Study of morphology of aerosol aggregates formed during co-pyrolysis of C3H8+Fe(CO)5

Formation of aerosol nanoparticles as well as carbon nanotubes and nanofilaments is studied during co-pyrolysis of iron pentacarbonyl and propane with argon as a carrier gas in a flow reactor. Gaseous intermediates from propane thermal decomposition (CH4, C2H6 and C3H4 and Fe(CO)5 conversion are monitored by gas chromatography and IR-spectroscopy, respectively. The aerosol morphology is studied by transmission electron microscopy (TEM) and high resolution TEM. The aerosol particle concentration and size distribution are measured by an automated diffusion battery. The crystal phase composition of particles is studied by x-ray diffractometry. The decomposition of the Fe(CO)5 + Ar mixture resulted in an iron aggregate formation composed of fine primary particles. In the case of lower pyrolysis temperatures, about 450 K, the primary particle mean diameter is about 10 nm, and consequently, the majority of the primary particles are superparamagnetic, thus forming compact aggregates. At intermediate pyrolysis temperatures in the range 800–1040 K the primary particle diameter is about 20–30 nm, and most of the particles are ferromagnetic in nature. The coagulation of these particles results in a chain-like aggregate formation. Finally, at temperatures higher than the Curie point (1043 K) the ferromagnetic properties vanish and the formation of compact aggregates is observed again. The co-pyrolysis of Fe(CO)5 and C3H8 mixed with Ar carrier gas resulted in aerosol aggregate structures dramatically different from those formed by iron pentacarbonyl pyrolysis. In particular, in the temperature range 1070–1280 K, we observed Fe3C particles connected by long carbon nanotubes (CNTs). The aggregate morphology is described in terms of a fractal-like dimension Df, which is determined from TEM images on the basis of a scaling power law linking the aggregate mass (M) and radius (R), M<RDf. The Fe3C–CNT aggregate morphology is a function of the inlet ratio between propane and iron pentacarbonyl concentrations [C3H8]0/[Fe(CO)5]0. At the low ratio of [C3H8]0/[Fe(CO)5]0<80 the fractal dimension of aggregates decreases (from 1.7 down to about 1) with the increasing ratio of inlet concentrations. This effect, as observed by TEM, is due to the increase in the mean nanotube length. Vice versa, in the range C3H8]0/[Fe(CO)5]0>80 the fractal aggregate dimension is higher for a larger ratio of [C3H8]0/[Fe(CO)5]0, which is explained by the larger thickness of growing nanotubes obtained for a relatively large propane concentration. The aggregate formation mechanism includes consecutive stages of iron aggregate formation due to Fe(CO)5 decomposition, carbon deposition on iron particles from C3H8 pyrolysis intermediates, carbon dissolution in iron particles, nanotube nucleation at the carbon concentration of about 60 at.% in Fe–C solution and disruption of the Fe–C aggregates into small pieces by the growing nanotubes.