Size Tunable Synthesis and Surface Chemistry of Metastable TiO2-Bronze Nanocrystals

The metastable titania phase, bronze, has great potential as photo-catalyst or as anode material in Li-ion batteries. Here, we report the first synthesis of colloidally stable, size-tunable TiO₂-bronze (TiO₂-B) nanocrystals, via a hydrothermal process. By employing Definitive Screening Design, the experimental parameters affecting the size and agglomeration of the nanocrystals are identified. The size is mostly determined by the reaction temperature, resulting in 3 – 8 nm NCs in the range of 130 °C – 180 °C. To avoid irreversible aggregation, short reaction times are desired and in this respect microwave heating proved essential due to its fast heating and cooling rates. The resulting nanocrystals are de-aggregated and stabilized in polar solvents using either positive or negative surface charges. In nonpolar solvents, steric stabilization is provided by long chain amines and carboxylic acids. Furthermore, we study this peculiar post-synthetic surface modification through solution ¹H NMR and elemental analysis. Surprisingly, we find ion-pairs of alkylammonium carboxylates bound to the surface, contrasting with earlier reports on carboxylic acid stabilized metal oxide nanocrystals. To rationalize this, a general framework of acid/base chemistry with metal oxide nanocrystals is constructed and discussed.