addition to the mentioned analyses, energy dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric-differential thermal analysis (TG-DTA) are used to detection of the chemical composition, the chemical bonds, and the thermal stability of nanofibers.

2. Experimental 2.1. Chemicals and preparation of samples Zirconyl chloride octahydrate (ZrOCl2.8H2O), polyvinylpyrrolidone (Mw ¼ 1,300,000), and absolute ethanol (C2H5OH  99.9%) were purchased from Merck company. ZrOCl2/PVP nanofibers were prepared by Electroris (eSpinner NF COeN/VI, Iran, www.anstco.com) with a high voltage 1e35 kV, followed by calcination using muffle furnace (FM 8P, Iran, farazmaco.com). First, a 0.0021 M zirconium precursor solution was prepared by adding weighted amount of zirconyl chloride into absolute ethanol-water (1:1 mass ratio) binary mixture. It was stirred continuously using a magnetic stirrer (Delta model HM-101) at

ambient temperature for some minutes to achieve a homogeneous solution. Then, 0.9 g PVP was added to previous mixture, and the resulting solution stirred for 12 h at ambient temperature. The polymer solution was transferred to a plastic syringe with a 22-gauge metal needle. Electrospinning was performed by applying high voltages (12e20 kV) in different tip to collector distances (10e20 cm). Ultimately, optimum conditions for preparation appropriate polymer nanofibers were adjusted to 20 kV and 10 cm. As-spun nanofibers were dried at 50
C for 30 min and calcined at different temperatures for an hour with a heating rate of 2
C/min to remove polymer and obtain ceramic nanofibers. The zirconia nanofibers were produced after polymer removal from the structure using a thermal treatment at 500, 600, 700 and 800
C, labeled as S5, S6, S7 and S8, respectively. 2.2. Instrumentations The morphology of both polymer and ceramic nanofibers was detected on scanning electron microscope AIS2100 of Seron Technology Company. Elemental composition of ceramic nanofibers was obtained from EDS equipped to the FESEM TESCAN MIRA3. The thermal decomposition of the ZrOCl2/PVP composite nanofibers was studied by thermal gravimetric analysis (TA Co.: SDT Q600) under air atmosphere from 34
C to 800
C with a heating rate of 10
C/min. The X-ray diffraction data were gathered using CuKa radiation (l ¼ 1.5406 Å), operated at 40 kV and 40 mA, on an X’PERT-PRO PMD diffractometer in BraggeBrentano geometry. Spectroscopic characterization was performed on Fourier transform infrared spectrometer (Bruker Alpha model), and the spectra were taken in a region between 4000 and 400 cm1 as KBr pellets. The absorbance of the samples was recorded by UVevis diffuse reflectance spectrophotometer (UV-DRS, SCINCO: S-4100). The PerkinElmer (Model: LS-55l, pulsed Xenon lamp) was used to measure the PL spectra. The ambient temperature magnetic property of samples was investigated using VSM, Magnetic Daghigh Kavir: MDKB measurements. 3. Results and discussion 3.1. Morphology and elemental analysis Morphological characterization of both polymer and ceramic nanofibers was studied by SEM. The diameter size distribution histograms were achieved through an image analyzing software (Digimizer). As shown in Fig. 1, the surface of as-prepared and calcined fibers is smooth and free of beads. After polymer decomposition and solvents evaporation, continuity and intact shape of nanofibers retain. With increasing calcination temperature, no dramatic alteration in diameter could be detected. The average diameter of the produced nanofibers was estimated by fitting a lognormal distribution. It was observed that the estimated average diameter would increase with increasing the calcination temperature due to coalescence of fibers. Each of calcined samples showed a reduction in average diameter of nanofibers from 127 nm (51.94% relative standard deviation (RSD), ZrO2/PVP) to 61 nm (26.55% RSD, S5), 69 nm (28.89% RSD, S6), 82 nm (30.36% RSD, S7), and 77 nm (25.18% RSD, S8). Reduction in RSD amount after calcination indicated the majority of nanofibers have a diameter size around the mean value. In order to get insight into the morphological properties, two samples, S5 and S8, were studied by FESEM technique, which the images and the diameter size distribution histograms were presented in Fig. 2. As is evident in Fig. 2(b), the crystallites are clearly

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Fig. 1. SEM micrographs and diameter size distribution of nanofibers (a) before calcination, (b, c, d and e) after calcination at 500, 600, 700 and 800
C, respectively.

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Fig. 2. FESEM image and diameter size distribution of S5 and S8 samples.

respectively. The EDX analysis proves the presence of zirconia elements. 3.2. TG-DT analysis

Fig. 3. Energy dispersive X-ray spectrum of sample S5.

detectable. As shown in Fig. 3, for the elemental composition study, EDX analysis was performed on the sample S5. The observed peaks in 0.27, 0.50, 1.76, 2.00 and 2.12 keV are corresponding to emission lines Ka1 of carbon, Ka1 of oxygen and Li, La1, Lb1 of zirconium,

The thermal behavior of the as-spun ZrO2 nanofibers was investigated based on the thermogravimetric/differential thermal analysis and differential scanning calorimeter. Fig. 4(a) depicts the weight loss and the first derivative of the weight loss as a function of temperature, from 34 to 800
C at 10
C/min in air atmosphere. The weight loss of polymer nanofibers occurs at three stages, represented by three peaks on the DTA curve. The first weight loss, starting at 41
C with mass reduction of 21%, was due to endothermic process, evaporation of trapped moisture, constitution water from ZrOCl2$8H2O and remained ethanol in the fibers, which is in agreement with DSC result. The second major weight loss of ca. 29% between 280 and 370
C was related to the disintegration of PVP. In Fig. 4(b), the exothermic sharp peak is also positioned approximately at the same temperature on the DSC curve. The last weight loss registered in the range 370e500
C is almost 31% and can be attributed to the crystallization of ZrO2, which is another exothermic happened event (Fig. 4(b)). As can be seen from the TGA curve, the onset temperature for PVP decomposition is 280
C. TGA results also confirm that ca. 19% of total weight retained at the end of the TGA test. Since considerable weight loss is not detected after the last step, the lowest calcination temperature to obtain ceramic ZrO2 nanofiber can be chose

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Fig. 4. (a) TGA combined with DTA curves, (b) DSC graph of ZrOCl2/PVP nanofibers.

Table 1 Summarization of the TG/DTA information of ZrOCl2/PVP nanofibers. Stage

Peak range (
C)

Peak position (
C)

Weight loss (%)

Dehydration process Decomposition of the organic components Crystallization of ZrO2

41e150 280e370 370e500

81.55 326.22 428.65

21 29 31

Fig. 5. The X-ray diffraction patterns of different samples.

at 500
C. The TG/DTA information of produced nanofibers is summarized in Table 1. 3.3. X-ray diffraction analysis The diffraction profiles were recorded in a step-scan mode for

the angular range 20e80
with a step size of 0.0260
and a step counting time of 43.0950s. The XRD diffractographs of the ZrO2 samples are displayed in Fig. 5, which confirm the polycrystalline nature of the nanofibers. A comparison of the recorded X-ray diffraction pattern with JCPDF cards numbered 00-013-0307, 00024-1164 and 00-003-0640 shows that the samples crystallized in

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the monoclinic, tetragonal and cubic with space groups P21/c, P42/ nmc and Fm3m, respectively. The percentage of different phases was obtained by the Rietveld refinement approach on the X-ray diffraction profiles employing the material analysis using diffraction (MAUD) program [39]. In this approach, the diffraction lines were simulated by applying the pseudo-Voigt function and space groups, symmetries information. Then, the simulated patterns were compared with the experimental ones. Finally, the background coefficients and structural details such as lattice constants, and atomic positions were refined through the least-squares procedure until the best match between the two diffraction profiles was achieved. As reported in Table 2, with an increase in the calcination temperature up to 800
C, monoclinic phase percentage ascends, and mutually cubic and tetragonal phases percentage descends. Also the crystallographic position of each elements of ZrO2 corresponding to their phase, are shown in Table 3.

Table 2 The phases percentage corresponding to Monoclinic, Tetragonal and Cubic of different samples. Sample

Monoclinic phase (%)

Tetragonal phase (%)

Cubic Phase (%)

S5 S6 S7 S8

7.57 29.31 40.51 82.60

62.31 65.38 55.64 15.35

30.12 5.31 3.85 2.05

Also, we utilized the whole powder pattern modeling (WPPM) approach using PM2K package to investigate the effect of calcination temperature on the crystallite size and size distribution of nanostructured zirconia nanofibers. The observed and refined XRD patterns for different samples are shown in Fig. 6. In the WPPM approach, all peaks in the diffraction profiles of polycrystalline materials are modeled simultaneously by considering instrumental effects, background, peak profile width, shape, and position [40,41]. Also, this model doesn’t use any profile function like Gaussian, Lorentzian, and Voigt functions. The lattice parameters of different phases and goodness of fit are listed in Table 4, separately. As its evident, the lattice constants and the position of elements for each sample are influenced by the thermal treatments due to temperature stresses/strain. Assuming the spherical shape of crystallites and fitting the lognormal size distribution function to X-ray diffraction data, the Fourier size coefficients can be written in terms of mean (m) and variance (s) of lognormal size distribution function. The volume (DV) and area (DS) weighted crystallite size can be determined by m and s according to the following equations:

2 DS ¼ expðm þ 2:5s2 Þ: 3

(1)

3 DV ¼ expðm þ 3:5s2 Þ: 4

(2)

which, the obtained parameters values are reported in Table 5.

Table 3 Crystallographic positions of the elements O, Zr in different samples. S5 Phase

Elements

X

Y

Z

Tetragonal

Zr O Zr O Zr O1 O2

0 0 0 0.25 0.24909593 0.11483818 0.520106

0 0.5 0 0.25 0.016337715 0.33248052 0.7075142

0 0.22645204 0 0.25 0.22399625 0.291776 0.4928303

Cubic Monoclinic

S6 Phase Tetragonal Cubic Monoclinic

S7 Phase Tetragonal Cubic Monoclinic

S8 Phase Tetragonal Cubic Monoclinic

Elements

X

Y

Z

Zr O Zr O Zr O1 O2

0 0 0 0.25 0.274637 0.0816232 0.4546731

0 0.5 0 0.25 0.035787642 0.32497093 0.75372463

0 0.20665546 0 0.25 0.21150231 0.3498164 0.49870157

Elements

X

Y

Z

Zr O Zr O Zr O1 O2

0 0 0 0.25 0.2737392 0.0636063 0.44673634

0 0.5 0 0.25 0.03640994 0.29193932 0.75152117

0 0.20709883 0 0.25 0.20968227 0.33648953 0.50605446

Elements

X

Y

Z

Zr O Zr O Zr O1 O2

0 0 0 0.25 0.27492073 0.069351494 0.44495976

0 0.5 0 0.25 0.03930212 0.330269 0.76039517

0 0.22365893 0 0.25 0.20900372 0.33490586 0.4851872

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Fig. 6. Experimental and refined patterns from WPPM refinement for different samples. The difference between two patterns can be seen under each profile.

Table 4 The lattice parameters of different phases and goodness of fit versus calcination temperature. Sample

Phase

a (Å)

b (Å)

c (Å)

a

b

g

GoF

S5

M T C M T C M T C M T C

5.250132 3.716638 5.150690 5.153549 3.670894 5.119596 5.138008 3.668925 5.094223 5.142754 3.670274 4.846694

5.279339 3.716638 5.150690 5.229647 3.670894 5.119596 5.199131 3.668925 5.094223 5.202777 3.670274 4.846694

5.369200 5.117331 5.150690 5.318886 5.084420 5.119596 5.311385 4.985491 5.094223 5.315960 4.978227 4.846694

90 90 90 90 90 90 90 90 90 90 90 90

97.24 90 90 98.97 90 90 99.05 90 90 99.13 90 90

90 90 90 90 90 90 90 90 90 90 90 90

1.15558

S6

S7

S8

3.4. FTIR spectroscopy 1.12762

1.14637

1.07905

Table 5 The values of m and s of lognormal distribution function and the area and volume weighted crystallite size for the various specimens. Sample

Phase

m

s

DS (nm)

DV (nm)

S5

M T C M T C M T C M T C

1.381884 1.638311 2.569424 2.126626 1.763680 2.402444 2.344977 2.140231 2.133420 2.775047 2.356959 2.444565

0.4199090 0.4792110 0.3101000 0.4488774 0.5427479 0.4164152 0.4622972 0.4925411 0.5740163 0.4740765 0.4909618 0.2579687

4.12 6.09 11.07 9.25 8.12 11.36 11.87 10.39 12.83 18.75 12.86 9.07

5.54 8.62 13.71 12.73 12.27 15.21 16.53 14.90 20.06 26.42 18.41 10.91

S6

S7

S8

The estimated crystallite size increased with temperature due to the crystal growth. The lognormal size distribution curves of different phases for calcined ZrO2 nanofibers are indicated in Fig. 7. It’s evident that the size distribution curves expanded towards the greater sizes by rising calcination temperature.

Fig. 8 shows the FTIR spectra of both as-spun and calcined nanofibers. All the spectra exhibited a broad band at approximately 3425 cm1, corresponding to the OeH stretching vibration of residual or chemisorbed water and hydroxyl groups. The absorption peaks labeled by the number located at 2960, 2929, and 2865 cm1 are assigned to the CeH stretching vibration of polyvinylpyrrolidone. Also, the lines at around 1728, 1624, 1460, and 1284 cm1 are respectively matched to carbonyl group (C]O) vibrations, CeC bond vibrations, CeH bending vibrations, and CeN bond vibrations of PVP [42]. As can be seen from the spectra, the absorbance peaks corresponding to OeH, CeH, C]O, CeC, and CeN bonds were weakened considerably after calcination; It implies the decomposition of PVP and organic materials from the structure by the thermal treatment. Due to occurrence of the metal oxides peaks in the short wavenumber region, we discuss the region below 900 cm1 separately. The observed peaks in this region clearly indicate the existence of ZrO2 after the thermal treatment (Fig. 9). The absorption peak of about 452 cm1 observed for S5 and S6 samples indicates ZreO bond vibrations in a cubic phase [43,44], which its intensity decreases at higher calcination temperature, and a new peak around 413 cm1 related to ZreO bond in monoclinic phase appears [45,46]. Absorption peak in 496 cm1 assigned to ZreO vibrations of m-ZrO2 was detected after calcination at 600
C [47], and it was strengthened with increasing calcination temperature. The broad observed peak in the first calcined

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Fig. 7. The lognormal size distribution function of zirconia nanofibers in various calcination temperatures.

specimen, centered at about 653 cm1, in the 557 to 751 cm1 region is due to zirconia vibrations in tetragonal phase, which it is resolved to three peaks about 580 [47], 670 [48], and 750 [46] cm1 representing m-ZrO2/t-ZrO2, t-ZrO2, and m-ZrO2 vibrations with the rise of calcination temperature, respectively. As can be seen, the absorption bands of the monoclinic phase vibrations are increased as a function of calcination temperature and its finding supports the XRD results in the X-Ray Diffraction analysis section. So, one can conclude that there is a good agreement between XRD and FTIR results. 3.5. Optical properties of ZrO2 nanofibers

Fig. 8. FTIR spectra of polymer and ceramic nanofibers in the range 4000e400 cm1.

The diffuse reflectance spectroscopy results of ZrO2 nanofibers are shown in Fig. 10. The absorption peak of ZrO2 nanofibers and the absorption edge clearly are blue shifted by increasing calcination temperature. The first derivative method has been used to calculate the optical band gap. In this method, the band gap can be estimated from the maximum of the first derivative of the absorbance data with respected to the photon energy (Fig. 11). The estimated optical band gap energy values corresponding to each sample are summarized in Table 6. As can be seen, the band gap of the ZrO2 nanofibers is increased by rising calcination temperature. This could be due to the higher crystallinity and leaving of the disorder surface structure. The PL spectra of samples were recorded at an excitation wavelength of 260 nm to get insight into excitonic energies and different recombination mechanisms of the ZrO2 fibers. Fig. 12 shows the PL spectra of ZrO2 nanofibers calcined at different calcination temperature. The PL spectra of zirconia exhibit intense peaks around 386e400 nm and 541e541.5 nm (Table 6). Due to the low dimensional structure and large surface/volume ratio of ZrO2 fibers, the PL emission bands can be originated from the nonstoichiometry created by the oxygen deficiency defects, anion

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Fig. 10. The absorbance spectra of different samples. The inset shows the absorbance curve versus the photon energy (eV).

 V o 4 V  o þe

Fig. 9. FTIR spectra of polymer and ceramic nanofibers in the range 900e400 cm1.

vacancies including electrons (F and Fþ centers), interstitial oxygen and Zr vacancy in ZrO2 that creates mid-gap trap states as surface defects [49e51]. Overall, oxygen vacancies as a common defect state are an inherent aspect of ZrO2 nanocrystals. The process for formation of oxygen vacancies and Zr4þ vacancies in zirconia structure can be expressed as [52]:

ZrO2 þ hn/Zr4þ þ O2 þ 4e

(3)

Zr 4þ þ e /Zr3þ

(4)

 2Oo / 2V 2þ o þ O2 þ 4e

(5)

Vo 4 2V o þ e

(6)

(7)

the Oo and Vo indicates an oxygen atom in a regular position and oxygen vacancy in the lattice, respectively. Upon excitation of ZrO2 by a photon (hn), the electrons are trapped by Vo and recombination centers (F) are created. A singly and doubly ionized oxygen vacancy is represented by V o, and V  o , respectively. A notable change in the PL intensity of the ZrO2 samples is attributed to the degree of crystal quality and also difference in the number of defect centers of the samples. However, in comparison with the PL intensity of other sample, enhanced intensity of the ZrO2 calcined at 800
C could be due to more crystallinity of sample which is in tune with the structural analysis. Broad and weak emission peaks were observed around 268e269.5 nm and 668e682 nm. The UV emission (268e269.5 nm) originates from optical excitation across the gap, inducing an electron-hole pair that can be correlated to the near band edge (NBE) transition and crystal quality of the sample [52]. High crystal quality favors the change in the emission intensity and the strong UV emission of the free exciton radiative recombination at near band edge at room temperature. The observed 668e682 nm emission may also be due to oxygen position perturbation into the zirconia structure [52]. 3.6. Magnetic properties of ZrO2 nanofibers In order to investigate the magnetic properties of the prepared samples, the VSM analysis was used at room temperature. Fig. 13(a) shows the variations of magnetization as a function of applied magnetic field and saturated hysteresis loop formation of different samples. Saturated hysteresis loop formation confirms the weak ferromagnetic nature of the samples. Also the enlargement of M  H curve in the low field region for samples is showed in Fig. 13(b). The first derivative of magnetization (M) with respect to H for ZrO2 nanofibers are plotted in Fig. 14. The values of saturation magnetization (Ms), the coercive force (Hc), the remanent magnetization (Mr) and the susceptibility (c), the first derivative of M with respect to H at H0, are summarized in Table 7.

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Fig. 11. The first derivative of the absorbance data versus energy for ZrO2 nanofibers calcined at different temperatures.

Table 6 The band gap value and the PL emission bands of different calcined nanofibers. Sample

Band gap (eV)

First peak

Second peak

Third peak

Fourth peak

S5 S6 S7 S8

4.12 4.08 4.23 4.26

269 268.5 268.5 268.5

400 391.5 386.5 386.5

541 541.5 542 541.5

682 684 673.5 668

The saturated magnetization decreases by increasing calcination temperature from 500 to 800
C in parallel with increment the monoclinic phase from ca. 8e83%. The value of saturation magnetization for sample S5 with the lowest percentage of monoclinic phase was estimated to be around 0.14 emu/g, which decreases to 0.0048 emu/g with rising calcination temperature at 800
C, that the monoclinic phase is dominant. It seems the existence of defects, especially oxygen vacancies, in the crystal lattice of nanofibers that improves the tetragonal phase formation, may be the major reason for the presence of ferromagnetic behavior. The same observation was reported by Ning et al. [38]. Also, in their another study, the existence of phase-dependent d0 ferromagnetism in ZrO2 thin films was discussed to understand the oxygen vacancy role on the ferromagnetic properties [53]. Hc shows apposite trend to Ms with increasing calcination temperature, which is expected from the Brown’s relation as Hc ¼ 2K/m0Ms where K is the first order anisotropy constant [54]. Higher anisotropy of particle makes spin of core and surface to align in different directions. 4. Conclusion

Fig. 12. The PL emission spectra of different samples.

In summary, ZrO2 nanofibers were successfully obtained by the electrospinning technique followed calcination at different temperature. The produced nanofibers were characterized by a wide range of methods (SEM, TG-DTA, XRD, FTIR, DRS and VSM) applied to characterized the morphology feature, structure, chemical, optical and magnetic properties. The SEM images shows formation of smooth and free of beads fibers with average diameter below 100 nm after calcination. A precise structure and microstructure analysis corresponding to Monoclinic, Tetragonal and Cubic phase were investigated using XRD analysis and utilizing Rietveld approach and whole powder pattern modeling (WPPM)

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Fig. 13. (a) Magnetization vs. magnetic field curve measured at room temperature of ZrO2 nanofiners, (b) Enlarged M  H curve of ZrO2 nanofiners.

Fig. 14. The first derivative of M with respect to H for ZrO2 nanofibers calcined at different temperatures.

Table 7 Obtained values of saturation magnetization, magnetic remanence, initial magnetic susceptibility and coercivity of calcined ZrO2. Sample

Ms 103 (emu/g)

Mr103 (emu/g)

c105 (emu/gOe) Hc (Oe)

S5 S6 S7 S8

141.17 60.12 39.02 4.78

7.07 6.53 5.20 1.86

32.27 11.73 7.16 1.37

38.68 127.33 133.57 188.28

technique, respectively. The chemical study based on the absorption bands formation corresponding to each crystalline phase is in good agreement with the XRD results. PL study shows an enhancement emission intensity of the ZrO2 calcined at 800
C that could be due to the degree of crystal quality and also the number of defect centers present in the sample. VSM measurements exhibited the room temperature ferromagnetism in ZrO2 nanofibers.

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S. Khalili, H.M. Chenari / Journal of Alloys and Compounds 828 (2020) 154414

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