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This work aims to investigate and characterize the photo-ignition phenomenon of MWCNT/ferrocene mixtures by using a continuous wave (CW) xenon (Xe) light source, in order to find the power ignition threshold by employing a different type of light source as was used in previous research (i.e., pulsed Xe lamp). The experimental photo-ignition tests were carried out by varying the weight ratio of the used mixtures, luminous power, and wavelength range of the incident Xe light by using selective optical filters. For a better explanation of the photo-induced ignition process, the absorption spectra of MWCNT/ferrocene mixtures and ferrocene only were obtained. The experimental results show that the luminous power (related to the entire spectrum of the Xe lamp) needed to trigger the ignition of MWCNT/ferrocene mixtures decreases with increasing metal nanoparticles content according to previously published results when using a different type of light source (i.e., pulsed vs CW Xe light source). Furthermore, less light power is required to trigger photo-ignition when moving towards the ultraviolet (UV) region. This is in agreement with the measured absorption spectra, which present higher absorption values in the UV-vis region for both MWCNT/ferrocene mixtures and ferrocene only diluted in toluene. Finally, a chemo-physical interpretation of the ignition phenomenon is proposed whereby ferrocene photo-excitation, due to photon absorption, produces ferrocene itself in its excited form and is thus capable of promoting electron transfer to MWCNTs. In this way, the resulting radical species, FeCp2 +∙ and MWCNT-, easily react with oxygen giving rise to the ignition of MWCNT/ferrocene samples.
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134
Photo-ignition process of multiwall carbon nanotubes and
ferrocene by continuous wave Xe lamp illumination
Paolo Visconti*1, Patrizio Primiceri1, Daniele Longo1, Luciano Strafella1, Paolo Carlucci1,
Mauro Lomascolo2, Arianna Cretì2 and Giuseppe Mele1
Full Research Paper Open Access
Address:
1Department of Innovation Engineering, University of Salento, Lecce
73100, Italy and 2Institute for Microelectronics and Microsystems -
IMM-CNR, Department of Lecce, University Campus, Lecce 73100,
Italy
Email:
Paolo Visconti* - paolo.visconti@unisalento.it
* Corresponding author
Keywords:
absorption spectra; CW Xe light source; metal nanoparticle ignitors;
multiwalled carbon nanotubes; photo-induced ignition
Beilstein J. Nanotechnol. 2017, 8, 134–144.
doi:10.3762/bjnano.8.14
Received: 20 July 2016
Accepted: 22 December 2016
Published: 13 January 2017
Associate Editor: R. Naaman
© 2017 Visconti et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
This work aims to investigate and characterize the photo-ignition phenomenon of MWCNT/ferrocene mixtures by using a continu-
ous wave (CW) xenon (Xe) light source, in order to find the power ignition threshold by employing a different type of light source
as was used in previous research (i.e., pulsed Xe lamp). The experimental photo-ignition tests were carried out by varying the
weight ratio of the used mixtures, luminous power, and wavelength range of the incident Xe light by using selective optical filters.
For a better explanation of the photo-induced ignition process, the absorption spectra of MWCNT/ferrocene mixtures and ferrocene
only were obtained. The experimental results show that the luminous power (related to the entire spectrum of the Xe lamp) needed
to trigger the ignition of MWCNT/ferrocene mixtures decreases with increasing metal nanoparticles content according to previ-
ously published results when using a different type of light source (i.e., pulsed vs CW Xe light source). Furthermore, less light
power is required to trigger photo-ignition when moving towards the ultraviolet (UV) region. This is in agreement with the
measured absorption spectra, which present higher absorption values in the UV–vis region for both MWCNT/ferrocene mixtures
and ferrocene only diluted in toluene. Finally, a chemo-physical interpretation of the ignition phenomenon is proposed whereby
ferrocene photo-excitation, due to photon absorption, produces ferrocene itself in its excited form and is thus capable of promoting
electron transfer to MWCNTs. In this way, the resulting radical species, FeCp2+ and MWCNT, easily react with oxygen giving
rise to the ignition of MWCNT/ferrocene samples.
134
Introduction
The photo-ignition process of carbon nanotubes (CNTs) was
observed for the first time accidentally by exposing single-wall
carbon nanotubes (SWCNTs) to the flash of an ordinary camera
[1]. Following this, studies [2] highlighted that this photo-effect
occurs in air for different types of SWCNTs prepared with dif-
ferent methodologies and weight percent of CNTs (in the range
Beilstein J. Nanotechnol. 2017, 8, 134–144.
135
50–90 wt %) with respect to the Fe metal catalyst mixed with
them. The authors conjectured that the ignition and combustion
occur when there is a local increase in temperature sufficient to
initiate the oxidation of carbon. Their interpretation was that
SWCNTs lend themselves to this photo-effect due to their black
color, which allows better absorbing of the visible flash
light and transmits the resulting thermal energy to the Fe
nanoparticles.
The local transient high temperature inside the nanotubes must
be at least 1500 °C in order for a structural reconstruction
process to take place. This results in a permanent change in the
SWCNT structure to a non-tubular structure, rather than merely
elastically deforming. Furthermore, this reconstruction also
happens after flash exposure even if ignition and combustion do
not occur. Previous research work has provided the following
results: the average light power required to ignite SWCNTs
with density of 0.2 g/cm3 is 100 mW/cm2, while for high densi-
ty, compact SWCNTs (>1 g/cm3), an average light power of
300 mW/cm2 was required [1,2].
Braidy et al. [3] focused their studies on the analysis of the
post-ignition of the sample, after exposing the SWCNTs to the
camera flash. By means of X-ray diffraction analysis of the
residual dust, they found that this solid material is mainly
composed of Fe2O3, with traces of Fe3O4, resulting from the Fe
catalyst. A TEM investigation was also performed revealing
that this material is composed of two different morphologies of
oxides: small nanoparticles trapped within a network of residual
SWCNT bundles and large randomly interconnected or fused
grains. This implies that temperatures higher than 1500 °C
were reached in localized points within the raw SWCNTs,
confirming the hypothesis put forward in [1] and [2].
Further experiments were carried out to determine the cause of
the photo-induced ignition in the SWCNTs [4]. In these tests,
SWCNTs (synthesized by the high-pressure carbon monoxide
process from Carbon Nanotechnologies, Inc., which utilizes Fe
as a growth catalyst for the SWCNTs) and Fe powder are
ignited if exposed to the camera flash. Under the same condi-
tions, the purified SWCNTs showed no reaction. The authors
postulated the following theory: photons emitted by the camera
flash are absorbed by catalytic metal nanoparticles causing very
high temperatures inside the sample; this occurs because Fe is
less conductive than SWCNTs. In fact, they suggested that the
heat is dissipated mostly in the CNT bundles with their high
conductivity and interconnection, whereas the Fe nanoparticles
are comparatively better as insulating heat. Therefore, the Fe
particles can store enough thermal energy to reach the right
temperature to oxidize. The authors supposed that the SWCNTs
provided a stabilizing support to the Fe particles, avoiding their
spontaneous ignition until they are exposed to a suitable ener-
getic stimulus (such as light energy of the camera flash).
Finally, according to the authors, the pyrophoric nature of Fe
nanoparticles present within CNT bundles is the main reason
the SWCNTs flash ignite, rather than any other physical/chemi-
cal characteristic of the MWCNTs. The results reported in [4]
for SWCNTs have been confirmed also for multiwall carbon
nanotubes (MWCNTs) in [5]. In fact, in this research work, the
MWCNTs showed the same stabilizing behavior as the metal
particles dispersed within them.
Assuming the role of metallic additives dispersed in the CNTs
is pivitol [1,2,4], Sysoev et al. [6] gave a qualitative description
of the processes that occur during photo-ignition of carbon
nanotubes. This analysis determined the roles and functionali-
ties of the different stages of combustion. At the first stage
(during flash ignition), the oxidation of metal impurities
dispersed in the CNT bundles takes place. At the next stage,
CNT heating by chemical reaction occurs. This stage lasts
1.2 ms, according to [1], and finishes with an explosion. The
explosion appears to occur as a consequence of the combustion
sources arising in separate segments initiating a combustion
wave. Tseng et al. [7] showed that the amount of catalyst (Fe
particles embedded in CNTs) plays a key role for ignition,
where a higher content of catalyst allows for easier ignition.
The main features of the SWCNT photo-ignition process (with
50% Fe content) using a camera flash and utilizing light sources
with two different pulse time durations were studied in [8]. In
addition, selected wavelength ranges of the incident light beam
were suitably obtained by using optical filters. The study high-
lighted that, regardless which of the optical filters were used,
the minimum energy needed for the ignition trigger only
depends on the light pulse duration. With a pulse duration of
0.1 ms, an energy of about 30–35 mJ for each pulse is required
to trigger the ignition of a loosely compact SWCNT sample
mixed with Fe, whereas with a pulse duration of 9 ms, about
80–90 mJ/pulse were needed to achieve the same results. The
authors also investigated on the minimum energy to ignite sam-
ples with various concentrations of Fe, using a pulse time dura-
tion of 9 ms. The reported results showed that a higher concen-
tration of Fe reduces the minimum energy needed to trigger
ignition [8,9]. The same results were also confirmed by other
published research works [10,11].
The use of nanostructured materials as a means for distributed
ignition and combustion improvement in propulsion applica-
tions (e.g., homogeneous-charged compression ignition (HCCI)
engines, liquid rocket fuel sprays and enhanced flame stabiliza-
tion in gas turbine engines) was patented in 2009 [12]. Since
then, the properties of nanostructured materials as ignition
Beilstein J. Nanotechnol. 2017, 8, 134–144.
136
agents have been studied [10,13-15]. In these works, the
combustion ignition agent was constituted of the combination of
metallic nanoparticles with CNTs mixed with different fuel
mixtures (i.e., hexane/acetone, ethylene/air). On the other hand,
ferrocene (FeCp2), whose molecular structure is shown in
Figure 1, was the first pure hydrocarbon derived from iron and
was accidentally discovered in 1951 [16]. Starting from its
discovery, many other chemical compounds derived from it
were synthesized and utilized in different research areas. The
chemistry of ferrocene and its derivatives is actually well
known as well as their chemical/physical features, which has
proven very useful for different technical applications. In fact,
nowadays, ferrocene and its derivatives mixed with other
gaseous or liquid materials find more and more use in several
areas [17] such as asymmetric catalysis, nonlinear optics [18],
electro-chemistry [19] (due to the quasi-reversible oxidation of
iron, e.g., Fe(II) oxide), and finally, in advanced fuel combus-
tion processes.
Figure 1: Ferrocene molecular structure (a) and its view as a 3D
model (b).
Ferrocene photochemistry has been analyzed in detail [20-24].
Ferrocene is usually quite stable under visible irradiation, how-
ever, chemical modification may occur in the presence of light,
or ferrocene may be used as an excited state quencher or photo-
sensitizer (i.e., for the catalysis of photo-chemically induced
reactions). From a fundamental viewpoint, the photo-chemical
behavior of ferrocene and its derivatives is a topic of increasing
interest. New applications are emerging in which ferrocene acts
as a redox center to allow the occurrence of energy and elec-
tron transfer processes.
This innovative research field was recently developed in
[25,26]. In more detail, the photo-induced ignition of MWCNTs
containing ferrocene metal nanoparticles by using a Xe flash
lamp was used to photo-ignite gaseous methane/air mixtures.
This resulted in a more rapid and homogeneous combustion
compared to ignition triggered by a traditional spark plug. In
their work, the authors used 20 mg samples of MWCNTs con-
taining 75 wt % ferrocene as ignition agents, and then added
this to the methane/air gaseous mixture. The experimental
results showed that the photo-induced ignition of a gaseous fuel
mixture triggered by MWCNTs determines a higher combus-
tion pressure gradient and a higher peak pressure with respect to
spark-induced ignition for all the tested methane/air ratios. In
addition, the high-speed camera images showed that the light-
induced ignition using MWCNT/ferrocene mixtures as ignition
agents leads to a more distributed homogeneous-like combus-
tion, and therefore, to a faster consumption of the gaseous fuel
mixtures without the formation of a discernible flame front.
In all reported research works, a pulsed flash lamp has always
been used to trigger the ignition of the CNTs enriched with
metal impurities. To our knowledge, a continuous wave (CW)
Xe lamp has never been employed for this purpose, thus no in-
vestigation into the photo-ignition phenomenon has been con-
ducted using this type of light source.
The aim of this research work is (i) to investigate and charac-
terize the photo-ignition phenomenon of the MWCNT/ferro-
cene mixture by using CW Xe light source in order to better
clarify the photo-induced ignition from a chemico-physical
point of view; and (ii) to evaluate the power/energy ignition
thresholds by using a different type of light source compared to
that used in the previously mentioned works.
The experimental photo-ignition tests were carried out using
varying weight ratios of mixtures, incident luminous power, and
wavelength range of incident Xe light. In addition, in order to
better understand the photo-induced ignition process, absorp-
tion spectra measurements of MWCNT/ferrocene mixtures and
ferrocene were carried out after proper sample preparation [28].
Experimental
The preparation procedure for obtaining reliable absorption
spectra of the nanostructured material using a UV–vis spec-
trophotometer will be described. Furthermore, the experimental
setup used to characterize the ignition of the MWCNT/ferro-
cene mixtures, the variable parameters such as MWCNT/ferro-
cene weight ratios, as well as the luminous power and wave-
length range of the CW Xe incident beam will be presented.
Material preparation method and absorption
spectra
The absorption spectra of ferrocene and the MWCNT/ferrocene
mixture were determined for comparison purposes. Concerning
the measurements on ferrocene, toluene was selected as solvent
because in order to obtain a homogeneous liquid solution, tolu-
ene appeared a better solvent compared to water. In Figure 2,
water and toluene are shown after adding ferrocene followed by
Beilstein J. Nanotechnol. 2017, 8, 134–144.
137
Figure 2: Ferrocene diluted in toluene (left) and ferrocene diluted in
water (right) after sonication operation.
sonication. In the glass on the left containing the ferrocene/tolu-
ene solution, the sample appears more homogeneous than in the
glass on the right containing the ferrocene/water solution.
The sonication operation, carried out employing the ultrasonic
sonicator Bandelin SONOREX™ SUPER, was performed for a
time duration variable from ten seconds up to five minutes on a
solution of toluene (8 mL) and different amounts of ferrocene
(as reported in Table 1). The sonication duration, although
varied over a broad temporal range, did not highlight any signif-
icant changes in the homogeneity of the solution, even for
higher ferrocene concentrations in toluene. In Table 1, the dif-
ferent amounts of ferrocene used in absorption spectra measure-
ments are reported.
Table 1: Solvent type and amount of ferrocene used in this work.
Sample
#ID
Solvent Volume
(mL)
Quantity of
ferrocene (mg)
Volume
ratio
#1 toluene 8 4.7 0.053%
#2 toluene 8 3.7 0.042%
#3 toluene 8 2.5 0.028%
#4 water 8 3.7 0.042%
After sonication, the solution was introduced into the spec-
trophotometer (JASCO, V-660 UV-VIS) to obtain the absorp-
tion spectra, which was normalized with respect to toluene and
is shown in Figure 3. For each concentration of ferrocene in tol-
uene, the absorbance curves present a considerable peak at
about 280 nm and a wider peak around 440 nm. Moreover, the
absorbance values increase with the ferrocene concentration.
The spectrum resulting from the ferrocene/water solution (also
reported in Figure 3), normalized with respect to water, does not
present significant peaks. This was expected because of the
insolubility of ferrocene in water.
To obtain the absorption spectra of the MWCNT/ferrocene
mixture, the following procedure was adopted. Initially a
material with weight of 9.5 mg (mixture of MWCNTs with
Figure 3: Absorption spectra of ferrocene diluited in toluene or water,
varying the ferrocene concentration in 8 mL of solvent.
75% ferrocene by weight) in 25 mL of toluene (equal to a
volume ratio of 0.029%) was used. To obtain a homogeneous
solution, an ultrasonic probe (Figure 4a) and a magnetic stirrer
(Figure 4b), were employed, each for ten minutes. The result
was a visible change in the solution from that shown in
Figure 4c to Figure 4d. The solution presented high opacity,
large agglomerates and very fast precipitation of solids. These
problems were not resolved by subjecting the sample to an addi-
tional ten minutes of sonication (Figure 4e).
Employing sodium dodecyl sulfate (SDS), the dispersion of the
mixture in the solvent was facilitated [28]. Using 250 mg of
SDS in 25 mL of water, equal to a volume ratio of 1%, the
ultrasonic probe and the magnetic stirrer were applied for ten
minutes. The solution then appeared more homogeneous
(Figure 5) than the previous ones, allowing for a reliable mea-
surement of the absorption spectrum of the mixture.
Three absorption tests were performed on the same solution: the
first test was carried out soon after the sonication treatment (ten
minutes) and magnetic stirring (ten minutes), providing, as
result, the Sample 1 curve shown in Figure 6. After one hour,
the same solution was again inserted into the spectropho-
tometer obtaining the red curve (Sample 2). Subsequently, after
a further sonication for ten minutes, a new measure was made
obtaining the green curve (Sample 3). The achieved results
show that the absorption of the MWCNT/ferrocene mixture in-
creases going from the visible toward ultraviolet region, as re-
ported in the literature [15,27,28].
Experimental setup for photo-ignition tests
A sketch of the experimental setup is shown in Figure 7. Using
this setup, the ignition tests of MWCNT/ferrocene mixtures,
Beilstein J. Nanotechnol. 2017, 8, 134–144.
138
Figure 4: Image of sonicator probe Sonopuls Bandelin HD 2070 during the sonication treatment (a) and magnetic stirrer (FALC F70 model) to
disperse the sample and obtain a more homogeneus solution (b). MWCNT/ferrocene mixture in toluene, before the treatment (c), after ten minutes of
sonication and other ten minutes of magnetic stirring (d) and after another sonication for ten minutes (e).
Figure 5: MWCNT/ferrocene mixture after adding SDS surfactant.
subjected to a continuous wave light source, were carried out.
The adopted CW Xe lamp is highlighted in red in the “lamp
management module” block. Then, the generated light was
reduced with a pinhole, properly filtered, and focused with suit-
able lenses for igniting the MWCNT sample placed in the
holder. In the experimental setup used to perform the ignition
tests (Figure 7 and Figure 8), the MWCNT/ferrocene sample
was located in a position indicated in Figure 8 as “focal point/
sample position”, at which the Xe light beam presents an almost
uniform, circular illumination area, having a diameter of about
9 mm (illumination area, S = 63.6 mm2). The Xe lamp, whose
emission spectrum is shown in Figure 9, is the LSB521
Ozone-free Arc lamps 150W Xe model from LOT-Quantum
Design.
Figure 6: Absorption spectra of MWCNT/ferrocene soon after sonica-
tion and magnetic stirring (Sample 1), after one hour (Sample 2) and
after a further sonication (Sample 3).
Along the optical path of the light beam generated by the lamp,
a small circular opening (pinhole), which partitions the periph-
eral parts of the light radiation, was introduced. In this way, a
Beilstein J. Nanotechnol. 2017, 8, 134–144.
139
Figure 7: Schematic of the experimental setup for photo-ignition tests of the MWCNT/ferrocene mixture using a CW Xe lamp.
Figure 8: Images of the experimental setup for photo-ignition tests:
lateral view (a) and front view (b).
more uniform luminous beam was obtained at the sample. The
optical filters were arranged on proper supports (filter holder in
Figure 8) placed after the pinhole. Two kinds of filters (Thor-
labs) have been employed: neutral density (ND) filters, which
attenuate the light radiation intensity of a given percentage
factor evenly in the whole wavelength range covered by the
lamp spectrum, and frequency selective filters, i.e., long pass
(LP) filters and short pass (SP) filters, which attenuate certain
wavelengths while others pass unchanged. Combining both LP
and SP filters it was possible to select a specific wavelength
range (bandpass filter) of the Xe light source. By means of a
laser power meter (model 407A, shown in Figure 10a), the
luminous power emitted by the CW Xe lamp was measured by
placing the sensor at the focal point where the MWCNT/ferro-
cene mixture is located (Figure 10b).
The thermopile detector, employed in the measurements, is
capable of measuring CW light power from a few mW up to
more than 20 W and can withstand 20 kW/cm2 average
CW power density. Readings are extremely precise because
the absorbance of the detector varies only by ±1% in the
range 400–1000 nm and by ±3% between 250 nm and 11 µm.
Since the detector diameter is equal to 18 mm (with an area
of 2.54 cm2), the whole illumination area of the light beam
(0.64 cm2) falls within it (Figure 10b).
Results
In the following, the results derived from the experimental mea-
surements are illustrated. Initially, the minimum power thresh-
olds necessary to trigger the ignition of the MWCNT/ferrocene
mixtures will be reported for different mixture weight ratios and
using the entire lamp spectrum. Subsequently, the light power
threshold necessary for triggering the ignition when changing
the wavelength range of the incident light on the sample will be
presented and analyzed. Finally, a chemico-physical interpreta-
Beilstein J. Nanotechnol. 2017, 8, 134–144.
140
Figure 9: Typical spectra of a Xe arc lamp. The emission spectrum of the 150 W LSB521 Ozone-free Xe light source used in the experimental setup
is highlighted.
Figure 10: Laser power meter, Spectra Physics, Analog 407A (a) and
view of the sensor positioned at the height of the focal point to
measure the light spot power (b).
tion of the results will be presented by comparing with the
results already reported in the literature.
Ignition process with varying mixture
composition and wavelength range
As reported in the literature, even if the CNTs are exposed to a
light pulse with lower power than that required to trigger the
photo-induced ignition, they are oxidized and undergo a total
structural reconstruction in the presence of air or inert gases or
in vacuum [1,2,7]. Therefore, each sample was illuminated only
once during the experiment, regardless of whether the sample
was ignited or not.
The purpose of the tests was to search for the minimum power
of light needed to trigger the ignition of the sample subjected to
the luminous flux. Using the setup of Figure 8, the ignition tests
were performed varying the weight ratio of MWCNT/ferrocene
from 4:1 to 1:4, using the whole lamp emission spectrum
(Figure 9) and making use of the optical attenuation ND filters
(for adjusting to the beam power level that reaches the sample).
The obtained results, reported in Figure 11, show that the
1:3 MWCNT/ferrocene mixture requires the least power to be
ignited (240 mW which corresponds to a power density
threshold of 377 mW/cm2). An uncertainty of about ±4% was
estimated, relative to the reported threshold light power values.
Figure 11: Light power thresholds for ignition of MWCNT/ferrocene
mixtures found by varying the weight ratio.
Beilstein J. Nanotechnol. 2017, 8, 134–144.
141
As already reported in the literature, the luminous energy value
needed to trigger the ignition of SWCNT/Fe decreases with in-
creasing Fe [7-11]. In this work, even if a different type of light
source (i.e., CW Xe lamp instead of pulsed Xe lamp) has been
used, a similar dependence between the ferrocene concentra-
tion and the minimum power needed for ignition was observed.
The best weight concentration of ferrocene was 75% with
respect to CNTs (MWCNT/ferrocene ratio 1:3).
As a second experiment, a MWCNT/ferrocene mixture with a
weight concentration of 1:3 was used, which was the mixture
requiring the lowest ignition power. Making use of frequency
selective optical filters (i.e., LP and/or SP filters), different
wavelength ranges of light were selected to illuminate the sam-
ple. For each wavelength range, the luminous power threshold
for ignition was found by using light attenuating ND filters.
In Figure 12, the luminous power thresholds obtained by em-
ploying SP filters (blue arrows) (each filter characterized by a
UV cut-off frequency equal to about 380 nm) or LP filters
(brown arrows) are reported. The measured power values
related to the illumination area (0.636 cm2) are highlighted in
green, while the calculated luminous power density values
(mW/cm2) are shown in the brackets in orange.
Figure 12: Light power thresholds for ignition using filters to select
specific wavelenght ranges: low (blue arrows) and high pass filters
(brown arrows).
In Figure 13, the luminous power thresholds and calculated
power density values relative to the combined use of SP and LP
filters (i.e., band pass filters) are reported. The results highlight
that in order to ignite the MWCNT/ferrocene mixture, less
power in the range 210–240 mW (namely 330–377 mW/cm2) is
needed in the UV region.
Figure 13: Light power thresholds for ignition using band pass filters.
Figure 14: Frames extracted from videos recorded during ignition
tests. The variable visible color of the incident light on the sample is
due to the different wavelenght ranges selected by the optical filters.
For each test, ignition was observed.
These results are consistent with the obtained absorption spec-
tra (shown in Figure 3 and Figure 6). In fact, the absorption
curves of ferrocene present a peak in the range 270–320 nm
(however, this is not usable in the ignition tests of Figure 12 and
Figure 13 because of the SP filter absorption in UV region) and
a less intense but wider peak around 440 nm. Moreover, a
higher absorption has been found for MWCNT/ferrocene mix-
tures illuminated by visible radiation with decreasing the
wavelength up to 400 nm. In Figure 14, some frames are
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142
Figure 17: Chemical reactions relative to photo-induced electron and energy transfer in MWNTs–FeCp2 nanocomposites occurring during the photo-
induced ignition process.
shown extrapolated from videos recorded during the ignition
tests.
In the following pictures reported in Figure 15, the time
evolution of the combustion process is shown when using
the full spectrum of the Xe-lamp light beam, i.e., without
any selective filters. The combustion flame is clearly visible
(after only 100 ms, third frame) and it occurs for the
1:3 MWCNT/ferrocene mixture weight ratio, with a luminous
power density equal or higher than 377 mW/cm2.
Figure 15: Captured pictures of the combustion process where the
MWCNT/ferrocene sample is ignited by illumination of the whole spec-
trum of the Xe lamp. The first frame refers to the time just before the
sample enters the white light beam path.
In Figure 16, the images of MWCNT/ferrocene samples before
and after the ignition process are shown. As reported in the lit-
erature, after ignition, the exposed samples display orange dots
or clusters because of the presence of ferrocene in the mixture
(more evident with increasing ferrocene concentration), indica-
tive of iron oxide particles [9,15].
Figure 16: Sample images of MWCNT/ferrocene mixture (with weight
ratio 1:3) positioned on the slide before (a) and after photo-induced
ignition process (b).
Discussion
A chemico-physical interpretation of
obtained results
One of the central aspects in CNT chemistry and physics is their
interaction via electron transfer. The intermolecular interac-
tions with electronic charge transfers between nanotubes and
ferrocene showed that this composite material can be used for
converting solar energy into energy to promote an electron
transfer by means of a photo-excitation process.
The chemical physical feature of the multiwalled carbon nano-
tube/ferrocene nanocomposites (MWCNT/FeCp2) responsible
for starting the ignition process could be ascribed to the charge-
separated state, MWCNT .–FeCp2 .+, as expected by photo-in-
duced electron transfer in the MWCNT–FeCp2 nanomaterial
hybrid. This leads us to postulate intermolecular electron
transfer to give MWCNT.–FeCp2.+, depending on the selected
wavelength range of the incident Xe light used to trigger the
photo-induced ignition process. From a chemico-physical point
of view, the suggested evolution of ignition process has been
summarized in the Figure 17.
As reported in Figure 17, the photo-induced process of charge
separation, promoted by UV–vis–IR light irradiation (photon
energy h∙ν = hc/λ with 380 nm < λ < 1000 nm, c speed of
light and h Planck constant), represents the key step of the
Beilstein J. Nanotechnol. 2017, 8, 134–144.
143
MWCNT/Fe ignition process. In this scenario, as already
widely reported in the literature, the presence of metal nanopar-
ticles (in this work FeCp2) allows for the photo-ignited reaction.
In particular, the photo-excitation of FeCp2 produces ferrocene
in its excited form FeCp2. which is capable of promoting elec-
tron transfer processes to MWCNTs. The radical species pro-
duced in this way, FeCp2.+ and MWCNT., easily react with
oxygen (O2) giving rise to a typical combustion reaction. Of
course, the opportune combination of the proper incident light
wavelength range and luminous power (as reported in Figure 12
and Figure 13) are important in order to ensure the success of
the MWCNT/ferrocene ignition process.
The importance of the presence of oxygen for ignition was
already observed in [2] where the authors reported that when
the sample is more dense, then higher power is required to
ignite the SWCNT material. They believe that the lack of
oxygen access and the loss of heat into the bulk of the denser
samples make their ignition more difficult. In more detail, if the
CNT material is more dense, many bundles are in contact re-
sulting in fast heat dissipation into the bulk. In conclusion,
lower light energy density is required for ignition when bundles
are separated and surrounded by oxygen [2].
Conclusion
In this work, a full analysis of the ignition process of MWCNT/
ferrocene mixtures, photo-ignited by using a CW Xe light
source, was carried out. Although a different type of Xe light
source (i.e., CW instead of pulsed Xe light, as was used in
previous research works) has been used, photo-induced ignition
was similarly obtained. The absorption spectra of MWCNTs/
ferrocene mixtures revealed higher absorbance values moving
from the visible toward UV region while with only ferrocene, a
significant peak in UV region at 280 nm and a slight peak
around 440 nm were obtained. The luminous power thresholds
for triggering ignition were found for different mixture weight
ratios varying the parameters of the CW Xe light incident on the
sample such as the luminous power and the wavelength range.
In order to select different wavelength ranges and to reduce the
light source power, selective optical filters (LP and/or SP) and
ND attenuating filters were employed. The results obtained by
irradiating the sample with the full Xe lamp spectrum show that
the MWCNT/ferrocene mixture with 1:3 weight ratio (i.e.,
75% ferrocene by weight) requires the least luminous power to
be ignited. Utilizing this weight ratio and selecting specific
wavelength ranges with selective filters, we found that photo-
induced ignition is obtained with lower luminous power in the
380–450 nm UV–vis range; this was in agreement with the ob-
tained absorption spectra. Finally, a chemico-physical interpre-
tation was given to explain the ignition phenomenon. The irra-
diating light on MWCNTs/FeCp2 samples produces ferrocene
in its excited form capable of promoting electron transfer pro-
cesses to MWCNTs. In this way, the formation of radical
species, which easily react with oxygen, triggers the combus-
tion process of the mixture.
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... In particular, the 1:3 weight ratio of HP-MWCNTs/FeCp 2 samples presents the lowest ignition threshold. As previously reported, the ignition threshold decreases as much as the ferrocene amount in the sample increases when continuous-wave Xe was used as the irradiation lamp [6]. ...
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Chapter
IntroductionFormation of Excited State OTM Complexes: Electronic Transitions and Spectral PropertiesPrimary Decay ProcessesClosing Remarks
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A mechanism for the flash ignition of a carbon nanotube (CNT) is proposed. At the first stage (during flash ignition) oxidation of an ultradispersed impurity of metals in the CNT occurs. At the second stage, heating by chemical reactions occurs. The final stage is the initiation of the combustion wave. Qualitative descriptions of processes at each stage are given.
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Nanostructured materials exhibit peculiar and unique properties unseen at the bulk level. One such behavior is the observed ignition of SWCNTs and others such as silicon nanowires and iron nanoparticles with an ordinary camera flash. In this paper, minimum ignition energy (MIE) of SWCNTs with an ordinary low-energy camera flash is reported. Experiments are focused on effects of incident pulsed-light exposure duration and wavelength on minimum ignition energy. Results indicate that lower energy/pulse is needed to initiate ignition when shorter flash duration is used. For example, at flash duration of ∼0.2 ms, it required 30–35 mJ/pulse to initiate ignition of as-produced fluffy samples in standard air, whereas at 7 ms duration, it needed 80–90 mJ/pulse to achieve the same result. Averaged intensities between 10 and 150 W/cm2 are needed to bring about ignition of SWCNTs, being a factor of 80 lower than cases where laser (pulsed and cw) is used in coal particles.