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

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.