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Simultaneous Reduction of Metal Ions by Multiple Reducing Agents Initiates the Asymmetric Growth of Metallic Nanocrystals
journal contribution
posted on 2015-09-02, 00:00 authored by Mahmoud A. MahmoudThermodynamically unfavorable metallic
nanocrystals can be prepared
only by the growth of the nanocrystals under kinetically controlled
experimental conditions. The common technique to drive the growth
of metallic nanocrystals under kinetic control is to adjust the rate
of the generation of metal atoms to be slower than the rate of deposition
of such atoms onto the surface of nanocrystal nuclei, which form in
the first step of the nanoparticle synthesis. The kinetically controlled
growth leads to the formation of seeds with crystal defects, which
are needed for the growth of anisotropic nanocrystals such as silver
nanodisks (AgNDs). The simultaneous multiple asymmetric reduction
technique (SMART) is introduced here to successfully prepare AgNDs
of controllable sizes and on a large scale within a few seconds. The
SMART is simply based on the simultaneous reduction of silver ions
with a strong reducing agent such as borohydride (redox potential
of 1.24 V) and a weak reducing agent such as l-ascorbic acid
(redox potential of 0.35 V) in the presence of a polyvinylpyrrolidone
capping agent. The random formation and deposition of silver atoms
by the two different reducing agents generated stacking faults in
the growing nanocrystal. The hexagonal close-packed {111} layers of
silver atoms were then deposited on the surface of the growing nanocrystal
containing stacked faults along the [111] plane. This initiated asymmetric
growth necessary for the formation of platelike seeds with planar
twin defects, which is required for the formation of anisotropic AgNDs.