The diffusion of Au in Ag2S from the inner core to the surface of Ag2S was reported. and a new nanocomposite of core-shell Pt@Ag2S and Au nanoparticles has been derived through this diffusion phenomenon. Ostwald ripening was observed by transmission electron microscopy during the characterization of the nanocomposite. This elucidated the mechanism of formation of semiconductor-metal heterostructures as a consequence of Au diffusion in Ag2S nanocrystals.