Methanol Gas-Sensing Properties of SWCNT-MIP Composites
© The Author(s). 2016
Received: 3 July 2016
Accepted: 5 October 2016
Published: 25 November 2016
The single-walled carbon nanotube (SWCNT)-molecularly imprinted powder (MIP) composites in this paper were prepared by mixing SWCNTs with MIPs. The structure and micrograph of the as-prepared SWCNTs-MIPs samples were characterized by XRD and TEM. The gas-sensing properties were tested through indirect-heating sensors based on SWCNT-MIP composites fabricating on an alumina tube with Au electrodes and Pt wires. The results showed that the structure of SWCNTs-MIPs is of orthogonal perovskite and the average particle size of the SWCNTs-MIPs was in the range of 10–30 nm. SWCNTs-MIPs exhibit good methanol gas-sensitive properties. At 90 °C, the response to 1 ppm methanol is 19.7, and the response to the interferent is lower than 5 to the other interferent gases (ethanol, formaldehyde, toluene, acetone, ammonia, and gasoline). The response time and recovery time are 50 and 58 s, respectively.
KeywordsSWCNT-MIP composites Gas-sensing properties Methanol gas
Perovskite oxides with ABO3 structure (A, rare earth; B, transition metal) have been shown to have excellent gas-sensing properties [1–5]. For some p-type ABO3 semiconductors that are prepared in air condition, their resistance will be decreased when they adsorb oxidizing gas while their resistance will be increased when they adsorb reducing gas. This special property could be employed in the application of gas sensor to detect different gas in the atmosphere. Zhang et al.  investigated the formaldehyde-sensing properties of Ag-LaFeO3 system and found that Ag-LaFeO3 sensors exhibited good performance to formaldehyde gas. Giang et al.  reported that the mixed potential sensor based on Pt/YSZ/SmFeO3 had very high sensitivity to NO2 at the operating temperature from 300 to 500 °C. Chen et al.  investigated the SmFe1 − xNixO3 sensors and showed that these sensors exhibited good performance to ethanol gas. It was showed that single-walled carbon nanotube (SWCNT) incorporation of Ag-LaFeO3 system lowered the operating temperature effectively and increased the sensing response to formaldehyde gas . Doroftei et al.  designed a La–Pb–Fe–O perovskite gas sensor of methanol, and the highest response of the sensor to 400 ppm methanol gas is 146.6 at 230 °C.
Molecular imprinting is a technique for the introduction of selective recognition sites into highly cross-linked polymeric matrices, in which the functionalized monomers are introduced into the polymer network via direct-template-assemble [11, 12]. Generally, polymerization acts as print molecule or template and forms the complex with the constituent monomers. When the templates are removed, the cavities that keep specific recognition sites are left in the polymeric structure. Today, molecular imprinting has been successfully used in the fields of enzyme-mimicking catalysts, chromatographic separation, biosensors, and chemical sensors [13–15]. However, most of the analyte molecules are organic macromolecule, such as tryptophan, caffeine, glutamic acid, and bisphenol, and there is no report on using the molecular imprinting technique in semiconductor oxides to recognize small organic molecules in addition to our previous work [16, 17]. SWCNTs have been the most actively studied materials in recent years due to their special nanostructure, large specific surface area, and excellent electrical properties . As reported, SWCNTs are very sensitive to the surrounding environment. The presence of O2, NO2, and NH3 gases and many other molecules can either donate or accept electrons, resulting in an alteration of the overall conductivity [19, 20]. SWCNTs become the ideal gas-sensitive materials because of their excellent properties. Recently, the combination of semiconductor metal oxide with SWCNTs has been explored to modify semiconductor metal oxide gas sensors, and many interesting findings have been obtained [21, 22].
In our previous work , a new methanol gas sensor was prepared by using molecularly imprinted powders (MIPs). The sensor has many good characteristics, such as selectivity, stability, and response-recovery characteristics, but its optimal operating temperature is still unsatisfactory (around 130 °C). In order to reduce the optimal operating temperature, MIPs were modified with SWCNTs. The results show that SWCNT-MIP composites exhibited excellent gas-sensing properties for methanol gas and the optimal operating temperature has been reduced to 90 °C.
Preparation of MIPs
All the chemical reagents used in the present work were obtained from commercial sources as guaranteed-grade reagents and used without further purification.
LaFeO3 of perovskite structure oxides was prepared by a sol-gel method. The detailed process was as follows: Fe(NO3)3·9H2O, La(NO3)3·6H2O, and citrate with mole ratio (Fe3 +)/(La3 +)/citrate = 1:1:1 were first dissolved in distilled water, and subsequently, polyethylene glycol (PEG) was added. Finally, the mixed solution was stirred at 80 °C for 8 h to get LaFeO3 sol. The functional monomer of methylacrylic acid (MAA) was mixed with methanol (which acts as a template molecule) in a reaction vial. Then, the cross-linking agent of LaFeO3 sol was added into the MAA solution with various molar ratios (x = MAA:LaFeO3 = 1:10, 4:10, 6:10, 8:10, 10:10), the radical initiator of azodiisobutyronitrile (AIBN) was added to the mixture, and the mixture was stirred for polymerization at 50 °C for 12 h under the protection of N2. After this, the resulting polymer was ground and dried at 80 °C to remove the template molecule completely.
Preparation of SWCNT-MIP Composites
The MIPs (x = 6:10) were modified with SWCNTs (w = SWCNTs:MIPs = 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50% weight ratio). SWCNTs and MIPs were treated by ultrasonic dispersion in the distilled water for 30 min. Subsequently, the mixture was put in a microwave chemical device for 2 h and then dried. The composites were finally obtained.
Fabrication of Gas Sensors
In the present work, the gas sensors were fabricated in the following process: the SWCNT-MIP composites were mixed with deionized water to form paste, and then coated onto the outside of an alumina tube with Au electrodes and Pt wires. A Ni-Cr alloy wire crossing the alumina tube was used as a resistor which ensured both substrate heating and temperature control. All the gas sensors were aged at operating temperature 150 °C for 170 h in air in order to improve their stability and repeatability. The gas response was defined as the ratio of the electrical resistance in gas (Rg) to that in air (Ra) .
X-ray diffraction (XRD, D/max23) with Cu Kα radiation (λ = 1.54056 Ǻ) and transmission electron microscope (TEM, JEM-2100) were used for the phase identification and morphology of the samples. The infrared spectra were identified by FTS-40 infrared spectrometer, and the sample was scanned from 4000 to 400 cm−1 with a KBr pellet method.
Results and Discussion
Gas-sensing properties of the MIPs with x = 6:10 (labeled “Sample-A” in the following text) and the SWCNT-MIP composite with w = 1.00% (labeled “Sample-B” in the following text) are better than those with x = 1:10, 4:10, 8:10, and 10:10 and those with w = 0.25, 0.50, 0.75 1.25, and 1.50%, respectively. So in this paper, we mainly discuss Sample-A and Sample-B.
Structure and Morphology of Gas-Sensing Materials
Gas-Sensing Performance of the Sensors
The gas-sensing mechanism of MIPs sensors has already been explained in our previous work . After the MIPs modified with SWCNTs, the gas-sensing mechanism is similar to that of MIPs, but the sensing properties of the MIPs could be enhanced by modifying with SWCNTs. (1) The optimal operating temperature of MIPs which modified with SWCNTs reduce effectively from 130 to 90 °C, and (2) the selectivity of SWCNT-MIP composites could be enhanced. The reasons are as follows. (1) SWCNTs can transport the electrons easily, which can reduce the resistance of the sensor. With lower resistance, the sensor can operate at a lower temperature. Thus, modifying with SWCNTs can reduce the operating temperature effectively. (2) The particle size of the MIPs is smaller by adding SWCNTs, so the specific surface area of the SWCNT-MIP composite is increased. Hence, there are much more methanol gas-adsorbing vacancies on the surface of the sensor, which lead to effective adsorption of methanol gas and result in enhancement of the selectivity.
Perovskite-type MIPs (Sample-A, MAA:LaFeO3 = 6:10) and SWCNT-MIP composite (Sample-B, SWCNTs:MIPs = 1.00%) have been prepared. The average particles size of MIPs (MAA:LaFeO3 = 6:10) and SWCNT-MIP composite (SWCNTs:MIPs = 1.00%) are about 50 and 20 nm, respectively. The MIP (MAA:LaFeO3 = 6:10) and SWCNT-MIP composite (SWCNTs:MIPs = 1.00%) sensors showed high gas sensing to methanol. Compare to MIP sensor, the SWCNT-MIP sensor has lower optimal operating temperature (90 °C) and high selectivity (to 1 ppm methanol, the response is 19.7 at the operating temperature of 90 °C, and to the other test gases, the responses are all lower than 5). These results indicate that the methanol gas-sensing properties of the sensor based on the MIPs can be improved by modifying with SWCNTs, and the SWCNTs modified MIPs is a feasible way for improving the gas-sensing properties of the MIP-based sensors.
The methanol gas-sensing properties of the SWCNT-MIP sample (SWCNTs:MIPs = 1.00%) is the best.
The response of SWCNT-MIP sample (SWCNTs:MIPs=1.00%) to 1.0 ppm methanol gas is 19.7 at 90 °C and lower than 5.0 to the other test gases.
This work was supported by the National Natural Science Foundation of China (Grant No. 51402257, 51262038,), the Natural Science Foundation of Yunnan Province, China (No. 2013FZ003), the Training Program for Young Teachers of Yunnan University, and the Fund Projects in Yunnan Province Department of Education (No. 2015Y009), the fund of the State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals, China (NO. SKL-SPM-201208).
JZ and QL conceived and designed the experiments. JZ, QZ, and CH performed the experiments and analyzed the data. YZ and QL wrote the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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