Preparation of superhydrophilic α-Fe2O3 nanofibers with tunable magnetic properties

Thin Solid Films 510 (2006) 271 – 274 www.elsevier.com/locate/tsf

Preparation of superhydrophilic a-Fe2O3 nanofibers with tunable magnetic properties Ying Zhu a,b, Jing Chang Zhang a, Jin Zhai b, Lei Jiang b,* b

a School of Science, Beijing University of Chemical Technology, Beijing 100029, PR China Center of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China

Received 4 February 2005; received in revised form 12 July 2005; accepted 1 September 2005 Available online 23 February 2006

Abstract The paper describes a simple, effective method for producing a-Fe2O3 nanofibers by electrospun poly(vinyl alcohol)/ferrous acetate composite nanofibers precursors and high-temperature calcination in air. The experimental results show that the morphology and crystalline phase of a-Fe2O3 nanofibres are influenced by the content of ferrous acetate in composite nanofibres and the calcination temperature. The a-Fe2O3 nanofibres can generate superhydrophilic surface displaying the contact angle of water as 0-. By controlling the calcination temperature of electrospun composite nanofibers in the air, the magnetic property of a-Fe2O3 nanofibres could be tuned from superparamagnetic to ferromagnetic. D 2006 Elsevier B.V. All rights reserved. PACS: 75.50.K Keywords: Electrospinning; Iron oxide; Magnetic properties; Hydrophilicity

1. Introduction Hematite (a-Fe2O3), the most stable iron oxide under ambient conditions, has been widely used as pigment [1], catalysts [2], gas-sensing material [3], and photoanode for possible photoelectrochemical cell [4]. There has been much interest in the investigation of synthetic methods and properties for nanosized a-Fe2O3 materials [5 – 8]. Herein, we report a novel and simple approach to the large-scale fabrication of a-Fe2O3 nanofibers by electrospinning. Electrospinning is a process in which high static voltages are used to fabricate fibers with diameter ranging from a few to several hundred nanometers, depending on the polymer and processing conditions [9,10]. Up to date, electrospinning technique has been used to prepare a wide variety of polymer, ceramic and composite nanofibers [11 – 18], which are of interest for a variety of applications in texturing, biomedical materials, sensing, membrane-based separation and actuators photovoltia calls. In this work, the a-Fe2O3 nanofibers were prepared using ferrous acetate (FeAc2) mixed with polyvinyl alcohol (PVA) as precursors. A series of a-Fe2O3 nanofibers with different * Corresponding author. E-mail address: [email protected] (L. Jiang). 0040-6090/$ - see front matter D 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.004

FeAc2 contents ranging from 20 to 50 wt.% have been fabricated by using electrospinning technique. It is proven that the magnetic property of the final nanofibers can be tuned from superparamagnetic to ferromagnetic properties by simply controlling the calcination temperature. Moreover, this nanofibers of superhydrophilic property enables water to spread so rapidly on them that it can be applied in filter, microfluidic device, biochip with fast-response time and self-cleaning surface. 2. Experimental details The electrospinning solution is obtained from a FeAc2 molecular precursor based on conventional sol – gel procedures. Aqueous PVA (Mw 88,000) solution (10 wt.%) is first prepared by dissolving PVA powder in deionized water (resistivity 18.2 MV cm), heating 80 -C with stirring for 1 h, and then cooling down to room temperature and stirring for 12 h. In a typical procedure, 10 g aqueous PVA solution of 10 wt.% is dropped into the solution of FeAc2 (1.0 g FeAc2 and 2.0 g H2O) with magnetic stirring, and the reaction proceeds in the water bath at 50 -C for 8 h. This mixture is aged at room temperature for about 12 h, then, a light brown sol solution is obtained to give the final electrospinning solution.

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As reported in the literature [16], the electrospinning solution was loaded into a hypodermic syringe pipette of 2 ml in volume with a stainless steel needle of 0.8 mm in diameter, in an electric field of 1.8 kV/cm. The syringe was placed perpendicularly so that a small drop could be maintained at the needle’s tip due to the surface tension of the solution. The metallic plate wrapped with aluminum foil served as a collector during electrospinning. The as-electrospun PVA/FeAc2 composite nanofibers were placed in a vacuum oven for 12 h at room temperature in order to remove the solvent residuals, then calcined for 4 h in air at 400, 600, 800 -C, respectively, and finally the a-Fe2O3 nanofibers were obtained. The heating rate was 20 -C min 1. The as-electrospun and calcined nanofibers or films were characterized using JEOL JSM-6700F field-emission scanning electron microscope (SEM) operated at an accelerating voltage of 3.0 kV. The X-ray diffraction (XRD) patterns of fibers were recorded using a Mac Science MXP-AHF18 diffractometer (Cu Ka radiation) at a scanning rate of 8-/min in 2h ranging from 15- to 80-. The infrared spectra were recorded using a Bruker EQUINOX55 FT-IR spectrophotometer with a resolution of 1 cm 1. The magnetic property of a-Fe2O3 nanofibers was measured using a superconducting interference device magnetometer with an applied field between 5 and 5 kOe. Contact angle (CA) were measured on Dataphysics OCA20 contactangle system at ambient temperature. 3. Results and discussion Fig. 1 shows the SEM photographs of various as-electrospun PVA/FeAc2 composite nanofibers prepared with different contents of FeAc2. As observed, the film of as-electrospun PVA/FeAc 2 composite nanofibers displays a fully interconnected pore structure, i.e., netlike microstructure of

micrometer magnitude, and the surface of the nanofibers is smooth and uniform, due to the amorphous nature of the PVA. The morphologies and diameters of the nanofibers are different for each sample, changed with increasing the content of FeAc2. Fig. 1(a – d) shows an increase in fibers’ diameter from 70 T 30 to 150 T 40 nm with the contents of FeAc2 increased from 20 to 50 wt.%. Junctions of nanofibers are observed in the sample containing 20 wt.% FeAc2 (Fig. 1a), however, the morphologies of nanofibers became more regular when the content of FeAc2 is above 30 wt.% (Fig. 1b –d). This phenomenon can be well understood: At lower concentration, wet electrospun nanofibers are difficult to dry during solidification process before they reach the collector, resulting in the junctions morphology as shown in Fig. 1a. In contrast, the as-electrospun composite nanofibers are mostly dried by the moment when they reach the collector at higher concentration of FeAc2, and then the uniform fiber morphology is obtained (Fig. 1b– d). These results indicate that the content of FeAc2 may largely influence the morphology of as-electrospun composite nanofibers because the solution viscosity and surface tension can be changed with the change of FeAc2 content. Fig. 2 shows the results of a-Fe2O3 nanofibers prepared by using PVA/FeAc2 composite nanofibers with 40 wt.% FeAc2 at the calcination temperature of 400, 600 and 800 -C, respectively. After calcination, it can be seen that the netlike microstructure and the nanofiber shape are completely maintained until 600 -C. In Fig. 2a, the photograph of the calcined a-Fe2O3 fibers with 40 wt.% FeAc2 exhibits shrinkage and roughness after calcinations at 400 -C, and the average diameter is reduced from ca. 120 T 20 to ca. 40 T 10 nm due to the decomposition of the PVA component. After calcination at 600 -C (Fig. 2b), it can be clearly seen that the a-Fe2O3 fibers have rougher surfaces, and the average diameter of fibers is ca. 60 T 20 nm, due to the crystallization related to the

a

b

c

d

Fig. 1. SEM images of the various electrospun PVA/FeAc2 composite fibers with different contents of FeAc2 (a) 20; (b) 30; (c) 40; (d) 50 wt.%.

Y. Zhu et al. / Thin Solid Films 510 (2006) 271 – 274

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a

b

Fig. 3. X-ray diffraction patterns of the nanofibers with 40 wt.% FeAc2 (a) electrospun PVA/FeAc2 composite fibers; (b) calcined 400 -C in air at 400 -C for 4 h; (c) calcined in air at 600 -C for 4 h; (d) calcined in air at 800 -C for 4 h.

c

Fig. 2. SEM images of as-calcined a-Fe2O3 nanofibers with 40 wt.% FeAc2 at different calcination temperature for 4 h (a) 400; (c) 600; (d) 800 -C.

formation of particulate morphology. After calcinations at 800 -C (Fig. 2c), the surface of the nanofibers is rough and the fibers are broken. Meanwhile, a number of large size particles or particle aggregates are found, the average diameter of which are ca. 130 T 50 nm, due to crystalline growth with increasing calcination temperature. Furthermore, PVA/FeAc2 composite nanofibers with different contents of FeAc2 also calcined at 600 -C in air, showing that the nanofibers with 20 wt.% FeAc2 are burnt into rod with a rough surface when the content of FeAc2 is low. The shape of a-Fe2O3 fibers has been maintained until the content of FeAc2 reached 40 wt.%. The morphology of aFe2O3 nanofibers are strongly affected by the calcinations temperature and the content of FeAc2. As a result, a-Fe2O3 nanofibers of different shape, size and surface microstructure could be obtained by controlling the calcinations temperature and the content of FeAc2. The a-Fe2O3 nanofibers were also characterized by X-ray diffraction spectra (XRD). Fig. 3 shows the XRD spectra of nanofibers in the as-electrospun PVA/FeAc2 with 40 wt.% FeAc2 (a), and the a-Fe2O3 nanofibers after the calcination of the corresponding PVA/FeAc2 nanofibers at 400 (b), 600 (c) and

800 -C (d), respectively. As shown in spectra a, a broad peak around 2h = 20- corresponds to the (101) plane of semicrystalline PVA in the PVA/FeAc2 composite nanofibers [19]. After calcinations at 400 -C (spectra b), the crystalline peak of PVA disappeared, and the diffraction can be indexed to (012), (104), (110), (024), (116) planes of a-Fe2O3 [20], respectively. No other peaks for impurities were observed. Notably, when the calcination temperature increased to 600 and 800 -C (spectra c and d ), all the peaks belonging to a-Fe2O3 are markedly sharpened up with high intensity, which suggests that the crystallinity of a-Fe2O3 phase is higher at high calcination temperature than that obtained at lower calcination temperature. The formation of a-Fe2O3 nanofibers with 40 wt.% FeAc2 is further supported by FT-IR spectra. In Fig. 4a, a broad peak at about 3400 cm 1 corresponds to H – OH stretch, and the peaks in the range of 1300– 1700 cm 1 correspond to the bending and stretching vibrations of the PVA [21]. As shown in Fig. 4b, these characteristic peaks of PVA were removed completely from the nanofibers after calcination at 400 -C, illustrating the decomposition of this organic matter. Besides, two peaks

Fig. 4. FT-IR spectra of nanofibers with 40 wt.% FeAc2 (a) electrospun PVA/ FeAc2 composite fibers; (b) calcined in air at 400 -C for 4 h.

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Fig. 5. The magnetic hysteresis (M-H) loops a-Fe2O3 with 40.0 wt.% FeAc2 calcined at different temperature (a) 400; (b) 600 -C.

around 584 and 358 cm 1 appeared, which can be ascribed to the metal-oxygen vibration of the a-Fe2O3 nanofibers [8]. Therefore the nanofibers obtained at the calcination temperature of 400 -C were pure a-Fe2O3 species. Fig. 5 shows the hysteresis loops of the a-Fe2O3 nanofibers with 40 wt.% FeAc2 calcined at 400 and 600 -C, respectively. The a-Fe2O3 nanofibers calcined at 400 -C have no remnant magnetization at zero magnetic field strength, which indicates superparamagnetism as can be seen in Fig. 5a. While for the magnetization curves of the nanofibers calcined at 600 -C shown in Fig. 5b, it is clearly seen that the a-Fe2O3 nanofibers exhibit hysteresis loop proving a superparamagnetic to ferromagnetic transition. We also observed from experiments that the saturation magnetization and coercivity of the a-Fe2O3 nanofibers calcined at 800 -C are larger than that of the aFe2O3 nanofibers calcined at 600 -C. It is well known that the magnetic properties are influenced by many factors, such as size, shape and defects in crystal structure [7]. As the average size of a-Fe2O3 crystals increased with increasing the calcination temperature, magnetic particles the a-Fe2O3 change from single domain to multi-domain, yielding the a-Fe2O3 nanofibers with their magnetic property changed from superparamagnetism to ferromagnetism. Therefore, the magnetic property of a-Fe2O3 nanofibers can be tuned by controlling the calcination temperature. The observation of the superhydrophilic surface was initiated by the contact angle measurements of a-Fe2O3 nanofibers film contacted with water. When a water droplet touch the film with 40 wt.% FeAc2 calcined at 400 and 600 -C, respectively, they spread quickly within hundreds of millisecond, resulting in a water contact angle (CA) of 0-. This observation can be explained by the Wenzel Equation [22]. This equation indicates that the water CA of the surface decreases with increasing surface roughness when the surface is composed of hydrophilic materials. As a polar material with water CA of 30- for the smooth surface by spin-coating [23], the microscale netlike of a-Fe2O3 nanofibers was believed to contribute to the large surface roughness, and therefore, resulting in the superhydrophilicity of a-Fe2O3 nanofibers film.

In summary, the superhydrophilic a-Fe2O3 nanofibers film with diameters of 30 – 90 nm was fabricated by electrospinning technique, using the as-electrospun PVA/ FeAc2 composite nanofibers as precursor in the calcinations process. The SEM, XRD, FT-IR results show that the morphologies of superhydrophilic a-Fe2O3 nanofibers are strongly affected by both the calcination temperature and the content of FeAc2. Moreover, superhydrophilic a-Fe2O3 nanofibers film with different shape, size and surface microstructure, especially with the tunable magnetic property, were obtained by controlling the calcinations temperature and the content of FeAc2. It is considered that the superhydrophilicity of a-Fe2O3 nanofibers film may be widely used as fast magnetic filter in the industrial applications. It is also indicated that the preparation method of a-Fe2O3 nanofiber films described here may provide a generic route for nanofibers film of favorite properties to be made from other superhydrophilic oxides. Acknowledgements The work is supported by the Special Research Foundation of the National Nature Science Foundation of China (20125102 and 20341003). References [1] R. Zboril, M. Mashlan, D. Petridis, Chem. Mater. 14 (2002) 969. [2] N. Mimura, I. Takahara, M. Saito, T. Hattori, K. Ohkuma, M. Ando, Catal. Today 45 (1998) 61. [3] J.S. Han, D.E. Davey, D.E. Mulcahy, A.B. Yu, Sens. Actuators, B, Chem. 61 (1999) 83. [4] A. Watanabe, H. Kozuka, J. Phys. Chem., B 107 (2003) 12713. [5] X. Wang, X. Chen, X. Ma, H. Zheng, M. Ji, Z. Zhang, Chem. Phys. Lett. 384 (2004) 391. [6] Y.Y. Fu, R.M. Wang, J. Xu, J. Chen, Y. Yan, A.V. Narlikar, H. Zhang, Chem. Phys. Lett. 379 (2003) 373. [7] T.P. Raming, A.J.A. Winnubst, C.M. van Kats, A.P. Philipse, J. Colloid Interface Sci. 249 (2002) 346. [8] D.H. Chen, X.L. Jiao, D.R. Chen, Mater. Res. Bull. 36 (2001) 1057. [9] J. Doshi, D.H. Reneker, J. Electrost. 35 (1995) 151. [10] D.H. Reneker, I. Chun, Nanotechnology 7 (1996) 216. [11] J.M. Deitzel, J.D. Kleinmeyer, J.K. Hirvonen, N.C. Beck Tan, Polymer 42 (2001) 8163. [12] S.W. Choi, S.M. Jo, W.S. Lee, Y.R. Kim, Adv. Mater. 15 (2003) 2027. [13] X. Wang, C. Drew, S.-H. Lee, K.J. Senecal, J. Kumar, L.A. Samuelson, Nano Lett. 2 (2002) 1273. [14] D. Li, Y. Xia, Nano Lett. 3 (2003) 555. [15] P. Viswanathamurthi, N. Bhattarai, H.Y. Kim, D.L. Lee, S.R. Kim, M.A. Morris, Chem. Phys. Lett. 374 (2003) 79. [16] H. Guan, C. Shao, S. Wen, B. Chen, J. Gong, X. Yang, Inorg. Chem. Commun. 6 (2003) 1302. [17] R.A. Caruso, J.H. Schattka, A. Greiner, Adv. Mater. 13 (2001) 1577. [18] F. Ko, Y. Gogotsi, A. Ali, N. Naguib, H. Ye, G.L. Yang, C. Li, P. Willis, Adv. Mater. 15 (2003) 1161. [19] Y. Nishio, R. St. John Manley, Macromolecules 21 (1988) 1270. [20] Powder Diffraction File, Joint Committee on Powder Diffraction Standards, ASTM, Swarthmore, PA, 1988, Card 32-469. [21] A. Takasu, H. Itou, M. Takada, Y. Inai, T. Hirabayashi, Polymer 43 (2002) 227. [22] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988. [23] M. Miyauchi, A. Nakajima, T. Watanabe, K. Hashimoto, Chem. Mater. 14 (2002) 2812.