Dear Manoj Yadav

The color of your cobalt nanoparticles will be probably grayish..........For detailed please see the article.

Materials Sciences and Applications, 2011, 2, 1307-1312

Dear Manoj Yadav

The color of your cobalt nanoparticles will be probably grayish..........For detailed please see the article.

Materials Sciences and Applications, 2011, 2, 1307-1312

Thank you...but bulk cobalt looks greyish ..and i guess Co NPs should look different in colour from them. 

The color will depend on the size: very small particles may have plasmonic resonance with visible light. When I have made them previously (big, r ~100 nm) the color is very dark grey, almost black.

Thank you @ Dr. Aaron Bloomfield for sharing experience ... Now, I have understood and I have done colloidal synthesis of a Co-NP sample which shows black colour in solution. But only concern which my supervisor was telling me that there may some cobalt oxide NP also. 

Hi Manoj,

I have been synthesizing Co NPs for some time now and my observation is that one has to try as much as possible to carryout the synthesize in an airless system otherwise the colour of the colloidal suspension may change indicating either oxidation or increase in size of the Co NPs

Yes, thanks sharing experience and what you said is correct, Lotanna!