Synthesis of oxide nanotubes on Ti13Nb13Zr alloy by the electrochemical method
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Surface modification of titanium alloys expands the range of their applicability in medicine, particularly in the form of various implants. Present work reports the results of the electrochemical formation of self-ordered oxide nanotubes on Ti13Nb13Zr alloy. Due to its relatively low Young modulus (77–79 GPa) this alloy can be attractive material for orthopedic application. The experiments were conducted in the (NH4)2SO4 + NH4F electrolyte at room temperature. Anodization of the alloy samples was carried out for 2 h under variable external potential U (in the range from 10 to 45 V) and the current versus time transients were recorded. Obtained surface morphology was investigated by the scanning electron microscopy and the X-ray techniques. The morphological parameters of the obtained nanotubes such as the inner (din) and outer (dout) diameters were determined. The tubes diameter dependence on the voltage of anodization process was derived. The dependence d[nm] = f(U)[V] was established at constant temperature 25 °C. It provides the basis for controlled oxide nanotubes layer growth. It was also demonstrated that these nanotubes exhibit photocatalytic activity.
KeywordsTi13Nb13Zr alloy Nanotubes Anodic oxidation
During oxidation of a number of metals, the formation of one-dimensional nanostructure of the oxide layer having the shape of nanotubes has been observed. These self-organized structures having well developed surfaces and high specific surface area attracted a lot of attention as potential materials used in photocatalysis , solar cells  sensors , hydrogen generation , wastewater treatment  and (perhaps most important) in biomedical field [6, 7]. It was found that titanium and its alloys can be appropriate implant materials, which can be used as orthopedic and dental implants as well as blood clotting agents . Oxide nanotubes formation on Ti can be obtained in the course of the electrochemical anodization process during which the size and shape of nanotubular arrays may be controlled to a large extend. However, during biomedical applications at least three problems must be taken into account.
First, and probably the most important, is connected with the biomechanical properties of the metal. The elastic modulus for Ti is about 100 GPa, while that of the human bones may vary (depending on the bone) between 2 and 30 GPa. This problem can be somehow avoided by the addition of alloying elements to titanium, which may decrease the elastic modulus of the alloy. The best result so far has been achieved by preparing the alloy Ti24Nb4Zr7.9Sn which exhibits elastic modulus equal to 46 GPa . The second problem is connected with the alloying element itself. It must be non-toxic. Thus, a number of potentially interesting elements from the point of view of biomechanical properties must be excluded since they transfer through the oxide layer into the human body may bring about unwanted and harmful effects. Needless to say, the corrosion process of the alloy in the environment of the body fluids must be known. Finally, the metallic surface often does not support the osseointegration. Attempts are being made to solve this problem with proper surface treatment and the formation of oxide nanotubular coatings is a possible solution.
Moreover, it is known that to enhance implant’s bonding with adjacent bone surface, the formation of hydroxyapatite (HAp) coating is helpful. Hydroxyapatite has the ability to form bond-like layer on the oxide surface, which bonds to the bone. Thus, there is a question how to influence the hydroxyapatite formation. One important factor in HAp formation is the surface area and roughness of the oxide layer. Tsuchiya et al.  studied the HAp formation on different TiO2 nanotube layers in SBF solutions. They confirmed that the presence of nanotubes (NT’s) on metal surface enhances the formation of hydroxyapatite layer. The best results were obtained for long nanotubes composed of anatase + rutile mixture. Kim et al.  and Oh et al.  found that in the presence of NaOH in SBF solution, the exchange between charged Na+ and Ca2+ ions may take place, accelerating HAp formation on the oxide surface.
Detailed study of Kim et al.  demonstrated that the overall process of hydroxyapatite formation is composed of three steps. At first, sodium titanate is formed on the surface which next converts into calcium titanate, and finally into apatite with bone like structure. This overall process is accompanied with the surface charge variation. Kar et al.  investigated electrodeposition process of HAp onto nanotubular TiO2. They used two-step process which consisted in potential pulsing followed by constant current flow. They found that hydroxyapatite growth was vertical with increased bond strength of the coating.
These experiments may suggest that by influencing nanotubes reactivity with ions in the solution one may facilitate the process of HAp formation which in all cases had something to do with the charge transfer. Therefore, one can imagine another process making use of opposite charge attraction. It is proved that TiO2 based nanomaterials exhibit extraordinary catalytic activity under the action of light . Irradiated TiO2 generates electron–hole pair:
first, to find out if increased content of Nb in Ti may affect the process of nanotubes growth rate and their formation
second, to derive nanotubes diameter versus voltage dependence since it is commonly known that the key factor controlling the tube dimension is the anodization voltage.
third, to find out if the presence of Nb or Zr oxides in NT’s layer may influence its photocatalytic activity, so characteristic for pure TiO2.
Chemical composition of the Ti13Nb13Zr alloy, wt% - manufacturer certificate
The morphology of the “pure” alloy sample as well as anodized samples was investigated by using field emission scanning electron microscope (FE-SEM) Hitachi SU-70. Chosen samples were studied by point and region analyses using EDS analysis (Thermo Scientific) attached to the SEM to obtain chemical composition of the phases. The point analyses were performed over five points or areas in each phase. Data acquisition and calculations of chemical composition were done by using NSS 3 software. Generally, for each element an error within 0.2 at% was ascribed. After investigation of the microstructure, each sample was subjected to X-ray diffraction (Rigaku Mini Flex II) using monochromatic Cu Kα radiation (0.15416 nm). XRD spectra were solved by using PDXL software (Rigaku, Japan) with ICDD PDF 2 + Release 2010 database. All measurements of nanotubes diameters were performed by using AxioVision v 4.8 software (Zeiss Germany).
The photocatalytic activity of oxides nanotubes layer was investigated by the 3 h degradation of methyl orange solution under UV light irradiation. To perform this experiments sample anodized at 20 V has been chosen. The UV light was provided by the 200 W mercury-xenon lamp (HAMAMATSU Lightningcure Spot light source LC8, Japan) with radiation wavelength of 365 nm. An 0.5 cm2 (geometrical area) nanotubes layer was immersed into 20 cm3 of aqueous methyl orange solution with a concentration of 10 mg/dm3 in the same cell which was used for anodization process. The solution was stirred using magnetic stirrer with rotation speed equal 150 rpm. The sample was left in the cell prior to UV illumination for 30 min in a darkness to achieve adsorption/desorption equilibrium. During the photocatalytic process, the absorbance of the MO solution was recorded every 30 min using an UV–Vis V-770 spectrophotometer (JASCO, Japan) with wavelengths in the 200–800 nm range. The absorption intensities before and after degradation as well as the degradation ratio of MO can be calculated as follows:
3 Results and discussion
The EDS analysis results (wt%) of nanotubes formed at 30 V
The comparison of these equations with the results obtained for Ti6Al7Nb alloy indicates that the oxide layer growth is faster for Ti13Nb13Zr alloy.
The electrochemical oxidation of Ti13Zr13Nb alloy in aqueous electrolyte containing fluoride ions results in a fine nanotubular structure determined by SEM morphology investigations. The nanotubes consist of mixed oxides of alloy components (SEM–EDS) and are found to be amorphous (XRD). It seems, that the best developed nanotubes are obtained for voltages in the range from 20 to 40 V. The increased amount of Nb in the alloy in fact did not affect the tube formation. Its influence had to be compensated by zirconium addition. We found previously that ZrO2 nanotubes can be formed easily on zirconium surface . The diameters of synthesized nanotubes depend linearly on applied anodizing voltage in the range from 10 to 45 V. Derived linear dependencies d = f(U) can be used in the future to synthesize nanotubes with expected geometrical dimensions under optimized conditions presented in this work. It was also demonstrated that these nanotubes exhibit photocatalytic activity which can be exploited during hydroxyapatite formation under light illumination.
This paper is supported by Polish Ministry of Science and Higher Education (Grant No. 220.127.116.119). Authors are grateful to Prof. Krzysztof Fitzner for valuable discussion.
- 16.J.A. Davidson, P. Kovacs, US Patent 5.169.597, 1992Google Scholar
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