Plasmonic sensing of catalytic nanoparticles
Conference poster, 2016
Studying nanoparticles for catalytic applications in situ is a challenge because their working environment is often harsh in terms of temperature, pressure and chemical reactivity. Nonetheless, the ability to study the chemistry of nanoparticles while reactions occur in or around them is of great interest in many fields, including heterogeneous catalysis, and indirect plasmonic sensing (INPS) has shown to be an effective strategy for this purpose. Nevertheless, INPS experiments are still hampered by the fact that, for instance, sample temperature changes (e.g. due to dissipated reaction heat or in transient experiments) can affect the measured plasmonic parameters and induce unwanted signals that complicate ambiguous measurements. To conceptually address this issue, we have previously demonstrated a method that utilizes indirect plasmonic sensing on specifically tailored asymmetrical heterodimers consisting of a symmetrical inert sensor paired with a smaller reactive particle. By monitoring the structures using two probing light polarizations simultaneously, changes in the small reactive particle can be effectively singled out and drift as well as temperature-induced plasmon shifts can be efficiently eliminated. The reason for this is the near-field coupling that occurs between nanoparticles placed close to each other when light polarized along the dimer long axis is applied. The fabricated structures are therefore dichroic and changes occurring in the small reactive particle adjacent to the inert element in the dimer are only detectable in the polarization parallel to the dimer axis. In this study oxidation and reduction of Cu nanoparticles has been studied in situ using plasmonic sensing both by directly monitoring the spectra of pure Cu structures and indirectly by monitoring the Cu particles using an inert Au sensor placed beside the reacting Cu particle. The reactions have been studied in a temperature range of 20-300°C, and in different gas concentrations. Using direct sensing there is a dramatic change in the plasmonic spectra when the particle oxidizes, however, the measured spectrum is also affected by changes in temperature which motivated us to use the indirect measuring scheme presented above. Using INPS we find that changes in the plasmonic response obtained from the Au-Cu dimers’ simultaneously monitored orthogonal polarization readouts stem from several contributions, including temperature changes, spill-over processes to the support, and the targeted oxidation of the Cu. Using the difference between the two polarization direction signals, however, we can then elegantly single out the sensor response stemming exclusively from the Cu oxidation and reduction process. The optical response of the Cu disks and the Au-Cu dimer system has then been compared with corresponding FDTD simulations that have shown good correlation. We will also report on our progress of applying the above strategy at the single nanoparticle level, to further expand the possibilities of single particle plasmonic nanospectroscopy for the characterization of catalytic processes at the individual nanoparticle level, as we recently demonstrated possible on the example hydrogenation of individual Pd nanocrystals with different size and shape. Combining these two methods creates an effective analysis platform for a wide set of chemical reactions with the possibility to be both accurate and selective.
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