Fabrication and characterization of Zinc oxide based thick and. thin film ethanol sensors doped with Aluminium Oxide

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1 Int. Journal of Applied Sciences and Engineering Research, Vol. 5, Issue 1, by the authors Licensee IJASER- Under Creative Commons License 3.0 Research article ISSN Fabrication and characterization of Zinc oxide based thick and thin film ethanol sensors doped with Aluminium Oxide Ruchika 1, Vijay Kumar Anand 1, Sood S.C. 1, Virdi G.S 2 1- Department of Electronics and Communication Engineering, Ambala College of Engineering and Applied Research- Ambala, Haryana, India 2- Director R&D GGS College of Modern Technology, Mohali, Punjab, India DOI: /ijaser Abstract: Ethanol gas is a hypnotic or sleep producer gas having toxic nature. The persons working on ethanol synthesis have great chances of being victims of respiratory and digestive track cancer. So there is a massive demand and rising challenges for monitoring ethanol gas. The present work is focused on the fabrication and characterization of pure and aluminum oxide doped n-type semiconducting zinc oxide based ethanol sensors prepared using thick and thin film technologies. Characterization of samples by XRD and SEM provided valuable information about the structural and morphological properties of nanoparticles. Doped and pure ZnO samples show good crystallinity having wurtzite hexagonal structure. In case of thick film sensor, sensitivity of pure ZnO towards 400 ppm ethanol gas is 56.53% and sensitivity of Al2O3 doped ZnO film is 80.34%. For thin film sensor, sensitivity of pure ZnO is 59.44% and sensitivity of Al2O3 doped ZnO is 94.29%. The sensitivity of Al2O3 doped ZnO is found to be more than the sensitivity of the pure ZnO. It is also observed that the sensitivity of thin film is better than the sensitivity of thick film. The response and recovery time is of the order (~6s) and (~ 20s) respectively. Keywords: Zinc oxide (ZnO), Aluminium oxide (Al2O3) thick film and thin film sensors. 1. Introduction Environmental protection and increasing demands to monitor the dangerous and inflammable gases in industry and home have attracted interests in developing gas sensors (Hongsith Niyom, 2006). The semiconductor metal oxide based gas sensors have become the most promising devices among the solid-state chemical sensors due to their small size, low cost, simple operation and good reversibility. Many semiconductor oxides such as ZnO, SnO 2, Fe 2O 3, In 2O 3, CeO 2, WO 3, TiO 2, Cr 2O 3 and CuO, have been used to detect the toxic and inflammable gases, such as CO, CO 2, H 2 S, LPG, NO X and ethanol. Zinc Oxide is a wide band gap semiconductor with a bandgap of 3.37 ev and has a large binding energy of 60 mev. It has wide range of applications such as, luminescent devices, chemical sensors. solar cells etc, whose conductivity can be modified by controlling the variation from stoichiometry and by doping. Appropriate doping can provide electronic defects that raise the influence of oxygen partial pressure on the conductivity (Santhaveesuk Theerapong, 2010). In this work, Al 3+ substitution on Zn 2+ was chosen due to the small ion size of Al 3+ (0.53Å) compared to that of Zn 2+ (0.74 Å).The aim of the present research work is to enhance the catalytic activity of ZnO using Al 2 O 3 as dopant which is a weak n-type semiconductor material with wide band gap (8.8 ev). Al 2 O 3 has applications due to its low conduction as compared to the single component oxide of ZnO and a lower power requirement for bulk material (Chou Shih Min, 2006). It is present in different crystalline form and has good thermal stability, and works as good catalyst with semiconductor when the gases such as ethanol come in contact with (Patil D.R., 2007). Ethanol is a sleep *Corresponding author ( ruchika1810@gmail.com) 81 Received on December, 2015; Published on February, 2016

2 producer gas having toxic nature (Hongsith Niyom, 2010). Heavy exposure or consumption of alcoholic beverages, mainly by smokers, increases the risk of cancer of the upper respiratory and digestive tracks. Among women, the chances of breast cancer increase with alcoholic consumption or exposure (Pandya Hardik Jeetendra, 2011). Those working on ethanol synthesis have great chances of being victims of respiratory and digestive track cancer. So there is a huge demand and rising challenges for monitoring ethanol gas at trace level. 2. Experimental works 2.1 Fabrication of Thick Film Sensor For the preparation of pure and doped ZnO nanopaowder, In beaker A1, 22 gm of zinc acetate was dissolved manually in 200cc of methanol (for doping add 1% aluminium chloride). This solution was kept in ultrasonic cleaner for 45 minutes at 50 C. A homogeneous solution was obtained. Initially the ph of this solution is 6. In beaker A2, 2M NaOH solution was prepared by dissolving 16 gm sodium hydroxide pellets in 200cc de-ionized water. This solution was kept in ultrasonic cleaner for 30 minutes at 50 C. NaOH solution was introduced in beaker A drop by drop with constant stirring. This will result in the formation of precipitates. Check the value of ph regularly. Stop when the value of ph exceeds 10. Keep this milky solution in ultrasonic cleaner for 1 hour at C. The resulting precipitates were filtered and washed several times with de-ionized water. Wash the precipitates with 5% acetic acid until the ph value becomes 6. Again wash them with de-ionized water. These resultants were dried in the oven for 24 hours at 100 C. The precipitates were collected and crushed manually with mortar and pestle for 15 minutes. The prepared composite material was sintered at 1000 C for 4 hours in air ambience. Sintering is done in order to remove organic vehicle (Shyju G. J., 2012). Finally, pure and Al 2 O 3 ZnO nanoparticles were obtained. The thixotropic paste was formulated by mixing the fine powder of the functional material with lead borosilicate glass frit (permanent binder) thoroughly in an acetone medium by using a mortar and pestle. The pastes were prepared by maintaining inorganic to organic materials ratio at 70:30. The inorganic part consisted of a functional material (doped and pure ZnO) and glass frit. The organic part consisted of 8% ethyl cellulose and 92% terpineol. The mixture of doped and pure ZnO powder, glass frit and ethyl cellulose was mixed by terpineol dropwise until a proper thixotropic property of paste was achieved. The formulated paste was screen printed (Wang Ping, 2012) on the glass substrates. The pattern was allowed to settle for 15 to 20 minutes in air. The printed samples were dried for 45 minutes in oven at C to evaporate terpineol and then fired at 600 C for 1 hour in the muffle furnace in air ambience and then attained at the room temperature. Firing is done to decrease the dense number of grain boundaries thus improving the crystallinity of the film. Metallic contacts of silver were deposited on the film by screen printing technique. 2.2 Fabrication of thin film sensor Thin films were deposited on glass substrate using chemical bath deposition technique (Bajpai Ritu, 2012). Firstly, a seed layer is deposited on the glass substrate which is used for the surface modification of the substrate and provides nucleation sights for the growth of nano-structures and also helps to enhance the density as well as the homogeneity of particles. Also, seed layer is adopted to improve the orientation of 82

3 ZnO films on glass substrates. For the deposition of seed layer, 0.01M Zinc acetate was mixed in 20cc of 2-methoxyethanol for 10 minutes manually and then mix it ultrasonically for half an hour. Add 5-6 drops of diethanolamine in it to stabilize the solution and give ultrasonic treatment for 1 hour. Filter this solution and coat it on the glass substrates by spin coating technique. Dry the samples in the oven for 10 minutes at 100 C. Finally, anneal the samples in the furnace at 250 C for 10 minutes and then at 500 C for 30 minutes. In this work Zinc oxide thin films were deposited on glass substrates by dipping into ammonium zincate bath kept at 90 C temperature (El-Sayed A. M., 2012.). The ammonium zincate bath was prepared in the following way. 0.01M zinc acetate was dissolved in DI water and was ultrasonically mixed for 30 minutes. For Al 2 O 3 doping add 5 mg Aluminium chloride in zinc acetate solution. Ammonium hydroxide was added drop by drop until the ph becomes 11 and then the solution was put in the ultrasonic for 60 minutes. This was followed by dipping the glass samples with the seed layer vertically into the solution. Keep it in the oven at 90 C for 3 hours. The samples were then allowed to cool. After cooling the samples were washed with DI water and dried in the oven at 100 C. 3. Structural and morphological analysis 3.1 XRD analysis In order to understand the structural properties of pure and Al 2 O 3 doped ZnO composite powder materials, X-ray diffraction analysis of the sintered powders were carried out in the range using Cuk α radiation. Figure 1 represents the XRD patterns of pure and Al 2 O 3 doped ZnO nanopowders. Figure 1(a) shows XRD patterns of pure ZnO material. The observed diffraction peaks are corresponding to the hexagonal wurtzite structure of ZnO and well matched with the RRUFF (R060027) reported data of ZnO (Muthuraja S, 2012). The sharp peaks of XRD attributed to ZnO material are observed to be polycrystalline in nature. The higher peak intensities of an XRD pattern are due to the better crystallinity of the sample. Figure 1(b) shows the XRD patterns of Al 2 O 3 doped ZnO composite material. In this composition, formation of only ZnO wurtzite phase is observed. The possible formations of Al 2 O 3 or ZnAl 2 O 4 phases are not detected. This might be due to the relatively low sintering temperature (~1000 C) at which addition of Al into Zn sites might not have occurred to a complete extent. It has been observed that (101) reflections are of maximum intensity, which indicates that ZnO films have preferred orientation in the (101) plane. It was also observed clear that the peak position corresponding to the plane (101) was shifted to lower value of 2θ, when aluminium oxide was incorporated. The grain size for pure ZnO sample varies from nm and the grain size of Al 2O 3 doped ZnO sample varies from nm. Hence grain size varies with aluminum oxide doping. 3.2 SEM analysis The surface morphology and chemical composition of the ZnO nanopowder was analyzed using a scanning electron microscope [SEM model JEOL 6010-LA]. SEM images in Figure 2 show the surface morphology of pure ZnO and Al 2 O 3 doped ZnO nanopowders. Figure 2(a) shows the micrograph of pure ZnO nanopowder. It consists of randomly distributed particles with smaller size and shape and with limited porosity. The particle size of films varies in between 182 nm to 206 nm. 83

4 Figure 1: XRD Patterns of (a) Pure ZnO and (b) Al 2 O 3 Doped ZnO Nanopowders Plane-view SEM investigation of Figure 2(b) reveals a porous structure of Al 2 O 3 doped ZnO nanopowder. Petal-shaped particles of various sizes in between 101 nm to 305 nm were observed in all samples. The majority of these particles come out several times larger than the average crystallite sizes calculated from X-ray diffraction data thus, indicating that most of the particles comprise multiple crystallites. No systematic variation in the microstructure of the ZnO films as a function of the doping concentration was observed. In Figure 2(b), the Al 2 O 3 additives present on the ZnO grains are at an optimum level leading to high porosity and large effective surface area available for the adsorption of oxygen species. The ZnO film doped with 1 wt. % Al 2 O 3 was observed to be the most sensitive to ethanol. On increasing the concentration of dopant, gas response becomes poor. This is due to the fact that if we increase the concentration of dopant than Al 2 O 3 particles present over ZnO surface would be greater than the optimum level, which reduces the porosity and resists reaching the target gas to the inter-grain boundary of Al 2 O 3 -ZnO. Hence, the film shows poor response to the ethanol gas. 84

5 (a) Figure 2: SEM Images of (a) Pure ZnO and (b) Al 2 O 3 Doped ZnO showing the crystallite size (b) 4 Gas sensing studies The gas sensing mechanism of the metal oxide based semiconductor gas sensors belong to the surface controlled type, which is based on the change in resistance of the semiconductor (Liao Wei-Zhen, 2013). The oxygen adsorbed on the surface directly influence the resistance of metal oxide based sensors. Oxygen is adsorbed on the oxide crystals surface as ions are formed by abstracting free electrons from the metal oxide based semiconductor gas sensors, reducing the electrical conductivity. The amount of oxygen adsorbed on the surface depends on working temperature, concentration of additives, particle size, and specific surface area of the sensor. In the aerial atmosphere where the partial pressure of oxygen is constant, oxygen is adsorbed on sensor surfaces of n-type ZnO in the form of O, O - and O 2-, depending on the temperature (Wang Ping, 2012). The oxygen adsorbed on the surface directly influence the conductance of metal oxide based sensors. The state of oxygen on the surface of ZnO sensor undergoes the reactions shown in Equations (1-4). (1) (2) (3) (4) When ethanol vapours react with these chemisorbed oxygen, a complex reaction will take place and convert ethanol into carbon dioxide and water. The possibility of a reaction of ethanol with the ZnO sensing layers can be explained as two oxidation states, where [O] represent the surface oxygen ions: (5) (The dehydrogenation to Acetaldehyde) 85

6 (6) (The dehydration to ethylene) The first reaction (5) taking place is a process initiating the oxidation by the dehydrogenation to CH 3 CHO intermediate, and the second reaction (6) is initiated by the dehydration to C 2 H 4. But the selectivity for the two reactions is initiated by the acid base properties of the oxide surface. The dehydrogenation process is more probable on the oxide surface with basic properties, while the dehydration is favoured on the acid surface. Finally intermediate products, acetaldehyde and ethylene, are subsequently reduced to CO 2 and H 2 O. This shows an n-type conduction mechanism (Muthuraja S., 2012). Thus on oxidation, single molecule of ethanol liberates twelve electrons in conduction band and results in decrease in resistance of the sensor. Increase in operating temperature causes oxidation of large number of ethanol molecules, thus producing very large number of electrons. Therefore, conductivity increases to a large extent. This work was initiated to study the gas sensing properties of pure and Al 2 O 3 doped ZnO thick and thin film sensors. As the gas sensors are based on resistive material used in the form of thick and thin film, an electrical characterization is related to the resistance measurements at various operating temperatures in ethanol vapours. On the basis of measured data, the sensitivity of thick and thin film sensors for a fixed gas concentration (400 ppm) in air ambient condition is calculated. Response time and recovery time are also observed. 4.1 Sensitivity Sensitivity is the ratio of resistance of the sensing element in the target gas to that in air. Sensitivity is highly dependent on film porosity, film thickness, operating temperature, presence of additives and crystallite size. From the measured resistance in air as well as in gas atmosphere, the sensitivity of gas was determined at particular operating temperature using the equation, S = 100 (7) Where, R a = Resistance of the film in air atmosphere, R g = Resistance of the film in gaseous atmosphere. Figure 3(a) shows the variation in the sensitivity of ethanol gas with the operating temperature of doped and pure ZnO thick film sensor. It is noted from Figure 3(a) that the pure ZnO thick film (56.53%) is observed to have poor response for ethanol gas (400 ppm) while 1% Al 2 O 3 doped ZnO thick film (80.34%) showed good response to ethanol gas at 300 C. The gas response increases with the increase in temperature until it reaches its maximum value, after that it starts decreasing with further increase in temperature. It has been observed that the increased sensitivity of thick film is due to the dopants or additives which enhance the chemisorptions of the film to specific ethanol gas. Also, on increasing the concentration of dopant, gas response becomes poor. This is due to the fact that if we increase the concentration of dopant than Al 2 O 3 particles present over ZnO surface would be greater than the optimum 86

7 level, which reduces the porosity and resists reaching the target gas to the inter-grain boundary of Al 2 O 3 -ZnO. Hence, the film shows poor response to the ethanol gas. Similarly from Figure 3(b) it is noted that that the pure ZnO thin film (59.44%) is observed to have less response for ethanol gas while 1% Al 2 O 3 doped ZnO thin film (94.29%) showed excellent response to ethanol gas at 300 C. Presence of Al 2 O 3 on ZnO grains in optimum amount leads to high porosity and large effective surface area available for the adsorption of oxygen species. 1% by wt. Al 2 O 3 doping was observed to be the most sensitive to ethanol than other films. It was observed that the sensitivity of thin film (94.29%) was better than the sensitivity of thick film (80.34%). The reason behind this is that the crystallite size of thin films is smaller than the crystallite size of thick films. Nanostructures deposited during the thin film deposition are much smaller than the nanostructures deposited during thick film deposition. (a) (b) Figure 3: Sensitivity of (a) Thick Film Sensor (b) Thin Film Sensor 4.2 Response time and recovery time Response time is the time taken for the sensor to attain 90% of the maximum increase in conductance on exposure to the target gas and the recovery time is the time taken by the sensor to get back 90% of the original conductance when the gas is switched is switched off. The response time and the recovery time of the Al 2 O 3 doped ZnO film samples were observed. The response was quick (~6s) to ethanol gas while the recovery was fast (~ 20s). The quick response may be due to faster oxidation of gas. Its high volatility 87

8 explains its quick response and fast recovery to its initial chemical status. 5. Conclusions Pure and Al 2 O 3 doped ZnO thick films have been prepared by screen printing technique and pure and Al 2 O 3 doped ZnO thin films have been prepared by chemical bath deposition technique. XRD and SEM provided valuable information about the structural and morphological properties of nanoparticles. Doped and pure ZnO samples show good crystallinity having wurtzite hexagonal structure. The XRD patterns reveal that the samples have (101) dominating peak indicating the strong preferred orientation along these planes. From SEM analysis, the topography of the sensing materials revealed a porous morphology. The crystallinity is improved with Al 2 O 3 doping on the glass substrate. Particle size of pure ZnO varies from 61 nm to 206 nm and particle size of Al 2 O 3 doped ZnO varies from 22 nm to 305 nm. These sensors operate at low working temperature i.e. 300 C. Sensitivity of pure and Al 2 O 3 doped ZnO thick film sensor is 56.53% and 80.34% respectively for 400 ppm ethanol gas. Similarly, sensitivity of pure and Al 2 O 3 doped ZnO thin film sensor is 59.44% and 94.29% respectively for 400 ppm ethanol gas. The sensitivity of Al 2 O 3 doped ZnO towards 400 ppm ethanol gas is more than the sensitivity of the pure ZnO. It is also observed that the sensitivity of thin film sensor is better than the sensitivity of thick film sensor. Also, for the fixed concentration of ethanol gas i.e. 400 ppm, the response is quick (~6s) while the recovery is fast (~ 20s). 6. References 1. Bajpai Ritu, Motayed Abhishek, Davydov Albert V., Bertness Kris A., and Zaghloul Mona E., Fellow, IEEE, UV-Assisted Alcohol Sensing With Zinc Oxide-Functionalized Gallium Nitride Nanowires, IEEE Electron Device Letters, 33(7). 2. Chou Shih Min, Teoh Lay Gaik, Lai Wei Hao, Su Yen Hsun and Hon Min Hsiung,2006. ZnO:Al Thin Film Gas Sensor for Detection of Ethanol Vapor, Sensors 2006, 6, El-Sayed A. M., Ismail F. M., Khder M. H., Hassouna M. E. M. and Yakout S. M., Effect Of CeO 2 Doping On The Structure, Electrical Conductivity And Ethanol Gas Sensing Prpperties Of Nanocrystalline ZnO Sensors, International Journal On Smart Sensing And Intelligent Systems, 5(3). 4. Hongsith Niyom and Choopun Supab, Enhancement of Ethanol Sensing Properties by Impregnating Platinum on Surface of ZnO Tetrapods, Institute of Electrical and Electronics Engineers Sensors Journal, 10, Hongsith Niyom, Choopun Supab, Mangkorntong Pongsri and Mangkorntong Nikorn, Ethanol Sensing Properties of Zinc Oxide Nanobelts Prepared by RF Sputtering, Bulletin of Materials Science, 30, Liao Wei-Zhen, Dai Ching-Liang and Yang Ming-Zhi, Micro Ethanol Sensors With a Heater Fabricated Using the Commercial 0.18 µm CMOS Process. 7. Muthuraja S, Govardhan K, Khan Nwaz, Roban Mohana, and Vijayaraghvan, Ethanol Sensor based on Dip Coated ZnO thick films,journel of Applied Sciences,3(5). 8. Pandya Hardik Jeetendra, Chandra Sudhir, Vyas Anoop Lal, MEMS-Based Ethanol Sensors Using Zinc Oxide Nanostructured Films, The Second International Conference on Sensor Device Technologies and Applications,

9 9. Patil D.R., Patil L.A. and Amalnerkar D.P., Ethanol gas sensing properties of Al 2 O 3 -doped ZnO thick film resistors, Bulletin of Materials Science, 30, Santhaveesuk Theerapong, Wongratanaphisan Duangmanee, and Choopun Supab, Enhancement of Ethanol Sensing Properties by Alloying Ti O2 With ZnO Tetrapods, Institute of Electrical and Electronics Engineers Sensors Journal, 10, Shyju G. J., Nagarani S., Dawn S., Roy Dharma, Sanjeeviraja C., Gas Sensing Properties of Semiconducting Metal Oxide Thin Films, Archieves of Applied Science Research, Wang Ping, Liu Dong-yan and Li Dong-sheng, Hydrothermal Synthesis of Different Zinc Oxide Nanostructures: Growth, Structure and Gas Sensing Properties, Material Transactions, 53(11),