Abstract

The solid solution Cr_2−xTi_xO₃ is an excellent gas sensor material, with stability of performance over the short and long-term and minor influences of variations of humidity. It is the first new material to be successfully commercialised in large-volume manufacture for sensing of hydrocarbons, volatile organic compounds (VOC), hydrogen and carbon monoxide at low (ppm) concentrations in air since the introduction of SnO₂ for this purpose in the 1960s. The phase limit is at x ≃ 0.3–0.4, above which a 2-phase mixture with CrTiO₃ is found. Substitution of Ti strongly decreases the electrical conductivity of the porous bodies studied. Surface high-valency Cr, assumed to be Cr^VI, whose proportion is decreased by Ti substitution, is detected by XPS. This effect, and the surface segregation of Ti, control the gas sensor behaviour. Defect models of the (0001) and (102) surfaces have been assessed by computational modelling: in the absence of Ti, one stable defect is a Cr^VI–V_Cr‴ pair, which is surface segregated at (0001) and contributes to the relatively high p-type conductivity shown by finely porous bodies of Cr₂O₃ at elevated temperature; with Ti addition, a stable defect, also surface segregated, is the complex (Ti˙_Cr)₃V_Cr‴. Distortion of the arrangement of surface oxygen above the Cr vacancy creates a possible binding site; the high-valency surface cation creates another. It is suggested that the two sites act in concert to promote the dissociation of oxygen and the surface reaction needed for gas sensing.