Facile and controllable route for nitrogen doping of carbon nanotubes composite membranes by catalyst-free chemical vapour deposition

Abstract

In this study, we present a facile approach that makes it possible to finely and selectively dope with nitrogen the structure of carbon nanotubes (CNTs) composite membranes. CNTs were grown inside nanoporous anodic alumina membranes (NAAMs) by a catalyst-free chemical vapour deposition (CVD) approach using different precursor mixtures containing carbon and nitrogen at different ratios. Different precursors were used to fabricate undoped and N-doped CNTs-NAAMs, with nitrogen content ranging from 0 to 7%. The morphology and chemical composition of the prepared composite membranes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The obtained results reveal that the C/N ratio in the precursor source plays a crucial role in the doping process and incorporation of nitrogen into the structure of CNTs. It was found that a minimum threshold of 5:1 C/N ratio is required to obtain N-doped CNTs structures. This fabrication route does not compromise the morphology and graphitic structure of doped CNTs, making it an optimal process to tune the properties of these composite membranes for different applications, including catalysis, molecular separation and sensing. In this work, the molecular transport properties of N-doped CNTs-NAAMs with different level of doping were assessed by studying the diffusion of two model molecules with different hydrophilic-hydrophobic and charge properties. The obtained results show that the diffusional flux of a particular dye molecule was significantly affected by the nitrogen content of CNTs deposited inside NAAMs, confirming that the transport properties and chemical selectivity of CNTs-NAAMs composites can be controlled by nitrogen doping. This approach presents a new avenue to tune the properties of CNTs-NAAMs composites membranes, opening new opportunities for these membranes to be used in separation applications.