Detection of a CO and NH3 gas mixture using carboxylic acid-functionalized single-walled carbon nanotubes
© Dong et al.; licensee Springer. 2013
Received: 31 October 2012
Accepted: 10 December 2012
Published: 4 January 2013
Carbon nanotubes (CNT) are extremely sensitive to environmental gases. However, detection of mixture gas is still a challenge. Here, we report that 10 ppm of carbon monoxide (CO) and ammonia (NH3) can be electrically detected using a carboxylic acid-functionalized single-walled carbon nanotubes (C-SWCNT). CO and NH3 gases were mixed carefully with the same concentrations of 10 ppm. Our sensor showed faster response to the CO gas than the NH3 gas. The sensing properties and effect of carboxylic acid group were demonstrated, and C-SWCNT sensors with good repeatability and fast responses over a range of concentrations may be used as a simple and effective detection method of CO and NH3 mixture gas.
KeywordsGas sensors Carboxylic acid functionalized Single-walled carbon nanotubes Mixture gas
Among several applications using carbon nanotubes (CNT) , chemical gas sensors are currently regarded as one of the most promising application due to their fast response and high sensitivity toward gaseous molecules at low operational temperatures. Although considerable theoretical efforts have been devoted to the study of the possible interaction of a broad variety of gas molecules including H2, NH3, NO2, O2, and CO with CNT [2–9], these gases are frequently found in the polluted air from modern big cities. Therefore, to commercialize gas sensors using CNT as sensing materials, sensing experiments should be performed in a mixed gas environment in order to take actual air characteristics into account. Sensing mixture-gas molecules is important for environmental monitoring, control of chemical processes, agriculture, and biological and me2dical applications. Upon exposure to gas molecules, the electrical resistance of single-walled carbon nanotubes (SWCNT) changes and the threshold voltage is shifted due to charge transfer between the semiconducting SWCNT and electron-withdrawing and electron-donating molecules. Theoretical calculations showed the binding energy of CO and NH3 to SWCNT, which indicate a weak charge transfer. The conductivity change may also be caused by contact between the electrode and SWCNT, and the contact between SWCNT [8–11]. Both CO and NH3 are toxic, and even a small amount of exposure for a given period could lead to fatality, where detection of the former can be difficult due to its characteristics, having no odor and color, while the latter becomes dangerous for the environment in an anhydrous state, flammable, and can form explosive mixtures with air, especially for agricultural industries [12–14]. The detection of the CO and NH3 gases was reported by Fu et al. and Kong et al., respectively [7, 15]. They suggested that the sensing characteristics of the CO and NH3 gases by carbon nanotubes are different for each gas. Additionally, CO gas molecules are adsorbed on carboxylic acid functionalities through weak hydrogen bonding, while the NH3 molecules are physically and chemically adsorbed and work as an electron donor like H2O . In this study, to incorporate mixture of gases as well as individual detection, a gas sensor using carboxylic acid-functionalized single-walled carbon nanotubes (C-SWCNT) was introduced for CO and NH3 gases. Also, comparisons will be made with conventional sensors highlighting improved characteristics.
High-purity SWCNT, purchased from Hanwha Nanotech, Inc. (Incheon, South Korea), are synthesized by the arc-discharge method, with purity of about 90%. The SWCNT have diameters between 1 and 1.2 nm and were very long (5 to 15 μm). For the experiments in this research, 100 mg of SWCNTs were dispersed in 100-mL deionized water (DI) water and sonicated for 2 h using bath sonicator (frequency 53 kHz, power 180 W). Then, nitric acid was added to the dispersion to reach 6 M acid concentration for highly carboxylic acid group functionalized. This dispersion was further sonicated for 4 h. The dispersion was filtered through polytetrafluoroethylene (PTFE) membrane (pore diameter 450 nm) and repeatedly washed with DI water. The resulting C-SWCNT film was easily peeled off from the PTFE membrane. The control C-SWCNT film was formed by filtering the aqueous C-SWCNT dispersion without nitric acid that has been sonicated for 6 h. The films were dried at 80°C in a vacuum and heat-treated in air at 200°C for 2 h. Then, the tube solution consisted of approximately 32 mg/L of individual C-SWCNT in a 0.6 wt% aqueous sodium dodecyl sulfate (SDS) solution. The C-SWCNTs were dispersed in DI water with the SDS which is used to obtain a stable colloidal suspension of C-SWCNTs. Dispersion of C-SWCNT was performed in a bath sonicator for 6 h and then centrifuged for 30 min at 4,500 rpm. This method is simple and classically employed to disperse C-SWCNT in deionized water with the help of commercially available SDS molecules . The steric repulsion force introduced by the surfactant overcomes the van der Waals attractions between the SDS-wrapped C-SWCNT surfaces. Wrapping nanotubes with SDS surfactant guarantees that tubes previously separated by sonication will no rejoin .
We experimentally found that the resistances of the C-SWCNT typically ranged from 4 to 5 kΩ, depending on the amount of C-SWCNT across the electrode pair. The flow rate of N2 and the concentration of gases (CO, NH3, and their mixture) were controlled by pneumatic mass flow controllers. The resistance change value was measured and stored by a source meter (Keithley 2400, Keithley Instruments, Inc., Cleveland, USA) and LabVIEW (National Instrument Corp., Austin, USA) software, respectively. Adsorbed gases were desorbed-vent with N2 flow.
Results and discussion
where Rg represents the resistance upon exposure to the test gases, and R0 is the initial resistance in the presence of N2. The carrier gas (N2) flux was maintained at 500 sccm throughout the experiment.
We conducted an experiment to get the response of the mixed gas consisting of electron-withdrawing and electron-donating gases. One gas had a faster response time and lower sensor response than the other. In our experiment, CO and NH3 were chosen as gases having a faster response time with weak bonding and faster sensor response with strong bonding, respectively. Previous studies reported individual detection of CO [6–8, 20] and NH3, where these sensors were using C-SWCNT bundle sensing layer, accordingly. As well as introducing mixture-gas detection capability, the C-SWCNT sensor fabricated in our study was more responsive even for individual detection, see Figures 3 and 4.
The C-SWCNT-based sensor was used to detect the change of resistance when the sensor was exposed to three types of gases. When the sensor was exposed separately to CO and NH3, the sensor resistance decreased and increased, respectively. However, in the case of a mixture of the gases, CO and NH3, the resistance was decreased due to an initial reaction of CO with the surface of the C-SWCNT in the gas mixture. The decrease of resistance in a cycle may be due to the adsorption of CO because the response of the CO was faster than that of the NH3. As the chemical reaction between NH3 and CO progressed, the resistance was gradually increased.
However, since we presume that the absorption on CO is much faster than that on NH3, absorbed CO gas firstly reacts with the C-SWCNT, followed by the reaction of NH3 gas which has a dominant and proper reaction in the total reaction. A comparison was made with conventional sensors, showing enhanced sensor response for individual detection. Also, selectivity for mixture-gas detection was explored, and this result clearly shows that a C-SWCNT-based gas sensor can be a good candidate for mixture-gas detection.
This work was supported by World Class University (WCU, R32-2009-000-10082-0) Project of the Ministry of Education, Science and Technology (Korea Science and Engineering Foundation) and partially supported by the Industrial Core Technology Development Program funded by the Ministry of Knowledge Economy (grant no. 10037394). This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (no. 2012R1A1A3013893). The authors thank the staff of Korea Basic Science Institute (KBSI) for the technical assistance.
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