I am synthesizing Cu doped Titania nanoparticles by in-situ doping. I prepared the stock solution and took sample on pH-2 and pH-7 by condensation with NH4OH . The pH value of stock was 0.58 and its color was bluish. The color of sample was white at pH-2 but it was again bluish at pH-7. Can anybody guide me why the variation in color is observed ?
It is a well known that the chemical nature of an electrolyte affects aggomerization of the NPs. When treated with NH4OH, the resulting solution has pH=7.0(Appxo.) As the absorptivity of the cations (NH4+) is affected by pH, there occurs the formation of a nonuniform disperse system with large agglomerates up to 5 micro meter size ofTiO2NP. At the same time, there develops zeta potential as it is a function of pH. This, in turn, varies absorptivity of the NH4+ cations by changing the dispersion conditions and the isoelectric point of the NP suspension.
The iso-electric point is the pH value at which the zeta potential is approximately zero. At a pH near the iso-electric point, the NPs are usually unstable as the particles tend to coagulate. Tables for iso-electric points for different materials are available with rutile it is around pH 4.So Rutile is unstable in the pH range from 2 to 6(Appxo).
Now discussion about the three p Hs and the changes shown in the colors is as follows:
As NP are unstable between 2-6 pH range, they do not absorb and look colorless.Below 2.0 (say 0.58) pH or above 6.0 (say7.0) pH, they exhibit intense blue color by undergoing Charge Transfer Transitions.
Apoint left: Small sized Cu (+2) remain embedded in the very large sized TiO2 agglomers ,e.g. are dormant between2.0-6.0 pH and are hardly unaffected ny NH4OH
Pls. delete "hardly" from above. Also ,it is "by" not " ny." .Typographical mistakes regretted
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