When performing a synthesis of CuO-doped TiO2 nanoparticles through sol gel process (alkoxide path; ethanol as solvent), a bluish green sol and a green intermediate product (dried gel) were obtained. I came across a few threads on forums and in wikipedia stating that the green intermediate product is actually ''basic copper nitrate'' (Cu2(NO3)(OH)3). However i fail to find any reliable literiture confirming this claim.
What is/could be the identity of this green intermiediate product? Can anyone explain in detail the reaction path of thermal decomposition of hydrated copper (II) nitrate?
The synthesis steps (rough description):
1) preparation of a sol --> bluish green sol
2) gelation --> bluish green gel
3) drying in fume box (24h around 18 °C) --> green (emerald green) crystal-like solid
4) drying in oven (24h at 120°C) --> dark green crystal-like solid
5) calcination (8h at 300°C) --> black crystal like solid (assumed to be CuO trapped in TiO2 network)
the products for each step during the synthesis are shown in the attach images
Dear Yu Han Tai,
have you ever thought of any copper(I) species?
I can only refer to one or the other experience where an alkaline copper(II) system discolored to green, especially when some copper (0) was present.
I could imagine, that some symproportioning could have taken place, causing the green colour.
If so, you could check it by adding oxidizing agents like hydrogen peroxide: When you get the original blue colour back, the presence of copper(I) is quite likely.
Interesting finding. Did you measure the IR spectrum of the green intermediate? CHN analysis and X-ray diffraction pattern would also be the great help. If you do analysis, provide data please, we can figure out together.
For the chemistry, have a look at Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 3, pp 284 - 286. It may be 90 years old, but the facts haven't changed. You should be able to find it somewhere online.
Also check out information on the minerals Gerhardtite and Rouaite, which are polymorphs of Cu2(NO3)(OH)3 - these are green. Copper(II) basic salts can be green or blue, depending on the proportion of copper to hydroxide (e.g. the basic carbonates: malachite - green vs azurite - blue).
Powder X-ray diffraction is the best way of identifying these species - hope that your product isn't too small in crystallite size to give a useful diffraction pattern.
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