Acid-base properties of hydroxyapatite(0001) by the adsorption of probe molecules: An ab initio investigation

The presence of both acidic and basic adsorption sites on the surface of hydroxyapatite [${\mathrm{Ca}}_{10}{\left({\mathrm{PO}}_4\right)}_6{\left(\mathrm{OH}\right)}_2$; HAP] is an interesting property for catalytic applications. Here, we report a density functional theory investigation of the adsorption properties of $\mathrm{CO}, {\mathrm{CO}}_2, {\mathrm{C}}_2{\mathrm{H}}_2, {\mathrm{CH}}_4, {\mathrm{H}}_2, {\mathrm{H}}_2\mathrm{O}, {\mathrm{NH}}_3, {\mathrm{SO}}_2$, and ${\mathrm{BCl}}_3$ on the $\mathrm{HAP}\left(0001\right)$ surface. All probe molecules have a lower energy when they are adsorbed in the region between the most exposed ${\mathrm{Ca}}^{2+}$ ion (electron acceptor) and a neighboring $\mathrm{PO}{{}_4}^{3-}$ group, where the $\mathrm{O}$ atoms (electron donor) contribute to the stabilization of the adsorbed molecule. By evaluating the redistribution of the electron density and the change of the atomic charges, the ${\mathrm{Ca}}^{2+}$ and $\mathrm{PO}{{}_4}^{3-}$ sites were identified as Lewis acidic and Lewis basic adsorption sites, respectively, which indicates that simultaneous acid-base interactions occur upon adsorption of all studied probe molecules. All adsorbates interact with the surface via atoms of opposing charges, which enhances the ionic character of molecular bonds by increasing the distinction between cationic and anionic charges within the molecule. Furthermore, molecules with greater ionic character show stronger interaction with the substrate and greater geometric deformation. Although most adsorbed molecules ($\mathrm{CO}, {\mathrm{CO}}_2, {\mathrm{C}}_2{\mathrm{H}}_2, {\mathrm{CH}}_4, {\mathrm{H}}_2, {\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{NH}}_3$) do not show substantial net charge transfer, polarization effects due to the redistribution of charge are observed upon adsorption of all probe molecules. The change in the work function increases linearly with the total change in the surface dipole moment for ${\mathrm{H}}_2\mathrm{O}, {\mathrm{NH}}_3, {\mathrm{SO}}_2$, and ${\mathrm{BCl}}_3$, while for the remaining systems, the magnitude of the work function change remains more uniform. By identifying the type of interaction between each probe molecule and the $\mathrm{HAP}\left(0001\right)$ surface, the present study contributes to the understanding of the acid-base properties of the $\mathrm{HAP}\left(0001\right)$ surface, which we elaborated in a short discussion based on the individual bond orders for the acidic and basic sites.

Figure 1

Bulk phases of stoichiometric hydroxyapatite, namely, the $P{2}_1/b$ monoclinic and $P{6}_3$ hexagonal phases. The side view of each phase emphasizes the orientation of the ${\mathrm{OH}}^{-}$ groups along the [0001] direction. Additional details on the crystallographic positions for the $P{6}_3$ structure are provided in the SM [34], Sec. $\mathrm{S}4$.

Figure 2

Orientation of the ${\mathrm{OH}}^{-}$ groups across the $\mathrm{HAP}\left(0001\right)$ slab model. The total energy relative to the lowest-energy configuration, $\mathrm{\Delta }{E}_{\mathrm{tot}}$, is given below each model. The work functions of the top and bottom surfaces are indicated as ${\mathrm{\Phi }}_{\mathrm{top}}$ and ${\mathrm{\Phi }}_{\mathrm{bottom}}$.

Figure 3

Schematic side view of HAP layers, in which $d_{\perp }^i$ indicates the interlayer distance relative to the chemical species $i=\{\mathrm{Ca},\mathrm{P},\mathrm{O},\mathrm{O}\left(\mathrm{H}\right)\}$ in the substrate.

Figure 4

Optimized lowest-energy configurations of adsorbed molecules on $\mathrm{HAP}\left(0001\right)$. The adsorption energy, $E_{\mathrm{ad}}$ (eV), is given below each structure. To facilitate visualization, the oxygen atoms in the molecules are depicted as light-green spheres.

Figure 5

Electron density difference isosurfaces ($0.01e{\AA }^{-3}$) for the lowest-energy adsorption configurations. Yellow and blue regions indicate accumulation and depletion of charge, respectively. $\mathrm{\Delta }{\rho }_z \left(e{\AA }^{-1}\right)$ is the plane-average electron density along the direction perpendicular to the surface, $z$ direction. The dashed red and blue lines indicate, respectively, the center of mass of the adsorbed molecule and the position of the most exposed ${\mathrm{Ca}}^{2+}$ on $\mathrm{HAP}\left(0001\right)$.

Figure 6

Relationship between work function and surface dipole changes. Dashed lines indicate the linear regression considering only the values for ${\mathrm{H}}_2\mathrm{O}, {\mathrm{NH}}_3, {\mathrm{SO}}_2$, and ${\mathrm{BCl}}_3$ adsorption systems, shown in blue. Solid lines show the effect of different surface coverages on the linear regression slope. Yellow, orange, and red lines represent $\mathrm{\Theta }=0.10$, 0.20, and 0.30 ML, respectively.

Figure 7

Local density of states (LDOS) averaged per atom for the lowest-energy adsorption structures. Blue, red, and pink lines refer, respectively, to vacuum-exposed $\mathrm{Ca}, \mathrm{O}$, and $\mathrm{P}$ atoms on the $\mathrm{HAP}\left(0001\right)$ surface, while black and green lines refer to atoms within the adsorbed molecules. The dashed vertical lines indicate the Fermi energy.