- Email: [email protected]
A novel strategy to synthesize cobalt hydroxide and Co3O4 nanowires
Journal of Physics and Chemistry of Solids 72 (2011) 904–907
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
A novel strategy to synthesize cobalt hydroxide and Co3O4 nanowires Waleed E. Mahmoud n,1, F.A. Al-Agel King Abdulaziz University, Faculty of science, Physics department, Jeddah, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2010 Received in revised form 20 April 2011 Accepted 24 April 2011 Available online 14 May 2011
Cobalt hydroxide ultra fine nanowires were prepared by a facile hydrothermal route using hydrogen peroxide. This method provides a simple, low cost, and large-scale route to produce b-cobalt hydroxide nanowires with an average diameter of 5 nm and a length of ca. 10 mm, which show a predominant well-crystalline hexagonal brucite-like phase. Their thermal decomposition produced highly uniform nanowires of cobalt oxide (Co3O4) under temperature 500 1C in the presence of oxygen gas. The produced cobalt oxide was characterized by X-ray diffraction, transmission electronic microscopy, and selected-area electron diffraction. The results indicated that cobalt oxide nanowires with an average diameter of 10 nm and a length of ca. 600 nm have been formed, which show a predominant wellcrystalline cubic face-centered like phase. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Electron microscopy D. Crystal structure
1. Introduction One-dimensional (1D) nanoscale materials such as nanotubes, nanobelts, nanorods, and nanowires have been prepared by different approaches, which can be classified into the following strategies [1,2]. First, the growth of 1D nanoscale materials is achieved in a hard template with well-confined structures, such as alumina, silica, block polymer, mica, and membranes [3,4]. Second, soft templates are used to produce 1D nanoscale materials [5,6]. Generally, in this process, surfactants are applied to stabilize the surface of nanonuclei and kinetically control the growth rates of various facets of nuclei. Third, the intrinsic structures are used to form 1D nanostructures [7,8]. Usually, the materials with hexagonal structure are favored to form 1D nanostructures under a suitable reaction condition. Fourth, vapor–liquid–solid (VLS) growth has been employed to prepare 1D nanostructures [9–11]. Recently, it was found that the lowtemperature process based on one or more strategies described above is more promising to prepare 1D nanoscale materials. Cobalt hydroxide nanostructure has attracted increasing attention recently because of its novel properties in technological applications [12–15]. In particular, cobalt hydroxide can be used to enhance electrochemical performance for enhancing the electrode conductivity and chargeability [16]. It is well known that cobalt hydroxide has two polymorphs: a- and b-Co(OH)2. These two phases are all-layered and have the same hexagonal structures,
n
Corresponding author. Tel.: þ966560860583. E-mail address: [email protected] (W.E. Mahmoud). 1 Permanent address: Suez Canal University, Faculty of Science, Physics Department, Ismailia, Egypt. 0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.04.014
except that the b form is isostructural with brucite-like compounds and consists of a hexagonal packing of hydroxy ions with Co (II) occupying alternate rows of octahedral sites [17]. a-Co(OH)2, however, is isostructural with hydrotalcite-like compounds that consist of stacked Co(OH)2 x layers intercalated with various anions in the interlayer space to restore charge neutrality. Spinel Co3O4 is an important p-type semiconductor due to its potential applications in ceramic pigments, solid-state sensors, energy storage as intercalation compounds, rotatable magnets, heterogeneous catalysts, and electrochromic devices [18–20]. Especially, it is very useful to prepare anisotropic antiferromagnetic nanostructures for studying the magnetic properties as a function of geometric structure [21]. Increasing interest has been generated with antiferromagnetic nanoparticles since the discovery of their potentials for quantum tunneling [22] and their applications in spin-valve systems [23]. Synthesis of cobalt oxide nanoparticles have been obtained by different methods as solvothermal, mechanochemical, reduction–oxidation, sol–gel, and polymer combustion, generating different morphologies like nanotubes, nanorods, nanocubes, and spherical particles [24–26]. Here, we employed a new approach to synthesize ultra fine cobalt hydroxide nanowires and their decomposition to Co3O4 nanowires. The structure and morphology of cobalt oxide nanowires were also investigated.
2. Experimental work For the synthesis of cobalt hydroxide nanowires, 1 g cobalt powder (10 mm, particle size) and 3 ml of aqueous ammonia ‘‘surfactant’’ were dissolved in 25 ml hydrogen peroxide and stirred for 30 min. This solution was transferred into 80 mL
W.E. Mahmoud, F.A. Al-Agel / Journal of Physics and Chemistry of Solids 72 (2011) 904–907
905
Fig. 1 shows typical XRD patterns of b-Co(OH)2 nanostructure. All diffraction peaks in this pattern can be indexed to the hexagonal cell of brucite-like b-Co(OH)2 with lattice constants a¼0.318 and c¼0.465 nm, which are consistent with the values in the literature
(JCPDS 30-0443). The (0 0 1) peak is taller and far narrower than other peaks in the reflections, which implies the highly preferentially oriented growth of the b-Co(OH)2 nanowires. Diffraction peaks of a-Co(OH)2 or other impurities were not observed, which indicates the high purity of the final products successfully synthesized under the current experimental conditions. Fig. 2 displays the FT–IR spectrum of the cobalt hydroxide nanostructures synthesized by hydrothermal method. It can be seen clearly from Fig. 2 that a sharp peak observed at 3630 cm 1 is assigned to the hydroxyl group in the brucite-like structure enhanced due to high basicity. The peaks in the region of 496–540 cm 1 can be assigned to metal oxygen vibrations and metal–OH bending vibrations in the brucite-like octahedron sheets [27]. Thus, the FTIR measurement also confirms the formation of a brucite-like structure. Fig. 3a is a typical SEM image of b-Co(OH)2 nanowires. As can be seen, the sample was composed of uniform ultra fine nanowire like structures with diameter of 5 nm and length of 10 mm. Fig. 3b gives the HRTEM image confirms the layer structure with a lattice spacing of 0.46 nm, in accordance with the (0 0 1) plane of brucite-like b-cobalt hydroxides. SAED pattern (inset of Fig. 3b) indicates the single crystal nature of hexagonal structures of brucite-like b-cobalt hydroxides. The use of aqueous ammonia is critical for the formation of nanowires. Without it, only cobalt hydroxide hexagonal nanosheets are obtained. Wang et al. [28] synthesized b-Co(OH)2 nanoplatelets via microwave hydrothermal process using potassium hydroxide at 140 1C for 3 h. The reason for formation of these hexagonal nanoplatelets is that the brucite-like b-Co(OH)2 has a layered structure comprised of sheets of hexagonally close-packed OH ions with Co(II) bonded to the six OH. The sheets are parallel to the (0 0 1) plane. These b-Co(OH)2 sheets are bonded to one-another by weak OH–OH dipole interactions. Therefore, brucite crystals have the tendency to grow into thin hexagonal platelets. In the present work, ammonia has been successfully used as the morphology directing agent for the synthesis of one-dimensional nanostructures. This ammonia modifies Ehrlich–Schwoebel barrier and the mobility of adatoms and enables the growth speed along the (0 0 1) plane, which has the lowest surface energy. If the growth along the [0 0 1] direction dominates, then nanowires often grow continuously with homogeneous diameters. Thus the shape of cobalt hydroxide could be considered in terms of growth kinetics, by which the fastest growing planes should disappear to leave behind the slowest growing planes as the facets of the product. This implies that the final shape of cobalt hydroxide could be controlled by introducing ammonia to change the free
Fig. 1. XRD of ultra fine cobalt hydroxide nanowire.
Fig. 2. FT–IR spectroscopy of ultra fine cobalt hydroxide nanowire.
teflon-lined stainless steel autoclave. The autoclave was sealed and introduced in oven maintained at 200 1C for 4 h, and then left to cool to room temperature. The resulted powders were filtered, washed with distilled water and ethanol to remove the ions possibly remaining in the final powders, and finally dried in vacuum at 60 1C for 2 h. The chemical equation of reaction process for fabricating cobalt hydroxide nanowires can be described as follows: 200 1 C,4h
Coþ H2 O2 !CoðOHÞ2 3 g of the as-prepared cobalt hydroxide nanopowders were introduced into ceramic crucible and put in an electric oven at 500 1C in the presence of oxygen gas (99.99%) at flow rate of 10 sccm (sccm denotes for standard cubic centimeter per minute) for 3 h to produce Co3O4 nanopowders. The chemical reaction process follows the following equation: 5001 C,3h
3CoðOHÞ2 þ 12O2 !Co3 O4 þ 3H2 Om The X-ray measurements were performed using Philips X’pert diffractometer supplied with copper X-ray tube (lka1 ¼1.5406 A1), nickel filter, graphite crystal monochromator, proportional counter detector, divergence slit 11, and 0.1 mm receiving slit. The working conditions were 40 kV and 30 mA for the X-ray tube, scan speed 0.051, and 2 s measuring time per step. For each measurement, a complete scan was made between 101 and 701 (2y1). To calibrate the measured Bragg 2y-angles, a standard reference material (SRM 640a) of pure Si powder was used. Field Emission electron microscopy (FE-SEM) was carried out using a JEOL 2010 high-resolution transmission electron microscope operated at 200 kV. To prepare the FESEM samples, purified nanopowders were first redispersed in ethanol and diluted, followed by placing a droplet of the solution onto a 400-mesh carbon-coated copper grid. The grid was then dried in desiccators for one day before imaging. Infrared spectra of the samples formed in KBr platelets were recorded with JASCO FT/IR 420 spectrometer.
3. Results and discussion 3.1. Cobalt hydroxide characterization
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W.E. Mahmoud, F.A. Al-Agel / Journal of Physics and Chemistry of Solids 72 (2011) 904–907
Fig. 3. a: SEM image of ultra fine cobalt hydroxide nanowire. b: HRTEM and SAED of ultra fine cobalt hydroxide nanowire.
Fig. 5. a: TEM image of cobalt oxide nanowires. b: HRTEM and SAED of cobalt oxide nanowire.
Fig. 4. Proposed mechanism of surfactant-directed cobalt hydroxide nanowire growth. The single crystalline seed particles have facets that are differentially blocked by the surfactant. Subsequent addition of metal ions and weak reducing agent lead to growth at the exposed particle faces.
energies of the various crystallographic surfaces and thus to alter their growth rates [29] as indicated in Fig. 4. 3.2. Cobalt oxide characterization Cobalt oxide nanowires were formed by the thermal decomposition of cobalt hydroxide under certain conditions. Cobalt hydroxide was annealed at 500 1C for 3 h in the presence of oxygen gas. Fig. 5a shows the morphology of the sample annealed at 500 1C for 3 h under O2, where one can see that the sample sustains nanowire like structures. The diameter of these nanowires is 10 nm and the length about 600 nm. The HRTEM image (Fig. 5b) indicates a highly crystalline character with a lattice spacing of 0.81 nm, corresponding to the value of the (1 1 1) plane
Fig. 6. XRD of cobalt oxide nanowire.
of the Co3O4 phase. The inset of Fig. 4b shows an SAED pattern taken from a mass of the Co3O4 nanowires; the pattern reveals the satisfactory crystallinity of the sample, which can be indexed to the face-centered cubic phase of spinel Co3O4. To investigate the structure of the sample further, the XRD pattern was analyzed, as shown in Fig. 6, where all reflection peaks at (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) can be indexed to the peaks of the Co3O4 phase. The lattice constant (a¼0.807 nm) is in
W.E. Mahmoud, F.A. Al-Agel / Journal of Physics and Chemistry of Solids 72 (2011) 904–907
good agreement with bulk Co3O4 (JCPDS file no. 78-1970). No peaks due to hydroxides were observed, indicating the complete decomposition of hydroxides under O2 atmosphere.
4. Conclusion In summary, we have developed a simple method for the synthesis of single-crystalline hexagonal b-Co(OH)2 nanowires in large quantities by choosing super critical water as oxidizing agent. This novel synthetic method can be carried out to synthesize other high-quality hydroxide nanowires, and it should have potential applications in future large-scale synthesis owing to its high yield, simple reaction apparatus, and low reaction temperature. Notably, single-crystalline face-centered cubic Co3O4 nanowires were selectively obtained by thermal decomposition of the single-crystalline b-Co(OH)2 nanowires in the presence of oxygen gas at 500 1C for 2 h. This strategy may become a general method for the fabrication of ultra fine nanowires for other metal oxides via calcination of the corresponding metal hydroxide nanowires under appropriate conditions. References [1] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. [2] D.J. Milliron, S.M. Hughes, Y. Cui, L. Manna, J. Li, L.W. Wang, A.P. Alivisatos, Nature 430 (2004) 190. [3] T. Thurn-Albert, J. Schotter, G.A. Kastle, N. Emley, T. Shiabaushi, L. KrusinElbaum, K. Guarini, C.T. Black, M.T. Tuominen, T.P. Russell, Science 290 (2000) 2126. [4] M. Wirtz, C.R. Martin, Adv. Mater. 15 (2003) 445.
907
[5] S.J. Park, S. Kim, S. Lee, Z. Khim, K. Char, T. Hyeon, J. Am. Chem. Soc. 122 (2000) 8581. [6] Y. Ma, L. Qi, J. Ma, H. Cheng, Adv. Mater. 16 (2004) 1023. [7] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430. [8] Y. Shao, J. Sun, L. Gao, J. Phys. Chem. C 113 (2009) 6566. [9] N.A. Barakat, M.S. Khil, F.A. Sheik, H.Y. Kim, J. Phys. Chem. C 112 (2008) 12225. [10] R. Xu, H.C. Zeng, Langmuir 20 (2004) 9780. [11] M. Verelst, T.O. Ely, C. Amiens, E. Snoeck, P. Lecante, A. Mosset, M. Respaud, J.M. Broto, B. Chaudret, Chem. Mater. 11 (1999) 2702. [12] F. Gu, C. Li, Y. Hu, L. Zhang, J. Cryst. Growth 304 (2007) 369. [13] B. Varghese, T.C. Hoong, Z. Yanwu, M.V. Reddy, B.V.R. Chowdari, A.T.S. Wee, T.B.C. Vincent, C.T. Lim, C.H. Sow, Adv. Funct. Mater. 17 (2007) 1932. [14] S.G. Kandalkar, J.L. Gunjakar, C.D. Lokhande, O.-S. Joo, J. Alloys Compd. 478 (2009) 594. [15] M. Casas-Cabanas, G. Binotto, D. Larcher, A. Lecup, V. Giordani, J.-M. Tarascon, Chem. Mater. 21 (2009) 1939. [16] E. Hosono, S. Fujihara, I. Honma, H. Zhou, J. Mater. Chem. 15 (2005) 1938. [17] L. He, Z. Li, Z. Zhang, Nanotechnology 19 (2008) 155606. [18] A.C. Co, J. Liu, I. Serebrennikova, C.M. Abel, V.I. Birss, J. Mater. Sci. 40 (2005) 4039. [19] W. Zhang, M. Han, Z. Jiang, Y. Song, Z. Xie, Z. Xu, L. Zheng, Chem. Phys. Chem. 8 (2007) 2091. [20] B. Varghese, Y. Zhang, L. Dai, V.B.C. Tan, C.T. Lim, C.-H. Sow, Nano Lett. 8 (2008) 3226. [21] T.-L. Lai, Y.-L. Lai, C.-C. Lee, Y.-Y. Shu, C.-B. Wang, Catal. Today 131 (2008) 105. [22] Z.-g. Guo, W.-m. Liu, Appl. Phys. Lett. 90 (2007) 193108. [23] H. Yang, J. Ouyang, A. Tang, J. Phys. Chem. B 111 (2007) 8006. [24] R. Al-Tuwirqi, A.A. Al-Ghamdi, N. Abdel Aal, A. Umar, Waleed E. Mahmoud, Superlattices Microstruct. 49 (2011) 416. [25] S.C. Petitto, E.M. Marsh, G.A. Carson, M.A. Langell, J. Mol. Catal. A: Chem. 281 (2008) 49. [26] A. Gulino, P. Dapporto, P. Rossi, G. Anastasi, I. Fragal_a, J. Mater. Chem. 14 (2004) 2549. [27] G. Sun, X. Zhang, M. Cao, B. Wei, C. Hu, J. Phys. Chem. C 113 (2009) 6948. [28] J. Wang, G. Du, R. Zeng, B. Niu, Z. Chen, Z. Guo, S. Dou, Electrochim. Acta 55 (2010) 4805. [29] J. Wang, L.Y. Zhang, P. Liu, T.M. Lan, J. Zhang, L.M. Wei, Eric Siu-Wai Kong, C.H. Jiang, Y.F. Zhang, Nano-Micro Lett. 2 (2010) 134.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
A novel strategy to synthesize cobalt hydroxide and Co3O4 nanowires Waleed E. Mahmoud n,1, F.A. Al-Agel King Abdulaziz University, Faculty of science, Physics department, Jeddah, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2010 Received in revised form 20 April 2011 Accepted 24 April 2011 Available online 14 May 2011
Cobalt hydroxide ultra fine nanowires were prepared by a facile hydrothermal route using hydrogen peroxide. This method provides a simple, low cost, and large-scale route to produce b-cobalt hydroxide nanowires with an average diameter of 5 nm and a length of ca. 10 mm, which show a predominant well-crystalline hexagonal brucite-like phase. Their thermal decomposition produced highly uniform nanowires of cobalt oxide (Co3O4) under temperature 500 1C in the presence of oxygen gas. The produced cobalt oxide was characterized by X-ray diffraction, transmission electronic microscopy, and selected-area electron diffraction. The results indicated that cobalt oxide nanowires with an average diameter of 10 nm and a length of ca. 600 nm have been formed, which show a predominant wellcrystalline cubic face-centered like phase. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Electron microscopy D. Crystal structure
1. Introduction One-dimensional (1D) nanoscale materials such as nanotubes, nanobelts, nanorods, and nanowires have been prepared by different approaches, which can be classified into the following strategies [1,2]. First, the growth of 1D nanoscale materials is achieved in a hard template with well-confined structures, such as alumina, silica, block polymer, mica, and membranes [3,4]. Second, soft templates are used to produce 1D nanoscale materials [5,6]. Generally, in this process, surfactants are applied to stabilize the surface of nanonuclei and kinetically control the growth rates of various facets of nuclei. Third, the intrinsic structures are used to form 1D nanostructures [7,8]. Usually, the materials with hexagonal structure are favored to form 1D nanostructures under a suitable reaction condition. Fourth, vapor–liquid–solid (VLS) growth has been employed to prepare 1D nanostructures [9–11]. Recently, it was found that the lowtemperature process based on one or more strategies described above is more promising to prepare 1D nanoscale materials. Cobalt hydroxide nanostructure has attracted increasing attention recently because of its novel properties in technological applications [12–15]. In particular, cobalt hydroxide can be used to enhance electrochemical performance for enhancing the electrode conductivity and chargeability [16]. It is well known that cobalt hydroxide has two polymorphs: a- and b-Co(OH)2. These two phases are all-layered and have the same hexagonal structures,
n
Corresponding author. Tel.: þ966560860583. E-mail address: [email protected] (W.E. Mahmoud). 1 Permanent address: Suez Canal University, Faculty of Science, Physics Department, Ismailia, Egypt. 0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.04.014
except that the b form is isostructural with brucite-like compounds and consists of a hexagonal packing of hydroxy ions with Co (II) occupying alternate rows of octahedral sites [17]. a-Co(OH)2, however, is isostructural with hydrotalcite-like compounds that consist of stacked Co(OH)2 x layers intercalated with various anions in the interlayer space to restore charge neutrality. Spinel Co3O4 is an important p-type semiconductor due to its potential applications in ceramic pigments, solid-state sensors, energy storage as intercalation compounds, rotatable magnets, heterogeneous catalysts, and electrochromic devices [18–20]. Especially, it is very useful to prepare anisotropic antiferromagnetic nanostructures for studying the magnetic properties as a function of geometric structure [21]. Increasing interest has been generated with antiferromagnetic nanoparticles since the discovery of their potentials for quantum tunneling [22] and their applications in spin-valve systems [23]. Synthesis of cobalt oxide nanoparticles have been obtained by different methods as solvothermal, mechanochemical, reduction–oxidation, sol–gel, and polymer combustion, generating different morphologies like nanotubes, nanorods, nanocubes, and spherical particles [24–26]. Here, we employed a new approach to synthesize ultra fine cobalt hydroxide nanowires and their decomposition to Co3O4 nanowires. The structure and morphology of cobalt oxide nanowires were also investigated.
2. Experimental work For the synthesis of cobalt hydroxide nanowires, 1 g cobalt powder (10 mm, particle size) and 3 ml of aqueous ammonia ‘‘surfactant’’ were dissolved in 25 ml hydrogen peroxide and stirred for 30 min. This solution was transferred into 80 mL
W.E. Mahmoud, F.A. Al-Agel / Journal of Physics and Chemistry of Solids 72 (2011) 904–907
905
Fig. 1 shows typical XRD patterns of b-Co(OH)2 nanostructure. All diffraction peaks in this pattern can be indexed to the hexagonal cell of brucite-like b-Co(OH)2 with lattice constants a¼0.318 and c¼0.465 nm, which are consistent with the values in the literature
(JCPDS 30-0443). The (0 0 1) peak is taller and far narrower than other peaks in the reflections, which implies the highly preferentially oriented growth of the b-Co(OH)2 nanowires. Diffraction peaks of a-Co(OH)2 or other impurities were not observed, which indicates the high purity of the final products successfully synthesized under the current experimental conditions. Fig. 2 displays the FT–IR spectrum of the cobalt hydroxide nanostructures synthesized by hydrothermal method. It can be seen clearly from Fig. 2 that a sharp peak observed at 3630 cm 1 is assigned to the hydroxyl group in the brucite-like structure enhanced due to high basicity. The peaks in the region of 496–540 cm 1 can be assigned to metal oxygen vibrations and metal–OH bending vibrations in the brucite-like octahedron sheets [27]. Thus, the FTIR measurement also confirms the formation of a brucite-like structure. Fig. 3a is a typical SEM image of b-Co(OH)2 nanowires. As can be seen, the sample was composed of uniform ultra fine nanowire like structures with diameter of 5 nm and length of 10 mm. Fig. 3b gives the HRTEM image confirms the layer structure with a lattice spacing of 0.46 nm, in accordance with the (0 0 1) plane of brucite-like b-cobalt hydroxides. SAED pattern (inset of Fig. 3b) indicates the single crystal nature of hexagonal structures of brucite-like b-cobalt hydroxides. The use of aqueous ammonia is critical for the formation of nanowires. Without it, only cobalt hydroxide hexagonal nanosheets are obtained. Wang et al. [28] synthesized b-Co(OH)2 nanoplatelets via microwave hydrothermal process using potassium hydroxide at 140 1C for 3 h. The reason for formation of these hexagonal nanoplatelets is that the brucite-like b-Co(OH)2 has a layered structure comprised of sheets of hexagonally close-packed OH ions with Co(II) bonded to the six OH. The sheets are parallel to the (0 0 1) plane. These b-Co(OH)2 sheets are bonded to one-another by weak OH–OH dipole interactions. Therefore, brucite crystals have the tendency to grow into thin hexagonal platelets. In the present work, ammonia has been successfully used as the morphology directing agent for the synthesis of one-dimensional nanostructures. This ammonia modifies Ehrlich–Schwoebel barrier and the mobility of adatoms and enables the growth speed along the (0 0 1) plane, which has the lowest surface energy. If the growth along the [0 0 1] direction dominates, then nanowires often grow continuously with homogeneous diameters. Thus the shape of cobalt hydroxide could be considered in terms of growth kinetics, by which the fastest growing planes should disappear to leave behind the slowest growing planes as the facets of the product. This implies that the final shape of cobalt hydroxide could be controlled by introducing ammonia to change the free
Fig. 1. XRD of ultra fine cobalt hydroxide nanowire.
Fig. 2. FT–IR spectroscopy of ultra fine cobalt hydroxide nanowire.
teflon-lined stainless steel autoclave. The autoclave was sealed and introduced in oven maintained at 200 1C for 4 h, and then left to cool to room temperature. The resulted powders were filtered, washed with distilled water and ethanol to remove the ions possibly remaining in the final powders, and finally dried in vacuum at 60 1C for 2 h. The chemical equation of reaction process for fabricating cobalt hydroxide nanowires can be described as follows: 200 1 C,4h
Coþ H2 O2 !CoðOHÞ2 3 g of the as-prepared cobalt hydroxide nanopowders were introduced into ceramic crucible and put in an electric oven at 500 1C in the presence of oxygen gas (99.99%) at flow rate of 10 sccm (sccm denotes for standard cubic centimeter per minute) for 3 h to produce Co3O4 nanopowders. The chemical reaction process follows the following equation: 5001 C,3h
3CoðOHÞ2 þ 12O2 !Co3 O4 þ 3H2 Om The X-ray measurements were performed using Philips X’pert diffractometer supplied with copper X-ray tube (lka1 ¼1.5406 A1), nickel filter, graphite crystal monochromator, proportional counter detector, divergence slit 11, and 0.1 mm receiving slit. The working conditions were 40 kV and 30 mA for the X-ray tube, scan speed 0.051, and 2 s measuring time per step. For each measurement, a complete scan was made between 101 and 701 (2y1). To calibrate the measured Bragg 2y-angles, a standard reference material (SRM 640a) of pure Si powder was used. Field Emission electron microscopy (FE-SEM) was carried out using a JEOL 2010 high-resolution transmission electron microscope operated at 200 kV. To prepare the FESEM samples, purified nanopowders were first redispersed in ethanol and diluted, followed by placing a droplet of the solution onto a 400-mesh carbon-coated copper grid. The grid was then dried in desiccators for one day before imaging. Infrared spectra of the samples formed in KBr platelets were recorded with JASCO FT/IR 420 spectrometer.
3. Results and discussion 3.1. Cobalt hydroxide characterization
906
W.E. Mahmoud, F.A. Al-Agel / Journal of Physics and Chemistry of Solids 72 (2011) 904–907
Fig. 3. a: SEM image of ultra fine cobalt hydroxide nanowire. b: HRTEM and SAED of ultra fine cobalt hydroxide nanowire.
Fig. 5. a: TEM image of cobalt oxide nanowires. b: HRTEM and SAED of cobalt oxide nanowire.
Fig. 4. Proposed mechanism of surfactant-directed cobalt hydroxide nanowire growth. The single crystalline seed particles have facets that are differentially blocked by the surfactant. Subsequent addition of metal ions and weak reducing agent lead to growth at the exposed particle faces.
energies of the various crystallographic surfaces and thus to alter their growth rates [29] as indicated in Fig. 4. 3.2. Cobalt oxide characterization Cobalt oxide nanowires were formed by the thermal decomposition of cobalt hydroxide under certain conditions. Cobalt hydroxide was annealed at 500 1C for 3 h in the presence of oxygen gas. Fig. 5a shows the morphology of the sample annealed at 500 1C for 3 h under O2, where one can see that the sample sustains nanowire like structures. The diameter of these nanowires is 10 nm and the length about 600 nm. The HRTEM image (Fig. 5b) indicates a highly crystalline character with a lattice spacing of 0.81 nm, corresponding to the value of the (1 1 1) plane
Fig. 6. XRD of cobalt oxide nanowire.
of the Co3O4 phase. The inset of Fig. 4b shows an SAED pattern taken from a mass of the Co3O4 nanowires; the pattern reveals the satisfactory crystallinity of the sample, which can be indexed to the face-centered cubic phase of spinel Co3O4. To investigate the structure of the sample further, the XRD pattern was analyzed, as shown in Fig. 6, where all reflection peaks at (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) can be indexed to the peaks of the Co3O4 phase. The lattice constant (a¼0.807 nm) is in
W.E. Mahmoud, F.A. Al-Agel / Journal of Physics and Chemistry of Solids 72 (2011) 904–907
good agreement with bulk Co3O4 (JCPDS file no. 78-1970). No peaks due to hydroxides were observed, indicating the complete decomposition of hydroxides under O2 atmosphere.
4. Conclusion In summary, we have developed a simple method for the synthesis of single-crystalline hexagonal b-Co(OH)2 nanowires in large quantities by choosing super critical water as oxidizing agent. This novel synthetic method can be carried out to synthesize other high-quality hydroxide nanowires, and it should have potential applications in future large-scale synthesis owing to its high yield, simple reaction apparatus, and low reaction temperature. Notably, single-crystalline face-centered cubic Co3O4 nanowires were selectively obtained by thermal decomposition of the single-crystalline b-Co(OH)2 nanowires in the presence of oxygen gas at 500 1C for 2 h. This strategy may become a general method for the fabrication of ultra fine nanowires for other metal oxides via calcination of the corresponding metal hydroxide nanowires under appropriate conditions. References [1] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. [2] D.J. Milliron, S.M. Hughes, Y. Cui, L. Manna, J. Li, L.W. Wang, A.P. Alivisatos, Nature 430 (2004) 190. [3] T. Thurn-Albert, J. Schotter, G.A. Kastle, N. Emley, T. Shiabaushi, L. KrusinElbaum, K. Guarini, C.T. Black, M.T. Tuominen, T.P. Russell, Science 290 (2000) 2126. [4] M. Wirtz, C.R. Martin, Adv. Mater. 15 (2003) 445.
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