I had synthesized ZnO nanoparticles by combustion method. The TEM gives the particle size in the range of 40- 50 nm. The relative reflectance of the particle based on optical spectrum gave a band gap of 2.5eV. Smaller the band gap larger should be the particle. Can anyone explain why am I getting a smaller band gap even though the particle size is small?
In general, the band gap can be different of material pure or doped using any ions such as Fe,Co.....etc. For if you have ZnO pure the band gap decreasing when the particle size increasing and which you can investigated by increases the temperature of heat treatment. this case when increases the size ,the absobance will decreasing (reduce the surface area) with enhancement of the reflectance, that is why the band gap decreases. however, when using the doping (not pure materiel)also the band gap decreasing by shifting the absorbance to other region(like from UV to visible) which the doping by the N-type or p-type effects on the fermi level energy. This effect shows the reducing the band gap especially in powder form. for more information ypu can search deeply about that.
best regards
well it's a little bit late but ... experimentally, the band gap decrease (red shift) with the decrease of particle size to a certain minimum value compared to the band gap energy of the bulk material. After the critical size, the band gap energy starts to increase as the "theory" express.
A plausible explanation for this effect in the band gap vs the particle size, is that the bulk defects induce a delocalization of the conduction band edge and create shallow/deep traps in electronic energy causing a red shift of the absorption spectra.
Check this article : Size dependency of nanocrystalline TiO2 on its optical property and photocatalytic reactivity exemplified by 2-chlorophenol. Applied Catalysis B: Environmental 68 (2006) 1–11
The authors study the effect of TiO2 nanoparticle size vs the band gap energy. I've also worked with ZnS and TiO2 nanoparticles and titanate nanotubes and the same effect was observed.
what could be the reason ,if the particle size as well as band gap decreases when ZnO is doped with the alkali metal ions??
i couldn't understand the question, but ... there are different factors that can influence the band gap shift. Ion size, %doping, type of ion, how the ion introduction change the structure, if it's too big, the structure could collapse and that could be reflected as slightly change of the band gap. There's the theory but to actually understand how dopping will affect the band gap you will have to test it, with different alkali metal ions as well as different % of doping.
Why the band gap is decreasing while adding the higher ion size..Can u please explain me what is the reason behind that?
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