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

Abstract

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.

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.…”

“…[13] These structural transformations are characteristic of a successful polarization process. Although the internal structure of the catalyst was exhaustively characterized in previous work by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction, [13,15] additional measurements are provided in Figure S1 for completeness. The crystallinity (χ c ) and crystallite size in the preferred (211) direction (L 211 ) of pristine HAp were 0.83 � 0.02 and 25 � 1 nm, respectively, while those of p-HAp were 0.96 � 0.03 and 75 � 3 nm, respectively.…”

“…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. [12,14,15] Recent studies showed that p-HAp catalyzes the conversion of N 2 , CO 2 and CH 4 gases into amino acids (glycine and alanine) when properly coated with aminotris(methylenephosphonic acid) (ATMP) and zirconyl chloride (ZC). [14,16] Moreover, CO 2 :CH 4 gas mixture (1 : 1) was converted into organic molecules in presence of p-HAp.…”

Polarized Hydroxyapatite: New Insights and Future Perspectives Through Systematic Electrical Characterization at the Interface

“…Interestingly, n →1 for all circuit elements of the two the HAp/tsp samples, which is in complete agreement with the recently proposed polarization mechanism. [ 3 ] Indeed, the fact that the charge carriers are delocalized over different crystal domains reduces the capacitive imperfections due to grain boundaries, leading to an ideal capacitor behavior. For this reason, C elements have been used instead of CPE for HAp/tsp.…”

“…Synthetic hydroxyapatite (HAp), Ca 5 (PO 4 ) 3 (OH), with enhanced electrical properties has been extensively explored. [ 3–5 ] As its open crystalline structure favors ion exchange, the incorporation of dopants into HAp is the most widely used approach to enhance the electrical properties of this ceramic. Thus, HAp‐doped composites with improved osteoblast capacity, [ 6 ] anisotropic osteogenesis (by inducing a piezoelectric behavior), [ 7 ] and antibacterial response [ 8 ] have been reported.…”