**2.1. Hydrothermal-seeded synthesis of mesostructured titania**

Synthesis of nanostructured titanium dioxides has been greatly explored and discussed. The hydrothermal-seeded synthesis route was inspired by a similar approach in preparing mesoporous aluminosilicates containing zeolite framework [5–8]. The auto-assembly zeolite seed concept was applied by creating anatase seeds to the synthesis of mesoporous titania. Basically, there are three main steps in the synthesis route, namely, anatase seed preparation, hydrothermal self-assembly, and template removal. Anatase phase was chosen as the nanocrystal seed due to its highest photocatalytic activity among other crystalline phases of titania. Instead, some studies showed that it is easy to obtain such phase at low temperature and short-time synthesis [9, 10]. Anatase is also well known as a kinetically favored crystalline phase of titania for most synthesis routes. In this study, the anatase seeds prepared via neutral and acidic route (**Figure 1**) are discussed. Hydrothermal interaction between block-copolymer surfactant and the anatase seeds is illustrated. Results on the adsorption of Ru-"black dye" onto selected powders are included.

for certain applications. A high-surface-area titania is beneficial as photoanode in photoelectrochemical solar cells to obtain efficient light harvesting due to the characteristics of dyemonolayer adsorption. However, high surface area is not enough for efficient photoanodes. The titania must also have high crystallinity of the photoactive phase, which is mostly anatase crystalline phase. The presence of microporosity is not favorable for the dye adsorption, hindering efficient dye adsorption. Previous studies have shown that there is a compromise between high surface areas with porosity and crystallinity of the anatase phase [1–3]. For multifunctional textiles, amorphous titania is favored due to strong adherence to the surface of the cellulose-based fabrics [4] compared to the crystalline phase of titania. Interestingly, 1D-nanostructured titania, such as nanorods or nanotubes, has shown notable photocatalytic

Here, synthesis route to obtain high-surface-area titania with full domain of anatase phase will be presented and discussed. Some results for the application of the resulted mesostructured titania for dye-sensitized solar cells (DSSC) will be included. Secondly, applications to functional coating for textile and wood are also deliberated by considering photocatalytic and hydrophilic/hydrophobic mechanism. Combining nanostructured titania and silica resulted in excellent antibacterial coatings. Recent results of nanorod titania and silica as antifouling

Firstly, mesostructured titania as photoanodes for photoelectrochemical solar cells that are famously called dye-sensitized solar cells (DSSC) is discussed. Hydrothermal-seeded protocol is offered as a recommended synthesis route to achieve the targeted photoactivity. Secondly, one-dimensional nanostructured titania is proposed for the photoanodes. Nanotube titania was synthesized through templated-hydrothermal method and has shown improved photoactivity than that of nanoparticles. Sonication is proposed to shorten the synthesis time to

Synthesis of nanostructured titanium dioxides has been greatly explored and discussed. The hydrothermal-seeded synthesis route was inspired by a similar approach in preparing mesoporous aluminosilicates containing zeolite framework [5–8]. The auto-assembly zeolite seed concept was applied by creating anatase seeds to the synthesis of mesoporous titania. Basically, there are three main steps in the synthesis route, namely, anatase seed preparation, hydrothermal self-assembly, and template removal. Anatase phase was chosen as the nanocrystal seed due to its highest photocatalytic activity among other crystalline phases of titania. Instead, some studies showed that it is easy to obtain such phase at low temperature and short-time synthesis [9, 10]. Anatase is also well known as a kinetically favored crystalline phase of titania for most synthesis routes. In this study, the anatase seeds prepared via neutral

**2. Synthesis routes to nanostructured titanium dioxides (mesostructure spherical nanoparticles, nanotubes)**

**2.1. Hydrothermal-seeded synthesis of mesostructured titania**

activity under the visible light irradiation.

446 Titanium Dioxide - Material for a Sustainable Environment

coating on wood are presented.

prepare nanotube titania.

The preparation of anatase seed is aimed at obtaining nanocrystals with less than 5 nm size, as the thickest wall obtained for mesoporous titania so far does not exceed 5 nm [11]. Hydrothermal technique was proposed to be the technique of choice to prepare the anatase seeds, due to its simplicity and its good reproducibility [9]. The resultant materials were then examined either by XRD or TEM for the presence of the anatase nanocrystals.

Two main routes of the seed preparation, as displayed in **Figure 1**, were proposed. It comprises the hydrothermal hydrolysis and condensation of titanium precursor at a neutral and an acidic condition. A mixture of ethanol in water was used as the hydrolysis media. Ethanol was introduced as a cosolvent to slightly slow down the hydrolysis and condensation rates. It was also chosen to obtain higher oxide content from the hydrolytic condensation of titanium (IV) tetraisopropoxide compared to the process using its parent alcohol [12]. Then, in the second route, acid is introduced to further retard the condensation [13, 14] and obtain clear solution seeds. **Figures 2** and **3** show the dark- and bright-field TEM images of the resulted anatase seeds resulted from neutral and acidic solution, respectively.

The seed suspensions were obtained after 4 h hydrothermal treatment at 80°C. The brightand dark-field TEM images of this seed as well as its selected area electron diffraction (SAED) pattern are presented in **Figure 2**. Diffuse ring SAED pattern indicates the formation of a very small polycrystalline material, which has been indexed as the anatase crystal phase of titania [1]. The presence of bright spots all over the sample region shows the uniform distribution of these crystal phases, while the magnified image in the dark-field TEM image presents the observed lattice strain from [101] anatase phase. Based on the TEM image, the crystal size is around 5 nm as designed. However, the seed solution was a milky solution. Anatase seeds obtained from neutral hydrothermal route can be recovered as a very light powder.

**Figure 1.** Flow diagram of the approach used in anatase seed preparation via hydrothermal technique.

The acidic route was proposed in order to obtain an ideal clear seed solution containing the desired crystalline phase as applied in zeolitic-aluminosilicate synthesis. Previous study has proven the importance of introducing acid to stabilize colloidal particles resulted during hydrolysis-condensation reaction and obtaining transparent solution [12, 15]. Variation on the acid concentration can be altered to obtain fewer amounts of HCl for such purpose. HCl is also known to accelerate the anatase nucleation [10], which will be beneficial for hydrothermal process at low temperature. Since the seed is a clear solution, TEM was used to examine the crystal size and phase by using dark-field/bright-field images at the same area and selected area electron diffraction, respectively.

l. The bright spots observed in the dark-field images confirm the formation of nanocrystals. From **Figures 2** and **3**, it can be seen that the presence of acidic strength resulted in smaller crystalline phase, which are averaged at 2.05 nm. As a result of such small crystallites, the SAED of the seed from acidic route (indexed as an anatase phase) is more diffuse compared to that of the anatase seed obtained from neutral route. The seed solution is hazy. Those two anatase seeds were then proceeded into step 2 hydrothermal synthesis which is mesoporous

**Table 1** lists the textural parameters of the resulted titania powders synthesized at various

tion–desorption isotherms as well as the corresponding pore size distribution (BJH model desorption branch) of calcined mesoporous titania obtained from hydrothermal interaction between neutral route-derived anatase seed and neutral aqueous surfactant solution. The second step of hydrothermal synthesis involved block copolymer, P123 as the pore template

pore size distribution (BJH model desorption branch) of calcined mesoporous titania obtained from hydrothermal interaction between neutral route-derived anatase seed and neutral aque-

From **Table 1** and **Figure 4**, it can be seen that the time alternation at given temperature does not change significantly the isotherm type, as well as the surface area and pore diameter. Type IV isotherm of typical mesoporous materials with well-defined hysteresis loops was obtained. However, the pore size distribution is much broadened at shorter (5 h) or longer (48 h) hydrothermal treatment, more likely due to insufficient interaction or disrupted interaction, respectively, between anatase seed and block copolymer micelles. On the other hand, effect of hydrothermal temperature is much pronounced on surface area, pore size, and pore volume as demonstrated in **Table 1** and **Figure 5**. Type IV isotherms with hysteresis loop corresponding to the ordered mesopore filling are evidenced. The relative pressure of pore filling increases with hydrothermal temperature. Thus, an increase of pore size and a decrease in the surface area with the hydrothermal temperature are observed. Pore size broadening is

adsorption–desorption isotherms as well as the corresponding

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condition of hydrothermal temperature and time, while **Figure 4** displays the N2

titania synthesis.

[1, 2]. **Figure 5** shows the N<sup>2</sup>

**Sample1 SBET (m2**

1

surfactant.

TTIP:P123:EtOH:H<sup>2</sup>

Hydrothermal 100°C, 5 h 101.00 0.26 7.86 Hydrothermal 100°C, 10 h 100.90 0.24 7.83 Hydrothermal 100°C, 20 h 106.00 0.23 7.72 Hydrothermal 100°C, 48 h 110.40 0.27 7.89 Hydrothermal 80 °C, 20 h 152.30 0.29 6.19 Hydrothermal 100°C, 20 h 106.00 0.23 7.72 Hydrothermal 150°C, 20 h 98.70 0.24 7.90

O = 1:0.034:10.55:431.6 (molar ratio precursor composition).

**Table 1.** Textural parameters of mesoporous titania resulted from neutral route anatase seed and neutral aqueous

ous surfactant solution.

**Figure 3** displays the bright- and dark-field TEM images of anatase seed resulted from the acidic route with molar ratio precursor composition of 1TTIP:26.10EtOH:34.06H<sup>2</sup> O:1.25HC

**Figure 2.** Bright-field TEM image with the respective SAED pattern (left) and the dark-field (right) [1] TEM image of anatase seed solution prepared hydrothermally at 80°C for 4 h and neutral condition.

**Figure 3.** Bright-field TEM image with the respective SAED pattern (left) and the dark-field (right) TEM image of anatase seed solution prepared hydrothermally in acidic solution at 80°C for 4 h.

l. The bright spots observed in the dark-field images confirm the formation of nanocrystals. From **Figures 2** and **3**, it can be seen that the presence of acidic strength resulted in smaller crystalline phase, which are averaged at 2.05 nm. As a result of such small crystallites, the SAED of the seed from acidic route (indexed as an anatase phase) is more diffuse compared to that of the anatase seed obtained from neutral route. The seed solution is hazy. Those two anatase seeds were then proceeded into step 2 hydrothermal synthesis which is mesoporous titania synthesis.

The acidic route was proposed in order to obtain an ideal clear seed solution containing the desired crystalline phase as applied in zeolitic-aluminosilicate synthesis. Previous study has proven the importance of introducing acid to stabilize colloidal particles resulted during hydrolysis-condensation reaction and obtaining transparent solution [12, 15]. Variation on the acid concentration can be altered to obtain fewer amounts of HCl for such purpose. HCl is also known to accelerate the anatase nucleation [10], which will be beneficial for hydrothermal process at low temperature. Since the seed is a clear solution, TEM was used to examine the crystal size and phase by using dark-field/bright-field images at the same area and

**Figure 3** displays the bright- and dark-field TEM images of anatase seed resulted from the

**Figure 2.** Bright-field TEM image with the respective SAED pattern (left) and the dark-field (right) [1] TEM image of

**Figure 3.** Bright-field TEM image with the respective SAED pattern (left) and the dark-field (right) TEM image of anatase

anatase seed solution prepared hydrothermally at 80°C for 4 h and neutral condition.

seed solution prepared hydrothermally in acidic solution at 80°C for 4 h.

O:1.25HC

acidic route with molar ratio precursor composition of 1TTIP:26.10EtOH:34.06H<sup>2</sup>

selected area electron diffraction, respectively.

448 Titanium Dioxide - Material for a Sustainable Environment

**Table 1** lists the textural parameters of the resulted titania powders synthesized at various condition of hydrothermal temperature and time, while **Figure 4** displays the N2 adsorption–desorption isotherms as well as the corresponding pore size distribution (BJH model desorption branch) of calcined mesoporous titania obtained from hydrothermal interaction between neutral route-derived anatase seed and neutral aqueous surfactant solution. The second step of hydrothermal synthesis involved block copolymer, P123 as the pore template [1, 2]. **Figure 5** shows the N<sup>2</sup> adsorption–desorption isotherms as well as the corresponding pore size distribution (BJH model desorption branch) of calcined mesoporous titania obtained from hydrothermal interaction between neutral route-derived anatase seed and neutral aqueous surfactant solution.

From **Table 1** and **Figure 4**, it can be seen that the time alternation at given temperature does not change significantly the isotherm type, as well as the surface area and pore diameter. Type IV isotherm of typical mesoporous materials with well-defined hysteresis loops was obtained. However, the pore size distribution is much broadened at shorter (5 h) or longer (48 h) hydrothermal treatment, more likely due to insufficient interaction or disrupted interaction, respectively, between anatase seed and block copolymer micelles. On the other hand, effect of hydrothermal temperature is much pronounced on surface area, pore size, and pore volume as demonstrated in **Table 1** and **Figure 5**. Type IV isotherms with hysteresis loop corresponding to the ordered mesopore filling are evidenced. The relative pressure of pore filling increases with hydrothermal temperature. Thus, an increase of pore size and a decrease in the surface area with the hydrothermal temperature are observed. Pore size broadening is


1 TTIP:P123:EtOH:H<sup>2</sup> O = 1:0.034:10.55:431.6 (molar ratio precursor composition).

**Table 1.** Textural parameters of mesoporous titania resulted from neutral route anatase seed and neutral aqueous surfactant.

**Figure 4.** N2 adsorption–desorption isotherms (A) and pore size distribution (B) of mesoporous titania synthesized from neutral anatase seed and neutral surfactant solution at various hydrothermal times at 100°C after calcination at 400°C.

At 80°C hydrothermal treatment, the low-angle XRD peak (data not shown) appears as a

**Figure 6.** Illustration of micelle-stabilized anatase seed aggregation mechanism during hydrothermal treatment.

hydrothermal temperature results in broad low-angle XRD peak, indicating possible mesopore distribution broadening. This fact is also evidenced by its pore size distribution obtained

On the other hand, anatase seed undergoes transformation into rutile crystalline phase at 150<sup>o</sup> C (data not shown). The high temperature results in rapid crystallization owing to a favored dissolution precipitation mechanism [10], which allows the fast transformation from anatase to rutile crystalline phase. It has been shown that anatase seed treated hydrothermally with aqueous block copolymer solution at 100°C for 20 h exhibits highest porosity with ordered mesopores and full anatase crystalline domain [1]. The TEM images of the resulted powder

From the TEM images, it is clear that block copolymer functions to aid pore organization without showing template mechanism as usually observed in micelle-templated silica or metal oxide using the same nonionic surfactant [11, 16–19]. The possible mechanism is assumed to be an aggregation mechanism over block copolymer-steric stabilized-anatase seed particles. EO20PO70EO20 is known to form spherical micelles, with the dense cores consisting of dehydrated PO blocks and hydrated EO blocks at the micellar surface (coronas), at critical micellar

The micelles are attached to the anatase seed via their protruded-EO chains in such a way covering the anatase surface, creating steric stabilization to the anatase seeds. Such interaction is likely driven by a surface charge potential between the micelles and anatase seeds. Control over pore formation and crystalline growth is attained by a corona of block copolymer micelles formed around anatase seed particles as illustrated in **Figure 6**. The subsequent

2θ, indicating less order mesopore structuration as implied by the

adsorption–desorption isotherm as well. Higher

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shoulder around 1.5o

can be found elsewhere [1].

using N2

presence of broad hysteresis loop in its N2

adsorption–desorption analysis.

concentration (cmc) of 0.03 wt% at 25°C in water [20, 21].

**Figure 5.** N2 adsorption–desorption isotherms and pore size distribution of mesoporous titania synthesized from neutral anatase seed and neutral surfactant solution at various hydrothermal temperatures for 20 h after calcination at 400°C.

prominent at 150°C hydrothermal treatment, which is likely due to phase separation of the block copolymer template at temperature higher than its *cloud point* (85°C for P123 in aqueous neutral solution) [16, 17]. At higher temperature than its *cloud point*, micelle-micelle interaction is getting more prominent as water becomes less effective solvent for the polyethylene oxide (PEO) chains, resulting in strong micelle-micelle aggregation. Therefore the micelle aggregate size is getting bigger, directing the formation of larger pores. The interaction is illustrated in **Figure 6**.

**Figure 6.** Illustration of micelle-stabilized anatase seed aggregation mechanism during hydrothermal treatment.

At 80°C hydrothermal treatment, the low-angle XRD peak (data not shown) appears as a shoulder around 1.5o 2θ, indicating less order mesopore structuration as implied by the presence of broad hysteresis loop in its N2 adsorption–desorption isotherm as well. Higher hydrothermal temperature results in broad low-angle XRD peak, indicating possible mesopore distribution broadening. This fact is also evidenced by its pore size distribution obtained using N2 adsorption–desorption analysis.

On the other hand, anatase seed undergoes transformation into rutile crystalline phase at 150<sup>o</sup> C (data not shown). The high temperature results in rapid crystallization owing to a favored dissolution precipitation mechanism [10], which allows the fast transformation from anatase to rutile crystalline phase. It has been shown that anatase seed treated hydrothermally with aqueous block copolymer solution at 100°C for 20 h exhibits highest porosity with ordered mesopores and full anatase crystalline domain [1]. The TEM images of the resulted powder can be found elsewhere [1].

From the TEM images, it is clear that block copolymer functions to aid pore organization without showing template mechanism as usually observed in micelle-templated silica or metal oxide using the same nonionic surfactant [11, 16–19]. The possible mechanism is assumed to be an aggregation mechanism over block copolymer-steric stabilized-anatase seed particles. EO20PO70EO20 is known to form spherical micelles, with the dense cores consisting of dehydrated PO blocks and hydrated EO blocks at the micellar surface (coronas), at critical micellar concentration (cmc) of 0.03 wt% at 25°C in water [20, 21].

prominent at 150°C hydrothermal treatment, which is likely due to phase separation of the block copolymer template at temperature higher than its *cloud point* (85°C for P123 in aqueous neutral solution) [16, 17]. At higher temperature than its *cloud point*, micelle-micelle interaction is getting more prominent as water becomes less effective solvent for the polyethylene oxide (PEO) chains, resulting in strong micelle-micelle aggregation. Therefore the micelle aggregate size is getting bigger, directing the formation of larger pores. The interaction is

anatase seed and neutral surfactant solution at various hydrothermal temperatures for 20 h after calcination at 400°C.

adsorption–desorption isotherms and pore size distribution of mesoporous titania synthesized from neutral

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

neutral anatase seed and neutral surfactant solution at various hydrothermal times at 100°C after calcination at 400°C.

illustrated in **Figure 6**.

**Figure 4.** N2

450 Titanium Dioxide - Material for a Sustainable Environment

**Figure 5.** N2

The micelles are attached to the anatase seed via their protruded-EO chains in such a way covering the anatase surface, creating steric stabilization to the anatase seeds. Such interaction is likely driven by a surface charge potential between the micelles and anatase seeds. Control over pore formation and crystalline growth is attained by a corona of block copolymer micelles formed around anatase seed particles as illustrated in **Figure 6**. The subsequent removal of the block copolymer leads to interstitial pore arrangements resulting in the formation of uniform and well-controlled mesopores.

capability with textural properties of the powders, large pore samples perform higher ability to adsorb the dye molecules. Therefore, it is confirmed that the presence of large pore with appreciable porosity is favorable for dye adsorption to sensitize titania photoanode in photoelectrochemical solar cells. The optimum peaks of the dye solution after adsorption have shifted to higher wavelengths due to possible dye oxidation during the measurement. The discussion on the DSSC performance will be on Section 3. Based on the textural properties and the resulted crystalline phase of anatase titania, mesostructured titania from hydrothermal-

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

NS = neutral anatase seed, NSNS = neutral anatase seed + neutral surfactant, NSAS = neutral anatase seed + acidic

O, NSAS-1 = 3.2 g P123/(40 mL H<sup>2</sup>

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

**/g) Isotherm/hysteresis loop type Dye adsorbed (%)**

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).

O + 4 mL HCl<sup>c</sup>

**SBET (m2**

NSNS-1 0.48 9.66, 60.60 146.50 II–IV/H1–H2 61.20 NSAS-1 0.54 12.61 118.70 IV/H2 81.28 P25 (Degussa) 0.10 74.2 55.74 II >99.00

seeded synthesis are suitable for photoanodes of DSSC.

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

adsorption. A commercial titania Degussa P25 was used as comparison.

**Dp (nm) BJHd**

**Sample Pore volume** 

**(cc/g)**

surfactant, NSNS-1 = 1.25 g P123/40 mL H<sup>2</sup>

properties.

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

**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


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

**Figure 7.** (A) N2 adsorption–desorption isotherms and (B) pore size distribution of mesoporous titania synthesized at various hydrothermal times from acidic route anatase seeds' neutral aqueous surfactant.

capability with textural properties of the powders, large pore samples perform higher ability to adsorb the dye molecules. Therefore, it is confirmed that the presence of large pore with appreciable porosity is favorable for dye adsorption to sensitize titania photoanode in photoelectrochemical solar cells. The optimum peaks of the dye solution after adsorption have shifted to higher wavelengths due to possible dye oxidation during the measurement. The discussion on the DSSC performance will be on Section 3. Based on the textural properties and the resulted crystalline phase of anatase titania, mesostructured titania from hydrothermalseeded synthesis are suitable for photoanodes of DSSC.
