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 gas 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.
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