Nanochannel induced advection with reaction-diffusion dynamics of reacting ions leads to self-assembly of ordered cylindrical structures

Abstract

We present a theoretical model, which elucidates the physical principles involved in the formation of very uniform CdS nanocylinders of different radii by combining the physics of flow, diffusion, self assembly, and aggregation of constituent particles. Very recent experiments report that when 0.1M solutions of CdCl2 and Na2S were allowed to mix through some anodised aluminium oxide (AAO) nanochannels, one observes the growth of an array of CdS nano-cylinders on only one end of the AAO template [A. Varghese and S. Datta, Phys. Rev. E 85, 056104 (2012)]. These cylinders have a pore along the center of the cylinder but closed at one end. The reaction happens only in the Na2S chamber, and growth of cylinders of uniform size and shape continues as long as the supply of the reactant molecules (CdCl2 and Na2S) is maintained. To try to understand the physics of the observed phenomenon, we propose a model where the Cd+2 ions exit the AAO-nanochannel to enter Na2S chamber with a finite velocity; these ions then react with the diffusing S?2 ions to form CdS, which then self assemble to form cylinders of uniform width and cross-section. The flow of Cd+2 out of the AAO nano-channel is the key symmetry breaking feature, which facilitates the formation of uniform cylindrical structures of CdS instead of a CdS precipitate. Since our model does not crucially depend on the chemical details of the reaction, this mechanism can be extended to self-assemble other structures of relevance.