Materials and Methods
Alumina square substrates 5x5 mm were used for the preparation of KP sensing devices. On the top of the substrate, a gold thin film was deposited to assure the ground connection of the metal oxide layer deposited on gold. While on the bottom of the substrate a meander of platinum was deposited to act as a heater, necessary for reaching the desired temperature on the sensing material deposited on the top of the gold layer. Thin films of p-type CuO were deposited by RF magnetron sputtering starting from a ternary oxide 2” target (delafossiteCuAlO2) at room temperature (RT), in inert (Ar) atmosphere at 100W RF power prepared by sintering, while nanowires were grown by thermal oxidation of those grown thin films in tubular furnace at 300oC for 12 hours in O2 (80%) and Ar (20%) environment (SEM images: figure 2). Before sensing tests samples were kept at 300°C in ambient air for two weeks to induce controlled aging. The surface morphology of the thin film deposited on the alumina substrates were investigated with a fieldemission scanning electron microscope (FE-SEM, LEO 1525), operating in the secondary-electron imaging mode with a 10-KeV accelerating voltage. Raman analysis was carried out with a modular micro-Raman confocal system from Horiba equipped with single monochromator (ihr320mst3) and Peltier cooled CCD camera. He:Cd laser at 442 nm was used as excitation source, along with interference filters on laser lines and edge filters on the signal. Spectra were collected on the instrument operating with 1800 l/mm grating, and 100X objective. The samples were mounted on a motor controlled X-Y stage. The materials used for sensing tests analyzed mounted on electrical TO5 case. The variations in surface contact potential difference (CPD) i.e. ο߶were recorded by a standard Kelvin probe 2mm in diameter, oscillating very near to the sample surface and controlled by a standard zero-locking Kelvin control system in a 1L stainless steel closed chamber. Test Chamber was placed at 20oC. Synthetic air was passed through drechsel bottle held in thermostatic bath and then through condensation vessel in order to favor the condensation of saturated vapors and finally was mixed with dry air in appropriate composition to get desired relative humidity before mixing with target gas. A humidity sensor (Viasala, Finland, Model: HMI 36) was mounted inside the chamber to measure the humidity of the mixture of gases inside the chamber and to control humid and dry air mixing. Test gases with certified composition were supplied by SIAD spa. Italy. Dry synthetic air was used as a carried gas to those target gases mixed by MKS instrument mass flow controller Model: 647B. The total flow inside the sensing chamber was fixed at 300sccm. The sensor temperature controlled by applying fixed voltage to the heater of the sensor by Thurlbly-Thandar PL330DP power supplies, typically 5.5-6.75V. CPD was measured by Keithley 580. Samples were heated at desired operating temperature inside the chamber keeping the humid air flow constant for several hours for getting thermal stabilization. After exposing the target gas to the sensor for 1hour in each step, air flow was restored to estimate the time for recovery of the baseline.