Gas Sensing Properties of Nanostructured Tin/Zinc Oxide Thin Films

Abstract

In the present world of industrialization, because of the production of harmful gases as a by-product of the factories and automobiles, sensors are essentially required in all aspects of life. In many industries, various harmful, toxic and explosive gases including H2, CH4, NOX, NH3, SOX etc. have become increasingly important as raw materials. In case of accidental leakage these gases can be life threatening also and hence it has become very important to develop highly sensitive gas sensors. Gas sensors based on semiconducting metal oxides have gained a lot of interest in last few decades. However, some unresolved issues that require special attention include the realization of gas sensors with low operating temperature, higher response towards low concentration of target gas, fast response and recovery speeds, low power consumption, low cost etc. Hydrogen is a colorless, odorless, explosive, and extremely flammable gas having low minimum ignition energy (0.017 mJ), high heat of combustion (142 kJ/g H2) and wide flammable range (4−75%), as well as a high burning velocity and detonation sensitivity. Hydrogen is envisioned as the most attractive basic and sustainable energy source for the future generation due to its high efficiency and renewable properties. Hydrogen gas is important in the synthesis of ammonia and methanol, the hydration of hydrocarbons, the desulphurization of petroleum products and the production of rocket fuels, in metallurgical processes etc. However, leakage of H2 may lead to some fatal accidents which can be prevented if detected timely. To monitor such leakages, the development of sensors with improved sensitivity and selectivity is essentially required. Recently, metal oxide semiconductor thin films have attracted considerable interest as gas sensing materials due to their higher specific surface area and smaller grain size than bulk materials, which can lead to higher response, lower operating temperatures and fast response processes. Moreover, nanocrystalline thin films are more compatible with current micro-fabrication processes and circuits. Therefore in this thesis thin films of Tin/Zinc oxide have been developed to investigate the gas sensing properties. Nowadays, investigation is directed, rather than to new sensors, to improve the gas sensing properties of existing sensors. Ideal gas sensor should present: high sensitivity towards target gas, high selectivity, high stability, high reliability and low sensitivity to humidity and temperature, robust, durable, short reaction and recovery time, easy calibration and have small dimensions. Unfortunately, creating a sensor that satisfies all these requirements adequately has not yet been realized and there is no universal decision for simultaneous optimization of all such sensors parameters. Therefore, one should always seek a compromise between these parameters of the developed gas sensor. Most of the above desired requirements are dependent not only on the used material, but also on the method used for its synthesis and specially on the additives present on the material surface and the method used for their addition. Consequently, in the present work Sol-Gel and PLD techniques have been exploited for the growth of nanocrystalline SnO2/ZnO thin films for the development of suitable gas sensor. Most of the tin/zinc oxide based gas sensors operate at quite high temperatures normally between 250 ºC to 450 ºC, which is inconvenient for using such sensors at high temperatures. Because high working temperature of the gas sensor is energy intensive, increases operating cost and requires high operational safety measures in an environment with explosive gases like hydrogen etc. Moreover, higher power consumption and short battery life time are also the other drawbacks of higher sensing temperature. Besides, high working temperature, high response for low concentration of target gas and good selectivity of the present sensors are also major concern. The modulation of electronic properties of sensing layer with suitable catalyst are expected to improve the sensing response characteristics to a great extent, which can be easily attained using thin film technology. Incorporation of optimum quantity of suitable catalyst with novel dispersal in sensing layer is expected to improve the sensing response, selectivity and operating temperature significantly. In the present work, attempt has been made to realize efficient H2 gas sensors based on semiconducting tin/zinc oxide thin films having suitable catalyst in optimum quantity and novel dispersal manner, showing high response at relatively low operating temperature for low concentration of gas. Undoped and doped SnO2/ZnO thin films sample series have been deposited by sol-gel technique and spin coating process on quartz substrates. The sample series of silver doped SnO2 nanocomposite thin films were successfully synthesized by chemical route. These deposited films were annealed at different temperatures (300 ºC, 350 ºC, 400 ºC, 450 ºC and 500 ºC). The structural analysis and surface morphology of as prepared silver doped SnO2 thin films were characterized by complementary techniques such as X-ray diffraction and scanning electron microscopy. The tetragonal structure of SnO2 was confirmed by XRD and, for all the annealed samples, the appearance of the sharp peaks and similarities in all XRD profiles in terms of the peak positions and the half peak width indicate a well-developed crystallinty. The crystalline domain size estimated from Sherrer diffraction formula and the average crystalline size was found to be ≈18.62 nm. It is evident from the SEM images and particle size distribution that by increasing the annealing temperature particle size increases, which is consistent with the XRD information. In the present work highly sensitive and selective gas sensor for hydrogen gas has been developed utilizing copper doped zinc oxide (Cu-ZnO) thin films. These films were fabricated using pulsed laser deposition (PLD) technique on quartz substrates with interdigitated platinum electrodes. The structural and surface characteristics of obtained films were investigated by X-ray diffraction technique and atomic force microscopy. The gas sensing properties of as grown films were tested by I-V measurements techniques using a gas sensing unit for different concentration of H2 and CH4 gases at substantially reduced operating temperatures of 50 ºC, 100 ºC and 150 ºC. It is observed that for 3% Cu-doping the optimum parameters are 1000 ppm of hydrogen gas at 150 ºC in the voltage range of 0-1volt. More importantly this sensor responds to 10 ppm of hydrogen gas even at low operating temperature i.e. 50 ºC. The novelty of this sensor is that the sensitivity for CH4 and H2 can be completely reversed if measurements are performed at 150 ºC as compared to 50 ºC or 100 ºC. In addition to above we have also studied undoped ZnO, and 1 to 8% Li-doped ZnO thin film sample series for hydrogen sensing and these films were characterized by an X-ray diffraction and scanning electron microscopy. The gas sensing measurements were performed by a PC connected to a Keithley 2401 low voltage source, picoammeter through GPIB interface and lab view v8.5 software. It is observed that highly resistive 2% Li-doped ZnO thin films exhibit optimum performance at 150 °C to a flow of 50sccm (standard cubic centimeters per minute) of hydrogen, whereas high lithium doping (8%) show excellent sensing performance at 50°C for the same flow of hydrogen. The response time of the 2% Li- doped ZnO films is about 55 seconds and recovery time is about 220 seconds.