Functionalized aluminosilicate nanotubes for photocatalysis

Rising energy demand and the need to reduce the use of fossil fuels in order to limit global warming have led to an urgent need for clean energy harvesting technologies. One emerging solution is to use solar energy to produce molecules of interest (H2, CO or CH4). The Holy Grail would be to produce them using only CO2 or water and the sun's photons. This task, accomplished daily by plants thanks to a fascinating self-assembled photosynthesis machinery, is still far from our technological grasp. Mature technologies do exist, right up to industrial pilot scale. However, they are still very energy-intensive. At the current state of the art, several dozen EPRs would be needed to provide the energy required to implement a European strategy for synthetic fuels.

Since Fujishima and Honda's pioneering discovery in 1972 of the photolysis of water using TiO2 as a catalyst, efforts have been made to identify inexpensive materials for photocatalytic reactions, and a large number of semiconductors have been investigated. Among them, 1D hollow nanostructures are promising due to their favorable properties: high and accessible specific surface areas, confined environments, long-distance electron transport and facilitated charge separation. However, they are obtained by complex preparation methods, with expensive or scarce elements (Co, Ir, Ta, Pt, La, Bi, W) and the photocatalytic mechanisms (linked to morphology) have not been fully clarified.

For several years now, we have been working at LIONS laboratory on 1D hollow nanotubes called imogolite. These are clays whose particularity lies not in their composition (O, Al, Si, the three most abundant elements in the earth's crust) but in their intrinsic curvature, which induces a permanent polarization of the wall. Predicted by DFT-type calculations, this has been demonstrated experimentally and quantified in our laboratory. Valence and conduction bands are spatially separated across the wall, promoting photo-induced charge separation (one of the key processes in photosynthesis), with holes moving to the inside of the tubes and electrons to the outside (which we have been able to demonstrate through radiation chemistry experiments). This charge separation phenomenon limits recombination reactions and enables oxidation and reduction reactions in the cavity and on the outer surface of the nanotubes, respectively.

These materials are therefore good candidates as nanoreactors for photocatalytic reactions. These nanosystems can be synthesized by sol-gel methods under mild conditions in large quantities to form stable, transparent dispersions. The wall polarization of these compounds can be modulated by modifying the internal surface chemistry. In particular, the latter can be made hydrophobic (surface covered with methyl groups), enabling the encapsulation of numerous organic molecules, typically dyes. It is also possible to double-functionalize this cavity by partially replacing some of the methyl groups with other chemical functions (amine, halogen, thiol, vinyl, phenyl).

In the laboratory, we have demonstrated that this clay is a nanoreactor for photocatalytic reactions (H2 production and CO2 reduction) under UV illumination. In order to obtain a useful photocatalyst, two aspects need to be further investigated: i) photon collection needs to be extended into the visible range. Imogolite has a high bandgap energy value (> 5 eV). To achieve this, one strategy being considered is to encapsulate and covalently graft dyes into the cavity, acting as antennae and enabling visible light photons to be collected according to their absorbance spectrum. Initial tests in our laboratory have demonstrated the covalent grafting of naphthofurazane-type molecules with imogolites possessing -NH2 groups in the cavity. Another strategy, already tested in our laboratory, involves grafting metal centers (gold nanoparticles) onto the external surface.

This coupling improves H2 production by a factor of 90 compared with ungrafted imogolite. Another consequence of this coupling is that the system also becomes active in the visible range. Although promising, gold is certainly not the most suitable metal from the point of view of developing more sustainable and environmentally-friendly systems. The use of other metal centers or reducing agents, grafted onto the outer surface of the tubes to enhance the photocatalytic properties of the samples, has not yet been demonstrated.

This thesis is funded by PEPR LUMA and is part of the SYNFLUX LUMICAL project, which brings together a consortium of 19 teams from all over France. The aim of this project is to develop innovative strategies for the more efficient collection and use of light in solar fuel production systems. The various teams in the consortium are developing both dyes (for the cavity) and reducing agents (for the external surface), which will be tested as part of this thesis. In addition, the consortium has platforms for the study of short-time energy transfer, as well as advanced characterization of samples and their operando behavior under light stress.

The aims of this thesis are: i) to synthesize imogolites with different internal functionalizations, and then to study the encapsulation and grafting of dyes into the cavity of these functionalized imogolites; ii) to study the grafting of different reducing agents onto the external surface. For both types of functionalization, time-resolved experiments will be carried out on the consortium's platforms, to gain a better understanding of energy transfer phenomena.  The final objective is iii) to study the photocatalytic properties of these systems (proton and CO2 reductions triggered by solar illumination). To this end, operando characterization experiments will be carried out.

Objectives

The project is divided into three main objectives: i) synthesize imogolites with different internal functionalizations, study the encapsulation and grafting of dyes in the cavity of these functionalized imogolites; ii) graft on the external surface different reducing agents and characterize the system, and iii) study the photocatalytic properties of these samples (proton and CO2 reductions triggered under solar illumination).

Task 1: synthesis of functionalized imogolites, encapsulation and grafting of dyes

The aim of this task is to graft dyes into the imogolite cavity. Based on the different molecules proposed by the consortium and the various possible functionalizations of the imogolite (amine, thiol, phenyl, halide), a selection of dye/chemical function couples will be made. The aim will be to synthesize the various functionalized imogolites, then encapsulate and graft the dyes.

Task 2: grafting reducing centers onto the external surface

The aim of this task is to graft various reducing centers onto the external surface of functionalized imogolites in order to improve the photocatalytic properties of these systems. Several centers are envisaged, either metallic (the aim being to vary the quantity and size of grafted nanoparticles) or phosphate-based reducing agents (supplied by the consortium teams). For the latter, it will be necessary to check that the coupling has no impact on the structure of the nanotubes.

Task 3: study of photocatalytic properties

Once the systems are complete (internal grafting of dyes and external grafting of reducing agents), their photocatalytic properties will be studied:
i) establishment of band diagrams;
ii) production of H2/CO or CH4 from solar radiation;
iii) study of the fate of the photogenerated electron using time-resolved spectroscopy (carried out on the consortium's platforms). More generally, energy transfer phenomena will be studied, for different samples, using time-resolved techniques available in the consortium. Various parameters of interest, such as nanotube length, will be studied, in order to better understand and optimize these processes. Photocatalysis operando experiments will also be carried out at the SOLEIL synchrotron.

Finally, the student will write his/her manuscript.

The various tasks (demonstration of dye grafting in the cavity and of reducing centers on the external surface, determination of electronic properties, study of reactivity) will be the subject of articles.

Method

A wide range of techniques will be used in the course of this thesis, both within the laboratory and on the various platforms accessible through the PEPR LUMA.

Firstly, the synthesized samples will be characterized by small-angle X-ray scattering, NMR, infrared/UV-vis/fluorescence spectroscopy, microscopy and more.

The electronic properties of these materials will be determined by UV-visible diffuse reflectance and XPS (laboratory and synchrotron).

The study of the fate of the photogenerated electron, and more generally of energy transfers in the samples, will be carried out using time-resolved spectroscopy experiments carried out on the platforms available in the consortium.

Reactivity experiments will be carried out using a solar simulator. The gases produced will be determined by gas micro-chromatography and gas chromatography-mass spectrometry. Finally, operando experiments to characterize the gases produced and understand the evolution of the catalyst will be carried out at the SOLEIL synchrotron.

Expected results

Development of sustainable nanomaterials for photocatalysis (H2 production and CO2 reduction).

Understanding of reaction mechanisms under illumination in confined environments.

Demonstrate grafting of dyes into the cavity.

Study energy transfer between the inside and outside of functionalized nanotubes.

Identification of the best photocatalytic system.

Study of nanosystem aging under illumination.

References

1. Pignié et al. “Experimental determination of the curvature-induced intra-wall polarization of inorganic nanotubes”, Nanoscale, 2021, 13, 19650-19662 (DOI: 10.1039/D1NR06462B)
2. Pignié et al. “Confined water radiolysis in aluminosilicate nanotubes: the importance of charge separation effects”, Nanoscale, 2021, 13, 3092-3105 (DOI: 10.1039/D0NR08948F)
3. Patra et al. “Inorganic nanotubes with permanent wall polarization as dual photo-reactors for wastewater treatment with simultaneous fuel production”, Environmental Science-Nano, 2021, 8, 2523-2541 (DOI: 10.1039/D1EN00405K)
4. Patra et al. “UV-Visible photo-reactivity of permanently polarized inorganic nanotubes coupled to gold nanoparticles”, Nanoscale, 2023, 15, 4101-4113 (DOI: 10.1039/D2NR05796D)
5. Farré et al.” A Blue Diketopyrrolopyrrole Sensitizer with High Efficiency in Nickel-Oxide-based Dye-Sensitized Solar Cells”, ChemSusChem, 2017, 10(12), 2618-2625 (DOI: 10.1002/cssc.201700468)
6. Romito et al. “Dye-Sensitized Photocatalysis: Hydrogen Evolution and Alcohol-to-Aldehyde Oxidation without Sacrifical Electron Donor”, Angewandte International Edition Chemie, 2024, 63(12), e202318868
7. Queyriaux et al. “Recent developments in hydrogen evolving molecular cobalt(II)–polypyridyl catalysts”, Coordination Chemistry Reviews, 2015, 304, 1-19 (DOI: 10.1016/j.ccr.2015.03.014)
8. Droghetti et al. “Recent findings and future directions in photosynthetic hydrogen evolution using polypyridine cobalt complexes”, Dalton Transactions, 2022, 51, 10658-10673

The thesis work will be carried out at the CEA/Saclay site (Université Paris Saclay) in the LIONS laboratory (Laboratoire Interdisciplinaire sur l'Organisation Nanométrique et Supramoléculaire). This laboratory has all the tools and skills needed to carry out this thesis work. It specializes in the synthesis and characterization of inorganic nanomaterials, as well as in reactivity studies (gas production measurements, determination of reaction mechanisms, etc.).

In addition, experiments are planned on the consortium's various platforms (time-resolved spectroscopy experiments, to better understand energy transfer processes) and at the SOLEIL synchrotron (operando gas production experiments with monitoring of catalyst evolution). 

The candidate must have a solid grounding in physical chemistry and materials chemistry.

The candidate must possess rigor, curiosity, a taste for experimentation and instrumentation, and a critical sense.

The candidate must be able to travel easily in France and adapt to different environments to work on the consortium's various platforms.