Synthesis of Titanium Dioxide Photoanodes by Anodization in a Sodium Chloride Electrolyte

Abstract

Titanium dioxide photoanodes were synthesized using the anodization method at low potentials in a 0.3 M sodium chloride electrolyte at room temperature. The morphology of the materials was analyzed with optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM); the crystalline phase of the synthesized materials was analyzed by X-ray diffraction and Raman spectroscopy; meanwhile, the hydrophobic–hydrophilic properties were assessed through contact angles measurements, and all the synthesized samples possess hydrophilic properties. The samples anodized at 15 and 20 volts exhibited the formation of titanium dioxide in the anatase phase.

1. Introduction

Since the pioneering work by Fujishima and Honda in 1973, in which they used titanium dioxide as a photoanode for water electrolysis reaction to produce hydrogen and oxygen [1], there has been extensive research into their applications in aqueous environments. Various fabrication techniques for titanium dioxide photoanodes have been explored, like deposition, microwave-assisted, hydrothermal, sol–gel, and anodization, among others [2,3,4].

Anodization is an electrochemical process used to form an oxide layer on metallic substrates. This method is particularly useful in industrial settings for enhancing corrosion resistance and improving paint and adhesive adhesion [5]. One notable advantage of synthesizing titanium dioxide films via anodization is the direct growth of the oxide film on the titanium substrate. Still, it requires the use of hazardous reagents such as hydrofluoric acid (HF) and ammonium fluoride (NH₄F) [6,7,8,9]. Other works have proposed the use of alternative electrolytes that do not contain fluoride ions, such as hydrochloric acid (HCl) [10,11,12], potassium bromide (KBr) [13], and sodium chloride (NaCl) [14,15]; these alternatives may mitigate risks associated with traditional reagents, the synthesis conditions and their resultant impact on the morphology of the titanium dioxide layers remain undefined. Further characterization of photoanodes produced using these electrolytes is essential to elucidate the variables and conditions that lead to their optimal performance, particularly for applications in the remediation of persistent organic pollutants in aqueous systems by photocatalyst.

In this context, the present work aims to contribute novel insights by exploring the synthesis of titanium dioxide films through anodization in NaCl-based electrolytes at low anodization potentials (4–20 V) and at room temperature. This approach contrasts with the majority of studies that utilize higher potentials and more aggressive electrolytes. Our findings provide valuable information about the feasibility and effects of using NaCl as an anodizing medium and demonstrate the potential of this mild, accessible method for the development of TiO₂ photoanodes.

2. Results and Discussion

Figure 1 shows the current density profiles of the anodization of titanium samples in a 0.3 M NaCl electrolyte. In Figure 1a, the current density observed at 4 V and 6 V is shown; the applied potential is below the threshold required for oxidation [16] and, therefore, does not lead to oxide formation. Figure 1b presents the current density observed at 8.5 V and 10 V. At 8.5 V, a current density of 52 mA/cm² was observed, which gradually decreased to approximately 0 mA/cm². At 10 V, an initial current density of 276 mA/cm² was recorded, and as the oxidation process progressed, the current density decreased to 26 mA/cm². In Figure 1c, increasing current curves are observed without reaching the formation/dissolution equilibrium. The anodization profile at 15 V and 20 V suggests a dissolution rate higher than the oxide formation rate, which prevents the growth of the film thickness.

The sample Ti-P0 shows that the plain Ti surface had some grooves and scratches due to mechanical grounding [17]. In the samples Ti-P4, Ti-P6, and Ti-P8.5, no significant morphological modifications were observed, indicating that the applied potential is insufficient to induce the oxidation reaction (see Figure S1 in the Supplementary Materials), C. Richter et al. observed similar results when anodizing titanium in electrolytes containing 0.4 M chloride ions and a pH ~1.5, using NH₄Cl as the chlorine source, along with 0.5 M oxalic, formic, trichloroacetic acids and gluconic and different concentrations of hydrochloric and sulfuric acid [16].

Optical micrographs of anodized samples at different voltages are illustrated in Figure 2. Starting with Figure 2b, Ti-P10, a significant change is observed—the formation of small white spots with an approximate diameter of 30 μm. These spots increase in both number and size in Figure 2c, Ti-P15, and Figure 2d, Ti-P20, indicating the development of a film on the substrate surface. This film becomes denser and more uniform in Ti-P20, marking a clear advancement in our understanding of the anodization process.

The Ti-P20 sample exhibits a highly rough surface with a porous morphology, suggesting the formation of an oxide layer with a sponge-like appearance. An irregular distribution of holes is observed across the surface, likely originating from a localized dissolution process of the oxide.

The random attack and etching of Cl⁻ ions on the surface of the anodized foil led to the formation of those “pitting corrosion”. This process is initiated when Cl⁻ ions disrupt the protective oxide layer of the titanium substrate; according to Hahn R. et al., these spots indicate the formation of the oxide layer. They used FE-SEM to zoom in on these locations and identified the formation of bundles of closely packed TiO₂ nanotubes prepared in ClO₄ and Cl-solutions [14].

The micrographs obtained by scanning electron microscopy are shown In Figure 3a, showing a plain Ti surface after mechanical grounding; it can be observed that this treatment generated fine grooves on the surface. Similar results were observed by Salih Durdu et al. when anodizing titanium plates using 180, 240, 400, 800, 1000, and 2000 SiC sandpapers [17]. Additionally, some imperfections resembling clusters presumably of the same metal as the plain Ti are present.

In contrast, Figure 3b, Ti-P10 surface exhibits the initial formation of an irregularly shaped rough film that partially covers the metal surface, aggregating around the imperfections present on the plate’s surface. Upon increasing the potential to 15 V, as shown in Figure 3c, the Ti-P15 sample exhibits a higher surface roughness compared to Ti-P10, maintaining the irregular morphology of the growing film. However, this layer displays fractures, scratches, and irregular regions that can be attributed to consolidated titania layers forming islands. The growth of a TiO₂ film on the substrate surface is corroborated by optical microscopy results, where Ti-P15 (Figure 2c) and Ti-P20 (Figure 2d) plates reveal a whitish coating that becomes more homogeneous as the oxide content increases on the metal substrate.

The micrograph of Figure 3d, the Ti-P20 sample, shows a smoother surface compared to the previous Ti-P15, with clusters scattered throughout the substrate. The clusters appear to have a granular and aggregated morphology, with irregularly shaped particles grouped.

The clusters appear to have a granular and aggregated morphology, with irregularly shaped particles grouped together. The background surface looks partially uniform or flat, with minimal texture, except where the clusters are present.

It is important to highlight that the morphology observed in this study is characteristic of anodization in chloride-based electrolytes, such as NaCl. In contrast, the formation of well-organized and vertically aligned TiO₂ nanotubes is typically observed in fluoride-containing electrolytes under higher anodization voltages [7,18]. These structural differences reflect the distinct mechanisms of oxide growth and dissolution in chloride versus fluoride media.

In Figure 4 of the Ti-P20 substrate, a homogeneous background is observed without the grooves present in the original plate, suggesting the formation of a uniform layer that covers the metal’s imperfections. On this surface, islands or nuclei of irregular sizes are evident, characterized by a porous structure with multiple fractures. These formations appear to originate from surface irregularities of the plates, thereby reducing their homogeneity. A similar morphology was observed by Narges Fathy Fahim et al. when anodizing titanium in the presence of chloride ions in a 0.1 M HClO₄ electrolyte at pH 1 [19].

To confirm the morphology observed by optical and scanning electron microscopy, the clean and anodized substrates were analyzed by atomic force microscopy. The resulting RMS roughness (Sq) values are indicated in

Table 1. RMS roughness (Sq) of the synthesized samples.

Sample	RMS Roughness (sq)
Ti-P0	51.72 nm
Ti-P10	125.8 nm
Ti-P15	53.02 nm
Ti-P20	79.09 nm

. The surface becomes more irregular as the imposed potential increases during the anodization process. However, no correlation is observed between the Sq and the imposed potential in the selected areas for this analysis, as the nucleation sites developed randomly.

The roughness values do not have a linear behavior concerning the anodizing potential applied to each substrate; this is because the oxidation reaction was not carried out homogeneously on the substrate surface; due to the heterogeneity of the substrate, likely, the grooves, clusters, or imperfections observed on the surface of the clean substrate acted as the initial seat of the reaction. At a potential of 10 V, the roughness, Sq, is twice that of the unmodified substrate, 51 nm, confirming the formation of a titanium dioxide layer on the titanium metal. The film’s roughness decreases at higher potential, while at 20 V, it increases again to 80 nm. The behavior of the surface roughness confirms the formation of a non-uniform layer, which is corroborated by the AFM image of material Ti-P15 shown Figure 5.

Figure 6 shows the EDX spectrum of Ti-P0 and Ti-P20 samples. In the starting substrate, Ti-PO, the weight percentage of titanium is 100%, characterizing an oxygen-free metallic surface, whereas, in the Ti-P20 sample, the oxygen content is 47%, its increase shows that an oxidation reaction is taking place on the surface of the substrate. Figure 7 summarizes the results of the EDX elemental analysis of the surface of the anodized substrates. The increase in oxygen concentration and the decrease in titanium atomic percentage accelerate with increasing potential used during the anodizing process.

Complementary to morphological properties, the wettability properties of the modified substrates and titanium oxide-free substrate were determined by measuring the contact angles, θ. Figure 8 shows the contact angle values as a function of the applied potential during the anodization process. The results show a clear trend towards smaller contact angle values, from approximately 74° to 5°, as the applied potential increases from 0 V to 20 V.

By convention, when θ < 90°, it is said that the liquid is wetting the solid, and the solid is hydrophilic in nature. On the other hand, when θ > 90°, it is said that the liquid does not wet the solid; in this case, the solid is considered hydrophobic [20]. According to these results, all the samples have hydrophilic properties.

No reports exist regarding the wettability of photoanodes fabricated in chlorinated electrolytes. However, Salih Durdu et al. observed that, upon anodizing titanium in fluoride-containing media, the unmodified titanium plate exhibited hydrophilic characteristics [17]. Increasing the anodization voltage resulted in photoanodes displaying more hydrophobic properties, correlating with an increase in surface roughness (Sa); atmospheric gases in the tubes initially create resistance for short periods after drop contact on the TiO₂ nanotube surface, preventing the surface from wetting [17].

According to the average contact angle values observed in

Table 2. Average contact angle values.

Sample	Contact Angles (°)
Ti-P0	74
Ti-P10	67
Ti-P15	42
Ti-P20	5

., the decrease in the contact angle is not directly related to the roughness, similar results have been reported for TiO₂ films deposited on glass, Si, and FTO [20]. This phenomenon is due to the formation of -OH groups, which give the film a high affinity for water molecules.

The X-ray diffractograms of TiP0, TiP-10, TiP-15, and TiP-20 samples are shown in Figure 9. In the TiP15 and TiP20 samples, it can be observed that titanium dioxide crystallized in its anatase form, which was identified by its characteristic signal at 25.3° [9,15,21]. The NaCl was used as the electrolyte, and according to the X-ray analysis, it is prevalent on the surface of the substrates. On the other hand, the titanium plate used as substrate shows only the signals corresponding to titanium [8,22,23].

The Intensity of the characteristic diffraction peak of metallic titanium on the clean substrate (T1-P0) shows an intense peak at 71°, which decreases as the electrochemical anodizing voltage increase. The reduction in intensity of this peak indicates that the anodizing reaction is rapidly modifying the surface of the metal and begins to form a layer of homogeneous titanium dioxide in the anatase phase. Figure 4, corresponding to the Ti-P20 sample, supports the idea that the titanium plate’s surface is uniformly covered by a homogeneous titanium dioxide layer, effectively concealing the original scratches. On this layer, surface protrusions are evident, corresponding to the imperfections in the original metallic substrate, as observed through optical microscopy (Figure 2d).

These results of TiO₂ formation were confirmed by Raman analysis on all the samples, which are shown in Figure 10. A summary of the Raman signals is presented in

Table 3. Raman shifts obtained and the associated crystalline structure.

Raman Signals (cm−1)	Crystalline Phase
139.6	Anatase [24,25,26]
393.4	Anatase [3,24,25,26]
513	Anatase [24,25,26]
639	Anatase [3,24,25,26]

; for the samples Ti-P15 and Ti-P20, well-defined Raman signals were observed at 139.6, 393.4, 513, and 639 cm⁻¹, which are associated with the anatase phase of titanium dioxide; the presence of rutile or brookite phases was not identified. The Ti-P10 sample exhibited a barely perceptible signal at 139.6 cm⁻¹, suggesting the onset of TiO₂ crystal nucleation. These results are consistent with the optical micrographs, where white spots observed from 10 V onward indicate the beginning of titanium dioxide crystal nucleation.

3. Materials and Methods

3.1. Electrochemical Anodization

Prior to anodization, titanium plates measuring 0.5 mm in thickness underwent a sequential polishing process using sandpaper with varying grit sizes: 350, 600, 1200, and 1500. After polishing, the substrates were thoroughly cleaned in a series of solvents—acetone, isopropyl alcohol, and deionized water—each subjected to ultrasonic cleansing for 5 min to ensure effective removal of contaminants. A defined geometric area of 1.1 cm² on each titanium plate was selected for the anodization treatment. The anodizing procedure was executed within a conventional three-electrodes electrochemical cell, wherein the titanium plates acted as the working electrodes, stainless steel plates functioned as the counter electrodes, and Ag/AgCl electrodes served as reference electrodes. The anodizing potentials evaluated during the process were 4, 6, 8.5, 10, 15, and 20 V, with a reaction time of 180 s, utilizing a 0.3 M sodium chloride aqueous electrolyte maintained at a pH of 6.6 and room temperature. An Autolab 302 N potentiostat/galvanostat provided power for the lower potentials (4, 6, 8.5, and 10 V), while a Haltronic power supply was employed for the higher potentials (15 and 20 V). Finally, the anodized samples were annealed at 500 °C for 3 h in a Thermolyne muffle furnace. The nomenclature for the anodized samples is summarized in

Table 4. As-prepared samples.

Anodizing Potential	Samples
0 volts	Ti-P0
4 volts	Ti-P4
6 volts	Ti-P6
8.5 volts	Ti-P8.5
10 volts	Ti-P10
15 volts	Ti-P15
20 volts	Ti-P20

.

3.2. Characterization

Optical microscopy images were conducted utilizing a Maxlapter optical microscope (Maxlapter, Shenzhen, China) to provide detailed imaging of the samples; meanwhile, the images of scanning electron microscopy were obtained in a microscope FEI NanoLab 200 (FEI Company, Hillsboro, OR, USA)of field emission. The contact angle was measured with the Theta Lite 101 Attension optical tensiometer (Biolin Scientific AB, Västra Frölunda, Sweden) using the sessile drop method. Raman spectra were collected using a B&W Tek MicroRaman i-Plus in a spectral range of 100–3000 cm⁻¹ using a laser line of 532 nm with a fluence of 6 mW/cm⁻² (B&W Tek, Inc., Plainsboro, NJ, USA). Atomic Force Microscopy (AMF) was performed on the sample using the contact mode in a TT-AFM Workshop instrument, using Si tips with a resonance frequency of ca. 170 kHz (AFM Workshop, Santa Barbara, CA, USA).

4. Conclusions

The effect of anodizing titanium plates in a sodium chloride electrolyte at different voltages was studied. The results obtained with optical, scanning electron, and atomic force microscopy showed that TiO₂ film formation starts from 10 V with synthesis conditions proposed and implemented, affecting the film morphology at a threshold of 10 V. Still, the best results are achieved at 20 V. Raman and XRD analysis show titanium dioxide crystals in the anatase phase are formed.

This study represents the first stage of our research. Future work will focus on evaluating the photoanodes prepared in this study for the degradation of a model organochlorine contaminant, aiming to assess their photoelectrochemical performance in practical environmental applications.