Conductometric assisted ZnO and polypyrole composite based NH3 gas sensor

Metal oxides are excellent choices as base materials in emerging technologies in the eld of Gas Sensors. In this work, an effort is made to prepare the ZnO and polypyrrole composite thin lm using sol-gel technique. Nanocomposite thins lm was prepared using spin coating on glass substrates for ammonia gas sensing. ZnO and polypyrrole sols were prepared using sol-gel technique and thin lms were prepared by using spin coater. The nanocomposite thin lms were prepared by varying the percentage composition of ZnO and Polypyrrole. Structural, optical and morphological properties of the prepared lms were done using XRD, UV-Visible and SEM studies. The lms were separately prepared on Inter-Digital Transducers for gas sensing applications. Gas sensing response characteristics of the prepared sensor were performed using laboratory. The sensing response of the prepared lms is observed and found to be maximum (~33) for the 70%PPy+30%ZnO lm at a relatively low operating temperature of about 150°C.


Introduction
Development of gas sensors is essential for the protection of environment and work places from harmful and hazardous gases [1]. Due to the increase in the emission of gases from automobiles, there is a sharp rise in air pollution. The alarming increase in the usage of various gases in day today life, either in a domestic kitchen or workplaces or industries especially of explosive or toxic nature has immensely increased the need for reliable gas sensors in the commercial market. The major attributes to look forward in a gas sensor are: (i) high sensing response, (ii) small recovery time, (iii) selectivity [2]. In the present work, focus has been done on the detection of ammonia (NH 3 ). This is because, NH 3 , in gaseous form is a highly harmful to human health, commonly produce as animals and human excreta at home, work places and industries in many ways [3,4]. The lower limit of NH 3 perception by smell by human beings is around 5 ppm, thus, it is important to develop e cient gas sensors which can monitor and detect the low concentration of (smaller than ,5 ppm) NH 3 leaks [5]. Researchers have put enormous efforts on designing the highly e cient NH 3 gas sensors [6,7].
Semiconductor metal oxides represent an interesting, and a varied class of materials that are suited for gas sensing applications for several types of gases like oxidizing gases, reducing gases, odorous gases etc.. Amongst the various metal oxides exploited for gas sensing, Zinc Oxide (ZnO) is the most suitable metal oxide for gas sensing applications having diverse semiconducting, piezoelectric and pyroelectric properties [8]. Conducting polymers is a new class of sensing material which have arisen interest in due to good electrical and optical properties [9]. Poly pyrrole (PPy) is mostly exploited material due to highly stable nature at high temperatures and ease in synthesis by using chemical methods [10]. Arora et al. have prepared PPy thin lms electrochemically for NH 3 sensing in the concentrations of 0.01-1% [11].
Another report is on chemically synthesized Ppy lms giving show response values 4-8% and response time of 4 min to detect NH 3 gas [12]. Nanocomposite thin lms have found enormous applications in the eld of gas sensing due to high sensing response at lower operating temperature and selectivity towards a target gas [13,14]. Wu et al.
have observed the improvement in selectivity towards the target NOx gas by making the nanomposites of ZnO and PPy prepared by the simple method of mechanical mixing [13]. Patil et al. designed a highly sensitive NO 2 gas sensor using ZnO and PPy nanocomposite thin lms [14]. Wang et al. have proposed a highly novel sensing platform of nano ber composites of PPy/TiO 2 -ZnO for detecting a small concentration of NH 3 [15].
Hence, the present manuscript focusses on the development of NH 3 gas sensor using the composite thin lms of ZnO and PPy by varying its compositional percentage.

Experimental Details
The nanocomposite thin lms were prepared using sol-gel and spin coating technique. PPy nanoparticles were synthesized by using chemical polymerization method and ZnO nanoparticles were synthesized using sol-gel method. The ZnO composites with PPy were prepared by adding ZnO (NPs) in different weight ratios (20% and 30%) in smooth agate mortar and pestle. The nanocomposite powder was put in m-cresol and stirred for 11 h to get casting solution. Thin lms were prepared on glass substrates by spin coating method at 3000 rpm for 40 s and dried on hot plate at 100°C for 10 min.
The prepared lms were characterized structurally, optically and morphologically using XRD, UV-Visible and SEM techniques respectively. XRD measurements were done in θ-2θ scan mode. For UV-Visible measurements, transmittance of the lms was recorded as a function of wavelength. SEM studies were done in non-contact mode.
Gas sensing measurements were done using indigenously assembled laboratory setup. The lms were separately prepared on Inter-digital Transducers for gas sensing measurements as shown in gure 1.
Platinum (Pt) Inter-Digital Electrodes were formed by using Photolithography on glass substrates. PPy and ZnO composite thin lm was prepared on the Pt Inter-Digital Electrodes substrates. The system consist of a chamber in which vacuum is created using rotary pump. The sensing system is placed on a temperature-controlled stage and xed concentration of gas is inserted inside the chamber. The variation in the resistance of the sensor is measured by the data acquisition system which is interfaced with computer. Selectivity measurements of the prepared sensor were also done by measuring the gas sensing response of the prepared sensor in the presence of the interfering agents. A xed concentration of interfering agents is inserted inside the chamber and corresponding response was measured.

Results And Discussion
XRD pattern of ZnO thin lm is shown in the inset of gure 2 which shows the perfect growth of highly caxis oriented lm. A well-de ned and sharp peak is observed at 34.1° indicating the (002) plane [10]. Figure 2 shows the X-ray diffraction pattern of PPy and ZnO composite thin lms by varying their composition. Both the XRD graphs show a broad peak at around 24.8° indicating their amorphous nature [11]. This broad peak may be attributed to the scattering offered by the PPy chains at the interplanar spacing [12].
The transmittance spectra of the composite thin lms in UV-Visible regime is shown in Figure 3(a). This indicates that with increase in the concentration of ZnO, transmittance increases. The transmittance spectra is exploited to calculate the band gap of the prepared thin lms using Tauc plot [16]. The variation of the square of the absorption coe cient with the energy (in eV) is shown in gure 3(b). The linear portion of the plot is extrapolated to cut the x-axis giving the value of band gap. The band gap increases minutely from 3.89 eV to 3.92 eV with increase in the percentage composition of ZnO from 20% to 30% [17,18].
Morphological study of the lm (70% PPy + 30% ZnO) is analyzed using Scanning Electron Microscopy technique ( gure 4). The average grain size of the lm is mentioned in the image is around 38 nm. The prepared lm is found to be rough and porous appropriate for gas sensing applications [19].
The variation of response of the prepared sensing matrix to the target NH 3 gas is shown in gure 5. Both the sensing response of the prepared lms (80%PPy + 20%ZnO and 70%PPy + 30%ZnO) shows maximum response at the operating temperature of 150 °C . The sensing matrix having composition 70%PPy + 30%ZnO gives higher sensing response of 33 as compared to the other composition. This is attributed to the two factors: (i) higher band gap and (ii) higher surface roughness and porosity.  . The presently prepared gas sensor demonstrates poor response towards 100 ppm of these interfering gases as compared to that obtained for 50 ppm of NH 3 gas (Figure 7). This further indicates that the presently prepared gas sensor is highly selective selective towards the target NH 3 gas.

Conclusion
Summarizing the above work, an effort has been made to prepare the ZnO and Poly pyrrole composite thin lm for the detection of NH 3 gas. ZnO and Poly pyrrole sols were prepared using sol-gel technique.
Two types of lms were prepared by varying the percentage composition of ZnO and PPy (80%PPy + 20%ZnO and 70%PPy + 30%ZnO). Nanocomposite thin lm was prepared using spin coating on glass substrates. Structural studies of the lms were done using XRD which showed the amorphous nature of the prepared lms. Band gap of the prepared lms was also estimated using UV-Visible transmittance studies. Band gap of the lm having composition 70%PPy + 30%ZnO was found to have higher band gap. Morphological study of this lms was alos done using SEM and showed the formation of highly rough and porous lm. Gas sensing studies were also performed of the prepared lms. A maximum sensing response of ~33 is obtained for ZnO/PPy nanocomposite thin lm (70%PPy + 30%ZnO) at a relatively low operating temperature of about 150°C.

Competing interests
The authors declare that they have no competing interests" in this section.

Funding
No funding received from any agency for the present work Authors' contributions APSG has devised the problem for the present work. All the literature survey and work plan of the present work has been done. Analysis of the performed work.
AP has done all the experimental work. Some part of the analysis of data has also been done AK has done the nal proo ng of the article and reviwed the article for publication. Scienti c inputs in the analysis of the data has also been given.