Anodic synthesis of TiO 2 nanotubes by step-up voltages

Abstract

Fabrication of TiO₂ nanotubes (NTs) has extensive application properties due to their high corrosion resistant and compatibility with biomedical applications, the synthesis of TiO₂ nanotubes over titanium has drawn interest in various fields. The synthesis of TiO₂ NTs using novel in-situ step-up voltage conditions in the electrochemical anodization process is recorded in this work. For manufacturing the NTs at 1 hour of anodization, the input potential of 30, 40 and 50 V was selected. With increasing step-up voltage during the anodization process, an improvement in the NTs was observed, favoring corrosion resistance properties. The surface of NTs enhances the structure of the ribs, raising the potential for feedback over time. XRD was used to analyze phase changes, and HR-SEM analyzed surface topography. Impedance tests found that longer NTs improved the corrosion resistance.

Keywords

Electrochemical synthesis scanning electron microscopy nano-indentation corrosion

Introduction

Materials engineering for biomedical applications is greatly encouraged to explore the biocompatible physical and chemical properties. The toxicity resulting from the direct interaction of blood ions with materials can be eliminated using surface modified materials that control the harmful chemical reaction. Modifying the material to possess a rough or porous surface elevates the material’s adhesion properties and wetting behavior.¹ Sol-gel, hydrothermal, electrospinning, electrochemical, template synthesis, etc., are the different types of fabrication techniques used in surface modification.^2,3 Electrochemical anodization is the best among accessible traditional mechanisms for surface modification.⁴ The template synthesis found that lesser effective in shape of output, thickness of shells and uniformity of finished components. Final fabricated passivation layer receives the poor mechanical strength and low corrosion resistivity on other synthesis method.⁵ Electrospinning had spinning jet instability and post heat treatment for improve the crystallinity of fabricated samples. Hydrothermal process required a long fabrication time for nanotubes.⁶ Overcome all there limitation electrochemical anodization is the best owing to simple fabrication technique and feasible method. The geometrical properties of nanotubes (NTs) diameter and length are tunable by changing anodization conditions such as input potential or current (constant voltage path or varying the input), electrolyte composition, and anodization process time. The mechanical, physical and chemical properties are the primary indicators to find a material’s applications.^7,8

The NT’s geometry plays an important role in photocatalytic activity,⁹ solar cells,¹⁰ cell adhesion^11,12 and advances application such as Photonic,¹³ Li-ion microbatteries.¹⁴ Increase the surface area of titanium with an extra porous layer or bamboo-type structure (TiO₂) demonstrated with ridges¹⁵ by involving the alternating-voltage (fluctuating conditions)¹⁶ has improved the materials mechanical chemical, and physical properties. The surface change in TiO₂ nanotubes ridges or extra tube lengths vertically aligned structures enhances surface wettability¹⁷ and corrosion resistances.¹⁶ The growth mechanism of TiO₂ nanotubes complex and few theories to explain nanotubes formation have been mathematical described.^18–20 The most general theory of field-assisted dissolution or dissolution equilibrium theory suggests that dissolution, directed and escalated by an input electric field, is the core of titanium nanotube preparation.¹⁸ Five stages of initiation of embryo mechanism and deviation and fitting of embryo growths were reported by Xufei Zhu et al.²¹ The boundary condition of the vibration model for single-wall carbon nanotubes has been derived. Different applied potentials and various times have been used to analyze the oxide barrier layer.²² Crystallization improving the material's chemical stability,²³ corrosion resistance,²⁴ and cell viability^25,26 have been demonstrated. Furthermore, the cell viability on titanium based materials has been considered on several aspects such as hydroxyapatite (HAp) deposition,²⁷ length and diameter of NTs,^26,28 etc.

Zhaoxiang Peng and Jiahua Ni have fabricated the TiO₂ nanotube under the step-up condition and they found that an increase in voltage profoundly damaged the nanotube structure after the rise in voltage.¹⁷ In this study, damage of nanotube in step-up condition occurs but doesn’t destroy. Arenas et al. successfully fabricate the TiO₂ nanotubes with different wt. % of NH₄F and step up conditions and achieves the bottle shape nanotubular structures.²⁹ However, the growth of nanotube on titanium material step-up condition delivers different surface morphologies. Freitas et al. reported that constant voltage and pulsed voltage impact with nanotube surface changes positively impacted nanotube surfaces. The input current flow rate influenced the material; rippled surface absorbed from voltages variation. The TiO₂ NTs with different structure morphologies using different step-up voltages and the effect of the anodized step-up process were reported.³⁰ Another team fabricated the two-layer NTs in three-electrode configurations.³¹

With additional stirrer setup, further applied potential, two types of rotation direction, it is reported to obtain different promising surface morphologies.³² However, the significant growth of NTs was obtained by different input potential,²¹ ramp voltage³³ and fluctuation of potential.³⁴ Though many researchers report synthesizing the nanotube by electrochemical anodization at constant input voltages and ramp voltages, only a few reports are available with the novel in-situ voltage up conditions. However, the surface modified with TiO₂ nanotubes are essential for biomedical applications.

Electrochemical fabrication of TiO₂ nanotubes with the novel in-situ voltage method was presented in this research work. Fabricated samples were characterized with the high-resolution scanning electron microscope (HR-SEM), X-ray Diffraction (XRD), Raman spectra of TiO₂ nanotubes, wetting behavior (contact angle measurement) and corrosion behavior was also analyzed.

Materials and methods

A combination of smooth and rough layers of TiO₂ was fabricated using the step-up anodization process. The electrochemical unit consists of a conventional two-electrode setup. The stainless steel 316 L plate serves as a cathode and CP titanium as an anode. The obtained samples were initially polished with a sandpaper grit size of 320, 600, 1000, and 2000 and then chemically etched using 1:4:5 hydrofluoric acid, nitric acid and DI water.^35,36 The etching time for all the samples was the 30 s. The anodization process was then carried out in two-stages. The voltage was manually tuned from 0 V to 30 V and was maintained for 30 min. Subsequently, the second anodization process, which was continuous, was done. In this step-up condition, the voltage was maintained at 40 V for 30 min of anodization time. Fabricated samples were washed with distilled water and then dried with air to remove the moisture. The samples thus obtained underwent heat treatment at 550 °C for 1 hour to get the crystallinity.

The figure (Figure 1) represents the anodization conditions. The anodization process are conducted in two-stage of anodization as Figure 1(a) 30 V-30min, 40 V-30min (sample - 1), (b) 30 V-45min, 40 V-15min (sample - 2), (c) 30 V-30min, 50 V-30min (sample - 3)and (d) 30 V-45min, 50 V-15min (sample - 4). All the experiments were carried out at room temperature.

Figure 1.

Schematic diagram of anodization condition.

Deposition of the hydroxyapatite layer and the behavior of the material in simulated body conditions were undertaken. For preparing artificially simulated body fluid which is similar to the ions in the human blood, NaCl (8.035 g), NaHCO₃ (0.355 g), KCl (0.225 g), K₂HPO₄.3H₂O (0.231 g), MgCl₂.6H₂O (0.311 g), 1.0 M – HCl (39 ml – if required for maintaining the pH value), CaCl₂ (0.292 g) and Na₂SO₄ (0.072 g) were added in 100 ml DI water with continuous stirring condition. Artificially simulated body fluid was prepared using the method reported in ref.³⁷

Results and discussion

Surface morphological results

The high-resolution scanning electron microscope (HR-SEM, Hitachi S-400) was utilized to observe the nanotube's surface morphology. Figure 2(a) to (d) shows the surface micrograph of TiO₂ nanotube fabricated on titanium plate by electrochemical anodization in the fluoride at stepped-up input potential voltages of different levels. Figure 2(a) shows the step-up condition of 30 V and 40 V each for 30 minutes of anodic anodization. Here, the morphology indicates that the initial anodization condition makes a minor rib structure on the nanotube’s surface.

Figure 2.

Anodized TiO₂ nanotubes under step-up condition (a) 30 V-30min, 40 V-30min (sample-1), (b) 30 V-45min, 40 V-15min (sample-2), (c) 30 V-30min, 50 V-30min (sample-3) and (d) 30 V-45min, 50 V-15min (sample-4).

With the increase in input potential, the rib structure starts to form in the NTs, as indicated by the arrow. Two surface morphologies are obtained using a single process: smooth surface in the first stage (bottom) and rough (Rib structure nanotube) in the second stage. The length of the nanotubes obtained was about 1 µm. During the second stage at 40 V, the ribs form sequentially. Initial input conditions favor the formation of rib structure over the nanotubes. Increased input potential condition helps in continuous formation due to increased charge transport rate in the second stage. Figure 2(b) shows the HR-SEM image of sample with initial voltage as 30 V for 45 minutes and second input potential as 40 V for 15 minutes. Increasing the input potential increases dissolution and the nanotubes form at different rib structures. An increase in the rib’s breadth is noticed while the length is 1.5 µm (approx.). However, the nanotube tip shows damage due to the rise in the input potential dissolution reaction. Figure 2(c) refers to a sample with 30 V for 30 minutes and then 50 V for another 30 minutes.

Normal anodization of 30 V shows growth with a smooth morphology while increasing the voltage to 50 V, and the nanotubes start to damage. Thus, nanotubes get damaged during the second part of anodization, but nanotubes' length increased to 2.5 µm (approx.). Figure 2(d) represents the anodization of 30 V for 45 minutes and 15 minutes for 50 V input potential voltages. Here, the length of the nanotube obtained was about 1.5 µm. Compared to Figure 2(b) and (c), nanotube end crack significantly reduces. These results of different morphologies are matched with the previous reports^15,18 from the surface morphological effects. We find that an increase in input potential after 30 minutes of anodization leads to crack or nanotubes damage. The 50 V increased sample results deliver that crack and damage in the extension.

XRD analysis

X-ray diffraction studies were done using Riganku set up Cu-K∝ radiation (λ = 1.5418°A) at a scanning rate of 0.1 step size for a range of 20°-90° (2θ), 40 kV of tube voltage and 20 mA of electric current were maintained for structural analysis. Further, the effect of annealing temperature on phase transition was analyzed.

Figure 3 gives the XRD phase analysis of pristine TiO₂ nanotubes fabricated under step-up conditions. The major TiO₂ nanotubes peaks (JCPDS file: 89-4921) indexed at 25.5°, 38.5°, 40.2°, 48.2°, 53.1°, 62.5°, 70.9° are shown in Figure 3.^38,39 The anatase and rutile phases of XRD JCPDS cards were identified files: Anatase: 21–1272; Rutile: 21–1276; and Brookite: 29–136038. Increase the intensity of the peak at 25.5° peak post-heat treatment shows an increase in crystallinity as also observed from the presence of ((101), (200)) peaks. It is noticed that brookite peaks form after annealing temperature of 450 °C for 1 hour and is similar to that annealed at 500 °C. However, the peak intensity increases after annealing at 550 °C.^35,39,40

Figure 3.

XRD patters of TiO₂ nanotubes obtained by different annealing condition.

Raman shift analysis

The crystallographic phases of TiO₂ nanotubes were analyzed by Raman spectroscopy (BRUKER RFS 27, resolution of 2 cm-1). Here, TiO₂ nanotube crystallinity is indicated with four vigorous peaks found at 146, 396, 514, 636 cm-1 refer Figure 4.^41,42 The inset in Figure 4 shows a detailed view of the region of interest, revealing anatase and rutile phases. The initial form of anatase was formed in 146 cm-1, followed by a 396 anatase phase of (A) B1g, another anatase phase of 514 cm-1 (A) A1g-B1g and (A) E1g anatase phases as indicated.⁴³ A or B mentioned as non-degradation mode in one dimention, E mentioned as double degradation in two dimention, T mentioned as triple degradation in three dimention and subscription 1 and 2 denoted as symmetric, non symmetric and principle symmetric axis all the notation were follow the group theriory. The subscription of g or u mentioned as the symmetric or anti symmetric refer to the inverison of coordinates. Hence, this spectroscopic analysis also reveals the phase transition because of annealing treatment and validates the XRD results.⁴³

Figure 4.

Raman spectra of TiO₂ nanotubes.

Raman spectra can observe that the peak at 146 cm-1 on the different samples dramatically increases in intensity with annealing. Also, the peak broadens due to the calcination of 550 °C. The enhancement in peak intensity is attributed to increasing crystalline nature this observation is consistent with the XRD results.

Wetting behavior

Different surface morphology of the nanotube was fabricated and the surface layer has improved the hydrophobic and hydrophilic condition of the samples. Figure 5 shows the wetting behavior of anodized samples using contact angle measurement. The contact angle can see that anodized samples 1,2,3 and 4 with nanotube length of 1,1.5, 2.5 and 1.6 µm respectively showed the contact angle degrees as 128°, 118°, 49, and 92.806° (Figure 5(a) to (d)). When voltage increased to 40 V, surface wettability yields 128° and 118°. The other samples, 3 and 4, with an increased voltage of 50 V, show a lower contact angle of 48.583° and 92.806°. The change in water contact is attributed to modifying the surface area by increasing the diameter and length of the NTs. Such highly hydrophilic surface morphologies would improve the osteoblast behavior and enhance the osseointegration.³⁶

Figure 5.

Wetting behavior of different surfaces (a) sample-1 (30 V and 40 V for 30 minutes each), (b) sample-2 (30 V and 40 V for 45,15 minutes respectively), (c) sample-3 (30 V and 50 V for 30 minutes each) and (d) sample-4 (30 V and 50 V for 45,15 minutes respectively).

Nano indentation

The mechanical behavior of the coated material surface strength is studied using (ISO 14,577) nanoindentation. Figure 6 illustrates the typical loading starting from point (a) and reaches the maximum load of 8 µN at point (b) with a constant load rate of 50 mN/s. After the load reaches the top end (b), the load is held for a few seconds from point (b) to (c). The holding process is stopped and the input load is unloaded using the same constant unload (50 mN/s) to measure the material’s plasticity (d).

Figure 6.

Nano indentation studies of four different TiO₂ nanotubes.

Table 1 shows the nanoindentation results of anodized samples with the step-up condition. Evaluation of the nanotube properties considers the following: deformation contact depth impression of the indenter on the nanotubes (h_c), the maximum load applied on the nanotubes (P_max), unloading curve slope (S) to measure the stiffness of the nanotube, contact of the indenter and nanotubes projected contact area, (h_max) the highest depth travel by the indenter, and E_r elastic modulus of the tip.

Table 1.

Summary of fabricated TiO₂ nanotubes nanoindentation results.

	Contact depthh_c(nm)	Max. indentation force P_max(µN)	Unloading curve slope S(µN/nm)	Contact areaA(nm²)	Max. penetrating depth h_max(nm)	Elastic modulus of tip Er(GPa)
1	637.54	7999.1	264.6	10.37E5	659.98	72.44
2	743.00	7999.9	255.9	13.84E5	766.94	60.97
3	704.78	7983.1	266.4	12.58E5	727.15	66.72
4	722.40	7998.7	257.3	13.12E5	745.70	62.95

For the nanoindentation test, an 8 µN load has been used as a maximum load for the sample analysis. The unloading curve (c)-(d) curved line and hr = h_max (w2 ≠ 0 – not a straight line) is not a pure plastic material. The penetration initially starts from the sample's surface area. From the initial indentation contact with the nanotubes deformation of the sample is the displacement begins to form the nanotube's surface. The maximum penetration depth achieved in the material can be determined by the hardness of the material⁴⁴ using equation (1).

For the nanoindentation test, an 8 µN load has been used as a maximum load for the sample analysis. The unloading curve (c-d) curved line and h_r= h_max (w2 ≠ 0 – not a straight line) is not a pure plastic material. The penetration initially starts from load carried by the sample from the initial indentation contact with the sample that deformation of the sample is the displacement of the sample begins to form the surface of the nanotubes. The maximum penetration depth achieved in the material can be determined by the hardness of the material⁴⁴ using equation (1).

H = \frac{P_{\max}}{A}

(1)

where, P_max is the maximum load applied to the material, h_max maximum depth penetration, and (A) is measured as the maximum measured contact between the indenter and specimen. For Berkovich indenter, A surface contact is expressed as a function of connection between specimen and indentation depth hf as shown in equation (2)^45–47

A = 3 \sqrt{3 h_{f}^{2}} \tan^{2} 65.3 = 23.96 h_{f}^{2}

(2)

Then h_f contact with the material can be expressed using equation (3) as

h_{f} = h_{\max} - ε \frac{p_{\max}}{S}

(3)

The h_max maximum displacement attained at the maximum load achieved (P_max), S represents the contact stiffness arising from maximum displacement $(S = (\partial p / \partial h) h_\max$ ). The rational indentation geometrical consideration derived is shown in equation (4)

h_{c} = h_{\max} - {(\frac{P}{\frac{\partial p}{\partial h}})}_{h_{\max}} = h_{\max} - {(\frac{p}{s})}_{h_{\max}}

(4)

Evolution of finding the stiffness S, the fitting of unloading data ‘power law’ can be expressed as following, $p = {m (h - h_{r})}^{n}$ here, the m, n is denoted as the fitting parameters. Besides, the maximum penetration of indenter and the relation between the (stiffness of maximum displacement) S and the elastic module can be written as shown in equation (5),

{S |}_{h_{\max}} = {(\frac{\partial p}{\partial h})}_{h_{\max}} = \frac{2 β}{\sqrt{π}} \sqrt{A_{\max}} E^{*}

(5)

Here, A_max indicates as the contact area projected in indenter and the material h_max, β variation in the geometry, E* reduced elastic module.⁴⁶

Table 1 shows the nanoindentation results of anodized samples with step up condition. Initially, the contact depth of the samples are high due to the low strength of the sample. Sample 1 delivers the minimum length indicating the increased strength of the sample. All the experiments reach the predetermined maximum load.

The unloading data could determine the rate of elastoplasticity/plastic deformation. The relation between indentation contact and the material surface indicates that a low penetration, high strength material (sample 1) delivers the contact surface (A). During the unloading process, the sample showing less penetration renders the high elastic modulus shown in the last column. However, the maximum strength material (high strength nanotube) contact depth and contact are of the materials that deliver high durability compared to others.^48,49

Initially, the contact depth of the samples is high due to the low strength of the sample. Sample 1 delivers the minimum length indicating the high strength of the sample. All the experiments reach the predetermined maximum load. The unloading data could determine the rate of elastoplasticity/plastic deformation. The relation between indentation contact and the material surface indicates that a low penetration sample, high strength material (sample 1) delivers the level contact surface (A). During the unloading process, the sample showing less penetration renders the high elastic modulus displayed in the last column.

Electrochemical studies

Electrochemical behavior in artificial simulated body fluid results for all samples are shown in Figure 7. Corrosion potential (E_corr) and corrosion current density (i_corr) are the two significant factors calculated from impedance spectroscopy. The substrate chemical potential is measured using E_corr, and is the thermodynamic parameter shown in Figure 7(a). If the corrosion resistance is high in tafel analysis the E_corr values become low and i_corr values are high.⁵⁰ The E_corr and i_corr values for samples 1-4 are -535, -246, -273, -262 mV and 1.98, 2.77, 3.03, 2.63 µA, respectively (Table 2). Corrosion potential decreases when the step-up condition increases but in corrosion current density condition, the values decrease in samples 1-3. The low values of i_corr and high value of E_corr suggest that the material has better corrosion resistance. The electrochemical impedance spectroscopy images are shown in Figure 7(b) and (c) with frequency versus magnitude and phase angle. An equivalent electrical circuit calculated the samples polarization resistance by fitting the (Inset: Figure 7(d)) along with the Nyquist plot. Figure 7(d-a_1, b_1, c₁) shows the solid electrolyte of simulated body fluid, anodized sample, and deposition of SBF solution, respectively.

Figure 7.

Electrochemical behavior of (a) potentiodynamic polarization curve, (b) impedance as function of frequency, (c) phase angle as a function of frequency and (d) Nyquist plot.

Table 2.

Effect of polarization analysis.

	I_corr (µA)	E_corr (mV)	Beta A (V/decade)	Beta C (V/decade)	Corrosion Rate (mpy)
Sample 1	1.98	–535	1.193	2.70E-01	2.55E-01
Sample 2	2.77	–246	1.402	4.88E-01	8.72E-01
Sample 3	3.03	–273	1.416	4.88E-01	3.91E-01
Sample 4	2.63	–262	2.302	4.88E-01	8.26E-01

The electrochemical impedance spectroscopy analysis was carried out and results are shown in Figure 7(d). According to the Nyquist plot large capacitive the circle is obtained in the sample with maximum length. The enlargement in loop indicates the high corrosion resistivity of material. The stable structure improves the corrosion resistance thus increasing the suitability in the bio-implantation environment. The corroded specimen and the SBF relation manifests in the equivalent electric circuit (EEC) shown in Figure 7(d). The impedance spectrum was fitted with the EEC with the use of gamry software interface 1010 Rs solution resistance, Rct and Qdl represent the polarization resistance the passive layer polarization resistance. Qdl2 and Rd2 fitted with the equivalent circuit, Qdl2 and Rd2 represent double layer and charge transfer resistance capacitance, respectively. By comparing all the Nyquist results, the maximum capacitive loop indicates the maximum corrosion resistances.

Table 3 shows the corresponding results obtained from the EEC fitting model. The improvement leads to further investigation of cell attachment for hydrophilic condition and the hydrophobic state samples were increasing the corrosion resistance.^51,52 Based on the previous summarized reports informs that increase the voltage with a reasonable anodization time delivers that different surface morphology of the nanotubes rib structure increases the corrosion resistance.^23,50,53

Table 3.

EIS parameters for different length and structured sample.

	Rs	Rct	Qdl	Rb1	Rf	Qdl2	Rb2	Goodness of Fit
Sample 1	205	1.47E + 04	9.16E-05	8.32E-01	388	1.46E-04	1	3.56E-04
Sample 2	204.8	8.19E + 03	6.80E-05	1.00	6.57E + 04	1.37E-03	4.91E-01	1.69E-03
Sample 3	210.4	3.01E + 03	1.31E-04	1.00	1.24E + 03	3.02E-04	5.83E-01	1.48E-03
Sample 4	187.8	7.99E + 05	8.53E-05	1.00	3.28E + 03	4.07E-05	9.31E-01	2.50E-03

Overall, incorporated in-situ voltage conditions and modified structures captivate 1) enhanced crystallinity, 2) good surface wettability, 3) Better mechanical property and 4) corrosion resistivity of the material. These promising features render this coating appealing suitable for the orthopedic application. Essential for preclinical examination, in-situ, bio-activity and animal study should be performed the interaction of coating performance with bones and tissues.

Conclusion

The conventional In-situ voltage condition used to synthesize the TiO₂ nanotubes with different surface morphologies are obtained. The evolution of rib structure was absorbed due to the voltage change with respect to time. The sudden voltage change enhances the oxygen evolution helps to form nanotubes that can absorb the rib structure can be absorbed. The existence of rib structure enhances the strength of nanotubes confirmed by nano-mechanical properties. Changes in wetting behavior reveals that samples are promising bioactive action. The corrosion results revel that short length nanotubes are poor in resist the corrosion. The enlarged nanotubes with different rib structures are informed that they withstand corrosion and promise corrosion resistance.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

R Narayanan

References

Sobieszczyk

Self-organized nanotubular oxide layers on Ti and Ti alloys. Adv Mater Sci 2009; 9: 14–17.

Zhao

Wang

Huo

, et al. Biomaterials the osteogenic activity of strontium loaded titania nanotube arrays on titanium substrates. Biomaterials 2013; 34: 19–29.

, et al. One-dimensional TiO₂ nanotube photocatalysts for solar water splitting. Adv Sci 2017; 4: 1–31.

Zhao

Cui

, et al. A mathematical model for the growth of anodic TiO₂ nanotubes under higher current density. J Electrochem Soc 2017; 164: E401–E407.

Bui

VKH

Van Tran

Moon

, et al. Titanium dioxide microscale and macroscale structures: a mini-review. Nanomaterials 2020; 10: 1190–1131.

Tan

Pingguan-Murphy

Ahmad

, et al. Advances in fabrication of TiO2 nanofiber/nanowire arrays toward the cellular response in biomedical implantations: a review. J Mater Sci 2013; 48: 8337–8353.

Ramos-Tonello

Lisboa-Filho

Arruda

, et al. Titanium dioxide nanotubes addition to self-adhesive resin cement: effect on physical and biological properties. Dental Mater 2017; 33: 866–875.

Yaghoubi

Taghavinia

Alamdari

, et al. Nanomechanical properties of TiO₂ granular thin films. ACS Appl Mater Interfaces 2010; 2: 2629–2636.

Hajjaji

Elabidi

Trabelsi

, et al. Bacterial adhesion and inactivation on Ag decorated TiO₂ -nanotubes under visible light: effect of the nanotubes geometry on the photocatalytic activity. Colloids Surfaces B: Biointerfaces 2018; 170: 92–98.

10.

Prakash

Review on nanostructured semiconductors for dye sensitized solar cells. Electron Mater Lett 2012; 8: 231–243.

11.

Cipriano

Miller

Liu

Anodic growth and biomedical applications of TiO₂ nanotubes. J Biomed Nanotechnol 2014; 10: 2977–3003.

12.

Benčina

Junkar

Zaplotnik

, et al. Plasma-induced crystallization of TiO₂ nanotubes. Materials 2019; 12: 626.

13.

David

TiO₂ self-assembled, thin-walled nanotube arrays for photonic applications. Materials 2019; 12: 1332.

14.

Fraoucene

Sugiawati

Hatem

, et al. Optical and electrochemical properties of self-organized TiO₂ nanotube arrays from anodized Ti-6Al-4V alloy. Front Chem 2019; 7: 1–9.

15.

Guan

Hymel

Wang

Growth mechanism and morphology control of double-layer and bamboo-type TiO2 nanotube arrays by anodic oxidation. Electrochim Acta 2012; 83: 420–429.

16.

Ahmadi

Mohammadi

Sadrnezhaad

SK.

Hydroxyapatite based and anodic titania nanotube biocomposite coatings: fabrication, characterization and electrochemical behavior. Surface and Coatings Technology 2016; 287: 67–75.

17.

Peng

Surface properties and bioactivity of TiO₂ nanotube array prepared by two-step anodic oxidation for biomedical applications. R Soc Open Sci 2019; 6: 181948.

18.

Huang

Ying

, et al. Split TiO₂ nanotubes − evidence of oxygen evolution during Ti anodization. Electrochem Commun 2019; 106: 106532.

19.

Cui

, et al. Terminated nanotubes: evidence against the dissolution equilibrium theory. Electrochem Commun 2018; 86: 80–84.

20.

Chen

, et al. Studies of oxide growth location on anodization of Al and Ti provide evidence against the field-assisted dissolution and field-assisted ejection theories. Electrochem Commun 2018; 87: 76–80.

21.

Chong

Jin

, et al. Theoretical derivation of anodizing current and comparison between fitted curves and measured curves under different conditions. Nanotechnology 2015; 26: 145603.

22.

Apolinário

Quitério

Sousa

, et al. Modeling the growth kinetics of anodic TiO₂ nanotubes. J Phys Chem Lett 2015; 6: 845–851.

23.

Rautray

Narayanan

Kwon

T-Y

, et al. Surface modification of titanium and titanium alloys by ion implantation. J Biomed Mater Res 2010; 93B: 581–591.

24.

Wang

Wan

Ren

, et al. Bioactivity of micropatterned TiO₂ nanotubes fabricated by micro-milling and anodic oxidation. Mater Sci Eng C 2019; 95: 114–121.

25.

Mazare

Totea

Burnei

, et al. Corrosion, antibacterial activity and haemocompatibility of TiO₂ nanotubes as a function of their annealing temperature. Corros Sci 2016; 103: 215–222.

26.

Barudin

NHA

Sreekantan

Thong

, et al. Studies of cell growth on TiO₂ nanotubes. Amr 2012; 620: 325–329.

27.

Sreekantan

Saharudin

Wei

LC.

Formation of TiO₂ nanotubes via anodization and potential applications for photocatalysts, biomedical materials, and photoelectrochemical cell. IOP Conf Ser: Mater Sci Eng 2011; 21: 012002.

28.

Yang

, et al. Nano size effects of TiO₂ nanotube array on the glioma cells behavior. Int J Mol Sci 2013; 1: 244–254.

29.

Hernández-López

Conde

De Damborenea

, et al. TiO₂ nanotubes with tunable morphologies. RSC Adv 2014; 4: 62576–62585.

30.

Shi

Liao

, et al. Fabrication of two-layer nanotubes with the pear-like structure by an in-situ voltage up anodization and the application as a drug delivery platform. J Alloys Compounds 2015; 647: 590–595.

31.

Liang

Wen

Liao

A visible-light-controlled platform for prolonged drug release based on Ag-doped TiO₂ nanotubes with a hydrophobic layer. Beilstein J Nanotechnol 2018; 9: 1793–1801.

32.

Bhattacharyya

Sarswat

Islam

, et al. Geometrical modifications and tuning of optical and surface plasmon resonance behaviour of Au and Ag coated TiO₂ nanotubular arrays. RSC Adv 2015; 5: 70361–70370.

33.

Lim

Zainal

Tan

, et al. Anodization parameters influencing the growth of titania nanotubes and their photoelectrochemical response. Int J Photoenergy 2012; 2012: 1–9.

34.

Y-C

Huang

F-M

Chang

Y-C.

Biodegradable polymer–silica xerogel composite microspheres for controlled release of gentamicin. J Biomed Mater Res. Part B, Appl Biomater 2005; 83: 340–344.

35.

Sivaprakash

Narayanan

Surface modification TiO2 nanotubes on titanium for biomedical application. Mater. Sci. Forum 2021. 1019: 157–163.

36.

Karthieka

Venkatasubbu

Prakash

Nanocomposite thick films of CsPbI3: PVDF-HFP on plastics for direct conversion low-dose X-ray sensor. Mater Sci Semiconductor Process 2020; 120: 105289.

37.

Kokubo and Takadama H. How useful is SBF in predicting in vivo bone bioactivity. Biomaterials 2006. 27: 2907–2915.

38.

Lai

Sreekantan

Photoelectrochemical performance of smooth TiO₂ nanotube arrays: effect of anodization temperature and cleaning methods. Int J Photoenergy 2012; 2012: 1–11.

39.

Prakash

Prasad

Ramasamy

, et al. Optical and electrical properties of mechanochemically synthesized nanocrystalline delafossite CuAlO 2. J Nanosci Nanotechnol 2008; 8: 4273–4278.

40.

Sivaprakash

Narayanan

Anodic synthesis of TiO2 nanotubes influence of water content and the analysis of the nano-mechanical application. J Bio Tribo Corros 2020; 6: 1–9.

41.

Cravanzola

Cesano

Gaziano

, et al. Sulfur-doped TiO₂: structure and surface properties. Catalysts 2017; 7: 214.

42.

Cravanzola

Cesano

Gaziano

, et al. Carbon domains on MoS₂/TiO₂ system via catalytic acetylene oligomerization: synthesis, structure, and surface properties. Front Chem 2017; 5: 1–12.

43.

Eder

Windle

AH.

Carbon-inorganic hybrid materials: the carbon-nanotube/TiO₂ interface. Adv Mater 2008; 20: 1787–1793.

44.

Oliver WC and Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J. Mater. Res. 1992; 7: 1564–1583.

45.

Tsongas

Tzetzis

Karantzalis

, et al. Microstructural, surface topology and nanomechanical characterization of electrodeposited Ni-P/SiC nanocomposite coatings. Appl Sci 2019; 9: 2901.

46.

Briscoe

Fiori

Pelillo

Nano-indentation of polymeric surfaces. J Phys D: Appl Phys 1998; 31: 2395–2405.

47.

VanLandingham

MR.

Review of instrumented indentation. J Res Natl Inst Stand Technol 2003; 108: 249–265.

48.

Lichinchi

Lenardi

Haupt

, et al. Simulation of berkovich nanoindentation experiments on thin films using finite element method. Thin Solid Films 1998; 312: 240–286.

49.

Bruce. A method for interpreting the data from depth-sensing indentation instrurnents. J Chem Inform Model 2011; 53: 1689–1699.

50.

Huang

Zhang

, et al. Nanotube-formed Ti substrates coated with silicate/silver co-doped hydroxyapatite as prospective materials for bone implants. J Alloys Compounds 2017; 697: 182–199.

51.

Rautray

Narayanan

Kwon

, et al. Accelerator based synthesis of hydroxyapatite by MeV ion implantation. Thin Solid Films 2010; 518: 3160–3163.

52.

Munirathinam

Pydimukkala

Ramaswamy

, et al. Influence of crystallite size and surface morphology on electrochemical properties of annealed TiO₂ nanotubes. Appl Surf Sci 2015; 355: 1245–1253.

53.

Praveenkumar

Venkatasubbu

Thangadurai

, et al. Nanocrystalline bismuth oxyiodides thick films for X-ray detector. Mater Sci Semiconductor Process 2019; 104: 104686.