It was demonstrated more than 30 years ago that a palladium - silicondioxide - silicon (MOS-) structure acted as a hydrogen sensor in air, with typical practical detection limits around 1 ppm of hydrogen. Experiments in UHV showed, that in an inert atmosphere, it should be possible to detect hydrogen pressures down to 10-14 torr.
The present communication addresses the limit for detection of a concentration step of hydrogen in (synthetic) air, i.e. during water production on the catalytic metal surface. The signal to noise ratio at a given hydrogen concentration was improved by using a (much) larger catalytic metal area than in the original devices. The composition of the catalytic metal gate was changed to a double layer of platinum and palladium. A measurement protocol using the initial rate of change of the hydrogen signal upon the introduction of the test gas was used. The large area device was interrogated with a chopped light beam inducing a photo capacitive current in the reversed biased MOS-structure. Hydrogen atoms adsorbed at the metal-oxide interface induce a charge or dipole layer, which shifts the photo capacitive current-voltage curve along the voltage axis.
We thus present results on an improved catalytic metal gate, where an added small concentration of hydrogen can be measured in a (large) background of hydrogen (around 0.1 -0.5 ppm) in the reference gas as. Detection limits of the order of 10 ppb of added hydrogen are observed. The experimental observations are discussed in terms of: a) the influence of the area of the catalytic surface on the signal to noise ratio, b) the flux of hydrogen towards the catalytic metal surface and c) the dissociation of hydrogen on a "completely" oxygen covered metal surface, with a sticking coefficient several orders of magnitude smaller than that of a clean surface.
In summary the new developments compared to the "classical" palladium gate hydrogen sensors are: i) larger detection area, ii) other composition of the metal gate, and iii) sensitive detection of the rate of change of the "voltage" shift. These changes, together with the pulsed test gas application, contribute to the possibility to detect 10 ppb hydrogen concentration steps in air, with an extrapolated detection limit close to 1 ppb.
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