Metal Substitution as the Method of Modifying Electronic Structure of Metal-Organic Frameworks.

Targeted modification of electronic structure is an important step in the optimization of metal-organic frameworks (MOFs) for photovoltaic, sensing, and photocatalytic applications. The key parameters to be controlled include the band gap, the absolute energy position of band edges, the excited state charge separation, and degree of hybridization between metal and ligand sites. Partial metal replacement, or metal doping, within secondary building units is a promising, yet relatively unexplored route to modulate these properties in MOFs. Therefore, in the present study, a general method for selecting metal dopant is worked out in theory and validated by experiment, retaining MIL-125 and UiO-66 as the model systems. Metal mixing enables targeted optimization of key electronic structure parameters. This method is applicable to any MOF architecture and can serve as a roadmap for future synthesis of MOFs with predefined properties

Unravelling the enhanced reactivity of bulk CeO 2 doped with gallium: A periodic DFT study

Doping CeO2 with gallium leads to promising materials with application in hydrogen purification processes for fuel cells. The bulk ceria?gallia is investigated by ab initio calculations. The outstanding reactivity is explained by important relaxations induced by gallium in the ceria host, having a strong impact in the electronic structure. As a result, the mixed oxide is found to be more reducible than the pure oxides in agreement with experimental data. It is thus possible to correlate structure and reactivity of the doped system on the molecular level, representing a step forward to the rational design of materials with selected properties.Fil: Quaino, Paola Monica. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; ArgentinaFil: Syzgantseva, Olga. Université Pierre et Marie Curie; FranciaFil: Siffert, Luca. Université Pierre et Marie Curie; FranciaFil: Tielens, Frederik. Université Pierre et Marie Curie; FranciaFil: Minot, Christian. Université Pierre et Marie Curie; FranciaFil: Calatayud, Monica. Universite de Paris; Franci

Stabilization of the Perovskite Phase of Formamidinium Lead Triiodide by Methylammonium, Cs, and/or Rb Doping

In this work we perform a computational study comparing the influence of monovalent cation substitution by methylammonium (MA(+)), cesium (Cs+), and rubidium (Rb+) on the properties of formamidinium lead triiodide (FAPbI(3))based perovskites. The relative stability of the desired, photoactive perovskite alpha phase ("black phase") and the nonphotoactive, nonperovskite delta phase ("yellow phase") is studied as a function of dopant nature, concentration and temperature. Cs+ and Rb+ are shown to be more efficient in the stabilization of the perovskite alpha phase than MA(+). Furthermore, varying the dopant concentration allows changing the relative stability at different temperatures, in particular stabilizing the alpha phase already at 200 K. Upon Cs+ or Rb+ doping, the corresponding onset of the optical spectrum is blue-shifted by 0.1-0.2 eV with respect to pure FAPbI(3

Strain relaxation and multidentate anchoring in n-type perovskite transistors and logic circuits

This is the author accepted manuscriptData availability: Source data are provided with this paper. Additional data related to this work are available from the corresponding authors upon request.Code availability statement: All codes (software) used in the calculation and visualization are publicly available and the condition of their usage in the publication is an appropriate citation.The engineering of tin halide perovskites has led to the development of p-type transistors with field-effect mobilities of over 70 cm2 V-1 s-1 . However, due to their background hole doping, these perovskites are not suitable for n-type transistors. Ambipolar lead halide perovskites are potential candidates, but their defective nature limits electron mobilities to around 3-4 cm2 V-1 s-1, which makes the development all-perovskite logical circuits challenging. Here, we report formamidinium lead iodide perovskite n-type transistors with field-effect mobilities of up to 33 cm2 V-1s-1 measured in continuous bias mode. This is achieved through strain relaxation of the perovskite lattice using a methyl ammonium chloride additive, followed by suppression of undercoordinated lead through tetramethyl ammonium fluoride multidentate anchoring. Our approach stabilizes the alpha phase, balances strain, and improves surface morphology, crystallinity, and orientation. It also enables low-defect perovskite–dielectric interfaces. We use 46 the transistors to fabricate unipolar inverters and eleven-stage ring oscillator

Dust in brown dwarfs and extra-solar planets IV. Assessing TiO2 and SiO nucleation for cloud formation modeling

Clouds form in atmospheres of brown dwarfs and planets. The cloud particle formation processes are similar to the dust formation process studied in circumstellar shells of AGB stars and in Supernovae. Cloud formation modelling in substellar objects requires gravitational settling and element replenishment in addition to element depletion. All processes depend on the local conditions, and a simultaneous treatment is required. We apply new material data in order to assess our cloud formation model results regarding the treatment of the formation of condensation seeds. We re-address the question of the primary nucleation species in view of new (TiO2)_N-cluster data and new SiO vapour pressure data. We apply the density functional theory using the computational chemistry package Gaussian 09 to derive updated thermodynamical data for (TiO2)_N-clusters as input for our TiO2 seed formation model. We test different nucleation treatments and their effect on the overall cloud structure by solving a system of dust moment equations and element conservation or a pre-scribed Drift-Phoenix atmosphere structure. Updated Gibbs free energies for the (TiO2)_N-clusters are presented, and a slightly temperature dependent surface tension for T=500 ... 2000K with an average value of sigma_infty = 480.6 erg 1/cm2. The TiO2-seed formation rate changes only slightly with the updated cluster data. A considerably larger effect on the rate of seed formation, and hence on grain size and dust number density, results from a switch to SiO-nucleation. Despite the higher abundance of SiO over TiO2 in the gas phase, TiO2 remains considerably more efficient in forming condensation seeds by homogeneous nucleation followed by heterogeneous grain growth. The paper discussed the effect on the cloud structure in more detail.Comment: accepted for publication in A&A (abstract abridged

Carrier Lifetimes and Recombination Pathways in Metal-Organic Frameworks

Understanding the excited-state charge carrier relaxation in metal-organic frameworks (MOFs) and revealing ways to alternate its rate are of primary importance for the development of novel hybrid photoactive materials with sufficiently long carrier lifetimes; in particular, shedding light on the main recombination pathways in this class of compounds is needed. Therefore, in this work the radiative and phonon-assisted nonradiative electron-hole recombination is investigated theoretically for a model MOF system, and the nonradiative pathway is demonstrated to be dominant even for a pristine defect-free material. Theoretically predicted electron-hole lifetimes are in line with the available experimental data, suggesting that the adopted methodology is suitable for prediction of carrier lifetimes and helpful for the interpretation of experimental data. Based on the obtained conclusions, the principles for modification of MOF geometrical and chemical structure, enabling the extension of carrier lifetimes, are formulated

Modélisation des oxydes métalliques hydrogénés (vers une compréhension des mécanismes de réduction)

Dans cette thèse, nous étudions l interaction de l hydrogène avec des oxydes métalliques. L impact de l hydrogénation sur leur stabilité, leurs propriétés structurales et électroniques est analysé grâce aux outils de la chimie quantique. Dans la première partie, nous considérons l hydrogénation des clusters les plus stables de l oxyde de titane (TiO2)n (n = 1 10), en s intéressant à la stabilité, à l état électronique des clusters hydrogénés et aux tendances de la localisation des électrons de réduction. Dans la deuxième partie, nous étudions l interaction de l hydrogène avec les surfaces stœchiométriques et réduites de ZrO2 monoclinique. Afin de proposer un modèle de la surface réduite de ZrO2, nous adressons la stabilité et la structure électronique des lacunes d oxygène superficielles. Ensuite, nous explorons les modes d adsorption et de dissociation de H2 sur les surfaces stœchiométriques et lacunaires. Par ailleurs, nous utilisons un modèle de cluster ZrO2 pour analyser sa réactivité par rapport à l hydrogène à l échelle moléculaire à différents niveaux de calcul (DFT,CCSD(T)). Dans la troisième partie nous modélisons les lacunes d oxygène dans le cristal massif des oxydes MgO, TiO2, ZrO2, HfO2, Ga2O3, CeO2, CeGaOx pour caractériser leur structure électronique et leurs propriétés d oxydoréduction. A ces fins, nous explorons les énergies de formation des lacunes d oxygène ainsi que la position du niveau électronique de défaut dans la structure des bandes et la localisation électroniquePARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Revealing the Surface Reactivity of Zirconia by Periodic DFT Calculations

ZrO₂ is known in chemistry to be a stable material. However, some technological applications point to specific redox reactivity related to surface oxygen composition. In this paper, ab initio calculations are used to characterize structure–reactivity relationships on monoclinic zirconia surfaces as regards thermodynamic stability, electronic structure, and chemical reactivity in reducing conditions. It is shown that the formation of different types of oxygen vacancies is possible on the surfaces of monoclinic zirconia. 2-Fold vacancies are found to induce an important structural relaxation. Moreover, the presence of O vacancies affects the surface electronic structure in two ways: either zirconium sites are reduced or F-centers are formed; the final state depends on the local geometry around the vacancy. Finally, some oxygen vacancy sites are found to be highly reactive toward dihydrogen, leading to its spontaneous dissociation and formation of dihydride species. These results reveal a rich and complex surface chemistry of zirconia with potential applications in many scientific fields

Preparation of Highly Porous Metal-Organic Framework Beads for Metal Extraction from Liquid Streams

Metal-organic frameworks (MOFs) offer great promise in a variety of gas- and liquid-phase separations. However, the excellent performance on the lab scale hardly translates into pilot- or industrial-scale applications due to the microcrystalline nature of MOFs. Therefore, the structuring of MOFs into pellets or beads is a highly solicited and timely requirement. In this work, a general structuring method is developed for preparing MOF-polymer composite beads based on an easy polymerization strategy. This method adopts biocompatible, biodegradable poly(acrylic acid) (PAA) and sodium alginate monomers, which are cross-linked using Ca2+ ions. Also, the preparation procedure employs water and hence is nontoxic. Moreover, the universal method has been applied to 12 different structurally diverse MOFs and three MOF-based composites. To validate the applicability of the structuring method, beads consisting of a MOF composite, namely Fe-BTC/PDA, were subsequently employed for the extraction of Pb and Pd ions from real-world water samples. For example, we find that just 1 g of Fe-BTC/PDA beads is able to decontaminate >10 L of freshwater containing highly toxic lead (Pb) concentrations of 600 ppb while under continuous flow. Moreover, the beads offer one of the highest Pd capacities to date, 498 mg of Pd per gram of composite bead. Furthermore, large quantities of Pd, 7.8 wt %, can be readily concentrated inside the bead while under continuous flow, and this value can be readily increased with regenerative cycling

Charge Transfer at the Hybrid Interfaces in the Presence of Water: A Theoretical Study

The presence of water molecules at the interfaces of dye-sensitized solar cells can hinder the excited-state charge transfer (CT), which constitutes a crucial step in solar energy harvesting by photovoltaic devices. To rationalize the impact of water adsorption on interfacial CT, this process is simulated within the time-dependent density functional theory in a model system formed by perylene-3-carboxylic acid and the TiO₂ (101) anatase surface. The adsorption of molecular water results in a moderate decrease of the CT efficiency, while dissociative adsorption of H₂O is shown to substantially reduce the electron accepting capacity of TiO₂. The amplitude of the effect depends smoothly on the amount of adsorbed water molecules, though distinct adsorption configurations contribute to it in different ways. The dissociation of the COOH anchor under the action of water species, simultaneous with the CT, results in an increased CT efficiency from the dye molecule to the TiO₂ surface