Abstract
Thick films of AR grade In2O3 were prepared by standard screen-printing technique. The gas sensing performances of thick films were tested for various gases. It showed maximum sensitivity to ethanol vapour at 350°C for 80 ppm concentration. To improve the sensitivity and selectivity of the film towards a particular gas, In2O3 sensors were surface-modified by dipping them in a solution of 2% nanosilver for different intervals of time. Obtained results indicated that spherical nano-Ag grains are highly dispersed on the surface of In2O3sensor. The surface area of the nano-Ag/ In2O3 sensor is several times larger than that of pure In2O3 sensor. In comparison with pure In2O3 sensor, all of the nano-Ag-doped sensors showed better sensing performance in respect of response, selectivity, and optimum operating temperature. The surface-modified (30 min) In2O3 sensor showed larger sensitivity to H2S gas (10 ppm) at 100°C. Nano silver on the surface of the film shifts the reactivity of film from ethanol vapour to H2S gas. A systematic study of gas sensing performance of the sensor indicates the key role played by the nano silver species on the surface. The sensitivity, selectivity, response, and recovery time of the sensor were measured and presented.
1. Introduction
Hydrogen Sulphide (H2S) is a colourless, toxic, flammable, and malodorous gas as sources from gasoline, natural gases, city sewage, volcanic gases, and hot springs with smells like rotten eggs. It can also be produced from bacterial breakdown of organic matter or wastes produced by humans and animals. Other sources are craft paper mills, tanneries, and petroleum refineries. H2S gas is badly harmful to human body and the environment. According to the safety standards established by American Conference Government Industrial Hygienists, the threshold limit value (TLV) defined for H2S is 10 ppm. Meanwhile the type of oil and natural gas is correlative with the concentration of H2S. The oil and natural gases mines can be found depending on the concentration of H2S. Therefore, the detection and monitoring of H2S are of high importance for both resource exploitation and human health. In the recent researchers, a number of semiconductor sensors have been found to be sensitive to H2S including WO3, In2O3, ZnO, and a few perovskite-type materials [1–9].
In general, metal oxide materials are rarely used as a single phase for sensors. And their gas sensing characteristics are usually well modified by adding a small amount of catalysts. Platinum and Palladium are well known active catalysts, which enhance the sensitivity against reducing gases [10]. Gold (Au), Silver (Ag) are also utilized to enhance the sensitivity of oxide materials [11]. It is believed that the catalysts promote chemisorptions process and thus increase the density of chemisorbed oxygen species, which are reaction centres for reducing gas molecules. The addition of second component in metal oxide semiconductor gas sensor either as bulk doping or as surface modification is one of the successful methods to optimize and improve the properties of gas sensors [12–17].
Semiconductor metal oxide-based gas sensors have been a subject of extensive research because of their use in detecting several toxic inflammable and odourless gases [18–23]. The sensing properties are based on the reaction between semiconducting metal oxides and gases in the atmosphere. The gas sensing mechanism involves the redox reaction at the surface, leading to the changes in the depletion layer of the grains that in turn changes the electrical resistance [24–28]. There are many possible reactions for the changes in electrical resistance; the most common reaction that leads to changes in the conductance is the adsorption of gases on the semiconducting surface. With the adsorption being a surface effect, great emphasis has been placed in the research towards the study of nanosized materials. In nanosized materials, a large fraction of the atoms is present at the surface, and hence, the surface properties become dominant. Hence, for the application of semiconducting metal oxide, as a sensing layer material of chemical gas sensors, nanosized particles are desirable. Indium oxide offers new advantages in designing metal oxide-based gas sensors, linked with an essential difference in electrophysical and chemical properties between SnO2 and In2O3. As a gassensing material, In2O3 has been extensively applied to detect O3, NO2 and CO [29–31]. In2O3 is known as to be an n-type semiconductor in its nonstoichiometric form, due to oxygen vacancy doping and has been extensively studied as a conductivity-based chemical sensor [32].
Due to advantages such as small size, low cost, simple operation, and good reversibility, the semiconductor sensors have become the most promising devices among the solid-state chemical sensors. Hence, the metal oxide gas sensing materials are widely investigated. Many semiconductor oxides such as SnO2, ZnO, Fe2O3, In2O3, WO3, and CuO have been explored to be detecting the polluting toxic and inflammable gases, such as CO, CO2, NOx, H2S, and ethanol [33–37].
In the present study, the nano-Ag is used for surface modification of In2O3 thick film. It has been described that the nano-Ag particles on the surface of In2O3 films by dipping technique improves the surface adsorption capability. As a result of the numerous interfaces between the nano-grains that interact with gas molecules can effectively improve the diffusion properties, while greatly increasing the gas sensitivity of the materials [38–40]. The nano-Ag used in the present work was prepared as described in the literature [41].
In this work, In2O3 thick films were prepared and deposited by screen printing on to glass substrates. The films surface were modified by dipping them into the solution of nano-Ag. The sensing performance of the films were tested by static gas sensing system for different gases such as H2S, CO, CO2, H2, LPG, NH3, Cl2, O2, and ethanol vapour.
2. Experimental
The AR grade powder of In2O3 was calcined at 1000°C for 6 h. Then In2O3 powder was milled for 2 hrs using planetary ball mill to obtain fine-grained powder. The thixotropic paste was formulated by mixing the fine powder of In2O3 with a solution of ethyl cellulose (a temporary binder) in a mixture of organic solvents such as butyl cellulose, butyl carbitol acetate, and terpineol. The weight ratio of the inorganic to organic part was kept at 75 : 25 in formulating the paste. This paste was screen-printed on a glass substrate in a desired pattern [42–44]. The sensors were fired at 550°C for 30 min. Silver contacts are made for electrical measurements. The preparation of nano-Ag colloidal solution was explained elsewhere [41]. The pure In2O3 sensors were surface-modified by dipping them into 2% nanosilver solution for different intervals of time: 5, 10, 15, 20, 30, and 45 min [45]. The oxidation state of nano-Ag in colloidal state is zero. These sensors were dried under IR lamp at 80°C for 1 h and followed by firing at 550°C for 30 minutes. During firing process nano-Ag° was converted into Ag2O (with oxidation state +1). The thickness of the sensors was measured by using the Taylor-Hobson (Talystep, UK) system and was observed in the range from 60 to 65 μm. The reproducibility of the sensors’ thickness was achieved by maintaining the proper rheology and thixotropy of the paste.
The sensing performance of the sensors was examined using a “static gas sensing system.” There were electrical feeds through the base plate. The heater was fixed on the base plate to heat the sample under test up to required operating temperatures. The current passing through the heating element was monitored using a relay operated with an electronic circuit with adjustable on-off time intervals. A Cr-Al thermocouple was used to sense the operating temperature of the sensor. The output of the thermocouple was connected to a digital temperature indicator. A gas inlet valve was fitted at one of the ports of the base plate. The required gas concentration inside the static system was achieved by injecting a known volume of a test gas using a gas-injecting syringe. A constant voltage was applied to the sensor, and the current was measured by a digital Pico ammeter. The air was allowed to pass into the glass chamber after every gas exposure cycle.
3. Results
The purity and crystallinity of the calcined powder structure were examined using X-ray diffractogram (XRD) with CuKα radiation (wavelength 1.5418 ). The average grain size was calculated according to Debye-Scherrer’s equation; it was observed to be 51 nm. Figure 1, depicts that the XRD patterns are indexed with pure In2O3 with cubic structure. The observed peaks in figure are matched well with (JCPDS Card no. 01-071-2194) reported data of In2O3 [46]. Figure 2, shows the micrographs of pure In2O3 and nano-Ag surface-modified In2O3 sensors. The comparison of these micrographs shows the interesting changes in morphology. Scanning electron microscopic (SEM) studies were carried out by using JEOL JSM 6360 (LA) Japan. Figure 2(a) depicts the SEM image of pure In2O3 sensor fired at 550°C. The sensor consists of voids and a wide range of grains with size distribution ranging from 0.25 μm to 0.5 μm, distributed nonuniformly. Figure 2(b) represents the SEM image of nano-Ag surface-modified In2O3 (30 min) sensor fired at 550°C. It shows a number of small particles distributed uniformly between the larger grains around the In2O3, which may be attributed to the presence of nano-Ag. The grain size range was observed to be from 0.05 μm to 0.5 μm. The presence of nano-Ag particles on the surface of the sensor alters the adsorption-desorption ability of the sensor. The altered surface morphology of the sensor shifts the affinity and reactivity of sensor towards the H2S gas. EDAX analysis of nanosilver surface-modified sensor shows the presence of nanosilver, and the appearance from the sensor indicates that the nanosilver particles are present at the surface of the In2O3 sensor.
(a)
(b)
The quantitative elemental analysis of the pure and nano-Ag surface-modified In2O3 sensors was carried out by using an energy dispersive spectrophotometer (EDS) JEOL-JED-2300 LA Japan. The constituent elements such as In, O, and Ag associated with various sensors are presented in Table 1. It is clear from Table 1 that the weight percentage of silver is increased with dipping time and reached to a maximum. The sensor with the dipping time of 30 min as we observed to be more oxygen deficient (55.03 wt.%). This oxygen deficiency may make the sample possible to adsorb a large amount of oxygen species.
Figure 3 shows the I-V characteristics of pure In2O3 sensor at room temperature. I-V characteristics are observed to be symmetrical in nature, indicating the ohmic nature of silver contact.
Figure 4 shows the dependence of electrical conductivity of pure In2O3 and surface modified by nano-Ag In2O3 sensors in air ambience. The electrical conductivity of these sensors goes on increasing with increase in temperature, indicating negative temperature coefficient (NTC) of resistance. This shows the semiconducting nature of the sensors.
Figure 5 shows the variation in the gas response of ethanol vapour (80 ppm) with operating temperatures ranging from 50°C to 450°C. It is noted from the graph that response increases with increasing temperature, attains a maximum at 350°C, and decreases with further increase in operating temperature.
Selectivity is defined as the ability of sensor to respond to a certain gas in the presence of other gases. Selectivity is another important parameter of a gas sensor. The sensor must have rather high selectivity for its application. Pure In2O3 sensor is examined for different gases at different operating temperatures, and the results are shown in Figure 6. The bar diagram indicatves selectivity of pure In2O3 sensor at 350°C to ethanol vapour against the other gases. The sensor is the most selective to ethanol vapour against the other gases.
Response time and recovery time are the basic parameters of the gas sensors. Which is defined as the time taken for the sensor to attain 90% of maximum change in resistance on exposure to gas is the response time. The time taken by the sensor to get back 90% of the original resistance is the recovery time [47]. The 90% response time and recovery time were attained within 6 s and 60 s, respectively. The very short response time is the important features of the nano-Ag surface-modified (30 min.) In2O3 sensor. The large recovery time would be due to lower operating temperature. At lower temperature species is more prominently adsorbed on the surface, and thus it is less reactive as compared to other species of oxygen, and .
The enhanced response of nano-Ag surface-modified sensors can be attributed to two factors. First the higher specific surface areas of the nano-Ag doped sensors can lead to increase in active surface area for gas sensing. Secondly, the nano-Ag dopant, as a catalyst, enhances the adsorption of gas molecules and accelerates the electron exchange between the sensor and test gas [48]. The two factors together contribute to the improvement of gas sensing properties of nano-Ag surface-modified In2O3 sensors. Furthermore, the maximum response is observed for nano-Ag surface-modified (30 min) In2O3 sensor, perhaps due to the largest amount of active reaction sites formed on the surface of In2O3 sensor by the random dispersion of nano-Ag [49].
4. Discussion
Since long it has been known that adsorption of reducing gas molecules results in decrease in electrical resistance of oxide material [50]. The In2O3 materials are characteristically n-type semiconductor due to nonstoichiometry associated with oxygen vacancy and/or metal excess which acts as donor states thus providing conduction electrons. However, the overall surface resistance of such films is generally influenced by chemisorptions (chemical adsorption) of oxygen from air on the surface and at the grain boundaries. The chemisorbed oxygen traps conduction electrons and remains as negatively charged species (, , or depending on temperature) on the surface [51]. The process results in an increase of surface resistance. In presence of reducing gases the trapped electrons are released due to the reaction between the gas molecules and negatively charged chemisorbed oxygen species resulting in decreasing in resistance of the materials. When the gas is removed from the sensor environment, the resistance again increases and the material recovered to original resistance.
Figure 7 shows the variation in the gas response of pure and nano-Ag surface-modified In2O3 sensors to H2S gas (10 ppm) with operating temperature ranging from 50°C to 400°C. The H2S gas response of pure In2O3 sensor fired at 550°C was 0.6 at 100°C while that of nano-Ag surface-modified (30 min) In2O3 sensor was 40010 at the same operating temperature. Therefore, the nano-Ag surface-modified of In2O3 sensors was observed to be excellent for H2S gas sensing than the pure In2O3 sensor.
The nano-Ag surface-modified (30 min) In2O3 sensor showed maximum gas response to H2S gas. The amount of nano-Ag incorporated onto the In2O3 sensor surface would be optimum to cover the surface uniformly, leading to the enhanced adsorption mechanism:
The gas response to H2S gas goes on increasing with increasing the amount of nano-Ag up to a certain limit, attains maximum gas response, and decreases with increasing the amount of nano-Ag. The largest gas response in case of the sample (30 min) may be because of more available sites (misfits). The surface of nano-Ag misfit regions enhances the oxygen adsorption on the surface. Thus the number of oxygen species adsorbed on the activated surface would be larger. The larger the number of oxygen species adsorbed, the faster the oxidation of H2S gas would be. The pure In2O3 sensor showed the highest response to ethanol vapour, while nano-Ag surface-modified sensor showed it to H2S gas. The alteration in nature of gas sensing response from ethanol vapour to H2S gas for the nano-Ag surface modified In2O3 sensor could be largely due to possibility of formation of silver sulphide more easily than formation of silver oxide. This would increase the conductance of the film crucially, enhancing gas response. The abrupt decrease of response for the sensors doped with more amount of Ag is probably due to the reduction of active sites correlated with the agglomeration of Ag grains. At higher concentrations, the nano-Ag would mask the entire base material and would resist the gas to reach to the surface active sites, so gas response would decrease further [52].
It is well known that the gas sensitivity is greatly influenced by the operating temperature and the amounts of additives. In order to determine the optimum operating temperature and additive amount, the gas responses of nano-Ag/In2O3 sensors with different nano-Ag content to 10 ppm H2S gas were measured at different operating temperatures. The gas response of pure In2O3 sensor to H2S gas was also measured for comparison. The results are shown in Figure 8. It can be seen obviously from Figure 8 that the pure In2O3 sensor has a poor response to H2S gas, while the nano-Ag surface-modified sensors with different amounts of Ag; all exhibit much higher gas responses than the pure one. Among the entire nano-Ag surface-modified In2O3 sensors, the one with dipping time 30 min (0.38 wt.%) shows the largest gas response to H2S gas, whilst the gas sensing property change trend is similar to the specific surface area change trend of the samples. Additionally, the gas responses of the sensors to H2S gas are also affected by the operating temperature. The operating temperature of all the nano-Ag surface-modified sensors is lower than that of the reported H2S gas sensors [48, 53, 54]. The lower operating temperature could lead to lower energy consumption, while it is one of current pursuits in solid-state gas sensors. Based on the above results, we can see that the nano-Ag/In2O3 sensors exhibit much better gas response and lower operating temperature. Figure 8, represents the bar diagram indicating selectivity of nano-Ag surface-modified In2O3 sensor dipped for 30 min, and the operating temperature is 100°C to H2S gas against the other gases. The sensor is the excellent selective to H2S gas.
Figure 9 shows the variation of gas response with dipping time and the amount of nano-Ag (wt%). The sensor dipped for 30 min showed the maximum gas response to H2S gas at operating temperature 100°C. At 30 min dipping time, the senor would find Ag (0.38 wt%) to be optimum. The optimum wt% of Ag (0.38) would cover the film surface uniformly. The lower gas response at higher operating temperature would be due to consumption of H2S gas by adsorbed oxygen. At higher wt%, the Ag surfactant would mask the base material and resist the gas to reach to the surface active sites, so that gas response decreases.
5. Conclusions
The following conclusions can be drawn from the experimental results (1)The In2O3 was found to be nonstoichiometric and oxygen-deficient material. It showed negative temperature coefficient in nature.(2)The pure In2O3 sensor showed the highest response to ethanol vapour (80 ppm) at 350°C.(3)The In2O3 sensors were modified by nano-Ag solution by dipping technique. The surface-modified sensors showed the best response to H2S gas (10 ppm).(4)The surface modification shifts the gas response of the sensor from ethanol vapour to H2S gas.(5)Since introduction of nano-Ag on the surface would alter the adsorption-desorption relationship of the sensor, the optimum dipping time was found to be 30 min.(6)The surface modification alters only the surface morphology of the sensors not the bulk properties.(7)The surface modification facilitated adsorption of larger number of oxygen ions on the surface could immediately oxidize the exposed H2S gas leading to faster response time of the sensor.(8)The surface-modified In2O3 sensor was observed to be excellent selective to H2S gas.(9)The quick response of the sensor could be attributed to larger oxygen deficiency in the surface-modified In2O3 sensor.
Acknowledgments
The authors are grateful to U.G.C., New Delhi and B.C.U.D., University of Pune for granting financial assistance to this project. D. N. Chavan is very much thankful to the Principals, Arts, Commerce and Science College, Nandgaon, NVPM’s A.C.S. College Lasalgaon, and K.T.H.M. College Nasik for providing laboratory facilities. Thanks are also due to the Department of Physics, Chemistry at University of Pune, and M.V. P. Samaj, Nasik for providing characterization facilities for this research work.