**Rare Earth‐Doped Anatase TiO2 Nanoparticles**

Vesna Ðorđević, Bojana Milićević and

Miroslav D. Dramićanin

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68882

#### **Abstract**

Titanium dioxide is a wide band‐gap semiconductor of high chemical stability, nontoxic‐ ity and large refractive index. Because of the high photocatalytic activity, anatase is a pre‐ ferred TiO2 form in many applications such as for air and water splitting and purification. Doping of TiO2 with various ions can increase the photocatalytic activity by enhancing light absorption in visible region and can alter structure, surface area and morphology. Also, by doping TiO2 with optically active ions, visible light via up‐ or downconversion luminescence can be produced. It is a challenge to optimize the synthesis procedure to incorporate rare earth RE3+ ions into the TiO2 structure due to large mismatch in ionic radii between the Ti4+ and RE3+ and because of the charge imbalance. Visible (VIS) and ultraviolet (UV) luminescence of several RE3+ ions can be obtained when incorporated into anatase TiO2 , also affecting microstructural characteristics of TiO<sup>2</sup> . It is of great importance to summarize publications on rare earth‐doped anatase TiO2 nanoparticles to find correct TiO<sup>2</sup> ‐RE combination to sensitize trivalent rare earths luminescence, as well as to predict or tune structural and morphological properties. A better understanding on these topics may progress the desired design of this kind of material towards specific applications.

**Keywords:** anatase, rare earth ions, photoluminescence, photocatalysis

## **1. Introduction**

Rare earth (RE) elements are sixth period elements in the periodic table, from 57La to 71Lu. Because of many similarities, such as ionic +3 charges and similar ionic radius, 39Y that also belongs to the III transition group and is positioned just above 57La is also often considered

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as a part of the RE group. Even though the group is regarded as rare earth elements, they are not particularly rare. However, they are costly but highly efficient for many technological applications, mainly in lighting and display devices. With the absence of 57La and 71Lu, RE atoms, all have incompletely filled 4f orbitals that are positioned in the inner shell of xenon [Xe: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 ] electron configuration, which are responsible for their emission properties. Since they are shielded by outer 5s2 and 5p6 orbitals, electrons from 4f orbitals do not participate in bonding and are only slightly affected by the surroundings of the ions. Ionic +3 charges are the most often, although some cases +2 and +4 can be stable as presented in **Table 1**. Electronic states are noted as 2*<sup>S</sup>*+1*L*<sup>J</sup> , where *L* is the orbital angular momentum, *S* is the spin angular momentum, and *J* is the total angular momentum, and cor‐ responding notations are also presented in **Table 1**. Lanthanide contraction makes significant decrease of ionic radii in the series with an increase in atomic number, and the values for six‐coordinated RE3+ are also presented in **Table 1**.

Laporte's selection rule states that electron transitions between 4f states are forbidden, but they become partially allowed when RE ions are incorporated in non‐symmetric sites [2, 3]. In that way, each ion has characteristic 4f energy levels with narrow‐emission lines that depend on the crystalline environment of the host material in the order of few hundred cm−1. The Dieke diagram is the energy‐level diagram of trivalent lanthanide 4f electrons of RE3+


**Table 1.** Outer electronic configurations of RE atoms and ions, outside of the [Xe] shell, ground‐state term of RE3+ and radii of 6‐coordinated *RE*3+ (taken from Ref. [1]).

incorporated in LaCl3 crystals, which can be found in the original or revised form, which is informative for many materials [4–7]. It schematically represents variations between ground‐ and excited‐level energies or rare earth ions, proposing emissions of almost any colour in visible spectra by using one, or a combination of various RE ions in hosts.

as a part of the RE group. Even though the group is regarded as rare earth elements, they are not particularly rare. However, they are costly but highly efficient for many technological applications, mainly in lighting and display devices. With the absence of 57La and 71Lu, RE atoms, all have incompletely filled 4f orbitals that are positioned in the inner shell of xenon

4f orbitals do not participate in bonding and are only slightly affected by the surroundings of the ions. Ionic +3 charges are the most often, although some cases +2 and +4 can be stable

momentum, *S* is the spin angular momentum, and *J* is the total angular momentum, and cor‐ responding notations are also presented in **Table 1**. Lanthanide contraction makes significant decrease of ionic radii in the series with an increase in atomic number, and the values for

Laporte's selection rule states that electron transitions between 4f states are forbidden, but they become partially allowed when RE ions are incorporated in non‐symmetric sites [2, 3]. In that way, each ion has characteristic 4f energy levels with narrow‐emission lines that depend on the crystalline environment of the host material in the order of few hundred cm−1. The Dieke diagram is the energy‐level diagram of trivalent lanthanide 4f electrons of RE3+

**Atomic number Name RE symbol Atom RE2+ RE3+ RE4+ 2S+1LJ Radii** *REVI*

5d1

5d1

66 Dysprosium Dy 4f10 6s2 – 4f9 4f8 11H15/2 0.912

67 Holmium Ho 4f11 6s2 – 4f10 – <sup>5</sup>

68 Erbium Er 4f12 6s2 – 4f11 – <sup>4</sup>

69 Thulium Tm 4f13 6s2 4f13 4f12 – <sup>3</sup>

70 Ytterbium Yb 4f14 6s2 4f14 4f13 – <sup>2</sup>

6s2

**Table 1.** Outer electronic configurations of RE atoms and ions, outside of the [Xe] shell, ground‐state term of RE3+ and

6s2 – [Xe] – <sup>1</sup>

6s2 – 4f2 4f1 <sup>3</sup>

6s2 4f4 4f3 4f2 <sup>4</sup>

6s2 – 4f4 – <sup>5</sup>

6s2 4f6 4f5 – <sup>6</sup>

6s2 4f7 4f6 – <sup>7</sup>

6s2 – 4f8 4f7 <sup>7</sup>

6s2 – 4f7 – <sup>8</sup>

– 4f14 – <sup>1</sup>

6s2 – 4f1 [Xe] <sup>2</sup>

] electron configuration, which are responsible for

orbitals, electrons from

*3+*  **[Å]**

, where *L* is the orbital angular

S0 1.032

F5/2 1.020

H4 0.990

I9/2 0.983

I4 0.970

H5/2 0.958

F0 0.947

S7/2 0.938

F6 0.923

I8 0.901

I15/2 0.890

H6 0.880

F7/2 0.868

S0 0.861

and 5p6

[Xe: 1s2

26 Titanium Dioxide

 2s2 2p6 3s2 3p6 4s2

3d10 4p6

their emission properties. Since they are shielded by outer 5s2

as presented in **Table 1**. Electronic states are noted as 2*<sup>S</sup>*+1*L*<sup>J</sup>

six‐coordinated RE3+ are also presented in **Table 1**.

57 Lanthanum La 5d1

58 Cerium Ce 4f1

59 Praseodymium Pr 4f3

60 Neodymium Nd 4f4

61 Promethium Pm 4f4

62 Samarium Sm 4f6

63 Europium Eu 4f7

64 Gadolinium Gd 4f7

65 Terbium Tb 4f9

71 Lutetium Lu 4f14 5d1

radii of 6‐coordinated *RE*3+ (taken from Ref. [1]).

5s2

4d10 5p6

Luminescent materials that absorb energy as light and do not emit it as heat, but as ultraviolet, visible or infrared (IR) light, are called phosphor materials. Typically, they are composed of insulating or semiconducting host material that is doped with activator ions. Phosphors with RE ions as activators are important materials that have found applications in artificial light, cathode‐ray tubes, vacuum fluorescent and field emission displays, solid‐state lasers, and so on [8]. It is now a custom to refer materials that have at least one dimension less than 100 nm as nanomaterials. The great number of atoms in top layers of nanoparticles significantly alters their optical properties; hence, it is justified to name nanostructured phosphors as a nanophosphors. Today, nanophosphors can be found in many forms, such as nanopowders, composites, coatings and thin films, giving new possibilities for application in bio‐imaging and various types of physical and chemical sensing [9–11].

Photoluminescence of RE ions can be induced by the absorption of light through host lattice (host, H) that is transferred to RE ion (activator, A), directly exciting A, or energy transfer from other exited ions (sensitizer, S) that are also incorporated in matrix. A schematic diagram showing direct and indirect excitations with energy transfer resulting in the emission of light or heat is presented in **Figure 1(a)**.

When RE ions are used as activators in phosphor materials, depending on the positions of energy levels in RE ion, two main energy conversion mechanisms can lead to radiative energy transfer that results in the emission of light, one being downconversion, and the other upconversion. As it can be seen in **Figure 1(b)**, the principal difference between the two is the difference in excited and emitted energies. As schematically presented, in downconversion process electrons are excited by higher‐energy photons compared to energy obtained from emission. In the process, prior to the emission of photons some energy is lost by non‐radiative transitions. Oppositely, in upconversion process electrons are excited by lower‐energy pho‐ tons compared to energy obtained from emission. In order to preserve energy conservation rule, more than one photon is necessary for either single‐ion excited‐state absorption process, or in energy transfer upconversion process where the second ion is the sensitizer ion.

In order to fully understand the processes of downconversion light emission, we refer to energy‐level diagram scheme presented in **Figure 2**. In honour of professor Alexander Jablonski, this type of energy diagrams is often called the Jablonski diagram. It qualitatively represents electronic energy levels as bolded horizontal lines and vibrational energy levels as a stack of horizontal lines in vertical energy diagram. Straight and wavy vertical arrows rep‐ resent transitions between the states, where straight arrow represents transition associated with photon, while wavy arrows represent non‐radiative transfers. A radiative decay process is a process in which electron releases some of its excitation energy as photon, while in a non‐ radiative decay excess energy is transferred into thermal motions, as vibration, rotation and translation processes, heat. Once an electron is excited through very quick process of absorp‐ tion of photon, into, for example, some vibronic state of second excited singlet state, there are several ways that energy may be dissipated. The first is through vibrational relaxation, a non‐radiative process that lowers energy of electron to the lowest excited singlet state, with or without non‐radiative internal conversion process, depending on the overlap of vibra‐ tional and electronic energy of different states. Next, a radiative process of energy transfer to ground singlet state is followed by emission of photons in terms of fluorescence. There is no change in multiplicity *S*<sup>1</sup> →*S*<sup>0</sup> , so the transition is spin allowed and consequently fast. Since there are a large number of vibrational levels in electronic states, transitions can result in a range of emitted wavelengths. There is also a probability of non‐radiative relaxations between

**Figure 1.** (a) Direct and indirect excitation with energy transfer resulting in emission of light or heat, by activators (A), hosts (H) and sensitizers (S). (b) Basic mechanisms of downconversion and upconversion luminescence.

**Figure 2.** Radiative and non‐radiative processes with corresponding approximate time interval of the processes in energy‐level diagram scheme. *S*<sup>0</sup> , ground singlet state; *S*<sup>1</sup> , *S*<sup>2</sup> , excited singlet states; *T*<sup>1</sup> , excited triplet state.

the singlet states (*S*<sup>1</sup> ⤳*S*<sup>0</sup> ). If in the process of dissipating of energy spin multiplicity changes by slower process of intersystem crossing, energy can be radiatively emitted from lowest excited triplet state to ground singlet state by phosphorescence *T*<sup>1</sup> →*S*<sup>0</sup> , or non‐radiatively by relaxations between the triplet and singlet states (*T*<sup>1</sup> ⤳*S*<sup>0</sup> ). Intersystem crossing and therefore phosphorescence are spin‐forbidden processes; nevertheless, by coupling vibrational factors into the selection rules transitions become partially allowed, and they are consequently much slower.

#### **2. Synthesis of rare earth‐doped anatase TiO2 nanoparticles**

are several ways that energy may be dissipated. The first is through vibrational relaxation, a non‐radiative process that lowers energy of electron to the lowest excited singlet state, with or without non‐radiative internal conversion process, depending on the overlap of vibra‐ tional and electronic energy of different states. Next, a radiative process of energy transfer to ground singlet state is followed by emission of photons in terms of fluorescence. There is no

there are a large number of vibrational levels in electronic states, transitions can result in a range of emitted wavelengths. There is also a probability of non‐radiative relaxations between

**Figure 1.** (a) Direct and indirect excitation with energy transfer resulting in emission of light or heat, by activators (A),

**Figure 2.** Radiative and non‐radiative processes with corresponding approximate time interval of the processes in

, excited singlet states; *T*<sup>1</sup>

, excited triplet state.

, *S*<sup>2</sup>

, ground singlet state; *S*<sup>1</sup>

hosts (H) and sensitizers (S). (b) Basic mechanisms of downconversion and upconversion luminescence.

, so the transition is spin allowed and consequently fast. Since

change in multiplicity *S*<sup>1</sup>

28 Titanium Dioxide

energy‐level diagram scheme. *S*<sup>0</sup>

→*S*<sup>0</sup>

TiO2 nanoparticles present several advantages for applications compared to their bulk counterparts. Their high‐surface‐to‐volume ratio, improved charge transport and lifetime, afforded by their dimensional anisotropy, allows efficient contribution to the separation of photo‐generated holes and electrons [12]. The properties of TiO2 depend on its crystal struc‐ ture, surface chemistry, dopants, doping levels, crystallization degree, size and morphol‐ ogy [13]. Hence, it is of great importance to control the particle size, shape and distribution of the synthetized TiO2 . To achieve desired characteristics, a variety of TiO2 nanostructures have been prepared, such as nanoparticles, nanotubes, nanorods, nanofibres, nanosheets and nanofilms. These structures can be synthetized through various preparation methods, such as sol‐gel, direct oxidation, micelle and inverse micelle techniques, sonochemical, hydrothermal/ solvothermal, microwave, chemical vapour deposition, physical vapour deposition and elec‐ trospray deposition [14–17]. Significant progress has been made in the last 10 years regarding new approaches to the preparation of TiO2 . These include doping TiO2 with optically active rare earth ions (RE). TiO2 can be considered as an 'unusual' matrix for doping with RE3+ ions due to the large mismatch of both charge and ionic radius between the dopant and the host constituent cations. It is a challenge even now to optimize the synthesis procedure in the way to efficiently incorporate RE3+ ions into TiO2 nanostructure and to obtain material with high crystallinity. Spectroscopic studies have showed that the RE ions can reside in the anatase in three different sites [18–20]. In nanopowders, substantial number of RE ions occupies the sites near the surface with the lowest point symmetry.

TiO2 occurs in three most abundant crystalline phases in nature: anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic). Rutile TiO2 is the most stable form, while ana‐ tase and brookite phases are metastable and can be transformed to rutile phase at higher temperatures. Even though rutile is denser and thermodynamically more stable than anatase, this significant temperature treatment is not favourable for the formation of nanoparticles with a diameter lower than 15 nm, which is a feature of anatase form TiO2 [21, 22].

*Sol‐gel* synthesis is the most common method for the preparation of RE‐doped TiO2 nanopar‐ ticles, being simple, cost‐effective and low‐temperature procedure, with the ability to fab‐ ricate nanostructure with high purity, homogeneity and controllable morphology. This synthesis includes the process of hydrolysis and poly‐condensation of Ti–OH–Ti or Ti–O–Ti bonds forming densely three‐dimensional structure that after heating changes from sol to gel, and after thermic treatment results in the form of oxide. Titanium source precursors can be alkoxides (such as titanium (IV)‐isopropoxide (TTIP), titanium (IV)‐butoxide (TBT)) or titanium (IV)‐chloride (TiCl4 ). RE ion precursors can be acid‐soluble oxides (RE2 O3 ) or water‐ soluble nitrates, acetates or chlorides (RE(NO3 )3 ·*x*H2 O, RE(CH3 COO)3 ·*x*(H2 O), RECl3 ).

In the method of hydrolysis of TTIP, products are characterized by low surface area, wide pore size distribution with contribution to pores of mesopores scale (<50 nm) [22]. The sol‐gel synthesis with a two‐step procedure of mixing precursor solutions was successfully used to obtain RE‐doped TiO2 [18, 23–36]. The gels obtained in such procedures undergo various temperature treatments, which are summarized in **Table 2**. In the method of hydrolysis of TiCl4 , which is another sol‐gel method for the preparation of RE‐doped anatase TiO2 , minor amounts of brookite phases are often present and slightly larger crystallite size compared to RE‐doped TiO2 from the titanium alkoxides is reported [13, 37].

*Hydrothermal* synthesis is a heterogeneous chemical reaction in the presence of an aqueous solvent, above room temperature (<200°C) in a closed system, where the pressure is elevated.



be alkoxides (such as titanium (IV)‐isopropoxide (TTIP), titanium (IV)‐butoxide (TBT)) or

)3 ·*x*H2

In the method of hydrolysis of TTIP, products are characterized by low surface area, wide pore size distribution with contribution to pores of mesopores scale (<50 nm) [22]. The sol‐gel synthesis with a two‐step procedure of mixing precursor solutions was successfully used to

temperature treatments, which are summarized in **Table 2**. In the method of hydrolysis of

amounts of brookite phases are often present and slightly larger crystallite size compared to

*Hydrothermal* synthesis is a heterogeneous chemical reaction in the presence of an aqueous solvent, above room temperature (<200°C) in a closed system, where the pressure is elevated.

> **Crystallite size\* (nm)**

, which is another sol‐gel method for the preparation of RE‐doped anatase TiO2

from the titanium alkoxides is reported [13, 37].

**Crystalline phase**

– – 400–700 A 8.14–79.1 25–117 3.26–6.4 [13, 18,

– – 500–800 A + R 14.1–101.8 0.59–17.94 4.68 [22–24, 29] – – 800–1000 R 32.7–100 0.34–16.7 – [22–24] Sc 2 500 A + B 16.6 – – [37] Sc 2 500–550 A 16.6–26.9 – – [13] Sc 2 600 A + R 45.0 – – [13] Sc 2 650–800 R 51.7–65.2 – – [13] Y 0.25–2 400–500 A 8.5–9.4 89.68–151 – [28, 30, 31] La 0.1–10 500 A 8.57–13.40 46.51–105.66 4.90–12.34 [25] La – 600 A + R 17.2 36.7 – [32] Ce 0.1–10 500 A 8.68–13.79 53.31–94.49 5.46–12.52 [24, 25] Ce 5 800 A + CeO2 – – – [24] Pr 0.25–1 400–650 A 9–20 77.5–134 – [28, 33, 40] Nd 0.05–4 400–700 A 10–20 7.5–75 – [24, 34, 40,

Nd 0.1–5 800 A + R 25 <1.0 – [24, 41]

Sm 0.3–3 420–700 A 5.8–12 50.78–95.9 5.20 [18, 29, 34,

Sm 0.3–0.5 800 A + R – 16.1–24.7 – [38]

Nd4 Ti9 O24

). RE ion precursors can be acid‐soluble oxides (RE2

O, RE(CH3

[18, 23–36]. The gels obtained in such procedures undergo various

**BET surface area (m2 /g)**

– – – [24]

COO)3

·*x*(H2

O3

).

O), RECl3

**Pore diameter (nm)**

**Refs.**

22–28, 38, 39]

41]

35, 38, 42]

) or water‐

, minor

titanium (IV)‐chloride (TiCl4

obtain RE‐doped TiO2

**Dopant ions Doping conc. (%)**

RE‐doped TiO2

TiCl4

30 Titanium Dioxide

soluble nitrates, acetates or chlorides (RE(NO3

**Calcination temperature (°C)**

Nd 0.1–5 900–1000 A + R +

**Table 2.** The sol‐gel synthesis conditions and major physicochemical properties of RE‐doped TiO2 nanostructures; A‐anatase, B‐brookite, R‐rutile.

The synthesis has been used to produce homogeneous, high‐purity, crystalline nanostruc‐ tured RE‐doped TiO2 with different morphologies: nanotubes, nanobelts, nanowires or spherical nanoparticles. Alkoxide Ti precursors and water‐soluble RE precursors are acti‐ vated by acids or bases prior to the temperature treatments in Teflon‐liners autoclave up to several days [28, 44–53]. Obtained precipitates should be washed to neutral pH [47] before calcination in order to gain well‐defined TiO<sup>2</sup> nanoparticles. Also, hydrothermal route can use the synthesized or commercial available TiO2 nanoparticle without the post‐calcination treatment [44, 46]. The main difference of *solvothermal* synthesis is using other solvents than water. The obtained samples are spherical nanoparticles with an average diameter of 16 nm and the doping process can be easily achieved without significant loss of dopants [54]. The main characteristics and major physicochemical properties of RE‐doped TiO2 nanostructures synthetized by hydrothermal and solvothermal are summarized in **Table 3**.

*Electrospinning method* can be employed to produce nanostructure RE‐doped TiO2 with fibre morphology and the average fibre diameter in the range of 35–80 nm. Typically, RE‐doped TiO<sup>2</sup>



**Dopant ions**

**Doping conc.** 

**Hydrothermal** 

**Calcination** 

**Crystalline** 

**Crystallite** 

**BET surface** 

**Morphology**

**Refs.**

**area (m2/g)**

**size (nm)**

**temperature (°C)**

**phase**

**treatment (°C)**

**(%)**

–

–

Y Y La La Pr Pr Pr Nd Sm Eu

0.25–0.5

130–200

400–500

A

8.6

133

Spherical particle

[28, 56]

(*d* = 5–15 nm)

Sub‐microspheres

[52]

(*d* = 300 nm)

Spindle particles

[53]

(*d* = 50–100 nm, *l*\* = up to

several μm)

Nanorods (*d* = 10–20 nm,

[53]

*l* = up to several μm)

Nanobelts (*w*\* = 200–400

[45]

nm, *l* = several μm)

1

150

500

A

16

–

Spherical particles

[54]

(*<sup>d</sup>*

∼ 16 nm)

0.3

80

–

0.3

80

–

A + R A + R

–

220

–

–

200

–

[51]

[51]

0.25

160

400

A

9.0

127

Spherical particle

[28]

(*d* = 5–15 nm)

0.25–2.0

100

400

A

5.04–6.22

155–170

Spherical particle

[55]

(*d* < 10 nm)

0.3

80

–

0.11–0.53

200

500

A + R A + R

–

–

–

22.32–24.38

69–86

Spherical particle

[44]

[51]

0.3

80

–

A + R

–

–

–

[51]

0.25

150–160

≤400

A

9.8

120–157

Spherical particle

[28, 47]

(*d* = 5–15 nm)

–

200

500

A + R

22.8

53–165

Spherical particle

[44, 51]

–

140–160

≤400

A

9.3–30

102–312.5

Spherical particle

[28, 47–49, 54]

32 Titanium Dioxide

(*d*\* = 10–30 nm)

**Table 3.** Hydrothermal and solvothermal synthesis conditions and major physicochemical properties of RE‐doped TiO2 nanostructures. nanofibres are fabricated with the use of polymer solvents of polyvinyl pyrrolidone (PVP) or polyvinyl alcohol (PVA), titanium alkoxides and RE chlorides or nitrates. Starting solutions in glass syringe with stainless‐steel needle are connected to a high voltage and electrospun in air at different tensions, needle‐target distances and feed rates [57–61]. In order to remove the poly‐ meric component and obtain nanocrystalline anatase, RE‐doped TiO2 , as‐spun nanofibres were calcined at 500°C. However, the pure phase of RE‐doped rutile TiO2 can be obtained after higher calcination temperature (>1000°C). The synthesis conditions and major physicochemical prop‐ erties of RE‐doped TiO2 nanostructures reported in the literature are summarized in **Table 4**.



nanofibres are fabricated with the use of polymer solvents of polyvinyl pyrrolidone (PVP) or polyvinyl alcohol (PVA), titanium alkoxides and RE chlorides or nitrates. Starting solutions in glass syringe with stainless‐steel needle are connected to a high voltage and electrospun in air at different tensions, needle‐target distances and feed rates [57–61]. In order to remove the poly‐

calcination temperature (>1000°C). The synthesis conditions and major physicochemical prop‐

PVP, TTIP – – 400–500 A [57, 58] PVP, TTIP – – 500–900 A + R 15.71–40 [57–59] PVP, TTIP – – 1000 R [57]

**Crystalline phase**

Y 1–2 500 A + R 11.35–13.8 [59]

Y 3 500 A 8.8 [59]

La 1 500–800 A 40 [57]

La 1 900–1000 A + R [57]

La 1 1100 R [57]

La 1 500 A [58]

La 1 700 A + R 12.51 [58]

Ce 1 500 A [58]

Ce 1 700 A + R 11.49 [58]

Nd 1 500 A [58]

Nd 1 700 A + R 10.2 [58]

Eu 1, 3 500–800 A 60, 70 [57]

Eu 1 900 A + R [57]

Eu2 Ti2 O7

Eu 3 900 A + R +

**Calcination temperature (°C)**

nanostructures reported in the literature are summarized in **Table 4**.

**Crystallite size** 

**(nm)**

, as‐spun nanofibres were

can be obtained after higher

**Fibre diameter** 

**Refs.**

[57]

**(nm)**

meric component and obtain nanocrystalline anatase, RE‐doped TiO2

calcined at 500°C. However, the pure phase of RE‐doped rutile TiO2

erties of RE‐doped TiO2

**Dopant ions Doping** 

**conc. (%)**

**Precursor materials**

34 Titanium Dioxide

PVP, TTIP, Y(NO3 ) 3

PVP, TTIP, Y(NO3 )3

PVP, TTIP, La(NO3 )3

PVP, TTIP, La(NO3 )3

PVP, TTIP, La(NO3 ) 3

PVA, TTIP, La(NO3 )3

PVA, TTIP, La(NO3 )3

PVA,,TTIP, Ce(NO3 )3

PVA, TTIP, Ce(NO3 )3

PVA, TTIP, Nd(NO3 ) 3

PVA, TTIP, Nd(NO3 ) 3

PVP, TTIP, Eu(NO3 )3

PVP, TTIP, Eu(NO3 ) 3

PVP, TTIP, Eu(NO3 ) 3

**Table 4.**The electrospinning synthesis conditions and major physicochemical properties of RE‐doped TiO2 nanostructures.

*Thermal plasma pyrolysis* is rarely used for the synthesis and preparation of RE‐doped TiO2 nanopowders, which enables highly crystallized and well‐dispersed nanoparticles due to the processing temperature (up to 1.0 × 104 K), rapid quenching rate at the plasma tail (~105–7 K/s) and very short residence time [62]. The advantage of this synthesis is that well‐dispersed and highly crystalline nanoparticles in a single processing step are obtained, without post‐annealing treatment. On the other hand, it promotes crystalliza‐ tion of several crystalline phases of TiO2 , and with small amount of RE dopants mixtures of anatase and rutile are formed, while at higher temperatures dititanate structures were also formed [62].

*Electrochemical* synthesis is a significant method in the preparation of TiO<sup>2</sup> nanotubes at substrates, providing the precise control of nanotube morphology, length and pore size, and the formation of thick walls at substrates. Electrolytes used in this procedure are fluo‐ rides, where the concentration strongly effects on the dimensions and pH on the thickness of TiO2 nanotubes [63, 64]. With anodic potential from 10 to 30 V, nanotubes with diam‐ eters between 15 and 200 nm are formed, and by cathodic electrochemical process RE ions are incorporated into the nanotubes. Also, magnetron‐sputtering method can be used to prepare RE‐doped TiO2 films [65] as well as evaporation‐induced self‐assembly method [66–69].

In order to investigate structural, morphological, photocatalytic and optical properties of RE‐doped anatase TiO2 nanopowders with a series of RE3+ ions (Pr, Nd, Sm, Eu, Dy, Tb, Ho, Er and Tm) at a fixed concentration of 1 at.%, the sol‐gel method has been used. To prepare samples, titanium (IV)‐isopropoxide, water, ethanol and nitric acid were mixed in 1:3:20:0.08 molar ratios and the synthesis procedure is schematically shown in **Figure 3** and given in Ref. [27].

**Figure 3.** Schematic representation of the sol‐gel synthesis with quantities of precursors used to prepare 3 g RE3+‐doped TiO2 nanopowders.

#### **3. The influence of rare earth doping on the stability of phase structure, surface area and morphology of anatase TiO2 nanoparticles**

that well‐dispersed and highly crystalline nanoparticles in a single processing step are obtained, without post‐annealing treatment. On the other hand, it promotes crystalliza‐

of anatase and rutile are formed, while at higher temperatures dititanate structures were

substrates, providing the precise control of nanotube morphology, length and pore size, and the formation of thick walls at substrates. Electrolytes used in this procedure are fluo‐ rides, where the concentration strongly effects on the dimensions and pH on the thickness

In order to investigate structural, morphological, photocatalytic and optical properties of

Er and Tm) at a fixed concentration of 1 at.%, the sol‐gel method has been used. To prepare samples, titanium (IV)‐isopropoxide, water, ethanol and nitric acid were mixed in 1:3:20:0.08 molar ratios and the synthesis procedure is schematically shown in **Figure 3** and given in

**Figure 3.** Schematic representation of the sol‐gel synthesis with quantities of precursors used to prepare 3 g RE3+‐doped

 nanotubes [63, 64]. With anodic potential from 10 to 30 V, nanotubes with diam‐ eters between 15 and 200 nm are formed, and by cathodic electrochemical process RE ions are incorporated into the nanotubes. Also, magnetron‐sputtering method can be used to

films [65] as well as evaporation‐induced self‐assembly method

nanopowders with a series of RE3+ ions (Pr, Nd, Sm, Eu, Dy, Tb, Ho,

*Electrochemical* synthesis is a significant method in the preparation of TiO<sup>2</sup>

, and with small amount of RE dopants mixtures

nanotubes at

tion of several crystalline phases of TiO2

also formed [62].

36 Titanium Dioxide

prepare RE‐doped TiO2

RE‐doped anatase TiO2

of TiO2

[66–69].

Ref. [27].

TiO2

nanopowders.

In most morphologies of calcinated TiO2 powders, anatase phase is stabile up to temperatures below 500°C. Anatase to rutile crystalline phase transformation occurs above this tempera‐ ture. In RE ions doped of anatase materials, the temperature of phase transformations shifts to higher values, suggesting the stabilization of anatase phase. As it can be seen in **Tables 2**–**4** in Section 2, phase transformations of RE‐doped anatase to rutile crystalline phase occur in the temperature range of 500–1000°C. There are three types of dominant nucleation modes in forming rutile from anatase, bulk, interface and surface, which lead to the phase transfor‐ mation. The proposed mechanisms affect the rate of grain forming and the density of rutile nucleation sites. The bulk nucleation of rutile particles is most likely to occur at tempera‐ tures above 500°C, when the grain boundary is surrounded by RE ions hindering the surface nucleation. The interface nucleation mode is dominant in the range of 550–680°C, when rutile particles with a larger crystallite size are formed on account of anatase particles, probably through aggregating of some anatase particles at the surfaces [70]. When calcination tempera‐ ture increases, the phase transformation is not completed because the surface region is still in the mixed phases of anatase and rutile, with increasing percentage of rutile particles. At the same time, the formation of multiphase RE‐titanate structures can also be noticed at higher temperatures, usually dititanates pyrochlore structures with a general formula of RE2 Ti2 O7 [22, 57, 60, 71]. The contribution of these structures increases with RE‐doping concentration [57], and it is more pronounced with RE ions with smaller ionic radius (heavier ions). When RE ions with a larger ionic radius occupy TiO2 lattice sites, ionic mobility is hindered and the possibility of forming other titanate phases is lower. The electrospinning sol‐gel route can be used to fabricate RE‐doped TiO2 with pure rutile phase at higher calcination temperature (>1000°C) without the formation of the RE2 Ti2 O7 phase [57].

The influence of doping TiO<sup>2</sup> with RE, where larger RE ions of different charge (+3) com‐ pared to Ti ions are introduced into the anatase phase, gives rise to substitutional defects and, consequently, the large decrease in the short lattice order, thus in the reduction of the crystallite size. With increasing the concentration of RE ions, amorphization of crystalline powders is expected. The contents of RE ions used in sol‐gel synthesis are usually in the 0.1–3% range, while further addition of RE ions (≥5%) effectively obstructs TiO<sup>2</sup> crystallinity owing to a lattice distortion, and remarkably reduces the crystallite size [22, 25]. The increase of doping concentration leads to a higher content of RE–O–Ti bonds that inhibit the growth of TiO2 crystal grains restricting the direct contact of anatase particles, shifts diffractions to lower 2 theta angles, and as a consequence of smaller crystallites, broadening of X‐ray dif‐ fraction (XRD) maxima [18, 55, 72, 73]. Even in undoped TiO2 , the anatase phase is reported to be thermodynamically stable at very low particle size. In respect to the particle size, it is reported that rutile phase can be formed when the crystallite size reaches a critical value of 12–20 nm [22]. Therefore, with the temperature increase, the crystallite size increases, which also favours anatase to rutile phase conversion. The influence of the incorporation of RE ions into the TiO2 is reflected in the reduction of the crystallite size that inhibits the transformation of anatase to rutile phase. Taking into account all possible RE‐doping effects on the stability of anatase phase, size and concentration of RE ion, applied synthesis method and calcination temperature, a number of parameters may be varied in an attempt to optimize desired TiO<sup>2</sup> powder structure and properties.

RE‐doped TiO2 nanopowders were prepared by the sol‐gel route using a series of RE (Pr, Nd, Sm, Eu, Dy, Tb, Ho, Er, Tm) oxides and titanium(IV)‐isopropoxide. The final calcination treatment is carried out at a temperature of 420°C for 2 h. XRD measurements were done on synthesized powders using Rigaku SmartLab instrument under the Cu Kα1,2 radiation, in a 2*θ*‐range from 10° to 120° in 0.02° steps, and are shown in **Figure 4**. The XRD patterns indexed according to the ICDD card No. 00‐021‐1272. These patterns consist of the charac‐ teristic, intense peaks corresponding to 101, 004, 200, 105, 211 and 204 main reflections from anatase phase TiO2 in all RE‐doped TiO2 nanopowders. There are no diffraction peaks of another crystalline phase of TiO2 (rutile or brookite), rare earth oxide phase or dititanate pyro‐ chlore structures. The analysis of relevant structural parameters was obtained using PDXL Integrated software, and calculated results are presented in **Table 5**. The average crystallite size of undoped TiO2 was determined to be 149.6 Å, which is a much higher value than for the doped ones, suggesting the decrease in crystallinity with doping with RE as a result of the RE–O–Ti bonds in doped TiO2 nanopowders. A consequence of the incorporation of larger RE ion compared to Ti ion (*r*(*T i VI* 4+ ) = 0.605 Å into anatase structure results in an increase in cell parameters that result in an increase of cell volume.

Mesoporous materials have important properties for potential applications, such as well‐ defined pore structure, uniform pores in the range between 2 and 50 nm and high surface area that provides a large number of active sites. Nevertheless, during the calcination treat‐ ment, TiO2 nanoparticles pass through the process of crystal growth and anatase‐to‐rutile phase transformation causing the collapse of the mesoporous framework and a decrease of surface area. Incorporation of RE ions into the TiO2 matrix has been presented as a potential strategy to overcome these disadvantages, with a possibility for thermal stability of the mesoporous structure and retarded decreasing of surface area of TiO2 nanoparticles at high temperatures [25]. Also, RE ion‐doped nanocrystalline TiO2 has a significant number of active sites at anatase wall, leading to different physicochemical properties compared to undoped TiO<sup>2</sup> nanoparticles.

One of the problems in the synthesis of mesoporous TiO2 is to achieve an appropriate bal‐ ance between the hydrolysis and condensation processes of the titanium precursor. A slow hydrolytic condensation could lead to a small surface area in pure mesoporous TiO2 , because small quantities of water influence the reactivity of titanium precursor materials, and affects polymerization of TiO2 [25]. On the other hand, higher reactivity of the titanium precursor towards hydrolysis and condensation leads to denser inorganic networks, which is promoted by the influence of hydrated RE precursors. In that way, relatively higher surface area and pore diameter are expected in RE‐doped TiO2 nanoparticles compared to undoped TiO2 [25]. In sol‐gel synthesis of anatase, TiO2 nanoparticles crystallize with a pore diameter in the range of 3.26–6.4 nm and the surface area in the range of 25–117 m2 /g [13, 18, 22–28, 38]. In the low‐concentration RE‐doped anatase TiO2 nanoparticles annealed at the intermediate tem‐ peratures, pores have almost the same size as in the undoped ones. However, relatively high doping concentrations of RE ions (up to 10%) induce significant change in pore size distribu‐ tion, indicating the significant process of filling the pores, additionally promoted at higher

of anatase phase, size and concentration of RE ion, applied synthesis method and calcination temperature, a number of parameters may be varied in an attempt to optimize desired TiO<sup>2</sup>

Nd, Sm, Eu, Dy, Tb, Ho, Er, Tm) oxides and titanium(IV)‐isopropoxide. The final calcination treatment is carried out at a temperature of 420°C for 2 h. XRD measurements were done on synthesized powders using Rigaku SmartLab instrument under the Cu Kα1,2 radiation, in a 2*θ*‐range from 10° to 120° in 0.02° steps, and are shown in **Figure 4**. The XRD patterns indexed according to the ICDD card No. 00‐021‐1272. These patterns consist of the charac‐ teristic, intense peaks corresponding to 101, 004, 200, 105, 211 and 204 main reflections from

chlore structures. The analysis of relevant structural parameters was obtained using PDXL Integrated software, and calculated results are presented in **Table 5**. The average crystallite

the doped ones, suggesting the decrease in crystallinity with doping with RE as a result of the

Mesoporous materials have important properties for potential applications, such as well‐ defined pore structure, uniform pores in the range between 2 and 50 nm and high surface area that provides a large number of active sites. Nevertheless, during the calcination treat‐

transformation causing the collapse of the mesoporous framework and a decrease of surface

to overcome these disadvantages, with a possibility for thermal stability of the mesoporous

ance between the hydrolysis and condensation processes of the titanium precursor. A slow

small quantities of water influence the reactivity of titanium precursor materials, and affects

towards hydrolysis and condensation leads to denser inorganic networks, which is promoted by the influence of hydrated RE precursors. In that way, relatively higher surface area and

peratures, pores have almost the same size as in the undoped ones. However, relatively high doping concentrations of RE ions (up to 10%) induce significant change in pore size distribu‐ tion, indicating the significant process of filling the pores, additionally promoted at higher

[25]. On the other hand, higher reactivity of the titanium precursor

wall, leading to different physicochemical properties compared to undoped TiO<sup>2</sup>

hydrolytic condensation could lead to a small surface area in pure mesoporous TiO2

nanoparticles pass through the process of crystal growth and anatase‐to‐rutile phase

in all RE‐doped TiO2

*VI*

parameters that result in an increase of cell volume.

structure and retarded decreasing of surface area of TiO2

One of the problems in the synthesis of mesoporous TiO2

of 3.26–6.4 nm and the surface area in the range of 25–117 m2

area. Incorporation of RE ions into the TiO2

[25]. Also, RE ion‐doped nanocrystalline TiO2

pore diameter are expected in RE‐doped TiO2

low‐concentration RE‐doped anatase TiO2

In sol‐gel synthesis of anatase, TiO2

nanopowders were prepared by the sol‐gel route using a series of RE (Pr,

nanopowders. There are no diffraction peaks of

matrix has been presented as a potential strategy

has a significant number of active sites at anatase

nanoparticles compared to undoped TiO2

nanoparticles annealed at the intermediate tem‐

nanoparticles crystallize with a pore diameter in the range

nanoparticles at high temperatures

is to achieve an appropriate bal‐

/g [13, 18, 22–28, 38]. In the

nanoparticles.

, because

[25].

(rutile or brookite), rare earth oxide phase or dititanate pyro‐

nanopowders. A consequence of the incorporation of larger

4+ ) = 0.605 Å into anatase structure results in an increase in cell

was determined to be 149.6 Å, which is a much higher value than for

powder structure and properties.

another crystalline phase of TiO2

RE–O–Ti bonds in doped TiO2

RE ion compared to Ti ion (*r*(*T i*

RE‐doped TiO2

38 Titanium Dioxide

anatase phase TiO2

size of undoped TiO2

polymerization of TiO2

ment, TiO2

**Figure 4.** XRD patterns of undoped TiO<sup>2</sup> and TiO2 doped with series of RE ions (RE = Pr, Nd, Sm, Eu, Dy, Tb, Ho, Er and Tm).


**Table 5.** XRD and BET results of undoped TiO2 and RE doped TiO2 . temperatures. For most of the RE ion‐doped anatase TiO2 nanoparticles, porosity can be pre‐ sented by unimodal distributions, while the bimodal distribution may occur in some cases of higher doping concentration of RE ions and higher calcination treatments, when their pore diameter exceeded 100 nm [38].

The adsorption isotherms of RE‐doped TiO2 nanoparticles prepared by sol‐gel route show type IV behaviour with the typical hysteresis loop. Undoped TiO2 often show tails in their hysteresis loops at higher relative pressure, which are usually attributed to wide distribution of mesopores with some percentage of macropores (>50 nm). With the increase in calcination temperature, the crystallite size increases, also resulting in the significantly larger average pore size, but also with reduction in surface area values. The RE‐doped TiO2 are characterized by high degree of pore‐size uniformity and a well‐defined narrow pore size distribution without any contribution of macro‐ pores. On the contrary to the undoped TiO2 , high surface area can be retained even at relatively high temperatures [22]. Different trends are observed in samples prepared by impregnation sol‐ gel synthesis based on the later addition of RE metals that can lead to blockage pores and the formation of agglomerations due to low dispersion over the surface. The comparison of surface areas reveals that the specific surface area decreases by adding the metal oxides on the surface [71, 74]. The pore diameter of the RE‐doped TiO2 nanoparticles prepared with co‐precipitation synthesis is larger and basically consists of some percentage of macropores (>50 nm). The for‐ mation of macroporous structure in the RE‐doped TiO2 nanoparticles was attributed to the agglomerations of TiO2 particles and higher calcination treatment, as already known that higher calcination temperature will facilitate the growth of grains, obviously the smaller pores endured much greater stress and collapsed first during the calcination treatment [32].

RE‐doped TiO2 prepared by hydrothermal route shows higher Brunauer, Emmett and Teller (BET) surface area values when compared to undoped TiO2 . Probably, the increase in the BET surface area with increasing the doping level of RE ions is a consequence of smaller crystallite size for RE‐doped TiO2 [28]. However, the lack of linear correlation between the crystallite size of TiO2 and the specific surface area may suggest that small amounts of RE<sup>2</sup> O3 were accu‐ mulated on the surface of TiO2 nanoparticles resulting in higher surface area [28].

The specific surface area of the synthesized materials estimated by BET method is summa‐ rized in **Table 5**. The significant influence of RE3+ ions in doped anatase TiO2 is obvious by the huge increase in the surface area of doped materials compared to the undoped one. The crystallite size and BET surface area have no linear correlation, suggesting a small amount of RE2 O3 accumulated on the surface of TiO2 . The result could also be discussed regarding agglomeration of nanoparticle which is unavoidable in this kind of synthesis.

Transmission electron microscopy (TEM) was performed in order to investigate the surface morphology of the undoped TiO2 nanopowder and nanopowders doped with the series of RE ions. RE‐doped TiO2 nanopowders were prepared by the sol‐gel method using the series of RE (Pr, Nd, Sm, Eu, Dy, Tb, Ho, Er and Tm) oxides and titanium(IV)‐isopropoxide, as previously discussed. The final calcination treatment is carried out at a temperature of 420°C for 2 h. As it can be seen from **Figure 5(A)**, the undoped sol‐gel anatase sample consists of densely aggregated crystalline nanoparticles of irregular shapes, and variable dimensions of about 10–20 nm in size. Using selected area electron diffraction (SAED) technique, local crystal structure was confirmed to be pure anatase phase. The ring pattern was indexed by ICDD card no. 00‐021‐1272 with rings that correspond to 101, 004, 200, 105, 211 and 204 main reflections, presented in **Figure 5(B)**. The presence of rings suggests polycrystalline sample, and the char‐ acteristic grainy appearance of the rings suggests that crystallites have a size of 20 nm or more, suggesting only few joint unit cells per particle.

temperatures. For most of the RE ion‐doped anatase TiO2

IV behaviour with the typical hysteresis loop. Undoped TiO2

diameter exceeded 100 nm [38].

40 Titanium Dioxide

The adsorption isotherms of RE‐doped TiO2

pores. On the contrary to the undoped TiO2

reduction in surface area values. The RE‐doped TiO2

[71, 74]. The pore diameter of the RE‐doped TiO2

agglomerations of TiO2

size for RE‐doped TiO2

mulated on the surface of TiO2

morphology of the undoped TiO2

RE ions. RE‐doped TiO2

RE‐doped TiO2

size of TiO2

of RE2 O3

mation of macroporous structure in the RE‐doped TiO2

(BET) surface area values when compared to undoped TiO2

accumulated on the surface of TiO2

sented by unimodal distributions, while the bimodal distribution may occur in some cases of higher doping concentration of RE ions and higher calcination treatments, when their pore

loops at higher relative pressure, which are usually attributed to wide distribution of mesopores with some percentage of macropores (>50 nm). With the increase in calcination temperature, the crystallite size increases, also resulting in the significantly larger average pore size, but also with

uniformity and a well‐defined narrow pore size distribution without any contribution of macro‐

high temperatures [22]. Different trends are observed in samples prepared by impregnation sol‐ gel synthesis based on the later addition of RE metals that can lead to blockage pores and the formation of agglomerations due to low dispersion over the surface. The comparison of surface areas reveals that the specific surface area decreases by adding the metal oxides on the surface

synthesis is larger and basically consists of some percentage of macropores (>50 nm). The for‐

calcination temperature will facilitate the growth of grains, obviously the smaller pores endured

surface area with increasing the doping level of RE ions is a consequence of smaller crystallite

and the specific surface area may suggest that small amounts of RE<sup>2</sup>

The specific surface area of the synthesized materials estimated by BET method is summa‐

the huge increase in the surface area of doped materials compared to the undoped one. The crystallite size and BET surface area have no linear correlation, suggesting a small amount

Transmission electron microscopy (TEM) was performed in order to investigate the surface

of RE (Pr, Nd, Sm, Eu, Dy, Tb, Ho, Er and Tm) oxides and titanium(IV)‐isopropoxide, as previously discussed. The final calcination treatment is carried out at a temperature of 420°C for 2 h. As it can be seen from **Figure 5(A)**, the undoped sol‐gel anatase sample consists of densely aggregated crystalline nanoparticles of irregular shapes, and variable dimensions of about 10–20 nm in size. Using selected area electron diffraction (SAED) technique, local crystal structure was confirmed to be pure anatase phase. The ring pattern was indexed by ICDD card no. 00‐021‐1272 with rings that correspond to 101, 004, 200, 105, 211 and 204 main reflections,

rized in **Table 5**. The significant influence of RE3+ ions in doped anatase TiO2

agglomeration of nanoparticle which is unavoidable in this kind of synthesis.

much greater stress and collapsed first during the calcination treatment [32].

particles and higher calcination treatment, as already known that higher

[28]. However, the lack of linear correlation between the crystallite

nanoparticles resulting in higher surface area [28].

prepared by hydrothermal route shows higher Brunauer, Emmett and Teller

nanoparticles, porosity can be pre‐

often show tails in their hysteresis

nanoparticles prepared by sol‐gel route show type

, high surface area can be retained even at relatively

are characterized by high degree of pore‐size

nanoparticles prepared with co‐precipitation

. The result could also be discussed regarding

nanopowder and nanopowders doped with the series of

nanopowders were prepared by the sol‐gel method using the series

nanoparticles was attributed to the

. Probably, the increase in the BET

O3

were accu‐

is obvious by

In **Figure 6**(**A–I**), TEM of RE‐doped TiO2 nanopowders is collected at different magnifica‐ tions, all showing a bar of 20 nm. All of the doped samples show agglomerated nanoparticles, only the estimated particles are smaller in size compared to the undoped sample.

**Figure 5.** Transmission electron micrograph of undoped TiO2 nanopowders recorded at magnification of ×67,000 (A), with corresponding selected area electron diffraction (B).

**Figure 6.** Transmission electron micrographs of RE‐doped TiO2 nanopowders at different magnification with bar of 20 nm: (A) TiO2 :Pr, (B) TiO2 :Nd, (C) TiO2 :Sm, (D) TiO2 :Eu, (E) TiO2 :Dy, (F) TiO2 :Tb, (G) TiO2 :Ho, (H) TiO2 :Er and (I) TiO2 :Tm.

#### **4. The influence of rare earth doping on photocatalytic activity of anatase TiO2 nanoparticles**

One of the main challenges in photocatalytic research is the increase of spectral sensitivity of TiO2 from ultraviolet (UV) to visible (VIS) spectrum. Incorporation of various RE ions into the anatase TiO2 can increase the photocatalytic activity by enhancing the light absorp‐ tion, adjustment of the phase structure, crystallinity, doping concentration, surface area and morphology. An overview of literature where RE‐doped TiO2 was used as a photocatalyst in respect to variables to experiments is given in **Table 6**. For detailed information about the type of artificial light source, time of illumination, as well as the percentage of dye degradation, the readers are advised to inquire the reference list provided in **Table 6**.



**Table 6.** RE‐doped TiO2 used as photocatalyst in recent photocatalytic studies.

**4. The influence of rare earth doping on photocatalytic activity of anatase** 

One of the main challenges in photocatalytic research is the increase of spectral sensitivity

tion, adjustment of the phase structure, crystallinity, doping concentration, surface area and

respect to variables to experiments is given in **Table 6**. For detailed information about the type of artificial light source, time of illumination, as well as the percentage of dye degradation, the

**temperature (°C)**

Sc 2 Sol‐gel 500 A + B Rhodamine B [37] Y 1.5 Sol‐gel 500 A Methyl orange [31] Y – Hydrothermal 150 A Methyl orange [47] Y 0.25 Hydrothermal 400 A Phenol [28] Y 0.3 Hydrothermal 400 A + R Phenol [51] La 0.3 Hydrothermal 400 A + R Phenol [51] La 1 Sol‐gel 550 A Direct blue dye

morphology. An overview of literature where RE‐doped TiO2

readers are advised to inquire the reference list provided in **Table 6**.

**Synthesis method Optimal calcination** 

Pr 0.3 Sol‐gel 450 A Herbicide

Pr 0.25, 0.5 Hydrothermal 400 A Methyl orange [55] Pr 0.3 Hydrothermal 400 A + R Phenol [51] Nd 0.3 Hydrothermal 400 A + R Phenol [51] Nd 1 Sol‐gel 550 A Direct blue dye

Sm 0.3 Sol‐gel 500 A Diuron [38]

Sm 1 Sol‐gel 500 A Methylene blue [42] Sm 1 Sol‐gel 550 A Direct blue dye

Eu 0.5–2.0 Sol‐gel 400 A Methylene blue [39] Eu 1 Sol‐gel 500 A Rhodamine B [71] Eu 1 Sol‐gel 420 A Crystal violet [27]

Sm 0.7 Sol‐gel 500 A Remazol red

from ultraviolet (UV) to visible (VIS) spectrum. Incorporation of various RE ions

can increase the photocatalytic activity by enhancing the light absorp‐

**Crystalline phase**

was used as a photocatalyst in

**Dye Refs.**

[75]

[33]

[75]

[29]

[75]

(DB53)

(DB53)

RB‐133

(DB53)

metazachlor

**TiO2**

42 Titanium Dioxide

of TiO2

 **nanoparticles**

into the anatase TiO2

**Dopant ion Optimal** 

**doping conc. (%)**

> Initially, when TiO2 is exposed to light, it produces two types of charge carriers: electrons (e<sup>−</sup> ) in conduction band and holes (h+ ) in valence band, as presented in **Figure 7(a)**. These e<sup>−</sup> /h+ pair generations follow the processes of charge separation and migration to the surface. At the surface, active species in valence band (*h*vb <sup>+</sup> ) reacts with adsorbed water producing OH• radical and proton (H+ ). At the same time, active species in conducting band (*e*cb <sup>−</sup> ) reacts with oxygen to produce active O2 •‐ radical. The radical reacts with the proton and produces OH2 • radical. When paired, the OH2 • radicals produce H2 O2 which degrades into two OH• radicals. The for‐ mation of OH• is crucial for the degradation of organic dye. However, the rate of recombina‐ tion of photogenerated e<sup>−</sup> /h+ pairs is very fast (few nanoseconds) and substantial number can be recombined with just the release of heat [76]. When RE‐doped TiO2 is used as photocatalyst, incorporation of RE ions into the TiO2 host creates charge imbalance. With increasing charge imbalance, more hydroxide ions are being adsorbed on the TiO2 surface. Hydroxide ions (OH<sup>−</sup> ) restrain the recombination of e<sup>−</sup> and h+ , and additionally react with holes to produce surface

hydroxyl radical (OH•), which substantially improve the photocatalytic degradation of dye [26, 28]. The main disadvantage in the application of anatase TiO2 as catalyst is dominant absorption in UV caused by its band gap (*E*<sup>g</sup> <sup>∼</sup>3.2 eV). One approach to enhance absorption in VIS is doping. In the means of energy, doping can alter absorption threshold to lower energies. Incorporation of RE ions into the TiO2 host modifies the band gap of TiO<sup>2</sup> with sub‐band‐gap energy levels of RE ions, as illustrated in **Figure 7(b)** [64, 77]. These energy levels offer elec‐ tronic transition from the TiO2 valence band to the empty RE ion sub‐band‐gap energy levels. These transitions require less energy than TiO2 valence‐to‐conduction band transition and can be induced by visible light. In that way, RE ions in the TiO2 host enhance the separation of e<sup>−</sup> and h+ , contributing to photocatalytic degradation of organic dyes [28].

The main focus on the photocatalytic activity of RE ions incorporated into the anatase TiO2 is the influence of RE‐doping concentration [23, 26, 28, 31, 46, 56, 61, 65]. On the other hand, reports of comprehensive investigation of the type of RE ions in TiO2 matrix, in order to predict the influence of dopants on the photocatalytic activity under UV and visible light, are scarce [51, 75, 78]. The results for photocatalytic activity of 1 at.% RE (RE = Pr, Nd, Sm, Eu, Dy, Tb, Ho, Er and Tm)‐doped anatase TiO2 nanopowders are presented in **Figure 8**. All of doped nanopowders were prepared in the same way, as presented in **Figure 3**. Methylene orange (MO) aqueous solution with a concentration of 5 mg/l was used in all experiments. Solutions were photocatalytically treated up to 4 h with 0.1 g of undoped‐ and RE‐doped TiO2 nanopowders. UV‐VIS light irradiation Ultra‐Vitalux 300 W, Osram lamp was used in all experiments in order to simulate the solar radiation. Absorptions of MO solution aliquots were measured after 0, 5, 10, 20, 30, 60, 90, 180 and 240 min of illumination. The results of photodegradation of MO, observed at a maximum absorbance of MO at 464 nm, for Ho‐doped TiO2 nanopowder, are presented in **Figure 8(a)**. The results of MO degrada‐ tion for all samples were calculated by *Degradation* (%) <sup>=</sup> [ (*C*0 \_ − *C*) *<sup>C</sup>*<sup>0</sup> ] <sup>×</sup> <sup>100</sup>%, where *C*<sup>0</sup> is the initial concentration of MO solution and *C* is the concentration of MO solution after 4 h, and is given in **Figure 8(b)**. These results show that the incorporation of the RE ions into the TiO2

**Figure 7.** (a) Basic photocatalytic mechanism under UV or visible light irradiation. (b) Modification of band gap with sub‐band‐gap energy levels of RE ions.

matrix may bring a positive effect on the photocatalytic activity of TiO<sup>2</sup> , as presented in **Figure 8(b)**. The reasons could be attributed to the synergetic effects of anatase phase sta‐ bility, reduced crystallite size, relatively large surface area, significant improvement of the separation rate of photogenerated e<sup>−</sup> /h+ pairs and efficient absorption of visible light due to sub‐energy levels of RE ions into the band gap of TiO2 .

**Figure 8.** (a) The concentration of MO solution as a function of irradiation time for Ho‐doped TiO2 used as photocatalyst, inset: the absorption spectra of MO after different illumination times. (b) Photocatalytic degradation of MO after 4 h for various RE‐doped TiO2 , with the fixed concentration of RE ions.
