Photocatalytic Activity of Titania Nanotube Coatings Enriched With Nanohydroxyapatite

Hydroxyapatite (HA) has the similar chemical composition as bone tissues mineral phase and therefore it attracts the interest of the scientific community in the medicine, material science, tissue engineering and implantology [1-4]. In my earlier investigations I used it to enrich the nanotube coating (TNT) on implants made of titanium alloy [5]. The inserting of TiO2 inner layer between very thin HA coating and Ti6Al4V substrate, the adhesion strength increase of the coating surface was possible [5,6]. Moreover, the addition of TiO2 inner layer should prevent the corrosion of the Ti alloy substrate and lead to achieve a surface free of cracks. The use of HA in the TNT/HA system allows also on the production of materials, which have a large surface area and high adsorption ability. In this application HA plays the role of a transparent or semi-transparent material to allow UV and visible radiation to pass through it and reach the true photocatalyst, which is TiO2 [7]. Biological and chemical molecules adsorption on the HA surface generally depends on its crystallite size and morphology of particles or coatings, which are in turn dependent on synthesis methods. Nonami et al. [8] soaked TiO2 powder in a simulated physiological solution containing phosphate ions for 1 h at 37°C. The 0.7 μm apatite film, consisting of crystals with plate-like shape, has formed on the approximately 0.3 μm thick TiO2 layer and in this form, it could adsorb contaminants without exposure to light. Then the contaminants were decomposed by TiO2 photocatalyst on exposure to light. Such photocatalytic TiO2/HA composites can be used to the air purification or play the role of the antimicrobial and antifungal coating with HA absorber. A facile two-step approach, which involves sol–gel process and calcination let to obtain Ag-TiO2/HA/Al2O3 bioceramic composite membrane was reported by Ma et al. [9]. In this composite membrane, HA component acted as a highly efficient bacterial adsorbent, while Ag-TiO2 provided powerful photocatalytic activity Escherichia coli. TiO2 /HA composite with mosaic structure was fabricated by Xie et al. [10] via a facile route without any structure-directing agent. Their studies results proved that increased photocatalytic activity of the composite resulted from the combination of adsorption capacity of HA and the high photocatalytic activity of TiO2 Liu et al. [11] synthesized HA-modified N-TiO2 composite using by wetchemical method and evaluated its photocatalytic activity by the decomposition of gaseous acetone under visible-light irradiation.


Introduction
Hydroxyapatite (HA) has the similar chemical composition as bone tissues mineral phase and therefore it attracts the interest of the scientific community in the medicine, material science, tissue engineering and implantology [1][2][3][4]. In my earlier investigations I used it to enrich the nanotube coating (TNT) on implants made of titanium alloy [5]. The inserting of TiO 2 inner layer between very thin HA coating and Ti6Al4V substrate, the adhesion strength increase of the coating surface was possible [5,6]. Moreover, the addition of TiO 2 inner layer should prevent the corrosion of the Ti alloy substrate and lead to achieve a surface free of cracks. The use of HA in the TNT/HA system allows also on the production of materials, which have a large surface area and high adsorption ability. In this application HA plays the role of a transparent or semi-transparent material to allow UV and visible radiation to pass through it and reach the true photocatalyst, which is TiO 2 [7]. Biological and chemical molecules adsorption on the HA surface generally depends on its crystallite size and morphology of particles or coatings, which are in turn dependent on synthesis methods. Nonami et al. [8] soaked TiO 2 powder in a simulated physiological solution containing phosphate ions for 1 h at 37°C.
The 0.7 μm apatite film, consisting of crystals with plate-like shape, has formed on the approximately 0.3 μm thick TiO 2 layer and in this form, it could adsorb contaminants without exposure to light. Then -the contaminants were decomposed by TiO 2 photocatalyst on exposure to light. Such photocatalytic TiO 2 /HA composites can be used to the air purification or play the role of the antimicrobial and antifungal coating with HA absorber. A facile two-step approach, which involves sol-gel process and calcination let to obtain Ag-TiO 2 /HA/Al 2 O 3 bioceramic composite membrane was reported by Ma et al. [9]. In this composite membrane, HA component acted as a highly efficient bacterial adsorbent, while Ag-TiO 2 provided powerful photocatalytic activity Escherichia coli. TiO 2 /HA composite with mosaic structure was fabricated by Xie et al. [10] via a facile route without any structure-directing agent.
Their studies results proved that increased photocatalytic activity of the composite resulted from the combination of adsorption capacity of HA and the high photocatalytic activity of TiO 2 Liu et al. [11] synthesized HA-modified N-TiO 2 composite using by wetchemical method and evaluated its photocatalytic activity by the decomposition of gaseous acetone under visible-light irradiation.
The results of these investigations revealed that the best photocatalytic activity was found for the 10%-HA-N-TiO 2 sample. This activity may arise from the synergism of adsorption ability of HA and titania photocatalytic activity manifested in generation

ARTICLE INFO abstract
Photocatalytic activity of titania nanotube coatings (TNT) enriched with nanohydroxyapatite (HA) has been studied on the basis of UV-induced degradation of methylene blue (water-soluble pollutant pattern). The impact of the TNT/HA composites morphology on their photocatalytic properties has been determined. The results of my works proved that in all studied TNT/HA samples hydroxyapatite increased the photocatalytic activity of titania nanoporous coatings. Obtained results pointed out the synergistic activity of HA nanocrystals deposited on the surface of titania nanotubular systems.
Keywords: Titania Nanotubes; Hydroxyapatite; Photocatalytic Activity; Methylene Blue Photodegradation; Morphology of oxygen-reactive species, their diffusion and reaction with the acetone molecules located on the HA. Beside the role as an adsorption material in the photocatalytic TiO 2 /HA composites, some recent studies showed that hydroxyapatite promotes photocatalytic degradation, supporting the real photocatalyst [7]. The activity of HA is assigned to the generation of active superoxide anion radicals (O 2˙− ) due to a change, which has place in the electronic state of the surface PO 4 3group under UV irradiation [12]. Mohamed et al. [13] studied photocatalytic activity of Pd-TiO 2 -HA composites, using applications of the TNT/HA system have been discussed in my earlier work [5]. The proven photocatalytic activity of these systems, which is presented here, will be their additional advantage, which can be used for example in the processes of surface sterilization of implants (possessing a surface layer in the form of a TNT/HA nanocomposite), using UV radiation.

Photocatalytic Activity Test
The photocatalytic activity of the produced titania coatings

Results and Discussion
Using the anodic oxidation of Ti6Al4V samples in 0.3 wt% aqueous HF solution it was possible to obtain nanotubular coatings The SEM image shows, that the density of the hydroxyapatite nanocrystals on the titanium alloy sample is the lowest (Figure 2).
According to received results, the change of the TNT coating surface nanoarchitecture (and also tubes diameter), as it has place in the case of titania dioxide nanotubes produced at different potentials, influences on the distribution density of produced hydroxyapatite nanocrystals. As the structure of the analyzed systems has been described in detail [5] and the applied synthesis procedure has not been changed -here I do not present structural studies, however I would like to point out that the titanium dioxide nanotubes obtained by anodic oxidation method of the titanium alloy are amorphous and the hydroxyapatite is in a crystalline form.     The same dependency is visible for Ti6Al4V and Ti6Al4V/ HA samples -also in this case the enrichment of the titanium alloy surface with HA nanocrystals increases its photocatalytic activity. The degree of methylene blue degradation, induced by UV irradiation, increased more than eight times for Ti6Al4V/ HA system in comparison to pure titanium alloy substrate. The studies of Ti6Al4V and Ti6Al4V/HA samples confirmed the clear photocatalytic activity of HA nanocrystals, which is in agreement with literature data [19]. The weak, but noticed, photocatalytic activity of Ti6Al4V sample can be attributed to the formation on its surface the natural titanium oxide passivation layer, which in fact is responsible for photocatalytic activity [20]. Among studied systems TNT/HA, the highest photocatalytic activity exhibited the TNT30/HA sample, i.e. the system formed by TNT layer (composed of nanotubes of biggest diameter, c.a. 140 nm) not fully covered by HA nanocrystals. In this case, the larger nanotubes diameter allows to get analysed solution into tubular interior, and thus into the interior of the layer, increasing the efficiency of MB decomposition.

Conclusion
In all studied samples the same relationship was noticed, i.e.
the clear increase of photocatalytic activity of TNT coatings covered by HA nanocrystals, in MB degradation. This can be explained by a synergistic acting of HA nanocrystals deposited on the surface of titania nanotubular systems. In this case, hydroxyapatite, which possess high adsorption ability, act as: a) Material allowing UV radiation to pass through it and reach the true photocatalyst, which is TiO 2 nanotube coating, as well as b) Real photocatalyst, which generates active entities, e.g.