Judy N. Hart

Influence of fluorine in the synthesis of apatites. Synthesis of solid solutions of hydroxy-fluorapatite

Hydroxy-fluorapatites (OH-FAps) occur biologically in teeth and form the basis for application as biomaterials. This work aims to synthesize a series of fluoride substituted calcium hydroxyapatites (OHAps) to determine how fluoride influences the synthesis and the resulting characteristics of solid solutions. OH-FAPs powders were synthesized with a chemical composition of Ca(10)(PO(4))(6)(OH)(2-x) F(x), with x=0.0, 0.4, 0.8, 1.2, 1.6 and 2.0. The synthesis of partially substituted OHAp yields materials with lower crystallinity and higher specific surface area than OHAp or fluorapatite (FAp). The smallest crystal size of 263A, occurs at less than 50% hydroxyl substitution with fluoride at x=0.4, and the highest surface area of 132m(2)/g occurs at x=0.8. Reaction kinetics occur faster at higher fluoride content, producing the expected Ca/P ratio of 1.67 only for x=2.0. X-ray and IR studies show that OH-FAPs are homogeneous solid solutions instead of mixtures of OHAp and FAp. The presence of a high fluoride concentration increases the driving force for crystal growth during the calcination process.

Improving density functional theory for crystal polymorph energetics

We show that the quality of density functional theory (DFT) predictions for the relative stabilities of polymorphs of crystalline para-diiodobenzene (PDIB) is dramatically improved through a simple two-body correction using wavefunction-based electronic structure theory. PDIB has two stable polymorphs under ambient conditions, and like Hongo et al. [J. Phys. Chem. Lett., 1, 1789 (2010)] we find that DFT makes wildly variable predictions of the relative stabilities, depending on the approximate functional used. The two-body corrected scheme, using Grimmes spin-scaled variant of second-order Møller-Plesset perturbation theory and any of the tested density functionals, predicts the α-polymorph to be more stable, consistent with experiment, and produces a relative stability that agrees with the benchmark quantum Monte-Carlo results of Hongo et al. within statistical uncertainty.

Energy minimization of single-walled titanium oxide nanotubes.

Different crystal structures have been proposed as a basis for titanium oxide nanotubes. We have used atomistic simulation techniques to calculate the relative stability of nanotubes with these different crystal structures. Our approach is to use energy minimization, where the total interaction energy is calculated with interatomic potentials based on the Born model of solids. The results reveal nanotubes with the trititanate structure to be the most stable (at unit activity for water). Indeed, nanotubes with the trititanate structure were found to be thermodynamically more favorable than bulk trititanate for nanotube diameters greater than approximately 8 nm. However, the formation of cross-linking bonds between layers of the trititanate structure occurred frequently; this problem was eliminated by replacing two out of three Ti(4+) ions with Ti(3+) ions, although this resulted in a higher energy. Of the structures that do not contain hydrogen, chiral nanotubes made from (101) sheets of anatase are the lowest in energy, suggesting that this is the most likely structure for nanotubes synthesized at low water chemical potential. In general, the stability of the nanotubes increased as the nanotube diameter increased.

GaP‐ZnS Solid Solutions: Semiconductors for Efficient Visible Light Absorption and Emission

GaP-ZnS solid solutions and multilayered structures have a tunable direct band gap in the energy range for absorption and emission of visible light. A direct band gap of around 2.0 eV, the optimum for photocatalysis of water splitting, is readily accessible with these systems.

Optical properties of zirconia ceramics for esthetic dental restorations: A systematic review

Statement of problem. Yttria‐stabilized tetragonal zirconia polycrystal has been used as a dental biomaterial for several decades because the fracture toughness and bend strength are increased by a stress‐induced transformation‐toughening mechanism. However, its esthetics are compromised by its poor translucency and grayish‐white appearance. Purpose. The purpose of the present systematic review was to assess information on the mechanical, chemical, and optical requirements of monolithic zirconia dental restorations. Material and methods. The following databases (2010 to 2015) were electronically searched: ProQuest, EMBASE, SciFinder, MRS Online Proceedings Library, Medline, Compendex, and Journal of the American Ceramic Society. The search was limited to English‐language publications, in vitro studies, experimental reports, and modeling studies. Results. The data from 57 studies were considered in order to review the intrinsic and extrinsic characteristics of zirconia and their effects on the optical properties. Conclusions. The materials and microstructural issues relevant to the esthetics and long‐term stability of zirconia have been considered in terms of monolithic restorations, while there also are restorations specifically for esthetic applications. Although zirconia‐toughened lithium silicate offers the best esthetic outcomes, transformation‐toughened zirconia offers the best mechanical properties and long‐term stability; cubic stabilized zirconia offers a potential compromise. The properties of these materials can be altered to some extent through the appropriate application of intrinsic (such as, annealing) and extrinsic (such as, shade‐matching) parameters.

Band‐Gap Control of Zinc Sulfide: Towards an Efficient Visible‐Light‐Sensitive Photocatalyst

The electronic properties of transition-metal-doped zinc sulfide (ZnS) have been investigated by using first-principles calculations. Transition-metal doping can allow electronic transitions at energies corresponding to visible-light wavelengths, thus potentially resulting in increased photocatalytic efficiency under sunlight. In particular, our calculations show that transition-metal atoms that produce little lattice strain, such as Co, Ni, Mn, and Fe, can be readily incorporated in ZnS. Due to their low formation energies and appropriate band energies, we predict that Ni- and Co-doped ZnS will be promising materials for photocatalytic hydrogen production.

Predicting crystal structures ab initio: group 14 nitrides and phosphides

Crystal structures are predicted for a range of group 14 nitrides and phosphides with 1 : 1 stoichiometry, following our method of starting from the known structures for a range of binary compounds and looking for trends in the preferred local bonding environments in the optimised structures. We have previously applied this method to predict the structures of carbon nitride and phosphorus carbide. Here, we use a similar approach to predict the structures of silicon and germanium nitrides and phosphides with 1 : 1 stoichiometry. We find that the local bonding environments in the preferred structures for the nitrides are the same as those for the 3 : 4 stoichiometry. For the phosphides, we have found several possible structures with similar energies. Structures containing hypervalent phosphorus must be considered as these are often low in energy, particularly for GeP; these have not been included in previous work. The greater tendency to form hypervalent phosphorus in GeP than SiP can be rationalised by considering the bond enthalpies for the two compositions.

Growth mechanism of ceria nanorods by precipitation at room temperature and morphology-dependent photocatalytic performance

Ceria (CeO2) nanorods have been prepared by simple short-term precipitation at room temperature for the first time using aqueous solutions based on Ce(NO3)3·6H2O and NaOH. TEM showed that (a) the two solutions alone yielded nanooctahedra of cross section ∼10 nm and (b) selective surface modification by isopropanol (IPA) played a significant role in the morphological development of approximately square nanorods of dimensions 4–5 nm width, 15–25 nm length, and [110] growth direction. DFT was used to assess surface energies and the interactions of the H2O and IPA molecules with the {111}, {110}, and {100} ceria surfaces. A growth mechanism on the basis of these adsorption energies and orientations is proposed and it depends on the favorable IPA adsorption energy. Its effect is twofold. First, it facilitates the formation of a {110} prism that alters the morphology from octahedral to spheroidal and then cuboidal. Second, the anisotropic electrostatic field in the electrical double layer, which is established by the oriented adsorption of the IPA molecule, is considered to facilitate the growth of the nanorod morphology. XPS data show that nanorods exhibit a greater concentration of Ce3+ (and associated oxygen vacancies) than do the nanooctahedra. The parameters determining the development of nanoparticle morphology are ranked in the order: packing density ≈ lattice spacing > IPA adsorption > H2O adsorption > surface energy. The present work suggests the applicability of crystallography considerations and DFT modeling to direct the crystal growth of specific morphologies.

Investigating the effect of UV light pre-treatment on the oxygen activation capacity of Au/TiO2

The potential for applying UV light pre-treatment to enhance the oxygen activation capacity of Au/TiO2 under ambient conditions was examined. Catalytic formic acid oxidation in an aqueous environment was employed as the test reaction. Pre-illuminating Au/TiO2 with UV light can amplify the catalytic formic acid oxidation rate by up to four times with the degree of enhancement governed by system parameters such as Au loading, pre-illumination time, and initial formic acid loading. X-ray photoelectron spectroscopy, photoluminescence spectroscopy and electrochemical assessment of the Au/TiO2 indicated light pre-illumination invokes photoexcited electron transfer from the TiO2 support to the Au deposits. The Au deposits then utilise the additional electrons to catalyse molecular oxygen activation and promote the oxidation reaction. Scanning tunneling spectroscopy analysis and first principle calculations indicated the Au deposits introduced new electronic states above the TiO2 valence band. The new electronic states were most intense at the Au–TiO2 interface suggesting the Au deposit:TiO2 perimeter may be the key region for oxygen activation. The current study has demonstrated that pre-illuminating Au/TiO2 with light can be used to augment reactions where oxygen activation is a critical component, such as for the oxidation of organic pollutants and for the oxygen reduction reaction in fuel cells or energy storage systems.

Enhancing bimetallic synergy with light: the effect of UV light pre-treatment on catalytic oxygen activation by bimetallic Au–Pt nanoparticles on a TiO2 support

UV light pre-treatment was examined as a means of enriching the catalytic activation of oxygen by bimetallic AuPt deposits loaded on TiO2. The rate of catalytic oxygen activation was assessed by monitoring the rate of formic acid oxidation in an aqueous system. A catalytic synergy was observed to exist for the bimetallic AuPt on TiO2 and was governed by the Au–Pt structure and ratio. The extent of the synergy was further enhanced upon UV light pre-treatment. Exceptional improvements in bimetallic catalysts are often simply attributed to a synergy effect, which is not necessarily well-understood. The Au–Pt bimetallic synergy and UV light pre-illumination phenomena were probed using high-end characterisation tools in conjunction with first principle calculations with the effects attributed to a combined influence of work-function difference and lattice mismatch between Au and Pt. Understanding the origin of bimetallic synergism is a critical step toward developing advanced catalysts.