New Paradigms for Active Site Engineering in Titanium Dioxide Photocatalysts

The common trends in the synthesis of new TiO2 photocatalysts are based on the premise that increasing the area of high index facets results in higher photocatalytic activity. Nevertheless, there exist a number of catalysts, that, due to their peculiar morphology which does not mirror the crystal symmetry, possess sites or properties which fall outside of the realm of this general paradigm. A new universe of possibilities becomes available by considering structures which have been deposited under conditions very far from equilibrium (i.e. plasmas), or which have received a post-synthesis process that alters the energetics or geometries of the sites. A new framework for understanding these materials and tools to categorize the types of sites they present is not yet in place. In this Thesis we consider three types of systems and with each one introduce a new concept in the synthesis of materials that could open research directions that transcend and complete the limitations of the single-crystal framework. We introduce the concept of parent morphologies for TiO2 nanorods that are produced by collapsing a nanotube structure. We analyze in detail the properties of acetaldehyde adsorption at environmentally relevant pressures setting the stage for practical applications, and use HRTEM to reveal the orientation of the crystallite within the rod. We show that a unique structure results from this procedure yielding (100) rods. We discuss the use of parent structures with different crystal orientations as a viable route to produce highly energetic metastable configurations. Next, we discuss the interaction between crystal domains in TiO2 thin films. We introduce the use of X-ray goniometry to analyze the degree of crystallite orientation in films deposited with magnetron sputtering, constituting the first analysis of crystallite orientation in TiO2 films for photocatalysis. We find the degree of orientation to be an excellent predictor of reactivity. We investigate organic-semiconductor active sites with the third system, TiO2 surfaces functionalized with salicylate derivatives. We analyze the adsorption of CO2 and track the electron dynamics using EPR. We demonstrate that the adsorption of CO2 creates a surface state and postulate its importance in the charge transfer to CO2.