Modified CuO surface appropriate for selective CO gas sensing at CuO/ZnO heterocontact

The surface properties of copper oxide (CuO) were discussed for considering the working mechanism of a CuO/ZnO heterocontact gas sensor having carbon monoxide (CO) gas selectivity. We focused on p-type semiconducting CuO, using sodium as a dopant, which is a key material for selective CO gas sensing and its surface properties were characterized by XPS, TPD and FT-IR analyses. It was found that the Cu/O ratio of the 1 mol% Na2CO3-added CuO surface was higher than that of pure CuO surface by XPS analysis. By TPD analysis, it was found that CO was adsorbed more strongly on 1 mol% Na2CO3-added CuO than on pure CuO. By the in situ IR measurement of CO adsorbed on the surface of 1 mol% Na2CO3-added CuO specimen under 300 degrees C, we found two asymmetric bands of CO, whereas an asymmetric streching CO2 band was found on IR measurement of pure CuO specimen. It was confirmed that Cu atoms on pure CuO surface exposed to CO at 300 degrees C changed from Cu2+ to Cu1+ or Cu-0, in contrast to Cu atoms on 1 mol% Na2CO3-added CuO surface, which changed to Cu2+. The electrical conductivity was measured as a function of temperature for pure CuO, CuO with 1 mol% Na2CO3 and ZnO specimens. The resistivity of ZnO was larger than that of pure CuO and CuO with 1 mol% Na2CO3 by three and six orders of magnitude at 250 degrees C, respectively. A working mechanism of CuO (Na)/ZnO heterocontact gas sensor was elucidated in order to explain its high CO selectivity.