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Abstract
Double-walled oxide nanotube structures are interesting for a wide range of applications, from photocatalysis to drug delivery. In this work, a progressive oxidation method to fabricate double-walled nanotube structures is reported in detail. The approach is based on the electrodeposition of metallic iron nanowires, in porous alumina templates, followed by a selective chemical etching, nanoscale Kirkendall effect, a fast oxidation and out-diffusion of the metallic core structure during thermal annealing. To validate the formation mechanism of such core-shell structure, chemical composition and atomic structure were assessed. The resulting hematite nanotubes have a high degree of uniformity, along several microns, and a nanoscopic double-walled structure.
Highlights
Iron oxides have attracted much interest for their potential application in nanotechnology[1,2,3,4]
Crystalline Fe NWs were grown inside porous anodic alumina (PAA) templates into pores of 35 nm diameter, typical from oxalic acid self-organized regime anodization (Fig. 1a)
Cross-section scanning electron microscopy (SEM) images revealed no defects along the PAA template and each wire was continuous and homogeneous (Fig. 1b)
Summary
Iron oxides have attracted much interest for their potential application in nanotechnology[1,2,3,4]. Quantum confinement in very thin nanostructures, such as nanotubes with extremely thin walls, can be used to tune the band edges and band gap of a semiconductor for photoelectrochemical applications[19] In this way, template-assisted methods are advantageously providing a highly flexible and controlled template that shapes the desired nanoparticle to the intended dimensions. A novel and scalable fabrication method of crystalline α-Fe2O3 double-walled NTs grown on porous anodic alumina (PAA) templates with controllable dimensions is here reported. 1D Fe nanostructures grown inside PAA templates were converted into the desired α-Fe2O3 phase with double walled NT morphology by combining a controlled selective chemical etching and the Kirkendall effect through an annealing route. The key role of the Kirkendall effect, to obtain double-walled nanostructures instead of reported single-walled, is here addressed
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