Electronic Structure of Ti substituted hydroxyapatite: TiHap

Hydroxyapatite (Hap), with the chemical formula Ca$_{10}$(PO$_{4}$)$_{6}$(OH)$_{2}$, is the main mineral constituent of mammal tooth enamel and bone and has become an important biomaterial with medical applications. Hap also attracts increasing interest for use in environmental adsorbents and catalysts due to its porous nature and highly active ion-exchange character. Ti-modified Hap (TiHap) has been proved to possess high affinity to organic molecules and bacteria as well as high photocatalytic activity for their oxidative decomposition. The concentration of Ti$^{4+}$ is a key factor controlling TiHap crystallinity and catalytic efficiency. Here we studied the sorption mechanism of Ti$^{4+}$ on Hap using Density Functional Theory within periodic slab models. Ti$^{4+}$ or (Ti(OH)$_{2}$)$^{2+}$, as the most likely ion exchange species with Ca$^{2+}$, were first considered in bulk Hap. A second charge compensated model considered includes both surface Ca ion vacancies and substitutional Ti$^{4+}$. To obtain insight of the energetic stability and microscopic crystal structure of TiHap, Ti substitution on different Ca sites and distributions at different atomic ratios are investigated in both bulk and surface models.