Chemiresistive Gas Sensors
Chemiresistive gas sensors detect gases through the change in electrical resistance of a sensing material, most often a semiconducting metal oxide such as TiO2 or SnO2. Adsorbed or reacting molecules trap or donate conduction electrons near the surface, so the resistance shifts with gas exposure; supported metal or metal oxide particles are often added to improve sensitivity and selectivity. See also Metal-support interactions and Oxygen vacancies in oxides.
Four-point probe study of TiO2(001) and Pd/TiO2(001) in UHV (2003)
McCormick, Kitchin, Barteau and Chen used a linear four-point probe together with Auger spectroscopy and STM in ultrahigh vacuum to study rutile TiO2(001) as a model oxygen sensor. Sputtering lowered both the surface O/Ti ratio (to about TiO1.5 after 2500 s) and the resistivity, while exposing a sputtered surface to 1.1 x 10-5 Torr O2 at 300 K raised the resistivity irreversibly, with a transient that was better fit by dissociative (second-order) than molecular (first-order) adsorption kinetics. Pd particles annealed to 573 K (5 to 12 nm) enhanced the response to 2.5 x 10-7 Torr O2, whereas as-deposited (1 to 6 nm) and 773 K annealed (10 to 23 nm) particles gave little enhancement, which the authors attribute to the number of Pd sites for O2 adsorption and spillover (McCormick et al. 2003). Summary: mccormick2003.