Regulated oxidation of nickel in multisegmented nickel-platinum nanowires: an entry to wavy nanopeapods.

Oxidation of solid metal nanoobjects is a versatile approach to generating hollow metal oxide nanostructures. The mechanism for the solid-to-hollow conversions has been attributed to the Kirkendall effect, which describes an unbalanced interdiffusion of a thermal diffusion couple. When a metal nanoobject is exposed to oxygen at elevated temperatures, the outward diffusion of the metal cations is much faster than the inward diffusion of the oxygen anions through the oxide layer. As a result, a flux of vacancies is injected into the interior and gradually accumulates into a single void. The hollow metal oxide nanostructures formed in most cases are symmetric with a uniform wall thickness. The oxidation behavior of some metal elements is exceptional. For example, the complete oxidation of Ni nanoparticles in air resulted in hollow NiO nanospheres with an off-centered cavity. Similarly, irregular nanotubes with a fluctuant wall thickness were obtained by oxidizing Ni nanowires at high temperatures. 6a] It is verified that the sufficient mobility of vacancies in Ni induces the localization of limited void nuclei at random positions, which disequilibrate the following outward mass transfer of unconsumed Ni cores. The difficulty in forming homogeneous hollow NiO nanostructures hints that the rapid self-diffusion of injected vacancies in the Ni cores is a rather uncontrolled process. To date, it is very difficult only by oxidation to convert solid Ni nanoobjects into hollow NiO nanostructures of high uniformity. However, morphological regularity is of extreme importance for the reliable and reproducible applications of NiO nanostructures in fields ranging from resistive switching memory to sensors. Herein, we report a novel route to fabricate highly uniform wavy Pt/NiO hollow nanopeapods by regulating the oxidation of Ni in predesigned multisegmented Ni/Pt nanowires. It is revealed that the Ni/Pt interface is an efficient platform for manipulating the oxidation behavior of Ni that intrinsically presents a random manner. Moreover, this interface-tailored low-temperature oxidation strategy can be employed as a unique but general entry to novel wavy metal/ oxide nanopeapods relative to the approaches existent for nanopeapod construction. The multisegmented Ni/Pt nanowires with tailored parameters were prepared in a three-electrode electrochemical cell by pulsed electrodeposition using porous anodic aluminum oxide membranes as templates (see the Supporting Information). After the removal of the alumina templates by alkaline etching, dispersed straight nanowires with a smooth surface were obtained (Figure 1a). A magnified SEM view (Figure 1b) displays that the nanowires consist of