18 Feb 2014

Revealing the Role of Catalysts in Carbon nanotubes and nanofibers by Scanning Transmission X-ray Microscopy

The identification of effective components on the atomic scale in carbon nanomaterials which improve the performance in various applications remains outstanding challenges. Here the catalyst residues in individual carbon nanotube (CNT) and carbon nanofiber (CNF) were clearly imaged with a concurrent characterization of their electronic structure by nanoscale scanning transmission X-ray microscopy. Except for prominent catalyst nanoparticle at the tip, tiny catalyst clusters along the tube (fiber) were detected, indicating a migration of the catalysts with the growth of CNTs (CNFs). The observation provides the direct evidence on the atomic metal in CNT for oxygen reduction reported in the literature. Interaction between catalysts (Fe, Ni) and CNTs (CNFs) at the tip was also identified by comparing the X-ray absorption spectra. A deep understanding of catalyst residues such as Fe or Ni in carbon nanomaterials is very vital to growth mechanism development and practical applications.

Carbon-based materials containing metals (such as Fe or Co) and nitrogen impurities have been considered to be good alternatives to precious-metal electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. Nitrogen doping in carbon nanotubes (CNTs) or graphene has been studied and shown to be a key role for the high ORR activity. However, because of the complex compositions in carbon materials such as the existence of amorphous carbon, catalyst residues, various structural defects and surface contaminations, the identification of effective components in carbon-based materials which improve the performance and the understanding of the nature of ORR catalytic sites on the atomic scale still remain outstanding challenges. For example, the extremely small amounts of irons originated from nanotube growth seeds were recently revealed to play very important role in facilitating the formation of catalytic sites and boosting the activity of the catalyst together with nitrogen species6. The iron and nitrogen atoms in the few-walled CNTs attached with nanoscale graphene sheets were clearly imaged by annular dark-field (ADF) and atomic-scale electron energy loss spectrum (EELS) imaging in aberration-corrected scanning transmission electron microscopy (STEM). It is thus important to explore the form of catalyst residues such as Fe or Ni in carbon nanomaterials with the identification of their electronic structure, which may help for the understanding of the growth mechanism of carbon nanomaterials and their functionality in practical applications.

Read the complete article in Nature: http://www.nature.com/srep/2014/140108/srep03606/full/srep03606.html

Reference: Gao, Jing, et al. "Revealing the Role of Catalysts in Carbon Nanotubes and Nanofibers by Scanning Transmission X-ray Microscopy." Scientific reports 4 (2014).  DOI: 10.1038/srep03606

SEM image of CNF grown with 20 nm Ni/Ti underlayer.

 

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