Encapsulation of copper and zinc oxide nanoparticles inside small diameter carbon nanotubes

• CuO and ZnO deposited in CNTs of 6–7 nm i.d. by ultrasound-assisted impregnation.
• CNT to be functionalized by thermal shock in air rather than by HNO3.
• Selectivity for intra-CNT location improved by selective washing procedure.
• Reduction of intra-CNT CuO gives polycrystalline Cu NPs with limited accessibility.
• Cu/ZnO aggregates intended to mimic the methanol synthesis catalyst prepared in CNTs.

Copper and zinc oxide nanoparticles have been reproducibly deposited into carbon nanotubes (CNT) of 6–7 nm internal diameter via simple impregnation techniques with different metal salts followed by thermal decomposition of the precursors and reduction in H2 in case of Cu. Oxygen functionalization via a gas-phase method involving thermal shocks was a critical step while traditional functionalization with nitric acid resulted in failures. Intra-CNT location of CuO particles could be proven by STEM images, and was examined by TEM for materials prepared by various routes. It was found that Cu and Zn oxide nanoparticles could be deposited throughout the whole interior CNT space. The filling capacity depended on the preparation conditions, on conditions of subsequent precursor decomposition, and on the inner diameter of the CNTs. After the reduction of the CuO nanoparticles, XRD, XAFS, and N2O reactive frontal chromatography indicated a bimodal particle size distribution due to the presence of agglomerates outside the CNTs. To enhance selectivity for endohedral location, a washing step with HNO3 with the inner CNT space blocked by xylene was applied to selectively remove aggregates in the outer space. Based on the best procedures for introduction of CuO and ZnO, a bimetallic CuZnO@CNT sample was prepared via a consecutive preparation route.

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