Characterization of Multi-Walled Carbon Nanotube Film Sensor and Ethanol Gas-Sensing Properties

Multi-wall carbon nanotubes (MWNTs) film-based sensor on the substrate of printed circuit board (PCB) with inter digital electrodes (IDE) were fabricated using layer-by-layer self-assembly, and the electrical properties of MWNTs film sensor were investigated through establishing models involved with number of self-assembled layers and IDE finger gap, and also its ethanol gas-sensing properties with varying gas concentration are characterized at room temperature. Through comparing with the thermal evaporation method, the experiment results shown that the layer-by-layer self-assembled MWNTs film sensor have a faster response and more sensitive resistance change when exposed to ethanol gas, indicated a prospective application for ethanol gas detection with high performance and low-cost.


Introduction
Gas detection plays a very important role in the various fields of atmospheric pollution monitoring, toxic gas or volatility poisonous chemical leak, power transformer diagnosis, human health diagnosis [1], [2]. Gas sensors based on nanomaterials are also emerging in a large way as the recent trend indicates the preference towards potential applications. Carbon nanotubes (CNTs) have attracted extensive attention because of their unique physical, chemical and electrical properties since their discovery by Iijima in 1991, nowadays they have been used as one of the ideal materials for gas sensors [3], [4]. Carbon nanotubes-based thin film can convert the information in relation with gas type and concentration into electrical signals when exposed to different gases, such as NO 2 , NH 3 , CH 4 , and humility sensor has also been mentioned [5][6][7][8][9][10][11].
Currently, the detection and sensing of ethanol gas is important for a variety of purposes including ethanol production, industrial chemical processing, fuel processing and use, societal applications, and physiological research on alcoholism. A large number of commercial ethanol measurement systems are available for several of these applications, but in general, these systems are operated at relatively high power levels and high-temperature, are expensive and bulky, and possess functionality that is more limited than required for a number of applications. Therefore, the use of CNTs-based devices for detection of ethanol vapor has received some attention for sensing in a smaller-size, inexpensive and portable way.  Figure 1.

Fabrication of Self-Assembled MWNT's Film Sensors
Interdigital electrodes are printed on printed circuit board with an outline dimension of 1×1cm, the IDE finger gap used are 200µm, 300µm, 400µm and 500µm, the electrode material is Ni/Cu. The Sketch of the thin films is shown as Figure 2, sensing material MWNTs and binding materials PDDA and PSS are fabricated on the substrate of PCB-IDE device. Here we used LbL self-assembly method to fabricate MWNT thin films, which is illustrated as Figure 3. First, immerse the preprocessed IDE device into solution PDDA for 10min, then rinse it with DI water and dry with nitrogen flow, followed by immersion in solution PSS for 10min, then rinse and dry. Repeat once again to form two precursor layers of PDDA/PSS on the substrate for charge enhancement. Afterwards, alternately immerse the IDE substrate into solutions PDDA and MWNTs for n cycles, the immersion time here used is 10 min for PDDA and 15 min for MWNTs, and intermediate rinsing with DI water and drying with nitrogen flow are required after each monolayer assembly to reinforce the interconnection between layers. Finally the multilayer films (PDDA/PSS) 2 (PDDA/ MWNTs) n are formed, where n is 1, 3, 5, 7, 9 ,11 here, and heat the self-assembled MWNTs film devices under 80 ºC for 2 hours as the last step.

Result and Analysis 3.1. Electrical Characterization of the Self-Assembled MWNT Film Sensors
The electrical properties of the fabricated MWNT film devices with different layer numbers are characterized by the data acquisition unit. Figure 4 plots the relationship of the film resistance versus self-assembled layer number and finger gap, and indicates that the film resistance decreases with the increasing of the assembled layers under the same IDE finger gap, and also shows that decreases with the increasing of the IDE finger gap under the same layer numbers. The curves in Figure 4 are fitted using the model f(x)=a/x 2 +b/x+c, where a, b, c are the undetermined coefficients, x represents the cycle numbers of self-assembled (PDDA/MWNTs) bi-layers. Table 1 indicates the calculated results for model fitting parameters. In order to further investigate the cross-influence of both finger gap and self-assembled layers on the film resistance, a three-dimensional numerical model f(x,y)=a/x 2 +b/x+cy+d(y/x)+e is employed, where a, b, c, d, e are the undetermined coefficients, x and y are the cycle numbers of self-assembled (PDDA/MWNTs) bi-layers and IDE finger gap respectively. The simulation results are shown in Figure 5, and the calculated results for the model parameters are listed in Table 2. Coefficient of determination R 2 is used here as a measure of how well the models fits the experiment data, which is determined as: where y i and ŷ i represent experimental measured values and predictive values of model, respectively, and m represents number of measuring points. R 2 , listed in the Table 1, shown the selected models for data fitting are appropriate, and so disclosed the relationships of the film resistance versus assembled layer number and IDE finger gap.

Characterization of Ethanol Gas-Sensing Properties
The ethanol gas-sensing experiments are carried out at room temperature, Figure 6 shows the experiment setup for ethanol gas-sensing properties, which, are characterized by using Agilent 34970A. Inject ethanol solution into the sealed beakerflask using a micropipettor and evaporates it completely through heating. Expose the self-assembled MWNTs film sensor (IDE finger gap d=200µm and layer number n=5) into ethanol gas, the adsorption of ethanol gas produced a measurable change in the MWNTs film resistance. The resistance responses to varying ethanol-gas concentration are recorded. Figure 7 shows the ethanol gas sensing response of the MWNTs film sensor in terms of sensitivity change with changing the concentration of ethanol gas. The sensitivity S (percentage change in resistance), is denoted as the ratio ∆R/R air , where ∆R is the difference of resistance of self-assembled MWNTs sensor in the presence of the ethanol gas and in dry air, R air is the resistance of the sensor in dry air. The results in Figure 7 illustrates that the sensitivity of the MWNTs film sensor increases along with the increasing of gas concentration, and reachs beyond 4% when exposing to ethanol gas concentration of 30 kppm.
Anthor alternative method known as thermal evaporation method is used to fabricate MWNTs sensors for making a comparision with LbL self-assembly method. In thermal evaporation method, the same MWNTs solution and PCB-IDE substrate are employed, and the MWNTs solution is driped on the substrate surface and then place it under 80 ºC for more than 2 hours for drying and forming a MWNT layer. Figure 8 shows the compressive results in response for the two methods when alternating expose to ethanol gas concentration of 30kppm and dry air for three cycles. From the curves in Figure 8 we find that the sensitivity response of LbL self-assembly method is more than two times bigger than that of thermal evaporation method. The response time of LbL self-assembly method is about 10s and the sensitivity reaches 4.1%, while those of evaporation method is around 15s and 1.98%, and both the   appearing as resistance increasing. Another reason perhaps is that the adsorption of ethanol molecules directly causes the swelling of the polymer and the increase of tunneling barriers between MWNTs, resulting in an increase of the film resistance.

Conclusion
We fabricated MWNT film devices on the substrate of PCB-IDE using LbL selfassembly as gas sensors, the electrical properties of MWNTs film devices were investigated through establishing models involved with number of self-assembled layers and IDE finger gap, and also its ethanol gas-sensing properties are characterized at room temperature. As shown by the experimental results, the LbL self-assembled MWNTs film sensor have a fast response and sensitive resistance change when exposed to ethanol gas, indicated a prospective application for ethanol gas detection with high performance and low-cost.