Rare-Earth-Modified Titania Nanoparticles: Molecular
Insight into Synthesis and Photochemical Properties
A molecular precursor approach to titania (anatase) nanopowders modified with different amounts of rare-earth elements (REEs: Eu, Sm, and Y) was developed using the interaction of REE nitrates with titanium alkoxides by a two-step solvothermal–combustion method. The nature of an emerging intermetallic intermediate was revealed unexpectedly for the applied conditions via a single-crystal study of the isolated bimetallic isopropoxide nitrate complex [Ti2Y(iPrO)9(NO3)2], a nonoxo-substituted compound. Powders of the final reaction products were characterized by powder X-ray diffraction, scanning electron microscopy–energy-dispersive spectroscopy, Fourier transform infrared, X-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence (PL). The addition of REEs stabilized the anatase phase up to ca. 700 °C before phase transformation into rutile became evident. The photocatalytic activity of titania modified with Eu3+ and Sm3+ was compared with that of Degussa P25 titania as the control. PL studies indicated the incorporation of Eu and Sm cations into titania (anatase) at lower annealing temperatures (500 °C), but an exclusion to the surface occurred when the annealing temperature was increased to 700 °C. The efficiency of the modified titania was inferior to the control titania while illuminated within narrow wavelength intervals (445–465 and 510–530 nm), but when subjected to a wide range of visible radiation, the Eu3+- and Sm3+-modified titania outperformed the control, which was attributed both to doping of the band structure of TiO2 with additional energy levels and to the surface chemistry of the REE-modified titania.
Introduction<!>Chemicals<!>Preparation of REE-Modified Nanopowders and the Molecular Intermediate<!>Characterization<!>Results and Discussion<!>Synthesis of Bimetallic Complex<!><!>Synthesis of Bimetallic Complex<!><!>PXRD<!><!>SEM and AFM<!><!>Vibration Spectrometry<!><!>Vibration Spectrometry<!><!>PL<!><!>PL<!>XPS<!><!>Photocatalysis<!><!>Photocatalysis<!>Conclusion<!><!>Accession Codes<!>Author Contributions<!>
<p>Titania (TiO2) is one of the most investigated semiconductor
nanomaterials, primarily because of its photocatalytic activity, chemical
stability, low toxicity, and facile synthesis.1,2 The
discovery of its ability to split water in the 1970s by Fujishima
and Honda3 encouraged intensive research
for applications in the photocatalytic degradation of organic pollutants4 and hydrogen production from water.5 The two most common phases of titania are anatase
and rutile. Anatase is considered to be the catalytically most active
phase.6 Still, the band gap of anatase
is ca. 3.2 eV, and UV radiation is therefore required to promote an
electron from the valence band (VB) to the conduction band (CB). This
makes photocatalysis in sunlight inefficient. To overcome this obstacle,
different strategies, including sensitizing and doping, have been
employed to make use of visible light for photocatalysis. In sensitizing,
an organic or inorganic compound is attached to the surface of titania.
This compound absorbs in the visible spectrum and can transfer an
excited electron into the CB of titania.7−9 This is used in dye-sensitized
solar cells.10 Doping of metals or nonmetals
into the titania lattice may generate intermediate energy levels between
the VBTiO2 and CBTiO2 and
redshift the absorption spectrum into the visible region. For substitutional
doping of titania, it is a requirement that the cation that replaces
Ti has a similar ionic radius.</p><p>There has been recent interest
in titania "doped"
with rare-earth elements (REEs).11−14 The f orbitals of the REE "dopants"
can act as electron traps, thereby decreasing the electron–hole
recombination rates, and facilitate transfer to surface hydroxide
groups to form reactive hydroxyl radicals. An increased efficiency
from Gd3+ doping was explained by its stable 4f7 configuration with half-filled f orbitals. Transfer of an electron
from VBTiO2 to one of the half-filled Gd f orbitals
would cause decreased stability. To regain the more stable configuration
of half-filled f orbitals, the excited electron will be kicked out
and has the possibility of reaching the particle surface.12,15 A suggested mechanism of this is that charge imbalance upon doping
and the locally created permanent dipoles would facilitate more efficient
charge separation upon excitation and subsequent charge transport.16 An additional benefit of REE "doping"
of titania is the formation of surface complexes between organic molecules
with free electron pairs (i.e., those containing O, N, or S) and the
REEs.17 The hydroxyl radicals on the particle
surface are responsible for the degradation of organic molecules,
and an improved surface adsorption capacity would result in more efficient
degradation.11,12,18 There appears to be an optimum amount of REE "dopants"
because at a certain concentration the catalytic activity starts to
decrease. This has been suggested to depend on the formation of recombination
centers and active site blocking.15,19,20 The atomic radii21 of
the lanthanide (Ln) ions (about 1.10–1.20 Å) are considerably
larger than that of Ti4+ (0.745 Å), and "doping"
may instead result in surface-bound REEs, REE oxides (RE2O3), mixed phases (e.g., REE titanates), or intercalation.
Different mixed-metal titanates have previously been demonstrated
to function as inorganic sensitizers by interfacial charge transfer
to titania,22,23 and it is likely that REE titanates
and REE oxides can do the same, although this appears to be less investigated.
It is generally observed that the addition of REE "dopants"
results in smaller average particle size and delayed anatase-to-rutile
phase transformation.20,24 Using conventional low-temperature
sol–gel processing for doping titania with Ln ions (and other
elements with substantially larger ionic radii) may lead to the segregation
of a lanthanide oxide phase on the titania surface,25,11 and the hydrothermal synthesis of REE-doped titania can result in
mixed anatase–rutile or anatase–brookite phases.26,27</p><p>In this work, we focused on the development of a molecular
precursor
approach to titania nanoparticles modified with three different REEs
(Y, Sm, and Eu) using a solvothermal method with subsequent annealing
at different temperatures. Our aim was to bring a molecular understanding
to how REE can be incorporated into the titania structure both in
the course of the reaction and with respect to their occurrence in
the final oxide products. Although yttrium is classified as a REE,
it has a somewhat smaller radius (1.04 Å for Y3+).
It was applied in isolation of the molecular model for the intermediate
species in the synthesis. Eu and Sm are positioned next to each other
in the Periodic Table and have similar ionic radii (1.087 and 1.098
Å, respectively, in their 3+ oxidation states). Photoluminescence
(PL) of Eu and Sm was employed to determine the positions of the Ln
ions in the resulting titania materials.</p><!><p>Titanium(IV) ethoxide (Sigma-Aldrich), titanium(IV)
isopropoxide (Sigma-Aldrich), Y(NO3)3·6H2O (99.8%, Aldrich), Sm(NO3)3·6H2O (99.9%, Aldrich), and Eu(NO3)3·5H2O (99.9%, Aldrich) were used as received. Ethanol (99.7%,
Solveco) was refluxed and distilled over metallic calcium, and isopropyl
alcohol (Sigma-Aldrich) was refluxed and distilled over lithium aluminum
hydride.</p><!><p>The appropriate amounts of REE nitrates (i.e., 2.5 or 5 mol % with
respect to Ti) were dissolved in anhydrous ethanol in Teflon containers.
Titanium(IV) ethoxide was then added under a nitrogen atmosphere in
a glovebox. The Teflon containers were placed in steel autoclaves
and treated as follows: ramped at 2.6 °C min–1 to 60 °C (held for 4 h) and then ramped at 3.3 °C min–1 to 160 °C (held for 20 h). Brownish transparent
liquids were obtained. After drying at 45 °C, brown-orange, amorphous
powders were produced. The powders were ground in an agate mortar
and annealed at different temperatures (400–800 °C in
increments of 100 °C). Ramping from room temperature to the final
temperature was set to 1 h, and this temperature was held for 2 h. Caution!The presence of nitrates can cause ignition
upon heating with small pieces that can be spread around! Heating
should be carried out in a closed porcelain cup and the door of the
oven kept properly closed until the end of the process! The
powders were cooled naturally to room temperature in the oven. The
different powders were designated as Ti-REEx-t, where REE is Y, Sm, or Eu, x is either
2.5 or 5 mol %, and t is the annealing temperature.</p><p>The molecular intermediate, Ti2Y(iPrO)9(NO3)2 (1), was synthesized by a solvothermal method. To 0.80 mL of anhydrous
toluene containing 0.15 equiv of Y(NO3)3·6H2O was added 0.51 mmol of Ti(iPrO)4 under a nitrogen atmosphere. The reaction mixture was placed
in a Teflon-lined steel autoclave, heated to 85 °C for 44 h,
and then naturally cooled to room temperature to obtain small colorless
needle-shaped crystals of compound 1. Crystallographic
details are presented in Table 1. The data were collected at room temperature using Mo KÎ±
radiation, λ = 0.71073 Å, with a Bruker D8 SMART Apex2
diffractometer. The structure was solved by direct methods. The coordinates
of the metal atoms were obtained from the initial solution, and those
of other atoms were found in subsequent Fourier syntheses. All non-H
atoms were refined first in isotropic and then in anisotropic full-matrix
approximations. H atoms were added via geometric calculation and included
in the final refinement in the isotropic approximation. Nitrate ligands
were severely disordered, and possible alternative configurations
of O atoms were introduced using partial occupation. Full details
of the structure solution and refinement are available from the Cambridge
Crystallographic Data Center at https://ccdc.cam.ac.uk/ using CCDC 2095265.</p><p>Compound 1 was not stable over time, even in a freezer,
but decomposed within a few days, possibly as a result of the redox
sensitivity of the iPrO ligands.</p><!><p>Powders were characterized by scanning
electron microscopy (SEM; Hitachi TM-1000 and Hitachi FlexSEM 1000II),
energy-dispersive X-ray spectrometry (EDS; Oxford Instruments), powder
X-ray diffraction (PXRD; Bruker D8 Apex II CCD diffractometer, Mo
Kα, λ = 0.71073 Å, with a graphite monochromator).
The PXRD data were treated and analyzed with the Bruker APEX2 program suite and EVA v12) and Fourier transform
infrared (FTIR; PerkinElmer Spectrum 100). For atomic force microscopy
(AFM), a Bruker Dimension Fastscan atomic force microscope with ScanAsyst
was used. Raman spectroscopy, PL, and photocatalysis were performed
at the NILPR (Romania). The luminescence measurements were carried
out using a Fluorolog 3 spectrofluorometer (Horiba) operated in a
fluorescence mode. The luminescence spectra were measured upon excitation
into the broad UV absorption of TiO2 (Sm) or Ln f–f
absorption transitions (Eu). A JEOL JEM-2100F transmission electron
microscope was used for transmission electron microscopy (TEM) imaging
at a 200 kV operating voltage. X-ray photoelectron spectroscopy (XPS)
was measured on a PHI Quantum 2000 spectrometer with monochromated
Al Kα radiation with a 45° angle of electron emission.
The survey scan spectra were recorded from 0 to 1100 eV (binding energy
range), using 224 eV pass energy and 0.8 eV step–1. The high-resolution XPS spectra for Ti 2p, O 1s, and C 1s were
performed with a pass energy of 55 eV and a step energy of 0.05 eV.
Electron and ion neutralization was used for all measurements. The
resulting spectra were analyzed by CasaXPS software.
All of the peaks were calibrated with adventitious C 1s with a binding
energy of 284.8 eV.</p><!><p>Using a molecular
precursor with known stoichiometry of metal ions,
i.e., a single-source precursor, is an attractive way to ensure homogeneous
distribution of the dopant in the oxide during hydrolysis/thermolysis,
compared to annealing of the separate oxides, carbonates, etc. Solution–combustion
synthesis is well reported, for instance, in the synthesis of different
bimetallic oxides using the polymeric precursor synthesis. A common
example is the complexation of metal ions with citric acid.28−30 Mixing the different metal ion–citric acid complexes then
provides a very homogeneous mixture of the metal ions in the reaction
mixture. Commonly, an oxidizer such as nitrate is also introduced.
During the annealing step, the organic part is combusted with the
emergence of a homogeneous mixed-metal oxide phase, often of high
phase purity. In this work, we wanted to explore a less common method
to REE-modified titania: combustion of solvothermally synthesized
intermediates from the reaction between titanium alkoxides and REE
nitrates, utilizing the alkoxide ligands as fuel and nitrate as an
oxidizer. However, only for yttrium could a bimetallic precursor (compound 1) be isolated when changing titanium(IV) ethoxide for titanium(IV)
isopropoxide, allowing one to shed light on the nature of chemical
intermediates.</p><!><p>Attempts were made
to synthesize REE-Ti mixed-metal (oxo)alkoxide complexes. These can
be employed as single-source precursors to obtain doped/bimetallic
oxides. The advantage here is an even distribution of elements already
present in the precursors, which can facilitate an even distribution
of the dopant in the final oxide, which is expected to lead to improved
photochemical properties. The synthesis of complexes between titanium(IV)
alkoxides and nitrates of Y, Eu, and Sm was attempted without introducing
any additional organic ligands.</p><p>From the solvothermal method,
one mixed titanium–yttrium alkoxide, compound 1, was obtained (Figure 1). Compound 1 crystallized in the triclinic space group P1̅ as transparent needle crystals. Crystallographic
data are given in Table 1. The crystals were moisture-sensitive and quickly hydrolyzed without
protection. Compound 1 has a triangular structure containing
a commonly observed Ti3O fragment31 but with one Ti exchanged for Y. Notably, the complex contains no
oxo bridges despite the presence of water from Y(NO3)3·6H2O. This is presumably due to the lower
solubility of complex 1 in the hydrophobic toluene solvent.
When adding more than 0.30 equiv of Y(NO3)3·6H2O to Ti, only white precipitates were obtained. Ti and Y are
linked via bridging isopropoxide groups. The average bond length for
terminal isopropoxide groups is 1.754 Å (sd = 0.0051 Å),
while the average bond length for the isopropoxide groups bridging
the two Ti centers is 2.109 Å (sd = 0.097 Å). The Ti–iPrO and iPrO–Y
bond lengths are asymmetric, with average bond lengths of 2.324 and
1.9735 Å for Y–iPrO and Ti–iPrO, respectively. A number of bimetallic
oxoalkoxide complexes between Ti and REEs have been reported.32−34 These, however, are commonly partially hydrolyzed, contain oxo bridges,
and are modified with ligands such as carboxylates. A structural analogue,
[Ti2Y(iPrO)9Cl2], to compound 1 was previously reported by Veith
and co-workers35 but produced starting
with anhydrous yttrium chloride.</p><!><p>(a) Molecular structure of compound 1. H atoms and
disordered O atoms in the nitrate groups have been omitted for clarity.
(b) Metal–oxo core of compound 1. Color code:
green, Ti; turquoise, Y; purple, N; red, O; gray, C.</p><!><p>We have previously investigated the hydrolysis of a range
of titanium
alkoxide complexes containing different organic ligands, including
the carboxylate, phenoxide, and phosphonate groups.36−38 In all cases,
the complexes hydrolyzed into titania (anatase). It was interesting
to investigate the hydrolysis product of compound 1 because
bimetallic titanium alkoxide complexes can serve as precursors of
both doped titania39 and bimetallic oxides.40 Crystals were treated with deionized water and
analyzed by high-resolution transmission electron microscopy (HRTEM).
Compound 1 formed uniform spherical crystallites belonging
to the nondoped anatase phase of titania, according to the lattice
fringe distance of 0.2 nm, which corresponds to the 200 plane of anatase
(Figure 2) and EDS
analysis. This approach was thus not suitable for the production of
target materials. On the contrary, annealing of dried powders produced
by solvothermal synthesis with subsequent drying and regrinding offered
doped samples of anatase after subsequent annealing, as described
in the experimental section.</p><!><p>HRTEM micrograph of 1 treated with
water. A few primary
particles with crystalline cores (anatase) are indicated. The diameter
of the particles is ca. 3 nm. Inset: Increased magnification of an
anatase nucleus.</p><!><p>The annealed
titania nanopowders were characterized
by PXRD, and the diffractograms for 400, 500, 600, 700, and 800 °C
are presented in Figure 3. Annealing at 400 °C was not enough to transform the amorphous
titania to anatase. All samples modified with 2.5 mol % REE had transformed
into titania during annealing at 500 °C. For the 5 mol % samples,
only Ti-Y5-500 had transformed into anatase, while Ti-Eu5-500 and
Ti-Sm5-500 were mostly amorphous but with traces of anatase. This
is consistent with previous reports that an increased amount of dopants
retards crystallization. The presence of Ti-O-REE bonds in the amorphous
particles would complicate crystallization into anatase and phase
transformation into rutile.24,41 The reason Ti-Y5-500
could transform into anatase at 500 °C but not Ti-Eu5-500 and
Ti-Sm5-500 may be a consequence of the size differences. Because Y3+ has a smaller ionic radius, reorganization into anatase
might be easier compared to that for the larger Sm3+ and
Eu3+. Both 2.5 and 5 mol % modified powders annealed at
600 °C displayed phase-pure anatase, while the annealed powders
at 700 °C mostly consisted of anatase but with traces of rutile.
At 800 °C, anatase is still the major phase, but it is evident
that transformation into rutile has begun. The anatase-to-rutile transformation
is generally initiated from the particle surface,42 and the presence of Ti-O-REE bonds on the surface would
aggravate this process. It is possible that the deposition of REE
oxides or REE titanates onto the titania particle surfaces was able
to help in stabilization of the anatase.</p><!><p>PXRD patterns of the
REE-modified titania nanopowders at different
annealing temperatures: (a) Ti-Y2.5; (b) Ti-Y5; (c) Ti-Eu2.5; (d)
Ti-Eu5; (e) Ti-Sm2.5; (f) Ti-Sm5. Red, magenta, black, green, and
blue represent annealing temperatures of (top to bottom) 800, 700,
600, 500, and 400 °C, respectively. Peaks labeled "a" and "r" belong
to the anatase and rutile phases, respectively.</p><!><p>The morphologies of the synthesized titania
nanopowders were analyzed by SEM. Micrographs of the powders used
in the catalysis studies are shown in Figure 4. They have the typical morphologies and
variable aggregate sizes of hydrothermally synthesized titania. This
is a result of hydrolysis caused by crystal water from REE nitrate
precursors. To investigate the distribution of Sm and Eu in the powders,
elemental mapping was performed. Sm and Eu were evenly distributed
in the samples, as can be seen for the 2.5 mol % nanopowders in Figure 1. The nanosized character was confirmed
by AFM imaging of Ti-Eu2.5-500 and Ti-Eu5-700 (Figures S2 and S3), with particle sizes in the range 20–30
nm for Ti-Eu2.5-500 and 30–50 nm for Ti-Eu5-700.</p><!><p>SEM micrographs
of the TiO2-REE powders used for catalysis:
(a) Ti-Eu2.5-500; (b) Ti-Sm2.5-500; (c) Ti-Eu2.5-600; (d) Ti-Sm2.5-600;
(e) Ti-Eu5-600; (f) Ti-Sm5-600; (g) Ti-Eu5-700; (h) Ti-Sm5-700. Scale
bars represent 50 Î¼m.</p><!><p>Annealed and amorphous titania
nanopowders were analyzed by FTIR and Raman spectroscopy. The FTIR
spectra for most of the annealed samples showed an absorption band
around 1615 cm–1, which is assigned to bending of
the Ti–OH bonds. The intensity of this absorption tends to
decrease with increased annealing temperature, which is expected because
increasing the temperature results in more condensation and dehydration
of the surface OH groups. A weak absorption around 2330 cm–1 may be due to the formation of various organic compounds during
the solvothermal reaction, and this signal appears do diminish with
increasing annealing temperature (Figure 5).</p><!><p>(a) FTIR spectra of amorphous Ti-Eu5 and Ti-Sm5
powders and reference
titania. Interpretations of the signals are indicated. (b) FTIR spectra
of the annealed titania nanopowders used for photocatalysis. Interpretations
of the signals are indicated.</p><!><p>The FTIR spectra of Eu- and Sm-modified titania powders before
annealing contain a strong absorption line at 1384 cm–1, which is assigned to the presence of nitrate (Figure 5a). During annealing of the
powders, the nitrate signal disappears, as is evident from Figure 5b. In the amorphous
titania, the intensities of the bending OH signals at 1615 cm–1 and stretching for Ti–OH between 3100 and
3500 cm–1 are higher because of the presence of
more of Ti–OH.</p><p>Raman spectra were collected for the powders
used in the catalytic
studies (Figure 6).
The spectra of the Eu-modified titania display four of the vibration
modes for anatase: 199 cm–1 (Eg), 401
cm–1 (A1g), 520 cm–1 (B1g), and 643 cm–1 (Eg)43 with no traces of rutile. Three of the Sm-modified
titania nanopowders display the same anatase signals, except Ti-Sm5-700,
the vibration modes of which belong to the rutile phase: 449 cm–1 (Eg) and 617 cm–1 (A1g).43 The PXRD pattern of Ti-Sm5-700
indicated anatase, with evidence of an initiated transformation into
rutile. The Raman spectrum might have been collected in a small area
with higher rutile content rather than being representative for the
whole sample.</p><!><p>Raman spectra of titania nanopowders with the addition
of (a) 2.5
and (b) 5 mol % REE. a = anatase and r = rutile.</p><!><p>Figures 7 and 8 show the room-temperature PL
emission and excitation spectra of Eu- and Sm-doped TiO2 annealed at 500 and 700 °C, respectively. Upon excitation at
465 nm corresponding to Eu 7F0–5D2 absorption, Eu luminescence displays emission bands
at 580, 590, 615, 655, and 705 nm corresponding to the Eu 5D0, 5D0–7F1, 5D0–7F1, 5D0–7F3, and 5D0–7F4 emission transitions,
respectively. Some of us have recently shown by using low-temperature
(80 K) site-selective excitation that Eu distributes as both discrete
and continuous substitutional centers on anatase Ti4+ lattice
sites as well as on the nanoparticle surfaces. As shown in Figure 7a, the emission of
Ti-Eu2.5-500 I is a convolution of narrow and broad emissions characteristic
of the substitutional center I and surface center, respectively.44 The Eu-TiO2 annealed at 700 °C,
Ti-Eu5-700, presents a weaker emission with a shape similar to that
of surface Eu (Figure 7b). The increase of the annealing temperature from 500 to 700 °C
leads to partial migration of Eu on the surface and on the rutile
lattice sites, even though the long-range cation arrangement remains
of the anatase type (Figure 3).45 The excitation spectra monitored
at 616 nm of Ti-Eu2.5-500 and Ti-Eu5-700 display only narrow Eu f–f
absorptions at 394, 414, 465, 525, and 532 nm corresponding to the 7F0, 1–5L6, 7F1–5D3, 7F0–5D2, 7F0–5D1, and 7F1–5D1 absorption transitions, respectively
(Figure 7b,d).</p><!><p>PL emission
and excitation spectra of Ti-Eu2.5-500 (a and b) and
Ti-Eu5-700 (c and d). The excitation and emission wavelengths are
465 and 616 nm, respectively.</p><p>PL emission
and excitation spectra of Ti-Sm2.5-500 (a and b) and
Ti-Sm5-700 (c and d). The excitation and emission wavelengths are
465 and 616 nm, respectively.</p><!><p>Upon excitation at 340 nm into the TiO2 absorption band,
the Sm luminescence displays emission bands centered at 582, 610,
662, and 725 nm corresponding to the Sm 4G5/2–6H5/2, 4G5/2–6H7/2, 4G5/2–6H9/2, and 4G5/2–6H11/2 emission transitions, respectively (Figure 8a,c). Both Ti-Sm2.5-500 and
Ti-Sm5-700 present similar emissions, which are close to that of the
substitutional center I, as identified by Avram and co-workers.44 However, the emission intensity of Ti-Sm5-700
is relatively lower, which signals a partial migration of Sm on the
nanoparticle surfaces and rutile Ti4+ lattice sites. The
excitation spectra of Ti-Sm2.5-500 and Ti-Sm5-700 monitored at 582.6
nm (corresponding to the maximum of the Sm 4G5/2–6H5/2 emission transition) are dominated
by the broad TiO2 UV absorption band, followed by weak
Sm f–f absorptions in the 400–550 nm spectral range
(Figure 8b,d). Different
to Eu and Sm luminescences are efficiently sensitized by TiO2 absorption.46</p><p>Upon excitation above
the TiO2 band gap, electrons are
excited from the VB to CB, generating holes in the VB. The electron–hole
recombination energy is nonradioactively transferred to the Eu/Sm
excited states via a phonon-assisted energy-transfer process.46 The efficiency of sensitization by TiO2 absorption may depend on the relative positions of the Eu/Sm ground
and excited states with respect to the VB and CB of the host lattice,47 which may explain the observed differences in
the luminescence behavior of the Eu and Sm dopants.</p><!><p>The valence states of Ti 2p and O 1s for anatase
and Eu-doped (5 mol %) TiO2 at different temperatures were
evaluated by the high-resolution XPS spectra in Figure 9. Table 2 summarizes the peak positions of Ti 2p, which were
obtained from the experimental data in Figure 9. Ti 2p spectra show two spin–orbit
peaks with a binding energy difference of 5.72 eV, which reveals that
TiO2 is present mainly as the Ti4+ oxidation
state.48 After Eu doping, the valence state
of the Ti cation can be partly reduced, as seen in the binding energy
of Ti-Eu-600, shifting to a lower binding energy than that of Ti-Eu-400
(Figure 9 and Table 2). For the O 1s spectra,
the peak around 529.2 eV can be assigned to oxygen from the lattice,
while the O 1s spectrum peak at a higher energy (∼530 eV) is
attributed to oxygen vacancies. The energy peak at ∼530 eV
is stronger after Eu doping, and the Ovacancy area peaks
of Ti-Eu-400, Ti-Eu-500, and Ti-Eu-600 increase by 19.4%, 12.6%, and
9.5%, respectively.</p><!><p>High-resolution XPS spectra of Ti 2p and O 1s of anatase
and Eu-doped
(5 mol %) TiO2 at different annealing temperatures.</p><!><p>The photocatalytic performance of the
modified powders was compared to that of the reference Degussa P25
titania (Figure 10). The degradation of 5 mM aqueous trimethylphenol (TMP) was determined
spectrophotometrically after irradiation at 445–465 and 510–530
nm and a wide range of visible light for 15 min. The remaining TMP
concentration was determined by high-performance liquid chromatography–UV/vis.
The Sm- and Eu-modified titania showed less degradation of TMP compared
to the P25 titania when irradiated with the narrow wavelengths. Interestingly,
when a wide-range visible lamp was used, the modified titania had
higher degradation of TMP compared to the P25 control. The overall
degradation yields for all treatments were generally low but increased
with prolonged radiation time. The observed differences in the catalytic
activity may be explained from the PL data. The PL spectra of the
Ti-Eu2.5-500, Ti-Sm2.5-500, Ti-Eu5-700, and Ti-Sm5-700 suggest the
coexistence of substitutional and surface bond REEs at lower annealing
temperatures for both dopants. When the annealing temperature was
increased to 700 °C, the substitutional Eu3+ is forced
to the surface while a portion of the substitutional Sm3+ is still present. From the similar ionic radii of the Eu3+ and Sm3+ ions, it could be expected that they would have
similar substitutional behavior in the anatase lattice. However, it
is indicated here that Sm3+ is a more effective volume
dopant compared to Eu3+. A combination of the volume doping
of anatase, broadening of the adsorption spectrum of TiO2 and contributing additional energy levels across the band gap, and
increased defect concentration on the surface contributed to the improved
activity under a wide range of visible irradiation. The different
observed doping behaviors may be part of the difference in the photocatalytic
activity between the different samples.</p><!><p>Photocatalytic degradation
of TMP using titania nanopowders modified
with Eu and Sm under different wavelengths: (a), 445–465 nm
LEDs; (b) 510–530 nm LEDs; (c) wide-range visible lamp. (d)
Pseudo-first-order reaction kinetics of TMP degradation under visible
irradiation.</p><!><p>As confirmed by the plot described
in Figure 10d, photocatalytic
oxidation of TMP followed
a pseudo-first-order reaction, ln(C0/C) = kt, where C0 is the initial concentration of the organic
molecule (mmol/L), C its concentration at a certain
reaction time, k the rate constant (min–1), and t the reaction time (min). The rate constants
calculated on the basis of this equation are presented in Table 3.</p><p>According to these data, the activity is influenced
by both the
nature and loading of the Ln element and calcination temperature.
Sm turned out to be more effective than Eu (low loading of Eu led
to smaller reaction rates than the parent TiO2 photocatalyst),
and an increase of the calcination temperature at 700 °C afforded
reaction rates for higher times compared to the catalysts calcined
at 500 °C. However, 700 °C is a broader, higher temperature,
producing changes in the catalyst structure. These kinetic results
also correlated well with the characterization results, namely, to
the defect concentration induced by doping with REEs, as evidenced
by the PL experiments.</p><p>The influence of the dopant and calcination
temperature is more
evident from the turnover frequency (TOF) values calculated by the
participation of Ti and rare metals (Table 4). Accordingly, the TOF values using REE
are much higher than those for Ti. However, such an efficiency can
be induced only by their association with Ti. Only REEs are not effective
in this photocatalytic process. The experimental error was below 1%.
All of these experiments were reproduced five times.</p><!><p>The aim of the present work was to synthesize REE (Y, Eu, and Sm)-modified
titania via molecular precursors. This solution–combustion
approach is expected to provide an even distribution of the dopants
in the final oxide phase. A bimetallic titanium–yttrium alkoxide
complex was isolated as a potential molecular intermediate for Y-doped
titania. This complex was very sensitive to moisture and formed anatase
nuclei upon hydrolysis in water according to HRTEM. No bimetallic
intermediates could be isolated for Sm or Eu. The addition of REEs
was found, in accordance with previous reports, to increase the required
temperature for transformation from amorphous titania to anatase and
also for the anatase-to-rutile transformation. The anatase phase was
stabilized against transformation to rutile until ca. 700 °C.
Photocatalytic studies revealed that the REE-modified powders were
more efficient photocatalysts compared to commercial P25 titania under
irradiation from a wide range of visible light. PL data of the REE-modified
titania indicated different substitutional behaviors for Eu3+ and Sm3+ ions, where Eu3+ appeared to be expelled
from the anatase lattice at a lower annealing temperature than Sm3+. This could rationalize part of the results found in the
difference in the photocatalytic activity.</p><!><p>Elemental mapping,
AFM micrographs, and crystal data
(PDF)</p><p>ic1c02134_si_001.pdf</p><!><p>CCDC 2095265 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.</p><!><p>The manuscript
was written through contributions of all authors. All authors have
given approval to the final version of the manuscript.</p><!><p>This work was
supported by the Swedish Research Council (Vetenskapsrådet) under
Grant 2014-3938. C.T. received no specific funding for this work.
B.C. acknowledges financial support from Unitatea Executiva pentru
Finantarea Invatamantului Superior, a Cercetarii, Dezvoltarii si Inovarii
(UEFISCDI) under Grant PCE 235.</p><p>The authors declare no
competing financial interest.</p>