Crystal properties without crystallinity? Influence of surface hydroxylation on the structure and properties of small TiO2 nanoparticles

Titania (TiO2) nanoparticles (NPs) are widely employed in applications that take advantage of their photochemical properties (e.g. pollutant degradation, photocatalysis). Here, we study the interrelation between crystallinity, surface hydroxylation and electronic structure in titania NPs with 1.4-2.3 nm diameters using all electron density functional theory-based calculations. We show how the distribution of local coordination environments of the atoms in thermally annealed quasi-spherical non-crystalline NPs converge to those in correspondingly sized faceted crystalline anatase NPs upon increasing hydroxylation. When highly hydroxylated, annealed NPs also possess electronic energy gaps with very similar energies and band edge orbital characters to those of the crystalline anatase NPs. We refer to the crystallite-mimicking non-crystalline annealed NPs as crystalikes. Small stable crystalike NPs could allow for photochemical applications of titania in the size range where crystalline anatase NPs tend to become thermodynamically unfavoured (<3-5 nm). Our work implies the anatase crystal structure may not be as essential as previously assumed for TiO2 NP applications and generally suggests that crystalikes could be possible in other nanomaterials.

Automated summary

In this study, the interrelation between crystallinity, surface hydroxylation, and electronic structure in titania nanoparticles (NPs) was investigated using density functional theory-based calculations. The results showed that the local coordination environments of the atoms in annealed non-crystalline NPs became similar to those in faceted crystalline anatase NPs with increasing hydroxylation. Highly hydroxylated annealed NPs also exhibited similar electronic energy gaps and band edge orbital characters to crystalline anatase NPs. These findings suggest that the anatase crystal structure may not be essential for TiO2 NP applications, and crystal-like NPs could exist in other nanomaterials.