Rutile TiO2 : photocatalysis and polarons
Titanium dioxide is one of the best studied photocatalytic materials with numerous proposed applications, ranging from water photooxidation for hydrogen production to cleanup of environmental pollutants and self-cleaning coatings. Photocatalytic methoxy splitting on rutile TiO2 (110) surfaces that leads to formation of formaldehyde has previously been observed in STM and TPD experiments. In this work we computationally study non-adiabatic excited-state dynamics of methoxy splitting on TiO2 surfaces. Such computations are expensive, and in order to perform them we developed an efficient methodology based on time-dependent density functional theory (TDDFT). We use this methodology to study the excited-state trajectories that lead to conversion of metoxy to formaldehyde. We examine in detail electron and hole trajectories that lead to this reaction. We observe the hole migrating to adsorbed methoxy molecule and driving the reaction, while the remaining electron polarizes the crystal lattice and settles in a polaronic state. Polarons are distinctive feature of the rutile titania. We study them in bulk rutile TiO2 both experimentally, using Raman spectroscopy of photo-excited samples, and computationally, with the TDDFT mixed quantum-classical method. From simulation we find that small polaron formation in rutile titania is a strongly non-adiabatic process with the characteristic time scale of about 55 fs. In both experiment and theory we observe an unexpected stiffening of the A1g phonon mode under UV illumination. We computationally analyze the polaron structure and explain the observed effect. The resulting form of the potential in respect to oxygen atoms and t2g-orbitals of central Ti atom offers a possible explanation for an anomalous temperature-dependence of the Hall mobility in rutile titania, which, in contrast to drift mobility, is decreasing with the increasing temperature.
Grigory Kolesov received his first Ph.D. in 2005 in computational biology at the Technical University of Munich. He recieved his second Ph.D. in physics in 2013 at the University of Wyoming where he modeled quantum dot sensitized solar cells using non-equilibrium Green's functions. He is currently a research associate at Harvard University. He studies non-adiabatic dynamics on surfaces and interfaces in photo-catalytic and other contexts. He is also developing methodology and code for simulating non-adiabatic excited-state dynamics and low-temperature phenomena in large systems using time-dependent density functional theory.
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