Electrochemical Gas Sensors: Technology and Applications

1. ECE699 – 004Sensor Device Technology Chapter 2 Electrochemical Sensors Fall 2018 George Mason University

2. 2.1 Introduction to EC sensors Conductimetric, potentiometric and amperometric An example

3. 2.2 Conductimetric sensors 3 configurations

4. The circuit model

5. Metal-insulating oxide-conducting sensing film

6. The contact resistance of a junction Richardson constant Current through the metal/semiconductor junction

7. The resistance of the active layer If it is semiconductor:

8. Types of conductimetric sensors

9. 3. Semicoducting metal oxide sensors

10. 3. Semicoducting metal oxide sensors • Variety of semiconducting metal oxide materials • Tunable conductivity • Can be functionalized to detect target chemical • Adaptive to different environment • Low cost Most frequently used: ZnO, SnO2, WO3, TiO2, CuO

11. N-type

12. P-type

13. Electrical properties • Scattering of carriers is dominated by phonon scattering - mobility • However, the carrier concentration increases exponentially with increasing temperature – greater than the decrease by mobility

16. Surface of metal oxide films

19. The surface potential barrier: The electrons need to overcome this barrier to contribute to the current

20. Surface/space charge capacitance

22. Measurement setup

24. Accumulation and inversion layers

26. Semiconductor metal oxide film in the following forms: • Single crystal • Amorphous • Polycrystalline • Compressed powders

27. Compressed powders

28. Gas-semiconductor film interactions Three gas adsorptions: • Physisorption: bonding is physical, is weak • Chemisorption: chemically bonded, the bonding is >2.5 times stronger • Ionosorption: bo bonding occurs, by an electrostatic interaction, a surface state capturing an electron. Focus on the transfer of charge from film to gas

29. Adsorption of O2 : • Ambient O2 picks Up an electron • O2 is broken into two oxygens on metal oxide surface, then attract e from metal oxide bulks

30. The reaction with a reducing agent R •  more often

31. In the case of carbon: The amount of ionsorbed oxygen decreased, barrier potential decreased, excess electrons is produced  metal oxide film resistance decreased

32. Reaction with an oxidizing gas The resistance of metal oxide film increased because electrons were used.

34. Interaction with O2  decrease free electrons • Interaction with H2  H2 donate electrons • Interaction with water  H+ ion attracted to the film  decrease O- in the metal oxide

35. 2.5 other Electrochemical gas sensors • Solid-state capacitive gas sensors • MOS capacitive gas sensors • Micromachine capacitive polymer gas sensors • Schottky diode type gas sensors • Based on MIS capacitive structure

36. MOS capacitive gas sensors Usually, • Oxidizing gases increase the barrier potential • Reducing gases decrease the barrier potential • The selectivity depends on the materials in the device • Gases can be absorbed in metal, oxide or semiconductor • The insulating layer should be thicker than 10nm to prohibit electron tunneling

37. Pt detect H2 containing gases

39. Capacitive polymer gas sensors The polymer is the chemical selective layers

40. Schottky diode gas sensors

42. Pt/TiO2 MS diode gas sensor

43. Mid term exam Prepare a 20-slides review and 6-page report on one of the following subject • Gas sensors on H2, CO, CO2 and O2 • Gas sensors on toxic gases: Nox, Sox, H2S and S-containing gases • Gas sensors on organic solvent gases