Metal Oxide Nanomaterials for Chemical Sensors

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 Materialund Geowissenschaften, Technische Universitaet Darmstadt, Darmstadt, Germany e-mail: gurlo@materials.tu-darmstadt.de 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 3 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 current of the conductivity exhibits a kink when plotted as a function of the time. In this change the slopes immediately before and immediately after the kink point are proportional to the partial pressure of oxygen. One can use this effect to relate a known and an unknown concentration of oxygen often even under conditions in which one has a mixture of gases...’’ (cited from Ref. [1]). In 1957, Heiland showed that the ‘‘well-conducting surface layer on zinc oxide crystals provides a new, very sensitive test for atomic hydrogen’’ [28] and Myasnikov demonstrated that ZnO films can be used as a highly-sensitive oxygen-analyzer [29]. Later he developed this ‘‘to the method of semicondutor probes’’, which allows for ‘‘studying free radical processes’’ and for detecting ‘‘free active particles and to measure their concentration under stationary and non-stationary conditions in gases and liquids’’ [30]. However, the conditions under which ZnO was able to operate as a ‘‘sensing device’’ were far from the real ambient conditions (and, accordingly, from a practical application); the ‘‘sensitive’’ effects were observed: (i) in vacuum conditions, exposed to oxygen or hydrogen, (ii) after ‘‘activation’’ or ‘‘sensitization’’ of the surface by heating in H2 and in UHV. The practical use of metal-oxide-based gas sensors in normal ambient conditions was not considered until 1962, when Seiyama et al. reported that a ZnO film can be used as a detector of inflammable gases in air [31] (see also [32]), and Taguchi claimed that a sintered SnO2 block can also work in the same way [33] (for the history of TGS (Taguchi Gas Sensor) sensors, see [34]). The latter 4 A. Gurlo