EFFECT OF ETHYLENE GLYCOL ON THE FORMATION OF TITANIUM (II) OXIDE NANOTUBE

ABSTRACT

The effects of varying hydrofluoric acid concentration (0.01, 0.02, 0.03, 0.09, 0.183 mol dm^-3), anodizing temperature, and applied current on the formation of titanium dioxide nanotubes in an electrolyte consisting of ethylene glycol and water were studied. SEM images of the nanotubes obtained at different voltages (50V, 70V, and 90V) and temperatures (4, 20, 40, and 70 °C) from the top and side were obtained and analyzed. It was found that the nanotubes formed in electrolytes with low fluoride ion concentrations had flat surfaces and high order, while higher temperatures led to faster formation of nanotubes. It was established that there is a chemical equilibrium between the oxidation of titanium ions and their dissolution under the influence of fluoride ions.

АННОТАЦИЯ

Изучено влияние изменения концентрации плавиковой кислоты (0,01, 0,02, 0,03, 0,09, 0,183 моль дм^-3), температуры анодирования и приложенного тока на формирование наружного и внутреннего диаметров и длины нанотрубок диоксида титана в электролите, состоящем из этиленгликоля и воды. Были сняты и проанализированы верхние и боковые СЭМ-изображения нанотрубок, полученные при различных напряжениях (50 В, 70 В и 90 В) и температурах (4, 20, 40, 70 °C). Нанотрубки, образовавшиеся в электролитах с низкой концентрацией ионов фтора, имеют плоскую поверхность и высокий порядок, а высокая температура приводит к более быстрому образованию нанотрубок. Установлено, что существует химический баланс между окислением ионов титана и их растворением под действием фторид-ионов.

Keywords: Anodizing, fluoride ions, constant current, voltage.

Ключевые слова: Анодирование, ионы фтора, постоянный ток, напряжение.

Introduction. Grimes and co-workers reported the formation of TiO₂ nanostructures by anodizing titanium substrates in aqueous fluoride electrolytes [1]. TiO₂ nanotubes not only have a larger surface area but also the presence of pores on the surface of the oxide layer results in the mobility of electrons, which makes TiO₂ nanotubes have great potential for use in optical and electronic devices such as solar cells [2-5], photocatalysis [6], and sensors [7-9]. All published articles have shown that the outer diameter and length of TiO₂ nanotubes have a significant impact on their performance. Over the years, three stages of electrolytes containing fluoride ions have been developed to increase the NT (nanotube) length (L): (i) aqueous HF electrolytes (L < ~500 nm) [1], (ii) aqueous fluoride electrolytes (L < ~6 μm) [10], and (iii) polar organic fluoride-containing electrolytes (L > 6 μm) [11–17]. Paulose and co-workers successfully formed 134 μm long nanotubes using an ethylene glycol-based electrolyte [16], and anodized titanium dioxide nanotubes up to 1000 μm long and 1 mm thick were obtained when the EG electrolyte contained ammonium fluoride (0.6%) and water (3.5%) [17].

In the formation of nanotubes, there is a chemical equilibrium between the dissolution of the TiO₂ barrier layer by fluoride ions in the solution (r_e) and the formation of an oxide layer (r_c).

The formation of the oxide layer is directly dependent on the size of rc and the length of the resulting nanotubes rl. However, the formation of TiO₂ is influenced more by the ratio re/rc than by their size. Obviously, if the ratio re/rc is less than or very close to unity, chemical dissolution is the main process, which leads to the dissolution of the Ti substrate in electrolytes containing fluoride ions, in which case nanotubes are not formed and only random pores are formed in the Ti substrate. The water content in polar organic solvents reduces the dissolution of the barrier layer, which increases the length of the nanotubes [14,15]. However, the fluoride concentration, anodizing temperature and applied potential difference can have different effects on the ratios r_c/r_e and r_e/r_c.

Experimental

A 0.1 mm thick titanium foil (99.8% purity, Sigma Aldrich) (10 mm × 40 mm). To remove various roughness and impurities from the surface of the foil, it was cleaned in a solution of HF:HNO₃:H₂O=1:1:2) for 15 minutes (GT Sonic-D 6, AC-220-240V). Then it was washed in distilled water for another 15 minutes and dried in the open air for one hour. After that, it was anodized in an electrolyte containing 2% water, 183 mol dm^-3 NH₄F (purity 96%, Sigma Aldrich) and 98% ethylene glycol, at 20 °C and 50 V for 2 hours (Model-SS-350M, S/N 21121062). Titanium foil was used as the anode and a graphite electrode was used as the counter electrode. The titanium foil immersed in the electrolyte has an area of 4 cm² (2 cm×1 cm×2 sides).

The results obtained and their discussion. SEM images of TiO₂ nanotubes (NTs) formed by anodizing in 0.183 mol dm-3 NH4F electrolyte dissolved in ethylene glycol and water at 20 °C and 50 V for 2 h are shown (Fig. 1 a,b,c). These SEM images show that the nanotubes are open at the top and closed at the bottom, and have a length of several nanometers. It can be seen that the outer diameters of the (NTs) are formed uniformly, but the formation of the inner oxide layer decreases towards the titanium substrate.

1. Effect of NH₄F concentration

Varying the NH₄F concentration on nanotube formation in TiO₂ significantly changes the r_e/r_c ratio. The current-time (I-t) curves of Ti substrates anodized at different NH₄F concentrations at 40 °C (50 V and 2 h) are given. The current decreased when the NH₄F concentration was 0.02 mol dm-3, and then the current remained at a constant value after the nanotubes were formed. It can be seen that the initial decrease in the current is due to the formation of a compact oxide layer and the formation of hexafluoride salts of titanium (Ti⁺⁴) ions with fluoride (F^-) ions in the solution (Fig. 2).

It can be seen from the (I-t) curves that for NH₄F concentration of 0.04 mol dm^-3, the current was initially constant for 10 seconds and gradually decreased. In addition, the ratio of chemical dissolution of the oxide layer/formation of the oxide layer was higher than 0.9 with increasing NH₄F concentration and for 0.183 mol dm^-3. SEM images of the formation of TiO₂ nanotubes are shown (Fig. 3). It can be seen that at low (0.183 mol dm^-3) NH₄F concentration, the TiO₂ nanotubes formed a continuous layer with some pores on the top (Fig. 3a) and that for high NH₄F concentration, no continuous layer was observed (Fig. 3b-c).

Figure 1. SEM images of TiO₂ nanotubes formed in ethylene glycol 2% H₂O and 0.183 mol dm⁻³ NH₄F electrolyte at 20 °C, 50 V and 2 h.

(a) top, (b) side and (c) cross-sectional views

The increase in the concentration of (F^-) ions in the nanotubes formed in 0.183 mol dm^-3 NH₄F leads to an increase in the number of charge carriers in the solution, which leads to an increase in the analytical signal. After a certain amount, the increase in the number of charge carriers in the solution and, therefore, the density of the charge transport paths causes the diffusion to slow down (Fig. 3c). It can be seen from the figure that with increasing NH₄F concentration (0.00, 0.05, 0.10, 0.15, 0.20 mol dm^-3) the outer diameter and length of TiO₂ nanotubes decrease (Fig. 2b). This is due to the increase in the amount of (F^-) ions in the solution.

Figure 2. Current-time curves of TiO₂ nanotubes anodized at 40 °C, 50 V and 2 h at different NH₄F concentrations (0.01, 0.02, 0.03, 0.09, 0.183 mol dm^-3)

(a). Effect of changing NH₄F concentration on the outer diameter and length of the nanotubes (b).

Analysis of the (I-t) curves in Figure 2 with the SEM images in Figure 3 shows that the change in the (I-t) curve indicates the formation at a high constant concentration. Two NH₄F concentration values were determined for the formation of TiO₂ nanotubes: low (F^- 0.01 mol dm^-3) and high (F^- 0.183 mol dm^-3). Increasing NH₄F concentration leads to a decrease in the re/rc ratio and the total oxide layer formation, i.e. NH₄F concentration affects rc more than re, and higher concentration leads to the formation of short-diameter nanotubes.

Figure 3. SEM images (a, b, c) of TiO₂ nanotubes anodized at 40 °C, 50 V, and different NH₄F concentrations (0.01, 0.04, 0.183 mol dm^-3) for 2 h.

Figure 4. SEM images (a, b, c) of TiO₂ nanotubes formed at low (F)^- concentration at 50 V, 1 h at different temperatures (4 °C, 20 °C, 70 °C).

 Effect of temperature on the outer diameter and length of nanotubes (d).

The (I-t) current-time diagram shows that low fluoride (F^-) concentrations lead to the formation of highly ordered TiO₂ nanotubes.

When the re/rc ratio is less than 0.35, no nanotubes are formed under the barrier layer.

When the r_e/r_c ratio is greater than 0.9, only random pores are observed on the anodized TiO₂ substrate. If the ratio re/rc is in the range of 0.35-.90, highly ordered nanotubes are formed.

If we pay attention to the formation of nanotubes carried out at 40 °C, 2 hours at 70 V, and at a low concentration of (F^-) ions, it can be seen that there were pores on the surface, but ordered nanotubes were formed. This is due to the increased movement of ions in the electrolyte at 70 V and the formation of pores for a short time, as can be seen from the SEM images (Fig. 5a). In the nanotubes carried out at 40 °C, 2 hours at 90 V, and at a low concentration of (F^-) ions, hexagonal pores were not formed due to the increase in charges and a decrease in their diffusion rate as a result of the increase in current strength (Fig. 5b). The effect of increasing voltage on the outer diameter and length of the nanotubes is shown in (Figure 5c).

It can be seen that the increase in the outer diameter and length of the nanotubes formed with increasing voltage is directly proportional to the increase in the outer diameter and length of the nanotubes. This is because the number of charged ions in the electrolyte increases.

Figure 5. SEM images of TiO₂ nanotubes formed at 70 V and 90 V at 40 °C, 2 h, and low F^- concentration (a, b).

Effect of voltage changes on the outer diameter and length of the nanotubes (c).

2. Modeling the formation of TiO2 nanotubes

The formation of the oxide layer (r_c) is linearly dependent on the concentration of NH₄F and temperature. According to the Arrhenius equation, the chemical dissolution of the barrier layer (r_e) and the formation of the oxide layer (r_c) can be expressed as follows:

Figure 6. (a) r_e/r_c ratio at different concentrations of NH₄F, (b) effect of temperature on the growth rate in the presence of low fluoride 0.01 M NH₄F; (c) effect of NH₄F concentration on the formation rate of TiO₂ nanotubes at 40 °C

                                             (1)

Where Ea is the activation energy, R is the universal gas constant, Tis the temperature, [F] is the concentration of NH₄F, and A is the exponential factor.

The chemical dissolution of the barrier layer (r_e) is the result of the diffusion of (F^-) ions in NH₄F and can be expressed as:

                                                  (2)

According to the Einstein-Stokes equation, it can be seen from the formula that the change in NH₄F concentration affects (r_e) less than (r_c). The change in temperature T is linearly related to the diffusion coefficient (D_T) of ions in the electrolyte.

                                                    (3)

Conclusion

In this work, the effects of fluoride concentration, anodization temperature, and applied current on the formation and size of TiO₂ nanotubes in an electrolyte consisting of ethylene glycol and water were comprehensively studied. Fluoride concentration and anodization temperature were found to be two important factors for controlling the formation of titanium nanotubes.

Electrolytes containing low fluoride concentrations cause the nanotubes to be uniform and highly ordered, while at high temperatures, the solution density increases and after a certain amount, the excess charge carriers increase, and therefore the density of the charge transport paths slows down the diffusion.

The time-dependent curve of the anodization time (I-t) allows us to predict the formation morphology of TiO₂ nanotubes. A model of the relationship between the oxidation of titanium (Ti⁺⁴) and the dissolution of the oxide under the influence of fluoride (F^-) ions was developed. Morphologies based on this model allow the determination of optimal conditions for TiO₂ growth.

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