On the formation mechanism of metal nanoparticle nanotubes

We have previously described nanoparticle nanotubes (NPNTs), i.e., tubular metallic nanostructures comprising coalesced nanoparticles (NPs), obtained by passing citrate-stabilized metal (Au, Ag, Pd) NP solutions through aminosilane-modified nanoporous alumina membranes. Here we show that the mechanism of NPNT formation involves two stages: (i) electrostatic binding of a monolayer of metal NPs to the amine groups on the membrane pore walls; and (ii) accumulation of NP multilayers and room-temperature coalescence to form solid nanotubes. Free-standing NPNTs are obtained by post-dissolution of the membrane template. An improved fabrication apparatus enabled evaluation of the role of drying and other preparation parameters on the NP coalescence and NPNT formation and structure. Intermittent drying during the NP accumulation stage is necessary for the formation of solid NPNTs, while a slow flow rate of the colloid solution through the membrane pores and reversal of the flow direction promote the formation of more uniform and longer NPNTs.