CRYSTAL GROWTH & DESIGN
Synthesis and Growth of Rodlike and Spherical Nanostructures CoSb3 via Ethanol Sol–Gel Method
2008 VOL. 8, NO. 1 208–210
Ying Chu, Xinfeng Tang,* Wenyu Zhao, and Qingjie Zhang State Key Laboratory of AdVanced Technology for Materials Synthesis and Processing, Wuhan UniVersity of Technology, Wuhan 430070, China ReceiVed December 18, 2006; ReVised Manuscript ReceiVed September 21, 2007
ABSTRACT: The synthesis and growth of rodlike and spherical nanostructures of CoSb3 compounds using an ethanol sol–gel method are reported. The characterization results indicate that the molar ratios of Sb/Co and C6H8O7/Co affect the phase composition, and only when Sb/Co ) 6 and C6H8O7/Co ) 4, the obtained powder is a single-phase CoSb3 compound. The crystalline morphology of CoSb3 compounds is composed of spherical nanostructures about 50 nm in diameter and nanorods about 30 nm in diameter and 100-200 nm in length. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) prove that nanorods grow along the [110] direction. The rodlike CoSb3 compound is ultimately produced due to the differences in growth rates between the crystal faces. The formation of a carbonyl coordination compound Co2(CO)8 between the Co atoms reduced by H2 and the CO formed by citric acid decomposition causes the differences in the growth rate. In the local spaces without CO, the CoSb3 nuclei become spherical. Introduction Skutterudite compounds with a general formula TX3 (T ) Co, Rh, or Ir and X ) P, As, Sb) are among the most promising thermoelectric (TE) materials because of their high carrier mobilities and high Seebeck coefficients. However, the figureof-merit ZT used to evaluate the performance of TE materials and conversion of the thermoelectricity of binary skutterudite is not favorable, owing to its relatively large thermal conductivity.1–3 Presently, the main approaches to improving the ZT value are doping, including filling and substitution, and engineering of the materials on a nanometer scale. The doping methods have been investigated widely,4–10 and a large improvement in the ZT value has been achieved but not large enough to allow commercial application. Recently, some research has indicated that engineering the TE materials on a nanometer scale can improve their ZT values and their conversion of thermoelectricity.11–18 When the characteristic length of the microstructure of TE materials matches the coherent length and the mean free path of electron (or phonon) waves, and especially when it is smaller than these, the density (DOS) of carriers, the phonon carrier effective mass, the phonon dispersion relationship, the carrier-phonon interaction, and the TE transport properties will be significantly different compared with those of bulk materials. Therefore, TE transport properties can be adjusted and the ZT value can be improved effectively by controlling the microstructures of CoSb3 compounds. Solid-state reaction11 and coprecipitation12,13 have been used for the synthesis of nanosized CoSb3 compounds. However, the particle size of the powder obtained by these methods is relatively large. On the other hand, powder obtained by the sol–gel process is small, homogeneous, and pure. Thus sol–gel is a promising method for synthesizing CoSb3 compounds with a very small size. The solvent of the sol–gel process is generally water, however, and it is very difficult to synthesize CoSb3 compound using a sol–gel process with a water solvent. In this paper, we have synthesized a CoSb3 compound with small and homogeneous particles by an ethanol sol–gel process with an * To whom correspondence should be addressed. Tel: +86-27-87662832. Fax: +86-27-87860863. E-mail:
[email protected]. Web address: www.whut. edu.cn.
ethanol solvent and a citric acid complexing agent. To the best of our knowledge, this is the first report on the synthesis of a CoSb3 compound by ethanol sol–gel. Experimental Procedures The initial reagents were CoCl2 · 6H2O, SbCl3, an ethanol solvent, and a complexing agent of citric acid, C6H8O7. The molar ratios of Sb/Co and C6H8O7/Co were 3-9 and 2-6, respectively. The cobalt and antimony chlorides and citric acid were added to the ethanol one at a time, and this mixture was stirred by a magnetic stirrer. After sufficient mixing, a transparent and homogeneous blue sol was obtained at ambient temperature. The sol was heated slowly to eliminate ethanol and to form a dry gel, and a black honeycombed solid was gradually obtained in air. Samples of ground powder honeycomb grinded were processed in a tube furnace under the controlled conditions. The reduction temperature, remaining time, and atmosphere were 773 K, 0.5 h, and pure hydrogen, respectively. The product was characterized and analyzed by X-ray powder diffraction (XRD, X′ Pert PROPANalytical with Cu KR radiation and PCDFWIN-ICDD database), transmission electron microscopy (TEM, JEM-2010), selected area electron diffraction (SAED), and energy-dispersive X-ray detection (EDX) (JEM-200).
Results and Discussion According to the chemical formula of CoSb3, the molar ratio of Sb/Co should be 3. However, the Sb/Co ratio of the initial materials needs to be a nonstoichiometric proportion because antimonide is highly volatile. In order to determine the optimal value of Sb/Co, we designed four samples with identical C6H8O7/Co ratios but different Sb/Co ratios of 3, 4.5, 6, and 9. Figure 1 shows the XRD patterns of samples with different Sb/ Co ratios but identical C6H8O7/Co after different processing steps. From the patterns in Figure 1, it can be seen that the honeycombed solid before reduction is amorphous, which may be beneficial for the subsequent thermal treatment due to its higher reaction activity. The phase compositions of samples with identical C6H8O7/Co ratios reduced at 773 K for 0.5 h under a flow of pure hydrogen change from Co-rich phases to Sb-rich phases with increasing Sb/Co ratios, as shown in Figure 1b-e. On the other hand, we find a similar tendency for the phase compositions of samples with identical Sb/Co ratios to change from Co-rich phases to Sb-rich phases with an increasing
10.1021/cg060924j CCC: $40.75 2008 American Chemical Society Published on Web 12/07/2007
Rodlike and Spherical Nanostructures CoSb3
Figure 1. XRD patterns of the samples with different ratios of Sb/Co after different processing steps: (a) honeycomb; (b, c, d, e) Sb/Co ratios of 3, 4.5, 6, and 9, respectively, reduced at 773 K for 0.5 h under a pure hydrogen flow.
Figure 2. TEM image of the powder obtained after 0.5 h at 773 K under a pure hydrogen flow.
C6H8O7/Co ratio. Only when Sb/Co ) 6 and C6H8O7/Co ) 4 did the powder obtained after 0.5 h at 773 K consist only of CoSb3 with skutterudite structure that can be indexed as a cubic cell, R ) 0.9034 nm, as shown in Figure 1d. Figure 2 is a TEM image of a sample with Sb/Co ) 6 and C6H8O7/Co ) 4 reduced at 773 K for 0.5 h under a pure hydrogen flow. The TEM image shows that the crystal of the CoSb3 compound has two kinds of morphology, spheres about 50 nm in diameter and nanorods about 30 nm in diameter and 100-200 nm in length. There are big differences between the morphologies of CoSb3 compounds obtained by ethanol sol–gel and solid-state reaction11 or coprecipitation,12,13 with the latter methods producing spherical grains. The EDX results indicate that the molar ratio of spherical Sb/Co is about 3, which is the same as the molar ratio of rodlike particles. It can be concluded that all the spherical and rodlike particles are CoSb3 compounds, which is consistent with the XRD results. Figure 3 shows the TEM and SAED patterns of the nanorod CoSb3 compound obtained after 0.5 h at 773 K; these show that the rods are single crystals with a cubic structure. The SAED patterns can be indexed as a cubic structure with cell parameter a ) 0.9087 nm, which is close to the result obtained by XRD. At the same time, it can be seen that the direction of electron diffraction is parallel to the axial zone [001] of the CoSb3 crystal and that nanorod growth occurs along the [110] direction. From the experiments performed on the nanorod, we can suggest a possible growth process for CoSb3 nanorods as shown
Crystal Growth & Design, Vol. 8, No. 1, 2008 209
Figure 3. TEM and SAED patterns of the nanorod along [001] direction.
Figure 4. Distribution of atoms in (A) CoSb3 crystal cell, (B) (010) j face) simulated by MS 3.0 software face, (C) (100) face, and (D) (110) and a possible growth process of CoSb3 nanorods.
in Figure 4. In this model, the tiny CoSb3 compound particle j (Figure has three main growth faces (010), (100), and (110) 4a). The distribution of atoms in the CoSb3 crystal cell and the three crystal faces was simulated by Material Studio (MS) 3.0 software, as shown in Figure 4A-D. From the simulated patterns, it can be observed that both the (010) and (100) faces j face is composed of only consist of Sb atoms, while the (110) Co atoms. As mentioned in the Experimental Procedures, the complexing agent is citric acid, C6H8O7, and the atmosphere for the heat treatment is hydrogen, so citric acid, C6H8O7, might be decomposed to CO when the atmosphere is oxygen-deficient during the heat treatment process. Before being combined to form CoSb3, the Co atoms obtained by H2 have a low valence (zero valence) and high activity, leading to formation of the carbonyl coordination compound Co2(CO)819–20 between Co atoms and CO (Figure 4b). The carbonyl coordination compound forms due to the structure of the Co atoms and the CO. The valence electron structure of a Co atom is 3d74s2, and it has an unfilled d electron orbit. CO not only can provide an electron for forming a σ coordinate bond but also can accept an electron from the Co atom forming a π bond. Both functions reinforce each other and produce a σ˜-π synergistic effect, which strengthens the stability of the coordination compound. The formation of the coordination compound reduces the reactivity j crystal of the Co atom, and thus the growth rate of the (110) face composed of Co atoms decreases (Figure 4c). On the other
210 Crystal Growth & Design, Vol. 8, No. 1, 2008
hand, the valence electron structure of the Sb atom obtained by H2 is 5s25p3, and this cannot be satisfied by forming carbonyl coordination compounds, that is, the formed body must have an unfilled valence orbit. Therefore, the reactivity of Sb atom and the growth rate of the (010) and (100) crystal faces cannot decrease due to formation of coordination compounds. The rodlike CoSb3 compound is then produced because of the growth rate differences between the crystal faces (Figure 4d). From Figure 2, it can be seen that the morphologies of the CoSb3 compounds obtained after 0.5 h at 773 K are not only rodlike but also spherical. This is because the citric acid content and the CO content resulting from its decomposition are finite in the course of the synthesis of CoSb3 by ethanol sol–gel. In the local spaces without CO, the Co atoms obtained by the H2 reduction cannot combine with CO and then the CoSb3 nucleus becomes spherical because the growth rates of the crystal faces do not change due to the formation of coordination compounds. The microstructure of a single-phase CoSb3 compound composed of spherical and rodlike components synthesized by ethanol sol–gel may significantly affect the TE transport properties. It will be investigated in our future expert. Conclusions In this paper, the synthesis and growth of rodlike and spherical nanostructures CoSb3 compounds through an ethanol sol–gel method have been investigated. The following conclusions can be made. (1) The initial reagents were CoCl2 · 6H2O and SbCl3, with an ethanol solvent and a citric acid complexing agent. The phase compositions of samples with identical C6H8O7/Co (Sb/ Co) ratio reduced at 773 K for 0.5 h under a flow of pure hydrogen changed from Co-rich phases to Sb-rich phases with an increasing Sb/Co (C6H8O7/Co) ratio. Only at Sb/Co ) 6 and C6H8O7/Co ) 4 did the powder obtained consist only of rodlike and spherical CoSb3 compounds. (2) TEM and SAED patterns indicate that the crystalline morphology of CoSb3 is composed of spheres about 50 nm in diameter and nanorods about 30 nm in diameter and 100-200 nm in length, and nanorod growth occurs along the [110] direction. The rodlike CoSb3 compound is produced due to the difference in the growth rate between crystal faces. The formation of the carbonyl coordination compound Co2(CO)8 between the Co atoms reduced by H2 and the CO decomposed by citric acid causes the difference in
Chu et al.
growth rate. In the local spaces without CO, the CoSb3 nuclei become spherical. Acknowledgment. This work was supported by a major International Cooperation Program of the NSF of China (Grant No. 50310353) and by a 973 Program (No. 2007CB607501).
References (1) Feldman, J. L.; Sing, D. J.; Mazin, L. L.; Mandrus, D.; Sales, B. C. Phys. ReV. 2000, B61, 9209–9212. (2) Sales, B. C. MRS Bull. 1998, 23, 15–21. (3) Nolas, G. S.; Morelli, D. T.; Tritt, T. M. Annu. ReV. Mater. Sci. 1999, 29, 89–92. (4) Sale, B. C.; Mandrus, D.; Willams, R. K. Science 1996, 272, 1325– 1327. (5) Nolas, G. S.; Takizawa, H.; Endo, T.; Sellinschegg, H.; Johnson, D. C. Appl. Phys. Lett. 2000, 77, 52–54. (6) Chen, L. D.; Kawahara, T.; Tang, X. F.; Goto, T.; Hirai, T.; Dyck, J. S.; Hen, W.; Uher, C. J. Appl. Phys. 2001, 90, 1864–1867. (7) Puyet, M.; Lenoir, B.; Dauscher, A.; Dehmas, M.; Stiewe, C.; Muller, E. J. Appl. Phys 2004, 95, 4852–4855. (8) Tang, X. F.; Chen, L. D.; Goto, T.; Hirai, T.; Yuan, R. Z. J. Mater. Res. 2002, 17, 2953–2959. (9) He, T.; Chen, J. Z.; Rosenfeld, H. D.; Subramanian, M. A. Chem. Mater. 2006, 18, 759–762. (10) Zhang, W.; Shi, X.; Mei, Z. G.; Xu, Y.; Chen, L. D.; Yang, J.; Meisner, G. P. Appl. Phys. Lett. 2006, 89, 1–3. (11) Yu, B. L.; Tang, X. F.; Zhang, Q. j.; Liu, T. X.; Luo, P. F.; Wang, J. 22nd Int. Conf. Thermoelectr. 2003, 101, 104. (12) Wang, M.; Zhang, Y.; Muhammed, M. Nanostruct. Mater. 1999, 12, 237–240. (13) Muhammet, S. T.; Stiewe, C.; Platzek, D.; Williams, S.; Bertini, L.; Muller, E.; Gatti, C.; Zhang, Y.; Rowe, M.; Muhammed, M. AdV. Funct. Mater. 2004, 14, 1189–1196. (14) Bertini, L.; Stiewe, C.; Toprak, M.; Williams, S.; Platzek, D.; Mrotzec, A.; Zhang, Y.; Gatti, C.; Müller, E.; Muhammed, M.; Rowe, M. J. Appl. Phys. 2003, 93, 438–447. (15) Christensen, M.; Iversen, B. B.; Bertini, L.; Gatti, C.; Toprak, M.; Muhammed, M.; Nishibori, E. J. Appl. Phys 2004, 96, 3148–3157. (16) Liu, H.; Gui, H. M.; Han, F.; Li, X.; Wang, J. Y.; Boughton, R. I. Cryst. Growth Des. 2005, 5, 1711–1714. (17) Ohta, H.; Kim, S. W.; Ohta, S.; Koumoto, K.; Hirano, M.; Hosono, H. Cryst. Growth Des. 2005, 5, 25–28. (18) Purkayastha, A.; Kim, S.; Gandhi, D. D.; Ganesan, P. G.; BorcaTasciuc, T.; Ramanath, G. AdV. Mater. 2006, 18, 2958–2963. (19) Odedra, A.; Wu, C. J.; Madhushaw, R. J.; Wang, S. L.; Liu, R. S. J. Am. Chem. Soc. 2003, 125, 9610–9611. (20) Kluge, O.; Finger, M.; Reinhold, J. Inorg. Chem. 2005, 44, 6494–6496.
CG060924J
