# Fabrication of TiO2 Nanotube by Electrochemical Anodization: Toward Photocatalytic Application

Authors: O. Zakir; R. Idouhli; M. Elyaagoubi; M. Khadiri; A. Aityoub; Y. Koumya; S. Rafqah; A. Abouelfida; A. Outzourhit
Journal: Journal of Nanomaterials
 (2020)

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## Abstract

In this study, a self-organized nanotubular titanium dioxide (TiO2) array was successfully produced by anodizing pure titanium in a mixture of glycerol, distilled water (8% vol.), and ammonium fluoride using a dual electrode system. The size control and distribution of the nanopores were performed in a DC voltage range varying from 30 V to 60 V. The diameter of TiO2 nanopores varies from 59 to 128 nm depending on the anodizing voltage. Energy-dispersive X-ray spectroscopy (EDX) analysis reveals that the as-prepared films are essentially composed of TiO2. According to the X-ray diffraction (XRD) and Raman spectroscopy analysis, the nanotubular arrays of TiO2 annealed at 600°C for 2 hours are composed of a phase mixture of anatase and rutile. Mott-Schottky analysis showed that the TiO2 nanotubes are consistent with an n-type semiconductor with a donor density of about 1017 cm-3. Preliminary results on the photocatalytic degradation of a pharmaceutical pollutant showed that the TiO2 nanotubes can be used as a promising material for application in wastewater treatment.


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## Body

## 1. Introduction

Recently, owing to the diverse application of titanium dioxide TiO2, a thorough research and experiments have been devoted to the preparation of titanium dioxide TiO2 and the considerable number of parameters determining its formation. There is a vast amount of literature on the titanium dioxide application such as photoelectrochemical water splitting [1], water [2, 3] and air purification [4], solar energy conversion [5], medical applications [6], gas sensors [7], and supercapacitors [8]. These applications rely on the specific semiconductor nature of TiO2, in particular anatase, which is an n-type semiconductor with a large band gap of about 3.2 eV [9].In the last years, several techniques have received considerable attention for elaborating TiO2 nanostructures with promising properties. The most often used methods include the sol-gel process [10, 11], the electrochemical anodization of titanium [12], the hydrothermal method [13], and sputtering [14]. Among these methods, titanium anodization is the most effective way to produce highly ordered nanoporous TiO2 films [15]. It has been now demonstrated that two types of TiO2 morphology can be obtained by electrochemical anodization: compact TiO2 films are generally obtained in fluoride free electrolytes, whereas porous films can be prepared in electrolytes containing fluoride ions [16, 17].The nanostructure of the pores on the titanium surface obtained by electrochemical anodization is strongly affected by several parameters, such as applied potential [12, 18, 19], anodization bath temperature [20], electrolyte composition [21], anodizing time [12, 19, 22], water content in the electrolyte [23, 24], and the fluoride ion concentration [19, 21, 25].Considerable attention has been directed to study the mechanism of formation of nanotubular TiO2 films by electrochemical anodization. Consequently, a number of theories based on field dissolution [26] and localized acidification at the pore bottom that increases chemical dissolution [27, 28] have been proposed to explain some aspects of the mechanism related to the TiO2 nanotube formation and growth.The synthesis and properties of one-dimensional tubular arrays have been widely investigated. Lockman et al. [18] reported that in a mixture of Na2SO4 (1 M) and 5 wt% NH4F, the diameters and lengths of the nanotubes increased with increasing anodization voltage from 10 V to 25 V and the average diameters of the nanotubes were about 80 nm, 70 nm, and 50 nm for anodization voltages of 20 V, 15 V and 12 V, respectively. Albu et al. [29] showed that the geometry of the nanotubular layer depends strongly on the applied potential and the fluoride concentration. However, Kulkarni et al. [24] studied the effect of anodization parameters on the morphology of the TiO2 nanostructure and the mechanism converting the as-formed nanopores to nanotubes.Very little work has been carried out on the anodization of titanium in glycerol. Indeed, Sreekantan et al. [30] have stipulated that in glycerol containing 6 wt% of ethylene glycol (EG) and 5 wt% NH4F, in the voltage range from 20 to 60 V, the uniform TiO2 nanotubes were reached with a voltage less than 30 V. For a potential up to 50 V, the structure of the anodized titanium tended to be irregular.This work reports on the effect of anodization voltage on the morphology of nanotubular TiO2 on the pure titanium surface and their electrochemical, structural, optical, and electrical properties. Different approaches are thoroughly investigated with the aim of producing a nanotubular titanium dioxide including current time transients, SEM along with EDX, X-ray diffraction, Raman spectroscopy, and impedance measurements.


## 2. Experimental

### 2.1. Chemicals

Titanium foil (99.99% pure, 2 mm thick), glycerol (99.8%, anhydrous), ammonium fluoride (98%), acetic acid (99.98%), HF (40%), H2SO4 (96%), and carbamazepine (99% purity) were purchased from Sigma-Aldrich (St-Louis, USA).


### 2.2. Elaboration and Characterization

To obtain reliable and reproducible results, the pure titanium sample underwent, before each test, a pretreatment consisting of polishing the electrode surface with an increasingly fine grade emery paper (SiC #400, #1200, #2000, and #4000), followed by rinsing with distilled water and then drying under airflow.Afterwards, the sample was electrolytically polished in a solution containing hydrofluoric acid (18%v/v), sulfuric acid (40% v/v), and acetic acid (42% v/v) at a voltage of 11 V for 30 s. Samples are then abundantly rinsed with distilled water.The anodization of pure titanium foils was carried out in an electrolytic bath consisting of a mixture of glycerol-distilled water (92 : 8%v/v) and ammonium fluoride (0.4 M) for 60 min, at a voltage ranging from 30 to 60 V, using a two-electrode cell with the titanium sample as the anode and a platinum electrode as the counter electrode.Later with the intention of enhancing the crystallinity of as-synthesized nanoporous TiO2 films, the anodized samples were annealed in a muffle furnace at 600°C for two hours with a heating ramp of 10°C/min.The electrochemical study was performed in a 0.1 M Na2SO4 solution using a conventional three-electrode cell consisting of an anodized TiO2 as the working electrode (WE), a saturated calomel electrode (SCE) as the reference electrode, and a platinum sheet (4 cm2) as the counter electrode [31]. The measurements were performed using a VoltaLab potentiostat (PGZ301) controlled by the VoltaMaster 4 software. To perform the EIS experiments at steady state, the rotational speed of the stirrer was set at 200 rpm. The temperature was controlled in jacketed glass at 293 K using a bath thermostat (±1°C). The Mott-Schottky (MS) analysis was performed at a frequency of 1 kHz in wide voltage range (from -1 V to 1 V/SCE).The morphology of the anodized samples and the elemental composition of the oxide films are examined by scanning electron microscopy (SEM) along with energy-dispersive X-ray spectroscopy (EDX). The crystal structure of the oxide films was determined by X-ray diffraction (XRD) (Rigaku, SmartLab SE, Cu Kα1, λ=1.5418Å) and Raman spectroscopy (Confotec MR520) with laser of wavelength λ=532nm. Pore diameter measurement is performed using the image processing software ImageJ.The irradiation system used is equipped with an Ultra-Vitalux lamp at 300 W with high-pressure tungsten filament. A double-jacketed glass reactor allowed maintaining the temperature at 25°C of the solution during the irradiation time.


## 2.1. Chemicals

Titanium foil (99.99% pure, 2 mm thick), glycerol (99.8%, anhydrous), ammonium fluoride (98%), acetic acid (99.98%), HF (40%), H2SO4 (96%), and carbamazepine (99% purity) were purchased from Sigma-Aldrich (St-Louis, USA).


## 2.2. Elaboration and Characterization

To obtain reliable and reproducible results, the pure titanium sample underwent, before each test, a pretreatment consisting of polishing the electrode surface with an increasingly fine grade emery paper (SiC #400, #1200, #2000, and #4000), followed by rinsing with distilled water and then drying under airflow.Afterwards, the sample was electrolytically polished in a solution containing hydrofluoric acid (18%v/v), sulfuric acid (40% v/v), and acetic acid (42% v/v) at a voltage of 11 V for 30 s. Samples are then abundantly rinsed with distilled water.The anodization of pure titanium foils was carried out in an electrolytic bath consisting of a mixture of glycerol-distilled water (92 : 8%v/v) and ammonium fluoride (0.4 M) for 60 min, at a voltage ranging from 30 to 60 V, using a two-electrode cell with the titanium sample as the anode and a platinum electrode as the counter electrode.Later with the intention of enhancing the crystallinity of as-synthesized nanoporous TiO2 films, the anodized samples were annealed in a muffle furnace at 600°C for two hours with a heating ramp of 10°C/min.The electrochemical study was performed in a 0.1 M Na2SO4 solution using a conventional three-electrode cell consisting of an anodized TiO2 as the working electrode (WE), a saturated calomel electrode (SCE) as the reference electrode, and a platinum sheet (4 cm2) as the counter electrode [31]. The measurements were performed using a VoltaLab potentiostat (PGZ301) controlled by the VoltaMaster 4 software. To perform the EIS experiments at steady state, the rotational speed of the stirrer was set at 200 rpm. The temperature was controlled in jacketed glass at 293 K using a bath thermostat (±1°C). The Mott-Schottky (MS) analysis was performed at a frequency of 1 kHz in wide voltage range (from -1 V to 1 V/SCE).The morphology of the anodized samples and the elemental composition of the oxide films are examined by scanning electron microscopy (SEM) along with energy-dispersive X-ray spectroscopy (EDX). The crystal structure of the oxide films was determined by X-ray diffraction (XRD) (Rigaku, SmartLab SE, Cu Kα1, λ=1.5418Å) and Raman spectroscopy (Confotec MR520) with laser of wavelength λ=532nm. Pore diameter measurement is performed using the image processing software ImageJ.The irradiation system used is equipped with an Ultra-Vitalux lamp at 300 W with high-pressure tungsten filament. A double-jacketed glass reactor allowed maintaining the temperature at 25°C of the solution during the irradiation time.


## 3. Results and Discussion

### 3.1. Current Time Transient Measurements

During the anodizing process, the variation of the current density as a function of time is monitored. Figure1 shows the anodizing current density/time curves during the anodization at different potentials. The observed transients share similar trends. Three stages are typically observed in these curves related to the typical current density-time curve of the porous oxide formed under constant voltage. The kinetic trend consists of an initial fast drop (stage A), a steady increase (stage B), and a quasisteady state current density (stage C).Figure 1
Current density/time curves obtained at different anodization potentials.At stage A, the anodizing process begins, and the current quickly decreases to a minimum value because of the formation of a high resistance compact oxide layer on the surface by the interaction of the Ti4+ ions with the oxygen O2- ions in the electrolyte according to the following reaction [32]:
(1)Ti+2H2O→TiO2+4e−+4H+At stage B, the current subsequently rises to a maximum as the pore formation progresses. This is due to the chemical dissolution of the oxide layer by fluoride ions that is promoted by an electric field created between the cell electrodes. Small pits are formed on the surface of the compact layer followed by the formation of a nanoporous structure according to the following reaction [32, 33]:
(2)TiO2+6F−+4H+→TiF6−+2H2OAt the final stage, the current density attains a constant value when a steady state is reached owing to the formation of TiO2 nanotubes [34].


### 3.2. Morphological and Composition Characterization

For the morphological analysis of TiO2 nanotubular layers formed on the titanium substrate at different potentials, a scanning electron microscope TESCAN VEGA 3 was used. SEM top images and cross-sections of nanotube arrays produced in glycerol-distilled water (92 : 8% v/v) and NH4F (0.4 M) for 60 min at different anodizing potentials are given in Figure 2.Figure 2
SEM top images and cross-sections of nanotube layers grown in glycerol-distilled water (92 : 8%v/v) and NH4F (0.4 M) for 60 min at different anodizing potentials: (a) 30 V, (b) 40 V, (c) 50 V, and (d) 60 V.
(a)(b)(c)(d)The electrochemical anodization of titanium under these conditions leads to the formation of an ordered nanotubular array on the titanium surface. For the anodized sample at 30 V, there are areas where the oxide formed is compact. In addition, other areas have an ordered distribution of medium-sized nanotubes of about 59 nm in diameter. For voltages above 30 V, the surface of the samples is made up of neat TiO2 nanotubes. The average diameters of the nanotubes are about 80 nm and 128 nm for an applied voltage of 40 V and 60 V, respectively.In addition, the lengths of the nanotubes were measured by scanning electron microscope observation of cross-sections of the films. The result showed that the length of the TiO2 nanotube obtained on the surface of the titanium metal increases with the increasing anodization potential to reach approximately 1.5 μm at 60 V.Figure3 shows the evolution of nanotubular internal diameter as a function of the anodizing voltage. It is clearly seen that the nanotube diameter increases linearly with the anodizing potential with a good correlation coefficient R2 that is close to 1 (0.97). It should be noted that for a potential of 30 V, the internal diameter of the nanotubes is about 59 nm, while it reached 128 nm for a potential of 60 V.Figure 3
The variation of the TiO2 nanotube diameters as a function of the anodizing voltage for 60 minutes in glycerol-distilled water (92 : 8% v/v) and NH4F (0.4 M).


### 3.3. Composition Analysis by EDX

For the elemental characterization of the obtained nanotubular layers, energy-dispersive X-ray analysis is conducted using an acceleration voltage of 10 kV. The characteristics of the emitted X-rays from the anodized substrate produced at different conditions are presented in Figure4. The EDX spectrum indicates the presence of the Tikɑ peak at 4.58 eV and O peaks at 0.5 eV, as well as F and C peaks in the anodized sample. The obtained percentages of Ti (26.5 at%) and O (41.4 at%) yield a Ti/O ratio of about 0.5 indicating that the structure of the formed oxide is TiO2 with the presence of fluorine that arises from the solution during the anodization process [35] and carbon which comes from the metallization process using graphite.Figure 4
EDS spectrum of TiO2 obtained at 50 V for 60 minutes in glycerol-distilled water (92 : 8% v/v) and NH4F 0.4 M.


### 3.4. XRD Structural Characterization

The crystal phases of as-prepared and annealed samples were identified by XRD using Cu Kα radiation. As show in Figure 5, the formed TiO2 nanotubular film consists of a mixture of anatase and rutile phases. The anatase phase was identified from the peaks at ca. 25.03°, 48.05°, 54.85°, and 55.30° characteristic of the planes (101), (200), (105), and (211), respectively, according to JCPDS file no. 21-1272. In addition, the rutile phase is revealed by peaks at 27.4°, 36.07°, 41°, 43.6°, and 56.9°, corresponding to planes (110), (101), (110), (111), (210), and (220) (JCPDS card no. 21-1276), respectively. XRD spectra of annealed oxides reveal that TiO2 peaks become more intense when the anodizing potential increases.Figure 5
XRD spectra of samples anodized at different anodization potentials in glycerol-distilled water (92 : 8%v/v) and NH4F 0.4 M.The crystallite size is an important factor to determine the stability of nanotubular TiO2 crystalline phases [1, 36]. The average crystallite size of nanotubular TiO2 samples is calculated using the Scherrer equation D=0.9λ/βCosθ, where D is the grain size, λ (1.548 Å) is the wavelength of X-ray radiation used, θ is the Bragg diffraction angle obtained from XRD peak, and β is the full width at half maximum of the diffraction peak [37]. Using the width of the (101) peak for anatase and the width of the (110) peak for rutile, crystallite sizes were obtained in the ranges 7.3 to 8.5 nm and 8.2 to 9.7 nm for anatase and rutile phases, respectively.On the other hand, the phase composition of TiO2 has a crucial impact on the photocatalytic activity. The effect of the anatase/rutile ratio is discussed in many controversial works. It is reported that a mixture of anatase and rutile phases was found suitable for photocatalytic oxidation of organic pollutants in water [38]. Nevertheless, Tayade et al. [39] suggested that only the anatase phase has attracted considerable attention as a photocatalyst for the chemical treatment of organic pollutants. In our conditions, the relative amounts of anatase and rutile were estimated at 82% and 18%, respectively. This result is similar to that for the commercial TiO2, Degussa P25, which exhibits a strong photocatalytic activity and has an anatase-rutile mass fraction of 80 : 20.


### 3.5. Structural Characterization by Raman Microscopy

The TiO2 films obtained at different anodizing voltages were also characterized by Raman spectroscopy after thermal annealing at 600°C. Figure 6 shows the Raman spectra of the different anodized samples at different anodization potentials in a glycerol-distilled water mixture.Figure 6
Raman spectra of anodized simples at different anodization potentials in glycerol-distilled water (92 : 8%v/v) and NH4F 0.4 M.For the as-anodized sample, we noticed the absence of Raman peaks indicating the amorphous nature of the formed oxide [40]. However, the Raman spectra of all annealed samples depict a similar trend and show a very intense band at 143 cm-1 and four others at 195, 394, 515, and 637 cm-1. These bands can be attributed to the vibrational modes Eg1,Eg2,B1g,A1g, orB1g(superposition ofA1gandB1g), andEg of anatase, respectively. The observed bands confirm the predominance of anatase as the crystalline phase in the structure [41]. However, peaks of about 447 and 619 cm-1 indicate the presence of a certain amount of rutile and their intensities increase with the anodizing potential. These vibration modes are shifted by 2 to 4 cm-1 due to the variation in grain size and stoichiometric defects present in TiO2 structures as reported by Bassi et al. [42]. These structural observations are in good agreement with the XRD analysis.


### 3.6. Mott-Schottky (MS) Analysis

Electrochemical capacitance measurements were carried out to characterize the semiconductor nature of the films and to determine the donor densityND of the formed TiO2 layers, as well as to estimate the flat band potential (EFB) of the anodized samples at different potentials. MS analysis was performed in Na2SO4 0.1 M at 1 kHz in the potential range of -1 to 1 V/SCE. Figure 7 shows the variation of 1/Cs2 as a function of potential E (V/SCE).Figure 7
Mott-Schottky plots obtained for the formed nanotubularTiO2 layers in 0.1 M Na2SO4 solution.As shown in Figure7, a quasilinear behavior of the variation of 1/Cs2 as a function of the potential was observed. All curves have a positive slope corresponding to an n-type semiconductor [43]. The parameters ND and EFB can be determined from the slope of the linear regions in the Mott-Schottky plot and its intersection with the x-axis, respectively, according to the Mott-Schottky relationship:
(3)1CS2=2ɛ0ɛSeNDE−EFB−kTe,where CS is the space charge layer capacitance, e the electron charge (1.6010−19C), ɛ0the permittivity of free space (8.8510−14Fcm−1), ɛS the dielectric constant of TiO2 which is assumed to be 100 F cm-1 [44], E the applied potential, k the Boltzmann constant (1.381023JK−1), and T the absolute temperature. The values of the ND and EFB of the different samples are summarized in Table 1.Table 1
ND and EFB of anodized samples at different potentials.
Tension (V)EFB (V/SCE)ND×10−17 (cm-3)30-0.131.9540-0.163.3750-0.102.4560-0.144.25Table1 shows clearly that the ND values increase with anodizing potential, and the values obtained are in the order of 1017 cm-3. The ND values obtained are in good agreement with those reported in the literature for nanotubular TiO2 [45]. Generally, the growth of the anodized oxide films is always nonstoichiometric with an excess/deficiency of metal cations or oxygen anions [40]. According to the Point Defect Model (PDM), the n-type semiconductor behavior of an anodic passive film indicates that the defects in all samples are due to the oxygen deficiencies and/or interstitial titanium ions [46]. Somehow, Peng [47] reported that the predominance of oxygen deficiency is due to the low formation energy of oxygen vacancy compared to the interstitial titanium.


### 3.7. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) technique was used to study the electrochemical behavior of the interface of TiO2 nanotubular film. Figures 8(a) and 8(b) present the Nyquist and Bode diagrams of the EIS data obtained for the TiO2 films with their fitting adjustments obtained using EC-Lab software with the suggested equivalent circuit (inset in Figure 8(a)).Figure 8
Diagrams of the EIS data obtained for the TiO2: (a) Nyquist representations and (b) Bode representations. The equivalent circuit used to fit the experimental impedance spectra is inserted into (a).
(a)(b)The Bode diagrams of Figure8(b) reveal the presence of more than two time constants in the lower and intermediate frequency ranges. This behavior can be attributed to the presence of an inner compact layer and an outer porous TiO2 nanotube layer [48]. In addition, the decrease in phase angles at a high frequency is related to the porous nature of the outer layer [35]. At intermediate frequencies, the spectrum log∣z∣ vs. log(freq) is a straight line with a slope ranging from -0.78 to -0.88 that exhibits capacitive behavior.The Nyquist diagrams shown in Figure8(a) represent incomplete semicircles at low frequencies. Different equivalent circuits have been proposed in the literature to model the TiO2/electrolyte interface [40, 49]. In our case, Nyquist spectra are adjusted using the following equivalent electrical circuit as Rs+C1/R1+Q2/R2+Q3/R3.The proposed equivalent circuit takes into account the different time constants. The TiO2/electrolyte interface can be represented by the Helmholtz capacitance (C1). As shown in the equivalent circuit, this capacitance (C1) is in parallel with a resistance coupled to the constant phase element (R2/Q2) and (R3/Q3), corresponding, respectively, to the porous (outer) and barrier (inner) layer. Constant phase elements (Q) are used to take into account the surface heterogeneity, nonideality of capacitance, and frequency dispersion [50, 51]. The values of the circuit elements are summarized in Table 2.Table 2
Parameters obtained by fitting the impedance spectra of the anodized samples at different applied potentials.
Rs(Ω×cm2)C1(μF/m2)R1(Ω×cm2)Q2(μF×sa−1)a2R2(kΩ×cm2)Q3(mF×sa−1)a3R3(kΩ×cm2)30 V7.7419.9127.2086.180.444.380.550.979.3540 V23.0823.7625.6395.420.451.310.110.7021.7650 V10.1347.897.6356.640.291.020.230.605.6760 V14.9923.522.7676.490.564.310.920.6819.65For all the studied potentials, the calculatedRs and C1 parameters vary slightly compared to the other parameters. It is interesting to note that the resistance of the barrier layer R3 is higher than R2 of the nanotubes. The a2 values of the tubular layers are ranging from 0.29 to 0.56, while the a3 values of the barrier layers are between 0.60 and 0.97 for all samples which can be associated with a distribution of relaxation times due to the heterogeneities at the surface. The a2 values indicate that the tubular layers of all samples have a nonideal capacitive behavior. Values in the order of 0.30 and 0.50 have been reported in the literature for porous materials [52].


### 3.8. Photocatalytic Activity

To value the photocatalytic activity of nanoporous TiO2 formed in our conditions, the TiO2 nanotube arrays formed at 60 V and annealed at 600°C are evaluated for the degradation of a pharmaceutical pollutant known for its photochemical stability, carbamazepine (CBZ) [49]. Figure 9 shows the degradation curve of CBZ in an aqueous solution with an initial concentration of 5 ppm in the presence of the TiO2 nanotubes and under simulated sunlight irradiation (300 W). As it can be clearly observed, the anodized TiO2 in these conditions can effectively degrade 96% of CBZ within 10 h of irradiation. This may be due to their large surface area the TiO2 nanotubes and to their crystalline size and phase composition. Under these anodization conditions, TiO2 nanotube arrays were characterized by a pore diameter of 128 nm, a tube depth of 1.5 μm, and a maximum donor charge of about 4.25 1017 cm-3. With this tube length, in combination with minimal radial dimensions, the incident illumination can be effectively captured near the surface of the nanotubes, providing facile separation of a photogenerated charge. Our results are promising, and further work is underway to study the influence of other parameters on the photocatalytic activity of titanium dioxide nanotubes.Figure 9
Photocatalytic activity of TiO2 nanotubular- (formed at 60 V and annealed at 600°C) simulated sunlight irradiation (300 W).


## 3.1. Current Time Transient Measurements

During the anodizing process, the variation of the current density as a function of time is monitored. Figure1 shows the anodizing current density/time curves during the anodization at different potentials. The observed transients share similar trends. Three stages are typically observed in these curves related to the typical current density-time curve of the porous oxide formed under constant voltage. The kinetic trend consists of an initial fast drop (stage A), a steady increase (stage B), and a quasisteady state current density (stage C).Figure 1
Current density/time curves obtained at different anodization potentials.At stage A, the anodizing process begins, and the current quickly decreases to a minimum value because of the formation of a high resistance compact oxide layer on the surface by the interaction of the Ti4+ ions with the oxygen O2- ions in the electrolyte according to the following reaction [32]:
(1)Ti+2H2O→TiO2+4e−+4H+At stage B, the current subsequently rises to a maximum as the pore formation progresses. This is due to the chemical dissolution of the oxide layer by fluoride ions that is promoted by an electric field created between the cell electrodes. Small pits are formed on the surface of the compact layer followed by the formation of a nanoporous structure according to the following reaction [32, 33]:
(2)TiO2+6F−+4H+→TiF6−+2H2OAt the final stage, the current density attains a constant value when a steady state is reached owing to the formation of TiO2 nanotubes [34].


## 3.2. Morphological and Composition Characterization

For the morphological analysis of TiO2 nanotubular layers formed on the titanium substrate at different potentials, a scanning electron microscope TESCAN VEGA 3 was used. SEM top images and cross-sections of nanotube arrays produced in glycerol-distilled water (92 : 8% v/v) and NH4F (0.4 M) for 60 min at different anodizing potentials are given in Figure 2.Figure 2
SEM top images and cross-sections of nanotube layers grown in glycerol-distilled water (92 : 8%v/v) and NH4F (0.4 M) for 60 min at different anodizing potentials: (a) 30 V, (b) 40 V, (c) 50 V, and (d) 60 V.
(a)(b)(c)(d)The electrochemical anodization of titanium under these conditions leads to the formation of an ordered nanotubular array on the titanium surface. For the anodized sample at 30 V, there are areas where the oxide formed is compact. In addition, other areas have an ordered distribution of medium-sized nanotubes of about 59 nm in diameter. For voltages above 30 V, the surface of the samples is made up of neat TiO2 nanotubes. The average diameters of the nanotubes are about 80 nm and 128 nm for an applied voltage of 40 V and 60 V, respectively.In addition, the lengths of the nanotubes were measured by scanning electron microscope observation of cross-sections of the films. The result showed that the length of the TiO2 nanotube obtained on the surface of the titanium metal increases with the increasing anodization potential to reach approximately 1.5 μm at 60 V.Figure3 shows the evolution of nanotubular internal diameter as a function of the anodizing voltage. It is clearly seen that the nanotube diameter increases linearly with the anodizing potential with a good correlation coefficient R2 that is close to 1 (0.97). It should be noted that for a potential of 30 V, the internal diameter of the nanotubes is about 59 nm, while it reached 128 nm for a potential of 60 V.Figure 3
The variation of the TiO2 nanotube diameters as a function of the anodizing voltage for 60 minutes in glycerol-distilled water (92 : 8% v/v) and NH4F (0.4 M).


## 3.3. Composition Analysis by EDX

For the elemental characterization of the obtained nanotubular layers, energy-dispersive X-ray analysis is conducted using an acceleration voltage of 10 kV. The characteristics of the emitted X-rays from the anodized substrate produced at different conditions are presented in Figure4. The EDX spectrum indicates the presence of the Tikɑ peak at 4.58 eV and O peaks at 0.5 eV, as well as F and C peaks in the anodized sample. The obtained percentages of Ti (26.5 at%) and O (41.4 at%) yield a Ti/O ratio of about 0.5 indicating that the structure of the formed oxide is TiO2 with the presence of fluorine that arises from the solution during the anodization process [35] and carbon which comes from the metallization process using graphite.Figure 4
EDS spectrum of TiO2 obtained at 50 V for 60 minutes in glycerol-distilled water (92 : 8% v/v) and NH4F 0.4 M.


## 3.4. XRD Structural Characterization

The crystal phases of as-prepared and annealed samples were identified by XRD using Cu Kα radiation. As show in Figure 5, the formed TiO2 nanotubular film consists of a mixture of anatase and rutile phases. The anatase phase was identified from the peaks at ca. 25.03°, 48.05°, 54.85°, and 55.30° characteristic of the planes (101), (200), (105), and (211), respectively, according to JCPDS file no. 21-1272. In addition, the rutile phase is revealed by peaks at 27.4°, 36.07°, 41°, 43.6°, and 56.9°, corresponding to planes (110), (101), (110), (111), (210), and (220) (JCPDS card no. 21-1276), respectively. XRD spectra of annealed oxides reveal that TiO2 peaks become more intense when the anodizing potential increases.Figure 5
XRD spectra of samples anodized at different anodization potentials in glycerol-distilled water (92 : 8%v/v) and NH4F 0.4 M.The crystallite size is an important factor to determine the stability of nanotubular TiO2 crystalline phases [1, 36]. The average crystallite size of nanotubular TiO2 samples is calculated using the Scherrer equation D=0.9λ/βCosθ, where D is the grain size, λ (1.548 Å) is the wavelength of X-ray radiation used, θ is the Bragg diffraction angle obtained from XRD peak, and β is the full width at half maximum of the diffraction peak [37]. Using the width of the (101) peak for anatase and the width of the (110) peak for rutile, crystallite sizes were obtained in the ranges 7.3 to 8.5 nm and 8.2 to 9.7 nm for anatase and rutile phases, respectively.On the other hand, the phase composition of TiO2 has a crucial impact on the photocatalytic activity. The effect of the anatase/rutile ratio is discussed in many controversial works. It is reported that a mixture of anatase and rutile phases was found suitable for photocatalytic oxidation of organic pollutants in water [38]. Nevertheless, Tayade et al. [39] suggested that only the anatase phase has attracted considerable attention as a photocatalyst for the chemical treatment of organic pollutants. In our conditions, the relative amounts of anatase and rutile were estimated at 82% and 18%, respectively. This result is similar to that for the commercial TiO2, Degussa P25, which exhibits a strong photocatalytic activity and has an anatase-rutile mass fraction of 80 : 20.


## 3.5. Structural Characterization by Raman Microscopy

The TiO2 films obtained at different anodizing voltages were also characterized by Raman spectroscopy after thermal annealing at 600°C. Figure 6 shows the Raman spectra of the different anodized samples at different anodization potentials in a glycerol-distilled water mixture.Figure 6
Raman spectra of anodized simples at different anodization potentials in glycerol-distilled water (92 : 8%v/v) and NH4F 0.4 M.For the as-anodized sample, we noticed the absence of Raman peaks indicating the amorphous nature of the formed oxide [40]. However, the Raman spectra of all annealed samples depict a similar trend and show a very intense band at 143 cm-1 and four others at 195, 394, 515, and 637 cm-1. These bands can be attributed to the vibrational modes Eg1,Eg2,B1g,A1g, orB1g(superposition ofA1gandB1g), andEg of anatase, respectively. The observed bands confirm the predominance of anatase as the crystalline phase in the structure [41]. However, peaks of about 447 and 619 cm-1 indicate the presence of a certain amount of rutile and their intensities increase with the anodizing potential. These vibration modes are shifted by 2 to 4 cm-1 due to the variation in grain size and stoichiometric defects present in TiO2 structures as reported by Bassi et al. [42]. These structural observations are in good agreement with the XRD analysis.


## 3.6. Mott-Schottky (MS) Analysis

Electrochemical capacitance measurements were carried out to characterize the semiconductor nature of the films and to determine the donor densityND of the formed TiO2 layers, as well as to estimate the flat band potential (EFB) of the anodized samples at different potentials. MS analysis was performed in Na2SO4 0.1 M at 1 kHz in the potential range of -1 to 1 V/SCE. Figure 7 shows the variation of 1/Cs2 as a function of potential E (V/SCE).Figure 7
Mott-Schottky plots obtained for the formed nanotubularTiO2 layers in 0.1 M Na2SO4 solution.As shown in Figure7, a quasilinear behavior of the variation of 1/Cs2 as a function of the potential was observed. All curves have a positive slope corresponding to an n-type semiconductor [43]. The parameters ND and EFB can be determined from the slope of the linear regions in the Mott-Schottky plot and its intersection with the x-axis, respectively, according to the Mott-Schottky relationship:
(3)1CS2=2ɛ0ɛSeNDE−EFB−kTe,where CS is the space charge layer capacitance, e the electron charge (1.6010−19C), ɛ0the permittivity of free space (8.8510−14Fcm−1), ɛS the dielectric constant of TiO2 which is assumed to be 100 F cm-1 [44], E the applied potential, k the Boltzmann constant (1.381023JK−1), and T the absolute temperature. The values of the ND and EFB of the different samples are summarized in Table 1.Table 1
ND and EFB of anodized samples at different potentials.
Tension (V)EFB (V/SCE)ND×10−17 (cm-3)30-0.131.9540-0.163.3750-0.102.4560-0.144.25Table1 shows clearly that the ND values increase with anodizing potential, and the values obtained are in the order of 1017 cm-3. The ND values obtained are in good agreement with those reported in the literature for nanotubular TiO2 [45]. Generally, the growth of the anodized oxide films is always nonstoichiometric with an excess/deficiency of metal cations or oxygen anions [40]. According to the Point Defect Model (PDM), the n-type semiconductor behavior of an anodic passive film indicates that the defects in all samples are due to the oxygen deficiencies and/or interstitial titanium ions [46]. Somehow, Peng [47] reported that the predominance of oxygen deficiency is due to the low formation energy of oxygen vacancy compared to the interstitial titanium.


## 3.7. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) technique was used to study the electrochemical behavior of the interface of TiO2 nanotubular film. Figures 8(a) and 8(b) present the Nyquist and Bode diagrams of the EIS data obtained for the TiO2 films with their fitting adjustments obtained using EC-Lab software with the suggested equivalent circuit (inset in Figure 8(a)).Figure 8
Diagrams of the EIS data obtained for the TiO2: (a) Nyquist representations and (b) Bode representations. The equivalent circuit used to fit the experimental impedance spectra is inserted into (a).
(a)(b)The Bode diagrams of Figure8(b) reveal the presence of more than two time constants in the lower and intermediate frequency ranges. This behavior can be attributed to the presence of an inner compact layer and an outer porous TiO2 nanotube layer [48]. In addition, the decrease in phase angles at a high frequency is related to the porous nature of the outer layer [35]. At intermediate frequencies, the spectrum log∣z∣ vs. log(freq) is a straight line with a slope ranging from -0.78 to -0.88 that exhibits capacitive behavior.The Nyquist diagrams shown in Figure8(a) represent incomplete semicircles at low frequencies. Different equivalent circuits have been proposed in the literature to model the TiO2/electrolyte interface [40, 49]. In our case, Nyquist spectra are adjusted using the following equivalent electrical circuit as Rs+C1/R1+Q2/R2+Q3/R3.The proposed equivalent circuit takes into account the different time constants. The TiO2/electrolyte interface can be represented by the Helmholtz capacitance (C1). As shown in the equivalent circuit, this capacitance (C1) is in parallel with a resistance coupled to the constant phase element (R2/Q2) and (R3/Q3), corresponding, respectively, to the porous (outer) and barrier (inner) layer. Constant phase elements (Q) are used to take into account the surface heterogeneity, nonideality of capacitance, and frequency dispersion [50, 51]. The values of the circuit elements are summarized in Table 2.Table 2
Parameters obtained by fitting the impedance spectra of the anodized samples at different applied potentials.
Rs(Ω×cm2)C1(μF/m2)R1(Ω×cm2)Q2(μF×sa−1)a2R2(kΩ×cm2)Q3(mF×sa−1)a3R3(kΩ×cm2)30 V7.7419.9127.2086.180.444.380.550.979.3540 V23.0823.7625.6395.420.451.310.110.7021.7650 V10.1347.897.6356.640.291.020.230.605.6760 V14.9923.522.7676.490.564.310.920.6819.65For all the studied potentials, the calculatedRs and C1 parameters vary slightly compared to the other parameters. It is interesting to note that the resistance of the barrier layer R3 is higher than R2 of the nanotubes. The a2 values of the tubular layers are ranging from 0.29 to 0.56, while the a3 values of the barrier layers are between 0.60 and 0.97 for all samples which can be associated with a distribution of relaxation times due to the heterogeneities at the surface. The a2 values indicate that the tubular layers of all samples have a nonideal capacitive behavior. Values in the order of 0.30 and 0.50 have been reported in the literature for porous materials [52].


## 3.8. Photocatalytic Activity

To value the photocatalytic activity of nanoporous TiO2 formed in our conditions, the TiO2 nanotube arrays formed at 60 V and annealed at 600°C are evaluated for the degradation of a pharmaceutical pollutant known for its photochemical stability, carbamazepine (CBZ) [49]. Figure 9 shows the degradation curve of CBZ in an aqueous solution with an initial concentration of 5 ppm in the presence of the TiO2 nanotubes and under simulated sunlight irradiation (300 W). As it can be clearly observed, the anodized TiO2 in these conditions can effectively degrade 96% of CBZ within 10 h of irradiation. This may be due to their large surface area the TiO2 nanotubes and to their crystalline size and phase composition. Under these anodization conditions, TiO2 nanotube arrays were characterized by a pore diameter of 128 nm, a tube depth of 1.5 μm, and a maximum donor charge of about 4.25 1017 cm-3. With this tube length, in combination with minimal radial dimensions, the incident illumination can be effectively captured near the surface of the nanotubes, providing facile separation of a photogenerated charge. Our results are promising, and further work is underway to study the influence of other parameters on the photocatalytic activity of titanium dioxide nanotubes.Figure 9
Photocatalytic activity of TiO2 nanotubular- (formed at 60 V and annealed at 600°C) simulated sunlight irradiation (300 W).


## 4. Conclusion

The electrochemical anodizing method was used to produce amorphous TiO2 nanotubes converted by annealing into crystalline nanotubes. The nanotubes of significantly different diameters were obtained in a voltage range from 30 V to 60 V. The variation in anodizing voltage did not modify the chemical composition of the TiO2. In addition, their crystalline structure shows the presence of a mixture of anatase and rutile phases. The electrochemical study reveals that the oxide formed under these conditions is an n-type semiconductor with a donor density on the order of 1017 cm-3. Our results are promising, and further work is underway to study of the photocatalytic properties of the produced titanium oxide nanotubes.



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