Gas sensors are essential for industry and for a wide range of applications. They are for examples applied in public safety, pollution monitoring, and various industrial processes. Among the different gas sensing technologies, semiconducting metal oxide-based gas sensors are the most popular because of their low price, high sensitivity, short response time, high stability and simple operation. In these gas sensors, because gas adsorption has a direct relationship with the surface area of the sensing material, a higher surface area will result in a higher sensing response. Therefore, along with simple synthesis methods, nanowires (NWs) have recently gained special attention for the realization of gas sensors. In this tutorial review, the synthesis of metal oxide NWs, the fabrication of gas sensors and their sensing mechanisms are discussed. Different gas sensors such as single NW, noble metal functionalized NWs, heterojunctions NWs, self-heating NWs, UV-activated NWs and core-shell NWs are presented. This tutorial review aims to provide a broad vision for the researchers and students working in this upcoming field.
2
I. Toxic gases and vaporsGases are intimately linked to life, as most of the living species continuously need to breathe air, which is basically a mixture of oxygen, nitrogen, argon, and other gases. In addition, many gases are used in our industrial era. For example, liquefied petroleum gas (LPG) is widely used in industry, as well as for cooking and heating purposes. 1 Even though LPG is not toxic, it is highly explosive. 2 Also, hydrogen gas is seen as the next "green fuel" and is currently used in fuel cells, although it is highly explosive. 3,4 In addition to explosive gases, the sources of toxic and pollutant gases have been significantly increased in the recent years, and there are many toxic gases in our atmosphere. 5 Toxic gases can cause harm in low levels over long periods of time (chronic exposure) or in higher concentrations over short periods of time (acute exposure). The threshold limit value (TLV) has been defined as the maximum concentration of a gas, which is allowed for repeated exposure without resulting in adverse health effects. 6 For example, the TLV values for CO, NO2 and H2S gases are 50, 3 and 10 ppm, respectively. 6 Based on the WHO (World Health Organization), air pollution is mainly due to toxic gases and caused around seven million premature deaths in 2012. 7 There are many toxic gases in our surrounding atmosphere. For example, carbon monoxide (CO) poisoning results in over 5000 deaths in the USA. 8 In Denmark, from 1995 to 2015, several hundred people passed away due to CO poisoning. 9 Also, in Iran, as a typical developing country, 836 deaths occurred in 2016due to CO poisoning. 10 CO has not any color, odor and taste, 11 and it has 240 times greater affinity for hemoglobin in comparison with oxygen. It forms carboxyhemoglobin, which leads to a reduced oxygen delivery to tissues and can cause tissue hypoxia. 8,12 Also, CO easily binds to cytochrome oxidase and leads to...