THE EFFECTS OF CRYSTALLITE GROWTH AND DOPANT MIGRATION ON THE CARBON-MONOXIDE SENSING CHARACTERISTICS OF NANOCRYSTALLINE TIN OXIDE-BASED SENSOR MATERIALS

Tin oxide nanocrystals, both pure and Cu2+ and Fe3+ doped, have been p repared by a sol-gel process. The response of these materials to carbo n monoxide in dry air has been investigated as a function of annealing temperature. The growth of the crystallites was monitored by XRPD fro m room temperature to about 900 degrees C and the response of the mate rial to CO was studied for materials annealed over that temperature ra nge. All of the materials were shown to respond to low concentrations of CO with a narrow peak in sensitivity at an operating temperature of about 200 degrees C. A good response to CO was also observed at an op erating temperature of about 400 degrees C. No improvements in selecti vity to CO were observed by the addition of either of the cation dopan ts. The sensitivity to CO was shown to decrease as crystallite size in creased. The addition of the metal cation dopants impeded crystallite growth. Our previously reported Cu K-edge and Fe K-edge EXAFS measurem ents, on the Cu2+ and Fe3+ doped materials respectively, showed the do pant cations to move from ordered Sn4+ substitutional lattice sites in the as-prepared materials to more disordered regions, most likely the surface regions, as the materials were annealed. This dopant migratio n begins at about 400 degrees C and is accompanied by a corresponding large decrease in response to CO at an operating temperature of about 200 degrees C (the peak in sensitivity). This is attributed to the mig ration of the dopants to the surface of the crystallites and speculati ve explanations are given. The reduction in response of the same mater ials at an operating temperature of about 400 degrees C is not so larg e, indicating the response mechanisms at 200 and 400 degrees C to be d ifferent.