Langmuir 2002, 18, 10503-10504
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Synthesis of Size-Tunable Silver Iodide Nanowires in Reverse Micelles Sheng Xu,† Haicheng Zhou,‡ Jian Xu,† and Yadong Li*,† Department of Chemistry, the Key Laboratory of Atomic and Molecular Nanoscience (China Ministry of Education), Tsinghua University, Beijing 100084, People’s Republic of China; and Department of Chemistry and Engineering, Shandong University of Technology, Zibo, Shandong 255000, People’s Republic of China Received August 23, 2002. In Final Form: October 14, 2002
Nanomaterials have attracted great interest for a few years due to their marvelous characteristics that are different from those of bulk materials. Recently, onedimensional nanoscale building blocks, such as nanotubes and nanowires, have been becoming the focus of attention because of their possible application in optics, electricity, and nanodevices.1-3 Many efforts have been made to synthesize nanomaterials in microemulsions or organized system.4-13 However, until now there was still a lack of information about the preparation of well-defined 1-D AgI semiconductor nanomaterials. As is well-known, silver halides are light sensitive, and this property forms the basis of photography. Particularly, highly sensitive films contain AgI. Besides, AgI is very effective in nucleating ice-crystals in supercooled clouds, thereby inducing the precipitation. These properties have made AgI commercially available for a long time. Much attention has been paid to the preparation and characterization of AgI nanoparticles.14-18 However, to the best of our knowledge, the preparation of AgI nanowires has not been reported in the literature. In this paper, we introduce a soft-templated method to synthesize AgI nanowires in reverse micelles under selected conditions. This work will offer opportunities to study the characteristics of 1-D nanostructured AgI semiconductors in prospective research. * To whom all correspondence should be addressed. E-mail:
[email protected]. † Tsinghua University. ‡ Shandong University of Technology. (1) Dekker, C. Phys. Today 1999, 52, 22. (2) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (3) Wang, X.; Li, Y. D. J. Am. Chem. Soc. 2002, 124, 2880. (4) Kim, F.; Kwan, S.; Akana, J.; Yang, P. D. J. Am. Chem. Soc. 2001, 123, 4360. (5) ; Kwan, S.; Kim, F.; Akana, J.; Yang, P. D. Chem. Commun. 2001, 447. (6) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (7) Qi, L. M.; Colfen, H.; Antonietti, M.; Li, M.; Hopwood, J. D.; Ashley, A. J.; Mann, S. Chem. Eur. J. 2001, 7, 3526. (8) Fendler, J. H. Chem. Rev. 1987, 87, 877. (9) Dixit, S. G.; Mahadeshwar, A. R.; Haram, S. K. Colloids Surf., A 1998, 133, 69. (10) Eastoe, J.; Wayne, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 800. (11) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (12) Rees, G. D.; Evans-Growing, R.; Hammond, S. J.; Robinson, B. H. Langmuir 1999, 15, 1993. (13) Pileni, M. P.; Taleb, A.; Petit, C. J. Dispersion Sci. Technol. 1998, 19, 185. (14) Micic, O. I.; Meglic, M.; Lawless, D.; Sharma, D. K.; Serpone, N. Langmuir 1990, 6, 487. (15) Schmidt, K. H.; Patel, R.; Meisel, D. J. Am. Chem. Soc. 1988, 110, 4882. (16) Chen, S. H.; Ida, T.; Kimura, K. J. Phys. Chem. B 1998, 102, 6169. (17) Wang, Y. H.; Mo, J. M.; Cai, W. L.; Yao, L. Z. J. Mater. Res. 2001, 16, 990. (18) Brelle, M. C.; Zhang, J. Z. J. Chem. Phys. 1998, 108, 3119.
Figure 1. TEM images of AgI nanowires prepared at [I-]/[Ag+] ) 3: (a) ω ) 10; (b) ω ) 11; (c) ω ) 11 (without n-pentanol). (d) Selected area in part c (outlined with a rectangle) with a higher magnification.
A typical synthesis of AgI nanowires was as follows. AgNO3 or KI solution was added to a cyclohexane/Triton X-100/n-pentanol system. After 15 min of vigorous agitation, equivalent volumes of two separate microemulsion solutions containing AgNO3 or KI were mixed rapidly, to give final molar ratios of [I-]/[Ag+] ) 3 and water content ω ) [H2O]/[Triton X-100] (molar ratio) ) 11. The resulting yellowish mixture was then laid aside for l day at room temperature in the dark. One drop of solution as-prepared was dispersed in 2 mL of ethanol via ultrasonic waves. Sample as-obtained was placed on a copper grid and was visualized with a Hitachi Modle-800 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. AgI nanowires prepared at ω ) 10 (Figure 1a) were uniform in diameter (40 nm) and length (up to 1∼2 µm), while at ω ) 11 the length of AgI nanowires (Figure 1b) could reach ∼8 µm and lead to an aspect ratio of ∼260. It indicates that water content ω, which determines the size of the droplet in reverse micelles, has an important influence on the shape and length of the product. Partly self-assembled AgI nanowires could be observed arraying parallel to each other and forming bundlelike aggregates (Figure 1a and b). In the following part, ω ) 11 was chosen unless otherwise specified. For comparison, slight curved AgI nanowires with diameters ranging from 40 to 100 nm were obtained in reverse micelles without the presence of n-pentanol, as shown in Figure 1c. It is known that the incorporation of short chain alkanols makes reverse micelles more stable. In addition, the presence of alkanols decreases the aggregation number of the surfactant molecules and the diameter of the reverse
10.1021/la0264623 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/27/2002
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Notes
Figure 2. Schematic illustration of proposed formation mechanism for AgI nanowires.
micelles.19 Hence, it is reasonable that relatively uniform nanowires with smaller diameters could be achieved with participation of n-pentanol (Figure 1a and b). It is noteworthy that many discrete Ag particles could be found along AgI wires under an electron beam for 1∼2 s, as shown in Figure 1c and d (with a larger magnification). Therefore, electron diffraction (ED) analysis for an individual AgI wire was difficult to perform. Nevertheless, the decomposition of light sensitive AgI suggests the possibilities of implementing 1-D silver nanostructures from AgI nanowires.20 To our surprise, AgI nanowires were unable to be obtained when the molar ratios of I- ions to Ag+ ions e 1, and mainly spherical particles were observed with average diameter of 30 nm. It was verified reproducibly that, only around [I-]/[Ag+] ) 3, relatively uniform and long AgI nanowires (Figure 1) could form. Other reactant molar ratios were also attempted. At molar ratio ∼ 2, short nanorods with average diameter 40 nm were observed coexisting with irregular flakelike particles; at molar ratio > 3, much wider rodlike particles (200-500 nm in diameter) with lower aspect ratios (below 10) appeared. In view of the different shape and size of AgI precipitation exhibited at different reactant molar ratios, the formation mechanism of AgI nanowires is proposed as follows (see Figure 2). Surfactants can form reverse micelles in nonpolar solvents, and water is readily solubilized in the polar core. At the appropriate formula and suitable conditions, the reverse micelles may array directionally to form rodlike aggregation, in which the water channel is able to restrict the growth of the nuclei. The reactants exchange, nucleate, and grow into final products. In aqueous solution, when the amount of I- ions exceeds that of Ag+ ions, the complex ions can be formed,21,22 such as AgI2-, AgI32-, Ag2I64-, Ag3I85-, ..., and so forth:
Ag+ + I- T AgInucleation AgI + I- T AgI2AgI2- + I- T AgI32... mAgI + nI- T AgmIm+nn-complexation mAgI T (AgI)mgrowth
In our experimental work, molar ratios of [I-]/[Ag+] ∼ 1 were found to be favorable to formation of AgI spherical nanoparticles. It can be assumed that the formation of elongated wires is not satisfied without complexation of reactant ions. At [I-]/[Ag+] ∼ 2, short nanorods emerged predominantly rather than spherical particles due to complexation, which slows the growth of nuclei to some extent. Further addition of I- to [I-]/[Ag+] ∼ 3 leads to more complexation; long AgI nanowires with relatively uniform diameters may preferably be formed within the nanochannel in reverse micelles (Figure 2). This is probably because these complex ions can be regarded as a reservoir of AgI to make the nuclei grow gradually along the rodlike micelles. At other molar ratios > 3, much wider rodlike particles were observed. It is possible that the tradeoff between the preferential growth rate of AgI wires and complexation in an aqueous solution of nanochannels plays an important role on the shape and size of the final products. [I-]/[Ag+] ) 3 at ω ) 11 may be an optimal condition for the synthesis of relatively uniform AgI products. Further experimental investigation needs to be pursued, as well as experimentation on revealing the nanostructure of the reverse micelle systems adopted herein. In conclusion, AgI nanowires were synthesized in quaternary or ternary microemulsions. By changing reactant molar ratios at selected ω values, the size of AgI nanowires is partly tunable. The formation mechanism is proposed tentatively, and detailed information needs to be explored. This simple method can be expected to synthesize 1-D nanomaterials of other silver halides and silver. Acknowledgment. This work was supported by NSFC (20025102, 50028201, 20151001), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China, and the State Key Project of Fundamental Research for Nanomaterials and Nanostructures. LA0264623 (19) Moulik, S. P.; Paul, B. K. Adv. Colloid Interface Sci. 1998, 78, 99. (20) Liu, S. W.; Yue, J.; Gedanken, A. Adv. Mater. 2001, 13, 656. (21) Leden, I. Acta Chem. Scand. 1956, 10, 812. (22) Leden, I. Acta Chem. Scand. 1956, 10, 540.
