The negative charge trapped in these oxygen species causes an upward www.selleckchem.com/products/CAL-101.html band bending and thus a reduced conductivity compared to the flat band situation. As shown in Figure 1 [23], when O2 molecules are adsorbed on the surface of metal oxides, they would extract electrons from the conduction band Ec and trap the electrons at the surface in the Inhibitors,Modulators,Libraries form of ions. This will lead a band bending and Inhibitors,Modulators,Libraries an electron-depleted region. The electron-depleted region is so called space-charge layer, of which thickness is the length of band bending region. Reaction of these oxygen species with reducing gases or a competitive adsorption and replacement of the adsorbed oxygen by other molecules decreases and can reverse the band bending, resulting in an increased conductivity.

O? is believed to be dominant at the operating temperature of 300�C450 ��C [5] which Inhibitors,Modulators,Libraries is the work temperature for most metal oxide gas sensors. Figure 2 schematically shows the structural and band model of conductive mechanism upon exposure to reference gas with or without CO When gas sensors exposure to the reference gas with CO, CO is oxidized by O? and released electrons to the bulk materials. Together with the decrease of the number of surface O?, the thickness of space-charge layer decreases. Then the Schottky barrier between two grains is lowered and it would be easy for electrons to conduct in sensing layers through different grains. However, the mechanism in Figure 1 is only suitable for n-type semiconducting metal oxides of which depletion regions are smaller than grain size.Figure 1.

Schematic diagram of band bending after chemisorptions of charged species (here the ionosorption of oxygen) EC, EV, and EF denote the energy of the conduction band, Inhibitors,Modulators,Libraries valence band, and the Fermi level, respectively, while ��air denotes the thickness …Figure 2.Structural and band models of conductive mechanism upon exposure to reference gas. (a) with or (b) without CO (adapted from [23,24]).4.?Factors Influencing the Sensitivity4.1. Chemical CompositionSemiconducting metal oxides have been investigated extensively at elevated temperatures for the detection of simple gases. [25] There are many parameters of materials for Carfilzomib gas sensor applications, for example, adsorption ability, catalytic activity, sensitivity, thermodynamic stability, etc. Many different metal oxide materials appear favorable in some of these properties, but very few of them are suitable to all requirements.

For this situation, more recent works focus on composite materials, such as SnO2-ZnO [26,27] Fe2O3-ZnO [28], ZnO-CuO [29] etc. In addition to binary oxides, there are numerous ternary, selleck bio quaternary and complex metal oxides, which are of interest of mentioned applications [30,31]. The combination of metal oxides and other components, for example, organic and carbon nanotubes, were also investigated much.