Nanoporöse gesputterte Platin-Iridium-Schichten für Anwendungen in der Medizin- und Energietechnik

• Nanoporous sputtered Platinum-Iridium-thinfilms for medical and energy applications

Ganske, Gerald; Mokwa, Wilfried (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2012)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2012

Abstract

Sputtering makes it possible to create thinfilms of only a few atom layers and to customize them for special applications by adjusting the deposition parameters. In this work interface-layers are deposited and characterized in biological systems as stimulation electrodes for neural cells and as catalysts in hydrogen fuel cells. First of all, highly porous platinum films were created by sputtering at a pressure of 9 Pa and low power of less than 100 W. These parameters are an ideal compromise between deposition rate, porosity and disordered crystal structure of the layers. Investigations on co-sputtered platinum-iridium-films (PtIr) showed that these films form homogeneous structures and no distinction between the separate layers is possible. It was demonstrated that these films obtain the crystal structure of Pt as well as the finer cauliflower-like structure of iridium, if the atoms reach the substrate surface only with their thermal energy. Furthermore, it was shown that the film composition reflects the sputtering power of the separate targets in a linear way. The structure of the films can be predicted by means of monte-carlo-simulation, which was verified by SEM-pictures. The ratio of the sputtering power can be used to control the amount of interface elements which was confirmed by electrochemical tests. Electrode materials for the stimulation of neural cells need a large electrochemically active surface that allows for an interface between electron and ion conductivity. Test on platinum, iridium and PtIr have shown that the films sputtered at the lowest impact energy do have the largest active surface as well as the largest charge delivery capacity (CDC). Iridium films show the highest CDC (48 mC/cm²), followed by platinum-iridium (2 mC/cm², 100 W power at both targets) and pure platinum (16 mC/cm²). This can be explained by the large surface area of iridium and its electrochemical activation process. Although PtIr layers also show an activation process, it is much smaller than for Ir films. For fuel cell catalyst applications a large active surface (45 cm²real/cm²geom at 9 Pa sputtering pressure) is also beneficial, which was shown for Pt-films. These films have a drastically increased kinetically limited current density (514 µA/cm²) due to the increased amount of [200]-oriented crystals. Tests in a fuel cell environment proved that sputtered films are superior to commercial chemical catalysts in the utilization of the used platinum. Tests with PtIr films with an amount of about 11% iridium proved an active surface area increase of 20% in regard to the optimized platinum layers. Also the oxygen reduction reaction in acid electrolyte current was sixfold higher for PtIr. Because of this, PtIr is even superior to sputtered pure Pt, which was shown in fuel cell tests. This can be explained by a reduced lattice spacing and an increased amount of more reactive crystals in [200]-direction. These explanations do agree with previously published data of other groups for chemically produced catalysts.

Institutions

• Chair of Materials in Electrical Engineering I and Institute of Materials in Electrical Engineering [611510]