**3.2. Photocatalytic properties of metals doped M-TiO2**

through a 6-days sol-gel synthesis showed mesopores with 6.86 nm diameter and a specific

simultaneous disappearance of the hexagonal mesophase. The sample showed indeed a size-

contacted with a fresh MB solution. Such superior photocatalytic behaviour was assigned

SBA-15 silica as hard template, the activity of the sample being again ascribed to a compromise between its high surface area and the percentage of anatase. The former parameter was

as a stabiliser showed better photocatalytic activity than Degussa P25 towards the degradation of 2,4-dichlorophenol, a toxic chlorinated compound produced by environmental transformations of some chlorinated herbicides and/or antimicrobial agents [56]. Several M-TiO2 were studied and compared to Degussa P25: the most efficient degradation was obtained

resulted stable after recycling. The photocatalytic behaviour of the studied materials was

anatase phase and a well-ordered mesoporous structure facilitating diffusion of reactants/

activity. Nonetheless, the sample calcined at 800°C, though occurring as pure anatase, showed both a partial collapse of mesoporous walls and the formation of larger particles, leading to a

A performance comparable with Degussa P25 towards the degradation of dimethyl phthal-

weak acidic solution of acetic acid [60]. Similarly to what mentioned before, both the high surface area and the crystallinity of the anatase phase contribute to the catalytic activity of

As a whole, although different experimental conditions are adopted during photocatalytic

favours better photocatalytic performances under UV radiation. However, this is just a gen-

soft-template route under high-intensity ultrasound irradiation showed better photocatalytic performance than P25 in the UV-assisted degradation of *n*-pentane in air (**Figure 6**).

20% brookite and 80% anatase, the formation of brookite being ascribed to the ultrasound

was not pure anatase, but a bi-crystalline material, containing ca.

tests carried out in different laboratories, the occurrence of pure anatase M-TiO<sup>2</sup>

eral rule, but several exceptions are observed. For instance, another M-TiO<sup>2</sup>

photocatalyst obtained through the EISA method by employing ethylene diamine

calcined at 700°C, whereas the performance of M-TiO<sup>2</sup>

g−1): according to the authors, a trade-off exists between the occurrence of pure

Satisfactory results concerning the photobleaching of MB were obtained with a M-TiO2

able MB degradation with respect to Degussa P25, especially after M-TiO<sup>2</sup>

likely responsible for a very efficient dye adsorption at the surface of M-TiO<sup>2</sup>

at higher temperatures decreased. Moreover, the performance of the M-TiO<sup>2</sup>

to both higher surface area and higher anatase content of M-TiO2

/h<sup>+</sup>

explained not only on the basis of a higher surface area (122 m2

ate (a persistent antiparasite) was obtained with M-TiO2

treatment adopted during the synthesis [61]. The M-TiO2

products. At higher calcination temperatures, i.e. 900°C rutile M-TiO<sup>2</sup>

g−1. Longer synthesis time led to a decrease of both values and to the

) recombination, which is slow in crystalline and defect-free

was recycled and

, the latter likely

using

calcined

usually

obtained by

photocatalyst

g−1) with respect to Degussa

prepared by using Pluronic P123 in

samples after calcination were

, formed, with a lower

with respect to Degussa

surface area of 284 m2

reduced the electron/hole (e−

in the presence of a M-TiO2

worst photocatalytic performance.

P25 [60].

130 Titanium Dioxide

materials [28].

P25 (ca. 50 m2

the sample.

The studied M-TiO2

A M-TiO2

Processes like doping and modification of TiO<sup>2</sup> are usually required in order to decrease the recombination rate of e<sup>−</sup> /h<sup>+</sup> pairs and to extend the absorption range towards the Vis [65]. The latter effect is particularly important in the perspective of exploiting solar light, especially for photocatalytic processes of environmental remediation that have to be carried out under solar illumination in view of actual large-scale applications [66, 67].

**Figure 6.** Rates of UV-assisted degradation of *n*-pentane as catalysed by two M-TiO2 (SM-1 and SM-2) obtained under high-intensity ultrasound irradiation are compared to the rate obtained by using Degussa P25 (after Ref. [61]). (Copyright 2002 American Chemical Society).

**Figure 7.** XRD patterns of the as-prepared and calcined M-TiO<sup>2</sup> (SM-1 and SM-2) obtained under high-intensity ultrasound irradiation: A denotes anatase; B denotes brookite [61]. (Copyright 2002 American Chemical Society).

As expected, the photocatalytic activity of M-TiO<sup>2</sup> is significantly improved after doping with metals and non-metals. Besides doping, other methods are used to modify the photocatalytic properties of M-TiO2 , like, for instance, the synthesis of solid solutions or of different TiO<sup>2</sup> composites with noble metals, other metal oxides and quantum dots (not addressed here) [5].

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 (vide infra).

A Fe-doped M-TiO2 was used to obtain composites with hollow glass microbeads: the latter were able to prevent aggregation of M-TiO2 NPs [69]. As expected, Fe doping induced 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 industry fields, like water purification.

A Fe-doped M-TiO2 (with 2.5 wt.% Fe content) obtained by direct synthesis resulted very active towards the catalytic degradation of Acid Orange 7 (a model molecule of azo dyes) not only under UV irradiation but also in dark conditions in the presence of H<sup>2</sup> O2 [70, 71]. The dark process was studied in detail, showing that since not all Fe species entered the M-TiO2 framework, surface Fe3+ species were very active in Fenton-like reaction. Though limited Fe leaching was observed, the Fe-doped M-TiO<sup>2</sup> was still active after reactivation in air. 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 the surface of the catalyst. In any case, the studied M-TiO<sup>2</sup> materials showed remarkable high specific surface area (150–120 m<sup>2</sup> g−1) and pure anatase NPs.

The effect of other metals used for doping is more complex. For instance, when Ce was used to produce doped thin films of M-TiO<sup>2</sup> [72], Ce species were likely located at the surfaces/grain boundaries of M-TiO<sup>2</sup> crystallites, due to the larger size of Ce3+/Ce4+ ions with respect to Ti4+ ions. High levels of Ce doping, instead, adversely affected the crystallisation of nanocrystalline anatase since Ce−O−Ti bonds at the grain boundaries inhibited crystallite growth.

The optimal amount of Ce doping corresponded to a ratio Ce/Ti = 0.3 mol%: the corresponding Ce-doped M-TiO2 showed a remarkable photocatalytic activity towards the degradation of MB (**Figure 8**). The enhanced photocatalytic activity of the Ce-doped M-TiO2 with Ce/Ti = 0.3 mol% was assigned to enhanced electron transport and oxygen storage capabilities in the presence of Ce, along with the highly nanocrystalline nature of the TiO<sup>2</sup> framework.
