**2. Supported TiO2 photocatalysts**

When the light is irradiated to the surface of the semiconductor, its absorption of photons provokes the photocatalytic reaction at the surface of the catalyst. Among many semiconductor materials, titanium dioxide (TiO2) nanoparticles have been widely studied for photocatalytic applications over the last two decades [11, 12]. TiO2 is relatively inexpensive, insoluble in water, and non-toxic. It can provide photogenerated holes with high oxidizing power because of its wide bandgap (3.2 eV) [13].

The common mechanism of the photocatalytic process of TiO2 material consists of the interfacial redox reactions of the generated holes and electrons when the TiO2 materials are irradiated by light with appropriate energy (**Figure 3**) [14, 15].

Most of the photocatalysts found in the literature are in the form of powders. Only a few supported photocatalytic systems have been reported, even though they have clear advantages from a practical point of view [16–18]. The most important benefit is that the separation of the supported photocatalysts from the reaction medium is simple, which minimizes the power requirements. In addition, they can be adapted to operate in flow-type continuous reactors [19–21]. Most recently, research was described by Fernández et al. [21]. They reported two methods of deposition of TiO2 powders on different substrates (glass, quartz, and stainless steel) and evaluated the photocatalytic activities of these supporting materials through the degradation of organic compounds (**Figure 4**).

The authors demonstrated the influence of coating methodology and photocatalytic activities. The results showed that titania deposited on quartz displays a similar photocatalytic activity to that of the powder form. Therefore, the result opens a new feature for a new advantage route to immobilize catalyst for flow reactor or batch reactor. Because filtration step to recover the catalyst always causes many drawbacks in water treatment.

Then some authors [22, 23] developed a new type of supported photocatalyst that consists of mixtures of noble metal nanoparticles and commercially available TiO2 nanoparticles (P25, Degussa-Evonik) deposited by dip-coating procedure on quartz substrate. The photocatalytic activity of the immobilized catalyst was evaluated by the degradation of malic acid. A comparison of the photocatalytic activity between supported TiO2 with the powder TiO2 Degussa P-25 shows slightly lower catalytic activity. This phenomenon could be explained by the total surface exposed of the catalyst to the light during the irradiation. However,

#### **Figure 3.**

*The mechanism of photocatalysis process of TiO2. Reproduced from Hoffman et al. [15]. (1) The formation of the electrons and holes by photon absorption. (2) The recombination of the generated electrons and holes. (3) The electron trapping of Ti (IV) at the conduction band to form Ti (III). (4) The hole trapping of titanol group at the valence band. (5) The oxidative pathways at the conduction band. (6) The reductive pathways at the valence band. (7) Photocatalytic oxidation reactions to form harmless compounds.*

**159**

**Figure 5.**

*Natarajan et al. [25].*

**Figure 4.**

*Supported-Metal Oxide Nanoparticles-Potential Photocatalysts*

spheres as a photocatalyst to treat the wastewater (**Figure 6**).

*SEM micrographs for the three TiO2/quartz samples. Reproduced from Hoffman et al. [15].*

*Schematic representation of photocatalytic reactor using TiO2 supported on quartz. Reproduced by* 

the result could be compensated by the advantage of eliminating the recovering

Lately, several works also reported the preparation of thin TiO2 film on quartz substrates (**Figure 5**) by coating the TiO2 sol using rotary evaporator [24], by dip-

Recently, Borges et al. [27] supported commercial TiO2 (Degussa P25) on glass

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

catalyst after the treatment.

coating [25], by spin coating [26].

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

the result could be compensated by the advantage of eliminating the recovering catalyst after the treatment.

Lately, several works also reported the preparation of thin TiO2 film on quartz substrates (**Figure 5**) by coating the TiO2 sol using rotary evaporator [24], by dipcoating [25], by spin coating [26].

Recently, Borges et al. [27] supported commercial TiO2 (Degussa P25) on glass spheres as a photocatalyst to treat the wastewater (**Figure 6**).

#### **Figure 4.**

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

When the light is irradiated to the surface of the semiconductor, its absorption of photons provokes the photocatalytic reaction at the surface of the catalyst. Among many semiconductor materials, titanium dioxide (TiO2) nanoparticles have been widely studied for photocatalytic applications over the last two decades [11, 12]. TiO2 is relatively inexpensive, insoluble in water, and non-toxic. It can provide photogenerated holes with high oxidizing power because of its wide

The common mechanism of the photocatalytic process of TiO2 material consists of the interfacial redox reactions of the generated holes and electrons when the TiO2

Most of the photocatalysts found in the literature are in the form of powders. Only a few supported photocatalytic systems have been reported, even though they have clear advantages from a practical point of view [16–18]. The most important benefit is that the separation of the supported photocatalysts from the reaction medium is simple, which minimizes the power requirements. In addition, they can be adapted to operate in flow-type continuous reactors [19–21]. Most recently, research was described by Fernández et al. [21]. They reported two methods of deposition of TiO2 powders on different substrates (glass, quartz, and stainless steel) and evaluated the photocatalytic activities of these supporting materials

The authors demonstrated the influence of coating methodology and photocatalytic activities. The results showed that titania deposited on quartz displays a similar photocatalytic activity to that of the powder form. Therefore, the result opens a new feature for a new advantage route to immobilize catalyst for flow reactor or batch reactor. Because filtration step to recover the catalyst always causes many draw-

Then some authors [22, 23] developed a new type of supported photocatalyst that consists of mixtures of noble metal nanoparticles and commercially available TiO2 nanoparticles (P25, Degussa-Evonik) deposited by dip-coating procedure on quartz substrate. The photocatalytic activity of the immobilized catalyst was evaluated by the degradation of malic acid. A comparison of the photocatalytic activity between supported TiO2 with the powder TiO2 Degussa P-25 shows slightly lower catalytic activity. This phenomenon could be explained by the total surface exposed of the catalyst to the light during the irradiation. However,

*The mechanism of photocatalysis process of TiO2. Reproduced from Hoffman et al. [15]. (1) The formation of the electrons and holes by photon absorption. (2) The recombination of the generated electrons and holes. (3) The electron trapping of Ti (IV) at the conduction band to form Ti (III). (4) The hole trapping of titanol group at the valence band. (5) The oxidative pathways at the conduction band. (6) The reductive pathways at* 

*the valence band. (7) Photocatalytic oxidation reactions to form harmless compounds.*

materials are irradiated by light with appropriate energy (**Figure 3**) [14, 15].

through the degradation of organic compounds (**Figure 4**).

**2. Supported TiO2 photocatalysts** 

bandgap (3.2 eV) [13].

backs in water treatment.

**158**

**Figure 3.**

*SEM micrographs for the three TiO2/quartz samples. Reproduced from Hoffman et al. [15].*

#### **Figure 5.**

*Schematic representation of photocatalytic reactor using TiO2 supported on quartz. Reproduced by Natarajan et al. [25].*

**Figure 7** shows the surface morphology substrate of the glass spheres without TiO2 particles. **Figure 7b** shows the TiO2 supported on the substrate and the homogeneous distribution of TiO2 on the surface of the glass sphere. The authors showed that photocatalytic treatment in photoreactor displays more advantages than in batch system for high volumes of industrial wastewater.

Later, several authors also used poly-vinylidene fluoride (PVDF) dual layer hollow fiber membrane as a support to immobilize TiO2 to treat pharmaceutical compound in wastewater (**Figure 8**) [28, 29].

#### **Figure 6.**

*Packed-bed photoreactor system and glass spheres photocatalytic bed in photoreactor. Reproduced by Borges et al. [27].*

*SEM image of the glass sphere (a), SEM image of TiO2 supported on the glass sphere (b). Reproduced by Borges et al. [27].*

**161**

photocatalyst.

**Figure 8.**

*Supported-Metal Oxide Nanoparticles-Potential Photocatalysts*

The author demonstrated that the TiO2 supported catalyst on PVDF membranes

*Images of the PVDF membranes: (a) outer surface, (b) full cross section, (c) partial cross section, and (d) EDX* 

Recently, TiO2 is widely combined with other metal oxides, such as ZrO2, SnO2, WO3, CeO2, ZnO, to improve the photocatalytic activity of TiO2 in the ultraviolet

For example, the composite of mixed oxide ZrO2-TiO2 material recently gains a great attention. By combining TiO2 with ZrO2, the surface acidity of the composite can be increased, hence improved the reactivity compared to the TiO2 [31]. In addition, the hydroxyl groups are located on the surface of the catalyst, where the holes are trapped which could improve the efficiency of the degradation of organic pollutant [32]. Therefore, the mixed oxide ZrO2-TiO2 has been widely studied for the photodegradation of toxic organic compounds [31–35]. Many researchers also focused on the fabrication of ZrO2-TiO2 composite thin film and study its photo-

Luo et al. has fabricated the ZrO2-TiO2 composite thin film on glass substrate using micro-arc oxidation process and used it for the degradation of rhodamine

light and improving its photocatalytic efficiency in the visible region.

catalytic efficiency under ultraviolet or visible irradiation [36–40].

improved the photo-transformation rate of wastewater compounds during the photocatalytic treatment. The author also claimed that the supported catalyst could be easily recycled without any separation systems or catalysis recovery technologies. It is already to know that TiO2 is used as a photocatalyst in the ultraviolet light region due to its wide bandgap. To improve its photocatalytic efficiency in the visible light, efforts have been made such as, doping TiO2 with anionic species (Fluorine, Sulfur, Nitrogen), or combining TiO2 with other metal oxides. The combination of TiO2 with other metal oxides can reduce the recombination effect of the electron-hole before they migrate to the surface of the material [30]. In addition, the composite of TiO2 with other metal oxides can be generated in the surface hydroxyl groups, which can trap holes after the irradiation process, which improve the separation of the electron-holes. In some cases, the composite of TiO2 with other metal oxides can enhance the crystallinity degree of TiO2 and increases the specific surface area of the composite which are two important properties for a

*images of TiO2 nanoparticles at the outer layer. Reproduced by Paredes et al. [29].*

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

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

#### **Figure 8.**

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

batch system for high volumes of industrial wastewater.

compound in wastewater (**Figure 8**) [28, 29].

**Figure 7** shows the surface morphology substrate of the glass spheres without TiO2 particles. **Figure 7b** shows the TiO2 supported on the substrate and the homogeneous distribution of TiO2 on the surface of the glass sphere. The authors showed that photocatalytic treatment in photoreactor displays more advantages than in

Later, several authors also used poly-vinylidene fluoride (PVDF) dual layer hollow fiber membrane as a support to immobilize TiO2 to treat pharmaceutical

*Packed-bed photoreactor system and glass spheres photocatalytic bed in photoreactor. Reproduced by Borges* 

*SEM image of the glass sphere (a), SEM image of TiO2 supported on the glass sphere (b). Reproduced by* 

**160**

**Figure 7.**

*Borges et al. [27].*

**Figure 6.**

*et al. [27].*

*Images of the PVDF membranes: (a) outer surface, (b) full cross section, (c) partial cross section, and (d) EDX images of TiO2 nanoparticles at the outer layer. Reproduced by Paredes et al. [29].*

The author demonstrated that the TiO2 supported catalyst on PVDF membranes improved the photo-transformation rate of wastewater compounds during the photocatalytic treatment. The author also claimed that the supported catalyst could be easily recycled without any separation systems or catalysis recovery technologies.

It is already to know that TiO2 is used as a photocatalyst in the ultraviolet light region due to its wide bandgap. To improve its photocatalytic efficiency in the visible light, efforts have been made such as, doping TiO2 with anionic species (Fluorine, Sulfur, Nitrogen), or combining TiO2 with other metal oxides. The combination of TiO2 with other metal oxides can reduce the recombination effect of the electron-hole before they migrate to the surface of the material [30]. In addition, the composite of TiO2 with other metal oxides can be generated in the surface hydroxyl groups, which can trap holes after the irradiation process, which improve the separation of the electron-holes. In some cases, the composite of TiO2 with other metal oxides can enhance the crystallinity degree of TiO2 and increases the specific surface area of the composite which are two important properties for a photocatalyst.

Recently, TiO2 is widely combined with other metal oxides, such as ZrO2, SnO2, WO3, CeO2, ZnO, to improve the photocatalytic activity of TiO2 in the ultraviolet light and improving its photocatalytic efficiency in the visible region.

For example, the composite of mixed oxide ZrO2-TiO2 material recently gains a great attention. By combining TiO2 with ZrO2, the surface acidity of the composite can be increased, hence improved the reactivity compared to the TiO2 [31]. In addition, the hydroxyl groups are located on the surface of the catalyst, where the holes are trapped which could improve the efficiency of the degradation of organic pollutant [32]. Therefore, the mixed oxide ZrO2-TiO2 has been widely studied for the photodegradation of toxic organic compounds [31–35]. Many researchers also focused on the fabrication of ZrO2-TiO2 composite thin film and study its photocatalytic efficiency under ultraviolet or visible irradiation [36–40].

Luo et al. has fabricated the ZrO2-TiO2 composite thin film on glass substrate using micro-arc oxidation process and used it for the degradation of rhodamine

B under ultraviolet irradiation [40]. The ZrO2-TiO2 composite thin film consists of three compounds: anatase, rutile, and ZrO2 phases, results that the generated electron can transfer from rutile to anatase. This phenomenon inhibits the recombination of the generated electron-hole pairs, thus improved the photocatalytic efficiency of the composite. The photodegradation of MB under UV light irradiation shows that the photocatalytic activity of ZrO2-TiO2 composite thin film is three times higher than that of the pure TiO2 thin film.

Alotaibi et al. have reported the preparation of ZrO2-TiO2 composite thin film on glass substrate using aerosol-assisted chemical vapor deposition [32]. The photocatalytic activity of the fabricated composite thin film was evaluated through the photodegradation of resazurin redox dye under Ultraviolet light irradiation (**Figure 9**). The composite shows an enhancement of photocatalytic activity compared to a pure TiO2 thin film fabricated by the same condition.

Tungsten oxide (WO3) is a common dopant in heterogeneous photocatalysis. In the last decade, WO3 was extensively combined with TiO2 to improve the photocatalytic activity of TiO2 in both UV and Visible light. Besides of using the catalyst in the powder form, the preparation of WO3-TiO2 film and its photocatalytic activity was extensively studied [41–44] due to its advantage of the recuperation way. The WO3-TiO2 film has been fabricated by several methodologies such as sol-gel and dip coating [45]; spin-coating [46]; solvothermal method combining magnetron sputtering [47]; or film on pyrex substrates by casting methodology [48].

For example, Fu et al. have fabricated the WO3-TiO2 film on quartz substrate by dip-coating synthesis [42]. The photocatalytic efficiency was evaluated by the degradation of 4-chlorophenol-4 CP, xenobiotic micropollutants, under the irradiation visible light. The result shows that by incorporation of WO3 into TiO2, the WO3-TiO2 film can shift the absorption band from near UV region to the visible region. Under visible light, for the degradation of 4-CP, the prepared composite film demonstrated a higher photocatalytic activity for than that of pure TiO2 film.

Recently, Adel et al. have prepared the WO3-TiO2 thin film on glass substrate by reactive chemical spraying and tested its photocatalytic activity under visible light. The results show that the photocatalytic thin film can degrade completely dye in textile, wastewater leading to cleaner processes [49].

#### **Figure 9.**

*SEM images of the (a) ZrO2, (b) TiO2 and (c) ZrO2-TiO2 composite films with the high magnification. The side on images—(d) ZrO2, (e) TiO2, and (f) ZrO2-TiO2 composite—shows the film thickness. Reproduced by Alotaibi et al. [32].*

**163**

synthesis methods [56].

*Supported-Metal Oxide Nanoparticles-Potential Photocatalysts*

(Pt-TiO2 and WO3-TiO2) and pristine TiO2 nanotubes [50].

under irradiation light can be summarized as follows [52]:

Spanu et al. have prepared Pt deposited on WO3-TiO2 nanotube arrays on Ti foil by sputtering method in order to improve the photocatalytic activity of the WO3- TiO2 system. The photocatalyst was used for the fabrication of H2 by photocatalysis. The Pt deposited on WO3-TiO2 nanotube arrays show highly enhanced photocatalytic H2 evolution efficiency comparing to other single-photocatalyst system such as

ZnO is a metal oxide with a broad energy band gap (3.37 eV), which is one of the best semiconductors in the last decade. Recently, ZnO is extensively used as a photocatalyst under UV or Visible light irradiation due to its outstanding electrical, mechanical, optical, and non-toxic properties. In addition, the production cost of ZnO is low cost comparing to the fabrication of other semiconductors [51].

The mechanism of photocatalysis process of ZnO to degrade organic compounds

( ) •. ZnO h H O ZnO H OH VB 2

( ) • ZnO h OH ZnO OH VB

( ) • ZnO e O ZnO O CB 2 <sup>2</sup>

O H HO 2• 2•.

( ) • ZnO e H O OH OH CB 2 2

H O O OH OH O . 2 2 2• • <sup>2</sup>

ZnO shares many similar properties with TiO2, including a similar band gap (see **Figure 1**). There have even been several examples of ZnO displaying a higher photocatalytic activity than TiO2 [53]. In addition, ZnO exhibits a better quantum efficiency because it can absorb a larger fraction of the solar spectrum than TiO2 [54] and its price is even lower than that of TiO2 [55]. Compared to TiO2, ZnO can be easily supported on different types of substrates by means of low-temperature

We already know that the photocatalytic efficiency of a photocatalyst is evaluated through the photogeneration of electron-hole pairs and their time-life. However, the main limitation of ZnO as a photocatalyst is the rapid recombination rate of photogenerated electron-hole pairs, which decreases the photocatalytic efficiency of ZnO. In addition, the use of ZnO as a photocatalyst is limited by the photocorrosion phenomenon. This process occurs because of the action of UV

ZnO ZnO e h ( CB ) ( VB ) → + − + (7)

+ + + → ++ (8)

+ − +→+ (9)

− − +→ + (10)

HO HO H O O 2• 2• 2 2 2 +→+ (12)

− − + →+ (13)

− − +→+ + (14)

• H O hv 2OH 2 2 + → (15)

Organic pollutants OH Intermediates. + →• (16)

2 2 Intermediates CO H O. → + (17)

− + + → (11)

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

**3. Supported ZnO photocatalysts**

Spanu et al. have prepared Pt deposited on WO3-TiO2 nanotube arrays on Ti foil by sputtering method in order to improve the photocatalytic activity of the WO3- TiO2 system. The photocatalyst was used for the fabrication of H2 by photocatalysis. The Pt deposited on WO3-TiO2 nanotube arrays show highly enhanced photocatalytic H2 evolution efficiency comparing to other single-photocatalyst system such as (Pt-TiO2 and WO3-TiO2) and pristine TiO2 nanotubes [50].
