**4. Conclusion**

of the Ag-TiO<sup>2</sup>

120 Photocatalysts - Applications and Attributes

Aramendia et al. [38] studied on the TiO<sup>2</sup>

of BPA on TiHAp and TiO<sup>2</sup>

than those of TiO<sup>2</sup>

TiHAp, and TiO<sup>2</sup>

The bandgap of TiO<sup>2</sup>

while that of pure TiO<sup>2</sup>

TiHAp, HAp, and TiO<sup>2</sup>

tase-TiO<sup>2</sup>

lization and creates a certain interaction with titanium.

state that forms the valence band of HAp-containing PO<sup>4</sup>

mental endocrine disrupting chemical) over TiHAp and P25 TiO<sup>2</sup>


/natural phosphate material synthesized by sol-gel

group.

obeyed the Langmuir adsorption equation, and the adsorption

. To explain for the results, the authors presented that the zeta potentials of

/HAp materials were mentioned in some publications because it

was broader (3.12 eV) [44]. However, the bandgap of Ti-substituted

powder measured by diffuse reflectance spectroscopy were 3.65,

did not show significant difference when the pH was about seven, suggest-

crystal-

/HAp

photocatalysts were studied

/HAp material synthesized by

and HAp gels. The binding energy values

region, low recombination rate of the electron-hole pair, and large BET-specific surface area.

The results show that the presence of natural phosphate makes a retardation of TiO<sup>2</sup>

process and tested the photocatalytic activity in the photo-oxidation process of propan-2-ol.

Nishikawa et al. [39] indicated that the difference in photocatalytic activity of the materials containing HAp and without HAp may be originated from the properties of the electronic

Li et al. [40] synthesized Ti-substituted hydroxyapatite (TiHAp) by the coprecipitation method. The adsorption and photocatalytic degradation of bisphenol A (BPA, an environ-

using liquid chromatography-mass spectrometry. The results indicated that the adsorption

capacity and photocatalytic degradation activity of BPA of TiHAp material were much higher

ing that the surface charge was not the reason for the different adsorption capacities of the particles. In addition, the specific surface area and average pore diameter of TiHAp and TiO<sup>2</sup> were comparable, so these would not lead to the different adsorption capacities either. In fact, TiHAp material is produced from a substitution of some Ca sites in HAp by Ti, which resulted in multiple Ti-OH groups on the TiHAp surface. Large amounts of phosphates and hydroxyls

decides the energy separation between valence and conduction bands, the quantum effect, as well as the effect of visible-light utilization, etc. The bandgap of HAp was reported in [41] to be 3.95 eV by photoluminescence measurement; meanwhile, in other reports, the bandgap of HAp was calculated to be around 4.51–5.4 eV [42, 43]. The bandgap of TiO<sup>2</sup>

composites calculated by UV-vis diffuse reflectance spectra was between 3.06 and 3.08 eV

hydroxyapatite evaluated by both experimental and theoretical methods was 3.65 eV [45]. Masato Wakamura et al. determined the effect of Ti substitution in HAp synthesized by the coprecipitation method on the bandgap which was compared with that of a typical ana-

>6, and 3.27 eV, respectively. The authors explained the increase of bandgap in TiHAp due to hybridizing of Ti 3d orbital with O 2p orbital and forming an internal state in the HAp bandgap, consequently, causing the absorption-edge lowering of TiHAp. Researching on the acetaldehyde gas decomposition of TiHAp by UV with VIS irradiation, the authors investigated the increase of activity comparing with when UV irradiation alone was used.

of Ca 2p, P 2p, and O1s levels are related to hydroxyapatite phase, whereas those of Ti 2p

Linh et al. [46] also indicated the shift of bandgap of TiO<sup>2</sup>

the hydrothermal process containing TiO(OH)<sup>2</sup>

photocatalytic powder. The experimentally obtained optical bandgap energies of

in the crystal lattice of TiHAp can adsorb the hydroxyls of BPA by hydrogen bonding.

In the photocatalytic TiO<sup>2</sup> /HAp composites, HAp plays the role of an adsorption of contaminations which is generally influenced by the physicochemical properties of HAp material. On the other hand, the promoted impacts of hydroxyapatite in the composites are experimentally collected in the specific cases including the absorption of radiation in the visible-light region, the small recombination rate of the electron-hole pair, and the high surface area of HAp component. However, the role of HAp in TiO<sup>2</sup> /hydroxyapatite photocatalytic materials has been continuously researched and evaluated to obtain the best composite in the photocatalytic field, as well as in other applications.
