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SnO2 film with bimodal distribution of nano-particles for low concentration hydrogen sensor: Effect of firing temperature on sensing properties
Materials Chemistry and Physics 133 (2012) 681–687
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SnO2 film with bimodal distribution of nano-particles for low concentration hydrogen sensor: Effect of firing temperature on sensing properties Salim F. Bamsaoud a,b,∗ , S.B. Rane c,∗∗ , R.N. Karekar d , R.C. Aiyer b,∗ ∗ ∗ a
Microwave and Thin Film Lab, Department of Physics, University of Pune, Pune 411007, India Center for Sensor Studies, Department of Electronics, University of Pune, Pune 411007, India Center for Materials for Electronics Technology, Off Pashan Road, Pune 411008, India d RCMF Transducer Lab, Department of Electronics, University of Pune, Pune 411007, India b c
a r t i c l e
i n f o
Article history: Received 11 July 2011 Received in revised form 7 January 2012 Accepted 18 January 2012 Keywords: Tin oxide Nano particles Single step thermal decomposition Hydrogen sensor
a b s t r a c t Spin coated sensor films of nano-SnO2 with bimodal nano-size particle distribution were obtained through thermal decomposition of SnCl2 using a simple and fast single step thermal decomposition technique. The deposition was done on soda-lime glass substrates using variable decomposition/firing temperature in the range of 400–600 ◦ C at the interval of 50 ◦ C. The effect of firing temperature on structural, micro-structural and optical properties was studied by X-ray diffractometer (XRD), atomic force microscopy (AFM) and UV–Vis respectively. AFM study of the films revealed a special type of microstructure i.e. conducting larger globules of 8–20 nm with smaller nanoparticles of ≤3 nm on the surface of the bigger globules. The films fired at 400 ◦ C exhibit the highest UV absorption peak compared to other samples. A highest response (Rair /Rgas = 24) to 300 ppm of H2 is recorded for films fired at 550 ◦ C and they exhibit sensitivity of ∼90 × 10−3 for concentration of H2 (1–20 ppm). These sensors also exhibit good stability of resistance at the optimized operating temperature of 265 ◦ C and in the presence of different relative humidity (RH = 20 − 100%). © 2012 Elsevier B.V. All rights reserved.
1. Introduction Currently a variety of techniques are being used for film deposition to get nano structures for different applications. Technology involvement in film deposition comes in the picture in order to achieve high reproducibility, simplicity, flexibility and preferably low cost. For scientific purposes and industrial applications, proper selection of deposition method is vital. Spray pyrolysis [1–3] and sol–gel (dip-coating and sol–gel spin coating) [4–7] are widely used in industrial applications because of simplicity and no requirement of highly vacuum based sophisticated equipments. In spray pyrolysis, a thin film is deposited by spraying a solution on a heated substrate where the constituents react to form a chemical compound. It is a single step process. To get required quality of films, the concentration of spray solution, input pressure of carrier gas, substrate temperature and distance between substrate and spray gun are to be optimized. Sol gel process gets its name from the two major stages involved: formation of a sol and subsequent
∗ Corresponding author. Tel.: +91 20 2569 2678; fax: +91 20 2569 1684. ∗∗ Corresponding author. Tel.: +91 20 2589 9273; fax: +91 20 2589 8180. ∗ ∗ ∗Corresponding author. E-mail addresses: [email protected] (S.F. Bamsaoud), [email protected] (S.B. Rane). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.01.052
attainment of a semi-rigid gel. The semi-rigid gel is used to deposit thin films by either dip [4,5] or spin-coating [6,7]. Tin oxide films with nano grain size are prepared by different deposition techniques and have been extensively studied for various applications, including catalyst support [8], transparent conducting electrodes [9], Li-ion battery anode materials [10,11], storage applications, solar cells [12,13] and gas sensors [14–23]. Tin oxide (SnO2 ) based gas sensors have been extensively investigated, after the charge-carrier concentration on the metal oxide surface response towards the composition of the surrounding atmosphere was understood [24]. This seems to depend on the surrounding atmospheric oxygen in the form of various chemical species; the pre-adsorbed oxygen molecules are present on the surface of the metal oxide. These chemical species of oxygen are formed when the atmospheric oxygen reacts with the metal oxide by taking electrons from its conduction band, resulting in increase in the resistance of metal oxide [23]. The response of the material to chemical species depends on microstructure of the sensing material, especially on the grain size [16,17,25]. Ansari et al. [16] have reported that H2 sensing response increases with decrease in size of SnO2 based thick film sensors where the crystallite size of the powder was in the range of 20–45 nm. Xu et al. [17] have reported that the gas response of SnO2 based fired elements increases sharply for particle size below 10 nm because of extension of Debye length throughout the grain.
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Stability of sensor film against various factors like temperature, age, humidity, etc. is very important. The presence of ambient humidity greatly influences the gas response characteristics of semiconductor metal oxide based sensor. The effect of humidity on the semiconductor metal oxide sensor depends on many factors viz. chemical compounds, nature of the solvent used in synthesis [26], and chosen dopants [27]. In this paper, we report a simple and fast method to synthesize nanosized SnO2 with special reference of the influence of firing temperature on the films optical, electrical and sensing properties. The stability of the sensors resistance with respect to time and humidity is also reported. The response for low H2 concentration (1–20 ppm) was studied. 2. Experimental techniques 2.1. Preparation of the solution for spin coating The solution was prepared at room temperature by partially dissolving 2 g stannous chloride (SnCl2 ), (Ranbaxy, AR Grade) into 8 ml of distilled water and stirred for half an hour. Then 4 ml of glacial acetic acid (Ranbaxy, AR Grade) was added into this SnCl2 mixture and stirred again in a closed beaker for 1 h at 90 ◦ C. The mixture brought to room temperature and was filtered to get a yellowish transparent solution. For thermal analysis, the SnCl2 solution was dried at 80 ◦ C for 12 h and obtained the powder. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the obtained powder was carried out using TA instruments (SDT-Q-600). The analyses were performed at a heating rate of 10 ◦ C min−1 in air flow and for the temperature between 50 and 990 ◦ C. 2.2. Preparation of the SnO2 films with different firing temperatures The substrates were pre-cleaned using a chromic acid solution, followed by rinsing with distilled water. Tin oxide (SnO2 ) films were prepared by spinning 0.5 ml of the prepared solution on a sodalime glass substrates (1 cm × 1 cm) using spin coating technique at a speed of 2000 rpm. The dried samples were heated at different temperatures between 400 and 600 ◦ C in step of 50 ◦ C. Additionally, the deposition was done on different substrates like quartz, silicon and corning glass. The firing temperature for these substrates was chosen to be 550 and 1000 ◦ C. In all the cases, the heating and cooling rate was kept to be 5 ◦ C min−1 . The firing of the film was carried out at 400 ◦ C to avoid the presence of unwanted species related to Sn. The development of micro-structural properties is expected to take place at the firing temperature of 400–600 ◦ C. 2.3. Physico-chemical analysis of the films fired at different temperatures The low angle X-ray diffraction (LAXRD) of fired tin oxide films was carried out using X-ray diffractometer (Bruker AXS D8 Advance ´˚ at grazing angle of model) using Cu K␣ radiation ( = 1.5418 A) 1◦ . Microstructure analysis of the film was carried out using AFM (JSPM-5200) and UV–VIS spectra of the film were recorded on spectrophotometer (Jasco V-670). 2.4. Gas sensing-firing temperature dependence and sensor specifications The gas sensing study of the fired films (fired at different temperatures) was carried out in an indigenously developed static system having chamber capacity of 26 l similar to that described by Ansari
et al. [16]. The gas concentration in the chamber increases by incorporating more gas through syringe. It may be noted that all the measurements were carried out in the atmospheric pressure. The sensing performance was observed only for individual gas and not the mixture of gases. The sensor response is defined as the ratio of the sample resistance in air (Rair ) to H2 gas ambient (Rgas ) at the same operating temperature. Rs =
Rair Rgas
The optimal operating temperature is defined as the one that offers highest response (Rs ) at a particular concentration of hydrogen (300 ppm was used for all samples). The response was studied for 1–20 ppm of H2 concentration. The response time, defined as the time required for a sensor to attain the 90% of change in resistance on exposure to a test gas. The recovery time is the time taken to get back 90% of the original resistance in air by opening the chamber to atmosphere. The measurement of the response and recovery time was done for 8 ppm of hydrogen. Further, dc resistance of sensor film (in air) as a function of operating temperature was measured to check the stability of the sensor resistance. The study was done under uncontrolled humidity conditions for six months and also under controlled relative humidity range of RH = 20–100%. The relative humidity is controlled by injecting humid or dry air in test chamber. The change in RH% is measured directly using a calibrated hygrometer (DIEHL-Thermotord Hgro). The data collected was for six samples. Similarly the stability in the form of dc resistance of the sensor film in air as a function of time was also measured at the optimal operating temperature of the sensor (265 ◦ C). In this part, the temperature at different places inside the test chamber was also measured and found to be 35 ± 5 ◦ C (except very near the sensor ∼ a few cm). For selectivity study the response towards carbon monoxide (CO) and liquefied petroleum gas (LPG) gases was measured at the optimal operating temperature of sensor. The experiments were repeatedly carried out at least three times for three different samples. The experimental error is calculated by taking the percentage of standard deviation in the measured response. 3. Results and discussion 3.1. Thermal analysis TGA and DTA results of the precursor material are shown in Fig. 1. TGA-curve consists of three different weight loss 7.2 wt%, 18.3 wt% and 13.4 wt% respectively at temperatures ranged 30–150 ◦ C, 150–200 ◦ C and 200–400 ◦ C. The corresponding DTA shows two exothermic peaks one is at 221 ◦ C (broader one) and the second sharp peak at 399 ◦ C. There was no significant weight loss beyond 400 ◦ C. The weight loss in the temperature range 30–150 ◦ C is due to the evaporation of free adsorbed water from the surface of the particles, which corresponds to an endothermic peak observed in the DTA spectra around 61 ◦ C. The second weight lost at the temperature range of 150–200 ◦ C ascribed to dehydration process to form SnCl2 . The third weight loss corresponds to decomposition of SnCl2 to form SnO2 and the corresponding exothermic peak is observed at 399 ◦ C. 3.2. X-ray diffraction analysis Fig. 2 shows XRD of spin coated films deposited on sodalime glass, fired at temperatures of 400, 550 and 600 ◦ C. All the observed peaks are corresponding to tetragonal phase of SnO2 as confirmed by the standard JCPDS data (No. 72-1147). The crystallite
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Fig. 1. Thermal (TG–DTA) analysis of the precursor materials.
size (D) from XRD data was calculated using the Scherer formula ´˚ is the X-ray wave length (D = 0.9/(ˇ cos )) [28]. Here (1.5418 A) where as ˇ is half width at full maxima (HWFM) of the XRD peaks. ˇ was obtained by fitting the peak data to a Gaussian, considering all the three peaks observed at (2 = 26.6, 33.8 and 51.7) and the variation of average crystallite size with firing temperature is given. The calculated average crystallite size varies from 5 to 16 nm for films fired at temperature of 400–600 ◦ C (see inset in Fig. 2). Below 400 ◦ C, the film shows almost amorphous structure. SnO2 powder, with different phases and particle size of 9 nm, is synthesized using two steps thermal decomposition method. The powder is reported to be explicitly synthesized from tin di-acetate [19–21]. In the present single step thermal decomposition method, tin oxide is formed directly (in film form) from tin chloride exclusively with tetragonal phase and TG–DTA shows no weight gain indicating no
formation of tin di-acetate in single step thermal decomposition method to synthesis tin oxide. 3.3. AFM analysis
Average crystallite size (nm)
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The micrographs obtained from AFM (Fig. 3a–c) show that all films (fired at different firing temperatures on soda-lime glass) are having bimodal size distribution of nanoparticles. The entire film surface is uniformly covered with small nano-sized particles (≤3 nm) which are grown on relatively bigger globules. The small grains are increasing in number with respect to firing temperature but highest in numbers in case of the films fired at 550 ◦ C. Apparently, a high surface roughness is observed in case of films fired at 550 ◦ C and that might be due to the existence of the maximum number of small grains (≤3 nm). When film is fired at 600 ◦ C, the
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Fig. 2. X-ray diffractogram of the spin coated SnO2 film on soda lime glass fired at (a) 400, (b) 550 and (c) 600 ◦ C for 40 min. The inset shows variation of average crystallite size with firing temperature.
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Fig. 3. Three dimensional views of AFM image for nano SnO2 deposited on soda-lime substrate at firing temperature: (a–c) 400 ◦ C, 550 ◦ C and 600 ◦ C respectively, whereas (d and e) on Si substrate at the firing temperature of 550 ◦ C and 1000 ◦ C respectively.
small grains are seen to be relatively less in number. AFM images clearly show that as the firing temperature increases, the size of the bigger globules (which could work as conducting base for the small grains) increases and they are having the size of 7, 13 and 18 nm for films of firing temperatures 400, 550 and 600 ◦ C respectively. To confirm the formation of the bimodal SnO2 system, films were deposited on silicon substrate fired at 550 and 1000 ◦ C. The AFM (Fig. 3d) micrographs of 550 ◦ C film shows similar results to the earlier cases (Fig. 3a–c) i.e. small grains are grown on relatively bigger spherical particles. It may be noted that the bigger globules on silicon are larger than those observed on glass substrate. Films deposited on silicon substrate fired at 1000 ◦ C (Fig. 3e), show no small grains located on the observed globules. These results, with the reduction of the small grains on soda-lime glass film fired at 600 ◦ C (Fig. 3c), indicate that with further increase in firing temperature ≥600 ◦ C, the smaller grains start immerging into the bigger globules and get completely immerged into globules at 1000 ◦ C. 3.4. Optical absorption spectra and firing temperature UV–Vis absorption spectra of nano-SnO2 films deposited on soda-lime glass substrates and fired at different temperatures (400–600 ◦ C), are shown in Fig. 4. The thickness of the films was measured to be ∼100–200 nm. In all the cases, the UV–Vis spectra show a strong optical absorption “peak” at ∼295 nm (∼4.2 eV, blue shifted corresponding to band gap energy of ∼3.6 eV). The film fired at 400 ◦ C shows relatively highest UV absorption peak which slightly decreased in intensity for films fired at 450 ◦ C. A drastic reduction with rather broad UV absorption peak is observed in case of films fired at 500–600 ◦ C. A Gaussian fitting was done to the UV absorption peaks and area under curves was calculated. The calculation shows that the area reduced from 94% for film fired 450 ◦ C to less than 40% for film fired at 600 ◦ C.
A strong UV absorption peak, with slight change in peak position, is observed when the deposition was done on quartz (Q1 ) and corning (C1 ) glass substrate (Fig. 4 inset). The films were fired at 550 ◦ C. Such a peak was observed by Dharmadhikari et al. [29] and Kumbhojkar et al. [30] for semiconductor nano-sized ZnS [28,29]. They have attributed it to the small size of the nanoclusters which is less than ZnS Bohr exciton radius (˛B = 2.5nm). Quantum dots or nanoclusters, having sizes comparable to bulk Bohr exciton radius (˛B ), exhibit discrete electron energy levels with high oscillator strength [30]. Lee et al. [31] reported a broad UV absorbance peak for SnO2 having nanoparticles radius of 1.5 nm which is smaller than the calculated Bohr exciton radius (˛B = 2.7nm) of SnO2 . In the present case with the help of effective mass approximation [31] the size of the small grains that observed in AFM images was estimated using the above optical absorption data. The estimated size (diameter) was found to be ≈2.7 nm. Apparently, the density of the small grains (≤3) slightly increases with firing temperature and reach maximum on films fired at 550 ◦ C. The UV absorption peaks could be because of the existence of these small grains (radius ∼1.4 nm) which grow on the bigger globules (as seen in AFM). The radius of the bigger globules on the film fired at 400 ◦ C is ≈3.5 nm which is of the order of SnO2 Bohr exiton radius. This radius size could probably make, along with the small grains, the films having a remarkable narrow size distribution (highest intensity in UV absorption peak) [32]. The broader UV absorption peaks that were recorded for films fired at temperature of 500 ◦ C and above could be related to size variation of the system i.e. bigger size of bigger globules (radius ≥ 60 nm) and small grains (radius ≥ 1.4 nm). In case of higher firing temperatures (≥600 ◦ C), the small grains start immerge into the bigger globules followed by reduction in the number of small grains and leads to a relatively bigger size with smoother surface of the bigger globules (Fig. 3e). As compared with films deposited on quartz substrate fired at 550 and 1000 ◦ C (inset of Fig. 4), the UV–Vis spectra of the film fired at
S.F. Bamsaoud et al. / Materials Chemistry and Physics 133 (2012) 681–687 S = Soda lime glass Q = Quartz C = Corning glass o 1 = 550 C o 2 = 1000 C
Absorbance (a.u)
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λ (nm) Fig. 4. UV–Vis of spin coated tin oxide films with different firing temperatures (400–600 ◦ C) on soda-lime. Inset gives substrate effect: Q1 -quartz, C1 -corning glass, S1 -soda lime glass for film fired at 550 ◦ C and Q2 -quartz at 1000 ◦ C.
1000 ◦ C (Q2 ) shows a small hump and no strong optical absorption peak at this firing temperature. The recorded UV–Vis data for film on quartz substrate fired at 1000 ◦ C could be because of the absence of small grains on the bigger globules. 3.5. Electrical characterization and resistive sensor performance 3.5.1. Optimization of operating temperature The main interest in the present work is to study the sensor performances for low concentration (1–20 ppm) of H2 . In our earlier work, we reported sensing of H2 in the wider concentration range [14]. Sensing properties of the fired films were checked for 300 ppm of H2 injected at the start into the test chamber. Fig. 5 shows gas sensing response (Rs ), for all films fired at different temperatures and at operating temperature range of (30–350 ◦ C). The inset gives data corresponding to the optimized film (fired at 550 ◦ C) for 1 ppm H2 . In all the cases, the highest response of films with different firing temperatures was observed at operating temperature of ∼265 ◦ C. Almost, the same optimal operating temperature is reported earlier for pure SnO2 based hydrogen
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Temperature ( C) Fig. 5. Sensing response of spin coated nano-particulate SnO2 film, fired at temperatures 400–600 ◦ C for H2 (300 ppm), variation in response with operating temperature. Inset is sensing response of film fired at 550 ◦ C for H2 (1 ppm).
gas sensors prepared by screen printing [33] and sol–gel spin coating [34]. These sensors were having particle size of 50 nm and 7.7 nm respectively. The lowest response (Rs = 17) was recorded for films fired at 400 ◦ C. Sensor response is increasing with firing temperature and reach the highest value (Rs = 24) for films fired at 550 ◦ C. With further increasing in firing temperature the response reduced to (Rs = 24) which was recorded for films fired at 600 ◦ C. Such a change in sensor response (Rs ) is attributed to the changes in particle size through surface to volume ratio and the enhancement in defect states present on the surface of film [16,17]. Variation of firing temperature shows a change in the distribution of bimodal system through two effects. First is the increase of globule size and second is on the number of small grains (Fig. 3). Higher is the number of smaller grains, better is the sensor response, higher is size of bigger globules, lesser is sensor response. From AFM studies (Fig. 3) it is clear that variation of firing temperature (400–500 ◦ C) results in increasing in the number of smaller grains up to 500 ◦ C. At higher temperature (≥600 ◦ C), the smaller grains start merging into bigger globules resulting in reduction in their number and sequentially decreasing sensor response. The highest number of smaller grains on films fired at 550 ◦ C make the sensor able to sense even 1 ppm at the operating temperature of 265 ◦ C (Rs is ∼1.15 (inset of Fig. 5)). The response of the present sensor is higher than our earlier reports [21] where the particle size of the sensing material was large (46 ± 2 nm) and response was reported for 10 ppm as lowest detectable concentration of H2 . The films with highest response were chosen to study further electrical properties (with optimum firing temperature 550 ◦ C and optimal operating temperature 265 ◦ C).
3.5.2. Sensitivity curve Fig. 6 shows concentration dependence of sensor response (Rs ) for six samples fired at the optimized firing temperature (550 ◦ C, operating temperature 265 ◦ C and at RH = 40–50%). The sensitivity increases almost linearly with the increase of the gas concentration from 1 to 20 ppm of H2 . The sensitivity of the sensor (the slope of the response curve) in H2 concentration (1–20) is found to be ∼90 × 10−3 . The linear increase in the sensitivity to low concentration of hydrogen may be attributed to the availability of sufficient number of sensing sites on the film due to the small grains to act upon the hydrogen.
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S1 S2 S3 S4 S5 S6
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removed is found to be ∼2 s. these measurements of response and recovery time of the sensor were recorded for 8 ppm of H2 .
3.5.3. Response and recovery time The dynamic response of the sensor was also investigated and the response characteristics to H2 are shown in Fig. 7. One can see that the time taken by the sensor element to reach 90% of the maximum response at optimum operating temperature of 265 ◦ C is about 6 s. The time taken by the sensor to come back once H2 gas is
3.5.4. Stability of the sensor resistance in air (Rair ) for sample fired at 550 ◦ C: effect of operating temperature and humidity Stability is recorded in terms of sensor resistance (Rair ) as a function of operating temperature over the period of six months
14 12 Rair (G Ohm)
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Fig. 7. Variation in the response due to exposure of H2 (8 ppm) at the optimal operating temperature 265 ◦ C and with RH = 40–50%.
Fig. 6. Sensor response (Rs ) with respect to hydrogen concentration (1–20 ppm) for six samples (S1 –S6 ).
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Fig. 8. Sensor resistance of the SnO2 film at different temperature (a) period of six months, (b) different relative humidity (RH = 20–95%) and (c) operating temperature of 265 ◦ C and RH = 90% for 10 h.
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~ 40 ppm of different gases 6
Response (R s)
5 4 3 2 1 0 H2
LPG
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Fig. 9. Sensor response of 40 ppm of H2 , CO and LPG at the operating temperature 265 ◦ C and RH = 40–50%.
(Fig. 8a). The samples were tested in normal air, enclosed in the glass dome for six months with uncontrolled changes in relative humidity (found to be in the range of 40–90%) during the period of study. All the samples show NTC behavior, however at initial stage (up to 50 ◦ C), small increase in resistance was noted which might be due to desorption of water [16]. During the mentioned period, the sensors show large variation in resistance at lower operating temperatures (35–150 ◦ C). As the operating temperature increases this change in sensor resistance reduces and almost vanishes above 210 ◦ C. Clearly, the sensor resistance shows stability above the operating temperature of 210 ◦ C in spite of the change in relative humidity. The change in the electrical resistance of the samples at low operating temperatures may be mainly attributed to the adsorption/desorption of surrounding water molecules during storage (related to ionic conduction of H+ and OH– ). To confirm the stability of sample resistance, the electrical resistance of the sensors was studied at different controlled relative humidity (RH %) for the operating temperatures range (30–325 ◦ C, see Fig. 8b). The resistances of the films change remarkably with change in RH% at operating temperature <150 ◦ C and negligibly above 210 ◦ C. Fig. 8c shows sensor resistance variations with time (10 h). The experiments was done at the optimal operating temperature (265 ◦ C) and in presence of RH = 90%. It was observed that initial at the first half an hour, there is decrease (with respect to the calculated average value of the resistance) in resistance value. Then it is getting stabilized for the remaining tested period. The sensor shows no long-time drift in its resistance, so does not upset the sensor calibration and it becomes possible to detect hydrogen at low concentration [6]. From the above observations (effect of humidity and aging), it can be concluded that, the sensors show stable results in their resistance values at the operating temperature of 265 ◦ C. The sensor was also tested for ∼40 ppm of LPG and CO individually at 265 ◦ C. The sensor response towards ∼40 ppm of H2 , LPG and CO was found to be 5.73 ± 2%, 1.47 ± 4% and 1.37 ± 2% respectively (Fig. 9). The sensor shows a negligible response towards the test gases (CO and LPG) comparing to four orders in sensor response magnitude towards H2 . The negligible achieved sensitivity towards CO and LPG is exhibiting a highly selective nature of the sensor towards H2 . 4. Concluding remarks Spin coated sensor film of nano-SnO2 with bimodal nanosize particle distribution were obtained through thermal
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decomposition of SnCl2 using a simple and fast single step technique. The deposition was mainly done on soda-lime glass using the decomposition/annealing temperature of 400–600 ◦ C. The films fired at 550 ◦ C exhibit the highest response towards 300 ppm of H2 and they showed a linear sensing response to H2 for low concentration (1–20 ppm) at the optimal operating temperature of 265 ◦ C. At this operating temperature, the sensor did not show any substantial response to CO, LPG, humidity and aging other than H2 . However, at low operating temperature (50 ◦ C) the sensor showed good response to humidity and therefore can be used as humidity sensor at low operating temperatures. The resistive and sensing properties are seen to be related to choice of firing and operating temperature. They are in turn governed by bi-modal particle structure which is obtained by the novel single step thermal decomposition method. Acknowledgments The author Salim Bamsaoud is grateful to Dr. Omar Abdullah Bamahsoon, Kingdom of Saudi Arabia for the encouragements and also the partial financial support to carry out the research work at Pune University, Pune, India. References [1] S.H. Park, Y.C. Son, W.S. Willis, S.L. Suib, K.E. Creasy, Chem. Mater. 10 (1998) 2389–2398. [2] A. Gurav, T. Kodasa, T. Pluym, Y. Xiong, Aerosol Sci. Technol. 19 (1993) 411–452. [3] M.N. Rahaman, Ceramic Processes and Sintering, 2nd ed., Marcel Dekker, New York, 2003, p. 102. [4] S. Shukla, P. Zhang, H.J. Cho, L. Ludwig, S. Seal, Int. J. Hydrogen Energy 33 (2008) 470–475. [5] G. Shuping, X. Jing, L. Jianqiao, Z. Dongxiang, Sens. Actuators B 134 (2008) 57–61. [6] A.Z. Adamyan, Z.N. Adamyan, V.M. Aroutiouniana, A.H. Arakelyan, K.J. Touryan, J.A. Turner, Int. J. Hydrogen Energy 32 (2007) 4101–4108. [7] J. Kaur, V.D. Vankar, M.C. Bhatnagar, Sens. Actuators B 133 (2008) 650–655. [8] P.W. Park, H.H. Kung, D.W. Kim, M.C. Kung, J. Catal. 184 (1999) 440–454. [9] T. Minami, Semicond. Sci. Technol. 20 (2005) S35–S44. [10] Y. Wang, F. Su, J.Y. Lee, X.S. Zhao, Chem. Mater. 18 (2006) 1347–1353. [11] S.J. Han, B.C. Jang, T. Kim, S.M. Oh, T. Hyeon, Adv. Funct. Mater. 15 (2005) 1845–1850. [12] E. Elangovan, K. Ramamurthi, J. Optoelectron Adv. Mater. 5 (2003) 45–54. [13] N.G. Park, M.G. Kang, K.S. Ryu, K.M. Kim, S.H. Chang, J. Photochem. Photobiol. A161 (2004) 105–110. [14] S.F. Bamsaoud, S.B. Rane, R.N. Karekar, R.C. Aiyer, Sens. Actuators B 153 (2011) 382–391. [15] M. Law, H. Kind, B. Messer, F. Kim, P.D. Yang, Angew. Chem. Int. Ed. 41 (2002) 2405–2408. [16] S.G. Ansari, P. Boroojerdian, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, S.K. Kulkarni, Thin Solid Films 295 (1997) 271–276. [17] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators B 3 (1991) 147–155. [18] S.G. Ansari, S.W. Gosavi, S.A. Gangal, R.N. Karekar, R.C. Aiyer, J. Mater. Sci.: Mater. Electron. 8 (1997) 23. [19] C. Agashe, A.D. Garaje, R.C. Aiyer, Int. J. Appl. Ceram. Technol. 5 (2008) 181–187. [20] A.D. Garje, R.C. Aiyer, Sensor Lett. 4 (2006) 380–387. [21] A.D. Garje, R.C. Aiyer, Int. J. Appl. Ceram. Technol. 4 (2007) 446–452. [22] P.S. More, Y.B. Khollam, S.B. Deshpande, S.R. Sainkar, S.K. Date, R.N. Karekar, R.C. Aiyer, Mater. Lett. 57 (2003) 2177–2184. [23] M. Batzill, Sensors 6 (2006) 1345–1366. [24] Z. Ling, C. Leach, R. Freer, Sens. Actuators B 87 (2002) 215–221. [25] A. Rothschild, Y. Komem, J. Appl. Phys. 95 (2004) 6374–6380. [26] G. Korotchenkov, V. Brynzari, S. Dmitriev, Sens. Actuators B 54 (1999) 197–201. [27] V.V. Malyshev, A.V. Pislyakov, Sens. Actuators B 123 (2007) 71–81. [28] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company, Reading, MA, 1956. [29] A.K. Dharmadhikari, N. Kumbhojkar, J.A. Dharmadhikari, S. Mahamuni, R.C. Aiyer, J. Phys.: Condens. Matter 11 (1999) 1363–1368. [30] N. Kumbhojkar, V.V. Nikesh, A. Kshirsagar, S. Mahamuni, J. Appl. Phys. 88 (2000) 6260–6264. [31] E.J.H. Lee, C. Ribeiro, T.R. Giraldi, E. Longo, E.R. Leite, Appl. Phys. Lett. 84 (2004) 1745–1747. [32] N.S. Pesika, K.J. Stebe, P.C. Searson, J. Phys. Chem. B 107 (2003) 10412–10415. [33] J. Ahna, J. Kimb, J. Park, M. Huh, Sens. Actuators B 99 (2004) 18–24. [34] J.W. Hammond, C. Liu, Sens. Actuators B 81 (2001) 25–31.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
SnO2 film with bimodal distribution of nano-particles for low concentration hydrogen sensor: Effect of firing temperature on sensing properties Salim F. Bamsaoud a,b,∗ , S.B. Rane c,∗∗ , R.N. Karekar d , R.C. Aiyer b,∗ ∗ ∗ a
Microwave and Thin Film Lab, Department of Physics, University of Pune, Pune 411007, India Center for Sensor Studies, Department of Electronics, University of Pune, Pune 411007, India Center for Materials for Electronics Technology, Off Pashan Road, Pune 411008, India d RCMF Transducer Lab, Department of Electronics, University of Pune, Pune 411007, India b c
a r t i c l e
i n f o
Article history: Received 11 July 2011 Received in revised form 7 January 2012 Accepted 18 January 2012 Keywords: Tin oxide Nano particles Single step thermal decomposition Hydrogen sensor
a b s t r a c t Spin coated sensor films of nano-SnO2 with bimodal nano-size particle distribution were obtained through thermal decomposition of SnCl2 using a simple and fast single step thermal decomposition technique. The deposition was done on soda-lime glass substrates using variable decomposition/firing temperature in the range of 400–600 ◦ C at the interval of 50 ◦ C. The effect of firing temperature on structural, micro-structural and optical properties was studied by X-ray diffractometer (XRD), atomic force microscopy (AFM) and UV–Vis respectively. AFM study of the films revealed a special type of microstructure i.e. conducting larger globules of 8–20 nm with smaller nanoparticles of ≤3 nm on the surface of the bigger globules. The films fired at 400 ◦ C exhibit the highest UV absorption peak compared to other samples. A highest response (Rair /Rgas = 24) to 300 ppm of H2 is recorded for films fired at 550 ◦ C and they exhibit sensitivity of ∼90 × 10−3 for concentration of H2 (1–20 ppm). These sensors also exhibit good stability of resistance at the optimized operating temperature of 265 ◦ C and in the presence of different relative humidity (RH = 20 − 100%). © 2012 Elsevier B.V. All rights reserved.
1. Introduction Currently a variety of techniques are being used for film deposition to get nano structures for different applications. Technology involvement in film deposition comes in the picture in order to achieve high reproducibility, simplicity, flexibility and preferably low cost. For scientific purposes and industrial applications, proper selection of deposition method is vital. Spray pyrolysis [1–3] and sol–gel (dip-coating and sol–gel spin coating) [4–7] are widely used in industrial applications because of simplicity and no requirement of highly vacuum based sophisticated equipments. In spray pyrolysis, a thin film is deposited by spraying a solution on a heated substrate where the constituents react to form a chemical compound. It is a single step process. To get required quality of films, the concentration of spray solution, input pressure of carrier gas, substrate temperature and distance between substrate and spray gun are to be optimized. Sol gel process gets its name from the two major stages involved: formation of a sol and subsequent
∗ Corresponding author. Tel.: +91 20 2569 2678; fax: +91 20 2569 1684. ∗∗ Corresponding author. Tel.: +91 20 2589 9273; fax: +91 20 2589 8180. ∗ ∗ ∗Corresponding author. E-mail addresses: [email protected] (S.F. Bamsaoud), [email protected] (S.B. Rane). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.01.052
attainment of a semi-rigid gel. The semi-rigid gel is used to deposit thin films by either dip [4,5] or spin-coating [6,7]. Tin oxide films with nano grain size are prepared by different deposition techniques and have been extensively studied for various applications, including catalyst support [8], transparent conducting electrodes [9], Li-ion battery anode materials [10,11], storage applications, solar cells [12,13] and gas sensors [14–23]. Tin oxide (SnO2 ) based gas sensors have been extensively investigated, after the charge-carrier concentration on the metal oxide surface response towards the composition of the surrounding atmosphere was understood [24]. This seems to depend on the surrounding atmospheric oxygen in the form of various chemical species; the pre-adsorbed oxygen molecules are present on the surface of the metal oxide. These chemical species of oxygen are formed when the atmospheric oxygen reacts with the metal oxide by taking electrons from its conduction band, resulting in increase in the resistance of metal oxide [23]. The response of the material to chemical species depends on microstructure of the sensing material, especially on the grain size [16,17,25]. Ansari et al. [16] have reported that H2 sensing response increases with decrease in size of SnO2 based thick film sensors where the crystallite size of the powder was in the range of 20–45 nm. Xu et al. [17] have reported that the gas response of SnO2 based fired elements increases sharply for particle size below 10 nm because of extension of Debye length throughout the grain.
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Stability of sensor film against various factors like temperature, age, humidity, etc. is very important. The presence of ambient humidity greatly influences the gas response characteristics of semiconductor metal oxide based sensor. The effect of humidity on the semiconductor metal oxide sensor depends on many factors viz. chemical compounds, nature of the solvent used in synthesis [26], and chosen dopants [27]. In this paper, we report a simple and fast method to synthesize nanosized SnO2 with special reference of the influence of firing temperature on the films optical, electrical and sensing properties. The stability of the sensors resistance with respect to time and humidity is also reported. The response for low H2 concentration (1–20 ppm) was studied. 2. Experimental techniques 2.1. Preparation of the solution for spin coating The solution was prepared at room temperature by partially dissolving 2 g stannous chloride (SnCl2 ), (Ranbaxy, AR Grade) into 8 ml of distilled water and stirred for half an hour. Then 4 ml of glacial acetic acid (Ranbaxy, AR Grade) was added into this SnCl2 mixture and stirred again in a closed beaker for 1 h at 90 ◦ C. The mixture brought to room temperature and was filtered to get a yellowish transparent solution. For thermal analysis, the SnCl2 solution was dried at 80 ◦ C for 12 h and obtained the powder. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the obtained powder was carried out using TA instruments (SDT-Q-600). The analyses were performed at a heating rate of 10 ◦ C min−1 in air flow and for the temperature between 50 and 990 ◦ C. 2.2. Preparation of the SnO2 films with different firing temperatures The substrates were pre-cleaned using a chromic acid solution, followed by rinsing with distilled water. Tin oxide (SnO2 ) films were prepared by spinning 0.5 ml of the prepared solution on a sodalime glass substrates (1 cm × 1 cm) using spin coating technique at a speed of 2000 rpm. The dried samples were heated at different temperatures between 400 and 600 ◦ C in step of 50 ◦ C. Additionally, the deposition was done on different substrates like quartz, silicon and corning glass. The firing temperature for these substrates was chosen to be 550 and 1000 ◦ C. In all the cases, the heating and cooling rate was kept to be 5 ◦ C min−1 . The firing of the film was carried out at 400 ◦ C to avoid the presence of unwanted species related to Sn. The development of micro-structural properties is expected to take place at the firing temperature of 400–600 ◦ C. 2.3. Physico-chemical analysis of the films fired at different temperatures The low angle X-ray diffraction (LAXRD) of fired tin oxide films was carried out using X-ray diffractometer (Bruker AXS D8 Advance ´˚ at grazing angle of model) using Cu K␣ radiation ( = 1.5418 A) 1◦ . Microstructure analysis of the film was carried out using AFM (JSPM-5200) and UV–VIS spectra of the film were recorded on spectrophotometer (Jasco V-670). 2.4. Gas sensing-firing temperature dependence and sensor specifications The gas sensing study of the fired films (fired at different temperatures) was carried out in an indigenously developed static system having chamber capacity of 26 l similar to that described by Ansari
et al. [16]. The gas concentration in the chamber increases by incorporating more gas through syringe. It may be noted that all the measurements were carried out in the atmospheric pressure. The sensing performance was observed only for individual gas and not the mixture of gases. The sensor response is defined as the ratio of the sample resistance in air (Rair ) to H2 gas ambient (Rgas ) at the same operating temperature. Rs =
Rair Rgas
The optimal operating temperature is defined as the one that offers highest response (Rs ) at a particular concentration of hydrogen (300 ppm was used for all samples). The response was studied for 1–20 ppm of H2 concentration. The response time, defined as the time required for a sensor to attain the 90% of change in resistance on exposure to a test gas. The recovery time is the time taken to get back 90% of the original resistance in air by opening the chamber to atmosphere. The measurement of the response and recovery time was done for 8 ppm of hydrogen. Further, dc resistance of sensor film (in air) as a function of operating temperature was measured to check the stability of the sensor resistance. The study was done under uncontrolled humidity conditions for six months and also under controlled relative humidity range of RH = 20–100%. The relative humidity is controlled by injecting humid or dry air in test chamber. The change in RH% is measured directly using a calibrated hygrometer (DIEHL-Thermotord Hgro). The data collected was for six samples. Similarly the stability in the form of dc resistance of the sensor film in air as a function of time was also measured at the optimal operating temperature of the sensor (265 ◦ C). In this part, the temperature at different places inside the test chamber was also measured and found to be 35 ± 5 ◦ C (except very near the sensor ∼ a few cm). For selectivity study the response towards carbon monoxide (CO) and liquefied petroleum gas (LPG) gases was measured at the optimal operating temperature of sensor. The experiments were repeatedly carried out at least three times for three different samples. The experimental error is calculated by taking the percentage of standard deviation in the measured response. 3. Results and discussion 3.1. Thermal analysis TGA and DTA results of the precursor material are shown in Fig. 1. TGA-curve consists of three different weight loss 7.2 wt%, 18.3 wt% and 13.4 wt% respectively at temperatures ranged 30–150 ◦ C, 150–200 ◦ C and 200–400 ◦ C. The corresponding DTA shows two exothermic peaks one is at 221 ◦ C (broader one) and the second sharp peak at 399 ◦ C. There was no significant weight loss beyond 400 ◦ C. The weight loss in the temperature range 30–150 ◦ C is due to the evaporation of free adsorbed water from the surface of the particles, which corresponds to an endothermic peak observed in the DTA spectra around 61 ◦ C. The second weight lost at the temperature range of 150–200 ◦ C ascribed to dehydration process to form SnCl2 . The third weight loss corresponds to decomposition of SnCl2 to form SnO2 and the corresponding exothermic peak is observed at 399 ◦ C. 3.2. X-ray diffraction analysis Fig. 2 shows XRD of spin coated films deposited on sodalime glass, fired at temperatures of 400, 550 and 600 ◦ C. All the observed peaks are corresponding to tetragonal phase of SnO2 as confirmed by the standard JCPDS data (No. 72-1147). The crystallite
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Fig. 1. Thermal (TG–DTA) analysis of the precursor materials.
size (D) from XRD data was calculated using the Scherer formula ´˚ is the X-ray wave length (D = 0.9/(ˇ cos )) [28]. Here (1.5418 A) where as ˇ is half width at full maxima (HWFM) of the XRD peaks. ˇ was obtained by fitting the peak data to a Gaussian, considering all the three peaks observed at (2 = 26.6, 33.8 and 51.7) and the variation of average crystallite size with firing temperature is given. The calculated average crystallite size varies from 5 to 16 nm for films fired at temperature of 400–600 ◦ C (see inset in Fig. 2). Below 400 ◦ C, the film shows almost amorphous structure. SnO2 powder, with different phases and particle size of 9 nm, is synthesized using two steps thermal decomposition method. The powder is reported to be explicitly synthesized from tin di-acetate [19–21]. In the present single step thermal decomposition method, tin oxide is formed directly (in film form) from tin chloride exclusively with tetragonal phase and TG–DTA shows no weight gain indicating no
formation of tin di-acetate in single step thermal decomposition method to synthesis tin oxide. 3.3. AFM analysis
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The micrographs obtained from AFM (Fig. 3a–c) show that all films (fired at different firing temperatures on soda-lime glass) are having bimodal size distribution of nanoparticles. The entire film surface is uniformly covered with small nano-sized particles (≤3 nm) which are grown on relatively bigger globules. The small grains are increasing in number with respect to firing temperature but highest in numbers in case of the films fired at 550 ◦ C. Apparently, a high surface roughness is observed in case of films fired at 550 ◦ C and that might be due to the existence of the maximum number of small grains (≤3 nm). When film is fired at 600 ◦ C, the
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Fig. 2. X-ray diffractogram of the spin coated SnO2 film on soda lime glass fired at (a) 400, (b) 550 and (c) 600 ◦ C for 40 min. The inset shows variation of average crystallite size with firing temperature.
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Fig. 3. Three dimensional views of AFM image for nano SnO2 deposited on soda-lime substrate at firing temperature: (a–c) 400 ◦ C, 550 ◦ C and 600 ◦ C respectively, whereas (d and e) on Si substrate at the firing temperature of 550 ◦ C and 1000 ◦ C respectively.
small grains are seen to be relatively less in number. AFM images clearly show that as the firing temperature increases, the size of the bigger globules (which could work as conducting base for the small grains) increases and they are having the size of 7, 13 and 18 nm for films of firing temperatures 400, 550 and 600 ◦ C respectively. To confirm the formation of the bimodal SnO2 system, films were deposited on silicon substrate fired at 550 and 1000 ◦ C. The AFM (Fig. 3d) micrographs of 550 ◦ C film shows similar results to the earlier cases (Fig. 3a–c) i.e. small grains are grown on relatively bigger spherical particles. It may be noted that the bigger globules on silicon are larger than those observed on glass substrate. Films deposited on silicon substrate fired at 1000 ◦ C (Fig. 3e), show no small grains located on the observed globules. These results, with the reduction of the small grains on soda-lime glass film fired at 600 ◦ C (Fig. 3c), indicate that with further increase in firing temperature ≥600 ◦ C, the smaller grains start immerging into the bigger globules and get completely immerged into globules at 1000 ◦ C. 3.4. Optical absorption spectra and firing temperature UV–Vis absorption spectra of nano-SnO2 films deposited on soda-lime glass substrates and fired at different temperatures (400–600 ◦ C), are shown in Fig. 4. The thickness of the films was measured to be ∼100–200 nm. In all the cases, the UV–Vis spectra show a strong optical absorption “peak” at ∼295 nm (∼4.2 eV, blue shifted corresponding to band gap energy of ∼3.6 eV). The film fired at 400 ◦ C shows relatively highest UV absorption peak which slightly decreased in intensity for films fired at 450 ◦ C. A drastic reduction with rather broad UV absorption peak is observed in case of films fired at 500–600 ◦ C. A Gaussian fitting was done to the UV absorption peaks and area under curves was calculated. The calculation shows that the area reduced from 94% for film fired 450 ◦ C to less than 40% for film fired at 600 ◦ C.
A strong UV absorption peak, with slight change in peak position, is observed when the deposition was done on quartz (Q1 ) and corning (C1 ) glass substrate (Fig. 4 inset). The films were fired at 550 ◦ C. Such a peak was observed by Dharmadhikari et al. [29] and Kumbhojkar et al. [30] for semiconductor nano-sized ZnS [28,29]. They have attributed it to the small size of the nanoclusters which is less than ZnS Bohr exciton radius (˛B = 2.5nm). Quantum dots or nanoclusters, having sizes comparable to bulk Bohr exciton radius (˛B ), exhibit discrete electron energy levels with high oscillator strength [30]. Lee et al. [31] reported a broad UV absorbance peak for SnO2 having nanoparticles radius of 1.5 nm which is smaller than the calculated Bohr exciton radius (˛B = 2.7nm) of SnO2 . In the present case with the help of effective mass approximation [31] the size of the small grains that observed in AFM images was estimated using the above optical absorption data. The estimated size (diameter) was found to be ≈2.7 nm. Apparently, the density of the small grains (≤3) slightly increases with firing temperature and reach maximum on films fired at 550 ◦ C. The UV absorption peaks could be because of the existence of these small grains (radius ∼1.4 nm) which grow on the bigger globules (as seen in AFM). The radius of the bigger globules on the film fired at 400 ◦ C is ≈3.5 nm which is of the order of SnO2 Bohr exiton radius. This radius size could probably make, along with the small grains, the films having a remarkable narrow size distribution (highest intensity in UV absorption peak) [32]. The broader UV absorption peaks that were recorded for films fired at temperature of 500 ◦ C and above could be related to size variation of the system i.e. bigger size of bigger globules (radius ≥ 60 nm) and small grains (radius ≥ 1.4 nm). In case of higher firing temperatures (≥600 ◦ C), the small grains start immerge into the bigger globules followed by reduction in the number of small grains and leads to a relatively bigger size with smoother surface of the bigger globules (Fig. 3e). As compared with films deposited on quartz substrate fired at 550 and 1000 ◦ C (inset of Fig. 4), the UV–Vis spectra of the film fired at
S.F. Bamsaoud et al. / Materials Chemistry and Physics 133 (2012) 681–687 S = Soda lime glass Q = Quartz C = Corning glass o 1 = 550 C o 2 = 1000 C
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λ (nm) Fig. 4. UV–Vis of spin coated tin oxide films with different firing temperatures (400–600 ◦ C) on soda-lime. Inset gives substrate effect: Q1 -quartz, C1 -corning glass, S1 -soda lime glass for film fired at 550 ◦ C and Q2 -quartz at 1000 ◦ C.
1000 ◦ C (Q2 ) shows a small hump and no strong optical absorption peak at this firing temperature. The recorded UV–Vis data for film on quartz substrate fired at 1000 ◦ C could be because of the absence of small grains on the bigger globules. 3.5. Electrical characterization and resistive sensor performance 3.5.1. Optimization of operating temperature The main interest in the present work is to study the sensor performances for low concentration (1–20 ppm) of H2 . In our earlier work, we reported sensing of H2 in the wider concentration range [14]. Sensing properties of the fired films were checked for 300 ppm of H2 injected at the start into the test chamber. Fig. 5 shows gas sensing response (Rs ), for all films fired at different temperatures and at operating temperature range of (30–350 ◦ C). The inset gives data corresponding to the optimized film (fired at 550 ◦ C) for 1 ppm H2 . In all the cases, the highest response of films with different firing temperatures was observed at operating temperature of ∼265 ◦ C. Almost, the same optimal operating temperature is reported earlier for pure SnO2 based hydrogen
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Temperature ( C) Fig. 5. Sensing response of spin coated nano-particulate SnO2 film, fired at temperatures 400–600 ◦ C for H2 (300 ppm), variation in response with operating temperature. Inset is sensing response of film fired at 550 ◦ C for H2 (1 ppm).
gas sensors prepared by screen printing [33] and sol–gel spin coating [34]. These sensors were having particle size of 50 nm and 7.7 nm respectively. The lowest response (Rs = 17) was recorded for films fired at 400 ◦ C. Sensor response is increasing with firing temperature and reach the highest value (Rs = 24) for films fired at 550 ◦ C. With further increasing in firing temperature the response reduced to (Rs = 24) which was recorded for films fired at 600 ◦ C. Such a change in sensor response (Rs ) is attributed to the changes in particle size through surface to volume ratio and the enhancement in defect states present on the surface of film [16,17]. Variation of firing temperature shows a change in the distribution of bimodal system through two effects. First is the increase of globule size and second is on the number of small grains (Fig. 3). Higher is the number of smaller grains, better is the sensor response, higher is size of bigger globules, lesser is sensor response. From AFM studies (Fig. 3) it is clear that variation of firing temperature (400–500 ◦ C) results in increasing in the number of smaller grains up to 500 ◦ C. At higher temperature (≥600 ◦ C), the smaller grains start merging into bigger globules resulting in reduction in their number and sequentially decreasing sensor response. The highest number of smaller grains on films fired at 550 ◦ C make the sensor able to sense even 1 ppm at the operating temperature of 265 ◦ C (Rs is ∼1.15 (inset of Fig. 5)). The response of the present sensor is higher than our earlier reports [21] where the particle size of the sensing material was large (46 ± 2 nm) and response was reported for 10 ppm as lowest detectable concentration of H2 . The films with highest response were chosen to study further electrical properties (with optimum firing temperature 550 ◦ C and optimal operating temperature 265 ◦ C).
3.5.2. Sensitivity curve Fig. 6 shows concentration dependence of sensor response (Rs ) for six samples fired at the optimized firing temperature (550 ◦ C, operating temperature 265 ◦ C and at RH = 40–50%). The sensitivity increases almost linearly with the increase of the gas concentration from 1 to 20 ppm of H2 . The sensitivity of the sensor (the slope of the response curve) in H2 concentration (1–20) is found to be ∼90 × 10−3 . The linear increase in the sensitivity to low concentration of hydrogen may be attributed to the availability of sufficient number of sensing sites on the film due to the small grains to act upon the hydrogen.
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0
22
60
120
H 2 consentration (ppm)
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removed is found to be ∼2 s. these measurements of response and recovery time of the sensor were recorded for 8 ppm of H2 .
3.5.3. Response and recovery time The dynamic response of the sensor was also investigated and the response characteristics to H2 are shown in Fig. 7. One can see that the time taken by the sensor element to reach 90% of the maximum response at optimum operating temperature of 265 ◦ C is about 6 s. The time taken by the sensor to come back once H2 gas is
3.5.4. Stability of the sensor resistance in air (Rair ) for sample fired at 550 ◦ C: effect of operating temperature and humidity Stability is recorded in terms of sensor resistance (Rair ) as a function of operating temperature over the period of six months
14 12 Rair (G Ohm)
0.10
10
R air (G Ohm)
240
Fig. 7. Variation in the response due to exposure of H2 (8 ppm) at the optimal operating temperature 265 ◦ C and with RH = 40–50%.
Fig. 6. Sensor response (Rs ) with respect to hydrogen concentration (1–20 ppm) for six samples (S1 –S6 ).
a
180
Time (Sec)
8
0.05
6 150
4
180
210
240
270
300
330
Temperature(T)
2 0 50
100
150
200
250
300
0
120
350
Temperature (ºC)
10.0 9.5 9.0 8.5 8.0 7.5
120
80
40
7.0 6.5
c R (M Ohm)
Rair (G Ohm)
b
RH % 20-30% 40-50 % 60-70 % 80-90% 90-100%
10.5
0
50
100
150
200
250
Temperature (ºC)
300
350
240
360
480
600
Time (min)
Fig. 8. Sensor resistance of the SnO2 film at different temperature (a) period of six months, (b) different relative humidity (RH = 20–95%) and (c) operating temperature of 265 ◦ C and RH = 90% for 10 h.
S.F. Bamsaoud et al. / Materials Chemistry and Physics 133 (2012) 681–687
~ 40 ppm of different gases 6
Response (R s)
5 4 3 2 1 0 H2
LPG
CO
Fig. 9. Sensor response of 40 ppm of H2 , CO and LPG at the operating temperature 265 ◦ C and RH = 40–50%.
(Fig. 8a). The samples were tested in normal air, enclosed in the glass dome for six months with uncontrolled changes in relative humidity (found to be in the range of 40–90%) during the period of study. All the samples show NTC behavior, however at initial stage (up to 50 ◦ C), small increase in resistance was noted which might be due to desorption of water [16]. During the mentioned period, the sensors show large variation in resistance at lower operating temperatures (35–150 ◦ C). As the operating temperature increases this change in sensor resistance reduces and almost vanishes above 210 ◦ C. Clearly, the sensor resistance shows stability above the operating temperature of 210 ◦ C in spite of the change in relative humidity. The change in the electrical resistance of the samples at low operating temperatures may be mainly attributed to the adsorption/desorption of surrounding water molecules during storage (related to ionic conduction of H+ and OH– ). To confirm the stability of sample resistance, the electrical resistance of the sensors was studied at different controlled relative humidity (RH %) for the operating temperatures range (30–325 ◦ C, see Fig. 8b). The resistances of the films change remarkably with change in RH% at operating temperature <150 ◦ C and negligibly above 210 ◦ C. Fig. 8c shows sensor resistance variations with time (10 h). The experiments was done at the optimal operating temperature (265 ◦ C) and in presence of RH = 90%. It was observed that initial at the first half an hour, there is decrease (with respect to the calculated average value of the resistance) in resistance value. Then it is getting stabilized for the remaining tested period. The sensor shows no long-time drift in its resistance, so does not upset the sensor calibration and it becomes possible to detect hydrogen at low concentration [6]. From the above observations (effect of humidity and aging), it can be concluded that, the sensors show stable results in their resistance values at the operating temperature of 265 ◦ C. The sensor was also tested for ∼40 ppm of LPG and CO individually at 265 ◦ C. The sensor response towards ∼40 ppm of H2 , LPG and CO was found to be 5.73 ± 2%, 1.47 ± 4% and 1.37 ± 2% respectively (Fig. 9). The sensor shows a negligible response towards the test gases (CO and LPG) comparing to four orders in sensor response magnitude towards H2 . The negligible achieved sensitivity towards CO and LPG is exhibiting a highly selective nature of the sensor towards H2 . 4. Concluding remarks Spin coated sensor film of nano-SnO2 with bimodal nanosize particle distribution were obtained through thermal
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decomposition of SnCl2 using a simple and fast single step technique. The deposition was mainly done on soda-lime glass using the decomposition/annealing temperature of 400–600 ◦ C. The films fired at 550 ◦ C exhibit the highest response towards 300 ppm of H2 and they showed a linear sensing response to H2 for low concentration (1–20 ppm) at the optimal operating temperature of 265 ◦ C. At this operating temperature, the sensor did not show any substantial response to CO, LPG, humidity and aging other than H2 . However, at low operating temperature (50 ◦ C) the sensor showed good response to humidity and therefore can be used as humidity sensor at low operating temperatures. The resistive and sensing properties are seen to be related to choice of firing and operating temperature. They are in turn governed by bi-modal particle structure which is obtained by the novel single step thermal decomposition method. Acknowledgments The author Salim Bamsaoud is grateful to Dr. Omar Abdullah Bamahsoon, Kingdom of Saudi Arabia for the encouragements and also the partial financial support to carry out the research work at Pune University, Pune, India. References [1] S.H. Park, Y.C. Son, W.S. Willis, S.L. Suib, K.E. Creasy, Chem. Mater. 10 (1998) 2389–2398. [2] A. Gurav, T. Kodasa, T. Pluym, Y. Xiong, Aerosol Sci. Technol. 19 (1993) 411–452. [3] M.N. Rahaman, Ceramic Processes and Sintering, 2nd ed., Marcel Dekker, New York, 2003, p. 102. [4] S. Shukla, P. Zhang, H.J. Cho, L. Ludwig, S. Seal, Int. J. Hydrogen Energy 33 (2008) 470–475. [5] G. Shuping, X. Jing, L. Jianqiao, Z. Dongxiang, Sens. Actuators B 134 (2008) 57–61. [6] A.Z. Adamyan, Z.N. Adamyan, V.M. Aroutiouniana, A.H. Arakelyan, K.J. Touryan, J.A. Turner, Int. J. Hydrogen Energy 32 (2007) 4101–4108. [7] J. Kaur, V.D. Vankar, M.C. Bhatnagar, Sens. Actuators B 133 (2008) 650–655. [8] P.W. Park, H.H. Kung, D.W. Kim, M.C. Kung, J. Catal. 184 (1999) 440–454. [9] T. Minami, Semicond. Sci. Technol. 20 (2005) S35–S44. [10] Y. Wang, F. Su, J.Y. Lee, X.S. Zhao, Chem. Mater. 18 (2006) 1347–1353. [11] S.J. Han, B.C. Jang, T. Kim, S.M. Oh, T. Hyeon, Adv. Funct. Mater. 15 (2005) 1845–1850. [12] E. Elangovan, K. Ramamurthi, J. Optoelectron Adv. Mater. 5 (2003) 45–54. [13] N.G. Park, M.G. Kang, K.S. Ryu, K.M. Kim, S.H. Chang, J. Photochem. Photobiol. A161 (2004) 105–110. [14] S.F. Bamsaoud, S.B. Rane, R.N. Karekar, R.C. Aiyer, Sens. Actuators B 153 (2011) 382–391. [15] M. Law, H. Kind, B. Messer, F. Kim, P.D. Yang, Angew. Chem. Int. Ed. 41 (2002) 2405–2408. [16] S.G. Ansari, P. Boroojerdian, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, S.K. Kulkarni, Thin Solid Films 295 (1997) 271–276. [17] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators B 3 (1991) 147–155. [18] S.G. Ansari, S.W. Gosavi, S.A. Gangal, R.N. Karekar, R.C. Aiyer, J. Mater. Sci.: Mater. Electron. 8 (1997) 23. [19] C. Agashe, A.D. Garaje, R.C. Aiyer, Int. J. Appl. Ceram. Technol. 5 (2008) 181–187. [20] A.D. Garje, R.C. Aiyer, Sensor Lett. 4 (2006) 380–387. [21] A.D. Garje, R.C. Aiyer, Int. J. Appl. Ceram. Technol. 4 (2007) 446–452. [22] P.S. More, Y.B. Khollam, S.B. Deshpande, S.R. Sainkar, S.K. Date, R.N. Karekar, R.C. Aiyer, Mater. Lett. 57 (2003) 2177–2184. [23] M. Batzill, Sensors 6 (2006) 1345–1366. [24] Z. Ling, C. Leach, R. Freer, Sens. Actuators B 87 (2002) 215–221. [25] A. Rothschild, Y. Komem, J. Appl. Phys. 95 (2004) 6374–6380. [26] G. Korotchenkov, V. Brynzari, S. Dmitriev, Sens. Actuators B 54 (1999) 197–201. [27] V.V. Malyshev, A.V. Pislyakov, Sens. Actuators B 123 (2007) 71–81. [28] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company, Reading, MA, 1956. [29] A.K. Dharmadhikari, N. Kumbhojkar, J.A. Dharmadhikari, S. Mahamuni, R.C. Aiyer, J. Phys.: Condens. Matter 11 (1999) 1363–1368. [30] N. Kumbhojkar, V.V. Nikesh, A. Kshirsagar, S. Mahamuni, J. Appl. Phys. 88 (2000) 6260–6264. [31] E.J.H. Lee, C. Ribeiro, T.R. Giraldi, E. Longo, E.R. Leite, Appl. Phys. Lett. 84 (2004) 1745–1747. [32] N.S. Pesika, K.J. Stebe, P.C. Searson, J. Phys. Chem. B 107 (2003) 10412–10415. [33] J. Ahna, J. Kimb, J. Park, M. Huh, Sens. Actuators B 99 (2004) 18–24. [34] J.W. Hammond, C. Liu, Sens. Actuators B 81 (2001) 25–31.