Photo-induced formation of hydroxyapatite on TiO2 synthesized by a chemical–hydrothermal treatment

The formation of hydroxyapatite (HAp) on TiO2 surfaces under continuous ultraviolet (UV) irradiation was investigated. Pure Ti substrates were chemically treated with H2O2/HNO3 at 353 K for 20 min to form a TiO2 gel layer. The specimens were then hydrothermally treated with an aqueous NH3 solution in an autoclave at 453 K for 12 h. An adhesive and sufficiently crystallized anatase-type TiO2 film could be synthesized on the Ti surface. The specimens were immersed in simulated body fluid in darkness or under UV irradiation with a centered wavelength of λ = 365 nm. Under dark conditions, a thin homogeneous HAp film was formed, with just a few spherical clusters of HAp. The UV illumination promoted the formation of HAp clusters, which may be due to the generation of functional Ti–OH or Ti–O groups on the TiO2 surface. On the other hand, the UV light produced electron-hole pairs in the TiO2, and the photogenerated holes that migrated to the surface repelled the Ca2+ ions in the solution. As a consequence, the UV irradiation suppressed the formation of a HAp thin film.