Nanoplasmonic Alloy Hydrogen Sensors

Sammanfattning: The hydrogen economy proposes hydrogen gas as the main energy carrier thanks to its high energy density and the possibility to produce it in a sustainable way without CO2 emission. However, the wide flammability range of hydrogen-air mixtures dictates that hydrogen sensors will be a mandatory accessory to any appliance or vehicle fueled by hydrogen. Exploiting a phenomenon occurring at the nanoscale, a new type of hydrogen sensor based on the strong interaction of light with metal nanoparticles has rapidly developed in the past years. These so-called nanoplasmonic hydrogen sensors rely on hydride-forming metal nanoparticles that sustain localized surface plasmon resonance (LSPR); a collective oscillation of electrons in the nanoparticles induced by irradiated light. The energy at which the resonance occurs depends on the permittivity, as well as size and shape of the nanoparticles. Since both size and permittivity change significantly when a metal transforms into a metal hydride upon absorption of hydrogen, this effect can be used to detect it. To this date, palladium (Pd) has been the prototype material for both fundamental studies related to hydrogen sorption mechanisms in metals and in next-generation hydrogen detection devices across all sensing platforms. Specifically for the hydrogen detection, however, pure Pd does not satisfy the required sensing performance standard due to its inherent hysteresis during hydrogen absorption and desorption and slow kinetics. Furthermore it is also prone to deactivation by species like carbon monoxide and nitric oxides. To address these limitations, in this thesis a new class of plasmonic hydrogen sensors based on noble metal alloy nanoparticles comprised of Pd, Gold (Au) and Copper (Cu) is explored. To enable such sensors, we first developed a nanofabrication method to produce alloy nanoparticles with precise control of their composition, size and shape. Investigating the fundamental properties of these alloy systems upon interaction with hydrogen, we found a universal correlation between the amount of hydrogen absorbed and the optical response, independent of alloy composition. Moreover, we demonstrated how segregation of Au atoms to surface of PdAu nanoparticles can be measured as a distinct change in the plasmonic response. Focusing on the optical hydrogen sensor application, we then studied in detail the performance of various PdAu, PdCu and PdAuCu alloys, as well as the use of thin polymer selective membrane coatings to prevent sensor deactivation by poisoning gases. As the main result, we created sensors with hysteresis-free sub-second response with sub-5 ppm sensitivity that meet or exceed stringent performance targets. To push the concept closer to application, we also demonstrated the integration of alloy nanoparticles with optical fibers for hydrogen sensing.