Self-assembly of monodisperse CdS nanocylinders with an axial pore

We present our conclusions of the investigation of the self-assembly and growth of an array of CdS nanotubes: a consequence of a fine balance of directed motion, diffusion, and aggregation of reacting Cd+2 and S-2 ions. In a previous communication [Kiruthiga and Chatterji, J. Chem. Phys. 138, 024905 (2013)], we identified the mechanism of an unexpected growth of a CdS nanocylinder of uniform radial cross section from the end of a nanochannel. Furthermore, the cylinder had a pore along the axis but was closed at one end. This unique phenomenon of self-assembly of very monodisperse CdS nanocylinders had been observed in a rather simple experiment where two chambers containing 0.1M CdCl2 and 0.1M Na2S solutions were joined by an array of anodized aluminium oxide (AAO) nanochannels [Varghese and Datta, Phys. Rev. E 85, 056104 (2012)]. Interestingly, the growth of CdS nanotubes was observed only in the Na2S chamber. The primary focus of our previous study was on identifying the principles governing the growth of a single nanotube at the exit point of a single AAO nanochannel. In this communication, we identify factors affecting the self-assembly of a nanotube in the presence of other similar neighboring nanotubes growing out from an array of closely spaced AAO nanochannel exit points, a scenario closer to the experimental situation. Our model is not Cd+2 or S-2 specific, thus our conclusions suggest that the experimental scheme can be extended to the self-assembly of a general class of reacting-diffusing A and B ions with A (in this case Cd+2 ions) selectively migrating out from a nanochannel. In particular, we note that after the initial growth of nanotubes for a period of time, there can arise a severe deficiency of B ions (S-2) near the AAO-nanochannel exits. The low concentration of B near the nanochannel exits impedes further growth of uniform CdS nanotubes. We further identify the parameters which can be tuned to obtain an improved crop of monodisperse nanotubes. Thereby we predict the necessary characteristics of reacting systems which can be self-assembled using suitable adaptations of experiments used to grow CdS cylinders.