Dear Colleagues,

During the past three decades, there have been efforts to develop chemical gas sensors based on solid-state technology. The potential advantages of these sensors are miniaturization, low cost, easy calibration and measurement. The market currently offers several types of gas sensors, however their functional parameters remain still unsatisfactorily.

The most widely studied are conductometric gas sensors.  Their principle of operation involves a change in the resistance of the sensor sensitive phase (usually metal oxide) upon exposure to a specific component of the gas atmosphere. The sensor’s performance is determined by the charge transfer between the adsorbed molecules and the sensor material. Chemisorption on a semiconducting material may be considered as the formation of local centers (donors or acceptors) that have a tendency to ionize, in consequence resulting in a charge transfer between the material and the adsorbed gas.

Successful development of the conductometric gas sensors for commercialization requires achieving four “S”: sensitivity, selectivity, short response time and stability. Among them, selectivity is the most important requirement in the conductometric sensors, because the main drawback is their poor selectivity. The important problem is effect of humidity on the sensor performance.

The high operating temperature makes material problems with long term stability and high costs of the sensor manufacture and maintenance. The desired challenge is decrease of the operating temperature (optimally to room temperature).

The promising trends in improving the gas sensors performance is to use metal oxide nanomaterials and heterostructures. Metal oxides-components of the chemical sensors can be used in monitoring gas atmosphere (gas sensors) and chemical compounds in liquid medium.

There are several types of the chemical sensors such as:

1. Electrochemical sensors (conductometric, potentiometric and amperometric).
   
   • i) Conductometric sensors are the most numerous. They use various metal oxides (binary, ternary etc.) and their solid solutions as sensitive material: SnO₂, ZnO, TiO₂, Fe₂O₃, NiO, CoO, VO₂, V₂O₅, WO₃, Cr₂O₃, Al₂O₃, In₂O₃, Ga₂O₃, SrTiO₃, BaTiO₃ etc.
   • ii) Potentiometric and amperometric sensors use as solid electrolytes mainly: stabilized zirconia: metal oxide-ZrO₂ or stabilized ceria: metal oxidse CeO₂ (metal oxide: Y₂O₃, Gd₂O₃, CaO, MgO), high temperature protonic electrolytes (acceptor doped Sr or Ba cerates and zirconates), β-alumina, nasicon, lisicon etc. Various metal oxides are used to manufacture of a sensing electrode.
2. Calorimetric (pelistors) sensors use mainly Al₂O₃ as insulator material.
3. Surface or bulk acoustic wave sensors use oxide perovskites such as LiNbO₃, LiTaO₃, BaTiO₃ as piezoelectric material and such oxides as ZnO, TeO₂ and SnO₂ to coating surface of the piezoelectric element.

Prof. Dr. Mieczyslaw Rekas
Guest Editor

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• conductometric gas sensors
• applications of chemical sensors
• metal oxides
• defect structure
• electrical properties
• chemisorption
• cross sensitivity
• operating temperature
• nanomaterials
• heterostructures
• sensor mechanism