HORÁK Pavel Institute of Chemical Technology Prague

Impedance Measurement of Gas Sensors with Nickel(II)- and Copper(II)- Oxide Active Layers

Co-authors KHUN Josef, VRŇATA Martin, BEJŠOVEC Václav, LAVRENTIEV Vasyl, VACÍK Jiří

The contribution deals with semiconductor gas sensor whose active layer is based on transition-metal oxides. Thin layers (50-100 nm) of nickel(II)- and copper(II)- oxide were deposited onto ceramic sensor substrates equipped with interdigital electrodes for signal reading. The deposition was carried out in two or three steps: (i) sputtering by means of Ar ion beam from pure (99.99%) metal targets, (ii) following thermal oxidation (400 °C for 5 h) in air, (iii) in some cases - sputtering of Pd catalyst to the surface. Then the impedance response of produced sensors (NiO, NiO+Pd, CuO, CuO+Pd) to 1000 ppm of hydrogen, 1000 ppm of methanol vapor and to saturated water vapor was measured. Impedance measurements were performed in the frequency range from 40 Hz to 100 MHz. The obtained data were depicted in Nyquist representation (i.e. imaginary vs. real part of complex impedance). These diagrams have a character of one complete and one incomplete semicircle, each of them corresponding to a parallel RC-element. It was proved, that both NiO and CuO behave like p-type semiconductors; the sensor impedance increases on exposure to reducing gases. The response to water vapor (the most common interferent) was negligible. The best sensitivity was achieved on NiO+Pd sensor - during detection of hydrogen (1000 ppm) the real part of complex impedance measured at 40 Hz increased from 120 to 350 Ω.