Diamond nanoparticles (nanodiamonds) are used in nanomedicine, quantum technologies, lubricants and advanced composites [1-2]. Our recent results show that nanodiamond can also act as a photocatalyst, enabling the production of hydrogen under solar illumination [3]. Despite its wide band gap, its band structure is adaptable according to its nature and surface chemistry [4]. Moreover, the controlled incorporation of dopants or sp2 carbon leads to the generation of additional bandgap states that enhance the absorption of visible light, as shown in a recent study involving our group [5]. The photocatalytic performance of nanodiamonds is therefore highly dependent on their size, shape and concentration of chemical impurities. It is therefore essential to develop a "tailor-made" nanodiamond synthesis method, in which these different parameters can be finely controlled, in order to provide a supply of "controlled" nanodiamonds, which is currently lacking.
This PhD aims to develop a bottom-up approach to grow nanodiamond using a sacrificial template (silica beads) to which diamond seeds < 10 nm are attached by electrostatic interaction. The growth of diamond nanoparticles from these seeds will be achieved by microwave-enhanced chemical vapor deposition (MPCVD) using a homemade rotating reactor available at CEA NIMBE. After growth, the CVD-NDs will be collected after dissolution of the sacrificial pattern. Preliminary experiments have demonstrated the feasibility of this approach with the synthesis of faceted <100 nm nanodiamonds (so called CVD-ND), as shown in the scanning electron microscopy image.
During the PhD work, the nature of the diamond seeds (ultra-small NDs [size ˜ 5 nm] synthesized by detonation or HPHT, or molecular derivatives of adamantane) as well as CVD growth parameters will be studied to achieve better controlled CVD-NDs in terms of crystallinity and morphology. Nanodiamonds doped with boron or nitrogen will be also considered, playing on the gas phase composition. The crystalline structure, morphology and surface chemistry will be studied at CEA NIMBE using SEM, X-ray diffraction and Raman, infrared and photoelectron spectroscopies. A detailed analysis of the crystallographic structure and structural defects will be carried out by high-resolution transmission electron microscopy (collaboration). CVD FNDs will then be exposed to gas-phase treatments (air, hydrogen) to modulate their surface chemistry and stabilize them in water. The photocatalytic performance for hydrogen production under visible light of these different CVD-NDs will be evaluated and compared using the photocatalytic reactor recently installed at CEA NIMBE.
References
[1] Nunn et al., Current Opinion in Solid State and Materials Science, 21 (2017) 1.
[2] Wu et al., Angew. Chem. Int. Ed. 55 (2016) 6586.
[3] Marchal et al., Adv. Energy Sustainability Res., 2300260 (2023) 1-8.
[4] Miliaieva et al., Nanoscale Adv. 5 (2023) 4402.
[5] Buchner et al., Nanoscale 14 (2022) 17188.

The use of composite materials in the space field has led to great weight improvements. To continue to achieve significant weight gain, composite cryogenic tank is the next technological application to reach by replacing the current metal alloy cryogenic propellant tanks. Lighter reinforced organic matrix composite materials (that are at least as efficient from a mechanical, thermal, chemical and ignition resistance point of view) are a realistic target to be reached that has been explored in recent years. Many research approaches tend to answer to this technological lock, but the potentialities of Carbon NanoTubes (CNTs) in terms of mechanical and physical properties, need to be explored deeper.
A first phase to assess the interest of CNTs for space applications (collaboration CNES/CEA/I2M/CMP Composite) was carried out to associate CNTs with a cyanate ester matrix in layered composite materials through three processes: (i) transfer of aligned CNTs mats by hot pressing (ii) dispersion of entangled CNTs mixed with resin, or (iii) growth of nanotubes aligned directly on the dry ply. Knowing mechanical and thermal loads, the aim is to demonstrate the efficiency of CNTs and influence of their characteristics with regard to damage tolerance of the material and consist in delaying the cracking process of the composite nearby the CNT layer and thus blocking the percolation of the cracking network which leads to the loss of tightness. For the preferred development process identified, the aim of this doctoral work is now to consolidate the material functionalisation with CNTs (shape, density, etc.) and the understanding of the mechanical behaviour (at room temperature and at low temperature) for the development of the layered material integrating CNTs.
Knowing the potential final application as cryogenic tank or for the improvement of structural materials sustainability in dual application, relevant tests will be performed to demonstrate the impact in terms of damage development and tightness in comparison with the same material without CNTs.