Complex plane impedance plot as a figure of merit for tin dioxide-based methane sensors

Sensors and Actuators B 119 (2006) 431–434

Complex plane impedance plot as a figure of merit for tin dioxide-based methane sensors S. Chakraborty, A. Sen ∗ , H.S. Maiti Central Glass and Ceramic Research Institute, Kolkata 700032, India Received 12 September 2005; received in revised form 21 December 2005; accepted 22 December 2005 Available online 25 January 2006

Abstract Thick film methane sensors have been fabricated from nanosized tin dioxide powder containing antimony oxide and palladium. The powder has been prepared by sonication-assisted simultaneous precipitation and the sensors made with this powder showed optimum resistance for device applications and good sensitivity towards methane. This contrasts with the thick film sensors prepared with the powder synthesized without sonication, which showed very high resistance at the operating temperature. The complex plane impedance spectroscopy of the sensors (both in air and in the presence of gas) can be a good indicator of the sensor quality. It has been observed that the nature of the complex plane impedance plot of the sensors fabricated by using powders synthesized through sonication-assisted simultaneous precipitation matches well with that of high-quality imported Figaro (Japan) sensors. © 2005 Elsevier B.V. All rights reserved. Keywords: Gas sensor; Impedance spectroscopy; Methane; Tin dioxide

1. Introduction The impedance spectroscopy [1] is a powerful tool for studying various materials like ionically conducting glasses, amorphous semiconductors, electronically conducting polymers, ionically conducting polymers and transition metal oxides. Impedance spectroscopy has also been employed to study gas adsorption behavior of semiconductor gas sensors like SnO2 , In2 O3 , Ga2 O3 and WO3 . Among the various materials, SnO2 has been widely studied for gas sensor [2] applications. SnO2 is an n-type semiconductor with a direct bandgap of 4 eV and an indirect band gap of 2.6 eV [3]. SnO2 is a nonstoichiometric oxide having oxygen vacancies and electron donor states. Normally, atmospheric oxygen becomes chemisorbed on the surface, consuming the free electrons as given below: O2 + 2e− → 2O− ads

(1)

O2 + e− → O2 − ads

(2)

Any reducing gas like methane, butane and hydrogen, if present in the ambient, produces a counter-reaction, where the reducing ∗

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gas reacts with the highly reactive chemisorbed oxygen, frees the bound electrons and increases the conductivity of the semiconducting oxide, thus generating a signal. Although semiconductor gas sensors based on SnO2 have already been in the market for a long time, the modifications [4–8] of the sensing properties, such as the sensitivity and selectivity, are still under way to meet their ever expanding demands in new applications. In the present study, nanosized SnO2 -based powder containing antimony and palladium has been prepared by a sonicationassisted simultaneous precipitation route. The impedance characteristics of the sensors (in air as well as in the presence of methane) in thick film form were compared with those made from powder prepared without sonication. The impedance spectra of Figaro (Japan) sensors were also studied for comparison. 2. Experimental A batch containing tin dioxide, 0.25% (by weight with respect to tin dioxide) antimony oxide and 10 wt% palladium was prepared by sonication-assisted simultaneous precipitation technique. The composition of the powder was selected [9] keeping in view the role of Pd as a catalyst to improve the sensitivity and the role of Sb as an n-type dopant to enhance the carrier

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rer formula [10] D=

Fig. 1. XRD patterns of powders prepared (a) by simultaneous precipitation and (b) by sonication-assisted simultaneous precipitation after calcination.

concentration. In sonication-assisted simultaneous precipitation technique, reagent grade SnCl2 ·2H2 O, Sb2 O3 and PdCl2 were used to prepare the composition. Initially, tin, antimony and palladium chloride solutions were prepared [9] and mixed in the calculated ratio and the mixture was taken in a beaker. The solution was sonicated (ultrasonic processor, Vibronics, 25 kHz, 250 W) for 30 min at a time and the warm solution was allowed to cool for 15 min. This sonication procedure was continued for 6 h. At the end of the reaction, a black precipitate was obtained. The precipitate was centrifuged, washed with distilled water and ethanol in a sequence. The precipitate was again sonicated in ethanol for 10 min and finally the ethanol was evaporated by slow heating to obtain a dry powder. The dried powder was calcined at 900 ◦ C for 2 h in air. Similar procedure, excluding the sonication step, was followed to make another batch (conventional simultaneous precipitation). The dried and calcined powders were examined by X-ray diffraction (XRD; Philips, PW, 1710; Cu K␣ radiation). The average grain size of the powders was calculated by using Scher-

0.9λ β cos θ

(3)

˚ (X-ray wave where D is the √ average grain size, λ = 1.541 A length), and β = (B2 − b2 ), B being the width of the diffraction peak at half maximum for the diffraction angle 2θ and b is the same for very large crystallites. The value of b was determined from the XRD of a large grained sample prepared by calcining the powder at a high temperature (1000 ◦ C). For gas sensing study, thick pastes of the powders were prepared in an aquous medium containing a small amount (1%) of PVA binder. The pastes were painted on the outer surface of thin alumina tubes (length 3 mm, outer diameter 2 mm and thickness 0.5 mm) with gold electrodes and platinum lead wires already attached to the ends by curing at a high temperature. Kanthal heating coils were placed inside the tubes. After applying the coatings, the assembly was dried in an oven followed by curing at 600 ◦ C. The electrical resistance of the coatings (thickness ∼ 100 ␮m) was measured at 350 ◦ C by using a digital multimeter (Solartron), a constant voltage/current source (Keithley 228 A) and an X–Y recorder. All the fired samples were initially preheated at 350 ◦ C for 72 h to achieve the desired stability before measurements. The impedance studies were made by using a Hioki 3532-50 LCR Hitester in the frequency range 42 Hz to 4 MHz. 3. Results and discussion The XRD patterns of the powders prepared by precipitation as well as sonication-assisted precipitation are given in Fig. 1(a) and (b). The XRD peaks of SnO2 were found to be shifted slightly due to solid solution of Sb2 O3 in SnO2 [9]. The peaks of PdO merged with those of SnO2 peaks [(1 0 1), (2 1 1)]. No peak of Pd was observed. From the XRD data, the lattice parameters of the sonochemically prepared powder were found to be ˚ and c = 3.1707 A, ˚ whereas those of conventiona = b = 4.7124 A ˚ and c = 3.171 A. ˚ ally precipitated powder were a = b = 4.7173 A Hence, as the lattice parameters are nearly the same for both powders, the broadening of the peaks for the sonochemically prepared powder (Fig. 1(b)) arises primarily due to the fine

Fig. 2. SEM images of the powders prepared (a) by sonication-assisted precipitation and (b) conventional precipitation.

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particle size of the powder. The average particle sizes of the powders determined by the Scherrer method were 72 nm and 144 nm for sonochemical and conventional routes, respectively. The SEM powder morphology indicates the presence of agglomerates in the powder prepared by conventional precipitation (Fig. 2(a) and (b)). The frequency dependent properties of an insulator are generally described by complex impedance plots, where the impedance Z* is given by: Z∗ = Z − iZ

(4)

Z and Z being the real and imaginary parts of the impedance, respectively. Roughly, it can be said [11] that a semi-circular arc in the complex impedance plot corresponds to a lumped R–C combination and a quarter circular arc corresponds to a combination of a lumped R and a distributed R–C element, such as the Warburg impedance. From the impedance spectra in Fig. 3(a) and (b), firstly, we observe that the resistance values of the sensors prepared from the sonication-assisted powder are much lower (∼50–100 k) than those prepared from the powder synthesized without sonication (∼50–100 M). Such behaviour can be understood by considering the role of agglomerates and its consequence in determining the grain boundary-controlled or neck-controlled morphology [12] in the sensor films. The percent response (S) of the films to 1000 ppm methane was calculated from the following relation: S=

R A − RG × 100 RA

(5)

where RA is the sensor resistance in air at 350 ◦ C (at this temperature optimum sensitivity is normally achieved) and RG is the sensor resistance in 1000 ppm methane at the same temperature. Table 1 shows the response values for the sensors made by using powders prepared through different routes. It is evident from Table 1 that the percent response value for the powder prepared by sonication-assisted simultaneous precipitation is higher than that for simultaneous precipitated powder. Secondly, we find a scatter in the low-frequency data (Fig. 3(a)) for sensors prepared from the powder synthesized through simultaneous precipitation. The low-frequency scatter reflects [13] weak grain-to-grain contact. Incidentally, the impedance plots of the sensors prepared from sonochemically precipitated powder (Fig. 3(b)) and those of Figaro sensors (Fig. 3(c)) are nearly identical. The high-frequency break in the impedance plots, which is more prominent in air, probably demarcates the grain and grain boundary/neck contributions. Table 1 Percent response values for sensors on exposure to 1000 ppm methane at 350 ◦ C Sensors

Percent response

Simultaneous precipitation Sonochemical Figaro (Japan)

67 88 90

Fig. 3. Two-probe ac impedance spectra for the gas sensors in air and in 1000 ppm CH4 gas. Insets enlarge the low-frequency ends of the impedance plots.

The low-frequency loop-like feature of the impedance plots in air (insets I2 of Fig. 3(b) and (c)) is probably due to the so-called negative capacitance effect. This may arise from a current rising slowly with time under a voltage step excitation [14], rather than from metal–semiconductor contact [15], because in the presence of gas, the low-frequency loop-like feature transforms to a line with a positive slope indicating a barrier effect (finite length diffusion phenomenon) [16]. The equivalent circuit for the impedance plots in the present case is given in Fig. 4, where Rg , Rgb are the grain and grain boundary resistances, and Cg and Cgb are the grain and grain boundary capacitances, respectively. The optimum values of resistance (Rg , Rgb ) and capacitance (Cg , Cgb ) obtained for the sensors under different conditions are given in Table 2.

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Table 2 Values of resistance (Rg , Rgb ) and capacitance (Cg , Cgb ) for sensors in air and in 1000 ppm methane Sensors

Condition

Simultaneous precipitation

Air Methane

Sonochemical Figaro (Japan)

Rg

Cg (pF)

Rgb

7.14 M 4.98 M

1.96 2.03

71.73 M 26.33 M

8.99 9.97

Air Methane

8.92 k 1.01 k

1.64 1.72

36.81 k 3.97 k

7.44 8.97

Air Methane

12.75 k 1.69 k

1.77 1.96

57.84 k 5.72 k

8.11 11.03

Fig. 4. Equivalent circuit used for analysis of the impedance plots.

As the atmosphere changes from air to methane, we can see Rg decreases, whereas Cg does not vary much. On the other hand, in presence of methane, Rgb decreases and at the same time Cgb increases. The increase in capacitance on exposure to gas is attributed to a reduction in the width of the depletion region [17]. 4. Conclusion Sonication-assisted simultaneous precipitation was employed to get nanosized tin dioxide-based powders. Thick film sensors, prepared with such powders, showed very good methane sensitivity and nearly equivalent properties to imported Figaro (Japan) sensors. It was observed that the complex plane impedance spectroscopy of the sensors can be used to characterize the sensors. Indeed, the complex plane impedance plots of Figaro sensors matched only with those of the sensors made with nanosized tin dioxide-based powders prepared by sonication-assisted simultaneous precipitation. Acknowledgement The authors are thankful to the Department of Science and Technology, Government of India, for financial assistance. References [1] J. Ross Macdonald (Ed.), Impedance Spectroscopy Emphasizing Solid Materials and Systems, John Wiley, New York, 1987, pp. 2–6. [2] R. Rella, A. Serra, P. Siciliano, L. Vasanelli, G. De, A. Licciulli, A. Quirini, Tin oxide based gas sensors prepared by the sol–gel process, Sens. Actuators B 44 (1997) 462–467. [3] R. Summitt, J.A. Marley, N.F. Borrelli, The ultraviolet absorption edge of stannic oxide (SnO2 ), J. Phys. Chem. Solids 25 (1964) 1465–1469. [4] W. Gopel, D. Schierbaum, SnO2 sensors: current status and future prospects, Sens. Actuators B 26–27 (1995) 1–12. [5] N. Yamozoe, N. Mura, Environmental gas sensing, Sens. Actuators B 20 (1994) 95–102.

Cgb (pF)

[6] S. Roy Morrison, Selectivity in semiconductor gas sensors, Sens. Actuators 12 (1987) 425–440. [7] Y. Matsuura, K. Takahata, Stabilization of SnO2 sintered gas sensors, Sens. Actuators B 5 (1991) 205–209. [8] F. Lu, Y. Liu, M. Dong, X. Wang, Nanosized tin oxide as the novel material with simultaneous detection towards CO, H2 and CH4 , Sens. Actuators B 66 (2000) 225–227. [9] K. Chatterjee, S. Chatterjee, A. Banerjee, M. Raut, N.C. Pal, A. Sen, H.S. Maiti, The effect of palladium incorporation on methane sensitivity of antimony doped tin dioxide, Mater. Chem. Phys. 81 (2003) 33–38. [10] A. Taylor, X-ray Metallography, John Wiley, New York, 1961, pp. 678, 686. [11] J.E. Bauerle, Study of solid electrolyte polarization by a complex admittance method, J. Phys. Chem. Solids. 30 (1969) 2657–2670. [12] N. Das, A.K. Halder, J. Mondal, A. Sen, H.S. Maiti, Sonochemically prepared tin-dioxide based composition for methane sensor, Maters. Lett. (2005) in press. [13] J. Gutierrez, L. Ares, M.C. Horillo, Use of complex impedance spectroscopy in chemical sensor characterization, Sens. Actuators B 4 (1991) 359–363. [14] E.F. Owede, A.K. Jonscher, Time- and frequency-dependent surface transport on humid insulators, J. Electrochem. Soc. 135 (1988) 1757–1765. [15] X. Wu, E.S. Yang, H.L. Evans, Negative capacitance at metal– semiconductor interfaces, J. Appl. Phys. 68 (1990) 2845–2848. [16] A.K. Jonscher, Analysis of the alternating current properties of ionic conductors, J. Mater. Sci. 13 (1978) 553–562. [17] G. Kiss, Z. Pinter, I.V. Perczel, Z. Sassi, F. Reti, Study of oxide semiconductor sensor materials by selected methods, Thin Solid Films 391 (2001) 216–223.

Biographies Shirshendu Chakraborty received an MTech in ceramic engineering from Banaras Hindu University in 2003. Presently, he has been working as a senior research fellow at Sensor & Actuator Section, Central Glass and Ceramic Research Institute, Kolkata, India. His current research interests are developing semiconducting oxide-based sensors for detection of toxic and combustible gases. Amarnath Sen obtained a PhD in materials science from Indian Institute of Technology, Kanpur, in 1986. Currently, he is the head of Sensor & Actuator Section, Central Glass and Ceramic Research Institute, Kolkata, India. His current research interests are processing and characterization of electronic ceramic materials like gas sensors and piezoelectric materials. Himadri Sekhar Maiti obtained a PhD in metallurgy from Indian Institute of Technology, Kanpur, in 1975. Currently, he is the director of Central Glass and Ceramic Research Institute, Kolkata, India. His current research interests are processing and characterization of electronic ceramic materials like hightemperature fuel cell, lithium battery and gas sensors.