Detection and measurement of ammonia concentrations is of considerable commercial and medical interest. Our interest lies in room temperature sensing of ammonia using high surface area metal oxide-based sensors. A quartz piezoelectric crystal detector with SnO2 evaporated onto the electrode as a sensor substrate has been developed for the detection of small mass changes caused by the selective adsorption of ammonia vapor from an air.
SnO2 thin films are grown on quartz substrates by physical vapor deposition. Au and Ag were used as electrodes. The morphology and the chemical composition of the films were studied by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).
SEM analysis showed that the deposited SnO2 thin films possess a porosity suitable to fabricate gas sensors. The stoichiometry and chemical state in a depth of the layers were studied by Ar+ ion sputtering. XPS studies revealed significant difference in the thickness and the stoichiometry of the deposited SnO2 films depending on the use of Au and Ag electrodes. In both cases the deposited film is non-stoichiometric and its molecular formula may be denoted as SnO2-x (0<x<1). The binding energies of Sn 3d peaks at 486.3 eV and 494.8 eV, and the binding energy of O 1s peak at 530.2 eV were observed, which are in good agreement with the reported data for SnO2.
The gas sensing properties were tested by measuring the mass loading when the SnO2 films were exposed to 100 -500 ppm NH3 vapor. The studies in this work confirmed the potential use of the SnO2-based detector as an air pollution sensor.