What actually happens is, after immediate formation of the nanoparticles they undergo the following process in very quick time.

1. Formation of clusters (few nanoparticles stick together due to high surface energy) 2. Clusters join to form aggregates and 3. Aggregates join to form agglomerates. And depending upon the properties and density of particles it leads to sedimentation of the whole precipitate. What we see are bigger particles. If you can break these aggregates with various techniques you can again form the colloidal suspension of primary nanoparticles. So the primary particles which form the aggregates are nanoparticles (invisible). Thats why in-situ stabilizers are used to keep the particles in nano form which cover their surface immediately after their formation and prevent the whole process explained above.

Particles remaining in the suspension can give the color to the suspension of invisible nanoparticles it depends upon their photo-physical properties (as you have not mentioned which nanoparticles you refer).

Dear Hitanshu Sachania:

It seems that your own explanation:

There is some color the clear liquid retains. Is it because there are some nanoparticles in there imparting this color and the precipitate is just agglomerates? "

Is correct !

Best Regards,    Leonid

Dear Hitanshu Sachania, appearance of color or turbidity are different phenomenon. Nanoparticles smaller than 100 nm may give color in water, but turbidity in water indicates presence of large particles or aggregates.

You can use DLS for size and size homogeneity whereas TEM or SEM may be used for visualization.

As a starting point, you can see sample in light microscope. If the problem is larger size or aggregation, you can get clues of it in light microscope.

Thank you all for your response. I just estimated the size of the particles (CuO Nanorods) with DLS and there were three intensity peaks (File attached). Maybe, the colour was due to the 5.859 nm particles present.

Basically during co-precipitation method, what you see is an agglomeration of several thousands of particles (you may call these as crystallites). A single particle can be distinctly visible under Transmission Electron Microscopy (TEM) or at least FESEM where you can zoom in for one or a few particles. However, you can estimate the size of your particles by simple XRD analysis. Its very easy. You can also directly measure the size by TEM analysis of the sample.

Hope this helps!

HItanshu,

Remember that the principle of DLS instrument is based on spherical particles and importantly as it is a light scattering method when there are colored samples there is absorption of light interfering with the measurements. Aggregation of bare that is non stabilized particles (rods in your case) happens during the measurements giving you multiple peaks. As you are using Malvern instrument in the software after each measurement you should check the expert advice option (quality of data).