Mechanistic Aspects of Shape Selection and Symmetry Breaking during Nanostructure Growth by Wet Chemical Methods

Abstract

The control of shapes of nanocrystals is crucial for using them as building blocks for various applications. In this paper, we present a critical overview of the issues involved in shape-controlled synthesis of nanostructures. In particular, we focus on the mechanisms by which anisotropic structures of high-symmetry materials (fcc crystals, for instance) could be realized. Such structures require a symmetry-breaking mechanism to be operative that typically leads to selection of one of the facets/directions for growth over all the other symmetry-equivalent crystallographic facets. We show how this selection could arise for the growth of one-dimensional structures leading to ultrafine metal nanowires and for the case of two-dimensional nanostructures where the layer-bylayer growth takes place at low driving forces leading to plate-shaped structures. We illustrate morphology diagrams to predict the formation of two-dimensional structures during wet chemical synthesis. We show the generality of the method by extending it to predict the growth of plate-shaped inorganics produced by a precipitation reaction. Finally, we present the growth of crystals under high driving forces that can lead to the formation of porous structures with large surface areas.

Shape-controlled electrochemical synthesis of Au nanocrystals in reline: control conditions and electrocatalytic oxidation of ethylene glycol

“…At low temperatures, the energy state of reaction system is relatively low, the nucleation and growth of NCs into the thermodynamically favored equilibrium morphologies are preferred under a thermodynamically controlled regime, hence nanoparticles grow into quasi-spherical nanostructures. 44 When the temperature of reaction reaches a certain level, such as 90 C, the nucleation and growth of NCs are facilitated under a kinetically controlled regime. Meanwhile, reline as a special shape-directing agent plays a key role in shape control for the nucleation and growth of NCs at high temperature, leads to an orienting growth of NCs.…”

Self-assembly of metal nanostructures on binary alloy surfaces

“…Self-assembly can be manifested in either complex equilibrium structures, e.g., reflecting competing interactions, or in far-from-equilibrium growth structures (1). The latter can be very different and more diverse than the equilibrium forms (2). Significantly, self-assembly provides a practical strategy for creating ensembles of nanostructures with unique size and shape dependent properties, a central goal of nanotechnology.…”

Temperature-dependent growth shapes of Ni nanoclusters on NiAl(110)

“…The former is common in liquid-phase nanoparticle and crystal growth, particularly in the presence of capping agents. [3][4][5] The latter is more common for vapor deposition on surfaces, although some degree of attachment limitation may persist (e.g., an inhibition of atoms to incorporate at highlycoordinated kink sites as they cross steps or move around corners). [6][7][8][9] There have been extensive efforts to control the formation of metal and semiconductor nanostructures, manipulating both size and shape.…”

“…For liquid-phase formation, typically growth is controlled by surfactants and capping agents, which can produce an extraordinary variety of far-fromequilibrium growth shapes, e.g., pencil, arrow, tree-shaped, and hyper-branched semiconductor nanostructures, 3,10 and polyhedral nanocrystals, nanorods, nanowires, platelets, etc., for fcc metal nanostructures. 4,5,11 Certainly, a general appreciation of the factors controlling non-equilibrium growth morphologies has been established, e.g., a "kinetic control hypotheses" and insights from Cahn's formulation of crystal growth. 4 However, there is typically a lack of detailed atomistic-level understanding of the growth kinetics even though this would facilitate guided formation of functional nanostructures with tailored morphologies and "tuned" properties.…”

“…4,5,11 Certainly, a general appreciation of the factors controlling non-equilibrium growth morphologies has been established, e.g., a "kinetic control hypotheses" and insights from Cahn's formulation of crystal growth. 4 However, there is typically a lack of detailed atomistic-level understanding of the growth kinetics even though this would facilitate guided formation of functional nanostructures with tailored morphologies and "tuned" properties.…”