Enhancing Titanium Stability in Fusayama Saliva Using Electrochemical Elaboration of TiO 2 Nanotubes

Titanium having low density, stability and biocompatibility, is one of the most promising biomaterial of the century even with the natural passive stratum [1,2], but building a nano-tube structure in the last decade using various procedure could lead also to an improvement of a quite large range of properties important in applied chemistry. The aim of the paper is to elaborate electrochemically TiO 2 nanotubes and to evaluate the stability increase of Titanium in Fusayama saliva changing the surface morphology from micro TiO 2 to TiO 2 nanotubes. Anodization at room temperature in a mixture of (NH 4 ) 2 SO 4 and NH 4 F, was the choice of nanotubes structures elaboration, and cyclic voltammetry was the procedure of stability evaluation. The surface analysis was performed using scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDAX), and atomic force microscopy before and after anodization.

Titanium having low density, stability and biocompatibility, is one of the most promising biomaterial of the century even with the natural passive stratum [1,2], but building a nano-tube structure in the last decade using various procedure could lead also to an improvement of a quite large range of properties important in applied chemistry.The aim of the paper is to elaborate electrochemically TiO 2 nanotubes and to evaluate the stability increase of Titanium in Fusayama saliva changing the surface morphology from micro TiO 2 to TiO 2 nanotubes.Anodization at room temperature in a mixture of (NH 4 ) 2 SO 4 and NH 4 F, was the choice of nanotubes structures elaboration, and cyclic voltammetry was the procedure of stability evaluation.The surface analysis was performed using scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDAX), and atomic force microscopy before and after anodization.
Keywords: TiO 2 nanotubes, Fusayama Saliva, cyclic voltametry, atomic force microscopy Titanium and its alloys are well-established biomaterials, successfully used in the biomedical domain, for the fabrication of implants due to their low density and good mechanical properties combined with their biocompatibility [1][2].
The high degree of biocompatibility of Ti alloys is ascribed to their ability to form stable and dense oxide layers consisting mainly of TiO 2 .The native oxide layers on Ti are usually 2-5 nm thick and are spontaneously rebuilt in most environments whenever they are damaged due to an aggressive pH or mechanical factors [3,4].In order to enhance oxide layer stability in bioliquids various procedures were elaborated [5,6] and tested [7,8] and building a TiO 2 nanostructure was one of the most recently one [9,10].Having, stability and biocompatibility, titanium is one of the most promising biomaterial of the century even with the natural passive stratum [3,4], but building a nano-tube structure could lead either to a improvement of a quite large range of properties important in tissue engineering as changing of hydrophilic -hydrophobic balance, changing nano-porosity and increasing of biocompatibility etc [11][12][13].
Nanostructure elaboration involved different methods, anodic oxidation being a convenient electrochemical method, easy to produce various oxide layers on titanium surfaces by adjusting the anodizing conditions, such as anodizing electrolytes, temperature, anodizing voltage.This present work, investigates the stability of self-organized nanotubes on titanium obtained by anodizing in a mixture of 1M (NH 4 ) 2 SO 4 and 0.5% NH 4 F, the testing corrosion solution being artificial saliva Fusayama.

Materials
Ti samples with 99.6% purity, were donated by Institute for Non-Ferrous and Rare Metals, Bucharest.The anodization electrolyte was a mixture of 1M (NH 4 ) 2 SO 4 and 0.5% NH 4 F and as testing solution for electrochemical stability was chosen artificial saliva Fusayama with a composition as following [14]: 0.4 g/L NaCl, 0.9 g/L KCl, 0.795 g/L CaCl 2 .2H 2 O, 0.69 g/L NaH 2 PO 4 , 1 g/L ureea.

Preparation of Ti substrates for anodizing
Round shape electrodes were prepared by sequential grinding with silicon carbide paper up to 2400 finishing, washed with distilled water and dried in air.The substrate samples were 1mm thick discs sliced from 10 mm diameter titanium rods.

Electrochemical procedures Nanotubes elaboration.
For the electrochemical anodizing of the Ti sheet, a DC power supply was used.Constant voltage anodizing was carried out using a two electrode configuration with a working electrode and a carbon counter electrode.The experiments were conducted at room temperature and applied a voltage of 20 V. The anodizing time was 2 h.

Corrosion testing
All electrochemical stability measurements, were conducted with an Voltalab PG 301 potentiostatic assemblies of three electrodes in a single-compartment cell using platinum as a counter-electrode and Ag/AgCl as reference electrode.Potentiodynamic polarization curves were carried out with a scan rate of 2 mV/s in the range from 800 to 4000 mV.

Surface analysis
Surface analysis have been investigated by scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDAX), and atomic force microscopy before and after anodizing.The surfaces changes in the topography and distribution of the chemical constituents were observed using Environmental Scanning Electron Microscope FEI/Phillps XL30 ESEM, with 0.7 Torr apparatus pressure and for composition an EDAX modules was attached.The surface analysis and roughness evaluation was completed with atomic force microscopy (AFM) measurements.The used equipment was an atomic force microscope from APE Research, Italia.

Stability of passive films
In the point of view of corrosion mechanism, in the case of titanium, the good corrosion resistance results from the Dedicated to Prof. Iulia Georgescu on the occassion of her 90th birthday formation of very stable passive film which is a mixture of oxides, but generally according to literature [4] the predominant is TiO 2 , the most stable oxide of titanium.
The initial event during the immersion of titanium in bioliquids, as saliva environment is the hydrolysis of the titanium dioxide and the establishing of the equilibrium surface-solution.The dissolution products released are neutral species like Ti(OH) 2 or hydroxocomplex like TiO(OH) 2 [15].Dissolution and repassivation of Ti evaluated from aspects of the film stability was subject of many electrochemical studies [16-18] , and the electrochemical parameters were supported with surface analysis data. Figure 1 compares typical potentiodynamic polarization curves recorded in artificial saliva based on Fusayama's solution both for titanium and Ti/TiO 2 nanotube layers obtained in 1M (NH 4 ) 2 SO 4 + 0.5 wt.% NaF.
On the voltammogram recorded on the titanium electrode, the anodic increasing of current density at about -0.2V corresponds to the onset of oxide film formation on the titanium surface, which has been expected to be essentially TiO 2 .Then the current remain almost constant at 5 µA/cm 2 until 1500 mV.The increasing in the current density observed at potentials above 1500 mV on the voltammogram could be attributed to the formation of Ti compounds such as NaTiPO 4 on the surface [19].
By comparison on the second curve corresponding to Ti / TiO 2 nanotube surface obtained in 1M (NH 4 ) 2 SO 4 + 0.5 wt.% NaF the polarization test was started at a cathodic potential relatively to the corrosion potential, so that the passive film at the surface was at least partially removed due to the highly reducing initial potentials.This expected starting of the voltammogram with a very high cathodic current density can be ascribed to Ti 4+ reduction combined with H + intercalation.In aqueous electrolytes the most likely mobile intercalation species are protons.Already Dyer and Leach [20] examined oxidized titanium and found that in aqueous electrolyte hydrogen enters the film under cathodic polarization and up to 85% of the material can be converted to TiOOH.
Then the oxide layer re-formation on the surface was started at about 0 V entails a partial stabilization of current density of 1 µA/cm 2 at potential above 300 mV, suggests that in this range of potentials a protective passive film is formed.This is in agreement with the obtained lower values of the corrosion current.
The quantification of corrosion for titanium and Ti/TiO 2 nanotube surfaces have been evaluate in term of corrosion current, I corr , corrosion potential E corr and polarization resistance, R p .Table 1 shows the corrosion parameter values which were estimated by Tafel curve in artificial saliva based on Fusayama's solution both for titanium and Ti/TiO 2 nanotube obtained in 1M (NH 4 ) 2 SO 4 + 0.5 wt.% NaF.
Comparing the obtaining values can be observed that the corrosion potential for Ti/TiO 2 nanotube electrode are anodical shifted from -328 mV to -122 mV by comparing with uncoated titanium surfaces.Also, it is clear from the calculated values that TiO 2 nanotube coatings increase the surface stability by reducing the corrosion current densities and increase the polarization resistance.

Morphological studies: SEM and AFM
The scanning electron micrographs revealed the nanotube structure of the oxide surfaces and the porous morphology developed after controlled electrochemical etching.
Figure 2 shows the SEM images of the nanotubes layer obtained in the electrolyte consisting of 0.5 wt.% NaF + 1 M Na 2 SO 4 .The applied potential was 20 V for 2 h.No magnetic agitation was conducted.Highly ordered TiO 2 nanotubes with a porous structure can be observed with an average diameter of almost 120 nm and a wall thickness of 20 nm and a somewhat rough surface.It is also clear to observe that the pore mouths are open at the top of the layer.
The EDAX analysis (table 2) present an atomic proportion of 1.27:1 for Ti:O indicating different kinds of defects in non-stoichiometric rutile TiO 2 such as: (i) Ti 3+ at a normal lattice position (electron compensated Ti 4+ , i.e. an extra electron in the 3d orbital), (ii) oxygen vacancy (V o ), (iii) oxygen vacancy with trapped electron (V o -) and (iv) oxygen vacancy with two trapped electrons (V o 2-) [21].These defects are formed during the formation/growth of the oxide layer.
Taking into account that in the surface composition F - ion is present, it is very possible that a part of titanium to be in complex with fluoride as was suggested in literature [22], the computed ratio Ti:O could be different.
Figure 3 represent the AFM pictures of the electrochemically etched surfaces in aqueous electrolyte (1 wt.% NaF + 0.5 M Na 2 SO 4 ) show the topography and profile of the Ti / TiO 2 nanotube surface.Basically surface roughness is defined as the change in the profile of the surface in which the height and the depth of ridges and valleys vary in the nanometer order.The Root Mean Square (RMS) parameter Sq, and The Roughness Average, Sa are 23 nm and 29 nm, respectively.1M (NH 4 ) 2 SO 4 and 0.5% NH 4 F highly ordered TiO 2 nanotubes was obtained.
The surface analysis as scanning electron microscopy and atomic force microscopy revealed a porous nanotube structure of the oxide surfaces with an average diameter of almost 120 nm, a wall thickness of 20 nm and a somewhat rough surface.
The nanotubes TiO 2 structure presents more stability in Fusayama artificial saliva comparing with the natural passive TiO 2 layer.

Conclusions
Using a simple anodic deposition with an applied a voltage of 20 V for 2 h at room temperature in a mixture of

Table 1
THE CORROSION PARAMETER FOR TITANIUM AND Ti/TiO 2 NANOTUBE