Metal Oxide Nanomaterials for Chemical Sensors
This book presents a state-of-the-art account and critical analysis of of the science and technology of metal oxide nanomaterials based chemical sensors. It documents the impact of nanotechnology on sensor science by leading research laboratories and the
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Insights into the Mechanism of Gas Sensor Operation Aleksander Gurlo
Abstract Since the development of the first models of gas detection on metaloxide-based sensors much effort has been made to describe the mechanism responsible for gas sensing. Despite progress in recent years, a number of key issues remain the subject of controversy; for example, the disagreement between the results of electrophysical and spectroscopic characterization, as well as the lack of proven mechanistic description of surface reactions involved in gas sensing. In the present chapter the basics as well as the main problems and unresolved issues associated with the chemical aspects of gas sensing mechanism in chemiresistors based on semiconducting metal oxides are addressed. ‘‘Sensors have a ‘life cycle’ consisting of preparation, activation, operation with deactivation and, possible, regeneration. Thus understanding the performance in terms of reaction and conductance mechanisms is only a part of the total understanding of a sensor.’’ Dieter Kohl, Sensors and Actuators 1989, 18, 71.
A. Gurlo (&) Fachbereich Material- und Geowissenschaften, Technische Universitaet Darmstadt, Darmstadt, Germany e-mail: [email protected]
M. A. Carpenter et al. (eds.), Metal Oxide Nanomaterials for Chemical Sensors, Integrated Analytical Systems, DOI: 10.1007/978-1-4614-5395-6_1, Springer Science+Business Media New York 2013
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A. Gurlo
1.1 Chemiresistors: From Semiconductor Surfaces to Gas Detectors Since the early 1920s numerous investigations have demonstrated the influence of the gas atmosphere on conductivity, free carrier mobility, surface potential, and work function on a number of semiconductors (see summary of early works in [1– 13]). This led to the understanding that the surface of semiconductors is highly sensitive to chemical reactions and chemisorptive processes [3, 14–20] and resulted finally in the ‘‘theory of surface traps’’ (Brattain and Bardeen [21]), ‘‘boundary layer theory of chemisorption’’ [10, 22, 23] (Engell, Hauffe and Schottky) and ‘‘electron theory of chemisorption and catalysis on semiconductors’’ (Wolkenstein [5–7, 24]). They laid also the theoretical foundations for the subsequent development of metal-oxide-based gas sensors. Although from this understanding to the use of semiconductors as gas sensors ‘‘was, in principle, a small step’’ [25], the idea of using the changes in conductivity of a semiconducting metal oxide for gas detection was not conceived until the middle of the 1950s. The earliest written evidence came in 1956, in the Diploma Thesis performed in Erlangen under supervision of Mollwo and Heiland and entitled ‘‘Oxygen detection in gases changes in the conductivity of a semiconductor (ZnO)’’ [26], the results discussed later in [1, 27]: ‘‘If one exposes a zinc oxide layer which has been given a previous heating at 500 K in a high vacuum to oxygen at a constant pressure, the conductivity falls very rapidly initially and more slowly later. If one then increases the oxygen pressure suddenly, the
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