As one of the most widely used domestic fuels, the detection of possible leakages of Liquefied Petroleum (LP) gas from production plants, from cylinders during their storage, transport and usage is of utmost importance. This article discusses a study of the response of undoped and chlorine doped electrodeposited n-type Cuprous Oxide (Cu 2O) films to of LP gas. Undoped n-type Cu 2O films were fabricated in an electrolyte bath containing a solution of sodium acetate and cupric acetate whereas n-type chlorine doped Cu 2O thin films were prepared by adding a 0.02 M cuprous chloride (CuCl 2) into an electrolyte solution containing lactic acid, cupric sulfate and sodium hydroxide. The n-type conductivity of the deposited films was determined using spectral response measurements. The structural and morphological properties of the fabricated films were monitored using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Due to doping, the overall conductivity of the chlorine doped n-type Cu 2O films increased by several orders of magnitude. The temperature dependent gas responses of both the undoped and chlorine doped n-type Cu 2O thin films to the LP gas was monitored by measuring the electrical resistance (R), and using the contact probe method at a constant gas flow rate of 0.005 ml/s. Upon exposure to gases, both doped and undoped films showed a good response to the gas by increasing/decreasing the electrical resistance by ΔR. The undoped n-type Cu 2O thin films showed a negative response (ΔR < 0) at all temperatures resulting in a maximum response around 85°C. However, the chlorine doped n-type Cu 2O thin films initially showed a positive response (ΔR > 0) to the LP gas which then reversed its sign to give a negative response which peaked at 52°C. The positive response shown by the chlorine doped Cu 2O films vanished completely at 42°C.
Environmentally hazardous gases are being released continuously to the atmosphere due to industrialization, increased human activities and the natural processes that take place as a result of drastic changes in the environment. Therefore, monitoring of environment has been of extreme importance for the safety and well-being of human and animal life and nature in general. Consequently, gas sensing has become an important area of research that leads to the development of highly responsive gas sensing devices capable of detecting minute amounts of gases of different types. One such gas that requires monitoring in the context of developing countries is the highly inflammable Liquefied Petroleum (LP) gas which is being used very widely as a domestic fuel. At most times leakage of LP gas from production plants or from cylinders occur during their storage, transport and usage. These leakages at most times can be found from the odor of the gas, however, by then a significant amount of gases may have leaked out to the surroundings. Gas sensor devices enable the early detection of such leakages thus preventing accidents and wastage while helping maintain a safer and cleaner environment.
With the pioneering work reported in 1962 by Seiyema et al. [
It has been known that the electrodeposition can be used to fabricate Cu2O thin films with both n-type and p-type conductivities [
Both undoped and chlorine doped n-type Cu2O thin films were deposited on Ti substrates, a process that has been well established in the research group [
A film sample was then enclosed in a gas sensing chamber made of stainless steel. Chamber contained two compartments; the top through which the gas was flown and the bottom where the heating element was housed. In order to measure the electrical resistance, contact probes were fixed on to the surface of the film sample which was placed on an asbestos heating platform. Externally, the probes were connected to a multimeter, which in turn was connected to a computer data logger (
All the measurements were made under atmospheric conditions by using a flow through technique. While maintaining a constant gas flow rate of 0.005 ml/s, films were exposed to LP gas for approximately 30 - 40 s. and then the gas flow was stopped. The sensing temperature was varied between 30˚C and 100˚C while monitoring the temperature with a thermocouple (type K) which was in contact with surface area of the substrate which was not covered by the Cu2O film. The temperature was controlled using a thermostat with a temperature controller. The electrical resistivity measurements were made using computer interfaced a Keithley 2100 digital multimeter. The measurements were taken over a period of approximately 120 s after the gas was sent in to the chamber and repeated when the temperatures were brought down to the room temperature. This procedure was repeated at different temperatures in order to determine the temperature that corresponds to the maximum gas sensitivity.
samples indicating that both films have uniform polycrystalline coverage. However, effect of doping has caused the average polycrystalline grain size to reduce towards nanoscale compared to that of the undoped Cu2O thin films in which the average grain size is in the micro scale. Through SEM measurements, it was found that the average polycrystalline grain size of chlorine doped Cu2O thin films showed a concentration dependence on CuCl2. It was observed that the increased CuCl2 concentration caused the average grain size to reduce gradually providing a larger effective surface area to interact with the gas molecules [
This decrease in the resistivity was quite significant when compared to the resistivity of undoped Cu2O thin
films fabricated electrochemically using the acetate bath which produced Cu2O thin films in which the resistivity was larger by over 5 orders of magnitude than the chlorine doped Cu2O thin films obtained with the 0.02 M CuCl2 precursor concentration in the electrolyte in the lactate bath.
As mentioned above, the gas sensing properties were measured by monitoring the resistance of the Cu2O thin films upon exposure to LP gas and its stoppage after a certain period. The response to the gases are represented as resistance vs. time curves and the magnitude of sensitivity vs. time curves where the magnitude of sensitivity is termed as
Here RLPG is the resistance of the film upon exposure to LP gas and Rair is the resistance of the film when it is under normal atmospheric conditions.
Response at Low TemperaturesThis behavior can be well understood in terms of the mechanism given by Shukla [
This reduces the resistance of the film until the LP gas flow in to the chamber is stopped. The stoppage of the gas flow causes atmospheric oxygen to gradually adsorb on the Cu2O film once again allowing the resistance to recover back to its value under atmospheric conditions.
In contrast, upon exposure to LP gas, the resistance of the chlorine doped Cu2O films increased (RLPG > Rair) initially showing a positive response and then showed a negative response (RLPG < Rair) as in the case of undoped
Cu2O films yielding two peaks in the magnitude of sensitivity vs. time graph.
Therefore, the first peak appears to have arisen due to the presence of chloride ions in the doped Cu2O films. Thus for the sake of clarity, the first peak is termed as the chlorine peak whereas the second peak is termed as the Cu2O peak. Furthermore, it can be seen that the chlorine peak reduces its intensity with increasing temperature disappearing completely at 42˚C whereas the intensity of the Cu2O peak continues to increase.
This behavior can be understood in the following manner. Overall reduction of the resistance of the chlorine doped thin films measured under atmospheric conditions is attributed to the increase of electron density in the conduction band of the Cu2O thin film which occurs due to the electrons coming from the donor impurity levels. Thus the doping concentration controls the overall resistivity of the Cu2O film. However, when the films are exposed to the LP gas, due to the formation of electrostatic interactions between induced dipoles of LP gas molecules with the chloride ions in the film, the electron pumping process to the conduction band is retarded causing the resistance to increase initially. When the interaction with chloride ions reaches its saturation, the adsorbed oxygen in the film starts to react with the LP gas molecules as stated above according to the Equation (2) causing the resistance to decrease similar to the undoped Cu2O films until the gas flow is stopped. It can also be seen that compared to the undoped Cu2O films, the initial response to the LP gas arising from the chlorine doped Cu2O films is quicker.
When the chlorine doped thin films are exposed to LP gas at temperatures > 42˚C, the absence of the chlorine peak indicates that the gas is not responsive to chloride ions present in the film sample due to the weakening of the dipole interactions between the LP gas molecules and the chlorides at higher temperature. Thus the contribution to the sensing behavior arises only from the adsorbed
when the LP gas was sent changing the film temperature in the range from 50˚C - 95˚C. Measurements showed that the intensity of the Cu2O sensitivity peak of the chlorine doped sample increased with temperature up to about 55˚C and decreased with further increase in temperature. By varying the temperature around 55˚C, it was found that the maximum sensitivity was resulted in at 52˚C. In contrast, the undoped Cu2O films showed the maximum sensitivity around 85˚C as shown by the
Above behavior was repeatedly observed when both doped and undoped Cu2O thin film samples were exposed to the LP gas indicating that doping affects the overall gas sensing behavior of the films also.
In conclusion, it can be clearly seen that the chlorine doping has altered the gas sensing behavior of n-type Cu2O thin films significantly. While chlorine doping causes the resistance of Cu2O films to go down drastically, the presence of chloride ions in the film has resulted in an additional sensitivity peak that exists closer to the room temperature. It can be interpreted that the doping has caused LP gas molecules to interact initially with more active chloride ions providing a faster positive response, i.e.
University Grant Commission of Sri Lanka is gratefully acknowledged for financial assistance provided through the research grant UGC/ICD/RG 2011. KNDB is thankful to the Open University of Sri Lanka, for granting leave to carry out this research study.