Template Synthesis of Nanotubes by Room-Temperature Coalescence of Metal Nanoparticles

Metal nanoparticle nanotubes (NPNTs) have been introduced by us as a new class of template-synthesized, nanoparticle-based nanotubes possessing unique features such as room-temperature preparation, highly corrugated wall structure, electrical conductivity, mechanical stability, and defined optical absorbance. The nanotubes are prepared by passing a citrate-stabilized metal (Au, Ag) colloid solution through the pores of an aminosilane-modified nanoporous alumina membrane. The nanoparticles (NPs) aggregate, forming multilayers on the pore walls, and undergo spontaneous room-temperature coalescence to afford solid, porous, multiwall metallic nanotubes. Self-sustained NPNTs are obtained by membrane dissolution. It is shown that the nanotubes are formed in two stages, i.e., NP accumulation and initial coalescence in the wet stage, and final solidification upon drying, both crucial to their formation. The NPNT synthetic scheme is extended here to the construction of composite NPNTs, i.e., formation of bimetallic Au−Pd NPNTs using a mixed colloid solution. High-resolution transmission electron microscopy (HRTEM) of single-metal and composite NPNTs indicates actual coalescence and creation of metallic interfaces between individual NPs, with lattice continuation that extends into the NP bulk.