I am looking for a (preferably) wet-chemical method like Sol-Gel, solvothermal or precipitation to coat a commercially available metal oxide powder (e.g. LiCoO2) with another metal oxide (e.g. LiMnO2, ...). There have been several articles about this topic, but almost every SEM image that I have seen so far shows only incomplete coatings (larger chunks or small particles on the surface or agglomerated composites). I want to apply a homogeneous coating with a thickness between 10-100 nm resulting in “real” Core-Shell structures.
The next challenge: The powders usually come with a particle size distribution of about 2-20 micrometers (mostly spherical morphology) and therefore cannot be dispersed in common solvents without additional surfactants. The most straight-forward way would be to reduce the particle size by milling, disperse the particles with suitable surfactants aiming for a stable sol followed by sol-gel coating. For practical reasons I would like to forego the additional step of milling if possible.
My questions:
1) Do you know any methods to generate core-shell particles starting with particles with a large particle size distribution (2-20 µm)? Are there coating methods which do not involve stable dispersions of microparticles (maybe mechanosynthesis)?
2) If not, can anybody recommend a synthesis approach to get uniformly coated particles starting with smaller particle sizes (0.5-2 µm)?
Further remarks:
1) I don’t have much use for SiO2, Al2O3 or TiO2 coatings (these are the most common sol gel coatings)
2) I know that it would be easier to start a core-shell synthesis from solution (e.g. by co-precipation), but this is another topic.
3) I assume, the best results could be accomplished by ALD, but this is also another topic.
4) For now, I do not care about the final particle size distribution, so nano particle synthesis could be an option.
What about the auto-compustion, you can dissolve lithium nitrate, manganese nitrate, polyethylene glycol 400 (PEG 400) in ethanol. The clear solution should be poured over the LiCoO2 microparticle with vigorous stirring. Then gradually increase the mixture temperature to 80c to evaporate all ethanol (solvent), the remaing nitrate salt will be dissolved in PEG 400 as a viscous liquid which can uniformally coating your microparticle. Finally when the temperature increase to 300c, auto-igination will occour and all nitate will convert into oxides with advantage of nanopores formation. (Caution, the temperature of auto-combustion is very high around 900c )
...mmm If you could have access to a BeneQ TFS 200 ALD reactor .
@ Ali: I also thought about the auto-combustion route. I only did that once and from what I can remember I got a pretty porous material. I wonder if I can get a uniform coating via this route. Apart from that, I really liked your idea with two different solvents and the transfer of the salts between them. Have you any journal article related to this synthesis or is this from your own lab experience?
@Franklin: Yes ALD makes it a lot easier and almost every "pretty" SEM image of core-shell particles is done by this method and not by a wet-chemical route...
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