Anisotropic Growth of Silver Crystals with Ethylenediamine

anisotropic growth behavior causing the wire formation is tentatively ascribed to stepwise capping of the specific surfaces of silver crystals by orga...
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Anisotropic Growth of Silver Crystals with Ethylenediamine Tetraacetate and Formation of Planar and Stacked Wires Hiroaki Imai,* Hitoshi Nakamura, and Tomoyuki Fukuyo

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 1073-1077

Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi, Kohoku, Yokohama 223-8522, Japan Received October 8, 2004;

Revised Manuscript Received January 18, 2005

ABSTRACT: The morphology of silver crystals grown at the liquid-liquid interface between silver nitrate and ascorbic acid solutions was investigated in the presence of various organic molecules. The addition of ethylenediamine tetraacetate changed the morphology from dendrites and plates into single-crystalline wires with a width of 1001000 nm. The wires were produced by elongation and stacking of planar silver crystals exhibiting {111} faces. The anisotropic growth behavior causing the wire formation is tentatively ascribed to stepwise capping of the specific surfaces of silver crystals by organic molecules having multiple carboxyl groups. Introduction An ordered arrangement of inorganic crystals in multiple scales is required for the integration and application of functional materials to various kinds of devices.1-6 In this way, the control of the morphology of nano- or microscale crystals is essential to achieve sophisticatedly assembled materials. In particular, the formation of nanoscale anisotropic shapes, such as wires and rods, has attracted steadily growing interest due to their novel properties and unique applications.7-9 Noble metal nanowires and nanorods were reported to be synthesized by using various methods, including templating, seeding, and capping.10-21 The formation mechanism of the anisotropic shapes is quite interesting because the crystal structures of those metals usually have an isotropic cubic system. The formation of twinned crystals was proposed to be at the core of the anisotropic growth of silver wires,17,18,21 whereas twins were not always observed for the nanowires reported in other papers. The self-assembly of nanoparticles with molecular chains as a soft template was also demonstrated to be important for the production of anisotropic structures.21 However, the general concept of nanowire formation has not been sufficiently established. Recently, we found that flowerlike and stringy clusters of silver crystals are prepared by the reduction of silver nitrate with ascorbic acid.22 The formation of fascinating morphologies was ascribed to the previous production of seed particles and subsequent crystal growth around the seeds. The morphology of the silver crystals prepared by this method was drastically changed into various shapes, including wires, by the addition of anionic species. This paper describes the anisotropic growth of silver crystals and the formation of planar and stacked wires at the liquid-liquid interface by the reduction of silver ions with ascorbic acid in the presence of additives having carboxy groups. We clearly observed the change in the morphologies at the interface between the precursor solutions because the reaction gently occurred with a process of diffusion. Since the * To whom correspondence should be addressed. Phone: +81 45 566 1556. Fax: +81 45 566 1551. E-mail: [email protected].

morphology varied with the concentration of anionic additives, the formation mechanism of the planar and elongated architectures is discussed from the restriction of the crystal growth along specific directions by capping with the molecules. Experimental Section Silver crystals were prepared by mixing aqueous solutions containing 1.12 M silver nitrate (AgNO3) and 0.77 M L-ascorbic acid (AsA) as a reducing agent at room temperature. The reaction was induced at the liquid-liquid interface between the precursor solutions sandwiched between two glass plates. The distance between the glass plates was fixed to ca. 1 mm with plastic spacers. The AgNO3 and AsA solutions were injected from each end, and contacted at the center of the glass plates. The resultant precipitates were filtered, washed with water and 1 M sodium hydroxide solution, and then dried at room temperature. We mainly investigated the effect of ethylenediamine tetraacetate (H2edta2-) by the introduction of its disodium salt dihydrate into the AsA solution. The concentration of H2edta2- was varied in the range of 1-200 mM. Poly(acrylic acid) and various kinds of organic acids, such as acetic acid, glutamic acid, aspartic acid, iminodiacetic acid, succinic acid, and diethylenetriamine pentaacetic acid, were also added into the AsA solution as modifiers. Scanning electron microscopy (SEM) was utilized to observe the morphology of the products with a Hitachi S-2150 and a Hitachi S-4700. X-ray diffraction (XRD) and electron diffraction were measured using a Rigaku RAD-C diffractometer with Cu KR radiation and a Philips TECNAI F20 field-emission transmission electron microscope, respectively. Fourier transform infrared (FTIR) absorption was carried out to characterize adsorbed additives on silver crystals by a BIORAD FTS-60. Thermal gravimetry (TG) was performed using a Seiko TGDTA 6500.

Results and Discussion When no additives were introduced, yellowish gray precipitates were formed at the liquid-liquid interface upon contact and mixing of two solutions containing AgNO3 and AsA. The precipitates were identified to be silver crystals by XRD patterns and exhibited dendritic forms as shown in Figure 1. In our previous report,22 it was concluded that the dendritic morphologies were formed by the radial growth of silver crystals around a

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Figure 1. SEM images of silver crystals grown in the absence of additives.

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Figure 4. SEM images of silver crystals grown with 10 mM H2edta2-.

Figure 5. SEM images of silver crystals grown with 200 mM H2edta2-.

Figure 2. An optical micrograph of silver wires grown from a white band at the liquid-liquid interface.

Figure 6. SEM images of planar wires of silver crystals grown far from the interface (25 mM H2edta2-). (a) Planar morphologies of silver wires. (b) A zigzag structure with an angle of 30°.

Figure 3. SEM images of silver crystals grown with 1 mM H2edta2-.

seed of silver ascorbate. A highly supersaturated condition near the liquid-liquid interface promoted the formation of the branching morphologies via a diffusionlimited growth mode.23 In the presence of H2edta2-, a white solid band was immediately formed at the liquid-liquid interface and prevented rapid mixing of two solutions. Then, yellowish gray silver wires were grown on the AgNO3 side of the white band as shown in Figure 2. The white precipitates were confirmed to be Ag2H2edta by XRD patterns.24 The morphology of silver crystals was obviously influenced by the H2edta2- concentration and the distance from the liquid-liquid interface. A great number of platy crystals were mixed with wires in the precipitates produced by the addition of 1 mM H2edta2- (Figure 3). Large plates were still observed in a mass of wires with 10 mM H2edta2- (Figure 4). In the presence of H2edta2- above 50 mM, almost all silver crystals were formed as thin wires. The width of the silver wires gradually decreased with increasing the H2edta2- concentration, and the minimum value obtained with 200 mM H2edta2- was ca. 100 nm (Figure 5). The silver wires dominantly exhibited a planar morphology (Figure 6a). In the wires produced far from the liquid-liquid interface, in par-

Figure 7. SEM images of stacked wires of silver crystals grown near the interface.

ticular, the planar shapes were mainly surrounded by smooth surfaces. The wires frequently bent and formed zigzag structures with an angle of 30° or 60° (Figure 6b). On the other hand, the silver wires produced near the interface showed stacked structures consisting of hexagonal thin plates (Figure 7). A number of round particles were also observed with the stacked wires.

Anisotropic Growth of Silver Crystals

Figure 8. An XRD pattern of silver crystals grown with 100 mM H2edta2-. The standard diffraction pattern of silver (JCPDS No. 4-0783) is shown as a reference.

Figure 9. FTIR spectra of silver wires and various reagents.

The top surface of the platy morphologies exhibiting a hexagonal habit was assigned to the {111} face of the cubic unit of silver crystal because the {111} peak in the XRD patterns was intensified by pressing the sample powder (Figure 8). The addition of H2edta2- changed the initial reaction and the morphology of the silver crystals grown in the solution. The formation of the white band consisting of Ag2H2edta at the liquid-liquid interface means that H2edta2- molecules chelated to silver ions before ascorbic acid captured the ions in the solution. A decrease in the reducing reaction rate of silver ions with the formation of the H2edta2--Ag+ complex would induce an isotropic polyhedral form, such as a truncated octahedron (cubooctahedron; the equilibrium form of a silver crystal) through reaction-limited crystal growth. However, we observed anisotropic plates and wires exhibiting the {111} faces of a silver crystal instead of the isotropic shape. Absorption bands around 1400 and 1600 cm-1 in FTIR spectra (Figure 9) indicate that the wires contained H2edta2- even after being washed with water. Moreover, the single peak at the 1400 cm-1 band suggests that the state of carboxy groups was similar to that of Ag2H2edta rather than Na2H2edta. On the other hand, the diffraction peaks assigned to Ag2H2edta were not observed in XRD patterns. Therefore, we presume the existence of H2edta2--Ag+ complexes attached on the surfaces of the silver nanowires. The content of H2edta2- was roughly estimated to be about 10 wt % from a weight loss on thermal gravim-

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Figure 10. Schematic illustrations of the equilibrium form, and platy and wire morphologies. (a) A cubooctahedron (the equilibrium form) surrounded by four pairs of {111} and three pairs of {100}. (b) A hexagonal plate sandwiched with a pair of {111}. The side walls are {110}. (c) Wires sandwiched with a pair of {111} and elongated along 〈211〉 and 〈110〉. The side walls are {110} and {211}, respectively. (d) A multilayered morphology by stacking of platy wires.

etry. This value is consistent with the amount of the organic molecules covering all the surfaces of the nanoscale wires. Usually, the most stable {111} faces, which are the close-packed hexagonal lattice of silver atoms, are assumed to be covered with H2edta2--Ag+ complexes. However, the anisotropic elongation is not simply explained by the restriction of crystal growth in the 〈111〉 directions because the equilibrium form of silver crystal is isotropically surrounded by four pairs of the {111} planes (Figure 10a). Figure 11a shows an electron diffraction image for the planar view of a wire. The indexes of the diffraction spots indicate the presence of a hexagonal unit in the wire as illustrated in Figure 11b. This fact suggests that the wires were mostly produced through elongation of a silver crystal in the 〈211〉 or 〈110〉 direction with {111} planes, as shown in Figure 11c. We assume that the anisotropic growth resulting in the formation of single crystalline plates and wires is attributed to stepwise capping of the crystal faces. A platy morphology is achieved by the restriction of crystal growth on a pair of the {111} faces by capping with H2edta2--Ag+ complexes (Figure 10b). Since the complexes on a surface were assumed to connect with each other, the capping would be autocatalytically built upon the specific face. Thus, only one pair of the most stable {111} faces which are selected in the initial stage of the crystal growth could expand with the capping. When the capping effect is relatively weak at a low H2edta2concentration, thin plates are formed with the capping of a pair of the {111} faces. The side walls of the plates are inferred to be surrounded by three pairs of {110} because the planes are vertically arranged to {111} and composed of a highly packed lattice. An increase in the concentration of H2edta2- would induce additional capping of the side faces. The capping of a pair of {110} faces results in the elongation to the 〈211〉 direction exhibiting uncapped {110} faces, as illustrated in Figure 10c. The bending angle of 30° on a zigzag structure (Figure 6b) suggests that the growth also occurred in

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Figure 12. SEM images of silver wires exhibiting a complex structure produced in a stirred system using the same precursor solutions.

Figure 11. An electron diffraction pattern of a silver wire (a) and schematic models of the crystal planes causing the diffraction spots (b) and estimated growth directions of silver wires (c).

the 〈110〉 direction. In this case, the capped side faces are deduced to be {211}. Consequently, an increase in the concentration of H2edta2- changes the growth behavior from two-dimensional expansion to onedimensional elongation in a specific direction with an increase in the capping effect. Further development of the capping behavior on the side walls leads to a decrease in the width and an increase in the frequency of bending of the wires. Stacking of hexagonal plates similar to nacre of seashells (Figure 7) was observed in the silver wires produced near the liquid-liquid interface. As described in the above section, the {111} surface of hexagonal plates was suggested to be covered with the H2edta2-Ag+ complexes. However, the crystal growth sequentially occurred even on the capped {111} face, and the stacked structure was formed at a relatively high supersaturated condition (Figure 10d). When the silver crystals were prepared by mixing of the precursor solutions in a stirring system, wires having more complicated structures (Figure 12) were prepared with an extremely high degree of supersaturation. On the other hand, the backbone of the wires is suggested to be the same as the platy ones because the wires are regularly bent with an angle of 30°. Platy habits were also observed in the dendrites shown in Figure 1. Silver plates were clearly obtained in a stirred system containing AgNO3 and AsA.18 These facts indicate that the capping of the {111} faces was also induced with ascorbic acid, whereas the capping strength of AsA is expected to be weaker than that of H2edta2-. We found that the presence of organic molecules having two carboxy groups, such as glutamic acid, aspartic acid, iminodiacetic acid, and succinic acid,

caused the formation of the platy shapes, although acetic acid never affected the morphology. However, the wires were not obtained with these molecules. On the other hand, diethylenetriamine pentaacetic acid and poly(acrylic acid) produced wires similar to those produced with H2edta2-. It has been reported that the addition of poly(methacrylic acid) induced silver nanowires.20 Therefore, the presence of multiple ligands is basically effective in the formation of plates and wires by the stepwise capping of the {111} and {110} or {211} faces. The difference of the effects is ascribed to the capping strength depending on the number of ligands. Conclusions Planar and stacked silver wires exhibiting {111} faces were produced in the presence of H2edta2-. The anisotropic growth was ascribed to stepwise capping of the specific faces of {111} and {110} or {211} with the H2edta2--Ag+ complex. The width of the wires decreased to ca. 100 nm as the H2edta2- concentration increased. Organic molecules having multiple carboxy groups were effective in the formation of the wire morphology in an aqueous solution system. References (1) Service, R. F. Science 2001, 293, 782-785. (2) Sun, X.; Zhang, Z.; Dresselhaus, M. S. Appl. Phys. Lett. 1999, 74, 4005-4007. (3) Hermanson, K. D.; Lumsdon, S. O.; Williams, J. P.; Kaler, E. W.; Velev, O. D. Science 2001, 294, 1082-1086. (4) El-sayed, M. A. Acc. Chem. Res. 2001, 34, 257-264. (5) Hu, J.; Li, L.; Yang, W.; Manna, L.; Wang, L.; Alivisatos, A. P. Science 2001, 292, 2060-2063. (6) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J.; Liever, C. M. Nature 2001, 409, 66-69. (7) Wu, Y.; Yan, H.; Huang, M.; Messer, B.; Song, J. H.; Yang, P. Chem. Eur. J. 2002, 8, 1261-1268. (8) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775-778. (9) Lee, S.-M.; Cho, S.-N.; Cheon, J. Adv. Mater. 2003, 15, 441444. (10) Song, J. H.; Wu, Y. Y.; Messer, B.; Kind, H.; Yang, P. J. Am. Chem. Soc. 2001, 123, 10397-10398. (11) Choi, J.; Sauer, G.; Nielsch, K.; Wehrspohn, R. B.; Go¨sele, U. Chem. Mater. 2003, 15, 776-779. (12) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z. Y. Adv. Mater. 1999, 11, 850-852.

Anisotropic Growth of Silver Crystals (13) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736-4745. (14) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 7, 617-618. (15) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80-82. (16) Nikoobakht, B.; El-sayed, M. A. Chem. Mater. 2003, 15, 1957-1962. (17) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765-1770. (18) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955-960. (19) Wei, G. D.; Nan, C. W.; Deng, Y.; Lin, Y. H. Chem. Mater. 2003, 15, 4436-4441.

Crystal Growth & Design, Vol. 5, No. 3, 2005 1077 (20) Zhang, D. B.; Qi, L. M.; Yang, J. H.; Ma, J. M; Cheng, H. M; Huang, L. Chem. Mater. 2004, 16, 872-876. (21) Hu, J. Q.; Chen, Q.; Xie, Z. X.; Han, G. B.; Wang, R. H.; Ren, B.; Zhang, Y.; Yang, Z. L.; Tian, Z. Q. Adv. Funct. Mater. 2004, 14, 183-189. (22) Fukuyo, T.; Imai, H. J. Cryst. Growth 2002, 241, 193-199. (23) (a) Oaki, Y.; Imai, H. J. Cryst. Growth 2003, 3, 711-716. (b) Imai, H.; Oaki, Y. Angew. Chem., Int. Ed. 2004, 43, 1363-1368. (c) Oaki, Y.; Imai, H. Langmuir 2005, 21, 863869. (24) Fiorucci, A. R.; Saran, L. M.; Cavalheiro, EÄ . T. G.; Neves, E. A. Thermochim. Acta 2000, 356, 71-78.

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