Global Journal of Engineering Science and Research Management

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1 OPTICAL NITROGEN DIOXIDE (NO2) GAS SENSOR USING ZNO THIN FILM Ravinder Gaur*, Himansu Mohan Padhy * Department of Electronics and Communication Engineering Biju Patnaik University of Technology Bhubaneswar, Odisha, India Department of Electronics and Communication Engineering Sohitorium Engineering College, Biju Patnaik University of Technology Bhubaneswar, Odisha, India DOI: 1281/zenodo KEYWORDS: SPR, Optical properties, gas sensor, dielectric, plasmons, thin film. ABSTRACT The present work includes the application of developed SPR setup for gas sensing at room temperature using a specially designed gas cell. The change in the optical properties of dielectric layers (ZnO) with adsorption of gases (NO2) in order to develop an optical sensor has been presented. The obtained results emphasize the applications of Surface Plasmon Resonance (SPR) setup towards the study of interaction of adsorbed gas molecules, with dielectrics and gas sensing. INTRODUCTION Conductometric gas sensors exploiting semiconductor sensing materials are exclusively used for fabricating gas sensors due to simple detection principle and easy sensor fabrication [1]. However, these types of sensors comes with few disadvantages such as requirement of high operating temperature which results in consumption of high power with a poor selectivity. This paves the way for realizing efficient gas sensors which can operate at room temperature which is provided by optical sensors [2]. Surface Plasmon Resonance (SPR) based sensors have several merits like simple fabrication, room temperature operation, fast response at lower concentration of toxic gases etc. Many researchers have exploited SPR based sensors in Kretschmann configuration for detecting toxic gases by coating a suitable sensing film on the plasmonic metal [3, 4].The deposition of gas sensitive layer on the noble metal surface (excites Surface Plasmons (SPs) at metal/dielectric interface) is the major requirement for SPR based gas sensors whose refractive index changes in contact with target gas in turn bringing the variations in resonance parameters which is in correlation with the interacting gas molecules concentration NO 2 is a highly toxic gas dangerously affects human health and also contributes the formation of photochemical smog. The major sources of the NO 2 gas includes automobiles, thunderstorms, gas boilers, industries etc[5]. Increase in the levels of NO 2 gas even after 20 pap concentration is injurious to living beings as it can affect the upper respiratory tract and lungs badly [6]. Thus, it an urgent requirement to fabricate highly efficient and sensitive NO 2 gas sensor. The SPR based NO 2 gas sensor requires the identification of a suitable sensitive coating. Wide band gap semiconductor materials such as SnO 2and ZnO are the suitable sensing materials which have been utilized extensively for realization of conductometric gas sensors for the recognition of a many toxic, dangerous and injurious gases which includesno 2, SO 2, CO, H 2S, CO 2etc. for domestic, commercial and industrial applications [7, 8, 9]. Amongst all the metal oxides, Zinc Oxide (ZnO) is an exclusive material having semiconducting, photoconducting, piezoelectric and pyroelectric properties suitable for applications in gas sensors[10]. Thus, in the present work ZnO has been chosen as a sensing material for detection of NO 2 gas utilizing SPR technique. In the present work, variation in optical properties of gas sensitive thin film(zno) with interaction of toxic gas(no 2) have been monitored. The ZnO film with suitable sensing properties was deposited on gold coated BK7 glass prisms using rfmagnetron sputtering technique. Surface plasmon resonance reflectance curves were obtained for different concentration of NO 2 gas for prism/au/zno system by varying incident angle. The prepared sensor structure is investigated for selectivity corresponding to other interfering and interacting gases. EXPERIMENTAL PROCEDURE Angular interrogation method was used to record the SPR reflectance in Kretschmann configuration where Au thin film (around 40 nm) was deposited on a BK-7 glass prism (right angled) to support the propagation of SPsat the [21]

2 prism/au interface. An indigenously assembled SPR measurement system in which a p-polarized light source (He- Ne laser light of wavelength 633 nm) was used to excite the SPs for prism/au/zno/air system. The light which is reflected from another face of the prism was monitored using a photodetector interface with the power meter with varying incident angle. The schematic of the laboratory assembled SPR measurement system specially designed for gas sensing applications is shown in figure 1 where gas cell especially designed is placed on the prism table. The gas cell is provided with a window to couple the Au-prism and gas sensitive (ZnO) films so that ZnO thin film is in the vicinity with the target gas molecules inside the gas cell. The complete scan of SPR reflectance curve was acquired by inserting a particular concentration of target gas of gas sensing measurements. For the transient measurements, photodetector is replaced by CCD where the whole prism/au/zno system is fixed at a stage where incident angle corresponds to the resonance angle and continuous variation in reflected intensity was noted with increasing gas concentration. In order for the sensor (prism/au/zno) to recover, a rotary pump is exploited to evacuate the chamber and fill in the fresh clean air. The selectivity of the sensor is also a crucial parameter which is observed by monitoring the interference with other gases including H 2, NH 3, LPG etc. Figure 1: Experimental setup of the Kretschmann configuration based SPR gas sensor using CCD All the technical information regarding the growth of ZnO thin film are given in Table 1 where the films were prepared by using rf magnetron sputtering with constant thickness of 200 nm on Au coated glass prism for sensing applications. [22]

3 Target Table 1: Technical details for the growth of ZnO thin film growth 2 inch diazinc target (~99.999% pure) Composition of deposition gases (Ar:O 2) 60:40 Deposition Pressure 20 mtorr Radio frequency Power 40 W In-situ annealing temperature 250 o C Thickness 200 nm RESULTS AND DISCUSSIONS Observation of SPR reflectance curves Figure 2 shows the experimental curves (symbols) obtained for prism/au/air and prism/au/zno/air systems at a wavelength = 633 nm. Narrow SPR reflectance curve represents low absorption losses along with the longer propagation length of SPs which is quantified by real and imaginary parts of refractive index i.e. n and k respectively [11]. Gold possesses stable optical and chemical properties making it the most preferred metal for SP excitation at metal-dielectric interface [12]. The prism/au/air system as evident from figure 2 shows the resonance angle at at R min (reflectance at the resonance position) of The continuous curve shown in figure 2 indicates the theoretically fitted curve obtained by Fresnel s equations [13].The estimated values of thickness of metal thin film and the dielectric constant of glass prism and air has been used in the fitting. The dielectric ) and refractive index (n + ik) values of metal thin films are valued by theoretically fitting the SPR data obtained experimentally (symbols in figure 2). The estimated values fairly correspond to the reported values [14]. The observed SPR reflectance curve (symbols)for prism/au/zno system at λ = 633 nm is shown in figure 2. The reflectance data were recorded by SPR setup in the angular interrogation mode using Kretschmann configuration. The SPR curve shows a shift in θ SPR at higher angles after integrating the thin film of ZnO with prism/au structure. The shift observed in θ SPR is due to the variation in interface from Au/air to Au/dielectric having another layer of ZnO of different dielectric constants. The experimentally observed SPR data is theoretically fitted using the Fresnel s equations while using the estimated value of the dielectric constant estimated for Au film (40 nm thin, i1.51) and varying the dielectric constant value of respective ZnO thin film as fitting parameters. For ZnO thin wavelength of λ = 633 nm to be 3.24 and 1.8 respectively[15]. [23]

4 Minimum Reflectance [Gaur* 6(1): January, 2019] ISSN Reflectance Theoretical prism/au/air prism/au/zno/air Angle ( o ) Figure 2: SPR reflectance curve for prism/au/air and prism/au/zno/air system sat 633 nm excitation wavelength Response to NO2 gas obtained by the prism/au/zno as-prepared structure The as-prepared sensor structure (i.e. prism/au/zno/air system)with200 nm thin ZnO film grown at 20 mt sputtering pressure is used for measuring the response towards NO 2 gas utilizing SPR technique. SPR reflectance data obtained for prism/au/wo 3 structure at λ =633 nm at room temperature on exposure to the increasing concentration of NO 2 gas from 1 ppm to 250 ppm are depicted in Figure 3(a). SPR reflectance curve for the prism/au/zno structure obtained in the air ambient (0 ppm) is also included in figure 3 (a) for comparison. Linear variation in SPR angle ( SPR) and minimum reflectance (R min) with various concentrations of NO 2 gas is plotted in figure 3 (b) as a function of NO 2 gas concentration and it was observed that the values of both θ SPR and R min was found to increase linearly from 44.8 to 58.5 and 0.32 to 0.73 respectively(figure 3 (b)). This is because of the change in the optical properties of sensing dielectric layer (i.e. ZnO thin film)in terms of refractive index due to adsorption of oxidizing NO 2 gas. The sensitivity of the NO 2 gas sensor using WO 3 sensing layer was estimated to be /ppm from the figure 3 (b). The basic mechanism behind the detection of toxic gases by SPR based optical gas sensors is explained clearly in figure 4. It is evident from the figure that SPR sensors works on the principle of change in SP dispersion conditions which may be because of difference in the refractive index of two things: bulk media (i.e. change of air to NO2 gas) and dielectric layer i.e. ZnO thin film [16]. Reflectance Theoretical 0 ppm 1 ppm 10 ppm 100 ppm 250 ppm Angle ( o ) (a) SPR ( o ) Concentration (ppm) (b) [24]

5 Figure 3: (a) Value of reflectance as a function of incident angle (i.e. SPR reflectance curve) for the prepared structure on interaction with NO2 gas of fixed concentrations varying from 1 to 250 ppm and (b) Linear change in SPR angle (θspr) and the value of minimum reflectance with different concentration of NO2 gas (calibration curve) Figure 4: Mechanism of prism/au/zno system based NO2 sensing Transient response of the prepared sensor structure: prism/au/zno The gas sensing measurements done in the dynamic mode is shown in figure 5 (a) indicates the change in reflectance of the prism/au/zno system fixed at the resonance position for varying concentrations from ppm- 250 ppm. The response cycle as evident from the figure is shown by the rise in the reflectance value (at fixed )to a stable high value (saturation) after the insertion of target gas molecules. From figure 5 (a), the sensor regains its original value of minimum reflectance (R min) just after removing the target gas (i.e. NO 2 gas) from the gas cell during recovery cycle. Figure 5 (b) depicts the linear change in the sensing response ( R) with the concentration of target gas (i.e. calibration curve of the sensor prism/au/zno). The selectivity response of the prepared sensor is shown in figure 6 where the gas sensing response is measured for different harmful gases likenh 3, LPG and H 2 of 250 ppm concentration. 2.0 Gas OFF 2.0 Change in Reflectance Gas ON Time (in sec) ppm 1ppm 5ppm 10ppm 50ppm 100ppm 150ppm 200ppm 250ppm (a) (b) Figure 5: Transient response for prism/au/zno structure having ZnO sensing layer with exposure to NO2 gas of varying concentration (-250 ppm). Change in reflectance Concentration (in ppm) [25]

6 Intensity (in a.u.) 1.5 H 2 NO x Time (in ms) {NH 3, LPG} Figure 6: Selectivity study of the prism/au/zno sensor on exposure to different interfering gases (NH3, LPG, H2) CONCLUSION SPR technique has been effectively used for monitoring the change in dielectric properties of semiconducting thin films (ZnO thin film) on exposure to NO 2 gas for fabricating an efficient gas sensor. High sensing response is obtained using ZnO thin film of thickness of about 200 nm with increase in concentration range from to 250 ppm of NO 2 gas. The prepared optical gas sensor indicates the quick response (1 s) and also the recovery towards the target gas. The sensitivity of the prepared sensor is found to be /ppm and selectivity studies were done with other gases like NH 3, LPG, H 2. ACKNOWLEDGEMENTS Authors are thankful to University of Delhi for providing facility to carry out the research work. REFERENCES 1. Sharma, M. Tomar, V. Gupta, Low temperature operating SnO 2 thin film sensor loaded with WO 3 micro-discs with enhanced response for NO 2 gas, Sensors and Actuators B: Chemical, 161, 2012, pp M. G. Manera, R. Rella, Improved gas sensing performances in SPR sensors by transducers activation, Sensors and Actuators B: Chemical, 179, 2013, pp P. I. Nikitin, A. A. Beloglazov, V. E. Kochergin, M. V. Valeiko T. I. Ksenevich, Surface plasmon resonance interferometry for biological and chemical sensing, Sensors and Actuators B: Chemical, 54, 1999, pp J. N. Wilde, M. C. Petty, J. Saffell, A. Tempore, L. Valli. Surface plasmon resonance imaging for gas sensing, Meas. Control, 30, 1997, pp K. Skalska, J. S. Miller, S. Ledakowicz, Trends in NOx abatement: A review, Science of The Total Environment, 408, 2010, pp M. Kampa, E. Castanas, Human health effects of air pollution, Environmental Pollution, 151, 2008, pp M. Kumar, A. Kumar, A. C. Abhyankar, SnO2 based sensors with improved sensitivity and response recovery time, 40, 2014, pp A. T. Mane, S. B. Kulkarni, S. T. Navale, A. A. Ghanwat, N. M. Shinde, J. H. Kim, V. B. Patil, NO 2 sensing properties of nanostructured tungsten oxide thin films, Ceramics International, 40, 2014, pp Y. Cao, D. Jia, R. Wang, J. Luo, Rapid one-step room-temperature solid-state synthesis and formation mechanism of ZnO nano rods as H 2S-sensing materials, Solid-State Electronics, 82, 2013, pp M. K. Hossain, S. C. Ghosh, Y. Boontongkong, C. Thanachayanont, J. Dutta, Growth of zinc oxide nanowires and nanobelts for gas sensing applications, Journal of Metastable and Nanocrystalline Materials, 23, pp , [26]

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