SnO2 thick films for room temperature gas sensing

SnO2 thick films for room temperature gas sensing applications Kamalpreet Khun Khun, Aman Mahajan, and R. K. Bedi Citation: J. Appl. Phys. 106, 124509 (2009); doi: 10.1063/1.3273323 View online: http://dx.doi.org/10.1063/1.3273323 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v106/i12 Published by the American Institute of Physics.

Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

Downloaded 29 Mar 2012 to 203.134.215.216. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

JOURNAL OF APPLIED PHYSICS 106, 124509 共2009兲

SnO2 thick films for room temperature gas sensing applications Kamalpreet Khun Khun, Aman Mahajan, and R. K. Bedia兲 Material Science Laboratory, Department of Physics, Guru Nanak Dev University, Amritsar 143005, India

共Received 28 May 2009; accepted 13 November 2009; published online 29 December 2009兲 Porous nanosized SnO2 powder has been synthesized by a simple nonaqueous sol gel method using SnCl2 · 2H2O and C2H5OH as precursors. Thermal stabilization of the gel is investigated by thermogravimetric/differential thermal analysis. SnO2 powder has been obtained by calcining the gel at 500 ° C for 3 h and studied for its structural properties using x-ray diffraction 共XRD兲, field emission scanning electron microscopy 共FESEM兲, and transmission electron microscopy 共TEM兲. XRD observations confirm the formation of rutile structured SnO2. On an average, 35 nm size particles have been found in TEM micrographs of SnO2 powder. FESEM of the powder reveals the formation of a porous network formed by weak aggregation of nanoparticles. An attempt has been made to fabricate gas sensor by depositing thick SnO2 films on glass substrate. Gas sensing studies show that the sensing response of SnO2 sensor toward ammonia is comparatively higher at room temperature as compared to that toward acetone and ethanol. © 2009 American Institute of Physics. 关doi:10.1063/1.3273323兴 I. INTRODUCTION

Nanostructured materials have drawn considerable attention in recent years because of their improved catalytic activity, stability, sensitivity, and optical and electrical properties as compared to conventional microcrystalline materials. The large surface to volume ratio of these materials enhance their gas response characteristics and hence make them highly suitable for gas sensing applications.1–4 Metal oxide semiconductors have been investigated extensively for sensing various types of vapors and toxic gases.5–9 SnO2 is the most widely studied sensor material as it has high chemical sensitivity, good stability, and a low production cost. Nanostructured SnO2 powder has been grown by different techniques such as homogeneous precipitation,10 hydrothermal route,11 high energy ball milling,12 sol gel method,13 sol gel combustion,14 and laser ablation.15 The method of preparation and experimental conditions are found to control the microstructure of the synthesized material and thus indirectly affect the performance of a gas sensing material.16 Among these, the sol gel method is found to be a convenient and simple technique for synthesizing homogeneous highly reactive powder using inexpensive precursors. The use of organic solvents in this method strongly influences particle size, shape surface, and assembly properties of the synthesized material.17 Thin and thick films of SnO2 have been deposited by using a variety of techniques, such as spin coating,18 dip spray pyrolysis,20 thermal oxidation,21 coating,19 22 23 sputtering, and screen printing for gas sensing applications. Thick film gas sensors based upon semiconductor oxide powders have certain advantages, such as low cost, simple construction, and high sensitivity over other type of gas sensors. It has been reported that SnO2 thin/thick films show good sensing response only at high operating temperaa兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0021-8979/2009/106共12兲/124509/5/$25.00

tures. Literature finds scant data for the preparation of SnO2 based sensors that can operate at room temperature. With this end in view, in this communication, the synthesis, structural, and gas sensing properties of porous nanosized SnO2 powder prepared by a nonaqueous sol gel method have been reported. An attempt has been made to fabricate inexpensive SnO2 based sensor with comparatively higher response at low operating temperature. II. EXPERIMENTAL

Porous nanosized SnO2 powder has been synthesized by a simple nonaqueous sol gel method using ethanol as a solvent. SnO2 sol was prepared by dissolving 0.033 mol of SnCl2 · 2H2O in 100 ml ethanol obtained from SigmaAldrich 共U.S.A兲. The mixture was well stirred and heated at 50 ° C in a closed vessel. The solution thus obtained was finally heated in an oven at 120 ° C until the solvent was completely evaporated. During this process, the solution appeared to change from a clear transparent sol to a thick yellowish gel. The gel so formed was finally calcined at 500 ° C for 3 h in a muffle furnace to obtain crystalline SnO2 powder. The thermal decomposition of the gel was studied in air atmosphere using Perkin Elmer 共Pyris diamond兲 thermal analyzer in the temperature range from 25 to 700 ° C. The crystal phases in the calcined powder were studied by x-ray diffractometer 共X’Pert Panalytical兲 operated at 40 kV, 30 mA over 20°–70° with a scan rate of 0.002° / s. The surface morphology and composition of the powder was investigated using field emission scanning electron microscopy 共FESEM兲共JEOL JSM 6700兲 with a beam voltage 25 kV. The shape and size of nanostructures were determined by transmission electron microscopy 共TEM兲共FEI Technai 30兲 operated at 300 kV. For the preparation of thick film, a known quantity of the synthesized SnO2 powder was thoroughly mixed with a few drops of diethanolamine and grinded in a mortar and pestle

106, 124509-1

© 2009 American Institute of Physics


124509-2

Khun Khun, Mahajan, and Bedi

J. Appl. Phys. 106, 124509 共2009兲

FIG. 1. TGA-DTA curve for SnO2 powder.

to obtain a fine paste. The paste was coated on to glass substrate and the film so obtained has been annealed at a temperature of 400 ° C for 1 h to remove the organic binder. The thickness of the film was monitored using depth profiler 共Dektek 3030 XT兲 and was found to be 32 ␮m. The sensor performance of thick SnO2 film was tested in a home built test chamber in the temperature range 25– 250 ° C. A known volume of the gas to be tested was introduced in the chamber and the resistance of the sensor was measured by Keithely electrometer 共6517A兲 as a function of time at various operating temperatures. The sensitivity of the sensor is calculated by the equation S=

冉

冊

Rg − Ra ⫻ 100 % , Ra

where Rg and Ra represent resistance in the presence of gas and air respectively. Here, response time of the sensor is calculated as the time required for the resistance to reach 90% of the equilibrium value after the test gas is introduced and the recovery time is measured as the time necessary for the sensor to attain a resistance 10% above the original value in air.

FIG. 2. XRD plot of SnO2 powder calcined at 500 ° C for 3 h.

weight losses are in good agreement with the TGA observations made on SnO2 and other metal oxides.24–27 No significant weight loss has been observed beyond 500 ° C indicating that SnO2 powder becomes thermally stabilized. Based on these observations, the optimum temperature of calcination is chosen to be 500 ° C. B. Structural characterization of SnO2 powder

The x-ray diffractogram of the SnO2 powder 共Fig. 2兲 calcined at 500 ° C for 3 h shows the presence of sharp and well resolved peaks. X-ray diffraction 共XRD兲 pattern shows 共110兲 as the dominant peak along with other peaks corresponding to reflection from 共101兲, 共200兲, 共211兲, 共310兲, and 共301兲 planes. These observations confirms the polycrystalline nature of the sample having rutile structured unit cell, dimensions a = b = 4.7334 Å, c = 3.184 Å, and unit volume 共V兲 of 71.3 Å3. These observations are closely in agreement with JCPDS data of SnO2.28. Energy dispersive x-ray analysis of synthesized SnO2 powder show that the sample contains 21.72% of oxygen and 78.28% of Sn by weight thus indicating the formation of stoichiometric SnO2 powder. TEM images of SnO2 powder 共Fig. 3兲 show the forma-

III. RESULTS AND DISCUSSION A. TGA-DTA analysis

Figure 1 shows the thermogravimetric-differential thermal analysis 共TGA-DTA兲 curves of the gel obtained at 120 ° C representing the variation of percentage weight loss and heat exchange taking place in the system as a function of temperature. An overall weight loss of 31.24% is noticed in the entire temperature range 25– 700 ° C. A weight loss of 7.35% and an endothermic peak at 78 ° C has been observed in the temperature range of 25– 100 ° C, which can be due to the loss of residual alcohol entrapped in the micropores of the powder. Subsequent heating to 300 ° C induces a weight loss of about 10.50% accompanied by a broad exothermic peak having maxima at 160 ° C, which may be attributed to the decomposition of the organic matter and onset of crystallization of the gel. A weight loss of 13.39% is observed in the TGA curve in the temperature range of 300– 500 ° C, which is characterized by a second exothermic peak initiating at 360 ° C. This peak results from the crystallization process accompanied by loss of carbon species. These three

FIG. 3. TEM micrographs of SnO2 powder calcined at 500 ° C for 3 h, magnified TEM micrograph is shown in inset.


124509-3

J. Appl. Phys. 106, 124509 共2009兲


(a)

FIG. 4. FESEM of SnO2 powder calcined at 500 ° C for 3 h.

tion of nearly spherulitic nanoparticles. From the magnified TEM image 关Fig. 3共b兲兴, average size of nanoparticles is measured and is found to be around 35 nm. Figure 4 shows the FESEM images of SnO2 powder, which show the presence of weakly aggregated nanosized particles arranged in the form of a highly porous network. It can be attributed to the release of excess amount of gases during the decomposition of the gel to form SnO2 powder, causing a large number of microscopic pores in the microstructure. (b)

C. Sensing characteristics

The thick SnO2 film prepared on to the glass substrate was exposed to 50 ppm of reducing gases such as ammonia, acetone, and ethanol at different operating temperatures to find out optimum operable temperature and sensitivity. The variation of sensitivity 共S兲 versus time for SnO2 based sensor at different operating temperatures in presence of gases are shown in Figs. 5共a兲–5共c兲. A wide range of sensitivity values from −100% to +694.4% has been found for SnO2 based sensor toward different reducing gases. In operating temperature range 25– 200 ° C, SnO2 film shows a positive value of sensitivity to ammonia gas due to the increase in resistance of the film in corresponding temperature range. Whereas, at an operating temperature of 250 ° C, a negative value of 共S兲 has been noticed, possibly due to the decrease in the resistance of film at this temperature. Similar type of switching in the response behaviour of metal oxide based sensor for ammonia has also been reported by Kim et al.29 In case of acetone and ethanol, a negative value of 共S兲 in the operating temperature range 25– 250 ° C has been observed. This may be assigned to the overall decrease in resistance of the film within the investigated temperature range. The sensitivity of SnO2 thick film at room temperature 共25 ° C兲 toward ammonia is found to be more than other available metal oxide based ammonia sensors.30–32 This can be due to formation of a porous microstructure, which in-

(c) FIG. 5. Sensing response of SnO2 thick films exposed to 50 ppm of 共a兲 ammonia, 共b兲 acetone, and 共c兲 ethanol.

creases the interaction between gas and the surface of the material and thus improves the gas sensitivity. The comparative sensing response of SnO2 thick films toward different gases as a function of operating temperature is shown in Fig. 6. Observations reveal that the sensitivity of SnO2 sensor for ammonia decreases with an increase in operating temperature, whereas for acetone and ethanol the


124509-4

J. Appl. Phys. 106, 124509 共2009兲


electrons are given back to the conduction band of SnO2 thereby reducing the depletion width and hence the surface resistance decreases. This change in the surface resistance of SnO2 is recorded as a measure of sensitivity for reducing gases. The oxidation of ammonia on the SnO2 surface may take place by the following possible reactions:35,36

FIG. 6. Comparative sensing response of SnO2 thick films toward different gases as a function of operating temperature.

value of sensitivity increases. This can be attributed to the fact that at high temperatures, the rate of desorption of ammonia becomes greater than the rate of adsorption, which can decrease the number of gas molecules interacting with the oxygen species, hence sensitivity decreases. Investigations indicate that SnO2 sensor practically show no response to 50 ppm acetone and ethanol at room temperature 共25 ° C兲, while a maximum gas response of 694.4% with response time of 175 s and recovery time 210 s has been observed for 50 ppm of ammonia. SnO2 sensor exhibits comparatively higher response to 50 ppm of acetone and ethanol at operating temperature of 250 ° C with response time of 35 and 40 s and recovery time of 45 and 75 s, respectively. These observations indicate that working temperature plays a vital role in determining the sensitivity and selectivity of the film toward a particular gas. It is also found that response and recovery time of SnO2 sensor decreases with increase in operating temperature. This can be due to the increase in chemical kinetics of the reaction at elevated temperatures. D. Sensing mechanism

The sensing mechanism of SnO2 films to reducing gases such as ammonia, acetone, and ethanol can be explained by the adsorption of oxygen on the SnO2 surface. Depending on the operating temperature, different oxygen species according to the following chemical reactions are formed:33,34 O2共gas兲 → O2共adsorbed兲, O2共adsorbed兲 + e → −

O−2 + e− → 2O− ;

O−2 ;

2NH3 + 3O− → N2 + 3H2O + 3e− ,

共1兲

4NH3 + 5O−2 → 4NO + 6H2O + 5e− .

共2兲

The phenomenon of increase in resistance of SnO2 film in presence of ammonia within operating temperature range 共25– 200 ° C兲 can be attributed to the predominance of reaction 共2兲. NO formed in reaction 共2兲, can be easily transformed into an oxidizing species NOx in the presence of oxygen, which increase the resistance of SnO2 film, where as reaction 共1兲 explains the decrease in resistance of SnO2 in the presence of ammonia at operating temperature of 250 ° C due to release of reducing N2 gas. The reaction mechanism of acetone and ethanol with SnO2 can be represented by the following equations:37,38 CH3COCH3 + 8O− → 3CO2 + 3H2O + 8e− ,

共3兲

CH3CH2OH + 6O− → 2CO2 + 3H2O + 6e− .

共4兲

The decrease in resistance of SnO2 film in presence of acetone and ethanol gases within operating temperature range 共25– 250 ° C兲 can be attributed to these reactions, which result in the formation of reducing CO2 gas. Thus, reaction mechanism is completely dependent on the availability of oxygen species and the interaction of the test gas. IV. CONCLUSIONS

Porous tin oxide powder consisting of nearly 35 nm wide spherulitic particles has been synthesized by a simple and inexpensive non aqueous sol gel method. FESEM studies indicate the formation of a highly porous microstructure with weakly agglomerated nanosized particles. Gas sensing studies of SnO2 thick film shows that the operating temperature and nature of oxygen species appear to strongly influence the response of SnO2 based films toward ammonia, acetone, and ethanol. Thick film SnO2 sensor showed a maximum sensitivity of 694.4% for 50 ppm of ammonia at room temperature. Therefore, it is concluded that the thick film prepared from the synthesized SnO2 can be considered as a good ammonia sensing material at room temperature with high sensitivity and selectivity. ACKNOWLEDGMENTS

T ⬍ 100 ° C,

T = 100 – 300 ° C.

The depletion region formed due to trapping of electrons by oxygen to form different species increases the surface resistance of SnO2 layer. When a test gas molecule of a reducing gases adsorbs on the SnO2 surface, it reacts with the dominant oxygen species and gives out electrons. These

Authors are thankful to IUAC, New Delhi for providing financial assistance for carrying out this project. Thanks are also due to the Director, RSIC Department, Punjab University, Chandigarh and IISC Department, IIT, Roorkee, India for providing XRD, FESEM, and TEM facilities. M. Franke, T. Koplin, and U. Simon, Small 2, 36 共2006兲. Y. Ma, K. J. Liao, and C. Y. Kong, J. Wide Bandgap Mater. 10, 113 共2002兲.

1 2


124509-5

N. Yamazoe, Sens. Actuators B 5, 7 共1991兲. A. Rothschild and Y. Komem, J. Appl. Phys. 95, 6374 共2004兲. 5 Z. Bai, C. Xie, M. Hu, S. Zhang, and D. Zeng, Mater. Sci. Eng., B 149, 12 共2008兲. 6 R. B. Kamble and V. L. Mathe, Sens. Actuators B 131, 205 共2008兲. 7 P. Samarasekara, N. T. R. N. Kumara, and N. U. S. Yapa, J. Phys.: Condens. Matter 18, 2417 共2006兲. 8 D. Manno, M. Di Giulio, T. Siciliano, E. Fillippo, and A. Serra, J. Phys. D: Appl. Phys. 34, 2097 共2001兲. 9 J. Zhu, O. K. Tan, Y. C. Lee, T. S. Zhang, B. Y. Tay, and J. Ma, Nanotechnology 17, 5960 共2006兲. 10 H. Yang, X. Song, X. Zhang, W. Ao, and G. Qiu, Mater. Lett. 57, 3124 共2003兲. 11 H. C. Chiu and C. S. Yeh, J. Phys. Chem. C 111, 7256 共2007兲. 12 Ü. Kersen and L. Holappa, Anal. Chim. Acta 562, 110 共2006兲. 13 F. Gu, S. F. Wang, M. K. Lu, G. J. Zhou, D. Xu, and D. R. Yuan, J. Phys. Chem. B 108, 8119 共2004兲. 14 L. Fraigi, D. G. Lamas, and N. E. Walsöe de Reca, Nanostruct. Mater. 11, 311 共1999兲. 15 G. Williams and G. S. V. Coles, J. Mater. Chem. 8, 1657 共1998兲. 16 M. R. Vaezi and S. K. Sadrnezhaad, Mater. Sci. Eng., B 140, 73 共2007兲. 17 M. Niederberger, G. Garnweitner, N. Pinna, and G. Neri, Prog. Solid State Chem. 33, 59 共2005兲. 18 T. R. Giraldi, M. T. Escote, A. P. Maciel, E. Longo, E. R. Leite, and J. A. Varela, Thin Solid Films 515, 2678 共2006兲. 19 S. Rani, M. C. Bhatnagar, S. C. Roy, N. K. Puri, and D. Kanjilal, Sens. Actuators B 135, 35 共2008兲. 20 C. Luyo, I. Fabregas, L. Reyes, J. L. Solis, J. Rodriguez, W. Estrada, and R. J. Candal, Thin Solid Films 516, 25 共2007兲. 21 J. G. Partridge, M. R. Field, J. L. Peng, A. Z. Sadek, K. Kalantar-Zadeh, J. Du Plessis, and D. G. McCulloch, Nanotechnology 19, 125504 共2008兲. 22 D. Haridas, V. Gupta, and K. Sreeniwas, Bull. Mater. Sci. 31, 397 共2008兲. 3 4

J. Appl. Phys. 106, 124509 共2009兲

Khun Khun, Mahajan, and Bedi 23

K. Jain, R. P. Pant, and S. T. Lakshmikumar, Sens. Actuators B 113, 823 共2006兲. 24 A. Jitianu, Y. Altindag, M. Zaharescu, and M. Wark, J. Sol-Gel Sci. Technol. 26, 483 共2003兲. 25 J. Zhang, J. Q. Hu, F. R. Zhu, H. Gong, and S. J. O’Shea, Sensors 3, 404 共2003兲. 26 K. Wu, C. Wang, and D. Chen, Nanotechnology 18, 305604 共2007兲. 27 E. Barrera-Calva, J. Mendez-Vivar, M. Ortega-Lopez, L. Huerta-Arcos, J. Morales-Corona, and R. Olayo-Gonzalez, Advances in Materials Science and Engineering 2008, 190920 共2008兲. 28 JCPDS Card No. 41-1445. 29 Y. S. Kim, S. C. Ha, K. Kim, H. Yang, S. Y. Choi, and Y. T. Kim, Appl. Phys. Lett. 86, 213105 共2005兲. 30 G. S. Trivikrama Rao and D. Tarakarama Roa, Sens. Actuators B 55, 166 共1999兲. 31 J. Zhang, S. Wang, M. Xu, Y. Wang, H. Xia, S. Zhang, X. Guo, and S. Wu, J. Phys. Chem. C 113, 1662 共2009兲. 32 A. Ghosh, Y. G. Gudage, R. Sharma, R. S. Mane, and S. H. Han, Sensors & Transducers Journal 98, 1 共2008兲. 33 C. M. Ghimbeu, J. Schoonman, M. Lumbreras, and M. Siadat, Appl. Surf. Sci. 253, 7483 共2007兲. 34 F. Hellegouarc’H, F. Arefi-Khonsari, R. Planada, and J. Amouroux, Sens. Actuators B 73, 27 共2001兲. 35 I. Jimenez, A. M. Vila, A. C. Calveras, and J. R. Morante, IEEE Sens. J. 5, 385 共2005兲. 36 D. H. Yun, C. H. Kwon, H. Hong, S. Kim, K. Lee, H. G. Song, and J. E. Kim, Proceedings International Solid State Sensors and Actuators Conference 共Transducers ’97兲 2, 959 共1997兲. 37 L. Qin, J. Xu, X. Dong, Q. Pan, Z. Cheng, Q. Xiang, and F. Li, Nanotechnology 19, 185705 共2008兲. 38 G. Neri, A. Bonnavita, G. Micalli, G. Rizzo, N. Pinna, M. Niederberger, and J. Ba, Sens. Actuators B 130, 222 共2008兲.
