**5. Optical properties of rare earth‐doped anatase**

hydroxyl radical (OH•), which substantially improve the photocatalytic degradation of dye

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.

energy levels of RE ions, as illustrated in **Figure 7(b)** [64, 77]. These energy levels offer elec‐

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,

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,

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

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

 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,

nanopowder, are presented in **Figure 8(a)**. The results of MO degrada‐

(*C*0 \_ − *C*)

host modifies the band gap of TiO<sup>2</sup>

valence band to the empty RE ion sub‐band‐gap energy levels.

valence‐to‐conduction band transition and can

nanopowders are presented in **Figure 8**. All

*<sup>C</sup>*<sup>0</sup> ] <sup>×</sup> <sup>100</sup>%, where *C*<sup>0</sup>

host enhance the separation of e<sup>−</sup>

as catalyst is dominant

with sub‐band‐gap

matrix, in order to

is the initial

[26, 28]. The main disadvantage in the application of anatase TiO2

, contributing to photocatalytic degradation of organic dyes [28].

reports of comprehensive investigation of the type of RE ions in TiO2

Incorporation of RE ions into the TiO2

These transitions require less energy than TiO2

Eu, Dy, Tb, Ho, Er and Tm)‐doped anatase TiO2

tion for all samples were calculated by *Degradation* (%) <sup>=</sup> [

be induced by visible light. In that way, RE ions in the TiO2

tronic transition from the TiO2

and h+

44 Titanium Dioxide

TiO2

for Ho‐doped TiO2

sub‐band‐gap energy levels of RE ions.

When light interacts with matter, the material can absorb, transmit or reflect some part of the light. Absorption spectroscopy is a method to measure absorption as a function of wavelength or frequency. Since light cannot penetrate opaque samples such as powders and other solids, it is reflected on the surface of the samples. Spectrometers with integrating spheres measure the change of reflected light of a surface and compare it to a standard, most often barium sulphate, which is taken to be 100% of reflected light. Then, the obtained value is relative reflectance, and the reflectance spectrum provides the information of interaction of light in the sample as a function of wavelength. In that manner, reflectance can be directly correlated with absorption. Nowadays, research‐grade spectrophotometers can combine detectors and extend detected light up to the near‐infrared region of 1400 nm.

Some of the absorbed light can subsequently be emitted as light, as was already discussed in Section 1. Then, the radiative processes can be observed by photoluminescence spectroscopy (PL). In steady‐state PL spectroscopy, we primarily refer to excitation and emission spectros‐ copy measurements obtained by a continual light source which emits a constant number of photons in time. Since exciting of electrons takes about 10–15 s−1, following energy dissipation, whether radiative of non‐radiative, is a much slower process so the number of excited elec‐ trons could be considered as constant. Absorption spectroscopy could suggest the wavelength that could be used to gain luminescence, but not all absorption result in emission. When we refer to the Jablonski diagram, it is obvious that absorption can occur to several excited singlet states, such as *S*<sup>1</sup> , *S*<sup>2</sup> , and so on, and expected emission normally occurs only from the lowest excited singlet or triplet states, *S*<sup>1</sup> and *T*<sup>1</sup> . In excitation spectrum, a single emission detection wavelength is chosen that corresponds to an expected band in the emission spectrum. The excitation source is then scanned through wavelength region, and the intensity of the emission at the single selected wavelength is scanned in a function of excitation wavelength. The output of absorption and excitation spectrum is not the same, although detected maxima (or minima) at the same wavelength suggest the same excited energy levels. In luminescence emission spectroscopy, a wavelength of exciting light is selected, and emission spectrum is obtained by detecting the intensity of the emitted light as a function of wavelength. In downconversion emission spectroscopy, emitted luminescence is recorded in the spectral range above the exci‐ tation wavelength to longer wavelengths, up to the region where luminescence is expected. It was then of interest to study the influence of rare earth doping on anatase nanoparticles by the interpretation of absorption (reflectance), excitation and emission spectroscopy methods.

Samples of RE‐doped anatase materials are in literature most often characterized by a posi‐ tioning of the threshold of absorption of doped samples and compared to the undoped ones. Even with the reduction of nanoparticles size after rare earth ions incorporation, the differ‐ ence in extrapolated slopes after Kubelka–Munk transformations in doped and undoped nanopowder samples should not be ascribed to quantum confinement effect, since particle sizes exceed the Bohr radius several times [18, 79]. Some modifications of materials density of states after the incorporation of trivalent rare earth ions are the most probable reason for small differences in observed band gaps, which is highly dependent on the synthesis procedure and the RE dopant. Kubelka‐Munk transformation of reflectance spectra of RE3+‐doped anatase TiO2 measured over the 360–440 nm spectral range is presented in **Figure 9**.

#### **5.1. Praseodymium**

The absorption of praseodymium ion in TiO2 hosts is reported in Refs. [28, 55, 80, 81]. From reflectance spectrum of TiO<sup>2</sup> :Pr presented in **Figure 10(a)**, absorptions of Pr3+ ions in TiO2 absorption edge are observed at approximately 445, 480 and 595 nm that could be attrib‐ uted to the transition from 3 H4 ground state to the 3 P2‐0 and 1 D2 excited states of the Pr3+ ions. Low wide absorption at around 1000 nm could be assigned to 1 G4 excited state. Excitation spectrum is recorded at a fixed emission wavelength of 493 nm in the range of 260–460 nm, presented in **Figure 10(b)**. Two wide excitations are observed at 325 and 447 nm. The excita‐ tion of 447 nm was used to obtain emission spectrum in the range of 475–780 nm. Even though the room temperature emission maxima are wide, several transitions can be assigned as fol‐ lows: 3 P0 →<sup>3</sup> H5 (493 and 536 nm), 3 P0 →<sup>3</sup> H6 (620 nm) and 3 P0 →<sup>3</sup> F2 (650 nm), as can be seen in **Figure 10(c)**. 1 D2 –3 H4 transition is not observed, suggesting high concentration of Pr3+ ions in TiO2 matrix, where cross‐relaxation between neighbouring Pr3+ ions occurs [82].

#### **5.2. Neodymium**

The absorption of neodymium ion in TiO2 hosts is reported in a spectral range up to 700 nm [41] and up to 1200 nm [34, 83]. From reflectance spectrum of TiO<sup>2</sup> :Nd presented in

**Figure 9.** Kubelka‐Munk transformation of reflectance spectra of RE3+‐doped anatase TiO2 measured over the 360–440 nm spectral range.

**Figure 10.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO<sup>2</sup> :Pr nanopowders.

**Figure 11(a)**, eight absorptions from ground 4 I9/2 to excited energy levels of Nd3+ ions in TiO2 are observed and assigned in energy‐level diagram in **Figure 11(b)**. Intense emission of Nd3+ can be obtained in the IR spectral range above 850 nm, **Figure 11(c)**. Three transitions from 4 F3/2 to its lower 4 I9/2, 4 I11/2 and 4 I13/2 are obtained with an excitation of 752 nm. The transitions correspond well with the reported data of Nd3+ in anatase matrix [34, 40, 84]. The position and shape of 4 F3/2→<sup>4</sup> I9/2 strongly suggest Nd‐doped TiO2 anatase sample, without the presence of other compositions of segregated neodymium oxide and neodymium titanate phases [34].

#### **5.3. Samarium**

that could be used to gain luminescence, but not all absorption result in emission. When we refer to the Jablonski diagram, it is obvious that absorption can occur to several excited singlet

wavelength is chosen that corresponds to an expected band in the emission spectrum. The excitation source is then scanned through wavelength region, and the intensity of the emission at the single selected wavelength is scanned in a function of excitation wavelength. The output of absorption and excitation spectrum is not the same, although detected maxima (or minima) at the same wavelength suggest the same excited energy levels. In luminescence emission spectroscopy, a wavelength of exciting light is selected, and emission spectrum is obtained by detecting the intensity of the emitted light as a function of wavelength. In downconversion emission spectroscopy, emitted luminescence is recorded in the spectral range above the exci‐ tation wavelength to longer wavelengths, up to the region where luminescence is expected. It was then of interest to study the influence of rare earth doping on anatase nanoparticles by the interpretation of absorption (reflectance), excitation and emission spectroscopy methods. Samples of RE‐doped anatase materials are in literature most often characterized by a posi‐ tioning of the threshold of absorption of doped samples and compared to the undoped ones. Even with the reduction of nanoparticles size after rare earth ions incorporation, the differ‐ ence in extrapolated slopes after Kubelka–Munk transformations in doped and undoped nanopowder samples should not be ascribed to quantum confinement effect, since particle sizes exceed the Bohr radius several times [18, 79]. Some modifications of materials density of states after the incorporation of trivalent rare earth ions are the most probable reason for small differences in observed band gaps, which is highly dependent on the synthesis procedure and the RE dopant. Kubelka‐Munk transformation of reflectance spectra of RE3+‐doped anatase

and *T*<sup>1</sup>

measured over the 360–440 nm spectral range is presented in **Figure 9**.

ground state to the 3

matrix, where cross‐relaxation between neighbouring Pr3+ ions occurs [82].

absorption edge are observed at approximately 445, 480 and 595 nm that could be attrib‐

spectrum is recorded at a fixed emission wavelength of 493 nm in the range of 260–460 nm, presented in **Figure 10(b)**. Two wide excitations are observed at 325 and 447 nm. The excita‐ tion of 447 nm was used to obtain emission spectrum in the range of 475–780 nm. Even though the room temperature emission maxima are wide, several transitions can be assigned as fol‐

(620 nm) and 3

, and so on, and expected emission normally occurs only from the lowest

. In excitation spectrum, a single emission detection

hosts is reported in Refs. [28, 55, 80, 81]. From

hosts is reported in a spectral range up to

G4

excited states of the Pr3+ ions.

(650 nm), as can be seen in

:Nd presented in

excited state. Excitation

:Pr presented in **Figure 10(a)**, absorptions of Pr3+ ions in TiO2

D2

P2‐0 and 1

P0 →<sup>3</sup> F2

transition is not observed, suggesting high concentration of Pr3+ ions in

states, such as *S*<sup>1</sup>

46 Titanium Dioxide

TiO2

lows: 3 P0 →<sup>3</sup> H5

TiO2

**Figure 10(c)**. 1

**5.2. Neodymium**

**5.1. Praseodymium**

reflectance spectrum of TiO<sup>2</sup>

uted to the transition from 3

D2 –3 H4

The absorption of praseodymium ion in TiO2

(493 and 536 nm), 3

The absorption of neodymium ion in TiO2

H4

Low wide absorption at around 1000 nm could be assigned to 1

P0 →<sup>3</sup> H6

700 nm [41] and up to 1200 nm [34, 83]. From reflectance spectrum of TiO<sup>2</sup>

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

excited singlet or triplet states, *S*<sup>1</sup>

In reflectance measurements presented in **Figure 12(a)**, significant absorptions of Sm3+ ion can be observed, with maxima positioned at around 480 nm, which corresponds to absorption into 4 G5/2, and several strong absorptions positioned at around 947, 1080 and 1230 nm. Room temperature excitation spectrum is in the range of 310–550 nm at a fixed emission at 585 nm shown in **Figure 12(b)**. Strong wide band below 400 nm, with maximum at about 365 nm, is characteristic for Sm3+ in TiO2 matrix that is assigned to charge transfer from the oxygen ligands in TiO2 to Sm3+ ion [18, 29, 34, 35]. Several smaller and combined excitations at around 411 and 476 nm could be assigned to 6 G7/2 or 6 P5/2 and 4 I13/2, respectively [18, 34]. In **Figure 12(c)**, room temperature emission spectrum in the range of 400–700 nm obtained after excitation into charge transfer at 365 nm showed only characteristic emissions from 4 G5/2→<sup>6</sup> H5,7,9/2 energy levels. It is worth mentioning that the same spectral features are obtained also with exciting directly into Sm3+ ion by excitation with 411 nm, with all the intensities decreased as expected from the excitation spectrum. No complete splitting of Stark components caused by ligand field that are obvious at room temperatures and are in correspondence with the literature is attributed to the large number of defect at the surface [18, 29, 34, 35, 42]. When directly excited, the enhancement of Sm3+ emission in TiO2 by codoping with silver dopant, caused by combined influence of plasmonic effects and sensitizing of Sm3+ emission by silver ions, is reported in TiO2 films [85].

#### **5.4. Europium**

The lowest excited level (5 D0 ) of Eu3+ ion is a non‐degenerate *(J* = 0) singlet level, along with crystal field non‐sensitive <sup>5</sup> D0 →<sup>7</sup> F1 transition and hypersensitive 5 D0 →<sup>7</sup> F2 emissions sim‐ plify the interpretation of emission spectra. Consequently, europium ion incorporated in

**Figure 11.** (a) Reflectance, (b) energy‐level diagram and (c) emission spectra of anatase TiO<sup>2</sup> :Nd nanopowders.

**Figure 12.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO<sup>2</sup> :Sm nanopowders.

various matrices is often used as a luminescent probe ion in photoluminescence spectros‐ copy [86–90]. In **Figure 13(a)**, after a sharp rise of absorption in UV spectral range below 400 nm, low‐intensity Eu3+ absorptions from 7 F0 →<sup>5</sup> D2 at around 465 nm and 7 F0 → <sup>5</sup> D1 at around 535 nm transitions are clearly observed. Those transitions are also present in exci‐ tation spectrum (**Figure 13(b)**) obtained with an emission fixed at 613 nm. Four dominant excitation bands originate from direct excitation of Eu3+ ions from ground 7 F0 level to 5 L6 (394 nm), 5 D3 (414 nm), 5 D2 (464 nm) and 5 D1 (532 nm) levels. By excitation into 5 L6 level, room temperature emission spectrum presented in **Figure 13(c)** clearly shows that emis‐ sions from 5 D0 →<sup>7</sup> F*J* (*J* = 0–4) transitions are centred at around 580, 593, 613, 653 and 702 nm, respectively. A small emission observed at 540 nm is emission from higher excited 5 D1 level. The positions and relative intensities of wide emissions are in correspondence with extensive literature data [18, 29, 42, 45, 52, 53, 63, 67, 69, 91]. In some presented results of low‐temperature site‐selective spectroscopy of the materials, three possible positions of Eu ion in TiO2 can be distinguished: Eu3+ can occupy Ti4+ site, it could enter into the interstitial site in the chain structure, or a third possible site for dopant cation is low‐symmetry‐dis‐ torted sites near nanoparticle's surface [18, 19, 91].

#### **5.5. Terbium**

into 4

48 Titanium Dioxide

ligands in TiO2

reported in TiO2

**5.4. Europium**

The lowest excited level (5

crystal field non‐sensitive <sup>5</sup>

is characteristic for Sm3+ in TiO2

411 and 476 nm could be assigned to 6

excited, the enhancement of Sm3+ emission in TiO2

D0

D0 →<sup>7</sup> F1

**Figure 12.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO<sup>2</sup>

**Figure 11.** (a) Reflectance, (b) energy‐level diagram and (c) emission spectra of anatase TiO<sup>2</sup>

films [85].

G5/2, and several strong absorptions positioned at around 947, 1080 and 1230 nm. Room temperature excitation spectrum is in the range of 310–550 nm at a fixed emission at 585 nm shown in **Figure 12(b)**. Strong wide band below 400 nm, with maximum at about 365 nm,

P5/2 and 4

room temperature emission spectrum in the range of 400–700 nm obtained after excitation

levels. It is worth mentioning that the same spectral features are obtained also with exciting directly into Sm3+ ion by excitation with 411 nm, with all the intensities decreased as expected from the excitation spectrum. No complete splitting of Stark components caused by ligand field that are obvious at room temperatures and are in correspondence with the literature is attributed to the large number of defect at the surface [18, 29, 34, 35, 42]. When directly

by combined influence of plasmonic effects and sensitizing of Sm3+ emission by silver ions, is

plify the interpretation of emission spectra. Consequently, europium ion incorporated in

G7/2 or 6

into charge transfer at 365 nm showed only characteristic emissions from 4

to Sm3+ ion [18, 29, 34, 35]. Several smaller and combined excitations at around

matrix that is assigned to charge transfer from the oxygen

) of Eu3+ ion is a non‐degenerate *(J* = 0) singlet level, along with

transition and hypersensitive 5

I13/2, respectively [18, 34]. In **Figure 12(c)**,

by codoping with silver dopant, caused

D0 →<sup>7</sup> F2

:Sm nanopowders.

G5/2→<sup>6</sup>

H5,7,9/2 energy

emissions sim‐

:Nd nanopowders.

Terbium ions often show a tendency to be stabilized by matrices in two valence states, +3 and +4. Only lower valence state is optically active in visible spectrum. The mixture of valences can additionally disturb crystallinity of matrices and introduce additional vacan‐ cies, and hence perturbations in energy states. In absorption spectra presented in **Figure 14(a)**, no clear absorption of Tb3+ ion can be resolved, but significant difference in absorption threshold of TiO2 is obvious, suggesting possible weak absorption of energy in the range below 500 nm. Some reports state no or very weak emission of Tb3+ ion in TiO2 matrix attributed to the mismatch of the energy levels of the <sup>5</sup> D4 ‐emitting state of Tb3+ with band gap of TiO2 [18, 29, 60, 69]. Nevertheless, as presented in **Figure 14(b**, **c)**, excitation and emission spectra are actually obtained. At an emission wavelength of 545 nm, excita‐ tion spectrum was measured in the range of 300–500 nm. Wide charge transfer band can be seen below 350 nm, and excitations of Tb3+ ion from 7 F6 ground level to 5 D4 excited level are observed at 484 nm, two excitations to 5 D2 368 nm and 5 D3 at 377 nm. When excited into 5 D4 excited energy level with 484 nm, emission spectrum in the range of 510–780 nm

**Figure 13.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO<sup>2</sup> :Eu nanopowders.

shows emission from 5 D4 to 7 F5 at 546 nm, 5 D4 to 7 F4 at 585 nm and 5 D4 to 7 F3 at 622 nm. The green emission at 546 nm is the dominant one. The findings are in good agreement with the literature [29, 60, 64, 69].

#### **5.6. Dysprosium**

Reflectance spectrum of Dy3+ ions into TiO2 presented in **Figure 15(a)** shows low‐wavelength bands of Dy3+ that overlaps with the absorption threshold of anatase at 450 and 470 nm and intense longer wavelength bands in the range of 700–1300 nm. Excitation spectrum of TiO2 :Dy3+ sample recorded at room temperature in the 300–500 nm range with a fixed emis‐ sion wavelength of 577 nm showed excitations corresponding to electron transitions from the Dy3+ ground states to the excited states: 4 K17/2 at 391 nm, 4 G11/2 at 425 nm, 4 I15/2 at 452 nm and 4 F9/2 at 472nm, **Figure 15(b)**. When excited with 425 nm, dominant luminescence is observed with two bands observed in the blue spectral region at 483 nm, which correspond to mag‐ netic‐dipole 4 F9/2→<sup>6</sup> H15/2 transition and in yellow spectral region at 580 nm, which correspond to electric‐dipole 4F9/2→<sup>6</sup> H13/2 transition, **Figure 15(c)**. A low‐intensity emission is observed in the red region at 674 nm that corresponds to 4 F9/2→<sup>6</sup> H11/2 transition. With literature proposing no luminescence from Dy3+ ion in anatase host [92], this finding shows that nanocrystalline anatase powders can actually host this ion that can successfully be excited and luminescence can be observed.

**Figure 14.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO2 :Tb nanopowders.

**Figure 15.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO2 :Dy nanopowders.

#### **5.7. Holmium**

shows emission from 5

**5.6. Dysprosium**

50 Titanium Dioxide

TiO2

netic‐dipole 4

can be observed.

4

the literature [29, 60, 64, 69].

D4 to 7 F5

Reflectance spectrum of Dy3+ ions into TiO2

Dy3+ ground states to the excited states: 4

the red region at 674 nm that corresponds to 4

**Figure 15.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO2

**Figure 14.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO2

F9/2→<sup>6</sup>

to electric‐dipole 4F9/2→<sup>6</sup>

at 546 nm, 5

D4 to 7 F4

green emission at 546 nm is the dominant one. The findings are in good agreement with

bands of Dy3+ that overlaps with the absorption threshold of anatase at 450 and 470 nm and intense longer wavelength bands in the range of 700–1300 nm. Excitation spectrum of

:Dy3+ sample recorded at room temperature in the 300–500 nm range with a fixed emis‐ sion wavelength of 577 nm showed excitations corresponding to electron transitions from the

K17/2 at 391 nm, 4

H15/2 transition and in yellow spectral region at 580 nm, which correspond

H13/2 transition, **Figure 15(c)**. A low‐intensity emission is observed in

F9/2 at 472nm, **Figure 15(b)**. When excited with 425 nm, dominant luminescence is observed with two bands observed in the blue spectral region at 483 nm, which correspond to mag‐

F9/2→<sup>6</sup>

no luminescence from Dy3+ ion in anatase host [92], this finding shows that nanocrystalline anatase powders can actually host this ion that can successfully be excited and luminescence

at 585 nm and 5

D4 to 7 F3

presented in **Figure 15(a)** shows low‐wavelength

G11/2 at 425 nm, 4

H11/2 transition. With literature proposing

:Dy nanopowders.

:Tb nanopowders.

at 622 nm. The

I15/2 at 452 nm and

Among all RE3+ ions doped in nanocrystalline anatase TiO2 powders in this work, Ho3+ has the most pronounced absorptions in VIS. As can be seen from **Figure 16(a)**, intense bands can be observed at 420, 456, 490, 542 and 645 nm and smaller intensity bands are observed at 890, 1150 and 1200 nm. In excitation spectrum at fixed emission wavelength of 554 nm presented in **Figure 16(b)**, several excitations centred at around 422, 452, 468 and 493 nm show several pos‐ sible energies for potential emission. As can be seen in **Figure 16(c)**, when excited with 452 nm, emission spectra in the range of 500–700 nm show dominant emissions from 5 F4 / 5 S2 → <sup>5</sup> I 8 transi‐ tions at about 545, 554, and 559 nm, and emission from 5 F5 → <sup>5</sup> I 8 transition with maximum cen‐ tred at 665 nm. Emissions from the same transitions can also be observed in samples sensitized with Yb3+ ions, when excitation wavelength was 980 nm that corresponds to the absorption of Yb3+ ions, and the mechanism of obtaining luminescence is upconversion [50].

#### **5.8. Erbium**

Absorptions of Er3+ ions in TiO2 matrices are reported in spectral range from UV up to 700 nm [26, 28], up to 800 nm [49], and when sensitized with Yb3+ ions up to 1200 nm [48]. All of the reported data correspond well with results presented in **Figure 17(a)**. Absorptions located at 452, 477, 491, 525, 655, 795 and 980 nm correspond to the transitions from 4 I15/2 to 4 F3/2, 4 F5/2, 4 F7/2, 2 H11/2 and 4 S3/2, 4 F9/2, 4 G9/2, 4 I11/2, respectively. In excitation spectrum shown in **Figure 17(b)**, with fixed emission of 565 nm, some low‐intensity excitations can be noticed at around 378, 410 and 453 nm. More pronounced excitations can be observed at 488 and 525 nm. In order to characterize emissions in the range of 520–700 nm, excitation wavelength of 488 nm was used, and the spectrum is presented in **Figure 17(c)**. From the combination of 2 H11/2→<sup>4</sup> I15/2 and 4 S3/2→ 4 I15/2 transitions, wide emissions can be observed in the range of 540–575 nm, as also reported in Refs. [42, 92].

#### **5.9. Thulium**

Absorption of thulium ion in the sample presented in **Figure 18(a)** shows small absorption at 470 nm, as well as stronger absorptions at 690, 795 and 1210 nm. Excitation spectrum with

**Figure 16.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO<sup>2</sup> :Ho nanopowders.

a fixed emission at 495 nm showed poor optical answer with some picks that most probably originate from defect, **Figure 18(b)**. In order to directly excite Tm3+ ion 470 nm excitation was used. Emission spectrum in the range of 490–780nm presented in **Figure 18(c)** shows shoulder of maximum at 495 nm originating from 1 G4 → <sup>5</sup> H6 transition and very low intensity of group of lines in the range of 650–670 nm that could be attributed to the <sup>1</sup> G4 →<sup>3</sup> F4 transition.

**Figure 17.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO<sup>2</sup> :Er nanopowders.

**Figure 18.** (a) Reflectance, (b) excitation and (c) emission spectra of anatase TiO<sup>2</sup> :Tm nanopowders.

### **6. Conclusion**

To conclude, the structure, morphology and optical properties of TiO2 nanoparticles may be substantially swayed by the addition of small quantities of RE3+ ions. Such nanostructures deliver new options to the already broad range of important TiO2 uses. In RE ion‐doped TiO2 , anatase phase is stabilized at medium temperatures since the temperature of phase trans‐ formations shifts to higher values. The reduction of the crystallite size is readily observed and doping induces mesoporous structure with enlarged specific surface in respect to one of undoped anatase TiO2 . Thus, the photocatalytic performance of nanopowder improves with the addition of RE3+ in small concentrations except for Pr3+ and Tb3+. Different rare earth ions cause TiO2 property changes of different magnitudes. Optical properties are altered too. The modification of materials density of states after incorporation of RE3+ ions in TiO2 causes changes in materials absorption which can be clearly evidenced from optical absorption spectra. Rare earth ions may be incorporated at three different sites in TiO<sup>2</sup> structure: they can substitute Ti4+ in the bulk of particle, enter vacancy site, but they at large reside near surface in low‐symmetry sites. In such cases, the characteristic RE3+ luminescence is observed in the case of doping with the following ions: Nd3+, Sm3+, Eu3+, Dy3+, Ho3+ and Er3+, while luminescence of low intensity is detected for Pr3+, Tb3+ and Tm3+.
