Superior electrochemical performance of CN_x nanotubes using TiSi₂ buffer layer on Si substrates

On-chip growth of vertically aligned nitrogen-containing carbon nanotube (C Nx NT) arrays was demonstrated. The nanotubes were grown by microwave plasma-enhanced chemical-vapor deposition on different types of silicon substrates (n,p, n+, p+) using a few nanometer thick Fe layer as a catalyst and a Ti buffer layer. The effects of the Ti thickness on the electrochemical (EC) characteristics of the C Nx NT arrays were studied. It was found that for a Ti thickness of 20 nm, while vertically aligned C Nx NTs were produced on all Si substrates, an almost ideal Nerstian behavior was observed only on highly conductive n+ and p+ substrates. As the Ti buffer thickness increased to 200 nm, good electrical contacts were established at the bottom end of the C Nx NTs and fast electron kinetics were then attainable on all kinds of Si substrates. Nevertheless, the use of thick buffer layers inhibited directional growth. Oxidation treatment of the catalyst Fe layer prior to nanotube growth proved efficient for achieving directional C Nx NT formation. Pretreatment of the Ti buffer layer at a temperature of 800°C, leading to the formation of Ti Si2, was appropriate for achieving simultaneously enhanced current density and fast electron kinetics comparable to those of C Nx NTs on bulk Ti electrodes. The Si-based micro-EC platform established in this work has superior current collection efficiency and is amenable for fundamental EC studies and energy applications.