Regulated Oxidation of Nickel in Multisegmented Nickel–Platinum Nanowires: An Entry to Wavy Nanopeapods

Abstract

Oxidation of solid metal nanoobjects is a versatile approach to generating hollow metal oxide nanostructures.[1] 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. [2, 3] 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.[4] 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. [1e, 5] Similarly, irregular nanotubes with a fluctuant wall thickness were obtained by oxidizing Ni nanowires at high temperatures. [1f, 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. [5, 6] 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. [7] 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. [8] 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). [8a, 9] After the removal of the alumina templates by alkaline etching, dispersed straight nanowires with a smooth surface were obtained (Figure 1 a). A magnified SEM view (Figure 1 b) displays that the nanowires consist of periodically stacked segments with different contrasts, which is indicative of a heterogeneous multisegmented structure. From a typical STEM image of a single nanowire and corresponding energy-dispersive X-ray (EDX) spectroscopic element mappings of Ni and Pt (Figure 1 c), it is concluded that...

Highly Ordered Hollow Oxide Nanostructures: The Kirkendall Effect at the Nanoscale

“…Since the surface diffusion coefficient is several orders of magnitude higher than its bulk counterpart, the oxidation process becomes faster. Similarly to most of works reported in literature,9, 13–15 the increase of the annealing temperature was found to enhance the reaction kinetics while preserving unchanged the fundamental mechanisms leading to the formation of hollow nanostructures (Figure S2).…”

“…Two years after the discovery of hollow oxide and sulfide nanoparticles,6 the concept of nanoscale Kirkendall effect was also demonstrated with 1D tubular nanostructures 7. Since then, this method has been adopted and applied by other groups to fabricate hollow nanostructures of different shapes and compositions including metal oxides,8–16 selenides,17 germanium,18 sulfides,19 and metals 20. However, it should be noted that most of the hollow nanostructures explored so far have been fabricated without any organization or periodic order.…”

“…However, it should be noted that most of the hollow nanostructures explored so far have been fabricated without any organization or periodic order. For example, oxide nanotubes were produced by oxidizing randomly dispersed metal nanowires prepared by electrodeposition in porous membranes 9, 12–15. Removing the porous membrane and dispersing the nanowires are serious issues which should be considered before the thermal oxidation since the vapor transport of oxygen to each single wire should not be hindered by the presence of the membrane or the agglomeration of wires.…”

“…This was confirmed after 2 min of thermal oxidation, where the increase of the size of the copper grains was observed (Figure 5b). In case of the field assisted diffusion mechanism, which is known to be dominant at low temperature, the electrons of the oxygen atoms adsorbed on the oxide shell and the ones of the copper atoms present within the inner part of the wire attempt to establish an equilibrium by tunneling via the oxide layer 13–15, 23. As a consequence, an electrical field is generated in the oxide layer, which enhances the diffusion of the metal and oxygen ions through the oxide shell.…”

Novel Three‐Dimensional Nanoporous Alumina as a Template for Hierarchical TiO2 Nanotube Arrays

“…Porous anodic aluminium oxide (AAO), made by electrochemical anodization of aluminium, has been extensively investigated in the past two decades for its widespread applications in the field of nanoscience and nanotechnology. Besides its use as photonic crystals,1, 2 sensors,3 bio‐separators4 and superhydrophobic supports,5, 6 porous AAO is very likely the most commonly used hard template, which allows fabrication of a broad spectrum of 1D nanostructures 7–20. In general, the anodization of aluminium has to be carried out under relatively mild conditions with a chemically or electrochemically polished mirror‐finished Al surface.…”