Anodic treatment of pure α-type Ti in a mixture of phosphoric and hydrofluoric acids results in the formation of a titanium oxide surface layer. The surface morphology is shown in Figures 1 and 2, and due to electrolyte composition, time, and applied potential, the surfaces are partly dissolved, resulting in pit formation.
The Ti surface recorded by SEM for different magnifications shows morphological changes from nano- to macroscale after electrochemical oxidization in 1M H3PO4 electrolyte with 2 pct HF (Figures 1(a) through (d)), 5 pct HF (Figures 1(e) through (h)), and 10 pct HF (Figures 1(i) through (l)). The anodic oxidation leads to formation of surface Ti-oxide through a partial surface dissolution and pit (craters) formation. The surface roughening and pit formation are correlated with grains and their boundaries ((d), (h), and (l)). The formed pits reflect the grains’ position (bottom of the pits) and the grain boundaries form hillocks (crater edges) in the surface morphology ((c), (g), and (k); (b), (f), and (j)). The higher HF content enlarged pits in the surface. In the nanoscale ((a), (e), and (i)), the small bright precipitations are visible, more densely packed for oxidation in the electrolyte of a higher HF content ((a), (e), and (i); (b), (f), and (j)). These precipitations in the form of nanoislands, which have a diameter in the range of 10 to 50 nm, participate in the formation of anodic oxide film and can be potentially correlated to an additional oxide or phosphate phase.
The AFM pictures of Ti surface morphology after 15 minutes of anodic oxidation in an electrolyte of 1M H3PO4 containing 2, 5, and 10 pct HF are shown in Figures 2(a), (b), and (c), respectively. The AFM results show that treatment in electrolytes of various HF content results in changes of surface morphology, from the spongy type to the well-defined circular-type pores. For 2 and 5 pct HF, the average pore diameter increases from 1 to 2 µm, respectively. For both cases, maximum pore diameters do not exceed 4 µm. After Ti anodic oxidation in an electrolyte containing 10 pct HF (c), the pore size maintains an average value close to 2 µm. The AFM results support and are consistent with those recorded by SEM.
The process undertaken in 1M H3PO4 electrolyte containing the addition of higher HF content raises the current density (Figure 3) and, thus, the dissolution rate; however, the surface morphology does not significantly change after treatment in the electrolyte consisting of 5 and 10 pct HF. In the anodic oxidation process, the oxide formation competes with oxide dissolution, especially at higher HF concentration. An increase of HF concentration from 2 to 10 pct results in a faster oxidation reaction. The roughness measured by AFM changes in accordance with anodic oxidation conditions (Table I). Anodic oxidation in 1M H3PO4 + 2 pct HF gives the highest roughness; however, taking into account the mean height of the profile, the oxidation in 1M H3PO4 + 2 pct HF and 1M H3PO4 + 10 pct HF, gives a promising surface for tissue in growth and proliferation. In the majority of commercial implants, their surfaces have Ra in the range 1 to 2 µm. Too high roughness can adversely affect implant bone interaction, leading to increased ion release, hindered cell adsorption, and increased stress.
The XRD study shows that the Ti provided by the supplier has a layer of native oxide, i.e., anatase-TiO2 (JCPDS No. 01-075-2552); however, Ti substrates (JCPDS No: 01-089-5009) dominate in the entire spectrum due to the low thickness of the native oxide layer (Figure 4(a)). The anodization results in the formation of thicker, albeit porous TiO2; hence, the signal from the Ti substrate (JCPDS No. 01-089-5009) is significant in the entire spectrum.
Treatment in 1M H3PO4 + 10 pct HF results in formation of more TiO2 (JCPDS No. 01-085-5943 for anatase (204) and 01-006-1890 for rutile (111)) and additional Ti(HPO4)2 (d) (JCPDS no. 00-032-1371). The H3PO4, as the major electrolyte component, provides the phosphorus ions, which are built into the surface. The HF, as the minor electrolyte component, provides the fluorine ions. The formed titanium dioxide–anatase-TiO2 and rutile-TiO2, as well as titanium hydrogen phosphate–Ti(HPO4)2, could be useful in the osseointegration process. Negatively charged ions (HPO42− and PO43− complexes) on the surface of titanium oxides contribute to calcium phosphate formation by attracting positively charged Ca2+ ions. Thus, phosphorous content in the surface may act positively on bone nucleation and growth and the osseointegration process, as phosphorous is a significant component of the bone. The enhanced concentration of phosphorous in the surface layer after anodization was also observed by Lee et al.
The surface state is very important for biomedical application; thus, the XPS measurements were carried out. The pure cleaned Ti surface was stored in UHV (5 × 10–11 mbar), and the XPS reference spectrum was recorded immediately after 1 minute in UHV (Figure 5(a)). After 2 hours of UHV (5 × 10–11 mbar) conditioning, the Ti surface adsorbed some portion of oxygen atoms from the remaining gases (Figure 5(b)). Some small peaks related to oxygen appear, but Ti-2p peaks are not shifted (Figure 5(b)). This finding means that chemisorption of the previously adsorbed oxygen does not occur.
The different anodic oxidation conditions have no significant effect on the XPS survey spectra (Figure 6). The samples were measured immediately after the transfer to the UHV chamber. In all electrochemically oxidized samples, the O-1s peak is related to oxygen, which is chemically bonded to the TiO2 layer. The presence of oxygen in the surface layer improves the wettability and initial stabilization of the implant. Additionally, an insignificant contamination with carbon was found, probably due to absorption of CO2 from the air.
Results of detailed studies of the positions of the Ti-2p3/2 and Ti-2p1/2 peaks for pure Ti (a) and for oxidized Ti (1M H3PO4 + 10 pct HF) (b) are presented in Figure 7. Peaks of Ti-2p for oxidized samples are significantly shifted (11 and 10.5 eV) in comparison to clean pure Ti. The position of Ti-2p3/2 (459 eV) and Ti-2p1/2 (464.5 eV) peaks for the porous anodic oxide layer indicates that stoichiometric TiO2 was formed. The position of Ti-2p peaks and exchangeable fission (5.5 eV) is in agreement with the XPS results of the anatase-TiO2 thin films obtained in References  and .
The valence band of Ti is affected by anodic oxidation. In all oxidized samples, the maximum of the valence band is significantly shifted to lower binding energy in comparison to clean pure Ti (Figure 8). This shift is enhanced for lower HF content in electrolyte used for anodic oxidation (a) and (b), whereas an increase in HF content leads to smaller shift (c). The anodic oxidation leads to formation of the TiO2 overlayer with a valence band very similar to that measured for bulk anatase-TiO2. An increase in HF content leads to an increase of stress and structural disorder of the TiO2 overlayer.
The corrosion resistance was investigated in Ringer’s electrolyte, which is a chloride solution. On the basis of polarization curves (Figure 9), corrosion current density (Icorr), passivation current density (Ip), and corrosion potential (Ecorr) were measured, and these values are shown in Table II. The untreated pure Ti shows very good corrosion resistance, with corrosion current density Icorr = 3.93 × 10–8 A/cm2. The anodic oxidation of Ti in 1M H3PO4 + 2 pct HF electrolyte slightly improves corrosion resistance, resulting in a decrease of Icorr to 2.94 × 10–8 A/cm2. The Ti anodically oxidized in an electrolyte of higher HF concentration 5 and 10 pct results in a slight decrease of the corrosion resistance with Icorr = 8.57 × 10–8 and 1.21 × 10–7 A/cm2, respectively. Taking into account the changes in the corrosion current density, it should be noted that during anodic oxidation, a porous, rough surface is formed, which finally leads to a larger surface area. The corrosion current density shown is designated with respect to the constant surface area of the nonanodized sample. Thus, in the case of anodized samples, in which the real surface area is larger (due to surface development), the real values of the corrosion current densities are higher. In fact, the corrosion resistance, determined on the basis of this parameter, may be slightly lower in the case of anodized samples, which is usually overlooked in publications. In our previous work, we showed that the surface area gain during anodization can reach a value of about a few percent for low voltage anodization conditions. In all presented oxidation conditions, the Ecorr is shifted toward more negative values, which means that the oxidation process of the anodized titanium proceeds more easily. In the case of Ecorr, this parameter is independent of the surface area; hence, it may be more useful in comparison of the samples’ corrosion resistance. Hence, the nonanodized sample of the smooth surface shows the best corrosion resistance. One more feature of the polarization curves may be useful for the characterization of corrosion resistance materials. It is a plateau in the anodic potential range. The wider and lower it is, the greater is the material tendency to passivate. However, in determining the Ip parameter, one should also bear in mind that the current density calculation was done with respect to the surface area of the nonanodized sample. The passivation current density Ip is lowest in the case of the sample oxidized in 1M H3PO4 + 2 pct HF (Table II). In the anodic oxidation process, shown previously, on the surface, passivating Ti-oxides are formed, which protect the material against corrosion attack. Taking into account the shape of the corrosion curves, it is obvious that the anodically oxidized samples have a characteristic current plateau in the passive range (Figures 8(b) through (d)), which was not observed on the untreated Ti (Figure 9(a)). The shape of the curves and the compromise corrosion parameters indicate good passivation and corrosion resistance of the anodically oxidized Ti, especially in the case of Ti electrochemically treated with the 1M H3PO4 + 2 pct HF electrolyte.
The bioadhesion was tested by means of application of wetting analysis (Table III), showing surface potential toward hydrophilic properties and cell attachment. In all cases of anodic oxidation, the wetting angle decreased in comparison to the untreated Ti, which indicates improved surface hydrophilicity. The best wettability was displayed by the sample anodized in the 1M H3PO4 + 2 pct HF electrolyte. The osteoblast culture, done using Nhost cells (Figure 10) on selected Ti anodized in the 1M H3PO4 + 2 pct HF electrolyte, shows cell attachment and proliferation that proceeds through formation of extended filopodia spreading through the surface. The rough surface with pits and high wettability acts positively on cell anchoring and proliferation. Osteoblast cells cover nearly 50 pct of the sample surface.
Titanium oxide layer, which covers Ti substrate, plays a key role in corrosion protection and biocompatibility. The surface porosity formed in the anodic oxidation may improve implant stability, especially in the early stage after implantation. Depending on the oxidation process conditions and the oxidation state of the surface, different types of titanium oxides may be formed, among which the TiO2 is thermodynamically most stable. In the crystalline form, TiO2 is found in three varieties: brookite, anatase, and rutile crystal structure. The last two are most useful in Ti surface biofunctionalization. In the anodic oxidation process, the material is subjected to etching (removal of the substrate material), oxidation (growth of the oxide layer), or both of these processes occurring simultaneously, depending on the processing conditions. The proper course of the oxidation process is determined by many factors, such as applied voltage, current density, electrolyte composition, its temperature, pH, or electrolyte mixing. Depending on the applied voltage, continuous, porous,[17,30] or nanotube oxide layers[17,31,32,33] may be formed on the titanium surface, which are characterized by different thickness, phase composition, morphology, and physicochemical properties. By designing the appropriate process conditions, it is possible to properly biofunctionalize the surface for implant applications. Quite a significant limitation of the anodic oxidation process, in which the porous structure is formed, is the pore diameter. As a rule, it does not exceed a dozen or so micrometers. Research conducted by Webster and Ejiofor shows that small surface nanofeatures also support bone growth. Anodic oxidation not only leads to formation of porous titanium oxide, but also to formation of surface TiO2 nanotubes or nanofibers. These surface nano-objects cause a good biological response of the tissue, e.g., better cell adhesion and differentiation. The growth of nanotubes proceeds in a few stages, one of which is the formation of nanopores. The formation of TiO2 surface nanotubes needs to maintain a narrow range of technological parameters to their growth and to prevent the delamination effect, which limits their application. Hence, a rather more common result of anodic oxidation is formation of an oxide porous surface layer, which is easier to obtain and apply. In most cases of titanium anodic oxidation, the use of phosphorus-containing electrolytes makes it possible to introduce this element into the implant surface layer (e.g., Ti(HPO4)2 in Figure 4(d)) and nucleation of calcium phosphates when soaking the material in the SBF solution. The addition of HF, which increases the current density, accelerates dissolution and enhances changes in substrate morphology. However, the fluorine ions that form TiF62− complexes dissolve titanium oxides. Thus, the dissolution process limits the thickness of the porous oxide layer. The tailored, porous, well wettable anodic titanium oxide surface layer should guarantee good and quick osseointegration with bone, which is obviously not provided by the native smooth titanium oxide layer. Among many surface treatment technologies, those based on electrochemical ones play a most important role in Ti biofunctionalization; hence, the wide interest in this area of research.