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

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

/PVP nanofibers (a) before calcination and (b) after calcination process at 500°C during 3 h [30].

eter 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.

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

and followed by calcination process at 500°C. The average diam-

 **nanostructured photoactivity**

has a wide range of applications, however

of PVP and amorphous TiO<sup>2</sup>

**Figure 9.** SEM image of TiO<sup>2</sup>

376 Titanium Dioxide - Material for a Sustainable Environment

**4. Strategies for improving TiO2**

As mentioned in the previous sections, 1D-TiO<sup>2</sup>

Incorporation selective doping of metal ion into 1D-TiO<sup>2</sup> nanostructured materials has been proven an efficient route to improve visible light absorption with hindered charge carrier recombination rate. The presence of transition metal ion in the structure of 1D-TiO<sup>2</sup> nanostructured 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 VB) of TiO<sup>2</sup> nanostructures [34, 35]. Wang et al. [36] investigated the effect of Fe, Mn and Co as dopants on the photoelectrochemical cell performance of TiO<sup>2</sup> nanorods. The maximum photocurrent density of 2.92 mA/cm<sup>2</sup> at 0.25 V vs. Ag/AgCl for Fe/TiO<sup>2</sup> , which is five times higher than that of undoped TiO<sup>2</sup> confirmed the presence of Fe into TiO<sup>2</sup> is the most favorable metal to improve the photocatalytic activity of TiO<sup>2</sup> compared to others. **Figure 10a** shows that the photocurrent density of Fe-TiO<sup>2</sup> is as high as 0.96 mA/cm<sup>2</sup> at 0.25 V vs. Ag/AgCl under visible light illumination (>420 nm). Incident-photon-to-current-conversion (IPCE) efficiency (up to 18%) measurements reveal that the Fe-TiO<sup>2</sup> nanorod sample significantly improves the photoresponse not only in the UV region but also in the visible light region, as illustrated in **Figure 10b**.

**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 1 M KOH solution [36].

Moreover, doping non-metal atoms such as nitrogen, sulfur carbon, boron and iodine can extend the absorbance to the visible region and improve the stability of materials through the simple and effective method [37]. For instance, TiO<sup>2</sup> /graphene composites were synthesized by Xing et al. [38] through hydrothermal method by decorating Ti3+ self-doped TiO<sup>2</sup> nanorods on boron-doped graphene sheets, in which NaBH4 acted as reducing agent and sources of boron dopant on graphene. The produced TiO<sup>2</sup> nanorods had the length of 200 nm with exposed (100) and (010) facets as shown in **Figure 11a**.

separation of electron-hole pairs (**Figure 11c**). All of the composites tested exhibited improved photocatalytic activities as measured by the degradation of methylene blue and phenol under visible light irradiation. This better photocatalytic activity was attributed to the synergistic

energy conversion system have received significant attentions. The charge transfer from one semiconductor to another with suitable band edge positions is thermodynamically favorable to increase the lifetime of the charge carriers thus promoting the interfacial charge transfer and

by successive ionic layer adsorption and reaction (SILAR) method for visible-light-driven hydrogen production and organic compound degradation. CdS has narrow bandgap (∼2.40 eV) and relatively high visible absorption coefficient of CdS enables its highly desirable use

dominating the charge separation. **Figure 10** shows the schematic diagram of charge transfer in CdS-TNTAs photocatalyst for visible-light photocatalysis. Therefore, this charge transfer can accelerate the separation of charge carriers and enhance the visible-light response and

 **nanostructures**

nanostructured materials are widely used as photocatalysts due to its high oxidation

water splitting, solar cells, supercapacitors and lithium batteries. This section is dedicated to

The first sensitization of large bandgap energy toward the visible region was reported in 1972 with ZnO semiconductor with the photoconversion efficiency of 1–2.5%. Gratzel et al. reported a breakthrough in the efficiency over 7% in 1991 using large surface area nano-crys-

**Figure 13a** and **b** displays a schematic presentation of DSSC and its operation principle. It

a monolayer of sensitizer in contact with iodide/tri-iodide redox electrolyte, which is sandwiched by second conductive glass covered with platinum as a counter electrode (CE). The

on the surface of FTO (F-doped tin oxide) glass substrate with thickness 5–20 μm and covered

helps to better absorb and attach dye on the surface of TiO<sup>2</sup>

most efficient DSSC had the highly mesoporous of anatase phase of TiO<sup>2</sup>

thin film, sensitizing with ruthenium complex. They explained that high surface

generation and Rh B degradation of CdS-TNTAs.

nanostructures have been paid much attention to photocata-

thin film as a working electrode (WE) or photoanode with

nanostructures in photocatalytic water splitting and dye sensitized

The fabrication, design, and tailoring of coupling other semiconductors like CdS, Cu<sup>2</sup>

and boron-doped graphene.

nanostructures to achieve better charge carrier separation in a light

One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices

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

O, CdSe,

379

nanotube arrays (TNTAs)

can hamper electron-hole recombination that

reduction into energy fuels, photocatalytic

thin film [40, 41].

which was coated

effect between Ti3+ self-doped TiO<sup>2</sup>

, etc. with 1D-TiO2

photocatalytic activity for H<sup>2</sup>

**5. Applications of 1D-TiO2**

and reduction ability. 1D-TiO2

the application of 1D-TiO<sup>2</sup>

**5.1. Dye sensitized solar cell**

includes nano-crystalline TiO2

solar cells (**Figure 12**).

talline TiO2

area of TiO<sup>2</sup>

 **with semiconductors**

catalytic efficiency [34, 39]. Zhu et al. [39] deposited CdS on TiO<sup>2</sup>

in solar applications. Coupling CdS with TiO<sup>2</sup>

lytic degradation of pollutants, photocatalytic CO<sup>2</sup>

**4.2. Coupling TiO2**

WO3

TiO2

The loading of TiO<sup>2</sup> nanorods on graphene sheets was characterized by TEM (**Figure 11b**), and confirmed TiO<sup>2</sup> nanoparticles were covalent bonded to GO, forming a composite favoring the

**Figure 11.** (a) FESEM of TiO<sup>2</sup> nanorods, (b) TEM images of TiO<sup>2</sup> /GR composite, and (c) schematic diagram of the charge transfer of TiO<sup>2</sup> -x/GR composite [38].

separation of electron-hole pairs (**Figure 11c**). All of the composites tested exhibited improved photocatalytic activities as measured by the degradation of methylene blue and phenol under visible light irradiation. This better photocatalytic activity was attributed to the synergistic effect between Ti3+ self-doped TiO<sup>2</sup> and boron-doped graphene.

#### **4.2. Coupling TiO2 with semiconductors**

Moreover, doping non-metal atoms such as nitrogen, sulfur carbon, boron and iodine can extend the absorbance to the visible region and improve the stability of materials through

sized by Xing et al. [38] through hydrothermal method by decorating Ti3+ self-doped TiO<sup>2</sup>

nanorods on graphene sheets was characterized by TEM (**Figure 11b**), and

nanoparticles were covalent bonded to GO, forming a composite favoring the

/graphene composites were synthe-

nanorods had the length of 200 nm

/GR composite, and (c) schematic diagram of the charge

acted as reducing agent and

the simple and effective method [37]. For instance, TiO<sup>2</sup>

378 Titanium Dioxide - Material for a Sustainable Environment

sources of boron dopant on graphene. The produced TiO<sup>2</sup>

The loading of TiO<sup>2</sup>

**Figure 11.** (a) FESEM of TiO<sup>2</sup>


transfer of TiO<sup>2</sup>

nanorods, (b) TEM images of TiO<sup>2</sup>

confirmed TiO<sup>2</sup>

with exposed (100) and (010) facets as shown in **Figure 11a**.

nanorods on boron-doped graphene sheets, in which NaBH4

The fabrication, design, and tailoring of coupling other semiconductors like CdS, Cu<sup>2</sup> O, CdSe, WO3 , etc. with 1D-TiO2 nanostructures to achieve better charge carrier separation in a light energy conversion system have received significant attentions. The charge transfer from one semiconductor to another with suitable band edge positions is thermodynamically favorable to increase the lifetime of the charge carriers thus promoting the interfacial charge transfer and catalytic efficiency [34, 39]. Zhu et al. [39] deposited CdS on TiO<sup>2</sup> nanotube arrays (TNTAs) by successive ionic layer adsorption and reaction (SILAR) method for visible-light-driven hydrogen production and organic compound degradation. CdS has narrow bandgap (∼2.40 eV) and relatively high visible absorption coefficient of CdS enables its highly desirable use in solar applications. Coupling CdS with TiO<sup>2</sup> can hamper electron-hole recombination that dominating the charge separation. **Figure 10** shows the schematic diagram of charge transfer in CdS-TNTAs photocatalyst for visible-light photocatalysis. Therefore, this charge transfer can accelerate the separation of charge carriers and enhance the visible-light response and photocatalytic activity for H<sup>2</sup> generation and Rh B degradation of CdS-TNTAs.

#### **5. Applications of 1D-TiO2 nanostructures**

TiO2 nanostructured materials are widely used as photocatalysts due to its high oxidation and reduction ability. 1D-TiO2 nanostructures have been paid much attention to photocatalytic degradation of pollutants, photocatalytic CO<sup>2</sup> reduction into energy fuels, photocatalytic water splitting, solar cells, supercapacitors and lithium batteries. This section is dedicated to the application of 1D-TiO<sup>2</sup> nanostructures in photocatalytic water splitting and dye sensitized solar cells (**Figure 12**).
