Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide.

In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.
Since 1962 it has been known that absorption or desorption of a gas on the surface of a metal oxide changes the conductivity of the material, this phenomenon being first demonstrated using zinc oxide thin film layers . The sensitivity of a surface to a gas can be as low as parts per billion (ppb) . It is highly desirable that metal oxide semiconductor sensors have a large surface area, so as to adsorb as much of the target analyte as possible on the surface, giving a stronger and more measurable response (especially at low concentrations). Advances in fabrication methods have enabled the production of low-cost sensors with improved sensitivity and reliability compared to those formed using previous methods. Production costs are kept low due to the simplicity of metal oxide semiconductor sensor devices. Their ability to be produced quickly and on a large scale with easily controllable processes makes them a desirable technology to exploit.

This paper aims to: (i) introduce the fundamental reasons for sensing gases, (ii) discuss sensor response mechanisms in metal oxide semiconductor sensors, and (iii) show how non-target gases can interfere with the response of such a sensor. The review will then present a summary of recent advances on sensors that have been developed for specific gases such as carbon dioxide and carbon monoxide, and in the final section discuss new approaches developed in our labs at University College London such as the ways in which zeolites can be used to increase specifity, selectivity and efficiency of sensors.