Undoped TiO2 showed a 3.2 eV bandgap while increasing % of Ag-doped TiO2 it is showing that the bandgap is reducing gradually. Is that something to do with crystallite size? the crystallite size is also decreasing with increasing doping %.
In another query, 3% Ag-doped TiO2 is showing around 2.8 eV bandgap while 3% Cu-doped TiO2 is showing 3.05 eV bandgap. What could be the possible reason?
For doping crystallite size may vary minutely, but that will not cause such significant change in the band gap. Basically any sort of doping changes the band-structure of the compound to an extent. Therefore, it is very much possible that, for certain systems, the electronic gap can decrease as well.
There is no standard qualitative means to study this. Rather, DFT is a good technique to quantitatively study the band-structure or at least DOS with different doping proportion. The experimental methods for such quantitative studies are expensive and hard to avail.
Furthermore, the fact that Ag as noble metal shows surface plasmon resonance, lowers/alters the band gap, too
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