We need to deposit gold nanoparticles from a water solution on a nickel mirror-like film of a rather large area, ~ 2 cm2. The problem is that when the water solution is deposited on the strongly hydrophobic Ni surface, it forms a droplet (or several individual droplets), but not a liquid film. The droplet keeps its original shape even if it is disturbed.
We tried the spin coating technique but it does not help since the droplets are thrown out of the surface even at relatively low rotation speeds. The simple static drying of the droplets is also not acceptable since (a) the surface remains spotty and (b) the particles aggregate on the surface during the drying.
Try electrospraying. Just like electrospinning, we use nano particle dispersion, instead of polymer solution in a syringe. Connect positive electrode of a high voltage DC power supply to the syringe tip and negative electrode to the nickel film. Place syringe on a syringe pump and adjust the flow rate (0.5 ml to 2 ml per hour) increase the voltage (10-30 kV). solvent evaporate during this process and nano particles deposit on the hydrophobic surface. I am not sure about the adhesive force between particles and the surface.
Alexander:
Have you tried a surfactant? Maybe your chemistry won't allow you to do that, but if you can, give that a try. Since your nanoparticles are already in aqueous solution, you should get a kind of 'inverse micelle' around the nanoparticle which might allow the particles to settle on the surface and allow the surface to properly 'wet'.
Try a 'small' surfactant, like SDS or CTAB. These are well characterized and easy to obtain.
Alexander;
I had a problem like that with an extremely hydrophobic coal sample and I used a small petri dish to have the surface completely submerged into the water. you may use a dish a bit bigger than your sample and fill it with water till you have Ni plate sunk.
or if it is applicable to your system you may use ethanol instead of water. It spreads easier than water
Try electrospraying. Just like electrospinning, we use nano particle dispersion, instead of polymer solution in a syringe. Connect positive electrode of a high voltage DC power supply to the syringe tip and negative electrode to the nickel film. Place syringe on a syringe pump and adjust the flow rate (0.5 ml to 2 ml per hour) increase the voltage (10-30 kV). solvent evaporate during this process and nano particles deposit on the hydrophobic surface. I am not sure about the adhesive force between particles and the surface.
Alexander,
Sometimes addition of just a few drops of ethanol to water solutions (in your case dispersion) can help with wetting. Also you can try to add some other water miscible and low-boiling solvents.
Olga
Alexander,
For improving the wetting ability of solution, try to use another solvent, otherwise use the surfactant as a dopant. In our previous work, we use zonyl (from Dupon) as a surfactant.
How thick should be your film? Do you want to have single sparse nanoparticles or a complete coverage of the substrate? Would your system allow for use of surfactants and binders? Organic additives can be removed later on with oxygen plasma.
I agree with Patrick and would suggest modifying the chemistry, if at all possible. Perhaps a liquid with a lower surface tension. I have found it nearly impossible to apply a uniformly coating with a liquid that dewets the surface. One technique that is useful for depositing nanospheres on Si wafers is the tilted-drain method (attached). It may be worth trying with your situation.
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I think any droplet method with evaporation even in small diameters will cause "coffee stain" effect on surface and ring shape deposition. try to wet the surface by reducing the surface tension.
Have you considered treating your metal surface with a short chain, e.g. n=3, dithiol first? You could then absorb the gold nanoparticles from aqueous solution. If you try this let me know if it works.
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