Understanding the Structural Dynamics of Polarons in Transition Metal Oxide Semiconductors by Vibrational Spectroscopy and Charge-Carrier Mobilities

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The generation of solar fuels by photocatalysis or photoelectrochemistry is certainly one of the most important and promising technological processes nowadays. Especially photocatalytic water splitting, where water is separated into gaseous oxygen and hydrogen, attracted a wide interest recently because it is envisaged as a key technology for energy storage. Remarkably, there is yet no photocatalyst available which can split water economically, despite the vast efforts in research over the last decades. One class of materials which can be used to drive photocatalytic water splitting are transition metal oxides. These oxides are semiconductors, where an electron is excited from the valence to the conduction band when a photon is absorbed. Thereby charge separation occurs and the resulting free electron and the electron hole can be used individually as reducing or oxidizing agent, respectively. These charge carriers are often trapped in transition metal oxides since they form polarons, where the atoms in the surrounding of the charge displace and thus create a potential minimum. When it comes to characterizing polaronic states on an atomistic level, theoretical methods are inevitable. However, such electronic structure calculations, which are usually based on hybrid DFT, can be ambiguous such that the exact geometry of a polaron significantly (and even qualitatively) depends on the chosen functional. Dynamically relevant properties, such as mobilities or electron transfer rate constants, obviously depend on the geometry of the polaron. A detailed knowledge of the exact polaronic structure is therefore crucial for following research projects. In this project, I propose an advanced theoretical simulation study of polaronic states in BiVO4 which is a leading candidate for efficient photocatalytic water splitting. Ab initio molecular dynamics simulations on the hybrid DFT level of theory are to be conducted of the hole polaron in BiVO4. From these simulations, vibrational spectra (IR and Raman) and charge-carrier mobilities are to be calculated. All simulations, including the calculation of the vibrational spectra, will be accelerated by machine learning which is absolutely necessary to obtain statistically converged data. Collaborations with renowned experimentalists are arranged, who will measure the corresponding vibrational spectra and mobilities. The aim of this project therefore is, to provide a benchmark for the electronic structure theory; in this case to definitively identify the correct electron hole polaron geometry in BiVO4, but the applied methods are transferable to any other metal oxide material as well. From the joint theoretical/experimental data, I will moreover gain a detailed understanding of the structural dynamics of the electron hole polaron in BiVO4. This could be of significant value to the community seeking to optimise solar-to-fuel conversion technologies as part of a low-carbon energy system.