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.