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Cagri Topal1 Andishaeh Dadgar1 Marimuthi Andiappan1 Kaan Kalkan1

1, Oklahoma State Univ, Stillwater, Oklahoma, United States

Copper is an attractive alternative for plasmonic photocatalysis due to its high abundancy as well as good spectral match of its Localized Surface Plasmon Resonance (LSPR) with solar radiation. In ambient conditions, Cu nanoparticles quickly oxidize to Cu/Cu2O core/shell structures. In this hierarchical structure, the native oxide, Cu2O, being a semiconductor of 2.2 eV band gap, has multiple roles. It allows for a longer lifetime for plasmon-excited hot electrons in its conduction band, resulting in more efficient electron transfer. While it is plasmons in the Cu core which strongly couple with solar photons and decay to hot electrons needed for catalytic activity, excited electron/hole generation may also occur in Cu2O under the effect of LSPR-enhanced nearfield. Other potential mechanisms in this core/shell structure are: i) Resonance Energy Transfer (RET) between Cu and Cu2O through plasmon-exciton dipolar coupling; and ii) Chemical Interface Damping (CID) at the Cu/Cu2O interface. To elucidate these open questions, here, we perform single nanoparticle scattering spectroscopy and electromagnetic simulations (FDTD). The Cu/Cu2O nanoparticles are synthesized by a modified polyol/microemulsion technique in the size range of 50-200 nm. The particles are immobilized on patterned glass slides, so scattering of a specific particle can be acquired in between sequential thermal oxidation steps (220 oC, 30 min) as the oxide thickness is increased systematically. The LSPR peaks can be conveniently resolved by single particle scattering spectroscopy in the absence of heterogenous broadening. Otherwise, the peaks are impossible to resolve from optical extinction spectra of the colloids. In larger nanoparticles, we observe the quadrupolar and hexapolar LSPR modes in addition to dipolar. By comparison of the peak widths for experiment and simulation, we infer the dipolar mode is by far more damped than the higher order modes. We explain the selective damping of the dipolar plasmon by its energy match with the Cu/Cu2O interface states, resulting in CID. While our simulations show increase of Mie scattering with thickening of the Cu2O shell, the single particle measurements reveal a systematic attenuation (i.e., ~4 times) during 90 min of thermal oxidation. Based on our simulations, we rule out reduction of the Cu core diameter as a cause for this attenuation of scattering. Therefore, an energy loss mechanism is inferred with increasing Cu2O thickness. This loss mechanism is not the surface-enhanced optical absorption in Cu2O, which is already included in the FDTD solver through imaginary component of the Cu2O dielectric function. We infer the attenuation of scattering is due to RET. Investigation is under way to validate RET in Cu/Cu2O nanostructures. The presence of RET should benefit plasmonic photocatalysis by providing an additional channel of energy transfer to catalytic electrons.

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