Cent. Eur. J. Chem. • 7(3) • 2009 • 410-414 DOI: 10.2478/s11532-009-0012-4
Central European Journal of Chemistry
Synthesis of Co3O4 nanoparticles by Oxidation-Reduction method and its magnetic characterization Research Article
T.Ozkayaa, A.Baykala*, Y.Koseoğlub, H.Kavasb Department of Chemistry, Fatih University, Hadimköy, 34500 Istanbul-Turkey a
Department of Physics, Fatih University, Hadimköy, 34500 Istanbul-Turkey b
Received 28 August 2008; Accepted 21 December 2008
Abstract: W ithout any surfactant, antiferromagnetic Co3O4 nanoparticles were synthesized successfully for the first time by means of an oxidation-reduction method with cobalt sulfate as starting material, which was oxidized to cobalt salt by NaNO3 after alkalinizing with NaOH. Morphological, structural, spectroscopic and magnetic characterization of the product were done by SEM, TEM, XRD, and VSM, respectively. The average crystallite size (on the base of line profile fitting method), D and σ, is estimated as 30 ± 6 nm. Some anomalous magnetic properties and their enhanced effect have been observed in Co3O4 antiferromagnetic nanocrystallites, including a bias field, coercivity, permanent magnetic moments and an open loop. These phenomena are attributed to the unidirectional anisotropy which is caused by the exchange coupling between AFM and FM layers, the existence of the spin glass like surface spins of Co3O4 nanoparticles due to size effects and surface-area effect. Keywords: Nanomaterials • Co3O4 • Spinels • Antiferromagnetism
© Versita Warsaw and Springer-Verlag Berlin Heidelberg.
1. Introduction In recent years, transition metal cobalt oxide (Co3O4) is known as a promising material due to its extensive applications in lithium-ion batteries, gas sensing, data storage, catalysis and electrochromic devices [1–11]. Co3O4 is very attractive for applications involving oxidation reactions because of the presence of mobile oxygen, such as the use of unsupported cobalt oxide as an active catalyst in air pollution control for the abatement of CO [12], NOx [13] and organic pollutants from effluent streams [14]. Co3O4 is an important magnetic p-type semiconductor, which belongs to the normal spinel crystal structure based on a cubic close packing array of oxide ions, in which Co(II) ions occupy the tetrahedral 8a sites and Co(III) ions occupy the octahedral 16d sites [15]. Extensive work has been done on preparation
of Co3O4 nanoparticles, including sol–gel [16,17], reduction/oxidation route [18], thermal decomposition [19,20], metal organic chemical vapor deposition (MOCVD) [21-23], chemical spray pyrolysis [24,25] etc. Previously, Co3O4 nanoparticles with various morphologies have been synthesized [26,27].
2. Experimental Procedures 2.1. Measurements
X-ray powder diffraction analysis was conducted on a Huber JSO-DEBYEFLEX 1001 Diffractometer (XRD) using Cu Kα (operated at 40 kV and 35 mA) radiation. FTIR transmission spectra were taken on Mattson Satellite Infrared Spectrometer in the range of 4000 to 400 cm-1. Samples for FTIR characterization were
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prepared by mixing 3 mg of sample powder with 100 mg of KBr, which was then ground and pressed into a transparent pellet with a diameter of 1 cm. Magnetic measurements were carried out with the Quantum Design Model 6000 Vibrating Sample Magnetometer (VSM) option for the Physical Property Measurement System (PPMS) and parameters like specific saturation magnetization (Ms), coercive force (Hc) and remanence (Mr) were deduced. Transmission Electron Microscopy (TEM) analysis was performed using a FEI Tecnai G2 Sphera Microscope. A drop of diluted sample in alcohol was dripped on the TEM grid. Following drying, the sample was loaded onto a TEM column for analysis. Particle size distribution was obtained from three micrographs after counting a minimum of 100 particles.
3. Results and discussion 3.1. XRD Analysis
XRD analysis performed on the nanoparticles obtained by two different methods revealed that the only phase observed was Co3O4 spinel oxide nanoparticles with ICDD card no of 42-1467. No secondary phases or impurities were observed. The mean size of nanoparticles was estimated from the diffraction pattern using Eq. 1 given in Wejrzanowski et al. [33] and Pielaszek [34]. The line profile, shown in Fig. 1, is fitted for 8 peaks (111), (220), (311), (222), (400), (422), (511), (440) and average crystallite size, D and σ, is estimated as 30 ± 6 nm.
2.2. Procedure
Cobalt sulfate heptahydrate, sodium nitrate and NaOH were dissolved in de-ionized H2O to prepare Co2+, NaNO3 and NaOH solutions that were 0.8, 0.8 and 3.2 mol L-1, respectively. Then a predetermined volume of NaOH that was pre-treated with N2 to remove oxygen under 10 min heating. When the temperature of the sodium hydroxide went up 60 - 100oC, a stoichiometric volume of cobalt sulphate solution was added dropwise at 10 mL min-1 rate into the three-neck flask while stirring. After exhausting the volume of sodium sulfate solution in 5 min, a predetermined sodium nitrate solution was dropped into the flask and the reaction was kept over night [28-30]. Alkalization reaction of ferrous ions has been studied extensively by Refait and Olowe [31,32] and they proposed a mechanism for the formation of magnetite, Fe3O4. Based on this mechanism we propose the following route for the formation of Co3O4: Co3O4 is formed as a result of a three step reaction that can be described as follows: i) formation of Co(OH)2 as a result of the reaction between CoSO4 and NaOH; ii) partial oxidation of cobalt hydroxide by NaNO3 in solution, and finally iii) dehydration reaction of cobalt hydroxide and cobalt oxyhydroxide. Detailed reactions for the suggested mechanism controlling the transformation of cobalt sulfate to the final phase of Co3O4 are given below in the order described above: CoSO4 + 2NaOH → Co(OH)2 + Na2SO4
(1)
3Co(OH)2 + NO3- → Co(OH)2 + 2CoOOH + H2O + NO2
(2)
Co(OH)2 + 2CoOOH → Co3O4 + 2H2O
(3)
Figure 1.
XRD pattern of Co3O4 nanoparticles with the line profiles fitting.
3.2. FTIR analysis
The FTIR spectrum given in Fig. 2 displays two distinct and sharp bands at 578 (ν1) and 662 (ν1) cm-1, which originate from the stretching vibrations of the metal-oxygen bond and also confirm the formation of Co3O4 spinel oxide [35-37]. As it was reported in Kurtuluş et al. [38], the ν1 band is characteristic of Co3+ vibration in the octahedral hole, and ν2 band was attributed to Co2+ vibration in tetrahedral hole in the spinel lattice. In the range of 4000 – 1000 cm-1, vibrations of CO32-, NO3- and moisture were observed.
Figure 2.
FTIR spectrum of Co3O4 nanoparticles. 411
Synthesis of Co3O4 nanoparticles by Oxidation-Reduction method and its magnetic characterization
3.3. TEM Analysis
Magnetic hysteresis measurements performed on Co3O4 nanoparticles prepared by Reflux method (Tev11b) are presented in Figs. 4 and 5. The room temperature M-H curve was linear with the field and has no coercivity and remenance (see Fig. 4). The sample can not reach the saturation even in the presence of 5 kOe magnetic field. Comparatively large coercivity and shifted hysteresis loops at 10 K are observed after field cooling as seen in Fig. 5. The sample was cooled from room temperature through the Co3O4 antiferromagnetic (AFM) Neel temperature to 10 K in the presence of an applied field. The M-H loop was measured in the +10 kOe to -10 kOe range. A small coercivity and shifted hysteresis loop from the origin is observed after field cooling. The loop was open up to 7.5 kOe and magnetization increased almost linearly with applied field at higher fields up to 10 kOe. Also the magnetization of the sample did
not reach saturation even in the presence of 10 kOe magnetic field. For a system with an open loop up to high applied fields indicates the existence of high surface anisotropy and a spin-glass-like surface layer. The considerable magnetization observed for FC curve indicates that the Co3O4 nanoparticles are in weakly ferromagnetic order. For bulk antiferromagnetic materials, due to the complete compensation of sublattice magnetizations, the net magnetization is zero. It is known that Co3O4 has normal spinel structure with antiferromagnetic exchange between ions occupying tetrahedral A sites and octahedral B sites [39]. Hence, the origin of weak ferromagnetism in our Co3O4 samples can be ascribed to the uncompensated surface spins and/or finite size effects [40-42] or the partly inverted spinel structure probably by local electron hopping [43]. On cooling a specimen through the antiferromagnetic (AFM) Neel temperature in the presence of an external applied field, the ferromagnetic (FM) layer displays unidirectional anisotropy resulting in a shift of the hysteresis loop from zero on the field axis by an amount of Heb called the exchange bias field [16]. The central portion of the FC loop at 10 K is shown in Fig. 6. The loop is broadened and shifted towards applied negative field, which exhibits the typical feature of an exchange bias system with an exchange bias field Heb = (HFC + HFC )/2 around -49 Oe for our sample. The C1 C2 exchange bias is an interfacial effect of the exchange coupling between AFM and FM layers which induces a unidirectional anisotropy of the FM layer. Both the loop shift and exchange bias field are measures of unidirectional exchange anisotropy. The observation of loop shift, enhanced coercivity, as well as exchange bias field indicate the existence of AFM core and FM surface spin in our Co3O4 nanoparticles, which can be attributed to the uncompensated surface spin due to the reduction of the coordination number at the surface of the AFM Co3O4 nanoparticles [16].
Figure 4.
Figure 5.
Fig. 3 presents a typical TEM micrography. As one can see, the magnetite nanoparticles mainly consisted of quadrate crystallites with a narrow size distribution. The average size of nanoparticles is around 30 – 45 nm, which is basically in agreement with the value calculated from XRD measurements. The product approximately displays a uniform morphology.
Figure 3.
TEM micrographs of Co3O4 nanoparticles.
3.4. Magnetic Measurements
412
M-H curve of Co3O4 nanoparticles at room temperature.
M-H curve of Co3O4 nanoparticles at 10 K.
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Figure 6. Central portion of FC hysteresis loop (at 10 K) at low field
Figure 7.
As for the opening of the loop, a spin-glass-like surface system with multiple spin configurations is believed to be its origin. According to Salaba et al. [44], the surface spins can be separated into two parts. One part is the frozen-in uncompensated spins which do not reverse during the field cycling. The other part is the free spins which can be aligned by an applied field. Thus, in one cycle of loop measurement, part of the magnetization (the frozen-in spins) is lost, leading to the opening of the loop. The existence of the spin-glass-like surface phase can be further confirmed by the training effect of the exchange bias field. Temperature dependence of magnetization of the sample in 200 Oe field-cooled (FC) case is shown in Fig. 7. As seen in FC curve, magnetization of the sample slightly increases when the temperature is decreased down to 45 K and strongly increases at lower temperatures down to 27 K reaching its maximum value at 27 K. Below 27 K it starts to decrease. This means that the sample showed superparamagnetic behavior above 45 K, ferromagnetic behavior between 25 K and 45 K and antiferromagnetic behavior below the transition temperature of 27 K which is less than that of the bulk value TN = 33 K [44,45]. This change of transition temperature may be ascribed to smaller particle size indicating a finite size effect of phase transition for the present system. For the higher temperature region, T ≥ 45 K, the plot of 1/M vs. T is linear indicating that the magnetization of product obeys the Curie–Weiss law with a negative θ value about 17 K.
4. Conclusion
showing an obvious loop shift and enhanced coercivity.
M-T curve of Co3O4 nanoparticles with 200 Oe applied field. The insert shows the 1/M vs. T curve.
Co3O4 nanoparticles were successfully synthesized for the first time by oxidation–precipitation method using cobalt sulphate as the starting material, and oxidized by NaNO3 after alkalizing with NaOH without any surfactant. XRD analysis revealed that the average particle size is 30 nm, which agrees well with the TEM based estimate of 30 - 45 nm. From magnetic measurements (M-H and M-T curves), paramagnetic behavior above 45 K, superparamagnetic behavior near the transition temperature, Tt, and the ferromagnetic behavior with slight hysteresis below Tt were observed. These behaviors should be due to the uncompensated surface spins on extremely small particle systems. A deviation of Neel temperature from the bulk value is observed which can be described by the theory of finite size scaling. An enhanced coercivity as well as a loop shift observed below Neel temperature under an applied field (FC case) are attributed to unidirectional anisotropy which is caused by the exchange coupling between AFM and FM layers. The opening of the loop up to high applied fields can be better explained by the existence of the spin glass like surface spins of Co3O4 nanoparticles.
Acknowledgement The authors are thankful to the Fatih University Research Project Foundation (Contract no: P50020803-2) and Turkish Ministry of Industry and Trade (Contract no: 00185.STZ.2007-2) for financial support for this study.
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References [1] G. Busca, F. Trifiro, A. Vaccari, Langmuir 6, 1440 (1990) [2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407, 496 (2000) [3] Z. Yuan, F. Huang, C. Feng, J. Sun, Y. Zhou, Mater. Chem. Phys. 79, 1 (2003) [4] M. Ando, T. Kobayashi, S. Lijima, M.J. Haruta, Mater. Chem. 7, 1779 (1997) [5] T. Maruyama, S.J. Arai, J. Electrochem. Soc. 143, 1383 (1996) [6] S.Weichel, P.J. Moller, Surf. Sci. 399, 219 (1998) [7] K. Kamata, Y. Lu, Y. Xia, J. Am. Chem. Soc. 125, 2384 (2003) [8] Y. Wang, L. Cai, Y. Xia, Adv. Mater. 17, 473 (2005) [9] M. Ando, T. Kobayashi, S. Iijima, M. Haruta, J. Mater. Chem. 7(9), 1779 (1997) [10] F. Svegl, B. Orel, M.G. Hutchins, K. Kalcher, J. Electrochem. Soc. 143, 1532 (1996) [11] T. Sugimoto, E. Matijevic, J. Inorg. Nucl. Chem. 41, 165 (1979) [12] Y.J. Mergler, J. Hoebink, B.E. Nieuwenhuys, J. Catal. 167, 305 (1997) [13] H. Hamada, Y. Kintaichi, M. Inaba, M. Tabata, T. Yoshinari, H. Tsuchida, Catal. Today 29, 53 (1996) [14] A.S.K. Sinha, V. Shankar, J. Chem. Eng. Biochem. Eng. 52, 115 (1993) [15] C. Mocuta, A. Barbier, G. Renaud, Appl. Surf. Sci. 56, 162 (2002) [16] F. Svegl, B. Orel, I. Grabec-Svegl, V. Kaucic, Electrochim. Acta 45, 4359 (2000) [17] M.E. Baydi, G. Poillerat, J.-L. Rehspringer, J.L. Gautier, J.-F. Koenig, P. Chartier, J. Solid State Chem. 109, 281 (1994) [18] Y.H. Ni, X.W. Ge, Z.C. Zhang, H.R. Liu, Z.L. Zhu, Q. Ye, Mater. Res. Bull. 36, 2383 (2001) [19] C. Pirovano, S. Trasatti, J. Electroanal. Chem. 180(1–2), 171 (1984) [20] S. Sakamoto, M. Yoshinaka, K. Hirota, O. Yamaguchi, J. Am. Ceram. Soc. 80, 267 (1997) [21] A.U. Mane, K. Shalini, A. Wohlfart, A. Devi, S.A. Shivashankar, J. Cryst. Growth 240, 157 (2002) [22] K. Shalini, A.U. Mane, S.A. Shivashankar, M. Rajeswari, S. Choopun, J. Cryst. Growth 231, 242 (2001) [23] M. Burriel, G. Garcia, J. Santiso, A. Abrutis, Z. Saltyte, A. Figueras, Chem. Vapor Depos. 11, 106 (2005) [24] R.N. Singh, J.F. Koenig, G. Poillerat, P. Chartier, J. Electrochem. Soc. 137, 1480 (1990)
414
[25] M. Hamdani, J.F. Koenig, P. Chartier, J. Appl. Electrochem. 18, 568 (1988) [26] R. Xu, H.C. Zeng, J. Phys. Chem. B 107, 926 (2003) [27] B.B. Lakshmi, C.J. Patrissi, C.R. Martin, Chem. Mater. 9, 2544 (1997) [28] T. Ozkaya, A. Baykal, H. Kavas, Y. Köseoğlu, M.S. Toprak, Physica B 403, 3760 (2008). [29] W. Yu, T. Zhang, J. Zhang, X. Qiao, L. Yang, Y. Liu, Materials Letters 60, 2998 (2006) [30] T. Ozkaya, “Synthesis and Characterization of M3O4 ((m = Fe, Co, Mn) Magnetic nanoparticles”, Master Thesis, Fatih University, İstanbul-Turkey, July (2008) [31] P.H. Refait, J.M.R. Génin, Corros. Sci. 34, 797 (1993) [32] A.A. Olowe, J.M.R. Génin, Corros. Sci. 32, 965 (1991) [33] T. Wejrzanowski, R. Pielaszek, A. Opalinska, H. Matysiak, W. Łojkowski, K.J. Kurzydłowski, Applied Surface Science 253, 204 (2006) [34] R. Pielaszek, Analytical expression for diffraction line profile for polydispersive powders, Appl. Crystallography (Proceedings of the XIX Conference, Krakow, Poland, 2003) 43 [35] H.K. Lin, H.C. Chiu, H.C. Tsai, Catal. Lett. 88, 169 (2003) [36] St.G. Christokova, M. Stayonava, M. Georgieva, D. Mehandjiev, Mater.Chem. Phys. 60, 39 (1999) [37] Y. Chen, Y. Zhang, S. Fu, Mater. Lett. 61, 701 (2007) [38] F. Kurtuluş, H. Güler, Inorganic Materials 41(5), 564 (2005) [39] W.L. Roth, J. Phys. Chem. Solids 25, 1 (1964) [40] T. Ambrose, C.L. Chien, Phys. Rev. Lett. 76, 1743 (1996) [41] R.H. Kodama, S.A. Makhlouf, A.E. Berkowitz, Phys. Rev. Lett. 79, 1393 (1997) [42] L. Néel, in: C. Dewitt, B. Dreyfus, and P.D. de Gennes (Eds.), Low Temperature Physics (Gordon and Beach, New York, 1962) 413 [43] C. Nethravathi, S. Sen, N. Ravishankar, M. Rajamathi, C. Pietzonka, Bernd Harbrecht, J. Phys. Chem. B 109, 11468 (2005) [44] E.L. Salabas, A. Rumplecker, F. Kleitz, F. Radu, F. Schüth, Nano Letters 6, 2977 (2006) [45] A. Salah, J. Makhlouf, J. Magn. Magn. Mater. 246, 184 (2002)
