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Synthesis and characterization of single-crystal Sb2S3 nanotubes via an EDTA-assisted hydrothermal route
Materials Chemistry and Physics 123 (2010) 236–240
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of single-crystal Sb2 S3 nanotubes via an EDTA-assisted hydrothermal route Guang-Yi Chen a , Wan-Xi Zhang a , An-Wu Xu b,∗ a b
School of Automotive Engineering, Dalian University of Technology, Dalian 116024, China Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 13 May 2009 Received in revised form 20 March 2010 Accepted 2 April 2010 Keywords: Nanostructures Chalcogenides Crystal growth Optical properties
a b s t r a c t Single-crystalline Sb2 S3 nanotubes have been successfully prepared by a simple hydrothermal method. It was found that ethylenediaminetetraacetic acid (EDTA) plays a key role in the formation of Sb2 S3 nanotubes. Without EDTA, only irregular particles with a size of several micrometers were produced. The morphology and structure of the obtained Sb2 S3 nanotubes were characterized by XRD, SEM, TEM, and EDS analysis in detail. UV–vis–NIR spectroscopy was further employed to estimate the band gap energy of the obtained products. The measurement of the optical properties revealed that the obtained nanotubes have a band gap of 1.55 eV. The obtained Sb2 S3 nanotubes may find potential applications in photoelectronic and solar energy because the experimental band gap is close to the optimum value for photovoltaic conversion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes in 1991 [1], extensive research has been carried out on one-dimensional (1D) tubular nanostructures owing to their unique size-dependent properties and remarkable potential applications in electronic, catalysis, biotechnology, separation, and so on [2–7]. Up to now, different types of nanotubes have been prepared by various approaches including vapor–liquid–solid, chemical vapor deposition, template-directed synthesis, and low-dimensional sacrificial precursors [8–11]. These strategies often require high temperature, special condition, and tedious procedures, and most of them are complicated and uncontrollable. Therefore, it is necessary to develop a simple and effective methodology to prepare 1D nanotubes. Recently, hydrothermal methods have emerged as powerful tools for the fabrication of nanotubes [12–15]. Compared to other strategies, the hydrothermal method has such advantages as: 1) reactions can take place in solution under mild conditions; 2) the approach is a typical one-step process, which need no tedious procedures or further purification of the products; 3) the products can be produced on a large scale, which is quite important with respect to technical applications. Antimony trisulfide (Sb2 S3 ) is a highly anisotropic V–VI group semiconductors and crystallizes in an orthorhombic crystal struc-
∗ Corresponding author. Fax: +86 551 3600724. E-mail address: [email protected] (A.-W. Xu). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.04.002
ture (pbnm space group), which is isostructural with Bi2 S3 [16]. Its direct band gap of 1.78–2.50 eV covers the visible and nearinfrared range of the solar spectrum energy. Sb2 S3 can find potential applications in many fields such as thermoelectric cooling devices, optoelectronics in the IR region, and solar energy conversion [17,18]. It has been demonstrated that the properties of antimony trisulfide are determined predominantly by their crystal structure, size and morphology. Therefore, the synthesis of Sb2 S3 materials with well-controlled size and shape is of great significance for their applications. Up to date, a variety of 1D nanostructures of Sb2 S3 such as nanorods [19–24], nanowires [25], microtubes [26], and nanoribbons [27] have already been synthesized by various methods. Considering the technical importance of this material, fabrication of Sb2 S3 with some inspired structures such as a tubular structure by a convenient and efficient method has always been a great interest. Herein, we demonstrate that single-crystal Sb2 S3 tubular nanostructure has been synthesized by a facile one-pot hydrothermal treatment in the presence of ethylenediaminetetraacetic acid (EDTA). 2. Experimental 2.1. Chemicals and synthesis All regents were analytical grade and used without any further purification. In a typical synthesis of Sb2 S3 nanotubes, 1 mmol of antimony potassium tartrate and 1 mmol of EDTA were first dissolved in 20 ml of water, then 2 mmol of thioacetamide (TAA) was added to the above solution under stirring. The resulting homogeneous solution was transferred to a 30 ml Teflon-line stainless steel autoclave and an appropriate amount of distilled water was added to 80% of the total capacity. The autoclave was sealed and maintained at 180 ◦ C for 12 h. After the reaction, the auto-
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clave was cooled to room temperature in air. The resulting black precipitate was filtered, washed with distilled water and absolute alcohol for several times, then dried at 60 ◦ C in air. 2.2. Characterization The X-ray powder diffraction (XRD) patterns of the sample were recorded on a Rigaku/Max-3A X-ray diffractometer with CuK␣ radiation ( = 1.54056 Å), the operation voltage and current maintained at 40 kV and 40 mA, respectively. Field emission scanning electron microscopic (FE-SEM) images were obtained with a JEOL JSM-6700F operated at 15.0 kV. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopic (HRTEM) images and the selected area electron diffraction (SAED) patterns were performed on a JEOL2010 microscope with an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDS) is attached to the JEOL 2010. Sample grids were prepared by sonicating powdered samples in ethanol for 20 min and evaporating one drop of the suspension onto a carbon-coated, holey film supported on a copper grid for TEM measurements. A UV–vis–NIR spectrophotometer (Solid Spec-3700 series) was used to record the absorbance spectra of the samples.
3. Results and discussion Synthesis of Sb2 S3 nanotubes was performed by hydrothermal treatment in the presence of EDTA. The XRD pattern of the asobtained Sb2 S3 samples is shown in Fig. 1. All diffraction peaks shown in Fig. 1 can be indexed as a primitive orthorhombic lattice of Sb2 S3 , which is in good agreement with the standard diffraction data (JCPDS Card File No. 42-1393). No other impurities were found in the samples, indicating that the products are pure stibnite Sb2 S3 . The morphology and size of the obtained products were investigated by scanning electron microscopy (SEM) measurements. The SEM images in Fig. 2 indicate that the as-prepared products are mainly composed of Sb2 S3 nanotubes with smooth surfaces and
Fig. 1. XRD pattern of the obtained Sb2 S3 sample.
open ends. Careful examinations under high magnification show that the crystals exhibit a tetragonal cross section rather than a circular cross section, and the outer diameters of the tubes are around one micrometer and lengths are up to tens of micrometers. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and electron diffraction (ED) were also used to characterize the microstructures of the products. A typical TEM image in Fig. 3a further demonstrates the hollow character of these nanotubes. The thickness of the wall is about 250 nm, and the inner diameters of the nanotubes are about 300 nm. Fig. 3b is the corresponding HRTEM image, having resolved
Fig. 2. SEM images of the as-obtained Sb2 S3 nanotubes grown by hydrothermal treatment at 180 ◦ C for 12 h. (a) At low magnification; (b, c) at high magnification.
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Fig. 3. TEM images (a) and HRTEM image (b) of the Sb2 S3 products obtained under hydrothermal treatment at 180 ◦ C for 12 h. (c) The SAED pattern taken from the [1 0 0] zone axis. (d) The EDS spectrum of the obtained Sb2 S3 nanotubes. Cu peak came from the TEM grid.
lattice fringes of (0 2 0) planes (d020 = 0.565 nm) and (0 0 1) planes (d001 = 0.385 nm), and thus the growth of the nanotube is along the [0 0 1] direction. The corresponding SAED pattern (Fig. 3c) taken from this nanotube can be indexed as an orthorhombic Sb2 S3 single crystal recorded from the [1 0 0] zone axis. Fig. 3d is the EDS spectrum taken from an area consisting of many nanotubes. Only Sb and S peaks are observed in this spectrum, suggesting that the sample is composed of Sb and S. Quantitative EDS analysis shows that the atom ratio of Sb/S is close to 2:3, giving the sample a composition of Sb2 S3 . The formation of Sb2 S3 nanotubes matches well with the general orientation of growth in one-dimensional nanostructures [9,28], which is largely determined by the anisotropic nature of the building blocks. This can be explained on the basis of a typical Sb2 S3 crystal structure (Fig. 4). It consists of infinite chains of (Sb4 S6 )n moieties running parallel to the c-axis that contain two types of Sb and three types of S atoms [29]. Among the three types of S atoms, one is divalent and two are formally trivalent. Within the chain, one of the trivalent sulfurs and the divalent sulfur are connected to antimony by strong covalent bonds. However, the third S is connected to the Sb of the second parallel running chain by weaker van der Waals bonds that are responsible for the rupture of the crystal. Thus, the rupture takes place parallel to c-axis direction in the 0 1 0 plane, where only van der Waals bonds are to be cleaved. Consequently, the break along the c-axis leads to the formation of the Sb2 S3 nanotubes. In fact, previous literature reports have already shown that Sb2 S3 tends to form 1D nanostructures, such as nanorods and nanobelts [19,21,30,31]. EDTA plays an important role in the formation of Sb2 S3 nanotubes. The previous studies demonstrated that EDTA was generally used as a structure-directing agent in the synthesis of 1D nanostructured materials [32,33]. It is well known that EDTA has
a considerable chelating ability and can combine with many metal ions [34]. When Sb3+ ions were introduced into the reaction solution, EDTA could combine with the Sb3+ ions to form Sb–EDTA complexes (the stability constant of this complex is about 8.24). The complexes might serve as a molecular template in control of the crystals growth [35,36]. When the reaction temperature was increased, S2− would react with Sb3+ to produce Sb2 S3 precipitates due to the strong complexing capability between S2− and Sb3+ ions. The formula for the formation of Sb2 S3 materials can be described
Fig. 4. The crystal structure of Sb2 S3 viewed along the [0 1 0] direction. The Sb–S covalent bonds are indicated by solid lines and the weak van der Waals bonds by the black dashed lines. The cleavage trace is demarcated by the red dashed line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 5. SEM image of the Sb2 S3 products obtained in the absence of EDTA under hydrothermal treatment at 180 ◦ C for 12 h.
Fig. 6. UV–vis–NIR absorption spectra recorded from Sb2 S3 nanotubes at room temperature.
as (Y represents EDTA):
out to provide information on the band structure of Sb2 S3 nanotubes. Typical UV–vis–NIR absorption spectrum recorded from Sb2 S3 nanostructures at room temperature is shown in Fig. 6, giving the absorption edge onset of the spectrum at about 800 nm. Therefore, the band gap of Sb2 S3 nanotubes can be estimated to be 1.55 eV, using the following equation [38]:
+
+
K(SbO)C4 H4 O6 · 12 H2 O ⇔ K + SbO + C4 H4 O6
2−
+ 12 H2 O
SbO+ + OH− ⇔ SbO(OH) SbO(OH) + H2 Y 2− ⇔ [Sb(OH)Y]2− + H2 O [Sb(OH)Y]2− + CH3 CSNH2 → Sb2 S3 ↓ Once the Sb2 S3 nanocrystals were formed during the hydrothermal process, EDTA would selectively bind to the specific surface of the preformed antimony sulfide, leading to the seeming restriction of crystal growth [37]. From Fig. 3c it can be seen that the Sb2 S3 nanotubes grow along the c-axis direction. So it can be concluded that the surface adsorption of EDTA on the (2 0 0) and (0 2 0) planes has a higher coverage than that on the (0 0 1) planes, which leads to the crystal growth direction along [0 0 1]. When the reaction was performed in the absence of EDTA, the final products are all particles in irregular shapes, as clearly seen in Fig. 5, even when other experimental conditions are kept the same. This is often appeared as particle aggregates, indicating that the initial formed nanoparticles had a strong tendency to aggregate without protective agents in the reaction system. Our study demonstrates that single-crystal Sb2 S3 tubular nanostructure can be synthesized by this facile one-pot hydrothermal treatment. The influence of the experimental conditions such as temperature, time and EDTA concentration on the morphology were investigated. It is found that with the reaction temperature increasing in a suitable range, the quantity and quality of Sb2 S3 nanotubes were increased and improved, but too high temperature (>220 ◦ C) would break the nanotubes to form irregular particles. The control experiments also indicated that an increase in the reaction time in a certain range is helpful for the formation of Sb2 S3 nanotubes, but the sample morphology did not change remarkably after 12 h. EDTA plays an important role in the formation of Sb2 S3 nanotubes. The Sb–EDTA complexes might serve as a molecular template in control of the crystals growth. When the reaction was performed in the absence of EDTA, the final products are all particles in irregular shapes (Fig. 5). Because one EDTA molecule can only coordinate with one Sb3+ ion, so the Sb2 S3 nanotubes did not change remarkably by increasing the amount of EDTA (>1 mmol). As one of the most important electronic parameters for semiconductor nanocrystals, optical absorption measurement was carried
˛Ephoton = A(Ephoton − Eg )1/2 where Ephoton is the corresponding phonon energy, ˛ is the absorbance coefficient and A is a constant. This is quite comparable to the values reported for Sb2 S3 nanorods and nanobelts of comparable dimensions [21,27]. It has to be mentioned that the nanotubes do not show a quantum confinement effect, a result also reported by other literature [21,27]. This may be due to the lower Bohr’s radius for this material. The present study opens a new avenue to low-cost synthesis of semiconductor Sb2 S3 nanotubes with potential applications in solar energy conversion and also for optical nanodevices operating in the near-infrared. Further study on its applications is on-going. 4. Conclusions In summary, we have introduced a new one-step approach to prepare Sb2 S3 nanotubes under hydrothermal treatment with EDTA acting as a structure-directing agent. The obtained products were characterized by XRD, SEM, TEM and EDS analysis in detail. The UV–vis–NIR absorption spectra give the absorption edge onset at around 800 nm. The EDTA-assisted hydrothermal method demonstrated in this paper may be extended to the fabrication of other chalcogenides 1D nanostructures. Acknowledgements Support from the National Natural Science Foundation of China (20971118), the 100 Talents program of the Chinese Academy of Sciences, the National Basic Research Program of China (2010CB934700) and Anhui Natural Science Foundation Key Project (ZD200902) is gratefully acknowledged. References [1] S. Iijima, Nature 354 (1991) 56. [2] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [3] R. Tenne, Nat. Nanotechnol. 1 (2006) 103.
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G.-Y. Chen et al. / Materials Chemistry and Physics 123 (2010) 236–240 Y.J. Xiong, B.T. Mayers, Y.N. Xia, Chem. Commun. (2005) 5013. M. Remskar, Adv. Mater. 16 (2004) 1497. C.R. Martin, P. Kohli, Nat. Rev. Drug Discov. 2 (2003) 29. S.B. Lee, D.T. Mitchell, L. Trofin, T.K. Nevanen, H. Soderlund, C.R. Martin, Science 296 (2002) 2198. E.P.A.M. Bakkers, M.A. Verheijen, J. Am. Chem. Soc. 125 (2003) 3440. Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Marers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353. J. Goldberger, R. Fan, P.D. Yang, Acc. Chem. Res. 39 (2006) 239. H.J. Niu, M.Y. Gao, Angew. Chem. Int. Ed. 45 (2006) 6462. A.W. Xu, Y.P. Fang, L.P. You, H.Q. Liu, J. Am. Chem. Soc. 125 (2003) 1494. J. Luo, H.T. Zhu, H.M. Fan, J.K. Liang, H.L. Shi, G.H. Rao, J.B. Li, Z.M. Du, Z.X. Shen, J. Phys. Chem. C 112 (2008) 12594. Q.X. Deng, M.D. Wei, X.K. Ding, L.L. Jiang, B.H. Ye, K.M. Wei, Chem. Commun. (2008) 3657. S. Hu, X. Wang, J. Am. Chem. Soc. 130 (2008) 126. S. Scavnicar, Z. Kristallogr. 114 (1960) 49. O. Savadogo, K.C. Mandal, Sol. Energy Mater. Sol. Cells 26 (1992) 117. M.S. Nair, Y. Pena, J. Campos, V.M. Garcia, P.K. Nair, J. Electrochem. Soc. 114 (1998) 2113. J.W. Wang, Y.D. Li, Mater. Chem. Phys. 87 (2004) 420. Y. Jiang, Y.J. Zhu., J. Phys. Chem. B 109 (2005) 4361. J. Ota, S.K. Srivastava, Cryst. Growth Des. 7 (2007) 343. C. Li, G. Shi, Y.L. Song, X.L. Zhang, S.Y. Guang, H.Y. Xu., J. Phys. Chem. Solids 69 (2008) 1829.
[23] Q.A. Zhu, M. Gong, C. Zhang, G.B. Yong, S. Xiang, J. Cryst. Growth 311 (2009) 3651. [24] Q.F. Han, L. Chen, M.J. Wang, X.J. Yang, L.D. Lu, X. Wang, Mater. Sci. Eng. B 166 (2010) 118. [25] Z.R. Geng, M.X. Wang, G.H. Yue, P.X. Yan, J. Cryst. Growth 310 (2008) 341. [26] J. Yang, Y.C. Liu, H.M. Lin, C.C. Chen, Adv. Mater. 16 (2004) 713. [27] Y. Yu, R.H. Wang, Q. Chen, L.M. Peng, J. Phys. Chem. B 110 (2006) 13415. [28] R.Q. Song, A.W. Xu, S.H. Yu, J. Am. Chem. Soc. 129 (2007) 4152. [29] V.P. Herzog, S.L. Harmer, H.W. Nesbitt, G.M. Bancroft, R. Flemming, A.R. Pratt, Surf. Sci. 600 (2006) 348. [30] X.W. Zheng, Y. Xie, L.Y. Zhu, X.C. Jiang, Y.B. Jia, W.H. Song, Y.P. Sun, Inorg. Chem. 41 (2002) 455. [31] Q. Xie, Z.P. Liu, M.W. Shao, L.F. Kong, W.C. Yu, Y.T. Qian, J. Cryst. Growth 252 (2003) 570. [32] S.M. Zhou, Y.S. Feng, L.D. Zhang, Mater. Lett. 57 (2003) 2936. [33] N. Wang, Q.F. Zhang, W. Chen, Cryst. Res. Technol. 42 (2007) 138. [34] D.E. Zhang, X.J. Zhang, X.M. Ni, J.M. Song, H.G. Zheng, Cryst. Growth Des. 7 (2007) 2117. [35] G.Z. Chen, S.X. Sun, X.Y. Song, Z.L. Yin, H.Y. Yu, C.H. Fan, W. Zhao, J. Mater. Sci. 42 (2007) 6977. [36] S.R. Gadakh, C.H. Bhosale, Mater. Chem. Phys. 78 (2002) 367. [37] F. Luo, W. Song, C.J. Jia, Cryst. Growth Des. 5 (2005) 137. [38] S. Tsunekawa, T. Fukuda, A. Kasuya, J. Appl. Phys. 87 (2000) 1318.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis and characterization of single-crystal Sb2 S3 nanotubes via an EDTA-assisted hydrothermal route Guang-Yi Chen a , Wan-Xi Zhang a , An-Wu Xu b,∗ a b
School of Automotive Engineering, Dalian University of Technology, Dalian 116024, China Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 13 May 2009 Received in revised form 20 March 2010 Accepted 2 April 2010 Keywords: Nanostructures Chalcogenides Crystal growth Optical properties
a b s t r a c t Single-crystalline Sb2 S3 nanotubes have been successfully prepared by a simple hydrothermal method. It was found that ethylenediaminetetraacetic acid (EDTA) plays a key role in the formation of Sb2 S3 nanotubes. Without EDTA, only irregular particles with a size of several micrometers were produced. The morphology and structure of the obtained Sb2 S3 nanotubes were characterized by XRD, SEM, TEM, and EDS analysis in detail. UV–vis–NIR spectroscopy was further employed to estimate the band gap energy of the obtained products. The measurement of the optical properties revealed that the obtained nanotubes have a band gap of 1.55 eV. The obtained Sb2 S3 nanotubes may find potential applications in photoelectronic and solar energy because the experimental band gap is close to the optimum value for photovoltaic conversion. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes in 1991 [1], extensive research has been carried out on one-dimensional (1D) tubular nanostructures owing to their unique size-dependent properties and remarkable potential applications in electronic, catalysis, biotechnology, separation, and so on [2–7]. Up to now, different types of nanotubes have been prepared by various approaches including vapor–liquid–solid, chemical vapor deposition, template-directed synthesis, and low-dimensional sacrificial precursors [8–11]. These strategies often require high temperature, special condition, and tedious procedures, and most of them are complicated and uncontrollable. Therefore, it is necessary to develop a simple and effective methodology to prepare 1D nanotubes. Recently, hydrothermal methods have emerged as powerful tools for the fabrication of nanotubes [12–15]. Compared to other strategies, the hydrothermal method has such advantages as: 1) reactions can take place in solution under mild conditions; 2) the approach is a typical one-step process, which need no tedious procedures or further purification of the products; 3) the products can be produced on a large scale, which is quite important with respect to technical applications. Antimony trisulfide (Sb2 S3 ) is a highly anisotropic V–VI group semiconductors and crystallizes in an orthorhombic crystal struc-
∗ Corresponding author. Fax: +86 551 3600724. E-mail address: [email protected] (A.-W. Xu). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.04.002
ture (pbnm space group), which is isostructural with Bi2 S3 [16]. Its direct band gap of 1.78–2.50 eV covers the visible and nearinfrared range of the solar spectrum energy. Sb2 S3 can find potential applications in many fields such as thermoelectric cooling devices, optoelectronics in the IR region, and solar energy conversion [17,18]. It has been demonstrated that the properties of antimony trisulfide are determined predominantly by their crystal structure, size and morphology. Therefore, the synthesis of Sb2 S3 materials with well-controlled size and shape is of great significance for their applications. Up to date, a variety of 1D nanostructures of Sb2 S3 such as nanorods [19–24], nanowires [25], microtubes [26], and nanoribbons [27] have already been synthesized by various methods. Considering the technical importance of this material, fabrication of Sb2 S3 with some inspired structures such as a tubular structure by a convenient and efficient method has always been a great interest. Herein, we demonstrate that single-crystal Sb2 S3 tubular nanostructure has been synthesized by a facile one-pot hydrothermal treatment in the presence of ethylenediaminetetraacetic acid (EDTA). 2. Experimental 2.1. Chemicals and synthesis All regents were analytical grade and used without any further purification. In a typical synthesis of Sb2 S3 nanotubes, 1 mmol of antimony potassium tartrate and 1 mmol of EDTA were first dissolved in 20 ml of water, then 2 mmol of thioacetamide (TAA) was added to the above solution under stirring. The resulting homogeneous solution was transferred to a 30 ml Teflon-line stainless steel autoclave and an appropriate amount of distilled water was added to 80% of the total capacity. The autoclave was sealed and maintained at 180 ◦ C for 12 h. After the reaction, the auto-
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237
clave was cooled to room temperature in air. The resulting black precipitate was filtered, washed with distilled water and absolute alcohol for several times, then dried at 60 ◦ C in air. 2.2. Characterization The X-ray powder diffraction (XRD) patterns of the sample were recorded on a Rigaku/Max-3A X-ray diffractometer with CuK␣ radiation ( = 1.54056 Å), the operation voltage and current maintained at 40 kV and 40 mA, respectively. Field emission scanning electron microscopic (FE-SEM) images were obtained with a JEOL JSM-6700F operated at 15.0 kV. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopic (HRTEM) images and the selected area electron diffraction (SAED) patterns were performed on a JEOL2010 microscope with an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDS) is attached to the JEOL 2010. Sample grids were prepared by sonicating powdered samples in ethanol for 20 min and evaporating one drop of the suspension onto a carbon-coated, holey film supported on a copper grid for TEM measurements. A UV–vis–NIR spectrophotometer (Solid Spec-3700 series) was used to record the absorbance spectra of the samples.
3. Results and discussion Synthesis of Sb2 S3 nanotubes was performed by hydrothermal treatment in the presence of EDTA. The XRD pattern of the asobtained Sb2 S3 samples is shown in Fig. 1. All diffraction peaks shown in Fig. 1 can be indexed as a primitive orthorhombic lattice of Sb2 S3 , which is in good agreement with the standard diffraction data (JCPDS Card File No. 42-1393). No other impurities were found in the samples, indicating that the products are pure stibnite Sb2 S3 . The morphology and size of the obtained products were investigated by scanning electron microscopy (SEM) measurements. The SEM images in Fig. 2 indicate that the as-prepared products are mainly composed of Sb2 S3 nanotubes with smooth surfaces and
Fig. 1. XRD pattern of the obtained Sb2 S3 sample.
open ends. Careful examinations under high magnification show that the crystals exhibit a tetragonal cross section rather than a circular cross section, and the outer diameters of the tubes are around one micrometer and lengths are up to tens of micrometers. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and electron diffraction (ED) were also used to characterize the microstructures of the products. A typical TEM image in Fig. 3a further demonstrates the hollow character of these nanotubes. The thickness of the wall is about 250 nm, and the inner diameters of the nanotubes are about 300 nm. Fig. 3b is the corresponding HRTEM image, having resolved
Fig. 2. SEM images of the as-obtained Sb2 S3 nanotubes grown by hydrothermal treatment at 180 ◦ C for 12 h. (a) At low magnification; (b, c) at high magnification.
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Fig. 3. TEM images (a) and HRTEM image (b) of the Sb2 S3 products obtained under hydrothermal treatment at 180 ◦ C for 12 h. (c) The SAED pattern taken from the [1 0 0] zone axis. (d) The EDS spectrum of the obtained Sb2 S3 nanotubes. Cu peak came from the TEM grid.
lattice fringes of (0 2 0) planes (d020 = 0.565 nm) and (0 0 1) planes (d001 = 0.385 nm), and thus the growth of the nanotube is along the [0 0 1] direction. The corresponding SAED pattern (Fig. 3c) taken from this nanotube can be indexed as an orthorhombic Sb2 S3 single crystal recorded from the [1 0 0] zone axis. Fig. 3d is the EDS spectrum taken from an area consisting of many nanotubes. Only Sb and S peaks are observed in this spectrum, suggesting that the sample is composed of Sb and S. Quantitative EDS analysis shows that the atom ratio of Sb/S is close to 2:3, giving the sample a composition of Sb2 S3 . The formation of Sb2 S3 nanotubes matches well with the general orientation of growth in one-dimensional nanostructures [9,28], which is largely determined by the anisotropic nature of the building blocks. This can be explained on the basis of a typical Sb2 S3 crystal structure (Fig. 4). It consists of infinite chains of (Sb4 S6 )n moieties running parallel to the c-axis that contain two types of Sb and three types of S atoms [29]. Among the three types of S atoms, one is divalent and two are formally trivalent. Within the chain, one of the trivalent sulfurs and the divalent sulfur are connected to antimony by strong covalent bonds. However, the third S is connected to the Sb of the second parallel running chain by weaker van der Waals bonds that are responsible for the rupture of the crystal. Thus, the rupture takes place parallel to c-axis direction in the 0 1 0 plane, where only van der Waals bonds are to be cleaved. Consequently, the break along the c-axis leads to the formation of the Sb2 S3 nanotubes. In fact, previous literature reports have already shown that Sb2 S3 tends to form 1D nanostructures, such as nanorods and nanobelts [19,21,30,31]. EDTA plays an important role in the formation of Sb2 S3 nanotubes. The previous studies demonstrated that EDTA was generally used as a structure-directing agent in the synthesis of 1D nanostructured materials [32,33]. It is well known that EDTA has
a considerable chelating ability and can combine with many metal ions [34]. When Sb3+ ions were introduced into the reaction solution, EDTA could combine with the Sb3+ ions to form Sb–EDTA complexes (the stability constant of this complex is about 8.24). The complexes might serve as a molecular template in control of the crystals growth [35,36]. When the reaction temperature was increased, S2− would react with Sb3+ to produce Sb2 S3 precipitates due to the strong complexing capability between S2− and Sb3+ ions. The formula for the formation of Sb2 S3 materials can be described
Fig. 4. The crystal structure of Sb2 S3 viewed along the [0 1 0] direction. The Sb–S covalent bonds are indicated by solid lines and the weak van der Waals bonds by the black dashed lines. The cleavage trace is demarcated by the red dashed line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 5. SEM image of the Sb2 S3 products obtained in the absence of EDTA under hydrothermal treatment at 180 ◦ C for 12 h.
Fig. 6. UV–vis–NIR absorption spectra recorded from Sb2 S3 nanotubes at room temperature.
as (Y represents EDTA):
out to provide information on the band structure of Sb2 S3 nanotubes. Typical UV–vis–NIR absorption spectrum recorded from Sb2 S3 nanostructures at room temperature is shown in Fig. 6, giving the absorption edge onset of the spectrum at about 800 nm. Therefore, the band gap of Sb2 S3 nanotubes can be estimated to be 1.55 eV, using the following equation [38]:
+
+
K(SbO)C4 H4 O6 · 12 H2 O ⇔ K + SbO + C4 H4 O6
2−
+ 12 H2 O
SbO+ + OH− ⇔ SbO(OH) SbO(OH) + H2 Y 2− ⇔ [Sb(OH)Y]2− + H2 O [Sb(OH)Y]2− + CH3 CSNH2 → Sb2 S3 ↓ Once the Sb2 S3 nanocrystals were formed during the hydrothermal process, EDTA would selectively bind to the specific surface of the preformed antimony sulfide, leading to the seeming restriction of crystal growth [37]. From Fig. 3c it can be seen that the Sb2 S3 nanotubes grow along the c-axis direction. So it can be concluded that the surface adsorption of EDTA on the (2 0 0) and (0 2 0) planes has a higher coverage than that on the (0 0 1) planes, which leads to the crystal growth direction along [0 0 1]. When the reaction was performed in the absence of EDTA, the final products are all particles in irregular shapes, as clearly seen in Fig. 5, even when other experimental conditions are kept the same. This is often appeared as particle aggregates, indicating that the initial formed nanoparticles had a strong tendency to aggregate without protective agents in the reaction system. Our study demonstrates that single-crystal Sb2 S3 tubular nanostructure can be synthesized by this facile one-pot hydrothermal treatment. The influence of the experimental conditions such as temperature, time and EDTA concentration on the morphology were investigated. It is found that with the reaction temperature increasing in a suitable range, the quantity and quality of Sb2 S3 nanotubes were increased and improved, but too high temperature (>220 ◦ C) would break the nanotubes to form irregular particles. The control experiments also indicated that an increase in the reaction time in a certain range is helpful for the formation of Sb2 S3 nanotubes, but the sample morphology did not change remarkably after 12 h. EDTA plays an important role in the formation of Sb2 S3 nanotubes. The Sb–EDTA complexes might serve as a molecular template in control of the crystals growth. When the reaction was performed in the absence of EDTA, the final products are all particles in irregular shapes (Fig. 5). Because one EDTA molecule can only coordinate with one Sb3+ ion, so the Sb2 S3 nanotubes did not change remarkably by increasing the amount of EDTA (>1 mmol). As one of the most important electronic parameters for semiconductor nanocrystals, optical absorption measurement was carried
˛Ephoton = A(Ephoton − Eg )1/2 where Ephoton is the corresponding phonon energy, ˛ is the absorbance coefficient and A is a constant. This is quite comparable to the values reported for Sb2 S3 nanorods and nanobelts of comparable dimensions [21,27]. It has to be mentioned that the nanotubes do not show a quantum confinement effect, a result also reported by other literature [21,27]. This may be due to the lower Bohr’s radius for this material. The present study opens a new avenue to low-cost synthesis of semiconductor Sb2 S3 nanotubes with potential applications in solar energy conversion and also for optical nanodevices operating in the near-infrared. Further study on its applications is on-going. 4. Conclusions In summary, we have introduced a new one-step approach to prepare Sb2 S3 nanotubes under hydrothermal treatment with EDTA acting as a structure-directing agent. The obtained products were characterized by XRD, SEM, TEM and EDS analysis in detail. The UV–vis–NIR absorption spectra give the absorption edge onset at around 800 nm. The EDTA-assisted hydrothermal method demonstrated in this paper may be extended to the fabrication of other chalcogenides 1D nanostructures. Acknowledgements Support from the National Natural Science Foundation of China (20971118), the 100 Talents program of the Chinese Academy of Sciences, the National Basic Research Program of China (2010CB934700) and Anhui Natural Science Foundation Key Project (ZD200902) is gratefully acknowledged. References [1] S. Iijima, Nature 354 (1991) 56. [2] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [3] R. Tenne, Nat. Nanotechnol. 1 (2006) 103.
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