Sulfidation of Cadmium at the Nanoscale Page: 2 of 25
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Since the first report in 2004,1 the formation of hollow nanoparticles via the
nanoscale Kirkendall effect has been observed in dozens of materials. 2- In the simplest
scenario, an elemental metallic nanocrystal reacts to form a compound nanoparticle;
hollow, polycrystalline nanoparticles result when the diffusion of the cationic species
outwards is faster than the inward diffusion of the anionic species. Some examples of
hollow particles formed in this way include oxides, sulfides, selenides, and phosphides of
cobalt, iron, molybdenum, and nickel. Fan, Gtsele, and Zacharias have written a recent
review outlining developments in this field,2 as well as describing criteria that can be
used to evaluate the possibility that a given system will form hollow particles by this
mechanism.
In materials with one or more macroscopic dimensions, like nanowires or films,
the Kirkendall effect leads to the formation of multiple voids at the metal-composite
interface.6' 7 In nanoparticles, a single void is typically observed in the center of the
particle when the reaction is complete, and the particles maintain an overall approximate
spherical symmetry through the course of the reaction.3' a In such spherical
nanostructures, the unreacted metal core remains in the center and is connected to the
hollow shell by thin filaments. '9 In a very recent report, Nakamura et al. already showed
that the incomplete oxidation of Ni nanoparticles leads to the formation of partially
hollow structures with a single void.1 On fully oxidized NiO particles, holes were
located at off-center positions and non-homogeneous shell thicknesses were obtained.
Here we explore the sulfidation of spherical cadmium crystals to form hollow CdS via the
nanoscale Kirkendall effect, Figure 1. Cadmium sulfide, having both direct band gaps in
the optical region and piezoelectric properties, is particularly suited for optical,
optoelectronic, and electromechanical applications. In the cadmium sulfidation reaction,
the metal self-diffusion time in the core is much lower than the self-diffusion time of the
metal ions through the shell. We show that the structures obtained from the partial
sulfidation of Cd particles contain a single off-center void region and a single off-center
region of unreacted Cd, both with a spherical cap geometry. Markedly, in spite of the
asymmetrical metal distribution in this new type of structure, a homogeneous growth of
the CdS shell is observed throughout the entire particle as the sulfidation proceeds to
completion. We measured the growth rates of CdS as a function of the reaction2
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Cabot, Andreu; Smith, Rachel; Yin, Yadong; Zheng, Haimei; Reinhard, Bjorn; Liu, Haitao et al. Sulfidation of Cadmium at the Nanoscale, article, May 22, 2008; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc900701/m1/2/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.