PORTEFEUILLE

The goal of this project is to establish an original framework to develop an energy efficient, environmental friendly and sustainable method for the synthesis of ammonia, one of the most commonly produced industrial chemicals. Indeed, the entire agro-food industry relies on ammonia to produce fertilizers in order to provide sufficient food to the world population. Ammonia (NH3) is obtained by splitting a nitrogen molecule N2; unfortunately, the two nitrogen atoms in that molecule are extremely strongly bound together, such that very high energy is required to break that bond and produce ammonia. Consequently, today, the synthesis of ammonia consumes approx. 2% of the world’s annual energy! On-going ammonia production requires both a very high pressure and a temperature, which are extremely energy greedy.

In this project, we aim at exploring a completely different route for splitting nitrogen molecules to produce ammonia, based on plasmon-assisted heterogeneous catalysis. This approach relies on two key phenomena. On the one side, nitrogen molecules will be adsorbed on a nanostructured surface, which will reorganize their energy landscape in a way such that the binding energy between both nitrogen atoms will decrease. This corresponds to the catalysis part and can be understood by a simple analogy: when a person is swimming freely in shallow water with all arms and legs stretched to float, it has a certain kinetic energy; when that person stands up on the ground in the water, its kinetic energy decreases. On the other side, we wish to harvest energy from light using plasmonic nanostructures in order to promote the nitrogen splitting process. Plasmonic nanostructures are made of specific metals such as gold, silver or aluminium. A metal is a good electric conductor because it possesses many free electrons that can be used to flow an electric current when a voltage is applied. In a plasmonic metal, this number of free electrons is so high that applying only light is sufficient to put the electrons in the metal in motion. By combining both effects, we do hope to be able to perform nitrogen splitting using much less energy than existing industrial processes.

This project aims at taking a completely fresh view on an existing industrial process by combining competences that stem from very different horizons and include nanotechnology, surface chemistry and electrochemistry.

We have successfully developed a sustainable green method for ammonia production through nitrogen splitting at room temperature under atmospheric pressure using plasmonic aluminium nanostructures and semiconductor photoelectrodes. While this demonstrates an original route for replacing energy-greedy processes currently used by the industry, unfortunately, our approach is not as efficient as we had anticipated. Although this prevents an immediate translation of our results to industry, asking this kind of highly challenging scientific questions is very important in the long term and the results of this project are significant on many respects. These results include 1) a clear vision of plasmonic near-field and hot electron effects for nitrogen splitting, 2) the production of ammonia at room temperature under atmospheric pressure and 3) the efficient harvesting of hot electrons from aluminium nanostructures. This project has also required the development of specific nanotechnologies, e.g. for the development of photoelectrodes consisting of plasmonic nanostructures over very large area, covering an entire wafer. The corresponding technology has been documented in details in the open literature and can be easily taken over by others in academia or industry, where it can be used for different products that rely on light harvesting by nanostructures.

This project has also provided a much clearer picture of the role plasmonic effects can take in photochemistry. Two main effects are generally considered in the literature: near-field coupling and hot electron generation; unfortunately, these effects are very difficult to distinguish, often leading to confusion on their respective strengths. Thanks to a combination of different electrochemical techniques and numerical simulations, we were able to tell both effects apart and quantify their respective strengths. Photoelectrochemical studies have clearly revealed the hot electron generation from plasmonic nanostructures. Although the external quantum efficiency is low for hot electron generation, those electrons are able to break the strong triple bond of the nitrogen molecule and to produce ammonia at a minimal rate. On the other hand, the efficiency of plasmonic near-field coupling was studied by depositing TiO2 on top of plasmonic nanostructures. It was found that the efficiency of the electron/hole pair generation in TiO2 is drastically increased when the surface plasmon resonance energy is located above the band gap energy of the TiO2. The same enhancement was observed for the ammonia production efficiency using photoelectrodes that combine TiO2 with plasmonic nanostructures. These results indicate successful nitrogen splitting at room temperature under atmospheric pressure.

Finally, we successfully improved the hot electron harvesting from plasmonic nanostructures by deposition of a thin, 2 nm, layer of platinum between the plasmonic nanostructures and TiO2. This thin platinum layer forms a Schottky barrier at its interface with TiO2, preventing the loss of electrons back into the plasmonic metal. Electrochemical photocurrent measurements clearly reveal that this new concept improves the hot electron extraction by two orders of magnitude.

M. Thangamuthu, Ch. Santschi, and O.J.F. Martin, “Photocatalytic ammonia production enhanced by a plasmonic near-field and hot electrons originating from aluminium nanostructures”, Faraday Discussions vol. 214, p. 399-415 (2019);
M. Thangamuthu, Ch. Santschi, and O.J.F. Martin, “Foolproof Langmuir Blodgett colloidal masks”, submitted to Langmuir (2019).