Figure 4
Sensor response to ethanol.
(a) The 3D plot of the sensitivity (S) to 1 ppm ethanol as a function of the working temperature and types of MHP. It clearly indicates that gas sensor has the highest sensitivity at a working temperature of 350 oC. And the sensor based on MHP 3 has higher sensitivity than other two sensors. (b) The sensitivities of sensors based on MHP 3, with monolayer, double-layer and triple-layer of NPA. Sensors with double-layer or triple-layer of NPA have lower sensitivity and larger error than sensors with monolayer of NPA. (c) Sensor response to ethanol with three different levels of concentrations: from 20 ppb to 100 ppb, from 100 ppb to 500 ppb and from 1 ppm to 5 ppm. The response time and recover time to ethanol are around 2 s with a concentration less than 100 ppb and decrease to around 1 s and less than 1 s when the concentration increase to 100 ppb – 500 ppb and 1 pm – 5 ppm respectively. Sensitivity to 20 ppb ethanol is 1.06. The sensitivity (S) has a linear relation to ethanol concentration (C) via linear fitting can be denote as S = 1 + 0.0023 × C. When the concentration increases to the level of several hundred ppb, sensitivity and concentration still have a good linear relationship as S = 1 + 0.0024 × C. When the concentration of ethanol increase to ppm level, there is a nonlinear relationship between sensitivity and concentration. Sensitivity tends to reach a constant value if the concentration keeps increasing.
