Professor Tiffany Walsh and her team from Deakin University is working with international collaborators to understand what makes certain kinds of nanoparticles good at catalysing chemical reactions. In industry, catalysts are pivotal to increasing the efficiency of production of commodity chemicals. Ideally, the catalysts themselves should be easy and safe to make, so a focus on environmentally-benign production approaches for catalysts is preferable.
In this work, published in the Journal of the American Chemical Society, the combined experimental and modelling teams devised and tested a "green", water-based approach to making catalytic gold nanoparticles. To do this, the team used biomolecules, namely peptides, as growth agents of the gold nanoparticles in water. The peptides wrap around the outside of the gold nanoparticles to stop their growth, which not only prevents the nanoparticles from aggregating in water and losing their catalytic potency, but, as the team found out, also helps to expose or cover-up different sites on the nanoparticle surface that are relevant to catalysis.
Different peptides were found to produce gold nanoparticles with very different catalytic properties and until now scientists were not sure of the reasons for this. By predicting the structure of the peptide/nanoparticle interface, Walsh's team were able to identify the peptide characteristics that led to enhanced catalytic performance.
Professor Walsh says "One peptide sequence will do a pretty good job of lowering the energy barrier for the reaction, and a different one won't do anything to lower the energy barrier. Depending how the peptides arrange themselves on the particle surface, we thought this had something to do with their catalytic behaviour."
To investigate the effect of different peptide structures on the nanoparticles, the team at Deakin simulated the structure of the nanoparticle and associated peptides in liquid water, at the atomistic level. Advanced simulation approaches are essential for this work and meant that each simulation was modelled using 16-32 copies of each system at once. Because each system comprises between 50,000 and 100,000 atoms, this molecular modelling approach becomes highly computationally intensive.
"That's where the need for lots of nodes on Raijin comes into play. We couldn't do these sophisticated simulations on such huge molecular systems without access to Raijin, otherwise it would be impossible. So for us to be able to make these connections between the catalytic behaviour and the structure of the peptide that we used, it was essential that we used the NCI," says Professor Walsh.
Next, the team will focus on replacing the gold in their studies with other materials such as iron oxide or copper sulfide. "Gold was the proof of concept, it has some useful properties from a physical and from a modelling point of view, but gold is expensive! Now that we've established a reliable way to tackle the problem using gold, we want to move to study transition-metal oxides. However, while they're cheap materials for industry, they're very challenging from a simulation point of view, so that's a major challenge for us."
This research is a first step in the "green" production of new kinds of catalytic nanoparticles. Learning about the production, structure and behaviour of these nanoparticles is the start of a process to understand how to make cheaper and cleaner catalysts in the future.