The characterization of a CuO/ZnO heterocontact-type gas sensor having selectivity for CO gas

An extensive characterization has been carried out for copper oxide (CuO)/zinc oxide (ZnO) heterocontacts having carbon monoxide gas selectivity. We have focused on p-type semiconducting CuO, using sodium as a dopant, which is a key material for CuO/ZnO heterocontacts having carbon monoxide gas selectivity. It is shown that this heterocontact responds quickly and selectively to CO. XRD results show that, by adding 1 mol% Na2CO3 to pure CuO, the monoclinic lattice constants a and b and the angle beta increase, but the lattice constant c decreases. The electrical conductivity is measured as a function of temperature for pure CuO and for CuO with 1 mol% Na2CO3. The resistivity of pure CuO is higher than that of CuO(Na) by three orders of magnitude at 250 degrees C. The capacitance is measured for pure CuO/ZnO and CuO(Na)/ZnO heterocontacts in pure dry air and in air with H-2 or CO. The increase of capacitance in response to CO is larger than the H-2 response in the CuO(Na)/ZnO heterocontact, but the reverse result is found in the pure CuO/ZnO heterocontact. It is concluded from XPS results that chemisorbed oxygen is less prevalent on the surface of the CuO(Na) than on the pure CuO surface.