When added to Distilled water (not degassed or deoxygenated) my 300 nm TiO2 particles finely disperse to form a milk white mixture. However, when almost anything is added to this mixture (copper nitrate, silver acetate, sodium acetate, naoh, hcl, ascorbic acid, and, NaBH4 solutions) the TiO2 particles form mm wide aggregates which quickly fall to the bottom of the container. Can anyone shed some light as to why this happens and how to inhibit this process? My task is to dope these nanoparticles with metals and I have been following papers on doing so but they dont mention this aggregation happening.
You are almsot certianly shifting the pH when you add any of those, er, additives. This will shift the zeta potential. If it drops below 30mV or thereabouts there won't be enough electrostatic repulse force to prevent aggreation. You have a few options:
1) Titrate to a region where the zeta potential is strong so a small change won't hurt
2) Functionalise the TiO2 particles / modify surface groups to enhance zeta potential
3) Att ligands or use a surfactant (most complicated approach)
Typical isoelectric points for TiO2 are between 3.5 (Anatase) and 6.5 (Rutile) - in case you were wondering.
In addition to workiing away from the isoelectric point (also often abbreviated as IEP): high salt concentration itself can lead to aggregation due to the strong screening of the electrostatic repulsion of the nanoparticles.
http://www.materials-talks.com/blog/2017/07/27/isoelectric-points-of-nanomaterials-qa/
TiO2 is one of my favorite materials! See the attached (slides 45 onwards) from an earlier 2008 webinar.
30 mV almost certainly isn't enough zeta potential for stability for 300 nm particles of TiO2 in suspension. Probably nearer 70 mV would be needed.
See:
November 3rd, 2015 Adhesion and cohesion http://tinyurl.com/zwb2wlh
As indicated above, changing the environment of the surrounding continuous phase will promote aggregation.
To see my enthusiasm for the material see:
August 11th 2011 Optical properties of the three forms of titanium dioxide http://tinyurl.com/pbgf9hj
Perhaps add a neutral surfactant, e.g., Triton x-100, but this may inhibit altering the surface of the particles.
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