Controllable growth of highly N-doped carbon nanotubes from imidazole: a structural, spectroscopic and field emission study

Abstract

Here we report the highest N-doping level achieved so far for carbon nitride (CNx) nanotubes by using a new precursor imidazole (as a dual supplier of carbon and nitrogen) together with ferrocene (as a catalyst). A controllable growth of aligned CNx nanotubes was achieved by simple CVD in a temperature range of 700–950 °C leading to nitrogen doping from 12.1 to 25.7 at%. The highest doping level was achieved at 850 °C, which is attributed to the abundance of C–N fragments at this decomposition temperature. Nitrogen doping level and the type of N-moieties were determined by XPS and EELS analyses. HRTEM analysis suggests that N-substituted graphitic units (gN) give rise to fullerenic curvature into the cylindrical graphite layers (leading to curled side-wall morphology), whereas pyridinic units (pN) break the graphitic continuity (leading to dangling bonds on the side walls). It is observed that the abundance of N-substituted graphitic units (gN) enhances the electrical conductivity of individual nanotubes by donating additional electrons to the CNT network. The field emission (FE) characteristics show that the increase of N doping from 19.6 at% to 25.7 at% effectively reduces the turn-on field (for 10 µA cm⁻²) from 1.92 V µm⁻¹ to 0.88 V µm⁻¹, whereas the threshold field (for 10 mA cm⁻²) decreases from 4.05 V µm⁻¹ to 2.32 V µm⁻¹. However, at high growth temperature (950 °C), the N-doping level decreases to 12.1 at%, thereby degrading the FE performance.