Berardo, E; (2015) Modelling the Excited State Properties of TiO2 Nanoparticles. Doctoral thesis , UCL (University College London).

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Abstract

Since the discovery in 1972 of photocatalytic water splitting on TiO2 electrodes, there has been a worldwide research effort focused on the study of this material. Recently, it was shown that the use of nanosized systems could overcome the usual TiO2 limitations, e.g. a too large band gap, and increase its efficiency in photocatalytic applications. This thesis involves the computational modelling of excited state properties of TiO2 nanoparticles in order to provide atomic scale insights into the optical and photocatalytic properties of these materials. In the first part of this thesis, accurate correlated wave function benchmarks (i.e. EOM-CC) are defined for the calculation of excited states of small TiO2 nanoparticles (< 1 nm in size). These results are then employed for the evaluation of the accuracy of different Time Dependent DFT (TDDFT) exchange-correlation (XC) potentials. The main conclusion is that standard TDDFT XC energy functionals (e.g. PBE, B3LYP) tend to underestimate charge transfer excitations, whereas long-range corrected (e.g. CAM-B3LYP) potentials accurately describe optical properties of TiO2 nanoparticles. Following this study, in an effort to close the gap between simple theoretical models and systems of experimental relevance, TD-DFT is used for the investigation of a range of optical and excited state properties for a rutile bulk-like particle with a diameter of approximately 2 nm. These studies reveal that this or smaller rutile nanoparticles are predicted to be thermodynamically unable to drive photocatalytic water splitting because of the strong self-trapping of free electrons and holes generated during the excitation process in these particles. Finally, the excited state lifetimes and the photo-reactivity of a small hydrated TiO2 particle were investigated by employing TD-DFT non-adiabatic excited state molecular dynamics (NAMD). This study corresponds to a first attempt of using TD-DFT touncover the initial steps of the photochemical water-splitting reaction catalysed by TiO2 nanoparticles. The results from this thesis suggest that in the case of the TD-B3LYP trajectories, the first steps of the water splitting reaction mechanism are found to be heterolytic in character.

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