**2. Synthesis of titanium dioxide and its derivatives**

The synthesis of titanium dioxide is one of the major research areas in 'green chemistry'. Titania is a chemically inert, thermally stable, insoluble, biocompatible, non-toxic material, and an excellent absorber of destructive UV radiation. Because of these properties, titanium dioxide and its derivatives have for some time enjoyed a great and still growing popularity in many applications; see **Figure 1** [1–4].

On an industrial scale, titanium dioxide pigments are obtained by two methods (see **Figure 2**) whose names refer to the substrate salts used:

• the sulphate method, in which TiO<sup>2</sup> is precipitated from a solution of ilmenite ore by concentrated sulphuric acid, leading to both rutile and anatase;

• the chloride method, in which titanium dioxide is obtained by oxidation of titanium tetrachloride (TiCl<sup>4</sup> ) obtained by reduction and chlorination of ilmenite ore; this method leads only to rutile.

Advanced Hybrid Materials Based on Titanium Dioxide for Environmental and Electrochemical... http://dx.doi.org/10.5772/intechopen.69357 145

**Figure 1.** Applications of titania-based materials.

than basic. Titanium dioxide is thermally stable: it loses oxygen only at a temperature of a few hundred degrees Celsius and under the influence of reducing agents (carbon, magnesium, hydrogen and halogens). Its melting point is 1825°C, while its boiling point is close to 2500°C. Above 400°C, a reversible change in colour to yellow takes place as a result of thermal expansion of the crystalline lattice. Above 1000°C, the oxide forms of titanium are formed, charac-

and the electrical conductivity changes. Titanium dioxide does not show activity towards

Titanium dioxide occurs in nature in three polymorphous varieties: tetragonal rutile, anatase and rhombic brookite. Anatase and rutile are of practical importance and are commonly used in many applications, while brookite is not used because of the instability of its structure [4–6]. The rapidly developing technologies for obtaining new functional materials based on titania are an especially important topic for both theoretical study and practical application. The continually growing requirements of different technologies require new directions to be sought in order to obtain materials with precisely designed physicochemical and structural properties. Hybrid

cochemical properties resulting from the effects of combining the characteristic behaviours of the individual compounds from which they are made. The presence of a foreign element in the matrix of pure titania can greatly affect the structural, textural, acid/base and catalytic properties [7]. The textural properties of the hybrid materials, such as pore size distribution, surface area, etc., are strongly dependent upon the conditions of synthesis, including the nature and composition of the precursors, solvent, complexing/templating agent, hydrolysis and calcination conditions [8]. Research into the production and potential applications of new functional materials based on titanium dioxide is only possible when the final materials have a strictly defined dispersive character, crystalline structure, morphology and porous structure [9–13].

The synthesis of titanium dioxide is one of the major research areas in 'green chemistry'. Titania is a chemically inert, thermally stable, insoluble, biocompatible, non-toxic material, and an excellent absorber of destructive UV radiation. Because of these properties, titanium dioxide and its derivatives have for some time enjoyed a great and still growing popularity in

On an industrial scale, titanium dioxide pigments are obtained by two methods (see **Figure 2**)

• the chloride method, in which titanium dioxide is obtained by oxidation of titanium tet-

) obtained by reduction and chlorination of ilmenite ore; this method leads

is precipitated from a solution of ilmenite ore by con-

constitute a new group of compounds exhibiting strictly designed physi-

, an undesirable colour change takes place

terised by a lower content of oxygen than in TiO<sup>2</sup>

**2. Synthesis of titanium dioxide and its derivatives**

many applications; see **Figure 1** [1–4].

• the sulphate method, in which TiO<sup>2</sup>

rachloride (TiCl<sup>4</sup>

only to rutile.

whose names refer to the substrate salts used:

centrated sulphuric acid, leading to both rutile and anatase;

living organisms [1–4].

144 Titanium Dioxide

systems based on TiO<sup>2</sup>

The use of the chloride process is on the increase. It is estimated that over half of the world's titanium dioxide is produced in this way. Although this process requires higher quality, previously enriched ore and more complex technology, it produces much less waste than the sulphate method, and the cost of production is also lower [1].

The properties of TiO<sup>2</sup> are determined by the morphology of its particles, the size of its crystals and its crystalline structure, which depend on the choice of method for its synthesis and final heat treatment [14]. Nanocrystalline TiO<sup>2</sup> particles are usually obtained by crystallisation (chemical precipitation) [15], the microemulsion method (reverse micelles) [16], the sol-gel method [17–21] or hydrothermal crystallisation [22–26]; see **Figure 2**. Each of these methods has its advantages and drawbacks, but a feature of all of them is the ability to obtain materials with strictly defined properties (**Table 1**).

Additionally, **Table 2** presents a comprehensive review of different methods of synthesis of titania-based materials.

### **2.1. Chemical precipitation**

Co-precipitation is a wet chemical method and is one of the oldest methods for obtaining nanometric materials. The most common precursors used in this method are salts: nitrates,

**Figure 2.** Synthesis of titanium dioxide.


**Table 1.** Advantages and disadvantages of the most commonly used methods for the synthesis of titanium dioxide.

 chlorates or chlorides, which dissolve in an appropriate solvent. Aqueous solutions are used most commonly, but the use of organic solvents is also possible. The precipitation reaction must be initiated; this may be done by changing the pH, concentration or temperature. Another way of starting the precipitation reaction is to carry out a reaction of hydrolysis, oxidation, reduction or complexation. Usually a base (potassium or sodium hydroxide) is added to the system containing the precursors of the oxide. The precipitation reaction itself involves reduction of the metal cation, and formation of a precipitate requires that the system reach saturation point. The precipitation method consists of three stages: nucleation, growth and agglomeration. To begin with, small crystallites are formed (nucleation), which over


 chlorates or chlorides, which dissolve in an appropriate solvent. Aqueous solutions are used most commonly, but the use of organic solvents is also possible. The precipitation reaction must be initiated; this may be done by changing the pH, concentration or temperature. Another way of starting the precipitation reaction is to carry out a reaction of hydrolysis, oxidation, reduction or complexation. Usually a base (potassium or sodium hydroxide) is added to the system containing the precursors of the oxide. The precipitation reaction itself involves reduction of the metal cation, and formation of a precipitate requires that the system reach saturation point. The precipitation method consists of three stages: nucleation, growth and agglomeration. To begin with, small crystallites are formed (nucleation), which over

**Table 1.** Advantages and disadvantages of the most commonly used methods for the synthesis of titanium dioxide.

**Method Advantages Disadvantages**

applications); defined morphological structure and high degree of particle dispersion

Homogeneous mixing of reactant precipitates reduces the reaction temperature; a simple direct process for the synthesis of fine material powders which are highly reactive in low-

consolidation is possible; smaller particle size and morphological control in powder synthesis; sintering at low temperature also possible; better homogeneity and phase purity than in traditional ceramic methods

Powders are formed directly from solution; it is possible to control particle size and shapes by using different starting materials and hydrothermal conditions; the resulting powders are highly reactive, which aid low-

An energy-efficient, environmentally friendly process; high-purity products can be synthesised; metastable and new phases can be accessed; simplified and precise control of the size, shape distribution, and crystallinity of the end product can be achieved via the adjustment of parameters such as reaction temperatures and time, the types of solvents,

Susceptibility to agglomeration of particles; high energy costs; large quantities of waste produced and requiring disposal

Not suitable for the preparation of a highly pure, stoichiometrically accurate phase; this method does not work well if the reactants have very different solubilities and precipitation rates; it does not meet universal experimental conditions for the synthesis of various types of

Raw materials for this process are expensive (in the case of metal alkoxides) compared with mineral-based metal ion sources; products have high carbon content when organic reagents are used in the preparative steps and this inhibits densification during sintering; since several steps are involved, close monitoring of the

Prior knowledge on the solubility of the starting materials is required; hydrothermal slurries are potentially corrosive; accidental explosion of the high-pressure vessel cannot be ruled out

The need for expensive autoclaves; safety issues during the reaction process; impossibility of observing the reaction process ("black box")

metal oxides

process is needed

Need to use high-quality substrates; production of only the rutile form, advanced technology; hazardous reaction environment, toxicity; danger of uncontrolled emission of chlorine gas

possibility of using low-quality ore

Sulphate method Ability to control the crystalline structure;

Chloride method High-quality products (including for medical

temperature sintering

temperature sintering

surfactants and precursors

Sol-gel route Low temperature processing and

Co-precipitation method

146 Titanium Dioxide

Hydrothermal route

Solvothermal method

Advanced Hybrid Materials Based on Titanium Dioxide for Environmental and Electrochemical... http://dx.doi.org/10.5772/intechopen.69357 147



**Product**

TiO2

**Starting materials**

Ti(OCH(CH3

C

H3

7

OH, NaOH, HNO

3

)2

)4 (TTIP),

**Conditions of synthesis**

Reaction: 80°C (5 h), then cooling

to room temperature,

ageing: 25°C for 24 h,

drying: 100°C for 12 h,

calcination: 200, 600 and 800°C

for 2 h

**Properties of obtained material**

Anatase-brookite (calcination at 200°C),

anatase-brookite-rutile (calcination at 600

and 800°C),

irregular clusters composed of spherical

nanomeric primary particles (200 nm—

pH = 2, 400 nm—pH = 9),

BET surface area: pH = 2—calcination

at: 200°C—186 m2/g, 600°C—48 m2/g,

pH = 4—calcination at: 200°C—109 m2/g,

600°C—42 m2/g

Anatase structure (400°C), rutile with a

–

[57]

small amount of

anatase (800°C), rutile (1000°C)

TiO2/SiO2 TiO2/ZrO2

Ti(OC(CH3

HNO3, C

H2

5

P123 and Macrogol 20000

(triblock copolymers),

ZrOCl2∙8H

TiO2, TiO2/ZrO2

TiOCl2, ZrO(NO3

cetyltrimethylammonium

bromide (CTAB), C

H2 OH 5

)2, NaOH,

Reaction: room temperature,

Irregular spherical agglomerates, crystalline

Photodecolorization

[60]

of the MB—sample

ZT8-600—94.1%

structure: 600°C—anatase TiO2 and

tetragonal ZrO2 crystals, 800°C—rutile and

anatase TiO2 and tetragonal ZrO2 crystals,

900°C—rutile TiO2 and tetragonal and

monoclinic ZrO2 crystals,

BET surface area: TT-600—88 m2/g, ZT8-

600—70 m2/g, ZT14-600—62 m2/g, ZT22-

600—63 m2/g, ZT32-600—61 m2/g

Amorphous structure of TiO2/Al

crystalline size: TiO2/Al

Al

O2

3

—4.1 nm,

BET surface area: TiO2/Al

ZrO2/Al

O2

3

—200 m

2/g

O2

3

—320 m

2/g,

O2

3

—4.9 nm, ZrO

2

/

TiO2/Al

ZrO2/Al

O2

—47% 3

O2

3

—99%,

O2 3

Ethanol conversion:

[61]

,

ZrO2/Al

TiO2/Al

O2 3

O2 3 and

2,4-Pentanedione,

Reaction: room temperature,

then 70°C

drying,

calcination: 500°C

n-butanol, alkoxides of the

respective metals

ageing: 80°C for 4 h,

drying: 100°C for 6 h,

calcination: 600-900°C for 2 h

O2

OH, Pluronic

)

)

4 (TBOT),

3

Si(OC

H2 5

HCl, Ti(OCH(CH3

)2

)

4 (TTIP)

)4 (TEOS), C

H2

5

OH,

Reaction: room temperature,

ageing: room temperature for 72 h,

drying: 80°C,

calcination: 1000°C for 2 h

Reaction: room temperature, after

Anatase structure,

Photodegradation

[59]

of Rhodamine B

(RhB)—90%

BET surface area: 149 m2/g

adding Zr precursor—80°C,

ageing: room temperature for 24 h,

drying: room temperature in air,

calcination: 800°C for 5 h

**Potential application**

Photodegradation

[55]

148 Titanium Dioxide

of methylene blue

(MB)—98%—samples

calcined at 200 or

600°C by pH = 2

 **Ref.**


**Table 2.** Synthesis of titania-based materials via different method.

time become more thermodynamically stable and larger (the growth stage); then as more time elapses, the permanent crystallites combine into persistent agglomerate forms (agglomeration). The properties of the final material are strongly influenced by the initial nucleation stage. An important part is also played by the particle agglomeration stage, which determines the morphological properties of the system [27, 28]. To obtain the final product, the dried precipitate is subjected to thermal treatment at the required temperature in an appropriate atmosphere.

Liu et al. [29] prepared titanium dioxide by five different methods: co-precipitation, the solgel route, hydrolysis, the hydrothermal method and sluggish precipitation. They investigated how the method used affected the physicochemical properties of the resulting TiO<sup>2</sup> . Synthesis of TiO<sup>2</sup> by the co-precipitation method was carried out using titanium tetrachloride, hydrochloric acid, hydrogen peroxide and ammonia. Titanium tetrachloride as a precursor of TiO<sup>2</sup> was added to hydrochloric acid and deionised water. The resulting mixture was maintained at a temperature below 10°C, and hydrogen peroxide was added. Finally, ammonia was added to the solution (pH = 10). The resulting sample was calcined at 500°C for 2 h. XRD analysis showed that this procedure of TiO<sup>2</sup> synthesis leads to a mixture of anatase and rutile phases with anatase predominant. The material synthesised by the co-precipitation method also demonstrated with good photocatalytic activity in the decomposition of helianthine (with absorbency equal to 0.25 and transmission equal to 60.0).

Muhamed Shajudheen et al. [30] synthesised titanium dioxide using titanium tetraisopropoxide as a Ti precursor and poly(vinyl pyrrolidone) (PVP) as a capping agent. To a mixture consisting of titanium tetraisopropoxide and propan-2-ol, PVP and then water were added. The resulting white precipitate was refluxed for 2 h and then stirred continuously for 1 day, followed by calcination at 800°C. The proposed co-precipitation method allows synthesis of the rutile phase of titania in a single-step process without impurities and other phases, as was confirmed by XRD and Raman spectroscopy.

To alter the physicochemical properties of titanium dioxide, Huang et al. [31] prepared SnO<sup>2</sup> /TiO<sup>2</sup> catalysts using five different preparation methods: the sol-gel method (SGM), the sol-hydrothermal method (SHM), the co-precipitation method (CM), a co-precipitationhydrothermal method (CHM) and the hydrothermal method (HM). They determined the impact of the methodology for obtaining SnO<sup>2</sup> /TiO<sup>2</sup> systems on the structure, chemical composition, particle sizes, specific areas, pore size distribution and energy band structure. The synthesis of SnO<sup>2</sup> /TiO<sup>2</sup> by the co-precipitation method was carried out using TiCl<sup>4</sup> and SnCl4 ∙5H<sup>2</sup> O, which were dissolved in deionised water. In the next step, the obtained solution was added to an aqueous solution of urea. At this stage of the synthesis, the reaction was carried out at a temperature of 80°C for 8 h. At the final stage, the resulting material was calcined at 550°C for 4 h. XRD analysis of the sample obtained by the co-precipitation method revealed the presence of diffraction peaks indicating a pure rutile structure. Moreover, peaks corresponding to SnO<sup>2</sup> were not present in the pattern. This may indicate that the Sn4+ ion was successfully incorporated into the crystal lattice sites of the titania to form uniform SnO<sup>2</sup> /TiO<sup>2</sup> solid solutions, or that the reflection bands attributed to SnO<sup>2</sup> overlapped with the crystalline plane of TiO<sup>2</sup> . Moreover, the synthesised sample demonstrated

**Product**

TiO2 TiO2/SiO2 and

Titanium(IV) n-butoxide,

tetraethylorthosilicate,

zirconium(IV) n-butoxide,

toluene

**Table 2.** Synthesis of titania-based materials via different method.

TiO2/ZrO2

HCl, TiF4, C

H3

7

OH, HF

5.5–44 h,

drying: in vacuum overnight,

calcination: 600°C for 90 min

Thermal treatment—300°C

to 9.0 nm,

BET surface area: TiO2/SiO

156 m2/g,

TiO2/ZrO

2

—from 95 to 106 m

2/g

2

—from 133 to

Anatase structure, crystallite size from 11.0

Conversion of

[74]

ethylene

TiO2/SiO to 32.4%,

TiO2/ZrO

to 39.5%

2

—from 22.2

2

—from 22.1

Thermal treatment—180°C for

**Starting materials**

**Conditions of synthesis**

**Properties of obtained material**

Anatase structure

**Potential application**

–

 **Ref.** [72]

150 Titanium Dioxide

good photocatalytic activity in the degradation of methyl blue. It was shown that the structure, crystallinity, dispersity, light adsorption properties and photocatalytic performance of SnO<sup>2</sup> /TiO<sup>2</sup> photocatalysts are critically dependent on the preparation method.

Another example of the use of a precipitation method to obtain TiO<sup>2</sup> /CeO<sup>2</sup> and TiO<sup>2</sup> /SnO<sup>2</sup> systems is reported by Yu et al. [32]. As precursors of titanium dioxide, cerium oxide and tin oxide, they used titanium(IV) sulphate(VI), cerium(III) nitrate(V) hexahydrate and tin chloride pentahydrate. The precipitating agent was an aqueous solution of ammonia. Aqueous solutions of the oxide precursors were stirred for 1 h until the components dissolved completely, and then, ammonia solution was added to the reaction mixture. The process was carried out at room temperature, and the pH of the reaction mixture was maintained at 10. After all solutions had been added in the appropriate quantities, the system was stirred for a further 3 h. The material was then subjected to an ageing process for 48 h. The resulting precipitate was dried at 105°C for 12 h and then calcined at 500°C for 6 h. Physicochemical analysis revealed specific surface areas of 108 and 59 m<sup>2</sup> /g respectively for the TiO<sup>2</sup> /CeO<sup>2</sup> and TiO<sup>2</sup> /SnO<sup>2</sup> systems. X-ray analysis showed the TiO<sup>2</sup> /CeO<sup>2</sup> system to have an anatase crystalline structure, while for TiO<sup>2</sup> /SnO<sup>2</sup> , the diffractogram contained peaks corresponding to rutile. In both cases, the crystalline structure of cerium or tin oxide was not observed. The catalytic properties of the systems were also investigated; they demonstrated excellent performance in the reduction of nitrogen oxides.

Zhang et al. [33] synthesised TiO<sup>2</sup> /ZrO<sup>2</sup> mixed oxide (with molar ratio = 1:1) by a co-precipitation method from TiCl<sup>4</sup> and ZrOCl<sup>2</sup> aqueous solutions, which were hydrolysed with ammonium hydroxide. The precursors of Ti and Zr were dissolved in deionised water, and then, HCl was added. Ammonia was added to the solution until pH = 10. Finally, the samples were dried at 110°C and then calcined at 500°C for 5 h. The Ti/Zr mixed oxide synthesised by a co-precipitation method was found to have a high specific surface area of 234 m<sup>2</sup> /g, which is linked to the amorphous structure of the material. No diffraction peaks characteristic of TiO<sup>2</sup> or ZrO<sup>2</sup> were detected in the obtained sample. Moreover, the TiO<sup>2</sup> /ZrO<sup>2</sup> mixed oxide exhibited good catalytic activity for the selective catalytic reduction of NO by NH3 .

### **2.2. Sol-gel method**

The sol-gel route is a wet chemical method and is a multi-step procedure involving both chemical and physical processes such as hydrolysis, polymerisation, gelation, drying, dehydration and densification. In a typical sol-gel process, a colloidal suspension or sol is obtained as a result of hydrolysis and polycondensation of precursors, which are usually inorganic metal salts or organometallic compounds such as metal alkoxides. Polycondensation and the loss of solvent lead to a transformation from the fluid sol to the solid gel phase [34–38]. The sol-gel method is based on hydrolysis and condensation of metal alkoxides or metal salts [39]. The process involves the reaction of a metal chloride with metal alkoxide or an organic ether, which is an oxygen donor, according to Eqs. (1) and (2):

$$\text{MCl}\_{\text{u}} + \text{M(OR)}\_{\text{u}} \rightarrow \text{ 2MO}\_{\text{u2}} + n\text{RCl} \tag{1}$$

$$\text{MCl}\_n + \text{(n/2)ROR} \rightarrow \text{MO}\_{n2} + n\text{RCl} \tag{2}$$

In these reactions, the formation of ≡M–O–M≡ type bonds is favoured by the condensation between ≡M–Cl and ≡M–OR, according to the reaction (3):

good photocatalytic activity in the degradation of methyl blue. It was shown that the structure, crystallinity, dispersity, light adsorption properties and photocatalytic performance of

systems is reported by Yu et al. [32]. As precursors of titanium dioxide, cerium oxide and tin oxide, they used titanium(IV) sulphate(VI), cerium(III) nitrate(V) hexahydrate and tin chloride pentahydrate. The precipitating agent was an aqueous solution of ammonia. Aqueous solutions of the oxide precursors were stirred for 1 h until the components dissolved completely, and then, ammonia solution was added to the reaction mixture. The process was carried out at room temperature, and the pH of the reaction mixture was maintained at 10. After all solutions had been added in the appropriate quantities, the system was stirred for a further 3 h. The material was then subjected to an ageing process for 48 h. The resulting precipitate was dried at 105°C for 12 h and then calcined at 500°C for 6 h. Physicochemical

In both cases, the crystalline structure of cerium or tin oxide was not observed. The catalytic properties of the systems were also investigated; they demonstrated excellent performance in

hydroxide. The precursors of Ti and Zr were dissolved in deionised water, and then, HCl was added. Ammonia was added to the solution until pH = 10. Finally, the samples were dried at 110°C and then calcined at 500°C for 5 h. The Ti/Zr mixed oxide synthesised by a co-precipita-

The sol-gel route is a wet chemical method and is a multi-step procedure involving both chemical and physical processes such as hydrolysis, polymerisation, gelation, drying, dehydration and densification. In a typical sol-gel process, a colloidal suspension or sol is obtained as a result of hydrolysis and polycondensation of precursors, which are usually inorganic metal salts or organometallic compounds such as metal alkoxides. Polycondensation and the loss of solvent lead to a transformation from the fluid sol to the solid gel phase [34–38]. The sol-gel method is based on hydrolysis and condensation of metal alkoxides or metal salts [39]. The process involves the reaction of a metal chloride with metal alkoxide or an organic ether,

*<sup>n</sup>* → 2 MO*<sup>n</sup>*/<sup>2</sup>

/CeO<sup>2</sup>

/ZrO<sup>2</sup>

.

/CeO<sup>2</sup>

/g respectively for the TiO<sup>2</sup>

, the diffractogram contained peaks corresponding to rutile.

mixed oxide (with molar ratio = 1:1) by a co-precipitation

aqueous solutions, which were hydrolysed with ammonium

system to have an anatase crystal-

and TiO<sup>2</sup>

/CeO<sup>2</sup>

/g, which is linked to the

mixed oxide exhibited good catalytic

+ *n*RCl (1)

or ZrO<sup>2</sup>

were

and

/SnO<sup>2</sup>

photocatalysts are critically dependent on the preparation method.

Another example of the use of a precipitation method to obtain TiO<sup>2</sup>

analysis revealed specific surface areas of 108 and 59 m<sup>2</sup>

and ZrOCl<sup>2</sup>

detected in the obtained sample. Moreover, the TiO<sup>2</sup>

activity for the selective catalytic reduction of NO by NH3

which is an oxygen donor, according to Eqs. (1) and (2):

MCl *<sup>n</sup>*

systems. X-ray analysis showed the TiO<sup>2</sup>

/SnO<sup>2</sup>

/ZrO<sup>2</sup>

tion method was found to have a high specific surface area of 234 m<sup>2</sup>

amorphous structure of the material. No diffraction peaks characteristic of TiO<sup>2</sup>

+ M (OR)

SnO<sup>2</sup>

152 Titanium Dioxide

TiO<sup>2</sup>

/SnO<sup>2</sup>

method from TiCl<sup>4</sup>

**2.2. Sol-gel method**

line structure, while for TiO<sup>2</sup>

the reduction of nitrogen oxides.

Zhang et al. [33] synthesised TiO<sup>2</sup>

/TiO<sup>2</sup>

$$\text{\#M-Cl} + \text{\#M-O-R} \rightarrow \text{\#M-O-M} \newline \text{\#+R-Cl} \tag{3}$$

In the reaction with ether (4), an alkoxide is formed as a result of a reaction of alcoholysis with ≡M–Cl:

$$\text{\#M-Cl} + \text{R-OR} \rightarrow \text{\#M-OR} + \text{R-Cl} \tag{4}$$

These reactions run slowly, and usually, the formation of inorganic oxide is favoured by elevated temperature in the range 80–150°C. The main reaction (5) between metal chloride and metal alkoxide takes place at room temperature and leads to a solution of metal chloroisopropoxide:

$$\text{\#M-Cl} + \text{\#M-OR} \rightarrow \text{\#M-OR} \# + \text{\#M-Cl} \tag{5}$$

The properties of materials obtained by the sol-gel method depend on such factors as pH, the presence of admixtures of other substances, the volume ratio of water to precursor, and the rate of stirring of the system. The pH also influences the size and shape of pores [38].

Advanced materials in a wide variety of forms, such as spherical or ultrafine shaped powders, fibres, thin film coatings, dense or porous materials including high-purity inorganic oxides, and hybrid (inorganic-organic) materials can be synthesised using the sol-gel method [40–43]. The sol-gel process is a useful synthetic method for the preparation of amorphous as well as structurally ordered products [44–49]. It is of particular interest because it gives very good compositional and morphological control over the product's properties, such as specific surface area, nanoparticle size and degree of aggregation. The precipitates or gels obtained by sol-gel processing are typically amorphous, exhibiting a fairly high specific surface area and are in some cases even (meso)porous. A transition from the amorphous to the crystalline phase is typically induced by thermal treatment at temperatures higher than 300°C, leading in most cases to a deterioration of the pore system and an increase in the particle size, associated with a decrease in the specific surface area. In sol-gel processing, for better control of the hydrolysis and condensation process, many different modifiers of alkoxide precursors can be used, including acetylacetone [50–52], acetic acid [50, 53] and other complex ligands.

Mesoporous TiO<sup>2</sup> nanocrystals were synthesised by Faycal Atitar et al. [54] using the sol-gel method in the presence of triblock copolymer as the structure directing agent. In the typical synthesis, triblock copolymer was first dissolved in ethanol, and the resulting mixture was added to CH3 COOH and HCl. Next, Ti(OC(CH3 )3 )4 (TBOT) was added to the mixture. The resulting TiO<sup>2</sup> nanocrystals were calcined at different temperatures (400, 500, 600, 700 and 800°C for 4 h) to demonstrate how their structural properties such as crystallite phases, morphology and mesoporosity affect the photocatalytic performance. X-ray analysis showed the samples to have a crystalline structure. The samples calcined at 400 and 500°C revealed the presence of anatase, but as the calcination temperature increased, the contribution of the anatase crystalline structure decreased in favour of rutile. All the synthesised materials were shown to be photocatalytically active, and the photocatalytic activity of mesoporous TiO<sup>2</sup> was strongly dependent on the final thermal treatment. The sample calcined at 500°C demonstrated higher activity in the decomposition of imazapyr (98%) and phenol (95%) compared with the commercially available Aeroxide TiO<sup>2</sup> (P-25). Based on the results obtained, the authors concluded that mesoporous titanium dioxide T-500 (calcined at 500°C) is an efficient material for the removal of organic pollutants from water.

The sol-gel route was also used by Mutuma et al. to obtain titanium dioxide [55]. The precursor used was titanium tetraisopropoxide (TTIP), which was dissolved in propan-2-ol and deionised water. The reaction mixture was heated to 80°C and maintained at that temperature for 5 h, after which it was cooled to room temperature. It was determined how the pH of the reaction system affected the physicochemical properties of the products. The pH was controlled by adding a precipitating agent in the form of a solution of sodium hydroxide or nitric(V) acid. The process was carried out at pH values of 2, 4, 7 and 9. The resultant systems were left to gel at room temperature for 24 h. The products were dried at 100°C for 12 h and then calcined at temperatures of 200, 600 and 800°C for 2 h. It was also determined how the process conditions influenced the crystalline and porous structures of the synthesised materials, which have a significant effect on their photocatalytic properties. X-ray analysis showed that titanium dioxide that had not undergone calcination had an anatase structure. Calcination at 200°C led to the appearance of brookite, although the intensity of the band corresponding to that crystallographic form decreased as the calcination temperature increased. When the calcination temperature increases to 800oC, bands corresponding to rutile appeared, indicating a transformation of anatase to rutile. In photocatalytic tests, the materials containing anatase-brookite (calcined at 200°C) or anatase-brookite-rutile (600 and 800°C) exhibited better photocatalytic properties than an anatase-rutile system (800°C). It was also found that the specific surface area of the products depends strongly on the pH of the reaction system. As the pH increased, the surface area of the synthesised materials decreased, irrespective of the calcination temperature. The experiments showed the systems with mixed crystalline structure to be an excellent photocatalytic material in the decomposition of non-biodegradable organic pollutants, for example from the textile industry.

Titanium dioxide powders were also prepared via the sol-gel method by Siwińska-Stefańska [56]. It was investigated how the conditions of preparation (addition of catalyst and chelating agent, temperature of calcination) affect the microstructural evolution, porous structure parameters and photocatalytic capability of the resulting TiO<sup>2</sup> powders. The results of dispersive analysis showed that an increase in the amount of catalyst used in the process of obtaining titanium dioxide results in an increase in particle diameter. Moreover, the diameter of particles tended to decrease with a decreasing quantity of chelating agent. The addition of chelating agent also caused significant changes in the crystalline structure and porous structure parameters of the resulting samples. The TiO<sup>2</sup> systems prepared by the sol-gel method with or without the addition of chelating agent exhibited relatively high photocatalytic activity in the decomposition of C.I. Basic Blue 9.

morphology and mesoporosity affect the photocatalytic performance. X-ray analysis showed the samples to have a crystalline structure. The samples calcined at 400 and 500°C revealed the presence of anatase, but as the calcination temperature increased, the contribution of the anatase crystalline structure decreased in favour of rutile. All the synthesised materials were shown to be photocatalytically active, and the photocatalytic activity of mesoporous TiO<sup>2</sup> was strongly dependent on the final thermal treatment. The sample calcined at 500°C demonstrated higher activity in the decomposition of imazapyr (98%) and phenol (95%) compared

authors concluded that mesoporous titanium dioxide T-500 (calcined at 500°C) is an efficient

The sol-gel route was also used by Mutuma et al. to obtain titanium dioxide [55]. The precursor used was titanium tetraisopropoxide (TTIP), which was dissolved in propan-2-ol and deionised water. The reaction mixture was heated to 80°C and maintained at that temperature for 5 h, after which it was cooled to room temperature. It was determined how the pH of the reaction system affected the physicochemical properties of the products. The pH was controlled by adding a precipitating agent in the form of a solution of sodium hydroxide or nitric(V) acid. The process was carried out at pH values of 2, 4, 7 and 9. The resultant systems were left to gel at room temperature for 24 h. The products were dried at 100°C for 12 h and then calcined at temperatures of 200, 600 and 800°C for 2 h. It was also determined how the process conditions influenced the crystalline and porous structures of the synthesised materials, which have a significant effect on their photocatalytic properties. X-ray analysis showed that titanium dioxide that had not undergone calcination had an anatase structure. Calcination at 200°C led to the appearance of brookite, although the intensity of the band corresponding to that crystallographic form decreased as the calcination temperature increased. When the calcination temperature increases to 800oC, bands corresponding to rutile appeared, indicating a transformation of anatase to rutile. In photocatalytic tests, the materials containing anatase-brookite (calcined at 200°C) or anatase-brookite-rutile (600 and 800°C) exhibited better photocatalytic properties than an anatase-rutile system (800°C). It was also found that the specific surface area of the products depends strongly on the pH of the reaction system. As the pH increased, the surface area of the synthesised materials decreased, irrespective of the calcination temperature. The experiments showed the systems with mixed crystalline structure to be an excellent photocatalytic material in the decomposition of non-biodegradable organic

Titanium dioxide powders were also prepared via the sol-gel method by Siwińska-Stefańska [56]. It was investigated how the conditions of preparation (addition of catalyst and chelating agent, temperature of calcination) affect the microstructural evolution, porous structure parameters

showed that an increase in the amount of catalyst used in the process of obtaining titanium dioxide results in an increase in particle diameter. Moreover, the diameter of particles tended to decrease with a decreasing quantity of chelating agent. The addition of chelating agent also caused significant changes in the crystalline structure and porous structure parameters of

(P-25). Based on the results obtained, the

powders. The results of dispersive analysis

systems prepared by the sol-gel method with or without the

with the commercially available Aeroxide TiO<sup>2</sup>

154 Titanium Dioxide

pollutants, for example from the textile industry.

and photocatalytic capability of the resulting TiO<sup>2</sup>

the resulting samples. The TiO<sup>2</sup>

material for the removal of organic pollutants from water.

In a report by Italian researchers [57], the sol-gel method was used to obtain an SiO<sup>2</sup> /TiO<sup>2</sup> system. The precursors of the dioxides were respectively TEOS and TTIP. First, TEOS was mixed with an organic solvent (ethanol) in the molar ratio TEOS:ethanol:water = 1:2:1. Hydrochloric acid was added to the mixture to maintain a pH of 1. After stirring for 6 h, TTIP was added to the system. The resulting sol was matured for 3 days to obtain a gel, which was then dried at room temperature for 7 days. The resulting materials then underwent calcination at temperatures of 600 and 800°C for 2 h. Detailed physicochemical analysis confirmed that the product consisted of titanium dioxide in rutile form and silicon dioxide. X-ray analysis showed that the system that had not been calcined had an amorphous structure, while calcination caused the formation of a crystalline structure. Calcination at 600°C leads to anatase, but when a temperature of 800°C is used, rutile appears. TEM microscopic images revealed a tendency for the agglomeration of particles in the samples. Variation in the molar ratio of the oxides was found not to have a significant effect on the morphology of the oxide system.

Siwińska-Stefańska et al. [58] reported the preparation of nano- and microstructured TiO<sup>2</sup> doped with Fe and Co by the sol-gel method and determined the effect of doping on the physicochemical properties of TiO<sup>2</sup> . The doped materials were found to contain particles of smaller diameter and lower homogeneity than pure TiO<sup>2</sup> . XRD analysis revealed that the addition of iron or cobalt to the titania preparation process has a significant effect on crystalline structure formation.

Fan et al. [59] prepared a mesoporous TiO<sup>2</sup> /ZrO<sup>2</sup> nanocomposite from titanium tetrabutoxide, ZrOCl<sup>2</sup> ∙8H<sup>2</sup> O, Pluronic P123 and Macrogol 20000 as double templates utilising the sol-gel method. In typical synthesis, to a solution of titanium tetrabutoxide and nitric acid, ethanol, Pluronic P123 and Macrogol 20000 were added. The resulting material was calcined for 5 h at 800°C. The structures and physicochemical properties of the products were determined by X-ray diffraction (XRD), Raman scattering studies and N<sup>2</sup> adsorption/desorption. The results proved that the use of double templates retarded the crystal phase transformation from anatase to rutile, and the obtained materials showed high thermal stability. Moreover, photocatalytic tests confirmed that the sample prepared with double templates exhibited higher photocatalytic activity in the decomposition of Rhodamine B (92%) than samples prepared with a single template (90 and 91%).

In another study, Shao et al. [60] obtained pure TiO<sup>2</sup> and TiO<sup>2</sup> /ZrO<sup>2</sup> system using the solgel method. The crystalline structure and particle shape and size were found to be strongly dependent on the calcination temperature and on the ratio of Zr to Ti. XRD analysis showed the crystalline structure of the synthesised materials to be significantly affected by the conditions of calcination. As the content of ZrO<sup>2</sup> increased, the intensity of the bands corresponding to anatase (TiO<sup>2</sup> ) decreased in favour of those corresponding to tetragonal ZrO<sup>2</sup> . When samples were treated at 800°C, the transformation of anatase to rutile was favoured, although a high content of zirconium dioxide retarded that effect. Further increase in the calcination temperature led to the transformation of tetragonal ZrO<sup>2</sup> to monoclinic.

Kraleva and Ehrich [61] obtained the oxide systems ZrO<sup>2</sup> /Al<sup>2</sup> O3 and TiO<sup>2</sup> /Al<sup>2</sup> O3 by the solgel route. They used 2,4-pentanedione as complexing agent and n-butoxide as solvent. The precursors of the component oxides were the alkoxides of the respective metals. The molar ratio of the precursors was 1:1. An appropriate quantity of the precursors was dissolved in the solvent, and the reaction system was stirred continuously for approximately 30 min. A complexing agent with a pH of 8 was then added, and the mixture was stirred for another 5 min. It was then heated to 70°C with the pH maintained at 8 for 10 min. Next, hydrolysis was performed by adding deionised water dropwise to the reaction mixture and stirring for 1 h. The sample was then left to cool at room temperature for 12 h, and then, the solvent was removed by pressure evaporation at 110°C. After drying, the system was calcined at 500°C. The resulting systems had high specific surface area (320 and 200 m<sup>2</sup> /g respectively for TiO<sup>2</sup> / Al2 O3 and ZrO<sup>2</sup> /Al<sup>2</sup> O3 ). The materials differed significantly in terms of crystallinity: the system containing titanium dioxide with aluminium oxide was completely amorphous, while that of zirconium dioxide with aluminium oxide had a crystalline structure. The synthesised materials were also shown to offer excellent performance as catalysts of the conversion of ethanol at 600°C, the products being H<sup>2</sup> and CO (syngas).

Kraleva et al. [62] used the sol-gel method to obtain TiO<sup>2</sup> /ZrO<sup>2</sup> systems with different contents of ZrO<sup>2</sup> (3, 6, 13 and 37% mol.). The synthesised materials were subjected to detailed physicochemical analysis. Analysis of their porous structure parameters showed that as the content of ZrO<sup>2</sup> increased, there was an increase in the BET specific surface area. X-ray spectroscopy revealed that the addition of zirconium dioxide also has a significant effect on the crystalline structure and the phase composition of the resulting oxide systems. The TiO<sup>2</sup> / ZrO<sup>2</sup> system obtained with a ZrO<sup>2</sup> content of 37% mol., calcined at 550°C, exhibited an amorphous structure. It was also observed that as the calcination temperature increased, diffraction bands appeared corresponding to srilankite—a mineral containing oxygen, titanium and zirconium.

### **2.3. Hydrothermal route**

The hydrothermal method is one of the most advanced techniques for obtaining metals and their oxides. Hydrothermal synthesis is a non-conventional method defined as crystal synthesis or crystal growth under high temperature and high pressure water conditions from substances which are insoluble at ordinary temperature and pressure (<100°C, <1 atm). Water may act both as a catalyst and as a component of the continuous phase during synthesis. Among the wet chemical preparation methods, the hydrothermal route has been recognised as an energy and time saver, with faster kinetics of crystallisation than classic co-precipitation or sol-gel methods. The hydrothermal method has proven to be an excellent method for the synthesis of powders, fibres, single crystals, monolithic ceramic bodies, and coatings on metals, polymers, and ceramics [63]. By adjusting simple parameters such as temperature, pressure or precursor concentration, it is possible to alter the characteristics of the product particles, e.g. crystalline phase and particle size. In the hydrothermal method, the temperature of crystallisation is usually lower than in a typical thermal process. The agglomeration of particles can be prevented by carrying out crystallisation under high pressure. The products obtained without calcination and grinding are of high quality. Using this method, it is possible to control the shape and size of particles; nonetheless, the process is slow and is unsuitable for use on an industrial scale [64].

Kraleva and Ehrich [61] obtained the oxide systems ZrO<sup>2</sup>

The resulting systems had high specific surface area (320 and 200 m<sup>2</sup>

Kraleva et al. [62] used the sol-gel method to obtain TiO<sup>2</sup>

Al2 O3

156 Titanium Dioxide

and ZrO<sup>2</sup>

tents of ZrO<sup>2</sup>

content of ZrO<sup>2</sup>

ZrO<sup>2</sup>

zirconium.

**2.3. Hydrothermal route**

/Al<sup>2</sup> O3

ethanol at 600°C, the products being H<sup>2</sup>

system obtained with a ZrO<sup>2</sup>

/Al<sup>2</sup> O3

). The materials differed significantly in terms of crystallinity: the sys-

/ZrO<sup>2</sup>

content of 37% mol., calcined at 550°C, exhibited an amor-

gel route. They used 2,4-pentanedione as complexing agent and n-butoxide as solvent. The precursors of the component oxides were the alkoxides of the respective metals. The molar ratio of the precursors was 1:1. An appropriate quantity of the precursors was dissolved in the solvent, and the reaction system was stirred continuously for approximately 30 min. A complexing agent with a pH of 8 was then added, and the mixture was stirred for another 5 min. It was then heated to 70°C with the pH maintained at 8 for 10 min. Next, hydrolysis was performed by adding deionised water dropwise to the reaction mixture and stirring for 1 h. The sample was then left to cool at room temperature for 12 h, and then, the solvent was removed by pressure evaporation at 110°C. After drying, the system was calcined at 500°C.

tem containing titanium dioxide with aluminium oxide was completely amorphous, while that of zirconium dioxide with aluminium oxide had a crystalline structure. The synthesised materials were also shown to offer excellent performance as catalysts of the conversion of

and CO (syngas).

physicochemical analysis. Analysis of their porous structure parameters showed that as the

troscopy revealed that the addition of zirconium dioxide also has a significant effect on the crystalline structure and the phase composition of the resulting oxide systems. The TiO<sup>2</sup>

phous structure. It was also observed that as the calcination temperature increased, diffraction bands appeared corresponding to srilankite—a mineral containing oxygen, titanium and

The hydrothermal method is one of the most advanced techniques for obtaining metals and their oxides. Hydrothermal synthesis is a non-conventional method defined as crystal synthesis or crystal growth under high temperature and high pressure water conditions from substances which are insoluble at ordinary temperature and pressure (<100°C, <1 atm). Water may act both as a catalyst and as a component of the continuous phase during synthesis. Among the wet chemical preparation methods, the hydrothermal route has been recognised as an energy and time saver, with faster kinetics of crystallisation than classic co-precipitation or sol-gel methods. The hydrothermal method has proven to be an excellent method for the synthesis of powders, fibres, single crystals, monolithic ceramic bodies, and coatings on metals, polymers, and ceramics [63]. By adjusting simple parameters such as temperature, pressure or precursor concentration, it is possible to alter the characteristics of the product particles, e.g. crystalline phase and particle size. In the hydrothermal method, the temperature of crystallisation is usually lower than in a typical thermal process. The agglomeration of particles can be prevented by carrying out crystallisation under high pressure. The products obtained without calcination and grinding are of high quality. Using this method, it is

(3, 6, 13 and 37% mol.). The synthesised materials were subjected to detailed

increased, there was an increase in the BET specific surface area. X-ray spec-

and TiO<sup>2</sup>

/Al<sup>2</sup> O3

/g respectively for TiO<sup>2</sup>

systems with different con-

by the sol-

/

/

Chae et al. [65] report the synthesis of titanium dioxide using titanium tetraisopropoxide (TTIP) as the precursor of TiO<sup>2</sup> in an ethanol–water mixture as solvent. TTIP was added dropwise to a mixture of ethanol, water and nitric acid with pH = 0.7. After being well stirred, the solution underwent a reaction in a hydrothermal reactor at 240–300°C for 4 h. X-ray diffraction (XRD) analysis showed that hydrothermal processing at 240–260°C leads to a product with the greatest crystallinity, containing an anatase crystalline phase and a small quantity of brookite. Increasing the temperature of the hydrothermal reaction above 260°C caused the formation of agglomerates of primary particles. The size of the particles was strongly influenced by the concentration of the titanium dioxide precursor and by the molar ratio of ethanol to water and less so by the temperature and time of the reaction. An increase in the ethanol:water molar ratio led to smaller particles; also, when that ratio exceeded 8, a less crystalline product was obtained, with a tendency for the formation of aggregates. An increase in the concentration of TTIP in the reaction mixture retarded the increase in particle size. Porous structure analysis confirmed that smaller particle sizes in the resultant materials corresponded to higher specific surface areas.

Zhang et al. [66] used the hydrothermal method to obtain TiO<sup>2</sup> nanowires with anatase crystalline structure. An appropriate quantity of white TiO<sup>2</sup> powder with anatase structure was placed in a teflon-lined autoclave, and 10 M NaOH was added up to 80% of the capacity of the reactor. The mixture was heated for 24 h at 200°C, and the product was then dried for 6 h at 70°C. The resulting material underwent detailed analysis using X-ray diffraction (XRD) and scanning, transmission and high-resolution electron microscopy (SEM, TEM, HRTEM). XRD analysis confirmed the very high purity of the product. SEM images showed the titanium dioxide to have the form of numerous nanowires with uncontaminated surfaces. It was also found that the product had an anatase crystalline structure. The obtaining of titanium dioxide in nanowire form was conditional on the use of NaOH, which acted as a "soft" matrix and on the high process temperature. A lower reaction temperature would favour the formation of titanium nanorods. Advantages of the reported process include its low cost, the high purity of the products, and the large number of TiO<sup>2</sup> nanowires produced.

Caillot et al. [67] carried out hydrothermal synthesis of TiO<sup>2</sup> /ZrO<sup>2</sup> oxide systems. It was determined how the process conditions affected the morphology, crystalline structure and specific surface area of the products. The precursors used were zirconium oxychloride (ZrOCl<sup>2</sup> ∙8H<sup>2</sup> O) and titanium tetrachloride (TiCl<sup>4</sup> ), which were added to a solution of ammonia water (NH3 ∙H<sup>2</sup> O). Hydrothermal processing took place at a temperature of 220°C under a pressure of 25 bar for 4 h. The resulting sample was dried at 120°C and finally calcined at 500°C for 10 h. Thermogravimetric analysis of the TiO<sup>2</sup> /ZrO<sup>2</sup> system following the hydrothermal process showed it to have high thermal stability. The diffractogram obtained for TiO<sup>2</sup> /ZrO<sup>2</sup> following calcination at 500°C indicated an amorphous structure. Porous structure analysis showed the oxide system to have a specific surface area of 209 m<sup>2</sup> /g.

Hirano et al. [68] investigated the catalytic properties and thermal stability of materials consisting of titanium dioxide and zirconium dioxide, obtained by the hydrothermal route from TiOSO<sup>4</sup> (titanium(IV) sulphate(VI)) and ZrCl<sup>4</sup> (zirconium tetrachloride). Solutions were placed in hydrothermal reactors and heated at a temperature of 200 or 240°C for 48 h. The precipitate was dried at 60°C. Samples were additionally heated for 1 h at temperatures ranging from 400 to 1000°C. Diffractograms obtained for samples following hydrothermal treatment at 240°C for 48 h indicate the increasing presence of the monolithic structure of ZrO<sup>2</sup> as the concentration of Zr in the initial solution increases. Diffraction bands corresponding to anatase are also visible. Transmission electron spectroscopy showed that the addition of ZrO<sup>2</sup> causes a decrease in the sizes of crystallites. The photocatalytic activity of the products was tested in the decomposition of methylene blue (MB) under ultraviolet radiation. The TiO<sup>2</sup> /ZrO<sup>2</sup> systems exhibited higher photocatalytic activity than a material consisting of pure TiO<sup>2</sup> .

### **2.4. Solvothermal method**

The solvothermal method is similar to the hydrothermal method, the difference lying in the type of solvent used: in the hydrothermal method it is water, while in the solvothermal method, it is a non-aqueous solvent. The range of temperatures used in the solvothermal method can be much greater than in the hydrothermal method and depends on the boiling point of the organic solvent used. In the solvothermal method, the control of the shape, size and crystallinity of TiO<sup>2</sup> particles is easier than in the hydrothermal method. The solvothermal method is considered a universal method for obtaining nanoparticles with a narrow range of size distribution. Using the solvothermal method, TiO<sup>2</sup> nanoparticles or nanotubes can be produced with or without a surfactant [69, 70].

Zhu et al. [71] described a method for obtaining mesoporous TiO<sup>2</sup> microspheres by a solvothermal route. The precursor used was titanium tetrabutoxide (TBOT), which was added to a solution of polyetherimide (PEI) and anhydrous alcohol. The resulting white suspension was transferred to an autoclave, where a reaction was carried out at 180°C for 24 h. The white precipitate was then washed with water and ethanol, dried for 6 h at 60°C, and calcined for 2 h at 400 or 500°C. The product was analysed using the XRD, SEM, TEM, HRTEM, XPS and BET techniques and UV-Vis absorption spectra. Photocatalytic activity was investigated based on the reaction of degradation of phenol and methyl orange (MO) under sunlight. Mesoporous anatase TiO<sup>2</sup> microspheres with high crystallinity were successfully obtained by the solvothermal method and exhibited high photocatalytic activity for both phenol and methyl orange.

The solvothermal method was used by Yang et al. [72] to synthesise titanium dioxide from titanium(IV) fluoride, which was dissolved in a mixture of deionised water and hydrochloric acid (used to stabilise the pH). The mixture was added, together with propan-2-ol and hydrofluoric acid, to a teflon-lined stainless steel autoclave. The reactor was placed in an electric oven at 180°C for between 5.5 and 44 h. X-ray analysis of the product showed the synthesised TiO<sup>2</sup> to have an anatase structure. The average particle size in the system was measured by scanning electron microscopy at 1.09 μm. Porous structure analysis showed the specific surface area of the titanium dioxide to be 1.6 m<sup>2</sup> /g.

Oshima et al. [73] used the solvothermal method to obtain TiO<sup>2</sup> nanoparticles. Here, a polymer gel was used, which enabled strongly dispersed and homogeneous particles to be obtained. First, polyvinyl alcohol (PVA) was dissolved in water at 70°C. The reaction mixture was then cooled to room temperature; next, the precursor of titanium dioxide [(NH<sup>4</sup> ) 8 (Ti<sup>4</sup> (C6 H4 O7)<sup>4</sup> (O<sup>2</sup> ) 4 ∙8H<sup>2</sup> O] was added, and water was evaporated off using microwaves. The resulting polymer gel was mixed with ethanol, which served as a solvent, and the mixture was placed in an autoclave and heated for 18 h at 230°C. Finally, the product was dispersed in water at 50–70°C. X-ray analysis showed that prior to the solvothermal process, the material had an amorphous structure, but the diffractograms obtained for the titanium dioxide following solvothermal treatment contained peaks indicating formation of the anatase crystalline structure. The particle size distribution was found by transmission electron microscopy to lie within the range 4.4–6.8 nm. Physicochemical analysis confirmed the soundness of the method used to obtain titanium dioxide, and that it leads to homogeneous particles without a tendency to form agglomerates.

TiOSO<sup>4</sup>

158 Titanium Dioxide

**2.4. Solvothermal method**

size and crystallinity of TiO<sup>2</sup>

anatase TiO<sup>2</sup>

TiO<sup>2</sup>

(titanium(IV) sulphate(VI)) and ZrCl<sup>4</sup>

48 h indicate the increasing presence of the monolithic structure of ZrO<sup>2</sup>

Transmission electron spectroscopy showed that the addition of ZrO<sup>2</sup>

of methylene blue (MB) under ultraviolet radiation. The TiO<sup>2</sup>

photocatalytic activity than a material consisting of pure TiO<sup>2</sup>

range of size distribution. Using the solvothermal method, TiO<sup>2</sup>

Zhu et al. [71] described a method for obtaining mesoporous TiO<sup>2</sup>

can be produced with or without a surfactant [69, 70].

face area of the titanium dioxide to be 1.6 m<sup>2</sup>

Oshima et al. [73] used the solvothermal method to obtain TiO<sup>2</sup>

in hydrothermal reactors and heated at a temperature of 200 or 240°C for 48 h. The precipitate was dried at 60°C. Samples were additionally heated for 1 h at temperatures ranging from 400 to 1000°C. Diffractograms obtained for samples following hydrothermal treatment at 240°C for

Zr in the initial solution increases. Diffraction bands corresponding to anatase are also visible.

sizes of crystallites. The photocatalytic activity of the products was tested in the decomposition

The solvothermal method is similar to the hydrothermal method, the difference lying in the type of solvent used: in the hydrothermal method it is water, while in the solvothermal method, it is a non-aqueous solvent. The range of temperatures used in the solvothermal method can be much greater than in the hydrothermal method and depends on the boiling point of the organic solvent used. In the solvothermal method, the control of the shape,

thermal method is considered a universal method for obtaining nanoparticles with a narrow

thermal route. The precursor used was titanium tetrabutoxide (TBOT), which was added to a solution of polyetherimide (PEI) and anhydrous alcohol. The resulting white suspension was transferred to an autoclave, where a reaction was carried out at 180°C for 24 h. The white precipitate was then washed with water and ethanol, dried for 6 h at 60°C, and calcined for 2 h at 400 or 500°C. The product was analysed using the XRD, SEM, TEM, HRTEM, XPS and BET techniques and UV-Vis absorption spectra. Photocatalytic activity was investigated based on the reaction of degradation of phenol and methyl orange (MO) under sunlight. Mesoporous

mal method and exhibited high photocatalytic activity for both phenol and methyl orange.

The solvothermal method was used by Yang et al. [72] to synthesise titanium dioxide from titanium(IV) fluoride, which was dissolved in a mixture of deionised water and hydrochloric acid (used to stabilise the pH). The mixture was added, together with propan-2-ol and hydrofluoric acid, to a teflon-lined stainless steel autoclave. The reactor was placed in an electric oven at 180°C for between 5.5 and 44 h. X-ray analysis of the product showed the synthesised

 to have an anatase structure. The average particle size in the system was measured by scanning electron microscopy at 1.09 μm. Porous structure analysis showed the specific sur-

/g.

gel was used, which enabled strongly dispersed and homogeneous particles to be obtained. First, polyvinyl alcohol (PVA) was dissolved in water at 70°C. The reaction mixture was then cooled

microspheres with high crystallinity were successfully obtained by the solvother-

(zirconium tetrachloride). Solutions were placed

/ZrO<sup>2</sup>

.

particles is easier than in the hydrothermal method. The solvo-

as the concentration of

causes a decrease in the

systems exhibited higher

nanoparticles or nanotubes

microspheres by a solvo-

nanoparticles. Here, a polymer

Supphasrirongjaroen et al. [74] used the solvothermal method to synthesise TiO<sup>2</sup> /SiO<sup>2</sup> and TiO<sup>2</sup> /ZrO<sup>2</sup> systems. It was investigated how the addition of Si or Zr affected the photocatalytic activity of the oxide system. Titanium tetraisobutanol (TNB), tetraethoxysilane (TEOS) and zirconium tetraisobutanol were used as sources of titanium, silicon and zirconium. Titanium dioxide with admixed SiO<sup>2</sup> and ZrO<sup>2</sup> was obtained by dissolving TEOS and zirconium tetraisobutanol in toluene. The resulting materials were placed in an autoclave (300°C, 2 h). The synthesis products were subjected to physicochemical analysis to determine how the process temperature affects the photocatalytic activity of the product. It was found that the samples treated at room temperature have higher photocatalytic activity. The process temperature was also found to have a significant effect on the specific surface area: in almost every case, the surface area was larger for the samples that had undergone calcination at 350°C. It was also found that the materials containing zirconium exhibited higher photocatalytic activity than those with silicon. The researchers concluded that the addition of an appropriate metal can improve the physicochemical properties of inorganic materials.

A study of the literature shows that research on the synthesis of advanced materials based on titanium dioxide is chiefly focused on the skilful control of processes (through appropriate choices of methods and conditions) serving to generate changes in the properties of those materials. Key factors include the selection of appropriate raw materials, optimization of the pH of the reaction system, modification of the relative quantities of reagents and selection of an optimum temperature for thermal processing. These process parameters make it possible to synthesise materials with controlled physicochemical and structural properties, including grain size and shape, degree of crystallinity, crystallite size and phase or surface composition, as well as chemical and thermal stability. Temperature has a particularly significant effect on the crystalline structure of materials based on TiO<sup>2</sup> , which in turn determines their potential applications. This applies both to the calcination temperature and to the conditions of synthesis. It is of particular interest to carry out reactions in hydrothermal or solvothermal conditions, leading to products not only having a precisely designed crystalline structure—with the use of a much lower temperature than in other conventional methods—but also exhibiting a unique and diverse morphology. These methods also enable greater control of the size and shape of particles. A fundamental weakness of these processes, however, is the difficulty of increasing their scale. When selecting an appropriate method for the synthesis of titanium dioxide or hybrid materials incorporating it, attention should be given to the possibility of obtaining those products on semi-industrial or full industrial scale. The transfer of optimum conditions of synthesis from the laboratory to larger-scale processes is often problematic and should continue to be the subject of intensive research.
