Plasmonic aluminum nanoparticles represent a naturally abundant, solution-phase, low-cost and non-toxic materials system and are a promising alternative to noble metal nanoparticles as photocatalytic enhancers for green chemical synthesis and degradation of environmental pollutants. We use transient absorption spectroscopy, in combination with simulations of phonon and thermal energy dissipation, to investigate the photophysical responses and energy-transfer dynamics in size-tuned plasmonic aluminum nanoparticles suspended in liquid isopropanol. The spectral response of all particle sizes to ultrafast excitation is characterized by a decrease in light transmission broadly across the visible and near infrared on a timescale that matches predictions for electron-lattice relaxation processes. Time-dependent bleaching of the interband transitions is largely isolated from spectral changes to the intraband transition and provides a window into electron-electron thermalization dynamics.  Fast thermal transfer rates to the solvent are observed for all particle sizes. Using a two-interface model, we find that the rapid thermal energy transfer is accounted for by the presence of a compact ~4 nm native oxide layer on the particles. Size-dependent phonon “breathing”/vibrational modes are also observed as oscillations in total cross-section. We find that both the oscillation frequency and damping rate increase as the diameter of the particles decreases. Based on results of finite element calculations, we attribute the damping strength and oscillation period observed to a combination of the non-crystalline nature of the native oxide shell and the presence of surface-bound ligands, both of which increase the vibrational mode damping rates relative to bare aluminum and aluminum particles with a bare crystalline oxide shell. These insights should guide future work on controlling energy transfer through the use of size- and surface-tuning in sustainable aluminum nanomaterials systems for applications in catalysis and sensing.

Size- and surface-dependent energy transfer dynamics in plasmonic aluminum nanoparticles

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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