Investigation on hydrogen sensing property of MWCNT/Pani nanocomposite films

Hydrogen sensing property of composite films of camphorsulfonic acid-protonated polyaniline (Pani) with different amounts of multiwall carbon nanotube (MWCNT) was investigated in this paper. The MWCNT/Pani composite films were deposited by a spin-coating method on both ITO and Au-interdigitated electrodes (Au-IDE) substrates. Sensor film characteristics were evaluated by monitoring the change in electrical resistance in the presence of hydrogen at room temperature. It was observed that all MWCNT/Pani composite films showed the better sensor indicators such as sensitivity, response and recovery times in comparison with pure Pani. It was found that sensor indicators were improved by increasing the MWCNT filler concentration in composite. Moreover, it was observed that Au-IDE substrate drastically increased sensor sensitivity in comparison with uniform ITO-coated glass at 4 wt% MWCNTs exposed to 0.4 vol% H2 in the air.


Introduction
Conducting polymers are of great interest in a variety of applications due to their low cost of fabrication and room-temperature operation [1][2][3][4]. Among the conducting polymers, polyaniline (Pani) is one of the most studied polymers because of its good environmental stability and interesting electronic properties which stem from protonic acid doping process [5][6][7][8][9]. Due to the fact that H 2 is explosive if it mixes with air in a wide flammable range of 4.65-93.9 vol% [10], many studies have been carried out to develop various kinds of H 2 gas sensors. Pani as an electrochemical sensor was used for detecting hydrogen gas based on the change in its electric resistivity resulting from an electrochemical reaction on the polymer surface [11,12]. However, Pani sometime seems to be unstable and present poor sensitivity due to moisture present in the environment and its strong affinity for volatile organic compounds [13].
Different studies have reported an improvement in conductivity as well as in both selectivity and sensitivity by incorporating specific nanoparticles (NPs) into the polymer [14][15][16][17].
In our previous work, it was found that incorporation of carbon nanotubes (CNTs) decreased the CNTs/Pani nanocomposite optical band gape and increased its electrical conductivity [18]. Srivastava et al. [19] had reported a higher response for CNTs/Pani nanocomposite film sensor in comparison with the pure Pani at room temperature for 2% hydrogen in the air. Furthermore, their study on the effect of electrode type and configuration showed that the interdigitated electrode (IDE) can strongly increase gas sensor sensitivity.
To our knowledge, in contrary to a wide investigation on NPs/Pani composite hydrogen sensor, few studies have been done on MWCNTs/Pani composite sensor. The main purpose of this work is to intensify and strengthen the role of MWCNTs/ Pani nanocomposite as a hydrogen gas sensor at room temperature. The effect of sensor electrode shape on sensor sensitivity is investigated too. In this study, MWC-NTs/Pani nanocomposite film was prepared via deposition of MWCNTs dispersed in protonated Pani at different concentrations by a spin coater.

Materials and methods
Synthesis of polyaniline (Pani) and MWCNTs/Pani nanocomposites was explained in our previous studies [6,18]. Pani film and MWCNTs/Pani nanocomposite films were deposited on clean ITO-coated glass substrates [10 × 10 mm 2 , ITO thickness: 80 nm, sheet resistance (Ω/sq): 8] using the spin-coating technique at a speed of 2500 rpm. The spin-coated film was dried at 60 °C in the vacuum for 1 h. The obtained nanocomposite films with 1, 2 and 4 wt% of MWCNTs were denoted as PC1, PC2 and PC3, respectively. Sample resistivity was measured by a two-point probe method (Au points of 0.2 mm coated one on the Pani film and the other one on the ITO as shown in Fig. 1a). Finally, one of the layers (PC3) was deposited at the same conditions on the Au-interdigitated electrodes (Au-IDE) substrate for evaluating the electrode configuration effect on the film sensitivity. Finger-type Au-interdigitated electrodes were patterned onto glass substrate of 10 × 10 mm 2 area by lithography as shown in Fig. 1b The prepared films were fixed into a homemade iron gas sensing chamber. The gas sensor setup is shown in Fig. 2. Resistance changes in the sensors in exposure to hydrogen gas were used to detect the sensing response of all samples [16].
Gas sensing measurements were taken under 0.4% H 2 gas in a closed chamber, using a 2401 source meter and a resistance signal of the gas sensing devices kept monitored at ambient temperatures. A mass flow controller (MFC) accurately measures and controls the mass flow rates of hydrogen.

Results and discussion
In our previous study [18], we put in evidence the presence of MWCNTs covered by polymer as core/shell nanocomposites on the ITO-coated glass using SEM image. Also, the presence of a large number of pores on the MWCNT/Pani nanocomposites suggested its application as a gas sensor.
The gas sensitivity of sensor was studied by calculating the variation in resistance of samples with time upon exposure of hydrogen at room temperature. The sensor response is defined as: response = R a /R g , where R a and R g are the resistance  in pure air and resistance in the presence of hydrogen gas, respectively. Figure 3a, b shows the resistance and response changes in Pani PC1, PC2 and PC3 thin films upon exposure to 0.4 vol% H 2 gas versus time at room temperature and air pressure. According to Fig. 3a, all samples indicate a reduction in resistance in the presence of H 2 gas and an increase in the absence of gas by cutting off the gas supply. Figure 3b shows a slight increase in response to increasing MWCNT amount in the composite. Gas sensing mechanism and change in the resistance can be explained by chemisorption and physisorption of H 2 molecules on the core/shell Pani-coated CNTs. As it is explained by Focke et al. [20], the conduction of Pani arises from the formation of unpinning polarons (radical cations) in the chain. The polarons in conductive Pani considered as disorder and defects in polymer structure are not accompanied by specific counterions; rather, they are subjected to the mean field created by a number of surrounding counterions. At this condition, the polarons can diffuse freely along the Pani chain and Pani backbone. The charge transport in Pani backbone is ensured via a hopping mechanism between adjacent chains. Figure 4 indicates the sensing mechanism of Pani in the presence of H 2 which consists of H 2 dissociation via interaction with free spins on adjacent polyaniline chains to form N-H bonds in metastable species. Figure 4a shows a segment of camphorsulfonic acid (CSA)-doped polyaniline chain. Figure 4b presents the formation of a bridge between two adjacent polyaniline chains in charged amine sites (-NH-) by hydrogen molecules. Consequently, hydrogen molecules dissociate and metastable polyaniline species containing a conventional ammonium ion are formed (Fig. 4c). Afterward, Pani via a transfer of charge gets transformed into polyaniline containing a neutral (unstable) ammonium species, which spontaneously decomposes to Fig. 4d by the formation of bipolaron lattice and with liberation of 0.5H 2 [21,22]. According to the proposed mechanism, charge transportation and the amount of carriers increase on Pani backbone after exposure to H 2 gas leading to faster charge transportation along the chain. Also, a decrease in potential barrier occurs between chains by carrier hopping from a chain to adjacent chain with the reduction in resistance. Other remark arising from Fig. 3a is a decrease in resistance with MWCNT amount increase in the core/shell Pani-coated MWCNTs. The enhanced electrical conductivity of the MWCNT-CSA-doped Pani composite over neat Pani is explained by -* electron interaction coupling detected in our previous work [15] between the MWCNTs and polymer chains, which facilitate charge transfer processes between the two components and enhance the conductivity with the increase in the concentration of the MWCNTs. Also coupling MWCNTs with high localization length with Pani results in enhancement of average localization length in Pani/ MWCNT composites compared to Pani which means higher conductivity [23][24][25].
Response time and recovery time, two important parameters for sensing characterization, are defined as the time required to reach 90% of the final change in resistance, when the gas is turned on and off, respectively. The shift in resistance (|R a -R g |), response value and response and recovery times are summarized in Table 1. The obtained values indicate an increase in response value and a decrease in response and recovery time by increasing the MWCNT content.
Sensitivity of all sensors, which is defined as S = |(R a -R g )/R a | × 100, is calculated and depicted in Fig. 5. Results show that variation of resistance and The other important parameters to be considered for a sensor are repeatability, sustainability and selectivity. For this purpose, simple PC3 presenting higher sensing properties has been considered. Figure 6a-c shows the repeatability, sustainability after 120 days and selectivity of PC3 sensor with 4% MWCNT, respectively. In Fig. 6b, slight changes in resistance of sensor after 120 days may be due to the change in environmental conditions and also absorption of oxygen molecules on the film layer. Figure 6c shows that PC3 is significantly more sensitive to H 2 rather than other gases. These outcomes indicate that PC3 is a good candidate for H 2 sensing. Comparing the results of this work with those of Refs. [16,26,27] in which MWC-NTs and Pani have been used as H 2 gas sensors (Table 2), one deduces that PC3 presents better sensing characteristics. Figure 7a shows the alteration in resistance of PC3 nanocomposite film on glasscoated ITO after exposure to different concentrations of H 2 gas ranging from 0.4 to 0.8 vol%. The response/recovery time and response value of PC3 for different vol% of H 2 gas are calculated from Fig. 7a and shown in Fig. 7b. It is clear from Fig. 7b that response time decreases and response value and recovery time increase by increasing H 2 content. According to collision theory, the rate of reaction increases by increasing the reactive concentration (hydrogen gas in our case) and increment in reaction kinetic causes a decrease in response time, while more reaction with film surface more time needs for desorption and removal of adsorbed gas, resulting in an increase in recovery time. Figure 8 shows the resistance and sensitivity of PC3 film on both ITO-and Au-IDE-coated glass substrates. We observe an increase in sensitivity from about 20% to about 60% when the substrate changes from ITO to Au-IDE.
The improvement in sensitivity can be due to the fact that the interdigitated geometry leads to the effective electrical contacts between sensitive layer and the electrodes and a wide contact area between the electrodes within the limited area  is established. A detailed study of the electrode configuration effect on gas sensing material parameters has been provided by Lee [28].

Conclusions
The effect of different amounts of MWCNTs dispersed in protonated Pani films on hydrogen sensing property has been studied. It is observed that the electrical conductivity of films increases with an increase in MWCNTs amount in CSA-doped Pani in the range of 2-4 wt% MWCNTs. The results show a significant amelioration in MWCNTs/Pani composite film sensitivity as well as response and recovery times as compared to neat Pani film when they are exposed to 0.4 vol% H 2 . Also, it is found that an increase in MWCNTs filler amount decreases both response and recovery times and improves film sensitivity. Although, at fixe filler concentration, response time decreases with an increase in hydrogen concentration, recovery time increases because of large number of hydrogen adsorption on sensor surface. With changing electrode configuration from uniform ITO-coated glass to Au-IDE type, film sensitivity shows a huge increase when it is exposed to hydrogen. Moreover, good repeatability, sustainability and selectivity of prepared MWCNTs/Pani composite films indicate their capability for hydrogen gas detection at room temperature.