Synthesis and Surface-Enhanced Raman Scattering Property of Pentagonal Dodecahedral Au Nanocrystals

The controlled synthesis of polyhedral Au nanocrystals (NCs) has attracted tremendous research interest in the past decades due to their promising applications in a variety of fields, such as plasmonics, sensors, electronics, catalysis, and surface-enhanced Raman scattering (SERS). Since the inherent plasmonic, electrical, and catalytic properties of Au NCs can be finely tuned by control over their shapes, numerous synthesis methods have been developed for the production polyhedral Au NCs with tailored morphologies, such as Platonic solids (tetrahedron, cube, octahedron, and icosahedron), cuboctahedron, decahedron, rod, and plate. To minimize the surface energy during the NC growth, these Au NCs are mostly enclosed by stable low-index {111} and/ or {100} facets. Recently, the synthesis of Au NCs with exposed high-energy facets, i.e., low-index {110} and highindex {hkl} (at least one index greater than 1), has been reported. NCs bound by high-energy facets have shown enhanced performance compared to conventional {111} and/ or {100}-faceted NCs due to the presence of high-density undercoordinated atoms on their surfaces as well as to their unique morphological characteristics. For instance, we reported that rhombic dodecahedral (RD) and hexoctahedral Au NCs exclusively bound by {110} and {321} facets, respectively, exhibited pronounced SERS activities. Nevertheless, it is still a great challenge to synthesize shapecontrolled NCs that are enclosed by high-index facets due to their high surface energy. Here we report for the first time on the synthesis of pentagonal dodecahedral (PD) Au NCs bound by high-index {hk0} facets. The PD Au NCs could be readily prepared by the reduction of Au precursors with N,N-dimethylformamide (DMF) in the presence of poly(vinyl pyrrolidone) (PVP). The pentagonal dodecahedron is a convex polyhedron with 12 pentagonal faces, where 3 faces meet at each corner. The surface facets of PD structures have been determined to be high-index {hk0}: the simplest one is {210} and other types are {320}, {410}, and {120}. The pentagonal dodecahedron has been known as a very unusual structure and crystalline materials with this structure have been rarely reported. Only the crystal of pyrite and some quasi crystals have this structure. To the best of our knowledge, the synthesis of metal NCs with PD structure has never been reported. Furthermore, the prepared PD Au NCs could be successfully applied in the field of SERS. Highly enhanced SERS signals could be obtained from various analytes with the PD Au NCs, which can be attributed to their high-surface energy facets. Figure 1(a) and (b) show representative lowand highmagnification field-emission scanning electron microscopy (FESEM) images of the as-prepared samples, demonstrating the formation of Au NCs with a well-defined structure. The sizes of Au NCs were in the range of 150-200 nm. The SEM image of NCs, shown in Figure 1(b), is closely matched by the 3D models of dodecahedra with different orientations (Figure 1(c)), indicating the successful generation of PD Au NCs enclosed by 12 pentagons under our experimental conditions. The X-ray diffraction (XRD) pattern of the PD Au NCs showed distinct diffraction peaks from the face centered cubic (fcc) structure of Au, revealing their crystalline nature (Figure 1(d)). Figure 2(a) shows the transmission electron microscopy (TEM) image of a PD Au NC recorded along the [110] direction. The corresponding convergent beam electron diffraction (CBED) pattern is also shown in Figure 2(b), which