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Synthesis and structural characterization of cobalt hydroxide carbonate nanorods and nanosheets
Journal of Alloys and Compounds 476 (2009) 739–743
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and structural characterization of cobalt hydroxide carbonate nanorods and nanosheets S.L. Wang, L.Q. Qian, H. Xu, G.L. Lü, W.J. Dong, W.H. Tang ∗ Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou, Zhejiang 310018, PR China
a r t i c l e
i n f o
Article history: Received 18 August 2008 Received in revised form 15 September 2008 Accepted 17 September 2008 Available online 7 November 2008 Keywords: Co2 (OH)2 CO3 Nanosheets PVP Hydrothermal method
a b s t r a c t Nanorods and nanosheets of new cobalt hydroxide carbonates have been fabricated through hydrothermal method using CoCl2 ·6H2 O and CO(NH2 )2 in the presence of polyvinylpyrrolidone (PVP). The products were characterized using Fourier transform infrared spectroscopy, elemental analysis, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy and transmission electron microscope/selected area electron diffraction. The nanorods and nanosheets crystallize in different crystal structures with the chemical formula of Co(OH)0.44 (CO3 )0.78 ·0.29H2 O and Co2 (OH)2 CO3 , respectively. For the hexagonal phase with the morphology of nanorods, its lattice parameters are: a = 10.335(5) Å, c = 3.124(1) Å, the possible space group is P622(177), and the number of chemical formula per unit cell is Z = 3. For the monoclinic phase with the morphology of nanosheets, its lattice parameters are: a = 9.362(1) Å, b = 12.175(5) Å, c = 3.281(4) Å, ˇ = 91.879(2), the possible space group is P21/a (14), and the number of chemical formula per unit cell is Z = 4. Our results indicate that the morphology and crystal structure of cobalt hydroxide carbonates are controlled by the reaction temperature, reaction time and PVP. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Low-dimensional nanoscale materials, such as nanorods, nanosheets, and nanotubes, due to their unique physical properties and potential applications as important components for nanoscale devices, have attracted a great deal of interest from researchers [1–3]. Cobalt oxide-based materials have many applications in catalysts, magnetics, electric transportation, sensors, and as absorbers of solar energy [4–8]. Metal hydroxide compounds, expressed using a general formula of Mx (OH)y (X)z ·nH2 O (M: metal, X: anion), are known as layered hydroxide metal salts (LHMS). Firstly, the morphology of the precursor may be effectively controlled by adjusting the interaction between the positively charged layers [M(OH)2−x ]x+ and anions [9]. Secondly, upon thermal decomposition of the precursor, the metal hydroxide compounds converted into metal oxide by topotactic transformation, the morphology of the metal oxides approximately maintain the morphology of their corresponding precursors. The topotactic reaction from hydroxide to metal oxide owing to the small packing mismatch between the adjacent oxide and hydroxide lamella was reported in other cases [10,11]. Xu and Zeng [12] reported a detailed investigation on the formation of cobalt basic carbonate compounds
∗ Corresponding author. Tel.: +86 571 86843222; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.096
[Co(OH)x (CO3 )0.5(2−x) ·nH2 O] with dimensional and morphological controls. Hoson et al. [13] fabricated films of brucite-type cobalt hydroxide with nanorod morphology and hydrotalcite-type cobalt hydroxide with nanosheet morphology by heterogeneous nucleation in a chemical bath using water and mixed solution of water–methanol as solvent, respectively. Zhao et al. [9] obtained 3D nanorods-based urchinlike and nanosheets-based flowerlike cobalt basic salt nanostructures. At the same time, some reported on the synthesis of cobalt basic salt with the morphology of nanobelts and dandelion by hydrothermal method [14,15]. As the complexity of the formed metal hydroxide compounds, it reveals the possibility that a new metal basic salt with new morphology or crystal structure may be prepared through adjusting the interaction between the anions and cations properly. During the nucleation and crystal growth processes, the preferential adsorption between surfactant and various crystallographic planes of the precursor could greatly reduce and/or enhance the growth rate along some directions [16,17]. In this paper, we focus on the selective interaction between PVP and various crystal faces of cobalt hydroxide carbonate, and successfully fabricated two novel kinds of cobalt hydroxide carbonate through a hydrothermal method using CoCl2 ·6H2 O and CO(NH2 )2 at 160 ◦ C in the presence of PVP. It is found that the two cobalt hydroxide carbonates with the morphology of nanorods and nanosheets crystallize in different crystal structures with hexagonal and monoclinic phase, respectively.
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Fig. 1. XRD pattern of the precursors: (A) 12 h, (B) 1.5 h and (C) XRD pattern of Co3 O4 obtained by thermal treatment of the precursor.
2. Experimental All chemical reagents were of analytical grade. In a typical procedure, 2 mmol CoCl2 ·6H2 O, 6 mmol CO(NH2 )2 and 1.005 g PVP were added to distilled water (40 mL) under stirring to form homogeneous transparent solution. The solution was then transferred into a stainless steel autoclave with a Teflon liner of 50 mL capacity, and heated in an oven at 160 ◦ C for 1.5 h, 3.5 h and 12 h, respectively. After the autoclave was air-cooled to room temperature, the resulting products was filtered, washed with distilled water and absolute ethanol for several times, then dried under vacuum at 60 ◦ C for 4 h. The crystallographic information of prepared samples was analyzed by powder X-ray diffraction (XRD) method using a Bruker AXS D8 DISCOVER X-ray diffractometer with Cu K␣ radiation ( = 1.5406 Å) at a scanning rate of 1◦ min−1 . Fourier transform infrared (FTIR) spectra were measured using the KBr method on a Fourier transform infrared spectrometer (FTIR Nicolet Avatar370). Each FTIR spectrum was collected after 40 scans at a resolution of 4 cm−1 from 400 cm−1 to 4000 cm−1 . The weight percentage of carbon, hydrogen and nitrogen in the prepared samples was measured in an EA-1112 elemental analyzer. Thermal behavior of samples was analyzed out using thermogravimetric method (TGA) in a TGA instrument Pyrisl. The samples were heated at a heating rate of 10 ◦ C min−1 from room temperature to 600 ◦ C. Scanning electron microscopy (SEM) images were taken on a JSM-5610LV scanning electron microscope. Transmission electron microscopy (TEM) with selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) were performed on JEM-2010 high-resolution transmission electron microscope. The acceleration voltage was 200 kV.
3. Results and discussion XRD analysis was adopted to analyze the crystal structure and phase composition of the products obtained. Fig. 1 shows the XRD patterns of the samples obtained at different synthetic conditions. Curve A is the XRD pattern of the precursor obtained by
hydrothermal reaction for 12 h in the presence of PVP, and the corresponding XRD pattern indexing results (Table 1) indicate that all of the diffraction peaks can be perfectly indexed to the monoclinic cobalt hydroxide carbonate phase with the lattice parameters of a = 9.362(1) Å, b = 12.175(5) Å, c = 3.281(4) Å, ˇ = 91.879(2), the possible space group is P21/a (14), and the number of chemical formula per unit cell is Z = 4. No impurities can be detected in this pattern. Curve B and Table 2 is the XRD pattern and the corresponding XRD pattern indexing results of the precursor obtained by hydrothermal reaction for 1.5 h in the presence of PVP, respectively. All of the diffraction peaks can be indexed to hexagonal cobalt hydroxide carbonate phase with the lattice parameters of a = 10.335(5) Å, c = 3.124(1) Å, the possible space group is P622(177), and the number of chemical formula per unit cell is Z = 3. Curve C is corresponding to the sample obtained by thermal decomposition and oxidization of the compound at 600 ◦ C for 5 h. All of the diffraction peaks can be indexed to crystalline cubic phase Co3 O4 with a lattice parameter of a = 8.079 Å, which is consistent with the stand value of a = 8.084 Å (JCPDS Card file No. 74-2120). The compositions of the precursors expected from XRD analysis, were investigated based on FTIR, TGA and elemental analysis. Fig. 2(a) presents the FTIR spectra of precursors A and B (precursors A and B were obtained by hydrothermal reaction for 12 and 1.5 h in the presence of PVP, respectively). The strong peaks at 3501 cm−1 and 3504 cm−1 are assigned to the stretching vibration of the O–H bond, v(OH), which indicates the presence of hydroxyl ions due to the metal-OH layer and/or water in the crystal [12]. Similarly, a shoulder at around 2927 cm−1 from hydrogen bonding in the interlayer indicates the presence of interlayer water molecules in precursor B, but the precursor A does not show this absorption [9]. The ı(HOH) mode water is also observed at 1646 cm−1 [13]. The peak at 3381 cm−1 is attributed to the O–H groups interacting with carbonate anions. The presence of CO3 2− in the precursors is evidenced by its vibration bands from middle to lower wave numbers. The peaks observed at 1549 cm−1 , 1401 cm−1 , 1350 cm−1 , 837 cm−1 , 1069 cm−1 , 768 cm−1 and 692 cm−1 (precursor A) are assigned to v(OCO2 ), v(CO3 ), C–O, ı(CO3 ), v(C O), ı(OCO) and (OCO), respectively [18]. The band at 971 cm−1 is ascribed to ı(Co–OH) bending modes while the band at 511 cm−1 is ascribed to w (Co–OH) [12]. In addition, the bands observed in the spectra of precursor B at 1504 cm−1 , 1394 cm−1 , 1045 cm−1 , 830 cm−1 , 748 cm−1 and 683 cm−1 , 528 cm−1 , 447 cm−1 are attributed to CO3 2− and Co–O, respectively [9,12]. On the other hand, absorptions of Cl− , NO3 − , NH3 or NH4 + , and NCO− or CN− are not detected. Moreover, TGA and elemental analysis results for as-prepared precursors are shown in Table 3. The CHN analysis also confirms the absence of nitrogen in the two precursors. Fig. 2(b) shows the TGA
Table 1 XRD pattern indexing results of the monoclinic cobalt hydroxide carbonate. Phase name Lattice parameters Cell volume Z Space group Crystal system
Co2 (OH)2 CO3 a = 9.362(1); b = 12.175(5); c = 3.281(4); ˛ = 90; ˇ = 91.879(2) 373.746(7) 4 P21/a (14) monoclinic
hkl
d (cal) (Å)
d (obs) (Å)
Intensity I/I0 (%)
hkl
d (cal) (Å)
d (obs) (Å)
Intensity I/I0 (%)
200 201 301 400 401 012 310
6.0964 5.1148 3.7221 3.0482 2.8996 2.6495 2.5534
6.0803 5.0967 3.7185 3.0412 2.8916 2.6437 2.5473
100 15 13 31 17 4 11
501 414 600 601 404 701 801
2.3552 2.1621 2.0321 1.9863 1.8611 1.7049 1.5045
2.3545 2.1578 2.0284 1.9825 1.8603 1.7094 1.5028
21 8 18 9 3 10 8
S.L. Wang et al. / Journal of Alloys and Compounds 476 (2009) 739–743
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Table 2 XRD pattern indexing results of the hexagonal cobalt hydroxide carbonate. Co(OH)0.44 (CO3 )0.78 ·0.29H2 O a = 10.335(5) Å c = 3.124(1) Å ˛ = 90 ˇ = 120 333.681(4) 3 P622(177) hexagonal
Phase name Lattice parameters Cell volume Z Space group Crystal system hkl
d (cal) (Å)
d (obs) (Å)
Intensity I/I0 (%)
hkl
d (cal) (Å)
d (obs) (Å)
Intensity I/I0 (%)
100 110 200 210 001 300 101 111 220 201 310 211 400
8.9444 5.1640 4.4722 3.3807 3.1211 2.9815 2.9469 2.6712 2.5820 2.5595 2.4807 2.2932 2.2361
8.9958 5.1721 4.4857 3.3842 3.1259 2.9827 2.9539 2.6730 2.5824 2.5609 2.4819 2.2946 2.2392
57 81 21 56 17 16 31 89 39 47 60 100 15
320 221 311 401 500 321 420 411 510 002 331 421 511
2.0520 1.9895 1.9420 1.8177 1.7889 1.7146 1.6920 1.6549 1.6065 1.5606 1.5073 1.4864 1.4284
2.0512 1.9899 1.9436 1.8205 1.7914 1.7156 1.6924 1.6550 1.6074 1.5621 1.5072 1.4883 1.4289
28 15 43 17 17 28 15 35 16 29 39 13 24
Fig. 2. (a) FTIR and (b) TGA curve.
curves of the two precursors in the temperature range of 40–600 ◦ C. No weight loss is found in Curve A at the temperature range of 40–280 ◦ C, due to the absence of interlayer water molecules. The weight loss between 280 ◦ C and 400 ◦ C is assigned to a simultaneous removal of hydroxyl and carbonate anions, and the total weight loss is about 23.19%. The TGA Curve B shows a distinctly different profile (Curve A). The first weight loss below 180 ◦ C is ascribed to the removal of the absorbed water, while the weight loss between 180 ◦ C and 280 ◦ C is assigned to the evaporation of the intercalated water molecules. The weight loss corresponding to the two steps in TGA are about 7.96% and 4.02%, respectively. The weight loss ranging from 280 ◦ C to 360 ◦ C is associated with the loss of hydroxyl and carbonate anions, while the weight loss between 360 ◦ C and 450 ◦ C is attributed to the continuing decomposition of some residual carbonate groups. The total weight loss is 37.56%, very close to the theoretical weight loss.
Fig. 3 displays the representative SEM and TEM images of the precursor A obtained by hydrothermal reaction for 12 h in the presence of PVP. The SEM images suggest that the precursor is composed of many rectangular-shaped nanosheets with lateral dimension of about 5 m × 12 m. The thickness of these nanosheets is estimated to be about 80–100 nm from SEM observation. Fig. 3c shows TEM image of a single nanosheet. The corresponding HRTEM image recorded from an individual nanosheet (Fig. 3d) clearly shows the well-resolved interference lattice fringe of about 0.304 nm that corresponds to the [4 0 0] crystal plane of cobalt hydroxide carbonate phase. The lattice parameters obtained from the SAED pattern of the nanosheet (the inset in Fig. 3d), which was taken along the [0 2¯ 1] zone axis, are in excellent agreement with those from the XRD method. Fig. 4a shows the SEM image of the precursor B obtained by hydrothermal reaction for 1.5 h in the presence of PVP. The SEM image indicates that the precursor is composed of many
Table 3 TGA and elemental analysis results for the two cobalt hydroxide carbonates. Sample
wC a
wH a
wN a
Chemical formula
Wtotal b
Wtotal c
Precursor A Precursor B
6.14 8.28
0.96 0.91
0 0
Co2 (OH)2 CO3 Co(OH)0.44 (CO3 )0.78 ·0.29H2 O
23.19 37.56
24.24 38.27
a b c
The weight percentage of carbon, hydrogen and nitrogen measured by CHN method. The total weight loss percentage obtained from TGA data. The theoretical weight loss percentage based on the obtained chemical formula.
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Fig. 3. (a) SEM image, (b) high-magnification SEM image, (c) TEM image and (d) HRTEM image and SAED pattern (the inset in d) for the Co2 (OH)2 CO3 .
nanorods with the diameter of about 50 nm. In accordance with the XRD results, the precursor B with the morphology of nanorods is confirmed to be hexagonal phase, while the precursor A with the morphology of nanosheets is confirmed to be monoclinic phase. Fig. 4d indicates the SEM image of Co3 O4 obtained by thermal treatment of the nanosheet-like precursor. The SEM image suggests that the morphology of the Co3 O4 approximately maintain the morphology of its corresponding precursor. The synthesis of the metal hydroxide carbonates usually involves precipitation of metal salts with an alkaline carbonate in the appropriate pH range of 7–9 [19,20], while these reactions are too rapid to control the crystal growth. Urea may be used as a good precipitator because the urea hydrolysis can provide both carbon-
ate and hydroxyl anions slowly to form cobalt hydroxide carbonate. The main reaction in the system can be expressed as follows: H2 NCONH2 + H2 O → 2NH3 + CO2 CO2 + H2 O → CO3
2− +
+ 2H
(1)
+
(2)
−
(3)
NH3 + H2 O → NH4 + OH
The formation of hexagonal cobalt hydroxide carbonate can formulate as: Co2+ + 0.44OH− + 0.78CO3 2− + 0.29H2 O ↔ Co(OH)0.44 (CO3 )0.78 ·0.29H2 O
(4)
Fig. 4. SEM image of the products obtained at 160 ◦ C in the presence of PVP: (a) 1.5 h, (b) 3.5 h, (c) 8 h and (d) SEM image of Co3 O4 obtained by thermal treatment of the nanosheet-like precursor.
S.L. Wang et al. / Journal of Alloys and Compounds 476 (2009) 739–743
The formation of monoclinic cobalt hydroxide carbonate can formulate as: 2Co2+ + 2OH− + CO3 2− ↔ Co2 (OH)2 CO3
(5)
To investigate the growth process of the precursor, we have proceeded the hydrothermal reaction at 160 ◦ C in the presence of PVP for different times. After the autoclave was heated for about 1.5 h, pink product becomes precipitated out of the solution, and the pH region in this solution is around 7. SEM image of the product obtained at this stage (shown in Fig. 4a) indicates that the product is composed of nanorods with diameters of about 50 nm, while the XRD results indicates the product is hexagonal cobalt hydroxide carbonate. The morphology of this product is identical with the product obtained in the same reaction condition except no PVP, which indicates that the PVP have no effect on the morphology of the product while the pH region of the reaction system is around 7. Fig. 4b shows the SEM image of the product obtained after the autoclave was heated for 3.5 h. The product shown in this figure is mixture of nanorods and nanosheets. With the increasing of pH region, the morphology of the product varies from nanorods to nanosheets partially under the influence of PVP, due to the unstable of the crystal with the morphology of nanorods. When the autoclave was heated for 8 h, the product is composed of rectangular-shaped nanosheets (shown in Fig. 4c), and the pH region of the reaction system is around 9. According to the XRD results, the product with the morphology of nanosheets is monoclinic cobalt hydroxide carbonate. When PVP was introduced, it is believed that the selective interaction between PVP and various crystallographic planes of cobalt hydroxide carbonate could greatly reduce the growth rate along some directions and/or enhance the growth rate along others [21]. Therefore, the pH region gradually increase with the reaction time prolonging, and the morphology of the precursor transforms from nanorods to nanosheets, while the crystal structure changes from hexagonal to monoclinic cobalt hydroxide carbonate under the influence of PVP and pH region. The difference in product morphologies may be related to the crystal structure of different cobalt basic salt nanostructures. Xu and Zeng stated that the carbonate anions may act as an inhibitor that selectively decreases the rate of crystal growth in the direction of the side planes of the rod [12]. Thus, the precursor forms to hexagonal cobalt hydroxide carbonate with the morphology of nanorods at the start. There may be a competition between CO3 2− and PVP during their inhibition and adsorption on different crystal faces, but the sorption of PVP will be predominant with the increase of pH region, and the selective interaction between PVP and various crystal faces of cobalt hydroxide carbonate greatly reduce the growth rate along [0 1 2] and enhance the growth rate along [2 0 0]. We also studied other influential factors on the preparation of the precursor. It was found that the change of the reaction temperature in a certain range (160–200 ◦ C) had no obvious effect on the obtained product with the morphology of nanosheet. Therefore, we may conclude that the precursor has a strong tendency to grow in a nanosheet-like structure at a higher temperature in the presence of PVP. Moreover, under the same experimental conditions, we also
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investigated the influences of other surfactants on the synthesis of the precursor. When the CTAB or SDBS was used as a surfactant, no nanosheet-like precursor was obtained. Only in the presence of PVP, will the uniform nanosheet-like precursor be obtained. This demonstrated that the PVP surfactant plays very important role in the fabrication of the nanosheet-like precursor. 4. Conclusions Nanorods and nanosheets of new cobalt hydroxide carbonate have been fabricated through hydrothermal method using CoCl2 ·6H2 O and CO(NH2 )2 in the presence of PVP. The Nanorods and nanosheets crystallize in different crystal structures with the chemical formula of Co(OH)0.44 (CO3 )0.78 ·0.29H2 O and Co2 (OH)2 CO3 , respectively. For the hexagonal phase with the morphology of nanorods, its lattice parameters are: a = 10.335(5) Å, c = 3.124(1) Å, the possible space group is P622(177), and the number of chemical formula per unit cell is Z = 3. For the monoclinic phase with the morphology of nanosheets, its lattice parameters are: a = 9.362(1) Å, b = 12.175(5) Å, c = 3.281(4) Å, ˇ = 91.879(2), the possible space group is P21/a (14), and the number of chemical formula per unit cell is Z = 4. The morphology and crystal structure of cobalt hydroxide carbonates are controlled by the reaction temperature, reaction time and PVP. Acknowledgement This work was supported by the National Nature Science Foundation of China (Grant No. 50672088 and 60571029). References [1] G.L. Wang, B. Tang, L.H. Zhuo, J.C. Ge, M. Xue, Eur. J. Inorg. Chem. 11 (2006) 2313–2317. [2] Z.H. Liang, Y.J. Zhu, X.L. Hu, J. Phys. Chem. B 108 (2004) 3448–3491. [3] C.J. Jia, L.D. Sun, Z.G. Yan, L.P. You, F. Luo, X.D. Han, Y.C. Pang, Z. Zhang, C.H. Yan, Angew. Chem. Int. Ed. 44 (2005) 4328–4333. [4] J. Llorca, P.R. Piscina, J.A. Dalmon, H. Homs, Chem. Mater. 16 (2004) 3573–3578. [5] Y.X. Wang, Y.J. Zhang, Y.M. Cao, M. Lu, J.H. Yang, J. Alloys Compd. 450 (2008) 128–130. [6] Z.Y. Yuan, F. Huang, C.Q. Feng, J.T. Sun, Y.H. Zhou, Mater. Chem. Phys. 79 (2003) 1–4. [7] M. Ando, T. Kobayashi, S. Lijima, M. Haruta, J. Mater. Chem. 7 (1997) 1779–1783. [8] R. Robert, S. Romer, A. Reller, A. Weidenkaff, Adv. Eng. Mater. 5 (2005) 303–308. [9] Z.G. Zhao, F.X. Geng, J.B. Bai, H.M. Cheng, J. Phys. Chem. C 111 (2007) 3848–3852. [10] M.G. Kim, U. Dahmen, A.W. Searcy, J. Am. Ceram. Soc. 71 (1988) 373–376. [11] M.J. Mckelvy, R. Sharma, A.V.G. Chizmeshya, R.W. Carpenter, K. Streib, Chem. Mater. 13 (2001) 921–926. [12] R. Xu, H.C. Zeng, J. Phys. Chem. B 107 (2003) 12643–12649. [13] E. Hoson, S. Fujihara, I. Honma, H. Zhou, J. Mater. Chem. 15 (2005) 1938–1945. [14] R.J. Yu, P.F. Tao, X.S. Zhou, Y.P. Fang, J. Alloys Compd. 461 (2008) 574–578. [15] B.X. Li, Y. Xie, C.Z. Wu, Z.Q. Li, J. Zhang, Mater. Chem. Phys. 99 (2006) 479–486. [16] Y.G. Sun, B. Mayers, T. Herricks, Y.N. Xia, Nano Lett. 3 (2003) 955–960. [17] J.G. Yu, X.F. Zhao, B. Cheng, Q.J. Zhang, J. Solid State Chem. 178 (2005) 861–867. [18] D.G. Klissurski, E.L. Uzunova, Chem. Mater. 3 (1991) 1060–1063. [19] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lojacono, G.J. Moretti, Mater. Chem. 1 (1991) 531–534. [20] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lojacono, G.J. Moretti, J. Chem. Soc., Faraday Trans. 88 (1992) 311–314. [21] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176–2178.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and structural characterization of cobalt hydroxide carbonate nanorods and nanosheets S.L. Wang, L.Q. Qian, H. Xu, G.L. Lü, W.J. Dong, W.H. Tang ∗ Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou, Zhejiang 310018, PR China
a r t i c l e
i n f o
Article history: Received 18 August 2008 Received in revised form 15 September 2008 Accepted 17 September 2008 Available online 7 November 2008 Keywords: Co2 (OH)2 CO3 Nanosheets PVP Hydrothermal method
a b s t r a c t Nanorods and nanosheets of new cobalt hydroxide carbonates have been fabricated through hydrothermal method using CoCl2 ·6H2 O and CO(NH2 )2 in the presence of polyvinylpyrrolidone (PVP). The products were characterized using Fourier transform infrared spectroscopy, elemental analysis, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy and transmission electron microscope/selected area electron diffraction. The nanorods and nanosheets crystallize in different crystal structures with the chemical formula of Co(OH)0.44 (CO3 )0.78 ·0.29H2 O and Co2 (OH)2 CO3 , respectively. For the hexagonal phase with the morphology of nanorods, its lattice parameters are: a = 10.335(5) Å, c = 3.124(1) Å, the possible space group is P622(177), and the number of chemical formula per unit cell is Z = 3. For the monoclinic phase with the morphology of nanosheets, its lattice parameters are: a = 9.362(1) Å, b = 12.175(5) Å, c = 3.281(4) Å, ˇ = 91.879(2), the possible space group is P21/a (14), and the number of chemical formula per unit cell is Z = 4. Our results indicate that the morphology and crystal structure of cobalt hydroxide carbonates are controlled by the reaction temperature, reaction time and PVP. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Low-dimensional nanoscale materials, such as nanorods, nanosheets, and nanotubes, due to their unique physical properties and potential applications as important components for nanoscale devices, have attracted a great deal of interest from researchers [1–3]. Cobalt oxide-based materials have many applications in catalysts, magnetics, electric transportation, sensors, and as absorbers of solar energy [4–8]. Metal hydroxide compounds, expressed using a general formula of Mx (OH)y (X)z ·nH2 O (M: metal, X: anion), are known as layered hydroxide metal salts (LHMS). Firstly, the morphology of the precursor may be effectively controlled by adjusting the interaction between the positively charged layers [M(OH)2−x ]x+ and anions [9]. Secondly, upon thermal decomposition of the precursor, the metal hydroxide compounds converted into metal oxide by topotactic transformation, the morphology of the metal oxides approximately maintain the morphology of their corresponding precursors. The topotactic reaction from hydroxide to metal oxide owing to the small packing mismatch between the adjacent oxide and hydroxide lamella was reported in other cases [10,11]. Xu and Zeng [12] reported a detailed investigation on the formation of cobalt basic carbonate compounds
∗ Corresponding author. Tel.: +86 571 86843222; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.096
[Co(OH)x (CO3 )0.5(2−x) ·nH2 O] with dimensional and morphological controls. Hoson et al. [13] fabricated films of brucite-type cobalt hydroxide with nanorod morphology and hydrotalcite-type cobalt hydroxide with nanosheet morphology by heterogeneous nucleation in a chemical bath using water and mixed solution of water–methanol as solvent, respectively. Zhao et al. [9] obtained 3D nanorods-based urchinlike and nanosheets-based flowerlike cobalt basic salt nanostructures. At the same time, some reported on the synthesis of cobalt basic salt with the morphology of nanobelts and dandelion by hydrothermal method [14,15]. As the complexity of the formed metal hydroxide compounds, it reveals the possibility that a new metal basic salt with new morphology or crystal structure may be prepared through adjusting the interaction between the anions and cations properly. During the nucleation and crystal growth processes, the preferential adsorption between surfactant and various crystallographic planes of the precursor could greatly reduce and/or enhance the growth rate along some directions [16,17]. In this paper, we focus on the selective interaction between PVP and various crystal faces of cobalt hydroxide carbonate, and successfully fabricated two novel kinds of cobalt hydroxide carbonate through a hydrothermal method using CoCl2 ·6H2 O and CO(NH2 )2 at 160 ◦ C in the presence of PVP. It is found that the two cobalt hydroxide carbonates with the morphology of nanorods and nanosheets crystallize in different crystal structures with hexagonal and monoclinic phase, respectively.
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S.L. Wang et al. / Journal of Alloys and Compounds 476 (2009) 739–743
Fig. 1. XRD pattern of the precursors: (A) 12 h, (B) 1.5 h and (C) XRD pattern of Co3 O4 obtained by thermal treatment of the precursor.
2. Experimental All chemical reagents were of analytical grade. In a typical procedure, 2 mmol CoCl2 ·6H2 O, 6 mmol CO(NH2 )2 and 1.005 g PVP were added to distilled water (40 mL) under stirring to form homogeneous transparent solution. The solution was then transferred into a stainless steel autoclave with a Teflon liner of 50 mL capacity, and heated in an oven at 160 ◦ C for 1.5 h, 3.5 h and 12 h, respectively. After the autoclave was air-cooled to room temperature, the resulting products was filtered, washed with distilled water and absolute ethanol for several times, then dried under vacuum at 60 ◦ C for 4 h. The crystallographic information of prepared samples was analyzed by powder X-ray diffraction (XRD) method using a Bruker AXS D8 DISCOVER X-ray diffractometer with Cu K␣ radiation ( = 1.5406 Å) at a scanning rate of 1◦ min−1 . Fourier transform infrared (FTIR) spectra were measured using the KBr method on a Fourier transform infrared spectrometer (FTIR Nicolet Avatar370). Each FTIR spectrum was collected after 40 scans at a resolution of 4 cm−1 from 400 cm−1 to 4000 cm−1 . The weight percentage of carbon, hydrogen and nitrogen in the prepared samples was measured in an EA-1112 elemental analyzer. Thermal behavior of samples was analyzed out using thermogravimetric method (TGA) in a TGA instrument Pyrisl. The samples were heated at a heating rate of 10 ◦ C min−1 from room temperature to 600 ◦ C. Scanning electron microscopy (SEM) images were taken on a JSM-5610LV scanning electron microscope. Transmission electron microscopy (TEM) with selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) were performed on JEM-2010 high-resolution transmission electron microscope. The acceleration voltage was 200 kV.
3. Results and discussion XRD analysis was adopted to analyze the crystal structure and phase composition of the products obtained. Fig. 1 shows the XRD patterns of the samples obtained at different synthetic conditions. Curve A is the XRD pattern of the precursor obtained by
hydrothermal reaction for 12 h in the presence of PVP, and the corresponding XRD pattern indexing results (Table 1) indicate that all of the diffraction peaks can be perfectly indexed to the monoclinic cobalt hydroxide carbonate phase with the lattice parameters of a = 9.362(1) Å, b = 12.175(5) Å, c = 3.281(4) Å, ˇ = 91.879(2), the possible space group is P21/a (14), and the number of chemical formula per unit cell is Z = 4. No impurities can be detected in this pattern. Curve B and Table 2 is the XRD pattern and the corresponding XRD pattern indexing results of the precursor obtained by hydrothermal reaction for 1.5 h in the presence of PVP, respectively. All of the diffraction peaks can be indexed to hexagonal cobalt hydroxide carbonate phase with the lattice parameters of a = 10.335(5) Å, c = 3.124(1) Å, the possible space group is P622(177), and the number of chemical formula per unit cell is Z = 3. Curve C is corresponding to the sample obtained by thermal decomposition and oxidization of the compound at 600 ◦ C for 5 h. All of the diffraction peaks can be indexed to crystalline cubic phase Co3 O4 with a lattice parameter of a = 8.079 Å, which is consistent with the stand value of a = 8.084 Å (JCPDS Card file No. 74-2120). The compositions of the precursors expected from XRD analysis, were investigated based on FTIR, TGA and elemental analysis. Fig. 2(a) presents the FTIR spectra of precursors A and B (precursors A and B were obtained by hydrothermal reaction for 12 and 1.5 h in the presence of PVP, respectively). The strong peaks at 3501 cm−1 and 3504 cm−1 are assigned to the stretching vibration of the O–H bond, v(OH), which indicates the presence of hydroxyl ions due to the metal-OH layer and/or water in the crystal [12]. Similarly, a shoulder at around 2927 cm−1 from hydrogen bonding in the interlayer indicates the presence of interlayer water molecules in precursor B, but the precursor A does not show this absorption [9]. The ı(HOH) mode water is also observed at 1646 cm−1 [13]. The peak at 3381 cm−1 is attributed to the O–H groups interacting with carbonate anions. The presence of CO3 2− in the precursors is evidenced by its vibration bands from middle to lower wave numbers. The peaks observed at 1549 cm−1 , 1401 cm−1 , 1350 cm−1 , 837 cm−1 , 1069 cm−1 , 768 cm−1 and 692 cm−1 (precursor A) are assigned to v(OCO2 ), v(CO3 ), C–O, ı(CO3 ), v(C O), ı(OCO) and (OCO), respectively [18]. The band at 971 cm−1 is ascribed to ı(Co–OH) bending modes while the band at 511 cm−1 is ascribed to w (Co–OH) [12]. In addition, the bands observed in the spectra of precursor B at 1504 cm−1 , 1394 cm−1 , 1045 cm−1 , 830 cm−1 , 748 cm−1 and 683 cm−1 , 528 cm−1 , 447 cm−1 are attributed to CO3 2− and Co–O, respectively [9,12]. On the other hand, absorptions of Cl− , NO3 − , NH3 or NH4 + , and NCO− or CN− are not detected. Moreover, TGA and elemental analysis results for as-prepared precursors are shown in Table 3. The CHN analysis also confirms the absence of nitrogen in the two precursors. Fig. 2(b) shows the TGA
Table 1 XRD pattern indexing results of the monoclinic cobalt hydroxide carbonate. Phase name Lattice parameters Cell volume Z Space group Crystal system
Co2 (OH)2 CO3 a = 9.362(1); b = 12.175(5); c = 3.281(4); ˛ = 90; ˇ = 91.879(2) 373.746(7) 4 P21/a (14) monoclinic
hkl
d (cal) (Å)
d (obs) (Å)
Intensity I/I0 (%)
hkl
d (cal) (Å)
d (obs) (Å)
Intensity I/I0 (%)
200 201 301 400 401 012 310
6.0964 5.1148 3.7221 3.0482 2.8996 2.6495 2.5534
6.0803 5.0967 3.7185 3.0412 2.8916 2.6437 2.5473
100 15 13 31 17 4 11
501 414 600 601 404 701 801
2.3552 2.1621 2.0321 1.9863 1.8611 1.7049 1.5045
2.3545 2.1578 2.0284 1.9825 1.8603 1.7094 1.5028
21 8 18 9 3 10 8
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Table 2 XRD pattern indexing results of the hexagonal cobalt hydroxide carbonate. Co(OH)0.44 (CO3 )0.78 ·0.29H2 O a = 10.335(5) Å c = 3.124(1) Å ˛ = 90 ˇ = 120 333.681(4) 3 P622(177) hexagonal
Phase name Lattice parameters Cell volume Z Space group Crystal system hkl
d (cal) (Å)
d (obs) (Å)
Intensity I/I0 (%)
hkl
d (cal) (Å)
d (obs) (Å)
Intensity I/I0 (%)
100 110 200 210 001 300 101 111 220 201 310 211 400
8.9444 5.1640 4.4722 3.3807 3.1211 2.9815 2.9469 2.6712 2.5820 2.5595 2.4807 2.2932 2.2361
8.9958 5.1721 4.4857 3.3842 3.1259 2.9827 2.9539 2.6730 2.5824 2.5609 2.4819 2.2946 2.2392
57 81 21 56 17 16 31 89 39 47 60 100 15
320 221 311 401 500 321 420 411 510 002 331 421 511
2.0520 1.9895 1.9420 1.8177 1.7889 1.7146 1.6920 1.6549 1.6065 1.5606 1.5073 1.4864 1.4284
2.0512 1.9899 1.9436 1.8205 1.7914 1.7156 1.6924 1.6550 1.6074 1.5621 1.5072 1.4883 1.4289
28 15 43 17 17 28 15 35 16 29 39 13 24
Fig. 2. (a) FTIR and (b) TGA curve.
curves of the two precursors in the temperature range of 40–600 ◦ C. No weight loss is found in Curve A at the temperature range of 40–280 ◦ C, due to the absence of interlayer water molecules. The weight loss between 280 ◦ C and 400 ◦ C is assigned to a simultaneous removal of hydroxyl and carbonate anions, and the total weight loss is about 23.19%. The TGA Curve B shows a distinctly different profile (Curve A). The first weight loss below 180 ◦ C is ascribed to the removal of the absorbed water, while the weight loss between 180 ◦ C and 280 ◦ C is assigned to the evaporation of the intercalated water molecules. The weight loss corresponding to the two steps in TGA are about 7.96% and 4.02%, respectively. The weight loss ranging from 280 ◦ C to 360 ◦ C is associated with the loss of hydroxyl and carbonate anions, while the weight loss between 360 ◦ C and 450 ◦ C is attributed to the continuing decomposition of some residual carbonate groups. The total weight loss is 37.56%, very close to the theoretical weight loss.
Fig. 3 displays the representative SEM and TEM images of the precursor A obtained by hydrothermal reaction for 12 h in the presence of PVP. The SEM images suggest that the precursor is composed of many rectangular-shaped nanosheets with lateral dimension of about 5 m × 12 m. The thickness of these nanosheets is estimated to be about 80–100 nm from SEM observation. Fig. 3c shows TEM image of a single nanosheet. The corresponding HRTEM image recorded from an individual nanosheet (Fig. 3d) clearly shows the well-resolved interference lattice fringe of about 0.304 nm that corresponds to the [4 0 0] crystal plane of cobalt hydroxide carbonate phase. The lattice parameters obtained from the SAED pattern of the nanosheet (the inset in Fig. 3d), which was taken along the [0 2¯ 1] zone axis, are in excellent agreement with those from the XRD method. Fig. 4a shows the SEM image of the precursor B obtained by hydrothermal reaction for 1.5 h in the presence of PVP. The SEM image indicates that the precursor is composed of many
Table 3 TGA and elemental analysis results for the two cobalt hydroxide carbonates. Sample
wC a
wH a
wN a
Chemical formula
Wtotal b
Wtotal c
Precursor A Precursor B
6.14 8.28
0.96 0.91
0 0
Co2 (OH)2 CO3 Co(OH)0.44 (CO3 )0.78 ·0.29H2 O
23.19 37.56
24.24 38.27
a b c
The weight percentage of carbon, hydrogen and nitrogen measured by CHN method. The total weight loss percentage obtained from TGA data. The theoretical weight loss percentage based on the obtained chemical formula.
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Fig. 3. (a) SEM image, (b) high-magnification SEM image, (c) TEM image and (d) HRTEM image and SAED pattern (the inset in d) for the Co2 (OH)2 CO3 .
nanorods with the diameter of about 50 nm. In accordance with the XRD results, the precursor B with the morphology of nanorods is confirmed to be hexagonal phase, while the precursor A with the morphology of nanosheets is confirmed to be monoclinic phase. Fig. 4d indicates the SEM image of Co3 O4 obtained by thermal treatment of the nanosheet-like precursor. The SEM image suggests that the morphology of the Co3 O4 approximately maintain the morphology of its corresponding precursor. The synthesis of the metal hydroxide carbonates usually involves precipitation of metal salts with an alkaline carbonate in the appropriate pH range of 7–9 [19,20], while these reactions are too rapid to control the crystal growth. Urea may be used as a good precipitator because the urea hydrolysis can provide both carbon-
ate and hydroxyl anions slowly to form cobalt hydroxide carbonate. The main reaction in the system can be expressed as follows: H2 NCONH2 + H2 O → 2NH3 + CO2 CO2 + H2 O → CO3
2− +
+ 2H
(1)
+
(2)
−
(3)
NH3 + H2 O → NH4 + OH
The formation of hexagonal cobalt hydroxide carbonate can formulate as: Co2+ + 0.44OH− + 0.78CO3 2− + 0.29H2 O ↔ Co(OH)0.44 (CO3 )0.78 ·0.29H2 O
(4)
Fig. 4. SEM image of the products obtained at 160 ◦ C in the presence of PVP: (a) 1.5 h, (b) 3.5 h, (c) 8 h and (d) SEM image of Co3 O4 obtained by thermal treatment of the nanosheet-like precursor.
S.L. Wang et al. / Journal of Alloys and Compounds 476 (2009) 739–743
The formation of monoclinic cobalt hydroxide carbonate can formulate as: 2Co2+ + 2OH− + CO3 2− ↔ Co2 (OH)2 CO3
(5)
To investigate the growth process of the precursor, we have proceeded the hydrothermal reaction at 160 ◦ C in the presence of PVP for different times. After the autoclave was heated for about 1.5 h, pink product becomes precipitated out of the solution, and the pH region in this solution is around 7. SEM image of the product obtained at this stage (shown in Fig. 4a) indicates that the product is composed of nanorods with diameters of about 50 nm, while the XRD results indicates the product is hexagonal cobalt hydroxide carbonate. The morphology of this product is identical with the product obtained in the same reaction condition except no PVP, which indicates that the PVP have no effect on the morphology of the product while the pH region of the reaction system is around 7. Fig. 4b shows the SEM image of the product obtained after the autoclave was heated for 3.5 h. The product shown in this figure is mixture of nanorods and nanosheets. With the increasing of pH region, the morphology of the product varies from nanorods to nanosheets partially under the influence of PVP, due to the unstable of the crystal with the morphology of nanorods. When the autoclave was heated for 8 h, the product is composed of rectangular-shaped nanosheets (shown in Fig. 4c), and the pH region of the reaction system is around 9. According to the XRD results, the product with the morphology of nanosheets is monoclinic cobalt hydroxide carbonate. When PVP was introduced, it is believed that the selective interaction between PVP and various crystallographic planes of cobalt hydroxide carbonate could greatly reduce the growth rate along some directions and/or enhance the growth rate along others [21]. Therefore, the pH region gradually increase with the reaction time prolonging, and the morphology of the precursor transforms from nanorods to nanosheets, while the crystal structure changes from hexagonal to monoclinic cobalt hydroxide carbonate under the influence of PVP and pH region. The difference in product morphologies may be related to the crystal structure of different cobalt basic salt nanostructures. Xu and Zeng stated that the carbonate anions may act as an inhibitor that selectively decreases the rate of crystal growth in the direction of the side planes of the rod [12]. Thus, the precursor forms to hexagonal cobalt hydroxide carbonate with the morphology of nanorods at the start. There may be a competition between CO3 2− and PVP during their inhibition and adsorption on different crystal faces, but the sorption of PVP will be predominant with the increase of pH region, and the selective interaction between PVP and various crystal faces of cobalt hydroxide carbonate greatly reduce the growth rate along [0 1 2] and enhance the growth rate along [2 0 0]. We also studied other influential factors on the preparation of the precursor. It was found that the change of the reaction temperature in a certain range (160–200 ◦ C) had no obvious effect on the obtained product with the morphology of nanosheet. Therefore, we may conclude that the precursor has a strong tendency to grow in a nanosheet-like structure at a higher temperature in the presence of PVP. Moreover, under the same experimental conditions, we also
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investigated the influences of other surfactants on the synthesis of the precursor. When the CTAB or SDBS was used as a surfactant, no nanosheet-like precursor was obtained. Only in the presence of PVP, will the uniform nanosheet-like precursor be obtained. This demonstrated that the PVP surfactant plays very important role in the fabrication of the nanosheet-like precursor. 4. Conclusions Nanorods and nanosheets of new cobalt hydroxide carbonate have been fabricated through hydrothermal method using CoCl2 ·6H2 O and CO(NH2 )2 in the presence of PVP. The Nanorods and nanosheets crystallize in different crystal structures with the chemical formula of Co(OH)0.44 (CO3 )0.78 ·0.29H2 O and Co2 (OH)2 CO3 , respectively. For the hexagonal phase with the morphology of nanorods, its lattice parameters are: a = 10.335(5) Å, c = 3.124(1) Å, the possible space group is P622(177), and the number of chemical formula per unit cell is Z = 3. For the monoclinic phase with the morphology of nanosheets, its lattice parameters are: a = 9.362(1) Å, b = 12.175(5) Å, c = 3.281(4) Å, ˇ = 91.879(2), the possible space group is P21/a (14), and the number of chemical formula per unit cell is Z = 4. The morphology and crystal structure of cobalt hydroxide carbonates are controlled by the reaction temperature, reaction time and PVP. Acknowledgement This work was supported by the National Nature Science Foundation of China (Grant No. 50672088 and 60571029). References [1] G.L. Wang, B. Tang, L.H. Zhuo, J.C. Ge, M. Xue, Eur. J. Inorg. Chem. 11 (2006) 2313–2317. [2] Z.H. Liang, Y.J. Zhu, X.L. Hu, J. Phys. Chem. B 108 (2004) 3448–3491. [3] C.J. Jia, L.D. Sun, Z.G. Yan, L.P. You, F. Luo, X.D. Han, Y.C. Pang, Z. Zhang, C.H. Yan, Angew. Chem. Int. Ed. 44 (2005) 4328–4333. [4] J. Llorca, P.R. Piscina, J.A. Dalmon, H. Homs, Chem. Mater. 16 (2004) 3573–3578. [5] Y.X. Wang, Y.J. Zhang, Y.M. Cao, M. Lu, J.H. Yang, J. Alloys Compd. 450 (2008) 128–130. [6] Z.Y. Yuan, F. Huang, C.Q. Feng, J.T. Sun, Y.H. Zhou, Mater. Chem. Phys. 79 (2003) 1–4. [7] M. Ando, T. Kobayashi, S. Lijima, M. Haruta, J. Mater. Chem. 7 (1997) 1779–1783. [8] R. Robert, S. Romer, A. Reller, A. Weidenkaff, Adv. Eng. Mater. 5 (2005) 303–308. [9] Z.G. Zhao, F.X. Geng, J.B. Bai, H.M. Cheng, J. Phys. Chem. C 111 (2007) 3848–3852. [10] M.G. Kim, U. Dahmen, A.W. Searcy, J. Am. Ceram. Soc. 71 (1988) 373–376. [11] M.J. Mckelvy, R. Sharma, A.V.G. Chizmeshya, R.W. Carpenter, K. Streib, Chem. Mater. 13 (2001) 921–926. [12] R. Xu, H.C. Zeng, J. Phys. Chem. B 107 (2003) 12643–12649. [13] E. Hoson, S. Fujihara, I. Honma, H. Zhou, J. Mater. Chem. 15 (2005) 1938–1945. [14] R.J. Yu, P.F. Tao, X.S. Zhou, Y.P. Fang, J. Alloys Compd. 461 (2008) 574–578. [15] B.X. Li, Y. Xie, C.Z. Wu, Z.Q. Li, J. Zhang, Mater. Chem. Phys. 99 (2006) 479–486. [16] Y.G. Sun, B. Mayers, T. Herricks, Y.N. Xia, Nano Lett. 3 (2003) 955–960. [17] J.G. Yu, X.F. Zhao, B. Cheng, Q.J. Zhang, J. Solid State Chem. 178 (2005) 861–867. [18] D.G. Klissurski, E.L. Uzunova, Chem. Mater. 3 (1991) 1060–1063. [19] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lojacono, G.J. Moretti, Mater. Chem. 1 (1991) 531–534. [20] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lojacono, G.J. Moretti, J. Chem. Soc., Faraday Trans. 88 (1992) 311–314. [21] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176–2178.