Introduction

Gas sensors are small devices, which can sense particular gas in a gaseous environment. Gas sensors based on various mechanistic principles, such as Schottky diode, optical, metal nano-wire, solid and polymer electrolyte, thermoelectric, surface acoustic wave, micro-cantilever, diamond-quartz crystal microbalance, thermal conductivity, and resistance-based, have been investigated in the literature. Semiconductor oxide sensors based on resistance-change mechanism are, however, the most popular ones among this group due to their simple operational principle and the simple measurement system associated with them. Hence, one of the primary goals of the present chapter is to review the current understanding of the working principle of the semiconductor oxide sensors based on resistance-change mechanism.

In the previous decade, different semiconductor oxides as gas sensors have been synthesized in different forms (thin and thick films, and pellets) using variety of techniques, such as spray pyrolysis, chemical vapor deposition, ionassisted deposition, sputtering, evaporation, arc plasma, metal organic deposition, and sol-gel, to sense different gases for environmental safety and pollution control. This chapter primarily focuses on reviewing the fundamentals of the sol-gel technique for synthesizing semiconductor oxide gas sensors, especially in the nanocrystalline form. The present chapter summarizes the gas sensing properties of different semiconductor oxides synthesized via sol-gel technique as reported in the last decade by different research groups around the world. In the beginning, the reports on the most popular sol-gel derived semiconductor oxide sensor are reviewed, followed by the summary of the gas sensing data reported for the other sol-gel derived nanocrystalline semiconductor oxides.

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