CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 5 2057–2060
Articles Controlled Solvothermal Synthesis and Structural Characterization of Antimony Telluride Nanoforks Shufeng Shi,† Minhua Cao,*,† and Changwen Hu*,‡ Department of Chemistry, Northeast Normal UniVersity, Changchun, Jilin, 130024, P. R. China, and Department of Chemistry, Beijing Institute of Technology, Beijing, 100081, P. R. China ReceiVed May 24, 2008; ReVised Manuscript ReceiVed February 10, 2009
ABSTRACT: High-quality Sb2Te3 nanoforks have been first successfully prepared by a simple wet chemical method under solvothermal conditions. Here, ethylene glycol has been used not only as a solvent, but also as a reducing agent. The nitric acid used has been proven to play an important role in the formation of such a unique shape. The shape and size of Sb2Te3 nanoforks can be well controlled by adjusting the experiment parameters. X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were used to characterize the as-synthesized samples. Introduction Nanostructures, such as nanorods, nanowires, and nanotubes, have received much attention because of their unique and interesting properties derived from their dimensional anisotropy and potential applications as interconnectors and functional building blocks for nanodevices.1 The binary chalcogenides of V-VI (A2VB3VI) are particularly important because of their applications in thermoelectric and optoelectronic devices.2 Moreover, they have also been found to be applied in other many fields, such as IR spectroscopy, paints, photoemitting diodes, and microware switches, etc.3 Both theoretical predictions and experimental results suggested that the nanostructured thermoelectric materials with a large thermoelectric figure of merit (ZT) could achieve high efficienvy solid-state thermoelectric energy conversion.4 Nanostructuring the thermoelectric materials can be considered as a successful strategy to gain a factorial enhancement in ZT due to both a high density of state and an increased phonon scattering or reduced lattic thermal conductivity in nanosystems.5 Therefore, more recently, there has been growing interest in synthesizing nanostructured thermoelectric materials by various methods, especially wet chemical methods. It is well-known that antimony telluride is a kind of thermoelectric material. It has attracted considerable fundamental and technological interest for decades because of its potential applications in minipower-generation systems and microcoolers, charge couple device (CCD) technology, and infrared detectors.6 Sb2Te3, with a narrow band gap, is a layered semiconductor * To whom correspondence should be addressed. E-mail: caomh043@ nenu.edu.cn. † Northeast Normal University. ‡ Beijing Institute of Technology.
with a tetradymite structure. It intrinsically possesses a high ZT due to its large Seebeck coefficient. Seebeck coefficient as high as 185 µV K-1 has been reported by Zhou before.7a It has been addressed that at a ZT of about 3, thermoelectric refrigerators or power generators can become competitive with conventional ones.7b This compound and its doped derivatives have the promising potential for near room-temperature thermoelectric material applications. For example, Venkatasubramanian reported that constructured Bi2Te3/Sb2Te3 superlattices exhibit a ZT of ∼2.4 at room temperature.1 Nanostructured Sb2Te3 with different sizes and shapes were originally synthesized mainly by the most popular metallorganic chemical vapor deposition,8 flash evaporation,9 or electrodeposition onto an indium tin oxide (ITO) substrate.10 However, these methods often suffer from the toxicity and high cost of organmetallic precursors or the removal of porous anodic alumina membrane (PAAM) templates. Recently, several green synthetic routes using stable inorganic compounds as precursors (for example, oxides and chlorides) have been developed in organic solvent systems11-14 (such as noncoordinating solvents) or a water-toluene two-phase system15 to get Sb2Te3 nanocrystals with a comparable quality, avoiding the use of extreme experimental conditions such as a glovebox or an inert atmosphere. In addition, most of the above investigations also used additional inorganic reducing agents, such as NaBH4, N2H4, and so on.16 Thus, it is important, but a challenge, to develop a simple route to prepare Sb2Te3 nanostructures. In this paper, we first successfully synthesized Sb2Te3 nanoforks via a surfactant-assisted solvothermal route. Here, ethylene glycol (EG) was used not only as a solvent but also as a reducing agent.
10.1021/cg800551u CCC: $40.75 2009 American Chemical Society Published on Web 03/13/2009
2058
Crystal Growth & Design, Vol. 9, No. 5, 2009
Shi et al.
Figure 1. The XRD pattern of the Sb2Te3 nanoforks.
Experimental Section Synthesis. All chemicals used were purchased without further purification. A typical experiment for Sb2Te3 nanoforks was conducted as follows: 0.0175 g of Sb2O3 was dissolved in 2 mL of HCl (36%) and a SbCl3 solution was obtained. Then, the SbCl3 solution, 0.0115 g of Te powder, 0.6 g of polyethylene glycol (PEG-20000), and 2 mL of HNO3 aqueous solution (5 M) were added into 60 mL of EG in a 80 mL Teflon-lined stainless steel autoclave under stirring conditions. The autoclave was maintained at 150 °C for 36 h and then cooled to room temperature in the air. The resulting solid product was collected and washed with deionized water and ethanol, and finally dried at room temperature for 12 h. Characterization. The resulting sample was characterized by X-ray powder diffraction (XRD) (Rigaku D-max-rA XRD with Cu-K radiation). The morphology and size of the samples were observed by field emission scanning electron microscopy (FE-SEM, JEOL 7500B), transmission electron microscopy (TEM, H-800), and high-resolution transmission electron microscopy (JEM-1010). X-ray photoelecton spectroscopy (XPS, ESCALAB 250) was used to confirm the oxidation state of the sample.
Results and Discussion The X-ray powder diffraction (XRD) pattern of the asprepared sample is shown in Figure 1. Those diffraction peaks marked with circles can be indexed to Sb0.405Te0.595 structure (JCPDS No. 45-1229). The Sb0.405Te0.595 structure could be ascribed to Sb2Te3 because its atom ratio of Sb/Te is 2:3. Those peaks marked with asterisks are possibly from Te-O compound. To further confirm the composition of the nanoforks, the surface analysis of the nanoforks was carried out by using X-ray photoelectron spectroscopy (XPS). The XPS data were collected in the constant analyzer energy mode at 20 eV. Mg KR (hγ ) 1253.6 eV) radiation was employed as the excitation source with an anode voltage of 15 kV and an emission current of 20 mA. The high-resolution spectra of the Sb3d and Te3d are obtained using C1s as the reference at 284.6 eV. Figure 2a shows a full/ wide scan spectrum of the sample. The high-resolution spectrum of Sb3d in Figure 2b shows a peak for Sb3d5/2 (529 eV). It is worth mentioning that this peak is a characteristic of the Sb3+ valence state, confirming that antimony in the sample is not in the +5 valence state. The Te3d5/2 and Te3d3/2 peaks are observed at 571.4 and 581.8 eV (Figure 2c), respectively, which was reported for Sb2Te3 in the literature.16 Additional peaks (574.4 and 584.9 eV) for Te3d, indicating a chemical environment different from Sb-Te, are ascribed to Te-O. The values are comparable to literature reports (576 (3d5/2) and 586 (3d3/2) eV).17 The presence of the above two peaks indicates the part oxidation of the nanoforks, which took place after exposure to the atmosphere. This result is in agreement with that from the XRD pattern.
Figure 2. XPS spectra of the product: (a) wide XPS spectrum, (b) highresolution spectrum of Sb3d, (c) high-resolution spectrum of Te3d.
The morphology of the sample was studied by field-emissiom scanning electron microscopy (FE-SEM). As shown in Figure 3a, the sample completely consists of uniform nanoforks, that is, two parallel rods with a connection in the middle of the rods. This kind of morphology is scarely reported in the literature.18 The connection has a length of about 500-800 nm. The parallel rods have a diameter of about 80-100 nm, and the length of the whole nanofork is up to 2.5-3 µm. A transmission electron microscopy (TEM) image further confirms the nanofork morphology (Figure 3b). A higher magnification TEM as shown in Figure 3c reveals that the surface of the nanoforks is rough. Figure 3d is a high-resolution TEM (HRTEM) image of the marked part in Figure 3c. HRTEM images recorded anywhere on the fork all show a perfect single-crystalline structure. The hexagonal diffraction spots in the fast Fourier transform (FFT) of the same lattice image further confirms the single crystalline nature and rhombohedral structure of the nanoforks. The influence of various experimental conditions on the morphology of the final product was investigated in detail and it was found that the nitric acid used was essential for the
Synthesis of Antimony Telluride Nanoforks
Crystal Growth & Design, Vol. 9, No. 5, 2009 2059
Figure 3. The images of the product: (a) the FE-SEM image, (b) the TEM image, (c) the high magnification TEM image, (d) HRTEM image, the inset is the corresponding FFT pattern.
Figure 5. TEM images of samples obtained with different reaction times: (a) 6 h, (b) 12 h, (c) 24 h, (d) 48 h.
Figure 6. Proposed growth mechanism for the formation of Sb2Te3 nanoforks.
2 Sb3++ 3Te2-)Sb2Te3
Figure 4. TEM images of the products after adding different quantities of nitric acid: (a) 0 mL, (b) 1.5 mL, (c) 2.5 mL, (d) 3 mL.
development of the nanofork structure. We changed the volume of HNO3 aqueous solution (5 M) to study its effect on the morphology of the forks, from 0 to 3 mL. If no nitric acid was added in the reaction system, only irregular nanorods were obtained as shown in Figure 4a. When 1.5 mL of the above nitric acid aqueous solution was used, nanoforks were formed. But the length of the nanoforks is short, as shown in Figure 4b. With the increase of the acid volume added (2.0-2.5 mL), uniform and long nanoforks were formed (Figures 3b and 4c). However, when excessive acid (>3 mL) was used, stubby nanoforks were obtained again. It is well-known that SbCl3 is very easy to hydrolyze in an aqueous solution. Therefore, in our synthesis, an organic solvent (EG) was chosen as a reaction system to stabilize SbCl3 effectively. On the other hand, EG exhibits reducibility,19 and it also served as a reducing agent for reducing Te into Te2-. The formation process of Sb2Te3 could be expressed by the following reactions:
Te + 2e ) Te2-
(1)
(2)
To understand whether the addition of PEG was necessary for the formation of nanoforks, we carried out an experiement in the absence of PEG. Similar nanofork structures have also been obtained, but the length of the nanoforks is shorter than that for the case in the presence of PEG (TEM image is not shown here). Acording to the above investigation, it can be deduced that the nitric acid used plays an important role, although the exact mechanism for the formation of the nanofork structures now is not clear. To further understand the growth mechanism of the nanoforks, the experiments with different reaction times were carried out to reveal the whole evolution process of the nanofork structures. As shown in Figure 5a, Sb2Te3 nanorods with a length of 800 nm were obtained when the reaction time was 6 h. Sb2Te3 nanoforks with the whole length of ca.1 µm appeared at 12 h (Figure 5b). After 24 h, the length of the nanoforks increased to 1.2-1.5 µm (Figure 5c). The well-defined nanoforks with ca. 2.5-3 µm long and sharp regular tips were formed at 36 h (Figure 3b). From the above results we can see that the increase of reaction time from 6 to 36 h induces the formation of a well-defined forklike morphology. However, when the reaction time was increased to 48 h, stubby nanoforks were formed again (Figure 5d). Therefore, on the basis of above experimental results and analysis, a feasible formation mechanism of this kind of nanostructures was proposed, as shown in Figure 6. At the initial stage of reaction, Te powder might be dissolved in nitric acid,20 and Te2- was produced from the reduction of Te powder by the EG. Then, tiny Sb2Te3 crystal nuclei formed through homogeneous nucleation. Further growth of these crystal nuclei resulted in the formation of shorter nanorods, which can be confirmed by Figure 5a. With the increase of the reaction time, the nitric acid existing in the solution began to corrode the nanorods along some preferred direction from the two tips of the nanorods. The main reason for the corrosion may be due to the relatively higher free energies of those crystal faces along
2060
Crystal Growth & Design, Vol. 9, No. 5, 2009
the preferred direction.21 When the reaction was prolonged, the corrosion further increased, resulting in the formation of nanoforks. Conclusions Single crystalline Sb2Te3 nanoforks have been successfully prepared in the EG system under solvothermal conditions. The fork-like nanostructures exhibit a special morphology in the nanomaterial research field. The EG solvent is used not only as a solvent, but also as a reductant. This simple synthesis route could be extended to prepare other semiconductor materials with interesting properties. Optimization of the thermoelectric properties through assembly or doping of the Sb2Te3 nanoforks may lead to novel thermoelectric materials and devices for future applications. Acknowledgment. This work was supported by the Natural Science Fund Council of China (NSFC, Nos. 20771022, 20671011, 20771022, and 20871016), the 111 Project (B07012), Key Laboratory of Structural Chemistry Foundation (KLSCF, No. 060017), the Huo Yingdong Foundation for financial support, and analysis and testing foundation of Northeast Normal University.
References (1) Alivisatos, A. P. Science 1996, 271, 933. (2) (a) Tritt, T. M. Science 1999, 283, 804. (b) Venkantasubramanian, R.; Siilvola, E.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597. (3) (a) Arivuoli, D.; Gnanam, F. D.; Ramasamy, P. J. Mater. Sci. Lett. 1988, 7, 711. (b) Abrinov, N. K.; Bankina, V. F.; Poretakaya, L. V.; Shelimova, L. E.; Skudnova, E. V. In Semiconducting II-VI and V-VI Compounds; Tybulewicz, A., Ed.; Plenum: New York, 1969, 186. (4) (a) Hicks, L. D.; Dresselhaus, M. S. Phys. ReV. B 1993, 47, 12727.
Shi et al. (b) Hicks, L. D.; Dresselhaus, M. S. Phys. ReV. B 1993, 47, 16631. (5) (a) Harman, T. C.; Taylor, P. J.; Spears, D. L.; Walsh, M. P. Electron. Mater. 2000, 29, L1. (b) Hicks, L. D.; Harman, T. C.; Sun, X.; Dresselhaus, M. S. Phys. ReV. B 1996, 53, R10493. (6) (a) Christian, P.; O,Brien, P. J. Mater. Chem. 2005, 15, 4949. (b) ElMandouh, Z. S. J. Mater. Sci. 1995, 30, 1273. (c) Das, V. D.; Soundararajan, N.; Pattabi, M. J. Mater. Sci. 1987, 22, 3522. (7) (a) Zou, H.; Rowe, D. M.; Min, J. J. Vac. Sci. Technol. A 2001, 19, 899. (b) Rowe, D. M., Ed.; CRC Handbooks of Thermoelectrics; CRC Press: Boca Raton, FL, 1995. (8) (a) Garje, S. S.; Eisler, D. J.; Ritch, J. S.; Afzaal, M.; O’Brien, P.; Chivers, T. J. Am. Chem. Soc. 2006, 128, 3120. (b) Giani, A.; Boulouz, A.; Delannoy, F. P.; Foucaran, A.; Boyer, A.; Aboulfarah, B.; Mzerd, A. J. Mater. Sci. Lett. 1999, 18, 541. (9) Patel, T. C.; Patel, P. G. Mater. Lett. 1984, 3, 2. (10) Leimkuhler, G.; Kerkamm, I.; Reinike-Koch, R. J. Electrochem. Soc. 2002, 149, C474. (11) Peng, X. G. AdV. Mater. 2003, 15, 459. (12) Yu, W. W.; Peng, X. G. Angew. Chem., Int. Ed. 2001, 41, 2368. (13) Joo, J.; Na, H. B.; Hu, T.; Yu, J. H.; Kim, Y. W.; Wu, F. X.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 11100. (14) Lim, W. P.; Zhang, Z.; Low, H. Y.; Chin, W. S. Angew. Chem., Int. Ed. 2004, 43, 5685. (15) Pan, D. C.; Jiang, S. C.; An, L. J.; Jiang, B. Z. AdV. Mater. 2004, 16, 982. (16) Wang, W. Z.; Poudel, B.; Yang, J.; Wang, D. Z.; Ren, Z. F. J. Am. Chem. Soc. 2005, 127, 13792. (17) Cheng, H. Y.; Jong, C. A.; Chung, R. J.; Chin, T. S.; Huang, R. T. Semicond. Sci. Technol. 2005, 20, 1111. (18) Andra´s, F.; Istva´n, P.; Kla´ra, H.; Imre, K. React. Kinet. Catal. Lett. 2006, 87, 263. (19) Mayers, B.; Xia, Y. AdV. Mater. 2002, 14, 279. (20) Marshall, H.; King, A. J.; Harr, T. Tellurium (IV) Oxide: (Tellurium Dioxide). In Inorganic Syntheses; McGraw-Hill Book Company: New York, 1950. (21) Zhu, W.; Wang, W. Z.; Xu, H. L.; Zhou, L.; Zhang, L. S.; Shi, J. L. Cryst. Growth. Des. 2006, 6, 2804.
CG800551U
