'Nanopowder' is an oxymoron - there are no free, independent, discrete particles < 100 nm in a powder. Thus you need laser diffraction, and not DLS, to measure the size distribution.
Another point is that DI water is about the worst system to obtain a stable dispersion.
It is possible to do better than you do with dispersing your nanoparticles in deionized water. Depending on the surface properties of the nanoparticles you could change ionic strength and pH so that dispersion improves. An important factor is concentration. This is because flocs as they are present now are less frequent at lower concentrations. Proper sonication is ofcourse a must.
Nevertheless, I agree with both respondees above that it is not a trivial thing to do: it requires skill and knowledge of the material at hand.
Alan F Rawle dear, thank you for this great information I have seen the video but i have only the DLS instrument and I am getting from the size quality report that my samples are polydispersed so what can I do to fix this problem ?
Dear, thank you so much but unfortunately, I don't have access to the Laser diffraction analyzer, I have only the DLS instrument and got polydisperse samples so I want to know how to get a ride from the polydispersity?
Ger Koper Dear, you mentioned an excellent point which is the concentration, but the size quality report didn't show anything about the concentration, so I think I have to correct the dispersity of my samples
I am new to this machine so I am relying on you to help me with this