Titanium dioxide (TiO2) surfaces and nanostructures are currently considered for many applications such as photocatalysis, biosensors, and nanomedicine. Most of them are in wet ambient or even bulk water so that water molecules adsorption on TiO2 surfaces and nanostructures is a key phenomenon that mediates the interaction with the environment, such as, for instance, light absorption in ambient conditions and biomolecules adsorption in biological moieties. With regards to photocatalysis, of particular relevance are the anatase
phase of TiO2, which predominates at the nanoscale, and the nanostructured brookite crystals. In these cases the photo-induced water splitting occurring at the surface is a key process that must be unveiled in bulk water environment in order to develop new technology strategies and applied science techniques for the future hydrogen economy.
State of the art density functional theory ab-initio atomistic modeling is nowadays the gold standard to study such systems in real environment on a quantum mechanical basis, with good compromise between accuracy and computational cost. Its great reliability makes this approach a sort of standard in-silico experimental technique.
The aim of the present project is to study, mainly using ab-initio modelling and quantum mechanical based molecular dynamics simulations (adiabatic and Car-Parrinello) the behaviour of various TiO2 surfaces and nanostructures in bulk water paying attention to the key phenomena of water induced surface reconstruction and both spontaneous or photo-catalysed water splitting in various thermodynamic conditions. We will especially focus on the anatase (101) and (001) surfaces, and brookite nano-particles and nano-rods.
The physics and physical chemistry of solid surfaces have been traditionally studied in vacuum or low coverage both from the experimental and the theoretical point of view due to the difficulty of access the observable relevant to the interface when the solid surface is in contact with a liquid solution. We know, however, that the behavior of the surface in real systems depends on a variety of phenomena occurring at the interface such as surface reconstruction, adsorption, charge transfer, polarization etc.. Experiments on oxide surfaces in water are even more complex due to the surface reactivity and the polar and directional nature of water molecules that make this solvent an ¿unicum¿ among all the liquid species. However this is the liquid a surface is in contact with in a large variety of applications. This is even more crucial in case the material is TiO2 that is considered in a wide range of applications especially at the nano-scale, such as photocatalysis and water splitting, implants in surgery, solid electrolytes, bio-medical applications and nano-medicine, nano-sensors etc.
The structure of the adsorbed water monolayer at low temperature and in vacuum conditions on TiO2 is experimentally accessible. Thousand of scientific articles have been published on the topic of 'anatase and water' in the least decade and more than two thousand on 'anatase(001)¿, yet some of the most basic details about the structure, stability and hydration activity of the anatase (001) surface are not settled.
In particular the behaviour of TiO2 surfaces and nanostructures in aqueous environment and room temperature where, as detailed previoulsy, most of the applications should operate, is still largely unknown.
With the present project we expect to increase the knowledge on the behavior of differently reconstructed TiO2 Anatase (001) surface in bulk water evidencing that this solid surface behaves differently depending on the thermodynamic state of the system, thus basically of the solvent. It is expected that clarifying this aspect might have an important impact on all those applications that are based on the interaction of titania surfaces and nano-structures with water in real environment, thus first of all those applications relying on the water splitting phenomena such as, for instance, photocatalyzed hydrogen production on titania surfaces.
It should be emphasized that the studying the behavior of titania surface reconstruction in bulk water has not been considered in the literature so far, except for the recent study published by the proponents cited in the section "Descrizione obiettivi progetto, conoscenza dello stato dell'arte nel tema specifico e impianto metodologico".
These results will be of beneficial impact also to understand the interaction of titania surfaces and nanostructures with biological matter, proteins, DNA etc., in biological moieties because such interactions are typically mediated by the first water layers of the solvent that, in turn, depend on the surface reconstruction and vice-versa.