Controlled uptake and release of metal cations by vanadium oxide nanotubes

Vanadium oxide nanotubes (C-n-VOx-NTs) contain alpha-monoamines (CnH2n+1NH2 with 4 less than or equal to n less than or equal to 22) as templates inte rcalated between crystalline VOx layers comprising multilayer tube wails. T he present study reveals that a large proportion of the amines can easily b e exchanged by metal cations. The tubular morphology is not affected by thi s reaction, but the distance between the VOx layers, i.e.,2.8 nm in C-12-VO xNTs, decreases in the reaction product to 0.9-1.2 nm, depending on the met al salt actually applied. Alkali (Na+, K+), alkaline-earth (Mg2+, Ca2+, Sr2 +), and transition-metal salts (Fe2+, Co2+, Ni2+, Cu2+) have successfully b een intercalated. This reaction is partly reversible since intercalated sod ium cations can be resubstituted by dodecylamine. This exchange produces ag ain C-12-VOx-NTs with the original inter-layer spacing. However, this relea se is successful only when sodium is complexed by a crown ether. Under thes e reaction conditions, even a cyclic uptake and release of Na+ and amine, r espectively, accompanied by a corresponding shrinkage and widening of the i nterlayer distance, is observed while the tubular structure is widely prese rved. Furthermore, a distinct selectivity of the metal-cation exchange has been observed.