Quantifying photochemistry at the nanoscale
Alan Bowman a, Alvaro Rodrí guez Echarri b c, Fatemeh Kiani a, Fadil Iyikanat b, Ted Tsoulos d, Joel Cox e, Ravishankar Sundararaman f g, Javier García de Abajo b h, Milad Sabzehparvar a, Can Karaman a, Giulia Tagliabue a
a Laboratory of Nanoscience for Energy Technologies, EPFL
b ICFO–Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology
c MBI–Max-Born-Institut, 12489 Berlin
d POLIMA–Center for Polariton-driven Light–Matter Interactions, University of Southern Denmark
e Danish Institute for Advanced Study, University of Southern Denmark
f Department of Materials Science & Engineering, Rensselaer Polytechnic Institute
g Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute
h ICREA–Institució Catalana de Recerca i Estudis Avançats
Proceedings of Catalyst Design Strategies for Photo- and Electrochemical Fuel Synthesis (ECAT23)
Keele, United Kingdom, 2023 December 4th - 5th
Organizers: Charles Creissen, Qian Wang and Julien Warnan
Poster, Alan Bowman, 023
Publication date: 10th October 2023

Driving chemical reactions optically represents a paradigm shift in our ability to synthesize materials. Plasmonic catalysis – using photoexcited hot carriers generated in metal nanoparticles to drive reactions – represents an exciting approach for chemical conversion. Importantly, the shape and size of the nanoparticle has been demonstrated to modulate the selectivity and efficiency of the reaction, allowing for novel reaction pathways to be found [1]. To realise the full potential of plamonic catalysis we need to understand the fundamental processes behind it. To this end I will present two complementary approaches, based on spectroscopy and scanning probe microscopy, that help unravel the microscopic dynamics of charge carriers in plasmonic metals.

Starting from gold monocrystalline flakes, we fabricated nanodisks to drive chemical reactions, with plasmonic resonances in the 600-800 nm wavelength range.  Using a combination of Scanning Electrochemical Microscopy and microscale optical characterization we were able to go beyond external quantum efficiencies and extract the internal quantum efficiency of the system, that is, the probability of a chemical reactions per absorbed photons. State-of-the-art density-functional-theory (DFT) parameterized calculations of the same nanostructures allowed us to calculate the fraction of photoexcited charge carriers that reach the exposed gold surfaces. Together these results allow us to find the probability of hot carrier charge transfer from a nanoparticle to a molecule as a function of electron and hole energy, a key and difficult to access parameter. For the example reaction of oxidation of ferrocyanide we show that the chemistry is dominated by high energy hot hole transfer [2].

Subsequently, we explored the potential of luminescence from gold as a non-invasive probe of hot carrier dynamics, and hence chemical reactions. We studied the luminescence from unpatterned monocrystalline gold flakes and modelled our results in DFT, enabling, for the first time, a complete understanding of this process. Our measurements show that pre-scattered hot carriers are responsible of a large fraction of the signal close to the excitation energy. Furthermore, for gold less than 40 nm thick quantum mechanical effects play a significant role in the luminescence spectrum, higher thicknesses than previously assumed. Overall, this work reveals that gold luminescence can be employed as a probe for nanoscale chemistry [3].

In summary, these projects realise new methodologies for probing plasmonic catalysis at the micro- and nanoscale, with implications for a wide range of chemical processes and materials.

ARB, MS, CK and FK acknowledge support of SNSF Eccellenza Grant PCEGP2-194181. ARB acknowledges SNSF Swiss Postdoctoral Fellowship TMPFP2_217040 and thanks Valeria Vento and Christophe Galland for the use of a commercial monocrystalline 200 nm gold sample, and Franky Esteban Bedoya Lora for the use of the Ocean Optics spectrometer. ARE, FI and FJGA acknowledge funding from the European Research Council (Advanced Grant No. 789104-eNANO), the Spanish MICINN (PID2020– 112625 GB-I00 and Severo Ochoa CEX2019-000910-S), the Catalan CERCA Program, and Fundaciós Cellex and Mir-Puig. JDC is a Sapere Aude research leader supported by VILLUM FONDEN (grant no. 16498) and Independent Research Fund Denmark (grant no. 0165-00051B). The Center for Polariton-driven Light—Matter Interactions (POLIMA) is funded by the Danish National Research Foundation (Project No. DNRF165). First-principles calculations were carried out at the Center for Computational Innovations at Rensselaer Polytechnic Institute.

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