Hi,

Centrifuging for large time like more than 12~20 hours in certain solvents like D-I water, ethanol or methanol will help to avoid agglomeration of particles. After that cleaning should be done in D-I water for several times. Also another thing that you should not put them longer time because after that they become agglomerates again into bigger particles sizes.

It is better to avoid aglomeration, eg by adding a suitable surfactant. In case of aglomerated sample, you can use ultrasonication to re-confirm the separation of particles. Centrifugation could get rid of the aglomerated part of the sample. This includ some loss of the prepared sample. This also means a better homogenity of the sample from the partcle sizes point of vew.

Thank you every body. Can you suggest a surfactant for TiO2 nanoparticles to avoid agglomeration again?

You can try the procedure used in this ref. DOI: 10.7763/IJMMM.2014.V2.91

They used Acetic Acid,  Oleic Acid  and Oley amine for the preparation of TiO nano-particles depending on chemical method. You can modify the procedure to be applied for your work targets.

Best Regards

Prof. Emad Al-Ashkar

The stages in getting a stable dispersion from a powder are wetting, separation, and stabilization in which the middle step is the important and difficult one. A surfactant is a wetting agent, not a stabilizer, typically. To stabilize TiO2 after separation (usually via sonication) normally phosphates (e.g. sodium hexametaphosphate) are added in the optimum concentration (typically 0.05 - 0.5 wt%). For any ionic additive, too much is as bad as too little and will cause destruction of the double layer and 'salting out'.

There's plenty of TiO2 experience in the paints and coatings industry.

Typical TiO2 sizes for the coatings industry (white pigment) are in the 300 nm range so this isn't considered 'nano' by ISO and ASTM definitions. A 'nano' TiO2 would be typically < 100 nm and would provide a clear, transparent, suspension often used for varnishes and UV absorption. Some optical properties can be found in:

August 11th 2011 Optical properties of the three forms of titanium dioxide

http://tinyurl.com/pbgf9hj

For aggregation and agglomeration issues see:

November 3rd, 2015 Adhesion and cohesion

http://tinyurl.com/zwb2wlh

Also take a look (Internet Explorer seems the only browser that will open this) - Slides 45 onwards:

Nov 11th, 2008. Dispersion and nanotechnology

http://www.brainshark.com/brainshark/vu/view.asp?pi=106756756&text=M021507