Effect of composition on electrical response to humidity of TiO2:ZnO sensors investigated by impedance spectroscopy

For impedance type sensors based on semiconducting metal oxides, the overall conduction mechanisms have a large influence on the magnitude and on the direction of the sensor signal variation. For humidity in particular, the diverse electronic/ionic charge transfer phenomena that take place at the semiconductor surface and porous structure, can be used to monitor and control this vital environmental characteristic. Various mechanisms have been proposed to explain the variations of electrical response to humidity. With the study of composite materials we look for a better sensitivity of these sensors, comparing to 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 materials conductivity, then the electronegativity of the occupying metal atoms may be used to regulate the sensitivity. In this paper, Ti, Zn oxide bulk sensors, using different proportions of Ti and Zn atoms, were prepared by a conventional sintering method, and the dependence of their complex impedance spectra, measured in the range 100 Hz to 10 MHz, on the relative humidity (RH), operating temperature and on the measuring frequency is shown and explained.