TiO2:WO3 composite humidity sensors doped with ZnO and CuO investigated by impedance spectroscopy

For impedance-type sensors based on semiconducting metal oxides, the overall conduction mechanisms strongly influence the magnitude and the direction of the sensor signal variation. For humidity, in particular, the electronic/ionic charge-transfer reactions that take place at the semiconductor surface can be used to monitor and control it. In recent years, various mechanisms have been proposed to explain the variations of electrical response to humidity. With the study of composite materials, we expect to obtain a better sensitivity of these sensors, when compared with the ones made out of only one metal oxide. This could be due to the fact that some of the positions initially occupied by the atoms of one of the metals are now occupied by atoms of the other metal: if a single covalent/ionic adsorption is decisive in the observed changes in the material's conductivity, then the electronegativity of the occupying metal atoms may be used to regulate the sensitivity. In this paper, TiO2:WO3 composite oxide bulk sensors, using 48.92 and 51.08% (w/w) of titanium and of tungsten atoms, respectively, doped with the same proportions of cooper and zinc oxides (7%), were prepared by a conventional sintering method, and their dependence of their complex impedance spectra, measured in the range 100 Hz–10 MHz, on the relative humidity (RH), operating temperature and on the measuring frequency is shown and explained