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Tetrathiafulvalene-Assisted Formation of Silver Dendritic Nanostructures in Acetonitrile Xiaqin Wang, Hideaki Itoh, Kensuke Naka,* and Yoshiki Chujo* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, 606-8501, Japan Received December 26, 2002. In Final Form: May 5, 2003 Silver dendritic nanostructures protected by tetrathiafulvalene (TTF) were synthesized via electron transfer from TTF to silver ions in acetonitrile, and the resulting positively charged TTF radical cations interacted with the surface of the silver dendrites. Irregularly shaped particles were visualized by scanning electron microscopy (SEM), while the silver dendritic nanostructures were found by transmission electron microscopy (TEM) due to the crystallization of the oxidized TTF after the removal of the solvent on substrates for SEM and TEM analyses. Energy-dispersive X-ray (EDX) analysis demonstrated that the particles were nanocomposites composed of the silver nanostructures and TTF. Silver and TTF were uniformly distributed in the individual particles. Both the feed molar ratio of AgNO3 to TTF and the addition of poly(vinylpyrrolidone) (PVP) as a stabilizer influenced the size and shape of the silver nanostructures. The X-ray powder diffraction, elemental, and SEM analyses of the isolated products by centrifugation indicated that the TTF-protected silver nanostructures were also formed because of the crystallization of oxidized TTF after the reaction mixture was incubated in the solution for a longer time than 5 days until the transparent solution transformed into a purple translucent dispersion. So crystallization of oxidized TTF played a major role for the formation of silver dendritic nanostructures during the evaporation of the solvent or the incubation of the silver dendrites in the solution.
Introduction Synthesis of noble metal nanostructures by various methods has received considerable interest in recent years for their uses as catalysts and in optical and electronic devices due to unusual chemical and physical properties different from those of bulk metals.1 The control of their sizes and shapes is essentially required, since their sizes, shapes, and morphologies dramatically influence and optimize the nanomaterial performance.2 Recent years have focused much attention on welldefined nanostructured noble metals such as nanorods, nanotubes, and nonowires, because well-defined sizes and shapes give rise to improved physical, mechanical, and electrical properties.3-6 Dendritic fractals have aroused scientists’ interests due to their attractive supramolecular structures. Selvan and co-workers7,8 observed gold dendritic nanostructures by employing vapor-phase polymerization of pyrrole onto solution cast films of polystyreneblock-poly(2-vinylpyridine). Silver dendrites were observed by an ultraviolet irradiation photoreduction technique with poly(vinyl alcohol) as a protecting agent.9 With the increased concentration of AgNO3, many convex areas appeared on the surface of silver nanorods and further developed into thick dendrites. Zhu et al.10 reported that * Corresponding authors: e-mail
[email protected]. (1) Han, M. Y.; Quek, C. H. Langmuir 2000, 16, 362. (2) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (3) Alivisatos, A. P. Science 1996, 271, 933. (4) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (5) Mohamed, M. B.; Volkov, V.; Link, S.; E1-Sayed, M. A. Chem. Phys. Lett. 2000, 317, 517. (6) Teranishi, T.; Miyake, M. Chem. Mater. 1998, 10, 594. (7) Selvan, S. T. Chem. Commun. 1998, 351. (8) Selvan, S. T.; Hayakawa, T.; Nogami, M.; Mo¨ller, M. J. Phys. Chem. B 1999, 103, 7441. (9) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z. Y. Adv. Mater. 1999, 11, 850. (10) Zhu, J.; Liu, S.; Palchik, O.; Koltypin, Y.; Gedanken, A. Langmuir 2000, 16, 6396.
silver nanoparticles with spheres, rods, and dendritic shapes had been prepared by a pulse sonoelectrochemical technique from the electrolysis of a solution of AgNO3 with nitrilotriacetate (NTA). With the same preparative method, the presence of deoxyribonucleic acid (DNA) also resulted in silver dendritic nanostructures.11 In the ultrasonically assisted templated synthetic processes of palladium and silver dendrites, noble metal ions were reduced by Raney nickel, of which the interspaces acted as a template.12 We have recently reported stable colloidal forms of π-conjugated polymer-protected metal nanoparticles.13,14 The reduction of metal ions by a π-conjugated poly(dithiafulvene) (PDF) as a strong electron donor led to metal nanoparticles, and the resulting oxidized PDF protected the metal nanoparticles. These results motivated us to use other strong electron donors as reducing agents to form metal nanostructures. Derivatives of tetrathiafulvalene (TTF) act as electron donors and form stable charge-transfer complexes with a variety of organic and inorganic acceptor species.15 Recently, we reported the initial studies on the preparation of silver dendritic nanostructures protected by TTF via reduction of silver ions with TTF in acetonitrile.16 This paper further investigates the nucleation and growth mechanism of the well-defined silver dendrites. A detailed research is carried out to reveal the influence factors of this kind of specific supramolecular nanostructures. (11) Zhu, J.; Liao, X.; Chen, H. Mater. Res. Bull. 2001, 36, 1687. (12) Xiao, J.; Xie, Y.; Tang, R.; Chen, M.; Tian, X. Adv. Mater. 2001, 13, 1887. (13) Zhou, Y.; Itoh, H.; Uemura, T.; Naka, K.; Chujo, Y. Chem. Commun. 2001, 613. (14) Zhou, Y.; Itoh, H.; Uemura, T.; Naka, K.; Chujo, Y. Langmuir 2002, 18, 277. (15) Roncali, J. J. Mater. Chem. 1997, 7, 2307. (16) Wang, X.; Naka, K.; Itoh, H.; Park, S.; Chujo, Y. Chem. Commun. 2002, 1300.
10.1021/la027070z CCC: $25.00 © 2003 American Chemical Society Published on Web 06/21/2003
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Scheme 1
Experimental Section Materials. AgNO3 was purchased from Nacalai Tesque, Inc., and tetrathiafulvalene (TTF) was obtained from Tokyo Kasei Kogyo Co. Ltd.. Both reagents were used without further purification. 2-Benzylidene-4-phenyl-1,3-dithiol (1) was prepared according to ref 17. Mp: 204-205 °C (lit. mp 197-199 °C).
Preparation. A typical preparation of silver dendritic nanostructures was made as follows. After AgNO3 (3.0 mg, 0.0176 mmol) was dissolved in 25 mL of acetonitrile at room temperature, 3.6 mg (0.0176 mmol) of TTF was added in the solution under vigorous stirring. The reaction mixture changed gradually from yellow to purple with stirring for 48 h and finally turned into a purple dispersion after 5 days. After centrifugation and desiccation, 1.6 mg of a purple product was obtained. The preparation of PVP-protected silver particles followed the same experimental procedures except that 27.0 mg of poly(vinylpyrrolidone) (PVP) (MW avg 40 000) was added as a stabilizer before the reaction. For preparation of 1-protected silver nanoparticles, AgNO3 (3.0 mg, 0.0176 mmol) was dissolved into 25 mL of a dimethyl sulfoxide (DMSO) solution of 1 (2.4 mg, 0.0176 mmol). The color of the transparent solution changed from light yellow to yellow under stirring. Characterization. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-100SX operated at an accelerating voltage of 100 kV. TEM samples were prepared by depositing 2 drops of a desired solution on a 200-mesh copper grid covered with a carbon film, removing the excessive solution with a Kimwipes wiper, and drying the TEM grid at room temperature. Scanning electron micrograph (SEM) images were obtained with a JEOL JSM-5600 operated at an accelerating voltage of 20∼25 kV. A JEOL JED-2300 was used as an energydispersive X-ray spectroscopy (EDX) detector. SEM and EDX samples were prepared by placing a single drop of a desired solution on a SiO2 substrate and then keeping the substrate in a vacuum desiccator for 6 h at room temperature to remove the solvent. UV-vis spectra were recorded on a Jasco-530 spectrophotometer. X-ray power diffraction (XRD) patterns were determined at a scanning rate of 0.02° s-1 in 2θ ranging from 30° to 80° on a Shimadzu X-ray diffractometer 6000 with highintensity Cu KR radiation (λ ) 0.151 478 nm). The silver dendrites for XRD measurement and elemental analysis were isolated from dispersed solution after stirring for 3 weeks by centrifugation and purified by repeated washing with acetonitrile. The samples were dried in vacuo at room temperature for 48 h. 1H and 13C NMR spectra were obtained on a JEOL JNM-EX270 spectrometer.
Results and Discussion 1. Preparation of TTF-Protected Silver Nanostructures. The reduction of silver ions by electron-rich TTF is schematically illustrated in Scheme 1. An electron transfer from TTF as an electron donor to silver ions resulted in the formation of metallic silver. The metallic silver was stabilized by the resulting oxidized TTF, which (17) Raap, R. Can. J. Chem. 1968, 46, 2251.
Figure 1. UV-vis absorption for the colloidal dispersion of the silver nanostructures in acetonitrile after stirring for 21 h. (a) Feed molar ratio of TTF to AgNO3 was 1.0. (b) Timedependent absorption evolution of the colloidal dispersion at 436 nm.
had dual purposes as a reducing agent and a stabilizer. Figure 1a shows the UV-vis absorption spectrum for the reaction mixture after 21 h of reaction. Three maximal absorptions appear at 436, 580, and 714 nm. Two evident absorption bands at 436 and 578 nm are due to radical cation monomers of TTF, and the one at 714 nm is typical for TTF radical cation dimers.18 Figure 1b focuses on the time-dependent evolution of the most intensive peak among the three maximal absorptions. The inset shows the increase of the absorption intensity at 436 nm with the reaction time, indicating the gradual generation of TTF radical cations from the redox reaction between TTF and silver ions. No surface plasmon resonance of silver nanoparticles appeared. The theoretically calculated value for the silver nanoparticles is 380 nm in an aqueous system, and the reported experimental results are ∼410 nm and even longer wavelengths as the nanoparticles become larger.19,20 Some surface-modified nanocolloids did not produce plasmon absorption bands that are characteristic of conventional metal colloids.21 Moreover, coreshell structured hybridized nanocrystals composed of polydiacetylene microcrystals (shell layer) and silver fine particles (core) provided the screen of Ag plasmon reasonance.22 From a TEM image in Figure 2, silver dendritic nanostructures are observed after TTF and AgNO3 reacted in acetonitrile for 21 h. The average diameter of the observed dendritic nanostructures was 2.70 ( 0.07 µm. Irregularly shaped particles were visualized by SEM after 21 h of reaction (Figure 3). The average particle size was 2.80 ( 0.16 µm, in good agreement with the result from the TEM picture. It should be noted that no such large particles were generated in the solution, because the (18) Torrance, J. B.; Scott, B. A.; Welber, B.; Kaufman, F. B.; Seiden, P. E. Phys. Rev. B 1979, 19, 730. (19) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (20) Kapoor, S. Langmuir 1998, 14, 1021. (21) Adachi, E. Langmuir 2000, 16, 6460. (22) Masuhara, A.; Kasai, H.; Okada, S.; Oikawa, H.; Terauchi, M.; Tanaka, M.; Nakanishi, H. Jpn. J. Appl. Phys. 2001, 40, L1129.
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Figure 2. TEM image of a silver dendritic nanostructure observed after 21 h of reaction. The initial feed molar ratio of TTF to AgNO3 was 1.0 (bar ) 200 nm).
Figure 3. SEM image of the silver dendrites. The same sample as shown in Figure 2 was measured.
reaction mixture after 21 h was still transparent. The formation of these particles proceeded on the substrates for TEM and SEM analyses during evaporation of the solvent. The composition of a typical silver nanostructure (see Figure S1, Supporting Information) was further probed by EDX analysis. An EDX spectrum shows two sulfur peaks (from TTF) as well as three Ag peaks (Figure S2, Supporting Information). From the distribution of Ag and sulfur in the individual particle by EDX analysis (see Figures S3 and S4, Supporting Information), both elements were uniformly distributed. These data gave the clue that the particles were nanocomposites consisting of Ag and TTF. The oxidized TTF possessed a positive charge and provided electrostatic interaction with the surface of the silver dendrites. 2. Formation of TTF-Protected Silver Nanostructures in Solution. The time-dependent growth of the silver dendrites incubated in the solution was suggested from the fact that the reaction system turned gradually from an initially yellow transparent colloidal solution to a purple translucent dispersion after 5 days. Finally, it formed large precipitates after 3 weeks. A purple solid product was obtained by centrifugation. No characteristic absorption for TTF was observed in the UV-vis spectrum of the filtrate (Figure 1b). The result means that all the oxidized TTF was incorporated into the silver precipitates.
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The crystal structure of the product was measured by XRD (see Figure S5, Supporting Information). Four Bragg reflection peaks are visible at 38°, 44°, 64°, and 78°. These peaks correspond to (111), (200), (220), and (311) crystalline planes of a fcc lattice, respectively, ensuring the formation of metallic silver.23 According to an elemental analysis of the isolated product, the TTF content in the product is 46 wt %. Irregularly shaped particles were observed by SEM for the isolated product by centrifugation after they were incubated in the solution for 9 days (Figure S6, Supporting Information). These results suggest that the TTF-protected silver dendritic nanostructures were formed gradually in the reaction solution when the solution was incubated for a longer time than 5 days until the transparent solution transformed into the translucent dispersion. 3. Time-Dependent Formation of Silver Dendrites. Figure 4 shows the time-dependent size growth and shape variation for the silver dendrites. Samples were prepared after 2 drops of the corresponding solutions were cast on the TEM grids. After 20 min of reaction, tiny dendrites with sizes of ∼260 nm were observed (Figure 4a). After 7 h, branches radiated and grew into larger sizes of ∼450 nm (Figure 4b). The size of dendrites increased into ∼3.4 µm after 71 h of reaction, as shown in Figure 4c. The growth of the silver dendrites with reaction time was due to the increased amount of generated radical cations of TTF and metallic silvers in the solution, which was also supported by the increased UV-vis absorption at 436 nm as shown in Figure 1b. Therefore, longer reaction time favors the formation of the silver dendrites with larger size via the evaporation of the solvent. 4. Effect of the Feed Molar Ratio of TTF to AgNO3. When the feed molar ratio of TTF to AgNO3 was increased to 1.8, a SEM image shows rodlike crystals with diameters of several hundred nanometers and lengths of several micrometers along with the spherical particles after 21 h of reaction (Figure 5). The rodlike crystals consisted mainly of TTF according to an EDX analysis, indicating the formation of a nanoscaled TTF-based crystallite. The TTFbased crystallite grew rapidly by π-stacks of TTF radical cations in the direction of the c-axis.24 When the initial feed molar ratio of TTF to AgNO3 was reduced to 0.8 and 0.5, the average sizes of the silver particles were 220 and 100 nm, respectively (see Figure S7, Supporting Information). The particle sizes decreased by more than 10 times compared with the size of the silver dendrites as shown in Figure 2, whose initial feed molar ratio of TTF and AgNO3 was 1.0. Insufficient TTF radical cations to construct the silver dendrites might account for the size and shape variation. 5. Role of TTF for the Formation of Silver Dendrites. Noble metal nanoparticles and well-defined nanostructures are generally synthesized in the presence of stabilizers, such as surfactants, metal ligands, block copolymers, and thiol derivatives, to control their sizes.1,7-11,15 When PVP was used as a stabilizer along with TTF (the feed molar ratio of TTF:AgNO3:PVP ) 1:1: 0.038), a TEM image of silver nanostructures (Figure S8, Supporting Information) shows the formation of silver particles with monodisperse size. The mean size is 200 nm and this monodisperse colloidal solution was stable without precipitation for more than 1 month, much more stable than the one without the addition of PVP as described in Figure 2. No silver dendritic nanostructures (23) Joint Committee on Powder Diffraction Standards. Diffraction Data File; JCPDS International Center for Diffraction Data: Swarthmore, PA, 1991. (24) Favier, F.; Liu, H.; Penner, R. M. Adv. Mater. 2001, 13, 1567.
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Figure 4. TEM images for time-dependent growth of the silver dendrites after (a) 20 min (bar ) 200 nm), (b) 7 h (bar ) 200 nm), and (c) 71 h (bar ) 500 nm) of reaction.
Figure 5. SEM image of the product when the feed molar ratio of TTF to AgNO3 was 1.8 after 21 h of reaction (bar ) 2 µm).
were formed in the presence of PVP because the interaction between the surface of the silver nanoparticles and PVP was strong enough to inhibit the formation of the silver dendrites. 2-Benzylidene-4-phenyl-1,3-dithiol (1) was used as a substitute for TTF to prepare silver nanostructures so as to identify the role of TTF, since the redox behavior of the sulfur-containing π-electron donor 1 (Epa ) 0.40 V vs Ag/ AgCl) is similar to that of TTF (Epa ) 0.39 and 0.81 V vs Ag/AgCl).25,26 A UV-vis spectrum of an acetonitrile solution containing AgNO3 and 1 (feed molar ratio of AgNO3 to 1 was 1.0) showed new absorption bands at 529 and 699 nm due to a radical cation of 1 (see Supporting Information). A TEM image of a sample after 21 h of reaction showed that the mean size of the silver nanoparticles was 1.8 nm, determined by counting 300 nanoparticles (see Supporting Information). The XRD pattern of the product isolated by evaporation indicated the generation of metallic silver. However, the surface plasmon resonance for the silver nanoparticles at 380 nm was not observed in the UV-vis spectrum. The absence of the surface plasmon resonance of the 1-protected silver nanoparticles indicated that the Ag nanoparticles are (25) Uemura, T.; Naka, K.; Gelover-Santiago, A.; Chujo, Y. Macromolecules 2001, 34, 346. (26) Naka, K.; Uemura, T.; Chujo, Y. Macromolecules 1999, 32, 4641.
smaller than the Mie-onset particle diameter and the absorption peak is flattened due to the large imaginary dielectric constant for small metal clusters.27,28 The transparent, yellow colloidal solution was stable without precipitation for more than 2 months. Dendritic fractals generally occurred in nonequilibrium growth phenomena.29,30 The formation of these random supramolecular nanostructures is usually explained by a diffusion-limited monomer-cluster aggregation (DLMCA) model, in which particles are released one by one from sites arbitrarily far from a central cluster, sticking irreversibly at first contact with the growing cluster. Because of their Brownian trajectories, which simulate diffusion, the particles cannot penetrate deeply into a cluster without intercepting a cluster arm. The arms effectively screen the interior from the flux of incoming particles. Therefore, growth occurs preferentially at exterior sites, resulting in mass fractal objects whose density decreases radially from the center of mass.31,32 TTF may not effectively stabilize the silver metals in the solution compared with PVP or 1, because the oxidized TTF tends to crystallize and diminishes its charge. PVP protects the surface of the silver metals through the oxygen atoms in the >CdO group to prevent the growth of the silver metals.33 Crystallization of the oxidized TTF on the surface of the silver particles might be prohibited due to the steric hindrance of PVP. Therefore, crystallization of the oxidized TTF played an essential role for the formation of the silver dendrites. The growth of the well-defined silver dendritic nanostructures might proceed in the interspaces of the resulting crystals of TTF. Conclusions The reduction of silver ions by equimolar TTF with strong electron-donating properties brought about the silver dendritic nanostructures with the resulting oxidized TTF protecting the metal surface to form the stable (27) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (28) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075. (29) Witten, T. A.; Sander, L. M., Jr. Phys. Rev. Lett. 1981, 47, 351. (30) Meakin, P. Phys. Rev. Lett. 1983, 51, 1119. (31) Witten, T. A.; Sander, L. M., Jr. Phys. Rev. Lett. 1981, 47, 1400. (32) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Harcourt Brace Jovanovich Publishers-Academic Press: London, 1990; p 197. (33) Huang, H. H.; Ni, X. P.; Loy, G. L.; Chew, C. H.; Tan, K. L.; Loh, F. C.; Deng, J. F.; Xu, G. Q. Langmuir 1996, 12, 909.
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nanocomposite. TTF-assisted formation of the silver dendritic nanostructures was caused by the crystallization of TTF radical cations during the removal of the solvent or long-time incubation in the solvent. The oxidized TTF possessed a positive charge and provided electrostatic interaction with the surface of the silver dendrites. The tendency to crystallize and diminish charge of the oxidized TTF facilitated the formation of the silver dendritic nanostructures. The silver dendritic nanostructures were proved to be nanocomposites consisting of Ag and TTF from the results of SEM, TEM, and EDX measurements. Electron transfer from TTF to Ag+ proceeded gradually in the solvent as clarified by the time-dependent evolution of UV-vis absorption of TTF radical cations. The size and shape control over the dendrites was realized by changing the feed molar ratio of TTF to AgNO3 and adding PVP as the stabilizer. The diffusion-limited monomer-cluster aggregation mechanism was proposed to explain the nucleation and growth of the dendritic nanostructures. It was concluded that TTF played an essential role during the formation of the well-defined dendritic nanostructures as verified by the other π-electron donor. This work proposes a new mechanism for dendrite formation and
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suggests a new concept for preparing a nanosized composite of metal nanostructures and π-electron organic molecules. This work opens up new vistas for making use of π-electron organic molecules to achieve other novel supramolecular nanostructures with unique properties as well. Acknowledgment. Dr. M. Tsujii and Professor T. Fukuda (Kyoto University) are gratefully acknowledged for the TEM micrographs. We are also indebted to Dr. T. Yazawa and Dr. K. Kuraoka (Osaka National Research Institute) for XRD analysis. We thank JEOL Co. for the EDX analysis very much. Supporting Information Available: Figures showing XRD, EDX, and SEM images of TTF-protected silver nanostructures, TEM images of the TTF-protected silver nanostructures from different feed ratios, PVP-protected silver nanoparticles, and 1-protected silver nanoparticles, and UV-vis spectrum of the 1-protected Ag colloidal solution. This material is available free of charge via the Internet at http://pub.acs.org. LA027070Z
