**5. Titania nanowires (TNWs)**

Photocatalytic activity of titania nanofibers, obtained by electrospinning method and annealed in 550°C, was studied with the use of dye degradation tests (basic blue 26, basic green 4, basic violet 4) [182]. The concentration of dye solution was measured in relation to UV irradiation time,

**Figure 5.** Results of MTT assay carried out on TNF coatings, obtained from different oxidation mixtures, with the use of

obtained by sol-gel method and for composite coatings consisting of titania nanofibers and TiO<sup>2</sup> nanoparticles. Doh et al. showed that after 3 h of UV illumination, 25.3% of basic blue 26 was degraded by titania nanofibers. This degradation efficiency was almost the same as for TiO<sup>2</sup> nanoparticles obtained by sol-gel method, for which the value was 23.7%. In comparison, the

as basic blue 26 was degraded by 78.7%. Rate constant calculated for titania nanofibers, titania nanoparticles, and composite material, on the base of simplified equation of the Langmuir-Hinshelwood kinetic model was equal adequately 15.7 × 10−4 min−1, 14.3 × 10−4 min−1, and 85.4 × 10−4 min−1. The values of kinetic rates obtained for composite material but during the degradation processes of dyes: basic green 4 and basic green violet were 81.2 × 10−4 min−1 and 67.4 × 10−4 min−1,

nanoparticles had high photocatalytic activity due to their high active surface area and due to complex pore structure. They stated that such materials could be suitable for the application to

oxidation with the use of hydrogen peroxide with or without simple inorganic compounds:

pollutant patters: acetone (A) and methylene blue (MB) [118]. Based on the same simplified equation of the Langmuir-Hinshelwood kinetic model, it was possible to calculate the kinetic

The values obtained during these studies are very close to those, obtained by Doh et al., however, titania nanofibers obtained in the process of chemical oxidation were not post annealed and they were not enriched by the titania nanoparticles [118, 182]. What should be pointed out clearly is the fact that the observed rate constants do not inform about the real nature of the appropriate processes. However, they provide basic information about change in the

nanofibrous coatings obtained during Ti/Ti alloy chemical

in 80°C, was analysed also on the base of degradation of two organic

nanofibers and TiO<sup>2</sup>

nanoparticles

nanoparticles) was much higher,

nanofibers and TiO<sup>2</sup>

using UV-Vis spectrophotometer. Additionally, the same tests were done for TiO<sup>2</sup>

murine fibroblasts L929 (adhesion after 24 h, proliferation after 72 h and differentiation after 5 days).

respectively. Authors concluded that composite materials consisted of TiO<sup>2</sup>

rates. The values of calculated kinetic rates are showed in **Table 3**.

photoactivity of composite material (TiO<sup>2</sup>

84 Application of Titanium Dioxide

the degradation of organic dye pollutant.

The photocatalytic activity of TiO<sup>2</sup>

SO4

HCl, NaCl, Na<sup>2</sup>

From the geometrical point of view, nanowires offer exceptional properties, such as flexibility and fatigue resistance, and the possibility to integrating them in large area with controlled pattern. Sometimes, nanofibers are also used to describe nanowires morphology, especially when nanowires (NW) are very long and not single crystalline. However, in order to achieve 1D NW morphology, it is crucial to obtain one rapid growth direction during the evolution of nanocrystals. This requirement is quite often fulfilled for some crystals, due to the strong anisotropic property of their crystal structures. For example, wurtzite crystals naturally have rapid growth along the [0001] direction and because of this fact NW is one of the preferred morphologies during self-assembly growth. For some other crystals, titania, for example, such anisotrophic behavior is less evident, and they demand additional support to create 1-D nanotopography [159]. Among auxiliary activities, the surface functionalization, introducing dislocations, applying catalysts, and increasing the building block concentration should be mentioned. More detailed strategies for crystal morphology control are given in review articles [182–186]. Plenty of TNW synthesis routes have been examined. Bottom up approaches include a large variety of solution- and vapor-based growth strategies [187–189]. Top down procedures, such as direct oxidation and electrochemical etching techniques, have also been explored for nanowires synthesis [190–192]. The widespread utilization of nanostructured TiO<sup>2</sup> , including TNW, is often hindered by the opposite demands for precise control of wellordered surface features and low-cost rapid production. Taking this into consideration, the synthetic routes, which engaged inexpensive techniques to produce nanowire coatings, are presented in this review.

Most of wet chemical methods require many steps, which increase production costs. Moreover, titania nanostructures, including nanowires, obtained in anodization process or by the electrospinning method, are amorphous, and they need to be annealed in order to endow them in high crystalline form. A method, which can be applied to the formation of titania nanowires, is the thermal oxidation of Ti/Ti alloy surface under a limited supply of oxygen, in argon atmosphere [193–195]. The exposure or pure titanium to argon, which contains ppm of oxygen, at a flow rate of 200 cm<sup>3</sup> /min for 8 h at 600°C resulted in the growth of titania nanowires of the length 50–400 nm. Such obtained nanowires posses the structure of rutile. What should be pointed out is the visible impact of the argon flow rate on the morphology of nanostructural TiO<sup>2</sup> . Increasing of the flow rate from 200 cm<sup>3</sup> /min to 1000 cm<sup>3</sup> /min caused the disappearance of nanowires. This effect was even more intensified by the increase of the temperature. Higher flow rate and higher temperature promoted growth of not nanowires but platelets and faceted oxide crystals. It can be concluded that the window of high aspect ratio nanowire growth is very narrow for pure titanium. The situation looks much better in case of titanium alloy—Ti6Al4V. The same procedure of thermal oxidation in the presence of argon both in low and high flow rates gives the same results—titania nanowires. However, the effect of increasing temperature is similar to adequate one for pure titanium. The optimal temperature of the titanium alloy oxidation in order to obtain nanowires is 700°C. At 800°C, a mixture of nanowires and well-faceted crystals is possible to obtain and at 900°C, only platelets and crystals are the results of process. The higher temperature impact on the formation of well-faceted equiaxed crystals indicates that one-dimensional growth at the low temperature is the result of oxidation reaction anisotrophy with the growth preferential on certain crystal faces. At higher temperatures, this anisotrophy decreases and the growth on other surface is promoted, allowing the formation of facetted crystals [194, 195].

My experience with the formation of nanowires, during the titanium direct oxidation, is similar to above mentioned. The use of lower argon flow rate (30 cm<sup>3</sup> /min) led to obtain well morphologically oriented nanowires at 475–550°C (**Figure 6a**). Above this temperature, maintaining the same gas flow, facetted crystals were obtained (**Figure 6b**). The addition of H<sup>2</sup> O2 vapors to the carrier gas caused that the formation of nanowires was much slower and less efficient (**Figure 6c**). Prolongation of the process time improved the quality of the nanowires, but only a little, but failed to get a system similar to those received for pure argon (**Figure 6d**).

**Figure 6.** SEM images of titania nanowires obtained in the process of thermal oxidation with the use of pure argon, at 450°C (a), at 600°C (b), and with the use of vapors Ar/H<sup>2</sup> O2 , at 500°C, but in 2 h (c) and in 6 h (d).

Daothong et al. found that in the presence of adequate organic vapour, titanium can be directly oxidized and TNWs were formed. They presented a size-controlled growth of titania nanowires in the presence of ethanol vapor at high temperature (650–850°C) and low pressure (~10 Torr). The nanowire length was proved to be directly proportional to the oxidation time, and their diameter was strictly connected with applied temperature [196].

Most of wet chemical methods require many steps, which increase production costs. Moreover, titania nanostructures, including nanowires, obtained in anodization process or by the electrospinning method, are amorphous, and they need to be annealed in order to endow them in high crystalline form. A method, which can be applied to the formation of titania nanowires, is the thermal oxidation of Ti/Ti alloy surface under a limited supply of oxygen, in argon atmosphere [193–195]. The exposure or pure titanium to argon, which contains ppm

nanowires of the length 50–400 nm. Such obtained nanowires posses the structure of rutile. What should be pointed out is the visible impact of the argon flow rate on the morphology

the disappearance of nanowires. This effect was even more intensified by the increase of the temperature. Higher flow rate and higher temperature promoted growth of not nanowires but platelets and faceted oxide crystals. It can be concluded that the window of high aspect ratio nanowire growth is very narrow for pure titanium. The situation looks much better in case of titanium alloy—Ti6Al4V. The same procedure of thermal oxidation in the presence of argon both in low and high flow rates gives the same results—titania nanowires. However, the effect of increasing temperature is similar to adequate one for pure titanium. The optimal temperature of the titanium alloy oxidation in order to obtain nanowires is 700°C. At 800°C, a mixture of nanowires and well-faceted crystals is possible to obtain and at 900°C, only platelets and crystals are the results of process. The higher temperature impact on the formation of well-faceted equiaxed crystals indicates that one-dimensional growth at the low temperature is the result of oxidation reaction anisotrophy with the growth preferential on certain crystal faces. At higher temperatures, this anisotrophy decreases and the growth on other surface is

My experience with the formation of nanowires, during the titanium direct oxidation, is simi-

morphologically oriented nanowires at 475–550°C (**Figure 6a**). Above this temperature, maintaining the same gas flow, facetted crystals were obtained (**Figure 6b**). The addition of H<sup>2</sup>

vapors to the carrier gas caused that the formation of nanowires was much slower and less efficient (**Figure 6c**). Prolongation of the process time improved the quality of the nanowires, but only a little, but failed to get a system similar to those received for pure argon (**Figure 6d**).

**Figure 6.** SEM images of titania nanowires obtained in the process of thermal oxidation with the use of pure argon, at

, at 500°C, but in 2 h (c) and in 6 h (d).

O2

. Increasing of the flow rate from 200 cm<sup>3</sup>

promoted, allowing the formation of facetted crystals [194, 195].

450°C (a), at 600°C (b), and with the use of vapors Ar/H<sup>2</sup>

lar to above mentioned. The use of lower argon flow rate (30 cm<sup>3</sup>

/min for 8 h at 600°C resulted in the growth of titania

/min to 1000 cm<sup>3</sup>

/min caused

/min) led to obtain well

O2

of oxygen, at a flow rate of 200 cm<sup>3</sup>

of nanostructural TiO<sup>2</sup>

86 Application of Titanium Dioxide

The aspect ratio and the shape of titania nanowires were described as resembling the needlelike shape of crystalline hydroxyapatite and collagen fibers found in the bone. Such environment was reported as the place, which ensures the proper cell organization, their vitality, and functionality. Studies on osteoblasts adhesion and growth assessment carried out by Tan et al. showed that osteoblast was able to adhere and spread on the nanowire coating [197]. Osteoblasts adhered to the surface, exhibited an oval shape on the first day and polygonal shape with some protruding lamellipodia on days 3–7. On day 14, osteoblasts formed intercellular network and indicated the cell-to-cell communication, by stretching out their filopodia toward each other. Osteoblasts growth was determined with the use of AlmarBlue assay, and it was proved that osteoblasts were able to proliferate on nanowires coatings. The cell number of osteoblasts on TNW coatings was increased significantly over the level of pure titanium sample. Degree of osteoblasts differentiation was investigated with the use of examination of intracellular ALP (key marker) activity. The test showed that until 7 days, the ALP level was significantly higher than for adequate pure titania coatings. Extracellular matrix mineralization of osteoblasts was evaluated on the base of Alizarin Reds. Also, this assay showed that greater number of discrete mineralized nodules in greater abundance was seen on the nanowire surface.

Our studies on the adhesion and proliferation of fibroblasts show that these processes proceed more efficiently on nanowires, but there are no significant difference between nanowire coatings and pure Ti/Ti alloy (**Figure 7**).

**Figure 7.** Adhesion (after 24 h) and proliferation (after 72 h and 5 days) of fibroblasts on the surface of titania nanowires obtained at different conditions of temperature and Ar rate flow: TNW1–TNW4 (475°C), TNW5–TNW8 (500°C), TNW1, TNW3, TNW5, TNW7 (30 cm<sup>3</sup> /min), TNW2, TNW4, TNW6, TNW8 (100 cm<sup>3</sup> /min).

Assuming the biological activity of all reviewed titania nanostructures, it can be stated that at the moment, titania nanotubes are the strongest used in biomedical applications, as the procedure of their fabrication is the most predictable and easy. However, the necessity of their further annealing may lead to the increase of the interest to systems, e.g., TNW, during the production of which, a specific crystalline structure is formed.

The photoactivity of titania nanowires should also be taken into the consideration as such activity gives the possibility to use these coatings in the process of UV-activated sterilization. Most of earlier reports showed the usefulness of titania nanowires rather in the processes of photoelectrochemical water splitting than in the degradation of organic pollutants. In case of sooner, the photoactivity of Au-decorated TiO<sup>2</sup> nanowires electrodes for photoelectrochemical water oxidation, was enhanced in the entire UV-VIS region by the manipulation of the shape of decorated Au nanostructures: nanoparticles and nanorods [198]. The titania nanowires photoactivity in the degradation of 4-chlorophenol was studied by Stengl et al. [199] They calculated the degradation rate constant assuming the reaction kinetic of the first order. The obtained values were in the range 0.0045–0.0083 min−1. So, they were comparable with those obtained for titania nanofibers. The authors noticed that the photocatalytic activity of the annealed samples gradually increased from the temperature of 350°C (0.0045 min−1) to 750°C (0.0129 min−1), and for the samples annealed to temperatures 900°C and 1000°C, respectively, the photoactivity decreased (0.0104 and 0.0083 min−1). They assumed that the initial photocatalytic activity growth for samples annealed in the range 350–750°C corresponds with enlargement of the anatase crystalline phase in consequence of annealing. The decrease of photocatalytic activity of the sample heated above 750°C, they associated this with the transformation of anatase to rutile phase and also with the lowering of surface area. In case of samples obtained in our lab during the thermal oxidation of titanium, in which the rutile phase was present, the calculated rate constants for the methylene blue photodegradation process were in the range 0.0001–0.0002 min−1, so much lower than in case of Ref. [199]. The rutile structure and the low surface area (see **Figure 6c** and **d**) were burdened of such low activity reason.

## **Author details**

Aleksandra Radtke

Address all correspondence to: aradtke@umk.pl

Faculty of Chemistry, Nicolaus Copernicus University in Toruń, Torun, Poland

#### **References**


[3] Ratner BD, Hoffman AS, Schoen FJ, Lemons JE, editors. Biomaterials Science—An Introduction to Materials in Medicine. New York; Academic Press, Elsevier; 2013. pp. 37-132

Assuming the biological activity of all reviewed titania nanostructures, it can be stated that at the moment, titania nanotubes are the strongest used in biomedical applications, as the procedure of their fabrication is the most predictable and easy. However, the necessity of their further annealing may lead to the increase of the interest to systems, e.g., TNW, during the

The photoactivity of titania nanowires should also be taken into the consideration as such activity gives the possibility to use these coatings in the process of UV-activated sterilization. Most of earlier reports showed the usefulness of titania nanowires rather in the processes of photoelectrochemical water splitting than in the degradation of organic pollutants. In case of sooner,

oxidation, was enhanced in the entire UV-VIS region by the manipulation of the shape of decorated Au nanostructures: nanoparticles and nanorods [198]. The titania nanowires photoactivity in the degradation of 4-chlorophenol was studied by Stengl et al. [199] They calculated the degradation rate constant assuming the reaction kinetic of the first order. The obtained values were in the range 0.0045–0.0083 min−1. So, they were comparable with those obtained for titania nanofibers. The authors noticed that the photocatalytic activity of the annealed samples gradually increased from the temperature of 350°C (0.0045 min−1) to 750°C (0.0129 min−1), and for the samples annealed to temperatures 900°C and 1000°C, respectively, the photoactivity decreased (0.0104 and 0.0083 min−1). They assumed that the initial photocatalytic activity growth for samples annealed in the range 350–750°C corresponds with enlargement of the anatase crystalline phase in consequence of annealing. The decrease of photocatalytic activity of the sample heated above 750°C, they associated this with the transformation of anatase to rutile phase and also with the lowering of surface area. In case of samples obtained in our lab during the thermal oxidation of titanium, in which the rutile phase was present, the calculated rate constants for the methylene blue photodegradation process were in the range 0.0001–0.0002 min−1, so much lower than in case of Ref. [199]. The rutile structure and the low surface area (see **Figure 6c** and **d**) were

nanowires electrodes for photoelectrochemical water

production of which, a specific crystalline structure is formed.

the photoactivity of Au-decorated TiO<sup>2</sup>

88 Application of Titanium Dioxide

burdened of such low activity reason.

Address all correspondence to: aradtke@umk.pl

Journal of Sciences. 2010;**8**:119-125

Faculty of Chemistry, Nicolaus Copernicus University in Toruń, Torun, Poland

[1] Williams D. On the nature of biomaterials. Biomaterials. 2009;**30**:5897-5909

[2] Mihov D, Katerska B. Some biocompatible materials used in medical practice. Trakia

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Aleksandra Radtke

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