For measuring particle size better to measure size of powder directly, e.g. by TEM.

Dear Farzad:Add the powder slightly oleic acid. Preferably with the expectation monolayer adsorption. Mix thoroughly, wait until it reacts with the zinc on the surface of the nanoparticles. Dispersed in water. Then the surplus is neutralized with ammonia.

There is a bad habit, try to measure something without thinking whether resulting info will be useful. On the way suggested by Yuri you will get info on the dispersion of the surface modified nanocomposite, which is fine if your task is dispersing the nano composite in water (you did not mentioned whether this would be your main task). If you want to characterize your nano composite powder you should use methods using the powder as is. Besides TEM you may apply BET measurement and analyse e.g. wetting behaviour.

With thanks to all, I have BET, SEM and XRD analysis of this sample, and all of them shows that the size of particles below 100 nm, And i need to measure the size distribution of sample, but the size distribution measured by DLS is about 1500 nm with 30 min sinication, because this sample not good dispersed in ethanol or deionized water. In addition the type of solvent is not very important.

With the dispersed sample you always will get only the distribution according to the degree of dispersion, i.e. something about the quality of dispersion. By the way, with DLS you will also be most sensitive to the courser fraction. So even when there is a larger fraction of nanoparticles you may not be able to detect them with DLS. For this you will need a method which separates the particles like analytical centrifugation.

Further it is very likely that your sample contains not only agglomerates but very strongly bound aggregates composed of primary particles. In principle it should be possible to derive size distribution from TEM with high efforts.

How do you prepared your composite, in gas or in liquid phase? If the latter is the case you should determine size distribution as long as in liquid phase.