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To verify theoretical model based on the surface depletion effect of oxide semiconductor in small crystallite, the SnO 2 particles of different crystallite size and donor density were prepared by controlling heat-treatment temperature and Fe 3+ doping concentration, respectively. In addition, Fe 3+ -doped SnO 2 was compared with Fe 2 O 3 -loaded SnO 2 to discuss the effect of donor density. The electrical resistance and sensor response of prepared SnO 2 films were measured in various partial pressures of oxygen and hydrogen. As results, both undoped-and Fe 3+ -doped SnO 2 showed the volume depletion in the oxygen concentration of more than 2.5% at 350 • C. The dependence of electrical resistance on oxygen partial pressure for smaller crystallite had steeper slope. Furthermore Fe 3+ -doping improved the sensor response to hydrogen, while the Fe 2 O 3 -loading did not work. Good agreement between experimental data the volume depletion theory was found.Semiconductor gas sensors using metal oxides have been used widely for various purposes such as safety, environments, amenity and so on. 1 In particular, SnO 2 is well known as an attractive material because of its high sensor response to reducing gases, long-term stability, robustness and low-cost. Such a high sensor response is significantly correlated with oxygen adsorbed on the SnO 2 surface as negatively charged species (typically O − ), which accompanied by the formation of depletion layers inside the SnO 2 particles. It was assumed that the thickness of depletion layer should increase with increasing the oxygen adsorption, and that it should be decreased by reacting with a flammable gas (H 2 ) as follows.Correspondingly, it was understood that the height of double Schottky barrier formed across the contact between grains changes, meaning the device resistance changes. Unfortunately, such model was unable to give any quantitative information about gas response. Shortcomings of the models were made clear recently. 2-4 Depletion in small semiconductor crystals is characterized by the occurrence of new type depletion (volume depletion) after conventional one (regional depletion), and inclusion of both types makes it possible to explain the receptor function and response to oxygen (air base), oxidizing gas (NO 2 ) and reducing gas (H 2 ). The equations derived theoretically by using both physical parameters of the semiconductor side and chemical parameters of the gases side appear to reproduce satisfactorily the sensing behavior to the above gases as well as the influence of changes in physical parameters such as crystallite size and donor density.About twenty years ago, remarkable crystallite size effects were found by preparing small crystallites of SnO 2 . 5 By decreasing the crystallite size, the resistances in air (base) as well as under exposure to H 2 or CO in air, R a and R g , respectively, increase sharply beyond a critical value, and the sensor response to H 2 or CO, R a /R g , also increased. Although such crystallite size effects seemingly appeared to m...
Methane is an important gas for domestic and industrial applications and its source is mainly coalmines. Since methane is extremely inflammable in the coalmine atmosphere, it is essential to develop a reliable and relatively inexpensive chemical gas sensor to detect this inflammable gas below its explosion amount in air. The metal oxides have been proved to be potential materials for the development of commercial gas sensors. The functional properties of the metal oxide-based gas sensors can be improved not only by tailoring the crystal size of metal oxides but also by incorporating the noble metal catalyst on nanocrystalline metal oxide matrix. It was observed that the surface modification of nanocrystalline metal oxide thin films by noble metal sensitizers and the use of a noble metal catalytic contact as electrode reduce the operating temperatures appreciably and improve the sensing properties. This review article concentrates on the nanocrystalline metal oxide methane sensors and the role of noble metals on the sensing properties.
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