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Novel Preparation of Snowflake-like Dendritic Nanostructures of Ag or Au at Room Temperature via a Wet-Chemical Route Xuping Sun*,† and Matthias Hagner‡ Fachbereich Chemie and Fachbereich Physic, UniVersita¨t Konstanz, UniVersita¨tsstrasse 10, D-78457 Konstanz, Germany ReceiVed May 23, 2007. In Final Form: July 1, 2007 In this letter we describe a novel but effective wet-chemical route for the simple preparation of snowflake-like dendritic nanostructures of Ag, which are homogeneous in size, carried out by directly mixing AgNO3 and p-phenylenediamine (PPD) aqueous solutions at room temperature. It reveals that such dendrites are aggregates of nanoparticles and highly crystalline in nature. It is found that the molar ratio of [PPD]/[Ag+] influences the final morphologies of the structures formed and that excessive PPD (the molar ratio of [PPD]/[Ag+] is higher than 1:1) is crucial to achieving dendrites. The possible dendritic growth mechanism is also predicted. This method can also be extended to the preparation of Au dendrites.
1. Introduction During the past years, investigation on noble metal nanostructures has become the subject of high interest in science because of their novel electronic, optical, thermal, catalytic, as well as other properties, and considerable attention has thus been paid to the synthesis and characterization of such structures.1 It has also been well-established that both physical and chemical properties of nanostructures are associated with their morphologies, and therefore the morphology-controlled synthesis of nanostructures is highly desirable.2 Among various specific nanostructures, dendritic fractals have attracted scientists’ interests because of their attractive supramolecular structures that can not only provide a framework for the study of disordered systems,3 but can also be used as catalysts whose activity and selectivity are strongly dependent on the morphology of the structures.4 To date, many methods have been developed for the preparation of dendritic structures of noble metal, including electrochemical or electroless metallic deposition,5 γ-irradiated deposition,6 dielectric breakdown,7 vapor-phase polymerization,8 ultraviolet irradiation photoreduction using poly(vinyl alcohol) as a protecting agent,9 solvothermal methods using poly(vinyl pyrrolidone) as an adsorption agent and architecture soft template,10 ultrasonic waveassisted hard template method,3 controlled seeding method,11 a * To whom correspondence should be addressed. E-mail: sun.xuping@ hotmail.com. † Fachbereich Chemie. ‡ Fachbereich Physic. (1) (a) Fendler, J. H., Ed. Nanoparticles and Nanostructured Films; VCH: Weinheim, Germany, 1998. (b) Klabunde, K. J., Ed. Nanoscale Materials in Chemistry; VCH: Weinheim, Germany, 2001. (c) Schmid, G. Chem. ReV. 1992, 92, 1709. (d) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (2) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (3) (a) Fleury, V.; Kaufman, J. B.; Hibbert, D. B. Nature 1994, 367, 435. (b) Xiao, J.; Xie, Y.; Tang, R.; Chen, M.; Tian, X. AdV. Mater. 2001, 13, 1887. (4) Chimenta˜o, R. J.; Kirm, I.; Medina, F.; Rodrı´guez, X.; Cesteros, Y.; Salagre, P.; Sueiras, J. E. Chem. Commun. 2004, 846. (5) (a) Martin, H.; Carro, P.; Hernandez Creus, A.; Gonzalez, S.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem. B 1999, 103, 3900. (b) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (c) Qiu, T.; Wu, X.; Mei, Y.; Chu, P. Appl. Phys. A 2005, 81, 669. (6) Wang, S.; Xin, H. J. Phys. Chem. B 2001, 104, 5681. (7) Niemeyer, L.; Pietronero, L.; Wiesmann, H. J. Phys. ReV. Lett. 1984, 52, 1033. (8) Selvan, S. T. Chem. Commun. 1998, 351. (9) Zhou, Y.; Yu, S.; Wang, C.; Li, X.; Zhu, Y.; Chen, Z. AdV. Mater. 1999, 11, 850. (10) Wei, G.; Nan, C.; Deng, Y.; Lin, Y. Chem. Mater. 2003, 15, 4436. (11) Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380.
micellar or mixed surfactant route,12 hydrolysis of poly(vinyl acetone)-assisted method,13 and so forth. More recently, Wen et al. successfully synthesized Ag nanodendrites through a simple surfactant-free method using a suspension of zinc microparticles as a heterogeneous reducing agent.14 However, to the best of our knowledge, a template- and surfactant-free wet-chemical route general to the preparation of snowflake-like dendritic Ag or Au nanostructures has not been reported in the literature up to now. On the other hand, phenylenediamine has recently been used by us as a kind of useful compound for the preparation of some interesting structures at room temperature through a simple wetchemical route. For example, the mix of o-phenylenediamine (OPD) and HAuCl4 aqueous solutions leads to the formation of poly(OPD) nanobelts,15 and the same system can be used for the preparation of large Au plates of nanometer thickness or spherical Au colloids.16 Similarly, the OPD-AgNO3 system can be used for the preparation of one-dimensional structures of assembled OPD oligomers or large spherical Ag colloids.17 The mix of H2PtCl6 and p-phenylenediamine (PPD) aqueous solutions has been used as an effective strategy for the facile preparation of submicrometer-scale, monodisperse, spherical colloids of organicinorganic hybrid materials, which has been attributed to the coordination-induced assembly of PtCl62- and PPD.18 In this letter we develop a simple wet-chemical method for preparing snowflake-like dendritic Ag or Au nanostructures. The formation of such dendrites occurs in a single process, carried out by directly mixing PPD and the corresponding metal salt aqueous solutions at room temperature. It is found that the dendrites are aggregates of Ag nanoparticles and are highly crystalline in nature. The influence of the molar ratio of [PPD]/ [Ag+] on the morphologies of the Ag structures is investigated, (12) (a) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635. (b) Zheng, X.; Zhu, L.; Wang, X.; Yan, A.; Xie, Y. J. Cryst. Growth 2004, 260, 255. (13) (a) Wu, W.; Pang, W.; Xu, G.; Shi, L.; Zhu, Q.; Wang, Y.; Lu, F. Nanotechnology 2005, 16, 2048. (b) Wu, W.; Shi, L.; Zhu, Q.; Wang, Y.; Pang, W.; Xu, G.; Lu, F. Nanotechnology 2006, 17, 1948. (14) Wen, X.; Xie, Y.; Mak, W. C.; Cheung, K. Y.; Li, X.; Renneberg, R.; Yang, S. Langmuir 2006, 22, 4836. (15) (a) Sun, X.; Dong, S.; Wang, E. Chem. Commun. 2004, 1182. (b) Sun, X.; Dong, S.; Wang, E. Chem. Sci. 2004, 1, C41. (c) Sun, X.; Dong, S.; Wang, E. Mater. Today 2004, 7, 10. (16) Sun, X.; Dong, S.; Wang, E. Angew. Chem., Int. Ed. 2004, 43, 6360. (17) (a) Sun, X.; Dong, S.; Wang, E. Macromol. Rapid Commun. 2005, 26, 1504. (b) Sun, X.; Dong, S.; Wang, E. J. Colloid Interface Sci. 2005, 290, 130. (18) Sun, X.; Dong, S.; Wang, E. J. Am. Chem. Soc. 2005, 127, 13102.
10.1021/la701519x CCC: $37.00 © 2007 American Chemical Society Published on Web 07/25/2007
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Figure 1. (a) Typical SEM image of the precipitate obtained at a 3:1 molar ratio of [PPD]/[Ag+]. (b) Locally magnified SEM image of a single dendrite. (c) EDS of the dendrites.
and it is found that excessive PPD (the molar ratio of [PPD]/ [Ag+] is higher than 1:1) is crucial to achieving dendrites. A possible mechanism to explain the growth of dendritic Ag structure is also predicted. 2. Experimental Section AgNO3 and PPD were purchased from Aldrich and used as received without further purification. Sample 1 was prepared as follows: In a typical experiment, 96 µL of 0.05 M PPD aqueous solution was added rapidly into 20 mL of 0.08 µM AgNO3 aqueous solution where the molar ratio of [PPD]/[Ag+] was 3:1, and the resulting solution was kept at room temperature. (Note: In order to evenly mix these two reactants, rigorous stirring was used during the addition of PPD). A rapid color change of solution was observed when the two solutions were mixed together, and a large amount of precipitate gradually formed. The resulting precipitate was collected by centrifugation, washed several times with absolute ethanol and water, and then suspended in water. The suspension thus formed was used for further characterization. Scanning electron microscopy (SEM) measurements were made on a Zeiss DSM 940 microscope operated at an accelerating voltage of 10 kV. Samples for SEM examination were made by placing a drop of the dispersion on an indium tin oxide (ITO) glass slide and air-dried at room temperature. The sample for X-ray photoelectron spectroscopy (XPS) characterization was prepared by placing 20 µL of the suspension on a glass slide and examining it on an ESCLAB MKII. Transmission electron microscopy (TEM) measurements were made on a Hitachi H-800 transmission electron microscope at an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of the suspension on a carbon-coated copper grid and drying the grid at room temperature. The X-ray diffraction (XRD) pattern was collected on a Japan Rigaku D/MaxRA X-ray diffractometer. The sample for XRD characterization was prepared by placing 20 µL of the suspension on a glass slide.
3. Results and Discussion Figure 1a shows a typical SEM image of the precipitate of sample 1 obtained at a 3:1 molar ratio of [PPD]/[Ag+]. It is clearly seen that the precipitate consists of a large quantity of snowflake-like dendritic nanostructures about 1.6 µm in size. A close view of a single dendrite reveals that it is an aggregate of very small nanoparticles, which is evidenced by a highmagnification SEM image of the dendrite as shown in Figure 1b. Energy dispersive X-ray spectroscopy is a chemical microanalysis
Figure 2. (a) Typical TEM image of a single dendrite and the corresponding SAED (inset). (b) XRD pattern of the bulk dendrites.
technique performed in conjunction with SEM by utilizing X-rays that are emitted from the sample during bombardment by the electron beam to characterize the elemental composition of the analyzed sample. The chemical composition of the dendrites was also examined by this analysis method. The energy-dispersive X-ray spectrum (EDS) collected from the dendrites shows the peak of Ag at about 3.00 together with several peaks of other elements that can be assigned to the ITO glass substrate used (Figure 1c). Also note that no peaks of C and N elements are observed. There observations indicate that the dendrites are not assemblies of PPD and Ag+ due to coordination interactions between these two components,18 but rather aggregates of Ag nanoparticles that are produced because of the chemical reduction of Ag+ by PPD.17 XPS involves irradiating a sample with X-rays of a characteristic energy and measuring the flux of electrons that leave the sample surface, and the energy spectrum for the ejected electrons thus acquired is a combination of an overall trend due to the transmission characteristics of the spectrometer, the energy loss processes within the sample, and the resonance structures that are derived from the electronic states of the material under analysis. Therefore, the XPS spectrum of these dendrites was collected to further identify the change in oxidation states for Ag after the reaction had occurred. Figure S1 (Supporting Information) shows the XPS spectrum corresponding to the Ag 3d spectrum region of the dendrites. Two peaks are observed at a binding energy of 368.5 and 374.1 eV, corresponding to the Ag3d3/2 and Ag3d5/2 spectrum regions, respectively, further indicating the formation of metallic Ag.19 The snowflake-like dendritic nature of the Ag nanostructure is further confirmed by the corresponding TEM image as shown in Figure 2a. The local crystalline nature of the dendrite was examined by electron beam microanalysis by TEM. It should be noted that the center part along each branch of the dendrite is too thick to be penetrated by the electron beam, and therefore we collected the selected-area electron diffraction (SAED) pattern (19) Briggs, D., Seah, M. P., Eds. Practical Surface Analysis; John Wiley and Sons: New York, 1983.
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Figure 3. Typical SEM images of the resulting precipitate obtained at (a) 1:1, (b) 1:6, (c) 1:12, and (d) 6:1 molar ratios of [PPD]/[Ag+], under conditions otherwise identical to those used for preparing sample 1. Table 1. The Molar Ratio of [PPD]/[Ag+] Used and the Morphology of the Corresponding Ag Product Obtained molar ratio of [PPD]/[Ag+] 1:1 1:6 1:12 3:1 6:1
morphology of the Ag product spherical structures consisting of nanoparticles irregular structures consisting of nanoparticles polyhedra nanoparticles snowflake-like, dendritic nanostructures consisting of nanoparticles snowflake-like, dendritic nanostructures consisting of nanoparticles
recorded by focusing an electron beam on the border of one branch (e.g., as indicated by A). The SAED pattern clearly reveals that a hexagonal symmetry diffraction spot pattern is generated, which indicates that the dendrite is single crystalline.9 The crystalline nature of the bulk dendrites was examined by XRD analysis. Figure 2b shows the XRD pattern collected. All the diffraction peaks observed can be assigned to the {111}, {200}, {220}, {311}, and {222} diffraction peaks of the cubic structure of metallic Ag, respectively, indicating that the dendrite is crystalline Ag.10 Note that the intensity ratio between the {111} and {220} diffraction peaks is higher than that of the standard file (JCPDS) (4.5 versus 2.5), which indicates that the dendrites are highly crystalline and abundant in {111} facets.4 To examine the influence of the molar ratio of [PPD]/[Ag+] on the formation of the Ag structures, we prepared four other samples with different molar ratios of [PPD]/[Ag+], under conditions otherwise identical to those used for preparing sample 1. Figure 3 shows typical SEM images of the as-prepared Ag structures. At a 1:1 molar ratio, we obtained spherical structures consisting of nanoparticles as shown in Figure 3a. When the molar ratio is decreased to 1:6, we obtained irregular structures consisting of nanoparticles as shown in Figure 3b. When the molar ratio is further decreased to 1:12, we only obtained polyhedra nanoparticles (Figure 3c). However, when excessive PPD (i.e., the molar ratio is higher than 1:1) is used in the synthesis, we obtained snowflake-like, dendritic nanostructures consisting of nanoparticles again. Figure 3d shows the structures obtained at a 6:1 molar ratio, clearly indicating the formation of dendrites about 1 µm in size. Table 1 summarizes the molar ratio of [PPD]/ [Ag+] used and the morphology of the corresponding Ag product obtained. These observations indicate that the molar ratio of [PPD]/[Ag+] plays an important role in determining the mor-
phologies of the Ag structures obtained, and excessive PPD is crucial to the formation of dendritic structures. Dendritic fractals are phenomena generally observed in nonequilibrium growth such as the growth of snowflakes, the aggregation of soot particles, and the solidification of metals. The deposition-diffusion aggregation model,20 the diffusionlimited aggregation model,21 and the cluster-cluster aggregation model22 have been widely used to interpret and analyze various fractal phenomena. The following experimental facts in our present synthesis should be noted: (1) The mix of PPD and AgNO3 aqueous solutions with a molar ratio above 1:1 leads to a rapid color change of solution due to the rapid redox between PPD and Ag+, but a lower molar ratio results in a slow color change, and it will take a longer time for the formation of precipitate; (2) only the use of excessive PPD can give dendrites; and (3) the dendrite is an aggregate of nanoparticles. On the basis of these observations, it is reasonable to suggest that both a rapid nucleation and growth kinetics of Ag nanoparticles and the excessive PPD molecules contribute mainly to the formation of the dendrites. Low-molar-mass additives or inorganic ions can influence crystal habit by a selective adsorption process leading to preferential growth inhibition for distinct crystal faces,23 and single crystals with complex morphologies such as flowerlike can be obtained when multiple steps are involved in such processes.24 OPD has also been used for the formation of compact micrometer-scale, single-crystalline Au nanoplates,16a wherein OPD serves as a reducing reagent and, at the same time, may serve as a low-molar-mass additive contributing to the formation of Au single crystals.23 Considering the single-crystalline nature of local Ag dendrites evidence by SAED, we suggest that the excessive PPD in the solution may function as a low-molar-mass additive leading to single-crystalline Ag nanoparticles. Furthermore, the fact that the aggregates of Ag nanoparticles with complex, snowflake-like dendritic morphology are also highly crystalline indicates that multiple steps are involved in the formation process of dendrites. We speculate that the formation of Ag dendrites involves the following stages: (1) AgNO3 is rapidly reduced by PPD to form Ag atoms when these two solutions are mixed together. Because the PPD used is excessive in the solution and AgNO3 is a powerful oxidant with high electropositive character of Ag+ (+0.799V), it is expected that all the Ag+ ions have been completely reduced at this stage, giving highly concentrated Ag atoms. (2) Highly concentrated Ag nanoparticles are rapidly formed by an initial nucleation phase in which tiny seed Ag particles precipitate spontaneously from solution, and a subsequent growth phase in which newly formed seeds capture free Ag atoms in the solution.25 During this stage, the excessive PPD may serve as an additive for the formation of single-crystalline Ag nanoparticles. Figure S2 (Supporting Information) shows the TEM image of the Ag products obtained with an elapsed time of 30 s from the time the PPD and AgNO3 aqueous solution are mixed, demonstrating that Ag nanoparticles are rapidly formed at the early stage during the formation of the Ag dendrites. The inset is the SAED showing a hexagonal symmetry diffraction spot pattern of the Ag nanoparticles thus formed, which indicates the single-crystalline nature of such Ag nanoparticles. (3) A crystal growth through PPD-mediated aggregation of Ag nanoparticles occurs.26 Each nanoparticle can (20) Barabasi, A. L.; Stanley, E. Fractal Concepts in Surface Growth; Cambridge University Press: Cambridge, 1995. (21) Witten, T. A., Jr.; Sander, L. M. Phys. ReV. Lett. 1981, 47, 1400. (22) Kolb, M.; Botet, R.; Jullien, R. Phys. ReV. Lett. 1983, 51, 1123. (23) Adair, J. H.; Suvaci, E. Curr. Opin. Colloid Interface Sci. 2000, 5, 160. (24) Co¨lfen, H.; Qi, L.; Mastai, Y.; Bo¨rger, L. Cryst. Growth Des. 2002, 2, 191. (25) deMello, J.; deMello, A. Lab Chip 2004, 4, 11N.
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Figure 4. A schematic diagram to illustrate the formation of Ag dendrites.
serve as an immobile “seed” in the solution. Some free nanoparticles are then launched from a random position far away and are allowed to diffuse toward the seed. Once they touch the seed, they are immobilized instantly on the empty surface of the seed, yielding an initial aggregate of Ag nanoparticles with the seed as the center and the attached nanoparticle as the arm. With elapsed time, other free nanoparticles will diffuse continually toward the aggregate and further be immobilized on the empty surface of the arm, forming a larger aggregate. This crystal growth process can be repeated until all free nanoparticles are depleted, leading to dendritic crystals. Actually, there is a mass of seeds distributed uniformly in the solution, and multiple aggregation events occur simultaneously in the respective regions of the solution. As a result, dendrites with limited size are obtained. During the formation of aggregates, the excessive PPD may be expected to dictate the oriental attachment of Ag nanoparticles, and thus highly crystalline aggregates of Ag nanoparticles are formed. Figure 4 shows a schematic diagram to illustrate the formation process of Ag dendrites in this synthesis. It is found that the present approach can also be used to selectively prepare Au dendrites. Figure 4 shows SEM images obtained at two different molar ratios of [PPD]/[Au3+]. At the molar ratio of 1:1, nearly spherical aggregates of Au nanoparticles are observed (Figure 5a). However, when the PPD is excessive in this synthesis, for example, at a molar ratio of 4:1, snowflakelike dendritic nanostructures are again formed (Figure 5b). These observations clearly indicate that this method is general toward the preparation of dendritic Ag and Au nanostructures. (26) Co¨lfen, H.; Mann, S. Angew. Chem., Int. Ed., 2003, 42, 2350.
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Figure 5. Typical SEM images of the resulting precipitate obtained at two different molar ratios of [PPD]/[Au3+]: (a) 1:1 and (b) 4:1.
4. Conclusions In conclusion, the direct mix of AgNO3 (or HAuCl4) and PPD aqueous solutions at room temperature has been proven to be an effective way for preparing snowflake-like dendritic Ag or Au nanostructures. It reveals that such dendrites are aggregates of nanoparticles and highly crystalline. The morphologies of the structures are heavily influenced by both the molar ratio and the concentrations of reactants. It suggests that excessive PPD is crucial to achieving dendrites. The possible dendritic growth mechanism is also predicted. Our observations are significant for the following reasons: (1) It presents a general route to simple fabrication of highly crystalline dendritic nanostructures of noble metals, including Ag and Au. (2) These dendritic structures may provide us a framework for the study of disordered systems.3 (3) It develops a new methodology for achieving Ag nanostructures with different morphologies for many applications such as in the field of heterogeneous catalysis and sensors.4,14 (4) It expands the use of phenylenediamine for interesting structure synthesis.15-18 Acknowledgment. X.S. thanks Prof. S. Mecking for being the research host and also appreciates the support from the Alexander von Humboldt Foundation. Dr. M. Krumova is appreciated for TEM characterization. Supporting Information Available: XPS spectrum of the Ag 3d region of the PPD/Ag dendrites and a TEM image of the Ag products obtained 30 s after PPD and the AgNO3 aqueous solution were mixed. This material is available free of charge via the Internet at http:// pubs.acs.org. LA701519X
