(706b) Synthesis of Graphene and Novel Carbon Nanostructures By Induction Heating of Nanocrystalline 3C-SiC Particles At Atmospheric Pressure

Authors: 
Jokiniemi, J., VTT Fine Particles
Torvela, T., University of Eastern Finland
Karhunen, T., University of Eastern Finland
Lähde, A., University of Eastern Finland
Miettinen, M., Fine Particle and Aerosol Technology Laboratory, Dept. of Environmental Science, University of Eastern Finland



Cubic silicon carbide (3C-SiC) has been shown to be a suitable substrate
for the growth of graphene (Suemitsu
et al. 2009). For
advanced applications, the uniform graphene film on
an insulating substrate is required. Thermal decomposition of SiC
at atmospheric pressure has been proposed to improve the homogeneity of the
formed graphene layers compared to vacuum conditions
(Emtsev et al. 2009). A considerable advantage of using SiC-based
methods to grow graphene is that it is not required
to transfer the as-grown graphene onto another
insulating substrate. We have studied the post-processing of previously
manufactured nanocrystalline 3C-SiC particles by
induction heating at atmospheric pressure (Miettinen
et al. 2011, Miettinen et al. 2013). Three precursor
powders with increasing crystallinity were synthesised at 1000 ºC, 1200 ºC and 1400 ºC by atmospheric
pressure chemical vapor synthesis and induction temperatures from 1900 ºC to
2600 ºC were used. 

Growth
of carbon sheets and spherical carbon particles was observed (Fig. 1). The
spherical carbon particles turned out to be new carbon structures, i.e. carbon nanoflowers (CNFs). Curved graphite layers grew from the Si-C
core, which decreased in size with increasing induction temperature and eventually
disappeared above induction temperature of 2200 ºC. To our knowledge the formation
of this kind of structures has not been reported previously. The growth of CNFs may
proceed through the cap formation mechanism proposed by Wang et al. (2007). The existence of C-C σ-bonds
perpendicular to the SiC surface and the persistence
of C-C π-bonds results in a repulsion in the cap center between two
adjacent dangling bond sites which is responsible for the formation of a stable
curved structure. The growth mechanism was probably induced by simple thermal
decomposition of the SiC surface followed by the
evaporation of Si. The number of the carbon layers in the nanoflower
structure decreased with increasing temperature from 11 at induction
temperatures below 2200 °C to 7 at the induction temperature of 2600 °C with
the precursor powder synthesized at 1000 ºC. The number of layers decreased
also with increasing crystallinity of the precursor
material and 5
layers were detected with the most crystalline precursor at the induction
temperature of 2050 °C. The carbon sheets were more
abundant when the precursor particles of higher degree of crystallinity
(SiC-1400) were used. The SAED analysis
indicated crystallinity of the carbon sheets and the X-ray diffraction analysis verified
the presence of crystalline carbon. Raman spectrum displayed sharp G and 2D bands with
intensity ratio 2D/G over four which is typical for single-layer, undoped graphene (Ferrari 2007). A very low intensity D band in
this spectrum indicates a highly ordered structure.


Fig. 1
Transmission electron microscopy images of the produced carbon sheets and nanoflowers (inserted is a larger magnification of the nanoflower)

The method
presented is easily scalable and opens up the possibility to produce the
advanced carbon structures that can be exploited as precursors for numerous applications, e.g.
deposition of graphene films or in biomedical
applications.

This work was supported by the
strategic funding of the University of Eastern Finland under the NAMBER
spearhead project.

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