Individual nanoparticles exist only in liquids (colloidal solutions and suspensions). When dry, nanoparticles form large agglomerates (sometimes several millimeters in size). This aglomeration is achieved by the influence of electrostatic forces: the smaller the particle, the stronger a bond between individual particles in a powder.

It exist no unagglomerated powder of single particles < 2000 nm.

In a SEM analysis, you can see that such particle lumps consist of individual nanoparticles (a good SEM can show particles below 100 nm).

Omar Z. Sharaf

As well as being invisible sub-100 nm particulate systems could not stay in a container - convection forces would blow them away...

You are correct in that solid-solid diffusion will prevent powders from containing individual, discrete, independent particles < 100 nm in size. See:

Dispersion and nanotechnology

https://www.brainshark.com/malvern/vu?pi=zHSznNgprz5chz0

At best we have a collection of sub- and post-micron fused aggregates (usually 'hard'; not disruptable by sonication) and agglomerates (usually 'soft'; disruptable by sonication). ISO is tending to prefer the term 'clusters' because of some confusion in the literature.

The solid-solid fusion in aggregates may still expose a large part of the surface area of small primary particle sized systems. If we have a specific surface area (SSA) > 60 m2/g for unit density then the derived Sauter Mean Diameter, D[3,2], is < 100 nm and would be classified as nano.

One route to get 'nano' in powders is to sterically separate active particles on an 'inert' (?) support. So, for example, a 5% Pd/C catalyst may have the Pd in sub-100 nm particles supported on the carbon surface and the SSA would reflect this.

Look up a paper that I wrote called 'Micron sized nano-materials' Powder Technology 174(1):6-9 (April 2007) DOI: 10.1016/j.powtec.2006.10.012

Omar Z. Sharaf

As Rudy Rucker stated: 'I think dry nanotechnology is probably a dead-end'.

Vadim Verlotski

Nanoparticles can exist in atomized form as well (solid-in-gas) e.g. decomposition of Ni(CO)4. They'll cease to be nano when they interact with other particles and surfaces, for example when collected.

Thank you, Dr. Alan F Rawle.

I enjoyed reading your paper (and watching most of your esteemed webinars, for that matter) one or two years ago, but I plan to go through it again with a new set of eyes in light of these new information.

I almost always use the terms "agglomerate" and "aggregate" as you have described. But I often notice when I'm reading through classic books (or pre-90s papers) on colloidal science (e.g., Hunter's, Hiemenz's, Russel's, Napper's) that the terms "flocculate" and "coagulate" are used in place of "agglomerate" and "aggregate", while "agglomerate" is actually used in the more general sense of the word in place of "cluster". Of course, many other combinations exist, which just compounds the problem for beginners like myself.

Omar Z. Sharaf

Thank you for your kind words.

Please find the attached paper. Dr. George wrote down a very nice paper.It might be useful for you....