Polyoxometalate clusters photosensitized with carbon-dots: a new generation of anode material for light assisted water-splitting – POMDOT

In order to overcome the issue of intensive use of fossil fuel energies, scientists need to rapidly propose new energy resources. For example, a fuel economy based on hydrogen is a major challenge. In particular, a sustainable approach relies on its synthesis from solar energy which appears as a green and renewable source without increasing atmospheric CO2 level. Original photo-electrochemical cells leading to water oxidation at the surface of a photo-anode and injecting fresh electrons in the external circuit until a platinum cathode for the proton reduction have been already designed. For example, several molecular photocatalysts have been already tested as materials for such photo-anodes. Among the different photocatalysts, polyoxometalates (POMs) constitute an original class of nanoscaled metal-oxygen polyanionic clusters which offer serious opportunities and promise great improvements. Indeed, their properties can be easily tuned by adjusting their structure and chemical composition, which in term of material processing is a real chance to design the best material for the best application. As a consequence, they have already been widely used for electro- and photocatalysis. Nevertheless, POMs are only useful in the UV-domain. Then, a strategy consists in the use of photosensitizers which absorb in the visible domain and which act as electron relays for POMs. In this proposal, we aim at coupling POMs to carbon dots (C-dots) used as photosensitizers. C-dots correspond to carbon nanocrystals which possess the same fascinating photophysical properties than quantum dots, but in contrast to them, they do not contain heavy metals, making them more safety and environmental friendly and exhibiting a high chemical stability. This project will be divided in three parts. Firstly, an important part of this project will be devoted to the synthesis of POMs and C-dots, and at the developing of electrostatic and covalent POMs-C-dots hybrids. In particular, we aim at the synthesis of C-dots having a substantial absorption in the visible domain and at the development of new strategies to functionalize C-dots, especially using diazonium salts method. The next step will be then devoted to the shaping of these hybrid materials on photo-electrodes. To test the capability of such photo-anodes, photocurrent measurements will be firstly performed in water to prove their ability towards the water oxidation. Then, photo-electrochemical cells will be built to perform water-splitting. Applications in photocatalysis towards organic pollutants degradation will be also investigated. Finally, in a last step, electron transfers between excited C-dots and POMs will be studied by several physical methods, in particular by electrochemical impedance spectroscopy, fluorescent measurements and microwave reflectance experiments in order to understand the mechanisms involved in the photo-electrochemical process.