These days, I do dye degradation experiment. I use mortar and pestle to grind bulk ZnO 10 min, the powders turn yellow. Then use that powder to do dye degradation, why it degrades slower than origina bulk one?
Yellowing of prior white crystalline zinc seemingly indicates the presence of sub-stoichiometric crystalline zinc oxide, what may possibly be due to a thermochromic (reversible) effect.
Thank you so much @ Carlos Araújo Queiroz, I checked that thermochromism of ZnO results from a minor loss of oxygen upon heating to temperatures around 800 °C. But I just grind manually in mortar for 10 mins, it changed yellow obviously by eye, can it explained by thermochromism?
According to many papers, like[1], loss of oxygen(yellow) of ZnO can enhance Photocatalytic Activity of ZnO, but in my case, it degraded slower than original white powder.
I am really confused by this, if anybody know anything, please help me. Thank you.
[1]Wang, Junpeng, et al. "Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO." ACS applied materials & interfaces 4.8 (2012): 4024-4030.
Dear Dr. Cai
Decreasing the size of a semiconducting particles like ZnO may have two opposite effects. First, decrease in size of the particle results in increased surface area and therefore, the activity should increase. But, on the other hand, lowering the particle sine of a material resulted into increase in its band gap. Band gap of ZnO is in the border area of of UV-visible range (388-414 nm depending on the type of crystal structures)and this will result in decrease in its activity for degrading a dye. Which factors dominate out of these two, will decide the change in activity of ZnO?. Only your observations can tell you.
it is known that as the particle size decreases in namomaterial region, the band gap increases. Grinding of ZnO may reduce the particle size and as a result of increased band gap, it will absorb in UV range rather than visible range. This may be probate reason for decreased activity in visible range, but one has to check the particle size of ZnO, before any final conclusion is made.
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