Preface

Gas sensors have become a highly researched area because they can detect and recognize different types of toxic gases and low concentrations of vapor compounds. This interest in gas sensing has attracted worldwide interest, because of several diverse applications. Nowadays, there are many tools and materials that can be employed to design a gas sensor system. However, improvements in gas sensing system performance have been substantially correlated with advances in nanotechnology. This book focuses on the fabrication and application of gas sensing systems. It covers the recent developments of different materials used to design gas sensors, such as conducting polymers, semiconductors, as well as layered and nanosized materials. The widespread applications of various gas sensors for the detection of gaseous compounds are also discussed. The book provides an overview of recent attributes of gas sensors and their applications to a broad audience, including beginners, graduate students, and specialists in both academic and industrial sectors. It contains seven chapters that describe the design, fabrication, different uses, applications, and attributes of gas sensors. The first chapter is related to metal oxide gas sensors such as ZnO and multiwalled carbon nanotubes as well as their composites. It describes the performance of fabricated sensors for sensing NO2, NH3, and CO gases. The second chapter discusses the design of microgas sensors using radiometric phenomena, which occurs due to the temperature difference in rarefied gases. The effects of primary factors and parameters are also described. The third chapter presents terahertz wave propagation in layered media based on waveguide and artificial material configurations to sense gas molecules, especially volatile organic compounds. Improvement in detection sensitivity and selectivity is also discussed with the aid of multilayer microporous polymer structures as a terahertz artificial material to adsorb vapor molecules. The fourth chapter is related to a review of the preparation of ZnO nanorods and their use in ethanol vapor sensing, where the efficiency of 1D ZnO nanostructures prepared by different techniques as a sensing material for gaseous compounds, especially ethanol, is discussed. Ethanol detection characteristics using a ZnO sensor reveals its efficiency in terms of electrical resistance, capacitance, and impedance. The fifth chapter describes the design and fabrication of films based on metal oxide materials for petroleum vaporous compounds. The sixth chapter discusses the synthesis and sensing mechanisms of different toxic chemicals using ZnO nanowire materials synthesized by thermal evaporation through a vapor transport method using a vapor liquid solid mechanism. The chapter also reveals the sensing performance of ZnO nanowires for CH4 gas with high sensitivity. The seventh chapter describes the contribution of conducting polymers as effective sensing materials to enhance their sensitivity, selectivity, and stability to detect gaseous compounds.

> **Sher Bahadar Khan, Abdullah M. Asiri and Kalsoom Akhtar** King Abdulaziz University, Saudi Arabia

**1**

**Chapter 1**

*Fatma Sarf*

**Abstract**

**1. Introduction**

day by day.

Silicon Valley in electronics [2].

Nanostructures

Metal Oxide Gas Sensors by

Recently, metal oxide gas sensors by nanostructures have stirred interest and have found their way in many applications due to their high sensitivity, material design compliance and high safety properties. Gas performance tests of n-type ZnO, Al-doped ZnO and ZnO/MWCNT structures toward different type gases from our previous studies have been reported. It is indicated that nanoparticle formations on the film surfaces, grain sizes, gas types and operating temperatures have a severe effect on the chemisorption/physisorption process. Low concentration detection, determination of grain size limit values and reducing operating temperature to room temperature are already obstacles on long-life sensitivity and long-term stability characters. Doping is an effective way to increase gas sensitivity with atomic surface arrangement and active gas adsorption sites, which are generated by doping atoms. However, C-based material/MO nanostructures are preferred than doped MO films with their working even at room temperature. Up to now, a lot of methods to improve the gas sensitivity has been proposed. With the help of the development of surface modification methods such as different types of doping and MO-C composite, sensitivity, which is the most important parameter

of sensor performance, can also be stable as well as increasing later on.

**Keywords:** metal oxide, gas sensor, toxic gas, doping, multiwalled carbon nanotube

Increased environmental pollution, numerous motor vehicles, factory wastes and urbanization factors have been the source of high increases in the release of toxic, explosive and flammable gases in the environment of developed countries. High rate of gas emissions has both a negative impact on human/animal health and it can also have bad consequences on the environment and natural resources from

With the start of the Industrial Revolution, the acceleration of coal and mine quarries caused a significant increase in deaths due to toxic gas. First, canaries were used in gas detectors in mines. The cost and difficulty of using different methods for determination of toxic gases have revealed the gas sensors. In 1815, British scientist H. Davy developed a gas meter called 'Davy's lamp' against methane gas [1]. In 1926, Johnson produced the first commercial catalytic, combustion gas sensor, and in 1929, the company they founded with Williams became the first company in
