Researches Of Precipitation Process Of Calcium Ions On Rutile (110) Surface With First-principles

When hard tissue functional substitutes of implant materials, such as titanium alloys are exposed to the body fluid bone hydroxyapatite is formed on their surface. However, the corresponding mechanisms of apatite nucleation and growth on substrates are not well understood. Recently the studies of deposition of hydroxyapatite in simulated body fluid show that the formation of hydroxyapatite is mainly due to the deposition of Ca²⁺ on substrates. The deposited Ca²⁺ combines with the group PO43- and formed amorphous structure. Finally it will form the hydroxyapatite. The microstructure of bone apatite is similar to the hydroxyapatite.The process of nucleation and growth is similar to that to the hydroxyapatite. Due to the fact that the passivation TiO₂ on implanted titanium alloy substrates, the deposition of Ca²⁺ is deposited on the TiO₂ surface. Therefore the investigation of Ca²⁺ deposited on TiO₂ surface is of great importance to the understanding of the mechanism and formation of bone apatite and hydroxyapatite on substrates.First, the experiment prepared the rutile TiO₂ GXRD analysis reveals that the (110) surface is the prior surface. The first-principles theory and program were used to compute surface energy of each surface of rutile TiO₂. The results show that rutile TiO₂ surface is the lowest energy surface. Based on the results we confirm that the rutile TiO₂ (110) is the surface on which Ca²⁺ is deposited.In this paper, the density functional theory (DFT), adopting CASTEP program package, the first-principles calculations using plane wave ultra-soft pseudo-potential and using of geometry optimization were used to analyze the precipitation of calcium ions on rutile (110) surface. The following results are obtained:(1) The Ca²⁺ deposited on the TiO₂ (110) surface. TiO₂ (110) surface is the lowest energy. The hydrated surface of TiO₂ (110) is a key factor of Ca²⁺ deposition.(2) The deposition on hydrated rutile (110) surface occurs after the complex of [Ca(H2O)5]2+ forms. The computation proved the complex of [Ca(H2O)5]2+ steadily exists in solution.(3) The deposition of Ca²⁺ on hydrated TiO₂ (110) surface is not spontaneous at thermodynamics. The drive force of deposition is the static gravitation of [Ca(H2O)5]2+ and hydrated TiO₂ (110) surface. During the deposition process [Ca(H2O)5]2+ is close to the hydrated TiO₂ (110) surface. The H2O molecule get out of the [Ca(H2O)5]2+ and entered the solution. The Ca²⁺ of the [Ca(H2O)5]2+ is separately combined to the bridge hydroxyl and terminated oxygen. Finally the [Ca(H2O)5]2+ is deposited on the TiO₂ (110) surface.