Metal oxide and silicate nanotubes: synthesis and hydrogen storage applications

Abstract

In the past two decades, an appreciation of the extraordinary properties of nanotubular materials has led to the discovery and investigation of many different nanotubes. A wide variety of nanotubes can be synthesised using scalable hydrothermal techniques, but understanding of the synthesis mechanisms is often limited. This research is concerned with manipulating the synthesis conditions of metal oxide and silicate nanotubes in order to improve understanding of the underlying synthesis mechanism, and investigating the properties of the nanotubes as hydrogen storage materials.

This thesis presents experimental results for the syntheses of aluminium silicate, nickel silicate and vanadium oxide multiwalled nanotubes under controlled hydrothermal conditions. A novel synthesis method at 220ºC, pH2 was developed for Al2Si2O5(OH)4 nanotubes through substitution of a mole fraction of SiO2 with GeO2 in the precursor SiO2 + Al(OH)3 suspension. An ideal Ni/Si molar ratio of 1.5 was demonstrated in the synthesis of Ni3Si2O5(OH)4 nanotubes at 195ºC. It was shown that increasing the concentration of NaOH widens the length distribution and increases the average length of the nanotubes. Variable temperature experiments with vanadium oxide nanostructures revealed a low temperature route for the synthesis of flexible elongated VOx nanosheets under reflux (90ºC) in an ethylenediamine-water mixture. The hydrothermal experiments revealed important details about the nanotube formation mechanisms, including the scrolling mode of nanosheets into nanotubes, which occurs in a specific crystallographic direction relative to the nanosheet growth axis.

Subsequent investigations into the room temperature stability of Al2Si2O5(OH)4 nanotubes under aqueous acidic and alkaline conditions revealed significant dissolution within 10 days in 1 mol dm-3 NaOH, H2SO4 and HCl solutions, initiated at the inner surface. The effect of acid or alkali concentration on the initial dissolution rate was measured, and the dissolution mechanism discussed. Acid treatment was shown to be an effective method for increasing nanotube surface area.

Investigations into the hydrogen adsorption properties of metal oxide nanotubes revealed weak adsorption of hydrogen at 77–298 K up to 150 bars (15MPa) pressure. Temperature-corrected adsorption isotherms for adsorbing H2TinO2n+1 (titanate), Ge-Al2GeO3(OH)4 and Ni3Si2O5(OH)4 nanotubes were compared with M3[Fe(CN)6]2 Prussian-blue analogues, and dimensions of the adsorbed hydrogen layer were derived using the Langmuir-Freundlich model.

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