PhD Topic : Study of photo(electro)catalytic properties of nanodiamonds for H2 production and CO2 reduction
TAmong nanoscale semiconductors, nanodiamonds (ND) have not been really considered yet for photoelectrocatalytic reactions. This originates from the confusion with ideal monocrystalline diamond featuring a wide bandgap (5.5 eV) that requires a deep UV illumination to initiate photoreactivity. At the nanoscale, ND enclose native defects (sp2 carbon, chemical impurities such as nitrogen) that can create energetic states in the diamond’s bandgap decreasing the light energy needed to initiate the charge separation. This is supported by a recent study that involved our group with combined experimental results and DFT calculations [1]. In addition, the diamond electronic structure can be strongly modified (over several eV) playing on its surface terminations (oxidized, hydrogenated, aminated) [2] which can open the door to optimized band alignments. Combining these assets, ND becomes competitive with other semiconductors toward photoreactions. We recently evidenced H2 production under solar illumination from water dissociation in the presence of nanodiamonds, with performances equivalent to those of P25 TiO2 nanoparticles [3]. The aim of this thesis project is to investigate the potential of nanodiamonds in producing solar fuels (H2 or CO2 reduction) by photo/electro-catalysis through a collaboration between the NIMBE team (Hugues Girard and Jean-Charles Arnault), which specializes in nanodiamond synthesis and SOLEIL team (Benedikt Lassalle, LUCIA), which has expertise in electrocatalysis and X-ray absorption spectroscopy (XAS) characterizations. Concerning H2 production, the work will focus on two main objectives: (i) gaining a better understanding of the mechanisms underlying the observed H2 production with nanodiamonds, and (ii) optimizing their performance by building heterostructures. Different surface chemistries and core structures will be studied, aiming to evaluate the influence of these parameters on the photocatalytic properties. Beyond hydrogenated and oxidized terminations, the work will focused on the influence of sp2 carbon promoted at the surface of the particles by high temperature annealing or within their core by electron irradiations. Surface and core modifications will undergo analysis using a range of complementary spectroscopic techniques, encompassing FTIR, Raman, and XPS. The modified ND will subsequently be dispersed within aqueous colloids and evaluated through DLS and Zetametry characterization before being tested toward H2 production. The second part of the thesis will explore the reduction of CO2 through (photo)electrocatalysis. In an initial approach, catalytic inks will be prepared using surface-modified nanodiamonds, following a technique already mastered by the SOLEIL team for other electrocatalysts (Nafion + carbon black). We will also explore a second approach involving the creation of porous ND structures. Such structures are fabricated using a process that combines an aerodynamic jet of nanodiamonds formed from a colloidal suspension with the simultaneous deposition of a matrix through PVD (Physical Vapor Deposition) [4]. By adjusting the ratios between the nanodiamonds and the matrix, it is possible to create a porous structure of nanodiamonds, bonded together by a solid conductive matrix while still leaving the particle’s surface accessible. This innovative approach is already well established at NIMBE for nanodiamonds. After conducting morphological and chemical characterizations of these catalytic inks and/or porous structures based on ND using SEM, XPS and XAS we will investigate their (photo)electrocatalytic performances under visible illumination toward CO2 reduction in terms of production rate and selectivity. Finally, the most efficient samples will be used for further developments using transition metal macrocyles, s cobalt or iron phthalocyanine [5].