**3.2.4 Wettability of TiO2 and self-cleaning materials**

During '90s, Fujishima and Heller independently reported the potential utilization of TiO2 in the development of self-cleaning ceramic materials (Heller, 1995; Watanabe et al., 1992). In these works, thin films of organic contaminants were photocatalytically oxidized on TiO2 coated surfaces. The typical oxidation-rate, evaluated in 1-5 picometers per day was sufficient to maintain clean the surface then in the case of low flux of contaminants. Typically several hundred mW/cm2 of UV light are available in the day-time and from the outdoor shade, which corresponds to about 1000 photons/cm2 per second. This is, of course, a very small quantity of energy, but very high if compared to the number of molecules adsorbed on the surface. This suggested the potential applications in the self-cleaning material technology. One of the first commercially applications was the self-cleaning glasscover for tunnel light. The largely used high pressure sodium lamps show an UV emission of about 3 mW/cm2, enough to keep efficient the photo-oxidative self-cleaning process.

During the same period, Frank and Bard applied TiO2 photocatalysis in the destruction of pollutants (Frank & Bard, 1997). Their preliminary studies on the oxidation of cyanide to cyanate, by using four different forms of TiO2 under xenon lamp, opened the way for the

In the last three decades, TiO2 powders have known wide applications into the recovery of water of industrial, agricultural or civil origin, as well as the decontamination of atmosphere and soil, through the mineralization of the pollutants, or at least their transformation into non-toxic compounds (Fox & Dulay, 1993; Gaya & Abdullaha, 2008; Hoffmann et al., 1995; Ravelli et al., 2011). Of course, the great advantage of these photocatalytic systems depends on the fact that they do not need the use of stoichiometric quantities of chemicals, potentially themselves polluting, but they act under light (often sunlight represents the best

Moreover, the high band-gap value of TiO2 allow to oxidize various organic substrates, such as hydrocarbons and their derivatives to lower molecular weight ozidized species and CO2, volatile organic compounds (VOCs) and nitrogen oxides (NOx) present in urban atmosphere

Photocatalytic decomposition reactions can be applied to the destruction of bacteria. Escherichia coli (*E. Coli*) cells are completely destroyed in one week under 1 mW/cm2 UV irradiation on TiO2 (Evans & Sheel, 2007; Fu et al., 2005; Liu et al., 2008; Sunada et al., 1998). Usually, an analogous anti-bacteria effect can be achieved in nearly 1 h under outdoor UV light intensity. However, the typical indoor UV light intensity is in the order of mW/cm2, thus the photocatalytic disinfection under indoor conditions require too long time to be considered useful from the practical point of view. The antibacterial function of TiO2 is strongly enhanced, even with weak UV intensity, using a fluorescent lamp assisted by the presence of Ag or Cu (Sunada et al., 2003). TiO2 photoactivity reaction assists the intrusion of the copper ions into the cell, which is probably the cause of the destruction of the *E. Coli*

Actually several nanostructured TiO2 derivatives are studied and applied as germicidal species, especially on the treatment of water contaminated with pathogenic microorganisms presenting a potential hazard to animals and human beings (Mccullagh et al.,

During '90s, Fujishima and Heller independently reported the potential utilization of TiO2 in the development of self-cleaning ceramic materials (Heller, 1995; Watanabe et al., 1992). In these works, thin films of organic contaminants were photocatalytically oxidized on TiO2 coated surfaces. The typical oxidation-rate, evaluated in 1-5 picometers per day was sufficient to maintain clean the surface then in the case of low flux of contaminants. Typically several hundred mW/cm2 of UV light are available in the day-time and from the outdoor shade, which corresponds to about 1000 photons/cm2 per second. This is, of course, a very small quantity of energy, but very high if compared to the number of molecules adsorbed on the surface. This suggested the potential applications in the self-cleaning material technology. One of the first commercially applications was the self-cleaning glasscover for tunnel light. The largely used high pressure sodium lamps show an UV emission of about 3 mW/cm2, enough to keep efficient the photo-oxidative self-cleaning process.

**3.2.2 Photodegradation of pollutants** 

(Carp et al., 2004).

**3.2.3 Anti-bacterial materials** 

colonies even under very weak UV light.

**3.2.4 Wettability of TiO2 and self-cleaning materials** 

2007; Skorb et al., 2008).

photodegradation of environmental pollutants.

choice) in the presence of oxygen (Malato et al., 2007).

Nowadays, various self-cleaning materials are used in commercial applications and intense researches are aimed to improve these materials (Parkin & Palgrave, 2005).

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 of Chubu International Airport in 2005.

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

#### **3.3 Applications of other semiconductors**

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

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

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 Degussa P25 (Hariharan, 2006).

Similarly to ZnO, nanostructured ZnS particles show good catalytic activity and are used in the removal of organic pollutants and toxic water pollutants (Hu, 2005).

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

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

Semiconductors in Organic Photosynthesis 89

oxygen-centred radical species are formed via oxygen reduction (Karraway, 1994 & Kormann, 1988). The first step of O2 action consists in the capture of the electron promoted

The role of O2 is not just that of scavenging the photogenerated electrons: it produces the socalled active oxygen species while the simultaneous oxidation of an organic substrate yields

(HROH)ads + 2( OH)s RO + 2H2O

Therefore, through a Kisch type B photocatalysis mechanism, coupling reactions of oxidation and reduction intermediates lead to products of partial or total oxidation,

. . . .

. . .

. .

As shown above, oxygen represents the source of highly reactive radical intermediates,

.

.

which are the "engine" of the photocatalyzed reaction mechanisms.

. .

. .

h

(e- / h+)

. O2 -

.OH + OH-

.

.

( OH)s

H2O2 + 2 OH-

. (1)

+ O2

(HROH)ads

HROOH + O2

RH + HROOH

RO + OH-

.

RO + OH-

RO + H2O

HROO

RH + H2O

(2)

(3)

RH + H+

in the CB by light irradiation of semiconductor surface (Eq. 1).

TiO2

O2 + e-

+ 2H2O

H2O2 + e-

RH2 + h+

RH2 + ( OH)s

RH + ( OH)s

RH + O2

HROO + RH2

HROO + e-

.

RH + O2 -

RH + HOO

HROO + HOO

.

depending on experimental conditions (Eq. 3) (Macyk & Kisch, 2001).

)s + h+

(OH-

. .

2 O2 -

radical intermediates (Eq. 2).

(s = surface, ads = adsorbed)

development of efficient photoelectrochemical devices, in which illuminated semiconductors are the process promoter (Mills & Le Hunte, 1997).

Often, platinum group metal, deposited on the surface of the semiconductor, are used to facilitate the reduction of water by the photogenerated electrons and platinum group metal oxides are often used to mediate the oxidation of water by the photogenerated holes. Of course, in the presence of a large band-gap semiconductors (TiO2 as example), with bandgap potential much higher than the oxygen one, there is a sufficiently large over-potential for the reaction to proceed readily without an oxygen redox catalyst.

#### **4. Photo-induced organic synthesis**

Photocatalysis in organic synthesis concerns the use of light to induce chemical transformations onto organic substrates which are transparent in the wavelength range employed. Radiation is absorbed by a photocatalyst whose electronically excited states induce electron- or energy-transfer reactions able to trigger the chemical reactions of interest. Significant examples of photocatalytic processes employed for synthetic purposes are oxidation and reduction processes, isomerization reactions, C-H bond activations, and C-C and C-N bond-forming reactions. The use of solar light as a reagent in oxidative catalysis is particularly relevant to realize innovative and economically advantageous processes for the conversion of hydrocarbons into oxygenates compounds and, at the same time, to move toward a "sustainable chemistry" that has a minimal environmental impact. The main reasons are because the sunlight represents a totally renewable source of energy; the photochemical excitation requires milder conditions than thermal activation and allows one to conceive shorter reaction sequences and to minimize undesirable side reactions. Generally, an important role in the photocatalyzed processes is played by O2. It is important to underline that the search for new catalysts capable of inducing the oxofunctionalization of hydrocarbons with this environmentally friendly and cheap reagent represents a major target from the synthetic and industrial points of view. On the basis of pure thermodynamic considerations, most organic compounds are not stable with respect to oxidation by O2. There are, however, kinetic limitations in this process mainly imposed by the triplet ground state of the O2 molecule, which is not consistent with the singlet states of many organic substrates. Activation of both O2 and the organic substrate may be achieved by photochemical excitation with light of the visible or of the near-ultraviolet regions (> 300 nm). The use of heterogeneous and organized systems is a suitable way to control efficiency and selectivity of catalytic processes through the control of the microscopic environment surrounding the catalytic centre. In particular, the nature of the reaction environment may affect numerous physical and chemical functionalities of the photocatalytic system, such as the absorption of light, the generation of elementary redox intermediates, the rate of competitive chemical steps, and the adsorption-desorption equilibria of substrates, intermediates, and final products. Moreover, another fundamental role of a solid support is to make the photocatalyst more easily handled and recycled.

#### **4.1 Role of O2**

In dispersed semiconductor photocatalytic processes oxygen acts as electron acceptor. On illuminated TiO2 surfaces, in the presence of air (O2), hydrogen peroxide and reactive

development of efficient photoelectrochemical devices, in which illuminated

Often, platinum group metal, deposited on the surface of the semiconductor, are used to facilitate the reduction of water by the photogenerated electrons and platinum group metal oxides are often used to mediate the oxidation of water by the photogenerated holes. Of course, in the presence of a large band-gap semiconductors (TiO2 as example), with bandgap potential much higher than the oxygen one, there is a sufficiently large over-potential

Photocatalysis in organic synthesis concerns the use of light to induce chemical transformations onto organic substrates which are transparent in the wavelength range employed. Radiation is absorbed by a photocatalyst whose electronically excited states induce electron- or energy-transfer reactions able to trigger the chemical reactions of interest. Significant examples of photocatalytic processes employed for synthetic purposes are oxidation and reduction processes, isomerization reactions, C-H bond activations, and C-C and C-N bond-forming reactions. The use of solar light as a reagent in oxidative catalysis is particularly relevant to realize innovative and economically advantageous processes for the conversion of hydrocarbons into oxygenates compounds and, at the same time, to move toward a "sustainable chemistry" that has a minimal environmental impact. The main reasons are because the sunlight represents a totally renewable source of energy; the photochemical excitation requires milder conditions than thermal activation and allows one to conceive shorter reaction sequences and to minimize undesirable side reactions. Generally, an important role in the photocatalyzed processes is played by O2. It is important to underline that the search for new catalysts capable of inducing the oxofunctionalization of hydrocarbons with this environmentally friendly and cheap reagent represents a major target from the synthetic and industrial points of view. On the basis of pure thermodynamic considerations, most organic compounds are not stable with respect to oxidation by O2. There are, however, kinetic limitations in this process mainly imposed by the triplet ground state of the O2 molecule, which is not consistent with the singlet states of many organic substrates. Activation of both O2 and the organic substrate may be achieved by photochemical excitation with light of the visible or of the near-ultraviolet regions (> 300 nm). The use of heterogeneous and organized systems is a suitable way to control efficiency and selectivity of catalytic processes through the control of the microscopic environment surrounding the catalytic centre. In particular, the nature of the reaction environment may affect numerous physical and chemical functionalities of the photocatalytic system, such as the absorption of light, the generation of elementary redox intermediates, the rate of competitive chemical steps, and the adsorption-desorption equilibria of substrates, intermediates, and final products. Moreover, another fundamental role of a solid support is

semiconductors are the process promoter (Mills & Le Hunte, 1997).

for the reaction to proceed readily without an oxygen redox catalyst.

to make the photocatalyst more easily handled and recycled.

In dispersed semiconductor photocatalytic processes oxygen acts as electron acceptor. On illuminated TiO2 surfaces, in the presence of air (O2), hydrogen peroxide and reactive

**4.1 Role of O2**

**4. Photo-induced organic synthesis** 

oxygen-centred radical species are formed via oxygen reduction (Karraway, 1994 & Kormann, 1988). The first step of O2 action consists in the capture of the electron promoted in the CB by light irradiation of semiconductor surface (Eq. 1).

$$\begin{array}{ccccc} \mathsf{T}\mathsf{O}\_{2} & \xrightarrow{\mathsf{hv}} & \text{(e}^{\ast}/\mathsf{h}^{\ast}) \\\\ \mathsf{O}\_{2} + \mathsf{e}^{\cdot} & \xrightarrow{\mathsf{hv}} & \text{O}\_{2}^{\cdot \cdot} \\\\ \mathsf{2}\ \mathsf{O}\_{2}^{\cdot \cdot} + 2\mathsf{H}\_{2}\mathsf{O} & \xrightarrow{\mathsf{h} \mathsf{H}\_{2}\mathsf{O}\_{2}} & \mathsf{H}\_{2}\mathsf{O}\_{2} + 2\mathsf{OH}^{\cdot} + \mathsf{O}\_{2} \\\\ \mathsf{H}\_{2}\mathsf{O}\_{2} + \mathsf{e}^{\cdot} & \xrightarrow{\mathsf{h} \mathsf{O}\_{2}} & \mathsf{\mathsf{O}}\mathsf{H} + \mathsf{O}\mathsf{H}^{\cdot} \end{array} \tag{1}$$

The role of O2 is not just that of scavenging the photogenerated electrons: it produces the socalled active oxygen species while the simultaneous oxidation of an organic substrate yields radical intermediates (Eq. 2).

RH2 + h+ (OH- )s + h+ (HROH)ads . RH2 + ( OH)s RH + ( OH)s (HROH)ads + 2( OH)s RO + 2H2O RH + H+ ( OH)s RH + H2O . . . . . . . (2)

(s = surface, ads = adsorbed)

Therefore, through a Kisch type B photocatalysis mechanism, coupling reactions of oxidation and reduction intermediates lead to products of partial or total oxidation, depending on experimental conditions (Eq. 3) (Macyk & Kisch, 2001).


As shown above, oxygen represents the source of highly reactive radical intermediates, which are the "engine" of the photocatalyzed reaction mechanisms.

Semiconductors in Organic Photosynthesis 91

activity and selectivity of the photocatalyst; in particular, selectivity toward benzaldehyde production increases as the particle size increases (Maira et al., 2001). These results can be explained considering that smaller particles are characterized by a larger surface area and also by a larger proportion of edges and corners which increases as size decreases. Edges and corners are expected to exhibit different catalytic and adsorption properties compared with the more planar surface sites. The involvement of Ti4+-O- radicals could be the reason of the increase of selectivity to benzaldehyde as the particle size of the TiO2 samples

Matra has found that size can affect the acid-base properties of surface hydroxyl groups (Martra, 2000). In the Merk TiO2 these groups are electron acceptor centres, while in Degussa P25 they present a nucleophilic character. These differences can explain the different selectivity toward benzaldehyde formation in the gas-phase photocatalytic oxidation of toluene, in particular, the lower selectivity observed with P25 is ascribed to a

Although semiconductor materials like as TiO2 have been extensively applied in the environmental field in order to mineralize organic pollutants, their application as tool for the organic synthesis has been also investigated. Recently, some reviews on this field have been published (Gambarotti, 2010; Pastori, 2009; Shiraishi, 2008; Anpo & Kamat, 2010). Herein we report some examples taken from the literature which can be used to take a view

The selective oxidation of hydrocarbons under mild conditions is an important industrial and scientific challenge because relatively cheap feedstocks are converted into highly value-

Phenol is an highly important chemical intermediate because it is used in a large number of different sectors including the production of phenolic resins, caprolactam, aniline, alkylphenols, diphenols, and salicylic acid (Weber et al., 2005). It is produced from benzene mainly through the cumene process (Hock process). Currently, in the USA and Western Europe, about 20 % of the total benzene production is consumed for the synthesis of phenol (Weissermel & Arpe, 1997). In the last years interests have aroused for the direct production of phenol from benzene, despite these proposed processes are still not effective and competitive if compared with the classic procedures (Bal et al., 2006; Guo et al., 2009; Tani et al., 2005; Tlili et al., 2009). Among them, photocatalytic processes, in particular based on the use of semiconductors, gained attention both in industriy and academy as an economically and environmentally favourable approach (Chen et al., 2009). Recently, Molinari and coworkers reported the direct benzene conversion to phenol in a hybrid photocatalytic membrane reactor (Molinari et al., 2009a). The authors used TiO2 Degussa P25 as catalyst to promote the oxidation of benzene to phenol in water under UV irradiation (λ = 366 nm, power = 6.0 mW cm-2). The particularity was the adoption of a membrane reactor having two compartments, one containing the aqueous phase, in which TiO2 and benzene were

**5. Organic synthesis in the presence of illuminated semiconductors** 

increases from 6 to 20 nm.

stronger adsorption of the aldehyde.

on the recent state of the art.

added products.

**5.1 TiO2 in organic photosynthesis** 

**5.1.1 Synthesis of phenol derivatives** 

#### **4.2 Redox potentials of semiconductors**

The relevant redox potential for some photocatalysts and H2O are compared in Fig. 7 (Kisch, 2001; Ravelli et al., 2011). An excited photocatalyst (C\*) oxidizes a substrate (S) when E(C\*) is more positive than the E(S), whereas there is reduction when E(C\*) is lower than E(S). For example as shown in Fig. 6, excited TiO2, as previously reported, is capable of causing water splitting, since the valence/conduction band-gap is sufficiently large for encompassing the H2O redox potentials (at least at low pH values) (Montalti, 2006). Single crystal catalysts in aqueous systems usually shift cathodically with higher pH value by approximately 0.06 V per pH unit as reported for TiO2 (Ward, 1983), CdS (White & Bard, 1985), and ZnS (Fan et al., 1983). In addition to this pH dependence, surface impurities, adsorbed compounds and the change to organic solvents may induce strong shift in the redox potential. In the case of CdS, the removal of traces of elemental sulphur and cadmium from the surface induces a cathodic shift of even almost 1 V! (Meissner & Memming, 1988).

Fig. 7. Band-gap and redox potentials (*vs*. NHE) at pH = 1 vs H2O at pH = 0 and 7

### **4.3 Influence of catalyst shape on photoactivity**

The rate of products formation is influenced by the specific surface area of the catalyst. Typically, two opposite effects play a key role: the rate of electron-hole recombination and the concentration of adsorbed substrates (Heller et al., 1987). The recombination rate increases linearly with surface area, and accordingly the reaction rate should decrease. On the contrary, there is a linear increase on the redox process rate due to increasing concentration of adsorbed substrates, which should also increase the product formation rate. Moreover, it is expected that, depending on the nature of semiconductor and substrates, the reaction rate may increase, decrease or remain constant with increasing surface area. For example, Reber founds that in the photoreduction of H2O, only CdS with very large (>> l00 m2/g) or very low (< 6.7 m2/g) specific surface areas produce hydrogen at a significant rate (Reber & Rusek, 1986).

The chemoselectivity may also depend on the surface area, usually related to the particle size. Maira and co-workers have studied the effect of particle size in the gas-phase photooxidation of toluene over TiO2. Particles from 6 to 20 nm have show to influence the

The relevant redox potential for some photocatalysts and H2O are compared in Fig. 7 (Kisch, 2001; Ravelli et al., 2011). An excited photocatalyst (C\*) oxidizes a substrate (S) when E(C\*) is more positive than the E(S), whereas there is reduction when E(C\*) is lower than E(S). For example as shown in Fig. 6, excited TiO2, as previously reported, is capable of causing water splitting, since the valence/conduction band-gap is sufficiently large for encompassing the H2O redox potentials (at least at low pH values) (Montalti, 2006). Single crystal catalysts in aqueous systems usually shift cathodically with higher pH value by approximately 0.06 V per pH unit as reported for TiO2 (Ward, 1983), CdS (White & Bard, 1985), and ZnS (Fan et al., 1983). In addition to this pH dependence, surface impurities, adsorbed compounds and the change to organic solvents may induce strong shift in the redox potential. In the case of CdS, the removal of traces of elemental sulphur and cadmium from the surface induces a

**4.2 Redox potentials of semiconductors** 

cathodic shift of even almost 1 V! (Meissner & Memming, 1988).

Fig. 7. Band-gap and redox potentials (*vs*. NHE) at pH = 1 vs H2O at pH = 0 and 7

The rate of products formation is influenced by the specific surface area of the catalyst. Typically, two opposite effects play a key role: the rate of electron-hole recombination and the concentration of adsorbed substrates (Heller et al., 1987). The recombination rate increases linearly with surface area, and accordingly the reaction rate should decrease. On the contrary, there is a linear increase on the redox process rate due to increasing concentration of adsorbed substrates, which should also increase the product formation rate. Moreover, it is expected that, depending on the nature of semiconductor and substrates, the reaction rate may increase, decrease or remain constant with increasing surface area. For example, Reber founds that in the photoreduction of H2O, only CdS with very large (>> l00 m2/g) or very low (< 6.7 m2/g) specific surface areas produce hydrogen at a significant rate

The chemoselectivity may also depend on the surface area, usually related to the particle size. Maira and co-workers have studied the effect of particle size in the gas-phase photooxidation of toluene over TiO2. Particles from 6 to 20 nm have show to influence the

**4.3 Influence of catalyst shape on photoactivity** 

(Reber & Rusek, 1986).

activity and selectivity of the photocatalyst; in particular, selectivity toward benzaldehyde production increases as the particle size increases (Maira et al., 2001). These results can be explained considering that smaller particles are characterized by a larger surface area and also by a larger proportion of edges and corners which increases as size decreases. Edges and corners are expected to exhibit different catalytic and adsorption properties compared with the more planar surface sites. The involvement of Ti4+-O- radicals could be the reason of the increase of selectivity to benzaldehyde as the particle size of the TiO2 samples increases from 6 to 20 nm.

Matra has found that size can affect the acid-base properties of surface hydroxyl groups (Martra, 2000). In the Merk TiO2 these groups are electron acceptor centres, while in Degussa P25 they present a nucleophilic character. These differences can explain the different selectivity toward benzaldehyde formation in the gas-phase photocatalytic oxidation of toluene, in particular, the lower selectivity observed with P25 is ascribed to a stronger adsorption of the aldehyde.
