**4. Supported WO3 photocatalysts**

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

represented by the following self-oxidation reactions [58–60]:

C60 fullerenes which become hybridized with ZnO [59].

catalyst against photocorrosion is required.

in photocatalytic decolorization efficiency [76].

where h<sup>+</sup>

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

( ) (2 n)

++ → + + (18)

+ + +→ + (19)

2 2 <sup>n</sup> ZnO 2 h nH O Zn OH ½ O nH + + − +

<sup>2</sup> ZnO 2 h Zn ½ O2

 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)

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 photo-

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

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

**164**

photocorrosion.

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 with other photocatalysts [84].

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 photocatalytic efficiency of WO3 [84, 87].

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

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 irradiation [103].

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

fabricated composite was deposited don fluorine-doped tin oxide (FTO) substrate [105]. The photocatalytic activity of the composite was tested by photocatalytic degradation of MB and Cr6+ in wastewater under UV illumination.

The supported WO3/C3N4 composite present higher photocatalytic activity on the decoloration of MB and the reduction of Cr6+ to Cr3+, compared to the photocatalytic activity of WO3 thin film.
