Nora H. de Leeuw

Magnesium incorporation into hydroxyapatite

The incorporation of Mg in hydroxyapatite (HA) was investigated using multinuclear solid state NMR, X-ray absorption spectroscopy (XAS) and computational modeling. High magnetic field (43)Ca solid state NMR and Ca K-edge XAS studies of a ∼10% Mg-substituted HA were performed, bringing direct evidence of the preferential substitution of Mg in the Ca(II) position. (1)H and (31)P solid state NMR show that the environment of the anions is disordered in this substituted apatite phase. Both Density Functional Theory (DFT) and interatomic potential computations of Mg-substituted HA structures are in agreement with these observations. Indeed, the incorporation of low levels of Mg in the Ca(II) site is found to be more favourable energetically, and the NMR parameters calculated from these optimized structures are consistent with the experimental data. Calculations provide direct insight in the structural modifications of the HA lattice, due to the strong contraction of the M⋯O distances around Mg. Finally, extensive interatomic potential calculations also suggest that a local clustering of Mg within the HA lattice is likely to occur. Such structural characterizations of Mg environments in apatites will favour a better understanding of the biological role of this cation.

Vacancy ordering and electronic structure of γ-Fe2O3 (maghemite): a theoretical investigation

The crystal structure of the iron oxide γ-Fe₂O₃ is usually reported in either the cubic system (space group P4(3)32) with partial Fe vacancy disorder or in the tetragonal system (space group P4(1)2(1)2) with full site ordering and c/a≈3. Using a supercell of the cubic structure, we obtain the spectrum of energies of all the ordered configurations which contribute to the partially disordered P4(3)32 cubic structure. Our results show that the configuration with space group P4(1)2(1)2 is indeed much more stable than the others, and that this stability arises from a favourable electrostatic contribution, as this configuration exhibits the maximum possible homogeneity in the distribution of iron cations and vacancies. Maghemite is therefore expected to be fully ordered in equilibrium, and deviations from this behaviour should be associated with metastable growth, extended anti-site defects and surface effects in the case of small nanoparticles. The confirmation of the ordered tetragonal structure allows us to investigate the electronic structure of the material using density functional theory (DFT) calculations. The inclusion of a Hubbard (DFT + U) correction allows the calculation of a band gap in good agreement with experiment. The value of the gap is dependent on the electron spin, which is the basis for the spin-filtering properties of maghemite.

Density Functional Theory Study of the Binding of Glycine, Proline, and Hydroxyproline to the Hydroxyapatite (0001) and (011̅0) Surfaces

In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue and in synthetic biomaterials, we have investigated the interaction of three constituent amino acids of the collagen matrix with two major hydroxyapatite surfaces. We have employed electronic structure techniques based on the density functional theory to study a range of different binding modes of the amino acids glycine, proline, and hydroxyproline at the hydroxyapatite (0001) and (0110) surfaces. We have performed full geometry optimizations of the hydroxyapatite surfaces with adsorbed amino acid molecules to obtain the optimum substrate/adsorbate structures and interaction energies. The calculations show that the amino acids are capable of forming multiple interactions with surface species, particularly if they can bridge between two surface calcium ions. The binding energies range from 290 kJ mol(-1) for glycine on the (0001) surface to 610 kJ mol(-1) for hydroxyproline on the (0110) surface. The large adsorption energies are due to a wide range of interactions between the adsorbate and surface, including proton transfer from the adsorbates to surface OH or PO(4) groups. Hydroxyproline binds most strongly to the surfaces, but all three amino acids should be good sites for the nucleation and growth of the hydroxyapatite (0110) surface at the collagen matrix.

Synthesis, characterization and DFT studies of zinc-doped copper oxide nanocrystals for gas sensing applications

Due to their unique properties, p-type copper oxide nanostructures have demonstrated promising potential for various applications, especially for the detection of ethanol vapour and other volatile organic compounds (VOCs). In this work a simple and cost-effective synthesis from chemical solutions (SCS) at low temperatures (≤80 °C) and rapid thermal annealing (RTA) process were used to grow zinc-doped copper oxide (ZnxCu1−xOy) nanostructures. The structural, morphological, vibrational, chemical, electronic and sensorial characteristics of ZnxCu1−xOy nanocrystallite layers obtained by using such an efficient approach based on both, the SCS and RTA processes, have been studied. The investigations demonstrated the possibility to tune sensitivity from VOC to H2, as well as an improved response and high selectivity with respect to hydrogen gas for ZnxCu1−xOy nano-crystalline thin films with x = 0.03. Density functional theory calculations showed that the charge transfer together with changes in the Fermi level facilitate H2 gas sensing, which is further enhanced by Zn doping. Hydrogen gas sensing with a high response and selectivity using p-type hybrid semiconductor nanostructures has been reported. An improved stability in humid air was observed by exposure of doped samples to rapid thermal annealing process for the first time. The experimental and calculation results provide an alternative to sensitive and selective detection of ethanol and hydrogen gases, which would be of particular benefit in the area of public security, industrial and environmental applications.

Electronic charge transfer between ceria surfaces and gold adatoms: a GGA + U investigation

We use density functional theory calculations with Hubbard corrections (DFT+U) to investigate electronic aspects of the interaction between ceria surfaces and gold atoms. Our results show that Au adatoms at the (111) surface of ceria can adopt Au(0), Au(+) or Au(-) electronic configurations depending on the adsorption site. The strongest adsorption sites are on top of the surface oxygen and in a bridge position between two surface oxygen atoms, and in both cases charge transfer from the gold atom to one of the Ce cations at the surface is involved. Adsorption at other sites, including the hollow sites of the surface, and an O-Ce bridging site, is weaker and does not involve charge transfer. Adsorption at an oxygen vacancy site is very strong and involves the formation of an Au(-) anion. We argue that the ability of gold atoms to stabilise oxygen vacancies at the ceria surface by moving into the vacancy site and attracting the excess electrons of the defect could be responsible for the enhanced reducibility of ceria surfaces in the presence of gold. Finally, we rationalise the differences in charge transfer behaviour from site to site in terms of the electrostatic potential at the surface and the coordination of the species.

Structure and dynamics of the hydrated magnesium ion and of the solvated magnesium carbonates: insights from first principles simulations

We report first principles molecular dynamics simulations based on the density functional theory and the Car–Parrinello method to study the structures and dynamics of the hydrated Mg2+ ion and of the solvated MgHCO3+ and MgCO3 complexes in aqueous solution. According to these simulations, the first hydration shell of the hydrated magnesium ion consists of six water molecules, whereas in the solvated magnesium bicarbonate and magnesium carbonate complexes the Mg2+ is mostly five-coordinated, which indicates that when coordinated to magnesium the HCO3− and CO32− anions reduce its the coordination sphere. Our simulations show that the structures of the most stable monomers of magnesium bi-carbonate and magnesium carbonate in solution are Mg[η1-HCO3](H2O)4+ and Mg[η1-CO3](H2O)4, i.e. the preferred hydration number is four, while the (bi-)carbonate is coordinated to the magnesium in a monodentate mode. The analysis of the exchange processes of the water molecules in the first and second hydration shell of Mg2+ shows that the HCO3− or CO32− ligands affect the dynamics of the magnesium coordination spheres by making its hydration shell more “labile”. Furthermore, molecular dynamics simulations of the non-associated Mg2+/Cl− pair in water suggest that, despite negligible differences in the coordination spheres of Mg2+, the chloride anion has a significant influence on the water exchange rates in the second hydration shell of Mg2+.

On the difficulties of present theoretical models to predict the oxidation state of atomic Au adsorbed on regular sites of CeO2(111).

The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO(2)(111) surface have been investigated in detail by means of periodic density functional theory-based calculations, using the LDA+U and GGA+U potentials for a broad range of U values, complemented with calculations employing the HSE06 hybrid functional. In addition, the effects of the lattice parameter a(0) and of the starting point for the geometry optimization have also been analyzed. From the present results we suggest that the oxidation state of single Au atoms on CeO(2)(111) predicted by LDA+U, GGA+U, and HSE06 density functional calculations is not conclusive and that the final picture strongly depends on the method chosen and on the construction of the surface model. In some cases we have been able to locate two well-defined states which are close in energy but with very different electronic structure and local geometries, one with Au fully oxidized and one with neutral Au. The energy difference between the two states is typically within the limits of the accuracy of the present exchange-correlation potentials, and therefore, a clear lowest-energy state cannot be identified. These results suggest the possibility of a dynamic distribution of Au(0) and Au(+) atomic species at the regular sites of the CeO(2)(111) surface.

A Density Functional Theory Study of the Interaction of Collagen Peptides with Hydroxyapatite Surfaces

Density functional theory calculations were applied to investigate the binding of four peptide strands, which are important in the collagen protein, to the bone and tooth mineral hydroxyapatite: amphiphilic PRO-HYP-GLY and HYP-PRO-GLY, and hydrophobic PRO-LYS-GLY and PRO-HYL-GLY. The particular peptide sequences are chosen for their different functional groups, containing (i) hydrophobic; (ii) uncharged polar; and (iii) charged polar side groups, thus allowing direct comparison of the general effect of these carboxylic acid and amine functional groups, as well as hydroxylation and charge, on their interactions with two major hydroxyapatite surfaces, (0001) and (0110). The calculated results are consistent with experiments, confirming that the terminal carboxyl groups and amine groups mainly contribute to the adsorption of the peptides to the hydroxyapatite surfaces and primarily to the (0110) surface rather than the dominant (0001) plane. Of the side groups in the tripeptide motifs representing the collagen protein, the -OH and positively charged -NH(3)(+) groups in particular bind strongly to the surfaces, and their presence should therefore promote hydroxyapatite growth.

Reduction of Carbon Dioxide to Formate at Low Overpotential Using a Superbase Ionic Liquid

A new low-energy pathway is reported for the electrochemical reduction of CO2 to formate and syngas at low overpotentials, utilizing a reactive ionic liquid as the solvent. The superbasic tetraalkyl phosphonium ionic liquid [P66614][124Triz] is able to chemisorb CO2 through equimolar binding of CO2 with the 1,2,4-triazole anion. This chemisorbed CO2 can be reduced at silver electrodes at overpotentials as low as 0.17 V, forming formate. In contrast, physically absorbed CO2 within the same ionic liquid or in ionic liquids where chemisorption is impossible (such as [P66614][NTf2]) undergoes reduction at significantly increased overpotentials, producing only CO as the product.

A computational investigation of stoichiometric and calcium-deficient oxy- and hydroxy-apatites

Computer modelling techniques have been employed to qualitatively and quantitatively investigate the dehydration of hydroxyapatite to oxyapatite and the defect chemistry of calcium-deficient hydroxyapatite, where a number of vacancy formation reactions are considered. The dehydration of hydroxyapatite into oxyhydroxyapatite is calculated to be endothermic by E = +83.2 kJ mol(-1) in agreement with experiment, where thermal treatment is necessary to drive this process. Calcium vacancies are preferentially charge-compensated by carbonate ions substituting for phosphate groups (E = -5.3 kJ mol(-1)), whereas charge-compensating reactions involving PO4 vacancies are highly endothermic (E > or =652 kJ mol(-1)). The exothermicity of the charge compensation of a Ca vacancy accompanied by a PO4/CO3 substitution agrees with their co-occurrence in natural bone tissue and tooth enamel. Our calculations of a range of defect structures predict (i) that calcium vacancies as well as substitutional sodium and potassium ions would occur together with carbonate impurities at phosphate sites, but that other charge compensations by replacement of the phosphate groups are unfavourable, and (ii) that the hydroxy ions in the channel are easily replaced by carbonate groups, but that the formation of water or oxygen defects in the channels is thermodynamically unfavourable. Calculated elastic constants are reported for the defect structures.