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

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

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

Impact of Ga–V Codoping on Interfacial Electron Transfer in Dye-Sensitized TiO₂

The improvement of charge transfer between an organic molecule and a semiconductor is an important and challenging goal in the fields of photovoltaics and photocatalysis. In this work, we present a time-dependent density functional theory investigation of the impact of Ga–V codoping of TiO₂ on the excited-state electron injection from perylene-3-carboxylic acid. The doping is shown to raise the charge-transfer efficiency for the highest possible surface dye uptake by ∼16%. The strength of the effect depends on the dopant-pair–dye separation, dopant concentration, and distribution of Ga, V atoms in TiO₂. The doping of the superficial level turns out to be more favorable than those in the bulk. The changes in electron injection dynamics are attributed to the modification of accepting semiconductor levels and hybridization profile between molecular and semiconductor states

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

Physical Factors Affecting Charge Transfer at the Pe-COOH–TiO₂ Anatase Interface

In this work we address the factors affecting the rate and the completeness of the excited state electron transfer from a COOH-anchored perylene molecule to the (101) anatase surface. To investigate the electron injection at the Pe-COOH–TiO₂ interface, a pure electron dynamics and a coupled electron–ion dynamics simulations are conducted within the real-time time-dependent density functional theory (RT-TDDFT) and RT-TDDFT-based Ehrenfest dynamics formalisms, respectively. The role of ionic dynamics, the influence of the adsorption mode, the surface coverage by the adsorbate, as well as the impact of the initial excitation energy on the charge transfer process are analyzed. The dissociative adsorption is shown to be less favorable for the charge transfer than the nondissociative one. The ionic dynamics turns out to have a limited effect on the total amount of the transferred charge, but it is responsible for the retardation of the electron transfer. The energy of initial excitation is revealed to be a determinant factor of the electron injection efficiency in the Pe-COOH–TiO₂ system: the excitation of an electron to the LUMO+1 molecular orbital, instead of the LUMO one, doubles the total amount of the transferred charge. Meanwhile, a complete CT can only be achieved in conjunction with specific coverage and slab thickness characteristics, with all other factors being fixed. A surface coverage ratio equal to or less than 0.25 molecule/nm² and a slab thickness of at least 4 TiO₂ layers, corresponding to 192 Ti sites per 1 molecule of chromophore, are required