Preparation and Characterization of Polycrystalline

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Journal of Nanoscience and Nanotechnology Vol. 11, 2514–2519, 2011

Preparation and Characterization of Polycrystalline La2Zr2O7 Ultrafine Fibres via Electrospinning Jinxian Wang, Xiangting Dong∗ , Qizheng Cui, Guixia Liu, and Wensheng Yu

RESEARCH ARTICLE

School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022, P. R. China Polyvinyl Pyrrolidone (PVP)/(Lanthanum nitrate and zirconium oxychloride) microfibres were fabricated by electrospinning. SEM micrographs indicated that the surface of the prepared composite microfibres was smooth, and the diameter of the microfibres was in the range of 2 m–5 m. X–ray diffractormetry (XRD) analysis revealed that the composite microfibres were amorphous in structure. La2 Zr2 O7 porous hollow ultrafine fibres (PHUF) were fabricated by calcination of the Delivered by Ingenta to: PVP/[La(NO3 3 + ZrOCl2 ] composite microfibres. The diameters of La2 Zr2 O7 PHUF were smaller Universidad Nacional Autonoma de Mexico Central Informacion Humanistica than those of the PVP/[La(NO3 3 + ZrOCl2 ](UNAM) composite -microfibres. The surface of Cientifica the ultrafine y fibres IP : 132.248.28.11 became coarse with the increase of calcination temperatures. La2 Zr2 O7 PHUF formed by nanopartiSat,temperature 26 Mar 2011 01:58:15 cles were acquired when calcination was above 800 C. Scanning electron microscope (SEM) analysis indicated that the diameter of the synthesized La2 Zr2 O7 PHUF was in the range of 500 nm–800 nm. XRD analysis revealed that the crystal structure of La2 Zr2 O7 PHUF was found to be of the fluorite type when calcination temperature was 800 C–900 C, and of the pyrochlore type at 1000 C. The latter is cubic in structure with space group Fd3m. The conductance of La2 Zr2 O7 PHUF is high to 5.19 S/cm2 and has no effect of frequency. Possible formation mechanism for La2 Zr2 O7 PHUF was preliminarily proposed.

Keywords: La, Zr, La2 Zr2 O7 , Porous Hollow Ultrafine Fibres, Electrospinning.

1. INTRODUCTION The science and technology of nanostructured materials is advancing at a rapid pace.1–3 Over the past decade, the preparation and functionalization of onedimensional nanostructured materials has become one of the most highly energized research fields.4–6 Onedimensional nanostructured materials, such as nanowires, nanorods, nanowhiskers and ultrafine fibres, have stimulated great interest due to their importance in basic scientific research and potential technological applications.7–9 They are expected to play an important role as both interconnects and functional components in the fabrication of nanoscale electronic and optoelectronic devices. In order to obtain these materials, various preparation methods have been developed including arc discharge,10 laser ablation,11 template,12 13 precursor thermal decomposition,14 and other methods.15–17 Electrospinning technique is widely applied to prepare polymers ultrafine fibres.18 Recently, some inorganic compounds ultrafine fibres have been prepared by electrospinning technique using electrospun ∗

Author to whom correspondence should be addressed.

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fibres of polymer/inorganic composite as the precursor.19–33 This processing involved the following steps: (1) Preparation of a sol with suitable inorganic precursor and proper polymer, and achieving the right rheology for electrospinning process; (2) Electrospinning of the solution to obtain fibres of polymer/inorganic precursors composite; (3) Calcinations of the composite fibres to obtain final oxide fibres. It is important, however, to control all of the above three stages in order to obtain high quality fibres with the desired final properties. Lanthanum zirconate has attracted much interest recently due to their specific optical, electrical, and catalytic properties.34–42 La2 Zr2 O7 were prepared by various methods, such as hydrothermal synthesis,43 44 hightemperature solid state reaction,34 35 45 sol–gel method,46–49 hydrazine method,50 51 wet mechanochemical treatment,52 electrospinning,53 etc. A few methods on the preparation of La2 Zr2 O7 nanocrystalline powder, films, fibres and coating were reported.47 53–59 However, to the best of our knowledge, there have been no reports on the preparation of La2 Zr2 O7 PHUF. In this paper, La2 Zr2 O7 PHUF were fabricated by calcination of the electrospun fibres of PVP/[La(NO3 3 + ZrOCl2 ] composite, and some new results were obtained. 1533-4880/2011/11/2514/006

doi:10.1166/jnn.2011.3574

Wang et al.

Preparation and Characterization of Polycrystalline La2 Zr2 O7 Ultrafine Fibres via Electrospinning

2. EXPERIMENTAL DETAILS 2.1. Chemicals Polyvinyl pyrrolidone (Mr ≈ 10000) was purchased from Tianjin Kermel Chemical Reagents Development Center. Lanthanum nitrate hexahydrate was obtained from Tianjin Guangfu Institute of Fine Chemicals. Zirconium oxychloride octahydrate was bought from the Third Chemical Reagents of Tianjin. All chemicals were analytically pure and directly used as received without further purification. Distilled water was used as solvent. Fig. 1. Schematic diagram of electrospinning setup.

2.2. Preparation of PVP/La(NO3 3 + ZrOCl2 ] Composite Sol

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performed with a JEOL JEM-2010 Transmission Electron Microscope (TEM). TG-DTA analysis was carried out with a SDT-2960 thermal analyzer made by AmerPVP/[La(NO3 3 + ZrOCl2 ] composite sol was prepared ican PERKIN-ELMER company in atmosphere, and the by dissolving 23.7348 g of PVP powders, 4.5380 g temperature-rising rate was 10 C/min. FTIR spectra of of La(NO3 3 · 6H2 O and 3.3773 g of ZrOCl2 · 8H2 O in the samples were recorded on BRUKER Vertex 70 Fourier 21.0939 g of distilled water, and stirring forDelivered 10 h, then by Ingenta transformto: infrared spectrometer made by Germany Bruker remaining motionlessly 2 h. Thus, a viscous(UNAM) gel company withInformacion a resolution of 1 cm−1 , and the specimen Universidad Nacionalfor Autonoma de Mexico - Central Cientifica y Humanistica 45% of PVP/[La(NO3 3 + ZrOCl2 ] composite containing for the measurement was prepared by mixing the sample IP : 132.248.28.11 4+ to Zr was 2011 PVP(wt%) and the molar ratio 1:1 of La3+Sat, with01:58:15 KBr powders and then the mixture was pressed into 26 Mar obtained for electrospinning processing. pellets, the spectrum was acquired in a wave number range from 4000 cm−1 to 400 cm−1 . The electrical characterization of the nanofibres was performed with an Agilent 2.3. Fabrication of PVP/[La(NO3 3 + ZrOCl2 ] E4980A LCR meter at room temperature and the pellets Composite Ultrafine Fibres and La2 Zr2 O7 for the measurement was pressed under pressure of 80 kN Porous Hollow Ultrafine Fibres for 5 min. The setup used for electrospinning was indicated in Figure 1. The viscous composite sol of PVP, La(NO3 3 3. RESULTS AND DISCUSSION and ZrOCl2 mixture was contained in a plastic capillary. A copper pin connected to a DC high-voltage generaIn order to investigate the lowest crystallizing temtor was placed in the sol, and the sol was kept in the perature and the transition of phases, the precursor syringe by adjusting the angle between syringe and the [PVP/(La(NO3 3 + ZrOCl2 composite fibres] and samples fixing bar. A grounded aluminum foil served as counter obtained by calcining precursor at different temperatures electrode and collector. A voltage of 20 kV was applied for 10 h were characterized by XRD, as indicated in to the sol and a sprayed dense web of fibres was colFigure 2. The PVP/[La(NO3 3 + ZrOCl2 ] composite fibers lected on the aluminum foil. The dense web of fibres was are amorphous in structure, only a broad peak was located PVP/[La(NO3 3 and ZrOCl2 ] composite ultrafine fibres. around 20 , it is the typical peak of the polymer. No The prepared composite fibres were dried initially at 70 C obvious diffraction peak of the sample at 300 C was for 12 h under vacuum, and then calcined at a heating rate observed. The polycrystalline La2 Zr2 O7 PHUF with sinof 120 C/h and remained for 10 h at 300 C, 800 C, gle phase was synthesized when calcination temperature 900 C and 1000 C, respectively. La2 Zr2 O7 PHUF were is in the range of 800–900 C, the crystal structure of obtained above 800 C. the prepared La2 Zr2 O7 was found to be of the fluorite type. It is a metastable phase.34 The diffraction peaks of crystallographic planes (111), (331), (511) were appeared 2.4. Characterization Methods at 1000 C. These peaks are characteristic peaks used to XRD analysis was performed with a Holland Philips Anadetermine whether pyrochlore-type structure is formed or lytical PW1710 BASED X-ray diffractometer using Cu not. Therefore, La2 Zr2 O7 with pyrochlore-type structure K1 radiation, the working current and voltage were 30 mA was formed at 1000 C, the d (spacing between crystal and 40 kV, respectively. Scans were made from 10 to lographic planes) values of La2 Zr2 O7 are consistent with 80 at the speed of 3 /min, and step was 0.05 . The those of JCPDS card 17–450. The obtained La2 Zr2 O7 is morphology and size of the fibres were observed with a of cubic system with space group Fd3m. S-4200 scanning electron microscope made by Japanese The SEM micrographs shown in Figure 3 revealed the morphology and size of the as-synthesized fibres, which Hitachi company. TEM analysis of the ultrafine fibres was

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From the TEM micrographs and SAED pattern shown in Figure 4, it was clearly seen that the La2 Zr2 O7 PHUF composed of nanoparticles were porous and hollow structure, and the average size of the nanoparticles was ca. 50 nm. These small nanopartices mutually connected to form the hollow ultrafine fibers. It was also shown from Figure 4 that the SAED pattern was made up of the regular diffraction dots, indicating that the La2 Zr2 O7 PHUF were crystalline structure. TG and DTA curves of the PVP/[La(NO3 3 + ZrOCl2 ] composite fibers were indicated in Figure 5. It was noted that there were mainly four stages of weight loss in the TG curve. The first weight loss step (10.28%) was in the range of 27 C to 156 C accompanied by a small endothermic peak near 65.71 C in the DTA curve caused Fig. 2. XRD patterns of samples: (a) PVP/[La(NO3 3 + ZrOCl2 ] comby the loss of the surface absorbed water or the residposite fibres, (b) 300 C, (c) 800 C, (d) 900 C, (e) 1000 C. ual water molecules in the composite fibers. The second weight loss step (34.43%) was noticed between 156 C varied strongly with the increase of calcination temperaand 388.96 C accompanied by a wide exothermic peak Delivered by Ingenta to: tures. The surface morphology of the PVP/[La(NO3 3 + near 334.43 C in the DTA curve due to the decomposiUniversidad Nacional Autonoma de Mexico (UNAM) Central Informacion Cientifica y Humanistica ZrOCl2 ] composite microfibres was very smooth, and the tion of the nitrate and side-chain of PVP. The third weight IP : 132.248.28.11 diameter of the composite microfibres was in the range loss step (19.86%) was observed in the range of 388.96 C Sat, 26 Mar 2011 01:58:15 of 2 m–5 m. The surface morphology of La2 Zr2 O7 to 447.39 C accompanied by a small exothermic peak ultrafine fibres became coarse with the increase of calaround 420.13 C in the DTA curve owing to the decomcination temperatures. La2 Zr2 O7 PHUF were acquired at position of the oxychloride. The last weight loss in the 800 C–1000 C. SEM analysis indicated that the diameter TG curve was 25.14% from 447.39 C to 534.77 C. In of the synthesized La2 Zr2 O7 PHUF was in the range of the DTA curve a sharp exothermic peak was located at 488.94 C. This is likely to be the oxidation combustion 500 nm–800 nm. The diameters of La2 Zr2 O7 PHUF were of the PVP main chain. And above 534.77 C, the TG and smaller than those of the PVP/[La(NO3 3 + ZrOCl2 ] comDTA curves are all stable, indicating that water, organic posite microfibers owing to the decomposition and evapocompounds, nitrate and oxychloride in the composite fibers ration of PVP and inorganic salts. volatilize completely and pure La2 Zr2 O7 ultrafine fibers could be obtained after 534.77 C. The total weight loss rate was 89.82%. Pure PVP, PVP/[La(NO3 3 + ZrOCl2 ] composite fibres and La2 Zr2 O7 PHUF (obtained by calcination of the PVP/[La(NO3 3 + ZrOCl2 ] composite fibres at 1000 C for 10 h) were investigated by FTIR, as shown in Figure 6. PVP (Fig. 6(a)) and PVP/[La(NO3 3 + ZrOCl2 ] composite fibres (Fig. 6(b)) had the identical spectra, but absorption peaks intensity of spectrum for PVP/[La(NO3 3 + ZrOCl2 ] composite fibres was lower than those of spectrum for pure

Fig. 3. SEM micrographs of the fibres obtained at different temperatures: (a) PVP/[La(NO3 3 + ZrOCl2 ] composite fibres, (b) 300 C, (c) 800 C, (d) 900 C, (e) and (f) 1000 C.

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Fig. 4. TEM images (a and b) and SAED (c) pattern of La2 Zr2 O7 ultrafine fibres at 1000 C for 10 h.


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Fig. 5.


TG–DTA curves of PVP/[La(NO3 3 + ZrOCl2 ] composite fibers.

Fig. 7. The frequency response of the capacitance of La2 Zr2 O7 coarse powder and nanofibres.

Fig. 6. FTIR spectra of PVP (a) PVP/[La(NO3 3 + ZrOCl2 ] composite fibres (b) and La2 Zr2 O7 ultrafine fibres (c).


Fig. 8. The frequency response of the conductance of La2 Zr2 O7 coarse powder and nanofibres.

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PVP. This resulted from the lower content of PVP in the PVP/[La(NO3 3 +ZrOCl2 ] composite fibres. All absorption is high to 5.19 S/cm2 , and has no effect of frequency. So peaks were attributed to PVP at 3426 cm−1 , 2956 cm−1 , the La2 Zr2 O7 nanofibres can be used as the wide band 1662 cm−1 , 1424 cm−1 , and 1290 cm−1 , corresponding to constant-conductance materials. Delivered by Ingenta to: the stretching vibrations of hydroxyl group ( O–H , C–H was illusFormation mechanism of Cientifica La2 Zr2 O7 PHUF Universidad Nacional Autonoma de Mexico - Central Informacion y Humanistica group (C=O , C–H bond ((UNAM) bond (C–H , carbonyl C–H , · 6H O, ZrOCl · 8H trated in Figure 9. La(NO 3 3 2 2 2 O and IP : 132.248.28.11 and C–N bond or C–O bond (C–N or C–O , respectively. PVP were mixed with distilled water to form sol with 26disapMar 2011 01:58:15 It is seen from Figure 6(c) that all peaks ofSat, PVP certain viscosity. PVP acted as template during the forpeared, and at low wave number range, new absorption mation process of La2 Zr2 O7 ultrafine fibres. La3+ , ZrO2+ , peak at 504 cm−1 appeared. This new absorption peak was − NO− were mixed with or absorbed onto PVP 3 and Cl ascribed to the vibration of metal-oxygen bonds, indicating molecules to form PVP/[La(NO3 3 + ZrOCl2 ] composLa2 Zr2 O7 was formed. The results of FTIR analysis were ite fibres under electrospinning. During calcination treatin good agreement with XRD results. − ment, water containing La3+ , ZrO2+ , NO− ions 3 and Cl The electrical properties of the La2 Zr2 O7 nanofibres in the composite fibres would remove to the surface of obtained by calcination of the PVP/[La(NO3 3 + ZrOCl2 ] the PVP/[La(NO3 3 + ZrOCl2 ] composite fibres and evencomposite fibres at 900 C for 10 h were investigated by tually evaporated from the composite fibres. Thus, the the measurement of capacitance and conductance. Figure 7 − La3+ , ZrO2+ , NO− 3 and Cl ions also remove to the surand Figure 8 showed the frequency response of the capaciface of the composite fibres brought by removed water. tance and conductance of the La2 Zr2 O7 coarse powder and With the increasing in calcination temperature, PVP and nanofibres, respectively. It is noted that the capacitance − NO− 3 would decompose and evaporated, and Cl was also of the La2 Zr2 O7 coarse powder and nanofibres decreases 3+ 2+ evaporated, La and ZrO were oxidized into La2 Zr2 O7 with the increasing of the frequency and the latter has a to form La2 Zr2 O7 crystallites, and many crystallites were greater reduction, but the conductance of the nanofibres combined into small La2 Zr2 O7 nanoparticles, then some


Wang et al.

20070402, 20060504), Key Research Project of Science and Technology of Ministry of Education of China (Grant No.207026), the Science and Technology Planning Project of Changchun City (Grant No. 2007045) and the Scientific Research Project of Environment Protection Bureau of Jilin Province (No. 2006–24).

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Received: 21 June 2009. Accepted: 20 April 2010.


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