Electrochemical Study of Anodized Titanium in Phosphoric Acid

&e anodization of the Ti-Cu (2%) alloy was carried out in a 5M H3PO4 solution for 2 minutes. &e obtained layers are characterized by XPS, X-ray diffraction, and Raman spectroscopy. &e results showed that the obtained films are composed of poorly crystallized TiO2 oxide. Electrochemical Impedance spectroscopy studies revealed that the thickness of the formed film increases with increasing anodization potential. Additionally, the resistance of charge transfer becomes higher when the anodization potential increases. &us, the Mott Schottky model revealed that the formed film is an n-type semiconductor. &e density of charge carriers is in good agreement with those found in the literature. Also, it is found that the flat-band potential increases with increasing treatment potential.


Introduction
Titanium and its alloy are highly solicited in many fields of use, especially in the area of medical implants [1][2][3][4]. Titanium is also used in submarines and aeronautical installations due to its physical properties and its lightness [5]. e resistance of titanium and its alloys in many acidic media makes them very useful in chemical industries [6][7][8][9]. is resistance against corrosion of titanium and its alloys is due to the formation of an oxide protective film. e nature and physical properties of the formed film depend on the mode of its formation. So, it is reported in the literature that the titanium oxide films exhibit either p-or n-type conductivity depending on their stoichiometry and the nature of the resulting defects [10,11]. Titanium oxide, especially TiO 2 , is widely used in photocatalyst application [12].
Further, it was found that, in most acidic media, titanium oxide exhibits high resistance to corrosion, and almost no anodic activity is observed in a wide range of applied voltage. However, it can act as catalytic support for the reduction reaction with a preferential affinity to the reduction of oxygen [13,14]. Titanium oxide was also investigated for its electrochromic properties [15] and is widely used in photovoltaic cells [15][16][17][18] and sensors [19][20][21].
Titanium oxide can be obtained by several techniques such as gel sol [22], thermal oxidation [23], radiofrequency sputtering [24], electrodeposition [25], and, in particular, anodic anodization of titanium and its alloys in different acidic or basic solutions. is technique seems promising since it is inexpensive and guarantees the reproducibility of the stoichiometry of the prepared films. e electrochemical study of titanium oxides obtained by different techniques presents different and very complex electrical circuits depending on the preparation technique of the oxide films. e objective of the present work is to prepare titanium oxide layers by anodizing Ti-Cu alloy (2%) in phosphoric acid at different potentials. ese layers will be characterized by X-ray diffraction, Raman spectroscopy, and energy dispersive X-ray spectroscopy (EDS). e oxidation state of the elements was studied by X-ray photoelectron spectroscopy. e surface morphology of the samples prepared was studied by scanning electron microscopy. e semiconductor nature of the formed films is revealed by Mott Schottky analysis, and an equivalent circuit was proposed by electrochemical impedance spectroscopy.

Materials and Methods
Anodization of Ti-Cu (2%) alloy was carried out in twoelectrode configuration, where titanium is the anode, and a sheet of highly pure platinum with a large surface was the cathode. e surface of the titanium alloy exposed to the solutions is 1 cm 2 . e anodization was performed with a stabilized tension DC power supply. e range of the applied potential extended from 20 to 35 V in 5 M phosphoric acid solution prepared from a 85% analytical-grade phosphoric acid.
Before anodization, the titanium samples were polished with different emery paper sizes (from 220 to 5000 grade). After, the samples were rinsed vigorously with tap water and distilled water followed by cleaning in an ultrasonic bath for 15 minutes.
After anodization, samples were characterized by X-ray diffraction to determine crystalline structures.
XPS was used to determine the oxidation states of titanium and oxygen on the surface of anodized samples. Additionally, the anodized samples were characterized by Raman spectroscopy with an excitation wavelength of 536 nm. e morphology of the obtained films and their compositions were determined by scanning electron microscopies coupled to an EDS analyzer. Some samples are annealed in an oven for five hours at 800°C. Impedance spectroscopy and the capacitance measurements at a frequency of 1 kHz were carried out using the type PZC 301 potentiostat/galvanostat in Na 2 SO 4 10 −1 M solution.

Results and Discussion
e anodized samples are colored, and their colors depend on the anodization potential. We start from a red-violet color at 20 V that becomes dark blue at 25 V then turns to a pale blue color at 35 V. e titanium alloy is coated with a colored layer after anodization. is allows broad use in the field of medical implants, given that we can know its location depending on the color of the implant [26].
During the anodization processes of samples in phosphoric acid 5 M, the evolution of the current density with time consists of a fast drop followed by stabilization at very low values close to zero, as shown in Figure 1.
It seems that the anodization time had no effect on the thickness since the composition of the film remained constant, as reported in Figure 2. Nevertheless, the thickness increased with increasing anodization voltage ( Figure 3). ese observations are in good agreement with phosphoric acid or other acidic media results reported in the literature [27,28].
When the titanium content decreases, the oxygen content increases as the anodization voltage increases. is suggests that the thickness of the films formed increases with the anodization voltage. However, when the voltage is fixed, the oxygen and titanium contents remain unchanged, and the color of the samples remains unchanged even if the anodization time varies.
From the diffractograms of anodized samples in phosphoric acid 5 M, the only peaks observed were attributed to titanium (file 00-001-1197). No peak characteristics of titanium oxide were observed, suggesting that the obtained oxide films are not crystallized. Further, it can be easily seen that the intensity of the Ti peaks decreases when the applied voltage increases, suggesting an increase in the thickness of the oxide films.
XPS measurements can determine the oxidation state of titanium in the formed films.   Advances in Materials Science and Engineering e Ti2p 3/2 peak is centered at 458.36 eV. Its width at half height is 1.56 eV. ese values show that the titanium is at oxidation state 4. ese characteristics are consistent with those of titanium in TiO 2 . Furthermore, the difference between the energy of Ti2p 3/2 and Ti2p 1/2 is around 5.7 eV. is value is characteristic of oxygen bond titanium in the TiO 2 oxide type [29,30]. e oxygen peak, shown in Figure 5(b), is large and can be deconvoluted to at least two subpeaks, centered at 530.6 eV and 532 eV. e first peak is attributed to the oxygen engaged in a Ti-O bond in TiO 2, and the second corresponds either to oxygen bound to water or adsorbed oxygen type O 2 [31,32]. Indeed, a strong release of oxygen was observed during the anodization. Figure 6 shows the Raman spectra of samples anodized at different voltages in 5 M H 3 PO 4 . We note the presence of two visible bands: the first is centered at 443 cm −1 and the other is centered at 608 cm −1 . is is in addition to a weaker, wider, and less pronounced band at 160 cm −1 .
To be able to attribute the different observed bands, we performed Raman analysis on the same samples after having been annealed at 800°C for 5 hours. Figure 7 shows the obtained spectrum for the sample anodized at 25 V. e bands observed for the annealed sample (blue line) correspond to those of the rutile phase of TiO 2 .
is is further confirmed by the X-ray diffraction spectrum of the anodized samples, which has been annealed for 5 hours at  Advances in Materials Science and Engineering 80°C ( Figure 8); it clearly shows that annealed films consist mainly of rutile TiO 2 phase. According to the literature, the bands appearing on the Raman spectrum (Figures 6 and 7) may be allocated as follows.
e band at 443 cm −1 corresponds to the Eg mode of the TiO 2 rutile, while the one centered at 608 cm −1 corresponds to the A1g mode of the TiO 2 rutile [33].

Advances in Materials Science and Engineering
However, other studies attribute this band to the anatase phase [40]. From the above, it can be concluded that, during anodization, a poorly crystallized rutile TiO 2 is formed.
After anodization of the samples in 5 M phosphoric acid, we recorded the evolution of the open-circuit potential as a function of time in 0.1 M of Na 2 SO 4 . As shown in Figure 9, we can note that the potential stabilizes at a noble value with increasing anodization potential and also protects the titanium surface alloy against corrosion better. After one hour, the potential tries to be fixed and we can subsequently perform Nyquist and Bode plots. From Nyquist representation, and as shown in Figure 10(a) inset, it can be concluded that there are no masked phenomena at high frequency.
e Bode representation, as presented in Figure 10(b), shows that the interfacial process is the same for all studied samples. Moreover, the observed phase shift is negative, suggesting a capacitive behavior of the equivalent electrical circuit. e peak observed at high frequency suggests the presence of a constant phase element (CPE) since the phase shift does not reach −90 (≈−70 rad). e existence of masked phenomena at low frequency cannot be excluded since the phase shift at low frequency (between 100 MHz and 1 Hz) is different from zero, and there is a small shoulder when log (frequency) is equal to −0.5.
e plot of Z Modulus as a function of the logarithm of the frequency shows the presence of two straight line segments with slopes α 1 and α 2 (Figure 11). e values of these parameters are regrouped in Table 1.
It can be observed that both parameters are less than one, while the values of α 1 are close to 0.5. is suggests that the equivalent circuit may include two CPEs with the presence of a Warburg diffusion regime (W) [41].
Indeed, the equivalent circuit capable of fitting the experimental curves (−imZ � f (ReZ)), with an acceptable correlation coefficient, is given in Figure 12.
In this diagram, R s is the solution resistance, R f is the resistance of the formed films, and R dt is the charge transfer resistance. Moreover, CPE f is the constant phase element relative to the formed films, CPE dl is the constant phase element relative to the charge transfer, and W is the Warburg impedance. e surface roughness of the samples may account for the CPE in the electrical equivalent circuit. Indeed, the SEM micrographs ( Figure 13) of a sample polished by emery paper show a rough surface, which leads to distributed elements and consequently to a nonideal capacity. e experimental curve and the best fit obtained by this equivalent circuit are presented in Figure 14. Figure 14 illustrates that there is good agreement between the theoretical and experimental curves. Additionally, we note that the metal/oxide/solution interface remains the same in the range of applied potential. Furthermore, the fitting parameters grouped in Table 2 further support the conclusions derived from the Bode representation. Using Mansfield law, we get (1) We have determined the film capacitance C f and the capacitance of the double layer C ct . e thickness d of the films is obtained from the relation C � εε 0 /d S, where ε is the relative permittivity of the formed film, εo is the absolute permittivity of the vacuum, and S is the surface area of the film.   Figure 12: Electrical equivalent circuit for the as-anodized Ti-Cu (2%) alloy. Experimental curve Fitted curve Figure 14: Nyquist representation and the best fit for the sample anodized at 30 V in 5M phosphoric acid. e parameters from the best fits are given in Table 2.   From Table 2, we can say that increasing anodization potential results in a decrease in the capacitance of the formed film and, consequently, to an increase in the thickness of the formed films.
Previous studies [42] have shown that the thickness (d) of the films obtained by anodizing titanium in H 3 PO 4 varies linearly as a function of the anodizing voltage (Va) according to the following equation: (2) To have this order of magnitude, we must choose a dielectric constant value of around 30 for the formed films.
is value has been reported in the literature for TiO 2 films from anodized titanium samples in aqueous media [20,43].
It is important to note that the capacity of the double layer is at least 300 times greater than that of the formed film. Consequently, the Mott Schottky plot of 1/c 2 as a function of V at constant frequency is relative to film capacitance.
To determine the charge carrier density N d , we used the following expression: where εr is the dielectric constant of the film, ε 0 is the vacuum permittivity (8.854 × 10 −14 F cm −1 ), e is the elementary charge, E fb is the flat-band potential, K is the Boltzmann constant (1.38 × 10 −23 J/K), and T is the absolute temperature.
In Figure 15, we present the evolution of 1/c 2 against potential in Na 2 SO 4 0.1 N at a frequency of 1 kHz. e curves have two inflection points, between which a large linear increase of 1/c 2 as a function of the potential E was observed. e slope of these lines is positive, indicating that the formed oxide TiO 2 is an n-type semiconductor. e slope of this line permits us to determine the values of charge carrier density Nd and the flat-band potential, which are summarized in Table 3 as a function of the anodization voltage. e increase of the charge carrier density with the anodization voltage can be related to the nature of defects of the oxide formed during anodization as summarized in Table 3. Additionally, the flat-band potential decreases as a function of the anodizing voltage; this is in perfect agreement with the increase of charge carrier numbers (i.e., a decrease in band bending).

Conclusion
Samples of titanium copper (2%) alloy were anodized in 5M phosphoric acid under potential control. e range of applied potential was extended from 20 to 35 V with a twoelectrode configuration. e obtained anodized samples were colored and easily distinguishable depending on their anodized potential. e chronoamperometric curves recorded during the anodization process showed an abrupt drop of the current with time due to the formation of a protective layer.
It is found that the thickness of the formed films increased with increasing anodizing potential. e formed films consisted of a poorly crystallized rutile TiO 2 , as shown by Raman, X-ray diffraction, and XPS and EDX measurements.
e electrochemical impedance spectra obtained in 0.1 M of Na 2 SO 4 , for the anodized samples showed that the charge transfer resistance increased with increasing applied potential. In addition, it was found from the electrochemical impedance study that the thickness of the formed film increases with increasing applied potential and that the relative permittivity of the formed oxide is approximately 30. e formed film is an n-type semiconductor, and the charge carrier density is in the range of 10 18 cm −3 .

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding the publication of this paper.

Supplementary Materials
e graphical abstract resume different techniques used in the present work. We used the anodization technique for obtaining oxide films under potential control, with two electrode devices. Obviously, the current density was also recorded. e obtained film was characterized before annealing and after being annealed with different characterization techniques such as X-ray diffraction, XPS, Raman shift, and also scanning electron microscopy coupled to EDS (right side of the graphical abstract).