Surface observation of nanotube-formed titanium by anodization in electrolyte containing hydroxyapatite nanoparticles

In this work, surface observation of nanotube-formed titanium by anodization in an electrolyte containing hydroxyapatite nanoparticles was carried out using FE-SEM and analyzed by XPS in order to determine changes in the chemical states of elements through depth profiling. The anodized layer consisted of three regions: (1) a TiO₂ layer adjacent to titanium_, (2) an intermediate barrier layer of TiO₂ nanotubes, and (3) a surface hydroxyapatite [HAp, Ca₁₀(PO₄)₆(OH)₂] layer. The nanotube-formed surface consisted of the anatase TiO₂ phase. The HAp layer that formed on the TiO₂ nanotube film grew vertically, and the electrodeposited HAp nanoparticles were distributed on the whole surface. From comparative analyses of the depth profile before Ar⁺ ion sputtering and after 4 keV Ar⁺ ion sputtering, the multilayer contained Ti, O, Ca, and P, and showed no noteworthy difference. The Ti-2p, O-1s, Ca-2p, F-1s, C-1s, and N-2s transitions were continuously detected in the multilayer as the sputtering time increased. The dominance of the Ti-2p and O-1 s peaks, both before and after the Ar⁺ ion sputtering, shows that the surface consists mainly of an oxidized TiO₂ layer. The Ca-2p and P-2p peaks were subsequently detected from HAp nanoparticles electrodeposited onto the TiO₂ nanotube film during an electrochemical reaction. The chemical states of calcium and phosphate in the multilayer were similar to those in HAp.