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Effect of palladium on gas sensing properties of Sn(Sb2O3)O2 nanoparticles synthesized by sonochemical processing at room temperature
Applied Surface Science 376 (2016) 290–297
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Effect of palladium on gas sensing properties of Sn(Sb2 O3 )O2 nanoparticles synthesized by sonochemical processing at room temperature
a r t i c l e
i n f o
Keywords: Nanostructure Sonochemical Pd effect Surface reaction Selectivity Sensor array
a b s t r a c t Palladium catalyzed Sn(Sb2 O3 )O2 nanoparticles prepared by the sonication assisted method exhibited a Pd dependent selectivity to butane as well as methane. Attempts have been made to correlate powder properties such as surface area, particle size, crystallite size and rate of agglomeration with sensor properties like resistance, percent sensitivity, response and recovery times. Sample with 3 wt% Pd exhibited the lowest rate of agglomeration amongst the prepared samples and around 70% sensitivity towards butane at 400 ◦ C operating temperature. 5 wt% Pd loaded sample, on the other hand, exhibited about 98% methane sensitivity at 350 ◦ C operating temperature. Results confirmed that either by varying the amount of palladium or by changing the operating temperature, it was possible to tune the selective sensitivity of the fabricated sensors towards either butane or methane. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tin dioxide (SnO2 ) has been studied extensively to develop gas sensors for both oxidizing and reducing gases [1]. The sensing characteristics of SnO2 depend mainly on the changes in the surface conductance in presence of reactive gases. Though commercial SnO2 -based thick film gas sensors have been available for quite a long time, their performance such as sensitivity, selectivity, response time and durability still need modification and improvement. In order to improve and enhance the selectivity and sensitivity of the sensors, small amounts of metal catalysts, such as, Pt, Pd, Au etc. as additives have been used to modify the surface reaction kinetics [1,2]. In addition, to increase the carrier concentration and conductivity of SnO2 , small amounts of Sb2 O3 has also been added as a dopant. The gas sensing properties of semiconducting oxides depend mainly on the chemical, electronic and surface characteristics of the active materials. In a simple way, the mechanism can be projected as an adsorption/desorption process of oxygen on the surface of the sensing material. At temperatures below 100 ◦ C, oxygen is physisorbed on the surface, but at higher temperatures, oxygen acts as an electron acceptor and forms oxygen anion adsorbates (O2 − or O− ) leading to a depleted surface layer due to electron transfer from surface to the adsorbed oxygen. When the sensor material is exposed to a reducing gas, electrons are re-introduced into the conduction band leading to a decrease in the resistance. In addition to the composition, this adsorption/desorption process depends, to a large extent, on the microstructure of the sensor materials. Thick or thin film SnO2 -based gas sensors have been studied extensively for a variety of toxic and explosive gases, such as CO, hydrocarbons, ammonia, H2 , nitrogen oxides and H2 S [1–6].
http://dx.doi.org/10.1016/j.apsusc.2016.02.246 0169-4332/© 2016 Elsevier B.V. All rights reserved.
SnO2 based sensor materials have been prepared by different methods such as auto-combustion, template assisted synthesis, coprecipitation and sonochemical processes and their gas sensing characteristics to butane and methane under different condition have been studied [7–10]. Studies on the influence of catalysts such as Pd and Pt on methane, butane and hydrogen sensitivity of antimony doped tin dioxide have also been made [11,12]. In this study, Sb-doped SnO2 nanoparticles [Sn(Sb2 O3 )O2 ] with different amounts of Pd were prepared by the sonochemical route and its response towards butane and methane sensing was checked. The properties of catalyzed powders were compared with those of pure and antimony doped SnO2 (0.25 wt% Sb2 O3 ) powders prepared similarly. The sonochemical route was adopted for the preparation of these oxide nanoparticles mainly due to its effectiveness and simplicity over other techniques [13].
2. Experimental details 2.1. Wet chemical synthesis of sample preparation Nanocrystalline SnO2 based powders were prepared by a sonication assisted simultaneous precipitation technique [10]. In this technique, the required amounts of SnCl2 ·2H2 O and Sb2 O3 in HCl medium were mixed thoroughly under sonication (Ultrasonic processor, model-PR 1000, OSCAR Ultrasonics, India, 1000 W) for 30 min. Subsequently PdCl2 solution corresponding to 1–5 wt% Pd respectively, (Pd is considered on metal basis) was added to the mixed solution. During sonication NH4 OH was added drop-wise to the reaction mixture as a precipitating agent. The slow addition of NH4 OH was found to be the controlling step in getting a homogeneous mixed precipitate. Sonication was continued for 2 h till the
Effect of palladium on gas sensing properties of Sn(Sb2 O3 )O2 nanoparticles synthesized by sonochemical processing / Applied Surface Science 376 (2016) 290–297
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Fig. 1. X-ray diffractograms of SnO2 based powders with different Pd contents (0–5%) along with those of pure and Sb doped SnO2 powder calcined at 950 ◦ C.
pH of the mixture reached around 9. The brownish-black mixed precipitate (blackness of the mixtures increased with the increase of the Pd content in the solution) was collected and centrifuged (at ∼6000 rpm). The precipitates were washed with distilled water and acetone in a sequence to remove the undesired anionic impurities and finally dried in a vacuum oven, to obtain the powders. The dried powders were calcined at 950 ◦ C for 2 h in air.
Fig. 2. Thermogravimetric responses of as-prepared SnO2 based powders with different Pd loadings: (a) 1 wt% (b) 2 wt% (c) 3 wt% (d) 4 wt% and (e) 5 wt% Pd.
where, ‘RA ’ and ‘RG ’ are the sensor resistances in air and in presence of the reactive gas (at the same operating temperature), respectively.
2.2. Powder characterization
3. Results and discussion
Phase identification of the calcined powders was carried out by powder X-ray diffraction (XRD) study on a Philips, PW1710 diffractometer with CuK␣ radiation ( = 1.5406 Å). Transmission electron microscopy (TEM) images were taken on a TECNAI G2 30 highresolution transmission electron microscope operating at 300 kV. The calcined powder was ultrasonically dispersed in acetone for 30 min. A small amount of the dispersion was dropped with the help of a micro-pipette onto the carbon coated copper grid (having 200 mesh size) for TEM examination. Morphology of the thick films (sensor surface coating) was monitored by a Field Emission Scanning Electron Microscopy (SUPRA; 35VP). Specific surface areas of all the calcined powders were determined by BET method (Gemini II 2370) using N2 adsorption–desorption isotherm.
3.1. Material characterization
2.3. Sensor prototype fabrication Thick paste of the powders were prepared in a non-aqueous medium containing a small amount of alumina gel as binder. The thick films (powder coating thickness of <30 m) were made on alumina tubes (length:diameter = 3 mm: 2 mm) applying the paste with a hand brush. The sensor packaging arrangement has been reported elsewhere [11,12]. 2.4. Measurements of electrical resistance and percent sensitivity of the sensor prototype Electrical resistance in air (Rair ) and in presence of gas (Rgas ) (Viz., n-butane and methane) and the percent response of the sensor coatings were measured at different operating temperatures (in an ambient humidity of ∼60–65%) by using a digital multimeter (Solartron) and a constant voltage/current source (Keithley 228A). Before the measurement, all the samples were initially heated at a constant temperature of 350 ◦ C for one week to achieve the desired stability required for the measurement. Percent response of the SnO2 based sensors in presence of different concentrations of analyte gas was calculated using the relation [10]: S = [(RA − RG )/RA ] × 100
(1)
In Fig. 1, the XRD patterns of the calcined powders containing 1–5 wt% Pd are presented along with those of pure SnO2 and SbSnO2 . Representative reflections of the tetragonal SnO2 are seen for both Sb-doped and Pd-catalyzed samples. The diffraction peaks are assigned to the cassiterite SnO2 having tetragonal rutile structure (JCPDS card no: 77-0447). No evidence of any secondary phase ascribable to SnO or Sb2 O3 is present. Comparison of the plots in Fig. 1 also shows the difference in the widths of the diffraction peaks of pure SnO2 and the catalyzed samples indicating increment in grain size with the addition of Pd compared to pure SnO2 . Some reflections in the XRD pattern contain overlapped peaks of both SnO2 and PdO. If we compare the intensities of the [1 1 0] and [1 0 1] reflections, the enhanced intensity of the [1 0 1] reflection of Pd catalyzed samples compared to pure SnO2 is indicative of merging of the [1 0 1] reflection of SnO2 with the [1 0 1] reflection of PdO. The ratio of the intensities of [1 0 1]/[1 1 0] reflections and their percent increase with respect to pure SnO2 were 37.34%, 37.36%, 37.40% 39.11%, and 41.47% for 1–4 and for 5 wt% Pd, respectively. This clearly indicates the gradual formation of PdO with increase in the Pd concentration. The calculated lattice parameters for Pd loaded samples were, a = 4.739 Å and c = 3.187 Å for 1% Pd, a = 4.738 Å and c = 3.187 Å for 2% Pd, a = 4.736 Å and c = 3.187 Å for 3% Pd, a = 4.735 Å and c = 3.186 Å for 4% Pd and a = 4.735 Å and c = 3.185 Å for 5% Pd, respectively. The calculated lattice constants were slightly different compared to the a = 4.755 Å and c = 3.199 Å values of pure SnO2 . The lattice volume of the samples varied in the order 72.33 > 71.57 > 71.54 > 71.48 > 71.43 > 71.40 [Å]3 with increasing concentration of Pd from 0 to 5 wt%. The average crystallite sizes of the powders calculated using the Debye-Scherrer formula [2] are given in Table 1. In the table, DBET , the particle size calculated from the specific surface area (SBET ) and crystallite size (DXRD ), calculated from the X-ray line broadening peaks (FWHM) are compared. The degree of agglomeration represented by DBET /DXRD ratio of the synthesized powders were also calculated. It is clear from Table 1 that an increase in Pd concentration initially results in reduction of the surface area of SnO2 -based powders, followed by an increase with increase in the Pd concen-
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Fig. 3. TEM image showing (a) the microstructure of 3% Pd loaded SnO2 based powder calcined at 950 ◦ C, (b) particle size distribution of (a) micrograph with normalized Gaussian distribution function, showing the mean size of the partcles, (c) SAED pattern with indexed phases of SnO2 and (d) a representative EDS pattern showing the elements present in the film excepting Cu, which originated from the Cu grid used for TEM study.
Table 1 Physical properties of SnO2 based powders with different Pd contents (0–5%) along with pure SnO2 powder. Samples (SnO2 + 0.25 wt% Sb2 O3 )
Surface area SBET (m2 /g)
Particle size DBET (nm)
Crystallite size DXRD (nm)
DBET /DXRD
[1 0 1/1 1 0] reflection ratio (XRD)
Pure SnO2 0 Pd (Sb-SnO2 ) 1 Pd 2 Pd 3 Pd 4 Pd 5 Pd
8 9 2 7 10 16 22
98 99 353 95 105 95 77
28 32 24 18 15 12 10
3.50 3.09 14.71 5.28 7.00 7.92 7.70
0.8369 0.8369 1.1494 1.1497 1.1499 1.1616 1.1840
tration. From the data shown in Table 1, it is also evident that, the crystallite size of the powders decreases with increase in the Pd content. There is a reduction in the crystallite size from 24 nm for 1% Pd sample to 10 nm for the 5% Pd sample. A comparison of the extent of agglomeration confirms that the sample with 1% Pd has the highest degree of agglomeration amongst the prepared samples. It is evident that an increased concentration of Pd could inhibit grain growth of sonochemically prepared Sb-SnO2 samples. A similar effect of Pd in controlling grain growth has been observed earlier by Rella et al. in their investigations on Pd-activated SnO2 films [14]. The effect of Pd concentration on the weight change exhibited by the as-prepared dried powders subjected to thermogravimetric analyses between room temperature and 900 ◦ C, at a heating rate of 10 ◦ C/min., is shown in Fig. 2. The weight changes observed around 100–200 ◦ C temperature region for all the powders correspond to the dehydration process. An indication of weight gain with increase in temperature is probably due to the oxidation of Pd to PdO. This change is clearly evident for all the samples, though a
shift in the reaction temperature is noticeable. This increase occurs at the highest temperature of 550–600 ◦ C for the sample containing the highest amount of Pd (5 wt%). Fig. 3(a–d) shows the representative bright field TEM micrographs of the 3% Pd catalyzed SnO2 based sample, which confirm the nanometric size of the particles. The micrograph shows a homogeneous distribution of the particles, which appear almost spherical to elongate in nature. A moderately high degree of agglomeration is also observed. Most of the particles appear to have a size range of about 7–11 nm and the distribution is represented in the histogram given in Fig. 3(b). The average particle size obtained by a Gaussian fit of the histogram was about 9.5 nm having a standard deviation of ±1.74 nm, which agreed well with that obtained by powder XRD data. The corresponding selected-area diffraction (SAD) pattern (Fig. 3(c)) indicates that the nanostructures are polycrystalline in nature and the diffraction rings from inner core to outside could be indexed to (2 0 0), (2 1 1), (2 1 2) and (4 2 0) planes of rutile SnO2 , respectively, matching well with JCPDS reflections
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Fig. 4. Resistance in air of pure SnO2 , SnO2 -Sb2 O3 and of different Pd loaded SnO2 Sb2 O3 samples at 350 ◦ C.
of card no. 77-0447. The EDAX analysis (Fig. 3d), confirmed the presence of Sn, Sb, Pd and O in the sample; the presence of Cu is attributable to the Cu-grid used in the TEM study. 3.2. Sensitivity measurements The measured electrical resistance (at 350 ◦ C) of pure SnO2 , Sb-SnO2 and Pd catalyzed Sb-SnO2 samples are shown in Fig. 4. With antimony doping there was a sharp decrease in resistance as expected followed by an increase due to the Pd addition. At low doping levels, Sb substitutes for Sn4+ into the structure of SnO2 as Sb5+ and at higher doping levels as Sb3+ [14–16]. The defects formed as a consequence of the Sb5+ doping create donor levels (n-type conductivity) and increase the carrier concentration in SnO2 . This was evident in the resistance measurement: a decrease in resistance was observed by the incorporation of 0.25 wt% Sb2 O3 into SnO2 . Variation of resistance with operating temperature of different Pd-loaded samples is shown in Fig. 5. From the figure it is observed that on Pd addition initial increase in the resistance up
Fig. 5. Variation of resistance in air with operating temperatures of different Pd loaded SnO2 -Sb2 O3 samples.
to 3 wt% of Pd is followed by decreased resistance with further increase in Pd-content up to 5 wt%. Such variation of resistance of SnO2 with variation in concentration of Pd is interesting and can probably be explained as follows. At a given temperature, the electrical resistance of pure SnO2 depends mainly on the concentration of adsorbed oxygen on the surface. In the case of Pd loaded samples, the resistance depends on the electronic interaction between PdO and SnO2. It has been verified by other workers [17–21] that PdO is more effective than Pd◦ in chemisorbing oxygen. Oxygen is adsorbed at the interface between PdO and SnO2 , when the sensor is exposed to air (schematically presented in Fig. 6). Such oxygen adsorbates could accept electrons and form electron deficient space-charge region around the SnO2 grains at the interface region. On exposure to a reducing gas, due to the reduction of PdO to Pd, such interaction disappears, resulting in a decrease of the resistance and a consequent effect on the gas sensitivity (Fig. 6). It is well known [1] that Pd exerts its catalytic activity via electronic
Fig. 6. Gas sensing mechanism at interface region: lowering of electrical resistance with depletion of adsorbed oxygen species.
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Fig. 7. Sensitivity (percent response) of Pd loaded (1–5 wt%) samples towards (a) 1000 ppm n-butane and (b) 1000 ppm methane gas.
interaction in contrast with spill-over mechanism for Pt and the sensitivity saturation is normally observed at a specific concentration. Matsushisma et. al. [22] studied the effect of Pd dispersion on SnO2 gas sensors and its effect on the gas sensing characteristics. Such appearance and disappearance of the electronic interaction was linked with the accompanying change in the oxidation state between PdO and Pd and was thought to play a key role for the Pd-promoted sensitizations. Garbowski et al. [23], proposed that strongly adsorbed oxygen on palladium particles form a surface palladium oxide layer resulting in a depletion of surface carriers. Hence, the increase in resistance of SnO2 based samples with Pd addition is expected, though, after an optimum Pd concentration, the resistance decreases [15]. The samples prepared by the room temperature sonochemical method exhibited resistance saturation at ∼3% Pd loading. By sonochemical method it was possible to disperse and control the grain growth and agglomeration of the Pd loaded particles. At higher Pd loadings i.e., beyond ∼3%, probably due to the increased agglomeration of the powder as evidenced by the DBET /DXRD data (Table 1), the particles having PdO-SnO2 contacts may be less compared to those with lower amounts of Pd. Moreover, there may be a possibility of the presence of more percolating Pd clusters resulting in a decrease in resistance. This could be the reason for the lower resistance exhibited by the 5% Pd catalyzed sample. In order to evaluate the effect of Pd concentration on the gas sensitivity, the response characteristics of the prepared sensors towards 1000 ppm n-butane and 1000 ppm methane at different operating temperatures were measured and are compared in Fig. 7(a) and (b), respectively. In all the cases, initially a steep rise in sensitivity was noticed upto 400/350 ◦ C followed by a decrease, indicating saturation of sensitivity at a particular temperature. In the case of butane gas Fig. 7(a), the temperature of the maximum sensitivity was observed at 400 ◦ C in all the cases. It was found that the sensors made by nanosized SnO2 based powders with 3 wt% Pd are more sensitive towards 1000 ppm n-butane than 1000 ppm methane. The maximum response towards butane was obtained at 400 ◦ C for the 3 wt% Pd loaded sample which was about 70%. On the other hand, for 1000 ppm methane gas (Fig. 7b), the maximum sensitivity was observed at 350 ◦ C, which was the optimized operating temperature reported for SnO2 based sensors [1,2,7]. In this case, 5 wt% Pd catalyzed sensors were more sensitive to 1000 ppm methane compared to 1000 ppm n-butane gas. At 350 ◦ C operating temperature, about 98% and 77% sensitivity were exhibited by 5%
and 3% Pd loaded samples, respectively, against 1000 ppm methane (Fig. 7b) whereas, 5 wt% Pd catalyzed sensor exhibited a sensitivity of 51%, and 3 wt% Pd loaded sample exhibited a sensitivity of around 70% against 1000 ppm n-butane gas (Fig. 7a), at the same operating temperature of 400 ◦ C. Fig. 8 shows the recovery time and nature of the sensors. Contrary to the sensitivity data, the 5% Pd catalyzed sample recovered at a faster rate compared to the rest of the samples. Being a light-weight gas and having smaller molecular diameter methane recovers faster (Fig. 8a) than butane of the same concentration (Fig. 8b). Fig. 9(a) and (b) represent the typical response-recovery curves of 3% and 5% Pd catalyzed SnO2 based sensors at 400 ◦ C and 350 ◦ C operating temperatures, respectively, against 1000 ppm methane and 1000 and 5000 ppm butane gases. The variation of sensor resistance against time was recorded upon exposure at fixed concentration level of gas for 20 s pulse of exposure time. This was done based on experimental observations of percent sensitivity variations between 3 s and 30 s gas adsorption time for 3 wt% and 5 wt% Pd loaded samples measured at 400 ◦ C and 350 ◦ C operating temperatures, respectively. The plots showed maxima at about 20 s response time. Thus, 20 s time was considered as the ‘response time’ i.e., the time taken for the sensors to read at least 90% of full-scale reading after being exposed to the given gas. From Fig. 9(a) and (b) it is clear that methane is sensed well by 5% Pd catalyzed sensor at 350 ◦ C operating temperature whereas, 3% Pd catalyzed sensor exhibits high butane sensitivity at 400 ◦ C operating temperature. Hence, an array of such sensors would be useful to detect both the gases simultaneously at a fixed operating temperature. Fig. 9 also displays the sensitivity changes with the concentration of the gas. With increased concentration of the exposed gas, sensitivity as well as response and recovery time also increases. Fig. 9(a) farther exhibits lowering of resistance of 3 wt% Pd catalysed sensor corresponding to ∼82% sensitivity against 5000 ppm n-butane gas with 55 s recovery time and ∼70% sensitivity against 1000 ppm n-butane with 35 s recovery time at 400 ◦ C operating temperature whereas, it exhibits only ∼60% sensitivity against 1000 ppm methane with 30 s recovery time. Fig. 9b shows the same for 5% Pd catalyzed sensor at 350 ◦ C operating temperature which exhibits ∼98% sensitivity against 1000 ppm methane with only 20 s recovery time, ∼43% sensitivity against 1000 ppm butane with 30 s recovery time and ∼52% sensitivity against 5000 ppm n-butane gas with 40 s recovery time. However,
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Fig. 8. Dependence of recovery time on the operating temperature of Pd loaded (1–5 wt%) samples towards (a) 1000 ppm n-butane and (b) 1000 ppm methane gas.
Fig. 9. Dynamic response-recovery curves of the sensors towards 1000 ppm methane, 1000 ppm n-butane and 5000 ppm n-butane gas for (a) 3 wt% Pd at 400 ◦ C and (b) 5 wt% Pd loaded SnO2 based sensors at 350 ◦ C operating temperature.
in both the cases the resistance output at the saturated response state distributes evenly and recover back almost to its original level, suggesting highly reproducible and reversible response nature of the sensors against the analyte gases. In all the cases the primary response times are very short, within 3–5 s. Such a short response time is really helpful for practical application purposes. Since sensitivity is a surface property, it is quite obvious that recovery time would increase with higher concentration of the exposed gas as the recovery kinetics slow down at high concentrations of surface adsorbates. In addition, at high gas concentrations, slow oxidation of the analyte gas also increases the recovery and response times of the sensors [24,25]. The microstructure of a typical fabricated sensor coating surface is shown in Fig. 10(a–c), with gradual magnifications (from 10 to 3 to 200 nm, respectively) indicating the porous nature of the samples which ultimately facilitate the sensing reactions. Based on the observations and discussions made above, it can be concluded that by tuning the operating temperature and Pd concentration, hydrocarbon gases (like methane, butane etc.) can be monitored selectively with the same sensor materials and electronic circuitry.
3.3. Sensing mechanism To understand the gas sensing behavior of the synthesized materials under this study, the probable ways by which the reactive gases are decomposed on the sensor surface may be considered as follows [7,26]: C4 H10 (g) ⇔ C4 H10 (physisorbed)
(2)
and the probable decomposition reaction would be, C4 H10 + 2PdO + e− → 2Pd◦ + C4 H8 O− + H2 O
(3)
Similarly, the decomposition reaction of methane will be, CH4 + 2PdO + e− → 2Pd◦ + CH2 O− + H2 O
(4)
The final oxidation products of decomposition of the hydrocarbon gases are CO2 and H2 O, but these reactions proceed through several intermediate steps. However, the overall reactions could be represented as, C4 H10 (g) + 13PdO → 4CO2 (g) + 5H2 O(g) + 13Pd
(5)
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Fig. 10. FESEM image of the sensor coating microstructure of 3 wt% Pd loaded sample with gradually high magnifications (from 1.00 KX to 30.00 KX) are shown in (a), (b) and (c).
(Enthalpy of reaction: H = −2877.6 kJ mol−1 ) CH4 (g) + 4PdO → CO2 (g) + 2H2 O(g) + 4Pd
(6)
for selective monitoring of methane and/or n-butane. The results of this study are likely to prove useful for developing sensor array suitable for simultaneous sensing of different gases.
H = −890.8 kJ mol−1 )
(Enthalpy of reaction: It is clear that the oxidation of butane is thermodynamically more favorable than methane. However, the decomposition reaction of butane and methane involve a series of intermediate reactions and is very slow at low temperatures. Though the reducing gases could penetrate through the surface and react with the sensor materials at low temperatures, the reduction of PdO to Pd as well as the decomposition of butane and methane are slow at lower temperatures, resulting in lower sensitivity at low temperatures. At higher temperatures (400 ◦ C and above), on the other hand, the gas at the outer surface could burn away easily, leaving most of the PdO particles intact inside, thus resulting in a loss of sensitivity. Hence, for thick film gas sensors, gas sensitivity is optimum at intermediate temperatures [22]. Though the reduction of PdO to Pd is slow at lower temperatures, it is the key factor in controlling the sensing mechanism in Pd catalyzed Sb-SnO2 samples at any given temperature. 5. Conclusion This work demonstrates a sonochemical synthesis technique for preparing palladium catalyzed SnO2 -Sb2 O3 nanoparticles suitable for fabrication of effective gas sensors for detecting butane and methane. Samples with 3 wt% Pd-loading exhibited around 70% sensitivity towards 1000 ppm butane at an operating temperature of 400 ◦ C, attributable to increased SnO2 -PdO contacts due to lower degree of agglomeration of the powder sample. On the other hand, at 350 ◦ C temperature 5 wt% Pd catalyzed sensor exhibited a sensitivity of ∼98% towards 1000 ppm methane. Thus, by simply varying the Pd content, sensors selective to two different gases can be developed. Consequently, either by modulating the operating temperature, or by varying the Pd concentration, the same sensor material and the same electronic circuitry could be used
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[23] E. Garbowski, C. Feumi-Jantou, N. Mouaddib, M. Primet, Catalytic combustion of methane over palladium supported on alumina catalysts: evidence of reconstruction of particles, Appl. Catal. A 109 (1994) 277–291. [24] S. Banerjee, P. Nag, S. Majumdar, P.S. Devi, High butane sensitivity and selectivity exhibited by combustion synthesized SnO2 nanoparticles, Mater. Res. Bull. 65 (2015) 216–223. [25] S. Majumdar, P. Nag, P.S. Devi, Enhanced performance of CNT/SnO2 thick Film gas sensors towards hydrogen, Mater. Chem. Phys. 147 (2014) 79–85. [26] I. Ray, S. Chakraborty, A. Chowdhury, S. Majumdar, A. Prakash, Ram Pyare, A. Sen, Room temperature synthesis of ␥-Fe2 O3 by sonochemical route and its response towards butane, Sens. Actuators B 130 (2008) 882–888.
Sanhita Majumdar Centre of Excellence for Green Energy and Sensor Systems (CEGESS), Indian Institute of Engineering Science and Technology (IIEST), Shibpur, Howrah 711103, West Bengal, India E-mail addresses: [email protected], [email protected]. 1 November 2015 5 February 2016 29 February 2016 Available online 3 March 2016
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of palladium on gas sensing properties of Sn(Sb2 O3 )O2 nanoparticles synthesized by sonochemical processing at room temperature
a r t i c l e
i n f o
Keywords: Nanostructure Sonochemical Pd effect Surface reaction Selectivity Sensor array
a b s t r a c t Palladium catalyzed Sn(Sb2 O3 )O2 nanoparticles prepared by the sonication assisted method exhibited a Pd dependent selectivity to butane as well as methane. Attempts have been made to correlate powder properties such as surface area, particle size, crystallite size and rate of agglomeration with sensor properties like resistance, percent sensitivity, response and recovery times. Sample with 3 wt% Pd exhibited the lowest rate of agglomeration amongst the prepared samples and around 70% sensitivity towards butane at 400 ◦ C operating temperature. 5 wt% Pd loaded sample, on the other hand, exhibited about 98% methane sensitivity at 350 ◦ C operating temperature. Results confirmed that either by varying the amount of palladium or by changing the operating temperature, it was possible to tune the selective sensitivity of the fabricated sensors towards either butane or methane. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tin dioxide (SnO2 ) has been studied extensively to develop gas sensors for both oxidizing and reducing gases [1]. The sensing characteristics of SnO2 depend mainly on the changes in the surface conductance in presence of reactive gases. Though commercial SnO2 -based thick film gas sensors have been available for quite a long time, their performance such as sensitivity, selectivity, response time and durability still need modification and improvement. In order to improve and enhance the selectivity and sensitivity of the sensors, small amounts of metal catalysts, such as, Pt, Pd, Au etc. as additives have been used to modify the surface reaction kinetics [1,2]. In addition, to increase the carrier concentration and conductivity of SnO2 , small amounts of Sb2 O3 has also been added as a dopant. The gas sensing properties of semiconducting oxides depend mainly on the chemical, electronic and surface characteristics of the active materials. In a simple way, the mechanism can be projected as an adsorption/desorption process of oxygen on the surface of the sensing material. At temperatures below 100 ◦ C, oxygen is physisorbed on the surface, but at higher temperatures, oxygen acts as an electron acceptor and forms oxygen anion adsorbates (O2 − or O− ) leading to a depleted surface layer due to electron transfer from surface to the adsorbed oxygen. When the sensor material is exposed to a reducing gas, electrons are re-introduced into the conduction band leading to a decrease in the resistance. In addition to the composition, this adsorption/desorption process depends, to a large extent, on the microstructure of the sensor materials. Thick or thin film SnO2 -based gas sensors have been studied extensively for a variety of toxic and explosive gases, such as CO, hydrocarbons, ammonia, H2 , nitrogen oxides and H2 S [1–6].
http://dx.doi.org/10.1016/j.apsusc.2016.02.246 0169-4332/© 2016 Elsevier B.V. All rights reserved.
SnO2 based sensor materials have been prepared by different methods such as auto-combustion, template assisted synthesis, coprecipitation and sonochemical processes and their gas sensing characteristics to butane and methane under different condition have been studied [7–10]. Studies on the influence of catalysts such as Pd and Pt on methane, butane and hydrogen sensitivity of antimony doped tin dioxide have also been made [11,12]. In this study, Sb-doped SnO2 nanoparticles [Sn(Sb2 O3 )O2 ] with different amounts of Pd were prepared by the sonochemical route and its response towards butane and methane sensing was checked. The properties of catalyzed powders were compared with those of pure and antimony doped SnO2 (0.25 wt% Sb2 O3 ) powders prepared similarly. The sonochemical route was adopted for the preparation of these oxide nanoparticles mainly due to its effectiveness and simplicity over other techniques [13].
2. Experimental details 2.1. Wet chemical synthesis of sample preparation Nanocrystalline SnO2 based powders were prepared by a sonication assisted simultaneous precipitation technique [10]. In this technique, the required amounts of SnCl2 ·2H2 O and Sb2 O3 in HCl medium were mixed thoroughly under sonication (Ultrasonic processor, model-PR 1000, OSCAR Ultrasonics, India, 1000 W) for 30 min. Subsequently PdCl2 solution corresponding to 1–5 wt% Pd respectively, (Pd is considered on metal basis) was added to the mixed solution. During sonication NH4 OH was added drop-wise to the reaction mixture as a precipitating agent. The slow addition of NH4 OH was found to be the controlling step in getting a homogeneous mixed precipitate. Sonication was continued for 2 h till the
Effect of palladium on gas sensing properties of Sn(Sb2 O3 )O2 nanoparticles synthesized by sonochemical processing / Applied Surface Science 376 (2016) 290–297
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Fig. 1. X-ray diffractograms of SnO2 based powders with different Pd contents (0–5%) along with those of pure and Sb doped SnO2 powder calcined at 950 ◦ C.
pH of the mixture reached around 9. The brownish-black mixed precipitate (blackness of the mixtures increased with the increase of the Pd content in the solution) was collected and centrifuged (at ∼6000 rpm). The precipitates were washed with distilled water and acetone in a sequence to remove the undesired anionic impurities and finally dried in a vacuum oven, to obtain the powders. The dried powders were calcined at 950 ◦ C for 2 h in air.
Fig. 2. Thermogravimetric responses of as-prepared SnO2 based powders with different Pd loadings: (a) 1 wt% (b) 2 wt% (c) 3 wt% (d) 4 wt% and (e) 5 wt% Pd.
where, ‘RA ’ and ‘RG ’ are the sensor resistances in air and in presence of the reactive gas (at the same operating temperature), respectively.
2.2. Powder characterization
3. Results and discussion
Phase identification of the calcined powders was carried out by powder X-ray diffraction (XRD) study on a Philips, PW1710 diffractometer with CuK␣ radiation ( = 1.5406 Å). Transmission electron microscopy (TEM) images were taken on a TECNAI G2 30 highresolution transmission electron microscope operating at 300 kV. The calcined powder was ultrasonically dispersed in acetone for 30 min. A small amount of the dispersion was dropped with the help of a micro-pipette onto the carbon coated copper grid (having 200 mesh size) for TEM examination. Morphology of the thick films (sensor surface coating) was monitored by a Field Emission Scanning Electron Microscopy (SUPRA; 35VP). Specific surface areas of all the calcined powders were determined by BET method (Gemini II 2370) using N2 adsorption–desorption isotherm.
3.1. Material characterization
2.3. Sensor prototype fabrication Thick paste of the powders were prepared in a non-aqueous medium containing a small amount of alumina gel as binder. The thick films (powder coating thickness of <30 m) were made on alumina tubes (length:diameter = 3 mm: 2 mm) applying the paste with a hand brush. The sensor packaging arrangement has been reported elsewhere [11,12]. 2.4. Measurements of electrical resistance and percent sensitivity of the sensor prototype Electrical resistance in air (Rair ) and in presence of gas (Rgas ) (Viz., n-butane and methane) and the percent response of the sensor coatings were measured at different operating temperatures (in an ambient humidity of ∼60–65%) by using a digital multimeter (Solartron) and a constant voltage/current source (Keithley 228A). Before the measurement, all the samples were initially heated at a constant temperature of 350 ◦ C for one week to achieve the desired stability required for the measurement. Percent response of the SnO2 based sensors in presence of different concentrations of analyte gas was calculated using the relation [10]: S = [(RA − RG )/RA ] × 100
(1)
In Fig. 1, the XRD patterns of the calcined powders containing 1–5 wt% Pd are presented along with those of pure SnO2 and SbSnO2 . Representative reflections of the tetragonal SnO2 are seen for both Sb-doped and Pd-catalyzed samples. The diffraction peaks are assigned to the cassiterite SnO2 having tetragonal rutile structure (JCPDS card no: 77-0447). No evidence of any secondary phase ascribable to SnO or Sb2 O3 is present. Comparison of the plots in Fig. 1 also shows the difference in the widths of the diffraction peaks of pure SnO2 and the catalyzed samples indicating increment in grain size with the addition of Pd compared to pure SnO2 . Some reflections in the XRD pattern contain overlapped peaks of both SnO2 and PdO. If we compare the intensities of the [1 1 0] and [1 0 1] reflections, the enhanced intensity of the [1 0 1] reflection of Pd catalyzed samples compared to pure SnO2 is indicative of merging of the [1 0 1] reflection of SnO2 with the [1 0 1] reflection of PdO. The ratio of the intensities of [1 0 1]/[1 1 0] reflections and their percent increase with respect to pure SnO2 were 37.34%, 37.36%, 37.40% 39.11%, and 41.47% for 1–4 and for 5 wt% Pd, respectively. This clearly indicates the gradual formation of PdO with increase in the Pd concentration. The calculated lattice parameters for Pd loaded samples were, a = 4.739 Å and c = 3.187 Å for 1% Pd, a = 4.738 Å and c = 3.187 Å for 2% Pd, a = 4.736 Å and c = 3.187 Å for 3% Pd, a = 4.735 Å and c = 3.186 Å for 4% Pd and a = 4.735 Å and c = 3.185 Å for 5% Pd, respectively. The calculated lattice constants were slightly different compared to the a = 4.755 Å and c = 3.199 Å values of pure SnO2 . The lattice volume of the samples varied in the order 72.33 > 71.57 > 71.54 > 71.48 > 71.43 > 71.40 [Å]3 with increasing concentration of Pd from 0 to 5 wt%. The average crystallite sizes of the powders calculated using the Debye-Scherrer formula [2] are given in Table 1. In the table, DBET , the particle size calculated from the specific surface area (SBET ) and crystallite size (DXRD ), calculated from the X-ray line broadening peaks (FWHM) are compared. The degree of agglomeration represented by DBET /DXRD ratio of the synthesized powders were also calculated. It is clear from Table 1 that an increase in Pd concentration initially results in reduction of the surface area of SnO2 -based powders, followed by an increase with increase in the Pd concen-
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Fig. 3. TEM image showing (a) the microstructure of 3% Pd loaded SnO2 based powder calcined at 950 ◦ C, (b) particle size distribution of (a) micrograph with normalized Gaussian distribution function, showing the mean size of the partcles, (c) SAED pattern with indexed phases of SnO2 and (d) a representative EDS pattern showing the elements present in the film excepting Cu, which originated from the Cu grid used for TEM study.
Table 1 Physical properties of SnO2 based powders with different Pd contents (0–5%) along with pure SnO2 powder. Samples (SnO2 + 0.25 wt% Sb2 O3 )
Surface area SBET (m2 /g)
Particle size DBET (nm)
Crystallite size DXRD (nm)
DBET /DXRD
[1 0 1/1 1 0] reflection ratio (XRD)
Pure SnO2 0 Pd (Sb-SnO2 ) 1 Pd 2 Pd 3 Pd 4 Pd 5 Pd
8 9 2 7 10 16 22
98 99 353 95 105 95 77
28 32 24 18 15 12 10
3.50 3.09 14.71 5.28 7.00 7.92 7.70
0.8369 0.8369 1.1494 1.1497 1.1499 1.1616 1.1840
tration. From the data shown in Table 1, it is also evident that, the crystallite size of the powders decreases with increase in the Pd content. There is a reduction in the crystallite size from 24 nm for 1% Pd sample to 10 nm for the 5% Pd sample. A comparison of the extent of agglomeration confirms that the sample with 1% Pd has the highest degree of agglomeration amongst the prepared samples. It is evident that an increased concentration of Pd could inhibit grain growth of sonochemically prepared Sb-SnO2 samples. A similar effect of Pd in controlling grain growth has been observed earlier by Rella et al. in their investigations on Pd-activated SnO2 films [14]. The effect of Pd concentration on the weight change exhibited by the as-prepared dried powders subjected to thermogravimetric analyses between room temperature and 900 ◦ C, at a heating rate of 10 ◦ C/min., is shown in Fig. 2. The weight changes observed around 100–200 ◦ C temperature region for all the powders correspond to the dehydration process. An indication of weight gain with increase in temperature is probably due to the oxidation of Pd to PdO. This change is clearly evident for all the samples, though a
shift in the reaction temperature is noticeable. This increase occurs at the highest temperature of 550–600 ◦ C for the sample containing the highest amount of Pd (5 wt%). Fig. 3(a–d) shows the representative bright field TEM micrographs of the 3% Pd catalyzed SnO2 based sample, which confirm the nanometric size of the particles. The micrograph shows a homogeneous distribution of the particles, which appear almost spherical to elongate in nature. A moderately high degree of agglomeration is also observed. Most of the particles appear to have a size range of about 7–11 nm and the distribution is represented in the histogram given in Fig. 3(b). The average particle size obtained by a Gaussian fit of the histogram was about 9.5 nm having a standard deviation of ±1.74 nm, which agreed well with that obtained by powder XRD data. The corresponding selected-area diffraction (SAD) pattern (Fig. 3(c)) indicates that the nanostructures are polycrystalline in nature and the diffraction rings from inner core to outside could be indexed to (2 0 0), (2 1 1), (2 1 2) and (4 2 0) planes of rutile SnO2 , respectively, matching well with JCPDS reflections
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Fig. 4. Resistance in air of pure SnO2 , SnO2 -Sb2 O3 and of different Pd loaded SnO2 Sb2 O3 samples at 350 ◦ C.
of card no. 77-0447. The EDAX analysis (Fig. 3d), confirmed the presence of Sn, Sb, Pd and O in the sample; the presence of Cu is attributable to the Cu-grid used in the TEM study. 3.2. Sensitivity measurements The measured electrical resistance (at 350 ◦ C) of pure SnO2 , Sb-SnO2 and Pd catalyzed Sb-SnO2 samples are shown in Fig. 4. With antimony doping there was a sharp decrease in resistance as expected followed by an increase due to the Pd addition. At low doping levels, Sb substitutes for Sn4+ into the structure of SnO2 as Sb5+ and at higher doping levels as Sb3+ [14–16]. The defects formed as a consequence of the Sb5+ doping create donor levels (n-type conductivity) and increase the carrier concentration in SnO2 . This was evident in the resistance measurement: a decrease in resistance was observed by the incorporation of 0.25 wt% Sb2 O3 into SnO2 . Variation of resistance with operating temperature of different Pd-loaded samples is shown in Fig. 5. From the figure it is observed that on Pd addition initial increase in the resistance up
Fig. 5. Variation of resistance in air with operating temperatures of different Pd loaded SnO2 -Sb2 O3 samples.
to 3 wt% of Pd is followed by decreased resistance with further increase in Pd-content up to 5 wt%. Such variation of resistance of SnO2 with variation in concentration of Pd is interesting and can probably be explained as follows. At a given temperature, the electrical resistance of pure SnO2 depends mainly on the concentration of adsorbed oxygen on the surface. In the case of Pd loaded samples, the resistance depends on the electronic interaction between PdO and SnO2. It has been verified by other workers [17–21] that PdO is more effective than Pd◦ in chemisorbing oxygen. Oxygen is adsorbed at the interface between PdO and SnO2 , when the sensor is exposed to air (schematically presented in Fig. 6). Such oxygen adsorbates could accept electrons and form electron deficient space-charge region around the SnO2 grains at the interface region. On exposure to a reducing gas, due to the reduction of PdO to Pd, such interaction disappears, resulting in a decrease of the resistance and a consequent effect on the gas sensitivity (Fig. 6). It is well known [1] that Pd exerts its catalytic activity via electronic
Fig. 6. Gas sensing mechanism at interface region: lowering of electrical resistance with depletion of adsorbed oxygen species.
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Fig. 7. Sensitivity (percent response) of Pd loaded (1–5 wt%) samples towards (a) 1000 ppm n-butane and (b) 1000 ppm methane gas.
interaction in contrast with spill-over mechanism for Pt and the sensitivity saturation is normally observed at a specific concentration. Matsushisma et. al. [22] studied the effect of Pd dispersion on SnO2 gas sensors and its effect on the gas sensing characteristics. Such appearance and disappearance of the electronic interaction was linked with the accompanying change in the oxidation state between PdO and Pd and was thought to play a key role for the Pd-promoted sensitizations. Garbowski et al. [23], proposed that strongly adsorbed oxygen on palladium particles form a surface palladium oxide layer resulting in a depletion of surface carriers. Hence, the increase in resistance of SnO2 based samples with Pd addition is expected, though, after an optimum Pd concentration, the resistance decreases [15]. The samples prepared by the room temperature sonochemical method exhibited resistance saturation at ∼3% Pd loading. By sonochemical method it was possible to disperse and control the grain growth and agglomeration of the Pd loaded particles. At higher Pd loadings i.e., beyond ∼3%, probably due to the increased agglomeration of the powder as evidenced by the DBET /DXRD data (Table 1), the particles having PdO-SnO2 contacts may be less compared to those with lower amounts of Pd. Moreover, there may be a possibility of the presence of more percolating Pd clusters resulting in a decrease in resistance. This could be the reason for the lower resistance exhibited by the 5% Pd catalyzed sample. In order to evaluate the effect of Pd concentration on the gas sensitivity, the response characteristics of the prepared sensors towards 1000 ppm n-butane and 1000 ppm methane at different operating temperatures were measured and are compared in Fig. 7(a) and (b), respectively. In all the cases, initially a steep rise in sensitivity was noticed upto 400/350 ◦ C followed by a decrease, indicating saturation of sensitivity at a particular temperature. In the case of butane gas Fig. 7(a), the temperature of the maximum sensitivity was observed at 400 ◦ C in all the cases. It was found that the sensors made by nanosized SnO2 based powders with 3 wt% Pd are more sensitive towards 1000 ppm n-butane than 1000 ppm methane. The maximum response towards butane was obtained at 400 ◦ C for the 3 wt% Pd loaded sample which was about 70%. On the other hand, for 1000 ppm methane gas (Fig. 7b), the maximum sensitivity was observed at 350 ◦ C, which was the optimized operating temperature reported for SnO2 based sensors [1,2,7]. In this case, 5 wt% Pd catalyzed sensors were more sensitive to 1000 ppm methane compared to 1000 ppm n-butane gas. At 350 ◦ C operating temperature, about 98% and 77% sensitivity were exhibited by 5%
and 3% Pd loaded samples, respectively, against 1000 ppm methane (Fig. 7b) whereas, 5 wt% Pd catalyzed sensor exhibited a sensitivity of 51%, and 3 wt% Pd loaded sample exhibited a sensitivity of around 70% against 1000 ppm n-butane gas (Fig. 7a), at the same operating temperature of 400 ◦ C. Fig. 8 shows the recovery time and nature of the sensors. Contrary to the sensitivity data, the 5% Pd catalyzed sample recovered at a faster rate compared to the rest of the samples. Being a light-weight gas and having smaller molecular diameter methane recovers faster (Fig. 8a) than butane of the same concentration (Fig. 8b). Fig. 9(a) and (b) represent the typical response-recovery curves of 3% and 5% Pd catalyzed SnO2 based sensors at 400 ◦ C and 350 ◦ C operating temperatures, respectively, against 1000 ppm methane and 1000 and 5000 ppm butane gases. The variation of sensor resistance against time was recorded upon exposure at fixed concentration level of gas for 20 s pulse of exposure time. This was done based on experimental observations of percent sensitivity variations between 3 s and 30 s gas adsorption time for 3 wt% and 5 wt% Pd loaded samples measured at 400 ◦ C and 350 ◦ C operating temperatures, respectively. The plots showed maxima at about 20 s response time. Thus, 20 s time was considered as the ‘response time’ i.e., the time taken for the sensors to read at least 90% of full-scale reading after being exposed to the given gas. From Fig. 9(a) and (b) it is clear that methane is sensed well by 5% Pd catalyzed sensor at 350 ◦ C operating temperature whereas, 3% Pd catalyzed sensor exhibits high butane sensitivity at 400 ◦ C operating temperature. Hence, an array of such sensors would be useful to detect both the gases simultaneously at a fixed operating temperature. Fig. 9 also displays the sensitivity changes with the concentration of the gas. With increased concentration of the exposed gas, sensitivity as well as response and recovery time also increases. Fig. 9(a) farther exhibits lowering of resistance of 3 wt% Pd catalysed sensor corresponding to ∼82% sensitivity against 5000 ppm n-butane gas with 55 s recovery time and ∼70% sensitivity against 1000 ppm n-butane with 35 s recovery time at 400 ◦ C operating temperature whereas, it exhibits only ∼60% sensitivity against 1000 ppm methane with 30 s recovery time. Fig. 9b shows the same for 5% Pd catalyzed sensor at 350 ◦ C operating temperature which exhibits ∼98% sensitivity against 1000 ppm methane with only 20 s recovery time, ∼43% sensitivity against 1000 ppm butane with 30 s recovery time and ∼52% sensitivity against 5000 ppm n-butane gas with 40 s recovery time. However,
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Fig. 8. Dependence of recovery time on the operating temperature of Pd loaded (1–5 wt%) samples towards (a) 1000 ppm n-butane and (b) 1000 ppm methane gas.
Fig. 9. Dynamic response-recovery curves of the sensors towards 1000 ppm methane, 1000 ppm n-butane and 5000 ppm n-butane gas for (a) 3 wt% Pd at 400 ◦ C and (b) 5 wt% Pd loaded SnO2 based sensors at 350 ◦ C operating temperature.
in both the cases the resistance output at the saturated response state distributes evenly and recover back almost to its original level, suggesting highly reproducible and reversible response nature of the sensors against the analyte gases. In all the cases the primary response times are very short, within 3–5 s. Such a short response time is really helpful for practical application purposes. Since sensitivity is a surface property, it is quite obvious that recovery time would increase with higher concentration of the exposed gas as the recovery kinetics slow down at high concentrations of surface adsorbates. In addition, at high gas concentrations, slow oxidation of the analyte gas also increases the recovery and response times of the sensors [24,25]. The microstructure of a typical fabricated sensor coating surface is shown in Fig. 10(a–c), with gradual magnifications (from 10 to 3 to 200 nm, respectively) indicating the porous nature of the samples which ultimately facilitate the sensing reactions. Based on the observations and discussions made above, it can be concluded that by tuning the operating temperature and Pd concentration, hydrocarbon gases (like methane, butane etc.) can be monitored selectively with the same sensor materials and electronic circuitry.
3.3. Sensing mechanism To understand the gas sensing behavior of the synthesized materials under this study, the probable ways by which the reactive gases are decomposed on the sensor surface may be considered as follows [7,26]: C4 H10 (g) ⇔ C4 H10 (physisorbed)
(2)
and the probable decomposition reaction would be, C4 H10 + 2PdO + e− → 2Pd◦ + C4 H8 O− + H2 O
(3)
Similarly, the decomposition reaction of methane will be, CH4 + 2PdO + e− → 2Pd◦ + CH2 O− + H2 O
(4)
The final oxidation products of decomposition of the hydrocarbon gases are CO2 and H2 O, but these reactions proceed through several intermediate steps. However, the overall reactions could be represented as, C4 H10 (g) + 13PdO → 4CO2 (g) + 5H2 O(g) + 13Pd
(5)
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Fig. 10. FESEM image of the sensor coating microstructure of 3 wt% Pd loaded sample with gradually high magnifications (from 1.00 KX to 30.00 KX) are shown in (a), (b) and (c).
(Enthalpy of reaction: H = −2877.6 kJ mol−1 ) CH4 (g) + 4PdO → CO2 (g) + 2H2 O(g) + 4Pd
(6)
for selective monitoring of methane and/or n-butane. The results of this study are likely to prove useful for developing sensor array suitable for simultaneous sensing of different gases.
H = −890.8 kJ mol−1 )
(Enthalpy of reaction: It is clear that the oxidation of butane is thermodynamically more favorable than methane. However, the decomposition reaction of butane and methane involve a series of intermediate reactions and is very slow at low temperatures. Though the reducing gases could penetrate through the surface and react with the sensor materials at low temperatures, the reduction of PdO to Pd as well as the decomposition of butane and methane are slow at lower temperatures, resulting in lower sensitivity at low temperatures. At higher temperatures (400 ◦ C and above), on the other hand, the gas at the outer surface could burn away easily, leaving most of the PdO particles intact inside, thus resulting in a loss of sensitivity. Hence, for thick film gas sensors, gas sensitivity is optimum at intermediate temperatures [22]. Though the reduction of PdO to Pd is slow at lower temperatures, it is the key factor in controlling the sensing mechanism in Pd catalyzed Sb-SnO2 samples at any given temperature. 5. Conclusion This work demonstrates a sonochemical synthesis technique for preparing palladium catalyzed SnO2 -Sb2 O3 nanoparticles suitable for fabrication of effective gas sensors for detecting butane and methane. Samples with 3 wt% Pd-loading exhibited around 70% sensitivity towards 1000 ppm butane at an operating temperature of 400 ◦ C, attributable to increased SnO2 -PdO contacts due to lower degree of agglomeration of the powder sample. On the other hand, at 350 ◦ C temperature 5 wt% Pd catalyzed sensor exhibited a sensitivity of ∼98% towards 1000 ppm methane. Thus, by simply varying the Pd content, sensors selective to two different gases can be developed. Consequently, either by modulating the operating temperature, or by varying the Pd concentration, the same sensor material and the same electronic circuitry could be used
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Sanhita Majumdar Centre of Excellence for Green Energy and Sensor Systems (CEGESS), Indian Institute of Engineering Science and Technology (IIEST), Shibpur, Howrah 711103, West Bengal, India E-mail addresses: [email protected], [email protected]. 1 November 2015 5 February 2016 29 February 2016 Available online 3 March 2016