If you know the point of zero charge of your nanomaterial, then you can either increase or decrease the pH to avoid being close to that point.

Bath sonication for 30min-60min can be a good method as well.

Vortex mixing has been tried as well.

You can have a look at this paper where they tried different dispersion methods:

Dickson, D.; G. Liu; C. Li; G. Tachiev, and Y. Cai. Dispersion and stability of bare hematite nanoparticles: Effect of dispersion tools, nanoparticle concentration, humic acid and ionic strength. Sci. Total Environ. 2012, 419 (0), 170-177.

What kind of nanocrystals? I think tha good solvent + magnetic stirring + ultrasounds is a good combination but be careful with ultrasounds since can destroy your nanofiller or matrix,

If ur nanomaterials is as nanoparticles then go to sonication for 30 min to 1 hr.

or you can use "Ultraturrax", but maybe also your formulation needs to be fine-tuned: many nanoparticles prefer aggregation rather than becoming dispersed, if no (or the wrong) dispersion aids are used.

I agree with Mr. Jon Trifol, the combination of techniques, he mentioned, can get stable nanosuspensions.... pH adjustment can decrease agglomeration as well... Ultrasonication time and power is an important factor towards achieving level of stability.... optimized sonication time and power must be used because excess sonication time has negative impact on the stability of nanosuspensions

Thanks to all. I am using silicon nano powders with large aggregations and need to remove aggregation in liquid media. I used ultrasonication for fragmentation, but it is inducing the oxidation. So I need to do fragmentation with controlled oxidation effect.

Then you may need to include some anti-oxidation agent (if not a real reducing agent) in your formulation, not only dispersion aids.

Dear Bernhard,

Thank your very much....

One possibility could be to disperse these crystals in an opportunely functionalised polymer. I've done it with silica and titania nanoparticles in epoxy resins by silanol functionalisation.

One simple possibility is to use pulsed ultrasonic perturbation (pulses of 10 to 15 seconds by 2 to 5 seconds of rest). You can combine this with the addition of antioxidants and some dispersant agent (since you are using a silicon nanopowder you could try 3 aminopropyltriethoxysilane.

I agree with Dr. Wessling, you probably need to discharge the crystals and "convince" them to spread apart in an opportune media rather than prefere to sggregate together. The aminopropyl bis triethoxy silan eproposed by Dr De Santis is one possible good solution.

If I understand you are dealing with the silicon in metallic state. If during the milling your silicon did not react with solvent, probably the surface of crystals are descompensated. Depending on the direction , , the intensity could be somewhat diferent. If you want to preserve the state of surface, may be you need to charge the crystal with e- to cause some repulsion (I do not know how to do that). If you do not mind if solvent react with surface may be you can test some byfuncional solvents like glicols or dicarboxylic acids or mixed functions. Or you mill in a solvent and suspend in another. Good luck! Tell us about!

If you know the point of zero charge of your nanomaterial, then you can either increase or decrease the pH to avoid being close to that point.

Bath sonication for 30min-60min can be a good method as well.

Vortex mixing has been tried as well.

You can have a look at this paper where they tried different dispersion methods:

Dickson, D.; G. Liu; C. Li; G. Tachiev, and Y. Cai. Dispersion and stability of bare hematite nanoparticles: Effect of dispersion tools, nanoparticle concentration, humic acid and ionic strength. Sci. Total Environ. 2012, 419 (0), 170-177.

You can keep them in the colloidal suspension by using various capping agents like suitable surfactants, polymers, etc

You can refer to the following publications:

1. Optical and structural properties silver nanoparticles, Radiation Physics and Chemistry 2005

2. Gamma Radiolytic Formation of Alloyed Ag-Pt Nanocolloids, Advances in Physical Chemistry 01/2011; 9.

Silicon particles, if handled in ambient and/or aqueous media, will very rapidly develop a layer of oxide on their surface. So for the practical purpose of dispersion you should think of silicon oxide, not silicon. SiOx surface is usually negatively charged, so by itself this is a good thing for you - the particles will repel each other. Was it not for the fact that water/aqueous mitigates this repusion and often turns it into attraction.

Besides, sometimes you can actually get "silicon bonding" of the type Si-O-Si, where two silicon atoms belong to two different particles.

Depending on the type of solvent that you have, I suggest using some sort of silane (e.g. HMDS) for surface passivation. Those are small, tough and graft very strongly on Si surfaces. Sonication and all other physical methods may only temporarily help; overnight the particles will re-agglomerate. Be careful with pH - basic media etches Si.

Metallic nanoparticles are unstable and have a tendency toaggregate. Therefore, the use of stabilizer is very significant to pre-vent the aggregation and to maintain the stability of AuNPs inaqueous solution.SDS

and CTAB are common surfactants that have been used as stabilizers

to prevent nanoparticle agglomeration because they provide

steric stabilization . polymer also can be used

I agree with Laura but you can also try dialysis method of separation if you know the size of the desired particles.

Tamilselvan,

are you sure that your starting material is nanopowder in reality. Are you starting with individual nanocrystal particles or with compact aggregates or loose agglomerates?

Dear Titus,

Thanks for your reply. Actually i am using silicon nano powders with loose aggregates.

I am not sure your application. If you have a large amount powder (>10 g), I like to use filtration methods. It is simple and quick to remove the aggregation. You can find 100 nm filtration membranes on the market. In filtration method, you have to check the compatibility between sample solvents and filtration membranes first. If samples contain a wide size distribution of particles, using different pore size of filtration membrane from large pore size to small size. Otherwise, the bigger particles will block the pores. For example, the size of particles ranging from 2 um to 50 nm can be filtrated to obtain 50 nm nanoparticles with a series of filtration membranes with 1 um, 0.7 um, 0.45 um, 0.22 um, and 0.1 um. During filtration process, you can add dispersing agents to avoid re-aggregation of nanoparticles.

Dear velusamy you can use probe sonicator to remove the aggregation of nanocrystals.

I believe Oil bath sonication with a probe sonicator will have good results.

I agree with Sunil, try sonication. And this post can give you some insights:

https://www.researchgate.net/post/What_methods_enhance_the_dispersion_stability_of_the_nanostructure_in_a_DI_water

It strongly depends on the application. For example, for catalysis in solution the best option is to initiate Leaching.

Leaching is a process of dynamic renewal of particles that suppresses aggregation.

J. Org. Chem., 2013, 78, 11117

http://dx.doi.org/10.1021/jo402038p

Aggregation of nanoparticles is a very common phenomenon, the most accepted method that your friends has suggested suits that is sonication but different media can be used during sonication like for few material acetone and lower alcohol media so depending upon your material viscosity and nature either go for more viscous material as media as your actual sample ,( difference in viscosity will amplify the damping sonic wave) and hence more segregation can be obtained.

Have you tried to functionalise the crystals or the liquids before the mixing of the two media?

If you give me more details I can try to help.

Magnetic stirring can also give good results

Dear Luca Prezzi ,

Thanks for your help. I didnt functionalise the silicon nano particle. While synthesizing silicon nano particles in chemical method, its agglomerating. But I need to remove this agglomeration for fabricating film.

you could try to add stabilizers like anionic or nonionic surfactants or polymers to the reaction mixture. You will have to play with stabilizer concentration.

TO THE BEST OF MY KNOWLEDGE SONICATION  USING ULTRASONICATOR  SHOULD HELPU.