Controlled Growth of Hexagonal Trigonal Selenium Microtubes Kun Tang, Dabin Yu,* Feng Wang, and Zirong Wang Laboratory of Optical and Nano-Scale Functional Materials, State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, Huangshan Road, Hefei, Anhui 230037, P. R. China
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 9 2159-2162
ReceiVed December 8, 2005; ReVised Manuscript ReceiVed June 15, 2006
ABSTRACT: Hexagonal selenium microtubes with a well-defined crystallographical structure were synthesized in the presence of n-hexadecyltrimethylammonium bromide (HTAB) at temperatures between 120 and 150 °C. The morphology of the products highly depended on the synthesis parameters, in particular, temperature and the concentration of HTAB, adjustment of which led to an obvious shape evolution of the products from rice-like rods to hexagonal rods, hexagonal tubes, and finally ribbed particles. The tubes were single crystals with average diameter of ca. 3.3 µm and length up to tens of micrometers. The results suggested that this should be an effective approach to the controlled growth of selenium microtubes, which may find applications in many fields. 1. Introduction Tubular micro- and nanocrystals with several different areas of contact (borders, inner and outer surfaces, and structured tube walls) that can be further functionalized in many ways, such as their usage as host materials, have been the subject of intensive research for many years.1 Among them, semiconductor tubes are of particular importance because of their intrinsic properties that exhibit wide practical and potential applications.2 Trigonal selenium (t-Se), a p-type semiconductor with well-known photoelectrical and biochemical properties,3 has been used in solar cells, rectifiers, photographic exposure meters, and even medical diagnostics.4 Over the past few years, t-Se nanorods and -wires have been generated by many strategies,5 but there are still few approaches to its tubular micro- and nanocrystals, for example, the hydrothermal method reported by Rao’s group,6 the sonochemical process by Li’s group,7 and the carbothermal chemical vapor deposition route by Zhang’s group.8 In this paper, we report the large-scale synthesis of t-Se microtubes by a new n-hexadecyltrimethylammonium bromide (HTAB)-controlled synthesis method under solvothermal conditions by using peroxide as oxidizer and HTAB as shape controller. This is a novel “from Se powder to tubes” route, in which Se powders were used as the Se source and reduced by sodium sulfite to form sodium selenosulfate, which was subsequently oxidized by peroxide to form elemental Se, and the growth of crystalline Se was controlled by adjusting temperature and the concentration of HTAB. Note that the synthesized Se microtubes were characterized by the nature of single crystals with a well-defined crystallographical structure, which, to our knowledge, has been seldom reported so far.
Figure 1. XRD pattern of the products. up to 100 mL with deionized water. The concentration of sodium selenosulfate was ca. 0.08 M. Growth of Se Microtubes. In a typical synthesis procedure of Se microtubes, 25 mL of deionized water and 10 mL of HTAB (50 mM) solution were added into a Teflon-lined stainless steel autoclave of 45 mL capacity, followed by the addition of 2.5 mL of freshly prepared sodium selenosulfate solution under vigorous stirring. H2O2 solution (0.1 M, 2.5 mL) was then added into the autoclave under stirring. The autoclave was sealed and heated at 140 °C in a furnace for 12 h. It was then allowed to cool naturally to room temperature. After that, a black product was generated, and it was washed with water and ethanol sequentially and collected for further characterization. Characterization. The XRD patterns were recorded on a Philips X’Pert PROSUPER X-ray powder diffractometer (λ ) 1.541 78 Å) in the 2θ range of 10-70°. TEM images and the corresponding selected area electron diffraction (SAED) patterns were taken using a HITACHI 800 instrument operated at an acceleration voltage of 200 kV. The scanning electron microscopy (SEM) images were taken on a LEO1530 scanning electron microscope.
2. Experimental Section Materials. Sodium sulfite (99.80%), Se powder (99.90%), nhexadecyltrimethylammonium bromide (HTAB, 98%), and peroxide (60%) were purchased from Shanghai Chemical Reagents Co, and all of them were used as received. Preparation of Sodium Selenosulfate Aqueous Solution. Na2SO3 (0.010 mol), Se powder (0.010 mol), and deionized water (10 mL) were mixed in a flask and refluxed at 98 °C for 4 h. After it cooled to room temperature, the mixture was filtered to remove the remained Se powder, and a clear solution was obtained. The solution was then quantitatively transferred into a 100-mL volumetric flask and was made * E-mail address:
[email protected].
3. Results and Discussion A typical XRD pattern of the products is shown in Figure 1, the peaks of which can be indexed to t-Se with the lattice parameters of a ) 0.4368 Å and c ) 0.4952 Å, which are consistent with those in the literature (PCPDF no. 6-362). This crystal structure is characterized by a symmetry, the parallel spiral chains of selenium atoms terminating at the corners and center of a regular hexagon, thus resulting in a very stable crystalline form of Se because the bonding within the chains in much stronger than the bonding between the chains.3 Compared to the peaks in the standard pattern, the (100) diffraction peak
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Figure 2. SEM images of (a) Se microtubes, (b) tubes with higher magnifications obtained from the area marked with a black pane as shown in panel a, (c) the end of an individual tube, and (d) the broken end of a tube.
Figure 3. (a) TEM image of a tube and the corresponding SAED pattern; (b) schematic illustration of the crystal structure of an individual tube.
abnormally showed the strongest intensity in the pattern, indicating that the product had a structure with preferential growth along the [001] orientation. Figure 2a shows a typical SEM image of the Se microtubes, indicating that the products consisted of a large quantity of microtubes with average diameter of ca. 3 µm and length up to tens of micrometers. Figure 2b shows the SEM image of Se tubes with higher magnifications obtained from the area marked with a black pane as shown in Figure 2a. As indicated by the black arrows, the tubes have opened ends. It is noteworthy that, as shown in Figure 2c, the outer surfaces of the tube formed a hexagonal prism, while the inner surfaces of the tubes were not very regular, as shown obviously by a tube with a broken end in Figure 2d. Although most of the tubes with thick walls (∼300 nm) were difficult for an electron beam to transmit, a few of them with relatively thin walls could still be confirmed by the contrast of TEM images as shown in Figure 3a. The insert in Figure 3a
Figure 4. SEM images of the samples obtained at a relatively low or much higher temperature with the concentration of HTAB fixed at 12.5 mM: (a) 100 °C; (b) 170 °C.
shows the SAED pattern of the corresponding tube, which was obtained by focusing the electron beam along the zone axis, the spots of which could be assigned to (003) and (110), respectively. Because the direction of (003) in the pattern is parallel to the tube axis, the tube is a single crystal with the
Controlled Growth of Hexagonal t-Se Microtubes
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Figure 5. SEM images of the samples obtained at the conditions with temperature fixed at 140 °C and with different concentrations of HTAB: (a) 5; (b) 10; (c) 20; (d) 50 mM. The inset in panel b shows the end of a hexagonal rod. Table 1. Various Se Particles Synthesized under Different Conditionsa [HTAB], mM
temp, °C
40 12.5 12.5
140 140 140 140 140 140 140 100 >160
shape/profile irregular particle rice-like rods hexagonal rods hexagonal tubes hexagonal tubes tubes, short rods ribbed particles needle-like rods short rods, irregular particle
average dimension, µmb 0.1 0.5 3 3.5 2 0.08
a The concentrations were calculated on the basis of the total volume of the reaction mixture, that is, the mixture of the solutions of HTAB and the reagents together with the water in the autoclave. The synthesis parameters were fixed as related in the Experimental Section, unless indicated otherwise in the table. b For tubes, this corresponds to the mean edge length; for rods, this corresponds to the diameters.
growth along the [001] direction, resulting from the highly anisotropic structure of Se, which is in good agreement with the analysis of the XRD pattern. In fact, such a growth feature of t-Se nanotubes has been found by several groups.8,9 Considering the shape, growth direction, and SAED pattern of the tube, as well as the nature of t-Se crystal structure, the regular hexagonal prism was, most probably, bounded by six symmetric facets, that is, (1h00), (010), (110), (100), (01h0), and (110), as schematicly shown in Figure 3b. The formation of Se microtubes was affected by various synthesis parameters, among which synthesis temperature and the concentration of HTAB were particularly crucial for the control of the morphology of Se particles, thus resulting in interesting combinations for the controlled synthesis of Se tubes (Table 1). It was found that the synthesis temperature played in an important role in the crystal growth of Se, but had no significant effect on the chemical reactions. The synthesis
reactions were described as in eqs 1 and 2, while the growth of
Na2SO3 + Se f Na2SSeO3
(1)
Na2SSeO3 + H2O2 f Se + Na2SO4 + H2O
(2)
Se tubes was mainly achieved on the basis of reaction 2. Upon addition of H2O2 solution into sodium selenosulfate aqueous solution, there appeared a brick red solution intermediately, originating from the reaction between selenosulfate ions and H2O2 to form elemental R-Se. Because R-Se is a less stable form, it will gradually turn into the more stable t-Se under hydrothermal conditions.10 That is, the synthesis reactions could be, in fact, performed at room temperature, but the growth of t-Se tubes should be achieved at a higher temperature, which effected not only the change of the crystal structure but the control of the morphology as well. The products were mainly dominated by needle-like rods at a relative low temperature (Figure 4a), while they were short hexagonal rods with poorly controlled shape at a much higher temperature (Figure 4b). Experimental facts showed that the desired temperature for the growth of the t-Se tubes ranged between 120 and 150 °C. As shape controller, HTAB was indispensable for the growth of the hexagonal Se crystals, in particular, those tubular ones. As shown in Table 1, with synthesis temperature fixed at 140 °C, an increase in the concentration of HTAB from 5 to 50 mM led to an obvious shape evolution of the products from rice-like rods (Figure 5a) to hexagonal rods (Figure 5b), hexagonal tubes (Figure 5c), and finally ribbed particles (Figure 5d). The hexagonal Se tubes could be synthesized in a wide concentration range of HTAB at a suitable temperature, while both the size and length of the tubes decreased obviously with the increase of the concentration. As an example, there is an obvious difference between the tubes shown in Figure 2 and the tubes shown in Figure 5c, which was attributed to the difference of the concentration of HTAB. It is noteworthy that
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when the concentration of HTAB was near 10 mM, the products began to form the shape with hexagonal prisms, when it ranged from 12 to 25 mM, the hexagonal tubes were produced, and when it was higher than 40 mM, there was a sharp shape change that the tendency of 1D growth disappeared but most of the particles still had obvious prisms (Figure 5d). It is known that there is a selective adsorption between surfactant and various crystallographic planes, thus resulting in the formation of many kinds of nanocrystals with well-defined morphologies in the presence of surfactant such as HTAB or cetyltrimethylammonium bromide (CTAB).11 Such adsorption has close relations with the concentration of HTAB.12 As a result, adjustment of the concentrations of HTAB led to an obvious shape evolution of Se crystals. Additionally, some other parameters such as synthesis time and the concentrations of the starting materials also affected the formation of Se crystals, but they were not the determining factors, compared to temperature and the concentration of HTAB. A desired molar ratio of H2O2 to sodium selenosulfate was ca. 1.2 to make reaction 2 perform completely, but too much superfluous H2O2 was also unfavorable for the synthesis due to its strong oxidation. 4. Conclusion In conclusion, hexagonal selenium microtubes with a welldefined crystallographical structure were, for the first time, synthesized on the basis of a new HTAB-controlled synthesis route. The products were characterized by various means (XRD, SEM, TEM, and SAED), showing that the tubes have the nature of a single crystal with growth along the [001] direction, and their regular hexagonal prisms were bounded by six symmetric facets, that is, (1h00), (010), (110), (100), (01h0), and (110). Adjustment of the synthesis parameters led to an obvious shape evolution of the products, exhibiting an effective route to Se crystals with designed morphologies. Considering the unique
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properties of t-Se, its microtubes with such a well-defined crystallographical structure should be of importance for both theoretical investigations and practical applications. Acknowledgment. We acknowledge the financial support from the Natural Science Foundation of Anhui Province (Grant 03044903). References (1) (a) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2447. (b) Dai, H. J.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley, R. E. Nature 1996, 384, 147. (c) Ajayan, P. M.; Zhou, O. Z. Top. Appl. Phys. 2001, 80, 391. (2) Lee, S. M.; Lee, Y. H.; Hwang, Y. G. Elsener, J.; Porezag, D.; Frauenheim, T. Phys. ReV. B 1999, 60, 7788. (3) Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. N. AdV. Funct. Mater. 2002, 12, 217. (4) Zingaro, R. A., Cooper, W. C., Eds.; Selenium; Litton Educational Publishing: New York, 1974, pp 12-28. (5) (a) Liu, X.; Mo, M.; Meng, J.; Qian, Y. T. J. Cryst. Growth 2003, 259, 144. (b) Gates, B.; Yin, Y.; Xia, Y. N. J. Am. Chem. Soc. 2000, 122, 12582. (c) Gates, B.; Myers, B.; Grossman, A.; Xia, Y. N. AdV. Mater. 2002, 14, 1749. (6) Gautam, U. K.; Nath, M.; Rao, C. N. R. J. Mater. Chem. 2003, 13, 2845. (7) Li, X.; Li, Y.; Zhou, W.; Chu, H.; Chen, W.; Li, I. L.; Tang, Z. Cryst. Growth Des. 2005, 5, 911. (8) Zhang, H.; Zuo, M.; Tan, S.; Li, G.; Zhang, S.; Hou, J. J. Phys. Chem. B 2005, 109, 10653. (9) Fan, H.; Wang, Z.; Liu, X.; Zheng, W.; Guo, F.; Qian, Y. T. Solid State Commun. 2005, 135, 319. (10) Wang, Z.; Chen, X.; Liu, J.; Yang, X.; Qian, Y. Inorg. Chem. Commun. 2003, 6, 1329. (11) (a) San, T. K.; Murphy, C. J. J. Am Chem. Soc. 2004, 126, 8648. (b) Yu D. B.; Yam, V. W. W. J. Am. Chem. Soc. 2004, 126, 13200. (c) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. N. Nano Lett. 2003, 3, 955. (d) Sun, Y.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 3892. (12) Handbook of Surface and Colloid Chemistry, 2nd ed.; Birdi, K. S., Ed.; CRC Press: London, 2003; pp 346-385.
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