IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 40 (2007) 2071–2082 doi:10.1088/0022-3727/40/7/032
Study of morphology of aerosol
aggregates formed during co-pyrolysis of
C
3
H
8
+Fe(CO)
5
N A Ivanova
1
, A A Onischuk
1
, S di Stasio
2
, A M Baklanov
1
and G A Makhov
1
1
Institute of Chemical Kinetics and Combustion SB RAS, 630090, Novosibirsk, Russia
2
Istituto Motori CNR Aerosol and Nanostructures Laboratory, Via Marconi 8, 80125 Napoli, Italy
E-mail: ivanova@ns.kinetics.nsc.ru
Received 3 April 2006, in final form 3 November 2006
Published 16 March 2007
Online at stacks.iop.org/JPhysD/40/2071
Abstract
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 (CH
4
,
C
2
H
6
and C
3
H
4
) 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 C
3
H
8
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 Fe
3
C particles connected by long carbon nanotubes (CNTs).
The aggregate morphology is described in terms of a fractal-like dimension D
f
, which is
determined from TEM images on the basis of a scaling power law linking the aggregate
mass (M) and radius (R), M∼R
D
f
. The Fe
3
C–CNT aggregate morphology is a function
of the inlet ratio between propane and iron pentacarbonyl concentrations
[C
3
H
8
]
0
/[Fe(CO)
5
]
0
. At the low ratio of [C
3
H
8
]
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 C
3
H
8
]
0
/[Fe(CO)
5
]
0
> 80 the fractal aggregate
dimension is higher for a larger ratio of [C
3
H
8
]
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 C
3
H
8
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.
0022-3727/07/072071+12$30.00 © 2007 IOP Publishing Ltd Printed in the UK 2071