**2. Template-based methods for the synthesis of ordered mesoporous titania: main features and critical issues**

#### **2.1. Hard-template methods: nanocasting**

Nanocasting (**Figure 1**, lower part) consists of using preformed (natural or synthetic) mesoporous solids to synthesise porous materials of different compositions: such method has been extensively applied to produce porous carbons, metal oxides and metal sulphides as negative replicas of preformed hard templates [23, 24].

Usually, nanocasting envisages three fundamental steps: (i) the precursor is infiltrated within the mesoporous channels of a porous solid (i.e. preformed mesoporous SiO2 or carbon); (ii) the precursor reacts forming a composite of the negative replica and the hard template and (iii) the hard template is removed (either chemically or thermally).

As a result, a negative replica is obtained of the hard template, which in the case of M-TiO<sup>2</sup> is mostly mesoporous SiO2 (e.g. SBA-15, SBA-16, MCM-41, MCM-48, KIT-6, FDU-12), mesoporous carbon (e.g. CMK-1, CMK-3), porous Al<sup>2</sup> O3 or polystyrene spheres [5].

Nanocasting allows obtaining M-TiO2 characterised by crystallinity and thermal stability, because it overcomes the problem of thermal collapse of the TiO2 framework, which may occur during phase transition. Some problems, however, may arise during the synthesis: for instance, if the precursor infiltration is performed in aqueous phase, some undesired precipitation and crystallisation of TiO2 in solution may lead to an incomplete filling of the hard-template pores, ultimately blocking the channels and avoiding any further precursor infiltration [15].

In order to avoid such undesired phenomenon, some parameters like precursor/template ratio, calcination temperature and immersion time have to be strictly controlled [25].

A careful evaluation of the weight ratio between the Ti alkoxide and the solid template [25] is crucial, as a too high amount of precursor would lead to formation of bulky material outside the hard-template pores, with consequent loss of surface area and poor precursor infiltration [25].

Conversely, some positive confinement effects due to the hard template in contact with TiO<sup>2</sup> allow obtaining anatase M-TiO2 at temperatures that are unusual for bulk TiO2 , for instance, with mesoporous silica KIT-6 as hard template, anatase (instead of rutile) M-TiO<sup>2</sup> formed by calcination at 750°C [25]. Such result is particularly sound since anatase, though characterised by a larger band gap than rutile (*E*<sup>g</sup> ≅ 3.2 eV and 3.0 eV for anatase and rutile, respectively), has a large surface area that is very useful for (photo)catalytic applications.

As for the soft-template routes discussed in the following paragraph, the choice of Ti precursor is the main issue. While metal nitrates and chlorides are successfully employed [26] for producing replicas of other metal oxides, Ti nitrates and chlorides are mostly unstable in water (or even when exposed to moisture).

Such problem has been successfully overcome by Yue et al. [27]: they prepared M-TiO2 negative replicas of both SBA-15 and KIT-6 mesoporous silica by pre-hydrolysing the Ti precursor (either Ti isopropoxide (Ti[OCH(CH3 )2 ]4 ) or tetrabutyl titanate (Ti[OC4 H9 ]4 ) and dissolving it in HNO3. The so-obtained acidic solution of Ti nitrate complexes was used for the infiltration step. Another interesting result was the formation of rutile M-TiO<sup>2</sup> at calcination temperatures as low as 100°C, ascribed to either the confinement effect [27] of KIT-6 mesoporous channels or the ability of nitrate ions to stabilise the rutile phase within the SiO2 walls. Indeed, when other Ti precursors are employed (like chloride, sulphate and isopropoxide), anatase M-TiO<sup>2</sup> preferentially forms and transition to rutile M-TiO2 is not observed at temperatures as high as 600°C [28].

**2. Template-based methods for the synthesis of ordered mesoporous** 

Nanocasting (**Figure 1**, lower part) consists of using preformed (natural or synthetic) mesoporous solids to synthesise porous materials of different compositions: such method has been extensively applied to produce porous carbons, metal oxides and metal sulphides as

Usually, nanocasting envisages three fundamental steps: (i) the precursor is infiltrated within

the precursor reacts forming a composite of the negative replica and the hard template and

As a result, a negative replica is obtained of the hard template, which in the case of M-TiO<sup>2</sup>

during phase transition. Some problems, however, may arise during the synthesis: for instance, if the precursor infiltration is performed in aqueous phase, some undesired precipitation and

In order to avoid such undesired phenomenon, some parameters like precursor/template

A careful evaluation of the weight ratio between the Ti alkoxide and the solid template [25] is crucial, as a too high amount of precursor would lead to formation of bulky material outside the hard-template pores, with consequent loss of surface area and poor precursor infiltration [25].

Conversely, some positive confinement effects due to the hard template in contact with TiO<sup>2</sup>

calcination at 750°C [25]. Such result is particularly sound since anatase, though characterised

As for the soft-template routes discussed in the following paragraph, the choice of Ti precursor is the main issue. While metal nitrates and chlorides are successfully employed [26] for producing replicas of other metal oxides, Ti nitrates and chlorides are mostly unstable in

Such problem has been successfully overcome by Yue et al. [27]: they prepared M-TiO2

negative replicas of both SBA-15 and KIT-6 mesoporous silica by pre-hydrolysing the Ti

O3

ultimately blocking the channels and avoiding any further precursor infiltration [15].

ratio, calcination temperature and immersion time have to be strictly controlled [25].

with mesoporous silica KIT-6 as hard template, anatase (instead of rutile) M-TiO<sup>2</sup>

has a large surface area that is very useful for (photo)catalytic applications.

(e.g. SBA-15, SBA-16, MCM-41, MCM-48, KIT-6, FDU-12), mesoporous

characterised by crystallinity and thermal stability,

or polystyrene spheres [5].

in solution may lead to an incomplete filling of the hard-template pores,

at temperatures that are unusual for bulk TiO2

≅ 3.2 eV and 3.0 eV for anatase and rutile, respectively),

or carbon); (ii)

, for instance,

formed by


framework, which may occur

is

the mesoporous channels of a porous solid (i.e. preformed mesoporous SiO2

(iii) the hard template is removed (either chemically or thermally).

because it overcomes the problem of thermal collapse of the TiO2

**titania: main features and critical issues**

negative replicas of preformed hard templates [23, 24].

**2.1. Hard-template methods: nanocasting**

mostly mesoporous SiO2

122 Titanium Dioxide

crystallisation of TiO2

allow obtaining anatase M-TiO2

by a larger band gap than rutile (*E*<sup>g</sup>

water (or even when exposed to moisture).

carbon (e.g. CMK-1, CMK-3), porous Al<sup>2</sup>

Nanocasting allows obtaining M-TiO2

Nanocasting is feasible also by using naturally occurring hard templates: a fascinating work reports on a nanocrystalline rutile TiO2 obtained by using the chitin scales present on the wings of a *Morpho* butterfly, which were coated with a uniform oxide film by adopting a computer-controlled surface sol-gel process [29]. In addition, the calcination process was a crucial step: when the composite was fired at 450°C, mostly anatase TiO<sup>2</sup> formed, whereas firing at 900°C mainly led to rutile, though some anatase was still present in the final product [29].

Hierarchical structures of TiO2 films (**Figure 2**) were obtained by combining PMMA (poly(methyl methacrylate)) microspheres and sol-gel chemistry, in the presence of an amphiphilic diblock copolymer as a structure-directing agent [30]. Poly(dimethylsiloxane) block-methyl methacrylate poly(ethylene oxide) (PDMS-bMA(PEO)) was used as a structure-directing agent for the preparation of the mesopore structure, whereas PMMA microspheres acted as a template for the micrometre-scale structure (**Figure 3**). Both the hard and the soft templates were then removed either by acetic acid or calcination, leading to a macro/mesoporous network, where the macropores generated by the hard template are supposed to favour mass transport phenomena and improve accessible surface area.

**Figure 2.** SEM image showing the occurrence of macro- and mesopores in a hierarchically structured TiO2 film. (Reprinted with permission from Ref. [30]. Copyright 2009 American Chemical Society).

**Figure 3.** Scheme of the procedure adopted to produce hierarchically structured TiO2 films. (Reprinted with permission from Ref. [30]. Copyright 2009 American Chemical Society).

Similarly, a macro/mesoporous TiO<sup>2</sup> was obtained by using poly(styrene-co-acrylic acid) colloidal spheres and triblock copolymer Pluronic P123 as macro- and mesoporous structure-directing agents [31], leading to a material with enhanced photoelectrocatalytic activity towards the removal of Rhodamine B.

#### **2.2. Soft-template methods**

The first report on an aqueous soft-templating route for the synthesis of M-TiO<sup>2</sup> dates back to 1995 [32]: soft-template methods employ different types of structure-directing agent, like charged surfactants (either anionic or cationic), neutral surfactants (e.g. alkylamines) and block copolymers [12, 14, 20, 32–38].

The crucial issue is the control of hydrolysis and condensation rates of the Ti precursor during the cooperative assembly between the structure-directing agent and the inorganic phase.

The second major problem is the thermal stability of the so-obtained M-TiO2 framework, especially during the high-temperature calcination step (required to remove the template), since undesired structural collapse and/or crystallisation may occur, leading to the loss of mesoporosity and/or the grain growth.

The Ti precursor stabilisation is particularly difficult when charged surfactants are used: in a pioneering work, Antonelli et al. used acetylacetonate as a ligand to decrease the reactivity of Ti isopropoxide through the formation of Ti acetylacetonate tris(isopropoxide) in the presence of an alkyl phosphate surfactant [32]. The proposed method showed, however, some limits: on the one side, it was not possible to completely remove phosphorous, likely due to the strong interaction between surfactant and M-TiO2 framework; on the other side, the method was unsuccessful when another charged surfactant (either anionic or cationic) was used.

In a subsequent work, Antonelli obtained a worm-like M-TiO<sup>2</sup> by using an amine as template [39]: the material was characterised by short-range order and relatively low thermal stability, as well as high specific surface area (≅700 m<sup>2</sup> g−1) and the possibility to tune pore dimensions by changing the length of the amine carbon chain (12–18 C atoms). At variance with the alkyl phosphate surfactant used in Ref. [32], where it was not possible to remove phosphorous completely by calcination, when an amine molecules is used, weaker H-bonding interaction occurs between the template and Ti oligomers, finally stabilising the latter species. The main drawback of this method was the longer ageing time required in order to obtain a stable material.

Other attempts with an amine as soft template were made, for instance, with hexadecylamine [40]: the basic molecule seemed to positively affect both hydrolysis and condensation of the Ti precursor, allowing an effective cooperative assembly between the organic and inorganic phase.

However, an effective development of M-TiO<sup>2</sup> was reached only with the advent of block copolymers and EISA method [20, 41]. Block copolymers are non-ionic surfactant (**Figure 4a**) consisting of distinct homopolymer subunits (blocks) linked by covalent bonds like the triblock copolymer known under the commercial name of Pluronic P123 (nominal chemical formula HO(CH2 CH2 O)20(CH2 CH(CH3 )O)70(CH2 CH2 O)20H).

Similarly, a macro/mesoporous TiO<sup>2</sup>

from Ref. [30]. Copyright 2009 American Chemical Society).

block copolymers [12, 14, 20, 32–38].

mesoporosity and/or the grain growth.

removal of Rhodamine B.

124 Titanium Dioxide

**2.2. Soft-template methods**

was obtained by using poly(styrene-co-acrylic acid) colloi-

dates back

films. (Reprinted with permission

framework,

dal spheres and triblock copolymer Pluronic P123 as macro- and mesoporous structure-directing agents [31], leading to a material with enhanced photoelectrocatalytic activity towards the

to 1995 [32]: soft-template methods employ different types of structure-directing agent, like charged surfactants (either anionic or cationic), neutral surfactants (e.g. alkylamines) and

The crucial issue is the control of hydrolysis and condensation rates of the Ti precursor during the cooperative assembly between the structure-directing agent and the inorganic phase.

especially during the high-temperature calcination step (required to remove the template), since undesired structural collapse and/or crystallisation may occur, leading to the loss of

The Ti precursor stabilisation is particularly difficult when charged surfactants are used: in a pioneering work, Antonelli et al. used acetylacetonate as a ligand to decrease the reactivity of Ti isopropoxide through the formation of Ti acetylacetonate tris(isopropoxide) in the presence of an alkyl phosphate surfactant [32]. The proposed method showed, however, some limits: on the one side, it was not possible to completely remove phosphorous, likely due to the strong

The first report on an aqueous soft-templating route for the synthesis of M-TiO<sup>2</sup>

**Figure 3.** Scheme of the procedure adopted to produce hierarchically structured TiO2

The second major problem is the thermal stability of the so-obtained M-TiO2

Such molecules have a high self-assembly capability and may form different mesostructures, depending on their concentration in an alcohol-rich solution [8, 42, 43]: at concentrations below the CMC (critical micelle concentration), each copolymer behaves as a free molecule. At the CMC, copolymer molecules tend to form spherical micelles, with the hydrophobic part in contact with the alcoholic solution and the hydrophilic part shielded within the micelle, in order to minimise the free energy. At concentration above the CMC, coalescence of spherical micelles into cylindrical ones occurs. If the copolymer concentration increases, phase separation may occur, with micelles self-assembling in hexagonal, cubic or lamellar mesophases.

Other types of synthesis imply the use of hydrophobic solvents like cyclohexane and diblock copolymers, known under the commercial name of Brij-n (**Figure 4c**): in those cases, inverse micelles form, with a hydrophilic core that acts as a nanoreactor for the polymerisation of TiO*<sup>x</sup>* . The surfactant is then removed by extractions/centrifugation/washing, and then a final calcination step brings about the total removal of the organic part as well as M-TiO2 crystallisation [44]. After calcination, M-TiO<sup>2</sup> NPs form aggregates with interparticle porosity (**Figure 4c**).

Concerning the morphology of the final material, it is possible to obtain films in a rather simple way, through the so-called EISA method coupled to the sol-gel technique [20, 43] in ethanol/water mixtures (**Figure 2**). An acid (i.e. HCl) is added to control the sol-gel chemistry of the Ti precursor, which forms Ti hydroxides and/or oligomers. The sol is then deposited on a solid substrate, and solvent evaporation is induced by regulating the relative humidity (RH).

**Figure 4.** Schemes of three synthesis routes that are possible by using as soft template: (a) a triblock copolymer, e.g. Pluronic P123. The optional addition of a swelling agent (like *n*-BuOH) allows tuning mesopore dimension [52]; (b) an ionic surfactant, like cetyltrimethylammonium bromide (CTAB). NBB resulting from partial condensation of the precursor may have a charged surface due to pH-dependent protonation of ≡Ti–OH to ≡Ti–OH<sup>2</sup> + and are stabilised by the negative charge of the anion [50, 51]; and (c) diblock copolymer, such as Brij-n, which forms inverse micelles in cyclohexane [44].

As the solvent evaporates, the concentration of both Ti species and the copolymer increases, and, simultaneously, the assembly of the inorganic and the organic phases is favoured. Induced evaporation also helps the removal of HCl (and other volatile species), improving the long-range order of the obtained film. Indeed, in a previous work where TiCl<sup>4</sup> was used as Ti precursor, the molecules of HCl likely induced some hydrolysis in the oxide framework, and rather disordered films were obtained [41].

The order of the final M-TiO<sup>2</sup> films obtained by the EISA method (**Figure 5**) depends on the amount of H2 O, HCl and RH and crystallisation process, as extensively discussed by Crepaldi et al. [20]. In particular, RH results to be a key parameter during the self-assembly step, as it both affects hydrolysis/condensation of Ti species and polarity of the PEO chain [20]. Moreover, RH was found to affect the order, thickness and transparency of the obtained film.

To further control the hydrolysis and condensation of the Ti precursor during the assembly process, addition of ligands like acetic acid [45] and acetylacetonate [46] usually leads to more ordered and/or more stable materials.

In the EISA method, the initial solution is prepared by dissolving anhydrous TiCl<sup>4</sup> into an alcohol-rich solution where the block copolymer has been pre-dissolved. The occurring reaction is

$$\text{TiCl}\_4 + \text{xEtOH} + y \to \text{TiCl}\_{4-x} \text{(OEt)}\_x \text{(OH)}\_y + (\text{x} - y) \text{HCl} \tag{1}$$

Concerning the Ti precursor, as TiCl<sup>4</sup> leads to the formation of HCl through reaction (1) with consequent pH lowering and disordering of the obtained solid [30], stabilised alkoxides are used either as such or mixed with TiCl4 itself [47, 48].

**Figure 5.** Scheme of the steps leading to the formation of M-TiO2 by means of the EISA method. (Adapted from Ref. [20]. Copyright 2003 American Chemical Society).

**Figure 4.** Schemes of three synthesis routes that are possible by using as soft template: (a) a triblock copolymer, e.g. Pluronic P123. The optional addition of a swelling agent (like *n*-BuOH) allows tuning mesopore dimension [52]; (b) an ionic surfactant, like cetyltrimethylammonium bromide (CTAB). NBB resulting from partial condensation of the

by the negative charge of the anion [50, 51]; and (c) diblock copolymer, such as Brij-n, which forms inverse micelles in

+

and are stabilised

precursor may have a charged surface due to pH-dependent protonation of ≡Ti–OH to ≡Ti–OH<sup>2</sup>

cyclohexane [44].

126 Titanium Dioxide

Ti alkoxides are more easy to handle with respect to TiCl4 , but they need some acid as a stabiliser to control the hydrolysis: besides providing acidic conditions, HCl also forms complexes with the Ti alkoxide. For instance, when Ti(OEt)<sup>4</sup> is used, the following reaction occurs:

$$\text{Ti} \left( \text{OEt} \right)\_4 + \left( 4 - x - y \right) \text{HCl} + y \text{H}\_2\text{O} \rightarrow \text{TiCl}\_{4 \rightarrow y} \text{(OEt)}\_x \text{(OH)}\_y + (4 - x) \text{EtOH} \tag{2}$$

Starting from different precursors, the same (partially hydrolysed) species TiCl4–*x*–*<sup>y</sup>* (OEt)*<sup>x</sup>* (OH)*<sup>y</sup>* form in the initial sol [49]. Such species are low-molecular-weight oligomers that, being resistant to hydrolysis, act as nanobuilding blocks (NBB) and cooperate with the hydrophilic portion of the copolymer micelles (for instance, by forming H-bonding with the PEO segment in the Pluronic surfactants).

Concerning NBB, their surface is pH-dependent, due to the protonation of ≡Ti─OH groups to ≡Ti─OH2 + , and in the presence of ionic surfactants, they can be stabilised by the charge of ions present in the solution [50, 51], as depicted in **Figure 4b**, where the ionic template may be hexadecyltrimethylammonium bromide (CTAB). Formation of the hybrid composite depends on the CTAB/Ti ratio and pH of the solution [50, 51]. After self-assembly and condensation, NBB are likely located between the micelle and the inorganic framework: the subsequent hydrothermal treatment can promote the condensation of such NBB, finally leading to a robust inorganic mesostructure.

In order to obtain larger mesopores, it is possible to use a swelling agent, like, for instance, an *n*-alkyl alcohol (*n*BuOH), which solubilises the hydrophobic/hydrophilic interface of the micelle, thereby causing it to swell (**Figure 4a**) [52]. The degree of swelling is proportional to the amount of alcohol, which not only acts as a swelling agent but, being likely located at the hydrophilic/hydrophobic interface, also stabilises the liquid crystal phase and determines its curvature at the interface with the inorganic mesostructure [52].

Calcination is the most effective process for the organic template removal, but it is also a crucial step, since the mesoporous network may collapse at high temperature, with consequent loss of specific surface area. Nonetheless, the thermal treatment also induces crystallisation of the initially amorphous material: the degree of crystallinity is of paramount importance for applications like photocatalysis; therefore, the temperature and time of the calcination have to be carefully controlled. A high degree of crystallinity is desirable, as it implies less surface defects, and therefore a better photocatalytic performance, as defects may act as recombination centres, lowering the photocatalyst performance. An opposite concomitant effect is the grain growth, favoured at high temperatures [5].

In order to obtain thermally stable materials, with suitable grain size and few surface defects, different post-synthesis thermal treatments have been proposed in the literature. For instance, it is possible to obtain cubic M-TiO2 with anatase phase stable up to 400°C obtained after 4 h calcination at 400°C (heating rate 1°C/min) [53]. This material was considered as a promising one for photocatalytic (and optoelectronic) applications, whereas the hexagonal M-TiO<sup>2</sup> was not stable above 250°C.

M-TiO2 thin films stable up to 600°C [54] due to the presence of rather thick walls (9.0–13 nm), where synthesised by using Pluronic F127 (a triblock polymer with chemical composition: PEO106PPO70PEO106, EO = ethylene oxide, PO = propylene oxide) as the structure-directing agent and tetrabutyl titanate as the precursor, in the presence of acetylacetonate [54]. According to the authors, the use of Pluronic F127 as the structure-directing agent favoured the formation of thick TiO2 framework.

Another work reported [55] on the preparation of a M-TiO2 stable up to 650°C obtained by a carbonisation step of the organic template with H2 SO4 followed by thermal treatment at 350°C in inert atmosphere (N2 ). Such process led to the formation of tubular C deposits that acted as stabilisers of TiO2 during the calcination at a high temperature carried out through different steps (first at 550 and 650°C in N<sup>2</sup> and then at 450°C in air to burn out the carbon). The final material was a highly ordered 2D hexagonal mesostructure, with a crystalline framework arising by the connection of anatase nanocrystals. The ordered M-TiO2 obtained had high surface area (∼193 cm<sup>2</sup> g−1), large pore volume (∼0.23 cm<sup>3</sup> g−1) high thermal stability (∼650°C) and uniform mesopore size (4.6–5.1 nm).

Stabilisation of M-TiO2 with larger mesopores (*Ø* > 5.0 nm) is still a challenge and requires some post-treatment methods. For instance, a modified EISA approach envisages the use of ethylene diamine molecules [56], which effectively protect the M-TiO<sup>2</sup> primary particles from collapsing finally delaying the phase transition of anatase to rutile. According to this method [56], a M-TiO<sup>2</sup> with large pore size (10 nm), high surface area (122 m<sup>2</sup> g−1) and thermal stability up to 700 °C was obtained.

Other factors controlling the phenomena occurring during the synthesis of M-TiO2 materials (not be addressed here) have been extensively reviewed in the literature [2, 5, 8].
