The Electrochemical Stability in NaCl Solution of Nanotubes and Nanochannels Elaborated on a New Ti-20 Zr-5 Ta-2 Ag Alloy

Nanotubular and nanochannels structures were fabricated via anodizing on a new alloy Ti-20Zr-8Ta-2Ag. A continuous coating of connected tubes/channels can be observed in the SEMmicrographs forming tubular structures with diameters in hundreds of nm, as well as smaller tubes, with diameters in tens of nm. In the case of nanochannels structure, the diameters are smaller and wall thicknesses significantly thinner than in nanotubes. Wettability measurements indicate a decrease of contact angles in both cases of nanotubes and nanochannels, but the increase of hydrophilic character is more significant in the case of nanochannels.The Tafel procedure and electrochemical impedance spectroscopy tests performed in NaCl 0.9% solution indicate a better stability for the nanostructured surfaces compared to untreated alloy, the surface with nanochannels offering higher corrosion resistance. Spectral UV-VIS determination has confirmed Ag metallic presence, opening the door for applications not only in tissue engineering but for water splitting and the photoreduction of CO 2 as well.


Introduction
Tailoring materials into nanostructure offers a large number of opportunities in the exploitation by enhancements of their properties.In this approach, surface modification of Ti alloys has potential for wide range of applications in tissue engineering, energy, and environment.New Ti bioalloys with nontoxic elements are required for the last generation of advanced orthopedic implants, which need to satisfy more restrictive requirements regarding corrosion and stability in order to extend their service life and to reach more performance.The Al and V substitution in commercial Ti bioalloys with more safety elements and better mechanical properties reducing elasticity modulus and implying values close to human bone (30 GPa) [1,2] involves important efforts.In this context the new alloys with Nb, Ta, and Zr [3,4] are considered safe and nonallergenic due to their corrosion resistance and compatibility with tissue, having also high mechanical resistance.Nowadays, Ta is the most corrosion resistant and most biocompatible metal in use.Ta is not affected by chemical attack at temperatures under 150 ∘ C (ASTM B 521-12) due to its oxides surface; Ta is very ductile but has a high melting temperature (3017 ∘ C) and a very high density (16.69 g/cm 3 ); there are no reports regarding harmful effect on working people health after industrial exposure; at the contact with tissues Ta is inert.There are only few papers about Ti-Ta alloys for orthopedic applications [5][6][7][8].Zhou and Niinomi [8] have elaborated binary Ti-Ta alloys with various amounts of Ta having high values for Young's Modulus, nonsuitable for orthopedic implants: Ti-30Ta alloy has a 70 GPa modulus, Ti-40Ta 82 GPa, and Ti-50Ta 89 GPa; only Ti-25Ta has a 64 GPa modulus and a good ratio strength/ modulus, compared to other Ti-Ta alloys.Some authors [9] have elaborated on the surface of Ti-Ta ( = 13, 25, 50, 80%) alloys nanotubes with increased bioactivity.These alloys have shown a biphase structure influencing the nanotubes 2 Journal of Nanomaterials homogeneity; the surface of Ti-80Ta was the only one where homogenous nanotubes have been formed.On the other alloys surface, the layers of nanotubes have been formed randomly, thus leading to an irregular surface.Other researchers [10] have studied the behaviour of three Ti-Ta ( = 30, 40, 50%) binary alloys in simulated physiological solutions (solutions 0.1 M NaCl and Ringer) finding that chemical activity is more visible in the case of alloys with higher Ta content.
Zr has demonstrated a good mechanical strength, high corrosion resistance, and biocompatibility.Zr is an isomorphic element which is soluble in both titanium ( and ) and tantalum [11][12][13].The alloy Ti-ZrNb [14] has good mechanical properties, elasticity, and machinability.Kobayashi et al. [15] have shown that Ti-50Zr has hardness 2.5 times larger than that of Ti or Zr.Other researchers have investigated binary Ti-Zr alloys with low content Zr (from 12% to 18%) and found that such alloys have suitable properties as implants [16][17][18].It is worth to mention that at tissue implant fibrous capsules formed around Ti-Zr alloys [19].The antibacterial effect of Ti-Zr alloys nanolayers and cell growth as a function of nanoarchitectures dimensions have been the object of several investigations as well [20,21].
Silver (density 10.54 g/cm 3 ) as a ductile and antibacterial noble metal is possible to enhance resistance to corrosion, electrochemical stability, mechanical properties, and antibacterial activity in binary alloys with Ti.Kang et al. [22] investigating three different alloys Ti-Ag ( = 1, 2, 4%) have observed a high antibacterial effect for the alloy Ti-4Ag for a 7-day period, a time when the alloy was biocompatible.Zhang et al. [23] determined excellent corrosion resistance, low ion release, and good biocompatibility for Ti-5Ag and Ti-20Ag alloys.Research had also been done on more complex alloy with Ta and/or Zr.Vasilescu et al. [24] had fabricated and evaluated electrodeposited nanolayers on a new Ti-15Ta-5Zr, while Li et al. have investigated biological properties of Ti-35Nb-5Zr [25].Bioactive ceramic coating was recently investigated as well for the Ti-15Ta-5Zr alloy [26].As it can be seen from the literature data, implants from Ti alloys with Ta, Zr, and Ag present controversial results regarding their properties.A way to avoid such a situation is elaboration of an innovator content of the alloying using small amount of Ta and Ag for keeping a low density of alloy (Ti has density 4.81 g/cm 3 ) and adding more Zr for mechanical resistance and hardness increase such as in the case of the present paper.The innovative alloying with Zr and Ag in the proper concentration has the role of keeping mechanical and anticorrosive properties in suitable limits inducing antibacterial protection via slow release of Ag + ions in surrounding tissue.In the present work, the formation and characterization of 1D nanoarchitectures on the Ti-20Zr-5Ta-2Ag alloy are subjected to the different conditions of anodizing treatment.The anodizing is recommended as a facile and cheap surface treatment and is investigated comparatively in this paper.The main aim is to find out the formation of surface nanostructures with best stability performance in NaCl solution.This stability is of significant importance for designing new strategies to enhance surface properties of materials with application in tissue engineering as well as in green chemistry.

Alloy Fabrication and Surface Characterization.
The alloy was obtained in as-cast state in a levitation furnace (Five Cells, CELLES MP 25HF) operating with a power of 25 kW, a magnetic field frequency of 215 kHz, and a melting temperature of about 2000 ∘ C. The alloy synthesis took place in two steps (melting and remelting) in a cold crucible, in levitation mode in inert gas (Ar) atmosphere, thus leading to a very homogeneous composition and avoiding the alloy contamination.
The alloy composition was determined with an X-ray fluorescence spectrometer (XEPOS 03); samples were cut from five different ingots and ground to a bright, smooth surface and then were exposed to X-ray flux of the spectrometer.The impurities level in the (as cast) titanium alloy complies with the limits of ASTM B367.In this situation an influence on electrochemical stability of the alloy is not expected.
The microstructure of the bare alloy and of the nanotubes and nanochannels were examined by scanning electron microscopy (SEM) in a FEI Quanta 3D FEG apparatus working at accelerating voltages between 5 and 20 kV equipped with secondary electrons (SE) and back scattering electrons (BSE) detectors and an energy dispersive X-ray (EDX) spectrometer for elemental chemical analysis.
For the nanochannels fabrication, the alloy samples were first grinded with SiC abrasive paper with roughness up to 1200.The samples were cleaned by ultrasonication in acetone and ethanol for 10 minutes each and then washed with deionized water.After drying, the samples were etched in an acid solution containing HF + H 2 SO 4 + H 2 O, rewashed with deionized water, and dried in atmosphere.The nanochannel formation was carried out by anodizing in a hot glycerol electrolyte containing 10%wt KHPO 4 in a two-electrode setup.As the working electrode the alloy was used and as counter electrode platinum.The anodizing was carried out at 180 ∘ C applying 50 V for 2 hours.
The self-ordered nanotubes were obtained by electrochemical oxidization of Ti-20Zr-5Ta-2Ag in glycerol solution with 4% H 2 O and 0.2 M NH 4 F. The alloy substrate was grinded with SiC abrasive paper with roughness up to 1200.Subsequently the alloy sample was ultrasonicated for 5 min and dried at room temperature.For the nanotubes growth the working (oxidizing) electrode was the Ti-20Zr-5Ta-2Ag alloy.Two rods of graphite represent the counter electrode.Between the Ti-20Zr-5Ta-2Ag and the graphite electrode a 40 V voltage was applied from a MATRIX MPS-7163 power source.
The surface characterization of the nanotubes and nanochannels was performed with the same FEI Quanta 3D FEG apparatus as the one used for the untreated alloy.The contact angles were determined for all samples using CAM 100 equipment and software at room temperature.Each value was the average of three determinations.

The Electrochemical Characterization.
The electrochemical characterization for nanotubes and nanochannels structures was performed with an Autolab PGStat 302N in a three-electrode configuration: a platinum counter electrode, an Ag/AgCl reference electrode, and the nanostructured Ti-20Zr-5Ta-2Ag substrate as working electrode.The electrochemical measurements and data processing were achieved with Nova 1.10 software.The testing solution for the electrochemical experiments was 0.9% NaCl saline solution and the temperature was 25 ∘ C.

Spectral UV-VIS Determinations.
For the spectroscopic measurements, a Perkin Elmer Lambda 900 equipped with an integrating sphere module was used, and the data was recorded by means of the UV WinLab 5.2.0.0646 software.
The alloy shows  +  biphase with Widmanstatten acicular microstructure (Figure 1(a)).It presents typical lamellas "colonies" of  and  phases, as a result of the slight variations in the distribution of the alloying elements.Interlamellar distances are in the order of few microns.EDX spectrum (Figure 1(b)) identified the constituent elements of the alloy: Ti, Zr, Ta, and Ag; quantitative elemental analysis determined the following (wt%): 71 ± 2% Ti; 20 ± 1% Zr; 6 ± 2% Ta; 2 ± 1% Ag.According to some unpublished results (XPS), yet, the native passive film on the Ti-20Zr-5Ta-2Ag alloy surface contains the protective oxides TiO 2 , ZrO 2 , Ta 2 O 5 , and metallic Ag.
Regarding wettability results the average contact angle from the three measurements of the surface before anodizing is 64.54, a value which is in the hydrophilic domain.

Surface Characterization of the 1D Nanoarchitectures on
the New Alloy.The nanostructured surface of the alloy Ti-20Zr-5Ta-2Ag is subject of Figures 2 and 3.
Hollow tubular structures with diameters in hundreds of nm can be observed in the SEM micrographs, as well as smaller tubes, with diameters in tens of nm, in the interstitial regions, forming a continuous coating of connected tubes/channels, with homogeneous walls thicknesses about 100 nm.
The elemental chemical analysis by EDX detected only the alloy elements, Ti, Zr, Ta, Ag, and O.The image taken with -contrast in BSE-SEM mode shows the distribution of Ag nanoparticles that decorates the oxide coating.
The small peak at 5 keV in the EDS corresponds to the K-beta line of titanium (Ti), which appears always accompanying the main K-alpha line at around 4.50.Only the K-alpha spectral lines have been marked on the spectra, for the sake of clarity.The sample shows the oxide coating with a structure of interconnected channels of varying diameters.The wall thickness in this sample is significantly thinner (even a few nm) than in the sample with nanotubes, with channels diameter varying from between a few tens of nm and a few hundreds of nm.Larger cavities can be observed where several channels converge.
The small bright dots in Figures 2(d) and 3(d) are silver nanoparticles on the surface of nanotubes or nanochannels.Ag is an element of the alloy treated electrochemically, the tubes or channels and the Ag nanoparticles are no longer part of the alloy, and the Ag nanoparticles decorate the structures formed electrochemically by anodic oxidation of the other metallic elements of the alloy (Ti, Zr, and Ta).The remaining alloy substrate still contains Ag as alloying element.
The average contact angle values for the surface with nanotubes and nanochannels are 61.92 and 29.41, respectively, indicating an increase of hydrophilic character in both cases.The increase of hydrophilic character is more significant in the case of nanochannels.

Electrochemical Stability of the Alloy before and after
Nanoarchitectures Electrodeposition.The Tafel plots of the bare alloy Ti-20Ta-5Zr-2Ag and of the alloy covered with nanotubes and nanochannels are presented in Figure 4.
Based on Tafel plots, electrochemical parameters as corrosion potential ( corr ), current density ( corr ), corrosion rate ( corr ), and polarization resistance have been computed and As can be seen from the main corrosion parameters of Table 1, nanostructured Ti-20Ta-5Zr-2Ag alloy in NaCl solution has better value for 1D nanoarchitectures, nanotubes, and nanochannels.As expected, the resistance of nanochannels structure is bigger as an expression of the significantly thinner dimension of walls as previously observed in the SEM images of Figure 3.
According to Table 1 values the nanochannels structure has the best stability and is in the first group of corrosion resistance class, entitled "perfect stable." Both the bare alloy and the nanotubes structure are in the second group of corrosion resistance, being "very stable." The range of stability taking into account the corrosion potential as well, which is more electronegative for bare alloy, is as follows: Nanochannels → Nanotubes → Bare alloy. ( Electrochemical impedance spectroscopy analysis is in good correlation with Tafel analysis confirming the same hierarchy of stability.Nyquist spectra (Figure 5) recorded in NaCl solution evinced larger semicircles with bigger radii and higher impedance data for the alloys with nanostructured surface in comparison with the bare one.
For quantitative interpretation of Nyquist spectra, different equivalent circuits were proposed for untreated and coated samples, as presented in Figure 5.In the proposed circuits,   represents the electrolyte solution resistance and parallel  ox and CPE ox elements are the resistance and constant phase element of the compact oxide layer.For treated samples the equivalent electric circuits contain a supplementary CPE parallel combination corresponding to the nanostructured oxide layers,  nc and CPE nc for nanochannels and  nt and CPE nt for nanotubes structures.
Electrical parameters of the proposed equivalent circuits obtained after fitting the results of EIS tests are presented in Table 2.
The resistance of the compact oxide layer is one order of magnitude higher for nanochannels as against untreated and nanotubes samples.The resistance of nanochannels layer is also better compared to nanotubes layer. coefficient of CPE ox shows an almost pure capacitive behavior for compact oxide layer of untreated sample ( is 0.93) while Journal of Nanomaterials  for the nanostructured samples it presents a quasi-similar pseudocapacitive behavior ( is 0.72 for nanochannels and 0.71 for nanotubes, resp.).
The impedance data from Table 2 confirm the fact that nanochannels surface is the most protective one.This aspect is also supported by the Tafel results.

Spectral UV-VIS Determinations.
The metallic Ag presence is confirmed in the UV-VIS diffuse reflectance.Specific bands associated with the Ag presence can be observed in the untreated sample at around 474 nm and 516 nm (Figure 6(a)).Both bands are associated with Surface Plasmon Resonance Phenomena and their position depends on the metallic Ag shape and size [29].The presence of the two peaks is mainly associated with the deposition matrix [29].The highest intensity is observed for the untreated alloy.Noble Ag is also deposited on the nanotubes and nanochannels, most likely due to an electron scavenging process of Ag + (Ω) ions [30].However, the decreased intensity indicates that for the nanostructures the presence of metallic Ag is reduced compared to the one of the bare alloy (the lowest content for the nanotubes, as highlighted in Figure 6(b)).
The nanochannels present a profile that is similar to the one of the bare substrate (Figure 6(b)).This indicates that the metallic Ag distribution is roughly similar.However, for the nanotubes (Figure 6(c)) a broad beak is observed that has a supplementary peak at 427 nm, with two shoulders at 476 nm and 507 nm.This result indicates an increased variability of the metallic Ag on the surface.The presence of metallic Ag alongside the nanostructures highlights properties of increased photocatalytic activity with possible applications for water splitting [31] and for the photoreduction of CO 2 and CH 4 [32].

Conclusions
The anodizing of a new Ti-20Zr-5Ta-2Ag alloy in specific conditions led to various types of 1D nanoarchitectures.The nanostructured surface covered with nanotubes and nanochannels has more favourable values for electrochemical parameters in NaCl 0.9% solution than those of the bare alloy.Both procedures, Tafel curves and electrochemical impedance spectroscopy, have established a range of stability with nanochannels in the first place.The surface with nanochannels is the one with the highest hydrophilic character as well.The untreated alloy has the higher contact angle indicating the lowest hydrophilic character.Based on the experimental data we can conclude that 1D nanostructures enhanced all corrosion parameters conferring superior performance to the Ti-Ta-Zr-Ag alloy and potential applications in health, energy, and environment.

Figure 5 :
Figure 5: Nyquist spectra for titanium alloy (a) untreated native oxide, (b) nanochannels, and (c) nanotubes and the representation of the nanostructures with equivalent circuits for (d) native oxide, (e) nanotubes, and (f) nanochannels, used for data fitting and proposed according to the impedance spectra.

Figure 6 :
Figure 6: The UV-VIS diffuse reflectance of the (a) untreated alloy, of the (b) nanotubes with the main SPR bands highlighted in the inset, and of the (c) nanochannels.

Table 1 .
The data establishes the class of corrosion resistance for bare alloy and for the nanostructured surfaces as well.

Table 2 :
EIS parameters for bare Ti-20Zr-8Ta-2Ag alloy and its modified surface with nanotubes and nanochannels.