Electrically conducting "Nanoparticle nanotubes" (NPNTs), the combination of nanotube geometry with nanoparticle morphology, are prepared by the assembly of gold or silver nanoparticles on the pore walls of a silane-treated nanoporous aluminamembrane template. Self-sustained NPNTs can be obtained by dissolving the template. For more information, see the Communication by Rubinstein and co-workers on the following pages.

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.

A novel scheme is presented for the synthesis of graded materials by electrodeposition in porous insulating templates. Lateral control of copper electrodeposition in nanoporous alumina membranes is achieved by application of a lateral potential gradient on a thin Au film evaporated on the membrane, used as the cathode. Formation of metal gradients in the membranes is shown to occur under conditions where essentially no gradient is formed on similar bare electrodes. This is attributed to the permanent resistivity of the thin Au film between the pores, which does not disappear upon Cu deposition, allowing a potential gradient to be maintained. Formation of a copper gradient in porous alumina membranes by uniform deposition followed by gradient dissolution is also demonstrated. These results establish the feasibility of controlled electrodeposition and gradient formation in nanoporous insulating templates.

Lateral control of template synthesis in nanoporous alumina membranes (NAMs) was previously shown by us to enable preparation of graded composite materials. Formation of thickness gradients of Cu was demonstrated using electrodeposition (or electrodissolution) of Cu in the NAM template under a lateral voltage drop applied to the working electrode. This approach is extended here to the formation of compositional gradients. The latter are achieved by electrochemical co‐deposition of two metals (Au and Pd) in the membrane pores from a mixed metal‐ion solution under a lateral potential drop, to form an alloy that shows a continuous lateral change of the Au/Pd ratio. Environmental scanning electron microscopy images of cross sections along the line of the applied voltage gradient show that the deposit height changes gradually, while local elemental analysis by energy dispersive spectrometry and X‐ray diffraction measurements confirm a continuous change of the alloy composition along the membrane matrix.

Goldige Röhrchen: Bei der Herstellung neuartiger stromleitender Röhren aus Goldnanopartikeln dienen die Poren von oberflächenmodifizierten nanoporösen Aluminiumoxidmembranen als Template. Die Nanopartikel lagern sich unter Flussbedingungen an die Porenwände an und aggregieren dort selbstorganisiert. Durch Auflösen der Membran erhält man die freien, formstabilen Nanoröhren (siehe Bild).