**3.1. Hydrothermal**

Hydrothermal is one of the most common methods to synthesis 1D-TiO<sup>2</sup> nanostructure due to simple setup, facile operation, and desirable results. Ever since Kasuga et al*.* [12] in 1988 showed the first evidence that oxide nanotubes can be obtained easily via chemical treatment, without the need for molds for replication or templates, much research has been carried out on the formation of 1D-TiO<sup>2</sup> nanostructures. In a typical hydrothermal synthesis, TiO2 or its precursors are dissolved in a concentrated aqueous acidic or alkaline solution and is implemented stainless steel at elevated temperature and pressures [13]. In the former method, the reactants are usually titanium salts with hydrochloric acid and the reaction normally leads to the formation of TiO<sup>2</sup> nanorods. In the latter method, the reactants are TiO2 nanoparticles and sodium hydroxide solution, which dissolution–recrystallization is always involved in this process and the products include nanotubes, nanowires, and nanobelts. **Figure 3** shows the various morphology of synthesized 1D-TiO<sup>2</sup> nanostructure with hydrothermal method [13–16].

**Figure 3.** Various morphologies of hydrothermally synthesized 1D*-*TiO2 nanostructures: (a) nanotube, (b) nanorods, (c) nanobelts, (d) nanowires, and (e) nanosheets*.*

**3. Synthesis of 1D-TiO2**

ried out on the formation of 1D-TiO<sup>2</sup>

normally leads to the formation of TiO<sup>2</sup>

hydrothermal method [13–16].

**3.1. Hydrothermal**

TiO2

TiO2

 **nanostructures**

Hydrothermal is one of the most common methods to synthesis 1D-TiO<sup>2</sup>

belts. **Figure 3** shows the various morphology of synthesized 1D-TiO<sup>2</sup>

were applied to synthesis various morphologies of 1D-TiO<sup>2</sup>

**Figure 2.** The principle of photocatalytic degradation over TiO<sup>2</sup>

370 Titanium Dioxide - Material for a Sustainable Environment

Several methods such as hydrothermal, vapor deposition, sol-gel, and electrospinning, etc.

photocatalyst [5].

nanorods, nanowires, nanobelts, nanosheets and nanofiber. In this section, we provide the comprehensive information related to hydrothermal method, which is widely used for manufacturing of small particles in the ceramic industry using aqueous or non-aqueous solution.

to simple setup, facile operation, and desirable results. Ever since Kasuga et al*.* [12] in 1988 showed the first evidence that oxide nanotubes can be obtained easily via chemical treatment, without the need for molds for replication or templates, much research has been car-

 or its precursors are dissolved in a concentrated aqueous acidic or alkaline solution and is implemented stainless steel at elevated temperature and pressures [13]. In the former method, the reactants are usually titanium salts with hydrochloric acid and the reaction

 nanoparticles and sodium hydroxide solution, which dissolution–recrystallization is always involved in this process and the products include nanotubes, nanowires, and nano-

nanostructures like nanotubes,

nanostructures. In a typical hydrothermal synthesis,

nanorods. In the latter method, the reactants are

nanostructure due

nanostructure with

Although the synthesis process seems simple, the preparation parameters including the choice of TiO<sup>2</sup> precursors, the hydrothermal condition (temperature, the concentration of reactants and hydrothermal duration), and post washing procedures play important role in the crystal structures and physicochemical properties of 1D-TiO<sup>2</sup> nanostructures [17]. The choice of initial raw materials such as anatase, rutile, brookite, and amorphous TiO<sup>2</sup> may affect the morphology of the resultant 1D-TiO<sup>2</sup> nanostructures but no systematic data is available. Yuan and Su [18] reported the effect of various TiO<sup>2</sup> precursors on the morphology of produced 1D-TiO2 nanostructures. Crystalline anatase or rutile or commercial P-25 as the raw materials formed titanium oxide nanotubes with diameter 10 nm in the range of reaction temperature of 100–160°C as illustrated in **Figure 4a** [18]. In addition, the surface area of product also affected by raw materials as surface areas of the produced nanotubes from commercial P-25 powder was higher than lab-made anatase TiO<sup>2</sup> . It noteworthy to mention that no nanotubes were identified when amorphous TiO<sup>2</sup> powders were the precursor with similar hydrothermal treatment in the presence of NaOH. **Figure 4b** shows that the product morphology was non-tubular needle-shaped fibers morphology in the presence of NaOH with concentration of 5–15 mol/l at the hydrothermal temperature range of 100–160°C. In addition, Nian et al. [19] synthesized anatase TiO<sup>2</sup> nanorods with a specific crystal-elongation direction through hydrothermal treatment of titanate nanotube suspensions under an acidic environment in the absence of surfactants or templates. They suggested that the transformation of the tube to rode is a result of local shrinkage of the tube walls to form anatase crystallites and the subsequent oriented attachment of the crystallites. Furthermore, the hydrothermal temperature strongly controls the morphologies of products. In addition, the increasing of hydrothermal temperature improves yield, length, and degree of crystallinity of nanotubes.

**Figure 4.** Synthesis of various morphologies of one-dimensional TiO<sup>2</sup> (a) nanotube and (b) nanofiber with crystalline anatase or rutile, amorphous TiO<sup>2</sup> , respectively, and (c) effect of increasing hydrothermal temperature on transfer morphology to nanoribbons [18].

range of compositions (from pure NaOH to pure KOH) and temperatures (from 50 to 110°C). The hydrothermal process is a cost-effective method with good dispersibility and high purity

**Figure 5.** SEM images of the nanotubes and nanoribbons synthesized hydrothermally at 180°C for different durations:

One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices

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

373

ignore its limitations, which diminish its wide applications. For instance, slow reaction kinetics result in long reaction, limited length of the nanotubes, and non-uniformed nanotube for

The coupling hydrothermal treatment with microwave heating, ultrasonication and a rotating autoclave on the reaction mixture can reduce the shortcomings of hydrothermal technique

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

Solvothermal method facilitates the synthesis of nanometer-sized crystalline TiO<sup>2</sup>

nanostructures. However, we cannot

powder

illustrating the great potential for formation of 1D-TiO<sup>2</sup>

large-scale application.

(a) 5, (b) 15, (c) 22, (d) 48, and (e and f) 72 h [20].

**3.2. Solvothermal method**

[17, 22].

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 treatment at below 100°C is not effective to transfer TiO<sup>2</sup> particles and a large amount of residual TiO2 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> nanostructures. As Yuan et al. reported that crystalline or amorphous TiO<sup>2</sup> powder mainly 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 yield of nanotubes with the maximum surface area of 350 m<sup>2</sup> /g can be synthesized with the 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 process on the morphology of 1D-TiO<sup>2</sup> nanostructures. All observed nanostructures, including nanosheets, nanotubes, nanofibers, and nanoparticles, have been mapped over a wide

**Figure 5.** SEM images of the nanotubes and nanoribbons synthesized hydrothermally at 180°C for different durations: (a) 5, (b) 15, (c) 22, (d) 48, and (e and f) 72 h [20].

range of compositions (from pure NaOH to pure KOH) and temperatures (from 50 to 110°C). The hydrothermal process is a cost-effective method with good dispersibility and high purity illustrating the great potential for formation of 1D-TiO<sup>2</sup> nanostructures. However, we cannot ignore its limitations, which diminish its wide applications. For instance, slow reaction kinetics result in long reaction, limited length of the nanotubes, and non-uniformed nanotube for large-scale application.

The coupling hydrothermal treatment with microwave heating, ultrasonication and a rotating autoclave on the reaction mixture can reduce the shortcomings of hydrothermal technique [17, 22].
