Crystal orientation points the way to improved hydrogen production

During my degree I always loved learning about crystallography, as it's a field where simple geometry on a microscopic scale can have big, visible effects. A classic example is the salt crystal pictured below – in sodium chloride the ions adopt a cubic arrangement, and this stays rigidly fixed even if you have the unimaginably huge number of ions it takes to be visible. As a result, single crystals of salt are cube-shaped no matter how big they are. This can result in widely varying properties in different directions, all depending on the geometry of those microscopic cubes of ions.

A team of researchers, including some from the University of Cambridge, have reported improved performance of a photocathode based on exactly that kind of variation (the original paper is here). Photocathodes convert incident light into an electric current, and they're of interest because they may one day allow the large-scale conversion of sunlight into fuels like hydrogen. That effectively allows the storage of solar energy for later use when the sun isn't out, and holds promise as a more environmentally friendly alternative to batteries.

The researchers produced a photocathode consisting of a single crystal of copper oxide (Cu₂O). Copper oxide has a cubic crystal structure not unlike salt, and the researchers tried three different orientations of the microscopic cubes to compare the resulting current. They found that along the [111] direction (that is, the main diagonal of the cubes) there was a 70% enhancement of the current compared to photocathodes made using alternative technologies; the performance was significantly worse along the cube edges or faces. The researchers suggest that Cu₂O nanowires along the [111] direction should be a target of further research.

It feels like magic to have such a massive change in the large-scale properties of a material based on a simple rotation, but it shows how powerful atomic (or ionic) structure is as a way of explaining the world around us!