the role of morphology and crystallographic structure of metal oxides

The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors G. Korotcenkov a,b, * a Korea Institute of Energy Research, Daejeon, Republic of Korea b Technical University of Moldova, Chisinau, Republic of Moldova Available online 18 March 2008 Abstract This review paper discusses the influence of morphology and crystallographic structure on gas-sensing characteristics of metal oxide conductometric-type sensors. The effects of parameters such as film thickness, grain size, agglomeration, porosity, faceting, grain network, surface geometry, and film texture on the main analytical characteristics (absolute magnitude and selectivity of sensor response (S), response time (t res ), recovery time (t rec ), and temporal stability) of the gas sensor have been analyzed. A comparison of standard polycrystalline sensors and sensors based on one-dimension structures was conducted. It was concluded that the structural parameters of metal oxides are important factors for controlling response parameters of resistive type gas sensors. For example, it was shown that the decrease of thickness, grain size and degree of texture is the best way to decrease time constants of metal oxide sensors. However, it was concluded that there is not universal decision for simultaneous optimization all gas-sensing characteristics. We have to search for a compromise between various engineering approaches because adjusting one design feature may improve one performance metric but considerably degrade another. # 2008 Elsevier B.V. All rights reserved. Keywords: Metal oxides; Polycrystalline; One-dimensional; Gas sensor; Sensor response; Morphology and crystallographic structure influence Contents 1. Introduction . 2 2. Structural parameters of metal oxides controlling gas-sensing characteristics . . 3 2.1. The role of sensor geometry and contacts . 3 2.2. The role of dimension factors in gas-sensing effects . . 7 2.2.1. The influence of thickness . . 7 2.2.2. Grain size influence . . 11 2.3. The role of crystallographic structure of metal oxides. 16 2.3.1. Crystal shape . . . 16 2.3.2. Surface geometry 20 2.3.3. Film texturing . . 22 2.3.4. Surface stoichiometry (disordering) . . . 23 2.4. The role of morphology and porosity of metal oxides. 24 2.4.1. Grain networks, porosity, and the area of inter-grain contacts . . 24 2.4.2. Agglomeration . . 28 2.5. Peculiarities of one-dimensional structure characterization. . . 31 3. Concluding remarks 31 Acknowledgements 35 References . . 35 www.elsevier.com/locate/mser A vailable online at www.sciencedirect.com Materials Science and Engineering R 61 (2008) 1–39 * Correspondence address: Korea Institute of Energy Research, Daejeon, Republic of Korea. E-mail address: ghkoro@yahoo.com. 0927-796X/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mser.2008.02.001 1. Introduction Conductometric (resistive) metal oxide sensors comprise a significant part of the gas sensor component market. While many different approaches to gas detection are available [1– 23], metal oxide sensors remain a widely used choice for a range of gas species [1–5,15,24–34]. These devices offer low cost, high sensitivity, fast response and relative simplicity, advantages that should work in their favor as new applications emerge, especially in the field of portable devices. The working principle of a typical resistive metal oxide gas sensor is based on a shift of the state of equilibrium of the surface oxygen reaction due to the presence of the target analyte. The resulting change in concentration of chemisorbed oxygen is recorded as a change in resistance of gas-sensing material. As an example, reducing gases (CO, H 2 ,CH 4 , etc.) lead to an increase of the conductivity for n-type semiconductors and a decrease for p- type material, respectively, whereas the effect of oxidizing gases (O 3 , etc.) is vice versa. The sensor response (sensitivity) of such devices, that is, the ability of a sensor to detect a given concentration of a test gas (analyte), is usually estimated as the ratio of the metal oxide electrical resistance (conductivity) (S = R gas /R air ,orR air /R gas ) measured in air and in an atmosphere containing the target gas. The rate of sensor response is described in such parameters as the response or recovery times, which characterize the time taken for the sensor output to reach 90% of its saturation value after applying or switching off the respective gas in a step function. Numerous materials have been reported to be usable for metal oxide sensors design including both single and multi- component oxides [15,31–33]. At that it has been established that materials in different structural states can be used in those resistive type gas sensors. These states include amorphous-like state, glass-state, nanocrystalline state, polycrystalline state, and single crystalline state. Each state has its own unique properties and characteristics that can affect sensor perfor- mance. However, in practice, nanocrystalline and polycrystal- line materials have found the greatest application in solid-state gas sensors [4,15,24,27,32,35–37]. Nanocrystalline and poly- crystalline materials have the optimal combination of critical properties for sensor applications including high surface area due to small crystallite size, cheap design technology, and stability of both structural and electro-physical properties. Typically amorphous-like and glassy materials are not stable enough for gas-sensing applications, especially at high temperature [32,38]. Single crystalline and epitaxial materials have maximum stability and therefore the use of materials in these sta tes for gas sensors may improve the temporal stability of the sensor. Unlike polycrystalline material, devices based on epitaxial and single crystalline materials will not be plagued with the problem of instability of grain size. However, the high cost and technological challenges associated with their deposition limit their general use in gas sensors. One-dimensional structures, which are single crystalline materials, can be synthesized using inexpensive, simple technology [24,39,40]. Wide use of one-dimensional structures is however impeded by the great difficulties required for their separation and manipulation [41,42]. During the synthesis process of one-dimension structures one may observe a considerable diversity in their geometric parameters. In polycrystalline and even nanocrystalline material we work with averaged grain size, while using one-dimension structures, each sensor is characteristic by the specific geometry of the one-dimensional crystal. Therefore, reproducibility of perfor- mance parameters for sensors based on one-dimension structures would depend on the uniformity of those structures. Unfortunately, the problem of separation, sizing, and manip- ulations of one-dimensional structures is not resolved yet. To achieve uniform sizing and orientation, new advanced technologies will need to be implemented, and these would be expensive and not accessible for wide use. There are a few interesting proposals for controlling one-dimensional structures [43], but they require further improvement for practical implementation. Thus, gas sensors based on individual one- dimension structures are not yet readily available commer- cially. Further, the manufacturing cost of sensors based on one- dimensional structures would far exceed that of polycrystalline devices. Based on what was said above, it becomes clear that in near future, polycrystalline materials would remain the dominant platform for solid-state gas sensors. Nano- and polycrystalline materials are very complicated objects for study, because the electro-conductivity of those materials depends on great number of factors [34,44–55]. Therefore, to specify optimal technologies for gas sensor manufacturing on the basis of such mater ials, it is necessary to expand our understanding of gas sensor mechanism in nano- and polycrystalline oxides. For example, it is necessary to establish the role of morphology and crystallographic structure in gas-sensing effects, because there is a lack of real, detailed, and integrated research establishing a connection between structural parameters of oxides and parameters of sensor response. One cannot find a large number of good works in this field. There are the works of Yamazoe and coworkers in the field of ceramic type sensors [26,44,56–62], in which a direct correlation between grain size of metal oxides and gas sensitivity of conductometric sensors was established; Ega- shira’s group [63–67] conducted a qualitative study of material porosity influence on sensor response; Morante’s group established a correlation between structural and gas-sensing properties of metal oxides [68–73], and papers of Korotcen- kov’s group conducted research in the field of thin film gas sensors [34,74–84]. Korotcenkov’s works emphasized the need for a broader approach for structural engineering of metal oxide films for solid-state gas sensors. While a lot of reviews and book chapters describe the working principle of metal oxide gas sensors in detail [1– 5,15,24–30,37,50], the aim of this review is to summarize the results highlighting the correlation between material structure and gas-sensing properties, and formulating some general conclusions typical for metal oxides. Earlier assessments of modeling morphological effects were made in Refs. [34,47,79,85]. The results used in the present review were obtained mainly with SnO 2 and In 2 O 3 -based gas sensors. These materials are the most studied metal oxides for gas sensor G. Korotcenkov / Materials Science and Engineering R 61 (2008) 1–392 applications [24,27,32,33,36,55,79–81,86–89] as well as the most commercially available. For example tin oxide is indeed the most popular material for gas-sensing due to its relatively low cost, its high sensitivity, and stability in different environments . The main focus in this review is on the analysis of undoped material. Consideration of electro-physical and catalytic proper- ties of a device with a second component would make the analysis too complicated. The introduction of the second component changes both the catalytic activity of base material and the chemical composition. New compounds or solid solutions with specific properties significantly different from the undoped material can be formed during metal oxide doping. Additives could also influence grain size, the shape of crystallites, bulk and surface stoichiometry, properties of intercrystalline barriers, and bulk electro-physical properties [34,74,87,90]. Additional possible effects of metal oxide doping includeformation of p–njunctions, the appearance of transitional areas and layers acting as catalytic filters, the changes in the valency of metal state, and others [91–95]. The analysis of those interrelated processes requires individual consideration. Some important conclusions regarding the influence of the second phase on structural, electro-physical and gas-sensing properties of metal oxides can be found in Refs. [25,33,34,57,71,74,93,95– 99,]. More detailed information about the effect of additives in metal oxide sensors can be obtained also from earlier reviews [24,25,35,46,49,50,52,57,101]. 2. Structural parameters of metal oxides controlling gas-sensing characteristics As it was indicated earlier, the fundamentals of resistive type sensor operation are based on the changes in resistance (or conductance) of the gas-sensing material as induced by the surrounding gas. The changes are caused by various processes, which can take place both at the surface and in the bulk of gas- sensing material [24,34,35,48,51,52,100–105]. Possible pro- cesses, which can control gas-sensing properties, are presented in Fig. 1. The possible consequences of these processes for surface and electro-physical properties of metal oxides are shown in Fig. 2. Research has confirmed that all processes indicated in Fig. 1, including adsorption/desorption, catalysis, reduction/reoxida- tion, and diffusion are relevant in gas sensors and influenced by structural parameters of the sensor material. This affirms that gas-sensing effects are structurally sensitive as well. Taking into account the complexity of the gas-sensing mechanism and its dependence on numerous factors, it becomes clear that we have to consider the influe nce of a great number of various structural parameters of metal oxide matrix on gas sensors’ parameters (see Fig. 3). It has been shown in Refs. [25,46,47,50,85] that the influence of the above-mentioned parameters on gas-sensing characteristics takes place through the changes in the effective area of inter-grain and inter-agglomerate contacts, energetic parameters of adsorption/desorption, number of surface sites, concentration of charge carriers, initial surface band bending, coordination number of metal atoms on the surface, etc. 2.1. The role of sensor geometry and contacts Fig. 4 shows some reported gas sensor electrode geometries. To make measurements on a semiconductor gas-sensing material it is possible to use a compressed pellet (see Fig. 4a), which may or may not be sintered, with metal electrodes on each face. This construction was used in Refs. [106,107] to obtain fundamental information on the tempera- ture dependence of conductance of tin dioxide. In a study of the competition between water and oxygen adsorption in tin dioxide [108], the electrode assembly consisted of two concentric tantalum cylinders with powdered tin dioxide Fig. 1. Diagram illustrating the processes, controlling the rate of sensor response. G. Korotcenkov / Materials Science and Engineering R 61 (2008) 1–39 3 between them (Fig. 4b). However, real devices usually have the sensing material presented as a thin (e.g. sputtered, vacuum- evaporated, or deposited as a result of chemical reactions) or thick (e.g. screen-printed) film on a substrate (Fig. 4 c–f) [109– 114]. Both electrodes can be fabricated together on the substrate before (Fig. 4g) or after (Fig. 4h) the sensing film is deposited. This provides great flexibility in the fabrication process, as it need not be compatible with the sensing material. At first approximation the sensor geometric parameters of length (L) and width (W) do not influence sensor response. The L/W ratio influences only sheet conductivity of the gas sensor (GS). As a rule, one needs to use inter-digital geometry of contacts (Fig. 4e) with small distance between contacts (L)in order to get small sheet conductivity. Appropriate adjustment of these design parameters can achieve acceptable value of gas sensor resistance suitable for further electronic processing. However, in reality the situation might be significantly different. First of all, the purely geometric effect arises because the film conductance does not change instantly or uniformly when the gas ambient changes: the gas must diffuse through the film, reacting with the particle surfaces as it does so. This leads to variations in local film conductance. A numerical simulation indicated, for example, that where a sensor is highly sensitive to the test gas, the sensitivity increased with electrode spacing when the electrodes were underneath the film, but decreased with spacing when the electrodes were deposited on top of the film [111]. If electrode spacing was decreased to less than the film thickness, it was possible to detect a les s-reactive gas in the presence of a more reactive one [111]. The possibility of exploiting these effects to produce self-diagnostic sensors has been considered in Ref. [110]: If two or more pairs of contacts with different separations are made on the sensor, then the Fig. 2. Diagram illustrating processes taking place in metal oxides during gas detection and their consequences for polycrystalline metal oxides properties. (Reprinted with permission from Ref. [105]. Copyright 2007: Elsevier). Fig. 3. Diagram showing structural parameters of metal oxides, which control gas-sensing properties. G. Korotcenkov / Materials Science and Engineering R 61 (2008) 1–394 conductance measured between any two pairs under a given set of conditions will be related by a known function, even though the individual values will, of course, change with test gas concentration. Thus, if the relationship observed deviates from this function, the sensor must be malfunctioning. For thin films discussed above-mentioned effect does not work. However, even in this case the inter electrode distance may be a strong influencing factor. For example, in Ref. [109] it was shown that the decrease of the distance betwee n inter- digitated electrodes from 400 mm to 200 mm may enhance the CO response in ceramic type SnO 2 -based sensors. Even greater differences in sensor parameters could appear when the distance between measurement electrodes in a sensor becomes less than some critical value. Such influences could be connected with the following factors: Electrode materials used (Pt, Pd, Au) are active catalysts with specific catalytic properties. As a result, in the area close to contact (spillover zones), electrode mater ials act as catalysts able to increase activity of gas-sensing metal oxides [97,98,115,116] (see Fig. 5). Spillover is a very important term in catalysis [117]. It is used as a shorthand description of the diffusion of adsorbed species from an active adsorbent to an otherwise inactive support. For instance, this could be the diffusion of atoms from active metal nanoparticles, where the dissociation is non-activated, to a support, where the dissociation directly from the gas phase is activated. Experimentally, this process was determined for hydrogen and oxygen [117]. Therefore, if the distance between contacts is comparable with the width of spillover zone (see Fig. 6b), the influence of geometric parameters of sensors on their gas- sensing characteristics would become noticeable. The width of spillover zone depends on the material and the nature of the detected gas. The influence of the contacts on sensor response with decreased length of the sensitive layer could become stronger because of another reason as well. At some distance the contact’s resistance could be comparable in magnitude or more than the resistance of the gas sensitive layer, especially in the atmosphere of reducing gases (for n-type semiconduct ors). In some cases, the potential barrier b etween the metal of the electrode and the gas-sensing oxide could be comparable to the potential barriers between the metal oxide grains. Under these circumstances, the chemical reactions between gas and metal– metal oxide interface could affect the total conductance of the sensor, even without the influence of the spillover effect [119]. Experimental data confirm both these effects [24,109,119– 123]. For example, Laluze et al. [120] found very large differences in the operation of sintered SnO 2 sensors fabricated using different electrodes. Fig. 7 illus trates how strong this influence could be. One can see that the change of electrode Fig. 4. Possible constructions of solid-state metal oxide sensors and topologies of measurement contacts. (Adapted with permission from Ref. [112]. Copyright 2005: Elsevier). Fig. 5. Schematic illustration of spillover effect at the SnO 2 surface at T oper < 180–210 8C. (Adapted with permission from Ref. [118]. Copyright 2003: Elsevier). G. Korotcenkov / Materials Science and Engineering R 61 (2008) 1–39 5 metal affects both the magnitude of the sensor response and the temperature position of the maximum sensitivity. Other studies also established that different electrode materials can affect sensor behavior [72,109,119,125].For example, in Ref. [125] the characteristics of SnO 2 based sensors with Pt, Au, and Pt–Au contacts were compared. It was shown that at approximately 550 8C, the conductance was about the same and independent on the electrode material. However, below 150 8C the conductance of the sensors with a Pt electrode was about three orders of magnitude higher than for those with Au electrodes. In Ref. [119] it was reported that for SnO 2 thick film sensors the conductance changes induced by H 2 and CO were very different for Pt and Au electrodes. The sensor with Pt electrodes was more sensitive to H 2 , whereas Au electrodes seemed to provide a better response to CO. In these experiments, an inter-digital electrode design with a 5 mm gap was used. The same effect was observed in Refs. [124,126]. In Ref. [126] it was shown that a chlorine detector, made from WO 3 and aluminum electrodes, had sensor response of about 400 for 1 ppm Cl 2 in air. The sensor response dropped to 1 with Pt electrodes. Response to humidity was also affected by the electrode material. In Refs. [109,125], it was established that the influence of water on the CO response of SnO 2 -based sensors is greater in the case of Au contacts, and lower in the case of Pt contacts. That the metal–semiconductor junction may be the main gas-sensing element responsible for the observed sensor response was confirmed in Ref. [127]. In this study, the effect of gap size on the sensor response to dilute NO 2 was investigated (see Fig. 8). Gap sizes in WO 3 microsensors were varied from 0.1 to 1.5 mm. It was found that the response to dilute NO 2 was unchanged for gap sizes larger than 0.8 mm, whereas below 0.8 mm the sensor response tended to increase with decreasing gap size. The sensitivity to 0.5 ppm NO 2 was as high as 57 at a gap size of 0.11 mm. For an explanation of the observed effect it was assumed that the contribution of Fig. 6. Diagram illustrating the role of spillover zones in thin film gas sensors. Fig. 7. Influence of electrode material on gas-sensing characteristics of SnO 2 sensors, fabricated on the base of thin films deposited by electrostatic spray pyrolysis. (Adapted with permission from Ref. [124]. Copyright 1999: Else- vier). Fig. 8. Sensitivities to dilute NO 2 of WO 3 microsensors as a function of gap size. WO 3 microsensors with micro-gap electrodes were fabricated by means of MEMS techniques (photolithography and FIB) and suspension dropping method. WO 3 powders were prepared by wet process. Powders were calcined at 400 8C for 3 h. (Adapted with permission from Ref. [127]. Copyright 2005: Elsevier). G. Korotcenkov / Materials Science and Engineering R 61 (2008) 1–396 resistance at the electrode–grain interface to the total sensor resistance becomes larger when the gap size is decreased. It was concluded that the resistance change at the electrode–grain interface is much larger than that at the inter-grain boundary when the microsensor is exposed to NO 2 . Thus, the sensitivity is increased with the decreasing gap size. It means that at small distances between contacts, the role of contact material is essential and should be considered in the design of gas sensors. Moreover, it is possible to control the sensing properties of semiconductor gas sensors simply by using different electrode materials. Optimum electrode material and electrode geometry could also be used to enhance the gas-sensing properties [25,111,119]. Using the electrodegeometry as a design parameter, onecould probe the variation of sensor signal with electrode position within the porous sensor body (see Fig. 4g and h). If the electrodes are closely spaced, the current for configuration shown in Fig. 4g probes only the base of the sensor layer, while the current probes the whole sensor layer for electrodes that are spaced sufficiently widely. For configuration shown in Fig. 4h we have other situation. The current can be pushed out into the layer by using narrow inter-digitated electrodes, and pulled down towards the base of the layer by using wide electrodes. Such a measurement could, for example, lead to a determination of the rate constant for the surface-catalyzed decomposition, which should be a characteristic parameter of the gas, the surface composition, and the temperature. To some degree, such measurements can beused to identify the gas [25]. Hoefer et al. [128] used an array of electrodes of differing width and separation to examine contact resistance effects in tin dioxide sensors. In its original form, they used the transmission line method for measuring the total resistance of a semiconductor sample as a function of electrode separation. The linear relation obtained allowed a determination of sheet resistance and contact resistance, while an additional ‘‘end resistance’’ measurement allowed an estimation of a further parameter: The ‘‘modified sheet resistance’’ of the film in the vicinity of the electrode [129]. It was shown that the modified sheet resistance displayed greater sensitivity to CO and NO 2 than either the sheet resistance itself or the contact resistance [128].In this case, wide electrodes with narrow spacing would produce the most sensitive detection. An array of electrodes varying in width and spacing (see Fig. 4f), but all using the same sensing material, could be used to resolve a mixture of CO, CH 4 ,NO 2 and water vapor into separate measurements of each component by first determining the relative sensitivity of the total resistance of each electrode pair to the individual gases [128]. Further, simulations have shown that a poorly reactive gas can be detected in the presence of a highly reactive gas if electrode placement and film thickness are chosen well [111]. In Ref. [107] it was found out that the lower detection limit can be improved by reducing the number of grains between the electrodes. The electrode material also affects the stability of the gas sensor. It was shown in Refs. [109,130] that Au electrodes are less stable as compared to Pt electrodes. Scanning electron microscopy (SEM) and resistance measurements, carrie d out in Ref. [130], have shown that platinum on an adhesion layer of titanium was stable up to 500 8C, while the changes in gold films with various adhesion layers were observed at noticeably lower temperatures. For example, gold on chromium starts degrading at as low a temperature as 250 8C. An increased diffusion coefficient and an inclination to form alloys are probably the reasons for such behavior of Au electrodes. If aluminum was to be used as an interconnect metallization, it was found that the maximum stability had contacts with an additional layer of platinum as the metal for making contact with the sensor material (metal oxide) and a barrier layer of titanium–tungsten between the aluminum and platinum. This combination was also usable up to 500 8C. Other layer structures show less thermal stability. Other important aspect of length’s influence appears when the distance between electrodes becomes less than the crystallite’s size (see Fig. 9c and d). In this case we could observe a situation, when the intercrystallite barriers stop affecting the gas-sensing effects, which could induce sig- nificant changes in sensor performance parameters or even loss of sensitivity. This implies that as the distance is decreased, the gas sensor mechanism could change. At sma ll distances only bulk grain effects would be present. It seems that a realization of this condition is impossible in near future for finely dispersed metal oxides. This principle can be realized only with single crystals, epitaxial films, and one- dimensional structures, where the grains and inter-grains boundaries do not exist. However, successful development of new advanced technologies [131] may make it feasible to produce sensors based on one individual grain. As it was shown in Ref. [112], state-of-th e-art electron-beam techniques can produce extremely narrow and closely spaced metallized lines with features of less than 10 nm in size. 2.2. The role of dimension factors in gas-sensing effects 2.2.1. The influence of thickness At present there are three main gas sensor design approaches: (1) – ceramics; (2) – thick film; and (3) – thin film [35]. Therefore, while analyzing the influence of thickness on sensor parameters, it is necessary to remember that for ceramics and thick film sensors the grain size does not depend on the thickness; rather it is determined by the conditions of synthesis and the thermal treatment parameters. The situation for thin film senso rs is fundamentally different. Th e grain size is determined directly by the thickness of the deposited film. The strength of that influence is shown in Fig. 10. The main regularities of film thickness influence on structural properties of SnO 2 and In 2 O 3 deposited by spray pyrolysis were discussed in Refs. [77–79,132]. The influence of film thickness (d) on sensor response to ozone and reducing gases for In 2 O 3 -based sensors fabricated using thin film technology is shown in Figs. 11–13.In 2 O 3 films in these experiments were deposited by spray pyrolysis. It is seen that the change of film thickness can lead to a change in both the magnitude and temperature position of the sensor response’s maximum. At that, the effect of thickness on gas- sensing characteristics was most pronounced for oxidizing gases. When the In 2 O 3 film thickness increases from 20 to G. Korotcenkov / Materials Science and Engineering R 61 (2008) 1–39 7 Fig. 10. The influence of the thickness of In 2 O 3 film deposited by spray pyrolysis on the grain sizes measured by (1) XRD; (2) AFM; and (3) TEM methods. (Reprinted with permission from Ref. [79]. Copyright 2004: Elsevier). Fig. 11. Thickness influence on sensor response to (1 and 2) ozone and (3) H 2 of In 2 O 3 thin films deposited by (1 and 3) spray pyrolysis and (2) sputtering. Results (2) were obtained immediately after In 2 O 3 photoreduction. For this purpose the samples were directly irradiated in vacuum by mercury pencil lamp for 20 min. (Adapted with permission from Refs. [79,81,133]. Copyright 2001 and 2004: Elsevier). Fig. 9. Diagram illustrating the influence of grain size on potential distribution along sensor. G. Korotcenkov / Materials Science and Engineering R 61 (2008) 1–398 400 nm, the gas response to ozone drops by more than a factor of 100 (Fig. 11). This drop in sensitivity can be rationalized by an increase in grain size [77,79] and a decrease of gas- permeability within the film. Due to high activity, ozone decomposition occurs on the top layer of the metal oxide film. Thus, thin films designs should be used for effective detect ion of oxidizing gases. Regarding the detection of reducing gases, especially hydrogen, the opposite effect occurs on the films. In general, thick films work better for hydrogen and other reducing gases. For example, thin In 2 O 3 films, deposited from diluted solutions, had lower sensitivity to hydrogen than thick films (Fig. 11, curve 3). The same effect was observed earlier for SnO 2 films [134], prepared by the spin-coating method. An explanation of this effect was presented in Refs. [59,64,110], where the diffusion- reactive model of gas sensitivity was developed. According to Ref. [64], the increased sensitivity to H 2 in thick films arises because H 2 has a much higher diffusion coefficient than oxygen. It is necessary to note that the sensitivity of sensors fabricated by thick film technology is dependent on film thickness as well. However, different authors have observed significantly different dependencies with thickness (see Fig. 12). Some reports have observed an increase in sensitivity to reduc ing gases with increased film thickness [137], while others have observed a loss in sensitivity [135], and one study observed that the sensor response would reach a maximum at some thickness [138]. Such disagreement demonstrates once again that gas sensitivity of metal oxides is dependent on many factors, which are hard to control. Because the depth of penetration of various gases into the oxide matrix depends on their diffusion coefficient and activity, the disposition of contacts (on top or below gas-sensing layer) starts playing an important role for ceramics or thick film sensors. This effect was studied in detail in Refs. [110,123], and was used for the determination of gas diffusion parameters into tin oxide. Research has shown that sensor characteristics, in particularly the gas nature influence on the temperature dependence of sensor response, are strongly dependent on the position of electrodes (see Fig. 14). Such a strong effect might be used for definition of the nature of detecting gas. At sufficient thickness the top layer of the sensing material could act as a filt er for certain gas molecules [139]. This effect could also explain the conclusion made in Ref. [140] regarding the H 2 response of the SnO 2 -based sensors with two types of noble metal (Au, Pt, and Pd) electrodes covering the surface of the tin oxide nanohole arrays. It was found that the temperature dependence of the sensor response differed between the sensors equipped with a pair of electrodes on both surfaces and the sensors equipped with a couple of inter-digital electrodes on one side. At that, the H 2 response of the sensors equipped with a pair of electrodes on both surfaces was much higher than that of the sensors equipped with inter-digital electrodes on one side. With increased film thickness, problems arise in using physical methods for the deposition of noble metals catalysts onto metal oxides [70,118]. In Refs. [62,72,125], this problem was studied for SnO 2 film doping by Pd and Pt. Some results are Fig. 12. Sensor response of SnO 2 -based sensors to reducing gases vs. film thickness, determined in various laboratories: (a) devices were fabricated by dropping and spinning of sol suspension over an alumina substrate attached with comb-type Au electrodes. SnO 2 powders were prepared by hydrothermal method; (b) SnO 2 films were deposited by (1) MOCVD method on alumina substrates with two Au electrodes, with following annealing at 600 8C for 15 h, and (2) by reactive DC sputtering with following annealing at 600 8C for 10 h. (Adapted with permission from Refs. [70,134,135]. Copyright 1994, 2001, and 2003: Elsevier). Fig. 13. SnO 2 film thickness influence on normalized S(T oper ) dependencies of sensor response to reducing gas. Sensors were fabricated on the base of thin films deposited by spray pyrolysis from SnCl 4 –water solution. (Reprinted with permission from Ref. [136]. Copyright 2001: Elsevier). G. Korotcenkov / Materials Science and Engineering R 61 (2008) 1–39 9 shown in Fig. 15. It was concluded that the optimal technical solution varied with the type of noble metal used as an additive [72]. Other important aspect of the influence of film thickness on gas sensor performance pertains to response and recovery times. The effect of film thickness on the time constants of sensor response to ozone and hydrogen are shown in Figs. 15 and 16. One can see that the time constants of sensor response increase as the film thickness increases. At that, the effect is more pronounced with oxidizing gas than for reducing gases. Response and recovery times for reducing gas exposure on In 2 O 3 -based thin film gas sensors increased nearly 10-fold as the film thickness changed from 20 to 400 nm (Fig. 16). Response times during ozone detection in a dry atmosphere changed almost two orders of magnitude (Fig. 17), although in humid atmosphere, this effect was weaker (see Fig. 17). Thus, gas sensor designers should decrease thickness to improve the sensing characteristics of metal oxide-based gas sensors. The use of thin films assures fast response and recovery. As shown empirically, there is not any diffusion limitation in response kinetics in thin-film devices [81]. The same conclusion was made by the authors of Ref. [143], studying gas-sensing properties of ZnO sensors fabricated by magnetron sputtering. It was found that sensors with minimal film thickness in the range 65–390 nm had the maximum response to CO and minimum response time. These results indicate that thin film gas sensors (d < 100 nm) will always be faster than thick (d > 100 nm) film gas sensors [63]. Another Fig. 14. Influence of electrode position (a) on gas sensitivity of SnO 2 -based thick film sensors (d $ 250–300 mm) loaded with 1.0 wt% of Pt or Pd to (b) H 2 and (c) CH 4 . Porous thick film sensors having (2) interior and (1) surface electrodes were fabricated on a porous mullite tube of 2 mm enter diameter and 1.7 mm inner diameter. SnO 2 powders had a surface area (S surf )of75m 2 /g). (Adapted with permission from Ref. [64]. Copyright 1998: Elsevier). Fig. 15. Scheme of the three doping methods (a), and influence of doping methods on the gas response of SnO 2 -based sensors (d $ 200 nm) to 100 ppm CO (T oper = 400 8C, RH = 40%). SnO 2 films and catalytic additives, (b) Pt and (c) Pd were deposited by reactive DC sputtering. 1, 2, and 3 correspond to methods of doping shown in figure. (Adapted with permission from Ref. [72]. Copyright 2003: Elsevier). G. Korotcenkov / Materials Science and Engineering R 61 (2008) 1–3910 [...]... oxides for different d, X and L is presented in Figs 9 and 20 It is clear that the width of the necks determines the height of the potential barrier for current carriers, while the length of the necks determines the depletion-layer width of the potential barrier It is necessary to note that the increase of the necks length increases the role of necks in the limitation of metal oxide conductivity, and. .. implications for the understanding of their nature This mechanism is expected to be an important one for such reducible oxides as TiO2, Fe2O3, SnO2, and ZnO, where shallow donor states provide a rise to a high density of electrons in the conduction band 2.4 The role of morphology and porosity of metal oxides 2.4.1 Grain networks, porosity, and the area of inter-grain contacts From an analysis of the numerous... band bending (eVs) of the metal oxide and the change of eVs at replacement of the surrounding gas Considering that the sensing properties of thin film sensors such as sensitivity, selectivity, and stability are strongly related to their microstructures and to the exact stoichiometry of their 23 surfaces, an accurate control of these parameters is extremely important for the production of sensors with reproducible... depended on the number of surface donors (oxygen vacancies), which determines the density of conduction band electrons The authors of Ref [242] assumed that the metal oxides act as reservoirs for oxygen; and the O2 diffusion may be a rate-limiting step in oxidation processes on these metal oxides Diffusion of oxygen molecules on a metal oxide surface plays a vital role in gas-sensing effects and therefore... However, it was impossible to find in the literature any correlation between the size of surface clusters and gas sensitivity At the same time the most recent research has shown that the process of surface clustering is structurally sensitive [213], and therefore the consequences of surface modification would depend on the surface structure of the used metal oxides The most important consequences pertain... with the height of potential barrier depending on the surrounding atmosphere In the frame of such approach the grain boundary space charge or band bending on inter-grain interfaces are the main parameters controlling the conductivity of nanocrystalline metal oxides The adequacy of above-mentioned model was estimated on the base of results obtained during the impedance spectroscopy of metal oxides At present,... [77], i.e the presence of a threshold temperature (Tst) below which the crystallites with fixed size remain stable, and the absence of t1/2 type grain size dependencies on time during thermal annealing Because the process of coalescence starts through the breaking of Me-atoms bonds with the lattice of metal oxides, in Ref [77] it was assumed that the formation energy of surface and bulk vacancies of Me-atoms... characterizing the temperature threshold of structural stability The more the energy of VIn and VSn formation is, the more stable is the lattice, i.e the temperature of possible grain coalescence is higher In spite of the fact that the observed process of grain growth in both In2O3 and SnO2 films takes place through diffusion of Me-atoms (In, Sn) following their incorporation in the lattice of a bigger... grain size and porosity control are the best ways for the improvement of metal oxide sensor response to oxidizing gases For reducing gas detection the requirements are more ambiguous and depend on both the sensor material and the nature of the detected gas For example in the case of thin film gas sensors the decrease in thickness, grain size, and degree of texture are the optimal approaches for the optimization... orientation and grain size dependent The decrease in crystal size in the nm range notably strengthens the crystallite shape influence on the adsorption properties Both the shape and the size of nanocrystals have a profound influence on the concentration of adsorbed species and on the type of bonding to the surface that takes place It is known that depending on the type of bonding, some chemical species . The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors G (disordering) . . . 23 2.4. The role of morphology and porosity of metal oxides. 24 2.4.1. Grain networks, porosity, and the area of inter-grain contacts .