Weather the TiO2 belongs to direct or indirect band-gap semiconductor?.
One of the reviewer mentioned that TiO2 is a Indirect band-gap semiconductor.
Please let me know.
There are several polymorphs of TiO2, in some the band gap is direct. in some indirect, and in some both. For details, consult [1].
[1] T Zhu, and S-P Gao, J. Phys. Chem. C 118, 11385 (2014).
Article The Stability, Electronic Structure, and Optical Property of...
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Dear Udayabhanu C Gowda ,
Behnam Farid is correct but allow me to elaborate using two figures.
The figure entitled "TiO2-band-structure.jpg" shows band structure diagrams for TiO2 in the rutile (left panel), anatase (center panel), and brookite (right panel) crystal structures (Article The electronic structure and optical response of rutile, ana...
). These diagrams plot the energy of the bands vs. the wave vector in reciprocal space (e.g. Γ). A direct band gap is when the highest energy point in the occupied or valence bands (blue) and the lowest energy point in the unoccupied or conduction bands (red) occur at the same wave vector. While it is hard to see unless you really zoom in on the Fermi energy, i.e. at Energy = 0 eV, rutile and brookite have direct band gaps of 1.88 eV and 1.86 eV at the Γ point and anatase has an indirect band gap of 1.94 eV from the M point in the valence band to the Γ point in the conduction band.Whether TiO2 adopts the rutile, anatase, or brookite crystal structure depends on the temperature and pressure of the environment. The figure entitled "TiO2-phase-diagram.jpg" shows the pressure-temperature phase diagram for TiO2 (Article Doping of TiO2 Polymorphs for Altered Optical and Photocatal...
). At ambient conditions, TiO2 prefers the anatase crystal structure and therefore will have an indirect band gap. If TiO2 is synthesized under higher temperature and/or pressure conditions, then TiO2 prefers rutile or brookite and therefore will have a direct band gap.I hope this was helpful. Please let me know if you have any other questions.
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