**3. Applications of** *n***-type semiconductors**

In the last decades photocatalytic processes induced by semiconductors have attracted the great interest, due to their low environmental impact. Nowadays, many *n*-type semiconductors are studied and applied in several application fields, such as energy production, smart-materials technology, environment depollution, chemical synthesis,

Semiconductors in Organic Photosynthesis 85

Nowadays, due to its most photoactivity, highest stability, low cost and non-toxicity, TiO2 is most probably the only photocatalyst suitable for heavy industrial applications (Hashimoto et al., 2005; Ravelli et al., 2011). In the last 30 years various research groups have developed several photocatalityc processes in which it plays a key role as catalyst. Generally those applications regard the photo-induced redox reactions of adsorbed substances, the photoinduced hydrophilic and/or hydrophobic behaviour of TiO2 itself and the use as white

Water reduction to H2 and oxidation to O2 requires that the bottom of the conduction band lies at a more negative potential than E°red(H+/H2), 0 V, and the top of the valence band at a more positive value than E°ox (H2O/O2), 1.23 V. The two-electron process from one mol of H2O to give one mol of H2 and half mol of O2 is termodinamically unfavored, DG° = 237 kJ/mol = 2.46 eV. Thus, the minimum energy required to drive the reaction corresponds to that of two photons of 1.23 eV, corresponding to l ca. 1000 nm, in the near infrared region. However, to overpass the activation barrier, higher photon energy is required in order to promote water splitting at a reasonable rate. Water splitting for hydrogen production aroused a great interest during the second oil crisis, in the 1970s. In this period Fujishima and Honda reported the possibility to easily obtain H2 and O2 using sunlight as energy

pigment from ancient times (Anpo & Kamat, 2010; Ohama & Gemert, 2011).

Fig. 6. Schematic representation of water splitting process at pH = 1

Nocera, 2007; Li et al., 2010; Maeda & Domen, 2007; Zhang et al., 2010).

Then, photocatalysis drew the attention of many researchers as one of the promising methods for hydrogen production. However, despite the high reaction efficiency, TiO2 can adsorb only the UV light present in a solar spectrum. This means that only about 3% of solar light was effective on the reaction and, in few years, from the productive technological point view, TiO2 photocatalysis lost its initial actractive. Thus, in the last 20 years, many efforts have been made in order to improve the photocatalytic performance of TiO2 by different approaches, such as doping/mixing with several other metals and semiconductors, like Pt, Cr, Ce, Ag, CdS, CdSe, use of zeolites and nanostructured TiO2 nano-arrays, and more (Balzani et al., 2008; Esswein &

**3.2 Applications of TiO2**

**3.2.1 Water splitting** 

source (Fig. 6) (Hashimoto et al., 2005).

whereas *p*-type semiconductors are rarely used because of their limited presence in nature and their usually too small band-gap (Palmisano et al., 2007a; Mills & Lee, 2002; Mills & Le Hunte, 1997). Certainly, the most studied and widely used is the titanium dioxide (TiO2), which can be considered the "king" of the photocatalysts.

### **3.1 TiO2: King of the photocatalysts**

Between the various semiconductors applied in photocatalysis, certainly TiO2 is the most used. The reasons for its widespread are due to its high environmental tolerance, large commercially availability and low price.

Its history began just in the late 60th dacades when Fujishima and Honda began to study the photoelectrolysis of water. The first paper, published in 1972, brought the world aware of its photo-catalytic potential (Fujishima & Honda, 1972). This important observation promoted extensive works focused on the solar energy conversion for the production of hydrogen from water. Moreover, it soon became apparent that novel redox reactions of organic and inorganic substrates could also be induced by band-gap irradiation of a variety of semiconductor particles, of sizes ranging from clusters and colloids to powders and large single crystals.

TiO2 is present in the three common allotropic forms: anatase, rutile and brookite: anatase and rutile belong to thetragonal crystal system, while brookite belongs to orthorhombic system (Linsebigler et al., 1995). Anatase and rutile are the two principal catalytic forms. Commercially available anatase is typically less than 50nm in size. These particles have a band-gap of 3.2 eV, corresponding to a UV wavelength of 385 nm. The adsorptive affinity of anatase for organic compounds is higher than that of rutile, and anatase exhibits lower rates of recombination in comparison to rutile due to its 10-fold greater rate of hole trapping. In contrast, though some exceptions exist, the thermodynamically stable rutile phase generally exists as particles larger than 200 nm. Rutile has a smaller band-gap of 3.0 eV with excitation wavelengths that extend into the visible at 410 nm. Nevertheless, anatase is generally regarded as the more photochemically active phase of titania, presumably due to the combined effect of lower rates of recombination and higher surface adsorptive capacity.

In the last decade, Gray and co-workers reported the enhanced photoactivity in the mixedphase (anatase and rutile) Degussa P25 TiO2 (Deanna et al., 2003). This fact is explained by the presence of small rutile crystallites, which creates a structure where rapid electron transfer from rutile to lower energy anatase lattice trapping sites under visible illumination, leads to a more stable charge separation. Transfer of the photogenerated electron to anatase lattice trapping sites allows holes that would have been lost to recombination to reach the surface. Subsequent electron-transfer moves the electron from anatase trapping sites to surface trapping sites, further separating the electron/hole pair. By competing with recombination, the stabilization of charge separation activates the catalyst and the rutileoriginating hole can participate in oxidative chemistry. Three main factors are employed in this increase in the photoactivity: (1) the smaller band-gap of rutile extends the useful range of photoactivity into the visible region; (2) the stabilization of charge separation by electron transfer from rutile to anatase slows recombination; (3) the small size of the rutile crystallites facilitates this transfer, making catalytic hot spots at the rutile/anatase interface. This process depends critically on the interface between the TiO2 phases and particle size. The atypically small size of the rutile particles in this formulation, and the intimate contact with anatase that the comparable size allows, are crucial to enhancing the catalyst activity.

### **3.2 Applications of TiO2**

84 Artificial Photosynthesis

whereas *p*-type semiconductors are rarely used because of their limited presence in nature and their usually too small band-gap (Palmisano et al., 2007a; Mills & Lee, 2002; Mills & Le Hunte, 1997). Certainly, the most studied and widely used is the titanium dioxide (TiO2),

Between the various semiconductors applied in photocatalysis, certainly TiO2 is the most used. The reasons for its widespread are due to its high environmental tolerance, large

Its history began just in the late 60th dacades when Fujishima and Honda began to study the photoelectrolysis of water. The first paper, published in 1972, brought the world aware of its photo-catalytic potential (Fujishima & Honda, 1972). This important observation promoted extensive works focused on the solar energy conversion for the production of hydrogen from water. Moreover, it soon became apparent that novel redox reactions of organic and inorganic substrates could also be induced by band-gap irradiation of a variety of semiconductor particles, of sizes ranging from clusters and colloids to powders and large

TiO2 is present in the three common allotropic forms: anatase, rutile and brookite: anatase and rutile belong to thetragonal crystal system, while brookite belongs to orthorhombic system (Linsebigler et al., 1995). Anatase and rutile are the two principal catalytic forms. Commercially available anatase is typically less than 50nm in size. These particles have a band-gap of 3.2 eV, corresponding to a UV wavelength of 385 nm. The adsorptive affinity of anatase for organic compounds is higher than that of rutile, and anatase exhibits lower rates of recombination in comparison to rutile due to its 10-fold greater rate of hole trapping. In contrast, though some exceptions exist, the thermodynamically stable rutile phase generally exists as particles larger than 200 nm. Rutile has a smaller band-gap of 3.0 eV with excitation wavelengths that extend into the visible at 410 nm. Nevertheless, anatase is generally regarded as the more photochemically active phase of titania, presumably due to the combined effect of lower rates of recombination and higher surface adsorptive capacity. In the last decade, Gray and co-workers reported the enhanced photoactivity in the mixedphase (anatase and rutile) Degussa P25 TiO2 (Deanna et al., 2003). This fact is explained by the presence of small rutile crystallites, which creates a structure where rapid electron transfer from rutile to lower energy anatase lattice trapping sites under visible illumination, leads to a more stable charge separation. Transfer of the photogenerated electron to anatase lattice trapping sites allows holes that would have been lost to recombination to reach the surface. Subsequent electron-transfer moves the electron from anatase trapping sites to surface trapping sites, further separating the electron/hole pair. By competing with recombination, the stabilization of charge separation activates the catalyst and the rutileoriginating hole can participate in oxidative chemistry. Three main factors are employed in this increase in the photoactivity: (1) the smaller band-gap of rutile extends the useful range of photoactivity into the visible region; (2) the stabilization of charge separation by electron transfer from rutile to anatase slows recombination; (3) the small size of the rutile crystallites facilitates this transfer, making catalytic hot spots at the rutile/anatase interface. This process depends critically on the interface between the TiO2 phases and particle size. The atypically small size of the rutile particles in this formulation, and the intimate contact with

anatase that the comparable size allows, are crucial to enhancing the catalyst activity.

which can be considered the "king" of the photocatalysts.

**3.1 TiO2: King of the photocatalysts** 

commercially availability and low price.

single crystals.

Nowadays, due to its most photoactivity, highest stability, low cost and non-toxicity, TiO2 is most probably the only photocatalyst suitable for heavy industrial applications (Hashimoto et al., 2005; Ravelli et al., 2011). In the last 30 years various research groups have developed several photocatalityc processes in which it plays a key role as catalyst. Generally those applications regard the photo-induced redox reactions of adsorbed substances, the photoinduced hydrophilic and/or hydrophobic behaviour of TiO2 itself and the use as white pigment from ancient times (Anpo & Kamat, 2010; Ohama & Gemert, 2011).

#### **3.2.1 Water splitting**

Water reduction to H2 and oxidation to O2 requires that the bottom of the conduction band lies at a more negative potential than E°red(H+/H2), 0 V, and the top of the valence band at a more positive value than E°ox (H2O/O2), 1.23 V. The two-electron process from one mol of H2O to give one mol of H2 and half mol of O2 is termodinamically unfavored, DG° = 237 kJ/mol = 2.46 eV. Thus, the minimum energy required to drive the reaction corresponds to that of two photons of 1.23 eV, corresponding to l ca. 1000 nm, in the near infrared region. However, to overpass the activation barrier, higher photon energy is required in order to promote water splitting at a reasonable rate. Water splitting for hydrogen production aroused a great interest during the second oil crisis, in the 1970s. In this period Fujishima and Honda reported the possibility to easily obtain H2 and O2 using sunlight as energy source (Fig. 6) (Hashimoto et al., 2005).

Fig. 6. Schematic representation of water splitting process at pH = 1

Then, photocatalysis drew the attention of many researchers as one of the promising methods for hydrogen production. However, despite the high reaction efficiency, TiO2 can adsorb only the UV light present in a solar spectrum. This means that only about 3% of solar light was effective on the reaction and, in few years, from the productive technological point view, TiO2 photocatalysis lost its initial actractive. Thus, in the last 20 years, many efforts have been made in order to improve the photocatalytic performance of TiO2 by different approaches, such as doping/mixing with several other metals and semiconductors, like Pt, Cr, Ce, Ag, CdS, CdSe, use of zeolites and nanostructured TiO2 nano-arrays, and more (Balzani et al., 2008; Esswein & Nocera, 2007; Li et al., 2010; Maeda & Domen, 2007; Zhang et al., 2010).

Semiconductors in Organic Photosynthesis 87

Nowadays, various self-cleaning materials are used in commercial applications and intense

During the studies on the self-cleaning surfaces, it was found a marked change in the water wettability of the TiO2 surface before and after UV light irradiation (Wuang et al., 1997). During the UV light exposition the contact angle of TiO2 coated surface showed a strong decrease from typically initial several tens of degrees (depending on the surface roughness) to nearly 0° (Sakai et al., 2003; White et al., 2003; Zubkov et al., 2005). This discover widened the commercial applications of TiO2 coated materials. The limitations of the self-cleaning efficiency, due to the low quantity of UV-light present in sunlight spectrum and outdoor shade, were resolved. The stains adsorbed onto TiO2 surface can be easily washed only by using water, because water is adsorbed between stain and the highly hydrophilic TiO2 surface. Moreover, even if the quantity of light is too low to decompose the stains, the surface is maintained clean by supplying water. Thus, outdoor coated surfaces can be kept clean by rainwater. Such called "Photocatalytic Building Materials'' have found use in outdoor application, for example, an exterior glass of 20000 m2 was installed in the terminal

The same UV-induced high hydrophilic behaviour of TiO2 coated surface has been applied in the development of anti-fogging treatment (Gan et al., 2007; Tricoli et al., 2009). Drops are formed when there is low affinity between water and the surface; on a highly hydrophilic

Despite the widespread use TiO2 in many applications, the relatively high cost of the photoactive anatase has proved to be uneconomical for large-scale water treatment operations. Thus, several other ways for the photocatalytic degradation of pollutants have

Because of their narrower band-gaps, metal calcogenides, such as CdS, ZnO and CdSe, show good sensitivities toward incident light in the visible spectrum. However, these narrower band-gap, make the semiconductor suitable of the photo-corrosion process, which can be

Because of its similar band-gap (3.2 eV), ZnO photocatalytic capacity should be comparable to that of TiO2. Due to its high light absorption in the region between 300 and 400 nm, ZnO is found to be as reactive as TiO2 toward the degradation of phenol (Dindar & Içli, 2001). However, as previously reported, photo-corrosion phenomenon can occurs under UV light, and this is one of the main reasons for the decrease of ZnO photocatalytic activity in aqueous solutions. Recently, ZnO nanoparticles have been reported as better photocatalysts in degrading common organic contaminants as compared to bulk ZnO and commercial TiO2

Similarly to ZnO, nanostructured ZnS particles show good catalytic activity and are used in

Enhanced photocatalytic activities may result from doping semiconductors by transition metals. These techniques generally influence the optical and electronic properties of the semiconductors, and can induce a shift of the optical absorption toward the visible region

Water splitting is another important field of application of semiconductor-sensitized systems. In this area, as already reported for TiO2, many efforts are aimed to the

partly suppressed by the addition of sulfide and sulfite to the contacting solution.

researches are aimed to improve these materials (Parkin & Palgrave, 2005).

surface, no water drops are formed but a uniform liquid thin film.

been explored (Chatterjee & Dasgupta, 2005; Mills & Le Hunte, 1997).

the removal of organic pollutants and toxic water pollutants (Hu, 2005).

of Chubu International Airport in 2005.

**3.3 Applications of other semiconductors** 

Degussa P25 (Hariharan, 2006).

(Pouretedal et al., 2009; Ullah & Dutta, 2008).
