Reliability studies of highly sensitive and specific multi-gas sensor based on nanocrystalline SnO2 film

Sensors and Actuators B 193 (2014) 484–491

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Reliability studies of highly sensitive and specific multi-gas sensor based on nanocrystalline SnO2 film C.A. Betty ∗ , Sipra Choudhury, K.G. Girija Chemistry Division, BARC, Trombay, Mumbai 400085, India

a r t i c l e

i n f o

Article history: Received 12 September 2013 Received in revised form 25 November 2013 Accepted 29 November 2013 Available online 7 December 2013 Keywords: Sensor reliability Multi-gas sensor Nanocrystalline SnO2 film Room temperature sensor SnO2 humidity effect

a b s t r a c t We had reported the real time detection of NH3 , H2 S and NO2 —each with specific transient response, using nanocrystalline thin film SnO2 sensor at room temperature in presence of other interfering gases, with sensitivity required for air quality monitoring. To ensure the reliable and continuous operation of the room temperature SnO2 thin film sensor, further sensing studies have been carried out under various deleterious conditions such as high humidity, in presence of various gas dissociation products and aging. The sensor did not show any change in the response towards high humidity levels up to 97% suggesting the reliability of the sensor. While SnO2 sensor showed recovery at room temperature with carrier gas flow after exposure to NH3 and H2 S, NO2 required heating up to 70 ◦ C for gas desorption. To explore the room temperature gas desorption possibilities after NO2 exposure, deNOx ification studies using NH3 injection was carried out. It showed only a transient effect on the sensor recovery. Sensitivity studies over a period of one year towards NH3 gas indicated the same sensitivity with an increase in response and recovery time indicating the durability of the sensor. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Due to the increasing growth of industries that uses various gases as raw materials/byproducts and the subsequent hazards, the detection and quantification of gaseous species in air at low cost is essential. These toxic gases cause environmental hazards such as acid rain, photochemical smog, and corrosion. To prevent or minimize the damage caused by atmospheric pollution, monitoring and controlling systems are needed that can rapidly and reliably detect and quantify pollution sources within the range of the regulating standard values. Some of the gaseous species to be observed are nitrous oxide (NO), nitrogen dioxide (NO2 ), hydrogen sulfide (H2 S), sulfur dioxide (SO2 ) and ammonia (NH3 ). Over the past few decades solid state gas sensors based on polycrystalline SnO2 thin films have become predominant for gas alarms in domestic, commercial and industrial premises [1–3]. Thin films of SnO2 make it possible to prepare devices with small size and low power consumption, which can easily be integrated in an array. Gas sensing generally involves a catalytic reaction (e.g., oxidation or reduction) of the gas or vapor by the surface of the sensor. Therefore, selective detection of each reducing/oxidizing gas is cumbersome. The key to success in developing a single functional gas sensing device is the technology of masking undesired functions

∗ Corresponding author. Tel.: +91 22 2559 0288; fax: +91 22 2550 5151. E-mail addresses: [email protected], betty [email protected] (C.A. Betty). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.11.118

such as cross sensitivity with other gas species. So far, selectivity has been achieved by chemically functionalizing the surface or by operating the sensor at corresponding elevated temperatures where the reaction is highly dominant. Sensors operated at high temperature, though specific, suffer from shorter life time and high thermal budget. Gas sensors for detecting air pollutants must be able to operate stably under deleterious conditions, including chemical and/or thermal attack. Besides in an explosive environment, the high-temperature operation of these oxide sensors is not favorable. Therefore room temperature operation is preferable, however, in the case of commercially available polycrystalline thin film sensors the bottleneck for room temperature operation lies in the dominant grain boundary barrier (Schottky diode) resistance. The physical properties limiting sensor devices fabricated in polycrystalline thin film can be readily overcome by exploiting nanoscale structures. Many techniques were adapted to deposit thin SnO2 films which are expected to have ultrahigh sensitivity and low temperature operation when the thickness of the active material is reduced to the order of Debye length (several nm) [4,5]. Nanowires with its well defined crystallographic directions as crystal surfaces and high crystallinity can be ideal choice, however, not many studies report room temperature operation. Gas response study of SnO2 nanowire and nanobelt (diameter 10–100 nm), at room temperature report lesser sensitivity for H2 S than their polycrystalline counter parts operating at about 150 ◦ C [6]. Yang et al. report maximum response (60% change) for 500 ppm CO at 250 ◦ C using SnO2 single crystal rutile phase nanowires of diameter about 100 nm [7].

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Recently sensing of reducing and oxidizing gases at room temperature has been reported on nanostructured SnO2 thin films (particle size 10–20 and 100 nm) wherein the sensing process depended on the morphology [8]. The proposed sensing mechanism is based on the partial or complete switching of the nanoparticle depletion regions on interaction with reducing or oxidizing gases. The room temperature NO2 sensor response and recovery of single crystalline nanoribbons requires only an external stimulus such as UV light exposure or passing of carrier gas [9,10]. Mani and Rayappan [11] reported highly selective room temperature NH3 sensing using spray deposited ZnO thin films attributing the high sensitivity to the increased surface area. However, for most of these sensors the underlying reasons for room temperature sensing phenomenon and high selectivity are not discussed. From the surface spectroscopic and X-ray diffraction studies on the geometric effects of SnO2 polycrystalline materials, Schierbaum et al. [12,13] have suggested models for explaining the high sensitivity observed for polycrystalline SnO2 against reducing gases. In the case of polycrystalline material, the resistance of the material depends on the barrier (of the order of twice the Debye length) between the grains. They have stated that Schottky barrier mechanism of the electron transport across the grain boundaries is valid only for grains larger than the Debye length of the electrons (micro granular film). For grains smaller than the Debye length (nano granular film) band bending at the surface of the grains transforms to overall shift of Fermi level in the grains. Shift of the Fermi levels suggests a larger resistivity of the material which changes sharply upon trace gas interaction resulting in higher sensitivity [14,15]. Since sensing is mainly a surface phenomena, the surface adsorbed species of SnO2 interact with reducing gases at high temperature leading to desorption of oxygen resulting in an increase in the conductivity of the film [1–4]. With oxidizing gases conduction band electrons are further removed resulting in a decrease in the conductivity of the film. Therefore the selective detection of target gases was a problem area. The method of preparation of the sensor material plays a role in manifesting the sensor characteristics. We had earlier reported that thin SnO2 films prepared by Langmuir Blodgett (LB) technique on quartz substrate detect ammonia, NO2 , H2 S and SO2 at room temperature with high sensitivity and selectivity [16–18]. The response was selective to each gas in the sense that while other reducing gases such as H2 S and SO2 showed an increase in current, our sensor showed a decrease in current for NH3 . Among many other gases studied only NO2 (which is a highly oxidizing gas) showed a decrease in current. We had also reported the distinctly different impedance responses towards the respective gases at room temperature in the presence of other residual gases using a single sensor without any complex electronics. There have been many attempts to detect various gases sensitively and selectively, however, many of these efforts have not yet reached commercial viability because of long term maintenance, cross sensing and stability issues. Many factors can lead to instability, like grain-size growth due to the high operating temperature, poisoning due to unknown chemical species or by the gas dissociation products, diffusion processes within the device, humidity effects etc. Metal interdiffusion in the sensor connections and between the catalysts and the other metallic parts of the sensors due to high operating temperature has been demonstrated to be a major cause of instability [3]. Compensation of time-variant ambient effects, such as temperature and chemical species, requires continuous monitoring of these effects and on-line correction of the sensor behavior. Majority of these problems could be avoided if the sensor operates at room temperature or at low temperature. Nanostructures, especially low temperature operated nanostructured thin film sensors are known to show instability effects due to surface contamination and humidity with time. Some of the contaminants of the sensor which one encounters while using gas

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Fig. 1. Schematic of the experimental setup.

sensor are due to H2 O and the target gas decomposition products like carbon, CO2 , SO3 and NOx . Surface poisoning of SnO2 affects the surface adsorption of the target gases and their subsequent detection [19]. It is reported that together with atmospheric oxygen the presence of humidity greatly influences the gas detection of bulk and nanostructured thin film sensors. Varghese and Malhotra have carried out a detailed study on humidity effects of ultrafine grained (4.5 and 9 nm) SnO2 thin films of thickness 180 nm [2]. In this study the SnO2 thin films have shown strong interaction with water vapour in the atmosphere at room temperature even at low humidity levels such as 5–40% RH. Extensive AC impedance and DC current studies carried out on SnO2 monocrystalline nanowires for CO and humidity sensing at various temperatures have indicated humidity effects even at 50% RH [20]. Accordingly, it is important to understand the role of water vapour in the sensing mechanism and operating life of SnO2 thin films consisting of nanocrystallites of the size less than twice the Debye length. Since the device is operating at room temperature, the present study addresses the issues of reliability in the response of the device over longer period due to humidity, cross sensitivity and aging effects. Besides, the recovery of our sensor towards H2 S and NH3 requires only ambient air flow. However, it requires heating at 70 ◦ C for the fast recovery after NO2 exposure. In order to have a room temperature operation for both the response and recovery, the present work also attempts to enhance the deNOx efficiency using NH3 –NO/NO2 reaction at room temperature. 2. Materials and methods SnO2 nanoparticle hin films were prepared on quartz substrates using LB technique, the details of preparation and characterization were found elsewhere [16,17]. The multilayer LB films were heated at 600 ◦ C in air to form SnO2 . FESEM micrographs of the SnO2 films were recorded using FESEM Hitachi, S-4700. For cross sectional SEM Model JEOL-JSM-6360, Japan was employed. Gas sensors were fabricated by depositing interdigitated gold electrodes using a shadow mask (5 electrodes each side) with 0.5 mm electrode width and 0.2 mm separation between the electrodes on SnO2 nanoparticle film. The sensors were fixed in a home made gas testing chamber (volume ∼350 ml) made of black painted glass with a cone-socket arrangement. Provisions were made using stop cocks for gas injection/release and evacuation as shown in Fig. 1. Sensor response of the films towards the target gases have been obtained by current vs. time measurement using Keithley electrometer. Impedance data have been measured over a frequency range of 1 Hz to 1 MHz using small AC signal of 20 mV and DC bias of 0.1 V using a frequency response analyzer FRA2 attached with a

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influence on the surface properties. The carrier concentration calculated from Mott–Schottky plot is ∼4 × 1016 cm−3 [17] which gives Debye length of ∼12 nm. The sizes of the nanocrystallites (∼8 nm from XRD, Ref. [16] and nano-grain size ∼20 nm as seen from Fig. 2a), which constitute the thin LB film, are less than twice the Debye length and therefore the grains are almost depleted of mobile charges by the surface adsorbed oxygen species. Though the film thickness is ∼250 nm, the layered structure of the nano-grain film ensures the possibility of the switching behavior of the layers from the partly depleted (during interaction with SO2 , H2 S) to totally depleted of carriers state (NH3 , NO2 ) upon exposure to target gases. Hence the major contribution towards bulk resistance of the nanograined thin film is from the charge carrier depleted nanograins and their grain boundaries. Thus the overall change in electrical conductivity of the thin film is dependent on the concentration of oxidizing and reducing agents in the surrounding atmosphere, the size of the SnO2 grains, size of the mesopores in the film which allows the diffusion of gases and the surface states due to the nonstoichiometry. 3.1. Humidity effects on the sensor performance

Fig. 2. (a) FESEM micrograph of mesoporous, laterally connected, nano-grained SnO2 thin film. The inset shows the size of the nano-grain as 20 nm. (b) Crosssectional SEM micrograph of nano-grained SnO2 thin film. The film thickness is ∼250 nm.

potentiostat (PGSTAT20, Echo Chemie, the Netherlands). The impedance spectra were recorded from 10 Hz to 1 MHz (in 7 min). To study the time dependent behaviour of the sensor structures towards different gases, measurements were carried out at two typical frequencies 477 Hz and 700 Hz. All impedance measurements were carried out at room temperature (25 ◦ C) and at atmospheric pressure without any carrier gas flow. In the start of the measurements stop cocks of the chamber containing ambient air were kept open. For each measurement, a base line was recorded under ambient air conditions. The stop cocks were closed after equilibration and known amount of various gases (NH3 , NO2 and H2 O) were injected using gas tight syringes. Measurements done immediately after injections are referred as ‘during’ in the impedance plots. The chamber was kept closed till the sensor response saturated, after which the stop cocks were opened for recovery. 3. Results and discussion The electrical conductivity of the polycrystalline SnO2 films depends mainly on the grain size, crystallinity and nonstoichiometry of the film. Fig. 2a shows the FESEM micrograph of the mesoporous, grain-to-grain connected SnO2 and layered thin film indicating the nano-grain size as about 20 nm. Cross sectional SEM micrograph of 113 multilayers of sintered nano-grained SnO2 film is shown in Fig. 2b with film thickness of ∼250 nm. XPS studies [16,17] on the thin SnO2 films have indicated the presence of oxygen vacancies on the surface which is due to nonstoichiometry. From the XPS spectrum about 31% weight has been estimated for adsorbed oxygen related surface states indicating a large

Detection of target gases using SnO2 thin film is generally carried out by measurement of DC current. Since SnO2 thin film is a polycrystalline material, the kinetics and thermodynamics of the target gas interaction mechanism is determined by the following factors: the presence of defects at the crystallite boundaries which constitute the grains, defects at the grain boundaries, the crystallite size and grain size, defects at the film surface–electrode contact interface, surface-reception properties and the adsorbate gas induced effects. Individual contributions of these components can only be discerned by AC impedance techniques since impedance response of a particular kinetic phenomenon depends on its characteristic time constants. We have observed that there is no measurable effect in DC or AC measurements due to injection of water vapour under ambient conditions in the chamber containing nano-grained thin film SnO2 sensor. Absence of any change in DC and AC impedance after injection of water into the chamber at ambient conditions suggests high reliability of the nano-grained thin film SnO2 sensor prepared by LB method. Simultaneously it is also presumed that in presence of ambient air injected water may remain either at the mouth of the stopcock or at the bottom of the chamber. To create an ambient infused with water vapour surrounding the sensor, the chamber was evacuated first to make the chamber free of physisorbed particles such as dust, air and water vapour etc. On subsequent water vapour injection, it is assumed that under vacuum there is no condensation taking place at the stop cocks. No significant change in impedance was observed till the humidity inside the chamber reached about 98%. This further confirms the superiority of the nano-grained thin film SnO2 sensors compared to other thin film sensors [2,3,6,7]. At about 99% relative humidity (i.e., when 30 ␮l water was dispersed under vaccum in the chamber and the chamber was left as it is without disturbance) the onset of impedance effect appeared after about 24 h (not shown) and disappeared after air was passed. Therefore to create a worst case condition, required amount of water vapour (70 ␮l) was injected to achieve super saturation conditions (relative humidity (RH) greater than 100% [21]). Fig. 3 represents the typical impedance response of sensor structure under 100% relative humidity conditions. The specific impedance response as in Fig. 3 was observed only when the relative humidity reached more than 99%. The impedance response under high humidity conditions indicates additional features such as diffusion in addition to the kinetically controlled charge transfer process (represented originally by R2 parallel Q1 as reported in Ref. [18]). Thus the first

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Fig. 3. (a) Effect of water vapour on ultrathin film SnO2 sensor under supersaturation conditions, measured (symbols) and fitted (line) data. Inset of the figure shows the equivalent circuit used for fitting the data. (b) Imaginary impedance vs. frequency plots of time dependent humidity effects.

semicircle in Fig. 3 represents kinetically controlled charge transfer process and the second semicircle in the low frequency range represents diffusion process. The inset shows the fitting of the impedance spectra, with an equivalent circuit model which had an additional R3 parallel W (Warburg impedance) representing the diffusion of transient ionic species across the grain boundaries in series with the original R2 parallel Q1 . Fig. 3b represents the imaginary impedance vs. frequency showing two peaks with the second peak appearing after 30 min. of water injection. The high frequency peak at around 30 kHz gives relaxation time constant of 5.4 ␮s and the low frequency peak around 25 Hz gives 6.2 ms. These findings are similar to that reported in the case of SnO2 thin films with ultrafine grains where the diffusion phenomena appears at relatively high humidity levels [2]. At room temperature and in ordinary atmospheric conditions, the highly active surface of a nanostructured tin oxide film is covered with physisorbed as well as chemisorbed gas species with the majority of these species being oxygen and water. Surface adsorbed species (O− ,O2 − ,OH among which O2 − is the predominant species at room temperature) on SnO2 thin/thick film surface results in the

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strong electro negativity of the surface [22,23]. Nano-grains have major fraction of the atoms on the surface as the surface states rather than the bulk states. We are suggesting that the large number of surface states (as estimated from the XPS spectrum, Ref. [8]) and suitable surface-reception properties give rise to the electro negativity on the surface. Compared to other nanocrystallite SnO2 thin films reported for gas sensing, the major difference for LB films is that these are porous films with ordered and aligned nano-grains in the form of layers, the size of the nano-grains of the order of Debye lengths, without any amorphous region between grains. The unique morphology of the layered and laterally connected nano-grains in the film as observed in the FESEM micrograph and data from XPS suggest that these films consists of large fraction of surface states compared to bulk, resulting in strong surface electro negativity. During chemisorptions of the adsorbates (Oxygen species) on the film surface, they saturate the dangling bonds of the surface atoms resulting in charge transfer between the adsorbates and surface atoms. This transfer manifests itself as an additional dipole field, the strength and direction of which determines the change in the electron affinity. The effect of electro negativity contributes to the width of the band gap. Pan et al. [24] have proposed general formulas for the electro negativity and bond ionicity of the binary compound semiconductors with the corresponding influences on the band gap. We have already reported about the increased band gap (3.95 eV) estimated from the optical absorption studies (reported in Ref. [9]) for the SnO2 thin film prepared by the LB method. This larger band gap as compared to the 3.6 eV reported for other thin SnO2 films suggest higher electro negativity of the film surface. The strong electro negativity of the nano-grained SnO2 thin film surface could be the reason for the absence of humidity effect even at relatively high humidity conditions. We have reported the absence of any response by thin SnO2 films towards H2 , O2 , CH4 , CO and NO [18]. On the other hand, the strong response of these sensors towards NH3 at ambient conditions due to electrostatic attraction of electropositive polar covalent molecule NH3 (partial positive charge on Hydrogen) which is a weak reducing gas at room temperature suggests the strong electro negativity of the surface [6]. Whereas, strong interaction observed with electro negative polar covalent molecules such as SO2 , H2 S and NO2 is due to strong reducing or oxidizing nature of these gases. SnO2 surface electro negativity also supports the fact that there was no sensitivity towards non-polar covalent H2 , O2 , CH4 , and less polar covalent CO, NO being weak reducing/oxidizing gases. Similarly, the strong electro negative SnO2 surface could repel the covalent H2 O molecules which are neither reducing nor oxidizing but negatively polarized (due to partial negative charge on Oxygen), leading to no significant effect (for RH < 99%) due to H2 O molecules at low concentrations. However, when water gets diffused into the film (when they are forced on to the surface by suction and the subsequent slow diffusion into the mesopores) at very high humidity levels, the humidity effects are observed. It is to be noted that only the nano-grained SnO2 films prepared by LB technique shows this insensitivity towards humidity effects suggesting better reliability of the sensor. The possible mechanism of water interaction on SnO2 film surface prepared by LB technique is discussed below. In presence of water vapour (when water gets diffused inside the nano-grained thin film under very high humidity conditions), chemisorption of water molecules can take place at room temperature [2]. The negatively charged oxygen of the water molecule gets attached to the positively charged cation site and one of the positively charged hydrogen atoms to the anionic site by electrostatic attraction forming two chemisorbed hydroxyl (OH) groups. The chemisorbed water can act as a donor and therefore, inject electrons into the conduction band of SnO2 as well as reduce the coverage of adsorbed oxygen species at the surface. Hence, the depletion layer between the nanocryatllites gets reduced and overall impedance

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a

b Fig. 5. AC impedance response of the sensor towards NO2 and NH3 observed at a frequency 700 Hz. Up arrows indicate the opening of the stop cock. Down arrows indicate the closing of the stop cocks followed by injection of respective gases.

Fig. 4. (a) DC response of nano-garined thin SnO2 sensor upon injection of NH3 . Yaxis represents the current ratio. Up arrows indicate the opening of the stop cock to let in the ambient air. (b) Imaginary impedance vs. frequency of nano-grained SnO2 thin film sensor before and after 6 ppm NH3 injection.

decreases radically. The appearance of second low frequency semicircle with a peak at 25 Hz indicates that the ions formed are able to diffuse through the grain boundaries towards the electrodes. Varghese and Malhotra [2] suggest that the second semicircle could be due to the free movement of H+ ions which is the dominant charge carrier in the physisorbed layer at high humidity levels. It is also known that at medium frequencies, the mobile ions will be able to move across the grain boundaries [25,26]. The size of the first semicircle as well as the second one reduces slowly with time indicating increased diffusion and reaction of water molecules with surface adsorbed species. Subsequent to water vapour exposure it is observed that recovery of the sensor structure takes place slowly by passing air or requires a low temperature (70 ◦ C, 2 h) heating for fast recovery. 3.2. Specific sensing of NH3 in presence of NO2 and deNOxification for room temperature recovery Fig. 4a shows that under equilibrium ambient conditions, NH3 response is a transient increase in DC current. 90% recovery took place within 30 min just by opening the stop cocks of the gas testing set up. The fast and reversible response of the sensor at room temperature towards NH3 with decrease in current, suggests a

relatively weak interaction with SnO2 surface. Even though the interaction is weak between the surface adsorbed NH3 and the SnO2 surface, it can affect the mobility of the charge carriers by increasing the charge carrier scattering resulting in a reduction in the current. The detailed chemical phenomenon that takes place on nano-grained SnO2 thin film surface during interaction with NH3 is explained in Ref. [18]. Normally in SnO2 thin films which is made of polycrystalline material, the grain–grain boundary play the major role where the activation energy for adsorption and subsequent desorption depends on the grain-grain barrier height and therefore it operates at higher temperature in order to overcome the barrier potential. The room temperature sensing of NH3 observed for nanocrystallite SnO2 thin films (Fig. 4a) indicates that a new type of physiochemical phenomenon might be responsible for this extraordinary gas sensing behavior. Such a type of gas sensing is possible via the passivation of dangling bonds or incomplete covalent bonds, or the available fast surface states caused by the large surface to volume ratio in nanocrystal systems. The target gases are likely to form temporary bonds with nanocrystal surface vacancies created due to the nonstoichiometric oxygen content. These bonds act as bridges for electron transfer between the gas and nanocrystal surface [27]. Besides, when the size of the crystals become the order of few Debye lengths, there is a an overall upward shift in the conduction band minimum and valence band maximum with a flat band condition for the grain boundaries instead of the Schotky-type barriers observed in the case of large grains [28]. The flat band condition between the grains indicates that at room temperature itself the SnO2 film is able to react with target gases whereas Schotkytype barrier would have warranted higher barrier activation energy provided by higher temperature. The imaginary impedance vs. frequency plots obtained before (ωmax = 868.5 Hz,  = 0.18 ms) and after 6 ppm NH3 (ωmax = 281.2 Hz,  = 0.57 ms) injection is shown in Fig. 4b. The activation energy estimated from the peak frequencies for NH3 sensing is 29.2 meV, confirming the room temperature sensing and recovery of the sensor. When the target gas interacts with the nano-grained thin SnO2 film surface, the electrochemical reactions that are taking place can be observed in the frequency range of 100–1000 Hz. Therefore we have chosen frequency <1000 Hz for impedance measurements and thus Fig. 5 represents the transient impedance response (real Z vs. time) of the sensor for NO2 and NH3 gas interactions, respectively,

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Fig. 6. AC impedance response of sensor to NH3 before and after NO2 interaction measured at 477 Hz. Down arrows indicate the closing of the stop cocks followed by injection of respective gases while up arrows indicate the opening of the stop cock.

measured at 700 Hz. Herein, first NO2 was injected whose interaction produced a fast impedance transient followed by a slow impedance increase as reported in Ref. [18]. However, when NH3 was injected before the recovery of the sensor from NO2 exposure we observed a faster transient with a decrease in impedance which is contrary to the normal response of our thin film sensor towards NH3 (Fig. 4a). Here also the recovery is faster with opening the stop cocks. Even though both gases (NH3 and NO2 ) when injected individually had shown an increase in impedance due to interaction with nano-grained thin SnO2 sensor, in presence of NO2 interaction products, NH3 show a different behavior as shown in Figs. 5 and 6. More specifically, we have earlier reported that the NO2 gas gives an impedance response (at <1000 Hz frequency) which involves a transient as seen in Fig. 5 that disappears within 300 s with a subsequent slow increase in impedance [18]. The impedance study over a range of frequencies (10 Hz to 1 MHz) had further shown indications of chemisorption effects which involve charge transfer between the film surface and NO2 gas, forming a transient species initially that transforms into stable new species. The study had helped to identify the transient species as NO with subsequent stable species formation of either [NO2 ] bridged with bridging oxygen vacancy or NO3 formation with O− surf . From the present study we have concluded that in presence of NO2 bridged oxygen vacancy the reaction with NH3 could be similar to the selective catalytic reduction reported on NO/NO2 NH3 systems with Vanadium or Iron based catalyst which are used for deNOx ification of diesel engines [29]. We have discussed the reaction mechanism based on the above presumption. After ammonia injection, it reacts with [NO2 ] bridged with oxygen vacancy, [NO2 ]− , releasing electrons which leads to the decrease in impedance as seen in Fig. 5. 2NH3 + 2[NO2 ]− → NH4 NO3 + N2 + H2 O + 2e−

(1)

Ammonium nitrate is formed as an intermediate at low temperature which on further decomposition gives NH3 resulting in an impedance increase. NH4 NO3 ↔ NH3 + HNO3

(2)

The transient with fast decrease in impedance shown in Fig. 5 after NH3 −NO/NO2 interaction suggests that [NO2 ]− (NO2 bridged with bridging oxygen vacancy that gets formed on NO2 exposure), which is highly electro negative is more probable to NO3 since the

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Fig. 7. DC response of the ultrathin SnO2 sensor towards NH3 and NO2. Down arrows indicate the injection of respective gases except for SO2 .

interaction with NH3 (polar covalent molecule with a net positive charge) is very fast. On interaction with NH3 there is release of electrons as in reaction (1) which leads to the decrease in impedance. The product formed could be an unstable one since there is fast recovery within 30 min. The r.h.s of reaction (2) suggests further NH3 production which explains the increasing impedance. Further the consistency of the transient response with NH3 before and after NO2 interaction is shown in Fig. 6. The consistent response observed for NH3 exposure with dissociation products of NO2 (formed by NO2 interaction on SnO2 nano-grained thin film) suggests the stable species as NO2 bridged with oxygen vacancy after NO2 exposure on nano-grained thin SnO2 sensor. The study has also indicated that the deNOx ification efficiency of nano-grained sensor is not significantly affected by injection of NH3 post NO2 exposure. Similar specific gas sensing studies using DC measurement was also carried out on nano-grained thin film SnO2 sensor and the results are shown in Fig. 7 [30]. The performance of the sensor over 8 h affirms the specific gas sensitivity and stability of the sensor. With NH3 (5 ppm), same sensitivity is observed for the first two instances. However, with SO2 the current increase is very large as expected in the case of other strongly reducing gases. In this case when SO2 (polar covalent but strongly reducing) interacts with O− , SO3 is formed giving back the electrons to the SnO2 conduction band increasing the current [18]. In the case of second SO2 injection it is to be noted that even in presence of SO2 , electropositive NH3 is attracted towards the surface and the signature response of the NH3 is observed. Further with highly oxidizing but polar covalent NO2 there is decrease in the current as explained in Ref. [18]. It is to be noted that after NO2 exposure, even though the sudden decrease in AC impedance upon NH3 injection and fast recovery is observed (as seen in Figs. 5 and 6), it is less significant in DC measurements. The response shown by both AC and DC measurements confirm a change in the charge carrier density that occurs as suggested in reaction (1). 3.3. Aging effects and long term stability of the sensor The long term reliability issues include possible drifts in the characteristics and/or responses of the sensors. It is known that the gas sensing performance of SnO2 thin films get degraded with various gas dissociation products and water vapour. Today’s commercially available gas sensors are getting regenerated since the device is operating at high temperatures. Since the SnO2

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Table 1 The reliable specifications of room temperature operating nano-grained thin film sensor. Property

Specifications

Drift in base current Sensitivity (towards NH3 ) S = ((RNH3 − Rair ) × 100)/Rair Response time Recovery time (after opening of stopcocks, without any carrier gas flow) Reproducibility after a year (towards NH3 )

2–3% 10–14% at 5 ppm of NH3 40 s 30 min for 90% recovery Sensitivity intact Response time Recovery time

Specificity and Sensitivity as change in current Gas Change in current NH3 decrease H2 No change No change H2 O No change CO Increase H2 S SO2 Increase Decrease (with a unique transient NO2 species observed in AC measurements)

200 s 1h

Gas concentration 1 ppm 6 ppm <97% RH 6 ppm <100 ppb <100 ppb <85 ppb

(∼100% RH). The reduced response of the nano-grained SnO2 thin film towards (<99%) humidity is due to the strong electro negativity of the film surface resulting in electrostatic repulsion of negatively polarized covalent H2 O molecules. The finding affirms the superiority of the nano-grained thin film sensor prepared by LB method over bulk and other nanostructured thin film sensors. The equivalent circuit for the interaction of H2 O with thin SnO2 film involves an additional R parallel W which represents the diffusion of H+ ions inside the thin film. Decrease in impedance observed on interaction of NH3 with the dissociation products of NO2 suggests that NO2 bridged with oxygen vacancy as the stable species on SnO2 surface after NO2 interaction. The reproducibility of the un-passivated sensor after one year with the same sensitivity further affirms the reliability of room temperature operating nano-grained thin film SnO2 sensor.

References Fig. 8. DC response of the sensor towards NH3 recorded initially and after one year. Y-axis represents the current ratio.

nano-grained thin film operates at room temperature, the regeneration of the device is not thermally activated. Therefore it becomes essential to monitor the performance of the sensor over a period of time. Repeated measurements were carried out on a single device that was exposed to various gases, over a period of 1 year and it showed the same response to ammonia indicating the stability and reproducibility of the sensor (Fig. 8). It was found that the response and recovery times were slower after one year. Incidentally, these devices were not packaged and were exposed to the ambient during this period. Exposure of the unpackaged sensor to ambient over a long time could be responsible for the slow recovery observed after a year. The accumulation of carbon and associated gas dissociation products can clog the film surface slowing down the response the sensor. Table 1 lists the reliable specifications of the room temperature operating nano-grained thin film sensor. 4. Conclusions Room temperature operating gas sensor based on nano-grained thin SnO2 film has been studied under various humidity conditions inside the chamber and adsorbed gas species on the film. Till 98% RH, no significant change in the impedance was observed when the sensor was exposed to water vapour. It has been observed that the typical response is a decrease in impedance which occurs after about half an hour in the case of very high humidity conditions

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Biographies Dr. Mrs. Betty obtained her M.Sc. in Physics in 1987. She joined Chemistry Division, Bhabha Atomic Research Centre, Mumbai as a scientist in 1989. She received her Ph.D. in Microelectronics from IIT, Bombay in 2003. Her area of expertise includes radiation effect studies on semiconductor devices, semiconductor physics and biochemical sensors. Dr. Sipra Choudhury received her M.Sc, Chemistry in 1991. She was awarded Ph.D. degree from National Chemical Laboratory, Pune, India in 1997. She joined BARC at 2002 as scientific officer. She is currently working on Langmuir Blodgett film, SnO2 based gas sensor etc. Mrs. K.G. Girija joined Chemistry Division, Bhabha Atomic Research Centre, Mumbai in 1987. She obtained her M.Tech. in Microelectronics in 1994 from IIT, Bombay. Her area of interest includes sensors based on CVD diamond and oxides.