Electrically Polarized Hydroxyapatite: Influence of the Polarization Process on the Microstructure and Properties

Abstract

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.

Enhanced CO2 Conversion into Ethanol by Permanently Polarized Hydroxyapatite through C−C Coupling

“…Exhaustive characterization of the catalytic activation through the polarization treatment has been carefully described in the literature, reporting Raman and SEM techniques as reliable control techniques to ensure proper comparison between reaction conditions. [13,15,16] The catalytic properties of p-HAp were found to be consequence of two different and distinguishable phenomena: [15] 1) the generation of vacancies inside the HAp lattice due to the sintering at 1000 °C, which is responsible of creating available charges; and 2) the specific orientation of the ions in the OH À columns that is due to the polarization and allows the delocalization of charges along all the independent crystalline domains, reinforcing the charge accumulation at the micrometric scale, particularly at grain boundaries.…”

“…[11] Indeed, the utilization of HAp in heterogeneous catalytic processes offers multiple advantages, such as high thermal stability, weak acid-base character and flexibility to adapt its chemical composition, changing from stoichiometric to non-stoichiometric ratios by replacing Ca 2 + by other divalent or trivalent cations, PO 4 3À by other anions or through OH À vacancies creation. The enhancement of the catalytic properties of p-HAp, which are obtained by applying a DC field of 3 kV/cm at 1000 °C to sintered HAp, [12,13] have been attributed not only to the OH À vacancies generated at high temperature but also to its capacitive behavior, which is due the accumulation of charge at the surface. Therefore, the properties imparted by the polarization treatment make possible to use p-HAp as an improved catalyst without the aid of any electrochemical setup.…”

Regulating the Superficial Vacancies and OH− Orientations on Polarized Hydroxyapatite Electrocatalysts

“…However, we recently demonstrated that Raman microscopy is a useful technique to quantify the relative amount of OH − groups in HAp and to measure interesting crystal lattice parameters, such as crystallinity or phase distribution. [ 12 ]…”

“…Such dehydration process, hereafter denoted calcination treatment, is also responsible for structural changes, affecting the crystallinity and mechanical properties of HAp. [ 12,13 ] Low generation of vacancies and suppression of carbonates present in the crystal lattice is expected to cause an increment of the crystallinity. Conversely, fast generation of vacancies, which can be favored by OH − abundant atmospheres at high temperatures, generates disorder or even transitions to other calcium phosphate salts, such as amorphous calcium phosphate or β‐tricalcium phosphate (βTCP), altering the final properties of HAp.…”

“…Conversely, fast generation of vacancies, which can be favored by OH − abundant atmospheres at high temperatures, generates disorder or even transitions to other calcium phosphate salts, such as amorphous calcium phosphate or β‐tricalcium phosphate (βTCP), altering the final properties of HAp. [ 12 ] Additionally, the higher the number of vacancies, the higher the mobility of OH − dipoles that are less affected by the electrostatic forces of nearby OH − in the same column. [ 14 ]…”