Selective deposition of metal nanoparticles inside or outside carbon nanotubes

Selective, localized deposition of metal nanoparticles inside or outside carbon nanotubes (CNTs) without any asymmetric functionalization is presented. The method is only based on the difference of surface tension of organic and aqueous solutions. Because of its lower surface tension (20 to 30 mN.m-1) compared to water (72 mN.m-1), the organic solvents better wets the surface of nanotubes and penetrates into its channel by capillarity. The precise control of the volume of each phase allows to fill the CNT with the organic phase and to cover its outer surface with the aqueous one. Hence, metal nanoparticles can be put either inside or outside the CNT just by choosing in which solvent to dissolve the metal precursor. SEM, TEM and 3D-TEM investigations show that selectivities between 75 and 85% can be reached with this technique. The nanoparticles are homogeneously dispersed and present a relatively narrow size distribution, centered on 5 nm. The strength of this method lies in that it can be used on the same batch of CNTs. It allows for the first time to selectively decorate either the inner or the outer surface of (i) opened CNTs, (ii) with the same diameter, (iii) without any other further step of functionalization than the classical treatment with nitric acid, i.e. both the inner and the outer walls present the same surface chemistry. This novel synthesis method allows accurate investigations of catalytic reactions carried out in a confined system, as well as the synthesis of new CNT-based materials