**2.2. Hydrothermal synthesis of 1D nanostructured titania**

The needs to get high surface area for DSSC and also photocatalysis encourage the search to utilize 1D-nanostructured titania, such as nanotube, nanorods, and/or nanofibers. Interest to the

**Figure 8.** Absorbance of ruthenium-"black dye" after various hydrothermal-derived mesoporous titania powder adsorption. A commercial titania Degussa P25 was used as comparison.


NS = neutral anatase seed, NSNS = neutral anatase seed + neutral surfactant, NSAS = neutral anatase seed + acidic surfactant, NSNS-1 = 1.25 g P123/40 mL H<sup>2</sup> O, NSAS-1 = 3.2 g P123/(40 mL H<sup>2</sup> O + 4 mL HCl<sup>c</sup> ).

**Table 3.** Ru-"black dye" adsorption onto selected calcined mesostructured titania along with their corresponding textural properties.

**Figure 7.** (A) N2

1

surfactant.

adsorption–desorption isotherms and (B) pore size distribution of mesoporous titania synthesized at

removal of the block copolymer leads to interstitial pore arrangements resulting in the forma-

**Table 2** tabulates the textural porosity of the resultant powders derived from hydrothermal interaction between acidic route anatase seeds with neutral aqueous surfactant. Subjecting acidic route anatase seed for longer hydrothermal treatment with acidic aqueous surfactant results in different effects to the porosity of the resultant calcined powders. Larger pores are created, while they present type IV isotherm with type H2 hysteresis loops (**Figure 7**). As predicted, prolonged hydrothermal treatment induces the formation of wide pore size distribution (**Figure 7** and **Table 2**). The crystalline phase is assigned as the anatase titania. Our study for higher acidity of the anatase seed

**Figure 8** shows the UV–Visible spectra of the Ru-"black dye" solution before and after overnight adsorption using the respective mesoporous titania powder synthesized hydrothermally from anatase seeds, as the adsorbents. Porosity characteristics of the samples are summarized in **Table 3**. It is clear that the powders resulted from anatase seed hydrothermal synthesis demonstrates significant adsorption of Ru-"black dye." Correlating the adsorption

] = AS-1 < AS-2~AS<AS-3.

**Table 2.** Textural parameters of mesoporous titania resulted from acidic route anatase seed and neutral aqueous

**/g,STP) VP (cm3 g−1) Pore diameter (nm)**

resulted in a mixture of crystalline phase of anatase and rutile titania (data not shown).

Hydrothermal 100°C, 40 h 156.70 0.46 9.70 Hydrothermal 100°C, 65 h 152.00 0.44 9.72 Hydrothermal 100°C, 90 h 127.30 0.47 12.52 Hydrothermal 100°C, 190 h 128.80 0.44 9.71

O; [H<sup>+</sup>

tion of uniform and well-controlled mesopores.

452 Titanium Dioxide - Material for a Sustainable Environment

**Sample1 SBET (m2**

TTIP:0.034P123:26EtOH:2.8HCl:170H<sup>2</sup>

various hydrothermal times from acidic route anatase seeds' neutral aqueous surfactant.

structure is mostly due to the surface area. Nanotube with open-ended tube structure may have surface area of ~400 m<sup>2</sup> /g [22]. Nanorods or nanofibers may only have 50 m<sup>2</sup> /g [23]. Therefore, the synthesis then is focused at obtaining titania nanotubes. Titania nanotubes can be achieved via alkaline hydrothermal method which is the most promising method, because of its simplicity and high reproducibility. Excess concentrated alkaline of 10 M NaOH is commonly used as the hydrothermal medium at low temperature of 110–150°C for 24 h to obtain sodium titanate nanotube. Washing with dilute acid and heating may lead the sodium titanate to transform into anatase crystalline phase via the formation of hydrogen titanate and TiO<sup>2</sup> (B) [23].

Using excessive amount of highly concentrated and corrosive base such as NaOH is not environmentally benign. This encourages several researches to alternate and study the effect of alkaline alteration. A vapor pressure by using NH<sup>3</sup> (aq) during the hydrothermal is one of the quests [24]. However, bundles of nanotubes could only be formed by the presence of KOH. It was found that only KOH and NaOH are the contributing alkaline medium for nanotube formation. The spherical titania will not transform into nanotube by the presence of LiOH<sup>2</sup> or NH3 . Successful route without NaOH(aq) in the solvent was proposed by Liu et al. [25] involving titania foil covered with NaOH as the titania source. They proposed scrolling mechanism induced by NH3 vapor pressure to transform titania nanosheets into nanotubes. NaOH was needed to make the titania nanosheets. Steps for the transformation of TiO<sup>2</sup> to nanotubular titanate can be summarized as follows: first, the dissolution of the TiO<sup>2</sup> sources, at the same time, to the growth of layered nanosheets of sodium trititanates. Secondly, nanosheets are curving and then wrapping into nanotubes [23].

The as synthesized products from hydrothermal method were titania with titanate crystalline phases, which were sodium trititanate or hydrogen trititanate, as depicted in **Figure 9** (right). These phases are not favored for the DSSC, due to its low photocatalytic activity [26]. Later, efforts to induce the anatase phase formation by acid washing and calcination are introduced. Care must be taken for using the acid, since too acidic medium exceeding the pH of 3 may ruin the tube structure [27, 28]. By applying calcination, the anatase crystalline phase starts to form at 200°C and complete at 500°C [29]. High temperature may lead to breaking up of nanotube structure [30]. Therefore, the optimum conditions for acid washing and calcination temperature are needed to obtain anatase TiO<sup>2</sup> with nanotube morphology.

In this research, the way to alter or reduce the amount of NaOH in the alkaline hydrothermal method was sought, by using NH<sup>3</sup> as the solvent combined by NaOH, at high concentration level for both solutions. Then, posttreatments, in particular the acid washing and calcination temperature, were introduced. The synthesis protocol for these nanotubes can be found elsewhere [31]. **Figure 9** shows the TEM images of the resulted titania synthesized using NaOH with (Na-Ti-DW) and without acid washing (Na-Ti-H), as well as using a mixed solution of NH3 and NaOH (1:3 molar ratio) with acid washing.

pattern and low-intensity diffraction, which identifies the contributions of amorphous material instead of polycrystalline materials. It has diffraction at crystal plane [011], [300], and [204] that belong to sodium trititanate phase. To ensure the materials crystallinity, XRD pattern was also taken to obtain the contributing crystalline phase of the sample. The XRD results show that the crystalline phase of sample Na-Ti-DW has strong peaks at 2θ ≈ 10.5°, 24°, 28°, 43°, and 48° similar to the XRD pattern by Sikhwivhilu et al. [24], with distinct peak at 2θ ≈ 10° and low-intensity peaks at 2θ ≈ 24°, 28°, and 48°. It is indicated that the as synthesized product has the similar crystalline phase, which is predominantly titanate phase. It is confirmed as sodium trititanate (PDF 31–1329). The sample Na-Ti-DW also bears amorphous, signified from the bland feature at around 2θ ≈ 30° and 60°, and anatase (PDF 21–1272) phase, identified from reflections at ~24° (d101) and 60° (d200). For Na-Ti-H, the XRD pattern shows the diminishing peaks of sodium trititanate phase. It is found that the synthesis of nanotubes with water washing treatment (Na-Ti-DW) exhibits titanate nanotube, while with dilute acid (Na-Ti-H),

**Figure 9.** Bright-field TEM images of sample (left), the corresponding SAED pattern (middle) of (A) Na-Ti-DW, (B)

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455


Na-Ti-H, and (C) 3:1 NaNH<sup>3</sup>

The TEM images (**Figure 9** left) show nanotube structure without the presence of other structures, such as nanorods or nanoparticles. Images of TEM show dispersed TiO<sup>2</sup> nanotubes with diameter around 3–5 nm and outer diameter of 8–12 nm. Nanotube lengths are ranging from 80 to 400 nm. The selected area electron diffraction (SAED) was taken to define the crystallinity of a certain area in the sample (**Figure 9** right). Sample Na-Ti-DW exhibits diffuse ring

Nanostructured Titanium Dioxide for Functional Coatings http://dx.doi.org/10.5772/intechopen.74555 455

structure is mostly due to the surface area. Nanotube with open-ended tube structure may have

the synthesis then is focused at obtaining titania nanotubes. Titania nanotubes can be achieved via alkaline hydrothermal method which is the most promising method, because of its simplicity and high reproducibility. Excess concentrated alkaline of 10 M NaOH is commonly used as the hydrothermal medium at low temperature of 110–150°C for 24 h to obtain sodium titanate nanotube. Washing with dilute acid and heating may lead the sodium titanate to transform into

Using excessive amount of highly concentrated and corrosive base such as NaOH is not environmentally benign. This encourages several researches to alternate and study the effect of

quests [24]. However, bundles of nanotubes could only be formed by the presence of KOH. It was found that only KOH and NaOH are the contributing alkaline medium for nanotube formation. The spherical titania will not transform into nanotube by the presence of LiOH<sup>2</sup>

involving titania foil covered with NaOH as the titania source. They proposed scrolling mech-

same time, to the growth of layered nanosheets of sodium trititanates. Secondly, nanosheets

The as synthesized products from hydrothermal method were titania with titanate crystalline phases, which were sodium trititanate or hydrogen trititanate, as depicted in **Figure 9** (right). These phases are not favored for the DSSC, due to its low photocatalytic activity [26]. Later, efforts to induce the anatase phase formation by acid washing and calcination are introduced. Care must be taken for using the acid, since too acidic medium exceeding the pH of 3 may ruin the tube structure [27, 28]. By applying calcination, the anatase crystalline phase starts to form at 200°C and complete at 500°C [29]. High temperature may lead to breaking up of nanotube structure [30]. Therefore, the optimum conditions for acid washing and calcination

In this research, the way to alter or reduce the amount of NaOH in the alkaline hydrothermal

level for both solutions. Then, posttreatments, in particular the acid washing and calcination temperature, were introduced. The synthesis protocol for these nanotubes can be found elsewhere [31]. **Figure 9** shows the TEM images of the resulted titania synthesized using NaOH with (Na-Ti-DW) and without acid washing (Na-Ti-H), as well as using a mixed solution of

The TEM images (**Figure 9** left) show nanotube structure without the presence of other struc-

diameter around 3–5 nm and outer diameter of 8–12 nm. Nanotube lengths are ranging from 80 to 400 nm. The selected area electron diffraction (SAED) was taken to define the crystallinity of a certain area in the sample (**Figure 9** right). Sample Na-Ti-DW exhibits diffuse ring

tures, such as nanorods or nanoparticles. Images of TEM show dispersed TiO<sup>2</sup>

was needed to make the titania nanosheets. Steps for the transformation of TiO<sup>2</sup>

bular titanate can be summarized as follows: first, the dissolution of the TiO<sup>2</sup>

. Successful route without NaOH(aq) in the solvent was proposed by Liu et al. [25]

vapor pressure to transform titania nanosheets into nanotubes. NaOH

with nanotube morphology.

as the solvent combined by NaOH, at high concentration

anatase crystalline phase via the formation of hydrogen titanate and TiO<sup>2</sup>

alkaline alteration. A vapor pressure by using NH<sup>3</sup>

are curving and then wrapping into nanotubes [23].

temperature are needed to obtain anatase TiO<sup>2</sup>

and NaOH (1:3 molar ratio) with acid washing.

method was sought, by using NH<sup>3</sup>

/g [22]. Nanorods or nanofibers may only have 50 m<sup>2</sup>

/g [23]. Therefore,

to nanotu-

sources, at the

nanotubes with

(B) [23].

(aq) during the hydrothermal is one of the

surface area of ~400 m<sup>2</sup>

454 Titanium Dioxide - Material for a Sustainable Environment

or NH3

NH3

anism induced by NH3

**Figure 9.** Bright-field TEM images of sample (left), the corresponding SAED pattern (middle) of (A) Na-Ti-DW, (B) Na-Ti-H, and (C) 3:1 NaNH<sup>3</sup> -Ti-H and XRD patterns (right) of Na-Ti-DW and Na-Ti-H.

pattern and low-intensity diffraction, which identifies the contributions of amorphous material instead of polycrystalline materials. It has diffraction at crystal plane [011], [300], and [204] that belong to sodium trititanate phase. To ensure the materials crystallinity, XRD pattern was also taken to obtain the contributing crystalline phase of the sample. The XRD results show that the crystalline phase of sample Na-Ti-DW has strong peaks at 2θ ≈ 10.5°, 24°, 28°, 43°, and 48° similar to the XRD pattern by Sikhwivhilu et al. [24], with distinct peak at 2θ ≈ 10° and low-intensity peaks at 2θ ≈ 24°, 28°, and 48°. It is indicated that the as synthesized product has the similar crystalline phase, which is predominantly titanate phase. It is confirmed as sodium trititanate (PDF 31–1329). The sample Na-Ti-DW also bears amorphous, signified from the bland feature at around 2θ ≈ 30° and 60°, and anatase (PDF 21–1272) phase, identified from reflections at ~24° (d101) and 60° (d200). For Na-Ti-H, the XRD pattern shows the diminishing peaks of sodium trititanate phase. It is found that the synthesis of nanotubes with water washing treatment (Na-Ti-DW) exhibits titanate nanotube, while with dilute acid (Na-Ti-H), it produces anatase crystalline phase. Diffraction at 2θ ≈ 24° is probably due to the shifts from peak at 25° of anatase (101) and titanate (011). Peak shifts are preferred due to the formation of nanotubes, which put strains on the bonding in sample. The corresponding Raman spectra published elsewhere [31] support the formation of anatase phase after acid washing.

The morphological structure for sample 3:1 NaNH<sup>3</sup> -Ti-H (**Figure 9C**) with 3:1 base ratio (NaOH:NH<sup>3</sup> ) was found to be similar with Na-Ti-H, as mostly all of the morphology of titania were open-ended nanotubes. The nanotubes are highly distributed (separated from each other), showing that the bundles of titania did not form. Yet a few nanosheets are visible in the image, confirming that the morphological changes did not perfectly occur. High amount of NH3 leads to the transformation from nanotubes into nanosheets and later to spherical structures [31]. Thus, nanotube preparation can be sought at NH3 ratio to NaOH of 1:3. Alteration to only using NH3 as the alkaline results in the formation of spherical nanoparticle titania with anatase crystalline phase [31].

Short-time synthesis of titania nanotube was proposed by applying mechanical or sonicationassisted stirring prior hydrothermal. The effect of various stirring times and hydrothermal treatments on the crystalline phases and morphology of the resulted titania has been studied [32]. It has been shown that the nanotube titania can be obtained after 5 h hydrothermal treatment at 150°C. The XRD patterns of the resulted powders showed the existence of a mixture of anatase and titanate crystalline phases with increased intensity of [200] as the stirring time increasing. At the longest stirring time, the existence of TiO<sup>2</sup> (B) was observed. Raman spectra have also confirmed the existence of both anatase and titanate crystalline phases. The high textural coefficient for [200] (TC200) has indicated oriented growth of one-dimensional anatase along [200]. All powders resulted at various stirring time were nanotubes, as confirmed by transmission electron microscope (TEM).

counter electrodes. It is observed that the thermally platinized counter electrode gave better DSSC performance with increased open-circuit voltage (VOC) and short-circuit current (ISC). The increased variables enhanced the cell efficiency to about 5%. This improvement can be attributed to the high surface area of platinum in thermally platinized counter electrode that will act as an efficient catalyst for iodine reduction [8]. The redox electrolyte shuttles the electron from the site of regeneration on the sensitized titania working electrode to the counter electrode to complete the electron cycle. During this process, the iodine must be reduced back to iodide at minimum energy loss on the counter electrode. However, the fill factor of this cell is much lower than that assembled with a sputtered Pt counter electrode. This may be caused by the loss of some scattered light, which may enhance the electron generation throughout the titania film. The possible recombination rate for this thermally platinized cell is slightly lower than that of the sputtered Pt counter electrode. Thus, it is expected to compromise the losses. In addition, this type of counter electrode produced a transparent DSSC. Illumination can be performed in both sides of the sandwiched cells that cannot be done by using sputtered Pt counter electrode. Comparing the performance of acid treated-acid route photoanode with neutral route, a higher efficiency was achieved for the latter photoanode [1]. High open-circuit voltage and short-circuit current are responsible for this performance, which may be caused by the presence of the full anatase domain characteristic of the powder precursor as previously demon-

**Figure 10.** I-V curves of mesoporous titania derived from neutral hydrothermal route using different platinization

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methods for counter electrodes: (A) light current and (B) the corresponding dark current curves [1].

**Table 4** shows the solar cell parameters of various titania photoanodes and natural dyes as sensitizers. Results on natural dyes as the sensitizers are not as high as the cells using ruthenium complex. Dried fruit of *joho*, bark of *tingi*, and *tegeran* were commonly used as dyes for traditional *Batik* clothes in Indonesia. The cells with *joho* and *tegeran* dyes have shown appreciable generated photovoltage and photocurrent compared to *tingi*. Both *joho* and *tegeran* dyes

strated (Section 1).

The next section will discuss the application of titanium dioxide for photoelectrochemical solar cells (DSSC) and multifunctional coatings for textiles and woods.
