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Accessing hot-spots in plasmonic nanoantennas

Periodic Reporting for period 1 - HOTSPOT (Accessing hot-spots in plasmonic nanoantennas)

Reporting period: 2015-05-01 to 2017-04-30

Bright times for photocatalysis

Photochemistry investigates the chemical effects of light. The first organic photochemical reaction was reported by Trommsdorff in 1834 when he described how crystals of R-santonin turn yellow and burst when exposed to sunlight. Only 15 years later, Pfizer and Erhart mixed that compound with almond toffee to create the first drug ever formulated in the USA. However, it was not until 2007 that the mechanism behind this reaction could be fully understood. This simple example shows the origin, uses and complexity of the interaction between light and molecules. In between, highly relevant processes to our daily life, such as vision and vitamin D activation, were also linked to light-driven chemical reactions. However, the most remarkable process of chemical transformations induced by light remains to be photosynthesis. The outstanding efficiency of plants for sunlight-into-energy conversion has highly inspired researchers across many different fields in order to understand how light is absorbed, transferred and storage in this system. In this project, I used the ability of plasmonic nanoparticles to absorb light and efficiently catalyse chemical reactions that by themselves will not proceed.

A simplified mechanistic description behind the photochemical reactions implies the molecular absorption of a given-frequency photon. Once excited, the molecular energy-landscape is modified and this can trigger new molecular bonds formation or dissociation. Sunlight-induced chemical reactions are then strongly limited to molecular species that can absorb photons with frequencies in the visible range of the spectrum, where the sun emits more efficiently. However, the absorption of abundant environmental and biological relevant molecules such as CO, CO2, H2, H2O, glucose, among others, falls out of this range of the spectrum. Even though, it is possible to perform light-induced reactions on non-absorbing molecular species via the presence of a photocatalytic material. Briefly, the light is absorbed by the catalytic material and its electronic structure is modified, creating energetic electron-hole pairs inside the material. These photo-generated reactive carriers can then be transferred to molecular species nearby inducing chemical reactions.

Indeed, light-induced chemical reactions on bulk catalytic metal surfaces (or photo-catalytic materials) have been explored for more than 50 years. Light absorption in the metal surface plays a key role in inducing photochemical transformations of adsorbed molecules. Our current ability to control both the absorption cross-sections and the energy of absorbed light by metal plasmonic nanoparticles opens new pathways for the manipulation of photochemical reactions. Physical phenomena associated to the localized surface plasmon resonances, such as energetic surface states and intensified electric fields, forces us to revisit our traditional understanding of photochemical reactions at metal surfaces. Long standing goals in the field – such as bond-selectivity and increased efficiency of photo-catalytic processes – might now be achievable, assisted by plasmonic nanoparticles. Along this project I have investigated the light-into-chemical energy conversion at the nanoscale by using metal plasmonic nanoparticles. In order to detect the places were these chemical reactions took place we used small gold nanopartilces to probe the reactive sites of plasmonic materials (see image attached of a plasmonic silver bow-tie antenna and two small gold nanoparticles used to track the chemical reaction).

The results of this MSCA project have helped us to design new efficient materials to convert sunlight energy into chemical energy; this means using sunlight for fuel generation, pollutants degradation, etc. Controlling chemical reactions with high efficiency and selectivity will revolution our current capabilities to mimic plants and their photosynthesis process.
The main objective of my project involved the efficient generation and transfer of hot-electrons at plasmonic interfaces followed by the modification of the hot-spots of plasmonic antennas with different molecules and/or nanomaterials.

Photocatalytic experiments due to transport of plasmonic hot-electrons derived in localised surface chemistry, as proposed originally. These experiments, which constitute the core of my project were carried out on silver antennas and the reaction that I monitored in order to probe this fact was the 4-NTP to 4-ATP redox conversion. These results allowed me to exploit the possibility to selectively functionalize nanomaterials with spatial resolution of 10s of nanometers,

Along the period covered by my fellowship strong impact and visibility were achieved. Results derived from my research has been published in more than 9 articles (7 published and 2 in preparation) in high-impact factor journals. The core publication of my fellowship (Cortes et al, Nat Comm 2017, 8, 14880) received a wide media cover, being highlighted in more than 15 media news (including Phys.org Science Newsline, Solar Daily, Nanotechnology Now, Imperial College News, etc).
Also during my fellowship the research was presented in more than 10 International Conferences.
My main objective in the project was to visualize the “reactive sites” in plasmonic nanoparticles, this means the nanoscale regions where the electron-transfer to molecules occurs in these systems. In order to achieve that I designed experiments involving plasmonic antennas, surface chemistry, charge transfer (redox) reactions and computational methods. The results of these experiments clearly showed that it is possible to perform local surface chemistry using the ability of the plasmonic particles to convert light into excited carriers (hot electrons) that are able to drive a chemical reaction. By the localization of the nanoscale regions where this process occurs, it is possible to design better and more efficient plasmonic catalytic materials to drive chemical reactions using visible light. These results set our current knowledge one step closer to understand and mimic plants in the light-into-chemical energy conversion. Also, it turns possible to efficiently use sunlight to drive chemical reactions by enhancing the absorption of light using reactive metal plasmonic nanoparticles.