Assemblage contrôlé de graphène et de nanotubes de carbone par transfert de films de tensioactifs pour le photovoltaïque

Cette thèse est dédiée à l'étude d'une nouvelle méthode de formation de films ultra-minces de nanomatériaux carbonés sur surface. Basée sur le transfert d'un film d'eau stabilisé par des tensioactifs, elle permet notamment la réalisation et l'étude de films de nanotubes de carbone et d'oxyde de graphène (GO) aux propriétés remarquables. L efficacité de l approche développée est prouvée au travers de l ajustement précis des caractéristiques des films. Pour l assemblage d objets bidimensionnels cette approche est particulièrement pertinente puisque la planéité des feuillets de GO est conservée quelle que soit leur taille. Les avantages de l approche ne se limitent pas à la réalisation de monocouches à morphologie contrôlée mais s étendent à la réalisation de films multicouches d épaisseur ajustée et de très faible rugosité. De plus, cette approche est modulable et permet le transfert de films de nano-objets sur des surfaces de différentes mouillabilités et de grandes dimensions (transfert à l échelle de wafers). L intérêt du graphène oxydé en tant qu analogue du graphene ne se justifie que par une désoxygénation (réduction) efficace du matériau idéalement complétée par une réparation de sa structure sp . Cette thèse aborde ces deux aspects. Les électrodes transparentes à base d'oxyde de graphène réduit (rGO) réalisées au cours de cette thèse sont parmi les plus performantes du domaine. Les résultats présentés incluent également un travail important sur les caractérisations électriques des feuillets individuels et des films de GO et de rGO. Ainsi, nous avons prouvé qu il est possible de mesurer leur conductivité sans contact, par voie électrochimique (Scanning Electrochemical Microscopy). Même si les performances des électrodes en rGO n'atteignent pas celles des électrodes en graphène, les films réalisés peuvent d ores et déjà être intégrés dans des dispositifs photovoltaïques. Nos travaux permettent de contribuer au domaine émergeant des cellules basées sur l hétérojonction entre film de nano-objets carbonés et silicium. Dans le cadre de cette thèse nous montrons en particulier que les analyses par Time Resolved Microwave Conductivity sont complémentaires des mesures effectuées à l échelle des cellules photovoltaïques, chacune permettant de caractériser, sous des angles différents, l efficacité de séparation des charges photo-induites. Les travaux réalisés au cours de cette thèse contribuent aux problématiques dépendantes d assemblage et d intégration des nano-objets carbonés dans des dispositifs en ouvrant de nombreuses perspectives dans ces domaines en rapide évolution.This thesis concerns the study of a new solution-based deposition method for the formation of ultrathin carbon nano-object films on surfaces. Based on the transfer of a surfactant-stabilized water film, this method enables the formation and the study of carbon nanotubes and graphene oxide (GO) films with remarkable properties. The efficiency of the developed approach is proven through the fine-tuning of the film properties. This method is particularly well-suited for the assembly of bidimensional nano-objects such as GO sheets, the flatness of which is preserved whatever their dimensions. The advantages of the approach are not limited to the morphological control of monolayer assemblies but extend to the realization of multilayer films of adjustable thickness and extremely low roughness. Besides, it enables the transfer of nano-object films on large (wafer-scale) surfaces of various wettability. The use of graphene oxide as an intermediate step toward graphene only makes sense if it is efficiently deoxygenated (reduced) and, ideally, repaired at the level of sp domains. This thesis addresses these aspects. The realized transparent electrodes made of reduced graphene oxide (rGO) are among the most efficient in this field. The presented results also include an important work on the electrical characterization of graphene oxide sheets and films. We notably prove that conductivity can be measured without contact by an electrochemical way using Scanning Electrochemical Microscopy. While the performances of rGO electrodes are below those of graphene electrodes, the studied films can already be integrated into photovoltaic devices allowing to contribute to the emerging field of solar cells based on carbon/silicon heterojunctions. We particularly demonstrate that Time Resolved Microwave Conductivity analysis and photovoltaic cell measurements are complementary. Each of these techniques allows evaluating the efficiency of the separation of photo-induced charges. This thesis contributes to the dependent problematics of nano-object assembly and nano-object integration into devices, which are central for the development of nanotechnologies based on the bottom-up strategy.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Photonic titanium dioxide film obtained from hard template with chiral nematic structure for environmental application

International audienceIn the present work, mesoporous TiO 2 with a photonic structure was elaborated using cellulose nanocrystals (CNCs) as a biotemplate by two-step hard template methods. This strategy enables to replicate the chiral nematic (CN) structure of the photonic films (biotemplate) in TiO 2 films. A series of iridescent CNCs films with different weight ratios of silica/CNCs composite photonic films were prepared via evaporation induced self-assembly (EISA) method. The films showed iridescent color and tuneable Bragg reflection wavelengths by solely changing the ratio between the silica and the CNCs biotemplate. Polarized optical microscopy (POM) performed on hydride SiO 2 /CNCs films showed a birefringence and typical fingerprint of chiral nematic structure. This birefringence was also observed for TiO 2 films obtained using SiO 2 films as a hard template, which suggested the transfer of the chiral nematic structure in TiO 2 materials. Afterwards, their optical, morphological and electronic properties were studied by scanning electron microscope (SEM), POM, energy-dispersive X-ray spectroscope (EDX) and time resolved microwave conductivity (TRMC). The photocatalytic activities were evaluated by following the phenol degradation using high performance liquid chromatography (HPLC). The results showed that the structuration of the TiO 2 film using a chiral nematic SiO 2 film as hard template enhances the photocatalytic performance compared to non-structured mesoporous TiO 2

Mise au point d'un réacteur photocatalytique pour la dépollution de l'eau.

L objectif de cette étude est de mettre au point une méthodologie de travail dédiée à la conception d un réacteur photocatalytique pour la purification des eaux. Notre ambition étant de passer de la photocatalyse comme processus chimique à la photocatalyse comme procédé de dépollution. Pour cela il nous a paru nécessaire de répondre à deux questions, à savoir : - Dans un tel système, quels seront les phénomènes physico-chimiques majeurs mis en jeux ? -Y-a-t il un moyen de prédire les performances de notre système ?Tout d abord, nous avons essayé de travailler sur des photocatalyseurs en suspension, donc non immobilisés sur un support. Le but est d éviter une étape de filtration couteuse et techniquement difficile. Ainsi différents types de dioxyde de titane ont été déposés sur des supports de différentes géométries.Le premier média obtenu était du dioxyde de titane commercial (P25 de Evonik et UV100 de Sachtleben) déposé sur des billes en verre de diamètre 2 et 4 mm, introduit dans un réacteur annulaire siège de la réaction de dégradation photocatalytique et éclairé par une seule source de lumière UV. La photoactivité de ce média a été testée sur du phénol, le polluant primaire modèle choisi dans cette étude. L hydroquinone et la benzoquinone, deux de ses principaux intermédiaires de dégradation ont également été analysés. L efficacité du dépôt a ainsi été calculée sur les différentes configurations obtenues. Les résultats ont été satisfaisants sans arriver à avoir un niveau de photoactivité comparable à celui du TiO2 en mode suspension. Nous avons donc cherché à améliorer les performances de notre système réactionnel en travaillant sur un support ayant une autre géométrie. Notre choix s est porté sur des mousses en alumine épousant parfaitement le vide réactionnel. Le même protocole expérimental a été appliqué à ces dernières. Les résultats de photodégradation ont montrés une durabilité meilleure que celle obtenue sur les billes en verre, mais aussi une photoperformance dépassant celle réalisée sur du dioxyde de titane en mode suspension.Afin de mieux comprendre les différences de photoactivité entre les supports, nous avons tenté de simuler, par la méthode Monte Carlo, la propagation de la lumière à travers les photocatalyseurs, qu ils soient en mode suspension ou déposés. Les résultats ont montrés que le dioxyde de titane en suspension avait le meilleur taux d absorption de la lumière comparé au TiO2 fixé sur les supports. Néanmoins l absorption se fait sur un volume très petit du réacteur et la meilleure répartition de l énergie lumineuse se trouve dans les mousses en alumine.Les données issues de ces simulations notamment, la LVRPA pour Local Volumetric Rate of Photon Absorption nous ont permis d entamer la dernière étape de notre travail à savoir la construction d un modèle prédictif des performances photocatalytiques du système réactionnel. Nous avons pour cela introduit un terme qui traduit la probabilité qu un photon absorbé par le photocatalyseur donne un radical hydroxyle. Les résultats obtenus par notre environnement mathématique ont montrés de bonnes corrélations avec les expériences et ont permis de tirer certaines conclusions quand à l amélioration de notre système photocatalytique.The objective of this work is to develop a methodology dedicated to the design of a photocatalytic reactor for water purification. The principle is to go from photocatalysis as chemical process to photocatalysis as remediation method.Iit necessary for that to respond to two questions: - In such a system, what are the major physico-chemical phenomena? - Is there a way to predict the performance of our system?First of all, we tried to work on suspended photocatalysts to avoid a costly filtration step and technical difficulties. Thus, different types of titanium dioxide were deposited on substrates of different geometries.The first obtained medium was commercial titanium dioxide (P25 from Evonik and UV100 from Sachtleben) deposited on glass beads of 2 and 4 mm, inserted into an annular reactor illuminated by a single UV light source. The photoactivity of this media has been tested on phenol, the primary pollutant model chosen in this study. Hydroquinone and benzoquinone, two of its main degradation intermediates were also analyzed. The deposition efficiency has been estimated on different configurations. The results were satisfactory but did not reach a level comparable to that of photoactivity of TiO2 in suspend mode. Thus, we sought to improve the performance of our reaction system working on a support having a different geometry. Our choice fell on alumina foams perfectly matching the vacuum in the reactor. The same experimental protocol was applied to the foam. The results have shown photodegradation durability better than that obtained on glass beads and also the photoperformance exceeding that of titanium dioxide in suspend mode.To understand better the photoactivity of our media, we simulated by the Monte Carlo method the propagation of the light through the different photocatalysts (suspend mode or deposited). The results showed that the titanium dioxide suspension had the best absorption of light compared to TiO2 fixed on media. Nevertheless, absorption was located on a very small volume of the reactor and a better distribution of the light energy occurs in the alumina foams.The data obtained from these simulations, including the LVRPA "Local Volumetric Rate of Photon Absorption", allowed us to achieve the final step of our work, which is the construction of a predictive model of photocatalytic performance of the reaction system. Thus, we have introduced a term that reflects the probability that a photon absorbed by the photocatalyst gives a hydroxyl radical. The results obtained by our mathematical environment have shown a good correlation with experiments and helped us to draw some conclusions for the improving of our photocatalytic system.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Morphology-dependent photocatalytic activity of octahedral anatase particles prepared by ultrasonication-hydrothermal reaction of titanates

Octahedral anatase particles (OAPs) were prepared by an ultrasonication (US) hydrothermal (HT) reaction of partially proton-exchanged potassium titanate nanowires (TNWs). The structural/physical properties of OAP-containing samples, including specific surface area, crystallinity, crystallite size, particle aspect ratio, composition and total OAP content, were analyzed. Photocatalytic activities of samples were measured under irradiation (>290 nm) for oxidative decomposition of acetic acid (CO2 system) and dehydrogenation of methanol (H-2 system) under aerobic and deaerated conditions, respectively. Total density of electron traps (ETs) was measured by double-beam photoacoustic spectroscopy (DB-PAS). Mobility and lifetime of charge carriers (electrons) were investigated by the time-resolved microwave conductivity (TRMC) method. The effects of synthesis parameters, i.e., HT duration, HT temperature and US duration, on properties and photocatalytic activities of final products were examined in detail. The sample prepared with 1 h US duration and 6 h HT duration at 433 K using 267 mg of TNWs in 80 mL of Milli-Q water exhibited the highest photocatalytic activity. It was found that change in HT duration or HT temperature while keeping the other conditions the same resulted in changes in all properties and photocatalytic activity. On the other hand, duration of US treatment, before HT reaction, influenced the morphology of both the reagent (by TNWs breaking) and final products (change in total OAP content); samples prepared with various US durations exhibited almost the same structural/physical properties evaluated in this study but were different in morphology and photocatalytic activity. This enabled clarification of the correlation between morphology and photocatalytic activity, i.e., the higher the total OAP content was, the higher was the level of photocatalytic activity, especially in the CO2 system. Although the decay after maximum TRMC signal intensity (I-max) was almost constant for all samples used in this study, photocatalytic activities were roughly proportional to Imax, which tended to be proportional to total OAP content. Assuming that Imax corresponds to the product of density of electrons in mobile shallow ETs and their mobility, the results suggest that OAP particles have beneficial shallow ETs in higher density and thereby the OAP content governs the photocatalytic activities. Thus, morphology-dependent photocatalytic activity of OAP-containing particles was reasonably interpreted by density of ETs presumably located on the exposed {101} facets

Sn-doped and porogen-modified TiO2 photocatalyst for solar light elimination of sulfure diethyle as a model for chemical warfare agent

International audienc

Preservation of the photocatalytic activity of TiO2 by EDTA in the reductive transformation of Cr(VI). Studies by Time Resolved Microwave Conductivity

P25 TiO2 was found to be deactivated after Cr(VI) (0.8 mM) photocatalytic experiments at pH 2 in the absence of EDTA, but this deactivation was not found in the presence of the organic compound (2 mM), which preserves the activity of the photocatalyst. The results were confirmed by TRMC measurements. The preservation of the activity by the presence of EDTA can be a general behavior of reducing oligocarboxylic acids present together with Cr(VI) in the photocatalytic system, where Cr(III) soluble complexes can be formed and Cr(III) deposition onto the TiO2 surface is prevented. The treatment of a set of diluted Cr(VI) solutions (0.8 mM) in successive experiments with the reused photocatalyst was more efficient for Cr(VI) removal in the presence of EDTA (2 mM, 700 s total irradiation time) than the use of a more concentrated sample (Cr(VI), EDTA) with the same photocatalyst submitted to a longer irradiation (900 s), indicating a good strategy for application. A change in the kinetics from a pseudo-first order at the lowest concentration to an almost zero order at the highest concentration takes place and is analyzed.Fil: Meichtry, Jorge Martin. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Colbeau Justin, Christophe. Universite de Paris XI. Laboratoire de Chimie Physique; Francia. Centre National de la Recherche Scientifique; FranciaFil: Custo, Graciela Susana. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); ArgentinaFil: Litter, Marta Irene. Universidad Nacional de San Martín. Instituto de Investigación en Ingeniería Ambiental; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

TiO2-photocatalytic transformation of Cr(VI) in the presence of EDTA: Comparison of different commercial photocatalysts and studies by Time Resolved Microwave Conductivity

The photocatalytic efficiency for Cr(VI) transformation in the presence of etilendiaminetetraacetic acid (EDTA) at pH 2 over various commercial TiO2 samples (Evonik P25, Cristal Global PC50, PC100 and PC500, Hombikat UV100, Fluka and Kemira S230) was evaluated. The decay was adjusted to a pseudo-first order kinetics in all cases and the photocatalytic activity of the samples was estimated from the kinetic constants. The order of the photocatalytic activity was PC500 ≅ UV100 > P25 ≅ PC100 > S230 ≅ PC50 ≫ Fluka and it strongly depends on the nature of the TiO2 samples, especially on the specific surface area, except for S230 and for P25. The formation of the Cr(III)–EDTA complex in solution as a product of the Cr(VI) photocatalytic reduction was confirmed by visible spectrophotometry. Simultaneous kinetics of Cr(VI) reduction and of Cr(III)–EDTA formation has been determined for the first time and the results indicate that evolution of both species are related. TRMC measurements were applied for the first time to a reductive heterogeneous photocatalytic reaction of a metal ion system, and the experimental results were adjusted to a power law decay, relating the data to those of the photocatalytic experiments.Fil: Meichtry, Jorge Martin. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Colbeau Justin, Christophe. Universite de Paris XI. Laboratoire de Chimie Physique; Francia. Centre National de la Recherche Scientifique; FranciaFil: Custo, Graciela Susana. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); ArgentinaFil: Litter, Marta Irene. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Seguridad Nuclear y Ambiente. Gerencia de Química (CAC); Argentina. Universidad Nacional de San Martín. Instituto de Investigación e Ingeniería Ambiental; Argentin