Preparation of Ag Stellar Dendrites: Modeling the Growth of Stellar

2,3-Bis(2-pyridyl)pyrazine was used as the growth-directing agent, forming hexagonal structures that served as the nuclei for the growth of stellar de...
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Preparation of Ag Stellar Dendrites: Modeling the Growth of Stellar Snowflakes Choon Hwee Bernard Ng and Wai Yip Fan* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 ABSTRACT: The synthesis of Ag stellar dendrites was achieved using a solution-phase method by modeling the growth of stellar snowflakes. In the synthesis, 2,3-bis(2pyridyl)pyrazine (dpp) acted as the growth-directing agent, forming hexagonal structures that served as the nuclei for the growth of stellar dendrites. Coupled with the careful control of experimental parameters to achieve a balance between reaction and diffusion rates, uniform and directional growth ensued, yielding symmetrical stellar dendrites represented by {111} facets. The influences of experimental conditions were investigated by analyzing the products formed under varied reaction and diffusion rates. The results were organized into an empirical chart that can serve as a visual reference to determine the experimental modifications required to prepare stellar dendrites. Time-dependence studies were carried out to elucidate the mechanism of growth into the unique dendritic patterns. SEM and HRTEM imaging of the intermediate structures revealed that the growth had occurred via oriented attachment of ∼10 nm Ag grains.

1. INTRODUCTION Dendritic crystals have been a subject of fascination for their elaborate structures characterized by symmetry formed from hierarchical branches that grow along specific crystallographic directions. A well-known example is snowflakes, which are single crystals of ice that form from water vapor. The crystallization of water vapor in the atmosphere is known to fashion crystals with an exotic array of morphologies, many of which exhibit unique dendritic patterns.1 This phenomenon has fueled efforts in constructing a detailed and accurate understanding of the mechanisms of the formation of such complex structures.2 Recently, the study of the growth of dendritic structures was revitalized by the prospect of commercial applications in nanoscale devices. These hierarchical structures, characterized by high porosity, high surface-area-to-volume ratios, and enriched with reactive edge and corner atoms, are reported to have important uses.3 In fact, studies have shown good promise on the use of dendritic nanostructures in catalysis,4 surface enhanced Raman spectroscopy (SERS),5 biosensing,6 hydrogen storage,7 and in the construction of fuel cells8 and superhydrophobic surfaces.9 Fundamentally, the preparation of dendritic nanostructures possesses scientific merits for they can provide a framework for the study of disordered systems.10 Over the years, an assortment of methods have been reported to yield dendritic nanostructures, which include hard templating,11 electrochemical deposition,12 galvanic replacement,13−15 solvothermal or chemical reduction in the presence of a protecting agent,16−18 and radiative reduction.19,20 Despite the plethora of methods reported, the formation of dendritic crystals with an overall symmetry that mirrors that of snowflakes has been rare.21−24 The widespread appearance of © 2014 American Chemical Society

fractal ice crystals in nature simply belies the complexity involved in the formation of ordered structures from disordered systems. From studies of the crystallization of water vapor, we can infer two important factors for the appearance of stellar snowflakes: (1) the formation of a hexagonal nascent structure and (2) the achievement of a delicate balance between crystal growth and mass transport.2 In this article, we demonstrate the successful preparation of stellar Ag dendrites by the reduction of silver nitrate (AgNO3) using L-ascorbic acid (AA) in the presence of 2,3-bis(2pyridyl)pyrazine (dpp) under ambient conditions. In the synthesis, dpp acted as a growth-directing agent in the formation of nascent hexagonal structures and a variety of experimental parameters were investigated to achieve the balance between the diffusion and reaction rates. Each crystal possesses a hierarchical dendritic structure with six primary branches radiating outward in an equatorial plane. In a similar fashion, secondary dendrites were found to grow above and below the equatorial plane, resulting in a 3D radiating structure that has an approximate D6h symmetry. The crystallographic relations and evolution mechanisms of the stellar dendrites were studied by high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM), and a possible mechanism of the formation of dendrites is discussed. Received: August 17, 2014 Revised: September 15, 2014 Published: September 17, 2014 6067

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Figure 1. (A)−(C) SEM images of as-prepared Ag stellar dendrites and (D) side view of the stellar dendrite, which reveals its three-dimensional structure. (E) XRD and (F) EDX spectra of the dendrites. XRD reflections are matched to fcc Ag (JCPDF No. 00-004-0783). The unlabeled peaks in the EDX plot are Cu signals that belong to the Cu grid used for nanoparticle deposition. vial into an ice bath and heating the reaction mixture to boiling. The reaction temperatures were measured using a thermometer to be 6−7 °C and 84−85 °C, respectively. For analysis of products at different reaction times, 3 μL of reaction mixture was sampled at the stated times, dropcast onto silica substrates, and dried rapidly under reduced pressure. 2.3. Instrumentation. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on the JEOL 3010 transmission electron microscope operating at 300 kV. Samples for TEM were prepared by drop-casting 5 μL of the aqueous dispersion onto carbon-Formvar coated copper grids (150 mesh), followed by drying in air. Scanning electron microscopy (SEM) was performed on the JEOL JSM-6701F scanning electron microscope operating at 5 kV. Samples for SEM were prepared by drop-casting 5 μL of the aqueous dispersion onto silica substrates, followed by drying in air. Powder X-ray diffraction (XRD) studies were performed using the Bruker D5005 diffractometer (Cu Kα, λ = 0.15418 nm) at a scan rate of 0.02 deg s−1. Concentrated aqueous dispersions of the samples were dropcast onto glass slides and dried in air. Ultraviolet−visible (UV−vis) spectroscopy was performed on the Shimadzu UV2600 spectrometer, using aqueous samples in 1 cm width quartz cuvettes.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ag Stellar Dendrites. The reagent-grade chemicals were obtained from Sigma-Aldrich and used without further purification. Ag stellar dendrites were synthesized by a solution-phase method. In a typical synthesis, 5 mL of aqueous silver nitrate (AgNO, 0.03 mmol) was mixed with 5 mL of ethanolic 2,3-bis(2-pyridyl)pyrazine (dpp, 0.04 mmol) under stirring (300 rpm) in a 20 mL sample vial. After 3 min, 5 mL of aqueous L-ascorbic acid (C6H8O6, 3 mmol) was introduced in one addition. The mixture turned olive green after a few seconds, and fine black particles were observed after 1 min. The mixture was allowed to stir for 1 h to ensure complete reaction. The black solid was obtained via centrifugation (5000 rpm, 30 min) and washed three times with water. The purified product was then redispersed in 5 mL of deionized water for characterization. 2.2. Controlled Experiments. Experiments investigating the effect of each reactant were conducted as above with a change in the concentration of the corresponding reactant by the stated amounts. Experiments investigating the effect of the mass transport rates were conducted as above with a change in the stirring rates to the stated values. Experiments investigating the effect of temperature were conducted by cooling the reaction mixture by immersing the reaction 6068

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3. RESULTS AND DISCUSSION 3.1. Structure and Characterization. Low-magnification SEM images of the reaction products illustrate the high yield and good uniformity of the stellar dendrites prepared in our synthesis (Figure 1A). Each stellar dendrite is made up of six primary branches that radiate from the nucleus in a twodimensional plane, spanning ∼5 μm in diameter. As shown in Figure 1B,C, sub-branches emerge at 60° to the parent branches with uniform alignments and spacings, rendering a periodic pattern. Each branch has a thickness of 80−100 nm. In a similar fashion, secondary dendrites grow above and below the equatorial plane resulting in a three-dimensional radiating structure that has an approximate D6h overall symmetry (Figure 1D). XRD measurements of the stellar dendrites gave peaks that can be assigned to the {111}, {200}, {220}, and {311} reflections of face-centered cubic (fcc) Ag (JCPDF No. 00-0040783) (Figure 1E). The absence of other peaks indicated the purity of the products. Another important observation is that the peak intensity ratio of {111} to {200} is ∼6, which is higher than that observed for polycrystalline Ag with randomly oriented grains, which is ∼2.5. This inflated ratio suggests that the Ag dendrites are abundant in {111} facets. Dispersive X-ray (EDX) spectroscopy showed only Ag signals, which confirmed the formation of pure Ag crystals (Figure 1F). HRTEM and selected area electron diffraction (SAED) analyses were carried out to determine the crystal orientation and growth direction of stellar dendrites. SAED of a branch of the dendrite (Figure 2A) showed a regular hexagonal pattern with spots that can be indexed to 1/3{422} and {220} Bragg reflections of fcc Ag (Figure 2B). The former is forbidden in the fcc system but has been reported for atomically flat surfaces of Ag and Au, which is attributed to the presence of twin planes in the {111} plane perpendicular to the electron beam.25

Similar patterns were observed for different regions of the dendrite, suggesting that the whole dendrite is a single crystal represented by {111} facets. Figure 2C,D shows HRTEM images of the tip of the main branch and side branch, respectively. In both images, clear lattice fringes with a spacing of 2.5 Å can be determined, which is in good agreement with the 3×{422} superlattice spacing of fcc Ag. The uniform lattice orientations observed for the different hierarchy of branches further indicate the single-crystalline nature of the dendrite. From HRTEM analyses, it can be inferred that the stellar dendrites had developed from preferential growth along the ⟨211⟩ directions. 3.2. Time Evolution Study. To better understand how the stellar dendrites are formed, SEM was used to monitor the products in the mixture at different reaction times. As shown in Figure 3, ∼10 nm spherical particles were formed in the very

Figure 3. SEM images showing the products isolated at different reaction times. Scale bars = 500 nm.

early stages of reaction (t < 5 s). By 10 s, the spherical particles had aggregated into larger particles with bulbous tips. As the reaction proceeded to 30 s, we observed the formation of hexagonal structures, with irregular surfaces that appeared to be residual features of the precursor particles. By 180 s, star-shaped crystals had formed from overgrowth of the corners of the hexagonal structures. These star-shaped crystals continued to grow uniformly along the six corners, with secondary branches appearing at ∼300 s. We also observed the aggregation of

Figure 2. (A) TEM image of a branch of the stellar dendrite. (B) SAED pattern obtained from a single branch. (C, D) HRTEM images of the tip of the main branch and side branch, respectively, as indicated by the circled regions in (A). 6069

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particles into secondary hexagonal structures on the surface of the star-shaped crystals. By 600 s, the crystals had developed well-defined stellar dendritic structures. There were no observable changes to the structure of the crystals beyond 600 s, which is likely due to the exhaustion of AgNO3 in the reaction mixture. 3.3. Effect of Reaction Conditions. 3.3.1. Concentration of dpp. Selective adsorption of low-molar mass additives or inorganic ions on different crystal facets is known to amplify kinetic-controlled growth and the manifestation of shapes that deviates from the thermodynamically favored equilibrium shape.26 In our synthesis, dpp is believed to be selectively adsorbed onto the {111} facets of the particles, which had two important influences. First, the inhibition of growth of {111} facets could promote the formation of hexagonal structures, which serve as the nuclei for the formation of stellar dendrites.25 Second, the stabilization of {111} facets could restrict crystal growth to along the set of ⟨211⟩ directions to yield {111}-represented structures. Experiments in which the concentrations of dpp were varied yielded observations that further indicated the role of dpp as a growth-directing agent. As shown in Figure 4A, when the concentration of dpp was reduced by half, coral-like dendrites were formed instead. The

Figure 5. SEM images of the products obtained at (A) 0.5× and (B) 2× of the optimal [Ag+], and (C) 0.5× and (D) 2× of the optimal [AA].

spherical aggregates with lumpy surfaces were obtained (Figure 5A,C). The greater polydispersity observed when the concentration of AA was reduced could be attributed to the longer nucleation times associated with lower reduction rates.29 In both cases, the diminished concentrations of Ag0 generated during the reaction lead to a lower supersaturation reached. At low supersaturations, crystal growth occurs under near equilibrium conditions, which leads to the products adopting more polyhedral forms, as observed in our case. In the experiments where the concentrations of Ag+ and AA were separately raised above their optimal concentrations, the products consisted of mainly irregular dendrites (Figure 5B,D). Under both conditions, the production of a large number of Ag particles results in extremely high supersaturations reached. At extremely high supersaturations, aggregation occurs more randomly, producing diffusion-limited aggregates with no observable crystallographic symmetry. 3.3.3. Stirring Rate and Temperature. We investigated the influence of mass transport by changing the stirring rates while maintaining the optimal concentrations of reactants. As observed in Figure 6A, the decrease in mass transport resulted in unequal growth of the dendrites, forming stellar dendrites

Figure 4. SEM images of the products obtained at (A) 0.5× and (B) 2× of the optimal [dpp].

appearance of coral-like dendrites has been reported to form from the reduction of AgNO3 with a large excess of AA and was observed to grow from bulbous seeds.27 Consistent with the report, bulbous seeds were observed in the early stages of our synthesis. However, in this case, the low concentration of dpp was not sufficient to effect the growth into hexagonal structures. Further growth occurs preferentially on the bulbous tips, leading to the formation of coral-like dendrites. When the concentration of dpp was increased by 2 times, the products consisted mainly of unsymmetrical dendrites (Figure 4B). The increase in dpp concentration could have led to increased adsorption on facets other than {111}, leading to less oriented growth. 3.3.2. Concentration of AgNO3 and AA. From studies of the formation of hierarchical structures, it is known that the morphology of a crystal is affected by the distance between the growth condition and the equilibrium state, i.e., the driving force for crystallization.28 For solution-phase syntheses, as in our case, the driving force is dictated by the degree of supersaturation reached. A series of controlled experiments were performed in the investigation of the optimal conditions for the formation of Ag stellar dendrites. Figure 5 summarizes the products obtained when the concentrations of AgNO3 and AA were varied. In the experiments where the concentrations of Ag+ and AA were separately reduced below their optimal concentrations,

Figure 6. SEM images of the products obtained under the different experimental conditions: stirring rates of (A) 100 rpm and (B) 900 rpm, and temperatures of (C) ∼7 °C and (D) ∼85 °C. 6070

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obtained. During the growth of the stellar dendrite, aggregation of particles on the surfaces of the hexagonal structures gave rise to secondary nuclei that underwent similar growth to yield secondary dendrites above and below the equatorial plane of the primary dendrite. As observed from the SEM images of the intermediates (Figure 8A), the growth of the dendrites is likely to have

with stubbed branches as well as isolated branches. On the other hand, the increase in mass transport leads to more isotropic and compact growth, yielding aggregates with sharp protrusions. A fraction of star-shaped aggregates could also be observed. Lastly, the effect of temperature was investigated through controlled experiments where the synthesis was conducted at ∼3 and 100 °C. At ∼3 °C, the reaction rate and mass transport were significantly lowered and the products observed were a mix of polyhedron particles hexagonal structures (Figure 6C). At 100 °C, coral-like dendrites were observed (Figure 6D). The high reduction and diffusion rates could have led to rapid growth of the bulbous seeds into coral-like dendrites before the hexagonal structures could form. Figure 7 summarizes the products obtained under varying supersaturation levels and mass transport rates. From the

Figure 8. SEM and HRTEM images of intermediate structures showing: (A) the attachment of 10 nm grains onto the growing dendrite and (B) an initially attached grain exhibiting a different crystallographic alignment to the nascent structure, as indicated by the marquee.

occurred via attachment of ∼10 nm Ag grains. These grain particles are transported to the growing structure by diffusion and become attached to the nascent structure once in contact. As shown in Figure 8B, the grains in some intermediate structures were not crystallographically aligned to the growing structures. This observation suggests that the grains had initially attached in a random fashion but underwent realignment before coalescing with the structure to give a single crystal abundant in {111} facets.30

Figure 7. Diagram illustrating the products obtained under different combinations of diffusion rates and supersaturation levels.

4. CONCLUSION Ag stellar dendrites were synthesized in good yield and high uniformity via the chemical reduction of AgNO3 with AA. The formation of stellar dendrites was achieved via the formation of hexagonal structures and subsequent uniform and ordered growth effected by a balance between the growth and diffusion rates. The addition of an optimal concentration of dpp was determined to be critical in the process. The products obtained for a spectrum of reaction and diffusion rates were elucidated via controlled experiments in which the reaction conditions were varied. The results were assembled into an empirical chart that is expected to be useful for determining the experimental modifications required to synthesize stellar dendrites. The synthesis of well-defined dendrites predominantly exposed with a uniform facet could lead to enhanced properties such as superior catalytic properties compared to their mixed-faceted counterparts.31 Furthermore, the ability to prepare symmetrical and well-defined dendrites has importance in research, providing systems for the study of self-assembly processes and the influence of dendrite structure on its physical and chemical properties.

results of the controlled experiments, it is evident that achieving the delicate balance between reaction and diffusion rates is crucial for the formation of symmetrical stellar dendrites. In this aspect, the diagram can serve as a visual reference to determine the experimental modifications required to prepare stellar dendrites. 3.4. Formation Mechanism. In the formation of the stellar dendrites, Ag+ was first reduced to Ag0 by AA, as illustrated by the equation C6H8O6 + 2Ag + → 2Ag 0 + C6H6O6 + 2H+

10 nm particles are formed from nucleation of Ag0 atoms. Because of the high surface energies of the small particles, they quickly aggregated into ∼200 nm bulbous particles. The preferential adsorption of dpp on the {111} facets on these bulbous seeds then directed their growth into hexagonal structures. As the hexagonal structures grew, the monomers surrounding it are consumed and a depletion zone is formed around it. The corners of the hexagonal structure that protrude into the region of higher concentration would grow faster than the edges, leading to the emergence of the six primary branches.22 Under the influence of dpp as well as the balance between diffusion and reaction rates, subsequent growth occurred preferentially along ⟨211⟩, giving rise to dendrites represented by {111} facets. Since the six primary branches experience the same growth conditions in the concentric depletion zone, a stellar structure with six uniform branches is

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 6567791691 (W.Y.F.). Notes

The authors declare no competing financial interest. 6071

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ACKNOWLEDGMENTS The project was supported by a National University of Singapore research grant under Grant No. 143-000-553-112. C.H.B.N. thanks NUS for a PGF research scholarship.

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REFERENCES

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