(ProQuest: ... denotes formulae omitted.)

1. Introduction

In the recent years, sensors have attracted a great deal of attention from scientists and engineers. Even in the near future it is expected to gain importance in view of the construction subsystems. Detection of various gases using solid-state chemistry has generated a great deal of interest, both in academia and in industry for environmental monitoring [1-2].

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 operable in ppm range of H^sub 2^S are in high demand for environmental safety and industrial control purposes.

Many H^sub 2^S gas sensors working on high temperatures are reported [3-9]. But very few are reported to work at room temperature. Gas sensors operating at room temperature are very important for electric power saving purpose. Hence research for new good gas-sensing materials operating at room temperature is continuously going on.

In this paper, we are reporting preparation of WO3 pellets by solid state route and their H2S gas sensing properties at room temperature.

2. Experimental

2.1. Preparation of Sintered Pellets OfWO^sub 3^

The WO^sub 3^ powder (purity ~ 99.99 %) was mechanically milled in an acetone medium using Fisher type electric agate pestle and mortar for 24 hours. The powder was then dried at 200 °C for 20 min. After drying, pellets of the powder were prepared by using a die under pressure of 7 tons for 10 sec with the help of a hydraulic press. The prepared pellets were sintered at 800 °C for 1 hour to increase the strength and to have compactness. Fig. 1 shows flowchart for the preparation of WO3 pellet by solid state route.

2.2. Study of Structural Properties of Prepared Pellets OfWO^sub 3^

The structural properties of the WO3 pellets were investigated using X-ray diffraction (XRD) technique. The X-ray diffraction patterns were recorded with a Rigaku diffractometer (Miniflex Model, Rigaku, Japan) having Cu K (? = 0.1542 nm). The thickness and diameter of the WO3 pellets were measured by a standard vernier caliper and micrometer screw gauge.

2.3. Details of Gas Sensing System

Fig. 2 shows schematic diagram of gas sensing system. The gas sensing studies were carried out using a static gas chamber to sense H^sub 2^S vapour in air ambient. The WO^sub 3^ pellets were used as the sensing elements. The sensing element was kept directly on a base in the gas chamber. The known volume of the H^sub 2^S gas was introduced in to the gas chamber pre-filled with air and it was maintained at atmospheric pressure. A constant voltage was applied to the sensor element through a potential divider arrangement as shown. The prepared WO^sub 3^ pellet i.e. sensor element was act as a lower branch resistor of potential divider arrangement as shown in Fig. 3.

The voltage across WO^sub 3^ pellet is takes as a base index for the measurement of sensitivity of the sensor element for the H^sub 2^S gas. The electrical voltage across the sensing element was measured by using a simple two probe configuration, before and after exposure to H2S gas using a sensitive digital multi meter (METRAVI 603).

The sensitivity (S) of the sensing element is defined as:

...

where V^sub a^ and V^sub g^ are the voltage values across the sensor element in air and in the presence OfH2S gas, respectively.

The sensing properties of the prepared WOa pellet for different regularly available gases in the atmosphere like LPG, NH3, ethanol, CO, CO2 etc was also investigated.

3. Results and Discussion

The X-ray diffraction measurements performed on the prepared WO3 pellets indicate that it is polycrystalline and the crystalline structure corresponds to the triclinic phase of WO3 (JCPDS Data card 83-0948). The diameter and thickness of the prepared WO3 pellets was lem and 2 mm respectively.

Response and recovery time are the basic parameters of the gas sensors. They are 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. Fig. 5 shows response-recovery curve of the prepared WO3 pellets at room temperature in response of H^sub 2^S gas (10 C.C.). It indicates that the WO3 pellet operating at room temperature shows response within -12 s after exposure to 10 c.c. H2S gas and recovers back within -45 s after removal of the H2S gas. The sensitivity OfWO3 pellet to H2S gas (10 c.c.) calculated from the response-recovery curve and the above mentioned formula is found to be ~ 10.49 %.

During the further H2S gas sensing experiments with the prepared WO3 pellet, the time dependence of the voltage across sensor element was monitored for the repeated exposure and removal of H2S gas. The exposure of the WO3 pellet to the H2S gas followed by the removal of the gas constitutes a single cycle of gas response study. The variation of voltage across theWO3 pellet operating at room temperature with the time when exposed to 10 c.c. H2S gas shown in Fig. 6. As can be seen from Fig. 6, the voltage value across the WO^sub 3^ pellet decreases from decreases from 9.05 V to 8.1 V, when exposed to 10 c.c. OfH^sub 2^S gas and after removal of the H^sub 2^S gas it shows the change in the voltage value back to 9.05 V. This constitutes the first cycle of the H^sub 2^S gas response study. On exposing again the same WO^sub 3^ pellet to H^sub 2^S gas , it is observed that the voltage value decreases from 9.05 V to 8.1 V and after removal of the H^sub 2^S gas it changes to 9.05V, which constitutes the second cycle of H2S gas response study. The similar changes in the voltage value were observed for further four cycles as shown Fig. 6. It is clear that the response and recovery characteristics are almost reproducible and rather quick when exposed to H2S gas.

Fig. 7 shows sensitivity verses concentration curve for WOa pellet in response to H2S gas in c.c. The prepared WO3 pellet was exposed to varying concentrations OfH2S gas (2-10 c.c.). It observed that the sensitivity increases linearly from 2-8 c.c. after that it remains constant.

The sensing properties of the prepared WOs pellet for different gases like LPG, NH3, ethanol, CO, CO2 etc was also investigated at room temperature. It is observed that the WO3 pellet sensor element was not shown any response to other gases. Thus the prepared WO3 pellet sensor element shows highest selectivity towards H2S gas at room temperature.

5. Conclusions

Following conclusions can be drawn from the experimental results

1. In this work pellets of WOa can be prepared by solid state route and characterized by XRD.

2. The H2S gas sensing properties of prepared WO3 pellets was investigated at room temperature and at different gas concentration.

3. The WOs pellet operating at room temperature exhibit excellent sensitivity, selectivity and repeatability towards H2S gas as compared to other gases like LPG, NH3, ethanol, CO, CO2 etc.

4. This study demonstrates the possibility of utilizing WO3 pellet as a sensor element for the detection OfH2S gas.

Acknowledgement

One of the authors (RSK) is thankful to the University Grants Commission, Pune, India, for the financial assistance provided for the Minor research project duration 2009-2011. The authors are grateful to Head, Principal and colleagues of G.D.M. Arts, K.R.N. Commerce and M.D. Science College, Jamner for providing laboratory facilities and for their encouragement. They are also thankful to Prof. Dr. P. P. Patii, Department of physical Sciences, N. M. University, Jalgaon.

References

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Author affiliation:

*Rajendra. S. Khadayate and Pruthwiraj. R. Patii

G. D. M. Arts, K. R. N. Commerce and M. D. Science College,

Jamner, Dist-Jalgaon, Maharashtra, India

Tel: +91 02580 233081, + 91 0257 2263437

E-mail: rskhadayate@yahoo.com

Received: 2 October 2011 /Accepted: 14 February 2012 /Published: 28 February 2012