Zinc oxide reduction in ethanol

Question

I'm trying to prepare a ZnO thin film deposited on a glass substrate. The precursor solution prepared by sol gel process through dissolving 1 M zinc acetate dihydrate $\ce{Zn(CH_3COO)_2.2H_2O}$ in ethanol with ethanolamine as stabilizer in 1:1 mol ratio with $\ce{Zn(CH_3COO)_2.2H_2O}$. I aged the solution for about 1-2 weeks in a closed glass container. But, after I actually deposited and annealed at 600 °C, the film's XRD measurement seems to show ZnO as well as zinc peak, with zinc somewhat showing stronger peak.

However I also have the one with precursor aged for only 1 day but annealed at 800 °C, this one show peak characteristic that of ZnO with no noticeable Zinc peak. Other samples I have are aged for 3-4 days added with 1-5at% Nickel doping and annealed at 600 °C, they also show strong Zinc peak along with ZnO peak.

In the ones with Ni doping, Zn peak relative to ZnO also seems to get stronger as Ni concentration increasing. It seems like nickel (or its derived compound) acts as catalyst in the reduction of ZnO. The ones with 3at% and 5at% nickel concentration also have solution with green-bluish turbid solution with white precipitates (this probably zinc) unlike 0at% and 1at%, which yields clear solution. This might support that nickel act catalyst.

So, I guess there must be more rate of reduction reaction of ZnO during aging process, or maybe some other reaction that produces Zinc. Maybe something happens during annealing as well, but that seems unlikely since ZnO seems to be stable at 800 °C.

The question:

1. Looking at the precursor solution, what possible reactions that would produce Zinc from ZnO, and how likely they are?

2. Is it possible that the reaction of producing Zinc happens during annealing? How likely they are? How much is the rate, can they be neglected?

3. Do Nickel doping contributes to the production of Zn? If yes, what mechanisms that seem most likely to affect the production of Zn?

Answer 1

As for your first question, I have personally had better luck only aging 24-48 hours (actually unsealed, just covered with Kimwipe to prevent contamination, but allowing for some of the solution to evaporate). We're getting good $\ce{ZnO}$ XRD peaks on this process, and are currently developing an SOP/Optimization for $\ce{ZnO}$ by sol-gel technique. I'll let you know when we publish.

When you say you're getting $\ce{ZnO}$ and Zinc peaks, do you mean you're seeing XRD peaks matching Zinc as crystalline metal? To your question precisely: "What possible reactions would produce $\ce{Zn}$ from $\ce{ZnO}$, and how likely are they?" Remember that your sol-gel is not necessarily $\ce{ZnO}$ yet. The reaction that produces $\ce{ZnO}$ begins with the dissociation of $\ce{Zn}^{2+}$ and $\ce2{CH_3COO}^{-}$ ions in ethanol, followed by a hydrolysis. As to how you're getting the extra electron to precipitate metal, I don't know. What we have seen has been the precipitation of $\ce{Zn(CH_3COO)_2}$ after about 96-120 hours after originally mixing. (We're pretty sure it's acetate because it smells like vinegar when we wash it out of the flasks). There is a very good article going over the chemistry and technique of $\ce{ZnO}$ sol-gel: article