Catalyst nanoparticles*

Hello to everybody,

we are synthesizing carbon nanotubes, and our catalyst is a mixture of Fe/Mo
supported by Al nanoparticles (we do this by mixing the Fe and Mo compounds
with the Al nanoparticles in a methanol solution). We place then a microdrop
of the liquid over a quartz surface, we leave it at 170 C degrees for 5' in
order to dry the solvent and then we run the CVD process.

To avoid the formation of aggregates of catalyst and to have 'real'
nanoparticles of catalyst over all the quartz surface, we leave the
Fe-Mo-Al-methanol catalyst solution to 24 hours of magnetic rinse and then
to 2 hours of ultrasounds, just before placing the microdrop of the catalyst
mixture over the quartz surface. Despite doing this, we find some areas
where we have strong aggregates of catalyst. Does anybody have any idea in
order to avoid the formation of these aggregates of catalyst?

Many thanks in advance

Are you spin-casting the catalyst solution onto the quartz substrate
surface? High shear always helps.

Three thoughts come to mind.

1) Add a very small concentration of ionic polymer to your
particulates dispersed in solvent. If the polymer adsorbs onto the
particles, the surface charge will prevent aggregation. Sonicate, add
polymer solution, sonicate. Obviously try both cationic and anionic
polymers to crate a Helmholtz layer. Nitrate or ammonium counterions
burn away cleanly. Chloride (halide) may cause metal volatility
problems. Alkali or alkaline earth cations may dope the catalyst
centers. This maybe good or bad.

Even oleic acid or sodium oleate may do it - dispersal of rod-milled
magnetite into colloidal ferrofluids.

2) Add detergent or surfactant at just barely more than its
critical micellar concentration (CMC) at temp. That wll make the
smallest micelles possible. There are cationic, anionic, and
non-ionic detergents. Gemini surfactants have remarkably low CMNCs.
Below the CMC you will have simple adsorption, as above.

3) Make up a batch (you may scale it down!) of one lb KOH pellets
dissolved in 4 gallons of 95% ethanol (preferably the biological stuff
denatured with isopropanol, not the standard stuff denatured with
kerosene, butanol, and whatnot). Store in a covered polyolefin
plastic 5 gallon pail (NEVER glass or any polyester plastic). Rubber
gloves and goggles! The bath eats through skin and destroys eyes on
contact.

Soak the quartz overnight at room temp. Remove and immediately plunge
into distilled water. Rinse while always kept immersed, then remove,
shake, immediately transfer to methanol, and keep immersed. That will
give you a very clean, very wettable surface to encourage spreading.
The surface will be alkaline. If you add a quick soak in 1 M HCl then
distilled water wash before changing solvents, the surface will be
neutral.

Not together, in separate experiments. If you mix cationic and
anionic syndets, you get precipitation. If you do it sequentially,
you may get layering aroud the metal particles.

Many thanks for the suggestions. We'll check them.

Are you spin-casting the catalyst solution onto the quartz substrate surface? High shear always helps.

We thought about using spin coating, but our solution is extremely
non-viscous, so we think it wouldn't help so much.

Thicken it with a soluble polymer (viscous during shear) or maybe 1%
fumed silica plus sonication to fully disperse (not viscous during
shear - though the time evolution of non_newtonian viscosity will be
interesting).

Adding fumed silia is a good experiment in any case. Did you ever
plant a flowerbed of very small seeds (e.g., Amaranthus)? You mix the
seeds with fine sand to get easy good dispersal.

First you need to disperse your particles better. With metal
nanoparticles, using aqueous solutions is very difficult even with
some of the best ammonium polymethacyrlate type dispersants or
electrostatic methods. Your organic solvents should work well though.
If you want to stick with methanol try some of the traditional powder
processing dispersants (ie menhaden fish oil or hypermer KD1 (made by
ICI surfactants)). You should only need less than a weight % to make
a significant improvement. Disolve your dispersant (polymer) in
solution, add your nanoparticles and disperse with an ultrasonic
probe. I have had good success dispersing carbon nanotubes in
toluene, etoh, and MEK with the fish oil in this manner. Remember if
you have good dispersion, you will not be able to see particles or
colloids with the eye. The solubility of the fish oil is improved
with about 20-40%xylene in meoh or etoh allowing the polymer chains to
stretch out and improve the steric barrier.

To thicken your solution for spin casting add some ethyl cellulose
(49% + ethoxyl content) until desired consistency is achieved
(approximately 10wieght% yields the viscosity of maple syrup).
Polyvinylbutyral works well too, but you might need a bit more to
reach equivalent viscosity of the ethyl cellulose. Further, if
absolutely clean burnout is required, use polypropylene carbonate
(trade name QPAC-40), this is the absolutely cleanest binder for
burnout which decomposes in air, vacuum, or inert. Of course you will
need to work with acetone or MEK solutions for decent solubility which
should also disperse your particles with the fish oil.

Yet one more final option is too use a higher viscosity solvent.
Alpha terineol is wonderful with a little oleic acid for the
dispersant. The alpha terpineol dries very slow, but levels
beautifully, and is quite non-toxic compared to other solvents. If
you need more body for spin coating, a % or so of ethyl cellulose will
do the trick.

Good luck

One possible hi-tech solution is to induce photofragmentation though a
high-power visible light laser. It really works !

If you elect to follow Ceraboy's recommendation and would like to evaluate our clean-burning QPAC-40 binder, please visit www.empowermaterials.com and send us a request. We believe our material is extremely well suited to nanoparticle applications. Thanks to Ceraboy for the mention.

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Shape of nanoparticle is more important while developing nanopartilce as catalyst.It's necessary to estimate it's surface area.Some chemical preparation and physical methods are used for growing several mettalic and binary alloy NP catalyst.Growth and catalytic reaction for synthesis of 1D nanostructure such as Zno nanowire and multiwall carbon nanotubes.