Development of a CVD plasma assisted process for the synthesis of spherical boron doped diamond core-shells and advanced characterizations – COCONUT

Boron Doped Diamond (BDD) emerges as an outstanding material for electrochemistry, electrocatalysis, photocatalysis and medicine. Wide band gap semiconductor with quasi-metallic behavior at high doping level, resilient to harsh chemical conditions, bioinert, this material gathers serious assets. Among all these applications of BDD, one emerges as crucial in the current context of climate change and concerns the recycling of CO2 through its re-use as useful chemicals and even fuels. Indeed, BDD has already been identified as an outstanding electrode for the electrochemical reduction of CO2 Furthermore, BDD has also been recently revealed as a solid source of solvated electrons in water, which opens the door to the selective CO2 into CO, not accessible with other materials. Both approaches, via pure electrochemistry or via photo-electro-chemical reduction, thus offer to BDD a great opportunity of development in the coming years.

Individual nano or sub-micron objects made of BDD would be of great benefit for these applications to (i) increase the effective surface of electrodes and (ii) break free from the size limitations of growth on planar wafers with conventional MPCVD process which is limited to few inches in diameter. However, today, it should be underlined that the state-of-the-art techniques to produce BDD particles consist mostly in the CVD growing of BDD films and their crushing into fine particles. This approach is time consuming, difficult to handle and limited in volume. In the LITCHI project, we propose a breakthrough approach, based on the synthesis of monodispersed spherical core/shell particles, starting from commercially available spherical cores of SiO2 and a shell of electrically conductive BDD of few tens of nm. We will develop an innovative CVD technology assisted by plasma dedicated to the individual coating of nano&micro particles with BDD, able to treat a large volume per batch (> 100 mg/h). Considering this innovating technology, the crushing step is suppressed, and the obtained core-shell diamond particles are perfectly spherical with a monodispersed size distribution adjustable by changing the size of the SiO2 core particles. We aim to provide to the scientific community a reliable source of BDD in dispersed form, which can be afterward used in suspension, their spherical shape further opening large possibilities of self-assembling into 2D or more complex 3D structures (like opals). Objectives of the project are (i) to design & develop a high production-yield synthesis reactor (>100 mg/h) with a technology which could be upscaled afterwards ensuring uniformity and reproducibility of the diamond coating (ii) to qualify the structural quality of BDD shells and the overall microstructure by a panel of advanced analysis techniques (iii) to investigate and evidence the performances of BDD core-shells regarding electrochemical and photochemical properties before (iv) evaluate their performances toward CO2 reduction. The first scientific challenge which will drive the development of the project will concern the BDD layer quality grown around the silica cores. This is why the project gathers expertises in innovative structural and physicochemical advanced characterizations (GEMAC, ONERA), nano-dedicated characterizations and electrochemistry (ILV). The second scientific challenge will concern the development of a scalable process (production over 100 mg/h for the prototype developed in LITCHI but scalable for further industrial exploitation) to synthesize this BDD core-shell, which respects the requirements described above for the BDD layers grown around SiO2 cores. Though, one strength of the project is to associate (i) an industrial partner able to provide innovative and tailored microwaves systems (SAIREM) with (ii) a long-term experienced team in the building of diamond CVD reactors, who already designed a home-made reactor able to treat by plasma nanoparticles in small quantities (<10 mg/h) (CEA LIST).