XPS and STM study of TiO 2 (110)-(1 × 1) surfaces immersed in simulated body fluid

Abstract

Rutile titanium dioxide (TiO2) ( 110)-(1×1) surfaces prepared in ultrahigh vacuum (UHV) were removed from the UHV, immersed in Hank's buffered salt solution (HBSS), and then re-introduced to the UHV to be examined for an atomic-level study of the osteoconductivity of TiO2. X-ray photoelectron spectroscopy (XPS) showed the adsorption of phosphate ions and divalent Ca ions from the HBSS. The density of the phosphate ions increased with the immersion time for up to 1 min and remained constant with further immersion time up to 1 week. Unlike in the case of the phosphate ions, the density of the Ca ions decreased with the immersion time after reaching the maximum value at 1 min immersion. Consequently, calcium phosphate was not formed on the (1×1) surface in the 1-week immersion. Scanning tunneling microscopy revealed molecule-sized spots covering the surfaces immersed for 1 min and longer. On the basis of density analysis by XPS, the spots were hypothesized to represent phosphate ions. Phosphate ions on the (1×1) surface are unable to undergo arrangement needed to facilitate the epitaxial growth of the brushite.

Dependence of calcium phosphate formation on nanostructure of rutile TiO2(110) surfaces

“…Such independence of the chemical state of the surface Ti atoms from adsorbates has been found for phosphate ions 27) as well as trimethyl acetic acid 29) , methanol 30) , and malonic acid molecules 31) .…”

“…That the peak was fitted with a single pair of spin-orbit split peaks indicates that the P atoms were present on the air-annealed surfaces in an identical chemical state. The elevation at 531.7 and 533.5 eV in the O 1s region spectra is attributed to the adsorption of phosphate ions 27) . The phosphate ions are coordinated to Ti atoms exposed on the airannealed surface 28) .…”

“…The brushite, which has been reported as a precursor of the hydroxyapatite, is a more likely candidate for calcium phosphate formed on the TiO2(110) surface 9,33,34) . The increase in CC-O peak intensity relative to the CC-C/C-H peak intensity is attributed to the adsorption of glucose 27) . Glucose is coordinated to the surface of TiO2 nanoparticles in monodentate or bridging form via the Ti-O bond 35) .…”

“…The formation of calcium phosphates on the air-annealed surface contrasts with the (11) surface prepared in UHV. Calcium phosphate was not formed on the (11) surface even after 1 week immersion in HBSS 27) . The scanning tunneling microscopy images revealed that phosphate ions were packed to form a monolayer with a (21) periodicity with respect to the (11) structure.…”

“…Thus, the lower density of OH groups on the O2-annealed surface may appear to explain the suppression of calcium phosphate formation on it. However, phosphate ions can be adsorbed on the (11) surface along Ti5c atom rows regardless of the OH groups 27) . Therefore, we could reasonably conclude that the structural change is responsible for the suppression of calcium phosphate formation.…”