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Growth of Sb2Se3 whiskers via a hydrothermal method
Materials Chemistry and Physics 82 (2003) 546–550
Growth of Sb2 Se3 whiskers via a hydrothermal method Debao Wang, Dabin Yu, Mingwang Shao, Jinyun Xing, Yitai Qian∗ Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 9 January 2003; accepted 14 April 2003
Abstract The growth of crystalline Sb2 Se3 whiskers has been conducted via a hydrothermal process. It was found that the organic additives added to the hydrothermal system had important influence on the growth behavior of Sb2 Se3 whiskers. The structure features of Sb2 Se3 whiskers were characterized by scanning electron microanalyzer and X-ray diffraction (XRD) techniques. The growth mechanism was discussed and the relationship between crystal morphologies and the crystal structure of Sb2 Se3 were discussed. © 2003 Published by Elsevier B.V. Keywords: Chalcogenides; Crystal growth; Chemical synthesis; SEM
1. Introduction The semiconducting V2 VI3 compounds have been studied extensively, mainly because some of these compounds present properties that make them important for technological applications in thermoelectric power conversion and in the fabrication of Hall effect devices [1]. Antimony triselenide, an important member of these V2 VI3 compounds, is a layer-structured semiconductor of orthorhombic crystal structure, and exhibits good photovoltaic properties and high thermoelectric power (TEP) which allow possible applications for optical and thermoelectronic cooling devices [2]. While most of the efforts on Sb2 Se3 chemistry have concentrated on preparing Sb2 Se3 thin films [2–4], Sb2 Se3 nanorods [5] and nanowires [6], information on the structural and physical features of bulk Sb2 Se3 itself is still insufficient to permit a clear interpretation between the growth modes of different morphologies. Only a few studies have focused on the mechanism of Sb2 Se3 crystal growth [7–8]. Classically, Sb2 Se3 single crystal was prepared by melting the elements of antimony and selenium in stoichiometric proportion at high temperature [7–8]. Microtublar single crystals of Sb2 Se3 were grown via a solvothermal process [9]. Thread-like Sb2 Se3 single crystals were grown from the gas phase in evacuated quartz ampuls [10]. The rate of crystallization of Sb2 Se3 crystals in aqueous solutions of NH4 Cl, NH4 Br, H2 Se, and Na2 S was studied via hydrothermal process at 300–470 ◦ C [11–12]. Hydrother∗ Corresponding author. Fax: +86-551-3607402. E-mail address: [email protected] (Y. Qian).
0254-0584/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/S0254-0584(03)00337-7
mal crystallization can offer a number of advantages, for example, the solubility, the diffusion process and the chemical reactivity of the reactants are greatly enhanced, while at the same time, the solvent itself behaves quite differently from what is expected under ambient conditions [13]. In the present work, we demonstrate the effectiveness of hydrothermal method to grow crystalline whiskers of Sb2 Se3 . In order to gain a better understanding of the growth mechanism, morphological studies were undertaken using SEM. In particular, the relationship between growth behaviors and the crystal structure of Sb2 Se3 was revealed.
2. Experimental All reagents were analytical-grade and used without further purification. In a typical procedure, 0.001 mol of SbCl3 was dispersed in concentrated ammonia solution (25–28%) under rapid agitation in the presence of appreciate amount of cetyltrimethylammonium bromide (CTAB) or diethylene glycol (DEG), followed by adding 0.0025 mol of Se powder. The mixture was transferred into a Teflon lined stainless steel container and filled with ammonia up to 90% of its capacity, then sealed and maintained at 190 ◦ C for 48 h. When the reaction was finished, the fiber-like products were collected and washed with water and absolute ethanol, respectively, then dried in a vacuum before characterization. The X-ray powder diffraction (XRD) patterns of the products were recorded by employing a Rigaku (Japan) D/Max r-A X-ray diffractometer with Cu K␣ radiation (λ = 0.154178 nm). Morphology and size information of
D. Wang et al. / Materials Chemistry and Physics 82 (2003) 546–550
Fig. 1. XRD patterns of Sb2 Se3 whiskers prepared in the presence of: (a) 5.0 × 10−3 mol/l CTAB; (b) 0.01 mol/l CTAB; and (c) 10% DEG.
the obtained Sb2 Se3 samples were observed on a Hitachi (X-650) scanning electron microanalyzer (SEM). X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB MKII X-ray photoelectron spectrometer using nonmonochromatized Mg K␣ radiation as the excitation source.
3. Results and discussion XRD patterns of the fiber-like crystals prepared in different conditions are shown in Fig. 1. The main reflection peaks are labeled and can be indexed to the corresponding orthorhombic phase of Sb2 Se3 with cell parameters a = 1.1637 nm, b = 1.1802 nm, and c = 0.3964 nm, which are very close to the data reported in the literature (JCPDS file No. 72-1184). All patterns exhibit abnormally strong intensity of those (h k 0) peaks of the orthorhombic phase Sb2 Se3 . Other relatively strong reflection peaks are absent or becoming extremely weak in these patterns in comparison with
547
the JCPDS data. The above results strongly indicate that the Sb2 Se3 whiskers are preferentially grown along the (0 0 1) direction. The SEM images of the as-prepared Sb2 Se3 samples are shown in Fig. 2. The images reveal that all the products consist of uniform whiskers with different size and morphologies. In the presence of 5.0 × 10−3 mol/l CTAB, sheet-like Sb2 Se3 whiskers with uniform size have been prepared (Fig. 2a). Fig. 2b exhibits the arrays of straight Sb2 Se3 whiskers grown in the presence of 0.01 mol/l CTAB. Most of these structures are 6–12 m in width, 0.6–1.0 m in thickness, and 1–3 mm in length, although some thinner whiskers (about 400 nm in diameter) are also be observed. Careful examinations under high magnification show that these Sb2 Se3 whiskers exhibit a narrow rectangle cross section rather than a circular cross section, which illustrates that Sb2 Se3 crystals prepared under this hydrothermal condition are also mainly sheet-like or belt-like whiskers. Fig. 2c shows wire-like Sb2 Se3 whiskers prepared in the presence of 10% DEG. The whiskers have a mean diameter of 300 nm and length of 1 0–20 m. The X-ray photoelectron spectra were carried out to identify the surface compositions of the as-prepared products. The XPS spectra of Se 3d region and Sb 3d region for Sb2 Se3 were shown in Fig. 3a and b. The values of the binding energy were calibrated using C ls peak (284.6 eV) as the internal standard. The peak at 53.6 eV (Fig. 3a) corresponds to the Se 3d transition. The weak peak at about 60 eV is assigned to oxidized Se species, in this case SeO2 or SeO3 2− . Since the position of Sb 3d5/2 binding energy (529 eV) is superposed with that of the O ls binding energy (530 eV), the peaks of Sb 3d3/2 were used to detect the binding energy of Sb, and it was measured at 539.4 eV by curve fitting (Fig. 3b). These values agree well with those reported in the literature [14]. The quantification of the XPS peaks gives the molecular ra-
Fig. 2. SEM images of Sb2 Se3 whiskers prepared in the presence of: (a) 5.0 × 10−3 mol/l CTAB, (b) 0.01 mol/l CTAB, and (c) 10% DEG.
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D. Wang et al. / Materials Chemistry and Physics 82 (2003) 546–550
Fig. 3. Typical XPS spectra of: (a) Se 3d region and (b) Sb 3d region for Sb2 Se3 .
tio of Sb:Se of 1.0:1.2, and the non-stoichiometric composition may result from the oxidization of selenium. In fact, all of the samples are ready to be oxidized due to exposure to air during the processing of samples. It is known that selenium dispropotionates in alkaline solution to give Se2− ions and these Se2− ions can react with the Sb3+ ions chelated by OH− to form Sb2 Se3 . Thus, the hydrothermal reactions of SbCl3 and Se in ammonia can be expressed as follows: Sb3+ (aq) + 3OH− → Sb(OH)3 −
3Se + 6OH → 2Se
2−
+ SeO3
(1) 2−
+ 3H2 O
2Sb(OH)3 + 3Se2− → Sb2 Se3 + 6OH−
(2) (3)
To get optimized hydrothermal condition of the formation of Sb2 Se3 , controlled experiments under different conditions have been conducted. On the basis of the principles of chemical equilibrium, high concentration of OH− ion, high temperature and the formation of Sb2 Se3 precipitates can promote the dispropotionation of selenium thermodynamically and/or kinetically. Concentrated ammonia and hydrothermal conditions (high pressure and temperature) meet the requirements. If the temperature was lower or the concentrated ammonia was diluted, the dispropotionation of Se would be limited, and the product would be a mixture of Sb2 Se3 whiskers, selenium powder, and antimony oxide, which can be confirmed by XRD. Fig. 1d indicates the appearance of traced element selenium in Sb2 Se3 whiskers (labeled by star, JCPDS file No.6-362) when ammonia was diluted with DEG. Sb2 Se3 is a layer-structured compound, and has an orthorhombic lattice, the unit cell of which is shown in Fig. 4 [7]. Considering the strongest bonds (Sb–Se separations from 0.2576 to 0.2777 nm), the structure consists of infinite chains parallel to the c or whisker axis. Pairs of these chains are fastened together along 21 screw axes to form larger chains through sets of Sb–Se bonds of length 0.298 nm.
These larger chains are, in turn, bonded into sheets roughly perpendicular to the b-axis through sets of Sb–Se bonds, which are 0.326 nm in length. Finally, the sheets are held together to make the crystalline solid through two sets of Sb–Se ‘bonds’, which are 0.346 and 0.374 nm long respectively. It is possible to think of this structure as made up of puckered sheets or planes of stoichiomtric composition running parallel to the c-axis, and more or less in the (0 1 0) directions. The binding between these sheets is considerably weaker than that within the sheets. This suggests that cleavage may take place more or less on (0 1 0) planes by breaking two sets of Sb–Se bonds of length 0.346 and 0.374 nm, and preferential growth in the (0 0 1) direction, which result in the formation of Sb2 Se3 whiskers with narrow rectangle cross section (sheet-like or belt-like whiskers). Considering set of bonds (0.326 nm) between
Fig. 4. Unit cell of Sb2 Se3 projected on (0 0 1).
D. Wang et al. / Materials Chemistry and Physics 82 (2003) 546–550
549
Fig. 5. SEM images of Sb2 Se3 whiskers: (a) cross section of a thick “whisker” shown in Fig. 2a–c the splitting of thinner whisker from the thick one, and (d) in the absence of organic additives.
Sb2 Se3 chins in one Sb2 Se3 sheet are longer than those within the chains, further cleavage may occur within these sheets, and thinner wire-like whiskers will be the result. This cleavage mechanism is supported by the SBM observation. Fig. 5a gives a cross section of one thicker Sb2 Se3 whisker shown in Fig. 2b. It can be seen clearly that it consists of a bundle of thinner whiskers each having a narrow rectangle cross section, which confirms that the thinner belt-like Sb2 Se3 whiskers come from the exfoliation of this kind of large whiskers. Fig. 5b shows the micrograph of the splitting of one thinner wire-like whisker from the sheet-like one (shown by arrow). In fact, more or less thinner wire-like whiskers were observed from SEM observation in Fig. 2a–c. More over, the formation of wire-like whiskers with a triangle cross section (as shown in Fig. 5c by arrow) can also be explained using this exfoliation mechanism considering the cleavages do not occur strictly perpendicular to (0 1 0) directions. This mechanism may be applied to explain the formation of one-dimensional crystals of other V2 V13 compounds with similar structures, such as nonorods, nanowires,
nanobelts, or micro-sized structures of these compounds. In fact the exfoliation process in polyol process had been reported for layer-structured Bi2 Se3 [15]. The presence of organic additives has much influence on the growth behavior of Sb2 Se3 . As stated in literature [16], the use of organic molecules, even in very small amounts, could help control the microstructures of inorganic solid products. The present study shows that organic additives favor the cleavage of Sb2 Se3 crystals, and the crystal morphology is well modified. It has been reported that CTAB [17] and DEG [18] can adhere or complex the surface of the particles and then can modify the surface of the products. After Sb2 Se3 nuclei formed in the hydrothermal process, the adsorption and de-adsorption of CTAB or DEG molecules on different surfaces of the nuclei may accelerate the crystal growth of Sb2 Se3 in (0 0 1) direction, which leads to the formation of uniform and long Sb2 Se3 whiskers. To verify it, further experiments were carried out. When other experimental conditions remained stable, but the organic additives were absent, the products were mainly sheet-like crystals
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D. Wang et al. / Materials Chemistry and Physics 82 (2003) 546–550
with no uniform size and low aspect ratio (Fig. 5d). If more DEG were added to the system, the dispropotionation of Se would be limited, and a reducing agent would be necessary to reduce selenium powder and Sb2 Se3 nanowires were obtained [6].
4. Conclusion In summary, Sb2 Se3 whiskers of 1–3 mm in length were prepared using hydrothermal method in ammonia solution. The formation of the belt-like and wire-like whiskers was thought to be the results of different exfoliation behaviors of Sb2 Se3 crystals. The as-prepared Sb2 Se3 whiskers might make it possible to measure the photovoltaic and thermoelectric properties of single Sb2 Se3 whisker and might find potential applications in construction of miniaturized optoelectric or thermoelectric device. Further researches on the size and morphology control of Sb2 Se3 whiskers in hydrothermal conditions were underway.
Acknowledgements Financial supports from National Natural Science Found of China and the 973 Projects of China are appreciated.
References [1] V.B. Nascimento, V.E. de Carvalho, R. Paniago, E.A. Soares, L.O. Ladeira, H.D. Pfannes, J. Electron Spectrosc. 104 (1999) 99. [2] K.Y. Rajpure, C.D. Lokhande, C.H. Bhosale, Mater. Res. Bull. 34 (1999) 1079, and references therein. [3] H.A. Zayed, A.M. Abo-Elsoud, A.M. Ibrahim, M.A. Kenawy, Thin Solid Films 247 (1994) 94. [4] A.P. Torane, C.H. Bhosale, J. Phys. Chem. Solids 63 (2002) 1849, and references therein. [5] J. Wang, Z. Deng, Y. Li, Mater. Res. Bull. 37 (2002) 495. [6] D. Wang, D. Yu, M. Shao, W. Yu, Y. Qian, Chem. Lett. (2002) 1. [7] N.W. Tideswell, F.H. Kruse, J.D. McCullouch, Acta Cryst. 10 (1957) 99. [8] D. Arivuoli, F.D. Gnanam, P. Ramasamy, J. Mater. Sci. Lett. 7 (1988) 711. [9] X. Zheng, Y. Xie, L. Zhu, X. Jiang, W. Song, Y. Sun, Inorg. Chem. 41 (2002) 455. [10] CA, 70: 41544u. [11] CA, 78: 49091e. [12] CA,110: 48745p. [13] A. Rabenau, Angew. Chem. Int. Ed. Engl. 24 (1985) 1026. [14] G.E. Muilenberg, C.D. Wager, W.M. Riggs, L.E. Davis, J.F. Moulder (Eds.), Handbook of X-Ray Photoelectron Spectroscopy, Perking-Elmer Corporation, New York, 1979. ˘ [15] M.I. Comor, M.D. Drami˘canin, Z. Rakoˆcevi´c, S. Zec, J.M. Nedeljkovi´c, J. Mater. Sci. Lett. 172 (1998) 1402. [16] S.I. Stupp, P.V. Braun, Science 277 (1997) 1242. [17] X.M. Sun, X. Chen, Z.X. Deng, Y.D. Li, Mater. Chem. Phys. 78 (2002) 99. [18] C. Feldmann, C. Metzmacher, J. Mater. Chem. 11 (2001) 2603.
Growth of Sb2 Se3 whiskers via a hydrothermal method Debao Wang, Dabin Yu, Mingwang Shao, Jinyun Xing, Yitai Qian∗ Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 9 January 2003; accepted 14 April 2003
Abstract The growth of crystalline Sb2 Se3 whiskers has been conducted via a hydrothermal process. It was found that the organic additives added to the hydrothermal system had important influence on the growth behavior of Sb2 Se3 whiskers. The structure features of Sb2 Se3 whiskers were characterized by scanning electron microanalyzer and X-ray diffraction (XRD) techniques. The growth mechanism was discussed and the relationship between crystal morphologies and the crystal structure of Sb2 Se3 were discussed. © 2003 Published by Elsevier B.V. Keywords: Chalcogenides; Crystal growth; Chemical synthesis; SEM
1. Introduction The semiconducting V2 VI3 compounds have been studied extensively, mainly because some of these compounds present properties that make them important for technological applications in thermoelectric power conversion and in the fabrication of Hall effect devices [1]. Antimony triselenide, an important member of these V2 VI3 compounds, is a layer-structured semiconductor of orthorhombic crystal structure, and exhibits good photovoltaic properties and high thermoelectric power (TEP) which allow possible applications for optical and thermoelectronic cooling devices [2]. While most of the efforts on Sb2 Se3 chemistry have concentrated on preparing Sb2 Se3 thin films [2–4], Sb2 Se3 nanorods [5] and nanowires [6], information on the structural and physical features of bulk Sb2 Se3 itself is still insufficient to permit a clear interpretation between the growth modes of different morphologies. Only a few studies have focused on the mechanism of Sb2 Se3 crystal growth [7–8]. Classically, Sb2 Se3 single crystal was prepared by melting the elements of antimony and selenium in stoichiometric proportion at high temperature [7–8]. Microtublar single crystals of Sb2 Se3 were grown via a solvothermal process [9]. Thread-like Sb2 Se3 single crystals were grown from the gas phase in evacuated quartz ampuls [10]. The rate of crystallization of Sb2 Se3 crystals in aqueous solutions of NH4 Cl, NH4 Br, H2 Se, and Na2 S was studied via hydrothermal process at 300–470 ◦ C [11–12]. Hydrother∗ Corresponding author. Fax: +86-551-3607402. E-mail address: [email protected] (Y. Qian).
0254-0584/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/S0254-0584(03)00337-7
mal crystallization can offer a number of advantages, for example, the solubility, the diffusion process and the chemical reactivity of the reactants are greatly enhanced, while at the same time, the solvent itself behaves quite differently from what is expected under ambient conditions [13]. In the present work, we demonstrate the effectiveness of hydrothermal method to grow crystalline whiskers of Sb2 Se3 . In order to gain a better understanding of the growth mechanism, morphological studies were undertaken using SEM. In particular, the relationship between growth behaviors and the crystal structure of Sb2 Se3 was revealed.
2. Experimental All reagents were analytical-grade and used without further purification. In a typical procedure, 0.001 mol of SbCl3 was dispersed in concentrated ammonia solution (25–28%) under rapid agitation in the presence of appreciate amount of cetyltrimethylammonium bromide (CTAB) or diethylene glycol (DEG), followed by adding 0.0025 mol of Se powder. The mixture was transferred into a Teflon lined stainless steel container and filled with ammonia up to 90% of its capacity, then sealed and maintained at 190 ◦ C for 48 h. When the reaction was finished, the fiber-like products were collected and washed with water and absolute ethanol, respectively, then dried in a vacuum before characterization. The X-ray powder diffraction (XRD) patterns of the products were recorded by employing a Rigaku (Japan) D/Max r-A X-ray diffractometer with Cu K␣ radiation (λ = 0.154178 nm). Morphology and size information of
D. Wang et al. / Materials Chemistry and Physics 82 (2003) 546–550
Fig. 1. XRD patterns of Sb2 Se3 whiskers prepared in the presence of: (a) 5.0 × 10−3 mol/l CTAB; (b) 0.01 mol/l CTAB; and (c) 10% DEG.
the obtained Sb2 Se3 samples were observed on a Hitachi (X-650) scanning electron microanalyzer (SEM). X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB MKII X-ray photoelectron spectrometer using nonmonochromatized Mg K␣ radiation as the excitation source.
3. Results and discussion XRD patterns of the fiber-like crystals prepared in different conditions are shown in Fig. 1. The main reflection peaks are labeled and can be indexed to the corresponding orthorhombic phase of Sb2 Se3 with cell parameters a = 1.1637 nm, b = 1.1802 nm, and c = 0.3964 nm, which are very close to the data reported in the literature (JCPDS file No. 72-1184). All patterns exhibit abnormally strong intensity of those (h k 0) peaks of the orthorhombic phase Sb2 Se3 . Other relatively strong reflection peaks are absent or becoming extremely weak in these patterns in comparison with
547
the JCPDS data. The above results strongly indicate that the Sb2 Se3 whiskers are preferentially grown along the (0 0 1) direction. The SEM images of the as-prepared Sb2 Se3 samples are shown in Fig. 2. The images reveal that all the products consist of uniform whiskers with different size and morphologies. In the presence of 5.0 × 10−3 mol/l CTAB, sheet-like Sb2 Se3 whiskers with uniform size have been prepared (Fig. 2a). Fig. 2b exhibits the arrays of straight Sb2 Se3 whiskers grown in the presence of 0.01 mol/l CTAB. Most of these structures are 6–12 m in width, 0.6–1.0 m in thickness, and 1–3 mm in length, although some thinner whiskers (about 400 nm in diameter) are also be observed. Careful examinations under high magnification show that these Sb2 Se3 whiskers exhibit a narrow rectangle cross section rather than a circular cross section, which illustrates that Sb2 Se3 crystals prepared under this hydrothermal condition are also mainly sheet-like or belt-like whiskers. Fig. 2c shows wire-like Sb2 Se3 whiskers prepared in the presence of 10% DEG. The whiskers have a mean diameter of 300 nm and length of 1 0–20 m. The X-ray photoelectron spectra were carried out to identify the surface compositions of the as-prepared products. The XPS spectra of Se 3d region and Sb 3d region for Sb2 Se3 were shown in Fig. 3a and b. The values of the binding energy were calibrated using C ls peak (284.6 eV) as the internal standard. The peak at 53.6 eV (Fig. 3a) corresponds to the Se 3d transition. The weak peak at about 60 eV is assigned to oxidized Se species, in this case SeO2 or SeO3 2− . Since the position of Sb 3d5/2 binding energy (529 eV) is superposed with that of the O ls binding energy (530 eV), the peaks of Sb 3d3/2 were used to detect the binding energy of Sb, and it was measured at 539.4 eV by curve fitting (Fig. 3b). These values agree well with those reported in the literature [14]. The quantification of the XPS peaks gives the molecular ra-
Fig. 2. SEM images of Sb2 Se3 whiskers prepared in the presence of: (a) 5.0 × 10−3 mol/l CTAB, (b) 0.01 mol/l CTAB, and (c) 10% DEG.
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D. Wang et al. / Materials Chemistry and Physics 82 (2003) 546–550
Fig. 3. Typical XPS spectra of: (a) Se 3d region and (b) Sb 3d region for Sb2 Se3 .
tio of Sb:Se of 1.0:1.2, and the non-stoichiometric composition may result from the oxidization of selenium. In fact, all of the samples are ready to be oxidized due to exposure to air during the processing of samples. It is known that selenium dispropotionates in alkaline solution to give Se2− ions and these Se2− ions can react with the Sb3+ ions chelated by OH− to form Sb2 Se3 . Thus, the hydrothermal reactions of SbCl3 and Se in ammonia can be expressed as follows: Sb3+ (aq) + 3OH− → Sb(OH)3 −
3Se + 6OH → 2Se
2−
+ SeO3
(1) 2−
+ 3H2 O
2Sb(OH)3 + 3Se2− → Sb2 Se3 + 6OH−
(2) (3)
To get optimized hydrothermal condition of the formation of Sb2 Se3 , controlled experiments under different conditions have been conducted. On the basis of the principles of chemical equilibrium, high concentration of OH− ion, high temperature and the formation of Sb2 Se3 precipitates can promote the dispropotionation of selenium thermodynamically and/or kinetically. Concentrated ammonia and hydrothermal conditions (high pressure and temperature) meet the requirements. If the temperature was lower or the concentrated ammonia was diluted, the dispropotionation of Se would be limited, and the product would be a mixture of Sb2 Se3 whiskers, selenium powder, and antimony oxide, which can be confirmed by XRD. Fig. 1d indicates the appearance of traced element selenium in Sb2 Se3 whiskers (labeled by star, JCPDS file No.6-362) when ammonia was diluted with DEG. Sb2 Se3 is a layer-structured compound, and has an orthorhombic lattice, the unit cell of which is shown in Fig. 4 [7]. Considering the strongest bonds (Sb–Se separations from 0.2576 to 0.2777 nm), the structure consists of infinite chains parallel to the c or whisker axis. Pairs of these chains are fastened together along 21 screw axes to form larger chains through sets of Sb–Se bonds of length 0.298 nm.
These larger chains are, in turn, bonded into sheets roughly perpendicular to the b-axis through sets of Sb–Se bonds, which are 0.326 nm in length. Finally, the sheets are held together to make the crystalline solid through two sets of Sb–Se ‘bonds’, which are 0.346 and 0.374 nm long respectively. It is possible to think of this structure as made up of puckered sheets or planes of stoichiomtric composition running parallel to the c-axis, and more or less in the (0 1 0) directions. The binding between these sheets is considerably weaker than that within the sheets. This suggests that cleavage may take place more or less on (0 1 0) planes by breaking two sets of Sb–Se bonds of length 0.346 and 0.374 nm, and preferential growth in the (0 0 1) direction, which result in the formation of Sb2 Se3 whiskers with narrow rectangle cross section (sheet-like or belt-like whiskers). Considering set of bonds (0.326 nm) between
Fig. 4. Unit cell of Sb2 Se3 projected on (0 0 1).
D. Wang et al. / Materials Chemistry and Physics 82 (2003) 546–550
549
Fig. 5. SEM images of Sb2 Se3 whiskers: (a) cross section of a thick “whisker” shown in Fig. 2a–c the splitting of thinner whisker from the thick one, and (d) in the absence of organic additives.
Sb2 Se3 chins in one Sb2 Se3 sheet are longer than those within the chains, further cleavage may occur within these sheets, and thinner wire-like whiskers will be the result. This cleavage mechanism is supported by the SBM observation. Fig. 5a gives a cross section of one thicker Sb2 Se3 whisker shown in Fig. 2b. It can be seen clearly that it consists of a bundle of thinner whiskers each having a narrow rectangle cross section, which confirms that the thinner belt-like Sb2 Se3 whiskers come from the exfoliation of this kind of large whiskers. Fig. 5b shows the micrograph of the splitting of one thinner wire-like whisker from the sheet-like one (shown by arrow). In fact, more or less thinner wire-like whiskers were observed from SEM observation in Fig. 2a–c. More over, the formation of wire-like whiskers with a triangle cross section (as shown in Fig. 5c by arrow) can also be explained using this exfoliation mechanism considering the cleavages do not occur strictly perpendicular to (0 1 0) directions. This mechanism may be applied to explain the formation of one-dimensional crystals of other V2 V13 compounds with similar structures, such as nonorods, nanowires,
nanobelts, or micro-sized structures of these compounds. In fact the exfoliation process in polyol process had been reported for layer-structured Bi2 Se3 [15]. The presence of organic additives has much influence on the growth behavior of Sb2 Se3 . As stated in literature [16], the use of organic molecules, even in very small amounts, could help control the microstructures of inorganic solid products. The present study shows that organic additives favor the cleavage of Sb2 Se3 crystals, and the crystal morphology is well modified. It has been reported that CTAB [17] and DEG [18] can adhere or complex the surface of the particles and then can modify the surface of the products. After Sb2 Se3 nuclei formed in the hydrothermal process, the adsorption and de-adsorption of CTAB or DEG molecules on different surfaces of the nuclei may accelerate the crystal growth of Sb2 Se3 in (0 0 1) direction, which leads to the formation of uniform and long Sb2 Se3 whiskers. To verify it, further experiments were carried out. When other experimental conditions remained stable, but the organic additives were absent, the products were mainly sheet-like crystals
550
D. Wang et al. / Materials Chemistry and Physics 82 (2003) 546–550
with no uniform size and low aspect ratio (Fig. 5d). If more DEG were added to the system, the dispropotionation of Se would be limited, and a reducing agent would be necessary to reduce selenium powder and Sb2 Se3 nanowires were obtained [6].
4. Conclusion In summary, Sb2 Se3 whiskers of 1–3 mm in length were prepared using hydrothermal method in ammonia solution. The formation of the belt-like and wire-like whiskers was thought to be the results of different exfoliation behaviors of Sb2 Se3 crystals. The as-prepared Sb2 Se3 whiskers might make it possible to measure the photovoltaic and thermoelectric properties of single Sb2 Se3 whisker and might find potential applications in construction of miniaturized optoelectric or thermoelectric device. Further researches on the size and morphology control of Sb2 Se3 whiskers in hydrothermal conditions were underway.
Acknowledgements Financial supports from National Natural Science Found of China and the 973 Projects of China are appreciated.
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