A facile route to Janus nanorods via redox-assisted ripening

We demonstrate that ripening can turn highly anisotropic Ag nanorods into Janus nanorods (lollipops), and eventually spherical nanoparticles. The divergent etching and growth on the Ag nanorods, and the morphology-specific ripening, are explained by expanding the definition of ripening. Using a silica shell as the mask and a marker, we study the dependence of the ripening on various reaction parameters. Oxygen, hydroquinone, and Ag+ ions are found to promote the ripening, judging from the extent of etching on one side of the nanorod. Basically, the redox chemistry provides an additional driving force for the oxidative etching as well as the reductive deposition, promoting the ripening. We believe that the underlying principle of this redox-assisted ripening could be generally applied to other systems.