**1.1 General description about photocatalysis**

Photocatalysis by semiconductors, such as those indicated above, is a wellestablished method for degrading organic contaminants in wastewaters. When the photon with energy greater than the band gap of the semiconductor is absorbed by the solid, an electron is excited from the valence band to the conduction band, resulting in an electron-hole pair. These exciting state conduction band electrons and valence band holes have several possible fates [1]:


In aqueous systems, these adsorbed species will correspond to water molecules, hydroxide ions, and oxygen molecules.

It is clear to know that in semiconductors, the generation of electrons and holes depends on the position of the energy levels. Besides, the redox potential of a donor species on the surface of the photocatalyst needs to be more negative (higher in energy) than the valence band position of the semiconductor to fill the electron vacancies. Similarly, acceptor molecules must have a more positive redox potential (lower in energy) than the conduction band. In view of this, one of the advantages of TiO2 and ZnO compared to other semiconductors is that their electronic structure is such that it allows both the reduction of protons and the oxidation of water [2]. This phenomenon can be appreciated in the redox potential diagram shown in **Figure 1**. Recently, the use of TiO2 and ZnO as photocatalysts has been thoroughly investigated.

Numerous conditions of a photocatalyst are required to achieve high photocatalytic efficiency. Firstly, the bandgap of the semiconductor must be higher than the redox potential of the H2O/OH• couple, and the material must be photo-stable. Secondly, the recombination of electron-hole pairs must be minimized. In other words, they need to be kept apart to allow time for the redox reactions to occur. Separation can be achieved by trapping electrons or holes in defects [4] or by using electrically conductive supports [5].

The holes in the photocatalyst valence band can oxidize the adsorbed water or hydroxide ions, while electrons in the conduction band can reduce molecular oxygen to superoxide anions [6]. These processes are summarized in the following equations:

$$\text{Semiconductor} + h\nu \rightarrow \text{e}^- + \text{h}^+\tag{1}$$

$$\text{H}^\* + \text{H}\_2\text{O}\_{\text{ads}} \rightarrow \text{OH} \bullet \text{}\_{\text{ads}} + \text{H}^\* \tag{2}$$

$$\text{\textbullet }\text{\textbullet}^{\text{}} + \text{\textbullet}\text{\textbullet}^{\text{}}\_{\text{\textbullet}\text{s}} \rightarrow \text{\textbullet}\text{H}\text{\textbullet}\_{\text{ads}}\tag{3}$$

$$\bullet^{-} + \bullet\_{2} \to \bullet\_{2} \bullet^{-} \tag{4}$$

$$\bullet \bullet \bullet^{-} + \text{H}^{+} \rightarrow \text{HO}\_{2} \\ \bullet \tag{5}$$

**157**

**Figure 2.**

**Figure 1.**

*(NHE). Reproduced from Ania et al. [3].*

*Gratzel et al. [2].*

pollutants into carbon dioxide and water.

photocatalysts are nowadays an important research area.

*Supported-Metal Oxide Nanoparticles-Potential Photocatalysts*

stability in many conditions, and the capability to create charge carriers when they were exposed under UV or visible light. The advantageous combination of the electronic structure, light absorption capacities, and excited lifetimes of metal oxides have provided of metal oxides has provided them possible for their application as photocatalyst. The photocatalysis employing metal oxides such as TiO2, ZnO, SnO2, and WO3 has demonstrated their efficiency in the degradation of various harmful

*Schematic diagram showing the processes involved in semiconductor photocatalysis. Reproduced from* 

*Band gaps (eV) and redox potentials for several semiconductors referred to the normal hydrogen electrode* 

The vast majority of the photocatalysts studied are in powder form with all the difficulties in handling and recovering that implies. Consequently, supported

*DOI: http://dx.doi.org/10.5772/intechopen.93238*

$$\text{O}\_2\bullet^- + \text{HO}\_2\bullet + \text{H}^\* \rightarrow \text{H}\_2\text{O}\_2 + \text{O}\_2 \tag{6}$$

The degradation of organic contaminants is commonly attributed to oxidation by hydroxyl radicals. This process is schematically described in **Figure 2**. This has led to the use of the term "Advanced Oxidation Processes," although there is evidence that in some systems, reductive pathways also operate [7].

Among *Advanced Oxidation Processes*, the photocatalytic processes are focused on the conversion of highly toxic organic to either less toxic organic compounds or CO2 and H2O [8]. When the photocatalytic reaction is implemented in the presence of O2, the catalyst plays two main roles: to scavenge the photogenerated electrons and to produce active oxygen species [9]. We already know that metal oxides can respond to both UV-light and visible light, depending on the energy band gap of the materials. Newly, the photocatalytic process used visible light is widely employed for environmental cleanup [10].

Recently, many metal oxides (TiO2, ZnO, SnO2, and WO3) have been widely used for the photocatalysis process. This is due to their abundance in nature,

*Supported-Metal Oxide Nanoparticles-Potential Photocatalysts DOI: http://dx.doi.org/10.5772/intechopen.93238*

#### **Figure 1.**

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

hydroxide ions, and oxygen molecules.

the redox potential of the H2O/OH•

electrically conductive supports [5].

In aqueous systems, these adsorbed species will correspond to water molecules,

It is clear to know that in semiconductors, the generation of electrons and holes depends on the position of the energy levels. Besides, the redox potential of a donor species on the surface of the photocatalyst needs to be more negative (higher in energy) than the valence band position of the semiconductor to fill the electron vacancies. Similarly, acceptor molecules must have a more positive redox potential (lower in energy) than the conduction band. In view of this, one of the advantages of TiO2 and ZnO compared to other semiconductors is that their electronic structure is such that it allows both the reduction of protons and the oxidation of water [2]. This phenomenon can be appreciated in the redox potential diagram shown in **Figure 1**. Recently, the use of TiO2 and ZnO as photocatalysts has been thoroughly investigated. Numerous conditions of a photocatalyst are required to achieve high photocatalytic efficiency. Firstly, the bandgap of the semiconductor must be higher than

Secondly, the recombination of electron-hole pairs must be minimized. In other words, they need to be kept apart to allow time for the redox reactions to occur. Separation can be achieved by trapping electrons or holes in defects [4] or by using

The holes in the photocatalyst valence band can oxidize the adsorbed water or hydroxide ions, while electrons in the conduction band can reduce molecular oxygen to superoxide anions [6]. These processes are summarized in the following equations:

h H O OH • H 2 ads ads

h OH OH • ads ads

O • H HO • 2 2

O • HO • H H O O 2 2 22 2

The degradation of organic contaminants is commonly attributed to oxidation by hydroxyl radicals. This process is schematically described in **Figure 2**. This has led to the use of the term "Advanced Oxidation Processes," although there is evi-

Among *Advanced Oxidation Processes*, the photocatalytic processes are focused on the conversion of highly toxic organic to either less toxic organic compounds or CO2 and H2O [8]. When the photocatalytic reaction is implemented in the presence of O2, the catalyst plays two main roles: to scavenge the photogenerated electrons and to produce active oxygen species [9]. We already know that metal oxides can respond to both UV-light and visible light, depending on the energy band gap of the materials. Newly, the photocatalytic process used visible light is widely employed

Recently, many metal oxides (TiO2, ZnO, SnO2, and WO3) have been widely used for the photocatalysis process. This is due to their abundance in nature,

dence that in some systems, reductive pathways also operate [7].

couple, and the material must be photo-stable.

Semiconductor e h *hv* − + +→+ (1)

+ + +→ + (2)

+ − + → (3)

− − + →2 2 e O O• (4)

− + + → (5)

− + + +→ + (6)

**156**

for environmental cleanup [10].

*Band gaps (eV) and redox potentials for several semiconductors referred to the normal hydrogen electrode (NHE). Reproduced from Ania et al. [3].*

#### **Figure 2.**

*Schematic diagram showing the processes involved in semiconductor photocatalysis. Reproduced from Gratzel et al. [2].*

stability in many conditions, and the capability to create charge carriers when they were exposed under UV or visible light. The advantageous combination of the electronic structure, light absorption capacities, and excited lifetimes of metal oxides have provided of metal oxides has provided them possible for their application as photocatalyst. The photocatalysis employing metal oxides such as TiO2, ZnO, SnO2, and WO3 has demonstrated their efficiency in the degradation of various harmful pollutants into carbon dioxide and water.

The vast majority of the photocatalysts studied are in powder form with all the difficulties in handling and recovering that implies. Consequently, supported photocatalysts are nowadays an important research area.
