DOI: 10.1021/cg100639s
Synthesis of Tetrahexahedral Gold Nanocrystals with High-Index Facets Do Youb Kim,† Sang Hyuk Im,*,‡ and O Ok Park*,†
2010, Vol. 10 3321–3323
†
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Korea, and ‡Korea Research Institute of Chemical Technology (KRICT), Korea
Received May 13, 2010; Revised Manuscript Received June 5, 2010
ABSTRACT: Tetrahexahedral (THH) gold nanocrystals bound to 24 high-index facets were synthesized through a facile colloidal chemistry by reducing HAuCl4 in the mixture solution of N,N-dimethylformamide and poly(vinylpyrrolidone) at 80 C. As-synthesized THH gold nanocrystals with Oh symmetry were single-crystals, mainly bound to 24 high-index {210} facets. The controlled slow reaction condition made the rhombic dodecahedral gold nanocrystals transform to THH gold nanocrystals with high-index facets.
*To whom correspondence should be addressed. E-mail: ookpark@ kaist.ac.kr (O.O.P.) and
[email protected] (S.H.I.). Telephone: þ82-42-3503923. Fax: þ82-42-350-3910. Web: http://stereo.kaist.ac.kr.
first is synthesis of gold seed nanocrystals, and the second is growth of them. Therefore, it is still challenging to synthesize the THH gold nanocrystals through a single step process (not seed mediated growth) in solution chemistry. Here we synthesized the THH gold nanocrystals with high-index facets through simple solution chemistry using N,N-dimethylformamide (DMF) both as a solvent and as a reducing agent in the presence of poly(vinylpyrrolidone) (PVP) as a surfactant. The as-prepared THH gold nanocrystals with Oh symmetry were single-crystal and bound to 24 high-index {210} (and {740}) facets. In our previous study, we have found that the gold nanocrystals could be transformed from rhombic dodecahedron enclosed by the most unstable {110} facets among the low-index facets to octahedron enclosed by the most stable {111} facets via rhombicuboctahedron by adjusting the concentration of water and poly(vinylpyrrolidone) (PVP) which could control the growth and etching of certain facets.9 Therefore, we speculate that the THH gold nanocrystal could be grown from the rhombic dodecahedron by suppression of etching and reduction of growth rate because the THH nanocrystal could be directly transformed by the selective addition toward {110} facets of rhombic dodecahedron without etching of edges. As a model system, in order to suppress the etching and growth reaction rate, no water and highly concentrated PVP in DMF solution were used, in contrast to the previous report, because the water increased the etching and overall growth reaction rate and the highly concentrated PVP could further reduce the growth reaction rate by slow addition of gold atoms. For this, 3.88 mL of DMF, 0.12 mL of 94.2 mM HAuCl4, and 8 mL of 2.47 M PVP (∼1750-fold of HAuCl4 concentration) in DMF were poured into a 50 mL capped vial and then heated to 80 C for ∼8 h in an oil bath (see the detailed synthetic method in the Supporting Information). Under the present reaction conditions, the reaction completion was prolonged to ∼8 h while the reaction was completed within several tens of minutes under relatively low concentrated PVP (∼920-fold of HAuCl4 concentration) solution. Low- and high-magnified scanning electron microscopy (SEM) images of the as-prepared THH gold nanocrystals are shown in Figure 1A and B. These images clearly indicate the formation of THH gold nanocrystals that are ∼300 nm in size, as obtained by means of simple solution chemistry. The yield of the THH gold nanocrystals exceeds 60%. A straightforward means of recognizing the THH structure with Oh symmetry is to consider the THH as a unit cube surrounded by six square pyramids at each square face, each of which has height of less than 1/2. The limiting cases are the cube and the RD, in which the height of the square pyramids are 0 and 1/2, respectively (Supporting Information, Figure S1). The X-ray diffraction (XRD) pattern of the THH gold nanocrystals shown in Figure 1C confirms that the structure
r 2010 American Chemical Society
Published on Web 06/14/2010
Shape and size control of metal nanocrystals has been one of the interesting research topics in the field of nanotechnology because their properties, including chemical, physical, and optical as well as catalytic activity, are strongly related to not only their size and composition but also their shapes.1 Particularly, synthetic methods for the shape-controlled gold nanocrystals have been studied intensively, owing to their potential applications in areas such as surface plasmonics, photothermal therapies, catalysts, and chemical and biological sensing fields.2 Although intensive studies in solution chemistry have been done to control the shape of gold nanocrystals, nanocrystals composed of lowindex facets such as the {111}, {100}, and even the {110} facets have been synthesized successfully in most cases due to the high surface energy of high-index facets.3 High-index facets have a much higher density of low-coordination-number stepped atoms, ledges, and kinks. Hence, they exhibit high activity and selectivity for chemical reactions.4 However, in solution chemistry, these high-index facets tend to disappear easily during the growth stage, as they are preferentially eliminated with the addition of atoms to minimize the surface energy of the produced nanocrystals. Therefore, it is very difficult and challenging to produce nanocrystals composed of high-index facets selectively. Thus far, metal nanocrystals with high-index facets have rarely been reported. Tian et al. initially reported that THH Pt nanocrystals prepared by an electrochemical method exhibited much higher (up to 400%) catalytic activity for small organic fuels due to their high-index facets.5 Since this report, hexoctahedral Pt, trapezohedral and concave hexoctahedral Pd nanoparticles, as well as Pt nanorods with high-index facets were prepared by the same method.6 Although the electrochemical method is powerful when used to prepare metal nanocrystals with high-index facets, the synthetic procedure is too complicated to mass-produce and nanocrystals grow only on a conductive substrate such as ITO (indium tin oxide) or GC (glassy carbon), which places a limitation on further applications. Ma et al. reported trisoctahedral gold nanocrystals enclosed with high-index facets prepared in a facile chemical method in which HAuCl4 was reduced by ascorbic acid in the presence of cetyltrimethylammonium chloride (CTAC).7 Recently, Ming et al. reported on the synthesis of THH gold nanocrystals through a seed mediated growth method in solution.8 They claimed that the THH gold nanocrystals could be synthesized by controlling the concentration of seed particle, shape regulating surfactant such as cetyltrimethylammonium bromide (CTAB), and pH. Namely, it is a two-step process: the
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Figure 1. (A) Low- and (B) high-magnification SEM image of the THH gold nanocrystals. (C) XRD pattern of the THH gold nanocrystals. (D) Ideal models of the THH in different orientations corresponding to those of the nanocrystals marked with the same numbers in part B.
of the produced metal nanocrystals includes a fcc (face centered cubic) gold structure (JCPDS 4-0784). The small peak around 56 derives from the silicon wafer that was used as a substrate for the XRD measurement. Figure 1D shows an ideal THH model with different orientations that correspond to the synthesized gold nanocrystals marked in Figure 1B. The crystal structure of the THH gold nanocrystals was further investigated by transmission electron microscopy (TEM) measurements. Figure 2A shows a TEM image of a single THH gold nanocrystal recorded along the [001] direction. The high-resolution TEM (HRTEM) image in Figure 2B recorded from the boxed area in Figure 2A shows a d-spacing of 0.20 nm for the adjacent lattice fringes; this corresponds to the {200} planes of the fcc Au (0.204 nm). The 4-fold symmetry of the selected area electron diffraction (SAED) pattern confirms that the THH gold nanocrystal is a single crystal (Figure 2C). The Miller indices of the exposed facets of the THH gold nanocrystal can be determined by measuring the adjacent angles between the facet projections along the [001] zone axis. Careful measurements recorded 127 and 143.8 angles, as shown in Figure 2A, which is consistent with the theoretical value of two angles, R = 126.9 and β = 143.1 between the {210} facets (Supporting Information, Figure S2). These results clearly indicate that the facets of the THH gold nanocrystal are mainly {210}. In particular, among all facets, the {210} facet has the highest density of broken bonds, at 8.94, whereas the density of broken bonds per area of unit cell for the {111}, {100}, and {110} facets of gold was 6.92, 8.00, and 8.48, respectively. This consequently reveals the most negative potential pzc of the zero charge.10 Furthermore, the {210} facet of gold has been recognized as not being reconstructed even at negative charges, unlike low-index facets.11 Although the major facet of the THH gold nanocrystals is {210}, another high-index facet of {740} was also simultaneously observed (Supporting Information, Figure S3). Figure 2D shows the atomic arrangement of the Au (210) surface, which is periodically constituted in (110) steps and (100) terraces. To address the growth mechanism of THH gold nanocrystals with high-index facets, the shape evolution of gold nanocrystals was examined in terms of the reaction time. The primary stage of the reaction can be recognized by the color change. At the initial stage, the reactant solution is light yellow in color and becomes transparent, which indicates the reduction of Au(III) to the Au(I) ion.12 The transparent reactant solution begins to show a light orange color, indicating the formation of gold nanocrystals. Finally,
Kim et al.
Figure 2. (A) TEM image of a single THH gold nanocrystal recorded along the [001] direction. (B) High-resolution TEM image recorded from the boxed area in part A. (C) Corresponding SAED pattern indicating the single-crystal structure of the THH gold nanocrystal. (D) Atomic model of the Au(210) plane consisting of the (100) and (110) subfacets.
Figure 3. SEM images of THH gold nanocrystals obtained at (A) 3 h 30 min, (B) 5 h 30 min, (C) 6 h 10 min, (D) 6 h 50 min, (E) 7 h 10 min, and (F) 7 h 40 min reaction times. All images have the same magnification.
the color of the reactant solution gradually darkens. At the early stage of the reaction after the formation of gold nanocrystals, relatively small (∼170 nm in diameter) rhombic dodecahedral gold nanocrystals were observed (Figure 3A). Here, for the synthesis of the THH gold nanocrystals, note that the reaction temperature was intentionally reduced to 80 C and the concentration of PVP was increased to ∼1750-fold of the HAuCl4 concentration in order to shrink the reaction rate, as the highindex facets are likely to disappear due to the preferential addition of gold atoms into these high-index facets due to their high surface energy under a relatively fast reaction condition. Under the slow reaction rate under these experimental conditions, the rhombic dodecahedral gold nanocrystals bound to the {110} facet are formed, possibly as a result of the stabilization introduced by DMF or its oxidation products, as reported in the literature and in earlier results reported by the authors.9,13 As the reaction proceeded, the small RD gold nanocrystals grew into larger (∼280 nm in diameter) nanocrystals with inflated surfaces (Figures 3B and C) and became nearly spherical in shape due to further growth at 6 h 50 min (Figure 3D). The rounded RD gold nanocrystals were then transformed into THH gold nanocrystals at 7 h 10 min, as easily recognized by the separation of each rhombic plane of the RD gold nanocrystals into two triangle planes (Figure 4E). Figure 4F shows a SEM image of gold nanocrystals obtained at 7 h 40 min, indicating the formation of well-defined THH gold nanocrystals (∼300 nm in diameter)
Communication with protruded vertices. This shape evolution as the reaction time progresses clearly shows that the THH gold nanocrystals were formed from the RD gold nanocrystals of which the growth rate along the Æ111æ and Æ110æ directions was faster than that along the Æ100æ direction (Supporting Information, Figure S4). It remains unclear as to whether the PVP is involved in the formation of RD gold nanocrystals in an early stage because the RD gold nanocrystals could be formed even without PVP.13 For the formation of RD gold nanocrystals, parameters related to the reaction rate, such as the reaction temperature and the amount of water added, appear to be the more dominant factors as compared to the PVP.9 However, for the synthesis of THH gold nanocrystals, both the reaction temperature and the PVP concentration seem to be crucial to form THH gold nanocrystals as a final product, as the THH gold nanocrystals could not be obtained by decreasing the concentration of PVP or by increasing the reaction temperature (Supporting Information, Figure S5) because the THH gold nanocrystals could be grown under the suppressed etching and growth reaction condition. In summary, THH gold nanocrystals bound to 24 high-index ({210} and {740}) facets were successfully synthesized by a facile, one-step colloidal method. As expected, the THH gold nanocrystals were transformed from RD gold nanocrystals due to the relatively fast growth in the Æ111æ and Æ110æ directions compared to that in the Æ100æ direction under suppressed etching and controlled slow reaction condition. Acknowledgment. This research was supported by the WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (No. R32-2008-000-10142-0).
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Supporting Information Available: Materials, detailed descriptions of experimental procedures, and additional data. This information is available free of charge via the Internet at http://pubs.acs.org.
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