This is a matter of particle size and light scattering. The scattered colour of the ground-up powder is a combination of internal light absorption (causing colour) and surface reflectance of the incident light. As the powder becomes finer surface reflectance reaches a higher proportion thus hiding the true yellow colour.
Hi Ethar Alawi, I wonder why your original zinc oxide (before grinding) is yellow at all. Pure ZnO is colorless (white), regardless of the particle size. So you should perhaps check or try to find out if the yellow color is due to some impurities. It is known that industrially manufactured ZnO sometimes contains lead impurities. Only upon heating, pure ZnO can change its color to lemon-yellow.
Ethar Alawi the reason could be the impurities from the mixed acids etc. try the washing by solvents and water repeatedly. you will get white ZnO free from impurities.
Zinc oxide is a white powder when it is in its purest form. When it is in its natural state, it is yellow due to the presence of impurities. When it is ground, the impurities are removed and the zinc oxide becomes white. This is because the grinding process breaks down the impurities and exposes the pure zinc oxide, which is white in color.
ZnO films/powders appear slightly yellowish when the ZnO compound is oxygen deficient. You heat treat, they will become white powders, and similar films should become transparent. I think with constant grinding you are developing some heat, and hence the yellowish powders are becoming white.
When ZnO is in its bulk form, the particles are relatively large and have a hexagonal crystal structure. These large particles have a bandgap of about 3.37 eV and exhibit a yellow color due to the presence of defects, such as oxygen vacancies and zinc interstitials. The defects in the bulk ZnO are responsible for the absorption of visible light, leading to the yellow color.
When ZnO is ground, the large particles are broken down into smaller particles with sizes in the range of nanometers. These smaller particles have a higher surface-to-volume ratio, which leads to an increase in the number of defects. The defects in the smaller particles can also affect the bandgap, which is shifted to a higher energy.
The smaller particles have a smaller bandgap, which results in a higher absorption of visible light, leading to the white color
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