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Sensing mechanism of Pd-doped SnO2 sensor for LPG detection
Solid State Sciences 11 (2009) 1602–1605
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Sensing mechanism of Pd-doped SnO2 sensor for LPG detection J.K. Srivastava*, Preeti Pandey, V.N. Mishra, R. Dwivedi Center for Research in Microelectronics, Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 February 2009 Received in revised form 8 June 2009 Accepted 12 June 2009 Available online 21 June 2009
In the present work, studies have been made to analyze the sensitivity, response, recovery time and sensing mechanism of Pd-doped thick film SnO2 sensor for detection of LPG. To achieve this, thick film Pd-doped (0.25 and 1% by weight in available Indium doped SnO2 thick film paste supplied by ESL, USA) along with an undoped (Indium doped) SnO2 sensors were fabricated on a 100 100 alumina substrate. It consists of a gas sensitive layer (doped SnO2), a pair of electrodes underneath the gas sensing layer serving as a contact pad for sensor. Also, a heater element on the backside of the substrate was printed for generating appropriate operating temperature at the substrate necessary for acquiring gas sensing properties. The sensor doped with 1% palladium showed the maximum sensitivity of 72% at 350 C for 0.5% concentration of LPG. Possible detailed sensing mechanism of Pd-doped SnO2 sensor for LPG detection has been proposed. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Thick film sensor Tin oxide Gas sensing mechanism Response and recovery time
1. Introduction In the recent years, atmospheric pollution has become a major global concern. Gases released from automobile exhausts and industries are severely polluting the environment. Thus, it has become imperative to develop sensors for the detection and also for the quantitative measurement of the toxic as well as explosive gases so that the exact concentration could be ascertained before some natural disaster may occur. This would also facilitate in on line monitoring of the hazardous gases [1] and providing a safe level of working environment. Tin oxide is known to be a potential material for gas sensor application as its conductivity/receptivity is modified in the presence of different oxidizing as well as reducing gases. Near room temperature the oxygen vacancies are frozen and isothermal conductance changes of a SnO2 device are due to chemisorptions [2]. The band gap of a clean SnO2 (110) face is free of intrinsic surface states. The extraction and injection of electrons by extrinsic surface acceptor or surface donors, respectively, are connected with the variation of a space charge layer. The electron concentration near the semiconductor surface varies with the density and occupancy of surface acceptors and donors. In a gas sensor, this density of surface states depends on surface reaction with ambient gases [3]. LPG is widely used in industry and domestic appliances. Metal oxide semiconductor gas sensors have been used for domestic gas leak detectors in house to produce an alarm at a given gas concentration [4,5].
* Corresponding author. Fax: þ91 0542 2366758. E-mail address: [email protected] (J.K. Srivastava). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.06.014
Recently, Abhilasha et al. [6] have reported the sensing mechanism of SnO2 sensor for reducing gases, but it doesn’t seem to be very supportive for the complete description of LPG. M.S. Wagh et al. [1] have reported sensing mechanism for LPG detection, using surface ruthenated SnO2 in which a few of the Sn–O linkages are believed to be replaced by grafting Ru–O linkages resulting in the change in adsorption–desorption kinetics. It is known that the operating principle of thick film tin oxide sensor is based on reactions of gas molecules with O 2 and O ions previously adsorbed on the sensor surface at an appropriate elevated temperature. The responses of these sensors are based on oxidation–reduction of gas species on the sensor surface, which modulates the conductivity of the sensor [7]. As tin oxide SnO2x, being a non-stoichiometric material, it is necessary to control its composition since the gas sensitivity of SnO2x changes with its composition and also the morphology [8].
2. Experimental Tin oxide sensors were fabricated by thick film screen printing technique as shown in Fig. 1. It consists of gas sensing layer (doped SnO2), a pair of electrodes underneath the gas sensing layer serving as a contact pad for sensors, and a heater element on backside of the substrate. A temperature sensor adjacent to the gas sensor is also incorporated to measure the sensor temperature. Alumina (96%) has been used as a substrate for sensor fabrication [9]. 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 (SnO2) has been taken as the base sensing material and further doped with Palladium (Pd) to modify
J.K. Srivastava et al. / Solid State Sciences 11 (2009) 1602–1605
Heater
SnO2 Sensing Layer
1603
70
Alumina Substrate
1% Pd- doped 0.25% Pd- doped
Silver Electrode
Fig. 1. Schematic of fabricated sensors.
its sensitivity for different gases. For the preparation of Pd-doped paste, dopants were added to base SnO2 paste in calculated amount (0.25% and 1% by weight) with cellulose based thinner. Palladium chloride (PdCl2) has been taken as starting material for palladium doping in SnO2 paste. Firstly, palladium chloride is weighed and mixed with tin oxide paste in a ball mill. The composition is thoroughly mixed by ball milling process for 8–10 h. The thermistor pattern is screen printed first (paste NTC 2413 ESL), dried at a temperature of 100 C and fired at 950 C. 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 backside of the substrate using silver palladium conductor paste (No. C1214, Heraeus, Gmbh) and is dried at the same temperature. The dried films are fired at 850 C. In the third step, Pd-doped and undoped tin oxide pastes were screen printed over the electrode pattern and the print was allowed to dry at a temperature 100 C for 20 min. The dried film was then fired in a thick film furnace (DEK model 840) in a set temperature profile with peak temperature zone of 550 C. The sensor was allowed to remain in this peak temperature zone for at least 20 min. Three sets of sensors were prepared i.e. undoped, 0.25% and 1% (by weight) Pd-doped and they were exposed to varying concentration of LPG in a locally developed test chamber having volume 2047 cm3 kept at metal base. The change in resistance of sensors is measured using KEITHELY 195A multimeter. The complete description about the used measurement setup has been given by V.N. Mishra and Agarwal [9].
Sensor Resistance (K ohm)
60
Undoped
50 40 30 20 10 0 0
50
100
150
200
250
Temperature (°C)
300
350
400
Fig. 2. Resistance variation of the sensors with temperature.
reading the chamber was flushed with clean air so that the sensors were allowed to stabilize towards its original resistance value. Fig. 3 shows the experimental plot of sensitivity versus the concentration of LPG gas for all the sensors. It is clear from Fig. 3, that 1% Pd-doped sensor shows better sensitivity towards LPG than undoped and 0.25% Pd-doped sensors at 350 C. Fig. 4 shows the variation of sensitivity with operating temperature for constant concentration (0.5%) of LPG, for each sensor. An attempt was also made to study the response and recovery time of the sensor for different concentrations of LPG and results are shown in Figs. 5 and 6. From these figures the response and recovery time were found to be least for 1% Pd-doped and maximum for undoped SnO2. 3.2. Response and recovery of the sensors Two measurements are usually required to measure the speed of response of a sensor. The first is called the response time tres which refers to the time needed to reach a stable sensor response after stepwise increase in the concentration. Hence, it measures the minimum time needed for complete sensor response. Usually the response time is defined as t90, i.e. the time needed to reach 90% of the complete sensor response after an increase in the concentration
3. Results 80
3.1. Gas response characteristics
0.25% Pd- doped
70
1% Pd-doped
60
Sensitivity
Initially, the 1%, 0.25% Pd-doped and undoped sensors were appropriately mounted in the test chamber containing only clean air and heated in steps without exposure of any gas. The resistance of the sensors was studied by measuring the sensor resistance at different temperatures in clean air. It was observed, that under static ambient condition, the magnitude of the resistance initially decreases due to excitation of conduction band electrons in the lower temperature range and then increases with temperature due to the adsorbed oxygen coverage [10], for all fabricated samples as shown in Fig. 2. The individual sensor resistance (1%, 0.25% Pddoped and undoped) was allowed to get stabilized at 350 C and the stabilized resistance was measured in clean air. The ratio Ra Rg/Ra has been defined as the sensitivity ‘S’ of the sensor, i.e. S ¼ Ra Rg/Ra100, where Ra is the resistance of the sensor in clean air and Rg is the resistance of the sensor after the gas exposure [9]. An estimated amount of gas (LPG) was injected into the test chamber and at each step the resistance of the sensor was allowed to stabilize and the resistance was recorded. After each set of
Undoped
50 40 30 20 10 0 0
1000
2000
3000
4000
5000
Concentration (ppm) Fig. 3. Response of sensors on exposure to LPG at 350 C.
6000
1604
J.K. Srivastava et al. / Solid State Sciences 11 (2009) 1602–1605
80
250 1% Pd doped Undoped
70
0.25% Pd doped
0.25% Pd- doped
50
Time (sec)
Sensitivity
Undoped
200
1% Pd-doped
60
40 30
150
100
20
50 10 0 0
50
100
150
200
250
Temperature (°C)
300
350
0
400
Fig. 4. Response of sensors on exposure to fixed concentration (0.5%) of LPG.
is accomplished in the test chamber. The second is the recovery time trec. It refers to the time which sensor needs to recover from an exposed condition to the clean air condition. The recovery time is defined similar to the response time. It is the time needed for 90% of the sensor response change after the gas exposure is completely removed from the chamber [11]. Figs. 5 and 6 show the variation in response and recovery time of sensors (1% Pd-doped, 0.25% Pd-doped and undoped) with variation in concentration (100–500 ppm) of LPG with time. Response and recovery time were measured. The response and recovery time of 1% Pd-doped, 0.25% and undoped SnO2 sensors at 500 ppm were(1 s, 78 s),(6 s, 123 s) and (8 s, 195 s) respectively. 3.3. X-ray analysis The XRD pattern of the 1% Pd-doped SnO2 powder is shown in Fig. 7 (the XRD patterns of 0.25% Pd-doped and undoped SnO2 powders were also very much alike hence only one pattern has been given and 2q values of the peaks of the different powders are given in Table 1 for comparison). Structural analysis was carried out by using X-ray diffractometer (Seifert, Germany, model-ID-3000).
0
100
200
300
400
500
Concentration (ppm)
600
Fig. 6. Variation in recovery time of 1% Pd-doped, 0.25% Pd-doped and undoped sensors with variation in concentration of LPG.
The observed pattern in both the cases has several peaks for different angles. All the reflection peaks in the X-ray diffraction pattern were indexed. The maxima of 110 peak or 100% intensity peak were observed nearly at 2q ¼ 26.43 in each case. The X-ray diffraction patterns are almost similar and the peak positions in each sample could be indexed with SnO2 cassiterite structure (JCPDS standard card no. 41-1445). The nature of the XRD pattern does not change with variation in concentration of Pd (from 0.25% to 1%). The absence of Pd peaks in XRD may be either due to small concentration of Pd as a separate phase in SnO2 or due to its solubility in SnO2 lattice and in the present study latter seems to be true due to small shift in 2q values of peaks of SnO2 (Table 1). The crystallite size found to be 21.3 nm for 0.25% Pd while 26.83 nm for 1% Pd-doped powders, which indicates that with higher doping concentration of Pd the average crystallite size increases. The particle size was calculated from X-ray diffraction pattern using Debye–Scherrer formula [12].
D ¼ k
l b cos q
25 120
1% Pd -doped 0.25% Pd -doped
20
Undoped
110
100
211
80
15
Intensity
Time (sec)
101
10
60
40 5 20 0 0
100
200
300
400
500
Concentration (ppm) Fig. 5. Variation in response time of 1% Pd-doped, 0.25% Pd-doped and undoped sensors with variation in concentration of LPG.
0 0
10
20
30
40
50
60
70
2 Theta Fig. 7. XRD pattern of the SnO2 powder with 1% Pd doping.
80
90
J.K. Srivastava et al. / Solid State Sciences 11 (2009) 1602–1605
1605
where D is the mean crystallite diameter, l is the X-ray wavelength (1.54056 Å), k is the Scherrer constant (0.89) and b is the full width at half maximum (FWHM) of the diffraction lines.
It is evident from the proposed reaction schemes that a large number of electrons are released upon LPG exposure to SnO2 surface particularly in the presence of O 2 which is responsible for a large conductivity increase. On doping the SnO2 with Pd, the improvement in the performance of the sensor towards reducing gases has already been reported [14–16] and can be explained by ‘spillover’ mechanism of oxygen atoms from the SnO2 surface. Palladium atoms act as a catalyst for dissociation of oxygen over the SnO2 surface, and thus, enhance the spillover of oxygen species over the SnO2 surface so that greater number of electrons can be withdrawn from the SnO2 bulk [17] and hence large number of electrons can be released upon exposure to LPG which will enhance the sensitivity of the sensor.
4. Discussion
5. Conclusion
4.1. LPG sensing mechanism of Pd-doped SnO2
It is concluded that fabricated sensor doped with palladium shows better sensitivity towards LPG gas than undoped sensor and maximum sensitivity is observed for 1% Pd doping. It is also inferred from the present study that with 1% palladium doping, response and recovery time of the sensor can be drastically reduced. Palladium doping (1%) shows a drastic reduction in the response time from 8 s to 1 s and reduction in recovery time from 195 s to 78 s as compared to undoped SnO2 sensor. We propose possible reaction scheme for lower and higher concentrations of LPG on Pd-doped SnO2 surface. The proposed reaction mechanism for LPG sensing clearly shows a release of large number of conduction electrons and thus these are responsible for higher sensitivity towards LPG.
Table 1 2q values of different peaks found in XRD pattern of 1% Pd-doped, 0.25% Pd-doped and undoped SnO2 powders. Peaks
110 101 211
2q values 1% Pd-doped
0.25% Pd-doped
Undoped
26.691 33.843 51.703
26.561 33.920 51.719
26.745 33.985 51.751
The LPG is a complex gas and the main constituents of LPG are methane, propane, butane, etc [9] where propane and butane dominate. Sensing mechanism of semiconducting gas sensors is based on the surface reaction of semiconducting oxide. In air, 2 molecular oxygen is chemisorbed in the form of O 2 , O or O2 depending on the operating temperature and depletes electron from the surface of these materials leading to reduction of conductivity [6]. Hydrocarbon is known to get dissociated on metal oxide surface before reacting with ionosorbed oxygen species. The response to LPG can be ascribed to the fact that hydrocarbon molecules with more carbon atoms dissociate and these dissociated species of a single molecule are expected to react with more electrons to the conduction band. This explains large change in electrical resistance observed for LPG. Hydrocarbon can dissociate either on the SnO2 surface or on the surface of deposited noble metal clusters. Subsequently, the hydrogen atom migrates to adsorption or reaction sites on the SnO2 surface. Thus, for the increase of conductivity two processes should be involved, first dissociation of hydrocarbons on the SnO2 surface and then dissociation of hydrogen molecule as adsorbed hydrogen molecules do not change the conductance. At moderate temperatures adsorbed hydrogen atoms act as donors. These donors provide additional electron and induce an accumulation layer [3] H D ðOlat ÞL L 4 ðOlat HÞL D eL
(1)
Since dominant constituents of LPG are butane and propane thereby reactions pertaining to the components must be considered. For butane and propane the activation of the C–H bond is the first crucial step in all oxidation reactions [13]. Once the first bond is broken, sequential reactions to carbon dioxide and water are relatively facile. The proposed reactions in the present study are given as: C4 H10 D 13OL / 4CO2 D 5H2 O D 13eL
(2)
L C4 H10 D 13=2OL 2 / 4CO2 D 5H2 O D 13=2e
(3)
C4 H10 D 13O2L / 4CO2 D 5H2 O D 26eL
(4)
C3 H8 D 10OL / 3CO2 D 4H2 O D 10eL
(5)
L C3 H8 D 5OL 2 / 3CO2 D 4H2 O D 10e
(6)
C3 H8 D 10O2L / 3CO2 D 4H2 O D 20eL
(7)
References [1] M.S. Wagh, G.H. Jain, D.R. Patil, S.A. Patil, L.A. Patil, Surface customization of SnO2 thick film using RuO2 as a surfactant for the LPG response, Sensors and Actuators B 122 (2007) 357–364. [2] S. Munnix, M. Schmeits, Electronic structure of tin dioxide surfaces, Physical Review B 27 (1983) 7624–7635. [3] D. Kohl, Surface processes in the detection of reducing gases with SnO2-based devices, Sensors and Actuators B 18 (1989) 71–113. [4] R. Lalauze, Z. Pijiolat, A new approach to selective detection of gas by a SnO2 solid state sensor, Sensors and Actuators B 5 (1984) 55–63. [5] T. Oyabu, T. Osawa, T. Kurbe, Sensing characteristics of tin oxide thick film gas sensor, Journal of Applied Physics 53 (11) (1982) 7125–7130. [6] Abhilasha Srivastava, Kiran Jain Rashmi, A.K. Srivastava, S.T. Lakshmikumar, Study of structural and microstructural properties of SnO2 powder for LPG and CNG gas sensors, Material Chemistry and Physics 97 (2006) 85–90. [7] I. Sayago, J. Gutierrez, L. Ares, J.I. Robla, M.C. Harillo, J. Getino, J.A. Agapito, The interaction of different oxidizing agents on doped tin oxide, Sensors and Actuators B 24/25 (1995) 512–515. [8] K.S. Yoo, N.W. Cho, H.S. Song, H.J. Jung, Surface morphology and gas sensing characteristics of SnO2x thin films oxidized from Sn film, Sensors and Actuators B 24/25 (1995) 474–477. [9] V.N. Mishra, R.P. Agarwal, Sensitivity, response and recovery time of SnO2 based thick film sensor array for H2, CO, CH4, and LPG, Microelectronics Journal 29 (1998) 861–874. [10] K. Ihokura, Tin oxide gas sensor for deoxidising gas, New materials and New Processes in Electrochem. Tech. 1 (1981) 43–50. [11] Patric Reichel, Dissertation on Development of a Chemical Gas Sensor system, 2005. [12] A. Taylor, X-ray Metallography, John Wiley, New York, 1961, pp. 678–686. [13] R. Burch, D.J. Crittle, M.J. Hayes, C–H bond activation in hydrocarbon oxidation on heterogeneous catalysts, Catalysis Today 47 (1999) 229. [14] Mukta V. Vaishampayan, Rupali G. Deshmukh, I.S. Mulla, Influence of Pd doping on morphology and LPG response of SnO2, Sensors and Actuators B 131 (2008) 665–672. [15] P. Manjula, L. Satyanarayan, Y. Swarnalatha, Sunkara V. Manorama, Raman and MASNMR studies to support the mechanism of low temperature hydrogen sensing by Pd doped mesoporous SnO2, Sensors and Actuators B 138 (2009) 28–34. [16] P. Menini, F. Parret, M. Guerrero, K. Soulantica, L. Erades, A. Maisonnat, B. Chaudret, CO response of a nanostructured SnO2 gas sensor doped with palladium and platinum, Sensors and Actuators B 103 (2004) 111–114. [17] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particle, Nano Letters 5 (2005) 667–673.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Sensing mechanism of Pd-doped SnO2 sensor for LPG detection J.K. Srivastava*, Preeti Pandey, V.N. Mishra, R. Dwivedi Center for Research in Microelectronics, Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 February 2009 Received in revised form 8 June 2009 Accepted 12 June 2009 Available online 21 June 2009
In the present work, studies have been made to analyze the sensitivity, response, recovery time and sensing mechanism of Pd-doped thick film SnO2 sensor for detection of LPG. To achieve this, thick film Pd-doped (0.25 and 1% by weight in available Indium doped SnO2 thick film paste supplied by ESL, USA) along with an undoped (Indium doped) SnO2 sensors were fabricated on a 100 100 alumina substrate. It consists of a gas sensitive layer (doped SnO2), a pair of electrodes underneath the gas sensing layer serving as a contact pad for sensor. Also, a heater element on the backside of the substrate was printed for generating appropriate operating temperature at the substrate necessary for acquiring gas sensing properties. The sensor doped with 1% palladium showed the maximum sensitivity of 72% at 350 C for 0.5% concentration of LPG. Possible detailed sensing mechanism of Pd-doped SnO2 sensor for LPG detection has been proposed. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Thick film sensor Tin oxide Gas sensing mechanism Response and recovery time
1. Introduction In the recent years, atmospheric pollution has become a major global concern. Gases released from automobile exhausts and industries are severely polluting the environment. Thus, it has become imperative to develop sensors for the detection and also for the quantitative measurement of the toxic as well as explosive gases so that the exact concentration could be ascertained before some natural disaster may occur. This would also facilitate in on line monitoring of the hazardous gases [1] and providing a safe level of working environment. Tin oxide is known to be a potential material for gas sensor application as its conductivity/receptivity is modified in the presence of different oxidizing as well as reducing gases. Near room temperature the oxygen vacancies are frozen and isothermal conductance changes of a SnO2 device are due to chemisorptions [2]. The band gap of a clean SnO2 (110) face is free of intrinsic surface states. The extraction and injection of electrons by extrinsic surface acceptor or surface donors, respectively, are connected with the variation of a space charge layer. The electron concentration near the semiconductor surface varies with the density and occupancy of surface acceptors and donors. In a gas sensor, this density of surface states depends on surface reaction with ambient gases [3]. LPG is widely used in industry and domestic appliances. Metal oxide semiconductor gas sensors have been used for domestic gas leak detectors in house to produce an alarm at a given gas concentration [4,5].
* Corresponding author. Fax: þ91 0542 2366758. E-mail address: [email protected] (J.K. Srivastava). 1293-2558/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2009.06.014
Recently, Abhilasha et al. [6] have reported the sensing mechanism of SnO2 sensor for reducing gases, but it doesn’t seem to be very supportive for the complete description of LPG. M.S. Wagh et al. [1] have reported sensing mechanism for LPG detection, using surface ruthenated SnO2 in which a few of the Sn–O linkages are believed to be replaced by grafting Ru–O linkages resulting in the change in adsorption–desorption kinetics. It is known that the operating principle of thick film tin oxide sensor is based on reactions of gas molecules with O 2 and O ions previously adsorbed on the sensor surface at an appropriate elevated temperature. The responses of these sensors are based on oxidation–reduction of gas species on the sensor surface, which modulates the conductivity of the sensor [7]. As tin oxide SnO2x, being a non-stoichiometric material, it is necessary to control its composition since the gas sensitivity of SnO2x changes with its composition and also the morphology [8].
2. Experimental Tin oxide sensors were fabricated by thick film screen printing technique as shown in Fig. 1. It consists of gas sensing layer (doped SnO2), a pair of electrodes underneath the gas sensing layer serving as a contact pad for sensors, and a heater element on backside of the substrate. A temperature sensor adjacent to the gas sensor is also incorporated to measure the sensor temperature. Alumina (96%) has been used as a substrate for sensor fabrication [9]. 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 (SnO2) has been taken as the base sensing material and further doped with Palladium (Pd) to modify
J.K. Srivastava et al. / Solid State Sciences 11 (2009) 1602–1605
Heater
SnO2 Sensing Layer
1603
70
Alumina Substrate
1% Pd- doped 0.25% Pd- doped
Silver Electrode
Fig. 1. Schematic of fabricated sensors.
its sensitivity for different gases. For the preparation of Pd-doped paste, dopants were added to base SnO2 paste in calculated amount (0.25% and 1% by weight) with cellulose based thinner. Palladium chloride (PdCl2) has been taken as starting material for palladium doping in SnO2 paste. Firstly, palladium chloride is weighed and mixed with tin oxide paste in a ball mill. The composition is thoroughly mixed by ball milling process for 8–10 h. The thermistor pattern is screen printed first (paste NTC 2413 ESL), dried at a temperature of 100 C and fired at 950 C. 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 backside of the substrate using silver palladium conductor paste (No. C1214, Heraeus, Gmbh) and is dried at the same temperature. The dried films are fired at 850 C. In the third step, Pd-doped and undoped tin oxide pastes were screen printed over the electrode pattern and the print was allowed to dry at a temperature 100 C for 20 min. The dried film was then fired in a thick film furnace (DEK model 840) in a set temperature profile with peak temperature zone of 550 C. The sensor was allowed to remain in this peak temperature zone for at least 20 min. Three sets of sensors were prepared i.e. undoped, 0.25% and 1% (by weight) Pd-doped and they were exposed to varying concentration of LPG in a locally developed test chamber having volume 2047 cm3 kept at metal base. The change in resistance of sensors is measured using KEITHELY 195A multimeter. The complete description about the used measurement setup has been given by V.N. Mishra and Agarwal [9].
Sensor Resistance (K ohm)
60
Undoped
50 40 30 20 10 0 0
50
100
150
200
250
Temperature (°C)
300
350
400
Fig. 2. Resistance variation of the sensors with temperature.
reading the chamber was flushed with clean air so that the sensors were allowed to stabilize towards its original resistance value. Fig. 3 shows the experimental plot of sensitivity versus the concentration of LPG gas for all the sensors. It is clear from Fig. 3, that 1% Pd-doped sensor shows better sensitivity towards LPG than undoped and 0.25% Pd-doped sensors at 350 C. Fig. 4 shows the variation of sensitivity with operating temperature for constant concentration (0.5%) of LPG, for each sensor. An attempt was also made to study the response and recovery time of the sensor for different concentrations of LPG and results are shown in Figs. 5 and 6. From these figures the response and recovery time were found to be least for 1% Pd-doped and maximum for undoped SnO2. 3.2. Response and recovery of the sensors Two measurements are usually required to measure the speed of response of a sensor. The first is called the response time tres which refers to the time needed to reach a stable sensor response after stepwise increase in the concentration. Hence, it measures the minimum time needed for complete sensor response. Usually the response time is defined as t90, i.e. the time needed to reach 90% of the complete sensor response after an increase in the concentration
3. Results 80
3.1. Gas response characteristics
0.25% Pd- doped
70
1% Pd-doped
60
Sensitivity
Initially, the 1%, 0.25% Pd-doped and undoped sensors were appropriately mounted in the test chamber containing only clean air and heated in steps without exposure of any gas. The resistance of the sensors was studied by measuring the sensor resistance at different temperatures in clean air. It was observed, that under static ambient condition, the magnitude of the resistance initially decreases due to excitation of conduction band electrons in the lower temperature range and then increases with temperature due to the adsorbed oxygen coverage [10], for all fabricated samples as shown in Fig. 2. The individual sensor resistance (1%, 0.25% Pddoped and undoped) was allowed to get stabilized at 350 C and the stabilized resistance was measured in clean air. The ratio Ra Rg/Ra has been defined as the sensitivity ‘S’ of the sensor, i.e. S ¼ Ra Rg/Ra100, where Ra is the resistance of the sensor in clean air and Rg is the resistance of the sensor after the gas exposure [9]. An estimated amount of gas (LPG) was injected into the test chamber and at each step the resistance of the sensor was allowed to stabilize and the resistance was recorded. After each set of
Undoped
50 40 30 20 10 0 0
1000
2000
3000
4000
5000
Concentration (ppm) Fig. 3. Response of sensors on exposure to LPG at 350 C.
6000
1604
J.K. Srivastava et al. / Solid State Sciences 11 (2009) 1602–1605
80
250 1% Pd doped Undoped
70
0.25% Pd doped
0.25% Pd- doped
50
Time (sec)
Sensitivity
Undoped
200
1% Pd-doped
60
40 30
150
100
20
50 10 0 0
50
100
150
200
250
Temperature (°C)
300
350
0
400
Fig. 4. Response of sensors on exposure to fixed concentration (0.5%) of LPG.
is accomplished in the test chamber. The second is the recovery time trec. It refers to the time which sensor needs to recover from an exposed condition to the clean air condition. The recovery time is defined similar to the response time. It is the time needed for 90% of the sensor response change after the gas exposure is completely removed from the chamber [11]. Figs. 5 and 6 show the variation in response and recovery time of sensors (1% Pd-doped, 0.25% Pd-doped and undoped) with variation in concentration (100–500 ppm) of LPG with time. Response and recovery time were measured. The response and recovery time of 1% Pd-doped, 0.25% and undoped SnO2 sensors at 500 ppm were(1 s, 78 s),(6 s, 123 s) and (8 s, 195 s) respectively. 3.3. X-ray analysis The XRD pattern of the 1% Pd-doped SnO2 powder is shown in Fig. 7 (the XRD patterns of 0.25% Pd-doped and undoped SnO2 powders were also very much alike hence only one pattern has been given and 2q values of the peaks of the different powders are given in Table 1 for comparison). Structural analysis was carried out by using X-ray diffractometer (Seifert, Germany, model-ID-3000).
0
100
200
300
400
500
Concentration (ppm)
600
Fig. 6. Variation in recovery time of 1% Pd-doped, 0.25% Pd-doped and undoped sensors with variation in concentration of LPG.
The observed pattern in both the cases has several peaks for different angles. All the reflection peaks in the X-ray diffraction pattern were indexed. The maxima of 110 peak or 100% intensity peak were observed nearly at 2q ¼ 26.43 in each case. The X-ray diffraction patterns are almost similar and the peak positions in each sample could be indexed with SnO2 cassiterite structure (JCPDS standard card no. 41-1445). The nature of the XRD pattern does not change with variation in concentration of Pd (from 0.25% to 1%). The absence of Pd peaks in XRD may be either due to small concentration of Pd as a separate phase in SnO2 or due to its solubility in SnO2 lattice and in the present study latter seems to be true due to small shift in 2q values of peaks of SnO2 (Table 1). The crystallite size found to be 21.3 nm for 0.25% Pd while 26.83 nm for 1% Pd-doped powders, which indicates that with higher doping concentration of Pd the average crystallite size increases. The particle size was calculated from X-ray diffraction pattern using Debye–Scherrer formula [12].
D ¼ k
l b cos q
25 120
1% Pd -doped 0.25% Pd -doped
20
Undoped
110
100
211
80
15
Intensity
Time (sec)
101
10
60
40 5 20 0 0
100
200
300
400
500
Concentration (ppm) Fig. 5. Variation in response time of 1% Pd-doped, 0.25% Pd-doped and undoped sensors with variation in concentration of LPG.
0 0
10
20
30
40
50
60
70
2 Theta Fig. 7. XRD pattern of the SnO2 powder with 1% Pd doping.
80
90
J.K. Srivastava et al. / Solid State Sciences 11 (2009) 1602–1605
1605
where D is the mean crystallite diameter, l is the X-ray wavelength (1.54056 Å), k is the Scherrer constant (0.89) and b is the full width at half maximum (FWHM) of the diffraction lines.
It is evident from the proposed reaction schemes that a large number of electrons are released upon LPG exposure to SnO2 surface particularly in the presence of O 2 which is responsible for a large conductivity increase. On doping the SnO2 with Pd, the improvement in the performance of the sensor towards reducing gases has already been reported [14–16] and can be explained by ‘spillover’ mechanism of oxygen atoms from the SnO2 surface. Palladium atoms act as a catalyst for dissociation of oxygen over the SnO2 surface, and thus, enhance the spillover of oxygen species over the SnO2 surface so that greater number of electrons can be withdrawn from the SnO2 bulk [17] and hence large number of electrons can be released upon exposure to LPG which will enhance the sensitivity of the sensor.
4. Discussion
5. Conclusion
4.1. LPG sensing mechanism of Pd-doped SnO2
It is concluded that fabricated sensor doped with palladium shows better sensitivity towards LPG gas than undoped sensor and maximum sensitivity is observed for 1% Pd doping. It is also inferred from the present study that with 1% palladium doping, response and recovery time of the sensor can be drastically reduced. Palladium doping (1%) shows a drastic reduction in the response time from 8 s to 1 s and reduction in recovery time from 195 s to 78 s as compared to undoped SnO2 sensor. We propose possible reaction scheme for lower and higher concentrations of LPG on Pd-doped SnO2 surface. The proposed reaction mechanism for LPG sensing clearly shows a release of large number of conduction electrons and thus these are responsible for higher sensitivity towards LPG.
Table 1 2q values of different peaks found in XRD pattern of 1% Pd-doped, 0.25% Pd-doped and undoped SnO2 powders. Peaks
110 101 211
2q values 1% Pd-doped
0.25% Pd-doped
Undoped
26.691 33.843 51.703
26.561 33.920 51.719
26.745 33.985 51.751
The LPG is a complex gas and the main constituents of LPG are methane, propane, butane, etc [9] where propane and butane dominate. Sensing mechanism of semiconducting gas sensors is based on the surface reaction of semiconducting oxide. In air, 2 molecular oxygen is chemisorbed in the form of O 2 , O or O2 depending on the operating temperature and depletes electron from the surface of these materials leading to reduction of conductivity [6]. Hydrocarbon is known to get dissociated on metal oxide surface before reacting with ionosorbed oxygen species. The response to LPG can be ascribed to the fact that hydrocarbon molecules with more carbon atoms dissociate and these dissociated species of a single molecule are expected to react with more electrons to the conduction band. This explains large change in electrical resistance observed for LPG. Hydrocarbon can dissociate either on the SnO2 surface or on the surface of deposited noble metal clusters. Subsequently, the hydrogen atom migrates to adsorption or reaction sites on the SnO2 surface. Thus, for the increase of conductivity two processes should be involved, first dissociation of hydrocarbons on the SnO2 surface and then dissociation of hydrogen molecule as adsorbed hydrogen molecules do not change the conductance. At moderate temperatures adsorbed hydrogen atoms act as donors. These donors provide additional electron and induce an accumulation layer [3] H D ðOlat ÞL L 4 ðOlat HÞL D eL
(1)
Since dominant constituents of LPG are butane and propane thereby reactions pertaining to the components must be considered. For butane and propane the activation of the C–H bond is the first crucial step in all oxidation reactions [13]. Once the first bond is broken, sequential reactions to carbon dioxide and water are relatively facile. The proposed reactions in the present study are given as: C4 H10 D 13OL / 4CO2 D 5H2 O D 13eL
(2)
L C4 H10 D 13=2OL 2 / 4CO2 D 5H2 O D 13=2e
(3)
C4 H10 D 13O2L / 4CO2 D 5H2 O D 26eL
(4)
C3 H8 D 10OL / 3CO2 D 4H2 O D 10eL
(5)
L C3 H8 D 5OL 2 / 3CO2 D 4H2 O D 10e
(6)
C3 H8 D 10O2L / 3CO2 D 4H2 O D 20eL
(7)
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