Density functional theory investigations of interactions in TiO2-based photocatalytic interfaces for water treatment

Recycling and re-use of water is essential for achieving United Nations’ sustainable development goal – 6 : “Ensure availability and sustainable management of water and sanitation for all”. Photocatalytic degradation can help provide clean water by destroying pollutants or converting them to less toxic form. To achieve this aim, we need to develop efficient photocatalysts, which in its turn requires thorough understanding of the properties of photocatalysts and their interactions with pollutants. This thesis reports our studies of TiO2 photocatalysts and TiO2-based interfaces, their optical properties and interactions with model pollutants and with surface modifiers, using dispersion-corrected density functional theory (DFT) calculations. Firstly, this thesis investigated interactions of the TiO2 anatase (101) surface with model pollutants: benzoic, nicotinic, salicylic and anthranilic acid. Different adsorption modes were investigated. Monodentate adsorbed configurations were found to be more stable than the bidentate structures for all the molecules studied. Thermodynamic probabilities of different adsorption configurations were analysed using the Boltzmann distribution, which showed dispersionstabilised tilted structures to be by far the most abundant. Dispersion was found to have a significant contribution to the adsorption energies, in addition to hydrogen bonding, in the most stable structures. Different methods of describing dispersion within DFT were compared, and the relative stabilities of TiO2/acid systems were found to be independent of the method of describing dispersion. Secondly, the effect of organic adsorbates as photosensitisers on TiO2 was investigated by modelling the optical properties of the TiO2/carboxylic acids interfaces. Aromatic carboxylic acids with additional functional groups, such as anthranilic and salicylic acids, were found to enhance the optical absorption of TiO2 in the visible region. Thus, adsorption of aromatic acids sensitises TiO2 photocatalysts to the visible region of the solar spectrum and enhances their light harvesting.