**5.1. Dye sensitized solar cell**

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-crystalline TiO2 thin film, sensitizing with ruthenium complex. They explained that high surface area of TiO<sup>2</sup> helps to better absorb and attach dye on the surface of TiO<sup>2</sup> thin film [40, 41]. **Figure 13a** and **b** displays a schematic presentation of DSSC and its operation principle. It includes nano-crystalline TiO2 thin film as a working electrode (WE) or photoanode with 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 most efficient DSSC had the highly mesoporous of anatase phase of TiO<sup>2</sup> which was coated on the surface of FTO (F-doped tin oxide) glass substrate with thickness 5–20 μm and covered

**Figure 12.** SEM image and schematic diagram of charge transfer in CdS-TNTAs photocatalyst for visible-light photocatalysis [39].

the atmosphere as major threats to the environment and human health. Hydrogen has been established as a clean energy carrier in many applications such as automotive, domestic heating, aircrafts and stationary power generation. Utilizing solar energy to split water into H<sup>2</sup>

**Figure 14.** (a) Structure of multilayer DSSC and (b) I-V curves of DSSC with single to five-layer photoelectrodes [42].

in terms of hydrogen production regardless of its limitations [43, 44]. The vertically oriented

due to their impressive vectorial 1D-channel pathways for fast electron transport in the axial direction. Shinde et al. [45] synthesized nanocomposite heterojunction photoanode involving

nanostructure are promising materials for photocatalytic solar hydrogen production

NT photoanode and CdS-NF/TiO<sup>2</sup>

cell. Amongst different metal oxide photocatalysts, TiO<sup>2</sup>

**Figure 15.** Surface FESEM images of (a) annealed TiO<sup>2</sup>

NT array, (d) annealed CdS on annealed TiO<sup>2</sup>

heterojunction of CdS NFs and TiO<sup>2</sup>

with different annealing conditions: (b) as-grown CdS on as-grown TiO<sup>2</sup>

NT array [45].

 in photoelectrochemical (PEC) cell is in fact one of the most promising technologies for hydrogen production. It would be interesting to combine solar energy and water in PEC cell to produce truly renewable and low environmental impact fuel on both large and small scale. A photocatalyst is the core of this system with some requirements like an optimal bandgap energy approximately 2 eV and a sufficient negative CB position [6]. Up to date, there has not been found any photocatalyst that meets all requirements for hydrogen production in the PEC

One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices

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381

is an attractive n-type photocatalyst


NT array, (c) as-grown CdS on annealed

NT array, and (e) the schematic of charge transfer mechanism in

and O2

1D-TiO2

TiO2

by a monolayer of sensitizer. To overcome the drawback of TiO<sup>2</sup> nanoparticles, a 1D-TiO<sup>2</sup> nanostructure is often applied as photoanode to enhance electron transfer ability. **Figure 14a** shows a novel TiO2 nanoparticles and TiO<sup>2</sup> nanotube (TNP/TNA) multilayer photoelectrode via a layer-by-layer assembly process to improve the DSSC performance as reported by Yang et al. [42]. The fabricated DSSC with multilayer photoelectrode has higher efficiency than the single-layer or bare DSSCs. The TNP/TNA four-layer photoelectrode provided a large surface area for dye adsorption with the highest photocurrent density (**Figure 14b**) and maximum photoconversion efficiency of 7.22% because of effective electron transport.

#### **5.2. Photocatalytic solar hydrogen production**

Energy dense fossil fuels are non-renewable source and the most coveted fuel that have ever been discovered, burning fossil fuels release such significant amount of greenhouse gases in

**Figure 13.** (a) Schematic diagram and (b) principle of operation and energy level of DSSC.

One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices http://dx.doi.org/10.5772/intechopen.72976 381

**Figure 14.** (a) Structure of multilayer DSSC and (b) I-V curves of DSSC with single to five-layer photoelectrodes [42].

by a monolayer of sensitizer. To overcome the drawback of TiO<sup>2</sup>

photoconversion efficiency of 7.22% because of effective electron transport.

**Figure 13.** (a) Schematic diagram and (b) principle of operation and energy level of DSSC.

nanoparticles and TiO<sup>2</sup>

**5.2. Photocatalytic solar hydrogen production**

380 Titanium Dioxide - Material for a Sustainable Environment

shows a novel TiO2

photocatalysis [39].

nanostructure is often applied as photoanode to enhance electron transfer ability. **Figure 14a**

**Figure 12.** SEM image and schematic diagram of charge transfer in CdS-TNTAs photocatalyst for visible-light

via a layer-by-layer assembly process to improve the DSSC performance as reported by Yang et al. [42]. The fabricated DSSC with multilayer photoelectrode has higher efficiency than the single-layer or bare DSSCs. The TNP/TNA four-layer photoelectrode provided a large surface area for dye adsorption with the highest photocurrent density (**Figure 14b**) and maximum

Energy dense fossil fuels are non-renewable source and the most coveted fuel that have ever been discovered, burning fossil fuels release such significant amount of greenhouse gases in

nanoparticles, a 1D-TiO<sup>2</sup>

nanotube (TNP/TNA) multilayer photoelectrode

the atmosphere as major threats to the environment and human health. Hydrogen has been established as a clean energy carrier in many applications such as automotive, domestic heating, aircrafts and stationary power generation. Utilizing solar energy to split water into H<sup>2</sup> and O2 in photoelectrochemical (PEC) cell is in fact one of the most promising technologies for hydrogen production. It would be interesting to combine solar energy and water in PEC cell to produce truly renewable and low environmental impact fuel on both large and small scale. A photocatalyst is the core of this system with some requirements like an optimal bandgap energy approximately 2 eV and a sufficient negative CB position [6]. Up to date, there has not been found any photocatalyst that meets all requirements for hydrogen production in the PEC cell. Amongst different metal oxide photocatalysts, TiO<sup>2</sup> is an attractive n-type photocatalyst in terms of hydrogen production regardless of its limitations [43, 44]. The vertically oriented 1D-TiO2 nanostructure are promising materials for photocatalytic solar hydrogen production due to their impressive vectorial 1D-channel pathways for fast electron transport in the axial direction. Shinde et al. [45] synthesized nanocomposite heterojunction photoanode involving

**Figure 15.** Surface FESEM images of (a) annealed TiO<sup>2</sup> NT photoanode and CdS-NF/TiO<sup>2</sup> -NT photoanodes prepared with different annealing conditions: (b) as-grown CdS on as-grown TiO<sup>2</sup> NT array, (c) as-grown CdS on annealed TiO2 NT array, (d) annealed CdS on annealed TiO<sup>2</sup> NT array, and (e) the schematic of charge transfer mechanism in heterojunction of CdS NFs and TiO<sup>2</sup> NT array [45].

CdS nanoflowers (NFs) and one-dimensional TiO<sup>2</sup> nanotube (TNT) arrays. An anodization method was employed for fabrication of TiO<sup>2</sup> nanotube (TNT) arrays and CdS NFs was decorated on the surface of TNT using hydrothermal method as shown in **Figure 15a**–**d**. As-grown CdS-NF/TiO<sup>2</sup> -NT array photoanode exhibited a 5.5-fold photocurrent enhancement in a polysulfide electrolyte compared to the pristine TiO<sup>2</sup> NT photoanode. Annealing of TiO<sup>2</sup> NTs as well as CdS NFs led to further improvement in the photocurrent owing to greater crystallinity, significantly higher visible light photon absorption and improved interface properties between CdS and TiO<sup>2</sup> . The better photocatalytic performance of CdS-NF/TiO<sup>2</sup> -NT was attributed to effective absorption of the visible light photons, leading to the photo-generation of electron-hole pairs and greater charge carrier separation as shown in **Figure 15e**.

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