Mechanism and growth kinetics of hexagonal TiO2 nanotubes with an influence of anodizing parameters on morphology and physical properties | Scientific Reports

Scientific Reports volume 14, Article number: 24721 (2024) Cite this article

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Hexagonal TiO2 nanotubes (hTNTs) mimic a honeycomb structure, indicating their high potential as implantable materials due to their superior mechanical, chemical, and biological properties. However, the fabrication of hTNTs with a hexagonal base and six rectangular sides poses significant challenges, underscoring the importance of this research. This study developed a novel sonoelectrochemical method for synthesizing uniform hTNTs and evaluated the influence of anodizing parameters on their morphology. The effects of electrolyte concentration (ethylene glycol 90–97.5% and ammonium fluoride 0.1–0.5 wt%) and anodizing parameters (time 5–90 min, potential 10–80 V) on the morphology (diameter and length) and physical properties (porosity, specific surface area, growth factor) of hTNTs were investigated using scanning electron microscopy and anodization analysis. The methodology enabled the synthesis of hTNTs with diameters ranging from 33 ± 3 nm to 203 ± 33 nm and lengths from 1.16 ± 0.04 μm to 20.93 ± 2.37 μm. The study demonstrated that the concentrations of ethylene glycol and ammonium fluoride influenced the diameter and length of hTNTs depending on the anodizing potential. Moreover, the anodizing potential significantly affected the diameter, while both potential and time impacted the length of hTNTs. The proposed method can modify material surfaces for diverse applications.

Surface quality is crucial for the long-term use of titanium (Ti) in various industrial sectors. Traditional manufacturing processes need to be improved to fulfill stringent industry requirements of titanium surface properties. Therefore, titanium surface modification techniques continually evolve, with nano-modification methods showing the most promising results in enhancing mechanical, corrosion, and biological properties1. Much of the research on titanium surface modification focuses on its use as a biomaterial in medicine, particularly as an implant material. One possibility of Ti nano-modification is a nano-porous hydroxyapatite-containing layer, which improves the bioactivity, hardness, corrosion resistance, and adhesion, growth, proliferation, and differentiation of cells2. Other methods of titanium modification include silver and zinc oxide nanoparticles due to their antibacterial properties3. Laser modification of the Ti surface allows the obtaining of nanopillars and nanotextured microcolumns, which improve the differentiation of human mesenchymal stem cells into an osteoblast lineage, reduce biofilm formation of bacterial cells, and reduce infection risk4. In turn, better tribological and corrosion properties of titanium can be achieved by using electroplated functionally graded nickel–Al2O3 nanocomposite coatings5. Among achievements in titanium surface modifications, titanium dioxide nanotubes (TNTs) have garnered significant interest6. Formed through anodization processes, they result in self-organized arrays of nanoscale pores on the titanium surface7. The unique structure of TNTs offers numerous advantages, including increased surface area, biocompatibility, enhanced osseointegration for bone implants, and potential photocatalytic applications8,9. Due to their good adsorption properties, TNTs are also used as a substrate for further modification, e.g. by deposition of hydroxyapatite or silver nanoparticles. Despite these advantages, reports indicate insufficient strength and adhesion of TNTs to the titanium substrate for implant surface application10,11. The low mechanical stability of TiO2 nanotubes may induce toxicity and severe immuno-inflammatory reactions due to abrasion of the implant surface and the release of harmful ions and layer splinters into the body. Challenges, such as achieving homogeneity12,13, enhancing mechanical strength10, and scaling up TNTs production14, have also been identified.

In recent years, a new type of titanium oxide has emerged. It has a hexagonal morphology. The hexagonal TiO2 described in the literature takes the form of the hexagonal arrangement of circular TiO2 nanotubes15,16,17 or the right hexagonal prism of TiO2 with a hexagonal base and six rectangular sides, referred in the literature as a TiO2 hexagonal nanotubes (hTNTs)18,19,20,21. Due to its specific geometry, hexagonal nano-TiO2 is very promising and confers novel properties compared to conventional TNTs. Hexagonal nano-TiO2 exhibits intriguing structural, mechanical, and electrochemical properties18. As of now, the mechanism behind the formation of TiO2 hexagonal nanotubes remains unclear.

To date, hTNTs have been formed by optimized anodization preceded by Atomic Layer Deposition (ALD), pre-texturing by focused ion beam milling22, using a pre-textured template made by ALD20, pulse anodization with both positive upper and lower limits23, sol electrolyte anodization21, one-step anodization24,25, two-step anodization19,26,27 and three-step anodization28.

The first research on the formation of the hTNTs was conducted in 2007 by Albu et al.26 and Macak et al.19 using two-step anodization. Anodizing was carried out at a high potential of 50–60 V. However, structures with a hexagonal outer wall but no hexagonal inner wall were obtained. Further studies used one-stage anodization of Ti foil in an organic solution containing fluoride ions22,24,25. Nevertheless, the result was a hexagonal outer wall with a circular inner wall of hTNTs. The most complicated hTNTs preparation methods are two-step anodization27,28 and three-step anodization29. Two-step anodizing consists of two anodizing processes in which the first produced oxide layer is removed by mechanical peeling27. In three-stage anodizing, the third anodizing was applied after removing the second oxide layer28. Following attempts to form hTNTs also included the presence of disodium edetate30 and ultrasonic waves during anodization. As a result, hexagonal TiO2 nanotubes with a hexagonal inner and outer wall were obtained. Among the remaining anodizing methods, electrolyte sol anodization31 and template methods20,22 were included. Among the methods mentioned, the hTNTs with the most homogenous and ideally hexagonal shape were obtained by one-step anodization25,32 and template methods19,20,22,24,25,33. The proposed template method negatively affects the mechanical and corrosion properties of hTNTs due to the orientation of hTNTs and the heterogeneity of the wurtzite-ZnO, anatase-TiO2, and cubic Zn2TiO4 phases20.

Therefore, the research aimed to develop a new surface modification of titanium by generating self-organizing hexagonal TiO2 nanotubes resembling a honeycomb structure. A thorough analysis of the anodization kinetics described the mechanism of hTNTs (hexagonal TiO2 nanotubes) formation via the sonoelectrochemical method. Furthermore, the influence of anodization parameters (potential, time, electrolyte concentration) on the morphological properties (diameter and length) and physical properties (porosity, specific surface area, growth factor) of hTNTs was determined.

Titanium foils (0.25 mm thickness, 99.7% purity, Sigma-Aldrich, USA) were cut into 5 mm (width) × 20 mm (height) × 0.25 mm (thickness) and degreased ultrasonically in acetone and distilled water for 10 min, then dried in nitrogen. The sonoelectrochemical anodization was carried out using ultrasonic waves (120 W, 45 kHz, VWR USC-T) by immersing 0.25 cm2 of the Ti foil in the electrolytic solution (100 ml). Electrolytic solution contained 0.1–0.5 wt% ammonium fluoride (NH4F, Chempur, \(\:\ge\:\)98.0%), 0.1 wt% EDTA disodium salt (Na2[H2EDTA], Chempur, \(\:\ge\:\)99.6%) and 90–97.5% ethylene glycol (Chempur, 99.0%). The formation of the hTNTs was carried out in a two-electrode system. Platinum foil (99.95% purity, Sigma-Aldrich, USA) was used as a counter electrode, and Ti foil as a working electrode. The anodization was performed for 40–90 min at an applied potential of 10–80 V using a laboratory power supply (STAMOS, S-LS-100). After the anodizing process, the Ti foil was rinsed with distilled water and dried in nitrogen. The influence of anodizing parameters on the diameter and length of hTNTs was tested in four experiments I-IV, the parameters of which are presented in Table 1. The effect of ethylene glycol (experiment I) and ammonium fluoride (experiment II) concentrations at 20, 50 and 80 V, as well as anodizing potential (experiment III) and time (experiment IV) were determined.

The formation of hexagonal titanium dioxide nanotubes (hTNTs) was conducted in a two-electrode system, with platinum foil (99.95% purity, Sigma-Aldrich, USA) as the counter electrode and titanium foil as the working electrode. Anodization was performed for 5 to 90 min at a constant applied potential of 50 V using the laboratory power supply Autolab PGSTAT302N (Metrohm, Herisau, Switzerland). The STAMOS S-LS-100 power supply was not used in these experiments. Anodization kinetics were recorded using Nova 2.1 software. After anodization, the Ti foil was rinsed with distilled water and dried under nitrogen.

The surface morphologies after anodization were analyzed using field emission scanning electron microscopy (FESEM, JEOL JSM-7600 F, Tokyo, Japan) that operated at 8 kV. The ideal structure of the hTNTs layer is represented by a closed-packed array of hexagonally arranged cells inclusive of hexagonal pores in the cell center (Fig. 1). FESEM images were used to measure the length (H), inner (Ø2), and outer (Ø1) diameter of a circle inscribed in a hexagon forming a hTNTs. In pore diameter calculations, pore shapes were assumed to be perfect hexagons. The H, Ø2, and Ø1 were measured using a PCSem at three locations for three samples to obtain 100 measurement points. Errors were estimated as standard deviation (SD) from the mean value of the measured parameter according to Eq. (1).

where \(\:{X}_{i}\) – measured value, \(\:X\) – mean of \(\:{X}_{i}\), \(\:\text{n}\) – number of data points.

Idealized structure of hexagonal titanium dioxide nanotubes (a) and a cross-sectional view of the anodized layer (b), where: t – wall thickness, Ø1 – outer diameter, Ø2 – inner diameter, H – length of hexagonal titanium dioxide nanotubes.

Based on microscopic analysis results, the wall thickness (t), porosity (P), total surface area of the hexagonal nanotube (Ai), pore density (in), and specific surface area (As) of hTNTs were estimated. These values were determined based on the equations for TNTs available in the literature32,33,34, considering the nanotubes’ hexagonal shape. The following expressions were used (2–6):

The growth factor was determined by dividing the value of the average length of hTNTs for a given anodizing potential by the value of this potential.

A comparative analysis of the circular and hexagonal shapes of titanium dioxide nanotubes (Fig. 2) was performed to verify the correctness of a hexagonal TiO2 nanotube synthesis method. According to the experimental TNTs synthesis model previously described by Nycz et al.35, circular TNTs (Fig. 2a) were formed in the anodizing process using 85% ethylene glycol with 0.65 wt% NH4F at 40 V for 1100 s. Hexagonal TNTs (Fig. 2b) were formed according to the example parameters of sonoelectrochemical anodization, i.e., anodization in 95% ethylene glycol with 0.3 wt% NH4F and 0.1 wt% Na2[H2EDTA] at 50 V for 60 min. Top-view scanning electron microscopy images presented in Fig. 2a indicated round outer and inner walls of nanotubes, while in Fig. 2b, the hexagonal shape of nanotubes. Another difference was the much smaller empty spaces between the walls of hTNTs compared to the distinct spaces seen with TNTs.

Currently, there is a lot of research on circular titanium dioxide nanotubes10,11,36,37,38,39, and only a few literature reports in the field of hexagonal titanium dioxide nanotubes15,16,17,19,26,32. The few available literature reports on hTNTs describe structures where either only the outer wall exhibits a hexagonal shape, or the walls lack distinctly outlined hexagons. In other reports, the hexagonal shape was determined by a specific arrangement of circular TiO2 nanotubes constituting the interior and forming the hexagonal area15,16,17. The developed method - sonoelectrochemical anodization - allowed for the synthesis of hTNTs with hexagonal inner and outer walls of titanium dioxide nanotubes.

FESEM images of the top-view of circular (a) and hexagonal (b) titanium dioxide nanotubes with empty spaces marked with red triangles and bottom-view of circular (c) and hexagonal (d) titanium dioxide nanotubes.

Mechanism of the hTNTs formation was analyzed based on the anodization curve and SEM images of Ti sample anodized in 95% ethylene glycol with 0.3 wt% NH4F and 0.1 wt% Na2[H2EDTA] at 50 V, and with the participation of ultrasounds. The course of the forming process and changes in the morphology of the TiO2 nanostructure during anodizing are shown in Fig. 3.

FESEM images for the anodization of titanium foil (a) and anodization kinetics (b). Images were taken at x30,000 magnification.

Anodizing of titanium in sonoelectrochemical anodization resulted in the formation of a titanium dioxide layer with oval-shaped pores observed for the first 30 min of anodizing. After 5 min of anodizing, the formation of a porous layer of titanium dioxide with a pore diameter of 63 \(\:\pm\:\) 15 nm was observed. After 10 min, the formed oxide layer took the form of TNTs with a diameter of 74 \(\:\pm\:\) 13 nm, and this form of TiO2 persisted for up to 30 min while the diameter of the TNTs increased to 95 \(\:\pm\:\) 12 nm. For longer times, from 40 to 90 min, the emerging oxide layer had hexagonal pores, and the hTNTs diameter did not change and took a value of approximately 116 \(\:\pm\:\) 14 nm. As the anodizing time increased from 5 to 30 min, the pore diameter of the titanium dioxide layer increased, and then after 40 min, hTNTs were formed, and their diameter remained constant.

In the first 10 min of anodizing, the length of hTNTs was 3243 \(\:\pm\:\) 221 nm. The length of hTNTs increased slightly after 30 min of anodizing to 3670 \(\:\pm\:\) 180 nm. For longer times, from 40 to 90 min, the emerging oxide layer has hexagonal pores, and the hTNTs length increased significantly from 7470 \(\:\pm\:\) 320 nm at 40 min to 20,930 \(\:\pm\:\) 2370 nm at 90 min. As the anodizing time increases, the length of the titanium dioxide layer on the titanium surface increases. The presented results disproved the Lebedeva et al. hypothesis40,41, according to which the hexagonal TNTs are a precursor of standard titanium dioxide nanotubes. Circular TNTs are, therefore, precursors of hexagonal TNTs.

For an anodization time of 70–90 min, nanograss was observed on the hTNTs surface. After 90 min of anodizing, degradation in the form of hTNTs cracks was observed. As reported by Tupala et al., at the high potentials the TNTs walls are not able to thin down anymore without collapsing, which contributes to the formation of cracks on the surface of hTNTs with longer anodizing time and the use of EDTA42.

During the formation of hexagonal titanium dioxide nanotubes, chemical reactions related to the formation and dissolution of the titanium dioxide layer occur. Initially, a thin TiO2 passive layer forms on the titanium surface due to applied potential and an aqueous acidic environment, according to Eq. (7). Subsequently, the TiO2 passive layer undergoes dissolution by F− ions and [H2EDTA]2− ions, as described by Eqs. (8)-(10). The passive layer dissolves due to the applied potential, the competition between F− ions and [H2EDTA]2− ions for complex formation with Ti4+, and the presence of ultrasound, which enhances ion mobility in the solution, directs the electrolyte to the electrode, consequently leading to a faster growth of hTNTs. In the final stage, the release of F− ions from [TiF6]2− complexes occurs through the stronger EDTA ligand complexation, as shown in Eq. (11) or (12).

1st stage43

2nd stage18,43

3rd stage18

Due to ongoing chemical reactions, i.e., the formation and dissolution of the TiO2 layer, changes in current conditions occurred. The anodic curve of hTNTs is presented in Fig. 4, along with the scheme of transformations occurring on the surface of anodized titanium. In the process of hTNTs formation, four stages I-IV were observed. In stage I, a dense TiO2 layer forming on the Ti surface (6) created a barrier, leading to a rapid decrease in current associated with the growth of the oxide layer thickness44. In stage II, F− ions and [H2EDTA]2− ions activated the oxide barrier layer (7–9), causing the dissolution of TiO2 and consequently leading to the formation of randomly distributed pores45. Simultaneously, in stage II, the current intensity increased due to the reduction in the thickness of the TiO2 barrier layer at the bottom of the pores. The increase in current intensity further deepened the pores, branching them and causing them to overlap. Properly chosen process conditions allow for an even distribution of current among the pores, leading to the spontaneous ordering of the porous layer.

The growing TiO2 layer started to form the nanotube template, and the current intensity decreased and stabilized. This marked the onset of stage III of the process, during which metallic Ti became trapped in the gaps between the nanotubes. This phenomenon was associated with the rapid dissolution of TiO2 by F− ions and [H2EDTA]2−, which were accelerated by ultrasonication and limited oxygen access to Ti. Furthermore, the reaction was facilitated by releasing F− ions from [TiF6]2− complexes (10, 11).

According to Tupala et al., EDTA increases the rate of dissolution of chemicals, which causes the nanotubes to elongate42. As the template’s nanotubes elongated, the nanotubes widened, resulting in an escalation of current intensity within the entrapped metallic Ti. During stage IV, the metallic Ti underwent rapid oxidation, and the resultant TiO2 was dissolved at an elevated pace, prompting the fusion of adjacent nanotube walls and the genesis of a matrix comprising hexagonal nanotubes. Subsequently, these nanotubes elongated progressively while the current intensity stabilized. Tupala et al.42 and Min et al.46 carried out the formation of TNTs using EDTA alongside other standard anodizing parameters, while Yousif et al.47 used ultrasounds during anodizing. In both cases, the effect of anodization was the production of circular TNTs, not hexagonal TNTs. Only Banerjee et al.30 using ultrasounds and EDTA simultaneously, produced hTNTs. Both factors, EDTA and ultrasounds, are necessary to transform TNTs into hTNTs during anodizing, which was confirmed in the conducted research18. Research indicates an increase in the growth rate, greater order, and cleaner surface of TNTs as a result of the use of ultrasound and EDTA during anodizing30,42,47. This is related to the increased rate of Ti dissolution due to the reaction with EDTA and its acceleration by ultrasound, which in turn leads to the widening of the nanotubes as they grow and the possibility of releasing Ti trapped between the TNTs walls and its dissolution by fluorine ions, and thus leads to the formation of hTNTs46. EDTA causes a larger specific surface area of hTNTs42.

A representative anodic curve for the formation of hexagonal titanium dioxide nanotubes. (I) The growth of the TiO2 barrier drastically decreases the current. (II) F− and [H2EDTA]2− ions cause the dissolution of TiO2, leading to the formation of randomly distributed nano-pits and cracks. The current increases. (III) nano-pits and cracks take the shape of a nanotube matrix. There is a decrease and stabilization of the current and entrapment of metallic Ti in the spaces between the nanotubes due to the rapid dissolution of TiO2 by F− and [H2EDTA]2− ions, which are accelerated by ultrasound and limited access of oxygen to Ti. (IV) metallic Ti undergoes rapid oxidation, and the resulting TiO2 is dissolved at a high rate, leading to the merging of adjacent nanotubes’ walls and forming a matrix of hexagonal nanotubes. These nanotubes continue to elongate, and the current stabilizes.

Changes in the diameter and length of hTNTs depending on the concentration of ethylene glycol in the electrolyte used for anodizing are shown in Fig. 5 and Online Resource Tab. A.1. The concentration of ethylene glycol was changed in the range of 90–97.5%. According to research on TNTs, the water content in the electrolyte should not exceed 10%, and the lower the water content, the more uniform the structure of the nanotubes48. For this reason, a water content of 2.5–10% was assumed. Other anodizing parameters remained constant: anodizing time (60 min), NH4F concentration (0.3 wt%) according to Table 1. The sonoelectrochemical anodization was performed under the potential of 20, 50 and 80 V. For the anodizing potentials of 20 and 50 V, hTNTs diameters were obtained with values of 50 \(\:\pm\:\) 9 nm and 109 \(\:\pm\:\) 13 nm, respectively, and no dependence of the concentration of ethylene glycol on the hTNTs diameter was observed (Fig. 5a). For 80 V, the diameter of hTNTs decreased with the increased in the concentration of ethylene glycol from 225 \(\:\pm\:\) 15 nm for 90% to 111 \(\:\pm\:\) 11 nm for 97.5%. For the potential of 80 V, the ethylene glycol concentration in the electrolyte influenced the hTNTs diameter. For anodizing at 20 V, a slight decrease in hTNTs length for ethylene glycol concentrations of 95 and 97.5% was observed from 3.503 \(\:\pm\:\) 0.139 μm for 90% to 3.087 \(\:\pm\:\) 0.150 μm for 97.5% (Fig. 5b). The increase in the length of hTNTs along with the increase in the concentration of ethylene glycol from 90 to 97.5% occurred for anodizing potentials of 50 V (from 6.327\(\:\pm\:\) 0.270 μm to 11.383 \(\:\pm\:\) 0.799 μm) and 80 V (from 16.933\(\:\pm\:\) 0.839 μm to 33.575 \(\:\pm\:\) 1.542 μm). The literature lacks data on the effect of ethylene glycol concentration on the parameters of hexagonal titanium dioxide nanotubes, but these data are available for circular titanium dioxide nanotubes. Researchers agree that with the decrease in the water content in the electrolyte, the diameter of the TNT decreases while the length increases, similarly to the thickness of the walls of the nanotubes15,49,50. Similarly to circular TNTs, the length of hTNTs increases with the increase of ethylene glycol concentration at potentials of 50 and 80 V. A clear dependence was not observed for 20 V.

An increase in the water content in the electrolyte containing ethylene glycol increases the conductivity of the electrolyte, which increases the electric field strength in the TiO2 barrier layer and accelerates the process of migration of H+, F− and [H2EDTA]2− ions50. In addition, with increasing water content, the viscosity of the electrolyte decreases, which accelerates the diffusion of ions.

The length of the nanotubes depends on the rate of their growth and the rate of chemical dissolution of their upper surface. The mobility of H+, F− and [H2EDTA]2− ions increases with increasing water content in the electrolyte. The importance of the electrochemical water oxidation process, in which both molecular oxygen and hydrogen ions are released, is also growing49.

The increase in the content and mobility of ions is the reason for the increase in the rate of chemical dissolution of the upper surface of the nanotubes, and this process begins to dominate over the process of their growth. As a result, as the electrolyte’s water content increases, the nanotubes’ final length decreases49.

Plots of changes in the (a) diameter of hTNTs and (b) length of hTNTs depending on the concentration of ethylene glycol (C2H6O2) at the sonoelectrochemical anodizing parameters: 20, 50 and 80 V, 60 min, 0.3 wt% NH4F and 0.1 wt% Na2[H2EDTA].

The effect of the concentration of fluorine ions derived from ammonium fluoride on the diameter and length of hTNTs is presented in the form of graphs in Fig. 6 and in Online Resource Tab. A.2. NH4F concentration was changed in the range of 0.10–0.50 wt% for potentials 20, 50 and 80 V. Other anodizing parameters remained constant: anodizing time (60 min), C2H6O2 concentration (95%) according to Table 1. When the anodizing potential was increased from 20 to 50 and 80 V, a decrease in the range of ammonium fluoride concentration at which hTNTs were formed was observed. hTNTs were successfully formed at NH4F concentration in the range of 0.20–0.50 wt% at 20 V, 0.25–0.40 wt% for 50 V, and only for a concentration of 0.30 wt% at 80 V. As the anodizing potential increases, the range of NH4F content in the electrolyte decreases, which leads to the formation of hTNTs during anodizing. Studies on the effect of the fluoride ion content in the electrolyte on the geometry of TNTs indicate that for a given anodizing potential, there is an optimal content of fluoride ions in the electrolyte, which allows obtaining the maximum dimensions of the structure with a high degree of order and homogeneity51,52,53. As reported by Yang et al.51, at too low a fluoride ion concentration of 0.10 wt% for 20 V, 0.20 wt% for 50 V, and 0.25 wt% for 80 V, a compact TiO2 layer was obtained, while at too high a concentration of F− ions, the ordered structure was destroyed, excessively etched, and as a consequence, the formed oxide layer cracked and separated from the titanium substrate. Maximum F− content values for 20 V are not specified. Both the tests for TNTs and the hTNTs experiments showed no effect of the concentration of fluoride ions in the electrolyte on the hTNTs diameter51,52. hTNTs with diameters were obtained: for 20 V 51 \(\:\pm\:\) 8 nm, for 50 V 106 \(\:\pm\:\) 14 nm, and for 80 V diameter of 150 \(\:\pm\:\) 30 nm. At 20 and 50 V, however, an increase in hTNTs length was observed with an increase in NH4F content. The length of hTNTs changed from 2.858\(\:\pm\:\) 0.046 μm to 4.750\(\:\pm\:\) 0.161 μm for NH4F concentrations from 0.20 to 0.50 wt% at 20 V and from 11.473 \(\:\pm\:\) 0.301 μm to 16.800\(\:\pm\:\) 0.954 μm for NH4F concentrations from 0.25to 0.40 wt% at 50 V. For 80 V, hTNTs were obtained only for 0.30 wt% NH4F with length of 31.3 \(\:\pm\:\) 4.25 μm.

Plots of changes in (a) diameter and (b) length of hTNTs depending on the concentration of ammonium fluoride at the sonoelectrochemical anodizing parameters: 20, 50, and 80 V, 60 min, 95% ethylene glycol and 0.1 wt% Na2[H2EDTA].

The effect of the anodizing potential on the hTNTs diameter and length is shown in Fig. 7 and Online Resource Tab. A.3. Figure 7b shows the growth factor for hTNTs length depending on the anodizing potential. In the tests, the anodizing potential was changed in the range of 10–80 V with the remaining constant anodizing parameters, i.e., ethylene glycol concentration 95%, ammonium fluoride concentration 0.3 wt%, anodizing time 60 min according to Table 1. Both the diameter and length of hTNTs increased with increasing potential (Fig. 7a). With the same other anodizing parameters, the highest anodizing potential produces the highest layer of hTNTs with the largest diameter. The growth factor of hTNTs is not significantly different for potentials 10–60 V, being higher at 70 V and highest at 80 V. The higher escalation factors at 80 V may be related to the critical breakdown potential for these parameters.

In the literature, many reports on the linear dependence of the diameter and length of TNTs on the anodizing potential can be found. This relationship was also confirmed in the case of hTNTs15,34. According to the data on the effect of anodizing potential on the length of TNTs, there is a linear relationship between the potential and the length of TiO2 nanotubes for low potentials (10–60 V)51. For potentials above 50 V, the researchers point to the critical breakdown potential above which the length of the nanotubes increases rapidly. Yang et al.51 also indicate the dependence of the value of critical breakdown potential on the concentration of NH4F in the electrolyte. The tests used 0.3 wt% NH4F and a faster increase in hTNTs length was observed for potentials of 70 and 80 V. Ozakan et al. found no dependence of the value of the TNTs growth factor on the potential at potentials of 10–50 V53.

Plots of changes in (a) diameter (line with left arrow) and length (line with right arrow) and (b) growth factor of hTNTs depending on the anodizing potential at the sonoelectrochemical anodizing parameters: 60 min, 95% ethylene glycol, 0.3 wt% NH4F and 0.1 wt% Na2[H2EDTA].

Table 2 presents the results of wall thickness (t), porosity (P), pore density (in), and specific surface area (As) of hTNTs for anodizing potentials of 10–80 V. t, P, in and As were determined according to the Eqs. (1–5). An increase in the value of t was observed with the increase in the anodizing potential. The increase in anodizing potential also caused a decrease in in. For P and As, an increase in the value was observed with the increase of the anodization potential, except for the potentials 20 and 60 V for \(\:{A}_{s}\) where the value decreased, for P, while at 20 V, the value decreased, while for the range of 40–60 V, the value of P is approximately constant.

A linear increase in wall thickness from the value of anodizing potentials was proven for TNTs34. According to Yahia et al., with the increase of the anodizing potential (20–50 V), the value of \(\:{i}_{n}\) decreases. This is confirmed by the effect of the anodizing potential in hTNTs. For potentials of 20–50 V, the wall thickness and the specific surface area increase with increasing anodizing potential34.

The effect of anodizing time on the diameter and length of hTNTs is shown in Fig. 8 and Online Resource Tab. A.4. Anodization time was changed in the range of 40–90 min. Other anodizing parameters remained constant: anodizing potential 50 V, C2H6O2 concentration 95%, NH4F concentration 0.3 wt% according to Table 1. The tests showed no changes in diameter and an increase in length of hTNTs with increasing anodizing time. The study of the effect of anodizing time on the length of hTNTs gave results analogous to those for TNTs, according to which the length of nanotubes increases with time54,55. Studies for TNTs indicate no dependence of the anodization time on the diameter of nanotubes, in these studies a slight effect of time on the diameter of hTNTs was observed.

Plots of changes in diameter (line with left arrow) and length (line with right arrow) of hTNTs depending on the anodizing time at the sonoelectrochemical anodizing parameters: 50 V, 95% ethylene glycol, 0.3 wt% NH4F and 0.1 wt% Na2[H2EDTA].

Table 3 shows the effect of anodizing time on diameter, length, wall thickness, porosity, pore density, and specific surface area of hTNTs. No significant changes in t and P were observed with increasing the anodizing time, As increased with a slight decrease in the value at 80 min, while for in an increase was observed with decreases in the value for 50 and 80 min of anodization time.

According to the available literature, with the increase of anodizing time from 0.5 to 3 h, there is an increase in Ø2, H, P and As and a decrease in in for TNTs56,57,58. For both TNTs and hTNTs, As increases with the anodizing time. For hTNTs, the dependence of P on anodization time, which was observed for TNTs, is missing. On the other hand, for in the dependencies are reversed, increase for hTNTs, and a decrease for TNTs.

In conclusion, this study represents a new controlled synthesis method of highly ordered hexagonal titanium dioxide nanotubes (hTNTs) through sonoelectrochemical anodization.

Based on top-view and button-view SEM pictures, the differences in morphology between hexagonal and circular titanium dioxide nanotubes were confirmed.

This study proposes a mechanism for the formation of hexagonal titanium dioxide nanotubes (hTNTs) using sonoelectrochemical anodization, which challenges the prevailing theory, positing hTNTs as precursors to circular TNTs. Advanced analysis of anodization kinetics demonstrated that a thin TiO2 passive layer initially forms on the titanium surface, rapidly decreasing current due to oxide layer growth. Subsequent activation of the oxide barrier layer causes TiO2 dissolution and pore formation. As the process continues, the porous layer transforms into nanotube templates, stabilizing current intensity.

The influence of anodizing parameters (potential, anodizing time) and electrolyte concentration (ethylene glycol concentration, ammonium fluoride concentration) on hexagonal titanium dioxide nanotube morphology was determined. The studies determined the ranges of these parameters for which the formation of hTNTs is possible. hTNTs are formed for ethylene glycol concentrations in the range of 90–97.5% and ammonium fluoride concentration in the range of 0.2–0.5 wt% at 20 V, 0.2–0.4 wt% for 50 V and only for a concentration of 0.3 wt% at 80 V. The anodizing potential in the range of 10–80 V allows to obtain hTNTs with a diameter of 30\(\:\:\pm\:\:\)5–169 \(\:\pm\:\) 56 nm, respectively, while the time must be longer than 30 min.

The anodizing parameters allow control of the diameter, length, wall thickness, porosity, pore density, and specific surface area of hTNTs. The effect of ethylene glycol and ammonium fluoride concentrations on the diameter and length of hTNTs depends on the applied anodizing potential.

The diameter, length, wall thickness, porosity, and surface area of hTNTs increased with the increase of the anodizing potential while the pore density decreased. A decreased diameter and increased length of hTNTs with increasing anodizing time were observed. There were no changes in wall thickness and porosity with increasing anodizing time; the specific surface area increased with decreasing anodizing time.

The raw data required to reproduce these findings cannot be shared at this time due to technical or time limitations. The data analyzed during the current study will be available upon reasonable request by contacting the corresponding author Katarzyna Arkusz at [email protected].

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Author Aleksandra Jędrzejewska is a doctoral student at the Doctoral School of Exact and Technical Sciences of the University of Zielona Gora.

This work was supported by the programs of the Polish Minister of Science and Higher Education under the name ”Regional Initiative of Excellence” in 2019–2023, project no. 003/RID/2018/19, funding amount 11 936 596.10 PLN, and under the program SPUB, project no. 40/530410/SPUB/SP/2022.

Aleksandra Jędrzejewska

Present address: The Doctoral School of Exact and Technical Sciences, University of Zielona Gora, Zielona Gora, 65-417, Poland

Department of Biomedical Engineering, Faculty of Mechanical Engineering, University of Zielona Gora, 9 Licealna Street, Zielona Gora, 65-417, Poland

Aleksandra Jędrzejewska & Katarzyna Arkusz

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Conceptualization: Aleksandra Jędrzejewska, Katarzyna Arkusz; Methodology: Aleksandra Jędrzejewska, Katarzyna Arkusz; Formal analysis and investigation: Aleksandra Jędrzejewska; Writing - original draftpreparation: Aleksandra Jędrzejewska; Writing - review and editing: Aleksandra Jędrzejewska, KatarzynaArkusz; Funding acquisition: Katarzyna Arkusz; Resources: Katarzyna Arkusz; Supervision: Katarzyna Arkusz;

Correspondence to Katarzyna Arkusz.

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Jędrzejewska, A., Arkusz, K. Mechanism and growth kinetics of hexagonal TiO2 nanotubes with an influence of anodizing parameters on morphology and physical properties. Sci Rep 14, 24721 (2024). https://doi.org/10.1038/s41598-024-76336-7

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DOI: https://doi.org/10.1038/s41598-024-76336-7

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