Soot and Nanomaterials Synthesis in the Flame

doi:10.4236/msce.2014.21001

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Journal of Materials Science and Chemical Engineering, 2014, 2, 1-6

Published Online January 2014 (http://www.scirp.org/journal/msce)

http://dx.doi.org/10.4236/msce.2014.21001

OPEN ACCESS MSCE

Soot and Nanomaterials Synthesis in the Flame

Z. Mansurov

Institute of Combustion Problems, Almaty, Republic of Kazakhstan

Email: [email protected]

Received October 2013

ABSTRACT

The general scheme of conversion of hydrocarbon fuels with new experimental data on the formation of fullere-

nes and graphenes taking into account the pressure effect is proposed for the fuel-rich flames. It is shown that

the formation of fullerenes is important to the corresponding spatial orientation of PAH, possible at low pres-

sures. The formation of hydrophobic soot surface on silicon and nickel substrates during combustion of pro-

pane-oxygen flame was studied. It is established that the hydrophobic properties are due to the presence of soot

particles in the form nanobeads. The photovoltaic properties of solar cells coated by nickel oxide nanoparticles

synthesized in counter flow propane-air flame. It is revealed that coated the surface of a silicon solar cell by

nickel oxide nanoparticles results in the increase in solar cell efficiency by 3%.

KEYWORDS

Soot; Nanomaterials; Hydrophobic; Nanoparticle

1. Introduction

The process of soot formation has been the object of nu-

merous investigations for more than 100 years [1,2], and

such investigations did not loose their significance to the

present day. This is explained, first of all, by the fact that

soot is an industrial product produced on a world scale in

the amount of 107 tons a year. The black (technological)

carbon is used as a filler of elastometers (90% of the

technological carbon is used for this purpose, and 2/3 of

it—in the production of tires) and has a wide application

in printers. However, soot is a carcinogenic pollutant of

the environment, formed as a result of the combustion of

hydrocarbon fuels in power plants and engines. For ex-

ample, diesel engines with direct fuel injection initially

transform approximately 10% - 20% of the fuel intro-

duced into the soot. Simultaneously with the soot forma-

tion, fullerenes and nanotubes are formed by the mecha-

nism competing with the mechanism of soot formation.

A knowledge of the conditions and mechanisms of for-

mation of soot, fullerenes, and nanotubes in a flame al-

lows one to change the combustion such that soot parti-

cles, fullerenes, or nanotubes are predominately formed.

At the present time a large number of experimental

data on the processes of soot formation have been accu-

mulated and different phenomenological models have

been proposed [3,4]. However, the mechanism of soot

formation is imperfectly understood yet. This is ex-

plained by the fact that even in simple cases, such as the

homogeneous pyrolysis of hydrocarbons, this process

includes a large number of rapid simultaneous reactions

leading to the formation of a new solid phase—soot par-

ticles (e.g., the time of transformation of methane with a

molecular mass of 16 a.m.u into the soot with the mo-

lecular mass more than 106 a.m.u makes 10−4 - 10−2 s).

Considerable interest of scientific and technical com-

munities to study production processes, structure and

properties of nanosized systems is caused by variety and

uniqueness of their practical applications. The small size

of structural components—typically up to 100 nm - de-

termines the difference in the properties of nanomaterials

from massive analogues. Flame is a self-sustaining sys-

tem in which hydrocarbons can be precursors of carbon

nanomaterials, and the heat released during combustion,

is a parameter of the process control. It is known that

PAH are nucleation centers of forming soot i.e . PAH can

be converted into either soot or fullerenes. The formation

of CNTs occurs in diffusion flames from the fuel side

and is initiated by transition metals particles.

The C60 and C70 fullerene ions were detected in flames

in 1987 and identified by the mass-spectrometry method

[5]. Howard et al. [6] have obtained large amounts of C60

and С70 in laminar premixed soot forming flames of

benzene and oxygen at low pressures. Unlike the evapo-

ration of graphite, in the fullerenes formed in flames the

ratio C70/C60 changes from 0.26 to 8.8 (in the case of

Z. MANSUROV

evaporation of graphite, this ratio changes from 0.02 to

0.18).

The original results on development of carbon nano-

materials of different functional application which were

obtained at the Institute of Combustion Problems.

2. Formation of Soot and Synthesis of

Fullerenes in Flame

The formation of fullerenes occurs at low pressures, and

corresponding space orientation which requires the ac-

count of steric factor is important here.

It should be noted, that formation of such elegant

molecule of С60 requires the necessary space orientation

of two molecules of С30. There are different formation

models of fullerenes С60 one of which is carried out by

zipper-me c h an is m.

Low pressures are the necessary conditions of such

mechanism. With increasing of pressure, i.e., transition

to the atmospheric and above, where triple collision take

place and coagulation of PAH occurs, with formation of

soot clusters. Howard has shown that the maximum of

fullerene formation shifted to the right relative to the

maximum of soot formation. At detailed examination by

him the formation of fullerenes from benzene flame is

shown that there is the second maximum at a distance of

70 mm from matrix of burner [6].

These data become the basis for development of alter-

native method for obtaining of fullerenes in the regime of

hydrocarbons combustion.

A series of experiments on the study of the yield of

fullerenes in a premixed benzene-argon-oxygen flame

exposed to a longitudinal electric field under the condi-

tions of a dark discharge, a corona discharge, and a glow

discharge [4,7] at C/O = 1.0, P = 40 torr, a benzene-flow

rate Q1 = 250 cm3/min, an oxygen-flow rate Q2 = 758

cm3/min, an argon-flow rate Q3 = 101 cm3/min (10% of

the combustion-mixture volume), V= 18.4 cm/s.

It has been stated that the negative polarity of the up-

per electrode is more favorable for the fullerene forma-

tion as compared to the positive one. The investigations

carried out under the conditions where the upper elec-

trode was positioned directly above a flame have shown

that, in this case, the yield of fullerenes increases. The

influence of the type of an electrode and the height of its

position above a flame on the formation of fullerenes was

investi gat ed for determining the conditions providing

their maximum yield. Electrodes in the form of a needle

and a ring were used. The investigations were carried out

at a negative polarity of the upper electrode under the

conditions where UH = 7 kV, H = 1 - 9 cm (with a step of

1 cm). A glow discharge appeared independently of the

type of an electrode at different heights of its disposition

above the flame. In this case, the average temperature of

the flame increased to T = 1200˚C (without a field, this

temperature was equal to T = 950˚C).

Thus, it has been stated that the yield of fullerenes in-

creases under the action of a glow discharge in the case

where an electrode (a needle or a ring) is positioned di-

rectly above flame and that a maximum yield of fullere-

nes is attained with the use of a ring electrode placed

above the central region of the flame front. The maxi-

mum yield of the fullerene С60 was β = 15% of the soot

formed.

3. Formation of Carbon Nanotubes in

Flames

The most promising way to produce carbon nanotubes,

according to Merchan-Merchan et al. [8], is the flame

method. In the synthesis of carbon nanoparticles using

flames, part of the fuel is consumed in heating of the

mixture, and part is used as a reactant, which makes this

method more cost-effective than methods based on the

use of electricity, pyrolysis of hydrocarbons or arc

evaporation of graphite.

The results of the study of a flat diffusion propane-

oxygen flame stabilized on an opposed-jet burner at at-

mospheric pressure are presented in [7]. Two opposed

flows formed the flat flame. The flame was surrounded

by an external nitrogen flow supplied from the burner

matrices. A solution of catalyst [Fe(CO)5 or an alcohol

solution of nickel nitrate] was sprayed by an ultrasonic

nebulizer and delivered through a metal nozzle into the

flame from the side of the fuel.

The resulting products were deposited on the walls of

the reactor and collected in traps with liquid nitrogen.

The temperature in the reactor was measured by a ther-

mocouple, and in the flame by an Iron Ultrimax pyrome-

ter.

It is evident from Figure 1 that the samples contain

soot agglomerates, among which metal particles are en-

countered. It was found that under certain experimental

conditions, well-ordered bundles of carbon nanotubes 20

- 30 nm in diameter formed.

4. Formation of Hydrophobic Soot in

Hydrocarbon Flames

Low surface-energy materials like amorphous carbon

(a-C) films are frequently used to modify surfaces in or-

der to control their wettability. The nanobeads are mor-

phologicall y similar to the carbon nanopearls synthesized

by Levesque and co-workers [9] through acetylene dis-

sociation at 700˚C on nickel catalyst nanoclusters. Puri et

al. have determined new ways of the synthesis of carbon

nanotubes in the fuel-rich diffusion flames, exposed to an

electric field during 2 - 10 min, on superhydrophobic

surfaces representing nanodimensional round amorphous

Z. MANSUROV

Figure 1. Electron micrographs of samples: (1) carbon

nanotube; (2) Ni in a carbon shell.

carbon particles deposited on a silicon substrate [10].

The formation of hydrophobic soot surface on silicon

and nickel substrates during combustion of propane-

oxygen flame was studied [11]. It is stated that the hy-

drophobic properties are due to the presence of soot par-

ticles in the form nanobeads. The schematic diagram of

the synthesis process is presented in Figure 2 and Figure

3 shows water droplets on superhydrophobic soot.

Studies have shown that carbon deposits on the plates

different in the morphological structure of deposited par-

ticles in different zones. In the central and middle zone

long chains of individuals formed in the form nanobeads

15 - 30 nm without applying an electric field, and 40 - 50

nm with an electric field. In the outer zone, regardless of

the conditions of combustion, there are coagulated ag-

gregates of soot particles with sizes 30 - 50 nm.

The results for the exploration of the soot formation of

hydrophobic surfaces on silicon substrates and nickel

during combustion of propane oxygen flame are listed.

The distance from the burner matrix and the substrate

was varied, the exposure time and the influence of the

electric field of different polarity and voltage. It is shown

that at the exposure of more than 4 minutes the soot with

hydrophobic properties is formed and a division of the

soot surface area occurs. The application of an electric

Figure 2. Schematic diagram of the synthesis process.

Figure 3. Water droplets on superhydrophobic soot.

field narrows the soot deposition on the substrate and in

diameter of 2.5 - 3 cm from the centre; the soot super

hydrophobic surface with a wetting angle of more than

170˚ is formed.

5. Formation of a Layered Graphene in the

Flames

The study of the formation of layered graphene films was

carried out in the propane-oxygen flame under the fol-

lowing conditions: flow rate of propane—219.1 cm3/min,

the flow of oxygen—381.2 cm3/min, corresponding to

the ratio of C/O = 0.86.

The studies were carried out both with the addition of

argon in benzene-oxygen mixture in an amount of 300 -

650 cm3/min and without argon. As catalytic substrates

used plates made of copper and nickel, are placed in the

fire .

Varied range of residence time of the substrate in the

flame: 5, 10, 20, 30, 40, 60 seconds, 5 and 10 minutes,

the angle varied substrate relative to the vertical axis of

the flame: α = 0˚, 30˚, 45˚, 60˚, 85˚. Flame temperature in

the experiments was in the range 900˚ - 950˚. Formed on

the substrate samples were examined for particulate

structures Raman spectrometer NTEGRA Spectra.

It was found that a substrate of nickel is more pre-

ferred for the synthesis of graphene films. A copper

catalyst substrate is an intensive formation of the amor-

phous structure of carbon black and copper oxides. Fur-

ther studies were conducted on a nickel substrate.

Z. MANSUROV

Figure 4 shows the Raman spectrum of the two layers

of graphene, when placed in a flame of nickel substrate at

an angle of 30 degrees to the vertical axis of the flame

yielded two layers (IG/I2D = 1,1), Raman spectrum is

shown in F ig ure 4(a). The further increase in the angle

of inclination of the substrate relative to the vertical axis

of the flame (over 30˚) leads to the increase in the mini-

mum number of graphene layers formed on a substrate -

from 5 to 10 (IG/I2D = 1,7 - 2,4), Figure 4(b).

On the basis of the data on synthesis of fullerenes,

carbon nanotubes, superhydrophobic soot and graphene

in the flame it is possible to modify the general scheme

proposed by H. Bockhorn [12] for rich fuel flames, namely

(a)

(b)

Figure 4. Raman spectra of graphene layers synthesized on

nickel substrate in propane-oxyg en-argon flame (C/O = 0.86;

t = 917˚C; τ = 5 min); (a) two layers (α= 30˚, IG/I2D = 1,1);

(b) five layers (α = 45˚, IG/I2D = 1,7).

to make a pressure-coordinate, which allows the forma-

tion of fullerenes at low pressures, and soot at high pre-

ssures. In addition the scheme was completed by gra-

phene formation as an intermediate product stage of gra-

phene formation (Fig ure 5).

6. Increase of the Power of Solar Elements

Using Nickel Oxide Nanoparticles

Synthesized in Flame

The main advantage of synthesis of nanoparticles in hy-

drocarbon flames is that average time of the full conver-

sion of fuel in a narrow zone of flame front is a few mil-

liseconds that provide nearly immediate formation of

expected product. Catalysts which are necessary for the

growth of nanoparticles of metals or their oxides can be

introduced. Different types and methods of introducing

catalysts into reaction flame zone allow influencing the

properties and sizes of nanoparticles being obtained [13].

This work presents the results of the investigations on

synthesis of nickel oxide nanoparticles in propane-oxy-

gen counter-flow flame [14]. The generated nanoparticles

are subsequently used to increase the efficiency of light

conversion in the solar cells.

The burner is positioned inside a hollow stainless steel

cylinder with the diameter of 150 mm and height of 82

mm. Two nozzles are installed on the axis of this cylin-

der opposite to each other. The nozzles are composed of

two cylinders with one cylinder inserted into another.

Oxidizer and fuel are supplied through the internal cyl-

inders from the opposite sides. In this study, propane was

used as a fuel and oxygen was used as an oxidizer. A

Figure 5. Modified scheme for soot, fullerenes and graphene

formation process in flames.

Z. MANSUROV

diameter of a visible flame front was 30 - 35 mm.

Nichrome wire with the diameter of 0.3 mm was used

as a substrate for the growth of nickel oxide nanoparti-

cles. X-ray fluorescent analysis of Nichrome wire shows

the following composition: Ni—60.27%, Fe—25.26%,

Cr—14.45% and Ti—0.0174%. Before nanoparticle

synthesis, Nicrome wire was pretreated for 20 minutes

with 25% solution of nitric acid.

Figure 6 shows scanning electron microscope (SEM)

images of Nichrome wire treated in oxygen zone of dif-

fusion counter flow propane-oxygen flame for the time

interval of 2 minutes.

The results of SEM studies show that the treatment of

nicrome wire with flame for 2 minutes leads to the for-

mation of nickel oxide nanoparticles on its surface with

an average size of 300 nm. High temperature and active

radicals formed in propane-oxygen flame interact with

nickel surface promoting the growth of nickel nanoparti-

cles. After coating of nickel oxide nanoparticle were on

the surface of solar cell on atomic power microscope.

The resulting image (Figure 7) shows that the size of

metal oxide nanoparticles on the surface of a solar cell

depends on the residence time in the flame.

Silicon solar cells with an active region of 1 cm2 were

used in this study. The solar elements being investigated

were made of monocrystalline silicon alloyed with boron

of p-type conductivity with specific resistance of 10

Ohm⋅cm and specific crystallographic orientation (100).

Silicon plates with the thickness of 300 µm were

smoothly-polished on both sides. Frontal n-layer was

formed by a thermal diffusion of POCl3 at the tempera-

ture of 900˚C for 20 minutes in inert medium. This

Figure 6. SEM images of a nichrome wire treated in the

oxygen zone of the diffusion counter flow propane-oxygen

flame.

(a)

(b)

Figure 7. Micrograph of nickel oxide nanoparticles on the

surface of the solar cell, 5 sec (a) 2 min (b).

resulted in the formation of p-n transition with the thick-

ness of ~0.3 µm. Back contact was formed by spraying

metallic aluminum in high vacuum with subsequent an-

nealing at 700˚C. Front side Ti-Ag contacts were applied

using photolithography. In-Ga contact was soldered di-

rectly to n-layer. The surface of the solar cell of such a

construction was covered with nickel oxide nanoparti-

cles.

To obtain uniform coating based on nanoparticles on

the surface of the solar cell, suspension of nanoparticles

in ethanol was preliminary created using an ultrasonic

bath. Up to 0.1 ml of suspension was necessary to coat

one solar element. Prior to coating, the measurements of

a short–circuit current and an open-circuit voltage of non-

coated solar element were carried out. Without changing

the conditions of the experiment short-circuits current

and open-circuit voltage were measured after application

of the coating.

The measurement unit consisted of the chamber cov-

ered with lightproof tissue from the outside to protect it

from mechanical dust and external sources of radiation.

Halogen lamp which stably radiated light similar to solar

spectrum was used as a source of light. Inside the cham-

ber there was a special support for fixing the solar cell.

The chamber allowed reproducible measurements with a

great accuracy irrespective of the substitution of a sample

being investigated. The measurements for each sample

were carried out with the time interval of 30 minutes for

3 hours. The values of current and voltage were regis-

tered using multimeter (with the accuracy of ±0.5%).

Z. MANSUROV

Power output of uncoated vs. coated solar cell with high

concentration of nickel oxide/methanol suspension. Bars

indicate a 95% confidence interval for measured values

(Figure 8).

The wavelength dependent light transmission of the

coating applied to the surface of the solar element is of a

great importance for its effective work. The spectra of

the transmission of the coating based on nickel oxide

nanoparticles to the surface of quartz substrate with the

concentration of 8 × 104 particle/cm2 were recorded in a

wavelength range from 400 to 1100 nm. The analysis

shows that in a short-wave region a slight decrease is

observed, because of light absorption in nanostructures

whereas in visible and long-wave regions transmission

coefficient reaches 93%.

Open-circuit voltage increased to 4% - 7%, the short-

circuit current increased to 20% - 28%, efficiency of the

solar cells increased by 2% to 3%, at a fill factor of the

element being equal to 0.75. The absence of other factors

which may cause the increase of the output power of the

solar cells means that original cause is the application of

nanoparticles.

We would like to note uniqueness of using the counter

flow of the burner to the opposing jets for the synthesis

of nanomaterials, which was created by Potter [15] and

Weinberg [16] to study the structure of the flame front.

Diffusion burner on the counter flow can be effec-

tively used for the production of carbon nanotubes in the

synthesis of the fuel, as well as for the introduction of

metal oxide nichrome wire into the zone of oxygen sup-

ply.

Figure 8. Power output of uncoated vs. coated solar cell

with high concentration of nickel oxide/methanol suspen-

sion.

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