Hi sridhar,

Basically, anatase and rutile structure differ from the arrangement of oxygen ion. Rutile has slightly distorted HCP (D4h-P42/mmm) sublattice oxygen ions and anatase has cubic close packed (D4h-I41/amd) sublattice oxygen ions. Both structures have edge sharing TiO6 octahedra, with two and four edges shared in rutile and anatase respectively.

Furthermore, the band-gabs of both anatase and rutile are indirect, although there is also a direct forbidden transition near 3eV in rutile. here valance band corresponds to the O 2p orbitals and the conduction band arises from Ti 3d orbitals. Due to a crystal field of six oxygen atoms, the 3d states split into two components namely t2g and eg, as mentioned above, the field is not exactly octahedral. this small oxygen deficiency considered as a impurity and provides n-type doping to the semiconducting material.

I hope this answer may useful to you.

thanks and regards

rajesh.r

The band structure for rutile (on the left) and anatase (on the right) TiO2 without doping. The maximum of the valence band is taken as the zero of energy.

Sanjines, R., Tang, H., Berger, H., Gozzo, F., Margaritondo, G., Levy, F., Electronic structure of anatase TiO2 oxide, Journal of Applied Physics 75 (1994) 2945-2951

Dear Sir,

Thank you very much for reply. I am not able to understand what u said.

Hi sridhar,

Basically, anatase and rutile structure differ from the arrangement of oxygen ion. Rutile has slightly distorted HCP (D4h-P42/mmm) sublattice oxygen ions and anatase has cubic close packed (D4h-I41/amd) sublattice oxygen ions. Both structures have edge sharing TiO6 octahedra, with two and four edges shared in rutile and anatase respectively.

Furthermore, the band-gabs of both anatase and rutile are indirect, although there is also a direct forbidden transition near 3eV in rutile. here valance band corresponds to the O 2p orbitals and the conduction band arises from Ti 3d orbitals. Due to a crystal field of six oxygen atoms, the 3d states split into two components namely t2g and eg, as mentioned above, the field is not exactly octahedral. this small oxygen deficiency considered as a impurity and provides n-type doping to the semiconducting material.

I hope this answer may useful to you.

thanks and regards

rajesh.r

I would accept the answers from Rajesh. Indirect band materials tend to have high anisotropy, while direct band gap materials have more isotropy. Therefore, Rajesh's answer is logic.

According to references, both anatase and rutile have tetragonal crystals (anisotropic) and should have indirect. But perhaps to different extremes, which justifies the difference.

Best wishes

Dear Rajesh Rajagopal,

Can you pls help me find a reference for your comment :

Basically, anatase and rutile structure differ from the arrangement of oxygen ion. Rutile has slightly distorted HCP (D4h-P42/mmm) sublattice oxygen ions and anatase has cubic close packed (D4h-I41/amd) sublattice oxygen ions. Both structures have edge sharing TiO6 octahedra, with two and four edges shared in rutile and anatase respectively.

Furthermore, the band-gabs of both anatase and rutile are indirect, although there is also a direct forbidden transition near 3eV in rutile. here valance band corresponds to the O 2p orbitals and the conduction band arises from Ti 3d orbitals. Due to a crystal field of six oxygen atoms, the 3d states split into two components namely t2g and eg, as mentioned above, the field is not exactly octahedral. this small oxygen deficiency considered as a impurity and provides n-type doping to the semiconducting material.

Why does anatase TiO2 show indirect band gap energy and rutile TiO2 show direct band gap energy? 

Dear Rajesh Rajagopal,

Can you please help me find a reference for your comment .