Fast Response PANI/MMT-rGO Polymer Nanocomposite Material for Sensing HCN Gas

: In the present work, we have developed a polymer based gas sensor. The polymer nanocomposites are synthesized by the chemical oxidative polymerization of aniline with ammonium persulfate and sulfuric acid. The fabricated sensor is able to achieve a sensing response of 4.56% for PANI/MMT-rGO at 2 ppm of hydrogen cyanide (HCN) gas. The sensitivity of the sensors PANI/MMT and PANI/MMT-rGO are 0.89 ppm -1 and 1.12 ppm -1 respectively. The increase in the sensitivity of the sensor may be due to an increase in the surface area provided by MMT and rGO which provided more binding sites for the HCN gas. The sensing response of the sensor increases as the concentration of the gas exposed increases but saturates after 10 ppm. The sensor recovers automatically. The sensor is stable and can work for 8 months.


Introduction
Hydrogen Cyanide (HCN) vapour is extremely hazardous to the living organism. HCN gas when inhaled it increases the intake level of oxygen by the cell [1][2][3]. The toxic level of the HCN gas is above 100 ppm and when exposed can kill a human being within 1 hr [4]. Bhopal gas tragedy in 1984 killed 3,787 innocent people in a single night. This tragedy could have been prevented if some warning alarm system (gas sensor) is installed. The detection of trace amounts of toxic gasses (ammonia, dimethyl methyl phosphonate (DMMP), carbon monoxide, carbon dioxide, nitrous oxide, HCN) is important to prevent a fatal accident. Thus fabrication and development of electronic noses at the micro-and nano-level are needed. Fabrication of gas sensors using nanostructures increases the sensitivity of the sensors. The increase in surface area due to nanoparticles increases the binding sites of the gas. A sensor is a device when receives a stimulus it responds with an electrical signal [5]. Chemical sensors work on the principle of change in resistance on exposure of gas. A standard sensor should satisfy the following characteristic features such as operation at room temperature, working in the ambient environment and no requirement of oxygen or air supply, no external stimulus is required, capacity to detect toxic gases at low concentration, high sensitivity and reproducibility, quick response and recovery, low cost and eco-friendly [6].
Conducting polymer-based gas sensors has numerous advantages over the metal oxide sensors such as high sensitivity, short response time, operation at room temperature and can be tuned by the nature of the dopant. The sensitivity of the polymer base gas sensor is high due to the large surface-to-volume ratio, compact in size, lightweight and easy to integrate with existing electronic system [7]. Many researchers around the globe pay attention to the polymer nanocomposite material (organic-inorganic) due to their unique property such as increases flexibility, improved surface hardness and heat resistance (due to inorganic components) [8][9][10].
Yang et al have reported the detection of HCN gas by quartz crystal microbalance (QCM) technique [11][12].
Here we are reporting for the first time the detection of HCN gas by chemi-resistance method having fast response. In the present work, we have synthesized the Polyaniline/MMT-rGO nanocomposite by chemical oxidative polymerization. The synthesized polymer nanocomposite material is characterized by SEM, FTIR, and XRD. We can achieve a sensing response of 4.56% for PANI/MMT-rGO at 2 ppm of hydrogen cyanide (HCN) gas. The sensor recovers back to the baseline after every exposure of HCN. The sensor is stable and is working for the past 9 months successfully.

Synthesis of reduce graphene oxide
Graphene oxide (GO) was synthesized from graphite powder (Sigma-Aldrich) by using modified Hummers Method and further reduced by hydrazine hydrate to form reduced graphene oxide (rGO) [13][14][15]. Synthesized rGO was filtered by using deionized (DI) water and methanol, and dried under vacuum. nanocomposite is dried at 60 0 C. The PANI/MMT polymer nanocomposites are prepared by without rGO [16][17][18] in the above process. image is taken using TEM Jeol microscope operating at accelerating voltage of 120 keV.

Gas Sensing Properties Measurement
Thin films of polymer nanocomposite is deposited over the flexible transparency sheet (25 mm x 0.5 mm) by a drop-cast method and dried at 45 0 C. Silver (Ag) paste is used to make electrodes for electrical measurements. The optical photographs of sensors flexibility is shown in Fig. 1 (a) and (b). The sensing activity of the sensor is conducted at simple home-made gas chamber of net volume 1 L. Different concentrations of HCN gas are introduced inside the chamber. The schematic diagram of the gas sensing assembly is shown in Fig. 1(c). The resistance of the sensor is measured when exposure to HCN using an LCR meter (Hioki 3232) in presence of dry nitrogen gas (N2), which gives final resistance and without HCN gives initial resistance. The sensing chamber is flushed with nitrogen gas before and after measurements.
The desired concentrations of gas are generated by static liquid distribution method [19,20].
where, C(ppm) is the desired target gas concentration, ρ is (g/mL) the density of the liquid (gas), V′ is the volume of liquid (μL), T temperature in Kelvin, M molecular weight of liquid (g/mol), and V volume of the chamber (L). Particular volume (μL) of analyte is injected into a chamber via a precision syringe. The gas chamber is flushed with the nitrogen (1000 sccm) gas before and after taking the reading.
The humidity present inside the chamber on the day of experiment was 34% and temperature of 30 0 C (inside the chamber temperature). The sensor response, R%, is defined by [21].
where, Ri is the initial resistance of the sensor in air, and Rf is the final resistance after exposure of HCN.
The sensitivity (S) is defined by the slope sensing response versus concentration of the target gas: Here ∆R and ∆C are changes in sensor response and concentration of gas.

Results
The graph of the FTIR studies of the GO, rGO, PANI, PANI-MMT and PANI/MMT-rGO is shown in Fig. 2   The XRD pattern of the rGO, PANI/MMT and PANI/MMT-rGO are shown in Fig. 2 (c) and (d).
The peak at 2θ = 11.24 0 corresponds to the (001) plane of GO having an interlayer spacing of 0.77 nm, due to interlamellar groups trapped between hydrophilic graphene oxide sheets. The low-intensity peak at 2θ = 43.27° having (100) plane is due to rGO, thus confirming a random packing of graphene sheets in rGO [24][25][26]. The planes correspond to (001), (100), (005), (110) and (300)  The TEM micrograph of the rGO reveals the formation of a single layer sheet structure as shown in Fig. 3 (a). PANI has tube-like structures as seen in SEM of micrograph in Fig. 3 (b). The average length and diameter of the PANI are found to be 250 nm and 50 nm respectively.

Gas Sensing Properties
The gas sensing study is carried out on the homemade chamber (1 L). 2 ppm concentration of different gases such as acetone, ammonia, benzene, hydrogen cyanide and xylene are introduced in the gas chamber containing the PANI/MMT gas sensor for the selectivity of the gas. The sensor is found to be more active to HCN having the sensing response of 3.5 % as compared to the other gases as shown in Fig. 4 (a). Similarly, PANI/MMT-rGO sensor is exposed to different gases. Here we have found that the sensing response of the PANI/MMT-rGO toward the HCN is 4.56% as compared to other gases as shown in Fig. 4 (b). Thus both sensors have a good response toward the HCN gas.
The sensor made of PANI alone when exposed to the 2 ppm concentration of HCN has a sensing response is 0.045%. The sensing response (0.05%) is slightly increased when the concentration of the HCN gas vapor is 4 ppm but the sensor becomes saturated after 6 ppm as shown in Fig. 5 (a). The sensor (PANI) is not fully recovering to the initial baseline. This may be due to the fact that the HCN molecules are permanently bonded to the polymer chain [28]. Both the sensors made up of PANI/MMT and PANI/MMT-rGO are exposed to 2 ppm, 4 ppm, 6 ppm, 8 ppm, and 10 ppm respectively. The sensing response of these sensors is calculated using equation (1) and shown in Table II. The sensing response of the sensor becomes 3.5% (2 ppm) for PANI/MMT which has higher value as compared to the PANI alone (0.045%). The sensor response further increases as the concentration of the gas increase as shown in Fig. 5 (b). This increase in the sensor response (PANI/MMT) as compare to PANI sensors may be due to the increase in the binding sites provided by the MMT. The sensing response for PANI/MMT-rGO is 4.56% at 2 ppm, which is more than the PANI/MMT (3.5%). This increase in the sensor   response is due to the increase in the surface area provided by rGO. In the case of graphene oxide, all the carbon atoms are available at the surface of the 2D sheet for binding with the exposed gas. Both the sensors PANI/MMT and PANI/MMT-rGO are recovering completely to the baseline. The response time of the sensor is defined as the time taken by the sensor to achieve 90% of the total sensor response. Fig. 6 (a)   The sensing response versus concentration graph is shown in Fig. 8 (a). From the graph, we have Both the sensors PANI/MMT and PANI/MMT-rGO are tested for 10 months by exposing 2 ppm concentration repeatedly, as shown in Fig. 9 (b). The sensing response of the sensor PANI/MMT remains constant for up to 6 months, but after which the sensing response becomes 3.25 %.
Similarly, the sensor PANI/MMT-rGO is stable up to 8 months and after that sensing response becomes 4.35 %. The fabricated sensors PANI, PANI/MMT and PANI/MMT-rGO are exposed to HCN gas at different relative humidity (RH). We have observed that the sensing response (S%) of the sensor increases as the RH increases but decreases after 40% of the relative humidity. Fig. 10 shows the graph between the sensing response with the RH when exposed to a 2 ppm concentration of HCN.
The sensor resistance is changed by the presence of humidity. In the above Fig. 10, we have observed an increase in the sensing response of the sensor as the RH% increases, this may be due to the decrease in electrical resistance of the sensing material. Inside the sensing material, the pores are previously filled with dry air are now filled with a water molecule. But after an RH% value of 40%, the sensing response of the sensors decreases. This is due to the absorption of more water by the sensing materials causing an increase in resistance. It also increases the separation between the polymer chains, thus causing hindering of the electron hopping process.
A similar phenomenon is also reported by Cavallo et al. [29].   [30]. Thus, it causes an increase in the resistance of the sensing material when exposed to the HCN gas vapour.

Conclusions
The polymer nanocomposite is synthesized and characterized with FTIR, XRD, TEM and SEM.
The TEM micrograph of the graphene shows the formation of sheet structures. The SEM micrograph of PANI shows the formation of a nano-tube of diameter 50 nm and length 250 nm.
The PANI is deposited over the entire surface of the MMT. In the case of the PANI/MMT-rGO, the rGO has encapsulated PANI/MMT. The peaks of the XRD pattern confirm the presence of MMT and rGO in the polymer composite. The sensor is exposed to different gases acetone, ammonia, benzene, hydrogen cyanide and xylene. The sensing material can detect HCN gas and give the highest sensing response. The sensing material PANI alone has a low sensing response of 0.05%. The sensing response of the sensing material increases as the MMT and rGO is added to the PANI. The PANI/MMT and PANI/MMT-rGo has the sensitivity of 0.89 ppm -1 and 1.1174 ppm -1 respectively. In both the sensors we observed that the sensing response of the sensor increases as the concentration of the gas exposed increases. The sensors recovered automatically within 21 s (PANI/MMT) and 25 s (PANI/MMT-rGO). The sensor's performance decreases after 6 months and 8 months.       Comaprision response time and recovery time of sensors when exposed to 2 ppm HCN gas.   Sensor responses to HCN (2 ppm) starting from different RH values