Experimental and computational study of the gas-sensor behaviour and surface chemistry of the solid-solution Cr2-xTixO3 (x <= 0.5)

The solid solution Cr2-xTixO3 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 similar or equal to 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 (10 (1) over bar2) surfaces have been assessed by computational modelling: in the absence of Ti, one stable defect is a Cr-VI-V-Cr'''dagger 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))(3) V-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.