Preparation and photocatalytic activity of Nd-modified TiO2 photocatalysts: Insight into the excitation mechanism under visible light

Article abstract of DOI:10.1016/j.jcat.2017.07.017

Titanium dioxide (TiO₂) nanoparticles (NPs) modified with neodymium (Nd) in the range between 0.1 and 1.0?mol% were prepared via the hydrothermal method. The samples obtained were characterized by diffuse reflectance spectroscopy (DRS), scannin

Full text of DOI:10.1016/j.jcat.2017.07.017

Journal of Catalysis 353 (2017) 211–222
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Preparation and photocatalytic activity of Nd-modified TiO2
photocatalysts: Insight into the excitation mechanism under visible light
Patrycja Parnicka a, Paweł Mazierski a, Tomasz Grzyb b, Zhishun Wei c, Ewa Kowalska c, Bunsho Ohtani c,
Wojciech Lisowski d, Tomasz Klimczuk e, Joanna Nadolna a,
⇑
a Department of Environmental Technology, University of Gdansk, 80-308 Gdansk, Poland
b Department of Rare Earths, Faculty of Chemistry, Adam Mickiewicz University in Poznan, 60-780 Poznan, Poland
c Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan
d Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland
e Department of Solid State Physics, Faculty of Applied Physics and Mathematics, Gdansk University of Technology, 80-233 Gdansk, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Titanium dioxide (TiO2) nanoparticles (NPs) modified with neodymium (Nd) in the range between 0.1
and 1.0 mol% were prepared via the hydrothermal method. The samples obtained were characterized
by diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), X-ray fluorescence
(EDX), Brunauer–Emmett–Teller (BET) method, X-ray powder diffraction analysis (XRD), X-ray photo-
electron spectroscopy (XPS) and photoluminescence spectroscopy (PL). The photocatalytic activity of
the obtained samples was evaluated by photodegradation of phenol in aqueous solution under ultravio-
let–visible (UV–Vis, k > 350 nm) and visible (Vis, k > 420 nm) irradiation. Experimental results showed
that the photocatalysts exhibited high photocatalytic activity under Vis light. The sample showing the
highest photoactivity under Vis irradiation was in the form of anatase; its surface area equalled
124 m2/g (1.16 times larger than that of pristine TiO2). The average crystal size was 10.9 nm, and it
was modified with 0.1 mol% of Nd3+ (28% of phenol was degraded after 60 min of irradiation). The pho-
Received 5 June 2017
Revised 18 July 2017
Accepted 19 July 2017
Keywords:
Heterogeneous photocatalysis
TiO2
Modified-TiO2
Rare earth metal
Neodymium
Visible light photoactivity
Phenol degradation
Action spectra analysis
tocatalytic tests of phenol degradation in the presence of scavengers confirm that eÀ and
were
responsible for the visible light degradation of organic compounds in the aqueous phase. Action spectra
analysis revealed that although Nd-modified TiO2 could be excited under visible light in the range of 400–
480 nm, the up-conversion process is not responsible for the degradation of phenol under Vis irradiation.
Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction
In particular, modification with rare earth (RE) metals, such as
neodymium (Nd3+), has a positive effect on photocatalytic activity
In recent years, various photocatalytic processes have been the
subject of intensive research because they belong to the most sus-
tainable and environmentally friendly technologies [1]. Titanium
dioxide (TiO2) is one of the most widely used semiconductors for
photocatalytic applications. The greater interest in TiO2 rests in
the fact that it is cheap, non-toxic, chemically and photochemically
stable and has a strong oxidation capacity [2]. However, despite
the promising properties, anatase TiO2 is sensitive only to UV light
due to a wide band gap (3.2 eV). Therefore, its application is lim-
ited because the UV region represents only 3–5% of the entire solar
spectrum [3]. Many studies have been performed to develop vari-
ous modifications of TiO2 to obtain photocatalysts sensitive to vis-
ible light (k > 400 nm) [4–10].
under visible light [11–14]. The beneficial influence on photocat-
alytic activity by titania modification with RE has been ascribed
to the f-orbitals of RE ions that can complex with various Lewis
bases (e.g., acids, alcohols, aldehydes, amines, and thiols) [15,16].
Thus, inclusion of rare earth ions into a TiO2 matrix could provide
means to concentrate the substrates at the semiconductor surface.
Moreover, modification with RE ions improves the separation effi-
ciency of photoinduced electron-hole pairs and prevents recombi-
nation [4].
The effect of Nd modification on the photocatalytic activity of
TiO2 has been reported in the literature. Štengl et al. synthesized
RE-doped TiO2 materials by precipitation method. They reported
that neodymium-doped TiO2 showed the highest photoactivity
under Vis irradiation for degradation of Orange II dye among other
RE-TiO2 photocatalysts (such as Pr3+, Ce3+, Ce4+, Eu3+, Sm3+, Gd3+
and La3+) [17]. Bokare et al. used Nd-TiO2 materials, synthesized
⇑
Corresponding author.
E-mail address: joanna.nadolna@ug.edu.pl (J. Nadolna).
http://dx.doi.org/10.1016/j.jcat.2017.07.017
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

212
P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
Table 1
by the sol-gel method, for degradation of methyl orange dye in an
aqueous solution under solar light irradiation. It was observed that
the dopant amount affected the photocatalytic activity of the pho-
tocatalysts. Nevertheless, the use of organic dyes as a model com-
pound should be avoided due to their absorption of visible light
and photosensitization of TiO2, which results in a complex mecha-
nism of dye degradation [18]. There are only a few papers reporting
photodegradation of other organic compounds. For example,
Gomez et al. tested photocatalytic activities of Nd-TiO2 photocata-
lysts, prepared at mild temperature by microwave-assisted treat-
ment, for the degradation of phenol and rhodamine B under UV–
Vis irradiation. It was found that the best photocatalytic activity
was obtained for an optimal Nd content of 0.06 mol%, resulting
in complete degradation of phenol in 120 min [19]. Xie et al. used
the chemical coprecipitation–peptization method to prepare Nd-
modified TiO2 nanoparticles. The experimental results indicated
improved photocatalytic activity of Nd-TiO2 for phenol pho-
todegradation compared to that of pristine TiO2 under visible light
irradiation [20].
Although many studies have reported on Nd-modified TiO2,
there is no available information regarding the origin of an excita-
tion mechanism of Nd-TiO2 under visible light. Some studies have
reported an interesting approach that the up-conversion process
can be responsible for photocatalytic activity of other RE-TiO2 sys-
tems under Vis light irradiation [21,22]. Nevertheless, up-
conversion was not discussed as a contributor to an improved exci-
tation mechanism of Nd-TiO2.
In the present work, we report the synthesis and photocatalytic
activity of neodymium-modified TiO2 photocatalysts prepared by
the hydrothermal method. The photocatalytic properties of all of
the samples under UV–Vis and Vis irradiation were investigated
by monitoring the degradation reaction of phenol in the aqueous
phase. To understand what type of oxygen species participate in
the degradation mechanism, a hydroxyl radical test with tereph-
thalic acid and the reactive oxygen species formation tests using
benzoquinone, silver nitrate, ammonium oxalate and tert-butanol
as scavengers were studied. The photocatalytic degradation path-
way of phenol was also examined. To explain the possible mecha-
nism of Nd-TiO2 photocatalyst excitation under visible light, the
action spectra measurements were investigated for select samples.
For the first time, the effects of irradiation wavelength on apparent
quantum efficiency have been studied by employing photodegra-
dation of phenol as a model pollutant.
Description and physicochemical characterization of Nd-TiO2 photocatalysts.
Sample label
Assumed content
of Nd3+ (mol.%)
SBET (m2/g)
Crystallite size (nm)
TiO2
None
0.1
0.25
0.5
107
124
141
146
160
18.8
10.9
8.8
6.8
4.7
0.1%Nd-TiO2
0.25%Nd-TiO2
0.5%Nd-TiO2
1.0%Nd-TiO2
1.0
The mixture was stirred for 30 min at room temperature. After
treatment at 120 °C for 4 h in a 200 mL autoclave (hydrothermal
process), the as-obtained samples were separated by centrifuga-
tion, washed three times with deionised water and ethanol, and
dried at 35 °C for 16 h. Finally, the samples were ground and cal-
cined at 450 °C for 2 h in a muffle furnace under air atmosphere
with a heating rate of 2 °C/min followed by grinding to obtain
the Nd-TiO2 powder. A description of the prepared photocatalysts
is shown in Table 1.
2.3. Characterization of Nd-TiO2
To characterize the photoabsorption properties of pristine and
modified photocatalysts, the diffusive reflectance spectra (DRS)
were recorded on
a Shimadzu UV–Vis spectrophotometer
(UV 2600) equipped with an integrating sphere. BaSO4 was used
as the reference. The obtained absorption spectra were recorded
in the range of 250–850 nm with a scanning speed of 200 nm/
min at room temperature. DRS were recorded and data were con-
verted by Kubelka–Munk (K-M) function to obtain absorption
spectra.
Nitrogen adsorption-desorption isotherms were carried out at
À197 °C (liquid nitrogen temperature) using a Micromeritics Gem-
ini V (model 2365). The surface area was determined according to
the standard Brunauer, Emmet and Teller (BET) method. All sam-
ples were degassed at 200 °C prior to nitrogen adsorption
measurements.
The morphology of Nd-TiO2 was determined using a Bruker
spectrometer-coupled Quantax 200 scanning electron microscope,
and the images were captured at 3000-fold magnification.
X-ray diffraction (XRD) was used to determine and verify the
crystalline structure of the obtained photocatalysts. XRD patterns
were recorded on a Rigaku diffractometer (RINT Ultima+) equipped
with a graphite monochromator using Cu Ka radiation (40 kV tube
2. Experimental section
voltage and 20 mA tube current). Measurements were performed
in a 2h range of 5–90°. Based on the results obtained, the particle
sizes of photocatalysts and XRD data were calculated using the
Scherrer’s equation.
2.1. Materials
Titanium isopropoxide (97%, TIP) and Nd(NO3)3Á6H2O (99.99%),
used as titanium and neodymium sources, were purchased from
Sigma–Aldrich, Poland. Isopropanol and nitric acid (65%) were pur-
chased from STANLAB, Poland. Ethanol, phenol and terephthalic
acid were purchased from POCh S.A, Poland. P25 from Evonik, Ger-
many was used as a standard and for comparison of the photocat-
alytic activity. Deionised water (0.05 mS) was used for all reactions
and treatment processes. All the chemicals were used as received
without further purification.
The surface composition of the samples was investigated by X-
ray photoelectron spectroscopy (XPS) on a PHI 5000 VersaProbeTM
(ULVAC-PHI) spectrometer with monochromatic Al K
a radiation
(h = 1486.6 eV). The high-resolution (HR) XPS spectra were col-
m
lected by the hemispherical analyzer at a pass energy of 23.5 eV,
an energy step size of 0.1 eV and a photoelectron take off angle
of 45° with respect to the surface plane. The binding energy (BE)
scale of all detected spectra were calibrated by normalizing the
large CAC peak in the C 1s region to 284.6 eV. The HR XPS spectra
were deconvoluted with Gaussian–Lorentzian-shaped profiles.
Photoluminescence (PL) properties in the range of 300–700 nm
were measured on Perkin Elmer limited LS50B spectrophotometer
equipped with a Xenon discharge lamp as an excitation source and
an R928 photomultiplier as a detector. The spectra were recorded
with an excitation wavelength of 315 nm directed on the sample
surface at an angle of 90°. PL spectra in the range of 400–
1000 nm were measured on a Princeton Instrument PIXIS:256E
2.2. Preparation of Nd-TiO2
The hydrothermal method was used for the preparation of Nd-
TiO2 nanoparticles. In a typical procedure, 136 mL of TIP was dis-
solved in 136 mL of isopropanol. Then, a mixture of 4.8 mL of
water, a certain amount of solid Nd(NO3)3Á6H2O, 4.8 mL of iso-
propanol and 1.6 mL of nitric acid were added to the TIP solution.

P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
213
Digital CCD Camera equipped with an SP-2156 Imaging Spectro-
graph and Opolette 355LD UVDM tunable laser as the excitation
source. The spectra were recorded with an excitation wavelength
of 350 nm.
Total Organic Carbon was carried using TOC analyzer Shimadzu
TOC-L.
phase under Vis (k > 420 nm), irradiation was conducted in the
presence of silver nitrate, ammonium oxalate, benzoquinone and
À
Å
tert-butanol as scavengers of eÀ, h+, O2Å and OH radicals, respec-
tively. In this case, ‘scavenger’ water solution was added to the
reactor together with phenol aqueous solution (C0 = 20 mg/L; 1:1
v/v). The experimental procedure is described in Section 2.4.1.
2.4. Photocatalytic experiments
2.4.4. Phenol decomposition under monochromatic irradiation (action
spectra measurements)
The photocatalytic properties of the obtained Nd-TiO2 samples
were determined in a two-reaction system, i.e., phenol decomposi-
tion under polychromatic UV–Vis and Vis irradiation. Furthermore,
a hydroxyl radical test under UV–Vis and Vis irradiation using
terephthalic acid was investigated and besides that the reactive
oxygen species formation test under Vis irradiation using benzo-
quinone, silver nitrate, ammonium oxalate and tert-butanol as
scavengers was also performed. Additionally, to calculate quantum
efficiency (action spectra measurements), phenol decomposition
was measured under monochromatic irradiation. The experimen-
tal procedures have been selected and described based on different
types of photocatalytic set-up and geometric of photoreactors.
Action spectra measurements were investigated for the selected
photocatalyst. Photocatalyst powder (50 mg) was suspended in
aqueous solution (5.0 mL) containing phenol (C0 = 20 mg/L) and
placed in a quartz cell and then irradiated at monochromatic wave-
lengths for 90 min (k = 400, 420, 440, 460, 480, 500, 520, 540,
560 nm), 180 min (k = 585, 645, 675, 745 nm) and 360 min
(k = 805 nm) using a diffraction grating-type illuminator (Jasco,
CRM-FD) equipped with
a 300 W Xenon lamp (Hamamatsu,
C2578-02). The light intensity was measured using an optical
power meter (HIOKI 3664). During the experiments, the reaction
mixtures were continuously stirred. During the irradiation, every
30 min a portion (0.2 mL) of the reaction mixture was withdrawn
with
a syringe and subjected to HPLC. The wavelength-
2.4.1. Phenol decomposition under polychromatic irradiation
dependent apparent quantum efficiency was calculated as the ratio
of the rate of electron consumption from the rate of benzoquinone
generation to the flux of incident photons, assuming that two pho-
tons are required according to the stoichiometry of this reaction.
The HPLC system was equipped with a WAKOSIL-II SC18 AR col-
umn (250 mm  4.6 mm). The flow rate was maintained at 1 mL/
min with a mobile phase composed of acetonitrile, water and
phosphoric acid (60/40/1 v/v/v) with a sample injection volume
Polychromatic irradiation was carried out for two irradiation
ranges, UV–Vis (k > 350 nm) and Vis (k > 420 nm), using a 150 W
Mercury lamp (Heraeus) and 1000 W Xenon lamp (Oriel 66021),
respectively. The experimental procedure under UV–Vis irradiation
was as follows: the photocatalysts (7.5 mg) were suspended in
phenol aqueous solution (15 mL, C0 = 20 mg/L) in boron-silicon test
tubes. The experimental procedure under Vis irradiation was as
follows: Nd-TiO2 powder (125 mg) was suspended in phenol aque-
ous solution (25 mL, C0 = 20 mg/L) in a quartz photoreactor. After
30 min of mixing (550 rpm) and aeration (5 dm3/h) in the dark,
the suspension was irradiated with a cut-off spectrum of light.
To avoid the stripping of the phenol the temperature of the suspen-
sion during photoirradiation was maintained at 10 °C using a ther-
mostatically controlled water bath. Samples of a 0.5 mL reaction
mixture were collected at regular time periods during irradiation
of 5 lL, and the detector was operated at 254 nm.
3. Results and discussion
3.1. Diffuse reflectance spectroscopy
and filtered through syringe filters (Ø = 0.2
l
m) to remove the sus-
To investigate the photoabsorption properties of the obtained
photocatalysts, the diffusive reflectance spectra (DRS) (of Nd-
modified TiO2 samples were investigated in the range of 250–
850 nm, and the results are shown in Fig. 1. Pristine TiO2 and
P25 were used as the reference samples. All prepared photocata-
lysts were white powders. Each sample has a broad intense absorp-
tion below 400 nm, which can be attributed to charge-transfer
related to electron excitation from the valence band to the conduc-
tion band in TiO2 [23–25]. P25 and as-prepared pristine TiO2 did
not exhibit any absorption in the visible region. All modified and
pristine TiO2 showed a redshift of their absorption compared to
that of P25. Moreover, compared with Nd-TiO2, unmodified TiO2
has a stronger absorption edge shift towards a longer wavelength.
This behaviour was described in other studies [4,12,26–29], and
the blueshift in the present work may be ascribed to the increase
in the band gap after modification of titanium dioxide. The proba-
bly reason for such a band gap energy increase was gradual move-
ment of the conduction band of TiO2 above the first excited state of
RE3+. Rare earth ions at the first excited state could interact with
electrons of the conduction band of TiO2, which would conse-
quently cause higher energy transfer from TiO2 to RE3+ ions. Fur-
thermore, the blueshift can also be assigned to effective quantum
size due to the decrease in the crystalline size [4]. Beyond that, it
can be seen that there are absorption bands located at 520, 585,
pended photocatalyst particles. Kinetics of phenol degradation by-
products and phenol concentration were determined by high-
performance liquid chromatography (HPLC, Shimadzu). The HPLC
system was equipped with
a
Kinetex C18 column
(150 mm  3 mm; particle size of 2.6
lm; pore diameter 100 Å)
and the SPD-M20 A diode array detector (k = 205 nm). The flow
rate was maintained at 0.4 mL/min with a mobile phase composed
of acetonitrile and water (7.5/92.5 v/v). The injection volume was
30 lL.
2.4.2. Hydroxyl radical generation efficiency
Terephthalic acid can produce the fluorescent compound 2-
hydroxyterephthalic acid by reaction with hydroxyl radicals. The
experimental procedure is described in Section 2.4.1. The photo-
catalysts were suspended in terephthalic acid aqueous solution
(C0 = 5 Á 10À4 mol/L). Samples of a 2 mL reaction mixture were col-
lected after 60 min irradiation and filtered through syringe filters
(Ø = 0.2 lm) to remove the suspended photocatalyst particles. Flu-
orescence spectra were measured at room temperature using a
Perkin Elmer limited LS50B spectrophotometer equipped with a
Xenon discharge lamp as an excitation source and an R928 photo-
multiplier as a detector. The obtained solution was measured with
the excitation wavelength of 315 nm.
745, 805 nm, which can be attributed to the transition from the
4
2.4.3. Reactive oxygen species generation efficiency
To investigate the role of the reactive oxygen species in the pho-
tocatalytic process, the photocatalytic activity test in the water
4I9/2 ground state to the excited states of the neodymium G7/2
,
2K13/2 and 4G5/2, 2G7/2 and 4S3/2, 4F7/2 and 4F5/2, 2H9/2 [24,30,31]. This
study showed that the intensity of neodymium absorption bands

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P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
Fig. 1. UV–Vis Kubelka–Munk absorption spectra of Nd-TiO2 photocatalysts compared to those of pristine TiO2 and P25.
increases with the amount of Nd3+, which is in agreement with the
available literature [24,29,32,33].
ions adsorbed either on the titania surface or placed inside the tita-
nia lattice [4,26,34].
The specific surface area and crystallite size of pristine and Nd-
modified TiO2 photocatalysts are shown in Table 1. All Nd-
modified samples showed a higher BET surface area than that of
the pristine TiO2 sample (107 m2 gÀ1). The surface area varied from
124 to 160 m2 gÀ1. An increase in the neodymium content from 0.1
to 1.0 mol% caused an increase in the BET surface area. The average
crystallite size of anatase was calculated using the Scherrer equa-
tion and ranged from 4.7 to 10.9 nm for Nd-TiO2. For pristine
TiO2, the crystallite size was 18.8 nm. It has been noted that crys-
tallite size decreases with increasing surface area. Literature
reports [26,27,35,36] have demonstrated that, because of the larger
radii of rare earth ions, they did not enter into the TiO2 crystal lat-
tice to substitute for Ti4+ but rather are distributed on the surface
of TiO2 forming the bond RE–O–Ti. Furthermore, RE–O–Ti bonds
around TiO2 particles could inhibit the growth of crystal grains,
thus reducing the specific surface area [37].
3.2. X-ray diffraction and BET surface area
The crystalline phase of pristine TiO2 and Nd-modified TiO2
nanoparticles was determined by X-ray powder diffraction. The
XRD patterns of samples are shown in Fig. 2. In all synthesized
photocatalysts, the figure presents a group of lines at 2h values
of 25.4°, 37.9°, 48.1°, 54.1°, 55.1°, 62.9°, 70.5° and 75.2°, which
are attributed to the anatase phase. This means that the phase
transformation of anatase to rutile did not occur despite the heat
treatment at 450 °C. In addition, the diffraction peaks of Nd2O3
are not observed in any of the as-prepared Nd-modified TiO2 pho-
tocatalysts. This indicated that the content of Nd3+ was too small
for detection, crystallites were very small and highly dispersed,
Nd3+ was present in the form of an amorphous phase or as Nd3+
3.3. Scanning electron microscopy
Scanning electron microscopy was used to characterize the
morphology of the obtained photocatalysts. Fig. 3 shows an SEM
image of pristine TiO2 and Nd-modified TiO2, and it is observed
that aggregation occurred in all samples. Pristine TiO2 particles
were found to be in the larger form of aggregates, unlike Nd-
modified TiO2. Neodymium ion modification preferably retarded
the aggregation and growth of well-dispersed particles. Correlation
between the extent of segregation with dosage of neodymium was
not observed. Moreover, particles of Nd-TiO2 were found to be
spherical in shape with a diameter in the range of 0.3–1 lm, and
the SEM-EDX analysis of the 0.25% Nd-modified TiO2 sample
revealed the presence and uniform distribution of neodymium in
the synthesized photocatalyst (Fig. 4). Furthermore, the SEM-EDX
was applied to determine the elements of samples. The average
elements content was calculated based on EDX analysis from the
area 1 Â 1
lm of each sample. The EDX analysis revealed the pres-
ence of titanium, oxygen, carbon and neodymium atoms in the
Fig. 2. X-ray diffraction patterns of pristine TiO2 and Nd-TiO2.
compositions of prepared samples (Table S1). Further information

P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
215
Fig. 3. SEM images of TiO2 and Nd-TiO2.
Fig. 4. SEM-EDX image of 0.25% Nd-TiO2.
about the composition of the surface photocatalysts was provided
by the XPS method.
defects in the titania lattice. It was reported that even faceted ana-
tase photocatalysts of high crystallinity and uniform morphology,
e.g., octahedral and decahedral anatase particles, contained a small
amount of Ti3+ [38,39]. The highest Ti3+ content was observed for
the 0.25% Nd-TiO2 sample (5.56%). The oxygen peak can be decon-
voluted into three peaks, where the most intense one at 529.7 eV
indicates the presence of oxygen in the titania lattice (TiO2)
[40,41]. Two other peaks at 530.2 eV and 531.2 eV can be related
to TiAO surface species (Ti-(OH)-Ti, TiOx) and AC@O, AOH groups,
respectively. It is known that the surface modification of titania
usually results in a decrease in the intensity of the last peaks
because modifiers replace the hydroxyl groups [28,39]. Conse-
quently, an increase in the content of oxygen in the form of TiO2
correlates with a decrease in the O/Ti ratio. In the present study,
a similar observation was noted for some Nd-TiO2 samples (a
decrease of O/Ti and an increase of Ti-Olatt contribution (Table 2)).
Therefore, it is proposed that a small number of neodymium ions
formed a surface complex as described in Fig. 6. The formation of
the complex between the f orbital of Nd with oxygen increases
the photocatalytic activity, providing an effective environment
for better interactions among the organic contaminants and hydro-
3.4. X-ray photoelectron spectroscopy (XPS)
The elemental contents and chemical character of elements in
the surface layer of pristine and Nd-modified TiO2 samples were
examined by XPS. The high-resolution (HR) XPS spectra of Ti2p,
O1s, C1s and Nd 4d were recorded for detected elements; titanium,
oxygen, carbon and neodymium, respectively. The deconvoluted
spectra are presented in Fig. 5 and the XPS data are summarized
in Table 2. The presence of neodymium was confirmed all modified
samples (see Nd 4d spectra in Fig. 5). The contents of neodymium
(in wt.%) agree well with a nominal amount of Nd used during all
samples preparation (Table 1). Deconvoluted Ti 2p, O 1s and C 1s
core level spectra reveal a chemical character of titanium, oxygen
and carbon species, respectively (Fig. 5 and Table 2). Two chemical
states of titanium were separated in the Ti 2p spectrum at the BE of
Ti 2p3/2 peak close to 458.4 eV and 456.8 eV, which can be identi-
fied as Ti4+ and Ti3+, respectively. The small amount of reduced tita-
nium (Ti3+) is usually observed in all titania samples due to oxygen

216
P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
Fig. 5. XPS spectra of pristine TiO2 and Nd-TiO2.
xyl radicals [42]. Deconvolution of the carbon peak results in three
peaks at BE 284.6, 286 and 288.7 eV due to the presence of carbon
in the form of CAC, CAOH and C@O, respectively. Usually, carbon
originates from the atmosphere and is always detected in all tita-
nia samples. Moreover, the precursor of titania (titanium iso-
propoxide) can be a source of carbon in the sample. A decrease
in the carbon content with an increase in neodymium content
(Table 2) may suggest the replacement of carbon on the titania sur-
face with neodymium.
properties of rare earth ions, resulting from electron transitions
within the 4f subshell and shielding properties of 5s and 5p outer
orbitals. Upon analyzing the spectrum, it may be noted that the
neodymium-modified TiO2 caused a decrease in intensity com-
pared to that of pristine TiO2. This indicates that Nd3+ effectively
inhibits the recombination of electron-hole pairs. In addition, it
can be seen that a small dose of Nd3+ can reduce the photolumines-
cence intensity, which suggests that the appropriate amount dop-
ing significantly inhibited the recombination of the
photogenerated charge carriers. Photoluminescent properties fol-
lows the order 0.1 < 0.25 < 1.0 < 0.5 < 0 mol% Nd3+
. When the
3.5. Photoluminescence spectroscopy
amount of neodymium is 0.1 mol%, the intensity reaches a mini-
mum value, indicating that the 0.1 mol% Nd-TiO2 has the highest
separation efficiency of electrons and holes. Based on available lit-
erature, it can be concluded that the effect of rare earth ions on
photoluminescence properties of TiO2 is ambiguous. Xiao et al.
[32] observed a decrease in intensity of PL with an increase in
the content of samarium (0.5 < 1.0 < 1.5 < 0 mol% Sm3+), whereas
according to research by Liqiang et al. [27], PL properties changed
Photoluminescence properties of prepared samples are pre-
sented in Fig. 7a. Under excitation by light at kex = 315 nm, pow-
ders showed a weak broad emission band with a maximum
intensity at around 350–700 nm with four bands. The first band
at approximately 420 nm was attributed to the TiO2 octahedral,
and the observed emissions at 440, 480 and 524 nm were associ-
ated with recombination of charge carriers. The maximum of the
emission band depends on the type of recombination taking place
and could shift in the whole spectral range [43]. It can be seen that
unmodified and Nd-TiO2 nanoparticles can exhibit obvious exci-
tonic PL signals with a similar curve shape, demonstrating that
Nd3+ modification does not give rise to new PL phenomena and
emission from Nd3+ ions is minimal in the presented range, which
is consistent with the literature and relates to the spectroscopic
properties of Nd3+ ions [18,27,32]. This is due to spectroscopic
in the order 0 < 5.0 < 0.5 < 3.0 < 1.0 mol% La3+
.
Photoluminescence spectra recorded in the wider range and
with the excitation wavelength kex = 350 nm (Fig. 7b) showed an
intense emission peak with
a maximum at approximately
880 nm. The observed emission relates to the 4F3/2 ? 4I9/2 elec-
tronic transition of Nd3+ ions. Furthermore, less intense emission
peaks were observed below 850 nm, resulting from the presence
of Nd3+ ions in the structure of the material. The emission of

P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
217
Fig. 6. Surface complex formation between Nd and TiO2.
Nd3+ ions is much more intense than that of TiO2 as a result of the
recombination mechanism (band with maximum intensity at
around 350–700 nm, see Fig. 7a). This is the reason for the direct
excitation of Nd3+ ions at 350 nm (through to 4I9/2 ? 5D1/2
,
2I11/2
,
5
5D5/2, and D3/2 transitions) into their higher excited states [44],
which was not observed with kex = 315 nm (Fig. 7a). Furthermore,
the spectral behaviour of Nd3+ is different from that of TiO2 due to
the spectroscopic properties of rare earth ions. The characteristic
narrow emission bands result from the electron transitions within
the 4f subshell and shielding properties of 5s and 5p outer orbitals.
By taking into consideration Nd3+-dependent photolumines-
cence of TiO2, we can also assume energy transfer from the TiO2
host compound to Nd3+ ions, which could explain the quenching
effect of Nd3+ ions on the TiO2 photoluminescence. Nd3+ ions are
prone to cross-relaxation (energy migration between Nd3+ ions);
therefore, samples with a concentration of Nd3+ ions above 0.25%
showed a decrease in luminescence intensity [45]. This process is
also responsible for the ratio between observed Nd3+ transition
bands from which 4F3/2 ? 4I9/2 is less sensitive for cross-
relaxation than others. Furthermore, the relatively low doping
level in which cross-relaxation becomes visible suggests the pres-
ence of Nd3+ clusters in the material or the existence of the Nd2O3
phase.
Unfortunately, up-conversion of Nd3+ ions was not detected
under excitation wavelengths in the Vis-NIR range. The phe-
nomenon of Nd3+ up-conversion is possible only in low phonon
host compounds such as fluorides or chlorides [46]. In oxides, the
mechanism of up-conversion is affected by properties of crystal
structure and multiphonon relaxation processes [46,47]. Addition-
ally, the TiO2 host compound may absorb the energy from the Nd3+
ions excited by NIR photons.
3.6. Photocatalytic activity of Nd-TiO2
The photocatalytic properties of Nd-TiO2 NPs were investigated
by observing the decomposition of phenol in an aqueous solution
under UV–Vis (k > 350 nm) and Vis (k > 420 nm) light. Pristine
TiO2 and P25 were used as reference samples. The results of phenol
photodegradation after 60 min of exposure to irradiation are
shown in Table 3 and Fig. 8. The highest UV–Vis activity was
observed for TiO2 modified with 0.5 mol% of Nd3+ with the
efficiency of phenol degradation reaching 38.54% after 60 min of
irradiation. However, all Nd-modified TiO2 is less active under
UV–Vis light in comparison to that of P25. This phenomenon could
be explained by the spectroscopic properties of RE3+ ions. RE3+
absorbed UV irradiation and emitted energy in the conversion
process of luminescence, lowered the excitation of TiO2 [28].
Fig. 8b shows the visible light photoactivity, which exhibits a
decreasing trend with an increasing dosage of neodymium
(0.1 > 0.25 > 0.5 > 1.0 mol% Nd3+). The photocatalysts 0.1 mol%
Nd-modified TiO2 exhibited the highest photocatalytic activity.
The efficiency of phenol decomposition measured after 60 min of
irradiation was 28.03%. It was found that all Nd-TiO2 revealed
higher photocatalytic activity than those of P25 and pristine TiO2
under visible light irradiation. The probably reason for the low
photocatalytic activity of the pristine sample results from the fact
that Nd3+ ions, acting as a Lewis acid, could act as an effective elec-

218
P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
Fig. 7. (a) Photoluminescence spectra under UV light (kex = 315 nm) of Nd-TiO2 photocatalysts compared to that of pristine TiO2. (b) Up-conversion spectra of the TiO2
samples modified with Nd3+ under excitation of a laser at kex = 350 nm.
Table 3
Efficiency of phenol degradation under UV–Vis and Vis irradiation and intermediate product formation under Vis light in the presence of Nd-TiO2.
Sample label
Phenol degradation after 60 min
UV–Vis (k > 350 nm) irradiation (%)
Phenol degradation after
60 min of Vis (k > 420 nm)
irradiation (%)
Amount of phenol after 60 min
of Vis irradiation (mg/L)
Amount of intermediate products after
60 min of Vis irradiation (mg/L)
Benzoquinone
Hydroquinone
Catechol
TiO2
24.48
28.84
34.48
38.54
34.09
94.08
4.74
19.05
14.39
15.51
15.58
18.11
18.68
Below detection limit
0.1%Nd-TiO2
0.25%Nd-TiO2
0.5%Nd-TiO2
1.0%Nd-TiO2
P25
28.03
22.47
22.09
9.45
0.54
0.51
0.50
0.09
–
1.28
1.00
1.01
0.22
–
0.74
0.70
0.65
0.21
–
6.60
Fig. 8. Photoactivity under (a) UV–Vis (k > 350 nm) and (b) Vis (k > 420 nm) light of Nd-TiO2 photocatalysts compared to those of pristine TiO2 and P25.
tron scavenger to trap the conduction band electron of TiO2 [34].
was no correlation between surface area and photoactivity. Which
suggests that there is an optimum doping content of Nd3+ ions in
TiO2 particles. Some studies showed that the larger BET surface
area of RE-TiO2 photocatalysts would be beneficial to achieve bet-
ter adsorption of reagents in aqueous suspension. However, it was
noticeable that a larger specific surface area of RE-TiO2 with a
higher RE ions dosage did not lead to a higher photocatalytic activ-
´
Xiao et al. [32] and Reszczynska et al. [48] in their study also
observed a decrease in photocatalytic activity under Vis irradia-
tion, increasing the amount of RE3+ (0.5 > 1.0 > 1.5 > 0 mol% Sm3+
and 0.25 > 0.5 > 0 > 1.0 mol% Er3+, respectively). Lower photoactiv-
ity could be explained by blocking of the TiO2 surface by Nd3+ ions
as well as to an increase in recombination of charge carriers. There

P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
219
ity, which might be limited by lower separation efficiency of
charge carriers [32].
irradiation is attributable to other forms of reactive oxygen species
À
Å
such as OÅ2 , H2O2, and OOH [12]. To confirm the role of generated
of the reactive oxygen species in the photocatalytic process, the
photocatalytic activity test in the water phase was conducted in
the presence of 0.1%Nd-TiO2 and scavenger. Silver nitrate, ammo-
nium oxalate, benzoquinone and tert-butanol were used as scav-
3.7. Detection of reactive species
À
To identify the possible mechanisms of photocatalytic degrada-
tion of phenol, especially to understand what type of oxygen spe-
cies are generated under ultraviolet and visible light
photocatalysis, tests with terephthalic acid (TPA) were performed.
Terephthalic acid can produce fluorescent compounds (2-
hydroxyterephthalic acid) by reaction with hydroxyl radicals
[49]. Thus, the intensity of the fluorescent band is proportional to
Å
engers of eÀ, h+, OÅ2 and OH radicals, respectively. The results of
phenol photodegradation presence of scavenger after 60 min of
exposure to Vis irradiation are shown in Fig. 10a. The phenol
degradation in the presence of silver nitrate and benzoquinone
caused
a negligible decrease in the photocatalytic efficiency
(approximately 3.75 and 5%, respectively). While the photocat-
alytic efficiency of phenol degradation in the presence of ammo-
Å
the amount of OH radicals produced in water. Pristine TiO2 and
nium oxalate and tert-butanol reached 23.87 and 25.02%,
À
blank sample (terephthalic acid) were taken as reference samples.
Fig. 9a shows the fluorescence spectra intensity change observed
after 60 min of irradiation of Nd-TiO2 under UV–Vis light. A series
of photocatalysts modified by neodymium has shown that the
introduction of Nd3+ increases the intensity of the bands compared
to that of an unmodified sample. This indicates that Nd3+ effec-
respectively. These results confirm the crucial role of eÀ and O2Å
in the photocatalytic degradation of phenol under Vis irradiation.
Results of HPLC analysis showed (Table 3) that the main interme-
diate product detected after 60 min of irradiation under visible
light was hydroquinone. Other identified intermediate products
included were catechol and benzoquinone. Based on the most
active sample (0.1%Nd-TiO2), it was observed that the concentra-
tion of products in the aqueous phase increased through 60 min
of irradiation (Fig. 10b). The degradation is slow with no selectivity
among the organic compounds, which are gradually decomposed
over time with exposure to Vis irradiation which means that differ-
ent by-products were produced and degraded at the same time.
Determination of Total Organic Carbon showed the beginning of
mineralization but at a very low level (results not shown).
Å
tively leads to an increase in the production of OH radicals on
the surface of the photocatalysts under UV–Vis light irradiation.
The obtained results show a correlation between the effectiveness
of the generated hydroxyl radicals exhibited and photocatalytic
activity under UV–Vis light. The intensity of the fluorescence spec-
tra varied as 0.1 < 0.25 < 1.0 < 0.5 mol% Nd3+, which responds to
photocatalytic activity. Published literature demonstrates that
degradation of pollutants in the UV/TiO2 system occurs by oxida-
tion of the chemical compounds by hydroxyl radicals [50]. Hydro-
xyl radicals attack the phenyl ring, leading to catechol and
hydroquinone as new intermediate products of degradation. Then,
the phenyl rings in these compounds are broken to give short-
chain organic compounds, such as formic acid, acetic acid and
finally carbon dioxide [51]. Fig. 9b shows the fluorescence spectra
intensity change observed after 60 min of irradiation of Nd- mod-
ified TiO2 under Vis light. Fluorescence spectra of pristine TiO2 and
Nd-TiO2 exhibited nearly the same intensity and this intensity is
very low compared with literature data [10]. The intensity of the
bands was much lower (approximately 20 times) than under
UV–Vis irradiation. This suggests that Nd-TiO2 modification does
4. Discussion of the mechanism
The up-conversion process involves excitation of a material
with lower energy photons, which stimulates emission of higher
energy photons [53]. According to energy-level diagrams
(Fig. 11a) showing the neodymium excitation under visible irradi-
ation, the up-conversion process can be achieved through a series
of processes such as ground state absorption (GSA) and excited
state absorption (ESA) [28,54]. To explain the possible mechanism
of neodymium-modified titania photocatalyst excitation under vis-
ible light, the photodegradation of phenol was investigated as a
function of irradiation wavelength. Based on quantum efficiency
measurements as a function of the excitation wavelength (action
spectra), it is possible to determine which aspect of light absorbed
by the photocatalyst is involved in photocatalytic reactions. The
Å
not lead to an increase in the production of OH radicals under
Vis irradiation. Most likely, the photodegradation process occurs
as a direct transfer of charge carriers from the photocatalyst to
the contaminant and subsequent radical reactions [51,52]. The
obtained results show that the photocatalytic activity under Vis
Fig. 9. Fluorescence spectral changes in solution of terephthalic acid under (a) UV–Vis (k > 350 nm) and (b) Vis (k > 420 nm) light irradiation.

220
P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
Fig. 10. (a) Photocatalytic decomposition of phenol in the presence of 0.1%Nd-TiO2 and scavenger after 60 min of visible (k > 420 nm) irradiation. (b) Efficiency of phenol
degradation and intermediate product formation under visible light (k > 420 nm) in the presence of 0.1%Nd-TiO2.
Fig. 11. (a) Simplified energy-level diagrams and excitation path for the up-conversion of emission at ultraviolet light Nd3+ under visible light excitation (k = 578 nm); GSA,
ground state absorption; ESA, excited state absorption. (b) Action spectrum of phenol oxidation on neodymium-modified TiO2 (Apparent quantum efficiency – squares and
circles) and absorption spectrum (K-M function – lines).
´
results for the sample 0.25% Nd-TiO2 and pristine TiO2 are shown
in Fig. 11b. The sample modified with 0.25 mol% was selected for
the analysis because it show the most intense emission band in
the visible light region (Fig. 7b). Action spectra of samples did
not resemble the respective absorption spectra (K-M function).
Photocatalytic activity under visible light is from irradiation rang-
ing from 400 to 480 nm. There is no activity observed for wave-
lengths (k = 525, 585, 745, and 805 nm) characteristic of the
presence of neodymium. These wavelengths should be responsible
for the up-conversion process. Based on the obtained measure-
ments (luminescence spectra and action spectra), it can be con-
cluded that the conversion process of visible to ultraviolet
irradiation does not have a significant effect on the photocatalytic
activity of the photocatalysts obtained. The up-conversion process
is thus likely not responsible for the degradation of phenol under
Vis irradiation. This may be due to the distance between the
ground and excited states of neodymium ions and affected by
properties of crystal structure and multiphonon relaxation pro-
cesses [46,47]. Different results have been obtained by
Castañeda-Contreras et al. [22], who reported that the process of
transforming visible to ultraviolet light by erbium ions is responsi-
ble for methylene blue photodegradation under 532 nm laser beam
light. Reszczynska et al. suggested that differences in the results
obtained (no up-conversion process of RE3+-TiO2) could be related
to the use of different radiation power sources power: xenon lamp
and laser [28,29].
Based on the obtained results, it could be suggested that neody-
mium ions can form complexes between the f-orbital of Nd with
oxygen and are also present as Nd3+ clusters and in the form of oxi-
des on the TiO2 surface. The concentration of the organic pollutant
at the photocatalyst could provide a mechanism for the enhanced
degradation by Nd-modified TiO2. Moreover, Nd-modified TiO2
may result in the formation of oxygen vacancies and surface
defects. This process can greatly suppress the recombination and
promotion of the separation of electrons and holes pairs, contribut-
ing to the enhanced photooxidation ability of holes, while the elec-
trons are easier to react with oxygen molecules to generate
another reactive oxygen species for degradation of the pollutant.
5. Conclusions
A hydrothermal method was used for the preparation of Nd-
modified TiO2 nanoparticles. SEM-EDX analysis revealed the pres-
ence and uniform distribution of neodymium in the synthesized

P. Parnicka et al. / Journal of Catalysis 353 (2017) 211–222
221
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Acknowledgments
Santamaría,
Microwave-assisted
mild-temperature
preparation
of
neodymium-doped titania for the improved photodegradation of water
contaminants, Appl. Catal. A Gen. 441–442 (2012) 47–53, http://dx.doi.org/
10.1016/j.apcata.2012.07.003.
This research was financially supported by Polish National
Science Centre (Grant No. NCN 2015/17/D/ST5/01331) and sup-
ported by the Foundation for Polish Science (FNP).
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
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