ETHANOL AND TRIMETHYL AMINE SENSING BY ZnO-BASED NANOSTRUCTURED THIN FILMS

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1 June, 0 ::pm WSPC/-IJN 00 st Reading International Journal of Nanoscience Vol. 0, No. (0) #.c World Scienti c Publishing Company DOI: 0./S0X00X ETHANOL AND TRIMETHYL AMINE SENSING BY ZnO-BASED NANOSTRUCTURED THIN FILMS. Introduction DURGAJANANI SIVALINGAM*,, JOHN BOSCO BALAGURU RAYAPPAN*,,, SAKTHIVEL GANDHI*,, SRIDHARAN MADANAGURUSAMY*,, RAJAN KALPOONDI SEKAR*, and UMAMAHESHWARI KRISHNAN*, *Centre for Nanotechnology and Advanced Biomaterials (CeNTAB) School of Electrical and Electronics Engineering School of Chemical and Biotechnology, SASTRA University Thanjavur, Tamil Nadu, India rjbosco@ece.sastra.edu Received Accepted Undoped and uorine (F)-doped nanostructured zinc oxide (ZnO) thin lms were deposited over glass substrates by spray pyrolysis technique using zinc acetate dihydrate with and without ammonium uoride as dopant in precursor solution respectively. The deposition conditions and the concentration of the dopant were optimized to obtain nanostructured ZnO thin lms. The lm obtained from 0.0 M of zinc acetate aqueous solution and 0% F sprayed at the rate of ml/min on preheated substrate kept at K yielded spherical shape well-connected grains, which has large surface to volume ratio. The structural and morphological studies of the lms were investigated by using X-ray di raction (XRD) and scanning electron microscope (SEM) respectively. The di raction peak positions in XRD con rmed the formation of highly crystalline ZnO lm with hexagonal wurtzite phase. Further sensing behavior of the lms towards various concentrations of volatile organic compounds (VOCs) such as ethanol and trimethyl amine (TMA) has been investigated at an optimized operating temperature of K and reported. Keywords: ZnO lms; spray pyrolysis; sensitivity; VOCs; o - avor. ZnO is a versatile, n-type metal-oxide semiconductor with excellent electrical and optical properties. Due to its stable thermal and chemical properties with non-toxic nature, it is found to have wide range of applications in optoelectronic devices, gas sensors, etc. For gas sensor, ZnO has been examined in various forms like thin lms, sintered pellets, thick lm, single crystal and heterojunctions. As, reaction at grain boundaries and depletion of charge carriers has strong in uence over the material transport properties, nanostructured ZnO attracted the attention of the researchers. Among many metal oxide semiconductor materials, ZnO is found to be a most promising one due to its low cost of production and good compatible nature which can sense VOCs and gases over a wide range. But to make use of nanostructured ZnO thin lm for food quality application, the sensitivity and selectivity of ZnO towards particular VOC/gas should be improved and this can achieved by adding suitable dopant

2 June, 0 ::pm WSPC/-IJN 00 st Reading D. Sivalingam et al However sensing properties of undoped ZnO thin lms toward gases and VOCs have been investigated; only few works were reported from the application point of view. Since ethanol and trimethyl amine (TMA) are the good markers for the detection of spoilage of food items like milk, sh, etc., due to bacterial contamination and genetic e ect in cow respectively. Hence in the present work, change in electrical resistance of undoped and F-doped nanostructured ZnO thin lms toward exposure of ethanol and TMA has been studied.. Materials and Method Undoped and F-doped nanostructured ZnO thin lms were deposited over a clean glass substrate using spray pyrolysis technique. The details about the experimental set up are found elsewhere in the literature. Zinc acetate dihydrate (.%, Merck) and ammonium uoride (.%, Merck) has been used as precursor to deposit undoped and F doped ZnO lm with the optimized deposition condition as listed in Table. The precursor solution concentration was optimized and xed as 0.0 M of zinc acetate dihydrate in the case of undoped ZnO. And 0% of ammonium uoride in 0.0 M of zinc acetate dihydrate for F doped ZnO lm. Structural characterization of the lms was carried out using X-Ray Di ractometer (XRD) with Cu K radiation of wavelength. Å (D Focus, Bruker, Germany) at the scanning rate of 0.0 /min and surface morphology image is obtained from eld emission scanning electron microscope (FE SEM, F, JEOL, Japan). Sensing property of the ZnO lms toward VOCs has been studied using a home built testing chamber of L capacity with digital thermostat-coupled compact heater and a septum provision to inject desired concentration of VOCs using micro-syringe. Change in electrical resistance of the lms was recorded using electrometer (Model A, Keithley, Germany) as a function of time during the process of injection and venting. Electrical contacts were made Table. Optimized deposition parameters. Distance between substrate and spray gun nozzle Angle of spray nozzle corresponding to the substrate Substrate temperature Spray time during each cycle Time interval between successive spray cm K 0 sec sec using thin copper wire and silver paste on the lm to obtain ohmic contact. The thickness of the lms was observed by cross-sectional SEM and is found to be 0 and nm for the undoped and F doped ZnO thin lms, respectively.. Results and Discussion A set of undoped and F doped ZnO thin lms were deposited on glass substrate by varying substrate temperature, precursor solution concentration and carrier gas pressure. It was observed that the lm obtained from 0.0 M of zinc acetate aqueous solution and 0% of ammonium uoride sprayed at a rate of ml/min on the preheated substrate kept at K yields spherical shape well-connected grains. The reproducibility of the lm was achieved for the above mentioned optimized conditions. The XRD patterns of as-deposited lms obtained is shown in Fig.. It shows that, the prepared undoped and doped lms are of polycrystalline in nature with hexagonal wurtzite structure. The peak positions are in well agreement with JCPDS and are indexed to (00), (00), (0), (0) and (0) plane. Absence of peak shift with reference to JCPDS indicates that the prepared lms are macro-strain free. Also F doped ZnO lm does not show any shift with respect to the peaks obtained in undoped ZnO. This indicates the doping of F atoms occurred without any lattice distortion, due to the similar ionic radii of O and F. Preferential growth along (00) axis similar to Rodriguez-Baez et al., has Intensity (arb. Unit) Fig.. 0% F doped ZnO Undoped ZnO Theta XRD of undoped and F doped ZnO

3 June, 0 ::pm WSPC/-IJN st Reading 00 Nanostructured ZnO Thin Films for Food Quality Discrimination Fig.. Sensitivity versus temperature of undoped and F doped ZnO toward 0 ppm of TMA and Ethanol. Fig.. SEM image of ZnO and 0% F doped ZnO. been observed in 0% F-doped ZnO lm which emphasize the fact that F doping has enhanced the growth along c-axis. Absence of phase segregation con rms the formation of binary metal oxide. Grain size (D) of undoped and F-doped ZnO lms were determined using Scherrer formula and is found be and nm respectively. This is comparatively agreed with the SEM image observed, as shown in Fig.. characterization of undoped and F doped ZnO towards various concentrations of ethanol and TMA has been carried out. Corresponding change in electrical resistance has been observed and is shown in Figs. and. It is observed from the response of undoped ZnO that, the electrical resistance of lm decreases as the concentration of ethanol and TMA increases. This is attributed by the following sensing mechanism,: O þ e ZnO! O C H OH þ O! CH CHO þ H O þ e.. Sensing characterization The sensitivity of undoped and F-doped ZnO lms were studied at various operating temperatures (0K K) and is shown in Figs. and. It was found that the sensitivity of lms toward TMA and ethanol at 0 ppm were found to show maximum response at K. Hence at an optimized operating temperature of K, further sensing ðþ ðþ ðch Þ N þ O! NO þ CO þ H O þ e ðþ Equation () indicates the oxidation reaction between ZnO surface and atmospheric air. Under this condition, the lm was exposed to VOCs which leads to the decrease in resistance due to the reaction as given in Eqs. () and ()

4 June, 0 ::0pm WSPC/-IJN 00 st Reading D. Sivalingam et al Fig.. Resistance versus concentration of undoped and F- doped ZnO towards Ethanol and TMA at an operating temperature of K. In the case of F doped ZnO, though F is a strong electronegative element and has a tendency to decrease the material resistance compared to undoped ZnO resistance, in the present scenario material resistance increases when it is exposed to reducing gases. This can be attributed to the fact that, addition of electrons on the surface of the lm due to the reducing gases are predominantly scattered at the grain boundaries.,,0, This is supported by the observed reduction in grain size of the F doped ZnO lm. From the observation, the sensitivity of undoped nanostructured ZnO thin lm shows better response and recovery towards TMA. When TMA is exposed to doped ZnO, increase in concentration of electrons due to reducing nature of TMA leads to enhanced Fig.. Response for ppm at K of undoped ZnO towards TMA and F doped ZnO towards ethanol. scattering at the grain boundaries. This results in competition between increase in electron concentration and increase in scattering. And in turn it prohibits the appreciable change in resistance towards TMA. But this competition is absent in undoped ZnO and hence the response is appreciable in pure ZnO lm. Whereas F-doped nanostructured ZnO thin lm shows better response towards ethanol. When ethanol is exposed to doped ZnO, high electronegative uorine sites restrict the ow of electrons injected by the reducing nature of ethanol. Hence the change in resistance is higher compared to undoped ZnO. This re ects in better sensitivity in doped than undoped ZnO lms. This trend may be validated by the fact that TMA dissociates with faster rate than ethanol on the surface of nanostructured pure and doped ZnO lm. Response and recovery time for these two cases are shown in Figs. and. This sensing trend may be utilized to specify the presence of reducing VOCs in an environment having mixture of gases. Hence the ne-tuned undoped and F doped nanostructured ZnO material can be used in the sensor array of an electronic nose for food quality discrimination.. Conclusion The prepared undoped and uorine doped nanostructured ZnO thin lms were investigated towards the sensing of ethanol and trimethyl amine. The sensitivity studies reveal that the undoped ZnO shows better response toward TMA and F-doped ZnO toward ethanol. Hence undoped and F-doped nanostructured ZnO lms can be used as an e ective sensor element for quality discrimination of food products like milk, sh, etc

5 June, 0 ::0pm WSPC/-IJN 00 st Reading Nanostructured ZnO Thin Films for Food Quality Discrimination Acknowledgments The authors wish to acknowledge Department of Science and Technology, New Delhi for their nancial support (Project ID/SEN//0). They also express their sincere thanks to SASTRA University, Thanjavur for extending infrastructural support to carry out the study. References. Z. C. Jin, I. Hamberg and C. G. Granqvist, J. Appl. Phys., ().. M. Suchea, S. Christoulakis, K. Moschovis, N. Katsarakis and G. Kiriakidis, Thin Solid Films, (00).. P. P. Sahay and R. K. Nath, Sens. Actuators B, (00).. K. L. Chopra, S. Major and D. K. Pandya, Thin Solid Films 0, ().. Ghosh and S. Basu, Mater. Chem. Phys., ().. S. Saito, S. Miyayama, K. Koumoto and H. Yanagida, J. Am. Ceram. Soc., ().. Z. L. Wang, Appl. Phys. A, (00).. B. P. J. de Lacy Costello, R. J. Ewen, P. R. H. Jones, N. M. Ratcli e and R. K. M. Wat, Sens. Actuators B, ().. N. Magana, A. Pavloub and I. Chrysanthakis, Sens. Actuators B, (00). 0. S. Ampuero and J. O. Bosset, Sens. Actuators B, (00).. N. E. Barbri, A. Amari, M. Vinaixa, B. Bouchikhi, X. Correig and E. Llobet, Sens. Actuators B, (00).. D. Perednis and L. J. Gauckler, J. Electroceram., 0 (00).. JCPDS File no.. B. D. Cullity and S. R. Stock, Elements of X-Ray Di raction, rd edn. Chapter (Addison-Wesley, ), p... M. de la, L. Olvera, A. Maldonado, R. Asomoza, O. Solorza and D. R. Acosta, Thin Solid Films, (00).. J. R. Baez, A. Maldonado, G. Torres Delgado, R. C. Perez, M. de la and L. Olvera, Mater. Lett., (00).. M. Ghosh and A. K. Raychaudhuri, Nanotechnology, (00).. R. L. Mishra, A. K. Sharma, R. K. Srivastava and S. G. Prakash, Invertis J. Sci. Technol., (00).. S. Roy and S. Basu, J. Mater. Sci.-Mater. Electron., (00). 0. K. Ellmer, A. Klein and B. Rech, Transparent Conductive Zinc Oxide, Basics and Applications in Thin Film Solar Cells, Vol. 0 (00). Springer series in materials science.. A. S. Juarez, T. Silver and A. Ortiz, Sol. Energy Mater. Sol. Cells, 0 ()