Functionalization of nanostructured substrates for energy conversion and storage

Afin de répondre au besoin de la société actuelle qui utilise toujours plus de moyens de transport et de dispositifs portables, les modes de production, de conversion et de stockage de l'énergie, sont en train de connaître de véritables mutations. Afin de créer des systèmes capables de générer une énergie maîtrisée et renouvelable, les nanosciences et nanotechnologies sont des domaines de premier plan. Le travail présenté dans ce manuscrit décrit la fabrication de structures, de taille nanométrique, organisées à grande échelle. La fonctionnalisation se fait par synthèse de films ou de particules par ALD. Des systèmes MIM sont synthétisés sur des structures ordonnées d'alumine poreuse. Les matériaux déposés en couches minces sont TiN, Al2O3 et HfO2. L'objectif est de fabriquer des nano-condensateurs à hautes performances pouvant être utilisés pour des applications de stockage de données, de mémoire ou pour le stockage d'énergie dans des petits dispositifs comme la technologie RFID.Dans une deuxième partie, des catalyseurs métalliques Pd/Ni sont déposés sur des membranes d'alumine pour l'électro-oxydation de l'acide formique. De la même manière, des nano-tubes de TiO2 fabriqués par oxydation anodique sont fonctionnalisés par des nano-particules de Pd pour l'électrooxydation de l'éthanol. Ces deux études systèmes peuvent conduire à leur utilisation comme catalyseurs au niveau de l'anode des piles à combustible liquide à combustion directe. Enfin, la dernière partie de ce travail consiste au dépôt par voie électrochimique de Cu2O, sur des nano-tubes de TiO2 qui servent de support. La jonction p/n ainsi fabriquée pourra servir pour la photo-conversion de l'énergie solaire.In order to meet the growing needs in today's society that requests more transportation and portable devices, energy production, conversion and storage systems are now experiencing real changes. To fabricate systems able to generate a controlled and renewable energy, nanoscience and nanotechnology are leading research fields. The work presented in this manuscript describes the fabrication of nanosized, large-scale organized structures. These nanostructures have been functionalized through film and particle synthesis using a chemical vapor deposition method: the Atomic Layer Deposition (ALD).Thus, metal/insulator/metal (MIM) systems are fabricated on highly-ordered high-aspect ratio porous alumina. The materials that have been deposited are TiN, Al2O3 and HfO2. The aim is to produce high performance nanocapacitors that can be used for data storage (DRAM) application or for energy storage in small devices such as RFID.In a second part, metallic catalysts such as Pd/Ni have been deposited on alumina membranes for formic acid electro-oxidation. Similarly, TiO2 nanotubes have been fabricated by anodic oxidation and they have been functionalized with Pd nanoparticles for ethanol electro-oxidation. Both studied systems can be used as anode catalysts in direct liquid fuel cells.Finally, the last part of this work has consisted in the study of Cu2O, as a p-type semiconductor, that has been electrochemically deposited onto TiO2 nanotubes (n-type semiconductor). The resulting p/n junction can be interesting for solar energy photoconversion

Atomic Layer Deposition of TiN/Al₂O₃/TiN Nanolaminates for Capacitor Applications

Abstract not Available.</jats:p

Atomic Layer Deposition of TiN/Al₂O₃/TiN Nanolaminates for Capacitor Applications

Atomic layer deposition (ALD) of nanolaminates within porous alumina membranes is a highly promising path to produce high performance capacitors. Such systems have shown their feasibility and their very good properties. However there are still many improvements that should be achieved. TiN/Al2O3/TiN nanolaminates appear to be good candidates for such devices. In this study, the optimized parameters for Al2O3 deposition were determined. The comparison of the chemical composition, crystal structure and morphology of TiN layers grown either by thermal or plasma-enhanced ALD was performed using various method such as TEM, SEM, XPS and electrical characterizations.</jats:p

Atomic Layer Deposition of i-Sb₂S₃/p-NiO Thin Layers into Anodic Alumina Membranes for Photoelectrochemical Water Splitting

A nanostructured photocathode was fabricated by atomic layer deposition (ALD) of NiO and Sb2S3 in nanoporous alumina. The fabrication, i.e. the successive ALD of NiO and Sb2S3, and the characterization of the arrays of coaxial NiO/Sb2S3 junction by electron microscopies are first reported. The optical and photoelectrochemical preliminary testing of the combination of such extremely thin absorber, Sb2S3, with a p-type semiconductor, NiO, are proposed in a second part. Although the photoconversion performances should be improved, the present work demonstrates validity of the concept.</jats:p

Atomic Layer Deposition of i-Sb₂S₃/p-NiO Thin Layers into Anodic Alumina Membranes for Photoelectrochemical Water Splitting

A system for the photoelectrochemical splitting of water into H2 and O2 must consist of components for light absorption, for the separate transport of electrons and holes, and for the separate catalytic turnover of the reduction and oxidation reactions. The integration of all functions into a monolithic device requires as a central element a multi-junction solar cell of exceptional character able to deliver at least 1.5 V [1]. Instead of this, we envision that both half-reactions (oxidation and reduction) could be performed at two separate photoelectrodes connected in series and each of which combines an intrinsic, light-absorbing layer with an appropriate transparent semiconductor. For applications, both materials should contain no rare or particularly toxic element. The transparent semiconductor provides a directionality for the separation of charge carriers by functioning as a hole barrier (if n-type) or electron barrier (if p-type). Its presence also enables the experimentalist to separate the functions of light absorption and charge transport, and optimize each of them separately. The thickness of each layer can indeed be set independently. We contend that this approach has a better promise than attempts to use a single semiconductor to absorb light and simultaneously carry current, a case where any change in the geometry affects both aspects. It relies on the capability to generate elongated nanostructures the “vertical” length of which defines the light absorption path length, whereas the charge carrier separation occurs on the “lateral” width. Nanostructured electrodes can indeed show a better charge carriers collection, a better light scattering, a quantum confinement inducing a better charge transfer, an appropriate band edge position, a surface area-enhanced charge transfer and a multiple exciton generation. It has been recently reported that Sb2S3 can be used as extremely thin absorber (ETA) in solar cell [2]. As shown in Figure 1, this ETA should therefore be combined with both p- and n-type materials such as NiO and TiO2, respectively. The present work is then focused on the fabrication of such nanostructured photocathode and its characterization. In order to create three-dimensional nanostructures, p-NiO and i-Sb2S3 thin films have been successively grown into anodic alumina oxide (AAO) membranes by atomic layer deposition (ALD). AAO offers the possibility to tailor the pore diameter and length while ALD allows an accurate control of the layer thickness even on such highly porous materials and produces high quality deposits. The appropriate temperature windows of the ALD reactions has been determined, as well as the films growth rates in optimized conditions. In some cases, the films have been subsequently be annealed in inert and reactive atmospheres so as to modify their crystalline structure and stoichiometry. These various samples have been characterized using numerous methods. The film thickness has been gained from spectroscopic ellipsometry. 
The chemical compositions have been identified by x-ray dispersive and x-ray photoelectron spectroscopies (EDS, XPS) and the morphology of the thin films has been observed by scanning and transmission electron microscopies (SEM, TEM). The crystal structure and quality have been determined by x-ray diffraction (XRD) and 
high-resolution TEM. Since morphology and crystalline structure of Sb2S3 is strongly dependent on the substrate, its deposition and annealing processes have been emphasized. Since the photocathode composition, structure and geometry is well controlled, the optical and photoelectrochemical properties have been investigated and optimized using cyclic voltammetry in the dark and under illumination as well as UV-visible absorption spectroscopy. It has been sought to uncover trends in the photoelectrochemical activity of the system (photocurrent density and photovoltage) and rationalize them in terms of optical and electrical functions. Due to the discontinuous nature of the ETA film, the electrochemical activity of the NiO underlayer could be detected. It has been found that such multilayered system exhibit a higher optical absorption when the Sb2S3 was deposited and it is further improved when it has been annealed. It demonstrates therefore the validity of the concept. [1] S. Y. Reece, J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers, D. G. Nocera, Science, 334, 645 (2011) [2]        H. Wedemeyer, J. Michels, R. Chmielowski, S. Bourdais, T. Muto, M. Sugiura, G. Dennler, J. Bachmann, Energy Environ. Sci., 6, 67(2013) Figure 1 <jats:p /

Ultra-rapid combustion synthesis of Na2FePO4F fluorophosphate host for Li-ion and Na-ion insertion

Exploring soft-chemistry synthesis of Fe-based battery cathode materials, we have optimized combustion synthesis as an ultra-rapid approach to produce Na2FePO4F fluorophosphate cathode. It yields nanoscale, carbon-coated target product by annealing (at 600 A degrees C) for just 1 min. The purity of the material crystallizing in the orthorhombic structure was confirmed by powder X-ray diffraction pattern and XPS analysis, while the morphology was studied by scanning electron microscopy. The as-synthesized material exhibits good electrochemical performance delivering a first discharge capacity of more than 70 mAh/g at C/10 rate versus both Li+/Li and Na+/Na, hence acting as an efficient host for both Li-ion and Na-ion insertion. Combustion synthesis can be employed as an economic route for synthesis and rapid screening of various phosphate-based insertion materials

Self-Ordered Sub-10 nm Nanoporous Anodic Alumina Membranes: A New Tool for Nanotechnology

Abstract not Available.</jats:p

Atomic Layer Deposition and Anodic Oxidation: A Good Tool Combination to Build Nanostructured Electrodes for Energy Applications

Anodic oxidation has been used for a longtime as a way to passivate metallic electrodes to prevent corrosion. It is the discovery of self-ordered nanoporous alumina membranes reported in 1995 [1] that has revealed its potential use for surface nanostructuring. Later, the anodic growth of TiO2 nanotubes (TiO2-nt) has considerably widened the field of applications of such approach since TiO2 exhibits many valuable properties [2]. It is now possible to use other metals or alloys to grow porous or tubular oxidized nanostructures. However to further improve the properties of such electrodes, it is necessary to functionalize their surface with other materials of interest. Among the various thin film deposition methods, Electrochemical Deposition (ED) and Atomic Layer Deposition (ALD) have shown a great ability to conformally coat porous structures exhibiting a high aspect ratio. The main advantages of ALD over ED is that it can be carried out onto non-conductive materials, it allows an accurate control of the thickness and it is usually more direct to grow oxides or nitrides. We report, here, three examples of nanostructured electrodes fabricated using anodic oxidation of Al and Ti in combination with ALD of active materials. The targeted applications are in the fields of electrocatalysis, Li-ion microbatteries and photoelectrochemical water splitting. Direct Ethanol Fuel Cells offer significant advantages due to ethanol non-toxicity and renewability and its high power density. TiO2 have been successfully used as replacement of C as catalyst support because it exhibits a good chemical stability and it can enhance the activity of the catalyst. After a brief reminding on the TiO2-nt fabrication and properties, the ALD of Pd nanoparticles into TiO2-nt array will be presented. The electrochemical activity toward ethanol oxidation has been tested in alkaline medium. Although a high and stable electroactivity has been measured further improvements such as annealing of the TiO2-nt and ALD of SnO2 onto the TiO2-nt have been proposed. As seen on Fig. 1a, the electrochemical response is at its highest when the tubes have been annealed and covered by SnO2. A 3D nano-architectured composite negative electrode has been fabricated in order to be used in microbatterie. It consists of TiO2-nt coated by a thin SnO2 film grown by ALD. TiO2-nt are known to exhibits very good insertion properties and the SnO2 coating that displays a high lithium insertion capability (max. theoretical capacity of 720 µAh/cm2·µm) is used to enlarge the capacity of the electrode. Such composite 3D structure increases therefore the active area, improves the kinetics of the system, facilitates the ion exchange at the electrode/electrolyte interface and, better accommodates the volume expansion induced by the Li insertion within the LixSn alloy. The ALD of SnO2 will be described in details. The influence of various parameters such as precursor nature, exposure sequence as well as reaction temperature on the deposit morphology, chemistry and crystalline structure will be presented. The electrochemical performances of such systems have been tested as function of the SnO2 film thickness and after thermal treatments that change the crystalline structure of the electrodes. The results show that TiO2/SnO2 delivers a promising capacity (~150 µAh/cm2) which is better than bare TiO2-nt (Fig. 1b). It demonstrated then that such 3D nano-architectured composite electrode opens valuable perspectives for microbatteries design. The last example consists of producing a nanostructured photocathode for water splitting in which the photogeneration and transport of the charge carriers do not occur in the same material. In order to create tailored 3D nanostructures, p-NiO and i-Sb2S3 thin films have been successively grown into nanoporous alumina by ALD (Fig. 1c). The photocathode composition, structure and geometry are well controlled. Then the optical and photoelectrochemical properties have been investigated and optimized using cyclic voltammetry in the dark and under illumination as well as UV-visible absorption spectroscopy. It has been sought to uncover trends in the photoelectrochemical activity of the system (photocurrent density and photovoltage) and rationalize them in terms of optical and electrical functions. [1] H. Masuda, K. Fukuda, Science, 268, 1466 (1995) [2] V. Zwilling, E. Darque-Ceretti, A. Boutry-Forveille, D. David, 
M. Y. Perrin, M. Aucouturier, Surf. Interface Anal., 27, 
629 (1999) Figure 1 <jats:p /