Gas phase synthesis and field emission properties of 3D aligned double walled carbon nanotube/anatase hybrid architectures

Abstract

A 3D hybrid architecture composed of macroscopic, vertically aligned CNT blocks which are formed via a metal catalyzed CVD process followed by deposition of TiO₂ on the CNT side walls in nanocrystalline or amorphous form is presented. The morphology of the deposited TiO₂ can be tailored by the deposition method employed. Depositing TiO₂ from the gas phase by employing the organometallic precursor Ti[OCH(CH₃)₂]₄ leads to formation of nanocrystalline anatase or rutile particles with a dense coverage on the surface and within the 3D CNT scaffold. Phase pure TiO₂ (anatase) is formed between 500 and 700 °C, while higher temperatures resulted in rutile modification of TiO₂. Below 500 °C, TiO₂ forms an amorphous oxide layer. At higher temperatures such initially formed TiO₂ layers segregate into particles which tend to crystallize. In contrast, when generating TiO₂ by oxidation of Ti metal which is deposited by vaporization onto the 3D CNT block array, and subsequently oxidized in air or controlled O₂ atmosphere this leads to a porous layer with a particular nanostructure on top of the CNT blocks. First studies of the fabrication and field emission of the new 3D CNT/TiO₂ hybrid cathodes display good and stable FE characteristics with onset fields for current density of 1 μA cm⁻² of 1.7 to 1.9 V μm⁻¹, while the average field enhancement factor is in the range between 2000 and 2500 depending on the O₂ base pressure during the measurements.