Pd/PdO functionalization of SnO2 nanowires and ZnO nanotetrapods

Tin oxide (SnO2) and zinc oxide (ZnO) nanostructures are widely studied because of their peculiar physical and chemical properties and the large number of possible application fields. Among these application, nanostructure-based chemoresistive gas sensing devices are very promising because they are considered faster and more stable than traditional thin or thick film sensors. Metallic oxide gas sensors are usually very sensitive towards a large number of gases and volatile organic compounds (VOCs), but unfortunately their response is characterized by very low selectivity (the capability to distinguish among different gases). Selectivity enhancements by adding palladium/palladium oxide (Pd/PdO) nanoparticles to traditional film-based gas sensors are widely reported in literature and they are generally obtained by co-deposition or co-synthesis techniques (in sputtering, sol-gel, etc). SnO2 nanowires and ZnO nanotetrapods have been grown on large areas by a combination of metal evaporation and controlled oxidation. Unfortunately Pd and PdO nanoparticles cannot be directly obtained in the same growth process used for the synthesis of SnO2 or ZnO nanostructures, because the large difference in evaporation rates of these different metals and oxides excludes the chance of preforming a co-evaporation process. So, a MOCVD (Metal Organic Chemical Vapour Deposition) process has been chosen in order to deposit Pd/PdO nanoparticles on the surface of oxide nanostructures. Palladium acetylacetonate, Pd(acac)2, has been evaporated and thermally decomposed, in presence of a co-reagent gas, on the substrates with SnO2 and ZnO nanostructures in different experimental conditions and, then, the obtained samples has been annealed in air and/or hydrogen in order to remove carbon residual and/or change the oxidation state of palladium nanoparticles. Samples morphology, structure and composition have been studied by means of SEM and TEM microscopy, EDS microanalysis and X-Ray diffraction. T