A method of forming bimetallic core-shell metal nanoparticles including a core of a first metal material and a shell of a second metal material, the method including photochemically producing metallic nanoparticle cores of the first metal material, and forming a shell of the second metal material around the cores. The shell can be formed by adding shell-forming precursor materials to a solution or suspension of the cores and photochemically forming the shells around the cores, or by separately photochemically producing metallic nanoparticles of the second metal material and mixing the metallic nanoparticles of the second metal material and the metallic nanoparticle cores to cause the metallic nanoparticles of the second metal material to form a shell around the metallic nanoparticle cores.