Dear all, if possible what is the metal oxide concerned by your investigation ? Please check the following documents. My Regards

https://doi.org/10.1111/j.1551-2916.2006.01005.x

https://doi.org/10.1002/anie.200704541

https://doi.org/10.1039/C4CC09277E

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5075760/

I have a question, why exactly you want surfactant-free routes. Many metal oxides are not soluble in common solvents like water, ethanol, methanol, chloroform etc. It depends on many factors such as hydrophilic or hydrophobic nature, enthalpy etc. It is a material property that it forms some electrostatic bonds, H-bonds, van der Waals interactions with particular solvents or not. This defines its solubility or dispersibility.

The sole purpose of surfactants or capping agents is to modify the chemical nature of the nanoscopic surface via appropriate functionalization, so that the dispersibiliy or solubility of the material can be controlled. So, if you are not modifying the surface, whatever synthetic technique you follow, dispersibility will not improve significantly.

Souvik Bhattacharjee

Thanks a lot for the answer.

We use the materials for catalytic applications (occurring on the surface of the material) and based on some studies, although still seemingly debatable, complete removal of capping agents or surfactants from the surface of the such oxide nano-particles is mostly very difficult. This, in some cases, affects the catalytic performance of the material depending on factors such as the catalytic reaction temperature, activation conditions, etc. So the whole idea of using surfactant-free routes is to avoid this.

I am also not sure if I am understanding you on the soluble part, but the idea of using this common solvents (water, ethanol, methanol, chloroform, etc.) was not to dissolve the metal-oxide nano-particles but to disperse them with the help of ultra-sonication. I was hoping there must be another way to modify the nanoscopic surface as you suggested, perhaps change the surface-charge/zeta-potential, without using capping-agents/surfactants.

Okay, now I understood your problem. And it is true that, most of the times, complete removal of surfactants is not possible. Also if they are removed, the dispersibility is again compromised and agglomeration-tendency increases. And yes, you can easily disperse nanoparticles by ultrasonication, but if it kept for long undisturbed, it'll precipitate gradually. I think, the best way is choosing an appropriate solvent for your material. Sometimes, pH of the system also matters significantly. You may also measure surface-charge or Zeta-potential of the nanoparticles in your solvent, if they are polar in nature. Specially it sometimes increases, if the size of the particles are reduced. But, I don't work in catalysis and cannot answer your question in this particular perspective.

Dear all, if possible what is the metal oxide concerned by your investigation ? Please check the following documents. My Regards

https://doi.org/10.1111/j.1551-2916.2006.01005.x

https://doi.org/10.1002/anie.200704541

https://doi.org/10.1039/C4CC09277E

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5075760/

Abdelkader BOUAZIZ

Thanks a lot! I will check them. The materials are nickel ferrites, cobalt oxide and iron oxide spinel.

Dear all, please check the following document. My Regards

https://doi.org/10.1039/C3RA40763B

In absence of surface-active substances, dispersibility of metal oxides depends on their nature, size, prehistory, morphology, as well as on their loading (concentration) and the solvent you are using. However, sometimes, some volatile and easily removable additives can help (at least partially) to disperse the nanopowder. You can add to your solvent (e.g. water) for example 5-10 vol % or more/less (it depends on your case) of pyridine (or something like this) and do ultrasonication for some time (it also depends on your case). You have also find the suitable loading (concentration) of the oxide in the solvent mixture. For example I could disperse ZnO nanoparticles in water with 10 % of pyridine. After casting and drying on a surface the additive was easily evaporated. I hope this can help you.

Dear Shaine Raseale

you may need to modify surface of the oxides by using polar groups as amino groups, in order to enhance dispersion in water.

You can extract some ideas from these articles:

Article Nano ZnO/amine composites antimicrobial additives to acrylic paints

https://www.sciencedirect.com/science/article/abs/pii/S1359836818309594

Michel Rahal

Thanks a lot. I will check it out!