Heteroepitaxial growth of core-shell and core-multishell nanocrystals composed of palladium and gold

Copper can still be epitaxially deposited on palladium nanocrystals to generate core-shell nanocubes despite their large lattice mismatch

Here we report the synthesis of Pd@Cu core-shell nanocubes via epitaxial growth, where the lattice mismatch is 7.1%. The synthesis involved the use of Pd seeds with different shapes (including cubes, cuboctahedra, and octahedra) for the epitaxial growth of Cu shells. Different from the conventional growth mode, Cu atoms initially nucleated only on a few of the many faces of a Pd seed, onto which more Cu atoms were continuously added to generate Cu blocks. Later, the Cu atoms also started to nucleate and grow on other faces of the Pd seed until the entire surface of the seed was covered by a Cu shell. As a result, the Pd seed was rarely located in the center of each core-shell structure. The final product took a cubic shape enclosed by {100} facets regardless of the type of Pd seeds used because of the selective capping of Cu(100) surface by hexadecylamine. The edge lengths of the Pd@Cu nanocubes could be tuned from 50 to 100 nm by varying the amount of Pd seeds while keeping the amount of CuCl(2) precursor.

Engineering "hot" nanoparticles for surface-enhanced Raman scattering by embedding reporter molecules in metal layers

Ag is deposited on the surface of Au nanoparticles functionalized with 4-mercaptobenzoic acid (MBA). Exceptionally strong surface-enhanced Raman scattering (SERS) signals are observed from the resulting colloid. Using SERS as a tool, evidence is obtained for the embedding of MBA inside the nanoscale metal layer.

Controllable synthesis of water-soluble gold nanoparticles and their applications in electrocatalysis and surface-enhanced Raman scattering

We report a facile method to synthesize water-soluble gold nanoparticles (AuNPs) using a biosurfactant sodium cholate as reducing reagents and protective groups in aqueous solution at ambient temperature. The diameters (13-70 nm) of uniform AuNPs can be readily adjusted by changing the initial molar ratio of sodium cholate to chloroauric acid (HAuCl(4)). Also, the alkaline condition of preparative solution is found to affect the size of as-synthesized AuNPs. This synthetic approach is one-step and "green". The obtained AuNPs exhibit a good electrocatalytic activity toward methanol oxidation. Meanwhile, the AuNPs thin films can serve as an efficient substrate for surface-enhanced Raman scattering (SERS). Furthermore, platinum nanoparticles (PtNPs) are also prepared by reducing sodium tetrachloro platinate hydrate with sodium cholate.

New Insights into the structure of Pd-Au nanoparticles as revealed by aberration-corrected STEM

Bimetallic nanoparticles of Au-Pd find important applications in catalysis. Their catalytic performance is directly related to the structure, alloy formation and variation of composition in the structure. A standard idea is that bimetallic nanoparticles can be either an alloy or a core shell structure. Our group has investigated the structure and composition of Pd-Au nanoparticles by using aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We reported previously that the nanoparticles are composed of an evenly alloyed inner core, an Au-rich intermediate layer, and a Pd-rich outer shell. The structure is more complicated than what simple models can predict. In this paper we report additional studies of this system wherein by carrying out spectral and chemical analysis (STEM*-EDAX, STEM-EELS) the interface structure can now be better identified and understood. Apart from the three-layered core-shell structures we have also been able to observe in some cases a four-layered core-shell structure as well. The entire core-shell structure is not rigid and there is indeed intercalation of Au-Pd into the other layers as well. In addition we have been able to locate stacking faults present in the nanoparticles. We also address the problem of the interface structure between the layers. By using nanodiffraction we have found that the whole structure of the nanoparticles becomes hcp in contrast to the bulk structure of Au or Pd.

Solution-phase synthesis of metal and/or semiconductor homojunction/heterojunction nanomaterials

The design and architecture of programmable metal-semiconductor nanostructures with excellent optoelectronic properties from metal and semiconductor building blocks with nanoscale dimensions have been a key aim of material scientists due to their central roles in the fabrication of electronic, optical, and optoelectronic nanodevices. This review focuses on the latest advances in the solution-phase synthesis of metal and/or semiconductor homojunction/heterojunction nanomaterials. It begins with the simplest construction of metal/metal and semiconductor/semiconductor homojunctions, and then highlights the synthetic design of metal/metal and semiconductor/semiconductor heterojunction nanostructures with different building blocks. Special emphasis is placed on metal/semiconductor heterojunction nanomaterials, which are the most challenging and promising nanomaterials for future applications in optoelectronic nanodevices. Finally, this review concludes with personal perspectives on the directions for future research in this field.