Pd-nanoalloys for hydrogen sensing: Multiscale modeling of thermodynamic and optical properties

Hydrogen sensing based on Pd nanoalloys has shown great promise in the past decades and could potentially be part of a solution that enables a safe future hydrogen economy. There are, however, remaining challenges related to, e.g., long-term stability and a need for further optimization of these systems. To efficiently span the possible combinations of alloyants, composition, and nanostructure geometry, computational methods are invaluable. This thesis focuses on two aspects of sensor optimization: surface segregation and hydrogen sensitivity, using multi-scale modeling approaches.Alloying with metals such as Au and Cu is necessary to overcome issues related to hysteresis and CO poisoning. At the same time, it introduces additional difficulties related to the chemical order such as surface segregation, which is directly related to long-term stability. In this thesis, the surface composition of Pd alloyed with Au or Cu is studied as a function of H2 pressure using Monte Carlo simulations based on cluster expansions parametrized against ab-initio calculations. For Pd-Au, an increased H2 concentration abruptly switches the surface from Au to Pd dominant. For Pd-Cu, the change with H2 concentration is much more gradual\ua0with non-monotonic tendencies, with an overall surplus of Pd in most conditions.The sensing principle is based on the shift in optical response upon H absorption. The magnitude of the sensor readout at a certain H2 pressure depends on nanoparticle geometry and alloy composition. In this thesis, extinction spectra are calculated for Pd-Au-H nanodisks using electrodynamic simulations and the corresponding H sensitivity is analyzed. It is found that the H sensitivity depends on the nanodisk diameter, mainly due to the interplay between a localized surface plasmon and an interband transition which becomes more apparent for smaller nanodisks

Computational Design of Alloy Nanostructures for Optical Sensing of Hydrogen

Pd nanoalloys show great potential as hysteresis-free, reliable hydrogen sensors. Here, a multiscale modeling approach is employed to determine optimal conditions for optical hydrogen sensing using the Pd-Au-H system. Changes in hydrogen pressure translate to changes in hydrogen content and eventually the optical spectrum. At the single particle level, the shift of the plasmon peak position with hydrogen concentration (i.e., the "optical" sensitivity) is approximately constant at 180 nm/c(H) for nanodisk diameters of greater than or similar to 100 nm. For smaller particles, the optical sensitivity is negative and increases with decreasing diameter, due to the emergence of a second peak originating from coupling between a localized surface plasmon and interband transitions. In addition to tracking peak position, the onset of extinction as well as extinction at fixed wavelengths is considered. We carefully compare the simulation results with experimental data and assess the potential sources for discrepancies. Invariably, the results suggest that there is an upper bound for the optical sensitivity that cannot be overcome by engineering composition and/or geometry. While the alloy composition has a limited impact on optical sensitivity, it can strongly affect H uptake and consequently the "thermodynamic" sensitivity and the detection limit. Here, it is shown how the latter can be improved by compositional engineering and even substantially enhanced via the formation of an ordered phase that can be synthesized at higher hydrogen partial pressures

Computational Design of Alloy Nanostructures for Optical Sensing of Hydrogen

Pd nanoalloys show great potential as hysteresis-free, reliable hydrogen sensors. Here, a multi-scale modeling approach is employed to determine optimal conditions for optical hydrogen sensing using the Pd-Au-H system. Changes in hydrogen pressure translate to changes in hydrogen content and eventually the optical spectrum. At the single particle level, the shift of the plasmon peak position with hydrogen concentration (i.e., the "optical" sensitivity) is approximately constant at 180 nm/c_H for nanodisk diameters >~ 100 nm. For smaller particles, the optical sensitivity is negative and increases with decreasing diameter, due to the emergence of a second peak originating from coupling between a localized surface plasmon and interband transitions. In addition to tracking peak position, the onset of extinction as well as extinction at fixed wavelengths is considered. We carefully compare the simulation results with experimental data and assess the potential sources for discrepancies. Invariably, the results suggest that there is an upper bound for the optical sensitivity that cannot be overcome by engineering composition and/or geometry. While the alloy composition has a limited impact on optical sensitivity, it can strongly affect H uptake and consequently the "thermodynamic" sensitivity and the detection limit. Here, it is shown how the latter can be improved by compositional engineering and even substantially enhanced via the formation of an ordered phase that can be synthesized at higher hydrogen partial pressures.Comment: 14 pages, 8 figure