Gas reception and signal transduction of neat tin oxide semiconductor sensor for response to oxygen
Introduction
Oxide semiconductor gas sensors, first proposed by Seiyama et al. [1] and Taguchi [2] a half century ago, have grown now to be important safety devices used in practice worldwide. Those are fabricated by stacking tiny crystallites (typically grains) of a metal oxide of n-type semiconductor, typically SnO2, In2O3 or WO3, mostly loaded or mixed with various kinds of additives as a sensitizer (e.g., PdO, Fe2O3 or Pt), a skeletal substance (Al2O3) or a binder (SiO2). Those detect various gases, reducing (e.g., H2, CO and CH4) to oxidizing (NO2 and O3), sensitively from a change in their electric resistance [3], [4], [5]. Those have been developed mostly through exploitations based on empirical knowledge and intuition, without being provided with proper concepts or theory to rely on.
It has long been recognized that the resistance of a sensor device in this group is determined by receptor function and transducer function [6], [7], as illustrated in Fig. 1, except for the third factor (utility factor) associated with the gas diffusion and reaction effect [8], [9]. The key issues concerned are electron transfer of each grain and electron transport between grains, respectively, as shown. Oxygen adsorption in form of O− and/or O2 −, changes with a change in oxygen partial pressure (PO2) or upon exposure to a reducing gas in air, accompanied by a change in the degree of electron depletion of grains. This in turn causes the resistance for electrons to transport between grains and thus the resistance of the whole device, too, to change accordingly. It was attempted for a long time to understand both functions by analogies to the working mechanisms of metal — semiconductor contact diodes (Schottky diodes), i.e., depletion in conventional scheme and transport of bulk electrons beyond a double-Schottky barrier, respectively [6], [7]. Unfortunately, this could not indicate more than whether the device resistance would increase or decrease with a change in gas ambient. It gave no satisfactory explanations to such important features typical to the gas sensors as the promotion of gas response with decreasing grain size (grain size effect) [10], [11] and the power laws held between responses and concentrations of target gases [12]. Later the grain size effect was explained semi-empirically in terms of the surface to volume ratio of constituent grains by Rothschild and Komen [13].
In order to overcome such deadlocks, we started investigating the two functions from a more basic standpoint. First, for the transducer function, the double Schottky barrier model was replaced by a simpler model in which the resistance of each contact between grains and hence that of the whole device as well could be assumed to be reciprocally proportional to the surface concentration of conduction electrons of grains. This replacement was found to lead to the power laws well recognized when grains were highly depleted of electrons [14]. Encouraged with this result, we further investigated the receptor function of tiny grains in shape of a sphere, column or plate by assuming the oxygen adsorption in the form of O− ions. As found, tiny grains are depleted of electrons in two stages, first following the conventional way, regional depletion and then switching to volume depletion [15]. Coupled with the transducer model, this receptor model was found to explain various features of gas responses including the grain size effect and the power laws [16], [17], [18]. In addition, the transducer model has been proven to be valid as long as uniform grains are brought into contact [19]. The receptor function has been extended later to include the formation of O− and/or O2 − ions [20]. Nevertheless, these treatments have turned out to be still insufficient to deal with the responses of sensors in practice, which are vulnerable to disturbances by moisture [21]. The surface chemistry of grains should be made finer for this purpose. It is remarked that our basic stance in these serial studies is to elucidate the kinds and roles of critical factors in determining the gas responses of oxide semiconductor gas sensors. For this purpose, sensor devices as well as constituent oxide particles (grains mostly) have been assumed to be simple and ideal as much as possible. There are many interesting researches which focus attention to the exotic shape of constituent crystals such as nano-rods [22], the exotic morphology of self-assembled crystals [23], or in the non-uniform distribution of donors inside constituent crystals [24]. For the time being, however, we consider it important to pay attention to more general features, not to peculiarities, in order to establish fundamental or theoretical standpoints for the oxide semiconductor gas sensors.
This paper aims at reporting summarily the mechanisms of gas reception and signal transduction revealed so far for a neat SnO2 sensor through these investigations. Although the target gas is confined to oxygen here, the mechanisms involved are basically in common with those involved for sensing other conventional gases as long as the role of oxide grains is concerned. In addition, elucidation of the response to oxygen is indispensable for understanding the meaning of the device resistance in air (base air resistance), an important base on which the response data to other target gases are acquired and discussed.
Section snippets
Semiconductor physics of small grains
For simplicity, we assume a thick film device consisting of uniform spherical grains of SnO2 with radius a, donor density ND and Debye length LD. In addition, oxygen is tentatively assumed to be adsorbed in the form of O− in this section.
Response behavior characteristic to the kinds of oxygen adsorbates
In order to formulate the response to oxygen, it is imperative to know the surface chemistry which correlates between the concentrations of anionic adsorbates of oxygen and PO2. Once the surface chemistry is available, one can formulate − QSC/q as a function of PO2. Then, using Eq. (3) or Eq. (6), the response to oxygen, R/R0, can also be expressed as a function of PO2. This treatment has to be carried out numerically in the stage of regional depletion because R/R0 is implicitly correlated with −
Remarkable influences of moisture
It is of great interest to see how well the theory of oxygen response developed above work under actual conditions. For some period of time after we began the present series of investigations, the oxygen response behavior observed was almost always well coincident with what could be expected from the formation of O− ions (Case 1 in Section 4), as reported elsewhere [16], [17], [18]. However, this turned out later to have resulted from the coexistence of a slight amount of moisture in the
Discussion
Oxygen adsorption has been found to be influenced drastically by moisture, resulting in the oxygen response behavior far more complex than anticipated. Nevertheless, the response behavior can be analyzed satisfactorily based on the proposed concepts on receptor function and transducer function, as demonstrated above. The adsorption constants data and the threshold pressures data, estimated or collected above, allow one to estimate how the base air response depends on PW. This has been done for
Conclusions
As shown above, the response of a neat SnO2 gas sensor to oxygen changes drastically with changes in ambient humidity (poisoning effects) as well as in pretreatment humidity (hysteresis effects). Nevertheless, the drastic changes of response can be analyzed quantitatively and satisfactorily on the basis of the schemes of gas reception and signal transduction we proposed recently. This gives a strong support to the gas sensing mechanism proposed for a gas sensor using tiny grains of an n-type
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