Question

The yellow colour of $ZnO$ produced on heating is due to

• A. Metal deficiency defect
• B. Extra positive ions present in interstitial sites
• C. Trapped electrons
• D. All of the above.

Answer 1

The correct option is C Trapped electrons

On heating to about ${800}^{\circ }$C $ZnO$, a few oxide ions looses electrons and escape as oxygen.
$ZnO\to Z{n}^{2+}+\frac{1}{2}{O}_2+2e^{-}$
Excess cations result in non-stoichiometric metal excess defect. Due to high temperature excess metallic cations $Z{n}^{2+}$ ions moves to interstitial sites. Thus, the formula becomes $Z{n}_{(1+\delta )}O$.
Free electrons balance the charge difference and occupy interstitial sites. Free electrons absorb blue light and emit complimentory yellow colour at high temperature. On cooling oxygen from atmosphere reacts with the free electrons to form oxide ions and the white colour of $ZnO$ returns.
The yellow colour is due to absorbtion of blue light by the free electrons. Movement of excess cations to interstitual site does not contribute to the yellow colour.
Hence, option (c) is the correct answer.