**3. Supported ZnO photocatalysts**

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

times higher than that of the pure TiO2 thin film.

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

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

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 sput-

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 demon-

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

*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* 

compared to a pure TiO2 thin film fabricated by the same condition.

tering [47]; or film on pyrex substrates by casting methodology [48].

strated a higher photocatalytic activity for than that of pure TiO2 film.

textile, wastewater leading to cleaner processes [49].

**162**

**Figure 9.**

*Alotaibi et al. [32].*

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

$$\text{ZnO} \rightarrow \text{ZnO} \left(\text{e}^-\_{\text{cB}}\right) + \left(\text{h}^\*\_{\text{VB}}\right) \tag{7}$$

$$\text{ZnO} \left( \text{h}^{\text{\textquotedblleft}} \text{h}^{\text{\textquotedblright}} \right) + \text{H}\_2\text{O} \rightarrow \text{ZnO} + \text{H}^{\text{\textquotedblleft}} + \text{OH}^{\text{\textquotedblright}} \tag{8}$$

$$\text{ZnO} \left(\text{h}^{\text{\text{\textdegree}}}\_{\text{VB}}\right) + \text{OH}^{\text{\textdegree}} \rightarrow \text{ZnO} + \text{OH}^{\text{\textdegree}} \tag{9}$$

$$\text{ZnO} \left( \text{e}^-\_{\text{cB}} \right) + \text{O}\_2 \rightarrow \text{ZnO} + \text{O}\_2 \text{"} \tag{10}$$

$$\rm O\_{2\*} ^{-} + H^{\*} \to HO\_{2\*} \tag{11}$$

$$\rm HO\_{2\*} + HO\_{2\*} \rightarrow H\_2O\_2 + O\_2 \tag{12}$$

$$\text{ZnO} \left( \text{e}^-\_{\text{cB}} \right) + \text{H}\_2\text{O}\_2 \rightarrow \text{OH}^\cdot + \text{OH}^- \tag{13}$$

$$\rm H\_2O\_2 + \rm O\_{2\*} \to \rm OH\_\* + \rm OH^- + O\_2. \tag{14}$$

$$\text{H}\_2\text{O}\_2 + \text{hv} \rightarrow 2\text{OH}^\* \tag{15}$$

Organic pollutants OH Intermediates. + →• (16)

$$\text{Intermediate} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O}.\tag{17}$$

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 synthesis methods [56].

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

radiation. As a consequence, the catalyst is partially dissolved, which gives rise to a dramatic decrease in catalytic activity [57]. The mechanism of this process is represented by the following self-oxidation reactions [58–60]:

$$\text{ZnO} + 2\,\text{h}^\* + \text{nH}\_2\text{O} \rightarrow \text{Zn(OH)}\_{\text{n}}^{\text{(2-n)\*}} + \text{h}\%\text{O}\_2 + \text{nH}^\* \tag{18}$$

$$\text{ZnO} + 2\,\text{h}^\* \to \text{Zn}^{2+} + \text{M}\_2\text{O}\_2 \tag{19}$$

where h<sup>+</sup> is the positive holes created by the action of UV radiation. Photocorrosion is the main obstacle to the use of ZnO as an effective photocatalyst. Therefore, significant efforts have been made to reduce the degradation of ZnO.

Beside TiO2 using as a photocatalyst, Jung et al. [61] studied the synthesis and photocatalytic activity of CuO-ZnO nanowires supported on stainless steel wire meshes (SSWM). They showed that CuO-ZnO structures supported on SSWM exhibit an enhanced photocatalytic activity with respect to catalysts using other supports, such as ITO. This result they attributed to the efficient charge separation of the electronhole pair favored by the SSWM support [61]. Another advantage of SSWM is its flexibility, which allows the mesh to be easily shaped to the desired configuration.

In general, the procedures used to achieve this involves the deposition onto the ZnO surface of: (a) silver nanoparticles [62–66]; (b) polyaniline monolayers [67], (c) graphitic carbon [68]; (d) Nafion films [69]; (e) AlSi nanoclays [70]; and (f) C60 fullerenes which become hybridized with ZnO [59].

Although the above modifications improve the photocatalytic stability of ZnO, some problems persist. For instance, Bessekhouad et al. [71] have reported that the photocatalytic activity of the doped materials is impaired by thermal instability and by an increase in the number of hole/electron recombination centers. Therefore, the development of novel methods that provide effective protection of the ZnO photocatalyst against photocorrosion is required.

An attempt must be made to increase catalytic activity under visible irradiation, since the solar spectrum contains a large fraction of visible light compared to UV radiation. Recently, the photocatalytic activity of ZnO in the visible region has been improved by various techniques, such as: (a) modification of the ZnO surface by non-metal element doping [72]; (b) transition metal doping [73, 74]; (c) the combination of ZnO with another semiconductor [75], etc. Of these methods, the coupling of different semiconductor photocatalysts offers great promise as it increases the charge separation of the electron-hole pairs, resulting in an increase in photocatalytic decolorization efficiency [76].

Recently, ZnO has been combined successfully with TiO2 [77], CdO [78], CdS [79], and WO3 [80]. CdO is a good candidate for coupling with ZnO due to its band gap, ~2.2 eV [78], so that CdxZn1-xO nanostructures are active in the visible range. In addition, under visible light the excited electrons from the conduction band (CB) of ZnO can be easily transferred to the CB of CdO since the ECB of CdO is lower than the ECB of ZnO. These transferal processes increase the excess of electrons in the conduction band of CdO, which shifts in the Fermi level of CdO [81], increasing its photocatalytic efficiency. To the best of our knowledge, CdO-ZnO has been always synthesized in powder form. The use of CdO-ZnO supported on a target substrate (such as SSWM) as a photocatalyst for dye degradation processes in UV or visible has reported by Vu et al. [82]. They also attempt to demonstrate in parallel that coupling CdO and ZnO may be also an excellent method to avoid photocorrosion.

**165**

**Figure 10.**

*Supported-Metal Oxide Nanoparticles-Potential Photocatalysts*

thin film as a photocatalyst in the visible region [84, 101].

Tungsten oxide (WO3) is a high-ranking material in photocatalysis [83]. The WO3 presents in several phases such as monoclinic, orthorhombic, triclinic, cubic, etc., but only the monoclinic phase exhibits the best photocatalytic efficiency. In addition, WO3 is a material with many advantages, such as harmless, high stability in acidic and oxidative ambient, and its cost fabrication cost is very low compared

It has been shown that the band gap energy of WO3 is varied from 2.5 to 3.0 eV [85, 86], leading WO3 can be used as a photocatalyst at the visible region. In recent times, there are many studies focused on the improvement strategies of the photo-

Recently, the researchers have proposed many strategies for the fabrication of WO3 thin film [88], such as sputtering deposition [89–92], aerosol-assisted chemical vapor deposition [93, 94], sol-gel spin-coating [95, 96], hydrothermal-assisted growth [97, 98], and surfactant-assisted spray pyrolysis [99, 100]. Many works focused on the photocatalytic efficiency of WO3, especially the studying of WO3

For example, the author fabricated a WO3 thin film with a thickness 500–600 nm

deposited on a quartz substrate by DC reactive magnetron sputtering [102]. The fabricated film WO3 used for the degradation of CH3CHO (acetaldehyde) under ultraviolet, standard fluorescence, and visible light. The result shows that WO3 film fabricated by sputtering can be a good photocatalyst under visible light region. To improve the separation of photogenerated charged and to increase the photocatalytic activity, many researchers combined other elements with the WO3 thin film. For example, Higashino et al. have fabricated a layer of WO3 on the Al-W allow coatings by selective solution and heat treatment (**Figure 10**). The formed thin film WO3 exhibits photocatalytic self-cleaning properties under the visible light

Takashima et al. have taken advantage of the multielectron reduction of Pt to improve the photocatalytic activity of WO3 thin film on W foil. The author fabricated Pt loaded WO3 thin film using a reactive DC magnetron sputtering technique or low damage reactive gas flow sputtering [104]. The formed Pt-WO3 thin film was used to photodegrade CH3CHO under visible light. The result shows that the fabricated thin film demonstrates excellent photodecomposition rates under visible light. Another approach to improve the separation of photogenerated charged, C3N4 is combined with WO3 film to form a heterojunction composite WO3/C3N4. The

*SEM images of the surface and cross-sectional of WO3. Reproduced by Higashino et al. [103].*

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

**4. Supported WO3 photocatalysts**

with other photocatalysts [84].

irradiation [103].

catalytic efficiency of WO3 [84, 87].
