Review
Metal oxides for solid-state gas sensors: What determines our choice?

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Abstract

The analysis of various parameters of metal oxides and the search of criteria, which could be used during material selection for solid-state gas sensor applications, were the main objectives of this review. For these purposes the correlation between electro-physical (band gap, electroconductivity, type of conductivity, oxygen diffusion), thermodynamic, surface, electronic, structural properties, catalytic activity and gas-sensing characteristics of metal oxides designed for solid-state sensors was established. It has been discussed the role of metal oxide manufacturability, chemical activity, and parameter's stability in sensing material choice as well.

Introduction

Numerous researches have shown that a characteristic of solid-state gas sensors is the reversible interaction of the gas with the surface of a solid-state material [1], [2], [3], [4]. In addition to the conductivity change of gas-sensing material, the detection of this reaction can be performed by measuring the change of capacitance, work function, mass, optical characteristics or reaction energy released by the gas/solid interaction [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Various materials, synthesized in the form of porous ceramics, and deposited in the form of thick or thin films, are used as active layers in such gas-sensing devices [17], [18], [19], [20], [21]. The read-out of the measured value is performed via electrodes, diode arrangements, transistors, surface wave components, thickness-mode transducers or optical arrangements. However, in spite of so big variety of approaches to solid-state gas sensor design the basic operation principles of all gas sensors above mentioned are similar for all the devices. As a rule, chemical processes, which detect the gas by means of selective chemical reaction with a reagent, mainly utilize solid-state chemical detection principles [2], [22].

Theoretically there are no limitations for using any materials for solid-state gas sensors design independently of their physical, chemical, structural or electrical properties. Thousands of results have been reported about the characteristics and performance of sensors based on different materials. At present, gas sensor's prototypes on the base of covalent semiconductors, semiconducting metal oxides, solid electrolytes, polymers, ionic membranes, organic semiconductors, and ionic salts have been already tested [5], [6], [7], [23], [24], [25], [26], [27], [28], [29], [30], [31]. However there are no evidences for assertion that all materials are equally effective for gas sensors applications. Therefore at such a big variety of materials, which can be used, the selection of optimal sensing material becomes key problem in both design and manufacturing of gas sensor with required operation parameters [8], [32], [33], [34], [35], [36].

For example, according to some earlier view on the problem of gas sensor design [1], the almost any metal oxide could be a basis for solid-state gas sensor. For this purpose we need only to prepare this metal oxide as a sufficiently fine dispersed porous substance with properties controlled by surface states. However, while requirements to elaborated gas sensors were getting stronger, and understanding of the nature of the gas-sensing effects was getting more fundamental [37], [38], [39], [40], [41], [42], [43], our conceptions of any material compatibility for gas sensor elaboration started changing. We began to understand that for implementation of all requirements, a material for solid-state gas sensors have to be possessed of specific combination of their physical–chemical properties, and not every material can be corresponded these requirements. According to [34], [44], [45] in order to be used in practice, a gas sensor should fulfil many requirements, which depend on the purposes, locations and conditions of sensor operation. Among the requirements, primarily important would be sensing performance-related ones (e.g., sensitivity, selectivity and rate of response) and reliability-related ones (e.g., drift, stability and interfering gases). These are all connected with the sensing materials used so that the selection and processing of the sensing materials (materials design) have key importance in research and development of gas sensors.

Of course this paper cannot include exhaustive reviews of all available solid-state gas sensors and materials aimed for application in these devices. At present there are three main types of solid-state gas sensor currently in large-scale use [46]. They are based on solid electrolytes (electrochemical sensors), on catalytic combustion (pellistors) and on resistance modulation of semiconducting oxides (conductometric or chemiresistance-based gas sensors). In this paper the main attention will be focused on the third type of solid-state sensors. The comparison of semiconductor gas sensors with another types of solid-state gas sensors is presented in Table 1.

The semiconductor gas sensors offer low cost, high sensitivity and a real simplicity in function; advantages that should work in their favor as new applications emerge. Moreover, the possibility of easily combining in the same device the functions of a sensitive element and signal converter and control electronics markedly simplifies the design of a sensor and constitutes the main advantage of chemiresistive-type sensors over biochemical, optical, acoustic, and other gas-sensing devices [22]. A sensing element of these sensors normally comprising a semiconducting material presenting a high surface-to-bulk ratio is deployed on a heated insulating substrate between two metallic electrodes. Reactions involving gas molecules can take place at the semiconductor surface to change the density of charge carriers available.

It is necessary to note that in spite of the simple working principle of chemiresistive gas sensor, the gas-sensing mechanism involved is fairly complex. The gas/semiconductor surface interactions on which is based the gas-sensing mechanism of chemiresistive gas sensors occur at the grain boundaries of the polycrystalline oxide film. They generally include reduction/oxidation processes of the semiconductor, adsorption of the chemical species directly on the semiconductor and/or adsorption by reaction with surface states associated with pre-adsorbed ambient oxygen, electronic transfer of delocalized conduction-band electrons to localized surface states and vice versa, catalytic effects and in general complex surface chemical reactions between the different adsorbed chemical species [22], [47], [48], [49], [50], [51]. Consequences of these processes for physical properties of metal oxides are shown in Fig. 1. The effect of these surface phenomena is a reversible and significant change in electrical resistance (i.e., a resistance increase or decrease under exposure to oxidizing and reducing gases respectively, referring as example to an n-type semiconductor oxide). This resistance variation can be easily observed and used to detect chemical species in the ambient. The influence of these surface chemistry phenomena on the sensor response may be understood on the base of the models discussed in Refs. [1], [2], [16], [42], [52], [53], [54], [55], [56], [57], [58], [59].

The above brief survey of mechanisms by which semiconducting oxides provide responses to changes in atmospheric composition emphasises the detailed electronic properties of the bulk and the reactivity of the solid surface and thus leads to an expectation that the characteristics of gas sensors will be strongly influenced by materials selection [2], [46]. Therefore, the principal issues involved in the role of materials in the semiconductor chemiresistive gas sensor are outlined below.

A systematic consideration of the desired parameters of materials for gas sensor applications indicates that the key properties, determining our choice, include the following: adsorption ability; electronic, electro-physical and chemical properties; catalytic activity; thermodynamic stability; crystallographic structure; interface state; compatibility with materials and technologies to be used in gas sensors fabrication; reliability, etc. [1], [2], [8], [34], [42], [43], [52], [53], [60]. Many different materials appear favorable in some of these properties, but very few of them are promising with respect to aggregate of all these requirements.

For this assertion confirmation let us examine a parameters of metal oxides, which can determine material's applicability for gas sensors design. Certainly this brief review could not cover all promising metal oxides, developed for gas sensors. In addition to binary oxides, there are numerous ternary, quaternary and complex metal oxides, which are of interest of mentioned applications [8], [32], [36], [54]. Therefore, for simplicity of analysis in this review the priority was given to examination of binary oxides. Inclusion the more complicated metal oxides in our review would make our task more difficult. Where it is possible, other materials, first of all polymers, would be analyzed. However it is necessary to note, that because of fundamental distinctions in metal oxides and polymers’ properties and gas response mechanism of sensors on their base, a comparative analysis for indicated materials could be conducted just for limited number of parameters, controlling gas-sensing effects.

Section snippets

Sensing material choice through their surface properties

It is known that operating characteristics of solid-state gas sensors, especially sensitivity, are controlled by three independent factors such as receptor (recognition) function, transducer function and peculiarities of sensor construction. Receptor function provides the ability of the oxide surface to interact with the target gas, and transducer function provides the ability to convert the signal caused by chemical interaction of the oxide surface into electrical signal [44]. Surface of metal

Band gap

Pretty big band gap (Eg) and small activation energy of the centers, responsible for metal oxide conductivity, is an optimal combination of parameters for the materials designed for semiconductor solid-state gas sensors. Such correlation of activation energies is necessary in order to avoid sensor's operation in the region of self-conductance. In this case the influence of surrounding temperature on sensor parameters is reduced. At that, as a rule, the higher operation temperature is, the

Thermodynamic stability

Materials, destined for gas sensors, working at high temperature, have to possess high thermo-dynamic stability. The better material's thermo-dynamic stability is, the higher are temperatures, at what chemical sensor with this material is able to work especially at the presence in atmosphere of reducing gases.

As far as is very important to have high thermal stability, one can judge on the base of results given in [126]. It was established that Fe2O3:Pt-based sensors operating at Toper  200–250 °C

The role of structure and technology in gas-sensing material choice

As it was wrote earlier the operating characteristics of solid-state gas sensors are determined by both receptor and transducer functions. The last function is very important, because it determines the efficiency of chemical interactions’ conversion into electrical signal. Usually this function is played by each boundary between grains, to which a double-Schottky barrier model or “neck” model can be applied [1], [2], [3], [8], [56], [85], [156]. The resistance depends on the surface potential

Outlook

As it follows from conducted analysis, the choice of a suitable material for gas sensors should be based on good gas response, low sensitivity to air humidity, high selectivity, low hysteresis, high stability of parameters over the time, all range of operation temperatures, thermal cycling, and on exposure to the various chemicals likely to be present in the environment [34], [182], [196]. Therefore desired efficiency of reactions, responsible for gas sensors’ sensitivity, it is necessary to

Acknowledgements

Author is thankful to Civilian Research Development Foundation (CRDF) and Moldovan Research and Development Association (MRDA) (Grant MO-E2-3054-CS-03), Supreme Council of the Republic of Moldova in the field of science and advanced technology (Contract 071), and NATO (Grant CLG 980670) for financial support of his scientific research.

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