Gas Sensors Based on Conducting Metal Oxides

Basic Understanding, Technology and Applications

Nicolae Barsan, Klaus Schierbaum, Ghenadii Korotcenkov

• Publisher: Elsevier
• Year: 2018
• Print ISBN:
  9780128112243
• eBook ISBN:
  9780128112250
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Gas Sensors Based on Conducting Metal Oxides: Basic Understanding, Technology and Applications focuses on two distinct types of gas sensors based on conducting metal oxides. Ion conduction, applied in so-called solid-state electrolytic sensors for one, and electronic conduction used in semiconductivity gas sensors for the other. The well-known ?–probe, a key component to optimize combustion in car engines, is an example of the former type, and the in-cabin car air-quality control SnO2 and WO2 sensor array stands for the semiconductivity type. Chapters cover basic aspects of functioning principles and describe the technologies and challenges of present and future sensors. - Provides reader background and context on sensors, principles, fabrication and applications - Includes chapters on specific technological applications, such as exhaust sensors, environmental sensors, explosive gases alarms and more - Presents a structured presentation that allows for quick reference of vital information

1

Introduction

Nicolae Barsan¹ and Klaus Schierbaum², ¹Institute for Theoretical and Physical Chemistry, University of Tübingen, Tübingen, Germany, ²Department of Materials Science, Institute of Experimental Condensed Matter Physics, University of Düsseldorf, Düsseldorf, Germany

Air components have become important metrics, not only for professionals, but also for public perception. A prominent example is the increase of nitric oxides in cities from the exhaust gases of diesel vehicles [1]. An increasing number of people understand how crucial “pure” breathing air is for one’s health and comfort.

In addition to particles of varying sizes, air contains many different gaseous components in different concentrations, shown in Fig. 1.1. Oxygen and water vapor (“moisture”) are life-essential gases present in the atmosphere. In addition, carbon dioxide—normally present in our air without harm—affects humans when having a high concentration, first causing tiring, then anesthetizing, and cab ultimately be lethal if its concentration exceeds the value of 8% over a period of 30–60 minutes [3].

Figure 1.1 The composition of air near the ground. Source: Adopted from B. Klose, Meteorologie, Springer-Lehrbuch, https://doi.org/10.1007/978-3-662-43622-6_2, Springer-Verlag Berlin Heidelberg. 2016 [2].

Other gases such as hydrocarbons, CO, NOx, NH₃, H₂, or SO₂ also present hazards: they are detrimental to health, an explosion hazard, and can even be lethal. News of carbon monoxide poisoning, often called the “silent killer,” in highly developed industrialized countries is regularly reported [4].

Specifically, there are a number of dangerous gases present in an industrial setting for which qualitative and quantitative detection methods are needed [5]. The field of sensor technology originates from the need for simple determination without the use of complex, analytical measuring methods and spectrometers in an industrial setting. Solid-state gas sensors based on metal oxides have been developed and used for five decades [6]. They have clear advantages: they are small, are highly sensitive allowing the detection of many different gases in the ppm or even the ppb range, they can be operated online, and they are cost-effective compared to methods using classical instrumental analysis.

Ideally, sensors should be cheap, allow mobile application, be easy to use with a fast response time, and be selective [7]. In reality, however, sensors—and not only the solid-state gas sensors with electron- and ion-conducting metal oxides treated in this book—often suffer from limited measuring accuracy, are not selective with the simultaneous occurrence of several gases and have problems with long-term stability [8]. Today, sensors are still far from this ideal, but it is justifiable to say that the considerable efforts in this field over the past few decades have led to substantial progress. A variety of materials and technologies have been developed and the fundamental understanding of the complex, mechanistic processes involved in the interaction of gases with the sensors and their surfaces has been constantly refined.

The central question has two parts: how the transport of charges (electrons and/or ions) and potential-generating processes are influenced by the molecules from the gas phase, and, in turn, which physical and chemical effects ultimately control the “communication” between the charge carriers and the molecules. As a result, there is a differentiation between the receptor and the transducer part of the sensor. This terminology can be explained using biosensors in which molecular receptors have been deposited as self-contained units on a physical, for example, optical or gravimetrically operating transducer. For solid-state gas sensors, the separation between the receptor and transducer is more difficult because the functions are interrelated. This becomes obvious from Fig. 1.2 which shows the basic types of sensors.

Figure 1.2 Schematic representations of solid-state gas sensors. (A) semiconductor, (B) mixed-potential, (C) microhot plate, (D) nanowire, and (E) TiO₂-based sensor. Further explanations are given in the text.

Fig. 1.2A shows a semiconductor gas sensor in a planar arrangement. The heater is deposited on the backside of an insulating support (substrate). The sensitive layer is typically heated to between 300°C and 500°C. The substrate contains a topside interdigital electrode on which the sensitive layer, for example, of tin dioxide, is applied. As can be schematically seen as the current between the grounded electrode and the voltage source is amplified. Changes in the resistance of the metal oxide layer as a result of variations in the surrounding gas composition are used as sensor signals. Reactions between atmospheric gases and the surface of grains in the sensitive layer result in changes of the current flow at the grain-to-grain boundaries (“percolation path”) [7].

Fig. 1.2B uses an ion-conducting metal oxide (“solid electrolyte”), frequently yttrium-stabilized zirconium dioxide, in which oxygen ions are mobile at temperatures above 500°C; the electron conduction can be neglected [8]. Different electrode materials allow two separate, partial reactions to take place at the working (or sensitive) and counter electrodes. One can distinguish between anodic oxidation (electron emission of a molecule) and a cathodic reduction (electron absorption of a molecule). It is important that the molecule to be detected is involved in the reaction at the working electrode and that the ions moving in the ionic conductor are involved in both reactions. The spatial separation of the two reactions does not have to be perfect; it is sufficient if they proceed with a sufficiently different rate on both electrodes. In this way, a potential is formed between the working electrode and the counter electrode. It is customary that the working electrode is the sensitive electrode and a molecule that is present in large excess (often oxygen) is reacted at the counter electrode. This arrangement is called a mixed-potential sensor; fuel cells of analogous construction are referred to as single-chamber fuel cells [9]. In the potentiometric variant of the solid-electrolyte sensor, the two porous electrodes of platinum are arranged separately from each other in two different “half cells” and the difference in the chemical potential of a molecule (oxygen in the lambda probe) or the partial pressures associated therewith, determines the potential difference [10]. In the amperometric variant, the mass flow of the molecules to be detected is limited by a small opening in a cover or membrane, and the charge carrier flow through the ion conductor is, thus, limited and itself a function of the partial pressure of the molecule to be detected [11]. In all these variants, a high temperature is necessary.

The high temperatures of the metal oxide-based sensor means that portable applications can hardly be realized. The constraint for power reduction and the need to reduce the dimensions of the sensors, coupled with the advantages of mass production at a low cost, drives the development and implementation of MEMS technology for gas sensors (Fig. 1.2C) [12]. The advantages of “micro heating plates” are obvious and will be discussed later in this Chapter 4. Nanotechnology pursues a completely different approach. Individual nanowires from the sensitive metal oxide between two electrodes are explored and developed (Fig. 1.2D). They are heated at the lowest electrical power in the microwave range. Their enormous surface-to-volume ratio is determined by the nanometer dimension of the cross section.

Another solution is to develop heaterless sensors, shown in Fig. 1.2E. Very porous titanium dioxide layers on a titanium support together with a porous graphite layer containing catalyst particles form a stacked structure in which the sensor signal is determined amperometrically, impedanceometrically, or calorimetrically [13,14]. This technology is still in the early stages, but the proof-of-concept for the detection of carbon monoxide and nitrogen dioxide has already been provided. This type of sensor may mark the transition to liquid electrolyte sensors, where the use of catalyst-loaded graphite layers as the electrode material is common, all of which are without external heating.

When working with and describing sensors, a standard terminology is used. Table 1.1 summarizes...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of contributors
• Series editor biography
• Editors’ biographies
• Preface to the series
• Preface to the volume
• Book outline
• 1. Introduction
• 2. Gas sensors based on oxygen ion conducting metal oxides
• 3. Basics of semiconducting metal oxide–based gas sensors
• 4. Technological realization of semiconducting metal oxide–based gas sensors
• 5. Application of SMOX-based sensors
• 6. Conclusion and outlook
• Index