I would say that it is nickel oxide.

Elias Henderson

Oxymoron in your question - nano combined with precipitation/sedimentation. If your particles were truly nano (< 100 nm), they'd not settle and you'd have a transparent (perhaps colored) liquid.

I concur that your precipitate is likely to be NiO combined with a hydroxide species, nominally Ni(OH)2. I avoided borohydride reduction like the plague during my Ph.D - everything gets contaminated with intractable B which is detrimental to applications such as catalysis (which may be your application...). The are 2 main factors to control here - concentration and stabilization. Low concentations favor small sizes. Reduce your concentrations by a factor of 10 or 100 and try again. I favor phosphate capping/stabilization agents - a low concentration (see the second webinar below - possibly in the 0.1% region). My favored reducing agent was 5 - 15% hydrazine hydrate (only water and nitrogen as the products).

Take a look at these webinars (registration required):

Metal colloids - their preparation, application and characterization

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W160218MetalColloids.html

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

It can't be nickel oxide, because it is run in reducing environment.

It will be Ni while the nanoparticles which you created will coaggulate and then settle. Sometimes, you have some "fine mist" appearance - then the agglomeration is relatively fine; sometimes "chunks" - then you have heavy agglomeration.

Why you don't dry the precipitate and look at it in SEM, elemental analysis and such?

I would assume you need some tensides in your reaction.

Bernhard Wessling

Of course, you are correct. My apologies to you and the poster of the question for any confusion. I must have been having a brain aberration...

And I made such systems in my Ph.D - too long ago (1970's) and I now only have timeshare on a single neuron...

The surface (5 - 10 atomic layers) of the particles will be fully oxidized (shown by ESCA/XPS). Small/nano size is favored by low concentrations of precursors.

Alan F Rawle No need to apologize, thanks, and my answer was more for Elias Henderson in order to give him some suggestion

Elias Henderson Bernhard Wessling

I'd still avoid borohydride reduction though.

Dear All,

Based on my experience, I would be inclined to say that the reaction product is agglomerates of nickel oxides. If the reduction of the salt with borohydride is not carried out in a deoxygenated argon atmosphere, nanoxides are always formed, even if there is still a reducing agent in the reaction solution.

Of course, I agree with the previous speakers that the product must be isolated and identified.

Good luck!

Hai EH

I think you are dealing with a problem of high pH. Citrate and PEG can raise the pH of your solution, making it harder for Ni to dissolve. I recommend lowering the pH of your reaction by adding a buffer such as citric acid or bicarbonate. You may also consider using a less reactive reducing agent, such as sodium format or sodium hydrosulfite. I hope this helps!