Recent developments in plasmon-assisted photocatalysis -- a personal perspective

Abstract

It has been known for many years that metallic nanoparticles can catalyse various chemical reactions, both in the dark and under illumination, through different mechanisms. In the last decade or so, many claims of plasmon-assisted "hot" electron driven catalysis of bond-dissociation reactions have been put forward. These claims were challenged in a recent series of papers, where both the underlying theory of "hot" electron generation and the use of specific experimental setups to discover them in chemical reactions were examined in detail. The conclusion that arose from these works is that as long as temperature gradients exist inside the system (as for typical experimental setups) a quantification of non-thermal effects is close to impossible. Instead, a standard thermal theory was shown to be capable of explaining the experimental findings quite accurately. Here, we review the central lines of thought that led to these conclusions, from a personal perspective. We lay out the key aspects of the theory, and point to the specific caveats one must be aware of in performing photo-catalysis experiments. Finally, we provide some future directions of study.Metal nanoparticles (NPs) are known to exhibit unusual optical properties, in particular, for having absorption and scattering cross-sections that exceed their physical size. This was predicted to enable a long list of potential applications in diverse fields 1 . Unfortunately, however, the strong absorption in the metal severely limits many of these applications 2,3 . Since the energy absorbed in the metal gets converted to heat almost in its entirety 4,5 , in more recent years, attention was diverted into applications that exploit the absorption, like high temperature chemistry, thermal treatment of cancer, water purification etc. [6][7][8][9][10] .Adding to these, one of the most promising such applications was predicted to be the speeding up of chemical reactions, usually referred to as plasmon-assisted photocatalysis. In particular, it was envisioned that the absorption of photons promotes the generation of a non-equilibrium (also known as non-thermal, or "hot") carrier distribution such that electrons in the high-energy tail of this distribution can tunnel out of the metal into high-energy orbitals of the surrounding molecules, and then catalyse the chemical reaction. This was usually referred to as the "hot carrier mechanism" (or "indirect" plasmon-assisted photocatalysis [11][12][13] ). We note that the term "hot carriers" is somewhat unfortunate, as it implies that the carriers are in thermal equilibrium (i.e. their distribution is Fermi-like) albeit with a temperature higher than the ambient (such that the system in general is out of equilibrium). Yet, the common practice in the field refers to "hot electrons" as having a distribution which is different than a thermal distribution with the ambient temperature (see, e.g., the title of Ref. [14]). Hereafter, we use the terms "non-thermal" and "hot" intermittently, as is accustomed in the literature.The "hot" ...