Ti-Modified Hydroxyapatites: Synthesis, Crystal Chemistry, and Photocatalytic Activity

The structural nature of photocatalytic properties of hydroxyapatites (HAp) synthesized from Ti-containing media (of Ti-modified hydroxyapatites) needs clarification. We synthesized hydroxyapatites from Ti-containing water solutions under various conditions and studied the received powder precipitations (before and after calcination at a temperature of 700 °C for 6 h) by a wide set of methods: powder X-ray diffraction (PXRD); vibrational, energy-dispersive X-ray, X-ray photoelectron, and diffuse reflectance spectroscopy; scanning electron microscopy, and gas phase photocatalytic activity tests. Analyzing the variations of unit cell parameters, we have proved that titanium is able to incorporate into the hydroxyapatite lattice in amounts of up to ∼10 wt % depending on synthesis conditions. At low concentrations (Ti/Ca ≤ 0.16), Ti⁴⁺ ions incorporate predominantly to the P site of hydroxyapatite structure. At higher concentrations (0.16 ≤ Ti/Ca ≤ 0.28), additional Ti⁴⁺ ions incorporate Ca sites. Besides, a significant share of titanium forms predominantly amorphous impurity phases, which transform into crystalline TiO₂ (anatase and rutile) upon calcination and are often undetectable by PXRD. The presence of crystalline titanium oxides (anatase and rutile) has a crucial effect on photocatalytic activity of calcined Ti-modified hydroxyapatites. Synthesized HAp/TiO₂ compositions are biocompatible heterostructured materials with photocatalytic activity comparable to that of commercial photocatalysts and could be recommended for medical–biological applications.