Zeta Potential reading shows a positive surface charge of ZnO np (pH 7.9) around +23mV. It is supposed to show negative charge. What may be the possible reason for a positive surafce charge of ZnO nanoparticle?
I came across to an article discussing the effect of pH and impurity on the surface charge of ZnO.Please check the following reference:
Journal of the European Ceramic Society 20(6):667-673 · May 2000
The excess positive charge would come from the ligands present to stabilize the ZnO nanoparticles.
The positive charge may be due to the stabilizing agent employed for the synthesis.
The pzc for ZnO is around 9.XX. Hence your measurement is correct. ZnO is positively charge below pH of arround 9. A detailed measurement can be found in the attached publication.
Article Synthesis and testing of ZnO nanoparticles for photo-initiat...
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