Séparation de charge induite par effet de courbure dans des nanotubes d'aluminosilicate; vers la photocatalyse

par Marie-claire PigniÉ (Pignie)

Projet de thèse en Chimie

Sous la direction de Antoine Thill et de Sophie Le caer.

Thèses en préparation à Paris Saclay , dans le cadre de École doctorale Sciences chimiques : molécules, matériaux, instrumentation et biosystèmes (Orsay, Essonne ; 2015-....) , en partenariat avec NIMBE - Nanosciences et Innovation pour les Matériaux la Biomédecine et l'Énergie (laboratoire) , Laboratoire Interdisciplinaire sur l'Organisation Nanométrique et Supramolécualire (equipe de recherche) et de Université Paris-Sud (établissement de préparation de la thèse) depuis le 01-10-2018 .


  • Résumé

    Les imogolites sont des nanotubes d'aluminosilicates naturels avec un diamètre monodisperse. Leur synthèse est simple et le laboratoire NIMBE possède une plateforme unique pour les produire à grande échelle. Leur formule chimique naturelle est (OH)3Al2O3SiOH: la surface interne est hydrophile couverte par des fonctions SiOH alors que la surface externe est formée par des fonctions aluminoles. Ces nanotubes se dispersent très bien dans l'eau. Grâce à leur taille nanométrique, ils ne diffusent pas la lumière de manière significative et forment des dispersions transparentes. Lors de la synthèse, il est possible de remplacer les fonction silanols internes par des fonctions Si-CH3, tout en préservant leur forme tubulaire et un diamètre monodisperse. Cela permet d'obtenir des espèces hybrides. Ces nanopores hydrophobes peuvent ainsi encapsuler des petites molécules organiques tout en dispersion dans l'eau. Ce nouveau matériau synthétique a donc des propriétés non explorées due à ses propriétés hydrophiles/hydrophobes. L'énergie de la bande interdite de ces matériaux a été calculées et vaut entre 3 et 4eV. Les imogolites sont donc des semi-conducteurs. De plus, des prédictions théoriques récentes ont montré que la courbure importante importante de la paroi des imogolites induit une déformation de la distribution de la densité électronique. Cela entraine la formation d'une densité de dipôles de surface d'environ 30mD. Å-2, qui joue un rôle crucial dans la séparation de la paire électron-trou. L'électron se dirige ainsi vers la surface externe des tubes tandis que les trous sont confinés à l'intérieur du tube. C'est pourquoi, une des applications possibles des imogolites est la photocatalyse. Cette propriété permettrait la catalyse de réactions de photoréduction à la surface des nanotubes. On note que ces matériaux sont économiquement intéressant grâce à leur haute surface spécifique (de l'ordre de 200 m2/g) et leur faible coût de production (Al et Si étant abondants et peu chers). De plus, grâce à la simplicité de la synthèse, la production d'imogolites devrait être possible à un coût de moins de 10$/kg. Ce matériau est donc unique pour sa morphologie et ses propriétés de surface, mais est aussi compétitif si comparé à des zéolites ou d'autres matériaux microporeux commercialisés. Néanmoins, jusqu'à maintenant, aucune validation expérimentale n'a été trouvé dans la littérature pour confirmer les prédictions théoriques sur l'énergie de bande interdite, la séparation de charge et la densité de dipôles de surface. Nous avons effectué des tests préliminaires en utilisant des colorants solvatochromiques. Ces derniers sont sensibles à la polarisation de leur environnement. Du Nile Red, molécule anisotropique, a ainsi été encapsulé dans les tubes hybrides. Le décalage observé du spectre d'émission de fluorescente a été expliqué par l'interaction dipolaire entre le Nile Red et la paroi de l'imogolite. Le but de cette thèse est d'étudier les propriétés des imogolites en tant que possible photocatalyseurs. Ainsi, l'imogolite hybride sera particulièrement étudié ainsi que les imogolites méthylés dopés avec des centres métalliques. Le dopage permet de diminuer l'énergie de la bande interdite et des nanotubes dopés au fer ont déjà été produit facilement. Les nanotubes seront synthétisés suivant les protocoles de synthèse optimisés pour limiter la production d'impuretés (hydroxydes d'aluminium et proto-imogolites). Les imogolites seront ensuite caractérisées minutieusement par diffusion des rayons X aux petits angles (SAXS) disponibles au NIMBE, spectroscopie infra-rouge (IR) et microscopie cryo-TEM. Des mesures RMN permettront d'apprécier la qualité de la structure locale et de suivre le dopage. Différentes techniques seront ensuite utilisées pour révéler et si possible quantifier l'effet de la densité de dipôles de surface prédite. Certaines de ces expériences nécessitent une grande quantité de matériau. Le NIMBE possède des brevets liés aux imogolites notamment aux imogolites hybrides. La production d'imogolites à grande échelle a été démontré lors du projet de valorisation NanoSaclay Labex PRODIGE. Quatre stratégies permettant de révéler les propriétés locales sont proposées. Les propriétés photocatalytiques des imogolites seront testées dans les demi-cellules électrochimiques et pour des mesures spécifiques à la photocatalyse. Ce travail ouvrira la porte pour proposer des cellules photoélectrochimiques innovatives et novatrices.

  • Titre traduit

    Curvature-induced charge separation in oxide semiconductor nanotubes: towards photocatalysis


  • Résumé

    Imogolites are natural aluminosilicate nanotubes with a well defined monodisperse diameter. They can be synthesized easily and NIMBE has a unique facility to obtain them on a large scale. Their natural chemical formula is (OH)3Al2O3SiOH: the hydrophilic inner surface is fully covered with SiOH groups while the outer surfaces consist of aluminol groups. These nanotubes are very well dispersed in water. Thanks to their nanometric size, they do not scatter light significantly and form transparent dispersions. By synthesis, it is possible to replace the internal SiOH groups by SiCH3 ones while preserving their tubular shape and monodisperse diameter: this enables producing hybrid materials. These hydrophobic nanopores are then able to trap small organic molecules, while being easy to handle in water. This completely new synthetic material has then many unexplored properties due to its hydrophilic/hydrophobic dual properties. The band gaps of these materials were calculated and were found to be in the 3-4 eV range. Imogolite is thus a semiconductor. Moreover, recent theoretical predictions have evidenced that the very high curvature of the 6 Å thick imogolite wall induces a deformation of the electronic density distribution and gives rise to a surface dipole density of about 30 mD. Å-2. This surface dipole would play a crucial role in the electron-hole separation, with the electron transferred towards the outer surface of the tubes; the hole remaining confined inside the tube. Therefore, one potential, but also very promising, application of imogolites is their use as photocatalysts. Indeed, such property would catalyze photoreduction reactions at the surface of disperse nanotubes. Note that these materials could be economically very interesting, due to their high specific area (around 200 m2/g) and to their low cost (Al and Si atoms being abundant and inexpensive). Moreover, due to the relative simplicity of the synthesis route, it is anticipated that an industrial production of imogolite would be possible at a cost lower than 10 $/kg. This makes this material both unique for its morphology and surface property, but also competitive when compared to zeolite or other commercial microporous materials. Nevertheless, up to now, no experimental validations of the theoretical predictions on the band gap, charge separation and surface dipole density can be found in the literature. However, we have performed some preliminary tests using solvatochromic dyes. Such dyes are indeed known to be sensitive to the polarization of their surrounding environment. For this purpose, we have encapsulated Nile Red, an anisotropic molecule, inside the hybrid nanotubes and the observed shift in the fluorescence emission spectra was explained by the dipole interaction of Nile Red with the imogolite wall. The purpose of the present PhD thesis is to investigate the properties of imogolites as potentially interesting photocatalysts. For this, we will focus on hybrid (methylated) imogolite (with the methyl groups inside the nanotubes) and on methylated imogolite doped with metal centers. Indeed, doping is known to lower the band gap values and doped hybrid nanotubes with iron were proven to be easily produced. The nanotubes will be synthesized with the latest optimized recipes enabling to get the lowest impurity rate (aluminum hydroxides, proto-imogolite). The nanotubes will be thoroughly characterized with Small Angle X-ray Scattering (SAXS) available in NIMBE, with infrared (IR) spectroscopy, using the recently published thin film technique, and with cryo-TEM microscopy. Additional NMR experiments will also be performed, as they are useful to assess the quality of the hybrid local structure and to follow the doping. We will then exploit several techniques to reveal, and, if possible, quantify the effect of the predicted surface dipole density at the local scale. Note that some of these experiments require a significant amount of material. NIMBE has a patent portfolio related to imogolite and in particular to hybrid imogolite, and the production of hybrid imogolite on a large scale facility has been demonstrated during the NanoSaclay Labex PRODIGE valorization project. Thus, the large scale facility production of imogolite will enable producing the large volumes of concentrated dispersion or significant amounts of powder. Four strategies enabling revealing the properties at the local scale are proposed by increasing order of complexity. Finally, the photocatalytic properties of imogolite will be tested in half-cell electrochemical experiments and in experiments dedicated to photocatalysis. Local scale experimental strategies: i) The quantification of the fluorescence lifetimes between free and encapsulated dye will enable understanding more precisely the conformation of the trapped molecules and the field effect due to the nanotube dipole surface density. Note that preliminary experiments have already been performed in the framework of the PhD thesis of Pierre Picot in the case of Nile Red. ii) The H2 production, measured by gas chromatography, and induced by irradiation with accelerated electrons, and also by a UV lamp will be quantified in order to assess the respective interest of the various imogolites as photocatalysts. Normal, hybrid and doped hybrid samples will be studied. NIMBE has all the facilities to perform such experiments, and has also the experience of many other systems (aluminum oxides, clay minerals, layered double hydroxides). We anticipate here an enhancement of dihydrogen production compared to similar clay minerals having a layered (and not tubular) structure. iii) The presence of a surface dipole density is expected to modify the redox properties of encapsulated molecules. To study this effect, we will use ferrocene. It is indeed a very classical redox probe used for instrument calibration. It has an interesting and easily reversible one electron redox reaction between Ferrocene (FeII) and ferrocenium ion (FeIII). We have already observed that ferrocene is small enough to enter the nanotube cavity. We will load the nanotubes by direct impregnation with powder, by liquid/liquid extraction or by powder impregnation with ferrocene sublimation. The localization of ferrocene will be assessed by inductively coupled plasma mass spectrometry (ICP/MS) and SAXS experiments. The redox change between ferrocene and ferrocenium will be monitored by UV-visible spectroscopy and by cyclic voltammetry under different pH and atmosphere conditions. iv) The band gaps of these materials (hybrid imogolite, doped materials, hybrid materials with ferrocene), as well as the positions of the valence and conduction bands will be characterized in details, especially by X-ray Photoelectron Spectroscopy (XPS) experiments. Indeed, the recently developed near ambient pressure (NAP) XPS or liquid jet XPS experiments enable measuring ionization energy levels from atoms of nanoparticles dispersed in water. Photoelectron spectroscopy from aqueous solutions has evolved from the largely qualitative observations of the past decade towards quantitative (exact) results obtained using ever increasing levels of theory. For example, it is now known that the ionization energy (IE) of O 1s electron is sensitive to its environment, even at small concentration (Olivieri et al, 2017). In the case of imogolite, taking advantage of the incident beam energy variation, we will be able to study the IE of O 1s located inside and around the nanotubes. The local potential is expected to split the normal bulk water O 1s line into two different IE energies. Assessment of photocatalytic properties: Having understood in details the local scale electrochemical properties of different imogolites and trapped organic molecules, their exploitation in an integrated device will be explored in collaboration with electrochemists. For instance, we could make an anode covered with imogolite to test its capacity to produce H2 in half-electrochemical cells. Then, new photoelectrochemical cells could be proposed. This work will open the door for proposing new and innovative photoelectrochemical cells.