Scattered like freckles across rooftops, canopied over parking garages, and painted over grassland, solar panels increasingly dominate the renewable energy landscape. Despite the potential capability of generating substantial amounts of electricity through solar cells, it becomes a more challenging problem to store this clean energy.
One promising option to store this energy is to convert it into chemical fuels via a process called "photocatalysis." As sunlight seeps into some devices, its energy can also be used to split hydrogen, a fuel source in itself, from oxygen in water – enabling downstream industrial processes that power heavy machinery, produce fertilizers, enable efficient transportation, and create cleaner burning fossil fuels. Such devices, though less ubiquitously known, are called solar fuel cells, which directly store energy carried by light as chemical compounds and are equally vital to generating scalable, long term energy solutions.
Critical to this process is a photocatalyst. This tiny piece of material makes the water splitting reaction efficient, and sets solar cells up as a leader in the renewable energy space.
Titanium dioxide (TiO₂), a naturally occurring, extremely common mineral, is a whitening agent often found in sunscreen, milk products, and toothpaste. It makes things look brighter, lighter, and fresher – the holy grail of retail. However, TiO₂ is more than just a pretty face – it has potential to serve as a readily available photocatalyst within solar fuel cells, and has been lauded as the material that may allow us to achieve artificial photosynthesis. Ubiquitous in the natural environment, it has the potential to drive down the cost of solar fuel cell production, enabling enhanced adoption of the technology.
While it may seem like something of a wonder drug for the energy crisis, TiO₂ is limited by how quickly electrons can come to its surface to efficiently split water molecules. Better understanding how this process works is critical to either optimizing the material for scaled use, or finding new materials with enhanced properties that can be applied to downstream energy applications.
Enter Feliciano Giustino, Director for the Center for Quantum Materials Engineering (CQME) at the Oden Institute for Computational Engineering and Sciences at The University of Texas at Austin, and Zhenbang Dai, a postdoctoral fellow with CQME. In a new study published in Proceedings of the National Academy of Sciences, titled ‘Identification of large polarons and exciton polarons in rutile and anatase polymorphs of titanium dioxide,’ they explore the properties of titanium dioxide in the context of polarons, a composite quasiparticle, which has been theorized to be critical in transporting electrons to the surface of a material, which enables catalytic chemistry (DOI: https://doi.org/10.1073/pnas.2414203121).
“There has been sustained interest in TiO₂ for decades, but the nature of polarons, which are charge carriers within the material, has remained poorly understood,” stated Giustino. “We wanted to understand their properties, and understand how they move in electric or magnetic fields.”
Traditional approaches to understanding large polarons within a material have been limited by the large computational costs that are associated with performing calculations surrounding them. Beyond this, understanding how polarons move within materials larger than 200-300 atoms (0.02% of the width of a human hair) was previously impossible computationally.