Morphological, structural, optical, and electrical study of nanostructured thin films: Charge transport mechanism of p-type Co3O4

Morphological, structural, optical, and electrical study of nanostructured thin films: Charge transport mechanism of p-type Co3O

In- and out-plane transport properties of chemical vapor deposited TiO2 anatase films

Due to their polymorphism, TiO2 films are quintessential components of state-of-the-art functional materials and devices for various applications from dynamic random access memory to solar water splitting. However, contrary to other semiconductors/dielectric materials, the relationship between structural/morphological and electrical properties at the nano and microscales remains unclear. In this context, the morphological characteristics of TiO2 films obtained by metal–organic chemical vapor deposition (MOCVD) and plasma-enhanced chemical vapor deposition (PECVD), the latter including nitrogen doping, are investigated and they are linked to their in- and out-plane electrical properties. A transition from dense to tree-like columnar morphology is observed for the MOCVD films with increasing deposition temperature. It results in the decrease in grain size and the increase in porosity and disorder, and subsequently, it leads to the decrease in lateral carrier mobility. The increase in nitrogen amount in the PECVD films enhances the disorder in their pillar-like columnar morphology along with a slight increase in density. A similar behavior is observed for the out-plane current between the low temperature MOCVD films and the undoped PECVD ones. The pillar-like structure of the latter presents a lower in-plane resistivity than the low temperature MOCVD films, whereas the out-plane resistivity is lower. The tree-like columnar structure exhibits poor in- and out-plane conductivity properties, whereas pillar-like and dense TiO2 exhibits similar in- and out-plane conductivities even if their morphologies are noticeably different

Structural, optical and electrical properties of MOCVD deposited Co3O4-TiO2 heterojunctions for the study of dihydrogen production by solar water splitting

Dans le contexte de réchauffement climatique global, l'hydrogène représente un vecteur d'énergie dont ni la combustion, ni l'utilisation dans une pile à combustible ne génère de gaz à effet de serre. La photolyse de l'eau est une des voies de production verte d'H2. Cette réaction peut être catalysée par l'emploi de TiO2, semi-conducteur de type n. Celui-ci n'absorbe cependant que la partie UV du spectre lumineux, et n'exploite donc que 5% du rayonnement solaire atteignant la Terre. L'ajout de Co3O4, un semi-conducteur de type p absorbant dans le visible, dans une architecture bicouche, permet d'élargir la gamme spectrale d'absorption et permettrait aussi la création d'une hétérojonction p-n, qui faciliterait la séparation des charges impliquées dans les réactions d'oxydo-réduction de la dissociation photocatalytique de l'eau. L'objet de cette thèse est l'étude des activités photocatalytiques d'hétérojonctions Co3O4/TiO2 déposées par MOCVD, via leurs propriétés microstructurales, optiques et électriques. L'étude montre que les couches de TiO2 sont composées d'anatase cristallisée, dont la morphologie varie de dense à colonnaire et poreuse lorsque la température de dépôt (Td) augmente. Les films les plus poreux montrent les meilleurs résultats de photogénération d'H2, ce qui peut être majoritairement attribué à la très grande surface spécifique, mais également à des aspects de structure de bandes et de diffusion de la lumière. Leur plus faible conductivité ne semble pas être un facteur dommageable prépondérant. La gamme de Td choisie pour les films d'oxyde de cobalt permet d'obtenir la phase Co3O4 spinelle, exclusivement. L'augmentation de l'épaisseur de ces films de ?30 à ?100 nm conduit à l'augmentation de leur conductivité. L'activité photocatalytique des films de Co3O4 n'est pas détectable. Le comportement semi-conducteur de type p est établi et il est démontré pour la première fois que le transport des charges a lieu à travers les grains par un mécanisme de Mott de saut à distance variable. Différentes hétérojonctions sont ensuite étudiées, en utilisant une couche de TiO2 colonnaire ou dense, surmontée d'une couche de Co3O4 de différentes épaisseurs. L'étude photocatalytique montre que sur TiO2 colonnaire, l'ajout d'une couche épaisse de Co3O4 diminue la quantité d'H2 photogénéré (mais la production reste supérieure à celle des monocouches de Co3O4). La photogénération d'H2 diminue davantage quand l'épaisseur du Co3O4 sur TiO2 colonnaire diminue. Par ailleurs, les hétérojonctions composées de Co3O4 épais sur TiO2 dense montrent le même effet. Ce comportement est discuté en fonction de la structure de bande des hétérojonctions.In the context of global warming, hydrogen represents an energy vector whose combustion or use in a fuel cell does not generate greenhouse gases. Water photolysis is one of the green production routes for H2. In this field, the well-known n-type semiconductor TiO2, has long been studied as a photocatalyst. Nonetheless, it only absorbs UV light; i.e. 5% of the solar radiation reaching the Earth. The addition of Co3O4, a p-type semiconductor absorbing in the visible range, in a 2-layer architecture, widens the absorption spectral range and, through the creation of a p-n heterojunction, facilitates charges separation for the redox reactions of water photolysis. The aim of this thesis is to study the photocatalytic behavior of different Co3O4/TiO2 heterojunctions deposited MOCVD, through their microstructural, optical and electrical properties. TiO2 films are composed of well-crystallized anatase. Various morphologies ranging from dense to columnar and porous are obtained by increasing the deposition temperature (Td) from 325°C to 500°C. Despite their lower conductivity, the most porous films show the best H2 photogeneration results, which can mainly be attributed to the large surface area, but also to peculiar aspects of their band structure and scattering phenomena. For the cobalt oxide films, the selected 400 - 500 °C Td range only yields the spinel Co3O4 phase. Increasing the thickness of these films from ? 30 to ?100 nm results in significant increase of their conductivity. The photocatalytic activity of Co3O4 films is not detectable. Different heterojunctions are then studied, using a columnar or dense TiO2 layer topped with Co3O4 layers of varying thicknesses. Addition of a thick layer of Co3O4 on columnar TiO2 decreases the amount of photo-generated H2 (but the production remains higher than that of the Co3O4 monolayers). On the other hand, when the thickness of the Co3O4 film on columnar TiO2 decreases, the production of H2 also decreases. In addition, heterojunctions composed of thick Co3O4 on dense TiO2 also show lower H2 production than TiO2 monolayers. A mechanism, based on the band structure of the heterojunctions is discussed to explain these results

Trimeric Cyclamers: Hierarchical High Z′ Crystal Engineering Based on Guest Structure and Basicity

Ureidosalicylic acid forms remarkable trimeric cyclamer-like structures with either three or six independent molecule in the asymmetric unit as a result of the excess of hydrogen bond donors and guest inclusion within the centre of the cyclamer

Cross-over study of electric and electronic properties versus microstructure of MOCVD processed TiO2 anatase films for solar water splitting

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TiO2 nanotree films for the production of green H2 by solar water splitting: From microstructural and optical characteristics to the photocatalytic properties

International audienceGreen H-2 production by solar water splitting relies entirely on the intrinsic properties of the photocatalyst. In this study the impact of these intrinsic properties on the photocatalytic activity of anatase TiO2, the quintessential component of state of the art photocatalytic systems was explored at the nanoscale. The exploration involved a holistic microstructural and optical characterization of fully crystallized anatase thin films synthetized by metalorganic chemical vapor deposition. A combination of electron microscopy, spectroscopic ellipsometry, and infrared spectroscopy revealed that when the deposition temperature increased, the morphology evolved from dense to porous and columnar nanostructures. Interestingly, the columns with a complex, tree-like nanostructure photogenerated 18 times more H-2 than the densest sample. This result shows that the beneficial effect of the morphological nano-complexification and crystallographic diversification of the exchange facets on the photocatalytic performance outweighs the detrimental aspects inherent to this evolution, namely the drop of the charge carrier transport and the increase of residual stress

Innovative ‘light-to-hydrogen’ all-solid multilayer device

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Innovative ‘light-to-hydrogen’ vacuum processed all-solid multilayered membrane-electrodes assemblies

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