I have 2 powder samples of same material (BiFeO3 based) with different particle size, first is around 50 n.m and second is around 200 n.m. My question is does particle size has a role on band gap or the band gap reduction is simply due to effect of dopant addition.
I have to admit that I don't know if the band gap is directly affected by the particle size. However, the spectrum you obtain may be strongly affected by the fact that the crystallites are optical anisotropic and that in the first case (50 nm) the crystallites can be considered as small compared to the wavelength of the band gap (i.e. < 1/10 of about 600 nm) whereas in the second case they are not. This can have a strong influence on the spectrum, see e.g.
Article Optical isotropy in polycrystalline Ba_ {2} TiSi_ {2} O_ {8}...
Band gap generally increase as particle size decreased I think so.
For addition to the above, a size of nanoparticles affects on the exciton energy, the latter determines the width of the band gap.
In the paper titled "Tunable bandgap in BiFeO3 nanoparticles: The role of microstrain and oxygen defects", the authors demonstrate a tunable bandgap from 2.32 eV to 2.09 eV in phase-pure BiFeO3 by controlling the particle size from 65 nm to 5 nm. They conclude that band gap decreases with decreasing particle size and is attributed to the competing effects of microstrain, oxygen defects, and Coulombic interactions.
I agree with all answers that reply , but before you study the effect of particle size on band gap have to be study the pure materiel and then doped materiel or modified which can give you clear explanation form which factor the effect size on band gap coming. After that you can refer the reason with how the affects occurred .
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