Abstract
Nanostructured SnO2 thin films were prepared by spraying tin chloride dihydrate onto the heated glass substrate at 250°C. The films were fired at 500°C. As-prepared thin films were studied using XRD and FESEM to know crystal structure and surface morphology. The average crystallite and grain size observed from XRD and FF-SEM was found to be less than 33 and 67 nm, respectively. The films sprayed for 30 min were observed to be most sensitive to H2S at 250°C. The results are discussed and interpreted.
1. Introduction
The n-type semiconductor material has been widely used in the gas detecting field during the last decades. The sensing properties of various semiconductor oxides, especially the SnO2-based material, have been extensively studied [1]. Gas sensing applications demand materials that have a quick response-recovery time and high response for trace level detection of various gases. Semiconducting tin oxide is found useful for various gas sensing applications and improves its sensitivity and selectivity with appropriate catalysts [2]. Several potential applications have been reported previously, such as a transparent conductive electrode for solar cells [3], a gas sensing material for gas sensors devices [4], photochemical and photoconductive device, liquid crystal display [5], gas discharge display, and lithium-ion batteries. There has been intensive research on improving the gas response and selectivity by controlling the particle size [6], nanostructures [7], sensing temperature [8], surface structure [9], and catalysts [10].
A variety of techniques have been used to prepare tin oxide (SnO2) thin films. These include spray pyrolysis [11], ultrasonic spray pyrolysis [12], chemical vapour deposition [13], activated reactive evaporation [14], ion-beam-assisted deposition [15], sputtering [16], and sol-gel [17] methods. Among these techniques, spray pyrolysis has proved to be simple, reproducible, and inexpensive, as well as suitable for large area applications. Besides the simple experimental arrangement, high growth rate and mass production capability for large area coatings make it useful for industrial as well as solar cell applications. In addition, spray pyrolysis opens up the possibility to control the film morphology and particle size in the nanometer range. As demonstrated [18] spray pyrolysis is a versatile technique for deposition of metal oxides.
Hydrogen sulfide is a toxic gas, often produced in coal, coal oil, or natural gas manufacturing. Even at low concentration it produces severe effects on the nervous system. Therefore, reliable sensors with low cost, low energy consumption having high sensitivity, selectivity, and operability in ppm range of H2S are in high demand for environmental safety and industrial control purposes. In general, SnO2 response towards H2S is reported at the higher temperatures; however, there are few references where a response at room temperature is also reported [19, 20].
We have tried to improve the H2S gas response by making nanostructured SnO2 thin film sensor and found that nanostructured SnO2 thin film gas sensor gives maximum response to H2S gas. In the present investigations, nanocrystalline SnO2 thin films with different spraying time of the solution were prepared by spray pyrolysis technique. Structural properties and grain sizes were studied using X-ray diffraction. Microstructure was studied using FESEM. These nanostructured SnO2 thin films were tested for sensing different gases and were observed to be most sensitive to H2S at 250°C.
2. Experimental
2.1. Preparation of Nanostructured SnO2 Thin Films
Figure 1 shows spray pyrolysis technique for preparation of nanostructured SnO2 thin films. Set-up consists of spraying chamber, spray nozzle (gun), compressor for carrier gas, heating system, and temperature indicator.
Nanostructured SnO2 thin films were prepared from aqueous solution of tin (II)chloride dehydrate (SnCl22H2O), Purified Merck) dissolved in deionized water to a concentration of 0.05 M for the preparation of thin films. The spray produced by nozzle was sprayed onto the ultrasonically cleaned glass substrates heated at °C. Various parameters such as nozzle-to-substrate distance, deposition time and flow rate of solution, deposition temperature, and concentration were optimized to get good-quality films. Thus the films with different spraying time of 10 min, 20 min, 30 min and 40 min were obtained and were referred to as sample S1, S2, S3, and S4, respectively. The as-prepared samples were annealed in air at 500°C for 1 h.
2.2. Characterization
In the present study nanostructured SnO2 thin films were characterized by X-ray diffraction (Miniflex Model, Rigaku, Japan). The microstructure of the films was analyzed using a field emission scanning electron microscope (FESEM, JEOLJED 6300). Gas sensing properties were measured using a static gas sensing system. The sensor performance on exposure of LPG, carbon dioxide, hydrogen, ammonia, ethanol, chlorine, and H2S was examined.
3. Results
3.1. X-Ray Diffraction Analysis
Figure 2 shows the X-ray diffractogram of nanocrystalline SnO2 thin film. The observed peaks are matching well with the standard JCPDS data of SnO2 [21]. The broad peaks may be due to the nanocrystalline nature of SnO2. The average grain size is calculated from Scherer’s formula, , where is average crystallite size, is X-ray wavelength (1.5418 Å), is FWHM of the peak, and is diffraction peak position. It was found to be 33 nm.
3.2. Surface Morphology
The microstructure of the prepared film was analyzed using a field emission scanning electron microscope (FE-SEM, JEOL.JED 6300).
Figure 3 shows the FESEM images, showing surface topography of most sensitive thin film samples. The morphology of the grains was roughly spherical in shape. The observed grain size was 67 nm.
3.3. Gas Sensing Performance of the Sensors
3.3.1. Gas Response with Operating Temperature
Gas response is defined as the change in conductance of the sample on exposure to gas to the original conductance. It is given by the relation where is the conductance of sensor in air and is the conductance of sensor in presence of gas.
Figure 4 shows the variation in response with the operating temperature to 600 ppm of H2S for S1, S2, S3, and S4 samples. For all the samples the H2S gas response increases with increase in operating temperature, reaches maximum ( for sample S3) at 250°C, and falls with further increase in operating temperature.
Response of sensors depends on two factors, namely, the speed of chemical reaction on the surface of the grains, and the speed of the diffusion of gas molecules to that surface. These are activation processes, and the activation energy of chemical reactions is higher. At low temperatures the sensor response is restricted by the speed of chemical reactions. At higher temperature the sensor response is restricted by the speed of the diffusion of gas molecules to that surface. At some intermediate temperature the speed values of two processes become equal, and at that point the sensor response reaches its maximum. According to this mechanism for every gas there is a specific temperature at which the sensor response attains its peak value.
Thus, in the present case the optimum operating temperature for nanostructured SnO2 films was 250°C. The temperature, which corresponds to a certain peak value, is a function of the kind of target gases and the chemical composition of the oxide, including additives and catalysts, and pure oxides are generally stable at lower temperatures.
3.3.2. Gas Response to H2S at Various Gas Concentrations
Figure 5 shows the gas response against H2S concentration at 250°C. It is observed that the response increases linearly as the H2S concentration increases from 100 to 600 ppm and then remains nearly constant with further increase in the H2S concentration. The linear relationship between the response and the H2S concentration at low concentrations may be attributed to the availability of sufficient number of sensing sites on the film to act upon the H2S. The low gas concentration implies a lower surface coverage of gas molecules, resulting into lower surface reaction between the surface adsorbed oxygen species and the gas molecules. The increase in the gas concentration increases the surface reaction due to a large surface coverage. Further increase in the surface reaction will be gradual when saturation of the surface coverage of gas molecules is reached. Thus, the maximum sensitivity was obtained at an operating temperature of 250°C for the exposure of 600 ppm of H2S.
3.3.3. Response and Recovery of the Sensor
The time taken for the sensor to attain 90% of the maximum decrease in resistance on exposure to the target gas is the response time. The time taken for the sensor to get back 90% of original resistance is the recovery time. The response and recovery of the nanostructured SnO2 thin film (S3) sensor on exposure of 600 ppm of H2S at 250°C are represented in Figure 6. The response is quick (4 s) and recovery is fast (9 s). The high oxidizing ability of adsorbed oxygen species on the surface nanoparticles and high volatility of desorbed by-products explain the quick response to H2S and fast recovery.
3.3.4. Comparison of H2S Sensing Performance of Reported SnO2 Sensor with sensors Prepared in the Present Work
(See Table 1).
4. Discussion
The gas response of any metal oxide semiconductor to a particular gas increases with the decrease in the size of nanocrystallites [28, 29] due to an increase in surface-to-volume ratio and therefore the reactivity. Grain sizes and microstructures of the sensor affect the gas sensing performance of the sensor. It was found that, if the grain size of the sensor material is sufficiently small, the area of active surface sites is larger, and the sensitivity and selectivity for a particular gas enhance largely. Nanostructured materials would be expected to show much better gas sensing performance as compared with the sensor fabricated from bulk materials [30, 31].
The resistance of the nanostructured SnO2 thin films decreases as gas flows into the test chamber and is adsorbed on the surface of the nanostructured SnO2. However, when nanostructured SnO2 thin films consist of nanograins in absence of H2S, the depletion layer would extend throughout the entire layer of nanostructured SnO2 on the film and its resistance would become strikingly large. In a H2S gas environment the depleted layer would shrink quickly as it obtains conduction electrons due to reaction between H2S adsorbed oxygen, and the resistance of the nanostructured SnO2 would experience a large change. The response to H2S can be explained in the following reaction:
5. Conclusion
Nanostructured SnO2 thin films could be prepared by simple and inexpensive spray pyrolysis technique. The structural and microstructural properties confirm that the as-prepared SnO2 thin films are nanostructured in nature. The SnO2 thin film of sample ( min spray time) was most sensitive to H2S gas and exhibited the response of to the gas concentration of 600 ppm at the temperature of 250°C. The sensor has good selectivity to H2S against different gases. The nanostructured SnO2 thin films exhibit rapid response–recovery, which is one of the main features of this sensor.
Acknowledgments
The authors are thankful to the University Grants Commission, New Delhi for providing financial support. Thanks are due to Principal, G. D. M. Arts, K. R. N. Commerce, and M. D. Science College, Jamner, for providing laboratory facilities for this work.