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Metal oxide nanostructures and their gas sensing properties: a review.

Sun YF, Liu SB, Meng FL, Liu JY, Jin Z, Kong LT, Liu JH - Sensors (Basel) (2012)

Bottom Line: When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called "small size effect", yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them.Besides, doping is also an effective method to decrease particle size and improve gas sensing properties.The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical and Automotive Engineering, Anhui Polytechnic University, Wuhu 241000, China. sunyufeng118@126.com

ABSTRACT
Metal oxide gas sensors are predominant solid-state gas detecting devices for domestic, commercial and industrial applications, which have many advantages such as low cost, easy production, and compact size. However, the performance of such sensors is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties. Lots of metal oxide nanostructures have been developed to improve the gas sensing properties such as sensitivity, selectivity, response speed, and so on. Here, we provide a brief overview of metal oxide nanostructures and their gas sensing properties from the aspects of particle size, morphology and doping. When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called "small size effect", yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them. In view of those reasons, nanostructures with many kinds of shapes such as porous nanotubes, porous nanospheres and so on have been investigated, that not only possessed large surface area and relatively mass reactive sites, but also formed relatively loose film structures which is an advantage for gas diffusion. Besides, doping is also an effective method to decrease particle size and improve gas sensing properties. Therefore, the gas sensing properties of metal oxide nanostructures assembled by nanoparticles are reviewed in this article. The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given.

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Schematic model of the effect of the crystallite size on the sensitivity of metal-oxide gas sensors: (a) D >> 2L; (b) D ≥ 2L; (c) D < 2L.
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f4-sensors-12-02610: Schematic model of the effect of the crystallite size on the sensitivity of metal-oxide gas sensors: (a) D >> 2L; (b) D ≥ 2L; (c) D < 2L.

Mentions: The “small size effect” of metal oxides has been reported by many publications [21–27]. As shown in Figure 4, a sensor is considered to be composed of partially sintered crystallites that are connected to their neighbors by necks. Those interconnected grains form larger aggregates that are connected to their neighbors by grain boundaries [28]. On the surface of the grains, adsorbed oxygen molecules extract electrons from the conduction band and trap the electrons at the surface in the form of ions, which produces a band bending and an electron depleted region called the space-charge layer. When the particle size of the sensing film is close to or less than double the thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably. Xu et al. explained the phenomena by a semiquantitative model [29]. Three different cases can be distinguished according to the relationship between the particle size (D) and the width of the space-charge layer (L) that is produced around the surface of the crystallites due to chemisorbed ions and the size of L is about 3 nm for pure SnO2 material in literatures [30–34]. When D >> 2L, the conductivity of the whole structure depends on the inner mobile charge carriers and the electrical conductivity depends exponentially on the barrier height. It is not so sensitive to the charges acquired from surface reactions. When D ≥ 2L, the space-charge layer region around each neck forms a constricted conduction channel within each aggregate. Consequently, the conductivity not only depends on the particle boundaries barriers, but also on the cross section area of those channels and so it is sensitive to reaction charges. Therefore, the particles are sensitive to the ambient gas composition. When D < 2L, the space-charge layer region dominates the whole particle and the crystallites are almost fully depleted of mobile charge carriers. The energy bands are nearly flat throughout the whole structure of the interconnected grains and there are no significant barriers for intercrystallite charge transport and then the conductivity is essentially controlled by the intercrystallite conductivity. Few charges acquired from surface reactions will cause large changes of conductivity of the whole structure, so the crystalline SnO2 becomes highly sensitive to ambient gas molecules when its particle size is small enough.


Metal oxide nanostructures and their gas sensing properties: a review.

Sun YF, Liu SB, Meng FL, Liu JY, Jin Z, Kong LT, Liu JH - Sensors (Basel) (2012)

Schematic model of the effect of the crystallite size on the sensitivity of metal-oxide gas sensors: (a) D >> 2L; (b) D ≥ 2L; (c) D < 2L.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3376589&req=5

f4-sensors-12-02610: Schematic model of the effect of the crystallite size on the sensitivity of metal-oxide gas sensors: (a) D >> 2L; (b) D ≥ 2L; (c) D < 2L.
Mentions: The “small size effect” of metal oxides has been reported by many publications [21–27]. As shown in Figure 4, a sensor is considered to be composed of partially sintered crystallites that are connected to their neighbors by necks. Those interconnected grains form larger aggregates that are connected to their neighbors by grain boundaries [28]. On the surface of the grains, adsorbed oxygen molecules extract electrons from the conduction band and trap the electrons at the surface in the form of ions, which produces a band bending and an electron depleted region called the space-charge layer. When the particle size of the sensing film is close to or less than double the thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably. Xu et al. explained the phenomena by a semiquantitative model [29]. Three different cases can be distinguished according to the relationship between the particle size (D) and the width of the space-charge layer (L) that is produced around the surface of the crystallites due to chemisorbed ions and the size of L is about 3 nm for pure SnO2 material in literatures [30–34]. When D >> 2L, the conductivity of the whole structure depends on the inner mobile charge carriers and the electrical conductivity depends exponentially on the barrier height. It is not so sensitive to the charges acquired from surface reactions. When D ≥ 2L, the space-charge layer region around each neck forms a constricted conduction channel within each aggregate. Consequently, the conductivity not only depends on the particle boundaries barriers, but also on the cross section area of those channels and so it is sensitive to reaction charges. Therefore, the particles are sensitive to the ambient gas composition. When D < 2L, the space-charge layer region dominates the whole particle and the crystallites are almost fully depleted of mobile charge carriers. The energy bands are nearly flat throughout the whole structure of the interconnected grains and there are no significant barriers for intercrystallite charge transport and then the conductivity is essentially controlled by the intercrystallite conductivity. Few charges acquired from surface reactions will cause large changes of conductivity of the whole structure, so the crystalline SnO2 becomes highly sensitive to ambient gas molecules when its particle size is small enough.

Bottom Line: When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called "small size effect", yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them.Besides, doping is also an effective method to decrease particle size and improve gas sensing properties.The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical and Automotive Engineering, Anhui Polytechnic University, Wuhu 241000, China. sunyufeng118@126.com

ABSTRACT
Metal oxide gas sensors are predominant solid-state gas detecting devices for domestic, commercial and industrial applications, which have many advantages such as low cost, easy production, and compact size. However, the performance of such sensors is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties. Lots of metal oxide nanostructures have been developed to improve the gas sensing properties such as sensitivity, selectivity, response speed, and so on. Here, we provide a brief overview of metal oxide nanostructures and their gas sensing properties from the aspects of particle size, morphology and doping. When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called "small size effect", yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them. In view of those reasons, nanostructures with many kinds of shapes such as porous nanotubes, porous nanospheres and so on have been investigated, that not only possessed large surface area and relatively mass reactive sites, but also formed relatively loose film structures which is an advantage for gas diffusion. Besides, doping is also an effective method to decrease particle size and improve gas sensing properties. Therefore, the gas sensing properties of metal oxide nanostructures assembled by nanoparticles are reviewed in this article. The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given.

Show MeSH
Related in: MedlinePlus