Surface reactions can be controlled by stoichiometry | Search

New research shows that mass properties have an unexpected effect on surface interaction. With wide-ranging applications such as catalysis, energy storage and structural engineering, this new mode of reaction control could have significant implications across applied sciences.

Although ubiquitous in everyday life, surfaces present an interesting chemical problem. We can define mass as atoms bound to their neighbors just like any other atom far from the surface. But surface atoms, they have dangling bonds which means they have very different properties, he explains Isabella Zlovarska, a materials scientist from the University of Wisconsin, USA. Surface interaction has therefore been traditionally understood as a consequence of how atoms interact upon inter-substrate interaction, with control of the interaction limited to structural and electronic modifications of the top layer atoms.

However, there are new simulations by Szlufarska and her colleague in Wisconsin Jianqi Shi revealed that some volumetric properties can also influence surface interactions. Using ab initio molecular dynamics, the duo modeled the hydrogen emission reaction on the surface of amorphous silicon carbide and, by changing the stoichiometry of the system, were able to fine-tune the elastic properties of the bulk material. During the reaction, the atoms below the surface must move to accommodate the surface intermediates, which means that more flexible bulk systems are better able to facilitate these reactions. This flexibility is directly dependent on the strength of the bond inside the structure: the carbon-rich system contains many strong carbon-carbon bonds, which makes the resulting bulk material rigid, while the silicon-rich system is more flexible due to the diffusion of weaker silicon – silicon bonds.

Then Szlufarska determined the effect of these huge modifications by measuring the interaction energy of each different stoichiometric system. “Jianqi came up with a really creative approach to these calculations that enabled us to separate the electronic energy contributions from the mechanical contributions arising from the stiffness,” she explains. By isolating these mechanical effects, Szlufarska and Xi demonstrated that the more solid carbon-rich system has a higher reaction energy barrier, due to the greater amount of energy required to deform the bulk structure as surface intermediates form.

It’s a great job,” comments Stephen Jenkins, a theoretical surface chemist from the University of Cambridge, UK. “But these mechanical contributions are probably still within the upper few atomic layers, so it’s still a surface effect on a large scale, rather than the whole bulk matter.” Szlufarska agrees that this is likely a local effect. “It’s usually within a few nanometers of the surface,” she explains. “What’s important is that this area has no surface characteristics.” Regardless of the scale, this could remain a valuable tool for surface chemists because mass properties are much easier and cheaper to control than surface properties.

“These computational methods have become so powerful that they can really tell the empiricist what is interesting to investigate,” he says. Andrzej KotarbaExperimental surfactant chemist from Jagiellonian University in Poland. “It’s an inspiring paper but the point is that nowadays you can verify simulations by experiment, and that’s actually what’s missing here.” The duo are keen to work alongside the experimentalists and hope that other researchers will take this idea and test it on their own systems. “We think this is a broader phenomenon,” Zlovarska says. Our plan is to find out how this can be applied to other materials or classes of materials, with a view to developing applications in catalysis. We hope it will open the doors to controlling chemical reactions in a new way.