**3.3. Photocatalytic properties of non-metal doped M-TiO2**

As expected, the photocatalytic activity of M-TiO<sup>2</sup>

**Figure 7.** XRD patterns of the as-prepared and calcined M-TiO<sup>2</sup>

ter were able to prevent aggregation of M-TiO2

ited Fe leaching was observed, the Fe-doped M-TiO<sup>2</sup>

the surface of the catalyst. In any case, the studied M-TiO<sup>2</sup>

industry fields, like water purification.

specific surface area (150–120 m<sup>2</sup>

tocatalytic properties of M-TiO2

different TiO<sup>2</sup>

132 Titanium Dioxide

(vide infra).

addressed here) [5].

A Fe-doped M-TiO2

A Fe-doped M-TiO2

M-TiO2

with metals and non-metals. Besides doping, other methods are used to modify the pho-

ultrasound irradiation: A denotes anatase; B denotes brookite [61]. (Copyright 2002 American Chemical Society).

Fe is one of the most used metals for doping: on the one side, it extends the absorption of TiO<sup>2</sup> in the Vis range; on the other side, Fe species present at the surface may give rise to Fentonlike reactions [68], finally enhancing the reactivity merely due to the photocatalytic process

a red shift of the absorption band. Consequently, an effective photodegradation of methyl orange (a model molecule of azo dyes) in aqueous solution was achieved under visible light (*λ* > 420 nm) irradiation, revealing the potential applicability of such nanocomposites in some

active towards the catalytic degradation of Acid Orange 7 (a model molecule of azo dyes)

The dark process was studied in detail, showing that since not all Fe species entered the

Preliminary results concerning the test reaction under UV-Vis illumination provided further support to this picture. The authors used the same synthesis protocol for V-doping, but in that case, it was not possible to obtain an actual doping, since all the V species resulted present at

g−1) and pure anatase NPs.

framework, surface Fe3+ species were very active in Fenton-like reaction. Though lim-

not only under UV irradiation but also in dark conditions in the presence of H<sup>2</sup>

composites with noble metals, other metal oxides and quantum dots (not

was used to obtain composites with hollow glass microbeads: the lat-

(with 2.5 wt.% Fe content) obtained by direct synthesis resulted very

is significantly improved after doping

(SM-1 and SM-2) obtained under high-intensity

NPs [69]. As expected, Fe doping induced

was still active after reactivation in air.

materials showed remarkable high

O2

[70, 71].

, like, for instance, the synthesis of solid solutions or of

Doping with non-metals (mainly C, N, S, F and I<sup>2</sup> [5]) is also supposed to extend TiO2 absorption towards the Vis range. The mechanisms responsible of such phenomenon are complex and usually related to either the narrowing of TiO2 band gap or the creation of intermediate steps within the band gap [66], due to the non-metal atoms substituting oxygen atoms in the framework.

**Figure 8.** Changes in absorption maximum of MB as a function of exposure time during photocatalytic tests carried out with Ce-doped M-TiO2 thin films. The best results are obtained with a Ce/Ti = 0.3 mol% composition (after Ref. [72]). (Copyright 2009 American Chemical Society).

N is one of the most used non-metals, as doping may be carried out rather simply by thermally treating M-TiO2 under NH3(g) flow or by heating a M-TiO<sup>2</sup> produced in solutions containing a N source (NH3 , urea, etc.).

The ultimate effect of N of the light absorption capacity of the sample is however complex and may be due to different processes:


All this notwithstanding, literature reports on N-doped M-TiO<sup>2</sup> materials with improved photocatalytic properties. For instance, by a template-free combustion method, a wormhole M-TiO2 was obtained, where N doping was due to the presence of urea during combustion [73]. The so-obtained N-doped M-TiO2 occurred as nanocrystalline anatase phase, showing high surface area (234 m2 g−1) and type-IV H3-mesoporosity.

Two photocatalytic reactions were studied, namely, Rhodamine B degradation and *p*-anisyl alcohol oxidation to *p*-anisaldehyde in aqueous solution under direct sunlight. Notwithstanding the high band gap (3.24 eV) of the N-doped sample, the good activity was assigned to a better utilisation of holes due to the low-charge diffusion barrier associated with wormhole mesoporosity along with the occurrence of crystalline NPs, finally confirming the importance of having an ordered mesoporous photocatalyst.

N-doped anatase M-TiO2 was prepared via soft-template route by using CTAB as template and by treating in NH3 (70%)/N<sup>2</sup> atmosphere the calcined samples. The material was characterised by small crystallite size, large surface area (420–126 m<sup>2</sup> g−1 for calcination temperatures in the 400–800°C range), high crystallinity and Vis light response. The N-doped anatase M-TiO2 photocatalysts showed much higher photocatalytic activity than N-doped Degussa P25 for the degradation of phenol under both UV and Vis light irradiation, owing to more oxidising hydroxyl radicals, which were the oxidative species mainly responsible for the degradation of phenol [74]. The authors concluded that the materials might be beneficial to solardriven applications in the photodegradation of organic pollutants.

However, the effect of doping on the photocatalytic performance is not straightforward, as it has been observed that N-doped TiO2 shows visible light-responsive photocatalytic activity but lower UV light-responsive photocatalytic activity. The visible light photocatalytic activity originates from new N 2*p* levels near the valence band. The oxygen vacancies and the associated Ti3+ species act as the recombination centres for the photoinduced e<sup>−</sup> /h<sup>+</sup> , finally reducing photocatalytic activity although contributing to Vis light absorbance [75].

### **4. Conclusions**

The production of mesoporous titania by template-assisted methods allows obtaining materials characterised by ordered mesoporous structure, controllable crystallinity, high surface area and tuneable pore size. Moreover, powder nanoparticles or films may be obtained, as well as hierarchical porosity materials.

From the point of view of photocatalytic applications, the type of crystalline phase and ordered mesopores are crucial factors: the former may be responsible of more efficient UV light absorption and slower electron-hole recombination rate; the latter positively affect diffusion processes and mass transfer phenomena. Nonetheless, the possibility of obtaining high surface area materials, also after calcination, positively affects any kind of heterogeneous catalytic process, besides photocatalytic ones, as mesoporous titania may be an efficient support for other types of catalytically active phases.

Several mesoporous titania materials reported by the literature have remarkable photocatalytic properties and are competitive, at least on a lab scale, with commercial samples. It must be considered, however, that template-assisted syntheses require high-cost reagents and/or energy-intensive steps, like calcination.

However, for applications requiring tailored photocatalysts, the progresses made on the side of synthesis procedures will surely allow the development of promising (photo)catalysts based on mesoporous titania.
