|Year : 2009 | Volume
| Issue : 6 | Page : 299-303
Nature of Dopants and Effects on Sensitivity of SnO 2 Based Gas Sensor
JK Srivastava, Preeti Pandey, VN Mishra, R Dwivedi
Department of Electronics Engineering, Center for Research in Microelectronics, Institute of Technology, Banaras Hindu University, Varanasi - 221 005, India
|Date of Web Publication||18-Jan-2010|
J K Srivastava
Department of Electronics Engineering, Center for Research in Microelectronics, Institute of Technology, Banaras Hindu University, Varanasi - 221 005
| Abstract|| |
In this work, a thick film SnO 2 sensor was fabricated on a 1" x 1" alumina substrate. It consists of a gas sensitive layer (SnO 2 ) doped with Pd and Pt; a pair of electrodes in array form underneath the gas sensing layers serves as a contact pad for sensor and a common heater element on the backside of the substrate. Alumina substrate (96%) has been used as a substrate for sensor fabrication. The fabricated sensor was tested for varying concentration of LPG and methane in a locally developed test chamber. The response of sensors to varying concentration of LPG and methane was studied at 350 C. The Pd-doped sensor was found to be more sensitive towards LPG and methane.
Keywords: Gas sensor, LPG, Pd, Sensitivity, Tin oxide, Thick film.
|How to cite this article:|
Srivastava J K, Pandey P, Mishra V N, Dwivedi R. Nature of Dopants and Effects on Sensitivity of SnO 2 Based Gas Sensor. IETE J Res 2009;55:299-303
| 1.Introduction|| |
Atmospheric pollution is a global issue. Gases from auto and industrial exhausts are polluting the environment. The sensors are required basically to quantify pollutants and monitor the working environment. Depending on the gas and its concentration in the atmosphere, electrical conductivity is different. Tin oxide is known as a potential material for a gas sensor application  . To achieve robustness and longevity in its role as a gas sensor, the SnO 2 is most commonly prepared in the form of a ceramic  which is sintered on to a substrate, usually of alumina. In operation, this substrate is heated by an electrically energized filament, the resistance of the actve material falls as the concentration of (combustible) contaminant gas rises. It is the change in resistance which is measurand  . Gas sensors based on semiconductor tin oxide and fabricated by the thick film (screen printing) technology offer the desirable characteristics of low cost, simplicity and process reproducibility. SnO 2 is an n-type, wide-band gap semiconductor  . The n-type behavior of SnO 2 is due to a deficit in oxygen. Donors are singly and doubly ionized oxygen deficient with donor states E d1 and E d2 at 0.03 eV below the conduction band , . Investigation has proven both donor levels to be completely ionized at the usual operation temperature of 200- 4008C , . The conduction band has its minimum at the Gpoint in the Brillouin zone and is a 90% tin s-like state. The valance band consists of a set of three bands (2+, 3+ and 5+). The valance band maximum is a G3+ state. In this way, SnO 2 has a direct band gap. According to the results of Barbarat et al  . A large contribution of Sn(s)-states is found at the bottom of the valence band between -7 and -5 eV. From -5 eV to the top of the valence band, Sn (p)-states contribution is decreasing, as the Sn(d)-states are occupying the top of the valence band. A large and extended contribution of the O (p)-states is found in the valance band.
Clearly, bonding between Sn and O is dominated by the p-states of the latter. Each anion in the unit cell is found to be bonded to the cations in a planar-trigonal configuration in such a way that the oxygen p-orbitals contained in the four atom plane, i.e. P x and P y , define the bonding plane. Consequently, the oxygen p-orbitals perpendicular to the bonding plane, i.e. P z orbital, have a non-bonding character are expected to form the upper valence levels  . The conduction band shows a predominant contribution of Sn(s) states up to 9 eV. For energies larger than 9 eV an equal contribution of Sn and O-states is found in the conduction band.
The sensing properties of SnO 2 sensors (sensitivity, selectivity and reproducibility) depend on several factors, mainly grain size and specific surface area. Sensing mechanism of semi conducting gas sensor is based on the surface reaction of semi conducting oxide. In air, molecular oxygen is chemisorbed in the form of O 2-, O- or O 2 2- depending on operating temperature and deplete electron from the surface, leading to reduction of conductivity. Upon exposure of the reducing gas to the sensor surface, the chemisorbed oxygen reacts with the reducing gas and electrons are subsequently reintroduced into the conduction band, leading to enhanced conductivity  . It is believed that the exposure of gas causes only the surface properties of the grain to change for nanosized sensor, especially when crystal dimension approaches thickness of charge depletion layer; the change in the properties of the whole grain, not just at the surface, is observed on the gas exposure.
In this work, two different thick film sensors doped with Pd and Pt were fabricated and the comparative study of their response towards the reducing gases-LPG and methane was carried out. An effort is made to study the effect of nature of dopants on the sensitivity of SnO 2 based gas sensor.
| 2.Experimental|| |
2.1 Screen Preparation
This involves the following steps:
(1) Master artwork preparation (2) Camera reduction (3) Image transfer.
The master artwork (which is 10 times of the actual pattern layout) pattern for gas sensor electrode, heating element, gas sensing layer, thermistor and thermistor contact pad has been prepared on a strippable Rubylith film using a manual coordinatograph. The master artwork for these patterns is reduced photographically to the actual pattern size. The reduced positive of the artwork has been used to prepare the screen using photolithography technique  .
2.2 Fabrication of Sensor Device
The schematic presentation of fabricated gas sensor is shown in [Figure 1] and cross sectional view is shown in [Figure 2]. Tin oxide was available in the form of indium doped tin oxide paste, supplied by Electro Science Laboratories (ESL3050, USA). This indium doped tin oxide paste (SnO 2 ) has been taken as the base sensing material. The doped pastes were prepared by adding 1% Pd and Pt (by weight) in base SnO 2 paste with cellulose based thinner. The thermistor pattern is screen printed first (paste NTC 2413 ESL), dried at a temperature of 100° C and fired at 9508C. In the second step, finger electrode pattern is screen printed using silver conductor paste (No. PD 6176, DuPont) and dried at a temperature of 100° C. Subsequently, a heater element is screen printed on the back side of the substrate using silver palladium conductor paste (No. C1214, Heraeus,Gmbh) which is dried at the same temperature. Now the dried screen printed films are fired at 850° C. In the third step, Pd-doped and Pt doped tin oxide pastes are screen printed over the electrode pattern and the print is allowed to dry at a temperature 100°C for 20 min. The dried film is then fired in a thick film furnace (DEK model 840) in a set temperature profile with peak temperature zone of 550°C.
The fabricated sensors are then exposed to varying concentration of LPG and methane in a locally developed test chamber of volume 2047cm 3 having placed at the metal base. The change in resistance of sensor is measured using KEITHELY 195A multimeter.
| 3.Sensor Response|| |
The ratio (R a - R g )/R a has been defined as the sensitivity 'S' of the sensor, , where Ra is the resistance of the sensor in clean air and Rg the resistance of the sensor after the gas exposure  .
| 4.Results and Discussion|| |
4.1 Gas Sensing Characteristics
The sensitivity of fabricated sensors was studied at different fixed temperatures (150° C-350° C) with varying concentration of LPG and methane. It was found that both the sensors possess peak sensitivity at an operating temperature of 350° C. The operating temperature is thus fixed at 350° C. The response of Pd-doped sensor at various concentrations (250 ppm- 2000 ppm) of LPG and methane at an operating temperature of 350° C is shown in [Figure 3] while response of Pt doped sensor for LPG and methane is shown in [Figure 4]. It is evident from the Figures that sensitivity of sensors increases initially with increase in concentration of LPG and methane and then attains the saturation value after some time. The Figures also indicate that both the sensors are more sensitive towards LPG than methane. [Figure 5] and [Figure 6] presents the comparative responses of Pd and Pt doped sensors for LPG and methane respectively at an operating temperature 3508C. It is evident from the Figures that the Pd-doped sensor is more sensitive towards chosen gases (LPG, methane) than Pt-doped sensor at 350°C.
LPG is a complex gas, and the main constituents of LPG are propane, propylene-butane, i-butane etc. Though, hardly any literature  presents mechanism of such gas, however, considering the potential constituents of LPG, it appears that the exchange reactions play the dominant role. However, it is likely that at higher operating temperature the hydrocarbon may dissociate resulting in hydrogen related reaction. Thus, sensitivity of Pd-doped tin oxide sensor for LPG is better. Due to n-type behavior of tin oxide the electrical conductivity increases with reducing gas LPG. LPG oxidation reaction over SnO 2 occurs at a lower temperature (350°C). In general, the reactions on the surface of SnO 2 gas can be summarized as 
R * + O2- __________ R * O 2 + e-
R * + O- --------- R * O + e-
Where R * represent the reducing gas. These two reactions correspond to oxidation of the reducing carriers. This process increases the carrier concentration and hence decreases the resistance on exposure to reducing gases.
For saturated hydrocarbons such as methane the activation of the C-H bond is the first crucial step in all oxidation reaction  . Once the first bond is broken, sequential reactions to carbon dioxide and water are relatively facile. Methane is more difficult to activate than higher hydrocarbons on oxide surfaces  which may be the cause of the lower response of the fabricated sensors towards methane.
4.2 Response Time of Sensors
Usually the response time is defined as t90 , i.e. the time it takes for 90% of the sensor response change, after an increase in the concentration is accomplished, in the test chamber. The response time of Pd doped sensor at 500 ppm for LPG and methane is one second and three seconds respectively whereas for Pt doped sensor it was found to be two and five seconds for LPG and methane respectively.
| 5.Conclusion|| |
It is concluded that fabricated sensors (Pd doped and Pt doped SnO 2 ) have different sensitivities to LPG and methane. Both the sensors are more sensitive towards LPG and show poor sensitivity towards methane i e. they have high selectivity for LPG and can be promising sensors for detection of LPG. It is also observed that Pd doped sensor is more sensitive than Pt doped sensor to the test gases (LPG and methane) at 350 o C.
| Author|| |
J. K. Srivastava received postgraduate degree from Dr. R.M.L.(Avadh) Univesity, Faizabad, India. At Present, he is working as a senior research fellow and pursuing his doctoral work at the Centre for Research in Microelectronics, Department of Electronics Engineering Institute of Technology , Banaras Hindu University,Varanasi, India. His field of interest is thick film gas sensors/sensor array.
Preeti Pandey has received her M.Sc. degree from Lucknow University. At Present, She is working as a senior research fellow and pursuing her doctoral work at the Centre for Research in Microelectronics, Department of Electronics Engineering ,Institute of Technology, Banaras Hindu University, Varanasi, India. His field of interest is MOS gas sensors.
V. N. Mishra received the B.E. degree from the University of Mysore in 1987, and the M.E. degree from the University of Roorkee in 1991. He joined the Department of Electronics and Computer Engineering, University of Roorkee, Roorkee as a Ph.D. student and worked there from May 1991 to July 1993. He joined the Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, Varanasi as a lecturer in 1993. Presently he is working as Reader in the same department in Microelectronics group. His research interest includes IC-compatible sensors, microelectronic devices and their applications.
R. Dwivedi received his Ph.D. degree in Electronics Engineering in 1978 from Banaras Hindu University, India. He worked as a lecturer in the Department of Electronics Engineering for about 6 years, and has been a reader in the same department since April 1986. He has more than 34 years of research experience in the area of microelectronics and solid-state device technology. He has over 130 research publications in the proceedings of symposia. He is currently working in the areas of sensors based on silicon and thick film technologies, MOS devices, and LSI/VLSI.
| References|| |
|1.||M.S. Wagh, G.H. Jain, R. Patil, S.A. Patil, and L.A. Patil. "Surface customization of SnO 2 thick films using RuO 2 as a surfactant for the LPG response", Sens. Actuators B 122, pp. 357-64, 2007. |
|2.||Ihokura K. "Tin oxide gas sensor for deoxydising gases", New mater, New Sci, Electrochem, Technol,143, 1981. |
|3.||Watson J. "The tin oxide gas sensor and its applications", Sensors Actuators 5, pp. 29-42, 1984. |
|4.||J. Robertson. "Electronic structure of SnO 2 , GeO 2 , PdO 2 , TeO 2 and MgF 2", J. Phys., C12 4767, 1979. |
|5.||C.G. Fonstad, and R.H. Rediker. "Electrical properties of high-quality stannicoxide crystal", J. Appl. Phys., 42, pp. 2911, 1971. |
|6.||S. Samson, and C.G. Fonsted. "Defect structures and electronic donor levels in stannic oxide crystal", J. Appl. Phys., Vol. 14, pp. 4618, 1973. |
|7.||Z.M. Jarzebski. "Physical properties of SnO 2 Materials II", J. Electrochem. Soc., 123N. 9 300C, 1976. |
|8.||J.S. Blackmose. "Semiconductor statistics", UK: Pergammon Press; 1962. |
|9.||P.H. Barbarat, S.F. Matar, and G. Le Blevennec. "First principles investigations of the electronic, optical and chemical bonding properties of SnO 2", J. Mater. Chem., Vol. 7, pp. 2547, 1997. |
|10.||A. Srivastava, K. Jain, Rashmi, A.K. Srivastava, and S.T. Lakshmikumar. "Study of structural and microstructural properties of SnO 2 powder for LPG and CNG gas sensore", Materials Chem. And Phy. 97, pp. 85-90, 2006. |
|11.||V.N.Mishra. "Development and characterization of SnO 2 -based gas sensors", Ph.D. Thesis. |
|12.||V.N. Mishra, and R.P. Agarwal. "Sensitivity, response and recovery time of SnO 2 based thick film sensor array for H 2 , CO, CH 4 and LPG", Microelectronics Journal 29, pp. 861-74, 1998. |
|13.||R. Burch, D.J. Crittle, and M.J. Hayes. "C-H bond activation in hydrocarbon oxidation on heterogeneous catalysts", Catal. Today, Vol. 47, pp. 229, 1999. |
|14.||R. Burcgh, M.J. Hayes, and J. Mol. Catal., Vol. 100, 13, 1995. |
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]