DFT Investigations of Formic Acid Adsorption on Single-Wall TiO₂ Nanotubes: Effect of the Surface Curvature

We carried out a theoretical study based on DFT calculations to provide a detailed characterization of the structural, electronic, and adsorption properties of single-walled TiO2 anatase nanotubes. We investigated nanotube models of increasing diameter, formally obtained by rolling a TiO2 anatase monolayer around the [1̅01] and [010] directions, giving rise to (n,0) and (0,m) nanotubes, respectively. We considered finite cluster models for both (n,0) and (m,0) TiO2 nanotubes, with diameters ranging from 5 to 30 Å, thus approaching realistic nanotube dimensions. Our results show that (n,0) tubes are lower in energy with respect to (0,m) tubes. For (n,0) tubes with diameters greater than 23 Å, the electronic energy and the band gap are almost converged with respect to the diameter length. We then investigated the adsorption of formic acid on the TiO2 nanotube sidewalls, as the simplest model of photosensitizers binding to the TiO2 surface, relevant to dye-sensitized solar cells. Adsorption of formic acid was investigated on (12,0) and (0,4) TiO2 nanotubes, optimizing two monodentate modes and one bidentate adsorption mode, and comparing the results to those obtained for a planar TiO2 surface. We find that while for a planar surface a bridged bidentate configuration is the more stable, the effect of the curvature in TiO2 nanotubes leads a monodentate configuration to be the more stable structure. These results are interpreted in terms of the peculiar electronic properties of TiO2 nanotubes and their implications for use of nanotubes in dye-sensitized solar cells are discussed

Magnetic Communication through Functionalized Nanotubes: A Theoretical Study

Functionalized nanotubes are good candidates to promote communication between paramagnetic centers at large distances through a highly delocalized π system. Our study using theoretical methods based on density functional theory predicts the presence of surprisingly strong coupling at very large distances for this kind of system. To reach such strong couplings the system has to fulfill two conditions, the presence of highly charged metal cations and a metallic character of the nanotube

Density Functional Study of the Dissociative Adsorption of Aromatic Molecules on the Si(100) Surface: On the Way from Benzene to Larger Polycyclic Hydrocarbons

Density functional calculations have been performed on possible mechanisms for the hypothetic C−H bond cleavage process of benzene chemisorbed on the Si(100) surface, in order to shed light on the analogous process on larger polycyclic aromatic hydrocarbons. We first identified the minima on the potential energy surface for the benzene adsorption on Si(100) and for the breaking of two C−H bonds, with formation of two Si−H bonds, and then we analyzed possible pathways for the C−H bond cleavage, looking for the transition states connecting the adsorption configurations to the final products of C−H breaking. We identified two adsorbed configurations of benzene from which the breaking of two C−H bonds can be accessible, i.e., the 1,2 tilted di-σ bonded configuration on top of a single dimer (T) and the 1,4 di-σ bonded configuration where benzene bridges two dimer rows (BR). The kinetically most favorable reactive channel on the T configuration involves the abstraction of two hydrogen atoms on the sp3 carbon atoms by the silicon atoms of an adjacent dimer, with an energy barrier of 22.0 kcal mol-1. Although seemingly low, such an activation energy is not expected to be accessible at temperatures below the onset of benzene desorption from this configuration, which requires 15.9 kcal mol-1. The kinetically most favorable reactive channel on the BR configuration, which has not been experimentally detected for the benzene molecule, involves the rupture of one Si−C bond, passing through an energy barrier of 29.8 kcal mol-1, and ends with the formation of a Si−H bond and a vertical phenyl unit anchored on a silicon dimer

Modeling Mesoporous Nanoparticulated TiO₂ Films through Nanopolyhedra Random Packing

We present an innovative methodology for the computer simulation of mesoporous nanoparticulated oxide films based on the random packing of faceted nanopolyhedra in the form of bifrustums, reproducing the experimentally observed TiO₂ anatase bifrustum shape. A computer simulation employing nanospheres as packing objects was preliminary considered to verify the validity of the developed procedure. The pore size distribution and other fundamental characteristics of the simulated films (porosity, radial distribution function, coordination number, contact number) are computed for nanospheres and nanopolyhedra simulated films. Our results show that the use of faceted bifrustums, while involving a computationally more demanding procedure, is essential to attain a reliable description of the morphology and local three-dimensional structure of mesoporous TiO₂ nanoparticles films

Ab Initio Simulation of the Absorption Spectra of Photoexcited Carriers in TiO₂ Nanoparticles

We investigate the absorption spectra of photoexcited carriers in a prototypical anatase TiO₂ nanoparticle using hybrid time dependent density functional theory calculations in water solution. Our results agree well with experimental transient absorption spectroscopy data and shed light on the character of the transitions. The trapped state is always involved, so that the SOMO/SUMO is the initial/final state for the photoexcited electron/hole absorption. For a trapped electron, final states in the low energy tail of the conduction band correspond to optical transitions in the IR, while final states at higher energy correspond to optical transitions in the visible. For a trapped hole, the absorption band is slightly blue-shifted and narrower in comparison to that of the electron, consistent with its deeper energy level in the band gap. Our calculations also show that electrons in shallow traps exhibit a broad absorption in the IR, resembling the feature attributed to conductive electrons in experimental spectra

Adsorption and Interfacial Chemistry of Pentacene on the Clean Si(100) Surface: A Density Functional Study

Density functional theory calculations have been performed on the main adsorption configurations of pentacene on the Si(100) surface and on the possible pathways for the following C−H bond cleavage. We considered possible candidates for all the orientations of pentacene experimentally observed with STM, i.e., on the top of silicon dimer rows, perpendicular to the dimer rows, diagonal to the dimer rows and between two adjacent dimer rows (“in between”). Our calculations indicate that the most stable adsorption configuration of pentacene on the Si(100) surface is the symmetric perpendicular structure with an adsorption energy of −128.3 kcal mol-1, with the in between structure 10.5 kcal mol-1 and the symmetric parallel structure 13.0 kcal mol-1 higher in energy. Transition states for the dissociation of C−H and formation of Si−H bonds from the main adsorption configurations of pentacene have been characterized and the corresponding energy barriers estimated. We identified two kinds of adsorbed configurations of pentacene from which the breaking of two C−H bonds can be accessible:  one on top of a silicon dimer row with one or both outer benzene rings di-σ−bonded through a [2 + 2] cycloaddition; one with one or more pentacene rings 1,4 di-σ-bonded across two dimer rows, such as the in between structure. The kinetically most favorable reactive channel is that from the in between configuration and involves the separate abstraction of two hydrogen atoms on the sp3 carbon atoms by the two silicon atoms of the two dimers bearing an unpaired electron, with an energy barrier of 29−30 kcal mol-1

Strong Antiferromagnetic Coupling at Long Distance through a Ligand to Metal Charge Transfer Mechanism

The use of large substituted oligoacenes with dicyanoamido groups as bridging ligands should allow one to design new dinuclear transition metal complexes with relatively strong exchange interactions at very long intermetallic distances. Theoretical methods based on density functional theory predict antiferromagnetic exchange coupling constants of around 200 cm-1 for a nonacene CrIII complex with an intermetallic distance of 33 Å, due to a ligand−metal charge-transfer mechanism. In contrast, the isoelectronic VII complex in which such electron-transfer process is not allowed shows a weak ferromagnetic coupling

Structural and Electronic Properties of Photoexcited TiO₂ Nanoparticles from First Principles

The structure and energetics of excitons and individual electron and hole polarons in a model anatase TiO₂ nanoparticle (NP) are investigated by means of Density Functional Theory (DFT) and Time Dependent (TD)-DFT calculations. The effect of the Hartree–Fock exchange (HF-exc) contribution in the description of TiO₂ NPs with unpaired electrons is examined by comparing the results from semilocal and hybrid DFT functionals with different HF-exc percentages, including a long-range corrected hybrid functional. The performances of TD-DFT and ground state (SCF) DFT approaches in the description of the photoexcited polaron states in TiO₂ NPs are also analyzed. Our results confirm that the HF-exc contribution is essential to properly describe the self-trapping of the charge carriers. They also suggest that long-range corrected functionals are needed to properly describe excited state relaxation in TiO₂ NPs. TD-DFT geometry optimization of the lowest excited singlet and triplet states deliver photoluminescence values in close agreement with the experimental data

Understanding the Solution Chemistry of Lead Halide Perovskites Precursors

Identifying the composition of the solvated iodoplumbate complexes that are involved in the synthesis of perovskites in different solution environments is of great relevance in order to link the type and quantity of precursors to the final optoelectronic properties of the material. In this paper, we clarify the nature of these species and the involved solution equilibria by combining experimental analysis and high-level theoretical calculations, focusing in particular on the DMSO and DMF solvents, largely employed in the perovskites synthesis. The specific molecular interactions between the iodoplumbate complexes and the solvent molecules were analyzed by identifying the most thermodynamically stable structures in various solvent solutions and characterizing their optical properties trough DFT and TD-DFT calculations. A comparison with the experimental UV–vis absorption spectra allows us to define  the number of iodide and solvent ligands bonded to the Pb2+ ion and the complex formation constants of the involved species