The syntheses, properties, and broad utility of noble metal plasmonic nanomaterials are now well-established. To capitalize on this exceptional utility, mitigate its cost, and potentially expand it, non-noble metal plasmonic materials have become a topic of widespread interest. As new plasmonic materials come online, it is important to understand and assess their ability to generate comparable or complementary plasmonic properties to their noble metal counterparts, including as both sensing and photoredox materials. Here, we study plasmon-driven chemistry on degenerately doped copper selenide (Cu2–x Se) nanoparticles. In particular, we observe plasmon-driven dimerization of 4-nitrobenzenethiol to 4,4′-dimercaptoazobenzene on Cu2–x Se surfaces with yields comparable to those observed from noble metal nanoparticles. Overall, our results indicate that doped semiconductor nanoparticles are promising for light-driven chemistry technologies.

One of the most exciting new developments in the plasmonic nanomaterials field is the discovery of their ability to mediate a number of photocatalytic reactions. Since the initial prediction of driving chemical reactions with plasmons in the 1980s, the field has rapidly expanded in recent years, demonstrating the ability of plasmons to drive chemical reactions, such as water splitting, ammonia generation, and CO2 reduction, among many other examples. Unfortunately, the efficiencies of these processes are currently suboptimal for practical widespread applications. The limitations in recorded outputs can be linked to the current lack of a knowledge pertaining to mechanisms of the partitioning of plasmonic energy after photoexcitation. Providing a descriptive and quantitative mechanism of the processes involved in driving plasmon-induced photochemical reactions, starting at the initial plasmon excitation, followed by hot carrier generation, energy transfer, and thermal effects, is critical for the advancement of the field as a whole. Here, we provide a mechanistic perspective on plasmonic photocatalysis by reviewing select experimental approaches. We focus on spectroscopic and electrochemical techniques that provide molecular-scale information on the processes that occur in the coupled molecular-plasmonic system after photoexcitation. To conclude, we evaluate several promising techniques for future applications in elucidating the mechanism of plasmon-mediated photocatalysis.

In this study, we showed that nanocrystals of the degenerately doped plasmonic semiconductor Cu 2−x Se show enhancement factors comparable to those of noble metals and can drive the dimerization of 4-NBT to give DMAB with similar ef f iciencies.

Plasmonic materials interact strongly with light to focus and enhance electromagnetic radiation down to nanoscale volumes. Due to this localized confinement, materials that support localized surface plasmon resonances are capable of driving energetically unfavorable chemical reactions. In certain cases, the plasmonic nanostructures are able to preferentially catalyze the formation of specific photoproducts, which offers an opportunity for the development of solar-driven chemical synthesis. Here, using plasmonic environments, we report inducing an intramolecular methyl migration reaction, forming 4-methylpyridine from N-methylpyridinium. Using both experimental and computational methods, we were able to confirm the identity of the N-methylpyridinium by making spectral comparisons against possible photoproducts. This reaction involves breaking a C–N bond and forming a new C–C bond, highlighting the ability of plasmonic materials to drive complex and selective reactions. Additionally, we observe that the product yield depends strongly on optical illumination conditions. This is likely due to steric hindrance in specific regions on the nanostructured plasmonic substrate, providing an optical handle for driving plasmonic catalysis with spatial specificity. This work adds yet another class of reactions accessible by surface plasmon excitation to the ever-growing library of plasmon-mediated chemical reactions.

Plasmonic materials are promising photocatalysts as they are well-suited to convert light into hot carriers and heat. Hot electron transfer is suggested as the driving force in many plasmon-driven reactions. However, to date there are no direct molecular measures of the rate and yield of plasmon-to-molecule electron transfer, or energy of these electrons on the timescale of plasmon decay. Here, we use ultrafast and spectroelectrochemical surface-enhanced Raman spectroscopy to quantify electron transfer from a plasmonic substrate to adsorbed methyl viologen molecules. We observe a reduction yield of 2.4 - 3.5 % on the picosecond timescale, with plasmon-induced potentials ranging from -3.1 to -4.5 mV. Excitingly, some of these reduced species are stabilized and persist for tens of minutes. This work provides concrete metrics toward optimizing material-molecule interactions for efficient plasmon-driven photocatalysis.

Photon upconversion is of great interest for improving the efficiency of silicon photovoltaic cells, for biological imaging, and for thermal management strategies. Currently, the vast majority of materials being developed for solar upconversion are composed of rare and expensive elemental compounds. Moving forward, the development of earth abundant, non-toxic materials that efficiently convert near infrared light into visible light would be ideal. Copper selenide-based materials meet these criteria, and are of great interest due to their unique thermoelectric and plasmonic properties. In particular, doped copper selenides (Cu2−xSe) have tunable near infrared localized surface plasmon resonances, large Seebeck coefficients, and low thermal conductivity, with a range of chemical and thermoelectric applications. Here, we observe another interesting application of this material in the upconversion of near infrared light from a silica xerogel film containing degenerately doped Cu2−xSe nanocrystals, with an onset flux of ∼ 1.96 ± 0.29 kW/cm^2 and at least 1% quantum yield. Our investigations suggest a plasmon-driven thermal mechanism likely plays a role in this upconversion process.