In-silico development of the potential of doped metal-oxide nanotubes as novel photo-catalysts for energy applications

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As the situation regarding the costs of energy production becomes more challenging, due to both resource limitation and the impact of an oil-fuelled economy on the climate, the potential relevance of new materials and technologies capable of converting sunlight into industrially viable forms such as electrical power or chemical fuels is paramount. Crucial to this energy conversion is the presence of photo-catalysts (PCs).PCs are compounds which comply with the following: first, they absorb light, whose energy (related to the frequency of its photons) is used to excite one electron (e) of a PC to a higher-energy state (e*) leaving one hole (h) in the pristine electronic state. Once a photon has been absorbed, and one e*-h pair created, several competing processes may take place: 1) the e*-h pair may induce decomposition of the compound; 2) the e*-h pair can release some of its energy into the nuclear vibrations before e* decays, by emission of one photon, to his pristine state thus recombining with h, or 3) e* and h can separately react with other molecules or enter an electric circuit. Thus, viable PCs are characterized by photo-stability, and an e*-h recombination rate slower than the time it takes to the e*-h pair to separately react with other species. Typically, the farther e* is created from h, the slower the e*-h recombination. The chemical elegance of PCs emerge in that, due to the absorbed photon energy, the e*-h pair is highly reactive and may separately reduce-oxidize reactants, thus triggering a redox chemical reaction which can be used to convert light into chemical energy i.e. fuels. If, as it happens for natural photosynthesis, the photon energy comes from the Sun, then PCs can make Sun energy available on Earth as fuels, thus contributing to energy production. Notably, the energy stored in the e*-h pair can in principle be used also for other energetically expensive applications, such as, for instance, the production of ammonia and its derivative, or the decomposition of industrial polluting waste into substances less hazardous for the environment.The research focuses on open-ended metal-oxide nanotubes (MONTs), which are macromolecular tubes made of Al (Ge), Si, O and H. The project builds on recent findings about the possibility to create e*-h pair on different sides of MONTs walls which should provide rather long e*-h recombination times as required for PCs. Preliminary simulations of these systems suggest that it should also be possible to insert other atoms (dopants) into MONTs walls. It is anticipated that the chemical nature of the dopant will influence the photo-catalytic performances of doped MONTs (dMONTs). The proposed research aims at identifying key aspects of the relationship between the structure of dMONTs and their photo-activity. This insight will then be used to optimise dMONTs in terms of side-dependent photo-catalytic performances with special focus on the enhancement of e*-h chemistry over e*-h recombination. The theoretical research will rely on solving the quantum mechanical equations of motion for the electrons and the nuclei of dMONTs in order to characterize their geometrical and electronic properties. This will allow to identify systems with maximum e*-h separation and the largest affinity to e*-h transfer to other species. Theoretical and experimental insight about the light energy required to generate e*-h pairs for specific dMONTS will also be obtained. Finally, simulation of the time evolution of e*-h in dMONTs will provide insight into the factors governing the competition between e*-h recombination, and e*-h transfer which will make it possible to selectively frustrate the former while enhancing the latter, thence boosting the photo-catalytic performance of dMONTs. This knowledge will open the way to possible dMONTs-based applications in many different areas important for our economy such as fuel production, industrially targeted PCs, and decomposition of polluting waste.