**3.2. Solvothermal method**

The yield of nanotubes increased with the hydrothermal temperature when the temperature was in the range of 100–150°C. The experimental results showed that hydrothermal treat-

was transferred to nanoribbons with very high yields (almost 100%) when hydrothermal temperature was in the range of 180–250°C with the NaOH concentration of 5–15 mol/l as shown in **Figure 4c** [18]. Moreover, the hydrothermal treatment duration has a strong effect on the morphological structure of the synthesized product; it also plays a major rule in the conversion of the nanotube structure into nanoribbons as reported by Elsanousi et al. [20]. **Figure 5** shows the effect of hydrothermal treatment duration (5–72 h) at the fixed temperature of 180°C on the morphology of the titanate nanotubes and nanoribbons. The hollow nanotubes were found out with an outer diameter of about 10 nm at treatment duration of 5 and 20 h. While further treatment duration up to 72 h was caused bundles of nanoribbons with widths ranging from 50 to 500 nm and lengths up to several tens of micrometers. Additionally, the experimental results illustrate that there are critical conditions depending on both the treatment duration and varying temperatures (120–195°C), possibly due to a critical pressure, which is needed to be reached so that the transformation process takes place. The concentration and type of alkaline solution also play an important role in the hydrothermal process. The increasing concentration of NaOH can accelerate the hydrothermal reaction owing to the enhancement of Ti (IV) dissolution and exfoliation rates of the precursors. High

NaOH concentration between 10 and 15 mol/l. The yield of nanotubes is very low when the NaOH concentration is lower than 5 M or as high as 20 M [18]. Bavykin et al. [21] investigated the influence of the binary NaOH/KOH aqueous mixture used in the hydrothermal

ing nanosheets, nanotubes, nanofibers, and nanoparticles, have been mapped over a wide

nanostructures. As Yuan et al. reported that crystalline or amorphous TiO<sup>2</sup>

 particles can be found in the product. In another perspective, increasing temperature could facilitate unidirectional crystal growth, leading to different morphologies of 1D-TiO<sup>2</sup>

particles and a large amount of residual

(a) nanotube and (b) nanofiber with crystalline

, respectively, and (c) effect of increasing hydrothermal temperature on transfer

/g can be synthesized with the

nanostructures. All observed nanostructures, includ-

powder mainly

ment at below 100°C is not effective to transfer TiO<sup>2</sup>

**Figure 4.** Synthesis of various morphologies of one-dimensional TiO<sup>2</sup>

anatase or rutile, amorphous TiO<sup>2</sup>

372 Titanium Dioxide - Material for a Sustainable Environment

morphology to nanoribbons [18].

yield of nanotubes with the maximum surface area of 350 m<sup>2</sup>

process on the morphology of 1D-TiO<sup>2</sup>

TiO2

Solvothermal method facilitates the synthesis of nanometer-sized crystalline TiO<sup>2</sup> powder at relatively low temperatures. Solvothermal reactions are similar to hydrothermal method while a non-aqueous solvent reacts under conditions of high pressure and mild temperature. This method shows promise for developing nanotechnologies. Organic solvents during solvothermal synthesis control the properties of products, corresponding to the structure. The various physical and chemical properties of selected solvent such as reactivity, the polarity, coordinating ability of the solvent, etc. affect the morphology and the crystallization of the final products. Furthermore, the influence of other reaction parameters such as temperature, stirring conditions, and co-solvent (water-ethanol, water-ethylene glycol) on the morphologies of the synthesized nanostructures (nanotubes, nanorods, nanowires, and nanoribbons), as well as their growth mechanism, have been explored [23]. Chen et al. [24] reported the

**Figure 6.** SEM images of various TiO<sup>2</sup> morphologies synthesized by solvothermal method: (a) nanosheets, (b) nanobelts, and (c) nanorods.

preparation of a single-layer polycrystalline anatase TiO<sup>2</sup> (SLP TiO<sup>2</sup> ) nanosheets **Figure 6a** with a porous structure through a simple solvothermal method by employing, rod-like titanyl sulfate, as the starting material, in the presence of glycerol, followed by a calcination process.

The structure and morphology were found to be dependent on the experimental conditions such as solvothermal reaction time, morphology of titanyl sulfate, and solvent type. Que et al. [25] successfully synthesized the nitrogen-fluorine co-doped TiO<sup>2</sup> nanobelts (**Figure 6b**) with anatase phase structure by the solvothermal method, which employs amorphous titania microspheres as the precursor. Results demonstrate a significantly enhanced photocatalytic degradation of methyl orange compared to commercial TiO<sup>2</sup> . Zhao and his co-workers [26] reported the synthesis of TiO<sup>2</sup> nanorod arrays (TNRs) directly on FTO glass (**Figure 6c**) through the solvothermal method, and thermal treatments. The results show that the crystal structure does not change due to thermal treatment. However, the surface morphology appears to change significantly from a thin amorphous layer to tiny crystallite spheres. All of these changes lead to a 39% improvement in the photoelectric conversion efficiency for the nanorod-based photoanode in dye-sensitized solar cells (DSSCs). These findings might be useful in photoelectrical applications of the solvothermal method.

The apparent 1-D morphology of TiO<sup>2</sup>

NRs grown on sapphire (100) substrate.

respect to temperature from 13 to 75 nm.

structures (usually single crystal TiO2

(**Figure 8a**), using titanium-tetraisopropoxide (TTIP, Ti(OC<sup>3</sup>

densely-packed rutile TiO<sup>2</sup>

**Figure 7.** HR-SEM images of TiO<sup>2</sup>

tively. The rutile TiO2

packed TiO<sup>2</sup>

high for mass production.


) and it can be classified in chemical vapor deposi-

nanorod via metal-organic chemical vapor deposition (MOCVD)

nanorods (**Figure 8b**) were grown with a very high density and exhib-

calcined at different temperatures (a) 400°C, (b) 500°C, and (c) 600°C [28].

One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices

http://dx.doi.org/10.5772/intechopen.72976

375

H7 ) 4 nano-

) as a source reagent at a

600°C, showing a similar diameter (about 76 nm); however, crystallite size increases with

**Figure 8.** (a) Schematic diagram of the cold wall MOCVD aperture and (b) SEM images of vertically aligned and densely

tion (CVD) and physical vapor deposition (PVD) [27]. Chen et al. [29] grown well-aligned

deposition temperature of 550°C and under an oxygen pressure of 1.5 and 5 mbar, respec-

ited uniform height. However, this method requires expensive equipment and the cost is too

In addition, nanofibers in different forms, such as core-shell hollow and porous nanofibers are produced with electrospinning method as one of the most conventional methods [30]. These structures of nanofibers can be utilized for new applications such as ultra-filtration, fuel cells, membranes, tissue engineering, catalysis and hydrogen storage. Electrospinning provides

Vapor deposition method has been developed to high degree of crystallinity 1D-TiO<sup>2</sup>
