J. Phys. Chem. C 2008, 112, 13163–13170
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Generalized Preparation of Porous Nanocrystalline ZnFe2O4 Superstructures from Zinc Ferrioxalate Precursor and Its Superparamagnetic Property Man Wang, Zhihui Ai,* and Lizhi Zhang* Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China ReceiVed: May 7, 2008; ReVised Manuscript ReceiVed: June 10, 2008
In this paper, porous nanocrystalline ZnFe2O4 “timber-like” superstructures were synthesized through the thermal decomposition of zinc ferrioxalate precursor prepared from metal sulfates and sodium oxalate without adding any additives. The resulting ZnFe2O4 superstructures were systematically characterized by X-ray powder diffraction (XRD), thermogravimetry and differential scanning calorimetry (TG-DSC), scanning electronic microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR). We found that the porous nanocrystalline ZnFe2O4 superstructures exhibited superparamagnetic properties at room temperature. These porous nanocrystalline ZnFe2O4 superstructures may be applied in magnetic devices. Meanwhile, this general approach could be extended to synthesize porous nanocrystalline CoFe2O4 and NiFe2O4 “timberlike” superstructures. Introduction In recent years, there has been considerable interest in the preparation of transition metal oxides based ferrites (AFe2O4) nanostructures because of their importance in the fundamental understanding of the physical processes as well as their proposed significant and insatiable applications in different fields.1-4 Among AFe2O4, zinc ferrite (ZnFe2O4) is one of the most widely studied materials.5-8 Particularly, various high-quality ZnFe2O4 nanostructures are well-known to possess a wide range of possible applications and properties such as magnetic behavior,5 electrical characteristics,9 semiconductor photocatalysis,2,10 absorbent material for hot-gas desulfurization,11,12 and so on. Various synthesis methods, such as sol-gel,11,13 coprecipitation,14 microemulsion,15 pulsed laser deposition,16 electrodeposition,8 hydrothermal method,17 urea combustion,4 and ultrasonic cavitation approach,18,19 have been developed to prepare ZnFe2O4 nanostructures. For example, Veith et al. synthesized nanostructured ZnFe2O4 by a sol-gel processing of a heterobimetallic alkoxide.13 Sivakumar and co-workers developed a new ultrasound assisted emulsion and evaporation protocol for the synthesis of ZnFe2O4 nanoparticles with narrow size distribution.18 The performances of ZnFe2O4 strongly depend on their stoichiometric ratio and microstructures, which vary with the preparation processes.18 Therefore, it is of great interest to develop new and simple routes toward the preparation of ZnFe2O4. As one of the wet chemistry methods, the precursor method could produce stoichiometric and single phase transition metal oxides on a large scale. Meanwhile, this method can obtain the target oxide at a lower temperature with less time than the conventional ceramic method.20 Metal ferrioxalates have proven to be the most suitable precursors for the preparation of ferrites. Although a detailed investigation has been made on the thermal decomposition of transition metal oxalates,21 we still lack the * To whom correspondence should be addressed. E-mail: Z.A.,
[email protected]; L.Z.,
[email protected]. Phone/Fax: +86-27-6786 7535.
Figure 1. Powder X-ray diffraction (XRD) patterns of the different samples. (a) zinc ferrioxalate precursors. ZnFe2O4 obtained by calcining the precursors in air at 773 K (b), 873 K (c), and 973 K (d).
information about the thermal decomposition of complex zinc ferrioxalates to get ternary metal oxides. Superparamagnetic materials do not retain any magnetization after the removal of the magnetic field.22,23 They are of great interest for many applications including magnetic resonance imaging (MRI), separation and purification, drug delivery, and magnetically induced hyperthermia.24 The most frequently used superparamagnetic materials are monoxides such as iron oxides (maghemite and magnetite),25,26 CoO,27 and NiO. However, superparamagnetic properties of ternary oxides such as spinel ZnFe2O4 nanostructures have not been studied intensively. Herein we develop a simple method to synthesize porous nanocrystalline ZnFe2O4 “timber-like” superstructures via the thermal decomposition of zinc ferrioxalate precursor and investigate the magnetic properties of the resulting porous nanocrystalline ZnFe2O4 superstructures. Moreover, this precursor method can be extended to prepare porous nanocrystalline
10.1021/jp804009h CCC: $40.75 2008 American Chemical Society Published on Web 07/31/2008
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Figure 2. DSC-TG curves of the zinc ferrioxalate precursor.
CoFe2O4 and NiFe2O4 “timber-like” superstructures, suggesting its generality. Experimental Section Synthesis. Porous nanocrystalline ZnFe2O4 superstructures were prepared with a zinc ferrioxalate precursor route using FeSO4 · 7H2O, ZnSO4 · 7H2O, and sodium oxalate (Na2C2O4) as starting materials. All reagents were of commercially available analytical grade and were used without further purification. Deionized water was used in all experiments. In a typical preparation, aqueous solutions of 0.1 M ZnSO4 · 7H2O and 0.2 M FeSO4 · 7H2O were prepared, respectively. A 100 mL aliquot of each solution was then mixed together at 353 K. An excess of 0.1 M Na2C2O4 solution was introduced into the above mixed solution under stirring at 353 K to produce a fawn zinc ferrioxalate precursor. The precursor was washed with distilled water several times and dried at 373 K for 24 h. The precursor was finally calcined in air at 773, 873, or 973 K for 2 h to obtain different ZnFe2O4 samples. Similarly, CoFe2O4 and NiFe2O4 samples were synthesized from transition metal ferrioxalate precursors by mixing equimolar quantities of aqueous solutions of FeSO4 · 7H2O, Na2C2O4 and the corresponding cobalt and nickel sulfates under vigorous stirring. The reaction mixtures were heated on a water bath to 353 K and allowed to cool slowly under stirring. Yellowish precipitations began to appear at this temperature. After cooling to room temperature, the precursors were filtered and washed with distilled water. CoFe2O4 and NiFe2O4 samples were obtained by calcining the corresponding precursors in air at 773 K for 2 h. Characterization. X-ray powder diffraction (XRD) patterns were obtained on a Bruker D8 Advance X-ray diffractometer with Cu KR radiation. Thermogravimetry and differential scanning calorimetry (TG-DSC) analysis was determined at a heating rate of 10 °C/min under air atmosphere. Scanning electron microscopy (SEM) images was performed on a LEO 1450VP scanning electron microscope. Transmission electron microscopy (TEM) study was carried out on a Philips CM-120 electron microscope. The samples for TEM were prepared by dispersing the final powders in ethanol; the dispersion was then dropped on carbon-copper grids. Furthermore, the obtained powders deposited on a copper grid were observed by a highresolution transmission electron microscope (HRTEM; JEOL JSM-2010 microscope) operating at 200 kV. X-ray photoemis-
sion spectroscopy (XPS) was recorded on a Kratos ASIS-HS X-ray photoelectron spectroscope equipped with a standard and monochromatic source (Al KR) operated at 150 W (15 kV, 10 mA). In the XPS measurements, all binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. FT-IR spectra were recorded on a Nicolet Nexus spectrometer with the standard KBr pellet method. The magnetic properties of the sample were measured with a Lake model 7300 vibrating sample magnetometer (VSM). Results and Discussions XRD Patterns of the As-Prepared Precursor and the Calcined Samples. The crystallinities and phase purities of the as-prepared precursor and the calcined products were examined by powder X-ray diffraction (XRD). Figure 1 shows the XRD patterns of the precursor as well as the samples after calcination at different temperatures. The pattern of the precursor matches well with the standard patterns of zinc iron oxalate (JCPDS file No. 49-1079). All of the peaks of the patterns of the calcined samples can be readily indexed to cubic ZnFe2O4 with spinel structure (JCPDS file No.79-1150), where the diffraction peaks at 2θ values of 30.1, 35.2, 42.8, 52.9, 56.5, and 67.8° can be ascribed to the reflection of (220), (311), (400), (422), (511), and (440) planes of the spinel ZnFe2O4, respectively. The average crystallite sizes were calculated from the width of the prominent (311) reflection using the Scherrer’s equation.28 The sizes of the ZnFe2O4 crystals are about 28 nm after calcination at 773 K and increase to 62 nm after calcination at 973 K. No peaks from other phases are detected, indicating high purity of the products. Thermal Analysis of the Precursor. The thermal analysis of zinc ferrioxalate precursors was studied in a static air atmosphere from ambient to 1273 K. Figure 2 presents the DSCTG curves of the zinc ferrioxalate precursor at a heating rate of 10 K/min. The elimination of six water molecules appears on DSC as an endothermic peak at 485.6 K with a weight loss of 19.97% in the TG curve (eq 1). Moreover, there is a rapid change in mass loss up to 35.45% in the TG curve, suggesting the removal of the carbon monoxide and carbon dioxide molecules and subsequent oxidative decomposition of the precursor into crystalline ZnFe2O4. This step corresponds to a strong exothermic peak centered at 576.8 K in the DSC curve. The exothermic peak at around 768.1 K with a weight loss of
Preparation of Porous Nanocrystalline ZnFe2O4 0.24% can be also observed, which could be ascribed to the crystallization of ZnFe2O4. Therefore, after dehydration the anhydrous precursor undergoes decomposition to yield a zinc iron intermediate in the temperature range from 448 to 563 K (eq 1). A subsequent oxidative decomposition of the zinc iron species leads to the formation of ZnFe2O4 in successive stages at about 768 K (eq 2). The thermal analysis indicates that crystalline ZnFe2O4 could be formed at around 773 K. This is consistent with XRD results.
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Zn2+ + 2Fe2+ + 3C2O42- + 6H2O f ZnFe2(C2O4)3 · 6H2O (1) ∆
ZnFe2(C2O4)3 · 6H2O 98 ZnFe2O4 + 2CO2 + 4CO + 6H2O (2) SEM Images of the Precursor and the Obtained ZnFe2O4. The morphology of the resulting nanocrystalline ZnFe2O4 was investigated by scanning electron microscopy (Figure 3). Figure
Figure 3. SEM images at low magnification (a) and high magnification (b) of the zinc ferrioxalate precursor; the SEM images at low magnification (c, d) and high magnifications (f) of the porous “timber-like” nanocrystalline ZnFe2O4 superstructures obtained by calcining the precursor at 773 K.
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Figure 4. TEM images of low magnification (a), high magnification (b) TEM, and HRTEM images (c) of the porous “timber-like” nanocrystalline ZnFe2O4 superstructures obtained by calcining the precursor at 773 K.
3a displays a representative overview of the precursor. It shows that the as-prepared precursor is completely composed of “timbers”. The high magnification SEM images in Figure 3b show that the surfaces of the “timbers” are very smooth with the width in the range of 1-3 µm and the length in the range of 10-20 µm, respectively. The overall morphology of the ZnFe2O4 obtained by the calcination of the precursor is shown in Figure 3c-e. Although many “timber-like” rods could be also found in the obtained ZnFe2O4 sample, these ZnFe2O4 rods are of porous structures, different from those in the precursor. Figure 3f is a representative higher magnification SEM image of the ZnFe2O4. We interestingly find that the porous “timber”
of ZnFe2O4 is formed through the agglomeration of numerous nanoparticles. Therefore, we conclude that the resulting ZnFe2O4 is a novel porous superstructure material of nanocrystals with several tens nanometers in size. TEM and HRTEM Images of the Obtained ZnFe2O4. Transition electron microscopy (TEM) and high-resolution transition electron microscopy (HRTEM) were used to further investigate the crystal structure and morphology of these porous nanocrystalline ZnFe2O4 supersturctures (Figure 4). TEM image confirms the nanocrystalline nature of the ZnFe2O4 sample (Figure 4a). The average particle sizes of the nanoparticles are in the range of tens of nanometers (Figure 4b), which are in
Preparation of Porous Nanocrystalline ZnFe2O4
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Figure 5. XPS spectra of the ZnFe2O4 obtained by calcining the precursor at 773 K, (a) survey of the sample, (b) Fe 2p, (c) Zn 2p, and (d) O 1s.
Figure 6. Infrared spectra of (a) the zinc ferrioxalate precursor, and (b) ZnFe2O4 obtained by calcining the precursor at 773 K.
agreement with the XRD result. The single-crystal nature of the ZnFe2O4 was revealed by HRTEM analysis (Figure 4c). The lattice spacing is about 2.55 Å between adjacent lattice planes of the nanocrystalline ZnFe2O4, corresponding to the distance between (311) crystal planes of the spinel phase of zinc ferrite (JCPDS file No. 79-1150).
Figure 7. Magnetic hysteresis loops for the ZnFe2O4 obtained by calcining the precursor at different temperatures.
XPS Spectra of the Obtained ZnFe2O4. In ZnFe2O4, Fe and Zn atoms exist in the samples with more than one chemical states (A-sites or B-sites), bringing about several different contributions with different binding energies in the XPS spectra.29 Therefore, X-ray photoelectron spectroscopy was used
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Wang et al. 1047.1 and 1024.2 eV can be attributed to Zn 2p1/2 and Zn 2p3/ respectively. This reveals the oxidation state of Zn2+ in the sample.4,30 Figure 5c shows the Fe 2p peaks at binding energies of 716.0 and 729.3 eV, with a shakeup satellite at 724.3 eV, which is consistent with that reported for Fe3+ in ZnFe2O4.29-31 Meanwhile, O 1s spectra of ZnFe2O4 were also recorded (Figure 5d). The broad peak of O 1s can be fitted by two peaks at binding energies of 533.8 and 535.4 eV. The dominant peak at 533.8 eV is characteristics of oxygen in metal oxide, and the other peak at around 535.4 eV suggested the presence of other components such as OH, H2O and carbonate species adsorbed on the surface. In addition, XPS analysis confirms that the ratio of Zn to Fe in the sample is very close to 1/2, agreeing with the formula of ZnFe2O4. FT-IR Spectra of the Precursor and the Obtained ZnFe2O4. The FT-IR spectrum was used to identify the functional groups on the surface of both the zinc ferrioxalate precursor and the porous nanocrystalline ZnFe2O4 superstructures (Figure 6). The two IR spectra display broadband at 3375 cm-1, which is believed to be associated with the stretching vibrations of hydrogen-bonded surface water molecules and hydroxyl groups. This broadband in the spectrum of the precursor is stronger than that of the obtained ZnFe2O4. Additionally, the bands at 1362 and 1312 cm-1 imply the existence of large numbers of residual hydroxyl groups. All these three bands become very weak in the IR spectra of the ZnFe2O4, 2,
Figure 8. Powder X-ray diffraction (XRD) patterns of the CoFe2O4 (a) and NiFe2O4 (b) obtained by calcining the corresponding precursors at 773 K in air.
to reveal the surface chemical compositions of the porous nanocrystalline ZnFe2O4 superstructures obtained from the calcinations of the precursor at 773 K (Figure 5). Figure 5a shows the survey spectra of the nanocrystalline ZnFe2O4. Elements of Zn, Fe, O, and adventitious C existed in the ZnFe2O4. As shown in the spectrum in Figure 5b, the peaks of
Figure 9. SEM images at low magnification (a) and high magnification (b) of the obtained CoFe2O4; SEM images at low magnification (c) and high magnification (d) of the obtained NiFe2O4. The samples were calcined in air 773 K.
Preparation of Porous Nanocrystalline ZnFe2O4 suggesting that the calcination process can remove most of the surface hydroxyl groups and adsorbed water. The band at 1625 cm-1 is assigned to the stretching vibration mode of CdO. The band around 600 cm-1 corresponds to intrinsic stretching vibration of metal cations at the tetrahedral site and the peak at 553 cm-1 is attributed to the Fe-O bond vibration of the samples.1,20 Comparing the two spectra, we notice that CdO stretching vibration at 1625 cm-1 and C-C bending modes at about 816 cm-1 disappear in the spectrum of nanocrystalline ZnFe2O4. The disappearance of these two absorption bands indicates a complete decomposition of the precursor after the calciantion process, confirming the results of XRD, SEM and XPS. Magnetic Properties of the Obtained Porous Nanocrystalline ZnFe2O4. The traditional bulk ZnFe2O4 belongs to the normal spinal type with antiferromagnetic properties below the ne´el temperature of about 10.5 K and behaves paramagnetically at room temperature.5-8 In bulk form, ZnFe2O4 is a normal spinel with Zn2+ ions located in the A-sites and Fe3+ ions in the B-sites, it behaves as an antiferromagnet at 10.5 K.5 To investigate the magnetic properties of these novel porous “timber-like” nanocrystalline ZnFe2O4 superstructures, hysteresis loops of the different ZnFe2O4 samples were measured by VSM. Figure 7 shows their magnetization curves taken at room temperature (300 K). Typical “S”-like shape of hysteresis loops were observed, indicating a superparamagnetic property of the obtained nanocrystalline ZnFe2O4 superstructures. This superparamagnetic property is evident by zero coercivity and remanance on the magnetization loop.5-8 These “S”-like shape loops can be divided into two parts: curves and lines. The lines are attributed to the antiferromagnetic parts of the samples, and the curves may contribute to the change of the inversion parameter with the particle size decreases to nanoscale, which is induced by the preparation techniques. We found that the hysteresis loops of the obtained nanocrystalline ZnFe2O4 superstructures could not be saturated with the available maximum field of 15 kOe. This is an indication of the presence of large anisotropy in the material. It was also found that the magnetization decreased with calcination temperature increase. This could be attributed to the growth of crystal size at higher temperatures and the redistribution of cations in the nanocrystalline ZnFe2O4.5,32 Some of Fe3+ ions is pushed to the tetrahedral sites, switching on the A-B superexchange interaction, and thus gives rise to ferrimagnetic ordering.32 These magnetic results indicate that the porous “timber-like” nanocrystalline ZnFe2O4 superstructures may be applied in magnetic devices. The Generality of the Preparation Method. Similarly, CoFe2O4 and NiFe2O4 samples were synthesized from transition metal ferrioxalate precursors by mixing equimolar quantities of aqueous solutions of FeSO4 · 7H2O, Na2C2O4 and the corresponding transition metal sulfates. As shown in Figure 8, the XRD patterns of the resulting calcined samples match well with the standard patterns of cubic cobalt ferrite (JCPDS file No. 22-1086) and cubic nickel ferrite (JCPDS file No. 3-875), respectively. By using Scherrer’s equation, we estimated the sizes of the CoFe2O4 and NiFe2O4 obtained by calcining the corresponding precursors at 773 K to be about 32 and 25 nm, respectively. Moreover, SEM images show that similar porous nanocrystalline CoFe2O4 and NiFe2O4 “timber-like” superstructures were also obtained through the thermal decomposition of the precursors (Figure 9). This suggests our precursor approach is general for the synthesis of porous nanocrystalline transition metal ferrites “timber-like” superstructures.
J. Phys. Chem. C, Vol. 112, No. 34, 2008 13169 Conclusions In summary, we have demonstrated a simple method to prepare porous “timber-like” nanocrystalline ZnFe2O4 superstructures via the thermal decomposition of zinc ferrioxalate precursor. The zinc ferrioxalate precursor was prepared from metal sulfates and sodium oxalate without adding any additives. The resulting porous nanocrystalline ZnFe2O4 superstructures possessed interesting superparamagnetic property at room temperature. Moreover, other transition ferrites including porous nanocrystalline CoFe2O4 and NiFe2O4 “timber-like” superstructures were also prepared by this approach. We believe this general precursor method could be extended to prepare other ternary metal oxides with porous superstructures. Acknowledgment. This work was supported by National Basic Research Program of China (973 Program) (Grant 2007CB613301), National Science Foundation of China (Grants 20503009, 20673041 and 20777026), Program for New Century Excellent Talents in University (Grant NCET-07-0352), the Key Project of Ministry of Education of China (Grant 108097), and Postdoctors Foundation of China (Grants 20070410935). References and Notes (1) Selvan, R. K.; Krishnan, V.; Augustin, C. O.; Bertagnolli, H.; Kim, C. S.; Gedanken, A. Chem. Mater. 2008, 20, 429. (2) Gao, F.; Chen, X. Y.; Yin, K. B.; Dong, S.; Ren, Z. F.; Yuan, F.; Yu, T.; Zou, Z. G.; Liu, J. M. AdV. Mater. 2007, 19, 2889. (3) Mathur, S.; Veith, M.; Ruegamer, T.; Hemmer, E.; Shen, H. Chem. Mater. 2004, 16, 1304. (4) Sharma, Y.; Sharma, N.; Rao, G. V. S.; Chowdari, B. V. R. Electrochim. Acta 2008, 53, 2380. (5) Li, F. S.; Wang, H. B.; Wang, L.; Wang, J. B. J. Magn. Magn. Mater. 2007, 309, 295. (6) Grasset, F.; Labhsetwar, N.; Li, D.; Park, D. C.; Saito, N.; Haneda, H.; Cador, O.; Roisnel, T.; Mornet, S.; Duguet, E.; Portier, J.; Etourneau, J. Langmuir 2002, 18, 8209. (7) Hochepied, J. F.; Bonville, P.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 905. (8) Roy, M. K.; Verma, H. C. J. Magn. Magn. Mater. 2006, 306, 98. (9) Ponpandian, N.; Narayanasamy, A. J. Appl. Phys. 2002, 92, 2770. (10) Meng, W. Q.; Li, F.; Evans, D. G.; Duan, X. J. Porous Mater. 2004, 11, 97. (11) Zhang, R.; Huang, J.; Zhao, J.; Sun, Z.; Wang, Y. Energy Fuels 2007, 21, 2682. (12) Kobayashi, M.; Shirai, H.; Nunokawa, M. Energy Fuels 2002, 16, 1378. (13) Veith, M.; Haas, M.; Huch, V. Chem. Mater. 2005, 17, 95. (14) Nordhei, C.; Mathisen, K.; Bezverkhyy, I.; Nicholson, D. J. Phys. Chem. C 2008, 112, 6531. (15) Yuan, Z. H.; Zhang, L. D. Mater. Res. Bull. 1998, 33, 1587. (16) Wakiya, N.; Muraoka, K.; Kadowaki, T.; Kiguchi, T.; Mizutani, N.; Suzuki, H.; Shinozaki, K. J. Magn. Magn. Mater. 2007, 310, 2546. (17) Yu, S. H.; Fujino, T.; Yoshimura, M. J. Magn. Magn. Mater. 2003, 256, 420. (18) Sivakumar, M.; Takami, T.; Ikuta, H.; Towata, A.; Yasui, K.; Tuziuti, T.; Kozuka, T.; Bhattacharya, D.; Iida, Y. J. Phys. Chem. B 2006, 110, 15234. (19) Nyutu, E. K.; Conner, W. C.; Auerbach, S. M.; Chen, C. H.; Suib, S. L. J. Phys. Chem. C 2008, 112, 1407. (20) Randhawa, B. S. J. Mater. Chem. 2000, 10, 2847. (21) Mohamed, M. A.; Galwey, A. K.; Halawy, S. A. Thermochim. Acta 2005, 429, 57. (22) Jordan, A.; Scholz, R.; Wust, P.; Fahling, H.; Roland, F. J. Magn. Magn. Mater. 1999, 201, 413. (23) Jordan, A.; Scholz, R.; Maier-Hauff, K.; Johannsen, M.; Wust, P.; Nadobny, J.; Schirra, H.; Schmidt, H.; Deger, S.; Loening, S.; Lanksch, W.; Felix, R. J. Magn. Magn. Mater. 2001, 225, 118. (24) Stjerndahl, M.; Andersson, M.; Hall, H. E.; Pajerowski, D. M.; Meisel, M. W.; Duran, R. S. Langmuir 2008, 24, 3532. (25) Brusentsov, N. A.; Nikitin, L. V.; Brusentsova, T. N.; Kuznetsov, A. A.; Bayburtskiy, F. S.; Shumakov, L. I.; Jurchenko, N. Y. J. Magn. Magn. Mater. 2002, 252, 378. (26) Hergt, R.; Hiergeist, R.; Hilger, I.; Kaiser, W. A.; Lapatnikov, Y.; Margel, S.; Richter, U. J. Magn. Magn. Mater. 2004, 270, 345. (27) Zhang, L. Y.; Xue, D. S.; Gao, C. X. J. Magn. Magn. Mater. 2003, 267, 111.
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