Palladium Nanotubes with Tailored Wall Morphologies

Abstract

Nanotubes have an outstanding potential to transport or store gases and fluids for fuel cells, energy conversion, catalysis, and drug release.[1±10] Nanotube walls with an internal fine structure and thus a high specific interface area should strongly enhance the reactivity as well as the capability for adsorption. Furthermore, the efficiency of transport processes within and across the nanotube walls should be improved. We report on a simple method to generate nanotubes with controllable wall morphology or porosity in the nanometer range. It consists of three steps: the formation of multi-component nanotubes; demixing to generate coexisting phases within the tube walls; and the controlled ripening of the phase morphology. Depending on the selected components, their concentrations, and the ripening stage, nanotubes with characteristic wall morphologies were obtained. Selective removal of one component yields residual nanotubes with a specific nanoroughness and a controllable porosity. This fundamental concept should be applicable to a broad range of materials. We will demonstrate this exemplarily by means of structured palladium nanotubes, since palladium nanoparticles are of considerable interest for catalysis, [11±14] sensor technology, [15] and hydrogen storage. [16] Decomposition in thin films on smooth substrates has been investigated extensively.[17] Spinodal demixing induced either by a thermal quench or by the evaporation of a solvent generates a co-continuous phase morphology with a characteristic length. Simultaneously, ripening occurs to reduce the initially large internal interface area. The phase morphology is strongly affected by substrate/film and film/air interfaces, as well as by confinement effects.[18±22] Several groups have used thin films structured by phase separation for devices such as transistors, [23] light emitting diodes, [24] or photodiodes.[25] It would be advantageous if nanotubes with such well-defined morphologies could be obtained in a controlled way. However, to date, the range of wall morphologies accessible by preparation methods such as self-assembly [1±5] and the use of templates [6±10] is limited to core±shell structures via consecutive synthetic steps. [6,10] Our approach is based on wetting porous matrices that exhibit high surface energies with multicomponent melts or solutions containing polymers.[26] A mesoscopic film of the wetting liquid covers the pore walls rapidly, since polymers adsorb avidly on high energy surfaces.[27] A kinetically stable stationary state is generated when all adsorption sites on the pore wall are occupied. Solidification of the wetting liquid at this stage results in the preservation of nanotubes. Decomposition can be induced by solvent evaporation during wetting or by thermal quenching. Ripening occurs after the onset of phase separation, as long as the solvent concentration is sufficiently high to prevent solidification, or if the system is annealed at temperatures where at least one component is liquid. Freezing of the ripening pro...

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