Sound interesting! First, you have to confirm the purity of ZnO phase by XRD and EDS. If you sure that you were use high purity precusors and clean environment, colouring of the heated ZnO powder may indicate the presence of defects which effect the electronic, optical and even the magnetic properties of synthesized product. Therefore, these defects may enhance the absorption peak and shift it to higher wavelenghts (UV-Vis spectrometer). I observed yollow colouring of my prepared ZnO nanorods.

please check out the article along with the thread below for more details:

https://www.researchgate.net/post/What_are_the_reasons_for_the_variations_in_colour_I_see_in_my_sol-gel_synthesized_ZnO_nanostructures_They_can_be_grey_white_yellowish-brown_or_pink/amp

Article Probing defects in chemically synthesized ZnO nanostrucures ...

may be it oxidized

Sound interesting! First, you have to confirm the purity of ZnO phase by XRD and EDS. If you sure that you were use high purity precusors and clean environment, colouring of the heated ZnO powder may indicate the presence of defects which effect the electronic, optical and even the magnetic properties of synthesized product. Therefore, these defects may enhance the absorption peak and shift it to higher wavelenghts (UV-Vis spectrometer). I observed yollow colouring of my prepared ZnO nanorods.

please check out the article along with the thread below for more details:

https://www.researchgate.net/post/What_are_the_reasons_for_the_variations_in_colour_I_see_in_my_sol-gel_synthesized_ZnO_nanostructures_They_can_be_grey_white_yellowish-brown_or_pink/amp

Article Probing defects in chemically synthesized ZnO nanostrucures ...

Dear Derman Akşit,

ZnO is ha igh vopor pressure compound. Heating in air or certain atmosphere (was mentioned in the original question) my lead formation of oxygen vacancies. This could alter the valence band and change the electronic and optical properties. The preferential loos of Zn is also possible. The quantify this, surface sensitive techniques are required. XRD and EDS will not detect defect formations with high precision. You may want to use XPS.

Regards

Engin

Hello dear Mohammed M. Obeid,

I've used zinc nitrate hexahydrate as precursor, and i've tried to synthesize it repeatedly. When the colour turned green after termal treatment, i couldn't be sure about the purity of the precursor. So i've decided to do my experiments with the same precursor in different brands. However, all of them are green and the little sweet green ZnOs are everywhere. Now, i'm looking forward the characterization results. :(

Dear Engin Ciftyurek,

Loss of Zn is a very interesting information for me. Can Zn and ZnO occur together in the air atmosphere when exposed to the termal treatment?

I guess I'll have to wait for the characterization results. :(

Dear Dr. Aksit:

During thermal treatment or calcination or sintering occurs grain growth, this phenomenon allow the diffusion of cationic species that were placed at grain boundary or grain surface. As function, of the diffusion of species and ZnO grain growthing, there is the development of color via substitutional defects, since a solid solution is formed.

Well, it is possible by hypothesis; some kind of contamination, the contaminant agent would be in the surface of a crucible, plate or walls of furnace used in thermal treatment to attain ZnO.

In the past, I work with some ZnO solid solutions.

In the above sense, ZnO doped with cobalt cations gives a clear green color to the ZnO that should be almost white color. Also, niobium doped ZnO gives a solid solution at minor fraction of cations but the color is defined green color. ZnO doping with manganese cations gives a yellow tonality with increase of doping fraction the color reach to reddish-brown.

It is possible a very, very great number of colors.

With best regards

Marcos Nobre

Dear Derman Akşit,

What is the temperautre regime you utilized ?

My working temperature was 500 Celcius degrees.

Dear Marcos Nobre,

Thank you for your answer. As you mentioned above, defects can be occured or there is a contamination that's why my product turns green repeatedly.

*** If there is a contamination, i'm sure it does not arise from the precursor.

*** "the contaminant agent would be in the surface of a crucible, plate or walls of furnace."... That's true. To check on the furnace, i cleaned them yesterday. Two days later i'll synthesize again. I hope it won't be a problem.

*** I used this green ZnO during the photocatalytic reaction, its activity was almost the same compared to white ZnO. On the other hand, i can not say anything about the structure of sample for now because i don't have the characterization results such as XRD, EDX... I have to wait for the results.

Dear Derman Akşit,

if the colour origins from contamination it could be from Cobalt resulting in Rinman's green with ZnO ( https://de.wikipedia.org/wiki/Rinmans_Gr%C3%BCn or https://en.wikipedia.org/wiki/Cobalt_green#Rinman's_green )

For the the metal content the best approach is ICP-MS or ICP-AES. You'll have exact value for the impurities.

Pure ZnO on heating turns yellow when hot white on cooling due to crystal defects followed by electronic excitation. But giving a green colour even when cold clearly indicates the presence of some cations like Co 3+, Cr4+, Fe2+ etc as Impurities, which may be detected by ICOPES.

Another idea: You always used the same furnace? I remember during my studies we had problems with a furnace because always during heating up some small crumbs of oxides from the heating wires fell into the crucibles or on the lids.

Dear Dr. Aksit:

During thermal treatment or calcination or sintering occurs grain growth, this phenomenon allow the diffusion of cationic species that were placed at grain boundary or grain surface. As function, of the diffusion of species and ZnO grain growthing, there is the development of color via substitutional defects, since a solid solution is formed.

Well, it is possible by hypothesis; some kind of contamination, the contaminant agent would be in the surface of a crucible, plate or walls of furnace used in thermal treatment to attain ZnO.

In the past, I work with some ZnO solid solutions.

In the above sense, ZnO doped with cobalt cations gives a clear green color to the ZnO that should be almost white color. Also, niobium doped ZnO gives a solid solution at minor fraction of cations but the color is defined green color. ZnO doping with manganese cations gives a yellow tonality with increase of doping fraction the color reach to reddish-brown.

It is possible a very, very great number of colors.

With best regards

Yedluri Anil Kumar

This is strange! A100% plagiarized answering is appeared in this post.

See the article ( https://de.wikipedia.org/wiki/Rinmans_Gr%C3%BCn or https://en.wikipedia.org/wiki/Cobalt_green#Rinman's_green )

Dear all,

We determined that "green ZnO" was caused by the impurity in the "furnace" and finally I managed to synthesize it as "white". Because of this contamination, accidently, I've learned a lot of new information about the catalysts. :) Thank you all for your valuable contributions.

It is nice that you have learnt much more by experiments on your question.

Thanks Derman Akşit for sharing the cause of ZnO coloring.

You're welcome. :) @Mohammed M. Obeid

Dear Dr. Aksit:

Nice job.

Congratulations by fast solution of the question.

Despite of my answer have some contribution ...” Well, it is possible by hypothesis; some kind of contamination, the contaminant agent would be in the surface of a crucible, plate or walls of furnace used in thermal treatment to attain ZnO. "..., your felling was very good.

As a whole, the ZnO.Co solid solution should exhibits a decreasing of electrical resistivity or increasing of the electrical conductivity. In this sense, a decrease of optical gap, 3.2 eV of ZnO, should be expected.

with best regards.

Marcos Nobre