Radiation Physics and Chemistry 93 (2013) 21–24
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Structural studies of magnetic Fe doped ZnO nanofibers A. Baranowska-Korczyc n, K. Fronc, J.B. Pełka, K. Sobczak, D. Klinger, P. Dłużewski, D. Elbaum Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, PL-02668 Warsaw, Poland
H I G H L I G H T S
The structural studies of Fe doped ZnO nanofibers (ferromagnetic at room temperature) were reported. Incorporating at. 10% Fe ions into ZnO did not modify the wurtzite structure of the nanofibers. The structural analysis (resolution about 1.5 nm) did not reveal any evidence of a second phase. The ferromagnetic signal came from Fe ions built-in ZnO crystals. The low activation energy for ZnO crystal growth was responsible for higher level of doping.
art ic l e i nf o
a b s t r a c t
Article history: Received 17 May 2012 Accepted 26 February 2013 Available online 23 March 2013
This work reports structural properties of room temperature ferromagnetic Fe doped ZnO nanofibers (NFs). The NFs were obtained by electrospinning and calcination in air. The input atomic ratio of Fe to Zn ions was about 0.1. The structural characterization was performed by X-ray (XRD) examination, using the synchrotron radiation, and Energy-Filtered Transmission Electron Microscopy (EFTEM) analysis. Incorporating Fe ions into ZnO does not affect the crystal structure of the wurtzite host. No clear evidence of the second phase (Fe, FeO, Fe2O3 or ZnFe2O4) was detected. Diameters of crystals responsible for the magnetic properties ranged from 3 to 10 nm. We did not observe any precipitates of the different phase with diameters equal or larger than 1.5 nm. It implies that the magnetic signal comes from Fe ions built-in ZnO crystals. No other crystals (besides ZnO crystals) were observed in this range of sizes. We propose that the low activation energy for the nanocrystals growth in NFs allows a large amount of doped ions to be built-in the ZnO crystals. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Electrospinning ZnO nanofibers ZnFeO
1. Introduction ZnO with its large exciton binding energy of 60 meV and wide band gap of 3.37 eV at room temperature is a promising nanomaterial for potential applications in electronics and optoelectronics (Morkoç and Özgür, 2008; Klingshirn, 2007). Moreover, ZnO could be doped with 3d transition metals to obtain diluted magnetic semiconductors (Dietl et al., 2000). Traditionally, popular semiconducting materials with magnetic properties are subject of interest in spin-electronics, especially ferromagnetic above room temperature Ohno et al. (2001). ZnO prepared through many different routes is frequently doped with various ions, such as Fe, Mn, Co and Ni (Dietl et al., 2000; Pan et al., 2008; Venkatesan et al., 2004; Coey et al., 2005; Seshadri, 2005; Yang et al., 2008A; Risbud et al., 2003). The ferromagnetism occurrence in semiconducting oxides after doping is not obvious and is still a controversial issue. The physical origin of ferromagnetism is a subject of an active research. The transition
metal-doped semiconducting oxides, including ZnO, are extensively studied by a number of methods. Although many growth techniques are employed to produce magnetic ZnO, electrospinning is still a new approach (Liu et al., 2012; Li et al., 2012). Electrospinning is gaining more attention due to its versatility, high-efficiency and low cost operation. Electrospun nanofibers can be obtained from various materials, made of polymers, composites or ceramics (Li and Xia, 2004; Ramaseshan et al., 2007). We have obtained the room temperature ferromagnetic Fe doped ZnO nanofibers by electrospinning and calcination in air. Our previous work presents extensive studies of these magnetic, electrospun Fe doped ZnO nanofibers (Baranowska-Korczyc et al., 2012). In this report, based on the new data, we discuss structural properties of the nanofibers and location of Fe ions, responsible for the magnetic signal, in the ZnO NFs.
2. Experimental n
Corresponding author. Tel.: þ48 228436601x3539. E-mail address:
[email protected] (A. Baranowska-Korczyc).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.02.038
Fe doped ZnO nanofibers were obtained by electrospinning technique followed by air calcination. The electrospinning was
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performed using two precursors: zinc acetate (C4H6O4Zn 2H2O, CHEMPUR) and iron acetate (CH3CO2)2Fe (Sigma Aldrich). They were dissolved in water solution of poly(vinyl alcohol) (PVA) (Mw ¼72 000, POCH) (wt. 10%). The PVA was prepared in distilled water at 60 1C and left for a few days to obtain a homogeneous solution. The input content of the doped Fe to Zn atoms was about 0.1. The electrospun NFs were calcined at 500 1C for 4 h in air to obtain ceramic nanofibers. More details of preparation of ZnO NFs, including the electrospinning process parameters, are presented in our previous work (Baranowska-Korczyc et al., 2012). Structural characterization of Fe doped ZnO nanofibers was investigated by using Transmission Electron Microscopy (TEM) (JEM2000EX), Energy-Filtered Transmission Electron Microscopy (EFTEM) (TITAN CUBED 80–300) and X-ray diffraction (XRD) techniques. The XRD measurements were performed using synchrotron radiation at the W1 beamline at DESY-HASYLAB with 2θ scan in the glancing incidence geometry (λ ¼1.54056 Å). The samples were tested with different densities of nanofibers. The density of nanofibers depended on the duration of electropsinning: the higher (collection time—30 min) and the lower (collection time—5 min). Two types of substrates were applied to collect nanofibers: square 300 mesh gold grids for EFTEM and TEM analysis and silicon wafers (111) for XRD measurements.
3. Results and discussion Fig. 1 (the bright field TEM image) reveals the fibrous structure of Fe doped ZnO nanofibers. The NFs consist of nanocrystal conglomerates (the dark field images, included in previous work) (Baranowska-Korczyc et al., 2012). The crystalline structure of Fe doped ZnO NFs was studied by XRD using a synchrotron radiation. The X-ray diffraction patterns confirm the presence of ZnO and reveal wurtzite crystal structure of NFs (Fig. 2) (JCPD: 36-1451). We did not observe any additional peaks connected with the presence in NFs of such compounds as: Fe, FeO, Fe2O3 or ZnFe2O4. The second phase crystals was not noted for all samples with various densities of nanofibers (Fig. 2). We obtained similar results as previously, using the powder diffractometer employing CuKα1 radiation (Philips X’Pert MPD Pro Alpha1). We did not observe the peaks from the different phase, in spite of the fact that the content of Fe ions was high. The input atomic ratio of Fe to Zn was about 0.1. The measurements recorded with 2θ scan in the glancing incidence geometry using synchrotron radiation should reveal evidence of the second phase, such as ZnFe2O4 cubic phase, if
Fig. 1. The bright field TEM image shows the fibrous structure of Fe doped ZnO nanofibers.
Fig. 2. Typical X-ray diffraction patterns of two densities of Fe doped ZnO nanofibers: the higher (collected for 30 min) (a) and the lower (collected for 5 min) (b).
present in the nanomaterial. Indeed, the sizes of crystals responsible for ferromagnetic signal in Fe doped nanofibers are sufficient to detect them by this technique. It was obtained previously as a result of zero field cooled (ZFC) and field cooled (FC) measurements. For blocking temperature TB ¼300 K, we obtained nanoparticle diameters in the range of between 3 and 10 nm (Baranowska-Korczyc et al., 2012). The crystals of this range of sizes are visible for above XRD technique. The second, the concentration of Fe ions in NFs was relatively high. Previously, we demonstrated confirmation of the intended amount of Fe ions in ZnO by EDX (Energy-dispersive X-ray Spectroscopy) characterization. Moreover, we examined our samples by TEM analysis. We observed electron diffraction pattern typical of wurtzite ZnO and not found any additional diffraction rings. These results imply that the magnetic signal comes from Fe ions built-in the ZnO lattice, because we do not observe any other crystals (besides ZnO crystals) above 3 nm. Recently, Seo et al. (2010), observed similar results on thin films of Zn1−xFexO, containing up to 0.07 Fe. The authors revealed absence of Fe clusters and they attributed the magnetic properties of these ZnO films to intrinsic ferromagnetism resulted from the Fe ion substitution for the ZnO crystalline Zn sites. Moreover, nanocrystalline Fe doped ZnO powder obtained by the sol–gel method, followed by calcination (similar method to ours), exhibited the wurtzite structure without any evidence of the second phase (Rattana et al., 2009). We confirmed also, based on EFTEM analysis, that incorporating Fe ions into ZnO does not affect the crystal structure of the host. In Fig. 3, we show EFTEM maps of Zn–M, O–K and Fe–L for Fe doped ZnO nanofiber. Zinc, oxide and iron atoms are homogenously distributed along the fiber. We did not observe any precipitates from the second phase with resolution of about 1.5 nm. As mentioned above, the sizes of crystals responsible for magnetic properties are of diameters in the range of between 3 and 10 nm. It implies that the magnetic signal in our material comes from Fe ions built-in ZnO crystals. However, presence of the additional crystals in the ZnO matrix, e.g. Fe2O3, FeO, ZnFe2O4 with diameter below 1.5 nm, not detectable by XRD and EFTEM techniques, is possible. The particles with size of less than 1.5 nm are comparable to the dimensions of the unit cell is therefore difficult to consider such objects as crystals. Previously, we performed cathodoluminescence (CL) study of ZnO and Fe doped ZnO nanofibers (Baranowska-Korczyc et al., 2012). CL spectra for doped and undoped nanofibers consisted of two emission bands. One, centered around 380 nm for ZnO and 400 nm for Fe doped nanofibers, is related to the interband
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Fig. 3. EFTEM maps of Zn–M (a), O–K (b) and Fe–L (c) Fe doped ZnO nanofiber.
recombination. The other one centered around 600 nm is a defect band and originates from oxygen vacancies (Cheng and Ma 2009). We observed that the maximum of CL spectra was shifted from 380 nm for undoped NFs to 400 nm for Fe doped NFs. This result indicates that some part of Fe ions could be build-in ZnO crystals and implies that we obtained a solid solution ZnxFe1−xO. Similar result was reported previously for ZnO doped with Fe (Alver et al., 2007; Soumahoro et al., 2010; Mari et al., 2010). Interestingly, we did not observe that XRD peaks are shifted, as a result of ZnFeO presence. Fe ions did not modify ZnO structure, because of the similarity of ionic radius of Zn2 þ and Fe2 þ . Cotton and Wilkinson (Gaus) noted that ionic radius of Zn2 þ is about 0.74 Å and the Fe2 þ is about 0.75 Å. Pan et al. (2012) reported that the ionic radius for Fe2 þ in ZnO host is about 0.76 Å. Due to the similarity of Fe and Zn ions atomic radii changes in crystal parameters are not visible in diffraction patterns of Fe doped ZnO nanofibers. Summarizing, in the Fe doped ZnO NFs, no clear evidence of the second phase can be confirmed, as could be expected according to the atomic ratio of Fe to Zn ions. This is consistent with previously reported results for similar size of ZnO nanocrystals doped with Co and Mn ions (Straumal et al., 2008; Straumal et al., 2009). Our results suggest that the process of crystal growth, especially its energy, has a significant influence on the level of doping. The crystal growth is thermally activated and it allows, by applying Arrhenius equation, to gain some insight of the process mechanism through determination of its activation energy (Park and Kim, 2009). The activation energy for ZnO crystal growth in our nanofibers was estimated to be about 12 kJ/mol (BaranowskaKorczyc et al., 2013). The determination of ZnO crystal average diameters were based on nanocrystals measured from dark field TEM images. The activation energy for electrospun ZnO nanofibers is one order of magnitude lower than the value for the epitaxial ZnO nanowires (about 150 kJ/mol) (Yang et al., 2008B) or bulk ceramics ZnO (more than 300 kJ/mol) (Tomlins et al., 1998). The low activation energy and relatively fast growth of the crystal allowed a large amount of doped ions to be built-in the ZnO crystals.
4. Conclusions We reported on structural studies of magnetic Fe doped ZnO nanofibers, obtained by electrospinning and calcination. Based on the results obtained in this and the previous work, we discussed structural properties of the nanofibers. Incorporating at. 10% Fe ions into ZnO does not modify the wurtzite crystal structure of the NFs. We examined our samples by XRD, TEM and EFTEM techniques and did not observe any Fe-rich magnetic nanoparticles (Fe, FeO, Fe2O3, ZnFe2O4). If they form in NFs, their size is below 1.5 nm and they are present in small number, thus are not responsible for the magnetic properties of Fe doped NFs. The ferromagnetic signal comes from crystals whose sizes are in the range of 3–10 nm and
are detectable using our methods. Moreover, the maximum of CL spectrum was shifted for the Fe doped ZnO nanofibers resulted from Fe ions built-in ZnO crystals. We suggest that the one order of magnitude lower activation energy for crystal growth in NFs, compared with ceramic nanostructures obtained by other method, is responsible for the higher level of doping in ZnO nanofibers.
Acknowledgments The research was partially supported by the European Union within European Regional Development Fund, through Grant Innovative Economy (POIG.01.01.02-00-008/08) and by the Ministry of Science and Higher Education (Poland), and Grant no. N518 424036. the European Regional Development Fund within the Innovative Economy Operational Program 2007–2013 No POIG.02.01-00-14032/08.
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