Predicting the growth of two-dimensional nanostructures

Abstract

Abstract:The ability to predict the morphology of crystals formed by chemical reactions is of fundamental importance for the shape-controlled synthesis of nanostructures.Based on the atomistic mechanism for crystal growth under different driving forces, we have developed morphology diagrams to predict regimes for the growth of two-dimensional crystals. By using controlled reactions for crystal growth in the absence of surfactants/capping agents, we demonstrate the validity of this approach for the formation of 2-D structures of Au, Ag, Pt, Pd andhydroxyapatite.An understanding of the external morphology of crystals and its relation to internal structure has been a topic of active study since the times of Kepler. It is well-recognised that the morphology of nanostructures affects their properties profoundly [1][2][3][4][5][6][7][8] . Twodimensional nanostructures in the form of platelets/sheets, nanoprisms and belts exhibit intriguing properties that have several potential applications [1][2][3][4]6,7,[9][10][11] . In spite of the availability of a number of methods [2][3][4][5][6][7][8][12][13][14][15] , the mechanism of formation of such structures remains elusive. Mechanisms based on preferential adsorption of surfactants 8,16 , oriented attachment 16 , soft templates 17,18 , aggregation of spherical particles 4 and kinetic control [6][7][8]18,19 have been proposed in the literature but do not satisfactorily explain the shape control. Here, we show for the first time that this problem can be analysed based on classical crystal growth concepts. Based on the atomistic mechanism for crystal growth under different driving forces 20,21 , we have developed morphology diagrams to predict conditions under which two-dimensional crystals form. By using controlled reactions for crystal growth in the absence of reducing and capping agents or by precipitation under controlled conditions coupled with detailed microstructural evidence, we demonstrate the validity of this approach for the formation of 2-D nanostructures of Au, Ag, Pt, Pd and hydroxyapatite. The analysis and experiments have important consequences for rational synthesis of two-dimensional nanostructures and answer some long-standing questions related to their growth. The generality of the analysis implies that it can be used to predict regimes of two-dimensional growth in a variety of systems ranging from crystals synthesized from a solution phase, from the vapor phase and inorganic phases formed by biomineralization.The formation of crystals from vapor or in a liquid phase proceeds by nucleation and growth mechanism with the driving force given by the associated volume free energy change. Based on the driving force, two distinct growth regimes can be identified. At large driving forces, the interface can move normal to itself leading to a continuous growth (Fig. 1A). At low driving forces, however, growth has to rely on the formation of steps and a lateral motion of steps on the surface [20][21][22] . It has been proposed that screw dislocations enable crysta...

Au2Sx/CdS Nanorods by Cation Exchange: Mechanistic Insights into the Competition Between Cation‐Exchange and Metal Ion Reduction

“…According to calculations, the formation of Au 2 S 3 is thermodynamically more favorable than the formation of Au 2 S. To compare this with the competing reaction of metal ion reduction, we also evaluated the free energy change for the formation of Au with water acting as the reducing agent in the medium by a method reported in literature. [33][34][35] We note that this free energy change is negative indicating that water can indeed reduce Au salt to metal at room temperature. However, our control experiments indicate that there could be signifi cant kinetic barriers and thus higher temperatures and/or a lowering of nucleation barrier by lowering the interfacial energy for heterogeneous nucleation may be needed for the formation of Au [ 36 ] (also see Supporting Information).…”

Two-Dimensional Colloidal Nanocrystals

“…However, not only the surface energy is important, the surface area also needs to be taken into account, and the NC will tend to minimize it too, resulting in truncated octahedrons enclosed by (111) and (100) facets. [46][47][48][49] These first examples show that the equilibrium shape of a NC cannot be determined only by the minimization of the surface area as it would always yield spherical-shaped NCs.…”

Formation of two-dimensional structures by tuning the driving force of chemical reactions: An interpretation of kinetic control

“…Growth of thin films, formation of ice crystals in the atmosphere [9], bone formation by biomineralization [10,11] and wet-chemical synthesis of nanostructures represent typical examples of diverse areas where such an understanding is vital. Wet-chemical methods are widely used to synthesize range of anisotropic nanostructures for variety of applications [12][13][14][15]. Anisotropic metal nanostructures have attracted considerable attention owing to the wide variety of plasmonic, magnetic and electronic properties that they exhibit.…”