In-situ investigations at ROBL-MRH

In-situ investigations at ROBL-MRH

Baehtz, C.; von Borany, J.; Wirth, C. T.; Bayer, B. C.; Hofmann, S.

The last few years have seen an increasing interest in in-situ investigations using different diffraction methods. This is one of the main applications of the Material Research Station of the Rossendorf Beamline (ROBL) at the ESRF. The beamline operates with a monochromatic beam in the energy range of 6 to 36 keV and an energy resolution of ΔE/E of 5*10-4. Equipped with a six circle goniometer, different high temperature chambers can be mounted and various diffraction experiments can be performed, e.g. under controlled reaction conditions. A detector bank behind the goniometer allows the use of additional heavy weight detectors. One field of interest is the synthesis of carbon nanotubes (CNT) by chemical vapor deposition. Here, CNTs were grown from iron nanoparticles acting as the catalyst. The particles were formed by annealing Fe thin films with thicknesses ranging from 1-10 nm. The Fe films were evaporated on Si substrates covered with a SiO2 or Al2O3 buffer layer respectively. The splitting was initiated by annealing the initial Fe films at a temperature of up to 760°C in an Argon/Hydrogen atmosphere at ~200 mbar. The reducing atmosphere also removed initially amorphous iron oxide layers. The splitting temperature depends on the buffer layer, and the particle size of the formed nanocrystals depends on the initial film thickness of the pristine material. Adding acetylene (C2H2) to the gas stream initiated CNT-growth. Two different reaction pathways were observed. A very high α/γ iron ratio at high temperature under reducing atmosphere led to the formation of iron carbide when acetylene is added. If the ratio is around 1, no carbide was observed during CNT synthesis. SEM images show a comparable yield of CNTs in both scenarios. Therefore, the catalytic activity of iron carbide in this process is doubtful [1]. The influence of the buffer layer will be discussed. Thick Ni films (>170 nm) can be used to synthesize graphene also by chemical vapour deposition [2]. Compared with the usual increase of the lattice parameter with temperature due thermal expansion (measured at 5*10-6 mbar), an unusual increase of the the lattice parameter is observed when acetylene is introduced (at 2*10-3 mbar). Also, Rietveld refinement does not reveal any structural changes. This effect can only be explained by cracking of the acetylene molecules on the surface and subsequent carbon storage on interstitials. Upon cooling to room temperature, the differences in lattice parameter are negligible. These examples show that in-situ investigations are a powerful and versatile tool to monitor the deposition of nanosized materials from chemical processes.

Keywords: synchrotron radiation; carbon nano tubes

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Publ.-Id: 14786