Gas sensors based on porous silicon and palladium oxide clusters

Palladium oxide/porous silicon (PdO/PS) structures were developed for studying their behavior with gas molecules as hydrogen, ammonia and some others similar molecules. A lot of work has been done in demonstrating the usefulness of palladium (Pd) in sensing hydrogen containing gas compounds. However there is not confidence if the main role in the sensing process is just Pd or palladium oxide (PdO). Therefore our work was directed to distinguish the role of PdO in the sensing process. The structures were based on porous silicon (PS) films prepared by anodization of silicon substrates. The PS films were prepared using several etching current densities for obtaining films with distinct porous densities. Pd clusters was deposited on PS films either by adding palladium chloride in the electrolytic bath or in a second process by an electroless step using an autocatalytic bath just after silicon anodization. After that the films were subjected to an oxidization process in an oxygen atmosphere to transform the Pd deposit into PdO clusters. The temporal response of films as resistive sensors are discussed in terms of a possible initial surface composition of the studied structures. The response of the PdO/PS sensor can be explained by considering the influence of PdO particles on the PS surface. The catalytic decomposition of hydrogen containing molecules by the PdO particles is a more efficient process than the use of metallic palladium ones. The objectives in this work are to demonstrate that metal oxide clusters on PS layers can improve sensitivity and also that it has effects on sensor selectivity.