J Supercond Nov Magn DOI 10.1007/s10948-014-2562-8
ORIGINAL PAPER
Triethanolamine Assisted Hydrothermal Synthesis of Superparamagnetic Co3O4 Nanoparticles and Their Characterizations S. Shafiu · A. Baykal · H. S¨ozeri · M. S. Toprak
Received: 27 March 2014 / Accepted: 10 April 2014 © Springer Science+Business Media New York 2014
Abstract We present, for the first time, on a facile route for the fabrication of highly crystalline Co3 O4 nanoparticles using trietanolamine (TEA) assisted hydrothermal synthesis route and single precursor. Synthesized material has been evaluated for its structural, morphological and magnetic properties using x-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM) techniques. The material has been identified as highly crystalline Co3 O4 , with superparamagnetic character due to size confinement. Estimated particle size from SEM is about 10 nm, which is close to the magnetic domain size (estimated from VSM as 8.4 ± 1.7 nm) and the crystallite size (estimated from XRD as 11 pm 4 nm), which reveal nearly single crystalline character of Co3 O4 nanoparticles. The suggested route is facile, which provides a good size control over the nanoparticles, and can be used for the fabrication of other ceramic materials. S. Shafiu Kano Univ. Science and Techn., Wudil, Kano State Nigeria S. Shafiu · A. Baykal () Department of Chemistry, Fatih University, 34500, B. C¸ekmece, Istanbul, Turkey e-mail:
[email protected] H. S¨ozeri TUBITAK-UME, National Metrology Institute, PO Box 54, 41470, Gebze-Kocaeli, Turkey M. S. Toprak Department of Materials and Nano Physics, KTH-Royal Institute of Technology, Kista-Stockholm, Sweden M. S. Toprak Department of Materials Science and Engineering, Yildirim Beyazit University, Ulus-Ankara, Turkey
Keywords Hydrothermal synthesis · Co3 O4 · Magnetic property · TEA
1 Introduction Due to their noble characteristics with regards to many properties such as magnetic and recyclable catalytic nature as well as their diverse applications in electronics devices, compared to their counter bulk materials, transition metal oxides nanoparticles remain among the most important materials for research [1]. However, among these oxides synthesis and characterization of spinel Co3 O4 , having an important magnetic p-type semiconducting behavior with an indirect band gap of 1.5 eV, is given a peculiar treatment due to its tremendous applications as a solid-state sensor, material in Li-ion rechargeable batteries, anode material and being used as heterogeneous catalyst, pigment, electrochromic sensor, and magnetic material as well as in the solar energy storage devices [2, 3]. Although, cobalt exist in different forms of oxides, cobalt (II) oxide (CoO) and cobalt (II, III) oxides (Co3 O4 ) are mostly of technological and scientific importance because of their excellent properties at nano scale [4]. It is known that Co3 O4 has normal spinel structure with antiferromagnetic exchange (AFM) between ions occupying tetrahedral A sites and octahedral B sites (AB2 O4 ). The Co3+ ions have no moment on B sites, while Co2+ ions on A sites have magnetic moment of 3.25 μB revealed from the neutron diffraction experiments [5]. The Neel temperature of Co3 O4 , being size dependent, was reported as 30 K deduced from the heat capacity measurements and 40 K from the susceptibility measurements [5, 6]. AFM nanoparticles can have a net magnetic moment due to presence of uncompansated spins [7, 8] or partly inverted spinel structure probably by local electron hopping [9], and
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thus, may show superparamagnetic behaviour . On the other hand, due to the freezing of spins at the surface, spin-glass behaviour may also arise within the particle [10–12]. In the literature, there are quite many articles supporting spin-glass and superparamagnetic behaviour of bare, coated and dispersed Co3 O4 NPs revealed by hysteresis, time dependance of magnetization and exchange bias measurements [13–16]. D. A. Resnick et al. showed the superparamagnetic Co3 O4 nanoparticles mineralized in Listeria innocua LDps (Listeria innocua Dps proteins) [17]. As of now, many root to synthesis of magnetic Co3 O4 have been reported in the literature, including microemulsion method [18], chemical spray pyrolysis [19], chemical vapour deposition [20, 21], sonochemical route [22, 23], coprecipitation method [24], microwave irradiation root [25], and mechanochemical processing [26], combustion method [27], sol–gel method [28], hydrothermal method [29, 30], thermal decomposition of cobalt precursors [31, 32], among others. Nevertheless, most if not all of these methods have some downside as far as the application is concern, for examples, time consumption, the use of toxic and inexpensive starting materials, the use of expensive equipment and the use of high synthetic temperatures as well as complicated synthetic mechanism are among the deprivations of the roots [33]. Furthermore, from the general knowledge, the synthesis of nano-materials occurred in three distinct steps which are; initiation, growth and termination steps. However, all these steps depend on the type of capping agent used and the temperature employed during the synthesis. Despite its difficulties in controlling the growth of the crystallinity of material during the synthetic time, hydrothermal synthesis root is among the most promising methods for the synthesis of nanoparticles. This is due to its tendency to synthesize highly crystalline materials which are not stable at melting point temperature and the ability to produce materials with high vapour pressure close to their melting points. This method is also most suitable for the production of large-scale crystalline nanomaterials under the control of their decomposition. The uses of organic ligands as capping as well as stabilizing or protecting agents during the synthesis of magnetic nanoparticles are usually required due to their significant influence in controlling the size, agglomeration and morphology of the resulting nanoparticle. These molecules (ligands) usually maintained some specialized functional groups bearing the molecules to coat the surface of magnetic nanomaterials, thereby decreasing the surface energy and control the grain growth as well as particle agglomeration [34]. Many results from literature revealed the use of some surfactants to avoid agglomeration for the purpose mentioned above for example, synthesis of magnetic nanoparticles assisted; sodium dodecyl sulphate (SDS), sodium oleate, PEG, and polymers containing thiol (SH),
carboxyl (COOH), and amino (NH2 ) groups as capping agents were all presented in literature [18, 35–37]. However, apart from being used as growth and particle size control, capping agents were found to provide an electrostatic and steric stabilization effects for the dispersion of nanoparticles in suspensions [38, 39]. Due to the strong coordinating property of the TEA, it can form very stable complex by coordinating with M2+ (Co2+ ) in alkaline medium. According to Ubale et al. [40], In the bulk reaction, homogeneous as well as heterogeneous reaction takes place. In the presence of complex, metal ions forms metalcomplex, which slowly de composes, producing favorable condition for the ion-by-ion condensation of ions on the substrate [41–43]. However without complex, the metal ions vigorously react and produce clusters in the solution giving cluster-by cluster deposition. According to Li et al. [38] this formed complex will avoid the formation of metal hydroxide (Co(OH)2). In relation to this Co3 O4 @ZnO was produced by our research group via thermal decomposition method in triethylene glycol (TEG) [39] In the present work, we report a facile trietanolamine (TEA) assisted synthesis of highly crystalline Co3 O4 via hydrothermal synthesis. The influence of this molecule (TEA) on the morphology, structure and magnetic properties of this material were fully investigated. To the best of our knowledge, no report on the synthesis of this material based on TEA assisted hydrothermal synthetic root is presented in literature.
2 Experimental 2.1 Chemicals All chemcials were of analytical grade (triethanolamine (TEA, ≥99.0 % (GC)), CoSO4 .7H2 O (≥98.0 %), NaOH, Ethanol (C2 H5 OH)) and obtained from Merck. They were used without further purification. 2.2 Instrumentations 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. Fourier transform infrared (FT-IR) spectra were recorded in transmission mode in the range 4000–400 cm−1 with a Perkin Elmer BX FT-IR infrared spectrometer. The powder samples were ground with KBr and compressed into a pellet prior to analysis. 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
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Fig. 1 X-ray powder diffraction pattern of Co3 O4 NPs prepared via TEA assisted hydrothermal synthesis
Fig. 3 FT-IR spectrum of Co3 O4 NPs prepared via TEA assisted hydrothermal synthesis
(H c) and remanence (Mr) were deduced. Scanning electron Microscopy, SEM, analysis was performed in order to investigate the microstructure and morphology of the sample, using an FEI XL40 Sirion FEG Digital Scanning Microscope. Samples were coated with gold at 10 mA for 2 min prior to analysis.
collected by centrifugation and washed several times with water and alcohol. The solid was dried at 60 ◦ C in a vacuum oven for 12 h and then annealed at 300 ◦ C for 5 h to remove the excess TEA.
3 Results and Discussion 2.3 Procedure 3.1 XRD Analysis The synthesis of Co3 O4 nanoparticles was performed as follows: To 12 ml of deionized water 0.8 g of NaOH and 7.8 g of triethanolamine (TEA) solution were mixed under continuous stirring. Subsequently, 1 mmol of CoSO4 .7H2 O was added into the above solution. The mixture was stirred for 30 min at room temperature and homogeneous solution was obtained, and the solution was transferred into 30 ml Teflon-lined stainless steel autoclave, sealed and maintained at 300 ◦ C for 12 h. After which the black solid product was
Figure 1 displays the XRD pattern of the product. The observed peaks in this pattern are fully matched with the cubic spinel structure with the indexed miller indices (hkl) of (220), (311), (400), (511), (440) (JCPDS card ?le No. 43- 1003) [35, 44]. Crystallite size of the product was calculated from the full width at half maximum (FWHM) of all the peaks using the Debye–Scherrer formula [45] 30
20
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Fig. 2 Thermodynamic modeling diagram showing the fraction, i.e. speciation, of Co2+ ions at room temperature
Fig. 4 M–H Curve of Co3 O4 NPs prepared via TEA assisted hydrothermal synthesis
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as 11±4 nm. Thermodynamic modeling was performed to determine the speciation of Co 2+ ions at room temperature; the resultant diagram as a function of pH is presented in Fig. 2. At room temperature, above pH 8, all of the cobalt ions are in hydroxideform, Co(OH)2 , as described in Eq. (1). At elevated temperatures, part of Co 2+ will oxidize to Co 3+ Eq. (2), which in turn will favor the formation of mixed hydroxides that transform to oxides via dehydration process Eq. (3). Co2+ (aq) + 2OH− (aq) → Co(OH)2(s)
(1)
Co2+ (aq) → Co3+ (aq) + e−
(2)
Co2+ (aq) +Co3+ (aq) +8OH− (aq) → Co3 O4(s) +4H2 O (3) Fig. 5 a–e SEM micrographs of Co3 O4 NPs, prepared via TEA assisted hydrothermal synthesis, at different magnifications. f EDX spectrum of synthesized material
The high phase purity of the synthesized material reveals the high reaction yield of the utilized process. 3.2 FT-IR Analysis Figure 3 presents the FT-IR spectrum of the product. The observed two distinct FT-IR bands at 575 (ν1 ) and 652 (ν2 ) cm−1 , which originate from the stretching vibrations of the metal-oxygen bond and confirm the formation of Co3 O4 spinel oxide [44, 46, 47]. The ν1 band is characteristic of Co3+ vibration in the octahedral site, and ν2 band is attributable to Co2+ vibration in tetrahedral site in the spinel lattice [48]. 3.3 VSM Analysis Magnetic characterization of synthesized Co3 O4 NPs has been performed at room temperature using vibrating sample
(a)
(b)
(c)
(d)
(e)
(f)
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magnetometer (VSM) and the magnetization curve is shown in Fig. 4. It was observed that Co3 O4 NPs have superparamagnetic behaviour due to the immeasurable remenance and coercivity together with unsaturated magnetization at high fields. These are typical features of superparamagnetic NPs and probably arise from presence of uncompansated spins, which are attributed to structural inhomogeneties and finite size effects. Magnetization of superparamagnetic NPs can be described by Langevin model. The average crystallite size can be calculated by fitting the Langevin function to the M-H hysteresis curve [49] as described by Eq. (4) as shown below; M = Ms
μH coth kB T
kB T − μH
(4)
where μH corresponds to energy of an magnetic moment in the field H and kB T stands for thermal energy of magnetic moments. Then, the average magnetic domain size of Co3 O4 NPs was estimated as 8.4 pm 1.7 nm. 3.4 SEM Analysis The morphology of synthesized Co3 O4 NPs were investigated with SEM, and few micrographs at different magnification are presented in Fig. 5. The SEM images reveal that the product consists of a large quantity of platelet-like structure, which have strongly aggregated most likley during the heat generated by the combustion of the organic residues under annealing temeparture. The platelets have edge length of 800–1000 nm and thickness of about 50–100 nm, Fig. 5e. A closer investigation of inset in Fig. 5e reveals that these platelets have formed by the collection/aggregation of smaller particles about 10 nm. The particle size is close to the magnetic domain size (estimated from VSM) and the crystallite size (estimated from XRD) which may indicate nearly single crystalline character of nanoparticles. EDX analysis, spectrum shown in Fig. 5f, confirmed the atomic composition of the material as Co/O : 3/4, in agreement with the crystalline material determined from XRD analysis.
4 Conclusion In this study, Co3 O4 nanoparticles with superparamagnetic property was synthesized via TEA-assisted hydrothermal route under mild conditions. The experimental results reveal that the sample is mainly composed of Co3 O4 nanoparticles, despite a single precursor for cobalt was used. We proposed a plausible reaction mechanism fort he formation of Co3 O4 . Crystallite size of the product was estimated from xrd data as 11 pm 4 nm and the average magnetic domain size of
Co3 O4 NPs was estimated as 8.4 ± 1.7 nm from VSM analysis. Synthesized Co3 O4 nanoparticles exhibit superparamagnetic behavior at room temperature. This novel feature (superparmagnetism) of Co3 O4 nanoparticles make them be used as “artificial catalyst” for biosensor, biocatalyst and biomedicine.
Acknowledgments This work was supported by Fatih University under BAP Grant no: P50021301 - Y (3146).
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