NANO LETTERS
Bottom-Up and Top-Down Approaches to the Synthesis of Monodispersed Spherical Colloids of Low Melting-Point Metals
2004 Vol. 4, No. 10 2047-2050
Yuliang Wang and Younan Xia* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 Received August 13, 2004; Revised Manuscript Received August 19, 2004
ABSTRACT We report two different, solution-based approaches that have allowed us to process metals with melting points below 400 °C as monodispersed spherical colloids, in copious quantities, and with diameters controllable in the range of 100 to 600 nm. Bismuth was selected as a typical example to demonstrate the concepts. The production of monodispersed bismuth particles was realized by either thermally decomposing bismuth acetate in boiling ethylene glycol (the bottom-up approach) or by emulsifying molten drops of bismuth in boiling di(ethylene glycol) (the top-down approach), followed by quenching with cold ethanol. Depending on the concentration of Bi precursor and the stirring rate, the diameters of these uniform spherical colloids could be readily varied from 100 to 600 nm. The synthetic protocols have also been extended to prepare uniform spherical colloids from other metals with relatively low melting points, and typical examples include Pb, In, Sn, Cd, and their alloys.
Production of monodispersed spherical colloids has been a challenging subject in the general area of colloidal science ever since the beginning of last century.1 Thanks to the efforts from many research groups, a number of materials can now be routinely synthesized as monodispersed spherical colloids. In recent years, these colloidal particles have been extensively exploited as building blocks or templates to fabricate threedimensional (3D) photonic crystals through the self-assembly route.2 Most studies in this area, however, have been constrained to dielectrics such as polystyrene and silica because only these materials can be readily processed as monodispersed samples, in copious quantities, and with suitable diameters. Owing to their high refractive indices, both semiconductors and metals are expected to provide some immediate advantages over the dielectric materials in terms of creation of wide, robust photonic band gaps.3 Although a number of semiconductors (e.g., CdS, ZnS, TiO2, and amorphous Se) have recently been synthesized as monodispersed samples and further crystallized into opaline lattices with stop bands located in the visible regime,4 it still remains a grand challenge to synthesize uniform spherical colloids of metals with diameters controllable in the range of 0.1-1 µm. Almost all published work related to the synthesis of monodispersed colloids of metals has been limited to the length scales below 100 nm.5 * Corresponding author. E-mail:
[email protected]. 10.1021/nl048689j CCC: $27.50 Published on Web 09/02/2004
© 2004 American Chemical Society
As demonstrated by computational studies,6 3D crystalline lattices constructed from spherical colloids of metals might exhibit complete photonic band gaps extending over the entire optical regime. However, experimental realization of such crystals has been met with limited success due to the lack of building blocks characterized by suitable compositions and diameters. Wiley et al. have recently made impressive progress by demonstrating a template-directed procedure for generating uniform spherical colloids from a number of metals or semi-metals.7 The use of a template, however, may inevitably increase the complexity of a synthetic procedure and greatly limit the quantity of spherical colloids that can be obtained in each run of synthesis. On the other hand, coating of silica or polystyrene beads with metal shells has also been explored with some success,8 albeit the core/shell particles are often troubled by problems such as incomplete coating, poor uniformity in shell thickness, and deviation from spherical shape due to surface roughness. Here we report two different, solution-based approaches; both of them have allowed us to process metals with melting points below 400 °C as monodispersed spherical colloids, with diameters controllable from 100 to 600 nm, and in copious quantities. Figure 1 schematically illustrates the procedures of these two synthetic approaches. Bismuth was selected as a typical example to demonstrate the concepts. Although it has been successfully prepared as colloidal particles using a number
Figure 1. Schematic illustration of two different approaches to monodispersed spherical colloids of metals with relatively low melting points (e.g., Bi, Pb, In, Sn, Cd, and their alloys). (A) The bottom-up approach, where a molecular precursor is decomposed to generate metal atoms that nucleate and grow into monodispersed colloids. (B) The top-down approach, where large drops of a metal are broken into smaller pieces and then transformed into uniform droplets by shear forces through a mechanism similar to conventional emulsification.
of chemical methods,9 most of the samples were either polydispersed in size/shape or too small to be used for fabricating photonic crystals. In the first approach (Figure 1A), bismuth acetate was added to ethylene glycol (EG) and refluxed under the protection of nitrogen for 20 min until the reaction was quenched by pouring the hot mixture into an ethanol bath (250 mL) that was purged with a continuous flow of nitrogen. Poly(vinyl pyrrolidone) (PVP, M.
M.)55,000, Aldrich) was added (together with the precursor compound) to the reaction mixture as a stabilizer for the colloids. The production of Bi atoms was believed to originate from thermal decomposition of bismuth acetate, a process that has been investigated in the solid state using techniques such as differential thermal analysis (DTA) and thermogravimetry (TG).10 These studies indicated that the minimum temperature required for the generation of elemental Bi was around 280 °C. Here we found that the acetate precursor could decompose at a much lower temperature (the bp of EG is 198 °C) probably because it was dissolved as molecular species in the solution phase. Due to the low melting point of bismuth (271 °C) and relatively small sizes, the resultant bismuth colloids were believed to exist in the form of spherical, liquid droplets in the boiling EG.11 As the hot reaction mixture was quenched by ethanol, the spherical shape of these droplets could be retained as a result of abrupt drop in temperature. For the second approach (Figure 1B), Bi powders (100 mesh, Aldrich) were directly added to boiling di(ethylene glycol) (DEG, with a bp of 241 °C) and melted to produce big drops. PVP was also added as a stabilizer. After the reaction mixture had been vigorously stirred and thus emulsified for 20 min, uniform spherical colloids of bismuth were obtained as the hot mixture was poured into a cold ethanol bath. For both syntheses, the immediate product, monodispersed spherical colloids of bismuth, could be harvested as black reddish precipitates by centrifuging the quenched hot reaction mixture from cold
Figure 2. Characterization of Bi spherical colloids synthesized using both bottom-up and top-down approaches. (A, B) SEM and TEM images of a sample (225(8 nm in diameter) that was prepared using the thermal decomposition route. In this case, 0.15 g of bismuth acetate and 0.2 g of PVP were dissolved in 20 mL ethylene glycol at a stirring rate of 800 rpm. The inset of (B) shows a typical SAED pattern taken from an individual sphere, suggesting that it was a single crystal. (C) X-ray diffraction pattern taken from a large quantity of Bi spherical colloids (the same batch as shown in Figure 2A). (D) SEM and TEM (inset) images of another sample (305(11 nm in diameter) that was prepared using the top-down route by emulsifying 0.070 g Bi powders together with 0.2 g of PVP in 20 mL di(ethylene glycol) at a stirring rate of 550 rpm. Note that the same level of monodispersity can be routinely achieved for both protocols. 2048
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Figure 3. Control over size for the Bi spherical colloids: (A) A plot showing the relationship between the diameter of Bi spherical colloids and the concentration of bismuth acetate. A fixed stirring rate of 800 rpm and 0.2 g PVP was used in all these preparations. The solid curve represents a cubic fitting. (B) A plot showing the relationship between the diameter of Bi spherical colloids and the stirring rates used in the emulsification process. All these studies involved the same amount of starting materials: 0.070 g Bi powders and 0.2 g PVP in 20 mL di(ethylene glycol). The solid line represents a hyperbola fitting curve. The insets show the SEM images of three typical samples with the following mean diameters: 197(5, 305(11, and 553(18 nm.
ethanol (which had been kept in a freezer with its temperature around -18 °C). Although these two protocols began with completely different raw materials (with the first one being bottom-up and the second one being top-down), the resultant spherical colloids of bismuth displayed the same level of monodispersity (with size variations well below 5%), crystallinity, and surface smoothness. Figure 2A shows the SEM image of a typical sample prepared using the thermal decomposition route. This image clearly illustrates the copiousness in quantity and the uniformity in size/shape that could be routinely accomplished in this synthesis. A closer look (the inset) revealed that the surfaces of these spherical colloids were featureless. Figure 2B shows a TEM image of the same batch of bismuth colloids whose surfaces were found to be covered by PVP shells of ∼10 nm in thickness. If necessary, the PVP coatings could be removed from the surfaces of colloids by washing the sample with an ample quantity of ethanol. Selected-area electron diffraction (SAED) pattern taken from an individual bismuth particle indicated that it was a single crystal. An X-ray diffraction (XRD) pattern (Figure 2C) taken from a bulk quantity of bismuth colloids also confirms the exclusive existence of pure Bi. Combined with the SAED pattern, a rhombohedral crystal structure was assigned to the assynthesized spherical colloids of bismuth. Figure 2D shows the SEM and TEM images of a second sample of Bi colloids that was prepared using the emulsification route. It is worth pointing out that the Bi spherical colloids exhibited the same Nano Lett., Vol. 4, No. 10, 2004
Figure 4. SEM and TEM images of monodispersed spherical colloids made of other low melting-point metals and alloys: (A) SEM and (B) TEM images of Pb spherical colloids synthesized via thermal decomposition of lead acetate in tetra(ethylene glycol). (C) SEM image of Pb spherical colloids prepared by emulsifying its powders in tetra(ethylene glycol). (D) TEM image of In spherical colloids prepared by emulsifying its powders in dodecane. (E) TEM image of Cd/Pb alloyed spherical colloids prepared by emulsifying a mixture of Pb and Cd powders (9:1 in molar ratio) in tetra(ethylene glycol). (F) Energy-dispersive X-ray (EDX) spectrum of the Pb/ Cd alloyed colloids. PVP was used for all these syntheses except for the preparation of In colloids in dodecane, where stearic acid was used as the stabilizer.
level of uniformity and range of sizes no matter how they were synthesized (bottom-up or top-down). For both synthetic methods, the size and monodispersity of the final product are believed to be the result of a balanced interplay between several parameters that include the concentration of bismuth precursor, the concentration of PVP and thus viscosity of the reaction medium, the stirring rate, and the temperature. As a result, the size of bismuth spherical colloids produced using these two approaches could be readily controlled by varying the concentration of precursor (i.e., bismuth acetate or bismuth powder) and the stirring rate involved in each run of synthesis. Figure 3A shows the relationship between the diameter of as-synthesized bismuth colloids and the concentration of bismuth acetate added to the reaction system (at the same stirring rate). The data points could be fitted using a cubic equation C∝ d3, where d is the average diameter and C is the total concentration of Bi added to the reaction solution. This result implies that the number of colloids was roughly the same even though different amounts of the bismuth precursor were involved. When the amount of bismuth powders was fixed, the diameter of resultant colloids was mainly determined by the stirring rate. Figure 3B shows the dependence of particle diameter (d) on the stirring rate (γ˘ ). According to the Taylor’s equation (d ≈ σ/ηeγ˘ ), which was developed for the conventional emul2049
sion system, the average diameter of liquid droplets should be inversely proportional to the shear stress ηeγ˘ or the stirring rate γ˘ .12 Our results seem to follow the same trend, as shown by the hyperbola fitting curve in Figure 3B. The insets in both plots show the SEM images of three typical examples. For comparison, these images were taken at the same magnification. All these samples are characterized by excellent monodispersity, with their size variations below
