Structural studies of sputtered noble metal catalysts on oxide surfaces

Abstract

Catalytically active noble metal additives are used in semiconductor gas sensors as supported catalysts in order to increase both the selectivity and the sensitivity of the sensors. This study considers the formation of nanoparticles from ultra-thin noble metal deposits on oxide surfaces during heating of the sputtered metal deposits. Ag, Au, Pd and Pt layers of various thicknesses were sputtered on SiO2 surfaces and on thin-film surfaces of semiconducting SnO2. The ultra-thin Ag, Au and Pt layers, contiguous after the sputtering, become discontinuous during the heating and form discrete metal nanoparticles. Because of the oxidation, the behaviour of Pd layers is very different from that of Ag, Au and Pt layers during heating. The structure of the metal layers was studied by atomic force microscopy and X-ray diffraction before and after the heat treatment. In addition, resistivity measurements were used to monitor the structural phenomena during the heat treatment. The agglomerated layer does not have the high conductivity of the contiguous film. A sudden conductivity decrease by many orders of magnitude was found at relatively low temperatures during the heat treatment. The conductivity decrease occurred within a narrow temperature range.