Revealing Hydroxyapatite Nanoparticle Surface Structure by CO Adsorption: A Combined B3LYP and Infrared Study

The adsorption of CO at hydroxyapatite (HA) surfaces has been studied by combining quantum mechanical modeling with experimental IR results. To model the adsorption, the hybrid B3LYP-D*, inclusive of dispersive interactions, has been adopted within the periodic boundary conditions, using the CRYSTAL09 program and a polarized Gaussian type basis set. Four HA surfaces have been investigated using slabs of finite thickness: two stoichiometric HA(001) and HA(010)R surfaces and two nonstoichiometric HA(010) in which the value of the Ca/P ratio was either higher (HA(010)_Ca-rich) or lower (HA(010)_P-rich) than the bulk value. Geometrical, energetic, and vibrational features of the adsorption process have been fully investigated, by considering CO coverage ranging from 1.5 to 6 CO/nm(2), respectively. By combining the results from the modeling study with experimental IR data, it was assessed that the vibrational features of adsorbed CO can be proposed as a potential tool for the recognition of types of surface terminations exposed by HA crystalline nanoparticles.