**3.3. Other synthesis method**

Sol-gel method is another solution-based growth technique, offering several major advantages for mass production of nanomaterials including low-cost, simple processing, and good scalability. This technique is typically conducted through two steps; sol preparation, including mixing the precursors such as metal organic compounds or inorganic metal salts through vigorous stirring to complete hydrolysis and polymerization reaction and gel preparation by removing solvent and converting the sol to a three-dimensional network [27]. This technique is mainly useful for synthesizing oxide ceramic nanomaterials from hydrolyzing titanium precursors. Through sol-gel process, TiO<sup>2</sup> NPs can be aligned following their crystal orientations and form NWs. For example, Rodríguez-Reyes et al. prepared nanocrystalline TiO<sup>2</sup> wires (**Figure 7a**–**c**) by the sol-gel method, using titanium isopropoxide (TIP) and acetic acid as a TiO2 sol modifier in alcohol solvent showed to be a successful synthesis route of Ti─O─Ti inorganic network with controlled properties [28].

**Figure 7.** HR-SEM images of TiO<sup>2</sup> calcined at different temperatures (a) 400°C, (b) 500°C, and (c) 600°C [28].

preparation of a single-layer polycrystalline anatase TiO<sup>2</sup>

et al. [25] successfully synthesized the nitrogen-fluorine co-doped TiO<sup>2</sup>

lytic degradation of methyl orange compared to commercial TiO<sup>2</sup>

useful in photoelectrical applications of the solvothermal method.

[26] reported the synthesis of TiO<sup>2</sup>

**Figure 6.** SEM images of various TiO<sup>2</sup>

374 Titanium Dioxide - Material for a Sustainable Environment

and (c) nanorods.

**3.3. Other synthesis method**

as a TiO2

precursors. Through sol-gel process, TiO<sup>2</sup>

inorganic network with controlled properties [28].

(SLP TiO<sup>2</sup>

morphologies synthesized by solvothermal method: (a) nanosheets, (b) nanobelts,

nanorod arrays (TNRs) directly on FTO glass (**Figure 6c**)

NPs can be aligned following their crystal orien-

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

with anatase phase structure by the solvothermal method, which employs amorphous titania microspheres as the precursor. Results demonstrate a significantly enhanced photocata-

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

Sol-gel method is another solution-based growth technique, offering several major advantages for mass production of nanomaterials including low-cost, simple processing, and good scalability. This technique is typically conducted through two steps; sol preparation, including mixing the precursors such as metal organic compounds or inorganic metal salts through vigorous stirring to complete hydrolysis and polymerization reaction and gel preparation by removing solvent and converting the sol to a three-dimensional network [27]. This technique is mainly useful for synthesizing oxide ceramic nanomaterials from hydrolyzing titanium

tations and form NWs. For example, Rodríguez-Reyes et al. prepared nanocrystalline TiO<sup>2</sup> wires (**Figure 7a**–**c**) by the sol-gel method, using titanium isopropoxide (TIP) and acetic acid

sol modifier in alcohol solvent showed to be a successful synthesis route of Ti─O─Ti

) nanosheets **Figure 6a**

nanobelts (**Figure 6b**)

. Zhao and his co-workers

**Figure 8.** (a) Schematic diagram of the cold wall MOCVD aperture and (b) SEM images of vertically aligned and densely packed TiO<sup>2</sup> NRs grown on sapphire (100) substrate.

The apparent 1-D morphology of TiO<sup>2</sup> -related nanowires was thermally stable from 400 to 600°C, showing a similar diameter (about 76 nm); however, crystallite size increases with respect to temperature from 13 to 75 nm.

Vapor deposition method has been developed to high degree of crystallinity 1D-TiO<sup>2</sup> nanostructures (usually single crystal TiO2 ) and it can be classified in chemical vapor deposition (CVD) and physical vapor deposition (PVD) [27]. Chen et al. [29] grown well-aligned densely-packed rutile TiO<sup>2</sup> nanorod via metal-organic chemical vapor deposition (MOCVD) (**Figure 8a**), using titanium-tetraisopropoxide (TTIP, Ti(OC<sup>3</sup> H7 ) 4 ) as a source reagent at a deposition temperature of 550°C and under an oxygen pressure of 1.5 and 5 mbar, respectively. The rutile TiO2 nanorods (**Figure 8b**) were grown with a very high density and exhibited uniform height. However, this method requires expensive equipment and the cost is too high for mass production.

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

single material, which can match all above criteria [1, 6, 32, 33]. Here, we reviewed the developed strategies on bandgap engineering of titania to extend light absorption into visible light through doping metal and non-metal ions and compositing with another semiconductor for synergic absorption and charge separation for enhanced utilization of solar energy [34].

One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices

proven an efficient route to improve visible light absorption with hindered charge carrier

structured materials increases the formation of Ti3+ ions, leading to improve photocatalytic activity, owing to the existence of more oxygen defects, which facilitate the efficient adsorption of oxygen on the titania surface. In addition, the substitution of metal ions into the TiO<sup>2</sup> induces visible light absorption because of introducing intraband state close to the CB or VB edge and charge transfer transition between the **d** electrons of the dopant and the CB (or

nanostructures [34, 35]. Wang et al. [36] investigated the effect of Fe, Mn and Co

confirmed the presence of Fe into TiO<sup>2</sup>

is as high as 0.96 mA/cm<sup>2</sup>

visible light illumination (>420 nm). Incident-photon-to-current-conversion (IPCE) efficiency

photoresponse not only in the UV region but also in the visible light region, as illustrated in

**Figure 10.** (a) Photocurrent density vs. applied potential curves of four nanorod photoanodes under visible light illumination >420 nm and (b) IPCE spectra of nanorod photoanodes measured at an applied bias of 0.6 V (vs. RHE) in

at 0.25 V vs. Ag/AgCl for Fe/TiO<sup>2</sup>

recombination rate. The presence of transition metal ion in the structure of 1D-TiO<sup>2</sup>

nanostructured materials has been

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

nanorods. The maximum

at 0.25 V vs. Ag/AgCl under

compared to others. **Figure 10a** shows

nanorod sample significantly improves the

, which is five times

is the most favorable

nano-

377

**4.1. Doping with metal and non-metal ions**

photocurrent density of 2.92 mA/cm<sup>2</sup>

that the photocurrent density of Fe-TiO<sup>2</sup>

higher than that of undoped TiO<sup>2</sup>

VB) of TiO<sup>2</sup>

**Figure 10b**.

1 M KOH solution [36].

Incorporation selective doping of metal ion into 1D-TiO<sup>2</sup>

as dopants on the photoelectrochemical cell performance of TiO<sup>2</sup>

metal to improve the photocatalytic activity of TiO<sup>2</sup>

(up to 18%) measurements reveal that the Fe-TiO<sup>2</sup>

**Figure 9.** SEM image of TiO<sup>2</sup> /PVP nanofibers (a) before calcination and (b) after calcination process at 500°C during 3 h [30].

a straightforward electrohydrodynamical mechanism to produce fibers with diameters less than 100 nm, even up to 5 nm. Under the influence of an electric field, a pendant droplet of the polymer solution at the spinneret is deformed into a conical shape. The post heat treatment is usually needed to remove the solvent and solidify the fiber structures. The viscosity, conductivity, and applied solvents, as well as the conformation and molecular weight of the polymer limit the electrospun ability of a polymer solution. Some polymers are not spinnable because of limited solubility in a proper solvent for electrospinning, having proper polar characteristics [31]. In addition, electrospinning is an efficient method for mass production however; the high resistance caused by the polycrystalline characteristics of the product nanofibers limits its applications. **Figure 9** shows that synthesized anatase nanofibers using electrospinning technique by Li and Xia [30]. They injected ethanol solution including poly(vinyl pyrrolidone) (PVP) and titanium tetraisopropoxide through a needle under a strong electrical field. The final product was the composite nanofibers with lengths up to several centimeters, consisting of PVP and amorphous TiO<sup>2</sup> and followed by calcination process at 500°C. The average diameter of nanofibers was varied from 20 to 200 nm owing to changing a number of parameters like ratio between PVP and titanium tetraisopropoxide, their concentrations in the alcohol solution, the strength of the electric field, and the feeding rate of the precursor solution.

#### **4. Strategies for improving TiO2 nanostructured photoactivity**

As mentioned in the previous sections, 1D-TiO<sup>2</sup> has a wide range of applications, however in this book chapter, we intend to address only the photocatalytic applications including photocatalytic degradation and photocatalytic solar hydrogen production. Nanostructure materials need to meet some requirements for photocatalytic application; (i) should possess a sufficiently large active surface area, (ii) have a broad light absorption band to utilize the full range of solar spectrum, and (iii) should be an effective charge carrier separation to transfer more electron and hole to the interface of the electrode/electrolyte. Nevertheless, there is no single material, which can match all above criteria [1, 6, 32, 33]. Here, we reviewed the developed strategies on bandgap engineering of titania to extend light absorption into visible light through doping metal and non-metal ions and compositing with another semiconductor for synergic absorption and charge separation for enhanced utilization of solar energy [34].
