Shaping metal nanocrystals through epitaxial seeded growth Page: 4 of 19
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Our understanding of the overgrowth process and the epitaxial interface of the Pt/Pd
cubes suggests that the defined structure of the Pt seed has a direct influence on the
controlled overgrowth of Pd from the epitaxial interface. Extending our control of the
directed overgrowth on Pt nanocubes to include shape control chemistry can allow for the
utilization of the conformal epitaxial growth to produce core-shell particles with other
well-defined shapes. Nitrogen dioxide is known to dissociate on Pd surfaces to give
adsorbed NO and adsorbed atomic oxygen. The NO desorbs between 230 and 300K
leaving behind adsorbed oxygen,33 which may interact selectively with the Pd surfaces of
the growing particles.34 By adding NO2 we were able to vary the growth rates along the
<100> and <111> directions to give cuboctahedrally (~90%, 36.4 nm measured body
diagonal, with 6% distribution) and octahedrally shaped Pd shells (~90%, 34.6 nm
measured 2D projection of body diagonal with 6% distribution, 40.8 nm calculated body
diagonal). The cuboctahedral particles imaged by SEM in Figure le have a more
spherical morphology than that shown by TEM (Fig. lf). A hexagonal projection for
particles oriented on their <111> zone axis can be readily seen in Figure lf. The SEM
image of the octahedra (Fig. li) shows particles that are significantly less spherical than
the cuboctahedra and have prominent vertices. Analysis by TEM (Fig. lj) also shows
particles that have a hexagonal projection, but unlike the cuboctahedra that pack
hexagonally with no overlap, the octahedra overlap at each of their vertices in a manner
characteristic of their geometry. Also visible in the TEM images are the presence of the
Pt seeds at the core of each structure.
Here we have used the decomposition of concentrated HNO3 in the presence of
concentrated HCl (aqua-regia) as an NO2 source, although similar results can be obtained
by bubbling NO2 gas through the reaction to give octahedrally shaped Pt/Pd particles
(Fig. S4). For all samples, the acid concentration was kept constant at 1 mM by the
addition of HNO3 to avoid any influence of pH on the reactions containing acidic aqua-
regia. The addition of aqua-regia or NO2 slows down the reduction rate considerably.
Comparative analysis of the UV-Vis spectra for the [TTA]2[PdBr4] complex formed prior
to reduction,35 and the complex following exposure to NO2(g), shows a shift in the
absorption maxima from 341 and 252 nm, to 320 nm with a shoulder at 250 nm after NO2
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Habas, Susan E.; Lee, Hyunjoo; Radmilovic, Velimir; Somorjai,Gabor A. & Yang, Peidong. Shaping metal nanocrystals through epitaxial seeded growth, article, February 17, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc896286/m1/4/: accessed December 10, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.