University of Texas at Austin

News

Researchers Uncover New Insights into Polarons in Titanium Dioxide

By Aira Balasubramanian, Joanne Foote

Published Dec. 4, 2024

Large hole polaron (orange surface) in the rutile phase of TiO2. Credit: Zhenbang Dai and Feliciano Giustino/UT Austin

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.  

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.

— Feliciano Giustino

“In the past, we were only able to study very small polarons,” said Dai. However, within the context of titanium dioxide, existing theories have postulated that the polarons within them are too large to effectively model or simulate.

“When we’re limited to systems that are smaller than the size of a coronavirus, we essentially preclude the possibility of including larger polarons. However, by modeling a system including hundreds of thousands of atoms, we’ve essentially introduced the possibility of finding larger objects," added Dai.

By improving the resolution at which polarons can be visualized at a scale supported by supercomputers, Dai and Giustino discovered two novel types of polarons in TiO₂  that are larger than those found before, confirming past experimentation in a manner that clarifies how polarons tend to move within the material. 

Building on these groundbreaking findings, the door to exploring how these newly discovered large polarons can be harnessed to improve the efficiency of photocatalysis in TiO₂ is opened.

Giustino and Dai hope to understand how to optimize their process in creating still images of polarons within titanium dioxide to create high-resolution movies that capture the dynamic nature of these particles. They also hope to advocate for a greater appreciation in the development of advanced computational techniques within the field of academic research to create powerful tools that can uncover new, previously inaccessible insights into materials chemistry.

As progress in this fields develops, scientists have the potential to optimize how electrons move to the surface in catalytic materials, enabling efficient reactions with the potential to revolutionize the energy space.  

___________________________________

This research was primarily supported by the Computational Materials Sciences Program funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, under award no. DE-SC0020129 (EPW development, calculations, and analysis). Part of this research was supported by the National Science Foundation, Office of Advanced Cyberinfrastructure under Grant No. {2103991} of the Cyberinfrastructure for Sustained Scientific Innovation program, and the National Science Foundation (NSF) Characteristic Science Applications for the Leadership Class Computing Facility program under Grant No. {2139536} (development of exciton polaron module). This research used resources of the National Energy Research Scientific Computing Center and the Argonne Leadership Computing Facility, which are DOE Office of Science User Facilities supported by the Office of Science of the U.S. Department of Energy, under Contracts No. DE-AC02-05CH11231 and DE-AC02-06CH11357, respectively. The authors also acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing access to Frontera and Lonestar6.