Recently, Black titania are reported in literature , We know Ti3+ ion is blue in colour and atlast only Ti3+ ion will generate in TiO2 lattice so how this originate Black colour. However concentration is increased it should not change Blue to Black.
Chemical reduction method using NaBH4 and Hydrogenated TiO2 both shows black colour at high defects concentration. I am curious to know as Ti3+ ion gives only blue colour so how Black colour originates ?
Article High-thermoelectric performance of TiO 2-x fabricated under ...
The absorbance from the Ti3+ sites is extremely broad. It absorbs more strongly in the red part of the spectrum, but still stretches all the way to the blue and UV end. See fig. 5 in this paper :
Article Visible-light-driven oxygen vacancies and Ti 3+ co-doped TiO...
Thus, at low concentration (or short optical path), red is absorbed, some blue makes it through, and it appears grey-blue to the human eye. But at high enough concentrations (over long enough optical paths), even blue light is fully absorbed, and it appears black.
-edit- forgot to mention, when the material is very strongly reduced, it can actually alter the crystal structure, so you get different absorptions. Again, figure 5 of the paper mentioned above shows this.
Probably different oxidation states of Ti occur + different oxygen vacancy density which can give different colors due to combination of light emiision-absorption efects.
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