Band gap of TiO2 is high and it absorb the UV light, and used in PV as well in Photocatalysts. Why is the recombination rate high in this?
Rutile and anatase,being the most discussed semiconductors(SC) have large band gaps values [3.06eV(direct).3.10 ( indirect); appxo.413 nm ]and[ 3.23 eV(only indirect); appxo.387 nm] respectively.
Though no single reason can ba assigned to account for the high or low values of band gaps of SC, yet it depends upon cumulative effect of factors like their crystal structures, temperature, particle size( NP/ bulk),electronetativties of the constituent atoms and the hybridization among them.Keeping factors like temperature and particle size constant, the remaing three factors are briefly explained as:
The band gap being the energy difference between the valence band and the conduction band, any factor which decreases valence band enery or increases the conduction band
will increase the band gap.
[I] Along [001] direction in TiO2 crystal structure, ,there are rows of six-fold Ti alternate with five- fold Ti coordinated atoms with one dangling bond perpendicular to the surface.Also, there are two kinds of oxygen atoms.Oxygens with the main surface plane are three fold coordinated as in the bulk while the so called bridging oxygens are two fold coordinated. Because of their coordinative unsaturation, they can be easily removed to result a very large number of point defects which lowers the valence band energy.
[II] Oxygen ,being the second highest EN element ,we would expect a considerable polar character which acts as a stabilizing factor for the valence band of TiO2.
[III] The involvement of d orbitals in the hybridization will again add to the lowering of valence band of TiO2.
Why high( e -h) rate of recombination?
In order to understand this point, we first know the the mechanism as to what happens during the production of (e -- h) in a reaction:
The species produced by the reduction of TiO2 during irradiation in the presence of strong (h) trapping agent like MeOHor TEA(triethanolamine) in Ar atmosphere which turnMeOH/TEA blue/ bluish grey due to the production of Ti^3. Crystalline defects are one of the most prabable sites for e -- h recombination. The quantitative estimation of these defects can be estimated by the density of Ti^3 by femtosecond laser spectroscopy.The following reactions may take place:
TiO2+e---------Ti^3+2O(-2)
2O(-2)+3h -----O2(-1)[super oxide]
Superoxide, being short lived, will at once take up electrons to give O(-2) while Ti^3 will take up the positive hole to form Ti^4 which reform TiO2.The reaction continues as long as light is available along withMeOH/TEA.
O2(-1) +3e----- 2O(-2)
Ti^3+h -----Ti^4
Ti^4+O2(-2)------ TiO2
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