Most chemists would describe titanium dioxide as an ionic compound composed of Ti4+ and two O2–. Not so, suggests a theoretical analysis: TiO2 is better described with the charge states Ti3+ and O1.5– 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 O2–, 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 O1.5–.
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.