Jian Liao, Guang-Zhong Xie, Hui-Ling Tai, Ya-Dong Jiang, Wei-Zhi Li,Yong Zhou, and Fang Xu

1.Introduction

Toluene is an easy-to-volatile colorless organic liquid at room temperature and belongs to one of the volatile organic compounds (VOCs).However, it does great harm to human body, which can cause neurasthenia and skin allergies, and long-term exposure to hazardous air containing toluene may cause cancer.Therefore, detecting toluene concentration in air is very important for the environmental monitoring.The semiconductor gas sensors usually using polycrystalline particles of metal oxides such as SnO2, WO3,ZnO and In2O3have been utilized for gas detection[1].However, compared with polymer sensitive materials, they have a drawback of cross-sensitivity that significantly reduces their selectivity against the measured gases[2].Another advantage of the sensor using polymer as sensitive materials is that, unlike the metal oxide sensors, the sensing operation occurs at room temperature, which lower the power consumption.Xie et al.[3]fabricated a sensor array composed of three kinds of polymers and carbon black to detect organic vapors at low concentration, of which the sensor responses is S=0.005 at 400 ppm for carbon black(CB)/polyethylene oxide (PEO) composites.Dong et al.[4]fabricated a sensor to detect low concentrations of toluene vapor, and the results showed that sensor responses of S=0.05 at 800 ppm for PEO/CB and poly butyl methacrylate (PBMA)/CB composites films.Matsuguchi et al.[5],[6]proposed other novel copolymer coatings for a QCM-based toluene vapor sensor.Small and portable chemical gas sensor array, which is inexpensive, reliable,and able to detect and identify a wide variety of chemical gases, has been attractive for many applications[7]-[10].Compared with polymer/CB composite thin film sensors,the polymer/multi-wall carbon nanotubes (MWNTs)composite sensors show very good performance with respect to sensitivity, response time, reproducibility, and long-term stability[11].The alignment of the MWNTs is very important for polymer/MWNTs composite film sensors, in particular several attempts to alignment nanotubes have been reported[12]-[14].Taking into account the cost and ease to use, an inexpensive and miniature of VOCs gas sensor is very necessary.

In this paper, poly 4-vinylphenol (P4VP) was used as the sensitive layer while MWNTs were used as a conductive layer.P4VP/MWNTs multi-layer sensitive films were deposited on interdigitated electrodes by airbrush technology to detect toluene vapor at room temperature.

2.Experimental

2.1 Materials

P4VP was purchased from Alfa Aesar.Tetrahydrofuran,toluene and MWNTs (Outer diameter ＞50 nm, length 10 μm to 20 μm) were purchased from Chengdu Organic Chemicals Co.Ltd, Chinese Academy of Sciences.All of these chemicals were guaranteed reagents, and utilized without further purification.

2.2 Preparation of Sensitive Films

Fig.1.Structure of toluene gas sensor.

In the experiment, the interdigitated electrode pairs(IDTs) with gold electrodes (width and gap of interdigitated electrodes were 50 μm) on a silicon substrate with a sensing area of 5×8 mm2were used.The structure of toluene gas sensor is shown in the Fig.1.The sensitive film was fabricated as follows.

(1) Preparation of P4VP/MWNTs two-layer films:0.6 ml MWNTs (0.2 wt% in water) was sprayed on the interdigital electrode, forming a layer of MWNTs conductive layer.Then 0.5 ml of P4VP (dissolved in THF)was sprayed onto the MWNTs layer, then dried in vacuum at 60°C for 24 h to form the two-layer films.

(2) Preparation of P4VP/MWNTs three-layer films:0.3 ml MWNTs (0.2 wt% in water) was sprayed on the interdigitated electrodes, forming a layer of MWNTs conductive layer.Then 0.5 ml P4VP (dissolved in THF) was sprayed onto the MWNTs layer, forming the bilayer films.Finally, 0.3 ml MWNTs (0.2 wt% in water) was sprayed on the P4VP film, then dried in vacuum at 60°C for 24 h to form the three-layer films.

2.3 Experimental Procedures

Concentrations of toluene vapor were controlled by the MF-3C liquid organic solvent dynamic gas distribution device.The changes of resistance caused by adsorption and desorption of toluene were monitored by Keithley 2700,and the data were received by a computer system via a GPIB interface board.The entire experiment was carried out at room temperature.

3.Result and Discussion

3.1 Morphologies of Sensitive Films

The scanning electron microscope (SEM) images of the surface of P4VP/MWNTs two-layer and three-layer films are shown in Fig.2 and Fig.3, respectively.The final dispersion state of MWNTs within the polymer resulted from a competition between vander Waals interactions among the CNTs and the viscous forces acting within the polymer solution.Low viscosity not only facilitates dispersion during film processing but also promotes CNT reagglomeration[15].The SEM images of the section of P4VP/MWNTs two-layer and three-layer films are shown in Fig.4 and Fig.5, respectively.It is that there are a lot of little “pores” between the P4VP layer and MWNTs layer.It is estimated that these amount of “pores” can contribute to adsorption and desorption.

Fig.2.SEM images of the surface of P4VP/MWNTs two-layer films.

Fig.3.SEM images of the surface of P4VP/MWNTs three-layer films.

Fig.4.SEM images of the section of P4VP/MWNTs two-layer films.

Fig.5.SEM images of the section of P4VP/MWNTs three-layer films.

3.2 Response of the P4VP/MWNTs Films Exposed to Different Concentrations of Toluene

The real time response curves of the sensors exposed to different concentrations of toluene are shown in Fig.6.It is found that the resistance of the sensor increases after the exposure to toluene vapor.The phenomenon can be interpreted by the “swelling effect”.The adsorption of vapor of a good solvent in the composites causes swelling of the matrix polymer, which increases the distance between the conductive particles, resulting in an overall increase in resistance[16].From Fig.6, it is estimated that the response time and the recovery time of P4VP/MWNTs two-layer films sensor are about 49 s and 41 s to 200 ppm toluene vapor, while those of P4VP/MWNTs three-layer films sensor are about 43 s and 40 s.(The response time and the recovery time are defined as 90% and 10% of the time for sensors to arrive at stable state after exposure to toluene vapor, respectively).The result shows that the P4VP/MWNTs three-layer film sensors have better response compared with the two-layer film sensors to different concentrations of toluene.The sensing properties of the multi-layer films are shown in Fig.7.It is found that the response has a good linearity with the concentration of toluene vapor.The sensor response of two-layer films to 100 ppm toluene vapor is R=0.17%, while sensor response of three-layer films to 100 ppm toluene vapor is about R=0.43% (R is defined as ΔR/R0, ΔR equals Rs-R0.Rsis the resistance value of the sensitive films exposure to toluene vapore gas and R0is the initial resistance value when exposure to nitrogen).It is also found that the slope of the fitting line of toluene concentration dependent sensing response in two-layer films is 0.00178 compared with 0.00407 in three-layer films, which means the sensitivity of three-layer films is better.However, the square of relative coefficient of the two-layer films is r2=0.99973 compared with r2=0.99902 in three-layer films, which meant that that linearity of the two-layer films was better than that of the three-layer films with respect to concentration in the range investigated.

The reason why the sensitivity of three-layer films is better is perhaps that the dispersion of MWNTs in the three-layer films is better than that of two-layer films which can be seen from Fig.2 and Fig.3.When the polymer absorbs toluene molecules, the “swelling effect” has less impact on the distribution of agglomerate MWNTs, so the resistance does not change significantly.Another reason maybe is that the P4VP in three-layer films has larger contact area than that of two-layer films, so the three-layer films have more “pores” in the contact area than two-layers films, and these “pores” can contribute to adsorption and desorption.The toluene molecules pass through the “pores”into the sensitive films.When pores are dispersed extensively, the gas has a fuller contact with the polymers,which leads to a sufficient adsorption of gas so as to cause a better sensing response.

The repeatability of the P4VP/MWNTs three-layer film sensors to 200 ppm toluene vapor at room temperature is shown in Fig.8.From this figure, it is found that the response curve of three-layer film sensors almost keeps stable with a little attenuation in five periods.

Fig.6.Real-time response curves of the P4VP/MWNTs multi-layer films to toluene at room temperature.

Fig.7.Sensing response of the multi-layer films and the linear fitting line with toluene concentration change ranging from 100 ppm to 500 ppm.

Fig.8.Repeatability curve of the P4VP/MWNTs three-layer films to 200 ppm toluene vapor at room temperature.

4.Conclusions

Sprayed multi-layer sensitive films were deposited on interdigitated electrodes to detect toluene vapor at room temperature.It is observed that the sensitivity of the three-layer P4VP/MWNTs films sensor iss better than that of two-layer P4VP/MWNTs films sensor.Linearity of the two-layer films is better than that of the three-layer films.It is demonstrated that both three-layer film sensors and two-layer films sensor have a good linearity with the concentration of toluene vapor.Additionally, the results exhibit a stable response with a little attenuation to 200 ppm toluene vapor at room temperature in five periods during the investigation on the response curve of three-layer film sensors.

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