X-Ray Photoelectron and Ion Scattering Spectroscopic Surface Analyses of Amorphous and Crystalline Calcium Phosphate Nanoparticles with Different Chemical Histories
Abundant hydration, diffusivity and volatile phase composition render the surface of calcium phosphates (CPs) a complex dynamic phenomenon. Three CP powders were analyzed using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) for their surface composition and fine structure: hydroxyapatite (HAp), amorphous CP (ACP) and a CP formed under the same conditions as ACP, but allowed to ripen into HAp (ACP→HAp). The XPS analysis showed that Ca/P atomic ratio in 2-10 nm thick layers of the nanoparticle surface was somewhat lower than the stoichiometric for all three CPs, but still lower for ACP and ACP→HAp than for HAp, indicating incongruent dissolution of all surfaces, but also a greater instability and higher solubility of ACP and ACP→HAp compared to HAp. Consequently, as indicated by both XPS and ISS, the amount of adventitious carbon bound to HAp was higher than on ACP or ACP→HAp. The binding energies of the most intense XPS lines of all three main atomic elements in HAp - O1s, P2p and Ca2p - were downshifted in HAp compared to the same binding energies in ACP and ACP→HAp, supporting the premise of similarity in the surface structure between the two CP powders with a common amorphous precursor and kinetic path of formation. The ISS analysis, which probes 1-2 topmost atomic layers of the surface, indicated a higher level of heterogeneity of oxygen states. This coincided with 40Ca accounting for over 80 % of atoms in this uppermost atomic layer of the surface of all three CPs. Ca-O bonds were particularly dominant in the topmost surface layer of ACP, where Ca/P atomic ratio was by an order of magnitude higher than in ACP→HAp or HAp. A thermodynamic explanation and a structural model of the surface, accounting for the overabundance of Ca2+ ions in the topmost layer and the overall depletion of it elsewhere, are provided in the discussion. The results of combined XPS and ISS analyses demonstrate a similarity between the surface of two different forms of HAp compared to ACP, but also a definite impression of the traces of the history of their formation on it.