- About this Journal ·
- Abstracting and Indexing ·
- Aims and Scope ·
- Annual Issues ·
- Article Processing Charges ·
- Articles in Press ·
- Author Guidelines ·
- Bibliographic Information ·
- Citations to this Journal ·
- Contact Information ·
- Editorial Board ·
- Editorial Workflow ·
- Free eTOC Alerts ·
- Publication Ethics ·
- Reviewers Acknowledgment ·
- Submit a Manuscript ·
- Subscription Information ·
- Table of Contents

Journal of Nanomaterials

Volume 2011 (2011), Article ID 525967, 5 pages

http://dx.doi.org/10.1155/2011/525967

## Preparation and Magnetic Properties of ZnFe_{2}O_{4} Nanotubes

^{1}Physics Department, Xinxiang University, Xinxiang 453003, China^{2}Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China

Received 19 April 2010; Revised 3 June 2010; Accepted 14 June 2010

Academic Editor: William W. Yu

Copyright © 2011 Yan Xu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Ordered ZnFe_{2}O_{4} nanotube arrays with the average outer diameter of 100 nm were prepared in porous anodic aluminum oxide template using an improved sol-gel approach. The morphology was studied by transmission electron and field emission scanning electron microscope. X-ray diffraction result shows that the nanotubes were polycrystalline in structure. The magnetic properties of the prepared ZnFe_{2}O_{4} nanotubes were also studied. The results show that the sample shows typical superparamagnetism at room temperature and obvious ferromagnetism below blocking temperature.

#### 1. Introduction

Spinel ferrites belong to a kind of magnetic materials that can be used in many areas, such as magnetic devices and switching devices [1–3]. Zinc ferrite (ZnFe_{2}O_{4}) is of interest not only to basic research in magnetism, but also has great potential in technological application, such as magnetic materials [4–10], gas sensors [11], catalysts [12], photocatalysts [13], and absorbent materials [14–18], described by the formula (A)[B]_{2}O_{4}. Spinel ferrites, which possess the cubic structure, are The (A) and [B] that indicate tetrahedral and octahedral cation sites in a face-centered cubic anion (oxygen) sublattice, respectively. Bulk ZnFe_{2}O_{4} has a normal spinel structure with Zn^{2+} ions in the A-site and Fe^{3+} ions in the B-sites. For bulk Zn-ferrite prepared by the conventional ceramic method, the inversion parameter *δ* equals zero (normal spinel). However, in contrast to bulk compound, the nanocrystalline ZnFe_{2}O_{4} system always shows up as a mixed spinel in which Zn^{2+} and Fe^{3+} ions are distributed over the A and B-sites. This cationic rearrangement leads to the formation of two magnetic sublattices, which is responsible for the enhanced magnetization displayed when compared with normal ZnFe_{2}O_{4} [19–21]. It is known that the bulk ZnFe_{2}O_{4} is a paramagnet at room temperature, however, magnetic order has been observed in its nanoparticles at room temperature [22–28], and similar reports of magnetic properties in ZnFe_{2}O_{4} ferrite thin films [29–33].

Up to now, only a few groups reported the preparation of ZnFe_{2}O_{4} nanotubes [34, 35]. There are a few reports of magnetic properties for ZnFe_{2}O_{4} nanotubes. Thus, the synthesis and magnetic study of ZnFe_{2}O_{4} nanotubes should be of substantial interest from both fundamental and applied perspective. Herein, we synthesized ZnFe_{2}O_{4} nanotube arrays by an improved sol-gel template method. The structure and morphology of the ZnFe_{2}O_{4} nanotubes were characterized and its magnetic properties were studied. Our results show that the sample has superparamagnetism at room temperature, and ferromagnetic below

#### 2. Experimental

Anodic aluminum oxide (AAO) templates with a pore diameter of about 100 nm were prepared by anodic oxidation of 99.99% pure Al foil in oxalic acid (1.2 M) under two-step anodizing process [36]. The precursor solution was prepared as follows. Fe and Zn with a molar ratio of 2 : 1 were dissolved in distilled water to form 0.045 M aqueous solution of nitrate. An amount of citric acid equal to was dissolved in the mixture solution as a surfactant. The pH value of the solution was adjusted to near 7 by adding ammonia, and then an amount of urea as complexing agent was added into the solution, the molar ratio of and urea was 1 : 10. All chemicals were of analytical grade and used without further purification. The AAO templates were immersed in the precursor solution for the desired time at 80°C. When the solution was heated, the pH value of the solution increased because of urea undergoing hydrolysis above 60°C, the combined with and formed negatively charged Fe_{2}Zn [(OH)* _{x}*](H

_{2}O)

*sol, which is similar to the previous report [37]. Meanwhile, the pore walls of the AAO were positively charged [38]. As coexistence of the charge interaction and the capillary action, it is reasonable that the nanotubes firstly formed near wall areas of the pores and then extended to the center area gradually. Subsequently, the AAO templates were taken out and placed into saturated HgCl*

_{y}_{2}solution to separate templates from the Al substrate. After rinsing with distilled water, the precursor in templates was heat treated in a tube furnace at 600°C for 10 h in air, and the arrays of ZnFe

_{2}O

_{4}nanotubes inside the AAO templates were obtained.

Field Emission Scanning Electron Microscope (FE-SEM) was performed by using a Hitachi S-4800× microscope operated at 10 kV. Structural and morphology data were collected by X-ray diffraction (XRD) using an Pertpro Philips diffractometer with radiation and transmission electron microscope (TEM, Hitachi H-600), respectively. The magnetic properties were measured by a vibrating sample magnetometer (VSM, Lakeshore 7300) and Quantum Design MPMS magnetometer based on superconducting quantum interference device (SQUID).

#### 3. Result and Discussion

Figure 1 shows the SEM, TEM images of ZnFe_{2}O_{4} nanotubes with diameters of 100 nm. It is apparent that the ZnFe_{2}O_{4} nanotubes have a uniform diameter. The mean outer diameter of these nanotubes is about 100 nm, corresponding to the diameter of channels in the AAO template. The thickness of tube wall is about 18 nm. The inset of Figure 1(b) shows the selective area electron diffraction (SAED) pattern of the ZnFe_{2}O_{4} nanotubes. The SAED pattern indicates that the ZnFe_{2}O_{4} nanotube is polycrystalline in structure. The pattern for the sample is well resolved in XRD patterns.

Figure 2 shows the X-ray diffraction pattern of the ZnFe_{2}O_{4} nanotubes. The reflection peaks are clearly distinguishable. The main peaks correspond to a spinel-type lattice (Fd3m) with lattice parameter Å. The lattice parameter is smaller than that of bulk ( Å) because of crystallite size reduction plus lattice disorder. No other obvious reflections were detected indicating inexistence of a second crystal phase.

The results of zero-field-cooled (ZFC) and field-cooled (FC) measurements of magnetization as function of temperature for the ZnFe_{2}O_{4} nanotubes are presented in Figure 3. In order to avoid the nonlinearity effect [6], these measurements were performed at a low field of 50 Oe. For antiferromagnetic materials, typical ZFC/FC curves show a sharp cusp at the Neel temperature. There is no such a cusp in the present case. However, the ZFC curve shows a peak appearing at about 90 K. This behaviour of ZFC is typical of a superparamagnetic system. The blocking temperature () of 90 K is higher than the Néel temperature of 10 K for the bulk. This may indicate that the samples are indeed the mixed spinels with the Curie temperature at least higher than the observed blocking temperature. Below the blocking temperature, the ZFC and FC curves significantly diverge and the ZnFe_{2}O_{4} nanotubes are in the ferrimagnetic state. Above *T _{B}*, the ZFC and FC curves coincide due to the fact that the nanotubes are at the superparamagnetic state.

Figure 4 shows the hysteresis loops measured at 10 K and 150 K. The temperatures are so chosen that one is well below and the other is above the blocking temperature (90 K) for the sample. The inset in Figure 4 is the magnified view of the *M*–*H *curves. The loops at 10 K and 150 K are quite different. At 150 K, the coercivity is near to zero, which shows that the sample is completely superparamagnetic. This is well expected, as for a superparamagnetic system the coercivity is zero. But at 10 K the coercivity is as high as 1420 Oe, and the remanence is obvious. This clearly shows ferrimagnetic coupling due to the A–O–B superexchange interaction and hence existence of a significant amount of cation inversion. Cation distribution has changed from normal to mixed spinel type, that is, some Fe^{3+} ions occupy the tetrahedral A-sites and switch on the A–B superexchange interaction. Up to 30 kOe, the magnetization is far from being saturated and still increases with increasing magnetic field, due to the presence of a linear and reversible contribution. Such behaviour indicates the presence of a paramagnetic phase, as it has already pointed out by other authors .

#### 4. Conclusion

In summary, we have prepared ZnFe_{2}O_{4} nanotube arrays with a diameter of 100 nm by a improved sol-gel template method. The ZnFe_{2}O_{4} nanotube has a uniform diameter with tube wall of about 18 nm. The sample shows typical temperature of 90 K. When the measured temperature below at 10 K, the coercivity is as high as 1420 Oe, and the remanence is obvious. These indicate that the nanocrystalline ZnFe_{2}O_{4} system shows up as a mixed spinel.

#### Acknowledgments

This work is supported by NSFC (Grant no. 50671046) and J0630313, Program for basic and advanced technology research of Henan Province.

#### References

- R. W. Chantrell and K. O’Grady, “The magnetic properties of fine particles,” in
*Applied Magnetism*, p. 113, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1994. View at Google Scholar - T. Nakamura, T. Tsutaoka, and K. Hatakeyama, “Frequency dispersion of permeability in ferrite composite materials,”
*Journal of Magnetism and Magnetic Materials*, vol. 138, no. 3, pp. 319–328, 1994. View at Google Scholar · View at Scopus - T. Tsutaoka, M. Ueshima, T. Tokunaga, T. Nakamura, and K. Hatakeyama, “Frequency dispersion and temperature variation of complex permeability of Ni-Zn ferrite composite materials,”
*Journal of Applied Physics*, vol. 78, no. 6, pp. 3983–3991, 1995. View at Publisher · View at Google Scholar · View at Scopus - H. Ehrhardt, S. J. Campbell, and M. Hofmann, “Magnetism of the nanostructured spinel zinc ferrite,”
*Scripta Materialia*, vol. 48, no. 8, pp. 1141–1146, 2003. View at Publisher · View at Google Scholar · View at Scopus - S. Bid and S. K. Pradhan, “Preparation of zinc ferrite by high-energy ball-milling and microstructure characterization by Rietveld's analysis,”
*Materials Chemistry and Physics*, vol. 82, no. 1, pp. 27–37, 2003. View at Publisher · View at Google Scholar · View at Scopus - A. Kundu, C. Upadhyay, and H. C. Verma, “Magnetic properties of a partially inverted zinc ferrite synthesized by a new coprecipitation technique using urea,”
*Physics Letters A*, vol. 311, no. 4-5, pp. 410–415, 2003. View at Publisher · View at Google Scholar · View at Scopus - K. Tanaka, M. Makita, Y. Shimizugawa, K. Hirao, and N. Soga, “Structure and high magnetization of rapidly quenched zinc ferrite,”
*Journal of Physics and Chemistry of Solids*, vol. 59, no. 9, pp. 1611–1618, 1998. View at Google Scholar · View at Scopus - F. Grasset, N. Labhsetwar, D. Li et al., “Synthesis and magnetic characterization of zinc ferrite nanoparticles with different environments: powder, colloidal solution, and zinc ferrite-silica core-shell nanoparticles,”
*Langmuir*, vol. 18, no. 21, pp. 8209–8216, 2002. View at Publisher · View at Google Scholar · View at Scopus - H. Deng, X. Li, Q. Peng, X. Wang, J. Chen, and Y. Li, “Monodisperse magnetic single-crystal ferrite microspheres,”
*Angewandte Chemie. International Edition*, vol. 44, no. 18, pp. 2782–2785, 2005. View at Publisher · View at Google Scholar · View at Scopus - M. H. Sousa, F. A. Tourinho, J. Depeyrot, G. J. Da Silva, and M. C. F. L. Lara, “New electric double-layered magnetic fluids based on copper, nickel, and zinc ferrite nanostructures,”
*Journal of Physical Chemistry B*, vol. 105, no. 6, pp. 1168–1175, 2001. View at Google Scholar · View at Scopus - X. Niu, W. Du, and W. Du, “Preparation and gas sensing properties of Zn${\text{M}}_{2}{\text{O}}_{4}$ (M $=$ Fe, Co, Cr),”
*Sensors and Actuators B*, vol. 99, no. 2-3, pp. 405–409, 2004. View at Publisher · View at Google Scholar · View at Scopus - J. A. Toledo-Antonio, N. Nava, M. Martínez, and X. Bokhimi, “Correlation between the magnetism of non-stoichiometric zinc ferrites and their catalytic activity for oxidative dehydrogenation of 1-butene,”
*Applied Catalysis A*, vol. 234, no. 1-2, pp. 137–144, 2002. View at Publisher · View at Google Scholar · View at Scopus - J. Qiu, C. Wang, and M. Gu, “Photocatalytic properties and optical absorption of zinc ferrite nanometer films,”
*Materials Science and Engineering B*, vol. 112, no. 1, pp. 1–4, 2004. View at Publisher · View at Google Scholar · View at Scopus - M. Kobayashi, H. Shirai, and M. Nunokawa, “Measurements of sulfur capacity proportional to zinc sulfidation on sorbent containing zinc ferrite-silica composite powder in pressurized coal gas,”
*Industrial and Engineering Chemistry Research*, vol. 41, no. 12, pp. 2903–2909, 2002. View at Google Scholar · View at Scopus - M. Kobayashi, H. Shirai, and M. Nunokawa, “Estimation of multiple-cycle desulfurization performance for extremely low-concentration sulfur removal with sorbent containing zinc ferrite-silicon dioxide composite powder,”
*Energy and Fuels*, vol. 16, no. 6, pp. 1378–1386, 2002. View at Publisher · View at Google Scholar · View at Scopus - M. Pineda, J. M. Palacios, E. García, C. Cilleruelo, and J. V. Ibarra, “Modelling of performance of zinc ferrites as high-temperature desulfurizing sorbents in a fixed-bed reactor,”
*Fuel*, vol. 76, no. 7, pp. 567–573, 1997. View at Google Scholar · View at Scopus - N.-O. Ikenaga, Y. Ohgaito, H. Matsushima, and T. Suzuki, “Preparation of zinc ferrite in the presence of carbon material and its application to hot-gas cleaning,”
*Fuel*, vol. 83, no. 6, pp. 661–669, 2004. View at Publisher · View at Google Scholar · View at Scopus - F. Tomás-Alonso and J. M. P. Latasa, “Synthesis and surface properties of zinc ferrite species in supported sorbents for coal gas desulphurisation,”
*Fuel Processing Technology*, vol. 86, no. 2, pp. 191–203, 2004. View at Publisher · View at Google Scholar · View at Scopus - V. Sepelak, M. Zatroch, K. Tkacova, P. Petrovic, S. Wibmann, and K. D. Becker, “Structure and properties of the ball-milled spinel ferrites,”
*Materials Science and Engineering A*, vol. 22, pp. 226–228, 1997. View at Google Scholar - S. A. Oliver, V. G. Harris, H. H. Hamdeh, and J. C. Ho, “Large zinc cation occupancy of octahedral sites in mechanically activated zinc ferrite powders,”
*Applied Physics Letters*, vol. 76, no. 19, pp. 2761–2763, 2000. View at Google Scholar · View at Scopus - B. Jeyadevan, K. Tohji, and K. Nakatsuka, “Structure analysis of coprecipitated Zn${\text{Fe}}_{2}{\text{O}}_{4}$ by extended x-ray-absorption fine structure,”
*Journal of Applied Physics*, vol. 76, no. 10, pp. 6325–6327, 1994. View at Publisher · View at Google Scholar · View at Scopus - C. N. Chinnasamy, A. Narayanasamy, N. Ponpandian, K. Chattopadhyay, H. Guérault, and J.-M. Greneche, “Magnetic properties of nanostructured ferrimagnetic zinc ferrite,”
*Journal of Physics Condensed Matter*, vol. 12, no. 35, pp. 7795–7805, 2000. View at Publisher · View at Google Scholar · View at Scopus - F. S. Li, L. Wang, J. B. Wang et al., “Site preference of Fe in nanoparticles of Zn${\text{Fe}}_{2}{\text{O}}_{4}$,”
*Journal of Magnetism and Magnetic Materials*, vol. 268, no. 3, pp. 332–339, 2004. View at Publisher · View at Google Scholar · View at Scopus - L. D. Tung, V. Kolesnichenko, G. Caruntu et al., “Annealing effects on the magnetic properties of nanocrystalline zinc ferrite,”
*Journal of Magnetism and Magnetic Materials*, vol. 319, no. 1–4, pp. 116–121, 2002. View at Publisher · View at Google Scholar · View at Scopus - C. Upadhyay, H. C. Verma, V. Sathe, and A. V. Pimpale, “Effect of size and synthesis route on the magnetic properties of chemically prepared nanosize Zn${\text{Fe}}_{2}{\text{O}}_{4}$,”
*Journal of Magnetism and Magnetic Materials*, vol. 312, no. 2, pp. 271–279, 2007. View at Publisher · View at Google Scholar · View at Scopus - H. Xue, Z. Li, X. Wang, and X. Fu, “Facile synthesis of nanocrystalline zinc ferrite via a self-propagating combustion method,”
*Materials Letters*, vol. 61, no. 2, pp. 347–350, 2007. View at Publisher · View at Google Scholar · View at Scopus - M. Atif, S. K. Hasanain, and M. Nadeem, “Magnetization of sol-gel prepared zinc ferrite nanoparticles: Effects of inversion and particle size,”
*Solid State Communications*, vol. 138, no. 8, pp. 416–421, 2006. View at Publisher · View at Google Scholar · View at Scopus - C. Yao, Q. Zeng, G. F. Goya et al., “Zn${\text{Fe}}_{2}{\text{O}}_{4}$ nanocrystals: synthesis and magnetic properties,”
*Journal of Physical Chemistry C*, vol. 111, no. 33, pp. 12274–12278, 2007. View at Publisher · View at Google Scholar · View at Scopus - S. Nakashima, K. Fujita, K. Tanaka, and K. Hirao, “High magnetization and the high-temperature superparamagnetic transition with intercluster interaction in disordered zinc ferrite thin film,”
*Journal of Physics Condensed Matter*, vol. 17, no. 1, pp. 137–149, 2005. View at Publisher · View at Google Scholar · View at Scopus - J. Chen, G. Srinivasan, S. Hunter, V. Suresh Babu, and M. S. Seehra, “Observation of superparamagnetism in rf-sputtered films of zinc ferrite,”
*Journal of Magnetism and Magnetic Materials*, vol. 146, no. 3, pp. 291–297, 1995. View at Google Scholar · View at Scopus - R. Nawathey, R. D. Vispute, S. M. Chaudhari et al., “Pulsed laser-induced vaporization from the surface of a binary oxide (zinc ferrite) and its implications for synthesis of thin films,”
*Journal of Applied Physics*, vol. 65, no. 8, pp. 3197–3204, 1989. View at Publisher · View at Google Scholar · View at Scopus - M. Taheri, E. E. Carpenter, V. Cestone et al., “Magnetism and structure of ${\text{Zn}}_{x}{\text{Fe}}_{3-x}{\text{O}}_{4}$ films processed via spin-spray deposition,”
*Journal of Applied Physics*, vol. 91, no. 10, Article ID 7595, 3 pages, 2002. View at Publisher · View at Google Scholar · View at Scopus - M. Bohra, S. Prasad, N. Kumar et al., “Large room temperature magnetization in nanocrystalline zinc ferrite thin films,”
*Applied Physics Letters*, vol. 88, no. 26, Article ID 262506, 2006. View at Publisher · View at Google Scholar · View at Scopus - G. Zhang, C. Li, F. Cheng, and J. Chen, “Zn${\text{Fe}}_{2}{\text{O}}_{4}$ tubes: synthesis and application to gas sensors with high sensitivity and low-energy consumption,”
*Sensors and Actuators B*, vol. 120, no. 2, pp. 403–410, 2007. View at Publisher · View at Google Scholar · View at Scopus - J. Fu, D. Gao, Y. Xu, Z. Yan, and D. Xue, “One-step process to fabricate Fe core/Fe-dimethylsulfoxide shell coaxial nanocables,”
*Chemistry of Materials*, vol. 20, no. 5, pp. 2016–2019, 2008. View at Publisher · View at Google Scholar · View at Scopus - G. Wu, L. Zhang, B. Cheng, T. Xie, and X. Yuan, “Synthesis of ${\text{Eu}}_{2}{\text{O}}_{4}$ nanotube arrays through a facile sol-gel template approach,”
*Journal of the American Chemical Society*, vol. 126, no. 19, pp. 5976–5977, 2004. View at Publisher · View at Google Scholar · View at Scopus - J. W. Diggle, T. C. Downie, and C. W. Goulding, “Anodic oxide films on aluminum,”
*Chemical Reviews*, vol. 69, no. 3, pp. 365–405, 1969. View at Google Scholar · View at Scopus - L. D. Tung, V. Kolesnichenko, G. Caruntu et al., “Annealing effects on the magnetic properties of nanocrystalline zinc ferrite,”
*Physica B*, vol. 319, no. 1–4, pp. 116–121, 2002. View at Publisher · View at Google Scholar · View at Scopus