Calcium Phosphate Deposition on Planar and Stepped (101) Surfaces of Anatase TiO2: Introducing an Interatomic Potential for the TiO2/Ca-PO4/Water Interface.

Titanium is commonly employed in orthopaedic and dental surgery, owing to its good mechanical properties. The titanium metal is usually passivated by a thin layer of its oxide, and in order to promote its integration with the biological tissue, it is covered by a bioactive material such as calcium phosphate (CaP). Here, we have investigated the deposition of calcium and phosphate species on the anatase phase of titanium dioxide (TiO2) using interatomic potential-based molecular dynamics simulations. We have combined different force fields developed for CaP, TiO2, and water, benchmarking the results against density functional theory calculations. On the basis of our study, we consider that the new parameters can be used successfully to study the nucleation of CaP on realistic anatase and rutile TiO2 nanoparticles, including surface defects.

Periodic modeling of zeolite Ti-LTA.

We have proposed a combination of density functional theory calculations and interatomic potential-based simulations to study the structural, electronic, and mechanical properties of pure-silica zeolite Linde Type A (LTA), as well as two titanium-doped compositions. The energetics of the titanium distribution within the zeolite framework suggest that the inclusion of a second titanium atom with configurations Ti-(Si)0-Ti, Ti-(Si)1-Ti, and Ti-(Si)2-Ti is more energetically favorable than the mono-substitution. Infra-red spectra have been simulated for the pure-silica LTA, the single titanium substitution, and the configurations Ti-(Si)0-Ti and Ti-(Si)2-Ti, comparing against experimental benchmarks where available. The energetics of the direct dissociation of water on these Lewis acid sites indicate that this process is only favored when two titanium atoms form a two-membered ring (2MR) sharing two hydroxy groups, Ti-(OH)2-Ti, which suggests that the presence of water may tune the distribution of titanium atoms within the framework of zeolite LTA. The electronic analysis indicates charge transfer from H2O to the Lewis acid site and hybridization of their electronic states.

Detection of Posner's clusters during calcium phosphate nucleation: a molecular dynamics study.

Hydroxyapatite (HA), the main mineral phase of mammalian tooth enamel and bone, originates in body fluids from amorphous calcium phosphate (ACP). ACP presents short-range order in the form of small domains with size of 0.9 nm and chemical formula Ca9(PO4)6, known as Posner's clusters. In this study, the aggregation and clustering of calcium and phosphate ions in water has been investigated by means of shell-model molecular dynamics simulations. Calcium phosphate aggregates form in solution with compositions and Ca coordination that are similar to those found in Posner's cluster, but the stoichiometry of these species is dependent on the ionic composition of the solution: calcium-deficient clusters in solutions with low Ca : P ratio; cluster containing protonated phosphate groups in neutral solutions; sodium ions partially substituting calcium in solutions containing a mixture of sodium and calcium ions. These Posner-like clusters can be connected by phosphate groups, which act as a bridge between their central calcium ions. The simulations of the aggregation in solution of calcium phosphate clusters are an unbiased and unequivocal validation of Posner's model, and reveal for the first time the structure and composition of the species that form during the early stages of ACP nucleation at a scale still inaccessible to experiment.

Density functional theory simulation of the adsorption of sulphur multilayers on Au(100).

The adsorption of sulphur multilayers on Au(100) has been studied using density functional theory (DFT) within the generalized gradient approximation (GGA). The first sulphur layer was adsorbed on the four-fold sites of the unreconstructed Au(100) surface forming a lattice. The experimental parameters of the lattice were reproduced taking into account the surface expansion of the topmost Au(100) layer. This expansion should occur when gold islands are formed after the lifting of hex-reconstruction, which allows the lateral movement of the gold atoms. The second sulphur layer, on top of the lattice, consisted of eight S atoms (octomer phase) in a quasi-rectangular arrangement. The structural optimization of the octomer phase was achieved in a specific spatial orientation with respect to the lattice. The analysis of Bader atomic charges and the projected density of states (PDOS) demonstrated that charge transfer from the Au(100) surface to the sulphur layers, sulphur chemisorption and sulphur-sulphur inter-layer mixing of electronic states control the formation of sulphur multilayers.

Sulfur dimers adsorbed on Au(111) as building blocks for sulfur octomers formation: A density functional study.

Experimental scanning tunneling microscopy (STM) studies have shown for more than two decades rectangular formations when sulfur atoms are deposited on Au(111) surfaces. The precursors have ranged from simple molecules or ions, such as SO2 gas or sulfide anions, to more complex organosulfur compounds. We investigated, within the framework of the Density Functional Theory, the structure of these rectangular patterns assuming them entirely composed of sulfur atoms as the experimental evidence suggests. The sulfur coverage at which the simulations were carried out (0.67 ML or higher) provoked that the sulfur-sulfur association had to be taken into account for achieving a good agreement between the sets of simulated and experimental STM images. A combination of four sulfur dimers per rectangular formation properly explained the trends obtained by the experimental STM analysis which were related with the rectangles' size and shape fluctuations together with sulfur-sulfur distances within these rectangles. Finally, a projected density of states analysis showed that the dimers were capable of altering the Au(5d) electronic states at the same level as atomic sulfur adsorbed at low coverage. Besides, sulfur dimers states were perfectly distinguished, whose presence near and above the Fermi level can explain both: sulfur-sulfur bond elongation and dimers stability when they stayed adsorbed on the surface at high coverage.