Category Oxide Semiconductors. for Solar Energy. Conversion

Acceptor-Doped Ti02

Chromium has been commonly applied as an acceptor-type dopant for TiO2. Its ionic radius (0.052 nm) is smaller than that of the host Ti4+ ion (0.068 nm).

The incorporation of trivalent ions, such as Cr, into the Ti sites of the TiO2 lattice leads to the formation of acceptors. These result in reducing the concentration of electrons leading, ultimately, to the conversion of n-type TiO2 into p-type TiO2.

[13] Equilibration of the TiO2-based solid solution with the gas phase of well – defined oxygen activity

• Incorporation of controlled amount of foreign ions into the TiO2 lattice, leading to the formation of donors and acceptors

• Controlled cooling, including controlled rate of cooling and well defined oxygen activity in the gas phase

[14] Hydrogen...

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Research Progress and Perspectives

The increasingly apparent effects of global warming and resulting climate changes are forcing us to increase the use of renewable energy, such as solar energy. The global efforts in this directions aim to develop new energy conversion systems. The TiO2-based semiconductors are the promising candidates for the development of solar energy conversion systems with versatile applications. However, the research in development of high-performance systems for the modern day technology is mul­tidisciplinary. Therefore, the progress in the area of photocatalysis requires bringing

SOLAR HYDROGEN

Area of the solar panel required to cover all Australia’s energy needs

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FIGURE 8...

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ALTERNATIVE APPLICATIONS

The photocatalytic effects of TiO2 have resulted in a wide range of its environmen­tally friendly applications (alternative to photocatalytic water purification and the generation of solar hydrogen fuel), including [1,126]: •

• Antiseptic Coatings for Sanitary Areas. The antiseptic properties of TiO2 coatings are related to the light-induced bactericidal effect. The antiseptic coating can be applied in sanitary areas and hospitals.

• Antifogging Coatings on Glasses. Antifogging activity is related to the superhydrophilicity of TiO2- coated surfaces.

• Deodorizing Coatings. Deodorizing effects are related to the strong oxida­tion power of TiO2 leading to destruction of airborne organic compounds.

• Biological Effects...

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Oxidation of Microorganisms

The change of the population of microorganisms in water depends on a number of factors, including:

• Temperature

• Intensity and energy of light

• Composition of the aqueous environment

• Alkalinity/acidity

• Concentration of oxygen and oxidants

• Exchange of mass (turbulence)

The reproducibility of data requires that all the factors influencing the photocata­lytic process are well defined (standardized).

The photocatalytic water disinfection may be represented schematically by the concentration of bacteria versus time in the presence and absence of photocatalyst, such as TiO2, and light (Figure 8.39):

• Absence of both light and TiO2 (Figure 8.39a). In this case the bacteria have the tendency to multiply.

• Presence of TiO2 and absence of light (Figure 8.39b)...

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Unresolved Problems

While the search for a high-performance photocatalyst intensifies, the technology at this stage is still remote from the commercial requirements. The major hurdles include:

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FIGURE 8.35 Kinetics of E. coli bacteria disinfection for undoped and N-doped TiO2, according to Liu et al. [10]. (Reproduced with permission from Y Liu, J Li, X Giu, C Burda, Novel TiO2 nanocatalyst for wastewater purification: Tapping energy from the sun, Water Science & Technology, 54 (2006) 47-54. Copyright IWA Publishing.)

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FIGURE 8.36 Reflectance spectra for undoped and N-doped TiO2, according to Liu et al. [10]. (Reproduced with permission form Y Liu, J li, X Giu, C Burda, Novel TiO2 nanocatalyst for wastewater purification: Tapping energy from the sun, Water Science & Technology, 54 (2006) 47-54...

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Doping with Aliovalent Ions

Doping of TiO2 with aliovalent ions is the current major research strategy in the development of high-performance photocatalysts. The aim is to increase absorption of sunlight by reduction of the band gap through the incorporation of aliovalent ions [8,9-11, 71,97]. A range of anions (nitrogen, fluorine, carbon) and cations has been applied for doping [98,99].

Liu et al. [10] reported that TiO2 doping with nitrogen results in an enhanced photocatalytic activity in the inactivation of E. coli bacteria and the decolorization of acid orange. Their data on the effect of nitrogen doping (>20 at% N) on pho­tocatalytic activity was compared to that of Degussa P25 (80% anatase and 20% rutile). The observed enhancement in inactivation of the E. coli bacteria is shown in Figure 8.35...

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Modification of Photocatalytic Properties

8.5.3.1 Deposition of Noble Metals

A substantial number of reports have been accumulated on the effect of noble metals, such as Pt, on the photocatalytic performance of TiO2 [81-92]. This data indicates that the photocatalytic activity depends on surface dispersion of Pt, forming small islets, and the related ratio of the Pt/TiO2 areas.

Sakthvitel et al. [91] reported photocatalytic oxidation of dichloroacetic acid at dif­ferent Pt load (the studies also included Au and Pd). The data reported by Sakthvitel et al. [91] on the effect of Pt on photocatalytic properties is represented in Figure 8.33. As shown in Figure 8.34, the Pt-activated TiO2 catalysts exhibit optimum per­formance at 0.8 wt% of the Pt load.

In most cases platinum results in enhanced performance, which is consistent with ...

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Cathodic Site

The primary photo-induced reaction at the cathodic site is the reduction of molecular oxy­gen (Figures 8.30 and 8.32) [83, 95-97]. The progress of the cathodic reaction depends on the supply of oxygen to cathodic sites. The concentration of electrons in n-type semi­conductors is relatively high. Consequently, the supply of oxygen to the cathodic site is critical for the performance of the photocatalyst [95]. Enhanced oxygen supply may be achieved by passing a gas rich in oxygen through water (aeration). Alternatively, an enhanced oxygen supply may be achieved by adding an oxidant to water.

Reduction of oxygen, which is the most important reactive agent at the cathodic site, leads to the formation of superoxide species:

O2 + e’^ O – (8.21)

The superoxide species may react with protons, le...

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