IISER Pune researchers address role of fluid dynamics in the mechanism by which nanostructures form, an often overlooked aspect in this area of research.
Size becomes important, especially when you are talking about making ever smaller devices which could have applications in electronics, engineering and medicine. And, when you are dealing with sizes in the nanoscale—meaning, sizes 1,000,000,000 times smaller than a meter—you end up having to redefine properties of materials, the same characteristics that you can exploit to generate a new product. Synthesizing materials in the nanoscale and with new, useful properties is therefore an area of research buzzing with activity. While there are several new nanostructures being synthesized and explored for their utility, little is known about the mechanism by which they are formed.
Recent results from researchers at IISER Pune point to a previously unexplored role of fluid dynamics in how nanostructures are made, which is an often overlooked aspect. Shouvik Datta, principal investigator of this study, and his graduate student Arthur Varghese chose to test an unconventional method of preparing nanostructures by setting up an array of nanometer sized chemical reactors.
The experimental set-up
The set-up was created as follows: two chemical reactants were placed in separate beakers connected by a horizontal bridge; at the centre of the bridge was placed a membrane with tiny (nanoscale) pores, referred to as nanochannels. The idea was to allow the reactants to flow and mix through these nanochannels and see if this leads to the formation of any solid nanostructures and how. This is in contrast to a single reagent/single component fluid used in traditional nanofluidics experiments.
“We were initially apprehensive that this set-up with two reactants and their solid by-product may eventually clog nanochannels of the membrane and not allow nanostructures to form or to grow. So we were pleasantly surprised when we saw formation of highly ordered arrays of cadmium sufide nanotubes on the membrane and that these nanotubes could extend horizontally up to several micrometers in length within few hours,” says Datta.
The authors then viewed these structures through an electron microscope and learnt that the cadmium sulfide nanotubes are formed only in a specific direction on one particular side of the membrane. “The directionality of nanotube growth caught our attention as this has not been described before in literature,” says Varghese describing his novel observation.
Unidirectionality of nanotubes
In order to understand this phenomenon, Varghese and Datta carried out simple experiments with the hypothesis that the capillary flow of a reagent, or the lack of it, across a nanochannel depends on acidity or basicity (pH) of the reactant. Thus the reactant (say A) that manages to flow faster through the channel as a result of lower resistance to its flow meets its partner (say B, which experiences a higher resistance to its flow through the channel) at the other end of the channel; A and B react to form cadmium sulfide nanotube that now begins to grow unidirectionally on one side of the membrane. The nanotube continues to grow as long as A flows through the channel, meets B at the end of the channel, and the two react to produce solid cadmium sulfide nanotubes.
Experimental evidences from the authors’ work suggested that the flow of sodium sulfide (representing B) is indeed much lesser than the flow of cadmium chloride (representing A). Now that the authors had a fair idea why these cadmium sulfide nanotubes were beginning to form on one particular side of these nanochannels, the next aspect to address was how.
Nanotubes imaged using a scanning electron microscope
Proposed model for nanotube growth
Looking closely at electron microscopy images, authors found that the top end of the tubes remains closed and it is at the bottom end—edges of nanotubes on the membrane surface—where all activity lies. As the authors had predicted, the bottom-end region seemed to be structurally weak considering the observation that some nanotubes were found to break apart from this joint alone.
“We observed a horizontal growth of nanotubes that are several micrometers long and are uniformly straight. This kind of sustained growth cannot be explained by standard theories of fluid dynamics, which usually predict that fluid flows in a laminar fashion at nanometer scales. Instead, continuous intermixing of reactants without clogging these nanochannels can be made possible only if there were chemically triggered convective instabilities (vortices) at liquid-liquid reactive interface,” says Datta suggesting that this work could prompt new experimental and theoretical research to understand further details of the collective role of chemical kinetics and fluid dynamical instabilities at the nanoscale. “Understanding such an involved role of physics of fluids in nanostructure formation may also be useful for biomedical applications and drug delivery,” Varghese adds.
Datta’s group has now initiated additional experiments and theoretical modeling of the growth mechanism and is also trying to understand certain unique optical and electronic properties of these nanotubes for optoelectronics applications. His collaborators at IISER Pune in this effort include research groups led by Apratim Chatterjee, Arijit Bhattacharyay and Prasenjit Ghosh.
This study titled “Directionally asymmetric self-assembly of cadmium sulfide nanotubes using porous alumina nanoreactors: Need for chemohydrodynamic instability at the nanoscale” has appeared in the journal Physical Review E, Vol 85, pg 056104, 2012 and is authored by Arthur Varghese and Shouvik Datta. The research was supported by Department of Science and Technology, India along with funds from IISER Pune.
-Reported by Shanti Kalipatnapu