Controlled Uptake and Release of Metal Cations by Vanadium Oxide Nanotubes

Abstract

Vanadium oxide nanotubes (Cn-VOx-NTs) contain α-monoamines (CnH2n+1NH2 with 4≤n≤22) as templates intercalated between crystalline VOx layers comprising multilayer tube walls. The present study reveals that a large proportion of the amines can easily be exchanged by metal cations. The tubular morphology is not affected by this reaction, but the distance between the VOx layers, i.e., 2.8 nm in C12-VOxNTs, decreases in the reaction product to 0.9 – 1.2 nm, depending on the metal salt actually applied. Alkali (Na+, K+), alkaline-earth (Mg2+, Ca2+, Sr2+), and transition-metal salts (Fe2+, Co2+, Ni2+, Cu2+) have successfully been intercalated. This reaction is partly reversible since intercalated sodium cations can be resubstituted by dodecylamine. This exchange produces again C12-VOx-NTs with the original inter-layer spacing. However, this release is successful only when sodium is complexed by a crown ether. Under these reaction conditions, even a cyclic uptake and release of Na+ and amine, respectively, accompanied by a corresponding shrinkage and widening of the inter-layer distance, is observed while the tubular structure is widely preserved. Furthermore, a distinct selectivity of the metal-cation exchange has been observed.