A series of SnO2-CO3O4 composite thick films (about 10 mum thick) containing 0-100% CO3O4 by mass of the composites were prepared from the component oxides through mixing by ball-milling for 24 h, screen-printing and sintering at 700degreesC for 3h. The composite films were found to exhibit n- or p-type response to CO and H-2 depending on the CO3O4 contents of the composites. The n-type response was exhibited at 200degreesC or above by SnO2-rich composites (CO3O4 content up to 5 mass%). The sensor response to both CO and H-2, defined as R-a/R-g, where R-a and R-g are electrical resistances in air and gas, respectively, was promoted strongly by the addition of small amounts of Co3O4 to SnO2, and the response at 250degreesC as correlated with the CO3O4 contents went through a sharp maximum at 1 mass% CO3O4. The p-type response, obviously originating from Co3O4 (p-type), was exhibited at 200degreesC or below by the composites containing 25-100 mass% Co3O4. Remarkably, sensor response to CO, defined as R-g/R-a, was larger with the 50% Co3O4 composite than with the neat Co3O4, while the reverse was true for the response to H-2. Thus fairly sensitive and selective detection of CO over H-2 was possible with the 50% Co3O4 composite. The physical state of the composites and the mechanisms of promotion of n- and p-type response were discussed. (C) 2003 Elsevier B.V. All rights reserved.
