Selectivity of gas sensor is very important. My question is, why do ZnO nanowires give selective sensing and which things affect the gas selectivity?
Selectivity of gases by sensors is based on sensing ability of the respective materials.
Usually, the non-covalent interactions between the gases molecules and the sensing materials are responsible for the enhanced selectivity.
It depends on many factors...such as, surface electrocatalytic information, change in metal-oxide bond energy, polarity/non-polarity of the surface/gases, presence of noble metal, sensor architecture and porosity, oxidation energy of reducing gases....etc .... Therefore no single reason..
In general，the sensing mechanism can be explained by the change in resistance caused by the adsorption of oxygen and reaction with test gas molecules on the surface of oxides. Therefore, the sensing properties of semiconductor oxides is usually influenced by their chemical composition, morphologies and microstruc-
tures. Except that, the properties of tested gases and operating temperature of gas sensors also have a influence on the selectivity of ZnO.
The sensing properties of ZnO nano-structures are usually dependent on their size, especially for nano-wires their diameter is very important, if it is less then 30 nm then there will be quantum confinement which can affect sensing properties much, The chemical composition and morphology also affects the sensing properties.
Selectivity in metal oxides is a tricky subject. Most metal oxides detect gases through oxygen chemisorption. Hence no matter what the gas, the mechanism is similar. Hence selectivity in metal oxides can only be achieved if the metal oxide is somehow preferentially made to react with a particular gas. Certain gas reactions need certain energy to proceed. This is usually achieved through changing operating temperature for different reactions to be favoured. Reducing the size (going nano-sized) usually increases the sensitivity/response and does not play a major part in selectivity. If the surface energy of the nanostructure is comparable to the gas reaction energy, that gas sensing is favoured. This could accidentally be the reason for observed selectivity in nanostructures. But tailoring the nanostructure to a particular surface energy or activation energy is very difficult. Other well-known technique is addition of dopants which act as reaction centres for particular gas reaction to take place (e.g. Pt or Pd for hydrogen selectivity).
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