Oxygen vacancies, the optical band gap (Eg) and photocatalysis ofhydroxyapatite: Comparing modelling with measured data

Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) is a calcium phosphate employed both in biomedicine and forenvironmental remediation. It is known that HAp can also be photocatalytic under UV light, probablydue to oxygen deficiencies, but the mechanism is unclear, and reported optical band gap energies varygreatly. For the first time we propose the mechanisms and precise kinds of vacancies which may causethe photocatalytic activity of HAp, and compare these theoretical data with our measured data on bothsamples of marine origin and commercial HAp powders. Density functional theory (DFT) (from firstprinciples calculations and Density of States (DOS) modelling) was used to calculate the optical bandgap energy (Eg) created by various possible oxygen vacancies in the HAp lattice: O from PO4, O fromOH, the loss of an entire OH group, or the simultaneous loss of O from PO4and an entire OH group. Themodelled values match the measured values very closely, suggesting that in non-photocatalytic HAp, ifany vacancies exist, they are O atoms from the OH group, resulting in a band gap of ∼5 eV in the UVCregion (not present in solar light at the Earth’s surface). However, in photocatalytic HAp, reduction fromthe combustion of an organic component at 1000◦C led to oxygen deficiency in the phosphate groups,probably in the O15 position, giving an Egof ∼3.45 eV, in the UVA region (present in sunlight). HeatingHAp with no organic component to 1200◦C also led to vacancies, of both an entire OH group and oxygenfrom PO4groups, which led to an intermediate Egvalue of ∼4 eV, on the boundary of the UVA-UVB regions.Absorption peaks were also predicted in the visible-light region with some types of vacancy.