Why does ZnO's color become yellow when it is hot? I have read that it is because of physical properties, but I need a more elaborate explanation.
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1$\begingroup$ I read on wikipedia that it is a thermochrome compound (see: en.wikipedia.org/wiki/Thermochromism ) $\endgroup$– user2117Commented Jan 9, 2014 at 17:05
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4$\begingroup$ If anyone is interested, this video shows the effect very well, and also touches on the explanation. $\endgroup$– Nicolau Saker NetoCommented Apr 2, 2015 at 15:53
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$\begingroup$ IT is most likely Thermally induced charge transfer between the inherently unstable oxide ion and the Zinc ion resulting in an electronic transition that absorbs blue light. Observe it under a photographic safe light it will not change color. $\endgroup$– jimchmstCommented Sep 7, 2023 at 21:05
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$\begingroup$ See en.m.wikipedia.org/wiki/F-center $\endgroup$– Ian BushCommented Sep 7, 2023 at 21:25
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5 Answers
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The thermochromism of $\ce{ZnO}$ results from a minor loss of oxygen upon heating to temperatures around 800 °C, i.e. a non-stoichiometric $\ce{Zn$_{1+x}$O}$ with $x = 7 \times 10^{-5}$ is formed.
Under air, this effect is reversible. Heating (and cooling) of the material while hooked up to a vacuum pump might result in a more persistent colour change.
answered Jan 9, 2014 at 14:16
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$\begingroup$ So heating causes $\ce{2O^{2-}}$ to be oxidized to $\ce{O2}$? That seems unlikely to me. Does something else happen to the $\ce{O^{2-}}$? $\endgroup$ Commented Mar 11, 2016 at 8:09
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3$\begingroup$ This may explain the chemical change but it doesn't explain the reason behind the colour change. What causes the colour? $\endgroup$ Commented May 23, 2016 at 18:43
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$\begingroup$ F centres are the reason for the colour see e.g. en.m.wikipedia.org/wiki/F-center $\endgroup$– Ian BushCommented Sep 7, 2023 at 21:24
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$\begingroup$ O= is endothermic I propose that Zn++ O= becomes Zn+O- This can be checked by heating in an inert, argon atmosphere. The color change should still be reversible. $\endgroup$– jimchmstCommented Sep 7, 2023 at 21:44
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When you heat $\ce{ZnO}$ then Oxygen leaves as $\ce{O2}$ leaving behind $\ce{Zn^{2+}}$ and 2 electrons. $\ce{Zn^{2+}}$ and the 2 electrons move to the interstitial sites of the crystal . This provides excess electrons in the crystal lattice of $\ce{ZnO}$. When light falls on these crystals then these electrons absorb a part of the light in the visible region and hence impart a yellow colour to the $\ce{ZnO}$.
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Most probably because the band gap closes, thus allowing electrons from the valence band to be elevated into the conduction band (by absorbing blue light, which makes the reflected color yellow).
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Upon heating ZnO, anionic vacancies are created and when an electron occupies that vacancy it imparts yellow colour (metal excess defect/f-centre).
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1$\begingroup$ It would be nice to flesh this answer out a bit, particularly on the color centers and their population vs temperature in thermodynamic equilibrium. $\endgroup$ Commented Mar 10, 2016 at 14:37
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In $\ce{ZnO}$, $\ce{Zn}$ is present in the 2+ oxidation state, and the d subshell has 10 electron, and the s subshell has 0. When $\ce{ZnO}$ is heated there is a transition of electron from d to s subshell, causing it to become yellow, and on cooling it becomes white.
