Revealing Hydroxyapatite Nanoparticle Surface Structure by CO Adsorption: A Combined B3LYP and Infrared Study (original) (raw)
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A Computational Study of the Properties and Surface Interactions of Hydroxyapatite
Ferroelectrics, 2013
Hydroxyapatite (HAP, Ca 10 (PO 4 ) 6 (OH) 2 ) was studied from first principles approaches using the local density approximation (LDA) method in combination with various quantum-chemical (QM) and molecular mechanical (MM) methods from HypemChem 7.5/8.0. The data then were used for studies of HAP structures, and the interactions of HAP clusters with ionic species such as citrates. Computed data show that HAP can co-exist in different phases at room temperature, as both hexagonal and monoclinic. Special interest is connected with the ordered monoclinic structure, which could reveal piezoelectric properties. Obtained data on HAP interactions with citrates show the formation of differing HAP nanostructure forms, depending upon the concentration of citrate present.
Surface energetics of the hydroxyapatite nanocrystal-water interface: a molecular dynamics study
Langmuir : the ACS journal of surfaces and colloids, 2014
Face-specific interfacial energies and structures of water at ionic crystal surfaces play a dominant role in a wide range of biological, environmental, technological, and industrial processes. Nanosized, plate-shaped crystals of calcium phosphate (CaP) with nonideal stoichiometry of hydroxyapatite (HAP, ideal stoichiometry Ca10(PO4)6(OH)2) comprise the inorganic component of bone and dentin. The crystal shape and size contribute significantly to these tissues' biomechanical properties. Plate-shaped HAP can be grown in the presence of biomolecules, whereas inorganically grown HAP crystals have a needlelike shape. Crystal morphology reflects the relative surface areas of the faces and, for an ideal inorganically grown crystal, should be governed by the surface energies of the faces with water. Interfacial energies and dynamics also affect biomolecule adsorption. Obtaining face-specific surface energies remains experimentally challenging because of the difficulty in growing large H...
2012
High-resolution transmission electron microscopy (HRTEM) and ab initio quantum-mechanical calculations of electronic structure were combined to investigate the structure of the hydroxyapatite (HA) (0 1 0) surface, which plays an important role in HA interactions with biological media. HA was synthesized by in vitro precipitation at 37 • C. HRTEM images revealed thin elongated rod nanoparticles with preferential growth along the [0 0 1] direction and terminations parallel to the (0 1 0) plane. The focal series reconstruction (FSR) technique was applied to develop an atomic-scale structural model of the high-resolution images. The HRTEM simulations identified the coexistence of two structurally distinct terminations for (0 1 0) surfaces: a rather flat Ca(II)-terminated surface and a zigzag structure with open OH channels. Density functional theory (DFT) was applied in a periodic slab plane-wave pseudopotential approach to refine details of atomic coordination and bond lengths of Ca(I) and Ca(II) sites in hydrated HA (0 1 0) surfaces, starting from the HRTEM model.
Physical Review Materials, 2021
The presence of both acidic and basic adsorption sites on the surface of hydroxyapatite [Ca 10 (PO 4) 6 (OH) 2 ; HAP] is an interesting property for catalytic applications. Here, we report a density functional theory investigation of the adsorption properties of CO, CO 2 , C 2 H 2 , CH 4 , H 2 , H 2 O, NH 3 , SO 2 , and BCl 3 on the HAP(0001) surface. All probe molecules have a lower energy when they are adsorbed in the region between the most exposed Ca 2+ ion (electron acceptor) and a neighboring PO 4 3− group, where the O atoms (electron donor) contribute to the stabilization of the adsorbed molecule. By evaluating the redistribution of the electron density and the change of the atomic charges, the Ca 2+ and PO 4 3− sites were identified as Lewis acidic and Lewis basic adsorption sites, respectively, which indicates that simultaneous acid-base interactions occur upon adsorption of all studied probe molecules. All adsorbates interact with the surface via atoms of opposing charges, which enhances the ionic character of molecular bonds by increasing the distinction between cationic and anionic charges within the molecule. Furthermore, molecules with greater ionic character show stronger interaction with the substrate and greater geometric deformation. Although most adsorbed molecules (CO, CO 2 , C 2 H 2 , CH 4 , H 2 , H 2 O, and NH 3) do not show substantial net charge transfer, polarization effects due to the redistribution of charge are observed upon adsorption of all probe molecules. The change in the work function increases linearly with the total change in the surface dipole moment for H 2 O, NH 3 , SO 2 , and BCl 3 , while for the remaining systems, the magnitude of the work function change remains more uniform. By identifying the type of interaction between each probe molecule and the HAP(0001) surface, the present study contributes to the understanding of the acid-base properties of the HAP(0001) surface, which we elaborated in a short discussion based on the individual bond orders for the acidic and basic sites.
Ferroelectrics 509 (2017) 105-112, 2017
Hydroxyapatite (HAp) has structural features that define its basic physical properties, which have an important role at the surface, and it is one of the most used materials in bone implants. In this work, we present a density functional modeling (DFT) study of HAp both as bulk and with special HAp models with various defects, especially oxygen vacancies in HAp surface layers, which can also determine photocatalytic properties, confirmed experimentally. The first-principles calculations of bulk and modified HAp were carried out using local basis (AIMPRO) and plane-wave (VASP) codes. Data obtained are analyzed using both approaches, and compared.
Surface properties of various powdered hydroxyapatites
Journal of Colloid and Interface Science, 2005
Electrophoretic mobilities of various synthetic and semisynthetic hydroxyapatites (Ca 10 (PO 4 ) 6 (OH) 2 , HAP) suspended in aqueous solutions have been measured as a function of pH and calcium concentration. The studied powders differ in particle size, crystallinity degree and surface contamination (carbonate). When equilibrated in mineral acids or bases, a large plateau of negative mobility is observed in the pH range 5-8, with increasing negative values at higher pH. Only in the case of the sample composed of nanoparticles, positive mobility obtains at pH < 8.9. When Ca 2+ is added, positive mobility values are observed for all samples, and a bell-shaped profile results as a function of pH. Two possible models are explored to describe the results: the Nernstian approach, which assumes solubility equilibrium and surface potentials determined by the three potential-determining ions (Ca 2+ , PO 3− 4 , and OH − ), and the surface complexation approach, based on the idea of negligible phase transfer of structural phosphate. The Nernstian model is inadequate, whereas a very simple surface complexation model based on the equations ≡Ca 5 (PO 4 ) + 3 = ≡Ca 4 (PO 4 ) − 3 + Ca 2+ , ≡Ca 4 (PO 4 ) − 3 + H + = ≡Ca 4 (PO 4 ) 2 (PO 4 H),
The Journal of Physical Chemistry C, 2015
Two types of biomimetic hydroxyapatite (HA) nanoparticles were prepared by acid−base neutralization reactions, using Ca(OH) 2 or Ca(CH 3 COO) 2 as a calcium source, to evaluate the effect of acetate anions on particle formation. High-resolution transmission electron microscopy observations provided evidence that in both cases nanoparticles are elongated along the c-axis, but to a more limited extent when prepared in the presence of acetates, and are mainly limited by {010} facets. IR spectra of nanoparticles containing adsorbed CO revealed that the actual termination of these are both of the {010}_Ca-rich and {010}_P-rich type, the latter being significantly more abundant for HA nanoparticles grown in the medium containing CH 3 COO − species. Moreover, these nanoparticles appeared to be more sensitive toward aggregative stacking by thermal treatment, resulting in a significant decrease in specific surface area, while retaining the size of primary particles.
A solubility and surface complexation study of a non-stoichiometric hydroxyapatite
Geochimica et Cosmochimica Acta, 2009
The dissolution and surface complexation of a non-stoichiometric hydroxyapatite (Ca 8.4 (HPO 4 ) 1.6 (PO 4 ) 4.4 (OH) 0.4 ), (HAP) was studied in the pH range 3.5-10.5, at 25°C in 0.1 M Na(Cl). The results from well-equilibrated batch experiments, potentiometric titrations, and zeta-potential measurements were combined with information provided by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The information from the analyses was used to design an equilibration model that takes into account dissolution, surface potential, solution and surface complexation, as well as possible phase transformations. The results from the XPS measurements clearly show that the surface of the mineral has a different composition than the bulk and that the Ca/P ratio of the surface layer is 1.4 ± 0.1. This ratio was also found in solution in the batches equilibrated at low pH where the dominating reaction is dissolution. In the batches equilibrated at near neutral pH values, however, the Ca/P ratio in solution attains values as high as 25, which is due to re-adsorption of phosphate ions to the HAP surface. The total concentration of protons as well as the total concentration of dissolved calcium and phosphate in solution were used to calculate a model for the dissolution and surface complexation of HAP. The constant capacitance model was applied in designing the following surface complexation model:
Ceramics International, 2017
We describe the temperature dependence of the vibrational, rotational and translational partition functions of the activated complex of hydroxyapatite (HAp). Computed data show that the vibrational modes have a larger contribution towards the partition function of HAp compared to the rotational and translational contributions. X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy have been applied for HAp in the temperature range from 730 °C to 1030 °C at steps of 100 °C. Temperature dependent density functional theory (DFT), B3LYP, and Ground State Hartree-Fock (HF) with 6-311G basis set calculations were also applied to HAp to calculate FTIR spectra, HOMO, and LUMO energies, and density of states (DOS), and the results have been compared to experimental findings. The present results underline that experimental measurements and theoretical calculations of unit cell parameters, and the intensities of most of the FTIR data for HAp are nearly independent on temperature.