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.