Anatase TiO2 is a photocatalyst used in water splitting and photo-degradation reactions. To improve the effectiveness of anatase, research has focused on controlling the crystal habit to deliberately expose particular facets that may be more reactive for particular processes. However, the crystal growth mechanism(s) that leads to a well-formed morphology is largely unknown, which makes the preparation of materials that are optimal for heterogeneous catalysis uncertain. At a fundamental level, the species in solution that condense to form the solid are critical to crystal growth. In particular, we are interested in the growth mechanism of anatase TiO2 films from titanium (IV) fluoride solutions that have been shown to be effective for water splitting and photo-oxdiation chemistry. To discover what species are present in aqueous solutions containing TiF4, variable temperature 19F and 47,49Ti NMR was used to study the sol-gel composition before and after hydrothermal synthesis. It was found that at least four Ti-F transition metal complexes exist in solution before synthesis including TiF62-, TiF5-, TiFOH, TiF4, and TiF3+. Interestingly, the TiF4 disproportionates into multiple species upon dissolution and all species except TiF62- exchange ligands (F- and/or solvent D2O) at rates comparable to the time scale of the NMR measurement. HF is formed during TiO2 synthesis and was observed in the product solution but not before synthesis. At low temperatures 271 K, the linewidths are narrowest allowing identification of individual transition metal complexes and their relative abundance ratios. This paper focuses on analysis of the NMR spectra to determine the species in solution that lead to the formation of highly oriented and crystalline TiO2 films.

19F and 47,49Ti NMR study of TiO2 synthesis solutions

248th National Meeting (2014)

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