posted on 2021-09-23, 19:39authored byMuhua Sun, Zhihua Cheng, Weiyin Chen, Matthew Jones
The vast majority of single crystalline
metal nanoparticles adopt
shapes in the Oh point
group as a consequence of the symmetry of the underlying face-centered
cubic (FCC) crystal lattice. Tetrahedra are a notable exception to
this rule, and although they have been observed in several syntheses,
their growth mechanism, and the symmetry-reduction process that necessarily
characterizes it, is poorly understood. Here, a symmetry breaking
mechanism is revealed by in situ liquid flow cell
transmission electron microscopy (TEM) observation of seeded growth
in which tetrahedra nanoparticles are formed from higher symmetry
seeds. Real-time observation of the growth demonstrates a kinetically
driven pathway during which rhombic dodecahedra nanoparticles transition
to tetrahedra through tristetrahedra intermediates, with an accompanying
surface facet evolution from {110} to {111} via {hhl}
(where h > l), respectively. On the basis of these data, we propose
a mechanism that relies on a rapid loss of inversion symmetry in the
initial stages of the reaction, followed by differential reactivity
of tips vs faces under conditions of relatively high supersaturation
and moderate ligand concentration. The application of these insights
to ex situ synthesis conditions allowed for an improved
yield of tetrahedra nanoparticles. This work sheds an important mechanistic
light on the crystallographic underpinnings of nanoparticle shape
and symmetry transformations and highlights the importance of single-particle
characterization tools for monitoring nanoscale phenomena.