Lanthanum nickel alloy catalyzed growth of nitrogen-doped carbon nanotubes by chemical vapor deposition

Abstract

Nitrogen-doped carbon nanotubes (N-doped CNTs) were synthesized by chemical vapor deposition using lanthanum nickel (LaNi₅) alloy as a catalyst and an acetylene–ammonia mixture as the carbon/nitrogen (C/N) precursor. The effects of experimental parameters such as temperature and time on the structure and yield of N-doped CNTs were studied. Transmission electron microscopy studies showed that with the increase of growth temperature and time, the C/N solubility, outer diameter and internal compartments of N-doped CNTs were increased progressively. The optimal conditions for the synthesis of N-doped CNTs were found to be 900 °C and 20 min. The elemental mapping of the catalyst tip confirmed that an intermetallic compound of lanthanum and nickel governs the growth of N-doped CNTs. X-ray photoelectron spectra revealed that the N content of CNTs varied between 3.5 and 6.9 at.% upon changing the growth temperature. Confocal Raman spectroscopy analysis showed the degree of graphitization dependence on the N doping level of CNTs. The growth of N-doped CNTs through the LaNi₅ alloy catalysis was discussed on the basis of both surface and bulk diffusion mechanisms. Finally, the effect of acid treatment on the N-content and electronic structure of as-synthesized samples was investigated. These acid treated N-doped CNTs are expected to be more suitable for electrochemical applications, such as supercapacitors and the oxygen reduction reaction in fuel cells.