(Invited) Plasmonic Enhancement of Electrochemical Reactions Using LSPR Phenomenon

Abstract

Using Localized Surface Plasmon Resonance Effect for Enhancing Electrochemical reactions has been reported earlier both in terms of direct electron injection/transfer (DET) in outer-sphere reductive processes as well as charge injection into semiconductors via plasmon-induced resonant electron transfer processes (PIRE). While the former (DET) is mostly influenced by the lifetimes of the ejected hot electrons and their rapid cooling at the interface. For inner sphere reductive processes via the LSPR phenomenon, direct charge injection into the LUMO states of the adsorbed species is required. In this regard, it is imperative to distinguish between the inter-band charge transfer of the adsorbed species because of exposure to photons. In contrast to these charge injection into semiconductors via plasmon-induced resonant electron transfer processes (PIRE) is less understood and more complex. In this presentation, we will use the well-known endothermic anodic oxygen evolution reaction (OER) in alkaline pH to showcase fundamental aspects of charge injection and its effect on the hole-driven OER mechanism. For this well know OER catalyst, layered double hydroxides of Ni with Fe and Co dopants will be used. Electrochemical enhancement of OER will be explained based on detailed structural motifs of the semiconductors its band structure and the resonance effect of Au induced by the LSPR effect. The presence of direct and Indirect bandgaps will be discussed in the context of three possible mechanisms: (i) Charge carriers are directly injected via LSPR into the semiconductor. The conduction band of the semiconductor is usually (-0.1 to 0.0 eV vs. NHE) and the valence band is between (2.00 and 3.5 eV) which corresponds to electron energy between -2.00 to 3.5 eV vs NHE. For LSPR nanoparticles SPR energy is between 1.0 and 4.0 eV. Also, the Fermi energy of LSPR is usually 0.0 vs. NHE. Hence LSPR only enables energetic electrons to be transferred from metals to semiconductors. It is the interface between LSPR and the semiconductor which is important. Charge injection is more prevalent when metal LSPR is of lower energy than the semiconductor. Direct electron transfer (DET). (ii) Transfer not involving direct electron injection but via (a) near field electromagnetic and (b) resonant photon scattering mechanism. This has been shown to work by adding a thin dielectric material between the LSPR and semiconductor. This would be most likely in the case of OER where surface localized plasmons will be the most important determinant as opposed to recombination events in the bulk of the semiconductor. This is most important for OER as it depends on the surface hole concentration. It should be noted that larger nanoparticles (>50 nm) have increased resonant photon scattering. Such a mechanism is more prevalent when we have an overlap between metal LSPR and semiconductor bands leading to plasmon induced resonant electron transfer (PIRET). (iii) When metal LSPR is in direct contact with the semiconductor all three phenomenon could be active.