8. Nanoscale Raman Spectroscopy

Heterogeneous metal-catalysis plays a significant role in organic synthesis as it lowers the activation barrier necessary to initiate a chemical reaction. In this field, mainly thermally activated catalysts are established whereby unwanted by-products are often an issue. Photocatalysts can be a promising alternative, especially, if they can be activated with visible light under ambient conditions. To promote the application and development of this rather young class of catalysts, a detailed understanding of their surface structure, processes at the interface, reaction kinetics and mechanisms on the atomic and molecular level is of great importance. Photocatalysts based on noble metal nanoparticles like gold and silver are highly attractive candidates in this field. When irradiated with the appropriate wavelength, resonant light absorption of the nanoparticles can lead to the excitation of localized surface plasmons (LSPs) and the generation of a highly amplified electromagnetic field in their immediate vicinity. This can be exploited to initiate charge-driven reactions with an energy transfer between metal and adsorbate as the so-called “hot electrons” or “hot holes.” (Zhang et al., Acc Chem Res 52:2506–2515, 2019; Wang et al., Appl Mat Today 15:305–314, 2019; Ren et al., RSC Adv 7:31189–31203, 2017; Linic et al., Nat Mater 14:567–576, 2015). Still, there are many fundamental questions unresolved, such as the catalyst-molecule electronic interactions and transfer and the structural (re)-organization of the adsorbates on the catalytic surface. Since the general aspects of plasmon-activated catalysis are discussed in another chapter of this book, in this chapter we focus mainly on the high lateral resolution aspects. The consequences for spectroscopy when using single plasmon particles as analytical probes will be considered specifically in the respective topics. If necessary, specific properties of the probe composition and its influence on the spectra and on the overall results of the reactions are also discussed.