STUDY ON ELECTRICAL AND OPTICAL PROPERTIES OF ZINC OXIDE SEMICONDUCTOR FOR GAS SENSOR APPLICATION

Abstract

Ammonia, ethanol, acetone, etc. are commonly found as toxic gases in most places. The real-time monitoring of these gases is essential because their excessive exposure may produce serious health problems. In recent times, several metal oxide semiconductors (MOS) have been exploited for gas detection. In this context, zinc oxide (ZnO) is considered one of the potential materials for its interesting properties such as non-toxicity, high thermal & chemical stability, and easy tunable electrical & optical behavior. High working temperature (>300 °C ), poor selectivity, and low sensitivity are some of its shortcomings. The operation at high temperatures degrades the sensor’s stability and consumes power. This study aims to enhance the sensing characteristics of ZnO-based sensors after utilizing strategies like metal and metal-metal doping into it. Herein, the ZnO and metal-doped films were prepared by using spin coating, spray pyrolysis, and doctor blade methods for an in-depth understanding of gas sensing. Its optical and structural characterizations were done by ultra violet visible (UV-Vis) spectrophotometer and X-ray diffraction (XRD) respectively. Surface morphology and elemental composition were studied using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. The gas sensing performances of ZnO-based sensors were measured using a homemade gas sensor setup. At first, spin-coated ZnO was tested for the detection of traces of ammonia, ethanol, acetone, methanol, and isopropanol at room temperature. Its XRD and SEM micrographs demonstrated the polycrystalline wurtzite phase with a grainy surface. The band gap was found to be 3.202 ± 0.023 eV. The sensitivity measurements revealed the highest response of 38.5 ± 0.6 with an exposure of 400 ppm of ammonia vapour, indicating its selectivity among the tested gases. The results of sensitivity measurements over multiple cycles showed the device’s good stability. The sensing capability here was found to be better than other similar works. Hence, the results obtained here will be helpful in the development of a low-cost, effective room temperature MOS gas sensor with a lower detection limit of 20 ppm which is below the Occupational Safety and Health Administration’s (OSHA) approved threshold. For acetone sensing, ZnO deposited on an fluorine doped tin oxide (FTO) substrate prepared by doctor blade was used. XRD and fourier transform infrared (FTIR) spectroscopy were used for phase purity and optical characterization of ZnO nanoparticles (ZnONPs) prepared from the co-precipitation method prior to deposit on the FTO substrate. The sensing measurements demonstrated the maximum value of gas response of 25.697 ± 0.012 at an operating temperature of 285 ± 7 °C for exposure of 800 ppm of acetone along with the rapid response and recovery. This operating temperature was found to be lower than the published values that were prepared by different methods. The response & recovery times were measured to be 39 sec and 79 sec, respectively. Sequentially, in other sets of experiments, the undoped ZnO, Fe-doped ZnO (Fe-ZnO), and Sn-doped ZnO (Sn-ZnO) films were used to detect ethanol vapours in the temperature range of 100-300 °C. The sensitivity measurements for 2% Fe-ZnO film showed the highest response of 40.91 ± 0.23 at the exposure of 400 ppm of ethanol at 260 ± 7 °C. The comparison with similar reported values confirmed its goodness. And 2% Sn-ZnO film showed the highest response of only 17.659 ± 0.011 for 400 ppm exposure at 220 ± 5 °C. This working temperature was found to be slightly lower than the published value. Interestingly, this also reports that 2% Sn-ZnO film was able to detect as small as 0.5 ppm of ethanol. The spin-coated Fe-Al co-doped ZnO sensors were also tested to monitor ethanol in the temperature range of 120 − 360 °C. The 1%Fe-1%Al-ZnO sample showed a very high value of the response, 152.304 ± 0.003 at the exposure of 400 ppm at 290 ± 7 °C. It is due to an increase in specific surface area which occurs due to the reduction of grain size after Fe-Al co-doping. The observed values of response and recovery times were 33 sec and 201 sec respectively at an operating temperature of 290 ± 7 °C. Hence, metal-metal co-doping is found to be a good strategy to improve the sensitivity of ZnO-based gas sensors. Finally, the effect of gate electrode potential on the ammonia sensing ability of ZnO at ambient temperature was also reported here. Required films were prepared by the spray pyrolysis method. The gas response of ZnO for 400 ppm of ammonia was increased from 30.292 ± 0.042 to 54.581 ± 0.062 on increasing the gate electrode potential from 0 to 24V. Hence, this will be a new finding to improve the gas response of future ZnO-based gas sensors.