In figure 4.12, we reported the response curves of Cu-doped NiO based sensor at a different operating temperature. The operating temperatures were ranging from 60 to 220°C. We can see from Figure 4.11 that the sensor shows high sensitivity about 102, 94.3, 77.1 % for operating temperatures of 140, 110 and 180°C towards 1000 ppm of ethanol, acetone, and methanol fumes respectively, which means that Cu doping at 3 at.% led to sensor sensitization which had a positive effect on operating temperature, which decreased significantly in the case of ethanol and acetone. But had a negative
(a) 03 at.% Co
effect in the case of methanol where its operating temperature increased significantly. Figure 4.12 (b) shows that the sensor has a higher sensitivity than that shown in Fig- ure 4.12 (a) for all vapors, which means that increasing the doping rate up to 12 at.% led to improved sensor sensitivity in general, where they are 110.9, 97.2 and 84.8 % for operating temperatures of 140, 110 and 180°C towards 1000 ppm of ethanol, acetone, and methanol vapors respectively.
Figure 4.12: Response curves of Cu-doped NiO based sensor towards methanol, etha-
nol and acetone (1000 ppm) at different operating temperatures.
4.5.3.2 Sensitivity
Figure 4.13 shows the dynamic responses of the Cu-doped NiO-based sensors to 1000 ppm of the ethanol, acetone and methanol vapors at the optimum operating tem- perature of each gas (vapor).
(a) 03 at.% Cu
The dynamic responses of the Cu-doped NiO based sensors to all vapors showed a better response than those obtained from the undoped and Co-doped NiO based sen- sors this in the cases of Cu doping at 3 at.% and 12 at.%. We also observed that sensor resistance increases when the sensor layer is exposed to all the vapors with a sensitivi- ty of 102, 94.3 and 77.1 %. For ethanol, acetone and methanol, respectively, in case of Cu doping at 3 at.% and 110.9, 97.2 and 84.8 %. For ethanol, acetone and methanol re- spectively in the case of Cu doping at 12 at.%. The maximum responses of Cu-doped NiO based sensors for ethanol, methanol and acetone are summarized in table 4.4.
Figure 4.13: Responses of Cu-doped NiO based sensors towards 1000 ppm of ethanol,
acetone and methanol at the optimum operating temperatures.
(a) 03 at.% Cu
4.5.3.3 Selectivity
Figure 4.12 shows a significant difference between the operating temperatures of the three vapors: 140°C for ethanol, 110°C for the acetone and 180°C for methanol, which means that the Cu-doped NiO based sensor can clearly distinguish the gases ac- cording to the operating temperature.
4.5.3.4 Response and recovery times
The response and recovery times were determined for the Cu-doped NiO based sensor from the figure 4.13. The response times are 67.6, 63.3, 74.2 seconds for etha- nol, acetone and methanol and recovery times are 109.5, 100.1 and 104 seconds for ethanol, acetone and methanol for sample doped with 3 at.% of Cu. For the second sample which doped with 12 at.% of Cu the response times are 72, 68.4 and 76.2 se- conds and recovery times are 103.1, 101.7 and 100.2 to ethanol, acetone and methanol. It is clear through the results of the samples that there is a clear decreasing in re- sponse times, unlike recovery times that have seen increased for all vapors (gases). This means that Cu doping has accelerated the gas reaction mechanism with the sensi- tive layer of the Cu-doped NiO thin films.
4.5.3.5 Detection limit
Figure 4.14 shows the variation of the sensitivity of Cu-doped NiO based sensor as a function of acetone, methanol and ethanol concentration ranging from 100 to 3000 ppm was studied at the optimum temperature for. The response increases linearly as concentration of vapors increased from 100 to 500 ppm. The slope of all the graphs decreased with concentration which is due to occurrence of saturation in the response.
The sensitivity of Cu-doped NiO thin film doped by 3 at.% of copper to 100 ppm is about 41 % for methanol vapor, 59 % for acetone vapor and 40 for ethanol vapor, while for the doping sample 12 at.% of copper the sensitivity of the sensor to 100 ppm in- creased to 43 % for methanol vapor and 65 % for acetone vapor and 24 % for ethanol vapor. This increase was due to the effect of copper doping on the morphological and electrical characteristics of the films, which improved the process of chemical and physical adsorption of oxygen and gas and thus improved sensitivity values. The sen- sitivity of our sensor remains significant even for the lowest concentrations lowest to 100 ppm for acetone, methanol and ethanol vapors.
Figure 4.14: Variation of the sensitivity of Cu-doped NiO based sensors as function of
concentration of vapors.
Table 4.4: Gas sensing performance of Cu-doped NiO based sensor towards methanol, ethanol and acetone vapors.
Material Target gas Sensitivity S%
Operating temperature (ºC) Response Time (S) Recovery Time (S) Detection Limit (ppm) Cu-doped NiO (3 at.%) Ethanol 102.0 140 67.6 109.5 <<100 Acetone 94.3 110 63.3 100.1 <<100 Methanol 77.1 180 74.2 104.0 <<100 Cu-doped NiO (12 at.%) Ethanol 110.9 140 72.0 103.1 <<100 Acetone 97.2 110 68.4 101.7 <<100 Methanol 84.8 180 76.2 100.2 <<100 (a) 03 at.% Cu (b) 12 at.% Cu
4.5.1.6 Stability
The figure 4.15 shows the sensitivity of Cu-doped NiO based sensors to 1000 ppm of acetone, ethanol and ethanol at the optimum operating temperature of each gas for 31 days. Figure (a) illustrates the instability of the sensor response especially after the ninth day of figure (a) and the seventh day of figure (b), which means that the sensor is poisoned by the effect of gas, air or moisture, and is likely to have the effect of air and humidity, where air and humidity and under fairly high operating temperatures may lead to the oxidation of a part of the sensitive outer layer of the film containing Easy oxidation copper ions which prevents the interaction of the gas molecules with the film's sensitive layer.
Figure 4.15: Stability characteristics of Cu-doped NiO based sensors.