Distribution and structure of N atoms in multiwalled carbon nanotubes using variable-energy X-ray photoelectron spectroscopy
ABSTRACT We investigated the inhomogeneous distribution of concentration and electronic structure of the nitrogen (N) atoms doped in the multiwalled carbon nanotubes (CNTs) by variable-energy X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure, and electron energy-loss spectroscopy. The vertically aligned N-doped CNTs on the substrates were grown via pyrolysis of iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), and nickel phthalocyanine (NiPc) in the temperature range 750-1000 degrees C. They usually have a bamboo-like structure, and the diameter is in the range of 15-80 nm. As the photon energy of XPS increases from 475 to 1265 eV, the N content increases up to 8 atomic %, indicating a higher N concentration at the inside of nanotubes. We identified three typed N structures: graphite-like, pyridine-like, and molecular N(2). The pyridine-like N structure becomes significant at the inner walls. Molecular N(2) would exist as intercalated forms in the vicinity of hollow inside. The XPS valence band analysis reveals that the pyridine-like N structure induces the metallic behaviors. The CNTs grown using NiPc contain the higher content of pyridine-like structure compared to those grown using FePc and CoPc, so they exhibit more metallic properties.
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ABSTRACT: Phosphorus–nitrogen doped multiwalled carbon nanotubes (CNxPy) were prepared using a floating catalyst chemical vapor deposition method. Triphenylphosphine (TPP), as phosphorus (P) precursor, was used to control the structure of the CNxPy. Transmission electron microscope (TEM) observation indicated that with the increase of TPP amount, the outer diameter and wall thickness of the CNxPy gradually increased, while their inner diameter decreased. TEM and backscattered electron imaging revealed that structural changes of the nanotubes could be attributed to the shape change of the catalyst particles, from conical for nitrogen-doped carbon nanotubes (CNx) to elongated for CNxPy, with the addition of TPP. X-ray photoelectron spectroscopy analysis demonstrated that the P content in CNxPy can reach as high as 1.9 at.%. Raman analysis indicated that CNxPy had a lower crystallinity than CNx.Carbon 12/2011; 49(15):5014–5021. DOI:10.1016/j.carbon.2011.07.018 · 6.16 Impact Factor
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ABSTRACT: Nitrogen-doped carbon nanotubes (CNx–NTs) were prepared using a floating catalyst chemical vapor deposition method. Melamine precursor was employed to effectively control nitrogen content within the CNx–NTs and modulate their structure. X-ray photoelectron spectroscopy (XPS) analysis of the nitrogen bonding demonstrates the nitrogen-incorporation profile according to the precursor amount, which indicates the correlation between the nitrogen concentration and morphology of nanotubes. With the increase of melamine amount, the growth rate of nanotubes increases significantly, and the inner structure of CNx–NTs displayed a regular morphology transition from straight and smooth walls (0 at.% nitrogen) to cone-stacked shapes or bamboo-like structure (1.5%), then to corrugated structures (3.1% and above). Both XPS and CHN group results indicate that the nitrogen concentration of CNx–NTs remained almost constant even after exposing them to air for 5 months, revealing superior nitrogen stability in CNTs. Raman analysis shows that the intensity ratio of D to G bands (ID/IG) of nanotubes increases with the melamine amount and position of G-band undergoes a down-shift due to increasing nitrogen doping. The aligned CNx–NTs with modulated morphology, controlled nitrogen concentration and superior stability may find potential applications in developing various nanodevices such as fuel cells and nanoenergetic functional components.Carbon 04/2010; 48(5-48):1498-1507. DOI:10.1016/j.carbon.2009.12.045 · 6.16 Impact Factor