Tartaric Acid Assisted Growth of Sb2S3 Nanorods by a Simple Wet Chemical Method Jyotiranjan Ota and Suneel Kumar Srivastava* Inorganic Materials and Nanocomposites Laboratory, Department of Chemistry, Indian Institute of Technology, Kharagpur, India
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 343-347
ReceiVed August 18, 2006; ReVised Manuscript ReceiVed October 26, 2006
ABSTRACT: Good quality nanorods of Sb2S3 have been synthesized by a simple wet chemical method under refluxing conditions. In this, tartaric acid has been successfully used as a complexing agent to grow these single-crystalline nanorods at a comparatively much lower temperature (115 °C) than reported earlier (180-200 °C). X-ray diffraction (XRD) and electron diffraction (ED) studies showed that the rods correspond to the pure orthorhombic phase of Sb2S3, the phase purity of which is further confirmed by energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS). A possible growth mechanism has been discussed on the basis of a series of transmission electron microscopy (TEM) studies of the product obtained at different durations. The effect of concentration of tartaric acid on the formation of Sb2S3 and its morphology has also been discussed. The morphology of the final product remained same for different sulfur sources used, though there is small change in dimension. The band gap was found to be 1.56 eV, suitable for photovoltaic applications. Introduction The unique and interesting properties of the materials in the nanoregime have attracted researchers to put forward their best efforts to develop the materials for future applications.1,2 Specifically, the one-dimensional nanostructures, nanotubes, -rods, -belts, -wires, etc., owing to their functional advantages over other forms of the materials, are more promising in this concern.3,4 The binary chalcogenides of V-VI (A2VB3VI) are very useful semiconductor materials having applications in thermoelectric and optoelectronic devices.5 Moreover, they find applications in IR spectroscopy, paints, photo-emitting diodes, microwave switches, etc.6 The band gap of a material determines its applicability as an optoelectronic material; therefore, the tailoring of the band gap is very helpful. The band gap of the Sb2S3 varies between 1.5 and 2.2 eV due to changes in crystallinity, size, and shape in the nanoregime.7 Therefore, various methods have been employed to synthesize this material on the nanoscale, especially in one dimension. It is a highly anisotropic material with a layered structure that crystallizes in purely orthorhombic phase.8 It has a ribbon-like polymeric structure in which SbS3 and SSb3 layers form interlocked pyramids, which makes this material anisotropic and helps in confined growth. So far, the different research groups have employed hydrothermal,9 sonochemical,10 single-source decomposition,11 and polyol-assisted routes12 to synthesize this material in either rod, ribbon, or wire form. Chen et al. used Sb2S3 powder to grow nanotubes of the same by a thermal evaporation method.13 However, till now the developments regarding the synthesis of this material are very little, especially related to the growth using simple chemical methods. In the recent past, much research has been focused on using templates for unidirectional growth of different materials. Among all other growth-directing agents, the use of a complexing agent to confine the growth in the desired direction remains an interesting aspect. Bi- or multidentate amines (ethylene diamine, triethanol amine),14 glycols (ethylene glycol),15 acids (such as ethylenediaminetetraacetic acid (EDTA),16 citric acid, and acetic acid),17 etc. have been used as complexing agents in synthesizing * To whom correspondence should be addressed. E-mail: sunit@ chem.iitkgp.ernet.in. Fax: 91-3222-755303. Tel: 91-3222-283334.
nanorods or tubes. These polyfunctional ligands coordinate with the inorganic ions to form complexes and also act as a soft template in the growth process. However, in the case of the group V-VI semiconductors, the use of the acidic ligands or complexing agents has not been widely studied, though there are some recent reports on synthesis of Bi2S3 nanorods using either bismuth citrate18 or EDTA.19 Deng et al. prepared nanorods of Bi2Te3 by a controlled oriented attachment using an EDTA complex as a template.20 The same group also obtained chainlike crystal structures of Bi2Te3, as well as Ag, in a hydrothermal process using EDTA or polyvinyl pyrrolidone (PVP).21 On the other hand, there have been a number of studies done regarding the deposition of thin films of various semiconductors using complexing agents.22 Bhosale et al. have reported the deposition of thin films of Sb2S3 using tartaric acid as a complexing agent and also studied the effect of concentration of the same.23 However, not many attempts have been made so far to exploit the use of tartaric acid for the growth of the nanorods in wet chemical methods. There is only one such report on formation of nanorod bundles, where tartaric acid has been used to completely dissolve SbCl3 in water.24 Moreover, most of the synthetic methods for the Sb2S3 nanorods under solvothermal conditions have been carried out in the temperature range of 180-200 °C. Hence, obtaining these nanorods at relatively much lower temperature, that too under refluxing conditions, is valuable from a chemist’s prospective and has remained always a challenge. Our group has been studying the possibilities of using various soft templates for synthesis of group V-VI semiconductors and previously reported the fabrication of Bi2S3 nanotubes through a micelle template assisted route.25 In the present work, we have successfully used tartaric acid to synthesize nanorods of Sb2S3 through a simple soft chemical route. Potassium thiocyanate has been used as a sulfur source for the first time in this work. The effect of concentration of the complexing agent has been studied carefully, and a possible reaction as well as growth mechanism for the formation of Sb2S3 is proposed. Experimental Section All the chemical reagents used in this work were of analytical grade and used without further purification. The reaction was carried out in
10.1021/cg0605537 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/24/2006
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Figure 1. XRD profile of the product indexed according to JCPDS file 42-1393. a 100 mL round-bottom flask where 1.5 g (10 mmol) of tartaric acid (TA) was added to 50 mL of water followed by 0.625 g (2.7 mmol) of SbCl3 with constant stirring till a clear solution was obtained. To this, 0.5 g (5 mmol) of potassium thiocyanate (KSCN) was added, and the whole solution was refluxed at 115 °C for 24 h. Finally, the dark brown precipitate was filtered and washed with water and ethanol. The resultant powder was dried under vacuum at 60 °C for 4 h and characterized. The same reaction was repeated using 5 and 0 mmol of TA, all other conditions remaining same. The phase analysis of the products were performed on a Philips PW1710 X-ray diffractrometer (40 kV, 20 mA) using Cu KR radiation (λ ) 0.154 18 nm) at a scan rate of 0.05°/s in the range 10-70°. X-ray photoelectron spectroscopy (XPS) was carried out on ESCALab MKII X-ray photoelectron spectrometer, using non-monochromatized Mg KR X-ray as the excitation source. The morphology of the sample was studied by scanning electron microscopy (SEM) using a JEOL JSM5800 at an accerating voltage of 20 kV. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and selected area electron diffraction (SAED) pictures were recorded on a JEOL 2100 electron microscope operating at 200 kV. Energy dispersive X-ray (EDX) analysis of the samples was carried out on an OXFORD INCA instrument attached to the transmission electron microscope in the scanning range of 0 to 20 kV to find out the chemical composition. A Perkin-Elmer Lambda20 spectrophotometer was used to get the absorption spectra of the sample using Ba2SO4 as a reference material.
Results and Discussion Figure 1 shows the X-ray diffraction (XRD) pattern of the product obtained under reflux conditions and is scanned for the presence of all possible phases. It shows the presence of sharp peaks, which could be indexed on the basis of pure orthorhombic phase of the Sb2S3 (JCPDS 42-1393). Absence of any other peak due to impurities indicates the purity of the product. The composition and purity of the product was further checked by XPS analysis, where high-resolution spectra of the Sb 3d and S 2p are obtained using C 1s as the reference at 284.6 eV. Figure 2a represents the full/wide scan spectra of the sample where no impurities could be detected, thereby further confirming the purity of the product. The high-resolution spectra of the Sb 3d in Figure 2b show the presence of two peaks for Sb3d5/2 and Sb 3d3/2 at 530.4 and 539.7 eV, respectively. It may be interesting to mention that these peaks are characteristics of the Sb3+ oxidation state confirming that the antimony is not in the +5 state. Figure 2c represents the high-resolution spectra of the S 2p with the peak at 162.2 eV. The peak positions for both Sb and S also matched very well with those in the literature.26 The ratio of Sb to S is found to be is 1:1.46 from the quantification of area of the respective peaks, which is very close to 1:1.5, an ideal value for the stoichiometry corresponding
Figure 2. XPS spectra of the product: (a) low resolution; (b) highresolution spectrum of Sb 3d; (c) high-resolution spectrum of S 2p.
to the Sb2S3. Hence, XPS also proves the stoichiometry and purity of the sample within some experimental error. Morphology of the product was studied by SEM analysis. Figure 3 represents the typical SEM micrograph of the sample, where rodlike morphology can be seen with a diameter in the nanometer range and length varying in some micrometers. The sample was characterized further by TEM to obtain more information on the structure and morphology and is shown in the Figure 4. The typical TEM image in Figure 4a shows a bunch of nanorods having diameters in the range of 50-150 nm and length of a few micrometers. The purity of a single nanorod was examined by EDX analysis, and the spectrum is given in Figure 4b. In the spectra, peaks for Sb, S, and Cu could be discerned. The peak for Cu is due to the copper grid over which sample has been mounted. The quantitative analysis gave the ratio of Sb/S as nearly 2:3 (Sb 41.25%; S 58.75%), further confirming the purity of the nanorods. One single nanorod of diameter around 50 nm and length ∼400 nm has been focused and is shown in Figure 4c. The SAED pattern of the demarcated portion of the nanorod is also shown in the inset. The parallely
Tartaric Acid Assisted Growth of Sb2S3 Nanorods
Figure 3. SEM image of the product obtained, showing a large number of nanorods.
Figure 4. TEM images of the Sb2S3 nanorods: (a) image at low magnification; (b) EDX spectrum of a single nanorod (inset shows the nanorod selected for the experiment); (c) TEM image of a single nanorod (inset is the SAED pattern obtained from the area demarcated on the nanorod); (d) HRTEM image of tip of a nanorod.
arranged bright spots confirm that the nanorods are single crystalline in nature, and these spots correspond to the orthorhombic phase of Sb2S3. A HRTEM image of the tip portion of a nanorod showing the clear fringes is displayed in Figure 4d. The interlayer spacing of 0.358 nm corresponds to the (130) plane of the Sb2S3. This suggests that growth occurs perpendicular to this plane along the tip of the nanorod, that is, toward the (001) direction. This matches well with the general orientation of growth in one-dimensional nanostructures3b and can be explained on the basis of a typical crystal structure of Sb2S3 as shown in Figure 5. It consists of chainlike (Sb4S6)n moieties running parallel to the 001 axis that contain two types of Sb and three types of S atoms.27 Out of the three types of sulfur atoms, two are formally trivalent and one is divalent. Within the chain, the divalent sulfur and one trivalent sulfur are connected to antimony by strong covalent bonds. However, the third sulfur is connected to the antimony of the second parallely running chain by weaker van der Waals bonds that are responsible for the cleavage of the crystal. Thus, the cleavage occurs parallel to the c-axis (001) in the 010 plane, where only
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Figure 5. Crystal structure of Sb2S3 projected on the (010) plane. The Sb-S covalent bonds are shown by solid lines and the weak van der Waals bonds by dotted lines. A possible cleavage site is demarcated by the red line.
Figure 6. TEM image of the products obtained at different reaction durations: (a) after 1 h, showing agglomerated nanoparticles; (b) after 3 h, showing presence of some rods along with the particles; (c) after 6 h, majority of the product being the nanorods with some particles; (d) only nanorods can be seen after 12 h.
van der Waals bonds are to be ruptured. As a result, Sb2S3 breaks easily along the c-axis, and this results in formation of a onedimensional structure, either as nanowires or nanorods. In order to understand the growth mechanism for the formation of the nanorods, the products obtained at different reaction durations have been analyzed through TEM images. Figure 6a represents a typical image of the product formed after 1 h and indicates the presence of agglomerated particles in the nanorange. Figure 6b represents the TEM image of the product obtained after 3 h. It clearly suggests that increase in reaction duration from 1 to 3 h induces a rodlike morphology, along with the particles in the product. The product obtained after 6 h contains a majority of the nanorods of very narrow dimension amidst of some particles, as is evident from Figure 6c. TEM images of the product recorded after 12 h shows the presence of only rodlike morphologies. The EDX analysis of the particles present in the sample obtained at both 1 and 3 h duration suggests a composition of the reaction product as SbxOySz. The
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Figure 7. XRD pattern of the product obtained for different concentrations of tartaric acid (TA): (a) product obtained without any TA added; (b) for 5 mmol of TA; (c) for 10 mmol of TA.
formation of a similar intermediate during preparation of Sb2S3 under hydrothermal conditions has also been reported by Yu et al.9 According to them, SbxOySz itself is unstable and gives rise to Sb2S3 when subjected to suitable reaction conditions. In our case, it is the reaction duration that facilitates the decomposition of SbxOySz to form Sb2S3 nanoparticles, which serve as nuclei in their subsequent growth as nanorods. The tartaric acid in the reaction mixture, which acts as a capping agent, is adsorbed on the surface of these nuclei and controls the overall growth kinetics. The growth of the nuclei is restricted only to specific facets due to hindrance of the capping agent, and driven by the anisotropic crystal structure, this results in the growth of the product along the c-axis in the direction of the 001 plane. The formation of nanoparticles and nanorods continued simultaneously with increase in the duration for some time. Therefore, a mixture of particles and rods are obtained in the product up to 6 h. However, with increase in the reaction duration, only nanorods are found at 12 h, and these are subsequently crystallized to give well crystalline nanorods after 24 h. In the present work, we have also studied the effect of concentration of the complexing agent (tartaric acid) on the phase purity and the final morphology of the product. Figure 7a,b,c shows XRD patterns of the products obtained for 0, 5, and 10 mmol of tartaric acid, respectively. It is observed from Figure 7a that the product obtained without using complexing agent consists of more intense peaks that are mainly due to Sb4O5Cl2 (JCPDS 70-1102), though a few peaks for Sb2S3 appeared. In Figure 7b, it can be seen that the peaks for Sb2S3 are developed though not fully crystallized. It also contains a few less intense peaks for Sb4O5Cl2. Interestingly, on increasing the concentration of the complexing agent to 10 mmol, wellcrystallized peaks only for the pure Sb2S3 are observed. These studies clearly demonstrate that the formation of Sb2S3 is largely dependent on the concentration of the complexing agent. A complex of the Sb3+ with the tetradentate acid is formed when SbCl3 is added to the aqueous solution of the tartaric acid.28 Release of Sb3+ ions from this complex takes place in a controlled manner, and the ions react with the sulfur ion obtained from KSCN to give an intermediate like SbxOySz, which subsequently forms Sb2S3. In the absence of complexing agent, SbCl3 is completely hydrolyzed to give Sb4O5Cl2, and perhaps
Figure 8. SEM image of (a) the product obtained for 5 mmol of the TA when the reaction was carried out under refluxing conditions and (b) the product obtained in the reaction being carried out in autoclave at 120-160 °C for 10 mmol of TA used.
the reaction conditions are not favorable for this to react with KSCN and form Sb2S3. Therefore, tartaric acid-Sb complex acts as a precursor for the source of Sb3+ and controls the reaction for the formation of Sb2S3 in rodlike morphology. Figure 8a is a typical SEM image of the product obtained for the reaction with 5 mmol of tartaric acid. It shows some rodlike morphology, along with the flakelike microstructures. Absence of proper concentration of the complexing agent might be the cause for this. However, the reaction is optimized for 10 mmol of the complexing agent leading to the formation of only nanorods as a final product. When same reaction was carried out in autoclave in the temperature range of 120-160 °C for 24 h, Sb2S3 was obtained. The morphology was rodlike but much bigger in dimension. The typical SEM micrograph in Figure 8b shows that the rods are 2-4 µm wide and their length is around 15-20 µm. The same reaction was repeated using either thiourea or thioacetamide as an alternative source of sulfur while keeping all the other conditions the same. In the case of thioacetamide, the nanorods are almost comparable in dimension to those obtained earlier using potassium thiocyanate. However, when thiourea is used as the sulfur source, the yield is comparatively less, though the morphology remained more or less identical. The decrease of yield in the case of thiourea can be explained on the basis of the thermodynamic stability of the respective intermediates formed during the reaction progress. The reaction of thiourea and thioacetamide in acidic medium has been widely
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optimum concentration of it is necessary for complete formation of the Sb2S3. No change in the morphology of the product is observed even by changing the sulfur source, though their dimensions differed slightly from one another. The band gap of the nanorods is found to be 1.56 eV, which is suitable for photovoltaic conversion. References
Figure 9. UV-visible spectrum obtained from Sb2S3 nanorods. Inset is the plot of (Rhν)2 with respect to hν.
studied for the deposition of thin films of different metal sulfides.22,29 According to Rajpure et al.,29a in an acidic medium thioacetamide forms an intermediate (CH3CSH2NH+) that subsequently gives H2S, which acts as the sulfur source. Thioacetamide and thiourea have a similar type of structure with only a methyl group substituted by amine in case of the latter. Therefore, a similar type of intermediate, that is, (NH2CSH2NH+), is also expected for thiourea. But the latter one (NH2CSH2NH+) is thermodynamically more stable due to resonance between the lone pair on NH2 group and the positive charge developed on S. As a result, the subsequent progress of the reaction is retarded in the case of thiourea, which results in a low yield of Sb2S3 compared with the thioacetamide. Sb2S3 is very important from an optoelectronic point of view due to its comparatively higher band gap among the other semiconducting materials of the same group. Thus, the optical properties of the nanorods have been studied by absorption spectroscopy, and the corresponding UV-visible absorption spectrum of the nanorods is shown in the Figure 9. The band gap is calculated from this by extrapolating the linear part of curve of (Rhν)2 with respect to hν (inset) on the latter axis and is found to be 1.56 eV. This is quite comparable to the values reported for nanoribbons and nanorods of comparable dimensions.9,30 It is also noted that the nanorods in our dimension range do not show a quantum confinement effect, a fact also established by these workers. This may be attributed to the lower Bohr’s radius for this material. However, the band gap is suitable for the material to be used for photovoltaic conversion. Conclusion In summary, Sb2S3 nanorods have been successfully synthesized at relatively much lower temperature (115 °C) under refluxing conditions using tartaric acid as a complexing agent. Tartaric acid acts as both a coordinating agent to make the SbCl3 soluble in aqueous medium and a capping agent for onedimensional growth. The phase purity of the nanorods has been confirmed by help of XRD, XPS, and EDX studies. The effect of concentration of the complexing agent on formation of Sb2S3 nanorods has also been shown by XRD, suggesting that an
(1) Rao, C. N. R.; Muller, A.; Cheetham, A. K. Chemistry of Nanomaterials; Wiley-VCH: Weinhein, Germany, 2004. (2) Burda, C.; Chen, Z.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (3) (a) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (b) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.;Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (4) (a) Ota, J. R.; Roy, P.; Srivastava, S. K.; Bihro, R. P.; Tenne, R. Nanotechnology 2006, 17, 1700. (b) Roy, P.; Srivastava, S. K. Cryst. Growth Des. 2006, 6, 1921. (5) (a) Tritt, T. M. Science 1999, 283, 804. (b) Venkantasubramanian, R.; Siilvola, E.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597. (6) 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; p 186. (7) Ibuke, S.; Yochimatsu, S. J. Phys. Soc. Jpn. 1955, 10, 549. (b) Nair, M. T. S.; Pena, Y.; Campos, J.; Garcia, V. M.; Nair, P. K. J. Electrochem. Soc. 1998, 141, 2113. (8) Wyckoff, R. W. G. Crystal Structure, 2nd ed.; Wiley: New York, 1964. (9) Yu, Y.; Wang, R. H.; Chen, Q.; Peng, L. M. J. Phys. Chem. B 2005, 109, 23312. (10) Wang, H.; Lu, N. Y.; Zhu, J. J.; Chen, H. Y. Inorg. Chem. 2003, 42, 6404. (11) (a) Yang, J.; Zeng, J. H.; Yu, S. H.; Yang, L.; Zhang, Y. H.; Qian, Y. T. Chem. Mater. 2000, 12, 2924. (b) Zie, G.; Qiao, Z. P.; Zeng, M. H.; Chen, X. M.; Gao, S. L. Cryst. Growth Des. 2004, 4, 513. (12) Hu, H.; Mo, M.; Yang, B.; Zhang, X.; Li, Q.; Yu, W.; Qian, Y. J. Cryst. Growth 2003, 258, 106. (13) Yang, J.; Liu, Y. C.; Lin, H. M.; Chen, C. C. AdV. Mater. 2004, 16, 713. (14) Wnag, W.; Geng, Y.; Qian, Y.; Xi, Y.; Liu, X. Mater. Res. Bull. 1999, 34, 131. (15) Shen, G.; Chen, D.; Tang, K.; Li, F.; Qian, Y. Chem. Phys. Lett. 2003, 370, 334. (16) Liu, J.; Li, K.; Wang, H.; Jhu, M.; Yan, H. Chem. Phys. Lett. 2004, 396, 429. (17) Gong, J. Y.; Yu, S. H.; Qian, H. S.; Luo, L. B.; Liu, X. M. Chem. Mater. 2006, 18, 2012. (18) Chen, R.; So, M. H.; Che, C. M.; Sun, H. J. Mater. Chem. 2005, 15, 4540. (19) Yu, H. S.; Yang, J.; Wu, Y. S.; Han, Z. H.; Xie, Y.; Qian, Y. T. Mater. Res. Bull. 1998, 33, 1661. (20) Deng, Y.; Nan, C. W.; Guo, L. Chem. Phys. Lett. 2004, 383, 572. (21) Deng, Y.; Nan, C. W.; Guo, L. Chem. Phys. Lett. 2003, 368, 639. (22) Roy, P.; Srivastava, S. K. Mater. Chem. Phys. 2006, 95, 235. (23) Rajpure, K. Y.; Lokhande, C. D.; Bhosale, C. H. Mater. Chem. Phys. 1997, 51, 252. (24) Lu, Q.; Zeng, H.; Wang, Z.; Cao, X.; Zhang, L. Nanotechnology 2006, 17, 2098. (25) Ota, J. R.; Srivastava, S. K. Nanotechnology 2005, 16, 2415. (26) Espinos, J. P.; Elipe, A. R.; Jumas, J. C.; Fourcade, J. O.; Morales, J.; Tirado, J. L.; Lavela, P. Chem. Mater. 1997, 9, 1393. (27) Herzog, V. P.; Harmer, S. L.; Nesbitt, H. W.; Bancroft, G. M.; Flemming, R.; Pratt, A. R. Surf. Sci. 2006, 600, 348. (28) Godakh, S. R.; Bhosale, C. H. Mater. Chem. Phys. 2002, 78, 367. (29) (a) Mane, R. S.; Sankapal, B. R.; Lokhande, C. D. Mater. Res. Bull. 2000, 35, 587. (b) Wang, S. Y.; Du, Y. W. J. Cryst. Growth 2002, 236, 627. (30) Yu, Y.; Wang, R. H.; Chen, Q.; Peng, L. M. J. Phys. Chem. B 2006, 110, 13415.
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