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.
Let's first list the requirements to obtain "real" core-shell structure. These are:
1- The core particles must be well dispersed before the coating. In this regard, the large particle size, unless it causes settlement, is preferred.
2- The shell material must be nucleated on the surface of the core material only, i.e. the nucleation must be full heterogeneous. We know that heterogeneous nucleation is preferred over the homogeneous one, however, if the interfacial surface energy between the core and shell materials is high enough, as compared with the surface energy of each, the homogeneous nucleation will be preferred and, hence, the shell material will not cover the core particles. If the thermodynamic data are not available for the system, experimental trials are needed. However, the use of finer core particles will enhance the heterogeneous nucleation and this will be in conflict with the "well dispersion" requirement. Also, treating the surface of the core with the suitable agents is another option.
3- The amount of the shell material must be sufficient to totally cover the surface of the core material. This can be approximately calculated using:
mass of the shell = theoretical density of shell material * the required thickness * the total surface area of the core materiel.
4- The thickness of the shell must be, as much as possible, uniform and even for all the particles. The solution treatment for prolonged time will be helpful for this purpose.
5- The obtained core-shell particles must be well dispersed. Additives may be required for the same.
Thank you for your replies. Does a susccesful coating really only depends on the surface energy of the materials? I was also looking at crystallographic database looking for materials with the same crystal structure.
Also, I would like to discuss the chemistry of the surfactants. I assume (correct me if I'm wrong) that an added surfactant to stabilize the suspension is located on the surface of the core material and hinders the nucleation of a shell. Except if the surfactant has a bifunctional character which coordinates on the core surface and also selectivly links to the shell to be formed.
For a start I was thinking to neclect the sol stabilization (because I already have a lot of experimantal parameters) and try to prevent the settling of the particles by stirring. The problem I see here is a higher grade of agglomeration as soon as the stirring stops.
Anyway, this is all theory. From all the papers that I have read during the past weeks I did not find one with SEM or TEM images proving that they have synthesized more than a few particles. At least this is what most authors are tending to do, showing one ore two particles (or even only the edge of one particle with high magnifaction). Does anybody have a recommandation here? Also, most papers related to core-shell are with nano-sized materials. Coating of commerically available microparticles often involve dispersing the particles in a coating sol and heating until the solvent is gone. These coating usually don't look homogenous...
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