Shape-Controlled Synthesis of Gold and Silver Nanoparticles

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Science  13 Dec 2002:
Vol. 298, Issue 5601, pp. 2176-2179
DOI: 10.1126/science.1077229


Monodisperse samples of silver nanocubes were synthesized in large quantities by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). These cubes were single crystals and were characterized by a slightly truncated shape bounded by {100}, {110}, and {111} facets. The presence of PVP and its molar ratio (in terms of repeating unit) relative to silver nitrate both played important roles in determining the geometric shape and size of the product. The silver cubes could serve as sacrificial templates to generate single-crystalline nanoboxes of gold: hollow polyhedra bounded by six {100} and eight {111} facets. Controlling the size, shape, and structure of metal nanoparticles is technologically important because of the strong correlation between these parameters and optical, electrical, and catalytic properties.

Metal nanoparticles play important roles in many different areas. For example, they can serve as a model system to experimentally probe the effects of quantum confinement on electronic, magnetic, and other related properties (1–3). They have also been widely exploited for use in photography (4), catalysis (5), biological labeling (6), photonics (7), optoelectronics (8), information storage (9), surface-enhanced Raman scattering (SERS) (10, 11), and formulation of magnetic ferrofluids (12). The intrinsic properties of a metal nanoparticle are mainly determined by its size, shape, composition, crystallinity, and structure (solid versus hollow). In principle, one could control any one of these parameters to fine-tune the properties of this nanoparticle.

Many metals can now be processed into monodisperse nanoparticles with controllable composition and structure (13) and sometimes can be produced in large quantities through solution-phase methods (14, 15). Despite this, the challenge of synthetically controlling the shape of metal nanoparticles has been met with limited success. On the nanometer scale, metals (most of them are face-centered cubic, or fcc) tend to nucleate and grow into twinned and multiply twinned particles (MTPs) with their surfaces bounded by the lowest-energy {111} facets (16). Other morphologies with less stable facets have only been kinetically achieved by adding chemical capping reagents to the synthetic systems (17–22). Here we describe a solution-phase route to the large-scale synthesis of silver nanocubes. Uniform gold nanoboxes with a truncated cubic shape were also generated by reacting the silver cubes with an aqueous HAuCl4 solution.

The primary reaction involved the reduction of silver nitrate with ethylene glycol at 160°C. In this so-called polyol process (23), the ethylene glycol served as both reductant and solvent. We recently demonstrated that this reaction could yield bicrystalline silver nanowires in the presence of a capping reagent such as poly(vinyl pyrrolidone) (PVP) (24). Subsequent experiments suggested that the morphology of the product had a strong dependence on the reaction conditions. When the concentration of AgNO3 was increased by a factor of 3 and the molar ratio between the repeating unit of PVP and AgNO3 was kept at 1.5, single-crystalline nanocubes of silver were obtained (25). Figure 1, A and B, show scanning electron microscope (SEM) images of a typical sample of silver nanocubes and indicate the large quantity and good uniformity that were achieved using this approach. These silver nanocubes had a mean edge length of 175 nm, with a standard deviation of 13 nm. Their surfaces were smooth, and some of them self-assembled into ordered two-dimensional (2D) arrays on the silicon substrate when the SEM sample was prepared. It is also clear from Fig. 1B that all corners and edges of these nanocubes were slightly truncated. Figure 1C shows the transmission electron microscope (TEM) image of an array of silver nanocubes self-assembled on the surface of a TEM grid. The inset shows the electron diffraction pattern obtained by directing the electron beam perpendicular to one of the square faces of a cube. The square symmetry of this pattern indicates that each silver nanocube was a single crystal bounded mainly by {100} facets. On the basis of these SEM and TEM studies, it is clear that the slightly truncated nanocube could be described by the drawing shown in Fig. 1D. The x-ray diffraction (XRD) pattern recorded from the same batch of sample (supported on a silicon substrate as in Fig. 1A) is also displayed inFig. 1D, and the peaks were assigned to diffraction from the (111), (200), and (220) planes of fcc silver, respectively. The lattice constant calculated from this pattern was 4.088 Å, a value in agreement with the literature report (a = 4.086 Å, Joint Committee on Powder Diffraction Standards file no. 04-0783). It is worth noting that the ratio between the intensities of the (200) and (111) diffraction peaks was higher than the conventional value (0.67 versus 0.4), indicating that our nanocubes were abundant in {100} facets, and thus their {100} planes tended to be preferentially oriented (or textured) parallel to the surface of the supporting substrate (26). The ratio between the intensities of the (220) and (111) peaks was also slightly higher than the conventional value (0.33 versus 0.25) because of the relative abundance of {110} facets on the surfaces of our silver nanocubes.

Figure 1

(A) Low- and (B) high-magnification SEM images of slightly truncated silver nanocubes synthesized with the polyol process. The image shown in (B) was taken at a tilting angle of 20°. (C) A TEM image of the same batch of silver nanocubes. The inset shows the diffraction pattern recorded by aligning the electron beam perpendicular to one of the square faces of an individual cube. (D) An XRD pattern of the same batch of sample, confirming the formation of pure fcc silver. a.u., arbitrary units.

The morphology and dimensions of the product were found to strongly depend on reaction conditions such as temperature, the concentration of AgNO3, and the molar ratio between the repeating unit of PVP and AgNO3. For example, when the temperature was reduced to 120°C or increased to 190°C, the product was dominated by nanoparticles with irregular shapes. The initial concentration of AgNO3 had to be higher than ∼0.1 M, otherwise silver nanowires were the major product. If the molar ratio between the repeating unit of PVP and AgNO3 was increased from 1.5 to 3, MTPs became the major product. Silver nanocubes of various dimensions could be obtained by controlling the growth time (25). Figure 2, A and B, show TEM images for 17- and 14-min growth times, and the nanocubes had a mean edge length of 115 ± 9 and 95 ± 7 nm, respectively.Figure 2C shows a TEM image of the sample that was synthesized using a lower concentration (0.125 M) of AgNO3 and a shorter growth time (30 min). The mean edge length of these silver nanocubes decreased to 80 ± 7 nm. Silver nanocubes with smaller sizes (∼50 nm, Fig. 2D) have also been obtained at a shorter growth time (25 min), although some of these particles have not been able to evolve into complete cubes. These demonstrations suggest that it is possible to tune the size of silver nanocubes by controlling the experimental conditions.

Figure 2

TEM images of silver nanocubes synthesized under different conditions. (A and B) The same as in Fig. 1, except that the growth time was shortened from 45 min to 17 and 14 min, respectively. (C and D) The same as in Fig. 1, except that the AgNO3 concentration was reduced from 0.25 to 0.125 M and the growth time was shortened to 30 and 25 min, respectively. Scale bars, 100 nm.

As illustrated by Wang (27), the shape of an fcc nanocrystal was mainly determined by the ratio (R) between the growth rates along <100> and <111> directions. Octahedra and tetrahedra bounded by the most stable {111} planes will be formed when R = 1.73, and perfect cubes bounded by the less stable {100} planes will result if R is reduced to 0.58. For the slightly truncated nanocubes illustrated in Fig. 1D, the ratioR should have a value close to 0.7. If PVP was not present, the silver atoms generated by reducing silver nitrate with ethylene glycol nucleated and grew into MTPs bounded by the most stable {111} facets (28). When PVP was introduced, it is believed that the selective interaction between PVP and various crystallographic planes of fcc silver could greatly reduce the growth rate along the <100> direction and/or enhance the growth rate along the <111> direction, and thus reduce R from 1.73 to 0.7. Both Fourier transform infrared and x-ray photoelectron spectroscopy measurements indicate that there exists a strong interaction between the surfaces of silver nanoparticles and PVP through coordination bonding with the O and N atoms of the pyrollidone ring (29, 30), although the exact bonding geometry and the nature of selectivity between different crystallographic planes are still not clear.

We have also exploited the use of these silver nanocubes as sacrificial templates to generate gold nanoboxes with a well-defined shape and hollow structure (31–33)Embedded Image(1)Based on this stoichiometric relationship, it was possible to completely convert silver nanocubes into soluble species and thus leave behind a pure solid product in the form of gold nanoboxes (34). Figure 3A shows an SEM image of silver nanocubes after they had reacted with an insufficient amount of HAuCl4, as calculated from Eq. 1. The black spots represent pinholes in their surfaces, where no gold had been deposited through the replacement reaction. It is believed that the existence of such pinholes allowed for the transport of chemical species into and out of the gold boxes until the reaction had been completed. The locations of these black spots implied that the replacement reaction occurred on the surface of a template in the following order: {110}, {100}, and {111} facets. This sequence was consistent with the order of free energies associated with these crystallographic planes: γ{110} > γ{100} > γ{111}(27).

Figure 3

SEM images of silver nanocubes (Fig. 1) after they had reacted with (A) 0.3 ml and (B) 1.5 ml of aqueous HAuCl4 solution (1 mM). As indicated by the black spots in (A), the {111} facets of gold nanoboxes were incompletely closed in the early stages of this replacement reaction, when HAuCl4 was in deficiency (as calculated from the stoichiometric equation). If excess HAuCl4 solution was added [as in (B)], the area of {111} facets could increase up to a maximum value at the expense of {100} and {110} facets. (C and D) Electron diffraction patterns of two gold nanoboxes with their square and triangular facets oriented perpendicular to the electron beam, respectively. Scale bars, 100 nm.

The gold nanoboxes shown in Fig. 3B self-assembled into a close-packed 2D array during sample preparation. The size of these gold boxes increased by ∼20% as compared with that of the silver templates. Such an increase in size was in agreement with the shell thickness calculated from stoichiometric and geometric arguments. The gold nanoboxes were finished with smooth surfaces, and most of them (>95%) were free of defects such as pinholes. Each box was bounded by two sets of facets (eight triangular facets and six square ones), and any one of these facets could lie against a solid substrate. The inset of Fig. 3B shows the SEM image of an individual box sitting on a silicon substrate against one of its triangular facets, illustrating the high symmetry of this polyhedral hollow nanoparticle. The crystallinity and structure of these gold nanoboxes were examined using electron diffraction. Figure 3, C and D, show the electron diffraction patterns obtained from two gold nanoboxes sitting on the TEM grids against one of their square and triangular faces, respectively. These diffraction spots suggest that each nanobox was a single crystal, with its square facets being indexed to {100} planes and triangular ones to {111} planes. On the basis of these observations, we believe that an epitaxial relationship might exist between the surfaces of the silver cubes and those of the gold boxes that greatly facilitated the transformation from the single-crystalline templates to the single-crystalline products. Minor reconstruction also occurred in the replacement process; for example, the {110} planes that were observed as ridges on the surfaces of silver cubes disappeared, and the areas of {111} and {100} facets were enlarged and reduced, respectively.

Silver nanocubes with controllable dimensions were synthesized by means of a modified polyol process that involved the reduction of silver nitrate with ethylene glycol in the presence of a capping reagent such as PVP. Although the fundamental basis of shape selectivity for this system has yet to be fully understood, it is believed that the selective adsorption of PVP on various crystallographic planes of silver played the major role in determining the product morphology. Uniform gold nanoboxes having a highly truncated cubic shape were also synthesized by reacting the silver nanocubes with an aqueous HAuCl4 solution. These silver and gold nanoparticles should find use in a variety of areas that include photonics, catalysis, and SERS-based sensing. This work and previous demonstrations from other groups (17–22) make it clear that chemical synthesis of metal nanoparticles with well-controlled shapes, sizes, and structures is a practical reality. The major requirement seems to be the selection of a capping reagent that is able to chemically modify various faces of a metal with an appropriate selectivity.

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