Chapter 5 Results and discussion – polymer/CNT stacked sensors
5.1. Sensing response of six toxic industrial gases
(a) Response of SAA/MWCNTs stacked sensor
The cycle room temperature sensing response ( △ R/R %) curves for SAA/MWCNTs stacked sensors under three different gas concentrations are shown in
Figs. 5-1(a) and 5-1(b) for two different gases (THF and CHCl3), respectively. The graphs illustrate a time-dependent change of parameter sensor response (△R/R %) representing sensitivity of the SAA/MWCNTs stacked sensors. The curves also show the reponses during gas inlet and outlet cycling periods, which is related to gas adsorption/desorption on the sensor. It indicates an advantage of quick repeating usage of the sensor without additional heating to desorp the gas. By comparing Fig.
1(a) with Fig. 1(b), the former gives a quick greater response due to lower polarity of both THF gas and SAA polymer. The interaction of CHCl3 and SAA polymer is slower due to a greater polarity difference of CHCl3 gas and polymer, but it shows to reach more easily to an equilibrium response value, which is more favour to determine the gas concentration. For SAA/MWCNTs stacked sensors, the average values of sensing response change for each gas cycling period are approximately 0.8
% and 0.2 % for THF and CHCl3 gases, respectively, as shown in Table 5-1. These values are defined at time reaching about 95% of total change during gas adsorption or desorption periods. As can be seen from the graphs, the time required is depending on the difference in polarity of gas with SAA polymer. A greater difference in polarity gives rise to a longer time. In other words, the response and recovery times are about 15–30 sec and 200-300 sec for CHCl3 and THF gases, respectively. In general, the response curves demonstrate good reversibility of adsorption/desorption processes, though there are some abnormal sharp peaks in the response curves probably due to partial destruction of some contacts between crossing nanotubes in SAA/MWCNT stacked sensing film(Yoon et al. 2006)
.
-0.2
Fig. 5-1 Effect of gas type and concentration on cycle sensing response (△R/R %) curves for SAA/MWCNTs stacked sensors: (a) THF and (b) CHCl3 gases,
respectively.
(b) Response of PMVEMA /MWCNTs stacked sensor
The cycle sensing response (△R/R %) curves for PMVEMA /MWCNTs stacked sensors under three different gas concentrations for six different gases: (a) MEK, (b) THF, (c) toluene, (d) Xylene, (e) CHCl3, and (f) CCl4 are presented in Figs. 5-2(a) to 5-2(c) and 5-3(a) to 5-3(c), respectively. The curves also show three cycling responses during the gas inlet and outlet periods. The figures demonstrate the similar
(a)
(b)
reversibility of adsorption/desorption processes, as compared with the SAA/MWCNT stacked sensor in the previous section, except a greater ( △ R/R)max value for SAA/MWCNTs stacked sensor under the same gas type. By comparing SAA/MWCNT with PMVEMA/MWCNT stacked sensors, the average values of the maximum sensing response change, (△R/R)max, for each gas cycling period are approximately 0.8 % vs 1.2% and 0.2 % vs 0.8% for THF and CHCl3 gases, respectively, as also shown in Table 5-1. In other words, the PMVEMA/MWCNT stacked sensor is more sensitive then SAA/MWCNT stacked sensor for these two gases. As to the response and recovery times, the PMVEMA/MWCNT stacked sensor gives the same value of 15–30 sec for CHCl3 gas and a faster value of 15-30sec instead of 200-300 sec for THF gases.
For other four gases (MEK, toluene, Xylene, and CCl4), there are no detectable response by SAA/MWCNT stacked sensor; but for the PMVEMA/MWCNT stacked sensor, the corresponding average maximum response values are about 0.2%, 0.4%, 0.6% and 0.2%, respectively, as show in Table 5-1. The faster response and recovery times are apparent and are in the range of 15–30 sec.
-0.2 curves for PMVEMA/MWCNTs stacked sensors: (a) MEK, (b) THF, and (c) toluene
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(c)
-0.8 curves for PMVEMA/MWCNTs stacked sensors: (a) xylene, (b) CHCl3, and (c) CCl4
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(c)
(c) Response of PMS/MWCNT stacked sensor
The cycle sensing response (△R/R %) curves for PMS/MWCNT stacked sensor under three different gas concentrations for six different gases: (a) MEK, (b) THF, (c) toluene, (d) xylene, (e) CHCl3, and (f) CCl4 are presented in Figs. 5-4(a) to 5-4(c) and Figs. 5-5(a) to 5-5(c), respectively. As shown in Table 5-1, the (△R/R)max values of this sensor are ranging from 0.3% to 1.9%, indicating their sensing sensitivities of the sensors. The minimum and maximum sensitivities are to sense MEK and THF gases, respectivily. It is found that the PMS/MWCNT stacked sensor gives the best sensing sensitivity amoung the eight stacked sensors. Another advantage of this sensor is that the reponse and recovery times are within acceptable range of 15 - 30 sec.
-0.2 curves for PMS/MWCNTs stacked sensors: (a) MEK, (b) THF, and (c) toluene
(c) (b) (a)
-0.2 curves for PMS/MWCNTs stacked sensors: (a) xylene, (b) CHCl3, and (c) CCl4
(c) (b) (a)
(d) Response of five other stacked sensors
The five other stacked sensors include PEA/MWCNT, PVP/MWCNT, P(VDC-AN)/MWCNT, HPMC/MWCNT, and PVBC/MWCNT. It is known that polymer selection for fabricating the stacked sensor within a sensing chip depends greatly on the distribution of their chemical functionalities to interact with test gases.
If the target polymers possess a broad distribution of chemical functionalities, then it will result in a broad distribution of sensing responses to determine the gas specificity.
From Table 5-1, it indicates that the sensing sensitivity is varied from sensor to sensor and from gas to gas. In other words, the eight stacked sensors on a chip in this work possess the ability to differentiate the gas type or gas specificity of the gas group, which consists of six toxic industrial gases. This is the best advantage of this sensor array.
(e) The bar chart and radar plots of the sensing chip for this gas group
Based on (△R/R)max values in Table 5-1, Figs. 5-6, 5-7, and 5-8 show the sensing response bar charts of the sensing chip with eight polymer/MWCNT stacked sensors. The testing group of gases in this case includes six toxic industry compounds (TIC) gas (MEK, THF, toluene, xylene, CHCl3, and CCl4) under three different concentrations. The corresponding sensing response radar plots are indicated in Figs.
5-9, 5-10, and 5-11, respectively.
From the bar charts, it is noted that the variations of sensing response with respect to gas concentration are nearly linear for most of the sensors, except PVP assisted sensor for toluene gas sensing. In other words, the most of the sensors in the chip can be used to determine the gas concentration. It is also noted that these bar charts or radar plots indicate different patterns under different gas types and
concentrations. In other words, selections of polymer group in the sensing chip in this work are good enough for determining the gas specificity of this gas group.
0
PVP PMVEMA PMS HPMC PEA PVBC
△R/R(%)
SAA P(VDC-AN) PVP PMVEMA PMS HPMC PEA PVBC
△R/R(%)
THF (9748ppm) THF (8346ppm) THF (6781ppm)
Fig. 5-6: Bar charts of the peak sensing response, (△R/R)max, of the sensing chip under different gas concentrations for (a) MEK, and (b) THF gases, respectively.
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(b)
(△R/R)max %(△R/R)max %
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SAA P(VDC-AN) PVP PMVEMA PMS HPMC PEA PVBC
△R/R(%)
SAA P(VDC-AN) PVP PMVEMA PMS HPMC PEA PVBC
△R/R(%)
Xylene (1417ppm) Xylene (708ppm) Xylene (354ppm)
Fig. 5-7: Bar charts of the peak sensing response, (△R/R)max, of the sensing chip under different gas concentrations for (a) toluene, and (b) xylene gases, respectively.
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(b)
(△R/R)max %(△R/R)max %
0
PVP PMVEMA PMS HPMC PEA PVBC
△ R /R (% )
SAA P(VDC-AN) PVP PMVEMA PMS HPMC PEA PVBC
△R/R(%)
CCl4 (5033ppm) CCl4 (2445ppm) CCl4 (2225ppm)
Fig. 5-8: Bar charts of the peak sensing response, (△R/R)max, of the sensing chip under different gas concentrations for (a) CHCl3, and (b) CCl4 gases, respectively.
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(b) (△R/R)max %(△R/R)max %
0 under different gas concentrations for (a) MEK, and (b) THF gases, respectively.
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(b)
%
%
0 under different gas concentrations for (a) toluene, and (b) xylene gases, respectively.
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%
%
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