Ferdinand Boucher

PhD Topic : Exploration of diamond-based nanomaterials for (sono)photocatalysis

Among nanoscale semiconductors, nanodiamonds (NDs) have only recently begun to be seriously considered for photocatalytic reactions, due to confusion with the ideal monocrystalline diamond, characterized by a wide bandgap (5.5 eV) requiring deep UV illumination (230 nm) to trigger its photoreactivity. At the nanometric scale, NDs have native defects (sp2 carbon, chemical impurities such as nitrogen) that introduce electronic states into the band gap, reducing the light energy required to initiate charge separation. This has now been confirmed by several studies, including one carried out by the NIMBE team, combining experimental results and DFT calculations [1, 2]. Furthermore, unlike other semiconductors, the electronic structure of nanodiamond can be significantly modified (over several eV) by modifying the surface chemistry of the nanoparticle (oxidized, hydrogenated, aminated), allowing optimal adjustment of the band alignments [3]. These characteristics make NDs competitive with other semiconductors in photocatalytic processes. For example, the NIMBE team recently demonstrated the production of H₂ under solar illumination from the dissociation of water in the presence of nanodiamonds, with performance comparable to that of TiO₂ nanoparticles [4]. A growing body of research is also focusing on the ability of nanodiamonds to photogenerate solvated electrons, in particular for the photocatalytic degradation of pollutants and the reduction of CO₂ to compounds of interest [1,5].

In this PhD topic, we aim to accelerate the development of nanodiamond-based ‘solar-to-X’ technologies by exploring sonochemical routes to improve both the surface properties of nanodiamonds and the photocatalytic process itself. The topic will thus bring together the expertise of the NIMBE team on nanodiamonds and the ICSM team on sonochemistry.

The first part of the work will concern the synthesis of nano-photocatalysts based on nanodiamonds. Various surface chemistries and the association of nanodiamonds with co-catalysts (semiconductors or metals) will be considered, with the aim of improving interfacial reactivity, light absorption and optimizing charge separation. Both ‘classical’ and ‘sonochemical’ syntheses are envisaged at the two facilities for the synthesis of these hybrid materials, in particular via the reduction of metal salts on the surface of nanodiamonds. Preliminary work carried out between ICSM and NIMBE teams has already shown that sonochemistry can be considered as an original alternative for modifying and controlling the surface chemistry of NDs in solution. The photocatalytic properties of these materials will initially be evaluated using the benches available at the NIMBE.

The second part of the work will focus on the sonophotocatalytic approach. During photocatalytic processes, it has been shown that the phenomenon of acoustic cavitation, generated by the additional propagation of ultrasound in solution, improves mass transfer by dispersing the catalytic particles and promotes additional reactive species, which can play a key role in H₂ production or CO₂ reduction. In addition, acoustic cavitation can be accompanied by the emission of light propagating from the UV to the near IR, likely to improve the photogeneration of charges in nanodiamonds [6]. A sonophotocatalytic cell will thus be developed at the ICSM, followed by studies aimed at evaluating the synergies between sonochemistry and photocatalysis for the production of H₂ or the reduction of CO₂ using these nanodiamond-based materials. This exploratory and innovative part will constitute the most innovative aspect of the PhD, which is particularly relevant to the use of nanodiamonds as photocatalysts.

Références
[1] F. Buchner, Early dynamics of the emission of solvated electrons from nanodiamonds in water, Nanoscale. 2022, 14, 17188.

[2] T. Yoshikawa, CO₂ reduction by visible-light-induced photoemission from heavily N-doped diamond nano-layer, Carbon 2024, 218, 118689.

[3] D. Miliaieva, Absolute energy levels in nanodiamonds of different origins and surface chemistries, Nanoscale Adv. 2023, 5, 4402–4414.

[4] C. Marchal et al., Oxidized detonation nanodiamonds act as an efficient photocatalyst to produce hydrogen under solar irradiation, Adv. Energy Sustain. Res. 2024, 5, 1-8.

[5] W.A. Maza, Degradation of perfluorooctanesulfonate (PFOS) by sub-bandgap irradiation of hydrogen-terminated nanodiamond, Appl. Catal. B Environ. 2023, 325, 122306.

[6] M.A. Ahmed, Advances in ultrasound-assisted synthesis of photocatalysts and sonophotocatalytic processes : A review, Iscience 2024, 27, 108583.