Sensing and identification of carbon monoxide using carbon films fabricated by methane arc discharge decomposition technique
© Akbari et al.; licensee Springer. 2014
Received: 5 May 2014
Accepted: 24 July 2014
Published: 19 August 2014
Carbonaceous materials have recently received attention in electronic applications and measurement systems. In this work, we demonstrate the electrical behavior of carbon films fabricated by methane arc discharge decomposition technique. The current-voltage (I-V) characteristics of carbon films are investigated in the presence and absence of gas. The experiment reveals that the current passing through the carbon films increases when the concentration of CO2 gas is increased from 200 to 800 ppm. This phenomenon which is a result of conductance changes can be employed in sensing applications such as gas sensors.
Continuous emission of carbon dioxide (CO2) and other greenhouse gases by industrial activities has been increased recently and has led to global warming. This calls for the need to develop low-cost, sensitive, resettable sensors that can be used to monitor the CO2 concentration in industrial exhaust gases [1–3].
Over the past few years, graphene and carbon nanotubes have become the center of attention in the sensor manufacturing technology [4–8]. Furthermore, their unique electrical properties such as tunable conductance and high charge mobility make them ideal for application as sensing medium in nanotechnology [9, 10]. In this paper, we have designed and developed a method for the fabrication of a carbon film material implementing high-voltage AC arc discharge [11–14]. In the proposed system, pure methane in atmospheric pressure is passed over the electrodes inside a Pyrex glass tube chamber where the carbon film fabrication process takes place [15–17]. Once the arc ignites between the graphite electrodes, the methane gas starts to decompose to its constituent species. At the end of this process, a fine soot of carbonaceous material remains between the two electrodes. The material produced this way is then checked through high-resolution optical microscopy as well as scan electron microscopy (SEM) to observe the material physical and structural characteristics. Once the carbon films are grown, the measurement process is carried out.
Arc discharge decomposition
Generally, when a voltage is applied to two electrodes, an electrical potential is created which tends to move electrons from the positive pole to the negative. This is what causes an electric flow of electrons or electric current through a wire or resistance. When there are no conductive wires and/or resistors connecting the two electrodes, i.e., there is either an insulating barrier or simply the ambient air between them, no flow of electrons occurs under normal circumstances for low voltages. In case of high-voltage arc discharge, when the voltage is increased, the methane between the electrodes is ionized. In this situation, the non-conductive medium breaks down and becomes conductive, allowing for the charge carriers to travel through it. This phenomenon occurs very fast and is usually accompanied by sparks and light emissions. As a matter of fact, the electrons inside the gap are accelerated with the applied voltage and cause electron impact ionization. When methane is present in the gap between the electrodes, it will be defragmented into carbon and hydrocarbon species. This electric arc discharge under flowing methane is then used in the experiment for carbon decomposition.
Operating parameters of carbon strands
At room environment
1 to 26 kV
200 to 800 ppm
Pure methane (99.99%)
Diagnostics of the carbon film
Inspection by scan electron microscopy
Among all types of carbon allotropes, only graphene, graphite, and CNTs show electrical conductivity. On the other hand, the carbon films also show conducting behavior. This implies that the grown carbonaceous materials belong to one of the above types of graphitized carbon. With reference to similar images from carbon materials published in the literature [19–21], it can be observed by comparison that the scanned material is composed of carbon.
Results of optical emission spectroscopy
The results of the evolved species in the second phase are different from initial ionization process of pure methane regarding the evolved species. In the second phase, the high peak belongs to C2 radical which also indicates that the concentration of C2 is much higher in the methane plasma than the other evolved species. The second spectrum also indicates the pyrolysis process of gaseous hydrocarbons that causes carbon deposition between electrodes. The evolved species consist of swan band C2 which appears at 516.75 nm and C2 at 590 nm, while the two peaks corresponding to hydrogen Hα and CH are absent.
Species of pure methane evolved during decomposition process
Excitation energy (eV)
Evolved in first phase
Evolved in second phase
Measurements of electrical characteristics
Results and discussion
In the presence of gas, higher values of current were read which proves the higher conductivity when the carbon films are subjected to gas. Also, as the concentration of gas was increased from 200 to 800 ppm, the current passing through the channel increased further. This phenomenon can be explained by the fact that gas molecules are adsorbed on the carbon film surface and will increase channel conductivity.
Values for parameters a, b, c, and d and the corresponding regressions
F(x) = aexp(bx) + cexp(dx)
7.859e + 5
−7.859e + 5
2.999e + 6
−2.999 + 6
A set of experiments were carried out to fabricate carbon films using high-voltage arc discharge methane decomposition method. High-resolution optical microscopy as well as OES and SEM imaging techniques were implemented to verify the fact that the substances obtained are carbonaceous materials. The carbon films were then used as the channel in an electrical circuit to measure their current-voltage characteristics. Among all types of carbon allotropes, only graphene, graphite, and CNTs show electrical conductivity. On the other hand, the carbon films also show conducting behavior. This implies that the grown carbon films belong to one of the above types of graphitized carbon. It was observed that higher currents pass through the channel when it is exposed to higher concentrations of gas. A mathematical model was obtained for the experimental results using MATLAB curve fitting tool. With the aid of this mathematical representation, it will be possible to characterize and predict the electrical behavior of the carbon films. This will provide a reliable mathematical model which can be used in gas sensing applications to minimize the need for conducting experimental studies.
The authors would like to thank Ministry of Education (MOE), Malaysia (grant Vot. No. 4 F382) and the Universiti Teknologi Malaysia (grant Vot. No. 07H56) for the financial support received during the investigation.
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