Scanning transmission X-ray microscopy and X-ray absorption near-edge structure studies of N-doped carbon nanotubes sealed with N2 gas

Abstract

N-doped carbon nanotubes (NCNTs) were synthesized and their electronic structures have been explored by X-ray absorption near-edge structure (XANES) spectroscopy. With a surface sensitive mode, XANES confirms the N-doping in NCNTs. However, with a more bulk sensitive detection mode of XANES, large amount of gaseous N2 have been found to be sealed in NCNTs, even in a high vacuum environment. The encapsulation of the ferrocene residues in carbon nanotubes had been revealed by scanning transmission X-ray microscopy (STXM), which may help for the N2 sealing. The results suggest that the easily sealed gas should be taken into consideration for CNT-based applications.

Synchrotron Soft X‐ray Absorption Spectroscopy Study of Carbon and Silicon Nanostructures for Energy Applications

“…N‐doped carbon nanotubes with excellent properties were widely used in fuel cells with excellent performance. Recently the electronic structures of NCNTs were explored by XANES . CNTs and NCNTs synthesized with the floating‐catalyst chemical vapor deposition method under similar conditions were studied by XANES in details .…”

“…The electronic structure difference revealed by the XANES spectra provided strong evidence to understand the enhanced ORR performance. Also in a recent report XANES measured by a surface sensitive mode (TEY) was used to confirm N‐doping in NCNTs . NCNTs were synthesized by CVD method which might introduce gaseous N 2 in the system.…”

“…In TEY mode only N atoms doped in CNT were detected. However, a large amount of N 2 (gas phase) could be found in the bulk sensitive TFY mode, which were sealed in NCNTs . When performing XANES experiments, the sample was in a ultrahigh vacuum chamber and the adsorbed gas could be removed.…”

“…To a limited extent the TEY and the TFY can be considered as proportional to the absorption cross section if the samples are dilute or thin, and are therefore often used for recording XANES spectra . Working in TEY mode XANES is a surface‐sensitive technique, whereas working in TFY mode XANES provides near‐surface and bulk sensitivity because fluorescence photons have longer escape depth than electrons . TFY spectrum is very useful for aqueous solutions by providing information about ultrafast charge‐transfer processes .…”

Imaging Catalysts at Work: A Hierarchical Approach from the Macro‐ to the Meso‐ and Nano‐scale

“…Strong changes due to the expansion/contraction of the lattice and the mobility of silica species were observed, just as in TEM studies of Cu/ZnO catalysts (Figure 16,6a), nanofibre growth over Ni‐particles (Figure 18,6b) and oxidation/reduction of SOFCs (Figure 19,111). A recent study of RuO 2 on MWCNTs has further demonstrated that spectroscopic insight into changes in valence state can be obtained on the nm scale121 and the technique is attractive for studies of CNTs in general 122. The STXM approach was further applied to zeolite materials at work123 while Meirer at al 124.…”