Exploration of Diamond-Based Nanomaterials for (Sono)photocatalysis: Applications in Hydrogen Production and CO2 Reduction
Nanodiamonds (NDs) are increasingly being studied as semiconductors for photocatalysis, thanks in particular to the very specific positions of their valence and conduction bands, which can be modulated. For example, it has recently been shown that NDs can produce hydrogen under sunlight with an efficiency similar to that of TiO2 nanoparticles. Other studies also show the possibility of photogenerating solvated electrons from certain NDs for CO2 reduction or the degradation of stubborn pollutants.
With a view to accelerating the development of nanodiamond-based ‘solar-to-X’ technologies, we propose in this thesis to integrate nanodiamonds as photocatalysts in a sonophotocatalytic approach. Acoustic cavitation, generated by ultrasound, can improve mass transfer by dispersing catalytic particles and can produce additional reactive species (hydroxyl radicals, superoxides). It also emits sonoluminescence, which can promote the photogeneration of charges and should limit the recombination of charge carriers.
The first phase of the work will focus on the synthesis of nanodiamond-based photocatalysts, exploring various surface chemistries and their association with co-catalysts. Both classical and sonochemical synthesis methods will be used, as preliminary studies have shown that sonochemistry can effectively modify the surface chemistry of NDs. The photocatalytic properties of these materials will first be evaluated, leading to the design of a sonophotocatalytic cell. Further studies will explore the synergies between sonochemistry and photocatalysis for hydrogen production or CO2 reduction. This thesis will be carried out as part of a collaboration between the NIMBE at the Saclay CEA centre and the ICSM at the Marcoule CEA centre.
Brines for metal recycling
Critical metals are essential for a range of technologies that are vital to reduce our carbon dioxide emissions. However, the global recycling rate for metals contained in electronic waste is below 20%, indicating that electronic waste is a relatively untapped source of metals. Additionally, it is increasingly urgent to develop effective processes for recycling waste from products like solar panels, as the volume of waste solar pannels generated is set to rise significantly in the coming years. Currently, conventional hydrometallurgical methods often rely on toxic aqueous solutions to dissolve metals, which presents substantial environmental challenges.
This project proposes an innovative alternative by using concentrated brines (aqueous salt solutions) to oxidize and dissolve metals. This thesis will investigate the fundamental properties of brines and their ability to dissolve metals through various techniques, particularly electrochemical methods. Artificial intelligence methods developed within the lab will be employed to screen a wide range of brines that would allow metal dissolution. Subsequently, brine-based recycling processes will be developed to recover metals from printed circuit boards and solar panels. Finally, metal separation and the treatment of used brines will be explored using membrane and electrochemical processes.
Understanding the mechanisms of direct CO2 hydrogenation using (Na,K)FeOx catalysts via theoretical-experimental coupling
In the context of climate change, we need to reduce our CO2 emissions by using less energy. Another approach is to capture, store and use CO2, with the aim of moving towards a circular carbon economy and, ultimately, defossilization. With this in mind, the direct hydrogenation of CO2 enables it to be transformed into molecules of interest such as hydrocarbons, via the coupling of the reverse water gas shift (RWGS) reaction and Fischer-Tropsch synthesis (FTS).
Computational operando catalysis has recently emerged as a reasoned alternative to the development of new catalysts, thanks to a multi-scale approach from the atom down to the active particle, to model catalyst selectivity and activity. New tools combining ab initio simulations (DFT) and molecular dynamics (MD) via machine learning algorithms bridge the gap between the precision of DFT calculations and the power of atomistic simulations. Current bifunctional catalysts (active for RWGS, and FTS) for direct CO2 hydrogenation are based on doped iron oxides (metal promoters).
The aim of this project is the theoretical study of Na-FeOx and K-FeOx catalysts doped with Cu, Mn, Zn and Co, in 4 stages: DFT simulations (adsorption energies, density of states, energy barriers, transition states), microkinetic modeling (reaction constants, TOF), construction of interatomic potentials by DFT/machine learning coupling, simulation of whole particles (selectivity, activity, microscopic quantities).
This theoretical study will go hand in hand with the synthesis and experimental measurements of the studied catalysts, and optimized catalysts emerging from the computational results. All the accumulated data (DFT, MD, catalytic properties) will be fed into a database, which can eventually be exploited to identify descriptors of interest for CO2 hydrogenation.
Magneto-ionic gating of magnetic tunnel junctions for neuromorphic applications
Magneto-ionics is an emerging field that offers great potential for reducing power consumption in spintronics memory applications through non-volatile control of magnetic properties through gating. By combining the concept of voltage-controlled ionic motion from memristor technologies, typically used in neuromorphic applications, with spintronics, this field also provides a unique opportunity to create a new generation of neuromorphic functionalities based on spintronics devices.
The PhD will be an experimental research project focused on the implementation of magneto-ionic gating schemes in magnetic tunnel junction’s spintronics devices. The ultimate goal of the project is to obtain reliable and non-volatile gate-control over magnetisation switching in three-terminal magnetic tunnel junctions.
One major challenge remains ahead for the use of magneto-ionics in practical applications, its integration into magnetic tunnel junctions (MTJ), the building blocks of magnetic memory architectures. This will not only unlock the dynamic control of switching fields/currents in magnetic tunnel junctions to reduce power consumption, but also allow for the control of stochasticity, which has important implications in probabilistic computing.
Study of electronic processes in nitride LEDs by electro-emission microscopy
Nitride LEDs are universally used for energy-efficient lighting. They are extremely efficient at low indium content and low current density, allowing to produce commercial white LEDs from a blue LED and a phosphor that absorbs blue and re-emits a broad spectrum in the visible range. However, nitride LEDs suffer from a drastic drop in efficiency at higher current densities and higher indium concentrations, for emission in the green or red. This is an obstacle to extending their use, in order to obtain higher efficiencies with less material, as well as better color rendering. These efficiency drops are partly due to an increase in three-particle Auger-Meitner processes, which are strongly impacted by local device heterogeneities, and can be reduced by specific engineering of structural defects in nitride materials. This thesis proposes to study the electronic processes in nitride LEDs in operando, using electro-emission microscopy. In particular, charge injection mechanisms in the active part of the LEDs and Auger-Meitner processes will be investigated and quantified. The spatial resolution of the technique will allow to characterize the role of heterogeneities (defects or alloy disorder) in the loss processes.
Design of plasmonic nanocomposite membranes for biomolecule detection
Detection of specific small biomolecules amounts is usually challenging. Recently, nanomaterials have provided new materials with interesting optical properties for such an application, especially plasmonic nanomaterials.
In this project, we propose the design of a specific type of nanocomposite made from the incorporation of plasmonic nanoparticles (NPs) within track-etched functionalized polymer membranes. The tuning of the material plasmonic response will be achieved by a controlled in situ NP synthesis directly within the membrane nanopores, through chemical and physico-chemical processes. Especially, the use of radiation (electron beam, ?-rays) to induce the in situ reduction of the metallic precursor will be studied. Ionizing beams (Swift Heavy ions) will also serve to structure the polymer matrix in nanoporous membrane with controlled nanoporosity. The relation between the composite nanostructure and its optical properties will be thoroughly investigated in order to determine the ideal material for biomolecule detection, which will be tested on model molecules such as proteins or virus-like particles (VLPs) in the final part of the project.
Experimental study of boundary layers in turbulent convection by diffusive waves spectroscopy
Turbulent convection is one of the main drivers of geophysical and astrophysical flows, and is therefore a key element in climate modeling. It is also involved in many industrial flows. Transport efficiency is often limited by boundary layers whose nature and transitions as a function of control parameters are poorly understood.
The aim of this thesis will be to set up a convection experiment to probe the dissipation rate in boundary layers in the turbulent regime, using an innovative technique developed in the team: multi-scattered wave spectroscopy.
Topological superconductivity and Fermi surface in spin-triplet superconductors
Topological superconductivity has become a subject of intense research due to its potential for breakthrough in the field of quantum information. Bulk systems are a promising possibility, with candidates found mainly among unconventional superconductors, which are also strongly correlated electron systems. Today, only a few candidate compounds for topological bulk superconductivity exists, and they are mostly uranium-based heavy fermion superconductors. UTe2 is one of the most prominent candidates. The topological properties of the superconductors depends crucially on the topology of the Fermi surface.
In this project we want to set up a novel technique (for our team) relying on a tunnel diode oscillator circuit. This techniques is very sensitive to quantum oscillations, and to be well both to high magnetic fields and to high-pressure studies. First experiments concentrate on the novel superconductor UTe2, where the Fermi surface is only partly known. In further studies the topological properties of the ferromagnetic superconductors UCoGe and URhGe will be revised.
Sub-critical crack growth in oxide glasses
Material failure is a concern for scientists and engineers worldwide. This includes oxide glasses, which are integral parts of building, electronics, satellites due to multiple advantageous features, including optical transparency, elevated mechanical and thermal properties, chemical durability, biocompatibility and bioactivity, etc. Despite this, oxide glasses have a significate drawback: they are inherently brittle. Oxide glasses are well known to undergo dynamic fracture (crack propagation velocity of ~km/s – as in the case of a glass crashing to the floor and shattering); yet, there is another fracture mode less noticeable that will be studied during this thesis, where crack fronts grow sub-critically. The growth of these crack fronts is aided by environmental parameters including atmospheric humidity and temperature, and the crack front velocity depends on the local stress felt by a crack tip, coined the stress intensity factor.
Currently, our experimental setup tracks the crack front position in time via a tubular microscope equipped with a camera. Post-analysis of images provides the crack front velocity and reveals the environmental limit K_e and region I. However, the current experimental setup cannot capture regions II and III. Several factors play into this limitation: elevated crack front velocity (10e-4 to 1500 m/s), sample size (5×5×25 mm^3), camera acquisition rates, etc.
In recent years, our team has used the potential drop technique to track the crack front velocity when v > 10e-4 m/s in PMMA. This technique involves the deposition of conductive strips on the sample surface. Subsequently, these lines are attached to a high frequency oscilloscope. As the crack front propagates through the sample, the lines are severed resulting in an increase in the electrical resistance. We now wish to adapt this technique to DCDC samples on oxide glasses. The thesis goal is the development and application of the potential drop techniques to DCDC samples. The challenge concerns the spatial temporal resolution (50 µm and 1 ns) in comparison to the crack tip velocity and sample size. The thesis student will take part in all the steps to realize the experiments: designing and depositing patterns (series of strips) on the glass surfaces using a cleanroom, running sub-critical cracking experiments in Region II and III, and analyzing data acquired during the experiment.
Development of solid porous siliceous supports for actinide sorption - Behaviour under irradiation
The aim of this research project is to study the densification of a mesoporous structure under the effect of irradiation damage produced by the presence of an actinide (238Pu) in the porous structure. To achieve this, siliceous materials based on mesoporous silicas modified by the addition of additive elements (B, Al, etc.) will be used. The purpose of adding these elements is to weaken the mesoporous structure in order to promote densification. The characteristics of the mesoporous structure (pore diameter, wall size, symmetry of the pore network) will be other parameters of the study. These materials will be functionalised with phosphonate ligands for actinide adsorption: thorium as a simulant in a preliminary stage, followed by plutonium. The final part of this work, which will continue beyond the thesis, will involve using various techniques (SAXS, BET, microscopy, etc.) to study the evolution of the mesoporous structure under the effect of irradiation damage as the material ages. This fundamental research work could have spin-offs in the field of nuclear waste conditioning materials: ageing of gels on the surface of nuclear glass, support material for decontaminating radioactive effluents. Part of the work will be carried out at CEA Marcoule's Atalante facility.