CODEN (USA): IJCRGG, ISSN: 0974-4290, ISSN(Online):2455-9555 Vol.10 No.7, pp 494-500, 2017
*1Mude K.M., 2Mude B.M., 3Raulkar. K. B, 4Yawale S.S.,5Yawale S.P.
Abstract : The gas sensitivity response of nano-metal oxide (ZnO) doped composites (ZnO/PANI) was studied. The chemicals used for the preparation of gas sensor were first calcinated at 800oC for 5 h. Composites of ZnO/PANI were prepared and multilayer sensor was developed using screen printing technique with Al2O3 as substrate on glass plate. The composites of ZnO and PANI were characterized by FTIR and XRD. The sensitivity was measured by measuring the electrical resistance in presence of CO2 gas which was found to be more for ZnO/PANI/Al2O3 multilayer sensor. It was found that response of multilayer sensor increases with increase in ppm concentration of CO2 gas. The entire phenomenon is discussed on the basis of gas adsorption on the surface of the sensor which arises due to charge transfer. Keywords: ZnO; screen-printing technique; CO2 gas sensor, sensitivity.
It is well known that the sensing properties of ZnO-based material depend on its chemical and physical characteristics, which are strongly dependent on the preparation conditions, dopant and grain size. This implies that the synthesis of the sensing material is a key step in the preparation of high-performance Metal oxide semiconductor (MOS) gas sensors. ZnO powders and films can be prepared by a variety of synthesis methods [1-5].
The present investigation mainly deals with the preparation of CO2 gas sensor ofZnO doped Polyaniline. It was found that ZnO system with Polyaniline shows more sensitivity to carbon dioxide gas.
A gas sensor is a device, which detects the presence of different gases in an atmosphere, especially those gases that might be harmful to living animals. The design of gas sensor technology has received considerable attention in recent years for monitoring environmental pollution. Tin dioxide based chemiresistors have high gas sensing response as compare to the chemiresistors based on conducting polymers but they are operated at high temperature (>200°C),whereas conducting polymers (CP) such as Polyaniline (PANI) doped with metal oxides shows better sensing response at room temperature[6].
Chemical synthesis of CP is usually performed by such oxidants as (NH4)2S2O8 or FeCl3 and is commonly used for the preparation of CPs, while electrochemical deposition is used mainly for deposition of CP films on conducting substrates. An advantage of this method is to control the film thickness by the charge passed through the electrochemical cell during the film growth. Other popular techniques for depositing thin films on various substrates are spin coating by a solution of a chemically synthesized CP, the deposition of one or more monomolecular layers of CP by Langmuir–Blodgett technique, or coating of substrates by bilayers of CP and opposed charged polymers by the layer-by-layer technique. CPs is multifunctional materials; it is not always possible to make a definite separation of their functions. Finally, the application of a combinatorial approach for synthesis and high-throughput screening of chemo-sensitive properties of CP is discussed. Polyaniline (PANI) is one such polymer whose synthesis does not require any special equipment or precautions. Conducting polymers generally show highly reversible redox behavior with a noticeable chemical memory and hence have been considered as prominent new materials for the fabrication of the devices like industrial sensors. The properties of conducting polymers depend strongly on the doping level, protonation level, ion size of dopant and water content. Conducting polymer PANI is prepared either by electrochemical oxidative polymerization or by the chemical oxidative polymerization method. The emeraldine base of PANI is an electrical insulator consisting of two amine nitrogen atoms followed by two amine nitrogen atoms. PANI (emeraldine base) can be converted into a conducting form by two different doping processes: protonic acid doping and oxidative doping. Protonic acid doping of emeraldine base corresponds to the protonation of the amine nitrogen atoms in which there is no electron exchange. In oxidative doping, emeraldine salt is obtained from leucoemeraldine through electron exchanges. The mechanism causing the structural changes is mainly recognized to the presence of -NH group in the polymer backbone, whose protonation and deprotonation will bring about a change in the electrical conductivity as well as in the color of the polymer. Considerable research effort is now directed towards the development of sensors and artificial noses and electronic tongues synthesis and characterization of Thin Films of Conducting Polymers for Gas Sensing applications. Based on conducting materials used for the detection of chemical vapors and gases and biological species was done [1].
In 100 ml solution of aniline (0.4 M)and 1M sulfuric acid; 100 ml solution of ammonium persulphate(0.5 M)was added drop wise with constant stirring at room temperature at normal condition. After completion of the oxidant addition, stirring was continued for further 2 h to insure completion of the reaction. During polymerization, the sequence of coloration of the reaction mixture was light blue, blue green and finally greenish black precipitate. This color indicates that the product is conducting emeraldine salt. The reaction mixture was kept overnight. Then it was filtered, washed with distilled water until the filtrate become colorless and finally with methanol to remove the impurities and oligomers. This Polyaniline is then used for active layers of Semiconductor Gas Sensors.
Fig. 1:Synthesis of PANI
ZnO and Al2O3 powders (AR grade) were calcinated at about 800 °C for 4-5 h and were crushed in mortal pestle to get fine powder. ZnO and PANI were characterized by XRD. XRD patterns of the samples were obtained using X-ray Diffractometer at Govt. VISH, Amravati. The diffraction pattern was in the terms of I vs2θ at continuous scan type at step size = 0.015o.
The ink or paste of the sample was prepared by using screen-printing (thick film technique) technique. The binder for screen-printing was prepared by thoroughly mixing 8 wt% butyl carbitol with 92 wt% ethyl cellulose. On chemically and ultrasonically cleaned glass plate, paste of Al2O3 was screen printed and it was kept for 24 h to dry it at room temperature and then heated at 140oC for 2.5 h to remove the binder. The Al2O3 layer provides mechanical support as well as high thermal conductivity. Paste of ZnO and ZnO mixed in proper stoichiometry was then screen printed on Al2O3 layer. Again plate was dried at room temperature for 24 h and binder was removed by heating it at 150oC for 2.5 h. Finally PANI layer was deposited on ZnO and doped with ZnO layer by screen printing, whole plate was dried and again binder was removed as above. Fabrication of multilayer sensor is shown in following fig. 1.
Finally on the top surface of the sensor, interdigited electrodes [26] were fabricated using conducting silver paste as shown in the Fig.1 (aandb). Thickness of ZnO and doped with ZnO layer and PANIlayers were recorded with the help digital micrometer (series 293, Japan) having resolution of + 0.001 mm and were found to be 10 μm and 7 μm respectively. To measure the sensitivity, electrical resistance was measured with the help of voltage drop method.
Fig. 2: (a) Fabrication of interdigited Electrodes (b) Actual photograph of interdigited electrodes (c) Circuit of resistance measurement using interdigited electrodes.
XRD of PANI and 80ZnO:20PANI (fig.2 a and b) showed that Polyaniline is amorphous in nature. A broad peak at 2θ = 26o was observed which is due to the scattering from PANI chains at the inter-planar spacing [28]. The average crystalline size of PANI was calculated by using Scherrer’s formula given by equation (1),
(1)
Where, D is the crystalline size, K is the shape factor and β is the full width at half maximum of diffraction angle in radians. The average crystallite size of PANI was found to be 101 nm.
The Polyaniline powderwas analyzed by FTIR. FTIR spectra showed the main characteristic peaks at
−1 −1 −1−1
761cmcorresponding to C-N bond, 1271 cmcorresponding to C-H deformation,1533 cmand 1459 cmcorresponding to the fundamental vibrations of Polyaniline . The peaks at 1640 cm−1 corresponding to C=C. The peak at 3411 cm −1 corresponds to the N-H bond. These peaks were observed in the present work for preparations using FeCl3 as oxidants and various dopants such as ZnO and ZnO. This confirms the formation of Polyaniline[12].
Fig. 4: FTIR pattern of (1) PANI, (2) PANI–ZnOnanocomposites and (3) ZnOnano particles.
The sensitivity of the sensor is given by equation (2),
Where, Rair and Rgas are the resistances of sensors in air and gas respectively.
From Fig. (5), multilayer structure of the sensor shows more sensitivity to Ammonia gas than that for pure ZnO and pure ZnO. Resistance of multilayer sensor was found to be decreasing with increase of carbon dioxide gas concentration and thereby sensitivity was increasing[10]. Maximum sensitivity was recorded for multilayer sensor at 80 ppm concentration of CO2.
Fig. 5: Variation of sensitivity with of CO2 gas concentration at room temperature. Sample Codes:
Sr. No. | Pure | Codes |
---|---|---|
1 | ZnO | P1 |
2 | PANI-ZnO | P2 |
3 | PANI | P3 |
Step responses for pure PANI &ZnO-doped PANI sensor for 60 and 80 ppm are shown in fig. 6. The time taken to reach 85% of the response when ppm of gas is changed is known as response time and time taken to reach 85 % of recovery when gas is turned off is known as recovery time. Response time (tres) and Recovery time (trec) are the two important parameters of the sensor. It was found that response time is 59 s and recovery time is 101 s for multilayer sensor at 80 ppm of CO2.i.e.ZnO-doped PANIsensor is fast.
Fig.(6) Step response of (1) pure PANI, (2)ZnO doped PANI sensor at room temperature
Rate of change of resistance of the sensor with respect to time defines the stability of the sensor. A sensor should be more stable for its better response. The changes in the resistance for multilayer sensor (80ZnO:20PANI)[13-14] and pure samples are shown in the fig. (7).
Rs M
From fig. (7), it is observed that resistance of multilayer sensor does not change drastically as that in case of pure samples. This shows that multilayer sensor is more stable than other.
From XRD and SEM characterization it is concluded that the crystallite size of 80ZnO:20PANI/PANI/Al2O3 multilayer is smaller and it is more porous and hence has greater surface area and therefore shows greater response to CO2 gas. Screen printing technique is the easiest for the preparation of sensor. 80ZnO:20PANI/PANi/Al2O3 multilayer sensor shows good stability than pure samples and dynamic response is also fast.
The author Mude K.M. is very much thankful to Head & Professor, Department of Physics, GovtVidarbha Institute of Science & Humanities, Amravati, and also thankful to Principal Dr. F. C Raughuwanshi, VidyabhartiMahavidyalaya& Dr. S.S. Yawale, Director, GovtVidarbha Institute of Science & Humanities, Amravati for providing necessary facility at the time of my research work.
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