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Large scale hydrothermal synthesis of β-Co(OH)2 hexagonal nanoplates and their conversion into porous Co3O4 nanoplates
Journal of Alloys and Compounds 478 (2009) 823–826
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Large scale hydrothermal synthesis of -Co(OH)2 hexagonal nanoplates and their conversion into porous Co3 O4 nanoplates Peiying Zhan ∗ Department of Chemistry, Tonghua Normal University, Tonghua, PR China 134002
a r t i c l e
i n f o
Article history: Received 13 October 2008 Received in revised form 3 December 2008 Accepted 13 December 2008 Available online 24 December 2008 Keywords: Electrode materials Nanostructured materials Chemical synthesis
a b s t r a c t -Co(OH)2 hexagonal nanoplates with a diameter of 200–400 nm and a thickness of 30–50 nm have been synthesized successfully by a template and surfactant-free hydrothermal route in a very simple system composed only of water, CoCl2 and NaOH, and many of them are regular hexagons with the angles of adjacent edges of 120◦ . This method is high yield, simple and environmentally benign. According to the systematical investigation of the reaction parameters (temperature, time and the concentration of NaOH), the optimal conditions to -Co(OH)2 hexagonal nanoplates were concluded, including the temperature of 120 ◦ C, the hydrothermal time of 3 h and the NaOH dosage of 5 mmol. A dissolution–recrystallization mechanism was put forward. Furthermore, the as-obtained Co(OH)2 hexagonal nanoplates can be easily converted into porous Co3 O4 nanoplates by calcining Co(OH)2 nanoplates in air at 500 ◦ C for 2 h. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Currently, researches on fundamental properties and practical applications of nanomaterials are attracting much attention [1]. In which, the size effect of nanocrystals was widely studied. In fact, the shape of nanomaterials has considerable influence on physical properties and is also important in many potential applications [2–4]. Therefore, numerous efforts have been made to explore various approaches for the preparation of nanoscale materials with different dimension and shape. In which, the preparation of disk-like or plate-like nanoparticles is now a new and interesting research focus. For example, silver and gold nanodisks or nanoplates with perfectly or truncated triangular shape, PbSe nanosheets, petal-like ZnO nanoplates have been synthesized successfully [5–8]. Cobalt hydroxides (Co(OH)2 ) can be used as additives to improve the electrochemical activity of alkaline secondary batteries [9] and the tribological property of lubricating oils [10]. It is also reported that cobalt hydroxide films show catalytic and reversible electrochromic properties [11–12]. Under flowing N2 atmosphere and with the assistance of ethylenediamine ligand, Co(OH)2 hexagonal sheets were synthesized by Sampanthar and Zeng [13]. By exfoliating as-synthesized layered double hydroxide (LDH) in formamide, hexagonal Co(OH)2 nanosheets with an average lateral size of 3–4 m and a thickness of a few tens of nanometers were obtained [14]. Under the direction of poly(vinylpyrrolidone) (PVP),
∗ Tel.: +86 435 3208079. E-mail address: zhanpy [email protected]. 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.025
hexagonal Co(OH)2 nanoplatelets were prepared by hydrothermal treating Co(NO3 )2 and NaOH in a mixture solvent of ethanol and distilled water [15]. In spite of the success mentioned above, the search for simple, high-yield, and environmentally benign methods to synthesize Co(OH)2 nanoplates is still an on-going process. Herein, we report a one-step, large scale hydrothermal synthesis of -Co(OH)2 hexagonal nanoplates in a very simple system composed only of water, CoCl2 and NaOH. And the effect of the reaction parameters, such as concentration of NaOH, hydrothermal temperature and time, was systematically investigated. Furthermore, the Co(OH)2 hexagonal nanoplates can be easily converted to porous Co3 O4 nanoplates by calcining Co(OH)2 nanoplates in air at 500 ◦ C for 2 h, which also has potential application in many fields because Co3 O4 is a kind of important material in ceramic pigments, solid-state sensors, energy storage as intercalation compounds, rotatable magnets, heterogeneous catalysts, and electrochromic devices [16–17]. 2. Experimental section In a typical experiment, 2 mmol CoCl2 and 5 mmol NaOH were dissolved in 40 mL distilled water. After 10 min stirring, the suspension was transferred into 45 mL Teflon-lined stainless steel autoclave and maintained at 120 ◦ C for 3 h, and then it was allowed to cool to room temperature naturally. A mass of precipitates were collected by centrifugation and washed with distilled water several times. The yield of the -Co(OH)2 nanoplatelets can be as high as 90% according to the amount of CoCl2 ·6H2 O used. To compare, parallel experiments were carried out with different amount of NaOH as well as different hydrothermal duration and temperature. Porous Co3 O4 nanoplates were obtained when the as-synthesized Co(OH)2 nanoplates were directly calcined at 500 ◦ C for 2 h in a muffle furnace. The as-prepared powder samples were characterized by X-ray powder diffraction (XRD) analysis on a Rigaku X-ray diffractometer with Cu K␣ radiation ( = 1.5406 Å). The morphologies and sizes of the as-obtained products were observed by transmission electron microscopy (TEM, Hitachi H-800). High-
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Fig. 1. The XRD patterns of the Co(OH)2 nanoplates: (A) as-obtained by hydrothermal treatment at 120 ◦ C for 3 h; (B) as-obtained without hydrothermal treatment.
resolution transmission electron microscopy (HRTEM) was performed with a JEM-3010 transmission electron microscope (300 kV). Chemical bonding information was studied with a Fourier transform infrared (FTIR) spectrometer. N2 adsorption was determined by BET measurements using an ASAP-2000 surface area analyzer.
3. Results and discussion The X-ray powder diffraction pattern of the as-prepared Co(OH)2 nanoplates is shown in Fig. 1A, and all the peaks can be indexed to pure brucite-like phase of -Co(OH)2 (hexagonal structure, JCPDS file no. 30-443). Compared with the standard pattern, the intensity of the (0 0 1) peak is unusually stronger than others, implying the preferential orientation of (0 0 1) on the surface. Fig. 2 shows the FTIR spectrum of the as-obtained Co(OH)2 nanoplates. A sharp peak observed at 3633 cm−1 is assigned to the hydroxyl group in the brucite-like structure. The peaks in the region of 490–540 cm−1 can be assigned to metal–oxygen vibrations and metal–OH bending vibrations in the brucite-like octahedron [18]. Thus, the FTIR measurement also confirms the brucite-like structure of the product. The morphology of the products was characterized by TEM. As shown in Fig. 3a, one can see that the cobalt hydroxide products are well-defined hexagonal platelets with a size in the range of 200–400 nm. Many of them are regular hexagons with the angles of adjacent edges of 120◦ as indicated by arrows in Fig. 3b. The surface of the nanoplates is the (0 0 0 1) plane of the hexagonal Co(OH)2 phase, which is consistent with the very strong intensity
Fig. 2. The FTIR spectrum of the Co(OH)2 nanoplates as-obtained by hydrothermal treatment at 120 ◦ C for 3 h.
Fig. 3. (a) Low-magnification and (b) high-magnification TEM images of the Co(OH)2 nanoplates obtained at 120 ◦ C for 3 h. The insets are SAED and HRTEM images, respectively.
of (0 0 1) peak in XRD pattern, and the angles of 120◦ may be those of the (1 0 − 1 0) and (0 1 − 1 0) planes. The suggestion is supported by selected area electron diffraction (SAED) pattern (the inset in Fig. 3a) with the [0 0 0 1] as zone axis, which exhibits the hexagonal structure of brucite-like -cobalt hydroxide. In addition to nanoplates, some rod-like nanostructures can also be found in both images, which should be some nanoplates that lie on their side and the corresponding HRTEM image (inset of Fig. 3b) indicates clear lattice fringes with a spacing of 0.46 nm, which is in good agreement with the d spacing value of (0 0 0 1) plane of the hexagonal -Co(OH)2 and also confirms the layer structure of the product. In addition, it is could be concluded that the thickness of the plates is 30–50 nm according the side-view image. To shed light on the formation mechanism of the Co(OH)2 nanoplates, their growth process has been followed by examining the products harvested at different intervals of 0, 1, 3, 6, 12, and 24 h hydrothermal treating at 120 ◦ C. Before hydrothermal treatment, namely 0 h reaction time, the product was primarily composed of palpus-like nanostructures coexisting with a few of hexagonal nanoplates, the size distribution of the nanopalpus was wide and many of them aggregated into flocculation (Fig. 4a). The corresponding XRD pattern (Fig. 1B) reveals that they were also brucite-like phase of -Co(OH)2 , but the crystallinity was a little poor compared with that of the product of 3 h hydrothermal treatment (Fig. 1A), indicating that hydrothermal treatment was
P. Zhan / Journal of Alloys and Compounds 478 (2009) 823–826
Fig. 4. TEM images of the product obtained after (a) 0 h and (b) 1 h hydrothermal reaction. (c) TEM image of the product obtained when 2.5 mmol NaOH was used.
absolutely necessary to the fabrication of hexagonal -Co(OH)2 nanoplates with narrow size distribution and better crystallinity. After 1 h hydrothermal reaction, the hexagonal nanoplates were the main product as presented in Fig. 4b. Whereas, when the hydrothermal process was extended to 6, 12 and 24 h, there was no obvious variety in product morphology compared to the sample of 3 h hydrothermal treatment, which demonstrates that too long
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hydrothermal duration is not necessary for the preparation of the hexagonal -Co(OH)2 nanoplates. Temperature dependent experiments were carried out under otherwise identical conditions. The product of 90 ◦ C was composed of 80% hexagonal nanoplates and 20% nanopalpus, indicating that the transformation from nanopalpus to nanoplates was not completed. Contrary wise, when the temperature exceeded 120 ◦ C, such as 150 and 180 ◦ C, hexagonal nanoplates were still the only product, suggesting that temperature also had no apparent effect on the morphology of the products when it was over 120 ◦ C. Moreover, the influence of NaOH was also investigated by adding different amount of NaOH into the reaction system with other conditions kept constant. When 2.5 mmol NaOH was used, the as-obtained product took on a color of dark-green, showing the formation of cobalt hydroxychloride as reported in literature [19] and TEM investigation indicated that the dark-green sample consisted of large scale irregular big nanosheets with diameter of several micrometers (Fig. 4c). On the other hand, increase of NaOH to 10, 20 and 40 mmol resulted in the same morphology to that of the case of 5 mmol NaOH. Therefore, too much NaOH was not necessary to the uniform and large scale generation of hexagonal Co(OH)2 nanoplates, too. Based on the above investigations, three conclusions can be ruled out as follows: (i) hydrothermal treatment is vital to obtain hexagonal Co(OH)2 nanoplates with better crystallinity and high yield, and it has been verified that hydrothermal method is a kind of powerful technique to the synthesis of uniform and perfect crystallized inorganic nanomaterials [20], (ii) the optimal conditions to -Co(OH)2 hexagonal nanoplates include the temperature of 120 ◦ C, the hydrothermal time of 3 h and the NaOH dosage of 5 mmol, and (iii) the hexagonal phase structure nature of -Co(OH)2 should be responsible for the formation of hexagonal nanoplates because of the little effect of reaction conditions on the morphology of final products. Thus, a dissolution–recrystallization mechanism, which is a thermodynamically driven process, is put forward to the formation of hexagonal -Co(OH)2 nanoplates. Under hydrothermal conditions, higher temperature and pressure increased the solubility of -Co(OH)2 precursor in water so that a highly supersaturated solution was formed; this was followed by a nucleation and crystallization process of nanocrystals. Owing to the hexagonal crystal phase nature of -Co(OH)2 , the nanocrystals as-formed were inclined to take on a shape of hexagonal. In addition, NaOH is a strong electrolyte, the hydroxyl functions of which can selectively adsorb and surround on (0 0 0 1) faces according to Wang et al [21], which leads to the limitation of crystal growth along 0 0 0 1 direction. As a result, -Co(OH)2 nanoplates with hexagonal shape came into being. When the temperature was at 90 ◦ C, it could not provide enough energy to complete the recrystallization process within 3 h and nanopalpus were left over. An increase to 120 ◦ C was sufficient for the transmission of the nano-palpus. Therefore, higher hydrothermal temperature, such as 150 ◦ C and 180 ◦ C, was not necessary. Similarly, 5 mmol NaOH was enough to restrain the growth along 0 0 0 1 direction and to ensure the formation of plate-like nanocrystals. Porous Co3 O4 nanoplates could be obtained by thermal decomposition of the as-synthesized precursor Co(OH)2 nanoplates at 500 ◦ C for 2 h. Fig. 5a shows the XRD patterns of as-obtained Co3 O4 nanoplates. The obtained diffractogram reveals that the samples could be perfectly indexed as cubic Co3 O4 with the lattice constant a = 8.08 Å, which are consistent with the values in the standard card (JCPDS Card no. 42-1467). No peaks from other phases are observed in this pattern. Typical TEM image of the porous Co3 O4 nanoplates is shown in Fig. 5b. It can be seen that there is an abundance of pores in the nanoplates, and that there is no significant change in the diameter of the nanoplates as compared to
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specific surface areas and porous nature of the Co3 O4 nanoplates, Brunauer–Emmett–Teller (BET) gas-sorption measurements were carried out. Nitrogen adsorption–desorption isotherm of these porous nanostructures is shown in Fig. 5c, and the inset therein is the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution plots. The isotherm can be categorized as being of type IV with a distinct hysteresis loop. The BET specific surface area of the samples is found to be 146 m2 g−1 , which is much higher than that (86.82 m2 g−1 ) of Co3 O4 nanowires obtained by thermal treatments Co(OH)2 nanowires [22]. A relatively narrow pore size distribution with maximum at around 2.6 nm (The average pore was 6.23 nm.) is also shown in the inset of Fig. 5c. The as-prepared Co3 O4 nanoplates with so large specific surface areas would find its promising applications in catalysis, sensing, Li-ion batteries, and field-emission and electrochromic devices, and so on. 4. Conclusion In summary, -Co(OH)2 nanoplates with hexagonal shape were successfully synthesized in one step via a template and surfactantfree hydrothermal route. The reaction system was very simple and composed only of water, CoCl2 and NaOH. The experiments with different hydrothermal temperature and time as well as NaOH concentration revealed that the optimal parameters to Co(OH)2 hexagonal nanoplates include the temperature of 120 ◦ C, the hydrothermal time of 3 h and the NaOH dosage of 5 mmol. A dissolution–recrystallization mechanism should be responsible for the formation of the hexagonal nanoplates. Thermal decomposition was employed to produce porous Co3 O4 nanoplatelets, which exhibited large specific surface areas and narrow pore size distribution and have promising applications in catalysis, sensing, Li-ion batteries, and field-emission and electrochromic devices, and so on. References
Fig. 5. (a) The XRD patterns, (b) TEM image, and (c) nitrogen adsorption–desorption isotherm of the porous Co3 O4 nanoplates obtained by calcining Co(OH)2 nanoplates in air at 500 ◦ C for 2 h.
their precursors. But the hexagonal shape is not well maintained. For the porous sample should be polycrystalline, the crystallite size of it has been estimated by the Scherrer equation. The average size of the nanoparticles is calculated to be about 8 nm by using the strongest peak (3 1 1) at 2 = 36.85. To investigate the
[1] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435–445. [2] W.U. Huynh, J.J. Dittmer, A.P. Alivisatos, Science 295 (2002) 2425–2427. [3] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. Elsayed, Science 272 (1996) 1924–1925. [4] H. Mattoussi, L.H. Radzilowski, B.O. Dabbousi, E.L. Thomas, M.G. Bawendi, M.F. Rubner, J. Appl. Phys. 83 (1998) 7965–7974. [5] L.Y. Wang, X. Chen, J. Zhan, Z.M. Sui, J.K. Zhao, Z.W. Sun, Chem. Lett. 33 (2004) 720–721. [6] X.Q. Zou, S.J. Dong, J. Phys. Chem. B 110 (2006) 21545–21550. [7] X. Wang, G. Xi, Y. Liu, Y. Qian, Crys. Growth Des. 8 (2008) 1406–1411. [8] C. Yan, D. Xue, L. Zou, J. Alloy. Compd. 453 (2008) 87–92. [9] P. Elumalai, H.N. Vasan, N. Munichandraiah, J. Power Sources 93 (2001) 201–208. [10] G.X. Chen, Z.S. Hu, J.X. Dong, L.G. Wang, Y. Peng, T. He, et al., Lubr. Eng. 57 (2001) 36–39. [11] M. Dinamani, P.V. Kamath, J. Appl. Electrochem. 305 (2000) 1157–1161. [12] N. Jozer, D.G. Chen, T. Buyuklimanli, Sol. Energy. Mater. Sol. Cells 52 (1998) 223–231. [13] J.T. Sampanthar, H.C. Zeng, J. Am. Chem. Soc. 124 (2002) 6668–6675. [14] R.Z. Ma, K. Takada, K. Fukuda, N. Iyi, Y. Bando, T. Sasaki, Angew. Chem. Int. Ed. 47 (2008) 86–89. [15] Y.L. Hou, H. Kondoh, M. Shimojo, T. Kogure, T. Ohta, J. Phys. Chem. B 109 (2005) 19094–19098. [16] T. He, D. Chen, X. Jiao, Y. Xu, Y. Gu, Langmuir 20 (2004) 8404–8408. [17] D. Barreca, C. Massignan, S. Daolio, M. Fabrizio, C. Piccirillo, L. Armelao, E. Tondello, Chem. Mater. 13 (2001) 588–593. [18] Y. Zhu, H. Li, Y. Kottypin, A. Gedanken, J. Mater. Chem. 12 (2002) 729–733. [19] Z. Zhao, F. Geng, J. Bai, H. Cheng, J. Phys. Chem. C 111 (2007) 3848–3852. [20] J. Yang, C. Li, Z. Quan, D. Kong, X. Zhang, et al., Cryst. Growth Des. 8 (2008) 695–699. [21] H. Xu, W. Wang, W. Zhu, Mater. Lett. 60 (2006) 2203–2206. [22] Y. Li, B. Tan, Y. Wu, J. Am. Chem. Soc. 128 (2006) 14258–14259.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Large scale hydrothermal synthesis of -Co(OH)2 hexagonal nanoplates and their conversion into porous Co3 O4 nanoplates Peiying Zhan ∗ Department of Chemistry, Tonghua Normal University, Tonghua, PR China 134002
a r t i c l e
i n f o
Article history: Received 13 October 2008 Received in revised form 3 December 2008 Accepted 13 December 2008 Available online 24 December 2008 Keywords: Electrode materials Nanostructured materials Chemical synthesis
a b s t r a c t -Co(OH)2 hexagonal nanoplates with a diameter of 200–400 nm and a thickness of 30–50 nm have been synthesized successfully by a template and surfactant-free hydrothermal route in a very simple system composed only of water, CoCl2 and NaOH, and many of them are regular hexagons with the angles of adjacent edges of 120◦ . This method is high yield, simple and environmentally benign. According to the systematical investigation of the reaction parameters (temperature, time and the concentration of NaOH), the optimal conditions to -Co(OH)2 hexagonal nanoplates were concluded, including the temperature of 120 ◦ C, the hydrothermal time of 3 h and the NaOH dosage of 5 mmol. A dissolution–recrystallization mechanism was put forward. Furthermore, the as-obtained Co(OH)2 hexagonal nanoplates can be easily converted into porous Co3 O4 nanoplates by calcining Co(OH)2 nanoplates in air at 500 ◦ C for 2 h. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Currently, researches on fundamental properties and practical applications of nanomaterials are attracting much attention [1]. In which, the size effect of nanocrystals was widely studied. In fact, the shape of nanomaterials has considerable influence on physical properties and is also important in many potential applications [2–4]. Therefore, numerous efforts have been made to explore various approaches for the preparation of nanoscale materials with different dimension and shape. In which, the preparation of disk-like or plate-like nanoparticles is now a new and interesting research focus. For example, silver and gold nanodisks or nanoplates with perfectly or truncated triangular shape, PbSe nanosheets, petal-like ZnO nanoplates have been synthesized successfully [5–8]. Cobalt hydroxides (Co(OH)2 ) can be used as additives to improve the electrochemical activity of alkaline secondary batteries [9] and the tribological property of lubricating oils [10]. It is also reported that cobalt hydroxide films show catalytic and reversible electrochromic properties [11–12]. Under flowing N2 atmosphere and with the assistance of ethylenediamine ligand, Co(OH)2 hexagonal sheets were synthesized by Sampanthar and Zeng [13]. By exfoliating as-synthesized layered double hydroxide (LDH) in formamide, hexagonal Co(OH)2 nanosheets with an average lateral size of 3–4 m and a thickness of a few tens of nanometers were obtained [14]. Under the direction of poly(vinylpyrrolidone) (PVP),
∗ Tel.: +86 435 3208079. E-mail address: zhanpy [email protected]. 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.025
hexagonal Co(OH)2 nanoplatelets were prepared by hydrothermal treating Co(NO3 )2 and NaOH in a mixture solvent of ethanol and distilled water [15]. In spite of the success mentioned above, the search for simple, high-yield, and environmentally benign methods to synthesize Co(OH)2 nanoplates is still an on-going process. Herein, we report a one-step, large scale hydrothermal synthesis of -Co(OH)2 hexagonal nanoplates in a very simple system composed only of water, CoCl2 and NaOH. And the effect of the reaction parameters, such as concentration of NaOH, hydrothermal temperature and time, was systematically investigated. Furthermore, the Co(OH)2 hexagonal nanoplates can be easily converted to porous Co3 O4 nanoplates by calcining Co(OH)2 nanoplates in air at 500 ◦ C for 2 h, which also has potential application in many fields because Co3 O4 is a kind of important material in ceramic pigments, solid-state sensors, energy storage as intercalation compounds, rotatable magnets, heterogeneous catalysts, and electrochromic devices [16–17]. 2. Experimental section In a typical experiment, 2 mmol CoCl2 and 5 mmol NaOH were dissolved in 40 mL distilled water. After 10 min stirring, the suspension was transferred into 45 mL Teflon-lined stainless steel autoclave and maintained at 120 ◦ C for 3 h, and then it was allowed to cool to room temperature naturally. A mass of precipitates were collected by centrifugation and washed with distilled water several times. The yield of the -Co(OH)2 nanoplatelets can be as high as 90% according to the amount of CoCl2 ·6H2 O used. To compare, parallel experiments were carried out with different amount of NaOH as well as different hydrothermal duration and temperature. Porous Co3 O4 nanoplates were obtained when the as-synthesized Co(OH)2 nanoplates were directly calcined at 500 ◦ C for 2 h in a muffle furnace. The as-prepared powder samples were characterized by X-ray powder diffraction (XRD) analysis on a Rigaku X-ray diffractometer with Cu K␣ radiation ( = 1.5406 Å). The morphologies and sizes of the as-obtained products were observed by transmission electron microscopy (TEM, Hitachi H-800). High-
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Fig. 1. The XRD patterns of the Co(OH)2 nanoplates: (A) as-obtained by hydrothermal treatment at 120 ◦ C for 3 h; (B) as-obtained without hydrothermal treatment.
resolution transmission electron microscopy (HRTEM) was performed with a JEM-3010 transmission electron microscope (300 kV). Chemical bonding information was studied with a Fourier transform infrared (FTIR) spectrometer. N2 adsorption was determined by BET measurements using an ASAP-2000 surface area analyzer.
3. Results and discussion The X-ray powder diffraction pattern of the as-prepared Co(OH)2 nanoplates is shown in Fig. 1A, and all the peaks can be indexed to pure brucite-like phase of -Co(OH)2 (hexagonal structure, JCPDS file no. 30-443). Compared with the standard pattern, the intensity of the (0 0 1) peak is unusually stronger than others, implying the preferential orientation of (0 0 1) on the surface. Fig. 2 shows the FTIR spectrum of the as-obtained Co(OH)2 nanoplates. A sharp peak observed at 3633 cm−1 is assigned to the hydroxyl group in the brucite-like structure. The peaks in the region of 490–540 cm−1 can be assigned to metal–oxygen vibrations and metal–OH bending vibrations in the brucite-like octahedron [18]. Thus, the FTIR measurement also confirms the brucite-like structure of the product. The morphology of the products was characterized by TEM. As shown in Fig. 3a, one can see that the cobalt hydroxide products are well-defined hexagonal platelets with a size in the range of 200–400 nm. Many of them are regular hexagons with the angles of adjacent edges of 120◦ as indicated by arrows in Fig. 3b. The surface of the nanoplates is the (0 0 0 1) plane of the hexagonal Co(OH)2 phase, which is consistent with the very strong intensity
Fig. 2. The FTIR spectrum of the Co(OH)2 nanoplates as-obtained by hydrothermal treatment at 120 ◦ C for 3 h.
Fig. 3. (a) Low-magnification and (b) high-magnification TEM images of the Co(OH)2 nanoplates obtained at 120 ◦ C for 3 h. The insets are SAED and HRTEM images, respectively.
of (0 0 1) peak in XRD pattern, and the angles of 120◦ may be those of the (1 0 − 1 0) and (0 1 − 1 0) planes. The suggestion is supported by selected area electron diffraction (SAED) pattern (the inset in Fig. 3a) with the [0 0 0 1] as zone axis, which exhibits the hexagonal structure of brucite-like -cobalt hydroxide. In addition to nanoplates, some rod-like nanostructures can also be found in both images, which should be some nanoplates that lie on their side and the corresponding HRTEM image (inset of Fig. 3b) indicates clear lattice fringes with a spacing of 0.46 nm, which is in good agreement with the d spacing value of (0 0 0 1) plane of the hexagonal -Co(OH)2 and also confirms the layer structure of the product. In addition, it is could be concluded that the thickness of the plates is 30–50 nm according the side-view image. To shed light on the formation mechanism of the Co(OH)2 nanoplates, their growth process has been followed by examining the products harvested at different intervals of 0, 1, 3, 6, 12, and 24 h hydrothermal treating at 120 ◦ C. Before hydrothermal treatment, namely 0 h reaction time, the product was primarily composed of palpus-like nanostructures coexisting with a few of hexagonal nanoplates, the size distribution of the nanopalpus was wide and many of them aggregated into flocculation (Fig. 4a). The corresponding XRD pattern (Fig. 1B) reveals that they were also brucite-like phase of -Co(OH)2 , but the crystallinity was a little poor compared with that of the product of 3 h hydrothermal treatment (Fig. 1A), indicating that hydrothermal treatment was
P. Zhan / Journal of Alloys and Compounds 478 (2009) 823–826
Fig. 4. TEM images of the product obtained after (a) 0 h and (b) 1 h hydrothermal reaction. (c) TEM image of the product obtained when 2.5 mmol NaOH was used.
absolutely necessary to the fabrication of hexagonal -Co(OH)2 nanoplates with narrow size distribution and better crystallinity. After 1 h hydrothermal reaction, the hexagonal nanoplates were the main product as presented in Fig. 4b. Whereas, when the hydrothermal process was extended to 6, 12 and 24 h, there was no obvious variety in product morphology compared to the sample of 3 h hydrothermal treatment, which demonstrates that too long
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hydrothermal duration is not necessary for the preparation of the hexagonal -Co(OH)2 nanoplates. Temperature dependent experiments were carried out under otherwise identical conditions. The product of 90 ◦ C was composed of 80% hexagonal nanoplates and 20% nanopalpus, indicating that the transformation from nanopalpus to nanoplates was not completed. Contrary wise, when the temperature exceeded 120 ◦ C, such as 150 and 180 ◦ C, hexagonal nanoplates were still the only product, suggesting that temperature also had no apparent effect on the morphology of the products when it was over 120 ◦ C. Moreover, the influence of NaOH was also investigated by adding different amount of NaOH into the reaction system with other conditions kept constant. When 2.5 mmol NaOH was used, the as-obtained product took on a color of dark-green, showing the formation of cobalt hydroxychloride as reported in literature [19] and TEM investigation indicated that the dark-green sample consisted of large scale irregular big nanosheets with diameter of several micrometers (Fig. 4c). On the other hand, increase of NaOH to 10, 20 and 40 mmol resulted in the same morphology to that of the case of 5 mmol NaOH. Therefore, too much NaOH was not necessary to the uniform and large scale generation of hexagonal Co(OH)2 nanoplates, too. Based on the above investigations, three conclusions can be ruled out as follows: (i) hydrothermal treatment is vital to obtain hexagonal Co(OH)2 nanoplates with better crystallinity and high yield, and it has been verified that hydrothermal method is a kind of powerful technique to the synthesis of uniform and perfect crystallized inorganic nanomaterials [20], (ii) the optimal conditions to -Co(OH)2 hexagonal nanoplates include the temperature of 120 ◦ C, the hydrothermal time of 3 h and the NaOH dosage of 5 mmol, and (iii) the hexagonal phase structure nature of -Co(OH)2 should be responsible for the formation of hexagonal nanoplates because of the little effect of reaction conditions on the morphology of final products. Thus, a dissolution–recrystallization mechanism, which is a thermodynamically driven process, is put forward to the formation of hexagonal -Co(OH)2 nanoplates. Under hydrothermal conditions, higher temperature and pressure increased the solubility of -Co(OH)2 precursor in water so that a highly supersaturated solution was formed; this was followed by a nucleation and crystallization process of nanocrystals. Owing to the hexagonal crystal phase nature of -Co(OH)2 , the nanocrystals as-formed were inclined to take on a shape of hexagonal. In addition, NaOH is a strong electrolyte, the hydroxyl functions of which can selectively adsorb and surround on (0 0 0 1) faces according to Wang et al [21], which leads to the limitation of crystal growth along 0 0 0 1 direction. As a result, -Co(OH)2 nanoplates with hexagonal shape came into being. When the temperature was at 90 ◦ C, it could not provide enough energy to complete the recrystallization process within 3 h and nanopalpus were left over. An increase to 120 ◦ C was sufficient for the transmission of the nano-palpus. Therefore, higher hydrothermal temperature, such as 150 ◦ C and 180 ◦ C, was not necessary. Similarly, 5 mmol NaOH was enough to restrain the growth along 0 0 0 1 direction and to ensure the formation of plate-like nanocrystals. Porous Co3 O4 nanoplates could be obtained by thermal decomposition of the as-synthesized precursor Co(OH)2 nanoplates at 500 ◦ C for 2 h. Fig. 5a shows the XRD patterns of as-obtained Co3 O4 nanoplates. The obtained diffractogram reveals that the samples could be perfectly indexed as cubic Co3 O4 with the lattice constant a = 8.08 Å, which are consistent with the values in the standard card (JCPDS Card no. 42-1467). No peaks from other phases are observed in this pattern. Typical TEM image of the porous Co3 O4 nanoplates is shown in Fig. 5b. It can be seen that there is an abundance of pores in the nanoplates, and that there is no significant change in the diameter of the nanoplates as compared to
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specific surface areas and porous nature of the Co3 O4 nanoplates, Brunauer–Emmett–Teller (BET) gas-sorption measurements were carried out. Nitrogen adsorption–desorption isotherm of these porous nanostructures is shown in Fig. 5c, and the inset therein is the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution plots. The isotherm can be categorized as being of type IV with a distinct hysteresis loop. The BET specific surface area of the samples is found to be 146 m2 g−1 , which is much higher than that (86.82 m2 g−1 ) of Co3 O4 nanowires obtained by thermal treatments Co(OH)2 nanowires [22]. A relatively narrow pore size distribution with maximum at around 2.6 nm (The average pore was 6.23 nm.) is also shown in the inset of Fig. 5c. The as-prepared Co3 O4 nanoplates with so large specific surface areas would find its promising applications in catalysis, sensing, Li-ion batteries, and field-emission and electrochromic devices, and so on. 4. Conclusion In summary, -Co(OH)2 nanoplates with hexagonal shape were successfully synthesized in one step via a template and surfactantfree hydrothermal route. The reaction system was very simple and composed only of water, CoCl2 and NaOH. The experiments with different hydrothermal temperature and time as well as NaOH concentration revealed that the optimal parameters to Co(OH)2 hexagonal nanoplates include the temperature of 120 ◦ C, the hydrothermal time of 3 h and the NaOH dosage of 5 mmol. A dissolution–recrystallization mechanism should be responsible for the formation of the hexagonal nanoplates. Thermal decomposition was employed to produce porous Co3 O4 nanoplatelets, which exhibited large specific surface areas and narrow pore size distribution and have promising applications in catalysis, sensing, Li-ion batteries, and field-emission and electrochromic devices, and so on. References
Fig. 5. (a) The XRD patterns, (b) TEM image, and (c) nitrogen adsorption–desorption isotherm of the porous Co3 O4 nanoplates obtained by calcining Co(OH)2 nanoplates in air at 500 ◦ C for 2 h.
their precursors. But the hexagonal shape is not well maintained. For the porous sample should be polycrystalline, the crystallite size of it has been estimated by the Scherrer equation. The average size of the nanoparticles is calculated to be about 8 nm by using the strongest peak (3 1 1) at 2 = 36.85. To investigate the
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