Smart designs of hydroxyapatite (HAp) materials with customized electrical properties are drawing increasing attention for their wide range of potential applications. Such enhanced electrical properties directly arise from the number and orientation of OH− groups in the HAp lattice. Although different polarization treatments have been proposed to enhance the final conductivity by generating vacancies at high temperatures and imposing specific OH− orientations through electric voltages, no direct measurement showing the evolution that OH− groups undergo has been described yet. In this article, the first direct empirical observation that allows the characterization of both the generation of vacancies and the polarization of OH− groups is reported. The mechanisms behind the electrical enhancement are elucidated allowing to distinguish between charge accumulation at the crystal grains, which is due to the formed vacancies, and charge accumulation in the boundaries of particles. In addition, a linear dependence between the number of vacancies and the superficial charge is observed. Therefore, it is demonstrated that the charge accumulation at the micrometric grain boundaries has a great impact on the catalytic properties of the thermally stimulated polarized HAp. These results will be used for further optimization of the catalyst properties.

Semipermanently polarized hydroxyapatite, named SP/HAp­(w), is obtained by applying a constant dc electric field of 1–10 kV/cm at 300–850 °C to the samples previously sintered in water vapor, while permanently polarized hydroxyapatite, PP/HAp­(a), is produced by applying a dc electric field of 3 kV/cm at 1000 °C to the samples sintered in air. SP/HAp­(w) has been used for biomedical applications, while PP/HAp­(a) has been proved to be a valuable catalyst for N2 and CO2 fixation. In this work, structural differences between SP/HAp­(w) and PP/HAp­(a) have been ascertained using Raman microscopy, wide-angle X-ray diffraction, scanning electronic microscopy, high-resolution transmission electron microscopy, and grazing incidence X-ray diffraction. Results prove the existence of crystal distortion in the form of amorphous calcium phosphate and β-tricalcium phosphate (β-TCP) phases close to the surface because of the atmosphere used in the sintering process. The existence of an amorphous layer in the surface and the phase transition through β-TCP of SP/HAp­(w) are the structural factors responsible for the differences with respect to PP/HAp­(a). Moreover, a superstructure has been identified in PP/HAp­(a) samples, which could be another structural factor associated with enhanced conductivity, permanent polarization, and catalytic activity of this material.

Conversion of CO2 into valuable chemicals is not only a very challenging topic but also a socially demanded issue. In this work, permanently polarized hydroxyapatite obtained using a thermal stimulated...

Design of hydroxyapatite (HAp) with customized electrical properties is of special interest for developing technological and biomedical applications with new improved functionalities. Polarized HAp, which is obtained by applying an external electric voltage at high temperature, has been successfully shown to be an alternative to doped‐HAp that is limited by the biocompatibility of the dopants used. However, many aspects about such new material remain scarcely studied, as for example the relationship between the polarization conditions and the resultant electrical enhancement, hinder a solid progress in its application. In this work, polarized HAp has been extensively characterized using electrochemical impedance spectroscopy by means of proposing a unified electrical equivalent circuit model with physical sense. This allows to explain the properties of such material by separating the bulk and the interface contributions. Moreover, the limits of the polarization mechanism have been explored, enabling a precise control on the electrical resistivity of polarized HAp above or below the intrinsic resistivity of nonpolarized HAp. Overall, necessary insights on the polarization treatment have been reported, opening an appealing avenue for generating new biomedical and technological applications based on dopant‐free polarized HAp.

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Hydroxyapatite (HAp) is a well-known ceramic material widely used in the biomedical field. This review summarizes the very recent developments on permanently polarized HAp (pp-HAp), a HAp variety with tuned...

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Hydroxyapatite (HAp) is a naturally occurring mineral form of calcium apatite of biomedical importance due to its similarity with human hard tissues in morphology and composition. Upon polarization at high temperature, applying 3 kV/cm at 1000 °C, the resulting polarized HAp (p-HAp) exhibits enhanced catalytic behavior due charge accumulation at the interface. More specifically, p-HAp was found to catalyse the conversion of mixtures of CO 2 (g) and CH 4 (g) into low carbon organic molecules and into amino acids when N 2 (g) was added to the mixture. In this work, we report how p-HAp facilitates the conversion of CO 2 (g) mainly in ethanol by means of forming CÀ C coupled bonds on its activated surface. After evaluation of a wide range of experimental conditions, we evidence the production of formic acid, methanol and formaldehyde (C1 products); ethanol and acetic acid (C2 products); and acetone (C3 product) from CO 2 (g) using moderate reaction conditions. Moreover, optimization of the reaction parameters led to a significant increase towards ethanol.