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Synthesis and Characterization of CdS Nanorods via Hydrothermal Microemulsion Peng Zhang and Lian Gao* State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China Received July 16, 2002. In Final Form: October 16, 2002
Introduction Nanoscale one-dimensional structures have attracted considerable attention due to their unique electronic, optical, and mechanical properties.1,2 The development of synthesis methods, control of morphology, and assembly of desired nanostructure still remain a challenge in this field.3-10 In recent years, synthesis of one-dimensional semiconductor materials such as nanowires, nanorods, or fibers has been the focus of research work.11-14 As an important II-VI semiconductor material, CdS nanocrystal has received considerable interest of researchers in the control of its morphology and size. Various approaches, such as the solvothermal route,15-17 a liquid crystal template,18 irradiation,19,20 polymer controlled growth,21 and electrodeposition on a porous template,22 have been applied to achieve one-dimensional CdS nanocrystals. As ideal media for the synthesis of nanoparticles, microemulsion systems have been widely employed to prepare superfine spherical particles.23,24 “Water-in-oil” * Corresponding author. E-mail:
[email protected]. Telephone: +86-21-52412718. Fax: +86-21-52413122. (1) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (2) Dekker, C. Phys. Today. 1999, 52, 22. (3) Pileni, M. P. Langmuir. 1997, 13, 3266. (4) Wang, J.; Deng, Z.; Li, Y. Mater. Res. Bull. 2002, 37, 495. (5) Xiong, Y.; Xie, Y.; Du, G.; Tian, X. J. Mater. Chem. 2002, 12, 98. (6) Chen, M.; Xie, Y.; Yao, Z.; Qian, Y.; Zhou, G. Mater. Res. Bull. 2002, 37, 247. (7) Lu, J.; Xie, Y.; Du, G.; Jiang, X.; Zhu, L.; Wang; Qian, Y. J. Mater. Chem. 2002, 12, 103. (8) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (9) Yu, Y.; Chang, S.; Lee, C.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (10) Yu, D.; Wang, D.; Meng, Z.; Lu, J.; Qian, Y. J. Mater. Chem. 2002, 12, 403. (11) Li, Y.; Liao, H.; Ding, Y.; Qian, Y.; Yang, L.; Zhou, G. Chem. Mater. 1998, 10, 2301. (12) Yu, S.; Wu, Y.; Yang, J.; Han, Z.; Xie, Y.; Qian, Y.; Liu, X. Chem. Mater. 1998, 10, 2309. (13) Shen, C.; Zhang, X.; Li, H. Mater. Sci. Eng. 2001, A303, 19. (14) Yang, C.; Awschalom, D. D.; Stucky, G. D. Chem. Mater. 2002, 14, 1277. (15) Yang, J.; Zeng, J.; Yu, S.; Yang, L.; Zhou, G.; Qian, Y. Chem. Mater. 2000, 12, 3259. (16) Wu, J.; Jiang, Y.; Li, Q.; Liu, X.; Qian, Y. J. Cryst. Growth 2002, 235, 421. (17) Chen, M.; Xie, Y.; Lu, J.; Xiong, Y.; Zhang, S.; Qian, Y.; Liu, X. J. Mater. Chem. 2002, 12, 748. (18) Li, Y.; Wan, J.; Gu, Z. Mater. Sci. Eng. 2000, A286, 106. (19) Mo, X.; Wang, C.; You, M.; Zhu, Y.; Chen, Z.; Hu, Y. Mater. Res. Bull. 2001, 36, 2277. (20) Nikitenko, S. I.; Koltypin, Y.; Mastai, Y.; Koltypin, M.; Gedanken, A. J. Mater. Chem. 2002, 12, 1450. (21) Zhan, J.; Yang, X.; Wang, D.; Li, S.; Xie, Y.; Xia, Y.; Qian, Y. Adv. Mater. 2000, 12, 1348. (22) Xu, D.; Xu, Y.; Chen, D.; Guo, G.; Gui, L.; Tang, Y. Chem. Phys. Lett. 2000, 325, 340. (23) Schwuger, M.; Stickdom, K.; Schomacker, R. Chem. Rev. 1995, 95, 849. (24) Gan, L. M.; Liu, B.; Chew, C. H.; Xu, S. J.; Chua, S. J.; Loy, G. L.; Xu, G. Q. Langmuir 1997, 13, 6427.
microemulsions contain nanosized water pools which are dispersed in a continuous oil medium and are stabilized by surfactant and cosurfactant molecules localized at the water/oil interface. The nanoscale water pools can provide ideal microreactors for the formation of nanoparticles.25,26 Quaternary microemulsions such as CTAB/water/hexane/ n-pentanol have certain advantages in preparing spherical nanoparticles.27 As far as the quaternary microemulsion is concerned, the molar ratio between water and surfactant (W) and that between alcohol and surfactant (P0), together with the surfactant molar concentration [S], are the three factors that determine the composition of the microemulsion. The size of the water pools within the microemulsion is a function of the water content (W). And all three factors have an influence on the dynamic behavior of the reverse micelles in microemulsion. Nanorods have been prepared in CTAB micellar solution via arrested precipitation.28 However, there are few reports concerning the formation of nanorods or nanowires in microemulsion systems. BaSO4 nanofilaments and BaCO3 nanowires have been prepared in microemulsions and reverse micelles, respectively, at room temperature after aging for several days. A formation mechanism of each sample has also been proposed.29,30 The hydrothermal method has been utilized in microemulsions as a treatment of the product to improve the crystallinity.31,32 There are also a few reports concerning hydrothermal microemulsions as a synthesis method, but only large particles were obtained.33 When temperature rises, the reverse micelles in microemulsion may be broken, resulting in fast nucleation of clusters in a random orientation and, consequently, a larger particle size.23,33 It is this process that limits the application of the hydrothermal method on microemulsions. In this work, we present hydrothermal microemulsion as a novel technique to synthesize cadmium sulfide nanorods with hexagonal phase at a relatively low temperature. On the basis of the as-observed tower-shaped intermediate state, the formation process of cadmium sulfide nanorods and the corresponding mechanism are discussed. Experiment All of the reactants and solvents are analytical-grade and used without any further purification. In a typical experimental procedure, 75 mL of a W ) 30, P0 ) 8.65, [CTAB] ) 0.1 M CTAB/ water/hexane/n-pentanol quaternary microemulsion was prepared at room temperature. The stock aqueous solution was prepared by dissolving carbamide with 0.1 M cadmium nitrate solution to a concentration of 0.3 M. Then 0.07 mL of carbon disulfide was added to the microemulsion. Afterward, the optically transparent stock microemulsion was poured into a 100 mL stainless Teflon-lined autoclave and maintained at 130 °C for 15 (25) Tojo, C.; Blanco, M. C.; Rivadulla, F.; Lopez-Quintela, M. A. Langmuir 1997, 13, 1970. (26) Giustini, M.; Palazzo, G.; Colafemmina, G.; Monica, M. D.; Giomini, M.; Ceglie, A. J. Phys. Chem. 1996, 100, 3190. (27) Agostiano, A.; Catalano, M.; Curri, M. L.; Monica, M. D.; Manna, L.; Vasanelli, L. Micron 2000, 31, 253. (28) Chen, C.; Chao, C.; Lang, Z. Chem. Mater. 2000, 12, 1516. (29) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 1819. (30) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. J. Phys. Chem. B 1997, 101, 3460. (31) Xu, S. J.; Chua, S. J.; Liu, B.; Gan, L. M.; Chew, C. H.; Xu, G. Q. Appl. Phys. Lett. 1998, 73, 478. (32) So, W. W.; Jang, J. S.; Rhee, Y. W.; Kim, K. J.; Moon, S. J. J. Colloid Interface Sci. 2001, 237, 136. (33) Liu, B.; Xu, G. Q.; Gan, L. M.; Chew, C. H.; Li, W. S.; Shen, Z. X. J. Appl. Phys. 2001, 89, 1059.
10.1021/la0206458 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/07/2002
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Figure 1. X-ray diffraction (XRD) patterns of CdS samples prepared at 130 °C for 15 h (a), 10 h (b), and 5 h (c), respectively. h. Finally, a yellow precipitate was obtained by drying the product in a vacuum at 70 °C. After being dissolved in ethanol via ultrasonication, the precipitate was centrifuged to form a pellet and washed with ethanol several times. Finally, the product was dried in a vacuum at room temperature and collected. To investigate the formation process of nanorods in the microemulsion system, we also prepared samples by taking the autoclaves out of the oven after the reaction was carried out for 5 and 10 h, respectively, and cooling them to room temperature immediately. The as-prepared powder sample was characterized by X-ray powder diffraction on a D/Max 2550V X-ray diffractometer with Cu KR irradiation at λ ) 1.5406 Å. Transmission electron microscopy (TEM) observations were performed using a JEOL 200CX transmission electron microscope. And high resolution transmission electron microscopy (HRTEM) was carried out using a JEOL 2010 electron microscope. Samples were deposited on thin amorphous carbon films supported by copper grids from ultrasonically processed ethanol solutions of the products. Energy dispersive spectrometry (EDS) was also performed on an OXFORD ISIS 300 to identify the exact elemental component of the prepared nanorods.
Results and Discussion The XRD patterns in Figure 1a show that the asprepared CdS sample at 130 °C for 15 h is in the hexagonal phase with cell constants a ) 4.125 Å and c ) 6.692 Å, which are very close to the values in the literature (JCPDS Card, File No. 41-1049). It is interesting that wurtzite CdS was obtained at such a low temperature. According to Rhee et al. the temperature at which a CdS nanocrystal changes from the amorphous phase to the hexagonal phase when treated with the hydrothermal method was up to 160 °C, and a nearly complete hexagonal phase was obtained at 240 °C.32 Figure 1b and c shows the XRD patterns of the CdS samples at 130 °C for 5 and 10 h, respectively. The relative intensity of the (002) peak increases with the prolonging of reaction time, which indicates the crystals are grown along the c-axis.15,17 Nanorods with diameters of 30-80 nm and lengths up to 170-1110 nm are observed in the TEM image (Figure 2a). In a much clearer image of the rods (Figure 2b), we find straight traverse stripes which may illustrate the growth progress of the crystals. To determine the detail of the stripes, high resolution transmission electron microscopy (HRTEM) was performed on a single rod, as shown in Figure 2c. The traverse stripes on the rods may be attributed to the defects during the growth process. We notice that the rod-shaped crystal is preferentially grown along the c-axis. The distance of the (001) lattice space is 0.67 nm, which is close to the c-value estimated from the XRD pattern. The elemental composition of the
Figure 2. (a) TEM image of CdS nanorods. The inset shows the corresponding electron diffraction pattern. (b) TEM image of several nanorods; clear traverse stripes are observed on rods. (c) HRTEM image of a single rod showing that the nanorod is a single crystal with a growing direction of (001). The traverse stripes are attributed to be the growing defects between original particles. The inset shows the corresponding electron diffraction pattern.
single rod is determined by energy dispersive spectrometry (EDS), which displays cadmium LR1 (3.13 keV), Lβ1 (3.31 keV), and Lβ2 (3.53 keV) peaks and sulfur KR1 (2.30 keV) peaks. The quantification calculation shows the ratio of Cd/S to be 50.80/49.20. These data clearly indicate that the as-prepared nanorods are exactly CdS. Figure 3 shows the morphologies of samples obtained under the same conditions for 5 and 10 h, respectively, which illustrate the intermediate states during the formation of nanorods. Interesting tower-shaped rods rather than straight nanorods are found in Figure 3a, which seem to be composed of many quasi-triangles linked together. However, the sharp edges of the triangles are greatly blurred in Figure 3b. And the final product (Figure 2) is nanorods with a smooth surface. The formation mechanism as described in the literature,28 preformation of rodlike micelles, finds it hard to explain the existence of intermediate tower-shaped rods with sharp edges in microemulsion systems. On the basis of our results, we prefer the directed aggregation mechanism in the formation process.29,30 It has been reported that triangular CdS nanocrystals with 10 nm sides were produced in reverse micellar solution.34 We propose that triangular crystals
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Figure 4. TEM image of the sample produced from microemulsion with the addition of n-dodecanethiol. Spherical (quasitriangular) particles are obtained. The inset shows the SAED image of the particles.
Figure 3. (a) TEM image of the sample produced at 130 °C for 5 h. The rod shows an interesting tower shape. The inset shows the SAED pattern of the several selected triangular crystals of the rod. (b) TEM image of the sample produced at 130 °C for 10 h with the sharp edge of the tower-shaped rod greatly blurred.
with sides of 30-50 nm are obtained during the early stage of the reaction. Then the crystals self-assemble along the c-axis to reduce the surface of the (001) plane due to its high chemical potential,35 which leads to the formation of the tower-shaped rods. With the slow release of S2from the chemical reaction, Cd2+ adsorbed on the (001) surface is selectively coordinated with S2-. The sharp edges of the tower-shaped rods gradually blur because of the further growth of CdS on the zigzag of the tower-shaped rods, as can be seen in Figure 3b. The growth stripes remaining on the final smooth nanorods represent the attachment site of each single particle. Pairs of rods may be coupled through the c-axis, leading to the increase of rod length. A certain rod composed of two rods with different diameters in Figure 2b provides evidence for this process. The velocity of the provision of S2- can greatly affect the directed aggregation. Among the reactants, (NH2)2CO is dissolved in water and CS2 is dispersed in oil, so that the reaction must be performed at the interface (34) Pinna, N.; Weiss, K.; Sack-Kongehl, H.; Vogel, J.; Pileni, M. P. Langmuir 2001, 17, 7982. (35) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389.
between the oil and water phases. This limited interfacial reaction slows down the reaction rate and the release of sulfur anion, allowing the growth of crystals at a moderate rate within the water pools of the microemulsion. The slow provision of sulfur anion promotes the self-assembly (directed aggregation) of the crystals, so as to decrease the area of the high potential (001) plane. When we selected thioacetamide as the sulfur source, only pyramid-shaped single crystals of CdS (with an average side of 200 nm) were obtained due to quick release of S2- and the resulting breaking down of the motivation of the self-assembly process. To further corroborate the assumption of directed aggregation, a certain amount of n-dodecanethiol was introduced into the stock microemulsion before heating. The strong chemical bonds between -SH and Cd2+ should greatly decrease the chemical potential of the (001) surface and inhibit the self-assembly. Then quasi-triangular particles of CdS nanocrystals should be anticipated in the resulting sample. Figure 4 shows monodispersed wurtzite particles (quasi-triangles exactly) with diameters of 3050 nm without any trace of nanorods. On the basis of these facts, we believe that although the details of the formation process need further investigation, the directed aggregation process can to some extent explain the formation process of nanorods in microemulsion. Conclusion This paper demonstrates that microemulsion, generally an ideal medium for the preparation of spherical nanoparticles, can also be used to prepare CdS nanorods under hydrothermal conditions. An interesting tower-shaped intermediate state clearly illustrates the formation process of nanorods. Slow reactions between carbamide and carbon disulfide are employed to provide the sulfur anion continuously. LA0206458
