DFT-Based Study of the First PCET of the OER on Large (001)-Exposing Anatase TiO2 Nanoparticles in a Water-Splitting Environment

This dissertation computationally studies the oxygen evolution reaction (OER) that occurs at the surface of titanium dioxide (TiO2) nanoparticles in water, specifically the first proton-couple electron transfer (PCET) of four sequential PCETs that the overall OER comprises. To do so, we first modelled TiO¬2 nanoparticles and developed a realistic passivation scheme for the nanoparticle’s surface in an aqueous environment. Additionally, we developed a fragment-based initial guess (FIG) methodology in order to make studying systems approaching real-life nanoparticle sizes computationally tenable. The FIG methodology allowed us to employ hybrid-DFT (B3LYP) calculations to study systems of around 5000 basis functions, or more than 500 atoms. Then, we simulated the PCET by selecting particular points along the PCET reactant pathway and optimizing the system at each of these dozen points. In order to make these optimizations occur in a tractable amount of time, they were run on the STAMPEDE supercomputer operated by the Texas Advanced Computing Center (TACC), implemented with an OpenMP/MPI hybrid parallelization. These optimizations provided us with a picture of the electronic structure changes that occur over the course of the PCET and allowed us to calculate theoretical overpotentials to the PCET, both of which inform our conclusion of the nature of the inefficiency of the solar water-splitting reaction. In short, our studies show the major source of inefficiency to be the thermodynamic instability of the intermediate oxygen species (a hydroxyl adsorbate) on the surface of the nanoparticle. Additionally, while the hole created by cationization does lower the overpotential of the PCET, we do not find that this is because the two happen concertedly, which would confer stability to the system and lower the total energy, but rather for some other reason. Additionally, we do not find that the hole gets filled by the transferring electron of the PCET, raising questions as to how the OER can continue to occur without the structure of the nanoparticle degrading.