Template-Free Synthesis of Self-Assembled Co3O4 Micro/Nanocrystals

Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 11, 1–9, 2011

Template-Free Synthesis of Self-Assembled Co3O4 Micro/Nanocrystals R. Amutha1
2 , S. Akilandeswari1
∗ , M. Muruganandham2 , Mika Sillanpää2
3 , Bashir Ahmmad4 , and Takahiro Ohkubo4 1

Department of Physics, Annamalai University, Annamalainagar 608002, India Laboratory of Applied Environmental Chemistry, University of Eastern Finland, Patteristonkatu 1, Mikkeli 50100, Finland 3 Faculty of Technology, Lappeenranta University of Technology, Patteristonkatu 1, FI 50100 Mikkeli, Finland 4 Department of Fundamental Material Science, Graduate School of Natural Science and Technology, Okayama University, Tsushima, Okayama 700-8530, Japan 2

Keywords: Cobalt Oxalate, Co3 O4 , Thermal Decomposition, Self-Assembly, Magnetic Properties.

1. INTRODUCTION Fabrication of nano/micro structured semiconductor materials with tunable morphologies have been the current topic of interest and also owing to their special properties such as large surface–volume ratio, increased activity, electronic and optical properties.1
2 It is well known that the size and shape controlled synthetic methodologies are of great interest in material chemistry. Thus, compounds with the same compositions but different morphologies are exhibiting remarkable differences in their properties.3 Recently, transition metal oxides have been widely studied because of its intriguing properties and unique technological applications.4 These materials possess the ability to adopt various oxidation states and electronic configurations. The mixed valence of these oxides reflects the possibility of electronic delocalization over the metal-oxygen framework, whereas the jahn teller effect of several of ∗

Author to whom correspondence should be addressed.

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their cations, such as Cu2+ , Co2+ or Mn3+ , favors the formation of anisotropic structures.5 Among these transition metal oxides, cobalt oxide is one of the most versatile magnetic materials, since it is a p-type antiferromagnetic oxide semiconductor with the highest Curie temperature, Tc = 1123  C and is being stable at low temperatures. In particular, the spin configurations of the cations are very complex for cobalt, so that complex magnetic transitions are often generated. Generally, CoO and Co3 O4 are the two stable forms cobalt oxide. Moreover, the Co3 O4 belongs to the normal spinel structure based on the cubic close packing arrays of oxide ions, in Co (II) ions occupy the tetrahedral 8a sites and Co (III) ions occupy the octahedral 16d sites.6 Thus, Co3 O4 have widely been investigated as electrode materials for lithium ion batteries, catalysts for water reduction and carbon monoxide oxidation, electro chromic materials, and gas sensors.7–13 The development of a rational synthetic strategy to grow Co3 O4 microrods with tunable morphologies still remains a great

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In this article, we have reported fabrication of various morphological porous Co3 O4 by thermal decomposition of cobalt oxalate at open atmospheric conditions. Uniform cobalt oxalate microrods and microneedles were synthesized without using any surfactants or templates in large scale. The cobalt oxalate preparation method was played crucial role on the crystal structure and its morphology. The as prepared cobalt oxalates and its corresponding cobalt oxides were characterized by using the thermogravimetric analysis, X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM) and nitrogen adsorption analysis. The XRD results indicated that the orthorhombic and monoclinic cobalt oxalates were formed in different experimental conditions. The influence of preparation method of cobalt oxalates and cobalt precursors on the final morphology has been investigated. The M–H loop of the Co3 O4 porous microrods and microneedles showed presence of paramagnetic properties at room temperatures. A plausible mechanism of both cobalt oxalates and Co3 O4 formation was proposed based on the experimental results.

Cobalt oxalate was synthesized by using the following procedure: About 11.9 g of cobalt sulfate heptahydrate (CoSO4 · 7H2 O) was dissolved into 100 mL of milli-Q water and heated to form a homogeneous solution. A stoichiometric amount of oxalic acid was dissolved in another 100 ml of water and heated to nearly its boiling point. The hot oxalic acid solution was then added into the hot solution of cobalt sulfate under occasional stirring. The formed pink precipitate was filtered off and washed with water and ethanol several times and then dried at 120  C for 2 h. While keeping the other conditions constant, experiments 2

The thermogravimetric-differential scanning calorimetric (TG-DSC) analyses were performed using a PerkinElmer thermogravimetry analyzer at the heating rate of 10  C/min. The X-ray diffraction pattern was recorded by using X’ Pert PRO PAN analytical diffractometer ( = 154178 Å) with the scanned angle from 10 to 100 . High resolution transmission electron microscope images were recorded using the FE-TEM, Philips CM-200 FEG(S) TEM-super twin instrument. Samples for HR-TEM were prepared by ultrasonically dispersing the samples into ethanol, then placing a drop of this suspension onto a copper grid and then drying it in air. The working voltage of TEM was 200 kV. The morphology of the product were examined by S-4800 field-emission scanning electron microscope (FE-SEM) equipped with an energy dispersive X-ray spectroscopy (EDX). Prior to SEM measurements, the samples were mounted on a carbon platform which was then coated by platinum using a magnetron sputter for 10 min. The plate containing the sample was then placed in SEM for the analysis with desired magnifications. The surface area, pore size, and pore volume of the sample was measured by nitrogen adsorption method using an Autosorp-1 Quantachrome instrument. The X-ray photoelectron spectra were collected on an ESCA-1000 X-ray photoelectron spectrometer (XPS), using MgK X-ray as

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Cobalt (III) sulfate heptahydrate, Cobalt (III) nitrate hexahydate, oxalic acid and ethanol were purchased from Sigma Aldrich, Finland. The analytical grade chemicals were used as received without further purification. In all experiments milli-Q (resistivity = 18.2  water was used.

The required amount of the synthesized cobalt oxalate precursor was decomposed in air at desired decomposition temperatures. The resultant product was washed with water and ethanol then dried at 120  C for 2 h.

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were also carried out at room temperature addition of both precursor solutions.

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challenge. Much effort has been made to synthesize various morphological Co3 O4 including spheres, rods, cubes, hollow spheres, bundles and urchin like structures.14–22 Many of these previous reports have revealed that the morphology of products can be controlled either the addition of surfactants or by using the homogeneous precipitation method. However, there is a difficulty of removal process in the former and high production of homogenous morphology in the later. Despite the numerous synthetic methods only few reports are available on the preparation of Co3 O4 microstructures without the use of structuredirecting templates or surfactant. The thermal decomposition of cobalt oxalates in air is the most commonly used technique for the preparation of cobalt oxides.23
24 Ahmed et al., demonstrated microemulsion process for the fabrication of Co and Co3 O4 from sub-micrometer rods of cobalt oxalate which were synthesized using CTAB and n-butanol as surfactant and cosurfactant respectively.25 The hydrothermal synthesis of cobalt oxalate has been reported for the fabrication of one dimensional array of Co3 O4 nanoparticles by Wang and his co-workers.26 The fabrication of Co3 O4 nanocrystals by the thermal decomposition of sol–gel derived oxalates were reported by Thota et al.27 However most of the synthetic procedures required the use of structure directing agents which forces to explore the simple, low cost synthetic methodology for large scale synthesis. We have designed a simple, template-free method to prepare Co3 O4 porous microrods by thermal decomposition of cobalt oxalate complex without using surfactants and additives. The proposed synthetic method is suitable for large-scale and low cost preparation.

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the excitation source. Magnetic measurements were done by using the quantum designed PPMS instrument.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Cobalt Oxalate Microrods Cobalt oxalate synthesis was performed under different experimental conditions by using oxalic acid with various cobalt precursors. The experimental results indicated that the preparation method of cobalt oxalates played a crucial role on the crystal phase and its morphology. It is interesting to note that the XRD patterns cobalt oxalate synthesized using different experimental conditions exhibited significantly different diffractograms.

Figure 1 shows the X-ray diffractograms of the cobalt oxalates prepared by using cobalt sulfate and oxalic acid. Generally, cobalt oxalate dihydrate exhibits two allotropic forms: -monoclinic with spacegroup C2/c, and -orthorhombic with Cccm. The XRD pattern of the samples in Figure 1(a) ascribed to the orthorhombic -cobalt oxalate with lattice constants a = 11855 Å: b = 5412 Å: c = 15578 Å which are quite consistent with the results of Aragon et al.28 No impurity peaks were found in the XRD spectra indicating the selective formation of cobalt oxalate. The FESEM images of the synthesized -cobalt oxalates by hot mixing method are shown in Figure 2(b). It revealed a rod like morphology with 80 nm average diameters and 40 m in length. It can be seen that the microrods consists of smooth surface structures.

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Fig. 2. FE-SEM images of cobalt oxalates prepared by (a) room temperature mixing of CoSO4 and oxalic acid (b) hot mixing of CoSO4 and oxalic acid, (c) hot mixing of CoNO3 and oxalic acid and Co3 O4 prepared by thermal decomposition of cobalt oxalate at 400  C for 2 h by using (d, e, f) hot mixing of CoSO4 and oxalic acid, (g, h, i) hot mixing of CoNO3 and oxalic acid.

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morphology. In order to investigate the effect of cobalt precursor on the morphology of cobalt oxalates, experiments were also performed by changing the cobalt precursor while maintaining the other conditions constant. It can be seen that the morphology of the cobalt oxalate is quite different while using the cobalt nitrate instead of cobalt sulphate as the cobalt source by hot mixing method. The FESEM picture of the cobalt oxalate prepared by using the cobalt nitrate is shown in Figure 2(c). The FESEM clearly revealed the formation of needlelike cobalt oxalate with the co-existence of small amount of bulletlike morphology. The needlelike cobalt oxalate possesses the sharp ends at both sides with a swelling at the central part. While comparing the bulletlike structures the needlelike structure is more in quantity. We speculated that the bulletlike structure may form in the early stages of decomposition process which were further self-assembled into needlelike morphology. It is reasonable that the variation in diffusion of nanoparticles during the addition process may cause the different morphologies and the bulletlike morphology may form due to the insufficiency in diffusion of nanoparticles.

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Figure 1(b) showed the XRD pattern of the cobalt oxalate fabricated by the room temperature mixing (without heating) of the same precursors as discussed before. The XRD pattern clearly showed that the reflections ascribed to the- monoclinic structure with P21 /m space group (a = c = 0662 nm, b = 0780 nm,  = 13157 ) corresponding to cobalt oxalate dihydrate which is in good agreement with Pujol et al., observation.29 Similarly, the morphology of the cobalt oxalates prepared by this method greatly varies from the hot mixing method as discussed before. The FESEM image of the cobalt oxalates prepared at room temperature mixing showed that the microsphere like morphology and was assembled by short rods as shown in Figure 2(a). The results indicated that the method of preparation of oxalate not only influences the crystal structures but also the final morphology of the cobalt oxalates. We have also performed some controlled experiments by changing the cobalt precursors and the results indicated that the cobalt precursor also influenced the final

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Fig. 5. XRD pattern of the cobalt oxides synthesized by thermal decomposition of cobalt oxalates at (a) 400  C (b) 500  C (c) 600  C.

TGA-DSC analysis of cobalt oxalate.

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Fig. 6. EDAX analysis of Co3 O4 microrods synthesized at 400  C for 2 h.

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RESEARCH ARTICLE Fig. 7. Cobalt oxide synthesized at different decomposition temperature and time (a) 300  C for 2 h (b) 500  C for 2 h (c, d) 400  C for 12 h and (e, f) 600  C for 2 h.

3.2. Synthesis of Cobalt Oxide The thermal behavior of synthesized -cobalt oxalate was investigated by using TG and DSC measurements. The TGA-DSC curve of the cobalt oxalates prepared by hot mixing method is shown in Figure 3. The decomposition clearly indicated two weight losses in the region starts at 120  C and 180  C. The first 19.9% of weight loss is attributed to the complete dehydration of two water molecules whereas the second one, in the temperature range between 180  C–400  C is ascribed to the J. Nanosci. Nanotechnol. 11, 1–9, 2011

decomposition of the cobalt oxalate into cobalt oxides. The anhydrous cobalt oxalate decomposed in air shows 48.2% weight loss which is close to the calculated weight loss of 50.9% mainly attributed to the formation of Co3 O4 .30 The DSC curve shows two endothermic peaks corresponding to the dominant mass losses as mentioned earlier. The results indicated that the calcinations at 400  C in air, all the cobalt oxalates were converted into cobalt oxide. Based on the thermal analysis, it was determined that 400  C is the suitable temperature to obtain Co3 O4 and therefore all decompositions were performed at 400  C. 5

Template-Free Synthesis of Self-Assembled Co3 O4 Micro/Nanocrystals

The XRD pattern of the Co3 O4 obtained by the calcination of cobalt oxalate at 400  C is shown in Figure 4. The thermal decomposition of cobalt oxalate results the face centered cubic Co3 O4 . All the peaks can be indexed to face centered cubic Co3 O4 with Fd3m space group and the calculated lattice parameter a = 8153 3 Å (JCPDS: 80-1544) which is quite consistent with the reported value.31 No impurity peaks were detected in the XRD pattern indicating the purity of synthesized cobalt oxide. Though the cobalt oxalate have different allotropic forms, the thermal decomposition of all cobalt oxalates yields similar face centered cubic cobalt oxide as evident from the XRD pattern. The XRD pattern of the cobalt oxides synthesized by thermal decomposition of cobalt oxalates at various decomposition temperatures is shown in Figure 5. It is obvious that the decomposition from 400  C to 600  C results cubic phase Co3 O4 which clearly indicated that the decomposition temperature is not influencing the crystal structure of the synthesized cobalt oxides. The energy dispersive X-ray (EDX) analysis of the microrods is shown in Figure 6. The EDX microanalysis demonstrated that the microrod consists of Co and O elements and the

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quantitative analysis confirmed the atomic ratio of Co: O is 1:1.32 is quite consistent with the stoichiometric ratio of the Co3 O4 . The FE-SEM images shown in Figure 2(d) displays the morphology of the cobalt oxides prepared by the thermal decomposition of cobalt oxalate microrod at 400  C for 2 h. The FE-SEM pictures clearly indicated that the synthesized cobalt oxide exhibits the identical morphology corresponding to the morphology of their parental cobalt oxalates as shown in Figure 2. The thermal decomposition of cobalt oxalates converted into cobalt oxides with the release of possible gases such as CO2 and CO. It is not surprising that both cobalt oxalates and oxides possess identical morphology because there is no byproduct except the above mentioned gases. It is obvious that the surfaces of the microstructures almost losses its smoothness and shows porous structure due to the release of CO and CO2 , as evident from Figure 2(f). The morphology of the cobalt oxide prepared under various decomposition temperatures and calcinations time is shown in Figure 7. It is observed that there is no variation in the morphology even at the prolonged calcination temperature and decomposition time indicating that the calcination temperature and time did not affect the morphology. However, nanoparticles fusion noted on the surface of microrods at higher decomposition temperatures due to high thermal energy. The X-ray photoelectron (XPS) spectra of the Co3 O4 microrods are shown in Figure 8. The Co 2p spectra showed two main peaks for 2p3/2 and 2p1/2 spin-orbit with the binding energies of 779.85 and 796.7 eV, respectively. The O 1s peak is often believed to be composed of two peaks related to two different chemical states of oxygen. The two main peaks observed at 528.8 eV and 531.9 eV can be ascribed to the oxygen species from the OH adsorbed onto the surface of the microrods and the oxide ions respectively.32 It was reported that cobalt (III) oxides were distinguished from cobalt (II) oxides by the absence of mulitelectron excitation satellites in the former.33 The low intense shakeup satellites peaks located

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Scheme 1. The schematic diagram of Co3 O4 microrod growth process.

above the main peak for ∼10 eV indicate the presence of pure Co3 O4 . All of the results clearly demonstrated the presence of Co3 O4 . To investigate the specific surface areas and the porous nature of the Co3 O4 microrods nitrogen adsorption measurements were performed. The nitrogen adsorption desorption isotherm of the cobalt oxide synthesized at 400  C for 2 h is shown in Figure 9. The isotherm of these porous microstructures and the corresponding

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BJH pore size distribution plots reflects the characteristics of macroporous materials. The BET (Brunauer–Emmett– Teller) specific surface area of Co3 O4 is found to be 11.6 m2 g−1 and the pore size distribution of Co3 O4 is mainly located at 34.3 nm which showed the presence of macroporous surface structure. 3.3. Formation Mechanism According to the classical theory of precipitation, at high supersaturations a fast nucleation occurs when the precursor monomers (atoms or molecules) reach a critical concentration.29 The nucleation is then followed by the growth of the nucleus to form primary particles. When the solution containing cobalt ions from the dissolution of cobalt sulfate in hot condition comes into contact with the solution containing hot oxalate ions, an immediate complex formation occurs as confirmed by the formation of pink color precipitation. Although the neutral CoC2 O4 form is insoluble in water, the precipitation reaction is slow, and it must overcome a nucleation barrier. Due to the thermal agitation it easily overcome the nucleation barrier and precipitated very fast.

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3.4. Magnetic Properties The temperature dependences of magnetization of the Co3 O4 microrods in zero-field-cooled (ZFC) and fieldcooled (FC) conditions are shown in Figure 11. The ZFC measurements were done by first cooling the sample down to 5 K in zero field and then raising the field to 10 mT and recording the magnetic moment when warming the sample (warming cycle). The FC measurement was done directly afterwards by cooling the sample in the 10 mT field (cooling cycle). The FC-ZFC plots showed the weak noisy signals and the curve shows the linear relationship of 1/M versus T indicating that the magnetizations of Co3 O4 8

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The cobalt oxalate microrods act as a template during the thermal decomposition growth of the Co3 O4 microrods which consisting of nanoparticles. According to the morphology evolution of Co3 O4 prepared by the thermal decomposition of various oxalates, we can propose the oriented attachment mechanism for the formation of Co3 O4 microrods. A simple scheme to visualize the growth process is given in Scheme 1. The identical diameters of nanoparticles as well as the microrods are the main characteristic of the oriented attachment.34 The main driving force for oriented attachment or aggregation of nanoparticles can generally be attributed to the tendency for reducing the high surface energy through the attachment among the primary nanoparticles. It can be observed that the growth each “microrods” is composed of large number of nanoparticles which form chainlike aggregates due to dipole–dipole interactions and then fuse gradually and recrystallize into microrods. According to Tang et al., the long range strong force that producing the chains of nanocrystals is dipole–dipole attraction.35 When two nanoparticles approach to each other closely, they are attracted by van der Waals forces. However, due to their thermal energy they can still rearrange to find the low-energy configuration represented by a coherent particle–particle interface.36 In the oriented attachment, particles were appeared to be fused almost end to-end along the longitudinal axis and form linear chains of nanoparticles. These chains of nanoparticles are self assembled to form the microrod morphology. The FESEM, TEM results clearly supported the above discussed mechanism. The attachment leads to a lowering of the surface energy after the elimination of highly curved surfaces of individual nanoparticle spheroids and it is an enthalpy favorable process.37 Figure 10(a) shows the TEM image of the single microrod which consists of nanoparticles. The SAED pattern displays the polycrystalline nature of the Co3 O4 (Fig. 10(b)). The HR-TEM (Fig. 10(d)) clearly indicated that the highly crystalline character of the synthesized Co3 O4 with a lattice spacing of 4.7 Å, corresponding to the value of the (111) plane of cubic Co3 O4 .

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microrod obey the Curie–Weiss law. The M–H hysteresis loop of the various morphological Co3 O4 synthesized under different conditions are presented in the Figure 12. The room temperature M–H curves of all samples are linear with the field and with no coercivity and remenance. The samples cannot reach the saturation even in the presence of 10 kOe magnetic field. For a system with an open loop up to high field indicates the existence of high surface anisotropy and a spin-glass-like surface layer.38 Srikala et al. reported the ferromagnetic behaviour of various morphological cobalt oxide such as nanospheres, nanodiscs and nanocubes.39 In contrast, no hysteresis was observed from our products and the loops are superimposed. The magnetization is almost linear with the field. It is reported that at higher temperatures, when the particle moments are free to thermal fluctuations, the ZFC and FC curves merge together.40 The magnetic properties of the materials have been believed to be highly dependent on the sample 1.5

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shape, crystallinity, and magnetization direction, and so on.41 The similar paramagnetic behavior with very small coercivity 38.471 Oe and retentivity 3.097×10−3 emu/g in cobalt oxide is reported by Pal et al.31 In our preparation method all the Co3 O4 synthesized under different conditions exhibited a linear magnetization curve attributed to the paramagnetic behavior at room temperature.

4. CONCLUSION In summary, we have successfully fabricated various morphological cobalt oxalates and corresponding its cobalt oxide by a simple template-free method. The simple industrially applicable synthetic methods offer new vistas for the well defined porous microrods synthesis. The morphology of cobalt oxalate is depends to the method of preparation of cobalt oxalates and also cobalt precursors. By simply changing the preparation method both orthorhombic and monoclinic cobalt oxalates could be prepared. The hot mixing method yields well defined rodlike structure where as room temperature addition did not yield similar morphology. The calcination temperature and time did not affect the Co3 O4 microrod morphology which was selfassembled by oriented attachment mechanism. The Co3 O4 synthesized at different experimental conditions showed pure paramagnetic behaviors at room temperature.

References and Notes 1. A. Henglein, Chem. Rev. 89, 1861 (1989). 2. A. Agfeldt and M. Gratzel, Chem. Rev. 95, 49 (1995). 3. R. N. Singh, J. F. Koenig, G. Poillerat, and P. Chartier, J. Electroanal. Chem. 314, 241 (1991). 4. M. C. Daniel and D. Astruc, Chem. Rev. 104, 293 (2004). 5. B. Raveau, J. Eur. Cer. Soc. 25, 1965 (2005). 6. C. Mocuta, A. Barbier, and G. Renaud, Appl. Surf. Sci. 56, 162 (2002). 7. F. Svegl, B. Orel, M. G. Hutchins, and K. Kalcher, J. Electrochem. Soc. 143, 1532 (1996). 8. W.-Y. Li, L.-N. Xu, and J. Chen, Adv. Func. Mater. 15, 851 (2005). 9. Y. Li, B. Tan, and Y. Wu, J. Am. Chem. Soc. 128, 14258 (2006).

Received: 2 September 2010. Accepted: 6 September 2010.

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RESEARCH ARTICLE

Acknowledgments: One of the authors R. A. gratefully acknowledges Professor A. N. Kanappan, Head of the Department, Department of Physics, Annamalai University, Annamalainagar, India. The Finnish Cultural Foundation, EU and City of Mikkeli are acknowledged for financial support.

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