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Metal oxide gas sensors (sensing mechanism)

 

                                                                       H.-J. Kim, J.-H. Lee, Sens. Actuators B 2014;192:607-627.

<Gas sensing mechanism of n- and p-type metal oxides>

                                                

- In n-type metal oxides such as SnO2, ZnO, In2O3, and WO3, the resistive electron depletion

  layers are formed near the surface by the adsorption of oxygen with a negative charge. Thus,

  their resistance is relatively high in the air atmosphere.

- When exposed to reducing gases, the surface oxygen ions oxidize the gas, in the meantime,

  the electrons of the oxygen go back to the electron depletion layer. Thus, their resistance starts to

  decrease.

- We can detect gases by comparing the resistance of the sensors before and after gas exposure. 

- In p-type metal oxides such as Co3O4, NiO, and Cr2O3, the hole accumulation layers are formed

  near the surface by the oxygen adsorption. Thus, their resistance is relatively low in the air and

  the resistance increases with gas exposure. 

 

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Gas sensors using nanostructures

 

                                                           J.-H. Lee, Sens. Actuators B 2009;140:319-336.

<Nanostructures for gas sensor application>

 

- In general, nanoparticles with high surface area and reduced grain size show high chemiresistivity

  to gases. However, they are readily agglomerated at the sensor operation temperatures of 200-400°C, 

  leading to the degradation of sensing performance with time.

- Nanostructures with abundant pores and small grain sizes are excellent platform materials to minimize

  the effect of agglomeration. Thus. they can offer high gas response and rapid response/recovery

  speeds without significant degradation of sensing performance even at high temperatures. 

- Gas sensors using nanostructures can be used in the indoor air quality monitoring, food freshness

  assessment, and breath analysis for disease diagnosis.

 

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Controlling gas selectivity of nanostructured metal oxides

 

<Strategies to achieve gas selectivity of metal oxide gas sensors>

 

- Despite the advantages of nanostructured metal oxide gas sensors, lack of gas selectivity hampers

  their practical applications in the current industry.

- To date, many efforts have been directed, which include the modulation of sensor temperatures,

  the loading of catalysts, and the utilization of gas filtering layers. Nevertheless, only limited achievements

  have been accomplished on gas selectivity.

- Our laboratory aims to develop high-performance gas sensors with exclusive gas selectivity capable of

  detecting trace amounts of target gases with negligible cross-responses to other interfering gases.