# Electrochemical Study of Anodized Titanium in Phosphoric Acid

Authors: M. Khadiri; M. Elyaagoubi; R. Idouhli; Y. Koumya; O. Zakir; J. Benzakour; A. Benyaich; A. Abouelfida; A. Outzourhit
Journal: Advances in Materials Science and Engineering
 (2020)

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## Abstract

The anodization of the Ti-Cu (2%) alloy was carried out in a 5M H3PO4 solution for 2 minutes. The obtained layers are characterized by XPS, X-ray diffraction, and Raman spectroscopy. The 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. Thus, the Mott Schottky model revealed that the formed film is an n-type semiconductor. The 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.


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## Body

## 1. Introduction

Titanium and its alloy are highly solicited in many fields of use, especially in the area of medical implants [1–4]. Titanium is also used in submarines and aeronautical installations due to its physical properties and its lightness [5]. The resistance of titanium and its alloys in many acidic media makes them very useful in chemical industries [6–9]. This resistance against corrosion of titanium and its alloys is due to the formation of an oxide protective film. The 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 TiO2, 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 [15–18] and sensors [19–21].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. This technique seems promising since it is inexpensive and guarantees the reproducibility of the stoichiometry of the prepared films. The 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.The objective of the present work is to prepare titanium oxide layers by anodizing Ti-Cu alloy (2%) in phosphoric acid at different potentials. These layers will be characterized by X-ray diffraction, Raman spectroscopy, and energy dispersive X-ray spectroscopy (EDS). The oxidation state of the elements was studied by X-ray photoelectron spectroscopy. The surface morphology of the samples prepared was studied by scanning electron microscopy. The semiconductor nature of the formed films is revealed by Mott Schottky analysis, and an equivalent circuit was proposed by electrochemical impedance spectroscopy.


## 2. Materials and Methods

Anodization of Ti-Cu (2%) alloy was carried out in two-electrode configuration, where titanium is the anode, and a sheet of highly pure platinum with a large surface was the cathode. The surface of the titanium alloy exposed to the solutions is 1 cm2. The anodization was performed with a stabilized tension DC power supply. The 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. The 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 Na2SO4 10−1 M solution.


## 3. Results and Discussion

The 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. The titanium alloy is coated with a colored layer after anodization. This 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 Figure1.Figure 1
Chronoamperometric curves obtained at different applied potential during 2 min, in 5 M phosphoric acid.It seems that the anodization time had no effect on the thickness since the composition of the film remained constant, as reported in Figure2. Nevertheless, the thickness increased with increasing anodization voltage (Figure 3). These observations are in good agreement with phosphoric acid or other acidic media results reported in the literature [27, 28].Figure 2
Amounts of titanium and oxygen at fixed polarization voltage (25 V) at various times.Figure 3
Amounts of titanium and oxygen during 2 min at various polarization voltages. Samples anodized at different applied voltage were characterized by X-ray diffraction. Figure4 illustrates the results.Figure 4
X-ray diffraction of anodized titanium in phosphoric acid 5 M obtained at different voltages for 2 min.∗Sample carrier.When the titanium content decreases, the oxygen content increases as the anodization voltage increases. This 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. Figures5(a) and 5(b) show the Ti2p and O1s XPS peaks of the oxide films obtained from the titanium-copper alloy anodized in phosphoric acid 5 M at 35 V under potentiostatic control.Figure 5
Ti2p XPS spectra for titanium anodized at 35 V for 2 min in 5 M phosphoric acid. (a) Ti2p spectra; (b) O1s spectra.
(a)
(b)The Ti2p3/2 peak is centered at 458.36 eV. Its width at half height is 1.56 eV. These values show that the titanium is at oxidation state 4. These characteristics are consistent with those of titanium in TiO2. Furthermore, the difference between the energy of Ti2p3/2 and Ti2p1/2 is around 5.7 eV. This value is characteristic of oxygen bond titanium in the TiO2 oxide type [29, 30].The oxygen peak, shown in Figure5(b), is large and can be deconvoluted to at least two subpeaks, centered at 530.6 eV and 532 eV. The first peak is attributed to the oxygen engaged in a Ti-O bond in TiO2, and the second corresponds either to oxygen bound to water or adsorbed oxygen type O2 [31, 32]. Indeed, a strong release of oxygen was observed during the anodization.Figure6 shows the Raman spectra of samples anodized at different voltages in 5 M H3PO4. 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. This is in addition to a weaker, wider, and less pronounced band at 160 cm−1.Figure 6
Raman spectra of different anodized samples at different voltages in H3PO4 5 M.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. Figure7 shows the obtained spectrum for the sample anodized at 25 V.Figure 7
Raman spectra obtained for anodized sample at 25 V before and after annealing at 800°C for 5 hours.The bands observed for the annealed sample (blue line) correspond to those of the rutile phase of TiO2. This is further confirmed by the X-ray diffraction spectrum of the anodized samples, which has been annealed for 5 hours at 80°C (Figure 8); it clearly shows that annealed films consist mainly of rutile TiO2 phase.Figure 8
X-ray diffraction of anodized samples annealed at 800°C for 5 Hours. •: Simple careers;Ti: titanium; R: rutile.According to the literature, the bands appearing on the Raman spectrum (Figures6 and 7) may be allocated as follows.The band at 443 cm−1 corresponds to the Eg mode of the TiO2 rutile, while the one centered at 608 cm−1 corresponds to the A1g mode of the TiO2 rutile [33].Other studies have suggested that the band observed at 146 cm−1 is relative to the B1g mode of rutile [34–39]. 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 TiO2 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 Na2SO4. 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.Figure 9
Open-circuit potential against time in 0.1 M Na2SO4.The Nyquist and Bode representations of the impedance spectra of the anodized Ti-Cu alloy (2%) at different voltages in H3PO4 5M for 2 min, obtained in 10−1M of Na2SO4 at open-circuit potential (OCP), are, respectively, shown in Figures 10(a) and 10(b).Figure 10
Electrochemical impedance spectroscopy of anodized samples, obtained in 10−1 M of Na2SO4 at OCP. ((a) Nyquist representation; (b) Bode representation).
(a)
(b)From Nyquist representation, and as shown in Figure10(a) inset, it can be concluded that there are no masked phenomena at high frequency.The Bode representation, as presented in Figure10(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. The peak observed at high frequency suggests the presence of a constant phase element (CPE) since the phase shift does not reach −90 (≈−70 rad). The 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.The plot ofZ Modulus as a function of the logarithm of the frequency shows the presence of two straight line segments with slopes α1 and α2 (Figure 11). The values of these parameters are regrouped in Table 1.Figure 11
log (z) against log (frequency) for sample anodized at 20 V in 5 M phosphoric acid.Table 1
α
1 and α2 values resorted from slopes of log (z) against log (frequency) at different anodization voltages.
Anodization voltage
20 V
25 V
30 V
35 V
α1
0.36
0.40
0.35
0.35
α2
0.81
0.80
0.84
0.83It can be observed that both parameters are less than one, while the values ofα1 are close to 0.5. This 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.Figure 12
Electrical equivalent circuit for the as-anodized Ti-Cu (2%) alloy.In this diagram,Rs is the solution resistance, Rf is the resistance of the formed films, and Rdt is the charge transfer resistance. Moreover, CPEf is the constant phase element relative to the formed films, CPEdl is the constant phase element relative to the charge transfer, and W is the Warburg impedance.The surface roughness of the samples may account for the CPE in the electrical equivalent circuit. Indeed, the SEM micrographs (Figure13) of a sample polished by emery paper show a rough surface, which leads to distributed elements and consequently to a nonideal capacity.Figure 13
Micrograph of Ti-Cu (2%) freshly polished with emery paper (until 5000 grade).The experimental curve and the best fit obtained by this equivalent circuit are presented in Figure14.Figure 14
Nyquist representation and the best fit for the sample anodized at 30 V in 5M phosphoric acid. The parameters from the best fits are given in Table2.Table 2
Electrochemical parameters for the electrical equivalent circuit obtained in Na2SO4 0.1 M
(V)
χ × 103
R
1 (Ω)
Q
f × 106 (F×cm−2)
a
1 (α2)
C
f × 106 (F×cm−2)
R
f (Ω)
Q
dl × 106 (F × cm−2)
a
2 (α1)
C
dl × 106 (F×cm−2)
R
ct (Ω)
S3 ( Ω × s−1/2)
20
25
35.33
1.28
0.92
0.6
745
29.80
0.42
132
101165
15011
25
50
30.07
1.22
0.90
0.58
1042
28.28
0.40
258
154531
23354
30
36
27.43
0.75
0.94
0.46
779
29.88
0.38
365
155282
36812
35
24
30.03
0.6
0.94
0.33
726
29.1
0.38
359
160000
61223Figure14 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)Ci=Qi×Ri1−α1/α.We have determined the film capacitanceCf and the capacitance of the double layer Cct.The thicknessd of the films is obtained from the relation C=εε0/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.From Table2, 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 H3PO4 varies linearly as a function of the anodizing voltage (Va) according to the following equation:(2)d=1.94×Va+5.2nm.To have this order of magnitude, we must choose a dielectric constant value of around 30 for the formed films. This value has been reported in the literature for TiO2 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/c2 as a function of V at constant frequency is relative to film capacitance.To determine the charge carrier densityNd, we used the following expression:(3)1C2=2εε0eNdE−Efb+KTe,where εr is the dielectric constant of the ﬁlm, ε0 is the vacuum permittivity (8.854 × 10−14 F cm−1), e is the elementary charge, Efb is the ﬂat-band potential, K is the Boltzmann constant (1.38 × 10−23 J/K), and T is the absolute temperature.In Figure15, we present the evolution of 1/c2 against potential in Na2SO4 0.1 N at a frequency of 1 kHz. The curves have two inflection points, between which a large linear increase of 1/c2 as a function of the potential E was observed. The slope of these lines is positive, indicating that the formed oxide TiO2 is an n-type semiconductor. The 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.Figure 15
Mott–Schottky curve obtained at 1 KHz, in 0.1 M of Na2SO4 sample anodized in phosphoric acid.Table 3
Electrical parameters of titanium oxide.
Applied potential
20 V
25 V
30 V
35 V
Nd (×1018 cm−3)
1.31
1.44
1.96
5.28
Efb (V)
−0.47
−0.42
−0.39
−0.12The 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 Table3. 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).


## 4. Conclusion

Samples of titanium copper (2%) alloy were anodized in 5M phosphoric acid under potential control. The range of applied potential was extended from 20 to 35 V with a two-electrode configuration. The obtained anodized samples were colored and easily distinguishable depending on their anodized potential. The 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. The formed films consisted of a poorly crystallized rutile TiO2, as shown by Raman, X-ray diffraction, and XPS and EDX measurements.The electrochemical impedance spectra obtained in 0.1 M of Na2SO4, 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. The formed film is an n-type semiconductor, and the charge carrier density is in the range of 1018 cm−3.



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