ZnO–SnO2 based composite type gas sensor for selective hydrogen sensing

Sensors and Actuators B 194 (2014) 389–396

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

ZnO–SnO2 based composite type gas sensor for selective hydrogen sensing Biplob Mondal a,∗∗ , Borat Basumatari a , Jayoti Das b , Chirosree Roychaudhury c , Hiranmay Saha d,∗ , Nillohit Mukherjee d a

Department of Electronics and Communication Engineering, Tezpur University, Assam 784028, India Department of Physics, Jadavpur University, Kolkata 700032, India c Department of Electronics and Telecommunication Engineering, Bengal Engineering and Science University, Kolkata 711103, India d Centre of Excellence in Green Energy and Sensors Systems, Bengal Engineering and Science University, Kolkata 711103, India b

a r t i c l e

i n f o

Article history: Received 18 August 2013 Received in revised form 19 December 2013 Accepted 22 December 2013 Available online 8 January 2014 Keywords: Hydrogen sensor ZnO–SnO2 composite material Selectivity Low temperature

a b s t r a c t This work reports the synthesis and detailed investigation on ZnO–SnO2 composite type hydrogen sensor prototype. The sensor material was structurally and morphologically characterized by X-ray diffraction technique and scanning electron microscopy, respectively. The gas sensing behaviour of the fabricated sensor prototype was investigated for varied concentration of test gases at different temperature. The cross-response of this sensor to other gases, viz. methane and carbon mono-oxide was also investigated, which showed good selectivity, excellent response and reproducibility to hydrogen at 150 ◦ C. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The oxide based semiconductor sensors have been widely used for detection of toxic, hazardous and combustible gases for the safety of human being and environmental control. Metal oxide semiconductor (MOS) in the form of micro/nano structures has received much attention because of the possible advantages like high sensitivity to small concentration of gases, low fabrication cost, long term stability and high potential to provide extremely small and low power consuming sensor. The popular sensing mechanism of metal oxide semiconductor gas sensor is based on the variation of electrical conductivity in presence and absence of the test gas. At ordinary situation, oxygen is adsorbed on the surface of metal oxide, thereby extracting electrons from the conduction band and forming a depletion layer. Upon exposure of the reducing gases, the negatively charged oxygen oxidizes the reducing gas and re-injects the electron back into the conduction band of the semiconductor. The reaction between the surface adsorbed oxygen species and target gas may result in a significant resistance change

∗ Corresponding author. Fax: +91 33 2668 2916. ∗∗ Corresponding author. E-mail addresses: [email protected] (B. Mondal), [email protected], [email protected] (H. Saha). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.12.093

which is detected to determine the presence and concentration of the test gas. It is established that metal oxide based sensors offers lots of advantages supporting its candidature as an excellent sensing material. However the selectivity of the sensor remains a challenging issue as they give similar response to a wide range of gases. Many researchers have suggested various ways to overcome the problem of cross-sensitivity in order to achieve selective sensor which may include, (a) addition of noble catalytic metal to promote the reaction to specific gas [1], (b) operating the sensor in dynamic mode [2], (c) surface modification [3] and (d) use of multi-compositional sensing film [4–13]. In this regard, composite type sensor utilizing two or multiple oxide material in single or multiple layers could prove to be an effective way to enhance the selectivity of the sensors. Indeed, various authors have reported selective and sensitive gas sensing characteristics of composite oxide material. This hybrid material includes ZnO–SnO2 nanofibers for ethanol detection [4], ZnO–Cr2 O3 nanorods for trimethylamine detection [5], CuO–SnO2 nanofibers for H2 S detection [6], TiO2 –SnO2 nanobelt for detection of VOC [7], ZnO–In2 O3 nanofibers for detection of trimethylamine [8], etc. A summary of various other composite type gas sensors along with their morphology, synthesis technique and target gases are presented in Table 1. Nevertheless, according to the knowledge of the authors, the report on such ZnO–SnO2 composite type sensor is very limited in number.

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Table 1 Summary of composite type gas sensor. Material

Morphology

Method of preparation

Target gas

Reference

ZnO–SnO2 ZnO–Cr2 O3 CuO–SnO2 TiO2 –SnO2 ZnO–In2 O3 SnO2 –ZnO, SnO2 –WO3 , ZnO–WO3 ZnO, CuO, Sm2 O3 doped SnO2 NiO–SnO2 SnO2 –In2 O3 CuO and ZnO doped SnO2

Nanofiber Nanorods Nanofiber Nanobelt Nanofiber Nanoparticle

Electrospinning Thermal evaporation Electrospinning Hydrothermal Electrospinning Combinatorial deposition Precipitation Electrospinning Electrospinning

Ethanol Trimethylamine H2 S Volatile organic compound (VOC) Trimethylamine Ethanol, acetone Chlorinated VOC Toluene Formaldehyde CO

[4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Nanofiber Nanofiber Microstructure

Both ZnO and SnO2 are n-type wide band semiconductor which have attracted remarkable attention as highly effective sensing material for gases like H2 [14,15], CO [16,17], CH4 [17,18], NO2 [19,20] and ethanol [21,22], etc. An optimal composition of ZnO and SnO2 is therefore expected to improve the selectivity and response of the sensor to a particular gas. Though, many literatures are available on hydrogen sensing by individual ZnO and SnO2 based sensor, as per our knowledge, no report is available on selective hydrogen sensing by ZnO–SnO2 composite type sensor. In this contribution, we report the synthesis of microdimensional ZnO–SnO2 composite sensor using a simple chemical route and a detailed discussion on its sensing mechanism. Focus was directed to the improvement of selectivity and response. Effect of annealing on the morphological change and sensing characteristics were also investigated and discussed.

Table 2 summarizes the annealing temperature for the sensor prototypes under study.

2. Experimental procedure

The schematic of the sensor device is shown in Fig. 1a. The sensor is designed to operate in a resistive mode which consists of ZnO–SnO2 composite material deposited over a Si/SiO2 substrate (1.5 cm × 1 cm). Pd–Ag contact electrodes were made on the top of the ZnO–SnO2 layer by e-beam evaporation technique. The contact dimension was 2 mm × 2 mm and the average distance between two lateral contacts was 5 mm.

2.1. Synthesis of ZnO–SnO2 micro/nanocomposites The ZnO–SnO2 composite sensing material was prepared by a simple chemical route using a solution of zinc acetate dihydrate (Zn(CH3 COO)2 ·2H2 O) and stannic chloride pentahydrate (SnCl4 ·5H2 O) in polyvinylalcohol (PVA) as capping agent. All the chemicals were commercially available and AR grade. To start with, 0.2 g of PVA was dissolved in 20 ml of distilled water (0.032 ␮M) and stirred continuously at 1500 rpm for 3 h while maintaining a temperature of 70–80 ◦ C, until a transparent solution was obtained. 0.3 g of zinc acetate dihydrate and 0.5 g of stannic chloride pentahydrate was added simultaneously to the PVA solution under continuous stirring at 100 ◦ C to produce a final solution of 27 ␮M Zn2+ and 28 ␮M Sn4+ in 20 ml 0.032 ␮M PVA. 1.0 M sodium hydroxide (NaOH) solution was then added drop wise until the pH of the entire solution becomes neutral. The final solution was stirred at 110–120 ◦ C for 1 h to get a homogeneous milky white solution. The resulting precursor was spin coated onto a cleaned 1 0 0 oriented Si/SiO2 substrate to form thin uniform film. In a typical procedure 0.25 ml of the solution was deposited using a micro-syringe and spun at 1200 rpm for 60 s. This process was repeated for 30 times and the substrate was dried in an oven at 110 ◦ C for 10 min. The deposited film was then annealed in air within a temperature range 400–700 ◦ C for 30 min in a quartz tube furnace to get the crystalline thin films containing the ZnO–SnO2 composite microstructures.

2.2. Characterization techniques The structure and the phase of the deposited materials (both as deposited and annealed) were determined by X-ray diffraction technique (RIGAKU-MINIFLUX), using Cu K␣1 X-radiation ˚ operated at 30 kV, 15 mA. The scan rate was ( = 1.540598 A) 1◦ /min, within 2 = 10–70◦ . The morphology of the films (S1–S4) was determined by scanning electron microscopy (JEOL JSM 6390LV, operating at 15 kV). The film thickness was determined using a surface profiler (Bruker Dektak XT). 2.3. Sensor architecture and testing method

Table 2 Sensor under study. Sensor

Annealing temperature (◦ C)

S1 S2 S3 S4

– 400 550 700

Fig. 1. Schematic of (a) sensor structure (b) sensor setup.

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The gas sensing properties of the proposed sensors were studied by two probe I–V measurements technique using custom made sensor characterization setup as shown in Fig. 1b and by varying the hydrogen gas concentration and measurement conditions, as well. The sensor was placed inside an enclosed test chamber of size ∼300 cm3 , fitted with heating arrangement that would maintain a uniform temperature (with an accuracy of ±3 ◦ C) inside the chamber. Ultra high pure hydrogen diluted with dry air is fed to the sensor prototype in appropriate concentration through Alicat® mass flow controllers (MFC). The total allowed gas flow to the test chamber was two standard litres per minute (SLPM). The humidity level was ∼70% inside the chamber before heating. However, the relative humidity was found to decrease with increasing temperature and subsequent dry air purging and at 100 and 250 ◦ C it was found to be 35 and 15, respectively. An electrometer (Agilent U1253B) was used to monitor the variation of sensor resistance. The sensor performance was examined within 1000–10 000 ppm of hydrogen gas in dry air at room temperature and also at higher temperatures (up to 300 ◦ C). 3. Results and discussion 3.1. Structural characterization by XRD The X-ray diffraction patterns of the as deposited and annealed ZnO–SnO2 composites are presented in Fig. 2a. From the XRD patterns, it is evident that the as spun film is amorphous in nature and the crystallinity of the films improves with increasing annealing temperature. Each XRD pattern of the annealed samples (S2–S4) shows the characteristic peak that matches the hexagonal phase of ZnO (JCPDS #36-1451) and tetragonal phase of rutile SnO2 (JCPDS #88-0287). It was also noted that, the polycrystalline nature of the films increased with increasing annealing temperature, which might be due to the additional thermodynamic stability gained by the various phases of ZnO and SnO2 , which was previously established by ab initio calculations by Lee et al. [23]. We therefore confirm that the final material is composed of a mixture of both ZnO and SnO2 , instead of any mixed oxide like ZnSnOx . The formation of a meta stable [ZnSn(OH)6 ] phase which occurs at 150–200 ◦ C is also confirmed from the XRD pattern shown in Fig. 2b. 3.2. Morphological analysis by SEM The microstructures of the as prepared and annealed films were analyzed by SEM. Fig. 3 shows the scanning electron micrographs of the as deposited film (S1) as well as the films annealed at 400 ◦ C (S2), 550 ◦ C (S3) and 700 ◦ C (S4). The SEM micrograph of the as deposited film shows no distinct morphology, however, good surface coverage was evident from this image. The formation of hexagonal rod like ZnO microstructures dispersed randomly into granular SnO2 nano-particles is evident from the SEM images (S2–S4). Significant amount of grain growth was also observed as an effect of high temperature annealing. The average diameter of the ZnO micro-rods was estimated to be ∼150 nm for S2, while that for S3 and S4 was ∼300 nm and ∼500 nm, respectively. On the other hand, the average diameter of the spherical SnO2 nano-particles was found to be in the range 50–90 nm for S2, S3 and S4. Significant growth by annealing in the diameter of SnO2 nano-particles was also observed from the SEM images (S2–S4). EDX analyses have been carried out to determine the concentration (in atomic %) of chloride ion within the four composites. The concentration was found to decrease with the annealing temperature in the order 10.52 > 6.66 > 1.84 > 0.60 for S1, S2, S3 and S4, respectively. It can be seen from the SEM images that the surface of the annealed samples (S2, S3 and S4) are very uneven due to the presence of c-axis

Fig. 2. X-ray diffraction pattern of samples (a) S1–S4 and (b) ZnSn(OH)6 . The peak indicated by “*” might be due to presence of impurity like Zn(OH)2 .

oriented ZnO rods along with spherical SnO2 grains, it was difficult to estimate the exact thickness of the films. However, the film thickness was found to vary within 600–800 nm, as obtained from the surface profiler. The growth mechanism of ZnO–SnO2 composite microstructure is initiated by the reaction between Zn2+ , Sn4+ and OH− ions. It is proposed that [24] a meta-stable cubic [ZnSn(OH)6 ] phase is first produced by the reaction between above three ions at 150–200 ◦ C. At higher temperature, this meta-stable [ZnSn(OH)6 ] decomposes and re-crystallizes to form ZnO–SnO2 composite [25,26]. The whole formation process is expressed by the following chemical equations: Zn2+ + 4OH− → [Zn(OH)4 ]2−

(1)

Sn4+ + 2OH− + [Zn(OH)4 ]2− → [ZnSn(OH)6 ] (at150–200 ◦ C)

(2)

[ZnSn(OH)6 ] → ZnO–SnO2 + 3H2 O(above400 ◦ C)

(3)

In order to establish the above mentioned growth mechanism for our case, we have annealed the as spun substrate at 150 ◦ C and

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Fig. 3. SEM images of as deposited (S1) and annealed sensors (S2–S4).

carried out SEM analysis, which clearly showed the cubic morphology of the meta-stable [ZnSn(OH)6 ] (Fig. 4a). This on further annealing at 400 ◦ C yielded the ZnO–SnO2 composite structure (Fig. 4b). Fig. 4 is showing the schematic presentation of the observed phenomenon.

3.3. Gas sensing prototype 3.3.1. Response study The gas sensing performance of the sensor was studied by measuring the conductance change of the sensing film in presence and absence of hydrogen. The percentage change in resistance of the sensor on gas exposure is defined as:

Response (s) =

R − R  gas air Rair

× 100

(a)

In ambient environment, oxygen species are adsorbed on the surface of metal oxide extracting electron from the conduction band of the semiconductor. Under this condition the ZnO–SnO2 film presents high resistance. When the metal oxide surface is exposed to reducing gas like H2 , the hydrogen molecules react with the

surface adsorbed oxygen species and re-injects electrons to the conduction band as per the following reaction [27]: H2(ads) + O− → H2 O(gas) + e− (ads)

(4)

The electron concentration of ZnO–SnO2 film thus increases. This in turn lowers the resistance of the film and hence sensor response increases. The response of the reported ZnO–SnO2 sensor prototypes annealed at 400 ◦ C (S2), 550 ◦ C (S3) and 700 ◦ C (S4) upon exposure to hydrogen (1000–10 000 ppm) in air at various operating temperature is presented in Fig. 5. All the sensors showed identical behaviour of higher response at higher concentration of hydrogen. The optimum operating temperature of the sensors was estimated to be approximately 150 ◦ C. At lower temperature the reaction energy required for the reduction reaction between the chemisorbed oxygen and hydrogen molecules is not sufficient and hence the sensor resistance is high. With the increase of the temperature the sensor response increases to a maximum value afterwards the response drops. Oxygen molecules are adsorbed on the surface of n-type metal oxide to form O2 − below 100 ◦ C, O− between 100 ◦ C and 300 ◦ C, and O2− species above 300 ◦ C [28]. The adsorbed oxygen in the form of O− makes the sensor material the most sensitive because this O− is the most reactive species among the three. When the sensor is operated between 100 and 300 ◦ C, the sensor

Fig. 4. Schematic of the formation process of ZnO–SnO2 composite. (a) SEM image of the meta-stable [ZnSn(OH)6 ] intermediate and (b) morphology of ZnO–SnO2 composite.

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Fig. 5. Sensor response at different operating temperature for (a) 1000, (b) 3000, (c) 5000, and (d) 10 000 ppm of H2 .

surface preferentially forms O− ions providing higher response. At temperature below 100 ◦ C and above 300 ◦ C, the semiconductor surface preferentially forms O2 − ions and O2− , respectively, and the response of the sensor is low. The thermal instability of PdAg at higher temperature may also contribute to the low response as temperature increases. Highest response was obtained for the sensor annealed at 700 ◦ C (S4), giving a maximum response of 90% to 10 000 ppm H2 when operated at 150 ◦ C. Under the same operating condition the response of the sensor annealed at 550 ◦ C (S2) and 400 ◦ C (S3) were only 82.5% and 52%, respectively. This behaviour is attributed to the fact that the sensor S4 presents highest resistance in air because of greater number of surface adsorbed oxygen compared to S2 and S3. As a consequence, when the sensor is exposed to the target gas; more electrons are released as a result of the reaction between surface adsorbed oxygen species (here O− ) and hydrogen, giving higher sensor response. A few reports on composite metal oxide based hydrogen sensors are available in literature, but none of them are on ZnO–SnO2 based composites. Mridha and Basak [29] reported hydrogen gas sensing property of p-CuO/n-ZnO thin film heterojunction. They obtained response as high as 266.5 for 3000 ppm of hydrogen at 300 ◦ C. However, below 250 ◦ C their sensor response was negligible. Wang et al. [30] reported improved hydrogen sensing property of p-NiO/n-SnO2 composite nanofiber based sensors with high response (∼130 for 10 000 ppm) and good response and recovery time (∼3 s), but the operating temperature was significantly high (320 ◦ C). In comparison, sensor developed by us gave relatively higher response of ∼50% to low concentration of hydrogen (1000 ppm) even at 150 ◦ C (Fig. 5a). Individual and comparative studies with pure ZnO and SnO2 as gas sensing materials have been done many times previously by various groups [14–22]. A dramatic improvement in response efficiency due to the formation of hetero-junction in composite ZnO–SnO2 sensor over sensor made of pure ZnO/SnO2 is observed in this study. Fig. 6 shows the responses of pure and composite

sensors, prepared using the technique reported here, to 10 000 ppm H2 . The response of the ZnO–SnO2 composite based sensor was highest (∼90%) at 150 ◦ C, which is approximately two times than that of the responses of pure ZnO (∼48%) and pure SnO2 (∼55%) based sensors. The response transient of the sensors to various concentrations (3000–10 000 ppm) of hydrogen operated at 150 ◦ C are presented in Fig. 7. The response time is defined as the time required for reaching 90% of the equilibrium value of the resistance after gas exposure and the recovery time is defined as the time required for the resistance to return to 10% below the original resistance in air after the test gas is out. Sensor resistance was seen to fall to an equilibrium value upon gas exposure and settles back to the original value when

Fig. 6. Response of pure and composite sensors to 10 000 ppm of H2 .

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Fig. 7. Response transient of the ZnO–SnO2 sensor to varied concentration of H2 at 150 ◦ C: (a) S2, (b) S3 and (C) S4.

the test gas is vented, a very good reproducibility is thus clearly observed. The dependence of the sensor response on the concentration of hydrogen was also noticed. With increasing hydrogen concentration, the sensor response was seen to increase. Sensor response along with response and recovery times for the sensors S2, S3 and S4 for 10 000 ppm hydrogen at an operating temperature of 150 ◦ C are summarized in Table 3. 3.3.2. Selectivity study Selective detection of specific gas remains a challenging issue for the commercial application of metal oxide based gas senor [25,31]. Metal oxides in its pure form are sensitive to wide range of gases. Selectivity can be tuned with the use of composite sensing material as discussed previously. Hwang et al. [32] reported the fabrication of ZnO–SnO2 core shell nano-wire sensor selective to ethanol. At 400 ◦ C the response of the sensor to 200 ppm H2 is ∼1.7 and ∼1.06 times higher compared to CO and CH4 . Illyaskutty et al. [33] also reported novel ZnO incorporated MoO3 thin film sensor exhibiting high sensitivity and selectivity to ethanol. The sensor response to 500 ppm H2 was approximately 0.6 and 0.3 times higher than CO and CH4 , respectively. In order to investigate the selectivity we have performed the sensing studies of the annealed ZnO–SnO2 composite sensor to other gases like methane (CH4 ) and carbon monoxide (CO). The response of the sensor to 10 000 ppm of these gases at 150 ◦ C is shown in Fig. 8a. It was seen that the response of the sensor to hydrogen was highest among the other tested gases. For the sensor annealed at 700 ◦ C, the response to hydrogen Table 3 Comparison between response magnitude, response time and recovery time for 10 000 ppm hydrogen at 150 ◦ C. Sensor annealing temperature (◦ C)

Response magnitude (%)

Response time (s)

Recovery time (s)

400 (S2) 550 (S3) 700 (S4)

52 82.5 90

65 75 60

80 90 75

was 90%, whereas the responses to methane and carbon monoxide were only 47% and 31%, respectively. The response of the ZnO–SnO2 sensor to hydrogen was approximately 2 and 3 times higher than methane and carbon monoxide respectively, which is higher than other reported values [32,33]. Similar is the case for the sensors S2 and S3. Selectivity of a sensor is defined as the ratio of the maximum response of interfering gas to the maximum response of the target gas [% Selectivity = (Sother gas /Starget gas ) × 100%] [34]. Fig. 8(b) shows the percentage selectivity of the sensor operated at 150 ◦ C to 10 000 ppm of each gas. It can be seen that the sensor is most selective to hydrogen. The response of the sensor to hydrogen was 2.9–3.5 times higher compared to carbon mono-oxide and 1.9–2.2 times higher than methane. It is also to be noted that the maximum selectivity was obtained for the sensors annealed at 550 ◦ C. The response transient of this sensor to (3000–10 000 ppm) hydrogen, carbon mono-oxide and methane operated at 150 ◦ C is presented in Fig. 9.

3.4. Sensing mechanism It is well known that gas sensing mechanism of metal oxide is governed by the modulation of electrical conductivity in presence and absence of test gas. In ambient metal oxide surface is covered with chemisorbed oxygen ions, preferentially, O2 − below 100 ◦ C, O− between 100 and 300 ◦ C and O2− above 300 ◦ C. As a consequence, electrons are extracted from the metal oxide surface and a space charge region is created leading to a decrease in carrier concentration and electron mobility. This increases sensor resistance. When a reducing gas is exposed to the sensor it reacts with the adsorbed oxygen ions releasing the trapped electron back to the conduction band. This in turn restores the original resistance of the sensor. In the present case, SnO2 –ZnO composite material forms a ‘n–n’ hetero-junction which might play an important role in the enhancement of the sensor response and selectivity. Similar result of sensor performance improvement using metal oxide hetero-structure is reported by various authors. Zeng et al.

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Fig. 8. Response of the sensor to H2 , CH4 and CO: (a) response magnitude and (b) percentage selectivity.

[7] fabricated SnO2 nanosphere functionalized TiO2 nanobelts by hydrothermal method. The sensor made of SnO2 –TiO2 showed 2–5 times enhanced sensitivity towards volatile organic compounds compared to sensor made of TiO2 nanobelt alone. Likewise, Sharma et al. [31] reported the development of WO3 loaded SnO2 thin film based sensor which showed about 4 times higher sensitivity to NO2 than bare SnO2 thin film based sensor. Selective TMA detection of SnO2 –ZnO nanocomposite sensor was also achieved by Zhang and Zhang [35]. The sensor gave higher response compared to bare SnO2 (∼35 times) and ZnO (∼7 times). When ZnO (˚ = 5.2 eV,  = 4.3 eV, Eg = 3.37 eV [36]) is coupled with SnO2 (˚ = 4.9 eV,  = 4.5 eV, Eg = 3.5 eV [36]) it forms n–n type hetero-junction. In such junction electron transfer can occur from semiconductor with low work function (SnO2 ) to the other with high work function (ZnO) until the Fermi levels equalize. This creates an electron depleted layer at the interface of ZnO and SnO2 which bends the energy band. The enhanced sensing performance of composite ZnO–SnO2 is attributed to the combined effect of the formation of depleted layer at the surface of individual ZnO–SnO2 as well as the formation of hetero-junction between ZnO and SnO2 grains. Fig. 10 shows the schematic of the band energy level of ZnO–SnO2 ‘n–n’ hetero-junction. The formation of two depleted layer, one at the surface of individual grain by the adsorption of oxygen species and other at the hetero-interface of ZnO and SnO2 , promotes higher oxygen adsorption on the sensor surface to a greater extent which might provide higher reaction sites [6–8]. The sensor thus presents a higher resistance in air and the resistance

Fig. 10. Schematic of the band energy level of ZnO–SnO2 hetero-junction used for gas sensing.

change upon gas exposure is increased to a greater extent resulting in higher sensor response. 4. Conclusion ZnO–SnO2 composite material based hydrogen sensor is fabricated by a simple chemical route. The structure, morphology and gas sensing property of the annealed ZnO–SnO2 samples were investigated. Effect of annealing on the micro-structural change and sensing characteristics were also discussed in detail. The fabricated sensor showed good selectivity and excellent response and reproducibility to hydrogen gas at a relatively lower temperature of 150 ◦ C. Maximum response of 90% was obtained when exposed to 10 000 ppm hydrogen at 150 ◦ C. The response of the sensor to hydrogen was 2.9–3.5 times higher than carbon mono-oxide and 1.9–2.2 times higher than methane. Acknowledgements One of the authors (B. Mondal) is thankful to Tezpur University for providing the material development and characterization facilities. Thanks are also due to the Department of Science and Technology (GOI) and CEGESS, BESU for funding, characterization and gas sensing facilities. References

Fig. 9. Response transient of the ZnO–SnO2 sensor annealed at 550 ◦ C to varied concentration of gases at 150 ◦ C.

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Biographies Biplob Mondal received his M.Tech degree in Electronics Design and Technology in 2007 from Tezpur University, India. He is pursuing Ph.D. under the guidance of Prof. H. Saha. He is presently working as an Assistant Professor in the Department of Electronics and Comm. Engg., Tezpur University. His current research interest includes micro-fabrication of sensors and sensing materials. Borat Basumatari received his B.Tech in Electronics from IMTS College of Engg. and Technology, WBUT, India. He has completed Master of Technology in Electronics Design and Technology from Tezpur University, India. Jayoti Das has received her Ph.D. degree from Jadavpur University. She is currently working as Assistant Professor in the Department of Physics, Jadavpur University. Her research interest includes sensors, electronic materials, etc. Chirosree Roychaudhury has received ME degree from Jadavpur University in Electron Device in 2003 and Ph.D. from Jadavpur University in MEMS based porous silicon pressure sensor 2007. She is now associated with Bengal Engineering and Science University as Assistant Professor in the Department of Electronics & Tele Communications Engineering. Her research field includes Biosensors, MEMS based pressure and conductivity sensor and VLSI based signal processing. Hiranmay Saha received his M.Tech degree in Radiophysics and Electronics from University of Calcutta in 1967 and Ph.D. degree in Solar Cells and Systems from Jadavpur University in 1977. He was former Chairman of Solar Energy Division (Eastern Region), the Ministry of New and Renewable Energy, Government of India and Advisor of WBREDA (Dept. of Power and N.E.S., Govt. of West Bengal). He is associated with Jadavpur University, one of the premier Engineering. Colleges of India as Professor in the Department of Electronics and Telecommunication Engineering and was the Coordinator of the University’s IC Design and Fabrication Centre. He is involved in high end Research & Development work in the field of Photovoltaic Cells, Sensors and VLSI based intelligent signal processing architectures and smart embedded systems. Currently he is the professor-in-charge of ‘the Centre of Excellence for Green Energy and Sensor Systems’, Bengal Engineering and Science University, WB. He is Fellow, IEE (UK), IETE and Member IEEE, NCS (WB), Dept. of Science & Technology (WB), WBREDA. Dr. Saha is the chairman of the Board of Directors of Agni Power & Electronics Pvt. Ltd. Nillohit Mukherjee has received Ph.D. in Chemistry from Bengal Engineering and Science University. He is currently associated with the Centre of Excellence for Green Energy and Sensor Systems, Bengal Engineering and Science University, as an Assistant Professor. He is actively involved in the research and development on thin film semiconductors, wide and narrow band gap semiconductors, nanomaterials and porous metal oxides for their applications in sensor technology and third generation solar cells.