Titanium dioxide’s elemental charge states reevaluated

Most chemists would describe titanium dioxide as an ionic compound composed of Ti⁴⁺ and two O^2–. Not so, suggests a theoretical analysis: TiO₂ is better described with the charge states Ti³⁺ and O^1.5– and has some covalent character to its bonds (J. Phys. Chem. Lett. 2017, DOI: 10.1021/acs.jpclett.7b00313).

TiO₂ is commonly used as a photocatalyst and in solar cells and battery electrodes. In these applications, understanding of TiO₂’s chemistry is largely based on the assumption that the charge states are Ti⁴⁺ and O^2–, and consequently that the titanium cannot be further oxidized nor the oxygen further reduced.

But no direct experimental evidence shows that those are the oxidation states. In fact, some computational quantum analysis has suggested that TiO₂’s electronic structure might not be as simple as assumed.

Inspired by that analysis, Daniel Koch and Sergei Manzhos of the National University of Singapore systematically explored the electronic structure of TiO₂ using multiple theoretical methods to study individual TiO₂ molecules as well as the mineral forms rutile and anatase.

In their new description of TiO₂, Ti³⁺’s remaining valence electron is localized within a radius that is shorter than half of the Ti–O bond length and has contributions from s and d states. On the basis of their calculations that the titanium in TiO₂ is Ti³⁺, they concluded that the oxygens must be O^1.5–.

The results suggest that the chemistry of TiO₂ may need to be reconsidered, particularly in redox systems that could involve additional oxidation of titanium and reduction of oxygen, the researchers say.