A multi-rate kinetic model for spontaneous oriented attachment of CdS nanorods

A multi rate kinetic model to explain the spontaneous oriented attachment of CdS nanorods in the presence of an amine is presented. The model demonstrates the reasons that elongation is restricted to a maximum of quadruple the starting rod length for rods of a certain aspect ratio (8 x 30 nm) with no elongation occurring for rods of a shorter aspect ratio. The rate constants for all possible attachment events are determined showing that elongation by attachment occurs sequentially by single rod addition alone. Both the reaction rate and the activation energy for subsequent attachment are found to increase as the rod lengthens. The increase in reaction rate correlates with increased dipole moment of longer rods which orients the rods end to end to maximize collision events. The two components of this reaction are alignment and fusion, with the former restricting attachment of low aspect ratio rods and the latter restricting attachment to higher aspect ratio rods. The model therefore predicts an aspect ratio “window” in which oriented attachment of nanorods is energetically possible.