On the Nanoscale: Where Size and Shape Matter
European chemists have discovered what one might think of as a butterfly effect in the world of nanoparticles. They have shown that the rate at which a stock solution is prepared can influence the final outcome when crystallizing cobalt-based nanoparticles and that the same phenomenon affects the morphology of other nanocrystals too.
For nanoscientists and technologists size and shape really do matter. On the nano scale – the previously latent domain between single atoms and molecules and the bulk phase – the properties of particles are not those of the everyday world, nor necessarily are they that of the unexpected quantum behavior of the smallest particles. They are somewhere in between and at once unique to that domain. As such, controlling particle diameter, shape, and morphology seem critical to controlling the way in which any given set of nanoparticles behaves. Thus, ways to ensure all particles in a batch grow to the same diameter and have the same crystalline morphology is often key to success in the laboratory and in fine-tuning the industrial potential of nanoparticles.
Of course, it is well known to those preparing and working with nanoparticles that such perfection in control is very difficult to achieve. Many approaches have been taken to increase uniformity and many separation techniques have been developed to filter off those nanoparticles in a batch that do not fit that uniform. Nevertheless, there is an almost endless list of parameters that must be finely controlled in every experiment. So, when what, at first glance, seems to be a neatly reproducible experiment involving lab-prepared stock solutions of [Co{N(SiMe3)2}2(THF)], hexadecylamine, and lauric acid actually generates very different batches of cobalt-based nanocrystals, any research team would be at some loss to explain what is going wrong.
Hidden Parameter: Rate of Stock Solution Preparation
A collaborative effort based in France, Greece, and Italy reports in the Journal of the American Chemical Society this month how they used precisely measured concentrations of those three reactants in their preparations, but obtained batches of nanoparticles with very different characteristics on several occasions. It almost sounds like some kind of weird reaction solution memory effect, an inorganic homeopathy one might think. Of course, chemists know that there are no memory effects and that homeopathy is based on various misconceptions about the nature of matter. So, if not inorganic homeopathy, then what?
The team was as diligent as the members could be in handling those stock solutions, in mixing them, in carrying out the reactions. However, what if they were to take a step backwards from the reactions themselves to how those stock solutions were prepared in the first place? Therein lies the rational explanation. They have now demonstrated that it is the rate at which the researcher adds the cobalt precursor to the ligand solution to make up that particular stock solution that influences the final outcome for any given batch of nanoparticles. The mixing of precursor and ligand solution, it seems, triggers a previously unrecognized side-reaction even at room temperature that consumes some of the lauric acid in the pre-nano brew.
The lauric acid is the primary stabilizing agent for the nanoparticles, but if some of it is consumed and converted to silyl ester than there will be inadequate stabilization. The rate it is consumed and thus the final concentration in the purportedly stock solutions is then compromised. The team describes this “innocent mixing” as perhaps underlying why many nanoparticle preparations fail to give the results desired despite the best efforts of the chemists involved. The team has now demonstrated the same effect with iron nanoparticle preparation.
This latent parameter, now exposed, might allow control to be taken over many otherwise errant nanocrystallizations. It might even be exploited to fine tune size and morphology in sophisticated experiments in which supposedly stock reagents are made in situ at precisely controlled rates.
- The Big Impact of a Small Detail: Cobalt Nanocrystal Polymorphism as a Result of Precursor Addition Rate during Stock Solution Preparation,
Nikos Liakakos, Benoît Cormary, Xiaojian Li, Pierre Lecante, Marc Respaud, Laurent Maron, Andrea Falqui, Alessandro Genovese, Laure Vendier, Spyros Koïnis, Bruno Chaudret, Katerina Soulantica,
J. Am. Chem. Soc. 2012.
DOI: 10.1021/ja304487b
