1,721,144 research outputs found
Birational sheets in reductive groups
Full text linkWe define the group analogue of birational sheets, a construction performed by Losev for reductive Lie algebras. For G semisimple simply connected, we describe birational sheets in terms of Lusztig–Spaltenstein induction and we prove that they form a partition of G, and that they are unibranch varieties with smooth normalization by means of a local study
Reactivity and energy level of a localized hole in liquid water
No full textWe study the reactivity and the energy level associated with the reduction of the H2O+ radical cation in liquid water by combining ab initio molecular dynamics simulations at the hybrid functional level, a grand-canonical formulation of solutes in aqueous solution, and nudge-elastic-band calculations. We demonstrate that this extremely oxidative solute promptly dissociates and calculate an energy barrier for the reaction of 0.06 eV, consistent with the short measured lifetimes. We calculate the energy level related to the H2O+/H2O redox couple with respect to the vacuum level and to the computational standard hydrogen electrode (SHE), using the thermodynamic integration method. This energy level is found to lie at 3.8 +/- 0.1 eV above the SHE level, in remarkable agreement with a previous estimate based on thermodynamical data. The implications of the present results for the mechanism of water splitting at the heterogenous semiconductor-water interface are discussed
Singlet fission in linear chains of molecules
No full textWe develop a model configuration interaction Hamiltonian to study the electronic structure of a chain of molecules undergoing singlet fission. We first consider models for dimer and trimer and then we use a matrix partitioning technique to build models of arbitrary size able to describe the relevant electronic structure for singlet fission in linear aggregates. We find that the multi-excitonic state (ME) is stabilized at short inter-monomer distance and the extent of this stabilization depends upon the size of orbital coupling between neighboring monomers. We also find that the coupling between ME states located on different molecules is extremely small leading to bandwidths in the order of similar to 10 meV. This observation suggests that multi-exciton states are extremely localized by electron-phonon coupling and that singlet fission involves the transition between a relatively delocalized Frenkel exciton and a strongly localized multi-exciton state. We adopt the methodology commonly used to study non-radiative transitions to describe the singlet fission dynamics in these aggregates and we discuss the limit of validity of the approach. The results indicate that the phenomenology of singlet fission in molecular crystals is different in many important ways from what is observed in isolated dimers. (C) 2014 AIP Publishing LLC
Strong Hole Trapping Due to Oxygen Dimers in BiVO4: Effect on the Water Oxidation Reaction
No full textWe present a study of hole bipolarons in BiVO4. We show that in the presence of two holes O-O dimers are formed, leading to strong charge trapping. While the formation of bipolarons in bulk BiVO4 requires overcoming a kinetic barrier, we find that these defects should be spontaneously formed at the surface of the material and its interface with water. Through molecular dynamics simulations, we study the effect of bipolarons on the water-splitting reaction and show that their presence may be especially beneficial in alkaline conditions
Hole diffusion across leaky amorphous TiO2 coating layers for catalytic water splitting at photoanodes
No full textWe investigate the mechanism of hole diffusion across leaky amorphous TiO2 (am-TiO2) layers. Through ab initio molecular dynamics simulations, we construct an atomistic model of am-TiO2 consistent with the experimental characterization. We first demonstrate that the oxygen vacancies do not occur in am-TiO2, as they can be assimilated by the amorphous structure upon structural rearrangement. Hence, their role in hole diffusion is ruled out. In contrast, O-O peroxy linkages are formed in pristine am-TiO2 upon injection of excess holes, with an associated defect level lying at 1.25 +/- 0.15 eV above the valence band of the material. We show that such linkages can provide a viable mechanism for hole diffusion in am-TiO2, as illustrated by a diffusion path of 1.2 nm with energy barriers lower than 0.5 eV in our atomistic model of am-TiO2
Effect of the Anchoring Group on Electron Injection: Theoretical Study of Phosphonated Dyes for Dye-Sensitized Solar Cells
No full textThe attachment chemistry of the chromophore onto the semiconductor surface influences the efficiency of electron injection in dye-sensitized solar cells (DSSCs). In this work, we study injection times for dyes that bind to the semiconductor surface via the phosphonic acid anchoring group and the effect on the injection time of different binding modes (molecular or dissociative, monodentate or bidentate) of phosphonic acid for both TiO2 rutile (110) and anatase (101) surfaces. We calculate electron injection times for a large set of organic dyes on TiO2 rutile (110) and anatase (101) surfaces for the most stable adsorption geometries of the phosphonic acid anchoring group, using a model based on partitioning the semiconductor chromophore system into fragments. We analyze the influence of the size and nature of the anchoring group on the injection times, performing calculations with larger models of the anchoring group (e.g., phenyl-phosphonic acid). Through the partitioning procedure, we are able to separate the effect of the binding geometry from other effects influencing the efficiency of the electron injection. The results show that dissociative bidentate adsorption modes generally lead to faster injection, compared to monodentate and molecular ones, similar to the results obtained earlier for analogous carboxylated dyes. Our results are in good agreement with experiments (where available), showing that our model is capable of predicting the effects of the anchoring groups and of different spacer groups on the injection times and is therefore suitable for designing new and more efficient chromophores
Redox levels in aqueous solution: Effect of van der Waals interactions and hybrid functionals
No full textWe investigate redox levels in aqueous solution using a combination of ab initio molecular dynamics (MD) simulations and thermodynamic integration methods. The molecular dynamics are performed with both the semilocal Perdew-Burke-Ernzerhof functional and a nonlocal functional (rVV10) accounting for van der Waals (vdW) interactions. The band edges are determined through three different schemes, namely, from the energy of the highest occupied and of the lowest unoccupied Kohn-Sham states, from total-energy differences, and from a linear extrapolation of the density of states. It is shown that the latter does not depend on the system size while the former two are subject to significant finite-size effects. For the redox levels, we provide a formulation in analogy to the definition of charge transition levels for defects in crystalline materials. We consider the H+/H-2 level defining the standard hydrogen electrode, the OH-/OH* level corresponding to the oxidation of the hydroxyl ion, and the H2O/OH* level for the dehydrogenation of water. In spite of the large structural modifications induced in liquid water, vdW interactions do not lead to any significant structural effect on the calculated band gap and band edges. The effect on the redox levels is also small since the solvation properties of ionic species are little affected by vdW interactions. Since the electronic properties are not significantly affected by the underlying structural properties, it is justified to perform hybrid functional calculations on the configurations of our MD simulations. The redox levels calculated as a function of the fraction alpha of Fock exchange are found to remain constant, reproducing a general behavior previously observed for charge transition levels of defects. Comparison with experimental values shows very good agreement. At variance, the band edges and the band gap evolve linearly with alpha. For alpha similar or equal to 0.40, we achieve a band gap, band-edge positions, and redox levels in overall good agreement with experiment. (C) 2015 AIP Publishing LLC
What Is the Best Anchoring Group for a Dye in a Dye-Sensitized Solar Cell?
No full textWe developed a computational procedure to screen many different anchoring groups used or usable to connect a dye to the semiconducting surface in a dye-sensitized solar cell. The procedure leads to a clear identification of the anchoring groups that bind strongly to the surface and facilitate the electron injection at the same time, providing clear-cut indications for the design of new dyes. The complicated interplay of factors that determine the final results (preferred adsorption mode, the anchor's effect on the dye's electronic structure, and dye semiconductor coupling) is illustrated through a few examples showing how chemical intuition can often be misleading in this problem
Evaluation of Photocatalysts for Water Splitting through Combined Analysis of Surface Coverage and Energy-Level Alignment
No full textTo examine whether suitable conditions occur for the water splitting reaction at their interfaces with liquid water, we determine the pH-dependent surface coverage for a series of semiconductors, including GaAs, GaP, GaN, CdS, ZnO, SnO2, and rutile and anatase TiO2. For this, we calculate acidity constants at surface sites through ab initio molecular dynamics simulations and a Grand Canonical formulation of adsorbates. The resulting pH values at the point of zero charge show excellent agreement with experiment and thereby support the validity of our approach. By combining information concerning the surface coverage with the alignment of the band edges with respect to the relevant redox levels, we scrutinize the potential of the considered semiconductors as photocatalysts and identify the corresponding optimal pH ranges for hydrogen and oxygen evolution. More specifically, our results indicate that GaN stands out among these semiconductors as the most promising candidate for the overall water splitting, with the potential of further improvement through alloying. With the surface coverage, our computational analysis brings a new descriptor that is currently beyond experimental reach
Oxygen defects in amorphous Al2O3: A hybrid functional study
No full textThe electronic properties of the oxygen vacancy and interstitial in amorphous Al2O3 are studied via ab initio molecular dynamics simulations and hybrid functional calculations. Our results indicate that these defects do not occur in amorphous Al2O3, due to structural rearrangements which assimilate the defect structure and cause a delocalization of the associated defect levels. The imbalance of oxygen leads to a nonstoichiometric compound in which the oxygen occurs in the form of O2- ions. Intrinsic oxygen defects are found to be unable to trap excess electrons. For low Fermi energies, the formation of peroxy linkages is found to be favored leading to the capture of holes. The relative +2/0 defect levels occur at 2.5 eV from the valence band. Published by AIP Publishing
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