**2.2. One-step synthesis of titania nanoflakes**

Anatase powders with sizes ranging from 5 to 165 nm were employed various synthesis methods. Regardless of fabrication processes, the optimum particle size range of 25–40 nm within all photocatalytic experiments processes were suggested by Almquist and Biswas [37]. The optimum photocatalysis is a function of competing mechanisms such as light absorption and scattering efficiency of the particles, as well as electron–hole pair combination and interfacial charge transfer. Therefore, we believe a one-step synthesis for titania nanomaterials with at least 1D close to the optimum size of photocatalyst will be a revolution of the titania photocatalyst fabrication. The one-step synthesis is spreading an oil phase consisting of titanium tetraisopropoxide and a low surface tension hydrocarbon at the surface of water to produce micrometer sized titania flakes having a thickness around 40 nm [38, 39]. Moreover, the thickness of nanoflakes could be tuned by changing the volume ratio of titania precursor and hydrocarbon. For instance, 40 nm titania flakes were successfully synthesized using a ratio of 1:8 of titania tetraisopropoxide to hydrocarbon. The reactions of forming TiO<sup>2</sup> in this research could be represented as follows:

$$\text{Ti} \left( \text{O} \text{C}\_3 \text{H}\_7 \right)\_4 \text{+4} \text{H}\_2\text{O} \rightarrow \text{Ti} \left( \text{OH} \right) \text{+4 C}\_3 \text{H}\_7 \text{OH} ... \text{[hydrolysis]} \tag{1}$$

$$\text{Ti(OH)}\_{4} \rightarrow \text{TiO}\_{2} + 2\text{ H}\_{2}\text{O...(condensation)}\tag{2}$$

Where d is the calculated grain size, λ is the wavelength of X-ray (Cu Kα 1.54 Å), β is the fullwidth at half-maximum intensity, and θB is the half of the diffraction peak angle. The grain size was determined to be 4 and 9 nm for synthesized and calcined flakes respectively (**Table 2**). High resolution transmission electron microscopy (HR-TEM) images revealed circular crystalline platelets of about 5–8 nm in diameter (**Figure 5**). The interference lattice fringes with separation distance of 0.35 nm corresponded to the interplanar spacing of the (101) planes for anatase [44]. The concentric diffraction rings in the select area diffraction mode indicated random orientation of individual grains over the both flakes which is consistent with the anatase (101), (004), (200), (105) for circles 1–4 respectively (the insets of **Figure 5**). On closer inspection, synthesized samples contained an amorphous layer can be seen surrounding the smaller circular crystallites (**Figure 5a**). After calcination, calcined samples developed some pores due to local rearrangement and growth of crystal grains (**Figure 5b**). Consequently, polycrystalline fine grains of anatase populated throughout the whole calcined flakes. The nitrogen absorption isotherms in conjunction with the Brunauer-Emmett-Teller (BET) model was used

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**Figure 1.** SEM images of titania nanoflakes: (a) synthesized samples (b) calcined samples (c, d) edge views of A and B

respectively. Images were obtained without conducting coating.

The resulting slurry was washed with Nanopure water and isoproponal thoroughly to remove impurities. Dried and well dispersed particles were collected by the supercritical fluid drying process [40]. The following heat treatment for phase transformation was conducted at 400°C. This synthesis has high potential to manufacture gram to kilogram quantities of nanomaterials. Scanning electron microscopy (SEM), laser diffraction analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), and physisorption techniques were used to further characterize these titania nanoflakes.

#### **2.3. Characterization of titania nanoflakes**

The surface morphology of synthesized titania nanoflakes was investigated using SEM images. The images revealed the major diameter was within the order of 10–20 μm and the thickness was approximately 40 nm (**Figure 1**). Agglomerations or aggregations were not observed under SEM review when comparing the received samples with and without calcination. Moreover, the thickness of calcined flakes did not changed by the heat treatment. Both the uncalcined and calcined flakes were transparent under the SEM. The surface morphology of titania nanoflakes were similar to exfoliated nanosheets and micro-arc oxidized flakes on titanium substrate (**Figure 2**) [25, 32]. Light scattering technology such as laser diffraction were also used to quantified the particle size distributions of synthesized and calcined flakes. Theoretically, laser diffraction analysis alone is not designed for characterizing size distribution of anisotropic particles. To have a better understanding of the particle morphology, conjunction with electron microscopic method is necessary. Comparable data of major dimension were obtained by two methods. The statistics data of particle size distribution by volume were shown in **Table 1**. The synthesized material has wide size distribution spanning from 1 to 100 μm with a D50 of 20.8 and 19.0 μm for the synthesized and calcined flakes respectively (**Figure 3**). Some larger flakes may crack or break during the re-crystallization and dehydration in terms of the volume distributions. The average aspect ratio of the flakes is 1:500 resulting from the calculation which divided mean size by thickness (**Table 1**). Powder X-Ray Diffraction (XRD) monitored the crystalline structure changes of the titania flakes during the heat treatment (**Figure 4**). The synthesized flakes show broadening and weak peaks which suggests the mixture of amorphous material with a presence of the anatase phase. Pure anatase phase [JCPDS: 21-1272] which is the most photoactive structure were identified by XRD with the seven diffraction peaks in the heat treated flakes [41–43]. No other common phase such as rutile were observed by XRD. Complete phase conversion of amorphous titania was confirmed by comparing the intensity characteristics peaks between calcined flakes and the commercial pure anatase standard materials. The average crystal grain size can be calculated by the Scherer equation:

$$d = \frac{k\lambda}{B\cos\theta\_{\delta}}\tag{3}$$

Where d is the calculated grain size, λ is the wavelength of X-ray (Cu Kα 1.54 Å), β is the fullwidth at half-maximum intensity, and θB is the half of the diffraction peak angle. The grain size was determined to be 4 and 9 nm for synthesized and calcined flakes respectively (**Table 2**). High resolution transmission electron microscopy (HR-TEM) images revealed circular crystalline platelets of about 5–8 nm in diameter (**Figure 5**). The interference lattice fringes with separation distance of 0.35 nm corresponded to the interplanar spacing of the (101) planes for anatase [44]. The concentric diffraction rings in the select area diffraction mode indicated random orientation of individual grains over the both flakes which is consistent with the anatase (101), (004), (200), (105) for circles 1–4 respectively (the insets of **Figure 5**). On closer inspection, synthesized samples contained an amorphous layer can be seen surrounding the smaller circular crystallites (**Figure 5a**). After calcination, calcined samples developed some pores due to local rearrangement and growth of crystal grains (**Figure 5b**). Consequently, polycrystalline fine grains of anatase populated throughout the whole calcined flakes. The nitrogen absorption isotherms in conjunction with the Brunauer-Emmett-Teller (BET) model was used

1:8 of titania tetraisopropoxide to hydrocarbon. The reactions of forming TiO<sup>2</sup>

*Ti* (*O C*<sup>3</sup> *H*7)<sup>4</sup> + 4 *H*<sup>2</sup> *O* → *Ti* (*OH*) + 4 *C*<sup>3</sup> *H*<sup>7</sup> *OH*…(*hydrolysis*) (1)

The resulting slurry was washed with Nanopure water and isoproponal thoroughly to remove impurities. Dried and well dispersed particles were collected by the supercritical fluid drying process [40]. The following heat treatment for phase transformation was conducted at 400°C. This synthesis has high potential to manufacture gram to kilogram quantities of nanomaterials. Scanning electron microscopy (SEM), laser diffraction analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), and physisorption techniques were used to

The surface morphology of synthesized titania nanoflakes was investigated using SEM images. The images revealed the major diameter was within the order of 10–20 μm and the thickness was approximately 40 nm (**Figure 1**). Agglomerations or aggregations were not observed under SEM review when comparing the received samples with and without calcination. Moreover, the thickness of calcined flakes did not changed by the heat treatment. Both the uncalcined and calcined flakes were transparent under the SEM. The surface morphology of titania nanoflakes were similar to exfoliated nanosheets and micro-arc oxidized flakes on titanium substrate (**Figure 2**) [25, 32]. Light scattering technology such as laser diffraction were also used to quantified the particle size distributions of synthesized and calcined flakes. Theoretically, laser diffraction analysis alone is not designed for characterizing size distribution of anisotropic particles. To have a better understanding of the particle morphology, conjunction with electron microscopic method is necessary. Comparable data of major dimension were obtained by two methods. The statistics data of particle size distribution by volume were shown in **Table 1**. The synthesized material has wide size distribution spanning from 1 to 100 μm with a D50 of 20.8 and 19.0 μm for the synthesized and calcined flakes respectively (**Figure 3**). Some larger flakes may crack or break during the re-crystallization and dehydration in terms of the volume distributions. The average aspect ratio of the flakes is 1:500 resulting from the calculation which divided mean size by thickness (**Table 1**). Powder X-Ray Diffraction (XRD) monitored the crystalline structure changes of the titania flakes during the heat treatment (**Figure 4**). The synthesized flakes show broadening and weak peaks which suggests the mixture of amorphous material with a presence of the anatase phase. Pure anatase phase [JCPDS: 21-1272] which is the most photoactive structure were identified by XRD with the seven diffraction peaks in the heat treated flakes [41–43]. No other common phase such as rutile were observed by XRD. Complete phase conversion of amorphous titania was confirmed by comparing the intensity characteristics peaks between calcined flakes and the commercial pure anatase standard materials. The average crystal grain size can be calculated by the Scherer equation:

*B*cos *θ<sup>B</sup>*

<sup>4</sup> → *Ti O*<sup>2</sup> + 2 *H*<sup>2</sup> *O*…(*condensation*) (2)

could be represented as follows:

44 Titanium Dioxide - Material for a Sustainable Environment

*Ti* (*OH*)

further characterize these titania nanoflakes.

**2.3. Characterization of titania nanoflakes**

*d* = \_\_\_\_\_\_ *<sup>k</sup>*

in this research

(3)

**Figure 1.** SEM images of titania nanoflakes: (a) synthesized samples (b) calcined samples (c, d) edge views of A and B respectively. Images were obtained without conducting coating.

**2.4. Applications of titania nanoflakes**

*2.4.1. Photocatalysis of titania nanoflakes*

**Figure 4.** XRD patterns of synthesized and calcined titania nanoflakes.

nanoparticles for water treatment is limited in practical application since it is very

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difficult to remove these ultrafine particles due to very small mass. Therefore, the conventional separation methods such as centrifuging, filtration, and sedimentation are difficult and expensive to implement. In addition, the efficiency of photodegradation by using nanoparticles is not very high because of the poor accessibility of the organic pollutants to catalyst surface caused by the agglomeration of particles [46]. Synthesizing larger flake-like titania with nanosized thickness will alleviate this problem. These titania flakes can be easily separated from the treated water by simply filtration or sedimentation. Because the flakes are nano thin, superior photocatalytic properties are retained due to high surface to volume ratios and short diffusion paths, which are favorable for the migration of electrons and holes. This reduces the

**Figure 3.** Volume based particle size distribution for synthesized and calcined titania flakes.

Using TiO<sup>2</sup>

**Figure 2.** Literature SEM images of titania flakes: (a) heated at 700°C (b) edge view [25] (c, d, e) micro-arc oxidation flakes in alkaline solution of 1.25 M, 2.5 M, and 5.0 M respectively [32].

to estimated the specific surface area of synthesized and calcined titania nanoflakes. Higher specific surface area usually leads to higher photoactivity from larger amount of adsorbed organic molecules at surface sites which enhance the reaction rate. However, it is usually simultaneously with higher amounts of crystal defects favoring recombination of charge carriers leading to lower photoactivity. Compared to a commercial photocatalyst, Degussa P25 [45], 2–6 times higher surface area were obtained by titania nanoflakes (**Table 3**). The photocatalytic activity of these flakes was investigated by performing dye degradation experiments under ultraviolet activation.


**Table 1.** Particle diameter statistics for synthesized and calcined titania flakes under investigation.

## **2.4. Applications of titania nanoflakes**

#### *2.4.1. Photocatalysis of titania nanoflakes*

to estimated the specific surface area of synthesized and calcined titania nanoflakes. Higher specific surface area usually leads to higher photoactivity from larger amount of adsorbed organic molecules at surface sites which enhance the reaction rate. However, it is usually simultaneously with higher amounts of crystal defects favoring recombination of charge carriers leading to lower photoactivity. Compared to a commercial photocatalyst, Degussa P25 [45], 2–6 times higher surface area were obtained by titania nanoflakes (**Table 3**). The photocatalytic activity of these flakes was investigated by performing dye degradation experiments

**Sample D10 (μm) D50 (μm) D90 (μm) Mean (μm) Standard deviation (μm)**

Synthesized flakes 5.2 20.8 81.6 39.1 58.9 Calcined flakes 5.1 19.0 51.8 24.7 20.9

**Table 1.** Particle diameter statistics for synthesized and calcined titania flakes under investigation.

**Figure 2.** Literature SEM images of titania flakes: (a) heated at 700°C (b) edge view [25] (c, d, e) micro-arc oxidation flakes

under ultraviolet activation.

in alkaline solution of 1.25 M, 2.5 M, and 5.0 M respectively [32].

46 Titanium Dioxide - Material for a Sustainable Environment

Using TiO<sup>2</sup> nanoparticles for water treatment is limited in practical application since it is very difficult to remove these ultrafine particles due to very small mass. Therefore, the conventional separation methods such as centrifuging, filtration, and sedimentation are difficult and expensive to implement. In addition, the efficiency of photodegradation by using nanoparticles is not very high because of the poor accessibility of the organic pollutants to catalyst surface caused by the agglomeration of particles [46]. Synthesizing larger flake-like titania with nanosized thickness will alleviate this problem. These titania flakes can be easily separated from the treated water by simply filtration or sedimentation. Because the flakes are nano thin, superior photocatalytic properties are retained due to high surface to volume ratios and short diffusion paths, which are favorable for the migration of electrons and holes. This reduces the

**Figure 4.** XRD patterns of synthesized and calcined titania nanoflakes.


**Table 2.** Grain size calculation by the Scherer equation for both nanoflakes.

probability of the recombination of photogenerated electrons and holes. At the same time, the common agglomeration problem caused by nanoparticles can also be mitigated and therefore maintain the advantages of micro and nanostructure. The photocatalytic efficiency of titania nanoflakes were investigated by UV-visible spectroscopy and degradation of methylene blue under UV irradiation. The typical light absorption of the semiconductor materials such as titania showed the sharp decrease in the diffuse reflectance in the UV region (**Figure 6**). Calcined sample demonstrated a blue shift of the onset of reflectance. In semiconductor physics, the general relation between the absorption coefficient and the band gap energy is given by

$$(al\nu\nu)^{\circ\circ} = \; \! \cdot h\nu - E\_{\circ} \tag{4}$$

methylene blue form water under UV light illumination. For comparison, Degussa P25 was used as a reference material. Among all tested samples, calcined flakes exhibited the highest photocatalytic efficiency on the dye degradation. A first order rate reaction was observed which suggested dye concentration is the limiting factor (**Figure 7**). In contrast, significant enhancement was observed when agitation was added to the system by continuously introducing air bubbles (**Figure 7b**). A pseudo first order reaction was obtained by adding turbulence resulted in much higher efficiency especially for the flake systems. The oxygen depletion during the photocatalysis process could be one possible explanation of these differences. The flake samples have much higher surface area than P25 and consisted of very small nanocrystallites which imply a large amount of defects (grain boundaries) from the results of XRD and TEM. Fast recombination of photoassisted electron and hole pairs preferentially occurs at these local defect sites and dominates the reaction. Without supplying oxygen to the system, the photocatalytic performance is therefore not proportional to surface area. However, dissolved oxygen may form superoxide radicals as the electron acceptor when applying oxygen to the system [48]. More importantly, eliminating excited conduction band electron would help to suppress the rate limiting step, fast recombination, and result in higher efficiency.

**/g) Specific pore volume (cm<sup>3</sup>**

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**/g)**

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on the photon energy for synthesized and

Anatase titanium dioxide is a promising negative electrode material for Li-ion batteries. However, the low intrinsic electrical conductivity and poor cycling performance have limited its application. Among all anatase titania samples, the calcined titania flakes performed higher rate capability, larger reversible capacity, and longer cycling stability [49]. The better

*2.4.2. Other applications: Li-ion battery and DSSC*

**Figure 6.** (a) Diffuse reflectance spectra and (b) the dependence of (αhν)<sup>2</sup>

calcined titania flakes.

**Sample Specific surface area (m<sup>2</sup>**

Synthesized nanoflakes 323 – Calcined nanoflakes 110 0.342

**Table 3.** Physisorption measurements of P25, synthesized and calcined titania nanoflakes.

Where m is an index depending on the nature of the electron transitions, α is the absorption coefficient, h is the Planck constant, ν is the frequency of electromagnetic radiation, and E is band gap energy of the semiconductor. The optical absorption energy of both nanoflakes were determined via extrapolation of (αhν)<sup>2</sup> versus hν plot (**Figure 6b**). The quantum confinement effect of higher crystallinity after calcinations and thin flaky morphology could result in an increase in band gap from 3.25 to 3.33 eV, i.e. a blue shift [47]. **Figure 6** compares the photocatalytic activity of P25, synthesized and calcined titania nanoflakes by removing the

**Figure 5.** HR-TEM images of titania nanoflakes (the SAD pattern as inset) (a) synthesized samples (b) calcined samples the diffraction rings are indexed as (1) 101 (2) 004 (3) 200 (4) 105 for anatase.


**Table 3.** Physisorption measurements of P25, synthesized and calcined titania nanoflakes.

methylene blue form water under UV light illumination. For comparison, Degussa P25 was used as a reference material. Among all tested samples, calcined flakes exhibited the highest photocatalytic efficiency on the dye degradation. A first order rate reaction was observed which suggested dye concentration is the limiting factor (**Figure 7**). In contrast, significant enhancement was observed when agitation was added to the system by continuously introducing air bubbles (**Figure 7b**). A pseudo first order reaction was obtained by adding turbulence resulted in much higher efficiency especially for the flake systems. The oxygen depletion during the photocatalysis process could be one possible explanation of these differences. The flake samples have much higher surface area than P25 and consisted of very small nanocrystallites which imply a large amount of defects (grain boundaries) from the results of XRD and TEM. Fast recombination of photoassisted electron and hole pairs preferentially occurs at these local defect sites and dominates the reaction. Without supplying oxygen to the system, the photocatalytic performance is therefore not proportional to surface area. However, dissolved oxygen may form superoxide radicals as the electron acceptor when applying oxygen to the system [48]. More importantly, eliminating excited conduction band electron would help to suppress the rate limiting step, fast recombination, and result in higher efficiency.

#### *2.4.2. Other applications: Li-ion battery and DSSC*

**Figure 5.** HR-TEM images of titania nanoflakes (the SAD pattern as inset) (a) synthesized samples (b) calcined samples

probability of the recombination of photogenerated electrons and holes. At the same time, the common agglomeration problem caused by nanoparticles can also be mitigated and therefore maintain the advantages of micro and nanostructure. The photocatalytic efficiency of titania nanoflakes were investigated by UV-visible spectroscopy and degradation of methylene blue under UV irradiation. The typical light absorption of the semiconductor materials such as titania showed the sharp decrease in the diffuse reflectance in the UV region (**Figure 6**). Calcined sample demonstrated a blue shift of the onset of reflectance. In semiconductor physics, the general relation between the absorption coefficient and the band gap energy is given by

**Sample θB (degree) d (nm)** Synthesized flakes 25.91 4.1 Calcined flakes 25.35 8.7

**Table 2.** Grain size calculation by the Scherer equation for both nanoflakes.

48 Titanium Dioxide - Material for a Sustainable Environment

(*h*)*<sup>m</sup>* = *h* − *Eg* (4)

Where m is an index depending on the nature of the electron transitions, α is the absorption coefficient, h is the Planck constant, ν is the frequency of electromagnetic radiation, and E is band gap energy of the semiconductor. The optical absorption energy of both nanoflakes

ment effect of higher crystallinity after calcinations and thin flaky morphology could result in an increase in band gap from 3.25 to 3.33 eV, i.e. a blue shift [47]. **Figure 6** compares the photocatalytic activity of P25, synthesized and calcined titania nanoflakes by removing the

versus hν plot (**Figure 6b**). The quantum confine-

the diffraction rings are indexed as (1) 101 (2) 004 (3) 200 (4) 105 for anatase.

were determined via extrapolation of (αhν)<sup>2</sup>

Anatase titanium dioxide is a promising negative electrode material for Li-ion batteries. However, the low intrinsic electrical conductivity and poor cycling performance have limited its application. Among all anatase titania samples, the calcined titania flakes performed higher rate capability, larger reversible capacity, and longer cycling stability [49]. The better

**Figure 6.** (a) Diffuse reflectance spectra and (b) the dependence of (αhν)<sup>2</sup> on the photon energy for synthesized and calcined titania flakes.

**Figure 7.** Photocatalytic decomposition of methylene blue by using synthesized and calcined flakes (a) without bubbling treatment (b) with bubbling treatment.

charge/discharge and rate capabilities resulted from the higher specific surface area of the flakes which leads to faster transportation between Li-ion and electron within the matrix of titania lattice (**Figure 8a**). Besides, the porous morphology of the calcined flakes provided extra space for the volume change during cycling and therefore significantly improved the cycling performance (**Figure 8b**).

Using the same deposition method to assemble the DSSC, integrally and closely bonded films resulted from better particle dispersion of titania flakes (**Figure 9**). In contrast, discontinuity of P25 nanoparticle layers were observed after the evaporation and sintering processes [50]. Improved energy conversion efficiency of DSSC could be attributed to two features of titania flakes: (1) Stronger adsorption of visible dyes from high specific surface area (2) Micron scale in two dimensions lead to stronger light scattering of visible light spectrum. The important IV characteristics of DSSC such as short-circuit current density (Isc) and open-circuit voltage (Voc) were found to be related to the thickness of the TiO<sup>2</sup> photoelectrodes. According to the calculations, calcined titania flakes demonstrated 5 times higher efficiency over the P25 photoelectrodes under the same thickness basis (7.4% vs. 1.2%) (**Figure 10**).

**3. Conclusions**

sunlight irradiation.

In summary, a high-aspect-ratio titania nanoflakes has been synthesized by the one-step modified surface hydrolysis. Compared to other methods for making low-dimensional nanomaterials, this spreading film process could continuously produce nanoflakes with a cost effective manner. These titania flakes could be easily separated from the treated water by simply sedimentation or filtration and therefore is very suitable for water purification application.

**Figure 10.** I-V characteristics of DSSCs made from P25 and calcined flakes of similar thickness under AM 1.5 simulated

**Figure 9.** SEM micrographs of sintered photoelectrodes made from (a) P25 nanoparticles (b) calcined titania nanoflakes.

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The major summary of applications were listed as follows: (1) Over 99% of methylene blue solution (50 μM) was degraded by the high aspect ratio calcined titania nanoflakes under UVA irradiation within 2 h, whereas not completely decomposition of dye solutions was achieved

**Figure 8.** (a) Rate capability and (b) specific discharge capacity comparison of CF-TiO<sup>2</sup> (calcined nanoflakes) and TiO<sup>2</sup> nanoparticles.

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**Figure 9.** SEM micrographs of sintered photoelectrodes made from (a) P25 nanoparticles (b) calcined titania nanoflakes.

**Figure 10.** I-V characteristics of DSSCs made from P25 and calcined flakes of similar thickness under AM 1.5 simulated sunlight irradiation.
