In one aspect, hollow nanoparticles are described herein. In some embodiments, a hollow nanoparticle comprises a metal shell and a cavity substantially defined by the shell, wherein the shell has a thickness greater than or equal to about 5 nm and the cavity has a curved surface. In another aspect, methods of making hollow nanoparticles are described herein. In some embodiments, a method of making hollow nanoparticles comprises forming a plurality of gas bubbles and forming a shell on the surface of at least one of the plurality of gas bubbles, wherein at least one of the gas bubbles is electrochemically generated. In another aspect, composite particles are described herein. In some embodiments, a composite particle comprises at least one nanoparticle and a polycrystalline metal shell substantially encapsulating at least one nanoparticle, wherein at least one surface of at least one nanoparticle is not in contact with the shell.
