The creation of Core-Shell Nanoparticles is not that simple as it always look like.

First of all you should say something about, how you try it and what kind of result you get.

Additionally you should also keep in mind, that not every system is able to form bimetallic structures. Based on the Educts (Salts and Surfactants + Solvent) you use, as well as the physical properties: surface tension of the metals and mixibility, you can produce bimetallic, so called janus or two seperate particles.

Finally if you have produced bimetallic nanoparticles once, they might be not able to stay bimetallic. Oxidation and surface effect can lead to segregation effect changing the properties