Complex Detection

Ammonia Vapor Detection

The DWCNTs sensors were first exposed to ammonia vapor. Since some water vapor is present in ammonia vapor exposure, a control experiment of water vapor exposure is

necessary. Because the ammonia concentration is very low, the amount of water vapor from ammonia solution approximates to it is from pure water in the same condition. In the following experiments we thus compare the signal of our devices when confronted to ammonia vapor or to the pure water vapor.

Raw DWCNTs Gas Sensor: When the raw DWCNTs sensor is exposed to ammonia and water, there are very distinct but low sensitivity as depicted in Figure 2.8. The black line is the response to water vapor and the red line is response to ammonia. This is one sensor expected to have high sensitivity, of which the resistance is ca. 91 KΩ, and the raw DWCNTs film-Au electrode connection is metallic.

Figure 2.8: A raw DWCNTs sensor’s response to water vapor (left)and ammonia solution vapor (right). R₀ is the initial resistance before exposure which is ca. 91 kΩ while R is the real-time resistance.

The relative resistance variation reaches less than 2% for pure water, while it reaches 7 % in the case of ammonia vapor.

There is thus a clear contrast in the signal between water vapor and ammonia solu- tion vapor exposure. The ammonia detection mechanism has been explained in Ref.[16]. However, it is difficult for pristine carbon nanotube to adsorb ammonia. The interac- tion between pristine carbon nanotube and ammonia is weak and induce small amount of charge transfer[17][18][19]. As a consequence the resistance variation remains small. However, when exposed to the ammonia, the Schottky barrier at junction of raw DWC- NTs film and Au electrode is also modulated by the ammonia molecules as explained in Ref.[16]. Therefor, a more significant signal appears. However, compared to functional- ized DWCNTs sensors, the raw DWCNTs sensors have very small sensitivity to either water or ammonia vapor.

A striking observation can also be made in Figure 2.8, the recovery is rather poor within the time set. When there is only water vapor, it recovered ca. 40% of the resistance variation from exposure. And when ammonia is added, it recovered only ca. 9% of resistance variation from exposure. Compared to functionalized DWCNTs sensors (shown in following sections), the recovery of raw DWCNTs sensor from water vapor and ammonia vapor exposure is limited. No matter the gas is water vapor or water vapor with ammonia, the recovery is far from as good as oxidized DWCNTs sensors and close but less good to aminated DWCNTs sensor.

To the conclusion, the raw DWCNTs film is sensitive to ammonia. Even in the environment where there is water vapor of more than 300 times concentration to the ammonia, the raw DWCNTs film is capable of distinguishing ammonia from water. However, with the presence of ammonia, the recovery quality is reduced dramatically.

Oxidized DWCNTs Gas Sensor: The oxidized DWCNTs sensors are first exposed to ammonia vapor in the same condition then the same control experiment with pure DI water vapor is performed. As being functionalized with carboxyl group, we expect higher sensitivity from oxidized DWCNTs to both water and ammonia vapor.

Like it is plotted in Figure 2.9, an oxidized DWCNTs sensor shows relatively high sensitivity to ammonia and water compared to raw DWCNTs sensors.

Compared to raw DWCNTs sensor, (Figure 2.8), the sensitivity is improved more than 2 times for both ammonia and water vapor. The relative resistance variation now reaches 5 % for pure water vapor and 16 % for ammonia vapor. The response time is also improved. There are clear different resistance evolution when the two kinds of sensors are exposed to ammonia and/or water vapor, which will be discussed in an independent section later.

Besides the improvement of sensitivity, the recovery from exposure is also improved compared to the raw DWCNTs sensor. It recovers ca. 46% of resistance variation from ammonia exposure, and ca. 50 % from water exposure for the oxidized DWCNTs sensor within the time set. This indicates that the oxidation is not only a way to improve DWCNTs sensitivity towards some chemicals but also a mean to ameliorate DWCNTs self-recovery from these chemical exposures.

aminated DWCNTs Gas Sensor: In Figure 2.10, the typical response of aminated DWCNTs gas sensor exposed to ammonia vapor in the same condition is presented. It is clear that the aminated DWCNTs gas sensor exhibits same behaviour as oxidized DWCNTs gas sensor to the ammonia vapor exposure: the functionalization increases DWCNTs sensitivity to ammonia vapor. In this case, it is more than 3 times higher than raw DWCNTs sensor’s sensitivity to ammonia vapor. The relative resistance variation reaches 23 % for ammonia vapor but remains as low as 3 % for pure water vapor.

This is a very interesting result that the amination improves DWCNTs sensitivity to ammonia while maintaining relative low sensitivity to water. From this point of view, this type of surface functionalization is an excellent candidate material for developing ammonia sensors.

Figure 2.9: The signal of an oxidized DWCNTs sensor to water (left) and ammonia (right) vapor. R₀ is the initial resistance before exposure which is ca. 14 kΩ while R is the real-time resistance.

For the recovery of exposure, compared to raw DWCNTs sensor, there is 25% recovery of resistance variation from ammonia vapor exposure, and 48% recovery from water vapor exposure within the time set. Though it is improved compared to raw DWCNTs sensor, the time span is expected rather long from the recovery curve. The recovery from water vapor exposure is close to the one of oxidized DWCNTs sensor.

2.4.3 Gas Detection Mechanism Discussion