Simple experimental procedures to distinguish photothermal from hot-carrier processes in plasmonics

Light absorption and scattering of plasmonic metal nanoparticles can lead to non-equilibrium charge carriers, intense electromagnetic near-fields, and heat generation, with promising applications in a vast range of fields, from chemical and physical sensing to nanomedicine and photocatalysis for the sustainable production of fuels and chemicals. Disentangling the relative contribution of thermal and non-thermal contributions in plasmon-driven processes is, however, difficult. Nanoscale temperature measurements are technically challenging, and macroscale experiments are often characterized by collective heating effects, which tend to make the actual temperature increase unpredictable. This work is intended to help the reader experimentally detect and quantify photothermal effects in plasmon-driven chemical reactions, to discriminate their contribution from that due to photochemical processes and to cast a critical eye on the current literature. To this aim, we review, and in some cases propose, seven simple experimental procedures that do not require the use of complex or expensive thermal microscopy techniques. These proposed procedures are adaptable to a wide range of experiments and fields of research where photothermal effects need to be assessed, such as plasmonic-assisted chemistry, heterogeneous catalysis, photovoltaics, biosensing, and enhanced molecular spectroscopy. Perfecting plasmonic processes for efficient catalysis Optimizing the production of charge carriers and heat from plasmonic nanoparticles will lead to more sustainable chemical reactions. Shining laser light onto plasmonic metal nanoparticles generates complex thermal and electromagnetic effects that can catalyze many useful reactions. It is important, but challenging, to distinguish direct laser heating of the nanoparticles from the production of ‘hot carriers’ – high-energy electrons and holes that can go on to activate redox reactions. Guillaume Baffou at Aix Marseille University in France, and co-workers, have reviewed existing and proposed experiments for gauging these two effects. The experiments, which include varying the power, beam size, polarization and wavelength of the incident laser light, improving measurements of sample temperatures using infrared cameras or thermocouples, and monitoring gas bubbles produced by heating, will enable researchers to make chemical procedures more efficient and sustainable.