FTIR studies on photocatalytic activity of Ti(IV)-doped calcium hydroxyapatite particles

Abstract

The photocatalytic activity of Ti(IV)-doped calcium hydroxyapatite (TiHap) particles with atomic ratio of Ti/(Ti+Ca) [XTi] from 0 to 0.20 prepared by a coprecipitation method, called as original particles, were examined by in situ Fourier-Transform IR (FTIR) measurement. The heat treated TiHap particles were also used after treated original particles at 650°C for 1h, called as heat treated particles. All the original particles were made up by rod-like particles with irregular size, while the heat treated particles were rounded by sintering and their sizes were enlarged in each XTi system. The TiHap particles were cation deficient particles with (Ca+Ti)/P atomic ratio ranging between 1.54 and 1.64. The XTi values of TiHap particles assayed showed approximately the dosage of Ti(IV) ions in each system. All the TiHap particles employed exhibited UV absorption below ca. 370nm, though the UV absorption characteristics did not vary by the heat treatment. However, the pure calcium hydroxyapatite with XTi=0 had no UV absorption. The IR spectra of the particles with XTi≥0.05 exhibited a new large peak at 3428cm−1 close to the peak at 3570cm−1 of OH− ions together with the bands at 3654–3682 or 3673 (surface POH groups) and 3737cm−1 (surface TiOH groups). The deuteration method revealed that the new band at 3428cm−1 could be assigned to the dislocated OH− ions by substituting Ti(IV) ions. The expansion of crystal structure along with c direction by the heat treatment was identified by the unit-cell dimension obtained from the XRD patterns. It was concluded therefore that the produced dislocated OH− ions by the heat treatment induce the photocatalytic activity of TiHap particles. The strong photocatalytic activity of TiHap particles was attributed to the dislocated OH− ions because they can produce OH radicals in the particles without water molecules by UV irradiation. Furthermore, FTIR and XRD measurements elucidated that the optimum temperature for inducing photocatalytic activity of TiHap is ca. 600°C.